Diatom Assemblages from the Daginsky Mud Volcano Sediments (Eastern Sakhalin) and Their Implication

1. Introduction

Mud volcanoes are geological structures formed as a result of the release of clay material mixed with water and gas through eruptive channels, forming a mud breccia on the surface [9,32,46,47,92]. Mud volcanism is confined to zones of development of Alpine folding, confined mainly to Cenozoic Molasses deflections [92]. Mud volcanoes often form in oil and gas regions and are important indicators in the search for oil and gas and other minerals, as well as an indicator of the geodynamic state of the earth's crust [32,46,47,78,80,92]. Mud volcanoes are characteristic of basins with a thick sedimentary cover and abnormally high reservoir fluid pressures and tend to the shelves of marginal seas and oceans [32]. In addition, mud volcanism is a potentially dangerous geological phenomenon [33,44] and a significant source of greenhouse gases [9,74,75]. In the Russian Far East, mud volcanoes are known in Kamchatka, the Kuril Islands and Sakhalin Island (Figure 1). In Kamchatka and the Kuril Islands, mud volcanoes and mudpots are associated with post-volcanic phenomena, with the solfataric stage of the activity of ordinary lava volcanoes [85]. The long-lived center of gas-fluid emanations associated with mud volcanism was recently discovered on the western slope of the Kuril Basin of Sea of Okhotsk [8].

Sakhalin Island is one of the few regions of Russia where classical mud volcanoes are common, through the outlets of which a multi-clastic mud mass saturated with water and gas is ejected – hill breccia [46,47]. The Yuzhno-Sakhalinsky, Pugachevsky, and Lesnovsky mud volcanoes located in the south of the Sakhalin Island are associated with Upper Cretaceous rocks [46,92], while the Daginsky Mud Volcano (DMV) located in the northeast of Sakhalin is confined to the Oligocene – Neogene deposits [80]. When studying the sediments of the DMV, numerous diatoms were discovered for the first time [75]. The diatom flora of the DMV sediments is characterized by high species richness (278 species and intraspecific taxa) [88]. A diverse diatom flora (131 species and varieties) was found in the phytoplankton and periphyton of the Daginsky hydrothermal springs [53], which are closely related to the DMV.

Information on diatoms of mud volcanoes is extremely scarce, although they were noted in the first microscopic studies of breccias of mud volcanoes in Italy in the 19th century [78]. The mass number of diatoms, represented mainly by Rhizosolenids (members of the family Rhizosoleniaceae), Thalassionema spp., Chaetoceros spp., found in the late Quaternary sapropels of the Napoli Mud Volcano in the Mediterranean Sea [60]. The age (5.6-4.6 Ma) of the mud volcano roots was determined from diatoms found in the breccia of the mud volcanoes of Baikal Lake [31]. Six diatom species were described in a water-silty suspension from the Bulganak mud volcanoes of the Kerch Peninsula of Crimea [69]. More data on the algal flora of geothermal springs in Kamchatka Peninsula and the Kuril Islands associated with active magmatic volcanism. Review and analysis of research on these sources has shown that up to 95% of the microalgae flora are diatoms [11,12,49,50,51,52,53,54]. The algal flora of geothermal springs consists mainly of freshwater, cold-water species that have adapted to high temperatures, but at a water temperature of more than 60° C, the species composition of the algal community significantly depleted. Diatom assemblages found in soils around the thermal springs of the Kamchatka Peninsula include 49 aquatic and terrestrial species [11]. 48 taxa of diatoms, among which benthic forms predominated, were found in the sediments of Craternaya Bay (Yankich Island, Central Kuril Islands), which is a crater of the active Ushishir Volcano [70]. Geothermal springs and mud volcanoes around the world are characterized by a diversity and a fairly high abundance of diatoms [e.g., 2,10,25,40,41,48,55,57,63,65,66,72,85]. Information on the diatom composition of sediments formed under extreme conditions is important for environmental knowledge and the reconstruction of paleoconditions in the geological past. The purpose of our paper is to determine the features of the diatom assemblages from the DMV and conditions for the formation of host sediments.

