Early Holocene sea-level jumps: evidence and open questions from the Adriatic Sea

Strong efforts have been done by the scientific community to reconstruct the last marine transgression and describe it through geophysical models that could match with the information observed in the field. However, because of the few available data, the eustatic curve is still affected by important uncertainties for the period before 8 ka BP. Thanks to its gentle slope, the continental shelf of the Northern Adriatic Sea is one of the very few areas in the Mediterranean that preserves a partial record of the sea-level variations occurred between Late Glacial and Early Holocene. These changes are documented by sediments and landforms formed in lagoon and paralic environments, which are strongly susceptible even to subtle variations of the relative sea level. We analysed the large dataset of CHIRP-Sonar profiles acquired in the Adriatic since 1990 by CNR-ISMAR during a series of oceanographic missions and the database of stratigraphic cores sampled in the off-shore and along the coastal plains of Northern Italy and Istria. A major issue is represented by the recognition and characterization of the periods of rapid increase of the sea level, which punctuated the Late Glacial and the first part of Holocene. These phases caused the abrupt submersion of large sectors of the former coast and alluvial plain, inducing dramatic environmental changes. In the study area, a large barrier-island system, which remnants can be found between -38 and -42 m Mean Sea Level (MSL), formed ca. 11-10 ka BP and was rapidly drowned in place soon after. Furthermore, important information about a sea-level jump occurred around 9.5-9.2 ka BP have been recently documented both in the deltaic plain of the Po River and in the submerged shelf at about -30 m MSL. This phase of transgression allowed to the Adriatic Sea to penetrate into the Gulf of Trieste, strongly transforming the area between Istria and the coast of Friuli. Another important phase of submersion is documented between 8.4-8.2 ka BP, when relative sea level in the area was around -15 m MSL. In the studied area the reconstruction of the sea-level evolution rises a major question, represented by the uncertainties related to the stratigraphic, geomorphological and environmental indexes which could allow the identification and quantification of sea-level jumps. Moreover, considering the lagoon and coastal environments, the decreases of the rate of the relative sea-level rise documented in the stratigraphic record could be the product of eustatic stillstands or, partly, also of the environmental changes induced by the episodic strong increase of the sedimentary input from the continent and the consequent basinward shifting of the coastline.