Arctic Ocean sediment thicknesses and upper mantle temperatures from magnetotelluric soundings

Published Arctic Ocean magnetotelluric soundings (periods <7200 s) are used to determine electrical conductances of sub-bottom materials (presumably sediments) under Wrangel Plain, Chukchi Plateau, Lomonosov Ridge, East Siberian Shelf and Nansen-Gakkel Ridge. From these conductances, sediment thicknesses are determined using a model for the electrical conductivity profile of conducting sub-bottom materials. The conductivity profile is obtained by combining Magara's empirical porosity-depth relation with Archie's Law and with a linear relation to describe the increase in seawater (filling pores) conductivity with temperature. From this model, up to 4 km of conducting materials are inferred beneath the Chukchi Plateau and Wrangel Plain locations. These thicknesses are consistent with determinations from seismic studies. Between 1 km and 4 km of sub-bottom conducting materials are derived for two East Siberian Shelf locations. Lomonosov Ridge, displaying a relief of 2.9 km, is believed to be wholly composed of conducting materials (sediments? volcanics?) since about 3.7 km of these materials are estimated for the axial portion of the Ridge. The Nansen-Gakkel Ridge has less than 0.5 km of conducting materials. Four-layer conductivity models which fit the observed long-period (> 7200 s) apparent resistivity curves of Trofimov and Fonarev have provided a range of depths to and conductivities of an upper mantle conductor. They reported depths of 165 km, 320 km and 325 km, and conductivities of 0.32 S/m, 0.43 S/m and 0.59 S/m respectively for Chukchi Plateau, Lomonosov Ridge and Wrangel Plain. The models studied here provide an uncertainty in estimating these depths (about ± 15 per cent) and conductivities (factor of 10). Temperatures in the upper mantle between depths of 100 km and 400 km are expected to be within 1100°C and 2000°C. It is not possible to derive upper mantle temperatures more precisely than this because of the large uncertainty in estimating the electrical conductivity and in assuming an ionic intrinsic conduction mechanism for an olivine upper mantle. No upper mantle conductor could be detected beneath the Nansen-Gakkel Ridge location.