Pulleniatina Obliquiloculata Research Articles

Effects of surface ocean conditions on deep‐sea calcite dissolution proxies in the tropical Pacific

Finding the ideal deep‐sea CaCO3 dissolution proxy is essential for quantifying the role of the marine carbonate system in regulating atmospheric pCO2 over millennia. We explore the potential of using the Globorotalia menardii fragmentation index (MFI) and size‐normalized foraminifer shell weight (SNSW) as complementary indicators of deep‐sea CaCO3 dissolution. MFI has strong correlations with bottom water [CO32−], modeled estimates of percent CaCO3 dissolved, and Mg/Ca in Pulleniatina obliquiloculata in core top samples along a depth transect on the Ontong Java Plateau (OJP) where surface ocean temperature variation is minimal. SNSW of P. obliquiloculata and Neogloboquadrina dutertrei have weak correlations with MFI‐based percent dissolved, Mg/Ca in P. obliquiloculata shells and bottom water [CO32−] on the OJP. In core top samples from the eastern equatorial Pacific (EEP), SNSW of P. obliquiloculata has moderate to strong correlations with both MFI‐based percent CaCO3 dissolved estimates and surface ocean environmental parameters. SNSW of N. dutertrei shells shows a latitudinal distribution in the EEP and a moderately strong correlation with MFI‐based percent dissolved estimates when samples from the equatorial part of the region are excluded. Our results suggest that there may potentially be multiple genotypes of N. dutertrei in the EEP which may be reflected in their shell weight. MFI‐based percent CaCO3 dissolved estimates have no quantifiable relationship with any surface ocean environmental parameter in the EEP. Thus MFI acts as a reliable quantitative CaCO3 dissolution proxy insensitive to environmental biases within calcification waters of foraminifers.

Millennial-Scale Planktic Foraminifer Faunal Variability in the East China Sea during the Past 40000 Years (IMAGES MD012404 from the Okinawa Trough)

High resolution planktic foraminifer fauna assemblage data are used to reconstruct the millennial-scale sea surface temperature (SST) variability of the past 40000 years at an IMAGES core site (MD012404) in the Okinawa Trough in the East China Sea (ECS). The fauna assemblages in core MD012404 are dominated by five species Globigerinoides ruber, Globigerina bulloides, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, and Globigerinita glutinata, which account for > 70% in relative abundance. Our Q-mode factor analysis decomposed the fauna abundance data into three factors, which indicate cold water mass, warm water mass, and possibly coastal water flow with low salinity in the ECS. The MD012404 fauna data show abrupt changes at ~16 kya, suggesting a return to a warmer climate or warm water intrusion of the Kuroshio into the Okinawa Trough since the Last Glacial Maximum (LGM). SST estimates based on the fauna assemblages of planktic foraminifers indicate a LGM cooling of 1 - 2C. A maximum cooling by 3 - 4C is observed in episodic, millennial-scale events in the glacial stages of the record. The SST record displays variability that closely tracks the structure of oxygen isotopes of stalagmites from Hulu Cave and ice cores from GISP 2 Dansgaard/Oeschger cycles and Heinrich events. Low salinity in the ECS is inferred based on MD012404 fauna SST and planktic foraminifer oxygen isotope records for the cold millennial-scale intervals, pointing to the Intertropical Convergence Zone (ITCZ) and/or East Asian monsoon as important factors driving SST and salinity in the subtropical western Pacific, both on orbital and suborbital time scales.

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.

A novel approach to dissolution correction of Mg/Ca–based paleothermometry in the tropical Pacific

Mg/Ca in planktonic foraminifers carries two main signals: calcification temperature and postdepositional test dissolution. Shell dissolution thus distorts water temperature reconstructions made with Mg/Ca in foraminifers. This problem could be resolved by quantifying the impact of carbonate dissolution on Mg/Ca with an independent, temperature‐insensitive deep‐sea calcite dissolution proxy, such as the Globorotalia menardii fragmentation index (MFI). To test the validity of this approach, we measured Mg/Ca in the tests of several planktonic foraminifers and MFI in core tops collected over a wide geographic region of the tropical Pacific and covering a wide range of deep‐sea calcite dissolution and seawater temperature. We confirm that Mg/Ca from different species have different susceptibility to temperature and dissolution. Mg/Ca in surface‐dwelling Globigerina bulloides is controlled by calcification temperature and is largely unaffected by carbonate dissolution estimated from MFI. In contrast, Mg/Ca in deeper dwelling G. menardii is minimally sensitive to temperature and dominantly affected by dissolution. Mg/Ca in Neogloboquadrina dutertrei and Pulleniatina obliquiloculata are significantly affected by both temperature and dissolution, and MFI can be effectively used to correct temperature estimates from these species for calcite dissolution. Additional variables besides temperature and dissolution appear to control Mg/Ca in Globorotalia tumida, and their identification is a prerequisite for interpreting elemental shell composition in this species. Combining down‐core measurements of Mg/Ca in multiple foraminifer species with MFI provides a powerful tool for reconstructing past changes in the upper water column temperature structure in the tropical Pacific.

