Autonomous Optics of the Ocean Biological Carbon Pump: PIC and particle birefringence along the GEOTRACES GP15 transect.

James K B Bishop, University of California Berkeley, Berkeley, CA, United States, Vinicius Jose Amaral, University of California Santa Cruz, Department of Ocean Sciences, Santa Cruz, United States, Phoebe J Lam, University of California Santa Cruz, Department of Ocean Sciences, Santa Cruz, CA, United States, Cristina Orrico, Sea-Bird Scientific, Philomath, OR, United States, Todd Wood, Lawrence Berkeley National Laboratory, Earth and Environmental Sciences Area, Berkeley, CA, United States and Thorsten Stezelberger, Lawrence Berkeley National Laboratory, Berkeley, CA, United States
Abstract:
We have developed a fast (10 Hz) low-power (~0.5W) sensor that uses cross-polarized transmitted light to detect birefringent particles in the water column. Calcite and Aragonite mineral polymorphs of CaCO3 which constitute particulate inorganic carbon (PIC) are far more birefringent than other minerals, thus the sensor’s signal is proposed as an optical proxy for PIC. The Sept. – Nov. 2018 GP15 transect (Alaska to Tahiti along 152°W) provided an opportunity to validate two birefringence sensors CTD deployed surface to >6000 m at 20+ stations against PIC determined in simultaneously collected size-fractionated (1-51 and > 51 µm) particulate samples from large volume in-situ pump deployments. Measured PIC ranged from 5 nM to 1000 nM along GP15 and was in systematic agreement with sensor profiles at many locations – but not all. In the surface layer, the sensor underestimated PIC by 30-50% - likely a packaging effect (live coccolithophores vs coccoliths). We found enhanced PIC concentrations (~100 nM) extending below 500 m near the subarctic front; furthermore, values below 10-20 nM were found in 100-200 m deep oxygen deficient waters near 11°N 152°W and at 200-300 m near 5°S, 152°W. Such data confirmed that the birefringence inherent optical property likely reflected the presence/dissolution of PIC and was uniquely different from fluorescence, scattering and beam attenuation coefficient. The sensor trended higher than measured PIC in deep nepheloid layers in the subarctic N Pacific where clays likely contributed to the signal. Puzzling, but reproducible peaks in birefringence were found at depths slightly deeper than the Chlorophyll maximum in the tropics in strong thermal gradients (near 25 °C) and which on occasion did not agree with PIC results. SEM imaging of samples and extensive thermal cycling of sensors have investigated whether or not these peaks are instrumental. At this writing, the sensor easily resolves 5-10 nM variations of PIC in the deep water column and we have a rich engineering and particulate matrix to refine the sensors measure of PIC.
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