2. Materials and Methods

2.1. Study Area

DMV was discovered and described by N.D. Tsitenko [86] in 1954 on the coast of the Nyisky (Daginsky) Bay. It differs from other mud volcanoes of Sakhalin in the absence of a central eruptive channel and explosive eruptions [80,86] and is called the Dagi mud volcanic manifestation [46,47] or the Dagi Geothermal System [75,81]. It consists of a group of small mud volcanoes, which are flooded and destroyed during high tides, and re-formed during low tides, so the location and number of volcanoes often changes. Mud volcanoes and thermal springs have a single cause of formation and activity, which is associated with the flow of hydrocarbon gas and water from the subsurface through tectonic fissures [86,87]. DMV is located in the tidal zone of the Nyisky Bay, separated from the Okhotsk Sea by a long sandy spit and several islands (Figure 2).

In the south, one of the largest rivers of Sakhalin, the Tym River, flows into it, in the north – the Dagi River. The hydrological regime of the bay is determined by the influence of river runoff and the action of tides. During the day, sharp changes in temperature, salinity and water level occur. The salinity in the strait areas of the Nyisky Bay reaches 30.7 psu, in the estuarine areas of the rivers, the salinity is minimal and does not exceed 1.0 psu [39]. The DMV waters and thermal springs are significantly affected by the surface waters of the Nyisky Bay and this obscures the true picture of the composition of waters coming from deep horizons [80]. The Dagi thermal springs belong to neutral or slightly alkaline terms (pH=7-8) with a temperature 25-55°C, and a sodium chloride composition with a mineralization of 1.2-9 g/l [59]. The temperature of the water and mud breccia of the small volcanos reaches 21.5 °C, and the temperature of mineral waters at their outlet reaches 36.5°C and 39.7°C [35,36], in the craters of thermal springs the water temperature was noted to 55°C [86].

Within geothermal field of DMV, there are about fifteen water and gas sources. The main component of gases is methane, formed presumably as a result of anaerobic decomposition of organic matter at depths of at least 2 km [75]. The geochemical features of the DMV silt are typical for near-continental terrigenous formations, the formation of which influenced by the active Garomai Fault and the associated natural outputs of thermogenic methane with a small proportion of microbial gases [81]. The DMV thermal mineral springs and muds characterized by high therapeutic quality [77].

The DMV is located in the area that is part of the North Sakhalin sedimentary basin, composed of the thick Cenozoic sedimentary sequences (Figure 3a,b), and is the largest industrial-oil and gas basin in the Okhotsk region [19,43].

Diatoms were found in Nutovskaya Formation (upper Miocene – Pliocene) of Dagi area; in underlying Okobykayskaya, Daginskaya, Ujninskaya. Daekhurinskaya, and Machigarskaya formations diatoms are absent [19,20]. On the adjacent shelf, diatoms occur in the deposits of the Nutovskaya and Okobykayskaya formations [94]. The Neogene deposits are everywhere overlain by Quaternary sediments, represented by modern floodplain, biogenic, lagoon, and eluvial-deluvial formations [79]. The geochemical features of the DMV sediments have characteristics typical for most of the continental terrigenous formations [81]. The concentration of arsenic (As) and silver (Ag) in the sediments obviously influenced by the nearby active Garomai Fault.

2.2. Sampling and treatment

Samples were collected by collaborators of the Laboratory of Gas Geochemistry of the Il’ichev Pacific Oceanological Institute of the Far East Branch of the Russian Academy of Sciences (POI FEB RAS) in the DMV area during field expeditions in 2001, 2012 and 2014. Eight sediment samples collected around the Kalmar gas-hydrothermal spring, the water temperature of which reaches 41°C [71], 5 samples in a gassing “bath” located about 150 m northwest of it (Figure 2; Table 1).

All samples were taken from the upper layer of sediments (0-1 cm), except for samples T1-3, T1-4 and T1-5, which were taken below the surface in the intervals of 10-11 cm, 30-31 cm and 40-41 cm, respectively. The selected sediments are represented by liquefied clayey silts of dark gray color with up to 70% organic remains (mainly, diatom algae).

