g-ray spectroscopy

We use a variety of naturally occurring (e.g., Pb-210, Be-7, Th-234) and artificial (Cs-137) radionuclides to study sediment accumulation and bioturbation in the coastal ocean. Measurements are made using a nearly identical pair of Canberra LEGe gamma-ray detectors. Samples (20-35 g dry weight) are ground to a fine powder using a mortar and pestle, weighed and measured, and then counted for 1-2 days. After a variety of steps that take into account detector efficiency and self-absorption, the activity (in Bq/kg or dpm/g) of each sample is plotted as a function of depth in the sediment.

How the activity vs. depth data are analyzed next depends on the question at hand. In the case of sediment accumulation (where we're typically using Pb-210 ), we fit a model that is a balance between advection (aka sediment accumulation) and radioactive decay. This model assumes that in the region of interest (> 10 cm below the sediment-water interface) biodiffusive mixing is negligible. The panels on the left (below) are examples of Pb-210 (diamonds) and Cs-137 (squares) profiles from two 110-m sites on the Eel River margin (from Wheatcroft & Sommerfield, in review). These profiles yield sediment accumulation rates of ~1.5 and 6 mm/yr. Can you tell which profile represents the slower rate?

If our goal is to estimate sediment bioturbation intensity (typically using Th-234), then the model used is a balance between decay and biodiffusive mixing. The reasoning here is that Th-234 is too short lived (24.1 d half life) to be affected, except in very unusual circumstances, by sediment burial. The panel on the right shows Th-234 data from three cores collected at a 20-m site on the Chianti margin (Adriatic Sea) as part of EuroSTRATAFORM. In this case all three profiles result in roughly the same biodiffusivity: ~ 5 cm^2/yr (pretty slow...).