Vienna Pee Dee Belemnite Research Articles

Deep‐burial alteration of early‐diagenetic carbonate concretions formed in Palaeozoic deep‐marine greywackes and mudstones (Bardo Unit, Sudetes Mountains, Poland)

AbstractCarbonate concretions formed in bathyal and deeper settings have been studied less frequently than those formed in shallow‐marine deposits. Similarly, concretions affected by catagenetic conditions have rarely been reported. Calcite concretions in deep‐marine mudstones and greywackes of the Bardo Unit (Sudetes Mountains, Poland) formed during early diagenesis and were buried to significant depths. Petrographic and geochemical (elemental and stable C and O isotopic) analyses document their formation close to the sediment–water interface, prior to mechanical compaction within the sulphate reduction zone and their later burial below the oil window. Although the concretions were fully formed during early diagenesis, the effects of increased temperature and interaction with late‐diagenetic interstitial fluids can be discerned. During maximum burial, the concretions underwent thorough recrystallization that caused alteration of fabric and elemental and O isotope composition. The initial finely crystalline cement was replaced by more coarsely crystalline, sheaf‐like, poikilotopic calcite in the concretions. These large calcite crystals engulf and partially replace unstable detrital constituents. The extremely low δ18O values (down to −21·2‰ Vienna Pee Dee Belemnite) in the concretions are the result of the increased temperature in combination with alteration of volcanic glass, both causing a significant 18O‐depletion of bicarbonate dissolved in the interstitial fluids. Recrystallization led to uniform O isotope ratios in the concretions, but did not affect the C isotope signature. The δ13C values of the late‐diagenetic cements precipitated in the greywacke and in cracks cutting through concretions imply crystallization in the catagenetic zone and decarboxylation as a source of the bicarbonate. These late‐diagenetic processes took place in a supposedly overpressured setting, as suggested by clastic dykes and hydrofractures that cut through both concretions and host rock. All of these features show how the effects of early and late diagenesis can be distinguished in such rocks.

Sedimentological and geochemical characteristics of a fluvial travertine: A case from the eastern Mediterranean region

AbstractA sedimentological and geochemical study was performed on the travertines in the southern part of the Uşak geothermal field, western Turkey, to assess the applicability of a fluvial tufa facies model in interpreting late Quaternary travertine deposits developed along the stream valleys that follow fault and fracture systems. Modern thermal (31 to 38°C) springs are found on the floor of the valley between 480 m and 520 m above sea‐level. The distribution and nature of travertine facies were determined from natural outcrops. Samples of the travertines and spring water were characterized using a range of geochemical and petrographic methods. Waterfall, slope and pool facies associations consist of various combinations of travertine facies and subordinate detrital facies. Waterfall and slope facies associations of the older deposits occur where the springs emerged onto a hillslope or topographic break. In contrast, the pool facies association developed in depressions or flat areas that were fed by thermal springs. The youngest generation (1·85 ka) precipitated at lower elevations than the older ones (147 to 153 ka). Stable carbon and oxygen isotope values of the Aksaz travertines range between +4·3‰ and +6·3‰ (Vienna Pee Dee Belemnite) and −12·6‰ and −7·2‰ (Vienna Pee Dee Belemnite), respectively. The high δ13C values suggest that the thermal waters were charged with isotopically heavy CO2 of deep origin. Based on palaeotemperature calculations, the temperatures of the palaeosprings are slightly higher (up to 44°C) than the present equivalents, but sometimes the temperature is lower, probably due to mixing with the stream water. Although the thermal waters occasionally are impeded by fluvial activity, travertine precipitation occurs in the protected parts of the Aksaz Stream valley. This contribution highlights the applicability of the fluvial facies model for tufa for the interpretation of travertine deposits worldwide.

Comparative studies of pelagic microbial methane oxidation within the redox zones of the Gotland Deep and Landsort Deep (central Baltic Sea)

Abstract. Pelagic methane oxidation was investigated in dependence on differing hydrographic conditions within the redox zone of the Gotland Deep (GD) and Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which indicates the transition between oxic and anoxic conditions, was characterized by a pronounced methane concentration gradient between the deep water (GD: 1233 nM, 223 m; LD: 2935 nM, 422 m) and the surface water (GD and LD < 10 nM). This gradient together with a 13C CH4 enrichment (δ13C CH4 deep water: GD −84‰, LD −71‰; redox zone: GD −60‰, LD −20‰; surface water: GD −47‰, LD −50‰; δ13C CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial methane consumption within the redox zone. Expression analysis of the methane monooxygenase identified one active type I methanotrophic bacterium in both redox zones. In contrast, the turnover of methane within the redox zones showed strong differences between the two basins (GD: max. 0.12 nM d−1, LD: max. 0.61 nM d−1), with a nearly four-times-lower turnover time of methane in the LD (GD: 455 d, LD: 127 d). Vertical mixing rates for both deeps were calculated on the base of the methane concentration profile and the consumption of methane in the redox zone (GD: 2.5 × 10–6 m2 s−1, LD: 1.6 × 10–5 m2 s−1). Our study identified vertical transport of methane from the deep-water body towards the redox zone as well as differing hydrographic conditions (lateral intrusions and vertical mixing) within the redox zone of these deeps as major factors that determine the pelagic methane oxidation.

