Abstract

The Merrimack River paleodelta formed during the late Pleistocene as post-glacial rebound produced a local low stand in sea level. Subsequent sea-level rise drowned and eroded the paleodelta, which is now reworked by a variety of processes. This study used benthic foraminifera as a biotic and environmental proxy to evaluate the sand and gravel resources of the paleodelta.

Nineteen box-core sediment samples were collected from the paleodelta along two east–west depth transects east of the Merrimack River mouth and off Plum Island, Massachusetts. From these samples, 62 species of benthic foraminifera were identified. When compared with distribution studies published over 60 years ago, some species have shifted landward in their distributions (e.g., Adercotryma glomeratum, Cibicides lobatulus, Reophax curtus, Trochammina squamata), while a few exhibited wider and more distal occurrences (e.g., Elphidium excavatum and Elphidium subarcticum). These differences point to variations in the Merrimack River outflow and its effects upon water column nutrient delivery, productivity, and organic matter flux over the past six decades. Other mitigating factors affecting benthic foraminiferal distributions may include increased seafloor disturbance from strong winter storms or fishing activity.

Species richness and evenness peak at the delta break, coincident with low species dominance at about the 50-m depth. Q-mode cluster analyses show three distinct assemblages: “shallow” (≤30-m water depth), “deep” (≥40 m), and “delta edge” (~50 m). There are no apparent correlations between foraminiferal distributions and deltaic bedforms or sediment type. This implies that foraminiferal distributions are controlled by environmental variables other than grain size, such as food availability.

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