Middle Pleistocene to Holocene coastal evolution of NW Sardinia (Mediterranean Sea, Italy)

摘要

The Middle Pleistocene-Holocene Sardinian stratigraphy has been revised to identify the roles played by sea and climate changes in controlling sedimentation. The succession, dated through Optically Stimulated Luminescence (quartz, K-feldspar) and ~(14)C (the most recent) was grouped into eight major strat-igraphic units mainly represented by coastal dunes, shallow marine, and alluvial systems. These units range in age from MIS 8 (300 ka) to MIS1 (6 ka). Little information is available for the period between MIS 8 and 6. The available data, however, allow some comments on climate changes over the last 125 ka. The last interglacial (MIS 5) is subdivided into five substages (e-a). MIS5e (Eemian) is considered the climate optimum of this interglacial, sea level was 4-6 m higher than today, and climate conditions were warmer and more humid. MIS 5c is the second high stand peak. Sea level was about 1.5 m above the modern level, and paleoclimatic and paleogeo-graphic conditions were similar to the present. During MIS 4, at the beginning of the glacial phase, a climatic deterioration occurred. Temperatures dropped by 6C° relative to MIS 5e in about 5 ka, and sea level fell about 60 m. This led to a progressive disruption of inland vegetation cover and to repeated valley slope denudation. The sea-level fall created the accommodation space for fan development in sheltered and cliffed areas, where debris flows developed and filled almost completely the terminal parts of the narrow coastal valleys/coves. In wider areas, however, coastal dune fields developed. Contemporaneous presence of alluvial fan and dune systems is associated with the local morphology that could mitigate or amplify moist conditions within a generally arid environment. During MIS 3, several climate fluctuations occurred (wet/arid, D/O-H events). However, the shallow Sardinian shelf was continuously sufficiently exposed to become the source area of bioclastic sands. These sands were blown inland, and formed extensive dunefield systems. Dunes dominated the west coasts of Sardinia under cold and relatively dry conditions. In some areas, these dunefield systems were eventually almost completely dismantled and reworked into water-flow dominated alluvial fans by catastrophic rainfall events. Thus, the local climate quickly switched from arid to humid conditions. No record of the last Glacial Maximum deposits, MIS 2, was observed along the NW Sardinian coast During the Holocene transgression (MIS1) coastal dune systems developed and progressively migrated inland, and temperature rose to reach the present-day condition.