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An understanding of past climatic variability at regional to local scales is mandatory for a proper evaluation of current and potential future repercussions from global climate change. The Southeast Asian tropics remain severely under-represented in paleoenvironmental and paleoclimatic reconstructions, although a few recent paleoclimate studies evaluated the variability, position and strength of the main Asian monsoon systems at different timescales from modern records to the last 15,000 years. Comparable data from Việt Nam, however, are scarce although Việt Nam and its Central Highlands are critically located at the intersection of the East Asian Summer Monsoon and the Indian Summer Monsoon with highly important socio-ecological consequences from changes in the climate system. This study presents a high-resolution sedimentary climate record from Ea Tyn Lake in Việt Nam’s Central Highlands covering the last 1250 years. Using geochemical and sedimentological proxies and principal component analysis, we reconstructed at least 12 drought events, some of which appear to be of supraregional significance as they coincide with historically documented droughts in India, China, and Cambodia. Beyond tracking short-term climate events, the Rb/Sr elemental ratio along the Ea Tyn Lake sediment sequence reflects long-term monsoon variability throughout the last millenium, which was previously only reconstructed via δ18O speleothem records from China and India. Our Ea Tyn Lake record shows that the East Asian Summer Monsoon was relatively strong between ~1000 and 1350 cal CE and weaker between 1350 and 1850 cal CE, followed by renewed intensification after ~1850 cal CE.

Global warming enhances atmospheric moisture loading and will likely affect the East-Asian monsoon system across Vietnam. The absence of a long written climate history from Vietnam creates a reliance on geological archives of past monsoon history and regional paleoenvironmental changes to provide a framework for evaluating current climatic trends. Biển Hồ lake (14°03′ N, 108°00′ E) is a volcanic crater (i.e. maar) in Vietnam's Central Highlands that has been accumulating sediment since the Pleistocene. Field campaigns between 2016 and 2018 recovered an abundance of gravity and piston sediment cores extending to a depth of ~15 m, covering approximately the last 30 ka BP. The paleoenvironmental interpretation of Pleistocene and Holocene sediment requires knowledge of modern lacustrine and sedimentary conditions, as well as the origin and transport pathways of the catchment material. Here, we focus on a high-resolution sedimentological and geochemical reconstruction of the recent environmental history - from 1950 AD to the present - based on sediment cores from Biển Hồ maar, in direct comparison with local and regional weather and historic records. The uppermost sedimentary record reflects a substantial anthropogenic influence such as deforestation, military use, crater breaching, dam and sill construction, and reforestation that strongly modified the maar's morphology and hydrology. A strong increase in sedimentation rate during the 1960–70s in Biển Hồ cores coincided with U.S. military activities and water withdrawal. A breach in the maar's rim and the connection to a new external reservoir in 1983-1984 AD increased Biển Hồ water level. Reforestation around most of Biển Hồ’s rim in the 1990s curbed erosion and sedimentation rate. Nutrient availability in Biển Hồ declined in the past ~10 years after the installation of a concrete sill between Biển Hồ and the reservoir to limit water exchange with the more nutrient-rich reservoir. This paper is a calibration study on modern sediment to provide an empirical basis for the interpretation of Biển Hồ’s deeper Holocene and pre-Holocene sedimentary record.

2H/1H ratios in animal biomass reflect isotopic input from food and water. A 10-week controlled laboratory study raised 48 mice divided in two generations (8 mothers Mus musculus and their offspring). The mice were divided into four groups based on the combination of 2H, 13C, 15N-enriched and non-enriched food and water. Glycine, the most common amino acid in bone collagen, carried the 2H, 13C, 15N-isotopic spike in food. ANOVA data analysis indicated that hydrogen in food accounted for ∼81 % of the hydrogen isotope inventory in collagen whereas drinking water hydrogen contributed ∼17 %. Air humidity contributed an unspecified amount. Additionally, we monitored 13C and 15N-enrichment in bone collagen and found strong linear correlations with the 2H-enrichment. The experiments with food and water indicate two biosynthetic pathways, namely (i) de novo creation of non-essential amino acids using hydrogen from water, and (ii) the integration of essential and non-essential amino acids from food. The lower rate of isotope uptake in mothers' collagen relative to their offspring indicates incomplete bone collagen turnover after ten weeks. The variance of hydrogen stable isotope ratios within the same cohort may limit its usefulness as a single sample proxy for archaeological or palaeoenvironmental research.

