SEDIMENTATION AND HYDROLOGIC CONTROLS ON ORGANIC AND INORGANIC CARBON DEPOSITION IN A KARSTIC WATERSHED

Land use change and climate induced variations in hydrologic systems could control sources, deposition, and cycling of organic and inorganic carbon in lotic sediments with potential feedbacks for regional and global climate change. To evaluate how C depositional patterns vary with time and space, we measured calcite contents, total organic carbon (TOC), total nitrogen (TN), C/N molar ratios (C:N), δ¹³C_OC and δ¹⁵N values in sediment cores from benthic sediments of the spring-fed Silver River. The river incises Holocene age and possibly lacustrine sediments that are from 2 to > 6 m thick. Near the headwater (Silver Spring), high C:N ratios and depleted δ¹³C_OC values indicate that terrestrial C3 vegetation was the predominant source of OC in benthic sediments, while downstream, low C:N ratios suggest a subequal mixture of algal and C3 plant contributions. These shifts in OC sources are more distinct in headwater sediments than downstream due to their proximity to highlands to the west. In general, deep sediments (>100-150 cm) at all sites show relatively low TOC contents and C:N ratios with elevated calcite contents suggesting quiescent, possibly lacustrian, conditions and minimal sedimentation that favored the deposition of carbonate-producing organisms. Elevated TOC contents, high C:N ratios and a shift to less negative δ¹³C_OC values at depths between 30 to 100 cm near the headwaters was most likely caused by increased sediment input from erosion of Eocene carbonate rocks and Miocene Hawthorn Group sediments from the western highlands. These characteristics are less common downstream, probably suggesting less influence of the highland sediments with distance. This erosion-deposition process may have filled a pre-existing lake to the east, exposed the carbonate and karstic Upper Floridan aquifer to the west, and blocked eastward flow to initiate discharge from Silver Spring. Eastward flow from the spring incised the old lake bed and thus formed the present Silver River. Our results suggest that the sources of OC and its link to carbonate deposition are strongly related to sediment erosion-deposition processes. This result reflects how depositional patterns and regional and global changes in surface-subsurface hydrology affect organic and inorganic carbon deposition.