BASAL RADIOCARBON AGE FROM BIG PEA PORRIDGE POND, MADISON, CARROLL COUNTY, NEW HAMPSHIRE, AND ITS RELATIONSHIP TO THE REGIONAL DEGLACIAL CHRONOLOGY

Big Pea Porridge Pond (BPPP) lies at 647 ft (197 m) asl (43.941 N, 71.188 W) in a broad depression within generally higher relief terrain in east-central New Hampshire. The area is underlain by Concord granite bedrock, which is veneered by diamicton and ice-contact stratified drift of variable thickness. We obtained a 10.2-m sediment core from the deepest pocket at the north side of the pond (14.6-m water depth) using a Livingstone sediment sampler. A broad shallow shoal just west of the deep pocket suggests a possible marginal position as wasting Laurentide ice retreated northwards.

A bulk sample of the deepest visible organic matter at 9.35-m core depth provided an AMS age of 12,125 +/- 50 14C yr BP (Beta-244299; 13C/12C = -23.9), calibrated to 13.84-14.15 ka (mean = 14.01 ka) using the CALIB 5.0.2 (Stuiver et al, 2004) on-line calculator (http://calib.qub.ac.uk/calib/). Detailed microscopic examination of sediments for extraction of chironomids suggests that an AMS age from bulk organic material might be possible from the 9.60-m depth, which would extrapolate to an age about 12,475 +/- 50 14C yr BP (14.32-14.73 ka; mean = 14.56 ka), but the sediments below 9.60 m are sterile. Although the lag time between deglaciation and deposition of the initial organic materials in lake basins in general cannot be known, we suspect that the lag time at BPPP might be less than a few hundred years.

Our basal age for BPPP is reasonably consistent with basal ages from other nearby sites, including Echo Lake (~10 km N, bulk AMS, 13.38-12.68 ka), Cushman Pond (~30 km NE, macro AMS, 15.38-15.72 ka), Lost Pond (~30 km NNW, bulk conventional, 13.93-14.37 ka), Surplus Pond (~75 km NNE, macro AMS, 14.03-14.18 ka), and Pond of Safety (~55 km NNW, macro conventional, 14.26-14.67 ka). Also, most of these lake- and pond-bottom basal ages appear to be consistent with the ice-marginal reconstructions of Ridge et al., (1999, 2001), which are based on varve, paleomagnetic, and 14C chronologies from proglacial lakes, including Glacial Lake Hitchcock.