Sediment accumulation and Holocene - OzCoasts

Sediment accumulation and Holocene evolution of the Fitzroy River lower floodplain
detailed chronology prior to 1958. A variation on the constant rate of supply
(CRS) model (Appleby, 2001) has been chosen to model excess 210 Pb in the
upper 60 cm. The chronology is established by fixing the age of sediment at 60
cm at 45 years, based on the 137 Cs profile, and assuming constant excess 210 Pb
flux since that time. Given the sporadic nature of flows required to deliver
sediment to the lagoon the assumption of constant excess 210 Pb flux is
questionable. However, inaccuracies in ages introduced by this assumption are
likely to be limited to just a few years, and longer-term (decadal) accumulation
rates are likely to be approximately correct. The calculated ages for the 0–60 cm
depth interval are listed in Table 14. The average accumulation rate over the 45-
year period since 1958 is 1.3 cm yr -1 (60 cm/45 years).
As noted above, distinct laminations are seen in the sediment profile, especially
in the interval spanning 46–60 cm. This interval also contains sections with
distinctly higher mean grain sizes. The 210 Pb- 137 Cs chronology dates these layers
as post 1958, and it is likely they span the period from the mid-1950s to the mid-
1970s, also coinciding with the Brigalow Development Scheme. This period
included a series of large floods in the lower Fitzroy River, and it is possible
these floods are responsible for the variable character of the sediment deposited
in this depth interval.
An interesting and important feature of the excess 210 Pb profile is the
reappearance of detectable excess 210 Pb at the base of the core in the region of
high organic matter (125–131 cm interval). The non-detection of excess 210 Pb in
the 60–125 cm interval probably represents the effects of high sediment fluxes
diluting excess 210 Pb activities below detection, rather than indicating that the
sediment is older than 120 years or so, the usual dating limit for 210 Pb. The fact
that significant activities of excess 210 Pb reappear at 125 cm suggests the age of
the base of the core is less than approximately 120 years, the maximum period
of time over which one would expect to be able to detect fallout 210 Pb.
OSL dating confirms the scenario described above (Table 16). The sediment just
below the first appearance of 137 Cs (64–69 cm) is dated at less than 70 years
(CLW-49/1, 45 ± 25 years), and the sediment in the organic rich layer that
contains detectable excess 210 Pb is dated at 120 ± 28 years (CLW-49/3).
Sediment at 120–125 cm is dated at 87 ± 10 years (CLW-49/2). Using this latter
date and the 137 Cs age of 45 years at 60 cm gives a sediment accumulation rate
of approximately 1.5 cm yr -1 for the first half of the 20 th century. This is similar to
the rate for the second half determined from 137 Cs, 1.3 cm yr -1 , based on 60 cm
of sediment accumulation in 45 years.
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