Glacial--Interglacial variations of the carbon cycle induced by climatic changes in Himalaya – GI-NOAH

conventional approach of pollen separation and analysis of the main ecological groups

Occurence of interglacial sedimentation at 8°N in the Bay of Bengal, i.e. in the middle part of a deep sea fan.

Tachambalath, A.P., France-Lanord, C., Galy, A., Charreau, J. (2021) Bengal fan record of silicate weathering of Himalaya through Neogene. Goldschmidt2021, 6996
Zorzi, C., Desprat, S., Galy, V., France-Lanord, C., Galy A., (2021) AGU Fall meeting 2021, Nouvelle-Orléans (USA). Glacial-Interglacial Vegetation and Hydroclimate Changes in the Ganges-Brahmaputra Basin Recorded in the Deep Sea Sediments from the Bengal Fan.

This project aims at a better understanding of the sensibility of the Earth to climate by building a record of carbon burial (organic and inorganic) induced by the erosion of the Himalaya at glacial-interglacial timescale. Continental weathering is the least understood process affecting non-linear feedback in the carbon cycle – climate link. Previous findings are ambiguous because they are based on marine proxies likely to be affected by other processes, such as the changes in the thermohaline circulation that are known to be important in the reorganisation of the carbon cycle between glacial and interglacial periods. Direct evidence of global significance can be retrieved in sedimentary basins dominated by terrigenous input and this has been recently achieved on the Bengal Fan, the biggest sedimentary basin. There, a 2-3 fold enhancement of Himalayan weathering after the last termination clearly indicates that 1) climate can modulate the terrestrial weathering, 2) tropics and high latitudes do respond differently, and 3) marine proxies of the continental weathering are not very precise but record changes in the continental weathering. However, the available record only covers the last change from glacial to interglacial conditions and might not be representative. In addition such record must be confirmed to avoid any sedimentary bias, but it shows the possibility to retrieve such signal from the chemistry of the detrital and authigenic phases and paved the way for a more systematic study.
This is now achievable thank to the IODP Exp. 354 that drilled the middle part of the Bengal Fan at 8°N and obtained a comprehensive record of the erosional history of the Himalaya. In particular, the sedimentation during the Pliocene, Pleistocene and Holocene is characterised by the localisation of the accumulation through the establishment of a channel. Several large levee units have been fully recovered with one representing the active Holocene channel and spanning the best part of last 100kyrs. We will perform detailed studies on this active levee and on the other drilled levees from the rest of the Quaternary. In particular, mineralogical and geochemical studies should provide information about the source of the eroded material and when compared with the Himalayan source rocks this will allow the quantification of the extend of chemical weathering at a given time. This includes sources (d13C and d18O of carbonates, Sr- Nd- and Pb-isotopes of the silicates), and weathering proxies (clay mineralogy, K/Al ratio, OH content of the silicate, amount of detrital carbonates) as well as tracers of paleo-vegetation and paleo-precipitation (d13C of organic carbon, dD of lipids). Moreover, we will use the hemipelagic sediments accumulated between levees to distinguish 1) glacial from interglacial sediments, and 2) marine hydrological variations from terrigenous input to the authigenic component. This will be achieved by acquiring sources (Sr- Nd- and Pb-isotopes), and weathering proxies (clay mineralogy, Nd- and Pb-isotopes in authigenic phases) in addition to stable isotopic compositions of foraminifera shells and paleomagnetic intensity on the hemipelagic sediments. Changes will be compared to independent tracers of the monsoon intensity or erosion fluxes especially the results of IODP Exp. 353 that constrain detailed paleoclimatic records in this region. Finally, the project will also generate a benchmark to quantify the anthropogenic perturbations on the modern Himalayan rivers.