Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratification, and circulation patterns

PALAEO-08325; No of Pages 18 Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

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Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratification, and circulation patterns Andrew Roark a,⁎, Ryan Flake a,1, Ethan L. Grossman a, Thomas Olszewski a, Joseph Lebold b, Debbie Thomas c, Franco Marcantonio a, Brent Miller a, Anne Raymond a, Thomas Yancey a a b c

Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, United States Department of Geology and Geography, West Virginia University, 330 Brooks Hall, 98 Beechurst Av., Morgantown, WV 26506, United States Department of Oceanography, Texas A&M University, College Station, TX, 77843, United States

a r t i c l e

i n f o

Article history: Received 11 March 2017 Received in revised form 10 June 2017 Accepted 11 June 2017 Available online xxxx Keywords: Oxygen isotopes Mg/Ca thermometer Carbon isotopes Superestuarine circulation Epicontinental sea Upwelling

a b s t r a c t Stable isotopic analyses of N 100 well-preserved Carboniferous brachiopod shells, representing two time slices from across North America, reveal systematic regional changes in environmental conditions. For both of the time slices studied, the Chesterian (latest Mississippian) and the Virgilian (latest Pennsylvanian), δ18O and δ13C show a stepwise decrease moving across the continent from modern-day west to east. Higher δ18O in the west reflects greater water depths, less freshwater input, and more direct upwelling influence from cool, openmarine waters than in the east. Modelling based on these regional isotopic data indicates salinity reductions on the order of ~1–4 psu in the Appalachian Basin relative to the Midcontinent Basin for the Virgilian time interval over a range of reasonable thermal gradient and δ18Ofw assumptions. The magnitude of this gradient is similar to those of modern-day Hudson Bay and the Panama Bight, which exhibit permanent haloclines driven by freshwater discharge. Considering existing paleoecological evidence for freshening, the lack of more dramatic benthic freshening suggests that the depth of a regional halocline was shallower than the depths inhabited by thickshelled stenotopic brachiopods, although other lines of evidence are necessary to verify the strength of the halocline. Estimated δ18O temperatures for Appalachian Basin brachiopods are ~3–5 °C cooler than those of coeval conodonts, due to either slightly warmer, fresher surface waters or uncertainty in the phosphate 18O paleothermometer. Despite discrepancies in calculated temperatures, the magnitude of the regional isotopic gradient derived from brachiopod and conodont proxies is similar after making appropriate glacioisotopic and water depth corrections. A decreasing δ13C trend toward the east (southern paleolatitude) likely reflects greater restriction and terrestrial influence near the sea's tropical shoreline. Trace element trends are less consistent and are affected by species and metabolism. Erratic variations in Mg/Ca concentrations, even among pristine samples of the same genera with similar δ18O values, and unrealistic Mg/Ca-derived temperatures justify continued caution in applying existing Mg/Ca thermometer equations to Paleozoic brachiopods. © 2017 Published by Elsevier B.V.

1. Introduction Regional salinity gradients in restricted marine settings exert a dominant influence on a variety of geologically important processes. These contrasts in salinity, both vertical and lateral, work alongside other key boundary conditions to control ecology and biological diversity, sedimentation, and organic matter production and preservation in

⁎ Corresponding author at: Chevron North America Exploration and Production Company, 1400 Smith St., Houston, TX 77002, United States. E-mail address: [email protected] (A. Roark). 1 Currently at ExxonMobil Production Company, 22777 Springwoods Village Parkway, Spring, TX, 77389, United States.

modern-day estuaries and continental seas (e.g. Tyson and Pearson, 1991; Robertson et al., 1993; Eyre and Balls, 1999; Hopkinson et al., 1999; Zettler et al., 2007; Algeo et al., 2008). A similar gradient may have helped control benthic redox conditions in the North American continental sea during the Pennsylvanian, permitting deposition of extensive black, dysoxic shale facies despite relatively shallow water conditions (Algeo et al., 2008; Algeo and Heckel, 2008). So-called “estuarine-type” circulation models have long figured into geological characterizations of the sea (cf. Heckel, 1977, 1991; Hatch and Leventhal, 1992; Olszewski and Patzkowsky, 2008) but are not universally accepted (e.g. Tyson and Pearson, 1991). Furthermore, interpretations of recently-collected conodont δ18O data have challenged whether significant regional salinity differences across the sea persisted into the Late

http://dx.doi.org/10.1016/j.palaeo.2017.06.009 0031-0182/© 2017 Published by Elsevier B.V.

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

Pennsylvanian (Joachimski and Lambert, 2015). Here, we present a compilation of new and previously published geochemical analyses of Virgilian (latest Pennsylvanian) brachiopod shells from across the North American continent, as well as data from the Chesterian (latest Mississippian) for comparison, to document regional changes and evaluate evidence for geochemical partitioning across the sea at benthic depths. Naming conventions for the North American sea have varied in published literature. To avoid confusion, we will refer to the North American continental sea during the Virgilian time slice as the Late Pennsylvanian Midcontinent Sea (LPMS) after Algeo and Heckel (2008). When referring to the Chesterian time interval, or speaking more generally about the Carboniferous Period as a whole, we use the terms “Carboniferous sea” or “continental sea.” 2. Geological background At its climax in the late Pennsylvanian, the North American Carboniferous sea stretched from the tropics, bounded to the south by the equator-parallel Alleghenian uplift belt, north and west into subtropical

latitudes (15°–20° N) to the edge of the craton, where the sea interfaced with the open Panthalassic Ocean (Fig. 1; Heckel, 1977; Algeo et al., 2008). Along its equatorial margin, the continental sea would have received a massive influx of freshwater, mostly as extensive runoff from the tropical mountain belt. This influx would have tapered to the north in the subtropical portion of the sea, which would have also been more proximal to ocean upwelling (Algeo and Heckel, 2008; Algeo et al., 2008). Heckel (1977) crystallized the latitudinal interplay between warmer continental and cooler oceanic water masses into the conceptual “quasiestuarine circulation” model for benthic dysoxia in the LPMS. Presumably, warmer, possibly less saline surface waters formed a lens over cooler marine waters advected from the open ocean. Resulting density stratification prevented the water column from overturning, thereby perpetuating low-oxygen conditions during highstands and leading to the deposition of black shales (Heckel, 1977). Algeo et al. (2008) and Algeo and Heckel (2008) reformulated this hypothesis as the “superestuarine circulation” model (Fig. 2), which more strongly emphasizes the role of salinity-driven stratification. The “superestuarine” model proposes that tropical freshwater influx from the continental

Fig. 1. Paleogeography of the North American continental sea during the late Pennsylvanian, showing sampling locations and average brachiopod δ18O values for the Virgilian time slice. Inset shows locations and names of the Appalachian Basin sampling sites. Asterisk indicates anomalous samples at FAIR locality collected by R. Flake. OH = Ohio; WV = West Virginia; PA = Pennsylvania. Paleogeography modified from Algeo and Heckel (2008) and Blakey (2011).

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

3

A

X 0 Halocline Thermocline

?

?

?

?

KS

MO

OH

Illinois Basin Midcontinent Basin Shumway Limestone Benthic Dysoxia No brachiopods

300

Midland Basin Palo Duro Basin

TX

WV

IL

OK

200

X’

PA IN

100

Depth (m)

rom

F al tori lt a u q E is t B e Mo

re istu Mo

Plattsmouth Limestone Mattoon Formation Oread Formation

Appalachian Basin Ames Marine Member Glenshaw Formation

Anadarko Basin 200 km

400

B

C 0

Depth (m)

100

X X’ ’

X

X’’

Midland Basin

Eastern Shelf Finis Shale

200

100 km

X’

300

400

Fig. 2. The “superestuarine” circulation model and paleobathymetry of sampling locations (modified in part from Algeo et al., 2008). A) Schematic continental transect through the LPMS. The “superestuarine” model proposes that high levels of tropical freshwater discharge into the sea spread to the north and west, forming a halocline that helped prevent water column overturning. Upwelling would have brought colder, poorly oxygenated water onto the midcontinent shelf, sustaining benthic anoxia despite relatively shallow water depths (Algeo and Heckel, 2008; Algeo et al., 2008). Labels indicate formation names for the sampled areas along the Virgilian transect. B) Index map. C) Transect showing Texas Eastern Shelf sampling region.

sea's southern coast spread out above the cooler, more saline marine water body, creating a stratified water column and helping maintain benthic dysoxia (Algeo et al., 2008; Algeo and Heckel, 2008). Several existing datasets support the “superestuarine” model's assertion that tropical freshwater entering the sea functioned as a dominant control over water column oxygenation levels. These lines of evidence include (1) increases in TOC and redox-sensitive trace elements in North American Carboniferous shales near inferred sources of freshwater runoff (e.g. Coveney et al., 1987; Algeo et al., 1997; Hoffman et al., 1998); (2) the presence of terrestrial organic matter in these shales (Coveney et al., 1987; Hoffman et al., 1998); (3) conodont distribution patterns suggesting heightened dysoxia near the paleoshoreline (Herrmann et al., 2015); and (4) a pronounced gradient in εNd, a proxy for continental weathering, in well-preserved conodonts from across the Carboniferous sea (Woodard et al., 2013). Oxygen isotope compositions of well-preserved fossil material can provide even more information. Oxygen isotopic data from inorganic fossil remains can identify paleoenvironmental freshening trends. Due to kinetic effects during evaporation and precipitation, freshwater δ18O is generally lower than that of marine water (e.g. Craig, 1961; Bowen and Wilkinson, 2002). Partially restricted marine bodies with high levels of freshwater input will thus have lower δ18Owater (e.g. Frölich et al., 1988), a change reflected in δ18O of contemporaneous biogenic carbonate and phosphate. Consequently, after controlling for temperature, which correlates inversely with δ18O, oxygen isotopic data can potentially quantify the degree of freshening (cf. Tao et al., 2013). Joachimski and Lambert (2015) applied this approach to deposits from the North American

Carboniferous sea, measuring δ18O of conodont apatite along a transect from the Appalachian Basin to the Midcontinent Basin for several time slices. Joachimski and Lambert (2015) found evidence for reduced salinity (i.e. decreases in δ18O in excess of predicted changes from temperature alone) in the Appalachian Basin relative to the Midcontinent Basin for most of these time slices. This result is consistent with stronger freshwater influence along the sea's Appalachian margin than in more offshore portions of the sea. However, according to Joachimski and Lambert (2015), this gradient was considerably reduced by the Virgilian (latest Pennsylvanian), suggesting that freshwater discharge into the sea was insufficient to maintain a halocline during this time interval. Here we present a compilation of new and previously published isotopic and trace element analyses of well-preserved brachiopod shells from across the LPMS during the anomalous Virgilian time slice. Calcite brachiopod shells generally precipitate in oxygen isotopic equilibrium with ambient seawater (Lowenstam, 1961; Parkinson et al., 2005; Brand et al., 2013, 2015) with some important caveats. Auclair et al. (2003), Yamamoto et al. (2010a, 2010b, 2011) and Cusack and PérezHuerta (2012) observed disequilibrium fractionation (vital effect) of 18 O in the outermost portion (mature as opposed to juvenile ontogenetic stage) of some modern brachiopods. However, these disequilibrium effects are nearly absent in non-punctate species like the ones used in the present study (Cusack and Pérez-Huerta, 2012). To investigate whether growth rates influenced shell δ18O of Carboniferous brachiopods, Roark et al. (2016) conducted high-resolution (sub-mm scale) sclerochronologies of Neospirifer brachiopods from the Appalachian Basin; data from these specimens are included in this study. Roark et al. found no evidence for ontogenetically-driven variations in shell

