Takuya Itaki

Depth-related radiolarian assemblage in the water-column and surface sediments of the Japan Sea

Plankton tows and surface sediments from the Japan Sea, a marginal sea of the northwestern Pacific, were examined to study the depth distribution of modern radiolarians. The depth distribution patterns between the living assemblage in the water-column and the thanatocoenose in surface sediments are similar to each other, indicating that the depth-related assemblage in sediments is closely related to the living depths of major species. Four clear restrictions of many species to discrete depth zones are recognized in the water-column, from the surface down to 2000 m: (1) upper subsurface-water species, which are abundant from 40 to 120 m (Larcopyle butschlii juvenile form, Spirocyrtis subscalaris, Spongodiscus resurgens and Lipmanella virchowii); (2) lower subsurface-water species, which are abundant from 40 to 300 m (Larcopyle butschlii adult form, Ceratocyrtis histricosa and Spongotrochus glacialis); (3) intermediate-water species, which have population maxima from 160 to 300 m (Ceratospyris borealis); and (4) deep-water species, which have maxima between 1000 and 2000 m (Cycladophora davisiana and Actinomma boreale). Only a few specimens of A. boreale were found in the water-column ranging from 1000 to 2000 m depth, although this is a dominant species in surface sediments from depths below 2000 m of which the dominance increases with water depth. A. boreale may live mainly in the bottom water. Cycladophora davisiana and A. boreale, which characterize the Japan Sea deep assemblage, are naturally shallow or intermediate dwellers in other oceans. On the other hand, the typical deep-dwellers (e.g. Cornutella profunda, Cyrtopera languncula and Peripyramis circumtexta) of other oceans have not been found in the Japan Sea. Such a peculiar deep assemblage is a result of the deep circulation system in the Japan Sea and the shallow depths of the sills around it.

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.

Land—sea linkage of Holocene paleoclimate on the Southern Bering Continental Shelf:

Detailed diatom records within surface and core sediments from the Southern Bering Continental Shelf (SBCS) reveal that the Holocene evolution of sea-ice distribution is associated with low pressure patterns. Holocene sea-ice distribution over the SBCS was mainly controlled by the location of the Aleutian Low. The corresponding paleoceanographic and paleoclimate conditions can be divided into three stages: (1) the early Holocene (before 7000 cal. yr BP) was characterized by extensive sea-ice distribution under two low-pressure cells, which covered the western Bering Sea and the Gulf of Alaska, respectively. (2) Between 3000 and 7000 cal. yr BP, the low-pressure system over the Gulf of Alaska became weak, causing total sea-ice mass over the SBCS to retreat. (3) In the past 3000 years, prevailing southwesterly winds over the SBCS due to the developing Aleutian Low have reduced further sea-ice cover on the SBCS. These paleoclimatic changes were probably a response to ENSO variation. The timings of water mass exchanges on the SBCS coincided with sea-level change along the Alaskan Peninsula. As a result, subsequent morphologic alterations have also influenced the paleoceanographic condition of the SBCS. The effect of the surface coastal water and bottom marine water on the SBCS intensified about 6000 cal. yr BP when sea level increased.

Polycystine Radiolarians in the Tsushima Strait in Autumn of 2006

Abstract. A total of 92 species or taxa of polycystine radiolarians were identified in depth-stratified plankton samples collected from the Tsushima Strait between Japan and Korea in autumn 2006. This assemblage can be divided into three groups: shallow eastern channel, shallow western channel, and bottom western channel. The distribution patterns are most likely related to different water masses. The western channel is influenced mainly by the Taiwan Current and coastal waters, which are characterized by low salinity and high nutrients, whereas water in the eastern channel is mainly from the Kuroshio Current. Cycladophora davisiana, which lives deeper than 500 m in the Japan Sea, was abundant in the western channel at 100–140 m. This suggests that the deeper microzooplankton in the Tsushima Strait are associated with colder and less saline water originating from the greater depths of the Japan Sea.

Jørgensen’s polycystine radiolarian slide collection and new species

E. H. Jørgensen’s radiolarian collection is stored at the Natural History Museum in Oslo and holds 93 slides, of these 76 are with plankton from the western and northern coast off Norway. Jørgensen described 43 new polycystine radiolarian species from this material, but did not assign any types. In this present study we have re-examined the collection, registered and photographed all slides and radiolarian specimens, and designated lectotypes and paralectotypes for many of Jørgensen’s species.

