Alan M. Seltzer

Nitrogen regulates primary productivity across much of the biosphere, yet fixed nitrogen loss remains poorly constrained because existing methods rely on indirect proxies or ex situ experiments. In this study, we show that natural abundances of the rare ¹⁵ N ¹⁵ N isotopologue of dinitrogen (N ₂ ) provide a direct tracer of biological N ₂ production across diverse aquatic environments. N ₂ produced by denitrification and anammox has a near-stochastic ¹⁵ N ¹⁵ N distribution [0 per mil (‰)], whereas atmospheric N ₂ carries a distinct ¹⁵ N ¹⁵ N excess (19‰), allowing the two sources to be quantitatively distinguished. Across aquifers, stratified lakes, coastal basins, oxygen minimum zones, and marine sediments, ¹⁵ N ¹⁵ N measurements reveal widespread nitrogen loss previously obscured by physical gas accumulation and nitrogen fixation. Natural ¹⁵ N ¹⁵ N abundances therefore provide a general framework for directly constraining fixed nitrogen loss across the aquatic nitrogen cycle.

Water stable isotopes (δ2H and δ18O) are widely used to reconstruct past hydrological changes and paleoclimate variability. A key limitation, however, is that fossil isotope records reflect not only local temperature but also changes in circulation, moisture sources, and hydrological processes along air-mass trajectories. Although modern monthly and interannual δ2H–δ18O–temperature relationships have been proposed to calibrate paleoclimate reconstructions, their applicability to longer timescales, such as glacial–interglacial transitions, remains uncertain. Here, we use fossil groundwaters worldwide as direct archives of past annual temperature and δ2H–δ18O co-variations over the last ∼35,000 years. Our global compilation of water stable isotope and noble gas temperature records (746 samples from 39 locations) shows that, while significant relationships occur in several regions, European fossil groundwaters exhibit the most consistent δ18O–temperature correlations. In Western Europe, δ18O variations during deglaciation were primarily temperature-driven, allowing δ18O to be used as a paleothermometer on glacial–interglacial timescales. In contrast, weaker correlations in North America, Asia, and Africa likely reflect stronger influences from atmospheric circulation, convection, and/or continental moisture recycling. For Western Europe, the observed relationships are consistent with a simple Rayleigh distillation model, although sites further inland seem to reflect more complex atmospheric processes. The mean European fossil groundwater-based δ18O–temperature slope, corrected for ice-volume effects since the LGM, is 0.48 ± 0.21 ‰ °C−1 (1 SD), comparable to modern monthly relationships for precipitation (0.29 ± 0.07 ‰ °C−1, 1 SD) but lower than modern interannual estimates (1.04 ± 0.30 ‰ °C−1, 1 SD). These results provide robust constraints for interpreting isotope-based climate archives and evaluating isotope-enabled climate models.

The source of volcanism in the Cameroon volcanic line remains unresolved. To better constrain the mantle components involved, we measure light noble gas (He–Ne–Ar) isotopes and, for the first time, heavy noble gas (Kr–Xe) isotopes in mantle-derived gases from São Tomé, using high-precision dynamic mass spectrometry (DMS). Our data show that the source of São Tomé volcanism is a plume-like mantle reservoir, which is enriched in ²²Ne relative to nucleogenic ²¹Ne when compared to samples originating from the convecting upper mantle, as represented by Mid-Ocean Ridge Basalts (MORBs). In contrast, the isotopic compositions of helium and xenon appear similar to those of the MORB mantle. DMS analysis of Xe isotopes reveals that volcanic gases are affected by diffusive transport fractionation in the subsurface, causing small enrichment in the light isotopes (e.g., ¹²⁸Xe). After correction for this secondary fractionation, we observe a slight, yet discernible, ¹³⁶Xe excess relative to the MORB mantle, attributable to spontaneous fission of ²³⁸U. We hypothesize that this excess fissiogenic Xe may arise from the ²³⁸U associated with recycled crustal component(s) in the São Tomé mantle source, potentially related to the influence of the HIMU-type mantle. The consistent ¹²⁹Xe/¹³⁶Xe (relative to air) observed in both MORB and plume-influenced mantle components is difficult to reconcile with the commonly accepted view that ¹³⁶Xe excesses originate from two distinct sources²³⁸U and ²⁴⁴Pu fission, respectivelyin these separate mantle reservoirs. Further investigation is needed to determine whether the apparent homogeneity in ¹²⁹Xe/¹³⁶Xe excess across the mantle, which contrasts with the distinct sources and evolutionary histories of other volatile elements (e.g., He, Ne, and N₂), is coincidental or indicative of an underlying process.

