Radiolarian assemblages in surface sediments along longitude 175°E in the Pacific Ocean

This study of 24 surface sediment samples along longitude 175°E between latitudes 48°N and 14°S in the western Pacific Ocean documents the relationship between the modern distribution of radiolarian assemblages and present oceanographic conditions. Ninety-two species were classified into eight clusters by using R-mode cluster analysis. Q-mode cluster analysis was used to distinguish five geographical zones, which were interpreted to represent the Tropical, Southern Subtropical, Northern Subtropical, Transitional, and Subarctic climatic zones. Species richness, diversity, and equitability are highest in the Tropical and Transitional Zones. We attempted to formulate new regression equations considering ecological information on the vertical distribution of radiolarian species. Transfer functions were derived from regression analysis of radiolarian assemblages to observed Sea Surface Temperature (SST) values. The transfer functions have a standard error of 1.16 °C in the estimate of winter SST, 0.81 °C in that of summer SST.     

Radiolarian-based transfer functions for estimating paleooceanographic conditions in the south Indian Ocean

A quantitative analysis of 37 radiolarian species in 58 deep-sea surface-sediment samples from the subtropical to the polar regions of the Indian Ocean produced four geographically distinct faunal assemblages (transitional, antarctic, subtropical, subantarctic). Geographic distributions of these assemblages coincide with present-day patterns of sea-surface temperature and water masses. The antarctic factor is almost exclusively found south of today's Antarctic Polar Front. Highest concentrations of the transitional factor are recorded at sites positioned between today's Subtropical Convergence and the Polar Front. The subtropical factor is dominant in sites north of today's Subtropical Convergence.Values of these four faunal assemblages in the surface-sediment samples were regressed onto present-day summer and winter temperatures of the surface waters overlying each of the core-top sites. Resulting transfer functions yield temperature estimates which compare favorably with observed (present-day) summer and winter sea-surface temperatures, with low standard errors of estimate (< ± 1.9°C) and no clear geographic pattern in maps of the residuals (difference between observed and estimated sea-surface temperature).     

The distribution of radiolarian assemblages in the modern and ice-age Pacific

Data on radiolarian abundances from several recent regional studies of the Pacific have been combined with new data from the temperate and tropical area to provide an ocean-wide view of radiolarian distributions. A Q-mode factor analysis of these data identified seven factors which have their areas of dominance in the following regions: tropics, western Pacific, subarctic, Antarctic, transitional zone, temperate regions, and the eastern central water masses. The distributions of these factors tended to follow those of surface water masses and major ocean currents. A more detailed analysis of the temperate and tropical region better delineated the complex flow and counter flow in this area. The relationship between the distribution of modern radiolarian assemblages and the surface circulation of the Pacific can be used to deduce the nature of oceanographic changes which occurred in the past. The modern radiolarian distributions are compared with those mapped at the 18, 000 B.P. level and reveal two major differences in the ice-age Pacific. The Tropical Factor, restricted primarily to the eastern half of the ocean in modern times, ranged across the entire ocean at 18, 000 B.P. and extended into the area of the western boundary currents. The Subarctic Factor, now found mainly in the western subarctic, expanded to the east and south at 18, 000 B.P. and had strong similarities to an assemblage found in the subantarctic area. The expansion of these two dominant assemblages was at the expense of the Western Pacific, Temperate, and Transitional Factors. These differences in the 18, 000 B.P. distributions are thought to be caused by an increase in the influence of arctic air masses in the North Pacific and by a general increase in the wind-driven zonal flow.     

