Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Hydrothermal vent fluids from Middle Valley, a sediment-covered vent field located on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186-281 °C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH₃ and organic compounds (C₁-C₄ alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C₁/(C₂ + C₃) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low temperature regions during recharge) as well as from thermogenic processes in higher temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and confirm that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower temperature fluids.     

Mobilization and transport of organic compounds from reservoir rock and caprock in geological carbon sequestration sites

Supercritical CO₂ (scCO₂) is a good solvent for organic compounds such as benzene, toluene, ethyl-benzene, and xylene (BTEX), phenols, and polycyclic aromatic hydrocarbons (PAHs). Monitoring results from geological carbon sequestration (GCS) field tests have shown that organic compounds are mobilized following CO₂ injection. Such results have raised concerns regarding the potential for groundwater contamination by toxic organic compounds mobilized during GCS. Knowledge of the mobilization mechanism of organic compounds and their transport and fate in the subsurface is essential for assessing risks associated with GCS. Extraction tests using scCO₂ and methylene chloride (CH₂Cl₂) were conducted to study the mobilization of volatile organic compounds (VOCs, including BTEX), the PAH naphthalene, and n-alkanes by scCO₂ from representative reservoir rock and caprock obtained from depleted oil reservoirs and coal from an enhanced coal-bed methane recovery site. Results showed that the extent of mobilization for the organic compounds was a function of the source rock. In fate and transport sand column experiments, moisture content was found to have an important influence on the transport of the organic compounds. In dry sand columns the majority of the compounds were retained in the column except benzene and toluene. In wet sand columns the mobility of the BTEX was much higher than that of naphthalene. Based upon the results determined for the reservoir rock, caprock, and coal samples studied here, the risk to aquifers from contamination by organic compounds appears to be relatively low; however, further work is necessary to fully evaluate the risks.     

Volatile C1-C7 organic compounds in surface sediments from Walvis Bay

C₁-C₇ volatile organic compounds were analyzed in three gravity cores taken from Walvis Bay shelf. The compounds detected included alkanes (methane, ethane, propane, i- and n-butane, and i- and n-pentane, and heptane), alkenes (2-methyl-2-butene, dimethylcyclopentenes, cyclohexene), oxygen containing compounds (2- and 3-methylfuran, 2,5-dimethylfuran, 2- and 3-methylbutanal and 3-pentanone), sulfur compounds (dimethylsulfide, thiophene, 2- and 3-methylthiophene) and aromatic compounds (benzene and toluene). In situ biological and low temperature chemical (less than 15°C) formation processes are proposed, possibly from marine terpene precursors. Subsequent to this work, these compounds were found to be widely distributed in surface gravity cores from other areas. Many of these compounds do not survive deeper burial. Furans, ketocompounds, and alkenes are generally not found in more than trace quantities in deeper (?10m subbottom) DSDP cores we have examined from other areas.     

Volatile C1C8 organic compounds in sediments from the Peru upwelling region

Volatile C₁C₈ organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C₄C₇ alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C₁, C₂, C₃, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C₄C₆ alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C₁C₇ alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).     

A novel procedure to detect low molecular weight compounds released by alkaline ester cleavage from low maturity coals to assess its feedstock potential for deep microbial life

In recent years the widespread occurrence of microorganisms was demonstrated in deep marine and terrestrial sediments. With this discovery inevitably the question of the potential carbon and energy sources for this deep subsurface microbial life arises. In the current study a new method for the investigation of low molecular weight (LMW) organic acids linked to the kerogen matrix is presented. These LMW organic acids form a potential feedstock for deep microbial populations. Twelve coal samples of different maturity (vitrinite reflectance (R₀) of 0.28–0.80%) from several coal mines on the North and South Island of New Zealand (NZ) were examined to assess the amount of bound LMW organic acids. Formate, acetate and oxalate were detected in significant amounts whereas the amounts of these compounds decrease with increasing maturity of the coal sample. This decrease of LMW organic acids mainly correlates to the phase of diagenetic alteration of the organic matter characterized by the release of oxygen containing compounds. Concomitantly, it coincides with temperature conditions assumed to be still compatible with microbial life in the deep subsurface. First assessments of the feedstock potential and generation rates of LMW organic acids indicate that the NZ coals investigated exhibit the potential to feed deep terrestrial microbial life with appropriate substrates over geological time spans.     

Hydrogen isotopic fractionation of petroleum hydrocarbons during vaporization: implications for assessing artificial and natural remediation of petroleum contamination

Compound-specific H isotope analysis has been used to monitor bioremediation of petroleum hydrocarbons. However, the success of this approach requires a full evaluation of the isotopic effects resulting from evaporation, because light petroleum hydrocarbons undergo both biodegradation and evaporation under natural conditions. The authors determined the H isotope fractionation of common volatile petroleum hydrocarbons, including the C₁₀–C₁₄ n-alkanes, MTBE (tert-butyl methyl ether), and BTEX (benzene, toluene, ethylbenzene, p-xylene and o-xylene) during progressive vaporization under simulated experimental conditions. A decrease in δD values for n-alkanes of up to 33.3‰ and up to 44.5‰ for BTEX compounds when 99% of these substances had evaporated was observed. The results also show that H isotope fractionation increases with n-alkane chain length. Such fractionation patterns are interpreted in terms of competition between the decreased intermolecular binding energy in D-enriched species, and the isotope effect due to the mass difference. In contrast to hydrocarbons, methanol and ethanol show H isotopic enrichment during vaporization, indicating that H-bonding, when present in organic molecules, plays a controlling role on the vapor pressure of different isotope species.     

In situ measurement of dissolved chloride in high temperature hydrothermal fluids

The ability to continuously monitor chemical properties of hydrothermal vent effluents for extended periods of time is essential to understanding dynamic processes responsible for the temporally variable nature of mid-ocean ridge hydrothermal systems. Although instruments do exist for some parameters, there has been no sensor capable of measuring the chloride concentration, an indicator of possible phase separation, on a real-time and long-term basis. In this article, we discuss the construction of a novel instrument which measures solution resistance as a proxy for chloride concentration. The sensor consists of four gold electrodes embedded in a cylindrical ZrO₂ ceramic housing. It has been successfully deployed in several high temperature vents at the Main Endeavour Field (MEF) on the Juan de Fuca ridge in the NE Pacific, and calibrated under simulated hydrothermal conditions ranging up to 380 °C and 300 bar. The in situ data clearly demonstrate a tidal influence on the effluent from some high temperature vents possibly relating to a subsurface mixing process involving non-seawater end-members. Non-tidal changes are used to constrain the sequence and type of controls operating on fluids circulating within the subsurface.     

Deep-sea smokers: windows to a subsurface biosphere?

Since the discovery of hyperthermophilic microbial activity in hydrothermal fluids recovered from "smoker" vents on the East Pacific Rise, the widely accepted upper temperature limit for life (based on pure culture data) has risen from below the boiling point of water at atmospheric pressure to approximately 115 degrees C. Many microbiologists seem willing to speculate that the maximum may be closer to 150 degrees C. We have postulated not only higher temperatures than these (under deep-sea hydrostatic pressures), but also the existence of a biosphere subsurface to accessible seafloor vents. New geochemical information from the Endeavour Segment of the Juan de Fuca Ridge indicative of subsurface organic material caused us to re-examine both the literature on hyperthermophilic microorganisms cultured from deep-sea smoker environments and recent results of microbial sampling efforts at actively discharging smokers on the Endeavour Segment. Here we offer the case for a subsurface biosphere based on an interdisciplinary view of microbial and geochemical analyses of Endeavour smoker fluids, a case in keeping with rapidly evolving geophysical understanding of organic stability under deep-sea hydrothermal conditions.     

Origin of organic matter in early solar system—V. Further studies of meteoritic hydrocarbons and a discussion of their origin

The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C₈ are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C₈, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH₄/C₂H₆ ratio is greater than 30, possibly greater than 700. Isoprenoids from C₁₇ to C₂₀ occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigenous.     

Metallogenesis in the Harlech Dome,North Wales: A fluid inclusion interpretation

A regional fluid inclusion study of Cu-Au (+Zn-Pb) mineralisation in the Harlech Dome area, North Wales, gives support to the concept of two distinct metallogenic episodes. The inclusion assemblages associated with the porphyry copper mineralisation at Coed-y-Brenin are consistent with a genetic model of early potassic-propylitic alteration overprinted by later phyllic alteration. High salinity fluids, normally characteristic of potassic alteration, are confined to the host rock quartz. The meteoric/hydrothermal system is closely linked to the emplacement of late-Cambrian diorites. Integrated fluid inclusion and mineralogical studies of the Gold-belt veins suggest that the mineralising fluids were probably dehydration waters released from weakly metamorphosed Cambrian and perhaps Precambrian sediments during hydraulic fracturing in a tensional zone at the close of the Caledonian orogeny. Localisation of economic concentrations of gold in veins at the level of the Clogau Formation is ascribed to a destabilisation of metal complexes caused by a change in fluid buffering from a pyrite-magnetite assemblage in the Lower Cambrian sediments to a pyrite-pyrrhotite-graphite assemblage in the Upper Cambrian sediments. Veining associated with the Coed-y-Brenin porphyry copper deposit and related breccia pipes can be distinguished from the copper-gold veins of the coextensive Dolgellau Gold-belt by the presence in the former of inclusions notably richer in CO₂. Furthermore the Gold-belt fluids have a distinctive low CO₂/CH₄+N₂+H₂ ratio.     

The lipid chemistry of an interfacial sediment from the Peru Continental Shelf: Fatty acids,alcohols, aliphatic ketones and hydrocarbons

A sample of the sediment-water column interface which lies on the continental shelf under the Peru upwelling regime, has been examined for fatty acids, fatty alcohols, ketones and hydrocarbons. Fatty acids were the most abundant compound class, ranging from C₁₂-C₂₄, with 16:0 as the major component (765.5 μg/g dry sediment). The alcohols were dominated by 3,7,11,15-tetramethylhexadeca-2-en-ol (phytol), with even-chain n-alcohols in the range C₁₄-C₂₀. The ketones consisted of C₃₇-C₃₉ di- and tri-unsaturated alken-2-ones and alken-3-ones. Both alkanes and alkenes were present in the hydrocarbon fraction; the alkanes ranging from C₁₃ — C₂₀ and comprising both straight chain and isoprenoid compounds; the alkenes consisting of isomeric pairs of C₂₅ branched trienes and tetraenes. The data indicate that the organic content has been contributed very largely from marine sources (probably mainly from phytoplankton and bacteria), showing little terrigenous influence. The presence of labile compounds such as polyunsaturated fatty acids (with two to six double bonds), implies that the sediment has undergone very little diagenetic alteration, and the lipids are probably largely unchanged from the state in which they actually reached the sediment. They may therefore serve as a useful baseline in assessing diagenesis in older sediments, where diagenetic transformations are more advanced.     

Occurrences and distributions of branched alkylbenzenes in the Dongsheng sedimentary uranium ore deposits,China

A series of branched alkylbenzene ranging from C₁₅ to C₁₉ with several isomers (2–5) at each carbon number were identified in sediments from the Dongsheng sedimentary uranium ore deposits, Ordos Basin, China. The distribution patterns of the branched alkylbenzenes show significant differences in the sample extracts. The branched alkylbenzenes from organic-rich argillites and coals range from C₁₅ to C₁₉ homologues, in which the C₁₇ or C₁₈ dominated. On the other hand, the C₁₉ branched alkylbenzenes dominated in the sandstone/siltstone extracts. The obvious differences of the branched alkylbenzene distributions between the uranium-host sandstones/siltstones and the interbedded barren organic-rich mudstones/coals probably indicate their potential use as biological markers associated with particular depositional environments and/or maturity diagenetic processes. Possible origins for these branched alkylbenzenes include interaction of simple aromatic compounds with, or cyclization and aromatization reactions of, these linear lipid precursors such as fatty acids, methyl alkanoates, wax esters or alkanes/alkenes that occur naturally in carbonaceous sediments. The possible simple aromatic compounds may include substituted benzenes, functionalized compounds such as phenols that are bound to kerogen at the benzyl position, and phenols that are decomposition products derived from aquatic and terrestrial sources. The distributions of methyl alkanoates and n-alkanes were found to be different between organic-rich mudstone/coal and sandstone/siltstone. From this result, it can be concluded that such differences of the alkylbenzene distributions were mainly resulting from the differences of organic precursors, although maturity effect and radiolytic alteration cannot be completely excluded.     

Geochemistry of C25 and C30 biogenic alkenes in sediments of the narragansett bay estuary

The distributions of a series of structurally related C₂₅ and C₃₀ biogenic alkenes in sediments of the Narragansett Bay estuary have been determined. The suite of alkenes detected differs both quantitatively and qualitatively from those previously reported in other estuanne and coastal regions. Four C₂₅ mono- and dienes and one C₃₀ diene comprise 73–91% of the total alkenes in all surface (upper 2.5–5 cm) sediments analyzed. However, significant geographic variations exist in the relative abundance of these five compounds throughout the estuary. A comparison of alkene concentrations with δ¹³C of the bulk sedimentary organic matter has shown that the geographic variations of some alkenes reflect the distribution of marine organic matter, suggesting a marine source for these compounds. The distributions of other alkenes are not similarly correlated. In particular, concentrations of the C₃₀ diene are relatively constant and exhibit no dependence on the origin of organic matter in these sediments. This distribution implies an in situ production of this alkene throughout the estuary. Analysis of several sediment cores reveals that alkene concentrations are generally highest at the surface and decrease to low, constant values within the upper 25 cm. An exception is the subsurface concentration of one C₂₅ diene, which exhibits an increase at the same depth in two separate upper bay cores.     

Reactivity of organic matter in aquifer sediments: geological and geochemical controls

Reduction rates in aquifers are commonly carbon limited, but little is known about the molecular composition and degradability of sedimentary organic matter (SOM) in aquifer sediments. The composition, source and degradation status of SOM in aquifer sediments of fluvio-glacial (Pleistocene) and shallow marine (Pliocene) origin, were determined using flash pyrolysis-gas chromatography/mass spectrometry. Incubation experiments (106 d) were used to assess the reactivity of SOM towards molecular oxygen. A dominance of lignin-derived components and long chain odd-over-even predominant alkanes indicate that terrestrial higher land plants were the main source of SOM even in the shallow marine sediments, while bacterial lipid-derived hopanoids and iso- and anteiso-C₁₅ and C₁₇ fatty acids indicate a minor contribution of microbial biomass. No compositional difference was observed between SOM present in the fine (<63 μm) and coarse fraction (63-2000 μm). A significant part of SOM was not present as low-molecular-weight compounds but was macromolecularly bound. For the fluvio-glacial sediments, a relatively higher abundance of resistant macromolecular compounds was in agreement with stronger signs of aerobic lignin, alkane and hopanoid oxidation. The more degraded status of SOM in the fluvio-glacial sediments was consistent with their significantly lower SOM mineralization (2-6%) during incubation, as compared with the shallow marine sediments (9-14%). The reactivity towards oxygen of SOM was controlled by the extent of past aerobic oxidation. Not the age of SOM, but the extent of oxygen exposure during syn- and postdepositional conditions seems most important in affecting the degradation status of SOM in aquifer sediments and thus their ability to reduce oxidants.     

Composition and origin of hydrothermal petroleum and associated lipids in the sulfide deposits of the Rainbow field (Mid-Atlantic Ridge at 36°N)

The lipid components in hydrothermal sulfide deposits from the Rainbow vent field (Mid-Atlantic Ridge at 36°N) were studied by gas chromatography/mass spectrometry. The Rainbow vent field is one of two known active hydrothermal systems related to abyssal circulation, where high-temperature fluids are formed during serpentinization of ultrabasic crustal rocks. The major amount of the extractable organic matter from the sulfides consists of normal and branched alkanes, UCM, PAHs, terpenoids, and fatty acids. The branched alkanes are comprised of unique gem-diethylalkane series, possibly from sulfide oxidizing bacteria, and biphytanes from archaea. The characteristic lipid and biomarker compounds found in the hydrothermal samples support a predominantly biological origin of the bitumens from the thermal transformation of the biomass of microorganisms (bacteria and archea) and minor macrofauna of this vent field. A search for molecular evidence for abiogenic thermocatalytic synthesis of organic compounds was negative. However, methane in the hydrothermal fluids and possibly a minor amount of the alkanes in the sulfides may be of an abiogenic origin in the Rainbow vent field.     

Qualitative fatty acid and n-alkane stratigraphy of the Lake Turkana Basin,Kenya

In order to improve understanding of the stratigraphy of the Lake Turkana Basin, one of the important sites in the evolution of early man, this study evaluates the usefulness of organic biological marker compounds, n-alkanes and fatty acids, for correlation of isolated sedimentary strata.Eighty-five paleosol samples were collected from well-defined sedimentary horizons in two regions (Area 103 and Area 130) of the Koobi Fora area of Lake Turkana. Results indicate that most of the organic matter present was derived from terrestrial plant waxes. In sediments where extensive diagenesis has occurred, microbial input of organic matter may have been substantial. Algae were either not an important source of organic matter, or their marker compounds have been removed or altered by degradative processes.The fate of the original paleosol organic matter has been governed to some extent by weathering processes, especially in Area 130. Weathering decreased the amount of extractable lipids, particularly fatty acids and the low molecular weight alkanes (C₁₇C₂₀); produced or retained relatively large amounts of alkanes greater than C₂₁ within a unimodal distribution; and lowered CPI values. Consequently, stratigraphic correlation by unique alkane and fatty acid distributions has been confined to short distances (many meters).Both n-alkanes and fatty acids have been retained better by association with clay minerals than by sand matrices. The alkane distribution of sandstones differs from that of clay organics in having a narrower carbon chain length distribution and lower CPI values. In Area 103, where weathering was less severe, compositional variations with stratigraphic position indicate that lipid material has been retained within each of the facies examined.     

Volatile organic compounds in Quaternary sediments

The distribution of volatile organic compounds in surface sediments varies widely with the depositional environment. Oxygen and sulfur compounds, alkenes and arenes, are more abundant than alkanes in strongly reducing sediments such as those from around Walvis Bay, West Africa. Among the compounds identified by GC-MS were toluene, benzene, 2-methyl-2-butene, 3,5-dimethylcyclopentene, 3-menthylfuran, 3-methylfuran, 2-methylbutanol, 3-methylbutanol, 3-pentanone, 2-methylthiophene, 3-methylthiophene and dimethylsulfide. Some of these compounds have the isoprene carbon skeleton.In a less reducing environment such as the Arabian Sea-Persian Gulf area, a more diverse alkene assemblage was observed including ethylene, propylene, pentenes, hexenes, cyclohexene, methylpentenes and dimethylcyclopentenes. Also, the sediments contained a few very specific alkane structures. The gem-dimethyl hydrocarbons (neopentane, neohexane, and neoheptane) were dominant among the branched hydrocarbons. The high concentration of volatile alkenes and oxygen and sulfur compounds is not observed in deep sea drilling samples at depths greater than about 100 m.     

Origin of the Kupferschiefer polymetallic mineralization in Poland

The Kupferschiefer ore series, between the Lower Permian (Rotliegendes) terrestrial redbeds/volcanics and the Upper Permian (Zechstein) marine sequence, is developed as dark-grey organic matter-rich and metal sulphide-containing deposits (reduced zone) and as red-stained organic matter-depleted and iron oxide-bearing sediments (oxidized zone?=?Rote Fäule). The transition zone from oxidized to reduced rocks occurs both vertically and horizontally. This zone is characterized by sparsely disseminated remnant copper sulphides within hematite-bearing sediments, replacements of copper sulphides by iron oxides and covellite, and oxide pseudomorphs after framboidal pyrite. These textural features and copper sulphide replacements after pyrite in reduced sediments imply that the main oxide/sulphide mineralization postdated formation of an early-diagenetic pyrite. Hematite-dominated sediments locally contain enrichments of gold and PGE. The Kupferschiefer mineralization resulted from upward and laterally flowing fluids which oxidized originally pyritiferous organic matter-rich sediments to form hematitic Rote Fäule areas, and which emplaced base and noble metals into reduced sediments. It is argued that long-lived and large-scale lateral fluid flow caused the cross-cutting relationships, expansion of the hematitic alteration front, redistribution of noble metals at the outer parts of oxidized areas, and the location of copper orebodies directly above and around oxidized and gold-bearing areas. The Rote Fäule may be a guide to favourable areas for both the Cu-Ag and new Au-Pt-Pd Kupferschiefer-type deposits.     

Chemical characterization of microbial-dominated soil organic matter in the Garwood Valley, Antarctica

Despite its harsh environmental conditions, terrestrial Antarctica contains a relatively large microbial biomass. Natural abundance carbon and nitrogen stable isotope signatures of organic materials in the dry valleys indicate mixed provenance of the soil organic matter (SOM) with varying proportions of contributions from lichens, mosses, lake-derived algae and cyanobacteria. Here we employed two complementary analytical techniques, biomarker measurements by gas chromatography/mass spectrometry and solution-state ¹H nuclear magnetic resonance spectroscopy, to provide further information at a molecular-level about the composition and possible source of SOM in the Garwood Valley, Antarctica. The predominance of branched alkanes and short-chain lipids in the solvent extracts indicates that the primary contribution to the SOM was microbial-derived. Chemical structures in the NaOH extracts from soils were also dominated by amide, peptides, and a CH₃-dominating aliphatic region that were characteristic of microbial signatures. Furthermore, the SOM in the Garwood Valley contained compounds that were different from those in the cyanobacteria-dominated mat from a nearby lake (including monoethyl alkanes and enriched side-chain protons). This observation suggests that easily degradable carbon sources from the nearby lake did not dominate the SOM, which is consistent with a fast turnover of the mat-derived organic matter found in the valley. This study highlights the important role of native soil microbes in the carbon transformation and biogeochemistry in terrestrial Antarctica.     

Experimental study of the hydrothermal reactivity of organic acids and acid anions: II. Acetic acid, acetate, and valeric acid

Organic acids and acid anions occur in substantial concentrations in many aqueous geologic fluids and are thought to take part in a variety of geochemical processes ranging from the transport of metals in ore-forming fluids to the formation of natural gas to serving as a metabolic energy source for microbes in subsurface habitats. The widespread occurrence of organic acids and their potential role in diverse geologic processes has led to numerous experimental studies of their thermal stability, yet there remain substantial gaps in our knowledge of the factors that control the rates and reaction pathways for the decomposition of these compounds under geologic conditions. In order to address some of these uncertainties, a series of laboratory experiments were conducted to examine the behavior of organic acids and acid anions under hydrothermal conditions in the presence of minerals. Reported here are results of experiments where aqueous solutions of acetic acid, sodium acetate, or valeric acid (n-pentanoic acid) were heated at 325°C, 350 bars in the presence of the mineral assemblages hematite + magnetite + pyrite, pyrite + pyrrhotite + magnetite, and hematite + magnetite. The results indicate that aqueous acetic acid and acetate decompose by a combination of two reaction pathways: decarboxylation and oxidation. Both reactions are promoted by minerals, with hematite catalyzing the oxidation reaction while magnetite catalyzes decarboxylation. The oxidation reaction is much faster, so that oxidation dominates the decomposition of acetic acid and acetate when hematite is present. In contrast to previous reports that acetate decomposed more slowly than acetic acid, we found that acetate decomposed at slightly faster rates than the acid in the presence of minerals. Although longer-chain monocarboxylic acids are generally thought to decompose by decarboxylation, valeric acid appeared to decompose primarily by “deformylation” to 1-butene plus formic acid. Subsequent decomposition of 1-butene and formic acid generated a variety of short-chain (≤C₄) hydrocarbons and moncarboxylic acids as well as CO₂. Valeric acid decomposition proceeded more rapidly (by a factor of 2) in the presence of hematite-magnetite-pyrite than with the other mineral assemblages, with the greater reaction rate apparently attributable to the effects of fluid chemistry. Valeric acid was observed to decompose at a substantially faster rate than acetic acid under similar conditions. The results suggest that decomposition of aqueous monocarboxylic acids may make a significant contribution to the conversion of petroleum to light hydrocarbons in natural gas and thermal fluids.