251-11 New Calcium Isotope Records Examined with Calcite Growth Models Reveal Persistent Biocalcification Stress during the PETM

Session: Phanerozoic Earth System Shifts in the Marine Sedimentary Record

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Abstract:

Large igneous province eruptions are thought to impact the global carbon cycle, seawater carbonate chemistry, and the stable calcium isotope (δ^44/40Ca) system. The Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), marked by a globally correlative negative δ¹³C excursion (CIE) at its onset, was an episode of pronounced ocean acidification (OA) and hypothesized biocalcification stress likely triggered by a rapid injection of volcanic CO₂. We present two new bulk carbonate δ^44/40Ca records for the PETM, one from a shallow epicontinental site in the Tarim Basin, east Tethys Ocean, and the other from IODP Site U1553, a deep-sea site in the South Pacific Ocean. High-precision TIMS measurements resolve comparable stratigraphic patterns that oscillate around different average δ^44/40Ca values, ranging from -1.56‰ to -0.92‰. The new records closely resemble previously published PETM planktic foraminiferal δ^44/40Ca records for the Pacific and Atlantic Oceans (Kitch et al., 2021), including a 0.25-0.35‰ positive δ^44/40Ca shift that peaks during the PETM. The positive Ca isotope excursion for the two Pacific sites and one Atlantic site begins before the onset of the CIE, while it coincides with the CIE at the Tarim Basin. We implemented calcite growth models that predict changes in Ca isotope fractionation as a function of saturation state, precipitation rate, and other variables (DePaolo, 2011; Nielsen et al., 2012) with global surface seawater [Ca²⁺] and [CO₃^2-] outputs from a cGENIE model previously applied to the PETM (Cui et al., 2011). With minimal assumptions, the calcite growth models successfully reproduce both the direction and magnitude of the observed positive δ^44/40Ca change. We interpret globally similar δ^44/40Ca signals, together with the numerical output, as evidence for the high-fidelity preservation of primary kinetic isotope effects deriving from biocalcification responses to environmental stress. Akin to inferences drawn from δ¹¹B records (Penman et al., 2014), our δ^44/40Ca records further show that OA and suppressed biocalcification were protracted throughout the ~170 kyr PETM interval (Röhl et al., 2007).

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

doi: 10.1130/abs/2025AM-8097

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