Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

We report δ⁷Li, Li abundance ([Li]), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite, and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical Archean continental crust. These data are compared with detrital zircons from the Jack Hills (Western Australia) with U–Pb ages greater than 3.9 Ga for which parent rock type is not known. Most of the TTG and sanukitoid zircon domains preserve typical igneous REE patterns and CL zoning. [Li] ranges from 0.5 to 79 ppm, typical of [Li] in continental zircons. Atomic ratios of (Y + REE)/(Li + P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. The δ⁷Li and trace element data constrain the genesis of TTGs and sanukitoids. [Li] in zircons from granitoids is significantly higher than from zircons in primitive magmas in oceanic crust. TTG zircons have δ⁷Li (3 ± 8‰) and δ¹⁸O in the range of primitive mantle-derived magmas. Sanukitoid zircons have average δ⁷Li (7 ± 8‰) and δ¹⁸O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid and TTG zircons indicate that high [Li] in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests the growth in proto-continental crust in magmas similar to Archean granitoids.     

Application of Plutonium Isotopes to the Sediment Geochronology of Coarse-Grained Sediments from Englebright Lake,California (USA)

The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (<63 μm). Correlations between cesium-137 and plutonium activities in fine-grained sediments (r = 0.81–0.98, p < 0.005) suggest that plutonium isotopes may be substituted for cesium isotopes in coarse-grained sediments where cesium is typically below detectable levels. Sediment accumulation rates calculated from grain-size normalized plutonium activity profiles ranged from 6 to 145 cm year^?1 in Englebright Lake and identified a sediment depocenter at the delta front upstream of Englebright Dam. Progradation of the delta front reflected changes in sediment supply from the watershed in response to flood events, whereas average annual accumulation responded to human impacts. This study extends the application of plutonium isotopes for sediment geochronology to aquatic environments dominated by coarse sediments and provides new information that contributes to a better understanding of the processes influencing sediment deposition in Englebright Lake.     

Cartographies of Stalin: Place,Scale, and Reputational Politics

In this article we explore the spatial variation of support for former Soviet leader Joseph Stalin in his home country, Georgia. This research contributes to the literature on reputational politics by highlighting the role of spatial, rather than only social, factors in the construction of public opinion about Stalin. We illustrate how geographic factors impact Georgians' perceptions of the Soviet dictator by examining various aspects of place at various scales—including distance to Gori, Stalin's birthplace, the history of urbanization, and economic and political indices. To this end we map attitudes toward the Soviet dictator and use a multilevel spatial regression technique to explain variance in support for Stalin across forty-seven districts of Georgia. This enables us to explore the impact of place on people's attitudes by examining (1) whether there are local and regional hotspots of Stalin admiration and, if so, (2) what the contextual explanations of these attitudes are.     

Influences of Natural and Anthropogenic Factors and Tidal Restoration on Terrestrial Arthropod Assemblages in West Coast North American Estuarine Wetlands

Compared to benthic and water-column invertebrate assemblages, considerably less is known about terrestrial arthropods inhabiting estuarine wetlands despite their importance to tidal wetland biodiversity and productivity. We also need to know more about how human modification of estuaries, including efforts to restore estuarine wetlands, affects these assemblages. To address this knowledge gap, we assembled data from multiple studies on terrestrial arthropod assemblages from 87 intertidal wetland sites in 13 estuaries along the west coast of North America. Arthropods were sampled between 1998 and 2013 with fallout traps deployed in wetlands for 1 to 3 days at a time. We describe patterns in the abundance and taxonomic composition of terrestrial arthropods and evaluate the relative ability of natural and anthropogenic factors to explain variation in abundance and composition. Arthropod abundance was highly variable. Vegetation assemblage, precipitation, and temperature best explained variation in arthropod abundance, while river discharge, latitude, and developed and agricultural land cover surrounding sampling sites were less important. Arthropod abundance rapidly achieved levels of reference wetlands after the restoration of tidal influence to leveed wetlands, regardless of surrounding land cover. However, arthropod assemblage composition was affected by the amount of developed land cover as well as restoration age. These results suggest that restoration of tidal influence to leveed wetlands can rapidly restore some components of estuarine wetland ecosystems but that recovery of other components will take longer and may depend on the extent of anthropogenic modification in the surrounding landscape.     

