PALEOMONSOON IN NW INDIA (SAMBHAR LAKE): AN ENVIRONMENTAL MAGNETIC APPROACH

Abstract

The Sambhar lake in Rajasthan, North-West India is one of the largest playa lakes and has preserved approximately 30 ka paleo-record^ in its sediments. To understand the paleomonsoon, since Holocene period, the present study was carried out by using environmental magnetism, a fast, non destructive and comparatively economic approach. The technique has earlier been applied successfully by various authors on marine and loess systems. In the present study, short (~ 2.25 m) and long cores (~20 m) were collected from the lake and analysed for various magnetic parameters. The concentration dependent magnetic parameter magnetic susceptibility (Xlf) and <*v<Kyv£- mineralogy show that the concentration of the magnetic fraction decreases towards the.Sambhar lake. Similarly the grain size sensitive bivariate ratio XARM vs. Xlf indicate the stable single domain fraction to be more dominant A in the catchment soils, -as compared to the moist lake bed sediments. The low value of the coercivity of remnance B0(Cr) (~40 mT) and S_ratio speculates -Vke. the dominance of ferrimagnetic minerals in catchment, river and lake sediments. Apart from the concentration and magnetic grain size, the mineralogy does not show much variation. The power regression relationship of the magnetic parameters such as Xlf, SIRM and XARM with concentrations lit dU.'*^-4^ of CaO and MgO speculates that dolomite precipitation often occurs in arid condition and show a low magnetic susceptibility and other parameters. The A /• 11 cut/w*ii* V *>» control of lithology over, magnetic parameters also advocate the use of environmental magnetism as a versatile tool to study various lacustrine conditions, which usually shows fluctuations with low values in arid phase. A A correlation of GRIP (Greenland) ice core with the long core of Sambhar lake and data of around 1000 years pf solar insolation with short core provide supporting strength for interpretation. On the basis of magnetic data, the core SLM-L2 was divided in to nine lithologic zones, zone-l (deeper) to zone-IX (towards the surface^ and the paleomonsoon links with the magnetic data have been described during (i) Pre-Holocene (30 ka-10 ka)' (ii) Holocene (10 A Ka- 1Ka) and, (iii) Millennium scale (1 Ka to present). The short core samples have been dated by 210Pb and OSL dating, whereas the age-dates in long cores have been assigned from the AMS dates published earlier in another core from the lake. In addition, grain size distribution at different depths of the sediments were measured and the apatite phosphorus fraction in the core sediments have been analysed and interpreted along with the magnetic data. The thesis is structured and presented as follows: Chapter 1 deals with an overall introduction of monsoon, paleomonsoon and some recent and previous paleomonsoon studies in North West India. The next section of this chapter deals with introduction of environmental magnetism and its utility for paleoclimatic and paleomonsoon reconstruction and sediment source. The chapter also out lines the A significance of the study. Chapter 2 provides information about the study area "The Sambhar Lake" and provides detailed information jsf the lake and is described with 3 figures and 1 table. II Chapter 3 deals with the sample collection and the various analyses carried out. The details of dating, magnetic parameters and phosphorus fractionation have been presented.JThe chapter contain 6Fig and 1tableJ ° •"•* Chapter 4 deals with the results and discussion, the first section of4he chapter involves determining the source of sediments in the Sambhar lake and spatial variations in magnetic mineralogy, concentration and grain size. The next section of this chapter involves the dating procedures by 210Pb and OSL methods. Various magnetic parameters in the lake sediments bave-been discussed in detail and monsoon variations have been discussed dyftng Pre- Holocene, Holocene and millennium time scalea This chapter is supported with 41 figures and 12 tables^ 0n** Chapter 5 points out the major conclusions of the study and is followed by the detailed listings of references cited in the texts.