Particle mixing rates in sediments of the eastern equatorial Pacific: Evidence from 210Pb, 239,240Pu and 137Cs distributions at MANOP sites

Particle mixing rates (D_B) calculated from excess ²¹⁰Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm²/y, with variation of a factor of 3–4 at a single site. Diffusion of the ²³⁶Ra daughter ²²²Rn may affect ²¹⁰Pb distributions under conditions of slow mixing and low ²¹⁰Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides ^239,240Pu and ¹³⁷Cs but no excess ²¹⁰Pb (relative to ²²⁶Ra). There is no clear relationship between ²¹⁰Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of ²¹⁰Pb mixing rates with those calculated from ^239,240Pu and ¹³⁷Cs distributions reveals better agreement for a pulse input of the fallout radionuclides (D_B = 0.03?0.4 cm²/y) than for continuous input at a constant rate (D_B = 0.1?1.6 cm²/y), although the Pu and ¹³⁷Cs data are better fit by the latter model. The agreement may be fortuitous because ^239,240Pu and ¹³⁷Cs appear significantly deeper than ²¹⁰Pb in at least one core. Tracer separation could be caused by particle size-selective mixing by the benthic fauna or by chemical mobilization. If the fallout radionuclides are scavenged from surface waters by large, organic-rich particles such as fecal pellets, their release and migration may result from decomposition of the carrier in surface sediments. Either a relatively unreactive form of Pu (e.g. oxidized Pu) has been released by this process or a one-dimensional model is inadequate to explain its observed penetration into the sediments. Activity ratios of ${}^{\mathrm{239,240}}\text{Pu}{}^{137}\text{Cs}$ in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of ^239,240Pu near the weapons test site at Christmas Island (2°N). The ${}^{\mathrm{239,240}}\text{Pu}{}^{137}\text{Cs}$ ratios and fluxes to the sediment (assuming constant input) at the siliceous ooze-red clay site are consistent with published sediment trap data from a nearby site. Thus if fallout radionuclide fluxes to the sea floor were higher in the past, both ^239,240Pu and ¹³⁷Cs have been released from sinking particles.