ARCUS Student Award | 2nd Annual Award

• Cuffey
• Ekwurzel
• Moosavi
• Person
• Index

Abstract

Recent evidence indicates erosion of the highly stratified Arctic Ocean halocline in the Eurasian Basin. To understand the evolution of the halocline observed in the 1990s, we must identify the freshwater source component distribution within the halocline and quantify any changes. Three icebreaker cruises during summer and fall provide tracer data between Fram Strait and Bering Strait: ARK IV/3 (FS Polarstern in 1987); ARCTIC91 (Oden); and AOS94 (CCGS Louis S. St-Laurent). Salinity, nutrients, oxygen, tritium, helium, and d¹⁸O data were used to evaluate the freshwater sources with salinity less than the Atlantic Water inflow through Fram Strait: Pacific Water, river runoff, and sea-ice meltwater. Similar tracer ages were found across the entire basin along halocline isopycnal surfaces. Halocline mean 3H/3He apparent age measured within 33.1 ± 0.3 psu is 4.3 ± 1.7 years and for 34.2 ± 0.2 psu is 9.6 ± 4.6 years. Isopycnal mixing is dominant within the halocline. However, there are lateral variations in the relative age structure suggesting, for example, slower flow over the Nansen-Gakkel Ridge. Sea-ice meltwater is the only freshwater component in the southern Nansen Basin (between Svalbard and the Nansen-Gakkel Ridge), consistent with Atlantic Water melting of sea-ice as an important process for local halocline formation. The river-runoff fraction results suggest the majority of Pechora, Ob, Yenisey, Kotuy, and Lena river waters do not flow off the shelf closest to their river deltas, but stay on the shelf and travel in a cyclonic circulation into the East Siberian Sea where most of the river-runoff flows off the shelf at the Mendeleyev Ridge. Where 1994 measurements overlap with 1991 data, we evaluate changes that occurred during this interval. At almost the identical station location (ca. 89°N, 145.5°E) over the Lomonosov Ridge, the 1994 surface Pacific Water Mass fraction dropped to 4% from over 18% measured in 1991. The river-runoff and sea-ice meltwater fractions seem to be in steady-state, at least at this central Arctic location and between 1991 and 1994. Variability in Pacific Water mass distribution over these three years is also evident when compared to the past several decades. ARCTIC91 and AOS94 Pacific Water fraction results document a decrease in extent of Upper Halocline Water, the traditionally defined core of the Pacific Water mass, compared to historical data. This decrease in an important source of fresher water has the potential to influence the salinity of Arctic Ocean water outflow through Fram Strait, which may influence deep water formation areas downstream.