Late Miocene to Holocene high-resolution eastern equatorial Pacific carbonate records: stratigraphy linked by dissolution and paleoproductivity
Authors: Lyle Mitchell , Drury Anna Joy , Tian Jun , Wilkens Roy , Westerhold Thomas 
Published: September 2019

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.