Characterisation of Andosols from Laacher See tephra by wet-chemical and spectroscopic techniques (FTIR, 27Al-, 29Si-NMR)

At 12,900 a BP, the eruption of the Laacher See volcano generated a new parent material for Holocene soil formation in parts of Western Germany. Weathering of these ashes commonly includes the formation of poorly crystalline minerals such as allophane, imogolite and ferrihydrite. Detection of these minerals in soil is difficult, yet an important task, because they may govern soil functions and processes, e.g., stabilisation of organic matter and nutrient availability. Therefore, we characterised three forested Andosols by a combination of wet-chemical and spectroscopic techniques including infrared and (27Al, 29Si) nuclear magnetic resonance (NMR) spectroscopy together with X-ray diffractometry. Deconvoluting the 29Si-NMR spectra revealed that 1.6 to 10.4% of total Si was present as allophanic compounds, which coincided with the amounts of oxalate-extractable Si. Since extraction methods are not completely selective, we observed a slight overestimation of allophanic Si estimated from oxalate extraction. Although the sites under study are located close to each other in similar relief positions and with similar vegetation, the combination of our results revealed varying amounts of loess in the parent materials and varying weathering intensity. High weathering intensities correlate with the amounts of allophane.