Late miocene—earliest pliocene glaciation in southern Argentina: implications for global ice-sheet history
Abstract In South America at latitude 47°S, till was deposited sometime between 7 m.y. and 4.6 m.y. ago, at a time when the local climate was colder than today's. During this same interval glaciers extended to sea level in southeast Alaska, and widespread cooling of the ocean surface in middle latitudes, worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred. From the last three events, major late Miocene expansion of the Antarctic Ice Sheet has been inferred, on the assumption that the history of North Atlantic ice rafting precludes the existence of Northern Hemisphere ice sheets until 3 m.y. ago. This is disputed, first because precipitation in Antarctica would probably have decreased at temperatures below today's, second because the Antarctic Ice Sheet cannot expand appreciably until buildup of Northern Hemisphere ice sheets has lowered sea level, third because virtually no late Miocene sediments are present at the Labrador Sea DSDP sites that are critical to the reconstruction of North Atlantic ice rafting history, and fourth because the scale of late Miocene glaciation in Alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east. If global ice sheet volume at the end of the Miocene was greater than it is today, by an amount that would have lowered sea level by several tens of meters, the excess ice did not accumulate in Antarctica, but in the Northern Hemisphere, chiefly in North America away from the Atlantic coast.