Spatial heterogeneity of organic carbon cycling in sediments of the northern Yap Trench: Implications for organic carbon burial

Abstract

The hadal trenches (>6000 m) are considered to be ultimate sinks of organic carbon (OC) from biosphere. However, highly spatial heterogeneity of geographic background and organic matter in different zones make significant influences on the OC cycling, and further the carbon burial. In this study, three sediment cores were drilled across the northern Yap Trench axis (9.8–9.9°N, 138.3–138.7°E). The elemental compositions, stable carbon isotopes of organic matter, mineral-specific surface area, and grain size composition were analyzed to investigate the spatial variations of organic sources, distribution, and its degradation in the northern Yap Trench. The surface sedimentary OC (SOC) contents in the northern Yap Trench increase from 0.25% at abyssal depth (4435 m) to 0.50% at hadal depth (~6674 m). Higher OC content (0.27% ± 0.08%) and sedimentation rate (5.8 cm kyr−1) occur in the station on the gradual oceanward slope (~4.2°) compared to those on the steep landward slope (~10°) (0.17% ± 0.13%, 0.52–1.8 cm kyr−1), suggesting enhanced accumulation of OC on the gradual slope. The allochthonous OC (i.e. marine phytoplankton-derived (55% ± 7%) and terrestrial (16% ± 8%)) dominates the SOC pool on the oceanward slope, whereas autochthonous microbe reworked OC contributes more than 50% in the steep landward slope, based on the evaluation of a three-end-member mixing model using δ13C composition and OC/N ratios as source markers. Correspondingly, radiocarbon age of SOC in the gradual slope is ~700 years younger than that in the steep slope with similar depth, indicating that the relatively fresh allochthonous OC is selectively deposited on the gradual slope. A multi-G degradation model demonstrates that OC degradation rates in the northern Yap Trench range from 0.00015 yr−1 to 0.00068 yr−1, which can be comparable with abyssal plain (3000–6000 m) and are lower than those of neritic environments (<1000 m). Mass accumulation model calculation shows that accumulation rate of surface OC in the northern Yap trench is ~2.7 × 10−2 g C m−2 yr−1, much lower than those from other deep ocean seafloors. This study suggests an inefficient SOC preservation within the northern Yap Trench, which might be induced by high labile marine phytoplankton-derived OC inputs and active heterotrophic microbial activity, and the results have important implications for better understanding the effects of sediment dynamics on OC cycling in hadal zones.