APPLICATION OF SCYTONEMIN AS AN ORGANIC GEOCHEMICAL ARIDITY PROXY IN MARINE AND LACUSTRINE SEDIMENTS

Desert biomes are prone to underrepresentation in the fossil record, as plant cover is sparse, animal hard parts are less common, and biomass in general is less abundant than in other terrestrial ecosystems. Further, the dearth of aquatic depositional environments makes desert plants and animal less likely to be preserved as fossils. However, following the model of wind-blown pollen, more continuous aridity records are derived from small eolian particles that carry molecular biomarkers eroded from desert microbiotic soil crusts. Our recent work on Holocene Black Sea sediments suggests that scytonemin, a pigment produced by colonial and filamentous cyanobacteria exposed to high levels of ultraviolet radiation, is such a molecular biomarker. By measuring scytonemin C and N stable isotope compositions in Black Sea sediments and comparing them with values for a range of modern environmental samples, we determined that the scytonemin in the Black Sea was indeed derived from cyanobacteria in desert microbiotic soil crusts. Scytonemin was particularly abundant in Subboreal sediments, which we interpreted as further evidence for an especially dry climate regime in the Black Sea region during that time.

We are currently turning to other sediment records to expand the utility of scytonemin as an aridity indicator. We detected reduced scytonemin with concentrations spanning three orders of magnitude in Holocene Great Salt Lake sediments, suggesting it is sensitive to environmental change. We are currently sampling for C and N isotopes to determine whether it derived from cyanobacteria living in the lake or in soils from the adjacent Great Basin Desert. Overall, scytonemin appears to be relatively recalcitrant in clay-rich sediments, supporting its potential as a biomarker to provide new geological records of past aridity.