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.

Subduction zones are the interface between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatiles are actively cycled between Earth reservoirs by plate tectonics. Helium is highly sensitive to mantle inputs and can be used to deconvolute mantle and crustal volatile pathways in arcs. We report He isotope and abundance data for 18 deeply-sourced gas seep samples in the Central Volcanic Zone (CVZ) of Argentina and the Southern Volcanic Zone (SVZ) of Chile. We use 4He/20Ne values to assess the extent of air contributions, as well as He concentrations. Air-corrected He isotopes from the CVZ range from 0.21 to 2.58 RA (n=7), with the highest value in the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt. 4He/20Ne values range from 1.7 to 546 and He contents range from 1.0 to 31 x 106 cm3STP/cm3. Air-corrected He isotopes from the SVZ range from 1.27 to 5.03 RA (n=7), 4He/20Ne values range from 0.3 to 69 and He contents range from 0.5 to 175 x 106 cm3STP/cm3). Taken together, these data reveal a clear southeastward increase in 3He/4He, with the highest values (in the SVZ) plotting below the nominal range of values associated with pure upper mantle He (8 ± 1 RA1), but approaching the mean He isotope value for arc gases of ~5.4 RA2. Notably, the lowest values are found in the CVZ, suggesting more significant crustal contributions to the He budget. The crustal thickness in the CVZ is up to 70 km, significantly more than in the SVZ, where it is just 35-45 km3. It thus appears that crustal thickness exerts a primary control on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the Andean Convergent Margin. These data agree well with the findings of several previous studies4-14 conducted on the volatile geochemistry along the Andean Convergent Margin, which suggest a much smaller mantle influence, presumably associated with thicker crust masking the signal in the CVZ. [1] Graham, 2002 [2] Hilton et al., 2002 [3] Tassara and Echaurren, 2012 [4] Hilton et al., 1993 [5] Varekamp et al., 2006 [6] Ray et al., 2009 [7] Aguilera et al., 2012 [8] Tardani et al., 2016 [9] Tassi et al., 2016 [10] Tassi et al., 2017 [11] Peralta-Arnold et al., 2017 [12] Chiodi et al., 2019 [13] Inostroza et al., 2020 [14] Robidoux et al., 2020