86Sr values over the Early–Middle Frasnian boundary (Late Devonian) recorded in well-preserved conodont elements from the Holy Cross Mountains, Poland

Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology 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 / p a l a e o

A positive trend in seawater 87Sr/86Sr values over the Early–Middle Frasnian boundary (Late Devonian) recorded in well-preserved conodont elements from the Holy Cross Mountains, Poland Eleanor H. John ⁎, Robert Cliff, Paul B. Wignall Department of Earth Sciences, School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK

A R T I C L E

I N F O

Article history: Received 07 November 2006 Accepted 22 April 2008 Keywords: Devonian Early–Middle Frasnian boundary Strontium isotopes Geochemistry Conodonts Holy Cross Mountains Poland

A B S T R A C T Over the Early–Middle Frasnian boundary interval (Late Devonian), the oceans experienced major changes in global carbon cycling and nutrient dynamics with a prominent positive carbon isotope shift at the base of the Palmatolepis punctata Zone (Middle Frasnian). The aim of this study was to construct seawater 87Sr/86Sr ratio curves for the Pa. transitans–Pa. hassi zonal interval using well-preserved conodont apatite from two sections in the Holy Cross Mountains, south-central Poland. The final curves suggest that seawater 87Sr/86Sr values were steady at ~ 0.70794–0.70799 during the transitans Zone but began to rise in the early punctata Zone towards a value of ~ 0.70814 in the late punctata Zone (an increase of ~ 0.0002). The increase in seawater 87Sr/ 86 Sr values over the Early–Middle Frasnian boundary appears to be the second of two prominent short-term rises in 87Sr/86Sr values, which led to an overall shift in values from a Givetian plateau of between ~0.70780 and 0.70785 to a Late Frasnian plateau of between 0.7080 and 0.7082. Correlation with a regional positive δ13C anomaly and evidence for eutrophication in the marine environment suggests that the causes of these phenomena may be linked and that the positive 87Sr/86Sr shift was caused by an increase in the continental flux of Sr to the oceans, i.e., the weathering flux. A possible explanation for an increase in the weathering flux is the denudation of a new Acadian–Eovariscan orogen, which would have been mostly weathered in the humid, warm tropical/equatorial region. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Detailed information about Devonian biogeochemical cycles has been largely restricted to the Frasnian–Famennian (Mid–Late Devonian) stage boundary interval. This interval is associated with one of the most severe mass extinction events of the Phanerozoic and has been linked to positive δ13C excursions in the marine record, bottom water anoxia, fluctuations in sea level and changes in climate (e.g., Wilde and Berry, 1986; Thompson and Newton, 1989; Buggisch, 1991; Joachimski and Buggisch, 1993; Becker and House, 1994; McGhee, 1996; Sepkoski, 1996; Hallam and Wignall, 1997; Joachimski et al., 2001; Bond et al., 2004). However, there were several intervals during the Devonian that were also associated, at least regionally, with apparent perturbations in carbon cycling, marine anoxia and faunal turnover but which are not linked to major extinction events (e.g., House, 1985; Streel et al., 2000; Sageman et al., 2003; Racki et al., 2004; Yans et al., 2007). House (2002) emphasised the need for further investigation into such “events” in order to properly under-

⁎ Corresponding author. Fax: +44 113 343 5259. E-mail address: [email protected] (E.H. John). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.04.031

stand Devonian palaeoenvironmental change and to put the Frasnian– Famennian extinction interval into its proper Devonian context. This study focuses on the interval spanning the Early–Middle Frasnian (E–MF) boundary, which corresponds to the boundary between the Palmotolepis transitans and Pa. punctata Zones (as formally fixed by the Subcommission on Devonian Stratigraphy, Becker and House, 1998; Ziegler and Sandberg, 2001; SDS Business Meeting, Leicester, July 2006). δ13Ccarbonate and δ13Corg data from the Holy Cross Mountains, Poland, and the Ardennes, Belgium, have revealed a major positive shift in δ13C near the base of the punctata Zone (of up to 5.85‰ in the Ardennes; Yans et al., 2007), which marks the beginning of a prolonged positive anomaly lasting until the late part of this zone (punctata Isotopic Event; Racki et al., 2004; Pisarzowska et al., 2006; Yans et al., 2007). This excursion coincides with evidence for a punctuated rise in sea level, transgression-related eutrophication, elevated primary productivity, dysoxia in the water column and prominent shifts in conodont biofacies, at least in the studied Polish sections (e.g., Johnson et al., 1985; Sandberg et al., 2002; Over et al., 2003; Racki et al., 2004; Pisarzowska et al., 2006; Sobstel et al., 2006). The E–MF boundary interval does not, however, coincide with a major extinction event or a major breakdown in carbonate production (Racki et al., 2004; Pisarzowska et al., 2006). It is not yet clear if the above changes were global in extent or restricted to the

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

south Laurussian shelf, but the δ13C data of Yans et al. (2007) and Ma et al. (2008-this issue) from Moravia and South China show a similar increase suggesting that the major perturbations in carbon cycling were indeed widespread. To better understand these changes, further investigation into biogeochemical cycles over the E–MF boundary interval is clearly needed. This study investigates changes in seawater 87Sr/86Sr values over the Palmatolepis transitans Zone–Palmatolepis hassi Zone interval using well-preserved conodonts from two well-studied Polish sections in order to better understand the processes behind the reported major perturbations in carbon cycling and the observed changes in ecosystem dynamics. The marine 87Sr/86Sr ratio is largely controlled by the amount and isotopic composition of Sr entering the oceans in rivers and groundwaters, which tends to have a higher 87Sr/ 86 Sr ratio than Sr entering the oceans from hydrothermal systems (average 87Sr/86Sr of modern rivers ~ 0.712, average 87Sr/86Sr of vent fluids ~0.7034; Palmer and Edmond, 1989). Thus, changes in ancient seawater 87Sr/86Sr ratios may reveal changes in continental weath-

167

ering, which also has implications for shallow-marine productivity and/or bottom water anoxia. 2. Materials and methods One of the major challenges in reconstructing ancient marine 87Sr/ Sr ratios is finding material that reliably records the primary isotopic signal of the ambient seawater in which it precipitated. Reconstructing Palaeozoic seawater 87Sr/86Sr values is particularly difficult because of the relative paucity of suitable material for analysis and the likelihood of diagenetic overprint in Palaeozoic rocks. Low-Mg brachiopod calcite is still considered to be the most reliable fossil material for measuring Palaeozoic marine Sr isotope ratios (Popp et al., 1986; Veizer et al., 1986; Brand, 2004) but the biostratigraphic resolution provided by brachiopods in rocks of this age is poor. Conodonts have long been proposed as a suitable alternative for this time period because they offer excellent biostratigraphical resolution (0.6 Myr average conodont zone duration in the most highly resolved 86

Fig. 1. a — Map showing the Late Devonian palaeogeography of Poland and location of the Holy Cross Mountains; b — Location of Wietrznia quarry in the context of Early–Middle Frasnian palaeogeography (a and b after Racki, 1993; Figs. 1 and 2, respectively). c — Sketch map of the Wietrznia quarry showing the location of Wietrznia I and the sampled sections (modified from Pisarzowska et al., 2006, Fig. 6).

