Geochemical Evidence Research Articles

Carbon pump dynamics and limited organic carbon burial during OAE1a.

Oceanic Anoxic Events (OAEs) are conspicuous intervals in the geologic record that are associated with the deposition of organic carbon (OC)-rich marine sediment, linked to extreme biogeochemical perturbations, and characterized by widespread ocean deoxygenation. Mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs, however, remain poorly constrained. In this work we reconstructed the BCP in the western Tethys Ocean across OAE1a (~120 Mya) using sediment geochemistry and OC mass accumulation rates (OCAcc ). We find that OCAcc were between 0.006 and 3.3 gC m-2 yr-1 , with a mean value of 0.79 ± 0.78 SD gC m-2 yr-1 -these rates are low and comparable to oligotrophic regions in the modern oceans. This challenges longstanding assumptions that oceanic anoxic events are intervals of strongly elevated organic carbon burial. Numerical modelling of the BCP, furthermore, reveals that such low OC fluxes are only possible with either or both low to moderate OC export fluxes from ocean surface waters, with rates similar to oligotrophic (nutrient-poor, <30 gC m-2 yr-1 ) and mesotrophic (moderate-nutrients, ~50-100 gC m-2 yr-1 ) regions in the modern ocean, and stronger than modern vertical OC attenuation. The low OC fluxes thus reflect a relatively weak BCP. Low to moderate productivity is further supported by palaeoecological and geochemical evidence and was likely maintained through nutrient limitation that developed in response to the burial and sequestration of phosphorus in association with iron minerals under ferruginous (anoxic iron-rich) ocean conditions. Without persistently high productivity, ocean deoxygenation during OAE1a was more likely driven by other physicochemical and biological factors including ocean warming, changes in marine primary producer community composition, and fundamental shifts in the efficiency of the BCP with associated effects and feedbacks.

Petrogenesis of the Wadi El-Faliq Gabbroic Intrusion in the Central Eastern Desert of Egypt: Implications for Neoproterozoic Post-Collisional Magmatism Associated with the Najd Fault System

The late Neoproterozoic gabbroic intrusion of the Wadi El-Faliq area in the central Eastern Desert of Egypt (north Arabian–Nubian Shield; henceforth, ANS) is a fresh, undeformed elliptical body elongated in a NW–SE trend following the main sinistral strike-slip faults of the Najd fault system. Mineralogical and geochemical evidence suggest that they were derived from hydrous tholeiitic mafic magmas with arc-like geochemical fingerprints resembling the post-collisional gabbroic intrusions in Saudi Arabia. Despite the arc-like signatures, their fresh and undeformed nature, together with the field relationships, indicates that the studied gabbroic intrusion post-dates the main collisional phase, supporting its emplacement after subduction ceased and during the post-collisional stage. As a result, the arc-like signatures were possibly transmitted from the earlier ANS subduction episode. Indeed, the high (La/Sm)N, and negative-Nb and positive-Pb anomalies suggest contributions from subduction components. Lithospheric delamination was possibly facilitated by the Najd faults and shear zones formed during the post-orogenic crustal extension associated with the Pan-African orogenic collapse. The delamination process could have generated a rapid upwelling and melting of the asthenosphere mantle. The melt-rock reaction process likely played an important role in the genesis of the studied rocks through the interaction of the asthenosphere melts with lithosphere mantle rocks during ascent. The HREE fractionation suggests a probable mixing between melts from both spinel- and garnet-bearing peridotites. We suggest that the Wadi El-Faliq gabbroic intrusion was likely emplaced due to the stretching and thinning of the lithosphere during the extensional tectonism following the Pan-African orogeny.

