Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments:: implications for detection of microbial CO2 signatures using δ13C-DIC

Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO2. There is preferential dissolution of Ca (CO3)-C-12 during hydrolysis, resulting in delta(13)C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as epsilon(carb). epsilon(carb) is greater on average for glacial sediments ((__)17.4%(0)) than for calcium carbonate ((__)7.8%(0)) for the < 63 mum size fraction, a sediment concentration of 5 g L-1 and closed system conditions at 5degreesC. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on delta(13)C-DIC at higher CaCO3,: water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO, at sediment concentrations of 5 g L-1. Atmospheric CO, dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least similar to 6 h after the start of the experiment at 5degreesC. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the delta(13)C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO2 signatures in DIC from delta(13)C-DIC data. Copyright (C) 2004 Elsevier Ltd.