Geochemistry Geophysics Geosystems最新文献

Dissolved neodymium (Nd) isotopes are key tracers for water mass mixing in the open ocean and for land-ocean exchanges in marginal seas. Hydrogenous Nd isotope ratios (expressed as ε_Nd) in cores north of the equator reflect values of the surrounding lithology and tend to vary in accordance with local rainfall records, indicating higher terrestrial input from intensified weathering/runoff. Here, we report fossil fish teeth derived ε_Nd from core MD05-2925 (9.3°S, 151.5°E, 1,661 m) in the western tropical Pacific, south of the equator, and explore its use as an indicator of hydroclimatic and weathering changes over the last 30 kyr. Data presented here reveal a pattern of change in ε_Nd that broadly corresponds to regional precipitation. Core MD05-2925 exhibited a decrease in ε_Nd (∼ε_Nd = −1.6) during Heinrich Stadial 1 (HS1) (a wet period at the core site) that could not be tracked to any immediate local sources. Subsequently, during the Bølling-Allerød (B-A) interstadial, characterized by drier conditions in the Southern Hemisphere, we observe an increase in ε_Nd values (∼ε_Nd = −0.37). We suggest that the decrease in ε_Nd observed during HS1 originates from the Australian coast under an intense rainfall regime, with this unradiogenic Nd being transported northward via the Upper Circumpolar Deep Water. Furthermore, we propose that variations in ε_Nd reflect changes in the local hydrological regime along the southern edge of the Intertropical Convergence Zone. This study highlights the effectiveness of hydrogenous Nd isotopes in nearshore environments as a potential indicator of regional hydrological changes.

The Svalbard archipelago is well-known for its outcropping geology, but its subsurface geometry and physical properties are less well constrained. To address this knowledge gap, a multidisciplinary approach has been applied to integrate potential field data with seismic reflection profiles, exploration boreholes and geology. We constructed five 2D gravity and magnetic forward models along pre-existing seismic lines in central Spitsbergen, utilizing newly acquired ground-borne gravity, helicopter-borne magnetic, and regional gridded potential field data. Specifically, we focused on characterizing the geophysical signature of the pre-Devonian basement composition and topography, position of the Billefjorden Fault Zone (BFZ), and the magmatic bodies related to the High Arctic Large Igneous Province (HALIP). Regional trends show a 15 km wide NNW-SSE trending magnetic and gravity high east of the BFZ and a long-wavelength elliptical magnetic anomaly of ca. 50 km diameter below central Isfjorden with no anomalous gravity response. We observe outcropping igneous rocks and interpret HALIP-related igneous sills on 2D seismic data in this region matching the short-wavelength signal. Our models show that most of the gravity signal is related to the arrangements of the sedimentary basins and the influence of the BFZ on the basement topography. Large lateral intra-basement magnetic susceptibility contrasts and a deep magnetic source were necessary to fit the magnetic signal, indicating basement heterogeneity and locally the influence of magmatic bodies.

Pockmarks are geomorphological depressions on the seafloor in various underwater environmental settings. They are commonly linked to fluid release from subsurface reservoirs; however, the processes involved, fluid types, and relative timing of their formation often remain enigmatic. This study investigates the spatio-temporal evolution of abundant pockmarks on the southwestern Chatham Rise, offshore New Zealand, to assess their timing and controlling mechanisms. High-resolution bathymetry and novel pseudo-3D sub-bottom imaging were used to characterize and quantitatively analyze over 1,300 pockmarks. Pockmark formation is temporally constrained to the seafloor and 16 distinct subsurface horizons with clusters of individual and composite features surrounded by exclusion zones without any pockmarks. Across adjacent horizons, these features can be genetically linked and form distinctive spatio-temporal patterns of embedded, stacked, or main and satellite pockmarks. Notably, pockmark horizons occur consistently above continuous, regional high-amplitude reflections in the Plio-Pleistocene succession, which are interpreted as terrigenous-rich sediments deposited at sea-level lowstands during glacial maxima. These observations indicate that pockmark onset in the study area is linked to sea-level rise, in contrast with more common assumptions relating their formation to gas release during sea-level fall. After their initiation, bottom currents erode and modify the pockmark morphologies, resulting in elongated and composite structures. These findings highlight the benefits of high-resolution 3D imaging for shallow subsurface analysis and offer new insights into episodic pockmark formation, with implications for understanding similar features globally.

