148-13 A Preliminary Investigation into Ecological and Size Variation of South Atlantic Planktic Foraminifera Over the Last 6 Million Years

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

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Abstract:

Ocean currents in the South Atlantic have changed significantly over the last 6 million years due to a cooling climate, but the expression of these changes within the South Atlantic Subtropical Gyre remains poorly understood. Planktic foraminifera are single-celled marine organisms with calcium carbonate shells and are sensitive to changes in ocean temperature, nutrient, and water column stratification, making them valuable proxies for reconstructing past oceanographic conditions. By analyzing these foraminifera assemblages, we gain insights into changes in ocean circulation and associated environmental changes over time.

Our study examines planktic foraminiferal community and size variation during the late Neogene and Quaternary at IODP site U1559 in the central South Atlantic Ocean. We have examined foraminifera assemblages and compared them with geochemical data such as Ca/Ti ratio and N₂ concentrations from the core. Populations of planktic foraminifera are classified into ecological groups that correspond to their depth habitats in the water column and trophic strategy. Our results show that foraminifera populations in this region, during the Miocene and early Pliocene epoch, are primarily composed of four ecogroups: mixed-layered (both symbiont- and non-symbiont-bearing), thermocline, and sub-thermocline assemblages. During the Pleistocene, however, the populations are characterized by the presence of five ecogroups, the fifth being high latitude assemblages, likely coinciding with short-lived glacial cycles. The mixed layer, non-symbiont-bearing foraminifera dominates during periods of increased bottom current intensity and nutrient supply, as evidenced by elemental Ca/Ti measurements and N₂ concentrations. These periods appear to correspond with a stronger global circulation during colder intervals, with enhanced overturning circulation likely driving stronger bottom currents. The increase in nutrient supply, as indicated by the proliferation of non-symbiont-bearing taxa, may be related to changes in nutrient delivery, either from a northward shift in the South Atlantic Subtropical Gyre or an increase in surface nutrients along the Subantarctic Front, which are entrained in the currents comprising the gyre. Additionally, dust flux from the adjacent continents could contribute to higher nutrient levels.

We hypothesize that global cooling, by influencing both current circulation and nutrient supply, is the primary driver of the observed shifts in the planktic community structure. Additionally, we are applying machine learning techniques to analyze foraminifera size variations, which will further contribute to understanding South Atlantic paleoceanographic changes.

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

doi: 10.1130/abs/2025AM-8088

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