ABSTRACT
The stable hydrogen isotope ratios (δD) of leaf wax n‐alkanes record valuable information on plant and ecosystem water relations. It remains, however, unknown if leaf wax n‐alkane δD values ...record only environmental variation during the brief period of time of leaf growth or if leaf wax n‐alkane δD values are affected by environmental variability throughout the entire lifespan of a leaf. To resolve these uncertainties, we irrigated Populus trichocarpa trees with a pulse of deuterium‐enriched water and used compound‐specific stable hydrogen isotope analyses to test if the applied tracer could be recovered from leaf wax n‐alkanes of leaves that were at different stages of their development during the tracer application. Our experiment revealed that only leaf wax n‐alkanes from leaves that had developed during the time of the tracer application were affected, while leaves that were already fully matured at the time of the tracer application were not. We conclude from our study that under controlled environmental conditions, leaf wax n‐alkanes are synthesized only early in the ontogeny of a leaf. Our experiment has implications for the interpretation of leaf wax n‐alkane δD values in an environmental context, as it suggests that these compounds record only a brief period of the environmental variability that a leaf experiences throughout its life.
The stable hydrogen isotope ratios (δD) of leaf‐wax n‐alkanes record valuable information on plant and ecosystem water relations. Here we show that under controlled environmental conditions leaf wax n‐alkanes are synthesized only early in the ontogeny of a leaf. Our experiment has implications for the interpretation of leaf wax n‐alkane δD values in an environmental context as it suggests that these compounds record only a brief period of the environmental variability that a leaf experiences throughout its life.
Hydrogen isotope ratios (2H/1H or δ2H) were measured in lipid biomarkers from algal, plant and microbial sources in sediment cores from a lake and a peat bog on the small, sparsely-inhabited ...Washington Island (4°43′N, 160°25′W) to assess central equatorial Pacific hydroclimate conditions during the last millennium. High δ2H values in lipids from a variety of biological sources indicate that the driest period of the last millennium occurred ∼1450–1600 CE, during the first half of the Little Ice Age (LIA; 1450–1850 CE). An Intertropical Convergence Zone (ITCZ) located south of its modern position, less frequent and/or weaker El Niño events, and/or a La Niña-like mean state in the tropical Pacific are potential causes for this drying at Washington Island. From ∼1600 to 1650 CE, an abrupt transition to modern-like low δ2H values in phytoplankton, plant, and microbial lipids occurred, signaling the establishment of a high-rainfall regime similar to that observed today. This transition coincided with increased ENSO variability, an excess of central-Pacific relative to eastern-Pacific El Niños, and a decline in the zonal SST gradient across the tropical Pacific, reflecting an El Niño-like mean state. The Medieval Warm Period (MWP; 900–1250 CE) was characterized by high δ2H values in lipids from phytoplankton (dinosterol, dinostanol), bacteria (hop-21-ene), and vascular plants (sitostanol), and by extension a drier climate relative to the modern lake. An increasing δ2H trend through the MWP in lipids from all sources implies drying as Northern Hemisphere temperatures declined from the early to the late MWP. This drying is hypothesized to have been driven by extensive volcanism in northern, southern, and tropical latitudes, all of which tend to cause zonal mean drying at the latitude of WI (5°N). Finally, the transition period between the LIA and MWP ∼1250–1450 CE was characterized by declining δ2H values of plant and microbial lipids in peat sediments, indicating a trend toward wetter conditions; in the absence of known internal or external climate forcings, this may have been a regional or local event. This study demonstrates that the application of compound-specific δ2H measurements of lipids from multiple biological sources and in multiple sedimentary archives from a single location can yield hydroclimate reconstructions with higher confidence than those based on single lipids. Such reconstructions are particularly important in the vast tropical Pacific, where few hydroclimate records exist.
