Ca as a proxy for Gulf of Mexico winter mixed-layer temperature: Evidence from a sediment trap in the northern Gulf of Mexico

Marine Micropaleontology 80 (2011) 53–61

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Marine Micropaleontology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r m i c r o

Research paper

Globorotalia truncatulinoides (dextral) Mg/Ca as a proxy for Gulf of Mexico winter mixed-layer temperature: Evidence from a sediment trap in the northern Gulf of Mexico Jessica W. Spear a, Richard Z. Poore a,⁎, Terrence M. Quinn b a b

U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, 600 Fourth Street South, Saint Petersburg, FL 33701, USA Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78758, USA

a r t i c l e

i n f o

Article history: Received 23 November 2010 Received in revised form 18 May 2011 Accepted 22 May 2011 Keywords: Globorotalia truncatulinoides Sediment trap Mg/Ca Gulf of Mexico Seasonality Planktic foraminifera Surface-mixed layer

a b s t r a c t Three years of weekly- to biweekly-resolved sediment-trap data show that almost 90% of the total flux of tests of the planktic foraminifer Globorotalia truncatulinoides to sediments in the northern Gulf of Mexico occurs in January and February. Comparison of δ18O from tests of non-encrusted Gl. truncatulinoides in sediment-trap samples with calculated calcification depths indicates that the non-encrusted individuals secrete their test in the winter surface-mixed layer, most likely at the bottom of the surface mixed zone. Mg/Ca-temperature estimates from non-encrusted Gl. truncatulinoides in sediment-trap samples are consistent with observed temperatures at the calcification depths inferred from the δ18O data. In contrast, Mg/Ca-temperature estimates from encrusted Gl. truncatulinoides in sediment-trap samples indicate the crust is formed in cooler (deeper) waters. A preliminary study in a core recovered near the sediment-trap site demonstrates that non-encrusted and encrusted forms of Gl. truncatulinoides in sediment samples show a similar offset in Mg/Ca values as observed in sediment-trap samples. A short (~100 years) Mg/Ca record from non-encrusted Gl. truncatulinoides indicates a warming trend that coincides with a warming trend in mean-annual sea-surface temperature recorded by Mg/Ca in Globigerinoides ruber (white) from the same core. These findings suggest Mg/Ca from non-encrusted Gl. truncatulinoides has clear potential as a proxy for past winter mixed-layer temperature. Published by Elsevier B.V.

1. Introduction Records of past climate variability are increasingly important in light of the current need to differentiate between natural and anthropogenic climate change (e.g., Cane, 2010) and to anticipate the impacts of climate warming related to increasing atmospheric CO2 concentrations. Recent studies have shown variability in low-latitude mean-annual sea-surface temperature (SST) to be surprisingly large over the past millennia, with SST changes on the order of 1–3 °C (Oppo et al., 2009; Richey et al., 2009). In addition, information on changes in seasonality is needed to help determine likely forcing mechanisms and possible feedbacks. For example, extreme winter temperatures, indicated by a comparison of Greenland mean-annual air temperature and summer-weighted snowline records, likely influenced climate throughout the northern hemisphere and tropics via winter sea-ice formation during numerous abrupt climate change events that spanned the last glaciation to the Holocene (Denton et al.,

⁎ Corresponding author. Tel.: + 1 727 803 8747; fax: + 1 727 803 2031. E-mail addresses: [email protected] (J.W. Spear), [email protected] (R.Z. Poore), [email protected] (T.M. Quinn). 0377-8398/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.marmicro.2011.05.001

2005). In addition, current climate change forecasts predict regional changes in seasonality that will likely impact North American precipitation via changes in moisture and latent heat flux from the Gulf of Mexico (Christensen et al., 2007). These studies highlight the need for more information on past records of regional and seasonal variability in contrast to records of hemispheric-wide averages (e.g., Mann et al., 2009). Many current marine climate reconstructions rely on the use of the Mg/Ca ratio in the calcite tests of planktic foraminifers as a temperature proxy because the amount of Mg substituted for Ca in the calcite of the foraminifer test is a function of temperature: i.e., Mg increases as temperature increases (Chave, 1954; Savin and Douglas, 1973; Nürnberg et al., 1996; Hastings et al., 1998; Lea et al., 1999). The use of the Mg/Ca temperature proxy can be complicated by a number of factors, including diagenetic addition or alteration of primary calcite and selective dissolution of high-Mg calcite. However, techniques and procedures have been developed to guard against these complications. The calibration equation for Mg/Ca to temperature is similar for all planktic foraminifers considered, and most calibration studies indicate ~ 9–10% change in Mg/Ca per 1 °C (e.g., Lea et al., 1999; Elderfield and Ganssen, 2000; Dekens et al., 2002). However, minor differences do occur between species, and therefore

