Abstract Quantifying the lipid biosynthesis rate of archaea in hot spring sediments is necessary to interpret the abundance, isotopic patterns, and environmental significance of archaeal lipid biosignatures, with implications for modern biogeochemical cycling and astrobiology. Here, we performed lipid hydrogen stable isotope probing (LH-SIP) experiments on whole sediments collected from two high-temperature, suboxic, circumneutral hot springs in Yellowstone National Park (USA) and El Tatio Geyserfield (Chile). We determined the incorporation of 2 H 2 O into intact polar lipids (IPLs) which provides a taxon- and metabolism-agnostic quantification of biosynthesis under near-natural conditions. We targeted isoprenoid glycerol dialkyl glycerol tetraether lipids (IPL iGDGTs) and recovered structures with 0 to 7 cyclopentyl rings from both springs. We observed minor 2 H-uptake into archaeal IPLs in spring sediments in Yellowstone, corresponding to decadal-scale apparent generation times (16 ± 3 years), and no uptake in El Tatio sediments (consistent with minimum generation times of 42 ± 21 years). We infer that net production of sedimentary IPL-iGDGTs is very slow, consistent with a combination of slow archaeal growth, persistence of older IPLs, lipid recycling, and/or contributions from recently sedimented planktonic biomass. These are the first direct, ex situ estimates of archaeal lipid production rates in terrestrial hydrothermal systems using LH-SIP incubations and provide critical constraints for interpreting archaeal lipids in ancient hot spring deposits. This research establishes a framework for assessing activity by slow-growing extremophilic archaea in hydrothermal environments and provides support for targeting hydrothermal deposits on Mars for biosignature detection efforts. Plain Language Summary Hot springs on Earth are important natural laboratories for understanding how signs of life might form and be preserved in hydrothermal environments on early Earth or Mars. In this study, we examine the rate of archaeal lipid biosignature production in sediments from two hot springs in Yellowstone National Park and the El Tatio Geyserfield in Chile. We used a method that measures new microbial production by tracing heavy hydrogen from labeled water as microbes incorporate that hydrogen into newly made lipids in their cell membranes. We found that archaeal lipids in hot spring sediments are produced very slowly, on timescales of decades. This result, along with the chemical stability of lipids and the rapid mineralization rate in hot springs, may allow these molecular biosignatures to be entombed and preserved in hot spring mineral deposits. These results help us better interpret ancient hydrothermal deposits on Earth and support the idea that slowly growing microbial communities could still leave detectable molecular traces in similar environments on Mars and other rocky planets. Key Points Lipid hydrogen stable isotope probing is applied to high temperature hot spring sediments for the first time In hot spring sediments, archaeal lipid production occurs on decadal timescales comparable to some marine sediments Active archaeal lipid synthesis in hot spring sediments supports targeting Martian hydrothermal deposits for biosignature detection

Methane is a potent greenhouse gas, an important energy source, and an important part of the global carbon cycle. The relative abundances of doubly substituted ("clumped") methane isotopologues (13CH3D and 12CH2D2) offer important information on the sources and sinks of methane. However, the clumped isotope signatures of microbially produced methane from different methanogenic pathways lack a systematic investigation. In this study, we provide a data set encompassing isotopic signatures of hydrogenotrophic, methylotrophic, acetoclastic, and methoxydotrophic methanogenesis. We find that a statistical "combinatorial effect" generates significant differences in 12CH2D2 compositions between hydrogenotrophic methanogenesis and the other pathways, while variations in the fractionation factors of clumped isotopologues result in differences in 13CH3D compositions between the methylotrophic, acetoclastic, and methoxydotrophic pathways. The energy yield of methanogenesis and the energy conservation approaches implemented by different microbial strains may also influence the isotope values of methane. Further analysis suggests that previously observed isotopic signatures of methane in freshwater environments are potentially due to mixing between hydrogenotrophic and other methanogenesis pathways. This study provides new experimental constraints on the isotope signatures of different microbial methanogenic pathways and evidence of the mechanisms responsible for the observed differences. This enables a better understanding of the sources and sinks of methane in the environment.