168

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

parts of the Devonian, 1 Myr average conodont zone duration over the entire Devonian period; Kaufmann, 2006). Conodonts are also abundant in a variety of marine facies, they contain relatively high concentrations of Sr (typically several thousand ppm; Kürschner et al., 1992; Holmden et al., 1996) and they provide their own measure of thermal alteration (CAI, conodont Colour Alteration Index). They are, however, believed to be more susceptible to diagenetic alteration and there are several guidelines (see below) that should be followed to avoid conodonts whose 87Sr/86Sr ratio has been altered by diagenesis (Bertram et al., 1992; Kürschner et al., 1992; Martin and Macdougall, 1995; Holmden et al., 1996; Ruppel et al., 1996; Ebneth et al., 1997; Trotter et al., 1999). Conodonts were chosen for analysis in this study due to the lack of suitable brachiopod material in known E–MF boundary sections and the discovery of conodonts with low CAI (b1.5) from limestone sections in the Holy Cross Mountains, Poland (Fig. 1), where they have been used in biostratigraphical and palaeoenvironmental studies (Pisarzowska et al., 2006; Sobstel et al., 2006). The conodonts chosen for analysis came from the Wietrznia Id-W (WId-W) and Wietrznia Ie (WIe) sections in the Wietrznia I Quarry in the Kielce region of the Holy Cross Mountains (designated after Pisarzowska et al., 2006, see

detailed descriptions of these sections therein). These foreslope successions were deposited on the Kielce carbonate platform, marked by the growth of the Dyminy Reef, on the southern part of the Laurussian shelf (Szulczewski, 1971; Racki, 1993). The same sections have already been investigated for changes in δ13Ccarbonate, δ13Corg and δ18O in conodont apatites by Pisarzowska et al. (2006) and Pisarzowska (2008), thus making it easy to directly compare changes in the different isotope age curves. The specimens were ultrasonically bathed in 18 MΩ water, dried and examined under binocular microscope. All specimens had a CAI of 1.5 or less and so could be considered further for 87Sr/86Sr analysis (Bertram et al., 1992; Kürschner et al., 1992). The conodonts were then selected on the basis of their morphology. Several studies have shown that the basal tissue of conodonts is more susceptible to diagenetic alteration than the ‘white matter’ and the highly crystalline hyaline apatite that make up the crown and compose the teeth-like denticles (Bertram et al., 1992; Holmden et al., 1996; Trotter et al., 1999). Thus, either coniform elements should be selected for analysis or as much basal tissue should be removed as possible prior to analysis. However, coniform elements are very scarce in the Devonian so those with large denticles (i.e., a high proportion of crown material) were mostly used

Fig. 2. SEM images of conodonts from the WId-W and WIe sections. a and b — Well-preserved conodont elements with much denticle material; c — well-preserved denticle fragment; d — conodont element displaying recrystallisation and adhering silicates (rejected for Sr isotope analysis), e — SEM image of polished block revealing internal microstructure of a conodont element.

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

and, where possible, individual denticles were removed from the main body using a fine blade. The selected conodonts were then examined under the SEM to check for secondary mineralisation and adhering matrix. Those with a smooth surface and evidence of original microstructure were assumed to be well-preserved and suitable for Sr isotope analysis (Fig. 2a–c) and those with obvious blocky crystal overgrowths, mineralised microfractures and/or large patches of silicate matrix were rejected (Fig. 2d). Polished blocks of eight conodonts were prepared and all showed clear retention of internal microstructure under the SEM including regular bands/laminations within crystalline hyaline apatite and pore spaces within white matter (Fig. 2e). Each well-preserved conodont fragment was ultrasonically bathed in acetone to remove the adhesive used to attach it to the SEM stub and then submerged in 0.5 ml 0.5% highest quality laboratory grade acetic acid for 12–16 h to remove the surface layer of apatite and expose the internal lamellae of the conodont element. This ‘leaching’ process has been recommended by several authors who have demonstrated that the core of the conodont element yields more reliable 87Sr/86Sr values than the outer layers which are more susceptible to incorporation of Sr from pore-waters (Martin and Macdougall, 1995; Ruppel et al., 1996; Trotter et al., 1999). This was tested at the University of Leeds by comparing the 87Sr/86Sr ratios of the ‘leached’ conodont element with those of its ‘leachate’ solution. The latter were consistently slightly higher than the former suggesting that the outer layers were more enriched in 87Sr, as would be expected given that pore-waters tend to have a higher 87Sr/86Sr than seawater due to the dissolution of clay minerals. This difference was only in the order of 5 × 10− 6, which is smaller than the minimum error in the sample values. However, this process was carried out as it has the added benefit of removing any remaining matrix. The conodonts were then rinsed and spiked with a small amount of highly 84Sr-enriched solution and dissolved in 3 M HNO3. Sr was extracted from the acid solution via a single pass elution through Eichrom Sr-resin using 0.05 M HNO3 UpA. The dried-down Sr extract was loaded onto a tungsten filament using a TaCl5 cocktail and analysed for its 87Sr/86Sr ratio in the Thermo-Finnigan Triton Thermal Ionisation Mass Spectrometer (TIMS). A mean value for SRM 987 (same as NIST 987) was 0.710273 ± 0.000003 (2σmean, n = 8). Each conodont sample value was then normalised to an assumed SRM 987 value of 0.710248 by subtracting 2.5 × 10− 5 from each measured ratio (McArthur, 1994). A correction for 87Rb did not affect the final 87Sr/86Sr value to six decimal places; in all cases, the 85Rb/86Sr was less than 3 × 10− 5. The final data were also corrected for ‘blank’ Sr using measurements from spiked blank runs, although these did not alter the ratios to six decimal places (all blank measurements yielded b10 pg Sr). The calculated error (presented as 2σmean) incorporated possible errors in a) blank and spike corrections (errors in the amount and composition of added spike), b) the measured value of 85 Rb/86Sr, c) assumed values of 87Rb/85Rb (0.386 ± 10% error) and d) the error in the average SRM 987 measurement used to normalise the data. A minimum 2σmean error of 0.00001 was adopted as this was the minimum error observed in the SRM 987 measurements. In most cases, the additional factors included in the error had an effect of no more than 0.00001 on the final 87Sr/86Sr values. 3. Results The 87Sr/86Sr values for the two sections are between ~0.70791 and ~0.70814 (Table 1; Fig. 3). The 2σmean errors are generally consistent between 1 × 10− 5 (applied minimum) and 3 × 10− 5, which is low considering that realistic limits of resolution for this technique are currently estimated to be 2.0 × 10− 5 based on measured interlaboratory differences in 87Sr/86Sr (McArthur and Howarth, 2004). The amount of Sr extracted from each sample varied considerably from 2 ng to over 1323 ng, probably due to natural variation in the size of

169

Table 1 Results of Sr isotope analyses on conodont denticles from the WIe and WId-W sections Sample

