Earth and Planetary Science Letters最新文献

All large planets in our Solar System have rings, and it has been suggested that Mars may have had a ring in the past. This raises the question of whether Earth also had a ring in the past. Here, we examine the paleolatitudes of 21 asteroid impact craters from an anomalous ∼40 m.y. period of enhanced meteor impact cratering known as the Ordovician impact spike, and find that all craters fall in an equatorial band at ≤30°, despite ∼70 % of exposed, potentially crater-preserving crust lying outside this band. The beginning of this period is marked by a large increase in L chondrite material accumulated in sedimentary rocks at 465.76 ± 0.30 Ma, which, together with the impact spike, has long been suggested to result from break-up of the L chondrite parent body in the asteroid belt. Our binomial probability calculation indicates that it is highly unlikely that the observed crater distribution was produced by bolides on orbits directly from the asteroid belt (P = 4 × 10^–8). We therefore propose that instead, a large fragment of the L chondrite parent body broke up due to tidal forces during a near-miss encounter with the Earth at ∼466 Ma. Given the longevity of the impact spike and sediment-hosted L chondrite debris accumulation, we suggest that a debris ring formed after this break up event, from which material deorbited to produce the observed crater distribution. We further speculate that shading of Earth by this ring may have triggered cooling into the Hirnantian global icehouse period.

The Cenomanian–Turonian boundary oceanic anoxic event (known as OAE 2, occurring 94.5–93.9 million years ago) provides an opportunity to clarify climatic forcing on marine environmental perturbations. OAE 2 has been extensively studied regarding its triggering mechanism and rates of marine deoxygenation in the proto-North Atlantic, Western Interior Seaway, Pacific, and European pelagic shelf. However, the detailed timing of the onset of ocean deoxygenation and the triggering mechanism behind the organic carbon burial leading to OAE 2 in the Neotethys Ocean remains less well-constrained. Here, we fill this gap by presenting high-resolution barium isotope (δ¹³⁸Ba) data from the highly expanded Tibet OAE 2 section spanning the Cenomanian–Turonian boundary. We observed a large negative δ¹³⁸Ba excursion that correlates with the positive δ¹³C shift. The onset of the negative δ¹³⁸Ba excursion precedes that of δ¹³C by an estimated 400 kyr, indicating that ocean deoxygenation began 400 kyr before OAE 2 in the Neotethys Ocean. The δ¹³⁸Ba values of the studied carbonates are significantly lower than those of surface seawater observed in the modern Atlantic and Pacific oceans, suggesting that export productivity levels in the pre-OAE 2 Neotethys Ocean were substantially lower than those in modern oceans. We thus provide new evidence that the burial of organic carbon during OAE 2 led to the observed positive δ¹³C excursion driven by extensive shallow-water anoxia, even when considering contributions from changes in marine primary productivity.

Ocean Acidification (OA) arises from the increase in atmospheric carbon dioxide concentration following the industrial revolution. The ecological and socio-economic consequences of OA were first identified around 10–15 years ago but remain poorly understood. This is particularly true in coastal regions where local processes can have dramatic consequences on pH trends through time, obscuring and compounding the long-term effects from rising atmospheric CO₂. Here we explore the possibility of generating long records of coastal ocean pH using the skeletons of widely distributed coralline algae (CA). The skeletons of these slow growing (<1 mm/year) taxa often contain micron-scale heterogeneities, making sampling for high-resolution climate reconstructions using bulk sampling techniques difficult. Here we use laser ablation coupled to inductively coupled plasma mass spectrometers to generate high-resolution 2D images of the element/calcium ratios and boron isotope composition (δ¹¹B) of a sample of Boreolithothamniom cf. soriferum from Loch Sween in Scotland, UK where we have been monitoring temperature since 2004 and pH during 2014. By carefully correlating the geochemical images with a scanning electron microscopy image we can segment them to remove the marginal portions of the skeleton, isolating the central growth axis to generate an age model and growth rate. The δ¹¹B-pH is significantly elevated above the seawater pH in Loch Sween (8.4 to 8.9 vs. 7.9 to 8.1) consistent with other CA that show internal pH elevation. On a seasonal scale, internal pH is negatively correlated with temperature and also exhibits a long-term decline. By removing this temperature effect, internal pH can be correlated to seawater pH during the 2014 monitoring period allowing us to reconstruct a seawater acidification trend from 2004 to 2018 of -0.018 pH units per year, 10x higher than open ocean trends but consistent with contemporaneous monitoring efforts of UK coastal waters. Reconstructed aqueous CO₂ suggests that prior to ∼2008 Loch Sween was a sink of CO₂ but after this date, particularly during the early summer, it was a substantial CO₂ source. Comparison of reconstructed aqueous CO₂ with a record of calcification rate of our sample of Boreolithothamniom cf. soriferum suggests this acidification and associated rise in local seawater pCO₂ may have freed this sample from carbon limitation leading to a recent increase in calcification.

