Developing an In-situ Sensor for High-frequency Measurements of Dissolved Inorganic Carbon, fCO2, and pH to Enable Fine-scale Studies of Seawater Carbonate Chemistry

Study of the marine CO₂ system is critical for understanding global carbon cycling and the impacts of changing ocean chemistry on marine ecosystems. A complete resolution of the marine CO₂ system requires simultaneous measurement of two out of four parameters: partial pressure or fugacity of CO₂ (pCO₂ or fCO2), dissolved inorganic carbon (DIC), total alkalinity (TA), and pH. Built on the success of the original development of the CHAnnelized Optical System (CHANOS), the CHANOS II has been redesigned to measure seawater DIC, fCO₂, or pH individually or paired DIC-pH or DIC-fCO₂ at high frequency (up to 1 Hz) up to 3000m water depth. This enables measurements of the CO₂ parameters at high temporal and spatial resolution from both stationary and mobile platforms (e.g., buoys, moorings, ROVs/AUVs, CTD rosettes). All parameters are measured based on spectrophotometric detection of solution [H⁺] using sulfonephthalein indicator. For DIC measurements, seawater is acidified and equilibrated with a pH-sensitive indicator across a CO₂ semi-permeable membrane via countercurrent flow, and solution pH in the CO₂-equilibrated indicator is then measured spectrophotometrically and used to derive total CO₂ (i.e., DIC) of the sample water. In-situ calibration of DIC may be performed with certified reference materials to ensure measurement quality. A second channel will independently and simultaneously measure either fCO₂ (with a method similar to DIC, but without sample acidification) or pH (using flow-through mixing between sample and indicator), allowing for complete resolution of the CO₂ system. Field results from stationary, surface time-series measurements during a suite of carbonate precipitation experiments in the Red Sea in July 2019 showed DIC precision and stability over 90 minutes of continuous, repeated measurements of within ~6 umol/kg before re-measuring optical reference to correct signal drift. In October 2019, CHANOS II will be deployed from the ROV Global Explorer and a CTD rosette during a mission to investigate sub-mesoscale variability of carbonate chemistry in relationship to the ecology of deep coral reefs near the West Florida shelf break, an effort among the first to map carbonate chemistry surrounding deep coral reefs in 3-D high resolution.