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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δ18O (mature and juvenile portions of the shells showed nearly identical δ18O values). Moreover, different brachiopod species that co-occur at the same sampling sites have nearly identical δ18O values (see discussion on oxygen isotope data below). It is highly unlikely that different species with vastly different sizes and shell structures would show such similar values if isotopic fractionation was affected by vital effects. Consequently, oxygen isotopic data from Pennsylvanian brachiopods like those sampled in this study provide a proxy for seawater temperature and δ18O at benthic depths and are a useful comparison to data from pelagic conodonts. Using carbonate shell material, as opposed to phosphate, permits the collection of carbon isotopic data, potentially providing further insight into environmental conditions. δ13C values of some Pennsylvanian and modern brachiopod shells likely do exhibit vital effects as indicated by interspecies differences that are maintained between regions (e.g., Grossman et al., 1993; Garbelli et al., 2014). Nevertheless, the high and consistent values seen in brachiopod shells and systematic regional trends argue for a dominantly environmental control on δ13C. In this study we sampled numerous localities within the Appalachian Basin, including from the extreme southeastern portion of the sea (West Virginia [WV], Pennsylvania [PA]), supplementing Joachimski and Lambert's (2015) Virgilian transect. Additionally, we present brachiopod data from a similar transect through the Chesterian (latest Mississippian) sea. For that time interval, we include data for the western margin of the sea (Arrow Canyon, Nevada), providing a record of environmental change beyond the midcontinent. 3. Samples and geologic setting For the Chesterian time slice, we collected samples from the Grove Church Formation in the Illinois Basin and Bird Spring Formation in Nevada (Fig. 3, Table 1). The Data Supplement includes stratigraphic columns and illustrations of the sampled horizons. During the late Mississippian, the Illinois Basin represented a relatively isolated tongue of marine water stretching into the North American continental interior, while the Bird Spring shelf, situated on the craton's northwestern coastline, interacted more directly with the open ocean (Poole and Sandberg, 1991; Blakey, 2011; Fig. 4). The Grove Church Formation, which records the final Mississippian marine incursion into the midcontinent, consists of interbedded fossiliferous shale and argillaceous limestone immediately beneath the Mississippian-Pennsylvanian unconformity (Norby, 1990; Treworgy, 1990). We

collected samples from a ~ 1.5 m interval, equivalent to Unit 4 of Weibel and Norby (1992), near the top of the Grove Church Shale in Johnson County, IL. The stratigraphic section exposed in Arrow Canyon, Nevada, defines the Global Stratotype Section and Point (GSSP) for the Mid-Carboniferous Boundary, containing a nearly complete section through the Mississippian-Pennsylvanian transition (Lane et al., 1990). Carbonate-rich shelf deposits dominate the late Mississippian portion of the section (e.g. Bishop et al., 2010). Jones et al. (2003) and Grossman et al. (2008) originally reported data for well-preserved brachiopods from argillaceous limestones over a ~1 m interval immediately below the Mississippian-Pennsylvanian boundary, within the BSb unit of the Bird Spring Formation (after Langenheim et al., 1962). This interval should be nearly time-equivalent to the upper Grove Church section; however, since the base of the Pennsylvanian section in the Illinois Basin is unconformable, it is possible that the Grove Church samples are slightly older than the samples from Arrow Canyon. For the Virgilian time interval, we collected samples from across North America, moving west from the Appalachian Basin through the Illinois Basin and the Eastern Shelf of the Midland Basin to the Midcontinent Basin (Fig. 1; Table 1). Unfortunately, we were unable to find pristine shells from the Virgilian section at Arrow Canyon. Cyclic changes in ice volume governed the extent of the LPMS, producing repetitive lithologic successions (“cyclothems”) in the sedimentary record (e.g. Wanless and Shepard, 1936; Busch and Rollins, 1984; Heckel, 1986, 2002). High-resolution conodont biostratigraphy permits the correlation of these packages across the continent (e.g. Heckel, 1994, 2013). Samples for this study come from the early Virgilian Oread cycle of Kansas and its equivalents (Figs. 1, 3). In the Midcontinent Basin, the Oread cycle consists of, in ascending order, the Leavenworth Limestone, Heebner Shale, Plattsmouth Limestone, and the Heumader Shale, each facies recording a successive eustatic stage as sea level rose and fell (Yang, 2003). The black Heebner Shale is generally assumed to represent maximum highstand conditions (cf. Heckel, 1977), although this interpretation is not universally accepted (e.g. Olszewski and Patzkowsky, 2003). Here we use data for well-preserved brachiopods from the Plattsmouth Limestone originally reported by Grossman et al. (1993). In the Illinois Basin, the Shumway cycle in the Mattoon Formation is chronostratigraphically equivalent to the Oread cyclothem (Heckel, 1994). Similar to the Oread, the Shumway cycle consists of a succession of facies reflecting changes in eustatic sea level. We collected samples

Fig. 3. Stratigraphic position of the Mississippian and Pennsylvanian units sampled; time scale divisions from Gradstein et al. (2012).

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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Table 1 Sampling localities. Locality

Locality description

County

State

Latitude

Longitude

Sourcea

Chesterian MBI CAN

Millstone Bluff locality, western Pope County, Illinois Arrow Canyon

Pope Clark

IL NV

37.482717 36.733300

−88.68825 −114.777800

F J

Monroeville, PA; more specific location not given n/a n/a Belle Valley PA and WV RR cut near mouth of Cross Creek New Concord Near Fairmont, WV Along Green Bag Road, Morgantown, WV I70 roadcut, 1 mile east of Middlebourne, Oxford Township West side of rt. 77 approx. 0.5 mi south of 821 exit SE of Belle Valley, SW1/4 SE1/4 sec 20 Noble township Tony Fasekas Farm, 1.5 mi south of New Concord, Union Township Along Shoal Creek, northern Effiingham County, IL Lake Jacksboro Spillway Northeast Coffee County, KS

Allegheny Athens Allegheny Noble Brooke Guernsey Marion Monogalia Guernsey Noble

PA OH PA OH WV OH WV WV OH OH

40.421181b 39.317219 40.571936 39.780917 40.309850b 39.982814 40.383517 39.197667 40.054075b 39.780917b

−79.788103b −82.101883 −79.790094 −81.553303 −80.599458b −81.693358 −80.6279 −79.933267 −81.320381b −81.553303b

CMP WVU WVU WVU CMP WVU FC, F, WVU FC, F, WVU OSU OSU

Guernsey Effingham Jack Coffee

OH IL TX KS

39.975977b 39.186900 33.237039 38.423000

−81.727061b −88.615383 −98.143791 −95.577000

OSU F G G

Virgilian A01 AMA AMP BELL BRK CONC FAIR GRR MID Nn-3 TFF SHCK FJTX COFF

a Sample source: FC = collected in the field by A. Roark; F = collected by R. Flake; CMP = from collections housed at the Carnegie Museum, Pittsburgh; WVU = from collection housed at West Virginia University; OSU = collection housed at Ohio State University; G = collected and originally reported by Grossman et al. (1991, 1993, 1996); J = collected and originally reported by Jones et al. (2003). b Coordinates inferred from locality description.

from the fossiliferous Shumway Limestone Member in its type area of Effington County, Illinois (cf. Scheihing and Langenheim, 1978). As with the Plattsmouth Limestone in Kansas, the Shumway Limestone is a marine limestone resting stratigraphically above the black and gray shale unit traditionally associated with the maximum highstand (e.g. Scheihing and Langenheim, 1978; Weibel, 1996; cf. Heckel, 1977). On the Eastern Shelf of the Midland Basin in north-central Texas, the Finis Shale is the lateral equivalent of the Oread Cyclothem (Boardman and Heckel, 1989). Isotopic data for brachiopod shells from the transgressive and regressive fusulinid and ammonoid depth zones of the Finis Shale are reported in Grossman et al. (1991). Well-preserved brachiopods are absent from the section in the deeper-water, conodontrich depth zone, which corresponds to the highstand (Grossman et al.,

1991). The Data Supplement includes illustrations of the stratigraphic columns and horizons sampled for the Midcontinent Basin, Illinois Basin, and Eastern Shelf localities. The Ames marine unit of the Glenshaw Formation comprises the Oread-equivalent section in the Appalachian Basin (Heckel, 1994). Unlike the central midcontinent, where Pennsylvanian strata were deposited on a dominantly marine, very low-relief platform (Olszewski and Patzkowsky, 2003), the Ames Member records an incursion of marine waters into a dominantly fluvial-deltaic basin (e.g. Al-Qayim, 1983; Fahrer, 1996). Regionally, the Ames thickens from a ~1 m, dominantly limestone section in southwest Ohio to a ~2–3 m-thick section of silty shale and argillaceous limestone in West Virginia and Pennsylvania, reflecting increased clastic influx and perhaps bathymetric deepening

Fig. 4. Paleogeography, sampling locations, and average brachiopod δ18O values for the Chesterian time interval. Paleogeography modified from Blakey (2011).

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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approaching the Alleghenian Uplift front (e.g. Al-Qayim, 1983; Saltsman, 1986; Lebold and Kammer, 2006). Locally, the Ames is even more variable, with outcrop sections occasionally more than doubling in thickness over relatively short (b10 km) distances (Al-Qayim, 1983). To capture some of the regional paleoenvironmental variability, we sampled the Ames from multiple localities across the basin, and from multiple horizons at two sites (Figs. 1, 5). We also utilized existing collections of specimens at the Carnegie Museum in Pittsburgh, PA, and at The Ohio State University in Columbus (Table 1). Roark et al. (2016) published a subset of the Appalachian Basin data. 4. Sample preparation and analytical methods 4.1. Sample screening and trace element analysis Initially, we visually examined shells for textural preservation and discarded shells if they showed noticeable Fe-oxide discoloration, pitting, or dissolution. Samples that passed this initial screening were rinsed and scrubbed in deionized water, dried in a glass desiccator overnight, and embedded in epoxy. Subsequently, we sectioned these specimens longitudinally, creating two billets per sample. We polished one billet using sequentially finer grits down to 0.3 μm alpha alumina to make the shell microtexture more visible; we then rinsed this polished billet in deionized water and reserved it for isotopic sampling. We mounted the second, unpolished billet onto a glass slide and cut the sample as a thin section, polishing the exposed thin section surface to 0.3 μm alpha alumina. We subjected specimen thin sections to an intensive three-part procedure to detect diagenetic alteration (cf. Grossman et al., 1991, 1996; Roark et al., 2016). Previously published samples (Grossman et al., 1991, 1993, 1996, 2008; Jones et al., 2003) underwent a similar set of screening procedures. Under a standard petrographic microscope, we examined and excluded samples showing disruptions of the primary crystal fabric, such as secondary mineral phases. Secondly, we examined specimens using cathodoluminescence microscopy (CL) with a Technosyn 8200 MKII cold cathode luminoscope. Images used a camera exposure time of 60 s, with beam conditions maintained at 200–300 nA and 10–15 kV. Orange luminescence in calcite results from lattice

defects that occur when Mn2+, a tracer for nonmarine diagenetic fluids, substitutes for Ca2 + (Machel et al., 1991). Iron can quench luminescence, thus shells must also be screened for high iron contents. Luminescence in fossil brachiopod shells, combined with selective screening for iron, has been consistently demonstrated to be a good proxy diagenetic alteration (e.g. Popp et al., 1986; Grossman et al., 1996; Samtleben et al., 2001). We culled luminescent samples. Thirdly, we conducted quantitative trace element analysis to screen for a panel of diagenetic indicators in select samples. For one sample subset, we analyzed thin sections for Ca, Mg, Mn, Fe, Sr, Na, S, and Al via wavelength dispersive spectroscopy on a Cameca SX50 electron microprobe. Analyses used standard beam conditions of 15 kV and a beam diameter of 20 μm. A second subset of samples was analyzed for Mg, Sr, Mn, and Fe using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). About 100 μg of shell powder were generated for each specimen using a handheld dental drill with a 0.5 mm burr. We reacted this powder with 2 mL of 2% HNO3 for 30 min prior to analysis on a Thermo-Scientific HR-ICP mass spectrometer. During diagenetic alteration of carbonates under burial and subsequently reducing conditions, Mn and Fe concentrations systematically increase and Na and S concentrations decrease (Brand and Veizer, 1980). Well-preserved brachiopod shells have concentrations of these elements falling within relatively narrow, predictable ranges (Grossman et al., 1996), whereas alteration typically leads to anomalous concentrations. Al and Si are tracers for precipitation of secondary silicate mineral phases, and concentrations exceeding trace levels (i.e. N1 mmol/mol Ca) could indicate alteration. We typically conducted 3– 5 microprobe spot analyses for each specimen. Roark et al. (2016) conducted much more detailed trace element sampling on some of the samples in order to elucidate seasonal variability. Previously published trace element datasets (Grossman et al., 1993, 1996) also utilized the electron microprobe following similar procedures. We nominally used the limits of the trace element ranges that Grossman et al. (1996) reported for unaltered brachiopods (Fe/Ca and Mn/Ca b 0.7 mmol/mol; S/Ca 4–12 mmol/mol; Na/Ca 8–26 mmol/mol) as screening criteria. However, no samples that passed cathodoluminescence and petrographic screening fell outside of these predicted ranges.

Fig. 5. Isotopic values for specimens from different horizons at two Appalachian Basin sampling sites, FAIR and GRR (see Fig. 1 for index map). “NL” signifies well-preserved specimens (“nonluminescent”), while “CL” indicates poorly-preserved (“cathodoluminescent”) specimens. Note that δ18O increases to the left.

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

4.2. Isotopic sampling methods For one subset of samples, we collected powders for isotopic analysis using a hand-operated dental drill with a 0.5 mm bur (Flake, 2011). We heated about 100–200 μg of powder overnight at 70 °C to remove moisture and then reacted the powder at 70 °C with phosphoric acid (specific gravity ≈ 1.925 g/cm3). The resultant CO2 was analyzed on a GasBench II coupled to a ThermoFinnigan Delta Plus XL isotope ratio mass spectrometer (IRMS). Average analytical precisions were ±0.1‰ (1σ) for δ18O and δ13C; analyses were calibrated using the NBS-19 standard (δ13C = +1.95‰ VPDB and δ18O = −2.20‰ VPDB). A second set of samples relied on two different isotopic sampling strategies. First, we serially microsampled large, well-preserved Neospirifer specimens along the axis of growth to resolve seasonal variability using a New Wave micromill. Roark et al. (2016) reported detailed procedures for these analyses and resultant data. Secondly, we manually sampled smaller-shelled Crurithyris and Composita specimens, as well as matrix and cement samples, under a binocular microscope using a dental pick whose tip had been filed down to a fine point. We weighed about 60 μg of powder for each sample into glass reaction vials, dried the powders overnight in an oven at 70 °C, and then reacted the powders in phosphoric acid (specific gravity ≈ 1.92) at 75 °C on a Kiel IV carbonate device coupled to a ThermoFinnigan MAT 253 IRMS. Average analytical precisions were ± 0.03‰ (1σ) for δ13C and ± 0.07‰ (1σ) for δ18O; we used the NBS-19 standard to calibrate these analyses. We report brachiopod isotopic data relative to the Vienna Peedee Belemnite (VPDB) standard. We will also discuss published δ18O values for conodonts and marine waters, which are reported relative to Vienna Standard Mean Ocean Water (VSMOW).

