Figure14,4. Reconsi ruction of the changes in PDB standard) across the Atlantic Ocean for the modern period (adapted from Kroopniek, 1984).

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Figure14,4. Reconsi ruction of the changes in PDB standard) across the Atlantic Ocean for the modern period (adapted from Kroopniek, 1984).

Other indicators may he used to estimate the nutrient content of ocean waters in the past. For instance, profiles of cadmium (Cd) dissolved in surface and deep waters are closely similar to those for phosphate (Broccker and Peng, 1982). Cd and P are removed efficiently from the surface ocean by organisms; these elements are incorporated into organic debris that sink. During their fall in the water column, these debris decompose partly in a nearly stoichiometric fashion. The general circulation of the ocean is then superimposed on this one-dimensional cycle to create signature variations in the chemical composition of occanic water masses. Boyle (1988a) showed that the Cd content of the calcific shells of foraminifera records that of the water in which these animals have lived and can be used as a tracer of deep water paleoctrculation.

Foraminifera] 5] yC and Cd data agree on the following features of the glacial oeean paleochemical distribution:

Below 2,5(H) m, most of the Atlantic basin is filled with Antarctic bottom water, so the relative contribution of water from the Southern Hemisphere as compared to that from the Northern Hemisphere is much greater than in the control case (Campin, 1997; Fichcfet ct al., 1994). However, sensitivity experiments with changes of =Ll%oin the glacial salinity boundary fields show circulation patterns ranging from one that is even stronger than the present day one to a near shutdown of the Atlantic deep sea circulation (Fichcfet et aL, 1994; Winguth et aL, 1999).

Modeling experiments provide new insights on changes in rates of both thermohaline circulation and ocean ventilation. Two parameters are computed: the actual age of the water, which is the time elapsed since water has been in contact with atmosphere, and the l4C age of the water, which is calculated from the difference of HC content between the deep water and the atmosphere. Both ages would be identical if the exchange of CO2 and ,4C was instantaneous at the air-sea interface. 'I here are significant differences between the modeled distribution of the HC age of the water and that of the actual age of the water that is only a function of the circulation intensity, because the 14c content of water in deep water sources depends not only on the oceanic circulation but also on the rate of 14C exchange at the air-sea interface (Figure 14,6), The differences between the t4C and the actual ages of water increase significantly when glacial boundary conditions are used to drive the model (Figure 14.7). In particular, a larger age shift is simulated for the newiv produced AABW, because the air-sea exchange of l4C was drastically reduced near Antarctica as the permanent sea-ice cover extended northward beyond the Antarctic divergence in the glacial run (Campin et al., 1999). The thermo-haline circulation therefore runs faster than suggested by ,4C measurements performed in benthic foraminifera having lived in Pacific deep water or AABW. In fact, similar rates of modern and last-glacial ocean advection of North Atlantic deep or intermediate water into the Circumpolar deep water have been inferred from mPa/230Th ratios of deep-sea sediments (Yu et al., 1996), The reduced ventilation of Southern Ocean surface water during LGM may also explain part of the discrepancy between benthic foraminifera <S13C and Cd content (Broeckcr, 1993),

14.3 Chcmieal Properties 14.3.1 pC02

Ice core studies have established that atmospheric carbon dioxide (CO2) was about 80 parts per million by volume (ppmv) lower during cold glacial climates than it is during interglacial times (Barnola et aL, 1987). The record of atmospheric CO? variation measured in the Vostok ice core indicates a strong similarity to other climate variables, such as global ice volume or south polar air temperature (Jouzel et aL, 1993). On this long time scale, the atmospheric CO? variation is dependent on the surface ocean pCOi, because there is 60 times more CO2 in the ocean than in the atmosphere, and 20 times more CO> in the ocean than in the continental biosphere (including soil). The evolution of atmospheric pCO^ during the last one million years may therefore be controlled mainly by the interaction of the cycle of biological production and decay of organic carbon with the vertical circulation of the ocean. This interaction depends on

Biosph Interaction

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