Release of dissolved organic carbon from coastal erosion into the southern Canadian Beaufort Sea

George.Tanski [ at ]


Arctic regions are highly vulnerable to climatic change processes and are currently undergoing the most rapid environmental transition experienced on Earth, at a pace that is expected to increase over the coming decades. Changing environmental conditions affect the sensitive ice-rich permafrost coasts in northern Canada that erode due to warmer climate, longer open water seasons and stronger storms. Coastal erosion in the Canadian Arctic that is among the highest in the world releases terrestrial organic carbon stored in ice-rich permafrost into the Arctic Ocean, which fosters the feedback mechanisms between carbon cycle and climate. The Yukon Coastal Plain is located in the western Canadian Arctic between the Mackenzie Delta and the Alaskan border and is characterized by the occurrence of ice-rich permafrost and large massive ground ice bodies. This ice contributes to facilitate coastal erosion, which is known to occur at a pace greater than in temperate regions during the short summer season. Erosion contributes to the release of large amounts of particulate organic carbon to the Arctic Ocean through the export of sediments. Additionally, large amounts of particulate and dissolved organic carbon (DOC) are released by rivers into the Arctic Ocean. Ground ice in permafrost also contains organic carbon in the dissolved state that will also be released to the ocean by coastal erosion, but the amounts of DOC present in the ground and eventually lost to the sea are unknown. It was therefore the objective of this thesis to quantify the amount of DOC present in massive ground ice and the amounts released by coastal erosion into the nearshore zone of the southern Beaufort Sea (Arctic Ocean). Several massive ground ice bodies and ice wedges, exposed by coastal erosion or thermal denudation, were sampled on Herschel Island and along the mainland coast of the Yukon Territory. In total, 41 samples of ice blocks were obtained from these bodies and analyzed in the laboratories of the Alfred Wegener Institute. DOC concentrations were determined on the melted solutions accompanied by a series of sedimentological and hydrochemical analyses, including ice and sediment content, pH and electrical conductivity. These values were then combined with existing datasets on coastal erosion, morphometry, and stratigraphy to calculate annual DOC fluxes into the Beaufort Sea. The DOC concentrations measured in massive ground ice bodies and ice wedges ranged between 1.0 and 19.5 mg/L. The calculated DOC stocks ranged between 0.09 and 0.24 g/m3 in massive ground ice for the whole Yukon Coastal Plain. The massive ground ice volume in coastal cliffs along the coastal plain was approximately 11 % and cliff heights ranged between 0.9 and 60.0 m. The average rate of erosion along the coastal stretch was 0.7 m/yr. Calculated DOC fluxes varied greatly, depending on the scenario of the computed DOCfluxes (25%-quartile, 50%-quartile or 75%-quartile of all conducted DOC measurements). A low-case scenario revealed a DOC flux of 148 kg/yr, a moderate-case scenario yielded 274 kg/yr and a high-case scenario gave a DOC flux of 466 kg/yr for the whole coast. DOC concentrations in ice wedges were up to eight times higher than in massive ground ice bodies. DOC in ice wedges was assumed to originate mainly from the presence of particulate organic carbon transported into polygon cracks during spring melt. For massive ground ice bodies, the origin of DOC seemed to depend on the genetic nature of the ice, as segregated ice or buried glacier ice. The DOC could have been introduced by water migration through the sediment and the interaction of basal glacier ice with subglacial sediments and/or could have been previously contained in glacier ice. DOC fluxes from the erosion of massive ground ice at the coast are much lower than both DOC fluxes from arctic rivers and fluxes of particulate organic carbon derived from coastal erosion. However, DOC released by coastal erosion is assumed to be more labile and could therefore be more bioavailable in the nearshore zone. DOC fluxes from massive ground ice seem to play only a minor role in the carbon budget of the Arctic Ocean. However, pore ice was not considered in this study and is assumed to be a greater source of DOC, because the interaction with the surrounding carbon-rich sediments due to freeze and thaw cycling is much stronger than for massive ground ice bodies.

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Tanski, G. (2013): Release of dissolved organic carbon from coastal erosion into the southern Canadian Beaufort Sea , Master thesis, Freie Universität Berlin / Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research.

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