Origin and emplacement of ultramafic–mafic intrusions in the Erro-Tobbio mantle peridotite (Ligurian Alps, Italy)

The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo–Mesozoic extension of the Europe–Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic–mafic intrusions. In this paper we present the results of field, structural and petrologic–geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt–rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (Ce_N/Sm_N = 0.006–0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle–melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo–An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3–5 kbar, i.e. 9–15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling.     

The chemistry of suspended particulate material in a highly contaminated embayment of Port Jackson (Australia) under quiescent,high-wind and heavy-rainfall conditions

This study investigated physico-chemical characteristics of the water column and chemistry of suspended particulate material (SPM) under quiescent, high-wind and high-wind/heavy-rainfall conditions in Homebush Bay, a highly contaminated embayment of Port Jackson (Australia) to distinguish source and possible adverse effects to benthic and pelagic animals. Mean concentrations in surficial sediment were <1, 14, 181, 141, 37, 290 and 685 μg g⁻¹ for Cd, Co, Cr, Cu, Ni, Pb and Zn, respectively. Sediment chemistry indicated these metals had multiple sources, i.e. the estuary, stormwater and industry. Mean total suspended solids (TSS) were 7, 17 and 20 mg L⁻¹ during quiescent, high-rainfall and heavy rainfall/high wind conditions, respectively, whereas SPM Cd, Co, Cr, Cu, Ni, Pb and Zn concentrations varied between 13–25, 166–259, 127–198, 38–82, 236–305 and 605–865 μg g⁻¹, respectively under these conditions. TSS and total water metal concentrations were lowest during quiescent conditions. High TSS and metal loads in surface water characterised high-rainfall events. Wind-induced resuspension contributed the greatest mass of SPM and metals to the water column. Benthic animals may be adversely affected by Pb and Zn in sediment. Total water Cu and Zn concentrations may pose a risk to filter-feeding animals in the water column due to resuspension of contaminated sediment.     

THE SEARCH FOR HUMBOLDT

HUMBOLDT'S COSMOS: Alexander von Humboldt and the Latin American Journey That Changed the Way We See the World. By Gerard Helferich. THE HUMBOLDT CURRENT: Nineteenth‐Century Exploration and the Roots of American Environmentalism. By Aaron Sachs.     

Dissolution rates of pure methane hydrate and carbon-dioxide hydrate in undersaturated seawater at 1000-m depth

To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH₄ and CO₂ hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10⁻² μm/s for the CO₂ and CH₄ hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO₂/m²s and 0.37 ± 0.03 mmol CH₄/m²s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH₄) or tens of mm (CO₂) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO₂ hydrates in marine carbon sequestration strategies.     

Fluid inclusion evidence for progressive folding during decompression in metasediments of the Voltri Group (Western Alps, Italy)

Quartz veins syntectonic to distinct folding events in metasediments from the Voltri Group (Ligurian Alps) were studied in order to compare fluid and structural evolution. Studied veins (VS1, VS2, VS3) pertain to three distinct generations of folds (F₁, F₂, F₃) that formed during the retrograde metamorphic evolution. Two types of fluids characterize the different generations of veins and are represented essentially by aqueo-carbonic mixtures of moderate salinity with decreasing densities (1.01–0.41 g/cm³). The chemical evolution is characterised by a progressive decrease of H₂O, from early fluids associated with opening of VS1 and VS2 (XCO₂≈0.08) to fluids related to VS3 formation (XCO₂≈0.3). The close match between the fluids in VS1 and VS2 suggests that the development of two superimposed systems of folds (F₁ and F₂ folds) occurs under very similar P–T conditions, during a progressive and continuous deformational event at glaucophanic and/or barroisitic metamorphic grade. A different evolution is outlined for the formation of VS3 during low greenschist grade. Successive isochores allow us to define a retrograde decompression path for the Voltri Group. Present results indicate that fluid inclusions are powerful markers to constrain the P–T–t conditions of different folding events.     

Assessing the impact of Celaque National Park on forest fragmentation in western Honduras

The effectiveness of parks as management regimes is much in debate. This study examines the effect of establishment of the Celaque National Park, Honduras, in 1987, on limiting deforestation through a comparison with the surrounding landscape using remote sensing, GIS and landscape pattern analysis. Pressure on the park region is found to relate spatially to the locations of towns and roads, with increasing deforestation in the landscape surrounding the park. In contrast, the park has been largely successful in maintaining forest cover. Although the extent of change within the park is not pronounced, the pattern of change is. Expansion of agriculture and coffee production have led to increasing pressure on the park boundaries, with as much as 25% of the landscape surrounding the park experiencing land cover change between 1996 and 2000. This has significant implications for the future of the park.     

Experimental and natural early diagenetic mobility of Sr and Mg in biogenic carbonates

Ammonium acetate dissolution experiments were performed on shell material of the modern bivalves, Crassostrea virginica (calcite) and Mercenaria mercenaria (aragonite). Their purpose was to determine the order of preferential dissolution of Sr, Mg and Ca; these results subsequently were compared with Sr, Mg and Ca data from other Recent as well as ancient mollusks.Results from these experiments suggest the following relative order of abundance of readily exchangeable Mg and Sr in biogenic carbonates: Mg(arag) > Mg(calc) > Sr(calc) > Sr(arag). It is apparent that incongruent dissolution of minerals with different solubilities cannot entirely explain the observed dissolution patterns for Sr, Mg and Ca in these biogenic carbonates. Secular changes in whole shell Mg and Sr concentrations for Recent and unrecrystallized fossil mollusks suggest an order of “ionic mobility” in diagenesis identical to the order of abundance for readily exchangeable ions found in the NH₄Ac dissolution experiments. It is concluded that this “ionic mobility” is due to a post mortem, early diagenetic (pre-recrystallization) approach to equilibrium with the surrounding chemical environment.     

A catalog of station coordinates for GEOS-C orbit determination

Center-of-mass coordinates for 57 tracking sites participating in the GEOS-C mission are given in a unified system having a common origin and scale. The positions were obtained either directly from analyses of satellite observations or from survey ties to colocated sites with coordinates previously determined by various investigators from optical, laser of doppler data. The uncertainty of the positions in nearly all cases is 5 m in each coordinate. Data reductions show that station coordinates of this quality introduce a rapidly changing error into the altitude of a satellite unless global tracking constrains the orbit.     

A water balance model to estimate climate change impact on groundwater recharge in Yucatan Peninsula,Mexico

ABSTRACT

The aim of this paper is to estimate the effect that climate change will have on groundwater recharge at the Yucatan Peninsula, Mexico. The groundwater recharge is calculated from a monthly water balance model considering eight methods of potential and actual evapotranspiration. Historical data from 1961–2000 and climate model outputs from five downscaled general circulation models in the near horizon (2015–2039), with representative concentration pathway (RCP) 4.5 and 8.5 are used. The results estimate a recharge of 118 ± 33 mm·year^–1 (around 10% of precipitation) in the historical period. Considering the uncertainty from GCMs under different RCP and evapotranspiration scenarios, our monthly water balance model estimates a groundwater recharge of 92 ± 40 mm·year^–1 (RCP4.5) and 94 ± 38 mm·year^–1 (RCP8.5) which represent a reduction of 23% and 20%, respectively, a result that threatens the socio-ecological balance of the region.