Reply on RC2

Abstract

With ongoing warming and sea ice loss, the Arctic Ocean and its marginal seas will likely become more hospitable to pelagic calcifiers, resulting in modifications of the regional carbonate cycle and the composition of the seafloor sediment. A substantial part of the pelagic carbonate production in the Arctic is due to the calcification of the dominant planktonic foraminifera species Neogloboquadrina pachyderma. To quantify calcite production and loss in the upper water layer by this important Arctic calcifier, we compile and analyse data from vertical profiles in the upper water column of shell concentration, shell sizes and weights of this species across the Arctic region during summer. Our data is inconclusive on whether the species performs ontogenetic vertical migration throughout its lifecycle, or whether individual specimens calcify at a fixed depth within the vertical habitat. The base of the productive zone of the species is on average located below 100 m and at maximum at 300 m and is regionally highly variable. The calcite flux immediately below the productive zone (production flux) is on average 8 mg CaCO3 m-2 d-1, and we observe that this flux is attenuated until at least 300 m below the base of the productive zone at the mean rate of 1.5 % per 100 m. Regionally, the summer production flux of N. pachyderma calcite varies by more than two orders of magnitude and the estimated mean export flux below the twilight zone is sufficient to account for about a quarter of the total pelagic carbonate flux in the region. These results indicate that estimates of the Arctic pelagic carbonate budget will have to account for large regional differences in production flux of the major pelagic calcifiers and confirm that substantial attenuation of the production flux occurs in the twilight zone.