Morphological and trace-elemental characteristics of various silica accumulations in soils of southern Portugal and Lanzarote for reconstruction of their genesis
Final Report Abstract
Main outcomes of the last project year are related to silica in soils with carbonate accumulation on Lanzarote island and to lateral silica transport from the Alto Alentejo into the Sado Basin in S Portugal. Silica in soils with carbonate accumulation on Lanzarote island. A chronosequence of soils developed on 180-270, 6000 and 40 000 year-old volcanic ashes on Lanzarote, showing progressive carbonate accumulation in their subsoils, was analyzed for carbonate content, HCl-extractable and NaOH-extractable Si. The samples were subjected twice to NaOH extraction, the first time to analyze Si from pedogenic amorphous silica, and the second time to quanfify Si released from partially dissolving volcanic ash particles, which was then subtracted from the amounts of Si found in the first extract. All analyses were carried out either on unsieved samples or on both fine and coarse fraction separately. Carbonate contents and amounts of HCl-soluble Si were only weakly related, indicating that Si is incorporated in carbonates in very variable proportions, which may be explained as follows. There are two different sources of carbonates in these soils; i. carbonates produced through weathering of calcium silicates and reaction with soil carbon dioxide, ii. carbonates contained in Saharan dust that becomes an increasingly important soil component with increasing soil age. Carbonate formation from calcium silicates (i.) involves concurrent release of silica, whereas carbonate dissolution from Saharan dust and subsequent precipitation (ii.) is not necessarily associated with silica dynamics. Very young soils (such as the 180-270 year-old soil) contain only small amounts of carbonates, nearly all of which are derived from Saharan dust and thus entirely present in the fine earth fraction. At the same time, weathering of easily weatherable volcanic glass leads to considerable silica release. Since both carbonate and silica dynamics occur concurrently in these young soils. Si is partly occluded in the developing carbonate precipitates. In older soils that contain much higher amounts of Saharan dust, large amounts of carbonates are present (particularly in the fine earth fraction) and may be dissolved and re-precipitated, largely independently from silica dynamics. Only where calcium released from ash particles forms carbonate precipitates on the particle surfaces, weak relationships between carbonates in the coarse fraction and HCl-soluble Si in the coarse fraction are observed, like in the 6000 year-old soil between 34 and 88 cm depth. If with time carbonates at shallow depth (like in the 180-270 year-old soil) are dissolved and reprecipitated at greater depth (like in the 40 000 year-old soil), the previously co-precipitated Si may not necessarily follow. This may be the reason for increased amounts of HCl-soluble Si in the upper part of the oldest soil. The similar values of HCl-soluble Si in the upper 10 cm in all three soils of this chronosequence may reflect co-precipitation of Si and carbonates during the early stages of soil formation and subsequent dissolution of near-surface carbonate precipitates in the older soils, whereas silica stays at the depth of its primary precipitation. This is however a first hypothesis that requires further directed studies for verification. Lateral silica transport from the Alto Alentejo into the Sado Basin in S Portugal in order to trace back silica pathways in the study area in southern Portugal to its sources, silcretes and parent rocks occurring in the area were analyzed for their trace element contents. For this purpose trace element contents were measured in the NaOH extracts of silcretes from soil profiles along an approximately 12 km transect from the boundary Serra de Portel / Sado Basin into the Sado Basin. Rock samples from the assumed silica source regions were subjected to complete combustion for analysis of Ba, Co, Cs, Nb, Pb, Rb, Sr, Y and Zr by ICP-MS, and Si, Al, Ca, Fe, K, Mg, Mn by ICP- OES. The rock samples of the assumed silica source areas showed considerable differences with regard to several trace elements. Elements that were abundant in only one of the rock samples were particulariy suitable as tracers. Lateral flows of silica were reconstructed based on trace element contents in the rock samples and trace element:Si ratios in the NaOH extract of the silcretes in the Sadon Basin. In this way several scenarios were developed, element by element. All scenarios confirmed lateral element flows from the Alto Alentejo into the Sado Basin. Interestingly, also manganese was found in the NaOH extracts in considerable amounts (as well as in most rock samples). Since manganese can be mobilized only through reduction, this is an additional indicator for perched water, which must have led to lateral water flow from the Alto Alentejo into the Sado Basin.
Publications
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(2009). Mikromorphologie verschiedener Kieselsäurephasen in Böden im Alentejo (Süd-Portugal). Gemeinsame Tagung der Kommissionen II und VII der DBG „Mineralogie und Chemismus der Oberflächen in Böden", 19.-20.2.2009 in Hannover, Berichte der DBG
Sauer, D., Stein, C, Zarei, M., Stahr. K.
