Does a large delta-fan sedimentary archive faithfully record floodplain vegetation composition?

Abstract

Large river systems have lately gained much attention in past as well as modern climate change studies because of their ability to transfer and sequester massive amounts of terrestrial organic matter (OM) in their delta-fan archives. Because of their long- uninterrupted- sedimentation history, the delta-fan repositories of the large rivers are also thought to be excellent paleo-climate archives. Being one of the largest, the delta-fan of the Ganges-Brahmaputra (G-B) River system has been extensively studied. OM-based paleo-vegetation proxy records, recovered from the floodplain archive, however, showed poor concurrence with the G-B delta-fan records. Using carbon isotopic composition (δ13C) of modern C3 and C4 plant produced OM, soil, and bedload sediments from the lower Gangetic plain, we have shown that riverine sediments and therefore delta-fan sedimentary archives fail to capture the floodplain vegetation composition in terms of C3–C4 plant abundance. The bias arises because of the faster removal of complete C4, and partial degradation of C3 plant derived carbon during OM transfer from plant to the soil, before its final storage in the delta-fan archives. Using a global compilation of riverine OM ages and their δ13C values, we propose that multiple deposition-erosion cycles of sediments, during its transportation through large River systems, possibly cause sedimentary OM ageing and play a key role in determining the quality of OM based proxy records. We, therefore, suggest that bulk OM ages are necessary to assess the quality of OM based proxy records when retrieving paleo-climate information from the large delta-fan archives.