Geochemistry Geophysics Geosystems最新文献

During the Eocene-Oligocene Transition (ca. 34 Ma), the Earth underwent a dramatic decline in atmospheric CO₂, global cooling, a deepening of the carbonate compensation depth (CCD), and the formation of a permanent ice sheet on Antarctica. The expansion of Antarctic glaciers eroded the underlying bedrock and increased the weathering flux to the ocean. However, the role silicate and carbonate weathering play in atmospheric CO₂ removal and the CCD through Ca²⁺ and alkalinity production is poorly understood. Magnesium isotopes (δ²⁶Mg) are fractionated during carbonate and clay mineral formation and can be used to quantify the relative flux from silicate and carbonate weathering. Here, we report the δ²⁶Mg composition of the carbonate, reactive (ferromanganese coatings), and residual (silicate) fraction of marine sediments from the Kerguelen Plateau (Ocean Drilling Program Site 738), near a major drainage system of the East Antarctic Ice Sheet, to explore the response of subglacial and shelf weathering to ice sheet expansion. The δ²⁶Mg of the carbonate fraction (−2.29‰ to −0.95‰), reactive fraction (−0.36‰ to 0.10‰), and residual fraction (−0.05‰ to 0.55‰) display similar values to surface-dwelling calcareous nannofossils, deep-water ferromanganese nodules, and Antarctic bedrock, respectively. Isotope fluctuations in all three phases suggest that the formation of the Antarctic ice sheet drove efficient chemical weathering of underlying silicate bedrock, which was rapidly transported to the Southern Ocean, resulting in further CO₂ drawdown, while a local sea-level low stand exposed carbonates on the Antarctic continental shelf to weathering, contributing to a deepening of the CCD.

Three voluminous inflated lobate lava flow complexes on the distal rifts of Axial Seamount are much larger than other known flows in the global spreading system. Each complex is 65–100 km², is up to 130 m thick, and is ∼3.0–4.6 km³, almost 100 times the volumes of historical Axial flows. These extraordinary flows are 5–7 times thicker than typical drained ponds in sheet flows. They thickened as impounded lava accumulated under chilled crusts. As flows expanded, molten interiors partially drained and flow tops collapsed. Levees built around collapses when interiors are repressurized. This formation sequence was preserved when the levee around one deep pond breached and drained the interconnected ponds. The complexes formed during moderately high-rate eruptions. Lavas from the south rift complex are plagioclase phyric mid-ocean ridge basalt (MORB) and those from the north rift complex are nearly aphyric and slightly more evolved. Glass compositions are similar to those of the summit and most rift lavas, implying that they resided in the summit magma reservoir where depleted ridge-derived magma and more enriched hot-spot-derived magma mixed. The distal south rift complex formed ∼1259 ± 119 years BP (or ∼691 CE; based on ¹⁴C dating of planktic foraminifera from core bases), a date that is statistically indistinguishable from the dates of phreatomagmatic deposits at the summit and formation of the present-day caldera. The north rift voluminous flows erupted ∼12,870 ± 173 years BP. The southwest complex, although partly mapped, remains unsampled, and is still older. Eruptions of these earlier voluminous lava complexes may also have coincided with prior caldera collapses.

This study presents a soil gas geochemical survey in the Kırcaoğlu and Reyhanlı regions of Hatay Province, southeastern Türkiye, following the 2023 Kahramanmaraş earthquake doublet. The aim was to identify concealed faults and assess seismic hazard through analysis of soil CO₂ flux, CO₂ and ²²²Rn concentrations, and carbon and helium isotopic compositions. A total of 98 sites were surveyed, and graphical statistical methods were used to establish geochemical anomaly thresholds. In Kırcaoğlu, two prominent gas anomaly zones were delineated with dominant NW-SE and NE-SW trends, likely representing buried faults linked to the Yesemek Segment, including one beneath the Reyhanlı Dam. In Reyhanlı, an east-west gas anomaly suggests a possible westward extension of the Reyhanlı Fault. These findings refine the region's structural framework and highlight seismic risks from buried faults. Isotopic analyses show CO₂ derives from biogenic and deep crustal reservoirs, with ⁴He/²⁰Ne and ³He/⁴He ratios confirming up to 7.1% crustal helium and <1% mantle helium. Heavier δ¹³C values and elevated crustal helium in Kırcaoğlu support deep gas migration along fault zones. Natural CO₂ emissions are estimated at 66 t/d in Kırcaoğlu and 60 t/d in Reyhanlı. Regionally, emissions from the Amik Basin (∼15,586 t/d) comprise ∼1.5% of Türkiye's daily anthropogenic CO₂. The overlap between gas anomalies and surface ruptures and liquefaction zones from the 2023 earthquakes confirms the effectiveness of soil gas surveys for buried fault detection. These results highlight the utility of soil gas geochemistry as a non-invasive tool for fault detection and seismic hazard assessment.

