Accretion and Canal Impacts in a Rapidly Subsiding Wetland. III. A New Soil Horizon Marker Method for Measuring Recent Accretion

Abstract

A new methodology used on a large scale is reported by which short-term (≤1 yr) marsh accretion rates were measured in saltwater and brackish marshes and compared to first-time measurements made in freshwater marshes. The stable rare-earth elements (REE) dysprosium and samarium were used for soil horizon markers that were collected by a cryogenic field coring method and detected by instrumental neutron activation analysis (INAA). Accumulation in saltwater marshes for 6 months was estimated to be 0.76±0.26 cm (n=11) and accumulation for 1 year was 1.29±0.49 cm (n=7). Accumulation in brackish marshes for 6 months was 0.51±0.34 cm (n=6) and for 1 year, 0.84±0.32 cm (n=10). These data from saline and brackish environments can be compared to first-time measurements of accumulation in a freshwater marsh of 1.53±0.66 cm (n=8) for 6-month accumulation and 2.97±0.92 cm (n=11) for 1-year accumulation. The cryogenic REE-INAA method for sampling and measuring 6-month and 1-year accretion is nonpolluting, does not alter natural marsh soil processes, and is effective in salt, brackish, and freshwater marshes. Additionally, the marker is essentially immobile, long lasting in the soil profile, and inexpensive to buy, apply, and sample. INAA analysis of the cores is expensive and time-consuming, yet the REE-INAA method yields accretion data, especially in freshwater habitats, that are obtainable in no other way. A comparison between short-term accretion and the presence or absence of man-made canals showed no statistically significant differences of accretion along transects from 0- to 50-m distance into brackish and saltwater marshes (no freshwater transects were established). Sediment depositions measured at 50 m into fresh, brackish, and saltwater marshes from natural or man-made waterways showed no statistically significant differences of accretion within each habitat over a 6-month or a 1-year time period.