Carbonate δ13C Values Research Articles

DYNAMICS OF A PALEOECOSYSTEM REEF ASSOCIATED WITH OCEANIC CHANGE IN CARBONATE SEDIMENTARY REGIME AND CARBON CYCLING (OXFORDIAN, SWISS JURA)

Abstract Herein we report an analysis of an Oxfordian (Upper Jurassic) paleoreef located in the Swiss Jura Mountains. The paleoreef is located in a Middle Oxfordian transitional interval in which sedimentation switched from marl-dominated to carbonate-dominated deposits. The paleoecosystem is composed of four successive fossil communities characterized by microsolenid corals and organisms that specialized in suspension feeding. Carbon isotopes measured from echinoid spine carbonates exhibit a positive trend from ∼1.0‰ to 2.5‰ in δ13C values from the base to the top of the paleoreef. Comparison of δ13C curves with organic matter and belemnites shows different patterns not compatible with a global variation of the carbon cycle. Similar fossil assemblages and stratigraphic sequences identical in age are found along the continental margin of the Tethys–Atlantic Ocean. This biolithostratigraphic succession corresponds to increasing δ13C values of marine and biogenic carbonates, to the transition from marl-domi...

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.

Stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of Proterozoic Lesser Himalayan rocks in central Nepal

In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in central Nepal, 441 new U/Pb isotopic ages of detrital zircons from the previously defined type Kuncha, Kushma, and Fagfog formations constrain the maximum possible depositional ages for these clastic units to be c. 1900, 1770, and 1810Ma, respectively. 182 detrital zircon U/Pb isotopic ages confirm the identity of another sample of the Fagfog Formation, and field relations indicate that this unit rests depositionally on the Kushma Formation with no obvious unconformity or lithologic difference. This interpretation, combined with the recognition that the term “Kushma Formation” historically has been applied incongruously to apparently different units in different parts of Nepal, leads us to recommend abandoning the term “Kushma Formation” and replacing it with “lower Fagfog Formation.” The overlying part of the Fagfog then becomes the “upper Fagfog Formation.” Because the 1770Ma constraint on the maximum depositional age comes from the lower part of the unit, the entire Fagfog Formation must have been deposited after 1770Ma. The depositional ages of both the lower and upper parts of the Fagfog must be younger than the Kuncha Formation because it was intruded by a granite at 1878±22Ma (2-sigma). Thus an unconformity lasting at least 86M.y. probably separates the Kuncha Formation (deposited between 1900 and 1856Ma, considering uncertainties) and the Fagfog Formation (deposited after 1770Ma). The lower Fagfog Formation is present in the Kali Gandaki region but apparently pinches out to the east because it is not present structurally beneath the Kathmandu nappe.Suggestions about the depositional age of the carbonate-dominated Malekhu Formation at the top of the Proterozoic part of the Lesser Himalayan series have ranged from Mesoproterozoic to late Paleozoic. These age estimates largely stem from correlation with parts of the Krol succession some 500km to the west in northwest India, and changing interpretations of the depositional age of the Krol rocks. 104 new measurements of carbon isotope abundances in carbonate from a 300m measured section through type exposures of the Malekhu Formation reveal a narrow range of δ13C values between −1.7 and +0.2‰ (VPDB) and a mean of −0.9±0.4‰ (1 standard deviation). These values are inconsistent with correlation with the Krol succession, which preserves large amplitude positive and negative excursions in δ13C values. Comparison with known marine carbonate δ13C values through time suggests deposition of the Malekhu Formation prior to c. 1300Ma. Upper and lower bounds for deposition of the Proterozoic part of the Lesser Himalayan series thus are c. 1300 and c. 1900Ma, though actual deposition likely occurred during only a portion of this interval. Further, a profound unconformity representing at least 900M.y. separates the Malekhu Formation from the depositionally overlying Carboniferous-Permian Sisne Formation. No upper Mesoproterozoic, Neoproterozoic, or lower Paleozoic rocks have been found in the Lesser Himalayan series in central Nepal. The carbon isotope stratigraphy of the Malekhu Formation in central Nepal matches that of the Buxa Formation exposed in the Ranjit window of Sikkim to the east, but does not match the chemostratigraphy of rocks correlated with the Buxa Formation in eastern Bhutan and Arunachal Pradesh, northeast India.

