Geochemistry of core sediments from the Middle Tagus alluvial plain (Portugal) since the last glacial: using background determination methods to outline environmental changes |
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Authors: | T M Azevêdo M A Gonçalves |
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Institution: | 1.Departamento de Geologia,Faculdade de Ciências da Universidade de Lisboa,Lisbon,Portugal;2.Geological Research Centre,University of Lisbon,Lisbon,Portugal;3.CREMINER/LA-ISR,Lisbon,Portugal |
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Abstract: | This work deals with the geochemistry of the sediments from the Santarém-Entre Valas (SEV) borehole, in the middle Tagus alluvial
plain in Portugal. Methods for background determination were used, notably the concentration–length and cumulative frequency
methods. Both methods were in excellent agreement for the elements of interest, namely Au, As, Fe, U, Cu, Pb, Zn, Co, and
Cr. With these results, five segments in the SEV borehole were identified to have higher than background values for at least
two or more of these elements. The most important segment spans at least 1,500 years during the Early Holocene Warm Period,
with almost all elements with anomalous values. This has been interpreted as the result of enhanced weathering of the host
rocks upstream as a consequence of deglaciation since the Last Glacial Maximum and general wetter conditions known to have
occurred in this region. No other segment of the core has shown a similar variety of elements with higher than background
values, but the upper segments correlate with the particular mineralogy on the sediments due to the development of an estuarine
environment at circa 3,000–4,000 cal. years BP. The presence of charcoal and charred organic matter during the beginning of
the Bronze Age characterises the segment with the first high values of Cu, but apart from deforestation and land use that
affected this region at the time, this anomaly is likely due to the chemical affinity of Cu to organic matter and not to anthropogenic
activities. Finally, the upper meter of the core sediments, spanning 812 years, have a clear anthropogenic signal for Cu (also
seen in other two boreholes) and systematic high values for Au (and Pb). Although Cu is surely an indication of secular agricultural
practices (vineyard sulphatation), Au (and Pb) might be the natural accumulation from weathering of ancient mine wastes scattered
in the Hercynian Massif upstream. |
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