Waterside convection and stratification control methane spreading in supersaturated Arctic fjords (Spitsbergen)

Abstract

Seasonally ice covered in the past, the fjords in West Spitsbergen turn into being perennially ice free in the present. This feedback to Arctic amplification of global warming changes gas fluxes at the atmosphere-ocean interface. Furthermore, in this Polar region, coupled feedbacks likely enhance Arctic amplification of global warming as numerous gas seepages provide evidence for active gas emissions at the sediment-water interface. We present a time series (2015–2017) of dissolved methane concentrations combined with hydrographic data in Adventfjorden and Tempelfjorden, two sub-fjords of Isfjorden located at the west coast of Spitsbergen. While both sub-fjords remained permanently supersaturated, we detected pronounced temporal and spatial variations in the methane excess level. Our study revealed that seasonal water transformations were key to seasonally changing methane pathways including potential sea-air flux (efflux). We suggest that a cascade of feedback processes, seasonally triggered by waterside convection and stratification, adjusts the amount of methane released and transported within fjord water. When sea ice was missing, strong winter cooling affected the methane supersaturation in contrary directions: first a drop and then a strong increase. In early winter, convective mixing favoured efflux, which reduced the supersaturation. Later in winter, the thermal convection resulted in a continuous overturning of the water column. When the thermal convection reached the bottom, sediment resuspension by turbulence increased, which in turn encouraged enhanced methane release. Subsequently transported along vertical isopycnals, methane from the bottom water reached the water-atmosphere interface. These coupled events created a steady state, simultaneously maintaining supersaturation and efflux. During the warm season, the fjord water became stratified and methane transport occurred mainly laterally in the bottom water. The seasonally changing hydrographic conditions strongly triggered the methane spreading in both sub-fjords and point to a switch between the atmosphere and ocean as main sinks in winter and summer, respectively. Upcoming variations in seasonality, i.e. warmer/cooler summer compared to colder/warmer winter will influence these pathways and the final fate of methane discharged into Arctic fjords.