No-analog planktonic foraminiferal faunas in the glacial southern South China Sea: Implications for the magnitude of glacial cooling in the western Pacific warm pool

Sea-surface temperature (SST) estimates in the sediment core MD01-2390 based on planktonic foraminiferal species abundances using five different transfer function techniques suggest nearly unchanged or unusually higher temperatures in the tropical southern South China Sea (SCS) during the Last Glacial Maximum (LGM) relative to modern temperatures. These results are in contrast to substantial cooling of 2–5 °C inferred by geochemical (U k' 37, Mg/Ca ratios) and terrestrial proxies from the western tropical Pacific region. Using multivariate statistics we show that the glacial southern SCS harboured unique planktonic foraminiferal assemblages that have no modern analogs. Analyses of faunal variation through the core reveal that planktonic foraminiferal assemblages responded to temperature changes inferred from Mg/Ca data but that this signal is subdued by superimposed variations in the relative abundance of Pulleniatina obliquiloculata and Neogloboquadrina pachyderma (dextral). These species occur in glacial samples at proportions that are not observed in the calibration data set. The glacial high abundance of N. pachyderma (dextral) are interpreted to reflect a seasonal (winter) inflow of cold surface water from the northeast via the Bashi Strait due to the combined effects of an intensified winter monsoon, a southward shift of the polar front and the eastward migration of the Kuroshio Current. In contrast, processes controlling the high relative abundances of P. obliquiloculata during the LGM may be unique to the southern SCS. We propose a scenario involving a stronger (winter) mixing or enhanced upwelling due to an intensified winter monsoon that prevented shallow-dwelling, warm indicators to establish larger populations during the LGM. Our results indicate that a no-analog behaviour of planktonic foraminifera faunas is responsible for the warm glacial conditions in this part of the western Pacific warm pool as implied by foraminiferal transfer functions and that a more significant surface cooling in the region as implied by terrestrial and geochemical (Mg/Ca ratios; alkenone unsaturation index) marine proxies is a more likely scenario.

Corroborating ecological depth preferences of planktonic foraminifera in the tropical Atlantic with the stable oxygen isotope ratios of core top specimens

Past variability in upper ocean thermocline depth is commonly estimated from the abundance of different species of planktonic organisms or the difference in oxygen isotopic composition between two species of planktonic foraminifera, one that lives in the mixed layer and one that lives in or near the thermocline. To test the latter relationships, we measured the oxygen isotopic composition of eight species of planktonic foraminifera (pink and white varieties of Globigerinoides ruber, Globigerinoides sacculifer without the final chamber, Orbulina universa, Pulleniatina obliquiloculata, Globorotalia menardii, Neogloborotalia dutertrei, and Globorotalia tumida) in surface sediment samples from 31 tropical Atlantic deep‐sea sediment cores. Bayesian analysis was used to compare measured oxygen isotopic compositions with their predictions based on modern data sets of annual temperatures and oxygen isotopic composition of ocean water in the upper 500 m at the core sites. Posterior probability densities for predictive model parameters were computed. Probability distributions of calcification depth for analyzed species corroborated their ecological preferences inferred from net tow and sediment trap data. Robustness of the habitat signals in core top specimens suggests that reconstructions of the entire upper ocean temperature profiles, not just their thermocline depth or temperature, might be possible.

