Foraminiferal oxygen and carbon isotopes during the last 34 kyr off northern Japan,northwestern Pacific

Oxygen and carbon isotopes of foraminifera were analyzed in core PC4, water depth 1366 m, off northern Japan, near the east side of the Tsugaru Strait (130 m depth) between the open northwestern Pacific Ocean and the Japan Sea. At present, the site is at the confluence of the Tsugaru Warm Current which flows eastwards out of the Sea of Japan through the Tsugaru Strait, the subarctic Oyashio Current and the subtropic Kuroshio Current. During the Last Glacial Maximum (LGM), the Oyashio Current penetrated further to the South and outflow from the Japan Sea was restricted by glacio-eustatic sea level lowering.The isotopic values of the planktic foraminifer Neogloboquadrina pachyderma (sinistral) and the benthic foraminifer Uvigerina akitaensis reflect rapid millennial-scale paleoceanographic changes between 34 and 6 ka. Hydrographic changes during deglaciation were related to events at high northern latitudes, but Holocene hydrographic changes were dominated by local effects, such as the development of the outflow of the Tsugaru Warm Current. High values of planktic δ¹⁸O during the LGM reflect the southward advance of the Oyashio Current. These values decreased by 0.3‰ from 19.4 to 18.9 ka, then increased by 0.5‰ at 18 ka, with highest values between 17.5 and 15 ka. The δ¹⁸O oscillations between 19.4 and 15 ka may reflect millennial-scale warm–cold oscillations during Heinrich event 1. Planktic microfossil data indicate that cold Oyashio waters flowed from the northwestern Pacific into the Japan Sea via the Tsugaru Strait between 17 and 16 ka, consistent with the occurrence of the highest planktic δ¹⁸O values in core PC4. Planktic δ¹⁸O values rapidly decreased by 0.9‰ at 15 ka, possibly reflecting the effects of both a rapid increase in fresh water flux and rising temperatures in the subarctic North Pacific. During the Younger Dryas, cold event planktic δ¹⁸O values increased by 0.5‰, followed by a gradual decrease by 1‰ from the early to middle Holocene, reflecting a gradual increase in eastward outflow via the Tsugaru Strait with sea level rise. Both planktic and benthic foraminiferal δ¹³C values oscillated between 34 and 10 ka, at relatively large amplitudes (about 0.5‰), then remained relatively stable during the last 10 kyr. Several negative planktic and benthic (∼ − 0.7‰) δ¹³C excursions were present in sediment dated between the precipitation of secondary carbonates during episodic methane release possibly associated with methane release from continental margin sediments.     

Paleoceanographic history around the Tsugaru Strait between the Japan Sea and the Northwest Pacific Ocean since 30 cal kyr BP

The paleoceanographic history around the Tsugaru Strait since 30.0 cal kyr BP was reconstructed using ecological and biogeographical habitats of diatom species and Q-mode factor analysis of diatom flora in six piston cores. At 30.0-17.5 cal kyr BP, a fall in sea level and the intensification of the Oyashio Current caused erosion of submarine sediments near the shore in the eastern area of the Strait and led to deposition of the third and second sequences of thinly laminated layers (TL3 and TL2) in the Japan Sea. At 17.5-11.5 cal kyr BP, cooling intensified around the Strait, producing an inflow of Oyashio water into the Japan Sea through the Strait. At 15.5-14.5 cal kyr BP, a cold-water mass around the Strait developed from subarctic sea-ice to arctic waters. At 13.0-11.2 cal kyr BP, the Younger Dryas cooling was clearly recorded by the increasing of oceanic cold-water diatom species in the eastern area of the Tsugaru Strait but not in the Japan Sea. At 15.5-13.0 cal kyr BP and 11.25-10.25 cal kyr BP, the warming around the Younger Dryas resulted in the deposition of thinly laminated layers off Shimokita in the eastern area of the Strait. At 9.5 cal kyr BP, the Oyashio Current water intruded into deep depths in the mixed water region off Sanriku. At 9.5-7.0 cal kyr BP, the fluctuation of 1.5-kyr intervals in the strength of the Tsushima Warm Current system in the Japan Sea was initiated.     

