Sea-Level Change and the Archaeology of Early Venice
Ammerman, A. J., McClennen, C. E., De Min, M., Housley, R., Antiquity
A city built on water with a long history of coping with environmental change, Venice today faces perhaps its greatest challenge ever. The first Doge of the Venetians, according to tradition, came to power in 697 AD. The last person to hold this position ruled until 1797, when Napoleon captured the city and dissolved the government of La Serenissima. The new threat to Venice in our time takes the form of acqua alta, the local name for an exceptionally high tide that floods, for several hours at a time, all but the most elevated areas of the city. Not only is everyday life disrupted by acqua alta but the salt water poses a direct threat to the physical fabric of the city. When the bricks in an old wall are exposed to sea water, they soak up salts whose accumulation hastens their decay. Tidegauge records reveal that such extreme high tides were comparatively rare during the first 30 years of this century. Both the frequency of acqua alta and the heights reached by the tides have increased in the last three decades (Pirazzoli 1982; 1983). The recent worsening of the situation is explained by a rising sea level, an eustatic component, in combination with the sinking of the land surface at Venice, a subsidence component (Gatto & Carbonin 1981; Sestini 1992). What are lacking in the literature on the problem so far are studies which look beyond recent trends in sea level and take a longer view. The reason for this can be traced in part to the previous lack of knowledge about the archaeology of Venice. Elsewhere in the Mediterranean (Flemming 1969; 1992), archaeological investigations have often made a contribution to local knowledge about changes in relative sea level over the last two thousand years. In contrast, Venice, as late as 1986, remained one of the few great historical centres in Europe with no urban archaeology. Fortunately, this situation has now changed (Ammerman et al. 1992; 1995; Tuzzato et al. 1993). There are eight sites in the Lagoon of Venice [ILLUSTRATION FOR FIGURE 1 OMITTED] where recent excavations have brought early levels of occupation to light. The main purpose of this report is to use this newly available evidence to consider the basic pattern of change in relative sea level at Venice over the last 6000 years.
By way of introduction, it is worth recalling at this point the previous attempt by Dorigo (1983) to reconstruct the height of early land surfaces in Venice. His approach (at a time before the recent advances in archaeology) was based upon two main assumptions:
1 the world-wide sea-level curve proposed by Fairbridge (1962: [ILLUSTRATION FOR FIGURE 6 OMITTED]) could be borrowed and applied to Venice and
2 the rate of subsidence at Venice was taken to be 14 cm per century (on the basis of short-term measurements in this century) and this value could be used as a constant for making extrapolations back to much earlier points in time.
This line of argument led to the inference that the land surface in the 2nd century AD stood more than 4 m above sea level (at the time) and the further claim that this land witnessed centuriation at the hands of the Romans. These claims, however, were soon questioned by Bosio (1984) simply on the grounds of ancient topography. In addition, there are problems with both of the assumptions made by Dorigo. Although it was regarded as a benchmark contribution in the 1960s, the curve put forward by Fairbridge (1961; 1962) represents a composite in which different times on the curve are drawn from coasts in different parts of the world (each with its own history of local earth movements). Thus, the pronounced fluctuations in his curve for the last 2000 years are now viewed, for the most part, as artefacts of sampling in space. Indeed, the whole notion of a single curve that would serve on a global scale - even in the case of carefully studied tide-gauge records covering the last 100 years (Pirazzoli 1986; Barnett 1990; Emery & Aubery 1991) - is no longer considered to be a tenable one. …