Stable isotope proxy records, such as speleothems, plant‐wax biomarker records, and ice cores, are suitable archives for the reconstruction of regional palaeohydrologic conditions. But the ...interpretation of these records in the tropics, especially in the Indian Summer Monsoon (ISM) domain, is difficult due to differing moisture and water sources: precipitation from the ISM and Winter Westerlies, as well as snow‐ and glacial meltwater. In this study, we use interannual differences in ISM strength (2011–2012) to understand the stable isotopic composition of surface water in the Arun River catchment in eastern Nepal. We sampled main stem and tributary water (n = 204) for stable hydrogen and oxygen isotope analysis in the postmonsoon phase of two subsequent years with significantly distinct ISM intensities. In addition to the 2011/2012 sampling campaigns, we collected a 12‐month time series of main stem waters (2012/2013, n = 105) in order to better quantify seasonal effects on the variability of surface water δ18O/δD. Furthermore, remotely sensed satellite data of rainfall, snow cover, glacial coverage, and evapotranspiration was evaluated. The comparison of datasets from both years revealed that surface waters of the main stem Arun and its tributaries were D‐enriched by ~15‰ when ISM rainfall decreased by 20%. This strong response emphasizes the importance of the ISM for surface water run‐off in the central Himalaya. However, further spatio‐temporal analysis of remote sensing data in combination with stream water d‐excess revealed that most high‐altitude tributaries and the Tibetan part of the Arun receive high portions of glacial melt water and likely Winter Westerly Disturbances precipitation. We make the following two implications: First, palaeohydrologic archives found in high‐altitude tributaries and on the southern Tibetan Plateau record a mixture of past precipitation δD values and variable amounts of additional water sources. Second, surface water isotope ratios of lower elevated tributaries strongly reflect the isotopic composition of ISM rainfall implying a suitable region for the analysis of potential δD value proxy records.
A potential human footprint on Western Central African rainforests before the Common Era has become the focus of an ongoing controversy. Between 3,000 y ago and 2,000 y ago, regional pollen sequences ...indicate a replacement of mature rainforests by a forest–savannah mosaic including pioneer trees. Although some studies suggested an anthropogenic influence on this forest fragmentation, current interpretations based on pollen data attribute the “rainforest crisis” to climate change toward a drier, more seasonal climate. A rigorous test of this hypothesis, however, requires climate proxies independent of vegetation changes. Here we resolve this controversy through a continuous 10,500-y record of both vegetation and hydrological changes from Lake Barombi in Southwest Cameroon based on changes in carbon and hydrogen isotope compositions of plant waxes. δ
13C-inferred vegetation changes confirm a prominent and abrupt appearance of C₄ plants in the Lake Barombi catchment, at 2,600 calendar years before AD 1950 (cal y BP), followed by an equally sudden return to rainforest vegetation at 2,020 cal y BP. δD values from the same plant wax compounds, however, show no simultaneous hydrological change. Based on the combination of these data with a comprehensive regional archaeological database we provide evidence that humans triggered the rainforest fragmentation 2,600 y ago. Our findings suggest that technological developments, including agricultural practices and iron metallurgy, possibly related to the large-scale Bantu expansion, significantly impacted the ecosystems before the Common Era.
The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic ...timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (δ34SSO4, δ18OSO4, and δ18OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO42– is derived mainly from a mixture of an evaporite source with uniform values of δ34SSO4 and δ18OSO4, and a sulfide source that contributes highly variable values of δ34S (−12.2 ‰ to +4.1 ‰) and δ18O (−17.7 ‰ to −1.6 ‰). Contributions of sulfide oxidation to riverine SO42– vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (∼104 years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (∼107 years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.
Silicate weathering and organic carbon (OC) burial in soil regulate atmospheric CO2, but their influence on each other remains unclear. Generally, OC oxidation can generate acids that drive silicate ...weathering, yet clay minerals that form during weathering can protect OC and limit oxidation. This poses a conundrum where clay formation and OC preservation either compete or cooperate. Debate remains about their relative contributions because quantitative tools to simultaneously probe these processes are lacking while those that exist are often not measured in concert. Here we demonstrate that Li isotope ratios of sediment, commonly used to trace clay formation, can help constrain OC cycling. Measurements of river suspended sediment from two watersheds of varying physiography and analysis of published data from Hawaii soil profiles show negative correlations between solid-phase δ7Li values and OC content, indicating the association of clay mineral formation with OC accumulation. Yet, the localities differ in their ranges of δ7Li values and OC contents, which we interpret with a model of soil formation. We find that temporal trends of Li isotopes and OC are most sensitive to mineral dissolution/clay formation rates, where higher rates yield greater OC stocks and lower δ7Lisoil values. Whereas OC-enhanced dissolution primarily dictates turnover times of OC and silicate minerals, clay protection distinctly modifies soil formation pathways and is likely required to explain the range of observations. These findings underscore clay mineral formation, driven primarily by bedrock chemistry and secondarily by climate, as a principal modulator of weathering fluxes and OC accumulation in soil.
