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phases and the main climatic variations recognized
in the European area during the Last Termination
(Bj¨rck et al. 1998) and Holocene can be attempted.
The FOC Synthem appears to correspond to
the Late Glacial Interstadial warming event
(Bølling-Allerød interval of Mangerud et al. 1974
and GI-1 episode of Bj¨rck et al. 1998) dated to
about 13-14 ka BP in the North Atlantic region
(Lowe et al. 1994; Bj¨rck et al. 1998) and to
15 ka BP in southern Europe (De Beaulieu et al.
1994).
The successive FOC-BEL erosional phase could
be related to the cold and dry Younger Dryas event
(GS-1: Bj¨rck et al. 1998) dated to c.11kaBPby
several authors (Broecker 1994; Lowe et al. 1994;
Bond 1995; Bj¨rck et al. 1998). The same Late
Glacial erosional phase documented in the Valdelsa
terraced sequence has been also recognized in some
Tuscan rivers (Arno River - Aguzzi et al. 2007;
Ombrone River - Biserni et al. 2005).
The BEL Synthem age corresponds to the
'Optimum Climatic' phase generally considered as
a wet and warm period. It seems to have been an
active period of carbonate deposition in Central
Italy since coeval calcareous tufa (c. 7-8 ka BP)
have been reported from the Adriatic side of North-
ern Apennine (Cilla et al. 1994; Calderoni et al.
1996).
The BEL-POG transition can be easily correlated
with the dry Sub-Boreal period. Evidence of this
period is largely reported from Central Italy. At
c. 4000 years BP (uncal. 14 C age) deposition of lime-
stone at low-stand lake-level has been found at
Accesa Lake (Magny et al. 2006). A sudden drop in
the amount of pollens from arboreal vegetation,
related to a climatic deterioration, has been recog-
nized in the same period at Lagaccione and
Mezzano lakes (Magri 1999; Ramrath et al. 1999).
The latest POG Synthem is characterized by the
near disappearance of carbonate deposition. Actu-
ally thin carbonate crusts are found only in the
immediate surroundings of only one of the low-
thermal springs active in Valdelsa (Bagnoli
spring). This drastic reduction of carbonate depo-
sition may be related to the same event described
in all the European area as 'tufa decline' (Goudie
et al. 1993). Causes of this decline dated to the
Sub-Atlantic Period (last 2.5 ka; Goudie et al.
1993) are not yet clear (for a full discussion see
Goudie et al. 1993; Dramis et al. 1999) but their
effects are evident also in the Valdelsa carbonate
area. The oldest Valdelsa Synthem (ABB) has not
yet been directly dated.
deposition/erosion cycles in Central Apennine
lakes during the Late Pleistocene, it seems likely
that a similar regime, primarily determined by the
amount of rainfall and consequent variations of
the hydrological balance, could have also occurred
during the last 30 ka in the relatively near Valdelsa
area. Under such a climatic regime, increase of
water discharge should enhance karstic dissolution
and consequently amplify tufa deposition because
of the increased availability of calcium carbonate
in stream waters. Conversely, reduction of rain-
fall would result in reduced limestone dissolution,
less calcium carbonate being dissolved in stream
waters, and consequently lower rates of tufa depo-
sition (Goudie et al. 1993; Pentecost 2005). In the
latter situation, calcareous tufa aggradations and
vegetation are reduced, while downcutting is
intensified. These conditions are reminiscent of
the Erhart's (1956) 'biostasie' and 'rhexistasie'
scenario where periods of biologically-mediated
landscape stability alternate with period of down-
cutting and erosion. Similar deductions derive
from Vandour (1986) and Goudie et al. (1993),
who infer a stable and extensive vegetation cover
associated with stable soils as the key factor control-
ling tufa deposition.
The close relation between climate and tufa
deposition has been stressed by many studies
(Srdoˇ et al. 1983; Henning et al. 1983; Carrara
1991; Griffiths & Pedley 1995; Bessan¸on et al.
1997; Martin-Algarra et al. 2003) showing that
tufa deposition reached a maximum during warm
and wet interglacial periods. Pedley (1990) observes
that Holocene tufas are better developed in humid,
temperate climates, whereas their growth is
slowed by cold conditions, and that semi-arid
regimes can rarely maintain the high water tables
necessary for sustained tufa deposition. Dissolution
rate in the recharge area, which ultimately controls
downstream tufa precipitation rates, can be affected
by reduced rainfall, increased evaporation, and
falling water tables (Ford & Pedley 1996). As evi-
denced by Evans (1999), the virtual absence of
tufas in the Oligocene Brule Formation (South
Dakota, USA) appears to result from palaeoclimatic
conditions, since an increase of aridity across the
Eocene-Oligocene boundary is well-documented
in that region.
The alternation of depositional and erosional
phases in the Valdelsa terraced succession appears
to have been strictly linked to the availability of
water that is, to a rainy regime, since radiometric
dating indicates that at least the older sequences
(CAL, FOC and probably ABB Synthems) were
deposited during a glacial period (O.I.S. 2). As a
consequence, air temperature would not have sig-
nificantly influenced the encrusting capacity of
these waters.
Main climatic regime
According to Giraudi's (2004) interpretation of the
climatic
control
in the development of
the
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