Modeling speleothem δC variability in a central Sierra Nevada cave using C and Sr/Sr

Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid-rock interaction in the host limestone, degassing of CO₂ and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water-rock interaction and soil processes in controlling stalagmite δ¹³C. This approach uses the dead carbon proportion (dcp) estimated from coupled ¹⁴C and ²³⁰Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water-rock interaction and soil processes in determining stalagmite δ¹³C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased δ¹³C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals.We use geochemical and isotope mixing models to estimate the host-carbonate contribution throughout the δ¹³C time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite. The degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators, with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals. Modeled δ¹³C values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite δ¹³C values, even during relatively wet intervals. These results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem δ¹³C records, particularly those from caves that formed in seasonal semi-arid to arid environments.     

Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245-0 ka) from the Niger and Congo deep-sea fans, however, reveal that C_org/N_tot ratios and isotopic signatures of bulk organic matter (δ¹³C_org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C_org/N_tot and δ¹³C_org (up to −17‰) were driven by generally nitrogen-poor but ¹³C-enriched terrigenous plant debris and SOM from C₄/C₃ vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C₃/C₄ vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C_org/N_tot and δ¹³C_org (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and ¹²C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C_org/N_tot and δ¹³C_org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO₂.     

Rayleigh distillation and the depth profile of C/C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC (δ¹³C_SOC) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC (C) and δ¹³C_SOC. Within 1 m of the soil surface, δ¹³C_SOC in the coarse-textured soil increases by 1.3 to 1.6‰, while δ¹³C_SOC from the fine-textured soil increase by as much as 3.8 to 5.5‰. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of δ¹³C_SOC in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of ¹³C in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of δ¹³C_SOC in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios.     

Loading and fate of particulate organic carbon from the Himalaya to the Ganga-Brahmaputra delta

We use the evolution of river sediment characteristics and sedimentary C_org from the Himalayan range to the delta to study the transport of C_org in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C_org content in order to evaluate the controls on C_org loading. Contributions of C3 and C4 plants are estimated from C_org stable isotopic composition (δ¹³C_org). We use the evolution of δ¹³C_org values from the Himalayan range to the delta in order to study the fate of C_org during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar C_org content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C_org loading. The general correlation between the total C_org content and Al/Si ratio indicates that C_org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ¹³C_org values. In bed sediments, δ¹³C_org values are compatible with a dominant proportion of fossil C_org derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ¹³C_org values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ¹³C_org values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ¹³C_org values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C_org coming from the Himalaya is oxidised and replaced by floodplain C_org. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.     

Episodes of late Holocene aridity recorded by stalagmites from Devil's Icebox Cave, central Missouri, USA

Two stalagmites from Devil's Icebox Cave, central Missouri, display similar δ¹³C and δ¹⁸O values and trends during the late Holocene. Positive δ¹³C excursions at 3.5-2.6 ka and 1.2-0.9 ka are interpreted to reflect drier conditions. These elevated stalagmite δ¹³C values could have plausibly been driven by increasing C₄ plant abundances over the cave or an increased contribution of bedrock carbon, both of which could reflect decreased effective moisture. A lack of corresponding oxygen isotopic anomalies during these intervals suggests that neither mean annual temperature nor the seasonality of precipitation changed concomitantly with dryness. Both of the δ¹³C excursions identified in our stalagmite record are roughly coincident with dry intervals from a number of sites located across the Great Plains.     

Stable isotopes of pedogenic carbonates from the Somma–Vesuvius area,southern Italy,over the past 18 kyr: palaeoclimatic implications

Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ¹⁸O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ¹⁸O of pedogenic carbonate recorded after this eruption. The δ¹³C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO₂, and the pedogenic carbonate. Carbon isotopes suggest prevailing C₃ vegetation and negligible mixing with volcanogenic or atmospheric CO₂. Copyright © 2000 John Wiley & Sons, Ltd.     

Stable isotopic insights into paleoclimatic conditions and alluvial depositional processes in the Kaiparowits Formation (Campanian,south-central Utah,U.S.A.)

