•Comparison of manual and automated solid phase extraction for lipid biomarkers.•Significant higher reproducibility using automated approach.•Reduction of errors also for stable isotope analysis ...using automated extraction.•Increase in efficiency due to unattended purification over night or weekend.
Effective recovery and high purity of lipid biomarkers are essential for compound-specific stable isotope analysis in a variety of fields ranging from hydrocarbon research, paleoclimatology, food and drug analysis and medicine. Solid-phase extraction (SPE) is the most common method for purifying organic compounds from complex mixtures. SPE constitutes the most labor-intensive part of laboratory work often limiting the number of samples that can be analyzed. Reliable, easy-to-use, automated methods could increase sample throughput as well as reproducibility. Here we introduce such a method using a Gilson ASPEC GX-271 system and test the separation quality, reproducibility, and efficiency in comparison to a classical manual SPE lipid purification procedure. Using multiple extractions of the same natural soil sample we show that the automated SPE is comparable in overall quality and slightly superior in reproducibility to a manual SPE. We demonstrate that stable hydrogen isotope measurements of n-alkanes purified using an automated SPE extraction showed significantly lower standard errors. Furthermore, the unattended operation of the system eases the purification of large sample sets. Generally, the automated SPE using the Gilson ASPEC GX-271 for lipid biomarker separation provides qualitatively and quantitatively accurate and reproducible results with more efficient purification of compounds than the manual method.
Leaf wax
n-alkanes from barley (
Hordeum vulgare) from a field in Switzerland exhibited changes in δD values on the order of 20‰ over a growing season, while source water (soil water) and leaf water ...varied by 40‰. Additionally the seasonal variability in δD values of leaf wax
n-alkanes of different barley leaves can only be found across different leaf generations (i.e. leaves that were produced at different times during the growing season) while
n-alkane δD values did not vary significantly within a leaf generation. Interestingly, δD values of
n-alkanes correlated best with the δD values of leaf water at midday of the sampling day but showed no significant correlation with soil water (e.g. precipitation) δD values. These results provide empirical evidence that leaf wax δD values record leaf water enrichment, and therefore integrate the isotopic effects of precipitation and evapotranspiration. Our results show that leaf wax
n-alkane δD values from grasses are ‘locked in’ early during leaf development and hence record the environmental drivers of leaf water enrichment, such as vapor pressure deficit (VPD). Our data have important implications for the interpretation of paleorecords of leaf wax δD. We suggest that leaf wax
n-alkane δD values from sedimentary records could be used to estimate changes in the degree of leaf water enrichment and hence VPD.
•δ18O and δD lapse rates from 4 to 6 km are ~4x steeper than lapse rates from 1 to 3 km.•δ18O and δD lapse rates vary seasonally with transitions in precipitation regimes.•δ18O, δD, and deuterium ...excess time series data track snow- and glacial melt.
The strong elevation gradient of the Himalaya allows for investigation of altitude and orographic impacts on surface water δ18O and δD stable isotope values. This study differentiates the time- and altitude-variable contributions of source waters to the Arun River in eastern Nepal. It provides isotope data along a 5000-m gradient collected from tributaries as well as groundwater, snow, and glacial-sourced surface waters and time-series data from April to October 2016. We find nonlinear trends in δ18O and δD lapse rates with high-elevation lapse rates (4000–6000 masl) 5–7 times more negative than low-elevation lapse rates (1000–3000 masl). A distinct seasonal signal in δ18O and δD lapse rates indicates time-variable source-water contributions from glacial and snow meltwater as well as precipitation transitions between the Indian Summer Monsoon and Winter Westerly Disturbances. Deuterium excess correlates with the extent of snowpack and tracks melt events during the Indian Summer Monsoon season. Our analysis identifies the influence of snow and glacial melt waters on river composition during low-flow conditions before the monsoon (April/May 2016) followed by a 5-week transition to the Indian Summer Monsoon-sourced rainfall around mid-June 2016. In the post-monsoon season, we find continued influence from glacial melt waters as well as ISM-sourced groundwater.
•Origin of particulate organic carbon in Himalayan river sediments constrained.•Bayesian end-member mixing approach robustly differentiates organic carbon sources.•Himalayan sediment and organic ...carbon yield are decoupled.•Decoupling due to spatial diversity of organic sources and erosion processes.•Organic carbon export dominated by input from Tibetan part of the catchment.
