Sea spray, trace elements, and decomposition patterns as possible constraints on the evolution of CH4 and CO2 concentrations and isotopic signatures in oceanic ombrotrophic bogs

摘要

Ombrotrophic, oceanic bogs in southern Patagonia have not yet been studied with respect to ongoing belowground organic matter decomposition. To obtain such information we analyzed three sites differing in precipitation and sea spray input and quantified concentration patterns and C-12/13 isotopic composition of CO2 and CH4 and iron, sulfur and trace metal contents that can influence decomposition. Concentrations of CO2 and CH4 increased with depth and reached 4,000-6,000 A mu mol L-1 of CO2 and 500-1,400 A mu mol L-1 of CH4. Chamber surface fluxes ranged from 40 to 62 mmol m(-2) day(-1) for CO2 and were not detectable for CH4 ( 0.2 mmol m(-2) day(-1)). Lowest gaseous C concentrations and fluxes occurred at the driest site under high sea spray input, which was accompanied by a higher degree of decomposition. Isotope fractionation factors alpha(c) ranged from 1.047 to 1.077 and suggested a predominance of hydrogenotrophic methanogenesis. The lower CH4 concentrations at one particular site may have been caused by a number of processes but isotope mass balances indicated a preferential loss of CH4 at all sites, especially at the site of lowest CH4 concentrations. Low CH4 concentrations were found under a high sea spray input and higher sulfate and reduced inorganic sulfur contents, suggesting a potential for attenuation of methanogenesis by sulfate reduction. All sites were characterized by very low Nickel concentrations of mostly 15 nmol L-1 and low concentrations of other essential trace elements that may further inhibit methanogenesis but also methanotrophy. The Patagonian sites fell within the reported range of CO2 and CH4 concentrations and depth patterns, and isotopic composition of the gases at northern sites despite different vegetation composition and seemingly lower surface fluxes. Fairly high sulfate and low trace element concentrations due to differences in atmospheric deposition may locally modify the decomposition patterns.