Holocene landscape development along the Portuguese Algarve coast – A high resolution palynological approach

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Abstract

Along the Algarve coast, multi-proxy investigations of four estuaries detail the Holocene landscape evolution through high-resolution pollen and sediment analyses. This approach focuses on palynological results and enables the reconstruction of differentiated human impact around the studied archives between the Neolithic and modern times. During the Chalcolithic and Phoenician periods anthropogenic land use increases clearly and triggers erosional processes in the catchment. The increased agricultural area and the change in land use methods since the beginning of the second Islamic epoch form the actual landscape. According to the palynological results, since 8200 cal BP highly variable climatic conditions in terms of moisture availability have occurred during the entire Holocene. Arid periods occurred during the early Holocene prior to 8000 cal BP, around 7000 cal BP and between 6400 and 6200 cal BP. During the mid Holocene, two further dry periods with very different duration could be identified between 5000 and 3300 cal BP and between 2800 and 2500 cal BP. Finally, during the late Holocene between 1300 and 1050 cal BP and between 700 and 500 cal BP, further arid periods appeared. In conclusion, climatic and anthropogenic influences caused erosion in the catchments and in this way silting up of the estuaries along the Portuguese Algarve coast.

Introduction

The landscape development of the Atlantic Iberian coast has been influenced by different factors, e.g., climatic and sea-level changes (Zazo et al., 2008) as well as high-energy events (Ruiz et al., 2005). Especially during mid and late Holocene times, human impact affected vegetation composition as well as geomorphological processes. Together with climate changes this controlled landscape development in this region. Although these driving factors have been studied during recent decades, their significance and relative importance for landscape development is still under debate. While Chester and James, 1991, Stevenson and Harrison, 1992, Stevenson, 2000, González-Sampériz and Sopena Vicién, 2002, Arteaga and Gonzalez, 2004, Schulz et al., 2004, and Delgado et al. (2012) argue that the long anthropogenic land use since the Chalcolithic could be responsible for changes in erosion and sedimentation processes, Dabrio et al., 1999, Dabrio et al., 2000, Zazo et al., 1999 and Santos and Sanchez Goñi (2003) focus on natural origins of these changes like climate and sea-level variations.

Many studies tried to unravel the anthropogenic and climatic contribution to landscape evolution along the west coast of Portugal. Palynological investigations have been used to document the landscape development of this area mostly based on estuarine sediments (Queiroz and Mateus, 1994, Santos and Sanchez Goñi, 2003, Van der Schriek et al., 2008). Studies on marine cores give an overview of vegetation changes along the western and southern Iberian coast covering the last Glacial and Holocene with a resolution between 100 and 300 years (Sánchez Goñi et al., 2000, Roucoux et al., 2005, Naughton et al., 2007, Fletcher and Sánchez Goñi, 2008, Combourieu Nebout et al., 2009). Only few studies have targeted the Portuguese Algarve south coast. Fletcher (2005) as well as Fletcher et al. (2007) investigated the entire Holocene in a temporal resolution of approx. 200 years for the middle and late Holocene periods in the Central Algarve. On the other hand Allen (2003) investigated the coastal wetland of Boca do Rio (West Algarve) for the last 1600 years with a temporal resolution of 25–30 years.

The base of this study is the hypothesis that environmental changes resulting from climatic variations are visible on a regional scale and thus within several archives at the same time, whereas effects from human impact should differ on a local scale. Hence, a high resolution multidisciplinary investigation of four estuaries along the Portuguese Algarve coast was conducted to allow the reconstruction of environmental changes based on sedimentological, geochemical, archaeological, palynological and micropalaeontological analyses (Hilbich et al., 2008, Teichner, 2008, Schneider et al., 2010, Höfer, 2014, Trog et al., 2013, Trog et al., 2015). The present study solely focuses on the results of palynological analyses and botanical micro remains. It aims to contribute to the understanding of the proportion and importance of the mentioned driving forces of environmental and landscape change on the coast of South Portugal.

Section snippets

Regional setting

The study area covers the coastal zone catchments between Faro and Lagos along the southern Portuguese Atlantic coast, the Algarve (Fig. 1). This region is presently characterized by a Mediterranean climate with a mean annual air temperature of 16.3 °C and annual precipitation which varies from 500 to 1000 mm (Loureiro and Coutinho, 1995). The majority (80%) of precipitation falls from October to March, partly as torrential rainfall (Rother, 1993, Teixeira, 2006). Vegetation can be described as

Field work

In total, 68 sediment cores (5 cm diameter) were drilled from the different estuaries using ramming drilling with a motor hammer. Most drill sites were aligned along longitudinal and transversal transects (Schneider et al., 2010, Trog et al., 2013). Core length varied between 3 and 11 m. For all cores, the stratigraphy was described and sketched in the field, colour was registered using the Munsell Colour Chart, and textural classes, sedimentary features, macroscopic organic content and the

Estuary of Ribeira de Quarteira (P01–5, Fig. 2)

Pollen zone A (5100–4510 cal BP) is characterized by relatively low values of Pinus (5%) while Quercus ilex type (17%) and Quercus robur type (33%) show higher values. Sporadic documented species are Ziziphus, Betula, Tilia, Alnus and Salix. Vegetation of maquis and garrigue is represented dominantly by Ericaceae (17%), Cistus type (4%) and Olea europaea (4%). Open ground vegetation is dominated by the occurrence of Artemisia (11%) and sporadic observations of Centaurea scabiosa. First presence

Interpretations and discussion

Estuaries are very complex and dynamic systems, but in areas without other useful archives they provide a unique chance for the reconstruction of landscape development. Therefore data sets need to be carefully interpreted and archives should be analysed with a temporal resolution of less than 100 years. In this way, hiatuses or redepositional phases can be identified. Following palynological indicators for human impact (Table 1, Behre and Kučan, 1994) the pollen composition of ecological groups

Conclusions

In coastal areas, the comparison of neighbouring estuaries in combination with high-resolution palynological and sedimentological analyses allows the interpretation of the combined effects of natural setting, climate change and human impact. During the early and middle Holocene, environmental changes in all estuaries at the same time rather point to a climatic influence while clear changes in single estuaries are probably caused by local anthropogenic land use. In addition, the correlation of

Acknowledgements

This research was funded by the German Research Foundation (DFG) (MA 1308/9-1). We thank the reviewers for the helpful comments. Many thanks to Dr Geert-Jan Vis (TNO, Geological Survey of the Netherlands) for help during field work and creative discussions, Prof. Dr Tomasz Boski (University Faro, Portugal) and his team for constructive ideas, the colleagues of the University of Jena: Dr Gerhard Daut for scientific support, Dr Thomas Kasper for critical and helpful comments and PD Dr Jussi Baade

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