Identifying resuspended particles using isotope ratios

Abstract

Three simple methods were developed to estimate the proportion of particles in lake water derived from resuspended material. These techniques use the different distributions of long and short-lived radioisotopes sorbed onto particles and were tested in the three basins of Lake Erie using 'Be, 13'Cs and 210Po/210Pb.While the concentration of 210Po on particles did not vary significantly in the lake, resuspended particles were characterized by high concentrations of 13'Cs and low concentrations of 7Be. The distribution of these radioisotopes is consistent with a simple mixing model in which the fraction of particles in the lake water derived from resuspension ranged from I 8 y0 to about 100 %. Higher concentrations of resuspended particles were found in deeper samples from the nepheloid layer and in the shallow western basin where thermal stratification was very weak.

Empirical evidence of the importance of sediment resuspension in lakes

“…Significant nepheloid layers often exist in these deep regions (Cornett et al 1994). Resuspension over larger areas may occur during a period of isothermal turnover resulting in ICM.…”

The 7Be/210Pbxs ratio as an indicator of suspended sediment age or fraction new sediment in suspension

“…Some 222 Rn escapes to the atmosphere, where it decays through a series of short-lived nuclides to 210 Ra. In contrast to this decadal-scale nuclide, the short half-life (t 1/2 = 53·3 days) of 7 Be allows its application to studies of erosion (Whiting et al, 2001;Wilson et al, 2003), sediment transport (Bonniwell et al, 1999;Matisoff et al, 2002b;Whiting et al, 2005), resuspension Robbins and Eadie, 1991;Cornett et al, 1994;Fitzgerald et al, 2001), and deposition (Sommerfield et al, 1999) on much shorter timescales. Beryllium-7 is a naturally occurring radionuclide, which is produced continuously in the upper atmosphere by cosmic ray spallation of nitrogen and oxygen.…”

A review of methods used to measure sediment resuspension

“…CS'~' (half life 30 years) and Be' (half live 50.3 days), may be used for measuring resuspension (Cornett et al, 1994). In general, Cs137 is accumulated in the bottom sediments (Santschi et al, 1990) and fresh material has lower activity.…”

“…On the other hand, Be' is removed quickly by natural decay in the bottom sediments, whereas its activity is higher in fresh material. Hence, resuspended particles are characterized by low Be' concentration (Cornett et al, 1994).…”