F. E. Maksimov

230Th/U chronology of ore formation within the semyenov hydrothermal district (13°31′ N) at the Mid-Atlantic ridge

A radiochemical study was carried out on massive sulfides from Semyenov hydrothermal district at the Mid-Atlantic Ridge. New and published results provide evidence that 230Th/U ages obtained for massive sulfides are reliable. The sulfide deposits from the West, North-West, North-East, and East hydrothermal sites at the Semyenov hydrothermal district were formed between ∼124 ka and ∼37 ka ago. The hydrothermal activity might have started in the eastern part of the district and moved to the west by episodic ore formation.

New data on chronology of landscape-paleoclimatic stages in northwestern Russia during the late glacial and Holocene.

Two lake and bog sediments have been thoroughly studied using palynological and radiocarbon dating methods. These are the Lembolovskoye Lake section located in the southern part of the Karelian Ithmus and the Mshinskoye bog section located in the southwestern part of the Leningrad province. The data obtained allow us to reconstruct the main features of the vegetation cover evolution, the chronology for the appearance and a real distribution of the main arboreal species from the south of the Leningrad province to the north, and to construct curves of the paleoclimate parameter changes for the area under study. Thirty-six (super 14) C dates were obtained for the Lembolovskoye Lake section (7 m thick). According to those dates, the organic gyttja formation in the lake began 9870+ or -170 BP. Spore-pollen spectra with high percentage of herbs, arborescent, and shrub-birch dated back to the Younger Dryas were found out in clay layers at a depth of 6.5 m. An appearance of spruce and alder pollen is dated at 6860+ or -120 and 7510+ or -150 BP, respectively. The maximal percentage of broad-leaved species falls on the first half of the Atlantic (AT-1). Thirty-two dates were obtained for the Mshinskoye bog section (6 m thick): from 60+ or -70 to 9520+ or -170 BP (the last date fixed the beginning of peat formation); 12 palinozones from the Preboreal to the Subatlantic were recognized there. The spruce and alder pollen began to appear 7520+ or -110 and 7670+ or -130 BP, respectively. The maximal amount of broad-leaved species is observed at 4690+ or -80 BP. The detailed reconstruction of changes in vegetation communities during the Late Glacial and Holocene was correlated with paleoclimatic characteristics, which have been reconstructed for the section under study by using the information from a statistical method of spore-pollen data processing.

First 230Th/U date of Middle Pleistocene peat bog in Siberia (key section Krivosheino, Western Siberia)

A 14C date older than 53900 yrs BP was obtained for the uppermost part of the buried peat bog in Krivosheino section (Middle Pleistocene of Western Siberia). These sediments also yielded 230Th/U dates of 195−9.1+10.8 ka using the leachate alone (L/L) and 204−13+17 ka using total sample dissolution (TSD) models. Peculiarities of 230Th/U dating are discussed. Palynological investigation of the buried peat bog together with underlying and overlaying sediments, and comparison with palynological data from Baikal and Elgygytgyn lakes revealed that the peat layer in Krivosheino section was formed at the end of Shirta Interglacial (Marine Isotopic-Oxygenous stages MIS-7), when climate conditions at all studied sites were more severe compared to the modern ones.

The first case study of 230Th/U and 14C dating of mid-valdai organic deposits

From the viewpoint of precision and reliability of radioisotopic dating, deposits whose quantitative age can be determined through several methods of geochronometry are of special interest. The mutually conforming finite 14C and 230Th/U dates of buried Neopleistocene organic deposits, taken from the Tolokonka section by the North Dvina River (100 km downstream from the city of Kotlas), have been obtained for the first time in Russia. The stratigraphical reference of these results to those obtained via the optically induced luminescence for upper and lower bedding layers has been established. The presented geochronometric data have allowed us to consider the age of oxbow lake organic deposits completely reliable and refer the time of their formation to the Tyrbei warming within the MIS-3. The applicability of the new version of the 230Th/U method for dating of interglacial and interstadial deposits, for the purpose of solving the Middle and Late Neopleistocene chronostratigraphy issues, is confirmed.

Stratigraphy of bottom sediments in the Mendeleev Ridge area (Arctic Ocean)

602 Two age models are available for bottom sediments cored in the Arctic Ocean: “older” [1–5] and “younger” [6–9]. The first of them assuming very low sedimentation rates is based on interpretation of the magnetization vector in sediments and finds of partic ular stratigraphically significant microfossils (index species and assemblages). The replacement of the pos itive residual magnetization in sedimentary sections by the negative one, which was correlated with the boundary between the Brunhes and Matuayma paleo magnetic epochs, was considered as the main criterion for developing the “older” model [3]. The second model assuming higher sedimentation rates is based on amino acid dates obtained for tests of planktonic foraminifers, optically stimulated luminescence dates obtained for quartz and feldspar grains, climatostrati graphic interpretations of oxygen isotope curves, con tents of ice rafted material, and Fe and Mn minerals in sediments. In this model, older foraminifers and diatoms are considered as being reworked and inter vals with reversed magnetization of sediments are cor related with episodes of the Brunhes, not Matuyama, epoch. Detailed analysis of sediment cores obtained recently throughout the entire Arctic Ocean has stim ulated most researchers to accept the “younger” model.

