Late Middle Ordovician environmental change and extinction: Harbinger of the Late Ordovician or continuation of Cambrian patterns? Mark E. Patzkowsky Leta M. Slupik Michael A. Arthur Richard D. Pancost Katherine H. Freeman Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802-2714
ABSTRACT Positive excursions in carbon isotope compositions of carbonate (~3‰) and organic carbon (~4‰–6‰) from the late Middle Ordovician (middle Caradocian) of the midcontinent and the eastern United States indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric pCO2, precipitating global cooling, although not necessarily ice-sheet formation. These climatic changes were associated with regional orogenic uplift, a relative rise in sea level, changes in epeiric sea circulation patterns, and carbonate platform destruction that led to regional extinction of marine benthos. The combination of sea-level rise, changing ocean circulation, and extinction in the middle Caradocian is similar to the suite of environmental changes described for Cambrian biomere boundaries, suggesting shared causes for these events. In contrast, middle Caradocian environmental changes are markedly different from the environmental patterns associated with the Late Ordovician mass extinction, despite the evidence for long-term cooling from the Middle to the Late Ordovician.
INTRODUCTION Comparative study of episodes in Earth history can lead to a greater understanding of the physical, chemical, and biological processes that underlie the Earth life system. For example, although the Late Ordovician glaciation and mass extinction were concentrated within the last million years of the Ordovician (Brenchley et al., 1995), environmental changes and related extinctions may have begun at least 15 m.y. earlier. Long-term positive shifts in δ18O and δ13C values of well-preserved Early to Late Ordovician brachiopods can be interpreted, at least in part, in terms of progressive cooling of shallow waters and progressive increases in productivity and rate of burial of organic carbon (Qing and Veizer, 1994), the latter perhaps acting to draw down atmospheric pCO2 (Kump et al., 1995). Other long-term environmental changes from the Middle to the Late Ordovician include a decrease in volcanism (Stillman, 1984) and an increase in orogenic activity (Richter et al., 1992). These events may also have combined to draw down pCO2 by lowering rates of CO2 outgassing and increasing the weathering sink for CO2, respectively (Kump et al., 1995). In the middle Caradocian, some 15 m.y. prior to the Late Ordovician mass extinction, global diversity of marine animals decreased and then rebounded into the Ashgillian before the precipitous decline at the end of the Ordovician (Sepkoski, 1995). Given the evidence for long-term changes to the Ordo-
vician global environment, do the middle Caradocian environmental and biotic changes represent the initial response of the global ecosystem as the Earth gradually shifted from a greenhouse to an icehouse world, or do they represent phenomena unrelated to the Late Ordovician mass extinction? Here we report δ13C values of carbonates and organic carbon associated with a middle Carado-
cian extinction event in the eastern United States. Our new stable isotope data coupled with other lithologic data suggest that in the middle Caradocian, a pattern of linked changes in relative sea level, epeiric sea circulation, and productivity associated with regional extinctions shares more similarities with Cambrian biomere boundaries than with the Late Ordovician mass extinction. This suggests similar causes for the Cambrian
Figure 1. Stratigraphic correlation chart for Iowa and Pennsylvania based on Sweet (1984), Kolata et al. (1996), Leslie and Bergström (1995), McVey and Huff (1995), and Thompson (1963). CSS refers to conodont composite standard section of Sweet (W. Sweet, 1996, personal commun.). M and D refer to Millbrig and Deicke K-bentonites, respectively. Base of P. tenuis zone correlates to M4-M5 sequence boundary of Holland and Patzkowsky (1996) and is stratigraphic level of major paleoceanographic changes and extinctions across eastern North America (Patzkowsky and Holland, 1993; Holland and Patzkowsky, 1996, 1997). Water depth curve reflects effects of local subsidence in Iowa and Pennsylvania that largely overwhelmed eustatic changes that are marked by a prominent unconformity (M4-M5 sequence boundary) at this stratigraphic level elsewhere in eastern United States. Number of brachiopod species in eastern United States declines across this interval, illustrating widespread extinction that affected all marine benthos.
Data Repository item 9750 contains additional material related to this article. Geology; October 1997; v. 25; no. 10; p. 911–914; 4 figures.
