Water-rich regions in Earth's deeper mantle are suspected to play a key role in the global water budget and the mobility of heat-generating elements. We show that ice-VII occurs as inclusions in ...natural diamond and serves as an indicator for such water-rich regions. Ice-VII, the residue of aqueous fluid present during growth of diamond, crystallizes upon ascent of the host diamonds but remains at pressures as high as 24 gigapascals; it is now recognized as a mineral by the International Mineralogical Association. In particular, ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and around the 660-kilometer boundary.
We use concepts drawn from the adaptive governance literature to examine challenges and opportunities for fire management in Alaska, where rising average summer temperatures over the past several ...decades are associated with statewide increases in wildland fire activity. Alaska’s unique interagency fire management structure, rapidly changing climate, and natural resource dependent communities provide a valuable context for study. Our research sought to understand (1) current and future fire management challenges and responses to those challenges; (2) governance structures and processes that act as enablers of and barriers to changes in management approaches; and (3) the institutionalization of new practices. We explored these questions in a qualitative analysis of 41 interviews with fire managers. Participants perceived protection of communities, enhancement of subsistence hunting opportunities, and protection of remote points on the landscape as the most pressing current management challenges, with protection of ecosystem carbon sinks as a possible future challenge. Interviewees identified existing bridging organizations and boundary-spanning work as enabling factors in the governance system. At the same time, they indicated that federal agency budgeting processes, prescriptive laws that mandate the protection of certain values, and divisions across fire and land management personnel and planning processes can inhibit effective responses to management challenges. We found evidence of several types of institutional changes, some underway, and some perceived as necessary in the future. Our research suggests that in a thick institutional context, existing institutions that serve to bridge across actors likely can be repurposed to meet new challenges, while more prescriptive institutions may be less adaptive to changing conditions. This work provides an empirical investigation of adaptive governance in a rapidly changing system and contributes to theory building on institutionalization by shedding light on the nuances and complexities of institutional work.
Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars ...differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U-Pb (uranium-lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Achondrites provide an opportunity to examine the igneous processes of differentiated bodies in our solar system. The recent discovery of several silica‐rich achondrites suggests that andesitic ...crusts were more common among planetesimals than previously thought, though the processes behind their emplacement are not well understood. Here, electron backscatter diffraction (EBSD) is used to investigate the igneous emplacement conditions of Erg Chech 002 (EC 002), a recently discovered ungrouped achondrite representing andesitic magmatism ~2 Myr after the formation of calcium–aluminum‐rich inclusions (CAIs). EBSD analyses of crystallographic preferred orientations (CPOs) for augite and plagioclase feldspar phenocrysts indicate that EC 002 exhibits a weak foliation CPO. Augite misorientation inverse pole figures (mIPF) indicate preferential slip along the (100)001 system with a distinct shift toward the {0kl}u0w system in plastically deformed grains. Our findings support the hypothesis that EC 002 was likely emplaced in the lower regions of a magmatic intrusion. Augite slip signatures suggest that EC 002 crystallization and emplacement were restricted to high temperatures (>800°C) and experienced at least two strain regimes. The distinct shift from a dominant (100)001 slip system, which corresponds to high temperatures (800–1050°C), to a 0klu0w slip system indicates an increased strain rate due to shock deformation (1–5 GPa) attributed to ejection by hypervelocity impact.
The Mars 2020/Mars Sample Return (MSR) Sample Depot Science Community Workshop was held on September 28 and 30, 2022, to assess the Scientifically‐Return Worthy (SRW) value of the full collection of ...samples acquired by the rover Perseverance at Jezero Crater, and of a proposed subset of samples to be left as a First Depot at a location within Jezero Crater called Three Forks. The primary outcome of the workshop was that the community is in consensus on the following statement: The proposed set of ten sample tubes that includes seven rock samples, one regolith sample, one atmospheric sample, and one witness tube constitutes a SRW collection that: (1) represents the diversity of the explored region around the landing site, (2) covers partially or fully, in a balanced way, all of the International MSR Objectives and Samples Team scientific objectives that are applicable to Jezero Crater, and (3) the analyses of samples in this First Depot on Earth would be of fundamental importance, providing a substantial improvement in our understanding of Mars. At the conclusion of the meeting, there was overall community support for forming the First Depot as described at the workshop and placing it at the Three Forks site. The community also recognized that the diversity of the Rover Cache (the sample collection that remains on the rover after placing the First Depot) will significantly improve with the samples that are planned to be obtained in the future by the Perseverance rover and that the Rover Cache is the primary target for MSR to return to Earth.
