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 137

effects should be recorded by the d 18 O of all sedi-
ment samples of this period. Thus, other type of
factors must have accounted for the anomaly of
sediment d 18 O in the last year.

the shape or size of calcite crystals nor the density
of the deposits presented definite seasonal vari-
ations. In some stromatolitic deposits, the sediment
from warm periods, compared to that from cool
periods, was thicker, lighter in colour, less dense,
and the thickness of the tube-like microbial bodies
was thinner. In spongy, moss- and alga-rich tufa,
seasonal

Conclusions

The 3.5-year monitoring period of the River Piedra
within the Monasterio de Piedra Natural Park
showed an intense tufaceous sedimentation in this
area of Mediterranean environmental conditions.
The Park environment enables seasonal control of
physical, hydrochemical and biological parameters,
characterization of different subenvironments and
facies, measurement of sedimentation rates and
analysis of stable isotope composition of water
and of the associated deposits. The different sedi-
mentation intervals were identified in the tufa
cross-sections on tablets by means of the thickness
measurements taken every six months with the
MEM.
The following conclusions can be drawn from
the six-monthly sedimentological and geochemical
study of the sedimentary record on tablets and
the analysis of the associated environmental par-
ameters monitored in this natural laboratory from
1999 - 2003:
(1) Five main types of fluvial facies were
distinguished: (1) Dense stromatolitic tufa, formed
in fast flowing water areas along the river bed; (2)
Dense to porous, commonly loose, massive tufa,
deposited in slow flowing water areas of the river;
(3) Porous, spongy, coarsely laminated tufa,
formed in stepped waterfalls with bryphytes, algae
and cyanobacteria; (4) Steep banded deposits devel-
oped in vertical waterfalls; and (5) Dense, hard
laminated deposits formed in caves. The seasonal
study from tablet records was made in the first
three facies.
(2) The most suitable tufa record for seasonal
studies was that of laminated facies formed in fast
flowing water areas because this facies had the
most continuous sedimentation record with the
highest rates, and the sedimentological and geo-
chemical features were seasonally more marked.
Although stepped waterfalls also record high sedi-
mentation rates, erosion events prevented a continu-
ous sedimentary record. Facies from slow water
flowing areas presented low sedimentation rates
and frequent erosion events.
(3) Erosion can locally alter the seasonal pattern
of tufa sedimentation rates. So can slight variations
of climatic conditions (e.g. of temperature) with
respect to normal conditions during some seasons.
(4) Sedimentological analysis revealed some
differences between sediments from warm and
cool periods, although no regular patterns exist,
apart from general thickness variations. Neither

differences of these features were

less clear.
(5) The d 18 O composition of the sediment
showed a rhythmic variation, with higher values in
cool periods and lower values in warm periods.
Mean d 18 O isotopic composition was 28.42‰
PDB for the warm periods and 27.67‰ PDB for
the cool periods. Given the small variability of
water d 18 O, these periodic variations in d 18 O com-
position were mainly caused by the fractionation
effects linked to stream temperature changes
between seasonal periods. Mean d 13 C isotopic com-
position was 27.45 PDB for the warm periods
and 27.49 PDB for the cool periods. The d 13 C
composition of the sediment did not show a clear
rhythmic pattern. This is attributed to changing con-
tributions of the fractionation processes on a local
scale during the studied periods.
(6) The calculated temperatures, for a theoretical
equilibrium precipitation, accord with the measured
temperatures and display the same seasonal trend.
As a consequence: (a) tufa d 18 O values can be used
to estimate relative paleotemperature variations in
fluvial carbonate deposits; and (b) laminated tufa
sediments can be useful as high-resolution records
of temperature change.

This work was funded by project REN3575/CLI of the
Spanish Government and European Regional Develop-
ment Fund, and forms part of the activities of the
Continental Sedimentary Basin Analysis Group (Arag ´ n
Government). We thank the Confederaci ´ n Hidrogr ´ fica
del Ebro for providing data on water discharge, and
the Stable Isotope Analysis Laboratory of the University
of Salamanca and the department of SEM of the University
of Zaragoza for scientific and technical assistance. We are
grateful to the Monasterio de Piedra Park management and
staff who allowed and facilitated the fieldwork. The editors
of the volume and two reviewers are thanked for their sug-
gestions and meticulous revision.

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