Photosynthetic pigments in sediments: development of applications in archaeology and compound-specific radiocarbon analysis

Photosynthetic pigments derived from oxygenic aquatic photoautotrophs are biosynthesised from dissolved carbon dioxide that reflects atmospheric concentrations of radiocarbon (14C). Sedimentary pigment signatures are not influenced by a terrestrial signal as terrestrial photosynthetic pigments are overwhelmingly destroyed by photo-oxidation. These properties have been exploited by this study to reveal the presence of archaeological water features and to radiocarbon date the timing of a geochemically significant event. A new approach for identifying archaeological structures suggested to represent former aquatic features has been developed. HPLC and LC-MSn analysis of sediment extracts from several suspected water features revealed the presence of photosynthetic pigment derivatives, thus providing evidence of the occurrence of photoautotrophic and heterotrophic aquatic organisms at the time the sediment was deposited. Chlorophyll derivatives diagnostic of heterotrophic communities and bacteriochlorophyll derivatives which provide information on photic zone anoxia and eutrophication have been detected in some sites. Thus, the detection of photosynthetic pigments in archaeological sediments provides a geochemical method for investigating the existence and evolution of water features in past landscapes. Photosynthetic pigments are ideal candidates for compound specific radiocarbon analysis (CSRA) as they have known primary sources of carbon. Sediments from Kirisjes Pond Antarctica, which have been previously radiocarbon dated using bulk organic material, were extracted and individual pigments isolated and purified by a preparative HPLC system that had been validated with test samples. The younger CSRA results obtained from each layer were more credible than bulk measurements due to the age sequence determined within the section examined, which led to differences in age of up to ca. 1500 years between measurements. CSRA of isolates was used to constrain the timing of a marine incursion to between 7736 and 4688 years before present. Comparison of 14C dates from algal and bacterial pigments suggest a reservoir effect of between ca. 5500 to 6000 years. Radiocarbon measurements of standards showed that no isotopic fractionation had occurred during preparative HPLC.