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Radionuclide studies indicate an historically hardy "conveyor belt."

The broad features of modern deep ocean circulation are often described as a thermo-haline "conveyor belt" initiated by the formation of North Atlantic Deep Water (NADW) as it sinks in the Norwegian, Greenland, and Labrador Seas and flows southward into the Circumpolar Deep Water (CPDW) around Antarctica. This general pattern of circulation affects global climate as it drives warm, low-latitude surface waters to the north to replace the sinking water. It also influences the sequestering of carbon dioxide into deep water, thereby regulating the carbon dioxide concentration of the atmosphere and the "greenhouse" effect on the earth's heat balance.

Changes in deep water circulation may have played an important role in controlling global climate during the geological past, especially during the last million years, a period characterized by cyclical climatic oscillations between ice ages and warm interglacial periods. Variations in the isotopic and chemical composition of calcite shells of foraminifera living on the sea-floor document significant changes in deep water circulation during the last ice age, when NADW was replaced by the Glacial North Atlantic Deep/Intermediate Water (GNAIW) flowing at shallower depth. However, the shells do not provide information on the rate at which this glacial water mass was produced or whether it even reached the Antarctic Ocean and mixed with CPDW.

In an effort to evaluate past changes in the rate of "conveyor belt" circulation and document whether GNAIW reached the CPDW, we are taking a new approach based on the partitioning of two radionuclides, protactinium 231 (231Pa) and thorium 230 (230Th). These two long-lived radionuclides (half-lives: 32,000 years and 75,000 years) are the decay products of two natural isotopes of uranium dissolved in seawater. Both are "particle-reactive," that is, they are rapidly removed to the seafloor by adsorption onto sinking particles. 230Th, with a very strong affinity for particles, resides in the water column for only a few decades, while the somewhat less particle reactive 23Pa resides in seawater for more than a century. In the modern ocean, deep water resides on average 200 years in the Atlantic before being exported into the CPDW. Because its mean residence time in seawater is 10 times shorter, 90 percent of the 230Th produced in Atlantic water is removed to the underlying Atlantic sediments. In contrast, with a residence time similar to that of deep water in the Atlantic, only 50 percent of the 231Pa produced is removed to Atlantic sediments. The remainder is exported with NADW into the CPDW for eventual deposit in the sediments of the Antarctic Ocean. Consequently, there is a deficit of 231Pa compared to 230Th in Atlantic sediments, and a surplus in sediments of the Antarctic ocean.

The contrast between the two oceans depends primarily on the flow rate of the deep thermohaline circulation. A significant decrease in the rate would result in a lesser 231Pa deficit in Atlantic sediments, while a surplus of 231Pa in Antarctic sediments would indicate addition of deep Atlantic water into CPDW. We measured 231Pa and 230Th in Atlantic and Antarctic sediments deposited during the last glacial maximum and compared it with the postglacial values. After correction for radioactive decay, we found very little difference between the two time intervals, indicating that GNAIW reached CPDW and there was essentially no change in the rate of the global "conveyor belt" circulation. This important information bears on our understanding of the influence of deep water circulation on the global climate and atmospheric carbon dioxide level during the last ice age.
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Copyright 1996 Gale, Cengage Learning. All rights reserved.

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Author:Francois, Roger; Yu, Ein-Fen; Bacon, Michael
Publication:Oceanus
Date:Mar 22, 1996
Words:580
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