Rivers transfer particulate organic carbon (POC) from eroding mountains into geological sinks. Organic carbon source composition and selective mobilization have been shown to affect the type and quantity of POC export, but their combined effects across complex mountain ranges remain underexplored. Here, we examine the variation in organic carbon sourcing and transport in the trans-Himalayan Kali Gandaki River catchment, along strong gradients in precipitation, rock type and vegetation. Combining bulk stable nitrogen, and stable and radioactive organic carbon isotopic composition of bedrock, litter, soil and river sediment samples with a Bayesian end-member mixing approach, we differentiate POC sources along the river and quantify their export. Our analysis shows that POC export from the Tibetan segment of the catchment, where carbon bearing shales are partially covered by aged and modern soils, is dominated by petrogenic POC. Based on our data we re-assess the presence of aged biospheric OC in this part of the catchment, and its contribution to the river load. In the High Himalayan segment, we observed low inputs of petrogenic and biospheric POC, likely due to very low organic carbon concentrations in the metamorphic bedrock, combined with erosion dominated by deep-seated landslides. Our findings show that along the Kali Gandaki River, the sourcing of sediment and organic carbon are decoupled, due to differences in rock organic carbon content, soil and above ground carbon stocks, and geomorphic process activity. While the fast eroding High Himalayas are the principal source of river sediment, the Tibetan headwaters, where erosion rates are lower, are the principal source of organic carbon. To robustly estimate organic carbon export from the Himalayas, the mountain range should be divided into tectono-physiographic zones with distinct organic carbon yields due to differences in substrate and erosion processes and rates.
It is becoming increasingly clear that China experienced significant hydrological changes during the mid‐to‐late Holocene transition—a period characterized by societal changes. However, the nature of ...the hydroclimate anomaly as well as the direct consequences on societies in Southeast China remains unclear. Here, we present a leaf wax record from the Shuizhuyang peat deposit in Southeast China spanning the Holocene. The δD values of C29 n‐alkane (δDC29) showed a large positive shift up to 24‰ from 4.7 to 3.8 ka, which changed independent of vegetation proxies and could not be solely explained by precipitation δD variations. It is thus most likely to reflect abrupt drying, which is probably shaped by a more El Niño‐like mean state in the tropical Pacific Ocean. We hypothesize that such a significant change in hydroclimate might have promoted the development of mixed rice and millet farming on the southeast coast of China.
Plain Language Summary
The mid‐to‐late Holocene transition with significant changes in climate and agriculture‐based civilization affords a valuable insight into abrupt climate changes and human adaption. Increasing studies have shown large shifts in hydrological regime in China during this critical transition. However, the hydrological expression and its potential impact on the agriculture practice in Southeast China remains unclear. In this study, we utilize the hydrogen isotope compositions (δD) of leaf waxes from the Shuizhuyang peat deposit on the southeast coast of China to reconstruct hydrological changes during the mid‐to‐late Holocene transition. We find that the mid‐to‐late Holocene transition on the southeast coast of China is characterized by an abrupt drying, which is primarily modulated by a more El Niño‐like mean state over the tropical Pacific Ocean. Such a significant change in hydroclimate might have promoted the development of mixed rice and millet farming on the southeast coast of China coeval to the mid‐to‐late Holocene hydrological shift.
Key Points
Increasing aridity on the southeast coast of China from 4.7 to 3.8 ka captured by the hydrogen isotope compositions (δD) of leaf waxes
The abrupt drying is mainly shaped by a more El Niño‐like state over the tropical Pacific Ocean during the mid‐to‐late Holocene transition
Such a shift in hydroclimate might have promoted the development of mixed rice and millet farming on the southeast coast of China
The compound-specific hydrogen isotope composition (
δD) of sedimentary
n-alkanes is increasingly used as a palaeoclimate proxy. To explore the origin of the hydrogen isotope signal of
n-alkanes from ...higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the
n-alkane composition and
δD values of 31 plant biomass samples (Birch (
Betula), Beech (
Fagus), Oak (
Quercus), Alder (
Alnus), Hornbeam (
Carpinus) and Myrtle (
Myrtus) as well as non-stomata containing
Sphagnum,
Cladonia and moss) from 14 sites along a climatic gradient from Northern Finland to Southern Italy. Eleven of the sites were previously investigated for the
n-alkane
δD values in adjacent lake sediments and are compared to the leaf wax
n-alkane
δD values.