Conodont zone

87

Sr/86Sr

WIe section WIe 233 WIe 225-1 WIe 225-2 WIe 209-1 WIe 209-2 WIe 205 WIe 195 WIe 172 WIe 171 WIe 169-1 WIe 169-2 WIe 105-1 WIe 105-2 WIe 105-3 WIe 60 WIe 48 WIe 27

hassi punctata punctata punctata punctata punctata punctata punctata punctata punctata punctata transitans transitans transitans transitans transitans transitans

0.7080797 0.7081396 0.7081486 0.7080956 0.7080457 0.7081087 0.7080620 0.7080207 0.7080387 0.7080067 0.7080128 0.7080886 0.7080057 0.7080178 0.7080620 0.7079516 0.7080157

0.000017 0.000030 0.0000147 0.000050 0.000013 0.000011 0.000030 0.000010 0.000010 0.000016 0.000009 0.000029 0.000025 0.000024 0.000010 0.000006 0.000030

WId-W section WId-W56 WId-W54 WId-W51-1 WId-W51-2 WId-W49 WId-W45-1 WId-W45-2 WId-W43-1 WId-W43-2 WId-W27-1 WId-W27-2 WId-W25-1 WId-W25-2 WId-W21-1 WId-W21-2 WId-W15-1 WId-W15-2 WId-W15-3

hassi punctata punctata punctata punctata punctata punctata punctata punctata transitans transitans transitans transitans transitans transitans transitans transitans transitans

0.7080650 0.7081346 0.7081086 0.7080360 0.7080636 0.7080297 0.7079816 0.7079804 0.7080017 0.7079407 0.7079587 0.7079407 0.7079667 0.7079127 0.7079207 0.7079427 0.7079437 0.7079820

0.000012 0.00001 0.000029 0.00005 0.000025 0.000013 0.000021 0.00001 0.000032 0.000012 0.000014 0.000016 0.000012 0.000014 0.000018 0.000011 0.000016 0.000009

2σmean

the conodont elements and the size of the denticle fragments used. Those containing less than ~ 2 ng Sr were rejected because of the larger errors associated with measurements on such small amounts of Sr but most (N70%) of the samples yielded between 30 and 200 ng Sr. The Sr isotope data from the two sections are presented in Fig. 3 against sedimentary logs constructed by Pisarzowska et al. (2006) and conodont biozones. Due to facies constraints, the standard index palmatolepid species, Pa. transitans and Pa. punctata, were scarce over the transitans–punctata zonal transition and their appearance was somewhat delayed in the later punctata Zone. Therefore, the appearance of ancyrodellid species, which are more abundant in these sections and could be correlated with the palmatolepid zones (with the aid of chemostratigraphic proxies), were used as alternative zonation markers (Pisarzowska et al., 2006). The Early Frasnian (Mesotaxis falsiovalis Zone) was identified by the association of Ancyrodella rugosa and the ‘late forms’ of A. alata and A. rotundiloba; the transitans Zone was identified by the appearance of A. africana and A. pramosica in great abundance; the appearance of A. gigas (forms 1 and 2) was used to mark the punctata Zone and the earlier part of the hassi Zone was identified by the appearance of A. curvata (Klapper, 1989; Sandberg and Ziegler, 1989; Racki and Bultynck, 1993; Pisarzowska et al., 2006). Thus, the Sr isotope age curves in this study span the early transitans–early hassi zonal interval. Gaps in the curve (e.g., for the late A. africana–A. pramosica interval; Figs. 7–8 in Pisarzowska et al., 2006) are present either because of a lack of conodonts or because those available were poorly preserved or lacked sufficient crown tissue. The A. africana–A. pramosica interval in the WId-W section is dominated by argillaceous sedimentary rocks (= Śluchowice Marly Level of Pisarzowska et al., 2006) and it is not

170 E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

Fig. 3. Seawater

87

Sr/86Sr curves for the WId-W and WIe sections with sedimentary logs and carbon isotope data from Pisarzowska et al. (2006, Fig. 7). Error bars denote 2σmean values.

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

recommended that conodonts from clastic lithologies are used for Sr isotope analysis (Ebneth et al., 1997; Veizer et al., 1997, 1999). The general trend observed in both curves is a gradual increase in 87 Sr/86Sr values of ~ 0.0002 from the early punctata Zone, culminating in a maximum in the late punctata Zone, and followed by a decrease into the earliest hassi Zone. This is clearest in the WId-W section curve in which 87Sr/86Sr values remain relatively constant in the transitans Zone (between ~ 0.70794 and ~ 0.70799) and begin a steady rise in the early punctata Zone towards a value of 0.708135 ± 0.00001 in the late punctata Zone (with A. gigas form 2). 87Sr/86Sr values then decrease by ~ 0.00007 to a value of 0.708065 ± 0.000012 in the early hassi Zone. This pattern is not as clear in the WIe section data but the curve does reproduce the overall trend of the WId-W section, albeit with more scatter in the transitans–early punctata data. A maximum value of 0.708148 ± 0.000014 (average of two datapoints) is reached in the late punctata Zone and values then fall to 0.708079 ± 0.000017 in the hassi Zone. 4. Discussion 4.1. A reliable, global signature? Given that the conodont material used for analysis stood up to rigorous screening for diagenetic alteration and that the measured values are consistent with existing data from this interval based on wellpreserved brachiopod calcite (Veizer et al., 1999; Van Geldern et al., 2006), the first choice material for reconstructing Palaeozoic seawater 87 Sr/86Sr values, it is reasonable to assume that the values are representative of original seawater. Whether or not the values are a global representative of the Devonian oceans depends, firstly, on whether the conodonts analysed lived in a marine environment with an open ocean connection and, secondly, whether the Late Devonian oceans were homogeneous with respect to their Sr isotope composition over the timescales investigated and thus whether the measured changes were only regional in extent. An open marine connection is supported by the regional geology and the fact that the fauna are typical of Late Devonian shallow-marine carbonate shelves (Pisarzowska et al., 2006). For the Late Devonian oceans to have been uniform with respect to their 87Sr/86Sr ratio, the oceanic residence time of Sr in the Late Devonian should have been greater than the time it took to mix the oceans. Unless the oceanic mixing time was considerably longer in the Devonian and the residence time of Sr much shorter than today, uniformity of oceanic 87Sr/86Sr is to be expected. However, as with any ancient time period, oceanic uniformity with respect to 87Sr/86Sr ratios should be confirmed by comparison with data from distant palaeogeographic locations. The lower resolution Sr isotope curve of Zheng and Liu (1997), based mostly on well-preserved primary micrites in South China (Longmen Mountains; see discussion in Ma et al., 2008-this issue), has one datapoint near the falsiovalis–transitans Zone boundary of 0.70792, one from the transitans Zone of 0.70799, and three consecutive values from the punctata–Early hassi zonal interval of 0.70804, 0.70806 and 0.70814. These values are generally consistent with our data and support the idea that the trend revealed in this study is not a regional one. The fact that the data in this study are also consistent with the datapoints of Veizer et al. (1999), which were measured in sections deposited on another part of the south Laurussian shelf (see Section 4.2), also supports this.

