Influence of terrestrial vegetation on leaf wax δD of Holocene lake sediments

The hydrogen isotope composition of terrestrial plant leaf wax in sediments is increasingly used as a paleoclimatic indicator. Modern calibration studies suggest that paleoclimatic interpretation of leaf wax δD values requires consideration of the differences in the apparent fractionation of hydrogen isotopes among different groups of plants. However, it is not common that paleoecological data are used to help interpret leaf wax δD profiles. Here we assess the relative importance of factors influencing millennial-scale shifts in δD values of n-alkanoic acids at Steel Lake (Minnesota, USA), an extensively studied site with independent records of vegetation composition, δD of input water to the lake, and evaporation. The δD values of the n-C₂₈ alkanoic acid (δD_C28) vary between −190 and −168‰, and do not correlate with δD of input water or the extent of evaporation. However, δD_C28 is negatively correlated with the δ¹³C values of the n-C₂₈ alkanoic acid (δ¹³C_C28). The correlation, along with pollen assemblage and carbonate δ¹³C records, suggests that Holocene shifts between forest and grassland and/or in the water use efficiency of C₃ plants influenced the stratigraphic variation in leaf wax δD. Thus, paleoecological information, such as that inferred from pollen assemblages and carbon isotopes of plant-derived compounds, may aid paleoclimatic interpretation of leaf wax δD profiles from lake sediments.     

Paleoenvironmental implications of taxonomic variation among δN values of chloropigments

Natural variations in the ratios of nitrogen isotopes in biomass reflect variations in nutrient sources utilized for growth. In order to use δ¹⁵N values of chloropigments of photosynthetic organisms to determine the corresponding δ¹⁵N values of biomass - and by extension, surface waters - the isotopic offset between chlorophyll and biomass must be constrained. Here we examine this offset in various geologically-relevant taxa, grown using nutrient sources that may approximate ocean conditions at different times in Earth’s history. Phytoplankton in this study include cyanobacteria (diazotrophic and non-diazotrophic), eukaryotic algae (red and green), and anoxygenic photosynthetic bacteria (Proteobacteria), as well as environmental samples from sulfidic lake water. Cultures were grown using N₂, NO₃⁻, and NH₄⁺ as nitrogen sources, and were examined under different light regimes and growth conditions. We find surprisingly high variability in the isotopic difference (δ¹⁵N_biomass − δ¹⁵N_{chloropigment}) for prokaryotes, with average values for species ranging from −12.2‰ to +11.7‰. We define this difference as ε_por, a term that encompasses diagenetic porphyrins and chlorins, as well as chlorophyll. Negative values of ε_por reflect chloropigments that are ¹⁵N-enriched relative to biomass. Notably, this enrichment appears to occur only in cyanobacteria. The average value of ε_por for freshwater cyanobacterial species is −9.8 ± 1.8‰, while for marine cyanobacteria it is −0.9 ± 1.3‰. These isotopic effects group environmentally but not phylogenetically, e.g., ε_por values for freshwater Chroococcales resemble those of freshwater Nostocales but differ from those of marine Chroococcales. Our measured values of ε_por for eukaryotic algae (range = 4.7-8.7‰) are similar to previous reports for pure cultures. For all taxa studied, values of ε_por do not depend on the type of nitrogen substrate used for growth. The observed environmental control of ε_por suggests that values of ε_por could be useful for determining the fractional burial of eukaryotic vs. cyanobacterial organic matter in the sedimentary record.     

Compound specific δD values of long chain n-alkanes derived from terrestrial higher plants are indicative of the δD of meteoric waters: Evidence from surface soils in eastern China

The stable carbon and hydrogen isotope composition of higher plant-derived long chain n-alkanes (δ¹³C_n-alkanes and δD_n-alkanes) from 45 surface soil samples (within well characterized vegetation zones) from eastern China (18°N–50°N) are reported. The weighted average δD_n-alkanes value for n-C₂₇, n-C₂₉ and n-C₃₁ in the samples and the annual average δD of meteoric water recorded at 12 weather stations proximal to the sampling sites show similar spatial variations. The δD of n-alkanes shows a gradual depletion in value with increasing latitude. The results demonstrate that, on a large spatial scale, the δD values of long chain n-alkanes derived from higher plants have the potential to record the δD of meteoric water, although many other factors can also influence the isotope values. There appears to be no apparent relationship between the δD of the n-alkanes extracted from the surface soil and the overlying vegetation type (i.e. forest/grassland or C₃/C₄ composition). Therefore, palaeoenvironmental studies utilizing δD_n-alkanes from higher plant-derived material in geological samples have the potential to provide additional information with regard to the past hydrological cycle.     

