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Gulf Stream density structure and transport during the past millennium

Abstract

The Gulf Stream transports approximately 31 Sv (1 Sv = 106 m3 s-1) of water1,2 and 1.3 × 1015 W of heat3 into the North Atlantic ocean. The possibility of abrupt changes in Gulf Stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries. Given the limited length of the instrumental record, our knowledge of Gulf Stream behaviour on long timescales must rely heavily on information from geologic archives. Here we use foraminifera from a suite of high-resolution sediment cores in the Florida Straits to show that the cross-current density gradient and vertical current shear of the Gulf Stream were systematically lower during the Little Ice Age (ad 1200 to 1850). We also estimate that Little Ice Age volume transport was ten per cent weaker than today’s. The timing of reduced flow is consistent with temperature minima in several palaeoclimate records4,5,6,7,8,9, implying that diminished oceanic heat transport may have contributed to Little Ice Age cooling in the North Atlantic. The interval of low flow also coincides with anomalously high Gulf Stream surface salinity10, suggesting a tight linkage between the Atlantic Ocean circulation and hydrologic cycle during the past millennium.

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Figure 1: Bathymetric map of Florida Straits showing core locations (red circles).
Figure 2: Foraminiferal δ 18O c data for Florida Straits cores.
Figure 3: Transport reconstruction for the Florida Current.
Figure 4: Comparison of Florida Current transport and surface water δ 18O w reconstructions.

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Acknowledgements

We thank O. Marchal, L. Keigwin, D. Oppo and J. McManus for suggestions. We also thank D. Ostermann, M. Jeglinski, P. Cerulli, S. Thorrold and S. Birdwhistle for technical support. We are grateful to the WHOI core lab for sample collection and archiving, the captain and crew of the R/V Knorr, and to the Sea Education Association for access to their vessel Westward. This work was supported by the US National Science Foundation.

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Correspondence to David C. Lund.

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Supplementary information

Supplementary Notes

This file contains Supplementary Methods and Supplementary Data. (DOC 86 kb)

Supplementary Figure 1

Stable isotope analyses for the Florida Straits cores were based primarily on Cibicidoides species from the >250 μm size fraction. (PDF 220 kb)

Supplementary Figure 2

Stable isotope analyses for the Florida Straits cores were based primarily on Cibicidoides species from the >250 μm size fraction. (PDF 190 kb)

Supplementary Figure 3

Quantification of the relationship between foraminiferal δ18O18Oc) and seawater σt. (PDF 199 kb)

Supplementary Figure 4

Comparison of core top foraminiferal and CTD density values from Dry Tortugas and Great Bahama Bank indicates the stable oxygen isotopic composition of the benthic foraminifera used in this study reliably record the density structure of the Florida Current. (PDF 143 kb)

Supplementary Figure 5

A shift in the level of no motion from 850 to 950 m during the Little Ice Age, but with the same shear, would produce a total volume transport of ~31 Sv, equal to the modern value. (PDF 122 kb)

Supplementary Figure 6

An isotopic depletion of the fresh-water end member results in a steeper δ18Ow-S slope (top) which in turn alters the polynomial fit to the δ18Oc -σt data in the range occupied by the Dry Tortugas and Bahamas benthic δ18Oc values (middle). The net effect is to increase density at Dry Tortugas more than the Bahamas, producing slightly higher transport values (~0.5 Sv) (bottom). (PDF 175 kb)

Supplementary Table 1

Dry Tortugas Radiocarbon Ages (DOC 85 kb)

Supplementary Table 2

Great Bahama Bank Radiocarbon Ages (DOC 109 kb)

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Lund, D., Lynch-Stieglitz, J. & Curry, W. Gulf Stream density structure and transport during the past millennium. Nature 444, 601–604 (2006). https://doi.org/10.1038/nature05277

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