Extant calcareous nannoplankton in the Australian Sector of the Southern Ocean (austral summers 1994 and 1995)

Living calcareous nannoplankton in the region between Australia and Antarctica are distributed in five assemblages associated with distinct physico-chemical properties of surface and subsurface water masses. Temperature and salinity ranges for living assemblages were 2–15.7°C and 33.7–35.56‰, respectively, with maximum cell densities for austral summer 1994 found at 9.63°C and 34.44‰, and for austral summer 1995 at 12.8°C and 35.17‰. Nutrients (phosphate, silicate and nitrate) increase poleward and vertically from surface to depth. Abundance and diversity of calcareous nannoplankton decrease in a poleward direction with major shifts located across both the Subtropical and the Subantarctic Fronts. Higher cell densities were found below 50 m equatorward of the Subtropical Front and above 50 m poleward of this front. Poleward of the Antarctic Divergence coccolithophores are absent from all samples. Three different morphotypes of Emiliania huxleyi were identified, one of which has a distribution associated with the Subtropical Front. Of the subordinate species Syracosphaera spp, Calciosolenia murrayi and Umbellosphaera tenuis dominate equatorward of the Subtropical Front with Syracosphaera spp and Calcidiscus leptoporus dominant poleward of this front. A peculiar community of weakly calcified species is recorded for the first time outside the Weddell Sea.     

The distribution of Recent benthic foraminifera in the Polar Front region, southwest Atlantic

Twenty-five core-top samples from the Maurice Ewing Bank (MEB) and Islas Orcadas Rise (IOR) were examined to determine the distribution of benthic foraminifera in the vicinity of the Polar Front in the southwest Atlantic Ocean. The Polar Front has a subsurface expression that effects the areal and depth distribution of benthic foraminifera in this region.Three faunal assemblages were identified by Q-mode factor analysis. The shallowest assemblage, dominated by Bulimina aculeata, is present from 1500 to 2600 m on the Maurice Ewing Bank and is associated with potential temperatures of 1.71-0.50°C, salinities of 34.74-34.70‰ and potential density values of 45.84–46.04 sigma-4. A second assemblage, dominated by Uvigerina peregrina, occurs in water depths from 2600 to 3100 m and is associated with potential temperatures of 0.40-0.26°C, salinities of 34.70-34.69‰ and potential density values of 46.05–46.07 sigma-4. The third assemblage is dominated by Nuttallides umbonifera, Ehrenbergina trigona and secondarily by Oridorsalis umbonatus and Pullenia bulloides (the N. umbonifera-E. trigona assemblage) is present form 2770 to 3120 m on the Islas Orcadas Rise. This assemblage is associated with potential temperatures of 0.36-0.14°C, salinities of 34.69-34.68‰, and potential density values of 46.06–46.09 sigma-4. Although the U. peregrina assemblage and the N. umbonifera-E. trigona assemblage overlap bathymetrically, they are present in waters of slightly different properties. The Bulimina aculeata assemblage is within the core of the Lower Circumpolar Deep Water (LCDW), while the other two assemblages occur within transition zones between the LCDW and Weddell Sea Deep Water (WSDW).The difference in the benthic foraminiferal assemblages at similar depths on the Islas Orcadas Rise and the Maurice Ewing Bank is the result of different water-mass regimes separated by the Polar Front.     

Response of nannoplankton to major changes in sea-surface temperature and movements of hydrological fronts over Site DSDP 594 (south Chatham Rise, southeastern New Zealand), during the last 130 kyr

A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocapsa are unique to this site, with E. huxleyi dominating over medium Gephyrocapsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora (E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocapsa, and the combined value of all medium Gephyrocapsa increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive. climatic trends during the last glacial cycle: (1) Syracosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Calcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.     

Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables

Canonical correspondence analysis of diatoms from surface sediment samples and oceanographic environmental variables shows that summer and winter sea-surface temperatures, water depth and winter sea-surface salinity are the main environmental factors affecting diatom distribution around Iceland. Of these, summer sea-surface temperature is the most important. Five diatom assemblages are distinguished and the distribution of these assemblages is clearly correlated with oceanic current patterns in the region. The sea-ice diatom assemblage is limited to the area where the East Greenland Current (Polar Water) has its strongest influence, and the cold diatom assemblage is basically controlled by the less cold East Icelandic Current (Modified Polar Water). The mixing diatom assemblage results from the interaction between the cold East Greenland and East Icelandic Currents and the warm Irminger Current. The warm diatom assemblage is located in the area dominated by the Irminger Current and may be used as an indicator of warm-water masses (Atlantic Water). The coastal diatom assemblage is the only one strongly influenced by both water depth and summer water temperatures.     

Movement of oceanic fronts south of Australia during the last 10 ka: interpretation of calcareous nannoplankton in surface sediments from the Southern Ocean

Preservation of calcareous nannoplankton in surface sediment samples from the Southern Ocean south of Australia and adjacent to New Zealand record a single assemblage. The dominant species are Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus. The assemblage varies little in abundance and diversity with minor correlation to present-day overlying surface water masses and oceanic fronts. Increase in abundance of H. carteri and C. pelagicus in the region of the Subtropical Front may reflect higher nutrients associated with this front. The assemblage, although altered by dissolution, represents a warmer climatic interval than present-day with the presence of preferentially dissolved, warm-water species preserved as far south as the Polar Front. The presence of warm-water species under sub-Antarctic waters at the Polar Front is interpreted as a relic population from the Holocene climatic optimum of 10–8 ka. The absence of coccoliths in sediments poleward of the Polar Front suggests an equatorward shift of this front following the climatic optimum, resulting in increased productivity of siliceous phytoplankton associated with the colder waters and increased dissolution of coccoliths. Movement of the Subtropical Front for the same interval is not recorded in the preserved coccoliths. The more heavily calcified form of E. huxleyi which dominates the living assemblage north of the Subtropical Front is subject to dissolution in this region and is poorly preserved in the sediment assemblage.     

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).     

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.     

Subtropical Front fluctuations south of Australia (45°09′S, 146°17′E) for the last 130 ka years based on calcareous nannoplankton

Calcareous nannoplankton assemblages from a Late Quaternary deep-sea core (GC07; 46°09′S, 146°17′E) south of Australia provide information on regional palaeoceanography and palaeoclimate changes in the Southern Ocean, in particular the movement of the Subtropical Front for the past 130 ka years. Marine Isotope Stages 1–5 are identified through changes in calcareous nannoplankton assemblages, supported by ¹⁴C dates, and oxygen isotope and %CaCO data.Two distinct assemblages are recognised: a warm water assemblage with higher abundances of Calcidiscus leptoporus, Emiliania huxleyi, Helicosphaera.carteri, Syracosphaera pulchra, Gephyrocapsa caribbeanica and Gephyrocapsa oceanica; and, a cold water assemblage with higher abundances of Gephyrocapsa muellerae and Coccolithus pelagicus. Alternation between these two assemblages downcore in GC07 reflect movement of the Subtropical Front across the location and can be correlated to Marine Isotope Stages (MIS) 1–5. Sediments with a cold water assemblage indicate the position of the Subtropical Front equatorward of the site when transitional to sub-antarctic waters were overlying the site. Conversely sediments with a warm water assemblage indicate the Subtropical Front was poleward of GC07 when warmer, subtropical waters were over the site. MIS 1 and 5 are interpreted as warmer than MIS 3 (based on species composition) with the Subtropical Front more poleward than for MIS 3. During MIS 3 the Subtropical Front is interpreted as adjacent to or immediately poleward of GC07. Some species including C. leptoporus and C. pelagicus show negative covariance and are considered to be reliable species in identifying glacial and interglacial intervals in this region.Comparison with established biostratigraphy based on calcareous nannoplankton showed the datum event for the reversal between E. huxleyi and G. muellerae of 73 ka in transitional waters is not applicable in this region. The reversal between these two species occurs between 48 and 30 cm downcore in GC07 with a ¹⁴C date of 11 020 year BP at 49–48 cm, i.e. the reversal event is younger than this date.     

The warm interglacial Marine Isotope Stage 31: Evidences from the calcareous nannofossil assemblages at Site 1090 (Southern Ocean)

Calcareous nannofossil assemblages have been investigated at Ocean Drilling Program (ODP) Site 1090 located in the modern Subantarctic Zone, through the Pleistocene Marine Isotope Stages (MIS) 34–29, between 1150 and 1000 ka. A previously developed age model and new biostratigraphic constraints provide a reliable chronological framework for the studied section and allow correlation with other records. Two relevant biostratigraphic events have been identified: the First Common Occurrence of Reticulofenestra asanoi, distinctly correlated to MIS 31–32; the re-entry of medium Gephyrocapsa at MIS 29, unexpectedly similar to what was observed at low latitude sites.The composition of the calcareous nannofossil assemblage permits identification of three intervals (I–III). Intervals I and III, correlated to MIS 34–32 and MIS 30–29 respectively, are identified as characteristic of water masses located south of the Subtropical Front and reflecting the southern border of Subantarctic Zone, at the transition with the Polar Front Zone. This evidence is consistent with the hypothesis of a northward shift of the frontal system in the early Pleistocene with respect to the present position and therefore a northernmost location of the Subantarctic Front. During interval II, which is correlated to MIS 31, calcareous nannofossil assemblages display the most significant change, characterized by a distinct increase of Syracosphaera spp. and Helicosphaera carteri, lasting about 20 ky. An integrated analysis of calcareous nannofossil abundances and few mineralogical proxies suggests that during interval II, Site 1090 experienced the influence of subtropical waters, possibly related to a southward migration of the Subtropical Front, coupled with an expansion of the warmer Agulhas Current at the core location. This pronounced warming event is associated to a minimum in the austral summer insolation. The present results provide a broader framework on the Mid-Pleistocene dynamic of the ocean frontal system in the Atlantic sector of the Southern Ocean, as well as additional evidence on the variability of the Indian–Atlantic ocean exchange.     

