Time-resolved,defect-hosted,trace element mobility in deformed Witwatersrand pyrite

The Pb isotopic composition of rocks is widely used to constrain the sources and mobility of melts and hydrothermal fluids in the Earth's crust. In many cases, the Pb isotopic composition appears to represent mixing of multiple Pb reservoirs. However, the nature, scale and mechanisms responsible for isotopic mixing are not well known. Additionally, the trace element composition of sulphide minerals are routinely used in ore deposit research, mineral exploration and environmental studies, though little is known about element mobility in sulphides during metamorphism and deformation. To investigate the mechanisms of trace element mobility in a deformed Witwatersrand pyrite(FeS_2), we have combined electron backscatter diffraction(EBSD) and atom probe microscopy(APM). The results indicate that the pyrite microstructural features record widely different Pb isotopic compositions, covering the entire range of previously published sulphide Pb compositions from the Witwatersrand basin. We show that entangled dislocations record enhanced Pb, Sb, Ni, Tl and Cu composition likely due to entrapment and short-circuit diffusion in dislocation cores. These dislocations preserve the Pb isotopic composition of the pyrite at the time of growth(~3 Ga) and show that dislocation intersections, likely to be common in deforming minerals, limit trace element mobility. In contrast, Pb, As, Ni, Co, Sb and Bi decorate a highangle grain boundary which formed soon after crystallisation by sub-grain rotation recrystallization.Pb isotopic composition within this boundary indicates the addition of externally-derived Pb and trace elements during greenschist metamorphism at ~2 Ga. Our results show that discrete Pb reservoirs are nanometric in scale, and illustrate that grain boundaries may remain open systems for trace element mobility over 1 billion years after their formation.     

CO_2 degassing and melting of metasomatized mantle lithosphere during rifting-Numerical study

:Reactivation of metasomatized mantle lithosphere may occur during continental extension,which is an important component of plate tectonics.The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO2.Therefore,partial melting of these metasomatized domains may play a crucial role in the global carbon cycle.However,little is known about this process and up until now few numerical constraints are available.Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting.CO2 degassing and melting.We test 4 lithospheric thicknesses:90,110,130 and 200 km with a 10 km thick metasomatized layer at the base using CO2 of 2 wt.%in the bulk composition.The carbonate enriched layer is stable below^3 GPa(>110 km)for a temperature of 1300℃;therefore,we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick.The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting,whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension.The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young(shallow)and of the North Atlantic Rift for old(thick)lithosphere.     

Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States

The origin and significance of pimple mounds (low, elliptical to circular dune-like features found across much of the south-central United States) have been debated for nearly two centuries. We cored pimple mounds at four sites spanning the Ozark Plateau, Arkansas River Valley, and Gulf of Mexico Coastal Plain and found that these mounds have a regionally consistent textural asymmetry such that there is a significant excess of coarse-grained sediment within their northwest flanks. We interpret this asymmetry as evidence of an eolian depositional origin of these mounds and conclude they are relict nebkhas (coppice dunes) deposited during protracted middle to late Holocene droughts. These four mounds yield optically stimulated luminescence ages between 2400 and 700 yr that correlate with well-documented periods of eolian activity and droughts on the southern Great Plains, including the Medieval Climate Anomaly. We conclude vegetation loss during extended droughts led to local eolian deflation and pimple mound deposition. These mounds reflect landscape response to multi-decadal droughts for the south-central U.S. The spatial extent of pimple mounds across this region further underscores the severity and duration of late Holocene droughts, which were significantly greater than historic droughts.     

Inorganic synthesis of Fe–Ca–Mg carbonates at low temperature

A set of free-drift experiments was undertaken to synthesize carbonates of mixed cation content (Fe, Ca, Mg) from solution at 25 and 70 °C to better understand the relationship between the mineralogy and composition of these phases and the solutions from which they precipitate. Metastable solid solutions formed at 25 °C which are not predicted from the extrapolation of higher temperature equilibrium assemblages; instead, solids formed that were intermediary in chemical composition to known magnesite–siderite and dolomite solid solutions. A calcite–siderite solid solution precipitated at 25 °C, with the percentage of CaCO₃ in the solid being proportional to the aqueous Ca/Fe ratio of the solution, while Mg was excluded from the crystal structure except at relatively high aqueous Mg/Ca and Mg/Fe ratios and a low Ca content. Alternatively, at 70 °C Mg was the predominant cation of the solid solutions. These results are compatible with the hypothesis that the relative dehydration energies of Fe, Ca and Mg play an important role in the formation of mixed cation carbonates in nature.     

Oxygen and carbon stable isotopes of modern land snail shells as environmental indicators from a low-latitude oceanic island

Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a low-latitude oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10-2160 m a.s.l.) in Tenerife Island (∼28°N) of the Canary Archipelago.Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ¹⁸O value of the shell (δ¹⁸O_shell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the δ¹⁸O value of rain (δ¹⁸O_rain). Calculated δ¹⁸O values of the snail body water (δ¹⁸O_body) derived from observed temperatures and δ¹⁸O_shell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.)59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ¹⁸O values for local rain water. The calculated δ¹⁸O_body values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the “altitude effect” observed for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ¹⁸O_shell values indicate lower temperatures, lower δ¹⁸O_rain values and possibly, higher rainfall totals. A positive correlation between the mean δ¹³C values of shells (δ¹³C_shell) and plants by site suggests that shells potentially record information about the surrounding vegetation. The δ¹³C_shell values varied between −15.7 and −0.6‰ (VPDB), indicating that snails consumed C₃ and C₄/CAM plants, where more negative δ¹³C_shell values probably reflects the preferential consumption of C₃ plants which are favored under wetter conditions. Individuals with more positive δ¹³C_shell values consumed a larger percentage of C₄ plants (other potential factors such as carbonate ingestion or atmospheric CO₂ contribution were unlikely) that were more common at lower elevations of the hotter and drier south facing transect. The relatively wide range of shell isotope values within a single site requires the analysis of numerous shells for meaningful paleoclimatic studies. Although small differences were observed in isotope composition among snail species collected at a single sampling site, they were not significant, suggesting that isotope signatures extracted from multi-taxa snail data sets may be used to infer environmental conditions over a broad range of habitats.     

Geochronology of Quaternary glaciations from the tropical Cordillera Huayhuash, Peru

The Cordillera Huayhuash in the central Peruvian Andes (10.3°S, 76.9°W) is an ideal mountain range in which to study regional climate through variations in paleoglacier extents. The range trends nearly north-south with modern glaciers confined to peaks >4800 m a.s.l. Geomorphology and geochronology in the nearby Cordillera Blanca and Junin Plain reveal that the Peruvian Andes preserve a detailed record of tropical glaciation. Here, we use ASTER imagery, aerial photographs, and GPS to map and date glacial features in both the western and eastern drainages of the Cordillera Huayhuash. We have used in situ produced cosmogenic ¹⁰Be concentrations in quartz bearing erratics on moraine crests and ice-polished bedrock surfaces to develop an exposure age chronology for Pleistocene glaciation within the range. We have also collected sediment cores from moraine-dammed lakes and bogs to provide limiting ¹⁴C ages for glacial deposits. In contrast to the ranges to the north and south, most glacial features within the Cordillera Huayhuash are Lateglacial in age, however we have identified features with ages that span 0.2 to 38 ka with moraine sets marking the onset of glacier retreat at 0.3 ka, 9–10 ka, 13–14 ka, 20–22 ka, and >26 ka. The range displays a pronounced east-west variation in maximum down-valley distance from the headwall of moraine crests with considerably longer paleoglaciers in the eastern drainages. Importantly, Lateglacial paleoglaciers reached a terminal elevation of 4000 m a.s.l. on both sides of the Cordillera Huayhuash; suggesting that temperature may have been a dominant factor in controlling the maximum glacier extent. We suggest that valley morphology, specifically valley slope, strongly influences down-valley distance to the maximum glacier extent and potential for moraine preservation. While regionally there is an extensive record of older (>50 ka) advances to the north (Cordillera Blanca) and to the south (Junin region), the apparent lack of old moraines in this locality may be explained by the confined morphology of the Cordillera Huayhuash valleys that has inhibited the preservation of older glacial geomorphic features.     

Early burial mud diapirism and its impact on stratigraphic architecture in the Carboniferous of the Shannon Basin,County Clare,Ireland

