Conodont apatite d18O signatures indicate climatic cooling as a trigger of the Late Devonian mass extinction Michael M. Joachimski* Werner Buggisch
Institute of Geology and Mineralogy, University of Erlangen, Schlossgarten 5, 91054 Erlangen, Germany
ABSTRACT The oxygen isotopic composition of conodont apatite from two Frasnian-Famennian boundary sections was measured in order to reconstruct variations in marine paleotemperatures during the late Frasnian mass-extinction event. The measured conodont apatite d18O values reveal two positive excursions with maximum amplitudes of 11‰ to 11.5‰ that parallel positive excursions in the carbonate carbon isotopic composition. The 13‰ excursions in carbonate d13C have been interpreted as consequences of enhanced organic carbon burial rate resulting in a decrease in atmospheric CO2 concentration. Climatic cooling as a potential consequence of lower atmospheric CO2 concentration is confirmed by the conodont apatite d18O records, which translate into cooling of low-latitude surface waters by 5–7 8C. Repeated cooling of the low latitudes during the late Frasnian had a severe impact on the tropical shallow-water faunas that were probably adapted to warm surface-water temperatures and severely affected during the late Frasnian crisis. These prominent variations in ocean-water temperature were stressful to the tropical shallowwater fauna and potentially culminated in low origination rates of new species, one of the major factors of the decline in diversity during the latest Frasnian. Keywords: Late Devonian, mass extinction, apatite, INTRODUCTION The Late Devonian faunal crisis represents one of the big five Phanerozoic mass-extinction events, shallow-water tropical faunas being severely affected. The Devonian stromatoporoidcoral reef ecosystem vanished in the latest Frasnian as a consequence of the decimation of the most important reef constructors, such as rugose and tabulate corals, as well as stromatoporoids. The ultimate cause of this extinction event is still a matter of debate. A bolide impact or comet showers were considered as potential triggers (e.g., Wang et al., 1991; McGhee, 1996). However, no unequivocal geochemical data were presented that could support the idea of an impact at the Frasnian-Famennian boundary. Climatic warming of the lowlatitudes was favored by Thompson and Newton (1988). In contrast, Copper (1977, 1986) favored cooling of the low latitudes as the cause of the extinction of many tropical shallow-water organisms and the spreading of taxa formerly restricted to high latitudes into low latitudes. However, no high-resolution paleotemperature data were yet available that could support either assumption. Paleotemperature estimates may be derived from the oxygen isotopic composition of skeletons if the primary isotopic signals are preserved. Low-magnesium calcitic brachiopod shells have been preferentially used to estimate paleotemperatures of Paleozoic seas (e.g., Veizer et al., 1999). However, because *Corresponding author: e-mail, joachimski@ geol.uni-erlangen.de.
18O/16O,
paleotemperature.
Paleozoic brachiopods were preferentially abundant in shallow-water deposits, it becomes difficult to construct high-resolution d18O records. Conodonts may be considered an alternative because they are abundant in shallow as well deeper marine deposits and represent one of the most important faunal groups for Devonian biostratigraphy. Conodonts are composed of carbonate-fluorapatite and are interpreted to represent tooth-like jaw elements of an extinct marine chordate animal, usually referred to as the agnathan hagfishes. Previous attempts to use conodonts for d18Oapatite investigations (Luz et al., 1984) were hampered mainly by their small size (the weight of individual conodont element is ,100 mg). By using a laser-based microsampling technique (Sharp and Cerling, 1996; Wenzel et al., 2000), we were able to determine the d18O of conodont microsamples (0.5–1.0 mg) from two Frasnian-Famennian boundary sections. The presented conodont apatite d18O data suggest that prominent changes in sea-surface temperature occurred during the latest Frasnian and identify climatic cooling as a potential major cause of the Late Devonian mass extinction. MATERIALS AND METHODS Conodont elements were extracted from two Frasnian-Famennian boundary sections known to be rich in conodonts. Both sections were deposited in the Paleotethys basin at tropical to subtropical paleolatitudes. The Behringha¨user Tunnel section (Northern Rheinische Schiefergebirge, Germany) expo-
ses reefal limestones of Givetian to Frasnian age at the base of the section and Frasnian to Famennian light gray cephalopod limestones, which are unconformably overlain by early Carboniferous siliceous shales. The Kellwasser horizons—bituminous horizons generally deposited under anaerobic conditions in deeper water settings at the base of the Late rhenana conodont zone and at the FrasnianFamennian boundary—are not developed. The occurrence of shallow-water benthos within the cephalopod limestones argues for a depositional setting within mixed surface waters above the aerobic-anaerobic interface. The Frasnian-Famennian boundary section studied in the Vogelsberg quarry (Thu¨ringische Schiefergebirge, Germany) is represented by reddish cephalopod limestones with two intercalated gray units that are biostratigraphically dated as corresponding to the Kellwasser horizons. The repeated change from reddish to gray limestones is interpreted as evidence for a decrease in bottom-water oxygenation. However, the observation that the mottled gray limestones show no lamination argues against anaerobic depositional conditions. We therefore assume that the succession exposed in the Vogelsberg section was deposited partially under dysaerobic conditions and in greater water depths by comparison to the succession developed at the Behringha¨user Tunnel. Conodonts (;1 mg) were dissolved in nitric acid and chemically converted to Ag3PO4 using a slightly modified version of the method described by O’Neil et al. (1994). The oxygen isotopic composition was measured on CO2 generated by heating Ag3PO4-graphite mixtures with an infrared laser using a ThermoFinnigan MAT 252 mass spectrometer (Wenzel et al., 2000). The d18O values are reported in per mil deviation relative to V-SMOW (Vienna standard mean ocean water). Accuracy and reproducibility of the measurements (1s # 0.2‰) were monitored by multiple analyses of NBS120c and several phosphate reference samples (for details see Wenzel et al., 2000). Stable isotope analyses of carbonate carbon were performed on the conodont-bearing limestone samples using a Kiel I device connected to a ThermoFinnigan MAT 252 mass spectrometer. The d13C values are reported in per mil relative to V-PDB (Vienna Peedee belemnite). Accuracy of the carbon isotope measurements was controlled by replicate analyses
q 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; August 2002; v. 30; no. 8; p. 711–714; 2 figures.
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Figure 1. Oxygen isotopic composition of conodont apatite and carbon isotopic composition of carbonates for FrasnianFamennian boundary sections studied in Rheinische Schiefergebirge (Germany). Horizontal bars give analytical reproducibility (1s) for d18O. V-SMOW is Vienna mean standard ocean water; V-PDB is Vienna Peedee belemnite.
of NBS19 and laboratory standards. Reproducibilty was better than 60.03‰ (1s). RESULTS Carbon isotope ratios in the Behringha¨user Tunnel and Vogelsberg sections show two positive excursions with maximum amplitudes of 13‰ (Fig. 1). The maximum enrichment of the lower excursion is observed in the lower part of the Late rhenana condont zone and can be correlated with the deposition of a lower gray horizon in the Vogelsberg section that stratigraphically corresponds to the Lower Kellwasser horizon. The second excursion in d13C starts near the base of the linguiformis conodont zone and reaches maximum values in the Early triangularis zone. The d13C values decrease during the Middle triangularis conodont zone. The oxygen isotopic composition of conodont apatite seems to follow the d13Ccarb curves with a minor time lag. In the Behringha¨user Tunnel section background d18O values are ;117.4‰ (V-SMOW) and increase to maximum values of 119‰ in the Late rhenana zone and 118.5‰ in the Middle triangularis zone. Phosphatic fish remains mostly show d18O values comparable to conodont apatite d18O values. Background d18O values in the Vogelsberg section are between 117.5‰ and 118‰ (V-SMOW) and increase to maximum values of 118.7‰ in the Late rhenana zone and 119.0‰ (V-SMOW) in the Middle triangularis zone. 712
