We are characterizing sediments on the Mississippi continental shelf in terms of density distributions in order to evaluate the hydrodynamic sorting processes that occur as sediment grains and associated organic matter move across the shelf and into the Mississippi River Canyon. To do so, we have fractionated sediments on the basis of density (using heavy liquids) and measured elemental, stable carbon and radiocarbon isotope compositions of density fractions. Our analyses are part of a collaborative project, with colleagues from Texas A&M University (T. Bianchi) analyzing lignin and pigments, Tulane University (Brent McKee) determining sediment character and deposition rates, and Virginia Institute of Marine Science (E. Canuel) measuring lipid biomarkers.

We (along with Drs. Canuel and Bianchi) are also examining historical changes in the organic carbon composition of sediments in key sub-environments of the Sacramento River- San Joaquin River Delta complex to better-understand how human activities have altered the flow of carbon. The Delta has been impacted by human-induced alterations common to many ecosystems globally, making it a model system for examining the role of human impacts on carbon flow in riverine-delta ecosystems. We are use a suite of isotopic and molecular tracers ("biomarkers") to characterize the sources, ages, and diagenetic state of sediment organic matter – thus inferring its nutritional value - representing past and present conditions in the Delta and its watershed. We are also measuring the radiocarbon content ("ages") of bulk carbon and carefully-selected biomarkers present in surface sediments and in select horizons of sediment cores to quantify the "ages" of carbon within the Delta. This information, used along with the biomarker distributions, will provide us with a tool for assessing how much of the carbon present in the Delta is "old", presumably refractory material of limited nutritional value to consumer organisms.