Geology Reference
In-Depth Information
Sodium carbonate fraction
This component is not normally determined in P
fractionation studies but was included here to
remove all the remaining P from the sample. It can
be regarded as 'recalcitrant P' and will include
occluded or strongly-bound mineral P plus any
organic P not extracted by other methods. It is gen-
erally regarded as 'biologically unavailable' and it
seen to be an insignificant fraction in all of the
samples, ranging from 1 - 6% of the total.
The carbonate fraction might be expected to
contain most of the phosphorus since it is by far
the greatest component of tufa, and phosphorus
co-precipitation readily occurs. However the
results demonstrated that in only one sample was
this the case. The older tufas contained a greater pro-
portion of P in the carbonate fraction and this
suggests that some additional P is sorbed or
co-precipitated after deposition. The process
appears to take place over a long time, because in
the case of the Palustriella facies, the carbonate
and other fractions remain similar over a period of
26 years. The 26-year old tufa sample was collected
from a deposit that began to form in 1983 and it was
evident that while there were no obvious intact
bryophyte remains within it, the gross composition
of the material had otherwise changed little. In
Malham Tarn, a marl lake less than 5 km from
these sites with waters derived from the same
limestone formation, calcium carbonate, the major
component of the sediment, contained only 14%
of the total phosphorus (Pentecost 1998). Here, the
total P content of the sediment was an order of mag-
nitude higher than that of the tufas. In the Tarn sedi-
ments however, the proportion of organic matter
was much higher owing to the retention of lake
seston.
Little is known of the form of the phosphorus in
these carbonates although it appears likely that most
exist as a calcium phosphate co-precipitate (House
1990). Studies of speleothem using NMR spec-
troscopy indicate the existence of monetite
(CaHPO
4
) and incorporated PO
4
ions in the calcite
lattice (Mason et al. 2007).
There has been much interest in the association of
phosphorus with calcium carbonate in freshwaters
(e.g. Koschel 1983; Istv
´
novics et al. (1989);
Talling & Parker 2002). Pettersson & Bostr ¨m
(1986) in a detailed study of phosphorus in the
sediments of calcareous Lake Balaton, Hungary,
estimated a phosphorus co-precipitation rate of
0.6 - 2.2 mgP/m
2
/day onto calcium carbonate. Esti-
mates may also be made for the tufa sites. Deposition
rates of tufa in the Yorkshire streams are known to
be about 3 mm/year. Given the total P values in
Table 1, total P removal rates to tufa would
amount to about 1 mgP/m
2
/day, although only
about 20% of this would be associated with the
calcite. Gordale Beck, one of the sampling sites,
has a 2 km length where tufa is deposited exten-
sively. With a width of about 3 m, a total P content
of c. 10 mg/l and a discharge of about 200 l/sec,
the daily P removal rate to tufa would be about
3 g. This is only about 4% of the total amount of P
entering with the stream, suggesting that P uptake
by tufa is unlikely to have a significant effect on
the stream biota. Similar values were obtained for
the other sampled stream, Waterfall Beck.
Discussion
There have been few reports of P analyses from
tufas and travertines. Demovic et al. (1972) reported
some values from Central Europe and in a previous
study of UK tufas, Pentecost (1993) found that the
UK mean P content was 210 ppm with no significant
difference between the levels in modern and fossil
material. These data were summarised in Pentecost
(2005) where an absolute range of 8 - 950 ppm was
noted. Similar amounts of P have been reported
from speleothem by Huang et al. (2001) and
Borsato et al. (2007). In an Italian speleothem dis-
playing annual laminations, it was found that phos-
phorus levels varied seasonally and were associated
with an organic-rich layer thought to represent an
autumnal infiltration phase, but there appear to be
no P fractionation studies of speleothems. In tufas,
organic matter content may also vary seasonally
owing to variations in plant productivity, but high
resolution studies of P levels have not yet been
made. House et al. (1986) found Ca:P ratios of
200 - 2500 in some calcareous stream deposits
depending on the total P in solution in one exper-
imental channel in the southern UK. These ratios
are far lower than those found here, which ranged
from 2400 - 8900.
River sediments generally contain much higher
levels of total P than tufas. A global survey by
Froelich (1988) found most to be in the range
1150 - 1250 ppm. The low values found in the tufa
probably reflect the small organic matter and clay
content, and the oligotrophic nature of the associ-
ated waters.
Fractionation studies of phosphorus were first
applied to soils and lake sediments to gain an under-
standing of the form of the element in these
materials and their availability to plants for
growth. The water-soluble fraction is considered
part of the 'biologically available sediment P'. It is
in fact the most available fraction to plants. In the
tufa, a small but significant proportion appears to
be present that could be available to aquatic
plants, particularly the bryophytes since they
develop rhizoids that can penetrate the surface
layers of the deposit.
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