Search for: All records

Abstract. Field investigations of the properties of heavily melted “rotten” Arcticsea ice were carried out on shorefast and drifting ice off the coast ofUtqiaġvik (formerly Barrow), Alaska, during the melt season. While noformal criteria exist to qualify when ice becomes rotten, the objectiveof this study was to sample melting ice at the point at which its structural andoptical properties are sufficiently advanced beyond the peak of the summerseason. Baseline data on the physical (temperature, salinity, density,microstructure) and optical (light scattering) properties of shorefast icewere recorded in May and June 2015. In July of both 2015 and 2017, smallboats were used to access drifting rotten ice within ∼32 km of Utqiaġvik. Measurements showed that pore space increased as icetemperature increased (−8 to 0 ^∘C), ice salinitydecreased (10 to 0 ppt), and bulk density decreased (0.9 to0.6 g cm⁻³). Changes in pore space were characterized with thin-sectionmicrophotography and X-ray micro-computed tomography in the laboratory. Theseanalyses yielded changes in average brine inclusion number density (whichdecreased from 32 to 0.01 mm⁻³), mean pore size (whichincreased from 80 µm to 3 mm), and total porosity (increased from0 % to > 45 %) and structural anisotropy (variable, withvalues of generally less than 0.7). Additionally, light-scattering coefficientsof the ice increased from approximately 0.06 to > 0.35 cm⁻¹ as the ice melt progressed. Together, these findings indicate thatthe properties of Arctic sea ice at the end of melt season are significantlydistinct from those of often-studied summertime ice. If such rotten ice wereto become more prevalent in a warmer Arctic with longer melt seasons, thiscould have implications for the exchange of fluid and heat at the oceansurface. 

Abstract The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO₂uptake. We studied how sea‐ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreakerOden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea‐ice microbial communities were exposed to elevated partial pressures of CO₂(pCO₂). We incubated intact, bottom ice‐core sections and dislodged, under‐ice algal aggregates (dominated byMelosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 μatm pCO₂for 10 d under different nutrient regimes. The results indicate that the growth of sea‐ice algae and bacteria was unaffected by these higher pCO₂levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO₂enrichment. These central Arctic sea‐ice microbial communities thus appear to be largely insensitive to short‐term pCO₂perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO₂and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea‐ice microorganisms even as the loss of Arctic sea ice continues.