Records of freshwater diatom fossils over several tens of thousands of years are incomparable because the time interval is much longer than the life span of common lakes. Sediment cores containing such long diatom records have been taken by the International Continental Scientific Drilling Program which incidentally suggests that global freshwater diatom biogeography became synchronized with glacial-interglacial cycles. In a local diatom community, abrupt extinction and arbitrary immigration instead of gradual and repetitive changes were observed. Diatoms changed their dominance in a lake not only as a result of changes in local environmental factors, but their geographic distribution controlled by regional community structure and limited dispersal of each species. Argument against ubiquitous dispersal arose in the last few decades based on comprehensive researches during the 1990s on diatoms as environmental indicators. Insights into diatom biogeography are required to interpret fossil diatom flora, especially those with large geographic distributions and longtime ranges such as glacial-interglacial cycles.
This paper reviewed diatom-based environmental reconstructions in coastal areas and upgraded so-called “environmental indicators.” After the definition of diatom species as environmental indicators by Kosugi (1988), many papers have referred to them when reconstructing paleo-environments by fossil diatom assemblages. However, in contrast, no one has upgraded them in the subsequent decades. Here we reorganized the environmental indicators following the recent flora work around the Japanese coast. The environmental indicators are upgraded as follows: common brackish species and “brackish water algae or plants (C2)” were removed from the environmental indicators. Eight taxa in “open sea (A)”, five taxa in “inner bay (B)”, two taxa in “marine water algae or plants (C1)”, four taxa in “marine water sand flat (D1)”, eight taxa in “marine water mud flat (E1), one taxon in “brackish water mud flat (E2)”, nine taxa in “lake (M)” and fourteen taxa in “high moor (P)” were added to the new list of environmental indicators.
The establishment of sea-level index points is essential to reconstruct sea-level changes. Diatom analysis of sediments has been applied to Holocene sea-level studies. In Japan, changes in diatom assemblages during a transition in sedimentary facies from marine to freshwater have been recognized to be the upper limit of marine facies, which provides direct evidence for the marine limit. In this paper, indicative meaning of the marine limit was reassessed, and was interpreted as a sea-level index point formed at a tidal level of mean high water spring. The horizon, at which autochthonous intertidal diatoms are present in great abundance, can yield a sea level index point along with its elevation and depositional age. Pseudopodosira kosugii is known to be a useful indicator for identifying past sea levels during the Holocene. Along most of the Japanese coast, this species occurs abundantly in the successive diatom assemblages of a sedimentary sequence formed as a result of the Holocene marine transgression (‘Jomon’ transgression). Past sea-level reconstructions using intertidal diatom indicators (P. kosugii and Terpsinoë americana) were made for the Tofutsu core collected in Hokkaido. A brief account of a diatom-inferred sea-level change during the last 8,000 years was also shown for the Harimanada coast on the border of the northeastern Seto Inland Sea.
Studies on diatoms as indicators of the past ocean environment are reviewed from two points of view. One is from diatom temperature indices derived from the dependence of diatom production on SSTs (sea surface temperatures), and the other is from the latitudinal shift of surface-water fronts inferred from changes in the biogeography of diatom species. By taking mainly two indices Td′ and Twt ratios as examples, their usefulness for paleo-SSTs reconstructions from Pliocene-Holocene cores is discussed, and by taking Oligocene-Holocene biogeography of a cosmopolitan diatom species Thalassionema nitzschioides sensu lato as an example, its usefulness as a proxy for delineating oceanic fronts is suggested.
