Notches in the Northern Adriatic Sea: Genesis and development

Abstract

The genesis and development of the submerged notch occurring along the Northern Adriatic limestone coasts, surveyed at depth ranging between −0.35 and −2.8 m and amplitude consistent with the local tide is discussed using new and published data on its elevation and data on limestone lowering rates, collected using a micro-erosion meter and a traversing micro-erosion meter. Coastal limestone lowering rates in the Northern Adriatic range between 0.008 and 1.170 mm/y and they are about eight times higher than inland limestone lowering rates. Data support the hypothesis that notch is presently carving, even if no present-day notch has been surveyed in the area.

The lack of a modern notch, and the presently submerged notch position, could be related to (1) a rapid still-acting tectonic downdrop or (2) an increase in weathering/erosion rates causing by different environmental conditions, e.g. during the so-called Medieval Warm Period, marked by more aggressive weathering, both chemically, biologically and/or mechanically, than present conditions, due to an increase in rainfall or variations in the acidity of seawater. Pros and cons of these hypotheses are discussed in the paper.

Introduction

Notches are coastal recesses extended along marine cliffs. They develop because of higher lowering rates in the intertidal zone rather than in the supratidal or subtidal zone. Notches cut on hard limestones represent one of the most useful geomorphological markers to study sea level changes. Despite their importance, their genesis is still not fully understood.

Notches can be divided into different types, according to their origin and shape. Lithological notches are due to differential erosion between layers at different resistance. They are not used as sea level markers. Abrasion notches are due to mechanical abrasion of sand and cobbles moved constantly in the surf zone (Kelletat, 2005). Their elevation can vary and they can develop between the subtidal zone (up to −10 m) and the high-tide level, so they are not used as sea level markers.

Tidal notches, common erosional features along limestone cliffs, are successfully used as sea level markers, because they develop only around mean sea level. The contribution of biological, chemical, mechanical weathering and erosion involved in the notch development is still under debate. Higgins (1980) suggested freshwater outflow as the main factor driving the genesis of tidal notches. Furlani et al. (2009) recognized higher lowering rates in correspondence with freshwater outflows, but most researchers (Trudgill, 1976, Torunski, 1979, Kelletat, 2005) indicate bioerosion as the most important genetic factor both in tropical and temperate regions (Trudgill et al., 1981).

Carobene, 1972, Carobene, 1978, Carobene and Pasini, 1982, Ferranti et al., 2006, Antonioli et al., 2006 and De Pippo et al. (2007) recognized the occurrence of tidal notches along the Mediterranean coasts. In contrast, along the Eastern Adriatic coast between the Gulf of Trieste and Dalmatian coasts, a continuous submerged tidal notch has been surveyed (Table 1, Fig. 1, Fig. 2). Tamino (1948) identified it in the Zadar area. Pirazzoli (1980) surveyed the presence of a well-carved notch between −0.5 and −0.6 m at Pag Island, and it has also been found by Fouache et al. (2000) in the Istrian and Qvarner area. Benac, 1989, Benac, 1992, Benac et al., 2004 and Faivre and Fouache (2003) studied Rijeka Bay, the Kornati area and Dalmatia (Faivre and Fouache, 2007). Benac et al. (2008) surveyed the Vinodol Channel and Bakar Bay. Antonioli et al., 2004, Antonioli et al., 2007 surveyed the occurrence of a well-carved submerged notch in the Gulf of Trieste and noted the absence of a present-day notch along the northeastern Adriatic coast. They noted that the submerged notch has roughly the same amplitude as the local tide. The authors suggest post-Roman co-seismic events to explain the present position of the notch, perhaps the Cissa Earthquake of AD 361 (Pirazzoli, 1980, Benac et al., 2004, Benac et al., 2008) for the Qvarner area or even more recent ones to explain local differences in the Gulf of Trieste (Furlani et al., in this issue).

The topic of limestone erosion rates and the resulting notch development in the Adriatic have been suggested by Torunski, 1979, Cucchi et al., 2006 and Furlani et al. (2009), using MEM and TMEM data. Torunski (1979) argues that lowering rates increase downward, and are at maximum in the subtidal zone due to biological activity. Furlani et al. (2009) suggest that intertidal rates are, on average, one order of magnitude higher than inland rates, and suggest that lowering rates are higher close to submarine springs and large fluvial inputs.

Recent surface changes have been measured at different sites of the Gulf of Trieste, both on natural limestone outcrops (Furlani et al., 2009) and on a rock removable slab (RRS) set in the intertidal zone (Furlani et al., 2010), which allows use of the TMEM (traversing micro-erosion meter) on vertical surfaces. The first year of slab exposure suggests that it lowered mainly in the supratidal zone, because of chemical weathering, but the authors underline the lack of data in the mid and subtidal zone because of biological colonization.

This paper focuses attention on the genesis and development of the submerged tidal notch occurring in the Adriatic Sea through the comparison of sea level change data (archaeological, sedimentological and geomorphological markers), surface change data and physical/chemical data. Considerations reported here on the origin and development of Adriatic notches, thanks to the vast repertoire of notch surveys along the whole Mediterranean coasts can be very useful also in other Mediterranean areas, although considering the peculiarity of Adriatic in comparison to other localities.