Daiki Nomura

The effect of sea‐ice growth on air–sea CO2 flux in a tank experiment

In order to clarify the CO2 exchange between the seawater and the overlying air during the sea-ice formation, we have carried out tank experiments in a low-temperature room. CO2 concentration above the sea-ice began to increase since the beginning of the sea-ice formation, and increased at larger rates with time and the decrease in air temperature. This increase of CO2 concentration in air was mainly caused by the increase in dissolved inorganic carbon concentration in the brine of the upper part of sea-ice, changes in CO2 solubility and dissociation constants of carbonic acid. The CO2 flux increased logarithmically with time, and reached a level of 2 × 10-4 to 5 × 10-4 g-C m-2 hr-1 at 50 mm ice thickness. We found that the CO2 flux was correlated well with the salinity and negatively with the volume of the brine in the upper part of the sea-ice. These suggested the larger role of the difference in partial pressure of CO2 between brine and air as compared to that of competitive change in the brine volume. Present results suggest the necessity to examine the CO2 exchange between the seawater and air in seasonal sea-ice areas.

Effects of snow, snowmelting and refreezing processes on air-sea-ice CO2 flux

The air-sea-ice CO2 flux was measured in the ice-covered Saroma-ko, a lagoon on the northeastern coast of Hokkaido, Japan, using a chamber technique. The air-sea-ice CO2 flux ranged from -1.8 to +0.5 mg C m -2 h -1 (where negative values indicate a sink for atmospheric CO2). The partial pressure of CO2 (pCO2) in the brine of sea ice was substantially lower than that of the atmosphere, primarily because of the influence of the under-ice plume from the Saromabetsu river located in the southeastern part of the lagoon. This suggests that the brine had the ability to take up atmospheric CO2 into the sea ice. However, the snow deposited over the sea ice and the superimposed ice that formed from snowmelting and refreezing partially blocked CO2 diffusion, acting as an impermeable medium for CO2 transfer. Our results suggest that the air-sea-ice CO2 flux was dependent not only on the difference in pCO2 between the brine and the overlying air, but also on the status of the ice surface. These results provide the necessary evidence for evaluation of the gas exchange processes in ice-covered seas.

Characterization of ikaite (CaCO3•6H2O) crystals in first year Arctic sea ice north of Svalbard

Abstract We identified ikaite crystals (CaCO3 ·6H2O) and examined their shape and size distribution in first-year Arctic pack ice, overlying snow and slush layers during the spring melt onset north of Svalbard. Additional measurements of total alkalinity (TA) were made for melted snow and sea-ice samples. Ikaite crystals were mainly found in the bottom of the snowpack, in slush and the surface layers of the sea ice where the temperature was generally lower and salinity higher than in the ice below. Image analysis showed that ikaite crystals were characterized by a roughly elliptical shape and a maximum caliper diameter of 201.0±115.9 μm (n = 918). Since the ice-melting season had already started, ikaite crystals may already have begun to dissolve, which might explain the lack of a relationship between ikaite crystal size and sea-ice parameters (temperature, salinity, and thickness of snow and ice). Comparisons of salinity and TA profiles for melted ice samples suggest that the precipitation/dissolution of ikaite crystals occurred at the top of the sea ice and the bottom of the snowpack during ice formation/melting processes.

Effect of glacial drainage water on the CO2 system and ocean acidification state in an Arctic tidewater‐glacier fjord during two contrasting years

In order to investigate the effect of glacial water on the CO2 system in the fjord, we studied the variability of the total alkalinity (AT), total dissolved inorganic carbon (CT), dissolved inorganic nutrients, oxygen isotopic ratio (d18O), and freshwater fractions from the glacier front to the outer Tempelfjorden on Spitsbergen in winter 2012 (January, March, and April) and 2013 (April) and summer/fall 2013 (September). The two contrasting years clearly showed that the influence of freshwater, mixing, and haline convection affected the chemical and physical characteristics of the fjord. The seasonal variability showed the lowest calcium carbonate saturation state (X) and pH values in March 2012 coinciding with the highest freshwater fractions. The highest X and pH were found in September 2013, mostly due to CO2 uptake during primary production. Overall, we found that increased freshwater supply decreased X, pH, and AT. On the other hand, we observed higher AT relative to salinity in the freshwater end-member in the mild and rainy winter of 2012 (1142 lmol kg21) compared to AT in 2013 (526 lmol kg21). Observations of calcite and dolomite crystals in the glacial ice suggested supply of carbonate-rich glacial drainage water to the fjord. This implies that winters with a large amount of glacial drainage water partly provide a lessening of further ocean acidification, which will also affect the air-sea CO2 exchange.

