Geochemistry of core sediments from the Middle Tagus alluvial plain (Portugal) since the last glacial: using background determination methods to outline environmental changes

Abstract

This work deals with the geochemistry of the sediments from the Santarem-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.