δ13C Values Of Organic Matter Research Articles

The Late Neogene distribution of C 3 -C 4 plants in the Himalayan foreland basin: Insight from the δ 13 C values and sedimentological architecture of the Siwaliks

Abstract Recent studies emphasize that in addition to climate-driven forces, sediment grain size and depositional setting with respect to mountain front significantly influenced the abundance of late Neogene C 3 -C 4 plants in the Himalayan Foreland Basin (HFB). The contrasting depositional settings of the Siwalik Group exposed across the western, central and eastern HFB therefore provide an ideal opportunity to understand the influence of sedimentary architecture on the distribution of C 3 -C 4 plants in paleolandscapes. Towards this end, we generate new δ 13 C soil carbonate data from Siwaliks of the Katilukhad region (12 Ma to 6 Ma) of Kangra sub-basin and synthesize these data with compiled sedimentological data and δ 13 C values of organic matter, soil carbonate and n -alkane data from western to eastern HFB Siwalik Group. Our comparison suggests that the rate and magnitude of positive shift in the 13 C/ 12 C ratios were higher in the floodplain-dominated Siwaliks. Despite an existing conducive climate in the late Neogene for the growth of C 4 plants, the channel-fill-dominated Siwaliks favored C 3 over C 4 plants in the eastern HFB. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7168691

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Chemolithoautotrophic Organic Matter Contributions to Subterranean Food Webs Dominated by Filter-feeders

Reconstruction of landscape and climate of the largest drainage basin in the Ladakh Range, NW Trans Himalaya during the last 7000 years

Landform geomorphology and glacial lake deposits in the largest drainage basin of the Ladakh Range i.e. Chang La-Tangtse basin were studied to infer their palaeoclimatic significance. The grain size, mineral magnetism, percentage loss on ignition (%LOI) and organic carbon stable isotope (δ13C) data in combination with total organic carbon (TOC) supported by AMS 14C (calibrated) dates of a 190 cm long section from the Chang La-Tsoltak palaeolake provides a climatic record since the last ∼ 7075 cal yr BP. The χlf, χARM, and SIRM values suggest that the catchment-derived palaeolake sediments predominantly contain magnetically “soft” minerals like magnetite and maghemite. The δ13C values range between −21 and −24 ‰ with an average of –22 ‰ which suggests a mixed C3-C4 plant signature and water stressed ecosystem. The relatively small variations in the δ13C values of organic matter in the entire lake profile suggest a stable climatic condition. The prominent effect of westerlies is seen between 7075 and 6040 cal yr BP with huge detrital influx at the lake bottom indicating a glacial advancement in the region. The affect of Mid-Holocene warm period is evident at 6040 cal yr BP with the advent of ISM. Paradigm shifts in the proxy values are observed at 5710, 4890, 3435, and 2800 cal yr BP. The influence of westerlies gradually reduces at 2800 cal yr BP. The landscape evolution and the climatic variations in the Trans-Himalaya are primarily governed by westerlies and do not correspond to the Indian monsoon variability records, particularly during the Mid-Holocene Thermal Maxima. Several other regions of the Ladakh Range also record similar climatic variations, indicating that the palaeolake sediments also reflect regional climate variations.

Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective.

The Bohai Sea, adjacent to the Northwest Pacific, is a semi-enclosed shallow-water marginal sea that was considered on a critical path of eutrophication and environmental degradation. To better understand the Bohai Sea metabolism-induced summertime dissolved oxygen (DO) decline, five field surveys were conducted between July 2019 and July 2021 to investigate the seasonal/interannual and spatial variations in DO, dissolved inorganic carbon (DIC), and stable isotopic composition of DIC (δ13CDIC). Although the water-mixing scheme was subject to spatial variation, a uniform apparent ratio of δ13CDIC versus apparent oxygen utilization was estimated at -0.0122‰ per μmol O2 kg-1 in the Bohai Sea in summer. Based on a three-endmember water-mixing model and the mass balance of DIC and its stable isotopic composition, the assumed uniform δ13C values of oxygen-consuming organic matter in the Bohai Sea DO-deficient areas was estimated to be -19.47 ± 1.85‰ in 2020 and between -20.6‰ and - 18.1‰ in 2021. This isotopic composition is very similar to the δ13C value of organic matter from marine diatoms, but different to that of terrestrial organic matter sources surrounding the Bohai Sea. Our results indicate that nearly all the organic matter consumed by community respiration in the Bohai Sea is produced in situ by marine plankton. To mitigate the seasonal DO shortage in the Bohai Sea, reduction of allochthonous nutrients is crucial.

