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

Abstract

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.

Distribution of Mercury in the Water-Suspended Matter-Bottom Sediments System of the Lake Onego Water Area

“…For the homogeneous silts below, the main mineral phases are the same, but their quantitative ratio is different. In the homogeneous silts, Mn hydroxides are present in smaller quantities, while Fe hydroxides are completely absent, because they are transformed into phosphates and carbonates of Fe in the diagenesis under reducing conditions [30]. The different ratio of the main mineral phases leads to various combinations of chemical elements by groups (Figure 7).…”

“…From the point of view of the material composition, the bottom sediments and the material of sedimentation traps are identical, with the exception of the cationic composition of illite and chlorite. As part of the bottom sediments, the main part of illite and chlorite is represented by ferruginous varieties that were formed directly in the lake [25,30]. On the dendrogram of the R-type cluster analysis for the sedimentation trap material, as well as for the bottom sediments, lithophilic elements form their own groups, the isolation of which is due to the occurrence of these elements in the silicate minerals.…”

“…At the same time, the highest concentrations are observed in the oxidized part of the bottom sediments. According to radiometric dating using nonequilibrium 210 Pb [30], an increase in mercury concentrations is observed in the bottom sediments with an age up to 150 years ago, while a sharp increase has been observed since 80 years ago.…”

“…According to [30], the lake bottom is composed (from top to bottom): Holocene lacustrine deposits-silt and sand (lH, where l is lacustrine), Upper Pleistocene depositsvarved clays (lgIII, where lg-limno-glacial), glacial and fluvial glacial deposits of the Upper Pleistocene-coarse sand with pebbles, clays with boulders, boulder loams (gIII; fIII, where g is glacial, f is fluvioglacial).…”