δD Values Of Precipitation Research Articles

Tropical Leaf Wax n‐Alkane and n‐Alkanoic Acid Reflect δD of Precipitation During Early Stages of Leaf Growth: Insights From an Isotope Labeling Experiment

AbstractThe hydrogen isotopic compositions of leaf‐wax n‐alkanes (δDalk) and n‐alkanoic acids (δDacid) reflect ambient climatic conditions (including precipitation δD values, δDprecip). However, the understanding of climatic conditions of exactly which period (i.e., early or entire period of the leaf's lifespan) these biomarkers preserve is still evolving. Previous studies on the δDalk and δDacid values, done only in extra‐tropical regions, mostly indicate that δDalk values are biased toward the early growing season whereas δDacid values are not biased toward any season. To decipher the seasonal bias in δDalk and δDacid records from tropics, we conducted a long‐duration experiment wherein deciduous and evergreen species were grown using normal water (δD = −2‰) during early stages of the leaf growth and later using isotopically labeled water (δD = 1,000‰). Our experiment revealed (a) in deciduous and evergreen species, δDalk and δDacid values reflect δDprecip during early stages of the leaf growth, (b) synchronous synthesis of n‐alkanes and n‐alkanoic acids, and (c) in deciduous species, minor incorporation of the previous year's photosynthates in leaf wax pool of the current year's mature leaves. Our study suggests that δDalk and δDacid records in the tropics are biased toward the climatic conditions prevailing during early stages of leaf growth. This bias should be considered while comparing the δDprecip values generated from leaf wax proxy records and isotope‐enabled atmospheric circulation models.

Biomarker Evidence for an MIS M2 Glacial‐Pluvial in the Mojave Desert Before Warming and Drying in the Late Pliocene

AbstractAncient lake deposits in the Mojave Desert indicate that the water cycle in this currently dry place was radically different under past climates. Here we revisit a 700 m core drilled 55 years ago from Searles Valley, California, that recovered evidence for a lacustrine phase during the late Pliocene. We update the paleomagnetic age model and extract new biomarker evidence for climatic conditions from lacustrine deposits (3.373–2.706 Ma). The MBT′5Me temperature proxy detects present‐day conditions (21 ± 3°C, n = 2) initially, followed by warmer‐than‐present conditions (25 ± 3°C, n = 17) starting at 3.268 and ending at 2.734 Ma. Bacterial and archeal biomarkers reveal lake salinity increased after 3.268 Ma likely reflecting increased evaporation in response to higher temperatures. The δ13C values of plant waxes (−30.7 ± 1.4‰, n = 28) are consistent with local C3 taxa, likely expanded conifer woodlands during the pluvial with less C4 than the Pleistocene. δD values (−174 ± 5‰, n = 25) of plant waxes indicate precipitation δD values (−89 ± 5‰, n = 25) in the late Pliocene are within the same range as the late Pleistocene precipitation δD. Microbial biomarkers identify a deep, freshwater lake and a cooling that corresponds to the onset of major Northern Hemisphere glaciation at marine isotope stage marine isotope stages M2 (3.3 Ma). A more saline lake persisted for ∼0.6 Ma across the subsequent warmth of the late Pliocene (3.268–2.734 Ma) before the lake desiccated at the Pleistocene intensification of Northern Hemisphere Glaciation.