Atmospheric pCO2 Reconstruction of Early Cretaceous Terrestrial Deposits in Texas and Oklahoma Using Pedogenic Carbonate and Occluded Organic Matter

Pedogenic carbonate samples collected from three Lower Cretaceous (Aptian–Albian) fossil localities in Texas and Oklahoma were analyzed to develop paleoatmospheric pCO2 estimates by measuring the stable carbon isotopes of pedogenic calcite and their co-existing occluded organic matter. Calcite δ13C values ranged from −10.9‰ to −4.4‰ while occluded organic matter δ13C values ranged from −27.3‰ to −21.1‰. These stable carbon isotope measurements combined with temperature (30 °C) and soil-respired CO2 concentration (839–6047 ppmV) values provided atmospheric pCO2 estimates ranging from 67 ppmV to over 1100 ppmV. These estimates show a significant increase in atmospheric pCO2 during the late Aptian followed by a decrease in atmospheric pCO2 during the late Aptian to early Albian transition period, roughly correlating with the OEA1b event. Given the lack of chronostratigraphic constraints of the Lower Cretaceous geologic units in the study area, these data provide further evidence for the approximate age of the units as well as pertinent paleoclimate insights into greenhouse climate conditions.

Contribution of deep-sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from radiocarbon and stable isotope geochemistry

Sulfate-driven anaerobic oxidation of methane (AOM) limits the release of methane from marine sediments and promotes the formation of carbonates close to the seafloor in seepage areas along continental margins. It has been established that hydrocarbon seeps are a source of methane, dissolved inorganic carbon, and dissolved organic carbon to marine environments. However, questions remain about the contribution of deep-sourced carbon from hydrocarbon seeps to the sedimentary organic carbon pool. In this study, we analyzed carbon quantity, radiocarbon content (as percent modern carbon, pMC), stable carbon isotopic compositions (as δ13C) of organic matter enclosed within seep carbonates from the Gulf of Mexico and the South China Sea to assess if sediment organic matter may be used as a proxy for methane seepage intensity. The δ13C values of organic matter (δ13Corg) exhibited a large range from −81.4‰ to −23.9‰. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% pMC, suggesting organic matter of the carbonates is a mixture of marine particulate organic matter (δ13C = −22‰ VPDB and 90% modern carbon) and biomass resulting from methane oxidation (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity and duration are the most important factors controlling the contribution of methane-derived carbon to the sedimentary column. This study reinforces the potential for using δ13C values of organic carbon to discern methane-rich environments in ancient sedimentary environments where authigenic carbonate is not present and to constrain the record of AOM through Earth history.

Differentiating early from later diagenesis in a Cretaceous sandstone and petroleum reservoir of the Cedar Mountain Formation, Utah