Nanogeochemistry of hydrothermal magnetite

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO₂, from below detection limit (bdl) to 3.1 wt%; Al₂O₃, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO₂, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na₂O, bdl–0.4 wt%; and K₂O, bdl–0.4 wt%. An exception is V₂O₃, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvöspinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.     

Groundwater recharge in a confined paleovalley setting,Northeast British Columbia,Canada

Hydrogeology Journal - Ancient river channels or subglacial drainage networks infilled with younger sediments can include significant deposits of highly permeable sands and gravels. Despite being...     

Use of fallout tracers 7Be, 210Pb and 137Cs to distinguish the form of sub‐surface soil erosion delivering sediment to rivers in large catchments

Fallout radionuclides (FRNs) ¹³⁷Cs and ²¹⁰Pb are well established as tracers of surface and sub‐surface soil erosion contributing sediment to river systems. However, without additional information, it has not been possible to distinguish sub‐surface soil erosion sources. Here, we use the FRN ⁷Be (half‐life 53 days) in combination with ¹³⁷Cs and excess ²¹⁰Pb to trace the form of erosion contributing sediment in three large river catchments in eastern Australia; the Logan River (area 3700 km²), Bowen River (9400 km²) and Mitchell River (4700 km²). We show that the combination of ¹³⁷Cs, excess ²¹⁰Pb and ⁷Be can discriminate horizontally aligned sub‐surface erosion sources (rilled and scalded hillslopes and the floors of incised drainage lines and gully ‘badland’ areas) from vertical erosion sources (channel banks and gully walls). Specifically, sub‐surface sources of sediment eroded during high rainfall and high river flow events have been distinguished by the ability of rainfall‐derived ⁷Be to label horizontal soil surfaces, but not vertical. Our results indicate that in the two northern catchments, erosion of horizontal sub‐surface soil sources contributed almost as much fine river sediment as vertical channel banks, and several times the contribution of hillslope topsoils. This result improves on source discrimination provided previously and indicates that in some areas erosion of hillslope soils may contribute significantly to sediment yield, but not as topsoil loss. We find that in north‐eastern Australia, scalded areas on hillslopes and incising drainage lines may be sediment sources of comparable importance to vertical channel banks. Previous studies have used the combination of ¹³⁷Cs, excess ²¹⁰Pb and ⁷Be to estimate soils losses at the hillslope scale. Here, we show that with timely and judicious sampling of soil and sediment during and immediately after high flow events ⁷Be measurements can augment fallout ¹³⁷Cs and ²¹⁰Pb to provide important erosion source information over large catchments. Copyright © 2013 John Wiley & Sons, Ltd.     

Hierarchical livelihood outcomes among co-managed fisheries

Collaborative management arrangements are increasingly being used in fisheries, yet critical questions remain about the conditions under which these are most successful. Here, we conduct one of the first comprehensive tests of Elinor Ostrom's diagnostic framework for analyzing social–ecological systems to examine how 16 socioeconomic and institutional conditions are related the livelihood outcomes in 42 co-management arrangements in five countries across the Indo-Pacific. We combine recent developments in both theory and modeling to address three key challenges among comparative studies of social–ecological systems: the presence of a large number of explanatory mechanisms, variables operating at multiple scales, and the potential for interactions among socio-economic and institutional factors. We find that resource users were more likely to perceive benefits from co-management when they are more involved in decisions, were aware that humans are causal agents of change in marine systems, were wealthier, were not migrants, were in villages with smaller populations and older co-management arrangements, and had clearly established boundaries. Critically, we quantify a number of key interactions between: wealth, dependence on marine resources, involvement in decision-making, and population size that have strong implications for co-management success in terms of livelihood benefits. This study demonstrates that context plays a critical but identifiable role in co-management success.