Reversed flow of Atlantic deep water during the Last Glacial Maximum

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Abstract

The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies1,2 and recent model simulations3 indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic 231Pa/230Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength4,5,6. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in 231Pa/230Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of 231Pa/230Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC—with a prominent southerly flow of deep waters originating in the North Atlantic—arose only during the Holocene epoch.

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Figure 1: Multi-proxy profiles of MD02-2594.
Figure 2: MD02-2594 versus North Atlantic records.
Figure 3: Transit time estimates.

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Acknowledgements

MD02-2594 and MD02-2588 sediment cores were provided by the International Marine Past Global Changes Study (IMAGES) and the Institute Polaire Français Paul Emile Victor (IPEV). TN057-21 and PS2489-2 samples were supplied by S. Barker and A. Martínez-Garcia. Financial support is acknowledged from the Ministerio de Ciencia e Innovación, Spain, through scholarship AP-2004-4278 to C.N., REN2002-01958 to G.M.-M., and grant CGL2007-61579/CLI and funds from the Comer Abrupt Climate Change Foundation to R.Z. P.M. acknowledges an ICREA Academia award by the Generalitat de Catalunya.

Author information

R.Z. and P.M. designed the study and supervised C.N. during his PhD.; R.Z. and I.R.H. participated in the retrieval of the sediment cores; C.N. and G.M.-M. sampled the cores; C.N. processed the samples for 231Pa/230Th with help from A.L.T., J.L.M., P.M. and G.M.H.; A.L.T., G.M.H. and C.N. performed the Pa/Th/U measurements and data processing; G.M.-M. performed foraminiferal δ18O and δ13C analyses; I.R.H. provided S̄S̄ data; C.N. analysed opal concentrations; C.N. and R.Z. wrote the paper. All authors contributed to the interpretation of the results and provided input to the manuscript.

Correspondence to César Negre.

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Supplementary information

Supplementary Information

The file contains a brief introduction on 231Pa/230Th as a water flow-rate proxy, Supplementary Figures 1-3 with legends, the data from the Agulhas Plateau and the equatorial Atlantic which help to interpret the MD02-2594 record; the equations used for the calculation of 231Pa/230Th ratios. The file also contains Supplementary Table 1 and additional references. (PDF 767 kb)

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Negre, C., Zahn, R., Thomas, A. et al. Reversed flow of Atlantic deep water during the Last Glacial Maximum. Nature 468, 84–88 (2010) doi:10.1038/nature09508

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