Snapshots of mean ocean temperature over the last 700 000 years using noble gases in the EPICA Dome C ice core

Together with the latent heat stored in glacial
ice sheets, the ocean heat uptake carries the lion’s share of
glacial–interglacial changes in the planetary heat content, but
little direct information on the global mean ocean temperature
(MOT) is available to constrain the ocean temperature
response to glacial–interglacial climate perturbations. Using
ratios of noble gases and molecular nitrogen trapped in the
Antarctic EPICA Dome C ice core, we are able to reconstruct
MOT for peak glacial and interglacial conditions during
the last 700 000 years and explore the differences between
these extrema. To this end, we have to correct the
noble gas ratios for gas transport effects in the firn column
and gas loss fractionation processes of the samples after ice
core retrieval using the full elemental matrix of N2, Ar, Kr,
and Xe in the ice and their individual isotopic ratios. The
reconstructed MOT in peak glacials is consistently about
3.3+-0.4°C cooler compared to the Holocene. Lukewarm
interglacials before the Mid-Brunhes Event 450 kyr ago are
characterized by 1.6+-0.4°C lower MOT than the Holocene;
thus, glacial–interglacial amplitudes were only about 50%of
those after the Mid-Brunhes Event, in line with the reduced
radiative forcing by lower greenhouse gas concentrations and
their Earth system feedbacks. Moreover, we find significantly
increased MOTs at the onset of Marine Isotope Stage 5.5 and
9.3, which are coeval with CO2 and CH4 overshoots at that
time.We link these CO2 and CH4 overshoots to a resumption
of the Atlantic Meridional Overturning Circulation, which is
also the starting point of the release of heat previously accumulated
in the ocean during times of reduced overturning.