Monitoring long-term bottom water temperature changes using fiber-optic sensing in submarine telecommunication cables

Long-term environmental monitoring of the deep ocean environment is crucial for better understanding the feedback processes between the oceans and Earth’s climate in the face of global warming. However, obtaining in-situ observations from the deep seafloor is difficult and costly. Use of laser reflectometry in optical fibers using existing submarine telecommunication cables can help bridge this knowledge gap. We performed distributed fiber-optic sensing using the BOTDR (Brillouin Optical Time Domain Reflectometry) technique, which is sensitive to mechanical strain (elongation/ shortening) and temperature changes, on a network of commercially operating telecom cables connecting the islands of the Guadeloupe archipelago in water depths of 10 - 700 m. Monitoring at regular 6 month intervals over the past 2.5 years reveals a seasonally adjusted two-year temperature change (delta T) of about +1.5°C  between 2022 and 2024 on the shallow carbonate platform (10 - 40 m water depth) south of Grande-Terre (Saint François), Guadeloupe. These sea-floor measurements are corroborated by satellite observations of the Sea-Surface-Temperature (SST) during the past three years, which document an identical temperature increase at the sea surface, in the same location (offshore Saint François). CTD measurements performed in 5 locations along the cables reveal well-mixed waters and no temperature stratification. A smaller temperature increase (0.2 - 1.0°C) is observed in deeper waters (300 - 700 m) between the islands over the same period (2022 - 2024). A new measurement campaign is planned in mid-March 2025 with BOTDR on the submarine cables and XBT (eXpendable Bathy Thermograph)  measurements at sea. Together with an additional field campaign performed (at a 3-month interval) in Sept. 2024 (during the maximum annual water temperature) the new campaign (performed during the minimum annual temperature and again at a 3-month interval) will fully constrain the seasonal variations in water temperature. These results can open the path for widespread use of submarine cables for long-term environmental monitoring of the seafloor.