Understanding the sensitivity of the Particulate Inorganic Carbon sensor to major particle phases

An optical sensor for particulate inorganic carbon (PIC) has been developed to enable routine high-resolution spatial and temporal observations of PIC concentration dynamics in the ocean water column using ships and autonomous platforms. Tests of the PIC sensor against particulate samples collected by large volume in-situ filtration during the GEOTRACES Inter-Calibration (July 2008 & 2009) cruises revealed factor of two differences in sensor response to PIC. Profiles at the Bermuda Atlantic Time Series (BATS) station as well as in north Atlantic Slope waters (Slope) had calibration factors of ~10nM/mV whereas profiles from the north Pacific gyre (SAFe) and the Santa Barbara Basin yielded calibration factors of 6nM/mV and 5nM/mV, respectively. We describe the relative influence of major particulate phases (opal, organic matter, and carbonate), particle concentration, and size distribution on PIC signal. At all four stations CaCO3 was dominated by the small size fraction (1-51um) and PIC sensor data closely follow the systematics of total CaCO3 concentration. It is evident that at stations BATS and Slope the percent weight of CaCO3 is much greater than the percent weight of silica. At station SAFe the percent weights of silica and inorganic carbon are extremely close in value, and at SBB the percent weight of silica is much greater than that of CaCO3. Ratios of percent weight CaCO3 to percent weight silica were 3:1 for BATS, 1.6:1 for Slope, 1:1 for SAFe, and 1:10 for SBB. Comparing these percent weights with the PIC calibrations it is apparent that the calibration sensitivity increases with increases in the percent weight of silica relative to CaCO3. Further calibration studies of the sensor are under way.