Earth and Planetary Science Letters最新文献

The quenched (rapidly-cooled) angrite meteorites, which formed in the inner Solar System, record large-scale planetary mixing in the first few Ma of Solar System history, and therefore, provide a unique opportunity to investigate the role of impacts in terms of water addition to the growing planetesimals. Here we investigate the H isotopic composition and H₂O abundance of relict olivine grains that survived impact melting within Asuka (A) 12,209 and compare them with impact melt-produced groundmass fractions using in-situ nanoscale secondary ion mass spectrometry (NanoSIMS). These analyses test if the angrite parent body (APB) acquired a CC-like H isotopic composition before early large-scale impact mixing and/or acquired volatiles by subsequent impact(s). Furthermore, we analyse the H isotopic composition and H₂O abundance of later-forming plutonic (NWA 4801), intermediate (NWA 10,463) and dunitic (NWA 8535) angrite meteorites to assess the role of impacts, in terms of volatile delivery, during the first 50 Ma of the inner Solar System history. The H isotopic composition of most quenched angrites appears to be affected by degassing. Consequently, we opt to use the weighted average δ_D of pyroxenes and olivines in the plutonic angrite, NWA 4801, to estimate the original composition of the APB (-235 ± 113 ‰ 1σ, n = 18), in agreement with recent studies on the hydrogen isotopic signatures of mineral-hosted melt inclusions in D'Orbigny and Sahara 99,555. Additionally, we use the H₂O abundances of NWA 4801 pyroxene (7.9 ± 1 µg/g 2σ) and olivine (6.1 ± 0.6 µg/g 2σ) to estimate the lower (85 to 110 µg/g) and upper (519 to 1089 µg/g) limits of the primitive APB mantle H₂O content, implying that the APB was one of the most hydrated bodies in the early inner Solar System. The similarity of δ_D/H₂O systematics in the relict olivine grains and groundmass olivine within A 12,209 argues against water delivery through impacts in the early inner Solar System. Overall, the non-carbonaceous reservoir in the inner Solar System appears to retain a single source of water, which isotopically resembles either water ice in carbonaceous chondrite parent bodies or fractionated nebula water.

The Doushantuo negative carbon isotope excursion (DOUNCE), which corresponds to the worldwide mid-Ediacaran Shuram-Wonoka anomaly (567∼574 Ma), is characterized by a significant shift in carbonate-carbon isotopic values (δ¹³C_carb) from ∼ 5‰ down to ∼ -12‰. It has been proposed to result from remineralizing a giant dissolved organic carbon (DOC) pool within the Ediacaran Ocean. However, direct geological proof confirming the existence and oxidation of such a large DOC reservoir is still scarce. The strong affinity of Hg for organic carbon in the surface environment could provide helpful information on the potential contribution of Hg-associated DOC to the DOUNCE. Here, Hg content and isotope values of marine sediments from the slope (Songtao) and basin (Fengtan) settings were measured and discussed with the previously published data from the shelf environment (Jiulongwan) in South China. The generally low Hg concentration and the absence of Hg/TOC (total organic carbon) anomaly within the DOUNCE carbonate from the proximal to distal sections argue against the massive contribution of DOC to the DOUCE. The gradually decreasing Δ¹⁹⁹Hg but increasing δ²⁰²Hg in the proximal depositions would be indeed ascribed to the enhanced terrestrial input and the photic zone euxinia (PZE), which might hinder the development and expansion of macroscopic multicelluar eukaryotes via H₂S poisoning. Our new Hg isotope data casts doubt on the DOC oxidation model, and contributions from other ¹²C-enriched reservoirs (e.g. recycled carbonatites) could be the cause of the most significant negative δ¹³C_carb excursion in Ediacaran.

Organic and inorganic geochemistry was integrated with sedimentology from the Eagle Ford Group in the U.S. Geological Survey Gulf Coast-3 drill core to investigate why organic matter enrichment and anoxia predate the Cenomanian–Turonian oceanic anoxic event (OAE-2) at the southern Western Interior Seaway (WIS). The relationship between the degree of organic matter sulfurization and preservation in the southern WIS is comparable to OAE-2 sites outside of the WIS, but enhanced organic matter sulfurization and preservation predated OAE-2, distinguishing the southern WIS from other OAE-2 localities. Persistent euxinia and organic matter sulfurization before and during the mid-Cenomanian event (MCE) facilitated maximum organic matter enrichment and preservation to coincide with extensive lateral sediment transport. The new perspective detailed here reconciles geochemical evidence of depositional euxinia with sedimentological evidence of sediment transport, which informs how organic-rich mudstones form in dynamic environments. After the MCE but before OAE-2, reducing conditions weakened as relative sea-level continued to rise, resulting in an oxygen-depleted environment that was prone to transient euxinia. The extent of organic matter sulfurization, preservation, and enrichment declined as euxinia became intermittent. As maximum sea-level was established during OAE-2, boreal water masses flowed into the southern WIS and unrestricted, oxygenated marine conditions developed, which resulted in degradation of marine organic matter and less organic enrichment.

