LATEST HOLOCENE SEDIMENTATION RATES AND SEA LEVEL HISTORY FOR THE TIDAL MARSHES OF THE POTOMAC AND RAPPAHANNOCK RIVERS, TRIBUTARIES TO CHESAPEAKE BAY

Instrumental tide gage records indicate that the modern rates of sea level rise in the Chesapeake Bay more than double the global average of 1.2 to 2.2 mm/year. The primary objective for this study is to establish a relative sea level history for the tidal reaches of the Potomac and Rappahannock River that will help us improve our understanding of bay sedimentary processes so we can better estimate the impact of future sea level rise as a result of 21^st century warming. Marsh cores were collected from Blandfield Point VA, Tappahannock VA, Potomac Creek VA, and Budd's Landing MD. The sedimentary facies include: 1) a lower unit of grey, organic poor clay with fine sand and silt layers, relatively high magnetic susceptibility values, and abundant brackish to marine microbiota; and 2) an upper unit of organic-rich clay and peat, relatively low magnetic susceptibility values, and abundant brackish to freshwater microbiota. Uncorrected AMS ¹⁴C measurements on terrestrial vegetation were used to constrain the geochronology and to establish sedimentation rates for the region. Sedimentation rates were highest at Potomac Creek ranging from ~3-4 mm per year in the lower clay facies with a substantial increase to ~8-10 mm/yr in the uppermost peat. Sedimentation rates calculated for Blandfield Point indicate ~1-2 mm/yr in the basal clays with an increase up-core to ~6-8 mm/yr in the uppermost peat. Foraminifera were used to identify meter-scale changes in relative sea level at Potomac Creek such that there is an apparent rise in sea level from ~2400-2000 YPB with a maximum at ~1500-1100 YBP. The paleoenvironmental trends recorded from the Chesapeake Bay tributaries suggest both climate and anthropogenic activities influence sedimentation patterns in the region. The meter-scale sea level cycle superimposed on the gradual rising trend for the Holocene may record a millennial warming trend coincident with the Medieval Warm Period. The warm humid conditions contributed to the transition from open bay estuarine conditions to enhanced peat production and marsh aggradation. The past several hundred years have been characterized by an abrupt increase in sedimentation and deterioration of salt marsh growth in response to aggressive agricultural practices in the Chesapeake Bay Watershed since the Industrial Revolution.