Micromorphology and mineralogy of the SEV core (Santarém, Portugal): evolution from a transitional to a continental sedimentary environment during the Holocene

The present work focuses on the soil micromorphology of a multi-disciplinary study performed on a 20 m-deep core taken in the alluvial plain of the Tagus river at Entre Valas, a few km upstream of Santarém (Portugal). The micromorphology was integrated with palynological, sedimentological and geochemical data. The SEV (Santarem Entre Valas) core is representative of the evolution of the Tagus River valley during the last 15,000 years, encompassing Late Glacial and Holocene sediments. The lowest part of the series corresponds to a deltaic/estuarine sedimentary system, with shallow freshwater backswamp ponds receiving occasional inputs of seawater during exceptionally high tides or storms. The upper part of the core represents the transition from the lower deltaic/estuarine sedimentary complex towards an alluvial plain during the middle and upper Holocene. No indicators of sedimentary hiatuses accompanied by pedogenesis were observed in thin-section, suggesting a rather ‘continuous’ sedimentation. At a depth of 3.5 m pedorelicts of eroded and re-deposited former topsoil horizons were observed. These are most likely related to a phase of topsoil erosion that could be the result of deforestation and heath-land expansion, which in the region are known to begin in the Bronze Age.