The Kaiparowits Formation contains an exceptionally rich history of tectonic, climatic, and biologic conditions within the Western Interior of North America during the Campanian. Here we reconstruct aspects of the southern Cordilleran foreland basin's paleohydrology using δ¹⁸O and δ¹³C values determined from unionoid bivalve shells and pedogenic carbonate nodules derived from a suite of lithofacies associations. Unionoid shells derived from fluvial deposits display average water δ¹⁸O estimates of −13.7‰ ± 2.1 (1σ) (VSMOW) and shell δ¹³C values of −4.0‰ ± 1.5 (VPDB), whereas pedogenic carbonate nodules display average values of −6.0‰ ± 0.5 and −8.7‰ ± 0.8, respectively. Unionoid shells derived from pond deposits fall in between the two other environments with average values of −9.5‰ ± 1.8 and −5.7‰ ± 2.1, in δ¹⁸O and δ¹³C values respectively. Water δ¹⁸O estimates are interpreted to represent high altitude runoff within river systems, low elevation precipitation within the basin onto floodplain soils, and varying degrees of mixing between these two components within floodplain ponds. δ¹³C values track the isotopic composition of dissolved inorganic carbon within river, soil, and pond waters with high values likely reflecting greater contribution from chemically weathered marine carbonates exposed in the hinterland and lower values reflecting greater contributions from the in situ degradation of plant matter. Up-section there is a shift to lower δ¹⁸O values and higher δ¹³C values in fluvially-derived unionoid shells that post-dates an incursion of the Western Interior Seaway, but coincides with a shift in sediment provenance, an increase in basin sedimentation rates, and a change to a more anastomosed-style channel morphology within the basin foredeep depocentre. By combining the isotopic patterns with previously published sedimentologic, climate model, and paleofloral records we find: 1) additional evidence for humid, wet, and potentially monsoonal conditions within the region, 2) support for a tectonic uplift event, potentially related to Laramide deformation, and 3) greater aggradation and overbank flooding within the alluvial system in response to the uplift event.     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.     

Chemical and isotopic tracers of the contribution of microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin

In this study, the geochemistry and origin of natural gas and formation waters in Devonian age organic-rich shales and reservoir sandstones across the northern Appalachian Basin margin (western New York, eastern Ohio, northwestern Pennsylvania, and eastern Kentucky) were investigated. Additional samples were collected from Mississippian Berea Sandstone, Silurian Medina Sandstone and Ordovician Trenton/Black River Group oil and gas wells for comparison. Dissolved gases in shallow groundwaters in Devonian organic-rich shales along Lake Erie contain detectable CH₄ (0.01–50.55 mol%) with low δ¹³C–CH₄ values (−74.68 to −57.86‰) and no higher chain hydrocarbons, characteristics typical of microbial gas. Nevertheless, these groundwaters have only moderate alkalinity (1.14–8.72 meq/kg) and relatively low δ¹³C values of dissolved inorganic C (DIC) (−24.8 to −0.6‰), suggesting that microbial methanogenesis is limited. The majority of natural gases in Devonian organic-rich shales and sandstones at depth (>168 m) in the northern Appalachian Basin have a low CH₄ to ethane and propane ratios (3–35 mol%; C₁/C₂ + C₃) and high δ¹³C and δD values of CH₄ (−53.35 to −40.24‰, and −315.0 to −174.6‰, respectively), which increase in depth, reservoir age and thermal maturity; the molecular and isotopic signature of these gases show that CH₄ was generated via thermogenic processes. Despite this, the geochemistry of co-produced brines shows evidence for microbial activity. High δ¹³C values of DIC (>+10‰), slightly elevated alkalinity (up to 12.01 meq/kg) and low SO₄ values (<1 mmole/L) in select Devonian organic-rich shale and sandstone formation water samples suggest the presence of methanogenesis, while low δ¹³C–DIC values (<−22‰) and relatively high SO₄ concentrations (up to 12.31 mmole/L) in many brine samples point to SO₄ reduction, which likely limits microbial CH₄ generation in the Appalachian Basin. Together the formation water and gas results suggest that the vast majority of CH₄ in the Devonian organic-rich shales and sandstones across the northern Appalachian Basin margin is thermogenic in origin. Small accumulations of microbial CH₄ are present at shallow depths along Lake Erie and in western NY.     

