δ13C Values Of Organic Carbon Research Articles

Sulfur sources for ultramafic‐hosted sulfide mineralization in the Tianzuo hydrothermal field, 63.5° E, South‐west Indian Ridge: Insights from sulfur and carbon isotopes

To date, discovered ultramafic‐hosted seafloor massive sulfide (UM–SMS) deposits are rare, remain poorly understood in terms of their genesis, and typically form along slow‐ and ultraslow‐spreading mid‐oceanic ridges. The Tianzuo hydrothermal field (THF) is the only UM–SMS deposit identified so far along the ultraslow‐spreading South‐west Indian Ridge (SWIR). This study presents new detailed mineralogical and S–C isotopic data for serpentinized peridotite, gabbro, serpentinite, and sulfide ore samples from the THF and to provide new insights into the processes that formed the deposit. Chromium‐reduced sulfur (CRS) and sulfate–sulfur (SS) in samples of serpentinized peridotite have positive δ34S values (up to 10.22‰ and 20.67‰, respectively). The CRS formed through thermochemical reduction of seawater sulfate during reactions between seawater and ultramafic rocks, whereas the SS most likely occurred as dissolved sulfate phases in seawater occupying the pore space of serpentinized peridotite after exposure at the seafloor. The serpentinite samples contain SS with δ34S values of 19.21‰–20.9‰, indicating infiltration by seawater that led to the deposition of seawater sulfate. The sulfide ores have acid‐volatile sulfur (AVS), CRS, and SS δ34S values of 10.99‰–12.07‰, 2.34‰–10.98‰, and −4.13‰ to 2.76‰, respectively, indicating that the AVS and CRS within the samples are dominated by sulfur reduced from seawater sulfate (e.g., anhydrite) and leached from the surrounding wall rocks, although the re‐oxidization of microbial‐reduced sulfur most likely generated the sulfate with low and negative δ34S values. The total carbon and total organic carbon δ13C values of the THF samples are negative and range from −21.99‰ to −0.34‰ and −26.7‰ to −21.02‰, respectively, further suggesting that the samples record microbial activity. Gabbro samples have strongly negative AVS (−10.69‰) and CRS (−7.2‰) δ34S values, indicating that microbial‐reduced sulfur derived from shallow levels in the crust was incorporated into the hydrothermal system that circulated through the deeper‐seated gabbroic units. Our results suggest that the ultramafic and gabbroic rocks, seawater, and microbial activity all provided sulfur to the THF, with well‐developed fractures and local magma supply probably being the key factors controlling the formation of this field and similar UM–SMS deposits elsewhere.

Downslope variation in hemipelagic sedimentation in an active margin basin: An example from the lower Pleistocene Kiwada and Takamizo formations on the Boso Peninsula, Japan

AbstractHemipelagites are widespread fine‐grained sediments found from shelf margin to deep‐water environments, and they are considered to have been formed as background sedimentation by vertical settling and slow lateral advection of clastic and biogenic fine‐grained particles. Downflow variation in hemipelagic sedimentation, however, is not yet clearly understood, and was investigated in the lower Pleistocene Kiwada and Takamizo formations in the Kazusa forearc basin on the Boso Peninsula, Japan. Mapping of three tephra key beds enabled detailed correlation of two hemipelagite beds over a distance of ca 30 km from shelf‐margin to basin‐plain environments. The sedimentation rates decreased gradually in the downslope direction, but increased locally in response to turbidite sedimentation. This downslope variation indicates that hemipelagic sedimentation is not necessarily constant and varies in response to progradation and retrogradation of deep‐water depositional systems, as well as additional input of fine‐grained sediments from turbidity currents. In contrast, the grain size, magnetic susceptibility, clay‐mineral composition, total organic carbon contents relative to total nitrogen contents (carbon/nitrogen), and δ13C values of organic carbon do not exhibit distinct downslope variation. This downslope consistency indicates that hemipelagites document temporal perturbations of geological and palaeoclimatic conditions in hinterlands, regardless of spatial and temporal variations in deep‐water depositional environments. The concentrations of total organic carbon, total nitrogen and five biomarkers were lower values in turbiditic mudstones and hemipelagites that formed above a turbidite bed than in those of the other hemipelagites. This variation suggests that an additional supply of fine‐grained sediments from turbidity currents likely diluted the organic matter in the hemipelagites. In addition, hemipelagites for which deposition was affected by turbidity currents exhibit slightly lower δ13C values, and may also document an additional contribution of terrestrial organic matter from turbidity currents.

