Related to some of the coldest intervals between D-O events were six distinctive events, named after paleoclimatologist Hartmut Heinrich, that are recorded in North Atlantic marine sediments as layers with a large amount of coarse-grained sediments derived from land (Figure 4). These layers, which are continuous across large areas of the North Atlantic, are evidence for both an increase in icebergs discharged from the Laurentide ice sheet in North America and a southward extension of cold, polar waters (Bond et al. 1992, Bond and Lotti 1995). Icebergs carry sand-sized grains eroded by ice sheets. When icebergs encounter warmer ocean water, they melt and drop their sediment load onto the sea floor. A southward extension of cold, polar waters allowed icebergs to travel farther before melting. Figure 4 shows that Heinrich events occurred less frequently than D-O events. D-O events repeated every several thousand years on average, while ~10,000 years elapsed between Heinrich events. Neither of the two types of events is strictly periodic, however.
What caused Heinrich and Dansgaard-Oeschger events?
The cause of these glacial events is still under debate. Currently, the leading hypothesis involves a slowdown of the ocean's thermohaline circulation. During the last glacial time, large ice sheets rimmed the North Atlantic (Figure 2). At certain times, these ice sheets released large amounts of freshwater into the North Atlantic. Heinrich events are an extreme example of this, when the Laurentide ice sheet disgorged excessively large amounts of freshwater into the Labrador Sea in the form of icebergs. Scientists have hypothesized that these freshwater dumps reduced ocean salinity enough to slow deepwater formation and the thermohaline circulation. Since the thermohaline circulation plays an important role in transporting heat northward, a slowdown would cause the North Atlantic to cool. Later, as the addition of freshwater decreased, ocean salinity and deepwater formation increased and climate conditions recovered.
This freshwater forcing hypothesis is supported by evidence for changes in deepwater formation. Measurements from deep sea sediments in the North Atlantic indicate that deepwater formation was strongly reduced during Heinrich events (Shackleton et al., 2000; Elliot et al., 2002; McManus et al., 2004). Evidence for reduced deepwater formation between D-O events is somewhat more ambiguous, but it is possible that the North Atlantic branch of the thermohaline circulation was weaker and somewhat shallower at those times (Elliot et al., 2002).
The initial trigger for freshwater releases has not yet been identified. One suggestion is that small, gradual changes in solar output could have influenced the timing of abrupt changes (Bond et al., 2001). Other ideas call upon natural oscillations of the ice sheets themselves (MacAyeal, 1993; Hulbe et al., 2004; Alley et al., 2006) or of ocean processes (Shaffer et al., 2004, Flückiger et al., 2006).
Even though Heinrich and D-O events seem to have been initiated in the North Atlantic, they had a global footprint. Some of the best evidence outside of the North Atlantic for these events comes from sediment cores in the Caribbean (Peterson et al. 2000) and Arabian Seas (Schulz et al. 1998), and in speleothems from Hulu Cave in China (Wang et al. 2001). These sites, along with others, are shown in Figure 5. The climate anomalies are consistent with a slowdown of the thermohaline circulation and reduced ocean heat transport into the northern high latitudes. During cold phases in the North Atlantic, large regions of North America and Eurasia became colder and drier. A southward shift of the tropical rain belt moistened many parts of the Southern Hemisphere. Antarctic ice cores show warming, consistent with a reduction of northward heat transport from the Southern Hemisphere.
Some important datasets related to Heinrich and Dansgaard-Oeschger events:
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