The hadal zone is an important and heterogeneous sink of black carbon in the ocean

Black carbon (BC), a highly condensed carbonaceous residue from incomplete combustion, is ubiquitous in the marine environment. Annual emissions from fossil fuel and biomass burning are substantial, with BC's refractory nature allowing it to reach remote ocean regions. Previous studies have documented BC in shallower marine zones (sublittoral, bathyal, abyssal), but its presence and significance in the hadal zone (6000–11,000 m depth) remained unknown. The hadal zone, comprising trenches formed at subduction zones, covers a small percentage of the seafloor (~1%) but a significant portion of the ocean's depth range (45%). Its unique topography, along with seismic activities and fluid dynamics, is thought to promote high organic carbon accumulation. Given this, and the relationship between BC and total organic carbon (TOC) in marine sediments, this study hypothesizes that significant BC accumulates in the hadal zone. Understanding hadal BC accumulation is crucial for assessing global carbon cycling and the impact of anthropogenic activities on the deep ocean.

Existing research highlights BC's long environmental lifetime and its transport to remote ocean locations. Studies have quantified BC burial fluxes in ocean sediments, revealing a higher BC/TOC ratio in deep-sea sediments compared to continental shelf sediments. The resistance of BC to degradation makes it an important component of slow-cycling carbon. However, while BC presence in shallower marine zones is established, data from the hadal zone were lacking before this study. A recent study examined dissolved BC in the Mariana Trench, suggesting cycling and aging similar to bulk dissolved organic carbon, contrasting the prevalent view of BC as a refractory, aged carbon pool. Research on hadal trenches has revealed elevated organic carbon accumulation rates, benthic biomass, and microbial activities, possibly due to the 'funneling' effect of trench topography and episodic events like earthquakes. This elevated organic matter accumulation in hadal trenches hinted at the potential for significant BC accumulation as well, setting the stage for the current investigation.

Sediment samples were collected from six hadal trench regions (Mariana Trench, Mussau Trench, New Britain Trench, Bougainville Trench, Kermadec Trench, Atacama Trench) and surrounding non-hadal sites. Black carbon was quantified using the chemothermal oxidation-375 (CTO-375) method. Stable (δ¹³C) and radioactive (Δ¹⁴C) carbon isotopes of BC and TOC were measured using isotope ratio mass spectrometry and accelerator mass spectrometry (AMS). δ¹³C helps distinguish organic carbon sources (terrestrial C3 plants, C4 plants, marine phytoplankton), while Δ¹⁴C indicates the contribution of biomass burning versus fossil fuel combustion to the BC pool. Using BC contents, δ¹³C and Δ¹⁴C signatures of BC and TOC, and ²¹⁰Pb-derived sedimentation rates, the study aimed to determine BC concentration, source, distribution, and its role in global ocean BC burial. Depth profiles of TOC and BC were investigated in two cores from abyssal and hadal sites to understand temporal variability. A two-endmember mixing model using Δ¹⁴C was applied to estimate the proportions of fossil carbon and biomass burning contributions to the BC pool. Global BC burial in the hadal zone was estimated by extrapolating findings from the five trenches with available sedimentation rate data to the total hadal zone area.

BC constituted a significant portion (10%) of the total organic carbon in hadal trench sediments. Isotopic signatures (δ¹³C and Δ¹⁴C) indicated that the BC primarily originated from terrestrial C3 plants and fossil fuels. The contribution of fossil carbon to the BC pool showed spatial heterogeneity, correlating with factors like distance from land, land cover, and regional socioeconomic development. The Atacama Trench exhibited the highest BC concentration, potentially due to a combination of high marine primary productivity, lack of vegetation cover, and proximity to a harbor city with significant anthropogenic activities. In contrast, the Bougainville and New Britain Trenches had lower BC/TOC ratios, suggesting a greater contribution of marine-derived organic carbon. Depth profiles in the Atacama Trench revealed variations in BC content and BC/TOC ratios, potentially influenced by mass-wasting events. A global BC burial rate of 1.0 ± 0.5 Tg yr⁻¹ was estimated for the hadal zone, significantly higher per unit area than the global ocean average. The two-endmember mixing model revealed variable contributions of fossil carbon and biomass burning to BC across the trenches, reflecting regional differences in land use and industrialization.

The findings demonstrate that the hadal zone is a previously unrecognized significant sink for black carbon in the global ocean. The high BC accumulation rates in the hadal zone, exceeding global average rates per unit area, highlight the importance of considering this deep-sea region in global carbon cycle assessments. The spatial heterogeneity in BC sources underscores the influence of various factors (anthropogenic activities, proximity to landmasses, land cover) on BC distribution and accumulation in the hadal environment. The observed isotopic offsets between BC and TOC point towards distinct sources and biogeochemical processes influencing BC distribution between dissolved and sedimentary phases. Further research is needed to explore the isotopic fractionation and fate of dissolved BC in the ocean. The findings also suggest that mass-wasting events and other episodic disturbances in hadal trenches might play an important role in the transport and deposition of BC.

This study reveals the hadal zone as an important, previously overlooked sink for black carbon in the global ocean. The high BC burial rate in the hadal zone relative to other ocean regions highlights the need to incorporate this deep-sea environment in global carbon cycle models. Future research should focus on refining BC burial estimates by accounting for episodic events and expanding sampling to a wider range of hadal trenches. Further investigation into the isotopic fractionation of BC during its journey from terrestrial sources to the deep sea is also warranted.

The study's estimates of global BC burial in the hadal zone are based on extrapolations from a limited number of trenches. While the selected trenches represent diverse geographic settings, broader sampling would improve the accuracy and generalizability of the global estimates. The influence of episodic events like earthquakes and mass wasting on BC transport and deposition needs further investigation. The application of the two-endmember mixing model for estimating fossil carbon and biomass burning contributions is based on certain assumptions that might affect the accuracy of these estimations.