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.

Stable carbon isotopic compositions and source apportionment of the carbonaceous components in PM2.5 in Taiyuan, China

To determine the stable carbon isotopic compositions (δ13C) and source apportionment of carbonaceous components in Taiyuan, PM2.5 samples were collected using medium flow samplers (TH-150C) during 4 months in 2014. The δ13C values of organic carbon (OC) and elemental carbon (EC) in PM2.5 were determined using an MAT-251 gas mass spectrometer with a dual inlet system. Moreover, δ13CEC values from industrial and domestic coal combustion were synchronously measured. In March, May, August, and December, the mean δ13COC values were −23.9 ± 0.9‰, −25.2 ± 0.2‰, −25.3 ± 0.6‰, and −22.9 ± 1.0‰, respectively; the corresponding δ13CEC values were −23.8 ± 0.8‰, −24.2 ± 0.5‰, −25.3 ± 0.4‰, and −23.8 ± 0.3‰; and the corresponding δ13CTC values were −23.8 ± 0.7‰, −24.8 ± 0.2‰, −25.3 ± 0.4‰, and −23.2 ± 0.6‰. Notable differences were observed in temporal distributions of stable carbon isotopic compositions of OC, EC, and total carbon (TC). The heavier δ13CEC values in December and March was caused by increased coal combustion and C4 plant burning during the heating season. The δ13CEC value was heavier than the δ13COC value in May, whereas the opposite was true in December. The δ13COC value may have been changed in atmospheric reactions during secondary organic aerosols formation in May and photochemical aging in December. There was almost no difference between δ13COC and δ13CEC in March and August. Source apportionment suggest that the EC sources in Taiyuan during the 4 months were different. EC in March and December was mainly due to fossil fuels (93.8%) and C4 plant burning (6.2%), EC in May was mainly affected by industrial coal combustion (50.0%) and vehicle exhaust (50.0%), and EC in August was the comprehensive result of fossil fuels (63.6%) and charcoal burning (36.4%). These results may enhance understanding of the sources of carbonaceous components in fine particles and may provide valuable data for environmental management.

Environmental perturbations during the latest Permian: Evidence from organic carbon and mercury isotopes of a coal-bearing section in Yunnan Province, southwestern China

The end-Permian mass extinction (EPME) is sudden, but prior to which, the ecological resilience has diminished. The Late Permian is an important geologic period for coal formation associated with extensive volcanic activities in South China and adjacent regions. Here we present a systematic analysis of organic carbon and mercury isotopes as well as elemental geochemistry and mineralogy of a coal-bearing section from the Lianying Coalfield in Yunnan Province, South China. The data show that organic carbon δ13C values are negatively shifted by −25.22‰ to −25.99‰ in pyroclastic tonstein partings, which are near-synchronous with the negative Eu anomaly (0.48–0.52), significant Hg enrichment (130–352 ppb) and near-zero Δ199Hg (−0.08–0.07‰). These C and Hg isotope anomalies and associated geochemical signatures, which are manifested by characteristic Al2O3/TiO2, Zr/TiO2, and Nb/Yb ratios, are likely caused by subduction or collision-related volcanism. Our study provides evidence for the occurrences of volcanism before the advent of EPME, which played a significant role in stressing the environment and consequently diminishing the ecological resilience in the latest Permian.