J.W. Spear et al. / Marine Micropaleontology 80 (2011) 53–61

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Fig. 1. (A) Location of the sediment trap mooring (inverted triangle) at 27°32.42 N, 90°19.35 W, and its proximity to the Pigmy Basin (black dot), 27°11.61 N, 91°24.54 W, in the northern Gulf of Mexico. (B) Encrusted (black) and non-encrusted (gray) Gl. truncatulinoides flux (tests m−2 day− 1) plotted versus each mid-week collection day during 7- to 14-day collection intervals for 2008–2010. The x-axis is the 18th day of each month. (C) January and February (87.6%) flux comprise the bulk of the total flux.

species-specific calibration equations generally provide a more accurate temperature estimate (Anand et al., 2003). In addition, the ecology of the planktic foraminifer being analyzed must be known to properly interpret the Mg/Ca record. Most proxy Mg/Ca planktic foraminifer records are based on analyses of Globigerinoides ruber (d'Orbigny) (white variety) for several reasons. G. ruber (white) is a surface dwelling symbiontbearing form that lives and calcifies in the upper 50 m (m) of the water column (Lidz et al., 1968; Bé et al., 1971; Tolderlund and Bé, 1971; Bé, 1982; Thunell et al., 1999; Anand et al., 2003). In addition, G. ruber (white) has a wide geographic range; it is common in sediments from tropical to temperate regions, and it is known to live year round in many areas (e.g., Deuser, 1987; Mohtadi et al., 2009; Storz et al., 2009; Tedesco et al., 2009; Wilke et al., 2009; Spear and Poore, 2011). More recently, Richey et al. (2007) showed that G. ruber (white) Mg/Ca values from northern Gulf of Mexico core–top samples, dated to post-A.D. 1950 age, yield temperature estimates equivalent to the modern mean-annual SST for the region. Thus, G. ruber (white) Mg/Ca records are typically interpreted as a proxy for mean-annual SST. Other species of planktic foraminifers are known to have different depth and seasonal preferences and thus may be useful for developing records of water column structure or for seasonal variation provided the habitat preference of the species is well constrained. Globorotalia truncatulinoides (d'Orbigny) is one species of planktic foraminifer that has potential for providing information on both water column structure and seasonal climate variability. Gl. truncatulinoides has a complicated life cycle, which involves substantial migration in the water column that is likely related to reproduction (e.g., Bé and Ericson, 1963; Deuser and Ross, 1989; Lohmann and Schweitzer, 1990). Plankton-tow data suggest reproduction in most areas usually occurs just below the euphotic zone in the fall/winter, when vertical mixing and deep convection is greatest. Juveniles then ascend to the euphotic zone, and from there grow to adult size before descending deeper in the water column, where they may continue secreting calcite (Bé and Ericson, 1963; Bé and Lott, 1964; Erez and Honjo, 1981; Hemleben et al., 1985; Lohmann and Schweitzer, 1990).

The test of Gl. truncatulinoides often has two distinct components that reflect depth changes during the life cycle. An inner component of primary calcite accounts for chamber growth and size (Lohmann, 1992), and this primary test is usually formed in the upper part of the water column (Bé and Ericson, 1963; Hemleben et al., 1985). A layer of secondary calcite, or crust, often overlies the primary test. The crust is added later in the life cycle as the foraminifer sinks into deeper and colder water (Bé and Ericson, 1963; Orr, 1967; Hemleben et al., 1985). Previous work on the chemistry of primary and secondary layers has shown an offset between the δ18O and Mg/Ca of the primary and secondary calcite (Vergnaud Grazzini, 1976; Mulitza et al., 1997: McKenna and Prell, 2004) reflecting the addition of secondary calcite (crust) at depth. Sediment-trap and plankton-tow studies indicate the flux of G. truncatulinoides to sediments is highest in winter to spring, but the details vary with location. Sediment-trap data from the Sargasso Sea (Deuser et al., 1981; Deuser and Ross, 1989) show the flux of Gl. truncatulinoides is highest from December to March. In the Mediterranean Basin, G. truncatulinoides is abundant from December to April (Vergnaud Grazzini, 1976). Plankton-tow and sediment-trap data from the northeast Atlantic indicate the peak flux of Gl. truncatulinoides to sediments occurs in January to March (Storz et al., 2009; Wilke et al., 2009), whereas the North Pacific sediment-trap data indicate the maximum production of Gl. truncatulinoides occurs in early March (Eguchi et al., 1999). In this study, we present data from a sediment-trap that show nonencrusted representatives of the planktic foraminifer Gl. truncatulinoides can be used as a proxy for temperature variations in the winter surfacemixed layer in the northern Gulf of Mexico. In addition, we conducted a short sediment-core pilot study that demonstrates the potential of this proxy for the reconstruction of past Gulf of Mexico winter mixedlayer temperature. 2. Methods and materials 2.1. Sediment-trap samples A sediment-trap mooring was deployed in early January 2008 in ~1150 m water depth at 27°32.42 N and 90°19.35 W (Fig. 1). The

Plate 1. Scanning electron microscopic (SEM) images of non-encrusted and encrusted specimens of Gl. truncatulinoides from sediment-trap and sediment-core material. Figs. 1–4 are from sediment-trap samples and Figs. 5–8 are from sediment-core PBBC-1. Figs. 1–2 and 5–6 are non-encrusted Gl. truncatulinoides; Figs. 3–4 and 7–8 are encrusted Gl. truncatulinoides. Scale bar for each figure is 200 μm.

