Deciphering source, degradation status and temporal trends of organic matter in a himalayan freshwater lake using multiproxy indicators, optically stimulated luminescence dating and time series forecasting

Abstract

The present study assessed source, degradation status, and temporal trends of organic matter up to ∼3700 years Before Present (BP) in Dal Lake sediment cores using multiproxy indicators, optically stimulated luminescence (OSL) dating, and time series forecasting. Bulk organic parameters such as total organic carbon and total nitrogen indicated their increasing trend in upper sediment layers from ∼476 to ∼73 years BP. Similarly, the C/N ratio revealed varying sources of organic matter with values ranging from 2.26 to 13.05, depicting an increasing trend up to ∼1811 years BP and decreasing significantly from ∼2002 years BP. Amino acid and hexosamine concentrations decreased with depth, dominated by aspartic acid, glycine, glutamic acid, alanine, glucosamine, and galactosamine in recent sediments (∼73 years BP) of the lake. The positive degradation index (DI) confirmed fresh organic matter in top sediment layers (∼396 years BP) of the lake, showing a strong positive correlation with reactivity index (RI) and total organic carbon. However, a negative correlation was observed between DI and RI with Ox/Anox index. Furthermore, the Gluam/Galam, Asp/β-Ala and Glu/γ-Aba ratios revealed enhanced organic matter degradation with age, indicating prokaryotic origins, microbial activity changes, and redox conditions within the lake. Multivariate analysis revealed increasing autochthonous and allochthonous inputs, domestic effluent discharges, agricultural runoff, redox conditions, and microbial processes as the possible contributing factors affecting the dispersal pattern of amino acids and nutrients in Dal Lake. Time series forecasting using the ARIMA model effectively confirmed the future temporal dynamics of amino acids, and organic parameters, indicating potential ecological shifts and external influences affecting the degradation of organic matter in Dal Lake. This work provides novel insights into the diagenetic alterations and biogeochemical processes essential for understanding the organic matter cycling and guiding the conservation efforts for Dal Lake and beyond.