Seasonal record from recent fluvial tufa deposits (Monasterio de Piedra, NE Spain): sedimentological and stable isotope data - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 135

Carbon and oxygen stable isotopes of the
tufa record

which corresponds to a stepped waterfall with
moss development, displayed a more irregular sea-
sonal d 18 O pattern and the isotopic differences
between warm and cool periods were smaller than
in the samples from fast flowing conditions
(Fig. 12). Waterfalls can produce a strong degassing
effect than can influence tufa d 18 O formed in water-
falls and in areas downstream of these (Andrews
et al. 1997). In the present study, this process does
not seem to have had a significant effect in the
tufa produced downstream of the waterfalls.
The periodic variations in d 18 O composition are
thought to be mainly caused by the fractionation
effects due to changes in temperature between the
warm and cool periods, as interpreted in other
fluvial systems that present little seasonal variation
in d 18 O composition of water. Similar conclusions
have been reported for other modern and subrecent
seasonal laminated tufas in semiarid conditions
(Chafetz et al. 1991; Matsuoka et al. 2001;
O'Brien et al. 2006; Kano et al. 2007), with differ-
ences between sediment from warm and cool
periods of about 1 - 1.5‰ PDB. These values are
close to those obtained in this study (mean d 18 O
difference is 0.75‰).
The small d 18 O variability and lack of a spatial
pattern among sediment from the three tablets in
fast flow conditions, along with the little d 18 O varia-
bility of water, leads to infer that seasonal changes
in d 18 O primarily reflect the water temperature
variations.
Exceptions seen in tablet 7 (warm period of 2000
and cool period of 2001 - 2002) might be related to
sampling problems, due to the common erosive pro-
cesses that make difficult the identification of the six-
monthly record in the stepped waterfalls (Figs 4b and
7). These problems might also account for the less
contrasted seasonal d 18 O differences in this sub-
environment. On the other hand, CO 2 degassing in
waterfalls can locally alter the sediment d 18 O, both
by varying the water d 18 O and the fractionation
process. In contrast, exceptions in tablets 2 and 5,
mainly the transition between the last two periods,
where the seasonal trend was not so evident, cannot
correspond to sampling errors and the explanation
of the anomaly is not clear.

The sedimentary record on tablets that allowed
identification of warm and cool depositional inter-
vals was sampled for analysis of stable isotope com-
position of sediment of every warm and cool period.
This record corresponded to laminated deposits
formed in fast flowing water areas and to moss-
and alga-rich, crudely laminated deposits formed
in stepped waterfalls. Four tablets were selected
for this purpose: three with laminated deposits and
one with moss and alga deposits. The total number
of intervals anlysed was 25: 14 ascribed to cool
periods and 11 to warm periods, which correspond
to those indicated by six-monthly thickness

measurements with the MEM.
The isotopic composition of the sediment ana-
lysed in the four tablets ranged, for the warm
period samples from 28.07 to 27.07‰ PDB for
d 13 C and from 28.81 to 27.92‰ PDB for d 18 O,
and for the cool period samples from 28.30 to
26.88‰ PDB for d 13 C and from 28.32 to
27.21‰ PDB for d 18 O. Mean isotopic composition
was d 13 C ¼ 27.45 ‰ and d 18 O ¼ 28.42‰ PDB
for the warm periods and d 13 C ¼ 27.49 and
d 18 O ¼ 27.67‰ PDB for the cool periods
(Table 4; Fig. 11). Although the fields of warm and
cool period samples overlap each other, the
samples are partially set apart into two fields by
d 18 O values. The difference in mean d 18 O values
between warm and cool periods was 0.75‰. In con-
trast, d 13 C values do not show separate isotopic fields
of the two periods. The difference in mean d 13 C
values between warm and cool periods was 0.04‰.
These d 18 O and d 13 C values are within the range
of other recent tufas (Andrews et al. 1997; Ihlenfeld
et al. 2003; Andrews & Brasier 2005). As in many
other fluvial carbonate systems, in the study case cor-
relation between d 18 O and d 13 C values was very
poor, which accords with fractionation processes
driven by different environmental parameters.

Oxygen isotopes
The d 18 O composition of the sediment of the four
tablets showed a rhythmic variation, with higher
values in cool periods and lower values in warm
periods (Fig. 12). Some exceptions to this pattern
occurred in some intervals of tablet 7, and in
tablets 2 and 5 the sediment of the warm period of
2002 had no isotopic variation or slightly higher
values than in the following cool period.
As for d 18 O variations among the three tablets
corresponding to fast flowing conditions (tablets 2,
3 and 5; Fig. 12), these were small and did not
show any spatial pattern for any of the warm and
cool periods. In contrast, samples from tablet 7,

Carbon isotopes
The d 13 C composition of the sediment of the four
tablets did not show a persistent rhythmic pattern
as that of d 18 O (Fig. 12). Only in tablet 3 lower
values of warm periods alternated with higher
values of cool periods. In the rest, the six-monthly
d 13 C variation was irregular, so that both high and
low values were recorded during cool and warm
periods. In addition, no significant variations in
d 13 C were seen between the two subenvironments