TRACKING SEDIMENTARY PARTICLES FROM TERRESTRIAL SOURCE TO MARINE SINK-- ORGANIC MATTER AS A SEDIMENTOLOGICAL TOOL

Geologists have long used the concentration and composition of organic matter in modern and ancient sediments as a tool for reconstructing depositional environments. These analyses, however, have focused almost exclusively on bulk samples and have thus not fully exploited the information that is available when the OM in separate size and density fractions is examined. These separate organic fractions may differ considerably in origin and will carry different types of environmental information. Wood fragments, for example, behave hydrodynamically like sand-sized particles, and can serve as useful tracers of riverine sediment dispersal on the continental margins. In contrast, organic matter associated with the clay-sized fraction consists not of discrete particles, but is strongly bound to the mineral grains. Clay-organic complexes are dynamic, undergoing transformations as particles are exposed and weathered from preexisting rocks in mountains, temporarily stored in terrestrial reservoirs including colluvium and soils, transported in fluvial systems, and delivered to and ultimately buried on continental margins. These transformations depend in large part the residence time of the particles in biologically active, near-surface horizons en route from source to sink. As a result, the amount and composition of clay-associated carbon in offshore sediments is a reflection of terrestrial processes, including rates of uplift and mass wasting, as well as marine processes of sediment transport and accumulation.