THE PALEO-ENVIRONMENTAL HISTORY OF THE ST. JONES ESTUARY, DE: PRE-HOLOCENE TO PRESENT

The importance of addressing the mechanisms for sustaining resources within the estuarine ecosystem in response to sea level rise has become a main focal point of coastal research. In order to assess ways by which the estuarine ecosystem may be conserved, it is essential to have long-term geologic information on the overall setting of the estuary through the Holocene. Past environments provide important constraints on future conditions within the estuary. The response of a marsh to rising sea level can be modeled from past transgressive sequences preserved in the subsurface sediments. This interdisciplinary study of the St. Jones estuary integrated high-resolution subsurface geophysical profiles (CHIRP sonar) and coring (both Dutch Auger and Vibra-coring). These methods yielded a three dimensional model of the marsh deposits, as well as the geometric characteristics of the Pleistocene incised-valley. The stratigraphic and lithologic data revealed the overall impact from changes in sea level, climate change, and anthropogenic disturbances have had upon this natural environment. Large-scale shifts in halophyte lithosomes and sediment lithologies relay the major alterations in the paleo-environment that have resulted from sea level change. Small scale changes in sediment lithologies and depositional settings directly resulted from adjustments in the path of the St. Jones River, variations in sedimentation rates, and inundation rates. The dominant effect that sea level has rendered upon this marsh can be viewed in the succession of three major lithosome deposits through the Holocene (a brackish- fringing marsh, a purely distichlis spicata marsh, and a Spartina alterniflora marsh). The present day St. Jones estuary shows large shifts in salt concentrations, longer periods of inundation, widespread slumping along the banks, and infilling of the tidal gut through increased sedimentation. These characteristics may represent the preliminary signs of the drowning of this estuarine environment in response to sea level rise. Normally without the integration of the background paleo-environmental data this presumption could not be truly supported. Through the full integration of the entire Holocene estuarine record, a detailed interpretation can be assessed upon the modern marsh environment and the dominant stressors that impact its evolution.