Significant Decrease in Sedimentary Organic Carbon Sequestration in the Lower Bengal Fan During the Last Sea Level Rise

Abstract

Deep-sea fans represent the largest sediment and organic carbon (OC) accumulation zones on Earth. However, variations of sedimentary OC sequestration in deep-sea fans during the last sea level rise have not been well evaluated. Here, a gravity core (4.24 m) retrieved from the inner flank of the active channel in the Lower Bengal Fan was analyzed for mineralogy, inorganic elements, total OC (TOC) and carbon isotopes (δ¹³C, Δ¹⁴C), and lignin phenols to reconstruct sources and accumulation rates of sediment and OC over the past 15 ka. The results showed significantly higher TOC accumulation rate (TOC_AR, 443 ± 221 mg/cm²/ka), terrestrial OC proportion (53 ± 5%), and burial efficiency (37 ± 8%) during sea-level lowstand (15–10 ka) than the following sea-level highstand (10–2 ka, 7 ± 2 mg/cm²/ka, 39 ± 6%, 22 ± 4%) due to considerable decline of terrestrial sediment and OC supply when the sea level was high. This was further evidenced by decreasing lignin content (0.46 ± 0.30 vs. 0.02 ± 0.02 mg/100 mg OC) and pre-depositional age (4,607 ± 300 vs. 2,650 ± 933 years). At 2–0 ka, slight increase in these parameters was most likely due to enhanced anthropogenic interference. The re-evaluated TOC_AR and burial efficiency for global deep-sea fans during the Holocene and the last deglaciation are higher than for deep-sea plains (>1,000 m) and upwelling regions, suggesting deep-sea fans are hotspots of OC sequestration. This study highlights the role of active channels of deep-sea fans in modulating OC biogeochemistry under global climate change.