The Effect of Past Saturation Changes on Noble Gas Reconstructions of Mean Ocean Temperature

Abstract

The ocean's immense ability to store and release heat on centennial to millennial time scales modulates the impacts of climate perturbations. To gain a better understanding of past variations in mean ocean temperature (MOT), a noble gas‐based proxy measured from ancient air in ice cores has been developed. Here we assess non‐temperature effects that may influence the atmospheric noble gas ratios reconstructed from polar ice and how they impact the temperature signal with an intermediate complexity Earth system model. We find that changes in wind speed, sea‐ice extent, and ocean circulation have partially compensating effects on mean‐ocean noble gas saturation, leading to a slight reduction of noble gas undersaturation at the Last Glacial Maximum (LGM). Taking these effects and ice core measurements into account, our model suggests a revised MOT difference between the LGM and pre‐industrial of −2.1 ± 0.7°C that is also in improved agreement with other independent temperature reconstructions.

A Systematic Role for Extreme Ocean‐Atmosphere Oscillations in the Development of Glacial Conditions Since the Mid Pleistocene Transition

“…We acknowledge that our estimation is crude, and substantial uncertainty remains as to why variations in MOT are so well aligned with those in AAT and whether this covariation held during other periods (S. Shackleton et al., 2021). Furthermore variations in mean‐ocean noble gas saturation (Pöppelmeier et al., 2023) and refinements in the fractionation corrections applied when calculating MOT (S. Shackleton et al., 2020) will likely mean current estimates of MOT will need revising in the future. Notwithstanding, our approach provides a first order estimate of MOT that allows us to test some of the basic ideas about changes in relative temperature, volume and salinity referred to above (Figure 4).…”

“…Data points are simulated using a Monte Carlo method (3,000 loops) to obtain uncertainties on the fitted parameters. Errors in MOT are taken from the original sources (mean error = ±0.3°C; although we note a recent estimate for the LGM increases this uncertainty to ±0.7°C, Pöppelmeier et al., 2023). We apply a constant error of ±0.5°C to the AAT stack.…”

“…Of relevance to our study it has been suggested that enhanced deep ocean stratification during MIS 4 could have played an important role in lowering atmospheric CO 2 and ushering in glacial conditions at that time (Adkins, 2013; Barker & Diz, 2014; Bereiter et al., 2012; Ferrari et al., 2014; Hines et al., 2021). Recent work (S. Shackleton et al., 2021) also suggests that Mean Ocean Temperature (MOT) during MIS 4 (−2.7 ± 0.3°C relative to the Holocene) was comparable with the LGM (−2.6 ± 0.2, Bereiter et al., 2018, recently updated to −2.1 ± 0.7°C (Pöppelmeier et al., 2023)). The transition from MIS 5 to MIS 4 occurred when northern summers were relatively cool (a function of low obliquity and positive precession parameter, meaning northern hemisphere summers coincided with aphelion; Figure 1), but unlike the gradual (∼10 kyr) changes in Earth's orbital geometry, the final descent into MIS 4 was abrupt (≤3 kyr; Barker & Diz, 2014; Cutler et al., 2003; Woillard, 1979) and involved extreme oscillations in ocean and atmospheric circulation patterns (Dansgaard‐Oeschger, DO events 19 and 20; Figures 1 and 2; Table S1 in Supporting Information ).…”

“…−2.1 ± 0.7°C (Pöppelmeier et al, 2023)). The transition from MIS 5 to MIS 4 occurred when northern summers were relatively cool (a function of low obliquity and positive precession parameter, meaning northern hemisphere summers coincided with aphelion; Figure 1), but unlike the gradual (∼10 kyr) changes in Earth's orbital geometry, the final descent into MIS 4 was abrupt (≤3 kyr; Barker & Diz, 2014;Cutler et al, 2003;Woillard, 1979) and involved extreme oscillations in ocean and atmospheric circulation patterns (Dansgaard-Oeschger, DO events 19 and 20; Figures 1 and 2; Table S1 in Supporting Information S1).…”

Global Ocean Cooling of 2.3°C During the Last Glacial Maximum

“…Notably, none of these Kr/N 2 versus Xe/N 2 offsets fall outside the reported uncertainty bounds. The mismatch of ice core data with the model of Pöppelmeier et al (2023) led the authors to include a wide uncertainty estimate in noble gas disequilibria (translating to a 0.7°C error bar in MOT) and to call for future studies to revisit disequilibrium in the LGM.…”