Delta fronts are often characterized by high rates of sediment supply that result in unstable slopes and a wide variety of soft‐sediment deformation, including the formation of overpressured and mobile muds that may flow plastically during early burial, potentially forming mud diapirs. The coastal cliffs of County Clare, western Ireland, expose Pennsylvanian (Namurian) delta‐front deposits of the Shannon Basin at large scale and in three dimensions. These deposits include decametre‐scale, internally chaotic mudstone masses that clearly impact the surrounding sedimentary strata. Evidence indicates that these were true mud (unlithified sediment) diapirs that pierced overlying strata. This study documents a well‐exposed ca 20 m tall mud diapir and its impact on the surrounding mouth‐bar deposits of the Tullig Cyclothem. A synsedimentary fault and associated rollover dome, evident from stratal thicknesses and the dip of the beds, define one edge of the diapir. These features are interpreted as recording the reactive rise of the mud diapir in response to extensional faulting along its margin. Above the diapir, heterolithic sandstones and siltstones contain evidence for the creation of localized accommodation, suggesting synsedimentary filling, tilting and erosion of a shallow sag basin accommodated by the progressive collapse of the diapir. Two other diapirs are investigated using three‐dimensional models built from ‘structure from motion’ drone imagery. Both diapirs are interpreted to have grown predominantly through passive rise (downbuilding). Stratal relationships for all three diapirs indicate that they were uncompacted and fluid‐rich mud beds that became mobilized through soft‐sediment deformation during early burial (i.e. <50 m, likely <10 m depth). Each diapir locally controlled the stratigraphic architecture in the shallow subsurface and potentially influenced local palaeocurrents on the delta. The mud diapirs studied herein are distinct from deeper ‘shale diapirs’ that have been inferred from seismic sections worldwide, now largely disputed.     

Response of unconfined turbidity current to deep‐water fold and thrust belt topography: Orthogonal incidence on solitary and segmented folds

Sea‐floor topography of deep‐water folds is widely considered to have a major impact on turbidity currents and their depositional systems, but understanding the flow response to such features was limited mainly to conceptual notions inspired by small‐scale laboratory experiments. High‐resolution three‐dimensional numerical experiments can compensate for the lack of natural‐scale flow observations. The present study combines numerical modelling of thrusts with fault‐propagation folds by Trishear3D software with computational fluid dynamics simulations of a natural‐scale unconfined turbidity current by MassFlow‐3D? software. The study reveals the hydraulic and depositional responses of a turbidity current (ca 50 m thick) to typical topographic features that it might encounter in an orthogonal incidence on a sea‐floor deep‐water fold and thrust belt. The supercritical current (ca 10 m sec^?1) decelerated and thickened due to the hydraulic jump on the fold backlimb counter‐slope, where a reverse overflow formed through current self‐reflection and a reverse underflow was issued by backward squeezing of a dense near‐bed sediment load. The reverse flows were re‐feeding sediment to the parental current, reducing its waning rate and extending its runout. The low‐efficiency current, carrying sand and silt, outran a downslope distance of >17 km with only modest deposition (<0·2 m) beyond the fold. Most of the flow volume diverted sideways along the backlimb to surround the fold and spread further downslope, with some overspill across the fold and another hydraulic jump at the forelimb toe. In the case of a segmented fold, a large part of the flow went downslope through the segment boundary. Preferential deposition (0·2 to 1·8 m) occurred on the fold backlimb and directly upslope, and on the forelimb slope in the case of a smaller fold. The spatial patterns of sand entrapment revealed by the study may serve as guidelines for assessing the influence of substrate folds on turbiditic sedimentation in a basin.     

Cryospheric hazards and risk perceptions in the Sagarmatha (Mt. Everest) National Park and Buffer Zone,Nepal

Glacial lake outburst floods (GLOFs) are among the most serious cryospheric hazards for mountain communities. Multiple studies have predicted the potential risks posed by rapidly expanding glacial lakes in the Sagarmatha (Mt. Everest) National Park and Buffer Zone of Nepal. People’s perceptions of such cryospheric hazards can influence their actions, beliefs, and responses to those hazards and associated risks. This study provides a systematic approach that combines household survey data with ethnography to analyze people’s perceptions of GLOF risks and the socioeconomic and cultural factors influencing their perceptions. A statistical logit model of household data showed a significant positive correlation between the perceptions of GLOF risks and livelihood sources, mainly tourism. Risk perceptions are also influenced by spatial proximity to glacial lakes and whether a village is in potential flood zones. The 2016 emergency remediation work implemented in the Imja Tsho (glacial lake) has served as a cognitive fix, especially in the low-lying settlements. Much of uncertainty and confusions related GLOF risks among locals can be attributed to a disconnect between how scientific information is communicated to the local communities and how government climate change policies have been limited to awareness campaigns and emergency remediation efforts. A sustainable partnership of scientists, policymakers, and local communities is urgently needed to build a science-driven, community-based initiative that focuses not just in addressing a single GLOF threat but develops on a comprehensive cryospheric risk management plan and considers opportunities and challenges of tourism in the local climate adaptation policies.