Climate forcing of the terrestrial organic carbon cycle during the last deglaciation: the Himalaya-Bengal fan example

Abstract:

Over geological timescales, atmospheric carbon dioxide concentrations are modulated by exchanges between atmospheric, oceanic and terrestrial reservoirs of carbon. Here we investigate whether climate change exerts a first-order control on the delivery of terrestrial organic matter to the coastal ocean by rivers and explore the consequences for the rate of C exchange between atmospheric, oceanic and terrestrial reservoirs of C. Specifically, we employ inorganic proxies of sediment source and composition, coupled with stable-isotope and radiocarbon measurements of terrestrial biomarkers delivered to the Bay of Bengal since the Last Glacial Maximum (LGM) to investigate climate-driven changes in the dynamics of terrestrial organic carbon (OC) export and burial in the world’s largest depocenter of sediment and OC.

Compound-specific stable hydrogen (δD) and carbon (δ¹³C) isotopic measurements of plant wax compounds from a series of cores from the channel-levee system of the Bengal Fan capture variations in the strength of the Indian summer monsoon and vegetation dynamics within the Ganges-Brahmaputra drainage basin over the past 21 kyrs. Specifically, a 35 ‰ shift in plant wax δD between the LGM and Holocene Climatic Optima, (HCO; 9-5 ka) indicates a change from weaker to stronger monsoon conditions over this time period. Likewise, compound-specific δ¹³C measurements demonstrate a ca. 4 ‰ shift from the LGM to the HCO, recording a large decline of C₄ plants in the basin during this period.

Residence times of organic matter within the Ganges-Brahmaputra drainage basin determined from compound-specific radiocarbon dating of plant wax compounds vary between ca. 800 and 8000 years over the past 21 kyrs. These calculated residence time show a strong correlation with climate and in particular with the intensity of the summer monsoon as inferred from plant wax δD values. This is illustrated by an order of magnitude decrease in residence time between the driest (Heinrich event 1) and wettest (HCO) intervals of our record. This difference reflects protracted storage of organic matter on land during relatively colder, drier periods. Thus, we have identified climate change as a driver of the rates and sources (e.g., deep soils vs. recent biomass) of terrestrial-OC export from the largest erosional system on earth.