5. Results and discussion 5.1. Oxygen isotope data 5.1.1. Virgilian Virgilian δ18O data show a systematic, stepwise decrease moving across the continent from Kansas (−1.4 ± 0.3‰ VPDB mean δ18O) to Illinois (−2.2‰ VPDB, 1 sample) then to Texas (−2.4 ± 0.3‰ VPDB) and finally to West Virginia (−3.7 ± 0.7‰ VPDB; Table 2; Figs. 1, 6). On initial observation, this trend appears to support the case for significant increasing freshwater influence in the midcontinent sea approaching the southern margin of the LPMS, a major implication of the superestuarine hypothesis of Algeo et al. (2008). However, in order to directly compare the Appalachian and Midcontinent samples, we must first correct for temperature and ice volume (“glacioisotopic”) effects. Brachiopods from the Midcontinent region, where dysoxic conditions eliminated benthic faunas during interglacials, come from limestones and gray shales formed during sea level regression (Scheihing and Langenheim, 1978; Weibel, 1996; Yang, 2003). Meanwhile, Appalachian samples were deposited during near-highstand conditions, the only time when near-marine salinities developed in the Appalachian Basin (Donahue and Rollins, 1974; Brezinski, 1983; Lebold and Kammer, 2006). Decreasing global ice volume during glacioeustatic highstands lowers global δ18Osw; combined with concomitant climatic warming, these changes result in lower recorded shell δ18O for highstand specimens (e.g. Emiliani, 1955; Shackleton and Opdyke, 1973). Fortunately, published isotopic data make it possible to correct for these ice volume and temperature differences. Joachimski et al. (2006) measured the glacial-interglacial change in recorded δ18O during the Oread Cyclothem (Virgilian samples in this study come from the Oread cycle and its lateral equivalents) by sampling conodonts from multiple horizons. Between the regressive limestone and highstand shale facies, δ18O decreased by 1.1‰. Consequently, we apply a glacioisotopic correction of −1.1‰ to our Midcontinent samples to estimate the “highstand-equivalent” environmental gradient across the continent.

7

Table 2 Average isotopic data by region and taxona. Age/locality/genera Chesterian Illinois Basin Illinois Anthracospirifer Inflatia Antler Foreland Basin Nevadab Anthracospirifer Composita Productid Virgilian Appalachian Basin Pennsylvania Crurithyris West Virginia Neospirifer Crurithyris Composita Ohio Neospirifer Crurithyris Illinois Basin Illinois Crurithyris Eastern Shelf Texasc Neospirifer Crurithyris Composita Midcontinent Basin Kansasd Neospirifer Crurithyris Composita

δ13C (‰)

SD

δ18O (‰)

SD

# specimens

# analyses

+1.2 +1.6 +0.6

0.8 (1.2) n/a

−2.8 −2.5 −3.4

0.5 (0.4) n/a

3 2 1

9 6 3

+3.6 +3.8 +3.2 +2.8

0.5 0.2 (1.3) n/a

−1.6 −1.7 −1.6 −0.7

0.5 0.5 (0.9) n/a

9 15 2 1

24 15 5 4

+1.9

1.1

−3.7

0.6

57

462

+2.2 +1.7 +2.5 +1.3 +1.8 +2.5 +2.7 +2.0

0.8 1.2 0.4 1.3 0.6 0.5 0.3 0.5

−3.8 −3.7 −3.7 −3.7 −3.7 −3.8 −3.9 −3.7

0.6 0.7 0.2 0.9 0.1 0.2 0.2 0.1

5 39 12 24 3 13 9 4

10 192 134 51 7 260 250 10

+3.2

n/a

−2.2

n/a

1

2

+4.2 +4.1 +4.0 +5.8

0.5 n/a 0.1 n/a

−2.4 −2.3 −2.5 −1.9

0.3 n/a 0.3 n/a

11 1 9 1

21 2 17 2

+4.4 +3.9 +4.5 +4.8

0.6 0.5 (1.8) 0.4

−1.4 −1.6 −1.4 −1.4

0.3 0.3 (0.3) 0.4

23 8 2 13

23 8 2 13

SD = standard deviation. a For localities with only two samples, the difference between the two samples is shown in parentheses instead of the standard deviation. Standard deviation is not reported for localities with only one sample. b All Nevada data originally reported by Jones et al. (2003) and Grossman et al. (2008). c All Texas data originally reported by Grossman et al. (1991). d All Kansas data originally reported by Grossman et al. (1993).

Even after correcting for glacioisotopic effects, δ18O for our Appalachian Ames samples is still significantly lower than for Midcontinent specimens. Average corrected δ18O for specimens from West Virginia is − 2.6‰ VPDB, 1.2‰ less than the average of − 1.4‰ VPDB average of our Kansas specimens. This remaining difference must result from a combination of 1) freshening influence in the Appalachian Basin and 2) cooling in the Midcontinent Basin due to greater water depths and upwelling. To determine a realistic estimate of freshening in the LPMS, we modeled the salinity in the Appalachian Basin based on our observed isotopic trends across a range of water depth and thermal gradient scenarios. It is possible to calculate salinity from an observed difference in measured δ18O using the mass balance equation,
  Sfw −Ssw  Sm ¼ Ssw þ ΔS ¼ Ssw þ δ18 Obr;m −δ18 Obr;sw  δ18 Ofw −δ18 Osw

ð1Þ

where S represents salinity, sw refers to values for open ocean water (no freshwater influence), fw refers to values for freshwater entering the sea, m refers to measured values, and br refers to brachiopod values. For our base case, we assume δ18Ofw = − 8‰ VSMOW, analogous to modern rainfall values along the mountainous tropical coastline of Papua New Guinea (Rozanski et al., 1993) and freshwater discharge into the Pacific Ocean off the coast of Panama (Tao et al., 2013).

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

Fig. 6. Regional oxygen isotope trends for conodonts (Joachimski and Lambert, 2015) and brachiopods (this study) during the Virgilian Oread interval. We apply a − 1.1‰ correction for “glacioisotopic” effects (temperature and ice volume changes related to glacioeustasy) to our Midcontinent Basin samples (sea level regression) in order identify environmental trends relative to our Appalachian Basin (highstand) samples.

Determining the isotopic contribution from water column depth and circulation is less straightforward. Precise depths across the LPMS are uncertain, although more recent estimates suggest values on the order of 10 s of meters (Algeo and Heckel, 2008) as opposed to 100 s of meters (cf. Heckel, 1977, 1994). Glacial-interglacial sea level fluctuations during the late Pennsylvanian were likely on the order of ~ 70 m–100 m (e.g. Adlis et al., 1988; Soreghan and Giles, 1989; Rygel et al., 2008). Meanwhile, lowstands resulted in subaerial exposure across much of the continent (e.g. Heckel, 1994). Consequently, water depth in the Midcontinent during the regressive portion of the cycle was significantly b 100 m. Abundant phylloid algae in the middle and upper parts of the Plattsmouth Limestone, where our Midcontinent Basin samples were collected, may indicate depths of 30 m or shallower, assuming an analog to green algae in modern marine settings (Yang, 2003; Algeo and Heckel, 2008). The temperature-water depth relationship in the LPMS is also uncertain. Modern tropical shelf settings with minimal upwelling influence such as the Caribbean coastline of Panama show very shallow seasonally averaged thermal gradients of ~0.01 °C/m in the upper 30 m of water, steepening to ~ 0.1 °C/m at greater depths (Locarnini et al., 2013). By contrast, tropical settings with direct upwelling of cold, open ocean water such as the Pacific coastline of tropical Central America show a steeper gradient of ~0.2 °C/m, decreasing to ~0.04 °C/m below 60 m. Algeo and Heckel (2008) have suggested that upwelling from the open ocean and the deeper Midland Basin was a major control on benthic redox conditions in the LPMS, but that the overall influx of deep waters was less than in modern upwelling zones. Abundant phosphatic nodules and concretions in Pennsylvanian shales of the Midcontinent offer direct evidence of nutrient-rich upwelling (Heckel 1977, 1991); however, because of the relatively inland position of the central LPMS and partial geographic barriers such as the ancestral Rocky Mountains between the LPMS and the open ocean, upwelling would have been weaker than in an open shelf setting. Consequently, the thermal gradient in the LPMS likely would have been intermediate between the Caribbean (no upwelling) and Pacific (direct upwelling) cases. It is possible to estimate the salinity difference between the Appalachian (App) and Midcontinent (Midcon) basins given an assumed

thermal gradient, water depth, and δ18Ofw. The glacioisotopicallycorrected δ18O gradient is a function of salinity and temperature: Δ18 OApp−Midcon ¼ f ðSÞ þ f ðT Þ

ð2Þ

In terms of differential equations, these functions may be expressed as follows: Δ18 OApp−Midcon ¼ ΔSApp−Midcon

dδ18 O dδ18 O þ ΔT App−Midcon dS dT

ð3Þ

ΔSApp− Midcon is the salinity difference between the two basins and dδ18 O dS

is the dependence of δ18Osw on salinity, which is determined by the isotopic composition of the freshwater influx into the sea (δ18Ofw). For example, for δ18Ofw of − 8‰ VSMOW, and assuming the salinities of pure freshwater and seawater to be 0 psu and 35 psu, respectively, dδ18 O dS

8 ¼ 35 ≈ 0:23. ΔTApp− Midcon is the temperature difference between 18

the two basins, and dδdT O is the constant temperature dependence of δ18O in calcite, ~ − 0.23‰/°C (e.g. Emiliani, 1955; Friedman and O'Neil, 1977). ΔTApp−Midcon can be further defined, ΔT App−Midcon ¼ z

dt dz

ð4Þ

dt where z is the water depth in the Midcontinent Basin and dz is the thermal gradient. Given the shallowness of the Appalachian Basin and the presence of the Cincinnati Arch as a barrier to cool upwelling from the Midcontinent (cf. Algeo and Heckel, 2008; Fig. 7), we assume the temperature of the Appalachian specimens to be equivalent to the surface water temperature. Assembling Eqs. (3) and (4) gives the combined form,

Δ18 OApp−Midcon ¼ ΔSApp−Midcon

dδ18 O dt dδ18 O þz dS dz dT

ð5Þ

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

9

Rearranging to solve for salinity gives the final form:

ΔSApp−Midcon ¼

Δ18 OApp−Midcon −z dδ18 O dS

dt dδ18 O dz dT

ð6Þ

Fig. 8 shows the results of salinity estimates for the Appalachian Basin over a range of possible water depths for the Midcontinent Basin and three thermal gradient scenarios. Table 3 lists the thermal gradient assumptions for each scenario. Temperature-depth surveys (Locarnini et al., 2013) used to generate these scenarios are included in the Data Supplement. Even for a gradual, Caribbean-type thermal gradient, and assuming a depth of only 20 m for the Midcontinent Basin, likely on the low end of reasonable estimates, the maximum salinity reduction for the Appalachian Basin is ~ 5.5 psu. The salinity reduction becomes much smaller when using less extreme thermal gradient and water depth assumptions. The “intermediate” thermal gradient scenario, an average of the extreme Caribbean and Pacific cases, predicts a salinity reduction of ~3.3 psu if the Midcontinent Basin water column depth was 30 m. These salinity estimates are highly sensitive to the assumed δ18Ofw. Higher δ18Ofw values (e.g., −6‰ VSMOW for Caribbean Panama; Lachniet, 2009) allow for a greater reduction in salinity if coupled with a relatively shallow depth for the Plattsmouth limestone in the Midcontinent Basin (Fig. 8b). However, mean precipitation δ18O values of greater than ~−7‰ VSMOW are unrealistic for mountainous tropical areas (elevation N 0.5 km; Bowen and Wilkinson, 2002). This modelling suggests that much of the glacioisotopically-corrected regional isotopic gradient for our Virgilian samples results from temperature-water column depth effects. Salinity in the Appalachian Basin may have been up to ~ 6 psu below that of the Midcontinent Basin if one employs a gradual thermal gradient, shallow water depth for the Midcontinent Basin, and high δ18Ofw values, but the actual salinity reduction was likely somewhat less given more reasonable assumptions. Intermediate thermal gradient, depth, and δ18Ofw estimates predict that salinity in benthic environments of the Appalachian Basin was ~1–4 psu below that of the Midcontinent Basin. 5.1.1.1. Appalachian Basin Regional Study. Our detailed analysis of specimens from different localities across the Appalachian Basin suggests

Fig. 7. Schematic transects across the LPMS illustrating the position of benthic brachiopod habitats during A) highstand and B) regression. Brachiopods that we sampled from the Midcontinent and Illinois basins were likely deposited during the regression immediately following the highstand in which Appalachian Basin samples were deposited. Paleobathymetric profile modified from Algeo et al. (2008). Precise water and habitat depths and the magnitude of glacial-interglacial sea level fluctuations are uncertain.