The sediment treatment and preparation of slides for microscopy as well as estimation of the diatom concentration (valves per gram of the air-dry sediment) were conducted based on the standard procedure [30,83]. The samples were enriched with used a heavy potassium-cadmium liquid (H₂O : CdI₂ : KI = 1 : 2.5: 2.25) with a density of 2.6 [30]. For the preparation of slides, a highly refractive MOUNTEX mounting resin (refractive index 1.67) (Darmstadt Germany) was used.

The diatoms were identified and enumerated with an IMAGER.A1 light microscope (magnification = x 1,000; Carl Zeiss AG, Oberkochen Germany) and imaged with an AxioCamMrC digital video camera (Carl Zeiss AG). The species were identified from the following main sources: [7,13,14,15,22,23,24,26,27,28,34,38,42,61,62,67,68,82,84] and specific taxonomic publications. The analysis of the ecological structure of diatom flora and assemblages was mainly carried out in relation to salinity: marine, brackish water, freshwater, and euryhaline; by habitat: benthic, planktonic, and benthic-planktonic. Ecological and biogeographic characteristics of diatoms are given mainly according to [4,29,45,51,54,73] and the above-mentioned sources. A taxonomic references of diatoms and silicoflagellates of the DMV sediments are given in [88].

3. Results

Concentration diatoms in sediments

The concentration of diatoms (valves per gram of air-dry sediment) in the DMV sediments varied considerably from 0.7 to 9.1 mln valves/g with an average of 4.8 mln valves/g (Figure 4; Supplementary Materials S1). The minimum abundance was observed in sample D01-1, taken near the boundary of the tidal zone of Nyisky Bay. The maximum concentration found in clay silts near the Kalmar gas-hydrothermal spring (sample T3-7) and a gassing "bath" located northwest of the Kalmar spring (samples T1-1, T1-2).

Diatom assemblages

Diatom assemblages from the studied sediments are similar in dominant and subdominant species and species composition in general. The assemblages contain from 63 to 141 species and intraspecific taxa of different biotopes (Supplementary Materials S2). Brackish water diatoms are diverse (18-36 taxa in the samples) and abundant, they dominate in almost all samples (33-56.3%) (Figure 5).

Benthic species Pinnunavis yarrensis (1.0-12.3%), Petroneis marina (2.8 -11.3%), Cocconeis scutellum (3.5-10.7%), as well as Coronia echeneis, Nitzschia linearis, Fallacia pygmaea, Delphineis kippae, Navicula maculata, Diploneis mereschkowskyi, D. interrupta, Scoliotropis latestriata, Rhabdonema arcuatum, Amphora proteus, A. libyca, Halamphora coffeiformis, Surirella brebissonii, Tryblionella compressa, T. granulata, T. punctata, T. hungarica, T. circumsuta (Figure 6 and Figure 7), which were found mainly as single specimens, more than half of which are alkaliphilic species.

Marine species (16-40 taxa) are also abundant and stably subdominate (20.3-46.7%). They are characterized by a large number (18.8-25%) of benthic-planktonic species Paralia sulcata, Odontella aurita, Actinoptychus senarius, Rhabdonema arcuatum et al. A noticeable number of planktonic species (12-17.1%) was observed only in samples T2-6, T3-7 and D12-6. Almost all assemblages also contain planktonic species inhabiting both the neritic and oceanic zones: Actinocyclus curvatulus, Coscinodiscus marginatus, Bacterosira bathyomphala, Thalassionema nitzschioides (Figure 8). Oceanic species Neodenticula seminae, Rhizosolenia hebetata, Сoscinodiscus obscurus, C. asteromphalus, Shionodiscus biporus occurred rarely. Benthic species are diverse (Cosmioneis grossepunctata, Navicula pennata, Petroneis glacialis, Tabularia fasciculata, Tryblionella granulata, T. compressa, et al.), but they are rare, their total amount in samples usually does not exceed 1.5% on average.