Determination of soil-groundwater systems recharge mechanism in the middle Inner Mongolia Plateau by isotopic tracers

The steppe located in the middle Inner Mongolia Plateau (IMP) has scarce water resources and fragile ecosystems and has an arid-semiarid monsoon characterized by low and uneven seasonal precipitation and high evaporation. Until now, problems relating to the hydrologic processes in the area remain poorly understood, and thus strategies for the exploitation and utilization of the area’s local water resource remain limited. This study aimed to investigate the recharge and discharge processes of soil water and the source and circulation of groundwater through geochemical methods. The local meteoric water line (LMWI) of our study area in the middle IMP was first proposed and compared with the local meteoric water line of Baotou in Inner Mongolia and Ulan Bator. The δ18O and δD values of precipitation showed an increasing trend from June to August and decreasing trend from August to late October. The H and O isotopes of soil water varied with time, location, and soil profile depth. Besides the geographical and climatic factors, the Cl concentrations of soil water can be affected remarkably by land use type and land cover change. In our monitoring period, variations in the Cl concentrations and δ18O and δD values of shallow soil water indicated that the evaporation of soil water was in the sequence August > June > October. Furthermore, the detailed evaporation proportions of soil water were estimated. The isotope compositions of groundwater plotted on the LMWI and there could be a rapid recharge mechanism of the groundwater.

Holocene and Last Interglacial climate of the Faroe Islands from sedimentary plant wax hydrogen and carbon isotopes

The Last Interglacial period (LIG) is Earth's most recent globally warm period and is analogous in some ways to projected future global warming. However, questions remain regarding the state of the climate during the LIG in the North Atlantic, a region that is extremely sensitive to changes in oceanic and atmospheric circulation. Here, we present hydrogen and carbon isotope (δD and δ13C) records from a suite of plant wax biomarkers preserved in Holocene and LIG lacustrine sediments from the North Atlantic Faroe Islands and interpret them as qualitative proxies for temperature and hydroclimate variability. These data are used to directly compare LIG and Holocene climate using the same proxy approaches from the same terrestrial location. Measuring multiple isotopes on multiple types of waxes elucidates the sources of homologous plant waxes. We deduce that the δD values of long-chain n-alkanes (C27–C33) and mid-chain n-alkanes (C23–C25) in these sedimentary archives reflect leaf water and lake water δD values, respectively, while the δD values for both long-chain (C28–C30) and mid-chain n-alkanoic acids (C24–C26) primarily represent lake water δD values. Plant wax-inferred δD values of precipitation during the early Holocene (10,100 to 8,200 cal yr BP) are ∼35‰ more positive than late Holocene values, and decline over the Holocene. δD-inferred hydrologic change and δ13C -inferred plant water use efficiency both indicate that the Faroe Islands became drier throughout the Holocene. Comparison with measurements from LIG plant waxes indicates that late LIG in the Faroe Islands was hydrologically similar to the early-to mid-Holocene (8,200 to 4,000 cal yr BP), with enriched precipitation isotopes and reduced evapotranspiration indicating a warmer, wetter environment.

Characteristics of Stable Isotopes in Precipitation and Moisture Sources in the Headwaters of the Yangtze River

Based on the stable isotopes of 73 precipitation samples continuously collected from May to October 2014 and related meteorological statistics in the Dongkemaldi Basin, the characteristics of δD, δ18O, and d-excess of precipitation, as well as the correlations between δ18O and the rainfall amount and air temperature were analyzed. The moisture sources were tracked by the HYSPLIT model to further estimate the contribution of different water vapor sources to the rainfall amount. The results showed that the range of δ18O and δD values varied from -26.5‰ to 1.9‰ and -195.2‰ to 34.0‰, respectively; meanwhile, the δ18O and δD values in precipitation fluctuated greatly with time in response to water vapor transport from different moisture sources of the Qinghai-Tibet Plateau. The slope and intercept of the Local Meteoric Water Line (LMWL) were both higher than those of the Global Meteoric Water Line (GMWL) and close to the LMWL in the northern area of the Qinghai-Tibet Plateau. The relationship between δ18O and δD in different precipitation types showed significant differences, which were mainly related to the source of water vapor and meteorological conditions during the process of precipitation formation. Because of the influence of local evaporation and the transport process of water vapor, the d-excess values of atmospheric precipitation were relatively large; the δ18O in precipitation had a significant amount effect, but had no temperature effect, thus indicating that the rainfall amount was more effective in controlling the stable isotope content of atmospheric precipitation than temperature. The modeled trajectory of vapor sources showed that water vapor of precipitation was mainly derived from the marine vapor carried by the southwest monsoon, local moisture, and the westerly water vapor, and their contributions to the rainfall amount were 43%, 36%, and 21%, respectively. The results of this study can contribute to further understanding of the atmospheric circulation characteristics and water cycle process of the Dongkemadi basin in the headwaters of the Yangtze River.