Many regional aquifer systems undergoing depletion contain “fossil” groundwater recharged under different climate conditions, raising questions on groundwater renewability. We examine the spatial distribution in groundwater ages estimated with multiple tracers covering different timescales, along with differences in paleoclimate and water table depths (WTD) in the Tucson Basin, Arizona, USA, to constrain recharge history. The presence of ³H and ⁸⁵Kr in upgradient wells indicates modern recharge, while ³⁹Ar, ¹⁴C, and ⁴He values show components of water several hundred to tens of thousands of years old in downgradient wells. In some wells, apparent ages on the orders of decades, centuries, and millennia are simultaneously indicated by ⁸⁵Kr, ³⁹Ar, and ¹⁴C activities, indicating that the flow system cannot be explained by a simple piston‐flow model. To disentangle mixing and recharge history signals, we apply an inverse modeling technique that couples lumped parameter modeling of residence times with (stable) noble gas reconstructions of recharge temperature and WTD. A mid‐Holocene maximum in WTD, reconstructed from Kr and Xe isotopes, suggests enhanced aridity associated with a reduction of wintertime precipitation, inferred from a shift in δ¹⁸O and δD. Colder temperatures were present during the Last Glacial Maximum, reflecting both climatic cooling and cold mountain front recharge. Recharge to the Tucson Basin has been continuous since the LGM, albeit with a likely reduction during the mid‐Holocene. Natural fluctuations in groundwater recharge have been small compared to those associated with groundwater extraction, indicating that pumping poses a larger threat to groundwater resources than climate change.

Billions of people rely upon groundwater for drinking water and agriculture, yet predicting how climate change may affect aquifer storage remains challenging. To gain insight beyond the short historical record, we reconstruct changes in groundwater levels in western North America during the last glacial termination (LGT, ~20 to 11 thousand years ago) using noble gas isotopes. Our reconstructions indicate remarkable stability of water table depth in a Pacific Northwest aquifer throughout the LGT despite increasing precipitation, closely matching independent Earth system model (ESM) simulations. In the American Southwest, ESM simulations and noble gas isotopes both suggest a pronounced LGT decline in water table depth in in response to decreasing precipitation, indicating distinct regional groundwater responses to climate. Despite the hydrologic simplicity of ESMs, their agreement with proxy reconstructions of past water table depth suggests that these models hold value in understanding groundwater dynamics and projecting large-scale aquifer responses to climate forcing.

The development of life on Earth has been enabled by its volatile-rich surface. The volatile budget of Earth’s surface is controlled by the balance between ingassing (for example, via subduction) and outgassing (for example, through magmatic and tectonic processes). Although volatiles within Earth’s interior are relatively depleted compared to CI chondrites, the total amount of volatiles within Earth is still substantial due to its vast size. However, the relative extent of diffuse degassing from Earth’s interior, not directly related to volcanism, is not well constrained. Here we use dissolved helium and high-precision argon isotopes combined with radiocarbon of dissolved inorganic carbon in groundwater from the Columbia Plateau Regional Aquifer (Washington and Idaho, USA). We identify mantle and crustal volatile sources and quantify their fluxes to the surface. Excess helium and argon in the groundwater indicate a mixture of sub-continental lithospheric mantle and crustal sources, suggesting that passive degassing of the sub-continental lithospheric mantle may be an important, yet previously unrecognized, outgassing process. This finding that considerable outgassing may occur even in volcanically quiescent parts of the crust is essential for quantifying the long-term global volatile mass balance.

Nitrogen plays a critical role in maintaining Earth’s hospitable surface environment over geological time. Despite our atmosphere being dominated by nitrogen, our understanding of how nitrogen was delivered to Earth and how subsequent planetary processes modified Earth’s nitrogen budget through time is currently lacking. Here, we report measurements of isotopologues of N₂ (Δ₃₀), along with ultrahigh precision measurements of Ar, Kr, and Xe isotopes, of hydrothermal gas samples from Yellowstone National Park. We show that δ¹⁵N variations are correlated with nonradiogenic Ar, Kr, and Xe isotope ratios, indicating that groundwater-derived nitrogen and noble gases in hydrothermal samples are fractionated by the same process as they diffuse through a rising column of magmatic CO₂. Notably, a similar correlation exists regardless of the degree of atmospheric contamination, suggesting that the δ¹⁵N of the Yellowstone mantle source is similar to the atmosphere (i.e., ∼0‰). Two component mixing models between Δ₃₀ and noble gases demonstrate that N₂/³⁶Ar (5.3 ± 0.7 × 10⁵) and ³⁶Ar/¹³⁰Xe (1611 ± 212) in the Yellowstone mantle source are lower and greater than the MORB mantle source, respectively, suggesting that contrary to previous findings, the plume mantle source has not been more efficiently overprinted by the addition of N₂- and Xe-rich recycled material. Conversely, we suggest that the similarity in δ¹⁵N and N₂/³⁶Ar between the Yellowstone mantle source and chondritic meteorites indicates that nitrogen and noble gases in the deep mantle reflect the composition of the material that initially formed Earth.

The distribution of helium isotopes in the upper kilometer of the water column along the GP15 section in the central Pacific reflects the large‐scale patterns of upwelling hydrothermal ³He in the tropics and sub‐polar gyre, tracing two important pathways whereby bottom water exits from the deep Pacific. Heavy noble gas saturation anomalies, particularly in the upper two hundred meters of the water column, are more strongly increased by seasonal radiative heating, while lighter noble gas saturation anomalies are increased more by air injection processes. A similar, seasonally persistent radiative heating feature was observed in the Equatorial Undercurrent, and appears to be replicated in climate system model simulations. The origin of this feature, however, remains a mystery. A heuristic component model explains the noble gas saturation anomaly distributions, separating the influences of air injection, barometric pressure and radiative heating/cooling. Results show cohesive spatial patterns consistent with where water masses originate, their circulation, and gas exchange dynamics in relation to their formation regions. Using this model, we diagnose the distribution of “non‐atmospheric” ⁴He in shallow waters, which parallels the helium isotope anomaly and silica distributions.