•Hydroclimate reconstructed from δ2H values of algal, plant & microbial biomarkers.•Lake & peat bog sediment cores from Washington Island, central equatorial Pacific.•Driest period of the last millennium was the early Little Ice Age 1450–1600 CE•Abrupt transition to modern high-rainfall regime occurred 1600–1650 CE
Atmospheric transport is an understudied mechanism for leaf wax hydrogen isotope applications that contributes to mobilizing and depositing these compounds on the surface of the Earth. While previous ...efforts have identified the importance of atmospheric leaf wax deposition in remote marine locations, the processes are not well constrained on land in temperate latitudes where lakes are common and sedimentary leaf wax hydrogen isotope values are an attractive tool for understanding past precipitation changes. This work presents results from a field study that was conducted in 2010 and 2011 at Hainich National Park, Germany in order to evaluate the quantity and sources of leaf waxes in the atmosphere. Aerosols were sampled at approximately weekly intervals inside the forest canopy, and n-alkane distributions and hydrogen isotope values were compared with those from major tree species surrounding the sampling site. Despite sampling in what was expected to be a major production center, the distribution and hydrogen isotope values of atmospheric n-alkanes bore little resemblance to those of the local vegetation. Comparison with local meteorological data and to 10-day and 36-h back air mass trajectories indicated shifting effects of winds and temperature, and that mesoscale transport processes were more important than long-range mechanisms. Back trajectories also highlighted source effects, with easterly winds coinciding with relatively lower leaf wax hydrogen isotope values from more continental regions. These results suggest that leaf wax aerosols average over spatial scales that exceed typical surface catchment areas for small lake systems, even in forested areas, yet that the area over which these compounds are derived is still relatively regional. Depositional fluxes were also estimated in order to assess the potential importance of atmospheric transport to sedimentary archives. Although difficult to constrain, these estimates suggest that atmospheric deposition may be non-negligible for lake systems in cases where inputs from rivers or surface runoff are limited. Together, these observations provide new insights on how leaf waxes from different sources are integrated during aeolian transport and the spatial scales over which these processes occur.
Trees and shrubs in tropical Africa use the C-3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C-4 cycle. Leaf-wax lipids from sedimentary archives ...such as the long-chain n-alkanes (e.g., n-C-27 to n-C-33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n-alkane delta C-13 values are often used to reconstruct past C-3/C-4 composition of vegetation, assuming that the relative proportions of C-3 and C-4 leaf waxes reflect the relative proportions of C-3 and C-4 plants. We have compared the delta C-13 values of n-alkanes from modern C-3 and C-4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C-3 vegetation cover (f(C3)) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C-3-dominated rain forest by C-4-dominated savanna. The C-3 plants analysed were characterised by substantially higher abundances of n-C-29 alkanes and by substantially lower abundances of n-C-33 alkanes than the C-4 plants. Furthermore, the sedimentary delta C-13 values of n-C-29 and n-C-31 alkanes from recent lake sediments in Cameroon (-37.4%) to 26.5%) were generally within the range of delta C-13 values for C-3 plants, even when from sites where C-4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C-3 and C-4 vegetation cover when using the delta C-13 values of sedimentary n-alkanes, overestimating the proportion of C-3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C-3 and C-4 plants. We therefore tested a set of non-linear binary mixing models using delta C-13 values from both C-3 and C-4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the f(C3) values as the minimum and maximum delta C-13 values are approached, and a hyperbolic function that takes into account the differences between C-3 and C-4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed delta C-13 values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert delta C-13 measurements on sedimentary n-alkanes directly into reconstructions of C-3 vegetation cover.
The Central Asian Pamir Mountains (Pamirs) are a high‐altitude region sensitive to climatic change, with only few paleoclimatic records available. To examine the glacial‐interglacial hydrological ...changes in the region, we analyzed the geochemical parameters of a 31‐kyr record from Lake Karakul and performed a set of experiments with climate models to interpret the results. δD values of terrestrial biomarkers showed insolation‐driven trends reflecting major shifts of water vapor sources. For aquatic biomarkers, positive δD shifts driven by changes in precipitation seasonality were observed at ca. 31–30, 28–26, and 17–14 kyr BP. Multiproxy paleoecological data and modelling results suggest that increased water availability, induced by decreased summer evaporation, triggered higher lake levels during those episodes, possibly synchronous to northern hemispheric rapid climate events. We conclude that seasonal changes in precipitation‐evaporation balance significantly influenced the hydrological state of a large waterbody such as Lake Karakul, while annual precipitation amount and inflows remained fairly constant.