The oldest seafloor massive sulfide deposits at the Mid - Atlantic Ridge: 230th/u chronology and composition

Abstract A geochronological and geochemical study on 10 samples of seafloor massive sulfides (SMS) from the inactive Peterburgskoye hydrothermal field at the Mid-Atlantic Ridge (MAR) was carried out. The 230Th/U ages of the SMS are the oldest for the Quaternary hydrothermal ores ever found at the ocean floor. According to them the hydrothermal activity at Peterburgskoye field started at least 170 ka and continued down to 63 ka. The oldest hydrothermal ores from this field consist mainly of pyrite and chalcopyrite and have geochemical properties typical for SMS associated with basalts.

Origin of high 234U/238U ratio in post-permafrost aquifers

Isotope composition and concentration (δ2H, δ18O, 234U/238U, Ar, 40Ar/36Ar, 3He/4He and 20Ne/4He) were measured in the groundwaters (Tomsk-7). Water has a distinct cold climate isotope signature δ2H = −127..−140, δ18O = −17.0..−18.2 in contrast of modern meteoric water δ2H = −117, δ18O = −15.9. Stable isotopes are in concord with noble gas temperature tNGT = 0..+4 °C and helium model ages t = 7−14 ka. Disequilibrium uranium 234U/238U up to 16 (activity ratio) obtained and explained of 234U leaching by the melt water during the permafrost degradation.

Geochronology of vegetation stages of south-east Baltic coast (Kaliningrad region) during the middle and Late Holocene

The raised bog sediments that have been continuously accumulated over time represent the most suitable natural object which enables us to reconstruct Late Glacial and Holocene vegetation and palaeoclimates. Bog peat consists of organic carbon formed in situ. It contains moss, plant fragments and microfossils that are necessary for the study of palaeovegetation and palaeoclimate. However, a successful study of palaeoenvironment can be carried out on the basis of investigation of a great quantity of samples along the whole peatbog thickness. In the present paper, the authors present the results of palynological, botanical investigations and radiocarbon dating of 31 peat samples taken from the raised bog Velikoye, located in the eastern part of Kaliningrad Region. The data obtained have enabled us to reconstruct the palaeovegetation, reveal the evolution of the bog and determine rate of peat formation at different evolutional stages over the last 7500 cal BP.

THE DEVELOPMENT OF LANDSCAPES OF THE SHKOTOVO PLATEAU OF SIKHOTE-ALIN IN THE LATE HOLOCENE

New data in the development of the basalt plateau landscapes under multidirectional changes of climate in the late Holocene is presented. Objects for paleolandscape reconstructions were lacustrinemarsh deposits of the Larchenkov marsh. The marsh arose on the site of a paleolake that has passed several stages of development. The greatest depth and area of the lake had about 3220–4000 14 С years BP in the conditions of warming and increasing moisture. At the fi nal stage of its existence, there have been periods of flooding and the shallowing with general tendency to active bogging. At the top of the sections of peat bogs, the volcanic ash B-Tm of Baitoushan volcano (the eruption 969 AD) was found. Stages in the development of landscapes were highlighted and the borders of their reconstructions were defined. Natural landscape-forming factors with the analysis of change rates of the biotic components were analyzed. Conclusions on stability of geosystems are made, and the contribution of global and regional paleoclimatic events is estimated. In forest vegetation of the plateau, the share of broadleaf species has increased during warming periods, the share of the Korean pine and dark coniferous, primarily spruce, has increased in the cold periods. In forest vegetation of the plateau, the share of broadleaf species has been increasing during warming periods, the share of the Korean pine and dark coniferous, primarily spruce, has been increasing in the cold periods. The expansion of dark coniferous forests had occurred in climatic conditions similar to modern – about 2550 14 С years BP; coniferous-deciduous forests had retreated to the edge of the plateau. The larch ( Larix komarovii ) had existed framing the paleolake during the all studied period and is a relic of the Last Glacial Maximum. The formation of modern landscapes with identifying their ages was analyzed.

The first uranium-thorium dating of the Middle Neopleistocene peat in West Siberia

For a sample from the roof of peat (0–2 cm) located in a reference section for the Middle Neopleistocene in West Siberia near the village of Krivosheino, we obtained an exorbitant value of 14C date ≥53.9 ka B.P. (LU-6024). In the peat proper, the 230Th/U dates obtained by the isochronous method are 195.2 ± 10.8/9.1 ka B.P. for the L/L model and 204.1 ± 17/13 ka B.P. for the TSD model. The palynospectra of the peat characterize, from the bottom up, birch forests with fir and spruce participation; then spruce forests with fir; next spruce-cedar forests, similar to the middle taiga subzone. In the clay, the following palynospectra have been examined: forb-gramineous grasslands and light forests with spruce and Betula fruticosa. In the upper part of clays, the palynospectra reflect the evolution of swampy, birch, light forests with spruce participation. The conclusion is made that the studied part of the section formed at the end of the Samarovo Ice Age and in the last third of the Taz Ice Age. The break in sedimentation related to the erosion contact covers a part of the Samarovo Ice Age and the first two-thirds of the Taz Ice Age.