171

Mountains, Germany (Fig. 7 of Veizer et al., 1999). Close correlation between Veizer et al.'s (1999) datapoints and those in this study is not possible but their values suggest that there was a steep rise in marine 87 Sr/86Sr ratios in the transitans Zone, slightly earlier than reported here, although this could be a result of correlation difficulties (Veizer et al., 1999, p. 70, acknowledge biostratigraphic uncertainties in their chosen sections). Published Sr isotope curves for the entire Devonian period suggest that the early Late Devonian witnessed an increase from an Eifelian– Givetian (Middle Devonian) 87Sr/86Sr plateau of ~ 0.7078–0.7079, to Late Frasnian–Famennian values of between 0.7080 and 0.70815 (Fig. 4; Veizer et al., 1999; Van Geldern et al., 2006). The positive trend over the transitans–punctata interval reported in this study seems to represent a prominent, rapid, short-term increase superimposed on this long-term rise. The data of Van Geldern et al. (2006; Fig. 4) suggest that there may have been an additional earlier short-term rise immediately prior to the Givetian–Frasnian boundary in which values increased by ~0.0001 from ~0.70782 to ~0.70792. Thus, rather than there being a gradual increase in seawater 87Sr/86Sr values over the early Late Devonian, it seems that the increase may have been concentrated in two short-term positive shifts: one at the end of the Givetian and the other at the start of the punctata Zone, with relatively constant values in the intervening falsiovalis and early transitans Zones (Fig. 4). The timescale over which the transitans–punctata increase occurred is uncertain and the duration of Devonian conodont zones is continually being revised. The duration of the 0.0002 excursion almost spans the entire punctata Zone. According to the timescale of House and Gradstein (2004), this zone has an approximate duration of ~1.8 Myr which would give a rate of increase of ~0.00011 Myr− 1. According to the revised timescale of Kaufmann (2006), based on biostratigraphic correlation and U–Pb ID-TIMS ages from ash bands, the punctata Zone is only ~ 0.6 Myr in duration, thus giving a much more rapid increase in 87Sr/86Sr (~0.00033 Myr− 1). To put both these estimates in context, the fastest known rate of increase in marine 87Sr/ 86 Sr ratios over the Phanerozoic is a rate of ~ 0.00014 Myr− 1 over the Late Permian–Early Triassic interval (McArthur et al., 2001). Interestingly, the interval of elevated 87Sr/86Sr values (early–late punctata Zone) corresponds to a major positive δ13C anomaly (an increase of +2–5‰ in δ13Corg and δ13Ccarbonate curves) observed in Poland, Belgium and China (Fig. 3; Racki et al., 2004; Pisarzowska et al., 2006; Yans et al., 2007; Ma et al., 2008-this issue; Pisarzowska, 2008), as well as regional evidence in the Holy Cross Mountains for: 1) intermittent sea-level rise (beginning with the IIc deepening event of Johnson et al., 1985, at the E–MF boundary), as exemplified by the retreat of the Dyminy Reef (Pisarzowska et al., 2006); 2) a shift to an impoverished conodont fauna, largely consisting of opportunistic genera (Sobstel et al., 2006); 3) evidence for eutrophication and intermittent anoxia in the water column (summarized in Pisarzowska et al., 2006); and 4) regional synsedimentary tectonics and block tilting (e.g., Szulczewski, 1971; Racki, 1998; Racki and Narkiewicz, 2000; Pisarzowska et al., 2006), thus possibly suggesting a common causal mechanism. 4.3. Interpretation of trends

4.2. Comparison with other Late Devonian

87

Sr/86Sr values

Putting the results into a Frasnian context is hindered by the lack of published seawater 87Sr/86Sr data for the Late Devonian. Veizer et al. (1997, 1999) have published the highest resolution data for the interval investigated in this study, with 11 datapoints in the falsiovalis–hassi zonal interval based on measurements of well-preserved secondary layer brachiopod calcite from sections in the Eifel

If we firstly assume that the trend is a global one, we must try to decipher the change in terms of changes in the fluxes of Sr to the oceans, on a global scale. The three main fluxes of Sr to the oceans are: (1) the continental flux where Sr is released from rocks via weathering and transported to the sea in rivers and groundwater; (2) the release of Sr during hydrothermal exchange of seawater Sr with that in midocean ridge basalt; and (3) the release of Sr during the diagenetic

172

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

Fig. 4. Seawater 87Sr/86Sr curve for the Devonian period showing the data of Van Geldern et al. (2006) and the datapoints from this study. Modified from Fig. 5 of Van Geldern et al. (2006). Conodont zones from Weddige (1996) and timescale from STG (2002).

alteration of buried carbonates by percolating fluids (e.g., Brass, 1976; Chaudhuri and Clauer, 1986; Palmer and Edmond, 1989; Francois and Walker, 1992). The weathering flux has an average 87Sr/86Sr ratio today of ~ 0.712 (Palmer and Edmond, 1989) but this ratio depends upon the proportions of the different rocks types being weathered globally (weathering of old sialic rocks: 87Sr/86Sr ~ 0.718–0.730; weathering of young sialic rocks: 87Sr/86Sr ~0.705; weathering of basalts: ~0.703; weathering of sedimentary carbonates: 87Sr/86Sr ~ 0.708) (e.g., Francois and Walker, 1992). The 87Sr/86Sr ratio of modern hydrothermal fluids, however, does not vary significantly between locations and is close to that of the mantle (~ 0.7034; Palmer and Edmond, 1989; Davis et al., 2003). Diagnostically, the diagenetic flux has an 87Sr/86Sr ratio close to that of the ocean (between 0.7068 and

0.7091; modern 87Sr/86Sr values ~ 0.7084; Elderfield and Gieskes, 1982). A sufficiently large change in the relative size and/or the isotopic composition of each flux may serve to alter the isotopic composition of Sr in the open ocean. Interpretation of the observed trend over the E–MF boundary interval in terms of changes in the global Sr budget is hindered by a lack of knowledge about Late Devonian controls on the size and ratio of the Sr fluxes (e.g., no information about rock types and weathering rates in major erosional belts). However, a brief assessment of plausible scenarios was made based on current understanding of the Sr budget and consideration of possible differences between the modern world and the Late Devonian. Rigorous modelling of the observed trend was not the objective of this study.

