Changing Freshwater Fluxes in the Arctic: A Tale of Melted Ice, River Runoff and the Bering Strait

The oxygen isotope composition of the upper Arctic Ocean halocline (salinity = 33.1, depth 100-200m) was established as early as the late 1980’s as having a δ¹⁸O value of approximately -1.1‰. More recent data however of the Bering Sea contributions to the upper halocline indicates a shift to a more negative isotopic delta value (~-1.6‰), suggesting an increase in freshwater fluxes through the Bering Strait. Runoff volume from Arctic rivers has also increased in the past century. Both data sets are consistent with observations of added freshwater inventories in the Canada Basin and Beaufort Gyre. Surface water analyses of oxygen isotopes furthermore indicate that a third source of freshwater, melted sea ice, is not as significant despite the accelerated recent loss of seasonal sea ice; in most cases melted sea ice provides no more than a few percent of freshwater contributions to surface seawater. The isotopic time-series of the upper Arctic halocline can be used as an independent index of freshwater flux changes through the Bering Strait. Several reasonable assumptions must be satisfied: 1) Sea ice melt contributions at the depth of the upper halocline must remain negligible, 2) 33.1 must remain the salinity of the brine-injected and influenced nutrient maximum, and 3) the isotopic composition of the freshwater end-member (-21.5‰) present in Bering Strait has not significantly changed. Given those conditions, balancing a simple end-member mixing model requires the volume of freshwater (including runoff and other meteoric water, but not sea ice melt) flowing through Bering Strait to have increased by ~45% over the past two decades to account for a change in the isotopic composition of the 33.1 salinity water from a δ¹⁸O value of approximately -1.1‰ to -1.6‰. This estimated increase is comparable with independent mooring measurements, which indicated that the Bering Strait freshwater flux rose from 2000–2500 km³ in 2001 to 3000–3500 km³ in 2011.