RESIDENCE TIMES OF MARSH FORAMINIFERAL ASSEMBLAGES: IMPLICATIONS FOR THE FORMATION OF LATEST HOLOCENE SEA-LEVEL SIGNALS

A primary obstacle of Holocene climate studies is the "noisy" nature of shallow-water foraminiferal assemblages that results from spatio-temporal variation of test inputs and preservation. We amplified signal/noise ratios of marsh assemblages at Bombay Hook National Wildlife Refuge (BHNWR, Smyrna, DE) using "artificially time-averaged" (ATA) assemblages, in which dead and live counts of the most abundant species were summed for all seasons (Journal of Foraminiferal Research 30:272-293). We further tested the utility of ATA assemblages by using them to extract decadal-to-centennial scale sea-level (SL) signals of the last ~600 years (based on radiotracer-derived accumulation rates) from an auger core (2 m length) taken in the low marsh. The BHNWR SL curve closely resembles one previously published for Clinton, CT, marshes, and implicates regional climatic forcing mechanisms.

The absence of calcareous hardparts precluded estimation of durations of time-averaging using carbon-14 dates. Duration of time-averaging were instead estimated using residence times (RTs) of assemblages, in which ATA dead counts were divided by ATA live counts. RTs of the surface mixed layer (0-20 cm), in which taphonomic processes are most intense, are typically <5 years. However, the SL rise recorded in the surface layer spans the last century, which suggests that RTs could be ~100 years. The discrepancy between RTs and the SL curve suggests that assemblages rapidly come into equilibrium with SL when it changes, but that after the SL signal is established in death assemblages, assemblages continue to turn over rapidly until burial beneath the surface layer.