Isotopic disequilibrium during uptake of atmospheric CO2 into mine process waters: implications for CO2 sequestration.

Dypingite, a hydrated Mg-carbonate mineral, was precipitated from high-pH, high salinity solutions to investigate controls on carbon fixation and to identify the isotopic characteristics of mineral sequestration in mine tailings. δ(13)C values of dissolved inorganic carbon content and synthetic dypingite are significantly more negative than those predicted for equilibrium exchange of CO(2) gas between the atmosphere and solution. The measured δ(13)C of aqueous carbonate species is consistent with a kinetic fractionation that results from a slow diffusion of atmospheric CO(2) into solution. During dypingite precipitation, dissolved inorganic carbon concentrations decrease and δ(13)C values become more negative, indicating that the rate of CO(2) uptake into solution was outpaced by the rate of carbon fixation within the precipitate. This implies that CO(2) gas uptake is rate-limiting to CO(2) fixation. δ(13)C of carbonate mineral precipitates in mine tailings and of DIC in mine process waters display similar (13)C-depletions that are inconsistent with equilibrium fractionation. Thus, the rate of carbon fixation in mine tailings may also be limited by supply of CO(2). Carbon sequestration could be accelerated by increasing the partial pressure of CO(2) in tailings ponds or by using chemicals that enhance the uptake of gaseous CO(2) into aqueous solution.

Rare earth elements in authigenic methane-seep carbonates as tracers for fluid composition during early diagenesis

Authigenic carbonates forming at an active methane-seep on the Makran accretionary prism mainly consist of aragonite in the form of microcrystalline, cryptocrystalline, and botryoidal phases. The δ13Ccarbonate values are very negative (− 49.0 to − 44.0‰ V-PDB), agreeing with microbial methane as dominant carbon source. The δ18Ocarbonate values are exclusively positive (+ 3.0 to + 4.5‰ V-PDB) and indicate precipitation in equilibrium with seawater at bottom water temperatures. The content of rare earth elements and yttrium (REE + Y) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution ICP-MS varies for each aragonite variety, with early microcrystalline aragonite yielding the highest, cryptocrystalline aragonite intermediate, and later botryoidal aragonite the lowest REE + Y concentrations. Shale-normalised REE + Y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation: Microcrystalline aragonite shows high contents of middle rare earth elements (MREE), reflecting REE patterns ascribed to anoxic pore water. Cryptocrystalline aragonite exhibits a seawater-like REE + Y pattern at elevated total REE + Y concentrations, indicating higher concentrations of REEs in pore waters, which were influenced by seawater. Botryoidal aragonite is characterised by seawater-like REE + Y patterns at initial growth stages followed by an increase of light rare earth elements (LREE) with advancing crystal growth, reflecting changing pore fluid composition during precipitation of this cement. Conventional sample preparation involving micro-drilling of carbonate phases and subsequent solution ICP-MS does not allow to recognise such subtle changes in the REE + Y composition of individual carbonate phases. To be able to reconstruct the evolution of pore water composition during early diagenesis, an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases. High-resolution analysis of seep carbonates from the Makran accretionary prism by LA-ICP-MS reveals that pore fluid composition not only evolved in the course of the formation of different phases, but also changed during the precipitation of individual phases.

Modern calcite-precipitating experiment of algae and isotope imbalance

To understand the geochemistry effect of perplasmic carbonic anhydrase (pCA) during the sediment of calcium carbonate, Nosioc calcicola Breb. was studied. The photosynthetic carbon fixation in the obturator was enhanced by CA catalytically converting HCO3 − to CO2 and facilitating CO2 supply to the cell, which accelerated the sedimentation rate, reaching 22.4 mg/day/Lwater in the obturator and producing isotope fractionation. By testing δ13C and δ18O values of calcium carbonate affected by Nosioc calcicola Breb. and the corresponding contrast, the δ13C and δ18O values of calcium carbonate affected by Nosioc calcicola Breb. are 1.46 and −7.48‰, respectively, and the δ13C and δ18O values of contrast are −12.61 and −8.99‰, respectively, which means that isotope fractionation is influenced by the photosynthesis. Therefore, the results suggest that when isotope datum are used for paleoclimatic reconstruction by using stalagmites, local environment factors must be considered carefully to avoid the effect caused by metabolism.

