WUWT readers may recall this recent story: New evidence of Younger Dryas extraterrestrial impact The story below provides much more detail about the Younger Dryas event and the split that has developed in the scientific community over the cause. I’ve added this graph below from NCDC to give readers a sense of time and magnitude of the event. – Anthony Watts
The Younger Dryas cold interval as viewed from central Greenland. From:
Quaternary Science Reviews Volume 19, Issues 1-5, 1 January 2000, Richard B. Alley
Guest Post By: Rodney Chilton www.bcclimate.com
A consideration of many other very important factors that should be considered as well as the “Black Mats and Nanodiamonds”
The genesis of the Younger Dryas stadial (cold interval) remains an enigma. The onset was both climatologically unexpected and extremely sudden. The two principle theories are diametrically opposed and the proponents of both deeply committed. The debate to date has primarily been centred on some unusual “black mat” deposits that may or may not be linked to a cosmic origin. What has been lacking in the wider discussion are all the other important features associated with the Younger Dryas. The following addresses many of these, in hopes of their inclusion in future debate.
AND NOW THE REST OF THE STORY:
The Younger Dryas onset remains a little understood event. The cause of the 1,300 year-long interval continues to be debated. There are two completely different theories that have split the scientific community. One group strongly endorses an overall slowing or complete stoppage of the Northern Atlantic Ocean circulation 13,000 years ago. The other camp maintains that a catastrophic event originating from the cosmos was the cause.
Following on the heels of the mostly milder Bølling and Allerød intervals (interstadials), there was an extremely sudden and severe climate reversal, this was the Younger Dryas, first detected from Danish pollen studies as long ago as the mid 1930’s. Pollen from the Dryas flower, an arctic species lends its name to this very cold interval. The Younger Dryas cold was first thought to have been confined to north-west Europe, with a possible extension to some other localities immediately surrounding the North Atlantic. More recently however, the cold climate shift is seen as world-wide in extent or nearly so.
The Younger Dryas appeared similar to earlier events known as Heinrich events that were prominent in the Pleistocene (approximately 70,000 to 14,000 years ago) (1). Their cause is not altogether clear, but marine cores, primarily in the north-east Atlantic are festooned with layers of sand, pebbles and rock (lithic materials). These deposits arrived in this area carried on “large armadas” of ice that upon melting deposited their lodes onto ocean bottoms. Rapid climate shifts have been linked to ice melt from sea ice and the large continental glaciers that surrounded the North Atlantic. Lower salinity meltwater is less dense than ocean water and tends to float as a freshwater cap over the marine waters, and this is perceived as associated with North Atlantic Ocean circulation disruption. The Younger Dryas is understood to be linked primarily with meltwater almost solely from the great continental ice sheets.
North Atlantic Ocean circulation has been likened to a great ribbon-like conveyor belt (2). Driven by temperature (thermo) and salinity (haline) differences, the thermohaline (THC) circulation is associated with the formation of North Atlantic Deep Water (NADW). The sinking of the NADW is alleged to result in the drawing north of warmer waters from southerly climes. This provides north-west Europe with its generally mild climate. However all of this is thought to change when the North Atlantic Ocean circulation slowed or stopped.
It has been proposed that a sudden immense amount of fresh water disrupted the THC approximately 13,000 years ago, and the most likely source was eastern North America’s Laurentide Ice Sheet (3). This particular scenario is presumed to have been affiliated with the relocation of freshwater outflow that had been exiting via the Mississippi River with entry into the Gulf of Mexico. Presumably, an alternate route, the more northerly St. Lawrence corridor became available as the Laurentide Glacier retreated (4). As time has passed however, this idea has largely been abandoned. Not only did salinity levels in the off shore waters adjacent to the St. Lawrence remain the same during the Younger Dryas (5), but the St. Lawrence route remained blocked by ice until well after the Younger Dryas ended (6).
Failure of the St. Lawrence River to deliver the melt has lead to alternative freshwater routes proposed. One of these involves the continent of Antarctica. The idea suggested here is that a significant increase in meltwater entry into world oceans took place approximately 14,300 to 14,600 years ago (7). An inundation known as “meltwater – pulse 1a” (mwp-1a) occurred with perhaps as much as 90% of the meltwater volume originating from Antarctica (8). This premise has the Antarctic melt as affecting the North Atlantic region, but with a significant delay (the bipolar see-saw concept where at least Antarctic climate is out of phase with the Northern Hemisphere). The eventual arrival of the Antarctica melt waters is seen then, as making the North Atlantic vulnerable to even modest amounts of meltwater (9). Presumably, the final threshold was crossed 13,000 BP, allowing the North Atlantic to become disrupted (10). Not all researchers share this view, as at least one study assigned a much different date for mwp-1a, and that was shortly before 13,800 BP (11). And although these scientists also conclude slowing or shutdown of the North Atlantic, the Antarctic as a source becomes questionable.