Application of IsoSource Model to Orbital-Scale Partitioning of Organic Matter Sources During the Last 200 ka: Case Study of the Mahanadi Basin (Bay of Bengal)

In the Mahanadi Basin (Site U1445), the mean δ13CSOM (i.e., δ13C of sedimentary organic matter) value is consistently higher during the glacial periods (–18.8‰) than during the interglacial periods (–19.4‰), which implies either more deposition of terrestrial organic matter from C4 plants or increased marine primary production (MPP). We attempted to quantify the relative contributions of organic matter by C3 plant, C4 plant, and MPP, using the three end-member simple mixing model (IsoSource). This study reports that their partitions are distinctly different between the interglacial (29%, 28%, and 43%) and glacial (22%, 40%, and 38%) periods, despite the model’s limitations. The contribution of allochthonous terrestrial organic matter overwhelms the production of autochthonous marine organic matter in the Mahanadi Basin, particularly during the glacial periods, emphasizing that the freshwater discharge by the Indian monsoon precipitation is a key process of the organic carbon cycle. Our results also verify that C4 plants were more distributed in the Himalaya foreland during the glacial periods under the decreased atmospheric pCO2 and humidity levels. Different contribution of terrestrial organic matter sources to the Mahanadi Basin depends on the orbital-scale Indian monsoon variability, highlighting the land–sea transfer of organic carbon.