ABSTRACT Previously published anomalous whole-rock stable isotopic values from the Poison Strip Sandstone Member of the Cretaceous Cedar Mountain Formation (CMF) of eastern Utah are of uncertain origins. This study investigated the diagenetic history and the processes responsible for these anomalous data. Accordingly, we integrated photomicroscopic techniques including polarized light microscopy, epifluorescence and cathodoluminescence (CL) imaging, micromilling of stable isotope samples, and fluid-inclusion heating and freezing measurements to this end. The key observations involved the microscopic mapping of calcite cement stratigraphy using CL imaging to permit the analysis of stable isotopes of calcite cements that crystallized during early and late diagenesis. The mapping of calcite cement zones of sufficient submillimeter size to mill out and isolate microgram-sized stable isotope samples enabled this discrimination. Early diagenetic calcite cements have the most positive δ18O values (-10 to -8.5‰ Vienna Pee Dee Belemnite [VPDB]) of all components. The pattern of δ13C and δ18O variation in this early diagenetic cement indicates affinities with early meteoric diagenesis previously documented in published literature on the CFM. The late diagenetic calcite cements yield the most negative δ18O values (-18 to -16‰ VPDB). We interpret the late diagenetic cements to be responsible for the anomalously low whole-rock δ18O values previously reported from the Poison Strip Sandstone Member. Our discoveries of bitumen in late-stage pore fillings and liquid petroleum in the fluid inclusions of late diagenetic calcite cements of the Poison Strip Sandstone Member explain the lower whole-rock organic matter δ13C values and anomalous Δ13C values reported from the unit. Comparatively lower carbonate δ18O and organic δ13C values originally derived from whole-rock analyses of samples from the Poison Strip Sandstone Member resulted from high-temperature basinal diagenesis (hydrothermal circulation and/or petroleum migration), rather than the alternative interpretation of early diagenesis related to a Cretaceous paleoclimatic perturbation. Our results are illustrative of methods to resolve the long-standing geologic problem of discriminating and characterizing products of early vs. late diagenesis in terrigenous clastic sedimentary strata.

Palaeofloral Investigation Based on Morphotaxonomy, Palynomorphs, Biomarkers and Stable Isotope from Lalmatia Coal Mine of Rajmahal Lower Gondwana Basin, Godda District, Jharkhand: An Inclusive Empirical Study

A rich Glossopteris dominated plant fossils assemblage is recovered from the Barakar Formation of Lalmatia coal mine, Hura coalfield of Rajmahal Basin, Godda district, Jharkhand, India. The assemblage includes Glossopteris, Gangamopteris and Noeggerathiopsis (leaf forms) along with Vertebraria (root form). There are 4 genera and 14 species of megafossils in the collection, which includes 10 species of Glossopteris, two species of Gangamopteris and one species of Noeggerathiopsis and Vertebraria each. The flora, although similar to that of the Barakar Formation of the Damodar Basin complex, exhibit unique characteristics and is of Artinskian to Kungurian in age. The palynoassemblage recovered in the section reveals the dominance of genus Scheuringipollenites and sub-dominance of Faunipollenites (=Protohaploxypinus) along with Parasaccites, Potonieisporites, Barakarites, Plicatipollenites, Striomonosaccites, Striatopodocarpites, Crescentipollenites, Verticipollenites, Rhizomaspora, Distriatites, Primuspollenites, Striasulcites, Lunatisporites, Guttulapollenites, Platysaccus, Microfoveolatispora, Microbaculispora, Horriditriletes, Tiwariasporis and Weylandites, which demonstrates an early Permian age (Artinskian). The palynoassemblage indicates that the sediments belong to the lower part of the Barakar Formation. Biomarker analysis suggests varying organic matter input during the deposition of the studied sequence. Diterpane compounds and megafossil evidences suggest Glossopteris were the dominant flora along with conifers. Bacterial alteration of the organic matter is inferred from the hopane compounds. The δ13C value of organic matter associated with the carbonaceous shale directly corroborates with the typical δ13C values of the Permian time and is suggestive of warm and humid climatic conditions.

Early Holocene Indian summer monsoon and its impact on vegetation in the Central Himalaya: Insight from δD and δ13C values of leaf wax lipid

Increase in rainfall during the early Holocene has been reported from different regions of Indian subcontinent, although the magnitude shows spatial variability with lower change in higher rainfall regions and vice versa. While the early-Holocene rainfall records are available from lowland areas of the Indian sub-continent, the record is poorly documented from the high-altitude Central Himalayan region where orography plays an important role in the spatial variability in the rainfall pattern. For the first time, δDC29, δ13CC29 values of alkane and δ13C values of organic matter from a relict lake of Benital area in the Central Himalaya have been used to reconstruct monsoonal rainfall and contemporary vegetation for last 10.5 ka with an emphasis on the early Holocene. The δDC29 values suggest that the early Holocene was characterized by a wet phase at ca. 9 ka with 25% higher rainfall compared to present while the middle-late-Holocene was relatively arid. The estimated increase in monsoonal rainfall for the early Holocene is the highest compared to the previous report from elsewhere. The magnitude of early-Holocene rainfall estimated in this study suggests that in addition to erosion of the bed-rocks at the head-water region, increased stream power reworked the older floodplain and resulted in 2.3 times higher sediment discharged into the Ganges-Brahmaputra (GB) delta compared to the present discharge. The δ13CC29 values of the lake sediments suggest a transition from woody to non-woody plant assemblages occurred at 7 ka. The poor correlation between δDC29 and δ13CC29 values suggests at higher rainfall, the δ13CC29 values of catchment vegetation were less responsive.

