What 4,000 Drifting Sensors Are Telling Us About Mining-Adjacent Waters

The Argo programme is one of the most important ocean monitoring systems ever built. Launched in 2000 as a collaboration between more than 30 countries, Argo has deployed over 4,000 autonomous floats that drift through the world's oceans, measuring temperature, salinity, and — in newer models — oxygen, pH, nitrate, and optical properties. The result is the first near-real-time global picture of the ocean interior.

How Argo Floats Work

An Argo float is a battery-powered cylinder about 1.5 metres long and 20 centimetres in diameter. It has no propulsion. It operates on a 10-day cycle: the float descends to a parking depth of 1,000 metres, drifts with the prevailing current, then descends further to 2,000 metres before slowly rising to the surface while its sensors record a continuous vertical profile. At the surface it transmits data via satellite — and then the cycle repeats.

Core Sensors (all Argo floats):

• Temperature (precision ±0.002°C)
• Salinity via conductivity (precision ±0.005 PSU)
• Pressure (depth to ±2.5 dbar)

BGC-Argo additions (biogeochemical floats):

• Dissolved oxygen (O₂)
• pH (ocean acidification indicator)
• Nitrate
• Chlorophyll-a (phytoplankton proxy)
• Particulate backscatter (proxy for organic carbon)

Why Argo Floats Matter for Deep-Sea Mining

Argo floats are the only instrument system with global coverage of the deep ocean interior at the depths where mining operations would occur. This makes them uniquely useful for:

Baseline monitoring. Before any mining begins, Argo data establishes what "normal" looks like: the typical temperature, salinity, and oxygen profiles at a given location, and the natural variability across seasons and years.

Sediment plume detection. When nodule mining equipment disturbs the seafloor, it generates sediment plumes that can rise hundreds of metres into the water column and travel laterally for hundreds of kilometres. BGC-Argo floats — measuring backscatter as a proxy for suspended particles — can detect anomalously turbid water that may signal a plume.

Chemical anomaly detection. Mining activities can release trace metals, dissolved organic material, and hydrogen sulphide from disturbed sediments. Oxygen and pH sensors can detect changes in water chemistry.

Drift tracking. Each Argo float drifts at its parking depth, tracing the path of the water mass it sits in. The drift trajectory is an accurate representation of how a passive particle — like a sediment grain or a larva — would be transported by ocean currents at that depth.

The Data on Abyssal Claims

Abyssal Claims displays the positions and recent profiles of Argo floats operating near ISA mining concessions. Each float shows its last recorded depth profile, any alarm events (anomalous readings beyond the expected range), and its drift path over the preceding 35 days.

The alarm thresholds are depth-adaptive — what counts as an anomalous reading at 200 metres differs from what is anomalous at 2,000 metres. Floats operating inside or near active mining zones with elevated backscatter, reduced oxygen, or pH anomalies are flagged for closer examination.

Limitations

Argo floats are not purpose-built mining monitors. Their spatial coverage is determined by oceanic circulation, not by where monitoring is needed. In the Clarion-Clipperton Zone — where most ISA mining contracts are concentrated — float density is lower than in heavily trafficked North Atlantic waters.

Despite these limitations, Argo represents the most comprehensive environmental monitoring dataset available for the deep ocean — and it is publicly accessible in near-real-time through the international Argo data centres.