In marine systems, ²¹⁰Po and ²¹⁰Pb disequilibria are being increasingly used to examine oceanic particle formation and export. Here, an updated assessment of current methods for determining ²¹⁰Po and ²¹⁰Pb activity in marine samples is provided and includes a complete description of the vast number of calculations and uncertainties associated with Po and Pb loss, decay, and ingrowth during sample processing. First, we summarize the current methods for the determination of ²¹⁰Po and ²¹⁰Pb activities in dissolved and particulate seawater samples and recommend areas for improvement. Next, we detail the calculations and associated uncertainties using principles of error propagation, while also accounting for radionuclide ingrowth, decay, and recovery. A spreadsheet reporting these calculations is included as a downloadable Web Appendix. Our analysis provides insight into the contributions of the relative uncertainty for each parameter considered in the calculation of final ²¹⁰Po and ²¹⁰Pb activities and gives recommendations on how to obtain the most precise final values. For typical experimental conditions in open seawater, we show that our method allows calculating ²¹⁰Pb activity with a relative uncertainty of about 7%. However for ²¹⁰Po activities, the final relative uncertainty is more variable and depends on the ²¹⁰Po/²¹⁰Pb activity ratio in the initial sample and the time elapsed between sampling and sample processing. The lowest relative uncertainties on ²¹⁰Po that can be obtained by this method is 6% and can only be obtained for samples with high ²¹⁰Po/²¹⁰Pb activity ratios (>1) that were rapidly processed.

The Arctic sea‐ice extent reached a record minimum in September 2012. Sea‐ice decline increases the absorption of solar energy in the Arctic Ocean, affecting primary production and the plankton community. How this will modulate the sinking of particulate organic carbon (POC) from the ocean surface remains a key question. We use the ²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb radionuclide pairs to estimate the magnitude of the POC export fluxes in the upper ocean of the central Arctic in summer 2012, covering time scales from weeks to months. The ²³⁴Th/²³⁸U proxy reveals that POC fluxes at the base of the euphotic zone were very low (2 ± 2 mmol C m⁻² d⁻¹) in late summer. Relationships obtained between the ²³⁴Th export fluxes and the phytoplankton community suggest that prasinophytes contributed significantly to the downward fluxes, likely via incorporation into sea‐ice algal aggregates and zooplankton‐derived material. The magnitude of the depletion of ²¹⁰Po in the upper water column over the entire study area indicates that particle export fluxes were higher before July/August than later in the season. ²¹⁰Po fluxes and ²¹⁰Po‐derived POC fluxes correlated positively with sea‐ice concentration, showing that particle sinking was greater under heavy sea‐ice conditions than under partially ice‐covered regions. Although the POC fluxes were low, a large fraction of primary production (>30%) was exported at the base of the euphotic zone in most of the study area during summer 2012, indicating a high export efficiency of the biological pump in the central Arctic.

In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO₂. Three contrasting east Antarctic polynyas were visited in late December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m², respectively) and net community production (5.3 and 4.6 mol C/m², respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m² and 1.8 mol C/m²). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas.