USING LASER GRAIN SIZE ANALYSIS TO INVESTIGATE CHANGES IN SURFACE CARBONATE PRODUCERS IN THE SOUTHEASTERN ATLANTIC (WALVIS RIDGE) ACROSS THE EOCENE-OLIGOCENE TRANSITION

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 the pelagic calcifier populations can significantly alter carbonate flux. Traditional methods to study carbonate production at the surface and burial at the seafloor, like CaCO3 weight percentage and carbonate mass accumulation rates, or separation of different sediment fractions prior analyses, might not distinguish the contribution of planktic foraminifera vs calcareous nannoplankton to the sedimentary carbonate fraction, while analyses that can determine plankton type can be time-consuming. Our goal is to develop a new method to reconstruct pelagic carbonate production using laser grain size analysis. Our approach allows detailed analysis of both coarse (>20 microns) and fine (<20 microns) fractions as well as size fractions within them, allowing usproviding the possibility to identify and quantify planktic foraminifera versus calcareous nannofossils in the samples examined. We tested this method by analyzing samples spanning the Eocene-Oligocene transition (33.7-34.5 Ma) from Ocean Drilling Program Site 1265 (Walvis Ridge; southeastern Atlantic Ocean; 3083 m water depth). Our findings reveal that laser grain size analysis captures variations in plankton groups. The data show the predominance (~80-90%) of calcareous nannoplankton (grain size: 1-20 microns) in the samples analyzed. Small foraminifera (grain size: 20-63 microns), medium foraminifera (grain size: 63-500 microns), and large foraminifera (grain size: 500-1000 microns) exhibit lower and more variable densities, typically under 10%. Although calcareous nannoplankton remain dominant throughout the record, foraminifera between 63 and 500 microns become more common in the early Oligocene.