Scytonemin and Past Aridity
I developed a new sedimentary biomarker for aridity while studying phytoplankton pigments in Black Sea sediments. Scytonemin, a pigment produced only by cyanobacteria, is preserved in abundance in Black Sea sediments, especially in the Subboreal interval (~5500-2000 BP). Scytonemin has particular ecological significance, as its production is stimulated by ultraviolet radiation, such as in desert soil crusts, microbial mats, and lichens and it has not been found in pelagic marine cyanobacteria. I found that C and N stable isotopes can be used to distinguish its source, and in the Black Sea, it came from microbial desert soil crusts that were probably eroded and transported by the wind. I also proposed that 7-methylheptadecane, a lipid found in the Black Sea and much older ancient sediments, is also produced primarily in desert soil crusts and like scytonemin it is transported via wind currents. This study published in Organic Geochemistry is the first to detect scytonemin in deep sea sediments, suggesting that it could be an excellent biomarker for cyanobacteria in deep time when UV exposure, at times, may have been significantly greater. I have a pending NSF proposal to greatly expand the use of scytonemin and 7-methylheptadecane as biomarkers of terrestrial climate change.
Predicted Cyanobacterial C and N Isotopes
I developed a method to purify and measure the stable isotopic composition of scytonemin using HPLC and nano-EA-IRMS. I analyzed samples from diverse sources, including desert soil crusts, lichens and black crusts on rocks, and found that scytonemin is consistently 13C and 15N depleted relative to total biomass (Above). This relationship allowed me to predict the isotopic composition of the cyanobacteria that produced scytonemin from Black Sea sediments (Right). By comparison with published biomass d13C and d15N values, I determined that scytonemin in the Black Sea most likely derived from desert microbiotic soil crusts. Please refer to Fulton et al., 2012 for more details.