Accretionary lapilli from the Sudbury impact event

Meteorite impact‐generated accretionary lapilli are not well studied. The recently discovered distal ejecta from the 1850 Ma Sudbury impact event contain abundant accretionary lapilli generated during the impact and deposited at great distances from the crater. We petrographically and geochemically examined lapilli from five sites (McClure, Connors Creek, Hwy 588, Pine River, and Grand Trunk Pacific, approximately 480–750 km from the center of the Sudbury structure). The compositions of quartz, K‐feldspar, calcite, biotite, and chlorite minerals are similar to each other in all of the samples, although the relative proportions of the minerals vary from site to site. The lapilli occur in a matrix of coarse‐grained quartz, carbonate, and feldspar grains. Zonation within lapilli appears to be due to grain size distribution rather than compositional variation. The inner zones are coarser grained than outer zones. The relative abundances of calcite, phyllosilicates, and feldspars are similar in each zone within individual lapilli. A meteoritic component is indicated by up to 1.8 ppb Ir in one lapillus from the Pine River site, and Ni and Cr ratios are on a chondritic trend line for many of the lapilli. Mechanisms previously proposed for accretionary lapilli formation seem inadequate to explain deposition of distal accretionary lapilli resulting from impact events. A new mechanism for upper atmospheric accretion is proposed, whereby ash ejected from impact events concentrates at altitudes of neutral buoyancy, where it then accretes and is deposited later than ballistically emplaced particles. Likely, multiple processes are taking place in the chaotic postimpact environment.     

Tidal disruption of globular clusters in dwarf galaxies with triaxial dark matter haloes

We use N -body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static Navarro, Frenk & White halo with a triaxial shape. We apply our models to five GCs spanning three orders of magnitude in stellar density and two in mass, chosen to represent the properties exhibited by the five GCs of the Fornax dSph. We show that only the object representing Fornax's least dense GC (F1) can be fully disrupted by Fornax's internal tidal field – the four denser clusters survive even if their orbits decay to the centre of Fornax. For a large set of orbits and projection angles, we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of an F1-like GC. Our simulations show that such debris appears as shells, isolated clumps and elongated overdensities at low surface brightness (≥26 mag arcsec⁻²), reminiscent of substructure observed in several Milky Way dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. The kinematics of the debris depends strongly on the progenitor's orbit. Debris associated with box and resonant orbits does not display stream motions and may appear 'colder'/'hotter' than the dSph's field population if the viewing angle is perpendicular/parallel to the progenitor's orbital plane. In contrast, debris associated with loop orbits shows a rotational velocity that may be detectable out to a few kpc from the galaxy centre. Chemical tagging that can distinguish GC debris from field stars may reveal whether the merger of GCs contributed to the formation of multiple stellar components observed in dSphs.     

Grooves on small saturnian satellites and other objects: Characteristics and significance

High-resolution images from the Cassini Imaging Science Subsystem (ISS) show parallel sets of grooves on Epimetheus and Pandora. Grooves have previously been observed on other satellites and asteroids, including Phobos, Gaspra, Ida, Eros, and minor occurrences on Phoebe. Sets of parallel grooves are so far observed only on satellites known or likely to be subject to significant tidal stresses, such as forced librations. Grooves on asteroids and on satellites not subject to significant forced librations occur in more globally disorganized patterns that may reflect impacts, varying internal structures, or even thermal stresses. The patterns and individual morphologies of grooves on the tidally-affected satellites suggest fracturing in weak materials due to tidal stresses and forced librations.     

Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?

Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ⁵⁶Fe values up to +0.35‰ relative to the solar system mean. On average, the δ⁵⁶Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ⁵⁶Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ⁵⁶Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ¹⁸O (0.9–1.5‰) than their host chondrites, and the same is true for Δ¹⁷O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an ¹⁶O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.     

Petrology and trace element geochemistry of Tissint,the newest shergottite fall

The fall and recovery of the Tissint meteorite in 2011 created a rare opportunity to examine a Martian sample with a known, short residence time on Earth. Tissint is an olivine‐phyric shergottite that accumulated olivine antecrysts within a single magmatic system. Coarse olivine grains with nearly homogeneous cores of Mg# >80 suggest slow re‐equilibration. Many macroscopic features of this sample resemble those of LAR 06319, including the olivine crystal size distribution and the presence of evolved oxide and olivine compositions. Unlike LAR 06319, however, no magmatic hydrous phases were found in the analyzed samples of Tissint. Minor and trace element compositions indicate that the meteorite is the product of closed‐system crystallization from a parent melt derived from a depleted source, with no obvious addition of a LREE‐rich (crustal?) component prior to or during crystallization. The whole‐rock REE pattern is similar to that of intermediate olivine‐phyric shergottite EETA 79001 lithology A, and could also be approximated by a more olivine‐rich version of depleted basaltic shergottite QUE 94201. Magmatic oxygen fugacities are at the low end of the shergottite range, with log fO₂ of QFM‐3.5 to ‐4.0 estimated based on early‐crystallized minerals and QFM‐2.4 estimated based on the Eu in pyroxene oxybarometer. These values are similarly comparable to other depleted shergottites, including SaU 005 and QUE 94201. Tissint occupies a previously unsampled niche in shergottite chemistry: containing olivines with Mg# >80, resembling the enriched olivine‐phyric shergottite LAR 06319 in its crystallization path, and comparable to intermediate olivine‐phyric shergottite EETA 79001A, depleted olivine‐phyric shergottite DaG 476, and depleted basaltic shergottite QUE 94201 in its trace element abundances and oxygen fugacity. The apparent absence of evidence for terrestrial alteration in Tissint (particularly in trace element abundances in the whole‐rock and individual minerals) confirms that exposure to the arid desert environment results in only minimal weathering of samples, provided the exposure times are brief.     

Multi-zone fusion crust formation and classification of the 2004 Auckland meteorite (L6, S5, and W0)

On June 12, 2004, a meteorite passed through Earth's atmosphere and landed under the television in the living room of a house in Auckland, New Zealand. Textural characteristics, the chemistry of olivine (Fa_23–24) and orthopyroxene (Fs_20.7), and the bulk rock triple oxygen isotopes (δ¹⁷O + 3.1; δ¹⁸O + 4.2‰) from the interior of the completely unweathered (W0) 1.3 kg meteorite, hereafter referred to as Auckland, suggest it to be a strongly metamorphosed fragment from the interior of a low iron ordinary chondrite (L6) parent asteroid. The occurrence of maskelynite but shock fracturing of olivine and pyroxene indicates Auckland experienced extreme shock metamorphism (S5), likely during Ordovician fragmentation of the asteroid parent. The fusion crust consists of three zones: (1) an innermost zone containing narrow Fe-Ni-S-bearing veins that migrated along pre-existing shock fractures in olivine and pyroxene; (2) a middle zone in which the meteorite partially melted to form a silicate glass and immiscible blebs of metal and troilite, and is accompanied by unmelted silicate minerals; and (3) an approximately 0.1 mm wide vesicular-rich outermost layer that largely melted, volatilizing sulfides, before quenching to form glass and olivine. Oxygen isotope values of the bulk rock and/or maskelynite of melted rim and modified substrate are 2–3‰ greater than the meteorite interior and indicate that up to 19% of terrestrial atmospheric O₂ was incorporated into the fusion crust during the formation. The fusion crust migrated inwards as ablation occurred, enabling melting, migration, and re-precipitation ± loss of sulfide and metal components, with the prominent glassy rim therefore forming from an already chemically modified zone.     

Coastal marsh response to historical and future sea-level acceleration

We consider the response of marshland to accelerations in the rate of sea-level rise by utilizing two previously described numerical models of marsh elevation. In a model designed for the Scheldt Estuary (Belgium–SW Netherlands), a feedback between inundation depth and suspended sediment concentrations allows marshes to quickly adjust their elevation to a change in sea-level rise rate. In a model designed for the North Inlet Estuary (South Carolina), a feedback between inundation and vegetation growth allows similar adjustment. Although the models differ in their approach, we find that they predict surprisingly similar responses to sea-level change. Marsh elevations adjust to a step change in the rate of sea-level rise in about 100 years. In the case of a continuous acceleration in the rate of sea-level rise, modeled accretion rates lag behind sea-level rise rates by about 20 years, and never obtain equilibrium. Regardless of the style of acceleration, the models predict approximately 6–14 cm of marsh submergence in response to historical sea-level acceleration, and 3–4 cm of marsh submergence in response to a projected scenario of sea-level rise over the next century. While marshes already low in the tidal frame would be susceptible to these depth changes, our modeling results suggest that factors other than historical sea-level acceleration are more important for observations of degradation in most marshes today.     

Calibrating a glaciological model of the Greenland ice sheet from the Last Glacial Maximum to present-day using field observations of relative sea level and ice extent

We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.     

Climatic and environmental controls on speleothem oxygen-isotope values

Variations in speleothem oxygen-isotope values (δ¹⁸O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem δ¹⁸O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem δ¹⁸O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on δ¹⁸O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of δ¹⁸O in the atmosphere include temperature and relative humidity through their role in the multiple isotope “effects”. Variability and modifications of water δ¹⁸O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO₃, resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO₃, and have implications for the ‘Hendy’ test. To assist interpretation of speleothem δ¹⁸O time series, quantitative and semi-quantitative δ¹⁸O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of δ¹⁸O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics.