Global calibration of ecological models for planktic foraminifera from coretop carbonate oxygen-18

A global dataset of coretop planktic foraminiferal δ¹⁸O_c is combined with a global database of seawater δ¹⁸O_w observations and ocean climatologies to determine robust optimum parameters for an ecological model for six commonly measured species and varieties. These parameters consist of the temperature ranges, optimum temperatures, depth habitat, and the amount of secondary calcification for Globigerinoides ruber (white), G. ruber (pink), Neogloboquadrina pachyderma (l), N. pachyderma (r), Globigerinoides sacculifer, and Globigerina bulloides. This approach produces ecological models and temperature ranges consistent with previous work, and manages to reproduce the coretop oxygen-18 carbonate values remarkably well. The standard error of modelled values of coretop calcite globally and for all species is 0.53‰, compared to an error of 1.2‰ when assuming annual average mixed layer equilibrium calcite.     

Planktic foraminiferal molecular evolution and their polyphyletic origins from benthic taxa

Phylogenetic analyses based on partial sequences of the small subunit (SSU) ribosomal (r) RNA gene have shown that the planktic and benthic foraminifera form a distinct monophyletic group within the eukaryotes. In order to determine the evolutionary relationships between benthic and planktic foraminifers, representatives of spinose and non-spinose planktic genera have been placed within a molecular SSU rDNA phylogeny containing sequences of the benthic suborders available to date. Our phylogenetic analysis shows that the planktic foraminifers are polyphyletic in origin, not evolving solely from a single ‘globigerinid-like’ lineage in the Mid-Jurassic, but derived from at least two ancestral benthic lines. The benthic ancestor of Neogloboquadrina dutertrei may have entered the plankton later than the Mid-Jurassic, and further investigation of related extant species should provide an indication of the timing of this event. The evolutionary origin of the non-spinose species Globorotalia menardii remains unclear. The divergences of the planktic spinose species generally support recent phylogenies based on the fossil record, which infer a radiation from a globigerinid common ancestor in the Mid- to Late Oligocene. The branching pattern indicates that there are possibly four distinct groups within the main spinose clade, with large evolutionary distances being observed between them. Globigerinoides conglobatus clusters strongly with Globigerinoides ruber and are divergent from Globigerinella siphonifera, Orbulina universa and Globigerinoides sacculifer.Conserved regions of the SSU rRNA gene show sufficient variation to discriminate foraminifers at the species level. Large genetic differences have been observed between the pink and white forms of Gs. ruber and between Ge. siphonifera Type I and II. The two types of Ge. siphonifera cannot be discriminated by traditional palaeontological methods, which has considerable implications for tracing fossil lineages and for the estimation of molecular evolutionary rates based upon the fossil record. The conserved regions show a high degree of sequence identity within a species, providing signature sequences for species identification. The variable regions of the gene may prove informative for population level studies in some species although complete sequence identity was observed in G. sacculifer and O. universa between specimens collected from the Caribbean and Western Pacific.     

The occurrence of two genotypes of the planktonic foraminifer Globigerinoides ruber (white) and paleo-environmental implications

Planktonic foraminifera provide a record of the upper ocean environment in the chemical and isotopic composition of individual shells. Globigerinoides ruber is a common tropical–subtropical planktonic foraminifer, and this species is used extensively for reconstruction of the paleo-environment. The different stable isotopic compositions of two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.), first identified in sediments, suggested that G. ruber s.s. was dwelling in the upper 30 m of the water column and G. ruber s.l. at greater depths. Plankton tows and sediment trap experiments provided additional evidence distinguishing the two morphotypes and their habitats and invited the question as to whether the two morphotypes could be distinguished genetically. In this study, using phylogenetic analysis of nuclear partial small subunit ribosomal DNA (SSU rDNA) gene sequences representing 12 new and 16 known sequences, we identified four genotypes within G. ruber white variation; one of which is a sister group of Globigerinoides conglobatus, whereas the three others were sister groups of the G. ruber pink variation. Moreover, these two major groups corresponded to morphological differences described as G. ruber s.l. and s.s., respectively. This genetic evidence corroborates differences between the two morphotypes in the isotope record, and it will contribute to a more precise reconstruction of the thermal structure of the water column.     