Linking carbonate cyclicity in platforms to depositional and diagenetic overprints: an example from the Lower Eocene Drunka Formation, west of Assiut–Minia stretch, Western Desert, Egypt

Early diagenetic modifications in carbonate sequences can be traced to fluctuations in relative sea level and, hence, sequence stratigraphy. These variations in carbonate cyclicity can have an impact on the spatial and temporal distribution of diagenetic alterations. This study aims to investigate the depositional and diagenetic attributes of early Eocene Drunka carbonate sequences in the Western Desert of Egypt as a function of changes in cyclicity and relative sea-level variations. Drunka Formation is a thick carbonate platform consisting of two members; the lower member (34 m thick) consists of four depositional shallowing-upward or emergence carbonate cycles that range in thickness from 4 to 10.75 m. Each cycle commences with a thin lime mudstone and/or algal skeletal wackestone and capped with thick oolitic–peloidal grainstone. These cycles represent nongradual cycles that signify no regular balance between subsidence and sedimentation rates. They indicate high-frequency sea-level fluctuations and/or short-time sea-level oscillations, accompanied with high production of carbonates. The upper member (99.75 m thick) comprises of emergence carbonate cycles that begin with a thick lime mudstone, followed by algal wackestone/packstone and capped by a thin bed of nummulitic algal packstone or peloidal grainstone (0.5 to 5 m thick). These cycles represent gradual cycles that denote regular relative vertical increase in sea level concurrent with increase in sedimentation rate. They resemble low-frequency sea-level fluctuations with high rate of subsidence outpacing the increase in sea-level rise. The drowning of carbonate platform is evidenced by increase in thickness of emergent carbonate cycles upward especially in the upper member, the increase of thickness in the lower parts of cycles (lime mudstone and wackestone) at the expense of cycle cap (packstone and grainstone), and the decrease of skeletal particles. The carbonate components of the grainstone lithofacies (topmost part of cycles) are characterized by luminescence characteristics and depleted oxygen isotopic values (−7.57 to −9.07 ‰ Vienna Pee Dee Belemnite (VPDB); av. = −8.32 ‰ VPDB) and carbon isotopic values (−4.90 to −6.45 ‰ VPDB; av. = −5.675 ‰ VPDB). These values reflect formation/stabilization under meteoric phreatic conditions. The coated grains, which show aggrading neomorphism, display oxygen isotopic values that range between −6.48 and −9.48 ‰ VPDB (av. = −7.98 ‰ VPDB). The close and relatively depleted δ18 O values of the base of cycles (lime mudstone, wackestone, and packstone) have probably suffered neomorphic stabilization in freshwater-dominated environment on the spatial and temporal distribution of diagenetic alterations.

Characteristics and Dolomitization of Upper Cambrian to Lower Ordovician Dolomite from Outcrop in Keping Uplift, Western Tarim Basin, Northwest China

Abstract:Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow‐marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include (1) micritic dolomites (MD1); (2) fine–coarse euhedral floating dolomites (MD2); (3) fine–coarse euhedral dolomites (MD3); and (4) medium–very coarse anhedral mosaic dolomites (MD4). Dolomite cement occurs in minor amounts as coarse saddle dolomite cement (CD1) that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of −9.7‰ to −3.0‰ VPDB (Vienna Pee Dee Belemnite); δ13C values of −0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values (−12.9‰ to −9.1‰ VPDB); slightly lower δ13C values (−1.6‰–0.6‰ VPDB); higher 87Sr/86Sr ratios (0.709165–0.709764); and high homogenization temperature (Th) values (98°C–225°C) and salinities (6 wt%−24 wt% NaCl equivalent). Limited data from dolomite cement shows a low Sr concentration (58.6 ppm) and high Fe and Mn contents (1233 and 1250 ppm, respectively). These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.

Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime

Emerald in the Mackenzie Mountains is hosted in extensional quartz–carbonate veins cutting organic-poor Neoproterozoic sandstones and siltstones within the hanging wall of a thrust fault that emplaced these strata above Paleozoic rocks. Isotopic compositions of water extracted from emerald are typical of evolved sedimentary sulphate brines. Fluid inclusion studies indicate two saline fluid populations: a CO2–N2-bearing, high-salinity brine (20.4–25.8 wt.% NaCl equivalent), and a gas-free, saline brine (7.6–15.3 wt.% NaCl equivalent). Both populations display evidence of post-entrapment volume changes. δ18OVSMOW (VSMOW, Vienna standard mean ocean water) values for emerald, quartz, and dolomite yield averages of 17.3‰ (±0.9), 19.6‰ (±1.5), and 18.1‰ (±1.0), respectively. Dolomite δ13CVPDB (VPDB, Vienna Pee Dee belemnite) averages –6.8‰ (±1.0). Two pyrite samples returned δ34SCDT (CDT, Cañon Diablo troilite) values of 5.1‰ and 11.2‰. Triply concordant mineral equilibration temperatures determined from mineral pair δ18OVSMOW equilibration (quartz–emerald, quartz–dolomite, emerald–dolomite) range from 380 to 415 °C. Depth calculations based on mineral pair isotope equilibration and typical geothermal gradient indicate vein formation at 6–11 km depth. A Re–Os isochron age of 345 ± 20 Ma from pyrite indicates that mineralization was contemporaneous with estimated ages of some northern Cordilleran Zn–Pb occurrences. Emerald mineralization resulted from inorganic thermochemical sulphate reduction via the circulation of warm basinal brines through siliciclastic, carbonate, and evaporitic rocks. These brines were driven along deep basement structures and reactivated normal faults during the development of a trans-tensional back-arc basin during the late Devonian to middle Mississippian. The Mountain River emerald occurrence thus represents a variant of the Colombian-type emerald deposit model requiring thermochemical sulphate reduction.

Lipid biomarkers in ooids from different locations and ages: evidence for a common bacterial flora

Ooids are one of the common constituents of ancient carbonate rocks, yet the role that microbial communities may or may not play in their formation remains unresolved. To search for evidence of microbial activity in modern and Holocene ooids, samples collected from intertidal waters, beaches and outcrops in the Bahamas and in Shark Bay in Western Australia were examined for their contents of lipid biomarkers. Modern samples from Cat and Andros islands in the Bahamas and from Carbla Beach in Hamelin Pool, Western Australia, showed abundant and notably similar distributions of hydrocarbons, fatty acids (FAs) and alcohols. A large fraction of these lipids were bound into the carbonate matrix and only released on acid dissolution, which suggests that these lipids were being incorporated continuously during ooid growth. The distributions of hydrocarbons, and their disparate carbon isotopic signatures, were consistent with mixed input from cyanobacteria together with small and variable amounts of vascular plant leaf wax [C27 -C35 ; δ(13) C -25 to -32‰Vienna Pee Dee Belemnite (VPDB)]. The FAs comprised a complex mixture of C12 -C18 normal and branched short-chain compounds with the predominant straight-chain components attributable to bacteria and/or cyanobacteria. Branched FA, especially 10-MeC16 and 10-MeC17 , together with the prevalence of elemental sulfur in the extracts, indicate an origin from sulfate-reducing bacteria. The iso- and anteiso-FA were quite variable in their (13) C contents suggesting that they come from organisms with diverse physiologies. Hydrogen isotopic compositions provide further insight into this issue. FAs in each sample show disparate δD values consistent with inputs from autotrophs and heterotrophs. The most enigmatic lipid assemblage is an homologous series of long-chain (C24 -C32 ) FA with pronounced even carbon number preference. Typically, such long-chain FA are thought to come from land plant leaf wax, but in this case, their (13) C-enriched isotopic signatures compared to co-occurring n-alkanes (e.g., Hamelin Pool TLE FA C24 -C32 ; δ(13) C -20 to -24.2‰ VPDB; TLE n-alkanes δ(13) C -24.1 to -26.2 -‰VPDB) indicate a microbial origin, possibly sulfate-reducing bacteria. Lastly, we identified homohopanoic acid and bishomohopanol as the primary degradation products of bacterial hopanoids. The distributions of lipids isolated from Holocene oolites from the Rice Bay Formation of Cat Island, Bahamas were very similar to the beach ooids described above and, in total, these modern and fossil biomarker data lead us to hypothesize that ooids are colonized by a defined microbial community and that these microbes possibly mediate calcification.

Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa

Small (0.3–1.8mm long), locally abundant, urn-shaped fossils within surface horizons of a paleosol in the 2.2Ga Hekpoort Formation near Waterval Onder, South Africa, are here described and named Diskagma buttonii Retallack gen. et sp. nov. The fossils are from fresh rock of a deep highway cutting, and have been metamorphosed to upper greenschist facies like their matrix. Despite metamorphic alteration, total organic carbon of the samples was 0.04% and its isotopic composition (δ13C) was −25.6±0.08‰ (two standard deviations) versus Vienna Pee Dee belemnite standard. Organic outlines of the fossils are also accentuated by recystallized berthierine and opaque oxides. The fossils are locally clumped within surface swales of a Vertisol paleosol, identified from characteristic penecontemporaneous deformation (clastic dikes between swales of mukkara structure) and from pronounced geochemical differentiation (phosphorus and copper strain-corrected mass-depletion characteristic of an oxidized biologically active soil). This paleosol's chemical composition is evidence of temperate humid climate (mean annual temperature 11.3±4.4°C, and mean annual precipitation 1489±182mm). Associated paleosols indicate atmospheric CO2 of 6640 (+12,880/−4293)ppm (0.6%) and 0.9–5% atmospheric O2. The best preserved examples of Diskagma are shaped like an urn with a flared rim, and closed below the flare. Observation of hundreds of specimens in thin section reveals substantial variation in growth (elongation) and decay (shredding and deflation). They had a hollow ellipsoidal interior that is unusually devoid of opaque debris, unlike the matrix. Diskagma is superficially comparable with lichens such as Cladonia (Ascomycota) and Geosiphon (Glomeromycota). Definitive reproductive structures remain unknown. They predate the oldest other likely fossil eukaryotes (1.9Ga) and fungi (1.5Ga), and current molecular clock estimates for eukaryotes (1.6Ga) and fungi (1.1Ga). Lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks. Although biological affinities of Diskagma are uncertain, these fossils reveal the general appearance of Paleoproterozoic life on land.

Genesis of the Au–Bi–Cu–As, Cu–Mo ± W, and base–metal Au–Ag mineralization at the Mountain Freegold (Yukon, Canada): constraints from Ar–Ar and Re–Os geochronology and Pb and stable isotope compositions

The genesis of mineralized systems across the Mountain Freegold area, in the Dawson Range Cu–Au ± Mo Belt of the Tintina Au province was constrained using Pb and stable isotope compositions and Ar–Ar and Re–Os geochronology. Pb isotope compositions of sulfides span a wide compositional range (206Pb/204Pb, 18.669–19.861; 208Pb/204Pb, 38.400–39.238) that overlaps the compositions of the spatially associated igneous rocks, thus indicating a magmatic origin for Pb and probably the other metals. Sulfur isotopic compositions of sulfide minerals are broadly similar and their δ34S (Vienna-Canyon Diablo Troilite (V-CDT)) values range from −1.4 to 3.6 ‰ consistent with the magmatic range, with the exception of stibnite from a Au–Sb–quartz vein, which has δ34S values between −8.1 and −3.1 ‰. The δ34S values of sulfates coexisting with sulfide are between 11.2 and 14.2 ‰; whereas, those from the weathering zone range from 3.7 to 4.3 ‰, indicating supergene sulfates derived from oxidation of hypogene sulfides. The δ13C (Vienna Peedee Belemnite (VPDB)) values of carbonate range from −4.9 to 1.1 ‰ and are higher than magmatic values. The δ18O (V-SMOW) values of magmatic quartz phenocrysts and magmatic least-altered rocks vary between 6.2 and 10.1 ‰ and between 5.0 and 10.1 ‰, respectively, whereas altered magmatic rocks and hydrothermal minerals (quartz and magnetite) are relatively 18O-depleted (4.2 to 7.9 ‰ and −6.3 to 1.5 ‰, respectively). Hydrogen isotope compositions of both least-altered and altered igneous rock samples are D-depleted (from −133 to −161 ‰ Vienna-Standard Mean Ocean Water (V-SMOW)), consistent with differential magma degassing and/or post-crystallization exchange between the rocks and meteoric ground water. Zircon from a chlorite-altered dike has a U–Pb crystallization age of 108.7 ± 0.4 Ma; whereas, the same sample yielded a whole-rock Ar–Ar plateau age of 76.25 ± 0.53 Ma. Likewise, molybdenite Re–Os model ages range from 75.8 to 78.2 Ma, indicating the mineralizing events are genetically related to Late Cretaceous volcano-plutonic intrusions in the area. The molybdenite Re–Os ages difference between the nearby Nucleus (75.9 ± 0.3 to 76.2 ± 0.3 Ma) and Revenue (77.9 ± 0.3 to 78.2 ± 0.3 Ma) mineral occurrences suggests an episodic mineralized system with two pulses of hydrothermal fluids separated by at least 2 Ma. This, in combination with geological features suggest the Nucleus deposit represents the apical and younger portion of the Revenue–Nucleus magmatic-hydrothermal system and may suggest an evolution from the porphyry to the epithermal environments.

Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO<sub>2</sub> in air

Abstract. The need for a unifying scale anchor for isotopes of CO2 in air was brought to light at the 11th WMO/IAEA Meeting of Experts on Carbon Dioxide in Tokyo 2001. During discussions about persistent discrepancies in isotope measurements between the worlds leading laboratories, it was concluded that a unifying scale anchor for Vienna Pee Dee Belemnite (VPDB) of CO2 in air was desperately needed. Ten years later, at the 2011 Meeting of Experts on Carbon Dioxide in Wellington, it was recommended that the Jena Reference Air Set (JRAS) become the official scale anchor for isotope measurements of CO2 in air (Brailsford, 2012). The source of CO2 used for JRAS is two calcites. After releasing CO2 by reaction with phosphoric acid, the gases are mixed into CO2-free air. This procedure ensures both isotopic stability and longevity of the CO2. That the reference CO2 is generated from calcites and supplied as an air mixture is unique to JRAS. This is made to ensure that any measurement bias arising from the extraction procedure is eliminated. As every laboratory has its own procedure for extracting the CO2, this is of paramount importance if the local scales are to be unified with a common anchor. For a period of four years, JRAS has been evaluated through the IMECC1 program, which made it possible to distribute sets of JRAS gases to 13 laboratories worldwide. A summary of data from the six laboratories that have reported the full set of results is given here along with a description of the production and maintenance of the JRAS scale anchors. 1 IMECC refers to the EU project "Infrastructure for Measurements of the European Carbon Cycle" (http://imecc.ipsl.jussieu.fr/).

Climatic and tectonic controls on sedimentation and erosion during the Pliocene-Quaternary in the Qaidam Basin (China)

The Qaidam Basin is an internally drained basin located in the northeastern Tibetan Plateau. Presently, over 50% of the basin floor exposes thick (>1 km) sections of Pliocene–Quaternary strata that are deformed by folding and faulting. We investigated this nearly continuous Pliocene–Quaternary sedimentary record for the effects of global climate change and deformation on basin sedimentation. New detailed stratigraphic, magnetostratigraphic, and stable isotope (δ 18 O, δ 13 C) data from the Pliocene Shizigou and Pleistocene Qigequan Formations along the southwestern flank of an intrabasin fold within the north-central Qaidam Basin are presented here. Strata reveal climatically controlled, meter-scale parasequences within shallow-lacustrine, marginal-lacustrine, and deltaic lithofacies. Paleocurrents shift from eastward at the base to southwestward for the majority of the section, but they abruptly shift toward the south-southeast in the upper 400 m. Twenty-two magnetozones constrain deposition between 5.2 Ma and ca. 0.8 Ma and reveal that sedimentation rates were fairly constant (474 ± 34 m/m.y.) from 5.2 to 3.0 Ma, after which time rates abruptly decreased to 154 ± 40 mm/yr before increasing again to ∼750 m/m.y. since 1.2 Ma. The δ 18 O values shift from relatively constant values (avg. −6.8‰, range −9.6‰ to −4.5‰ relative to Vienna Peedee belemnite [VPDB]) to less negative values (avg. -1.2‰, range -1.2‰ to -2.7‰ VPDB) between 3.1 and 2.6 Ma and to extremely variable values (avg. –2.9‰, range −8.3‰ to 4.0‰) after ca. 2.6 Ma. The post–2.6 Ma extreme variability in stable isotopes reflects the same timeframe of enhanced climatic cyclicity associated with Northern Hemisphere glaciation. The δ 13 C values remain relatively constant (average −4.0‰, range −5.7‰ to -1.0‰) until ca. 0.9 Ma, when the values increase to -0.3‰ (range -1.0‰ to 1.5‰) VPDB. The appearance of growth strata at 3.0 Ma, shallow-water, evaporite deposition after 2.6 Ma, and the observation of paleoyardangs (buried, wind-sculpted landforms) within lake-marginal strata at 2.4 Ma imply that emergence of the adjacent anticline was followed by the shallowing and partitioning of the lake basin and subaerial exposure and erosion of marginal lake sediments. These data reflect a significant change to a more arid climate in the Qaidam Basin between ca. 3.1 Ma and 2.6 Ma, overlapping with the onset of significant Northern Hemisphere glaciations and basin-floor deformation. Lake-level cycles were on ∼230,000 yr frequency from 2.6 Ma to 1.2 Ma, before increasing to ∼12,000 yr frequency, suggesting increased aridity and broad, subaerial exposure of lake sediments after the Pliocene-Quaternary transition. Deformation of intrabasin sediments by at least 3.0 Ma caused uplift of the basin floor, which provided an environment rich in friable material for wind deflation of the Qaidam Basin, a likely source for sediments on the Chinese Loess Plateau and nutrients for the Pacific Ocean carried by westerly winds. By 2.6 Ma, deformation of the Qaidam Basin created closed depressions that facilitated evaporite sedimentation that continues today. Coeval intrabasin deformation, combined with increasing aridity after 3.1 Ma, thus controlled both deposition and erosion within the region.