Influence of the North Atlantic Oscillation on δD and δ18O in meteoric water in the Armenian Highland

The stable hydrogen (δD) and oxygen (δ18O) isotope composition of modern meteoric water are influenced by changes in global temperature, Rayleigh distillation processes, and moisture transport history. However, on a global scale the relative influence of these different meteorological variables is spatially heterogeneous. We collected daily precipitation samples from meteorological stations in the Armenian Highlands for a period of one year in order to determine the constraints on δD and δ18O and relationship to broader climatic conditions. These data are coupled with HYSPLIT back-trajectory analysis to determine how these compositions relate to moisture source and transport path. Multivariate analysis shows the strongest relationship between δD and δ18O and climate variables is temperature at the sampling site. Along with temperature, the intensity of the North Atlantic Oscillation, a pattern of atmospheric circulation which determines the strength and location of Northern Hemisphere midlatitude westerlies, is significant (p < 0.05) for both δ18O and δD values of precipitation. This correlation between NAO and isotopes in precipitation is also observed in four nearby GNIP stations from 2008 to 2015. Negative NAO phases are associated with weakened westerlies, which allows precipitation enriched in heavy isotopes from the Mediterranean and southerly sources to penetrate into the Armenian Highlands, whereas positive NAO phases are associated with stronger westerlies bringing more negative precipitation from the Black Sea. This relationship is most significant (p < 0.001) during the winter and spring months (December to May). These data demonstrate that paleo-archives of δD and δ18O in the Armenian Highlands could serve as a tool to measure changes in the NAO in the past, particularly when coupled with independent measurements of temperature.

Plant water use strategies in the Shapotou artificial sand-fixed vegetation of the southeastern margin of the Tengger Desert, northwestern China

Stable oxygen and hydrogen isotopic compositions (δ18O and δD) of plant xylem water and its potential water sources can provide new information for studying water sources, competitive interactions and water use patterns of plants. The contributions of different water sources to three plants, Hedysarum scoparium (HS), Caragana Korshinskii (CK) and Artemisia ordosica(AO), were investigated in the artificial sand-fixed vegetation of Shapotou, the southeastern margin of the Tengger Desert of northwestern China, based on meteorological data and δ18O and δD values of precipitation, groundwater, soil water and xylem water of HS, CK and AO. Our results indicated that water content throughout the soil profile and their competition balance for water uptake during different growth season. The results indicate that these sand-fixed plants have developed into a relatively stable stage and they are able to regulate their water use behavior as a response to the environmental conditions, which reinforces the effectiveness of plantation of native shrubs without irrigation in degraded areas.

The effect of Indian Summer Monsoon rainfall on surface water δD values in the central Himalaya

AbstractStable 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.

Stable isotope composition of precipitation at different elevations in the monsoon marginal zone