Kinematic analysis of ductile shear zones is an important method to interpret the dynamic evolution of many tectonic and magmatic processes on Earth, such as orogeny, rifting, and plutonism. Despite decades of study, kinematic vorticity analysis is an underutilized tool to interpret the dynamic drivers of shear zones. Determination of the kinematic vorticity number (W_k) quantifies the relative contributions of pure- versus simple-shear strain in shear zones. Here we systematically investigated W_k from mylonitic shear zones associated with metamorphic core complexes (MCCs) developed across the central and southern North American Cordillera using electron backscatter diffraction (EBSD) data that allows consistent and reproducible results from a large number of recrystallized grains. We investigated samples from four distinct MCC systems and the structural aureole of a Cordilleran pluton to investigate their kinematics. We find that most MCC samples display pure-to-general-shear strain (average 70 % pure shear), consistent with significant bulk shear-zone shortening (>80 % shortening) observed in many of the MCC systems. Such strain patterns are remarkably similar to deformation observed around plutons that were forcefully emplaced as diapirs. To further validate and investigate these results, we constructed the W_k field of these geologic processes using numerical simulations to highlight that pure shear kinematics are more common with diapirism and coupled wallrock deformation compared with discrete detachment-involved normal fault systems. These observations support that buoyant doming can be a viable end-member process to form the investigated MCCs. Our results also suggest that pure shear deformation may be a diagnostic strain characteristic for diapir-like crustal processes, and therefore W_k analyses could be used to test similar processes like dome-and-keel models for Early Earth.

Effusive to violently explosive eruptions of crystal-rich magmas are frequently found in volcanic records. The competing effects of rheological stiffening of magma in the presence of crystals and magma overpressure build-up in the presence of bubbles typically control the volcanic explosivity. The bubble growth exerts extensional stress on its wall, i.e., the melt+crystal matrix surrounding it. However, the rheology of crystal-rich magma under such extension along with the effect of crystals on bubble growth, are poorly understood. From analog experiments, this study finds that crystalline magma exhibits yield stress and power-law rheology with broadly comparable values under extensional and shear deformation. The pressure loss due to the presence of yield stress can significantly affect bubble growth in magma. Using bubble growth model in crystallizing magma, this study shows that the yield stress in melt+crystal matrix surrounding bubbles can exceed gas overpressure, preventing bubble growth. The model parameter search exhibits three regimes of bubble growth in crystallizing magmas for a wide range of magma decompression and crystallization rates during effusive to explosive volcanic eruptions. In the yield stress-limited regime, a complete halt in bubble growth can occur at a relatively small viscosity of crystal-rich basaltic magma (∼10⁶ Pa s), and depending on the crystalline system, at a crystal volume as low as ∼30%. On the other hand, at relatively higher magma decompression rates, significant magma expansion associated with relatively rapid bubble growth, even at a relatively high normalized crystal content of >90%, could cause magma fragmentation and eruption explosivity. This study demonstrates that small changes in eruption conditions, such as magma decompression rates and crystallization rates, can cause significant changes in bubble growth dynamics with implications for transitions in volcanic eruption styles.

