Geog 372 Global Climate Change
Ruddiman, Chapter 13 Images
The Last Deglaciation
Timing of retreat of the North American Ice Sheets, beginning ~ 18 ka, ending around 6 ka, lasting 12,000 years. And of course, Greenland is still melting.
Radiocarbon dating and Thorium/Uranium dating of Caribbean corals revealed the discrepancy in ages over time. The record also shows the rate of sea level rise over the past 20,000 years in response to the melting ice sheets.
Deglaciation occurred in two pulses, interrupted by a cold period, the Younger Dryas, during which melting continued at a much slower pace.
Evidence of freshwater flowing into the oceans from delta O-18 records of calcium carbonate shells of plankton. The glacial meltwater is depleted in O-18.
The Younger Dryas. Evidence from Polar forams, tree pollen in Scotland, fossil insects in England, all record steep drop in temperature over Europe that lasted over 1,000 years. Note that New England also suffered a return to glacial conditions during this time.
Evidence from Greenland ice cores of the abrupt onset and ending of the Younger Dryas. At ~15,000 years ago, sudden increases in snow accumulation occurred ahead of the return to glacial conditions on the ice sheet by around 13,000 years ago. The return to warm deglacial conditions also occurred abruptly with an increase in snow accumulation around 11,700 years ago. Windblown dust is also an indicator of glacial conditions.
Testing the Broecker hypothesis that fresh meltwater flowing into the North Atlantic stopped the thermohaline circulation and caused the Younger Dryas. Why does Ruddiman think this is not the cause of the Younger Dryas?
If this is not the cause, then what did cause the Younger Dryas? More importantly, could meltwater from the present day Greenland Ice Sheet cause a shutdown in the thermohaline circulation again, as a result of global warming?
How did astronomical configurations begin the deglaciation? What sorts of feedbacks likely enhanced the rate of deglaciation? Note that Present orbital conditions, and therefore solar radiation input today, is very similar to conditions during the last glacial maximum (LGM).
Positive Feedback processes:
Likely Laurentide ice sheet thinned by surging and by ice streams delivering ice bergs and melt water to the North Atlantic. Ice along the margins then melted rapidly in the depressed basins at lower latitudes. Thus the ice sheet area did not decrease but ice volume did.
Glacial retreat and the formation of proglacial lakes along the ice margins. As the glaciers melted back the depressed crust began to slowly rebound, tilting the proglacial lakes to the north.
Lake Agassiz -- One enormous proglacial lake!
Deglacial flooding of coastlines worldwide. This had significant impact on people as well as animals as land bridges were flooded, closing off access between continents and from large islands.
The Missoula flood deposits.
Giant ripples in the land, too large to be seen on the ground. Note scouring of basalt cliffs in the background.
Early to mid-Holocene insolation and seasonality. Ice volume reaches modern levels, CO2 reaches 280 ppm.
Effects on monsoons in Africa and South Asia in response to the high insolation levels, peaking by 8,000 years ago.
Vegetation-moisture feedback. High summer insolation peaked at about 9,000 years ago. Wetter soils and increased vegetation provided positive feedback, bringing more moisture farther into the continent of Africa.
Models versus Observed data, Spruce and Oak