T183 Assessing the Fidelity of Geochemical Signals in Deep Time: Primary, Authigenic, and Diagenetic Signals in Proxy Data

Thursday, 29 October 2020
10:00 AM - 12:00 PM
Meeting Rooms

CALCIUM ISOTOPE VARIABILITY ACROSS ANCIENT CANDIDATE OCEAN ACIDIFICATION EVENTS

Paper number: T183-6
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

CO2 emissions during large igneous province (LIP) eruptions are expected to affect biocalcification by acidifying seawater, decreasing [CO32-], and lowering carbonate saturation states. Marine Ca isotope (δ44/40Ca) records may provide insight into biocalcification crises because rate-dependent effects govern Ca isotope fractionation during carbonate mineral formation. However, input/output flux imbalances and early diagenesis can produce similar signals. To resolve various controls, we compared five high-precision TIMS δ44/40Ca records spanning candidate ocean acidification events: the end-Permian mass extinction (EPME, ~252 Ma), the Cretaceous Oceanic Anoxic Events (OAE1a and OAE2, ~120 and ~94 Ma), the end-Cretaceous mass extinction (ECME, ~66 Ma), and the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). The EPME exhibits a large (~0.6‰) and rapid negative δ44/40Ca excursion. The other four records show similar patterns, with positive excursions (~0.10 – 0.15‰) that begin before major boundaries indicated by the onset of δ13C excursions, negative shifts after the boundaries, and positive recoveries. The apparent similarities are striking given that the four records represent different archives (bulk sediments and fossils), mineralogies (calcite and aragonite), and event durations (~0.2 to 1 Myr). These data are consistent with preservation of primary kinetic isotope effects and provide less support for a diagenetic origin. In particular, diagenesis of molluscan aragonite drives Sr/Ca and Ca isotope ratios in the opposite direction compared to bulk sediments and calcite microfossils. The ECME δ44/40Ca record correlates with sedimentological indicators of saturation state. PETM foraminiferal δ44/40Ca show no correspondence with sedimentological indicators of diagenesis. In the case of OAE1a, bulk carbonate δ44/40Ca and [Sr], as well as δ44/40Ca and δ88/86Sr, strongly correlate and yield slopes expected for variable mass-dependent fractionation. Noting that atmospheric CO2 provides a first-order control on the carbonate geochemistry of seawater, we hypothesize that the similar δ44/40Ca patterns reflect a causal connection between LIP eruptions and biocalcification feedbacks. The EPME record appears anomalous and likely reflects additional effects.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355894

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