DISCUSSION The interpretation of conodont d18O data with respect to seawater temperatures requires that conodont apatite was precipitated in oxygen isotopic equilibrium with seawater, that the d18O signals did not reequilibrate during diagenesis, and it depends on knowledge concerning the life habitat of the conodont animal. Because the conodont animal became extinct in the Late Triassic, it is difficult to prove that conodont apatite was precipitated without any vital fractionation effect. That most analyzed phosphatic fish remains from the Behringha¨user Tunnel section are comparable in d18O to conodont apatite and that modern fish crystallize apatite in apparent isotopic equilibrium with seawater (Kolodny et al., 1983) suggest that conodont apatite was probably formed in isotopic equilibrium with ambient seawater. Diagenetic reequilibration may potentially alter the primary oxygen isotope signals. It is generally accepted that the oxygen isotopic composition of dense microcrystalline tooth enamel or conodont apatite has a high potential to preserve its primary signature. This is in contrast to strontium isotope compositions, and oxygen isotopic compositions of structurally bound carbonate or hydroxyl groups that are known to be affected by diagenetic processes (e.g., Holmden et al., 1996; Kohn et al., 1999). The issue of conodont life habitat is more difficult to address. Different conodont biofa-
cies are recognized from the Frasnian and Famennian and the distribution of conodonts is interpreted to have been controlled by water depth and distance from the shoreline (e.g., Ziegler and Sandberg, 1990). For example, all species of Palmatolepis are abundant in openmarine pelagic deposits, whereas representatives of Icriodus preferentially occur in shallowmarine sediments. However, it is not known whether Palmatolepis was thriving in nearsurface waters, in the deeper water column, or near the sediment-water interface. Oxygen isotope analyses of Palmatolepis and Icriodus subsamples from the Behringha¨user Tunnel section gave comparable d18O values, suggesting that both genera were thriving in the same environment. Because Icriodus is confined to shallow environments and thus to the uppermost water column, we argue that Palmatolepis was thriving in surface waters of the open ocean. Consequently, the conodont apatite d18O values are interpreted to represent surface-water d18O signals and may thus be used to calculate sea-surface paleotemperatures. Paleotemperatures are calculated by means of the phosphate paleotemperature equation given by Kolodny et al. (1983). However, these calculations require an estimate of the oxygen isotopic composition of Late Devonian seawater. The Frasnian to early Famennian interval is generally characterized as a warm equable climatic time period; there is no eviGEOLOGY, August 2002
Figure 2. Comparison of sea-level changes (Johnson et al., 1985), composite carbon isotope record (Joachimski and Buggisch, 1993; Joachimski et al., 2002; this study), variations in sea-surface temperature (SST) based on conodont apatite d18O (dotted line—Vogelsberg section), and major biotic events across Frasnian-Famennian boundary. KW is Kellwasser horizon; V-SMOW is Vienna standard mean ocean water: V-PDB is Vienna Peedee belemnite.
dence for significant ice sheets. Accordingly, a d18O value of Devonian seawater of 21‰ seems plausible. However, the oxygen isotopic composition of well-preserved Devonian brachiopod shells varies on average between 25‰ and 26‰ (Veizer et al., 1999), and these d18O values translate into unreasonably high paleotemperatures, if a Devonian seawater d18O value of 21‰ is assumed. As a consequence, Veizer et al. (1999) assumed that Paleozoic or Devonian seawater must have been depleted in 18O. The Devonian conodont and fish apatite d18O values reported in this study (17.1‰–19.1‰) agree relatively well with d18O values measured on fish remains from Early Cretaceous to early Eocene deposits formed at low latitudes (17.8‰–20.1‰; Kolodny and Raab, 1988). This observation is remarkable and contrasts the assumed secular change in the oxygen isotopic composition of seawater (Veizer et al., 1999). Because our conodont apatite d18O data seem not to support a significant decrease of Devonian seawater d18O in comparison to Mesozoic or Cenozoic seawater, a Late Devonian seawater d18O value of 21‰ is assumed. With this estimate, calculated paleotemperatures for the late Frasnian and early Famennian in the Behringha¨user Tunnel section are ;32 8C (Fig. 2). The recorded positive excursions in d18O translate into a temperature decrease of 5–6 8C to minimum values of ;26 8C. Measured conodont apatite d18O values from the Vogelsberg section give paleotemperatures of ;30 8C, the positive excursions in d18O translating into a temperature decrease of a maximum of 5 8C (Fig. 2). The calculated paleotemperatures indicate that (1) the assumption of a Devonian seawater d18O value of 21‰ V-SMOW results in relatively realistic tropical surfacewater temperatures, and (2) two significant cooling events occurred during the latest Frasnian. GEOLOGY, August 2002
