High-resolution studies of the eastern equatorial Pacific, ODP shipboard scientific party, leg 138

Abstract. Coherent variation in CaCO3 burial is a feature of the Cenozoic eastern equatorial Pacific. Nevertheless, there has been a long-standing ambiguity in whether changes in CaCO3 dissolution or changes in equatorial primary production might cause the variability. Since productivity and dissolution leave distinctive regional signals, a regional synthesis of data using updated age models and high-resolution stratigraphic correlation is an important constraint to distinguish between dissolution and production as factors that cause low CaCO3. Furthermore, the new chronostratigraphy is an important foundation for future paleoceanographic studies. The ability to distinguish between primary production and dissolution is also important to establish a regional carbonate compensation depth (CCD). We report late Miocene to Holocene time series of XRF-derived (X-ray fluorescence) bulk sediment composition and mass accumulation rates (MARs) from eastern equatorial Pacific Integrated Ocean Drilling Program (IODP) sites U1335, U1337, and U1338 and Ocean Drilling Program (ODP) site 849, and we also report bulk-density-derived CaCO3 MARs at ODP sites 848, 850, and 851. We use physical properties, XRF bulk chemical scans, and images along with available chronostratigraphy to intercorrelate records in depth space. We then apply a new equatorial Pacific age model to create correlated age records for the last 8 Myr with resolutions of 1–2 kyr. Large magnitude changes in CaCO3 and bio-SiO2 (biogenic opal) MARs occurred within that time period but clay deposition has remained relatively constant, indicating that changes in Fe deposition from dust is only a secondary feedback to equatorial productivity. Because clay deposition is relatively constant, ratios of CaCO3 % or biogenic SiO2 % to clay emulate changes in biogenic MAR. We define five major Pliocene–Pleistocene low CaCO3 % (PPLC) intervals since 5.3 Ma. Two were caused primarily by high bio-SiO2 burial that diluted CaCO3 (PPLC-2, 1685–2135 ka, and PPLC-5, 4465–4737 ka), while three were caused by enhanced dissolution of CaCO3 (PPLC-1, 51–402 ka, PPLC-3, 2248–2684 ka, and PPLC-4, 2915–4093 ka). Regional patterns of CaCO3 % minima can distinguish between low CaCO3 caused by high diatom bio-SiO2 dilution versus lows caused by high CaCO3 dissolution. CaCO3 dissolution can be confirmed through scanning XRF measurements of Ba. High diatom production causes lowest CaCO3 % within the equatorial high productivity zone, while higher dissolution causes lowest CaCO3 percent at higher latitudes where CaCO3 production is lower. The two diatom production intervals, PPLC-2 and PPLC-5, have different geographic footprints from each other because of regional changes in eastern Pacific nutrient storage after the closure of the Central American Seaway. Because of the regional variability in carbonate production and sedimentation, the carbonate compensation depth (CCD) approach is only useful to examine large changes in CaCO3 dissolution.

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