Paleoenvironmental changes in the northern shelf of the Sea of Okhotsk during the Holocene

The combined micropaleontological (spores and pollen, diatoms, benthic foraminifers), lithologic, and isotopic-geochemical analysis of sediments from the northern shelf of the Sea of Okhotsk recovered by hydrostatic corer from the depth of 140 mbsl elucidated environmental changes in this part of the basin and adjacent land areas during the last 12.7 thousands cal. years. Geochronological scale of the core is established using the acceleration mass-spectrometry method for radiocarbon dating of benthic Foraminifera tests. The first insignificant warming in the northern part of the sea after glaciation occurred in the mid-Boreal time (9.6 ka ago) but not at the onset of the Holocene. The strongest warming in the region took place in the mid-Atlantic epoch to reach climatic optimum in the second half of the Subboreal (6 to 2.5 ka ago). A cooling in the northern shelf and adjacent land areas is established at the beginning of the Subatlantic (2.5 ka). A comparison of results obtained for Core 89211 with dated hydrological and climatic changes in central and southern parts of the Sea of Okhotsk (Gorbarenko et al., 2003, 2004) is used for a high-resolution analysis of climatic fluctuations in the study region and other areas of the basin during deglaciation and the Holocene.     

Deep structure and seismotectonics of the Southern Sea of Okhotsk region along a profile from southern Sakhalin to the southern Kuril Islands

The results of deep seismic profiling through Southern Sakhalin, the southern Sea of Okhotsk, and the Southern Kuril Islands allowed the identification of deep fault zones, the hypocenter locations, the features of the stress state, and the types of seismic dislocations at the earthquake sources. The east side of the fault was upthrown relative to the west side beneath the southern part of the Tatar Strait and Sakhalin Island and led, as a result of multiple thrusting events along the fault over the geologic history, to the rise and 5-8 km displacement of the seismic boundaries. The true uplift of the Greater Kuril arc block was determined using the focal mechanism solutions. The seismoctectonics and present-day dynamics of the crustal blocks were estimated using a detailed joint analysis of the position of the structural boundaries at the seismic section and the seismotectonic movements according to the earthquake focal mechanisms.     

Stratigraphic and structural characteristics of the Romanian Black Sea shelf

The Western Black Sea basin opened during Cretaceous times by back-arc rifting in association with a north dipping subduction at the rear of the Cretaceous–Early Tertiary Pontide volcanic arc. The sedimentary wedge developed on the shelf of the Romanian Black Sea sector reflects a complex interplay between large scale rifting, uplift of the orogenic flanks, large-scale post-rift subsidence and sea level changes. We examine the detailed structural configuration of this sector for a regional correlation with the adjacent offshore in Ukraine and Bulgaria. The evolution of the western Black Sea basin started in the Albian–Cenomanian times, when two extensional phases with significantly different directions (N–S and subsequently E–W) lead to the formation of a complex interplay between isolated blocks organised in horsts and grabens generally deepening eastwards. Superposition of normal faults footwall blocks from the two extensional episodes generated a deeply subsided area with enhanced accommodation space, i.e., the Histria Depression, and, consequently, recorded a larger thickness of Paleogene sediments in the post-rift stage. (Re)activation of faults and associated folding reflects repeated inversion during the Late Cretaceous–Oligocene times, associated with subsequent periods of non-deposition and/or erosion during moments of basin fill exposure. These periods of inversion recorded in the Black Sea are controlled by coeval orogenic deformations taking place in the Balkans, Pontides and the Crimean thrust belt. Sea level fluctuations during the Neogene and late Alpine tectonics in the neighbouring orogens caused massive sedimentation followed by sediment starvation and/or significant erosion. Large thicknesses of sediments accumulated during the Pontian, presumably associated with an extensional episode deepening the distal parts of the basin and with differential compaction structures. The interpretation of a high-quality seismic dataset combined with published data allowed the correlation of major structural units and lineaments defined onshore towards the Carpathians with the ones deeply buried below the western Black Sea basin sediments. Unit correlations are furthermore used to derive an integrated tectonic image of the western Black Sea area.     

East Sakhalin island arc paleosystem of the Sea of Okhotsk region

It has been established that volcanic rocks of the Schmidt, Rymnik, and Terpeniya terranes are fragments of the compound Early to Late Cretaceous-Paleogene East Sakhalin island arc system of the Sea of Okhotsk region. This island arc paleosystem was composed of back-arc volcano-plutonic belt, frontal volcanic island arc, fore-arc, inter-arc, and back-arc basins, and the Sakhalin marginal paleobasin. The continental volcanic rocks dominate in the back-arc volcano-plutonic belt and frontal volcanic island arc. The petrochemical composition of basalts, basaltic andesites, andesites, and trachytes from the frontal island arc formed in submarine conditions are typical of oceanic island arc or marginal sea rocks (IAB). The petrochemical composition of volcanic rocks from the island arc structures indicates its formation on the heterogeneous basement including the continental and oceanic blocks.     

