ALBERT

Description: We integrate benthic foraminiferal stable isotopes, X-ray fluorescence elemental ratios, and carbonate accumulation estimates in a continuous sedimentary archive recovered at International Ocean Discovery Program Site U1443 (Ninetyeast Ridge, Indian Ocean) to reconstruct changes in carbonate deposition and climate evolution over the interval 13.5 to 8.2 million years ago. Declining carbonate percentages together with a marked decrease in carbonate accumulation rates after ~13.2 Ma signal the onset of a prolonged episode of reduced carbonate deposition. This extended phase, which lasted until ~8.7 Ma, coincides with the middle to late Miocene Carbonate Crash, originally identified in the eastern equatorial Pacific Ocean and the Caribbean Sea. Inter-ocean comparison reveals that intense carbonate impoverishment at Site U1443 (~11.5 to ~10 Ma) coincides with prolonged episodes of reduced carbonate deposition in all major tropical ocean basins. This implies that global changes in the intensity of chemical weathering and riverine input of calcium and carbonate ions into the ocean reservoir were instrumental in driving the Carbonate Crash. An increase in U1443 Log(Ba/Ti) together with a change in sediment color from red to green indicate a rise in organic export flux to the sea floor after ~11.2 Ma, which predates the global onset of the Biogenic Bloom. This early rise in export flux from biological production may have been linked to increased advection of nutrients and intensification of upper ocean mixing, associated with changes in the seasonality and intensity of the Indian Monsoon.

Description: During the late Miocene, global cooling occurred alongside the establishment of near-modern terrestrial and marine ecosystems. Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at mid to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital-resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. To test if an atmospheric pCO2 decrease may have driven this cooling, we also present new paleoclimate model simulations under three atmospheric pCO2 scenarios (300 ppm, 420 ppm and 560 ppm; in the range suggested by existing pCO2 proxy records). The data contains Mg/Ca ratios (mmol/mol) measured in tests of a planktic foraminifera species living in the mixed layer (Trilobatus trilobus), together with reconstructed Sea Surface Temperatures. Samples are from the revised shipboard splice from Site U1443 (equatorial Indian Ocean) retrieved during International Ocean Discovery Program Expedition 353. Data span the late Miocene-earliest Pliocene (9 to 5 Ma) with a mean resolution of 5.5 kyr and allow reconstruction of SSTs on secular to orbital timescales. Sea Surface Temperatures are reconstructed with the Dekens et al., (2002) T. sacculifer calibration equation from Pacific Ocean, including a basin-specific dissolution correction. Correction for variation of Mg/Ca ratio of seawater are also calculated following the scenario from Higgins & Schrag (2015) and the approach from Tierney et al. (2019). ±1σ and ±2σ uncertainties linked to analytical, calibration, and age model errors are estimated via a bootstrap Monte Carlo procedure using the Paleo-Seawater Uncertainty Solver (PSU solver, Thirumalai et al., 2016) in Matlab.

Description: In situ measurements of Mg/Ca, Zn/Ca, Mn/Ca, and Ba/Ca in Globigerinoides bulloides and Globigerina ruber from southwest Pacific core top sites and plankton tow are reported and their potential as paleoproxies is explored. The modern samples cover 20° of latitude from 34°S to 54°S, 7-19°C water temperature, and variable influence of subantarctic (SAW) and subtropical (STW) surface waters. Trace element signatures recorded in core top and plankton tow planktic foraminifera are examined in the context of the chemistry and nutrient profiles of their modern water masses. Our observations suggest that Zn/Ca and Mn/Ca may have the potential to trace SAW and STW. Intraspecies and interspecies offsets identified by in situ measurements of Mg/Ca and Zn/Ca indicate that these ratios may also record changes in thermal and nutrient stratification in the upper ocean. We apply these potential proxies to fossilized foraminifera from the high-resolution core MD97 2121. At the Last Glacial Maximum, surface water Mg/Ca temperature estimates indicate that temperatures were approximately 6-7°C lower than those of the present, accompanied by low levels of Mn/Ca and Zn/Ca and minimal thermal and nutrient stratification. This is consistent with regional dominance of SAW and reduced STW inflow associated with a reduced South Pacific Gyre (SPG). Upper ocean thermal and nutrient stratification collapsed during the Antarctic Cold Reversal, before poleward migration of the zonal winds and ocean fronts invigorated the SPG and increased STW inflow in the early Holocene. Together with reduced winds, this favored a stratified upper ocean from circa 10 ka to the present.

Description: Data from Enhanced late Miocene chemical weathering and altered precipitation patterns in the watersheds of the Bay of Bengal recorded by detrital clay radiogenic isotopes in Paleoceanography and Paleoclimatology. Marine sediment samples from the late Miocene (approximately 9 to 5 million years ago) were taken from International Ocean Discovery Program (IODP) Site U1443 located on the top of the Ninety East ridge in the southern Bay of Bengal. Samples were taken along the composite sediment section or splice and the age model based on shipboard calcareous nannofossil biostratigraphy and magnetostratigraphy was refined using the stable carbon isotopes of benthic foraminifera. The detrital silicates of the clay size fraction were isolated and measured for their radiogenic Sr, Nd, and Pb isotope composition by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to investigate the provenance of clays produced by weathering in the surrounding watersheds.

Description: The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the last glacial maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of northern hemisphere glaciation (iNHG) ~3.5 to 2.5 Ma, is less clear. Here, we present new records from IODP Site U1313 (41°N) spanning ~2.8-2.4 Ma to trace the influence of Subarctic Front waters above this mid-latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater δ18O using paired Mg/Ca-δ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis, and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganisation in the mid-latitude North Atlantic analogous to the LGM. Our findings suggest that LGM-like processes sensu stricto cannot be invoked to explain interglacial-glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest nutrient injection from cold-core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the mid-latitude North Atlantic from 2.7 Ma.