Benthic Foraminiferal Response to Reduced Oceanic Acidification and Global Warming During the Miocene Monterey Excursion, Congo Fan, Southeastern Atlantic

The Early-Middle Miocene was a period of global warming and increased carbon burial as evidenced by the Monterey global positive carbon isotope shift from 18 - 14 Ma (Vincent and Berger 1985), which culminated in the deposition of massive carbonates and has been linked to recorded atmospheric CO2 lows at this time. We have collected the first ever record of benthic δ18O and δ13C from the Miocene of the Congo Fan, and report on the benthic foraminiferal response to these isotopic shifts from BP wells spanning the Early-Middle Miocene (21 - 14 Ma). Oligocene sediments from the Congo Fan consist almost entirely of agglutinated foraminifera due to deposition below a locally raised CCD. During the Early Miocene a gradual and persistent increase in the percentage of calcareous foraminifera is mirrored by our record of increasing benthic δ13C (from Cibicidoides spp.) at this site, suggesting reduced bottom-water acidification and a lowering of the CCD. Faunas record well- oxygenated bottom waters (~1000 m paleodepth) from 21 - 16 Ma. A dramatic shift in the shallow infaunal morphogroup at ~16 Ma (high abundances in Bulimina elongata, Brizalina aff. barbata, Uvigerina aff. carapitana) records lower oxygen and a probable expansion and strengthening of the oxygen minimum zone lasting for at least 1 my. This is coincident with cooling in this location, adding evidence for raised colder bottom-waters. We postulate that global cooling at this time (e.g. Zachos et al. 2001) was responsible for increasing the strength of the polar front, and in turn strengthening offshore winds at this location affecting an increase in upwelling and surface water productivity.