A Temperature-Dependent Positive Feedback on the Magnitude of Carbon Isotope Excursions

The decrease in the average magnitude of carbon isotope excursions in marine carbonates over Phanerozoic time is a longstanding unresolved problem. In addition, carbon isotope excursions commonly co-occur with oxygen isotope excursions of the same sign, implying the existence of a longstanding link between organic carbon burial fluxes and temperature. It was proposed that this connection was provided by the thermodynamic relationship between temperature and microbial respiration rates – changes in temperature drive changes in organic carbon remineralization rate and organic carbon burial efficiency. Such a mechanism provides the logic for a positive feedback affecting the magnitude of both climate changes and carbon isotope excursions. Here, we employ feedback analysis to quantify the strength of this mechanism with modifications to a simple carbon isotope mass balance framework. We demonstrate that the potential strength of this feedback is large (perhaps several permil) for plausible ranges of historical climate change. Furthermore, our results highlight the importance of the surface temperature boundary condition on the magnitude of the expected carbon isotope excursion. Comparisons of our model predictions with data from the terminal Eocene and Late Ordovician (Hirnantian) greenhouse–icehouse climate transitions suggest that these excursions might be substantially explained by such a thermodynamic microbial respiration feedback. Consequently, we hypothesize that the observed pattern of decreasing excursion magnitude toward the present might be explained at least, in part, by a decrease in the mean temperature of environments of organic carbon burial driven by long-term climate and paleogeographic trends. SOMMAIRELa diminution de l'amplitude moyenne des excursions des isotopes du carbone dans les carbonates marins au fil du Phanérozoïque est une énigme de longue date.  On note en outre que les excursions des isotopes du carbone coexistent couramment avec des excursions isotopiques de même signe de l'oxygène, ce qui implique l'existence d'un lien de longue date entre les flux d’enfouissement du carbone organique et la température.  On a suggéré que ce lien découlait de la relation thermodynamique entre la température et les taux de respiration microbienne - les changements de température déterminent le taux de reminéralisation du carbone organique et l’efficacité de l’enfouissement du carbone organique.  Un tel mécanisme peut expliquer la rétroaction positive affectant à la fois l'ampleur des changements climatiques et les excursions des isotopes du carbone.  Dans le cas présent, nous utilisons l'analyse de la rétroaction pour quantifier la robustesse de ce mécanisme avec des modifications d’un simple bilan de masse des isotopes du carbone.  Nous démontrons que la robustesse potentielle de cette rétroaction est forte (peut-être plusieurs pour mille) dans les gammes plausibles du changement climatique historique.  De plus, nos résultats mettent en évidence l'importance de la condition aux limites de la température de surface sur l'ampleur de l'excursion isotopique du carbone attendue.  Les comparaisons des prédictions de notre modèle avec les données de la fin de l'Éocène et de la fin de l’Ordovicien (Hirnantien) des transitions climatiques à effet de serre-effet/de glaciation permettent de penser que ces excursions pourraient être correctement expliquées par une telle rétroaction de la thermodynamique de la respiration microbienne.  Par conséquent, nous émettons l'hypothèse que la tendance observée de diminution de l'ampleur de l’amplitude des excursions du passé vers le présent peut s'expliquer, au moins en partie, par une diminution de la température moyenne du milieu d'enfouissement du carbone organique engendrée par des tendances climatiques et paléogéographiques à long terme

Emerging biogeochemical views of Earth's ancient microbial worlds

This work was supported by the NASA Astrobiology Institute under Cooperative Agreement No. NNA15BB03A issued through the Science Mission Directorate (TWL), a Natural Environment Research Council Fellowship (NE/H016805/2) (AZ), and National Science Foundation grants (EAR-0951509, OCE-1061476, EAR-1124389, and OCE-1155346) and a Packard Fellowship (DAF).Microbial processes dominate geochemical cycles at and near the Earth’s surface today. Their role was even greater in the past, with microbes being the dominant life form for the first 90% of Earth’s history. Most of their metabolic pathways originated billions of years ago as both causes and effects of environmental changes of the highest order, such as the first accumulation of oxygen in the oceans and atmosphere. Microbial processes leave behind diverse geochemical fingerprints that can remain intact for billions of years. These rock-bound signatures are now steering our understanding of how life coevolved with the environments on early Earth and are guiding our search for life elsewhere in the universe.PostprintPeer reviewe

