The Winchcombe fireball—That lucky survivor

On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South-West England near the town of Winchcombe. We reconstruct the fireball's atmospheric trajectory, light curve, fragmentation behavior, and pre-atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s⁻¹). A near-catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near-Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.     

Sedimentary filling characteristics of the South China Sea oceanic basin,with links to tectonic activity during and after seafloor spreading

ABSTRACT

Based on approximately 11,000 km of seismic reflection data collected across the South China Sea oceanic basin, we describe the sedimentary filling characteristics of the basin since its Oligocene opening, as well as connections between this history and contemporaneous regional tectonic events. The seismic lines are spaced ~50 km apart, and the data are tied to International Ocean Discovery Program (IODP) Expedition 349 drilling data. Basin filling occurred in three phases, with basin-wide mean sedimentation rates increasing through time. During the Oligocene to middle Miocene, sediments accumulated primarily in the northern East and Northwest Sub-basins, with a mean basin-wide sedimentation rate of 8 m/m.y. The presence of these deposits over deep basement floor indicates that seafloor spreading initiated in these northern regions. During the late Miocene, deposition occurred primarily in the Northwest Sub-basin and partly in the southern East Sub-basin, with a mean basin-wide sedimentation rate of 30 m/m.y. Basin filling during this time seems to have been linked to slip reversal of the Red River Fault and collision of the North Palawan Block with the Luzon Arc. During the Pliocene and Pleistocene, sediments accumulated rapidly in the northeastern and southern East Sub-basin and the Southwest Sub-basin. The mean basin-wide sedimentation rate was 70 m/m.y. Basin filling during this phase seems to have been associated with the Taiwan and North Palawan collisions, SCS subduction along the Manila Trench, and Tibetan Plateau uplift. Gravity flow deposits predominate throughout the basin fill.     

Dantu's mineralogical properties – A view into the composition of Ceres' crust

Impact crater Dantu not only exhibits a very complex geological history but also shows an exceptional heterogeneity of its surface composition. Because of its location within a low‐lying region named Vendimia Planitia, which has been proposed to represent an ancient impact basin, Dantu possibly offers a window into the composition of Ceres’s deeper crust, which apparently is enriched in ammonia. Local concentration of carbonates within Dantu or its ejecta blanket may be either exposed or their emplacement induced by the Dantu impact event. Because carbonates can be seen along Dantu's crater walls, exposed due to recent slumping, but also as fresh spots or clusters of spots scattered across the surface, the deposition/formation of carbonates took place over a long time period. The association of several bright spots enriched in carbonates with sets of fractures on Dantu's floor might be accidental. Nevertheless, its morphological and compositional similarity to the faculae in Ceres’s prominent impact crater Occator including its hydrated state does not exclude a cryo‐volcanic origin, i.e., upwelling of carbonate‐enriched brines influenced by H₂O ice in the subsurface. Indeed, an isolated H₂O ice spot can be identified near Dantu, which shows that ice still exists in Ceres’s subsurface at midlatitudes and that it can exist on the surface for a longer period of time.     

Time‐lapse electrical resistivity imaging of solute transport in a karst conduit

The use of electrical resistivity surveys to locate karst conduits has shown mixed success. However, time‐lapse electrical resistivity imaging combined with salt injection improves conduit detection and can yield valuable insight into solute transport behaviour. We present a proof of concept above a known karst conduit in the Kentucky Horse Park (Lexington, Kentucky). A salt tracer solution was injected into a karst window over a 45‐min interval, and repeat resistivity surveys were collected every 20 min along a 125‐m transect near a monitoring well approximately 750 m downgradient from the injection site. In situ fluid conductivity measurements in the well peaked at approximately 25% of the initial value about 3 h after salt injection. Time‐lapse electrical resistivity inversions show two broad zones at the approximate conduit depth where resistivity decreased and then recovered in general agreement with in situ measurements. Combined salt injection and electrical resistivity imaging are a promising tool for locating karst conduits. The method is also useful for gaining insight into conduit geometry and could be expanded to include multiple electrical resistivity transects. Copyright © 2015 John Wiley & Sons, Ltd.     

The inception and growth of leucosomes: microstructure at the start of melt segregation in migmatites

The beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites. Most arrays of leucosomes record a more advanced stage where flow dominates over segregation. However, the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta‐argillite from the metatexite zone (>800 °C) of the contact aureole around the Duluth Complex, Minnesota. The rock contains 2.4 modal% leucosome in a matrix consisting of 40.5% in situ neosome and 57.1% cordierite + plagioclase framework. The domainal microstructure in the matrix is a pre‐anatectic feature resulting from the bulk composition. Terminal chlorite reactions produced a large volume of cordierite which, with plagioclase, formed a framework that enclosed patches of biotite + quartz + plagioclase ± K‐feldspar. Upon melting, these fertile domains became patches of in situ neosome. Plagioclase in the neosome is less sodic than in the leucosome, hence segregation of melt occurred during crystallization, not melting. Segregation was delayed because the cordierite + plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started. The leucosomes are small (i.e. they are microleucosomes) and display a systematic progression in morphology as length and aspect ratio increase from ~1 to 19 mm and from ~2.5 to >30 respectively. Small equant micropores form first, and in places these coalesce into small (~1 mm, aspect ratio ~2.5), isolated, blunt‐ended, elliptical microleucosomes. In the next stage, micropores develop ahead of, and at ~45° to the left and right of the blunt tip of a microleucosome; one of these develops into an elliptical leucosome and an en echelon array of either a left‐ or right‐stepping elliptical microleucosome forms. Each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix. Next, microleucosomes of greater length (>4 mm) and aspect ratio (>5) form when the bridges of cordierite + plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag‐shaped microleucosome. Finally, still longer microleucosomes with greater aspect ratios (~30) are formed by the joining of zigzag arrays. Such a progression is characteristic of the way ductile fractures grow. The segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome, which drew melt to the growing fracture, thereby creating a microleucosome. The microleucosomes are filled arrays of ductile fractures. Melt was contiguous only between microleucosomes and adjacent patches of in situ neosome. The length‐scale of segregation was ~5 mm, the size of a typical patch of in situ neosome, and restricted by the surrounding impermeable cordierite + plagioclase framework. The melt in the microleucosome was the most fractionated and the last to crystallize. All microleucosomes contain entrained minerals as a consequence of their mechanism of growth. Rupture of the bridges resulted in the entrainment of pre‐anatectic phases. However, microleucosomes that cross patches of in situ neosome are also contaminated with peritectic phases that were transported with the melt.     

