VNIIOkeangeologia carried out a fieldwork in Chukchi Sea in 2006 on board the “Shuya” vessel under the State program of “third generation” geological mapping scaled 1:1000000. The methodology included standard kit of techniques, seismoacoustic profiling, sonar survey, sampling by grab and piston corers, and shallow drill coring that used an original technology developed in Donetsk Technical University, Ukraine [Kalinichenko et al., 2001]. Importantly, three boreholes were drilled, near Schmidt Cape, 5.5 m deep, at the southern edge of Wrangel Island (12 m), and in the southern part of Chukchi Sea, down to 3.5 m into the sediments.

The latter hole due to its negligible depth did not heave the batch of Holocene deposits. Tow others cut three seismostratigraphic units, Upper Pleistocene – Holocene, Eopleistocene (?), and Pliocene. A distinct unconformity is observed between the Eopleistocene (?), and Pliocene seismostratigraphic units, and the Eopleistocene (?) deposits lie on an eroded top of the Pliocene rocks. Upper Pleistocene – Holocene and Eopleistocene (?) seismostratigraphic units are represented by fine marine silts and clays with mollusk shells and rare small pebbles. The Pliocene complex is built of sand and sandy silt with pebble and gravel and numerous fragments of burnt lumber.

46 samples from the first, and 114, from the second, boreholes were selected for paleomagnetic study. In the samples from the upper parts of both sections a component of direct polarity is identified. We refer this part of the sequence to the Brunes epoch. A reverse magnetization zone begins from the 3 m depth in the first, and 7 m, in the second boreholes. This likely corresponds to the Matuyama orthozone. In the upper part of this orthozone, some non-uniformities are observed in both sections: in section 1, a brief interval of frequent alternation of polarity is observed, and in section 2, a short zone of direct magnetization. Perhaps this can be related to a part of Jaramillo microzone of the Global Magnetic-Stratigraphic Scale.

Spore and pollen spectra from both boreholes show a twofold structure of the upper part of the sediment cover of the Chukchi Sea shelf. The relatively lower part of the sequence is believed to have formed in a warm environment in Late Pliocene and Eopleistocene (?), when the deposition area was occupied by pine-and-birch woods with Siberian pine, fur, alder, willow, hazel, and deciduous species. Moisture-loving species typical for swamps dominated among the herbs, but wood-free areas were generally rare. The climate was warmer than now. Such spore-and-pollen complex could be compared with those of Pestsovskaya Formation of Chukotka Peninsula [Petrov, 1966] and Kolvinskaya Formation of the Timan-Urals region [Zarkhidze, 1992]. The upper part of the sequence was formed in Late Pleistocene and the Holocene when a forest-tundra and tundra vegetation occupied the considered territory.

The microfauna analysis of samples showed that foraminifers were absent in the lower parts of the sections of both boreholes: below 3 m in the first, and below 7 m, in second borehole. Thus, a continental or subcontinental genesis was proposed for the lower sediment batches, i.e. for the Pliocene unit and the bottom part of Eopleistocene (?) unit. Microfauna samples from the rest of Eopleistocene (?) deposits show from 6 to 11 species, cold-water Nonionidae, Retroelphidium, and Cribroelphidium definitely prevailing. Numerous agglutinated forms and underdeveloped shells are found. Such species as Elphidium origonense, Retroelphidium selseyense, Sigmomorphina sp., Quinqueloculina longa indicative of marginal layers between Pliocene and Eopleistocene at the north of Chukotka Peninsula occur in the said samples. The samples from the Upper Pleistocene – Holocene deposits include from 15 to 34 species. Arctic speies predominate, Guttulina lactea (Walk. Et Jacob), Buccella troizkyi Gud., Cribrononion obscurus Gud, Haynesina orbicularis (Brady) as well as some others – Arctic-Boreal (Buccella frigida (Cushm.), Cribrononion incertus (Will.), Nonionella auricular (H.-A.-et Earl.), Retroelphidium atlanticum Gud. And so forth). Boreal-Arctic and boreal species are notably more scarce.

