Development of a deep-sea in-situ anion analyzer for marine biogeochemical cycle observation

A novel deep-sea in-situ anion analyzer (DIAA) is proposed and developed for long-term and continuous observation of marine biogeochemistry and ecology. The DIAA is designed to analyze the concentrations of various anions in seawater, such as F⁻, Cl⁻, Br⁻, NO₃⁻, SO₄²⁻, and PO₄³⁻, based on ion chromatography (IC) technology. The DIAA consists of a control circuit module, a conductivity signal acquisition module, and a flow analysis module. The control circuit module is installed in a sealed pressure-resistant vessel, while the conductivity signal acquisition module and the flow analysis module are housed in a sealed pressure-balanced vessel. The two vessels are capable of functioning at a water depth of up to 4000 m. A seawater in-situ dilution device is designed to solve the issue that the concentrations of Cl⁻ and SO₄²⁻ in seawater exceeds the IC detection limit. Moreover, an in-situ calibration device enables the DIAA to operate subsea for a long time without recovery. In addition, the DIAA can diagnose and isolate faults automatically. The operation status can be monitored remotely on the shore, and the scientific data can be acquired in real-time. During the sea trial, the DIAA was connected to Monterey Accelerated Research System (MARS) seafloor observatory at a water depth of approximately 900m and operated for six months successfully. The measurement accuracy and the long-term stability of the instrumentation have been verified.