Salt marsh paleoecology

Jeff Donnelly

Recent studies indicate that both climate warming and increases in the rate of sea-level rise (SLR) in New England over the last 150 years are unprecedented in at least the last 1000 years. Through the use of macrofossil and stratigraphic analysis of marsh peat, I examine how the distribution of New England salt marsh communities, which are intrinsically linked to the magnitude, frequency, and duration of tidal inundation, responded to this recent, roughly three-fold increase in the rate of SLR.

Plant macrofossils from sediment cores across modern plant community boundaries at Rumstick Cove and Nag Creek Marshes provided a 2500-year record of marsh community composition and documented the migration of cordgrass into the high marsh. The combination of Pb-210 and Cs-137 dating revealed that the initiation of cordgrass migration occurred in the late nineteenth century and continued through the twentieth century.

For example at Nag Creek Marsh, located on Prudence Island in Narragansett Bay, cordgrass has migrated landward, replacing marsh hay and resulting in a wedge of peat at the surface dominated by cordgrass remains (Figure 1). The wedge of cordgrass peat tapers landward from a thickness of 60 cm at the marine edge of the marsh and contains significantly more tidal-borne mineral sediment than the underlying marsh-hay peat. Marsh-hay-dominated peat extends to a depth of over 2 meters. The initiation of this time-transgressive unit (core NC1) dates to approximately the 1870s. The transition to cordgrass in core NC8 occurred around 1960. Today the transition between cordgrass and marsh hay exists close to the location of core NC6. Similar results are documented for Rumstick Cove Marsh in Barrington, Rhode Island.

The correspondence in time of this recent acceleration in sea-level rise with climate records of a warming planet and theoretical models of the impacts of increased "greenhouse" gas concentrations suggests a link between proposed human-induced climate change and accelerated rates of coastal inundation. Long-term records of marsh-community structure can be used to determine whether or not similar transgressive events have occurred in the recent geologic past.

Figure 1: Stratigraphic cross-section of Nag Creek Marsh transect (Donnelly and Bertness, 2001). Radiocarbon dates and maximum and minimum calibrated calendar age ranges at two standard deviations are displayed for two samples from NC7. White arrows represent the interval at which Cs-137 becomes detectable and is inferred to have been deposited in 1954. Black arrows represent the interval deposited in 1900 based on accretion rates derived from Pb-210 activity. The age of the oldest and most seaward transition from marsh hay or spike grass peat to cordgrass was estimated by extending the accretion rate derived from Pb-210 to this horizon in cores NC1. The extrapolation of Pb-210 accretion rates to this contact indicates that the initiation of cordgrass migration occurred about 1876 in NC1.

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