de Souza, Gregory F.Hennekam, RickVance, DerekChiu, Chun Fung...
12页
查看更多>>摘要：The elemental concentrations and isotope compositions of molybdenum (Mo) and uranium (U) are commonly used for the reconstruction of past global and local redox conditions, and recent studies using both elements have revealed the potential of their paired application. However, such studies have generally focused either on modern marine sediments or on relatively low-resolution reconstructions of deep-time paleo-redox conditions. Here, we present high-resolution profiles (every 0.2-0.3 kyr) of Mo and U elemental and isotope compositions for anoxic organic-rich sediments of Eastern Mediterranean sapropels S5 and S7. The new Mo-U data reveal the processes leading to descent into basinal euxinia in more precise and systematic detail than lower resolution datasets focused on either Mo or U only. During the intensification from anoxic non-sulfidic to persistently euxinic conditions, delta Mo-98 and delta U-238 covariation systematics exhibit two stages. We identify the Mo-U isotope signature of the early transition from anoxic non-sulfidic to mildly euxinic conditions in the first stage, characterized by a rise in U isotope ratios (from -0.3 parts per thousand to +0.2 parts per thousand +/- 0.05 parts per thousand) controlled by the depth within the sediment of the uranium reduction-accumulation front. As the water column turns persistently euxinic, delta Mo-98 values approach the seawater value for both sapropels, but delta U-238 evolves to different final values in the two sapropels. We interpret these differences as reflecting different redox potentials and/or different degrees of restriction of these two sapropel events, and the more gradual response of U sequestration to redox variation in comparison with threshold behavior of Mo. The findings presented here show temporal patterns in delta Mo-98 and delta U-238 on relatively short timescales that suggest the combined use of these proxies at high resolution allows detailed reconstruction of local redox and hydrographic conditions. (c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Qin, ZhaoyuanOliveira, Elson P.Girardi, Vicente A. V.Wang, Chong...
11页
查看更多>>摘要：The concurrence of both global orogenesis and large igneous provinces (LIPs) from 2200-1600 Ma remains enigmatic. This apparent contradiction of top-down and bottom-up geodynamics occurred during assembly of Earth's oldest-known supercontinent. We present a multidisciplinary study combining the dismembered portions of major LIP worldwide and correlate them across supercontinent Columbia to the same plume center (Xiong'er of the North China craton). The succession of LIPs is then used to trace the oldest established hotspot track associated with a pulsating plume constraining ancient plate motion. Subduction-controlled plume generation can explain mantle upwelling occurring where subducted slabs forming the supercontinent had previously sunk to the base of the mantle and advected the Xiong'er mantle plume due to return flow. Tectonic motion detected by the hotspot track can account for both those regions of the supercontinent with and without LIPs. Such historical constraints on the interaction between plumes and subduction are critical in linking bottom-up and top-down tectonic processes at the dawn of the supercontinent cycle. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：We report 534 detrital apatite (U-Th)/He thermochronometric ages collected along a > 400 km latitudinal transect along the Antarctic Peninsula (AP). We use the dataset to evaluate the relative roles of climate change and tectonics on the timing of km-scale fjord incision. We find that the onset of km-scale topographic change occurred more than 15 million years after the initiation of glaciation in the region, and generally coincided with the arrival times of the spreading ridge at different latitudes. These results indicate that tectonically-initiated rock uplift accelerated rates of erosion during long glacial conditions at the AP. We infer that rock uplift primarily affected glacial erosion rates by increasing topographic relief, and thereby increasing ice motion and its capacity to erode. These are among the first empirical observations to reveal that a tectonic history, and its control on the regional topography of a landscape, has influenced glacial erosion rates over geologic timescales. (C)& nbsp;2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Understanding the subsurface processes that generate volcanic unrest, including surface deformation, earthquakes, temperature increase, and gas emissions, is essential to improve the forecasting of volcanic eruptions. Volcanic gases exsolved from magma reservoirs can transfer heat towards the surface when the system is open, or pressurize the volcano and lead up to eruptions when the system is closed. Hence, the nature of the observed precursory signals is greatly dependent on whether exsolved volatiles accumulate or escape. In this study, we develop a two-dimensional finite element model to calculate the thermal and poroelastic responses of a volcano to gases that exsolve from depth and migrate to the surface through a pre-existing fractured conduit. This model is explored through a set of sensitivity tests to quantify the controls of gas fluxes and permeability on geophysical observables; and is used to interpret surface deformation (GPS), ground temperature, and seismicity data recorded before the 2006 eruption of Augustine volcano, Alaska, by utilizing the Ensemble Kalman Filter data assimilation technique and Coulomb stress calculations. Our results show that the permeable transfer of gas through a fractured conduit can yield a measurable thermal anomaly at the surface for at least one year before the eruption, consistent with ground- and remote sensing-based data. Moreover, gas flux increased about ten times around three months before the eruption, which might have accelerated hydrothermal alteration and reduced permeability of the conduit by several orders of magnitude, thus accumulating gases inside the volcanic edifice, generating surface deformation, and triggering volcano-tectonic earthquakes. Eventually, failure of the sealed pathways due to high overpressure led to the eruption. Multi-physical numerical models coupling gas flow with host rock deformation and heat transfer are useful tools to understand the triggering mechanisms of volcanic eruptions driven by volcanic gases. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：Several studies have revealed small heterogeneities in the relative abundance of 182W, the radiogenic nuclide of short-lived 182Hf (t1/2 = similar to 9 Ma), in terrestrial rocks. Whereas the majority of Archean rocks display 182W excesses relative to bulk silicate Earth, many young ocean island basalts show small 182W deficits, in particular if they are sourced from deep-rooted mantle plumes. The origin of this anomaly is still ambiguous, proposed models focus on core-mantle interaction or the presence of reservoirs in the lower mantle that have been isolated since the Hadean. In order to evaluate the role of upper mantle reservoirs, we report the first 182W data for intraplate basalts where a deep plume origin is still debated (Ascension Island, Massif Central, Siebengebirge and Eifel) and intraplate volcanic rocks associated with either plume or subduction zone environments (Italian Magmatic Provinces) and compare them to new data for basalts that have a deep mantle plume origin (La Reunion and Baffin Island). The proto-Iceland plume basalts from Baffin Island have uniform and modern mantle-like mu 182W of around 0 despite extremely high (3He/4He). In contrast, basalts from both volcanic edifices from La Reunion span a range from modern upper mantle values to deficits as low as mu 182W = -8.8 ppm, indicating a heterogeneous source reservoir. The mu 182W in all other intraplate volcanic provinces overlap the composition of modern upper mantle to within 3 ppm. The absence of resolvable 182W anomalies in these intraplate basalts, which partially tap the lithospheric mantle, suggests that primordial components are neither present in the central and southern European lithosphere nor in the European asthenospheric reservoir (EAR). The general absence of 182W anomalies in European plume-related basalts can either be explained by a shallow mantle source or by the absence of isotopically anomalous and isolated domains in the deep mantle beneath the northern hemisphere, as also suggested by geophysical evidence. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
查看更多>>摘要：Recent findings of evolved martian crustal materials by orbiters and rovers challenged the longstanding notion of an only-tholeiitic basaltic crust. Models exploring intraplate magmatic processes like fractional crystallization show that unrealistic amounts of solid accumulation are sometimes required for the formation of martian evolved materials, including evolved materials analyzed by the Curiosity rover at Gale crater. Assimilation and fractional crystallization (AFC) play an important role in the diversification of magmas on Earth, but has not been explored for its role in the generation of evolved martian compositions. In this study, we conduct a series of thermodynamic AFC models using the Magma Chamber Simulator (MCS) model interfacing with the Rhyolite-MELTS algorithm to better understand the controls on assimilation and whether it contributed to the evolution of the martian crust. We find that AFC models replicate evolved Gale crater targets effectively at different pressures and water contents. AFC likely contributed to martian crustal formation, particularly evolved compositions. Higher crustal temperatures likely lead to a higher volume of evolved magmas formed by AFC processes on early (rather than modern) Mars. AFC may have also occurred during the Amazonian, especially in the fractured upper layers of crust, although in lower volume than during the Noachian. (c) 2022 Elsevier B.V. All rights reserved.
Caudron, CorentinSoubestre, JeanLecocq, ThomasWhite, Robert S....
11页
查看更多>>摘要：Applying seismic interferometry and network covariance matrix-based analyses to detect and locate the source of volcanic tremor during the 2010 Eyjafjallajokull effusive flank and explosive-effusive summit eruptions has provided new insights into this iconic event. The tremor source locations derived from the network covariance matrix approach were spatially distinct during the two eruptions. The tremor was radiated between the surface and 5-6 km depth during the effusive flank eruption, including an apparently progressive upward migration in early April 2010, but was strictly confined to the surface during the summit eruption. Each phase of the summit eruption left a distinct fingerprint in the seismic records. Effusive phases radiated continuous tremor between 0.6 and 5 Hz, whereas explosive phases produced tremor in a more pulsating fashion over a wider frequency band (0.2-10 Hz). A period of intermittent tremor bursts (called banded tremor) on 15 April, associated with formation of a new vent at the summit, was most likely generated by magma-gas-meltwater interaction within a subglacial enclosure. The banded tremor ceased following an abrupt draining of the newly formed subglacial cauldron, resulting in a large slurry glacial meltwater flood (jokulhlaup). This study highlights the importance of new data processing methodologies for future monitoring of volcanic tremor in real-time. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：A detailed magnetostratigraphic study, linked to a new latest Permian U-Pb ID-TIMS age, was undertaken on the continental Shichuanhe section (SCH) in North China in order to provide a magnetic polarity scale for the Late Permian-early Middle Triassic interval. Tilt-corrected mean directions of the characteristic remanent magnetization pass the reversal test and correspond to a site paleolatitude of 18.1N during the Early Triassic, consistent with previous results from the North China Block. The magnetostratigraphy shows close similarity with previous studies, allowing interregional correlations with both marine and non-marine records. Normal magnetozone SCH3n, constrained by an absolute age of 252.21 +/- 0.15 Ma from an ash bed 3.5 m below its base, is unambiguously correlated to the earliest Triassic normal magnetochron LT1n. Our newly established magnetostratigraphic framework and published carbon isotope chemostratigraphy, indicate that the Permian-Triassic Boundary is ca. 8 m above the base of SCH3n (within the middle part of the Sunjiagou Formation) at SCH. The overlying reverse polarity dominated interval (SCH3r-SCH5r) ranges to the middle Liujiagou Formation, and straddles an interval from the mid-Griesbachian to mid-Smithian. The base of the Olenekian is provisionally located in the lower part of the Liujiagou Formation, near the base of magnetozone SCH5n. The succeeding thick normal magnetozone SCH6n persists into the upper Heshanggou Formation, with the inferred Smithian-Spathian boundary located in the upper part of the Liujiagou Formation. The transition from reverse magnetozone SCH6r to the overlying normal magnetozone SCH7n, coincides with a clear erosional contact with the base of the Ermaying Formation. Consequently, magnetozone SCH7n is matched to the Early Anisian magnetochron MT3n, with the Olenekian-Anisian boundary interval missing. Our new timescale provides additional magnetostratigraphic constraints on the timing of the terrestrial ecological crisis in North China, which is found to lie within reverse magnetozone SCH2r (equivalent with reverse magnetochron LP3r), a level some 270 +/- 150 kyrs before the main marine extinction, that falls in the overlying normal magnetochron LT1n. (c) 2022 Elsevier B.V. All rights reserved.
Carrapa, BarbaraDeCelles, Peter G.Ducea, Mihai N.Jepson, Gilby...
11页
查看更多>>摘要：The Central Andes represent the archetypical Cordilleran orogenic system, with a well-developed continental volcanic arc and some of the thickest crust on Earth. Yet the relative contributions of shortening and magmatic additions to crustal thickening remain difficult to quantify, which hinders understanding processes of crustal evolution in continental arcs. Cerro Aconcagua, the highest mountain in the Americas and a relict Miocene stratovolcano resting on 55 km-thick crust, is the ideal natural laboratory to address this issue in subduction-related magmatic arcs because it preserves a multimillion year record of magmatism and deformation within the Aconcagua fold-thrust belt. Estimates of paleo-crustal thickness in the Andes can be made using the geochemistry of subduction-related magmatic rocks, or minerals crystallized within them. This study applies a geochemical proxy approach for crustal thickness estimates to detrital syntectonic deposits of the Santa Maria Conglomerate derived from the Aconcagua stratovolcano to reconstruct paleo-crustal thickness of the Andes at this latitude. Detrital zircon trace-element data from ashes intercalated in the conglomerate, combined with previously published paleo-crustal thickness data, indicate crustal thicknesses of similar to 35 km ca. 38 Ma and similar to 44 km ca. 12 Ma, requiring similar to 11 km of crustal thickening after ca. 12 Ma to achieve present-day crustal thickness of similar to 55 km. In the absence of significant magmatism since ca. 10 Ma at this location, we show that more than half of the crustal thickening after 12 Ma, corresponding to 2 km of uplift, was achieved by Miocene shortening. Our study also reveals significant differences in crustal thicknesses between the Central Andes and the southern Central Andes which we speculate may be due to southward crustal flow during the last similar to 20 My. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The Yellow River is one of the largest suppliers of sediments and organic carbon (OC) to the ocean. Previous studies have revealed that OC transported by the Yellow River largely derives from the erosion of the Chinese Loess Plateau, which is dominated by pre-aged soil carbon and could be efficiently preserved in marine sediments. Here, we used ramped oxidation radiocarbon analysis (RPO-C-14) to characterize the age and reactivity distribution of OC in two Yellow River suspended sediment samples and six Bohai Sea and Yellow Sea (BS-YS) surface sediments from a transect along the sediment transport pathway. RPO-C-14 independently characterizes the full spectrum of OC thermal stability and isotope compositions to reveal the source, age and reactivity structure of OC transported by the Yellow River and preserved in Chinese marginal sea sediments. We calculated the activation energy (E) distribution-a proxy for bonding environment and by extension reactivity-which, combined with C-14 and stable carbon isotope (delta C-13) compositions, reveals OC origin and stability. Our data suggest that 96% of OC in Yellow River suspended sediments is biospheric and weathered petrogenic, while unweathered petrogenic OC only accounts for 4% which is almost an order of magnitude lower than the fossil OC estimates (32%) based on compound specific C-14 analysis. RPO data reveal the prevalence of aged biospheric loess OC in the Yellow River. We use delta C-13, C-14 and RPO-derived activation energy data to quantify the contribution of terrestrial OC to surface sediments in the BS-YS. The resulting estimates of terrestrial OC proto-burial efficiencies yield an average value of 89 +/- 30%, revealing overall very high terrestrial OC preservation in the BS-YS. Additionally, and somewhat counter intuitively, we find that the preservation of terrestrial OC decreases with increasing E. This pattern may arise from an enhanced preservation of a pre-aged C-4 plants derived fraction of the loess-derived OC associated with secondary clays characterized by smaller grain size and higher surface area. Alternatively, the high E component of the Yellow River OC might comprise partially weathered petrogenic carbon, undergoing further mineralization during transport from rivers to marginal sea sediments via marine organic matter priming. (c) 2022 Elsevier B.V. All rights reserved.
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