North Atlantic mid-latitude surface-circulation changes through the Plio-Pleistocene intensification of northern hemisphere glaciation

This is the final version. Available from American Geophysical Union (AGU) / Wiley via the DOI in this record.All new data presented are available at https://doi.pangaea.de/10.1594/PANGAEA.892805The 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.Funding for this research was provided by IODP France (CTB) and the German Research Foundation (DFG) (grant OF 2544/2 to OF). IB is grateful to the UK IODP for financial support for shipboard and post-cruise participation in IODP Exp. 306. CTB, KT, TDG, LV, CS, and ME acknowledge OSU Pythéas. MMR acknowledges support by the USGS Land Change Science Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. PAW acknowledges NERC UK IODP NE/F00141X/1 and a Royal Society Wolfson Merit Award

North Atlantic Midlatitude Surface-Circulation Changes Through the Plio-Pleistocene Intensification of Northern Hemisphere Glaciation

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) at ~3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program 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 reorganization in the midlatitude 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 that nutrient injection from cold‐core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma.t. Funding for this research was provided by IODP France (C. T. B.) and the German Research Foundation (DFG) (grant OF 2544/2 to O. F.). I. B. is grateful to the UK IODP for financial support for shipboard and post-cruise participation in IODP Exp. 306. C. T. B., K. T., T. D. G., L. V., C. S., and M. E. acknowledge OSU Pythéas. M. M. R. acknowledges support by the USGS Land Change Science Program

Patterns and mechanisms of early Pliocene warmth

About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions

ENSO-like forcing on oceanic primary production during the Late Pleistocene

International audienceLate Pleistocene changes in oceanic primary productivity along the equator in the Indian and Pacific oceans are revealed by quantitative changes in nanoplankton communities preserved in nine deep-sea cores. We show that variations in equatorial productivity are primarily caused by glacial-interglacial variability and by precession-controlled changes in the east-west thermodine slope of the Indo-Pacific. The precession-controlled variations in productivity are linked to processes similar to the Southern Oscillation phenomenon, and they precede changes in the oxygen isotopic ratio, which indicates that they are not the result of ice sheet fluctuations. The 30,000-year spectral peak in the tropical Indo-Pacific Ocean productivity records is also present in the Antarctica atmospheric CO2 record, suggesting an important rote for equatorial biological productivity in modifying atmospheric CO2

Environmental controls of size distribution of modern planktonic foraminifera in the equatorial Indian ocean: A calibration study

International audiencePalaeoceanographic studies often rely on microfossil species abundance changes, with littleconsideration for species traits (e.g. size) that could be related to environmental changes. Wehypothesize that whole-assemblage and species-specific planktonic foraminifera (PF) testsize could be good predictors of environmental variables, and we test this using an EquatorialIndian Ocean (EIO) core-top sample set (62 viable samples). We use an automated imagingand sorting system (MiSo) to identify PF species, analyze morphology and quantifyfragmentation using machine learning techniques. Machine accuracy was confirmed bycomparisons with human classifiers. Data for 25 mean annual environmental parameterswere extracted from modern databases and, through Exploratory Factor Analysis andregression models, we investigate the potential of PF size, at the assemblage and specieslevel, for reconstructing oceanographic parameters in the Indian Ocean. Within our tropicaldataset, we find that SST is not a significant driver of assemblage size, although thermoclinedwelling species Globorotalia inflata and Globorotalia truncatulinoides show a significantrelationship with temperature. Our analyses indicate that deep carbonate ion concentrationand core depth may be important factors influencing PF size, especially in species that arelarge-sized or bear calcite crusts such as Globigerinoides conglobatus, Globorotaliamenardii, and Neogloboquadrina dutertrei. We propose that PF population size couldpotentially be useful to reconstruct bottom water carbonate concentrations and sea surfacetemperature. This approach will be tested on a new downcore record from the Arabian sea(ODP Site 722) during key Pleistocene glacial-interglacial transitions, where existing seasurface temperature and other paleo-reconstructions will allow meaningful comparisons