The carbonate system of Amur Bay (sea of Japan) under conditions of hypoxia

In the course of the expedition of the Pacific Oceanological Institute in August 2007, extensive hypoxia was found in the near-bottom layer of the Amur Bay water mass. The hypoxia’s formation was immediately reflected in the values and distribution of the carbonate parameters in the near-bottom waters of the bay. The maximum values of the carbon dioxide partial pressure, the dissolved inorganic carbon, and the total alkalinity were associated with the areas of the minimum oxygen content. The microbial destruction of the dead phytoplankton greatly increased the partial pressure of carbon dioxide, which was over 2000 μatm in the hypoxia centers at a depth of about 20 m. At the same time, the carbon dioxide partial pressure in the surface waters of Amur Bay was considerably lower than that in the atmosphere. Hence, the bay’s aquatic area was a sink for the atmospheric CO₂ despite its high content in the near-bottom waters. It was shown that the excess alkalinity associated with the hypoxia sites in the near-bottom layer of water was caused by the sulfate reduction proceeding in the upper layer of the sediment.