Euryhaline species (4-15 taxa) are characterized by the dominance of the benthic species Metascolioneis tumida (8-22.5%), as well as benthic Navicula digitoradiata, Anomoeoneis sphaerophora, Auliscus sculptus, Prestauroneis crucicula, benthic-planktonic alkaliphilic Caloneis bacillum, Achnanthes brevipes, and planktonic Thalassiosira lacustris, T. hyperborea, Ditylum brightwellii, and Cyclotella meneghianiana.

Freshwater species (12-40 taxa) were found in all samples, but they were found mostly in single specimens. They are predominantly represented by cosmopolitan alkaliphilic planktonic (Aulacoseira italica, A. islandica), and benthic (Gyrosigma acuminatum, Diploneis ovalis, Eunotia praerupta, Pinnularia viridis, Amphora copulata et al.) species. In addition, freshwater-brackish water, alkaliphilic species of Nitzschia clausii, N. thermalis, and Cyclotella atomus were found. A high diversity (30–40 taxa) and a significant abundance (12.7–14.3%) of freshwater diatoms were observed in sediments near the edge of the intertidal zone (sample D01-1) and near the Kalmar gas-hydrothermal spring (D12-1). Some freshwater diatoms are alkaliphilic (Amphora lybica, Eunotia bilunaris, et al.) or indifferent to pH (Fragilaria capucina, Pinnularia borealis, Stauroneis phoenicenteron, Ulnaria ulna et al.) and salinity (Caloneis silicula, Nitzschia amphibia, Diploneis ovalis, Gyrosigma acuminatum et al.) species (Supplementary Materials S2). The species diversity of freshwater diatoms is probably associated with the influence of Nelbutta River runoff, which flows into the Nyisky Bay at this place. It cannot be ruled out that these species can inhabit the DMV waters, which are characterized by low salinity and weak alkalinity.

Extinct species are characteristic of all assemblages. They are represented by 25 species, among which are mainly marine species of Eupyxidicula schenckii, Stephanogonia hanzawae, Ikebea tenuis, Denticulopsis lauta, D. hyalina, D. katayamae, Thalassiosira marujamica, extinct in the Miocene, and species of Eupyxidicula zabelinae, Eustephanias inermis, Porosira punctata, Cosmiodiscus intersectus, Coscinodiscus pustulatus, Thalassiosira insigna, T. nidulus, extinct in the Pliocene [5,17,19,37,76,93] (Figure 8), and 2 freshwater species Aulacoseira canadensis and A. praegranulata var. praeislandica f. praeislandica extinct in the Miocene and Pliocene, respectively [3,22]. Their content in the assemblages is insignificant (0.3-4.3%).

Thus, diatom assemblages from the studied DMV sediments are similar and characterized mainly by species Metascolioneis tumida, Paralia sulcata, Odontella aurita, Pinnunavis yarrensis, Petroneis marina, Cocconeis scutellum, and Navicula digitoradiata, which prevailed in them. They contain diatoms of various ecological groups (marine, brackish water, euryhaline, and freshwater) and extinct species. These groups differ in the number of species and abundance in the studied samples. Brackish water and marine species are numerous in diatom assemblages, accounting for 33-64% and 20.3-46.7%, respectively. Freshwater and euryhaline species are usually found in single specimens, but their total number in assemblages is quite significant: 3.0-14.3% and 3.0-13.1%, respectively. Extinct species are diverse, they are encountered constantly, but in single specimens.

Silicoflagellates

In addition to diatoms, silicoflagellates (Dictyochophyceae) Distephanus octangulatus, Stephanocha speculum var. minuta, and Octactis speculum were found (Table 2). The findings of silicoflagellates, typical inhabitants of marine plankton, in the sediments of the DMV, the salinity of the surrounding waters of which often does not exceed 1 psu [39], can be explained by transportation with seawater during tides. The species Octactis speculum can inhabit desalinated marine waters [21], which is proved by recent finds of this species in the apex of the Ulban Bay (Sea of Okhotsk), where the salinity of the waters does not exceed 8 psu [91]. This species is characteristic of Neogene deposits of Sakhalin Island [37,76] and could get into sediments with water-mud masses when passing through the sedimentary sequences.

