The oxygen isotope composition of planktonic foraminifera from the Cariaco Basin,Venezuela: Seasonal and interannual variations

Material collected during a three-year sediment trapping experiment in the Cariaco Basin, Venezuela (January 1997 to December 1999) is used to examine both temporal and inter-species variability in the oxygen isotope composition of planktonic foraminifera. Specifically, this study compares the oxygen isotope composition of six species of planktonic foraminifera (Globigerinoides ruber (pink), Globigerina bulloides, Neogloboquadrina dutertrei, Orbulina universa, Globorotalia menardii and Globorotalia crassaformis) with the climatology and hydrography of the region, and evaluates the application of each species for use in paleoceanographic reconstructions. The isotope results are consistent with known depth habitats for all six species. The lowest δ¹⁸O values (− 1 to − 2‰) were measured on G. ruber (pink) and G. bulloides, two species that live in the surface mixed layer. Values for deeper-dwelling species such as N. dutertrei, G. menardii and G. crassaformis are higher, predominantly ranging from 0 to − 0.5‰. Temperature estimates derived using species-specific paleotemperature equations indicate that G. ruber (pink) accurately estimates sea surface temperatures (SSTs) throughout the year, while G. bulloides temperature estimates are similar to measured surface temperatures only during the upwelling season (January–April). For the remainder of the year, the δ¹⁸O-derived temperatures for G. bulloides typically are lower than the measured SST. Although the maximum flux of all species occurs during upwelling, the flux-weighted annual mean isotopic composition of the six species indicates that only G. bulloides is biased towards this season. Therefore, we conclude that the sediment δ¹⁸O record of G. ruber (pink) is most suitable for estimating past values of mean annual SST, while G. bulloides provides information on conditions during spring upwelling. The depth of calcification of N. dutertrei varies seasonally in response to changes in the depths of the thermocline and chlorophyll maximum. As a result, the δ¹⁸O difference between G. ruber (pink) and N. dutertrei provides an estimate of the annual surface to thermocline temperature gradient in the basin.     

The oxygen isotope composition of planktonic foraminifera from the Guaymas Basin,Gulf of California: Seasonal,annual, and interspecies variability

Sediment trap samples collected over a seven-year period (February 1991–October 1997) from Guaymas Basin in the Gulf of California were used to study the oxygen isotope composition of five species of planktonic foraminifera, Globigerinoides ruber (white), Globigerina bulloides, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardii. The δ¹⁸O data were analyzed for temporal and interspecies variability and were compared to local hydrography to evaluate the use of each species in reconstructing past oceanographic applications. The two surface dwelling species, G. ruber and G. bulloides displayed the lowest δ¹⁸O values (～ 0.0 to ? 5.0‰), while δ¹⁸O values for the thermocline dwelling N. dutertrei, P. obliquiloculata, and G. menardii were higher (～ 0.0 to ? 2.0‰). The δ¹⁸O of G. ruber most accurately records measured sea surface temperatures (SSTs) throughout the year. G. bulloides δ¹⁸O tracks SSTs during the winter–spring upwelling period but for the remainder of the year records slightly colder, subsurface temperatures. The difference between the δ¹⁸O of the surface dwelling species, G. ruber and G. bulloides, and that of the thermocline dwelling species, N. dutertrei, P. obliquiloculata, and G. menardii, was used to estimate the surface to thermocline temperature gradient. During the winter these δ¹⁸O differences are small (～ 0.50‰) reflecting a well-mixed water column. These interspecies δ¹⁸O differences increase during the summer (～ 1.90‰) in response to the strong thermal stratification that exists at this time of year.     

Reconstruction of Indian monsoon variability over the past 230,000 years: Planktic foraminiferal evidence from the NW Arabian Sea open-ocean upwelling area

We determined the faunal composition and total number of tests (#/g) of planktic foraminifera (> 125 μm) in core KH00-05 GOA 6 near Oman in order to decipher monsoon-induced variability of oceanographic productivity in the open-ocean upwelling area in the northwest Arabian Sea. The core contains a continuous record of sedimentation over the last 230 kyr, with the age model based on oxygen isotope and accelerator mass spectrometry ¹⁴C dates. We focused on species (Globigerina bulloides and Globigerinita glutinata) typical for SW monsoonal upwelling and species typical for NE monsoon conditions (Neogloboquadrina incompta, Neogloboquadrina dutertrei, Globigerinoides ruber, and Globigerinoides sacculifer). The changes in relative abundance of these monsoonal indicators suggest that the open-ocean upwelling area was dominated by the SW monsoon during interglacial periods, but by the NE monsoon during glacial periods.Increases in total test abundance during glacial periods confirmed that the NE monsoon rather than SW monsoon contributes largely to planktic foraminiferal productivity in this area. We argue that three types of circumstances resulted in high productivity, with nine high productivity events occurring at a 23-kyr frequency. The first type caused high productivity events at 102 and 199 ka (interglacial periods), characterized by the dominance of upwelling species, indicating high productivity during strong SW monsoons, correlated with high July insolation at 45° N. An exceptional high productivity event occurred at 37 ka during interglacial marine isotope stage (MIS) 3, with contributions from both SW and NE monsoons. The second type of high productivity event occurred at 61, 147, and 175 ka, during glacial periods, characterized by dominance of NE monsoon species, and correlated with low January insolation at 45° N. In addition, a high productivity event at 85 ka (interglacial period) also was induced by enhanced NE monsoons. The last two high productivity events occurred during transitional periods from glacial to interglacial (MIS 6/5.5 and 2/1), were characterized by the replacement of NE monsoon species with upwelling species, and corresponded to abrupt climate warming, suggesting that they are related to both accelerated SW monsoon systems and reduced NE monsoon systems.     

Paleoceanographic conditions in the western Caribbean Sea for the last 560 kyr as inferred from planktonic foraminifera

Faunal analyses of planktonic foraminifera and upper-water temperature reconstructions with the modern analog technique are studied and compared to the magnetic susceptibility and gamma ray logs of ODP Core 999A (western Caribbean) for the past 560 kyr in order to explore changes in paleoceanographic conditions in the western Caribbean Sea. Long-term trends in the percentage abundance of planktonic foraminifera in ODP Core 999A suggest two hydrographic scenarios: before and after 480 ka. High percentage abundances of Neogloboquadrina pachyderma and Globorotalia inflata, low abundances of Globorotalia menardii and Globorotalia truncatulinoides, low diversity, and sea-surface temperatures (SST) under 24 °C are typical characteristics occurring from 480 to 560 ka. These characteristics suggest a “shallow” well-oxygenated upper thermocline and the influx of nutrients by either seasonal upwelling plumes and/or eddy-mediated entrainment. The second scenario occurred after 480 ka, and it is characterized by high and fluctuating percentage abundances of Neogloboquadrina dutertrei, G. truncatulinoides, G. menardii, Globigerinita glutinata, Globigerinella siphonifera, and Globigerinoides ruber; a declining trend in diversity; and large SSTs. These characteristics suggest a steady change from conditions characterized by a “shallow” thermocline and chlorophyll maximum to conditions characterized by a “deep” thermocline (mainly during glacial stages) and by more oligotrophic conditions. The influence of the subtropical North Atlantic on the upper thermocline was apparently larger during glacial stages, thus favoring a deepening of the thermocline, an increase in sea-surface salinity, and a dramatic reduction of nutrients in the Guajira upwelling system. During interglacial stages, the influx of nutrients from the Magdalena River is stronger, thus resulting in a deep chlorophyll maximum and a fresher upper ocean. The eddy entrainment of nutrients is the probable mechanism responsible of transport from the Guajira upwelling and Magdalena River plumes into ODP 999A site.     

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.     

