Description: Current estimates of global marine primary production range over a factor of two. Improving these estimates requires an accurate knowledge of the chlorophyll vertical profiles, since they are the basis for most primary production models. At high latitudes, the uncertainty in primary production estimates is larger than globally, because here phytoplankton absorption shows specific characteristics due to the low-light adaptation, and in situ data and ocean colour observations are scarce. To date, studies describing the typical chlorophyll profile based on the chlorophyll in the surface layer have not included the Arctic region, or, if it was included, the dependence of the profile shape on surface concentration was neglected. The goal of our study was to derive and describe the typical Greenland Sea chlorophyll profiles, categorized according to the chlorophyll concentration in the surface layer and further monthly resolved profiles. The Greenland Sea was chosen because it is known to be one of the most productive regions of the Arctic and is among the regions in the Arctic where most chlorophyll field data are available. Our database contained 1199 chlorophyll profiles from R/Vs Polarstern and Maria S. Merian cruises combined with data from the ARCSS-PP database (Arctic primary production in situ database) for the years 1957–2010. The profiles were categorized according to their mean concentration in the surface layer, and then monthly median profiles within each category were calculated. The category with the surface layer chlorophyll (CHL) exceeding 0.7 mg C m−3 showed values gradually decreasing from April to August. A similar seasonal pattern was observed when monthly profiles were averaged over all the surface CHL concentrations. The maxima of all chlorophyll profiles moved from the greater depths to the surface from spring to late summer respectively. The profiles with the smallest surface values always showed a subsurface chlorophyll maximum with its median magnitude reaching up to three times the surface concentration. While the variability of the Greenland Sea season in April, May and June followed the global non-monthly resolved relationship of the chlorophyll profile to surface chlorophyll concentrations described by the model of Morel and Berthon (1989), it deviated significantly from the model in the other months (July–September), when the maxima of the chlorophyll are at quite different depths. The Greenland Sea dimensionless monthly median profiles intersected roughly at one common depth within each category. By applying a Gaussian fit with 0.1 mg C m−3 surface chlorophyll steps to the median monthly resolved chlorophyll profiles of the defined categories, mathematical approximations were determined. They generally reproduce the magnitude and position of the CHL maximum, resulting in an average 4% underestimation in Ctot (and 2% in rough primary production estimates) when compared to in situ estimates. These mathematical approximations can be used as the input to the satellite-based primary production models that estimate primary production in the Arctic regions.

Description: Mineral ballasting enhances carbon export from the surface to the deep ocean; however, little is known about the role of this process in the ice-covered Arctic Ocean. Here, we propose gypsum ballasting as a new mechanism that likely facilitated enhanced vertical carbon export from an under-ice phytoplankton bloom dominated by the haptophyte Phaeocystis. In the spring 2015 abundant gypsum crystals embedded in Phaeocystis aggregates were collected throughout the water column and on the sea floor at a depth below 2 km. Model predictions supported by isotopic signatures indicate that 2.7 g m−2 gypsum crystals were formed in sea ice at temperatures below −6.5°C and released into the water column during sea ice melting. Our finding indicates that sea ice derived (cryogenic) gypsum is stable enough to survive export to the deep ocean and serves as an effective ballast mineral. Our findings also suggest a potentially important and previously unknown role of Phaeocystis in deep carbon export due to cryogenic gypsum ballasting. The rapidly changing Arctic sea ice regime might favour this gypsum gravity chute with potential consequences for carbon export and food partitioning between pelagic and benthic ecosystems.