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Earth and Planetary Science Letters
North-Holland
Earth and Planetary Science Letters

North-Holland

0012-821X

Earth and Planetary Science Letters/Journal Earth and Planetary Science LettersSCIEIAHCIISTP
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    Polarity-reversal subduction zone initiation triggered by buoyant plateau obstruction

    Riel, N.Rosas, F. M.Duarte, J. C.Schellart, W. P....
    14页
    查看更多>>摘要：Oceanic lithosphere worldwide is younger than ca. 200 Myr, suggesting that it must have been globally recycled by the recurrent formation of new subduction zones since the existence of subduction on Earth. However, postulated subduction zone initiation processes remain difficult to explain in many cases, and the specific geodynamic conditions under which these might occur are still largely unknown. We here use numerical models driven by the internal force balance of a subduction system to better understand the (geo)dynamics governing (intra-oceanic) polarity-reversal subduction zone initiation. This initiation mode assumes that the birth of a new subduction zone could be triggered by buoyant plateau-obstruction of a pre-existent one dipping in the opposite direction. Our work provides a new insight on the key geodynamic conditions governing this type of subduction zone initiation and discusses their general compliance with known natural examples. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    Upstream migration of avulsion sites on lowland deltas with river-mouth retreat

    Li, JiaguangGanti, VamsiLi, ChenglongWei, Hao...
    9页
    查看更多>>摘要：River deltas are fertile, populous landscapes that grow through river avulsions-episodic channel-jumping events. Historically, avulsions on deltas occurred at persistent locations, causing some of the deadliest recorded floods. Climate change and human activities are threatening to drown deltas but it is unknown how avulsions will respond because they occur infrequently on large, lowland deltas. Here, we use a low-gradient river delta formed on the margin of a lake in the Qaidam Basin, China, as a natural laboratory to explore how lowland deltas will respond to river-mouth retreat from accelerated relative sea-level rise. Using satellite imagery from 1973 to 2010 C.E., we identified and analyzed the response of 6 lobe-scale avulsions on the Sulengguole River delta to the seasonal expansion of the North Huoluxun Lake in the Qaidam Basin. We show that the seasonal lake-water area increase caused punctuated river-mouth retreat. In response, avulsion sites migrated upstream at a commensurate rate such that the avulsion length-streamwise distance of avulsion site to the river mouth-remained consistent and scaled with the backwater lengthscale, similar to large, lowland deltas. Results indicate that the drowning of lowland deltas from accelerated relative sea-level rise will shift avulsion hazards tens-to-hundreds of kilometers upstream, exposing new upstream communities to the risk of catastrophic flooding. (C) 2021 Published by Elsevier B.V.
      原文链接:
    • NSTL
    • Elsevier

    Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments

    Kipp, Michael A.Tissot, Francois L. H.
    13页
    查看更多>>摘要：Uranium isotopes (delta U-238) have quickly become one of the most widely-used redox proxies in paleoceanographic studies. The quantitative power of the delta U-238 proxy derives from the long marine residence time of uranium and the dominance of reduction in fractionating uranium isotopes during removal from seawater. The seawater delta U-238 value is therefore sensitive to the size of the anoxic sink, and by extension, the area of the seafloor overlain by anoxic waters. Leveraging the ability of carbonates to record and retain the seawater delta U-238 value, and the ubiquity of carbonate sediments in the geologic record, numerous studies have quantified seafloor anoxia across ocean anoxic events, mass extinctions, and global climatic changes. In most cases, forward models of marine uranium isotope mass balance have been used, illustrating potential histories of seafloor anoxia during these events. Here we show that there are multiple ways in which such forward modeling can lead to spurious inferences of anoxia, including (i) the poor sensitivity of the delta U-238 proxy when fractional anoxia is high, and (ii) the inherent bias in generating illustrative forward model outputs in stratigraphic sections with expected anoxic intervals. We thus explore inverse modeling approaches to constrain the most likely history of seafloor anoxia for a given delta U-238 dataset, and ultimately develop a framework for doing so using Bayesian inference via Markov Chain Monte Carlo simulation. We show that this approach can recover simulated trends, and further reconstruct marine anoxia for eight published delta U-238 datasets. We find that some previous interpretations of anoxic seafloor extent were inaccurate, either because steady state was improperly assumed, or because the illustrative forward models used were poor fits to the data. In order to overcome these issues in future work with the delta U-238 redox proxy, we have made this model publicly available, and also offer suggestions for the judicious use of forward models. By building on this framework, the future quantification of marine anoxia during transient environmental perturbations can be performed consistently, thereby facilitating robust comparison of anoxic extent between events. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations

    Sproson, Adam D.von Strandmann, Philip A. E. PoggeSelby, DavidJarochowska, Emilia...
    13页
    查看更多>>摘要：The Ordovician (similar to 487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It was followed by the Silurian (similar to 443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale (> 5 parts per thousand) carbon isotope (delta C-13) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (Os-187/Os-188) and lithium (delta Li-7) isotope measurements of marine sedimentary rocks that cover four Silurian delta C-13 excursions. Osmium and Li isotope records resemble those previously recorded for the Hirnantian glaciation suggesting a similar causal mechanism. When combined with a new dynamic carbon-osmium-lithium biogeochemical model we suggest that astronomical forcing of the marine organic carbon cycle, as opposed to a decline in volcanic arc degassing or the rise of early land plants, resulted in drawdown of atmospheric CO2, triggering continental scale glaciation, intense global cooling and eustatic sea-level lows recognised in the geological record. Lower atmospheric pCO(2) and temperatures during the Hirnantian and Silurian glaciations suppressed CO2 removal by silicate weathering, driving Os-187/Os-188 and delta Li-7 variability, supporting the existence of climate-regulating feedbacks. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    Evolving morphology of crustal accumulations in Earth's lowermost mantle

    Li, MingmingMcNamara, Allen K.
    11页
    查看更多>>摘要：Subducted oceanic crust is one of the major sources of compositional heterogeneity in Earth's mantle. It has been proposed that subducted oceanic crust may accumulate at the bottom of the Earth's mantle and cause the seismically-observed, Large Low Shear Velocity Provinces (LLSVPs). Testing this hypothesis requires a better understanding of the morphology of the crustal accumulations in the lowermost mantle. Here, through geodynamic modeling experiments, we find that thick subducted oceanic crust could accumulate into thermochemical piles with a height up to similar to 1000 km above the core-mantle boundary (CMB), and the crustal accumulations typically have chemically fuzzy top boundaries and stratified interiors. As the oceanic crust thins with mantle cooling, it becomes more difficult to accumulate on the CMB, and the previous crustal accumulations gradually become smaller in size and gain sharp top boundaries and relatively homogeneous interiors. Our results suggest that if the present-day LLSVPs are mainly caused by the accumulations of subducted oceanic crust, they may be produced in the early hotter Earth when subducted oceanic crust may be thicker, and they may start with chemically fuzzy top boundaries and stratified interiors and gradually develop into chemically sharper top boundaries and more homogenized interiors. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    Multiphase rheology as a cause for stick-slip like melt extraction

