Development and Field Application of a Deep-Sea In Situ Multispectral Spectrometer in Cold Seep Environments

Deep-sea hydrothermal vents and cold seeps release chemically reactive fluids, containing dissolved gases, anions, metal species, and labile organic compounds. Concentration gradients in these species drive carbon–sulfur–metal coupling and sustain chemosynthetic ecosystems but are easily altered during shipboard recovery. An in situ approach is therefore required to capture these linked processes directly on the seafloor. We developed RaLL, a deep-sea multispectral system, that integrates conventional Raman spectroscopy, laser-induced breakdown spectroscopy (LIBS), laser-induced fluorescence, and surface-enhanced Raman scattering (SERS) into a unified optical architecture. A shared excitation/collection design allows these modalities to probe, at essentially the same seafloor location, dissolved gases, anions, metals, and organic components. Laboratory testing verified stable Raman quantification of dissolved species, elemental detection in aqueous standards by LIBS, and cloth-based fluorescence/SERS assays for representative organic and nucleic-acid-related targets. Deployed at the Haima cold seep (∼1400 m), RaLL distinguished dissolved, free, and hydrate-bound CH4; detected H2S and SO42– in venting fluids; and profiled pore waters to resolve the sulfate–methane transition zone and anaerobic methane oxidation without coring. These results demonstrate colocated, process-level, multispecies chemical sensing in a deep-sea extreme environment and highlight multispectral in situ spectroscopy as a viable path toward routine seafloor biogeochemical monitoring.