Articles | Volume 13, issue 7
Biogeosciences, 13, 2093–2109, 2016

Special issue: Integrated perspectives on biological and geological dynamics...

Biogeosciences, 13, 2093–2109, 2016

Research article 08 Apr 2016

Research article | 08 Apr 2016

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

Janna Just1,2, Norbert R. Nowaczyk3, Leonardo Sagnotti4, Alexander Francke1, Hendrik Vogel5, Jack H. Lacey6, and Bernd Wagner1,2 Janna Just et al.
  • 1Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany
  • 2Collaborative Research Centre 806 – Our Way to Europe, University of Cologne, Cologne, Germany
  • 3Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
  • 4Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
  • 5Institute of Geological Sciences & Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 6NERC Isotope Geosciences Facilities, British Geological Survey, Nottingham, UK

Abstract. The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before  ∼  320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments.

While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, 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-lake processes.

Short summary
The magnetic record from Lake Ohrid reflects a strong change in geochemical conditions in the lake. Before 320 ka glacial sediments contain iron sulfides, while later glacials are dominated by siderite. Superimposed on this large-scale pattern are climatic induced changes in the magnetic mineralogy. Glacial and stadial sediments are characterized by relative increases of high- vs. low-coercivity minerals which relate to enhanced erosion in the catchment, possibly due to a sparse vegetation.
Final-revised paper