Lanternfish (Myctophidae) Zoogeography off Eastern Australia: A Comparison with Physicochemical Biogeography

In this first attempt to model the distributions of a mesopelagic fish family at this scale in the eastern Australian region (10°S to 57°S), lanternfish species occurrence data spanning a period from 1928 to 2010 were modelled against environmental covariates. This involved: (1) data collation and taxonomic quality checking, (2) classification of trawls into “horizontal” (presence-absence) and “oblique” (presence-only) types, and classification of vertical migration patterns using existing literature and the species occurrence database, (3) binomial GAMs using presence-absence data for representative temperate, subtropical and tropical species to examine depth interactions with environmental covariates and refine the selection of environmental layers for presence-only MAXENT models, (4) Presence-only MAXENT modelling using data from all trawls and the reduced environmental layers, and (5) Multivariate analysis (area-wise and species-wise) of the resulting matrix of logistic score by geographic pixel. We test the hypothesis that major fronts in the region (Tasman Front, Subtropical Convergence, Subantarctic Front) represent zoogeographic boundaries. A four-region zoogeographic scheme is hypothesised: Coral Sea region, Subtropical Lower Water region, Subtropical Convergence/South Tasman region and Subantarctic region. The Tasman Front, Subtropical Convergence and Subantarctic Front represented zoogeographic boundaries. An additional boundary at ∼25°S (coined the ‘Capricorn’ boundary) was adopted to delineate the Coral Sea from Subtropical Lower Water regions. Lanternfish zoogeographic regions are congruent with some aspects of two prevailing physicochemical biogeographic schema in the region, but neither of these schema alone accurately predicts lanternfish distributions. As lanternfishes integrate vertical ocean processes, the hypothesised lanternfish zoogeography may represent a useful model for a generalised pelagic biogeography that should be tested for other oceanic groups.     

Influence of the frontal zones on ichthyoplankton and mesopelagic fish assemblages in the Crozet Basin (Indian sector of the Southern Ocean)

One of the aims of oceanographic campaign MD 68/SUZIL, carried out in austral autumn 1991 in the Indian sector of the Southern Ocean and its adjacent subtropical waters, was to investigate the influence of hydrography on the ichthyoplankton and mesopelagic fish assemblages in the Crozet Basin. It appears that, in contrast to other sectors of the Southern Ocean, the main biogeographical barriers are the Subantarctic Front and the Agulhas Front which appear to be vertical convergence fronts. The importance of the Antarctic Polar Front and the Subtropical Front as barriers to fish seems to be minimized in this area because of its particular hydrological features, such as the lack of a subantarctic zone, the maximum current intensity of the Subantarctic Front between these fronts, and their structures — they are horizontal convergence fronts.     

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.     

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.     

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.     

Polychaete Distribution on the Chatham Rise,Southwest Pacific

Macrobenthic samples Were collected along 3 north-south transects at 244-1394 m depth across the Chatham Rise, eastern New Zealand, to examine the distribution of polychaete assemblages in the region of the Subtropical Convergence (STC). Two main assemblages were identified: at 244-663 m depth occurring mainly on the crest of the Rise, and at 802-1394 m on the slopes of the Rise. Community composition differed., however, between the northern and southern slopes of the Rise. The southern slope supported higher faunal densities and a greater proportion of surface deposit feeders. This latitudinal difference may reflect variability in organic flux across the STC.     

Diatom constraints on the position of the Antarctic Polar Front in the middle part of the Pliocene

The relative percentages of diatom taxa in 5 deep-sea cores (DSDP 266, ODP 699A, ODP 747A, ODP 751 A, and Eltanin Core 50-28) from the Southern Ocean are determined for an interval centered on 3.1 to 2.9 Ma in the middle part of the Pliocene. This climatically warm interval, which is being studied by the PRISM Project of the U.S. Geological Survey, coincides with a proposed interval of major deglaciation of East Antarctica. The maximum southerly position of the Antarctic Polar Front between 3.1 and 3.0 Ma is inferred from these diatom studies, the presence of calcareous nannofossils in the sediments, and sedimentologic and micropaleontologic information from the literature. It is suggested that the Antarctic Polar Front may have migrated by as much as 6 ° of latitude further to the south in the southeastern Atlantic and Indian Oceans during this Pliocene warm interval but probably lay close to its present day position in the southwest Atlantic and Drake Passage. Summer sea surface temperatures are inferred to have been no more than 3 °–4 °C warmer than present at latitudes between 55 ° and 60 °S.     

