HR: 17:00h
AN: PP44A-05 INVITED     [Abstracts]
TI: The Inter-Ocean Salinity Gradient-Holocene vs. Glacial
AU: * Stott, L
EM: stott@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AU: Cannariato, K
EM: Kevin Cannariato [cannaria@usc.edu]
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089 United States
AB: In a previous investigation of LGM and Holocene surface water $\delta$$^{18}$O$_{SMOW} ($\delta$w)and temperature variability within the western tropical Pacific we documented higher $\delta$w values ($\sim$0.5% higher than the ice volume component) during the LGM and during stadials, implying increased salinities at times of tropical cooling (Stott et al., 2002). We have also recently documented a systematic decrease in SSTs and $\delta$w during the late Holocene between 8$^{o}$N and 10$^{o}$S in the western tropical Pacific that indicates surface salinities have decreased progressively by about 1psu within the Pacific Warm Pool since the early Holocene (Stott et al., 2004). The progressive decrease in western Pacific salinities during the Holocene tracked shifts in the ITCZ in response to changing orbital forcing. We hypothesized that shifts in the ITCZ during millennial-scale climate shifts disrupted the exchange of moisture between the Atlantic and Pacific and affected the salinity contrast between the two ocean basins. If so, it is possible that the hydrologic cycle has had a primary role in modulating thermohaline circulation during large climate oscillations. However, on the basis of the Pacific records alone we can not assess whether the trends we observed in the Pacific were accompanied by opposing changes in the Atlantic. In this study combined planktonic foraminiferal oxygen isotope and Mg/Ca paleothermometry to reconstruct the oxygen isotope composition of tropical surface waters ($\delta$w) in the Atlantic during the last Glacial and Holocene in order to assess whether there has been a redistribution of salt between the two oceans over last 20 thousand years. We focus on the subtropical regions of the tropical Atlantic and the western tropical Pacific in this comparative study because these two regions should be sensitive to changes in the vapor exchange between the two basins. Diminished freshwater exchange between the Atlantic and the Pacific when the ITCZ is in a more northerly position would tend to decrease salinities in the Atlantic and increase them in the Pacific. Samples of Globigerinoides ruber (white) were taken every centimeter through 8 box cores collected at sites along the mid-Atlantic ridge between 20$^{o}$N and 20$^{o}$S. The carbonate sequences contain well preserved foraminifera but the sedimentation rates at these locations are low ($\sim$ 1.5cm/ka). However, we showed in a previous study that despite the low sedimentation rates, the amount of mixing has not significantly effected the oxygen isotopic contrasts between glacial and Holocene samples (Stott and Tang 1996). Therefore, although it is not possible to resolve discrete millennial-scale features from the geochemical records, it is possible to resolve whether there has been a systematic shift in the dw and hence salinity. In the subtropical North and South Atlantic G. ruber Mg/Ca temperatures were 1.5$^{o}$cooler during the LGM compared to the late Holocene. We observe that at each of the sites within the subtropical north and south Atlantic the Holocene-LGM dw contrast averages 1%, which is equivalent to the ice volume effect. On the basis of these records we conclude that there was not a systematic change in the salinity of the mid ocean gyres in the North and South Atlantic during the LGM. Furthermore, these data do not support the hypothesis that shifts in the ITCZ during the LGM and during the Holocene affected the salinity contrast between the two ocean basins.
DE: 4267 Paleoceanography
DE: 1655 Water cycles (1836)
DE: 1724 Ocean sciences
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting