The potential for molecular hydrogen (H-OH- groundwaters bearing up to 4.05 μmol⋅L-1 H2 , 3.81 μmol⋅L-1 methane (CH4) and 946 μmol⋅L-1 sulfate (SO42-) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H2 oxidation coupled to SO42- reduction. In shallower, oxidized Mg2+-HCO3- groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ13C and δD of CH4 (up to 23.9 ‰ VPDB and 45 ‰ VSMOW , respectively), indicated microbial CH4 oxidation, particularly in Ca2+-OH- waters with evidence of mixing with Mg2+-HCO3- waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

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.