High resolution palaeoenvironmental analyses of coastal wetland sediments from South East Sicily

Abstract

This study examines the sedimentology of salt marshes and lagoonal sediments from coastal wetland settings in south east Sicily. Palaeoenvironmentasl tudies have been carried out to assess the recent evolution of disturbed coastal wetlands, and to examine the sensitivity of these depositional settings in recording historical environmental change. The evolution of recent coastal wetland environments in the region can be related to anthrpogenic disturbance phases in drainage catchments due to changes in land-use. Archaeo-historical changes to the coastal topography are identified, in relation to the development of present-day coastal wetland areas in south east Sicily. The impact of 19th early 20th century salt workings is recognised as a major and continuing factor in the condition of present day wetlands. Measurement of 210Pb, 137Cs, pollen content, major and trace elements, loss on ignition and other sedimentological features generates valuable information on depositional processes that have occurred over the last 100-150 years. Sub-surfaces edimentological changes recognised in cores extracted from shallow sediment sequences (< 50 cm) can be related to coastal wetland land-use changes and hydrological flood events. A 30 cm depth sediment sequence analysed from the Mulinello estuary records the interaction between estuarine channel processes, following embankment construction and the variable influence of catchment generated flood episodes. A clearly identifiable change in accretion and core composition occurred during the mid-late 1940's and early 1950's which coincides with recorded peaks in monthly rainfall totals. The impact of marsh development, flooding and recovery is highlighted by the variable abundance of dominant pollen types. A marked increase of ruderal pollen types during periods of channel-dominated deposition is contrasted with an abundance of halophyte pollen during low-energy phases of organic marsh sedimentation. The two lagoonal cores from Pantano Piccolo record successive hydrological changes, due almost certainly to artificial enclosure in the late l8th to early 19th century. The apparent change in water levels dramatically affected marginal salt marsh communities. Artificial impoundment generated a lagoonal setting conducive to the accumulation of local and extralocal sources of pollen, reflecting the re- establishment of salt marsh vegetation. Although largely separated from catchment-overland flow patterns during the 20th century, large magnitude rainfall events were recorded in lagoonal accretion patterns. 21OPb-derived sediment accretion rates and estimated pollen accumulation rates have enabled the response and sensitivity, of coastal wetland and nearby plant communities to phases of disturbance and recovery, to be determined. Due to artificial impoundment of the lagoon, fringing-halophyte communities and organic accretion migrated outwards to occupy their present marginal position. To aid the interpretation of pollen encountered in estuarine and lagoonal settings, soil samples from nearby land surfaces around Pantano Piccolo were analysed for pollen content. Soil surfaces reflected the dominance of gravity fallout from surface vegetation and the accumulation of pollen from regional-atmospheric sources. The reliability of the multi-proxy approach and palaeoenvironmental analyses used, indicate that coastal wetlands in south east Sicily have evolved in a dynamic system of punctuated equilibria, due to climatic events and human activity over the last 100- 150 years. Their current status reflects the continued pressure on coastal systems by anthropogenic development and recent conservation measures.