Synchronous and time-transgressive Neogene radiolarian datum levels in the Equatorial Indian and Pacific Oceans

Fifty radiolarian events of early Pleistocene and Neogene age were identified in an E-W transect of equatorial DSDP sites, extending from the Gulf of Panama to the western Pacific and eastern Indian Oceans. Our objective was to document the degree of synchroneity or time-transgressiveness of stratigraphically-useful datum levels from this geologic time interval. We restricted our study to low latitudes within which morphological variations of individual taxa are minimal, the total assemblage diversity remains high, and stratigraphic continuity is well-documented by an independent set of criteria. Each of the five sites chosen (503, 573, 289/586, 214) was calibrated to an “absolute” time scale, using a multiple of planktonic foraminiferal, nannofossil, and diatom datum levels which have been independently correlated to the paleomagnetic polarity time scale in piston core material. With these correlations we have assigned “absolute” ages to each radiolarian event, with a precision of 0.1–0.2 m.y. and an accuracy of 0.2–0.4 m.y. On this basis we have classified each of the events as either: (a) synchronous (range of ages <0.4 m.y.); (b) time-transgressive (i.e., range of ages > 1.0 m.y.); and (c) not resolvable (range of ages 0.4–1.0 m.y.).Our results show that, among the synchronous datum levels, a large majority (15 out of 19) are last occurrences. Among those events which are clearly time-transgressive, most are first appearances (10 out of 13). In many instances taxa appear to evolve first in the Indian Ocean, and subsequently in the western and eastern Pacific Ocean. This pattern is particularly unexpected in view of the strong east-to-west zonal flow in equatorial latitudes. Three of the time-transgressive events have been used to define zonal boundaries: the first appearances ofSpongaster pentas, Diartus hughesi, andD. petterssoni. Our results suggest that biostratigraphic non-synchroneity may be substantial (i.e., greater than 1 m.y.) within a given latitudinal zone; one would expect this effect to be even more pronounced across oceanographic and climatic gradients.We anticipate that the extent of diachroneity may be comparable for diatom, foraminiferal, and nannofossil datum levels as well. If this proves true, global “time scales” may need to be re-formulated on the basis of a smaller number of demonstrably synchronous events.     

Radiolarian distribution and the late pleistocene history of the Southeastern Indian Ocean

A factor analysis, of radiolarian assemblages in 36 trigger-core tops from the Southeast Indian Ocean (between latitudes 40° to 65° S and longitudes 80° to 120° E) gives three temperature-related factors and one related primarily to near-bottom processes. A paleoecologic equation was written relating the surface factor values to the present-day observed sea-surface temperatures for summer and winter. By means of a regression technique, paleotemperatures were estimated at the level of the last glacial maximum (18, 000 years B.P.) in five Eltanin cores and for the last 600, 000 years in core E45-74. The average temperature difference along a longitudinal transect of cores crossing the Antarctic Convergence was 2.2° colder 18, 000 years ago than today. The estimated sea-surface paleotemperatures in E45-74 showed high correlations with radiolarian species diversity indices, the Subantarctic and Antarctic factor values, percent of CaCO₃, δ¹³O, number of radiolarian fragments and percent Antarctissa strelkovi. Based on paleotemperatures and factor values down this core, it is estimated that the Antarctic Convergence was displaced northward to the position of E45-74 six times in the last 600, 000 years.     

Late Quaternary history of atmospheric and oceanic circulation in the eastern equatorial Pacific

Four radiolarian assemblages have been defined in recent seafloor sediments of the equatorial Pacific Ocean. The distribution of these assemblages corresponds to the modern pattern of oceanic circulation and water mass structure in this region: the eastern Pacific shallow permanent thermocline and the Equatorial Undercurrent; Peru Current upwelling and the oxygen minimum; the subtropical water mass; warm western tropical water and the North Equatorial Countercurrent. In twelve cores chosen to transect the region both longitudinally and latitudinally, the distribution of these four assemblages has been reconstructed for six time-intervals during the last 127,000 years: 18,000 B.P. (glacial Stage 2); 36,000 B.P. and 52,000 B.P. (interstadial Stage 3); 65,000 B.P. (glacial Stage 4); 82,000 B.P. and 120,000 B.P. (interglacial Stage 5). Atmospheric and oceanic circulation changes through time have been inferred from the reconstructed microfossil assemblage distributions. Changes in assemblage distributions indicate that variations in intensity, direction and mean position of the tradewinds caused marked changes in the oceanic circulation patterns through the last glacial cycle.Near the end of interglacial Stage 5, the disappearance of the North Equatorial Countercurrent from the eastern Pacific suggests that the mean position of the tradewinds was shifted to the south approximately 5° of latitude relative to the modern position, so that the Northeast trades prevented the flow of the North Equatorial Countercurrent into the eastern Pacific. Near the end of interstadial Stage 3, a change in wind direction occurred from predominantly zonal winds, which enhance equatorial divergence and surfacing of the Equatorial Undercurrent, to more meridional winds, which enhance coastal upwelling associated with the Peru Current.In the tropical Pacific Ocean, late Quaternary changes in atmospheric and oceanic circulation are linked with times of continental ice sheet growth in the Northern Hemisphere (i.e., the interglacial-to-glacial transitions across oxygen isotope stage boundaries $\text{5}\text{4}$ and $\text{3}\text{2}$). The major changes in circulation seem to occur a few thousand years in advance of the glacial episodes, at or near periods of ice sheet growth. This relationship indicates that changes in atmospheric circulation in the tropics led and influenced the development of conditions suitable for polar and continental ice sheet growth in the Northern Hemisphere.     

