Gamma spectrometry of deep-sea cores and sediment accumulation rates

Deep-sea sediment accumulation rates can be determined by nondestructive gamma ray spectrometry with about as much confidence and with far less effort than by radiochemical alpha spectrometry. Both methods are based on the decay of ²³⁰Th within the sediment column, sometimes normalized against²³²Th; with the gamma ray technique, the isotopes actually measured are ²¹⁴Bi, a daughter of ²³⁰Th, and ²⁰⁸Tl, a daughter of ²³²Th. Comparison of alpha and gamma measurements on four cores from the South Indian Basin shows that near the tops of some cores ²¹⁴Bi is not in equilibrium with ²³⁰Th, partly because of diffusion of the intervening daughter, ²²⁶Ra. If ²¹⁴Bi is assumed to be equilibrium with ²²⁶Ra, a value for the diffusion coefficient of radium in sediments is determined to be in the order of 10^-9 to 10^-8 cm²s^-1, in agreement with previous results. Because such zones of diffusion are shallow compared to the total region of ²³⁰Th decay in the sediment, there is little effect on the apparent sediment accumulation rate as determined by the decrease in the ²¹⁴Bi/²⁰⁸Tl activity ratio down and core.