The influence of grain size and mineralogical composition of terrestrial material inputs on organic carbon sequestration in the Bengal Fan since the last deglaciation

Abstract

Exploring the intricate dynamics of organic carbon (OC) in marine sediments is essential for understanding global carbon cycling. This study examines how abrupt climate events, monsoonal variations, and mineral composition influence the delivery and burial of terrestrial organic matter (OM) in the western Bay of Bengal (BoB), focusing on the Bengal Fan (BF). Grain size and mineralogical composition have been critical in transporting and depositing terrigenous material, thereby affecting subsequent OC sequestration. Fine-grained sediments, notably those in the 2.5–10 μm range, are conducive to the widespread distribution and sequestration of terrestrial OM, facilitated by the force of monsoonal currents. In contrast, larger particles, particularly in the 30–46 μm range, tend to impede OC deposition. The mineralogy of the sediments also has a significant impact. Illite, for instance, is beneficial for OC transport and accumulation as it shields OM from microbial degradation and oxidative processes. Conversely, smectite is detrimental to OC preservation. Its high permeability, poor adsorptive capabilities, and association with OM-degrading environments can result in the loss of OC during transport and sedimentation, particularly in more distal ocean settings. The interaction between lithogenic materials and OM from both marine and terrestrial sources highlights the complexity of sedimentation processes in the BF. This study underscores that mineral composition and grain size characteristics played a crucial role in controlling OC sequestration. Before 9.5 ka, terrestrial OM was a substantial contributor to OC deposition in association with marine OM, driven by more vigorous monsoons and active river channels during lower sea levels. However, after 9.5 ka, the rapid rise in sea level altered river channels and delta dynamics, leading to reduced sedimentation rates and a decreased OM accumulation in the western BoB. During the Holocene Climate Optimum (HCO: 9.5–5.5 ka), the increased input of freshwater from intense summer monsoons led to the stratification of the water column, which reduced upwelling and primary productivity, which, in turn, led to less OC accumulation and preservation. The study highlights the importance of the summer monsoon in transporting terrestrial and marine OM to the deep ocean, emphasizing its crucial role in regulating the global carbon budget.