Climatic cycles recorded in the Middle Eocene hemipelagites from a Dinaric foreland basin of Istria (Croatia)

Climatic cycles recorded in the Middle Eocene hemipelagites from a Dinaric foreland basin of Istria (Croatia)Middle Eocene hemipelagic marls from the Pazin-Trieste Basin, a foreland basin of the Croatian Dinarides, display repetitive alternations of two types of marls with different resistance to weathering. This study focuses on the chemical composition, stable isotopes, and palynomorph content of these marls in order to better understand the nature of their cyclic deposition and to identify possible paleoenvironmental drivers responsible for their formation. The less resistant marls (LRM) have consistently lower carbonate content, lower δ18O and δ13C values, and more abundant dinoflagellate cysts than the more resistant marls (MRM). We interpret these differences between the two marl types to be a result of climatic variations, likely related to Milankovitch oscillations. Periods with wetter climate, associated with increased continental runoff, detrital and nutrient influx produced the LRM. Higher nutrient supply sparked higher dinoflagellate productivity during these times, while reduced salinity and stratification of the water column may have hampered the productivity of calcareous nannoplankton and/or planktonic foraminifera. In contrast, the MRM formed during dryer periods which favoured higher carbonate accumulation rates. This study provides new information about the sedimentary record of short-scale climate variations reflected in wet-dry cycles during an overall warm, greenhouse Earth.

Initial results of comparing cold-seep carbonates from mussel- and tubeworm-associated environments at Atwater Valley lease block 340, northern Gulf of Mexico

Chemosymbiotic macrofauna (such as mussels and tubeworms) and authigenic carbonates are typical of many hydrocarbon seeps. To address whether mussels and tubeworms could impact the sediment geochemistry of their habitat where authigenic carbonates are precipitated, a comparative study of petrographic and geochemical features of the authigenic carbonates from mussel- and tubeworm-associated environments at hydrocarbon seeps in Atwater Valley lease area block 340 (AT340) of the Gulf of Mexico was undertaken. Both mussel- and tubeworm-associated carbonates are dominated by high-magnesium calcite (HMC) and aragonite, and two tubeworm-associated carbonate samples have minor amounts of dolomite. The δ13C values of all carbonates are low, ranging from −60.8‰ to −35.5‰ PDB. Although there is much overlap, surprisingly the δ13C values of mussel-associated carbonates are generally higher than those of tubeworm-associated carbonates (−51.8‰ vs. −54.8‰ for an average of over 60 subsamples). It is suggested that (1) carbon isotopic vital effect of seep mussels and tubeworms, (2) fluid physical pumping of mussels, and (3) release of sulfate by tubeworm roots may be responsible for the relatively lower δ13C values of tubeworm-associated carbonates. It has been suggested that the heterogeneities in mineralogy and stable carbon isotope geochemistry of the seep carbonates may be attributed to the activity of macrofauna (mussels and tubeworms) and associated microbes. Our observations also suggest that at AT340 the geochemical evolution of seep macrofauna is from a mussel-dominated environment to a mixed mussel-tubeworm environment, and finally to a mostly tubeworm-dominated environment. This evolution is controlled mainly by the habitat, e.g., hydrocarbon seep flux.

Pedogenic Carbonates in Texas: Stable-Isotope Distributions and Their Implications for Reconstructing Region-Wide Paleoenvironments

Abstract Pedogenic carbonates are ubiquitous in paleosols through geologic history, and have been widely used to speculate on paleoclimatic conditions during their formation. They are widespread and well developed in large areas of Texas. A majority are of late Quaternary age and have formed across a range of climatic conditions on various types of substrates. To evaluate the usefulness of pedogenic carbonates as environmental indicators, this study investigates regional-scale trends in stable-isotope compositions of pedogenic carbonates across approximately 900 km of Texas. The δ13C values of 49 pedogenic carbonate profiles (182 analyses) on various host strata (e.g., limestone, igneous rocks), range from −10.6‰ to +0.7‰ V-PDB. Most δ13C values are evenly distributed between −9‰ and −1‰, and the δ18O values range from −9.1‰ to +0.2‰ V-PDB, mainly between −6‰ and −2‰. There is no correlation between carbon and oxygen isotopic values (R2 = 0.0002). Pedogenic carbonates in the Texas Gulf Coastal Plains, Southern High Plains, and west Texas exhibit higher δ13C values than those in south and central Texas, which corresponds to the spatial change in vegetation between C4-dominated and C3-dominated plants. The marked increase in δ13C values of the pedogenic carbonates from the Tertiary strata into the overlying Quaternary strata confirms the invasion of a C4 biomass into the Southern High Plains since the early Pliocene. From east to west, the δ18O values of late Quaternary pedogenic carbonates decrease gradually, a trend that mimics the spatial variation of δ18O values of modern meteoric water primarily due to the increased distance from the Gulf of Mexico (source of most of the meteoric water). In contrast to the regional trends, the stable-isotope values of some pedogenic carbonates developed on marine limestone and calcareous alluvium in central Texas and south Texas have isotopic signatures inherited from the host strata. In addition, some of the δ13C values of pedogenic carbonate nodules in the Gulf Coastal Plains have been affected by saline groundwater. Soil carbonates formed in the shallow parts of the profile, such as petrocalcic horizons, may have also been influenced by the atmospheric CO2. Nevertheless, climatic trends are readily apparent in the isotopic data, thus demonstrating their potential value in region-wide paleoclimatic analyses.

Geochemical conditions for the preservation of recent aragonite-rich sediments in Mediterranean karstic marine lakes (Mljet Island, Adriatic Sea, Croatia)

Conditions for the preservation of recent aragonite-rich sediments during early diagenesis in two semi-enclosed Mediterranean karstic seawater lakes on the island of Mljet (Adriatic Sea) were examined. The concentrations and stable isotope compositions of carbonate and sedimentary organic matter, as well as the geochemical parameters in pore water were measured. It was found that the smaller lake (Malo Jezero) receives considerably more terrestrial detritus than the larger lake (Veliko Jezero). A decrease in carbonate δ13C values with depth indicated a rather intensive transfer of organically derived C into the carbonate pool by diagenetic recrystallisation, masking the changes in carbonate δ13C caused by increasing amounts of aragonite. Dissolution of calcite as a result of CO2 released from the decomposition of organic debris and the upward diffusive flux of dissolved inorganic carbon were together responsible for up to 24% of the dissolved inorganic carbon added to the pore water. This indicated locally occurring carbonate dissolution, irrespective of its saturation state in the bulk sediment. Despite the larger input of terrigenous material into Malo Jezero, the carbonate content in the sediment was much higher than in Veliko Jezero, indicating greater authigenic aragonite production. As magnesium calcite accounted for most of the carbonate dissolution, aragonite preservation in the sediment is favoured.

Comparison of δ13C and 14C activities of CO2 samples combusted in closed-tube and elemental-analyzer systems

Abstract For combustion of organic samples, a closed tube-combustion (CTC) method is generally used, but this method requires much time and labor. The AMS facility at the Center for Chronological Research, Nagoya University, has established a CO2 production and purification system using an elemental analyzer (EA) connected to cryogenic traps. This system enables faster combustion and CO2 purification of samples than does the conventional CTC method. We compared measured carbon yields, δ13C values and 14C activities between EA and CTC samples of carbon standards of various sample quantities. This newly developed EA system had low background 14C activity due to sample preparation; the background activity was similar to that observed for the CTC method. The EA system also showed high precision and accuracy for δ13C and 14C measurements. The EA sample data obtained from conventional quantities (>1 mg carbon) of standards showed good agreement with the results obtained by the CTC method and with the consensus values, though background contamination was detected for samples containing

Fatty-acids and their δ 13C characteristics of seep carbonates from the northern continental slope of Gulf of Mexico

Here we reported the fatty-acids and their δ 13C values in seep carbonates collected from Green Canyon lease block 185 (GC 185; Sample GC-F) at upper continental slope (water depth: ∼540 m), and Alaminos Canyon lease block 645 (GC 645; Sample AC-E) at lower continental slope (water depth: ∼2200 m) of the Gulf of Mexico. More than thirty kinds of fatty acids were detected in both samples. These fatty acids are maximized at C16. There is a clear even-over-odd carbon number predominance in carbon number range. The fatty acids are mainly composed of n-fatty acids, iso-/anteiso-fatty acids and terminally branched odd-numbered fatty acids (iso/anteiso). The low δ 13C values (−39.99‰ to.32.36‰) of n-C12:0, n-C13:0, i-C14:0 and n-C14:0 suggest that they may relate to the chemosynthetic communities at seep sites. The unsaturated fatty acids n-C18:2 and C18:1Δ9 have the same δ 13C values, they may originate from the Beggiatoa/Thioploca. Unlike other fatty acids, the terminally branched fatty acids (iso/anteiso) show lower δ 13C values (as low as −63.95‰) suggesting a possible relationship to sulfate reducing bacteria, which is common during anaerobic oxidation of methane at seep sites.

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

AbstractBulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.

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.

PREDICTION OF ATMOSPHERIC 13CO2 USING PLANT CUTICLE ISOLATED FROM FLUVIAL SEDIMENT: TESTS ACROSS A GRADIENT IN SALT CONTENT

Abstract In order to test the assertion that the carbon-isotopic composition of the ancient atmosphere (δ13CO2) can be reconstructed from the carbon-isotopic composition of fossil terrestrial plant tissues across a variety of environments, the δ13C value of land-plant tissues isolated from modern fluvial sediments was compared to that of today's atmosphere. Plant stem and leaf fragments were isolated from organic carbon-rich sediments of the Black River in Jamaica, which drains a basin containing only C3 ecosystems. Sediment was sampled at 12 sites along a dissolved salt-content gradient, from the coastal plain to near its mouth, which allowed evaluation of the effect of salt influence on the organic carbon-isotope signature. Many properties of the sediment varied systematically with salt content (e.g., mass-percent carbonate, abundance of palynomorphs, δ13C and δ18O values of carbonate), confirming a significant and increasing marine influence closer to the mouth of the river. The δ13C value of total org...

Patterns and magnitude of deep sea carbonate dissolution during Eocene Thermal Maximum 2 and H2, Walvis Ridge, southeastern Atlantic Ocean

Eocene Thermal Maximum 2 (ETM2 or H1; ∼53.7 Ma) represents a short‐lived warming episode, associated with the injection of a large mass of 13C‐depleted carbon into the ocean‐atmosphere system. The mass of injected carbon, the extent of deep sea dissolution, and the amount of warming during ETM2 appear to be approximately half of those documented for the Paleocene‐Eocene thermal maximum (PETM, ∼55.5 Ma), but the pattern of lysocline migration during ETM2 has not yet been documented sufficiently to decipher potential differences in carbon sources and sequestration mechanisms. We present high‐resolution carbonate dissolution and bulk stable isotope records across ETM2 and the successive H2 event (∼53.6 Ma) on a common age model for four sites along the Walvis Ridge depth transect (1500 to 3600 m paleowater depth) to assess lysocline evolution. The onset of ETM2 is characterized by multiple, depth‐dependent transitions of carbonate dissolution (up to ∼96% of the total flux), associated with rapid depletions in bulk carbonate carbon (up to ∼1–1.5‰) and oxygen (up to ∼0.7–1.5‰) isotope values. H2 shows a ∼0.7‰ negative carbon isotope excursion, with a coeval decrease in δ18O of ∼0.5‰ and ∼80% of carbonate dissolution. During ETM2, the lysocline recovered within ∼30 ka. We attribute this rapid recovery to terrestrial CaCO3 neutralization through enhanced chemical weathering of carbonates in soils and rocks. According to theory, carbonate dissolution was lower after recovery than prior to ETM2, indicating carbonate ion oversaturation and a deeper position of the lysocline. Spectral analysis indicates that the changes in carbonate dissolution and δ13C values were precession paced, implying that weathering feedbacks and short‐term perturbations in the carbon cycle were important in determining early Eocene background and hyperthermal ocean [CO32−] conditions.