Spatial Grain Size Sorting in Eolian Ripples and Estimation of Wind Conditions on Planetary Surfaces: Application to Meridiani Planum, Mars

The landscape seen by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum is dominated by eolian (wind-blown) ripples with concentrated surface lags of hematitic spherules and fragments. These ripples exhibit profound spatial grain size sorting, with well-sorted coarse-grained crests and poorly sorted, generally finer-grained troughs. These ripples were the most common bed form encountered by Opportunity in its traverse from Eagle Crater to Endurance Crater. Field measurements from White Sands National Monument, New Mexico, show that such coarse-grained ripples form by the different transport modes of coarse- and fine-grain fractions. On the basis of our field study, and simple theoretical and experimental considerations, we show how surface deposits of coarse-grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces. Activation of Meridiani Planum coarse-grained ripples requires a wind velocity of 70 m/s (at a reference elevation of 1 m above the bed). From images by the Mars Orbiter Camera (MOC) of reversing dust streaks, we estimate that modern surface winds reach a velocity of at least 40 m/s and hence may occasionally activate these ripples. The presence of hematite at Meridiani Planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments; however, the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

Sedimentary Iron Cycling and the Origin and Preservation of Magnetization in Platform Carbonate Muds, Andros Island, Bahamas

Carbonate muds deposited on continental shelves are abundant and well-preserved throughout the geologic record because shelf strata are difficult to subduct and peritidal carbonate units often form thick, rheologically strong units that resist penetrative deformation. Much of what we know about pre-Mesozoic ocean chemistry, carbon cycling, and global change is derived from isotope and trace element geochemistry of platform carbonates. Paleomagnetic data from the same sediments would be invaluable, placing records of paleolatitude, paleogeography, and perturbations to the geomagnetic field in the context and relative chronology of chemostratigraphy. To investigate the depositional and early diagenetic processes that contribute to magneitzation in carbonates, we surveyed over 500 core and surface samples of peritidal, often microbially bound carbonate muds spanning the last not, vert, similar 1000 yr and deposited on top of Pleistocene aeolianites in the Triple Goose Creek region of northwest Andros Island, Bahamas. Sedimentological, geochemical, magnetic and ferromagnetic resonance properties divide the sediment columns into three biogeochemical zones. In the upper sediments, the dominant magnetic mineral is magnetite, produced by magnetotactic bacteria and dissimiliatory microbial iron metabolism. At lower depths, above or near mean tide level, microbial iron reduction dissolves most of the magnetic particles in the sediment. In some cores, magnetic iron sulfides precipitate in a bottom zone of sulfate reduction, likely coupled to the oxidation of decaying mangrove roots. The remanent magnetization preserved in all oriented samples appears indistinguishable from the modern local geomagnetic field, which reflects the post-depositional origin of magnetic particles in the lower zone of the parasequence. While we cannot comment on the effects of late-stage diagenesis or metamorphism on remanence in carbonates, we postulate that early-cemented, thin-laminated parasequence tops in ancient peritidal carbonates are mostly likely to preserve syn-depositional paleomagnetic directions and magnetofossil stratigraphies

A Stratified Redox Model for the Ediacaran Ocean

The Ediacaran Period (635 to 542 million years ago) was a time of fundamental environmental and evolutionary change, culminating in the first appearance of macroscopic animals. Here, we present a detailed spatial and temporal record of Ediacaran ocean chemistry for the Doushantuo Formation in the Nanhua Basin, South China. We find evidence for a metastable zone of euxinic (anoxic and sulfidic) waters impinging on the continental shelf and sandwiched within ferruginous [Fe(II)-enriched] deep waters. A stratified ocean with coeval oxic, sulfidic, and ferruginous zones, favored by overall low oceanic sulfate concentrations, was maintained dynamically throughout the Ediacaran Period. Our model reconciles seemingly conflicting geochemical redox conditions proposed previously for Ediacaran deep oceans and helps to explain the patchy temporal record of early metazoan fossils

Sulfur and oxygen isotope insights into sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea (5 m depth) hydrothermal venting off Milos Island provides an ideal opportunity to target transitions between igneous abiogenic sulfide inputs and biogenic sulfide production during microbial sulfate reduction. Seafloor vent features include large (>1 m2) white patches containing hydrothermal minerals (elemental sulfur and orange/yellow patches of arsenic-sulfides) and cells of sulfur oxidizing and reducing microorganisms. Sulfide-sensitive film deployed in the vent and non-vent sediments captured strong geochemical spatial patterns that varied from advective to diffusive sulfide transport from the subsurface. Despite clear visual evidence for the close association of vent organisms and hydrothermalism, the sulfur and oxygen isotope composition of pore fluids did not permit delineation of a biotic signal separate from an abiotic signal. Hydrogen sulfide (H2S) in the free gas had uniform δ34S values (2.5 ± 0.28‰, n = 4) that were nearly identical to pore water H2S (2.7 ± 0.36‰, n = 21). In pore water sulfate, there were no paired increases in δ34SSO4 and δ18OSO4 as expected of microbial sulfate reduction. Instead, pore water δ34SSO4 values decreased (from approximately 21‰ to 17‰) as temperature increased (up to 97.4°C) across each hydrothermal feature. We interpret the inverse relationship between temperature and δ34SSO4 as a mixing process between oxic seawater and 34S-depleted hydrothermal inputs that are oxidized during seawater entrainment. An isotope mass balance model suggests secondary sulfate from sulfide oxidation provides at least 15% of the bulk sulfate pool. Coincident with this trend in δ34SSO4, the oxygen isotope composition of sulfate tended to be 18O-enriched in low pH (75°C) pore waters. The shift toward high δ18OSO4 is consistent with equilibrium isotope exchange under acidic and high temperature conditions. The source of H2S contained in hydrothermal fluids could not be determined with the present dataset; however, the end-member δ34S value of H2S discharged to the seafloor is consistent with equilibrium isotope exchange with subsurface anhydrite veins at a temperature of ~300°C. Any biological sulfur cycling within these hydrothermal systems is masked by abiotic chemical reactions driven by mixing between low-sulfate, H2S-rich hydrothermal fluids and oxic, sulfate-rich seawater

Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate : insights from paired S and O isotopic analyses

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (#57548-ND2) to D.F. for partial support of this research and from the Estonian Research Council (#PUT611, #PRG836) to O.H and A.L.Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.PostprintPeer reviewe

Depositional and diagenetic constraints on the abundance and spatial variability of carbonate-associated sulfate

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (#57548-ND2) for partial support of this research.Marine carbonate rocks are composed, in varying abundance, of cement, micrite, abiotic grains and fossils, which can provide information about the physical and chemical environments in which they formed. Geochemical analyses of these carbonates are not always interpreted alongside the wealth of geologic (including petrographic) information available, resulting in potentially faulty reconstructions of biogeochemical and environmental conditions. These concerns have prompted closer scrutiny of the effect of depositional lithofacies and diagenesis on carbonate proxies. Here, we have combined X-ray Absorption Near Edge Structure (XANES) spectroscopy and μ-X-ray Fluorescence (μ-XRF) imaging to map the speciation and abundance of sulfur in carbonate petrographic thin sections in Upper Ordovician carbonates from Anticosti Island, Canada and early Silurian carbonates from Gotland, Sweden, across multiple depositional facies. Lithofacies and fossil communities between Anticosti Island and Gotland are similar, which allows for comparison of changes in the dominant S species and their abundance in separate basins, associated with variations in (glacio)eustatic sea level. Sulfide abundance is greatest in mudstone, wackestone and packstone facies, where interstitial micrite hosts abundant pyrite. Sulfate abundance, as carbonate-associated sulfate (CAS), varies within individual fossil fragments, as well as within the same fossil phylum and is particularly high in unaltered brachiopods. In contrast, sulfate abundance is generally very low in micrite (near the detection limit) and generally arises in situ from sulfide that has been oxidized as opposed to true CAS. In different cement fabrics, sulfate abundance is greatest in drusy, pore-filling cements. Organic sulfur compounds are also detected and, although low in abundance, are mostly found within micrite. The detection and characterization of both inorganic sulfur and organic sulfur compounds provides a platform to understand early processes of biomineralization. This approach will broaden our understanding of the source of inorganically bound sulfate in ancient carbonates, as well as the effect of depositional setting and diagenesis on CAS incorporation, (re)mobilization, and ultimate abundance in sedimentary carbonates. Additionally, this work has implications for the CAS isotopic value of individual carbonate components that may affect interpretations of stratigraphic variability of numerous CAS sections throughout Earth history.PostprintPeer reviewe

Records of carbon and sulfur cycling during the Silurian Ireviken Event in Gotland, Sweden

Field and stable isotope work was supported by an Agouron Institute grant to DAF and WWF as well as a Packard Fellowship and a Hanse-wissenschaftskolleg Fellowship awarded to DAF.Early Silurian (∼431 Ma) carbonate rocks record a ca. 4.5‰ positive excursion in the stable isotopic composition of carbonate carbon (δ13Ccarb). Associated with this isotopic shift is a macroevolutionary turnover pulse known as the ‘Ireviken Event’. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (∼0.6‰) in the stable isotopic composition of carbonate oxygen (δ18Ocarb). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in δ18Ocarb but show positive excursions of equal magnitude in both carbonate (δ13Ccarb) and organic (δ13Corg) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7‰ positive excursion in carbonate-associated sulfate (δ34SCAS) and a ca. 30‰ positive excursion in pyrite (δ34Spyr). The increase in δ34Spyr values is accompanied by a substantial, concomitant increase in stratigraphic variability of δ34Spyr. The relatively constant offset between the δ13Ccarb and δ13Corg excursions throughout the Ireviken Event could be attributed to increased organic carbon burial, or possibly a change in the isotopic composition of CO2 sources from weathering. However, a positive correlation between carbonate abundance and δ13Ccarb suggests that local to regional changes in dissolved inorganic carbon (DIC) during the shallowing-upward sequence may have been at least partly responsible for the observed excursion. The positive excursion recorded in δ34SCAS suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir. An inverse correlation between CAS abundance and δ34SCAS supports the notion of decreased sulfate concentrations, at least locally, consistent with a concomitant increase in pyrite burial. A decrease in the offset between δ34SCAS and δ34Spyr values during the Ireviken Event suggests a substantial reduction in the isotopic fractionations (εpyr) expressed during microbial sulfur cycling and pyrite precipitation through this interval. Decreased εpyr and the concomitant increase in stratigraphic variation in δ34Spyr are typical of isotope systematics observed in modern shallow-water environments, associated with increased closed-system behavior and/or oxidative sedimentary reworking during early sediment diagenesis. While the isotopic trends associated with the Ireviken Event have been observed in multiple locations around the globe, many sections display different magnitudes of isotopic change, and moreover, are typically associated with local facies changes. Due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the Ireviken Event, and their relationship to changes in local depositional environment, we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes.PostprintPeer reviewe