Landfast sea ice in the Bothnian Bay (Baltic Sea) as a temporary storage compartment for greenhouse gases

Abstract

Although studies of biogeochemical processes in polar sea ice have been increasing, similar research on relatively warm low-salinity sea ice remains sparse. In this study, we investigated biogeochemical properties of the landfast sea ice cover in the brackish Bothnian Bay (Northern Baltic Sea) and the possible role of this sea ice in mediating the exchange of greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) across the water column–sea ice–atmosphere interface. Observations of total alkalinity and dissolved inorganic carbon in both landfast sea ice and the water column suggest that the carbonate system is mainly driven by salinity. While high CH4 and N2O concentrations were observed in both the water column (up to 14.3 and 17.5 nmol L–1, respectively) and the sea ice (up to 143.6 and 22.4 nmol L–1, respectively), these gases appear to be enriched in sea ice compared to the water column. This enrichment may be attributable to the sea ice formation process, which concentrates impurities within brine. As sea ice temperature and brine volume decrease, gas solubility decreases as well, promoting the formation of bubbles. Gas bubbles originating from underlying sediments may also be incorporated within the ice cover and contribute to the enrichment in sea ice. The fate of these greenhouse gases within the ice merits further research, as storage in this low-salinity seasonal sea ice is temporary.

Photophysiological responses of bottom sea-ice algae to fjord dynamics and rapid freshening

“…We exclude for example the age of sea ice as a major factor explaining the differences in bulk-ice salinities as suggested for example for multi-year ice versus firstyear ice (e.g., Lange et al, 2015) or drainage with increasing ice age (Cox and Weeks, 1988), given the similar age of the ice present. We therefore used the low bulk salinity as strongest indicator for low surface water salinity impact during ice formation as known from other Arctic sites (e.g., Vonnahme et al, 2021) or brackish surface water sea ice systems like the Baltic Sea (e.g., Geilfus et al, 2021). Furthermore, it is plausible that a freshwater layer in TF1 was not detected in our study, due to wave propagated mixing, which has previously been measured in TF (Rabault et al, 2016;.…”