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Fig. 2. Raw Mg/Ca (mmol/mol) data from non-encrusted Gl. truncatulinoides (open triangles) and encrusted Gl. truncatulinoides (black triangles). X-axis is mid-week collection date for each 7- to 14-day collection period. The ~ 1 mol/mol average offset in non-encrusted and encrusted Gl. truncatulinoides Mg/Ca values (dashed lines) for January and February 2008–2010 indicates the primary calcite is secreted in warmer (shallower) water than the secondary calcite crust.

McLane Mark 78 automated sediment trap was positioned at 700 m of water depth on the mooring to guarantee the collection of deeper dwelling species of planktic foraminifers (e.g., Globorotalia spp.). The trap is equipped with 21 collection cups that are mounted on a rotating plate that is programmed to rotate every 7 to 14 days. Each cup contains a 3.7% buffered (sodium borate) formalin solution to poison and preserve the samples. Each trap sample represents a 1- to 2-week collection period and we recovered and redeployed the trap every 3–6 months. A gap in sampling occurred from late May to late September 2009 due to scheduling problems. Nine samples representing the weeks of March 10, March 31, May 1, October 22,

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November 19, and December 10 of 2009 and January 3, February 7, February 14, and February 21 of 2010 were not recovered due to loss of the cups during deployment/recovery. During visits to the trap site, we collected conductivity–temperature–depth (CTD) measurements using a Sea-Bird Electronics SBE-9plus. We collected water samples at depth intervals of 25 m from the surface to 75 m. Samples below 75 m were collected at the oxygen-minimum zone and generally every 100 m from the oxygen-minimum zone to the trap funnel at 700 m. Sediment-trap samples were wet split into four aliquots using a precision rotary splitter at the University of South Carolina and then stored in buffered de-ionized water and refrigerated. A quarter split

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Fig. 3. Salinity (A) and in-situ δ18Oseawater (B) in the upper 150 m and ~ 1150 m (insets) measured at the sediment-trap site in January 2008 (blue) and 2009 (red). Salinity and temperature are from CTD casts. δ18Oseawater measured from discrete Niskin bottle samples (filled circles). Temperature and δ18Oseawater plots were used to calculate equilibrium δ18Ocalcite profiles for January 2008 and 2009 shown in panel C. Measured δ18O of non-encrusted Gl. truncatulinoides from 2008 (blue triangles) and 2009 (red triangles) are plotted on the respective calculated equilibrium profiles (C). Panel D shows in-situ temperature in the top 150 m and ~ 1150 m (inset) plotted with temperature calculated from Mg/Ca of non-encrusted Gl. truncatulinoides samples (blue and red triangles) using the multi-species calibration equation of Anand et al. (2003). The calculated temperatures are plotted at the calcification depth indicated in panel C and suggest the non-encrusted forms of Gl. truncatulinoides are calcifying at the base of the winter surface-mixed layer, near the top of the thermocline. The error in our Mg/Ca-temperature estimates is ±1.22 °C and was determined by combining errors from analytical (0.01 °C) and intra-sample precision (0.46 °C) and error reported by Anand et al. (2003) for the multi-species calibration equation (1.13 °C).

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encrusted specimens followed the same criteria used to separate the sediment-trap samples (Plate 1, Figs. 5–8). About 30 individuals from each group were picked for isotopic and elemental analysis using the same methods outlined above for the sediment-trap material. We increased the size range for core samples due to the paucity of nonencrusted forms. All analyses were conducted on the right-coiling variety since Gl. truncatulinoides from both sediment-trap and core materials were exclusively right coiling.

was wet sieved over a 150-μm sieve and subsequently wet picked for all foraminifers. Gl. truncatulinoides in the 300–425 μm size fraction were picked from the January and February samples. The separation of encrusted and non-encrusted specimens was based on morphology observed visually with a light microscope. About 30 individuals were separated into groups of clearly non-encrusted and encrusted specimens for Mg/Ca and isotopic measurements and we did not include transitional specimens (Plate 1, Figs. 1–4). We used this size fraction because it was the average size of Gl. truncatulinoides from January and February samples. Each group of ~30 individuals was ultra-sonicated for ~ 5 s in methanol, dried in a 50 °C oven, and weighed (see Table 1 in supplementary information). Non-encrusted Gl. truncatulinoides were split into equal aliquots of typically 14 individuals for minor elemental and stable isotope measurements. Samples for minor elemental analysis were gently crushed between glass plates to remove chamber fill and subjected to an extensive cleaning process following steps outlined in Barker et al. (2003), appropriate for sediment-trap samples (McConnell and Thunell, 2005; Mohtadi et al., 2009). Briefly, this cleaning method involves multiple MilliQ water and methanol rinses to remove clays, an oxidation step to remove organic material, and a weak acid rinse to remove any adsorbents. Samples for stable isotope analysis were gently broken open and cleaned with a 1% buffered peroxide solution in a hot water bath for 40 min, rinsed numerous times with MilliQ water, dried in a 50 °C oven, and pulverized between two glass plates to maximize homogenization of the material.