Dinosterol δD values in stratified tropical lakes (Cameroon) are affected by eutrophication

In freshwater settings, dinosterol (4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (δD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested δD values of dinosterol in both particulate organic matter (POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the δD values of dinosterol correlated with lake water δD values, confirming a first order influence of source water δD values. However, we observed that sedimentary dinosterol was D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface (E_h OAI). The observations suggest that paleohydrologic reconstruction from δD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water δD values, with more positive dinosterol δD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C₃₄ botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and ββ-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with E_h OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol δD values to evaluate the effect of lake trophic status on sedimentary dinosterol δD values, as a basis for accurately reconstructing tropical lake water δD values.     

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of δD values of individual n-alkanes and δ<Superscript>18</Superscript>O and δ<Superscript>13</Superscript>C values of carbonates

For the first time ¹⁸O and ¹³C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for ¹⁸O and ¹³C values of dolomitized carbonates and D values of individual n-alkanes. ¹⁸O_carb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The ¹³C_carb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since ¹³C_carb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, ¹⁸O_carb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high ¹⁸O_carb and low ¹³C_carb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C₁₆ and n-C₁₈ which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C₂₇, n-C₂₈, n-C₂₉ and n-C₃₀, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. D_water values, reconstructed using the n-C₁₆ alkane and ¹⁸O _water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.     

“Background” δ34S values of Kupferschiefer sulphides in Poland: pyrite-marcasite nodules

Regional background ³⁴S values of pyrite-(marcasite) nodules throughout the Zechstein basin in Poland have been measured to help estimate the proportion of externally derived sulphur in the Kupferschiefer Cu-Ag ores. The ³⁴S values of the 17 FeS₂ nodules measured range widely, from -25.2 to -51.9%., similar to the previously published -28 to -43%. range in disseminated pyrite in the Kupferschiefer. The wide variation cannot be attributed to pyrite versus marcasite mineralogy, amount of contained chalcopyrite or sphalerite, carbonate versus shale host rock, early versus late formation, percent of included calcite, or to size, shape, or texture. There is also no relation with proximity to the centres of copper mineralization in southwestern Poland where sulphides are typically isotopically heavier. The ³⁴S values do, however, vary directly with percent of host-rock fragments included in the nodules. Repeat samples that were washed with acid or hot water show the same wide variation, indicating that contamination by sulphate sulphur in the host rock is not a factor. Neither is organic sulphur because of its small volume. Instead, the sulphur composition may be fundamentally controlled by the formation mechanism of the nodule, whereby ³⁴S-rich sulphide is preferentially concentrated, possibly replacing anhydrite lenses. Alternatively, a network of host rock inclusions might act as a more accessible conduit for later, ³⁴S-rich fluids to infiltrate the nodule and add to earlier, ³⁴S-poor pyrite.In the ore deposits, higher ³⁴S values of ore nodules suggest less indigenous sulphur in limestone than shale lithologies. An isotopic temperature of 61 °C from a chalcopyrite-galena pair agrees with other estimates of <105°C. Higher values in ore nodules/veinlets than in adjacent disseminations, and the calculated ³⁴S_py value from a pyrite-bornite mixture support the idea that metal-bearing ³⁴S-rich fluids penetrated the Kupfer-schiefer through a network of fractures.Contribution to IGCP Project 254 Metalliferous Black Shales     

Chemistry,isotope values (δD, δ18O, δ34SSO4) and temperatures of the water inflows in two Gotthard tunnels,Swiss Alps