Radiolaria as a reflection of environmental conditions in the eastern and southern sectors of the Indian Ocean: A new statistical approach

Cluster analysis and species abundance plots of radiolarian abundance counts from core tops from the eastern Indian Ocean between 12° S and 31° S, and the southern Indian Ocean between 31° S and 62.5° S, demonstrate the existence of environmentally-related provinces supporting distinct taxa assemblages. These provinces are closely associated with currents in the eastern sector of the Indian Ocean and with fronts in the southern sector.The radiolarian assemblages correlate strongly with salinity-normalised total alkalinity (NTA) at the sea-surface, with temperature, salinity, and density from the sea-surface to 300 m, and with dissolved oxygen and nitrate and phosphate concentrations from the sea-surface to 100 m. Palaeo-reconstructions of these parameters at the sea-surface have been made for six Last Glacial Maximum (LGM) samples from five eastern Indian Ocean cores. The LGM sea-surface temperature estimates are comparable with those based on planktonic foraminiferal counts of the same samples obtained by other researchers. The reconstructions show that, since the LGM, density increased markedly along the Western Australian coast south of 20° S but changed little further from the Western Australian coast. By contrast, phosphate concentrations were marginally lower than modern values along the Western Australian coast south of 20° S but more than twice modern values in the other LGM samples.The utility of various regression and calibration techniques is discussed. It is concluded that, probably due to the effects of differences in radiolarian habitat, ocean currents, and/or environmental gradients, only one method, weighted averaging — partial least squares, is reliable in a study area of this size and complexity. If other methods are to be used, the study area must be partitioned into at least two separate regions with the major split between the eastern and southern sectors of the Indian Ocean.     

Early Pleistocene glacial-interglacial radiolarian assemblages from the eastern equatorial Pacific

Polycystine radiolaria from ODP Hole 677A in the eastern equatorialPacific were examined at isotopically identified Early Pleistoceneglacial maxima and minima. Two distinct radiolarian assemblagesare recognized, characterizing glacial and interglacial optima.The Glacial Assemblage is characterized by high abundances ofTheocalyptra davisiana, Botryostrobus auritus, Anthocyrtidiumzanguebaricuim andHexacontium enthacanthum. The InterglacialAssemblage is characterized by Tetrapyle octacantha, Octapylestenozoa and Theocorythium vetulum. A comparison of these fossilassemblages with modern radiolarian distribution suggests thatthe Glacial Assemblage represents intensified upwelling of coldadvected water via the Eastern Pacific Boundary Current, whilstthe Interglacial Assemblage indicates climatic ameliorationin the eastern equatorial Pacific, with the prevalence of warm(>21C) tropical/subtropical surface waters. The recognitionof these radiolarian assemblages could be successfully appliedto studies of adjacent east Pacific areas where other palaeoecologicalindicators are lacking.     