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

Because the diagenetic flux has a 87Sr/86Sr ratio close to that of the ocean in which it originally precipitated and is likely to have been an order of magnitude smaller than the continental and hydrothermal flux (the modern diagenetic flux of Sr to the oceans is ~5–7% of the total oceanward Sr flux; Jones and Jenkyns, 2001), it is unlikely that changes in the diagenetic flux alone could produce a significant rise in oceanic 87Sr/86Sr values. A change in the hydrothermal flux could have theoretically caused the observed increase, either through a decrease in the size of the flux or an increase in its 87Sr/86Sr ratio. However, modern estimates suggest that the hydrothermal flux is only 10–35% the size of the continental flux and so, unless the hydrothermal flux was significantly greater during the Late Devonian, a very large change would have been needed to have caused such an increase in what should be a global trend (Wolery and Sleep, 1988; Palmer and Edmond, 1989). Indeed, the E–MF boundary interval is actually linked to a time of pulsed eustatic rises in sea level, which indirectly suggests an increase in sea-floor spreading rates, and thus an increase in hydrothermal circulation, at mid-ocean ridges, rather than a decrease (Johnson et al., 1985; Sandberg et al., 2002; Over et al., 2003). Given that the isotopic composition of mid-ocean ridge basalt is believed to be relatively constant at ~0.7025–0.703, a temporal increase in the 87 Sr/86Sr of vent fluid could only occur if the efficiency of the exchange of seawater Sr with that in basalt decreased. Even if there were zero exchange between seawater Sr and basaltic Sr, the vent fluid would simply have the same 87Sr/86Sr composition as seawater and thus would not affect the oceanic 87Sr/86Sr ratio. Thus, a change in the continental flux is the most attractive explanation of the observed increase in 87Sr/86Sr values. Indeed, the weathering flux dominates the global Sr budget and so changes in this term exert the major control on seawater 87Sr/86Sr ratios (Palmer and Edmond, 1989; Francois and Walker, 1992; Richter et al., 1992). Changes in the size of the weathering flux over time are also inevitable as they can be caused by variations in a wide variety of factors including tectonic activity, continent configuration, climate and vegetation cover. The seawater 87Sr/86Sr record has often been used to track the tectonic evolution of the earth with periods of rising 87Sr/86Sr values corresponding to periods of intense uplift and rapid denudation of newly formed orogens. Whether or not the transitans–punctata interval, specifically, represents a short-lived episode of enhanced tectonic uplift is poorly constrained because of inaccuracies in dating techniques. However, there is worldwide evidence for Late Devonian orogenic activity with the incipient collision of the Laurussian, Gondwanan, Kazakhstanian and Siberian plates (the Acadian–Eovariscan orogeny). The continental margins of these plates were uplifted and deformed to form several orogenic belts including the Ellesmerian–Svalbardian belt in the Arctic (e.g., Thorsteinsson and Tozer, 1970), the central Asian belt (e.g., Sengör and Nataliin, 1996), the North African Variscide belt (e.g., Echarfaoui et al., 2002), parts of the Appalachian orogenic belt (e.g., Murphy and Keppie, 1998), the Southern Uralian belt (e.g., Matte, 1995) and the Variscan belt in Europe (e.g., Tait et al., 1997; Tribovillard et al., 2004) with minor subduction-related uplift events in Western America, South America and Eastern Australia (Averbuch et al., 2005). Using stratigraphical techniques, Thorsteinsson and Tozer (1970) suggested that Ellesmerian uplift began towards the end of the Middle Devonian, and Echarfaoui et al. (2002) reported that the major compressional phase in Morocco folded all beds older than the Late Frasnian. Averbuch et al. (2005) suggested a radiometric age of major Acadian–Eovariscan metamorphism of ~ 360–380 Ma, which could encompass the E–MF boundary interval based on recent geochronological timescales (House and Gradstein, 2004; Kaufmann, 2006). Thus, the increase in values across the E–MF boundary interval possibly occurred due to a rapid and widespread pulse in tectonic uplift at this time and the longer term increase in seawater 87Sr/86Sr values may reflect the longer term increase in orogenesis. Most of the major mountain belts

173

were in tropical or equatorial regions where chemical weathering would have been accelerated in the warm and humid climate, thus releasing more continental Sr (and nutrients) to the oceans. Furthermore, erosion of a newly formed orogen may also serve to exhume old sialic basement rocks with very high 87Sr/86Sr ratios, thus adding to the amount of the 87Sr-rich flux entering the oceans. Climate change is an attractive explanation for short-term variations in seawater 87Sr/86Sr ratios as climatic variables can change greatly on very short timescales compared to other terms and can affect both the rate of chemical weathering and/or the ability to transport the weathered Sr to the sea (i.e., the amount of runoff/runout). An increase in global temperature may cause an increase in the rate of chemical weathering of Sr-bearing minerals thus releasing more Sr (and nutrients) to rivers and groundwaters; an accelerated hydrological cycle, usually linked to climatic warming, would have the additional effect of increasing runoff as well as enhancing chemical weathering rates. The Frasnian is believed to have been a time of greenhouse climates with sea surface temperatures around 25 °C during the early Frasnian and a warming trend towards the Frasnian–Famennian boundary (Brand et al., 1989; Becker and House, 1994; Joachimski et al., 2004; Van Geldern et al., 2006; Simon et al., 2007). Chemical weathering rates and rates of runoff/runout are thus likely to have been high during the Frasnian stage. Existing δ18Ocalcite, δ18Oapatite and modelling data for the Late Devonian suggest that the transitans–punctata interval lay at the beginning of the warming trend but no marked climatic event over the E–MF transition which could have exacerbated continental weathering has yet been detected, mainly because of the low resolution of the palaeoclimate proxy data for this specific interval. However, a weak cooling trend over the E–MF boundary interval was interpreted from δ18Oapatite data (Pisarzowska, 2008), suggesting that elevated greenhouse conditions and/or an accelerated hydrological cycle were not responsible for the rise in seawater 87Sr/86Sr values. Several authors have proposed that the influence of vascular plants on global weathering rates (and global nutrient dynamics) increased during the Late Devonian because of an accompanying increase in pedogenesis (Algeo et al., 1995; Berner, 1997). Although vascular plants were present on land in the Silurian and Early Devonian (e.g., Edwards, and Richardson, 2004), major colonisation of the land by vascular plants occurred during the Middle Devonian and would have been accompanied by widespread soil formation (Edwards and Berry, 1991). The Late Devonian also saw the rise of tree-sized vascular plants (namely Archaeopteris) which would have greatly increased the depth of the soil profile and caused more intense plant-related chemical weathering in their larger root systems. Following the model of Algeo et al. (1995), perhaps the E–MF boundary interval corresponded to a period of enhanced pedogenesis and chemical weathering in soils leading to an increased flux of Sr and nutrients to the sea. The precise timing of these changes in plant development, however, is not known and whether an increase in soil development could have caused a large enough increase in continental weathering is questionable. It is of course possible that the observed trend was not caused by a change in a single variable, but by a combination of smaller changes in the aforementioned fluxes. Also, in their modelling of changes in the different ratios/fluxes for the Jurassic, Waltham and Gröcke (2006) showed that smoothly changing parameters can cause dramatic changes in the seawater 87Sr/86Sr curve without changing greatly themselves because a critical threshold was crossed. It is therefore not possible to rule out the possibility that the increase in oceanic 87Sr/ 86 Sr over the E–MF boundary interval was merely a product of gradual changes in more than one of the terms in the global Sr flux. 5. Conclusions 87

Sr/86Sr ratios were measured in well-preserved conodont elements from two Early–Middle Frasnian boundary sections in the Wietrznia I quarry of the Holy Cross Mountains, south south-central

