Revised chronology and stable isotopic (Carbon and Nitrogen) characterization of Lahuradewa lake sediment (Ganga-plain, India): Insights into biogeochemistry leading to peat formation in the lake

Abstract

The Lahuradewa lake provided evidences for the earliest rice cultivation in central Ganga plains at ~8.3 ka BP using depth profiles of phytoliths and paddy field diatoms.Chronology of the lake profile was earlier constrained by six conventional radiometric 14 C dates.Organic matter was abundant throughout the Holocene and was expected to have recorded productivity changes controlled by monsoonal changes and/ or in-situ lakebiogeochemistry.To deduce these changes, here we measured total organic carbon (TOC) nitrogen (N) and sulphur (S) its concentration and their stable isotopes (δ 13 C TOC , δ 15 N and δ 34 S) in twenty eight sedimentary layers of the Lahuradewa lake.Besides, we also measured six additional 14 C dates by Accelerator Mass Spectrometry (AMS) to refine the chronology.All six AMS 14 C ages were found to be in excellent agreement with earlier reported radiometric 14 C dates.Using δ 13 C TOC and δ 15 N variations, depositional history of the lake could be sub-divided into four phases (i) pre-agricultural or peat zone (~10,899 to 7,451 year BP) (ii) early-agricultural (~7,450 to 2,623 year BP) (iii) deteriorating monsoon (~2,633 to ~1,228 year BP) and (iv) the modern phase (~1,228 to present day).The pre-agricultural phase (black organic muddy sediment between 2.3 to 2.8 m; peat zone) was found to have strikingly higher TOC and N contents (~28% and ~2% respectively) with significantly enriched δ 13 C TOC values (-17.5±0.5‰),lower δ 15 N values (2.1±0.2‰) and δ 34 S (9.6±0.9‰) is found only in this zone.We surmise that this pre-agricultural phase sequestered capacious amounts of atmospheric C (and N) by developing Botrycoccus algae, which produces natural hydrocarbons.Such algal growths in wetlands promote carbon sequestration (soaking greenhouse CO 2 gas) and act later as source(s) of renewable hydrocarbon biofuels.