3.4. Variations of nutrient concentrations in seawater-origin ice
Although nutrient concentrations decreased as the snow fraction increased in the topmost sea ice, the distributions of nutrient concentrations differed among the different nutrient components (Figure 5). Therefore, we here evaluate the effects of primary production, organic matter remineralization, and denitrification on nutrient concentrations in both first-year and multi-year ice. We first compared salinity to nutrient concentrations (Figure 7). Sea-ice salinity is a conservative component that only changes through physical processes such as brine drainage or exchange with under-ice water. Therefore, we investigated deviations from a dilution line based on the salinity and nutrient concentrations of the under-ice water collected during JARE60 to understand the biological processes affecting the nutrient distributions in the sea ice. Data plotting above the dilution line indicate nutrient supply due to the remineralization of organic matter by heterotrophic organisms and bacteria in the sea ice or from external sources (e.g., the atmosphere); conversely, data points below the dilution line indicate nutrient uptake by ice algae during primary production (e.g., Thomas et al., 1995).
Si(OH)4 concentrations in first-year ice locate significantly below the dilution line (Figure 7a), whereas those in multi-year ice are closer to or above the dilution line (Figure 7b). The distributions of chl.a concentrations in the sea ice (Figure 4d, h) indicate the presence of ice algae, which would mainly be diatoms (Fragilariopsis sp. and Pleurosigma sp. (photo in 4c, d) based on the cell counting for ice algae community assemblage in this study. Indeed, in first-year ice, primary production by diatoms consumes a large amount of Si(OH)4 (Tréguer & De La Rocha, 2013), and the slow dissolution of diatom frustules should limit the resupply of Si(OH)4. In contrast, in multi-year ice, the more protracted dissolution of diatom frustules should resupply Si(OH)4, eventually balancing the consumption of Si(OH)4 by primary production. However, in core JARE57_P31.5 (the deepest core studied), Si(OH)4concentrations plot well above the dilution line (Figure 7b), indicating that the resupply of Si(OH)4 by dissolution of diatom frustules exceeded consumption.
Consistent with a previous sampling at the same location (Nomura et al., 2018), PO43− concentrations plot above the dilution line in both first-year and multi-year ice (Figure 7c, d), suggesting resupply by the remineralization of organic matter. The difference in the resupply rates of PO43− and Si(OH)4 is due to the fact that the remineralization of PO43− is faster than the dissolution of Si(OH)4 (Fripiat et al., 2017).
NO3 concentrations in most first-year and multi-year ice cores plot below the dilution line (Figure 7e, f). This is expected for first-year ice, in which primary production consumes NO3, as with Si(OH)4. In contrast, remineralization in multi-year ice should resupply NO3, although our data do not reflect this. Therefore, we also measured NO2 concentrations (Figure 7g, h), which plot close to the dilution line for first-year ice and are shifted upward for multi-year ice. From this result, we expect that denitrification resupplied NO2 over longer periods in the multi-year ice. Indeed, denitrifying bacterial species has been confirmed in Antarctic ice (Staley & Gosink, 1999), which consume NO3 and supply NO2 (Rysgaard et al., 2008). To better understand the effect of denitrification on the nutrient balance, we calculated DIN:P, the ratio of dissolved inorganic nitrogen (DIN, i.e., NO3 + NO2) to PO43− (Figure 8). In most cores, DIN:P values were lower than the Redfield ratio (16:1) and the diatom nutrient uptake ratio (12:1; Takeda, 1998), which is thought to reflect the transformation of NO3 and NO2 to NO, N2O, and N2 by denitrification. The value of N*, which has been suggested as a proxy for denitrification, is calculated as follows (Gruber & Sarmiento, 1997; Nishioka et al., 2021; Yoshikawa et al., 2006):
N* = ([NO3] − 16 [PO43−] + 2.9) × 0.87. (3)
Figure 9 shows N* profiles of multi-year ice cores JARE56_NMS and JARE57_P31.5. In the lower parts of these cores, N* values decreased to negative values, reaching a minimum of −12.3 μmol L−1in core JARE57_P31.5. Chl.a concentrations are also negatively correlated with N*. Previous studies have reported that denitrification occurred in oxygen-depleted and organic matter-rich parts of the ice column (Rysgaard et al., 2008). It is therefore reasonable that denitrification occurred at depths where high chl.a concentrations indicate abundant organic matter. To our knowledge, because multi-year ice formed a long time ago in this area (about 40 years ago) (Higashi et al., 1982; Ushio, 2006), salinity is low (Figure 2c), and it is unlikely that brine channels physically drain the ice. Therefore, the air and brine pockets in multi-year ice has remained isolated and disconnected from exchanges with the atmosphere and under-ice water.
To examine the possibility of oxygen exchange through the brine channel network in the sea ice, we focused on the brine volume fraction, a proxy of sea-ice permeability. Calculating brine volume fraction requires the ice temperature and salinity; however, we could not obtain realistic ice temperature data due to the difficulty of inserting a temperature sensor in the sea ice, especially in low-salinity, snow-origin ice. The ice temperature increased during coring due to friction, and the air temperature was often positive (Nomura et al., 2011a), precluding accurate measurement of the sea ice temperature. Therefore, for the brine volume calculation, we assumed the temperature of the sea ice to be −2 to 0 °C, a reasonable estimate given the positive air temperature. We obtained brine volume fractions, estimated following Leppäranta and Manninen (1988), of 2.4 ± 2.6% (mean and standard deviation) at all depths in the multi-year ice. Typically, sea ice is considered permeable when the brine volume fraction exceeds 5–7.5% (Golden et al., 1998; Pringle et al., 2009; Zhou et al., 2013). Therefore, the multi-year ice collected in this study was impermeable, precluding any exchanges through brine channels. Therefore, gas components such as oxygen must have been consumed in a closed system within the ice by heterotrophic organisms and bacteria. Furthermore, the high density of the hard, low-salinity superimposed ice suggests that gases could not be exchanged with the atmosphere, consistent with previous studies that were unable to detect gas fluxes over superimposed ice at the same location (Nomura et al., 2012, 2013). Therefore, oxygen in the isolated interior sea ice could only have been depleted by heterotrophic organisms and bacteria, promoting denitrification in multi-year ice (Figure 9).
Based on our results, we developed a conceptual illustration for the evolving nutrient concentrations in multi-year fast ice (Figure 10). Immediately after ice formation, when nutrient concentrations have not yet been affected by biological processes, the ratio of salinity to nutrients in the brine discharged from the growing sea ice should plot along the dilution line. Then, during the first year, nutrient uptake by primary production of ice algae decreases nutrient concentrations in the ice while the salinity is reduced by brine convection. Over multiple years, remineralization by the degradation of organic matter resupplies nutrients to the ice while salinity continues to decrease. Once ice algae have exhausted the nutrient supply and decompose, and brine exchange with seawater is established at the bottom of the ice column, the nutrient concentrations surpass the dilution line (Figure 10). In addition, denitrification decreases NO3 concentrations and modifies the speciation of nitrogen within the ice.