Keratose sponges in ancient carbonates – A problem of interpretation

AbstractReports of diverse vermiform and peloidal structures in Neoproterozoic to Mesozoic open marine to peritidal carbonates include cases interpreted to be keratose sponges. However, living keratose sponges have elaborate, highly elastic skeletons of spongin (a mesoscopic end‐member of a hierarchical assemblage of collagenous structures) lacking spicules, thus have poor preservation potential in contrast to the more easily fossilized spicule‐bearing sponges. Such interpreted fossil keratose sponges comprise diverse layered, network, amalgamated, granular and variegated microfabrics of narrow curved, branching, vesicular–cellular to irregular areas of calcite cement, thought to represent former spongin, embedded in microcrystalline to peloidal carbonate. Interpreted keratose sponges are presented in publications almost entirely in two‐dimensional (thin section) studies, usually displayed normal to bedding, lacking mesoscopic three‐dimensional views in support of a sponge body fossil. For these structures to be keratose sponges critically requires conversion of the spongin skeleton into the calcite cement component, under shallow‐burial conditions and this must have occurred prior to compaction. However, there is no robust petrographic–geochemical evidence that the fine‐grained carbonate component originated from sponge mummification (automicritic body fossils via calcification of structural tissue components) because in the majority of cases the fine‐grained component is homogenous and thus likely to be deposited sediment. Thus, despite numerous studies, verification of fossil keratose sponges is lacking. Although some may be sponges, all can be otherwise explained. Alternatives include: (i) meiofaunal activity; (ii) layered microbial (spongiostromate) accretion; (iii) sedimentary peloidal to clotted micrites; (iv) fluid escape and capture resulting in bird's eye to vuggy porosities; and (v) moulds of siliceous sponge spicules. Uncertainty of keratose sponge identification is fundamental and far‐reaching for understanding: (i) microfacies and diagenesis where they occur; (ii) fossil assemblages; and (iii) wider aspects of origins of animal clades, sponge ecology, evolution and the systemic recovery from mass extinctions. Thus, alternative explanations must be considered.

Geochemical record of methane seepage in carbon cycling and possible correlation with climate events in the Qiongdongnan basin, South China Sea

At cold seeps, sulfate-driven anaerobic oxidation of methane (SD-AOM) is the dominant biogeochemical process, which can significantly affect the marine carbon-sulfur cycle. In this paper, multiple sedimentary indicators in core QS-1 were reported for methane seepage from Haima seep of the South China Sea (SCS). By combining total carbon (TC), total organic carbon (TOC), total sulfur (TS), total inorganic carbon (TIC), as well as the carbon and oxygen isotope compositions of benthic foraminifera Uvigerina spp. In sediments, our aim was to explore the geochemical evidence on methane seepage activities in this area. We identified three methane release events (MREs) with different intensities and timescales during MIS 1–2, characterized by particular anomalies of negative δ13CTIC (as low as −41.42‰) and δ13CU.spp (as low as −5.50‰), high contents of TS and high S/C ratios, simultaneously. High S/C ratios and negative values of δ13CTIC effectively indicate the intense SD-AOM and SD-AOM-related process in core QS-1. Three MREs, occurred at high or low sea level, may due to the climate-driven increase of hydrostatic pressure and seafloor temperature. The insights for sediment carbon-sulfur trace element, carbon isotopes of TIC contents and benthic foraminifera have obvious implications for tracing past methane seepages and its possible link with climate changes.

Geochemical evidence for the Eocene surface uplift of the southern Lhasa subterrane, southern Tibet and the implications