2.3. Stable isotope and Mg/Ca analyses Stable isotope mass spectrometry was performed at the Analytical Laboratory for Paleoclimate Studies at the Jackson School of Geosciences, University of Texas at Austin using a ThermoFinnigan Delta Plus XL dual-inlet mass spectrometer with an attached Kiel IV carbonate preparation device. We report isotopic data on the ViennaPee Dee Belemnite (VPDB) scale calibrated with the international standard NBS-19. Analytical precision (1σ) for the stable isotope measurements is 0.05‰ for δ 18O based on over 14 NBS-19 standards analyzed along with the foraminifer samples. Minor elemental analysis was performed at the U.S. Geological Survey, Saint Petersburg, Florida using a Perkin Elmer Optima 7300 dual-view inductively coupled plasma-optical emission spectrometer (ICP-OES). The analytical precision for this study is 0.36% root-mean standard deviation, based on a calibrated ICP-OES solution. The average precision for Mg/Ca was ±0.12 mmol/mol based on ~65% replicate and triplicate sample measurements (n = 20).

2.2. Sediment samples 2.4. δ 18O of seawater Non-encrusted and encrusted Gl. truncatulinoides were picked from the 300–500 μm size fraction from samples in the top 6 cm (n = 10) of sediment-core PBBC-1 from nearby Pigmy Basin (27°11.61 N, 91°24.54 W, water depth 2259 m). The identification and separation of Gl. truncatulinoides into groups of non-encrusted and

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Depth (mm) Fig. 4. Raw Mg/Ca (mmol/mol) (inside y-axes) data from G. ruber (white) (top panel) and non-encrusted Gl. truncatulinoides (middle panel) from PBBC-1 converted to temperature (outside y-axes) using the G. ruber (white) and multi-species calibration equation, respectively (Anand et al., 2003). The G. ruber Mg/Ca-temperature record is considered to represent mean-annual SST and shows a clear trend of increasing SST over the past ~ 100 years (Richey et al., 2007). The Gl. truncatulinoides Mg/Ca-temperature record indicates a trend of increasing winter surface-mixed layer temperature, similar to the record of mean-annual SST. Error in the G. ruber and Gl. truncatulinoides Mg/Ca-temperature estimates is ~±1 °C. The triangle on the x-axis denotes the radiocarbon date in the 0–60 mm interval of PBBC-1.

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minimize bubbles, subsequently poisoned with 100 μL of HgCl2, sealed, and kept refrigerated. The oxygen isotopic composition of seawater (δ 18Osw) was determined by continuous flow measurements on a Delta Plus XL mass spectrometer following automated CO2–water equilibration using a Finnigan GasBench II. Isotopic values, reported relative to Vienna-Standard Mean Ocean Water (VSMOW), were standardized with analyses of VSMOW and secondary standards. Analytical precision, determined by replicate analysis of seawater samples, was on average 0.05‰. 2.5. Temperature estimates There are currently no calibration equations specific to the environmental conditions and foraminifer populations of the Gulf of Mexico available to convert Mg/Ca to temperature. There are several alternative calibration equations available to convert non-encrusted Gl. truncatulinoides Mg/Ca to temperature. McKenna and Prell (2004) developed two Gl. truncatulinoides-specific equations based on electron-probe analysis of primary and secondary calcite of encrusted Gl. truncatulinoides from Indian Ocean core–tops. Anand et al. (2003) developed a multi-species equation for planktic foraminifers and species-specific equations for 12 different planktic foraminifers, including Gl. truncatulinoides. We chose to use the Anand et al. (2003) multi-species calibration equation, Mg/Ca = 0.38 exp (0.09 ⁎ T), because the equation is based on paired Mg/Ca and δ 18O data from 10 planktic foraminifer species from 6 years of sedimenttrap material and a long time series of hydrographic data from the same locality. Note that the equation used in this study is based on specimens in the 350–500 μm size fraction and may not be appropriate for the size fractions used in this study. The three equations specific to Gl. truncatulinoides yielded temperature estimates similar to the multi-species equation. Recent studies strongly suggest planktic foraminifer Mg/Ca is affected by salinity variability (e.g., Ferguson et al., 2008; Kisakürek et al., 2008; Arbuszewski et al., 2010). However, the extent to which salinity affects Mg/Ca is not well understood. Culture work indicates a small influence (2–8% increase in Mg/Ca per salinity unit) (Nürnberg et al., 1996; Lea et al., 1999; Kisakürek et al., 2008; Dueñas-Bohòrquez et al., 2009), while core–top studies suggest a much larger influence (15–30% increase in Mg/Ca per salinity unit) (Ferguson et al., 2008; Arbuszewski et al., 2010). In addition, the salinity effect is disputed by studies that show that the post-deposition of high-Mg calcite on foraminifer tests can account for the higher than expected Mg/Ca values in specimens from the highly saline Red Sea and Mediterranean Sea (Hoogakker et al., 2009; van Raden et al., 2011). Since there is no current consensus as to the extent to which G. ruber (white) Mg/Ca from the Gulf of Mexico is affected by salinity and no information on the influence of salinity on Gl. truncatulinoides, we did not apply a salinity correction to either Mg/Ca record. Furthermore, a salinity correction for both the G. ruber and Gl. truncatulinoides Mg/Ca records would not change their respective trends or our interpretation. 3. Results and discussion 3.1. Gl. truncatulinoides—seasonal distribution and depth habitat The abundance of Gl. truncatulinoides in northern Gulf of Mexico sediment-trap samples is summarized in Fig. 1. The data show that the production and delivery of encrusted and non-encrusted forms of Gl. truncatulinoides to the sediments occur predominantly in winter with almost 90% of the flux occurring in January and February (Table 1). Mg/Ca were measured from encrusted and non-encrusted forms in January and February samples from 2008 to 2010. The Mg/Ca analyses show a clear separation between encrusted and non-encrusted individuals (Fig. 2). Non-encrusted Gl. truncatulinoides Mg/Ca are

