3.3. Variations of nutrient concentrations in snow-origin ice
In Section 3.2, we showed that in multi-year ice columns, nutrient
concentrations were lower in snow-origin ice than in columnar ice (Table
3). To quantify the effect of snow on sea-ice nutrient concentrations,
we calculated the snow fraction in snow-origin ice (Eqs. 1 and 2) to
examine the relationship between nutrient concentrations and snow
fraction (Figure 5). Although the regressions differed between
nutrients, nutrient concentrations tended to decrease with increasing
snow fraction. This is due to the purity of the air near Antarctica,
being far from areas of human activity, compared to that in the Northern
Hemisphere. Therefore, the atmospheric transfer and deposition of
nutrients onto sea ice is limited in Antarctica, and the top surface of
the sea ice is replaced by clean, snow-origin ice. In contrast, in the
Northern Hemisphere, atmospheric transfer and deposition of polluted
snow is known to be a potential source of nutrients for sea ice
(Granskog & Kaartokallio, 2004; Granskog et al., 2003; Kaartokallio,
2001; Krell et al., 2003; Nomura et al., 2010; 2011b; Rahm et al.,
1995).
Although nutrient concentrations were generally negatively correlated
with snow fraction (Figure 5a, b, d),
NO3− concentrations notably differed,
with the negative correlation between snow fraction and
NO3− concentration being markedly low
compared to the other nutrients (Figure 5c). Because most of the sea-ice
samples in this study were collected near Syowa Station, it is possible
that this difference reflects slight pollution from local human
activities, such as exhaust from the research facility and/or
snowmobiles. Therefore, greater NO3−concentrations might be detected (Table 3) in areas of greater snow
fraction (Figure 5). However, many
NO3− data points plot near zero at
snow fractions of 30–80% (Figure 5c), potentially indicating that
NO3− was selectively removed by sea
ice, which can be explored by considering denitrification.
Denitrification is a reaction in which
NO3− is used by bacteria in anoxic
conditions to decompose organic substances within sea ice, converting
NO3− to
NO2−, NO, and eventually
N2O and N2 (Kaartokallio, 2001; Nomura
et al., 2010; 2011b; 2018; Rysgaard & Glud, 2004; Rysgaard et al.,
2008; Staley & Gosink, 1999). Snow-origin ice with small snow
fractions, i.e., containing some, albeit small, amounts of seawater,
must be older than snow-origin ice not containing any seawater (e.g.,
superimposed ice) because it is formed at an earlier stage of ice
growth. Therefore, we consider that denitrification occurred in the
older parts of the ice column where the snow fraction was low (Figure
5c; see Section 3.4 for details).
Due to the formation of snow-origin ice, the snow fraction in multi-year
ice increased year-by-year; the mean snow fractions in the top 200 cm of
multi-year ice gradually increased from about 88% in 2015 to 100% by
2018 (Figure 6). Therefore, our sea-ice core samples record a steady
increase in the contribution of snow to sea ice. Although we have not
collected multi-year ice cores at distant locations within the same year
(Figure 1c) and thus have not been able to unequivocally determine that
there is no spatial bias in our sampling, the nutrient concentrations do
not differ greatly between sampling locations, and the trends and
profiles are similar from year to year (Figures 2, 4).