This dataset contains bin-averaged optical particle measurements from Biogeochemical Argo floats. Full description of data and methodology is contained in the manuscript submitted to Science entitled "Particle fragmentation exerts strong control on biological sequestration of CO2 by the oceans". Optical measurements are particulate backscattering and chlorophyll fluorescence, and each have been partitioned into large (>100 µm) and small (<100 µm) size classes for use in estimating the rate of fragmentation of large, sinking partiles. The data cover 34 high-latitude open-ocean mesopelagic sinking particle plumes in the supolar North Atlantic and the Southern Ocean observed at 1 m vertical resolution and 2-5 day temporal resolution by profiling floats over a 30-day period. Each 30-day period is divided into five temporal bins of six days each. Vertical binning is at 50-meter intervals from the 250-950 m.