CLIMATE FORCING OF THE TERRESTRIAL ORGANIC CARBON CYCLE DURING THE LAST DEGLACIATION: THE HIMALAYA-BENGAL FAN EXAMPLE

Over geological timescales, the size of the atmospheric CO₂reservoir is partly controlled by variations in the rate of exchange between C reservoirs (atmosphere, ocean, solid Earth), which in turn result in changes in the residence times of C within each of these pools. We investigate whether climate change exerts a first-order control on the fluvial delivery of terrestrial organic matter to the coastal ocean and explore the consequences for the rate of C exchange between reservoirs. Specifically, we employ 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 the Bengal Fan capture variations in the strength of the Indian summer monsoon and vegetation dynamics within the Ganges-Brahmaputra (GB) drainage basin over the past 21,000 years. 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, 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 OC within the drainage basin determined from compound-specific radiocarbon dating of plant wax compounds vary between ca. 800 and 8000 years since the LGM and show a strong anti-correlation with climate, in particular with the intensity of the summer monsoon .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. Furthermore, given that the GB accounts for as much as 20% of the global burial flux of terrestrial biospheric OC, climate-driven changes in the carbon-export dynamics of this system can have important feedbacks on climate at a global scale.