This paper reviews the recent usage of diatom fossil analysis as an aid for paleoseismology. In tsunami geology, preliminary identification of tsunami deposits is often based on finding anomalous sand deposits in peaty or muddy deposits. The anomalous layers are then diagnosed using several criteria such as regional-scale inundation, grading of the particle size, and floral and faunal fossils within the deposits. During the interpretation of the deposits, marine diatoms are utilized to eliminate other processes such as flood. To understand the diatom assemblages within the anomalous deposits, many papers have described modern, historical, and prehistoric tsunami deposits. In the early stage of such a study, diatoms within tsunami deposits were regarded as chaotic assemblages that had been composed of broken valves. However, in recent studies such as those of the 2004 Sumatra and 2011 Tohoku tsunamis, diatom assemblages in the deposits did not follow such a pattern. As a result, we can recognize various types of assemblages and realize there is no specific criterion for evaluating event deposits based on diatom analysis. In reconstructing crustal deformation accompanied with earthquakes, diatoms are used mainly for the amount of vertical coastal deformation. For example, diatom micropaleontology was used at southwestern Washington sites to estimate subsidence ranging from 0.5 to at least 1.0 m during six of the seven great earthquakes. In another example, detailed land-level reconstructions using diatoms especially just before the earthquake had occurred have been done in southern Alaska. In the reconstructions, a few decimeters of land-level changes were attributed to preseimic deformation in earthquake cycles. Despite these successful examples, the research area still has the problem of lacking information on the early stage of taphonomy and diagenesis via modern analogues.
Diatom assemblages in the Kameyama Formation of the Mid-Pliocene Tokai Group and distributed in Tsu City, Mie Prefecture, central Japan are investigated. They consist of 109 taxa in 49 genera, and are characterized by an mixture of both freshwater and marine diatoms. Among them, freshwater planktonic species Aulacoseira praeislandica (including A. praeislandica f. curvata) is predominant, and the marine species Schuettia annulata is subordinate. The mixed flora may suggest that the Kameyama Formation was deposited in a lake-like condition, which was invaded intermittently by seawater in the Mid-Pliocene. This marine invasion may be caused by both local and global settings: this area was located at the southernmost end of the Tokai Group sedimentary basin, and the global climate at this time was warm and the sea level was high.
Diatom analysis has been commonly used as a tool for reconstructing coastal paleoenvironments, however there still remain some fundamental problems. One major problem is the incorporation of allochthonous diatom frustules in early stages of the taphonomic process, which changes the composition of the living and fossil diatom assemblages. Another is chemical dissolution and physical breakage of diatom frustules during the taphonomic process, which alters the taxonomic composition of dead or fossil assemblages. In order to clarify the taphonomic process, it is necessary to study crystallography and physical mineralogy, and also to understand sedimentology from the viewpoint of diatom frustules as fine silica sediment particles. In addition, to identify soluble species and allochthonous components, it is necessary to study the taphonomic process at each stage and the relationship between the dissolution of diatom frustules and changes in groundwater pH or Eh in particular. On the other hand, we should present a list of total taxa used in the study for reconstructing paleo-coastal environments by diatom assemblages in order to understand the selective preservation of each taxon.
Time-series variation of diatom sidocoenosis and sinking flux was studied at Station K2 (47°N, 167°E, 5,280 m in water depth) in the western Subarctic Pacific during two study periods (March 2005–June 2006, and November 2009–June 2010).During the first period, the level of diatom flux was similar to those previously observed on the pelagic side of the study area, and the dominant species was Neodenticula seminae, a representative pelagic diatom in the Subarctic Pacific. In the second period, the diatom flux often became much higher than that in the first period, which was due to high dominance of coastal diatoms such as Chaetoceros spp., Thalassiosira nordenskioeldii, and Fragilariopsis spp. Ocean color remote sensing detected a broad water area with high chlorophyll concentration along the east coast of the Kamchatka Peninsula reaching close to Station K2 in May 2009 and 2010. Accordingly, the maximum diatom flux was detected in mid-June 2009. A similar correspondence was previously observed at Station KNOT in 1999.
This is the first comprehensive report on Japanese Gyrosigma spp. with special reference to their taxonomy and distribution. They are very conspicuous in permanent slides because usually they have large valves with characteristic shapes. However they have not been well studied probably because of their paucity in most samples. In total 30 taxa were identified from samples collected from various localities throughout Japan. The following five taxa are newly reported from Japan: G. acuminatum var. curta, G. acuminatum var. lacustre, G. plagiostomum, G. simile, and G. spencerii var. curvula.
An investigation of freshwater Ellerbeckia species in Japan was undertaken with three taxa, E. arenaria f. arenaria, E. arenaria f. teres and E. sp., reported herein. Recent E. arenaria f. arenaria was found at one site in Lake Yamanaka, recent E. arenaria f. teres was also found at one site in Lake Akan, fossil E. arenaria f. teres was found in Early Miocene to Pleistocene sediments from 12 localities and E. sp. was found in Early Miocene sediment. All three taxa are illustrated using LM and SEM images, with an additional stereomicroscope photograph for one taxon.
A new Mesodictyon species is described from the upper Miocene marine sediments of the Nomura Formation, distributed in the Tsugawa area of Aga Town, Niigata Prefecture, Japan. The new taxon exhibits the main characteristics of the genus, with cribra inside the areolae, no valve face fultoportulae and a single rimoportula on the mantle, but has mantle fultoportulae with three satellite pores and areolae with small external projections similar to the basal parts of volae, which distinguishes it from any other Mesodictyon species reported to date. The species is described herein as Mesodictyon japonicum sp. nov., and is illustrated using both LM and SEM photographs. The species has a restricted stratigraphic range in the late Miocene; it first appeared at around 8.6 Ma, became common at 8.2 Ma and disappeared at around 5.5 Ma. Although it has been found in marine sediments, the species is presumed to be a freshwater diatom because it exhibits a similar occurrence pattern to that of the accompanying Aulacoseira species.
Actinocyclus hiramakiensis sp. nov. is described from Early Miocene freshwater sediment of the Hiramaki Formation located in southern Gifu Prefecture, Japan. The species has a round valve face with concentric undulation, radial areola rows usually irregular or loosely packed with areolae in valve center, areolae with external and internal vela, a pseudonodulus located on the valve face/mantle boundary as well as some stalked rimoportulae with long tubes and labia running parallel to the tube axes. The new species has characteristics similar to those of the genus Actinocyclus, but in other ways it differs from existing Actinocyclus species. Light and scanning electronic microscope photographs are provided along with the description.
The present work provides a first insight into the structure of epiphytic diatom assemblages inhabiting some dominant vegetation of the Huwaiza marsh, which is one of the most ecologically important wetlands in the Middle East. A total of 74 taxa belonging to 38 genera were recorded, of which Cocconeis placentula var. euglypta and Fragilaria pulchella predominate. Epiphytic diatom assemblages in Huwaiza marsh are a mixture of truly freshwater taxa (oligohalobous, 39%), brackish water taxa (mesohalobous, 33%) and marine forms (polyhalobous, 12%). Marine taxa have probably been transported into the marsh from the sea.
Morphological characteristics of a planktonic diatom Chaetoceros minimus were described from a brackish-water lake, Lake Nakaumi, Shimane Prefecture, Japan. This taxon is small in size with a single plastid, and has weakly silicified valves. The cell body is almost cylindrical in shape; 4.5–12 µm long in pervalvar axis and 2.0–4.0 µm in diameter. The valves of the frustule are similar, asymmetrically conical in shape with tops located axisymmetrically or asymmetrically to the pervalvar axis. A single long seta (0.5–50 µm) arises from the top of the valve face margin, and runs almost parallel (weakly bent) to the pervalvar axis. Ellipsoidal resting spores were observed, which are 3.5–7.5 µm long in pervalvar axis and 2.2–3.0 µm in diameter. Two types of cell sizes of Chaetoceros minimus are distinguishable (large and small),which suggests a relationship between cell size and water quality (salinity and/or water temperature). In this study, cells from Lake Nakaumi belong to the small type. Moreover, thin and thick cell-walled types are recognized from Lake Nakaumi. It is presumed that this dimorphism in shape corresponds to water temperature and/or cell cycle (vegetative or resting cell).