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.