The vertical distribution of Radiolaria in the waters surrounding Japan

Geographic and vertical distribution patterns of living Radiolaria are closely related to the characters of the water column. We studied living Radiolaria in samples collected at closely spaced depth intervals in the waters surrounding Japan in order to understand their vertical distribution and its controlling factors. Such information is needed to reconstruct past vertical water mass structure. The Japanese Islands are surrounded by the marginal Japan Sea and the North Pacific Ocean, and from approximately 46° N to 26° N. They thus extend across a large latitudinal range and are exposed to a similarly wide range of environmental conditions, inducing the warm Kuroshio, Tsushima, and Tsugaru Currents and the cold Oyashio Current.We performed plankton tows (mesh size 63 mm) from late May to early June 2002 at eight sites, sampling the upper 200 m of the water column, at seven depth intervals.We recognized five radiolarian assemblages: the Upper Surface Assemblage (0–40 m), the Lower Surface Assemblage (40–80 m), the Surface Assemblage (0–80 m), the Subsurface Assemblage (80–200 m), and the Lower Surface–Subsurface Assemblage (40–200 m) groups. Pseudodictyophimus gracilipes and Tetrapyle octacantha (juvenile) show “tropical submergence”, i.e. they live at the surface in high latitudes and at greater depth in low latitudes. Four taxa live at specific temperature and salinity: Acrosphaera spinosa, Larcopyle butschlii, Challengeron diodon, and Stichocorys seriata. Four taxa are associated with characteristic temperatures: Neosemantis distephanus, Arachnocorys umbellifura, Antarctissa sp. 1, and Saccospyris conithorax.     

Radiolarian distribution in the water column,southern Gulf of California,and its implication in thanatocoenose constitution

The geographical and depth distributions of five radiolarian plankton assemblages, defined through Q-mode Factor Analysis of 24 plankton samples, demonstrate a direct correspondence to the distribution of oceanographic water masses in the southern Gulf of California. Two assemblages are associated exclusively with water masses of the mixed-layer, two represent only subsurface water masses, and one is important in both surface and subsurface environments. The Lithomelissa thoracites group and Peridium spinipes are predominant species in water that is formed at the surface of the Guaymas Basin and subsequently is subducted southward into the Carmen Basin. Plagiocantha panarium is the predominant species in upwelled surface water at the Carmen Basin, while Tetrapyle octhacanta and Botryostrobus scutum follow the incursion of the Subtropical Water Mass into the Gulf of California through the Pescadero and Farallon Basins. Druppatractus variabilis and Porodiscus sp. B occur conspicuously in the upper layer of the Pacific Intermediate Water between approximately 400 and 700 m water depth. Statistical correlation between plankton and sediment assemblages supports previous evidence that the radiolarian thanatocoenose in the Gulf of California is composed primarily of species from the surface mixed-layer. Two exceptions to this are observed in the data. First, the upwelling plankton assemblage was not well correlated to the sediment assemblage, and second, input to the sediment during glacial stages appears to be dominated by subsurface forms.     

Radiolarian-based sea surface temperatures and paleoceanographic changes during the Late Pleistocene–Holocene in the subantarctic southwest Pacific

A high-resolution record of radiolarian faunal changes from Site Y8 south of the Subtropical Front (STF), offshore eastern New Zealand, provides insight into the paleoceanographic history of the last 265 kyrs. Quantitative analysis of radiolarian paleotemperature indicators and radiolarian-based sea surface temperature (SST) estimates reveal distinct shifts during glacial–interglacial (G-I) climate cycles encompassing marine isotope stages (MIS) 8–1. Faunas at Site Y8 are abundant and diverse and consist of a mixture of species typical of the subantarctic, transitional and subtropical zones which is characteristic of subantarctic waters just south of the STF. During interglacials, diverse radiolarian faunas have increased numbers of warm-water taxa (~ 15%) while cool-water taxa decrease to ~ 11% of the assemblage. Warmest climate conditions occurred during MIS 5.5 and the early Holocene Climatic Optimum (HCO) at the onset of MIS 1 where SSTs reach maxima of 12.8 and 12.9 °C, respectively. This suggests that temperatures during the HCO were comparable to the Eemian, one of the warmest interglacial intervals of the Late Quaternary. Glacials are characterized by less diverse radiolarian faunas with cool-water taxa increasing to 49% of the assemblage. Coolest climate conditions occurred in MIS 4 and 2 where SSTs are reduced to 5.4 °C and 4.3 °C, respectively. Radiolarian faunal changes and SST estimates clearly identify major water masses and oceanic fronts in the offshore eastern New Zealand area. During warmest MIS 5.5 and early MIS 1 substantial influence of northern-sourced Subtropical Surface Water (STW) is evident at Site Y8. This implies southward incursions of STW around the eastern crest of Chatham Rise with the STF displaced towards higher latitudes and spinning off eddies as far south as Campbell Plateau. Additionally, increased flow of the Southland Current (SC) might have enhanced the local occurrence of warm-water radiolarians derived from the subtropical Tasman Sea. Coolest glacials are marked by a strong inflow of cool, southern-sourced waters at Site Y8 indicating a more vigorous flow along the Subantarctic Front (SAF).     