Summary The conventional paradigm for generation of economically viable petroleum reserves involves thermal cleavage of C-C bonds within sedimentary organic matter. However, there is evidence for alternative chemical pathways yielding non-microbial methane generation at low temperatures. In addition, assessing the role of water in methane generation at temperatures lower than those required for it to form by thermal cracking represents a critical step for evaluating processes likely involved in catalytic methanogenesis. Previous pyrolysis experiments of source rocks in the presence and absence of water have yielded insights on the role of water in CH4 generation at a range of maturities including late-stage thermogenesis. Our studies explore the role of catalytic pathways in CH4 generation by experiments simulating maturation of immature source rocks at temperatures insufficient to achieve thermal cleavage of C-C bonds. These hydrous maturation experiments demonstrate that immature shales can generate CH4 at temperatures (80–100 °C). They establish the potential to yield δ13C values

The radiation and extinction of East African vertebrate fauna have been linked to key climatic changes that occurred around 2.8, 1.7, and 0.5 Ma. This study tests whether vegetation and diet changes were linked to these climatic fluctuations, focusing on the climatic event at 1.7 Ma, by analyzing the stable isotope composition of fossil herbivore teeth from the Olduvai Gorge's stratigraphic units Upper Bed I, Lower Bed II, and Upper Bed II, which span ~1.83–1.33 Ma, in order to detect shifts in herbivore diets from C3 to C4 plants in conjunction with their drinking water isotope geochemistry. C4 grass expansion is known to be favored by drier conditions, and this study tests whether herbivore diets permanently changed to being C4 dominant at this key time period. Crocodile teeth are also tested to determine if they may be used as paleoclimatic indicators, since large crocodiles prey on terrestrial herbivores in wetland settings. Vertebrate bioapatite carbon and oxygen isotope compositions are found to show similar trends as those of pedogenic carbonates (carbonate nodules, or concretions), which reflect a drying environment that was changing from C3 woodland dominated habitats to mixed woodlands and grasslands from Bed I to Lower Bed II times. While a return to more humid conditions in Upper Bed II might be expected to lead to a reversal to C3 plants, herbivores in this study retained mostly the drought-resistant C4 plant diet. These results indicate that vegetation and ecosystems passed through a threshold to C4 domination that did not revert to C3 despite partial reversals in climate that occurred later. Crocodile teeth were not useful in tracking C4 plant expansion, but the oxygen isotopes from modern crocodiles are able to show seasonal cycles from wet to dry seasons, and Bed I crocodiles indicate a degree of annual seasonal changes during that time as well.

The cause of rapid hydrological changes in the tropical West Pacific during the last deglaciation remains controversial. In order to test whether these changes were triggered by abrupt climate change events in the North Atlantic Ocean, variations in precipitation during the last deglaciation (18–10 ka) were extracted from proxy records of chemical weathering and terrigenous input in the western Philippine Sea (WPS). The evolution of chemical weathering and terrigenous input since 27 ka was reconstructed using the chemical index of alteration (CIA), elemental ratios (K/Al, TOC/TN and Ti/Ca), δ13Corg, terrigenous fraction abundance and flux data from International Marine Global Change Study Program (IMAGES) core MD06-3054 collected on the upper continental slope of eastern Luzon (northern Philippines). Sediment deposited during the Last Glacial Maximum (LGM) shows weathering equal to or slightly greater than Holocene sediment in the WPS. This unusual state of chemical weathering, which is inconsistent with lower air temperatures and decreased precipitation in Luzon during the LGM, may be due to reworking of poorly consolidated sediments on the eastern Luzon continental shelf during the LGM sea-level lowstand. Rapid changes in chemical weathering, characterized by higher intensity during the Heinrich event 1 (H1) and Younger Dryas (YD) and lower intensity during the Bolling-Allerod (B/A), were linked to rapid variations in precipitation in the WPS during the last deglaciation. The higher terrigenous inputs during the LGM relative to those of the Holocene were controlled by sea-level changes rather than precipitation. The terrigenous inputs show a long-term decline during the last deglaciation, punctuated by brief spikes during the H1 and YD related to sea-level rises and rapid precipitation changes in the WPS, respectively. The proxy records of chemical weathering and terrigenous input from eastern Luzon suggest high rainfall during the H1 and YD events, consistent with inferred rainfall patterns based on Fe/Ca records from offshore Mindanao. Rapid precipitation changes in the WPS did not coincide with migrations of the Intertropical Convergence Zone (ITCZ) but, rather, were related to state shifts of the El Nino-Southern Oscillation (ENSO) during the last deglaciation. Based on proxy records and modeling results, we argue that the Atlantic meridional overturning circulation (AMOC) controlled rapid precipitation changes in the tropical West Pacific through zonal shifts of ENSO or meridional migration of the ITCZ during the last deglaciation. Our findings highlight the dominant role of the North Atlantic Ocean in the tropical hydrologic cycle during the last deglaciation.

Over the past century, numerous discoveries throughout East Africa have advanced our understanding of hominin evolution and provided substantive evidence that climatic and environmental variability played a critical role in evolutionary developments. Stratigraphic records with high temporal resolution aid in testing evolutionary hypotheses that invoke changes in climate and environment at various timescales as drivers of hominin evolution. Olduvai Gorge, Tanzania has a rich history of hominin fossil discoveries from its ~2.0 Ma sedimentary sequence, which includes multiple deep lake intervals. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered a sequence of sediment cores that provide an extensive record of the Pleistocene sedimentary history, including paleolacustrine systems in this area. With >94% core recovery, these cores are ideally suited for high-resolution analyses of organic geochemical proxies and provide an exceptional opportunity to build on previously published outcrop-based paleoenvironmental data from Olduvai. The OGCP core 2A section between 76.6 and 86.9 m depth is considered as a time-stratigraphic equivalent of strata that host the hominin fossils OH24 and OH56 at the Olduvai DK archaeological site. This depositional interval age-bracketed between ~1.88 Ma (Bed I Basalt) and ~1.85 Ma (Tuff IB) was sampled for organic geochemical analyses. The carbon isotopic composition of organic matter (δ13CTOC) from this section varies between values representative of more forested and open grassland ecosystems over ~21 kyr. This observation is consistent with the Milankovitch precession cycle driving the availability of water, which was previously observed in lower resolution studies of outcrop samples. Complementary organic geochemical proxies provide further evidence of these shifts in environmental conditions and record sub-Milankovitch scale changes superimposed on the precession cycle. This suggests the occurrence of short-term fluctuations in the environments inhabited by hominins, which opens new lines of investigation on how environmental scenarios may affect particular evolutionary mechanisms that drove various evolutionary responses.

The Santa Barbara Basin (SBB) preserves a high-resolution sediment record that is varved down to ca. 1700 CE and laminated for at least 2500 years. The distributions of 25 polycyclic aromatic hydrocarbons (PAHs) were investigated in a suite of samples from sediment cores representing different depositional scenarios – varves/laminae, terrigenous flood layers, and marine turbidites. The concentrations of these PAHs were determined using gas chromatography–mass spectrometry in selected ion mode and evaluated in terms of their distributions, diagnostic ratios, and loadings recognized by principal component analysis (PCA). Two biogenic PAHs, perylene and retene, provided the greatest variation in their downcore concentration values. Perylene concentrations increased downcore exhibiting markedly higher abundance in sediments older than 300 CE, consistent with the interpretation that perylene is formed diagenetically in reducing sediments. A modest increase in perylene concentrations in sediments from the late 1960s may be associated with flood-derived transport of higher amounts of degraded terrestrial organic matter from fire-denudated landscapes into the coastal environment. The markedly higher concentrations of retene in flood layers and turbidites relative to varves/laminae suggest that these lithologies are more effective in transporting pyrogenic PAHs and can be interpreted as a proxy for wildfires in SBB. Changes in key diagnostic ratios reveal the shift from a dominance of petrogenic PAHs in older sediments to a strong pyrogenic signal beginning in the latter half of the 20th century. Statistical analysis using PCA of the PAH distributions in the samples revealed that modern sediments showed a strong, positive relationship with PC1, which represents the greatest variation among the samples, whereas flood layers showed a strong relationship with the second major variable (PC2). Thus, statistical approaches provide the capability to elucidate modern, anthropogenic signatures within the complex PAH distributions of SBB and also establish a PAH profile that characterizes the lithological signature of flood layers.

The paper presents the first systematic geochemical study on the content of natural gas (methane, heavier alkanes, CO2), and isotopic composition of methane dissolved in natural springs in Romania, from various tectonic and geological settings. The dissolved methane concentrations in the investigated areas range between 0.08 and 16,359 μg/L. Based on isotopic analyses, the origin of methane in the gas mixture released by the investigated springs has been inferred. The highest concentrations of methane have been measured in the Transylvanian Basin and Carpathian Foredeep. Ethane in relatively high concentrations has been detected in Herculane and Calimanesti-Caciulata areas. Very high concentrations of dissolved CO2 were determined in the Eastern Carpathians volcanic chain area. Lower amounts of dissolved CO2 were detected in the Carpathian Foredeep, Southern Carpathians, Transylvanian Basin, and Pannonian Basin. The measured values of δ13C-CH4 range between − 67‰ and + 4‰. The methane from the investigated areas is biotic, microbial in the Transylvanian Basin, Pannonian Basin, and Moldavian Platform, and thermogenic in the Carpathian Foredeep. We report for the first time the presence of a presumably abiotic methane component in Romania, from geothermal sources, in the areas of Ciomadul Volcano and Herculane area.

International audience; Of the most abundant elemental constituents of organic matter, hydrogen has received the least attention due to its exchangeability with ambient water. The development of steam equilibration using two isotopically distinct waters can enable the user to calculate the proportion of exchangeable H and generate a δ2H value of the non-exchangeable H. Three fossil faunal assemblages from Europe were selected in order to investigate collagen δ2H with respect to body mass, diet and to compare chronological periods in the Quaternary. δ15N values in each suite of fauna show trophic level enrichment, a trend which was also observed in δ2H values. In the case of predatory species with lower δ15N values than other carnivore species, the use of hydrogen isotopes could help to infer the consumption of prey with lower δ15N values rather than a significant dietary contribution of plant foods. For example, a cave bear which exhibits low δ2H values are consistent with low δ15N and therefore point to an herbivorous diet and not omnivory. The combination of δ2H, with δ15N and δ13C, from archaeological and ecological studies of humans and fauna could help reconstruct prey preference; which is especially important when trying to distinguish the quantity of marine versus freshwater resources in diets. Relative to dietary influences, climate variability seems to be a minor control on δ2H in the faunal assemblages studied here.

Recent observations suggest that karst landscapes may be an unaccounted sink for atmospheric CH4, but questions remain about the processes contributing to sub-atmospheric CH4 mole fractions in caves. The CH4 dynamics associated with karst environments were studied over 18 months at 6 locations in Buckner Cave, Southern Indiana by measuring the mole fractions and stable isotopic composition of CH4 and carbon dioxide (CO2). CO2 mole fractions were used to infer seasonal changes in airflow. Samples were obtained on a monthly basis. CH4 mole fractions ranged from 1.9 ± 0.1 ppm near the cave entrance to 0.1 ± 0.1 ppm in the more remote parts of the cave. The carbon and hydrogen stable isotopic compositions of CH4 in the cave ranged from − 58.7 to + 7 ‰ (VPDB) and − 170 to + 10 ‰ (VSMOW), respectively. The isotopic data suggest that the CH4 dynamics of Buckner Cave can be described by a seasonally variable mixing system in which atmospheric CH4 enters primarily through the main entrance and is subsequently consumed by methanotrophs. Additionally, at least two smaller CH4 sources are evident when air has been stagnant. The results suggest that subterranean karst cavities are an important sink for atmospheric CH4.

Small lakes in northern latitudes represent a significant source of CH4 to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH4 budgets are lacking as are measurements of δ13C-CH4 and δ2H-CH4. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH4 and corresponding δ13C-CH4 and δ2H-CH4 in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH4 flux was highly variable, ranging from an average flux of 7 mg CH4 m−2 d−1 in the deep-water zone to 154 mg CH4 m−2 d−1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH4 flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH4 emissions varied widely throughout the system, with δ13C-CH4 ranging from −66.2 to −55.5 ‰, and δ2H-CH4 ranging from −345 to −258 ‰. Carbon isotope composition of CH4 in ebullitive flux showed wider variation compared to net flux, ranging from −69.2 to −49.2 ‰. Dissolved CH4 concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH4 in the hypoxic deep water coincided with decreasing dissolved O2 concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ13C-CH4 and δ2H-CH4. The most depleted 13C- and 2H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ13C and δ2H observations of CH4, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH4 in deeper sediments were consistent with mixing/transition between CH4 production pathways, indicating a higher contribution of the CO2 reduction pathway. The large spatial variability in fluxes of CH4 and in isotopic composition of CH4 throughout a single lake system indicates that the underlying mechanisms controlling CH4 cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH4 emissions in these systems. Future studies of whole-lake CH4 budgets should consider this significant spatial heterogeneity.

Microbial oxidation of methane (CH4) plays a central role in carbon cycling in Arctic lakes, reducing potential CH4 emissions associated with warming. Isotopic signatures of CH4 (δ13C and δ2H) are indicators of microbial oxidation, wherein the process strongly enriches 13C and 2H in residual CH4. We present δ13C and δ2H measurements obtained from sampling the water column and sediment for dissolved CH4 from three, small Arctic lakes in western Greenland under both open-water and ice-covered conditions from 2013 to 2014. Despite substantial variations in aquatic chemistry among the lakes, δ13C and δ2H of CH4 suggested that CH4 was produced predominantly by acetoclastic methanogenesis in the littoral sediments and hydrogenotrophic methanogenesis in the profundal sediments in all of the lakes. Surprisingly large variations for both δ13C and δ2H of CH4 were observed, with δ13C extending from −72‰ to +7.4‰ and δ2H from −390‰ to +250‰. The CH4 isotopic values reported here were significantly more enriched (p

Species-specific responses of plant intrinsic water-use efficiency (iWUE) to multiple environmental drivers associated with climate change, including soil moisture (θ), vapor pressure deficit (D), and atmospheric CO2 concentration (ca ), are poorly understood. We assessed how the iWUE and growth of several species of deciduous trees that span a gradient of isohydric to anisohydric water-use strategies respond to key environmental drivers (θ, D and ca ). iWUE was calculated for individual tree species using leaf-level gas exchange and tree-ring δ13 C in wood measurements, and for the whole forest using the eddy covariance method. The iWUE of the isohydric species was generally more sensitive to environmental change than the anisohydric species was, and increased significantly with rising D during the periods of water stress. At longer timescales, the influence of ca was pronounced for isohydric tulip poplar but not for others. Trees' physiological responses to changing environmental drivers can be interpreted differently depending on the observational scale. Care should be also taken in interpreting observed or modeled trends in iWUE that do not explicitly account for the influence of D.

