Sumner D Y
Geology Department, University of California, Davis, CA 95616.
Astrobiology. 2001 Spring;1(1):57-70. doi: 10.1089/153110701750137431.
Analysis of the carbon isotopic composition of carbonates is a valuable tool for studying microbial processes and looking for evidence of life. Microbial fixation of CO2 and conversion of organic carbon to CO2 can produce measurable delta 13C shifts in a microbial mat environment. Diffusion modeling demonstrates that substantial isotopic shifts can develop within the mat and in the diffusion boundary layer in the fluid when CO2 fixation is rapid and prolonged for several hours. Carbonates that precipitate during rapid CO2 fixation can preserve these microbially produced isotopic shifts. However, continued precipitation during intervals when respiration dominates or after the cessation of active microbial growth commonly dilutes autotrophic isotopic signatures. Thus, preserved isotopic signatures rarely reflect the magnitude of isotopic shifts within the mat. Interpretation of observed isotopic shifts in microbial mat carbonate depends on fully characterizing ambient delta 13C and eliminating other origins for isotopic shifts. The carbon isotopic composition of reservoirs can vary substantially, both on Earth and on other planets. Characterizing the reservoir composition and any changes through time is critical to evaluating microbially induced shifts. In addition, careful evaluation of non-microbial causes for shifts in isotopic composition is essential for a reliable interpretation. Complicating processes include recrystallization, calcite precipitation over extended periods of time, variable precipitation rates and water chemistry, and mixing of carbonates having different isotopic signatures.
碳酸盐碳同位素组成分析是研究微生物过程和寻找生命证据的宝贵工具。在微生物席环境中，微生物对二氧化碳的固定以及有机碳向二氧化碳的转化会产生可测量的δ13C变化。扩散模型表明，当二氧化碳固定迅速且持续数小时时，微生物席内部以及流体中的扩散边界层会出现显著的同位素变化。在快速二氧化碳固定过程中沉淀的碳酸盐可以保留这些微生物产生的同位素变化。然而，在呼吸作用占主导的时间段内或活跃微生物生长停止后继续沉淀，通常会稀释自养同位素特征。因此，保存下来的同位素特征很少能反映微生物席内部同位素变化的幅度。对微生物席碳酸盐中观察到的同位素变化的解释取决于全面表征环境δ13C并排除同位素变化的其他来源。储层的碳同位素组成在地球和其他行星上都可能有很大差异。表征储层组成及其随时间的任何变化对于评估微生物引起的变化至关重要。此外，仔细评估同位素组成变化的非微生物原因对于可靠的解释至关重要。复杂的过程包括重结晶、长时间的方解石沉淀、可变的沉淀速率和水化学性质，以及具有不同同位素特征的碳酸盐的混合。