The Qilian Mountains in western China are an important ecological and security barrier and are the primary water source for inland river basins. In this study, we collected precipitation samples and meteorological data from 4 sampling sites at different elevations on the northern slope of the Qilian Mountains from October 2016 to October 2017. The purpose was to analyse spatial and temporal variations of the isotope composition of precipitation and to evaluate the influence of temperature, elevation, monsoon circulation and continuous rainout processes on the measured values. The results show that the isotopic composition exhibit obvious seasonal variations; the higher δ18O values occur in summer and autumn, and the lower values occur in winter and spring, while the lower d-excess values occur in spring and summer and the higher d-excess values occur in autumn and winter. Stable isotope values varied with elevation. δ18O and δD values decreased with increasing elevation, and d-excess increased with increasing elevation. The average annual elevation gradient for δ18O was −0.26‰/100 m, δD was −1.77‰/100 m, and d-excess was 3‰/100 m, and the elevation gradient in summer was higher than in winter. The slope and intercept of LMWL increased with increasing elevation. The elevation gradients for slopes and intercepts are 0.07/100 m and 1/100 m, respectively. The temperature effect is most significant below 0 °C. Local moisture recycling and sub-cloud evaporation also play an important role. Moisture sources are mainly controlled by westerly circulation. Influenced by moisture from the Asian monsoon, the δ18O and δD values of precipitation show a weak “precipitation effect” in July and August, and the values gradually become more negative with continued rainout. This study improves knowledge of the isotope evolution of precipitation in the Qilian mountains, and lays the foundation for further research on isotope hydrology in cold and arid regions.

Isotopic composition of atmospheric precipitation and its tracing significance in the Laohequ Basin, Loess plateau, China

Based on the δ18O and δD values in precipitation and related meteorological parameters in the Chinese Loess Plateau (CLP) from June 2014 to April 2016, the effects of temperature and precipitation amount effect on stable isotopic compositions were analyzed, as well as the tracing significance of deuterium excess.The results show that the Local Meteoric Water Line (LMWL) was defined as δD = 7.08δ18O + 4.73(R2 = 0.99), with a slope of <8, indicating that a non-equilibrium evaporation process occurred when the drops fell below the cloud base in the arid and semi-arid environment. The temperature effect was defined as δD = 0.4536T–46.937.44 (r = 0.113, n = 150, p = 0.168), which shows a weak positive correlation between temperature and isotopic composition. Meanwhile, the precipitation amount effect was defined as δD = −0.7883P–34.152 (r = −0.223, n = 150, p = 0.011), which shows a weak negative correlation between temperature and isotope composition. The slopes in cross plotting δD - δ18O records were 8.3, 7.7, 7.4, and 6.3, displaying a decreasing trend in the temperature gradient of T ≤ 0 °C, 0 < T ≤ 10 °C, 10 < T ≤ 20 °C and T > 20 °C, respectively. The amount effect is significant (δD = −7.0946P + 48.686, r = −0.548, n = 20, p = 0.012) in the precipitation range from 10 to 20 mm. The average deuterium excess values in winter were 24‰ and 25.3‰ for 2015 and 2016, respectively; however, the low values of 3.1‰ and 5.5‰ occurred in the summers of 2014 and 2015, respectively. These results suggested that intense sub-cloud evaporation decreased the deuterium excess values in the Chinese Loess Plateau under the climate conditions of the rainy season with high temperatures. However, deuterium excess values of snow in wintertime (LT-1, LT-2) could be reflective of the reliable characteristics of atmospheric vapor for weak sub-cloud evaporation and surface evaporation.

Late Holocene changes in vegetation and atmospheric circulation at Lake Uddelermeer (The Netherlands) reconstructed using lipid biomarkers and compound‐specific δD analysis

ABSTRACTWe reconstructed middle to late Holocene changes in atmospheric circulation patterns and vegetation in north‐west Europe by applying novel geochemical techniques to the sediment record of Lake Uddelermeer (The Netherlands). A comparison of higher plant‐derived leaf wax n‐alkane distributions archived in the lake sediments with those in living plant material, combined with palynological analysis, indicates that the vegetation immediately surrounding the lake became more open at 3150 cal a BP, while the regional vegetation responded more gradually and ∼650 years later. Our record of the hydrogen isotopic composition of plant leaf waxes (δDwax) shows a deuterium enrichment starting from 3500 cal a BP, which we interpret as a change in atmospheric circulation. A similar δDwax record from nearby Meerfelder Maar (Germany) shows an opposite trend around this time, which could be explained by a change in sea‐level pressure resembling a negative North Atlantic Oscillation phase. This could account for depleted δD values of precipitation at Meerfelder Maar, while confounding factors related to the more maritime position of Uddelermeer cause the opposite shift there.

Precipitation stable isotope records from the northern Hengduan Mountains in China capture signals of the winter India–Burma Trough and the Indian Summer Monsoon

This project reports results of the first precipitation stable isotope (δ18O and δD) time series produced for Qamdo in the northern Hengduan Mountains in the southeastern Tibetan Plateau. The data showed that the fluctuations of precipitation stable isotopes at Qamdo during the different seasons revealed various moisture sources. The westerlies and local recycling moisture dominated at the study area before the pre-monsoon and after the post-monsoon seasons, which resulted in similar trends of both precipitation stable isotopes and temperature. The marine moisture was transported to the northern Hengduan Mountains by the winter India–Burma Trough combined with convection. Consequently, stable isotopes in subsequent precipitation were occasionally observed to decrease suddenly. However, δ18O and δD values of precipitation at Qamdo were lower during the monsoon period and the duration of those low values was longer because of the effects of the Indian Summer Monsoon and the strengthening convection. Our findings indicate that the effects of seasonal precipitation differences caused by various climate systems, including the winter India–Burma Trough and Indian Summer Monsoon, need to be considered when attempting to interpret tree-ring and ice core records for the Hengduan Mountains.

A high-resolution hydrogen isotope record of behenic acid for the past 16 kyr in the northeastern United States

D/H ratios of leaf lipids from lacustrine aquatic macrophytes have been shown to record precipitation δD values, which can reflect precipitation-weighted mean annual temperature (MAT) in the northeastern United States. Here we report a high-resolution hydrogen isotopic record from Little Pond, Massachusetts, USA, which we compare with other paleoclimate data from the region, including a similar δD record from Blood Pond, Massachusetts. Together the two datasets provide a >16 ka record of δD variability in the region, affording new insights into Holocene climate history. First, the long-term trends in δD correlate significantly with regional temperatures inferred from alkenone records from nearby areas of the North Atlantic and lake-level inferred changes in precipitation and evaporation. The long-term δD trends reflect a period of maximum regional warmth at ca. 8–6 ka after the collapse of the Laurentide Ice Sheet. Second, unlike the positive relationship between temperature and δD observed over the long-term and during early events like the Younger Dryas, we find that a series of warm and dry events at 4.9–4.6, 4.2–3.9, 2.9–2.1, and 1.3–1.2 cal kya BP coincide with negative δD excursions from the long trends. These events were likely driven by summer drought dynamics.

Influence of the Indian monsoon and the subtropical jet on climate change on the Tibetan Plateau since the late Pleistocene

Precipitation atop the Tibetan Plateau (TP) is delivered by the Indian summer monsoon, the Asian summer monsoon, and weather systems associated with the subtropical westerly jet. Variations in the relative importance of the monsoon systems and the westerly jet are hypothesized to have occurred at decadal, millennial and glacial-interglacial scales. However, paleoclimate observations based on explicit climate proxies are still scarce, limiting our understanding of the mechanisms of Holocene climate variability on the Tibetan Plateau (TP). Here we present three independently dated compound specific hydrogen isotope records of sedimentary leaf waxes from lakes on the TP, Bangong Co, Lake Qinghai and Linggo Co. The leaf wax δD records reflect isotopes in precipitation, and we combine these observations with existing isotopic and hydrological data to investigate variations in the influence of the summer monsoon and the westerly jet on the moisture budget of the TP since the Late Pleistocene. δD values of precipitation at all three lakes were relatively positive during the Late Pleistocene indicating a weakened summer monsoon. During the early and mid-Holocene, δD values of precipitation at the three lakes were relatively negative, suggesting the importance of summer monsoon. During the middle to late Holocene, δD values at Bangong Co and Lake Qinghai gradually increased with superimposed episodes of short term of δD variability. However, at Linggo Co in the northern TP, periods of more positive δD values of precipitation correspond to wetter intervals inferred from lake level high stands, and likely reflect variations in moisture associated with the westerly jet. Thus, the δD records at Linggo Co imply the lesser importance of summer monsoon moisture in the hydrologic budget of the northern TP. Collectively, the hydrogen isotope records at these three lakes document millennial and centennial scale variations in the strength of the summer monsoon systems and concurrent changes in the westerly jet. Furthermore, millennial-scale fluctuations in the δD records at the three lakes during the middle to late Holocene suggest episodes of reduced summer monsoonal moisture delivery to these regions, and correspond with intervals of cool sea surface temperatures in the North Atlantic.

Different hydrogen isotope fractionations during lipid formation in higher plants: Implications for paleohydrology reconstruction at a global scale.

Leaf wax δDn-alkane values have shown to differ significantly among plant life forms (e.g., among grasses, shrubs, and trees) in higher plants. However, the underlying causes for the differences in leaf wax δDn-alkane values among different plant life forms remain poorly understood. In this study, we observed that leaf wax δDn-alkane values between major high plant lineages (eudicots versus monocots) differed significantly under the same environmental conditions. Such a difference primarily inherited from different hydrogen biosynthetic fractionations (εwax-lw). Based upon a reanalysis of the available leaf wax δDn-alkane dataset from modern plants in the Northern Hemisphere, we discovered that the apparent hydrogen fractionation factor (εwax-p) between leaf wax δDn-alkane values of major angiosperm lineages and precipitation δD values exhibited distinguishable distribution patterns at a global scale, with an average of −140‰ for monocotyledonous species, −107‰ for dicotyledonous species. Additionally, variations of leaf wax δDn-alkane values and the εwax-p values in gymnosperms are similar to those of dicotyledonous species. Therefore, the data let us believe that biological factors inherited from plant taxonomies have a significant effect on controlling leaf wax δDn-alkane values in higher plants.

Effect of aridity on δ13C and δD values of C3 plant- and C4 graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa)

The observation that the hydrogen isotope composition (δD) of leaf wax lipids is determined mainly by precipitation δD values, has resulted in the application of these biomarkers to reconstruct paleoclimate from geological records. However, because the δD values of leaf wax lipids are additionally affected by vegetation type and ecosystem evapotranspiration, paleoclimatic reconstruction remains at best semi-quantitative. Here, we used published results for the carbon isotope composition (δ13C) of n-alkanes in common plants along a latitudinal gradient in C3/C4 vegetation and relative humidity in Cameroon and demonstrated that pentacyclic triterpene methyl ethers (PTMEs) and n-C29 and n-C31 in the same soil, derived mainly from C4 graminoids (e.g. grass) and C3 plants (e.g. trees and shrubs), respectively. We found that the δD values of soil n-C27, n-C29 and n-C31, and PTMEs correlated significantly with surface water δD values, supporting previous observations that leaf wax lipid δD values are an effective proxy for reconstructing precipitation δD values even if plant types changed significantly. The apparent fractionation (εapp) between leaf wax lipid and precipitation δD values remained relatively constant for C3-derived long chain n-alkanes, whereas εapp of C4-derived PTMEs decreased by 20‰ along the latitudinal gradient encompassing a relative humidity range from 80% to 45%. Our results indicate that PTME δD values derived from C4 graminoids may be a more reliable paleo-ecohydrological proxy for ecosystem evapotranspiration within tropical and sub-tropical Africa than n-alkane δD values, the latter being a better proxy for surface water δD values. We suggest that vegetation changes associated with different plant water sources and/or difference in timing of leaf wax synthesis between C3 trees of the transitional class and C3 shrubs of the savanna resulted in a D depletion in soil long chain n-alkanes, thereby counteracting the effect of evapotranspiration D enrichment along the gradient. In contrast, evaporative D enrichment of leaf and soil water was significant enough to be recorded in the δD values of PTMEs derived from C4 graminoids, likely because PTMEs recorded the hydrogen isotopic composition of the same vegetation type.

