Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 5
Presentation Time: 3:15 PM


NATHAN, Stephen A.1, LECKIE, R. Mark2 and MABEE, Stephen B.1, (1)Office of the Massachusetts State Geologist, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003, (2)Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003,

During the late Pleistocene the Merrimack River paleodelta formed as post-glacial rebound produced a local low stand in sea level. Drowned as sea level rose, the paleodelta is now being reworked by a variety of processes. This study uses benthic foraminifera as a biotic and environmental proxy to study the sand and gravel resources of the paleodelta.

Nineteen sediment samples were collected from the paleodelta along two east-west transects east of the Merrimack River. From these samples nearly 6000 benthic foraminifera, representing 62 species, were collected and identified. Although dissolution compromised the preservation of calcite tests within six samples, the resulting data is robust and allows for numerous conclusions to be drawn. Specifically, benthic foraminifera become more common distally and specific species inhabit specific areas of the paleodelta. Distribution patterns of some species have changed significantly since the late 1940s, with some species migrating landward, others, seaward. Distributions of some taxa differ significantly between the two transects, both in the present day and from the past. These differences may point to the influence of, and changes in, the Merrimack River outflow upon water column nutrient delivery, productivity and food availability over the past 60 years.

Species diversity and evenness peak at the delta break, coincident with low species dominance at 50 meters water depth. Q-mode cluster analyses show three distinct assemblages, “shallow” (≤30 meters water depth), “deep” (≥40 meters), and “delta edge” (50 meters). There is no apparent correlation between foraminiferal distributions and deltaic bedforms, and in turn, sediment type. This implies that foraminiferal distributions are controlled by other environmental variables such as food.

In summary, benthic foraminiferal assemblage analyses complement geophysical techniques. Benthic foraminifera can also help assess the marine impact of, e.g., mining sediment resources, watershed development, pollution, rising sea level, and increased fishing.