ORIGINS, TRANSPORT, AND FATE OF PLANT-DERIVED BIOMARKERS IN A SMALL RIVER CATCHMENT IN THE UPPER PENINSULA OF MICHIGAN (USA)

Rivers provide a pathway for terrestrial organic carbon dispersal and integration into sediments and are an important component of the global carbon cycle. Understanding the cycling of terrestrial organic carbon can be challenging but terrestrial plant biomarkers can provide an indication of the source of organic carbon in both modern and ancient sediments. These same biomarkers are also useful paleovegetation proxies in geologic sediments where fossils are absent. However, the dispersal and deposition of plant biomarkers in modern river systems, especially for compounds that provide taxonomic specificity such as di- and triterpenoids (diagnostic for conifers and angiosperms, respectively) is poorly constrained. To better resolve the modes of organic carbon transport within fluvial and riparian systems, we characterized plant biomarker transport in a small river basin within a mixed angiosperm-conifer forest. Here we assess the transport of biomarkers in river systems from seven sites from the headwaters to Lake Superior along the Miners River (~20 km pathway). All samples contained long-chain n-alkyl lipids, sterols, diterpenoids (abietane and pimarane classes), and triterpenoids (oleanane, ursane, and lupane classes). With the exception of a soil sample taken at a depth of 30 cm, triterpenoids are found in higher concentrations than diterpenoids in riparian soils and river sediments. Biomarker compositions in riparian soils, point bar, and overbank deposits are similar to the surrounding vegetation, albeit much lower in concentration. The composition of di- and triterpenoids in the river-suspended particulate organic carbon is similar in composition to the surrounding vegetation and soils. We developed a method to isolate biomarkers in the dissolved organic carbon fraction in river waters using solid-phase extraction and the preliminary data suggests that di- and triterpenoids are transported as dissolved organic carbon, however concentrations are lower than in the particulate organic carbon fraction. Our results from the Miners River can better define terrestrial organic matter cycling in small river catchments. Defining these modes of transport is critical so that plant-derived biomarkers in fluvial and marine sediments can be used as quantitative paleovegetation proxies.