Methane is a major greenhouse gas and a key component of global biogeochemical cycles. Microbial methane often deviates from isotope and isotopolog equilibrium in surface environments but approaches equilibrium in deep subsurface sediments. The origin of this near-equilibrium isotopic signature in methane, whether directly produced by methanogens or achieved through anaerobic oxidation of methane (AOM), remains uncertain. Here, we show that, in the absence of AOM, microbial methane produced from deep-sea sediments exhibits isotopolog compositions approaching thermodynamic equilibrium due to energy limitation. In contrast, microbial methane from salt marsh and thermokarst lakes exhibits significant hydrogen and clumped isotopic disequilibrium due to high free-energy availability. We propose that clumped isotopologs of methane provide a proxy for characterizing the bioenergetics of environments for methane production. Together, these observations demonstrate methane clumped isotopes as a powerful tool to better understand the relation between methane metabolisms and the energy landscape in natural environments.

The stable hydrogen isotope composition (δ2H) of lipid biomarkers can track environmental processes and remain stable over geologically relevant time scales, enabling studies of past climate, hydrology, and ecology. Most research has focused on lipids from the domain Eukarya (e.g., plant waxes, long-chain alkanes), and the potential of prokaryotic lipid biomarkers from the domain Archaea to offer unique insights into environments not captured by eukaryotic lipids remains unclear. Here, we investigate the H-isotope composition of biphytanes in Sulfolobus acidocaldarius, a model thermoacidophile and obligate heterotroph. We conducted a series of experiments that varied temperature, pH, shaking rate, electron acceptor availability, or electron donor flux. From these experiments, we quantified the lipid/water H-isotope fractionation (2ϵL/W) values for core biphytane chains derived from tetraether lipids. The 2ϵL/W values are consistently negative (-230 to -180 ‰) and are relatively invariant across all experiments despite the 20-fold change in doubling times and 2-fold change in lipid cyclization. The magnitude and relative invariance of 2ϵL/W values are consistent with studies on other heterotrophic archaea and suggests archaeal lipids may be faithful recorders of the δ2H composition of growth water. Our study highlights the potential of archaeal lipid 2ϵL/W as a hydrological proxy, offering new insights into environments where traditional proxies, such as plant-derived lipids, are not available, including extreme environments and extraterrestrial settings.

Aerobic methane oxidation (AeOM) is an important biological sink of methane on Earth. Stable isotopes are critical tools in tracking the sources and sinks of Earth’s surface methane budget. However, the major factors that influence the two multiply-substituted (clumped) isotope signatures of AeOM, Δ13CH3Dand Δ12CH2D2 , are not well known. Here we quantify the influence of kinetics as a function of temperature and different methane monooxygenase (MMO) enzymes (modulated by copper) on the isotopologue concentrations of residual methane by the obligate aerobic methanotroph, Methylococcus capsulatus (Bath). We observe deviations from traditional closed-system distillation (Rayleigh) fractionation during exponential growth at high oxidation rates. We model this as a reservoir effect controlled by the ratio of oxidation rate in the cells to transport rate of methane into the cells, where environmental temperature affects both rates. We also test whether clumped isotope fractionation values vary for the particulate versus soluble MMOs, but the results show minimal differences. We further determine that the back reaction (re-equilibration) of methane with medium water is unlikely. Together, the observations and model demonstrate that at low oxidation-to-transport ratios, the clumped isotope signatures follow canonical Rayleigh fractionation, whereas at high ratios, more positive Δ12CH2D2 values result, deviating from simple Rayleigh-like trajectories. This study shows that the methane oxidation-to-transport ratio is a critical influence on clumped isotope signatures of AeOM that should be considered when interpreting the isotopic data of natural methane samples in both open and closed systems.

