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192 M. Vacchi et al. / Earth-Science Reviews 155 (2016) 172–197
that it was difficult to robustly support a tectonic component in the cur- contribution in this area may be a good explanation for the misfit be-
rent elevation of the index points. However, the RSL record in this re- tween RSL index points and the predicted curve. However, in the early
gion is based on a restricted number of index and limiting points and and mid Holocene, the sole index point from Is Mistras closely matches
further investigations are required to better assess the mid to late- the RSL prediction (Fig. 8, #8) while lagoonal index points from Cagliari
Holocene evolution of this area. and Oristano (Antonioli et al., 2007) are significantly lower than the
Furthermore, our database indicates that the major western Medi- minimal predicted curve. Such a misfit could be related to some dating
terranean subsidence trends are recorded in the Ebro Delta and the problems (both in Is Mistras and in Cagliari) or with compaction (even if
northwestern Adriatic Sea. The comparison between the elevation of the early Holocene record is based on beachrocks and basal samples). At
the index points from the Ebro Delta and the predicted RSL changes the moment, we cannot robustly assess the mid to early Holocene RSL
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(Fig. 6, #2) indicates subsidence rates of ~1 mm a between ~6.0 and history in this sector of the western Mediterranean and additional
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~3.5 ka BP, rising to ≥ 2mm a during the last ~2.5 ka BP. This subsi- datapoints are required.
dence pattern is chiefly controlled by the increase in sediment loading Similarly, in region #7, beachrock samples (virtually incompress-
following the RSL stabilization at ~7.0 ka BP. The increasing subsidence ible) are found significantly lower than the predicted curve, between
rates recorded in the last ~2.5 ka BP are most likely linked to the transi- ~2 and ~7.1 ka BP. Here, the magnitude of the misfit(significantly great-
tion from estuarine to deltaic environments at the river-mouth er than the remaining regions, especially in the Tyrrhenian Sea) calls for
(Törnqvist, et al., 2008; Maselli and Trincardi, 2013) with a consequent alternative explanations mainly focused on the quality of the RSL data-
increase of sediment loading. Furthermore, recent studies in Mediterra- points. Disparities arising from radiocarbon dates of beachrock bulk ce-
nean deltaic contexts have revealed a negative correlation between sub- ment are not unusual (e.g., Vousdoukas et al., 2007), especially for sam-
sidence rates and time, translating the rapid compaction of the latest ples during the early 1980s (like most of the beachrock of this region,
Holocene deposits. Compaction rates progressively decline with the in- Nesteroff, 1984). Results of recent coring in the Gulf of Sagone (mid-
creasing age of sediments (Marriner et al., 2012a). The mechanistic ex- eastern Corsica) placed the RSL above -2.7 m MSL at ~3.8 ka BP and in-
planation for this pattern appears consistent with the rapid compaction dicate that sea-level rise in the last ~2.5 ka BP was within 1 m (Ghilardi,
of the youngest delta sediments that undergo the most important phase 2015). These data suggest RSL was significantly higher than the one in-
of volume loss, through dewatering and oxidation of organic material, dicated by the beachrock samples, at least during the late Holocene.
during earlier periods following deposition (Becker and Sultan, 2009). As stated at the beginning of this section, the combined effects of
Our data corroborated, using a larger dataset, the general subsidence glacio and hydro isostatic components in the western Mediterranean
trend in the northwestern portion of the Adriatic Sea (Fig. 10,#16,#18) resulted in a continuous rise in RSL during the Holocene.
