IMPACT OF LAND-USE CHANGE ON CARBON BURIAL ON THE NORTHERN CALIFORNIA SHELF

During the past century, the flux of sediments and organic carbon from small mountainous watersheds to the continental margins has been greatly altered by human activities including timber harvesting and land clearance for agriculture. Numerous studies have examined the impacts of enhanced terrestrial erosion on soil productivity and water quality. Less is known, however, about the impacts of increased sediment delivery on processes in offshore marine environments. We investigated a 3.5 m long core from the continental shelf offshore from the Eel River in northern California to assess how a dramatic increase in sediment yield in one watershed has affected OC burial. The climate, steep terrain and geologic characteristics of the Eel watershed contribute toward a naturally high erosion rate. Human activities have substantially increased this rate during the past 50 years, however, contributing toward an order of magnitude increase in sediment accumulation rate and carbon burial on the shelf.

Organic carbon in various particle size fractions in the Eel system carries different signatures of land-use change. In the sand-sized fraction, discrete wood fragments deposited on the shelf between about 1000 and 1950 A.D. had average 14C ages of 400 years, reflecting long residence times in old-growth forests. Wood fragments deposited in the past 50 years, however, have post-modern 14C ages, signaling deforestation and contributions from much younger vegetation. OC bound to the clay-sized fraction on the shelf similarly reflects terrestrial processes. Kerogen carbon comprises about half of the clay-bound OC buried both before and after 1950. In this tectonically active setting, anthropogenic change has had no discernable impact on the survival of this ancient, recalcitrant carbon. The loading (OC/SA) of modern terrestrial and marine OC on the particles, however, shows the effects of the recent increase in sediment yield. A 30% reduction in OC/SA after 1950 reflects the shorter exposure times of particles in both soils and the surface mixed layer of the seabed.