Rapid Holocene hydrologic change along boreal treeline revealed by δ13C and δ18O in organic lake sediments, Northwest Territories, Canada

Analysis of ¹⁸O_cellulose, ¹³C_{organic matter}, and ¹³C_cellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched ¹³C_org and ¹³C_cell, caused by elevated lake productivity, declining [CO_2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted ¹⁸O_cell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with ¹³C_org/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO_2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.This is the 14th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers     

Elemental (C,N, H and P) and stable isotope (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) signatures in sediments from Zeekoevlei,South Africa: a record of human intervention in the lake

We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ¹³C_organic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ¹⁵N_organic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ¹⁵N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l⁻¹) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.     

Ostracode carbonate δ18O- and δ13C-signature of hydrological and climatic changes affecting Lake Neuchâtel,Switzerland, since the latest Pleistocene

Floating and grounded peat plateaus were studied in fens in the Yukon Territory (Canada). The peat deposit may be over 4 m thick and consists of a lower bed of aquatic peat overlain by humic fen peat, mesic fen peat and woody peat. Permafrost in the grounded peat plateaus is older than the 1200 year old White River Ash, whereas permafrost in the floating peat plateau is younger.Peat accumulation rates since 1200 years B.P. were greater in the fens (85–100 cm) than on the surface of the peat plateaus (25–55 cm). Where the peat plateau is free-floating, it will persist until the climate changes, causing the icy core to thaw. Where the peat plateau is frozen to the mineral substrate, it slowly drowns since the fen peat accumulates faster than the woody peat. This drowning results in degradation of the landform independently of the climate. Only degradation of floating peat plateaus can be used to identify climatic changes.This publication is the first paper in a series of papers presented at the session on Past Climatic Change and the Development of Peatlands at the ASLO and SWS Meetings in Edmonton, Canada, May 30–June 3, 1993. Dr. P. Kuhry and Dr S. C. Zoltai are serving as Guest Editors.     

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

We explored the use of carbon and nitrogen isotopes (¹³C and ¹⁵N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four ²¹⁰Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and ²²⁶Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low ¹³C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher ¹³C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the ¹⁵N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both ¹³C and ¹⁵N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that ¹³C and ¹⁵N reflect shifts in historic lake trophic state.     

Potential of δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N of cladoceran subfossil exoskeletons for paleo-ecological studies

Stable isotope analyses on cladoceran subfossil exoskeletons retrieved from sediment cores could allow the reconstruction of past changes in lake food webs provided the δ¹³C and δ¹⁵N values of the exoskeletons reflect those of the organisms’ whole body. The relationships between the C and N stable isotope compositions of the exoskeletons and those of the whole body were investigated for two freshwater cladoceran taxa (Bosmina sp. and Daphnia sp.) from modern samples. The C and N stable isotope compositions of the exoskeleton and those of the whole body were strongly correlated. Exoskeleton δ¹³C was similar to the whole body δ¹³C for both taxa. Daphnia exoskeletons were strongly depleted in ¹⁵N (−7.9‰) compared to the whole body. Stable isotope analyses were thereafter performed on cladoceran remains from five downcore samples from Lake Annecy, France. Results showed that Bosmina δ¹⁵N values increased by more than 4‰, between the early twentieth and twenty first centuries. Such changes might be the result of changes in nitrogen sources or cycling in the lake and/or of major shifts in Bosmina trophic position within the lake food web. This study sets up the potential of stable isotope analyses performed on cladoceran subfossil remains for paleo-ecological purposes.     

Holocene paleoclimate inferred from stable isotope (δ18O and δ13C) values in Sphagnum cellulose,Mohos peat bog,Romania

Journal of Paleolimnology - We measured stable isotopes (δ18O and δ13C) in Sphagnum cellulose that was extracted from a long peat core drilled in the ombrotrophic Mohos peat bog, Ciomadul...     

A 15.4-ka paleoclimate record inferred from δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N of organic matter in sediments from the sub-alpine Daping Swamp,western Nanling Mountains,South China

We inferred past climate conditions from the δ¹³C and δ¹⁵N of organic matter (OM) in a sediment core (DP-2011-02) from the sub-alpine Daping Swamp, in the western Nanling Mountains, South China. In the study region, a 1000-m increase in altitude results in a ~0.75‰ decrease in δ¹³C and a ~2.2‰ increase in δ¹⁵N. Organic carbon stable isotope (δ¹³C) values of the dominant modern vegetation species, surface soils, and the core samples taken in the swamp exhibit a strong terrestrial C3 plant signature. Comprehensive analysis of the core indicates both terrestrial and aquatic sources contribute to the OM in sediment. Temperature and precipitation are most likely the critical factors that influence δ¹³C: warm and wet conditions favor lower δ¹³C, whereas a dry and cool climate leads to higher δ¹³C values. Higher δ¹⁵N values may result from greater water depth and increased primary productivity, promoted by large inputs of dissolved inorganic nitrogen, induced by high surface runoff. Lower δ¹⁵N values are associated with lower lake stage and reduced productivity, under drier conditions. Therefore, stratigraphic shifts in these stable isotopes were used to infer past regional climate. Measures of δ¹³C and δ¹⁵N in deglacial deposits, in combination with total organic carbon (TOC) and nitrogen (TN) concentrations, the TOC/TN ratio, coarse silt and sand fractions, dry bulk density and low-frequency mass magnetic susceptibility, reveal two dry and cold events at 15,400–14,500 and 13,000–11,000 cal a BP, which correspond to Heinrich event 1 and the Younger Dryas, respectively. A pronounced warm and wet period that occurred between those dry episodes, from 14,500 to 13,000 cal a BP, corresponds to the Bølling–Allerød. The δ¹³C and δ¹⁵N data, however, do not reflect a warm and wet early Holocene. The Holocene optimum occurred between ~8000 and 6000 cal a BP, which is different from inferences from the nearby Dongge cave stalagmite δ¹⁸O record, but consistent with our previous results. This study contributes to our understanding of climate-related influences on δ¹³C and δ¹⁵N in OM of lake sediments in South China.     

Leaf wax δ2H and varve-thickness climate proxies from proglacial lake sediments, Baffin Island, Arctic Canada

We present a multiproxy paleoclimate record using leaf wax hydrogen isotopes (??²H_wax) and varve thickness from Arctic proglacial lake sediments. We also provide one of the first evaluations of the utility of ??²H_wax as a paleoclimate proxy in Arctic proglacial lakes. We compare varve thickness and ??²H_wax at sub-decadal resolution from 1948 to 2004 AD, and at sub-centennial resolution from 1450 to 2004 AD. Varve thickness and ??²H_wax both contain large interannual variability and are anti-correlated during the late twentieth century, suggesting that both proxies respond rapidly, but by different mechanisms, to catchment-scale forcings. At longer time scales, varve thickness exhibits a strong response to Little Ice Age cooling (1661?C1827 AD in this record) but does not show evidence for twentieth century warming recorded throughout the Arctic. ??²H_wax does record regional-scale temperature changes, with more ²H-depleted values during the Little Ice Age and an abrupt change to more ²H-enriched values in the twentieth century. This corresponds well with a recent Arctic-wide temperature reconstruction in which the seventeenth century is the coldest interval, and the twentieth century is the warmest interval. Our results suggest that ??²H_wax is a promising proxy that can be applied at high resolution in proglacial Arctic lakes.     

Biomarker and compound-specific δ<Superscript>13</Superscript>C evidence for changing environmental conditions and carbon limitation at Lake Koucha,eastern Tibetan Plateau

A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC₂₃) and δ¹³C values of TOC and nC₂₃ indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C₂₀ highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C₂₅ HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC₁₇-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and δ¹³C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The δ¹³C values of nC₂₃ range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ¹³C value of the inorganic carbon source may vary, the offset between the δ¹³C values of nC₂₃ and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.     

Environmental changes in northern New Zealand since the Middle Holocene inferred from stable isotope records (δ15N, δ13C) of Lake Pupuke

Maar lakes in the Auckland Volcanic Field are important high-resolution archives of Holocene environmental change in the Southern Hemisphere mid-latitudes. Stable carbon and nitrogen isotope analyses were applied on bulk organic matter and the green alga Botryococcus from a sediment core from Lake Pupuke (Auckland, North Island, New Zealand) spanning the period since 7,165?cal.?year BP. The origin of organic matter was established using total-organic?Ccarbon-to-nitrogen ratios (TOC/TN) as well as organic carbon (??¹³C_OM) and nitrogen (??¹⁵N) isotope composition of potential modern sources. This approach demonstrated that the contribution of allochthonous organic matter to the lake sediment was negligible for most of the record. The sedimentary TOC/TN ratios that are higher than Redfield ratio (i.e. >7) are attributed to N-limiting conditions throughout the record. Variations of nitrogen and carbon isotopes during the last 7,165?years are interpreted as changes in the dominant processes in the lake. While epilimnetic primary productivity controlled isotope composition before 6,600?cal.?year BP, microbial processes, especially denitrification and methane oxidation, caused overall shifts of the ??¹⁵N and ??¹³C values since the Mid-Holocene. Comparisons with climate reconstructions from the Northern Island suggest that changes in the wind-induced lake overturn and a shift to more pronounced seasonality were the most likely causes for lake-internal changes since 6,600?cal.?year BP.     

Taxon-specific δ13C analysis of chitinous invertebrate remains in sediments from Strandsjön,Sweden

Taxon-specific stable carbon isotope (δ¹³C) analysis of chitinous remains of invertebrates can provide valuable information about the carbon sources used by invertebrates living in specific habitats of lake ecosystems (for example, sediments, water column, or aquatic vegetation). This is complementary to δ¹³C of sedimentary organic matter (SOM), which provides an integrated signal of organic matter produced in a lake and its catchment, and of diagenetic processes within sediments. In a sediment record from Strandsjön (Sweden) covering the past circa 140 years, we analyzed SOM geochemistry (δ¹³C, C:N_atomic, organic carbon content) and δ¹³C of chitinous invertebrate remains in order to examine whether taxon-specific δ¹³C records could be developed for different invertebrate groups and whether these analyses provide insights into past changes of organic carbon sources for lacustrine invertebrates available in benthic and planktonic compartments of the lake. Invertebrate taxa included benthic chironomids (Chironomus, Chironomini excluding Chironomus, Tanytarsini, and Tanypodinae), filter-feeders on suspended particulate organic matter (Daphnia, Plumatella and Cristatella mucedo), and Rhabdocoela. δ¹³C of chironomid remains indicated periodic availability of ¹³C-depleted carbon sources in the benthic environment of the lake as δ¹³C values of the different chironomid taxa fluctuated simultaneously between ?34.7 and ?30.5 ‰ (VPDB). Daphnia and Bryozoa showed parallel changes in their δ¹³C values which did not coincide with variations in δ¹³C of chironomids, though, and a 2–3 ‰ decrease since circa AD 1960. The decrease in δ¹³C of Daphnia and Bryozoa could indicate a decrease in phytoplankton δ¹³C as a result of lower lake productivity, which is in accordance with historical information about the lake that suggests a shift to less eutrophic conditions after AD 1960. In contrast, Rhabdocoela cocoons were characterized by relatively high δ¹³C values (?30.4 to ?28.2 ‰) that did not show a strong temporal trend, which could be related to the predatory feeding mode and wide prey spectrum of this organism group. The taxon-specific δ¹³C analyses of invertebrate remains indicated that different carbon sources were available for the benthic chironomid larvae than for the filter-feeding Daphnia and bryozoans. Our results therefore demonstrate that taxon-specific analysis of δ¹³C of organic invertebrate remains can provide complementary information to measurements on bulk SOM and that δ¹³C of invertebrate remains may allow the reconstruction of past changes in carbon sources and their δ¹³C in different habitats of lake ecosystems.     

A study of the δ13C offset between chironomid larvae and their exuvial head capsules: implications for palaeoecology

We conducted a rearing experiment with the chironomid species Chironomus riparius to assess the relationship between the δ¹³C values of chironomid larvae and the δ¹³C values of their exuvial head capsules. Our experiment was also designed to study the extent of the trophic fractionation factor (Δ¹³C) under different dietary conditions. Three food sources were used (Tetramin, oats and corn), covering a range in δ¹³C values of 14.55 ‰. For each of the four successive instars, carbon isotope ratios were measured in larval tissues and head capsules. This approach highlighted the variability in δ¹³C for both larvae and their head capsules during larval development. Once the larvae reached the 3rd instar, their δ¹³C values were stabilised and did not significantly differ from their food δ¹³C (Δ¹³C = 0 ‰). It is probable that the variability in the δ¹³C offset during larval development reflected a difference in the carbon turnover for the chironomid cuticle compared with the whole body. At the 4th instar, the δ¹³C offset did not significantly differ between the three food sources and was ?0.9 ± 0.2 ‰. The proposed Δ¹³C and δ¹³C offset values can be considered as a first step for the reconstructions of the chironomid larvae paleo-diets with the aim of deciphering the different organic carbon sources supporting chironomid larvae productions. However, the influence of the environment (e.g. temperature, oxygen), other food sources (e.g. different nutritive values) as well as taxonomy (i.e. other chironomid species) should be assessed to strengthen the robustness of these results.     

Growth and physiological responses of larch trees to climate changes deduced from tree-ring widths and δ13C at two forest sites in eastern Siberia

Tree-ring chronologies of ring width and stable carbon isotope ratios (δ¹³C) over the past 160 years were developed using living larch trees at two forest sites, each with different annual precipitation, in eastern Siberia: Spasskaya Pad (SP) (62°14′N, 129°37′E); and Elgeeii (EG) (60°0′N, 133°49′E). Intrinsic water-use efficiency (iWUE) was derived from tree-ring δ¹³C. The physiological responses of the larch trees to climate varied between these sites and over time. Ring widths correlated negatively with summer temperatures at SP, where summer precipitation is lower than at EG, probably due to temperature-induced water stress. Since the 1990s, however, the negative effect of warming has been more severe at EG, where the productivity of larch trees is higher than at SP. A greater reduction of larch tree growth and higher increase rate of iWUE at EG reflects greater temperature-induced water stress, which is incident to the larger forest biomass. Our results suggest that effect of increase in atmospheric CO₂ on larch tree growth is not sufficient to compensate for temperature-induced water stress on larch growth in eastern Siberia and differences in precipitation and forest productivity largely affect the larch tree response to changing climate in eastern Siberia.     

Glacier meltwater runoff process analysis using δD and δ<Superscript>18</Superscript>O isotope and chemistry at the remote Laohugou glacier basin in western Qilian Mountains,China

Stable hydrogen and oxygen isotope has important implication on water and moisture transportation tracing research. Based on stable hydrogen (δD) and oxygen (δ¹⁸O) isotope using a Picarro L1102-i and water chemistry (e.g. major ions, pH, EC and TDS) measurement, this study discussed the temporal variation and characteristics of stable hydrogen and oxygen isotope, chemistry (e.g. TDS, pH, EC, Ca²⁺, Mg²⁺, Na⁺ and Cl^-) in various water bodies including glacier meltwater runoff, ice and snow, and precipitation at the Laohugou glacier basin during June 2012 to September 2013. Results showed that δD and δ¹⁸O in the meltwater runoff varied obviously with the temporal change from June to September, showing firstly increasing trend and then decreasing trend, with the highest values in July with high air temperature and strong glacier melting, which could indicate the temporal change of glacier melting process and extent. Variations of δD and δ¹⁸O in the runoff were similar with that of snow and ice on the glacier, and the values were also above the GMWL, which probably implied that the glacier runoff was mainly originated from glacier melting and precipitation supply. The glacier meltwater chemical type at the Laohugou glacier basin were mainly composed by Ca-Na-HCO₃-SO₄ and Ca-Mg-HCO₃-SO₄, which also varied evidently with the glacier melting process in summer. By analyzing the temporal change of stable hydrogen and oxygen isotope and chemistry in the melting period, we find it is easy to separate the components of the snow and ice, atmospheric precipitation and melt-runoff in the river, which could reflect the change process of glacier melting during the melting period, and thus this work can contribute to the glacier runoff change study of large-scale region by stable isotope and geochemical method in future.     

Palaeoclimate inferred from δ18O and palaeobotanical indicators in freshwater tufa of Lake Äntu Sinijärv,Estonia

We investigated a 3.75-m-long lacustrine sediment record from Lake Äntu Sinijärv, northern Estonia, which has a modeled basal age >12,800 cal yr BP. Our multi-proxy approach focused on the stable oxygen isotope composition (δ¹⁸O) of freshwater tufa. Our new palaeoclimate information for the Eastern Baltic region, based on high-resolution δ¹⁸O data (219 samples), is supported by pollen and plant macrofossil data. Radiocarbon dates were used to develop a core chronology and estimate sedimentation rates. Freshwater tufa precipitation started ca. 10,700 cal yr BP, ca. 2,000 years later than suggested by previous studies on the same lake. Younger Dryas cooling is documented clearly in Lake Äntu Sinijärv sediments by abrupt appearance of diagnostic pollen (Betula nana, Dryas octopetala), highest mineral matter content in sediments (up to 90 %) and low values of δ¹⁸O (less than ?12 ‰). Globally recognized 9.3- and 8.2-ka cold events are weakly defined by negative shifts in δ¹⁸O values, to ?11.3 and ?11.7 ‰, respectively, and low concentrations of herb pollen and charcoal particles. The Holocene thermal maximum (HTM) is palaeobotanically well documented by the first appearance and establishment of nemoral thermophilous taxa and presence of water lilies requiring warm conditions. Isotope values show an increasing trend during the HTM, from ?11.5 to ?10.5 ‰. Relatively stable environmental conditions, represented by only a small-scale increase in δ¹⁸O (up to 1 ‰) and high pollen concentrations between 5,000 and 3,000 cal yr BP, were followed by a decrease in δ¹⁸O, reaching the most negative value (?12.7 ‰) recorded in the freshwater tufa ca. 900 cal yr BP.     

Geochemistry of waters and brines from the Salinas Grandes basin,Córdoba,Argentina. II. Gypsum dissolution-calcite precipitation,and brine evolution

Rivers and streams originating in the surrounding mountainous area are the major sources of salt in the Salinas Grandes basin (Córdoba, Argentina). These rivers infiltrate when they reach the sandflat or in the fringes of the mudflat, feeding springs which often form shallow lakes. Presently, the lakes are distant from the playa edge, thus allowing inflow waters to dissolve ancient (Pleistocene?) evaporite beds. In the sandflat environment, two dominant types of water have been recognized (SO ₄ ^2? -Cl^?-HCO ₃ ^? -Na⁺, and Cl^?-SO ₄ ^2? -HCO ₃ ^2? -Na⁺), both considered as original members of the brine in the saline complex. Two main sources of solutes were distinguished, one related to the waters supplied by the southern sector and another to waters of the eastern sector. As a result of the chemical evolution in the playa environment, all brines belong to the neutral type (Cl^?-SO ₄ ^2? -Na⁺). Following Hardie and Eugster's (1970) model, waters from the southern sector should evolve towards an alkaline brine (Cl^?-SO ₄ ^2? -HCO ₃ ^? -Na⁺), whereas those from to the eastern sector should evolve towards a neutral one (Cl^?-SO ₄ ^2? -Na⁺). A computer simulation was carried out to model the chemical evolution of source waters. The results obtained by this methodology showed the same dichotomy (alkaline vs. neutral) established by Hardie and Eugster's (1970) model. The deficit in alkalinity could not be explained by any of the mechanisms published until now. Gypsum dissolution is the most likely mechanism which accounts for the chemical evolution of the waters investigated. When such a process is included in the computations, the Ca²⁺ supplied by gypsum beds generates an increase in the ion activity product (aCa⁺²·aCO ₃ ^2? ) and produces a significant change in the 2Ca⁺²/(2CO ₃ ^2? +HCO ₃ ^? ) ratio, switching from values less than 1 to values greater than 1. This process determines the precipitation of calcite, and leads to a decrease in alkalinity, which in turn would explain the existence of a neutral brine in the saline complex. An intermediate salinity brine was detected in the mudflat, which, according to the model (Hardie and Eugster, 1970), should evolve towards a SO ₄ ^2? -free neutral brine (Cl^?-Na⁺-Ca²⁺). The absence of this type of brine may be explained through mixing processes.     

