Eocene-Oligocene transition iceberg discharge reconstruction for the Falkland Plateau, using process length of Operculodinium centrocarpum as a surface-water density proxy.

Abstract

The cause of the onset of the Eocene-Oligocene transition is still disputed, as is the effect of the glaciation that happened right after. We use a process length Operculodinium centrocarpum as a salinity proxy to detect freshwater fluxes resulting from iceberg discharge. These icebergs originate from calving ice shelves, indicating a marine-terminating ice sheet on the Antarctic continent. Palynological slides from DSDP Leg 71 Site 511 are used for the reconstruction. This core is located at the North Scotia Ridge, south-west Atlantic, connected to the Scotia Sea and thus the iceberg alley. Optically, four Operculodinium morphotypes are distinguished, based on their process length. Average process lengths in the record that spans the EOT is compared to bulk and thermocline d18O records from the same site, for stratigraphic purposes and to potentially detect salinity changes in the difference between bulk and thermocline d18O. Beside that, the process length is compared to the abundance of Phthanoperidinium cysts that indicate freshwater conditions, and to a TEX86 reconstruction of the sea surface temperature. We found low density sea-water conditions immediately following the EOT, probably as a result of iceberg melting. Therefore, the Antarctic ice sheet was more likely to be 130% than 60% of today’s ice volume during the early Oligocene. Combining morphology change with SST data showed that the changes in density have been mostly due to temperature until Oi1, and mostly due to salinity after Oi1, thus relating to ice volume on the Antarctic continent.