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linked to the cosmic ray flux, a parameter that varies with solar activity and changes
in the Earths magnetic field as well as altitude and latitude (Kaste et al.
2002
; Walling
2013
). On a global scale, production is about three to five times higher at the poles
than the equator. Regionally, Wallbrink and Murray (
1994
) found that more than
90% of
7
Be is deposited as wet deposition and, thus, annual fallout varies primarily
in response to annual precipitation amounts. For a given precipitation amount, annual
fluxes are lower for lower latitudes (Walling
2013
).
Although it is assumed that
7
Be is strongly bound to particulate matter upon
reaching the soil, few studies have actually examined this postulate. Taylor et al.
(
2012
), however, found by means of laboratory experiments that
7
Be was rapidly
sorbed to unsaturated soil particles and remained attached to soils under commonly
observed field conditions.
7
Be differs, however, from
137
Cs and
210
Pb in two impor-
tant ways (Table
3.1
). First, it has a short half-life of 53.3days. Second, in part due
to its short half-life, it is concentrated in the upper most soil layers, generally within
the upper 1 cm (Fig.
3.3
e) within undisturbed soils. Within this shallow zone, it typ-
ically exhibits an exponential downward decrease in concentrations. In agricultural
areas, this exponential downward trend will occur in soils that have not be recently
ploughed because of its continuous flux to the soil surface, but will be non-detectable
in recently tilled soils as a result of mixing and dilution with deeper soil particles
devoid of
7
Be (Walling
2013
).
3.3 Applications
3.3.1 Use as a Geochemical Tracer to Determine Sediment
Provenance
As was the case for other types of geochemical fingerprinting parameters, FRNs
have been applied to determine the provenance of suspended, channel, and reser-
voir materials (Nagle et al.
2007
; Huisman and Karthikeyan
2012
; Kwan Kim et al.
2013
). However, their vertical distribution in surface soils makes them particularly
well suited to estimate the relative contribution of sediment derived from surface
and subsurface sources. More specifically, in undisturbed areas, maximum concen-
trations of
137
Cs
210
Pb
ex
and
7
Be occur at, or in the case of
137
Cs, immediately
below the ground surface. Concentrations subsequently decrease exponentially typ-
ically reaching non-detectable levels within about 10, 20, and 30 cm of the surface
for
7
Be
,
210
Pb
ex
, and
137
Cs, respectively. The overwhelming majority of the total
inventory, then, is confined to the upper few centimeters of the Earth's surface. As a
result, surface materials eroded by sheet or shallow-rill processes exhibit high con-
centrations of the FRNs, whereas subsurface materials eroded from gullies, channel
or bank deposits, dirt roads, or any other barren surface where the upper few centime-
ters of sediment have been stripped away will often contain little or no detectable
FRNs (Wallbrink and Murray
1993
; Olley et al.
2013
).
,
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