The stable hydrogen isotope composition of archaeal lipids is emerging as a potential paleoenvironmental proxy, adding to the well-established application of plant leaf wax-derived n-alkanes in paleohydrological reconstruction. A handful of studies reported relatively invariant and depleted hydrogen isotope compositions for archaeal lipids despite the range of different organisms and growth conditions explored. However, how modes of metabolism and physiological state (growth phase) affect the hydrogen isotope signatures of archaeal lipids remains poorly understood, limiting our ability to interpret archaeal lipid biomarker records from the environment. Here we conducted water isotope label experiments with a metabolically flexible and well-studied model archaeon Archaeoglobus fulgidus and quantified the hydrogen isotope fractionation between lipids and water in response to different carbon substrates and electron donor-acceptor pairs at different growth phases. The 2H/1H fractionation between lipids and water (epsilon L/ W) was overall negative. Both carbon metabolism and growth phase affected the magnitude of isotope fractionation in A. fulgidus; however, the changes in epsilon L/W values were relatively subtle where they ranged from -283 to -229 parts per thousand across all tested conditions, overlapping with the ranges observed for other archaea in previous studies. Isotope flux-balance model results suggest that >= 80 % and >= 50 % of lipid-bound H in A. fulgidus cultures directly reflect water isotope compositions (i.e., not via organic substrate or H2) during autotrophy and heterotrophy, respectively. The model results also suggest two main mechanisms of consistent 2H depletion observed in A. fulgidus tetraethers as well as other archaeal lipids reported in previous studies: 1) isotopic re-equilibration via upstream isomerization reactions involving C5 units and 2) downstream double bond reduction catalyzed by a flavoenzyme geranylgeranyl reductase. These results are consistent with previous isotope flux-balance model results for a different archaeon. Finally, we synthesized available data to compare epsilon L/W patterns across all three domains of life: Eukarya, Archaea and Bacteria. Because they vary fundamentally in lipid biosynthesis pathways, we present comparative discussions in pairs, focusing on the shared biochemical mechanisms among isoprenoid lipids and potential signals of metabolic adaptations across prokaryotic lipids. Emerging patterns between diverse archaeal and eukaryotic isoprenoid lipids are consistent with the two proposed mechanisms for 2H depletion identified (isomerization and final saturation). The patterns between archaeal isoprenoids and bacterial fatty acids suggest that the general state of energy limitation may also contribute to large, negative values of epsilon L/W observed in prokaryotic lipids. Altogether, these findings lend further support for the potential of archaeal lipid epsilon L/W as a paleohydrological proxy and provide a broader insight into the 2H/1H fractionation mechanisms potentially shared among prokaryotic and eukaryotic lipid biomarkers.

The stable hydrogen isotope composition of archaeal lipids is emerging as a potential paleoenvironmental proxy, adding to the well-established application of plant leaf wax-derived n-alkanes in paleohydrological reconstruction. A handful of studies reported relatively invariant and depleted hydrogen isotope compositions for archaeal lipids despite the range of different organisms and growth conditions explored. However, how modes of metabolism and physiological state (growth phase) affect the hydrogen isotope signatures of archaeal lipids remains poorly understood, limiting our ability to interpret archaeal lipid biomarker records from the environment. Here we conducted water isotope label experiments with a metabolically flexible and well-studied model archaeon Archaeoglobus fulgidus and quantified the hydrogen isotope fractionation between lipids and water in response to different carbon substrates and electron donor-acceptor pairs. The 2H/1H fractionation between lipids and water (ϵL/W) was overall negative, ranging from -280 to -226 ‰, and overlapped with the ranges observed for other archaea in previous studies. Isotope flux-balance model results suggest that ≥80 % and ≥50 % of lipid-bound H in A. fulgidus cultures originate from water during autotrophy and heterotrophy, respectively. The model results also suggest the final saturation during isoprenoid lipid biosynthesis catalyzed by a flavoenzyme geranylgeranyl reductase likely contributes to the large 2H/1H fractionation observed in this study, consistent with isotope flux-balance model results for a different archaeon. Finally, we synthesized available data to compare ϵL/W patterns across all three domains of life: Bacteria, Archaea and Eukarya. Emerging patterns between archaeal and eukaryotic lipids are consistent with the notion of highly fractionating geranylgeranyl reductase, and the patterns between archaeal and bacterial lipids suggest that the general state of energy limitation may also contribute to large, negative values of ϵL/W observed in prokaryotic lipids. Altogether, these findings lend further support for the potential of archaeal lipid ϵL/W as a paleohydrological proxy and provide a broader insight into the 2H/1H fractionation mechanisms potentially shared among prokaryotic and eukaryotic lipid biomarkers.

