Maria A. Zenina

The East Siberian Sea (ESS) is one of the least studied seas in terms of ostracod fauna. Ostracods are sensitive organisms to environmental changes, and detailed information on their ecology is still required. To fill this knowledge gap, we studied 33 meiobenthic dredge samples collected from the western part of the ESS and the Chaun Bay together with 17 grab samples taken from the eastern part of the sea. Quantitative analyses of the ostracod assemblages demonstrate that the river-influenced western part of the ESS is inhabited by low diverse and impoverished fauna consisting of the taxa which are able to adapt to different environmental conditions. In the isolated Chaun Bay sheltered from significant riverine influence, the ostracod assemblages contain species that prefer more stable conditions. The predominance of living specimens over dead ones and individual valves points to strong carbonate dissolution that is more pronounced in the western ESS than in the Chaun Bay. The formation of such conditions might be related to the high content of dissolved carbon dioxide resulting from bacterial remineralization of in situ produced and land-derived organic matter in the bottom sediments and low pH near the seabed. Numerous ferromanganese crusts were found on the ostracod valve surfaces and inside the shells from the Chaun Bay.

Palaeoecoligical reconstructions of predelta area of the North Caspian Sea were based on the multidisciplinary study of the core from the Rybachya borehole containing the Holocene deposits. Geochemical, grain size, mollusk fauna, diatom and ostracod analyses were performed, and the radiocarbon dating was carried out. The structure of deposits reflects palaeogeographic events of different scales including the inception of a palaeoincision in the Lower Khvalynian deposits, accompanied by erosion of the Upper Khvalynian deposits; its development during the Mangyshlak regression and gradual filling during the multistage Holocene Neocaspian transgression. The Holocene age of the deposits filling the palaeodepression is confirmed by radiocarbon dates - 8070 ± 110 and 7020 ± 140 cal. BP. The paleontological data indicate the interchange of tranquil and dynamic water regimes and the quasicyclical change in the conditions of the water basin, from brackish to freshwater and to marine during the stage of sedimentation, corresponding to the presentday conditions on the North Caspian shelf.

Benthic foraminiferal assemblages were studied in twenty surface sediment samples collected in the western part of the East Siberian Sea in the framework of the expedition of the R/V Ivan Kireev in 2004. Rose Bengal treatment made it possible to separate “living” and dead individuals. Taxonomic analyses reveal that “living” calcareous foraminifers dominate in the study region with the exception of the New Siberian Islands area and area to the northeast from the Indigirka River mouth where agglutinated tests constitute up to a half of the assemblage. Little amount of dead individuals points to a rapid dissolution of calcareous tests immediately after death of the organism. Taxonomic composition of the studied assemblages is similar to river-proximal benthic foraminiferal assemblage previously determined for the Kara Sea. In the East Siberian Sea, Elphidium clavatum, Elphidiella groenlandica, Haynesina orbiculare, E. incertum and Lagenammina atlantica are the most common species. Gordiospira arctica, Buccella frigida, E. bartletti, Reophax curtus, R. scorpiurus and polymorphinids are less abundant taxa. As a result of the statistical analysis, four areas with specific BF assemblages have been distinguished: the area of the Indigirka River mouth, the zone affected by the Siberian Coastal Current, the area located to the north of the Siberian Coastal Current and zone situated close to the New Siberian Island archipelago together with an area located to the northeast from the mouth of the Indigirka River. Such a distribution of benthic foraminiferal assemblages in the study region seems to be related to the distance of the river discharge area, type of sediments and fresh organic matter content.

This paper is the first one to investigate the stable oxygen isotope values of modern ostracods sampled across the whole area of the Caspian Sea. Six taxa were analyzed for δ¹⁸O_ost. The correlation between δ¹⁸O_ost and some parameters of the aquatic environment, such as water temperature, salinity and δ¹⁸O_water, was examined, providing the basis for reliable paleoenvironmental reconstructions in the future using oxygen isotope data. The results of 105 δ¹⁸O_ost measurements taken from 76 sites are consistent with the temporal and spatial indicators of the bottom water masses, as impacted by regional hydrography and physical processes. It has been proven that the δ¹⁸O of Caspian ostracods is mainly determined by a species-specific isotopic offset with an average value of + 1‰. The corresponding inference allows to successfully identify the properties of the water mass, where the carapace has been formed, and also enables to recognize the climatical signal in the δ¹⁸O records. The vital offset for the mixed ostracod samples was found to be equivalent to the average species-specific isotopic offset calculated on the mean annual and winter temperatures of the Caspian Sea water; it equals to + 1 (± 0.5) ‰. The regression analysis showed correlations to temperature and salinity parameters that may also facilitate the prediction of water-mass characteristics when being applied to core sediment records. The predicted temperatures correspond to actual measured temperatures at the site of collection, and the oxygen isotope composition of the studied species can be a reliable indicator of water temperatures corresponding to the period of shell calcification. However, in the context of the Caspian Sea environmental changes during the Quaternary period, such attempts of calculating paleotemperatures may lead to false results, since the paleotemperature equation and its variations are only applicable to a water body in its steady state.

