Holocene palaeoecology and floodplain evolution of the Muge tributary, Lower Tagus Basin, Portugal

The lower Muge valley, a tributary of the lower Tagus River (Portugal), features an important archaeological record of Mesolithic shell midden sites. Archaeological research has long assumed that tidally influenced valley floor environments in the immediate locality of the sites provided a rich food resource, attracting Mesolithic settlement. To date there has been little attempt to use palaeoenvironmental records to reconstruct Holocene floodplain evolution in the Lower Tagus valley. The cultivated freshwater lower Muge floodplain is locally underlain by ∼11 m of homogeneous fine-grained sediments and peat, comprising buried floodplain environments contemporary with Mesolithic occupation (∼6200–4800 cal BC). Pollen and foraminifera analyses demonstrate that fine-grained deposition, forced by sea-level rise, commenced ∼6200 cal BC in an estuarine setting. The lower Muge floodplain experienced maximum tidal influence ∼5800–5500 cal BC. Subsequently, sediment supply rates overtook the decreasing rate of sea-level rise and fluvial environments expanded. The pollen record may suggest regional desiccation from ∼5000 cal BC. Estuarine environments disappeared suddenly ∼3800 cal BC when freshwater wetlands were established. Although the initiation of Mesolithic settlement is shown to coincide with the beginning of tidal influence, site abandonment does not match with any major environmental change. Sea-level still stand (∼2600 cal BC) has been linked to valley floor stabilisation and soil formation. Alder floodplain woodland developed prior to ∼230 cal BC and was cleared, probably during Roman times, for agriculture. Renewed deposition after ∼230 cal BC may relate to internal mechanisms or to human impact upon the catchment vegetation.