Fig. 8. Predicted relative salinities in the Appalachian Basin, based on isotopic data in our Virgilian transect, over a range of possible Midcontinent Basin water column depths. A) Several Midcontinent Basin thermal gradient scenarios. These curves assume a constant δ18Ofw of − 8‰ VSMOW. The “no upwelling” and “direct upwelling” scenarios are approximations seasonally-averaged temperature-depth surveys along the Caribbean and Pacific coastlines, respectively, of modern-day Panama (Locarnini et al., 2013). Full surveys and locations are reported in the Data Supplement. For the “no upwelling” case, the thermal gradient is 0.01 °C/m from 0 m to 30 m depth, and then steepens to 0.1 °C/ m below 30 m. In the “direct upwelling” case, the thermal gradient is 0.06 °C/m in the upper 20 m, and then steepens to 0.2 °C/m from 20 m to 60 m before reaching a more gradual 0.1 °C/m below 60 m. The “Intermediate Upwelling” scenario is an average of the two extreme cases, and is likely the closest approximation to conditions is the LPMS. The gradient is 0.03 °C/m from 0 m to 20 m, 0.13 °C/m from 20 m to 50 m, and 0.08 °C/ m below 50 m. B) Predicted salinities using a range of tropical δ18Ofw values. Our base assumption is − 8‰ VSMOW based on modern mountainous tropical coastlines (Rozanski et al., 1993; Lachniet, 2009; Tao et al., 2013).

slight freshening in the far southeastern portion (PA) of the LPMS. Average δ18O values for sites in the central Appalachian Basin fall within a narrow range between − 3.7‰ VPDB and − 4.0‰ VPDB with no clear regional trends (Table 2; Fig. 1). The most extreme paleo-southeastern locality (AMP) has a low average δ18O of −4.2‰ VPDB, possibly indicating a salinity reduction on the order of 1 psu below the regional average. Meanwhile, the paleo-westernmost sampling location, RED/AMA, averages − 3.6‰ VPDB. However, it is important to note that variations among different specimens at individual sites (e.g. GRR) is similar to the total 0.6‰ range between the eastern and western extremes, and some of this apparent regional trend could be an artifact of small numbers of specimens from some sites. We recovered pristine samples from multiple horizons at two sites, FAIR and GRR. Both sites show a trend toward increasing δ13C and δ18O near the top of the section (Fig. 5). A separate sampling excursion collected additional specimens at FAIR, but did not record the horizon sampled. Given the heterogeneous nature of the Appalachian Basin

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

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A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx Table 3 Thermal gradient assumptions for salinity modela. Scenario/depth Direct upwelling Pacific Analog 0 m–20 m 20 m–60 m 60+ m No upwelling Caribbean Analog 0 m–30 m 30+ m Intermediate upwelling Average of extreme scenarios 0 m–20 m 20 m–50 m 50+ m

Thermal gradient (°C/m)

−0.06 −0.2 −0.04

−0.01 −0.1

−0.03 −0.13 −0.08

a Thermal gradient assumptions for the direct upwelling and no upwelling scenarios are approximations of present-day marine temperature-depth surveys (Locarnini et al., 2013). The Intermediate Scenario assumptions approximate a synthetic temperature-depth curve created by averaging the two extreme scenarios. Full surveys and locations are reported in the Data supplement.

during the deposition Ames Member (e.g. Al-Qayim, 1983), and the fact that Ames brachiopods were deposited in near-highstand conditions (Lebold and Kammer, 2006), these δ18O changes are likely the result of shifting local environmental conditions over time rather than stratigraphic effects. Two sets of specimens within the Appalachian Basin, samples from site A01 and a subset of the FAIR samples, show δ18O values significantly higher than the regional average (Fig. 1), possibly indicating rare, episodic periods of aridity (net evaporative isotopic enrichment) in the basin. Aridity in the tropical LPMS would have been abnormal, but desiccation cracks and other indicators of seasonal drying in the paleosol immediately below the Ames (Joeckel, 1995) suggests that this situation was locally possible. These occasional anomalous specimens underscore the importance of detailed and widespread sampling to capture the full range of environmental variability and to distinguish local effects from regional trends. Similarly, Rosenau et al. (2014) found δ18O variations as large as 2.6‰ among coeval conodonts collected from stratigraphically-equivalent Pennsylvanian highstand shales in the Illinois Basin, although most specimens fell within a narrower range. 5.1.2. Chesterian Chesterian δ18O data provide insight into environmental changes approaching the northwestern margin of the North American Carboniferous sea. These data come with some inherent caveats. Pangean tropical climate was likely wetter during the Mid-Carboniferous than the late Pennsylvanian (e.g. Phillips and Peppers, 1984; DiMichele et al., 2001, 2009). This wetter climate would have resulted in more equatorial moisture entering the late Mississippian sea than the LPMS; thus, geographic salinity trends observed in one of these time periods may not apply directly to the other. Additionally, the North American sea underwent significant geographical changes between the Mississippian and late Pennsylvanian, increasing in size and water mass connectivity and drifting northward by ~ 5° latitude (e.g. Blakey, 2011; Figs. 1, 4). These changes likely impacted surface circulation and the degree of freshening and upwelling in the sea. Despite these caveats, Chesterian δ18O values suggest similar geochemical trends to the Virgilian time interval. Between the Illinois Basin and the Bird Spring shelf, average δ18O of well-preserved brachiopods increased by ~ 1.4‰, slightly greater than the ~ 1.2‰ glacioisotopically-corrected increase in brachiopod δ18O between the Virgilian Appalachian Ames and Midcontinent Oread sections (Table 2). This change is consistent with greater upwelling influence, and/or diminished freshwater input in the Nevada section compared with the more inland Illinois Basin. Semi-arid conditions on the Bird Spring shelf for much of the Carboniferous (Bishop et al., 2010) may have

permitted net-evaporation in this area, which also would have increased δ18O. Due to its location on the margin of the continent, the Bird Spring Shelf may also have been more directly exposed to upwelling of cool marine waters than the Illinois Basin, a change that would have lowered brachiopod shell δ18O. An increase of 1.4‰ δ18O implies cooling of ~ 6 °C; such cooling is possible in modern continental shelf settings influenced by upwelling such as the tropical eastern Pacific (Fiedler et al., 1991; D'Croz and O'Dea, 2007). Most likely, though, the Chesterian trend results from a combination of cooling due to upwelling and diminished freshwater influence due to aridity along the continental margin (cf. Bishop et al., 2010), rather than temperature alone. 5.1.3. Comparison to the conodont δ18O record The Virgilian Appalachian Basin offers an optimal setting to compare oxygen isotope paleotemperatures derived from brachiopod carbonate and conodont phosphate. In this basin, unaltered brachiopods (this study) and conodonts (Joachimski and Lambert, 2015) were deposited at nearly the same time (highstand) and in relatively shallow water depths (cf. Algeo and Heckel, 2008), limiting the possibility of a significant temperature difference between benthic brachiopod and pelagic conodont habitats. Joachimski and Lambert (2015) report an average conodont δ18O of + 18.4‰ VSMOW for the Ames maximum flooding surface. Using the Pucéat et al. (2010) paleotemperature equation for biogenic apatite and assuming δ18Osw to be 0‰ VSMOW, the value used by Joachimski and Lambert (2015), the estimated conodont temperature is ~ 37.3 °C. Average δ18O for unaltered Ames highstand brachiopods in this study is −3.7‰ VPDB, equivalent to a temperature of ~33.5 °C via Hays and Grossman's (1991) quadratic approximation of the O'Neil et al. (1969) paleotemperature equation, again assuming δ18Osw to be 0‰ VSMOW. This apparent 3 °C offset between brachiopod and conodont proxies is significant, especially considering the high (~0.5 °C) resolution of the δ18O thermometer. Lower-than-modern ice volumes during Pennsylvanian highstands may have reduced δ18Osw by ~1‰ (cf. Joachimski et al., 2006). Using −1.0‰ VSMOW for δ18Osw instead of 0‰, conodont and brachiopod temperatures become 33.0 °C and 28.3 °C, an even larger offset. The cause of this apparent discrepancy between brachiopod and conodont data is unclear. A portion of the disagreement could result from slightly warmer temperatures and/or lower salinities in surface conodont habitats relative to the benthic zone. However, the Appalachian Basin was relatively shallow (water depths b50 m in highstands) and sheltered from upwelling by the Cincinnati Arch (Algeo and Heckel, 2008). Modern tropical seas without upwelling zones show minimal temperature variation in the upper ~ 30 m of the water column (Locarnini et al., 2013). Diagenetic effects are also unlikely, since brachiopod specimens were meticulously screened for alteration, and conodont tests are highly resistant to chemical alteration (e.g. Pucéat et al., 2010). Alternatively, uncertainties in the phosphate δ18O thermometer could account for a significant portion of the disagreement between brachiopod and conodont temperatures. Unlike carbonate δ18O paleotemperature equations (e.g. Shackleton, 1974; Friedman and O'Neil, 1977; Erez and Luz, 1983; Kim and O'Neil, 1997), which tend to converge on similar values, there has been considerable disagreement among published phosphate equations (e.g. Longinelli and Nuti, 1973; Kolodny et al., 1983; Lécuyer et al., 1996, 2013; Pucéat et al., 2010). Numerous researchers have discussed the complications with phosphate δ18O calibration at length (e.g. Pucéat et al., 2010; Lécuyer et al., 2013). In one relevant case study, Lécuyer et al. (2013) measured δ18O in brachiopod shells and teeth from sharks living in the same depth zone of the modern Mediterranean Sea. When applying the Pucéat et al. (2010) paleotemperature equation to the phosphate data, Lécuyer et al. (2013) found an apparent ~5 °C offset between the coeval brachiopod and shark tooth specimens, similar to the brachiopod-conodont discrepancy in the present study. Using the new phosphate thermometer that Lécuyer et al. (2013) proposed, instead of the Pucéat et al. (2010)

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

equation, decreases estimated temperatures for Virgilian Appalachian Basin conodonts (Joachimski and Lambert, 2015) to 34.6 °C for δ18Osw of 0‰ VSMOW or 30.1 °C for δ18Osw of − 1‰ VSMOW. This change would reduce the temperature difference between coeval brachiopods and conodonts to b 2 °C, resulting in close agreement between these two independent δ18O thermometers. Because the Joachimski and Lambert (2015) and Pucéat et al. (2010) data were produced in the same laboratory, the Pucéat et al. equation should provide the most accurate temperatures in this case. However, confirmation of this phosphate 18O paleothermometer will require additional study. Regionally, Joachimski and Lambert (2015) noted a ~ 0.5‰ decrease in conodont δ18O in the Appalachian Basin relative to the Midcontinent Basin during the Virgilian (Fig. 6). This amount of total regional variation is consistent with that of our Virgilian brachiopod transect, once corrected for water depth and glacioisotopic effects. Under our Intermediate Upwelling thermal gradient scenario, assuming δ18Osw to be −8‰ VSMOW and a water depth of 30 m in the Midcontinent Basin, our calculations suggest that ~0.7‰ of the difference between Appalachian and Midcontinent Basin brachiopods would result from regional freshening. Such close agreement in regional trends between brachiopod and conodont data testifies to the reliability of both proxies. 5.2. Carbon isotope data 5.2.1. Virgilian data Like δ18O, δ13C of well-preserved brachiopods in the Virgilian transect decreases from high values in the Midcontinent Basin (+ 4.4 ± 0.6‰ VPDB) and Texas (+ 4.2 ± 0.5‰ VPDB) to progressively lower values in Illinois (+ 3.2‰ VPDB, 1 specimen) and the Appalachian Basin (+ 1.9 ± 1.1‰ VPDB; Table 2), consistent with a systematic regional gradient. Glacioisotopic effects on the global δ13C record are inconsistent (e.g. Shackleton and Kennett, 1975; Shackleton et al., 1983) and likely would have played a secondary role relative to environmental influences. While offsets in measured δ13C between different taxa at the same site likely indicate some metabolic effects (cf. Grossman et al., 1991, 1993), the overall regional trend observed here (and in Grossman et al., 1991) is robust and similar in magnitude regardless of the species examined, suggesting a strong overriding environmental trend. One probable explanation for the regional decline in δ13C is a combination of increasing restriction and terrestrial influence approaching the paleo-southeastern edge of the LPMS. High rates of respiration in the warm, shallow shelf environment, combined with oxidation of terrestrial organic matter derived from stream discharge and restricted circulation, would have boosted the concentration of low-δ13C respired CO2 in the water column. Similar processes occur in the modern Florida Bay and Bahama Banks, lowering δ13CDIC by as much as 4‰ below typical surface seawater values (Patterson and Walter, 1994). Measured δ13C of mollusk shells from the south Florida region reflects this shift (Lloyd, 1964). In addition to ferrying terrestrial organic matter for oxidation, concentrated riverine input to restricted water bodies can also carry the products of reactions between soil-respired CO2 and the bedrock, further lowering the δ13C of water column DIC (e.g. Diz et al., 2009). Water mass restriction and relative terrestrial influence would have both increased systematically moving from the Midcontinent Basin to the Appalachian Basin, potentially explaining the isotopic trend. 5.2.2. Chesterian Chesterian samples show a decline in δ13C moving from the continental margin in Nevada to the Illinois Basin, similar to the regional decline δ13C of the Virgilian samples. Mean δ13C for Anthracospirifer, the only brachiopod genus present at both sites, declines from + 3.9 ± 0.2‰ VPDB (Nevada) to +1.7 ± 1.2‰ VPDB (Illinois; Table 2). One possible explanation for this trend, as with the Virgilian data, is higher water column concentrations of low-δ13C metabolic CO2 in the restricted, shallow interior (Illinois Basin) part of the continental sea relative to the more openly marine western (Nevada) portion. Consistent with this