Plain Language Summary
Lakes in arid Central Asia are particularly susceptible to the rise and fall of lake levels as a consequence of climatic changes. To evaluate drivers behind this phenomenon, we developed a record of humidity and lake levels throughout the last 31,000 years from a high‐altitude lake in the Pamir Mountains. Herefore, we combined hydrological and ecological reconstructions with climate model experiments. Results show that neither the enhanced inflow by melting glaciers nor the significantly increased precipitation amount was responsible for higher lake levels during the studied interval. Instead, reduced summer evaporation during cold episodes was the major trigger for lake transgressions. These fluctuations were driven by changes in radiative forcing (i.e., insolation and hence temperature change) as a consequence of changes in the Earth's orbit around the Sun. As such, our results suggest that a significant impact on lake levels in arid regions is also to be expected by the current anthropogenically driven global warming.
Key Points
Proxies for hydroclimate and catchment ecology show insolation‐driven trends, with higher δD values during the LGM similar to outputs from climate models
Reduced summer evaporation during cold episodes increased water availability
Increased summer moisture caused higher lake levels at 31–30, 28–26, and 17–14 kyr BP coinciding with northern hemispheric rapid climate events
Rainfall in the central Andes associated with the South American Monsoon and the South American Low-Level Jet results from orographic effects on atmospheric circulation exerted by the Andean Plateau ...and the Eastern Cordillera. However, despite its importance for South American climate, no reliable records exist that allow decoding the evolution of thresholds and interactions between Andean topography and atmospheric circulation, especially regarding the onset of humid conditions in the inherently dry southern central Andes. Here, we employ multi-proxy isotope data of lipid biomarkers, pedogenic carbonates and volcanic glass from the Eastern Cordillera of NW Argentina and present the first long-term evapotranspiration record. We find that regional eco-hydrology and vegetation changes are associated with initiation of moisture transport via the South American Low-Level Jet at 7.6 Ma, and subsequent lateral growth of the orogen at 6.5 Ma. Our results highlight that topographically induced changes in atmospheric circulation patterns, not global climate change, were responsible for late Miocene environmental change in this part of the southern hemisphere. This suggests that mountain building over time fundamentally controlled habitat evolution along the central Andes.
•δ2H values of alkanes proved to be a useful tool to discriminate petroleum mixtures.•n-Alkanes (>C20) showed a higher 2H-enrichment with the input from terrigenous source material to oil ...mixtures.•Combination of δ13C and δ2H values of n-alkanes provided an improved resolution to unravel petroleum mixtures.•δ2H values of n-alkanes from source rock extracts were mainly controlled by the δ2H of biosynthetic precursors.
In order to resolve oil mixtures and for oil-source correlation purposes, both hydrogen (δ2H) and carbon isotope compositions (δ13C) of n-alkanes and acyclic isoprenoid alkanes have been determined in petroleum mixtures and extracts from Jurassic source rocks of distinct organic matter type, depositional setting and lithology in the South Viking Graben, Norway. δ2H values vary from −80 to −140‰ for n-alkanes and from −130 to −165‰ for pristane (Pr) and phytane (Ph) in oil and condensate samples. At the same maturity level, in the main oil generative window, individual n-alkanes from most oils and condensates show a 2H-enrichment with n-alkane chain length by 15 to 50‰. With the increase in the estimated input of hydrocarbons from terrigenous source material to petroleum mixtures, the n-alkanes (>C20) exhibit a higher 2H-enrichment while n-alkanes (C10-C14) show a higher 13C-enrichment. δ2H values of n-alkanes (>C20) are useful to discriminate mixed oils that were not well differentiated by δ13C data. δ2H values of individual n-alkanes from oils suggest a possible exchange of C-bound H with other hydrogen during their generation from precursor molecules. We find that δ2H values of n-alkanes from Jurassic source rock extracts, at the onset of oil generation, were mainly controlled by the isotopic composition of biosynthetic precursors and varied from −100 to −150‰. Combination of carbon and hydrogen isotope compositions of n-alkanes proved to be a useful tool to discriminate Jurassic petroleum mixtures from distinct sources in the Norwegian South Viking Graben.
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.
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.
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