Hydrothermal experiments were conducted to evaluate the kinetics of H-2(aq) oxidation in the homogeneous H-2-O-2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 degrees C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H-2(aq) and O-2(aq) are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H-2(aq)-O-2(aq) equilibrium become slower with decreasing temperature, allowing excess H-2(aq) to remain in solution. Sluggish reaction rates of H-2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H-2(aq) and O-2(aq), even at temperatures lower than 40 degrees C due to H-2(aq) persistence in the seawater/vent fluid mixtures. For these H-2-O-2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H-2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.

The presence of graphite in natural environments is linked to the redox and thermal conditions of C-H-O fluid/graphite equilibrium in hydrothermal veins and metasomatic contacts. A time-series experimental study was performed to investigate the graphite undersaturated C-H-O system at 600 degrees C and 1000 MPa, and with f(o2) ranging from highly reducing (10(-23)) to highly oxidizing (10(4)). A nonvolatile intermediate carbon phase exhibiting the Raman spectral features of poorly ordered graphite was formed as the system evolves toward equilibrium as a function of run duration. The thermometric empirical expressions using the G and D bands in the spectra of graphite failed to accurately estimate the experimental temperature. Thus, the existing Raman geothermometers appear inadequate to address graphite formation under conditions of metastable equilibria and to account for kinetic effects such as, for example, the degree of crystallinity. The presence of poorly ordered graphitic carbon at all the redox conditions investigated suggests that the disordered structure of the mineral attains an extensive thermodynamic stability field, and that it may be more readily deposited than crystalline graphite. Metastable graphitic carbon could, therefore, function as a precursor and substrate for the formation of the well-ordered phase. Such metastable graphite may provide an intermediate state that facilitates subduction of carbonaceous material, while imposing constraints on the formation mechanisms and the C-13/C-12 isotopic systematics of deep seated carbonaceous fluids and minerals such as diamonds.

In order to evaluate the oxidation effect of dissolved hydrogen peroxide and the catalytic role of iron oxides on the kinetics of formic acid decarboxylation, a series of flow-through hydrothermal experiments was conducted at temperatures ranging from 80 to 150 degrees C and pressures of 172-241 bar. delta C-13 composition of residual HCOOH(aq) was also monitored to examine kinetic isotope effects associated with oxidation processes. Our results reveal that decomposition of H2O2(aq) in presence of magnetite follows pseudo first order kinetics, highly enhanced relative to the homogeneous H2O2(aq)-HCOOOH(aq)-H2O system, which possibly reflect synthesis of hydroxyl radicals ((OH)-O-center dot) through Fenton processes. The kinetic rate constants of HCOOH(aq) decarboxylation to CO2(aq) are also elevated relative to those previously measured in H2O2(aq) free experiments. However, reaction kinetics are slightly slower in the case of H2O2(aq) aqueous solutions coexisting with magnetite than in the absence of mineral phases. This behavior is attributed to the possible formation of Fe-bearing hydroxyl formate aqueous species that could serve as stable transition states leading to a decrease in the activation entropy of formic acid decomposition.

A series of experiments has been conducted in the H-2-D-2-D2O-H2O-Ti-TiO2 system at temperatures ranging from 300 to 800 degrees C and pressures between similar to 0.3 and 1.3 GPa in a hydrothermal diamond anvil cell, utilizing Raman spectroscopy as a quantitative tool to explore the relative distribution of hydrogen and deuterium isotopologues of the H-2 and H2O in supercritical fluids. In detail, H2O-D2O solutions (1: 1) were reacted with Ti metal (3-9 h) in the diamond cell, leading to formation of H-2, D-2, HD, and HDO species through Ti oxidation and H-D isotope exchange reactions. Experimental results obtained in situ and at ambient conditions on quenched samples indicate significant differences from the theoretical estimates of the equilibrium thermodynamic properties of the H-D exchange reactions. In fact, the estimated enthalpy for the H-2(aq)-D-2(aq) disproportionation reaction (Delta H-rxn) is about -3.4 kcal/mol, which differs greatly from the +0.16 kcal/mol predicted for the exchange reaction in the gas phase by statistical mechanics models.

A series of hydrothermal diamond-anvil cell experiments have been conducted to evaluate the role of supercritical water on the isotopic equilibrium between H/D methane isotopologues at 600-800 degrees C and 409-1622 MPa. Raman spectroscopy was deployed to investigate the distribution of HID isotopic molecules formed during hydrothermal decomposition of Si5C12H36 in H2O-D2O aqueous solutions. To this end, the intensities of the fundamental vibrational C-H and C-D modes of deuteromethanes were employed to determine the thermodynamic properties of isotope exchange reactions between HID isotopologues and to constrain the methane D/H molar ratios. By adjusting the initial volume ratios of silane/H2O-D2O, reactions in the CH4-D2O-H2O system were monitored for gaseous and supercritical-water phases. Discreet differences between the equilibrium constants, describing the relationship between the CH3D-CH2D2-CHD3-CH4 species dissolved in supercritical water or present as a homogeneous gas phase, are revealed. The bulk D/H methane composition in the liquid-system is also twice that of the D/H molar ratios recorded in the gas-bearing system. Accordingly, condensed-phase isotope effects are inferred to play a key role on the evolution of H/D isotopologues, likely induced by differences in the solubility of the isotopic molecules driven by the excess energy/entropy developed during mixing of non-polar species in the H2O-D2O structure. Our experiments show that isotope fractionation effects need to account for the presence of condensed matter (e.g., melts, magmatic fluids), even at conditions at which theoretical models suggest minimal (or nonexistent) isotope exchange, but comparable to those within the Earth's interior.

Shallow-water hydrothermal vents are extreme environments that share many characteristics with their deep-sea analogs. However, despite ease of access, much less is known about the geochemical dynamics of these ecosystems. Here, we report on the spatial and temporal geochemical dynamics of a shallow-water vent system at Paleochori Bay, Milos Island, Greece. Our multi-analyte voltammetric profiles of dissolved O-2 and hydrothermal tracers (e.g. Fe2+, FeSaq, Mn2+) on sediment cores taken along a transection in hydrothermally affected sediments indicate three different areas: the central vent area (highest temperature) with a deeper penetration of oxygen into the sediment, and a lack of dissolved Fe2+ and Mn2+; a middle area (0.5 m away) rich in dissolved Fe2+ and Mn2+ (exceeding 2 mM) and high free sulfide with potential for microbial sulfide oxidation as suggested by the presence of white mats at the sediment surface; and, finally, an outer rim area (1-1.5 m away) with lower concentrations of Fe2+ and Mn2+ and higher signals of FeSaq, indicating an aged hydrothermal fluid contribution. In addition, high-frequency temperature series and continuous in situ H2S measurements with voltammetric sensors over a 6-day time period at a distance 0.5 m away from the vent center showed substantial variability in temperature (31.6 to 46.4 degrees C) and total sulfide (488 to 1329 mu M) in the upper sediment layer. The analysis of these data suggests that tidal and wind forcing, and abrupt geodynamic events generate intermittent mixing conditions lasting for several hours to days. Despite substantial variability, the concentration of sulfide available for chemoautotrophic microbes remained high. However, the availability of electron acceptors originating from seawater might be more intermittent, which in turn has an effect on the reestablishment of the white mats after wave-induced disturbances. Our results emphasize the importance of transient events in the development of chemoautotrophic communities in the hydrothermally influenced sediments of Paleochori Bay. (C) 2013 Elsevier B.V. All rights reserved.