Water inflows in the Gotthard Highway Tunnel and in the Gotthard Exploration Tunnel are meteoric waters infiltrating at different elevations, on both sides of an important orographic divide. Limited interaction of meteoric waters with gneissic rocks produces Ca–HCO₃ and Na–Ca–HCO₃ waters, whereas prolonged interaction of meteoric waters with the same rocks generates Na–HCO₃ to Na–SO₄ waters. Waters circulating in Triassic carbonate-evaporite rocks have a Ca–SO₄ composition. Calcium-Na–SO₄ waters are also present. They can be produced through interaction of either Na–HCO₃ waters with anhydrite or Ca–SO₄ waters with a local gneissic rock, as suggested by reaction path modeling. An analogous simulation indicates that Na–HCO₃ waters are generated through interaction of Ca–HCO₃ waters with a local gneissic rock. The two main SO₄-sources present in the Alps are leaching of upper Triassic sulfate minerals and oxidative dissolution of sulfide minerals of crystalline rocks. Values of δ³⁴S_SO4 < ∼+9‰ are due to oxidative dissolution of sulfide minerals, whereas δ³⁴S_SO4 >∼+9‰ are controlled either by bacterial SO₄ reduction or leaching of upper Triassic sulfate minerals. Most waters have temperatures similar to the expected values for a geothermal gradient of 22°C/km and are close to thermal equilibrium with rocks. However relatively large, descending flows of cold waters and ascending flows of warm waters are present in both tunnels and determine substantial cooling and heating, respectively, of the interacting rocks. The most import upflow zone of warm, Na-rich waters is below Guspisbach, in the Gotthard Highway Tunnel, at 6.2–9.0 km from the southern portal. These warm waters have equilibrium temperatures of 65–75°C and therefore constitute an important low-enthalpy geothermal resource.     

Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (ε_a) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of ²H and ¹⁸O associated with evaporation of lake waters. The most negative ε_a values represent signatures least affected by aridity; these values were −125 ± 5‰ for tropical evergreen and dry forests, −130‰ for a temperate broadleaf forest, −120 ± 9‰ for the high-altitude tropical páramo (herbs, shrubs and grasses), and −98 ± 6‰ for North American montane gymnosperm forests. Minimum ε_a values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid δD variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax δD measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax δD and carbonate-δ¹⁸O measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau.     

Resolving the bulk δN values of ancient human and animal bone collagen via compound-specific nitrogen isotope analysis of constituent amino acids

Stable nitrogen isotope analysis is a fundamental tool in assessing dietary preferences and trophic positions within contemporary and ancient ecosystems. In order to assess more fully the dietary contributions to human tissue isotope values, a greater understanding of the complex biochemical and physiological factors which underpin bulk collagen δ¹⁵N values is necessary. Determinations of δ¹⁵N values of the individual amino acids which constitute bone collagen are necessary to unravel these relationships, since different amino acids display different δ¹⁵N values according to their biosynthetic origins. A range of collagen isolates from archaeological faunal and human bone (n = 12 and 11, respectively), representing a spectrum of terrestrial and marine protein origins and diets, were selected from coastal and near-coastal sites at the south-western tip of Africa. The collagens were hydrolysed and δ¹⁵N values of their constituent amino acids determined as N-acetylmethyl esters (NACME) via gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). The analytical approach employed accounts for 56% of bone collagen nitrogen. Reconstruction of bulk bone collagen δ¹⁵N values reveals a 2‰ offset from bulk collagen δ¹⁵N values which is attributable to the δ¹⁵N value of the amino acids which cannot currently be determined by GC-C-IRMS, notably arginine which comprises 53% of the nitrogen unaccounted for (23% of the total nitrogen). The δ¹⁵N values of individual amino acids provide insights into both the contributions of various amino acids to the bulk δ¹⁵N value of collagen and the factors influencing trophic position and the nitrogen source at the base of the food web. The similarity in the δ¹⁵N values of alanine, glutamate, proline and hydroxyproline reflects the common origin of their amino groups from glutamate. The depletion in the δ¹⁵N value of threonine with increasing trophic level indicates a fundamental difference between the biosynthetic pathway of threonine and the other amino acids. The δ¹⁵N value of phenylalanine does not change significantly with trophic level, reflecting its conservative nature as an essential amino acid, and thus represents the isotopic composition of the nitrogen at the base of the food web. Δ¹⁵N_Glu-Phe values in particular are shown to reflect trophic level nitrogen sources within a food web. In relation to the reconstruction of ancient human diet the contribution of marine and terrestrial protein are strongly reflected in Δ¹⁵N_Glu-Phe values. Differences in nitrogen metabolism are also shown to have an influence upon individual amino acid δ¹⁵N values with Δ¹⁵N_Glu-Phe values emphasising differences between the different physiological adaptations. The latter is demonstrated in tortoises, which can excrete nitrogen in the form of uric acid and urea and display negative Δ¹⁵N_Glu-Phe values whereas those for marine and terrestrial mammals are positive. The findings amplify the potential advantages of compound-specific nitrogen isotope analysis in the study of nitrogen flow within food webs and in the reconstruction of past human diets.     

Petroleum geochemistry of the Potwar Basin, Pakistan: 1. Oil-oil correlation using biomarkers, δC and δD

Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil-oil correlation of the area is established. Gas chromatography-mass spectrometry (GC-MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C₃₀ diahopane/C₂₉Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C₁₉-tricyclic and C₂₄-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ¹³C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C₂₃-tricyclic/C₃₀ hopane, C₂₈-tricyclic/C₃₀ hopane, total tricyclic terpanes/hopanes and C₃₁(R + S)/C₃₀ hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C₄₁₊) and steranes.Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/n-C₁₇, Ph/n-C₁₈ and low API gravities.     

Constraints on hydrological and paleotemperature variations in the Eastern Mediterranean region in the last 140 ka given by the δD values of speleothem fluid inclusions

Speleothem fluid inclusions potentially provide vital data on the paleohydrological conditions in glacial and interglacial periods. We show here that δD analyses of fluid inclusions from speleothems in three caves in the Eastern Mediterranean region (Israel) provide a basis for understanding hydrological and temperature variations in the last 140 ka. Using measured δD fluid inclusion values and temperature ranges constrained by the present-day Mediterranean Meteoric Water Line (MMWL) and the global MWL, we show a strong compatibility between the land and marine temperature records, with the highest land temperatures occurring at 120–130 ka (17–22°C) and the present (18°C) and the lowest temperatures during the LGM, about 10°C lower than present. Interglacial waters are constrained to follow the MMWL and most glacial waters also plot close to the MMWL, with a slight shift towards the MWL. However, during two remarkable brief periods at the termination of the LGM, the waters plot on the MWL, suggesting that both sharp cooling and an increase in relative humidity above the EM Sea reduced d-excess values to those of the global system.     

Variations in composition and δ18O values within the Kaffo albite—Riebeckite granite of Liruei Complex,Younger Granites of Nigeria

The peralkaline Kaffo albite—riebeckite granite is an albitised, low-temperature intrusion in Liruei Complex, one of the oldest of the ring-complexes in the Younger Granite province of Nigeria. Analyses of borehole samples from different parts of the intrusion show it to be compositionally heterogeneous, especially in respect of Si, Al, Na, K and F distribution and this, in part, can be correlated with the variable degree of albitisation. Isotopically the granite is a normal plutonic type with δ¹⁸ O values of $\text{+ 8.1 ± 0.2‰}$, and albitisation does not seem to have been accompanied by exchange of isotopes between albitising fluid and the granite. Co-existing riebeckitic-arfvedsonite and aegirine pairs from borehole samples show extreme enrichment in Na and Fe; the amphibole also shows considerable substitution of Fe by Ti, Zn and Mn, and of OH by F. Isotopically the amphibole and pyroxene are different from others, having low, variable δ¹⁸ O values (+5.3–+6.4 and $\text{+4.4–+5.1‰}$, respectively), and the fractionation value Δ Px — Am is always large, negative and constant (—$\text{1.2 ± 0.2‰}$). The low δ¹⁸ O values are considered to be due to special features of the crystal chemistry of the alkali amphiboles and pyroxenes, and the spread of each set of values may be due to sub-solidus isotope exchange between the minerals and albitising fluid.     

Positive correlation between salinity and n-alkane δ13C values in the mangrove Avicennia marina

Carbon isotope ratio (δ¹³C) values of lipid biomarkers from plants can be used to assess water use efficiency and to reconstruct environmental conditions in the past. We assessed the effect of salinity on the δ¹³C values for leaf wax n-C₃₁ and n-C₃₃ alkanes, bulk leaf matter and leaf total lipid extracts from Avicennia marina (gray mangrove) trees growing along the Brisbane River estuary in Queensland, Australia. We observed an increase in 0.19 ± 0.053‰ (R² 0.61, p 0.008) and 0.16 ± 0.052‰ (R² 0.55, p 0.01) per salinity unit for the two n-alkanes, respectively, and of 0.087 ± 0.028‰ (R² 0.41, p 0.009) for whole leaves per salinity unit, indicating that water use efficiency of A. marina increased with the salt content of water. There was no correlation between δ¹³C values of total lipid extracts and salinity, perhaps because of a decrease in lipid concentration at higher salinity or because of varying contributions of different lipid classes to the extract. The robust relationship between salinity and δ¹³C values of leaf wax lipids provides a means of quantitatively reconstructing past salinity from carbon isotope ratios of mangrove lipid biomarkers in sediments. When paired with measurements of the hydrogen isotope ratio values of the same compounds, the approach should facilitate quantitative reconstruction of the hydrogen isotope composition of environmental water. In order for the method to successfully reconstruct past salinity and water isotopes, a mangrove source for leaf wax would need to be confirmed by palynological or other evidence, or the isotopic composition of a more source specific biomarker, such as taraxerol, would need to be measured.     