Assessing the strength of the monsoon during the late Pleistocene in southwestern United States

Improved predictions of drought require an understanding of natural and human-induced climate variability. Long-term records across glacial–interglacial cycles provide the natural component of variability, however few such records exist for the southwestern United States (US) and quantitative or semi-quantitative records of precipitation are absent. Here we use the hydrogen isotope (δD) value of C28n-alkanoic acid in lacustrine sediments of Pleistocene age to reconstruct δD values of precipitation in northern New Mexico over two glacial–interglacial cycles (∼550,000–360,000 years before present) and obtain a record of monsoon strength. Overall, reconstructed δD values range from −53.8‰ to −94.4‰, with a mean value of −77.5 ± 8‰. Remarkably, this variation falls within the measured present-day summer monsoonal and winter weighted means (−50.3 ± 3‰ and −106.4 ± 20‰ respectively), suggesting that processes similar to those of present time also controlled precipitation during Marine Isotope Stage (MIS) 13 to 10. Using the δD summer monsoonal and winter mean values as end-members, we interpret our reconstructed δD record of precipitation as a direct, and semi-quantitative, indicator of monsoon strength during MIS 13 to 10. Interglacial periods were characterized by greater monsoon strength but also greater variability compared to glacial periods. Pronounced cycles in the strength of the monsoon occurred during interglacial periods and in general were positively correlated with maximum mean annual temperatures. Our estimates of monsoon strength are supported by independent proxies of ecosystem productivity, namely, TOC, δ13C of TOC and Si/Ti ratio and warm pollen taxa Juniperus and Quercus. Interglacial variability in the strength of the monsoon resembles a response to the land-sea surface temperature contrast (LSTC) except for the early part of MIS 11. During this period, LSTC would have remained relatively strong while monsoonal strength decreased to a minimum. This minimum occurred following the warmest interval of MIS 11, suggesting a more complex driving of monsoon strength during warm periods. In addition, this period of monsoon minimum coincided with a core section of mud-cracked sediments that suggest low monsoonal precipitation was an important factor in the onset of drought. Our estimates of monsoon strength represent a record of natural variability in the region that is relevant to present time, in particular the variability during interglacial MIS 11, which is considered an analog for the current interglacial. Our results suggest that natural variability can cause significant reductions in monsoonal precipitation with the implication of a potentially adverse effect from sustained warming.

Environmental significance and hydrochemical processes at a cold alpine basin in the Qilian Mountains

Isotope geochemistry of precipitation, river water, well water and spring water in the Babao River basin, China, during different seasons showed that regional runoff was dominated by precipitation in summer and autumn and by underground water in winter and spring. The δ18O and δD values of precipitation and spring water displayed a clear altitude effect. Water from the main stream and tributaries displayed a converse altitude effect in the summer-wet season. The isotopic composition of the various water sources was not well correlated with elevation during the winter-dry season. The spatial distribution of the stable isotope signals of river water demonstrated the influence of rainfall inputs, evaporation and variable flow. An end-member mixing model used for hydrograph separation indicated that in the summer-wet season, the Babao River water was recharged by precipitation (71.9 %), spring water (11.7 %) and well water (16.4 %), whereas the winter-dry season recharge was sourced from spring water (28.2 %) and well water (71.8 %).

