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Persistent export of 231Pa from the deep central Arctic Ocean over the past 35,000 years

Abstract

The Arctic Ocean has an important role in Earth’s climate, both through surface processes1 such as sea-ice formation and transport, and through the production and export of waters at depth that contribute to the global thermohaline circulation2,3. Deciphering the deep Arctic Ocean’s palaeo-oceanographic history is a crucial part of understanding its role in climatic change. Here we show that sedimentary ratios of the radionuclides thorium-230 (230Th) and protactinium-231 (231Pa), which are produced in sea water and removed by particle scavenging on timescales of decades to centuries, respectively4, record consistent evidence for the export of 231Pa from the deep Arctic and may indicate continuous deep-water exchange between the Arctic and Atlantic oceans throughout the past 35,000 years. Seven well-dated box-core records provide a comprehensive overview of 231Pa and 230Th burial in Arctic sediments during glacial, deglacial and interglacial conditions. Sedimentary 231Pa/230Th ratios decrease nearly linearly with increasing water depth above the core sites, indicating efficient particle scavenging in the upper water column and greater influence of removal by lateral transport at depth. Although the measured 230Th burial is in balance with its production in Arctic sea water, integrated depth profiles for all time intervals reveal a deficit in 231Pa burial that can be balanced only by lateral export in the water column. Because no enhanced sink for 231Pa has yet been found in the Arctic, our records suggest that deep-water exchange through the Fram strait may export 231Pa. Such export may have continued for the past 35,000 years, suggesting a century-scale replacement time for deep waters in the Arctic Ocean since the most recent glaciation and a persistent contribution of Arctic waters to the global ocean circulation.

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Figure 1: Locations of seven box cores in a transect across the Arctic Ocean.
Figure 2: 231Paxs /230Thxs activity ratio records from Arctic box cores.
Figure 3: Average Holocene, deglacial and late glacial 231Pax/230Thxs activity ratios.

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Acknowledgements

We thank R. Poore for sharing radiocarbon dates for box cores 28 and 32. We thank O. Marchal, R. Newton, L. F. Robinson, D. McCorkle, E. Boyle, H. Oates and M. Jackson for discussions. The manuscript was improved by the comments of A. Thomas. This work was supported in part by the Comer Science and Education Fund and by US NSF grants OCE-0402565 (J.F.M.), OCE-0550637 (J.F.M.), OCE-0902985 (J.F.M.), AGS-0936496 (J.F.M.) and ARC-0520073 (W.B.C.).

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S.S.H. and J.F.M. designed the study with input from W.B.C. S.S.H. carried out core sampling, sediment digestions and instrumental analyses, and wrote the majority of the paper. S.B.-L. contributed to sediment digestion and analytical laboratory procedures. J.F.M. contributed to writing and revising the paper. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Sharon S. Hoffmann.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1– 4, which illustrate comparisons of 231Pa/230Th ratios to Th-normalized sediment fluxes, and new and published Arctic coretop 231Pa/230Th data, a Supplementary Discussion, which gives further background about the isotopes and cores presented and additional references. (PDF 1463 kb)

Supplementary Tables

This file contains Supplementary Tables 1-5, which present coordinates for seven box cores, radiocarbon and calibrated ages used in constructing age models, radionuclide data and instrumental error bars for all nuclide measurements, biogenic silica contents for selected samples, and replicate 231Pa and 230Th concentrations used to construct typical external 1-sigma error bars for 231Pa/230Th ratios. (XLS 268 kb)

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Hoffmann, S., McManus, J., Curry, W. et al. Persistent export of 231Pa from the deep central Arctic Ocean over the past 35,000 years. Nature 497, 603–606 (2013). https://doi.org/10.1038/nature12145

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