Mixing and cycling of uranium, thorium and 210 Pb in Puget Sound sediments

Details Mixing and cycling of uranium, thorium and 210 Pb in Puget Sound sediments Mixing and cycling of uranium, thorium and 210 Pb in Puget Sound sediments

Oct 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1949c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 48, Issue 10, pp.1949-1963

Computer Science

Sound

5

Scientific paper

Activity profiles of excess 234 Th, excess 210 Pb, 232 Th, 230 Th, 234 U and 238 U, and 228/232 Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of 228 Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and 210 Pb in this coastal zone. Comparison of excess 234 Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column. Surface sediment horizons with excess 234 Th have no excess 228 Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of 228 Th due to loss of soluble 228 Ra from pore water to the overlying water persist to 20-30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound. Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. 232 Th and 230 Th activities and 230/232 Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic 230 Th. Steady state 210 Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved 210 Pb in sea waters adverting into Puget Sound. Excess 234 Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess 210 Pb profiles. At least six of eight 234 Th profiles show that mixing within the 210 Pb-defined surface mixed layer is depth dependent. In three profiles, 234 Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the 234 Th data are consistent with 210 Pb profiles determined for the same sediments, strongly suggesting that 234 Th and 210 Pb are mixed equivalently and in a multilayered manner.

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