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Mediterranean Bioconstructions Along the Italian Coast 87

leading to a quick growth of these bioherms. However, the accumulation rates
1
appeared low (0.006–0.83mmyear ), oscillating greatly with different depths
and time periods (Ballesteros, 2006; Garrabou and Ballesteros, 2000; Sartoretto
1
et al., 1996; Teixido ´ et al., 2011). The highest rates (0.20–0.83mmyear )
were recorded between 8000 and 5000 years BP. In more recent times, appre-
ciable radiocarbon accumulation rates of 0.11–0.42mmyear 1 have been
recorded only for bioherms growing in relatively shallow waters (10–35m
depth) (Bertolino et al., 2013), while an average growth rate of about
1
0.15mmyear was measured on coralligenous pillars from the Ionian Sea
(Bertolino et al., 2017b), a rate comparable with that recorded by Di
1
Geronimo et al. (2002) for similar pillars (0.27mmyear ) in the southern
Ionian Sea. Radiocarbon dating in bioherms from different localities of the
Ligurian Sea indicated average growth rates ranging from of 0.03 to
0.05mmyear 1 over 3000 years (Bertolino et al., 2013). In some cases, the
growth of the bioherms showed an incoherent and chaotic temporal growth
pattern (Bertolinoetal.,2017a), with older layers overlapping younger ones.
This phenomenon could be related to favourable periods for carbonate
deposition, alternating with partial destruction phases due, for example, to
intense mud deposition after episodes of intense floods (Fig. 8).
In the northern Adriatic continental shelf, biogenic frameworks are
generally superimposed on hard bottoms of still debated origin. Marine
sediments may be consolidated by methane-related calcium carbonate
cementation, thanks to seepage of CH 4 -rich fluids, observable near many
offshore reefs (Gordini et al., 2012). Meandering-shaped morphologies
and the distributions of many reefs, especially those off Chioggia, suggest
the importance of Pleistocenic rivers, Holocene tidal channels and beach
bars which are initial substrate for current coralligenous build-ups. The
lithification of these sandy palaeochannels is estimated to have started from
7000 years BP, likely due to the interaction between marine and less saline
fluids related to onshore freshwater discharge at sea through a sealed water
table (Tosi et al., 2017).
Sabellaria bioconstructions, which are formed from tubes made by sand
grains, are mostly ephemeral. They strictly depend on the balance between
bottom sediment features and water movement, supplying sand and particles
for tube building and growth. Sabellaria larvae show preferential settlement
either onto living bioconstructions or onto their ruins, so ensuring reef
persistence and recovery. Hence, the age of these bioconstruction greatly
exceeds the age of the living worms, since the reef may repeatedly
follow a cyclical progression of settlement, growth and destruction phases