Field-based cavity ring-down spectrometry of δ13C in soil-respired CO2

Measurement of soil-respired CO2 at high temporal resolution and sample density is necessary to accurately identify sources and quantify effluxes of soil-respired CO2. A portable sampling device for the analysis of δ13C values in the field is described herein. CO2 accumulated in a soil chamber was batch sampled sequentially in four gas bags and analysed by Wavelength-Scanned Cavity Ring-down Spectrometry (WS-CRDS). A Keeling plot (1/[CO2] versus δ13C) was used to derive δ13C values of soil-respired CO2. Calibration to the δ13C Vienna Peedee Belemnite scale was by analysis of cylinder CO2 and CO2 derived from dissolved carbonate standards. The performance of gas-bag analysis was compared to continuous analysis where the WS-CRDS analyser was connected directly to the soil chamber. Although there are inherent difficulties in obtaining absolute accuracy data for δ13C values in soil-respired CO2, the similarity of δ13C values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the δ13C data were obtained by the WS-CRDS techniques presented here. Field testing of a variety of tropical soil/vegetation types, using the batch sampling technique yielded δ13C values for soil-respired CO2 related to the dominance of either C3 (tree, δ13C=−27.8 to−31.9 ‰) or C4 (tropical grass, δ13C=−9.8 to−13.6 ‰) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept δ13C values of soil-respired CO2 were typically<0.4 ‰ for analysis of soils with high CO2 efflux (>7–9 μmol m−2 s−1).

Stable isotope profiling in moderate marine bryozoan colonies across the Isthmus of Panama

In the tropics, upwelling of cold, deep water is the principal source of major seasonal fluctuations in temperature. Along the tropical eastern Pacific (TEP) coast of the isthmus of Panama, seasonal upwelling induces corresponding drops in temperature. Upwelling does not occur along the southwestern Caribbean (SWC) coast of the isthmus. Our goal was to use these oceanographic differences to test the use of stable isotope profiles of free-living modern cupuladriid bryozoans as a method for quantifying paleo-seasonality. We determined O and C stable isotope values from micromilled carbonates profiled along the growth axis in three colonies of Cupuladria exfragminis Herrera-Cubilla, Dick, Sanner and Jackson, 2006 from the upwelling Gulf of Panama in TEP and three colonies of Cupuladria surinamensis Cadee, 1975 from the non-upwelling Bocas del Toro Archipelago in SWC. Pacific colonies had inter-colony [delta]18Ocarb values ranging from -2.1%o to -0.2%o on the international Vienna Pee Dee Belemnite scale, whereas SWC colonies ranged from -1.7%o to -0.6%o. Pacific colonies consistently reveal cyclical trends in [delta]18Ocarb that are absent in the Caribbean colonies. Based on published measurements of temperature, salinity, and [delta]18Osw, the <=2.5 yrs of cyclicity seen in the Pacific colonies reflects a combination of seasonal freshening and seasonal upwelling of colder water. This preliminary study suggests the potential for more exploration of bryozoans as a source of paleoclimate proxies.

Formation of carbonate concretions in surface sediments of two mud mounds, offshore Costa Rica: a stable isotope study

The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The d18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (d18Oporefluid = 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18Oporefluid &5 %) in Mound 11. A positive correlation between d13Ccarbonate and d18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (d13Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13Cporefluid &-70 %). Furthermore, the d18Oporefluid composition, with values up to ?4.7 % Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boronenriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).

HYDROTHERMAL CARBONATES OF THE SCHWARZWALD ORE DISTRICT, SOUTHWESTERN GERMANY: CARBON SOURCE AND CONDITIONS OF FORMATION USING 18O, 13C, 87Sr/86Sr, AND FLUID INCLUSIONS

