Computer Science
Scientific paper
May 1985
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49.1195c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 49, Issue 5, pp.1195-1210
Computer Science
16
Scientific paper
Particle mixing rates ( D B ) calculated from excess 210 Pb gradients in sediments of the east equatorial Pacific range from 0.04 to 0.5 cm 2 /y, with variation of a factor of 3-4 at a single site. Diffusion of the 236 Ra daughter 222 Rn may affect 210 Pb distributions under conditions of slow mixing and low 210 Pb flux to the seafloor, as shown by a siliceous ooze-clay core which contained the fallout radionuclides 239,240 Pu and 137 Cs but no excess 210 Pb (relative to 226 Ra). There is no clear relationship between 210 Pbderived mixing rates and sediment type, accumulation rate or organic carbon flux to the sediments. Comparison of 210 Pb mixing rates with those calculated from 239,240 Pu and 137 Cs distributions reveals better agreement for a pulse input of the fallout radionuclides ( D B = 0.03-0.4 cm 2 / y ) than for continuous input at a constant rate ( D B = 0.1-1.6 cm 2 / y ), although the Pu and 137 Cs data are better fit by the latter model. The agreement may be fortuitous because 239,240 Pu and 137 Cs appear significantly deeper than 210 Pb 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 239,240 Pu / 137 Cs in the sediments decrease with increasing north latitude, and the trend reflects higher fluxes of 239,240 Pu near the weapons test site at Christmas Island (2°N). The 239,240 Pu / 137 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 239,240 Pu and 137 Cs have been released from sinking particles.
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