Benthic foraminiferal assemblages were studied in twenty surface sediment samples collected in the western part of the East Siberian Sea in the framework of the expedition of the R/V “Ivan Kireev” in 2004. Rose Bengal treatment allows us to separate “living” and “dead” individuals. Taxonomic analyses reveal that “living” calcareous foraminifers dominate in the study region with the exception of the New Siberian Islands area and area to the northeast from the Indigirka river mouth where agglutinated tests constitute up to a half of the assemblage. Little amount of dead individuals points to a rapid dissolution of calcareous tests immediately after death of organism. Taxonomic composition of the studied assemblages is similar to river-proximal benthic foraminiferal assemblage previously determined for the Kara Sea. In the East Siberian Sea, the most commonly occurred species are E-lphidium clavatum, Elphidiella groenlandica, Haynesina orbiculare, E. incertum и Lagenammina atlantica. Less abundant species are Gordiospira arctica, Buccella frigida, E. bartletti, Reophax curtus, R. scorpiurus and polymorphinids. As a result of statistical treatment, four areas with specific BF assemblages have been distinguished: area of the Indigirka River mouth, zone affected by Siberian Coastal Current, area located to the north of the Siberian Coastal Current and zone situated close to the New Siberian Island archipelago together with an area located to the northeast from the Indigirka river mouth. Such a distribution of BF assemblages in the study area seems to be related to the distance of the river discharge area, type of sediments and fresh organic matter content.

Mollusk fauna is an important component of the Caspian Sea ecosystem alongside ostracods and diatoms. These faunal proxies are essential indicators of hydrological shifts reflecting global and regional climate changes. Adding lithological, geochemical, and geochronological (radiocarbon) data, we revealed paleogeographic events of different scales recorded in the sequence of the Rybachya core from the North Caspian Sea. Here, we present the reconstruction of Mangyshlak paleovalley sediments during the Holocene multi-stage Neocaspian transgression, reflecting global and regional climate changes varying in scale and direction. The determined age of paleovalley-fill sediments, 8070 ± 110 cal yr BP and 7020 ± 140 cal yr BP, suggests that sedimentation processes with extended warming and humidification started later and lasted longer than was assumed earlier. Biological proxies indicate quasi-cyclic variability and shifts from brackish to freshwater conditions throughout the studied interval. Rybachya core was obtained from the early Khvalynian deposits. The Mangyshlak flow formed the depression and eroded the late Khvalynian deposits, which we did not observe in the core structure. It possibly collapsed into paleodepression and acted as a host material for the freshwater lentic faunal association. During the Holocene, we detected a transition from a tranquil water regime to a more dynamic one during the paleovalley gradual filling, followed by marine conditions typical for the modern Caspian Sea.

The lithological study of Recent sediments, as well as planktonic foraminiferal tests and macrobenthos therein, within transverse sections of four submarine canyons in the northern sector of the Argentine Patagonia gravitite–contourite system revealed a considerably more complex facies variability within the canyons than on the Perito Moreno Terrace between them. The facies variability in canyons is controlled by a combination of along-slope contour currents and down-canyon gravity flows. Two southern canyons contain the channel facies of terrigenous glauconite-rich sand that separate the flank facies of sand with a high content of foraminiferal tests. Two northern canyons include the facies of mixed sediments with a high content of <0.1 mm fraction, whose analogs are lacking on the surrounding terrace. They were likely deposited from the down-canyon gravity flows of dense suspension in the still-water bottom environment. Attached macrobenthos with calcareous skeletal elements (commonly corals) inhabits the mainly ice- or iceberg-rafted coarse rock fragments in all canyons and on the surrounding Perito Moreno Terrace.

Distribution of 57 ostracod species is investigated in the Quaternary sediment samples from the borehole RBH-16, drilled at the north-eastern Black Sea shelf edge off the town of Anapa. Among them, 52 species of the Caspian type and 5 species of the Mediterranean type were found. Four distinct assemblage zones are identified: the Caspian type one typical for Chaudian basin (Early Neopleistocene) (I) in interval 15.55–14.11 m; the Caspian type one dominating from the Late Pleistocene to the early Holocene (II), according to species composition closed to Khvalynian basin, at the depth 13.4–3.84 m (from 13.6 to 8.7 cal. ka BP), the mixed one of Caspian type and Mediterranean type species (III) in interval 3.84–0.95 m (younger than 8.7 cal. ka BP), and the assemblage with progressively increasing dominance of Mediterranean species (IV) in interval above 0.75 m (after 7.4 cal. ka BP).

Micropaleontological studies of the Black Sea, including ostracod records, have suggested that early Holocene salinity values were between ~5 and 10 practical salinity units (psu), contrasting with present values of 18–22 psu. However, more precise paleoenvironmental reconstructions based on ostracod assemblages require additional information related to their modern ecological affinities. This study uses modern species information collected from samples with living fauna to interpret the fossil Holocene assemblages of two sediment cores, Ak-2575 and Ak-521, collected from the northeastern outer shelf of the Black Sea. A total of 37 ostracod species are recorded in the fossil assemblages, with 2 related to freshwater/oligohaline environments, 23 from Caspian-type environments, and 12 from environments similar to the Mediterranean. Three distinct assemblage zones are identified from the Caspian type dominating in the early Holocene up to 7.4 cal ka BP, a mixed assemblage of Caspian type and Mediterranean type from 7.4 to 6.8 cal ka BP, and a progressive dominance of Mediterranean species from 6.8 cal ka BP. It is very likely that the dominant control of ostracod species occurrence during the period up to ~6.8 cal ka BP is salinity. A range of factors including temperature, biotope, and sedimentation rates influenced the species distribution over the last 6.8 cal ka BP.