The distribution of terrestrial
n-alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake. Evidently,
n-alkane distributions within the same species at different sites were found to be variable. These findings complicate the use of certain
n-alkane distributions in sediments as a palaeoenvironmental indicator.
δD values of plant biomass
n-alkanes show a significant positive correlation with the precipitation
δD values. Biomass
n-alkane
δD values are in general less negative than the associated sedimentary
n-alkanes of terrestrial origin, possibly due to the stronger evapotranspiration during the summer, when sampling took place. This suggests, that
n-alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn, when most leaves are incorporated into the sediment. The variability of the
δD values from non-stomata containing plants is much higher than for broadleaf trees, since stomata can regulate water loss through evapotranspiration.
The average chain length (ACL) of
n-alkanes from deciduous tree leaves increases from North to South. With increasing ACL the hydrogen isotopic fractionation (
ε) between source water and
n-alkanes also increases.
Our results prove that terrestrial
n-alkane
δD values record the
δD value of precipitation, modified by the amount of isotope enrichment in the leaf water due to plant anatomy (stomata vs. no stomata) and site specific meteorological conditions, such as evapotranspiration, relative humidity and soil moisture. Therefore, terrestrial
n-alkanes in lake sediments can deliver information on changes in these parameters over geological timescales. In lake sediments where aquatic
n-alkanes, such as
n-C
17 record the lake water
δD value, the isotopic difference between terrestrial and aquatic
n-alkanes could therefore be used to reconstruct changes in evapotranspiration.
Leaf wax n-alkanes are long-chain hydrocarbons that can persist in sedimentary records over geological timescales. Since their hydrogen isotopic composition (expressed as a δD value) can be ...correlated to the δD values of precipitation, leaf wax n-alkane δD values have been advocated as new and powerful proxies for paleohydrological research. The exact type of hydrological information that is recorded in the δD values of leaf wax n-alkanes remains, however, unclear. In a companion paper we provide experimental evidence showing that the δD values of leaf wax n-alkanes of angiosperm plants grown under controlled environmental conditions not only reflect δD values of precipitation – as has often been assumed – but that evaporative deuterium (D)-enrichment of leaf water has an additional critical effect on their δD values. Here we present a detailed observational study that illustrates that evaporative D-enrichment of leaf water also affects the δD values of leaf wax n-alkanes in plants from natural ecosystems along a 1500km climate gradient in Northern Australia. Based on global simulations of leaf water D-enrichment we show that the effects of evaporative D-enrichment of leaf water on leaf wax n-alkane δD values is relevant in all biomes but that it is particularly important in arid environments. Given the combined influence of precipitation δD values and leaf water D-enrichment we argue that leaf wax n-alkane δD values contain an integrated signal that can provide general hydrological information, e.g. on the aridity of a catchment area. We also suggest that more specific hydrological information and even plant physiological information can be obtained from leaf wax n-alkanes if additional indicators are available to constrain the plant- and precipitation-derived influences on their δD values. As such, our findings have important implications for the interpretation of leaf wax n-alkane δD values from paleohydrological records. In addition, our investigations open the door to employ δD values of leaf wax n-alkanes as new ecohydrological proxies in contemporary plant and ecosystem sciences.
Isabel Lake is a hypersaline crater-lake on Isabel Island, Mexico, situated in the eastern tropical Pacific, an area highly sensitive to hydrological changes. Today, annual rainfall mostly occurs ...during the wet season, from June to October, when the northern edge of the Intertropical Convergence Zone (ITCZ) extends over the island. In order to evaluate the potential of sedimentary lipid biomarker signatures as a proxy of past hydroclimatic variability we have performed a calibration analysis comparing changes in biomarker distribution in the upper 16cm of the sediment core with a regional instrumental data set. Annual laminations present in the sediment sequence allow for precise chronological control (1942–2006). More than 80 different lipid compounds were identified in the sediment and could be assigned to three major groups of source organisms: (1) algal populations; (2) a mixed community of ciliates, bacteria and cyanobacteria; and (3) photosynthetic sulfur bacteria. We found that the observed changes in the relative contribution of the different lipid biomarkers to the sediment record were determined by the regional rainfall variability over the last 65years. The planktonic community of Isabel Lake was highly sensitive to salinity fluctuations related to rainfall variability; seasonal precipitation results in freshwater input into the lake, driving an annual algal bloom and a relative decrease in the abundance of the more halotolerant populations of (cyano)bacteria and ciliates. Consequently, the concentration ratio between the two most abundant biomarkers in the Isabel Lake sediments, n-alkyl diols and tetrahymanol (which we define as the DiTe index), representing algal and ciliate planktonic populations, respectively, was significantly correlated with the seasonal rainfall anomaly (r=0.68, p<0.01). We propose that the DiTe index is a proxy of changes in the aquatic ecosystem of Isabel Lake and, by extension, regional hydrological changes in a sensitive climatic area of the eastern tropical Pacific.