•Connections between silicate weathering and organic carbon burial in soil explored.•Lithium isotope ratios of sediment correlate with organic carbon content.•Bedrock chemistry strongly impacts clay formation and OC storage potential.
The hydrogen isotope composition of leaf wax biomarkers (delta.sup.2 H.sub.wax) is a valuable tool for reconstructing continental paleohydrology, since it serves as a proxy for the hydrogen isotope ...composition of precipitation (delta.sup.2 H.sub.pre). To yield robust palaeohydrological reconstructions using delta.sup.2 H.sub.wax in marine archives, it is necessary to examine the impacts of regional climate on delta.sup.2 H.sub.wax and assess the similarity between marine sedimentary delta.sup.2 H.sub.wax and the source of continental delta.sup.2 H.sub.wax . Here, we examined an aridity gradient from hyperarid to humid along the Chilean coast. We sampled sediments at the outlets of rivers draining into the Pacific as well as soils within catchments and marine surface sediments adjacent to the outlets of the studied rivers and analyzed the relationship between climatic variables and delta.sup.2 H.sub.wax values. We found that apparent fractionation between leaf waxes and source water is relatively constant in humid and semiarid regions (average: -121 0/00). However, it becomes less negative in hyperarid regions (average: -86 0/00) as a result of evapotranspirative processes affecting soil and leaf water .sup.2 H enrichment.
•Different patterns of soil leaf wax isotopes in eastern vs western Pamirs.•Western/central Pamirs: soil isotopes correlated with altitude and longitude.•Isotopic lapse rate vs altitude is lower than ...the global average.•Eastern Pamirs: multiple local effects lead to higher δ2H values.•Local control on soil isotopes important for interpretation of paleorecords.
Understanding the control mechanisms that affect soil leaf wax hydrogen isotopes is crucial when interpreting these data in the context of paleoclimatic reconstructions. Within a pilot study we aimed to evaluate the regional controls on leaf wax δ2H in the Central Asian Pamir Mountains (Tajikistan), a region characterized by pronounced hydroclimatic gradients. Soil and tap water samples were collected along a ca. 1000 km west-east-gradient, spanning ca. 3100 m of altitude difference. The results give evidence of variable environmental control on soil leaf wax δ2H values in different sub-regions of the study area. In the western and central Pamirs, the plant lipids mirror the signal of a mostly riverine water source, with an isotopic signature mainly controlled by winter and spring precipitation. With ca. –1.3‰ change per 100 m altitude, soil leaf wax derived isotopic lapse rates in these samples are lower than in other Asian high-altitude settings and the global average, which confirms earlier findings from surface water analysis from the Pamirs. In contrast, soil samples from the shielded Karakul basin showed significantly higher δ2H values. Those most likely reflect the more positive isotope signal of a predominant summer precipitation in the arid eastern Pamirs, with potential additional enrichment due to local vapor recycling and soil- and leaf water evapotranspiration. While the predominant water source (precipitation vs river water) plays an important role on a local scale, the results further highlight the significance of regional climate effects on isotopic signals incorporated into leaf waxes.
Summary
Understanding the long‐term impact of projected climate change on tropical rainforests is critical given their central role in the Earth's system. Palaeoecological records can provide a ...valuable perspective on this problem. Here, we examine the effects of past climatic changes on the dominant forest type of Southeast Asia – lowland dipterocarp forest.
We use a range of proxies extracted from a 1400‐yr‐old lacustrine sedimentary sequence from north‐eastern Philippines to determine long‐term vegetation responses of lowland dipterocarp forest, including its dominant tree group dipterocarps, to changes in precipitation, fire and nutrient availability over time.
Our results show a positive relationship between dipterocarp pollen accumulation rates (PARs) and leaf wax hydrogen isotope values, which suggests a negative effect of drier conditions on dipterocarp abundance. Furthermore, we find a positive relationship between dipterocarp PARs and the proxy for phosphorus availability, which suggests phosphorus controls the productivity of these keystone trees on longer time scales. Other pollen taxa show widely varying relationships with the abiotic factors, demonstrating a high diversity of plant functional responses.
Our findings provide novel insights into lowland dipterocarp forest responses to changing climatic conditions in the past and highlight potential impacts of future climate change on this globally important ecosystem.
See also the Commentary on this article by Utteridge 240: 2176–2178.
Hydrogen isotope values (δD) of sedimentary aquatic and terrestrial lipid biomarkers, originating from algae, bacteria, and leaf wax, have been used to record isotopic properties of ancient source ...water (i.e., precipitation and/or lake water) in several mid- and high-latitude lacustrine environments. In the tropics, however, where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes, calibration studies for this proxy are not yet available. To close this gap of knowledge, we sampled surface sediments from 11 lakes in Cameroon to identify those hydro-climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers. Here we present a robust framework for the application of compound-specific hydrogen isotopes in tropical Africa. Our results show that the δD values of the aquatic lipid biomarker n-C17 alkane were not correlated with the δD values of lake water. Carbon isotope measurements indicate that the n-C17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis. We demonstrate that the δD values of the n-C29 alkane were correlated with the δD values of surface water (i.e., river water and groundwater), which, on large spatial scales, reflect the isotopic composition of mean annual precipitation. Such a relationship has been observed at higher latitudes, supporting the robustness of the leaf-wax lipid δD proxy on a hemispheric spatial scale. In contrast, the δD values of the n-C31 alkane did not show such a relationship but instead were correlated with the evaporative lake water δD values. This result suggests distinct water sources for both leaf-wax lipids, most likely originating from two different groups of plants. These new findings have important implications for the interpretation of long-chain n-alkane δD records from ancient lake sediments. In particular, a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources. Our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and/or growth form of organisms and therefore, the source of hydrogen used during lipid biosynthesis.