Late Quaternary Protoperidinium cysts as indicators of paleoproductivity in the northern Arabian Sea

The reliability of organic-walled cysts of the heterotrophic dinoflagellate Protoperidinium as paleoproductivity indicators and the influence of bottom water oxygenation on cyst preservation is assessed by using Arabian Sea records of the past 125 kyr as a natural laboratory. Multidisciplinary geochemical, micropaleontological and palynological datasets are integrated to analyze the relationship between Protoperidinium cyst concentrations and other paleoproductivity proxies. Differential preservation potential is quantified in order to establish threshold oxidative degradation values for a possible application of quantitative Protoperidinium cyst records in paleoenvironmental reconstructions. Results indicate that variations in Protoperidinium cyst concentration closely correspond to other marine productivity and/or upwelling proxies. Although oxygenation will lead to significant cyst degradation, and thus decreased concentrations, down-core patterns in Protoperidinium cyst concentration still primarily reflect changes in sea surface productivity. In view of differential preservation among dinoflagellate cysts, down-core variations in relative abundance of Protoperidinium should be treated with caution.     

Millennial-scale variability in western tropical Atlantic surface ocean hydrography during the early Pliocene

We use high-resolution oxygen isotope data (δ¹⁸O) from planktonic foraminifera in the western tropical Atlantic Ocean (Ocean Drilling Program Leg 154, Site 925) to investigate millennial-scale climate variability during an interval of relative climate warmth, the early Pliocene. For this purpose, we have chosen a 100-kyr-long time interval from 4.13 to 4.24 Ma and subsampled it to obtain an average time step of 800 yr. We reconstruct changes in upper ocean hydrography using the δ¹⁸O values of Globigerinoides sacculifer, a mixed-layer dweller, and Neogloboquadrina dutertrei, a thermocline dweller. Their oxygen isotopic difference (Δδ¹⁸O) is taken as a measure of the mixed layer to thermocline thermal gradient. Time series analysis indicates that significant concentration of variance exists in the G. sacculifer and N. dutertrei at sub-Milankovitch periods of between 8 and 4 kyr, and in the Δδ¹⁸O record between 13 and 8 kyr. Wavelet analysis illustrates that the suborbital variance is only present in the record when the amplitude of the precessional signal is large between 4.24 and 4.20 Ma. In this particular portion of the record, however, we observe positive δ¹⁸O excursions in the individual δ¹⁸O time series. Thus, the suborbital periods evident in the spectra may reflect harmonics associated with asymmetrical time series. Because the excursions only occur when precession forcing is also strong, we suggest that there is a relationship between the proxy records and climate, although, we cannot conclude that it is cyclical in nature. The Δδ¹⁸O record on the other hand is characterized by positive as well as negative excursions. We observe significant concentration of variance close to half precession during the portion of the record when precession forcing is also strong, which we believe reflects a close, although nonlinear, response of the western tropical surface ocean to low-latitude insolation forcing.     

Sea surface temperature changes during May and August in the western Arabian Sea over the last 22 kyr: Implications as to shifting of the upwelling season

In the western Arabian Sea (WAS), the highest seasonal sea surface temperature (SST) difference presently occurs between May and August. In order to gain an understanding on how monsoonal upwelling modulates the SST difference between these two months, we have computed SST for the months of May and August based on census counts of planktonic foraminifers by using the artificial neural network (ANN) technique. The SST difference between May and August exhibits three distinct phases: i) a moderate SST difference in the late Holocene (0–3.5 ka) is attributable to intense upwelling during August, ii) a minimum SST difference from 4 to 12 ka is due to weak upwelling during the month of August, and iii) the highest SST difference during the last glacial interval (19 to 22 ka) with high Globigerina bulloides % could have been caused by the occurrence of a prolonged upwelling season (from May through July) and maximum difference in the incoming solar radiation between May and August. Overall, variations in the SST difference between May and August show that the timing of intense upwelling in the Western Arabian Sea over the last 22 kyr has been variable over the months of June, July and August.     

