| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February, 2006 Benthic and planktonic foraminiferal δ18O (δ18Oc) from a suite of well-dated, high-resolution cores spanning the depth and width of the Straits of Florida reveal significant changes in Gulf Stream cross-current density gradient during the last millennium. These data imply that Gulf Stream transport during the Little Ice Age (LIA: 1200-1850 A.D.) was 2-3 Sv lower than today. The timing of reduced flow is consistent with cold conditions in Northern Hemisphere paleoclimate archives, implicating Gulf Stream heat transport in centennial-scale climate variability of the last 1,000 years. The pattern of flow anomalies with depth suggests reduced LIA transport was due to weaker subtropical gyre wind stress curl. The oxygen isotopic composition of Florida Current surface water (δ18O w) near Dry Tortugas increased 0.4% during the course of the Little Ice Age (LIA: ~1200-1850 A.D.), equivalent to a salinity increase of 0.8-1.5 psu. On the Great Bahama Bank, where smface waters are influenced by the North Atlantic subtropical gyre, δ18Ow increased by 0.3% during the last 200 years. Although a portion (~0.1%) of this shift may be an artifact of anthropogenically-driven changes in surface water ΣCO2, the remaining δl8Ow signal implies a 0.4 to 1 psu increase in salinity after 200 yr BP. The simplest explanation of the δ18Ow data is southward migration of the Atlantic Hadley circulation during the LIA. Scaling of the δ18Ow records to salinity using the modern low-latitude δ18Ow-S slope produces an unrealistic reversal in the salinity gradient between the two sites. Only if C180 w is scaled to salinity using a high-latitude δ18Ow-S slope can the records be reconciled. Changes in atmospheric 14C paralleled shifts in Dry Tortugas δ18Ow , suggesting that variable solar irradiance paced centennial-scale Hadley cell migration and changes ...
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