Tracing pathways and transformations of particulate organic carbon from landscape sources to oceanic sinks is commonly done using the isotopic composition or biomarker content of particulate organic ...matter (POM). However, similarity of source characteristics and complex mixing in rivers often preclude a robust deconvolution of individual contributions. Moreover, these approaches are limited in detecting organic matter transformations. This impedes understanding of carbon cycling. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT‐ICR‐MS) can simultaneously identify many molecular formulas from mixtures of organic matter, and provide direct information on its compositional variability. Here, we investigate how FT‐ICR‐MS can give insight into POM dynamics on a landscape scale, focusing on the trans‐Himalayan Kali Gandaki River, Nepal. Using molecular information, we identify source tracers in the solvent extractable lipid fraction of riverine POM, finding up to 102 indicative molecular formulas for individual sources. Further, we assess molecular transformations of the lipid fraction of POM during its transfer from litter into topsoil, and onwards into the river. A large number of shared mass formulas and a well‐preserved isoprenoidal patterns suggest efficient incorporation of litter into topsoil. In contrast, we observe a selective loss of mass formulas and a preferential export of formulas with low double bond equivalents and a low nominal oxidation state of carbon after organic matter entrainment in the river. Our results demonstrate the potential of FT‐ICR‐MS for source‐to‐sink studies, allowing detailed organic matter source characterization and discrimination, and tracking of molecular transformations along organic matter pathways spanning different spatial and temporal scales.
Plain Language Summary
The transfer of organic matter (OM) by rivers from landscape sources into the ocean followed by its burial in marine sediments is an important carbon sink. Therefore, OM is often traced along this journey using its isotopic or biomarker composition. But contributions of OM sources to river sediments can be difficult to estimate because of similar source characteristics, mixing of many sources and changes of the molecular composition along the way. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT‐ICR‐MS) is a novel method able to identify many molecular formulas from OM mixtures at once providing direct information about their molecular composition. Here, we investigate how FT‐ICR‐MS contributes to understanding the transport and transformation of particulate OM focusing on a Himalayan river in Nepal. We use the molecular information to identify tracers for individual OM sources in the landscape. We then assess molecular transformations during the transfer of litter into topsoil, and onwards into the river. Our data suggest efficient incorporation of litter into topsoil, but we observe a selective loss of molecular formulas upon entrainment of sources into the river. Our results reveal that FT‐ICR‐MS is useful for detailed source characterization and tracking of molecular transformations along OM pathways.
Key Points
Organic matter sourcing and transformations in a Himalayan river studied by FT‐ICR‐MS measurements of solvent extractable lipids
Identification of up to 102 indicator mass formulas for different organic matter sources in the landscape using indicator species analysis
Mass formulas preserved during incorporation of litter into topsoil but selectively lost during entrainment of sources into the river
Globally, changes in stable isotope ratios of oxygen and hydrogen (δO18 and δD) in the meteoric water cycle result from distillation and evaporation processes. Isotope fractionation occurs when air ...masses rise in elevation, cool, and reduce their water-vapor holding capacity with decreasing temperature. As such, δO18 and δD values from a variety of sedimentary archives are often used to reconstruct changes in continental paleohydrology as well as paleoaltimetry of mountain ranges. Based on 234 stream-water samples, we demonstrate that areas experiencing deep convective storms in the eastern south–central Andes (22–28° S) do not show the commonly observed relationship between δO18 and δD with elevation. These convective storms arise from intermontane basins, where diurnal heating forces warm air masses upward, resulting in cloudbursts and raindrop evaporation. Especially at the boundary between the tropical and extra-tropical atmospheric circulation regimes where deep-convective storms are very common (∼26° to 32° N and S), the impact of such storms may yield non-systematic stable isotope-elevation relationships as convection dominates over adiabatic lifting of air masses. Because convective storms can reduce or mask the depletion of heavy isotopes in precipitation as a function of elevation, linking modern or past topography to patterns of stable isotope proxy records can be compromised in mountainous regions, and atmospheric circulation models attempting to predict stable isotope patterns must have sufficiently high spatial resolution to capture the fractionation dynamics of convective cells.
•We report δO18 and δD values from >250 water samples across the central Andes.•δO18 values range from −0.6 to −12‰ (VSMOW), with no distillation south of 26° S.•We show that convective storms overprint isotope systematics in rainfall.•Derive satellite temperature lapse rates and perform isotope modeling.•Highlight the effects of storms on the present and past isotope proxy records.
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