Effect of plant life form on relationship between δD values of leaf wax n-alkanes and altitude along Mount Taibai, China

We present δD_wax values from different forms of plants and soils, and δD_sw values from soil water along the northern slope of Mount Taibai, China. The results show a highly negative linear correlation of the δD_wax values for soils with altitude (R² 0.74) and we observed the same correlation for δD_sw values of soil water with altitude (R² 0.68). The δD_wax of living plants behaves like the soil, but does not exhibit a significant linear correlation with altitude (R² 0.11). The δD_wax values of woody plants and grasses also show a similar trend with respect to altitude with significant and no linear correlation, respectively (R² 0.50 for woody plants and 0.17 for grass), which suggest that the “altitude effect” can not be well documented for the δD_wax values of living plants, which may be due to differences in plant type and/or evapotranspiration controlled by the plant microclimate. The ε_wax-sw values of woody plants, grasses and soil show minor fluctuations with altitude. However, the ε_wax-sw and δD_wax values of woody plants are roughly 51‰ and 50‰ more positive, respectively, than those of grasses, suggesting that an “altitude effect” could be documented in the δD_wax of woody plants and grasses, with each responding independently to changes in precipitation along the altitude transect. Additionally, the ε_wax-sw values of soil are relatively constant with altitude, suggesting that the altitudinal change in the proportions between woody plant and grass input to soils will likely change the relationship between the δD_wax values of soil n-alkanes and altitude.     

Wet-dry seasonal and spatial variations in the δC and δO values of the modern endogenic travertine at Baishuitai, Yunnan, SW China and their paleoclimatic and paleoenvironmental implications

Seasonal and spatial variations in the δ¹³C and δ¹⁸O values of the modern endogenic (thermogene) travertine deposited in a calcite-depositing canal at Baishuitai, Yunnan, SW China were examined to understand their potential for paleoclimatic and paleoenvironmental implications. The sampling sites were set in the upstream, middle reach and downstream of the canal, and the modern endogenic travertine samples were collected semimonthly to measure their δ¹³C and δ¹⁸O values. It was found that both δ¹³C and δ¹⁸O values of the endogenic travertine were low in the warm rainy season and high in the cold dry season, and correlated with each other. The low δ¹⁸O values in warm rainy season were mainly related to the higher water temperature and the lower δ¹⁸O values of rainwater, and the low δ¹³C values are caused by the dilution effect of overland flow with low δ¹³C values in the warm rainy season and the reduced CO₂-degassing of canal-water caused by the dilution effect of the overland flow. The linear negative correlation between the travertine δ¹⁸O (or δ¹³C) values and rainfall amount may be used for paleo-rainfall reconstruction if one knows the δ¹⁸O (or δ¹³C) values of the fossil endogenic travertine at Baishuitai though the reconstruction was not straightforward. It was also found that there was a progressive downstream increase of the δ¹⁸O and δ¹³C values of the travertine along the canal, the former being mainly due to the preferential evaporation of H₂¹⁶O to the atmosphere and the latter to the preferential release of ¹²CO₂ to the atmosphere during CO₂-degassing. However, the downstream increase of the travertine δ¹⁸O and δ¹³C values was less intensive in rainy season because of the reduced evaporation and CO₂-degassing during the rainy season. To conclude, the downstream travertine sites could be more favorable for the paleo-rainfall reconstruction while the upstream travertine sites are more favorable for the paleo-temperature reconstruction. So, this study demonstrates that endogenic travertine, like epigenic (meteogene) tufa, could also be a good candidate for high-resolution paleoclimatic and paleoenvironmental reconstruction.     

