San Sebastian, Spain : Sociedad de ciencias aranzadi zientzi elkartea
Abstract
The northeastern coast of Spain is characterized by Mesozoic-Cenozoic sedimentary rocks forming high cliffs interrupted by short, narrow estuaries that are separated from the open sea by sandbars, beaches and dune deposits. The morphology and extent of the different estuarine sedimentary environments are constantly altered by erosion and deposition of sediments, and they are sensitive to even small changes in sea level. The Holocene transgressive episode caused the deposition of large volumes of well preserved estuarine sediments that can be studied to understand environmental and sea-level changes during this interglacial. The Bilbao estuary was originally the most extensive estuarine area on the Cantabrian coast of northern Spain. The modern estuary is 15 km long, an average of 100 m wide, and its channel depth ranges from 2 m in the upper estuary to 9 m at the mouth. The estuary is formed by the tidal part of the Nervion river, although four other rivers (Kadagua, Asua, Galindo and Gobelas) discharge into the main course. The tidal channel discharges into a wide marine bay called El Abra (average 3.5 km wide; up to 30 m deep). Tides are semidiurnal, with ranges between 4.6 m (spring tides) to 1.2 m (neap tides). The first iron and steel industry was opened on the middle estuary over reclaimed marshes as early as 1854. Since then, the natural features of the Bilbao estuary have been dramatically modified by urban, industrial and port developments. The exploitation of abundant local iron ore led to the early industrial development of Bilbao in the mid-19th century. The original estuary was rapidly reduced in size through land reclamation to form a tidal channel. This was isolated by dyking from its original intertidal areas, to allow a navigable watercourse from the city to the open sea. Today the Bilbao estuary is a largely artificial system which bears little resemblance to the original estuary. It has been calculated that the total amount of the original estuarine surface lost through human activity is approximately 1,000 ha. During the last 150 years, the Bilbao estuary has received wastes from many sources (mineral sluicing, industrial wastes and urban effluents) which have significantly degraded the environmental quality of the estuary. At present, the Bilbao estuary is the most polluted coastal area of northern Spain. A significant decrease in the flux of organic matter and heavy metal contaminants has occurred over the last decades, however, due to the implementation of environmental protection policies, the improvement in waste-treatment systems and the closure of some major factories during recent periods of economic recession. Furthermore, estuarine regeneration is currently being undertaken as part of a Revitalization Strategic Plan. Despite these improvements, the unremoved contaminated sediments from the intertidal areas may act as a long-term source of heavy metals to the aquatic environment through sediment mechanical reworking (e.g. dredging, shipping) and oxidation of anoxic sediments. Thus, these pollutants may continue procedures have been implemented. To reconstruct the palaeoenvironmental evolution of the Bilbao estuary since its formation following the postglacial rise of sea level, it have been analyzed the foraminiferal assemblages contained in eleven boreholes taken in the reclaimed areas of the middle and lower parts of the estuary. Integration of these results with similar data from previously studied boreholes has allowed us to reconstruct the environmental development of the Bilbao estuary during the Holocene. This environmental development has been organized into different systems tracts following a sequence-stratigraphic interpretation. Radiocarbon dating and palynological studies helped to locate in time the different palaeoenvironments and depositional episodes identified in the boreholes. Sediments that compose the estuarine infill range from basal fluvial coarse materials (Lateglacial), almost barren of foraminifera, that are interpreted as the lowstand systems tract (LST). This system tract is followed upwards by lower and middle Holocene transgressive materials (TST; 8500-3000 years cal BP), with upper Holocene regressive sediments on top (HST; 3000 years cal BP-19th century). This general sedimentary sequence is best represented in the middle estuarine area, and its sedimentary and microfaunal features are variable depending on its palaeogeographic setting: increasing muddy sediments and absence of open-marine elements are characteristic of the upper estuary, whereas increasing sandy sediments and reworking of the basal fluviatile materials are typical of the lower estuary. Comparison of these data with other sedimentary sequences from different coastal areas in the Bay of Biscay allows to reconstruct a general picture of the Holocene sea-level changes and coastal evolution in this area for the last 8500 years. On the other side, six cores retrieved from the Bilbao estuary tidal flats have been studied in order to evaluate the historical impact of the long lasting record of anthropogenic inputs, which have led to the disappearance of the indigenous microfauna and to the significant pollution of the sediments. Three different zones can be identified in the recent sedimentary record, reflecting initially the preindustrial estuarine conditions with abundant and diverse foraminiferal assemblages and baseline levels of metals, followed by the industrial-period sedimentary record, when high concentrations of metals in the estuarine environment allowed the development of abundant foraminiferal assemblages during the older industrial zone (period 1850-1950) that disappeared during the younger industrial zone (period 1950-2000) due to complete defaunation of the Bilbao estuary caused by minimum oxygen levels during this period. Obtained data provide important information in planning the restoration of the Bilbao estuary, because sediments have proven to be an important storage reservoir for pollutants and microfossils. Finally, the distribution and abundance of benthic foraminifera (together with geochemical analysis of heavy metals) from surface sediment samples (1997- 2003) and sediment cores collected from the intertidal flats of the Bilbao estuary have been studied to determine recent environmental contitions of this estuary. Furthermore a foraminiferal sampling network has been created in order to evaluate the future progress of the current regeneration schemes on a seasonal and anual basis. Pb-210 and Cs-137 determinations have also been undertaken to provide a chronology for pollutant inputs and ecological changes in the Bilbao estuary
