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 12 CO3 during hydrolysis, resulting in 13 C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as carb. carb is greater on average for glacial sediments (17.4‰) than for calcium carbonate (7.8‰) for the 63 m size fraction, a sediment concentration o f5gL 1 and closed system conditions at 5°C. 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 13 C-DIC at higher CaCO3: water ratios and is significant during at least the firs t6ho fcarbonate dissolution driven by atmospheric CO2 at sediment concentrations o f5gL 1 . Atmospheric CO2 dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the 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 13 C-DIC data. Copyright © 2004