The highest relative contribution of marine bacterial organic matter occurred during the persistently euxinic interval before and during the MCE. The relative input of algal organic matter increased as the depositional conditions became less reducing after the MCE but before OAE-2. Prasinophyte green algae contributed to the consistent dominance of C₂₈ steranes preceding OAE-2, which is becoming a widely observed feature across the southern and central WIS. A sharp drop in the C₂₈ sterane relative abundance reflects a decline in the abundance of prasinophytes as oxic depositional conditions developed during OAE-2. Relative abundances of 2-methylhopanes increased during OAE-2, which is a common signature of OAEs, suggesting a modified nitrogen cycle in the offshore southern WIS despite oxic depositional conditions during OAE-2.

Export of dissolved carbon from rivers connects terrestrial and oceanic carbon cycling, and represents a key component of global carbon budgets. In this study, we measured the concentrations of major ions, the sulfur isotopic composition of sulfate, and the stable (δ¹³C_DIC) and radioactive (Δ¹⁴C_DIC) isotopic composition of the dissolved inorganic carbon (DIC) in the upper Changjiang (Yangtze) River, China. Their spatio-temporal variabilities give constrains on carbon cycling across catchments with diverse physiographic conditions (e.g. climate, hydrology, geology, land-use change). Daily DIC fluxes were modeled using a hydro-chemical approach and showed significant temporal variations agreeing with the measured data, and supporting a conservative mixing behavior in river channel, similar to variations in anions and cations concentrations. However, the modeled (δ¹³C_DIC) were always higher (up to 3.1‰, and 0.8‰ on average) than the measured values. In addition, Δ¹⁴C_DIC would suggest a much lower carbonate contribution (8 to 33%) than concentration mixing relationship (40 to 64%). From the chemical weathering reaction front to the riverine transport, DIC can be processed with little net change in concentration but isotopic exchange and it can also exchange with the soil and/or atmospheric CO₂ as an open system. The openness parameter (ψ) uses the Δ¹⁴C_DIC to estimate the extent of such exchange, ranging from 0 (fully closed system) to 1 (fully open system) and varied from 0.26±0.10 to 0.86±0.06. ψ showed a strong relationship with drainage mean elevation, likely to represent the elevation-induced climate variation (i.e. precipitation and temperature) but also the proportion of cropland in the drainage area. All of these parameters are correlated and point toward a biological impact on ψ. In addition, modern DIC yield, estimated from Δ¹⁴C_DIC, is positively correlated to ψ. This study demonstrates that Δ¹⁴C_DIC measurements can greatly underestimate the contribution of carbonate-derived carbon because of exchange (estimated by ψ) especially in high biological-productivity areas.

Pulsing volcanotectonic cycles characterized by short-lived (10 s Myr) switches in magmatic and tectonic patterns have been broadly identified in active margins. However, the specific mechanism that causes these switches remains ambiguous, i.e., whether the subduction continuity and/or terrane arrival (accretion, underthrusting, or subduction of buoyant continental/oceanic blocks with a thicker crust than their surrounding oceanic plate) plays a crucial role in controlling the observed volcanotectonic cycles remains controversial. Here, by modeling subduction processes involving the sequential arrival of buoyant terranes, we show that 1) in scenarios where the oceanic plate is weakly coupled with terranes, the entraining of the terrane into the subduction induces slab breakup to occur between the partially subducted terrane and its adjacent oceanic slab, with the terrane then rebounding and moving away from the trench. This evolution causes switches in the magmatic and tectonic patterns within the overriding plate. 2) In these models, the exposed terrane materials can preserve characteristic pressure-temperature-time trajectories, i.e., nearly isothermal compression to isothermal decompression and/or isobaric heating after partial exhumation. 3) slab breakup does not guarantee the occurrence of a trench jump; only when the terrane has a medium scale (∼300 km) will a new trench tend to develop prominently behind the rebounded terrane. 4) in scenarios where the composite slab (terrane and oceanic portions) resists yielding deformation and the terrane density is close to that of the oceanic plate (<0.6% density contrast), continuous subduction will occur. In this latter scenario, slab deformation (revealed by subduction angle and slab curvature), instead of slab breakup, will control the magmatic and tectonic patterns in the overriding plate. By further comparing model results with observations, we demonstrate that intermittent subduction interrupted by the subduction of terranes can be a tectonic driver for episodes of compression-to-extension transformations and magmatism (or piston-like volcanotectonic cycles) in two representative accretionary belts — with or without trench jumps (exemplars in south-central-Tibet and eastern-Mediterranean). In contrast, continuous subduction with strongly coupled upper and subducting plates could have contributed to similar cycles in the example without accretion (e.g., the Altiplano in the Central Andes). Therefore, over 10 s of Myr, the scale and frequency of terrane arrivals could essentially control the specific motion pattern of the subducting plate, creating the observed short-lived volcanotectonic switches at these three subtypes of active margins.

