A Shift to Melted Sea Ice From Runoff as the Major Component of Chukchi Shelf Open Water Freshwater Fractions, 1993-2013

Abstract:

The freshwater fraction of water that is derived from melted sea ice has increased significantly on the Chukchi shelf relative to runoff within the past decade, based upon analysis of salinity and δ¹⁸O mixing lines from a number of research cruises from 1993-2013. The shift to summertime dominance of melted sea ice (freshwater end-member δ¹⁸O >-10 per mil) relative to runoff occurred within the past ten years with a transition primarily observed from runoff dominance to sea ice melt after 2004. This shift is localized to the Chukchi shelf and does not reflect large amounts of melted sea ice flowing north through Bering Strait, which still largely transports a freshwater component with runoff origins (freshwater end-member δ¹⁸O ~-20 per mil). These observations have implications for understanding high latitude shelf biogeochemical cycling as melted sea ice carries much lower fractions of dissolved organic carbon (DOC) than runoff, allowing for greater light penetration, including through melt ponds in sea ice, and potential changes in productivity. Lower alkalinity and buffering capacity in melted sea ice compared to runoff will also increase the vulnerability of shelf organisms to water column acidification. Melted sea ice, with low DOC relative to runoff can dominate the freshwater budget in Chukchi shelf waters even under apparently continuous ice coverage. The higher transmission of light through melted sea ice with low DOC may be in part responsible for recent reported under-ice blooms on the Chukchi shelf. Since these blooms occur in waters with the freshwater budget dominated by melted sea ice, they can reasonably interpreted as being part of a continuum with other ice melt-associated blooms and not independent of sea ice retreat and dissolution.