Sediment-laden sea ice in southern Hudson Bay: Entrainment, transport, and biogeochemical implications

During a research expedition in Hudson Bay in June 2018, vast areas of thick (>10 m), deformed sediment-laden sea ice were encountered unexpectedly in southern Hudson Bay and presented difficult navigation conditions for the Canadian Coast Guard Ship Amundsen. An aerial survey of one of these floes revealed a maximum ridge height of 4.6 m and an average freeboard of 2.2 m, which corresponds to an estimated total thickness of 18 m, far greater than expected within a seasonal ice cover. Samples of the upper portion of the ice floe revealed that it was isothermal and fresh in areas with sediment present on the surface. Fine-grained sediment and larger rocks were visible on the ice surface, while a pronounced sediment band was observed in an ice core. Initial speculation was that this ice had formed in the highly dynamic Nelson River estuary from freshwater, but δ^{18}O isotopic analysis revealed a marine origin. In southern Hudson Bay, significant tidal forcing promotes both sediment resuspension and new ice formation within a flaw lead, which we speculate promotes the formation of this sediment-laden sea ice. Historic satellite imagery shows that sediment-laden sea ice is typical of southern Hudson Bay, varying in areal extent from 47 to 118 km2 during June. Based on an average sediment particle concentration of 0.1 mg mL^{–1} in sea ice, an areal extent of 51,924 km2 in June 2018, and an estimated regional end-of-winter ice thickness of 1.5 m, we conservatively estimated that a total sediment load of 7.8 × 106 t, or 150 t km^{–2}, was entrained within sea ice in southern Hudson Bay during winter 2018. As sediments can alter carbon concentrations and light transmission within sea ice, these first observations of this ice type in Hudson Bay imply biogeochemical impacts for the marine system

Sea-air CO2 exchange in the western Arctic coastal ocean

The biogeochemical seascape of the western Arctic coastal ocean is in rapid transition. Changes in sea ice cover will be accompanied by alterations in sea-air carbon dioxide (CO2) exchange, of which the latter has been difficult to constrain owing to sparse temporal and spatial data sets. Previous assessments of sea-air CO2 flux have targeted specific subregional areas of the western Arctic coastal ocean. Here a holistic approach is taken to determine the net sea-air CO2 flux over this broad region. We compiled and analyzed an extensive data set of nearly 600,000 surface seawater CO2 partial pressure (pCO2) measurements spanning 2003 through 2014. Using space-time colocated, reconstructed atmospheric pCO2 values coupled with the seawater pCO2 data set, monthly climatologies of sea-air pCO2 differences (ΔpCO2) were created on a 0.2°latitude × 0.5°longitude grid. Sea-air CO2 fluxes were computed using the ΔpCO2 grid and gas transfer rates calculated from climatology of wind speed second moments. Fluxes were calculated with and without the presence of sea ice, treating sea ice as an imperfect barrier to gas exchange. This allowed for carbon uptake by the western Arctic coastal ocean to be assessed under existing and reduced sea ice cover conditions, in which carbon uptake increased 30% over the current 10.9 ± 5.7 Tg C (1 Tg = 1012 g) yr-1 of sea ice-adjusted exchange in the region. This assessment extends beyond previous subregional estimates in the region in an all-inclusive manner and points to key unresolved aspects that must be targeted by future research