Analysis of Satellite and In Situ Optical Proxies for PIC and POC During GEOTRACES GP15 and GP17-OCE Transects From the Subarctic North Pacific to the Southern Ocean

The biological pump, a fundamental process governing atmospheric CO₂, rapidly transfers particulate inorganic and organic carbon (PIC and POC) from surface waters to the deep sea but is inherently highly variable in space and time, and thus poorly observed. Here we synthesize PIC and POC data from satellites, CTD-profiled optical sensors (birefringence and transmissometer), and from in situ pumps samples from GEOTRACES transects spanning 20,000 km from the North Pacific to Southern Ocean. High resolution profile data from PIC sensors revealed strong subsurface maxima in the deepest euphotic zone waters of oligotrophic gyres; furthermore, data showed high concentrations of PIC penetrating to >500 m south of the Subarctic Front (45°N–35°N), at the equator, and north of the Antarctic Polar Front (45°S–55°S) indicating high carbon export in these regions. We developed a new temporal/spatial interpolation scheme for satellite data that improved matchups with ship observations. North of the Antarctic Polar Front (APF), PIC sensor data was generally well aligned with sample PIC; however, a positive bias of satellite PIC was found in poor retrieval regions. South of the APF, both satellite and birefringence sensor greatly overestimated PIC by factors of >25 and 12, respectively, compared to sample PIC which averaged 15 nM. The unanticipated discovery of a non-carbonate particle birefringence source coupled with a microscopic investigation of pump samples leads us to hypothesize that internal reflection within bubbles and/or cellular structures of heavily silicified colony-forming diatoms (Fragilariopsis and Pseudo-nitzschia) is the cause for anomalous birefringence and adds to backscattered satellite radiances.