Pedogenic carbonates archive modern and past precipitation change – A transect study from soils and loess-paleosol sequences from northern Iran

Stable carbon and oxygen isotope compositions in soil carbonates and in soil organic matter provide insights into (paleo) climatic conditions, especially in arid and semiarid regions where carbonate is abundant in soils. We here investigate δ13C and δ18O isotope composition of bulk carbonate, pedogenic carbonate nodules and organic carbon from modern soils developed along a precipitation gradient (Entisol, Inceptisol, Mollisol and Alfisol) and paleosols formed within loess units in northern Iran. δ13C values of organic carbon in the modern soils ranged from −19.5‰ in the arid and semiarid regions to −24‰ in the more humid regions with forest vegetation. δ13C values of bulk carbonate in the modern soils ranged from −12.1‰ at the wettest sites to −0.8‰ at the drier sites and thus, a 550 mm increase in precipitation leads to a ~11‰ decrease in δ13C values of bulk carbonate. In the modern soils, δ13C values of pedogenic carbonate nodules in the semiarid areas show higher values by ~3–4‰ more than the regions with forest vegetation. Unlike δ13C values of pedogenic carbonate nodules the δ18O values of pedogenic carbonate nodules of modern soils in this study did not display pronounced changes across the sections. The change in δ13C of carbonates and organic carbon is consistent with the present above-ground vegetation, consisting of a mosaic of habitats with differences in the proportion of C3 and C4 vegetation tending to larger C4 contribution with increasing aridity. The δ13C values of carbonates coupled with climate data of the region confirm seasonal bias in the formation of carbonate nodules in the modern soils seen in studies elsewhere. The δ13C values of pedogenic carbonate nodules in the paleosols range from −10‰ in humid areas to −7.2‰ in semiarid areas. The trends in δ13C composition in modern soils are also visible in the δ13C composition in the paleosols, suggesting that in the foothills of the Alborz and Iranian loess plateau, a precipitation gradient existed also in the last interglacial.

Formation of Authigenic Low-Magnesium Calcite from Sites SS296 and GC53 of the Gulf of Mexico

Authigenic low-magnesium calcite (LMC)—a mineral phase that should precipitate in calcite seas rather than today’s aragonite sea—was recently discovered at the seafloor of the Gulf of Mexico (GoM) at water depths of 65 m (site SS296) and 189 m (site GC53). This study investigates the mineralogical, petrographic, and geochemical characteristics of LMC from both sites to reveal its formation process. The δ18O values of LMC from site SS296 cluster in two groups (−0.6‰ to 1.7‰; 6.3‰ to 7.5‰) and the presence of cone-in-cone texture in the samples with lower δ18O values suggest precipitation at higher temperatures and greater depth. Low δ18O values of LMC from site GC53 ranging from −9.4‰ to −2.5‰ indicate an influence of meteoric waters during formation. LMC at both sites reveals a wide range of δ13C values (−17.4‰ to 2.6‰), indicating various carbon sources including seawater and/or organic matter. This interpretation is further supported by the δ13C values of organic carbon extracted from the LMC lithologies (δ13Corg: from −26.8‰ to −18.9‰). Relatively low Sr concentrations of LMC samples regardless of variable 87Sr/86Sr ratios, ranging from 0.707900 to 0.708498 for site GC53 and from 0.709537 to 0.710537 for site SS396, suggest the exchange of Sr between pore fluids and ambient sediments/rocks. The observed wide range of 87Sr/86Sr ratios and the enrichment of Fe and Mn in LMC is in accordance with pore fluids deriving from the dissolution of Louann salt. Overall, this study reveals that the formation of LMC at sites SS296 and GC53 was favored by the presence of low Mg/Ca ratio pore fluids resulting from salt dissolution in subsurface environments when sufficient dissolved inorganic carbon was available. These results are essential for understanding the formation of marine LMC at times of an aragonite sea, highlighting the role of formation environments—open environments close to or at the seafloor vs. confined subseafloor environments typified by pore waters with a composition largely different from that of seawater.

The sources and biogeochemical cycling of carbon in the Wudalianchi UNESCO Geopark volcanic system in Northeast China.