The diatoms were found only in the upper part of the borehole 2 section above 5.5 m. These are represented mainly by marine species typical for modern Arctic seas or by redeposited shells of extinct Neogene species. Diatoms are not abundant in Eopleistocene (?) deposits, just accidental shells of marine cold-water Arctic-Boreal species are reported – Thalassiosira gravida (spores), T. nordenskioeldii. Bacterosira bathyomphala and some others, and also some relatively warm-water species (Coscinodiscus radiatus, Thalassiosira anguste-lineata, etc.). This is common for marine Eopleistocene diatom associations of Enmakai Formation of Northern Chukotka. The amount of diatoms rapidly increases in the Upper Pleistocene and Holocene sediments cut by the boreholes. Dominant are the species that commonly occur in modern Arctic plankton (Thalassiosira gravida+T.antarctica, T.nordenskioeldii, T.hyperborea and others), as well as ice-hosted marine diatoms (Fossula arctica, Fragilariopsis oceanica, Attheya septentrionalis and so on), which naturally evidence for an ice coverage of Arctic seas.

The distribution of organic carbon and carbonate carbon in the sedimentary sequence as described in Borehole 2 correlates well with the paleomagneitc, microfauna, and spore-pollen data. Thus, the organic carbon and carbonate carbon rates within the 0-316 cm interval correspond to background values for modern terrigenous Holocne deposits of the Arctic shelf and are, respectively, 0.06% and 0.72%. Down the section, a natural diagenetic loss of carbon takes place from 0.95% to 0.51%.

In the 450-540 cm interval, with the generally constant carbonate level, the organic carbon drastically increases up to 1.82% that can be explained by change of marine environment to continental one and related increase of income of humus sedimentary material. Another rapid increase of the organic carbon is observed in the 613-623 cm interval and can be interpreted by shallow-water continental sedimentation environment that is confirmed by ostracoda abundance in the said interval.

Further down (>8 m), the organic carbon soars up again (1-3%) and so does the carbonate carbon (0.14-0.33%). We believe this documents a change of facies that could be related to the Holocene climatic optimum combined with sedimentation in continental shallow basins with considerable income of humus organics.

Our data generally support the stratigraphic models of Neogene–Quaternary sedimentary cover suggested for adjacent areas of Chukotka Peninsula [Petrov, 1966], Wrangel Island [Gualtieri et al., 2003], and East-Siberian Sea shelf [Puminov, 1981]. Still, the sections of Pliocene-Quaternary deposits dated based on paleomagnetic and biostratigraphic data were obtained for the first time at the Chukchi Sea shelf.

1.             Gualtieri L., Vartanyan S., Brigham-Grette J., Anderson P.M. Pleistocene raised marine deposits on Wrangel Island, northeast Siberia and implications for the presence of an East Siberian ice sheet. Quaternary Research, 2003, vol. 59, p. 399-410.

2.      Kalinichenko O.I., Karakozov A.A., Zybynskiy P.V. New methods and technology of marine drill system for drilling on shelf. Scientific proceedings, Donetsk State University, Geological series. Vol. 36, 2001, p. 144-148.

3.      Petrov O.M. Stratigraphy and mollusk fauna from Chukotka Peninsula Quaternary deposits. GIN RAS Proceedings, Vol. 155, Moscow, 1966, 252 p.

4.      Puminov A.P. Stratigraphy of Cenozoic sediments of East Russian Arctic Shelf. In: Geology and mineragenya of the Arctic part of USSR. 1981, p. 7-27.

5. Zarkhidze V.S. Paleogene and Neogene history of the Arctic Ocean. Geological history of the Arctic in Mezozoic and Cenozoic times. Vol. 2, St. Petersburg, 1992, p. 6-28.

Reference to this abstract: 

 Gusev E., Rekant P., Popov V., Iosifidi A., Polyakova E., Derevyanko L., Anikina N., Litvinenko I., Klyuvitkina T., Usov A. Pliocene-Quaternary sediments of Chukchi sea shelf. Arctic Palaeoclimate and its Extremes (APEX), abstract volume, 2008, p. 14-16.