Mechanisms controlling the distribution of helium and neon in the Arctic seas: the case of the Knipovich Ridge

Abstract

Helium concentration and isotopic ratio in the ocean are influenced by four sources: the mantle, atmosphere, crust and tritium decay. In addition, the concentration of dissolved gases can be altered by air injection and ice-related mechanisms such as brine rejection, sea-ice melting and glacial melting, especially in the high-latitude seas. The Knipovich Ridge, a northern continuation of the Mid-Atlantic Ridge (74–79°N, 6–8°E), is potentially affected by all the sources and mechanisms mentioned above. This suggests that the ridge should be a good place to investigate these mechanisms. During the Knipovich 2000 expedition, water samples were collected for helium and neon analysis along the axis of the ridge. Although Mn and CH 4 results suggest the existence of hydrothermal activity in the ridge, we could not detect a significant increase of helium isotopic ratio ( 3He/ 4He) at corresponding water depths. Instead, the bottom waters in the northern section of the ridge were supersaturated by up to 40% for 4He. The helium isotopic ratio of 1.3×10 −6 indicates the dominance of atmospheric helium. Using He and Ne saturation anomalies, we further explored the mechanisms affecting the distribution of helium and neon in the ridge. Around 5% supersaturation of He and Ne at most deep waters is explained by air injection and brine rejection. However, the large excess in the northern ridge was attributed to the input of 2.5% glacial meltwater, possibly originating from the glacier-covered islands, Svalbard.