consistently higher than Mg/Ca from encrusted forms by 1.00 ± 0.12 mmol/mol in 2008 samples, 0.99 ± 0.25 mmol/mol in 2009 samples, and 1.20 ± 0.12 mmol/mol in 2010 samples (Table 2). The Mg/Ca data indicate that in the northern Gulf of Mexico Gl. truncatulinoides grows its primary test in the upper part of the water column before descending and secreting a secondary crust in deeper and cooler waters, consistent with plankton-tow studies (Bé and Ericson, 1963; Hemleben et al., 1985; Lohmann and Schweitzer, 1990). Relatively little direct information is available on the depth distribution of Gl. truncatulinoides in the water column of the northern Gulf of Mexico. Thus, we infer the depth of calcification for nonencrusted Gl. truncatulinoides using available in-situ δ 18Oseawater and seawater temperature measurements at the sediment-trap site to calculate the equilibrium δ 18Ocalcite depth profiles for January 2008 and 2009 (Fig. 3, C). The equilibrium δ 18Ocalcite depth profiles were calculated using a rearrangement of the Orbulina low-light equation (Bemis et al., 1998): δ 18Oc = [(T − 16.5) − (4.8 ⁎ (δ 18Osw − 0.27))] / −4.8. We measured δ 18O in Gl. truncatulinoides from January 2008 and 2009 samples and plotted the measured δ 18O at the depth indicated by the calculated equilibrium δ 18Ocalcite depth profile. For example, the δ 18O measurements from January 2008, −0.57‰ and −0.47‰, indicate calcification depths of 97 and 108 m of water depth, respectively. Assuming that Gl. truncatulinoides calcifies in equilibrium with ambient seawater (Erez and Honjo, 1981; Deuser and Ross, 1989; Lončarić et al., 2006), the data indicate a calcification depth no deeper than ~ 120 m for non-encrusted Gl. truncatulinoides during January 2008 and 2009. The minimum error of our calcification depth estimates is on average ~±14 m. We determined this error by combining the error from analytical analysis (0.05‰), intra-sample precision (0.08‰), and the Orbulina low-light equation (0.16‰) (Bemis et al., 1998), and then converting the δ 18O error (±0.18‰) to depth by comparing the measured δ 18O ± 0.18‰ to the equilibrium δ 18Ocalcite depth profiles for January 2008 and 2009. To cross-check the depth assignments based on δ18O we then calculated temperatures for non-encrusted Gl. truncatulinoides from January 2008 and 2009 samples using the measured Mg/Ca and the multi-species calibration equation of Anand et al. (2003). The calculated temperatures are plotted on the observed temperature profile at the depth inferred from matching measured δ 18O to the calculated equilibrium δ 18Ocalcite depth profile (Fig. 3, D). For example, the Mg/Ca data from January 2008 (2.93 mmol/mol and 2.86 mmol/mol, which converts to 22.7 °C and 22.4 °C, respectively) are plotted at the calcification depths indicated by the paired δ 18O data at 97 and 108 m respectively. The third Mg/Ca-temperature estimate (22.5 °C) from January 2008 is not plotted because we did not have enough material for stable isotopic measurement, so we were unable to estimate the calcification depth of specimens from that sample. However, the 22.5 °C temperature estimate is consistent with a depth habitat within the winter surface-mixed layer. The calculated temperatures at inferred calcification depth are comparable to the observed temperatures from the CTD casts and support the interpretation that non-encrusted Gl. truncatulinoides are calcifying within the surface-mixed layer. The depth preference indicated by the comparison of measured δ 18O and equilibrium δ 18Ocalcite depth profiles suggests that Gl. truncatulinoides calcify near the base of the surface-mixed layer. The bottom of the surface-mixed layer in the Gulf of Mexico often coincides with the deep chlorophyll maximum, a subsurface maximum in the concentration of chlorophyll that is associated with the pycnocline (Hobson and Lorenzen, 1972). Previous studies have shown Gl. truncatulinoides and other planktic foraminifers occur in greater abundance at the deep chlorophyll maximum than in surrounding seawater (Fairbanks et al., 1980; Fairbanks and Wiebe, 1980; Ravelo et al., 1990). A comparison of measured δ 18O Gl. truncatulinoides from plankton tows in the southeast Atlantic to

