Fallout Radionuclides - Application of Geochemical Tracers to Fluvial Sediment - page 79

fill. Quantitatively, they calculate sedimentation rates by rearranging the CIA model
described above such that (Appleby and Oldfield 1992 ):

A 0 e − ʻ r

A = (3.8)

where r is the mass flux, given in g cm − 2 d − 1 , and m is the cumulative dry mass of
sediment per unit area above the lower boundary (g cm − 2 ) (i.e., above the buried
layer). This particular model assumes a constant initial concentration of 7 Be such
that its rate of accumulation is proportional to the mass of sediment accumulated. The
7 Be activity in the cores, then, should decrease with depth monotonically where this
assumption is met. Fisher et al. ( 2010 ) note that care should be taken when applying
the model as its results represent short-term depositional rates that are constrained
by the assumptions inherent in the CIA model, and are not necessarily indicative of
erosional or depositional rates that occur at other sites spread across the channel bed.
Although it is widely accepted that vertical trends in 7 Be activity can pro-
vide important insights into the time and nature of sediment accumulation and
remobilization processes, the resulting trends may be complicated by grain filtra-
tion along coarse (gravel) bed streams. Grain filtration is the process in which fine
particles are moved downward into the interstitial pore spaces of a stable, coarser
sedimentary layer at the channel surface (Gooseff et al. 2006 ; Haynes et al. 2009 ).
In a study of the regulated and unregulated streams in Vermont and New Hampshire,
Gartner et al. ( 2012 ) found that 7 Be activity in some cores varied in a non-systematic
manner with depth. They interpreted these profiles to result from channel bed scour
and fill processes similar to that suggested by Fitzgerald et al. ( 2001 ) in which higher
activity layers could be buried by low activity layers (and vice versa); the two lay-
ers consisting of sediment from different sources. However, Gartner et al. ( 2012 )
suggested that downward decreasing 7 Be activities observed in a number of their
cores was due to filtration where 7 Be activity was attenuated with increasing depth,
rather than the continuous accumulation and burial of 7 Be tagged sediment. Thus,
the systematic downward decrease in activity can result from both a continuous
accumulation of sediment and/or filtration. Exactly which process is of most impor-
tance can probably be deciphered for most rivers on the basis of the stability and
grain size of the bed material. Filtration is likely to be predominant along rivers with
coarse-grained bed material that is relatively stable during low- to moderate-flood
events.

3.5 Use of Fallout Radionuclides as an Age Dating Tool

Fluvial deposits associated with floodplains, terraces, riparian wetlands, and reser-
voirs contain an historical record, often extending over hundreds to thousands of
years, of sediment source, production, transport, and storage within a catchment.
While this archive is often discontinuous and incomplete, it can be effectively
unraveled to gain insights into a wide range of environmental problems such as
the effects of natural and anthropogenic disturbances (deforestation, wildfires, urban