Calcite dissolution along a transect in the western tropical Indian Ocean: A multiproxy approach

Three paleocarbonate ion proxies, size index, planktonic foraminifera shell weight, and calcite crystallinity, have been employed here to a set of core top samples from the western tropical Indian Ocean in the water depth ranges from 1086 to 4730 m. All three proxies complement each other well and reveal that calcite dissolution starts to affect planktonic foraminifera from 2250 m depth onward and intense calcite dissolution begins around 3900 m depth in this sector of the tropical Indian Ocean. Three planktonic foraminifera species, Globigerinoides sacculifer, Pulleniatina obliquiloculata, and Neogloboquadrina dutertrei, show wide variability in shell weights from core top sediments in the depth range of 3300 to 3400 m bathed by similar bottom water CO3=* concentrations. This variability is attributed to shell calcification, and shell weights are controlled by CO3= concentration in the surface waters. Further, surface water CO3= concentrations override milder dissolution effects experienced by samples from shallower water depths bathed by similar bottom water CO3=* concentrations.

Seasonal variations of modern planktonic foraminifera in the South China Sea

Although foraminiferal census and isotope analyses from sediment cores have been widely applied to paleoceanographic reconstructions in the South China Sea (SCS), their modern counterparts, i.e. foraminiferal shells from sediment traps and plankton tows, have been the subjects of few studies in this region. In this investigation, relative abundances of the most popular species used to reconstruct late Quaternary tropical sea-surface conditions have been studied in plankton tows from the SCS in order to offer better constraints on the interpretation of downcore records. In addition to the seasonal differences, the spatial variations in the faunal assemblage are evident from south to north in the SCS basin based on contemporary tows. For example, the species Globigerinella aequilateralis is found to be relatively dominant in the southern (lower latitude) basin but its relative abundance is significantly higher in planktonic tows than in sediments, indicating its proneness to burial dissolution. Pulleniatina obliquiloculata, an index taxon for the Kuroshio Current, is only abundant in the northern part of the SCS basin. The measured oxygen isotope compositions (δ18O) of Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, and P. obliquiloculata all show a good relationship with temperature in the mixed layer (Tmix). The carbon isotope compositions (δ13C) also display a positive distribution pattern relative to Tmix, but with a less significant correlation in winter months than δ18O. Differences of δ18O between shallower (G. sacculifer and G. ruber) and deep dwellers (N. dutertrei and P. obliquiloculata) accurately record the temperature gradient between the surface mixed layer and 100-m water depth, supporting application of this proxy to the reconstruction of upper water-column stratification.

Living planktonic foraminifera from the northwestern Pacific Ocean: SSU rDNA sequences and their paleoceanographic significance

Abstract Living planktonic foraminifera (PF) samples from the Okinawa Trough of the northwestern Pacific Ocean were taken for DNA analysis. The SSU rDNA sequences of two PF species, Globigerina sp. and Pulleniatina obliquiloculata collected at Station WP01, were obtained and compared with those from the southwestern Pacific Ocean. Only small differences (<0.7%–1.2% for P. obliquiloculata, and 0.3% for Globigerina sp.) were found between samples from the north- and south-western Pacific Ocean areas and this molecular evidence supported that these micropaleontogical species are the same species, which implies that the West Pacific Ocean circulation system influences the planktonic foraminiferal gene communication.

Comment on “Anomalous radiocarbon ages for foraminifera shells” by W. Broecker et al.: A correction to the western tropical Pacific MD9821‐81 record