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and late Middle Ordovician events and emphasizes the unique nature of the Late Ordovician mass extinction. LATE MIDDLE ORDOVICIAN CARBON ISOTOPE DATA Analysis of middle Caradocian samples from Pennsylvania and Iowa reveal major δ13C excursions in carbonates and organic matter that coincided with paleoceanographic changes and extinctions. The middle Caradocian in central Pennsylvania is marked by a shift from shallowramp carbonate mudstones and skeletal wackestones and packstones of the Nealmont Formation to deep-ramp carbonate mudstones and shales of the Salona Formation (Fig. 1). Carbonisotope values of micrites from a well-exposed outcrop in central Pennsylvania indicate a positive shift of ~3.0‰ across this interval (Fig. 2). Data from an additional correlative section over 30 km away (see Data Repository material1) show remarkable similarity in δ13C values, magnitude of shift, and even in some of the smallerscale variability, arguing that this represents a primary signal within the basin. In addition, there is no systematic relationship between δ13C and δ18O values (see footnote 1) of the micrites, suggesting that δ13C values are largely unaltered by diagenesis. Moreover, δ18O values vary narrowly from –6.0‰ to –5.0‰, well within the range of values reported for Ordovician skeletal carbonates presumed to retain their primary isotopic signatures (Qing and Veizer, 1994). Minor diagenetic alteration of micrites probably occurred in early postdepositional stages by marine waters, which are unlikely to reset primary δ13C values (Banner and Hanson, 1990). The δ13C values of organic carbon (δ13Corg ) from central Pennsylvania exhibit a positive 4‰ shift (Fig. 2); however, the maximum rate of change of δ13Corg occurs stratigraphically higher (base of P. tenuis zone) than the maximum rate of change of δ13C values in carbonates (δ13Ccarb ). Middle Caradocian strata in Iowa record a lithologic shift from the shallow-water carbonates of the Platteville Formation to the mixed carbonates and fine-grained siliciclastics of the Decorah Group (Spechts Ferry, Guttenburg, and Ion formations), which were deposited below fairweather wave base (Fig. 1). Previous analyses of whole-rock carbonates and organic matter from a core in northeast Iowa (Hatch et al., 1987; Ludvigson et al., 1996) found positive shifts in δ13C values of both carbonate and organic carbon (Fig. 2). Analyses indicate that the shift in wholerock δ13C values (~3‰) is carried largely by the micrites, whereas the δ13C values of brachiopod shells change little. Ludvigson et al. (1996) inter1GSA Data Repository item 9750, analytical methods and geochemical data, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail:
[email protected].
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preted this as indicating a locally stratified seaway: The micrites formed in near-surface productive waters, while the brachiopods lived in benthic environments below the mixed layer. A large (~6‰) positive excursion occurs in the δ13Corg (Hatch et al., 1987; Ludvigson et al., 1996). On the basis of the K-bentonite correlation and biostratigraphic evidence (Fig. 1), the positive excursions in the δ13C values of carbonates and organic carbon are widespread, extending from the midcontinent into the Appalachians (Fig. 2). The magnitude of the excursions recorded in carbonates is approximately the same in the two areas studied, but the baseline values are offset by about 2‰. The δ13Corg profiles from Pennsylvania and Iowa do not exhibit a similar 2‰ offset, although the magnitude of the positive excursion in Pennsylvania is about 2‰ less than in Iowa. High sampling density in the stratigraphically expanded Pennsylvania section indicates that the timing of the shift in δ13Ccarb occurred earlier than the shift in δ13Corg, a pattern that is not evident in the more sparsely sampled Iowa section. PALEOCEANOGRAPHIC INTERPRETATION Late Middle Ordovician environmental changes in North America have previously been interpreted as a response to the combined effects of the Taconic orogeny and a eustatic sea-level rise (Patzkowsky and Holland, 1993; Holland and Patzkowsky, 1996, 1997), and to global cooling associated with the onset of glaciation (Lavoie, 1995; Pope and Read, 1997). Lithologic changes in eastern North America include a switch from tropical-type to temperate-type carbonates, an increase in abundance and distribution of fine-grained siliciclastic sediment, and more widespread distribution of phosphatic rocks occurring abruptly across a major sequence boundary (M4-M5 sequence boundary of Holland and Patzkowsky [1996], which coincides with the base of the P. tenuis zone). These lithologic changes suggest increased turbidity and nutrient levels, and decreased water temperature, which all acted to inhibit carbonate production (Patzkowsky and Holland, 1993; Lavoie, 1995; Holland and Patzkowsky, 1996, 1997; Pope and Read, 1997). These environmental changes resulted in the destruction of the carbonate platform and elimination of habitats in eastern North America, which led to the extinction of many North American taxa and severe restrictions in the geographic ranges of others (e.g., Patzkowsky and Holland, 1993, 1996; Frey, 1995). The middle Caradocian δ13C excursions indicate that the sea-level rise and changes in oceanographic conditions were associated with major perturbations to the carbon cycle. The similar magnitude of the shifts in δ13Ccarb in Iowa and Pennsylvania suggests a cratonwide, if not global, control of the 13C content in carbonate, and probably reflects an increase in global organic-carbon
Figure 2. δ13C values of carbonate (δ13Ccarb ) and organic carbon (δ13Corg ) (PDB is Peedee belemnite) plotted versus stratigraphic position for Iowa (Ludvigson et al., 1996) and Pennsylvania (Reedsville, Pennsylvania section). Position of Millbrig K-bentonite in Iowa core (Cominco Millbrook Farms SS-9 core) was projected from nearby core and outcrop (D. R. Kolata, 1995, personal commun.).