Sea-surface temperature changes in the North Pacific during the Late Miocene

Twenty-seven radiolarian species and species groups are identified and used in this study. Their stratigraphic ranges span from Late Miocene to modern times and they appear to have distributions which have changed little over this interval of time. Census data for these 27 taxa in surface sediment samples are used to define six assemblages (Q-mode factors). The distribution of these assemblages are similar to modern water-mass distributions and can be statistically related to modern sea-surface temperatures.Census data on these same 27 taxa have been collected in the Upper Miocene intervals of six Pacific sites. These faunal data are described in terms of the modern radiolarian assemblages; and based on the multiple regression equations relating the modern assemblages to modern temperatures, estimates of Late Miocene temperatures are made. The estimated temperatures indicate that there is an overall cooling trend in the later part of the Miocene (magnetic Chron 8 to Chron 5), punctuated by several distinct cooling events. In August the low-latitude sites appear to be slightly cooler, and the mid-latitude sites warmer than in modern times. In February both the low- and mid-latitude sites appear to be warmer than modern temperatures at the same locations. The relatively warm temperatures near the base of the interval studied may be associated with the passage of warm tropical Atlantic waters into the eastern Pacific via the Isthmus of Panama. The general cooling trend seen in these records is thought to be associated with the gradual closure of these straits and the increased influence of the high southern latitudes.     

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.     

Diatoms in northeast Pacific surface sediments as paleoceanographic proxies

Fossil diatom total abundances (# valves/g) in 54 surface-sediment samples from the northeast (NE) Pacific Ocean reflect the position of high primary production associated with coastal upwelling and that possible biases associated with dilution or dissolution are small. Diatom species assemblages, defined by Q-mode factor analysis in 30 samples with abundant diatoms, are related to modern oceanographic properties. Five statistical assemblages, given by five specific diatom species and/or groups, are related to upwelling (Chaetoceros spores), subtropical (Thalassionema nitzschioides), subarctic (Rhizosolenia hebetata), transitional (Neodenticula seminae) and freshwater (freshwater diatoms) ecological environments. These factors are significantly correlated with primary productivity, temperature, nutrient concentrations and salinity, although the strongest relationship is that between diatom assemblages and productivity. However, it is not possible to distinguish between coastal and open-ocean (curl-driven) upwelling based on Chaetoceros spores relative percentages by themselves or on the floral factors.     

Phytoplankton production and biomass at frontal zones in the Atlantic sector of the Southern Ocean

A high resolution study of chlorophyll a and primary production distribution was carried out in the Atlantic sector of the Southern Ocean during the austral summer of 1990–91. Primary production (¹⁴C assimilation) and photosynthetic capacity levels at frontal systems were among the highest recorded during the cruise (2.8–6.3 mgC·m^–3·h^–1, and 1.3–4.7mgC·mgChl a ^–1·h^–1, respectively). Blooms at ocean fronts were strongly dominated by specific size classes and species. This suggests that the increase in biomass was probably the result of an enhancement of in situ production by selected components of the phytoplankton assemblage, rather than accumulation of cells through hydrographic forces. This hypothesis is supported by the high variability of photosynthetic capacities at adjacent stations along the transects. Blooms (ca 2.7–3.5 mg Chl a·m^–3) were found at three oceanic fronts (the Subtropical, Subantarctic and Antarctic Polar Fronts) during the early summer. These were equivalent to, or denser than, blooms in the Marginal Ice Zone and at the Continental Water Boundary. Seasonal effects on phytoplankton community structure were very marked. In early summer (December), netphyto-plankton (>20 m) was consistently the major component of the frontal blooms, with the chain-forming diatoms Chaetoceros spp. and Nitzschia spp. dominating at the Subantarctic and Antarctic Polar Fronts, respectively. During late summer (February), nanophytoplankton (1–20 m) usually dominated algal communities at the main frontal areas. Only at the Antarctic Polar Front did netphytoplankton dominate, with the diatom component consisting almost exclusively of Corethron criophilum. An early to late summer shift of maximum phytoplankton biomass from north to south of the Antarctic Polar Front was observed. Spatial covariance between silicate levels and water-column stability appeared to be the main factor controlling phytoplankton production at the Antarctic Polar Front. Low silicate concentrations may have limited diatom growth at the northern edge of the front, while a deep mixed layer depth reduced production at the southern edge of the front.