LATE PLEISTOCENE STABLE ISOTOPE RECORD OF NORTHERN INDIA LACUSTRINE SEDIMENTS

We present a record of δ¹³C_inorganic, δ¹⁸O, δ¹³C_organic, and C/N ratios obtained from lacustrine sediments from a 25 meter exposure of a relict lake in the Himalaya of Northern India. The chronology has been preliminarily defined by two OSL dates of 16 ka and 11 ka from the bottom and top of the core, respectively. Thirteen additional AMS ¹⁴C dates are in progress. C/N ratios are low (<8) for the entire core, suggesting that organic material in the lake was derived from aquatic plants. Values of δ¹³C_organic average approximately -24‰ throughout the samples, which suggests that organic carbon material was derived from aquatic C3 plants and that no major vegetation changes occurred during this interval.

Values of δ¹⁸O_VPDB show an overall increasing trend from -15‰ to -10‰. The data suggest two major periods with unique isotopic signatures that can be interpreted in terms of paleoclimate and paleohydrology of the lacustrine environment. The first period, assuming a constant sedimentation rate, extends from ~16 ka to ~14 ka, during which time the lake received a decreasing contribution of glacial meltwater. The oxygen isotope values steadily increase from -15‰ to -13‰ during this time interval, after which there is a sharp increase to -10‰. The second period, from ~14 ka to ~11 ka, is characterized by several abrupt oscillations where isotopic values vary from ~ -10‰ to -14%.

Environmental conditions can be divided into two similar periods based on values of δ¹³C_inorganic. During the first period carbon isotope values show an increasing trend from -4‰ to -1.5‰, after which isotopic values remain relatively constant at ~ -2.0‰. A decreasing contribution of glacial meltwater during the first period would result in a decrease in the amount of nutrients brought into the lake, which would consequently lower the productivity and lead to heavier isotopic values for carbon.