burial rate. Evidence for enhanced burial exists in the widespread black shales (e.g., Utica Shale) that were deposited in foreland basins during the onset of the Taconic orogeny, as well as the numerous Caradocian black shales that occur in other parts of the world (Leggett, 1980). The ~2‰ offset in the δ13Ccarb between Iowa and Pennsylvania reflects either different isotopic compositions of the local water masses or diagenesis. Although diagenesis cannot be completely ruled out, petrographic and geochemical examination of the Iowa (Ludvigson et al., 1996) and Pennsylvania (see above) samples indicate that they probably preserve primary δ13C values, and it is difficult to imagine a diagenetic mechanism that would shift one of the sections by 2‰, yet leave the form of the excursion intact. It is more likely that the 2‰ offset reflects regional variations in the isotopic composition of seawater, since variations of more than 2‰ can occur in the surface waters of modern carbonate platforms (Patterson and Walter, 1994). Such variations in surface water characteristics may have a simple explanation. The late Middle Ordovician paleogeography of North America (Fig. 3) indicates that basins of the midcontinent were separated from the Appalachian basin by a series of islands and shoals that inhibited exchange of waters between regions. Differences in the δ13C values of riverine input derived from the Transcontinental arch and the Taconic highlands may have produced localized water masses with distinct isotopic compositions. In addition, given the general southeasterly wind field for this part of the globe during the Middle Ordovician, GEOLOGY, October 1997
craton may have been enhanced by changing circulation patterns related to the development of the Taconic foreland basin (Patzkowsky and Holland, 1993; Holland and Patzkowsky, 1996, 1997). In light of this comparison, the end-Ordovician mass extinction stands out not only in its magnitude and impact on the global biota, but also in its association with a unique suite of catastrophic changes to the global environment.
Figure 3. Late Caradocian paleogeography modified from Witzke (1990) and Kolata et al. (1996). IA is approximate position of the Iowa core. PA is approximate position of the central Pennsylvania outcrops.
shore-parallel winds along the Transcontinental arch would have resulted in offshore transport of surface waters and the upwelling of nutrient-rich waters along the coast (Parrish, 1982). These upwelled waters would have been depleted in 13C, contributing to the offset between the two regions. The southern margin of Laurentia, occurring in the lee of the Taconic highlands and away from the northern landmass, would have seen very different wind and ocean circulation patterns that were not conducive to upwelling, except perhaps along the eastern side of the Nashville Dome (Holland and Patzkowsky, 1997). Although this simple explanation of oceanic circulation awaits confirmation by a quantitative epeiric sea circulation model (cf. Slingerland et al., 1996), the possibility of upwelling in Iowa is consistent with previous hypotheses (Hatch et al., 1987; Witzke, 1987; Ludvigson et al., 1996). The positive shift in δ13Corg at the base of the P. tenuis zone begins later than the positive shift in δ13Ccarb. The positive shift in organic carbon can reflect a variety of factors such as increased productivity, decreased dissolved aqueous CO2 in surface waters, and/or a change in the taxonomic composition of the source of the organic material (Hatch et al., 1987). A decrease in the difference in δ13C values between carbonate and organic carbon (∆13C) (Fig. 4) suggests a productivitydriven decrease in isotope fractionation during photoautotrophic carbon fixation due to increased growth rates and/or a decrease in dissolved CO2 (aq) in surface waters (Bidigare et al.,
Figure 4. Difference in δ13C values of carbonates and organic carbon (∆13C) in sections from Iowa and Pennsylvania. GEOLOGY, October 1997
