Structure of Late Pleistocene and Holocene Sediments in the Petrozavodsk Bay, Lake Onego (NW Russia)

Here, we present new results from seismic, geological, and geochemical studies conducted in 2015–2019 in the Petrozavodsk Bay of Lake Onego, NW Russia. The aims of these investigations were to (i) to characterize the structure of Quaternary deposits and (ii) provide new evidence of modern geodynamic movements and gas-seepage in Holocene sediments. The structure of the recovered deposits was composed of lacustrine mud, silt and sands from the Holocene, limno-glacial clays (varved clays) from the Late Glacial–Interglacial Transition, and glacial deposits (till) from the Late Pleistocene. The thickness of these deposits varied in different parts of the bay. Many pockmarks created by gases escaping and reaching sediment-water interface were observed in these deposits. Such pockmarks can play a significant role in the geochemical and biological processes in the bottom sediment surface, and gases that escape might modify the physicochemical characteristics of the environment.

On closed-form expressions for the approximate electromagnetic response of a sphere interacting with a thin sheet — Part 1: Theory in the frequency and time domain

In mineral exploration and geologic mapping of igneous and metamorphic terranes, the background is often dominantly resistive. The most important electromagnetic interaction is between a discrete conductor and an overlying sheet of conductive overburden (e.g., glacial clays or weathering products of the basement rocks). To enable the electromagnetic modeling of these common situations, here I provide closed-form expressions for the approximate electromagnetic response of a sphere embedded in highly resistive rocks and interacting with an overlying thin sheet. The sphere is assumed to be dipolar and excited by a locally uniform field. The expressions in the time and frequency domains are represented as sums of complete and incomplete cylindrical functions. New asymptotic approximations are provided for the efficient evaluation of the required incomplete cylindrical functions. The frequency-domain formulas are validated by numerical transformation to the time domain and comparison to the time-domain solution.

The chronostratigraphy of Holocene sediments from four Lake Superior sub-basins1Laboratory for Stable Isotope Science (LSIS) Contribution 264.

Radiocarbon dates, rhythmite occurrences, mineralogy, grain size, and stable isotopic compositions of detrital calcite obtained for four sediment cores from Lake Superior have been used to produce a chronology for these fine-grained deposits over the last ∼10 500 cal BP (calibrated years before A.D. 1950). Most of the rhythmites have been interpreted as annual laminations (varves) based on systematic variations in the carbonate content of rhythmite couplets and correlation of the rhythmite packages with varve sequences reported previously for Lake Superior sediments. Glacial sediments in these cores, which consist of red and grey clay, range in age from ∼10 500 to 8800–8700 cal BP. Distinctive carbon and oxygen isotopic compositions of allochthonous calcite in the red versus grey glacial clays point to different sources for this detritus. Final termination of glacial meltwater input into the Lake Superior Basin occurred between 9000 and 8700 cal BP. The significant decrease in sediment carbonate content over this time is a convenient marker for the start of postglacial sedimentation across the Lake Superior Basin.