The biogeochemical cycling and response mechanisms of carbon within the Wudalianchi UNESCO Global Geopark were characterized by the isotopic compositions of dissolved inorganic carbon (δ13CDIC) and dissolved organic carbon in ground and surface (lake) waters and their relating carbon isotopic composition of soil (δ13CSOC) and sediment organic carbon (δ13Corg). In addition to mantle-derived CO2, the oxidation of organic matter was prevalent in shallow groundwater during the summer. Their associated degassing of CO2 produced higher pCO2 values than in autumn or winter and elevated δ13CDIC values. In summer, DIC in the epilimnion showed a wide range of δ13CDIC from - 8.4 to 2.6‰. Waters in open-lake areas with relatively positive δ13CDIC values and the low levels of pCO2 were primarily influenced by CO2 degassing. Photosynthesis elevated the δ13CDIC values and led to minimal pCO2 levels in closed lake areas. Isotopically, δ13Corg was found to be positively related to δ13CSOC. In addition, lake bed sediments generally had lower concentrations and larger δ13C values of organic carbon than the surrounding soils. These results suggest that 12CO2 derived from the degradation of sediment was preferentially utilized by phytoplankton in the epilimnion during photosynthesis. The remaining 13C-rich organic matter was retained in the sediment. Since 2000, δ13Corg increased in lake 3 over time, reflecting the input of sewage and land use changes associated with a resort used for tourism. The values of δ13Corg in lake 5, distant from the resort, did not change substantially, indicating minimal human impacts.

Distribution of fresh carbon in different particle-size aggregates of different soil type

In order to study the effects of application of fresh carbon on distribution in soil aggregate fractions of red soil and lime concretion black soil, the stable carbon isotope (δ13C) technique was used, and explore the dynamic variation and distribution of soil organic carbon(SOC) in soil aggregates during 13C-labelled rice straw decomposition by an indoor incubation experiment. This experiment was put under incubation at 28°C for 120 days, which included two treatments: CK (no straw) and Str (added with 1% straw), all samples were separated into four aggregate-size classes (>2000, 2000~250, 250~53, <53 μm) by wet sieving in the different incubation period, while soil aggregates in different size fraction and organic carbon in soil aggregates were determined. The results showed that the δ13C value of organic carbon in different particle-size water-stable aggregates of Str treatment was significantly higher than that of CK treatment, the δ13C value varied greatly both in red soil and lime concretion black soil, but the turnover rate of native SOC was slow and the degradation degree was relatively close. The quantity of new organic carbon supplied by rice straw found in different levels of aggregates followed a same increasing order of 2000~250 μm < (>2000 μm) < (<53 μm) < 250~53 μm in red soil and lime concretion black soil at 120 days, which further indicates that the added fresh carbon was mainly accumulated in small-sized soil aggregates. The result shows that the addition of rice straw could increase soil organic carbon content in all sizes of aggregates, which provide theory basis for soil quality improvement and organic carbon recycle.

Degassing of organic carbon during regional metamorphism of pelites, Wepawaug Schist, Connecticut, USA

A comprehensive understanding of the degassing systematics of organic carbon (OC) during regional metamorphism is necessary to evaluate the role that metamorphism plays in the global carbon cycle. In this study, weight percentages and δ13C values of OC were measured in 70 samples of metapelites from the Wepawaug Schist, Connecticut, where classic Barrovian metamorphism occurred and graphitic OC is widespread. Relative to low-grade chlorite + biotite zone rocks, our mass balance analysis shows that OC in the metapelites underwent progressive loss from −14% (−0.06 g OC per 100 g rock) in the garnet zone, through −21% (−0.09 g/100 g) in the staurolite zone, to −26% (−0.11 g/100 g) in the kyanite zone. The average δ13C values in different metamorphic zones (ranging from −14.74‰ to −16.24‰) are all much higher than normal organic material in marine sediments, and increase slightly from the chlorite + biotite zone to the garnet zone and decrease slightly at higher metamorphic grades. Organic carbon degassing in the form of CH4 during the late stage of diagenesis or in the earliest stages of metamorphism could produce this significant 13C enrichment. Under the assumption that the 13C enrichment is caused by graphite degassing during the lowest-grade metamorphism (chlorite zone or lower), the degassing profile of OC during the regional metamorphism is reconstructed by combining the δ13C and OC mass change data. The computed results indicate that graphitic OC in the Wepawaug Schist probably underwent considerable loss at lowest-grade metamorphic conditions, ranging from ~−40% to ~−90% (or from −0.23 g OC per 100 g rock to −2.8 g OC per 100 g rock), and remained relatively inert at higher grades. Based on the mass balance analysis, δ13C systematics, and exploratory modeling results, this study argues that the lowest-grade or pre-metamorphic stages would be the more efficient OC liberators, and that the degassing potential of OC in the major stages of Barrovian metamorphism appears to be much more restricted. Additional independent studies are required to decipher the early degassing of OC after the deposition of organic matter, which could in turn help better constrain the degassing of OC during regional metamorphism.

Deposition of trace metals in sediments of the deltaic plain and adjacent coastal area (the Neretva River, Adriatic Sea)

The deposition and distribution of trace metals in surface sediments of the karstic, microtidal and low-wave energy environment of the Neretva River delta and the adjacent coastal region were investigated. These processes were studied in relation to the sedimentological characteristics of the deposits, their surface physico-chemical properties, i.e., specific surface area (SSA) and cation exchange capacity (CEC), and the content, characteristics and origin of sedimentary organic matter (SOM). The results indicate two principal mechanisms governing the spatial distribution of trace metals in the delta plain and adjacent marine surface sediments. Firstly, the presence of SOM in the delta plain freshwater sediments exerts a significant effect on their surface properties, thus influencing the distribution of trace metals, particularly Cd. In the Neretva Channel, the highest share of the terrestrial component of SOM (estimated from δ13C values of organic carbon) was found in the proximity of the river mouth. This coincides with the markedly elevated concentrations of Pb, Cu and Cd, indicating their association with organic substances of terrestrial origin. Secondly, the spatial distributions of Ni, Co and Zn in the surface marine sediments of the Neretva Channel are closely related to the sedimentation dynamics and deposition pattern of the river-borne fine-grained particles. The strong correlation between the concentrations of Ni, Co and Zn and the content of Al, Fe and Mn indicates their binding on Mn and Fe oxide and oxyhydroxide coatings at the clay mineral surfaces. The results indicate a low level of anthropogenic metal contamination in the investigated area.

Stable isotopic signatures of the modern land snail Eremina desertorum from a low-latitude (hot) dry desert—A study from the Petrified Forest, New Cairo, Egypt

This study was conducted on recent desert samples—including (1) soils, (2) plants, (3) the shell, and (4) organic matter from modern specimens of the land snail Eremina desertorum—which were collected at several altitudes (316–360m above sea level) from a site in the New Cairo Petrified Forest. The soils and shellE. desertorum were analyzed for carbonate composition and isotopic composition (δ18O, δ13C). The plants and organic matterE. desertorum were analyzed for organic carbon content and δ13C. The soil carbonate, consisting of calcite plus minor dolomite, has δ18O values from −3.19 to −1.78‰ and δ13C values −1.79 to −0.27‰; covariance between the two values accords with arid climatic conditions. The local plants include C3 and C4 types, with the latter being dominant. Each type has distinctive bulk organic carbon δ13C values: −26.51 to −25.36‰ for C3-type, and −13.74 to −12.43‰ for C4-type plants.The carbonate of the shellE. desertorum is composed of aragonite plus minor calcite, with relatively homogenous isotopic compositions (δ18Omean=−0.28±0.22‰; δ13Cmean=−4.46±0.58‰). Most of the δ18O values (based on a model for oxygen isotope fractionation in an aragonite-water system) are consistent with evaporated water signatures. The organic matterE. desertorum varies only slightly in bulk organic carbon δ13C values (−21.78±1.20‰) and these values suggest that the snail consumed more of C3-type than C4-type plants. The overall offset in δ13C values (−17.32‰) observed between shellE. desertorum carbonate and organic matterE. desertorum exceeds the value expected for vegetation input, and implies that 30% of carbon in the shellE. desertorum carbonate comes from the consumption of limestone material.

