Environmental magnetism of Lake Ohrid (Balkans) – Rock magnetic proxies for lacustrine and terrestrial environmental changes over the past 640 ka

Abstract:

Magnetic properties of lake sediments can provide valuable information on terrestrial environmental conditions. The composition of detrital magnetic minerals provides information on erosion and pedogenesis in the catchment. However, depending on the trophic state of the lake, water depth and stratification, oxygen supply is often limited and leads to dissolution and neo-formation of magnetic minerals.

Variations in bulk magnetic and geochemical properties of a sediment core (ICDP site 5045-1) from Lake Ohrid, mimic climatic changes on glacial-interglacial and millennial time scales. During extremely cold glacials, low accumulation of organic matter and enhanced mixing of the water-column coincides with the preservation of syn-sedimentary greigite whereas in sediments deposited during less severe glacials and interglacials, greigite is absent. “Non-greigite” glacial sediments are characterized by high concentration of high-coercivity magnetic minerals, which relates to enhanced erosion of soils due a retreat in vegetation. During interglacials magnetite dominates the magnetic mineral assemblage, most likely corresponding to detrital particles originating from physically weathered rocks. Millennial-scale variations of magnetic properties are superimposed on the general glacial-interglacial pattern. Higher contributions from high-coercivity minerals correspond to phases of low summer-insolation. We propose that also on these time scales retreated vegetation resulted in enhanced soil erosion. Our study demonstrates that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land, as well as intra-lake processes.