A global-scale map of isoprene and volatile organic iodine in surface seawater of the Arctic, Northwest Pacific, Indian, and Southern Oceans

Isoprene (C5H8) and three volatile organic iodine compounds (VOIs: CH3I, C2H5I, and CH2ClI) in surface seawater were measured in the western Arctic, Northwest Pacific, Indian, and Southern Oceans during the period 2008–2012. These compounds are believed to play an important role in the marine atmospheric chemistry after their emission. The measurements were performed with high time-resolution (1–6 h intervals) using an online equilibrator gas chromatography mass spectrometer. C5H8 was most abundant in high-productivity transitional waters and eutrophic tropical waters. The chlorophyll-a normalized production rates of C5H8 were high in the warm subtropical and tropical waters, suggesting the existence of a high emitter of C5H8 in the biological community of the warm waters. High concentrations of the three VOIs in highly productive transitional water were attributed to biological productions. For CH3I, the highest concentrations were widely distributed in the basin area of the oligotrophic subtropical NW Pacific, probably due to photochemical production and/or high emission rates from phytoplankton. In contrast, the lowest concentrations of C2H5I in subtropical waters were attributed to photochemical removal. Enhancement of CH2ClI concentrations in the shelf-slope areas of the Chukchi Sea and the transitional waters of the NW Pacific in winter suggested that vertical mixing with subsurface waters by regional upwelling or winter cooling acts to increase the CH2ClI concentrations in surface layer. Sea-air flux calculations revealed that the fluxes of CH2ClI were the highest among the three VOIs in shelf-slope areas; the CH3I flux was highest in basin areas.

Evidence for significant protein-like dissolved organic matter accumulation in Sea of Okhotsk sea ice

Abstract Absorption and fluorescence of chromophoric dissolved organic matter (CDOM) in sea ice and surface waters in the southern Sea of Okhotsk was examined. Sea-water CDOM had featureless absorption increasing exponentially with shorter wavelengths. Sea ice showed distinct absorption peaks in the ultraviolet, especially in younger ice. Older first-year sea ice had relatively flat absorption spectra in the ultraviolet range. Parallel factor analysis (PARAFAC) identified five fluorescent CDOM components, two humic-like and three protein-like. Sea water was largely governed by humic-like fluorescence. In sea ice, protein-like fluorescence was found in considerable excess relative to sea water. The accumulation of protein-like CDOM fluorescence in sea ice is likely a result of biological activity within the ice. Nevertheless, sea ice does not contribute excess CDOM during melt, but the material released will be of different composition than that present in the underlying waters. Thus, at least transiently, the CDOM introduced during sea-ice melt might provide a more labile source of fresher protein-like DOM to surface waters in the southern Sea of Okhotsk.

An artificial pool experiment in Antarctic sea ice: effects of sea ice melting on physical and biogeochemical components of pool water

Abstract We performed an artificial pool experiment in the Antarctic multi-year land-fast ice to examine and simulate the effect of sea ice melting on physical and biogeochemical components of the sea ice field. The input of snow and ice meltwater resulted in warmer, low salinity water at the surface of the pool and probably stratification of the less dense water. Current speed measurements also pointed to water stratification within the pool. Rapid phytoplankton growth in the pool resulted in drastic decreases in concentrations of dissolved inorganic carbon and nutrients (NO3- and Si(OH)4) in the surface waters of the pool, particularly depleted for NO3-. There was high correlation between variations of dissolved inorganic carbon and nutrient concentrations, but the apparent uptake ratios of these components deviated from that generally applied to marine phytoplankton. The sequence of changes in the physical and biogeochemical components of the pool water suggests that the onset of rapid phytoplankton growth was closely related to the water stratification, which provided stable conditions for phytoplankton bloom even though the supply of nutrients from under-ice water would have declined.

