Organo-mineral interactions in the floodplain govern the stability of buried organic carbon in continental margins

The global carbon cycle is largely controlled by the drawdown of atmospheric CO₂ by plants and preservation of organic carbon at the continental margins. In the context of Himalayan rivers, previous studies explored the fate of terrestrial organic carbon (C_org)without much consideration of its preservation within the floodplains. We undertook a novel approach to investigate the spatio-temporal preservation of C_org in floodplain paleosols, which form an intermediate between the source of C_org and their subsequent deposition in the continental margin. Towards this, we sampled five 35 m long sediment cores spanning the entirety of the Ganga River Floodplain (GRF). Carbon isotopic composition of C_org and soil carbonates (SC) (δ¹³C_org and δ¹³C_SC) and oxygen isotopic composition of SC (δ¹⁸O_SC) along with soil texture, Al and Fe oxides (Al_ox and Fe_ox) were used as predictors (n=158) of C_org preservation. The Random Forest Regression (RFR) model with the built-in feature importance tool was used to disentangle the dominant predictor of C_org across all the study sites. Our results suggest that in the upper stretch of GRF, C_org is low and preservation was predominantly controlled by the vegetation type (C3/C4) with grasslands accruing more C_org than forests. In contrast, in the lower stretch of GRF, the preservation was dominantly controlled through the formation of Al_ox and Fe_ox organo-mineral complexes, with the resultant C_org being one-order higher compared to upper stretches. Previous studies suggested that rapid burial predominantly acted as a major controlling factor on the sustenance of C_org in Bay of Bengal. However, our results along with similar Al/Si vs. C_org correlations within the lower GRF compared to the previously reported values from riverine suspended load and shelf sediments suggest that the floodplains transformed the labile C_org into stable organo mineral aggregates at lower stretch of GRF before it was deposited into the Bay of Bengal. We suggest that protection of C_org in floodplain is an importantstep towards its preservation at continental shelf. In the context of the Himalayan river system and the amount of C_org effectively preserved, the role of floodplains has profound implications for the global carbon cycle.