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consist of massive or laminated micrite and spar
calcite with calcite filamentous microbial remains.
No tablets were installed in this subenvironment.
(5) Dense, hard, laminated spar and micrite
calcite deposits formed in caves (subenvironment
f). Commonly the laminae are micrometre to
millimetre-thick and include calcite microbial
remains. The present surfaces of the caves include
mats of mosses, liverworts, bacteria and cyanobac-
teria that are slowly coated by calcite. The tablet
showed a very thin, discontinuous, greenish and
pinkish laminae of microbial origin. Some bryo-
phyte boundstones may form at the outer part of
the caves associated with facies 4.
Mineralogy
Mineralogical analyses were performed on the sedi-
ment sampled at the same time than water in areas
adjacent to the tablets. X-ray diffraction analysis
showed that the sediment invariably consisted of
low-magnesium calcite (average Mg content ¼
1.99% M), with quartz and ocasionally phyllosili-
cates and dolomite as trace components. No signifi-
cant variations in mineralogy were observed through
space or time.
Oxygen isotopes of water
The water d
18
O values showed little variation either
among subenvironments or downstream (Table 4,
Figs 11 & 12). Values ranged from - 9.08 to
- 7.90‰ SMOW in the cool periods and from
- 8.78 to - 8.08‰ SMOW in the warm periods.
Oscillation for each site was also small: standard
deviations were lower than 5% of the mean value.
Values of warm periods seem to be slightly
heavier than those of cool ones: mean values were
- 8.65‰ for cool periods and - 8.43‰ for warm
periods. This trend is opposite to the one displayed
by d
18
O composition of the sediment from tablets
(Figs 11 & 12).
These compositional changes can be related to
the common seasonal variation of the isotopic com-
position of rainfall in continental sites, with lower
values in cool periods than in warm ones (Leng &
Marshall 2004). Higher temperatures can also
have an influence through the loss of
16
O due to
an increase of evaporation. Water input in the
River Piedra comes mainly from groundwater, as
shown by the contrast between the marked seasonal
character of rainfall and the almost constant water
discharge. Thus, the water isotopic composition
should only change due to the limited variability
of the aquifer isotopic signature and to the small
changes produced by the runoff supply, whose
relevance is small with respect to the groundwater
supply.
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