Long-term sediment accumulation rates and organic carbon burial on the middle Eel shelf northern California /

by 1976- Perkey, David Wayne

Perkey, David Wayne. Long-term sediment accumulation rates and organic carbon burial on the middle Eel shelf: northern California. (Under the direction of Elana L. Leithold and Neal E. Blair) The Earth’s continental margins are currently the primary site of organic carbon burial. Over geologic time the burial and preservation of organic carbon plays a significant role in regulating atmospheric composition. The efficiency at which organic carbon is buried is influenced by many factors that include dissolved oxygen levels, sedimentation rates, and organic carbon composition. The Eel River shelf off of northern California is a well-studied system that is ideal for exploring the interplay between some of these factors Examination of a recent piston core taken from the middle Eel shelf off the coast of northern California revealed that sediment accumulation rates over the last five decades are elevated when compared to the previous 1900 years. 14C- dating of woody debris and clay organic carbon indicate that accumulation rates range between 0.9-1.4 cm/yr over the later half of the 20th century. However, prior to 1964-1955 sediment accumulation rates were 0.15±0.02 cm/yr. A change in core stratigraphy was also observed at a depth corresponding to the middle 20th century. Modern shelf sediments were finer grained and dominated by more frequent flood deposits when compared to sediments deposited before the later half of the past century. Despite the recent increase in preserved flood layers off shore of the Eel River, correlation between reconstructed river discharge of the nearby Sacramento River and the Eel River revealed that current levels of discharge are not unique over the past 2000 years. Therefore increased precipitation is not thought to be the key factor for the change in sedimentation on the middle shelf. Instead, the peak of logging activity in the basin was correlated to change in sedimentation. 14C-ages of woody debris recovered from the piston core revealed that old growth wood was present only in shelf sediments below a depth that corresponds to the 1955-1964 era. Therefore land use alteration is a key reason for the change in sediment accumulation rates on the middle Eel shelf. The 6-11 fold increase in modern sediment accumulation rates was linked to changes in the content of organic carbon being buried on the shelf. Unlike other studies, the majority of the organic carbon being buried on the Eel shelf was found within the coarse ( > 25µm) fraction. Modern mean %OC of this fraction was twice the mean %OC prior to 1955-1964. Analysis of the sedimentary organic carbon associated with clay particles revealed organic carbon loading (OC/SA) to be 30% higher when accumulation rates were lower. An isotopic mass balance of the organic carbon sources loaded on the clays showed that approximately half (40-60%) of the OC was from a kerogen source. This is true for samples deposited before and after the change in sedimentation rates. Furthermore, the increase in %OC associated with lower sedimentation rates is due to higher amounts of marine and terrestrial OC being preserved on the clay particles in response to longer residence times in both the regolith and benthic mixed layer. LONG-TERM SEDIMENT ACCUMULATION RATES AND ORGANIC