Drivers of methane variability in Arctic ponds

<p>Arctic ponds are significant sources of methane, but their overall contribution to pan-Arctic methane emissions is still uncertain. Ponds come in different sizes and shapes, which are associated with different stages of permafrost degradation. Methane concentrations and fluxes show large spatiotemporal variability. To better understand this variability, as a first step towards upscaling pond methane emissions, we studied 41 ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the geomorphology, vegetation cover as well as on key physical and chemical properties of the ponds and combined them with meteorological data.</p><p>The ponds are divided into three geomorphological types with distinct differences in methane concentrations: water-filled degraded polygon centers, water-filled interpolygonal troughs and larger collapsed and merged polygons. These ponds exhibit mean surface methane concentrations (with standard deviation) of 1.2 ± 1.3 μmol L-1, 4.3 ± 4.9 μmol L-1 and 0.9 ± 0.7 μmol L-1 respectively, while mean bottom methane concentrations amount to 102.6 ± 145.4 μmol L-1, 263.3 ± 275.6 μmol L-1 and 17.0 ± 34.1 μmol L-1. Using principle components and multiple linear regressions, we show that a large portion of spatial variability can be explained by the ponds’ shape and vegetation. Merged ponds have the least relative vegetation cover, and they also tend to be better mixed, both of which explains the lowest methane concentrations and the lowest variability in these ponds. Vegetation covers larger fractions in polygon centers and troughs, leading to a larger methane variability. Finally, troughs, as they are underlain by ice wedges, exhibit more pronounced stratification and the highest methane concentrations. More results will be presented at the conference.</p>

Formation of sea ice ponds from ice-shelf runoff, adjacent to the McMurdo Ice Shelf, Antarctica

Ponds that form on sea ice can cause it to thin or break-up, which can promote calving from an adjacent ice shelf. Studies of sea ice ponds have predominantly focused on Arctic ponds formed by in situ melting/ponding. Our study documents another mechanism for the formation of sea ice ponds. Using Landsat 8 and Sentinel-2 images from the 2015–16 to 2018–19 austral summers, we analyze the evolution of sea ice ponds that form adjacent to the McMurdo Ice Shelf, Antarctica. We find that each summer, meltwater flows from the ice shelf onto the sea ice and forms large (up to 9 km2) ponds. These ponds decrease the sea ice's albedo, thinning it. We suggest the added mass of runoff causes the ice to flex, potentially promoting sea-ice instability by the ice-shelf front. As surface melting on ice shelves increases, we suggest that ice-shelf surface hydrology will have a greater effect on sea-ice stability.