Late Pliocene Glaciation of Southern Greenland and Ice/Ocean Interactions

Greenland is home to the only persistent northern hemisphere ice-sheet, existing for much of the past 3.5 million years and yet likely to disappear if global temperatures rise another 2-3°C. The best analogue for the ice-sheet's current vulnerable state is likely near the time of its inception in the late Pliocene, when global surface temperatures are thought to have been 2-3°C higher than today. Although few proxy records exist for the pre-Quaternary, IODP Expedition 303 site U1307 captures the late-Pliocene warm period adjacent to the southern Greenland ice sheet. The sand fraction lithology in the Late Pliocene is dominated by well-sorted granitic material, likely sourced via runoff from the nascent southeastern ice sheet. Interpreted on a paleomagnetic reversal age model, this material shows 100 ka cyclicity between 3.6-3.1 Ma, with the exception of a period from 3.44-3.36 Ma. In that window, all lithologic evidence of material eroded from Greenland bedrock disappears. Poor carbonate preservation in conjunction with consistent silicious microfossil presence suggests benthic hypoxia driven by reduced regional deep-water ventilation, and may reflect a shutdown of the Atlantic Meridional Overturning Circulation (AMOC) through this interval. The appearance of glacial ice-rafted debris immediately prior supports the brief existence (<2 ka) of a marine-terminating ice margin at 3.45 Ma; freshwater input to the surface ocean associated with the disappearance of this larger ice sheet may have triggered changes in regional ocean circulation. Recovery from apparent benthic hypoxia begins at 3.36 Ma, accompanied by lithologic evidence for regrowth of the ice sheet. This episode of apparent AMOC shutdown provides insight on the feedbacks that exist between southern Greenlandic ice-sheet behavior and the AMOC.