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.     

Oxygen and carbon isotope studies in recent foraminifera from the southwest Indian ocean

Eighteen species of planktonic foraminifera have been analyzed for their oxygen and carbon isotopic composition in five Recent samples of deep-sea sediment from the southwest Indian Ocean; one sample of glacial age and one mid-Holocene sample were also studied. On the basis of oxygen isotopic composition three groups are recognized. Species in the first group (Globigerinoides ruber, G. sacculifer and G. conglobatus; G. Globigerina rubescens and Globigerinita glutinata) calcity in the near-surface Tropical Water, so that the oxygen isotopic composition of their test carbonate may be used to indicate surface temperature. Species in the second group (Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Orbulina universa, Globigerinella siphonifera and Sphaeroidinella dehiscens) are associated with the sub-surface high-salinity Subtropical Water, so that their oxygen isotope composition indicates trends in the temperature of this water mass. The third group (the species of Globorotalia) calcity in the deeper Central Water. The average oxygen isotopic composition of each Globorotalia species is more or less constant over the range studied and does not reflect the surface temperature trend.The carbon isotopic composition of three species (Globigerina rubescens, Globigerinoides ruber and Globigerinita glutinata indicate departure from isotopic equilibrium by at least 3%₀. Among the remaining species the variation of carbon isotopic composition with depth (where depth is inferred from the temperature estimated from oxygen isotopic composition) implies that N. dutertrei, P. obliquiloculata and G. siphonifera occupy the shallow subsurface oxygen minimum, while the deeper-dwelling globorotaliids approach the deeper oxygen minimum. Hence it is possible, despite scatter among the data, to discern the pattern of oxygen content with depth in the overlying water masses from an examination of oxygen and carbon isotopic composition among foraminiferal species in the sediment. This promises to be an exciting new tool for palaeo-oceanographic investigations.     

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.     

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.     

Habitats, abundance patterns and isotopic signals of morphotypes of the planktonic foraminifer Globigerinoides ruber (d'Orbigny) in the eastern Mediterranean Sea since the Marine Isotopic Stage 12

The chemical composition of shells of the planktonic foraminifer Globigerinoides ruber (white) is frequently used to determine past sea surface conditions. Recently, it has been shown that arbitrarily defined morphotypes within this species exhibit different chemical and isotopic signatures. Here, we investigate the occurrence through time and in space of morphological types of G. ruber (white) in late Quaternary and Holocene sediments of the central and the eastern Mediterranean Sea. In 115 samples representing two distinct time intervals (MIS 1–2 and MIS 9–12) at ODP Site 964 and the piston core GeoTü-SL96, we have defined three morphological types within this species and determined their relative abundances and stable isotopic composition. A quantitative analysis of morphological variation within G. ruber (white) in four samples revealed that the subjectively defined morphotypes occupy separate segments of a continuous and homogenous morphospace. We further show that the abundance of the morphotypes changes significantly between glacials and interglacials and that the three morphotypes of G. ruber show significant offsets in their stable isotopic composition. These offsets are consistent within glacial and interglacial stages but their sign is systematically reversed between the two Sites. Since the isotopic shifts among the three G. ruber morphotypes are systematic and often exceed 1‰, their understanding is essential for the interpretation of all G. ruber-based proxy records for the paleoceanographic development of the Mediterranean during the late Quaternary.     

Modelling the temperature dependent growth rates of planktic foraminifera

The temperature influence on foraminifera growth rate was analysed using a mechanistic formulation that take into account enzyme inactivation at extreme temperatures. Growth rates are calculated using available published and unpublished laboratory culture experiments for eight species, including Neogloboquadrina pachyderma (sinistral and dextral forms), Neogloboquadrina dutertrei, Globigerina bulloides, Globigerinoides ruber, Globigerinoides sacculifer, Globigerinella siphonifera and Orbulina universa. Modeled growth formulas readily reproduce the observed growth patterns for all species. Similar growth patterns are observed for the species that have the same symbiotic algae G. ruber, G. sacculifer, and O. universa. However, different growth patterns are observed for herbivorous species (Neogloboquadrina genus) compared to carnivorous species with or without symbionts. Our growth estimates correspond well to in situ observations from both plankton tows and sediment traps. These estimates will help to improve the quantification of the effects of environmental parameters on foraminifera species distribution and abundance.     

