The biogeochemistry of 210Pb and 210Po in fresh waters and sediments

Abstract

The focus of this work was geochemical cycling of 210Pb in lakes, including the water column, sediments, and their interactions with each other. A special goal was to elucidate processes that might influence the distribution and fluxes of the radionuclide in ways that could effect 210Pb sediment dating. A mass balance for the epilimnion showed that 210Pb inputs by precipitation were matched by outputs on settling particles, indicating that direct uptake by bottom sediments was inconsequential. Below the epilimnion, vertical eddy diffusion was calculated by the heat flux gradient method including corrections for both radiant heating and heat loss to sediments. Vertical mixing was very low because of stability imparted by a steep temperature/density gradient extending right to the sediment water interface. Anoxic conditions caused remobilization of reduced iron, which reprecipitated at the oxycline and returned to the bottom via settling. 210Pb followed the same pattern except that, at the interface, it was scavenged rather than precipitated. Below the zone of precipitation, both 210Pb and iron distributions could be described by a model consisting of constant release from anoxic sediments, horizontal transport, and simple dilution in the water column. Cycling of 210Po was complicated by unidentified additional factors. A finite difference model (SEDIMIX) was used to find the combination of sedimentation and Fickian redistribution that provided the best fit to the 210Pb sediment data. Sedimentation rates were found to increase linearly with overlying water depth. The magnitude of the Fickian component of 210Pb transport was equal to calculated rates of pore water diffusive flux, which is probably more important than sediment mixing in this lake. 210Pb, 210Po, and ancillary geochemical parameters were measured on the solid fraction and pore waters of two cores taken from the deepest basin in August and September. The radionuclides were two orders of magnitude higher than in overlying water and had steep concentration gradients that could support substantial diffusive fluxes. Fe, Mn, S(II), and alkalinity did not have similar gradients. 210Pb partition coefficients ranged from 1500 to 15000, decreasing with depth, and seemed to be controlled by sorption on iron oxides. Remobilization to the water column apparently comes from a thin layer of iron-rich floc near the sediment water interface. Deeper in the cores, diffusive transport can cause redistribution of 210Pb to an extent that can affect 210Pb dating.