During the mid-Cretaceous period, the global subsurface oceans were relatively warm, but the origins of the high temperatures are debated. One hypothesis suggests that high sea levels and the continental configuration allowed high-salinity waters in low-latitude epicontinental shelf seas to sink and form deep-water masses1,2,3. In another scenario, surface waters in high-latitude regions, the modern area of deep-water formation, were warmed through greenhouse forcing4, which then propagated through deep-water circulation. Here, we use oxygen isotopes and Mg/Ca ratios from benthic foraminifera to reconstruct intermediate-water conditions in the tropical proto-Atlantic Ocean from 97 to 92 Myr ago. According to our reconstruction, intermediate-water temperatures ranged between 20 and 25 ∘C, the warmest ever documented for depths of 500–1,000 m. Our record also reveals intervals of high-salinity conditions, which we suggest reflect an influx of saline water derived from epicontinental seas around the tropical proto-North Atlantic Ocean. Although derived from only one site, our data indicate the existence of warm, saline intermediate waters in this silled basin. This combination of warm saline intermediate waters and restricted palaeogeography probably acted as preconditioning factors for the prolonged period of anoxia and black-shale formation in the equatorial proto-North Atlantic Ocean during the Cretaceous period.
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We are grateful to the Leg 207 Shipboard Scientific Party, W. Hale, N. Westphal, K. Noeske and S. Feller. D. Panten, N. Andersen and H. Erlenkeuser are thanked for measuring stable isotopes. Our paper benefited from discussions with H. Brumsack, A. Forster, P. Hardas, A. Hetzel, J. Mutterlose, R. Norris and P. Sexton. This research used samples provided by the Ocean Drilling Program. ODP is sponsored by the US National Science Foundation and participating countries under the management of Joint Oceanographic Institutions. Financial support for this study was provided by the German Research Foundation (to O.F., J.E. and H.K.), KAKENHI (to K.M.) and UK ODP NERC (to P.A.W.).
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Friedrich, O., Erbacher, J., Moriya, K. et al. Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean. Nature Geosci 1, 453–457 (2008). https://doi.org/10.1038/ngeo217
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