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.     

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.     

Establishment of the western Pacific warm pool during the Pliocene: Evidence from planktic foraminifera, oxygen isotopes, and Mg/Ca ratios

This study presents new evidence of when and how the Western Pacific Warm Pool (WPWP) was established in its present form. We analyzed planktic foraminifera, oxygen isotopes, and Mg/Ca ratios in upper Miocene through Pleistocene sediments collected at Deep Sea Drilling Program (DSDP) Site 292. These data were then compared with those reported from Ocean Drilling Program (ODP) Site 806. Both drilling sites are located in the western Pacific Ocean. DSDP Site 292 is located in the northern margin of the modern WPWP and ODP Site 806 near the center of the WPWP. Three stages of development in surface-water conditions are identified in the region using planktic foraminferal data. During the initial stage, from 8.5 to 4.4 Ma, Site 806 was overlain by warm surface water but Site 292 was not, as indicated by the differences in faunal compositions and sea-surface temperature (SST) between the two sites. In addition, the vertical thermal gradient at Site 292 was weak during this period, as indicated by the small differences in the δ¹⁸O values between Globigerinoides sacculifer and Pulleniatina spp. During stage two, from 4.4 to 3.6 Ma, the SST at Site 292 rapidly increased to 27 °C, but the vertical thermal gradient had not yet be strengthened, as shown by Mg/Ca ratios and the presence of both mixed-layer dwellers and thermocline dwellers. Finally, a warm mixed layer with a high SST ca. 28 °C and a strong vertical thermal gradient were established at Site 292 by 3.6 Ma. This event is marked by the dominance of mixed-layer dwellers, a high and stable SST, and a larger differences in the δ¹⁸O values between G. sacculifer and Pulleniatina spp. Thus, evidence of surface-water evolution in the western Pacific suggests that Site 292 came under the influence of the WPWP at 3.6 Ma. The northward expansion of the WPWP from 4.4 to 3.6 Ma and the establishment of the modern WPWP by 3.6 Ma appear to be closely related to the closure of the Indonesian and Central American seaways.     

Fertility tracers and monsoon forcing at an equatorial site of the Somali Basin (Northwest Indian Ocean)

Temporal changes in biological and isotopic tracers have successfully been used to monitor monsoon variability in the Arabian Sea. Convincing evidence that fertility indicators, such as the relative abundance of the planktic foraminifer Globigerina bulloides, and SiO₂ and Ba fluxes, can monitor monsoon changes, has also been published. Time series of the ¹³C content of the thermocline foraminifer Neogloboquadrina dutertrei, and of an Upwelling Radiolarian Index (URI), have been used to reconstruct upwelling changes in the Somali Basin at 5 °N for the last 160 kyr. In order to establish a reference site for comparison with other upwelling and/or fertility records from the same area, a time series of 8 climatic and fertility proxies (¹³C content of N. dutertrei and Globorotalia menardii, percentages of G. bulloides and of a thermocline foraminiferal group, minimal δ¹⁸ values of Globigerinoides sacculifer, N. dutertrei and G. menardii) are investigated in the spectral domain for the last 360 kyr, at a site located at the equator and outside the Somali upwelling centers (Core MD 85668).Chronostratigraphies for these records are developed by correlation of oxygen isotope record of G. sacculifer to the Martinson standard stacked benthic oxygen isotope record. The temporal resolution of the data, is between 2 and 5 kyr.Cross spectral comparisons with ETP (Eccentricity + Tilt + Precession-composite signal) were used to estimate coherences in conjunction with phase relationships and to quantify relationships between fertility and climatic indicators. The timing of fertility indicator cycles at the equator shows little (obliquity band) or no (other orbital bands) coincidence with monsoon proxies of the Arabian Sea. At the equator, fertility responses are dissociated, and present important leads or lags with ETP signal at the three orbital bands (with the exception of the thermocline foraminifera, in phase with maximum eccentricity). Due to significant leads with ETP in the timing of ¹³C minima and URI cycles, no relationships with the Northern Hemisphere radiation can be deduced. At the obliquity and precessional frequency bands, thermocline foraminifera cycles display significant lag with ETP and ice volume minima, which suggests that radiation is not the sole forcing mechanism. Global climatic and ice volume changes may be an important forcing mechanism for these fertility proxies. The G. bulloides record is more complicated with cycles in phase with Arabian Sea monsoon proxies at the obliquity band, and ¹⁸O minima at the precession frequency.It appears that, at the equator, the southwest monsoon winds are not the main force driving radiolarian and foraminiferal productivity.     