Effect of sub-cloud evaporation on the δ~(18)O of precipitation in Qilian Mountains and Hexi Corridor,China

The sub-cloud evaporation effect refers to the evaporation process for raindrops that fall from the cloud base to the ground, which is usually accompanied by depleted light isotopes and enriched heavy isotopes in the precipitation. Based on 461 event-based precipitation samples collected from 12 weather stations in the Qilian Mountains and the Hexi Corridor from May to August of 2013, our results indicated that sub-cloud evaporation has a great influence on the δ¹⁸O of precipitation, especially in small-amount precipitation events. In May, June, July, and August the δ¹⁸O composition was enriched by 35%, 26%, 39%, and 41%, respectively, from the cloud base to the ground. This influence clearly strengthened with temperature rise, from the Qilian Mountains to the Hexi Corridor. When falling raindrops are evaporated by 1.0% in the Qilian Mountains and the Hexi Corridor, the composition of δ¹⁸O would be enriched by 1.2% and 2.6%, respectively. Temperature dominated the sub-cloud evaporation in the Qilian Mountains, whereas relative humidity controlled it in the Hexi Corridor. These results provide new proofs of the evolutional process of stable isotopes in precipitation in arid regions.     

Effects of chemical pretreatments on δ<Superscript>18</Superscript>O measurements,chemical composition,and morphology of chironomid head capsules

Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains can be used to reconstruct past lake water δ¹⁸O and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms of pyrolyzates show that NaClO₂ causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially after KOH treatment at high concentration (28%) and temperature (100°C). Scanning electron microscopy confirmed that a proteinaceous matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in head capsules pretreated with NaClO₂. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid δ¹⁸O values. Our results suggest that deproteination results in a relative increase of chironomid δ¹⁸O, whereas removal of chitin leads to decreased δ¹⁸O values. We therefore discourage the use of acids or prolonged (≥1 h) exposure to hot alkali (70°C) prior to chironomid δ¹⁸O analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid δ¹⁸O measurements. Caution and standardization are required when pretreating samples for chironomid δ¹⁸O analysis to ensure reliable, comparable, and reproducible results.     

δ~(18)O, δD and d-excess signatures of ground ice in permafrost in the Beiluhe Basin on the Qinghai-Tibet Plateau,China

In this paper, stable isotope(δ~(18)O, δD) investigations were completed in ground ice from a deep borehole in the Beiluhe Basin on northern Qinghai-Tibet Plateau to unravel the isotopic variations of ground ice and their possible source water. The δ~(18)O and δD of ground ice show distinctive characteristics compared with precipitation and surface water. The near-surface ground ice is highly enriched in heavier isotopes(δ~(18)O and δD), which were gradually depleted from top to bottom along the profile. It is suggestive of different origin and ice formation process. According to isotopic variations, the ice profile was divided into three sections: the near-surface ground ice at 2.5 m is frozen by the active-layer water which suffered evaporation. It is possible that ground ice between 3 and 4.2 m is recharged by the infiltration of snowmelt. From 5 to 6 m, the ground ice show complex origin and formation processes. Isotopic variations from 6 to 11.1 m and 20.55 m indicate different replenishment water. The calculated slope of freezing line(S=6.4) is larger than the experimental value(5.76), and is suggestive of complex origin and formation process of ground ice.