SEDIMENTS FROM THE LOWER SADO BASIN: A SOURCE OF INFORMATION CONCERNING OUR LATE MESOLITHIC ANCESTORS
STUDENT SEMINAR IN BIODIVERSITY AND EVOLUTION
Some 8000 years ago, the lower Sado valley was occupied by late Mesolithic peoples who were relatively high consumers of marine resources. Despite the unknown extension of the marine influence, it is fairly evident that sea level rise in the Early Holocene resulted in flooding of the pre-incised Sado valley. Did sea water reach the area of the Sado valley occupied by Mesolithic people influencing local biodiversity? Did these peoples exploit shellfish in the vicinity of their settlements, or did they collect them elsewhere closer to the mouth of the Sado river?
To characterize the environmental conditions of the Lower Sado valley experienced by Mesolithic people 8000 years ago, a 690 cm sediment core was collected in the alluvial plain of Sado river. The cored sediment was measured for magnetic susceptibility, macroscopically described and sub-sampled. Sub-samples have been studied for sedimentology (texture, organic and calcium carbonate contents), geochemistry (organic carbon, nitrogen,δ13C and δ15N) and microfossil (calcareous nanoplankton and foraminifera) proxies.
All the analysed proxies show a major difference in the depositional environment at ~350cm depth. Between the core base to approximately 350cm this Sado section was influenced by marine/brackish water and sedimentation is compatible with a marginal intertidal flat. The marine influence decreases further up the core and above c. 350cm the fluvial/terrestrial influence dominates the sedimentation pattern. The top 200cm corresponds to the aggradation of an alluvial plain.
The lack of objective dating of the base of the core prevents us from assessing whether this section of the Sado valley was under marine influence during the Mesolithic occupation, but the analyses made on the sediment core collected at this section of the Sado river are providing data to understand events that occurred in the middle/late Holocene history of this section of the valley.
Graduated in Geology in 2011 from the University of Lisbon, Ana Maria Costa completed her MSc in 2008 in Dynamic Geology (Lisbon University). The main objective of this work was to understand the role of submarine canyons in transporting sediments from land to the deep sea, including recent sediments enriched in anthropically derived metals. In 2011 Ana Maria became integrated LARC and subsequently ENVARCH as a BI in Geoarchaeology. She is currently involved in two main projects. In one – leading to a PhD thesis – Ana Maria intends to reconstruct the environment experienced by late Mesolithic hunter-gatherer groups in the Sado river area 8000 years ago by analyzing environmentally sensitive proxies in sediment cores collected in the Sado alluvial plain. The other project – Lisbon Stories (not financed) – has as its main objective to “tell stories” about Lisbon and the surrounding areas through time also using the sediments as a principal source of information. Several samples have been collected in the Lisbon waterfront in sites containing archaeological finds. The study of these sediments will help to reconstruct the dynamics of the Tagus river system over time, and will provide information about the relation of the human population with the river at different times in the past, at different places and in undertaking different activities.
Image credits: Ana Maria Costa