4. Discussion

The concentration of diatoms in sediments indicate mainly the productivity of surface waters [29,64,90]. The diatom content in the DMV sediments (0.7–9.1 mln valves/g) is higher than in the sediments of the Sakhalin Bay of Northern Sakhalin (0.03–3.5 mln valves/g) [56], Academy Bay (0.7-5.8 mln valves/g) [91], Aniva Bay of South Sakhalin, Lunsky and Nabilsky bays of Eastern Sakhalin (about 2.0 mln valves/g) [64], comparable to that in Terpeniya Bay of South Sakhalin (2–10 mln valves/g) [64], but lower than in the sediments of the productive northern shelf – 5–15.3 mln valves/g and the central part Sea of Okhotsk, where the concentration of diatoms reaches from 60 mln valves/g [90] to 152 mln valves/g [64]. Therefore, despite the extreme conditions, it should be noted that the concentration of diatoms in the DMV sediments is quite high and indicates the high productivity of the waters of the Nyisky Bay and the main role of these waters in the formation of DMV sediments.

A significant abundance and diversity of brackish water and marine species in all the studied samples indicates a main influence of marine waters on the formation of the diatom assemblages of DMV sediments. The dominant and constantly occurring species of the different ecological groups are mainly characteristic of coastal marine, lagoon, and estuarine waters, and tidal shoals [e.g., 6,58,68,89].

To find out which diatom flora develops in similar conditions, we compared the diatoms of DMV sediments and thermal springs of the Kamchatka Peninsula, Sakhalin Island, and the Kuril Islands (Kunashir, Shiashkotan, Yankich), studied by T. Nikulina with co-authors [49,50,51,52,53,54] and L. Ryabushko & V. Tarasov [70]. The comparison showed that a noticeable number of identical species with DMV diatoms is found only in diatom assemblages of thermal springs located on the sea coast. Maximum number of same species is found in the Dagi thermal springs associated with DMV (Figure 9).

They contain 75 identical species, including diatoms of different ecological groups – freshwater, brackish water and marine. A significant number of identical species are found in the diatom flora of the thermal springs of Stolbchaty Cape (36 species) and Alekhin Bay (36 species) of Kunashir Island, located on the coast of the Sea of Okhotsk [49,53], and Craternaya Bay (27 species) of Yankich Island [70], connected to the Rikord Strait by a narrow shallow channel. These are mainly freshwater (Eunotia bilunaris, Gomphonema parvulum, Hantzschia amphioxys, Fragilaria vaucheriae, Rhoicosphenia abbreviata, Tabellaria flocculosa, Ulnaria ulna, Pinnularia borealis, Ctenophora pulchella, Aulacoseira granulata) and brackish water (Cocconeis scutellum, Surirella brebissonii) species. The minimum number of identical species (7) is noted in the thermal spring from the caldera of the Golovnin Volcano (the southern part of the Kunashir Island), the temperature in which varies from 60° to 103°C [49,53].

Almost all above mentioned thermal springs as well as DMV sediments contain freshwater Rhoicosphenia abbreviata, Nitzschia amphibia, Gomphonema parvulum, Diploneis ovalis, Eunotia bilunaris, Epithemia adnata, Frustulia vulgaris, Ulnaria ulna, euryhaline Caloneis bacillum, Anomoeoneis sphaerophora, and brackish water Tryblionella apiculata, Amphora libyca species. All of these species are alkaliphilic cosmopolitans. Most of these species dominate the diatom assemblages of the thermal springs of Sakhalin (Dagi), the Kuril Islands, and Kamchatka [49,50,51,52,53,54]. The algal flora of geothermal springs consists mainly of cold-water species that have adapted to high temperatures [49,51,53]. The true thermophiles Pinnularia acidojaponica, P. acidophila, and Nitzschia thermaloides, typical of thermal springs [49,51,53], are not found in DMV sediments. Part of the freshwater species (27 taxa) of the DMV sediments are common for the diatom flora of Dagi River, which includes 86 species and intraspecific taxa [45].

