Sediment cores were immediately returned to the USGS Woods Hole Coastal and Marine Science Center after collection. They were placed in a refrigerator for 1 to 3 days, then split vertically, sectioned at 1 or 2 cm intervals, frozen, and then freeze dried for 7 days until sediment weights did not change further. Dry bulk density was determined as the dry weight of a known volume of sample. Approximately 5 grams of dried sediment sample was blended and homogenized prior to sealing in a jar for a minimum of three weeks and then placed on a planar-type gamma counter for 24 to 48 hours to measure 7Be, 137Cs, 210Pb, and 226Ra at 477, 662, 46.5 and 352 kiloelectronvolts (KeV) energies respectively (Canberra Inc., USA). Detector efficiency was determined from EPA standard pitchblende ore in the same geometry as the samples. Excess 210Pb was calculated as the decay-corrected difference between total 210Pb and supported 210Pb (considered to be equal to 226Ra). Activities of 7Be, 137Cs, and excess 210Pb were decay corrected to time of collection, using their respective half-lives. Suppression of low energy peaks by self-absorption was corrected for according to Cutshall and others, 1983. Gamma spectroscopy detection limits were determined in APTEC software for each sample; refer to the attribute accuracy section of this metadata for further details. Values reported are above this limit, while values below are reported as 0. Core sections not analyzed are reported as blank cells. Sediment ages and accretion rates were calculated with the Constant Rate of Supply (CRS) excess 210Pb age model, a variant on the advection-decay equation (Appleby and Oldfield, 1978; Goldberg, 1963). This model assumes that 210Pb supply to the sediment surface is constant through time, but allows for changing sedimentation rates, in addition to decay, to control the down-core activity of excess 210Pb. The common form of the CRS model, as derived by Appleby and Oldfield (1978), solves for age based on the distribution of excess 210Pb in the sediment record. Prior to application of the age model, excess 210Pb profiles were evaluated to ensure they were sufficiently resolved to apply the CRS model without bias towards ages that are too old or accretion rates that are too low at depth (Binford, 1990). Negative values of excess 210P occur where 226Ra is greater than 210Pb-total. These sections are not included in the age-model calculations. All gamma analyses were ongoing from 2015 and completed in 2018.
Appleby, P.G., and Oldfield, F., 1978, The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment: Catena, v. 5, issue 1, p. 1–8, https://doi.org/10.1016/S0341-8162(78)80002-2
Binford, M.W., 1990, Calculation and uncertainty analysis of 210 Pb dates for PIRLA project lake sediment cores: Journal of Paleolimnology, v. 3, issue 3, p. 253-267, https://doi.org/10.1007/BF00219461
Cutshall, N.H., Larsen, I.L., and Olsen, C.R., 1983, Direct analysis of 210 Pb in sediment samples—Self-absorption corrections: Nuclear Instruments and Methods in Physics Research, v. 206, issues 1–2, p. 309–312, https://doi.org/10.1016/0167-5087(83)91273-5
Goldberg, E.D., 1963, Geochronology with 210 Pb, in Miller, J.A., convener, Radioactive dating: International Atomic Energy Agency Symposium on Radioactive Dating, Athens, Greece, November 19-23, 1962, [Proceedings], p. 121-131.