Geochemistry Geophysics Geosystems最新文献

The relationship between the lithosphere and the mantle during the supercontinent cycle is complex and poorly constrained. The processes which drive dispersal are often simplified to two end members: slab pull and plume push. We aim to explore how lithosphere thickness and viscosity during supercontinent assembly may affect the interaction of deep mantle structures throughout the supercontinent cycle. We consider supercontinental lithosphere structure as one of many potential processes which may affect the evolution of upwellings and downwellings and therefore systematically vary the properties of continental and cratonic lithosphere, respectively within our 3D spherical simulations. The viscosity and thickness of the lithosphere alters the dip and trajectory of downwelling material beneath the supercontinent as it assembles. Focusing on Pangea, we observe that plumes evolve and are swept beneath the center of the supercontinent by circum-continental subduction. The proximity of these upwelling and downwelling structures beneath the supercontinent interior varies with lithosphere thickness and viscosity. Where slabs impinge on the top of an evolving plume head (when continental and cratonic lithosphere are thick and viscous in our simulations), the cold slabs can reduce the magnitude of an evolving plume. Conversely, when the continental lithosphere is thin and weak in our simulations, slab dips shallow in the upper mantle and descend adjacent to the evolving plume, sweeping it laterally near the core-mantle boundary. These contrasting evolutions alter the magnitude of the thermal anomaly and the degree to which the plume can thin the lithosphere prior to breakup.

Solid Earth CO₂ outgassing, driven by plate tectonic processes, is a key driver of carbon cycle models. However, the magnitudes and variations in outgassing are poorly constrained in deep-time. We assess plate tectonic carbon emissions and sequestration by coupling a plate tectonic model with reconstructions of oceanic plate carbon reservoirs and a thermodynamic model to quantify outfluxes from slabs and continental arcs over 1 billion years. In the early Neoproterozoic, our model predicts a peak in crustal production and net outgassing from 840 to 780 Ma that corresponds to a contemporaneous pulse in large igneous province eruptions. The Sturtian and Marinoan glaciations (717–635 Ma) correspond to a low in mid-ocean ridge outgassing, while the following Ediacaran global warming coincides with a rise in net atmospheric carbon influx, driven by an increase in plate boundary and rift length. The Cambrian, Silurian/Devonian and Triassic Jurassic hothouse climates are synchronous with a reduction in carbon sequestration flux into oceanic plates, increasing net outgassing. In contrast, the Early Cretaceous hothouse climate is accompanied by a pronounced increase in mid-ocean ridge outgassing. Both the Early Ordovician cooling and the late Paleozoic ice ages coincide with a significant decrease in net atmospheric outgassing, driven by an increase in carbon sequestration. The late Cenozoic glaciation is associated with a long-term decrease in mid-ocean ridge and rift degassing, and a pronounced increase in carbon flux into pelagic carbonate sediments. Our tectono-thermodynamic carbon cycle model provides a new foundation for future long-term climate and geochemical cycling models.

Oxidative weathering of organic carbon in sedimentary rocks is a major source of CO₂ to the atmosphere over geological timescales, but the size of this emission pathway in Earth's past has not been directly quantified due to a lack of available proxy approaches. We have measured the rhenium isotope composition of organic-rich rocks sampled from unweathered drill cores and weathered outcrops in south Texas, whose stratigraphic successions can be tightly correlated. Oxidative weathering of more than 90% of the organic carbon and ∼85% of the rhenium is accompanied by a shift to lower rhenium isotope compositions in the weathered outcrops. The calculated isotope composition of rhenium weathered from the initial bedrock for individual samples varies systematically by ∼0.7‰ with different fractions of rhenium loss. This variation can be empirically modeled with isotope fractionation factors of α = 1.0002–1.0008. Our results indicate that the isotope composition of rhenium delivered to the oceans can be altered by weathering intensity of rock organic matter and that the rhenium isotope composition of seawater is sensitive to past oxidative weathering and associated CO₂ emissions.

Active and relic marine methane-seep sites are widely distributed globally and are distinguished by distinctive geology, biogeochemistry, and ecosystems. The discovery of methane-seep sites in the Krishna-Godavari (K-G) basin has created exciting new opportunities for methane-seep research in the Bay of Bengal. In this study, we document the occurrence of authigenic carbonates, including micro-crystalline aragonite crust (arg-crusts) admixed with chemosynthetic shells and high-magnesium carbonate tubular structures (HMC-tube), from the methane-seep site SSD-045/4 in the K-G basin. The δ¹³C values of HMC-tubes (−54.5 to −46.2‰) and arg-crusts (−57.6 to −34.8‰) indicate biogenic methane as the likely carbon source. Enhanced porewater alkalinity driving carbonate precipitation may be attributed to microbial-mediated SO₄²⁻-AOM processes. Additionally, δ¹³C values (−35.2 ± 8‰) of the residual organic matter within the carbonates suggest a contribution of methanotrophic bacterial biomass. The δ¹⁸O_carb values of HMC and aragonite indicate methane hydrate degassing and crystallization pathways, respectively. Pelloid-filled burrows suggest the reworking of shallow HMC deposit by burrowing organisms, whereas the polyphase cementations (aragonite and HMC) within burrows indicate early and burial diagenetic pathways. The wide range in ΣLREE/ΣHREE ratios and Ce_anom values in arg-crusts reflect micro-spatial variations in redox conditions, likely due to cementation occurring in both open and closed diagenetic systems. In contrast, more constrained Ce_anom values and ΣLREE/ΣHREE ratios in HMC tubes suggest persistent sulfidic conditions. Overall, these findings provide insights into the pathways of carbonate formation at the K-G basin methane-seep site, highlighting the complex interplay of microbial processes, fluid dynamics, and diagenetic alterations.

