Climatic control on the occurrence of high-coercivity magnetic minerals and preservation of greigite in a 640 ka sediment sequence from Lake Ohrid (Balkans)

Abstract

The bulk magnetic mineral record from Lake Ohrid, spanning the past ca. 640 ka, shows a strong relationship to environmental conditions on glacial-interglacial and millennial time scales. During extremely cold glacials, a lower accumulation of organic matter and likely enhanced mixing of the water-column coincides with the presence of greigite, whereas greigite is absent in sediments deposited during less severe glacials. Those "non-greigite" glacial sediments are characterized by high concentration of high-coercivity magnetic minerals, which relates to enhanced erosion of soils that had formed during the preceding interglacials. In contrast, magnetite dominated magnetic mineral assemblages characterize interglacial deposits and most likely originate from detrital particles of physically weathered rocks. Superimposed on the glacial-interglacial behavior are millennial scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Likewise to the process on glacial-interglacial time-scales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights 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 and intra-lacustrine processes.