Utilizing small coastal mini-basins for multi-decadal, high-resolution estuarine geochronologies

Abstract:

Dynamic processes that impact estuarine sediments (mixing, erosion) typically result in poor sediment record preservation in these environments. Therefore, establishing high resolution records of sediment flux and source to estuaries is a major challenge. Sediment cores from mini-basins directly adjacent to estuarine systems may provide an alternative for establishing the estuarine geochronology. As a recently-established mini-basin with historically high rates of sedimentation, Cape Lookout Bight, NC presents an ideal coastal environment to capture a long-term (century) record of sedimentation. In 2010, a 4.6 m long core was extracted from the deepest portion of the basin. Through lithologic description, grain-size, water content and radio-isotope analysis (excess ²¹⁰Pb) conducted at 1-cm intervals, a high-resolution geochronology was established. Basin formation was initiated by spit elongation that captured the marine shelf environment. A transition in sediment source from marine-dominant deposition (storm-induced over-wash events) to estuarine-dominant deposition was identified. This transition reflects the increased trapping efficiency of fine estuarine mud due to the changing geomorphology of the mini-basin. An abrupt increase in the rate of estuarine sedimentation from a moderate (~3 cm/yr) to a high (~10 cm/yr) rate of sedimentation occurred around 1984. After careful reconstruction of basin geomorphology, this increased rate of sedimentation cannot be attributed to internal basin dynamics, but instead is the result of increased estuarine sediment flux from the upper estuary. Unlike records collected in the central estuary, which are prone to modification and can be incomplete, this work demonstrates that rapidly accreting mini-basins have the potential to collect high-resolution records of estuarine sedimentation when placed in the context of changing geomorphology, sediment source and flux of the estuarine system.