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 marine (Kelly et al., 1998, doi:10.1016/S0031-0182(98)00017-0; Bralower, 2002, doi:10.1029/2001PA000662; Crouch et al., 2001, doi:10.1130/0091-7613(2001)0292.0.CO;2) and terrestrial biota (Bowen et al., 2002, doi:10.1126/science.1068700), as well as significant changes in ocean chemistry (Dickens et al., 1997, doi:10.1130/0091-7613(1997)0252.3.CO;2; Zachos et al., 2005, doi:10.1126/science.1109004) and circulation (Kennett and Stott, 1991, doi:10.1038/353225a0; Pak and Miller, 1992, doi:10.1029/92PA01234). 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 warming (Bice and Marotzke, 2002, doi:10.1029/2001PA000678). 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.

Sediment depth is given in mbsf. Further relevant datasets see: Pak and Miller (1992) doi:10.1594/PANGAEA.52489. Benthic foraminiferal d13C data for Site 690 published by Thomas and Shackleton (1996) were excluded from this analysis (total = 28 data points, out of 91 published data points). These data were systematically offset towards more positive values (~0.5 per mil to 1 per mil) from the data published by Kennett and Stott 1990, Stott et al., 1990 and Thomas et al, 2002.

Supplement to: Nunes, Flavia; Norris, Richard D (2006): Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period. Nature, 439(7072), 60-63