Distribution and modification of diatom assemblages in and around a warm core ring in the western North Pacific Frontal Zone east of Hokkaido

Spatial distribution relationships between diatom assemblagesand water types were investigated in and around a 2-year-oldwarm core ring in the western North Pacific Polar Frontal Zoneoff Hokkaido, Japan. From the oceanographic and diatom data,principal component analysis and a clustering technique identifiedfour water types and four diatom assemblages. The BackgroundAssemblage had a low standing stock and was distributed overthe entire study area, including the core water of the ring.This assemblage was associated with water conditions that werenot highly suitable for diatom growth. A Cold Assemblage, probablylinked to submerging Oyashio Water, was found in sub-surfacewaters. Its standing crop was rather high. The two other assemblagesalso had relatively high standing stocks: one, the StratifiedAssemblage, was found around the nutricline in the sub-surfacelayer and the other, the Warm Streamer Assemblage, in a warmstreamer flowing along the outer edge of the warm core ring.We argue here that the Stratified Assemblage emerged from theBackground Assemblage because of favorable local conditions(nutrient level), and, conversely, that in the downstream ofthe warm streamer, the Warm Streamer Assemblage reversed tothe Background Assemblage because the nutrient supply was depleted.     

Modern benthic foraminifera on Banco Chinchorro,Quintana Roo,Mexico

Benthic foraminiferal assemblages may be used as environmental indicators on Banco Chinchorro, an isolated carbonate platform off the eastern coast of the Yucatan Peninsula. Seventy-seven species from 44 genera were identified in 14 surface sediment samples, which were collected along an E–W windward-to-leeward transect across the platform. A total of 15,493 foraminiferal tests (max. 1,200 tests per sample) were investigated and served as the basis of this study. As many taxa range throughout several platform zones, assemblages are better environmental indicators as compared to individual species. Four foraminiferal assemblages were identified using statistical methods including (1) a Homotrema assemblage, which occurs at the windward platform margin, (2) an Archaias-Homotrema assemblage which is found on the leeward margin and on platform interior coral patch reefs, (3) a Quinqueloculininae-Archaias-Rosalina assemblage of the western platform, and (4) an Archaias-Quinqueloculininae assemblage characteristic of the eastern platform interior. Environmental factors which influence variation in foraminiferal distribution and diversity on Banco Chinchorro include exposure to waves and currents and substrate (plant and algal growth). Sediment transport does not play a major role in Banco Chinchorro based on the observation that there are only limited amounts of taxa found outside their typical habitats, and, that mean grain-size and sorting of foraminiferal tests do not exhibit clear patterns. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ordovician radiolaria and conodonts from the peri-Gondwanan Karatau-Naryn microplate (Sarydzhaz, eastern Kyrgyzstan)

