Seminars and Events
Timescales of plant wax storage and transport in river systems: what’s in an age?
Constraining the residence time of terrestrial organic matter in river basins has broad implications for our understanding of the global carbon cycle as variations in the rate of carbon exchange between different continental reservoirs and the marine reservoir affects the atmospheric CO2 budget over relatively short timescales. Furthermore, adequate paleo-environmental interpretation of the composition of terrestrial biomarkers deposited in river-dominated ocean margins relies on a mechanistic understanding of how environmental signatures acquired on land are transferred to marine sediments. The timescales of plant wax storage and transfer in river systems is of particular importance as it defines the practical temporal resolution at which plant wax can be used to reconstruct paleo-environments. Using compound-specific stable hydrogen (δD) and radiocarbon (14C) isotopic compositions of terrestrial plant waxes preserved in the channel-levee system of the Bengal Fan, we reconstructed variations in the strength of the Indian summer monsoon and attendant consequences for the rate of C exchange between reservoirs since the Last Glacial Maximum. Long-chain fatty acids display radiocarbon age offsets varying between 680 and 6,180 14C years, reflecting their storage in soils within the Ganges-Brahmaputra (G-B) system prior to deposition in the Bengal Fan. Furthermore, these show a strong correlation with climate, in particular with the intensity of the summer monsoon, revealing protracted storage of organic matter on land during drier periods. Using 14C derived from nuclear weapons testing as a tracer, we then couple numerical models with leaf-wax fatty acid stable isotope and 14C signatures in a sediment core from the head of the canyon feeding the Bengal Fan in order to estimate fatty acid age distribution in the modern G-B watershed. We show that 79-83% of the leaf-wax fatty acids in this core are sourced from continental reservoirs that store OC for an average of 1,000-1,200 calendar years, while the remainder has an average age of 15 years. Based on this age distribution, we conclude that the high reservoir age offsets observed during the late glacial period are best explained by a large residence time increase of the slow-cycling component. Overall, our data indicate that weaker monsoons characteristic of the late glacial promoted protracted storage of organic carbon in soils. Thus, we have identified hydroclimate change as a driver of the rates of C exchange between the atmosphere and biosphere, demonstrating the potentially globalscale feedbacks of climate-driven changes in terrestrial C storage and export dynamics.