New optically stimulated luminescence ages provide evidence of MIS3 and MIS2 eolian activity on Black Mesa, northeastern Arizona, USA

Eolian deposition on the semiarid southern Colorado Plateau has been attributed to episodic aridity during the Quaternary Period. However, OSL ages from three topographically controlled (e.g. falling) dunes on Black Mesa in northeastern Arizona indicate that eolian sediments there were deposited in deep tributary valleys as early as 35–30 ka, with most sand deposited before 20 ka. In contrast, the oldest OSL ages for sand sheets fall within the Pleistocene-Holocene climatic transition (~ 12–8 ka). Thus most eolian sediment accumulated on Black Mesa under climatic conditions that were in general cooler, moister, and more variable than today, not more arid, pointing to a considerable increase in sediment supply.     

Logjams as a driver of transient storage in a mountain stream

We use four stream segments along a wood-rich, pool–riffle mountain stream in the Southern Rockies of Colorado, USA to examine how spatial variations in wood load and variations in discharge during and after the snowmelt peak flow influence the magnitude of surface and subsurface transient storage. Segments range in complexity from a single channel with no large wood to an anabranching channel with closely spaced, channel-spanning logjams. Discharges at which transient storage was assessed range from base flow to snowmelt peak flow. To explore these relations, we used 10 geomorphic variables representing channel morphology and bed substrate, four wood-related variables representing wood load and associated backwater storage, and two measures of skewness from instream and bulk electrical conductivity breakthrough curves during tracer tests. Instream curves reflect surface and subsurface transient storage, whereas bulk curves primarily represent subsurface transient storage. Higher values of skewness indicate greater retention, and we used the values here as a metric of increased transient storage. Although limited sample size restricts the power of our results, our findings suggest that stream segments with greater instream large wood loads have more and larger pools, greater storage of fine sediment and particulate organic matter, and higher values of skew from instream conductivity. The results also suggest that the presence of instream wood, rather than changes in channel morphology associated with wood, is the most important driver of transient storage. This implies that river management designed to foster transient storage should focus on retaining instream large wood. We did not find significant correlations between geomorphic or wood-related variables and the skew estimated from bulk conductivity, which may reflect the relatively thin alluvium present in the field area and the prevalence of surface transient storage in this system.     

A threatened world city: the benefits of protecting London from the sea

This paper describes the options appraisal undertaken in the UK within the major TE2100 project to investigate the future of protecting London from flooding from the sea. An economic analysis, within a Benefit-Cost framework complemented by Multi-Criteria Analysis, shows that improving the existing flood defences and, in 2070, constructing a new Thames Barrier downstream from the existing one are the “front runner” options for tackling the increase in flood risk that is anticipated in the future. Both sensitivity and scenario analysis have little effect on option choice. Uncertainties inevitably remain, however, when looking so far ahead, but it is clear that continuing to protect this area from the sea is highly cost-beneficial. Also the very high standard of protection now, and the robustness of the existing flood defence assets, mean that major new interventions will not be needed for some time (i.e. until c. 2070). We therefore have time to monitor the situation, carefully plan measures to maintain and enhance the existing defences, and to seek to restrain the growth of risk in the Estuary and in London through carefully designed and implemented resilience-building flood plain management measures. Rather than having to rush to new engineering works, because we have not anticipated what is needed but are forced to respond hastily to a “crisis” situation, the adaptive approach that is now possible is a key legacy of the TE2100 project.     

大陆破裂分离和海洋地壳的诞生：IODP所关注的科学问题

大陆破裂分离和新洋壳的诞生是板块构造最希望了解的基本科学问题,也是IODP下阶段工作的重要目标。2006年9月,来自6个国家的51位科学家在瑞士Pontresina开会,讨论目前关于大陆破裂与分离、沉积盆地形成的认识现状,     

Search for sulfates on the surface of Ceres

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile‐rich protoplanets like Ceres. Sulfates, present on water‐bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low‐pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H₂O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO₄·6H₂O by X‐ray powder diffraction. Long‐term (>40 h), continuous exposure of MgSO₄·nH₂O (n = 0, 6, 7) to low pressure (10^?3–10^?6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO₄·6H₂O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO₄·6H₂O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.