Multibeam Sonar Technology and Geology to Interpret Ancient Harbor Subsidence off Crotone Peninsula, Italy
Multibeam sonar (MBS) technology was developed to examine in great detail large stretches of the seafloor surface, providing accurately positioned and excellent 2-D and 3-D images of features as small as a few centimeters or covering areas as large as hundreds of square meters. In the present study, such high quality images obtained in plan and oblique views are used primarily for archaeological purposes as related to geological (sedimentological and tectonic) parameters. Presented here as example is an area on Calabria’s Ionian coastal margin, the Capo Colonna-Punta Scifo shelf platform located off the Crotone peninsula. We show how multibeam sonar can help solve geoarchaeological problems with respect to identification, origin, and subsidence of seafloor structures that were once positioned near a former coastline but now rest at a considerable depth of 12.5-13 m below sea level. MBS images, coupled with diver observations, indicate the structures are of human construction, not of natural origin, and were once part of a now-submerged Greek harbor facility. These structures are positioned on a seafloor that subsided by ~10 m since late or post-Greek time; submergence occurred not by mass gravity flow processes, but by a number of tectonic pulses over time that most probably involved lowering by extensional faulting and possible strike-slip events. Linear structural features observed on the seafloor show axial trends similar to those mapped on land, indicating that both subaerial and submerged areas of this Calabrian Arc sector were modified tectonically to the present time. Multibeam data show that this Crotone shelf platform has been subject to considerable structural offset and subsidence on Calabria’s Ionian margin since ancient Greek time.
During the past half century, there has been an increased number of projects conducted in offshore coastal and shelf settings in different world regions and this has fostered closer research collaboration between marine geologists and archaeologists. The hammer, camera, compass and tape measure commonly used by diving geologist are often supplemented by equipment to recover subsurface sediment, including drills and vibracores. In addition to these are high-resolution subbottom seismic and side-scan systems, ROVs and submersibles that are also included in the earth scientist’s tool kit, especially when working in deeper settings. To the above, one should add multibeam sonar, a widely available technology that details the seafloor surface, a zone usually poorly defined by subbottom geophysical profiles due to the indistinct seismic signal usually obtained at and just beneath the water-bottom interface. To date, this technology remains underutilized by marine scientists attempting to resolve seafloor surface and geoarchaeological problems in marine environments. The present study provides an example of the use of the MBS system applied specifically to a problem of archaeological site identification in what can be defined as a zone of extensive subsidence off Calabria’s Crotone peninsula, southern Italy (fig. 1).
I – Purpose of Study
MBS technology was developed after World War II as an acoustical observational method for naval research projects that required considerably more precise and realistic images of the ocean floor than had previously been available by single beam systems. Rendering of three-dimensional visualization of bottom surface features obtained rapidly over broad areas is a particularly attractive aspect of the technology, and for this reason it is now widely used for systematic seafloor mapping and varied offshore hydrographic purposes (HugHes Clarke et al., 1996; lurton, 2002). This acoustic system, one that records a broad swath of seafloor data, has been commonly employed since the 1970s by the offshore oil and gas industry. It is also increasingly applied to harbor management studies, marine civil engineering projects, fishery surveys and other purposes where it is essential to depict seafloor features in detail and accurately record their position and depth. Examples of the now diverse and practical applications of MBS are provided on search engines.
MBS, sometimes in conjunction with side-scan sonar, has been increasingly used during the past 3 decades for offshore archaeological exploration on shallow continental shelves, in deeper marine environments, and also in estuaries, lagoons and lakes. These swath acoustic systems have been particularly valuable for examination of recent shipwrecks and other anthropogenic debris such as airplanes, military armament and construction materials (Mayer et al., 2007; Dean et al., 2007), as well as ancient shipwrecks now resting or partially buried on the seafloor (Foley et al., 2009). Most recently, MBS has proven helpful in the investigation of once subaerially-emerged coastlines and discovery of associated archaeological sites and materials that are now submerged, sometimes to considerable depths, in aqueous environments (royal, 2008; niCkerson et al., 2010).
A case study is presented here in which MBS is applied to a geoarchaeological problem on the Ionian continental shelf off the coast of Calabria, southern Italy. Attention is paid primarily to a sector positioned seaward of Capo Colonna and Punta Scifo (fig. 1), where features of questioned origin lie in a geologically complex and recently modified tectonic sector off the Crotone peninsula (fig. 2). Databases used are those collected in 2005 by RPM Nautical Foundation (royal, 2008). The investigated seafloor relief features, including anthropogenic targets of interest, were originally discovered by means of MBS analysis in an area southwest of the Capo Colonna headland and about 250 m south of Punta Scifo (royal, 2008, his seafloor target coded AE/AF in his figs. 2 and 9). This AE/AF site on the inner continental margin (site 14 in fig. 2) was also visually examined in 2005 by divers and interpreted by Royal as part of a now submerged harbor facility of probable Greek Archaic age. This was based on construction style and comparisons with associated features and settings as illustrated and discussed at length later in this article. It comprises target AE, a long narrow structure attributed by him to be a breakwater. Northeast of the breakwater structure, there are also two large square and flat-surfaced blocks (target AF) interpreted by Royal as piers. These AE/AF structures now rest on the seafloor at a depth of about 12.5 to 13 m below present mean sea level (m.s.l.). Royal proposed that AE and AF were once part of a harbor facility that functioned from ca. 700 to 300 B.C., and then were submerged, perhaps during, or following, late Greek to Roman time.