11

interpretation, analyses of radiogenic Nd, a proxy for continental weathering, have suggested that the North American sea had developed a strong geochemical gradient by the latest Mississippian (Woodard et al., 2013). Consequently, both δ18O and δ13C data suggest similar overall environmental trends–increasing restriction and freshwater influence in the continental interior–in the Virgilian and Chesterian time intervals. 5.3. Trace element data Concentrations of the trace elements Mg, Sr, Na, and S in the Virgilian specimens do not show strong, consistent regional trends apart from species-dependent variations (Fig. 9; Table 4). Average trace element concentrations for brachiopods of the same taxon do vary within and between sampling sites; however, regional trends are inconsistent, and variations between taxa far eclipse regional variability. This type of inter-specific variability, such as elevated concentrations of Mg and S in Crurithyris, conforms to previous trace element characterizations of well-preserved brachiopods (e.g. Grossman et al., 1996) and likely reflects differences in metabolism or habitat preference. Chesterian data show systematic regional differences in Mg/Ca and Sr/Ca, which both increase from the Bird Spring Shelf to the Illinois Basin (Fig. 9, Table 4), but these differences (3.7 mmol/mol for Mg/Ca and 0.3 mmol/mol for Sr/Ca) fall within the range observed in the Virgilian samples and other Paleozoic marine specimens (e.g. Grossman et al., 1991, 1996). Perhaps the observed variability in Chesterian samples results from minor variations in water chemistry between basins. 5.3.1. Implications for the Mg/Ca paleothermometer in Paleozoic brachiopods If Mg/Ca ratios can be used as a unique paleothermometer as some have suggested (e.g Klein et al., 1996; Perez-Huerta et al., 2008), Mg/ Ca temperatures and brachiopod δ18O values can be used to estimate seawater δ18O and ultimately ice volume. Unfortunately, geochemical data in this study do not support the application of existing Mg/Ca thermometer equations to ancient brachiopod specimens. Compared with δ18O temperatures, Mg/Ca temperature estimates (1) generally show wider scatter at individual localities, (2) display much greater speciesdependent variability, (3) are unrealistically cool for tropical marine settings, and (4) do not accurately reflect expected regional trends (Fig. 10, Table 5). Perez-Huerta et al. (2008) and Powell et al. (2009) used the mollusk-derived paleothermometer equation of Vander Putten et al. (2000; T(°C) = (Shell Mg/Ca ratio + 0.63 (± 0.29))/ 0.70(±0.02)) to estimate temperatures of precipitation for Recent and ancient brachiopod specimens. Applying this equation to our West Virginia Crurithyris data yields an unreasonably large range of temperatures, from ~ 6 °C for the samples with the lowest measured Mg/Ca to ~ 16 °C for samples with the highest Mg/Ca (Table 5). Admittedly, Vander Putten et al. (2000) noted that Mg/Ca-temperature covariance was not consistent in all portions of the shells sampled. The “Global Brachiopod Mg Line” (GBMgL) of Brand et al. (2013; T(°C) = 14.386 + 15.428log(0.1*shell Mg/Ca ratio)), an empirical relationship between temperature and shell MgCO3 of modern marine brachiopods, yields temperatures for our West Virginia Crurithyris data ranging from ~8 °C to ~ 14.5 °C. For the same specimens, total δ18O temperatures range from ~32 °C to 38 °C (using seawater δ18O = 0‰, a high value considering maximum flooding and freshening, and the equation of Hays and Grossman, 1991). The range in Mg/Ca values is even wider when considering specimens of different genera from the same site, despite virtually identical δ18O values. Moving eastward from the Midcontinent Basin, mean Mg/Ca temperatures do not show a consistent warming trend expected with shallowing and deglaciation, and individual taxa frequently show opposing or counter-regional trends (Table 5). Mg/Ca temperatures are also unexpectedly cool for tropical marine environments, and well below GCM-based mean annual temperature estimates of N 20 °C for tropical Pangea (e.g. Peyser and Poulsen, 2008). Brand et al.

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Fig. 9. Crossplots illustrating trace element and oxygen isotopic data for the Virgilian and Chesterian time slice. Note the wide spread and lack of clear regional trends in Mg/Ca data, as well as the lack of correlation between Mg/Ca and δ18O, suggesting that shell Mg/Ca is a poor proxy for temperature. In general, species-related trace element trends overwhelm any systematic regional variation.

(2013) also present a revised δ18O thermometer to account for variations in shell Mg, using a + 0.17‰ δ18O per mol % MgCO3 correction factor. Because of the low Mg/Ca concentrations in our specimens (b10 mmol/mol Mg/Ca or b0.1 mol% MgCO3), this correction is not necessary for reporting δ18O temperatures in this manuscript. Several aspects of the trace element and isotopic data in this study point to metabolism and other species-dependent factors as significant drivers of shell Mg/Ca. First, Mg/Ca values for the Virgilian specimens show no correlation with shell δ18O, a proxy for temperature (Fig. 9), suggesting that Mg concentrations in these shells do not reflect equilibrium with ambient seawater. Mg/Ca and δ18O should vary inversely if both thermometers are functioning (cf. Mitsuguchi et al., 1996; Klein et al., 1996). In the Chesterian samples, Mg/Ca and δ18O appear to covary on a regional scale, but show no significant relationship at individual sites or among members of the same species (Fig. 9). Secondly, erratic variations in shell Mg/Ca, which lead to unexpectedly wide

ranges in reconstructed Mg/Ca temperatures (see discussion above), potentially implicate influence from biological processes, whose rates may vary through ontogeny. Finally, shell Mg/Ca does not uniformly reflect expected glacial and water depth-related temperature changes. For instance, Mg/Ca for Virgilian Crurithyris is slightly higher in Texas samples (7.3 mmol/mol) compared with Appalachian Basin samples (6.9 mmol/mol), opposite of the cooling trend expected from greater water depths and expanded ice volumes during deposition of the Eastern Shelf specimens. Unreasonably cool Mg/Ca temperature estimates, reflecting lower-than-expected shell Mg concentrations, might be another indication of disequilibrium. However, low shell Mg/Ca values could also occur if the seawater Mg/Ca ratio during the Carboniferous was significantly lower than in recent oceans, a possibility suggested by Dickson (2002). Studies of modern and ancient brachiopod specimens support the hypothesis that shell Mg/Ca do not accurately reflect seawater

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Table 4 Average trace element data by region and taxona. Age/locality/genera Chesterian Illinois Basin Illinois Anthracospirifer Inflatia Antler Foreland Basin Nevadab Anthracospirifer Composita Productid Virgilian Appalachian Basin Pennsylvania Crurithyris West Virginia Neospirifer Crurithyris Composita Ohio Neospirifer Crurithyris Illinois Basin Illinois Crurithyris Eastern Shelf Texasc Crurithyris Composita Midcontinent Basin Kansasd Neospirifer Crurithyris Composita

Mg/Ca

SD

Sr/Ca

SD

S/Ca

SD

Na/Ca

SD

7.6(9,24) 6.7(2,6) 9.5(1,3)

1.8 1.7 n/a

1.4(9,24) 1.2(2,6) 1.7(1,3)

0.2 0.1 n/a

– – –

n/a n/a n/a

– – –

n/a n/a n/a

3.2(9,24) 3.1(6,15) 3.3(2,5) 4.3(1,4)

0.5 0.4 (0.6) n/a

0.8(9,24) 0.9(6,15) 0.6(2,5) 0.9(1,4)

0.1 0.1 (0.5) n/a

– – – –

n/a n/a n/a n/a

– – – –

n/a n/a n/a n/a

5.5(43,581)

2.1

1.1(43,581)

0.3

13.8(26,581)

4.1

8.6(26,581)

3.5

6.1(2,8) 5.7(32,201) 3.9(12,145) 7.0(19,55) 2.6(1,1) 4.6(9,372) 3.7(6,360) 6.4(3,12)

{2.6} 2.3 0.8 2.0 n/a 1.4 0.3 0.4

1.5(2,8) 1.1(32,201) 0.8(12,145) 1.4(19,55) 0.6(1,1) 1.0(9,372) 0.8(6,360) 1.4(3,12)

{0.3} 0.3 0.1 0.2 n/a 0.3 0.1 0.3

16.4(2,8) 14.2(15,167) 8.2(3,120) 15.8(12,47) – 12.6(9,372) 9.6(6,360) 18.5(3,12)

{3.8} 3.8 1.4 2.3 n/a 4.9 1.8 3.0

11.5(2,8) 9.4(15,167) 4.1(3,120) 10.8(12,47) – 6.6(9,372) 4.1(6,360) 11.7(3,12)

{1.0} 3.0 0.7 1.2 n/a 3.9 0.6 1.3

9.3(1,1)

n/a

1.5(1,1)

n/a

–

n/a

–

n/a

6.8(5,52) 7.3(4,46) 4.6(1,6)

2.2 2.1 n/a

– – –

n/a n/a n/a

17.7(5,52) 21.1(4,46) 3.9(1,6)

8.0 2.8 n/a

9.8(5,52) 10.5(4,46) 6.9(1,6)

1.7 0.7 n/a

3.9(17) 3.0(7) 4.6(1) 4.6(9)

2.5 0.5 n/a 3.3

0.8(17) 0.9(7) 1.2(1) 0.7(9)

0.3 0.2 n/a 0.3

– – – –

n/a n/a n/a n/a

– – – –

n/a n/a n/a n/a

SD = Standard deviation. a Trace element concentrations reported as mmol/mol. The number of specimens analyzed, followed by the total number analyses, is shown is parentheses () adjacent to the trace element concentration. Standard deviation not reported for cases where only one specimen was analyzed. In cases where only two specimens were analyzed, the difference between the averages for the two specimens is shown in brackets {}. b All Nevada data originally reported by Jones et al. (2003) and Grossman et al. (2008). c All Texas data originally reported by Grossman et al. (1996). d All Kansas data originally reported in Grossman et al. (1993).

temperature. Buening and Carlson (1992) and Brand et al. (2013), among others, have noted metabolically-driven variations in Mg/Ca ratios in recent brachiopod specimens associated with factors such as growth rate. In fact, based on laboratory experiments, equilibrium composition should be ~ 5 to ~ 12 mol% MgCO3 at 5 °C to 40 °C in modern seawater (Mucci, 1987). Note that Brand et al.'s “Global Brachiopod Mg Line” excludes high-latitude specimens from high productivity regions, while incorporating high Mg calcite specimens from low latitudes. This has the effect of steepening the temperature dependence of Mg content. High-resolution sclerochronology studies of Paleozoic brachiopods by Roark et al. (2016) failed to find a consistent relationship between Mg/Ca and seasonal δ18O fluctuations, further hinting that shell Mg concentrations may not fully record environmental changes, though such a relationship was observed by Mii and Grossman (1994) in a Pennsylvanian Neospirifer sampled in fine detail. Given the uncertainties surrounding metabolic influences on shell Mg/Ca, as well as the possibility of temporal changes in the seawater Mg/Ca ratio, we urge extreme caution in applying existing Mg/Ca thermometer formulations to Paleozoic brachiopods. 5.4. Implications for “superestuarine” circulation and the persistence of the regional halocline Although data in this study may only indicate limited freshening in benthic habitats over the studied portion of the LPMS, these observations by no means preclude the persistence of a halocline at shallower depths. Joachimski and Lambert (2015), observing a relatively weak conodont δ18O trend across the Virgilian sea, concluded that drying

climatic conditions in the late Pennsylvanian (cf. Phillips and Peppers, 1984; Kosanke and Cecil, 1996) had so diminished freshwater input into the LPMS as to virtually eliminate the halocline. However, this conclusion conflicts with a comprehensive suite of paleontological and sedimentological data from the Ames marine unit supporting strong freshwater influence in the easternmost (PA) portion of the sea (Brezinski, 1983; Saltsman, 1986; Merrill, 1993; Lebold and Kammer, 2006). These studies all demonstrate a progressive, systematic shift from stenohaline faunal assemblages (e.g. crinoids, Neospirifer brachiopods) to brackish fauna (mollusks, Neochonetes brachiopods) approaching the southeastern edge of the LPMS, beyond the area sampled in this study. This faunal gradient persists even through the maximum highstand (Brezinski, 1983; Lebold and Kammer, 2006). Most likely, these data do reflect a significant, regional decrease in salinity. Unfortunately, thick-shelled brachiopod fauna suitable for isotopic analysis (e.g. Neospirifer, Crurithyris, Composita; cf. Grossman et al., 1993) are limited to waters with near-marine salinities (Stevens, 1971; Lebold and Kammer, 2006). Independent lines of evidence, such as the persistence of stratification-driven dysoxic episodes in the LPMS until the latest Pennsylvanian (Schultz and Coveney, 1992; Schultz, 2004), and a lack of evidence for seasonal drying in sclerochronological (Roark et al., 2016) and fossil tree ring (Chaloner and Creber, 1990) studies from the latest Pennsylvanian Appalachian Basin region, suggest that high levels of freshwater influx into the LPMS remained a dominant control on water column conditions through the end of the Carboniferous. Moreover, the magnitude and distribution of our inferred salinity gradient is consistent with that of modern inland seas with estuarine-

