A MILLENNIAL RECORD OF MARSH ACCRETION IN THE TIDAL REACHES OF THE POTOMAC RIVER

Tide gauge records in the Chesapeake Bay region, eastern US, indicate that relative sea-level rise over the last century is due to a combination of melting land ice (mass changes), ocean thermal expansion (ocean volume changes), and subsidence, mainly due to glacio-isostatic adjustment. However, there has been little research to establish a millennial record of marsh accretion to quantify pre-20^th century rates of Late Holocene sea level rise in the proximal-central estuary of the Potomac River downstream of Washington DC. Our primary objective is to establish a baselinephysical, biological, and geochronological record of marsh deposits spanning the past three millennia.

Three marsh cores were collected from Mattox Creek, Rosiers Creek, and Wilkerson Creek; tidal creeks adjacent to the Potomac River that are just north or south of Colonial Beach, Virginia. The cores range in thickness from 5.0-6.7 meters in length and comprise two primary lithofacies of basal grey clay and an upper organic-rich peat and clay. The grey clay lithology ranges in Total Organic Matter (TOM) from 4-20%and has highlyvariable magnetic susceptibility intensity peaks. In contrast, the alternating organic rich peat and clay ranges in TOM from 14-82% and has negligible variability with respect to magnetic susceptibility values. Microfossils extracted from the cores include an association of marsh and estuarine foraminifera that include Ammoastuta inepta, Miliammina fusca, Trochammina inflata, Jadammina macrescens, and Ammobaculites. An age model is in preparation pending the results of AMS¹⁴C of woody matter, peat, and skeletal calcite.

Our prior work indicates that the tidal marshes record steadily deepening estuarine conditions transitioning to relatively rapid marsh progradation at the approximate onset of the medieval warm period. Marsh aggradation was initiated at this time due to increasing humidity and favorable conditions for higher marsh productivity. Alternations between peat and clay in the uppermost core record millennial and centennial-scale climate oscillations such as the Little Ice Age.