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FIGURE 6.8 A typical vertical temperature, salinity, and density profile from the tropical oceans.As the amount of calcium carbonate secreted at depths below the upper mixed layer (generally -100 m) increases, so the lsO content of the test carbonate increases. Isotopic temperature estimates from foram tests that have undergone gametogenic calcification at depth are considerably lower than those obtained from living foraminifera collected in the upper mixed layer (~0.2%o per degree Celsius).

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to this problem is to compare the 8lsO of forams in core-top assemblages or in sediment traps with oceanographic conditions in the overlying water column. For example, studies of 8lsO in forams collected in sediment traps at depth in the Sargasso Sea, compared to the temperature profile in the overlying water column have shed light on both the seasonal occurrence of forams and the relationship between depth habitat and 8lsO in the foram tests (Deuser and Ross, 1989). This revealed, inter alia, that Globorotalia truncatulinoides adds calcite to its test at a depth of -800 m, resulting in a sample mean 8lsO that does not correspond to SSTs but to water temperatures at -200 m. The sediment trap data also demonstrate that many species only occur at particular times of year. For example, P. obliquiloculata and Neogloboquadrina dutertrei live in the upper 100 m and give 8lsO values that are representative of the winter mixed layer, whereas Globigerinoides ruber (pink variety), which also live in the mixed layer (upper 25 m), occur only in summer months, resulting in different values of 8lsO. This complication has some benefit; there is the potential of using seasonally occurring species of known depth habitat to reconstruct past changes in seasonality (and mixed layer depth) from-foram assemblages (Reynolds-Sautter andThunell, 1989).

One additional complexity in using 8lsO as a paleotemperature indicator is that 8180 is also strongly related to salinity (Fig. 6.9). Hence any change in salinity due to large-scale dilution effects (because of ice sheet melting) or to local changes in the precipitation-evaporation (P-E) relationship will also be recorded in foraminifera so affected (Duplessy et al, 1991). This can be put to good use in order to estimate salinity changes at the sea surface, providing all other effects can be determined independently. Thus, Duplessy et al. (1993) assessed local salinity changes off Portugal during the last deglaciation by first estimating SSTs (from micropaleontological transfer functions; see

FIGURE 6.9 Relationship between 8'80 and salinity in oceanic surface waters, based on modern water samples (Broecker, 1989, using data of H. Craig).

FIGURE 6.9 Relationship between 8'80 and salinity in oceanic surface waters, based on modern water samples (Broecker, 1989, using data of H. Craig).

Section 6.4) and taking into account the influence of ice sheet meltwater on surface salinity and oceanic 8lsO. By comparing the 8lsO record "expected" from such variations with the observed record, they argue that the resulting series of differences must be due to local changes in salinity. In spite of the uncertainties in such an approach, the results strongly indicate that salinity was high during the time of maximum ice sheet melting (Meltwater Pulse 1, "mwp-IA," around 12 ka B.P.; Fairbanks, 1989) due to either local changes in P-E or to a shift in water masses (Fig. 6.10). The same conclusions were reached in studies of other cores from the North Atlantic and Norwegian Sea, providing no support for the notion of a low salinity lid on North Atlantic circulation during massive low latitude meltwater events, which might have been expected to inhibit the production of North Atlantic Deep Water (NADW). However, when meltwater was primarily discharged to the North Atlantic via the St. Lawrence River drainage, salinities were reduced, even though the overall freshwater discharge rates were lower. Apparently, additional feedbacks involving reductions in evaporation, and/or increased precipitation, or reduced poleward advection of salty subtropical water, led to lower surface salinities in the North Atlantic, and a corresponding

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