Is the Noble Gas-Based Rate of Ocean Warming During the Younger Dryas Overestimated?

Noble gases in ice cores enable reconstructions of past mean ocean temperature. A recent result from the clathrate-containing WAIS Divide Ice Core showed tight covariation between ocean and Antarctic temperatures throughout the last deglaciation, except for the Younger Dryas interval. In the beginning of this interval, oceans warmed at 2.5 degrees C/kyr-three times greater than estimates of modern warming. If valid, this challenges our understanding of the mechanisms controlling ocean heat uptake. Here we reconstruct mean ocean temperature with clathrate-free ice samples from Taylor Glacier to test these findings. The two records agree in net temperature change over the Younger Dryas, but the Taylor Glacier record suggests sustained warming at the more modest rate of 1.1 +/- 0.2 degrees C/kyr. We explore mechanisms to explain differences between records and suggest that the noble gas content for the Younger Dryas interval of WAIS Divide may have been altered by a decimeter-scale fractionation during bubble-clathrate transformation. Plain Language Summary Oceans have taken up most of the additional heat trapped by greenhouse gases, mitigating the current rate of surface warming. In order to understand changes in ocean heat uptake over time, we use atmospheric noble gases measured in ice cores to estimate past ocean temperature change. This method works because the amount of noble gases dissolved in seawater changes with temperature. A recent ocean temperature reconstruction identified a 700-year interval during the transition from the last ice age to the current warm period when oceans warmed three times faster than they are currently warming. This result challenged our understanding of how oceans warm as an ice age ends. We tested this finding with a new ice core record and found that ocean warming during this interval occurred at a rate that is comparable to today, which is more consistent with our understanding of ocean heat uptake. We suggest that the noble gas record in the original ice core was altered by a process that affects how atmospheric gases are distributed in ice and is unrelated to ocean temperature change. From these findings we suggest caution in interpreting noble gas records in ice cores where this process may occur.