Holocene sea level rise research using archaeological site

Abstract

: In this paper the main problems in acquiring, computing and evaluating past sea level data from archaeological indicators are discussed in order to recognize recent eustatic variations. Many prehistoric peoples and historic civilisation developed during the Holocene along the Mediterranean coasts and were influenced by the natural sea level rise; those peoples could occupy coastal sites during sea level stands but the following rise, at a rate greater than one meter per century, compelled them to leave coastal planes and sea caves. Several information about the former coastal landscape could be drawn from ancient historical and geographical texts (Poems, Histories, Geography, Itineraries, portolanos and maps) and then comparing them with present conditions and with archaeological traces. In order to obtain reliable and reproducible data from geoarchaeological indicators it is necessary to fix some qualifications. First of all the archaeological remains must be datable with enough accuracy in a radiometric, AMS or archaeological way; then the altitude must be referred in one only reliable reference system (for instance the International Terrestrial Reference System); at last the isostatic, tectonic and sedimentary dynamic patterns must be known very well to understand the way that eustatic and terrestrial components affected sea level variations. The meaning of an archaeological indicator depends on its primary elevation on sea level. In fact each remain could only have been placed above or below sea level, so it could represent only a one-sided datum: for instance an emerged object could only indicate that the former sea level must be lower than the same object's present altitude increased of the amount of the great high tide. Hence an emerged indicator means an "upper allowable limit" of the former sea level as well as a submerged indicator means a "lower allowable limit". All remains of on land human activity which are placed in the lower part of the archaeological site and are also lower than present high-tide level could be reliable geoarchaeological indicators of sea level rise. In prehistoric coastal sites most common objects are tombs, hearths, paintings, stone tools, middens, walking surfaces, paths and way of access to sea caves; in marine settlement, such as Greek and Roman towns, the most useful remains are those of ancient structures as quarries, building foundations, paving, mosaic floorings, thermae, aqueducts, water tanks and sewer's canalization. In ports and fishing tanks {piscinae) there are often both emerged and submerged indicators functionally related to sea level, so that comparing their one-sided data (upper and lower limits) the ancient sea level must be included in an interval of a few centimeters; most common remains of this kinds are quays, slipways, piers, breakwaters, channels and tunnels, bottom planes of sluice-gates and platforms. In the Thyrrhenian Sea the great tides (occurring just after the sygyzy, twice a month) are within +20 and -20 cm on MSL and it could mean that marine structures have been planned according to that tide amplitude, to preserve their function even during great tides. It is more difficult to think that the structures could operate during the extreme tides, that exceed ±50 cm on MSL; however extreme tides (occurring about once a year) are less frequent than sea-storms so they couldn't represent a greater hazard. Geomorphologic and paleoclimatic studies about archaeological sites in Thyrrhenian's sea caves, accompanied with radiometric and AMS dating, point out some data concerning the Holocene sea level rise: during Mesolithic time (about 10 kyr BP) the sea level was about 45-50 meters lower than today's, between Mesolithic and Neolithic time (about 8.5 kyr BP) the sea risen to -15 meters and finally reached -10 meters during early Neolithic time (about 7 kyr BP). Geoarchaeological studies about Etruscan and Roman remains along the Latium coast (central Thyrrhenian Sea) point out some more eustatic data for historic time: in IV century b.C. (about 2.3 kyr BP) sea level was 1.70 meters lower than today, then in AD (1.95 kyr BP) sea level risen to -0.35 meters and reached -0.10 meters in the II century AD (about 1.75 kyr BP). In the last centuries the sea level probably continued oscillating within a few decimeters around present level, according to minor climatic variations, as tide gauge's data pointed out. The correct interpretation of geoarchaeological data is very useful in paleoclimatic researches based on eustatic curve reconstruction; however it is necessary to give the best attention in collecting, referring and evaluating these kind of data. At last it is necessary to study the details of the dynamic geology in order to separate terrestrial and eustatic components of sea level variations.