Abyssal NE Atlantic benthic foraminifera during the last 15 kyr: Relation to variations in seasonality of productivity

Benthic foraminiferal faunas (> 63 μm) and stable isotopes from the last 15 kyr were studied in BENGAL programme (high-resolution temporal and spatial study of the BENthic biology and Geochemistry of a north-eastern Atlantic abyssal Locality) kasten core 13078#16 from the Porcupine Abyssal Plain, NE Atlantic (48°49.91 N, 16°29.94 W, water depth 4844 m). Changes occurred in the accumulation rates, species composition, diversity, and stable isotopes during the last 15 kyr. Today, the area is strongly influenced by seasonal inputs of phytodetritus following the spring blooms in surface water primary productivity. Variations in the relative abundance of the two most abundant species, Epistominella exigua and Alabaminella weddellensis, which today show significant increases in abundance with the presence of phytodetritus on the sea-floor, are interpreted as resulting from changes in the seasonality of productivity. Seasonal productivity was higher during the Holocene than during the last deglaciation and Younger Dryas, probably coinciding with the retreat of the polar front to higher latitudes. This hypothesis is consistent with simultaneous decreases in the percentage of the polar planktic foraminifera Neogloboquadrina pachyderma (s), and increases in the percentage of Globigerina bulloides, a warmer water planktic foraminifera indicative of phytoplankton blooms and enhanced productivity. The relative abundance of the ‘phytodetritus species’ (E. exigua and A. weddellensis) covary between 14.7 and 8.1 kyr, but not between 7.8 and 1.2 kyr. Major decreases in the numbers per gram and accumulation rates of planktic and benthic foraminifera occurred at ∼ 12–8.5 kyr and at ∼ 4 kyr which correspond to decreases in the % sediment coarse fraction and published data on inorganic carbon contents suggesting that dissolution may have increased at these times. Relationships between benthic foraminiferal faunas and benthic stable isotope records suggest no simple relationship between faunal abundances and test isotope chemistry. For example, the abundances of phytodetritus species do not show strong correlations with either the δ¹³C values of E. exigua or the Δδ¹³C _{E. exigua − P. wuellerstorfi} record, which have previously been suggested as indicative of seasonality of productivity.     

Foraminiferal oxygen and carbon isotopes during the last 34 kyr off northern Japan, northwestern Pacific

Oxygen and carbon isotopes of foraminifera were analyzed in core PC4, water depth 1366 m, off northern Japan, near the east side of the Tsugaru Strait (130 m depth) between the open northwestern Pacific Ocean and the Japan Sea. At present, the site is at the confluence of the Tsugaru Warm Current which flows eastwards out of the Sea of Japan through the Tsugaru Strait, the subarctic Oyashio Current and the subtropic Kuroshio Current. During the Last Glacial Maximum (LGM), the Oyashio Current penetrated further to the South and outflow from the Japan Sea was restricted by glacio-eustatic sea level lowering.The isotopic values of the planktic foraminifer Neogloboquadrina pachyderma (sinistral) and the benthic foraminifer Uvigerina akitaensis reflect rapid millennial-scale paleoceanographic changes between 34 and 6 ka. Hydrographic changes during deglaciation were related to events at high northern latitudes, but Holocene hydrographic changes were dominated by local effects, such as the development of the outflow of the Tsugaru Warm Current. High values of planktic δ¹⁸O during the LGM reflect the southward advance of the Oyashio Current. These values decreased by 0.3‰ from 19.4 to 18.9 ka, then increased by 0.5‰ at 18 ka, with highest values between 17.5 and 15 ka. The δ¹⁸O oscillations between 19.4 and 15 ka may reflect millennial-scale warm–cold oscillations during Heinrich event 1. Planktic microfossil data indicate that cold Oyashio waters flowed from the northwestern Pacific into the Japan Sea via the Tsugaru Strait between 17 and 16 ka, consistent with the occurrence of the highest planktic δ¹⁸O values in core PC4. Planktic δ¹⁸O values rapidly decreased by 0.9‰ at 15 ka, possibly reflecting the effects of both a rapid increase in fresh water flux and rising temperatures in the subarctic North Pacific. During the Younger Dryas, cold event planktic δ¹⁸O values increased by 0.5‰, followed by a gradual decrease by 1‰ from the early to middle Holocene, reflecting a gradual increase in eastward outflow via the Tsugaru Strait with sea level rise. Both planktic and benthic foraminiferal δ¹³C values oscillated between 34 and 10 ka, at relatively large amplitudes (about 0.5‰), then remained relatively stable during the last 10 kyr. Several negative planktic and benthic ( − 0.7‰) δ¹³C excursions were present in sediment dated between the precipitation of secondary carbonates during episodic methane release possibly associated with methane release from continental margin sediments.     

Diatom-derived SSTs (Td′ ratio) indicate warm seas off Japan during the middle Holocene (8.2–3.3 kyr BP)

Regression analysis was performed between the ratio of warm- and cold-water diatoms (Td′ ratio) in 123 surface sediment samples in the northwest Pacific Ocean and the mean annual sea surface temperatures (SSTs) (°C) at the core sites, in order to calibrate this proxy for paleo-temperatures. Equations to derive annual SST (°C) from diatom analysis were somewhat different for the Tohoku area (y = 6.5711⁎ × ^0.273, r = 0.89946) than for the Japan Sea (y = 5.4069⁎ × ^0.26841, r = 0.89088). The annual paleo-SSTs (°C) in the Tohoku Area were in general higher than in the Japan Sea despite lower Td′ values, because the warm-water species Fragilariopsis doliolus is abundant only in the Tsushima Warm Current in the Japan Sea. The reliability of the equations is supported by our understanding of the ecology of recent diatoms. Td′-derived annual paleo-SSTs (°C) agree with alkenone-derived summer paleo-SST (°C) at a site off central Japan in the northwestern Pacific. In a south-to-north transect of cores around Honshu and Hokkaido, paleo-SSTs (°C) decreased when the Kuroshio and Tsushima Warm Currents weakened. The middle Holocene (8.2–3.3 cal kyr BP) was warmer by 1–2 °C than earlier and later parts of the Holocene, and mean annual paleo-SSTs (°C) show rhythmic fluctuations with durations of 1000 years and 400–500 years. These new data show that annual SSTs were warm in the northwestern Pacific at times that the northeastern Pacific was cold. In addition, oceanic warm- and cold-water species defined by Kanaya and Koizumi [Kanaya, T., Koizumi, I., 1966. Interpretation of diatom thanatocoenoses from the North Pacific applied to a study of core V20–130 (studies of a deep-sea core V20–130, part IV). Sci. Rep. Tohoku Univ., 2nd series (Geol.) 37, 89–130], and Koizumi et al. [Koizumi, I., Irino, T., Oba, T., 2004. Paleoceanography during the last 150 kyr off central Japan based on diatom floras. Mar. Micropaleontol. 53, 293–365] are illustrated.     

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.     

Paleotemperature response to monsoon activity in the Japan Sea during the last 160 kyr

Paleo-sea-surface temperatures in the northeastern- and southeastern-parts of the Japan Sea were reconstructed for the last 160 kyr using alkenone temperatures (U^K′₃₇-temperatures). U^K′₃₇-temperatures at two sites show distinct glacial–interglacial changes during the last 160 kyr except for the interval corresponding to middle MIS 3 to MIS 2. On orbital-timescales, U^K′₃₇-temperature tends to be high during MIS 5e, MIS 5c, and MIS 5a, which coincides with the intervals of stronger East Asian summer monsoon activity. The amplitude of temperature fluctuations in the Japan Sea is significantly higher than those in the neighboring seas. We suggest that the SST variation was amplified by the increasing source water (Kuroshio water) temperature and the changes in the volume transport of the Tsushima Warm Current (TWC) and/or the north–south oscillation of the sub-polar front position within the Japan Sea. Millennial-scale temperature fluctuations in the Japan Sea show that the temperature at the northern site was higher than that at the southern site during warmer periods of MIS 5, which is called “temperature reversal.” By analogy with modern oceanography, the temperature reversal could reflect the enhanced volume transport of the TWC and the spatial relationship between the studied site and the branches of the TWC, which is an essential factor in north–south temperature reversal around the eastern Japan Sea. Temperature drops were found at 114 ka, 111 ka, 93 ka, 87 ka, and 77 ka in MIS 5. Those events were associated with an increase in organic carbon and alkenone contents and can be correlated with the abundance peaks of ice-rafted debris (IRD) at Site GH05-1208 in the northern Japan Sea, suggesting that the surface water was cooled by enhanced mixing and consequent upwelling in a stronger winter monsoon regime.     

A size analysis of planktic foraminifera from the Arabian Sea

Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 μm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 μm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125–355 μm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in <125 μm fractions Dentigloborotalia anfracta, Tenuitella compressa, Tenuitella iota, Turborotalita quinqueloba and the immature specimens of larger species. Consequently, the choice of the sieve mesh size strongly determines the percent composition of the assemblage and in turn the paleoceanographic interpretations based on these counts. Species richness and the Shannon diversity increase with decreasing sieve mesh size, while equitability generally decreases with decreasing size. In the water column approximately 60% of the fauna (>100 μm) is present in the 100–125 μm fraction and 1–6% is larger than 250 μm. In samples representing a settling flux (sediment trap and sediment samples) 29–57% of the fauna is present in the 100–125 μm fraction, while 6–23% is larger than 250 μm. Size frequency distributions of the dextral Neogloboquadrina complex (= Neogloboquadrina dutertrei and Neogloboquadrina pachyderma + P–D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 μm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 μm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.     