Stable isotope characterisation of co-existing carbonates from the Holocene Gallocanta lake (NE Spain): palaeolimnological implications

The sedimentary succession of Gallocanta lake, a closed saline lake located in the Iberian Range (NE Spain), documents two successive lacustrine stages: (1) brackish lake stage and (2) shallow saline lake stage. The saline stage corresponds to the present-day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance. The carbonates of the brackish lake stage have relatively constant δ18O values, however, they are rather high (δ18ODo = 2.4‰ and δ18OCc = 4.5‰ mean values) suggesting a hydrologically closed lake with a long residence time of the waters. δ18O values of carbonates from the saline stage vary greatly, and are lighter than in the previous stage (δ18ODo = 0.5‰, δ18OCc = −0.7‰, δ18OMgs = −2.3‰ mean values). These carbonates also precipitated in a hydrologically closed lake, but in equilibrium with a lake water of more variable isotopic composition. The δ13C values for carbonates of both stages reflect a mixing of different pools of carbon, but during saline stage δ13C values have been more controlled by the equilibrium of the lake waters with atmospheric CO2. During the current stage, calcite and dolomite precipitate in Gallocanta lake mainly during spring and summer, although dolomite precipitation is more favoured towards the summer. Magnesite precipitates at the beginning of autumn, when the first rainfall re-dissolves the saline surface crust, producing saline waters with a high Mg2+ content. The isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates. It is considered that this could be due to two factors: either because there have not been many extremely dry years (like the year 2005) during the development of the lake, or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals.

Prediction of atmospheric δ13CO2 using fossil plant tissues

Reconstruction of the carbon isotope composition of atmospheric CO2 is critical to the understanding of long‐term global carbon cycling. We have suggested that the δ13C value of land plant carbon (δ13Cp) preserved in the geologic record should reflect the δ13CO2 at the time during which the plants grew (δ13Ca), based on a meta‐analysis of modern plant data. Here we present the results of laboratory experiments designed to quantify the relationship between plant tissue δ13C and δ13CO2 values under varying environmental conditions, including differential pCO2 ranging from 1 to 3 times today's levels. As predicted, plants grown under elevated pCO2 showed increased average biomass compared to controls grown at the same temperature. Across a very large range in δ13Ca (≈24‰) and pCO2 (≈740 ppmv) we observed a consistent correlation between δ13Ca and δ13Cp (p < 0.001). We show an average isotopic depletion of −25.4‰ for aboveground tissue and −23.2‰ for belowground tissue of Raphanussativus L. relative to the composition of the atmosphere under which it formed. For aboveground and belowground tissue, grown at both ∼23°C and ∼29°C, correlation was strong and significant (r2 ≥ 0.98 and p < 0.001); variation in pCO2 level had little or no effect on this relationship. These results validate our initial conclusion that in the absence of environmental stress, plant δ13C primarily reflects atmospheric δ13CO2 linearly across pCO2 levels; the demonstrated excellent correlation in δ13Ca and δ13Cp suggests a high level of predictive power across varying environmental conditions.

The Cu Stockwork and Massive Sulfide Ore of the Feitais Volcanic-Hosted Massive Sulfide Deposit, Aljustrel, Iberian Pyrite Belt, Portugal: A Mineralogical, Fluid Inclusion, and Isotopic Investigation

