Sediment Coring

On August 11, 2006, four vibracores were collected at sites 2, 3, 4, and 6 in Salt Pond and adjacent areas (Figure 1, Figure 3, and Figure 11) to document submarine-sediment lithology and to allow pore-water samples to be recovered from sediments that did not yield ground water during piezometer sampling. Coring was performed from a contract boat supplied and operated by TG&B Marine Service, Inc., and outfitted for such operations with a support frame, moon pool, and both hydraulic and pneumatic drive heads (Figure 13). A total of 63 4-cm-thick sediment samples were collected at 30-cm intervals from the four cores. Cores were collected in aluminum casing (irrigation pipe, 8-cm diameter) and split, described (Figure 14), photographed, sampled, and discarded in the field. Pore water was extracted from a subset of the core samples in the laboratory by using a hydraulic press and a Manheim-type sediment squeezer (Manheim et al., 1994); salinity (Figure 11) was measured by refractometer due to small sample volumes. Samples were also processed for grain size and one radiocarbon analysis. Core lithologic descriptions and pore-water salinity values are shown in Table 7 and Table 8. Images of core sections can be viewed in the sediment-core images gallery.

Figure 13: The contract boat used for collecting vibracores.

Figure 14: Vibracores were returned to shore immediately after collection.

The longest core (607 cm) collected at site 3 near the center of Salt Pond (water depth approximately 9 m) recovered glacial sand and gravel directly overlain by 115 cm of fine-grained lacustrine sediments and 257 cm of estuarine sediments. The salinity of pore water in the core dropped gradually over its length from a peak at 60 cm of 36 ppt down to 4 ppt just above the glacial sediments. Given the shallow depth of the connecting channel, the thickness of the estuarine deposits relative to the lacustrine deposits (more than twice as thick) was unexpected as the pond would have only been breached within the last 2,000 years according to published sea-level curves for the region (Bratton, 2007b). The fresh kettle pond, however, would have likely existed for about 15,000 years prior to the marine invasion. These observations suggested that either 1) estuarine sedimentation rates were much higher than lacustrine sedimentation rates, or 2) the breaching took place earlier than would be expected based on modern sill elevations relative to sea level. A radiocarbon age of 1800 +/- 30 years was determined by accelerator mass spectrometry on a small twig collected 13 cm below the lacustrine/estuarine transition in the core, consistent with the hypothesis of elevated estuarine sedimentation rates rather than anomalously early breaching of the pond.

The core from the farthest offshore site (site 6) in Salt Pond Bay recovered 4.8 m of estuarine sediments, including fine-grained estuarine deposits above and below a layer of clean medium sand 1.8 m thick, likely deposited in a tidal channel. Pore-water salinity dropped to 3 ppt at a depth of 1.5 m below the sediment surface just above the sand layer, climbed to 12 ppt just below the sand layer, and then declined to 5 ppt by the bottom of the core.