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

Abstract

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 °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°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.

Fractionation of O<sub>2</sub>/N<sub>2</sub> and Ar/N<sub>2</sub> in the Antarctic ice sheet during bubble formation and bubble-clathrate hydrate transition from precise gas measurements of the Dome Fuji ice core

“…The residuals of the data from the filtered curves are up to ~5 and ~3 ‰ for δO2/N2 and δAr/N2, respectively. The large scatters below the BCTZ have also been found in the WAIS Divide (Shackleton et al, 2019), EDML and EDC (Lüthi et al, 2010) ice cores. The pair differences of δO2/N2 and δAr/N2 are 1 -2 ‰ just below the BCTZ (~1200 m), and they decrease with depth to < 0.1 ‰ (Fig.…”

“…Very high δO2/N2 scatters have also been reported from the Vostok (Suwa and Bender, 2008a), GISP2 (Kobashi et al, 2008) and WAIS Divide (Shackleton et al, 2019) ice cores, with numerous positive values (higher than the atmosphere). The previous data were affected by the post-coring gas loss, thus they were interpreted only as artifacts created by the preferential gas loss from extremely N2-rich air bubbles (Bender, 2002;Ikeda-Fukazawa et al, 2001;Kobashi et al, 2008;Shackleton et al, 2019). In our data, relatively few samples (six and ten samples for δO2/N2 and δΑr/N2, respectively) show positive values, and even the samples with twice the surface removal (20 mm) show large scatters.…”

“…In the bubble-clathrate hydrate transition zone (BCTZ: e.g., 450 -1200 m at Dome Fuji, (Narita et al, 1999), extreme gas fractionations on the order of several 100 ‰ can occur between individual bubbles and clathrate hydrates due to lower dissociation pressure and larger permeation coefficient of O2 relative to N2 (e.g., Chazallon et al, 1998;Ikeda-Fukazawa et al, 2001). The microscopic gas fractionation, combined with the post-coring gas-loss fractionation, creates high variabilities of δO2/N2 and δAr/N2 from gas measurements in the BCTZ (Bender, 2002;Kawamura et al, 2007;Kobashi et al, 2008;Lüthi et al, 2010;Shackleton et al, 2019). Below the BCTZ, the variabilities of δO2/N2 and δAr/N2 decrease with depth over several hundred meters, possibly due to diffusive smoothing (Lüthi et al, 2010;Bereiter et al, 2014;Shackleton et al, 2019).…”

“…The microscopic gas fractionation, combined with the post-coring gas-loss fractionation, creates high variabilities of δO2/N2 and δAr/N2 from gas measurements in the BCTZ (Bender, 2002;Kawamura et al, 2007;Kobashi et al, 2008;Lüthi et al, 2010;Shackleton et al, 2019). Below the BCTZ, the variabilities of δO2/N2 and δAr/N2 decrease with depth over several hundred meters, possibly due to diffusive smoothing (Lüthi et al, 2010;Bereiter et al, 2014;Shackleton et al, 2019). The understanding of the smoothing process has been insufficient because of the lack of high-resolution gas data for a proper numerical simulation (Bereiter et al, 2014).…”

Earth’s radiative imbalance from the Last Glacial Maximum to the present

“…The most notable difference between the PRI reconstruction using EDC and WD is a sharper YD peak in WD. However, recent work by Shackleton et al (33) suggests that the OHC increase during the YD based on WD may be anomalously high, which would reconcile the two records.…”