Distribution Of Radiolarians Research Articles

Comparison of glacial and interglacial oceanographic conditions in the South Atlantic from variations in calcium carbonate and radiolarian distributions

Temperature estimates produced by a radiolarian-based transfer function, factor distributions of radiolarian assemblages, and variations in calcium carbonate were used to reconstruct the oceanographic conditions in the South Atlantic during the last glacial maximum (18,000 yr B.P.). This study suggests that while the position of the Subtropical Convergence at 18,000 yr B.P. was very similar to its present position, the Antarctic Polar Front shifted northward 1° to 3° of latitude in the eastern South Atlantic and 3° to 5° of latitude in the western South Atlantic. The largest temperature changes occurred in the subantarctic region and along the eastern portion of the Subtropical Gyre.

A transfer function for estimating paleoceanographic conditions based on deep-sea surface sediment distribution of radiolarian assemblages in the South Atlantic

A quantitative analysis of radiolarian species in 57 deep-sea surface sediment samples from the South Atlantic Ocean produced four geographically distinct assemblages (tropical, polar, gyre margin, and subtropical). The distributions of these assemblages or factors coincide with present-day patterns of sea-surface temperatures or water masses. These four assemblages were used to construct a transfer function relating radiolarian distribution in the surface sediments to present-day winter and summer temperatures using standard regression techniques. As a test of the quality of this transfer function, temperatures were estimated on surface sediment samples from the eastern South Pacific. The temperatures produced by the transfer function compared favorably with the observed (present-day) winter and summer sea-surface temperatures at these sites.

Radiolarian Distribution, Diversity, and Density in Water Column and Holocene Sediments of Gulf of Mexico and Adjacent Waters: ABSTRACT

Approximately 200 radiolarian species have been collected and identified from the water column and Holocene sediments of the Gulf of Mexico and adjacent seas using Nansen closing nets, DUCA high-speed plankton nets, water bottles, bottom grabs, and gravity and box cores. None of the identified species are endemic to the Gulf of Mexico. Most species appear to be endemic to or indicative of tropical surface water, subtropical underwater, North Atlantic central water, subantarctic intermediate water, and North Atlantic deep water. These water masses enter the Gulf of Mexico through the Yucatan Channel. Living radiolarian diversities (number of species present) and densities (number of individuals/cu m of water filtered) are generally low in waters over the continental shelves in the Gulf of Mexico. Diversities and densities are highest in the surface waters of the open gulf, peak at about 100 m, and decrease to minimum values with increasing water depth. Radiolarians are low in diversity and density in most shelf, slope, and basin-floor Holocene sediments. The fossil record for radiolarians in gulf sediments is characterized by sparse occurrences in surficial Holocene sediments, absence from subsurface Holocene to mid-Miocene sediments, and moderate occurrences in many mid-Miocene and older sediments. Conditions of hypersalinity and/or anoxia appear to enhance radiolarian preservation in fossil sediments in the Gulf of Mexico. Previously unsampled subsurface Holocene to mid-Miocene sediments deposited under these conditions should be useful in future studies of radiolarian biostratigraphy and paleo-oceanography in the gulf. End_of_Article - Last_Page 1598------------

