TY - JOUR
T1 - Sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in Early to Middle Ordovician greenhouse oceans
AU - Thompson, Cara K.
AU - Kah, Linda C.
N1 - Funding Information:
Funding for this project was provided by the National Geographic Society (NGS 7866-05 to Kah), the National Science Foundation ( NSF-EAR 0745768 to Kah), and the American Chemical Society ( ACS-PRF 48166 to Kah), along with student grants from Sigma Xi, the Geological Society of America, and SEPM (to Thompson). We give special thanks to Ricardo Astini and Fernando Gomez (University of Cordoba) and Geoff Gilleaudeau (University of Tennessee) for help in conducting field work in Argentina; and Lisa Pratt and Anna Synkiewicz (Indiana University) and Zheng-hua Li and Michael Peretich (University of Tennessee) for help with isotopic and elemental analyses. Critical review by two anonymous reviewers greatly improved this manuscript.
PY - 2012/1/1
Y1 - 2012/1/1
N2 - Despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal Neoproterozoic Era, Late Cambrian records point to the persistence of deep-water anoxia and potential for development of euxinic conditions. The Late Cambrian SPICE (Steptoean Positive Carbon-Isotope Excursion) event, however, is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere. Here, we present high-resolution carbon and sulfur isotope profiles from Early to Middle Ordovician carbonate rocks from the Argentine Precordillera and Western Newfoundland to constrain oceanic redox conditions in the post-SPICE world. Marine C-isotope profiles record relatively stable behavior (excursions <3‰) that is characteristic of greenhouse climates. Marine S-isotope profiles record short-term (<10 6yr), rhythmic variation superimposed over a longer term (~10 7yr) signal. Substantial isotopic heterogeneity between average S-isotope values of different sections (15-25‰) suggests the Ordovician marine sulfate reservoir was not well mixed, indicating a low marine sulfate concentration (likely <2mM or less than 10% modern). Short-term variation (7‰ excursions over 1Myr) is consistent with a small sulfate reservoir size and is best explained by the rhythmic oxidation of a deep-water reactive HS - reservoir. Greenhouse intervals, such as that represented by the Ordovician ocean, are often associated with deep-water anoxia, and the presence of a persistent, deep water HS - reservoir that is fed through bacterial sulfate reduction (BSR) is not unexpected. A broadly sympathetic relationship between carbon and sulfur isotope systems over long time scales (~10 7yr) suggests that the extent of deep-ocean euxinia was moderated by changes in organic productivity, which fueled BSR and production of reduced sulfide species. By contrast, short-term (<10 6yr) sulfur isotope variation appears to be decoupled from the marine carbon-isotope signal. We suggest that this apparent decoupling reflects a combination of elevated pCO 2 during greenhouse times-which acts to dampen C-isotope response-and relatively small-scale fluctuations in organic productivity that affected the position of the marine oxycline and the balance of HS - production and reoxidation.
AB - Despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal Neoproterozoic Era, Late Cambrian records point to the persistence of deep-water anoxia and potential for development of euxinic conditions. The Late Cambrian SPICE (Steptoean Positive Carbon-Isotope Excursion) event, however, is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere. Here, we present high-resolution carbon and sulfur isotope profiles from Early to Middle Ordovician carbonate rocks from the Argentine Precordillera and Western Newfoundland to constrain oceanic redox conditions in the post-SPICE world. Marine C-isotope profiles record relatively stable behavior (excursions <3‰) that is characteristic of greenhouse climates. Marine S-isotope profiles record short-term (<10 6yr), rhythmic variation superimposed over a longer term (~10 7yr) signal. Substantial isotopic heterogeneity between average S-isotope values of different sections (15-25‰) suggests the Ordovician marine sulfate reservoir was not well mixed, indicating a low marine sulfate concentration (likely <2mM or less than 10% modern). Short-term variation (7‰ excursions over 1Myr) is consistent with a small sulfate reservoir size and is best explained by the rhythmic oxidation of a deep-water reactive HS - reservoir. Greenhouse intervals, such as that represented by the Ordovician ocean, are often associated with deep-water anoxia, and the presence of a persistent, deep water HS - reservoir that is fed through bacterial sulfate reduction (BSR) is not unexpected. A broadly sympathetic relationship between carbon and sulfur isotope systems over long time scales (~10 7yr) suggests that the extent of deep-ocean euxinia was moderated by changes in organic productivity, which fueled BSR and production of reduced sulfide species. By contrast, short-term (<10 6yr) sulfur isotope variation appears to be decoupled from the marine carbon-isotope signal. We suggest that this apparent decoupling reflects a combination of elevated pCO 2 during greenhouse times-which acts to dampen C-isotope response-and relatively small-scale fluctuations in organic productivity that affected the position of the marine oxycline and the balance of HS - production and reoxidation.
KW - Carbon isotopes
KW - Euxinia
KW - Greenhouse
KW - Ordovician
KW - Sulfur isotopes
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U2 - 10.1016/j.palaeo.2011.10.020
DO - 10.1016/j.palaeo.2011.10.020
M3 - Article
AN - SCOPUS:84855831866
SN - 0031-0182
VL - 313-314
SP - 189
EP - 214
JO - Palaeogeography, Palaeoclimatology, Palaeoecology
JF - Palaeogeography, Palaeoclimatology, Palaeoecology
ER -