Understanding global carbon cycle perturbations of early Paleogene using size-fraction-specific isotopic variations in open marine bulk carbonates

Abstract:

Carbon isotope (δ¹³C) records from marine sediments have been extensively used in chemostratigraphic correlation across ocean basins and during different geologic time intervals. The early Paleogene interval in particular has received exceptional attention because negative carbon isotope excursions (CIEs) documented in the sedimentary record, e.g. at Paleocene Eocene Thermal Maximum (PETM), ca ~56 Ma, are believed to reflect significant perturbations in the global carbon cycle during the warmest interval of the Cenozoic era. However, while bulk carbonate δ¹³C values show robust correlations across widely separated marine sedimentary sections, the magnitude of CIEs and absolute values of δ¹³C vary from one site to another. Moreover, bulk carbonates in open marine environments are an ensemble of different components, each with a distinct isotopic composition. Consequently, a complete interpretation of the bulk δ¹³C record requires an understanding of how these components co-evolved. In this study, we dissect sediments from the early Paleogene interval (58-50 Ma) at ODP Site 1209 on Shatsky Rise, north-central Pacific Ocean, to investigate how an evolving bulk carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for their δ¹³C and δ¹⁸O compositions, first as bulk carbonate and then as individual size fractions. We find a significant increase in coarse fraction abundance across the PETM, driven by a changing community structure of calcifiers, which modulates the size of the CIE at Site 1209 and makes it distinct from those recorded at other deep marine sites. These results highlight the importance of biogeography in the marine stable-isotope record, especially when isotopic excursions are driven by climate- and/or carbon-cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as a conventional proxy for carbonate dissolution. This study, therefore, holistically presents the effect of varying community structure of marine calcifiers across space and time on carbon isotope excursions preserved in the marine sedimentary archive of the early Paleogene.