The Apollo basin, located in the northeastern part of the South Pole-Aitken (SPA) basin, represents one of the Moon's most significant geological features, offering profound insights into the lunar interior structure, the effects of the SPA impact, and the history of lunar crust evolution. This study presents an in-depth geological analysis of the Apollo basin region, revealing the distribution of rock types and compiling a comprehensive geologic map that correlates with the lithologic and geochemical properties of the area. Utilizing the characteristics and compositional provenance of the geologic units, we have constructed schematic cross-sections that elucidate the interior structure and stratigraphic evolution of the Apollo basin region. Despite excavations of the SPA and Apollo impacts, the anorthositic crust of this area was not entirely removed and has been uplifted to shallow depths, making it more susceptible to exposure by subsequent impacts. Additionally, upper mantle material, characterized by ultramafic, low-Ca pyroxene, was excavated by the SPA impact and is present in the impact melt/breccias of the Apollo basin. After the formation of the Apollo basin, multiple mare units were emplaced over a period potentially spanning ∼1.5 billion years, with the oldest of these maria being superposed by substantial postdating basin ejecta. The results of this study strengthen our understanding of the geology and evolution of the Apollo and SPA basins and offer valuable insights for interpreting the exploration and sample analysis results of the Chang'e-6 mission.

Understanding the factors that govern past fluid circulation in tectonically active and/or hydrocarbon-rich basins is crucial for elucidating present-day fluid-flow scenarios. We investigate the circulation of paleo-fluids in the extensional-transtensional Val d'Agri Basin (southern Italy), home to a giant oil field and significantly affected by both natural and human-induced seismicity. Our aim is to understand how faulting and the variable thickness of the clay-rich tectonic mélange, which constitutes the seal of the hydrocarbon reservoir, influenced past fluid flow under different tectonic regimes. To achieve this, we combined multiscale structural observations with isotope (C, O, clumped, and Sr) and Rare Earth and Yttrium (REY) analyses of fault-related calcite mineralizations. Using analytical methodologies that allow the analysis of sub-milligram samples for carbonate clumped isotopes, we provided a detailed characterization of the variability in precipitation temperatures and composition of parental fluids in both space and time. Our results reveal five main types of parental fluids, ranging from meteoric to intraformational and deep crustal, which were differently involved in the tectonic evolution of the Val d'Agri Basin. During orogenic shortening, vertical fluid circulation was mostly limited and compartmentalized, whereas post-orogenic extensional faulting promoted the ascent of deep fluids. Our findings indicate that the sealing properties of the mélange were likely enhanced locally by increased thickness but were also compromised by fault activity and associated seismic events. Fluid circulation in the study area has been influenced by the prevailing tectonic regime (compressive vs. extensional), stratigraphic-structural architecture, properties of impermeable horizons, and seismic events. The model proposed for the Val d'Agri Basin elucidates past processes that are useful for understanding current fluid circulation in the basin itself and can be applied to other basins where fluid circulation is partly manipulated by human activities.