Western equatorial Pacific planktic foraminiferal fluxes and assemblages during a La Niña year (1999)

A correlation between foraminiferal community dynamics and environmental conditions may provide a basis for establishing paleoclimatic proxies. We studied planktic foraminiferal shell fluxes and assemblages in samples collected in three time-series sediment trap deployments in the western equatorial Pacific under La Niña conditions from January to November 1999. Eleven species contributed about 90% of the total flux in all traps. Two sites (MT1, MT3) in the Western Pacific Warm Pool region (WPWP) were characterized by common occurrences of the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei. Site MT5 farther to the east in the equatorial upwelling region had common occurrences of Globigerina bulloides, Globigerinita glutinata, and Pulleniatina obliquiloculata. Very high abundances of G. bulloides and G. glutinata at MT5 indicate that equatorial upwelling (EU) occurred during the 1999 La Niña. The two western sites have similar assemblage compositions, but MT1 ( 135°E) has the highest fluxes (up to  3800 tests m^− 2 day^− 1), whereas MT3 ( 145° E) has fluxes below  2200 tests m^− 2 day^− 1. Relatively high fluxes (up to  3000 tests m^− 2 day^− 1) occur at site MT5 ( 176° E), where upwelling occurred.The differences in faunal composition in the WPWP and EU might be attributable to differences in the way in which nutrients are supplied to the phytoplankton: large amounts of suspended material are supplied to the WPWP by advection of waters passing through the coastal region of an archipelago, whereas upwelling of nutrient-rich waters enhances primary production in the EU. At the westernmost site in the WPWP, a peak in the G. bulloides flux coincided with southward flow of the New Guinea Coastal Current (NGCC) in late February, but the highest G. ruber flux coincided with northward flow of this current in late May. Thus, the differences in species dominance at this location may be caused by monsoon-driven variability in the flow direction of the NGGC.     

Distribution and ecology of planktonic foraminifera from the seas around the Indonesian Archipelago

Planktonic foraminiferal assemblages in 50 core-top samples from the western and southern areas of the Indonesian Archipelago and 29 core tops retrieved northwest of Australia were grouped using cluster analysis. These assemblages make it possible to sub-divide the studied area in five provinces: 1/ the Banda/Java region (I); 2/ the Timor region (II); 3/ the Java upwelling region (III); 4/ the Indian monsoon Sumatra region (IV), and 5/ the NW Australia margin region (V). The foraminiferal assemblage groups reflect differences in sea-surface temperature, salinity, thermocline depth, and nutrient supply between these five provinces. These differences are related to surface circulation patterns. The carbonate dissolution is rather intense compared to that in other areas of the eastern Indian Ocean. Within the studied area, the strongest dissolution occurs in samples from the Java upwelling region, with the lysocline level rising above ∼2800 m. The increase in abundance of Globigerina bulloides at 10–8 ka BP in core SHI-9034 (the Java upwelling region) corresponds to the decrease in core SHI-9006 (the Banda/Java region) which indicates an intensification of upwelling in relation to a strengthened southeastern monsoon over the studied area.     

Living Neogloboquadrina pachyderma sin and its distribution in the sediments from Oman and Somalia upwelling areas

Microfaunal analysis of plankton nets collected offshore Oman/Yemen and sediment traps offshore Somalia shows that Neogloboquadrina pachyderma sin is abundant during the SW monsoon upwelling. The sediment traps recorded the highest flux (136 specimens m⁻² day⁻¹) and the highest relative frequency (3.55% of the total foraminifera assemblage) of the species during this period. During the intermonsoon it became less abundant and decreased in size, and only very few N. pachyderma sin were found in the water column during the NE monsoon. Sediment trap and Recent sediment data collected along a downslope transect off Somalia show that the species frequency decreases offshore. The highest concentrations of N. pachyderma sin in plankton nets off Oman/Yemen were found at a depth of 300–500 m. However, the oxygen isotope compositions of N. pachyderma sin at the depth of 300–500 m and from the surface 8 m are identical, and also similar to that of Globigerina bulloides which reflects properties of the upper 25 m of the water column. The carbon isotope values are relatively consistent within the studied samples from both areas except for the specimens from the surface water samples offshore Oman/Yemen. The data show that the δ¹⁸O ratio of N. pachyderma sin is in equilibrium with the ambient water while the δ¹³C values are at least 0.8‰ lower. We infer that N. pachyderma sin in the Arabian Sea reproduces, grows and calcifies in the upper 25 m of the water column, but possibly descends into deeper waters later during its life cycle.     