Ground ice recharge via brine transport in frozen soils of Victoria Valley, Antarctica: Insights from modeling δO and δD profiles

Soils in the McMurdo Dry Valleys, Antarctica contain ice and considerable amounts of salt. Ice often occurs at shallow depth throughout the Dry Valleys and other areas of hyperarid permafrost, notably on Mars. This common occurrence of shallow ice is enigmatic; however, since according to published sublimation models it should disappear relatively quickly (at rates of order 0.1 mm a⁻¹) due to vapor loss to the atmosphere. This loss may be offset by recharge from snowmelt infiltrating and freezing in the soil. Herein, we present a first quantitative estimate of this recharge based on measured vertical profiles of δD and δ¹⁸O that reveal considerable detail about the sources and sinks of ice. We model these profiles, taking into account the salt content and a soil temperature record along a 1.6 m depth profile of ∼10 ka old ice-cemented soils in Victoria Valley, Antarctica. The stable isotopes of ice are enriched in heavy isotopes at the top of the ice cement (20 cm depth); both δD and δ¹⁸O values plotted against depth exhibit a concave upward curve. At depth, the isotope composition is similar to that of Lake Victoria and modern meteoric water. The concave shape of the isotope profile is suggestive of downward advection-dispersion of snowmelt water enriched in heavy isotopes into the ice cement. Our advection-dispersion model, coupled with field data, enables us to quantify the advective flux and dispersion of melt water into the ice. The advective velocity and dispersion coefficient depend on the time since advection began and the ice-to-brine ratio; they are, respectively, of the order of 10⁻¹¹-10⁻¹⁰ m s⁻¹ and 10⁻¹²-10⁻¹¹ m² s⁻¹. These values suggest that over the ∼10 ka time period, a total of 190 mm water infiltrated into the ice-cemented ground. The isotope composition and deuterium excess values of the uppermost ice cement can be modeled from snowmelt water enriched in salts using open system-Rayleigh fractionation. To develop the isotopic signature of the upper ice cement requires evaporation of ∼95% of the snowmelt water. Based on 190 mm brine infiltrating into the soil requires an initial total of ∼4 m of snowmelt water. This corresponds to ∼0.4 mm a⁻¹ suggesting that, under the current climate condition, water from snowmelt is sufficient to compensate modeled sublimation rates, and therefore conserve ground ice in Victoria Valley.     

Neutron diffraction study of aluminous hydroxide δ-AlOOD

We have determined the position of deuterium atoms in δ-AlOOD by neutron powder diffraction at ambient pressure. As previously reported by theoretical and experimental studies, the deuterium atoms are located in the tunnel formed by the chains of AlO₆ octahedra. The data are best fit with the P2₁ nm structure, producing bond lengths of D–O1 of 1.552(2) Å, O2–D of 1.020(2) Å and O1–O2 of 2.571(2). This study confirms that the hydrogen bond is asymmetric at ambient conditions in agreement with recent single-crystal synchrotron study for δ-AlOOH.     

Li, Be, B concentrations and δ7Li values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece)

Li, Be, B and δ⁷Li SIMS analyses of plagioclase phenocrysts from the 1040–1941 Niki dacite lava (Nea Kameni, Santorini, Greece) exhibit varied processes. From their anorthite contents alone, the crystals may be segregated into four main types: type-N shows the normal decline in An during crystallisation (An_62–40); type-O has only oscillatory zoning accompanied by resorption surfaces (An_58–39); type-C is complex with high-An cores (subtype C1: An_64–58, subtype C2: An_88–73) and normal rims (An_55–42). Type-A plagioclase with high An content (An_92–82) is found within mafic enclaves. On the basis of their Li concentrations, type-O crystals may be subdivided into subtype O1 with flat Li concentration profiles and subtype O2 with decreasing Li concentration from core to rim. The concentrations of Be and B of all four types show a negative correlation with anorthite content (An), but Li concentration profiles differ amongst the different plagioclase types. Types N and O1, and the cores of type-C, are equilibrated in Li concentration. Types O2 and A, and the mantles of type-C display an initial enrichment in Li, probably from volatile influx into the melt. Consistent with the propensity towards equilibrium with the melt, these crystals display dramatic rim-ward declines in Li concentration. All analysed plagioclase crystals, except for the xenocrystic type-A, have nearly the same Li, Be and B concentrations at their rims. These coincide with the composition of plagioclase microlites in the groundmass, thereby affording estimates of plagioclase-melt partitioning for the light elements: K _Li = 0.19–0.28, K _Be = 0.24–0.38 and K _B = 0.007–0.009. δ⁷Li profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to ~39 ‰, revealing preferential kinetic diffusion. This may have been driven by Li loss from the melt, most likely through degassing during decompression, perhaps in the course of magma ascent to subsequent eruption. Considering the rapid diffusion of Li in plagioclase, in situ phenocryst analyses may yield useful information about processes leading up to, or even causing, eruptions.