Ballasting by cryogenic gypsum enhances carbon export in a Phaeocystis under-ice bloom

J E Wollenburg,P Assmy,C Dybwad,I Peeken,L Rossmann,G Nehrke,F Gázquez-Sanchez,E.-M Nöthig,A Nikolopoulos,J Matthiessen,C Katlein,F Bruyant,D A Wolf- Gladrow,M Beaulieu,M Babin

doi:10.1038/s41598-018-26016-0

Abstract

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.

Highlights

Concurrent with the transformation of the Arctic sea ice from a thick, multi-year to a thinner, first-year ice cover, recent observations have detected phytoplankton blooms beneath snow-covered sea ice early in the season[13] and below ponded ice during the melt period[14]
We report cryogenic gypsum ballasting of an under-ice phytoplankton bloom dominated by the haptophyte Phaeocystis north of Svalbard: this is the first report of cryogenic gypsum ballasting
We report here for the first time on the ballasting of a Phaeocystis bloom by cryogenic gypsum released from melting sea ice and representing a previously unknown role of this mineral in enhancing deep carbon export in ice covered areas of the world ocean

Summary

Introduction

Concurrent with the transformation of the Arctic sea ice from a thick, multi-year to a thinner, first-year ice cover, recent observations have detected phytoplankton blooms beneath snow-covered sea ice early in the season[13] and below ponded ice during the melt period[14]. The fate of these under-ice blooms is unknown but their occurrence suggests that there is more organic material available for ice-associated mineral ballasting under the new sea ice regime. We report cryogenic gypsum ballasting of an under-ice phytoplankton bloom dominated by the haptophyte Phaeocystis north of Svalbard: this is the first report of cryogenic gypsum ballasting

Methods

Results

Conclusion