Carbon and Nitrogen Burial and Response to Climate Change and Anthropogenic Disturbance in Chaohu Lake, China.

Lakes are a crucial component of the global carbon and nitrogen cycle. As a trend of enhanced human activities and climate change, the mechanisms of burial remain poorly understood. In this study, diverse biogeochemical techniques were applied to analyze the temporal variation of organic carbon and nitrogen burial rates in Chaohu Lake. The results showed that burial rates have ranged from 9.39 to 35.87 g C m−2 yr−1 for carbon and from 1.66 to 5.67 g N m−2 yr−1 for nitrogen since the 1860s. The average rates were 19.6 g C m−2 yr−1 and 3.14 g N m−2 yr−1 after the 1970s, which were significantly higher than the rate before the 1970s, showing an increasing trend. The decrease of C/N ratios as well as organic matter δ13C values indicates that the major organic matter source in sediment has been algal production since the 1970s. The increase of δ15N values indicated that the promotion in productivity was stimulated by nutrient input from sewage and agricultural runoff. The burial rates of organic carbon and nitrogen were significantly positively related to socio-economics and temperature, indicating that Chaohu Lake will become an increasing carbon and nitrogen pool under conditions of enhanced human activities and intensive precipitation.

High‐resolution paleoclimatic records spanning the past 30 cal ka BP inferred from Qionghai Lake sediments in south‐west China: Insights from geochemical investigations and grain‐size characteristics

There have been few high‐resolution investigations covering the last glacial and Holocene from Sichuan, Yunnan, or south Asia area in the south‐west monsoon domain region. High‐resolution analyses of lake sediments retrieved from Qionghai Lake, Sichuan Province, were conducted using a combination of variables including total organic carbon (TOC) and total nitrogen (TN) concentration, low frequency magnetic susceptibility (χLF), particle size, carbonate content, and δ13C value of organic matter (δ13Corg), to provide a new perspective on the detailed paleoclimate history of south‐west China spanning the past 30 cal ka BP. Our findings suggest that organic matter in the Qionghai Lake was mainly derived from microorganisms, aquatic macrophytes, and terrestrial C3 plants. From 29.1 to 23.2 cal ka BP, low carbonate contents, high median grain size (Md values), and χLF values indicated a warm and wet climate. During the period 23.2 to 15.4 cal ka BP, low χLF, Md values, and high carbonate contents suggested a decrease in temperature and humidity. The cold and dry Last Glacial Maximum (LGM) and Heinrich 1 event (H1) were captured between 21.6 and 19.4 cal ka BP and 17.6 and 15.8 cal ka BP, respectively. During the interval 15.4 to 9.7 cal ka BP, low level of χLF values and Md values, together with relatively high carbonates, demonstrated that a dry climate prevailed in this period. Since 9.7 cal ka BP, the climate was warm and humid in the early to mid‐Holocene and shifted towards cold and dry condition in the late Holocene. A cold and dry period at 8.5 to 8.1 cal ka BP and warm and wet interval at 6.6 to 4.6 cal ka BP were identified, corresponding to the 8.2‐ka cold event and Holocene climatic optimum (HCO), respectively. These climate changes may reflect variations of the intensity of the south‐west summer monsoon, primarily in response to the Northern Hemisphere solar insolation on a millennial scale induced by orbital forcing. The position of the Intertropical Convergence Zone (ITCZ) may add a complementary effect on the evolution of the monsoon strength in the Holocene.

