Understanding bulk sediment stable isotope records in the Eastern Equatorial Pacific, from seven million years ago to present‐day

Daniele Reghellin, Gerald R. Dickens, Helen K. Coxall, and Jan Backman

Abstract
Stable isotope (δ13C and δ18O) records of bulk marine sediment carry information on past carbon cycling and oceanography, but origins and interpretations remain uncertain because such signals represent mixtures of different biogenic components, each with potential offsets from primary parameters. Studies of Neogene sediment from the Eastern Equatorial Pacific (EEP) exemplify this issue, because stable isotope records of bulk sediment and foraminifera at different sites exhibit similarities and differences in absolute value that somehow relate to depositional age. Here we measure δ13C and δ18O of bulk carbonate, two fine grain fractions (<63 and <20 μm), mixed species planktic and benthic foraminifera, and foraminifera fragments from sediments deposited over four time intervals within the last 7 Ma at ODP Site 851. These data can be compared to published δ13C and δ18O records of multiple single species planktic foraminifera from the same site and benthic foraminifera from an adjacent site. Bulk sediment δ13C and δ18O records represent a mixed signal dominated by reticulofenestrid coccolith calcite, but modified by variable amounts of different foraminifera. Similarities and differences between stable isotope records result from temporal changes in water chemistry and temperature, depths of calcite precipitation, and vital effects that impact fractionation of various biogenic components. The remarkable correlation of bulk stable isotope records within the EEP suggests that several factors change collectively over time across a broad oceanographic region. Ideally, multiple stable isotope records coupled with other proxy measurements might lead to an internally consistent paleoceanographic perspective of the EEP since the late Miocene.

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