    Vigneresse, Jean-LouisCenki, Benedicte
    9页
    查看更多>>摘要：When partial melting is parametrized in numerical thermo-mechanical models in a straightforward manner, it directly relates to variations in temperature. However, melt extraction is, to our knowledge rarely taken into account in these models, even if melt extraction and escape have a key influence on the mechanical behaviour of the entire system. Here, we propose a first step towards a numerical simulation for melt extraction in partially molten portions of the continental crust. We use a Lagrangian description, considering a discrete mineral distribution at sample scale. A cellular automaton mimics the mobile melt phase moving from cell to cell. An infinite source and sink for the melt and lateral periodic boundaries avoid melt accumulation. Time is represented by a proxy through successive iterations. The major controlling factors are the initial bulk melt percentage and a threshold controlling melt escape. We test pure and simple shear conditions to simulate strain partitioning between the melt and its matrix. The parameters, provided they do not vary, determine a regular or steady state amount of melt extraction after a few iterations. In contrast, for varying parameters during a single run, the quantity of melt extracted at each step presents alternate variations in amplitude (sigma(mu)) around an average value. Variations are neither periodic in the sense of a sinusoidal wave, nor of equal amplitude. Melt extraction behaves similar to a stick-slip motion in dry friction. Since we cannot have access to the prediction of melt amount, w test the origin of the sporadic motion. We examine variations of the input parameters (initial melt phi, permeability zeta, applied stress sigma or tau). The transient effects mark by spikes (Delta mu) with two contrasted situations. Return to a steady state is exponential, similar to the discharge of a capacitor. Nevertheless, a first mode occurs when the changes are made on separate runs, reinitializing the parameters between them. These experiments show a linear dependence of the variation in amplitude of sigma(mu) with the parameters. Alternatively, when the changes are successive during the same run, then sigma(mu) decreases accordingly. The exponential variation is interpreted as a memory effect or hysteretic behaviour of melt extraction. Similarly, the amplitude of the stiction spikes (Delta(mu)) does not show the same behaviour when permeability increases or decreases. By analogy with friction, the change from sticking to slipping is interpreted as a manifestation of pinning and depinning, leading to an avalanche-like phenomena. Mechanically they correspond to a change in friction velocity. Physically, it points to the change in the free energy of the system, with a first derivative akin to velocity and a second derivative akin to acceleration. During melting, the mechanical barriers correspond to the energy variations due to chemical reactions between melt and crystals. Depinning corresponds to enhanced melting whereas pinning links with back reactions leading to garnet growth. The sporadic melt extraction links dry friction to poroelastic flow, or more generally to a consequence of two-phases rheology. Hence the sporadic motion of the weak phase originates from interactions with its matrix. Hence the flow (velocity) suffers modulations due to interactions with chemical potential (back-reaction, melting), entropy (ordering through dislocation creep), and strain (aperture of porous flow). Such interactions should be considered when dealing with quantitative mush (melt+ matrix) modeling (rejuvenation, extrusion). (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    The roles of microlites and phenocrysts during degassing of silicic magma

    Caceres, FranciscoScheu, BettinaColombier, MathieuHess, Kai-Uwe...
    11页
    查看更多>>摘要：Silicic magmas span a wide range of eruptive styles between explosive and effusive, and transitions between these styles are commonplace. Yet the triggers of switches in eruptive style remain poorly understood. Eruptions are mostly driven by degassing of magmatic water and their eruption style effusive or explosive - is likely governed by the efficiency of outgassing as well as magma ascent rate. Microlites and phenocrysts are often purported to promote heterogeneous bubble nucleation and outgassing, both key variables in the degassing dynamics that become crucial in controlling the eruptive style. Here, in order to shed light on the role of nature, size and abundance of crystals on degassing of silicic magma, we experimentally investigate (heating-induced) vesiculation in a multiphase (microliteand phenocryst-bearing), low-water content, and bubble-free, natural rhyolite. The experiments were conducted at magmatic temperature (similar to 900-1100 degrees C) and atmospheric pressure in an optical dilatometer. Our results indicate that microlites exert a large influence on bubble nucleation while the effect of phenocrysts is subordinate. Amongst the microlite phases, bubbles nucleate more easily on FeTi oxides than other mineral phases. Bubble coalescence and connectivity are, in contrast, enhanced by the phenocrysts, more than microlites, in low-crystallinity magma. Comparing the bubble textures of the post-experimental samples with those produced in a phenocryst-free and microlite-poor silicic magma, we observe that the phenocryst- and microlite-bearing magma exhibits significantly more bubble coalescence and connectivity, as well as higher bubble number densities. These findings help to constrain the roles that pre- and syn-eruptive crystalline phases may play in degassing processes during ascent of silicic magma. (C) 2021 The Author(s). Published by Elsevier B.V.
      原文链接:
    • NSTL
    • Elsevier

    Receiver function mapping of the mantle transition zone beneath the Western Alps: New constraints on slab subduction and mantle upwelling

    Liu, DongyangZhao, LiangPaul, AnneYuan, Huaiyu...
    11页
    查看更多>>摘要：To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    A short-lived oxidation event during the early Ediacaran and delayed oxygenation of the Proterozoic ocean

    Chen, BoHu, ChunlinMills, Benjamin J. W.He, Tianchen...
    9页
    查看更多>>摘要：The Ediacaran Period was characterised by major carbon isotope perturbations. The most extreme of these, the similar to 570 Ma Shuram/DOUNCE (Doushantuo Negative Carbon isotope Excursion) anomaly, coincided with early radiations of benthic macrofauna linked to a temporary expansion in the extent of oxygenated seawater. Here we document an earlier negative excursion (the similar to 610 Ma WANCE (Weng'An Negative Carbon isotope Excursion)) anomaly in the Yangtze Gorges area, South China, that reached equally extreme carbon isotope values and was associated with a similar degree of environmental perturbation. Specifically, new uranium isotope data evidence a significant, but transient, shift towards more oxygenated conditions in tandem with decreasing carbon isotope values, while strontium and sulfur isotope data support an increase in continental weathering through the excursion. We utilize a biogeochemical modelling approach to demonstrate that the influx of such a weathering pulse into an organically-laden, largely anoxic ocean, fully reproduces each of these distinct isotopic trends. Our study directly supports the hypothesis that a large dissolved marine organic pool effectively buffered against widespread oxygenation of the marine environment through the Proterozoic Eon, and in doing so, substantially delayed the radiation of complex aerobic life on Earth. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier

    Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

    Marshall, Edward W.Ranta, EemuHalldorsson, Saemundur AriCaracciolo, Alberto...
    13页
    查看更多>>摘要：Enriched mantle heterogeneities are widely considered to be generated through subduction, but the connections between specific subducted materials and the chemical signatures of mantle heterogeneities are not clearly defined. Boron is strongly isotopically fractionated at the surface and traces slab devolatilization, making it a potent tracer of previously subducted and recycled materials. Here, we present high-precision SIMS boron concentrations and isotope ratios on a comprehensive suite of quenched basaltic glasses from all neovolcanic zones in Iceland, two rhyolite glasses, and a set of primitive melt inclusions from central Iceland. Boron isotope ratios (delta B-11) in Icelandic basalts and melt inclusions range from -11.6 parts per thousand to -1.0 parts per thousand, averaging -4.9 parts per thousand, which is higher than mid-ocean ridge basalt (MORB; delta B-11 =-7.1 parts per thousand). Because the delta B-11 value of the Icelandic crust is low, the high delta B-11 compositions of the Icelandic lavas are not easily explained through crustal assimilation processes. Icelandic basalt glass and melt inclusion B/Ce and delta B-11 values correlate with trace element ratio indicators of the degree of mantle partial melting and mantle heterogeneity (e.g. Nb/Zr, La/Yb, Sm/Yb), which indicate that the boron systematics of basalts are controlled by mantle heterogeneity. Additionally, basalts with low B/Ce have high Pb-206/Pb-204, further indicating mantle source control. These correlations can be used to deduce the boron systematics of the individual Icelandic mantle components. The enriched endmember within the Iceland mantle source has a high delta B-11 value and low B/Ce, consistent with the composition of "rehydrated" recycled oceanic crust. The depleted endmember comprises multiple distinct components with variable B/Ce, likely consisting of depleted MORB mantle and/or high He-3/He-4 mantle and two more minor depleted components that are consistent with recycled metasomatized mantle wedge and recycled slab gabbro. The compositions of these components place constraints on the devolatilization history of recycled oceanic crust. The high delta B-11 value and low B/Ce composition of the enriched component within the Iceland mantle source is inconsistent with a simple devolatilization process and suggests that the recycled oceanic crust component may have been isotopically overprinted by B-rich fluids derived from the underlying hydrated slab lithospheric mantle (i.e. "rehydration"). Further, the B/Ce and delta B-11 systematics of other OIBs can be used to constrain the devolatilization histories of recycled components on a global scale. Globally, most OIB B/Ce compositions suggest that recycled components have lost >99% of their boron, and their delta B-11 values suggest that rehydration may be a sporadic process, and not ubiquitous. (C) 2021 Elsevier B.V. All rights reserved.
      原文链接:
    • NSTL
    • Elsevier