Stromatolites of the 3.00 Ga Nsuze Group and the 3.33 Ga Mendon Formation (both in South Africa) provide potential sources of information about microbial life bracketing one of the oldest known intervals of continent formation and atmospheric evolution. We are using x-ray fluorescence micrscopy to map the distributions of trace clastically-derived and/or redox-active elements in stromatolitic sediments from each unit in order to interpret growth processes and chemical paleoenvironments. Results will contribute to our understanding of early microbial evolution and ecology during a time of major environmental change.