The benthic environment of the Aleutian Trench region: Sediment provenance, organic carbon sources and deposition dynamics

Sedimentological, mineralogical, chronological and basic biogeochemistry document that the Aleutian Trench region consists of four distinct regions, each with their respective benthic habitat, deposition dynamics, and material sourcing: the Northern Aleutian Slope (NS), Aleutian Trench – Axis (AT-A), Aleutian Trench – South Slope (AT-SS), and Southern Aleutian Slope (SS). The carbon pool is characterized by pyrolysis derived hydrogen and oxygen indices (HI and OI), radionuclides (Δ¹⁴C) and stable isotopes (δ¹³C-values), clearly varying among the respective regions. The NS has elevated total organic carbon (TOC) content, including distinct imprint of fresh, marine-derived organics originating from upwelling in the Gulf of Alaska with subsequent westward transport by the Alaskan Stream, sustaining high remineralization rates, with well-defined infaunal burrow structures in the sediment. Stations at the AT-A have a 2.5–24.1 cm thick, glacially derived fluid mud layer (FML) deprived of infauna, blanketing the original sediment surface. Chronological markers document that the infilling of the FML happened since the mid 1950′s and now extends to at least one fifth of the entire Aleutian Trench axis. The FML contains a mixture of fresh, marine-derived and recalcitrant terrigenous organics, with elevated microbial mineralization rates, presumably enhanced by microbial priming and efficient microbial degradation of otherwise recalcitrant organics. The AT-SS and SS have low TOC contents, exhibit very low microbial remineralization rates, with only sparse infaunal imprints. Our investigations document that the Aleutian Trench forms a unique hadal environment, while simultaneously confirming that hadal trenches generally act as depocenters with intensified microbial activity, sustained by various sources of organics.