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.

Biosignatures link microorganisms to iron mineralization in a paleoaquifer

Research Article| August 01, 2012 Biosignatures link microorganisms to iron mineralization in a paleoaquifer Karrie A. Weber; Karrie A. Weber * 1School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588, USA2Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68583, USA *E-mail: kweber2@unl.edu. Search for other works by this author on: GSW Google Scholar Trisha L. Spanbauer; Trisha L. Spanbauer 1School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588, USA †Current address: Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68583, USA. Search for other works by this author on: GSW Google Scholar David Wacey; David Wacey 3Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, WA 6009, Australia4School of Earth and Environment, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Matthew R. Kilburn; Matthew R. Kilburn 3Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar David B. Loope; David B. Loope 2Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68583, USA Search for other works by this author on: GSW Google Scholar Richard M. Kettler Richard M. Kettler 2Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68583, USA Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (8): 747–750. https://doi.org/10.1130/G33062.1 Article history received: 28 Nov 2011 rev-recd: 08 Mar 2012 accepted: 12 Mar 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Karrie A. Weber, Trisha L. Spanbauer, David Wacey, Matthew R. Kilburn, David B. Loope, Richard M. Kettler; Biosignatures link microorganisms to iron mineralization in a paleoaquifer. Geology 2012;; 40 (8): 747–750. doi: https://doi.org/10.1130/G33062.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Concretions, preferentially cemented masses within sediments and sedimentary rocks, are records of sediment diagenesis and tracers of pore water chemistry. For over a century, rinded spheroidal structures that exhibit an Fe(III) oxide–rich exterior and Fe-poor core have been described as oxidation products of Fe(II) carbonate concretions. However, mechanisms governing Fe(III) oxide precipitation within these structures remain an enigma. Here we present chemical and morphological evidence of microbial biosignatures in association with Fe(III) oxides in the Fe(III) oxide–rich rind of spheroidal concretions collected from the Jurassic Navajo Sandstone (southwest United States), implicating a microbial role in Fe biomineralization. The amount of total organic carbon in the exterior Fe(III) oxides exceeded measured values in the friable interior. The mean δ13C value of organic carbon from the Fe(III) oxide–cemented exterior, δ13C of −20.55‰, is consistent with a biogenic signature from autotrophic bacteria. Scanning electron micrographs reveal microstructures consistent with bacterial size and morphology, including a twisted-stalk morphotype that resembled an Fe(II)-oxidizing microorganism, Gallionella sp. Nanoscale associations of Fe, O, C, and N with bacterial morphotypes demonstrate microorganisms associated with Fe(III) oxides. Together these results indicate that autotrophic microorganisms were present during Fe(III) oxide precipitation and present microbial catalysis as a mechanism of Fe(III) oxide concretion formation. Microbial biosignatures in rinded Fe(III) oxide–rich concretions within an exhumed, Quaternary aquifer has broad implications for detection of life within the geological record on Earth as well as other Fe-rich rocky planets such as Mars, where both Fe(II) carbonate and Fe(III) oxide–rich concretions have been identified. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early Eocene warming events and the timing of terrestrial faunal exchange between India and Asia