[1] The effort to obtain high-resolution paleoceanographic data often leads to continental margin settings where the flux of clastic sediments produces higher sediment accumulation rates. However, on continental margins the possibility for down-slope transport of reworked sediment is a concern. This is particularly true in tectonically active settings such as the western Pacific, which includes the maritime islands of the Philippines and the adjacent archipelago. The continental margins of the western tropical Pacific are strategically located within the Pacific Warm Pool, and detailed paleoceanographic information from this region is a valuable asset in efforts to improve our understanding of what factors contribute to large and sometimes abrupt changes in global climate. The acquisition of data sets such as sea surface temperatures from the tropical Pacific warm pool through the last glacial cycle has indeed added measurably to a growing array of data from lowlatitude locations. Certainly, there is much left to be learned from the high deposition rate records located in continental margin settings. However, over the past 2 years, there have been concerns that one of the high deposition rate sequences in the western tropical Pacific, R/V Marion Dufresne core MD98-2181, is composed of down-slope transport of reworked material or slumping [Broecker et al., 2006]. Because this site has provided one of the highest-resolution data sets of sea surface temperature and salinity information for the Pacific warm water pool through the last glacial cycle [Stott et al., 2002, 2004], it is important to assess the stratigraphic integrity of the sequence and to identify where there may be problems with the record. [2] The MD98-2181 core was collected from a depth of 2114 m on the continental slope south of Mindanao Island. During the last glacial and Holocene sediments accumulated at this site at rates of between 50 and 80 cm/kyr making this an ideal location to collect high-resolution paleoceanographic information from the Pacific Warm Pool [Stott et al., 2004]. We have worked on the MD98-2181 core for the past eight years, conducting stable isotope and Mg/Ca measurements on planktonic and benthic foraminifers in an effort to document the timing and the magnitude of sea surface temperature and salinity changes in the western tropical Pacific Warm Pool through the last glacial cycle [Stott et al., 2002, 2004]. In doing so we have obtained C ages for 42 planktonic foraminifer samples from the Holocene and late glacial portions of the core (Table 1 and Figure 1). The majority of our MD98-2181 samples were taken from u-channel subcores. These subcores were sectioned continuously at 2 cm increments in Stott’s laboratory and the samples were washed in buffered DI water over a 63 mm sieve to remove fine clays. The planktonic Globigeriniodes ruber used for stable isotope and Mg/Ca measurements and also for C determinations was picked from the >150 mm fraction whereas Globigerinoides sacculifer was picked from the >250 mm fraction. In some instances other more robust planktonic species have also been C dated including, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, which were picked from the >250 mm fractions. M. Rincon, who has been Stott’s laboratory manager for the past 15 years, picked each sample used in these studies. The G. ruber dO stratigraphy for MD98-2181 is shown in Figure 2. [3] The late glacial to Recent dO stratigraphy from this core matches other high-resolution sequences from this region with the exception of a 20 cm interval between 941 and 961 cm. The samples from the 941–961 cm interval exhibit dO values that are clearly higher than adjacent intervals immediately above and below (Figure 2). The interval centered at 950 cm would have been deposited during the last deglaciation and the apparent timing of the anomalously high dO values would put it close to the time of the Younger Dryas. For this reason it was not initially clear whether these values represented the presence of reworked glacial age material or if the values were indicative of a brief return to glacial-like conditions during the deglaciation. The intervals directly below 961 cm do not exhibit such high dO values as those at 950 cm and the isotope stratigraphy appears to closely match that of other high-resolution records from the western tropical Pacific region [Stott et al., 2004]. [4] In 2003, W. Broecker approached Stott about a collaboration that involved C dating of planktonic and PALEOCEANOGRAPHY, VOL. 22, PA1211, doi:10.1029/2006PA001379, 2007 Click Here for Full Article

Microdistribution of Mg/Ca, Sr/Ca, and Ba/Ca ratios in Pulleniatina obliquiloculata test by using a NanoSIMS: Implication for the vital effect mechanism

The Mg/Ca ratio within foraminiferal calcareous tests (shells) is widely used to reconstruct past seawater temperature. However, recent studies reported that the organic components within a test affect the Mg/Ca distribution. In this study, we have measured the Mg/Ca, Sr/Ca, and Ba/Ca ratios within the planktonic foraminifera Pulleniatina obliquiloculata by using a NanoSIMS (secondary ion mass spectrometer (SIMS)), which has excellent spatial resolution (∼1 μm) and allows us to compare the distribution of chemical compositions with that of the organic components. Element compositions show banding distributions composed of alternately higher and lower values of those elemental ratios. The Mg/Ca ratios, previously considered to be mainly controlled by calcification temperature, show larger variations than the values expected from the seawater temperature at the habitat depth of P. obliquiloculata. Comparison of the elemental distribution with the test microstructure reveals that the bands of high Mg/Ca and Sr/Ca ratios correspond with layers of the organic components. Such coincidence suggests that the organic components strongly affect the Mg/Ca and Sr/Ca ratios within a test. In spite of the heterogeneous distribution, temperature estimated from the averaged Mg/Ca ratio within a test is consistent with seawater temperature at the habitat depth of P. obliquiloculata, indicating that whole Mg/Ca ratio of foraminiferal test may be useful as paleotemperature proxy. In contrast to the Mg/Ca ratio the heterogenity in Ba/Ca ratio, which previously has been considered to be mainly controlled by the ambient seawater composition, is not fully matched with the layers of the organic compositions. Although the organic components concentrate Ba, other unknown factors appear to also cause heterogenity in Ba incorporation.

