The impacts of clay minerals on the quantification of black carbon in marginal sediments

Sedimentary black carbon (BC), formed during incomplete combustion of biomass and fossil fuels, represents a chemically refractory carbon pool and an important long-term sink in the global carbon cycle. Coastal margins are major repositories of terrestrial organic carbon (OC) including BC, yet estimates of BC sequestration in these regions remain highly uncertain. Most existing datasets rely on the CTO-375 method, which isolates BC based on thermal resistance following acid pretreatment. However, this approach does not fully account for mineral-associated OC, particularly OC stabilized by clay minerals, potentially leading to systematic overestimation of BC.

Here, we investigate the extent and controls of this methodological bias by comparing BC contents obtained using the conventional CTO-375 protocol (HCl pretreatment only) and a modified protocol that includes HF treatment to remove clay minerals along a transect of Eurasian marginal seas. BC contents measured without HF treatment were consistently higher, with proportional overestimations (ΔBC) ranging from 39% to 94% and showing pronounced regional variability. ΔBC correlated positively with smectite content, but not with grain size, indicating that clay mineral composition, rather than total clay abundance, governs BC overestimation.

These findings indicate that the measurement of BC content in sediments should take the mineral composition into consideration, otherwise the climatic negative feedback associated with sedimentary BC burial would be overestimated.