INTERFACE DISSOLUTION CONTROL OF THE C-14 PROFILE IN MARINE SEDIMENT

The process of carbonate dissolution at the sediment-water interface has two possible end-member boundary conditions. Either the carbonate particles dissolve mostly before they are incorporated into the sediment by bioturbation (interface dissolution), or the vertical mixing is rapid relative to their extermination rate (homogeneous dissolution). In this study, a detailed radiocarbon profile was determined in deep equatorial Pacific sediment that receives a high rate of carbonate supply. In addition, a box model of sediment mixing was used to simulate radiocarbon, carbonate content, and excess thorium profiles that result from either boundary process following a dissolution increase. Results from homogeneous dissolution imply a strong, very recent erosional event, while interface dissolution suggests that moderately increased dissolution began about 10,000 years ago. In order to achieve the observed mixed layer radio-carbon age, increased homogeneous dissolution would concentrate a greater amount of clay and Th-230 than is observed, while for interface dissolution the predicted concentrations are too small. These results together with small discontinuities beneath the mixed layer in Th-230 profiles suggest a two-stage increase in interface dissolution in the deep Pacific, the first occurring near the beginning of the Holocene and the second more recently, roughly 5000 years ago.