We investigate full vector paleomagnetic changes recorded in high-resolution sediments of Petermann Fjord, North Greenland, deposited over the last 6 kyr, in the context of the recent rapid changes in the geomagnetic field. A Paleomagnetic Secular Variation (PSV) stack (inclination, declination, and relative paleointensity) was reconstructed using four marine sediment cores with an independent age model constrained by seven radiocarbon ages. Magnetic investigations demonstrate that the paleomagnetic signal is carried by low coercivity ferrimagnetic minerals and is well reproduced in all cores, attesting to the quality and reliability of the paleomagnetic recording of these sediments. This signal is broadly consistent in directional changes with distant records in North America and the northern North Atlantic at centennial and millennial timescales, and has millennial scale intensity variations that are consistent with model predictions. The offset between a magnetization age determined through comparison with a northern North Atlantic PSV reference curve, GREENICE, and the radiocarbon age model indicates either a reasonable lock-in depth of magnetization (∼11 cm from the coretop) or centennial-scale reservoir age variation through time in the fjord. Reconstructed virtual geomagnetic pole (VGP) migration for the last 6 kyr shows that the recent migration of the magnetic North Pole is consistent with secular paleomagnetic variations on geologic timescales. Our results suggest that magnetic field intensity variations (temporal and spatial) are linked to magnetic flux lobe dynamics and influence the VGP migration.

The minor and trace element compositions of biogenic carbonates such as foraminifera are important tools in paleoceanography research. However, most studies have focused primarily on samples with element to calcium (El/Ca) ratios higher than the El/Ca range often found in benthic foraminifera. Here, we systematically assess the precision and accuracy of foraminifera elemental analysis across a wide range of El/Ca especially at relatively low ratios, using a method on a Thermo Scientific iCAP Qc quadrupole Inductively Coupled Plasma Mass Spectrometer (ICP-MS). We focus on two benthic foraminifera species, Hoeglundina elegans and Cibicidoides pachyderma, and prepared a suite of solution standards based on their typical El/Ca ranges to correct for signal drift and matrix effects during ICP-MS analysis and to determine analytical precision. We observe comparable precisions with published studies at high El/Ca, and higher relative standard deviations for each element at lower El/Ca, as expected from counting statistics. The overall long-term analytical precision (2σ) of the H. elegans-like consistency standard solutions was 6.5%, 4.6%, 5.0%, for Li/Ca, Mg/Ca, Mg/Li, and 6.4%, 10.0%, 4.2% for B/Ca, Cd/Ca, Sr/Ca. The precision for H. elegans-like Mg/Li is equivalent to a temperature uncertainty of 0.5–1.1°C. Measurement precisions were also assessed based on three international standards (one solution and two powder standards) and replicate measurements of H. elegans and C. pachyderma samples. We provide file templates and program scripts that can be used to design calibration and consistency standards, prepare run sequences, and convert the raw ICP-MS data into molar ratios.

Variations in speleothem calcium isotope ratios (δ⁴⁴Ca) are thought to be uniquely controlled by prior carbonate precipitation (PCP) above a drip site and, when calibrated with modern data, show promise as a semi-quantitative proxy for paleorainfall. However, few monitoring studies have focused on δ⁴⁴Ca in modern cave systems. We present a multi-year comparative study of δ⁴⁴Ca, carbon isotopes (δ¹³C), and trace elemental ratios from cave drip waters, modern calcite, and host rocks from two cave systems in California—White Moon Cave (WMC) and Lake Shasta Caverns (LSC). Drip water and calcite δ⁴⁴Ca from both caves indicate PCP-driven enrichment, and we used a simple Rayleigh fractionation model to quantify PCP variability over the monitoring period. Modern calcite trace element and δ⁴⁴Ca data positively correlate at WMC, but not at LSC, indicating a shared PCP control on these proxies at WMC but not at LSC. At both WMC and LSC, we observe an inverse relationship between PCP and rainfall amounts, though this relationship is variable across individual drip sites. Our modeled data suggest that WMC experiences ∼20% more PCP than LSC, consistent with the fact that WMC receives less annual rainfall. This work supports speleothem δ⁴⁴Ca as an independent constraint on PCP that can aid in the interpretation of other hydrologically sensitive proxies and provide quantitative estimates of paleorainfall. Additional, long-term monitoring studies from a variety of climate settings will be key for understanding δ⁴⁴Ca variability in cave systems more fully and better constraining the relationship between PCP and rainfall.

Previous geophysical studies in the New England Appalachians identified a ∼15 km offset in crustal thickness near the surface boundary between Laurentia and the accreted terranes. Here, we investigate crustal structure using data from a denser array: New England Seismic Transects experiment, which deployed stations spaced ∼10 km apart across the Laurentia-Moretown terrane suture in northwestern Massachusetts. We used receiver function (RF) analysis to detect P to SV converted waves and identified multiple interfaces beneath the transect. We also implemented a harmonic decomposition analysis to identify features at or near the Moho with dipping and/or anisotropic character. Beneath the Laurentian margin, the Ps converted phase from the Moho arrives almost 5.5 s after the initial P wave, whereas beneath the Appalachian terranes, the pulse arrives at 3.5 s, corresponding to ∼48 and ∼31 km depth, respectively. The character of the RF traces beneath stations in the middle of our array suggests a complex transitional zone with dipping and/or anisotropic boundaries extending at least ∼30 km. This extension is measured in our profiles and perpendicular to the suture. We propose one possible crustal geometry model that is consistent with our observations and results from previous studies.