Sediment provenance and geomorphic development of soils in a blackwater ecosystem, South Carolina Coastal Plain

Abstract

Mineral soil is a conundrum on southeastern U.S. blackwater floodplains because these freshwater ecosystems are known for slow-flowing streams bearing little suspended sediment. Investigations on soils along this riparian system revealed two distinct geomorphic surfaces, the modern swamp floodplain (classified as Inceptisols according to Soil Taxonomy) and slightly elevated landforms (intra-swamp terraces) with more developed soils (Alfisols and Ultisols). This study examined soil characteristics and sediment provenance on the Black River floodplain and its tributaries in the Atlantic Coastal Plain of South Carolina to determine the properties and pedogenic relationship between these two geomorphic surfaces and to propose a theory for the geomorphic evolution for soils on these landforms. Spatial characterization of the soils will benefit ecologists researching the biological aspects of blackwater ecosystems such as the concentration and flux of dissolved organic carbon (DOC). Results showed terrace soils had aquic moisture regimes while floodplain soils had peraquic moisture regimes. Floodplain surface horizons were organic carbon-rich (5% to 25% OC) but seldom with enough to qualify as organic soil material. Discontinuities occurred within the soils of both landforms indicating two distinct sedimentological environments. The fine-earth fraction of soil profiles on both landforms generally had upper strata composed mainly of silt and clay indicative of wash load sediments underlain with sandy strata indicative of bed load deposits. Fine clay distribution and fine clay:total clay ratios (FC/TC) showed argillic horizon development in the terrace subsoils but not in the floodplain soils. Micromorphology supported argillic horizons for the terraces, but also revealed modest clay accumulation in pores and channels of the floodplain soils. These floodplain clay features are interpreted to have formed during a period of subaerial weathering. The sandy strata were likely deposited during the beginning of the transgressions following the down cutting that occurred during the regressions and sea level lows. Silt fraction comparisons between the blackwater soils and soils on other landscapes in the region suggest the silts and clays likely were deposited in an estuarine environment late in the transgressions.