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Particulate inorganic carbon (PIC) plays a major role in the ocean carbon cycle impacting pH, dissolved inorganic carbon, and alkalinity, as well as particulate organic carbon (POC) export and transfer efficiency to the deep sea. Remote sensing retrievals of PIC in surface waters span two decades, yet knowledge of PIC concentration variability in the water column is temporally and spatially limited due to a reliance on ship sampling. To overcome the space–time gap in observations, we have developed optical sensors for PIC concentration and flux that exploit the high mineral birefringence of CaCO 3 minerals, and thus enable real-time data when deployed operationally from ship CTDs and ARGO-style Carbon Flux Explorer floats. For PIC concentrations, we describe a fast (10 Hz) digital low-power (∼0.5 W) sensor that utilizes cross-polarized transmitted light to detect the photon yield from suspended birefringent particles in the water column. This sensor has been CTD-deployed to depths as great as 6,000 m and cross-calibrated against particulates sampled by large volume in situ filtration and CTD/rosettes. We report data from the September–November 2018 GEOTRACES GP15 meridional transect from the Aleutian Islands to Tahiti along 152°W where we validated two prototype sensors deployed on separate CTD systems surface to bottom at 39 stations, many of which were taken in nearly particle-free waters. We compare sensor results with major particle phase composition (particularly PIC and particulate aluminum) from simultaneously collected size-fractionated particulate samples collected by large volume in situ filtration. We also report results from the June 2017 California Current Ecosystem-Long Term Ecological Research (CCE-LTER) process study in California coastal waters where high PIC levels were found. We demonstrate that the PIC concentration sensor can detect PIC concentration variability from 0.01 to >1 μM in the water column (except in nepheloid layers) and outline engineering needs and progress on its integration with the Carbon Flux Explorer, an autonomous float. 

Abstract. Over the past decade, the GEOTRACES and wider trace metalgeochemical community has made substantial contributions towardsconstraining the marine cobalt (Co) cycle and its major biogeochemicalprocesses. However, few Co speciation studies have been conducted in theNorth and equatorial Pacific Ocean, a vast portion of the world's oceans byvolume and an important end-member of deep thermohaline circulation.Dissolved Co (dCo) samples, including total dissolved and labile Co, weremeasured at-sea during the GEOTRACES Pacific Meridional Transect (GP15) expedition along the 152∘ W longitudinal from 56∘ N to20∘ S. Along this transect, upper-ocean dCo (σ0<26) was linearly correlated with dissolved phosphate (slope = 82±3, µmol : mol) due to phytoplankton uptake and remineralization.As depth increased, dCo concentrations became increasingly decoupled fromphosphate concentrations due to co-scavenging with manganese oxide particlesin the mesopelagic. The transect revealed an organically bound coastalsource of dCo to the Alaskan Stream associated with low-salinity waters. Anintermediate-depth hydrothermal flux of dCo was observed off the Hawaiiancoast at the Loihi Seamount, and the elevated dCo was correlated withpotential xs3He at and above the vent site; however, the Loihi Seamountlikely did not represent a major source of Co to the Pacific basin. Elevatedconcentrations of dCo within oxygen minimum zones (OMZs) in the equatorialNorth and South Pacific were consistent with the suppression of oxidativescavenging, and we estimate that future deoxygenation could increase the OMZdCo inventory by 18 % to 36 % over the next century. In Pacific Deep Water(PDW), a fraction of elevated ligand-bound dCo appeared protected fromscavenging by the high biogenic particle flux in the North Pacific basin.This finding is counter to previous expectations of low dCo concentrationsin the deep Pacific due to scavenging over thermohaline circulation.Compared to a Co global biogeochemical model, the observed transectdisplayed more extreme inventories and fluxes of dCo than predicted by themodel, suggesting a highly dynamic Pacific Co cycle. 

The goal of the EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign is to develop a predictive understanding of the export, fate, and carbon cycle impacts of global ocean net primary production. To accomplish this goal, observations of export flux pathways, plankton community composition, food web processes, and optical, physical, and biogeochemical (BGC) properties are needed over a range of ecosystem states. Here we introduce the first EXPORTS field deployment to Ocean Station Papa in the Northeast Pacific Ocean during summer of 2018, providing context for other papers in this special collection. The experiment was conducted with two ships: a Process Ship, focused on ecological rates, BGC fluxes, temporal changes in food web, and BGC and optical properties, that followed an instrumented Lagrangian float; and a Survey Ship that sampled BGC and optical properties in spatial patterns around the Process Ship. An array of autonomous underwater assets provided measurements over a range of spatial and temporal scales, and partnering programs and remote sensing observations provided additional observational context. The oceanographic setting was typical of late-summer conditions at Ocean Station Papa: a shallow mixed layer, strong vertical and weak horizontal gradients in hydrographic properties, sluggish sub-inertial currents, elevated macronutrient concentrations and low phytoplankton abundances. Although nutrient concentrations were consistent with previous observations, mixed layer chlorophyll was lower than typically observed, resulting in a deeper euphotic zone. Analyses of surface layer temperature and salinity found three distinct surface water types, allowing for diagnosis of whether observed changes were spatial or temporal. The 2018 EXPORTS field deployment is among the most comprehensive biological pump studies ever conducted. A second deployment to the North Atlantic Ocean occurred in spring 2021, which will be followed by focused work on data synthesis and modeling using the entire EXPORTS data set.