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Fig. 10. Histogram of Mg/Ca and δ18O temperature estimates for the Virgilian specimens. Calculated δ18O temperatures on this histogram chart are raw values, uncorrected for glacioisotopic influences. δ18O temperatures generally show minimal differences between species and less scatter at individual localities than Mg/Ca temperatures. After correcting for ice volume, the δ18O thermometer yields reasonable tropical temperatures and reflects the expected cooling trend moving from the Appalachian Basin to the Midcontinent Basin. Mg/ Ca temperature estimates show a strong species-dependent bias and are unreasonably cool for shallow tropical seas. Expected regional trends are less apparent than with the δ18O thermometer.

type circulation. Hudson Bay maintains a permanent halocline driven by freshwater input along the sea's coastlines; surface and benthic-level salinities decrease approaching the shorelines and increase in the central part of the sea, away from the coasts (Algeo et al., 2008; Barber, 1967; Prinsenberg, 1986). This distribution matches that of our inferred salinity gradient for the LPMS (decreasing salinity in the proximal Appalachian Basin and increasing salinity in the more distal Midcontinent Basin). Outside of relatively small regions where riverine influx is

concentrated, such as the mouths of tributaries and the partially landlocked James Bay to the south, the magnitude of the proximal-distal salinity gradient at 50 m depth is only ~3 psu across Hudson Bay (Barber, 1967), within the range of our salinity gradient estimates for the LPMS. The high-latitude Hudson Bay has drastically different climatic boundary conditions than the tropical LPMS would have; however, Algeo et al. (2008) estimated similar total basin volume to freshwater discharge ratios for both bodies. In the modern-day Panama Bight in the Pacific, an

Table 5 Comparison of paleotemperature estimates (°C). Age/locality/genera

Chesterian Illinois Basin Anthracospirifer Inflatia Antler Foreland Anthracospirifer Composita Productid Virgilian Appalachian Basin Neospirifer Crurithyris Composita Illinois Basin Crurithyris Eastern Shelf Crurithyris Composita Midcontinent Basin Neospirifer Crurithyris Composita a b c

Mg/Ca(a)a

Mg/Ca(b)b

δ18Oc

Max

Min

Mean

SD

Max

Min

Mean

SD

Max

Min

Mean

SD

11.8 14.4

9.3 n/a

n/a n/a

n/a n/a

12.6 14.0

10.8 n/a

n/a n/a

n/a n/a

28.6 31.7

26.5 n/a

n/a n/a

n/a n/a

6.0 5.9 7.1

4.6 4.9 n/a

5.3 n/a n/a

0.5 n/a n/a

5.3 7.4 8.8

7.4 5.9 n/a

6.4 n/a n/a

0.8 n/a n/a

22.7 25.4 19.0

26.1 21.2 n/a

23.7 n/a n/a

2.7 n/a n/a

8.8 15.8 4.6

4.6 6.4 n/a

6.3 10.8 n/a

0.9 2.8 n/a

10.4 14.7 5.4

5.4 8.0 n/a

7.8 11.7 n/a

1.1 1.9 n/a

36.0 38.3 34.3

32.3 32.2 33.0

33.9 34.9 33.6

1.1 1.4 0.7

14.1

n/a

n/a

n/a

13.9

n/a

n/a

n/a

25.9

n/a

n/a

n/a

13.3 7.5

7.6 n/a

11.4 n/a

7.6 n/a

13.5 9.2

9.3 n/a

12.1 n/a

2.1 n/a

29.6 24.4

24.4 n/a

27.2 n/a

1.6 n/a

6.1 7.5 17.0

4.3 n/a 2.7

5.2 n/a 7.4

0.6 n/a 4.7

7.6 9.2 15.2

4.8 n/a 0.4

6.3 n/a 7.6

1.0 n/a 5.0

25.2 21.9 25.5

20.3 21.8 19.1

22.9 n/a 21.8

1.5 n/a 1.7

Paleotemperature equation of Vander Putten et al. (2000). Global Brachiopod Mg line equation of Brand et al. (2013). Oxygen isotope paleotemperature equation from Hays and Grossman (1991).

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average salinity gradient of ~3 psu is observed from nearshore Columbia (~30 psu) N1000 km offshore (33 psu), associated with a distinct halocline in the upper 100 m (Fiedler and Talley, 2006). In contrast, the modern Baltic Sea shows a much more dramatic salinity gradient due to higher levels of net freshwater discharge and limited deep water circulation with the open ocean; salinity in the inlandmost portions of the Baltic Sea approaches freshwater levels (Algeo et al., 2008). Our isotopic data as well as distribution patterns of marine fauna (e.g. Lebold and Kammer, 2006) show no evidence of such drastic salinity variations in the LPMS. As opposed to a complete elimination of salinity stratification in the Virgilian LPMS, we believe that a more reasonable interpretation, considering all relevant datasets, is that the halocline persisted through this time but had shallowed such that its base was generally above benthic and conodont habitats (Fig. 7). It is possible to roughly estimate the depth of the halocline based on faunal distribution patterns. Brezinski (1983) mapped the extent of the Crurithyris-associated faunal assemblage in the Ames Member in Pennsylvania; this boundary approximately delineates the shoreward limit of relatively marine salinities in the sea. Moving farther up the shelf, one encounters the brackish, mollusk-associated assemblage indicating diminished salinity and high levels of terrigenous influx (Brezinski, 1983; cf. Lebold and Kammer, 2006). If the paleoshoreline occurred near the easternmost depositional limit of the Ames marine unit and the Appalachian Basin reached its maximum depth of ~ 30 m (Algeo et al., 2008) by the base of Brezinski's (1983) Composita biofacies (slightly NW of our FAIR and GRR sampling sites), the approximate paleo-shelf dip would have been ~ 0.014°. This relatively shallow dip is similar to that found on the edges of the modern Gulf of Carpentaria, which Algeo et al. (2008) cite as the closest modern geographic analogue to the LPMS. In northeast West Virginia, the eastern limit of the Crurithyris facies occurs ~ 60 km from the paleoshoreline, implying a depth of at least ~ 15 m for the base of the reduced-salinity plume in this area (Brezinski, 1983; Fig. 11). Algeo et al. (2008) estimate the depth of the halocline in the LPMS to have been 15–30 m below the surface; the Crurithyris-estimated depth falls on the lower end of that range. Presumably, marine conodonts and benthic brachiopods would have inhabited deeper, more saline waters and thus δ18O sampled from the biogenic phosphate and carbonate of these organisms would not have recorded the full regional freshening signal. Consequently, it is probable that some form of halocline, though perhaps shallower than in earlier, more humid climate regimes, remained in play even into these waning days of the Carboniferous icehouse. 6. Conclusions Well-preserved Chesterian and Virgilian brachiopod shells sampled from across the North American continent show a systematic, progressive decrease in δ18O and δ13C moving from present-day west to east.

15

For oxygen isotopes, this transition results primarily from weakened upwelling, increased freshening, and a shallower water column approaching the restricted southeastern portion of the continental sea. The trend in δ13C for both time slices likely reflects greater restriction and heightened terrestrial runoff. After correcting for glaciosotopic effects and employing reasonable water depth assumptions, Virgilian δ18O trends suggest a salinity reduction of ~1–4 psu in the Appalachian Basin relative to the Midcontinent Basin. This gradient is consistent with that across the main portion of the modern-day Hudson Bay, which experiences year-round salinity stratification, and with that across a N 1000 km distance of the Panama Bight. Given the plethora of existing paleoecological evidence for brackish conditions in the extreme southeastern portion of the Appalachian Basin during the Virgilian, data in this study are consistent with the persistence of high freshwater input capable of sustaining a halocline across the LPMS into the latest Carboniferous, though the strength of the halocline was likely less than in the Middle Pennsylvanian. Additional data are necessary to estimate the degree of vertical salinity contrast in the sea. Inferred δ18O paleotemperatures from Virgilian Appalachian Basin brachiopods in this study are cooler than those of coeval conodonts, indicating either slightly warmer, fresher surface waters or calibration problems between the phosphate and carbonate thermometers. However, the magnitude of the regional δ18O trend across the LPMS, ~0.5–0.7‰ after correcting for ice volume, temperature, and water depth effects, is similar for both proxies. A lack of correlation between Mg/Ca and δ18O or inferred water depth; large, species-dependent offsets in Mg/Ca; and large spreads in Mg/Ca concentrations even among coeval brachiopods of the same species, yielding unrealistic, extreme ranges in inferred Mg/Ca temperatures, justify continued skepticism of the application of existing Mg/Ca paleotemperature equations to ancient brachiopods. Acknowledgments We thank Michael Joachimski and Lance Lambert for helpful discussion. Scott Elrick and Joe Devera (Illinois Geological Survey), Albert Kollar (Carnegie Museum, Pittsburgh), Greg Nadon (Ohio University) and Dale Gnidovec (The Ohio State University) provided assistance in gathering samples from the field and existing collections. Texas A&M University's Stable Isotope Geosciences Facility (SIGF) staff, headed by Chris Maupin, ensured successful processing of isotopic samples, while Ray Guillemette operated the electron microprobe for trace element analyses at Texas A&M's Department of Geology and Geophysics. Art Kasson and Luz Romero assisted in processing ICP-MS trace element analyses at Texas A&M's Department of Geology and Geophysics. National Science Foundation grant EAR-0643309 and the Michel T. Halbouty Chair in Geology at Texas A&M funded this research. Phil Heckel and an anonymous reviewer provided constructive feedback during the editing process. Additional financial support for A. Roark was provided through a 2-year Chevron fellowship from the Berg-

Fig. 11. Probable depth of halocline along the southeastern edge of the LPMS based on the transition between brackish and marine fauna in the Ames Member identified by Brezinski (1983). These schematic representations indicate that the halocline may have extended to ~ 15 m depth near the shoreline assuming a uniformly-dipping platform (0.014°). Consequently, a functioning halocline could have covered much of the Virgilian sea without strongly impacting brachiopod isotopic values. Shoreline position was estimated based on the furthest eastern extent of Ames outcrops.