J.W. Spear et al. / Marine Micropaleontology 80 (2011) 53–61 Table 1 Sample bottle schedule and flux of non-encrusted and encrusted Gl. truncatulinoides (tests m−2 day− 1) during 2008–2010. Gl. truncatulinoides flux Open

Close

14-Jan-08 21-Jan-08 28-Jan-08 4-Feb-08 11-Feb-08 18-Feb-08 25-Feb-08 3-Mar-08 10-Mar-08 17-Mar-08 24-Mar-08 31-Mar-08 7-Apr-08 14-Apr-08 21-Apr-08 28-Apr-08 5-May-08 12-May-08 19-May-08 26-May-08 2-Jun-08 9-Jun-08 16-Jun-08 23-Jun-08 30-Jun-08 7-Jul-08 14-Jul-08 21-Jul-08 31-Jul-08 7-Aug-08 14-Aug-08 21-Aug-08 28-Aug-08 4-Sep-08 11-Sep-08 18-Sep-08 25-Sep-08 2-Oct-08 9-Oct-08 16-Oct-08 26-Oct-08 2-Nov-08 9-Nov-08 16-Nov-08 23-Nov-08 30-Nov-08 7-Dec-08 14-Dec-08 21-Dec-08 28-Dec-08 4-Jan-09 9-Jan-09 16-Jan-09 23-Jan-09 30-Jan-09 6-Feb-09 13-Feb-09 20-Feb-09 27-Feb-09 6-Mar-09 20-Mar-09 27-Mar-09 10-Apr-09 17-Apr-09 24-Apr-09 8-May-09 15-May-09 22-May-09 27-Sep-09 4-Oct-09 11-Oct-09 25-Oct-09 1-Nov-09

21-Jan-08 28-Jan-08 4-Feb-08 11-Feb-08 18-Feb-08 25-Feb-08 3-Mar-08 10-Mar-08 17-Mar-08 24-Mar-08 31-Mar-08 7-Apr-08 14-Apr-08 20-Apr-08 28-Apr-08 5-May-08 12-May-08 19-May-08 26-May-08 2-Jun-08 9-Jun-08 16-Jun-08 23-Jun-08 30-Jun-08 7-Jul-08 14-Jul-08 21-Jul-08 28-Jul-08 7-Aug-08 14-Aug-08 21-Aug-08 28-Aug-08 4-Sep-08 11-Sep-08 18-Sep-08 25-Sep-08 2-Oct-08 9-Oct-08 16-Oct-08 23-Oct-08 2-Nov-08 9-Nov-08 16-Nov-08 23-Nov-08 30-Nov-08 7-Dec-08 14-Dec-08 21-Dec-08 28-Dec-08 4-Jan-09 8-Jan-09 16-Jan-09 23-Jan-09 30-Jan-09 6-Feb-09 13-Feb-09 20-Feb-09 27-Feb-09 6-Mar-09 13-Mar-09 27-Mar-09 3-Apr-09 17-Apr-09 24-Apr-09 1-May-09 15-May-09 22-May-09 27-May-09 4-Oct-09 11-Oct-09 18-Oct-09 1-Nov-09 8-Nov-09

Duration 7 7 7 7 7 7 7 7 7 7 7 7 7 6 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 4 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 5 7 7 7 7 7

Non-encrusted

Encrusted

63 101 87 76 32 51 8 33 10 0 1 1 1 1 0 1 0 0 1 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 31 14 24 109 7 32 61 44 5 14 10 5 3 4 3 2 4 2 3 0 0 0 0 0

74 352 158 270 63 59 66 25 13 5 2 0 2 3 4 5 1 4 9 2 1 1 2 2 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 24 54 93 115 149 46 515 194 19 55 21 5 22 2 7 11 2 1 4 2 0 1 0 0 0 (continued on next page)