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.     

Environmental and climatic changes in the central Mediterranean Sea (Siculo–Tunisian Strait) during the last 30 ka based on dinoflagellate cyst and planktonic foraminifera assemblages

A high resolution micropalaeontological study of the core MD 04-2797 CQ recovered in the Sicilian–Tunisian Strait provides insights into the paleoclimatic history of the Mediterranean Sea at the transition between the western and eastern basin over the last 30 ka. Using the analysis of dinoflagellate cyst and planktonic foraminiferal assemblages, we reconstruct the paleoenvironmental changes that took place in this region. High abundances of cold temperate dinocyst species (Nematosphaeropsis labyrinthus, Spiniferites elongatus, Bitectatodinium tepikiense) and the polar planktonic foraminifera Neogloboquadrina pachyderma (left coiling) reveal three major cooling events synchronous with North Atlantic Henrich events 1 and 2 (H1 and H2) and the European and North Atlantic Younger Dryas event. During the Holocene, the presence of warm dinocyst species (Spiniferites mirabilis and Impagidinium aculeatum) and planktonic foraminifera (Globorotalia inflata and Globigerinoides ruber), reflects a significant increase of sea surface temperatures in the western Mediterranean basin, but a full warming was not recorded until 1500 years after the onset of the Holocene. Moreover, our results show that the Holocene was interrupted by at least four brief cooling events at ~ 9.2 ka, ~ 8 ka, ~ 7 ka and ~ 2.2 ka cal. BP, which may be correlated to climatic events recorded in Greenland ice cores and in the Atlantic Ocean.     

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.     

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.     

Glacial Holocene environment of the southeastern Okhotsk Sea: evidence from geochemical and palaeontological data

Environmental conditions and productivity changes in the southeastern Okhotsk Sea have been reconstructed for the last 20 ka using planktonic and benthic foraminiferal oxygen isotope records and calcium carbonate, organic carbon and opal content data from two sediment cores. Species variability in benthic foraminiferal and diatom assemblages provides additional palaeoceanographic evidence. AMS radiocarbon dating of the sediments and oxygen isotope stratigraphy serve as the basis for the age models of the cores for the last 20 ¹⁴C kyr and for correlation between environmental variations in the Okhotsk Sea, and regional and global climate changes. Benthic foraminiferal assemblages in the two cores (depth 1590 and 1175 m) varied with time, so that we could recognise seven zones with different species composition. Changes in the benthic foraminiferal assemblages parallel major environmental and productivity variations. During the last glaciation, fluxes of organic matter to the sea floor showed strong seasonal variations, indicated by the presence of abundant A. weddellensis and infaunal Uvigerina spp. Benthic foraminiferal assemblages changed with warming at 12.5–11 and 10–8 ¹⁴C kyr BP, when productivity blooms and high organic fluxes were coeval with global meltwater pulses 1A and 1B. Younger Dryas cooling caused a decline in productivity (11–10 kyr BP) affecting the benthic faunal community. Subsequent warming triggered intensive diatom production, opal accumulation and a strong oxygen deficiency, causing significant changes in benthic fauna assemblages from 5.26–4.4 kyr BP to present time.     

The Late Glacial–Holocene record from Central Adriatic Sea: Paleoenvironmental reconstruction based on benthic foraminiferal assemblages

New micropaleontological data coming from three cores collected on the meso-Adriatic continental shelf (Vasto area) are studied. Comparisons to foraminiferal assemblages and radiocarbon dates previously collected from cores in the San Benedetto del Tronto and Tremiti areas allow the correlation of patterns observed in shallow water areas with those in deeper parts of the basin. We focused on the response of benthic foraminifera during the Holocene high-stand, corresponding to the installation of the recent sedimentary and trophic system. An influence of anthropogenic impacts cannot be ruled out; its effects consist of a depletion of oxygen level with a consequent modification of the structure of benthic foraminiferal assemblages. During the glacial/post glacial cycle, three phases, characterized by a total of six foraminiferal biofacies including different species assemblages were recognized. The first phase, from 14 kyr BP to 11 kyr BP, corresponds to the Bölling/Alleröd and Younger Dryas cold event, before the Holocene sea-level rise. During this phase, the continental shelf was characterized by an infralittoral environment with productive waters owing to the proximity of the Po river delta at the edge of the Mid-Adriatic Deep. The second phase, from 11 kyr BP to 4 kyr BP, represents the Holocene sea-level rise and is characterized by a condensed sedimentation spread over the entire basin. The third phase corresponds to the Holocene high-stand, during which time the modern current system became established. During this phase, the eastern portion of the shelf underwent to the central part of the mud-belt, corresponding to the sub-recent conditions. Recent eutrophication resulting from human activities over the last few centuries is evidenced by frequency fluctuations of typically opportunistic taxa such as Nonionella turgida and Epistominella vitrea.     

Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation: Evidence from the sedimentation rate and planktonic foraminiferal record

Well-dated, high-resolution records of planktonic foraminifera and oxygen isotopes from two sediment cores, A7 and E017, in the middle Okinawa Trough reveal strong and rapid millennial-scale climate changes since ∼ 18 to 17 thousand years before present (kyr B.P.). Sedimentation rate shows a sudden drop at ∼ 11.2 cal. kyr B.P. due to a rapid rise of sea level after the Younger Dryas (YD) and consequently submergence of the large continental shelf on the East China Sea (ECS) and the retreat of the estuary providing sediment to the basin. During the last deglaciation, the relative abundance of warm and cold species of planktonic foraminifera fluctuates strongly, consistent with the timing of sea surface temperature (SST) variations determined from Mg/Ca measurements of planktonic foraminifera from one of the two cores. These fluctuations are coeval with climate variation recorded in the Greenland ice cores and North Atlantic sediments, namely Heinrich event 1 (H1), Bølling–Allerød (B/A) and YD events. At about 9.4 kyr B.P., a sudden change in the relative abundance of shallow to deep planktonic species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough, which was synchronous with a rapid sea-level rise at 9.5–9.2 kyr B.P. in the ECS, Yellow Sea (YS) and South China Sea (SCS). The abundance of planktonic foraminiferal species, together with Mg/Ca based SST, exhibits millennial-scale oscillations during the Holocene, with 7 cold events (at about 1.7, 2.3–4.6, 6.2, 7.3, 8.2, 9.6, 10.6 cal. kyr BP) superimposed on a Holocene warming trend. This Holocene trend, together with centennial-scale SST variations superimposed on the last deglacial trend, suggests that both high and low latitude influences affected the climatology of the Okinawa Trough.     

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.     

Varieties of a single cosmopolitan diatom species associated with surface water masses in the North Pacific

A time series of 65 sediment trap samples, representing 48-month changes in diatom flora, and 62 sediment core-tops from the western North Pacific Ocean were analyzed. Varieties of a single diatom species were found to be characteristic of two surface water masses bounded by the Kuroshio Current. Thalassionema nitzschioides var. incurvata, var. inflata and var. parva dominate in warm saline waters in the Subtropical Gyre, while T. nitzschioides sensu stricto is abundant in less-saline waters northwest of this current. Time series data on percent abundances for the three T. nitzschioides varieties in the T. nitzschioides complex are compared with path variations of the current south of Japan. Inter-annual fluctuations in percent abundance are linked to changes in surface water masses over the sediment traps in association with migration of the Kuroshio paths. This finding makes the percent abundance a useful tool for deciphering path migrations of the Kuroshio Current in the geologic past. Down-core analyses of two sediment cores collected beneath the modern Kuroshio Current were performed. A sharp increase in percent abundances from 12 to 9 kyr BP, with an abrupt change around 11 kyr BP, records the previously reported transition of the Kuroshio Current from a predominantly offshore path to its modern path. Path migrations of the Kuroshio Current may be inferred from percent abundances of these diatoms, which comprise a new proxy for delineating the paleoclimate of the northwestern Pacific in relation to changes in northward heat transport from the western equatorial Pacific Ocean by the western boundary current.     

