Mineral carbonation of peridotite fueled by magmatic degassing and melt impregnation in an oceanic transform fault.

Most of the geologic CO2 entering Earth's atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO2 and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite-talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul's transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO2(aq) concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ13C of -3.40 ± 0.04‰, and the enrichment of CO2 in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO2. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul's Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO2. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO2-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO2.

Changing surface ocean circulation caused the local demise of echinoid Scaphechinus mirabilis in Taiwan during the Pleistocene-Holocene transition.

Abundant fossil specimens of Scaphechinus mirabilis, now occurring mostly in temperate waters, have been found in the Toukoshan Formation (Pleistocene) in Miaoli County, Taiwan. Environmental changes leading to its extirpation (local extinction) have thus far been elusive. Here, we reconstruct past environmental and oceanic conditions off northwest Taiwan by analyzing clumped isotopes, as well as stable oxygen isotopes, of well-preserved fossil echinoid tests collected from the Toukoshan Formation. Radiocarbon dates suggest that these samples are from Marine Isotope Stage 3 (MIS 3). Paleotemperature estimates based on clumped isotopes indicate that fossil echinoids were living in oceanic conditions that range from 9 to 14 °C on average, comparable with the estimate derived for a modern sample from Mutsu Bay, Japan. Notably, this temperature range is ~ 10 °C colder than today's conditions off northwest Taiwan. The substantially lower temperatures during ~ 30 ka (MIS 3) compared to the modern conditions might be due to the rerouting of surface currents off northwest Taiwan when the sea level was ~ 60 m lower than today, in addition to the cooling caused by a lower atmospheric CO2 level during the Last Glacial Period. Colder waters brought here by the China Coastal Current (CCC) and the existence of shallow subtidal zones termed "Miaoli Bay" (mainly located in the present-day Miaoli county) during MIS 3 plausibly sustained generations of S. mirabilis, yielding tens of thousands of fossil specimens in the well-preserved fossil beds. The likely extirpation driver is the drastic change from a temperate climate to much warmer conditions in the shallow sea during the Pleistocene-Holocene transition.

Determination of the spatial distribution of wetting in the pore networks of rocks.

HYPOTHESIS: The macroscopic movement of subsurface fluids involved in CO2 storage, groundwater, and petroleum engineering applications is controlled by interfacial forces in the pores of rocks. Recent advances in modelling these systems has arisen from approaches simulating flow through a digital representation of the complex pore structure. However, further progress is limited by difficulties in characterising the spatial distribution of the wetting state within the pore structure. In this work, we show how observations of the fluid coverage of mineral surfaces within the pores of rocks can be used as the basis for a quantitative 3D characterisation of heterogeneous wetting states throughout rock pore structures. EXPERIMENTS: We demonstrate the approach with water-oil fluid pairs on rocks with distinct lithologies (sandstone and carbonate) and wetting states (hydrophilic, intermediate wetting, and heterogeneously wetting). FINDINGS: Fluid surface coverage the within rock pores is a robust signal of the wetting state across varying rock types and wetting states. The wetting state can be quantified and the resulting 3D maps can be used as a deterministic input to pore scale models. These may be applied to multiphase flow problems in porous media ranging from soil science to fuel cells.

Geochronological and geochemical data from fracture-filling calcites from the Lower Pedraforca thrust sheet (SE Pyrenees).

U-Pb dating using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), δ13C, δ18O, clumped isotopes and 87Sr/86Sr analysis, and electron microprobe have been applied to fracture-filling calcites and host carbonates from the Lower Pedraforca thrust sheet, in the SE Pyrenees. These data are used to determine the type and origin of migrating fluids, the evolution of the palaeohydrological system and timing of fracturing during the emplacement of this thrust sheet, as described in the article "From hydroplastic to brittle deformation: controls on fluid flow in fold and thrust belts. Insights from the Lower Pedraforca thrust sheet (SE Pyrenees)" - Marine and Petroleum Geology (2020). The integration of these data is also used to compare the fluid flow evolution of the Southern Pyrenees with that of other orogens worldwide and to generate a fluid flow model in fold and thrust belts. At a more local scale, the U-Pb dataset provides new absolute ages recording the deformation in the Lower Pedraforca thrust sheet, which was previously dated by means of indirect methods such as biostratigraphy of marine sediments and magnetostratigraphy of continental deposits.

Effects of oxygen plasma ashing treatment on carbonate clumped isotopes.

RATIONALE: For clumped isotope analysis (Δ47 ), hydrocarbon and organic molecules present an important contaminant that cannot always be removed by CO2 purification through a Porapak-Q trap. Low-temperature oxygen plasma ashing (OPA) is a quick and easy approach for treatment; however, the impact of this treatment on the original carbonate clumped isotope values has never been fully studied. METHODS: We tested the isotopic impact of OPA using three natural samples with a large range of initial Δ47 values. Crushed and sieved (125 µm mesh) samples were placed into a Henniker Plasma HPT-100 plasma system and treated at a flow rate of 46 mL/min and a power of 100 W at a vacuum of 0.2 mbar for 10, 20, 30 and 60 min before clumped isotope analysis using two MAT 253 isotope ratio mass spectrometers modified to measure masses 44-49. RESULTS: OPA treatment for 30 min or more on calcite powder samples has the potential to alter the clumped isotopic composition of the samples beyond analytical error. A systematic positive offset is observed in all samples. The magnitude of this alteration translates to a temperature offset from known values ranging from 4°C to 13°C. We postulate that the observed positive offset in Δ47 occurs because the bonds within lighter isotopologues are preferentially broken by plasma treatment, leading to an artificial increase in the 'clumping' value of the sample. CONCLUSIONS: We recommend that any laboratory performing OPA treatments should reduce the runs to 10-20 min or carry out successive runs of 10 min followed by sample stirring, as this procedure showed no alteration in the initial Δ47 values. Our results validate the use of OPA for clumped isotope applications and will allow future research to use clumped isotopes for challenging samples such as oil-stained carbonates, bituminous shales or host rocks with very high organic carbon content.

Reducing contamination parameters for clumped isotope analysis: The effect of lowering Porapak™ Q trap temperature to below -50°C.

RATIONALE: Carbonate clumped isotope thermometry examines the thermodynamic preference of 13 C-18 O bonds to form within the carbonate crystal lattice. The 13 C18 O16 O isotopologue in analyte CO2 has a natural abundance of 44.4 ppm necessitating stringent purification procedures to remove contaminant molecules that may produce significant isobaric effects within range of the mass 47 isotopologue. Strict purifications of analyte CO2 are thus required as well as reliable contamination indicators. METHODS: CO2 purification was carried out by vacuum cryogenic purification through a static trap packed with Porapak™ Q (PPQ). The correlation between mass excesses on m/z 47, 48 and 49 in CO2 produced by acid digestion of 12 natural samples was measured by isotope ratio mass spectrometry (IRMS). CO2 from two contaminated carbonate samples was then purified at PPQ trap temperatures between -25 and -65°C and measured by IRMS to determine changes in mass excesses on m/z 47, 48 and 49. Finally carbonate standards, Carrara marble (CM) and ETH3, were purified at PPQ trap temperatures of -35 and -60°C to identify isotopic fractionation associated with lowering trap temperature. RESULTS: The correlation between mass excesses on m/z 47, 48 and 49 is determined to be sample dependent. Lowering the PPQ trap temperature to -60°C has a 78% success rate in decreasing Δ48offset , a measure of sample contamination, to within an acceptable range (<1.5 ‰). Lowering the PPQ temperature in purification of CM and ETH3 is associated with decreases in the δ13 C and δ18 O values as a result of isotopic fractionation. We demonstrate that we can correct for fractionation at a trap temperature of -60°C. CONCLUSIONS: Lowering the temperature of the Porapak Q trap to -60°C results in improved sample cleaning. It is possible to correct for fractionation in δ13 C and δ18 O values at lower PPQ trap temperatures using identically prepared standards. This result has important connotations for laboratories using similar sample preparation methods.

Community software for challenging isotope analysis: First applications of 'Easotope' to clumped isotopes.

