Glacial-interglacial sea water isotope change near the Chilean Margin as reflected by δ²H of C37 alkenones

Abstract. Hydrogen isotopic compositions of C37 long chain alkenones (δ²H_C37) have been proposed as a proxy for past seawater salinity and applied in several regions. However, previous studies were based solely on a single core and often suggested unlikely large changes in salinity. Here we present a new δ²H_C37 record, in combination with oxygen isotopes of benthic foraminifera from the same samples, from a sediment core from the Chilean Margin (ODP site 1235). The observed negative shift in δ²H_C37 of 20 ‰ during deglaciation was identical to that of the nearby located, but deeper, ODP core 1234, suggesting a regionally consistent shift in δ²H_C37. This shift translates into a hydrogen isotope shift of the surface seawater (∆δ²H_SW) of ca. 14 ‰, similar to glacial-interglacial reconstructions based on other δ²H_C37 records. The reconstructed bottom seawater oxygen isotope change based on benthic foraminifera during the last deglaciation is approximately 0.8 ‰, in line with previous studies. When translated into ∆δ²H_SW using the modern open-ocean waterline this would suggest a shift of ca. 5 ‰, smaller than the reconstructed surface seawater shift based on alkenones. The higher change in surface δ²H_SW suggests that the surface water experienced more freshening during the Holocene than bottom waters either due to increased freshwater input or reduced evaporation, or a combination of the two.