174

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175

Poland. The two isotope curves suggest that oceanic 87Sr/86Sr values were quite constant during the transitans Zone but began to rise in the early punctata Zone. By the later stages of the punctata Zone, values had increased by ~ 0.0002 to a maximum of ~ 0.70814. This increase appears to represent a sharp positive shift in the Devonian seawater 87 Sr/86Sr curve, superimposed on a longer term increase over the Frasnian before values level off in the Late Frasnian. This interval of increasing seawater 87Sr/86Sr values coincides with a major positive δ13C anomaly in Poland, Belgium and China, and with regional evidence for intermittent sea-level rise, marine eutrophication and prominent changes in conodont faunas. The most plausible explanation for the observed rise in 87Sr/86Sr values is an increase in the continental weathering flux of Sr to the ocean, possibly caused by an episode of intense tectonic uplift associated with the onset of the Acadian–Eovariscan Orogeny. The orogens formed in this episode of uplift would have been mostly weathered in the warm, humid tropical and equatorial regions where chemical weathering rates would have been higher. Exhumation of old sialic basement may have further contributed to rising marine 87 Sr/86Sr values. Developments in vascular plants and soil formation, climatic warming and/or an accelerated hydrological cycle may have also added to the enhanced chemical weathering rates and this could be reconciled through detailed investigation of palaeoclimatic proxies. Acknowledgements Many thanks to the Natural Environment Research Council (NERC) for the studentship award and to Dr. Christine Rogers for her support in the laboratory. Additional thanks to Dr. Jared Morrow, Dr. Robert van Geldern and Prof. Finn Surlyk whose comments helped improve this manuscript. References Algeo, T.J., Berner, R.A., Maynard, J.B., Scheckler, S.E., 1995. Late Devonian oceanic anoxic events and biotic crises: “rooted” in the evolution of vascular land plants. GSA Today 5, 64–66. Averbuch, O., Tribovillard, N., Devleeschouwer, X., Riquier, L., Mistiaen, B., van VlietLanoe, B., 2005. Mountain building-enhanced continental weathering and organic carbon burial as major causes for climatic cooling at the Frasnian–Famennian boundary (c. 376 Ma)? Terra Nova 17, 25–34. Becker, R.T., House, M.R., 1994. Kellwasser Events and goniatite successions in the Devonian of the Montagne Noire with comments on possible causations. In: Königshof, P., Werner, R. (Eds.), Willi Ziegler-Festschrift II. Cour. Forsch.-Inst. Senckenberg, vol. 169, pp. 45–77. Becker, R.T., House, M.R., 1998. Proposal for an international substage subdivision of the Frasnian. SDS Newslett. 15, 17–22 http://sds.uta.edu/Newsletter15/nl15body.htm. Berner, R.A., 1997. The rise of plants and their effect on weathering and atmospheric CO2. Science 276, 544–546. Bertram, C.J., Elderfield, H., Aldridge, R.J., Conway Morris, S., 1992. 87Sr/86Sr, 143Nd/144Nd, and REEs in Silurian phosphatic fossils. Earth Planet. Sci. Lett. 133, 239–249. Bond, D., Wignall, P.B., Racki, G., 2004. Extent and duration of marine anoxia during the Frasnian–Famennian (Late Devonian) mass extinction in Poland, Germany, Austria and France. Geol. Mag. 141, 173–193. Brand, U., 2004. Carbon, oxygen and strontium isotopes in Palaeozoic carbonate components: an evaluation of original seawater-chemistry proxies. Chem. Geol. 204, 23–44. Brand, U., Legrand-Blain, M., Streel, M., 1989. Global climatic changes during the Devonian–Mississippian: stable isotope biogeochemistry of brachiopods. Palaeogeogr. Palaeoclimatol. Palaeoecol. 75, 311–329. Brass, G.W., 1976. The variation of the marine 87Sr/86Sr ratio during Phanerozoic time: interpretations using a flux model. Geochem. Cosmochim. Acta. 40, 721–730. Buggisch, W., 1991. The global Frasnian–Famennian “Kellwasser Event”. Geol. Rundsch. 80, 49–72. Chaudhuri, S., Clauer, N., 1986. Fluctuations of isotopic composition of strontium in seawater during the Phanerozoic eon. Chem. Geol. (Isot. Geosci. Sect.) 59, 293–303. Davis, A.C., Bickle, M.J., Teagle, D.A.H., 2003. Imbalance in the oceanic strontium budget. Earth Planet. Sci. Lett. 211, 173–187. Ebneth, S., Diener, A., Buhl, D., Veizer, J., 1997. Strontium isotope systematics of conodonts: Middle Devonian, Eifel Mountains, Germany. In: Geldsetzer, H.H.J., Joachimski, M.M. (Eds.), Geochemical Event Markers in the Phanerozoic. Palaeogeogr. Palaeoclimatol. Palaeoecol., vol. 132, pp. 79–96. Echarfaoui, H., Hafid, M., Aït Salem, A., 2002. Seismic structure of the Doukkala Basin, Paleozoic basement, Western Morocco: a hint for an Eovariscan fold-and-thrust belt. C.R. Geosci. 334, 13–20.