1997). A productivity increase is consistent with previous interpretations (Hatch et al., 1987; Witzke, 1987; Ludvigson et al., 1996; Patzkowsky and Holland, 1993; Holland and Patzkowsky, 1997) and may have been driven by the aforementioned upwelling of nutrient-rich waters and an influx of nutrients from erosion of the Trancontinental arch and the Taconic highlands. However, in Pennsylvania, there are no independent indicators of surface-water productivity changes across this interval. Thus, the coeval decreases in ∆13C in both sections may indicate a change in isotope fractionation of marine algae that involved significant pCO2 drawdown, perhaps triggered by increasing burial of organic carbon (the δ13Ccarb increases prior to the ∆13C change) and which could be the cause of the inferred climatic cooling across this interval. The observed decreases in ∆13C greater than ~1‰ suggest an upper limit on pCO2 of ~8–10 × current levels. We emphasize this interpretation depends on the physiological properties of the dominant phytoplankton and their growth rates, which are currently unknown for the Ordovician (Bidigare et al., 1997). COMPARISON WITH LATE ORDOVICIAN The middle Caradocian events share some similarities with Late Ordovician environmental changes: a coincidence of positive excursions in δ13C values, fluctuations in sea level, and extinctions. There are, however, major differences in the relative timing of these events (Brenchley et al., 1995). The Late Ordovician positive shift in δ13C values of carbonates and organic carbon and associated extinctions occurred during a sea-level fall. The subsequent rise in sea level and second wave of extinctions were marked by a large negative shift in δ13C values, a pattern opposite to what is seen in the middle Caradocian. The Late Ordovician events were marked by a large positive shift in δ18O values of articulate brachiopod shells, reflecting in part growth of the Gondwanan ice sheet. The δ18O values of central Pennsylvania carbonates show no positive shift, although in other areas, Caradocian isotope data from brachiopod shells suggest increases in δ18O that may reflect global cooling (Qing and Veizer, 1994; Ludvigson et al., 1996), or that cooler waters may have invaded the southern margin of Laurentia (Railsback et al., 1990). The spread of cooler waters across the margin of the Laurentian
COMPARISON WITH CAMBRIAN EVENTS More striking are the similarities of the middle Caradocian environmental and biotic changes with those described for the Cambrian biomeres. Recent documentation of relative deepening of environments associated with a positive shift (~1‰) in δ13C values of carbonates across the Pterocephalid-Ptychaspid (Late Cambrian) biomere boundary suggests that those extinctions were caused by a rise in sea level, oceanic overturn, and spread of anoxic waters across the craton (Saltzman et al., 1995). Moreover, the end-Cambrian extinctions have been linked to biogeographic changes brought on by a major rise in sea level (Westrop and Ludvigsen, 1987) and this boundary also shows a positive shift in δ13C values of carbonates of ~1‰ (Ripperdan and Miller, 1995). The Cambrian environmental changes and associated extinctions are remarkably similar to the middle Caradocian patterns and present an extinction scenario for the marine realm that may have also operated at other times in the Phanerozoic (e.g., Cenomanian-Turonian boundary; Arthur et al., 1988). The ultimate drivers of these ecosystem perturbations, however, may not be the same in all cases: orogenic uplift associated with the Taconic orogeny plays a central role in the middle Caradocian environmental changes, but orogeny is not known to be associated with the Cambrian events. Ultimately, more comparative studies of environmental changes and extinction pulses at all scales of magnitude and geographic extent will be necessary to recognize shared causes as well as unique patterns in the history of life. ACKNOWLEDGMENTS We thank D. Walizer for technical support in the Stable Isotope Biogeochemistry Lab at Penn State, D. Hollander for initial isotopic analyses of organic carbon, G. Ludvigson for a preprint of a manuscript and related isotope data, and T. D. Frank, S. M. Holland, J. T. Parrish, and M. C. Pope for reviewing the manuscript. REFERENCES CITED Arthur, M. A., Dean, W. E., and Pratt, L. M., 1988, Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/ Turonian boundary: Nature, v. 335, p. 714–717. Banner, J. L., and Hanson, G. N., 1990, Calculation of simultaneous isotopic and trace element variations during water-rock interaction with applications to carbonate diagenesis: Geochimica et Cosmochimica Acta, v. 54, p. 3123–3137. Bidigare, R. R., and 14 others, 1997, Consistent fractionation of 13C in nature and in the laboratory: 913
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