D-O and Bond Cycles

Dansgaard-Oeschger (D-O) events and Bond cycles are both abrupt climate change events that occured within the last glacial period. The existence of D-O events was first discovered in ice cores from Greenland by famous climatologists Willi Dansgaard and Hans Oeschger. D-O events follow a characteristic pattern of an abrupt warming followed first by a period of gradual cooling and then a more rapid cooling. This is followed by a more stable cool period and then another D-O event. Figure 1 shows a model of the D-O events in Greenland and in Antarctica. In the model for Greenland it is easy to see the rapid warming, slow cooling pattern of the D-O events. Below the models are the actual ice core data from Greenland and Antarctica. 

Figure 1. D-O event model and data.  (http://www.realclimate.org/index.php/archives/2006/11/revealed-secrets-of-abrupt-climate-shifts/)

Figure 2 shows the 20 most recent D-O events in the top plot. In this plot it is more difficult to see the typical pattern that we expect of D-O events, but especially in the earliest events (19 & 20) it is still possible.

Figure 2. D-O and Heinrich events over the last 80,000 years. (http://www.ncdc.noaa.gov/paleo/abrupt/data3.html)

The oxygen stable isotope record from the GISP2 ice core in Greenland (top) and a record of ice-rafted material from a North Atlantic deep-sea core (bottom)

It is thought by some scientists that D-O events occur approximately every 1500 years, but the cycle is under debate as sometimes they occur every 3000 or 4500 years. There were 25 D-O events that occurred during the last glacial period. Currently, it is unclear what causes the D-O events, but it is hypothesized that a slowing of the ocean’s thermohaline circulation might be involved.  


Bond cycles occur over a longer scale than D-O events and are characterized by Heinrich events. Bond cycles were discovered by Gerard Bond in sediment cores form the north Atlantic. When he was comparing the sediment records to the ice core records Bond noticed that not all of the D-O events warmed to the same temperature. After a particularly large warming the next few D-O events would each be cooler than the previous one. After the D-O events got progressively colder for a few events in a row there would be another large warming. These cycles of progressively cooler D-O events are called Bond cycles.

An important feature of Bond cycles is the large warming at the end of each cycle, which is preceded by a Heinrich event. Over the course of a Bond cycle the ice sheet near the Hudson Bay would increase in size, growing quickly during the cold periods between D-O events. As the ice grew thicker it would trap the heat given off by the Earth like a blanket. Eventually, the ice would grow so thick that it trapped enough heat to melt the bottom layer of ice, forming a layer of water over the bedrock. The ice sheet then could slide easily over the water layer and expel a massive wave of icebergs into the north Atlantic. This purge of ice from the Hudson Bay is known as a Heinrich event. If we look back at Figure 1 the lower plot shows the Heinrich events. The Bond cycles are more difficult to see than the D-O events in the top plot, but if we look closely at Heinrich events H4 and H3 it is possible to see that they are in line with the end of a series of progressively cooler D-O events.

It is possible that the growth and expulsion of ice from the Hudson Bay might explain the progressive cooling and quick warming of the Bond cycle. When there is ice and snow covering the ground it reflects much of the Sun’s radiation is reflected back from the surface. The amount of radiation that is reflected is known as albedo. Due to the high albedo of ice and snow the surface temperature is cooler, which in turn leads to more ice buildup and higher albedo. This phenomenon could account for the slow cooling portion of the Bond cycle. When the ice was expelled from the Hudson Bay the exposed ground has a lower albedo than snow, meaning it would absorb more solar radiation, warming the area. 

Photo of Glacier Calving (at right) from wikipedia


References 

Alley, R.B. The Two Mile Time Mahcine. Princeton: Princeton University Press, 2000. Print.

"Heinrich and Dansgaard-Oeschger Events." NOAA Paleoclimatology. N.p.. Web. 7 May 2013. <http://www.ncdc.noaa.gov/paleo/abrupt/data3.html>. 

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