Whether the crust of southern Tibet has been thickened during the Eocene and if so, the mechanisms of crustal thickening in this region need to be further investigated. This study presents geochronological and geochemical data for Eocene (45–41 Ma) granitic plutons, including the Aima diorites in the eastern part and Huo'er pluton (granites and mafic enclaves) in the western part of the southern Lhasa subterrane. The Aima diorites show adakitic affinities, such as moderate SiO 2 contents and relatively high Sr/Y values, with depleted 87 Sr/ 86 Sr (i) (0.704039–0.704054), εNd (t) (+2.9 to + 3.3) isotopic compositions, indicating that they are derived from a juvenile continental crust. The Huo'er host granites have high SiO 2 and low MgO contents. Meanwhile, they are characterized by the enriched initial 87 Sr/ 86 Sr (i) (0.707899–0.708618), εNd (t) (from − 6.4 to − 5.3) and variable zircon εHf (t) (from − 6.8 to + 5.0) values. These geochemical characteristics and the presence of mafic enclaves demonstrate that the Huo'er host granites were the result of melting of crust (both ancient and juvenile) of southern Lhasa subterrane with the involvement of mantle-derived materials. The Huo'er mafic enclaves display an igneous texture and contain acicular apatite, suggesting that they are quenching of mafic magmas that have co-mingled with the host granites. The mafic enclaves have arc-like trace-element patterns, together with their enriched 87 Sr/ 86 Sr (i) (0.707608–0.707826), εNd (t) (from − 4.8 to − 4.6) and zircon εHf (t) (from − 4.0 to − 1.7) isotopes, revealing that they were derived from an enriched lithospheric mantle source. Taking the regional tectonic evolution into consideration, we propose that the Eocene Aima diorites and Huo'er pluton are the products of the last stage of magmatism related to break-off of the subducted Neo-Tethyan oceanic slab. Our study of these two plutons, along with results from previous studies, suggests that the crust of the southern Lhasa subterrane was thickened during the Eocene. The thickened crust was mainly caused by underplating of mafic magmas and, to a lesser extent, by thrust-related accretion of continental crust of the Indian terrane to the Lhasa terrane. We further propose that the Eocene thickened crust of southern Lhasa subterrane has triggered the surface uplift of this region during the Eocene, which may have non-negligible impact on the Cenozoic evolution of the Indian Monsoon system. • The Eocene Aima diorites and Huo'er pluton were in response to the slab break-off. • The crust of the southern Lhasa subterrane became thickened during the Eocene. • Magma underplating and accretion of Indian crust contributed to crustal thickening. • The Eocene surface uplift is synchronous with intensification of the Indian Monsoon.

Geochemical evidence of illuvial processes in clay-rich soils on limestones in a humid temperate climate

In environments with high precipitation, eluvial–illuvial processes play a key role in pedogenesis. Vertical translocation of clay leads to the formation of an argic horizon. Such soils are of global importance and formed on different rock substrates under different climatic conditions. However, the pedogenesis of clay-rich soils developed on limestone in humid temperate climates has not been sufficiently investigated. Therefore, this study aimed to evaluate the geochemical and mineral compositions and properties of Luvisols (10 profiles) from the Dinaric karst region and identify the geochemical indicators of eluvial–illuvial processes. The geochemical and mineral differences between the A, E, and Bt horizons could be explained by the pedogenic processes of biorecycling, migration, isomorphic substitution in minerals, surface adsorption to clay or Al/Fe oxide minerals, and precipitation from the soil solution. Illuviation is evident in the downward increase in Al2O3, Fe2O3, MgO, K2O, Ba, Co, Cr, Ga, Ni, Sc, Th, U, V, Y, and REE. Despite differences in the geochemical characteristics of the 10 studied profiles, eluvial–illuvial processes outweighed the heterogeneity of the soils from different sites and the probable contribution of allochthonous material to the soil. We established new textural and geochemical indicators that could explain the differences among horizons and profiles in light of the polygenetic origin of soil material and pedological processes.

Rare earth elemental and Sr isotopic evidence for seawater intrusion event of the Songliao Basin 91 million years ago