The relationship between the uplift of the Tibetan Plateau and the aridification of eastern Asia during the middle to late Eocene is still controversial, primarily due to the lack of high-resolution chronological frameworks for global isochronous comparisons. Here, we present a new high-resolution astrochronology for the Eocene Suonahu Formation in the Qiangtang Basin on the Tibet Plateau, which has been precisely dated using a secondary ion mass spectrometry U‒Pb age of 46.57 ± 0.30 Ma. This astrochronology establishes a detailed and continuous timescale of 7.61 ± 0.5 Myr duration, spanning the period from 42.08 ± 0.5 Ma to 50.36 ± 0.5 Ma. Sedimentary noise modeling revealed cycles of 1.2 Myr that are anti-phase to the 1.2 Myr obliquity cycle of the obliquity/total power (O/T) curve. This suggests that variations in lake level are modulated by these 1.2 Myr obliquity cycles. The change in lake level is inversely related to global sea level trends, reinforcing the hypothesis that groundwater dynamics are a significant driver of lake level fluctuations. The data further highlight the importance of orbital forcing in controlling hydrological cycling in eastern Asia. The new astrochronology further confirms the onset of aridification of in eastern Asia around 45.5 Ma, which is evidenced in the Suonahu Formation by the cyclic deposition of gypsum and an increase in chlorite content. Cyclostratigraphic analysis indicated that gypsum-siltstone/mudstone alternations were paced by 100 kyr short-eccentricity cycles. Thus, the late Eocene aridification of eastern Asia was driven both by the uplift of the Tibetan Plateau, which restricted northwards moist transport, and by orbital forcing and global cooling, which affected evaporation and precipitation patterns and intensities. This study highlights the complex interplay between tectonic processes and orbital forcing as key drivers of hydrological cycling and climate during the Eocene greenhouse-to-icehouse transition.

Chondrules, the principal high-temperature component of chondritic meteorites, may represent the fundamental building blocks of the terrestrial planets. The mass-independent isotope compositions of chondrules can be used to investigate their origins, as well as their subsequent transport and storage in the protoplanetary disk, which are weakly constrained. Debate surrounds whether mass-independent variability among chondrules arises from isotopically distinct precursor dust or small-scale addition of anomalous phases such as calcium-aluminium-rich inclusions (CAIs) and ameboid olivine aggregates (AOAs). Previous investigations employed isotope tracers that are concentrated in refractory inclusions (such as Ti), rendering them vulnerable to potential "nugget effects" arising from the presence of these anomalous phases and hindering their effectiveness as tracers of precursor dust compositions. An isotope tracer evenly distributed among silicates and thereby less sensitive to local additions from refractory inclusions, is essential to distinguish precursor dust compositions from minor additions of these phases. To address this challenge, we measured the mass-independent Si isotopic composition of chondrules from the carbonaceous Vigarano-type (CV) chondrites Allende and Leoville. Distinct isotopic signatures are observed in chondrules with different petrographic textures. Non-porphyritic chondrules exhibit ³⁰Si deficits akin to differentiated inner disk planetesimals, suggesting early formation within the inner disk (<1 Myr) before transportation to the CV accretion region in the outer disk. Conversely, porphyritic chondrules display a wide range of silicon isotope compositions, including both non-carbonaceous-like values and those exceeding bulk CV chondrites. Notably, non-porphyritic chondrules with substantial porphyritic igneous rims show compositional variations within individual chondrules, whereby cores retain ³⁰Si-depleted signatures while rims record more positive ³⁰Si compositions. Our findings show that contributions from isotopically anomalous refractory condensates cannot be the primary cause of mass-independent variability among chondrules in CV chondrites. Instead, we find that the observed compositional diversity in porphyritic chondrules results from the recycling of inner disk chondrules following the accretion of CI-like dust from the outer Solar System.

The thermal conductivity of iron and its alloys are critically important to understand conductive heat flow and dynamo action within planetary cores, however the effect of sulfur alloying is poorly understood. We have measured and computed the thermal conductivity of FeS at high pressures and temperatures using experimental techniques and first-principles calculations. Experimental conditions range from 19-116 GPa and up to 3000 K. Computations ranged from 20-150 GPa and up to 4000 K. Over this range of conditions, theory shows that FeS is in a low to intermediate spin state with finite moments at least up to 40 GPa. We obtain thermal conductivity κ from 15 W m⁻¹ K⁻¹ at 1000 K to 69 W m⁻¹ K⁻¹ at 4000 K from first-principles calculations, and values of 14(5)-20(10) W/m/K from experimental measurements at temperatures above 1500 K and high pressures. In both cases the effect of structure and pressure is small. We find that FeS is metallic, but a poor metal at the conditions investigated. As a result, sulfur-rich core compositions are compatible with available observational constraints on the cessation time of the Martian dynamo.