The timing of initiation of continent-continent collision between Asia and India is controversial, but this major tectonic event is generally thought to have occurred in the Early Eocene, ca. 50 Ma. New and independent data from strontium isotopes, stable carbon isotopes, microfossil biostratigraphy, and mammal fossils from an Early Eocene marginal marine sequence (Cambay Shale) at the Vastan Lignite Mine of western India indicate that terrestrial faunal exchanges, and therefore continental collision, between Asia and the Indian subcontinent took place before 53.7 Ma. This age coincides with the second Eocene Thermal Maximum (ETM2), a short-lived warming episode that followed the Paleocene-Eocene Thermal Maximum (PETM) ca. 55.5 Ma. Our data also document, for the first time, a clear record of the ETM2 in terrestrial organic material from a low-latitude site, which is represented by a 3‰−4‰ carbon isotope excursion (CIE) in lignite and dispersed organic carbon δ13C values. The magnitude of the CIE at this location closely matches that observed in marine cores from the Arctic Ocean, which supports an interpretation that this hyperthermal event, though of lower magnitude, was similar in character to that of the PETM, being a global phenomenon that affected both terrestrial and marine ecosystems.

Cottonballs, a unique subaqeous moonmilk, and abundant subaerial moonmilk in Cataract Cave, Tongass National Forest, Alaska

The Tongass National Forest is known for its world-class karst features and contains the largest concentration of dissolutional caves in Alaska. Within these karst systems exist unusual and possibly unique formations exhibiting possible biological origin or influence. Cataract Cave is an example of such a system. This cave hosts a unique depositional setting in which so-called “cottonballs” line two permanent pools. The cottonballs are a calcitic deposit heavily entwined within a mass of microbial filaments. They are juxtaposed with extensive subaerial calcitic moonmilk wall deposit of a more conventional nature but of an extraordinary thickness and abundance. Both the cottonballs and moonmilk are composed of microcrystalline aggregates (0.20 wt.%) compared to the cottonballs (0.12 wt.%). However, the cottonballs are dominated by monocrystalline needles, whereas the moonmilk is mainly composed of polycrystalline needles. The microbial environments of both displayed similar total microbial cell counts; however, culturable microbial counts varied between the deposits and among the various media. For both, in situ cultures and isolates inoculated in a calcium salt medium produced calcium carbonate mineralization within biofilms. Geochemical variations existed between the deposits. Moonmilk displayed a slightly higher abundance of organic carbon (0.20 wt%) compared to the cottonballs (0.12 wt%). Stable isotopic analysis revealed that the moonmilk (δ13C = -1.6‰) was isotopically heavier compared to the cottonballs (δ13C = -8.1‰) but both are lighter than the host rock (δ13C = +1.1‰). However, the organic carbon δ13C values of both deposits were similar (δ13C = -27.4 and –26.7‰) and isotopically lighter compared to other overlying surface organic carbon sources. Due to the similarities between the deposits, we infer that both the cottonballs and moonmilk are subject to a set of related processes that could collectively be accommodated by the term “moonmilk”. Thus, the cottonball pool formation can be characterized as a type of subaqueous moonmilk. The differences observed between the moonmilk and cottonballs may be largely attributable to the changes in the depositional environment, namely in air or water.

Sediment Organic Carbon in Todos Santos Bay, Baja California, Mexico

Sediment grain size and organic carbon (OC) data collected over the past 50 years, together with δ13C values of OC in recently collected samples, were analyzed to improve understanding of sediment OC distribution and abundance in Todos Santos Bay. Sediments in the submarine canyon at the mouth of the bay and in quiet-water locations along the shore are fine grained, high in OC, and have generally low δ13C values; sediments in high-energy environments are low in OC and have high δ13C values. A bivariate isotopic mixing model indicates that none of the sediments contain >50% terrigenous OC (average ~30%), and that the terrigenous OC content of the sediments is a small proportion of the OC content of local soils. Sediment OC composition is apparently controlled by energy-related sorting and deposition, oxidation of much of the original terrigenous OC, and replacement of some terrigenous OC by marine OC.

