Testing the Cenozoic multisite composite δ18O and δ13C curves: New monospecific Eocene records from a single locality, Demerara Rise (Ocean Drilling Program Leg 207)

Abstract

Until recently, very few high‐quality deep ocean sedimentary sections of Eocene age have been available. Consequently, our understanding of Eocene paleoceanography has become heavily reliant on “composite” records patched together from multiple sites in different ocean basins and generated using multiple taxa (potential sources of “local” noise in the global signal). Here we test the reliability of the early to middle Eocene composite δ18O and δ13C stratigraphies (Zachos et al., 2001) by generating new monospecific records in benthic foraminiferal calcite from a single locality, Demerara Rise, in the tropical western Atlantic (Ocean Drilling Program Leg 207). We present new stable isotope correction factors for commonly used Eocene benthic foraminiferal species. We find that interspecies isotopic offsets are constant across the isotopic range, supporting the notion that the inconstant intertaxa offsets reported elsewhere result from mixing species within genera. In general, the δ18O stratigraphy from Demerara Rise supports the validity of the Eocene δ18O composite, while revealing a temporary warming punctuating middle Eocene cooling. This warming may correspond to the so‐called “Middle Eocene Climatic Optimum” previously documented in the Southern Ocean. The composite and Demerara Rise records for δ13C differ substantially. By removing the intersite and intertaxa sources of uncertainty in δ13C, we obtain a clearer picture of carbon cycling during the Eocene. Secular change in interocean δ13C gradients through the Eocene reveals that intervals of climatic warmth (especially the early Eocene) are associated with very small water mass ageing gradients.