Themobility ofmetals in soils and subsurface aquifers is strongly affected by sorption and complexation with dissolved organic matter, oxyhydroxides, clay minerals, and inorganic ligands. Humic substances (HS) are organic macromolecules with functional groups that have a strong affinity for binding metals, such as actinides. Thorium, often studied as an analog for tetravalent actinides, has also been shown to strongly associate with dissolved and colloidal HS in natural waters. The effects of HS on the mobilization dynamics of actinides are of particular interest in risk assessment of nuclear waste repositories.

Natural hydrothermal vent environments cover a wide range of physicochemical conditions involving temperature, pH and redox state. The stability of simple biomolecules such as amino acids in such environments is of interest in various fields of study from the origin of life to the metabolism of microbes at the present day. Numerous previous experimental studies have suggested that amino acids are unstable under hydrothermal conditions and decompose rapidly. However, previous studies have not effectively controlled the redox state of the hydrothermal fluids. Here we studied the stability of glutamate with and without reducing hydrothermal conditions imposed by 13 mM aqueous H-2 at temperatures of 150, 200 and 250 degrees C and initial (25 degrees C) pH values of 6 and 10 in a flow-through hydrothermal reactor with reaction times from 3 to 36 min. We combined the experimental measurements with theoretical calculations to model the in situ aqueous speciation and pH values. As previously observed under hydrothermal conditions, the main reaction involves glutamate cyclizing to pyroglutamate through a simple dehydration reaction. However, the amounts of decomposition products of the glutamate detected, including succinate, formate, carbon dioxide and ammonia depend on the temperature, the pH and particularly the redox state of the fluid. In the absence of dissolved H-2, glutamate decomposes in the sequence glutamate, glutaconate, alpha-hydroxyglutarate, ketoglutarate, formate and succinate, and ultimately to CO2 and micromolar quantities of H-2(aq). Model speciation calculations indicate the CO2, formate and H-2(aq) are not in metastable thermodynamic equilibrium. However, with 13 mM H-2(aq) concentrations, the amounts of decomposition products are suppressed at all temperatures and pH values investigated. The small amounts of CO2 and formate present are calculated to be in metastable equilibrium with the H-2. It is further proposed that there is a metastable equilibrium between glutamate, glutaconate, alpha-hydroxyglutarate, ketoglutarate and H-2. The key redox-sensitive step is the reaction of alpha-hydroxyglutarate to alpha-ketoglutarate, which is effectively inhibited by the elevated H-2 concentrations, which in turn dramatically lowers the amounts of all decomposition products including ammonia. Theoretical calculations of the metastable thermodynamic equilibrium between glutamate and ketoglutarate are consistent with the experimentally determined effects of reducing conditions. These findings establish that redox state is as important a variable as temperature and pH in affecting the stability of amino acids under hydrothermal conditions. It is suggested that when natural hydrothermal fluids contain enough dissolved H-2, the stability of amino acids may be enhanced in fluids at least on short time scales. In turn, this result suggests that reducing hydrothermal environments may have been favorable for assembling the building blocks of biomolecules in the origin of life. Furthermore, in present day hydrothermal vents the microbial ecosystems may in part be supported by the availability of metastable amino acids through heterotrophic metabolism. Published by Elsevier Ltd.

The potential for chemical evolution of complex organic molecules such as peptides in hydrothermal environments requires the persistence of the component amino acids under such conditions. Here, we show experimentally that the redox state (activity of H-2) of the aqueous fluids plays a key role in the stability of glutamic acid during hydrothermal processes. The results demonstrate that highly reducing redox conditions imposed by elevated concentrations of dissolved H-2 suppresses the oxidative decomposition of glutamic acid at elevated temperatures. Our experimental data support proposals that amino acids may persist, albeit metastably, under geochemically relevant hydrothermal conditions. The reduced nature of deep-sea vent fluids might have been a critical parameter in sustaining the needed ingredients for the origin of life on the early Earth, and may currently play a role in facilitating the persistence of biomolecules supporting heterotrophic microbial communities in modern near-seafloor hydrothermal environments. Published by Elsevier B.V.