ALBERT

Beschreibung: Planetary science: Uninhabitable martian clays? Nature Geoscience 5, 683 (2012). doi:10.1038/ngeo1560 Author: Brian Hynek Clay minerals on Mars have been interpreted as an indication for a warm, wet early climate. A new hypothesis proposes that the minerals instead formed during brief periods of magmatic degassing, diminishing the prospects for signs of life in these settings.

Beschreibung: Significant contribution to climate warming from the permafrost carbon feedback Nature Geoscience 5, 719 (2012). doi:10.1038/ngeo1573 Authors: Andrew H. MacDougall, Christopher A. Avis & Andrew J. Weaver Permafrost soils contain an estimated 1,700 Pg of carbon, almost twice the present atmospheric carbon pool. As permafrost soils thaw owing to climate warming, respiration of organic matter within these soils will transfer carbon to the atmosphere, potentially leading to a positive feedback. Models in which the carbon cycle is uncoupled from the atmosphere, together with one-dimensional models, suggest that permafrost soils could release 7–138 Pg carbon by 2100 (refs , ). Here, we use a coupled global climate model to quantify the magnitude of the warming generated by the feedback between permafrost carbon release and climate. According to our simulations, permafrost soils will release between 68 and 508 Pg carbon by 2100. We show that the additional surface warming generated by the feedback between permafrost carbon and climate is independent of the pathway of anthropogenic emissions followed in the twenty-first century. We estimate that this feedback could result in an additional warming of 0.13–1.69 °C by 2300. We further show that the upper bound for the strength of the feedback is reached under the less intensive emissions pathways. We suggest that permafrost carbon release could lead to significant warming, even under less intensive emissions trajectories.

Beschreibung: Mantle flow deflected by interactions between subducted slabs and cratonic keels Nature Geoscience 5, 726 (2012). doi:10.1038/ngeo1553 Authors: Meghan S. Miller & Thorsten W. Becker Oceanic lithosphere is rapidly recycled into the mantle through subduction, an important part of the dynamic evolution of the Earth. Cratonic continental lithosphere, however, can exist for billions of years, moving coherently with the tectonic plates. At the Caribbean–South American Plate margin, a complex subduction system and continental transform fault is adjacent to the South American cratonic keel. Parallel to the transform fault plate boundary, an anomalous region of seismic anisotropy—created when minerals become aligned during mantle flow—is observed. This region of anisotropy has been attributed to stirring of the mantle by subducting slabs. Here we use seismological measurements and global geodynamic models adapted to this unique region to investigate how mantle flow, induced by subduction beneath the Antilles volcanic arc, is influenced by the stiff, deep continental craton. We find that three components—a stiff cratonic keel, a weak asthenospheric layer beneath the oceans and an accurate representation of the subducted slabs globally—are required in the models to match the unusual observed seismic anisotropy in the southeast Caribbean region. We conclude that mantle flow near the plate boundary is deflected and enhanced by the keel of the South American craton, rather than by slab stirring.

Beschreibung: Episodic tremor and slow slip potentially linked to permeability contrasts at the Moho Nature Geoscience 5, 731 (2012). doi:10.1038/ngeo1559 Authors: Ikuo Katayama, Tatsuya Terada, Keishi Okazaki & Wataru Tanikawa Slow earthquakes occur at the plate interface in subduction zones. These low-frequency tremors and slow-slip events are often located at about 30 km depth, near the boundary between the crust and mantle (Moho) on the overriding plate. Slow earthquakes occur on fault patches with extremely low frictional strength. This weakness is generally assumed to result from increased pore-fluid pressures and may be linked to the release of fluids from the descending plate. Here we propose that a contrast in permeability across the Moho results in the accumulation of water and the build-up of pore-fluid pressure at the corner of the mantle wedge that overlies the subducting plate. We use laboratory measurements of permeability to show that gabbroic rock layers in the crust are two orders of magnitude less permeable than serpentinite layers in the underlying hydrated mantle rocks. Inserting our experimental data into a numerical model that simulates pore pressure evolution across the Moho, we show that the pore-fluid pressure at this boundary can be as high as lithostatic pressure. We suggest that water released from the descending plate is trapped at the corner of the mantle wedge owing to this permeability barrier, and then causes the localized slow earthquakes that are triggered by fault instabilities.

Beschreibung: Elevation-dependent influence of snow accumulation on forest greening Nature Geoscience 5, 705 (2012). doi:10.1038/ngeo1571 Authors: Ernesto Trujillo, Noah P. Molotch, Michael L. Goulden, Anne E. Kelly & Roger C. Bales Rising temperatures and declining water availability have influenced the ecological function of mountain forests over the past half-century. For instance, warming in spring and summer and shifts towards earlier snowmelt are associated with an increase in wildfire activity and tree mortality in mountain forests in the western United States. Temperature increases are expected to continue during the twenty-first century in mountain ecosystems across the globe, with uncertain consequences. Here, we examine the influence of interannual variations in snowpack accumulation on forest greenness in the Sierra Nevada Mountains, California, between 1982 and 2006. Using observational records of snow accumulation and satellite data on vegetation greenness we show that vegetation greenness increases with snow accumulation. Indeed, we show that variations in maximum snow accumulation explain over 50% of the interannual variability in peak forest greenness across the Sierra Nevada region. The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited mid-elevations, between 2,000 and 2,600 m. In situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.

Beschreibung: Planetary science: Ubiquitous late veneer Nature Geoscience 5, 591 (2012). doi:10.1038/ngeo1551 Author: James Brenan Iron-loving elements are thought to have been added to Mars, Earth and the Moon after core formation. An analysis of meteorites formed in the first two to three million years of Solar System history suggests that a similar late veneer was added elsewhere too.

Beschreibung: Palaeontology: Slowed by sulphide Nature Geoscience 5, 593 (2012). doi:10.1038/ngeo1566 Author: Katja Meyer The Triassic–Jurassic period extinction marked a rapid turnover in the marine realm. Biomarkers in marine rocks suggest that the development of sulphidic conditions in the early Jurassic delayed marine recovery.

Beschreibung: Tectonics: Reanimating eastern Tibet Nature Geoscience 5, 597 (2012). doi:10.1038/ngeo1564 Author: Michael E. Oskin The high eastern Tibetan Plateau was thought to have formed from an inflow of material from the lower crust. The cooling histories of rocks exposed at the plateau margin, however, reveal protracted, episodic growth, suggesting that faulting also played a role.

Beschreibung: Links between early Holocene ice-sheet decay, sea-level rise and abrupt climate change Nature Geoscience 5, 601 (2012). doi:10.1038/ngeo1536 Authors: Torbjörn E. Törnqvist & Marc P. Hijma

Beschreibung: Archaean biogeochemistry: Unexpectedly abiotic Nature Geoscience 5, 598 (2012). doi:10.1038/ngeo1561 Author: Boswell Wing Sulphur cycling on early Earth is commonly linked to microbial activity. However, sulphur isotope values from 3.2–3.5-billion-year-old rocks indicate a central role for the breakdown of volcanic sulphur dioxide by ultraviolet radiation instead.