Redox system of the bottom sediments of the western Kara Sea

Abstract

The geochemical characteristics of bottom sediments from the western Kara Sea, including the estuary of the Ob River and regions outside the zone strongly affected by the river discharge (west of the Yamal Peninsula, Novaya Zemlya Trough, and St. Anna Trough) were considered in the context of reduction–oxidation (redox) processes in seawater, bottom sediments, and pore water. The content of organic matter controls the mechanism and scale of diagenesis, which results in the redistribution of Mn, Fe, and other elements along the sedimentary profile forming redox stratification in the regions of interest. The most significant manifestation of diagenesis in the bottom sediments is the dissolution of reactive Mn and Fe compounds, diffusion in pore water, and formation of an oxidized surface layer varying in thickness from one millimeter to 0.5 m. The appearance of ferromanganese nodules was observed in the oxidized sediments. An ochre-colored oxidized layer often observed in the sedimentary sequence was formed owing to oxygen input by benthic organisms and horizontal water inflow along aquifers connected to the surface or represents a rudimentary unit resulting from nonuniform sedimentation in the past periods. Most of the Kara Sea sediments are reduced and contain iron sulfides, which give gray and black colors to the sediments, and methane, often in cavities. Even the Kara Sea regions farthest from the Ob and Yenisei estuaries exhibit the influence of river discharge on the lithological properties of sediments, OM supply, and biological production. The geological transect from the mouth of the Ob River to the St. Anna Trough is characterized by a zoning typical of the river–sea mixing region in the south and relatively isolated conditions in the north.