Data for Modern Soil Chronometry using Fallout Radionuclides
Description
We describe environmental gamma spectrometry data for >700 soil samples collected from >35 high-resolution quantitative soil profiles spanning global sites. The data are collected for the purpose of modern soil chronometry based on fallout radionuclides (FRNs) 7Be and 210Pb, using the Linked Radionuclide Accumulation model (LRC). Cumulative gamma counting time for samples in the database exceeds 6.5 years. This is a living database to be augmented as data become available and corrected with improvements in data reduction or identification of errors. Versions and changes will be indexed. Special attention is paid to measurement uncertainties in the dataset, as well as how atmospheric or excess 210Pb is defined in both geochemical and mathematical terms for use in the LRC model. Basic familiarity with gamma spectrometry and radionuclide decay chains is assumed.
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2. Methods 2.1. Instrumentation Data are collected with six Mirion (formerly Canberra) Broad Energy intrinsic germanium detectors (BEGe) with carbon fiber endcaps and low background cryostats which are cooled with liquid nitrogen. The detectors are shielded with ultra-low background lead. Data acquisition is performed with Mirion DSA-1000 or Lynx digital signal processors running with Genie 2K software v.3. The specific BEGe detector design is important because it provides excellent efficiency at low gamma energies for 210Pb acquisition without sacrificing efficiencies at high energies. Carbon fiber provides good transmission of the low energy 210Pb emission which is otherwise attenuated by an aluminum endcap. Samples are typically run using four-day count times. Of two detector models, BEGe 3830 (38 cm2 detector active area, 30 mm thickness) and larger BEGe 5030 (50 cm2), the latter saves one day of counting and accomplishes in three days what the 3830 accomplish in four. 2.2. Sample preparation and geometry Foliage, litter, and soil samples are oven dried at 60 ℃. Bulk soils are sieved or hand-picked to remove clasts and debris >2mm in diameter. Fine roots >1 mm diameter or >2 cm in length are removed by hand. Foliage and leaf litter (Oi and Oe horizon) are typically milled to <1 mm using a Wiley mill with stainless steel blades. Soil samples are loosely disaggregated as necessary by agate mortar and pestel and packed into 110 cm3 polyethylene petri dishes with snap closure. The petris are sealed with common electrical tape and typically rested for >2 weeks to allow for 222Rn ingrowth, although we will show that this approach does not quantitatively retain Rn.