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Holocene environmental change in southern Portugal

Holocene environmental change in southern Portugal

The absence of lakes and freshwater wetlands in southern Portugal has constrained the use of traditional pollen analytical studies from which inferences can be made about the history of climate and human activity based on the region's vegetation history. Alternative pollen-bearing sediments, such as estuarine material, have the potential to provide palaeoecological data, both in terms of the wider vegetation history of the estuarine catchments and the changing nature of the ecology of the estuarine wetlands themselves. However, sediment accumulation in estuaries is influenced by factors such as changing sea-level, possible tectonic activity, sediment accommodation space and input from both terrestrial and marine sources. These factors, is turn, may reflect both the climatic and anthropogenic history of the area. It is a challenge, therefore, to discriminate between the different environmental signals contained within these types of sediment.

In southern Portugal, several rivers drain into the Atlantic which provide long depositional sequences, notably the Guadiana, Arade and Boina. For the Guadiana, sedimentation during the Holocene has resulted in accumulation of some 40 m of material. For the Arade and Boina, Holocene sediments account for approximately 15 m of core material. Research by William Fletcher, for his PhD (2005), presented pollen and sedimentary data for the periods 13,000 to 2,000 cal BP from these estuaries. Taxonomic precision and rigorous discrimination of wetland and non-wetland taxa demonstrate the existence of early to mid-Holocene woodland vegetation characterised by Pine (Pinus), evergreen and deciduous oaks (Quercus species) and associated understorey shrubs and herbs. From about 5,000 cal BP arboreal pollen in the Guadiana core declined in importance, while Quercus coccifera-type pollen and moor and heath taxa, such as Ericaceae and Cistaceae, increased. This suggests development of tall maquis communities. The timing and pollen compositional changes are similar to events elsewhere in the Iberian Peninsula and, in some respects, to the wider Mediterranean region. Whether these are climatically- or anthropogenically-induced has been the subject of considerable debate. From sites across the Mediterranean, aridification and/or forest clearance have been proposed as drivers of vegetation change. For the Algarve sites, Fletcher's work recognises that development of maquis is consistent with a major expansion of human activity into the uplands and hinterlands of southern Portugal. Unfortunately, the pollen signal from the Arade and Boina cores is poorly established from mid-Holocene times onwards.

Pollen and sedimentary analysis of a short (3 m) core from the Boca do Rio wetlands, draining the Ribeiro de Vale Barao west of Lagos, provides palaeoecological detail for the past 2,000 years. The early part of the record is characterised by coarse grained sediments and the presence of dinoflagellate species typical of estuarine conditions, such as Lingulodinium machaeropherum and Operculodinium centrocarpum. Local archaeological evidence also supports the proposition of a shallow estuary open to waves and tidal conditions in Roman times. Roman occupation of the region was at its most developed during the third and fourth centuries AD and Roman sites within the Boca do Rio catchment suggest that the river was navigable for several kilometres inland.

The dinoflagellate record suggests that a transition to a lower energy environment occurred in more than one phase, but from about 1100 cal BP sediment grain size becomes finer indicating an environment less dominated by marine conditions and more dominated by fluvial input. The mass specific magnetic susceptibility of the sediments increases suggesting inwash of eroded topsoil from the catchment; the proposition of a detrital origin for this comes from the significant correlation of susceptibility with the fine silt fraction of the sediment, typical of inwashed material. For the same period, the wetland pollen record is dominated by Chenopodiaceae taxa typical of saltmarsh environments. The presence of pollen from poor pollen producers such as members of the Cistaceae family, together with Ericaceae and Oleaceae, suggest the slopes surrounding the wetlands were dominated by typical Mediterranean maquis vegetation communities. An analysis of the origin of the mineral magnetic particles in the sediment record indicates a local, as opposed to regional, provenance for both sediment and pollen. This, together with the influence of a limestone lithology on the hydrological regime of the rivers draining into Boca do Rio, suggests that the record of probable land-use change is restricted to the immediate areas of the wetlands rather than being a regional signal.

The sedimentary record of the Boca do Rio wetlands is interrupted by a 5 cm layer typical of tsunami material and which is believed to originate from the 1755AD Lisbon earthquake. Radiocarbon dating of underlying material indicates that some 700 years of sediment was removed by the outgoing tsunami wave. Following the tsunami, a fresh-water and brackish wetland community existed for about 60 years in the lower reaches of the Ribeiro de Vale Barao. This was replaced by more saline conditions either as a result of lake desiccation or breaching by the river. The pollen record does not suggest gradual lake infill.

The use of less traditional sources of pollen evidence, such as estuarine sediments, has proved invaluable in southern Portugal for reconstructing vegetation and environmental history, despite the complexities of their palaeoecological record. This allows the record of vegetation and land-use change to be extended across the Iberian Peninsula to the furthest south-west in Europe. It also provides corroboration for the reconstruction of landscape stability which has formerly been derived from fluvial sediments. Fortunately sedimentary records from estuarine environments have been retrieved before loss of some of these sites as a result of recent infrastructural and tourist development in the region.

Publications

  • Allen, HD (2003) A transient coastal wetland: from estuarine to supratidal conditions in less than 2000 years - Boca do Rio, Algarve, Portugal Land Degradation and Development, 14, 265-283
  • Allen, HD (2003) Response of past and present Mediterranean ecosystems to environmental change Progress in Physical Geography, 27(3), 359-377
  • Fletcher, WJ (2005) Holocene Landscape History of Southern Portugal. Unpublished PhD thesis. University of Cambridge

Funding

William Fletcher's PhD research was supported by a NERC studentship (2001-04).

People

  • Harriet Allen
  • William Fletcher - now a postdoctoral researcher at Département de Géologie et Océanographie:
    UMR 5805 EPOC - OASU
    Université Bordeaux 1
    Avenue des Facultés
    33405 TALENCE CEDEX
    FRANCE