A 600 year record magnetic record from Lake Hallwil, Switzerland
Abstract
The magnetic mineralogy of lacustrine sediments is sensitive to environment conditions. Climate and redox conditions influence the mineralogy and the grain size of iron-bearing phases in lakes. This study examines changes in the magnetic mineralogy in Lake Hallwil over the past 600 years. The lake lies at 449 mASL on the central Swiss Plateau, and the geology of the catchment basin is characterized by Quaternary glacial deposits. The lake has become progressively eutrophic over the past 100 years, and oxygen has been pumped into the deepest part of the lake during summer since.1987. A freeze core was taken in 1998. Samples that cover 2 cm of core were sliced from a section and freeze-dried. Sediments older than the twentieth century contain a low coercivity phase, in which the saturation isothermal remanent magnetization (SIRM) is reached below 200 mT. Except for four turbiditic layers, the remanence carrier is relatively uniform between 1400 and 1900 AD. Variations in intensity of the anhysteretic remanent magnetization (ARM) and SIRM are due to changes in concentration of the low coercivity phase. Turbiditic layers are characterized by lower susceptibility and remanent intensities. Sediments deposited since 1900 show a decrease in intensity of susceptibility, ARM and SIRM, and a minimum value for these parameters is found between 1931 and 1994. IRM acquisition curves are dominated by a low coercivity phase, but a higher coercivity phase is also present in this time interval. These results will be compared with magnetic and geochemical results from other lakes from the central Swiss Plateau. They all show that before the twentieth century, the magnetic signal is dominated by fine-grained magnetite. With progressive eutrophication coarser grained magnetite and hematite are the main magnetic carriers; paramagnetic iron phases are also present in the form of iron sulfides.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8552H