Diagenesis and geochemistry of porites corals from Papua New Guinea: Implications for paleoclimate reconstruction

Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post-depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, δ¹⁸O, and δ¹³C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and δ¹⁸O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and −5.2 to −8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in δ¹⁸O of calcite relative to coral aragonite is a function of the δ¹⁸O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from −2.5 to −10.4%. The variability of δ¹³C in secondary calcite reflects the amount of soil CO₂ contributing ¹³C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on δ¹⁸O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral δ¹⁸O-SST is relatively small (−0.2 to 0.2°C per percent calcite). We show that large shifts in δ¹⁸O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions.     

Intensified mid-Holocene Asian monsoon recorded in corals from Kikai Island, subtropical northwestern Pacific

Coupled records of Sr/Ca and oxygen isotope ratios (δ¹⁸O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ¹⁸O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ¹⁸O_seawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ¹⁸O_seawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.     

Guano-derived δ13C-based paleo-hydroclimate record from Gaura cu Musca Cave,SW Romania

The δ¹³C values of 23 unevenly spaced guano samples from a 17-cm long clay sediment profile in Gaura cu Musc? Cave (GM), in SW Romania, made it possible to preliminarily characterize the Medieval Warm Period summer hydroclimate regime. The beginning of the sequence (AD 990) was rather wet for more than a century, before becoming progressively drier. After a brief, yet distinct wet period around AD 1170, drier conditions, with a possible shift from C₃ to a mixed C₃-dominated/C₄ type vegetation (2 ‰ lower δ¹³C values), prevailed for almost half a century before the climate became colder and wetter at the onset of the Little Ice Age, when bats left the cave. The guano-inferred wet and dry intervals from the GM Cave are mirrored by changes in the color and amount of clay accumulated in the cave. They also agree well with reconstructions based on pollen and charcoal from peat bogs and δ¹³C and δ¹⁸O on speleothems from other Romanian sites. Overall, these results indicate that the δ¹³C of bat guano can provide a sensitive record of the short-term coupling between local/regional climate and the plant–insect–bat–guano system.     

Tracking the fate of dung-derived carbohydrates in a temperate grassland soil using compound-specific stable isotope analysis

Carbohydrates are major organic components of dung and are likely to contribute substantially to increased carbon stocks in manured soils. To investigate this hypothesis, a field-scale experiment was conducted on a temperate grassland site in Devon, UK. C₄ dung (bulk δ¹³C value ?12.6‰) was applied to a temperate grassland C₃ soil (bulk δ¹³C value ?30.3‰) in April and the surface soil beneath cow pats sampled at seven dates over a year. Total carbohydrates were extracted as their monosaccharide components and analysed as the alditol acetates using gas chromatography. The δ¹³C values of the major monosaccharides glucose (?11.5 ±0.6‰), xylose (?10.4 ±0.4‰), arabinose (?10.4 ±0.5‰) and galactose (?8.3 ±1.6‰) extracted from the C₄ dung via acid hydrolysis were indicative of their source. Their weighted mean δ¹³C value was ?10.8‰, 1.8‰ more ¹³C-enriched than the bulk dung value. The δ¹³C values of individual monosaccharides recovered by acid hydrolysis in the 0–1 cm and 1–5 cm soil horizons beneath C₄ cow pats, compared with control soils determined over 372 days, allowed assessment of the extent of incorporation and fluxes of dung-derived monosaccharides. A maximum of 60% of the dung C in soil was derived from carbohydrates after 56 days, declining to around 20% after 372 days. Incorporation dynamics varied between monosaccharide species. Glucose, xylose and arabinose behaved in a similar manner because of their predominantly plant cell wall derived provenance in the dung, whilst dung-derived galactose and mannose appeared to have a microbial source in the soil. The dynamics of total dung-derived monosaccharides in the top 5 cm was comparable to incorporation and flux of bulk dung C, previously estimated using bulk δ¹³C values. The movement of dung-derived carbohydrates into the soil was inequivalent between the 0–1 cm and 1–5 cm horizons. The lack of a significant difference in concentration, but the evidence for the persistence of dung-derived monosaccharides in soil based on δ¹³C values, indicated replacement of existing pools in the soil, suggesting that the ability of this particular soil to sequester further C derived from carbohydrates was limited.     