Perspectives in reconnaissance for coastal-marine placers on southern and western continental shores of Okhotsk Sea

The resource base for the Russian tin industry is surveyed district by district, with emphasis on deposit geology and mineralogy, world and domestic market conditions, production linkages between mines and processing facilities, and exploration activity. Current Russian output is less than one-half the former Soviet total of the mid-1980s, a reflection of difficult economic conditions, divergent interests of mine producers and the management of the Novosibirsk tin smelter, and declining ore tenor at certain major deposits. Major deposits now are found exclusively in Russia's Far East economic region, often in remote and climatically inhospitable locations. Unlike most other major producing countries, the bulk of Russia's output comes from lode deposits rather than placers. A concluding section outlines factors expected to affect future tin demand in Russia and describes efforts to identify potentially promising nontraditional tin ore deposits.     

Paleoecology of late-glacial peats from the bering land bridge, Chukchi Sea shelf region, northwestern Alaska

Insect fossils and pollen from late Pleistocene nonmarine peat layers were recovered from cores from the shelf region of the Chukchi Sea at depths of about 50 m below sea level. The peats date to 11,300−11,000 yr B.P. and provide a limiting age for the regional Pleistocene-Holocene marine transgression. The insect fossils are indicative of arctic coastal habitats like those of the Mackenzie Delta region (mean July TEMPERATURES = 10.6–14°C) suggesting that 11,000 yr ago the exposed Chukchi Sea shelf had a climate substantially warmer than modern coastal regions of the Alaskan north slope. The pollen spectra are consistent with the age assignment to the Birch Interval (14,000–9000 yr B.P.). The data suggest a meadow-like graminoid tundra with birch shrubs and some willow shrubs growing in sheltered areas.     

Biostratigraphy of Upper Quaternary sediments in the southern part of the Oceanology Institute Rise,Sea of Okhotsk

The diversified analysis (data on radiocarbon ages, diatoms, planktonic and benthic foraminifers) of sediments from the central part of the Sea of Okhotsk (core from a depth of 1186 m) made it possible to define five sedimentary units corresponding to five marine oxygen-isotope stages MIS 5-MIS 1 (the last 120 ka) and provide their thorough paleontological characteristic, using different microfossil groups. Microfossil assemblages demonstrate variations in their abundance, distribution of individual species, and representatives of different ecological groups through the section.     

Holocene Siliceous Sediments in the Sea of Okhotsk

The mineral and chemical compositions and physical properties of diatomaceous clayey–siliceous sediments in the Sea of Okhotsk are studied. Absolute masses of silica accumulation are determined. Their compositional model based on the silica content is similar to that of Late Jurassic and Olenekian–middle Anisian cherts of the Sikhote Alin region. The thickness of the Holocene siliceous unit and the absolute mass of siliceous deposits depended on bioproductivity in the upper water column and the seafloor topography. Absolute masses of SiO₂ am (0.05–5.7 g cm^–2 ka^–1) and SiO₂ fr (0.5–11.6 g cm^–2 ka^–1) are minimal on seamounts and maximal in depressions near foothills. These values match absolute masses of SiO₂ fr accumulations in Triassic and Late Jurassic basins of the Sikhote Alin region (0.33–3 g cm^–2 ka^–1). Comparison of the composition and absolute masses of silica shows that Triassic and Late Jurassic siliceous sequences of Sikhote Alin could be accumulated in the marginal marine basin near a continent.     

New data on the structure of the Sea of Okhotsk bottom

The reinterpretation of the time and deep sections along CDP Profile 1632 revealed different previously unknown structural features: (1) a buried bar and a Late Pliocene regional unconformity, the Bol’sheretsk and Lebed subaerial shield volcanoes, and the Koni-P’yagina and Magadan megadikes at its level; (2) morphological evidence for the nappe structure of the Okhotsk Arch (homoclinal thrust slices of the acoustic basement with ramp half-grabens in the frontal part and a pull-apart zone with a solitary diapir in the Kol’skii Trough, which divides the arch into two allochthonous megablocks with opposite vergence of their thrusts), which is related to a divergent sheet decollement, probably, at the M discontinuity.     