Vertical distribution of polycystine radiolarians in the central North Pacific

Stratified opening/closing net tows in the upper 2000 m of the central North Pacific reveal the major features of the vertical zonation of polycystine radiolarians. Depth profiles of total radiolarian densities show high values at the surface, sharp decrease to a minimum at about 50 m, a maximum exceeding surface levels at about 100 m, and a decrease then to very low values at about 300 m which persist through the rest of the sampled water column. Individual species profiles indicate that the following zones can be identified: (1) A surface zone characterized by a sharp subsurface decrease from high densities at the surface. Densities are an order of magnitude lower by depths of 50–100 m. The rate of decrease resembles the temperature decrease in the thermocline. (2) An upper subsurface maximum zone characterized by species which are absent or very scarce at the surface and reach a pronounced maximum at about 50–100 m. Hydrographic features within the zone are the shallow salinity minimum and the oxygen maximum. A regional chlorophyll maximum occurs at about the same depth as the radiolarian maximum. (3) A lower subsurface maximum zone distinguished from the upper one by occurrence of the maximum about 100 m deeper. Species in this zone are generally scarce at the depth of the upper maximum while species in the upper maximum are scarce at the depth of this maximum. This zone is below the shallow salinity minimum and within the depth of the subjacent salinity maximum which characterizes the habitat of other (tripylean) radiolarians in this area. (4) A deep zone characterized by species which are never abundant but which are consistently absent from the upper few hundred meters. The North Pacific Intermediate Water, previously identified as a tripylean habitat, lies in the depth range of these species, though they may also extend deeper. Despite their lower densities, these species are conspicuous in deep-sea surface sediments.

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 CaCO 3, δ 13O, 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.

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.

Distribution of living polycystine radiolarians in the Gulf of Mexico and Caribbean Sea, and comparison with the sedimentary record

Plankton tows and bottom samples from the Gulf of Mexico and Caribbean Sea were examined to study the distribution, ecology, and preservation of modern radiolarians. Abundances of radiolarians from this region, an area of low primary productivity, are low. Diversities are variable in the surface waters; diversity indices decrease with increasing water depth, indicating that fewer niches are occupied by radiolarians in deep water. Living radiolarians are confined to specific water masses, some species ranging between two masses, and can be used as indicators of these water masses in ecologic or paleo-oceanographic studies. The record of radiolarian tests present in the sediment is dissimilar to the record of living radiolarians in the water column, but is similar to the record of empty tests falling through the water column. This indicates that a considerable length of time is involved in the settling of radiolarian tests. It also confirms the validity of ecologic interpretations based on a study of sediments only.

Relation of radiolarian distributions to subsurface hydrography in the North Pacific

Based on opening-closing net plankton collections, the vertical and horizontal distributions of some triplyean radiolarian species have been plotted for the eastern North Pacific. In a mid-ocean profile of the upper 400 m, there emerge: (1) a zone including subarctic near-surface waters and extending southward at greater depths, (2) a narrow zone to the south concentrated above the first zone, and (3) a zone yet farther south ranging from the surface to a lower boundary that shoals northward. These zones correspond generally to the following respective hydrographic features: (1) the Subarctic Water Mass and the Intermediate Water, centered on a salinity minimum, (2) an unnamed mid-latitude zone characterized by a salinity maximum above the salinity minimum and (3) the warm, high salinity, Central Water Mass. Toward the North American continent, distributions of species from the two more northern (and deeper) zones overlap as the respective water masses become mixed in the California Current. The southernmost (and shallowest) zone retains hydrographic and biologic continuity as a surface layer at the outer edge of the California Current. Semi-closed circulation systems in the hydrographic zones provide physical and biological continuity within the zones with minor interchange among them. The horizontal distributions of these radiolarians in the upper 150 m are comparable to those of other kinds of plankton in the same area, and they are generally consistent, hydrographically, with previously reported distributions of the same species in other oceans.

Radiolarians as possible chronometers of continental drift

The theory of continental drift which has recently received strong support from studies of palaeomagnetism, provides a basis for an experimental approach to historical oceanography. The present North Atlantic ocean may have originated by widening of a rift first opened in Jurassic or Cretaceous time. The widening may have been steady but is more likely to have proceeded by intermittent activity during which much hot magma was abubdantly extruded from underwater rifts. Powerful hydrothermal interaction should have led to enrichment of the world ocean with silicate and other substances of biological importance. Simultaneously, the ocean should have been warmed somewhat.The conditions should have favoured radiolarians which like warmth and require silica. Although radiolarians have proved extremely resistant to evolutionary change, they showed great development in the Jurassic and also in the upper Eocene.The taxonomy and distribution of radiolarians may well provide the means of dating major cataclysms during the separation of North America from Europe.