INTERGLACIAL PLEISTOCENE PERIPLATFORM CARBONATE δ13C AS AN ANALOGUE FOR INTERPRETING ANCIENT CHEMOSTRATIGRAPHIES

The δ¹³C of shallow carbonates preserved in the rock record frequently is used to reconstruct changes to Earth’s ancient carbon cycle and as a tool for chemostratigraphic correlation. Shallow carbonate sediments from the most recent Pleistocene glacial cycles present a unique opportunity to constrain the sensitivity of shallow δ¹³C to changes in local conditions (e.g., shelf area and restriction, the distribution of biomineralizer populations with different mineralogies and vital effects, input of terrestrial organic matter) while global seawater δ¹³C_DIC remains stable. However, the geochemistry of Recent shallow carbonate strata largely was overprinted by karst and meteoric diagenesis during the last glacial sea level lowstand. To understand how shallow carbonate geochemistry has changed during the Pleistocene, we instead must look to the periplatformal slope and proximal basins. These deep environments serve as a refuge for carbonate produced on the shelf and contain a record of shallow carbonate geochemistry that persists across glacial cycles. We study 21 short piston cores from around Bahamian platforms to quantify differences in banktop production and geochemistry between the Holocene interglacial and the last interglacial (LIG) periods. We find that LIG surface conditions produced banktop carbonate with elevated mean δ¹⁸O compared to the Holocene, and that higher LIG sea level likely increased export of banktop carbonate to the periplatform. We show that despite a significant reorganization of the LIG banktop surface environment, the δ¹³C of bulk banktop carbonate export remained constant between the LIG and the Holocene. We discuss a variety of hypotheses to explain the Pleistocene periplatform δ¹³C and δ¹⁸O records. Our results have implications for interpreting and correlating ancient chemostratigraphies crucial to the study of the coevolution of life and environment.