Pyrogenic PAHs Have Different Biogeochemical Fates in the Eastern Indian Ocean

Understanding the fate of polycyclic aromatic hydrocarbons (PAHs) in the deep ocean is crucial for elucidating the biogeochemical cycle of organic carbon under anthropogenic influences. In this study, surface sediments were collected from the deep sea of the Eastern Indian Ocean (water depth: 2161-4545 m) and analyzed for 29 semivolatile organic compounds (SVOCs), including parent PAHs and their alkylated derivatives, as well as source biomarkers. The target SVOCs (∑₂₉SVOCs: 23.0-183 ng/g, ∑₁₆PAHs: 11.3-93.3 ng/g) were mainly from pyrogenic sources, namely coal combustion, traffic emissions, and wood burning. The contributions from wood burning and coal combustion exhibited distinct trends with increasing total organic carbon contents, suggesting different dominant biogeochemical behaviors. Major fractions of PAHs from wood burning can be biodegraded or photodegraded, leading to a depletion-dominated fate in the water column. Conversely, PAHs from coal combustion showed an accumulation-dominated fate via their sedimentation due to their persistence and hydrophobicity. This study highlights the distinct biogeochemical fates of PAHs from biomass or fossil fuel combustion in deep oceans and has implications for the marine cycle of refractory organic carbon under anthropogenic impacts.