Morphological,geochemical, and ecological differences of the extant menardiform planktonic foraminifera Globorotalia menardii and Globorotalia cultrata

Taxonomic consistency is the prerequisite for any foraminiferal study. In particular, interpretation of planktonic foraminiferal geochemical data requires consistent selection of monospecific tests due to varying habitats of different species. In order to exemplify the impact of slightly varying morphological characteristics on geochemical signatures, we investigated three basic shape dimensions, test weights, stable isotope values, and Mg/Ca ratios of Globorotalia menardii and Globorotalia cultrata, which are often grouped or referred to as synonyms. Along with visual distinction, encrusted G. menardii is distinguished from shiny G. cultrata by its on average doubled test weight, when referring to the same maximum diameter. More elongated and flattened G. cultrata calcifies within the deeper mixed layer, which applies also for the initial calcification of G. menardii. Final encrustration of G. menardii after vertical migration is suggested to take place close to the bottom of the seasonal thermocline. Combined Mg/Ca ratios versus calcification–temperature calibration provides a reliable tool for monospecific thermal reconstructions for the seasonal thermocline (G. menardii) and the deeper mixed layer (G. cultrata).     

Using species-specific paleotemperature equations with foraminifera: a case study in the Southern California Bight

Species-specific paleotemperature equations were used to reconstruct a record of temperature from foraminiferal δ¹⁸O values over the last 25 kyr in the Southern California Bight. The equations yield similar temperatures for the δ¹⁸O values of Globigerina bulloides and Neogloboquadrina pachyderma. In contrast, applying a single paleotemperature equation to G. bulloides and N. pachyderma δ¹⁸O yields different temperatures, which has been used to suggest that these species record the surface-to-thermocline temperature gradient. In Santa Barbara Basin, an isotopically distinct morphotype of G. bulloides dominates during glacial intervals and yields temperatures that appear too cold when using a paleotemperature equation calibrated for the morphotype common today. When a more appropriate paleotemperature equation is used for glacial G. bulloides, we obtain more realistic glacial temperatures. Glacial–interglacial temperature differences (G–I ΔT) calculated in the present study indicate significant cooling (8–10°C) throughout the Southern California Bight during the last glacial maximum (LGM). The magnitude of glacial cooling varies from 8°C near the middle of the Southern California Bight (Tanner Basin and San Nicolas Basin) to 9°C in the north (Santa Barbara Basin) and 9.5–10°C in the south (Velero Basin and No Name Basin). Our temperature calculations agree well with previous estimates based on the modern analog technique. In contrast, studies using N. pachyderma coiling ratios, U^k′₃₇ indices, and transfer functions estimate considerably warmer LGM temperatures and smaller G–I ΔT.     

Surface water conditions in the Northern Benguela Region (SE Atlantic) during the last 450 ky reconstructed from assemblages of planktonic foraminifera