Radiolarian Assemblages in Surface Sediments of the Japan Sea

Abstract. Radiolarian assemblages in 69 surface sediment samples from the Japan Sea were moderately diversified, consisting of about 100 taxa in total, although only a few species accounted for a large proportion of most assemblages. First, the assemblages were often dominated by deep-dwelling species: Cycladophora davisiana, Actinomma leptodermum, A. boreale, A. langii, and adult forms of Larcopyle buetschlii. These species were restricted to great depths and were characteristic of the cold, oxygen-rich deep water that fills deep basins of this marginal sea, the so-called Japan Sea Proper Water. Second, although the observed Japan Sea assemblages included some subtropical elements, such as the Dictyocoryne and Euchitonia groups, Didymocyrtis tetrathalamus and the Tetrapyle octacantha group, many of the major temperate and subarctic elements of the North Pacific were essentially excluded. The semi-enclosed topography of the Japan Sea is most likely responsible for the dominance of certain subtropical surface dwellers as well as for the near-absence of transitional and cool water species from corresponding latitudes of the North Pacific. Q-mode cluster analyses of the relative abundance data of the radiolarian species distinguished three faunal provinces that reflect the modern surface water circulation and the distributions of the upper water masses, including the Tsushima warm current and the Liman cold current. These results suggest that the radiolarian assemblages are strongly related to the present hydrography of the Japan Sea and can therefore be used as environmental proxies in this region.

New northwest Pacific radiolarian data as a tool to estimate past sea surface and intermediate water temperatures

A new radiolarian data set for transfer function estimates of past sea surface temperature (SST) and intermediate water temperature was developed in this study for the northwestern Pacific Ocean covering a region from 1° to 50°N and 120° to 167°E. We analyzed 87 sediments surface samples, selected 30 species and/or species group found in shallow water for estimating past summer SST and 17 species and/or species group found in the intermediate water for estimating past intermediate water temperature. Since the intermediate water temperature changes greatly between 200 and 500 m, our estimates provide values at 500 m because temperatures are relatively stable between 500 and 1000 m. In this context, we estimated past summer SST and intermediate water (at ~500 m) temperature within an error margin of 0.9 and 1.2°C, respectively. A test of the accuracy of our transfer functions, conducted on core samples provided by IODP Expedition 346 Site U1429 in the northern East China Sea, showed that the reconstructed summer SSTs fluctuated between 17.2 and 26.5°C in selected late Pleistocene sequences. These temperatures corresponded to modern winter and summer SST, respectively, which highlights the ability of our new database to accurately reconstruct summer SST. The reconstructed intermediate water temperature fluctuates between 3 and 8°C, which corresponds to the observed temperature range at depths of ~500 m at high and midlatitudes, respectively.

Per Theodor Cleve: a short résumé and his radiolarian results from the Swedish Expedition to Spitsbergen in 1898

Cleve’s material from the Swedish Expedition to Spitsbergen in 1898 has been re-examined and lectotypes and some paralectotypes have been established for three spumellarian species, eight nassellarian species, and three phaeodarian species. In addition, important species in the Cleve collection, including three spumellarian species, 10 nassellarian species and three phaeodarian species have been illustrated and commented on. Some of Cleve’s identifications were erroneous, and we have carefully discussed these in our re-evaluation of Cleve’s species concepts.

Radiolarian Biostratigraphy from Middle Miocene to Late Pleistocene in the Japan Sea

Abstract. In the Integrated Ocean Drilling Program (IODP) Exp. 346, sampling by drilling was conducted at seven sites (U1422–U1427 and U1430) in the Japan Sea. Radiolarians in moderately well preserved states were found in most samples throughout the sequence in varying abundance. Forty-one radiolarian datum events were identified in this study, and the radiolarian zonation that best divides the middle Miocene to Pleistocene sequences with updated ages of radiolarian datum events (estimates based on the geomagnetic polarity time scale (GTS) 2012) was applied to the sedimentary sequences in the Japan Sea. Here, four new radiolarian zones are proposed for the Quaternary of the Japan Sea, and one zone is slightly revised to adjust for differences among other zones. The sequences collected at the sites extended from the Pleistocene Ceratospyris borealis Zone to progressively deeper zones as follows: Site U1427, four zones to the Pleistocene Schizodiscus japonicus; Site U1422, six zones to the late Pliocene Hexacontium parviakitaense Zone; Sites U1423, U1424 and U1426, eight zones to the early Pliocene Larcopyle pylomaticus Zone; and Sites U1425 and U1430, fourteen zones to the middle Miocene Eucyrtidium inflatum Zone. The absence or extremely rare occurrence of Stylatractus universus and E. matuyamai indicates that S. universus lived in the deep water of the northwestern Pacific and had not been able to migrate into the Japan Sea across the Tsugaru Strait since the Pliocene.

Response of the Bering Sea to 11‐year solar irradiance cycles during the Bølling‐Allerød

Previous studies find decadal climate variability possibly related to solar activity, although the details regarding the feedback with the ocean environment and ecosystem remain unknown. Here, we explore the feedback system of solar irradiance change during the Bolling-Allerod period, based on laminated sediments in the northern Bering Sea. During this period, well-ventilated water was restricted to the upper intermediate layer, and oxygen-poor lower intermediate water preserved the laminated sediment. An 11-year cycle of diatom and radiolarian flux peaks was identified from the laminated interval. Increased fresh meltwater input and early sea-ice retreat in spring under the solar irradiance maximum follow the positive phase of Arctic Oscillation which impacted the primary production and volume of upper intermediate water production in the following winter. Strength of this 11 year solar irradiance effect might be further regulated by the pressure patterns of Pacific decadal oscillation and/or El Nino-Southern Oscillation variability.