A new view on gold speciation in sulfur-bearing hydrothermal fluids from in situ X-ray absorption spectroscopy and quantum-chemical modeling

Despite the common belief that Au^I complexes with hydrogen sulfide ligands (H₂S/HS⁻) are the major carriers of gold in natural hydrothermal fluids, their identity, structure and stability are still subjects of debate. Here we present the first in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of the stability and structure of aqueous Au^I–S complexes at temperatures and pressures (T–P) typical of natural sulfur-rich ore-forming fluids. The solubility of native gold and the local atomic structure around the dissolved metal in S–NaOH–Na₂SO₄–H₂SO₄ aqueous solutions were characterized at temperatures 200–450 °C and pressures 300–600 bar using an X-ray cell that allows simultaneous measurement of the absolute concentration of the absorbing atom (Au) and its local atomic environment in the fluid phase. Structural and solubility data obtained from XAFS spectra, combined with quantum-chemical calculations of species geometries, show that gold bis(hydrogensulfide) Au(HS)₂⁻ is the dominant Au species in neutral-to-basic solutions (5.5  pH  8.5; H₂O–S–NaOH) over a wide range of sulfur concentrations (0.2 < ΣS < 3.6 mol/kg), in agreement with previous solubility studies. Our results provide the first direct determination of this species structure, in which two sulfur atoms are in a linear geometry around Au^I at an average distance of 2.29 ± 0.01 Å. At acidic conditions (1.5  pH  5.0; H₂O–S–Na₂SO₄–H₂SO₄), the Au atomic environment determined by XAFS is similar to that in neutral solutions. These findings, together with measured high Au solubilities, are inconsistent with the predominance of the gold hydrogensulfide Au(HS)⁰ complex suggested by recent solubility studies. Our spectroscopic data and quantum-chemical calculations imply the formation of species composed of linear S–Au–S moieties, like the neutral [H₂S–Au–SH] complex. This species may account for the elevated Au solubilities in acidic fluids and vapors with H₂S concentrations higher than 0.1–0.2 mol/kg. However, because of the complex sulfur speciation in acidic solutions that involves sulfite, thiosulfate and polysulfide species, the formation of Au^I complexes with these ligands (e.g., AuHS(SO₂)⁰, Au(HS₂O₃)₂⁻, Au(HS_n)₂⁻) cannot be ruled out. The existence of such species may significantly enhance Au transport by high T–P acidic ore-forming fluids and vapors, responsible for the formation of a major part of the gold resources on Earth.     

A primordial origin for the atmospheric methane of Saturn’s moon Titan

The origin of Titan’s atmospheric methane is a key issue for understanding the origin of the saturnian satellite system. It has been proposed that serpentinization reactions in Titan’s interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan’s planetesimals before its formation. Here, we point out that serpentinization reactions in Titan’s interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan’s water ice 30% lower than the value likely acquired by the satellite during its formation, based on Cassini observations at Enceladus. Alternatively, production of methane in Titan’s interior via radiolytic reactions with water can be envisaged but the associated production rates remain uncertain. On the other hand, a mechanism that easily explains the presence of large amounts of methane trapped in Titan in a way consistent with its measured atmospheric D/H ratio is its direct capture in the satellite’s planetesimals at the time of their formation in the solar nebula. In this case, the mass of methane trapped in Titan’s interior can be up to ∼1300 times the current mass of atmospheric methane.     

Radial thresholding to mitigate laser guide star aberrations on centre-of-gravity-based Shack–Hartmann wavefront sensors

Sodium laser guide stars (LGSs) are elongated sources due to the thickness and the finite distance of the sodium layer. The fluctuations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots, and generate spurious optical aberrations in closed-loop adaptive optics (AO) systems. According to an analytical model and experimental results obtained with the University of Victoria LGS bench demonstrator, one of the main origins of these aberrations, referred to as LGS aberrations, is not the centre-of-gravity (CoG) algorithm itself, but the thresholding applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed 'radial thresholding', is presented here, cancelling out most of the LGS aberrations without altering the centroid measurement accuracy.     

Natural variations of Se isotopic composition determined by hydride generation multiple collector inductively coupled plasma mass spectrometry

Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of the isotopic composition of Se in geological samples. Se is chemically purified before analysis by using cotton impregnated with thioglycollic acid. This preconcentration step is required for the removal of matrix-interfering elements for hydride generation, such as transitional metals, and also for the quantitative separation of other hydride-forming elements, such as Ge, Sb, and As. The analyte is introduced in the plasma torch with a continuous-flow hydride generation system. Instrumental mass fractionation is corrected with a “standard-sample bracketing” approach. By use of this new technique, the minimum Se required per analysis is lowered to 10 ng, which is one order of magnitude less than the amount needed for the N-TIMS technique. The estimated external precision calculated for the ⁸²Se/⁷⁶Se isotope ratio is 0.25‰ (2σ), and the data are reported as delta notation (‰) relative to our internal standard (MERCK elemental standard solution). Measurements of Se isotopes are presented for samples of standard solutions and geological reference materials, such as silicate rocks, soils, and sediments. The Se isotopic composition of selected terrestrial and extraterrestrial materials are also presented. An overall Se isotope variation of 8‰ has been observed, suggesting that Se isotopes fractionate readily and are extremely useful tracers of natural processes.     

High-pressure,low-temperature metamorphism in Alpujarride Units of southeastern Betics (Spain)Métamorphisme de haute pression et basse température dans les unités Alpujarrides du Sud-Est de la cordillère Bétique (Espagne)

We present the first occurrences of high-pressure, low-temperature ferro-magnesiocarpholite-bearing mineral assemblages associated to quartz segregations in the Alpujarride units of southeastern Betics (Sierra de Almagro, Sierra de los Pinos and Sierra Cabrera). Thermobarometric results show that the carpholite-bearing rocks underwent the same P–T conditions in the three outcrops, i.e. 8–10 kbar, 350–400 °C. Metamorphic and structural data allow us to conclude that these rocks belong to the same Alpujarride unit. In the Sierra de Almagro, tectonic units with carpholite-bearing rocks overlie low-pressure, low-temperature Alpujarride units, then forming a stack with an inverted tectono-metamorphic sequence, as observed in the central and western part of the Alpujarride complex. The preservation of carpholite-bearing assemblages in these rocks implies that no significant temperature increase occurred during the exhumation history. To cite this article: G. Booth-Rea et al., C. R. Geoscience 334 (2002) 857–865.     

Les « Porphyroı̈des » de Bretagne méridionale : une unité de HP–BT dans la chaı̂ne hercynienneEvidence for HP–LT Hercynian metamorphism within the ‘Porphyroı̈des’ of South Brittany (France)

The so-called ‘Porphyro??des’ Unit of South Brittany is located below units marked by an early HP–LT event (blueschists from the Île de Groix) and above units marked by Upper Carboniferous HT metamorphism. P–T estimates in the Porphyro??des (at Belle-Île-en-Mer), using the phengite–chlorite thermobarometer, indicate pressures around 8 kbar and temperatures of 350–400 °C. These new estimates suggest that the early tectonic history of the ‘Porphyro??des’ should be linked to that of the overlying Blueschists, and imply a much larger areal extension of the HP–LT domain in this part of the Hercynian Belt. To cite this article: F. Le Hébel et al., C. R. Geoscience 334 (2002) 205–211.     

Coastal Precipitation Regimes in Kenya

Kenya is under the influence of the seasonal reversal of the Indian ocean monsoons. However, its coastal belt, up to about 50 km inland, exhibits original climatic features. Hierarchical clustering of mean monthly rainfall for a large number of stations, particularly in the south-eastern region, strongly differentiate a maritime climate characterised by enhanced and delayed "long rains" and no dry season in the northern summer. Most of these rains fall at night or in the morning. Using daily rainfall data and twice-daily surface and upper-air wind observations, monsoon-breeze interactions and their role on April–August coastal precipitation are assessed. Rain spells common to the whole Kenya coast are associated to a slight weakening of the sea breeze, a strong easterly or south-easterly wind anomaly over most of Kenya at around 850–700 hPa, and sea surface temperatures greater than air temperatures.     

Evidence of a major environmental change recorded in a macrotidal bay (Marennes-Oléron Bay, France) by correlation between VHR seismic profiles and cores

New, very high-resolution (25 cm) seismic profiles have revealed the internal architecture of the infilling of a macrotidal bay, the Marennes-Oléron Bay, France. Within this geometry, a major seismic unconformity has been correlated with core data. This correlation provides evidence that the seismic unconformity corresponds to a sharp grain-size decrease. Both seismic and core data indicate that this change of grain size can be interpreted as a record of a recent (around 1,000 years b.p.) decrease in hydrodynamical energy with time and/or a larger supply of fine-grained material. This recent environmental change can be related to natural infilling of the Marennes Oléron Bay, and to tidal prism decrease, increasing human activities (e.g. land reclamation, deforestation, agricultural land use) and climate fluctuations during the late Holocene, such as the transition between the cold period of the Dark Ages (1,550–1,050 years b.p.) and the Medieval warm period (1,050–550 years b.p.).     

Effects of epibionts on Sabellaria alveolata (L.) biogenic reefs and their associated fauna in the Bay of Mont Saint-Michel

Biogenic structures built by ecosystem engineers such as corals, bivalves, polychaetes, and sea grasses provide habitat for benthic vertebrates and invertebrates. The polychaete Sabellaria alveolata is an important foundation species whose reef structure adds topographic complexity and high levels of biodiversity to the otherwise low-relief, low diversity, soft-bottom environments in the Bay of Mont Saint-Michel, France, where the largest such reef formations in Europe are found. In this bay, reefs are being increasingly colonised by oysters (Crassostrea gigas) from local aquaculture operations and by green algae (Ulva sp.) due to the increasing inputs of nitrates from terrestrial origin. The purpose of this study was to investigate the possible impacts of epibiotic oysters and green algae on the S. alveolata population and reef community structure in the Bay of Mont Saint-Michel, France. Univariate and multivariate comparisons of macrofauna were conducted for five reef types: controls (no epibionts), low oyster density, high oyster density, green algae, and oyster and green algae. Results showed that all the three reef types with oysters had significantly higher species richness and diversity values than control and algae-only reef types. Pairwise ANOSIM and SIMPER comparisons of controls versus the four reef types with epibionts revealed that all three of the reef types with oysters were significantly different from controls, but there was no significant difference between controls and algae-only reef types. A striking feature of the reef comparisons is that no single species in this species-rich system contributed more than 8.86% to the dissimilarity between the reef types. Thus, k-dominance curves for species abundances were not effective in revealing differences among the reef types. Our results demonstrate that recent anthropogenic inputs of oysters affect the reef species assemblage more strongly than algal epibionts. In addition, epibionts, especially green algae, alter S. alveolata population structure, causing a reduction in new recruits that over the long run may cause significant damage to the reef structure itself. These results are a first step towards understanding anthropogenic threats to S. alveolata reefs and may be useful in the development of strategies for their protection and management.