Loess in southwestern Sweden. Determining the genesis of the silt-rich layer rödfemman in Svartedalen nature reserve

Sammanfattning: The presence of loess in Sweden is relatively unexplored and has only been found in a few places. Loess acts as a paleoclimatic proxy, which in times of changing climates, is highly relevant. In 1992, Svensson wrote a master thesis on a thin and patchy silt-rich soil layer found in Svartedalen nature reserve in southwestern Sweden, in which it was proposed that the layer has an aeolian origin. In this master thesis, the sample area of Svensson was revisited and expanded. The aim was to determine the layer’s genesis, age of deposition, and potential source as well as paleo-wind directions. Grain-size analysis showed a clear modality in the coarse silt region, though many samples had bimodal distributions. The findings fit well with other studies in analogous settings, where the bimodality has been interpreted as a result of mixing from either syn- or postdepositional processes. The layer is interpreted as loess due to its high silt content, Svartedalen’s location above the highest shoreline, together with the layer’s position in the stratigraphic record and the prevailing erosion from the last glaciation. Svensson (1992) proposed the nearby Hålt delta as the source for the silt which could not be supported in this thesis. Instead it is suggested that the silt has multiple sources in the shape of glaciofluvial deposits, sequentially activated with the retreating ice margin, or directly from the ice itself. The loess has been mixed during deposition or reworked after, shown by the grain-size distribution. OSL ages showed burial ages between 1.08 - 6.8 ka. This either reflects the date of deposition or bleaching due to reworking. An older date of 11.2 ka was derived when one sample was remeasured as this thesis was being written. A maximum age model was applied and the new date implies that the sediment has been reworked after deposition.