HR: 1330h
AN: PP52A-0957    [PDF]
TI: Late Holocene variability in Florida Current surface density: Patterns and possible causes
AU: * Lund, D
EM: dlund@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop 24, Woods Hole, MA 02543 United States
AU: Curry, W
EM: wcurry@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop 24, Woods Hole, MA 02543 United States
AB: Planktonic foraminiferal $\delta$$^{18}$O records from three well-dated, high sedimentation-rate cores near the Florida Keys (24.4 N, 83.3 W) exhibit synchronous millennial-scale oscillations during the late Holocene. The cores span three different time intervals: the last 1000 years sampled at 20 year spacing; the last 2800 years at 40 year spacing; and the last 5200 years at 20 year spacing. If the 0.2-0.3 per mil isotopic shifts represent only changes in temperature, SSTs in the Florida Straits varied by 1-$2\deg$C over the past 5200 years. The largest increases in $\delta$$^{18}$O are centered at approximately 200, 2000, 3200, and prior to 4000 calendar years BP. High $\delta$$^{18}$O in the Florida Current during the Little Ice Age correlates with low sea-surface temperatures (SST) in the Sargasso Sea (Keigwin, 1996) and off the coast of West Africa (Site 658; deMenocal et al., 2000). An interval of low $\delta$$^{18}$O from 500 to 1800 yr BP is synchronous with the Medieval Warm Period at Site 658 but leads the MWP in the Sargasso Sea. Earlier intervals of enriched $^{18}$O at ~2000, 3200, and prior to 4000 correlate with low SSTs off the coast of West Africa. Sargasso Sea and Florida Straits SSTs tend to decrease during the negative phase of the North Atlantic Oscillation (NAO), while the opposite is true off the coast of West Africa. Synchronous cooling across the subtropical gyre is therefore difficult to explain using interannual NAO patterns (deMenocal et al., 2000). On decadal timescales, however, most of the gyre cools during the positive NAO phase (Visbeck et al., 2003). Simulations of a long-term reduction in solar irradiance create a North Atlantic SST pattern similar to that observed during the LIA (Rind and Overpeck, 1993; Shindell et al., 2001), though the simulated changes are generaly smaller than observed. Changes in Atlantic meridional overturning may have forced the SST shifts. Cold intervals at 200, 3200, and 4300 years BP are contemporary with ice-rafting events in the North Atlantic (Bond et al., 2001). Suppressed meridional overturning may have caused N. Atlantic ice-rafting events and cooler Gulf Stream temperatures in the Florida Straits. In the subtropical gyre, however, variations in proxy-derived SSTs are greater than the SST anomalies predicted from published models of reduced North Atlantic Deep Water production. Warming south of Newfoundland during the LIA (Keigwin and Pickart, 1999) is also inconsistent with a simple change in NADW production. While the Florida margin $\delta$$^{18}$O records display similar variability at millennial time scales, on centennial time scales the similarities are lost. Spectral analysis of the 5200-year record exhibits significant power at 360, 190, 130, 90, and 50 years. Two records that span the past 2800 years lack significant coherence at these periods, but this result is highly sensitive to the age model employed.
DE: 1620 Climate dynamics (3309)
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting