Geochemical signals of biological activity

There are two main types of geochemical proxy used to identify former life. These are biomarker molecules and isotopic fractionations. Biomarker molecules are specific molecules preserved in sediments as molecular fossils, which are unambiguous indicators of their biological precursors. Some of the most successful 

Most biological reaction pathways discriminate against heavy carbon 13C and become enriched in the lighter stable isotope 12C. So for example in photosynthesis, 13C is discriminated against during the fixation of C02 so that plants and autotrophic microbes tend to 

The isotopic fingerprinting of photosynthesis. Photosynthesis is currently an important means of fixing inorganic carbon and is used by plants, algae, bacteria, and Archaea. A particularly important enzyme (catalyst) in this process is Rubisco, the official shortened name for a molecule with a much more intricate systematic name. The reactions which take place in this process operate according to the Calvin-Benson Cycle and lead to isotopic fractionations of up to 30%o. 

C3 pathway Green plants and algae Rubisco (form ij -30 

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