ORGANIC MATTER AND STABLE ISOTOPIC RECORD IN NAINITAL LAKE SEDIMENTS IN KUMAUN HIMALAYAS, INDIA

Nainital lake in the Kumaun Himalayan region in northwest India is one of the most frequented tourist destinations in India. The lake is under stress from human induced activities, and lacustrine sediments represent ideal conditions for studying various paleoenvironmental changes in the watershed.

We retrieved a 45 cm long core from the deepest part of the lake and sliced it at 2 cm intervals. Elemental concentrations of C, N, and S and their ratios (C/N, C/S), isotopic signature, and specific biomarkers (n-alkanes and pigments) were measured in the sedimentary organic matter. 210Pb dating in the core represents an age of ca. 85 years, and a sedimentation rate of about 5.0 mm/yr. TOC in sediments range from 2-5%, total N is 0.2-0.5%, and S is 13-25 mg/g. The low (11-15) C/N ratio coupled with low values for Carbon Preference Index (1-3) and Terrestrial Aquatic Ratio (0.3-2.9) indicate the dominance of algal derived matter in these sediments. The ä13C and ä15N values in sedimentary organic matter are -26.1‰ to -28.8‰ and 3.8‰ to 8.6‰, respectively. The lake shows nutrient deficient conditions, which favors cyanobacterial fixation. This is consistent with the high (1 to 4 mmol/g) concentration of different cyanobacterial pigments. Sulfate reduction is prevalent in the sediments, and ä34S ranges from 1 to 3‰. The main source of sulfate is from an underground spring and regional lithology. Sediment characteristics in the mid-section of the core suggests higher anthropogenic input (mainly from agricultural activities; 1950-80s), which affects the trophic state of the lake. Preventive measures adopted in recent years (e.g., aeration) have affected the sediments and ongoing biogeochemical processes in the lake.