Development of an autonomous dissolved inorganic carbon sensor for oceanic measurements

Since the industrial revolution the CO2 concentrations in the atmosphere have increased from280 ppm to over 400 ppm, and each year the oceans take up approximately 25% of the annually emitted anthropogenic CO2. This increase in CO2 in the oceans has had a measure able impact on the marine carbonate system, and the resultant increase in the acidity of the ocean is a potential stressor for a range of ecosystems. In order to fully quantify the marine carbonate system there are four variables that can be measured, these are dissolved inorganic carbon (DIC), pH, total alkalinity and partial pressure of CO2. By measuring two of the four variables the others can be determined. Of these variables DIC is the only one without either an underway or in situ sensor, despite being one half of the preferred pairs for observing the carbonate system. To address this technological gap and increase the measurement coverage there is a clear need for an autonomous sensor capable of making quality measurements while having a robust, small physical size, and low power requirements. Presented here are the results of developmental work that has led to a full ocean depth rated autonomous DIC sensor, based on a microfluidic “Lab On Chip” (LOC) design. The final version of the DIC LOC sensor operates by acidifying < 1 ml of seawater, converting the DIC to CO2, which is diffused across a gas permeable membrane into an acceptor solution. The CO2 reacts with the acceptor resulting in a conductivity drop that is measured using a Capacitively Coupled Contactless Conductivity Detector (C4D). Each measurement takes ~15 minutes and the sensor can be set up to perform calibrations in situ. Laboratory testing demonstrated this system has a precision of < 1 µmol kg-1. The sensor was deployed as part of a large EU project aiming to detect a simulated sub-seabed leak of CO2. Over multiple deployments in the North Sea the sensor collected data used to locate the leak. A number of field tests have established the sensor has a precision of < 10 µmol kg-1. This work has demonstrated that this sensor offers potential to fill the current technological gap and collect data that will enhance understanding of the marine carbonate system