Tectonic evolution of the southern margin of Laurasia in the Black Sea region

The Black Sea region comprises Gondwana-derived continental blocks and oceanic subduction complexes accreted to Laurasia. The core of Laurasia is made up of an Archaean–Palaeoproterozoic shield, whereas the Gondwana-derived blocks are characterized by a Neoproterozoic basement. In the early Palaeozoic, a Pontide terrane collided and amalgamated to the core of Laurasia, as part of the Avalonia–Laurasia collision. From the Silurian to Carboniferous, the southern margin of Laurasia was a passive margin. In the late Carboniferous, a magmatic arc, represented by part of the Pontides and the Caucasus, collided with this passive margin with the Carboniferous eclogites marking the zone of collision. This Variscan orogeny was followed by uplift and erosion during the Permian and subsequently by Early Triassic rifting. Northward subduction under Laurussia during the Late Triassic resulted in the accretion of an oceanic plateau, whose remnants are preserved in the Pontides and include Upper Triassic eclogites. The Cimmeride orogeny ended in the Early Jurassic, and in the Middle Jurassic the subduction jumped south of the accreted complexes, and a magmatic arc was established along the southern margin of Laurasia. There is little evidence for subduction during the latest Jurassic–Early Cretaceous in the eastern part of the Black Sea region, which was an area of carbonate sedimentation. In contrast, in the Balkans there was continental collision during this period. Subduction erosion in the Early Cretaceous removed a large crustal slice south of the Jurassic magmatic arc. Subduction in the second half of the Early Cretaceous is evidenced by eclogites and blueschists in the Central Pontides and by a now buried magmatic arc. A continuous extensional arc was established only in the Late Cretaceous, coeval with the opening of the Black Sea as a back-arc basin.     

Postglacial land uplift patterns of south Sweden and the Baltic Sea region

Comparison of the land uplift pattern for the last 10,300 years, shown by studies of raised shorelines of the Baltic Ice Lake, with the pattern of present-day land uplift of Fennoscandia, shows that significant regional changes of uplift pattern have taken place. Some of these changes seem to be related to a halt in ice retreat during the Younger Dryas cold stage. It is also probable that some observed anomalies in the present uplift are not of glacio-isostatic origin, but are possibly related to structures in the lower lithosphere and upper mantle or large scale tectonics.     

Origin of diamictons on the Barents Sea shelf

Occurrence conditions and lithological-paleontological characteristics of two diamicton units developed in the southeastern segment of the Barents Sea shelf are considered. It is shown that they are lithologically analogous to the present-day (undoubtedly aqueous) diamicton mud. Correlation between bulk densities of both varieties and consolidation degree (consistency) is described by a single regression equation. We present pieces of evidence contradicting the widespread point of view about the diamicton as a glacial till moving along the seafloor. They are primarily represented by paleontological remains, mainly foraminiferal assemblages. It is shown that dislocations observed in cores and traditionally considered as glaciotectonic movements are confined to fault zones in the acoustic basement and they represent fragments of secondary neotectonic structures. Ridges and hills mapped previously as glacial accumulation morphostructures reflect a relict topography of the recent subaerial shelf development not masked by sedimentation so far.     

Prospects for gold and other economically valuable minerals in volcanogenic formations of northern Sea of Okhotsk coastal region

We review data for the Tuva–Mongolia Massif and show that this massif was not derived from the Siberian Craton.     

Miocene-pleistocene volcanism of deep-water basins of the Sea of Japan and Sea of Okhotsk

This paper reports the results of petrographic and geochemical studies of the Miocene-Pleistocene volcanic rocks that accompanied the formation of the deep-water basins of the Sea of Japan and Sea of Okhotsk. The geochemical types of these rocks, their geodynamic settings, and their derivation from different magmatic sources were determined. The marginal-sea basaltoids of the Sea of Japan are derivatives of fluid-enriched mantle (EMI), while the volcanics of the Kurile basin were generated from mantle enriched in continental crust (EMII). In spite of the different conditions of their genesis, they share some common geochemical features, in particular, their calc-alkaline signatures. These traces of the influence of sialic crust on magma generation confirm the development of the basins of both these seas on continental basement.     

Depths of the Sea of Okhotsk sedimentary basin in the Cenozoic

The biofacies analysis of benthic foraminifers in sediments of the Sea of Okhotsk paleobasin revealed the presence of typical abyssal species in their Oligocene-Miocene assemblages that are missing in the Pliocene and Pleistocene section. The development of the abyssal fauna in the Sea of Okhotsk was determined by its relatively large depths (>2000 m) and intense water exchange with the Pacific Ocean. The Sakhalin folding phase at the Neogene-Quaternary transition resulted in the uplifting of the Japan-Kurile cordillera and separation of deep basins of the Japan and Okhotsk seas from the ocean, which was responsible for the formation of unfavorable conditions for migration and existence of the Pacific abyssal fauna. The taxonomic similarity and general tendencies in the development of the Neogene benthic foraminifers common for the Japan and Okhotsk seas imply the lack of narrow and shallow thresholds between these basins similar to the present-day Nevel’skoi, La Pérouse, and Sangar straits. Such bottom topography stimulated the intense northward water flow, which determined the similarity between the benthic foraminiferal assemblages of the Japanese and Okhotsk paleobasins.