Planktonic foraminiferal records from Site 1083 (ODP Leg 175) were used to investigate changes in surface water conditions in the Northern Benguela Region over the past 450 ky. The assemblages of planktonic foraminifera are dominated by four species: sinistral coiling Neogloboquadrina pachyderma, dextral coiling N. pachyderma, Globigerina bulloides and Globorotalia inflata. Besides, tropical species deliver a small contribution to the assemblage.The most prominent temporal variations, displayed by N. pachyderma (s+d), represent changes in the coastal upwelling and the presence of cold, nutrient rich waters over the core site. Neogloboquadrina pachyderma (s+d) shows cyclic variability in the eccentricity and, to a less extent, precession frequencies. The changes indicate increased upwelling intensity in glacial maxima and precession maxima, and correlate well with the wind-strength record of Stuut et al. (2002). During glacial maxima, steep temperature gradients over the Southern Hemisphere caused strong SE trade winds and strong upwelling. Precession maxima cause a weak monsoonal circulation, more zonal SE trade winds, strong coastal upwelling, and nutrient-rich surface waters over ODP Site 1083.Advection of Angola Current (AC) surface water into the Walvis Basin, indicated by the tropical species, occurs when the Angola Benguela Front (ABF) is positioned southward. Occasionally, this happened during glacial maxima, as can be explained with the reconstructed and predicted meridional movements of the ABF. The amount of AC water was never sufficient to suppress the marine biological production at the core site.The contribution of Benguela Current (BC) water, reflected by Globorotalia inflata, is greatly determined by the upwelling. In periods of strong upwelling, the BC influence is suppressed.In several glacial substages, the temperature of the upwelling South Atlantic Central Water (SACW) may have been increased, as suggested by the dominance of Neogloboquadrina pachyderma (d) in the upwelling record. This phenomenon may be due to intensified subduction in the central South Atlantic that induces the formation of SACW, or to larger contributions of Eastern SACW to the upwelling water.Around 250–200 ky BP, a long-term shift to higher productivity occurred that is absent in the upwelling record. It was accompanied with a transition from a precession and obliquity variability to an eccentricity dominated variability in the Globorotalia inflata (BC) record. The shift was probably connected to a long-term southward shift of the circumpolar oceanic frontal systems south of the African continent.     

Surface-water cooling and salinity decrease during the Middle Miocene climate transition at Southern Ocean ODP Site 747 (Kerguelen Plateau)

Paleoceanographic variability at southern high latitude Ocean Drilling Program (ODP) Site 747 was investigated in this study through the interval which spans the Middle Miocene Climate Transition (MMCT). Between 15.0 and 12.2 million years ago (Ma), foraminiferal δ¹⁸O records derived from both benthic (Cibicidoides spp.) and planktonic taxa (Globorotalia praescitula and Globigerina bulloides) reveal a history of changes in water column thermal and salinity structure and a strong imprint of seasonality. Prior to the MMCT, in the interval between 14.35 and 13.9 Ma, G. bulloides displays relatively high δ¹⁸O values similar to those of G. praescitula, interpreted to indicate weakening of the thermocline and/or increased seasonality with cooler early-spring and/or late-fall temperatures. Following this interval, G. bulloides δ¹⁸O values diverge significantly from benthic and G. praescitula values, with G. bulloides values remaining relatively low for at least 600 kyr following the benthic foraminiferal δ¹⁸O shift during the MMCT at ~ 13.9 Ma. This divergence in δ¹⁸O records occurs in direct association with the Mi3 cooling and glaciation event and may suggest: (1) a strengthening of the vertical temperature gradient, with greater cooling of deep waters than surface waters, (2) changes in the depth habitat of G. bulloides, (3) changes in the dominant season of G. bulloides calcification, (4) modification of surface-water δ¹⁸O values in association with enhanced sea-ice formation, (5) increased surface-water carbonate ion concentration, and/or (6) a significant decrease in surface-water salinity across the MMCT. The first of these possible scenarios is not likely, particularly in light of recent Mg/Ca evidence for significant surface-water cooling in the Southern Ocean associated with the MMCT. Of the remaining possibilities, we favor a change in surface salinity to explain the observed trends in δ¹⁸O values and hypothesize that surface salinity may have decreased by up to 2 salinity units at ~ 13.9 Ma. In this scenario, the development of a lower-salinity Antarctic surface layer coincided with regional cooling of both surface and deep waters of the Southern Ocean during the Mi3 glaciation of East Antarctica, and contributed into the dominance of Neogloboquadrina spp. between 13.8 and 13.2 Ma. Additionally, the distinct patterns observed in planktonic foraminiferal δ¹⁸O records spanning the MMCT correspond with changes in the vertical δ¹³C gradient between planktonic and benthic foraminiferal records and major changes in planktonic foraminiferal assemblages at Site 747, providing further evidence of the environmental significance of this climatic transition.     

