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)
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 ferrimag...
| Main Authors: | , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
|
| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/2093/2016/bg-13-2093-2016.pdf |
| Summary: | 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.
<br><br>
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. |
|---|---|
| ISSN: | 1726-4170 1726-4189 |