SEDIMENTARY RECORDS OF HUMAN-INDUCED LANDSCAPE CHANGE AT NEAR-STREAM AND BACKSWAMP LOCATIONS, SOUTH GEORGIA

Human occupation and land clearing commonly promotes soil erosion in uplands and associated deposition down-valley, causing flood plain surfaces to aggrade and altering riparian wetlands. Sedimentary records of these changes vary spatially reflecting the timing and magnitude of disturbance and the geomorphic controls on landscape development and recovery. We illustrate these relationships by analyzing 14 vibracore (up to 4.68 m long) from in-channel and valley side locations, and 4 percussion core (up to 2.19 m) from backswamp/pond settings recovered from Providence Canyon State Park (PCSP) in southwest GA. PCSP consists of several canyons, up to 60 m deep that formed in response to land clearing following European settlement (ca. 1800). Cores from canyon floor locations penetrate through modern alluvium into Cretaceous sediments that contain buried soil profiles indicative of stable valley-bottom surfaces that existed before the canyons formed. Sediments eroded from the canyons and valley sides have since aggraded the valley floor creating complex deposits of alluvium that are normally graded and contain erosive bases lined with iron stone fragments, organic detritus, and large (10 cm diameter) kaolin balls all of which indicate repeated episodes of incision and aggradation. Conversely, more stable backswamp locations reveal subtle transitional contacts between pre-disturbance valley bottom sediments and overlying quiet-water mud. Transition zones contain organic detritus from trees killed during initial flooding, as well as discrete sand lenses associated with back-flooding from the main stream.

Although sedimentary records of human-induced change are easily recognized by sharp contacts between buried soil profiles and overlying alluvium near the stream, radiometric chronologies are more readily established in backswamp locations, where we find excellent agreement between 210Pb dates for backflooding sand lenses and historical rainstorms records. Furthermore, 210Pb mass sedimentation rates are well explained by land use changes in historical air photographs. However, repeated incision, highly variable lithologies, and the penetration of modern root systems into sandy valley-bottom alluvium limit the effectiveness of 14C and likely 210Pb and 137Cs dating at many riparian locations nearer the river.