The vertical distribution of Radiolaria in the waters surrounding Japan

Geographic and vertical distribution patterns of living Radiolaria are closely related to the characters of the water column. We studied living Radiolaria in samples collected at closely spaced depth intervals in the waters surrounding Japan in order to understand their vertical distribution and its controlling factors. Such information is needed to reconstruct past vertical water mass structure. The Japanese Islands are surrounded by the marginal Japan Sea and the North Pacific Ocean, and from approximately 46° N to 26° N. They thus extend across a large latitudinal range and are exposed to a similarly wide range of environmental conditions, inducing the warm Kuroshio, Tsushima, and Tsugaru Currents and the cold Oyashio Current.We performed plankton tows (mesh size 63 mm) from late May to early June 2002 at eight sites, sampling the upper 200 m of the water column, at seven depth intervals.We recognized five radiolarian assemblages: the Upper Surface Assemblage (0–40 m), the Lower Surface Assemblage (40–80 m), the Surface Assemblage (0–80 m), the Subsurface Assemblage (80–200 m), and the Lower Surface–Subsurface Assemblage (40–200 m) groups. Pseudodictyophimus gracilipes and Tetrapyle octacantha (juvenile) show “tropical submergence”, i.e. they live at the surface in high latitudes and at greater depth in low latitudes. Four taxa live at specific temperature and salinity: Acrosphaera spinosa, Larcopyle butschlii, Challengeron diodon, and Stichocorys seriata. Four taxa are associated with characteristic temperatures: Neosemantis distephanus, Arachnocorys umbellifura, Antarctissa sp. 1, and Saccospyris conithorax.     

Distribution and abundance of the planktic foraminifer Neogloboquadrina pachyderma in sea ice of the Weddell Sea (Antarctica)

Summary Sea ice cores were obtained from eleven fast ice stations and one floe in the Weddell Sea, Antarctica in January–February 1985. All cores from the north eastern part of the Weddell Sea contained numerous living and dead planktic foraminifers of the species Neogloboquadrina pachyderma (Ehrenberg), while cores drilled in southern parts were barren of foraminifers with one exception. Foraminiferal abundances were variable, with numbers up to 320 individuals per liter melted sea ice. Distribution of foraminifers appears to be patchy, parallel cores taken less than 30 cm apart contained numbers which varied considerably. On the other hand, three cores taken on a transect each more than 3 km apart showed striking similarities. In general, small dead tests were found in the upper parts of the sea ice cores while large living individuals mainly occurred in lower sections. Abundant diatoms probably serve as a food source for the foraminifers. Correlation of foraminiferal abundance with salinity, chlorophyll and nutrient profiles are inconsistent. The possible mechanism of incorporation of N. pachyderma into the ice is discussed.     

Environmental changes off North Iceland during the deglaciation and the Holocene: foraminifera, diatoms and stable isotopes

A combined study of foraminifera, diatoms and stable isotopes in marine sediments off North Iceland records major changes in sea surface conditions since about 15 800 cal years (yr) BP. Results are presented from two gravity cores obtained at about 400 m water depth from two separate sedimentary basins on each side of the submarine Kolbeinsey Ridge. The chronology of the sedimentary record is based partly on AMS ¹⁴C dates, partly on the Vedde and the Saksunarvatn tephra markers, as well as the historical Hekla AD 1104 tephra. During the regional deglaciation, the planktonic foraminiferal assemblages are characterised by consistently high percentages of sinistrally coiled Neogloboquadrina pachyderma. However, major environmental variability is reflected by changes in stable isotope values and diatom assemblages. Low δ¹⁸O values indicate a strong freshwater peak as well as possible brine formation by sea-ice freezing during a pre-Bølling interval (Greenland Stadial 2), corresponding to the Heinrich 1 event. The foraminifera suggest a strong concurrent influence of relatively warm and saline Atlantic water, and both the foraminifera and the diatoms suggest mixing of cold and warm water masses. Similar but weaker environmental signals are observed during the Younger Dryas (Greenland Stadial 1) around the level of the Vedde Ash. Each freshwater peak is succeeded by an interval of severe cooling both at the beginning of the Bølling–Allerød Interstadial Complex (Greenland Interstadial 1) and during the Preboreal, presumably associated with the onset of intense deep water formatiom in the Nordic Seas. The Holocene thermal optimum, between 10 200 and about 7000 cal years (yr) BP, is interrupted by a marked cooling of the surface waters around 8200 cal yr BP. This cold event is clearly expressed by a pronounced increase in the percentages of sinistrally coiled N. pachyderma, corresponding to a temperature decrease of about 3°C. A general cooling in the area is indicated after 7000–6000 cal yr BP, both by the diatom data and by the planktonic foraminiferal data. After a severe cooling around 6000 cal yr BP, the planktonic foraminiferal assemblages suggest a warmer interval between 5500 and 4500 cal yr BP. Minor temperature fluctuations are reflected both in the foraminiferal and in the diatom data in the upper part of the record, but the time resolution of the present data is not high enough to pick up details in environmental changes through the late Holocene.