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This article has been retracted at the request of Publisher/Editor for the below details:

Authors submitted a revised paper due to some errors appearing in the paper. Rather than a corrigendum being posted for this original paper, a second version was published: Earth Planet. Sci. Lett. 628 (2024) 118587, https://doi.org/10.1016/j.epsl.2024.118587. We need to retract at least one version of this paper.

The knowledge of dynamics and retreat patterns of marine-based ice streams under multiple stressors are of foremost importance for predicting Antarctic Ice Sheet response to climate changes. The Holocene palaeoglaciological record of former ice streams draining the northeast Antarctic Peninsula can elucidate the influences of changes in atmospheric and oceanic circulation and sea-level oscillations on the ice thinning and grounding line retreat. Here, terrestrial cosmogenic nuclide (TCN) dating of erratic boulders across the James Ross Island group sheds light on the pattern and timing of the ice recession along the two main arteries of the palaeo-ice drainage: Croft Trough and Prince Gustav Channel. The approach of using paired ¹⁰Be-²⁶Al nuclides enabled an assessment of cosmogenic isotope inheritance and complex burial-exposure history, notably on the high-altitude volcanic mesas. The TCN ages suggest that the Prince Gustav Channel Ice Stream was thinning from at least ∼12 ka, with subsequent separation of the Antarctic Peninsula and James Ross Island ice masses by 10–8 ka. The transition from grounded ice to open marine conditions in the Croft Trough occurred rapidly at 8.6–7.2 ka, following the Early Holocene Warm Period, concomitant with eustatic and relative sea-level rise and incursions of warmer circumpolar waters. Grounding line retreat was possibly further accelerated by buoyancy response of thinning ice stream to low-gradient bed topography. The lessons of rapid deglaciation of James Ross Island palaeo-ice streams may provide analogues for recent or future intensification of pressures on Antarctic glaciers.

Halokinetic deformations synchronous with salt deposition are processes already suggested in several salt giants including the Lower Cretaceous salt deposits of the Santos Basin in Brazil. However, the dynamics of syn-depositional deformation has never been studied in a coherent depositional and structural framework. This study investigates well data and high-resolution 3D seismic images in the evaporitic Ariri Formation of the Santos Basin to understand the intra-salt deformations, estimate their timing and establish a link with the depositional setting of the evaporites. The seismic data shows numerous examples of intra-salt onlaps, erosive surfaces and sedimentary wedges highlighting the occurrence of deformations during salt deposition. Two dominant processes have initiated salt creep: the density contrasts between a dense upper anhydrite-rich unit and a less dense lower halite-dominated unit and the gravitational system promoted by the late-rift subsidence. Updip, extension prevailed during salt deposition and favored a boudinage of the Ariri Formation. Downdip, early contraction favored the development of well-developed intra-salt minibasins. Deformation occurring during salt deposition and induced topographic relief likely influenced the hydrological conditions. It probably led to intra-salt dissolution/reprecipitation processes and hyper-saline conditions favoring the precipitation of tachyhydrite deposits in localized intra-salt depocenters. Eventually, post-salt minibasins systematically developed above syn-salt depocenters, indicating that syn-salt deposition deformation had a strong impact on post-salt evolution. This new understanding of the intra-salt deformation in the Santos Basin paves the way for new interpretations of halokinetic deformations reported elsewhere in the South Atlantic or in other salt-giant provinces.

During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe²⁺-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential.

Seismic swarms represent clusters of seismicity without large mainshocks. While they occur naturally, they can also be induced by human activities, particularly during reservoir hydraulic stimulations. A striking feature of seismic swarms is the migration of their hypocenters. The seismic front, initially attributed to fluid diffusion, has more recently been understood as the result of the propagation of a fluid-induced aseismic slip. Close to the center of the swarm, a seismic back-front is commonly admitted after the injection end, but a low density of events is also observed during the injection period. In our investigation, based on a compilation of 22 swarms of both natural or anthropogenic origin, we aim to explore the existence and origin of a seismic back-front. Interestingly, we observe a post-injection back-front only in rare cases, where a rapid fluid pressure decrease is imposed at the injection point. Conversely, a back-front during the injection period is always observed in both types of swarms. Consequently, the back-front cannot be reliably used to infer the end of injection, as commonly done for natural swarms. Moreover, the occurrence of this back-front during injection is linked to an increase in the minimum magnitude of seismic events. We interpret the vanishing of the seismicity close to the injection point as a consequence of the increase in earthquake nucleation length with increasing fluid pressure. With a substantially enhanced capability for detecting small events, it may become feasible to use this back-front as a means of monitoring injection pressure, even in the context of natural swarms.