Abstract
As a powerful in situ detection technique, Raman spectroscopy is becoming a popular underwater investigation method, especially in deep-sea research. In this paper, an easy-to-operate underwater Raman system with a compact design and competitive sensitivity is introduced. All the components, including the optical module and the electronic module, were packaged in an L362 × Φ172 mm titanium capsule with a weight of 20 kg in the air (about 12 kg in water). By optimising the laser coupling mode and focusing lens parameters, a competitive sensitivity was achieved with the detection limit of SO42− being 0.7 mmol/L. The first sea trial was carried out with the aid of a 3000 m grade remotely operated vehicle (ROV) “FCV3000” in October 2018. Over 20,000 spectra were captured from the targets interested, including methane hydrate, clamshell in the area of cold seep, and bacterial mats around a hydrothermal vent, with a maximum depth of 1038 m. A Raman peak at 2592 cm−1 was found in the methane hydrate spectra, which revealed the presence of hydrogen sulfide in the seeping gas. In addition, we also found sulfur in the bacterial mats, confirming the involvement of micro-organisms in the sulfur cycle in the hydrothermal field. It is expected that the system can be developed as a universal deep-sea survey and detection equipment in the near future.
Highlights
In recent years, the attempt of applying Raman spectroscopy to marine geochemical detection has achieved satisfactory results
These systems have been deployed on remotely operated vehicle (ROV) for in situ detection of sediment pore water [9,10], deep-sea minerals [11,12], bacterial mats [13], methane hydrates [14,15,16], and the fluid of hydrothermal vents or cold seeps [17,18]
In marine sediments, sulfate ions can be converted to sulfur ions by the combined action of sulfate-reducing and methane-oxidising bacteria and form insoluble minerals with elements such as cobalt and cadmium in the environment. This results in a dramatic reduction of sulfate ions in the pore water, especially for the cold seep and hydrothermal vent areas, which are rich in methane or hydrogen sulfide
Summary
The attempt of applying Raman spectroscopy to marine geochemical detection has achieved satisfactory results. Raman systems independently developed by the Ocean University of China (OUC) and the French Research Institute for Exploitation of the Sea (IFREMER) [4,5,6,7,8] These systems have been deployed on ROVs for in situ detection of sediment pore water [9,10], deep-sea minerals [11,12], bacterial mats [13], methane hydrates [14,15,16], and the fluid of hydrothermal vents or cold seeps [17,18]. This paper presents our attempt at constructing an easy-to-operate Raman system for deep-sea surveying and shows preliminary detection results obtained by this system in ocean survey applications
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