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

Mallory Cecile Ringham, Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, Aleck Zhaohui Wang, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, Frederick N. Sonnichsen, Woods Hole Oceanographic Institution, Applied Ocean Physics & Engineering, Woods Hole, MA, United States, Steve Lerner, Woods Hole Oceanographic Institution, Woods Hole, MA, United States and Kate Morkeski, The Ecosystems Center, MBL, Woods Hole, MA, United States
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.