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)

Norbert R Nowaczyk,Hendrik Vogel,Janna Just,Bernd Wagner,Alexander Francke,Jack H Lacey,Leonardo Sagnotti

doi:10.5194/bg-13-2093-2016

Abstract

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.

Highlights

Rock-magnetic properties of sediments can be used to reveal changing input of the lithogenic fraction and can serve as records of past environmental change
A major change is visible around ca. 320 ka corresponding to the transition between Marine Isotope Stage (MIS) 9 and MIS 8
The number of investigated scanning electron microscopy (SEM) samples is relatively low, the results suggest that a high MDFARM appears closely related to or at least accompanied by the general presence of iron sulfides, while maximum SIRM / χ is associated with the coarse-grained sulfide nodules

Summary

Introduction

Rock-magnetic properties of sediments can be used to reveal changing input of the lithogenic fraction and can serve as records of past environmental change. The primary magnetic mineral assemblage of detrital origin is often overprinted by post-depositional alteration (Hounslow and Maher, 1999; Nowaczyk et al, 2013; Roberts et al, 1996) The latter results from changing redox conditions at the lake/sea floor and in subsurface sediments leading to dissolution of iron oxides and formation of ferrimagnetic iron sulfides, such as greigite and pyrrhotite (Demory et al, 2005; Froelich et al, 1979; Karlin and Levi, 1983; Rowan et al, 2009; Sagnotti, 2007; Vasiliev et al, 2007) or paramagnetic minerals, such as pyrite, siderite, and vivianite (Dong et al, 2000; Karlin and Levi, 1983)

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