The Distribution of Size-Fractionated Particulate Organic Carbon Concentrations along a Meridional Transect in the Pacific

The U.S. GEOTRACES GP15 Pacific Meridional Transect along 152°W from Alaska to Tahiti crossed many oceanographic regimes, including coastal Alaskan waters, the iron-limited eastern Subarctic Pacific gyre, the transition zone chlorophyll front, the north and south subtropical Pacific gyres, and the iron-limited equatorial upwelling zone. Satellite retrievals of surface chlorophyll and particulate organic carbon (POC) concentrations along this transect show strong variations, but the subsurface expression of this surface biomass variability is not well constrained. Estimates of particle and POC distributions and flux from an Underwater Vision Profiler (UVP5) on previous cruises along a similar line have revealed intriguing features that suggest localized regions of efficient particle transfer to depth near the subarctic front and in the equatorial zone (e.g., 1). As far as we know, however, the UVP5 estimates have not been validated with direct measurements of POC. Here, we present full water column measurements of POC concentrations in size-fractionated (1-51 µm, >51 µm) samples collected during large volume in-situ McLane pump deployments at 20 stations along the GP15 transect. Particle beam attenuation coefficient was simultaneously measured using a CTD deployed WETLabs C-Star transmissometer that was carefully cleaned and calibrated over the 2 months of the experiment. Beam attenuation coefficient (beam-c) accuracy of ± 0.003 m^-1 and cast to cast precision of better than 0.001 m^-1 was achieved for the entire transect. POC—beam-c comparisons have been made previously, such as during the JGOFS expeditions, but have generally focused on the upper 1000 m. Here we present the first large scale surface to bottom comparison. We also examine the variability in concentration and size partitioning of POC over the transect, compare to contemporaneous satellite estimates of POC, and look for evidence of localized regions of efficient POC transfer to depth.

1. R. Kiko et al., Biological and physical influences on marine snowfall at the equator. Nature Geoscience 10, 852 (2017).