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Hughes Center for Petroleum and Sedimentary Systems at Texas A&M University. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.palaeo.2017.06.009. References Adlis, D.S., Grossman, E.L., Yancey, T.E., McLerran, R.D., 1988. Isotopic stratigraphy and paleodepth changes of Pennsylvanian cyclical sedimentary deposits. PALAIOS 3, 487–506. Algeo, T.J., Heckel, P.H., 2008. The Late Pennsylvanian Midcontinent Sea of North America: A review. Palaeogeogr. Palaeoclimatol. Palaeoecol. 268 (3–4), 205–221. Algeo, T.J., Heckel, P.H., Maynard, J.B., Blakey, R.C., Rowe, H., 2008. Modern and ancient epeiric seas and the super-estuarine model of marine anoxia. In: Holmden, C., Pratt, B.R. (Eds.), Dynamics of Epeiric Seas: Sedimentological, Paleontological, and Geochemical Perspectives. vol. 48. Geological Association of Canada Special Publication, pp. 7–38. Algeo, T.J., Hoffman, D.L., Maynard, J.B., Joachimski, M.M., Hower, J.C., Watney, W.L., 1997. Environmental reconstruction of anoxic marine systems: core black shales of Upper Pennsylvanian midcontinent cyclothems. In: Algeo, T.J., Maynard, J.B. (Eds.), Cyclic Sedimentation of Appalachian Devonian and Midcontinent Pennsylvanian black shales: Analysis of Ancient Anoxic Marine Systems–A Combined Core and Field Workshop: Joint Meeting of Eastern Section AAPG and The Society for Organic Petrography (TSOP), Lexington, Kentucky, September 27–28, 1997, pp. 103–147. Al-Qayim, B.A., 1983. Facies and depositional environment of the Ames marine member (Virgilian) of the Conemaugh Group (Pennsylvanian) in the Appalachian Basin. (PhD. thesis). University of Pittsburgh (306 p.). Auclair, A.C., Joachimski, M.M., Lécuyer, C., 2003. Deciphering kinetic, metabolic and environmental controls on stable isotopic fractionations between seawater and the shell of Terebratalia transversa (Brachiopoda). Chem. Geol. 202, 59–78. Barber, F.G., 1967. A contribution to the oceanography of Hudson Bay: Canada Marine Sciences Brach. Manuscript Report Series. vol. 4 (69 p.). Bishop, J.W., Montañez, I.P., Osleger, D.A., 2010. Dynamic Carboniferous climate change, Arrow Canyon, Nevada. Geosphere 6 (1), 1–34. Blakey, R.C., 2011. Paleogeographic and Geologic Evolution of North America. http:// cpgeosystems.com/nam.html (last accessed June 2016). Brand, U., Veizer, J., 1980. Chemical diagenesis of a multicomponent carbonate system: trace elements. J. Sediment. Petrol. 50 (4), 1219–1236. Boardman II, D.R., Heckel, P.H., 1989. Glacial-eustatic sea-level curve for early Late Pennsylvanian sequence in north-central Texas and biostratigraphic correlation with curve for midcontinent North America. Geology 17 (9), 802–805. Bowen, G.J., Wilkinson, 2002. Spatial distribution of δ18O in meteoric precipitation. Geology 30 (4), 315–318. Brand, U., Azmy, K., Bitner, M.A., Logan, A., Zuschin, M., Came, R., Ruggiero, E., 2013. Oxygen isotopes and MgCO3 in brachiopod calcite and a new paleotemperature equation. Chem. Geol. 359, 23–31. Brand, U., Azmy, K., Griesshaber, E., Bitner, M.A., Logan, A., Zuschin, M., Ruggiero, M., Colin, P.I., 2015. Carbon isotope composition in modern brachiopod calcite: a case of equilibrium with seawater? Chem. Geol. 411, 81–96. Brezinski, D.K., 1983. Developmental model for an Appalachian Pennsylvanian marine incursion. Northeast. Geol. 5 (2), 92–99. Buening, N., Carlson, S.J., 1992. Geochemical investigation of growth in selected recent articulate brachiopods. Lethaia 25 (3), 331–345. Busch, R.M., Rollins, H.B., 1984. Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units. Geology 12 (8), 471–474. Chaloner, W.G., Creber, G.T., 1990. Do fossil plants give a climatic signal? J. Geol. Soc. 147, 343–350. Coveney Jr., R.M., Leventhal, J.S., Glascock, M.D., Hatch, J.R., 1987. Origins of metals and organic matter in the Mecca Quarry Shale member and stratigraphically equivalent beds across the Midwest. Econ. Geol. 82, 915–933. Cusack, M., Pérez-Huerta, A., 2012. Brachiopods recording seawater temperature–a matter of class or maturation? Chem. Geol. 334, 139–143. Craig, H., 1961. Isotopic variations in meteoric waters. Science 133 (3465), 1702–1703. Dickson, J.A.D., 2002. Fossil echinoderms as monitor of the Mg/Ca ratio of Phanerozoic oceans. Science 298 (5596), 1222–1224. DiMichele, W.A., Montañez, I.P., Poulsen, C.J., Tabor, N.J., 2009. Climate and vegetational regime shifts in the late Paleozoic ice age earth. Geobiology 7, 200–226. DiMichele, W.A., Pfefferkorn, H.W., Gastaldo, R.A., 2001. Response of late Carboniferous and early Permian plant communities to climate change. Annu. Rev. Earth Planet. Sci. 29, 461–487. Diz, P., Jorissen, F.J., Reochart, G.J., Poulain, C., Dehairs, F., Leorri, E., Paulet, Y.-M., 2009. Interpretation of benthic foraminiferal stable isotopes in subtidal estuarine environments. Biogeosciences 6, 2549–2560. Donahue, J., Rollins, H.B., 1974. Paleoecological anatomy of a Conemaugh (Pennsylvanian) marine incursion. In: Briggs, G. (Ed.), Carboniferous of the Southeastern United States: Geological Society of America Special Paper. 148, pp. 153–170. D'Croz, L., O'Dea, A., 2007. Variability in upwelling along the Pacific shelf of Panama and implications for the distribution of nutrients and chlorophyll. Estuar. Coast. Shelf Sci. 73 (1–2), 325–340. Emiliani, C., 1955. Pleistocene temperatures. J. Geol. 63 (6), 538–578.

Erez, J., Luz, B., 1983. Experimental paleotemperature equation for planktonic foraminifera. Geochim. Cosmochim. Acta 47 (6), 1025–1031. Eyre, B., Balls, P., 1999. A comparative study of nutrient behavior along the salinity gradient of tropical and temperate estuaries. Estuaries 22 (2A), 313–326. Fahrer, T.R., 1996. Stratigraphy, Petrography and Paleoecology of Marine Units Within the Conemaugh Group (Upper Pennsylvanian) of the Appalachian Basin in Ohio, West Virginia, and Pennsylvania. (PhD. Thesis). The University of Iowa, Iowa City (298 p.). Fiedler, P.C., Philbrick, V., Chavez, F.P., 1991. Oceanic upwelling and productivity in the eastern tropical Pacific. Limnol. Oceanogr. 36 (8), 1834–1850. Fiedler, P.C., Talley, L.D., 2006. Hydrography of the eastern tropical Pacific: a review. Prog. Oceanogr. 69, 143–180. Flake, R.C., 2011. Circulation of North American Epicontinental Seas During the Carboniferous Using Stable Isotope and Trace Element Analyses of Brachiopod Shells. (M.S. Thesis). Texas A&M University, College Station (63 p.). Friedman, I., O'Neil, J.R., 1977. Compilation of stable isotope fractionation factors of geochemical interest. In: Fleischer, M. (Ed.), Data of Geochemistry, sixth ed U.S. Geological Survey Professional Paper 400-kk (12 p). Frölich, K., Grabczak, J., Rozanski, K., 1988. Deuterium and oxygen-18 in the Baltic Sea. Chem. Geol. 72, 77–83. Garbelli, C., Angiolini, L., Brand, U., Jadoul, F., 2014. Brachiopod fabric, classes and biogeochemistry: implications for the reconstruction and interpretation of seawater carbonisotope curves and records. Chem. Geol. 371, 60–67. Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M., 2012. The Geologic Time Scale 2012: Amsterdam. Elsevier (1144 p.). Grossman, E.L., Mii, H.S., Yancey, T.E., 1993. Stable isotopes in Late Pennsylvanian brachiopods from the United States: implications for Carboniferous paleoceanography. Geol. Soc. Am. Bull. 105 (10), 1284–1296. Grossman, E.L., Mii, H.S., Zhang, C., Yancey, T.E., 1996. Chemical variation in Pennsylvanian brachiopod shells–diagenetic, taxonomic, microstructural, and seasonal effects. J. Sediment. Res. 66 (5), 1011–1022. Grossman, E.L., Yancey, T.E., Jones, T.E., Bruckschen, P., Chuvashov, B., Mazzullo, S.J., Mii, H.-S., 2008. Glaciation, aridification, and carbon sequestration in the Permo-Carboniferous: the isotopic record from low latitudes. Palaeogeogr. Palaeoclimatol. Palaeoecol. 268, 222–233. Grossman, E.L., Zhang, C., Yancey, T.E., 1991. Stable-isotope stratigraphy of brachiopods from Pennsylvanian shales in Texas. Geol. Soc. Am. Bull. 103, 953–965. Hatch, J.R., Leventhal, J.S., 1992. Relationship Between Inferred Redox Potential of the Depositional Environment and Geochemistry of the Upper Pennsylvanian (Missourian) Stark Shale Member of the Dennis Limestone, Wabaunsee County, Kansas, U.S.A. vol. 99, no. 1–3 pp. 65–82. Hays, P.D., Grossman, E.L., 1991. Oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate. Geology 19:441–444. http://dx.doi.org/10.1130/00917613(1991)0192.3.CO;2. Heckel, P.H., 1977. Origin of phosphatic black shale facies in Pennsylvanian cyclothems of mid-continent North America. Am. Assoc. Pet. Geol. Bull. 61 (7), 1045–1068. Heckel, P.H., 1986. Sea-level curve for Pennsylvanian eustatic marine transgressive-regressive depositional cycles along midcontinent outcrop belt, North America. Geology 14 (4), 330–334. Heckel, P.H., 1991. Thin widespread Pennsylvanian Black Shales of Midcontinent North America: A Record of a Cyclic Succession of Widespread Pycnoclines in a Fluctuating Epeiric Sea. 58. Geological Society, London, Special Publications, pp. 259–273. Heckel, P.H., 1994. Evaluation of evidence for glacio-eustatic control over marine Pennsylvanian cyclothems in North America and consideration of possible tectonic effects. In: Dennison, J.M., Ettensohn, F.R. (Eds.), Tectonic and Eustatic Controls on Sedimentary Cycles: Society of Economic Paleontologists and Mineralogists Concepts in Sedimentology and Paleontology. vol. 4, pp. 65–105. Heckel, P.H., 2002. Observations and Constraints on Radiometric Dating of the Pennsylvanian Succession in North America and Its Correlation With Dates From Europe: Newsletter on Carboniferous Stratigraphy. vol. 20 pp. 10–14. Heckel, P.H., 2013. Pennsylvanian stratigraphy of the Northern Midcontinent Shelf and biostratigraphic correlation of cyclothems. Stratigraphy 10, 3–39. Herrmann, A.D., Barrick, J.E., Algeo, T.J., 2015. The relationship of conodont biofacies to spatially variable water mass properties in the Late Pennsylvanian Midcontinent Sea. Paleoceanography 30 (3), 269–283. Hoffman, D.L., Algeo, T.J., Maynard, B., Joachimski, M.M., Hower, J.C., Jaminski, J., 1998. Regional and stratigraphic variation in bottomwater anoxia in offshore core shales of Upper Pennsylvanian cyclothems from the eastern midcontinent shelf (Kansas), U.S.A. In: Schieber, J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones I: Stuttgart, Schweizerbart'sche, pp. 243–269. Hopkinson, C.S., Giblin, A.E., Tucker, J., Garritt, R.H., 1999. Benthic metabolism and nutrient cycling along an estuarine salinity gradient. Estuaries 22 (4), 863–881. Joachimski, M.M., Lambert, L.L., 2015. Salinity contrast in the US Midcontinent Sea during Pennsylvanian glacio-eustatic highstands: evidence from conodont apatite δ18O. Palaeogeogr. Palaeoclimatol. Palaeoecol. 433, 71–80. Joachimski, M.M., von Bitter, P.H., Buggisch, W., 2006. Constraints on Pennsylvanian glacio-eustatic sea-level changes using oxygen isotopes of conodont apatite. Geology 34 (4), 277–280. Joeckel, R.M., 1995. Paleosols below the Ames Marine Unit (Upper Pennsylvanian, Conemaugh Group) in the Appalachian Basin, U.S.A.: variability on an ancient depositional landscape. J. Sediment. Res. A65 (2), 393–407. Jones, T.E., Grossman, E.L., Yancey, T.E., 2003. Exploring the stable isotope record of global change and paleoclimate: The mid-Carboniferous GSSP (Arrow Canyon, Nevada) and the Ural Mountains, Russia. Geol. Soc. Am. Abstr. Programs 35 (6), 254 (September 2003). Kim, S.-T., O'Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. Geochim. Cosmochim. Acta 61 (16), 3461–3475.

