RELATIONSHIPS BETWEEN INTERTIDAL FLAT SEDIMENTS AND SURROUNDING SURFICIAL GEOLOGY: AN EXAMPLE FROM BOSTON HARBOR

A collaborative project has focused on the restoration and enhancement of Boston Harbor soft shell clams (Mya arenaria), including the characterization of the intertidal flat sediments in which they live. Here, we test the hypothesis that surficial geologic deposits surrounding intertidal flats influence grain size distributions, and indirectly affect organic matter and carbonate accumulation. Short sediment core samples (n=91) were taken from ten Boston Harbor flats and homogenized prior to grain size and loss on ignition analyses. Surficial geology was interpreted from previously published studies.

Grain size results support that the surrounding surficial geology has a control upon the sediment characteristics of the modern flat environments. For instance, there are two flat sites within the backbarrier system of Nantasket Beach, Hull. Hull East is proximal to the barrier, and Hull West is distal to the barrier, but in proximity to a drumlin. Hull East sediments have a coarser median grain size and higher kurtosis (2.3φ and 1.8, respectively) as compared to Hull West (3.0φ and 1.0, respectively), which can be explained by mature beach sands vs. drumlin tills as the source materials. An additional example that supports the hypothesis is found in data from two Quincy sites. These locations are the only two in the study that are adjacent to sand and gravel-dominated kames. In addition, the adjacent Wollaston Beach is annually enriched with sand by the city of Quincy. As a result, the median grain size (2.1φ and 1.4φ) is higher than any of the other ten sites, and the percent mud component of the two sites (2.4%, 3.0%) is lower due to the lack of fine sediment supply to this stretch of shoreline. Strong correlations are noted between median grain size of sediment and organic matter and carbonate content, where fine sediments tend to be associated with higher organic matter and higher carbonate content. It is interesting to note that despite the presence of Boston Blue Clay, the grain size observed at sites below the late Pleistocene marine limit (n=6) are not statistically different from those above the mean limit. Combined with recent studies suggesting a relationship between sediment type and juvenile clam survival, these results suggest that the surficial geology may be a useful tool for planning juvenile clams stocking locations.