Summary: Hydrogen (H) isotope ratio (δ2H) analyses of plant organic compounds have been applied to assess ecohydrological processes in the environment despite a large part of the δ2H variability observed in plant compounds not being fully elucidated. We present a conceptual biochemical model based on empirical H isotope data that we generated in two complementary experiments that clarifies a large part of the unexplained variability in the δ2H values of plant organic compounds. The experiments demonstrate that information recorded in the δ2H values of plant organic compounds goes beyond hydrological signals and can also contain important information on the carbon and energy metabolism of plants. Our model explains where 2H‐fractionations occur in the biosynthesis of plant organic compounds and how these 2H‐fractionations are tightly coupled to a plant's carbon and energy metabolism. Our model also provides a mechanistic basis to introduce H isotopes in plant organic compounds as a new metabolic proxy for the carbon and energy metabolism of plants and ecosystems. Such a new metabolic proxy has the potential to be applied in a broad range of disciplines, including plant and ecosystem physiology, biogeochemistry and palaeoecology.

Although the Late Devonian extinctions were amongst the largest mass extinction events in the Phanerozoic, the causes, nature and timing of these events remain poorly restrained. In addition to the most pronounced biodiversity loss at the Frasnian–Famennian (F–F) boundary and the end Famennian, there were also less extensively studied extinction pulses in the Middle to Late Givetian and the Frasnian. Here we used a combination of palynological, elemental, molecular and stable isotope analyses to investigate a sedimentary record of reef-systems from this time period in the Canning Basin, Western Australia. The acquired data generally showed distinct variations between sediments from (i) the time around the Givetian–Frasnian (G–F) boundary and (ii) later in the Frasnian and indicated a distinct interval of biotic stress, particularly for reef-builders, in the older sediments. Alterations of pristane/phytane ratios, gammacerane indices, Chlorobi biomarkers, δDkerogen and chroman ratios describe the change from a restricted marine palaeoenvironment with an anoxic/euxinic hypolimnion towards a presumably open marine setting with a vertically mixed oxic to suboxic water column. Simultaneous excursions in δ13C profiles of carbonates, organic matter (OM) and hydrocarbons in the older sediments reflect the stratification-induced enhancement of OM-recycling by sulfate reducing bacteria. Alterations in sterane distributions and elevated abundances of methyltrimethyltridecylchromans (MTTCs) and perylene indicate an increased terrigenous nutrient input via riverine influx, which would have promoted stratification, phytoplankton blooms and the development of lower water column anoxia. The detected palaeoenvironmental conditions around the G–F boundary may reflect a local or global extinction event. Our data furthermore suggest a contribution of the higher plant-expansion and photic zone euxinia to the Late Devonian extinctions, consistent with previous hypotheses. Furthermore, this work might contribute to the understanding of variations in Devonian reef margin and platform-top architecture, relevant for petroleum exploration and development in the global Devonian hydrocarbon resources.