Petrogenesis of lacustrine dolostone is closely related with paleo-lake water conditions. Here we report the high spatial-resolution petrographic and geochemical results of a lacustrine dolomite nodule from the Qingshankou Formation, the Songliao Basin. Sedimentary and elemental signatures confirm the protogenetic origin of this nodule and its effectiveness in recording geochemical characteristics of paleo-lake water during dolomitization. The low Y/Ho ratios, middle rare earth element (MREE) enrichment and subtle positive Eu anomalies within the nodule indicate a fresh water source. However, the Sr isotope values in the core of the nodule (0.7076–0.7080) are close to contemporaneous seawater (0.7074), yet different from the modern river (0.7120) and the host black shale (0.7100). On the premise of excluding the influence of hydrothermal fluids, the significantly low strontium isotope values of the lacustrine dolomite might be caused by seawater interference during dolomitization. Our findings demonstrate that lacustrine dolomite within black shales is not only a faithful tracer of diagenetic water environment, but also a novel and easily identified mineralogical evidence for episodic seawater intrusion event (91 Ma) in the Songliao Basin, which supplements other paleontological and geochemical evidence.

Himalayan zircons resurface in Sumatran arc volcanoes through sediment recycling

Understanding the processes of subducted sediment recycling in subduction zones is vital to decipher Earth’s crust-mantle interactions. This study uses along-arc geochemical variations and zircon U-Pb-Hf-O isotopes of Quaternary arc basalts and andesites on Sumatra Island, Indonesia to assess the mode of sediment recycling in subduction zones. The Hf-O isotopes of inherited zircons of the basalts and andesites near the Toba Caldera indicate that some of them were derived from subducted terrigenous sediments mainly sourced from the (eastern) Himalaya. Hybridization of the subducted sediments with the mantle also accounts for the enriched Sr-Nd isotopic compositions of arc volcanic rocks near the Toba Caldera. Thermodynamic modeling indicates that the subducted sediments did not melt on the slab surface. Rather, geochemical evidence supports their formation as diapirs that rise buoyantly through the hot mantle wedge and contribute to ~30 to 45% of the magma source of the arc volcanic rocks near Toba.

Geochemical Evidence for Diachronous Uplift and Synchronous Collapse of the High Elevation Variscan Hinterland

AbstractCompeting end‐member models for the late Paleozoic Variscan orogeny (ca. 360‐290 Ma) alternatively suggest moderate 2–3 km elevations underlain by relatively thin crust (&lt;50 km) or a thick crust (&gt;55 km) that supported high 4–5 km elevations. We tested these models and quantified the crustal thickness and elevation evolution of the Variscan orogeny using igneous trace element geochemical proxies and geochronologic data. These data suggest that thick crust (55–70 km) capable of supporting 3–5 km elevations developed diachronously from east to west between ca. 350 and 315 Ma. Crustal thinning occurred from ca. 315 to 290 Ma across the orogen. Crustal thickness and elevation changes at ca. 340‐325 and 315‐290 Ma correspond with increases in silicate weathering recorded by Sr and Li isotopes, consistent with models in which silicate weathering of the Variscan orogen contributed to global cooling associated with the late Paleozoic ice age.

Heterogeneous orogenic lithospheric mantle beneath the North Qaidam orogen: Geochemical evidence from syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U Pb ages and Hf O isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon U Pb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn -exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ 18 O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn -exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn -exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn -exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone. • Syn-exhumation/post-collisional mafic magmatic rocks in the North Qiadam orogen. • They were derived from a heterogeneous orogenic lithospheric mantle. • The mantle source was metasomatized by melts from the subducted continental crust. • These rocks record crust-mantle interaction in the continental subduction channel.

The petrogenesis of chemically zoned, phonolitic, Plinian and sub-Plinian eruptions of Somma-Vesuvius, Italy: Role of accessory phase removal, independently filled magma reservoirs with time, and transition from slightly to highly silica undersaturated magmatic series in an ultrapotassic stratovolcano

