CHARACTERISATION OF ORGANIC MATTER IN SEDIMENTS OF KUMAUN HIMALAYAN LAKES

Abstract

Characterisation of organic matter in lake sediments provides a variety of geochemical proxies to understand the source and depositional history of organic matter in lacustrine environments. The four lakes namely Nainital, Bhimtal, Sattal and Naukuchiatal, in the Kumaun Himalayan region are one of the most frequented tourist destinations in India, and may represent ideal conditions for studying environmental records. The elemental concentrations of TOC, N and S with their atomic C/N, C/S ratios, isotopic concentrations (8I3C, 534S and 5I5N) and specific biomarkers (n-alkanes, pigments and PAHs) were measured in different segments of lake bottom core samples, varying in length between 35 and 45 cm. The cores represent an age of ca. 85 years in Nainital, 72 years in Bhimtal and 87 years in Sattal, with sedimentation rates of approximately 5.0 mm yr"1, 4.8 mm yr*1 and 4 mm yr"' respectively. The TOC measured in all the four lakes, show anoxic microbial degradation of organic matter to have played an active role in controlling the TOC. The TOC values show that primary productivity increased, since the past few decades. The Naukuchiatal lake shows higher values (between 3 and 6%) of TOC, followed by Sattal lake, whereas the Nainital and Bhimtal lakes show lower (between 1 and 5% and 3 and 0.8% respectively) TOC contents. The total sulfur in Nainital and Bhimtal show variations, probably due to associated anoxic microbial degradation of organic matter. Atomic C/N (below 12), low CPI (between 0.1 and 2.9) and TAR (between 0.4 and 2.7) values, indicate dominance of algal organic matter in all the four lakes. Measurements made in all the lakes for short chain n-alkanes (Ci5j i7> 19), suggest dominance of algal organic matter. Furthermore, isotopic values of 8I3C and 515N (between -31%o and -24%o and between 8.6%o and -0.23%o respectively) show methanogenesis and ammonia degassing associated with high sulfate reduction in Bhimtal and Nainital lake sediments. All the four lakes show nutrient (nitrogen) deficit conditions, which favor cyanobacterial fixation, resulting in high concentration of cyanobacterial pigments in the lake sediments. The dominance of cyanobacterial pigments (filamentous sp.) in the lake, indicate higher concentration of phosphate. Concurrently, high (0.8 to 0.3 and 0.1 to.Ol mmolg"' respectively) concentration of chlorophyll a and chlorophyll b in Nainital lake show increased productivity. Further CD: TC ratio in the Nainital lake sediments represents higher productivity and anoxic condition. The higher (3.8%o) isotopic values in sulfate show, the main source of sulfate in the Nainital lake to be from underground spring. The PAHs measured in Bhimtal, Naukuchiatal and Sattal represent the main source of the polyaromatic hydrocarbons in the lake to be from domestic burning of fuels (wood and coal), petroleum from vehicular traffic, and may be also from the frequent forest fires during summer season. The present thesis on "Characterisation of organic matter in sediments of Kumaun Himalayan Lakes" is divided into five chapters. Chapter 1 includes the background of the biogeochemical proxies, with special emphasis on factors affecting TOC, C/N ratio, isotopes and specific biomarkers. Literatures on previous works on global lakes, carried out by various authors worldwide are also discussed. Chapter 2 deals with description on the geological setting of the lakes, bathymetry and sampling location. Chapter 3 deals with the methodologies followed for sample analyses. Results of the sample analyses, interpretations of various proxies for individual lakes and the relevant discussions are presented in Chapter 4. Chapter 5 outlines the major conclusions derived from the present study, followed by all references cited.