a b s t r a c t Molecular hydrogen produced through iron oxidation during formation of serpentine and magnetite can sustain terrestrial subsurface ecosystems. The Fe 3+ in serpentine partitions into octahedral and tetrahedral sites differently as serpentinization proceeds, and tetrahedral Fe 3+ is present toward the end of serpentinization. We map Fe oxidation states in a serpentinite to determine the degree to which serpentinization progressed and where hydrogen production has been maximized to assess habitability at an abandoned chrysotile mine in Norbestos, Quebec, in association with the Canadian Space Agency's Mars Methane Analogue Mission. We also analyzed stable isotopes of carbon and oxygen in carbonates to constrain the conditions of water-rock interaction during serpentinization. Iron oxidation and coordination was determined through field imaging of rock walls with a visible hyperspectral imager (420-720 nm), and samples collected from imaged rocks and elsewhere in the mine were imaged in the laboratory (420-1100 nm). Sample chemistry, mineralogy, and oxidation state were determined with laboratory measurements of visible through mid-infrared reflectance spectra, major element chemistry, mineralogy, and Mossbauer spectroscopy. Mapping with hyperspectral imaging of outcrops and hand samples shows that tetrahedral Fe 3+ is common in serpentinites at this site, and results are confirmed through other measurements. Major element chemistry and mineralogy are consistent with serpentine plus minor carbonate. Carbonate samples show an exceptional range in δ 13

The air in subterranean karst cavities is often depleted in methane (CH4) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH4 because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14 % of earth’s continental surface, but observations of CH4 concentrations in cave air are limited to localized studies in Gibraltar, Spain, Indiana (USA), Vietnam, Australia, and by incomplete isotopic data. To test if karst is systematically acting as a global CH4 sink, we measured the CH4 concentrations, δ13CCH4, and δ2HCH4 values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO2 concentrations, δ13CCO2, and radon (Rn) concentrations to support CH4 data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH4 concentrations in at least one location compared to their local atmospheric backgrounds. CH4 concentrations and δ13CCH4 and δ2HCH4 values suggest that microbial methanotrophy within caves is the primary CH4 consumption mechanism as the atmosphere exchanges with subsurface air. The pattern of δ13CCH4 and δ2HCH4 values along CH4 concentration gradients in cave air provides evidence for incomplete oxidation by methanotrophy. Only 5 locations from 3 caves showed elevated CH4 concentrations compared to the atmospheric background and could be ascribed to local CH4 sources from sewage and outgassing swamp water. Several associated δ13CCH4 and δ2HCH4 values point to carbonate reduction and acetate fermentation as biochemical pathways of limited methanogenesis in karst environments and suggest that these pathways occur in the environment over large spatial scales. Our data show that karst environments function as a global CH4 sink. Estimates of CH4 flux in karst landscapes are needed in order to include the subterranean CH4 sink in climate models.

An international project developed, quality-tested, and determined isotope-delta values of 19 new organic reference materials (RMs) for hydrogen, carbon, and nitrogen stable isotope-ratio measurements, in addition to analyzing pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and IAEA-600 (caffeine). These new RMs enable users to normalize measurements of samples to isotope-delta scales. The RMs span a range of delta H-2(VSMOW-SLAP) values from -210.8 to +397.0 mUr or parts per thousand, for delta C-13(VPDB-LSVEC) from -40.81 to +0.49 mUr and for delta N-15(Air) from -5.21 to +61.53 mUr. Many of the new RMs are amenable to gas and liquid chromatography. The RMs include triads of isotopically contrasting caffeines, C-16 n-alkanes, n-C-20-fatty acid methyl esters (FAMEs), glycines, and L-valines, together with polyethylene powder and string, one n-C-17-FAME, a vacuum oil (NBS 22a) to replace NBS 22 oil, and a H-2-enriched vacuum oil. A total of 11 laboratories from 7 countries used multiple analytical approaches and instrumentation for 2-point isotopic normalization against international primary measurement standards. The use of reference waters in silver tubes allowed direct normalization of delta H-2 values of organic materials against isotopic reference waters following the principle of identical treatment. Bayesian statistical analysis yielded the mean values reported here. New RMs are numbered from USGS61 through USGS78, in addition to NBS 22a. Because of exchangeable hydrogen, amino acid RMs currently are recommended only for carbon- and nitrogen-isotope measurements. Some amino acids contain C-13 and carbon-bound organic H-2-enrichments at different molecular sites to provide RMs for potential site-specific isotopic analysis in future studies.