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the Mid-Atlantic Ridge (Original)


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Carolyn Silver
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« Reply #30 on: July 28, 2008, 10:14:10 pm »

Variability During the Last Ice Age: Dansgaard-Oeschger Events

Abrupt changes did not begin during the Younger Dryas. Throughout the last glacial period (60,000 to 20,000 years ago), abrupt warming and cooling events, called Dansgaard-Oeschger or D-O events occurred in the North Atlantic. Greenland ice core records reveal that during the last glacial, the climate system abruptly shifted into, and then back out of, warm, close-to-interglacial conditions 23 times. Each oscillation consisted of gradual cooling followed by an abrupt warming.

Related to some of the coldest D-O intervals were distinctive events, recorded in North Atlantic marine sediments, of changes in the delivery of icebergs to the ocean and the amount of ice-rafted sand transported southward by the icebergs.

These Heinrich events in the sediment record resulted from changes in ocean circulation and iceberg melting, and were clear indications that cold polar waters extended farther south, carrying ice-rafted material from northern regions ( Bond et al. 1992, Bond & Lotti 1995). The events may have been accompanied by an influx of freshwater into the North Atlantic, through increased melting. Scientists have hypothesized that reduced deepwater formation may have accompanied these dramatic, but temporary, shifts of the Earth's climate. This is currently an area of active research (Maslin et al. 1995).

Cariaco Basin Sediment and GISP2 Ice Core Comparisons

Figure 19a. Abrupt climate events called Dansgaard-Oeschger events are found in Greenland ice cores, and some other locations such as the Cariaco Basin in the Caribbean Sea. Warm (interstadial) events are numbered in the ice core (red). Less negative numbers in the oxygen isotope ratio indicate warmer conditions in Greenland. In the Cariaco Basin sediment cores (green), highly reflective sediment layers indicate light green mud, and signals ocean climate and circulation associated with low plankton productivity. The data are significant because they reveal ocean-wide climate changes occuring within a century or less, altering the temperatures in the far North Atlantic, and the sea surface conditions close to the equator. In both regions, conditions appear to flip back and forth between two different states.



More recently, Bond and colleagues (Bond et al. 2001) have correlated the events in the North Atlantic with changes in solar output (the latter derived from proxy records in ice cores and tree rings). Their conclusion is that small, gradual changes in solar output crossed thresholds in the climate system, and that changes in thermohaline circulation resulted in abrupt shifts in the Earth's climate system.

Like the Younger Dryas, these events have had a hemispheric to global footprint. They were seen in sediment cores off the coast of Africa (Zhao et al. 1995), off the coast of Venezuela (Peterson et al. 2000), in the Arabian Sea (Schulz et al. 1998), and in Hulu Cave in China (Wang et al. 2001). The magnitude of change outside the North Atlantic, and more generally the geographic extent of abrupt change in temperature and precipitation during the last glacial, are currently topics of intense research.

http://www.ncdc.noaa.gov/paleo/abrupt/data_glacial2.html
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