RATIONALE: The measurement of complex isotope systems, notably the multiply substituted isotopologues of CO2 derived from carbonates, is challenging from a mass spectrometric point of view, but it is also time consuming and difficult from a data reduction and normalization perspective. Dedicated software often lags behind and currently limits fast, reliable and reproducible data analysis and inter-laboratory reproducibility. METHODS: We have developed new community software 'Easotope' using Java and the Eclipse framework. The objectives were to reduce and normalize complex isotopic data easily using a program that could run on multiple platforms, with a central database to store data and constants, an open architecture giving end users a complete view of the data processing steps, and a permissions system allowing the administrator to empower each user in proportion to their expertise. RESULTS: Easotope is now freely available to download, and comprises both a server and a client executable. The server can be run either on a remote machine accessible via the internet, or on a localhost. The client allows users to access the server, and to enter and manipulate data. Easotope currently supports full data storage, data processing and data normalization for bulk isotopes of carbon and oxygen, and for clumped isotopes. CONCLUSIONS: Easotope greatly simplifies data processing, reducing processing time to less than a second compared with 30 min when done manually. The software also ensures consistency in data reduction and normalization both within a laboratory and between laboratories. Easotope is designed with the ability to implement other isotopic systems in the future. Copyright © 2016 John Wiley & Sons, Ltd.

Environmental precursors to rapid light carbon injection at the Palaeocene/Eocene boundary.

The start of the Palaeocene/Eocene thermal maximum--a period of exceptional global warming about 55 million years ago--is marked by a prominent negative carbon isotope excursion that reflects a massive input of 13C-depleted ('light') carbon to the ocean-atmosphere system. It is often assumed that this carbon injection initiated the rapid increase in global surface temperatures and environmental change that characterize the climate perturbation, but the exact sequence of events remains uncertain. Here we present chemical and biotic records of environmental change across the Palaeocene/Eocene boundary from two sediment sections in New Jersey that have high sediment accumulation rates. We show that the onsets of environmental change (as recorded by the abundant occurrence ('acme') of the dinoflagellate cyst Apectodinium) and of surface-ocean warming (as evidenced by the palaeothermometer TEX86) preceded the light carbon injection by several thousand years. The onset of the Apectodinium acme also precedes the carbon isotope excursion in sections from the southwest Pacific Ocean and the North Sea, indicating that the early onset of environmental change was not confined to the New Jersey shelf. The lag of approximately 3,000 years between the onset of warming in New Jersey shelf waters and the carbon isotope excursion is consistent with the hypothesis that bottom water warming caused the injection of 13C-depleted carbon by triggering the dissociation of submarine methane hydrates, but the cause of the early warming remains uncertain.

The Palaeocene-Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records.

The Palaeocene-Eocene thermal maximum (PETM) is characterized by a global negative carbon isotope excursion (CIE) and widespread dissolution of seafloor carbonate sediments. The latter feature supports the hypothesis that the PETM and CIE were caused by the rapid release of a large mass (greater than 2000Gt C) of 12C-enriched carbon. The source of this carbon, however, remains a mystery. Possible sources include volcanically driven thermal combustion of organic-rich sediment, dissociation of seafloor methane hydrates and desiccation and oxidation of soil/sediment organics. A key constraint on the source(s) is the rate at which the carbon was released. Fast rates would be consistent with a catastrophic event, e.g. massive methane hydrate dissociation, whereas slower rates might implicate other processes. The PETM carbon flux is currently constrained by high-resolution marine and terrestrial records of the CIE. In pelagic bulk carbonate records, the onset of the CIE is often expressed as a single- or multiple-step excursion extending over 10(4) years. Individual planktonic shell records, in contrast, always show a single-step CIE, with either pre-excursion or excursion isotope values, but no transition values. Benthic foraminifera records, which are less complete owing to extinction and diminutive assemblages, show a delayed excursion. Here, we compile and evaluate the individual planktonic shell isotope data from several localities. We find that the most expanded records consistently show a bimodal isotope distribution pattern regardless of location, water depth or depositional facies. This suggests one of several possibilities: (i) the isotopic composition of the surface ocean/atmosphere declined in a geologic instant (<500yr), (ii) that during the onset of the CIE, most shells of mixed-layer planktonic foraminifera were dissolved, or (iii) the abundances or shell production of these species temporarily declined, possibly due to initial pH changes.