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.     

The source and aftershocks of the Altai (Chuya) earthquake of 2003

Results derived from processing of data of field observations in the epicentral zone of the Altai earthquake of 2003 are presented. Accurate location of aftershocks showed that their spatial distribution is in good agreement with the source rupture outcropping on the Earth’s surface. According to aftershock data, the source had a length of 75 km and a maximum width of 20 km and extended in the vertical direction for 17 km; the displacement in the source averaged 2.5 m. Clusters identified in the aftershock cloud define variously dipping planes. The location of hypocenters by regional, national, and global seismic networks is not accurate enough to examine spatial features of the aftershocks.     

Integrated Geological–Geophysical and Oceanographic Research in the South China Sea: Cruise 88 of the R/V “Akademik M.A. Lavrentyev”

Oceanology - Abstract—The paper gives brief results of comprehensive studies in the South China Sea obtained from a joint Russian–Vietnamese expedition in November 2019 (cruise 88 of...     

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.     

New data on the structure of the sedimentary section on the Ita Mai Tai Guyot (Magellan Seamounts,Pacific Ocean)

The geological-prospecting investigations carried out by scientists from the Yuzhmorgeologiya State Scientific Center in the eastern segment of the Magellan Seamounts of the Pacific during the cruise of the R/V Gelendzhik in 2003–2010 brought new data on the topography of the Ita Mai Tai Guyot and its Lower Cretaceous-Pleistocene sedimentary section. The guyot is characterized by its complex irregular morphology with angular outlines being complicated by numerous relief mesoforms: spurs, scarps, and volcanic cones and domes. It is shown that the sedimentary cover was formed during a long period. The analysis of the organic macro- and microfaunal remains from the sedimentary rocks made it possible to define transgressive phases in the guyot’s development: the Albian-Cenomanian, the Late Campanian-Maastrichtian, and the late Paleocene-early Eocene. The oolitic limestone sequence recovered by DSDP Hole 202 is determined to be the Albian in age. Based on the original and published data, the geological development of the guyot was analyzed in comparison with other similar structures of the Magellan Seamounts.     

Mineralogical and geochemical indicators of anoxic sedimentation conditions in local depressions within the Sea of Okhotsk in the late Pleistocene-Holocene

This paper reports specific mineralogical and geochemical characteristics of deposits from the local depressions of the Derugin Basin. They were formed in an environment with periodic changes from oxic to anoxic conditions and show evidence for the presence of hydrogen sulfide in bottom waters. The deposits of this type can be considered as a modern model for ancient ore-bearing black shale associations. Compared with typical metalliferous black shale sequences, which are characterized by high contents of organic matter, the sediments described here are depleted in the elements of the organophilic association (Mo, Ni, Cu, Zn, V, and U) but have higher Mn contents.     

Upper Campanian-lower Maastrichtian planktonic foraminifers from Govorov Guyot (Magellan Seamounts,Pacific Ocean)

The planktonic foraminiferal assemblage from foraminiferal limestone (ooze) dredged from the summit of one of guyots in the Magellan Seamount system of the Pacific is dominated by one-keeled species belonging to the genus Globotruncanita. The taxonomic composition of the assemblage correlates host rocks with the upper Campanian-lower Maastrichtian. One species and one subspecies are described as new taxa.     

The structure of the incoherent sediments in the Ita Mai Tai Guyot (Pacific Ocean) based on geoacoustic profiling data

The results of geoacoustic studies carried out by the acoustic MAK-1M system in the southeastern Ita Mai Tai Guyot (Magellan Seamounts, Pacific Ocean) are discussed. The obtained geoacoustic sections characterize the incoherent part of the sedimentary sequence with high resolution. The geoacoustic stratification is well correlative with the deep-sea drilling data: the acoustic units are reliably correlated with the lithostratigraphic units defined in the cores of DSDP holes 200 and 202. Three geoacoustic units are distinguished in the sedimentary section (from the top downward): unit I corresponding in age to the terminal Miocene (Messinian)-Quaternary; unit II correlated with the Late Miocene (Tortonian); unit III accumulated in the Early Eocene-Early Miocene. The acoustic basement is composed of reefogenic limestones that were formed at different stages of the guyot’s development through the Aptian-Turonian to Late Paleocene-Eocene. The sonar images allow the distribution of the incoherent carbonate sediments to be contoured along the periphery of the summit’s plateau.     

Dynamics of planktonic foraminiferal community in the Sea of Japan during the last 2 My

Based on the analysis of the foraminiferal assemblages in 118 sediment samples from the 210-m-long core of ODP Hole 798 drilled near Honshu Island, it is shown that the structure of their tanathocoenoses experienced three large reorganizations during the last 1.9 My, namely 0.68, 1.21, and 1.36 My B.P. The first reorganization coincides with the Matuyama-Brunhes boundary marked by the magnetic polarity reversal. The second reorganization corresponds to the sharp and progressive temperature fall caused by general climate deterioration. The third event is poorly interpretable thus far. Using the K-average method, the assemblages were differentiated into two types differing in their taxonomic structures. The replacement of one type of community with another occurred simultaneously with the cooling onset 1.21 My B.P. Spectral analysis of the variations in the integral characteristics of the taxonomic structure revealed three groups of cycles with average periods of 136, 59, and 33 ky. These estimates are one-third higher as compared to the duration of the Milankovitch climatic cycles. The species structure of the foraminiferal community is shown to be well consistent with the rank model of geometric series.