Chrysymenia tigillis sp. nov. (Rhodymeniales, Rhodophyta) from the Sultanate of Oman, with a census of currently recognized species in the genus Chrysymenia

Chrysymenia tigillis sp. nov. is described on the basis of a few specimens collected from Dhofar, Sultanate of Oman. The new species is known only from southern Oman, a region of the northern Arabian Sea that is strongly impacted by the upwelling from the summertime monsoon. It is distinguished from other species of the genus by the simple nature of the blades, their dimensions (to 75 cm in length and to 21 cm in width), the rough, bumpy surface of the blades, and the presence of internal struts connecting the inner sides of the blades. A census of the currently recognized species in the genus Chrysymenia is provided.     

Western equatorial Pacific productivity and carbonate dissolution over the last 550 kyr: Foraminiferal and nannofossil evidence from ODP Hole 807A

We analyzed foraminiferal and nannofossil assemblages and stable isotopes in samples from ODP Hole 807A on the Ontong Java Plateau in order to evaluate productivity and carbonate dissolution cycles over the last 550 kyr (kilo year) in the western equatorial Pacific. Our results indicate that productivity was generally higher in glacials than during interglacials, and gradually increased since MIS 13. Carbonate dissolution was weak in deglacial intervals, but often reached a maximum during interglacial to glacial transitions. Carbonate cycles in the western equatorial Pacific were mainly influenced by changes of deep-water properties rather than by local primary productivity. Fluctuations of the estimated thermocline depth were not related to glacial to interglacial alternations, but changed distinctly at ∼ 280 kyr. Before that time the thermocline was relatively shallow and its depth fluctuated at a comparatively high amplitude and low frequency. After 280 kyr, the thermocline was deeper, and its fluctuations were at lower amplitude and higher frequency. These different patterns in productivity and thermocline variability suggest that thermocline dynamics probably were not a controlling factor of biological productivity in the western equatorial Pacific Ocean. In this region, upwelling, the influx of cool, nutrient-rich waters from the eastern equatorial Pacific or of fresh waters from rivers have probably never been important, and their influence on productivity has been negligible over the studied period. Variations in the inferred productivity in general are well correlated with fluctuations in the eolian flux as recorded in the northwestern Pacific, a proxy for the late Quaternary history of the central East Asian dust flux into the Pacific. Therefore, we suggest that the dust flux from the central East Asian continent may have been an important driver of productivity in the western Pacific.     

Palaeoceanographic record of the last deglaciation in the Strait of Sicily

The palaeoceanographic evolution of the Levantine waters during the last deglacial time is investigated using the sedimentary record of a deep sea core, CS 70-5, from the Linosa basin (35° 44.4′N/13° 11.0′E, 1486 m water depth). Radiocarbon dating and oxygen isotope stratigraphy based on¹⁸O changes inGlobigerina bulloides allow us to recognize and to date the different steps of the deglaciation. These steps are synchronous with those reported in the North Atlantic, but correspond to a δ¹⁸O decrease of higher amplitude than in the Alboran sea or in the North Atlantic Ocean. Major faunal events permit the establishment of a local biozonation which differs from those reported either for the Alboran sea or the Eastern Mediterranean basin. Major breaks in the faunal assemblages occurred during Termination I_A, within the first step of the deglaciation around 14 kyr B.P., and near 10.6 kyr B.P. within the Younger Dryas. The onset of the last deglaciation induced important changes in the characteristics of the Levantine and Atlantic water masses which occupied the Strait of Sicily. The δ¹³C records ofGlobigerina bulloides andCibicidoides pachyderma indicate that the modifications observed in the assemblages of deep faunas are controlled by the oxygenation of the water column. δ¹³C records ofGlobigerina bulloides are similar throughout the West Mediterranean as well as in sediments located below the present Mediterranean outflow. The distributional pattern as well as its δ¹³C record suggest that this species could be a good recorder of the upper Levantine waters and, more precisely, of the mixing layer of the overlying Atlantic waters with the Levantine ones.Major influxes of Atlantic waters during Terminations I_A and I_B could have slowed down the vertical mixing of the different water layers present in the Strait of Sicily and caused a decrease in the oxygenation of the water column. During Termination I_B the effect of Atlantic influxes was reinforced by the occurrence of a low salinity layer in the eastern basin which led to the stagnation of the deep waters. The two episodes of decreased oxygenation in the Levantine waters also favored the precipitation of inorganic magnesium calcite.     