Diagenetic carbonates, metamorphic carbonates, primary hydrothermal carbonates, and secondary remobilized carbonates (including sinters) from the Schwarzwald ore district in SW Germany formed in various tectonic settings and hydrothermal environments over a period of almost 300 Ma. They were investigated in order to define sources of carbon, dispersion of carbon during fluid-rock interaction processes and, where possible, to specify geochemical fingerprints for carbonates formed during different processes and in different geochemical and tectonic environments. For this purpose, 335 samples of calcite, ankerite, dolomite, siderite, and strontianite from 92 localities in 46 mining areas in the Schwarzwald were analyzed for their carbon and oxygen stable isotope, radiogenic strontium isotope, and trace element (including REE) concentrations and compared to analyses from all potential carbon sources available in this region. These include graphite and rare marbles of the crystalline basement, Permian calcrete from redbed sedimentary rocks (Rotliegend) overlying the crystalline basement, and Triassic carbonates from sediments of higher stratigraphic levels (Muschelkalk). Hydrothermal carbonates mostly formed due to fluid-mixing of hot ascending brines with cool sediment-sourced formation water. Fluid inclusions record temperatures of formation between 100 and 150 °C for most primary calcites. The mixed fluid from which they formed was a highly saline brine of around 25 wt.% salinity, containing NaCl and CaCl 2 in similar proportions. Before mixing, the deep brine was in equilibrium with graphite of the basement and contained, as main carbon species, H 2 CO 3 of very low C-isotopic values [around −16‰, Vienna Pee Dee Belemnite (V-PDB)], whereas the sediment-sourced formation water contained HCO 3 − with higher C-isotopic values (around +2‰, V-PDB). We find that graphite and Triassic carbonates in variable proportions (which are mainly related to variations during the fluid mixing process) are the carbon sources for primary calcite, dolomite of the Permian calcrete for primary ankerite, and the Triassic carbonate sediments for the primary ankerite mineralization of the area between Waldkirch and Feldberg. At some localities, remobilization and reprecipitation appears to have taken place without addition of external solutes, as Sr and C show no difference in their isotopic composition between primary and secondary carbonates. The oxygen isotopic composition of secondary carbonates is invariably more positive than that of primary ones, reflecting lower formation temperatures. One very conspicuous type of secondary calcite, which forms olive-green stubby scalenohedra, was dated for the first time using the U-Pb isochron method. Its Neogene age represents uplift and erosional denudation of the Schwarzwald and corresponds well with its remobilized C and O isotope signature. The carbonates in the Schwarzwald hence reflect discontinuous addition of carbon from surface sediments to the crystalline basement through time involving fluid-rock interaction and fluid mixing processes.

Seepage of methane at Jaco Scar, a slide caused by seamount subduction offshore Costa Rica

Methane (CH4) concentrations and CH4 stable carbon isotopic composition (d13CCH4 ) were investigated in the water column within Jaco Scar. It is one of several scars formed by massive slides resulting from the subduction of seamounts offshore Costa Rica, a process that can open up structural and stratigraphical pathways for migrating CH4. The release of large amounts of CH4 into the adjacent water column was discovered at the outcropping lowermost sedimentary sequence of the hanging wall in the northwest corner of Jaco Scar, where concentrations reached up to 1,500 nmol L-1. There CH4-rich fluids seeping from the sedimentary sequence stimulate both growth and activity of a dense chemosynthetic community. Additional point sources supplying CH4 at lower concentrations were identified in density layers above and below the main plume from light carbon isotope ratios. The injected CH4 is most likely a mixture of microbial and thermogenic CH4 as suggested by d13CCH4 values between -50 and -62 % Vienna Pee Dee Belemnite. This CH4 spreads along isopycnal surfaces throughout the whole area of the scar, and the concentrations decrease due to mixing with ocean water and microbial oxidation. The supply of CH4 appears to be persistent as repeatedly high CH4 concentrations were found within the scar over 6 years. The maximum CH4 concentration and average excess CH4 concentration at Jaco Scar indicate that CH4 seepage from scars might be as significant as seepage from other tectonic structures in the marine realm. Hence, taking into account the global abundance of scars, such structures might constitute a substantial, hitherto unconsidered contribution to natural CH4 sources at the seafloor.

Organic Geochemical Composition of the Georgia Kaolins: Insights into Formation and Diagenetic Conditions

AbstractMost previous studies of the kaolin deposits in the southeastern United States have focused on their mineralogy and petrology to understand better the depositional and diagenetic environments of the kaolins. Many studies suggest, however, that much of the information held within the minerals was changed during extensive post-depositional groundwater and microbial alteration. Organic δ13C and biomarker analyses were used, therefore, to provide further information on the nature of the original sediments, the depositional environment(s), and the amount of diagenetic alteration that has occurred in Georgia kaolin deposits.Two different types of kaolin can be discerned, based on their total organic carbon contents: organic-lean kaolin and lignitic kaolin. The bulk organic δ13C in the Georgia kaolins ranges from ~−26 to −19% (VPDB, Vienna Pee Dee Belemnite standard), with a noticeable enrichment in 13C with decrease in organic carbon concentration. The lean kaolins are by far the more dominant types, with an organic-matter composition primarily of C16–C22n-alkanes, C16 and C18 fatty acids, and unresolved complex mixtures. Lignitic kaolin has a distinctly different organic matter (OM) composition. The lignitic material is primarily C15–C33n-alkanes with a greater abundance of C23–C31n-alkanes and lesser amounts of resinous and microbial constituents along with the oxidized forms of the saturated lipid fractions.Biomarker data suggest that the lignitic material is primarily terrestrially derived from conifers with minor input from microbial lipids. The OM in both types of kaolin shows strong signs of microbial decomposition that yield the organically lean kaolins. The oxidation of the detrital organic matter would subsequently yield organic acids that would have exerted significant influence on the mineralogy and metal mobility.