Late Quaternary sediments on the outer shelf of the Korea Strait and their paleoceanographic implications

Sedimentological and micropaleontological characteristics of core sediments from the outer shelf of the Korea Strait, which connects the northern East China Sea and the East Sea (Sea of Japan), were investigated to elucidate the paleoceanographic environment, especially the timing of the Kuroshio inflow, since the last glacial maximum. The core sediments, containing continuous records of the last 15,000 years, are characterized by a relatively high mud content (more than 50%, on average) and well-developed tide-influenced sedimentary structures. Their mineralogy suggests that the material originated from the paleo-Nakdong River system, which extended across the shelf of the Korea Strait during low sea-level periods. Planktonic foraminifers reveal a series of well-defined changes in paleoceanographic conditions during the late Pleistocene–Holocene. Down-core variations in the abundance of four foraminiferal assemblages, i.e., cold, coastal, tropical–subtropical, and Kuroshio water groups comprising characteristic planktonic species, suggest the occurrence of a distinct paleoenvironmental change in the surface water at 7,000 years b.p., i.e., from 15,000 to 7,000 years b.p., the area was influenced by coastal waters whereas since ca. 7,000 years b.p., it has been under the influence of open-sea water related to the Kuroshio Current flow, associated with both higher temperature and higher salinity. In particular, Pulleniatina obliquiloculata increased markedly in abundance at this time, documenting the inflow of the Kuroshio into the study area. These data indicate that the coastal water stage terminated at ca. 7,000 years b.p. when the warm Kuroshio and its major branch, the Tsushima Current, began to flow into the East Sea, as is the case today. The intrusion of the Tsushima Current through the Korea Strait after ca. 7,000 years b.p. resulted in abrupt changes in sedimentation rates and a dramatic increase in abundance of the Kuroshio indicator species, P. obliquiloculata.

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.

The Holocene Pulleniatina Minimum Event revisited: Geochemical and faunal evidence from the Okinawa Trough and upper reaches of the Kuroshio current

The Holocene Pulleniatina Minimum Event (PME) is characterized by a very low abundance of the planktonic foraminifer Pulleniatina obliquiloculata between ∼ 4.5 and 3 ka. The PME occurs widely in the Okinawa Trough and the South China Sea, and can be correlated throughout this area; it has been related to variability in the Kuroshio current. To further explore the nature of the PME, we studied cores obtained from the southern Okinawa Trough and the upper reaches of the Kuroshio current. Faunal census data indicate that all cores record the PME between ∼ 4.5 and ∼ 3 ka. The relative abundance of Neogloboquadrina dutertrei is negatively correlated to that of P. obliquiloculata in the southern Okinawa Trough, but not in the sites at the upper reaches. Mg/Ca and δ 18O measurements on Globigerinoides ruber shells from the southern Okinawa Trough indicate that there was no change in sea surface temperature or sea surface salinity during the PME. The vertical structure of the water column as reconstructed by multispecies δ 18O and δ 13C profiles shows no consistent anomalies in the southern Okinawa Trough and western Philippine Sea during the PME. These observations suggest that: (1) the PME was not restricted to marginal seas, but widespread in the western North Pacific. (2) The high abundance of N. dutertrei during the PME in the Okinawa Trough may be a result of higher food-availability in the absence of P. obliquiloculata. (3) No distinctive, consistent anomalies in the paleoceanographic proxies are associated with the PME, implying there were no changes in hydrography and productivity. The absence of a linkage between faunal variation and paleoceanographic proxies indicates that we do not yet understand what causes changes in planktonic foraminiferal assemblages. This lack of understanding implies that we cannot always trust fauna-based paleothermometry at millennial timescales.