Snapshot of cooling and drying before onset of Antarctic Glaciation

The late Eocene to Oligocene transition (EOT) witnessed a major ice advance on Antarctica. Because of the paucity of accessible outcrops and difficult drilling logistics, little is known about hydrological conditions in the Antarctic Peninsula during the late Eocene prior to the major ice advance. Here we study hydrological conditions with proxy evidence from marine sediment core NBP0602A-3C, adjacent to the tip of the Antarctic Peninsula, with sediments dated to 35.9±1.1 Ma providing a snapshot of conditions prior to the EOT. We reconstruct conditions during the latest Eocene based on plant leaf wax hydrogen isotopic evidence (δDwax) paired with previously-published evidence from pollen assemblages, and compared to climate model experiments simulating conditions before, during and after the transition. The pollen from late Eocene sediments of NBP0602A-3C indicate a Nothofagidites (southern beech) dominated landscape. In the same sediments, δDwax values average −202±7‰ (1σ, n=22) for the C28n-alkanoic acid. Uncertainty around the appropriate net fractionation between water and wax (ϵwax/w) provides the largest degree of uncertainty in paleoprecipitation δD reconstructions and we therefore use two reasonable fractionations −100 and −60‰ to constrain the likely range of average precipitation δD values to between −113 and −151‰. Model experiments confirm that isotopic compositions show low sensitivity to temperature changes at the tip of the Antarctic Peninsula, in comparison to much larger changes in the continental interior. Across the interval sampled, a decline in pollen abundance from 105 to 103 gdw−1 represents cooling and drying towards more tundra-like conditions. The 30‰ trend in δDwax values towards more positive values can best be explained by smaller fractionations as the vegetation shifted from forest to tundra, with greater water stress. Model results are broadly consistent with the data and quantify the cooling and drying across the full EOT from ca. 7 to 2 °C and 700 to 600 mma−1 with an isotopic shift in δD of only about −15‰ at the tip of the Antarctic Peninsula. Pollen and leaf wax proxies reconstruct vegetation and hydrological conditions prior to the transition, and model experiments through the EOT provide proxy-groundtruthed quantification of terrestrial transformations.

D/H ratios of methoxyl groups of the sedimentary organic matter of Lake Holzmaar (Eifel, Germany): A potential palaeoclimate/-hydrology proxy

Stable hydrogen isotope ratios (δD values) of methoxyl groups in lignin and pectin from a variety of plant types from different locations have been shown to mirror δD values of precipitation, with a mean uniform isotopic fractionation. Since δD in precipitation is mainly influenced by climatic conditions, including temperature, evaporation and precipitation amount, methoxyl groups of organic matter have been proposed as a potential palaeoclimate proxy. Here, we measured the δD values of the methoxyl groups of sedimentary organic matter derived from sediment core segments of Lake Holzmaar (Eifel, Germany). The sediment core covers the entire Holocene and part of the Late Glacial (until 13,650years BP). Since the sedimentary organic matter was indicated by organic source proxies of predominantly phytoplanktonic origin, we considered the δD values of the methoxyl groups as a proxy for the δD value of the lake water. Presently and probably during most of the Holocene, the δD value of precipitation was primarily controlled by temperature (2–4.5‰/°C) as well as in isotopic equilibrium with the δD value of the lake water. We used these modern temperature sensitivities of the Eifel region together with the measured δD values of the methoxyl groups of the sedimentary organic matter to evaluate temperature changes recorded for the Holocene. The resulting mean temperature variance was found to be consistently less than 1.8–4°C and is broadly in line with that reported from the NGRIP ice core for the same period. The transition from the Holocene to the Late Glacial period was accompanied with more negative δD values of the methoxyl groups (∼13‰ for the Younger Dryas). Besides the association with temperature we assumed a higher susceptibility of the δD value of Lake Holzmaar to evaporative enrichment during the Younger Dryas which could serve as an explanation for discrepancies reported for existing palaeoclimate/-hydrology records of the Eifel region. Our findings indicate that the δD values of methoxyl groups have great potential for assisting with palaeoclimate and palaeohydrology, particularly when the organic matter is well characterized and of uniform origin.