South Greenland Ice-Sheet Response to the Mid-Pliocene Climatic Optimum

Abstract:

The last time atmospheric carbon dioxide concentration approached present day levels was likely during the mid-Pliocene climate optimum 3.3-3.0 Ma. Estimates of sea level from the mid-Pliocene optimum vary widely and contain a significant component of glacial isostasy and dynamic topography, precluding a firm estimate of global ice volume. Here we present new data constraining the southern Greenland ice sheet (GIS) across the mid-Pliocene optimum from IODP Site U1307 located on the Eirik Ridge south of Greenland. Magnetic susceptibility dramatically decreases at ~3.3 Ma, recovering at ~3.05 Ma. Magnetic parameters on silt-size grains (Mrs/Ms) show this decrease likely reflects the loss of silt sourced from the Precambrian terranes of south Greenland, which would suggest the loss of the south GIS. The sand fraction reflects the degree of iceberg rafting with increases at an ~40 ka period. Planktic oxygen isotopes decrease during iceberg rafting events to values lower than the Holocene. An extended interval of low sand occurs from ~3.3 Ma to ~3.2 Ma, which suggests cessation of iceberg rafting. Planktic oxygen isotopes increase during this interval. Assuming a relatively warm climate, this increase could reflect the incursion of high 18-O subtropical waters into the Labrador Sea. The return of iceberg rafted sediments after ~3.2 Ma suggests the regrowth of the GIS with marine-terminating margins. Our new preliminary record points to a GIS collapse during the mid-Pliocene climatic optimum, but that ice-free conditions only lasted for ~100 ka before the return of a volatile ice sheet for the remainder of the mid-Pliocene climatic optimum.