Exercise Overview - Part 2 |
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Thus deep sea sediments can be analyzed for forams and other single-celled marine organisms which indicate changes in ocean temperature which ultimately corresponds to changes in global climate.
The polar bear of planktonic foraminifera, Neogloboquadrina pachyderma, is found only in polar regions and comes in two forms, differentiated by its coiling direction. The left-coiling variety, which runs counterclockwise when viewed on its aperture, is found in greatest abundance in the coldest waters of the North Atlantic and is nearly absent from regions where surface waters are warmer than 8°C. The right-coiling variety is almost never present in the colder waters, but dominates the N. pachyderma population in warmer waters. Thus this coiling direction and abundance change at 8°C makes the species especially useful for tracing the past position of the 8°C isotherm. The location of this isotherm is closely related to the extent of deep water convection occurring in the North Atlantic.
Past variations in the abundance of N. pachyderma have documented that ocean circulation is closely linked to the rapid changes in climate observed throughout much of the last glacial interval on earth and that many of these rapid oscillations in climate occur on time scales as short as several decades. Oscillations in N. pachyderma abundance document large geographic swings in the position of the sea surface temperature fronts in the North Atlantic Ocean, swings that are apparently synchronous with large, rapid changes in air temperature (fluctuations of more than 12°C in several decades) as recorded by changes in the chemistry of ice in cores extracted from the Greenland ice sheet. These fluctuations in N. pachyderma abundance document the incursions of warm salty waters into the high latitude regions where deep water production occurs. Cooling of this salty water increases its density, causing it to sink and form deep water that flows southward at depth. This process releases heat from the surface water, causing the North Atlantic region to warm by nearly 5°C.
The extent to which we understand the link between climate change and ocean circulation depends to a great extent on these geological records because measurements of atmospheric conditions and oceanic convection are reliable for only about the last 50 years. Though there are clearly identifiable links emerging from the atmospheric and oceanic records for this recent period, the magnitude of the changes are small compared to the large amplitude oscillations seen in the geological record. Likewise, future monitoring of changes in ocean convection and climate may also be small and will take place on time scales of 10 years or longer, thus making it difficult to identify the causes of the changes. Adding the larger, more dramatic changes seen in the geological record to the history of ocean circulation will help to constrain the factors that cause alterations in ocean circulation and increase our chances for improved prediction of future climate. But our geological reconstructions will only be a good as our understanding of the ecological factors governing today’s biota.
From Ground-Truthing the Paleoclimate Record Sediment Trap Observations Aid Paeoceanographers are strongly linked to temperatures in the region.
William B. Curry, Chairman, Geology and Geophysics Department
Dorinda R. Ostermann, Research Associate, Geology and Geophysics Department
Woods Hole Oceanographic Institition
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