Radiolarian biogeography in surface sediments of the eastern Indian Ocean

We analyzed recurrent groups of Radiolaria in 74 core top samples from a transect through the eastern Indian Ocean in order to supplement our previous results from the western Indian Ocean (Johnson and Nigrini, 1980). We now identify six distinct recurrent groups and nine radiolarian assemblages in the combined data set of 120 samples; this extended sample coverage has led to several re-interpretations of the oceanographic significance of the radiolarian distribution patterns. Assemblage boundaries closely reflect the presence of major oceanographic fronts and surface currents including the South Equatorial Divergence, Subtropical Gyre, Subtropical Convergence, and Antarctic Convergence. At least four major aspects of the assemblages in the eastern transect are notably different from those in the western transect, leading to a marked east-west asymmetry in faunal distribution patterns across the Indian Ocean. The assemblage formerly associated with strong upwelling near the Arabian Peninsula is present throughout the Bay of Bengal as well, and is interpreted to reflect high salinity and low oxygen in the subsurface waters of the Indian Ocean north of the Equator. A new assemblage has been identified associated with the westward-flowing Pacific water into the eastern Indian Ocean in low latitudes, and may be a potential stratigraphic and paleoclimatic marker for times of low sea level when this westward near-surface flow was shut off (i.e., glacial maxima). An extensive region in the core of the subtropical gyre between 25°S and 35°S is relatively barren of Radiolaria, yet is marked by a characteristic assemblage distributed asymmetrically, perhaps reflecting the lack of a strong boundary current off the west coast of Australia. Assemblage boundaries in the vicinity of the eastward circumpolar flow are not strictly zonal, and may indicate significant deviations from the mean eastward flow as a necessary condition for conservation of potential vorticity when the flow encounters topographic irregularities.     

Distribution of recent planktonic foraminifera in surface sediments of the Mediterranean Sea

The Mediterranean Sea is a partillay isolated ocean where excess evaporation over precipitation results in large east to west gradients in temperature and salinity. Recent planktonic foraminiferal distributions have been examined in 66 surface sediment samples from the Mediterranean Sea. In addition to mapping the frequency distribution of 16 species, the faunal data has been subjected to cluster analysis, factor analysis and species diversity analysis. The clustering of species yields assemblages that are clearly temperature related. A warm assemblage contains both tropical and subtropical elements, while the cool assemblage can be subdivided into cool-subtropical, transitional and polar-subpolar groupings. Factor analysis is used to delineate the geographic distribution of four faunal assemblages. Factor 1 is a tropical-subtropical assemblage dominated by Globigerinoiden ruber. It has its highest values in the warmer eastern basin. Transitional species (Globorotalia inflata and Globigerina bulloides) dominate factor 2 with highest values occurring in the cooler western basin. Factor 3 reflects the distribution of Neogloboquadrina dutertrei and is considered to be salinity dependent. Subpolar species dominate factor 4 (Neoglobuquadrina pachyderma and G. bulloides), with highest values occurring in the northern part of the western basin where cold bottom water is presently being formed. The Shannon-Weiner index of species diversity shows that high diversity exists over much of the western basin and immediately east of the Strait of Sicily. This region is marked by equitable environmental conditions and relatively even distribution of individuals among the species. Conversely, in areas where temperature and salinity values are more extreme, diversity values are lower and the assemblages are dominated by one or two species.     

Planktonic foraminifera from the eastern Indian Ocean: distribution and ecology in relation to the Western Pacific Warm Pool (WPWP)

Faunal assemblages, principal component (PCA), canonical correspondence (CCA), and factor analysis are applied to planktonic foraminifera from 57 core-top samples from the eastern Indian Ocean. The foraminiferal lysocline occurs at 2400 m north of 15°S where carbonate dissolution is induced by the Java upwelling system, and occurs deeper south of 15°S where carbonate dissolution is characteristic of the oligotrophic regions in the Indian Ocean. Dissolution effects, the February standing stock at the time of collection of the plankton-tow material, and different production rates explain the different foraminiferal assemblages found between plankton-tow and core-top samples. Core-top samples are differentiated by PCA into four groups — Upwelling, Western Pacific Warm Pool (WPWP), Transitional, and Southern — that are related to environmental variables (temperature, salinity and nutrients); all environmental variables follow a strong latitudinal component as indicated by the CCA analysis. Similarly, three assemblages are recognized by factor analysis: Factor 1 (dominated by Globigerinoides sacculifer, G. ruber, Globigerinita glutinata and Globorotalia cultrata), factor 2 (dominated by Globigerina bulloides and Globorotalia inflata) and factor 3 (dominated by Neogloboquadrina dutertrei) explain more than 92% of the variance, and are related to sea-surface temperature, thermocline depth and nutrient levels. The seasonal influence of the Java upwelling system supplies nutrients, phyto- and zooplankton to the oligotrophic eastern Indian Ocean (factor 1). South of 24°S, a deep chlorophyll maximum, a deep euphotic zone, a deep thermocline, SSTs below 22°C, and brief upwelling pulses seem to explain factors 2 and 3. The ratio of G. sacculifer and N. dutertrei, two mutually excluding species, appears to indicate the southern boundary of the WPWP. This ratio is applied to core Fr10/95-11 to demonstrate past shifts of the southern boundary of the WPWP.     

Multivariate analysis and quantitative paleoecology of benthic foraminifera in surface and Late Quaternary shelf sediments,northern Canada

Benthic foraminifera in surface sediments from northeastern Canada and the Beaufort Shelf were studied to determine the quantitative relationship between recent assemblage distributions and modern water depth, temperature and salinity. Factor analysis shows that 7 factor assemblages account for 92% of the variance between samples. Multiple linear regression shows that different factors are significantly related to water depth, August bottom water temperature or salinity. Multiple correlation coefficients for depth, temperature and salinity are 0.94, 0.72 and 0.92, respectively. Labrador fjords contain arenaceous faunas dominated by eitherSaccammina atlantica andReophax fusiformis or byEggerella advena. Regression analysis shows that theSaccammina-Reophax factor is most strongly related to salinity. TheEggerella factor is related to depth and temperature. A fauna dominated byTrochammina nana andBuliminella elegantissima is correlated with warm, deep water in Davis Strait. Calcareous assemblages dominated byElphidium excavatum orCassidulina reniforme are found in Arctic bays and shelf areas. These factor assemblages are strongly related to temperature and depth;Cassidulina is also related to salinity. AnIslandiella helenae fauna on the inner Labrador Shelf is strongly correlated with depth and salinity. ANonionellina labradorica fauna in deep shelf basins is closely related to salinity.The multiple regression model for the sea-bed data estimates water depth, August bottom water temperature and salinity within confidence limits of ± 67 m, 1.3°C and 1.0‰. Paleoecological transfer functions were used to quantify environmental changes recorded by the foraminifera in two piston cores on a transect across the Labrador Shelf. Paleo-depth estimates obtained from the core data produce relative sea-level curves which are similar in shape to theoretical curves for this region. Paleo-salinity and -temperature estimates for offshore Core 12 show that the change from a postglacialNonionellina fauna to a late glacialE. excavatum fauna is accompanied by a decrease of about 2‰ in salinity and 4°C in temperature. Paleoecological estimates for fjord Core 111 show that the change from a modernSaccammina-Reophax fauna to a mid-HoloceneI. helenae fauna corresponds to a salinity increase of 3–5‰ and temperature increase of 1–2°C.     

Planktonic ciliates in the northwestern Indian Ocean: their abundance and biomass in waters of contrasting productivity

The abundance and biomass of planktonic ciliates in the northwesternIndian Ocean ranged from 31 l^–1 and 0.1 µg C l^–1in oligotrophic open-ocean waters to 823 l^–1 and 1.2 µgC l^–1 in more productive waters of the equator, northernArabian Sea and Gulf of Oman. ³Present address: British Antarctic Survey, High Cross, MadingleyRoad, Cambridge, CB3 OET, UK     

Distribution of myctophid resources in the Indian Ocean

Mesopelagics are one of the largest under-exploited marine resources with wide distribution in the world oceans. Lanternfishes are the key members of mesopelagic communities and the total resource in the world oceans is estimated at 600 million tons. Lanternfishes belong to the family Myctophidae which comprises of about 250 species in 35 genera. Myctophids account for about 75% of total global catch of small mesopelagic fishes. They are known to exhibit diel vertical migration, concentrating during the day time between 400 and 1,000 m, and between 5 and 100 m, during the night. In this paper, an attempt is made to review the existing information on the occurrence and distribution of myctophid resources in the Indian Ocean. 137 myctophid species have been reported from the entire Indian Ocean. Studies in the Arabian Sea have indicated that the area is rich in the midwater fish stocks dominated by myctophids with an estimated potential of 100 million tons.     

Late Quaternary deep and surface water mass evolution in the northeastern Indian Ocean inferred from carbon and oxygen isotopes of benthic and planktonic foraminifera

New planktonic and benthic foraminiferal stable isotope records from core YDY05 (northeastern Indian Ocean) provide new insights into paleoceanographic changes in the northeastern Indian Ocean since the last glacial period. The distinct δ¹⁸O decrease was observed since the beginning of the deglaciation to the mid-Holocene (∼8–6 kyr BP), possibly reflecting a reduction in surface salinity in the central Bay of Bengal (BoB) water, which probably resulted from strengthened precipitation, concurrent enhanced river discharge and rising sea-level, related to the intensification of Indian Summer Monsoon (ISM). Variations in benthic δ¹³C and δ¹³C_{Planktonic-Benthic} in our core site reflect significant variations in source water characteristics over the LGM-Holocene. The large δ¹³C_{Planktonic-Benthic} offset during the glacial period suggests a more sluggish deep water circulation, and lower δ¹³C_{Planktonic-Benthic} from the deglaciation to the Holocene suggests an enhanced deep water circulation in the central BoB. The drastic depletion in benthic δ¹³C during the glacial period suggests a significant reduction of North Atlantic Deep Water (NADW) intrusion and a progressive influx of Antarctic Bottom Water (AABW) and ¹²C-rich Circumpolar Deep Water (CDW) into the central BoB. In contrast, since the deglaciation, the central BoB experienced a drastically increased intrusion of better ventilated and ¹³C-rich NADW. The differences in benthic δ¹⁸O between the LGM section and the Holocene exceeds the ice volume effect by ∼0.5‰, providing further evidence that the deep water mass of the central BoB was influenced by the less dense NADW, instead of the AABW, since the last deglaciation.     

The surface ocean productivity response of deeper water benthic foraminifera in the Atlantic Ocean

We test the relationship of deep sea benthic foraminiferal assemblage composition to the surface ocean productivity gradient in the low latitude Atlantic Ocean using 81 surface sediment samples from a water depth range between 2800 and 3500 m. The samples are selected so that the surface ocean productivity gradient, controlling the flux of organic carbon to the seabed, will be the most important environmental variable. The first two principal components of the assemblage data account for 73% of data variance and are clearly linked to the productivity gradient across the Atlantic. These components show that under higher productivity the assemblages contain a higher abundance of Uvigerina peregrina, Melonis barleeanum, Globobulimina spp. and other taxa with probable infaunal microhabitats. Alabaminella weddellensis, a species linked to episodic phytoplankton debris falls, is also important in these assemblages. As productivity decreases there is a regular shift in assemblage composition so that low productivity assemblages are dominated by Globocassidulina subglobosa and several Cassidulina species along with Epistominella exigua. We hypothesize that these taxa are epifaunal to very shallow infaunal since nearly all organic carbon oxidation occurs near the sediment-water interface in low productivity settings. Discriminant function analysis of the foraminiferal assemblages, with groups selected on the basis of surface ocean productivity, shows clear separation among five productivity levels we used. This analysis demonstrates that productivity variations have a strong influence on assemblage composition. Finally, we used two groups of samples from the Rio-Grande Rise representing water depths from 2007 to 2340 m and 2739 to 3454 m to test for effects produced by changing water depth. All these samples are from a low productivity region and represent nearly identical environmental conditions. Although the low productivity nature of all the Rio-Grande Rise samples is obvious, there are assemblage differences between our depth groups. We cannot account for the assemblage differences with changes in organic carbon flux, dissolution effects or other physical/chemical properties of the ocean. Thus there are as yet unidentified factors related to water depth which cause some assemblage variation in the low productivity setting we investigated.     

Bacterial diversity in surface sediments from the Pacific Arctic Ocean

In order to assess bacterial diversity within four surface sediment samples (0–5 cm) collected from the Pacific Arctic Ocean, 16S ribosomal DNA clone library analysis was performed. Near full length 16S rDNA sequences were obtained for 463 clones from four libraries and 13 distinct major lineages of Bacteria were identified (α, β, γ, δ and ε-Proteobacteria, Acidobacteria, Bacteroidetes, Chloroflexi, Actinobacteria, Firmicutes, Planctomycetes, Spirochetes, and Verrucomicrobia). α, γ, and δ-Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria were common phylogenetic groups from all the sediments. The γ-Proteobacteria were the dominant bacterial lineage, representing near or over 50% of the clones. Over 35% of γ-Proteobacteria clones of four clone library were closely related to cultured bacterial isolates with similarity values ranging from 94 to 100%. The community composition was different among sampling sites, which potentially was related to geochemical differences.     

Lower Cretaceous benthic foraminifera from DSDP Site 263: micropalaeontological constraints for the early evolution of the Indian Ocean

Analysis of 66 samples from DSDP Site 263 (Cores 263-4R-4 to 263-29R-4) reveals a unique faunal composition with a predominance of agglutinated taxa, many of them previously unrecorded from any other DSDP and ODP Indian Ocean sites. A total of 66 agglutinated and 31 calcareous taxa are documented and five new species are described: Hippocrepina gracilis n.sp., “Textulariopsis” elegans n.sp., Aaptotoichus challengeri n.sp., “Gaudryinopsis” pseudobettenstaedti n.sp. and “Gaudryina” cuvierensis n.sp. Three assemblages are recognized based on changes in the composition of dominant taxa and occurrences of stratigraphically important species: (1) a high-diversity Valanginian to Barremian Bulbobaculites-Recurvoides Assemblage (Cores 263-29R to -18R), comprised of numerous elongate agglutinated forms, rare nodosariids, and variable numbers of tubes and ammodiscids; (2) a moderately diverse Aptian to Albian Rhizammina-Ammodiscus-Glomospira Assemblage (Cores 26318R to -7R) with highly fluctuating numbers of the nominate taxa and Haplophragmoides, Trochammina, Verneuilinoides spp., and Vemeuilina howchini; (3) a very low diversity Albian or younger Assemblage (Cores 263-6R to -4R) containing sparse agglutinated foraminifera, rare nodosariids and rotaliids. We interpret the assemblages as shelf to lower slope and consider them to reflect a deepening palaeobathymetry as the Cuvier margin subsided after the initial breakup of East Gondwana during the Valanginian. Our interpretation is in sharp contrast with initial palaeodepth estimates of less than 100 m, as well as with original chronostratigraphic interpretations based on foraminifera and nannofossils which correlated the base of the recovered interval with the Aptian. The absence of many cosmoplitan forms, despite high diversity, suggests strong faunal differentiation in the Austral realm or endemisn within the Cuvier Basin during the Early Cretaceous.     

Distribution of epiplanktonic Chaetognatha along a transect in the Indian Ocean

Chaetognaths were identified and counted in 23 samples collectedin the west Indian Ocean along a transect from the Gulf of Adento the Cape of Good Hope, in February–March 1967. Althoughthe hydrographic front located at 5°S does not seem to representa barrier for the distribution of the majority of the species,some of them are in a preferential area south or north of thefront. The principal component analysis indicates the locationswhere the more intense overlapping of species takes place. Itis important to mention the occurrence of S.pseudoserratodentatain the proximity of Madagascar Island.     

Distribution of arenaceous foraminifera in depth profiles of the Southern Ocean (kerguelen plateau area)

Recent to subrecent benthonic arenaceous foraminifera from two areas of the Southern Ocean have been investigated with regard to their depth distribution and agglutination patterns. Strong differences exist between the arenaceous faunas from (a) diatomaceous sediments west of the Kerguelen Plateau and (b) glacial marine sediments from the slope of the Antarctic continent. Major ecologic influences are seen in the effect of sedimentological factors (especially the clay content of the sediment), in the saturation level of calcium carbonate and in the nutrient condition of the biotope. Since these factors are a function of geographic position and related water masses, the main differences of deep-sea faunas are primarily induced by the overall situation of an oceanic area rather than by depth or oceanographic factors like temperature or salinity. Availability of colloidal iron compounds also seems to be a limiting factor for distinct arenaceous species.     

印度洋深海沉积物石油烃降解菌分离、鉴定与多样性分析

烃类物质在海洋环境中广泛分布,深海可能含有特殊的烃降解微生物。本研究通过对西南印度洋中脊与印度洋中部深海沉积物中石油降解菌的富集培养和分离鉴定,从6个站点的样品中共分离得到800株菌,通过BOX-PCR去重复菌株后,对其中183株菌进行了16S rRNA基因序列分析,发现这些菌分属于23个属;其中,γ-变形菌纲的食烷菌属(Alcanivorax)和放线菌纲的微杆菌属(Microbacterium)占优势。此外,还发现了食烷菌属2个潜在的新种、假海栖菌属(Pseudooceanicola)1个潜在新种。高通量测序结果证明,富集菌群中γ-变形菌纲是优势菌,主要包括食烷菌属、盐单胞菌属(Halomonas)、海杆菌属(Marinobacter)等。结合可培养菌与高通量测序结果,食烷菌属、盐单胞菌属、海杆菌属、交替假单胞菌(Pseudoalteromonas)、海源菌属(Idiomarina)与微杆菌属(Microbacterium)是沉积物样品中常见的石油烃降解菌,迪茨氏菌属(Dietzia)、红球菌属(Rhodococcus),假单胞菌属(Pseudomonas)、赤杆菌属(Erythrobacter)与副球菌属(Paracoccus)等可能也参与了烃的降解。     