59

Table 1 (continued) Gl. truncatulinoides flux Open

Close

Duration

Non-encrusted

Encrusted

8-Nov-09 22-Nov-09 29-Nov-09 13-Dec-09 20-Dec-09 27-Dec-09 10-Jan-10 17-Jan-10 24-Jan-10 28-Feb-10

15-Nov-09 29-Nov-09 6-Dec-09 20-Dec-09 27-Dec-09 3-Jan-10 17-Jan-10 24-Jan-10 7-Feb-10 7-Mar-10

7 7 7 7 7 7 7 7 14 7

0 0 0 0 3 21 70 156 259 16

0 0 0 0 1 2 207 98 192 38

A gap in sampling occurred from late May to late September 2009 due to scheduling problems. Nine samples representing the weeks of March 10, March 31, May 1, October 22, November 19, and December 10 of 2009 and January 3, February 7, February 14, and February 21 of 2010 were not recovered due to loss of the cups during deployment/recovery.

expected δ 18O in equilibrium with seawater at the depth of maximum shell concentrations indicates Gl. truncatulinoides is calcifying where it occurs in greatest numbers (Lončarić et al., 2006). When combined, the δ 18O and Mg/Ca data indicate the nonencrusted forms of Gl. truncatulinoides secrete their tests within the surface-mixed layer, most likely at the base of the surface-mixed layer. Thus, the Mg/Ca of non-encrusted Gl. truncatulinoides in sediments from the northern Gulf of Mexico has the potential to be a proxy for winter surface-mixed layer temperature. Analyses of Mg/Ca in non-encrusted Gl. truncatulinoides in sediment samples should provide information on past winter surface-mixed layer change.

Table 2 Raw Mg/Ca and δ18O data for non-encrusted (NC) and encrusted (C) Gl. truncatulinoides in the 300–425 μm size fraction from 2008 to 2010 winter sediment-trap material. Mid-week collection date

Type

Average Mg/Ca (mmol/mol)

18-Jan-08

NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C

2.87 1.98 2.93 1.83 2.86 – 2.93 – 2.69 1.73 3.58 2.41 3.16 1.83 2.72 1.92 2.99 2.00 2.81 1.84 3.21 1.93 3.07 2.15 2.79 2.13 2.85 1.58 2.77 1.50 2.79 1.72

25-Jan-08 1-Feb-08 8-Feb-08 15-Feb-08 22-Feb-08 13-Jan-09 20-Jan-09 27-Jan-09 3-Feb-09 10-Feb-09 17-Feb-09 24-Feb-09 14-Jan-10 21-Jan-10 31-Jan-10

1σ

0.17 0.13 0.18 0.12 0.14

0.11 0.05 0.27 0.38 0.19 0.06

δ18O (‰ VPDB)

1σ

− 0.59

0.16

− 0.47

0.07

− 0.38

0.30

− 0.52

0.14

− 0.25 − 0.63

0.02

− 0.39

0.07

− 0.44 0.03 − 0.49 0.15

− 0.55

0.05

0.01

− 0.21

0.01

− 0.27

0.02 0.04 0.05 0.01 0.03

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J.W. Spear et al. / Marine Micropaleontology 80 (2011) 53–61

3.2. G. truncatulinoides—sediment record To test the use of Gl. truncatulinoides Mg/Ca as a proxy for past winter surface-mixed layer temperature in Gulf of Mexico sediments we analyzed Gl. truncatulinoides in samples from the top few centimeters of core PBBC-1, which was recovered from the Pigmy Basin near the sediment-trap site (Fig. 1). We selected this core for a pilot study for several reasons. The core has a well-constrained chronology. The age model is based on 7 accelerator mass spectrometer radiocarbon dates over the 59 cm long box core. The youngest radiocarbon date (at 1.5– 2.0 cm) suggests the top two centimeters of the core contains nuclear bomb carbon. Thus, the top of the core is likely post 1950 A.D. The 7 dates indicate a uniform accumulation rate over the entire interval (R2 = 0.996, Richey et al., 2007, supplemental material). We cannot assign an absolute calendar age to the top of the core, but we are confident that the top of the core is post 1950 A.D., and that the interval we included in our study spans about 100 years. Mg/Ca data from the surface dwelling planktic foraminifer G. ruber (white variety) are available from the core (Richey et al., 2007) and initial inspection found that samples from the top portion (upper 6 cm) of the core contain fairly abundant and well-preserved representatives of both encrusted and non-encrusted Gl. truncatulinoides. Thus, adequate material was present to analyze both encrusted and non-encrusted forms. Analyses of Gl. truncatulinoides in the PBBC-1 sediment samples reveal a distinct separation of Mg/Ca values between non-encrusted and encrusted forms with higher Mg/Ca values occurring in nonencrusted forms. The difference varies from a high of 0.95 mmol/mol to a low of 0.33 mmol/mol (average offset = 0.63 ± 0.23 mmol/mol; Table 3). These results indicate that Mg/Ca values from encrusted and non-encrusted forms of Gl. truncatulinoides in the sediment samples reflect the different calcification depths of primary and secondary calcite as they do in the sediment-trap samples. The Mg/Ca and resulting temperature estimates using the equation of Anand et al. (2003) from non-encrusted Gl. truncatulinoides in samples from the top 6 cm of core PBBC-1 are compared to the G. ruber (white) Mg/Ca and resulting SST estimates from the same samples in Fig. 4. The Mg/Ca-SST estimates from the planktic foraminifer G. ruber (white variety) are interpreted as a record of mean-annual SST (Richey et al., 2007). Although details differ, both the Gl. truncatulinoides and G. ruber Mg/Ca records show a general warming trend over the last ~100 years. However, the overall temperature change indicated by Gl. truncatulinoides Mg/Ca (~2.5 °C)