Carbon mass-balance modelling and carbon isotope exchange processes in dynamic caves

Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of ¹³C and ¹²C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ¹³C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO₂, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO₂ concentrations and tested against the carbon isotope data. Air advection forces winter pCO₂ to drop in the cave air to ca. 500 ppm from a summer peak of ca. 1500 ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4 days. The process of cave ventilation forces degassing of CO₂ from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. ¹³C-enrichment of dripwater whose δ¹³C_DIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ¹³C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO₂ evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic ¹³C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO₂ between cave water and cave air. This implies that speleothem δ¹³C values may not be simply interpreted either in terms of hydrology or soil processes.     

Nine thousand years of upper montane soil/vegetation dynamics from the summit of Caratuva Peak,Southern Brazil

Biodiversity loss, climate change, and increased freshwater consumption are some of the main environmental problems on Earth. Mountain ecosystems can reduce these threats by providing several positive influences, such as the maintenance of biodiversity, water regulation, and carbon storage, amongst others. The knowledge of the history of these environments and their response to climate change is very important for management, conservation, and environmental monitoring programs. The genesis of the soil organic matter of the current upper montane vegetation remains unclear and seems to be quite variable depending on location. Some upper montane sites in the very extensive coastal Sea Mountain Range present considerable organic matter from the late Pleistocene and other from only the Holocene. Our study was carried out on three soil profiles (two cores in grassland and one in forest) on the Caratuva Peak of the Serra do Ibitiraquire (a sub-range of Sea Mountain Range – Serra do Mar) in Southern Brazil. The δ¹³C isotopic analyses of organic matter in soil horizons were conducted to detect whether C₃ or C₄ plants dominated the past communities. Complementarily, we performed a pollen analysis and ¹⁴C dating of the humin fraction to obtain the age of the studied horizons. Except for a short and probably drier period (between 6000 and 4500 cal yr BP), C₃ plants, including ombrophilous grasses and trees, have dominated the highlands of the Caratuva Peak (Pico Caratuva), as well as the other uppermost summits of the Serra do Ibitiraquire, since around 9000 cal yr BP. The Caratuva region represents a landscape of high altitude grasslands (campos de altitude altomontanos or campos altomontanos) and upper montane rain/cloud forests with soils that most likely contain some organic matter from the late Pleistocene, as has been reported in Southern and Southeastern Brazil for other sites. However, our results indicate that the studied deposits (near the summit) are from the early to late Holocene, when somewhat wetter and warmer conditions (since around 9000 cal yr BP) enabled a stronger colonization of the ridge of Pico Caratuva by mainly C₃ plants, especially grassland species. However, at the same time, even near the summit, the soil core from the forest site already presented the current physiognomy (or a shrubby/elfin or successional forest), indicating that the colonization of the neighboring uppermost saddles and valleys were probably populated mainly by upper montane forest species.     