It is of note that these same features were subsequently reinterpreted by another archaeologist, D. Bartoli (2010), not as anthropogenic in origin but as naturally deposited rock strata, with royal’s (2008) proposed breakwater attributed to a possible beachrock origin. Bartoli based his conclusions in part on the long narrow shape of the rock structure and absence of ceramic sherds and other human artifacts associated with it; he also calls attention to the considerable seafloor depth on which the associated AE/AF features occur. One of his main arguments is that it would have been unlikely that the AE/AF targets identified by royal (2008), and attributed to quarried stone, could have been submerged to a much greater depth than that of the quarry which supposedly supplied the calcarenite blocks used to build it. Bartoli contends that if the blocks had indeed been obtained from such a quarry, it would have been necessary to transport them from a source locality well to the north of where the AE/ AF structures now lie, i.e. one likely positioned much closer to the present shoreline. By this reasoning, he proposes that the source quarry, which he locates in the now-submerged Bay of Punta Scifo (Bartoli, 2010, his p. 405 and fig. 6), was of roughly equivalent age (Archaic to Hellenistic Greek). However, this proposed quarry at ~6 m below m.s.l. is at a much shallower depth than the AE/AF structures. Bartoli (2010, his p. 406) thus remains “skeptical that an Archaic breakwater could be located 13 m deep.”
Several pertinent questions as to the origin of the AE/ AF targets thus need to be addressed. Were these structures positioned south of Punta Scifo actually of human construction, i.e. once part of a harbor installation built along a former coastline during the Greek Archaic period, perhaps as early as 2800-2700 years ago? Were they still above sea level as recently as the 4th century BC as suggested on the basis of what is known of ancient Greek Kroton and its associated sanctuary of Hera (Juno) Lakinia, of which vestiges remain on the Capo Colonna headland (CerCHiai et al., 2004)? Or, are the structures, especially the AE target, formed of beachrock and therefore of natural origin? In either case, beachrock or breakwater, it becomes necessary herein to explain how such features normally associated with coastal to shallow marine settings could have been lowered to their present considerable seafloor depth (13-12.5 m) in a time-span as brief as ~2300 years. Our investigation pursues these matters of origin and submergence using a multi-pronged geoarchaeological approach. It takes into consideration (1) direct observational records on the seafloor obtained by divers, including photographs and direct measurements of the AE/AF structures of interest in the study area. This is needed primarily to address the question of whether the AE/AF targets are of human construction or natural origin. If this question can be resolved, then (2) the direct diver records of the ocean floor are to be integrated with images derived from the 2005 MBS datasets that have been reprocessed specifically for this present study by applying up-dated stateof-the-art electronic software systems. It would also be useful (3), to identify and interpret the numerous small (few cm) to large (more than 100 m long) physical (sedimentary, tectonic), non-archaeological features on the seafloor of the study area; these had not received attention during the first analysis of MBS records by royal (2008), or in the subsequent studies by Bartoli (2008, 2010). For this exercise, (4) an attempt is made to compare some of the physical features recorded by MBS on the submerged shelf margin with physical features identified on land in the proximal Crotone peninsula. This approach may perhaps provide some new insight on seafloor subsidence, or absence of such movement, in this area.
Our investigation pursues these matters of origin and submergence using a multi-pronged geoarchaeological approach. It takes into consideration (1) direct observational records on the seafloor obtained by divers, including photographs and direct measurements of the AE/AF structures of interest in the study area. This is needed primarily to address the question of whether the AE/AF targets are of human construction or natural origin. If this question can be resolved, then (2) the direct diver records of the ocean floor are to be integrated with images derived from the 2005 MBS datasets that have been reprocessed specifically for this present study by applying updated state-of-the-art electronic software systems. It would also be useful (3), to identify and interpret the numerous small (few cm) to large (more than 100 m long) physical (sedimentary, tectonic), non-archaeological features on the seafloor of the study area; these had not received attention during the first analysis of MBS records by royal (2008), or in the subsequent studies by Bartoli (2008, 2010). For this exercise, (4) an attempt is made to compare some of the physical features recorded by MBS on the submerged shelf margin with physical features identified on land in the proximal Crotone peninsula. This approach may perhaps provide some new insight on seafloor subsidence, or absence of such movement, in this area.