Abstract
Chemical weathering of the continents is central to the regulation of atmospheric carbon dioxide concentrations, and hence global climate1,2. On million-year timescales silicate weathering leads to the draw-down of carbon dioxide1, and on millennial timescales chemical weathering affects the calcium carbonate saturation state of the oceans and hence their uptake of carbon dioxide2. However, variations in chemical weathering rates over glacial–interglacial cycles remain uncertain. During glacial periods, cold and dry conditions reduce the rate of chemical weathering3, but intense physical weathering3,4 and the exposure of carbonates on continental shelves due to low sea levels5,6 may increase this rate. Here we present high-resolution records of the lead isotope composition of ferromanganese crusts from the North Atlantic Ocean that cover the past 550,000 years. Combining these records with a simple quantitative model of changes in the lead isotope composition of the deep North Atlantic Ocean in response to chemical weathering, we find that chemical weathering rates were two to three times lower in the glaciated interior of the North Atlantic Region during glacial periods than during the intervening interglacial periods. This decrease roughly balances the increase in chemical weathering caused by the exposure of continental shelves, indicating that chemical weathering rates remained relatively constant on glacial–interglacial timescales. On timescales of more than a million years, however, we suggest that enhanced weathering of silicate glacial sediments during interglacial periods results in a net draw-down of atmospheric carbon dioxide, creating a positive feedback on global climate that, once initiated, promotes cooling and further glaciation.
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Acknowledgements
We thank G. Henderson for providing the crusts analysed in this paper; C. Hawkesworth and T. Elliott for constructive comments on an earlier draft of this manuscript; C. Coath for laboratory assistance; and B. Charlier for help with micro-drilling. Support for this research was provided by a NERC fellowship to G.L.F. and a NERC Joint Infrastructure Fund grant to the Department of Earth Sciences, University of Bristol.
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Supplementary Information
This file contains three sections – Supplementary Methods, Supplementary Figures and Supplementary Tables. The Supplementary Methods section provides additional information regarding the techniques used for the isotopic analysis of Pb in ferromanganese crusts using laser ablation. The Supplementary Figures section contains four images which show the location of the Pb isotope tracks and plots of 206Pb/204Pb and 208Pb/204Pb against age for each crust. The Supplementary Tables section contains three tables detailing the Pb isotopic data for crusts ALV 539, BM1969.05 and TR079 D-14. (PDF 609 kb)
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Foster, G., Vance, D. Negligible glacial–interglacial variation in continental chemical weathering rates. Nature 444, 918–921 (2006). https://doi.org/10.1038/nature05365
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DOI: https://doi.org/10.1038/nature05365
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