Paleosols of the Permian-Triassic: proxies for rainfall, climate change and major changes in terrestrial tetrapod diversity

ABSTRACTStable carbon isotope analysis of coexisting soil calcite and organic matter sampled from modern, California soil profiles representing 18 different U.S. Department of Agriculture (USDA) official soil series yields 51 paired calcite–organic matter δ13C values (Δ13Ccc-om values). These paired values correspond to atmospheric pCO2 estimates ranging from less than ∼100 to 2200 ppmV using standard assumed soil pCO2 concentrations at temperatures spanning the typical range of modern soil calcite crystallization. Nevertheless, there is a strong negative correlation of Δ13Ccc-om values with mean annual precipitation (MAP) among these modern sites, offering a potential means to estimate paleo-MAP values from calcite and associated, occluded organic matter in paleosols. One hundred and twenty-five Δ13Ccc-om values from lower Permian–lower Middle Triassic paleosol profiles in the southwestern U.S.A., northwestern China, and Africa are presented as a proxy for paleorainfall over ∼85° of paleolatitude. The results indicate substantial spatiotemporal variation in MAP and appear to reflect the development and strengthening of global megamonsoonal atmospheric circulation associated with assembly of the supercontinent Pangea. The data also indicate extremely low rainfall values in regions with endemic tetrapod fossil assemblages, suggesting that climate change was a principal driver of biogeography, biodiversity, and evolution of tetrapod faunas across continental landscapes during Permo-Triassic time.Citation for this article: Tabor, N. J., C. A. Sidor, R. M. H. Smith, S. J. Nesbitt, and K. Angielczyk. 2018. Paleosols of the Permian-Triassic: proxies for rainfall, climate change, and major changes in terrestrial tetrapod diversity; pp. 240–253 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. Journal of Vertebrate Paleontology 37(6, Supplement).

Geoarchaeology and paleoecology of the deeply stratified Richard Beene site, Medina River valley, South-Central Texas, USA

A geoarchaeological investigation that included soil-stratigraphic and paleoecological analyses was conducted at Richard Beene, a deeply stratified open-air site associated with an alluvial terrace of the Medina River in south-central Texas. The sequence of cultural and alluvial deposits at the site is one of the most complete records of Holocene human occupation and sedimentation documented in North America, and is very important to the interpretation of the cultural history of the Inner Gulf Coastal Plain and adjacent areas of the Southern Plains. The most salient ecological characteristics of the site are its riparian setting and location in the ecotone between North America's western grasslands and eastern woodlands. The cultural record spans the past 10,000 years and is buried in 14 m of fine-grained flood deposits with multiple buried paleosols. Excavations totaling 730 m2 sampled 20 stratigraphically distinct archaeological components in valley fill beneath the Applewhite terrace, yielding more than 80,000 artifacts, including flakes, tools, bones, mussel shells, and fire-cracked rocks left by Archaic and Late Pre-Columbian hunter-gatherers. Seven primary stratigraphic units and five laterally traceable buried paleosols were identified at the site. The oldest stratigraphic unit consists of coarse-grained channel deposits that accumulated soon before 33,000 14C yr B.P. Most of the units, however, consist of fine-grained alluvium that accumulated between ca. 10,000 and 1000 14C yr B.P. The δ13C values of organic matter from the site's soil-stratigraphic sequence indicate increased C4 production during the Younger Dryas (11,000–10,000 14C yr B.P.), and C4 productivity generally increased throughout the Holocene, culminating in peak warm intervals at ca. 5000 and 2000 14C yr B.P. Although during relatively dry periods grasses expanded at the expense of trees, a mosaic of trees and grasses (i.e., a savannah) typical of a grassland-forest ecotone prevailed at the site throughout the Holocene. Such a food-rich environment characterized by an abundance of deer and geophytes, as well as mussels from the Medina River accounts for repetitive human occupation spanning at least 10,000 years.