Cambrian-Ordovician sedimentary sequences crop out extensively in the area surrounding the Sarydzhaz River, a remote mountainous region situated in the eastern part of Kyrgyzstan. These sequences, composed essentially of fine siliciclastic lithologies, were deposited on a passive margin of the peri-Gondwanan Karatau-Naryn microplate. Palaeontological constraints for the age of these sequences are rare. Recent efforts of geological mapping in the area discovered reasonably well-preserved radiolarian and conodont faunas. The occurrence of conodonts was mentioned previously, but no taxonomic details were ever reported. Two conodont assemblages are identified from the Oldzhobai Formation; the first is dominated by Paracordylodus gracilis, which can be correlated with the upper Tremadocian to lower Floian; the second assemblage is more diverse and characterized by Prioniodus elegans, suggesting a mid Floian age. This is the first discovery of Ordovician Radiolaria from Kyrgyzstan. In spite of its rather moderately good to poor preservation, the fauna is interesting in many respects, especially due to the presence of a new radiolarian species (Inanigutta (?) kyrgyza nov. sp.). The latter is easily identifiable by the distinctive pore pattern of its cortical shell and has therefore the potential to become in the future a biostratigraphic marker species.     

Spatial and temporal distributions of epiphytic diatoms growing on Thalassia testudinum Banks ex König: relationships to water quality

The spatial and temporal distributions of the epiphytic diatom flora on Thalassia testudinum was described within the Florida Bay estuary and at one Atlantic site east of the Florida Keys over a 1-year period. Species of the genus Mastogloia dominated the epiphytic diatom flora (82 out of 332 total species). Nonmetric Multidimensional Scaling (NMDS) and Analysis of Similarity (ANOSIM) revealed four distinct spatial assemblages and two temporal assemblages. Eastern and western Florida Bay assemblages were identified within the estuary. The eastern diatom assemblage was characterized by high relative abundances of Brachysira aponina and Nitzschia liebetruthii, while the western assemblage was characterized by the abundance of Reimerothrix floridensis, particularly during summer. Two diverse and distinct marine assemblages, one located in the Gulf of Mexico along the western edge of Florida Bay and the other behind the Florida reef tract in the Atlantic Ocean, were also identified. Analysis of the spatial distribution of diatoms and water quality characteristics within Florida Bay suggest that these assemblages may be structured by salinity and nutrient availability, particularly P. The Gulf of Mexico and the western Florida Bay assemblages were associated with higher water column salinities and TP concentrations and lower DIN concentrations and TN:TP ratios relative to the eastern Florida Bay assemblage. The temporal variation in diatom assemblages was associated with water temperature, though temporal indicator species were few relative to the number of spatial indicators. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Protist community composition in the Pacific sector of the Southern Ocean during austral summer 2010

Knowledge about the protist diversity of the Pacific sector of the Southern Ocean is scarce. We tested the hypothesis that distinct protist community assemblages characterize large-scale water masses. Therefore, we determined the composition and biogeography of late summer protist assemblages along a transect from the coast of New Zealand to the eastern Ross Sea. We used state of the art molecular approaches, such as automated ribosomal intergenic spacer analysis and 454-pyrosequencing, combined with high-performance liquid chromatography pigment analysis to study the protist assemblage. We found distinct biogeographic patterns defined by the environmental conditions in the particular region. Different water masses harbored different microbial communities. In contrast to the Arctic Ocean, picoeukaryotes had minor importance throughout the investigated transect and showed very low contribution south of the Polar Front. Dinoflagellates, Syndiniales, and small stramenopiles were dominating the sequence assemblage in the Subantarctic Zone, whereas the relative abundance of diatoms increased southwards, in the Polar Frontal Zone and Antarctic Zone. South of the Polar Front, most sequences belonged to haptophytes. This study delivers a comprehensive and taxon detailed overview of the protist composition in the investigated area during the austral summer 2010.     