A feature of DMV diatom assemblages is a diversity of extinct species (total 25 taxa). Among them there are species with a limited age range: Eupyxidicula zabelinae (8.0-2.3 Ma), Porosira punctata (8.4-1.5 Ma), Thalassiosira insigna (7.9-5.0 Ma), Actinocyclus ingens (18.4-7.7 Ma), Denticulopsis lauta (15.9-13.1 Ma), D. hyalina (14.9-13.1 Ma), D. praedimorpha (12.4-11.5 Ma), and D. katayamae (9.3- 8.5 Ma), [5,93]. The percentage of extinct diatoms in the sediments is insignificant (0.8-4.3%). Most of the found extinct species (e.g., Porosira punctata, Eupyxidicula zabelinae, Thalassiosira insigna, T. nidulus, T. marujamica, Eustephanias inermis et al.), are typical for the Nutovskaya Formation of the Late Miocene – Pliocene of the Eastern Sakhalin shelf [17,94]. The outcrops of this formation are common in the Dagi region, but diatoms in these rocks have not been observed [19]. The extinct diatoms are also characteristic of the Mayamrafskaya and Matitukskaya formations of the Late Miocene to Pliocene in the Schmidt Peninsula [18,37], located in the very north of Sakhalin. Therefore, it is assumed that the fossil diatoms could get into DMV sediments as a result of drainage Nutovskaya Formation extended in the Dagi area and adjacent shelf. Denticulopsis lauta, D. hyalina, and D. praedimorpha, which are zonal species of the Middle Miocene zones [93], as well as Actinocyclus ingens, Ikebea tenuis, and Stephanogonia hanzawae, characterize the Middle Miocene deposits of the Sakhalin Island [e.g., 1,18,76]. Diatom assemblages of the Middle Miocene were identified in the well of drilled on the Veninskaya area of adjacent shelf. The assemblage of the Denticulopsis hyalina Zone (14.9-13.1 Ma) characterizes the lower part of the Okobykayskaya Formation (1080-1280 m interval), and the assemblage of the Denticulopsis praedimorpha Zone (12.9-11.5 Ma) – the upper part of the formation (720-1000 m interval) [94]. However, diatoms of these diatom zones are not found in the same formation in the Dagi area of Sakhalin [16,20]. The data obtained suggest that the source of the fossil Miocene diatoms is the Okobykayskaya Formation, which is located below the sea floor at depths of 1280-720 m of the adjacent shelf. It is assumed that the roots of the volcano are located approximately at these depths or deeper and are located under the shelf. The source of gases and water-and-mud masses is probably the Veninskoye gas-condensate field, located on the adjacent shelf. Thus, in DMV sediments, extinct species could presumably have come from the underlying Neogene sediments of the Okobykaiskaya and Nutovskaya formations with gas, mud, and water flows. It cannot be excluded that the fossil diatoms may have been introduced into the sediments of the DMV sediments by rivers and streams as a result of the erosion of the sediments of these formations. However, among the diatoms of the Dagi River and other Sakhalin rivers flowing in the areas of the Neogene sedimentary rocks, no extinct species were found [45,54]. The findings of extinct diatoms in DMV sediments confirm the existence of an active fluid dynamic system that drains oil and gas complexes of the Dagi area [75] and adjacent shelf.

5. Conclusions

The diatom assemblages from DMV sediments are distinguished by species diversity, a mixed ecological composition, and extinct species. They are mainly characterized by the predominance of Metascolioneis tumida, Paralia sulcata, Odontella aurita, Pinnunavis yarrensis, Petroneis marina, Cocconeis scutellum, and Navicula digitoradiata. The diversity and abundance of brackish water and marine species indicates the predominant influence of sea waters of Nyisky Bay on the formation of DMV sediments. The diverse freshwater species were mainly introduced into sediments with river runoff, but it is likely that some of them, cosmopolitan alkaliphilic species, are likely inhabitants of geothermal springs. Species characteristic of thermal springs are poorly represented and insignificant. No true thermophilic species were found. The extinct marine species redeposited from the Okobykaiskaya (Middle Miocene) and Nutovskaya (Upper Miocene–Pliocene) formations of the adjacent shelf, from where they were carried with gas-water-mud masses due to an active fluid-dynamic system. The presence of extinct species from the underlying Neogene sediments is the most typical for diatom assemblages of the DMV.