Abstract: Organic molecules preserved in sedimentary rocks are generally regarded as robust recorders of the biology and chemistry of ancient environments. Here I will outline our recent efforts to extract, quantify, and measure the isotopic composition of compounds derived from chlorophyll molecules in organic-rich rocks. Their nitrogen stable isotope compositions are of particular interest because they record fingerprints of ancient ocean biogeochemistry that are not found in modern seas. Hypotheses for these observations include enhanced ammonium recycling and redox control on the dominant nitrogen fixing enzyme, both of which have implications of for the evolving environments and metabolic pathways of microbial life. I will further discuss the prospect for extending these methods into the Precambrian.