藏南泽当雅鲁藏布缝合带中的三叠纪放射虫

西藏南部泽当西金鲁村附近发现的一套硅质岩含有两个放射虫化石组合：Capnuchosphaera triassica组合和Pseudostylosphaera nazaroui组合。这些放射虫化石可以同中三叠世拉丁期至晚三叠世卡尼期放射虫动物群对比,这套硅质岩为一外为岩体,包括在由高劈理化泥岩组成的混杂岩基质中,这是雅鲁藏布江缝合带三叠纪放射虫的首次报道。     

Surface lithofacies,biofacies, and diatom diversity patterns as models for delineation of climatic change in the southeast Atlantic Ocean

Distribution of diatom species in surface sediments of the southeast Atlantic Ocean is regulated by present-day oceanographic and hydrodynamic processes. Five assemblages (vectors) defined by factor-vector analysis, reflect different environments and conditions. Assemblage A is a high diversity flora associated with the nutrient-rich, relatively cold waters south of the Polar Front in the diatom ooze belt. Occurrence of this flora on Maud Rise and vicinity may reflect periodic occurrence of a polynya in that area. Assemblage B is a reworked assemblage that dominates the southern portion of the study area. Productivity there is low, reflecting sea ice cover during most of the year. The area north of the Polar Front is dominated by Assemblage C, whose characteristic species reflect the relatively warm Subantarctic Surface Water. Winnowing and frustule breakage have altered Assemblage D (found in three isolated samples) by removing relatively delicate forms leaving lag deposits of more robust species. Assemblage E is a low diversity stress flora, reflecting unstable, unpredictable environments along the Polar Front, Antarctic slope, and the northern boundary of winter sea ice. These sites are characterized by the sinking of cold water. Downcore analysis of cores lying adjacent to the Polar Front and the diatom ooze—pelagic clay boundary show evidence for past climatic variation. The low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality, make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations. The position of the Polar Front has migrated at least three times within the last 0.015 m.y. B.P. Within the last 0.3 m.y. B.P. warm maxima have occurred at approximately 0.0, 0.015, 0.125 and 0.3 m.y. B.P.     

Modern foraminiferal distribution and diversity in two atolls from the Maldives,Indian Ocean

Foraminifera from the coral-reef lagoons of two atolls are used to investigate the distribution and diversity of assemblages in the central Indian Ocean. Eight assemblages and 270 species of foraminifera are identified. Three assemblages are reefal and dominated by Amphistegina and Calcarina. Only one lagoon assemblage is present in both atolls, which is characterized by abundant Ammonia sp. 1 and smaller miliolid foraminifera. In Ari Atoll, abundant Amphistegina sp. 1, Operculina ammonoides, Amphistegina radiata and Nummulites venosa characterize one lagoon assemblage and abundant Neoeponides bradyi and Textularia cf. T. cushmani characterize the other. In Rasdhoo Atoll, abundant Textularia gr. foliacea and Spiroloculina nummiformis characterize one lagoon assemblage and abundant Textularia cf. T. cushmani and Textularia sp. 5 characterize the other. The assemblages compare well with sedimentological characteristics, which is also reflected in their distributions. Species diversity in the Maldives appears to be higher than the western Indian Ocean region due to the stronger influence of central Indo-Pacific faunas. It is not however, as high as the central Indo-Pacific region, which may be due to lower habitat diversity.     

Distribution patterns of diatom surface sediment assemblages in the Laptev Sea (Arctic Ocean)

The modern diatom distribution in the Laptev Sea, Arctic Ocean, was investigated in 89 surface sediment samples. Diatom concentrations are relatively low showing values between 0.01×10⁶ and 6.7×10⁶ valves per gram dry sediment. Based on a factor analysis using seventeen taxa or taxa groups five diatom surface sediment assemblages can be defined: the ice diatom assemblage of the central region of the Laptev Sea, the Chaetoceros assemblage of the eastern and southeastern shelf, the Thalassiosira antarctica assemblage of the continental slope and deep sea, the freshwater diatom assemblage in the vicinity of river mouths and deltas, and the Thalassiosira nordenskioeldii assemblage which shows a patchy occurrence on the central Laptev Sea shelf. The distribution pattern of diatom assemblages in surface sediment is significantly related to oceanographic conditions of surface water masses. The main factors controlling the distribution of diatoms in the Laptev Sea are the riverine freshwater input during the summer which strongly affects the salinity conditions, and the sea-ice extent. Furthermore, the composition of the Thalassiosira antarctica assemblage of the continental slope is largely influenced by dissolution and lateral transport processes.