Climatic cooling is predicted as result of a drawdown of atmospheric CO2 concentration as a consequence of an enhanced burial of organic carbon (Joachimski et al., 2002). Two positive excursions in the carbonate carbon isotope record with maximum amplitudes of 13‰ were interpreted as evidence for an increase in the organic carbon burial fraction on total carbon burial during the deposition of the Kellwasser horizons (Joachimski and Buggisch, 1993). The measured d13C patterns in the Behringha¨user Tunnel and Vogelsberg sections correspond to carbon isotope records reported from other Frasnian-Famennian boundary sections (Joachimski et al., 2001, 2002) and give further evidence for the global significance of this pattern. Assuming that the weathering flux from the continents and the riverine isotopic composition did not change, the late Frasnian 13‰ shifts require that the organic carbon burial fraction increased from 20% to 30%. Taking into account the oceanic carbon reservoir size and the carbon fluxes given by Kump (1991), these increases in the organic carbon burial rate result in an excess organic carbon sedimentation of 1.6 3 1018 and 4.6 3 1018 mol C for the Lower Kellwasser and the Frasnian-Famennian transition, respectively (Joachimski et al., 2002). If the volcanic CO2 degassing did not change significantly, the excess organic carbon burial is expected to have lowered CO2 concentrations in surface waters, and, because surface-water CO2 content is on a global scale in equilibrium with the atmospheric CO2 concentration, climate cooling is expected as an ultimate consequence. We argue that the increase in the organic carbon burial rate resulted in a drawdown of atmospheric CO2 concentration and culminated in the observed climatic cooling. This interpretation is supported by the comparison of the onsets in the positive shifts in d13Ccarb and d18Oapatite. The increase in d13Ccarb
predates the positive shift in d18Oapatite in both excursions in the Behringha¨user Tunnel and in the lower excursion in the Vogelsberg section. The earlier increase in d13Ccarb relative to d18Oapatite may suggest that a certain quantity of organic carbon had to be buried before atmospheric pCO2, and consequently surfacewater temperatures, decreased. The calculated 5–7 8C decrease of tropical sea-surface temperatures seems large in comparison to the assumed low-latitude cooling of 4–8 8C during the Pleistocene glaciations (Guilderson et al., 1994; Beck et al., 1997). The deposition of the Kellwasser horizons correlates with short-term transgressiveregressive cycles and a significant sea-level fall that occurred at the Frasnian-Famennian boundary (Fig. 2). The formation and melting of ice caps during the latest Frasnian as a consequence of an enhanced burial of organic carbon and concomitant climatic cooling could have caused these short-term sea-level changes (Buggisch, 1991). In this case, the icevolume effect on d18O of oceanic waters would reduce the calculated decrease in tropical paleotemperatures. However, no glacial deposits of Frasnian to early Famennian age have yet been reported. IMPLICATIONS FOR THE LATE FRASNIAN MASS EXTINCTION It is assumed that 50%–66% of all Frasnian genera vanished during the Late Devonian mass-extinction event (McGhee, 1996). Detailed studies of various faunal groups showed that low origination rates coupled with high extinction rates were probably responsible for the mass extinction (e.g., McGhee, 1988; Feist, 1991; Copper, 1998). It is noteworthy that tropical and subtropical pelagic and shallowwater ecosystems were heavily affected, whereas faunal groups living in deeper waters, high latitudes, or on continents seem to have been less affected. For example, 91% of brachiopods genera living in low-latitude shallow waters vanished (McGhee, 1996), whereas brachiopods from the cool-water Malvinokaffric faunal province were only removed at a 27% level (Copper, 1977). Ahermatypic rugose corals living in the deeper water column seem not to have been affected (Sorauf and Pedder, 1986). A bloom of glass and lithistid sponges—generally known for adaptation to cold waters—is reported from shallow-water deposits in New York State (McGhee, 1982) and other areas (Geldsetzer et al., 1993). The occurrence of labechiid stromatoporoids in the early Famennian was interpreted by Stearn (1987) as evidence for cold-water tolerance. Le´thiers and Raymond (1991) suggested that benthic ostracodes—a faunal group heavily affected by the late Frasnian extinction event— were probably adapted to very warm paleo713
temperatures and were affected by a dramatic decrease in the surface-water temperatures during the latest Frasnian. In summary, the late Frasnian extinction pattern seems to indicate that faunal groups adapted to warm tropical temperatures showed high losses, whereas groups tolerating cooler water temperatures were less affected and were invading shallow-water environments formerly occupied by tropical faunal groups. Climatic cooling of the low latitudes may severely affect stenothermal tropical taxa. Changing temperature will force organisms potentially adapted to very warm tropical temperatures during the latest Frasnian to adjust their physiology in order to compensate for this environmental change. Temperature has a major influence on the physiology of marine organisms in that it affects diffusion processes and reaction rates. According to Clarke (1993, p. 514) ‘‘the fate of a species depends not only on how well it can adjust its physiology to a new set of physical circumstances, but on how much this costs in energetic terms.’’ Species that are less effective with respect to a temperature compensation or are energetically less efficient will be at a selective disadvantage (Clarke, 1993). As a consequence, repeated paleotemperature changes during the latest Frasnian may have severely stressed faunal groups of the low-latitude ecosystems, and thus may have culminated in the low origination rates that seem to be the characteristic feature of the Late Devonian extinction event. We therefore argue that repeated and significant temperature changes during the latest Frasnian may be the key to the question concerning ‘‘the inhibiting factor that caused the cessation of new species originations’’ (McGhee, 1988, p. 256). ACKNOWLEDGMENTS We thank B. Wenzel and D. Lutz for help with conodont apatite d18O analyses. This study was financially supported by the Deutsche Forschungsgemeinschaft (projects Bu 312/31-1, Bu 312/31-2, and Bu 312/31-3). We acknowledge helpful reviews by P. Copper and C. Le´cuyer. REFERENCES CITED Beck, J.W., Recy, J., Taylor, F., Edwards, R.L., and Cabioch, G., 1997, Abrupt changes in early Holocene tropical sea surface temperatures derived from coral records: Nature, v. 385, p. 705–707. Buggisch, W., 1991, The global Frasnian-Famennian ‘‘Kellwasser Event’’: Geologische Rundschau, v. 80, p. 49–72. Clarke, A., 1993, Temperature and extinction in the sea: A physiologist’s view: Paleobiology, v. 19, p. 499–518.
714
Copper, P., 1977, Paleolatitudes in the Devonian of Brazil and the Frasnian-Famennian extinction event: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 21, p. 165–207. Copper, P., 1986, Frasnian-Famennian mass extinction and cold water oceans: Geology, v. 14, p. 835–839. Copper, P., 1998, Evaluating the Frasnian-Famennian mass extinction: Comparing brachiopod faunas: Acta Palaeontologica Polonica, v. 43, p. 137–154. Feist, R., 1991, The Late Devonian trilobite crisis: Historical Biology, v. 5, p. 97–214. Geldsetzer, H.H.J., Goodfellow, W.D., and McLaren, D.J., 1993, The Frasnian-Famennian extinction event in a stable cratonic shelf setting: Trout River, Northwest Territories, Canada: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 104, p. 81–95. Guilderson, T.P., Fairbanks, R.G., and Rubenstone, J.L., 1994, Tropical temperature variations since 20 000 years ago: Modulating interhemispheric climate change: Science, v. 263, p. 663–665. Holmden, C., Creaser, R.A., Muehlenbachs, K., Bergstro¨m, S.M., and Leslie, S.A., 1996, Isotopic and trace elemental systematics of Sr and Nd in 454 Ma biogenic apatites: Implications for paleoseawater