Coccolith distribution patterns in surface sediments of Equatorial and Southeastern Pacific Ocean

This study aims to contribute to a more detailed knowledge of the biogeography of coccolithophores in the Equatorial and Southeastern Pacific Ocean. Census data of fossil coccoliths are presented in a suite of core-top sediment samples from 15°N to 50.6°S and from 71°W to 93°W. Following standard preparation of smear slides, a total of 19 taxa are recognized in light microscopy and their relative abundances are determined for 134 surface sediment samples. Considering the multivariate character of oceanic conditions and their effects on phytoplankton, a Factor Analysis was performed and three factors were retained. Factor 1, dominated by Florisphaera profunda and Gephyrocapsa oceanica, includes samples located under warm water masses and indicates the occurrence of calcite dissolution in the water column in the area offshore Chile. Factor 2 is related to cold, low-salinity surface-water masses from the Chilean upwelling, and is dominated by Emiliania huxleyi, Gephyrocapsa sp. < 3 μm, Coccolithus pelagicus and Gephyrocapsa muellerae. Factor 3 is linked to more saline, coastal upwelling areas where Calcidiscus leptoporus and Helicosphaera carteri are the dominant species.     

印度洋表层海水石油降解菌的多样性分析

【目的】为了研究印度洋石油降解菌多样性,并获得新的石油降解菌。【方法】本研究通过印度洋表层海水样品采集、以柴油与原油1:1混合物作为碳源,从中富集、分离筛选石油降解菌,并通过PCR-DGGE对13个站点富集菌群的菌群结构进行分析。【结果】通过形态观察、生理生化反应和16SrRNA分析,共得到共29个属的51株不同的细菌,它们主要是属于α亚群和γ亚群。其中,Alcanivorax属(占18%),Novosphingobium属(占10%)和Marinobacter(占6%)Thalassospira(占6%)为主要的优势菌。通过生态多样性分析表明,Shannon-Winner指数(H)为4.57968,说明其具有较高的多样性;均匀度指数(E)为0.8664771,表示其分布比较均匀。单菌实验表明,49株具有石油降解能力其中,Sinomonas,Knoellia,Mesoflavibacter等属的细菌为首次发现有降解能力。DGGE分析表明Alcanivorax属的细菌是印度洋表层海水中的重要石油降解菌。【结论】本研究首次揭示了印度洋表层海水中石油降解菌的多样性,并获取了若干在海洋石油污染生物修复中具有应用前景的降解菌。     

Distribution of surface zooplankton and seabirds across the Southern Ocean

Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.     

Distribution of living planktonic foraminifera in the Ross Sea and the Pacific sector of the Southern Ocean (Antarctica)

In order to determine the factors controlling the distribution of planktonic foraminifera as a proxy for reconstruction of paleoenvironments, we present data on live assemblages collected in the Southern Ocean. Plankton tows and hydrographic measurements were taken in the upper 400 m of the water column at different sites in the Ross Sea (site B) and at the Polar Front of the Pacific Ocean (site O) during austral summers from 1998 to 2003.Based on qualitative micropaleontological observations we discriminated between Neogloboquadrina pachyderma dextral (dex) and N. pachyderma sinistral (sin). In addition for N. pachyderma (sin) we distinguished four morphs: the first one (1) has a thickened test and depressed sutures; the second morph (2) is represented by specimens characterized by a subspheric and heavily encrusted test; the third morph (3) has a thin and lobate walled test; the fourth one (4) represent the juvenile stage of N. pachyderma (sin) and is characterized by a smaller average size.The microfauna collected in the Ross Sea (site B) is characterized by the dominance of N. pachyderma (sin) (morphs 1 and 2), whereas low occurrences of Turborotalia quinqueloba, N. pachyderma (dex) and Neogloboquadrina dutertrei were noted in the first 50 m of the water column. The water column at this station is characterized by a marked and shallow stratification and a marked thermocline during the sampling season.At the ocean station (site O), the assemblage shows increasing diversification: T. quinqueloba, G. bulloides, N. pachyderma (dex) and few specimens of Globigerinita uvula characterize the planktonic microfauna. There is a predominance of non-encrusted morphs and juvenile specimens (3 and 4). At this station the mixed surface layer is deeper than in the Ross Sea (60–70 m), the pycnocline and the thermocline less marked.The depth and the intensity of the Deep Chlorophyll Maximum (DCM) influence foraminiferal distribution: N. pachyderma (sin) shows abundance peaks at or just below the DCM while G. bulloides peaks above the DCM. Coiling direction of N. pachyderma seems to be not controlled exclusively by Sea Surface Temperature (SST): probably the two coiling types are genetically different.Results document that diversity of planktonic foraminifera, number of specimens and variations in test morphology are related to regional differences in water properties (temperature, salinity, and DCM depth).     