Quaternary upper ocean thermal gradient variations in the South China Sea: Implications for east Asian monsoon climate

The east Asian monsoon climate has an overwhelming influence on local climatic conditions of the South China Sea (SCS). Here we present results from a 3‐year study of modern sediment traps from the central SCS in which the percentage of opal, the fluxes of Pulleniatina obliquiloculata and Globigerinoides ruber, the organic carbon flux, and primary productivity all show highest values during winter when the mixed layer depth is deepest. The δ18O difference (Δδ18O(P‐G)) between the subsurface dwelling P. obliquiloculata and the mixed layer dwelling G. ruber in core top samples from 12 piston cores and one Ocean Drilling Program (ODP) site in the SCS is large, ∼0.6‰ in a cold eddy area with a shallow mixed layer off Vietnam, but is reduced in deep mixed layer settings lacking cold eddies. We also present 1.56 Myr‐long isotope records of G. ruber and P. obliquiloculata from ODP site 1143 in the southern SCS that reveal upper ocean thermal gradient variations during the Quaternary. Lower Δδ18O(P‐G) values indicate increased mixed layer depths over most glacial episodes, likely due to stronger winter monsoon winds. In contrast, higher Δδ18O(P‐G) values during interglacial intervals suggest decreased mixed layer depths and thus weaker winter monsoon winds. Comparisons of Δδ18O(P‐G) with the percentage of P. obliquiloculata at site 1143 and the percentage of herbs and opal flux at site 1144 demonstrate that glacial‐interglacial variations were strongly influenced by stronger and weaker east Asian winter monsoons during glacials and interglacials, respectively. Therefore the upper ocean thermal gradient variations in the SCS have been consistently affected by the east Asian winter monsoon since at least the beginning of the Pleistocene.

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.

SEDIMENT TRAP RESULTS FROM THE JAPAN TRENCH IN THE KUROSHIO DOMAIN: SEASONAL VARIATIONS IN THE PLANKTIC FORAMINIFERAL FLUX

Time-series sediment traps were deployed at two depths at Station JT (Japan Trench) along the northern margin of the Kuroshio current system domain from March 1991 to May 1992. Total (>125 μm) foraminiferal fluxes were 334–1028 shells/m 2 /day in the shallow trap (1200 m) and 657–2130 shells/m 2 /day in the deep trap (3700 m). A total of 35 planktic foraminiferal species belonging to 13 genera were identified. The assemblage was dominated by Globigerina bulloides, Globigerinita glutinata, Globigerinoides ruber and Neogloboquadrina dutertrei, whose fluxes varied seasonally. Globigerina bulloides and Globigerina quinqueloba occurred during the early spring phytoplankton bloom. These taxa were replaced by Globigerinoides ruber, Globigerinoides sacculifer, N. dutertrei and Pulleniatina obliquiloculata during the late spring to early autumn, with the development of a sharp thermocline and a deep chlorophyll a maximum just below the thermocline. The maximum shell flux of Globigerinita glutinata, Globigerina falconensis, Globorotalia inflata and Globorotalia truncatulinoides occurred during the winter near the 18 °C isotherm in well-mixed surface water that resulted from the extensive vertical mixing induced by the winter monsoon. Temporal changes in the path of the Kuroshio current system caused by its meandering appeared to be the dominant flux control factor, based on the correlation between the meander types of the Kuroshio and the timing of large fluxes in the deep trap. During the trap experiments, major meanderings were observed from September to October 1991 and from mid-April to May 1992. During these two periods, prominent flux peaks of Globigerina bulloides and Neogloboquadrina incompta were observed in both shallow and deep traps. The major meander of the Kuroshio current system was near Station JT during these periods, and the nearshore side of the Kuroshio Front, which is characterized by high productivity in the euphotic zone, was very close to Station JT. These results suggest that these two species were produced on the nearshore side of the Kuroshio Front and collected in the sediment trap. The temporal occurrences of N. incompta at Station JT associated with the meanders of the Kuroshio are consistent with the high abundance of N. incompta in the surface sediment record from beneath the modern cold water mass off Kishu and Enshu-nada, between the Japanese coast and the axis of the Kuroshio current system.

Japan Sea planktic foraminifera in surface sediments: geographical distribution and relationships to surface water mass

Modern planktic foraminifera in 51 surface sediments from the Japan Sea, a marginal sea of the western North Pacific, were studied to reveal the relationships between geographical distribution and surface water masses in the Japan Sea. Twenty-four species belonging to 10 genera were identified, of which nine species, namely Neogloboquadrina incompta, Neogloboquadrina pachyderma, Globigerina quinqueloba, Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerinoides tenellus, and Globigerinita glutinata are predominant. We recognized four geographical distribution patterns of these dominant species that are related to hydrographic conditions in the Japan Sea. The transitional water formed by the mixture between the warm Tsushima Current and cold waters in the Japan Sea is optimal for N. incompta, while the distribution of N. pachyderma is matched with cold water in the northern Japan Sea. Globigerina quinqueloba and G. bulloides appear to be associated with less saline, nutrient-rich river water from the Changjiang (Yangtze River), and G. ruber, N. dutertrei, P. obliquiloculata, G. tenellus, and G. glutinata can be regarded as indicators of Tsushima Current water.