► Calibration analysis of lipid biomarker in a hypersaline lake (1942–2006). ► Rainfall variability determined the relative contribution of lipids. ► n-alkyl diols:tetrahymanol derivatives correlated with seasonal rainfall, r=0.68, p<0.01. ► DiTe index proposed as proxy of hydroclimatic variability.
Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, δ¹⁸O, of precipitation is associated with environmental conditions, ...cellulose δ¹⁸O should be as well. However, plant physiological models using δ¹⁸O suggest that cellulose δ¹⁸O is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose δ¹⁸O values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i) quantify the impacts that the primary climatic drivers humidity (ea) and air temperature (Tair) have on cellulose δ¹⁸O values in different tropical ecosystems and ii) determine which environmental signal is dominating cellulose δ¹⁸O values. Our results revealed that ea and Tair equally influence cellulose δ¹⁸O values and that distinguishing which of these factors dominates the δ¹⁸O values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of ea and Tair on the δ¹⁸O values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose δ¹⁸O values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that δ¹⁸O values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences.
•Hydrogen isotopic fractionation for Potamogeton grown in a lake and lab measured.•δD values for plants collected from a lake unaffected by groundwater.•δD values of plants grown in the lab became ...lower at higher salinity.•Average ε value −136±9‰ for freshwater samples.
Sedimentary lipid biomarkers have become widely used tools for reconstructing past climatic and ecological changes due to their ubiquitous occurrence in lake sediments. In particular, the hydrogen isotopic composition (expressed as δD values) of leaf wax lipids derived from terrestrial plants has been a focus of research during the last two decades and the understanding of competing environmental and plant physiological factors influencing the δD values has greatly improved. Comparatively less attention has been paid to lipid biomarkers derived from aquatic plants, although these compounds are abundant in many lacustrine sediments. We therefore conducted a field and laboratory experiment to study the effect of salinity and groundwater discharge on the isotopic composition of aquatic plant biomarkers. We analyzed samples of the common submerged plant species, Potamogeton pectinatus (sago pondweed), which has a wide geographic distribution and can tolerate high salinity. We tested the effect of groundwater discharge (characterized by more negative δD values relative to lake water) and salinity on the δD values of n-alkanes from P. pectinatus by comparing plants (i) collected from the oligotrophic freshwater Lake Stechlin (Germany) at shallow littoral depth from locations with and without groundwater discharge, and (ii) plants grown from tubers collected from the eutrophic Lake Müggelsee in nutrient solution at four salinity levels. Isotopically depleted groundwater did not have a significant influence on the δD values of n-alkanes in Lake Stechlin P. pectinatus and calculated isotopic fractionation factors εl/w between lake water and n-alkanes averaged −137±9‰ (n-C23), −136±7‰ (n-C25) and −131±6‰ (n-C27), respectively. Similar ε values were calculated for plants from Lake Müggelsee grown in freshwater nutrient solution (−134±11‰ for n-C23), while greater fractionation was observed at increased salinity values of 10 (163±12‰) and 15 (−172±15‰). We therefore suggest an average ε value of −136±9‰ between source water and the major n-alkanes in P. pectinatus grown under freshwater conditions. Our results demonstrate that isotopic fractionation can increase by 30–40‰ at salinity values 10 and 15. These results could be explained either by inhibited plant growth at higher salinity, or by metabolic adaptation to salt stress that remain to be elucidated. A potential salinity effect on δD values of aquatic lipids requires further examination, since this would impact on the interpretation of downcore isotopic data in paleohydrologic studies.
PDF