The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE.

AbstractThe stable hydrogen isotope composition of archaeal lipids is emerging as a potential paleoenvironmental proxy, adding to the well-established application of plant leaf wax-derivedn-alkanes in paleohydrological reconstruction. A handful of studies reported relatively invariant and depleted hydrogen isotope compositions for archaeal lipids despite the range of different organisms and growth conditions explored. However, how modes of metabolism and physiological state (growth phase) affect the hydrogen isotope signatures of archaeal lipids remains poorly understood, limiting our ability to interpret archaeal lipid biomarker records from the environment. Here we conducted water isotope label experiments with a metabolically flexible and well-studied model archaeonArchaeoglobus fulgidusand quantified the hydrogen isotope fractionation between lipids and water in response to different carbon substrates and electron donor-acceptor pairs. The2H/1H fractionation between lipids and water (εL/W) was overall negative, ranging from –280 to –226 ‰, and overlapped with the ranges observed for other archaea in previous studies. Isotope flux-balance model results suggest that ≥80 % and ≥50 % of lipid-bound H inA. fulgiduscultures directly reflect water isotope compositions (i.e., not via organic substrate or H2) during autotrophy and heterotrophy, respectively. The model results also suggest the final saturation during isoprenoid lipid biosynthesis catalyzed by a flavoenzyme geranylgeranyl reductase likely contributes to the large2H/1H fractionation observed in this study, consistent with previous isotope flux-balance model results for a different archaeon. Finally, we synthesized available data to compare εL/Wpatterns across all three domains of life: Bacteria, Archaea and Eukarya. Emerging patterns between archaeal and eukaryotic lipids are consistent with the notion of highly fractionating geranylgeranyl reductase, and the patterns between archaeal and bacterial lipids suggest that the general state of energy limitation may also contribute to large, negative values of εL/Wobserved in prokaryotic lipids. Altogether, these findings lend further support for the potential of archaeal lipid εL/Was a paleohydrological proxy and provide a broader insight into the2H/1H fractionation mechanisms potentially shared among prokaryotic and eukaryotic lipid biomarkers.

Hydrogen (δD) and oxygen (δ 18 O) isotopic ratios are strongly correlated in precipitation over time and space, defining the meteoric water line, and the slope of this δD‐δ 18 O relationship reflects covariations of deuterium excess (d‐excess) with δD or δ 18 O. This δD‐δ 18 O line provides a tool for inferring hydrologic processes from the evaporation source to condensation site. Here, we present δD‐δ 18 O relationships on seasonal and annual timescales for daily precipitation, snow pits, and a 15‐m ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD‐δ 18 O slopes are less than 8 (summer = 7.70; winter = 7.77), while the annual slope is greater than 8 (8.27). We suggest that intra‐season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than 8. The summer line has a greater intercept (higher d‐excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d‐excess primarily to contributions of vapor sublimated from the Greenland Ice Sheet and other terrestrial sources. High sublimated moisture proportions result in a large separation between seasonal δD‐δ 18 O lines, and thus high annual slopes. Inter‐seasonal weighting of precipitation amount also influences annual slopes because slopes are weighed by the number of storms each season. Using snow pit measurements, we demonstrate that precipitation isotopic signals translate to the snowpack. We generate indices to determine Sublimation Proportion Index and Precipitation Weighting Index, and find that annual Owen core δD‐δ 18 O line slopes are significantly related to these indices, demonstrating that these factors are recorded in ice cores.

