Tracing the source of deep water in the Arctic Ocean with 17Oexcess of dissolved O2

Abstract:

The ¹⁷O_excess of dissolved O₂ (¹⁷Δ) in the ocean is a unique property which is useful for telling apart O₂ produced by marine photosynthesis (bio-O₂) from atmospheric O₂. Unlike O₂ concentration, ¹⁷Δ is not affected by respiration and thus behaves conservatively in the deep sea. In general, ¹⁷Δ in the oceanic mixed layer is low due to the dominance of air-sea gas exchange. In contrast, in the Arctic mixed-layer ¹⁷Δ is higher because sufficient light penetrates through the sea-ice cover and drives photosynthesis, but air-sea gas exchange is retarded by sea ice cover. We have preliminary ¹⁷Δ data from depth profiles in the Eurasian and Makarov basins. In both, the fraction of bio-O₂ is about 20 % in the surface mixed layer. However, the vertical distribution beneath the mixed layer at the two stations is substantially different. In the Makarov Basin there is a layer of Pacific Water centered at about 100 m, which enters the Arctic Ocean through Bering Strait and is modified as it flows across the wide Chukchi and Siberian shelves. It has a strong maximum in ¹⁷Δ, equivalent to ~30% bio-O₂. ¹⁷Δ then decreases through the underlying halocline to a minimum between 500 and 700 m, which lies within the Barents Sea Branch of Atlantic Water (BSBW) indicating ~15% bio-O₂. At the Eurasian Basin station, ¹⁷Δ decreases from the mixed layer through the halocline reaching a minimum at the temperature maximum of Atlantic Water. This temperature maximum marks the core of the Fram Strait Branch of Atlantic Water (FSBW). ¹⁷Δ then increases to a maximum indicating ~20% bio-O₂ between 500 and 700 m. The BSBW is produced as Atlantic Water flows through the shallow Barents Sea becoming denser than FSBW and enters the Eurasian Basin through the Santa Anna Trough beneath the FSBW. Our ¹⁷Δ measurements suggest that waters of Pacific and Atlantic origin that transit across the wide Arctic continental shelves acquire a high ¹⁷Δ signal indicative of photosynthesis in ice covered water.