Table 3 Raw Mg/Ca and δ18O data for non-encrusted (NC) and encrusted (C) Gl. truncatulinoides in the 300–500 μm size fraction from sediment core PBBC-1. Depth (mm)

Type

n

Mg/Ca (mmol/mol)

7.5

NC C NC C NC C NC C NC C NC C NC C NC C NC C NC C

33 33 33 15 36 15 30 15 28 15 32 15 26 15 31 15 23 15 16 15

2.57 1.68 2.39 1.44 2.33 1.67 2.33 1.77 1.99 1.66 2.00 1.61 2.24 1.34 1.92 1.47 1.88 1.41 1.92 1.27

12.5 17.5 22.5 27.5 37.5 42.5 47.5 52.5 57.5

δ18O (‰ VPDB)

is slightly larger than the change indicated by the G. ruber (white) Mg/Ca (~ 1.5 °C). Direct comparison of instrumental and sediment records can be challenging because sediment samples represent material accumulated over a number of years. For example, the age model for PBBC-1 indicates each 0.5 cm sample represents approximately 20 years. Bioturbation can cause additional mixing. However, the trends shown in Fig. 4 are consistent with available long-term sea-surface temperature data from the Gulf of Mexico. We used data from the most recent version of the Extended Reconstructed Sea-Surface Temperature (ERSST.v3b) (NOAA_ERSST_V3 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/) compilation to calculate mean-annual SST in the 2° × 2° degree grid centered near the location of PBBC-1 and found it increased by about 1 °C between 1900 and 2010. The change in winter (January–February) SST calculated from ERSST is also about 1 °C, but the difference between the increase calculated from ERSST and the 2.5 °C Mg/Ca-temperature estimate is within the error of the Mg/Ca-temperature estimates. Our results suggest that Mg/Ca of non-encrusted Gl. truncatulinoides is a promising proxy for assessing past winter surface-mixed layer temperature.

4. Summary and conclusions Analyses of foraminifer assemblages in sediment-trap samples from the northern Gulf of Mexico show that Gl. truncatulinoides is abundant in the winter months and essentially absent during the rest of the year. Almost 90% of the production and delivery of tests of Gl. truncatulinoides to the sediments of the northern Gulf of Mexico occur in January and February. Mg/Ca and δ 18O analyses of the tests of Gl. truncatulinoides from the sediment-trap samples combined with measured in situ water column temperature and δ 18Oseawater indicate that non-encrusted forms of Gl. truncatulinoides calcify within the winter surface-mixed layer, whereas encrusted forms calcify in deeper and cooler waters. Since Mg/Ca in planktic foraminifer tests is a function of temperature at the time the test is formed, these results indicate that Mg/Ca from non-encrusted Gl. truncatulinoides can be used as a proxy for temperature variation in the winter surfacemixed layer. Results from a pilot study of Gl. truncatulinoides from a core recovered near the sediment-trap site are consistent with the results from the sediment-trap samples and show the potential of nonencrusted Gl. truncatulinoides Mg/Ca as a proxy for Gulf of Mexico winter surface-mixed layer temperature. A short record of past winter surface-mixed layer temperature variability from non-encrusted Gl. truncatulinoides Mg/Ca reveals a warming trend similar to that observed in a proxy record of mean-annual SST from G. ruber (white) Mg/Ca from the same samples.

0.08 − 0.58 − 0.30 − 0.12 − 0.09 − 0.29 − 0.15

Acknowledgments We thank Eric Tappa, Don Hickey, and Chris Reich for assistance in collecting sediment-trap material; Elizabeth Gordon for the δ 18Osw data; Adis Muslic, Kathryn Richwine, and Emily Wallace for laboratory support; Jen Flannery and Chris Maupin for instrumental support; and Julie Richey, Ben Flower, and two anonymous reviewers for helpful suggestions. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

− 0.11 0.07

Appendix A. Supplementary data

0.04

Supplementary data to this article can be found online at doi:10. 1016/j.marmicro.2011.05.001.

J.W. Spear et al. / Marine Micropaleontology 80 (2011) 53–61

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