Peculiarities of the present-day morpholithogenesis on the Laptev Sea Shelf: Semenovskaya shoal (Vasema Land)Doklady Earth Sciences

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Authors
O. V. Dudarev, A. N. Charkin, N. E. Shakhova, I. P. Semiletov, V. I. Sergienko, I. I. Pipko, S. P. Pugach, D. V. Chernykh
Year
2015
DOI
10.1134/S1028334X15050116
Subject
Earth and Planetary Sciences (miscellaneous) / Earth and Planetary Sciences (all)

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ISSN 1028334X, Doklady Earth Sciences, 2015, Vol. 462, Part 1, pp. 510–516. © Pleiades Publishing, Ltd., 2015.

Original Russian Text © O.V. Dudarev, A.N. Charkin, N.E. Shakhova, I.P. Semiletov, V.I. Sergienko, I.I. Pipko, S.P. Pugach, D.V. Chernykh, 2015, published in Doklady Akademii

Nauk, 2015, Vol. 462, No. 2, pp. 223–229. 510

The Late Quaternary evolution of the largest shelf zone in the World Ocean, the East Siberian Shelf Zone (ESSZ), is characterized by several major events that significantly affected the natural environment in the region. The most considerable changes occurred dur ing the Sartanian regression 18–16 ka B.P., when the shoreline shifted to the presentday 100m depth level.

On the dried shelf, loessice (yedoma) deposits formed under the cryogenic morpholithogenesis conditions of dry cold climate [1, 2]. The postglacial Holocene transgression, which started about 12 000 years B.P., led to flooding of the coastal plains and formation of islands in the elevated places. This is how icy islands such as Vasema Land, Sannikov Land, and Andreev

Land formed [2, 3]. According to the curve of gla cioeustatic oscillations for the East Arctic seas, Vasema

Land became an island about 8000–7000 years B.P.

About 5000–6000 years B.P., the sea level became sta ble and thermal and hydrodynamical abrasion acti vated in the coastal zone [4]. As early as 1815, only a shoal existed where earlier the island was. The shoal was known to hydrographers and named Semen ovskaya, and two small islands (Vasil’evskii and

Semenovskii) also existed there, but by 1950 they had also turned into shoals [3].

The aim of the present work is to reveal the pecu liarities of the presentday morpholithogenesis in

Semenovskaya shoal (SS), which is the largest remain der of the Late Pleistocene subaerial relief, with mini mal depths of 0.8–1.0 m [1]. The presentday location of the 10m isobath in the shallowwater zone is 130 km northeast of the Lena River delta; the length of this zone is 85 km, and the area is about 3000 km2.

Earlier, we obtained new data on the presentday state of Diomid shoal—another remainder of the Late

Pleistocene subaerial relief, along with SS, but smaller by a factor of 330 in area [5]. Study of these objects is important for understanding the factors controlling the rates of subaqueous permafrost degradation in the

ESSZ in the context of destabilization of gas hydrates hosted in the permafrost stratum [6, 7] and involve ment of organic matter from yedoma deposits into the contemporary biogeochemical cycle [8–10]. The present work is based on the results of an oceano graphic expedition carried out in September 2005 on board the Auga light vessel by the Laboratory for Arctic

Research, Pacific Oceanological Institute, Far East

Branch, Russian Academy of Sciences (POI FEB

RAS), in collaboration with the International Arctic

Research Center (IARC), University of Alaska, Fair banks, United States. To provide navigation safety in the shallowwater zone of the studied area and to col lect the maximum amount of data, a 140km section was made between slopes of Semenovskaya (station no. 111) and Vasil’evskaya (station no. 112) shoals, along the course of 230° (Fig. 1). The submarine relief was studied by using a FURUNO onboard naviga tional echosounder. The thermohaline structure of the water column was measured using an SBE 19plus

CTD probe. The bottom core was sampled using a Van

Veen grab sampler, and suspended matter was settled on filters with a pore diameter of 0.4 µm. Analytical studies of sediments were made in the laboratories of

POI FEB RAS and IARC [5, 9, 11].

Seafloor Relief

The seafloor profiles of the upper parts of the SS orthogonal slopes (depths are less than 15 m) are quasisymmetric relative to each other. The northeast ern slope is slightly convex and gentler compared to the southwestern one, with the surface bend located at the hypsometric level of 8.8 m. The southwestern slope is steeper ad concave, and its surface is at the hypso metric level of 7.2 m.

Peculiarities of the PresentDay Morpholithogenesis on the Laptev Sea Shelf: Semenovskaya Shoal (Vasema Land)

O. V. Dudareva, b *, A. N. Charkina, b, N. E. Shakhovab, c, I. P. Semiletova ,b, c, Academician V. I. Sergienkod,

I. I. Pipkoa, b, S. P. Pugacha, and D. V. Chernykha

Received November 24, 2014

DOI: 10.1134/S1028334X15050116 a Ilyichev Pacific Oceanological Institute, Far East Branch,

Russian Academy of Sciences, Vladivostok, Russia b Tomsk National Research Polytechnic University, Tomsk,

Russia c University of Alaska, Fairbanks, United States d Institute of Chemistry, Far East Branch, Russian Academy of Sciences, Vladivostok, Russia *email: dudarev@poi.dvo.ru

OCEANOLOGY

DOKLADY EARTH SCIENCES Vol. 462 Part 1 2015

PECULIARITIES OF THE PRESENTDAY MORPHOLITHOGENESIS 511

The relatively gentle slopes of the rugged relief with a depth difference of 0.9–1.3 m probably reflect a series of submarine bars formed in the zone of wave breaking. Steep slopes of relief depressions are most likely produced by the exaration activity of hum mocks. Expeditions of the Laboratory for Arctic

Research POI FEB RAS in 2008–2012 revealed mul tiple exaration furrows produced by hummock bot toms and keels [10]. Starting from 15 m depth, slopes become gentler and then gradually pass into the accu mulativeerosion submarine plain.

Thermohaline Structure of Water

In the studied area, the thermohaline structure of water is determined by interaction between river and shelf waters. Along the entire extent of the section from north to south, the salinity (S) and temperature (T) of surface water varied in the intervals of 12.6– 16.6‰ and 3.4–2.5°C, respectively (horizontal gradi ents were 0.04‰/km and 0.01°С/km, respectively).

The layer of the seasonal halocline was deepened down to 15 m (the hypsometric level of the lower part of the SS slope), and, as a result, the water structure was homogeneous at smaller depths. Nearbottom shelf waters were located below 20 m depth (S = 31.6‰, T = 1.9°С), while on the plain south of the shoal we found residual waters left over from last win ter and possessing negative temperature (S = 32.0‰,