Methane pathways within sea ice and seawater in the Arctic Ocean: an observational study

Abstract

This PhD thesis highlights the role of the cycles of sea ice formation and melt on the methane (CH4) pathways in the Arctic Ocean, considering the release of dissolved CH4 into the ocean as an alternative pathway to the ice-air CH4 flux. In this dissertation, CH4 concentration and its stable carbon isotopic signature (13C-CH4) were combined with oceanographic data, and the stable oxygen isotopic signature of seawater (18O-H2O) as a water mass tracer to follow the pathways of CH4. The CH4 pathways were traced within sea ice and at the sea ice-seawater interface in the Arctic Ocean and on an Arctic shelf region, to infer its potential sources and main sinks at the sea ice-ocean-atmosphere interface. Methane is a potent greenhouse gas contributing to global warming. Because of a phenomenon called Arctic amplification (warming is twice as large in the Arctic than in the global mean), the most notable changes due to global warming are currently happening in the Arctic. Arctic amplification of global warming is causing a rapid decrease in summer sea ice extent in the Arctic Ocean. Thin and more fragile first-year ice (FYI), which is more susceptible to oceanic and atmospheric forcing, is replacing complex structured and thick multi-year ice. Sea ice contributes to control the gas exchange between the ocean and the atmosphere. Hence, changes in the ice regime are likely to influence the CH4 pathways in the Arctic Ocean. In Publication I, we analyzed the CH4 concentration and the 13C-CH4 in both sea ice and seawater. The expedition PS106.1 took place on a drifting ice floe north of Svalbard during late spring 2017. We report on different storage durations of CH4 within FYI and ridged/rafted ice and supersaturation (CH4 excess) in surface waters. We found that the ice types and/or structures influence the fate of CH4. During the early melt season, when basal melting starts and the ice still holds impermeable ice layers at its surface, CH4 released from sea ice into the seawater is the predominant pathway. The excess of CH4 in surface waters depends on the degree of ice melt, which regulates the amount of CH4 ultimately diluted by meltwater via mixing. We suggest that sea ice loaded with CH4 acts as a source of CH4 for polar surface waters during late spring. In late summer, melting ice forms a low salinity freshwater layer that enhances the water stratification, but it also contributes to CH4 dilution. Therefore, this led to the question of what is the impact of meltwater discharge on the marine CH4 pathways. In Publication II, we present the effects of glacier and ice melt on marine CH4 pathways on the Northeast Greenland (NEG) continental shelf, a site of intense water mass transformation involving both glacier dynamics and sea ice processes. The expedition PS109 took place on the NEG continental shelf during the transition from late summer to early autumn 2019. Based on measurements of CH4 concentration, the 13C-CH4, the water mass tracer 18O-H2O, and the physical properties of the water masses, we constrained the marine CH4 pathways to infer its potential sources and main sinks. We report on CH4 excess in surface waters influenced by sea ice, i.e., in the winter mixed layer and meltwater layer. We determined that this CH4 excess is induced by brine release during sea ice formation in the previous winter season. In the following summer, the CH4 excess is sustained throughout the melt season with the help of an enhanced stratification, which restricts the efflux from the ocean surface into the atmosphere. This CH4 excess is susceptible to dilution by surrounding shelf waters influenced by different meltwater types, as the predominant pathway. In deeper waters uninfluenced by sea ice, the basal glacial meltwater further dilutes the CH4 excess, indicating that the ocean acts as a sink for this excess on the NEG continental shelf. We suggest that CH4 dilution by meltwater needs to be considered as a marine CH4 sink. In Publication III, we investigated the effect of freeze-up on CH4 inclusions in sea ice. Based on measurements of the CH4 concentration in sea ice and seawater on the drifting ice floe during the MOSAiC drift expedition 2019-2020, we followed the CH4 pathways at the ice-seawater interface during the freeze-up phase in autumn, to determine the fate of CH4 in the early stages of ice growth. We detected different stages of CH4 undersaturation in the cold halocline layer and the surface mixed layer, indicating on one side ongoing CH4 oxidation within seawater, and on the other, a decoupling between the ocean surface and the atmosphere. We traced the uptake of dissolved CH4 in seawater into permeable sea ice during the onset of freezing. During the ongoing freeze, surface ice shifts from permeable layers into impermeable, and the CH4 concentration at its surface decreases. The residual CH4 in sea ice remains stored in disconnected brine pockets, where CH4 concentration is controlled by brine dynamics. This PhD thesis advances the understanding of the CH4 exchange between Arctic sea ice, seawater, and atmosphere and demonstrates that during late spring when basal sea ice melting starts, dissolved CH4 released from sea ice into the seawater is the predominant pathway. We determine that sea ice loaded with CH4 acts as a source for supersaturation in surface waters. During late summer, different types of meltwater create a dilution of the CH4 excess in sea ice-influenced water. We suggest CH4 dilution as a marine CH4 sink in polar regions restricting sea-air flux. Finally, the extended fraction of FYI in the Arctic Ocean will influence the CH4 pathways at the sea ice-ocean-air interface and more meltwater discharge may enhance the sink capacity of the ocean.