Sulfate availability and the geological record of cold-seep deposits

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• journal_title：Geology
• Contributor：Thomas F. Bristow ; John P. Grotzinger
• Publisher：Geological Society of America
• Date：2013-07-01
• Format：text/html
• Language：en
• Identifier：10.1130/G34265.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：7
• firstpage：811
• section：Articles

摘要

<p id="p-1">Cold-seep deposits are the remnants of ancient chemosynthetic ecosystems that derive energy from microbial anaerobic oxidation of methane (AOM) using seawater sulfate. They provide a physical record of a microbial process that plays a critical role in regulating biospheric methane. Although highly p>13p>C-depleted kerogen suggests that AOM dates back 2.7 b.y., puzzlingly, the oldest reported cold seeps only appear at 635 Ma and lack carbon isotopic signals (<–30‰ Peedee belemnite) that are diagnostic of AOM in examples younger than 350 Ma. Using a one-dimensional biogeochemical reaction-transport model, we confirm that these discrepancies are an expected consequence of changes in seawater chemistry. More specifically, sub-millimolar (mM) to millimolar seawater sulfate concentrations ([SO₄p>2–p>]_SW) and elevated concentrations of dissolved inorganic carbon that characterized seawater through much of the Precambrian limited AOM-driven carbonate supersaturation and p>13p>C depletion, making seep carbonates less likely to form and more challenging to identify. Moderate p>13p>C depletions observed in 420–370-m.y.-old cold-seep carbonates (independently identified by fossil assemblages and contextual and textural observations) indicate [SO₄p>2–p>]_SW < 5 mM in this interval. This is significant because low [SO₄p>2–p>]_SW has been linked to widespread ocean anoxia in the early Paleozoic, an environmental condition thought to have influenced the evolution, extinction, and recovery of early animals. p>