We report fallout radionuclide (FRN) and major/trace element (MTE) contributions to bulk atmospheric deposition in Hanover, NH, USA (43.7022°N, 72.2896°W). Deposition of 7 Be, 210 Pb, and SO 4 covary [ R 2 > 0.4, n = 461] but are discriminated by production sources, depositional mechanisms, meteorological controls, MTE associations, and seasonal biases. 7 Be is dominated by rainout (78% of total deposition), recharged by long‐range transport (+23% over mean, o.m.), influenced by stratosphere‐troposphere exchange (+9% o.m.), and solar activity (−2% per doubling of sunspot count). Correlation with particulate nitrogen (+9% per doubling of N) indicates 7 Be affinity for biogenic aerosols. 210 Pb is dominated by dry + washout deposition (54% of total) and convective storms (+107% o.m), depleted in marine moisture sources (−133% o.m.), correlated with S (+9% per doubling of S), and biased to autumn with Mn, Hg, and V (+7% o.m.). Coincident long‐term declines in S and 210 Pb (−14%, −4% per year) suggest co‐scavenging by PbSO 4 . 7 Be: 210 Pb ratios increase asymptotically with precipitation through the dry‐washout‐rainout transition and recharge of 7 Be. At the global scale, 7 Be: 210 Pb increases with precipitation for North American/European sites due to recharge of 7 Be in midlatitude storm belts [ R 2 = 0.64, n = 31]. Conversely, 7 Be: 210 Pb is independent of precipitation for Southeast/East Asian sites where 7 Be recharge is low [ R 2 = 0.01, n = 40]. Globally, 7 Be: 210 Pb ratios in dry deposition reflect resuspended aerosols with mean age of ca. 200 days, contributing <5% of 210 Pb deposition. Different aerosol populations contributing to FRN deposition across spatial and temporal scales should emerge as a focus in terrestrial 7 Be, 210 Pb, and 10 Be tracer applications.

We report fallout radionuclide (FRN) and major/trace element (MTE) contributions to bulk atmospheric deposition in Hanover, NH USA (43.7022° N, 72.2896° W). Deposition of 7

Methane generated by microorganisms is most often depleted in the doubly substituted isotopologue 12CH2D2 relative to the stochastic reference distribution. To constrain the controls on depleted Δ12CH2D2 values, we experimentally isolated the root cause with microorganisms that produce methane from methylphosphonate via the C-P lyase pathway. This mechanism of methane production preserves the three hydrogens from methylphosphonate and adds one hydrogen from water. When maintaining the same methylphosphonate source, but varying the D/H composition of growth medium water, we observed significant shifts in methane Δ12CH2D2 values, but little to no change in Δ13CH3D values. We reproduced these observations with a model that considers only the combinatorial isotope effect. The variation in Δ12CH2D2 values of product methane resulted from the differences in D/H between reactants water and methylphosphonate. This work validates the hypothesis that combinatorial effects can strongly influence methane Δ12CH2D2 values, and must be considered for low temperature, abiotic or biotic systems where methane hydrogen is derived from multiple reservoirs.

Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.

We develop a one-dimensional (1-D) steady-state isotope marine boundary layer (MBL) model that includes meteorologically important features missing in models of the Craig and Gordon type, namely height-dependent diffusion and mixing, lifting to deliver air to the free troposphere, and convergence of subsiding air. Kinetic isotopic fractionation results from this height-dependent diffusion that starts as pure molecular diffusion at the air–water interface and increases with height due to turbulent eddies. Convergence causes mixing of dry, isotopically depleted air with ambient air. Model results fill a quadrilateral in δD–δ18O space, of which three boundaries are defined by (1) vapor in equilibrium with various sea surface temperatures (SSTs), (2) mixing of vapor in equilibrium with seawater and vapor in subsiding air, and (3) vapor that has experienced maximum possible kinetic fractionation. Model processes also cause variations in d-excess of MBL vapor. In particular, mixing of relatively high d-excess descending and converging air into the MBL increases d-excess, even without kinetic isotope fractionation. The model is tested by comparison with seven data sets of marine vapor isotopic ratios, with excellent correspondence. About 95 % of observational data fall within the quadrilateral predicted by the model. The distribution of observations also highlights the significant influence of vapor from nearby converging descending air on isotopic variations within the MBL. At least three factors may explain the ∼5 % of observations that fall slightly outside of the predicted regions in δD–δ18O and d-excess–δ18O space: (1) variations in seawater isotopic ratios, (2) variations in isotopic composition of subsiding air, and (3) influence of sea spray.

