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

Abstract

Abstract Transmissometers record high-frequency, high-sensitivity measurements of beam attenuation due to particles (beam c P ), 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 c P 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/ c P optical ratio thus shows promise in discriminating regions of elevated Fe(OH) 3 content at high vertical spatial resolution.