Interglacial History of a Palaeo-lake and Regional Environment: A Multi-proxy Study of a Permafrost Deposit from Bol’shoy Lyakhovsky Island, Arctic Siberia

Chironomid, pollen, and rhizopod records from a permafrost sequence at Bol’shoy Lyakhovsky Island (New Siberian Archipelago) document the development of a thermokarst palaeo-lake and environmental conditions in the region during the last Interglacial (MIS 5e). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool initially existed at the site. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in the basin, which could in turn induce thermokarst and create a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. The chironomid record points to three successive stages during the history of the lake: (1) a peatland pool; (2) a pond (i.e., shallower than the maximum ice-cover thickness); and (3) a shallow lake (i.e., deeper than the maximum ice-cover thickness). The trend of palaeo-lake development indicates that intensive thermokarst processes occurred in the region during the last Interglacial. Shrub tundra communities with Alnus fruticosa and Betula nana dominated the vegetation during the interglacial optimum. The climate was moister and warmer than present. The results of this study suggest that quantitative chironomid-based temperature reconstructions from Arctic thermokarst ponds/lakes may be problematic due to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblages.     

中国东部苏鲁榴辉岩超基性岩褶皱带的演化及其与大别(中国)和柯切塔夫(哈萨克斯坦)杂岩的对比(英文)

苏鲁褶皱带形成于元古宙 (2 2 33～ 185 5Ma)典型优地槽构造环境 ,主要由石榴橄榄岩、石榴辉石岩、榴辉岩等侵入岩 (柯石英深度相地幔岩浆房中形成 )和它们的火山沉积建造围岩一起经褶皱、变质而形成。变质作用经历了先蓝片岩相 (前花岗岩 )后片麻岩混合岩相过程。由于变质作用的不规律性 ,苏鲁褶皱带可分为 2个构造带 :(1)东部构造带 (蓝片岩 )和 (2 )西部构造带 (片麻岩混合岩 )。根据A·都城秋穗所识别的变质带系统 ,可将其作为一个双变质带。东部构造带以出现许多块状、条带状榴辉岩辉石岩橄榄岩组合的残余岩块为特征 ,其中还残留着高压的矿物 (石榴石、绿辉石、柯石英 ) ,而且有被混合岩和各种交代岩替代的显著标志。在中生代 ,苏鲁元古褶皱带受造山作用的影响活化 ,导致许多花岗岩体的侵入 ,使交代岩广泛发育。     

Faults in the Baikal region: morphostructural and structure-genetic features (case study of the Buguldeika fault junction)

Lineament analysis is applied to map the pattern of the Obruchev fault system in the Buguldeika Village area, where several fault zones (Olkhon, Primorsky, Prikhrebtovyi, Buguldeika, and Kurtun) make up a junction. As inferred from the predominant directions of genetically related lineaments, the Olkhon, Primorsky, and Prikhrebtovyi faults originated under NW-SE extension and compression. The extension and compression settings within these zones are reconstructed by analysis of lineaments that have prominent and poor geomorphic expression, respectively. However, the pattern of lineaments well expressed in the surface topography within a weakly deformed block corresponds to reverse slip, while that of poorly pronounced lineaments corresponds to left-lateral strike slip. The study confirms the idea that the latest extension (rifting) stage in the Baikal region reactivated fault zones but did not deform blocks. The blocks store record of residual deformation produced by previous settings of regional compression and shear. The obtained results agree with earlier tectonophysical analysis of faults and fractures in the area and prove the applicability of the suggested approach to map the fault patterns and reconstruct their respective stress settings in areas that underwent multiple deformation events of different ages.     

Composition of the suspended particulate matter at the Severnaya Dvina River mouth (White Sea) during the spring flood period

The grain-size and mineral composition of the suspended particulate matter (SPM) of the Severnaya Dvina River mouth is studied, as well as the content of several lithogenic elements in the SPM during the spring flood in May 2004. The data published on the composition of the riverine SPM in the White Sea basin are very poor. The spring flood period when more than half of the annual runoff is supplied from the river to the sea in a short time is understood more poorly. The report considers the comparison results for the grain-size compositions of the SPM and the bottom sediments. The data of laser and hydraulic techniques of the grain-size analysis are compared. The short-period variations of the SPM concentration and composition representing two diurnal peaks of the tide level are studied. It is found that the SPM is mainly transferred during the spring flood as mineral aggregates up to 40 μm diameter. The sandysilty fraction of the riverine SPM settles in the delta branches and channels, and the bulk of the fine pelitic matter is supplied to the sea. The mineral and chemical composition of the Severnaya Dvina River SPM is determined by the supply of substances from the drainage basin. This substance is subjected to intense mechanic separation during the transfer to the sea. The key regularities of the formation of the mineral composition of the SPM during the flood time are revealed. The effect of the grain-size composition of the SPM on the distribution of the minerals and elements studied in the dynamic system of the river mouth are characterized.     

南乌拉尔玛克苏托夫榴辉岩蓝片岩杂岩体的岩石学

玛克苏托夫 (Maksyutov)榴辉岩是乌拉尔—昆仑—祁连—秦岭—大别山元古代褶皱带之一部分。榴辉岩呈团块或布丁夹于片麻岩内 ,由于显生宙与花岗岩侵入有关的退变质叠加影响 ,时代变化自元古代至下古生代。榴辉岩矿物组分恒定但不同程度受绿帘蓝片岩相退变质叠加 ,榴辉岩中石榴石以富铁为主 (f=50 %～ 75% ) ;单斜辉石为绿辉石 Jd40 .3～ 52 .2。据 Krogh石榴子石—单斜辉石温度计 ,榴辉岩形成温度为 1 0 60～ 686℃ ,压力为 2 2 .3× 1 0 5～ 1 4.1 6× 1 0 5k Pa;PT轨迹具岩浆 (榴辉岩浆 )自上地幔向下地壳上侵的特点。     

On the elemental composition of suspended matter of the Severnaya Dvina River (White Sea region)

New data on the elemental composition of the Severnaya Dvina River, the largest one in the White Sea region, are presented. The elemental composition of the river water in May, the period of the snowmelt flood, is similar to the upper layer of the Earth’s continental crust due to the active erosion of the earth material in the catchment area. In August, the period of the summer low water, the impact of biogenic components increases and elevated concentrations of Cd, Sb, Mn, Zn, Pb, and Cu are observed. At other times, no significant pollution by heavy and rare-earth elements is registered.     

Chromium solubility in MgSiO3 ilmenite at high pressure

The crystal structure and chemical composition of crystals of (Mg_1?x Cr _x )(Si_1?x Cr _x )O₃ ilmenite (with x = 0.015, 0.023 and 0.038) synthesized in the model system Mg₃Cr₂Si₃O₁₂–Mg₄Si₄O₁₂ at 18–19 GPa and 1,600 °C have been investigated. Chromium was found as substitute for both Mg at the octahedral X site and Si at the octahedral Y site, according to the reaction Mg²⁺ + Si⁴⁺ = 2Cr³⁺. Such substitutions cause a shortening of the <X–O> and a lengthening of the <Y–O> distances with respect to the values typically observed for pure MgSiO₃ ilmenite and eskolaite Cr₂O₃. Although no high Cr contents are considered in the pyrolite model, Cr-bearing ilmenite may be the host for chromium in the Earth’s transition zone. The successful synthesis of ilmenite with high Cr contents and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can help in the understanding of the seismic velocity and density profiles of the transition zone and the constraining composition and mineralogy of pyrolite in this area of the Earth.     

X-ray single-crystal and Raman study of knorringite,Mg3(Cr1.58Mg0.21Si0.21)Si3O12, synthesized at 16 GPa and 1,600 °C

The crystal structure of a knorringite-type compound, Mg₃(Cr_1.58Mg_0.21Si_0.21)Si₃O₁₂, synthesized in a multi-anvil press at P = 16 GPa and T = 1,600 °C, was refined from single-crystal X-ray diffraction data up to R = 2.36 % for 314 independent reflections. Garnet was found to be cubic and have space group Ia $\overline{3}$ d, with the unit cell parameters a = 11.5718 (1) Å, V = 1,549.54 (2) Å³. The knorringite crystal studied contains 21 mol% of majorite end-member. The structural characterization of knorringitic garnet is important because the study of its thermodynamic constants provides new constraints on thermobarometry of peridotitic garnet assemblages of the lowermost upper mantle. The Raman spectra of synthetic knorringite have been obtained for the first time.     

Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data

Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg₃Al₂Si₃O₁₂ (Prp)-Na₂MgSi₅O₁₂ (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi₂O₆ (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na₂CO₃ are presented. Prp-Na₂MgSi₅O₁₂ join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na₂O) and excess Si (3.05–3.15 f.u.). Maximal Na₂O concentrations (1.5 wt % Na₂O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na₂MgSi₅O₁₂ component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na₂O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na₂O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na₂CO₃ system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na₂O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na₂O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with higher sodium concentrations (>1 wt % Na₂O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors.     

The 35th Session of the International Geological Congress,Cape Town, 2016

The results of the 35th Session of the International Geological Congress held in 2016 in Cape Town (South Africa) are discussed in this paper. The major scientific problems discussed at the most popular sections are considered. Among them are the early stages in the evolution of the Earth, studies of mineral resources worldwide, and the problems of the deep structure of the Earth, as well as modern fields of mineralogy.