Optical observation of particles and responses to particle composition in the GEOTRACES GP16 section

Transmissometers record high-frequency, high-sensitivity measurements of beam attenuation due to particles (beam cP), enabling determination of POC and SPM concentrations (via regression). Modern instruments have unique sensitivities to particle loads of various composition, but can exhibit unresolved thermal effects: even well-calibrated and monitored instruments exhibit asymmetry in upcast and downcast traces. On the US GEOTRACES EPZT transect, three independently deployed 650 nm, 25-cm pathlength (CStar) transmissometers and a broad-angle turbidity meter (Seapoint) were deployed at up to 36 stations. We report a procedure for improving agreement in upcast and downcast transmissometry via simple thermal modeling of instrument temperature using CTD temperature traces. Observed particle features from thermally corrected beam cP and turbidity include surface-derived biogenic phases, hydrothermal plumes, benthic nepheloid layers, and low-oxygen particle maxima. Optical responses to particles from a wide range of oceanographic regimes are examined against measured particle composition. In contrast to the transmissometers, the turbidity meter expresses a noted sensitivity to Fe(OH)3. The derived turbidity/cP optical ratio thus shows promise in discriminating regions of elevated Fe(OH)3 content at high vertical spatial resolution.