Autonomous Sensor for <i>In Situ</i> Measurements of Total Alkalinity in the Ocean

Total alkalinity (TA) is one of the measurable parameters that characterize the oceanic carbonate system. A high temporal and spatial frequency in TA data can lead to better measurements, modeling, and understanding of the carbon cycle in aquatic systems, providing insights into problems from global climate change to ecosystem functioning. However, there are very few autonomous technologies for <i>in situ</i> TA measurements, and none with field demonstrations below 2 m depth. To meet this need in marine observing capabilities, we present a submersible sensor for autonomous <i>in situ</i> TA measurements to full ocean depths. This sensor uses lab-on-a-chip technology to sample seawater and perform single-point open-cell titration with an optical measurement. It can carry multiple calibration materials on board, allowing for routine recalibration and quality checks in the field. The sensor was characterized in the laboratory and in a pressure testing facility to 600 bar (equivalent to 6 km depth) and deployed in a shallow estuary, on a lander at 120 m depth, and on an autonomous underwater vehicle. With a demonstrated precision and accuracy regularly better than 5 μmol kg^-1 in field deployments, this sensor has the potential to dramatically expand our ability to perform long-term autonomous measurements of the marine carbonate system.