Land surface models and Earth system models that include Arctic landscapes must capture the abrupt hydrological transitions that occur during the annual thaw and deepening of the active layer. In this work, stable water isotopes (δ2H and δ18O) are used to appraise hydrologically significant transitions during annual landscape thaw at the Barrow Environmental Observatory (Utqiaġvik, Alaska). These hydrologically significant periods are then linked to annual shifts in the landscape energy balance, deduced from meteorological data and described by the microclimatic periods: Winter, Pre‐Melt, Melt, Post‐Melt, Summer, and Freeze‐Up. The tight coupling of the microclimatic periods with the hydrological transitions supports the use of microclimatic periods as a means of linking polygonal surface water hydrology to meteorological datasets, which provides a mechanism for improving the representation of polygonal surface water hydrology in process‐based models. Rayleigh process reconstruction of the isotopic changes revealed that 19% of winter precipitation was lost to sublimation prior to melting and that 23% of surface water was lost to evaporation during the first 10 days post‐melt. This agrees with evaporation rates reported in a separate study using an eddy covariance flux tower located nearby. An additional 17% was lost to evaporation during the next 33 days. Stable water isotopes are also used to identify the dominant sources of surface water to various hydrogeomorphological features prevalent in polygonal terrain (a lake, a low centre polygon centre, troughs within the rims of low centre polygons, flat centre polygon troughs, a high centre polygon trough, and drainages). Hydrogeomorphologies that retained significant old water or acted as snow drifts are isotopically distinct during the Melt Period and therefore are easily distinguished. Biogeochemical changes related to the annual thaw are also reported and coupled to the hydrological transitions, which provides insight into the sources and sinks of these ions to and from the landscape.

The hydrogen and oxygen isotopic composition of ice cores from Summit, Greenland, has provided invaluable information about variations in past climate. However, interpretations of these isotopic data have been made despite a paucity of direct isotopic studies of Summit precipitation. We provide insight to such interpretations by examining the annual cycle of deuterium excess (d-excess) in precipitation samples from Summit and by considering the climatic controls on the annual cycle. Precipitation was collected daily from July 2011 to September 2014 at heights of 1, 2, and 4m. The isotopic composition of precipitation sampled at 4m above the snow surface is free of contamination from blowing snow. Precipitation d-excess is high in the summer and low in the winter, a pattern opposite to that found at most high-latitude locations, where summer d-excess is low relative to winter. Low winter d-excess values at Summit can be explained by varying degrees of Rayleigh distillation of moisture sourced from isotopically similar marine sources. However, the observed summer d-excess maximum at Summit is anomalously high compared with other Arctic locations, and we propose that this is due to high d-excess moisture contributed by sublimation of surface snow on the Greenland Ice Sheet. We demonstrate the plausibility of this hypothesis through simple isotopic mass balance calculations, analyses of cloud heights, and back trajectories to identify moisture sources. We show that Rayleigh distillation, sublimation, and the phase of the d-excess annual cycle are all important factors that should be considered in ice core d-excess interpretations.

Abstract. The meteoric water line, defined by the correlation of hydrogen (δD) and oxygen (δ18O) values, is one of the earliest described characteristics of precipitation isotopic variations. However, spatial and temporal variations in the slope of this line are less studied. The slope of the δD-δ18O relationship is coupled with how d-excess covaries with δD or δ18O, and may provide an integrated tool for inferring hydrologic processes from the evaporation to condensation site. We present a study of δD-δ18O relationships on seasonal and annual timescales for event-based precipitation and a 15-meter ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD-δ18O slopes are less than eight (summer = 7.71; winter = 7.77), while the annual slope is greater than eight (8.27). We suggest intra-season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than eight. The summer line has a greater intercept (higher d-excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d-excess to contributions of vapor sublimated from the Greenland Ice Sheet. Higher sublimated moisture proportions in summer cause larger separations between seasonal δD-δ18O lines, and thus higher annual slopes. Intra-seasonal distributions of precipitation amount also influence annual slopes because slopes are weighed by the number of storms each season. We generate indices to quantify sublimation proportion (SPI) and precipitation distribution (PDI), and find that annual Owen core slope measurements are significantly related to these indices, demonstrating that sublimation and precipitation distribution represent important climate conditions recorded in ice cores.