Ordinary chondrites account for the majority of the described meteorites on Earth. To expand the toolbox of analytical techniques available to describe such specimens, this study evaluates the ...application of a previously described fayalite determination method by X‐ray diffraction (XRD) to equilibrated ordinary chondrites. A suite of ordinary chondrites, ranging from petrologic type 4 to 6, and types H, L, and LL were analyzed by both XRD and electron probe microanalysis. A comparison of the results shows good agreement between the two methods with an R2 of 0.95 and better agreement for homogenous ordinary chondrites above petrographic grade 4. The differences between the two methods can largely be attributed to analytical uncertainty, as well as differences between point and bulk sampling techniques. These differences were used to identify two polymict breccia samples, Peace River and Northwest Africa 10946. Of note is the effect of exposure of the ordinary chondrites to room temperature and humidity conditions after sample preparation (powdering) and the impact on measured fayalite content by XRD. As such, it is recommended that XRD analyses of meteorites be performed immediately after sample preparation.
The feldspar minerals occur in a wide variety of lithologies throughout the Solar System, often containing a variety of chemical and structural features indicative of the crystallization conditions, ...cooling history and deformational state of the crystal. Such phenomena are often poorly resolved in micrometre-scale analyses. Here, atom probe tomography (APT) is conducted on Ca-rich (bytownite) and Na-rich (albite) plagioclase reference materials, experimentally exsolved K-feldspar (sanidine), shock-induced plagioclase glass (labradorite-composition), and shocked and recrystallized plagioclase to directly test the application of APT to feldspar and yield new insights into crystallographic features such as amorphisation and exsolution. Undeformed plagioclase reference materials (Amelia albite and Stillwater bytownite) appear chemically homogenous, and yield compositions largely within uncertainty of published data. Within microstructurally complex materials, APT can resolve chemical variations across a ~ 20 nm wide exsolution lamella and define major element (Na, K) diffusion profiles across the lamella boundaries, which appear gradational over a ~ 10 nm length scale in experimentally exsolved K-feldspar NNPP-04b. The plagioclase glass within the Zagami shergottite shows no heterogeneity in the distribution of major elements, although the enrichment of Fe, Mg and Sr in the bulk microtip points to at least minor incorporation of surrounding phases (pyroxene), and with that supports a shock-melt origin for the glass (maskelynite). The recrystallization of feldspar during post-shock annealing, such as in poikilitic shergottite NWA 6342, appears to induce a range of chemical nanostructures that locally effect the composition of the material. These findings demonstrate the ability of APT to yield new insights into nanoscale composition and chemical structures of alumniosilicate phases, highlighting an exciting new avenue with which to analyse these key rock-forming minerals.
Ocean acidification (OA), caused by the dissolution of increasing concentrations of atmospheric carbon dioxide (CO2) in seawater, is projected to cause significant changes to marine ecology and ...biogeochemistry. Potential impacts on the microbially driven cycling of nitrogen are of particular concern. Specifically, under seawater pH levels approximating future OA scenarios, rates of ammonia oxidation (the rate-limiting first step of the nitrification pathway) have been shown to dramatically decrease in seawater, but not in underlying sediments. However, no prior study has considered the interactive effects of microbial ammonia oxidation and macrofaunal bioturbation activity, which can enhance nitrogen transformation rates. Using experimental mesocosms, we investigated the responses to OA of ammonia oxidizing microorganisms inhabiting surface sediments and sediments within burrow walls of the mud shrimp Upogebia deltaura. Seawater was acidified to one of four target pH values (pHT 7.90, 7.70, 7.35 and 6.80) in comparison with a control (pHT 8.10). At pHT 8.10, ammonia oxidation rates in burrow wall sediments were, on average, fivefold greater than in surface sediments. However, at all acidified pH values (pH ≤ 7.90), ammonia oxidation rates in burrow sediments were significantly inhibited (by 79–97%; p < 0.01), whereas rates in surface sediments were unaffected. Both bacterial and archaeal abundances increased significantly as pHT declined; by contrast, relative abundances of bacterial and archaeal ammonia oxidation (amoA) genes did not vary. This research suggests that OA could cause substantial reductions in total benthic ammonia oxidation rates in coastal bioturbated sediments, leading to corresponding changes in coupled nitrogen cycling between the benthic and pelagic realms.
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