Trophic relationships in apex predators in an estuary system: A multiple-method approximation

The trophic level and ecological role of species in the ecosystem are determined by their feeding habits. While the trophic level and niche width are characteristic at population level, trophic overlap indicates the interaction between populations of a community. Three apex predators coexist in Uruguayan waters: Franciscana dolphin (Pontoporia blainvillei), South American sea lion (SASL, Otaria flavescens) and South American fur seal (SAFS, Arctocephalus australis). The two otariids consume different main prey species and have different isotopic niche, whereas feeding habits of Franciscana dolphin in Uruguayan waters remain practically unknown. This study aimed to determine Franciscana diet composition by stomach content and stable isotope (δ13C and δ15N) analyses, and to assess trophic overlap in diet composition as well as in isotopic niche space among the three apex predators. Diet composition of each species was estimated by Bayesian mixing models with prior information on the diet of predators. Moreover, Bayesian ellipses were used to define the isotopic niche space and overlap among the three species. Diets among predators varied: diet of SAFS was composed mainly by a few pelagic prey species, diet of SASL was more diverse and included pelagic and benthic preys, and for Franciscana dolphin the diet was mainly composed by coastal prey species. The SAFS had the largest isotopic niche area compared to the Franciscana and the SASL. The isotopic niche of the SASL did not overlap with those of the other two species. Although the Franciscana and the SAFS overlap in isotopic space, they do not overlap in feeding areas; this is because particulate organic matter δ13C values increase from inner Rio de la Plata to the estuary and the Atlantic Coast. Here, the relevance of using different sources of information to reach strong conclusions about feeding habits of marine predators and about co-use of resources is shown, mainly in environments where variation in δ13C can be related to diverse reasons.

Sharp differences in the δ13C values of organic matter and carbonate encrustations but not in ambient water DIC between two morphologically distinct charophytes

The stable carbon isotope composition of the carbonate encrustations (δ13CCARB), organic matter (δ13CORG) and the dissolved inorganic carbon (δ13CDIC) in the ambient lake waters were analysed for two common but morphologically different and, thus, representing different growth forms of Chara species. We hypothesized that the relationships between δ13CCARB, δ13CORG and δ13CDIC are species specific and related to the different growth forms of the studied charophytes. For each species (Chara tomentosa and Chara globularis), 10 individuals and water samples from above the macrophytes were collected in five lakes at three sites per lake in mid-summer. Opposing shifts were found between δ13CCARB and δ13CDIC values with 13C enrichment in C. tomentosa and 13C depletion in C. globularis. In addition, C. globularis exhibited more negative values of δ13CORG than C. tomentosa, even under similar conditions. The δ13CCARB and δ13CORG values were positively correlated in both species, but δ13CCARB and δ13CDIC as well as δ13CORG and δ13CDIC were positively correlated in C. tomentosa only. The differences found result from the different proportions between 13C and 12C in DIC used as a CO2 source for photosynthesis, which is linked to the different growth forms represented by the two studied charophytes and, thus, are species specific as we hypothesized.

Seasonality and paleoecology of the late Cretaceous multi-taxa vertebrate assemblage of "Lo Hueco" (central eastern Spain).

Isotopic studies of multi-taxa terrestrial vertebrate assemblages allow determination of paleoclimatic and paleoecological aspects on account of the different information supplied by each taxon. The late Campanian-early Maastrichtian “Lo Hueco” Fossil-Lagerstätte (central eastern Spain), located at a subtropical paleolatitude of ~31°N, constitutes an ideal setting to carry out this task due to its abundant and diverse vertebrate assemblage. Local δ18OPO4 values estimated from δ18OPO4 values of theropods, sauropods, crocodyliforms, and turtles are close to δ18OH2O values observed at modern subtropical latitudes. Theropod δ18OH2O values are lower than those shown by crocodyliforms and turtles, indicating that terrestrial endothermic taxa record δ18OH2O values throughout the year, whereas semiaquatic ectothermic taxa δ18OH2O values represent local meteoric waters over a shorter time period when conditions are favorable for bioapatite synthesis (warm season). Temperatures calculated by combining theropod, crocodyliform, and turtle δ18OH2O values and gar δ18OPO4 have enabled us to estimate seasonal variability as the difference between mean annual temperature (MAT, yielded by theropods) and temperature of the warmest months (TWMs, provided by crocodyliforms and turtles). ΔTWMs-MAT value does not point to a significantly different seasonal thermal variability when compared to modern coastal subtropical meteorological stations and Late Cretaceous rudists from eastern Tethys. Bioapatite and bulk organic matter δ13C values point to a C3 environment in the “Lo Hueco” area. The estimated fractionation between sauropod enamel and diet is ~15‰. While waiting for paleoecological information yielded by the ongoing morphological study of the “Lo Hueco” crocodyliforms, δ13C and δ18OCO3 results point to incorporation of food items with brackish influence, but preferential ingestion of freshwater. “Lo Hueco” turtles showed the lowest δ13C and δ18OCO3 values of the vertebrate assemblage, likely indicating a diet based on a mixture of aquatic and terrestrial C3 vegetation and/or invertebrates and ingestion of freshwater.

Stable carbon isotope biogeochemistry of propionate and acetate in methanogenic soils and lake sediments

In anoxic environments, degradation of organic matter (OM) results in strong fractionation of carbon isotopes, with formation of 13C-depleted CH4. Propionate and acetate are important products of OM fermentation. Propionate is further fermented to acetate. Acetate is a direct precursor of CH4, the remainder usually being produced from H2-mediated CO2 reduction. There is a paucity of data for the turnover of acetate and, even more so, propionate. We therefore analyzed the δ13C values of organic carbon, propionate, acetate and the methyl (Me) group of acetate (acetate-Me) during the production of CH4 in anoxic incubations of various flooded and non-flooded soils and various lake sediments. Incubation in the presence of CH3F, which inhibits CH4 production from acetate, allowed exclusion of isotope effects during aceticlastic methanogenesis. Despite the variation inherent in the wide diversity of sample type and origin, the data collectively showed that the δ13C value of acetate was only marginally different (−2±5‰) from that of OM, while propionate was depleted in 13C relative to total acetate (−6±5‰). Acetate-Me was generally depleted in 13C relative to total acetate (−8±5‰). Thus, isotopic enrichment factors during the degradation of OM to total propionate and acetate were much smaller than those during hydrogenotrophic and aceticlastic methanogenesis or the intramolecular difference in δ13C between the carboxyl (CO2H) and Me of acetate, so that the δ13C value of OM may be used as a proxy when data for acetate are not available.

Influence of migration distance, maturity and facies on the stable isotopic composition of alkanes and on carbazole distributions in oils and source rocks of the Alpine Foreland Basin of Austria

The Austrian part of the Alpine Foreland Basin has been studied previously by organic geochemical methods to establish oil–oil and oil–source rock correlations. The lateral distance of oil fields to the position of mature source rocks (Lower Oligocene Schöneck and Eggerding formations) beneath the Alpine nappes in the south suggests long distance migration up to 50km. Minor compositional trends in W–E direction were obtained reflecting differences in depositional environment (facies) and maturity of potential source rocks. Therefore, the oil and source rock samples are thought to provide valuable insights into carbazole distributions in relation to migration distance, maturity and facies. Compound specific carbon isotope analyses on n-alkanes and isoprenoids reflect the W–E trend, already seen within the total hydrocarbon fractions. The observed trend toward lighter δ13C values of alkanes from oil fields in a W–E direction are consistent with lower δ13C values of organic matter in unit c and similar differences obtained from sediments of the Schöneck Formation at wells Oberhofen 1 (W) and Molln 1 (E). The δ13C patterns of n-alkanes and isoprenoids are in agreement with data obtained from the potential marine source rocks (Schöneck Formation) of the oils. The hydrogen isotopic composition of short chain n-alkanes (C15–19) of most oil samples are lower than the data obtained from the extracts of the marine Permian Kupferschiefer of the Polish Zechstein Basin at similar maturation stages. Only the oils from the western oil fields yield δD values of n-alkanes comparable to those obtained from marine source rocks. The data imply brackish water conditions during deposition of ‘unit c’ of the Schöneck Formation. Differences in the position of δD of isoprenoids in relation to the respective maturation trend line are most probably related to calibration errors. Alkylated carbazoles and benzocarbazoles are only present in sufficient amounts for quantification in oils sampled from the eastern fields. Rock extracts of the Schöneck Formation from drill cores Molln 1 (5200m depth, 0.92% Rr) and from the marginally mature (ca. 0.6% Rr) samples from well Oberhofen 1 (4300m depth) obtained sufficient amounts of carbazoles. The significantly different concentrations of benzocarbazoles in the extracts (around 10μg/g at Oberhofen 1; > 50μg/g at Molln 1) are related to maturity differences, whereas the predominance of 1-methylcarbazole in the Oberhofen samples is inferred to reflect differences in source rock facies. In the oils from the eastern deposits, neither the concentration ratios of methylcarbazoles, nor the benzocarbazole ratio show indications for the influence of facies differences. Maturity of oil expulsion is estimated on the basis of MPI 1 (Rc=0.76–0.88%), confirmed by sterane isomerization parameters. The results argue for a minor influence of maturity differences on the benzocarbazole ratios of the oil samples. The obtained ratios and the reservoir source fractionation parameters, calculated from the data of Molln 1 source rocks and the respective oils, show a systematic decrease with the estimated migration distance. The results argue for the applicability of benzocarbazoles for the assessment of relative differences in migration distances in oil plays characterized by minor differences in source rock facies and maturity during oil expulsion.

