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Organic chemistry and magnetic susceptibility to characterize Late Mesolithic (ca. 8400-7000 cal BP) palaeoenvironmental conditions in the Sado Estuary, Portugal
Ana Maria Costa  1, 2, 3@  , Maria Da Conceição Freitas  2, *@  , Manel Leira  2, *@  , Rogério Mota  4, *@  , Ana Cristina Araújo  1, 5, *@  , Mariana Diniz  5, *@  , Klaus Reicherter  6, *@  , Felix Teichner  7, *@  , Pablo Arias  3, *@  
1 : Laboratório de Arqueociências - Direção-Geral do Património Cultural  (LARC - DGPC)
Calçada do Mirante à Ajuda, n10A 1300-418 Lisboa -  Portugal
2 : Instituto Dom Luiz  (IDL)
Faculdade de Ciências, Universidade de Lisboa, Edifício C6, Piso 3, Campo Grande, 1749-016 Lisboa -  Portugal
3 : Instituto Internacional de Investigaciones Prehistóricas de Cantrabria [Santander]  (IIIPC)  -  Website
Edificio Interfacultativo Avda. de los Castros,s/n Tel. 942 202090 E-39005 Santander Cantabria -  Espagne
4 : Laboratório Nacional de Engenharia Civil  (LNEC)
Av. do Brasil, 101, 1700-066 Lisboa -  Portugal
5 : Centro de Arqueologia da Universidade de Lisboa  (UNIARQ)
Universidade de Lisboa, Alameda da Universidade, 1600-214 Lisboa -  Portugal
6 : Inst. of Neotectonics and Natural Hazards, RWTH Aachen University
Lochnerstr. 4-20, 52056 Aachen -  Allemagne
7 : Philipps-Universität Marburg, Vorgeschichtliches Seminar
Biegenstraße 11, 35037 Marburg -  Allemagne
* : Corresponding author

Late Mesolithic hunter-gatherer communities (ca. 8400-7000 cal BP; ca. 6450-5050 cal BC) occupied the inner areas of the Tagus and Sado rivers (Portugal) during the Early-Middle Holocene transition. At that time both river valleys were flooded by marine ingression due to the rapid Holocene sea-level rise that occurred until ca. 7000 cal BP, forming large estuarine areas with favourable conditions for occurrence of marine/estuarine species.

The palaeoenvironmental characterization of the Tagus estuarine system shows that, during the Mesolithic occupation, shell middens were placed in the proximity of saltmarsh environments and near the upstream limit of tidal influence. The deceleration of the sea-level rise at ca. 7000 cal BP led to a relative increase in the fluvial sediment supply. However, despite the fluvial influence, estuarine conditions prevailed in the area of the Tagus that was occupied during the Late Mesolithic (e.g. Muge tributary) until ca. 6000 cal BP, long after the disappearance of these hunter-gatherer communities.

In the Sado estuary, the information is scarce thus far. Electrical Resistivity Tomography (ERT) profiles were performed to characterize the Late Quaternary palaeovalley morphology. Results revealed a deep channel reaching ca. 40 m below mean sea level (MSL) while tributaries accomplished shallower valleys. Analyses performed in sediment cores collected in the Sado channel show that estuarine conditions prevailed in the area until ca. 3300 cal BP at Arapouco (the most downstream studied area ca. 50km upstream the river mouth), and were identified ca. 4400 cal BP at Laxique (the more upstream studied area ca. 65 km upstream the river mouth).

According to the ERT profile, the Carrasqueira tributary palaeovalley, located near the Arapouco shell midden, reached ca. 15 m depth near its mouth. A 13.5 m sediment core (Arez) was collected on the alluvial plain of the Carrasqueira stream at ca. 2 m above MSL with the aim of characterizing the palaeoenvironmental conditions in the Sado valley during the Mesolithic occupation and its evolution through the Holocene.

In this work we present the preliminary results of the multi proxy analysis (texture, magnetic susceptibility and organic chemistry) performed on the Arez core combined with 14C dating.

Radiocarbon dates performed on bulk organic material at 10.5 m (median value of 8860 cal BP) and 7.5 m (median value of 7120 cal BP) below MSL indicate that sedimentation occurred since the Early Holocene and the sedimentary column covers the timespan of the Late Mesolithic occupation.

Modern materials such as plastic occur on the top 50 cm of the core pointing to recent anthropogenic influence probably related with rice planting known to occur in the valley at least since the 18th century.

The sediment is essentially composed by mud (<10% of sand).

Magnetic susceptibility (MS) values lower than 50x10-5 SI were measured in almost all samples, with exception of a peak reaching ca. 560x10-5 SI at ca. 10 m below MSL and of several (n=10) other peaks reaching values between ca. 100x10-5 SI and 200x10-5 SI scattered over the sedimentary column. The higher MS values are most probably the result of the input of terrestrial magnetic material brought by the fluvial system to the studied area during periods of higher river flow.

Stable carbon isotopes (δ13C; varying between -26.1‰ and -23.0‰) and the C/N ratio (varying between 10.8 and 17.0) point to the prevalence of estuarine conditions in the entire sedimentary column until, at least, 2 m core depth (top 2 meter sample analysis still on-going). The estuarine conditions are accessed by comparison with values determined in different environments of the present-day estuary. δ13C presents higher values between the core base (10.5 m below MSL) and ca. 7.5 m below MSL pointing to important environmental changes since ca. 7100 cal BP most probably as a response to the deceleration of the sea-level rise.

Although estuarine conditions are known to occur at upstream areas of the Sado valley, the maximum extension of the estuary is still unknown.

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