1051 / 2019-08-05 20:17:21

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

6、海洋地球科学 > 专题5：从流域到深海：沉积物的产生，搬运和埋藏

Hadal trenches are considered ultimate sinks of organic carbon (OC) that falls from the upper waters. However, remarkable habitat heterogeneity within hadal trenches may lead to diverse OC dispersal and further impact preservation and burial of sedimentary OC (SOC). The spatial heterogeneity of OC cycling in hadal trenches remains poorly understood. In this study, elemental and stable carbon isotopic composition, specific surface area and grain size composition of sediments in three sediment cores collected across the Yap Trench axis were analyzed. This study aimed to investigate the spatial variation of sources, distribution and degradation of OC in this steep trench, and to better constrain the effects of sediment dynamics on OC cycling in hadal trenches. In general, OC contents in surface sediments of the Yap Trench increased from 0.25% at abyssal depth to 0.50% at hadal depth. In keeping with higher moisture contents and lower dry density of sediments on the oceanward slope, mean OC content in sediments on the gradual oceanward slope (0.27% ± 0.08%) was higher than on the landward slope (0.17% ± 0.13%), suggesting stronger enrichment ability of OC on the oceanward slope. Based on a three-end-member mixing model using δ13C and C/N as source markers, three SOC sources were identified in the Yap Trench. Allochthonous OC, including marine phytoplankton-derived (55% ± 7%) and terrestrial (16% ± 8%), was dominant on the oceanward slope, whereas more than 50% of OC was benthic-microorganism-derived in the landward slope, indicating that most fresh allochthonous OC was delivered to the oceanward slope. A multi-G degradation model revealed OC degradation rates in northern Yap Trench ranged from 0.0002 to 0.011 yr−1, increased with water depth, were comparable with neritic and hadal environments, and were slightly higher than those of the abyssal plain. By employing a mass accumulation model, accumulation rate and settling flux of surface OC in the northern Yap trench were calculated as about 2.0 × 10−1 g C m−2 yr−1 and 2.4 × 109 g C yr−1, respectively, similar to other deep ocean seafloors. This study confirmed inefficient SOC preservation within the northern Yap Trench, which was attributed to high labile marine phytoplankton-derived OC inputs and active heterotrophic microbial activity.