Rates of sediment reworking at the HEBBLE site based on measurements of Th-234, Cs-137 and Pb-210

Abstract

Th-234, Cs-137 and Pb-210 measurements have been made on ten cores from the HEBBLE site on the Nova Scotian continental rise. Th-234 mixing coefficients from HEBBLE sediments show substantial lateral variability with values ranging from 1 to 33 cm 2 yr −1. These mixing coefficients are one to two orders of magnitude greater than values from typical continental-rise and abyssal sediments because of high densities of benthic macrofauna. Th-234 data from HEBBLE cores indicate that particles at the sediment—water interface are mixed to depths of 1–5 cm on a 100-day time scale. Cs-137 and Pb-210 data indicate that on time scales of 30–100 yrs surface sediments are reworked to depths ranging from 1 to 12 cm. Based on Th-234 profiles from two HEBBLE cores collected less than 200 m apart during consecutive years, no temporal variability in mixing rate could be resolved. In some HEBBLE sediments there is good agreement between the Th-234 mixing coefficient (100-day time scale) and the Pb-210 mixing coefficient (100-yr time scale). In other HEBBLE cores, however, Th-234 data indicate that the Pb-210 profile has not reached steady state. If steady-state solutions are used to model these non-steady-state profiles, the calculated Pb-210 mixing coefficient may underestimate the true mixing coefficient by as much as an order of magnitude.