Edwards, D., Berry, C.M., 1991. Silurian and Devonian. In: Cleal, C.J. (Ed.), Plant Fossils in Geological Investigations: The Palaeozoic. Ellis Horwood Series in Applied Geology Ltd., UK, pp. 117–153. Edwards, D., Richardson, J.B., 2004. Silurian and Lower Devonian plant assemblages from the Anglo-Welsh Basin: a palaeobotanical and palynological synthesis. Geol. J. 39, 375–402. Elderfield, H., Gieskes, M., 1982. Sr isotopes in interstitial waters of marine sediments from Deep Sea Drilling Project cores. Nature 300, 493–497. Francois, L.M., Walker, C.G., 1992. Modelling the Phanerozoic carbon cycle and climate: constraints from the 87Sr/86Sr isotopic ratio of seawater. Am. J. Sci. 292, 81–135. Hallam, A., Wignall, P.B., 1997. Mass Extinctions and their Aftermath. Oxford University Press, Oxford. 320 pp. Holmden, C., Creaser, R.A., Muehlenbachs, K., Bergstrom, S.M., Leslie, S.A., 1996. Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies. Earth Planet. Sci. Lett. 142, 425–437. House, M.R., 1985. Correlation of mid-Paleozoic ammonoid evolutionary events with global sedimentary perturbations. Nature 313, 17–22. House, M.R., 2002. Strength, timing and cause of mid-Palaeozoic extinctions. In: Racki, G., House, M.R. (Eds.), Late Devonian Biotic Crisis: Ecological, Depositional and Geochemical Records. Palaeogeogr. Palaeoclimatol. Palaeoecol., vol. 181, pp. 5–25. House, M.R., Gradstein, F.M., 2004. The Devonian Period. In: Gradstein, F., Ogg, J., Smith, A. (Eds.), A Geologic Time Scale 2004. Cambridge Univ. Press, Cambridge, pp. 202–221. Jones, C.E., Jenkyns, H.C., 2001. Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous. Am. J. Sci. 301, 112–149. Joachimski, M.M., Buggisch, W., 1993. Anoxic events in the late Frasnian — causes of the Frasnian–Famennian faunal crisis? Geology 21, 675–678. Joachimski, M.M., Ostertag-Henning, C., Pancost, R.D., Strauss, G., Freeman, K.H., Littke, R., Sinninghe Dammste, J.S., Racki, G., 2001. Water column anoxia, enhanced productivity and concomitant changes in δ13C and δ34S across the Frasnian–Famennian boundary (Kowala — Holy Cross Mountains/Poland). Chem. Geol. 175, 109–131. Joachimski, M.M., van Geldern, R., Breisig, S., Day, J., Buggisch, W., 2004. Oxygen isotope evolution of biogenic calcite and apatite during the Middle and Upper Devonian. Int. J. Earth Sci. 93, 542–553. Johnson, J.G., Klapper, G., Sandberg, C.A., 1985. Devonian eustatic fluctations in Euramerica. Geol. Soc. Amer. Bull. 96, 567–587. Kaufmann, B., 2006. Calibrating the Devonian Time Scale: a synthesis of U–Pb ID-TIMS ages and conodont stratigraphy. Earth Sci. Rev. 76, 175–190. Klapper, G., 1989. The Montagne Noire Frasnian (Upper Devonian) conodont succession. In: McMillan, N.J., Embry, A.F., Glass, A.F. (Eds.), Devonian of the World. Vol. III, Palaeontology, Palaeoecology and Biostratigraphy. Canadian Society Petroleum Geologists Memoir, vol. 14, pp. 449–468. Kürschner, W., Becker, R.T., Buhl, D., Veizer, J., 1992. Strontium isotopes in conodonts: Devonian–Carboniferous transition, the northern Rhenish Slate Mountains Germany. Ann. Soc. Géol. Belg. 115, 595–621. Ma, X.P., Wang, C.Y., Racki, G., Racka, M., 2008-this issuee. Facies and geochemistry across the Early–Middle Frasnian transition (Late Devonian) on South China carbonate shelf: comparison with the Polish reference succession. In: Racki, G., Joachimski, M.M., Morrow, J.R. (Eds.), A Major Perturbation of the Global Carbon Budget in the Early–Middle Frasnian Transition (Late Devonian). Palaeogeogr. Palaeoclimatol. Palaeoecol. Martin, E.E., Macdougall, J.D., 1995. Sr and Nd isotopes at the Permian/Triassic boundary: a record of climate change. Chem. Geol. 125, 73–99. Matte, P., 1995. Southern Uralides and Variscides: compilation of their anatomies and evolutions. Geol. Mijn. 74, 151–166. McArthur, J.M., 1994. Recent trends in strontium isotope stratigraphy. Terra Nova 6, 331–358. McArthur, J.M., Howarth, R.J., 2004. Strontium isotope stratigraphy. In: Gradstein, F., Ogg, J., Smith, A. (Eds.), A Geologic Time Scale 2004. Cambridge Univ. Press, Cambridge, pp. 96–105. McArthur, J.M., Howarth, R.J., Bailey, T.R., 2001. Strontium isotope stratigraphy: LOWESS Version 3: best fit to the marine Sr-isotope curve for 0–509 Ma and accompanying look-up table for deriving numerical age. J. Geol. 109, 155–170. McGhee, G.R., 1996. The Late Devonian Mass Extinction: The Frasnian/Famennian Crisis. Columbia Univ. Press, New York. Murphy, J.B., Keppie, J.D., 1998. Late Devonian palinspastic reconstruction of the Avalon–Meguma terrane boundary: implication for terrane accretion and basin development in the Appalachian orogen. Tectonophysics 284, 221–231. Over, D.J., Hopkins, T.L., Brill, A., Spaziani, A.L., 2003. Age of the Middlesex Shale (Upper Devonian, Frasnian) in New York State. In: Königshof, P., Schindler, E. (Eds.), MidPalaeozoic Bio- and Geodynamics. The North Gondwana–Laurussia Interaction. Cour. Forsch.-Inst. Senckenberg, vol. 242, pp. 217–223. Palmer, M.R., Edmond, J.M., 1989. The strontium isotope budget of the modern ocean. Earth Planet. Sci. Lett. 92, 11–26. Pisarzowska, A. 2008. Geochemia stabilnych izotopów węgla i tlenu na pograniczu dolnego i środkowego franu (dewon górny) na obszarze południowego szelfu Laurussii. Ph.D. Thesis, Silesian Univ., Sosnowiec, Poland. Pisarzowska, A., Sobstel, M., Racki, G., 2006. Conodont-based stratigraphy of the Early– Middle Frasnian transition on South Polish carbonate shelf. In: Baliński, A., Olempska, E., Racki, G. (Eds.), Biotic Aspects of the Early–Middle Frasnian Eventful Transition. Acta Palaeont. Pol., vol. 51, pp. 609–646. Popp, B.N., Podosek, F.A., Brannon, J.C., Anderson, T.F., Pier, J., 1986. 87Sr/86Sr ratios in Permo-Carboniferous sea water from the analyses of well-preserved brachiopod shells. Geochim. Cosmochim. Acta 50, 1321–1328. Racki, G., 1993. Evolution of the bank to reef complex in the Devonian of the Holy Cross Mountains. Acta Palaeont. Pol. 37, 87–182.