THE CARBON ISOTOPE ORGANIC GEOCHEMISTRY OF EARLY ORDOVICIAN ROCKS FROM THE ANNASCAUL FORMATION, COUNTY KERRY

Ireland is well known to geologists as containing some of the thickest successions of Early Ordovician (485—470 Ma; Walker et al 2012) sedimentary rocks in the world. The carbon stable isotope compositions (δ¹³C value) of similarly aged rocks have been reported for only very few places in the world (i.e., Argentina, southern China, and southern France), and no such analyses have been performed on the Early Ordovician, organic-rich rocks of Ireland. Here we report the δ¹³C values of bulk organic material and organic isolates recovered from the Annascaul Formation of Southwestern Ireland. Members of the Annascaul Formation spanning the Early Ordovician were sampled at multiple sites within five localities on the Dingle Peninsula, County Kerry. Mean bulk organic δ¹³C values for the organic matter within rocks of the Farranacarriga, Tinal, Illaunglass, Bealacoon, and Killelton Members ranged from — 28.7‰, (Farranacarriga) to — 22.1‰ (Illaunglass); the average δ¹³C values of organics isolated from the Farranacarriga and Bealacoon Members were — 29.0 and — 28.4‰, respectively. No statistical difference was observed between the δ¹³C value of isolates and the δ¹³C value of bulk sediment from which organics had been isolated (P= 0.85, Farranacarriga, N = 6; p = 0.81, Bealacoon, n = 5; paired Mest). The δ¹³C values we present here agree well with the previously published 5 l3 C values for Early Ordovician organic carbon (average = –28.4, –25.4, –25.5‰) from Argentina, southern China, and southern France (respectively). Our new data from the Annascaul Formation are ~3.3‰ higher than the average value reported for δ¹³C values of organic matter of marine origin for the same period, raising the possibility that terrestrial bryophytes (or other terrestrial photosynthesizers) contributed to these Early Ordovician sediments. Further equivocal evidence is provided by the abundant organic macerais found within the Farranacarriga Member with possible vitrinite origin. The δ¹³C values of palynomorphsized organic isolates from the Annascaul Formation reflect a marine origin for these organisms, consistent with their previous identification as acritarchs. Given the recognition of the earliest thalloid macrofossils and land-plant cryptospores in the Middle Ordovician sediments of the Appalachian basin (USA) and Argentina, respectively, our results highlight the Early-Middle Ordovician boundary as a potentially crucial time of terrestrial ecosystem expansion and development.

δ13C analysis of bulk organic matter in speleothems using liquid chromatography–isotope ratio mass spectrometry

The determination of δ13C values in speleothems is of considerable importance in palaeoenvironmental research, but has focussed solely on analysis of the carbonate. Here we demonstrate a new method for analysing the δ13C values of organic matter (OM) trapped in speleothems, utilising flow injection liquid chromatography–isotope ratio mass spectrometry (LC–IRMS). Developmental analysis using a homogenised speleothem powder showed that the method is robust, with repeated digests and analyses having an average standard deviation of 0.1‰. Dilution tests with samples of 4–23μg total organic carbon (TOC) show relatively small linearity effects, with the overall standard deviation across a peak response range of 1700–9000mV being 0.2‰.