We present new observations on the dynamics and locations of deep mantle reservoirs derived from the ages and compositions of Voyager Seamount Chain lava flows. The previously unexplored Voyager Seamount Chain trends NW–SE between the Mid-Pacific Mountains and the Northwestern Hawaiian Ridge. Volcanic samples were recovered from the chain during the Ocean Exploration Trust expedition NA134. The lava flows are alkalic to highly alkalic in composition. Ages ranged from 81 to 86 Ma (n = 8), with the oldest ages in the NW and an age-progression toward the SE. The Voyager age-progression continues southward through the Northern Line Islands region until at least 69 Ma. Mantle flowlines using absolute plate motion models indicate that the Voyagers were emplaced near the modern Marquesas Hotspot location approximately 86–69 Ma. The Sr-Nd-Pb-Hf isotope systematics show the influence of an EMII component and overlap the compositions of Pliocene volcanism from the Marquesas Islands, consistent with the plate motion age model. These data imply a long-lived plume as the source of the Voyager seamounts and the Marquesas. However, the lack of a clear and continuous seamount chain between the 86–69 Ma Voyager Seamount Chain and the 6–0 Ma Marquesas Islands implies that the mantle plume displays variable buoyancy flux over time. The surface expression of this mantle reservoir experienced a potential hiatus of up to ∼60 m.y. These new data indicate that the mantle beneath the Marquesas Islands region has been discontinuously producing age-progressive, EMII-like hotspot volcanism since at least the Late Cretaceous.

Fluids can modify the mechanical properties of rocks, including shear strength and strain behavior. We investigate the timing and magnitude of seismic events during fault motion in strike–slip systems across the Peloritani Mountains (northeastern Sicily) and Aeolian Archipelago using GNSS and seismological data analysis. Results reveal a strain partitioning along the already known crustal–scale NNW–SSE trending right–lateral transtensional deformation zone across the Peloritani Mts and its offshore extension up to Vulcano Island (defined as the Aeolian–Tindari–Letojanni Fault System, ATLFS), and WNW–ESE to NW–SE right–lateral transfer zones located in the western and central sectors of the Aeolian Archipelago. During 2021, the eastern block of the ATLFS underwent a significant velocity increase relative to the fixed western block, varying from 1.6 ± 0.28 mm/y (pre–2021 baseline) to 3.3 ± 0.99 mm/y. The acceleration of the eastern block of the ATLFS was accompanied by increased seismic strain release. It temporally correlated with the fastest ground inflation on Vulcano Island (central Aeolian Archipelago), which in turn coincided with the highest CO₂ flux emission on the island. This correlation, along with evidence of gas emissions in the Peloritani Mts, suggests that enhanced fluid pressure lubricated fault surfaces, thereby facilitating slip along the ATLFS. The fluid–induced slip acceleration was sustained for 9 months and was marked by frequent low–magnitude earthquakes.

Bulk (whole sample) magnetic properties of three-hundred-sixty-two samples from five North Atlantic Ocean sediment cores are compared to the magnetic properties of five grain-size fractions separated from the same samples. The relative abundance and magnetic properties of the size fractions are highly variable both within a core and among locations. Clay often dominates the sediment texture but contains low concentrations of magnetic minerals whose composition varies little from site-to-site. Silts are more magnetically variable, contain much higher ferrimagnetic mineral concentrations, are magnetically coarser than clays, and retain strong imprints of their geological source. The medium silt size fraction is a strong source discriminator, dictates bulk sediment magnetic property variations, and helps characterize a new magnetic source fingerprint for Laurentia. In contrast, bulk sediment magnetic properties are an average of the properties of the individual fractions weighted by the magnetic concentration of those fractions and their abundance. Bulk sample averaging is observed across magnetic methods, which mutes individual grain-size property variations that convolve source signatures and transport driven texture variations. Grain-size-specific approaches can help to restrict textural variance to isolate source signatures, track sediment texture changes independently of magnetic variation and provenance changes, facilitate process-based interpretations, and provide sedimentology-based insights into these important paleo-archives and their recorded paleo-environmental changes.