Bromoform concentrations in slush-layer water in Antarctic fast ice

Abstract Bromoform concentrations in water of the slush layer that developed at the interface between snow and sea ice were measured during the seasonal warming in Lützow-Holm Bay, East Antarctica. Mean bromoform concentration was 5.5 ± 2.4 pmol l-1, which was lower than that of the under-ice water (10.9 ± 3.5 pmol l-1). Temporal decrease in bromoform concentrations and salinity with increasing temperature of the slush water suggest that the bromoform concentrations were reduced through dilution with meltwater input from the upper surface of sea ice. In contrast, bromoform concentrations in the under-ice water increased during this period while the salinity of the under-ice water decreased. It is speculated that the sea ice meltwater input contained high bromoform concentrations from the brine channels within the sea ice and from the bottom of the ice that were contributed to the increased bromoform concentrations in the under-ice water.

Upwelling of Macronutrients and Dissolved Inorganic Carbon by a Subglacial Freshwater Driven Plume in Bowdoin Fjord, Northwestern Greenland

In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming a plume near the calving front. To elucidate the effects of this discharge on nutrient and dissolved inorganic carbon transport to the surface in these fjords, we conducted observational studies on Bowdoin Glacier and in its fjord in northwestern Greenland during the summer of 2016. Our results provide evidence of macronutrient and dissolved inorganic carbon transport from deep in the fjord to the surface in front of the glacier. This transport is driven by plume formation resulting from subglacial freshwater discharge and subsequent upwelling along the glacier calving front. The plume water is a mixture of subglacial freshwater and entrained fjord water. The fraction of glacial meltwater in the plume water is ~14% when it reaches the surface. The plume water is highly turbid because it contains substantial amounts of sediment derived from subglacial weathering. After reaching the surface, the plume water submerges and forms a turbid subsurface layer below fresher surface water at densities of 25.0 to 26.5 σθ. Phytoplankton blooms (~6.5 μg/L chlorophyll a) were observed near the boundary between the fresher surface and turbid subsurface layers. The bloom was associated with a strong upward NO3 + NO2 flux, which was caused by the subduction of plume water. Our study demonstrated that the subglacial discharge and plume formation at the front of Bowdoin Glacier play a key role in the availability of nutrients and the subsequent growth of phytoplankton in the glaciated fjord.

Influence of Sea Ice Crack Formation on the Spatial Distribution of Nutrients and Microalgae in Flooded Antarctic Multiyear Ice

Cracks are common and natural features of sea ice formed in the polar oceans. In this study, a sea ice crack in flooded, multiyear, land-fast Antarctic sea ice was examined to assess its influence on biological productivity and the transport of nutrients and microalgae into the upper layers of neighboring sea ice. The water inside the crack and the surrounding host ice were characterized by a strong discoloration (brown color), an indicator of a massive algal bloom. Salinity and oxygen isotopic ratio measurements indicated that 64–84% of the crack water consisted of snow meltwater supplied during the melt season. Measurements of nutrient and chlorophyll a concentrations within the slush layer pool (the flooded layer at the snow-ice interface) revealed the intrusion of water from the crack, likely forced by mixing with underlying seawater during the tidal cycle. Our results suggest that sea ice crack formation provides conditions favorable for algal blooms by directly exposing the crack water to sunlight and supplying nutrients from the under-ice water. Subsequently, constituents of the crack water modified by biological activity were transported into the upper layer of the flooded sea ice. They were then preserved in the multiyear ice column formed by upward growth of sea ice caused by snow ice formation in areas of significant snow accumulation. Plain Language Summary Formation of cracks in sea ice affects the environment associated with biological production and biogeochemical cycling in the surface ocean of sea ice systems. Because cracks are likely to form frequently within the sea ice during the season of ice melting and ice breaking, the contributions of cracks to biological production and biogeochemical cycling may be significant in ice-covered oceans. In the future, the melting of sea ice in polar oceans will strongly affect the output of biogeochemical parameters trapped within sea ice and their use in primary and secondary production within surface oceans. In the case of multiyear, land-fast ice, biogeochemical parameters that accumulate within the ice would be discharged abruptly to ocean surface waters when the multiyear ice breaks up.