Mind the gap: non-biological processes contributing to soil CO2 efflux

Received 15 April 2014 and accepted 14 November 2014Widespread recognition of the importance of soil CO2 efflux as a major source of CO2 to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO2 efflux focusing on processes of soil CO2 production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO2 efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO2 degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO2 transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO2 efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO2 efflux. Traditionally soil CO2 efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO2 concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO2 sources to quantify this flux on a global scale. Biogeochemical models should include biological and nonbiological CO2 production processes before we can predict the response of soil CO2 efflux to climate change. Improving our understanding of the processes involved in soil CO2 efflux should be a research priority given the importance of this flux in the global carbon budget.The Ministry of Economy and Competitiveness of Spain, Project CGL2011-24748 supported this work.Peer reviewe

Microbial generation of economic accumulations of methane within a shallow organic-rich shale

ALTHOUGH methane of bacterial origin is ubiquitous in marine and feshwater sediments, economic accumulations of bacterial gases occur mainly at depths of several kilometres in Tertiary basins that had high sedimentation rates(1'2). Here we present an integration of geochemical and isotopic data from gas and water extracted from the Upper Devonian Antrim shale, along the northern margin of the Michigan basin, which demonstrates that significant volumes of bacterial gas have been generated in organic-rich shales at depths of less than 600 metres. The Antrim shale is mainly a self-sourced reservoir, in contrast to conventional gas deposits that have migrated from a source to a reservoir, and has become one of the most actively exploited gas reservoirs(3) in the United States. The gas-forming processes operating at shallow depths in the Antrim shale are noe unique(4) and an understanding of these processes should lead to the identification acid development of other economic, non-conventional gas deposits around the world.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62808/1/383155a0.pd

Geochemistry of the magmatic-hydrothermal fluid reservoir of Copahue volcano (Argentina): insights from the chemical and isotopic features of fumarolic discharges

This study present the chemical (inorganic and organic) and isotopic compositions (d13C-CO2, d15N, 3He/4He, 40Ar/36Ar, d13C-CH4, dD-CH4, and dD-H2O and d18O-H2O) of gases from fluid discharges located at the foot of Copahue volcano. Gas samples were collected during 6 campaigns carried out from 1976 to 2012. Gas composition is typical of hydrothermal fluids from volcanic areas, since it consists of dominant CO2 and relatively high concentrations of H2S, H2, CH4 and N2. The isotopic ratios of He are the highest observed for a Southern American volcano to date (R/Ra up to 7.94). This feature is not common for gases from a classic arc-like setting, and is possibly related to an extensional regime subdued to asthenospheric thinning. The CO2/3He ratios (from 1.4 to 8.8×109) slightly exceeding that of MORB gases, and the d15N values (+5.3 to +5.5 ? vs. air) point to an occurrence of an additional crustal source for CO2 and N2. Gas discharges of the northern sector of the volcanic edifice are likely produced by mixing of hydrothermal gases with fluids from a shallow source permeating through local fault systems. Gas geothermometry based on chemical reactions characterized by slow kinetics, such as those involving the CO2-CH4 redox pair, are quenched at temperatures (~260 °C) and redox conditions [log(XH2/XH2O) = -2.8)] consistent with those measured in the goethermal wells. On the contrary, the C3H6-C3H8 pair, H2 and CO tend to re-adjust at decreasing temperatures and more oxidizing conditions [log(XH2/XH2O) ≤ -3.4] in the uprising vapor phase. The hydrothermal reservoir is mainly recharged by meteoric water whose isotopic signature is modified by water-rock interactions. The N2/He ratios measured in 2006-2007 were significantly lower than those of 2012, possibly due to variations of N2-bearing species in sediments interacting with the magmatic source. Considering that the R/Ra values of the 2006-2007 period were significantly higher than those measured in 2012, such compositional variation may also be explained by the injection of fresh N2and 3He-rich magma that triggered the 2000 eruption. This hypothesis, although speculative since no geochemical data of fumaroles are available from 1997 to 2006, implies that a geochemical monitoring of inert gas compounds discharged from the hydrothermal emissions could be used to detect the occurrence at depth of injections of new magma batches.Fil: Tassi, Franco. Università degli Studi di Firenze; ItaliaFil: Agusto, Mariano Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Vaselli, Orlando. Università degli Studi di Firenze; ItaliaFil: Chiodini, Giovanni. Istituto Nazionale di Geofisica e Vulcanologia; Itali