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

Fjords in Chilean Patagonia are highly dynamic systems shaped by land-derived inputs, oceanic exchange, and volcanic 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. Geological emissions of H 2 S from the seabed and microbial sulfate reduction may contribute to the observed H 2 S accumulation, enhancing and sustaining euxinic conditions, thereby strongly influencing the basin’s biogeochemical cycles. 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.