A mineralogical, major, LA-ICP-MS trace element mineral chemistry and bulk-rock geochemical study of juvenile samples of the Mercato, Avellino, Pompeii and Pollena eruptions, collected in stratigraphically and volcanologically well-characterized sections of the Somma-Vesuvius stratovolcano (Roman Magmatic Province), along with reference data on the 1944 CE and the Pomici di Base eruptions, highlights the compositional variability of bulk-rock and glass from leucite phonotephrite to garnet-bearing phonolite . The latter products have extreme fractionation of trace elements (e.g., La/Yb n = 126, Zr/Y = 89, Zr/Hf =78, Nb/Ta = 40; Th/U = 2.3), very low Sc, V, Y, HREE and very high As, Tl, Cs, Pb, Th and U. The Pomici di Base products, older than the eruptions described above, range from leucite-bearing shoshonites to trachytes , are devoid of garnet and belong to an independent liquid-line-of-descent, having also different fractionation between trace elements (e.g., La/Yb n = 15; Zr/Y = 12.4, Zr/Hf = 50, Nb/Ta = 15.6; Th/U = 3 in the Pomici di Base trachytes). A marked chemical variability of the observed phases is found. The geochemistry of garnet, amphibole , clinopyroxene and other phases shows wide variations of concentrations and elemental ratios (e.g., La/Yb n up to 520 in the sadanagaite coexisting with garnet). The magmatic evolution is dominated by low-pressure, oxidized, nearly closed-system fractional crystallization of clinopyroxene, plagioclase , leucite, ±magnetite, ±biotite, ±olivine and apatite in the transition tephrite-phonotephrite, and of potassic sanidine (±hyalophane), Fe-clinopyroxene, melanite garnet±Fe-amphibole in the transition tephriphonolite-phonolite. Mineralogic and geochemical evidence and modelling points out the existence of independent, zoned magma batches throughout the activity of the stratovolcano, which possibly started to crystallize at similar depths. The Somma-Vesuvius magmas thus evolved in shallow independent reservoirs with respect to those of the neighbouring volcanic complexes (Campi Flegrei, Ischia) as shown, for instance, by the contrasting compositional trends of clinopyroxene and amphibole, and have very limited evidence of crustal contamination (and/or carbonate assimilation). • Pollena, Pompeii, Avellino and Mercato are zoned eruptions with phonolites • The magmas which formed the various eruptions evolved independently • Different garnets dominate HREE, Y and HFSE; amphibole is strongly HREE-poor • The phonolites and their mineral compositions have no tendency to become peralkaline • F, Cl and H 2 O dominate the volatile budget

Tough to digest: first record of Teleosauroidea (Thalattosuchia) in a regurgitalite from the Upper Jurassic of north‐eastern Italy

AbstractPostcranial remains of a small teleosauroid from the Upper Jurassic of north‐eastern Italy are described in detail. The specimen, discovered in 1980 on a slab of Rosso Ammonitico Veronese (RAV Fm.; Bajocian–Tithonian), is represented by partially articulated thoracic, sacral and anterior caudal vertebrae, fractured and displaced osteoderms and pelvic girdle elements. Additional bone elements overlap one another in a densely packed cluster. The peculiar arrangement of this specimen is here interpreted as the result of regurgitation of the animal carcass from a pelagic predator or scavenger. Microstructural and geochemical evidence from bone and surrounding matrix support this hypothesis. A dearth of phosphorus in the matrix near the bone mass corroborates the interpretation of the specimen as a regurgitalite rather than a coprolite. Calcareous nannofossil and stratigraphic data allow us to assign the fossil to the latest Kimmeridgian or earliest Tithonian. The specimen is here tentatively referred to the teleosauroid subfamily Aeolodontinae based on the alternating pattern of the osteoderm ornamentation; this attribution is supported by stratigraphical and palaeoenvironmental evidence. If confirmed, this represents the first occurrence of Aeolodontinae in Italian outcrops of the western Tethys margin and represents the first marine crocodylomorph described as a bromalite.

The heterogeneous orogenic lithospheric mantle: whole rock and mineral geochemical evidence from early Paleozoic mafic intrusives in the Qilian orogen, China

The heterogeneous orogenic lithospheric mantle: whole rock and mineral geochemical evidence from early Paleozoic mafic intrusives in the Qilian orogen, China

Black shale–gray shale transitions in a Late Devonian shale succession, Central Appalachian Basin (Northern Ohio): Sedimentary and geochemical evidence for terrestrial organic matter input driving anoxia events

In a 140 m-thick sediment core through Upper Devonian shale units in the Central Appalachian Basin, gray shale units (TOC ≤1%) and black shale units (TOC ≤9%) are separated by “intercalated zones” (ICZs) of thinly interbedded gray and black shales. In accordance with Walther’s law of facies successions, ICZs imply environments producing black shales were coeval and adjacent to environments producing gray shales. To understand these facies transitions, we studied event layer sedimentology, organic geochemistry, and trace elements. This study found black shales to be shallow-water deposits (with tempestites and wave-modified turbidites), associated with significant terrestrial input (with abundant hyperpycnites, and terrestrial-derived humic and fulvic acids). The shales were deposited under varying redox conditions, with extensively bioturbated intervals alternating with intervals preserving intact micro-lamination, and demonstrating significant variations in trace element V/Cr, Ni/Co, and Th/U ratios. These observations are reconciled by attributing shale TOC values to sediment dynamics of deltas draining the Neo-Acadian Highlands. Progradation of individual delta distributary lobes resulted in localized offshore regions receiving increased terrestrial nutrient input and causing freshwater-induced water column stratification and oxygen stress (similar to the “dead zones” of many modern deltas), producing black shales. In contrast, gray shales formed between deltas or after delta distributary lobe abandonment. The intercalated black shale-gray shale transition intervals (ICZs) are interpreted to represent dynamic water and sediment conditions during the time interval of delta distributary lobe avulsion events. This model contrasts with previous models calling for basin-wide anoxia or assigning black shales to basinal deposits.

Impact of natural weathering on source rocks: Organic and inorganic geochemical evidence from the Triassic Chang 7 outcrop profile in Tongchuan of the Southern Ordos Basin (China)

Exposure of buried source rocks at or near the surface causes organic matter (OM) to be subjected to loss through weathering impacts. A detailed organic and inorganic investigation of the natural weathering of the Triassic Chang 7 Yishicun Profile in Tongchuan City of the Ordos Basin revealed pronounced changes in bulk, molecular, mineral, and elemental compositions. In this study, systematic and visible variations caused by natural weathering were shown in the amount and composition of both organic (kerogen and extractable organic matter [EOM]) and inorganic matter (minerals and elements). The results indicate that natural weathering could substantially affect the characteristics of source rocks at a depth of 2 m from the surface, resulting in a 41% decrease in total organic carbon (TOC) and a 75% loss of EOM. Furthermore, changes were noted in kerogen elemental compositions, fractional compositions of EOM, and some geochemical parameters from Rock-Eval pyrolysis. The mineral and major, trace, and rare earth element (REE) compositions of the outcrop samples were also analysed and compared. The weathering of source rocks was mainly due to the influence of surface water leaching and biodegradation of the OM. This resulted in the oxidation of kerogen and the preferential depletion of low-carbon-number n-alkanes and low-molecular-weight high-water-solubility biomarkers. This has led to bias in petroleum geochemical interpretation based on these organic geochemical parameters. The major elemental composition indicated that the outcrop profile was moderately weathered. Moreover, some commonly used palaeoenvironmental geochemical proxies, including U/Th, AU, Mo, Sr/Ba, Sr/Cu, and P/Ti, display varying degrees of weathering-related alteration. However, other palaeoenvironmental proxies such as V/(V + Ni), Ni/Co, V/Cr, δU, and Ceanom experienced little variation and remained reliable for environmental interpretation. Thus, petroleum resource evaluation and sedimentary environmental analysis of source rocks based on outcrop samples should consider the effects of natural weathering.

Supervolcanic resurfacing in northwestern Arabia Terra, Mars

An area roughly 9 × 105 km2 within northwest Arabia Terra contains several depressions interpreted to be supervolcanic calderas, contained within a chemical province considered consistent with large-scale igneous processes. Despite the underlying global significance to climate and geology, the supervolcanic hypothesis is yet to be tested with comprehensive compositional or geophysical analyses. Here we present geochemical evidence consistent with regional-scale supervolcanic resurfacing, with associated eruptions capable of degassing a climate-altering ∼108 kg of sulfur phases. Through gravitational modeling, we find evidence of low-density pyroclastic loading within this region, and a low elastic thickness, suggestive of a higher heat flow during the eruptive process. Our geochemical observations within this region reinforce its compositional uniqueness compared to contemporaneous volcanoes.

Meteorite impact at Ramgarh, India: Petrographic and geochemical evidence, and new geochronological insights

The impact origin of Ramgarh crater in India has been a topic of debate for several decades due to its conspicuous morphology, with recent studies confirming the structure as a complex impact crater. In this work, we present the results from an integrated petrographic and geochemical study to evaluate the impact origin of the crater, in conjunction with introduction of a new geochronological data for the crater through zircon U Pb systematics. Petrographic studies of the target Vindhyan sedimentary rocks show the presence of shock fabrics such as planar fractures, decorated planar deformation features, brecciation, melted grain boundaries, diaplectic glass, and vitrified matrix. Geochemically, the lithounits have high concentrations of Cr, Ni, Cu, and the corresponding chondrite normalized values show notably elevated levels than crustal average. The elevated levels of Cr, Ni and Cu in the target rocks indicate its interaction with an extra-terrestrial object. The positive correlation between Ni and Co, higher concentration of Cr (9–623.35 ppm), and more importantly the average Ni/Cr (~0.006- ~ 1.3), Ni/Co (~1.8 to ~22) and Cr/Co (~57–~368) ratios in the target lithounits of Ramgarh, point to a differentiated achondrite being the possible impactor. The zircon U Pb geochronological data from two samples in this study do not show significant Pb-loss and U enrichment. However, the relatively younger age of 528–395 Ma, depicted by quartzite sample (AN-4), along with the discordance and distinguishable Pb-loss, might hint that the impact event at Ramgarh have occurred between this timeframe. This age is considerably closer to the Precambrian-Cambrian Boundary (543 Ma) and, it also encompasses the age corresponding to the mid-Ordovician Meteor Event (OME) (467.5 ± 0.28 Ma). As this study infers that the potential impactor at Ramgarh is a differentiated achondrite and not a L-chondrite, the OME age can be ignored, further narrowing the age of Ramgarh impact structure. Our study also confirms the Ramgarh structure as an impact crater, and constrains the possible timing of the impact. • Tracking the impact event at Ramgarh Crater, NW India • Shock features in quartz grains mainly includes decorated PDFs, and melting. • Whole rock, trace and REE geochemistry reveals impact event. • Zircon U Pb ages of 528 to 395 Ma, which is the first convincing age for this crater

Intensive ocean anoxia and large δ13Ccarb perturbations during the Carnian Humid Episode (Late Triassic) in Southwest China

The Carnian Humid Episode (CHE) represents a dramatic dry to wet climate transition in the Late Triassic. Manifestations of this climate shift and its associated biological and environmental responses are not fully understood. Here, we carried out carbonate carbon isotope, trace metal, and pyrite framboid analyses at Wolonggang in southwest China to trace palaeoenvironmental changes during this critical interval. The CHE at Wolonggang is marked by the development of fine laminated carbonaceous siltstones and black shales overlying the intensely bioturbated Zhuganpo limestone deposited in the latest Julian 1. This lithological change is accompanied by two negative δ 13 C carb excursions, a strong enrichment of redox-sensitive trace metals U and Mo, and the development of abundant small pyrite framboids with an average diameter < 5 μm. The sedimentary and geochemical evidence collectively suggest a sharp decrease in carbonate production and a major carbon cycle disturbance, coinciding with the onset of intense ocean anoxia and the mass mortality of burrowing faunas. Unlike other areas in the western Tethys, the aftermath of the CHE at Wolonggang is characterized by long-lasting ocean anoxia, with weak carbonate production persisting at least to the late Tuvalian. This is evidenced by the deposition of finely laminated silty carbonates, marls, and carbonaceous siltstones with rare benthic fossils in the Wayao Formation. Our study suggests that anoxic-euxinic environments widely developed along the southwest margins of the Yangtze Platform and in the Nanpanjiang Basin, probably contributing to the loss of marine faunas during the CHE. • Sedimentary and bio-chemostratigraphic investigations were carried out in a key Carnian section in SW China. • Carbon cycle instability was seen during the Carnian Humid Episode. • Anoxia was basin-wide and long-lasting during the CHE and its aftermath. • Warming and anoxia played key roles in the loss of marine faunas during Carnian.

Reduced Late Bombardment on Rocky Exoplanets around M Dwarfs

Ocean-vaporizing impacts of chemically reduced planetesimals onto the early Earth have been suggested to catalyze atmospheric production of reduced nitrogen compounds and trigger prebiotic synthesis despite an oxidized lithosphere. While geochemical evidence supports a dry, highly reduced late veneer on Earth, the composition of late-impacting debris around lower-mass stars is subject to variable volatile loss as a result of their hosts’ extended pre-main-sequence phase. We perform simulations of late-stage planet formation across the M-dwarf mass spectrum to derive upper limits on reducing bombardment epochs in Hadean-analog environments. We contrast the solar system scenario with varying initial volatile distributions due to extended primordial runaway greenhouse phases on protoplanets and the desiccation of smaller planetesimals by internal radiogenic heating. We find a decreasing rate of late-accreting reducing impacts with decreasing stellar mass. Young planets around stars ≤0.4 M ⊙ experience no impacts of sufficient mass to generate prebiotically relevant concentrations of reduced atmospheric compounds once their stars have reached the main sequence. For M-dwarf planets to not exceed Earth-like concentrations of volatiles, both planetesimals, and larger protoplanets must undergo extensive devolatilization processes and can typically emerge from long-lived magma ocean phases with sufficient atmophile content to outgas secondary atmospheres. Our results suggest that transiently reducing surface conditions on young rocky exoplanets are favored around FGK stellar types relative to M dwarfs.

Crust sequences of the Xigaze ophiolite in South Tibet: The water effect on crust accretion at a slow-spreading oceanic ridge and its implications

The tectonic setting of the Xigaze ophiolite of southern Tibet has been debated ever since it was first described 1981. It lies in the central part of the Yarlung Zangbo but differs from adjacent ophiolite by having a relatively thin crustal section, an oceanic core complex and complex geochemistry. It is overlain by radiolarian cherts suggesting and open ocean environment but also show geochemical evidence of suprasubduction zone magmatic activity. Our detailed study of this massif shows that the crustal sequence was formed by three stages of magma injection derived from a common parental source. Stage 1 is represented by heavily altered and deformed gabbros that form dikes and pods in the upper part of the peridotites, whereas stage 2 includes dolerite sills and extruded basalts, which form >80% of the crustal section. Finally, stage 3 is composed of dolerite dikes and gabbro intrusions, which crosscut rocks of stage 1 and 2 at high angles. The youngest rocks of stage 3 are plagioclase wehrlite intrusions, which are characterized by having anomalously high calcium plagioclase (An = 99–100) which was preceded by clinopyroxene. We suggest that the multiple magma injection were formed during tectonic extension and magma-fluid interaction at a slow-spreading rift in a distal fore-arc environment during the Early Cretaceous. Our study implies that caution must be exercised in applying subduction initiation models to ophiolite genesis, particularly when the geochemistry of the rocks contradicts the field observations and stratigraphic evidence. • Provides new information on the crustal structure of slow-spreading ridges. • Detachment faults served as fluid conduits changing the fractional crystallization. • Geochemical changes do not necessarily reflect the change of tectonic setting. • Both ridge spreading and subduction dynamics were active in a SSZ environment.