studies: Earth and Planetary Science Letters, v. 142, p. 425–437. Joachimski, M.M., and Buggisch, W., 1993, Anoxic events in the late Frasnian—Causes of the Frasnian-Famennian faunal crisis?: Geology, v. 21, p. 675–678. Joachimski, M.M., Ostertag-Henning, C., Pancost, R., Strauss, H., Freeman, K.H., Littke, R., Sinninghe Damste´, J.S., and Racki, G., 2001, Water column anoxia, enhanced productivity and concomitant changes in d13C and d34S across the Frasnian-Famennian boundary (Kowala– Holy Cross Mountains/Poland): Chemical Geology, v. 175, p. 109–131. Joachimski, M.M., Pancost, R.D., Freeman, K.H., Ostertag-Henning, C., and Buggisch, W., 2002, Carbon isotope geochemistry of the Frasnian-Famennian transition: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 181 (in press). Johnson, J.G., Klapper, G., and Sandberg, C.A., 1985, Devonian eustatic fluctuations in Euramerica: Geological Society of America Bulletin, v. 96, p. 567–587. Kohn, M.E., Schoeninger, M., and Barker, W.W., 1999, Altered states: Effects of diagenesis on fossil tooth chemistry: Geochimica et Cosmochimica Acta, v. 63, p. 2737–2747. Kolodny, Y., and Raab, M., 1988, Oxygen isotopes in phosphatic fish remains from Israel: Paleothermometry of tropical Cretaceous and Tertiary shelf waters: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 64, p. 59–64. Kolodny, Y., Luz, B., and Navon, O., 1983, Oxygen isotope variations in phosphate of biogenic apatites, I. Fish bone apatite—Rechecking the rules of the game: Earth and Planetary Science Letters, v. 64, p. 398–404. Kump, L.R., 1991, Interpreting carbon-isotope excursions: Strangelove oceans: Geology, v. 19, p. 299–302. Le´thiers, F., and Raymond, D., 1991, Les crises du
De´vonien supe´rieur par l’e´tude des faunes d’ostracodes dans leur cadres pale´ogeographiques: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 88, p. 133–146. Luz, B., Kolodny, Y., and Kovach, J., 1984, Oxygen isotope variations in phosphate of biogenic apatites, III. Conodonts: Earth and Planetary Science Letters, v. 69, p. 255–262. McGhee, G.R., 1982, The Frasnian-Famennian extinction event: A preliminary analysis of Appalachian marine ecosystems, in Silver, L.T., and Schultz, P.H., eds., Geological implications of impacts of large asteroids and comets on the Earth: Geological Society of America Special Paper 190, p. 491–500. McGhee, G.R., 1988, The Late Devonian extinction event: Evidence for abrupt ecosystem collapse: Paleobiology, v. 14, p. 250–257. McGhee, G.R., 1996, The Late Devonian mass extinctions: New York, Columbia University Press, 303 p. O’Neil, J.R., Roe, J.L., Reinhardt, E., and Blake, R.E., 1994, A rapid and precise method of oxygen isotope analysis of biogenic phosphate: Israel Journal of Earth Sciences, v. 43, p. 203–212. Sharp, Z.D., and Cerling, T.E., 1996, A laser GCIRMS technique for in situ stable isotope analyses of carbonates and phosphates: Geochimica et Cosmochimica Acta, v. 60, p. 2909–2916. Sorauf, J.E., and Pedder, A.E.H., 1986, Late Devonian rugose corals and the Frasnian-Famennian crisis: Canadian Journal of Earth Sciences, v. 23, p. 1265–1287. Stearn, C.W., 1987, Effect of the Frasnian-Famennian extinction event on the stromatoporoids: Geology, v. 15, p. 67–71. Thompson, J.B., and Newton, C.R., 1988, Late Devonian mass extinction: Episodic climatic cooling or warming, in McMillian, N.J., et al., eds., Devonian of the world: Canadian Society of Petrologists and Mineralogists Memoir 14, p. 29–34. 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., and Strauss, H., 1999, 87Sr/86Sr, d13C and d18O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59–88. Wang, K., Orth, C.J., Attrep, M., Jr., Chatterton, B.D.E., Hou, H., and Geldsetzer, H.H.J., 1991, Geochemical evidence for a catastrophic biotic event at the Frasnian/Famennian boundary in south China: Geology, v. 19, p. 776–779. Wenzel, B., Le´cuyer, C., and Joachimski, M.M., 2000, Comparing oxygen isotope records of Silurian calcite and phosphate—d18O compositions of brachiopods and conodonts: Geochimica et Cosmochimica Acta, v. 64, p. 1859–1872. Ziegler, W., and Sandberg, C.A., 1990, The Late Devonian standard conodont zonation: Courier Forschungsinstitut Senckenberg, v. 121, p. 1–115. Manuscript received December 26, 2001 Revised manuscript received April 23, 2002 Manuscript accepted April 30, 2002 Printed in USA
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