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Presented By: Earth and Environmental Sciences

Smith Lecture - Dr. Andrew Jacobson, Northwestern University

Detecting Deep Time Ocean Acidification Events with Stable Ca and Sr Isotopes

For decades, Earth Scientists have hypothesized that CO2 outgassing during Large Igneous Province eruptions can acidify seawater and suppress biocalcification. To test this hypothesis, our group has worked over the past decade to generate high-precision Ca isotope (δ44/40Ca) records spanning multiple candidate ocean acidification (OA) events, including the Permian-Triassic boundary (P-Tr), Ocean Anoxic Event 1a (OAE1a), Ocean Anoxic Event 2 (OAE2), the Cretaceous-Paleogene boundary (K-Pg), and the Paleocene-Eocene Thermal Maximum (PETM). Notable patterns emerge when the records are aligned relative to operationally defined event boundaries/onsets. The P-Tr record exhibits a large and rapid negative δ44/40Ca excursion, whereas all other records display similar signals, including both negative and positive shifts before the boundaries, negative excursions after the boundaries, and positive recoveries. The consistently repeated pattern is striking given that the records represent different depositional settings, archives, mineralogies, and durations. Complementary analyses of high-precision stable Sr isotope ratios (δ88/86Sr) offer a rich but underutilized hypothesis testing framework for constraining the fidelity and meaning of carbonate δ44/40Ca signals. The δ44/40Ca and δ88/86Sr values for OAE1a define a line with a slope of 0.19, which is a value predictable from kinetic mass-fractionation theory. Interestingly, δ44/40Ca and δ88/86Sr values for a dolostone deposited after the Marinoan Snowball Earth event and a limestone deposited before the Sturtian Snowball Earth event also yield lines with slopes of 0.19. Preliminary B isotope ratios (δ11B) for OAE1a covary with existing δ44/40Ca values in ways anticipated for carbonate system control. For OAE2, foraminiferal δ44/40Ca values increase as tests become smaller and malformed. When the K-Pg dataset is plotted as δ44/40Ca vs. Sr/Ca and linearly regressed, the intercept at Sr/Ca = 0 (“pure aragonite”) yields a δ44/40Ca value of -1.59‰, which is equivalent to the equilibrium fractionation factor recently measured in bench-scale aragonite precipitation experiments. This seminar will interrogate hypotheses for explaining these and other observations, including early diagenesis, shifts in the Ca and Sr isotope compositions of seawater due to input-output flux perturbations, and kinetic isotope effects (KIEs) during primary carbonate formation. Research is ongoing. If a KIE hypothesis prevails, then the evidence may illuminate the candidate OA events as volcanically triggered biocalcification crises. Implications exist for several first-order topics, including, but not limited to, linkages between solid and surface Earth processes, mass extinctions, OA dampening mechanisms, biomineralization, and development of decarbonization strategies for mitigating the anthropogenic climate crisis, such as artificial ocean alkalinization.

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