New observations of the distribution, morphology and dissolution dynamics of cryogenic gypsum in the Arctic Ocean

Abstract. To date, observations on a single location indicate that cryogenic gypsum (Ca[SO4]⚫2H2O) may constitute an efficient but hitherto overlooked ballasting mineral enhancing the efficiency of the biological carbon pump in the Arctic Ocean. In June–July 2017 we sampled cryogenic gypsum under pack ice in the Nansen Basin north of Svalbard using a plankton net mounted on a remotely operated vehicle (ROVnet). Cryogenic gypsum crystals were present at all sampled stations, which suggested a persisting cryogenic gypsum release from melting sea ice throughout the investigated area. This was supported by a sea ice backtracking model, indicating that gypsum release was not related to a specific region of sea ice formation. The observed cryogenic gypsum crystals exhibited a large variability in morphology and size, with the largest crystals exceeding a length of 1 cm. Preservation, temperature and pressure laboratory studies revealed that gypsum dissolution rates accelerated with increasing temperature and pressure, ranging from 6 % d−1 by mass in polar surface water (−0.5 ∘C) to 81 % d−1 by mass in Atlantic Water (2.5 ∘C at 65 bar). When testing the preservation of gypsum in formaldehyde-fixed samples, we observed immediate dissolution. Dissolution at warmer temperatures and through inappropriate preservation media may thus explain why cryogenic gypsum was not observed in scientific samples previously. Direct measurements of gypsum crystal sinking velocities ranged between 200 and 7000 m d−1, suggesting that gypsum-loaded marine aggregates could rapidly sink from the surface to abyssal depths, supporting the hypothesized potential of gypsum as a ballasting mineral in the Arctic Ocean.