Black Carbon in Deep-Sea Seamount Sediment Cores: Vertical Variation and Non-negligible Char Black Carbon

Abstract

Deep-sea sediments (>1000 m) are often considered to be the ultimate sink for black carbon (BC), and the long-term buried BC in these sediments is believed to potentially provide a negative feedback effect on climate warming. The burial flux of BC in marine sediments is predominantly estimated based on soot BC (SBC) in most studies, frequently ignoring the contribution of char BC (CBC). While this methodology may result in an underestimation of the BC burial flux, the precise extent of this underestimation is yet to be determined. This study used the benzene poly(carboxylic acid) (BPCA) method and chemothermal oxidation (CTO) method to analyze CBC and SBC in four deep-sea sediment cores from the Zhongnan seamount in the South China Sea, respectively. The CBC content increased from 0.026 ± 0.010% at the seamount upper part (1432 m) to 0.039 ± 0.012% at the seamount foot (4278 m), constituting approximately 25 to 42% of the SBC content. The content disparity between CBC and SBC diminishes as depth increases. In deep-sea sediments, biogeochemical factors influence the variation of CBC molecules with depth. In the seamount middle-upper part (1432 and 2465 m), highly condensed CBC gradually accumulated along the core downward profile. In the sediment core profile of the seamount middle-lower part (3497 m), benzenetricarboxylic acid and benzenetetracarboxylic acid content decreased while the BC condensation degree rose, i.e., less condensed CBC was preferentially consumed. Afterward, CBC molecules reached a relatively stable state at the seamount foot. This study reveals that CBC possesses the capacity for long-term carbon sequestration in deep-sea sediments, and its content is not negligible.

Inferring the Molecular Basis for Dissolved Organic Matter Photochemical and Optical Properties

“…Polycyclic aromatic hydrocarbons (PAHs), known for their association with anthropogenic DOM and black carbon, are increasingly garnering attention in DOM research. − PAHs are composed of multiple aromatic rings fused together, which results in extended π conjugation and electron delocalization and makes them strong photosensitizers, with more conjugated PAHs exhibiting longer excited-state lifetime . While PAHs typically have low solubility in water; PAHs present in DOM are typically substituted with oxygen-containing functional groups [e.g., hydroxyl (−OH), carbonyl (CO), or carboxyl (−COOH) groups], which increases their solubilities and enhances their photoactivity. ,, Up to 10% of the global DOC pool has been estimated to be composed of compounds with these signatures .…”