Spatial and Temporal Changes in Marsh Vertical Accretion Rates at Monie Bay: Implications for Sea-Level Rise

Abstract

Spatial and temporal changes in vertical accretion rates and sediment characteristics were investigated in a large submerged upland, estuarine marsh on the Eastern Shore of Chesapeake Bay. Eighteen cores were collected from sites spanning shoreline, tidal channel-side and interior marsh environments. Accretion rates over various time scales (~30 years, ~100 years, and ~200 years) were determined using ¹³⁷Cs, ²¹⁰Pb, and pollen geochronologies. Marsh-wide, long-term vertical accretion rates have averaged about 3.0 mm/yr for the last two centuries, about the rate of submergence for the area over the last half century recorded by tide-gauges. But this average figure masks considerable spatial variability (ranging from 0.15 to 0.63 mm/yr), even within the same overall depositional environment. Shoreline and channel-side sites, where grain size and bulk density analyses indicate greater allochthonous mineral sediment inputs, generally were characterized by the higher vertical accretion rates, although at some sites sediment collapse from the weight of recent overwash horizons yielded lower than average rates of vertical accretion for this environment. Loss-on-ignition analyses show that spatial relations in vertical accretion are not constant. The balance between organic and inorganic accumulation shifts over time as storms come and go and hydraulic conditions in tidal channels change. Interior marsh sites, displaying the most consistent sediment characteristics with depth, tend to be the most stable areas in terms of vertical accretion budgets after initial colonization by marsh plants. Temporal changes in marsh vertical accretion rates were also equally variable, with rates integrated over a century or more sometimes less than half those based on the last few decades. Substantial decadal departures are nevertheless embedded in these long-term accretion trends, presumably reflecting equally short-term fluctuations in sea-level rise. These departures plus the evidence of incipient autocompaction in relatively recent sediments suggest that employing very short-term estimates of vertical accretion fate can potentially over-estimate marsh accretionary response to future sea-level rise.