Beschreibung: 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.

Beschreibung: This dataset includes orbital-resolution benthic foraminiferal (Cibicidoides wuellerstorfi) δ18O and δ13C measurements, a new orbitally tuned age model, millennial-resolution XRF scanning elemental data, and a discrete XRF calibration sample set measured by ICP-MS - all from IODP Site U1443 (Expedition 353) in the equatorial Indian ocean spanning the late Miocene interval 9-5 Ma.

Beschreibung: 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.