Factors determining the spatial distribution of gas-charged sediments in the continental shelf off southeastern Korea

A high-resolution seismic survey, piston coring, and water column measurements were conducted on the continental shelf off southeastern Korea to examine the gas-charged sediments and gas seepage in the water column. We examined the factors controlling the spatial distribution of the acoustic blanking zone in recent Korea Strait shelf mud (KSSM) deposits along the southeastern shelf. The molecular composition, methane, hydrogen, and carbon dioxide isotope values of headspace gases, and the geochemical composition of core sediments, were measured in 11 piston cores. Dissolved methane concentrations in the water column were observed at 6 of the 11 coring sites.Acoustic blanking of the recent KSSM deposits was limited to areas with water depths between 30 and 105m along the continental shelf, except line L-13 (133m depth). The flanks of the acoustic blanking zone can be classified as distinct, disturbed, and step types based on their configurations. The highest methane concentration in the headspace gases was about 2.7%, 2–4m below the seafloor, which matched the top of the acoustic blanking zone. No obvious methane was found in piston cores obtained from the gas-free zone. The δ13CCH4 ranged from −56.5 to −106.3‰ relative to Vienna Pee Dee Belemnite (V-PDB), and the δDCH4 ranged from −194 to −207‰ relative to Vienna Standard Mean Ocean Water (V-SMOW). These isotope values show that methane is produced by the microbial reduction of CO2. CH4 oxidation results in a significantly heavier carbon isotope fraction (from –91.2 to –56.5‰ in the 2m below the seafloor) with constant δ13CCO2 isotope fractionation. No obvious differences were seen in the total carbon (TC; 2.438–3.378%), total nitrogen (TN; 0.159–0.548%), total organic carbon (TOC; 1.139–2.078%), or CaCO3 (2.825–15.258%) in the continental shelf, whereas the TOC was high and the CaCO3 low distant from the Nakdong River. Distinct dissolved methane anomalies (21.87ppm) were seen in the water column in the acoustic blanking and gas seepage zones. This level was about 15 times higher than the dissolved methane level (1.5ppm) in the water column in the gas-free zone. These results indicate that the acoustic blanking zone accumulates biogenic methane. Methane seepage was observed in the water column throughout the acoustic blanking zone of the continental shelf off southeastern Korea.

Origin and fate of acetate in an acidic fen

Acetate is a central intermediate in the anaerobic degradation of organic matter, and the resolution of its metabolism necessitates integrated strategies. This study aims to (1) estimate the contribution of acetogenesis to acetate formation in an acidic fen (pH ~ 4.9), (2) assess the genetic potential for acetogenesis targeting the fhs gene encoding formyltetrahydrofolate synthetase (FTHFS) and (3) unravel the in situ turnover of acetate using stable carbon isotope pore-water analysis. H(2)/CO(2)-supplemented peat microcosms yielded (13)C-depleted acetate (-37.2‰ vs. VPDB (Vienna Peedee belemnite standard) compared with -14.2‰ vs. VPDB in an unamended control), indicating the potential for H(2)-dependent acetogenesis. Molecular analysis revealed a high diversity and depth-dependent distribution of fhs phylotypes with the highest number of operational taxonomic units in 0-20 cm depth, but only few and distant relationships to known acetogens. In pore waters, acetate concentrations (0-170 μM) and δ(13)C-values varied widely (-17.4‰ to -3.4‰ vs. VPDB) and did not indicate acetogenesis, but pointed to a predominance of sinks, which preferentially consumed (12)C-acetate, like acetoclastic methanogenesis. However, depth profiles of methane and δ(13)C(CH4) revealed a temporarily and spatially restricted role of this acetate sink and suggest other processes like sulfate and iron reduction played an important role in acetate turnover.

Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The delta Ca-44/40 ratios of the southwestern Dongsha area samples ranged from 1.21aEuro degrees to 1.52aEuro degrees and the ratio of the Baiyun Sag sample was 1.55aEuro degrees of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The delta C-13 values of the carbonates of the southwestern Dongsha area varied between -49.21aEuro degrees and -16.86aEuro degrees of the Vienna PeeDee Belemnite (VPDB) standard and the delta O-18 values ranged from 2.25aEuro degrees to 3.72aEuro degrees VPDB. The delta C-13 value of the Baiyun Sag sample was 2.36aEuro degrees VPDB and the delta O-18 value was 0.44aEuro degrees VPDB. The delta C-13 values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of delta C-13 values suggesting a dominant methane carbon source for the light samples and mixtures of delta C-13 values for the heavier samples, with possibly an organic or seawater carbon source. The delta O-18 values indicated that there is enrichment in O-18, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high delta Ca-44/40 values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revealed a clear correlation between the carbon and calcium isotope composition, indicating a link between the amount and/or rate of carbonate precipitation and methane contribution to the bicarbonate source. The results of the three stable isotope systems, mineralogy and petrography, show that mineral composition, the geochemical environment of authigenic carbonates and carbon source can control the calcium isotope fractionation.