Temperature-dependent oxygen and carbon isotope fractionations of biogenicsiderite

Isotopic compositions of biogenic iron minerals may be used to infer enviro nmental conditions under which bacterial iron reduction occurs. The major g oal of this study is to examine temperature-dependent isotope fractionation s associated with biogenic siderite (FeCO3). Experiments were performed by using both mesophilic (< 35 degreesC) and thermophilic (> 45 degreesC) iron -reducing bacteria. In addition, control experiments were performed to exam ine fractionations under nonbiologic conditions. Temperature-dependent oxygen isotope fractionation occurred between biogeni c siderite and water from which the mineral was precipitated. Samples in th ermophilic cultures (45-75 degreesC) gave the best linear correlation, whic h can be described as 10(3) ln alpha (sid-wt) = 2.56 X 10(6) T-2 (K) + 1.69 . This empirical equation agrees with that derived from inorganically preci pitated siderite by Carothers et al. (1988) and may be used to approximate equilibrium fractionation. Carbon isotope fractionation between biogenic si derite and CO2, based on limited data, also varied with temperature and was consistent with the inorganically precipitated siderite of Carothers et al . (1988). These results indicate that temperature is a controlling factor f or isotopic variations in biogenic minerals examined in this study. The temperature-dependent fractionations under laboratory conditions, howev er, could be complicated by other factors including incubation time and con centration of bicarbonate. Early precipitated siderite at 120-mM initial bi carbonate tended to be enriched in O-18. Siderite formed at < 30 mM of bica rbonate tended to be depleted in O-18. Other variables, such as isotopic co mpositions of water, types of bacterial species, or bacterial growth rates, had little effect on the fractionation. In addition, siderite formed in ab iotic controls had similar oxygen isotopic compositions as those of biogeni c siderite at the same temperature, suggesting that microbial fractionation s cannot be distinguished from abiotic fractionations under conditions exam ined here. Copyright (C) 2001 Elsevier Science Ltd.