Abstract
Ocean acidification in modern oceans is linked to rapid increase in atmospheric CO2, raising concern about marine diversity, food security and ecosystem services. Proxy evidence for acidification during past crises may help predict future change, but three issues limit confidence of comparisons between modern and ancient ocean acidification, illustrated from the end-Permian extinction, 252 million years ago: (1) problems with evidence for ocean acidification preserved in sedimentary rocks, where proposed marine dissolution surfaces may be subaerial. Sedimentary evidence that the extinction was partly due to ocean acidification is therefore inconclusive; (2) Fossils of marine animals potentially affected by ocean acidification are imperfect records of past conditions; selective extinction of hypercalcifying organisms is uncertain evidence for acidification; (3) The current high rates of acidification may not reflect past rates, which cannot be measured directly, and whose temporal resolution decreases in older rocks. Thus large increases in CO2 in the past may have occurred over a long enough time to have allowed assimilation into the oceans, and acidification may not have stressed ocean biota to the present extent. Although we acknowledge the very likely occurrence of past ocean acidification, obtaining support presents a continuing challenge for the Earth science community.
Highlights
Introduction and BackgroundEarth-surface change associated with the largest mass extinction in Earth history, the end-Permian extinction, which killed approximately 90% of all marine species 252 million years ago, is recognised to have a complex of components [1]
The ratio between the important two stable Ca isotopes (44/40Ca) shifts negative with increased 40Ca input derived from terrestrial weathering since almost all Ca is 40Ca; such weathering implies an increase in atmospheric CO2, so a negative Ca isotope excursion should be compatible with increased ocean acidity
Despite the likely occurrence of ocean acidification in Earth’s past history, and the valuable compilations of fossil and isotope data, to our knowledge no observations exist of cases where acidification can be unequivocally demonstrated, with the possible exception of the Palaeocene-Eocene
Summary
Introduction and BackgroundEarth-surface change associated with the largest mass extinction in Earth history, the end-Permian extinction, which killed approximately 90% of all marine species 252 million years ago, is recognised to have a complex of components [1]. A debate is developing in the Earth sciences about possible effects of one of these components, ocean acidification, in the process of mass extinction, including the end-Permian event, e.g., [2]. Some authors favor ocean acidification as a process associated with mass extinctions, e.g., [3]. There has not yet been an assessment of the quality of the sedimentary evidence of ocean acidification through the end-Permian mass extinction interval, a gap that this paper aims to fill. Key features are examined, followed by discussion of the potential for ocean acidification in associated with the extinction. We conclude that the evidence is currently equivocal and investigation of past ocean acidification associated with mass extinctions is still in its early stages
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