How Reliable is the Bulk δ13C value of Soil Organic Matter in Paleovegetational Reconstruction?

Abstract:

Carbon isotope ratios of soil/paleosol organic matter (δ¹³C_SOM) have been used to reconstruct abundance of C₃-C₄ plants survived in the landscape as the δ¹³C value of C₃ (-27‰) and C₄ (-12.5 ‰) plants are distinctly different. In an attempt to reconstruct the abundance of C₃ and C₄ plants, δ¹³C_SOM have been measured from three soil profiles developed on flood plain of the Gangetic plain, Mohanpur, West Bengal, India. Satellite images reveal that the investigated sediments have been deposited in an oxbow lake setting of the river Ganges. The total organic carbon content of the profile ranges from 0.9% to 0.1%. The δ¹³C_SOM values mostly range from -19.2‰ to -22‰ except a rapid positive excursions of ~5‰ at 1.5 m depth showing enriched value (-14.2‰) in all the three profiles. Based on mass balance calculation using the δ¹³C values of C₃ and C₄ plants, the δ¹³C_SOM in the Gangetic plain indicate presence of both C₃ and C₄ plants in the floodplain. However, characterization of alkanes separated from lipids extracted from the same soil organic matter reveals dominant preferences in short carbon chain (C₁₄, C₁₆, C₁₈, C₂₀) with a little preferences for higher chain (C₂₉, C₃₁, C₃₃). Interestingly, n-alkanes at 1.5 m depth shows very high concentration in short chain n-alkanes. Since the lower chain n-alkane represents aquatic productivity or intense bacterial decomposition and higher chain indicates the contribution from C₃-C₄ plants, the data from the investigated sedimentary profile shows contribution mostly from aquatic vegetation with a little contribution from terrestrial plants. This implies that before using bulk δ¹³C_SOM value for reconstruction of C₃-C₄ plants from soil/paleosol, characterization (molecular level) of soil organic matter is required