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Post-glacial cooling 8,200 Years Ago


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Image of outburst flood from glacial lake

Jökulhlaup (outburst flood from a glacial lake) at Hubbard Glacier in Alaska on August 14, 2002. From USGS.

Following the end of the last glacial period about 11,500 years ago, the Earth's climate system began to look and behave more like it does today. The large continental ice sheets shrank, sea level rose, temperatures ameliorated, monsoons grew in strength. Around 8,200 years ago, however, a surprising event occurred. The 8.2 ka event, as it is now known, was first discovered in the Greenland ice core GISP2, where high-resolution analyses indicate that over two decades temperature cooled about 3.3°C in Greenland (Alley et al., 1997; Kobashi et al., 2007). The entire event lasted about 150 years (Thomas et al., 2007; Kobashi et al., 2007) and then temperatures warmed, returning to their previous levels.

How widespread was the 8.2 ka event?


8.2 ka climate anomalies

Figure 7. Map of climate anomalies during the 8.2 ka event identified by Morrill and Jacobsen (2005) from published paleoclimate records and using a statistical test to objectively identify anomalies. Sites marked by an open circle did not record a statistically-significant anomaly, while those marked by a closed circle did.

The spatial extent of the 8.2 ka event is currently under debate. There is clear evidence from lake and ocean sediments that European climate was affected, with temperatures dropping about 2°C (Figure 7). Paleoclimate records from other parts of the world are more sparse, which makes it difficult to draw solid conclusions. Some evidence exists from speleothems, ocean sediments and an ice core that parts of the tropics became drier. Shrinking of tropical wetlands in a drier climate might also explain the 10-15% drop in atmospheric methane that is recorded in air bubbles of Greenland ice cores (Alley et al., 1997). There are too few high-resolution records from the Southern Hemisphere to determine whether climate changed there. Temperatures might have become somewhat warmer in Antarctica, but this conclusion is controversial.

What caused the 8.2 ka event?


Image of Hudson Bay at 8.2 ka

Figure 8.Geography of the Hudson Bay region just prior to the 8.2 ka event. From Clarke et al. (2003).

The cause of the 8.2 ka event remained a puzzle for some time, but scientists have recently begun to converge upon a consistent picture. In the early Holocene, as the large ice sheets of the last glacial period were wasting away, a large lake formed south of the Hudson Bay from meltwater. It was dammed to the north by a remnant of the Laurentide ice sheet (Figure 8). At some point prior to 8.2 ka, this dam failed catastrophically, releasing the waters of Lake Agassiz to the Hudson Bay and downstream to the Labrador Sea (Barber et al., 1999). Once in the Labrador Sea, these freshwaters changed the density structure of the ocean and slowed deepwater formation and the thermohaline circulation. Recent research published by Ellison et al. (2006) provides evidence for this freshening of the surface waters and weakening of the thermohaline circulation.

Some important datasets related to the 8.2 ka event:

  • Hu et al. (1999), sediment data from Deep Lake, Minnesota
  • Von Grafenstein et al. (1998), stable isotope data from Ammersee in Germany
  • Hughen et al. (2000), sediment grayscale from core PL07-58PC in the Cariaco Basin
  • Dean et al. (2002), varve record from Elk Lake, Minnesota
  • Lachniet et al. (2004), speleothem δ18O and δ13C measurements from Venado Cave, Costa Rica
  • Ellison et al. (2006), sediment data from Core MD99-2251 in the North Atlantic
  • Thomas et al. (2007), high resolution δ18O measurements from Greenland ice cores
  • Kobashi et al. (2007), methane and δ15N measurements from the GISP2 ice core

Next: End of the African Humid Period


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