The Variscan Feitais volcanic-hosted massive sulfide deposit in the Aljustrel district of the Iberian Pyrite Belt consists of 55 million metric tons of Zn-Pb-Cu massive sulfide overlying a Cu-rich stockwork. The massive ore is overlain by up to 30 m of feldspar-phyric, rhyolitic volcaniclastic rock and locally by a jasper and/or chert layer up to 15 m thick. The massive sulfide orebody consists dominantly of pyrite, sphalerite, galena, chalcopyrite, tetrahedrite-tennantite, arsenopyrite, and bournonite, together with minor quartz, chlorite, sericite, carbonate, and barite. The orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated, locally silicified, felsic volcanic rock, the upper 30 to 60 m of which contains chalcopyrite-quartzchlorite-sericite-carbonate‐bearing stockwork vein(let)s that prior to deformation were at a shallow angle to the base of the massive orebody. Chloritized footwall rocks extend up to 20 m below the Cu stockwork zone and are underlain by up to 50 m of quartz-sericite-pyrite‐altered rhyolitic rock. The stockwork veins also contain pyrite, tetrahedrite-tennantite, sphalerite, and arsenopyrite. Pyrite, both in stockwork and massive ore, locally displays partly recrystallized framboidal, reniform, and cellular textures. Two generations of quartz, Q1 and Q2, and carbonate in the stockwork veins contain primary (in growth zones) and pseudosecondary fluid inclusions, with homogenization temperatures of 270° to 315°C and salinities of 2.2 to 8.1 wt percent NaCl equiv. The δ34S(CDT) values of massive and stockwork ores range from ‐15.4 to +4.7 (mean, ‐2.8) and ‐11.2 to +11.9 (mean, ‐0.4) per mil, respectively, the lowest values from colloformtextured pyrite. With no evidence of oxidation of sulfide sulfur during mineralization, the most negative values indicate an origin by biogenic reduction of seawater sulfate. The δ13C(PDB) values for carbonates, ‐7.5 to ‐13.7 and +9.3 to ‐14.3 per mil in massive and stockwork ore, respectively, indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon. There are no significant lateral or vertical variations in S isotope values in sulfides or C-O isotope values in carbonates, either in massive or stockwork ore. The δ18O(SMOW) values for quartz in stockwork and massive sulfide are 11.6 to 13.9 and 16.7 to 17.9 per mil, respectively. Coexisting, and texturally contemporaneous, carbonate and quartz in stockwork veins are not in isotopic equilibrium, indicating that the C-O isotope values may have been reset. The δ18O values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4.2 to 5.2 per mil. Barite from the hanging wall and massive ore yields δ34S values (21.9‐27.9‰) equal to or slightly higher than those of coeval seawater; 87Sr/86Sr ratios (0.708438‐ 0.709063) are slightly more radiogenic than those of coeval seawater (0.7080‐0.7085), and much more radiogenic than those of coeval volcanic rocks (0.703304‐0.706642), probably representing mixtures between seawater Sr and radiogenic Sr in fluids sourced in the crustal pile. Deposition of the massive sulfide on the sea floor is suggested by its stratiform nature, the stronger alteration of footwall relative to hanging-wall rocks, the stockwork system terminating sharply at the base of the massive sulfide, the presence of sedimentary-like textures in the massive sulfide, the absence of replacement fronts, and the presence of framboidal and other sea-floor depositional textures indicative of fluid quenching. The sheetlike form, lack of rubble mounds and chimneys, scarcity of barite, reduced mineral assemblage, and metal zoning distinguish Feitais from Kuroko-type deposits. It shares most of the characteristics of those Iberian Pyrite Belt deposits for which a brine-pool origin has been proposed based on fluid inclusion data,

Isotopic variability in the intertidal acorn barnacle Semibalanus balanoides : a potentially novel sea-level proxy indicator

Abstract We report variations in the δ 13 C and δ 18 O values of barnacle skeletal carbonate as well as the δ 13 C and δ 15 N value of tissue from specimens of the acorn barnacle S. balanoides , as a function of elevation within its living range on the Isle of May, Scotland. Individuals were sampled over a 3.50 m range at 0.25 m intervals (1.00–4.50 m above ordnance datum). Carbonate δ 18 O values (2.44±0.13‰ [1σ], n=45) and tissue δ 15 N values (8.17±0.42‰, n=15) do not vary systematically with elevation. The δ 13 C value of shell carbonate increases with elevation by c. 1‰ (total range: −0.77‰ to +0.63‰), but the variability between samples at the same elevation suggests that this relationship will be of limited use in constraining palaeo-elevation. By contrast, tissue δ 13 C values show systematic variation with elevation, increasing by c . 8‰ (total range: −19.36‰ to −8.77‰) with increasing elevation. These results suggest that there is potential to use the tissue δ 13 C values to determine the elevation of a Fixed Biological Indicator (FBI) such as S. balanoides within its living range. If this is also true of the organic matrix of the carbonate skeleton, and if this organic matrix is preserved in Holocene FBIs, then the potential exists to use carbon isotopes to more precisely constrain the palaeo-elevation of FBIs within their living range and hence palaeo sea-level. The small range of carbonate δ 18 O values suggests that oxygen isotopes in FBIs can be used to constrain water δ 18 O values if an independent estimate of temperature is available, or temperature if an independent estimate of salinity is available.