The formation of sea ice in the Arctic Ocean, as well as other physical processes such as injection of air and rapid cooling, plays a crucial role in determining the physical and chemical properties of its waters, which in turn drive the circulation in the Arctic [1]. Such processes can be constrained by conservative tracers which are biologically and chemically nonreactive, such as the noble gases. The full suite of stable noble gases (He, Ne, Ar, Kr and Xe) have been measured for the first time in the Arctic Ocean—along with CFC-12, SF6, and other transient tracers—during the Ventilation and Anthropogenic Carbon in the Arctic Ocean (VACAO) project of the wider Synoptic Arctic Survey 2021 (SAS21) [2]. The noble gas profiles indicate a water column strongly influenced by rapid cooling and excess air injection, with a surface signature characteristic of solute rejection by sea ice formation.We have compared multiple Arctic Ocean gas exchange models (based on similar models used in the Antarctic by Loose et al. [3] and in the Labrador Sea by Hamme et al. [4]) to constrain the fractions of Arctic water composed of Pacific, Atlantic and sea ice melt-derived origin waters, as well as the amount of sea ice being formed and air being injected into the water via bubbles. These parameters are estimated using a χ­2-minimisation procedure, where the misfit between fitted parameters and data is minimised. Preliminary results indicate a non-negligible sea ice term in the equations describing gas saturation anomalies in the Arctic.Another key goal of VACAO is to use transient tracers to study the ventilation timescales of the Arctic Ocean, with application towards the study of the efficacy of its CO2 solubility pump/storage. CFC-12/SF6 is one tracer pair with which this is attempted; water dating with this pair requires knowledge of the concentration history of the tracers in the surface water, for which there are no direct measurements. Thus, the physical gas exchange parameters modeled from the noble gas data can be used in conjunction with observed atmospheric histories to more accurately describe the surface water histories of CFC-12 and SF6. This in turn can be used to better constrain the Transit Time Distribution parameters used when dating Arctic waters [5]. Comparisons with other VACAO age tracer data (39Ar and 129I/236U) may act as validation tools to this “correction” to CFC-12/SF6 dating.References for Abstract[1] Rudels, B., and Carmack, E. 2022. Arctic ocean water mass structure and circulation. Oceanography, 35(3–4), 52–65 pp.[2] Snoeijs-Leijonmalm, P. and the SAS-Oden 2021 Scientific Party (2022). Expedition Report SWEDARCTIC Synoptic Arctic Survey 2021 with icebreaker Oden. Swedish Polar Research Secretariat. 300 pp.[3] Loose, B., Stammerjohn, S., Sedwick, P., & Ackley, S. (2023). Sea ice formation, glacial melt and the solubility pump boundary conditions in the Ross Sea. Journal of Geophysical Research: Oceans, 128, e2022JC019322.[4] Hamme, R. C., Emerson, S. R., Severinghaus, J. P., Long, M. C., & Yashayaev, I. (2017). Using noble gas measurements to derive air-sea process information and predict physical gas saturations. Geophysical Research Letters, 44, 9901–9909 pp.[5] Jeansson, E., Tanhua, T., Olsen, A., Smethie, W. M., Rajasakaren, B., Ólafsdóttir, S. R., & Ólafsson, J. (2023). Decadal changes in ventilation and anthropogenic carbon in the Nordic Seas. Journal of Geophysical Research: Oceans, 128, e2022JC019318.

Estimates of climate sensitivity rely in part on the magnitude of global cooling during the Last Glacial Maximum (LGM). While ice cores provide reliable LGM temperatures in high-latitude regions, proxy records of sea-surface temperature (SST) disagree substantially in the low latitudes (1-3), and quantitative low-elevation paleotemperature records on land are scarce. Filling this gap, noble gases in groundwater record land surface temperatures via their temperature-dependent solubility in water (4), a direct physical relationship uncomplicated by biological and chemical processes (5-6). Individual groundwater noble gas studies (e.g. 7-8) have shown 5-7 °C LGM cooling, in line with some proxy data (e.g. tropical snowline reconstructions) but larger than notable low-latitude SST reconstructions considering land-sea cooling ratios. To date, limited spatial coverage and the use of different physical frameworks to determine temperature from noble gas data has prevented a comprehensive estimate of low-latitude LGM cooling from noble gases in groundwater. Here we compile four decades of groundwater noble gas data from six continents, all interpreted using a consistent physical framework (9). We evaluate the accuracy of the “noble gas paleothermometer” by comparing noble gas derived temperatures in late Holocene groundwater with modern observations. From LGM noble gas data, we find that the low-elevation, low-to-mid-latitude land surface cooled by 5.8 ± 0.6 °C during the LGM (9). The ratio of our land cooling estimate to a recent SST reconstruction (1) that found 4.0 °C cooling over the same low latitude band is consistent with the inter-model mean land-sea cooling ratio of 1.45 °C from PMIP4 simulations (10). Together, these recent land- and sea-surface LGM temperature reconstructions indicate greater low-latitude cooling and thus climate sensitivity than prior studies, with implications for projections of future climate. 1) Tierney et al. (2020). Nature. 2) CLIMAP Project Members (1976). Science. 3) MARGO Project Members (2009). Nat. Geosci. 4) Jenkins et al. (2019). Mar. Chem. 5) Aeschbach et al. (2000). Nature. 6) Kipfer et al. (2002). Rev. Mineral. Geochem. 7) Stute et al. (1995). Science. 8) Weyhenmeyer et al. (2000). Science. 9) Seltzer et al. (2021). Nature. 10) Kageyama et al. (2021). Clim. Past.