Abstract
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.
Highlights
Integrated Ocean Drilling Program (IODP) Expedition 320/321 drilled eight sites in the tropical Pacific in 2009
We report data from seven eastern equatorial Pacific drill sites, four with XRF scans along the sediment splice (IODP sites U1335, U1337, and U1338 and ODP site 849) and four with CaCO3 data estimated from spliced gamma ray attenuation (GRA) wet bulk density profiles (ODP sites 848, 849, 850, and 851; see Methods section for the process)
We have developed site-to-site correlations at the meter scale and a composite age model for seven ODP and IODP drill sites from the eastern equatorial Pacific in order to study the evolution of eastern equatorial Pacific sedimentation
Summary
Integrated Ocean Drilling Program (IODP) Expedition 320/321 drilled eight sites in the tropical Pacific in 2009. New chronostratigraphy from the PEAT drilling (e.g., Channell et al, 2013; Holbourn et al, 2014; Kochann et al, 2016; Drury et al, 2017, 2018) would be used to refine and update equatorial Pacific age models Another important objective from the PEAT expeditions was to identify major changes and causes of carbonate storage in the eastern equatorial Pacific. Large-scale flow of abyssal waters southward along the western flank of the East Pacific Rise (Johnson and Toole, 1993; Hautala, 2018) causes the western flank to be bathed by a common bottom water and to share a common signal of abyssal change Both surface and abyssal processes interact to drive these common CaCO3 profiles across the equatorial region. Since CaCO3 variability causes large changes in sediment bulk density in the eastern tropical Pacific (Mayer, 1979, 1991; Reghellin et al, 2013), the extensive geographic scale of coherent CaCO3 profiles produces a distinctive set of seismic reflection horizons (Bloomer and Mayer, 1997; Tominaga et al, 2011) linked with major paleoceanographic change in the Pacific Ocean (Mayer et al, 1986)
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