186-9 Calcium Isotopes Link Ocean Acidification to Aptian/Albian Foraminiferal Extinctions

Abstract:

The second-largest extinction event in the evolutionary history of planktic foraminifera occurred at the Aptian/Albian boundary, ~113.2 million years ago. Around 70% of planktic species disappeared in the latest Aptian, while just two species with small and thin-walled tests survived into the Albian. In contrast, benthic foraminifera appear largely unaffected. This extinction has been linked to environmental perturbations associated with Oceanic Anoxic Event 1b. Volcanic CO₂ emissions from the Kerguelen Plateau may have caused ocean acidification, driving extinctions of planktic foraminifera with more heavily-calcified tests.  

We constructed high-precision calcium isotope (δ^44/40Ca) records for planktic and benthic foraminifera and bulk carbonate sediments across the Aptian/Albian boundary at Deep Sea Drilling Project Site 511 (Falkland Plateau, South Atlantic). We also present the first δ^44/40Ca measurements of secondary calcite, manually separated from the chambers of infilled benthic foraminifera. The Ca isotope composition of marine carbonates may track biocalcification rates through time. Slower calcification rates yield higher δ^44/40Ca values, while faster rates yield lower values. Biocalcification rates, in turn, depend on the carbonate saturation state of seawater. Thus, δ^44/40Ca measurements may help to identify episodes of ocean acidification in the geologic past.

Benthic foraminiferal δ^44/40Ca values (measured for infill-absent or dissected tests) display a distinct sequence of negative and positive excursions, mirroring δ^44/40Ca records of other geologic acidification events. Bulk sediments show similar negative-to-positive shifts, occurring in parallel with benthic δ^44/40Ca variations. Secondary calcite yields δ^44/40Ca values akin to bulk sediment, suggesting that authigenic precipitation occurred under closed system conditions. These observations indicate that the major excursions in our δ^44/40Ca records reflect primary kinetic isotope effects. In planktic foraminifera, δ^44/40Ca values increase dramatically across the Aptian/Albian boundary. We interpret this excursion, one of the largest documented in the Phanerozoic, as evidence for a major decrease in the calcification rates of planktic foraminifera. The excursion also coincides with substantial decreases in planktic diversity and test size. Therefore, geochemical and paleobiological observations collectively support the hypothesis that ocean acidification drove extinctions of planktic foraminifera at the Aptian/Albian boundary. The consequent decline in surface-water calcification would have enhanced ocean alkalinity, potentially buffering the effects of acidification on deep-sea benthic foraminifera and fostering the formation of secondary calcite.

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