WASHINGTON ‘ Long at war with the sea, the city of Venice today faces perhaps its greatest challenge ever: The rate of sea-level rise is accelerating, a new archaeological study says.
The research also shows that people inhabited the Lagoon of Venice as far back as the third century and have struggled with its rising tides since then, ferrying in soil to build up land levels. And, the study’s lead author says, the city on water may not be saved by the current proposed remedy ‘ a series of mobile floodgates with a price tag of several billion dollars.
The research, which sent two American professors into tiny channels of the lagoon in a small boat sometimes deep into the night, is the first study of environmental archaeology ever undertaken in the Venice lagoon. It has drawn a picture of the world hidden beneath the lagoon and calculated the pattern of change in relative sea level going back to 4000 B.C. At that time relative sea level stood about 16 feet below its current level.
The results of the project are reported in the June issue of Antiquity by archaeologist Albert Ammerman and geologist Charles McClennen, both of Colgate University, and Maurizia de Min of the Superintendency of Monuments in Venice and Rupert Housley of the University of Glasgow, Scotland. Ammerman’s work is partly funded by the National Geographic Society.
Despite a legend that the Lagoon of Venice was unoccupied until the founding of the city in A.D. 421, Ammerman found evidence of a major effort to settle and build up land there two centuries earlier. The inhabitants of this western outpost of the Byzantine Empire transported soil on boats to shore up their watery domain and built structures that compensated for rising tides. For example, by the 11th century, when the mosaic floors of the great churches at Torcello and San Marco were laid, the ground level already had been raised more than 6 feet at the two sites.
Everyday life has long been disrupted by acqua alta ‘ the local name for an exceptionally high tide that floods all but the most elevated areas of the city for hours at a time. Damage caused by the phenomenon is notorious, some of it happening insidiously as sea salt is absorbed by the bricks in old buildings, hastening their decay. Both the frequency and the heights of acqua alta have increased in the last three decades. The problem is exacerbated by a combination of sea level rising worldwide and the sinking of the land surface in Venice.
Specially tailored acoustical studies combined with new dating techniques enabled the scientists to determine that relative sea level rose rather slowly in Venice ‘ about 3 inches per century ‘ between 4000 B.C. and A.D. 400. After that, however, the average rate jumped to nearly 5 inches per century and has accelerated this century. These wide variations in the rate over the centuries ‘ previously unknown ‘ make predicting future sea level changes risky.
Past attempts at archaeology and geology in Venice had difficulty pinpointing the natural boundary at the base of the lagoon, crucial to investigating early life there. By employing a small boat, Ammerman and McClennen were able to get into shallow channels never explored systematically before.
Using a state-of-the-art acoustical sub-bottom profiling device, the scientists mapped the lagoon and then took deep core samples over the side of the boat and at five archaeological sites on islands in the lagoon. New carbon-14 dating technology helped establish the chronology of the lagoon and the archaeological sites.
In light of the city’s long history, it is remarkable that almost no modern archaeology had been done there before 1987, Ammerman said. However, the city poses a serious challenge to archaeologists: Some of the early levels of occupation are buried 7 feet below modern sea level, making them extremely difficult to reach. Even routine tasks posed problems. Water around the Church of San Lorenzo was often so churned up by water taxis heading for the airport that the scientists had to wait to take measurements at lunch time, when the taxis took a few hours off. ‘Sometimes we worked late into the night, just to get the right conditions,’ Ammerman said.
Besides new evidence on the lagoon environment, the study has turned up a wealth of artifacts: coins from Roman times, amphorae, lamps, domestic pottery, glass cups, decorated bone combs and the remains of a small boat, dated to the fifth century.
But it’s the sea-level trend over 60 centuries that holds the most value for the future. A costly mobile gate system now under serious consideration by Venetian authorities would completely close off the lagoon when in use and is based on scant data, Ammerman said. ‘The data they’re using don’t look far enough back in time or far enough into the future,’ he said. ‘If anything, the rate of sea level change is accelerating in our time. And global warming over the next 100 years will only make the situation worse. What the past is telling us is that the best strategy for Venice is the oldest one: Just keep topping off the ground or you’re going to lose it.’
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July 14, 1999