21-6 Using Laser Grain Size Analysis and SEM imaging to Investigate Changes in Surface Carbonate Producers in the Southeastern Atlantic (Walvis Ridge) across the Eocene-Oligocene Transition

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

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Pelagic carbonate production by open-ocean organisms such as planktic foraminifera and calcareous nannoplankton plays a pivotal role in the global carbon cycle, accounting for about half of annual carbonate production. Changes in pelagic calcifier populations can significantly alter carbonate flux and deep-sea sediment composition. Traditional methods for studying carbonate production and burial, such as CaCO₃ weight percentage and carbonate mass accumulation rates, cannot distinguish the relative contributions of different plankton groups to deep-sea sedimentation. While sieving can help separate size fractions, it is time-consuming and lacks finer resolution. Given that deep-sea carbonate sediments are almost entirely biogenic, fractioning them by size offers a means to approximate the relative contributions of different calcifiers. Here, we present results from a method aimed at reconstructing the role of plankton groups in pelagic carbonate production using laser grain size analysis, which allows rapid, detailed quantification of both coarse (>20 μm, largely planktic foraminifera) and fine (<20 μm, largely calcareous nannoplankton) carbonate fractions, as well as trends within those fractions. We applied this method to samples spanning the Eocene–Oligocene Transition (33.7–34.5 Ma) from Ocean Drilling Program Site 1265 (Walvis Ridge, SE Atlantic, 3083 m water depth). Calcareous nannoplankton dominated throughout the interval, comprising ~80–90% of the carbonate fraction. Foraminifera remained minor (<10%), with small, medium, and large size classes contributing modestly but variably. A peak in foraminiferal abundance occurs in the late Eocene, followed by a decline in the early Oligocene. SEM imaging was used to confirm our findings and assess preservation. Despite high and relatively constant carbonate concentrations, SEM images reveal extensive foraminiferal damage, with large, dissolution-resistant species such as Globigerinatheka dominating the late Eocene. An increase in planktic foraminifera in Oligocene samples, especially small specimens, indicates improved preservation, consistent with trends at other South Atlantic sites. These findings demonstrate that laser grain size analysis, supported by SEM imaging, can quantify subtle changes in foraminiferal dissolution even in intervals with constant carbonate concentrations, providing more nuanced insights than bulk %carbonate or qualitative preservation estimates alone.

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

doi: 10.1130/abs/2025AM-9465

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