21-3 Calcium Isotopes Link Ocean Acidification to Extinctions of Planktic Foraminifera at the Aptian-Albian Boundary

Session: Insights from Microfossils and Their Modern Analogs: From Traditional to Emerging Approaches

Presenting Author:

Jonathan Chen

Authors:

Chen, Jonathan¹, Jacobson, Andrew D², Wan, Chuyan³, Waldeck, Anna⁴, MacLeod, Kenneth G.⁵, Balestra, Barbara⁶, Huber, Brian T.⁷, Sageman, Bradley B.⁸

(1) Department of Earth, Environmental, and Planetary Sciences, Northwestern University, Evanston, IL, USA, (2) Department of Earth, Environmental, and Planetary Sciences, Northwestern University, Evanston, IL, USA, (3) Department of Earth, Environmental, and Planetary Sciences, Northwestern University, Evanston, IL, USA, (4) Department of Geosciences, Pennsylvania State University, University Park, PA, USA, (5) Department of Geological Sciences, University of Missouri, Columbia, MO, USA, (6) Department of Environmental Science, American University, Washington, DC, USA, (7) National Museum of Natural History, Smithsonian Institution, Washington, DC, USA, (8) Department of Earth, Environmental, and Planetary Sciences, Northwestern University, Evanston, IL, USA,

Abstract:

The Aptian-Albian Boundary Interval (AABI, ~113 Ma) marks one of the largest turnovers in the evolutionary history of planktic foraminifera. During the latest Aptian, ~70% of planktic species—most bearing large, ornamented, and heavily-calcified tests—went extinct. Just two planktic species with small, thin-walled, and microperforate tests survived the extinction. This turnover occurred within Oceanic Anoxic Event 1b (OAE1b). Volcanic CO₂ emissions from the Kerguelen Plateau may have caused ocean acidification (OA) and the extinction of species that required heavily-calcified tests. To investigate paleoceanographic changes across the AABI, we generated high-precision calcium isotope (δ^44/40Ca) records of planktic and benthic foraminifera, as well as bulk carbonates, from Deep Sea Drilling Project Site 511 (southern South Atlantic). We also separated and analyzed secondary calcite spar isolated from chambers of infilled benthic foraminifera. Our guiding hypothesis is that kinetic isotope effects (KIEs) govern δ^44/40Ca values, with slower biocalcification rates yielding higher values and faster rates yielding lower values. Calcification rates depend on the calcite saturation state of seawater, among other variables. Thus, the δ^44/40Ca proxy can help identify ancient episodes of OA. Our δ^44/40Ca records of bulk carbonate sediments and well-preserved benthic test fragments both display distinct negative-to-positive shifts at the boundary but are offset by ~0.6‰. Similar shifts have been documented in δ^44/40Ca records of other OA events. Calcite spar yielded δ^44/40Ca values similar to those for bulk carbonates, suggesting that precipitation of infilling occurred under closed-system conditions and near chemical equilibrium. Meanwhile, planktic δ^44/40Ca values increase by ~0.6‰ across the Aptian-Albian boundary, peaking at values that exceed those for calcite spar. All Ca isotope shifts coincide with decreases in the size and abundance of planktic foraminifera and with increases in the abundance of agglutinated and weakly-calcified benthic taxa. These results support mixed layer OA as the cause for selective extinctions of planktic foraminifera at the AABI. The sudden decline in planktic calcification would have increased alkalinity in the surface ocean, potentially buffering subsequent effects of OA on deep-sea benthic foraminifera and contributing to authigenic calcite formation on the seafloor. Finally, our results further demonstrate that deep-sea microfossils and bulk carbonates can serve as high-fidelity archives of primary Ca isotope signals driven by KIEs, capturing biocalcification responses through geologic time.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6558

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