E.H. John et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 166-175 Racki, G., 1998. Frasnian–Famennian biotic crisis; undervalued tectonic control? Palaeogeogr. Palaeoclimatol. Palaeoecol. 141, 177–198. Racki, G., Bultynck, P., 1993. Conodont biostratigraphy of the Middle to Upper Devonian boundary beds in the Kielce area of the Holy Cross Mountains. Acta Geol. Pol. 43, 1–33. Racki, G., Narkiewicz, M., 2000. Tektoniczne a eustatyczne uwarunkowania rozwoju sedymentacji dewonu świętokrzyskiego. Przegl. Geol. 48, 65–76. Racki, G., Piechota, A., Bond, D., Wignall, P., 2004. Geochemical and ecological aspects of lower Frasnian pyrite-ammonite level at Kostomłoty (Holy Cross Mountains, Poland). In: Racki, G., Narkiewicz, M. (Eds.), Multidisciplinary Event Approaches to the Devonian Stratigraphic Record. Geol. Q., vol. 48, pp. 267–282. Richter, F.M., Rowley, D.B., DePaolo, D.J., 1992. Sr isotope evolution of seawater: the role of tectonics. Earth Planet. Sci. Lett. 109, 11–23. Ruppel, S.C., James, E.W., Barrick, J.E., Nowlan, G., Uyeno, T.T., 1996. High-resolution 87Sr/ 86 Sr chemostratigraphy of the Silurian: implications for event correlation and strontium flux. Geology 24, 831–834. Sageman, B.B., Murphy, A.E., Werner, T.P., Ver Straeten, C.A., Hollander, D.J., Lyons, T.W., 2003. A tale of shales: the relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian basin. Chem. Geol. 195, 229–273. Sandberg, C.A., Ziegler, W., 1989. New standard conodont zones and early Ancyrodella phylogeny across Middle–Upper Devonian boundary. In: Walliser, O.H., Ziegler, W. (Eds.), Contributions to Devonian Palaeontology and Stratigraphy. Cour. Forsch.Inst. Senckenberg, vol. 110, pp. 195–230. Sandberg, C.A., Morrow, J.R., Ziegler, W., 2002. Late Devonian sea-level changes, catastrophic events, and mass extinctions. In: Koeberl, C., MacLeod, K.G. (Eds.), Catastrophic Events and Mass Extinctions: Impacts and Beyond. Geol. Soc. Am. Spec. Paper, vol. 356, pp. 473–487. Sengör, A.M., Nataliin, B., 1996. Palaeotectonics of Asia. In: Yin, A., Harrison, T.M. (Eds.), The Tectonic Evolution of Asia. Cambridge Univ. Press, Cambridge, pp. 486–640. Sepkoski, J.J., 1996. Patterns of Phanerozoic extinction: a perspective from global data bases. In: Walliser, O.H. (Ed.), Global Events and Event Stratigraphy in the Phanerozoic. Springer, Berlin, pp. 35–51. Simon, L., Goddéris, Y., Buggisch, W., Strauss, H., Joachimski, M.M., 2007. Modeling the carbon and sulphur isotope compositions of marine sediments: climate evolution during the Devonian. Chem. Geol. 246, 19–38. Sobstel, M., Makowska-Haftka, M., Racki, G., 2006. Conodont ecology in the Early– Middle Frasnian transition (Late Devonian) of Holy Cross Mts, southern Poland. In: Baliński, A., Olempska, E., Racki, G. (Eds.), Biotic Aspects of the Early–Middle Frasnian Eventful Transition. Acta Palaeont. Pol., 51, pp. 719–746. Stratigraphic Table of Germany (STG, 2002), 2002. German Stratigraphic Commission. GeoForschungsZentrum (GFZ), Potsdam. ISBN 3-00-010197-7. Streel, M., Caputo, M.V., Lobozial, S., Melo, J.H.G., 2000. Late Frasnian–Famennian climates based on palynomorph analyses and the question of the Late Devonian glaciations. Earth Sci. Rev. 52, 121–173. Szulczewski, M., 1971. Upper Devonian conodonts, stratigraphy and facial development in the Holy Cross Mts. Acta Geol. Pol. 21, 1–129. Tait, J., Bachtadse, V., Franke, W., Sofel, H., 1997. Geodynamic evolution of the European Variscan fold belt: paleomagnetic and geological constraints. Geol. Rundsch. 86, 585–598. Thompson, J.B., Newton, C.R., 1989. Late Devonian mass extinction: episodic climatic cooling or warming? In: McMillan, N.J., Embry, A.F., Glass, D.J. (Eds.), Devonian of

175

the World, Vol. III, Palaeontology, Palaeoecology and Biostratigraphy. Canadian Society Petroleum Geologists Memoir, vol. 14, pp. 29–34. Thorsteinsson, R., Tozer, E.T., 1970. Geology of the Arctic Archipelago. In: Douglas, R.J.W. (Ed.), Geology and Economic Minerals of Canada. Geological Survey of Canada, Economic Geology Report No. 1, pp. 547–590. Tribovillard, N., Averbuch, O., Devleeschouwer, X., Racki, G., Riboulleau, A., 2004. Deep water anoxia over the Frasian–Famennian boundary (La Serre, France): a tectonically induced oceanic anoxic event? Terra Nova 16, 288–295. Trotter, J.A., Korsch, M.J., Nicoll, R.S., Whitford, D.J., 1999. Sr isotope variation in single conodont elements: implications for defining the Sr seawater curve. In: Sepagli, E. (Ed.), Seventh European conodont symposium: Studies on conodonts. Boll. Soc. Paleont. Ital., 37, pp. 507–514. Van Geldern, R., Joachimski, M.M., Day, J., Jansen, U., Alvarez, F., Yolkin, E.A., Ma, X.-P., 2006. Carbon, oxygen and strontium isotope records of Devonian brachiopod shell calcite. In: Buggisch, W. (Ed.), Evolution of the System Earth in the Late Palaeozoic: Clues from Sedimentary Geochemistry. Palaeogeogr. Palaeoclimatol. Palaeoecol., vol. 240, pp. 47–67. Veizer, J., Fritz, P., Jones, B., 1986. Geochemistry of brachiopods: oxygen and carbon isotopic records of Paleozoic oceans. Geochim. Cosmochim. Acta 50, 1679–1696. Veizer, J., Buhl, D., Diener, A., Ebneth, S., Podlaha, O.G., Bruckschen, P., Jasper, T., Korte, C., Schaaf, M., Ala, D., Azmy, K., 1997. Strontium isotope stratigraphy: potential resolution and event correlation. In: Geldsetzer, H.H.J., Joachimski, M.M. (Eds.), Geochemical Event Markers in the Phanerozoic. Palaeogeogr. Palaeoclimatol. Palaeoecol., vol. 132, pp. 65–77. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. In: Allègre, C.J., Arndt, N.T., Bickle, M.J., et al. (Eds.), Earth System Evolution: Geochemical Perspective. Chem. Geol., vol. 161, pp. 59–88. Waltham, D., Gröcke, D.R., 2006. Non-uniqueness and interpretation of the seawater 87 Sr/86Sr curve. Geochim. Cosmochim. Acta 70, 384–394. Weddige, K. (Ed.), 1996. Devon-Korrelationstabelle [Devonian Correlation Table]. Senckenberg. Lethaea, vol. 76, pp. 267–289. Wilde, P., Berry, W.B.N., 1986. Role of oceanographic factors in the generation of global bioEvents. In: Walliser, O. (Ed.), Global Bio-Events. Springer-Verlag, Berlin, pp. 75–91. Wolery, T.J., Sleep, N.H., 1988. Interactions of geochemical cycles with the mantle. In: Gregor, C.B., Garrels, R.M., Mackenzie, F.T., Maynard, J.B. (Eds.), Chemical Cycles in the Evolution of the Earth. Wiley-Interscience, New York, pp. 77–103. Yans, J., Corfield, R.M., Racki, G., Préat, A., 2007. Evidence for major perturbation of carbon cycle in the middle Frasnian punctata conodont zone. Geol. Mag. 144, 263–270. Zheng, R.C., Liu, W.J., 1997. Carbon and strontium isotopic effects of the Devonian sequence in the Longmen Mountains area. Geol. Rev. 43, 264–272 (in Chinese with English abstract). Ziegler, W., Sandberg, C.A., 2001. Utility of palmatolepids and icriodontids in recognising Upper Devonian series, stage and possible stage boundaries. In: Bultynck, P. (Ed.), Subcommission on Devonian Stratigraphy. Recognition of Devonian Series And Stage Boundaries in Geological Areas. Cour. Forsch.-Inst. Senckenberg, vol. 225, pp. 335–347.