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Abstract 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 (FragilariopsisandPseudo‐nitzschia) is the cause for anomalous birefringence and adds to backscattered satellite radiances. 

Abstract The oceanic biogeochemical cycling of iron is globally important yet difficult to fully understand due to the many chemical processes involved. There is potential to use scandium, which has a similar ionic size and charge density to trivalent iron but lacks redox cycling, as a simpler analog for specific parts of the iron cycle, if we can sufficiently develop our understanding of scandium's reactivity. Here we move closer to this understanding. We look at particle reactivity and solubility through a 24‐hr incubation experiment: 5 nmol/kg of dissolved scandium and/or iron were added to filtered and unfiltered California Current System water. Particulate scandium formed only in the unfiltered treatments, at a quantity unlikely to have been taken up biologically. This is the first direct observation of scavenging of scandium, an attribute shared with iron. Our results also serve as the first test of scandium solubility in seawater: 1.9 nmol/kg of dissolved scandium was stable in the filtered treatment, 50 times more than the highest natural concentrations so far observed. This indicates that, in contrast to iron, scandium's oceanic cycling is unlikely to be influenced by solubility limits. We also compare particulate depth profiles: labile particulate iron was disproportionally higher than that of scandium in shelf‐influenced samples, likely due to iron reductively dissolving in the sediments, which scandium cannot do, and then precipitating in oxic seawater. Due to this combination of behaviors, our results suggest that paired observations of scandium and iron may help distinguish between iron sourced from sediment resuspension and reductive dissolution.