Calcareous tufa as indicators of climatic variability: a case study from southern Tuscany (Italy) - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 267

related to the activity over time of these thermal
springs appears to be supported by their areal distri-
bution. In fact, as the small Bagnoli spring, the
Caldane di San Marziale and Vene di Onci springs
are also located upstream of the segment of the
Elsa River Valley where carbonates were deposited.
It is possible to assume that the Pleistocene calcar-
eous deposits occurring in southern Valdelsa Basin
were connected with thermal springs that in part
are now dried up or buried by the overlying
calcareous tufa.
Chronological dating
The existing chronology of the Valdelsa carbonate
succession was estimated to span from Late Pleisto-
cene to present on stratigraphic evidence (Capez-
zuoli & Sandrelli 2004). During this period,
significant changes of the landscape occurred.
Several stages of erosion and fluvial-palustrine
re-establishment have been recognized.
With the aim to date the different depositional
events, four U/Th dates (performed at CERAK
Laboratory, Belgium) were obtained from compact,
micritic layers collected in the different terraces,
while four 14 C dates (performed with AMS method-
ology at Beta Laboratory, USA) were obtained from
wood and organic-rich layers of different terraces
(Table 1). Unfortunately the U/Th dating failed
because three of the samples (T1, T2, T4) were
affected by contamination from detrital or colloidal
elements, as evidenced by the 230 Th/ 232 Th isotopic
ratio and by the 230 Th/ 234 U isotopic ratio higher
than 1 (Table 1). Detrital contamination is one of
the main constraints of the U/Th dating and that
often occur (Garnett et al. 2004). The fourth
sample (T3), not affected by contamination, did
provide however a low 230 Th/ 232 Th isotopic ratio
attesting for an unreliable age (Table 1).
On the contrary the results of the four AMS 14 C
dates (Table 2) provide evidences to constrain the
time interval of development of the synthems, and
record their cyclic evolution and high incision
rates of the valley. These four 14 C datings, per-
formed on wood and organic matter found in the
rare organic-rich layers, commonly located at the
base of the carbonate bodies, are consistent with
their relative stratigraphic positions and suggest a
regular evolution, from the older to the younger
terraces, of the Valdelsa sequence (Table 2). The
reliability of absolute dating moreover appears to
be confirmed by the range of variability of d 13 C
values measured in the four samples (Table 2)
reflecting the extent of isotopic fractionation in
the fossil material after deposition. According to
Walker (2005, 17-55) the extent of isotopic frac-
tionation ranges close to the mean isotopic