Provenance of pre-obduction Eocene turbidites from New Caledonia is used to better constrain their geodynamic context and inform debate on subduction polarity. Chemical compositions of detrital clinopyroxenes in arenites are compared against potential sources. Basaltic source modeling using cpx/rock partition coefficients confirms E-MORB origins from the allochthonous Poya Terrane. Detrital zircon populations in the Bourail Flysch are similar to those of the autochthonous Upper Cretaceous sedimentary cover. In contrast, a predominance of early Eocene zircons in the Nepoui-Koumac Flysch suggests derivation from the supra-subduction dyke system of the Peridotite Nappe. Results were compared with other arenite components (bioclasts, oxides, sulfides, zircon) and whole-rock compositions of rock fragments and blocks of the breccia/olistostrome in the upper part of the turbidites. Together, the data set allows identification of multiple successive sources and processes. A syntectonic character for the flysch basins is inferred from the pre-turbidite unconformity and provenance evolution. An abundance of shallow-water bioclasts throughout the succession indicates the formation and continuous destruction of a rimming carbonate platform. The existence of two previously identified types of turbidite basins is confirmed by the characteristics of the Bourail (foreland) and the Nepoui-Koumac (wedge-top) flysch successions. These basins were located on the northern Norfolk Ridge (lower plate) and ultramafic allochthon (accretionary wedge), respectively, representing elements of a foreland basin. These observations are inconsistent with a hypothesized connection between New Caledonia and a continent-directed Pacific subduction zone. Together with all available geologic data, the results of this investigation confirm the model of northeast- or east-dipping pre-obduction subduction.

The presence of fluid plays a critical role in controlling the fault slips and the seismogenesis in subduction zones. However, the origins and pathways of the fluid remain poorly understood. In this study, we examined the Inuyama sequence in the Mino belt of Japan, which is an on-land analog of a cold subduction zone such as the Japan Trench. In this region, white chert layers are intercalated within red chert layers, and the white chert layers were originally fluid conduits within subducting sediments along the plate interface according to the mass balance of the precipitated silica that constitutes the white chert. Trace element concentrations and Sr and Nd isotopic compositions of the chert layers suggest that the seawater expelled from subducting sediments due to compaction was the main carrier of silica. The temperature for the silica precipitation was approximately 60°C based on the O isotopic composition of the white chert layers, equivalent to the depth condition of approximately 5 km below the seafloor based on the analogy with the modern Japan Trench. As the major element concentrations suggest that the compaction and dehydration of the surrounding sediments could not fulfill the fluid volume required for supplying silica to fluid conduits, there may be trench-parallel fluid migration to one location where silica was precipitated. Furthermore, considering the fluid volume that would lead to silica precipitation, the permeability of the fluid conduit could be higher than that of the typical subducting siliceous sediments, suggesting that fluid flow toward shallow depths was transient.

Long-term tephrostratigraphies of volcanic islands such as the Azores are often limited to young and incomplete subaerial records. Here, we present a Pleistocene-Holocene marine tephra archive around the eastern islands of the Azores based on 22 marine gravity cores. We provide a new extensive data set comprising major element glass compositions of 350 marine tephra layers and 26 tephra samples collected from subaerial outcrops on São Miguel Island. The marine tephra record is dominated by trachytic glass compositions with lesser amounts of trachy-basaltic/basaltic materials. We categorize the marine deposits into four facies: primary fall (F1), primary flow (F2), eruption-related secondary flow (F3), and non-eruption-related secondary flow deposits (F4). Cores collected close to the islands contain a high proportion (>50%) of volcaniclastic material deposited during secondary processes. Using machine-learning algorithms, we assign potential volcanic sources to individual tephra layers. Furthermore, our spatial characterization of primary/secondary deposits demonstrates the importance of carefully selecting core locations during sea-going campaigns, as the preservation and distribution of primary and reworked tephras considerably differ depending on several factors (e.g., on paleo-wind directions, island topography, seafloor bathymetry, and distance from source volcanoes). We also report the occurrence of mafic primary fall tephras tens to hundreds kilometers away from the closest island, indicating a potential hazard from mafic Plinian eruptions in the Azores. This study aims to improve the estimation of the eruptive frequency of the archipelago and opens new pathways for more detailed studies concerning long-term volcanic cyclicity in the region.

Sulfate salts are deposited most commonly as evaporites on Earth; however, this is not their only origin. Pyrite oxidation during subsurface weathering is another common process on Earth that also produces a suite of sulfate salts—including the acidic phases jarosite and alunite—in sedimentary deposits. On Mars, the occurrence of sedimentary (non-vein-forming) sulfate salts has typically been interpreted as the result of an environmental evaporitic process. While these processes certainly occurred on Mars, we observed several specific occurrences of crystal pseudomorphs in the sedimentary strata preserved in Gale Crater that are likely nonevaporitic. These occurrences are not associated with bedded precipitates that might indicate a primary origin, nor are they associated with diagnostic sedimentary structures, including desiccation cracks, breccias, and tepee structures, that might indicate an early diagenetic origin. The sporadicity of these crystals implies a localized post-depositional process, which we propose to be the oxidation of sulfide-bearing minerals such as pyrite and pyrrhotite. If post-depositional pyrite and pyrrhotite oxidation took place in Gale Crater strata as it does perennially in terrestrial examples, it would have generated a significant amount of acidity that could dissolve a large proportion of any carbonates in contact with the same fluids.

South Sister volcano, Oregon Cascade Range, USA, has repeatedly erupted rhyolite since ca. 40 ka. The youngest such eruptions are the ca. 2 ka Rock Mesa and Devils Chain rhyolites, erupted several hundred years apart from two multi-vent complexes separated by 3–6 km. Fe-Mg interdiffusion models of orthopyroxene rims from both rhyolites produce timescales up to several-thousand years, but dominantly decades-to-centuries. Notably, the timescales of step-normal zoned orthopyroxene rims (i.e., normally zoned with a steep chemical gradient) from the Rock Mesa rhyolite are longer than those of reversely zoned crystals, whereas the Devils Chain produced mostly decadal timescales for both zoning types. Despite the proximity and broadly similar products of these episodes, their respective timescales indicate distinct sequences of events leading up to each eruption. The Rock Mesa timescales record centuries of magma chamber growth followed by decades of predominantly magma rejuvenation, reorganization, and destabilization. In contrast, the Devils Chain episode was preceded by a single episode of coupled rhyolite extraction, rejuvenation, and hybridization. Rare, high-An plagioclase cores and evidence of reheating implicate cryptic emplacement of mafic magma at the base of the rhyolite reservoirs. However, the diffusion timescales do not unequivocally support a single magma recharge event that affected both. Fluid fluxing and the reorganization of melt into buoyant magma chambers likely provided the source of increasing pressurization that initiated each eruption after several decades. Geodetic models of ongoing deformation west of South Sister could consider these processes in addition to magma emplacement.

Fe-oxidizing microorganisms in deep-sea hydrothermal vent environments are often used as analogs for primordial life on Earth. In fact, Earth's oldest purported microfossils are preserved as hematite filaments in a jasper rock dated between 4,160 and 4,280 million years and are thought to have originated in a seafloor hydrothermal environment. However, the kinds of post-depositional processes that can alter their morphologies are not well-known, which has implications for recognizing morphologies of bona fide microbial origin in the deep-time rock record. Here, we show that more than 10 morphological types of filamentous Fe-oxyhydroxide microstructures occur in Fe-oxide specimens from deep-sea hydrothermal vents in the southwest Indian Ocean, including thick and thin filaments with the following morphologies: parallel-alignments, branching and pectinate-branching, curved and straight, hollow to tubular, twisted and coated with botryoidal silica. Botryoidal silica mineralization is documented on several filament morphotypes and exhibits pattern with spheroidal twins, circular concentricity, and cavities, whereas their elemental composition is dominated by Si, Fe, Mn, C, with minor S and halogens. Such patterns and substances point to an origin from chemically oscillating reactions, which provide a novel abiotic model based on C, Fe, Mn, S, and halogen redox reactions in colloidal silica, to explain occurrences of botryoidal minerals grown onto deep-sea filamentous Fe-oxyhydroxide microstructures. The documented filamentous morphologies and new model for silica botryoid formation help to understand abiotic carbon cycling in marine and lacustrine environments, ancient filaments preserved in the geological record, as well as a basis to seek similar structures in deep-space settings.

A detailed study of palagonitization in rocks from Deception Island—one of Antarctica's most active volcanoes—has been performed to advance our understanding of this alteration process. A detailed petrographic (optical and SEM), mineralogical (XRD), and mineral and glass spot geochemistry (EDS and EMP) characterization has been conducted on pyroclastic samples. Palagonitization occurred at 80–100°C and involved (a) initial glass to palagonite transformation by congruent glass dissolution and precipitation, followed by (b) palagonite maturation resulting in increasing crystallization into an assemblage of dominant smectite with minor illite, zeolites and Ti-bearing oxides. During the first stage, an optically amorphous phase is formed with an estimated average density of 1.7–1.8 g/cm³ and a very early mineralogical control on its composition indicating nucleation at the nm-scale. Major elements are typically leached except for Ti, which behaves as immobile throughout palagonitization. Palagonite maturation occurs in an open system (variable element depletion and supply) and is controlled by an interplay between crystal nucleation and growth, overall mass balance, and local equilibration between crystals and fluid. Mass balances control palagonite porosity and density. Highly local physicochemical conditions (e.g., fluid chemistry or water-rock ratio) play a major role in the chemical and mineralogical composition and evolution of palagonite. Variability of these controls at the microscale produces a large variability in palagonite characteristics even at the intraclast scale. Glass composition has not been observed to play a significant role. Textures observed in several samples indicate the contribution of microbial activity to glass alteration.