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The rapidly expanding and energy-intensive production from the Canadian oil sands, one of the largest oil reserves globally, accounts for almost 12 % of Canada's greenhouse gas emissions according to inventories. Developing approaches for evaluating reported methane (CH4) emission is crucial for developing effective mitigation policies, but only one study has characterized CH4 sources in the Athabasca oil sands region (AOSR). We tested the use of 14C and 13C carbon isotope measurements in ambient CH4 from the AOSR to estimate source contributions from key regional CH4 sources: (1) tailings ponds, (2) surface mines and processing facilities, and (3) wetlands. The isotopic signatures of ambient CH4 indicate that the CH4 enrichments measured at the site were mainly influenced by fossil CH4 emissions from surface mining and processing facilities (56 ± 18 %), followed by fossil CH4 emissions from tailings ponds (34 ± 18 %) and to a lesser extent modern CH4 emissions from wetlands (10 ± CH4 emissions and show that this method can successfully distinguish wetland CH4 emissions. In the future, the isotopic characterization of CH4 sources and measurements from different seasons and wind directions are needed to provide a better source attribution in the AOSR.

Paleolimnological evidence indicates the ancient Maya transformed terrestrial ecosystems by felling forest vegetation to construct large civic-ceremonial centers and to expand agriculture. Human settlements influenced lacustrine environments but the effects of Maya activities on aquatic ecosystems remain poorly studied. Here we analyzed a sediment core from Lake Petén Itzá, Guatemala, to infer paleoenvironmental changes resulting from Maya occupation of the archaeological site of Nixtun-Ch’ich’. Increases in charcoal and fecal stanol concentrations indicate Maya occupation of the Candelaria Peninsula by the late Early Preclassic period. Geochemical proxies reveal a period of lake ecosystem alteration during construction and expansion of the city’s urban grid in the Middle and Late Preclassic periods. Depopulation of the city in the Terminal Preclassic resulted in a decline in lake trophic state. Whereas previous studies of Petén waterbodies have indicated depressed lacustrine primary production, the core collected near Nixtun-Ch’ich’ shows evidence of ancient Maya lake ecosystem deterioration.

We present the first spatially resolved distribution of the [Formula: see text] signature of wetland methane emissions and assess its impact on atmospheric [Formula: see text]. The [Formula: see text] signature map is derived by relating [Formula: see text] of precipitation to measured [Formula: see text] of methane wetland emissions at a variety of wetland types and locations. This results in strong latitudinal variation in the wetland [Formula: see text] source signature. When [Formula: see text] is simulated in a global atmospheric model, little difference is found in global mean, inter-hemispheric difference and seasonal cycle if the spatially varying [Formula: see text] source signature distribution is used instead of a globally uniform value. This is because atmospheric [Formula: see text] is largely controlled by OH fractionation. However, we show that despite these small differences, using atmospheric records of [Formula: see text] to infer changes in the wetland emissions distribution requires the use of the more accurate spatially varying [Formula: see text] source signature. We find that models will only be sensitive to changes in emissions distribution if spatial information can be exploited through the spatially resolved source signatures. In addition, we also find that on a regional scale, at sites measuring excursions of [Formula: see text] from background levels, substantial differences are simulated in atmospheric [Formula: see text] if using spatially varying or uniform source signatures. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Greenhouse gas (GHG) emissions from permafrost organic carbon decomposition in lakes and rivers can accelerate global warming. We used radiocarbon (14C) measurements to determine the predominant sources of dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and methane (CH4) in five thermokarst lakes and three rivers in an area of widespread permafrost degradation in Northern Quebec to assess contributions from thawing permafrost and other old carbon (fixed before CE 1950) reservoirs. We compared emission pathways (dissolved gas and ebullition), seasons (summer and winter), and surface soil type (mineral and peat soils). Modern carbon (fixed after CE 1950) was the dominant source of DOC, DIC, and CH4 of non‐peatland aquatic systems, while POC and sediment carbon were predominantly fixed in the last millennia. In the peatland systems, modern and permafrost carbon were important sources of DOC, lake DIC, lake ebullition CO2, and lake dissolved CH4. In contrast, POC, lake ebullition CH4, and river DIC were dominated by millennial‐old carbon. In winter, the 14C age of DOC, DIC, and POC in the peatland lakes increased, but the 14C age of dissolved CH4 did not change. Our results point to a clearly older overall carbon source for ebullition CH4 relative to dissolved CH4 in the peatland lakes, but not the non‐peatland lakes. The younger ages of diffusive CH4 and DIC relative to DOC and POC in all lakes suggest that recent primary productivity strongly influences the large total lake CH4 emissions in this area, as diffusion fluxes greatly exceed ebullition fluxes.

We present the first spatially varying map of the δD-CH4 signature of wetland methane emissions and model its impact on atmospheric δD-CH4. The δD-CH4 signature map is derived by relating the δD-H2O of precipitation to the measured δD-CH4 of methane wetland emissions at a variety of wetland types and locations. Since the δD-H2O of precipitation is highly latitude-dependent, including this spatial variation has the potential to have a large impact on the distribution of δD-CH4 observed in the atmosphere. This latitude-dependence means that wetland emissions at different latitudes can have very different impacts on atmospheric δD-CH4, which could provide a useful way to constrain the location of wetland methane emissions in future inverse modelling studies. Here, we assess the implications for model studies on the differences that arise by treating δD-CH4 wetland source signatures as globally uniform rather than accounting for the large spatial variation. We also assess the potential for δD-CH4 to provide an independent constraint on wetland emissions over the more abundant and widely measured δ13C-CH4.

Lakes and ponds can be hotspots for CO₂ and CH₄ emissions, but Arctic studies remain scarce. Here we present diffusive and ebullition fluxes collected over several years from 30 ponds and 4 lakes formed on an organic‐rich polygonal tundra landscape. Water body morphology strongly affects the mixing regime—and thus the seasonal patterns in gas emissions—with ice‐out and autumnal turnover periods identified as hot moments in most cases. The studied thermokarst lake maintained relatively high ebullition rates of millennia‐old CH4 (up to 3405 ¹⁴C YBP). Larger and deeper kettle lakes maintained low fluxes of both gases (century to millennium‐old), slowly turning into a CO₂ sink over the summer. During winter, lakes accumulated CO₂, which was emitted during the ice‐out period. Coalescent polygonal ponds, influenced by photosynthesizing benthic mats, were continuous CO₂ sinks, yet important CH₄ emitters (modern carbon). The highest fluxes were recorded from ice‐wedge trough ponds (up to 96 mmol CO₂ equivalent m⁻² d⁻¹). However, despite clear signs of permafrost carbon inputs via active shore erosion, these sheltered ponds emitted modern to century‐old greenhouse gases. As the ice‐free period lengthens, scenarios of warmer and wetter conditions could favor both the production of CO₂ and CH₄ from thawing permafrost carbon, and CH₄ production from recently fixed carbon through an atmospheric CO₂₋to₋CH₄ shunt at sites in which primary production is stimulated. This must be carefully considered at the landscape scale, recognizing that older carbon stocks can be mineralized efficiently in specific locations, such as in thermokarst lakes.

There is growing interest in developing spatially resolved methane (CH4) isotopic source signatures to aid in geographic source attribution of CH4 emissions. CH4 hydrogen isotope measurements (δ2H-CH4) have the potential to be a powerful tool for spatial resolution of CH4 emissions from freshwater environments, as well as other microbial sources. This is because microbial δ2H-CH4 values are partially dependent on the δ2H of environmental water (δ2H-H2O), which exhibits large and well-characterized spatial variability globally. We compiled a comprehensive global dataset of paired CH4 δ2H and δ13C measurements from freshwater environments, including wetlands, inland waters, and rice paddies, comprising a total of 131 different ecosystems, and compared these with measurements and estimates of δ2H-H2O. We found that the estimated δ2H of annual precipitation (δ2Hp) explained approximately 35 % of the observed variation in δ2H-CH4, and that the relationship between δ2H-CH4 and δ2Hp led to significant differences in the distribution of freshwater δ2H-CH4 between the northern high latitudes (60–90º N) relative to other global regions. Residual variability in δ2H-CH4 is partially explained by differences in the dominant methanogenic pathway and CH4 oxidation, as inferred from carbon isotope fractionation between CH4 and carbon dioxide (αC). Our results imply that hydrogenotrophic methanogenesis is characterized by a steeper slope of δ2H-CH4 vs. δ2Hp than acetoclastic methanogenesis, a pattern that is consistent with previous predictions. Biogeochemical sources of variability in δ2H-CH4 are reflected in apparent differences between different freshwater ecosystems, with relatively high values in rivers and bogs, and low values in fens and rice paddies, although more data is needed to verify whether these differences are significant. To estimate how changes in the spatial distribution of freshwater emissions would influence global atmospheric CH4 isotopic measurements, we developed a bottom-up mixing model of global CH4 δ2H and δ13C sources, including spatially resolved signatures for freshwater CH4 sources. This model implies that changes in high-latitude freshwater CH4 emissions would have an especially strong influence on global source δ2H-CH4. We estimate that global CH4 emissions sources have a combined δ2H value of −277±8 ‰, which is consistent with top-down estimates based on atmospheric measurements. In contrast our estimated δ13C value of −56.4±1.4 ‰ is not consistent with atmospheric measurements, suggesting possible errors in either emissions inventories or estimates of sink fluxes and isotopic fractionations. Overall our results emphasize the value of δ2H-CH4 measurements to help constrain atmospheric CH4 budgets.

The long-term effects of deforestation on tropical forest soil carbon reservoirs are important for estimating the consequences of land use on the global carbon cycle, but are poorly understood. The Maya Lowlands of Mexico and Guatemala provide a unique opportunity to assess this question, given the widespread deforestation by the ancient Maya that began ~4,000 years ago. Here, we compare radiocarbon ages of plant waxes and macrofossils in sediment cores from three lakes in the Maya Lowlands to record past changes in the mean soil transit time of plant waxes (MTTwax). MTTwax indicates the average age of plant waxes that are transported from soils to lake sediments, and comparison of radiocarbon data from soils and lake sediments within the same catchment indicates that MTTwax reflects the age of carbon in deep soils. All three sediment cores showed a decrease in MTTwax, ranging from 2,300 to 800 years, over the past 3,500 years. This decrease in MTTwax, indicating shorter storage times for carbon in lake catchment soils, is associated with evidence for ancient Maya deforestation. MTTwax never recovered to pre-deforestation values, despite subsequent reforestation, implying that current tropical deforestation will have long-lasting effects on soil carbon sinks.

The release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH₄) clumped isotope (Δ₁₈) and ¹⁴C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ₁₈ values are negatively correlated with CH₄ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH₄ Δ₁₈ and the fraction modern carbon. These correlations imply that CH₄ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ₁₈ values, are not positively correlated with CH₄ flux estimates, highlighting the likely importance of environmental variables other than CH₄ production rates in controlling ebullition fluxes.

Faecal stanols offer an exciting opportunity to determine population change in the past but the controls of their concentrations and ratios within lake sediments are not well understood. We present...

Investigation of the biology and biochemistry of the deep subsurface provides invaluable information regarding the limit of life in extreme environments and its role in the global carbon cycle. It has been observed that subsurface microbial CH4 can form in apparent isotopic equilibrium, both with respect to methane clumped isotopic species and D/H fractionation with respect to coexisting water. This observation fostered the suggestion that methanogenic metabolisms in energy-starved environments can operate through slow and reversible enzymatic reactions. Here we present isotopic data including a vertical profile of clumped isotopic indices of methane from Paleozoic-aged pore waters in an aquiclude system from the Michigan Basin. We show evidence of both internal isotopic equilibrium of methane and intermolecular H-isotopic equilibrium between methane and co-occurring non-gaseous n-alkanes. Various mixing and microbial metabolic models were tested and allowed us to identify the possibility of the production of methane at thermodynamic equilibrium from the syntrophic degradation of sedimentary n-alkanes at ultra-slow rates. Significance Statement The recent ability to measure the clumped isotopic composition of methane has fostered new ways of observing the deep subsurface biogeochemistry and has been proposed as a new independent geothermometer when methane is formed at or near isotopic equilibrium. In this manuscript, we present the first continuous Paleozoic sedimentary profile of clumped isotopes in pore methane (i.e. directly sampled from the pores of tight sedimentary rocks) from a paleo-bioreactor in the subsurface and show that a syntrophic metabolic network between fermentative bacteria and mainly acetotrophic methanogens can thrive in an energy limited environment.

The analysis of faecal stanols in lake sediment cores offers a novel opportunity to reconstruct human population change, assuming that variability in faecal stanol concentration is a reliable proxy for relative human populations. The ancient lowland Maya of Mesoamerica represents an important ancient society whose demographic dynamics in many locations remain uncertain. We apply the faecal stanol proxy to a sediment core retrieved from a lake adjacent to the archaeological site of Itzan, an ancient population centre in the southwestern Maya lowlands. The sedimentary faecal stanol record from Laguna Itzan implies substantial centennial- and millennial-scale changes in local human populations from 3300 cal years BP to the present. Variability in faecal stanol concentrations is broadly consistent with archaeological evidence for regional societal change across the Maya lowlands, but also implies an earlier presence of humans at this site than is currently indicated in the Itzan archaeological record. We find evidence for high-frequency variability in coprostanol concentrations during the Maya Preclassic period, which we infer represents centennial-scale shifts in settlement patterns associated with changes in agricultural and land use patterns. Given Preclassic-period faecal peak stanol concentrations, we observe lower-than-expected Classic-period faecal peak stanol concentrations, and these may partly be a result of either use of human waste for fertiliser or reduced soil erosion or both. Three periods of inferred population decline are associated with palaeoclimate evidence for a drying climate, specifically during the Terminal Classic (1220-1050 cal yr BP) and the Protoclassic 2 (1860-1670 cal yr BP), as well as the less well-studied drought between 3330 and 2900 cal yr BP during the Early to Middle Preclassic periods. An additional decline and hiatus in coprostanol input coincides with a period of anomalously wet climate in the Late Preclassic. These linkages suggest that climatic change and variability could have played a role in demographic change at multiple points in the evolution of Maya civilisation. Our work shows that faecal stanols are valuable proxies for past human population dynamics, and their relation to climatic change, in Mesoamerica.

Paleoclimatologists have discovered abundant evidence that droughts coincided with collapse of the Lowland Classic Maya civilization, and some argue that climate change contributed to societal disintegration. Many archaeologists, however, maintain that drought cannot explain the timing or complex nature of societal changes at the end of the Classic Period, between the eighth and eleventh centuries ce. This review presents a compilation of climate proxy data indicating that droughts in the ninth to eleventh century were the most severe and frequent in Maya prehistory. Comparison with recent archaeological evidence, however, indicates an earlier beginning for complex economic and political processes that led to the disintegration of states in the southern region of the Maya lowlands that precedes major droughts. Nonetheless, drought clearly contributed to the unusual severity of the Classic Maya collapse, and helped to inhibit the type of recovery seen in earlier periods of Maya prehistory. In the drier northern Maya Lowlands, a later political collapse at ca. 1000 ce appears to be related to ongoing extreme drought. Future interdisciplinary research should use more refined climatological and archaeological data to examine the relationship between climate and social processes throughout the entirety of Maya prehistory.

Changes in terrestrial environments across the Cretaceous-Paleogene boundary, including plant ecology and carbon and water-cycling, remain poorly defined. Fluvial sediments spanning the Cretaceous-Paleogene (K-Pg) boundary of southern Saskatchewan, Canada contain well preserved plant wax n-alkanes that provide a means of reconstructing changes to plant ecology and carbon and water cycling during this mass extinction event. We measured n-alkane carbon (δ13C) and hydrogen (δ2H) isotope ratios in two sedimentary sections and applied established fractionation factors to estimate the isotopic compositions of precipitation and bulk sedimentary organic matter sources. We also analyzed the distribution of n-alkanes as an indicator of the relative abundance of aquatic and terrestrial plants. We find a consistent shift towards a greater relative abundance of longer-chain n-alkanes across the boundary, implying a persistent increase in the relative abundance of terrestrial plants in the sedimentary basin. This is consistent with an increase in birch and elm palynomorphs immediately above the boundary. We hypothesize the extinction of all large herbivores at the boundary may have facilitated this transition to a terrestrial angiosperm dominated flora immediately after the boundary. We also find that the region was characterized by isotopically light precipitation, with δ2H values between −95‰ to −160‰, but do not observe evidence for major millennial-scale changes in regional precipitation isotopic composition spanning the boundary. n-Alkanes derived from both aquatic and terrestrial plants at one site display an upward trend in δ13C values of approximately 2‰ across the K-Pg boundary. This suggests millennial-scale local or global carbon-cycle variability altering either plant carbon isotope fractionation or the carbon isotope composition of dissolved inorganic carbon and atmospheric CO2. Overall our results suggest that carbon and water cycle changes associated with the K-Pg impact in terrestrial environments in western Canada were short-lived, but ecological shifts in plant communities were longer-lasting.

A growing body of paleoclimate data indicates that periods of severe drought affected the Maya Lowlands of southeastern Mexico and northern Central America, especially during the Terminal Classic period (ca. 800–950 CE), raising the possibility that climate change contributed to the widespread collapse of many Maya polities at that time. A broad range of paleoclimate proxy methods have been applied in the Maya Lowlands and the data derived from these methods are sometimes challenging for archeologists and other non-specialists to interpret. This paper reviews the principal methods used for paleoclimate inference in the region and the rationale for climate proxy interpretation to help researchers working in the Maya Lowlands make sense of paleoclimate datasets. In particular, we focus on analyses of speleothems and lake sediment cores. These two paleoclimate archives have been most widely applied in the Maya Lowlands and have the greatest potential to provide insights into climate change impacts on the ancient Maya. We discuss the development of chronologies for these climate archives, the proxies for past climate change found within them, and how these proxy variables are interpreted. Finally, we present strategies for improving our understanding of proxy paleoclimate data from the Maya Lowlands, including multi-proxy analyses, assessment of spatial variability in past climate change, combined analysis of climate models and proxy data, and the integration of paleoclimatology and archeology.

© 2015 Elsevier Ltd. Sources of methane to sedimentary environments are commonly identified and quantified using the stable isotopic compositions of methane. The methane "clumped-isotope geothermometer", based on the measurement of multiply substituted methane isotopologues (13CH3D and 12CH2D2), shows promise in adding new constraints to the sources and formational environments of both biogenic and thermogenic methane. However, questions remain about how this geothermometer behaves in systems with mixtures of biogenic and thermogenic gases and different biogenic environments. We have applied the methane clumped-isotope thermometer to a mixed biogenic-thermogenic system (Antrim Shale, USA) and to biogenic gas from gas seeps (Santa Barbara and Santa Monica Basin, USA), a pond on the Caltech campus, and methanogens grown in pure culture. We demonstrate that clumped-isotope based temperatures add new quantitative constraints to the relative amounts of biogenic vs. thermogenic gases in the Antrim Shale indicating a larger proportion (~50%) of thermogenic gas in the system than previously thought. Additionally, we find that the clumped-isotope temperature of biogenic methane appears related to the environmental settings in which the gas forms. In systems where methane generation rates appear to be slow (e.g., the Antrim Shale and gas seeps), microbial methane forms in or near both internal isotopic equilibrium and hydrogen-isotope equilibrium with environmental waters. In systems where methane forms rapidly, microbial methane is neither in internal isotopic equilibrium nor hydrogen-isotope equilibrium with environmental waters. A quantitative model of microbial methanogenesis that incorporates isotopes is proposed to explain these results.

A two-part archaeological and limnological study of the Malian Lakes Region has revealed the high research potential of the region. The exploratory reconnaissance of the Gorbi Valley, on the eastern edge of Lake Fati, identified, mapped and sampled eight new sites. The results of the survey suggest a long duration occupation of the Gorbi Valley, as well as possible connections with the populations of the Inland Niger Delta and southeastern Mauritania. The Lake Fati core represents the first lake sediment core from the western Sahel. This 5.4 m sediment core contains a continuous record of lake mud from 10.43 to 4.66 kyr BP. Analysis of the core reveals that Al and Si abundances are decoupled following a deposition of 16 cm of sand at 4.5 ka BP, with Al decreasing and Si increasing rapidly. This period of sand deposition is significantly younger than that of the transition at 5.5 ka BP recorded in marine cores from ODP site 658, taken off the Mauritanian coast, potentially extending the timeframe in which dune systems were stable and lake systems were at their highstand. While highlighting the need for more localized climate chronologies and archaeological investigations, this study may shed light on circumstances surrounding the initial colonization and further development of the Lakes Region.

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800–950 C.E.). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands but does not explain more pronounced and earlier societal disruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ∼200 C.E. to 500 C.E., during the Terminal Preclassic and Early Classic periods. Plant wax carbon isotope records indicate a decline in C_4 plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water-conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.

© 2018 The Author(s). Methane clumped-isotope compositions provide a new approach to understanding the formational conditions of methane from both biogenic and thermogenic sources. Under some conditions, these compositions can be used to reconstruct the formational temperatures of the gas, and this capability can be applied to common subsets of both biogenic and thermogenic systems. Additionally, there are examples in which clumped-isotope compositions do not reflect gas-formation temperatures but instead mixing effects and kinetic phenomena; such kinetic effects also occur in common and recognizable subtypes of biogenic and thermogenic gases. Here we review the use of methane clumped-isotope measurements for understanding the origin of methane in the subsurface. We review methane clumped-isotope measurements from numerous biogenic and thermogenic natural gas reservoirs.We then place these measurements in the context of common frameworks for identifying the formational conditions of methane including the use of methane δ13C and δD values and C1/C2-3ratios. Finally, we propose a framework for how methane clumped isotopes can be used to identify the origin of methane accumulations.

Earthquakes occurring naturally or induced by human activities can damage surface and subsurface infrastructure. Oil and gas wells represent a category of subsurface infrastructure that can act as leakage pathways connecting oil and gas reservoirs, groundwater aquifers, and the atmosphere. The integrity of these wells can be compromised through a wide range of processes and contribute to groundwater contamination, greenhouse gas emissions, and air quality degradation. We estimate the increase in such subsurface leakage potential due to seismic activity through geospatial analysis of 579,378 oil and gas well and 196,315 earthquake (magnitudes greater than 1.0) locations in Oklahoma, California, and British Columbia. We perform density-based clustering analysis and point density mapping using ArcGIS. We combine the well and earthquake point density maps to identify hot spots of joint high well and earthquake densities. We find that oil and gas wells and earthquakes are clustered in space, with densities reaching ∼60 wells per km2 and ∼40 earthquakes per km2 in California. There are at least two hot spots where these clusters overlap in each state/province. In Oklahoma and British Columbia, the hot spots are more correlated with earthquake densities; while, in California, the hot spots are more correlated with well densities. Our findings indicate the need to investigate the role of earthquakes on wellbore leakage through additional analysis of earthquake characteristics, wellbore attributes, improved data collection, and empirical field studies for all oil and gas wells, including those that are abandoned. In particular, large scale geospatial analysis establishing the scope of the problem and empirical field studies focusing on identified hot spots are needed to understand potential environmental impacts of earthquakes, especially those induced by oil and gas activities.

Leaf-wax hydrogen isotope composition (δDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δDwax in the tropics remain poorly understood. We measured δDwax and the stable carbon isotope composition of leaf-waxes (δ13Cwax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δDwax. In this region the estimated hydrogen isotope composition of meteoric water (δDw) varies by only 25‰, and variability in δDw does not explain the relatively large variance in δDwax (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (ewax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in ewax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ13Cwax, provides a secondary control on ewax/w for n-alkanoic acids (en-acid/w). Low δ13Cn-acid values are associated with high en-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between δ13Cn-alkane and en-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.

The early Eocene (ca. 55–48 Ma) encompasses one of the warmest intervals of the past 65 m.y. and is characterized by an unusually low equator-to-pole thermal gradient. Recent proxy studies suggest temperatures well in excess of 30 °C even at high latitudes, but confl icting interpretations derived from different types of data leave considerable uncertainty about actual early Eocene temperatures. A robust comparison among new paleotemperature proxies may provide insight into possible biases in their temperature estimates, and additional detail on the spatial distribution of temperatures will further resolve the early Eocene meridional temperature gradient. We use a suite of paleotemperature proxies based on the chemistry of bivalve shell carbonate and associated sedimentary organic matter from the United States Gulf Coastal Plain to constrain climate at a subtropical site during this key interval of Earth history. Oxygen isotope and clumped isotope analyses of shell carbonate and two tetraether lipid analyses of sedimentary organic carbon all yield temperatures of ~27 °C. High-resolution, intraannual oxygen isotope data reveal a consistent, large range of seasonal variation, but clumped isotope data suggest that seasonality is due primarily to precipitation, not to temperature. These paleotemperature estimates are 2–3 °C warmer than the northern Gulf of Mexico today, and generally consistent with early Eocene temperature estimates from other low and mid-latitude locations, but are signifi cantly cooler than contemporaneous estimates from high southern latitudes.