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.