Seasonal Variation in Shell Calcification of Planktonic Foraminifera in the NE Atlantic Reveals Species-Specific Response to Temperature,Productivity, and Optimum Growth Conditions

Using shells collected from a sediment trap series in the Madeira Basin, we investigate the effects of seasonal variation of temperature, productivity, and optimum growth conditions on calcification in three species of planktonic Foraminifera. The series covers an entire seasonal cycle and reflects conditions at the edge of the distribution of the studied species, manifesting more suitable growth conditions during different parts of the year. The seasonal variation in seawater carbonate saturation at the studied site is negligible compared to other oceanic regions, allowing us to assess the effect of parameters other than carbonate saturation. Shell calcification is quantified using weight and size of individual shells. The size–weight scaling within each species is robust against changes in environmental parameters, but differs among species. An analysis of the variation in calcification intensity (size-normalized weight) reveals species-specific response patterns. In Globigerinoides ruber (white) and Globigerinoides elongatus, calcification intensity is correlated with temperature (positive) and productivity (negative), whilst in Globigerina bulloides no environmental forcing is observed. The size–weight scaling, calcification intensity, and response of calcification intensity to environmental change differed between G. ruber (white) and G. elongatus, implying that patterns extracted from pooled analyses of these species may reflect their changing proportions in the samples. Using shell flux as a measure of optimum growth conditions, we observe significant positive correlation with calcification intensity in G. elongatus, but negative correlation in G. bulloides. The lack of a consistent response of calcification intensity to optimum growth conditions is mirrored by the results of shell size analyses. We conclude that calcification intensity in planktonic Foraminifera is affected by factors other than carbonate saturation. These factors include temperature, productivity, and optimum growth conditions, but the strength and sign of the relationships differ among species, potentially complicating interpretations of calcification data from the fossil record.     

Fluxes and isotopic composition of planktonic foraminifera off Hainan Island,northern South China Sea: Implications for paleoceanographic studies

Planktonic foraminifera collected from a sediment trap deployed off Hainan in the northwestern South China Sea (SCS-NW) between July 2012 and April 2013 were studied to evaluate their seasonal variability and ecology as well as to infer the factors controlling their shell fluxes. The total planktonic foraminifera flux, as well as the fluxes of the dominant species (Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei), showed three distinct maxima during SW-monsoon in August 2012, the SW-NE intermonsoon in October 2012 and the NE-monsoon in December 2012–February 2013. These periods were characterized by upwelling, aerosol fallout, and intense wind mixing, respectively, from which the foraminiferal assemblages benefitted, as indicated by the close correlation between wind speed, sea surface temperature (SST), chlorophyll a concentration (Chl-a), δ¹⁸O of G. ruber and the shell fluxes. The correlation also suggests that temperature and food availability might have been the primary drivers of the observed changes in foraminiferal abundance. The offset between the SST deduced from flux-weighted of G. ruber δ¹⁸O and annual mean SST is only ∼0.3 °C, much lower than ∼5.2 °C between the summer and winter temperature, indicating a balanced seasonality bias in the shell flux. The linear regression between the satellite-derived sea surface temperature and G. ruber δ¹⁸O reveals the strong potential of this species, at least in the studied region, as an ecological indicator for past oceanic environments.     

