The relative contributions of weathering and aeolian inputs to postglacial formation of Mediterranean alpine loess

Abstract

Between the southern margin of the European loess belt and Sahara Desert, thin and irregularly distributed loess deposits occur in Mediterranean mountains. During the most recent deglaciation, along the Pleistocene-Holocene boundary, the deposition of glacial, periglacial and outwash sediments, was the main local source of Mediterranean alpine loess, whereas proximal alluvial planes comprised a secondary source. The mid-Holocene termination of African Humid Period and subsequent aridification of Sahara Desert occurred simultaneously with a change of the regional climate from Atlantic to Mediterranean-dominated, characterized by frequent episodes of southerly winds. This resulted to a change of the loess source, as deflation of quartz rich silts enriched in Zr during intense episodes of Sahara dust transport became more dominant. Here, a 32cm loess profile from the Plateau of Muses (PM), below the summit of Mount Olympus, Greece, is investigated on the basis of grain size, mineralogy, environmental magnetism and geochemistry. Comparisons of loess samples with glacial and periglacial deposits, enables us to differentiate relative contributions of local sources and allochthonous aeolian inputs. Calcite sand rich in feldspars makes up the glacial and periglacial clast free matrix. In contrast, PM loess is composed by clay and fine silt fractions with minor calcite sand contributions. The mineralogical matrix of loess contains quartz, phyllosilicates and mixed layer clays, while its geochemical composition contains high amounts of detrital Fe-Ti oxides and aeolian transported Al and Zr. Based on the multi-proxy approach applied here, the loess profile is partitioned in three layers. Holocene average deposition rates (~2.5 cm/ka) broadly agree with modern Sahara dust deposition (~2.0 cm/ka) and long-term postglacial Mediterranean mountain denudation rates (~0.5 cm/ka). Such low rates provided ample time for post depositional modifications, such as decalcification, deferrification and removal of K, evident from the trends of chemical weathering proxies Ca/Sr, Fe/Ti and K/Rb, respectively.