already reported by Antonioli et al. (2009). South of the Po Delta, the ob- The sole exception is represented by the mid-Holocene highstand in
served subsidence is the sum of sediment compaction and long-term southern Tunisia (Gulf of Gabes) that, according to our record, occurred
negative vertical motions related to the neotectonic framework of the between ~6.0 and ~5.0 ka BP and did not exceed 1.5 m above MSL. How-
area (e.g., Ferranti et al., 2006; Antonioli et al., 2009). Comparison be- ever, Morhange and Pirazzoli (2005), on the basis of 14 C dating of a
tween the elevation of basal and intercalated index points from the subtidal shell (Petricola sp.) and vermetus (Vermetus triqueter), suggest
same area (and thus with comparable tectonics), gives insights into a highstand at ~1.9 m during the same period. More recently, OSL dating
the compaction-related subsidence. We attempted this comparison in of a beachrock outcrop in the Gulf of Gabes suggested a RSL at 1.4±0.4
Codigoro and Comacchio (Appendix A, Cibin and Stefani, 2009; Sarti m MSL at ~6.0 ka BP (Mauz et al., 2015a). This estimate, coupled with
et al., 2009) and the calculated subsidence rates are ~0.5 mm a -1 for our result, seems to suggest that the magnitude proposed by
thelast~9.5kaBP. Morhange and Pirazzoli (2005) is slightly overestimated and may be af-
Human-induced subsidence explains the significant scatter in the fected by levelling errors with respect to the biological MSL.
index points from Venice lagoon (region #16, Fig. 10) especially during Both the absence of significant historical seismicity and the long-
the last 2.0 ka (Serandrei-Barbero et al., 2006; Antonioli et al., 2009). In term tectonic stability (Fig. 2) suggest that the southern Tunisia high-
general, an assessment of the variability in subsidence trends in the stand is purely of isostatic origin (Stocchi et al., 2009; Mauz et al.,
northwestern sector of the Adriatic Sea is challenging. A comparative 2015a). The geomorphology of the inner part of the Gulf of Gabes and
study, using de-compacted coring data, long-term vertical movements the continental levering effects (e.g. the extensive upward deflection
and the geometry of local faults would significantly improve the under- of continental interiors and the broad subsidence of ocean basin,
standing of the Holocene RSL evolution of this area. Mitrovica and Milne, 2002) play a key role in the occurrence of this
There is a small shift in the RSL curve between the eastern border of highstand. In fact, due to the wide and shallow (≤30 m depth) continen-
the Rhone Delta and the remaining sites of southern France (region #3, tal shelf (Sammari et al., 2006), the inner part of the Gulf of Gabes acts as
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Fig. 6). The Rhone river shows major progradation rates (≥150 m a ) a pure continental margin where the levering effects are not
during the last 1.7 ka BP leading to significant sediment loading, espe- counterbalanced by the ocean loading. Thus, the subsidence driven by
cially on the eastern part of the delta (Maselli and Trincardi, 2013). the water loading in the remaining part of the western Mediterranean
However, some localized tectonic subsidence has been evoked as a is not able to offset the highstand in this region.
cause for this peculiar RSL pattern (Vella and Provansal, 2000; Vella The timing of this highstand (~5.5 ka BP) suggests a negligible role of
et al., 2005). These authors excluded compaction-related subsidence Northern hemisphere ice sheets due to the mutual cancellation of the
because most of the index points are basal peat samples that directly meltwater contributions from the Laurentide and Fennoscandian ice
overlie the uncompressible Pleistocene substratum. Nonetheless, sheets after ~7 ka BP (Stocchi and Spada, 2009; Stocchi et al., 2009).
whether it is local tectonics or major sediment loading that has caused Conversely, our record improved and refined the RSL reconstruction
the subsidence, the eastern Rhone data-points are not indicative of the provided by Stocchi et al. (2009) and Mauz et al. (2015a) confirming
RSL history of southern France. that the Tunisian high-stand is compatible with the melting history of
An intriguing open issue is represented by the RSL history of region the remote Antarctica ice sheet (Bentley, 1999; Stocchi and Spada,
#7 (northern Sardinia and southern Corsica, Fig. 7) and #8 (southwest- 2009; Stocchi et al., 2009).
ern Sardinia, Fig. 8). Here, both lagoonal and beachrock index points are
placed significantly below the minimal predicted curve, especially for 6.3. RSL variability along the western Mediterranean basin
the mid-Holocene. Both islands are considered to have been tectonically
stable since the last interglacial (Ferranti et al., 2006) and ongoing ver- In order to better assess the spatial variability of Holocene RSL
tical movements are negligible (Serpelloni et al., 2013). As for the mid- changes along the western Mediterranean basin, we tried to minimize
southern Adriatic and Ionian Seas, the underestimation of the isostatic both the local vertical movements and the effects of compaction in