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.     

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.     

The carbon isotopic composition of the eastern Mediterranean planktonic foraminifera orbulina universa and the phenotypes of globigerinoides ruber

The carbon isotopic composition of three phenotypes of Globigerinoides ruber, which deposit their shells at three different depths in the water column, reflects the changes in δ¹³C of the dissolved bicarbonate. The variations are in agreement with the interpretation of increased biological activity in the surface waters compared to the deeper waters. The carbon isotopic composition of Orbulina universa is not in isotopic equilibrium with the dissolved bicarbonate, even though the oxygen isotopic composition of this species is deposited in isotopic equilibrium.     

Late Neogene planktonic foraminifera of the Cibao Valley (northern Dominican Republic): Biostratigraphy and paleoceanography

An assemblage of planktonic foraminifera is described from 125 samples taken from the Cercado, Gurabo, and Mao Formations in the Cibao Valley, northern Dominican Republic. The primary objectives of this study are to establish a biochronologic model for the late Neogene of the Dominican Republic and to examine sea surface conditions within the Cibao Basin during this interval. The Cercado Formation is loosely confined to Zones N17 and N18 (∼ 7.0–5.9 Ma). The Gurabo Formation spans Zones N18 and N19 (∼ 5.9–4.5 Ma). The Mao Formation is placed in Zone N19 (∼ 4.5–3.6 Ma). Changes in the relative abundances of indicator species are used to reconstruct sea surface conditions within the basin. Increasing relative abundances of Globigerinoides sacculifer and Globigerinoides ruber, in conjunction with a decreasing relative abundance of Globigerina bulloides, suggests the onset of increasing sea surface temperature and salinity in conjunction with diminishing primary productivity at ∼ 6.0 Ma. Abrupt increases in the relative abundances of G. sacculifer and G. ruber at ∼ 4.8 Ma suggest a major increase in sea surface temperature and salinity in the early Pliocene. The most likely mechanism for these changes is isolation of the Caribbean Ocean through progressive restriction of Pacific–Caribbean transfer via the Central American Seaway. Periods of high productivity associated with upwelling events are recorded in the upper Cercado Formation (∼ 6.1 Ma) and in the middle Mao Formation (∼ 4.2 Ma) by spikes in G. bulloides and Neogloboquadrina spp. respectively. The timing of major increases in sea surface salinity and temperature as well as decreasing productivity (∼ 4.8 Ma) and periods of upwelling (∼ 6.1and 4.2 Ma) in the Cibao Basin generally corroborate previously suggested Caribbean oceanographic changes related to the uplift of Panama. Changes in sea surface conditions depicted by paleobiogeographic distributions in the Cibao Basin suggest that shoaling along the Isthmus of Panama had implications in a shallow Caribbean basin as early as 6.0 Ma. Major paleobiologic changes between ∼ 4.8 and 4.2 Ma likely represent the period of final closure of the CAS and a nearly complete disconnection between Pacific and Caribbean water masses. This study illustrates the use of planktonic foraminifera in establishing some paleoceanographic conditions (salinity, temperature, productivity, and upwelling) within a shallow water basin, outlining the connection between regional and localized oceanographic changes.     

Distribution of some planktonic foraminifera species in deep-sea cores from the red sea and their relation to eustatic sea-level changes

Distribution patterns of planktonic foraminifera in four sediment cores from the Red Sea are studied. The most common species are Globigerinoides ruber, G. sacculifer, Globigerinella siphonifera and Orbulina universa. G. ruber and G. sacculifer show opposite trends of distribution in the sediment cores. Abundance of the foraminifera during the glacial periods suggests that the connection of the Red Sea to the Indian ocean was not completely interrupted and the salinity conditions were not extreme.However, higher salinities appear to have existed in the northern Red Sea, where most of the planktonic foraminifera that occur in the southern Red Sea are absent. It is inferred that the salinity in the southern Red Sea during the glacial period was less than 50%, whereas higher salinity might have existed in the north where the influence of the Indian Ocean was minimal.