programme tuesday, july, 31 - Université de Caen Basse Normandie

Tidalites 2012, 8th International Conference on tidal environments, Caen, France – Abstract book
“DEEP TIME ON A TIDAL FLAT”: THE ANCESTRAL ICHNOASSOCIATIONS OF THE
MODERN GRADO LAGOON (NORTHERN ADRIATIC, ITALY)
Andrea BAUCON, Fabrizio FELLETTI
DIPARTIMENTO DI SCIENZE DELLA TERRA, Università di Milano, 20133, Milano, Italy,
andrea@tracemaker.com
The Northern Adriatic Sea is a shallow, semi-enclosed sea lying on continental crust, being
an ideal model for past epicontinental seas (Fig. 1A). In addition, recent studies suggests that its
benthic subtidal ecosystem closely resembles Paleozoic-style ecology (McKinney, 2003, 2007;
McKinney and Hageman, 2006). Although some objections were raised on the mentioned
ecological aspects (Zuschin and Stachowitsch, 2009), the Northern Adriatic Sea is unanimously
considered one of the few modern epicontinental seas comparable to some Paleozoic–Mesozoic
shelves (McKinney, 2003, 2007; McKinney and Hageman, 2006; Zuschin and Stachowitsch,
2009).
The Grado-Marano lagoon is the northernmost transitional system of this peculiar
geobiologic scenario. Here, barrier-island systems with vast tidal flats support a complex mosaic
of sedimentary environments, which offer varied habitats for tracemaking organisms (Baucon,
2008). Intriguingly, the intertidal flats are characterized by ancestral bioturbation styles coexisting
with modern, Thalassinoides-dominated ones:
• Microbial mat ichnoassociation. Intertidal microbial mats are characterized by horizontal,
gently winding burrows without branching (Helminthoidichnites; tunnel diameter: 1 mm). These
traces accurately follow the interface between the organic-rich and the mineral-rich layer of
microbial mats (Fig. 1B). The observed Helminthoidichnites reflect the mining of the microbial mat
from underneath: ‘undermat mining behaviour’ sensu Seilacher (1999). Similar behavioural
strategies were particularly common within Proterozoic microbial mats, before of the Agronomic
Revolution (Seilacher, 1999). Obviously, the Helminthoidichnites from Grado correspond to
Proterozoic-style behaviour but the tracemakers are modern: insect larvae (undetermined
Diptera). Intertidal insects (Heterocerus flexuosus) are also the authors of Macanopsis, an
unbranched burrow with a lower clavate chamber and a convolute, tapered neck (maximum
penetration depth: 5 cm; Fig. 1C).
• Rippled sands ichnoassociation. Rippled sands are dominated by deep (20-30 cm)
U-shaped burrows (Arenicolites) produced by sipunculans (Fig. 1D). Vertical burrows with
constructional lining (Skolithos) are common especially on longshore bars; the
suspension-feeding polychaete Megalomma is the tracemaker. With the exception of localized
clusters with dense Thalassinoides, crustacean burrows are very rare. These elements closely
resemble pre-Jurassic ichnoassociations, when decapod crustaceans were not dominating
shallow marine environments.
The mentioned ancestral features are explained by the peculiar intertidal environment of the
Grado lagoon. Indeed, the studied microbial mats are characterized by extreme conditions: high
cohesiveness, prolonged emersion time and noxious phosphate content. For these reasons, they
represent a rich trophic niche available only to few non-marine specialists. On the other hand,
rippled sands provide less extreme conditions, i.e. relatively high turbulence and low nutrients.
However, decapod crustaceans prefer to settle within the adjacent sheltered, organic-rich
habitats.
The described ichnologic features, united to the peculiar physiographic and ecological
context, make Grado an ideal analogue for past peritidal ichnological systems. Paraphrasing
McKinney (2007), the Grado ichnosite allows to study “deep time on a tidal flat”.
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