Linkages between surface and deep circulations in the southern Japan Sea during the last 27,000 years: Evidence from planktic foraminiferal assemblages and stable isotope records

We analyzed planktic foraminiferal assemblages, oxygen and carbon isotope records, and the presence or absence of laminations to reconstruct the paleoenvironments of the southern Japan Sea since the last glacial period. Data were collected from two well-dated cores. One core (water depth 999 m) included thinly laminated mud layers, the other (water depth 283 m) contained nonlaminated sediments. Tephrochronology and accelerator mass spectrometry ¹⁴C dating of 14 horizons revealed that the two cores contained continuous records of the last 27 cal kyr. A total of 13 planktic foraminiferal species belonging to six genera were identified in down-core samples. The typical indicators of the Tsushima Current water, Globigerinoides ruber, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerinoides tenellus, and Globigerinita glutinata occurred since 9.3 cal kyr BP. Neogloboquadrina incompta, which was the dominant species in the Tsushima Current region of the modern Japan Sea, first occurred at 8.2 cal kyr BP and dominated the assemblage since 7.3 cal kyr BP. These results clearly indicate that the warm Tsushima Current started to inflow into the Japan Sea at 9.3 cal kyr BP, and the modern surface conditions in the southern Japan Sea were essentially established at 7.3 cal kyr BP. Our data and comparison of the presence or absence of laminated sediments in three locations from the southern Japan Sea suggest that deep circulation during the deglacial period was weaker than that at present. In addition, deep circulation in the modern Japan Sea, which supplies oxygen-rich water to the entire basin, started probably in association with the first inflow of the Tsushima Current beginning at 9.3 cal kyr BP.     

Comparison of seasonal flux variations of planktonic foraminifera in sediment traps on both sides of the Ryukyu Islands,Japan

Variations in the fluxes of planktonic foraminifera were analyzed based on sediment trap data collected over a period of 10 months, from October 1994 to August 1995, at stations JAST01 and JAST02, located on both sides of the Ryukyu Islands, Japan. Station JAST01 (latitude 27°23′N, longitude 126°44′E) was deployed at a depth of 1000 m in the Okinawa Trough, East China Sea, along the flow axis of the Kuroshio Current, west of the islands, whereas station JAST02 (latitude 25°4′N, longitude 127°34′E) was deployed at a depth of 3000 m in the Ryukyu Trench, along the western margin of the northwest Pacific, east of the Islands. The total planktonic foraminiferal fluxes (TFFs) in the eastern station were high in winter, when the surface-water column was vertically well mixed. In contrast, the TFF peaks did not show significant seasonality in the western station, but values fluctuated with the northwest–southeast oscillation of the Kuroshio axis. Among the 36 planktonic foraminiferal species identified in the trap samples, Globigerina bulloides, Globigerinita glutinata, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerinoides ruber, Globigerinoides sacculifer and Globigerina falconensis exhibited a greater shell flux. On both sides of the Ryukyu Island Arc, the fluxes of G. ruber and G. sacculifer increased synchronously with the seasonal warming of surface waters, whereas that of G. falconensis increased during winter, when the water column was vertically well mixed as a result of the NW monsoon. In contrast to these species, which exhibited similar seasonal flux variation patterns at both stations, the species G. bulloides, G. glutinata, N. dutertrei, and P. obliquiloculata displayed different flux variations on both sides of the islands. These fluxes seem to be regulated by phytoplankton productivity, which is controlled by the vertical structure of the water column on the eastern Ryukyu Trench side and by the oscillation of the Kuroshio axis on the western Okinawa Trough side. Lateral transport of suspended planktonic foraminiferal shells to the Okinawa Trough might exist, but is not prominent enough to wipe out the original features of the planktonic foraminiferal flux. The species G. falconensis is an indicator of winter mixing on both sides of the Ryukyu Islands, and may possibly be used as a proxy to trace the intensity of the paleo-winter monsoon.