An exceptional analogue for the study of the causes and consequences of global warming occurs at the Palaeocene/Eocene Thermal Maximum, 55 million years ago. A rapid rise of global temperatures during this event accompanied turnovers in both marine1,2,3 and terrestrial biota4, as well as significant changes in ocean chemistry5,6 and circulation7,8. Here we present evidence for an abrupt shift in deep-ocean circulation using carbon isotope records from fourteen sites. These records indicate that deep-ocean circulation patterns changed from Southern Hemisphere overturning to Northern Hemisphere overturning at the start of the Palaeocene/Eocene Thermal Maximum. This shift in the location of deep-water formation persisted for at least 40,000 years, but eventually recovered to original circulation patterns. These results corroborate climate model inferences that a shift in deep-ocean circulation would deliver relatively warmer waters to the deep sea, thus producing further warming9. Greenhouse conditions can thus initiate abrupt deep-ocean circulation changes in less than a few thousand years, but may have lasting effects; in this case taking 100,000 years to revert to background conditions.
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We thank C. Charles for discussions and for assistance with the mass spectrometer at SIO, and P. Worstell for assistance in the laboratory. This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by the National Science Foundation and the US Science Support Program (to RDN). Author Contributions F.N. performed the data acquisition, and manuscript preparation; R.D.N. was responsible for project planning, manuscript revision and financial support. Both authors contributed equally in data analysis and interpretation.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
This table contains all stable isotope data from benthic foraminifera presented in this paper. The table also includes the age estimate for each sample, based on the chronology developed for this study. (XLS 88 kb)
List of references for Supplementary Table 1. (DOC 21 kb)
This table contains the average carbon isotope value for each site for each time interval along with standard deviation and number of data points used for each average. The table also presents basin averages and interbasinal aging gradients with standard deviation. (XLS 23 kb)
This table lists the data excluded from the deep ocean circulation analysis with a description of why the data were deemed unreliable. (DOC 24 kb)
This is a summary of the biostratigraphic datums available for correlation of the core of the carbon isotope excursion, and why they were deemed unsuitable for correlating the sites presented in this study. (DOC 32 kb)
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Nunes, F., Norris, R. Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period. Nature 439, 60–63 (2006). https://doi.org/10.1038/nature04386
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