Paleoenvironmental changes in the northern area of the East China Sea during the past 42,000 years

A 33.65-m-long IMAGES (MD982195) core with very high sedimentation rate (ca. 80 cm/kyr), retrieved from the northern part of the East China Sea, was analyzed at 10-cm intervals (about 125 years) for oxygen–carbon isotope of Globigerinoides ruber (sensu stricto), sea surface temperature (SST) from alkenones (U 37′ K), and principal component analysis of planktonic foraminiferal assemblage. The general pattern of the δ 18O curve, resembling the alkenone SST curve, is similar to the standard open ocean oxygen isotope curve. It reveals many light δ 18O peaks apparently corresponding to Dansgaard–Oeschger (D–O) cycles as observed in Greenland ice cores for the last 42 ka. The alkenone SST at the core top is 24 °C, which is close to the modern annual mean SST around the core site. On the other hand, SST during the last glacial maximum (LGM) was about 5 °C lower than that in the present day. Alkenone-derived SST does not correlate with the light negative δ 18O peaks corresponding to the D–O cycles. The oxygen isotope ratios for the surface waters, based on oxygen isotopes of G. ruber and alkenone SSTs, indicate approximately 1.0‰ increase on average during the LGM and many light peaks at the warm D–O events. This suggests that fresh water was supplied to the core site during these events. The meandering of the westerlies during the cold and warm D–O cycles would have caused an atmospheric tele-connection between East Asia and Greenland. The core was divided into four periods based on Q-mode factor analysis of the planktonic foraminiferal assemblage, i.e. (1) a strong upwelling period from 42 to 24 ka, indicated by abundant occurrence of Globigerina bulloides, (2) a cold and low salinity water mass period from 24 to 14 ka, suggested by increase of Neogloboquadrina pachyderma, Neogloboquadrina incompta, and Globigerina quinqueloba, (3) a transitional period from cold to warm water masses from 14 to 8 ka, as indicated by the increase of warm water species such as Pulleniatina obliquiloculata and Globigerinoides ruber, and (4) the present-day warm Kuroshio Current condition after 8 ka. The Kuroshio Current flowed into the ECS since the last 42 ka and weakened during the last glacial maximum (LGM) due to the expansion of the coastal water in the northern part of the ECS.

Amino acid geochronology of individual foraminifer (Pulleniatina obliquiloculata) tests, north Queensland margin, Australia: A new approach to correlating and dating Quaternary tropical marine sediment cores

In this study, we demonstrate the utility of amino acid geochronology based on single‐foraminiferal tests in Quaternary sediment cores from the Queensland margin, Australia. The large planktonic foraminifer Pulleniatina obliquiloculata is ubiquitous in shelf, slope, and basin sediments of north Queensland as well as pantropical oceans. Fossil tests are resistant to dissolution, and retain substantial concentrations of amino acids (2–4 nmol mg−1 of shell) over hundreds of thousands of years. Amino acid D and L isomers of aspartic acid (Asp) and glutamic acid (Glu) were separated using reverse phase chromatography, which is sensitive enough to analyze individual foraminifera tests. In all, 462 Pulleniatina tests from 80 horizons in 11 cores exhibit a systematic increase in D/L ratios down core. D/L ratios were determined in 32 samples whose ages are known from AMS 14C analyses. In all cases, the Asp and Glu D/L ratios are concordant with 14C age. D/L ratios of equal‐age samples are slightly lower for cores taken from deeper water sites, reflecting the sensitivity of the rate of racemization to bottom water temperature. Beyond the range of 14C dating, previously identified marine oxygen‐isotope stage boundaries provide approximate ages of the sediments up to about 500,000 years. For this longer time frame, D/L ratios also vary systematically with isotope‐correlated ages. The rate of racemization for Glu and Asp was modeled using power functions. These equations can be used to estimate ages of samples from the Queensland margin extending back at least 500,000 years. This analytical approach provides new opportunities for geochronological control necessary to understand fundamental sedimentary processes affecting a wide range of marine environments.