Sources and transportation of suspended matter and sediment in the southern Yellow Sea: Evidence from stable carbon isotopes

The concentrations of total suspended matter (TSM) and the compositions of organic stable carbon isotopes of TSM and bottom sediments were analyzed to study the sources of TSM and sediments and the transportation processes. For this study, 284 TSM samples and 64 sediment ones taken from 67 stations along 7 transects and in 5 layers were collected in the southern Yellow Sea on the cruise in May, 1998. The main sediment transportation pattern in the southern Yellow Sea was obtained by analyzing the distribution characteristics of TSM concentration and particulate organic carbon δ13C values. It was confirmed from the pattern that the bottom layer plays a more important role than the surface one in the transportation processes of terrigenous material to the central deep-water area of the southern Yellow Sea. The Yellow Sea circulation is an important control factor in determining the sediment transportation pattern in the southern Yellow Sea. The carbon isotope signals of sedimentary organic matter confirmed that the main material in sediments with high sedimentation rate in the Shandong subaqueous delta originated from the modern Yellow River. The terrigenous sediments in the deep-water area of the southern Yellow Sea are mainly from the abandoned Yellow River and the modern Yellow River, and a small portion of them are from the modern Yangtze material. The amount of terrigenous material from Korean Peninsula and its influence range are relatively limited. The conclusions derived from TSM and stable carbon isotopes were further confirmed by another independent material source tracer-PAHs.

δ13C values of organic carbon in cropland and forest soils in Japan

The carbon content and natural 13C abundance (δ13C) of soil organic carbon were determined for Japanese cropland and nearby forest soils by continuous flow CN analysis with mass spectrometry. The δ13C values of organic carbon from cropland soils in the Hokkaido District ranged between −27 and −24‰, suggesting that soil organic carbon originated mainly from C3 plants, while the δ13C values from those in Main Island, Shikoku, and Kyushu Districts ranged between −25 and −17‰, indicating that up to 70% of soil organic carbon originated from C4 plants (presumably Miscanthus sinensis). The δ13C values of organic carbon from Imaichi and Tanegashima forest soils showed that carbon was largely of C3 plant origin in the surface soils but in deeper horizons the values were strongly affected by the organic carbon of C4 plant origin. Sugarcane cultivation for 36 y in Tanegashima cropland soils had resulted in the increase of the soil δ13C values to around −15‰. In Shizuoka Prefecture, the δ13C values of soil organic carbon of non-volcanic ash Andosols under forests at Iwata and Makinohara were within the ranges of C3 plant origin. The values in volcanic ash Andosols at Fuji indicated that the soil carbon in the surface horizons mainly originated from C3 plants while that in deep soil horizons largely from C4 plants. Tea cultivation for more than 40 y had led to a decrease of the δ13C values at Fuji by 2-4‰. In the Miyako Island sugarcane fields, the δ13C values of the plow layers were apparently affected by sugarcane, a C4 plant species, compared to the subsoils where a large fraction of carbon originated from C3 forest plants. The shift from forest to sugarcane cultivation markedly decreased the carbon content and increased the δ13C values of soil organic carbon from −27-−25‰ to −17-−14‰ within 25 y.

Paleoenvironment in Dae-Am San High Moor in the Korean Peninsula

We discuss paleoenvironmental changes at the Dae-Am San high moor, located near the Demilitarized Zone at 38°N. This area has been reported to be the only high moor in the Korean peninsula. The 14C age of the bottom sediment (75–80 cm in depth) at this site is about 1900 BP. Since the radiocarbon ages for the intervals at 50–55 cm and 75–80 cm were almost the same, we conclude that the deep layers (55–80 cm) in the high moor were all part of the original soil. Low organic C and N contents in the deeper layers support this inference. The 50–55-cm layer consists of sandy material with very low organic content, suggesting erosion from the surrounding area. The surface layer (0–5 cm) was measured as 190 BP, and the middle layer (30–35 cm) was 870 BP. The bulk sedimentation rate was estimated to be about 0.4 mm yr−1 for the 0–30-cm interval. The δ13C value of organic carbon in the sediments fluctuated with depth. The δ13C profile of the Dae-Am San high moor may be explained by climatic changes which occurred during the Little Ice Age and Medieval Warm Period.