Back-arc spreading: Trench migration, continental pull or induced convection?

Mechanisms of the opening of back-arc systems are analyzed. Limited focal mechanisms of intraplate earthquakes are used to determine the stress regime of an overriding plate. Preliminary analyses show that compressive deviatoric stresses exist in the plate except near the spreading center. Based on this observation “trench suction” does not appear to be the primary force that drives back-arc spreading, since it will result in tensional deviatoric stresses within the overriding plate. Even though “continental pull” is able to satisfy the stress requirements, it does not appear to be a likely mechanism either because of the initiation and subsequent symmetric spreading difficulty associated with such a mechanism. The mechanism we favor is the one that involves the induced convective current in the mantle wedge immediately above the slab. Calculations show that the induced flow is able to generate sufficient stress to break up the overriding lithosphere if the tectonic stresses of the region are favorable. Both trench suction and continental pull may help to provide such a favorable tectonic stress regime.     

Microscopic physical biomarkers in carbonate hot springs: implications in the search for life on Mars.

Physical evidence of life (physical biomarkers) from the deposits of carbonate hot springs were documented at the scale of microorganisms--submillimeter to submicrometer. The four moderate-temperature (57 to 72 degrees C), neutral pH springs reported on in this study, support diverse communities of bacteria adapted to specific physical and chemical conditions. Some of the microbes coexist with travertine deposits in endolithic communities. In other cases, the microbes are rapidly coated and destroyed by precipitates but leave distinctive mineral fabrics. Some microbes adapted to carbonate hot springs produce an extracellular polymeric substance which forms a three-dimensional matrix with living cells and cell remains, known as a biofilm. Silicon and iron oxides often coat the biofilm, leading to long-term preservation. Submicrometer mineralized spheres composed of calcium fluoride or silica are common in carbonate hot spring deposits. Sphere formation is biologically mediated, but the spheres themselves are apparently not fossils or microbes. Additionally, some microbes selectively weather mineral surfaces in distinctive patterns. Hot spring deposits have been cited as prime locations for exobiological exploration of Mars. The presence of preserved microscopic physical biomarkers at all four sites supports a strategy of searching for evidence of life in hot spring deposits on Mars.     

Test of three methods to detect the overbank flow from water level time‐series analysis

In this study, we propose to identify morphological and hydraulic characteristics related to overbank flows in the water level time‐series available at many gauged stations. The results obtained at 13 river stations (the catchment sizes vary from 10 to 1700 km² with contrasted geology, land use and rainfalls regime) show that overflow mechanisms at the river‐reach scale can be systematically identified in the water level frequency distribution estimated with the peaks‐over‐threshold (POT) method. A first level (Lts1) was in the range of the incipient flooding onto the floodplain. Even if the definition of this level is variable in terms of flooded area at the reach scale, this method can be useful in providing a first estimation of the bankfull level for many gauged stations, without complex and costly field surveys. A second level (Lts2) was systematically detected on average 38 cm above the topographic flat floodplain elevation. The Lts2 inflection in the water‐level frequency distribution is assumed to reflect a composite effect of catchment hydrology and local hydraulics and channel geometry, without possibility to make a clear distinction between both processes at the moment. The local or reach scale effect would probably play an essential role in the frequency distribution as flood attenuation at catchment scale may explain the inflection Lts2 at only three sites. In light of the knowledge acquired in laboratory flumes with compound channels, most of the time Lts2 level would correspond to the level of resumption of flow both in the main channel and the floodplain. Once this method is validated in various physiographic contexts, it should apply to many hydrometric stations for both synchronic (e.g. regional analysis) and diachronic analysis (e.g. evolution over time of the bankfull discharge) to evaluate anthropic impacts on river morphology. Copyright © 2010 John Wiley & Sons, Ltd.     

East Asian monsoon intensification promoted weathering of the magnesium-rich southern China upper crust and its global significance

The Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO_2 consumption process.Here,we performed a first-order estimate of the CO_2 consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization.Our results show that an increase in long-term CO_2 consumption by silicate weathering varies from 0.06×10~(12)to 0.87×10~(12)mol yr~(-1)depending on erosion flux reconstructions,with an~50%contribution of Mg-silicate weathering since the late Oligocene.The organic carbon burial flux is approximately 25% of the contemporary CO_2 consumption by silicate weathering.The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China,which would contribute to the rapid decline in atmospheric CO_2 during the late Oligocene and the Neogene rise in the seawater Mg content.If this climatic reorganization was mainly induced by the Tibetan Plateau uplift,our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active.     

Fractures,hydrothermal alterations and permeability in the Soultz Enhanced Geothermal System

Borehole studies of the Soultz-sous-Forêts granite are dedicated to deep geothermics. The hydraulic properties of the reservoir are mainly controlled by the occurrence of some altered cataclastic shear zones showing a low natural permeability characterized by the occurrence of brines. Those zones show a fracture cluster organisation with sealed fractures of various types (post-filled joints, sheared fractures, veins). The main hydrothermal deposits observed within the permeable zones are geodic quartz, carbonates, illite and more locally sulphides. The fracture wall–rocks are intensely transformed: dissolution of igneous minerals, crystallization of new minerals, porosity and permeability increase. It is important to characterize the newly-formed minerals in order to choose the reagents used to improve the permeability of the exchanger by chemical stimulations. This article represents a synthesis of the studies completed by the authors between 1990 and 2008 on the fracture networks, hydrothermal alterations and mineral crystallizations they induced and data about the flow pathways in the exchanger.     

Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

In rivers draining the Himalaya-Tibetan-Plateau region, the ²⁶Mg/²⁴Mg ratio has a range of 2‰ and the ⁴⁴Ca/⁴²Ca ratio has a range of 0.6‰. The average δ²⁶Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ²⁶Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ²⁶Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ²⁶Mg (−1.31‰) and (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated values in rivers and rock reported so far demonstrating that both riverine and bedrock values may show greater variability than previously thought.Although rivers draining TSS limestone have the lowest values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ²⁶Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ²⁶Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in ²⁶Mg/²⁴Mg ratios between silicate rock and solute Mg reflects the ²⁶Mg/²⁴Mg isotopic fractionation factor () between silicate and dissolved Mg during incongruent silicate weathering. The value of of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ²⁶Mg value of −0.46‰ and an average value of 0.5‰, distinct from riverine values for δ²⁶Mg but similar to riverine values. Although rivers draining each major rock type have and δ²⁶Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments.The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle.     

Multi-proxy climate and environmental records from a Holocene eutrophic mire,southern taiga subzone,West Siberia

Palaeoenvironmental reconstructions from peat are strongly focused on ombrotrophic mires, but this study demonstrates that eutrophic mires can also be used. A multi-proxy approach was applied to a eutrophic mire on a floodplain terrace in the southern taiga of West Siberia. The results of the reconstruction were considered in the wide geographic context of the surrounding regions, including Siberia and Central Asia. Different palaeoecological proxies (analysis of plant macrofossils, testate amoebae, oribatid mites, molluscs, peat humification, ash content and spectral characteristics of humic acids) were used in this study. The results of different proxies showed a high level of consistency among themselves, which allowed for a robust interpretation of Holocene mire development. Throughout the ~7800 years history of the mire, there was a high level of surface wetness. The presence of mineral matter in the peat between 7800 and 5100 cal. a BP indicates regular flooding caused by the intensive fluvial activity, apparently resulting from increased precipitation. This was followed by a trend towards a gradual decrease in surface wetness from conditions of high surface moisture (stagnant water) between 5100 and 3000 cal. a BP to present day conditions of moderate surface moisture with a water table slightly below the mire surface. This pattern is consistent with the well-documented long-term trend from palaeoecological records throughout the taiga and arctic zones in West Siberia and central arid Asia. Our data further support the idea that the westerlies were the dominant driver of climate for the southern taiga of West Siberia during the Middle to Late Holocene.     

Planning for resettlement: building partnerships for,by, and with Indigenous peoples

Efforts in the United States to plan or implement relocation in response to climate risks have struggled to improve material conditions for participants, to incorporate local knowledge, and to keep communities intact. Mixed methodologies of community geography provide an opportunity for dialogue and knowledge-sharing to collaboratively diagnose the challenges of climate adaptation led by communities. In this article, we advance a participatory practice model for the co-creation of knowledge initiated during a two-day workshop with members from the Biloxi-Chitimacha-Choctaw Tribe from Isle de Jean Charles in Louisiana, Yup’ik people from Newtok Village in Alaska, and researchers from the MIT Resilient Communities Lab. Building on prior scholarship of indigenizing climate change research, this article shares the experience of the workshop to support knowledge exchange and dialogue, with the goal of understanding how to build participatory and non-extractive community-academic partnerships. We reflect on the community values and principles used to guide this workshop to inform more inclusive and co-produced research partnerships, and pedagogies that can improve and assist the self-determination of groups impacted by climate change. Workshop presentations and discussions highlight interconnected themes of resources, systems & structures, regulatory imbalance, and resilience that underpin climate resettlement. We reflect on the narratives presented by members of both Indigenous tribes and NGO partners that illustrate the shortcomings of resettlement planning practices past and present as perpetuating existing inequality. In response to this structured knowledge exchange, we identify potential roles for community-academic partnerships that aim to improve the equity of existing resettlement models. We propose approaches for incorporating traditional knowledge into the pedagogy, discourse, and practice of academic planning programs.

     

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy

We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C₂H₂ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10⁻⁸ M_⊙ yr⁻¹  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.