Natural volcanic glasses are well represented in the geologic record, and typically contain near-ideal single-domain particles required for standard Thellier-type absolute paleointensity experiments. Young (<∼50–100 ka) glasses have been demonstrated to reliably record Earth's magnetic field. However, it is unclear how the magnetic mineralogy and magnetization might change with age as the metastable glass structure relaxes. Here, we attempt to systematically address issues surrounding glass relaxation and devitrification. We subjected a set of natural basaltic and rhyolitic glasses to controlled annealing experiments at temperatures between 200°C and 400°C and assessed how the magnetic properties and glass structure (as assessed by the glass transition temperature, T_g) change over time. We compare the results to bulk magnetic properties and T_g for a suite of volcanic glasses spanning over seven orders of magnitude in age. Annealed samples show an increase in isothermal remanent magnetization acquisition, a decrease in coercivity, and basaltic samples show an increase in unblocking temperatures. The results are consistent with a coarsening of pre-existing magnetic particles rather than precipitation of new oxides. The natural data are more difficult to interpret, but trends in average parameters are consistent with a coarsening of magnetic particles in some—but not all—samples with age, and this appears to be accompanied by a reduction in T_g. While the annealing experiments take place under many different thermodynamic conditions compared to naturally aged samples, we suggest caution when using geologically older glasses for paleointensity analyses.

Mantle-derived magma flux has a first-order control on long-term volcanic productivity, volatile cycling, and crustal growth in convergent margins. However, the factors controlling it remain unclear. We used a simplified, 3D conceptualization of an intraoceanic subduction zone and petrologic constraints on mantle melting to calculate mantle-derived magma flux from the “bottom up”, and test the sensitivity of mantle-derived magma flux to a variety of input variables. Estimates of mantle-derived flux from our model can be compared to existing “top down” models based on erupted volumes and crustal growth models and is also a jumping-off point from which more complex models of mantle-derived magma flux at a variety of scales may be developed. We find that the total volume of mantle available to melt exerts the most significant control on mantle-derived magma flux between different arcs. At a given arc, convergence rate and the extent of melting have the greatest impact on mantle-derived magma flux. Variation in flux caused by variations in orthogonal convergence rates within the Aleutians may cause variability in mantle-derived magma flux along-arc.

Paleomagnetic studies typically assume that the long-term, time-averaged geomagnetic field behaves as a geocentric axial dipole (GAD). While paleodirectional data over the past five million years generally agree with GAD predictions, mid-to-low latitude paleointensity records fail to show GAD, with high values from Hawaii. Possible causes include experimental biases, non-dipole field contributions, and uneven temporal sampling. In this study, we conduct alternating field and thermal demagnetization measurements, as well as paleointensity experiments, on 12 late Pleistocene (∼0.2–0.5 Ma) lava flows from northern Hainan Island (∼20°N, ∼110°E) of the Brunhes normal polarity chron. Eleven sites yield stable paleomagnetic directions (D = 9.1°, I = 24.3°, α₉₅ = 4.0°), defining a virtual geomagnetic pole (VGP) at 78.7°N, 237.7°E, with VGP dispersion of 12.9°. Paleointensity results from four qualified sites range from 28.8 to 48.9 μT (mean = 37.8 ± 6.9 μT), which are in agreement with the Brunhes Hawaiian data from the same latitude. The directional and intensity results from Hainan are consistent with previous studies at similar latitudes but deviate from GAD predictions. Our results suggest that the high paleointensity values observed in the Hawaiian region may result from differences in age distributions compared with records from other latitudes. Considering these temporal differences, the observed non-GAD characteristics at mid-to-low latitudes may partly reflect comparisons between time-averaged field properties over distinct geological intervals.

The Armenian Highlands, a tectonically active segment of the Arabia-Eurasia collision zone, exhibit widespread Quaternary volcanism, rapid uplift, and intense seismicity. However, the lithospheric processes driving these phenomena remain poorly understood. We present the first P- and S-wave body-wave tomography model of Armenia's crust and uppermost mantle, derived from regional earthquakes recorded by both permanent and temporary seismic networks. Our model reveals that beneath the Armenian Highlands, the crust is underlain by a pronounced low-velocity anomaly in the uppermost mantle (∼40 km depth), indicative of asthenospheric upwelling and melt generation. This anomaly spatially correlates with monogenetic volcanic fields (e.g., Gegham and Syunik), supporting magma ascent through thermally weakened conduits. The lithosphere adheres to a “crème brûlée” rheological model, characterized by a seismogenic brittle layer (10–25 km depth, marked by high-velocity anomalies) atop a ductile lower crust and uppermost mantle (with negative Vp and Vs anomalies). Beneath the Gegham Plateau, a thinner brittle high-velocity layer coincides with elevated seismicity at 10–20 km depth, suggesting magmatically induced earthquakes rather than purely tectonic strain release. Collectively, this work establishes a framework for understanding collision-related magmatism in comparable orogens.