THE ORIGIN OF MAGNETIC MINERALS IN THE LAKE KHUBSUGUL SEDIMENTS (MONGOLIA)

Abstract

The aim of this paper is to discuss the results of studying the magnetic properties of sediments in the Khubsugul Lake (Mongolia). The samples of the sediments were collected at interval of ~0.04 m from the cores of the hole drilled to a depth of 53 m in the central part of the lake. Magnetic hysteresis data were obtained for 1026 rock samples using a coercitive spectrometer. The most typical samples were used to derive thermomagnetic curves for induced magnetization. These measurements revealed high variations in the magnetic properties of the sediments, probably caused by drastic changes in the paleoclimate and tectonics. The samples were classified into two groups in terms of their magnetic values. The samples of the first group showed higher magnetic values, including their magnetic hardness, and those of the second group were found to be less magnetized and were classified as magnetically soft. In terms of their thermal magnetization curves the samples were found to contain magnetite, Ti magnetite, and magnetic iron sulfide (greigite). The magnetic data, as well as the substantial homogeneity of the magnetite in the first group of samples, prove that they contain biologically induced magnetite (BIM) and greigite (BIG). The samples of the second group contain mainly allothigenous magnetic grains, represented mainly by magnetite. The sources of these grains were the volcanic rocks surrounding the lake. The varying concentrations of the two types of the magnetic material in the samples suggest variations in the ratio of the allothigenous and authigenous materials in the sediments. The decline of the biogenic magnetic component in the sediments suggests the predomination of the allothigenous component caused by the growth of the erosion at the expense of monsoon or tectonic activity. The drastic changes in the contents of the biogenic magnetic component in the sediments suggest high variations in the sediment accumulation rate. This factor need be taken into consideration in the creation of a time scale for the sediments of this lake.