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx Klein, R.T., Lohmann, K.C., Thayer, C.W., 1996. Bivalve skeletons record sea-surface temperature and δ18O via Mg/Ca and 18O/16O ratios. Geology 24 (5), 415–418. Kolodny, Y., Luz, B., Navon, O., 1983. Oxygen isotope variations in phosphate of biogenic apatites, I. Fish bone apatite-rechecking the rules of the game. Earth Planet. Sci. Lett. 64 (3), 398–404. Kosanke, R.M., Cecil, C.B., 1996. Late Pennsylvanian climate changes and palynomorph extinctions. Rev. Palaeobot. Palynol. 90 (1–2), 113–140. Lachniet, M.S., 2009. Sea surface temperature control on the stable isotopic composition of rainfall in Panama. Geophys. Res. Lett. 36 (3) (5 p.). Lane, H.R., Brenckle, P.L., Baesemann, J.F., Richards, B., 1990. The IUGS boundary in the middle of the Carboniferous: Arrow Canyon, Nevada, USA. Episodes 22 (4), 272–283. Langenheim Jr., R.L., Carss, B.W., Kennerly, J.B., McCutcheon, V.A., Waines, R.H., 1962. Paleozoic section in Arrow Canyon Range, Clark County, Nevada. Am. Assoc. Pet. Geol. Bull. 46 (5), 592–609. Lebold, J.G., Kammer, T.W., 2006. Gradient analysis of faunal distributions associated with rapid transgression and low accommodation space in a Late Pennsylvanian marine embayment: biofacies of the Ames Member (Glenshaw Formation, Conemaugh Group) in the northern Appalachian Basin, USA. Palaeogeogr. Palaeoclimatol. Palaeoecol. 231, 291–314. Lécuyer, C., Amiot, R., Touzeau, A., Trotter, J., 2013. Calibration of the phosphate δ18O thermometer with carbonate-water oxygen isotope fractionation equations. Chem. Geol. 347, 217–226. Lécuyer, C., Grandjean, P., Emig, C.C., 1996. Determination of oxygen isotope fractionation between water and phosphate from living lingulids: potential application to paleoenvironmental studies. Palaeogeogr. Palaeoclimatol. Palaeoecol. 126, 101–108. Lloyd, R.M., 1964. Variations in the oxygen and carbon isotope ratios of Florida Bay mollusks and their environmental significance. J. Geol. 72, 84–111. Locarnini, R.A., Mishonov, A.V., Antonov, J.I., Boyer, T.P., Garcia, H.E., Baranova, O.K., Zweng, M.M., Paver, C.R., Reagan, J.R., Johnson, D.R., Hamilton, M., Seidov, D., 2013. World Ocean Atlas 2013, volume 1: temperature. In: Levitus, S. (Ed.), NOAA Atlas NESDIS. In: Mishonov, A. (Ed.) vol. 73 (40 pp). Longinelli, A., Nuti, S., 1973. Revised phosphate-water isotopic temperature scale. Earth Planet. Sci. Lett. 19 (3), 373–376. Lowenstam, H.A., 1961. Mineralogy, O18/O16 ratios, and strontium and magnesium contents of recent and fossil brachiopods and their bearing on the history of the oceans. J. Geol. 69 (3), 241–260. Machel, H.G., Mason, R.A., Mariano, A.N., Mucci, A., 1991. Causes and emission of luminescence in calcite and dolomite. In: Barker, C.E., Kopp, O.C. (Eds.), Luminescence Microscopy: Quantitative and Qualitative Aspects: SEPM Short Course. 25, pp. 9–25. Mii, H.S., Grossman, E.L., 1994. Late Pennsylvanian seasonality reflected in the 18O and elemental composition of a brachiopod shell. Geology 22 (7):661–664. http://dx.doi. org/10.1130/0. Merrill, G.K., 1993. Late Carboniferous paleoecology along a tectonically active basin margin: Ames member near Huntington, West Virginia. Southeast. Geol. 33 (3), 111–129. Mitsuguchi, T., Matsumoto, E., Abe, O., Uchida, T., Isdale, P.J., 1996. Mg/Ca thermometry in coral skeletons. Science 274, 961–963. Mucci, A., 1987. Influence of temperature on the composition of magnesian calcite overgrowths precipitated from seawater. Geochim. Cosmochim. Acta 51, 1977–1984. Norby, R.D., 1990. Biostratigraphic zones in the Illinois Basin. Interior Cratonic Basins. vol. 51. American Association of Petroleum Geologists Memoir, pp. 179–194. O'Neil, J.R., Clayton, R.N., Mayeda, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys. 51 (12), 5547–5558. Olszewski, T.D., Patzkowsky, M.E., 2003. From cyclothems to sequences: the record of eustasy and climate on an icehouse epeiric platform (Pennsylvanian-Permian, North American midcontinent). J. Sediment. Res. 73 (1), 15–30. Olszewski, T.D., Patzkowsky, M.E., 2008. Icehouse climate and eustasy recorded in a low-latitude epeiric platform: alternating climate regimes in the PennsylvanianPermian succession of the North American continent. In: Holmden, C., Pratt, B.R. (Eds.), Dynamics of Epeiric Seas: Sedimentological, Paleontological, and Geochemical Perspectives. 47. Geological Association of Canada Special Publication, pp. 229–245. Patterson, W.P., Walter, L.M., 1994. Depletion of 13C in seawater ΣCO2 on modern carbonate platforms – significance for the carbon isotopic record of carbonates. Geology 22, 885–888. Parkinson, D., Curry, G.B., Cusach, M., Fallick, A.E., 2005. Shell structure, patterns, and trends of oxygen and carbon stable isotopes in modern brachiopod shells. Chem. Geol. 219 (1–4), 387–406. Perez-Huerta, A., Cusack, M., Jeffries, T.E., Williams, C.T., 2008. High resolution distribution of magnesium and strontium and the evaluation of Mg/Ca thermometry in recent brachiopod shells. Chem. Geol. 247, 229–241. Peyser, C.E., Poulsen, C.J., 2008. Controls on Permo-Carboniferous precipitation over tropical Pangea: a GCM sensitivity study. Palaeogeogr. Palaeoclimatol. Palaeoecol. 268 (3– 4), 181–192. Phillips, T.L., Peppers, R.A., 1984. Changing patterns of Pennsylvanian coal-swamp vegetation and implications of climatic control on coal occurrence. Int. J. Coal Geol. 3 (3), 205–255. Powell, M.G., Schöne, B.R., Jacob, D.E., 2009. Tropical marine climate during the late Paleozoic ice age using trace element analyses of brachiopods. Palaeogeogr. Palaeoclimatol. Palaeoecol. 280 (1–2), 143–149. Poole, F.G., Sandberg, C.A., 1991. Mississippian paleogeography and conodont biostratigraphy of the western United States. In: Cooper, J.D., Stevens, C.H. (Eds.), Paleozoic Paleogeography of the Western United States II: Pacific Section SEPM. vol. 67, pp. 107–136.

17

Popp, B.N., Anderson, T.F., Sandberg, P.A., 1986. Brachiopods as indicators of original isotopic compositions in some Paleozoic limestones. Geol. Soc. Am. Bull. 97 (10), 1262–1269. Prinsenberg, S.J., 1986. Salinity and Temperature Distributions of James Bay: Elsevier Oceanography Series. vol. 44 pp. 163–186. Pucéat, E., Joachimski, M.M., Bouilloux, A., Monna, F., Bonin, A., Motreuil, S., Morinière, P., Hénard, S., Mourin, J., Dera, G., Quesne, D., 2010. Revised phosphate-water fractionation equation reassessing paleotemperatures derived from biogenic apatite. Earth Planet. Sci. Lett. 298, 135–142. Roark, A., Grossman, E.L., Lebold, J., 2016. Low seasonality in central equatorial Pangea during a late Carboniferous highstand based on high-resolution isotopic records of brachiopod shells. Geol. Soc. Am. Bull. 123 (3–4), 597–608. Robertson, A.J., Daniel, P.A., Dixon, P., Alongi, D.M., 1993. Pelagic biological processes along a salinity gradient in the Fly delta and adjacent river plume (Papua New Guinea). 13 (2–3), 205–224. Rosenau, N.A., Tabor, N.J., Herrmann, A.D., 2014. Assessing the paleoenvironmental significance of Middle-Late Pennsylvanian conodont apatite δ18O values in the Illinois Basin. PALAIOS 29, 250–265. Rozanski, K., Araguds-Araguds, L., Gonfiantini, R., 1993. Isotopic patterns in modem global precipitation. In: Swart, P.K., Lohman, K.C., McKenzie, J., Savin, S. (Eds.), Climate Change in Continental Isotopic Records - Geophysical Monograph. vol. 78. American Geophysical Union, Washington, D.C., pp. 1–36. Rygel, M.C., Fielding, C.R., Frank, T.D., Birgenheier, L.P., 2008. The magnitude of Late Paleozoic glacioeustatic fluctuations: a synthesis. J. Sediment. Res. 78 (8), 500–511. Saltsman, A.L., 1986. Paleoenvironments of the Upper Pennsylvanian Ames limestone and associated rocks near Pittsburgh, Pennsylvania. Geol. Soc. Am. Bull. 97 (2), 222–231. Samtleben, C., Munnecke, A., Bickert, T., Pätzold, J., 2001. Shell succession, assemblage and species dependent effects on the C/O-isotopic composition of brachiopods — examples from the Silurian of Gotland. Chem. Geol. 175 (1–2), 61–107. Scheihing, M.H., Langenheim Jr., R.L., 1978. Lingulid, rhynchonellid and spiriferid brachiopods from the Shumway Cyclothem, Mattoon Formation, Pennsylvanian of Illinois. Ill. State Acad. Sci. Trans. 71 (2), 165–182. Schultz, R.B., 2004. Geochemical relationships of Late Paleozoic carbon-rich shales of the Midcontinent, USA: a compendium of results advocating changeable geochemical conditions. Chem. Geol. 206, 347–372. Schultz, R.B., Coveney Jr., R.M., 1992. Time-dependent changes in Midcontinent Pennsylvanian black shales, U.S.A. Chem. Geol. vol. 99, 83–100. Shackleton, N.J., 1974. Attainment of Isotopic Equilibrium Between Ocean Water and Benthonic Foraminifera Genus Uvigerina: Isotopic Changes in the Ocean During the Last Glacial. vol. 219. Centre National de la Recherche Scientifique Colloques Internationaux, pp. 203–209. Shackleton, N.J., Kennett, J.P., 1975. Paleotemperature History of the Cenozoic and the Initiation of Antarctic Glaciation: Oxygen and Carbon Isotope Analyses in DSDP Sites 277, 279, and 281 in DSDP. vol. 29 pp. 739–755. Shackleton, N.J., Opdyke, N.D., 1973. Pacific core V28-238: oxygen isotope temperatures and ice volumes on a 105 year scale. Quat. Res. 3 (1), 39–55. Shackleton, N.J., Imbrie, J., Hall, M.A., 1983. Oxygen and carbon isotope record of East Pacific core V19-30: implications for the formation of deep water in the late Pleistocene North Atlantic. Earth Planet. Sci. Lett. 65, 233–244. Soreghan, G.S., Giles, K.A., 1989. Amplitudes of Late Pennsylvanian glacioeustasy. Geology 27 (3), 255–258. Stevens, C.H., 1971. Distribution and diversity of Pennsylvanian marine faunas relative to water depth and distance from shore. Lethaia 4 (4), 403–412. Tao, K., Robbins, J.A., Grossman, E.L., O'Dea, A., 2013. Quantifying upwelling and freshening in nearshore tropical American environments using stable isotopes in modern gastropods. Bull. Mar. Sci. 89 (4), 815–835. Treworgy, J.D., 1990. Kaskaskia Sequence: Mississippian Valmeyeran and Chesterian series. Interior Cratonic Basins. vol. 51. American Association of Petroleum Geologists Memoir, pp. 125–142. Tyson, R.V., Pearson, T.H., 1991. Modern and ancient continental shelf anoxia: an overview. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. vol. 58. Geological Society Special Publication, pp. 1–24. Vander Putten, E., Dehairs, F., Keppens, E., Baeyens, W., 2000. High-resolution distribution of trace elements in the calcite shell layer of modern Mytilus edulis: environmental and biological controls. Geochim. Cosmochim. Acta 64 (6), 997–1011. Wanless, H.R., Shepard, F.P., 1936. Sea level and climatic changes related to Late Paleozoic cycles. Geol. Soc. Am. Bull. 47 (8), 1177–1206. Weibel, C.P., Norby, R.D., 1992. Paleopedology and conodont biostratigraphy of the Mississippian-Pennsylvanian boundary interval, type Grove Church Shale area, southern Illinois. Recent Advances in Middle Carboniferous Biostratigraphy–A Symposium. 94. Oklahoma Geological Survey Circular, pp. 39–49. Weibel, C.P., 1996. Applications of sequence stratigraphy to Pennsylvanian strata in the Illinois Basin. In: Witzke, B.J., Ludvigson, G.A., Day, J. (Eds.), Paleozoic Sequence Stratigraphy: Views From the North American Craton: Geological Society of America Special Paper. 306, pp. 331–339. Woodard, S.C., Thomas, D.J., Grossman, E.L., Olszewski, T.D., Yancey, T.E., Miller, B.V., Raymond, A., 2013. Radiogenic isotope composition of Carboniferous seawater from North American epicontinental sea. Palaeogeogr. Palaeoclimatol. Palaeoecol. 370, 51–63. Yamamoto, K., Asami, R., Iryu, Y., 2010a. Carbon and oxygen isotopic compositions of modern brachiopod shells from a warm-temperate shelf environment, Sagami Bay, central Japan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 291 (3–4), 348–359. Yamamoto, K., Asami, R., Iryu, Y., 2010b. Within-shell variations in carbon and oxygen isotope compositions of two modern brachiopods from a subtropical shelf

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009

18

A. Roark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2017) xxx–xxx

environment off Amami-o-shima, southwestern Japan. Geochem. Geophys. Geosyst. 11 (10) (17 p.). Yamamoto, K., Asami, R., Iryu, Y., 2011. Brachiopod taxa and shell portions reliably recording past ocean environments: toward establishing a robust paleoceanographic proxy. Geophys. Res. Lett. 38 (5 p.). Yang, W., 2003. The Late Pennsylvanian (Virgilian) Oread cyclothem–an introduction. In: Yang, W., Bruemmer, M., Turner, M., Summervill, M., Skelton, L. (Eds.), Field

Guidebook: Sedimentology and Stratigraphic Architecture of Upper Pennsylvanian Oread Cyclothem in Shelf-to-Delta Transition Zone, SE Kansas and NE Oklahoma. Kansas Geological Survey, 2003 Spring Field and Subsurface Workshop, pp. 1–7. Zettler, M.L., Schiedek, D., Bobertz, B., 2007. Benthic biodiversity indices versus salinity gradient in the southern Baltic Sea. Mar. Pollut. Bull. 55, 258–270.

Please cite this article as: Roark, A., et al., Brachiopod geochemical records from across the Carboniferous seas of North America: Evidence for salinity gradients, stratificatio..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2017), http://dx.doi.org/10.1016/j.palaeo.2017.06.009