The impact of climate changes during the Holocene on vegetation in northern French Guiana

The impact of climatic changes that occurred during the last glacial maximum and the Holocene on vegetation changes in the Amazon Basin and the Guiana Shield are still widely debated. The aim of our study was to investigate whether major changes in vegetation (i.e. transitions between rainforests and C₄ savannas) occurred in northern French Guiana during the Holocene. We measured variations in the δ¹³C of soil organic matter at eight sites now occupied by forest or savannah. The forest sites were selected to cover two regions (forest refugia and peneplains) which are thought to have experienced different intensities of disturbance during the latest Pleistocene and the Holocene. We found that none of the forest sites underwent major disturbances during the Holocene, i.e. they were not replaced by C₄ savannahs or C₄ forest savannahs for long periods. Our results thus suggest that tropical rainforests in northern French Guiana were resilient to drier climatic conditions during the Holocene. Nevertheless, geographical and vertical variations in the ¹³C of SOM were compatible with minor changes in vegetation, variations in soil processes or in soil physical properties.     

Fingerprinting of soil organic matter as a proxy for assessing climate and vegetation changes in last interglacial palaeosols (Veldwezelt, Belgium)

Soil characteristics in palaeosols are an important source of information on past climate and vegetation. Fingerprinting of soil organic matter (SOM) by pyrolysis-GC/MS is assessed as a proxy for palaeo-reconstruction in the complex of humic layers on top of the Rocourt pedosequence in the Veldwezelt-Hezerwater outcrop (Belgian loess belt). The fingerprints of the extractable SOM of different soil units are related to total organic carbon content, δ¹³C and grain-size analysis. Combined results indicate that the lower unit of the humic complex reflects a stable soil surface, allowing SOM build-up, intensive microbial activity and high decomposition. Higher in the profile, decomposition and microbial activity decrease. This is supported by a shift in the isotopic signal, an increased U ratio and evidence of wildfires. Although the chemical composition of the extracted SOM differed greatly from recent SOM, fingerprinting yielded detailed new information on SOM degree of decomposition and microbial contribution, allowing the reconstruction of palaeo-environmental conditions during pedogenesis.     

Sources of carbon isotope variation in kangaroo bone collagen and tooth enamel

The stable carbon isotopic composition (expressed as δ¹³C) of herbivore remains is commonly used to reconstruct past changes in the relative abundance of C₄ versus C₃ grass biomass (C₄ relative abundance). However, the strength of the relationship between herbivore δ¹³C and C₄ relative abundance in extant ecosystems has not been thoroughly examined. We determined sources of variation in δ¹³C of bone collagen and tooth enamel of kangaroos (Macropus spp.) collected throughout Australia by measuring δ¹³C of bone collagen (779 individuals) and tooth enamel (694 individuals). An index of seasonal water availability, i.e. the distribution of rainfall in the C₄ versus C₃ growing seasons, was used as a proxy for C₄ relative abundance, and this variable explained a large proportion of the variation in both collagen δ¹³C (68%) and enamel δ¹³C (68%). These figures increased to 78% and 77%, respectively, when differences between kangaroo species were accounted for. Vegetation characteristics, such as woodiness and the presence of an open forest canopy, had no effect on collagen or enamel δ¹³C. While there was no relationship between collagen δ¹³C and kangaroo age at death, tooth enamel produced later in life, following weaning, was enriched in ¹³C by 3.5‰ relative to enamel produced prior to weaning. From the observed relationships between seasonal water availability and collagen and enamel δ¹³C, enrichment factors (ε^∗) for collagen-diet and enamel-diet (post-weaning) were estimated to be 5.2‰ ± 0.5 (95% CI) and 11.7‰ ± 0.6 (95% CI), respectively. The findings of this study confirm that at a continental scale, collagen and enamel δ¹³C of a group of large herbivores closely reflect C₄ relative abundance. This validates a fundamental assumption underpinning the use of isotopic analysis of herbivore remains to reconstruct changes in C₄ relative abundance.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

Environmental significance of C/C and O/O ratios of modern land-snail shells from the southern great plains of North America

¹³C/¹²C and ¹⁸O/¹⁶O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east-west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ¹⁸O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ¹³C values ranged from −13.2‰ to 0.0‰. δ¹⁸O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ¹⁸O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ¹⁸O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ¹³C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ¹³C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C₃, C₄) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems.