Potential Remineralization of Terrestrial Organic Matter in the Sediments of the New Britain Trench Revealed by Optical and Molecular Properties of the Water-Extracted Dissolved Organic Matter

Abstract

Hadal trenches have recently been recognized as hotspots for organic carbon burial and degradation in the deep sea owing to its distinctive “funnel-shaped” topography and frequent tectonic activity. In this study, we analyzed dissolved organic carbon (DOC) concentration, optical properties, and molecular compositions of water-extractable organic matter (WEOM) in the sediment samples collected along two transects spanning diverse marine environments of the New Britain Trench (NBT) area. A significant positive correlation between DOC concentrations and total organic carbon (TOC) contents in sediments emphasizes the crucial role of organic matter (OM) supply for DOC production in the hadal environment. In addition, the optical parameters (e.g., fluorescence index, a₃₅₀/DOC) suggest remarkable influences of terrestrial OM on the composition of dissolved organic matter (DOM) in the NBT sediments. Indeed, the correlations between the stable carbon isotopes of sedimentary TOC (δ¹³C) and related optical parameters (SUVA₂₅₄, a₃₅₀/DOC, S_R and terrestrial humic-like) indicate that microbial degradation of terrestrial OM has an important impact on the sedimentary DOM. Moreover, analysis of DOM molecular compositions showed increased aromaticity and double bond equivalents double bond equivalent in the western landward core and two axis cores, also suggesting enhanced microbial degradation of terrestrial OM. The input of terrestrial OM into the NBT has led to production of aromatic, refractory, and high molecular weight DOM. Our findings have implications for understanding the fate of terrestrial OM in the deep ocean.