Euxinic conditions and altered biogeochemical cycles in a Patagonian fjord influenced by tectonic activity

Fjords in Chilean Patagonia are highly dynamic systems shaped by land-derived inputs, oceanic exchange, and tectonic activity. Prior to the elaboration of the present article, no in-depth investigation had been undertaken into the anoxic or euxinic conditions of fjords in this region. Consequently, the present research represents an interdisciplinary oceanographic approach to studying Quitralco Fjord (45.6° S, 73.1° W; 2022–2025) and provides the first evidence of a volcanically influenced euxinic fjord in Chilean Patagonia. A subsurface anoxic layer, beginning between 90 and 120 m and extending to the basin floor (~ 160 m), was shown to exhibit elevated temperatures and high concentrations of H ₂ S, consistent with inputs of volcanically derived fluids. A bubble-like acoustic scattering and the detection of CH ₄ within this layer suggest an external input of the gas into the water column. Although largely stagnant, this layer shifted vertically over time, likely driven by interannual deep-water renewal. Within the euxinic layer, nitrate was completely depleted, while high phosphate (20 µM) and ammonium (25 µM) concentrations indicated an active sulfur cycle. A pronounced deep fluorescence maximum was also detected in the dark, anoxic basin, attributed to fluorescent dissolved organic matter (fDOM) dominated by two humic-like components (C1 _245(350)-440 and C3 _270(400)-505 ) with high aromaticity. Microbial community composition changed markedly across the redox gradient, while geochemical and microbiological fingerprints exhibited shifts in metabolic potential through the water column. The geological emissions of H ₂ S from the seabed likely enhance and sustain the euxinic conditions, thereby strongly influencing the biogeochemical cycles of the basin. Overall, the present study reveals a previously unrecognized link between volcanic activity and fjord biogeochemistry, documenting for the first time the development of euxinic conditions in a Patagonian fjord in Chile.