New autonomous sensors for in situ measurements of marine carbonate chemistry parameters

Since the beginning of the industrial revolution the ocean has become more acidic due to uptake of anthropogenic CO₂ from the atmosphere, a process that is projected to continue under current scenarios. Monitoring ocean acidification requires characterization of the marine carbonate system in high spatiotemporal resolution which cannot be achieved through ship-based measurements alone. Small, fast, low power sensors could address this need when coupled with coastal monitoring stations, autonomous vehicles or moorings. The National Oceanography Centre, UK, and industry partners have been developing in situ carbonate chemistry sensors for autonomous ocean observations on stationary and moving platforms. Our autonomous lab-on-chip sensor technology, originally designed for spectrophotometric nutrient measurements, has been adapted for autonomous measurements of pH, total alkalinity (TA), total dissolved inorganic carbon (DIC).

The pH sensor uses a spectrophotometric technique based on a pH indicator dye. The TA and DIC sensors both start by acidifying a small volume of seawater and passing the acidified sample along a gas-permeable membrane where the generated CO₂ is driven across the membrane into a sodium hydroxide (NaOH) solution. The DIC is determined from the conductivity change in the NaOH solution. The TA is determined via the open-cell single-point titration method from spectrophotometric pH measurement in the acidified, degassed seawater via a pH-sensitive dye in the acid titrant.

Here, we present these recent developments, including initial validation data from the pH, TA, and DIC sensors. Results from laboratory tests, coastal deployments, (~1 m depth) and lander-based sensor deployments (~100 m depth) will demonstrate the current capability of this technology. We will also provide an overview of the technology development roadmap and considerations for deployment on different platforms.