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