Diatoms from surface sediments of the northern part of Lake Tanganyika

227 Diatom taxa were observed in the surface sediments of the northern part of Lake Tanganyika, including 1 new to science: Amphora tanganyikae. The diatom community of these sediments is mainly composed of benthic organisms while planktonic diatoms are rather rare. Many brackish-water and a few marine organisms were observed. Cosmopolitan organisms (77.1%) dominate the diatom flora but tropical, tropical African and African taxa are also well represented (22.9%)Deceased.Deceased.     

Paleoceanographic interpretations of coccoliths and oxygen-isotopes from the sediment surface of the southwest Indian Ocean

The distribution of forty-four coccolithophore species in one hundred deep-sea core-tops from the southwest Indian Ocean is described. Three coccolith assemblages have been recognised (Maputo, Agulhas Current and deep water) by the relative abundances of four ecologically significant coccolithophore species (Gephyrocapsa oceanica, Emiliania huxleyi, Calcidiscus leptoporus and Umbilicosphaera sibogae). Their biogeographical distribution appears to be related to water temperature, nutrient concentration and dissolution.The degree of preservation of coccoliths and foraminifera indicates that the carbonate lysocline lies somewhere between 3500 and 4000 m, resulting in the concentration of dissolution-resistant microfossils below this depth.Stable oxygen isotope ratios of the planktonic foraminiferal species Globigerinoides sacculifer range between −1.5 to −1.0‰ PDB (equal to 22.8–25.1°C) and occur in a narrow band on the sea floor beneath the “A” route of the Agulhas Current.These values are about 0.5 per mil PDB lighter than samples analyzed on either side of this band and can be explained by the Agulhas Current's elevated temperature at the ocean surface of 2–3°C. Thus an oxygen isotope imprint of the Agulhas Current exists beneath it on the sea floor.The Agulhas Current is probably the major factor influencing sedimentation, sediment-distribution patterns and geological features in the study area. At present it is voluminous and fast flowing, possibly eroding sediments up to 2500 m below the surface.The oxygen-isotope ratios and nannoplankton counts obtained in this study indicate, however, that the majority of samples are most probably recent or at least not older than 85,000 years. This implies that sediments are accumulating on the ocean floor and that the Agulhas Current does not have a pronounced erosional influence, at least in areas from which cores were retrieved for this study.     

Fourier shape analysis of Globorotalia truncatulinoides from late Quaternary sediments in the southern Indian Ocean

Fourier shape analysis has been used to determine the morphological variation within the planktonic foraminifera,Globorotalia truncatulinoides from the southern Indian Ocean. Measured specimens are from 20 Recent core top samples distributed from 22°S (central gyre water mass) to 47°S (subantarctic water mass). Fourier analysis determined that the two most significant components of shape within this species are elongateness and conicalness which are characterized by harmonics 2 and 3, respectively. Mean harmonic 2 values are significantly correlated with the percentage of sinistral coiled individuals in each sample (r = +0.91). Mean values for harmonic 3 have a correlation coefficient with water temperature and salinity at 200 m of r = +0.85 and +0.88. Specimens become less elongate and more conical in a traverse from south to north.Fourier shape analysis of planktonic foraminifera was then shown to be useful in investigations of late Quaternary paleoceanographic problems. Four piston cores were examined from the southern Indian Ocean. Total faunal and oxygen isotope analyses provided the stratigraphic framework. The results indicated that the central gyre water mass remained relatively stable during glacial stages. The region of the Subtropical Convergence was strongly affected, however, by the northward migration of the Polar Front.