Page 12 - 15 New data on the Holocenic sea-level

134 F. Antonioli et al. / Global and Planetary Change 34 (2002) 121–140
platforms is restricted to the intertidal zone and, rarely, always sealed by Serpulids at the time these are
to the uppermost part of the infralittoral zone; (3) the
platforms are easily accessible for sampling; (4)
Vermetids can be precisely dated by radiocarbon
techniques. The sampled Vermetid reefs have a max-
imum thickness of 40 cm, and their 14C ages indicate
that they started to grow since a few centuries from
the Present (Table 2).
Antonioli et al. (1999b) described the biological
characteristics and sampling zone of the Dendro-
poma reef of Capo San Vito. The range of the
semidiurnal tide at San Vito is 0.12 m; in flat
coastal areas, Dendropoma platforms, 5 – 10 m
wide, coalesce to form a single, uninterrupted rim
stretching for several kilometers. Each platform is
20– 40 cm thick, and the upper 8 –10 cm consist of
living organisms. The upper, living part lies at
mean sea level and, consequently, it is exposed
during low tide and submerged during high tide. A
vertical section of the reef was obtained in order to
make morphological observations and sampling for
radiocarbon dating feasible. Radiocarbon dating was
carried out on the most ancient, fossil part of the
reef, which is located at 30– 40 cm below present
sea level. Two types of sampling were performed:
Vermetids were both directly detached from the
reef, and collected on the beach 30 to 100 m from
the shoreline. The inner margin of the terrace,
attributed to the Tyrrhenian transgression, has been
measured by a Total Station (Nikon), with the zero
being the surface of the reef.
Submerged speleothems have been successfully
used to gain information on sea-level fluctuations
through time (Lundberg and Ford, 1994). These
carbonate deposits are particularly useful to recon-
struct sea-level changes when they exhibit both con-
tinental and marine layers (sensu Antonioli et al.,
1998b). On the coasts of the Capo San Vito Promon-
tory, submerged speleothems do not occur at depths
exceeding 5 m below sea level and do not show the
association of marine and continental layers. On the
contrary, the speleothem sampled at Marettimo, which
comes from the submerged cave at ‘‘Punta Martini
Cape,’’ exhibits marine organic overgrowth (Fig. 10).
Dating was carried out on both continental layers of
speleothems and marine deposits, i.e. on shells of
Lithophaga lithophaga, which are among the first
marine organisms that colonize speleothems, and are
reached by the sea, marking in this way the sea-level
rise (Antonioli and Oliverio, 1996).
Lithophaga, as all others biological indicators used
in sea-level rise analysis (Balanus, Vermetid, Litho-
phyllum, Serpulid and others), obviously records the
age of the arrival and the starting of growth of the
shells. The altitude of this retrieval records a sea-level
value which is slightly lower than the effective value.
This is due to the years necessary to develop the
CaCo3 of the shell. In order to achieve the important
results on the raising of the sea level during Holocene,
using and dating Tahiti corals, Bard et al., 1996,
wrote: ‘‘these assemblages are diagnostic of a high
water energy reef front at depths less than 6 m below
mean sea level.’’ Cabioch and Montaggioni, 1999,
discuss the error margins about habitat and palae-
obathymetry on thirty species of corals, coralline
algae and vermetid, supplying the bathymetry (with
metric and decimetric precision) within which the
various species are developed. Antonioli et al.,
2001, in reference to Serpulids and Lithophaga that
overgrowth the submerged speleothems in Italian
coasts, demonstrate that the error between the sea
level and the altitude of the sampled shells should be
less that one induced from 14C analysis.
Table 2 shows absolute ages obtained from Verme-
tids, continental portion of speleothems from the Capo
San Vito Promontory and from the Marettimo cave
and Lithophaga shells from the Marettimo submerged
speleothem only. The results (which span through
hundred years ago for Vermetids, and thousand years
BP for speleothems) have been used to reconstruct the
Late Holocene sea-level rise curve, the validity of
which has been assessed on the base of the verified
neotectonic constraints (Section 3.2). From these, the
average post-Tyrrhenian uplift rate obtained for the
coastal sector D (northern Castelluzzo Plain) is 0.032
mm/year. Since the Vermetid reefs sampled from
sector D yielded radiocarbon ages of the order of
hundred years, by taking into account the assumed
average uplift rate, it can be inferred that neotectonics
just slightly contributed to the vertical dislocation of
the present-day Vermetid reef height and, thus, affect
the palaeo-sea level datum with a quite small error
(order of 0.01 m at max.), which is well under the
range of about 8 cm introduced on accuracy by the
living belt thickness of Dendropoma.