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

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

Late Quaternary variations in water mass in the Shatsky Rise area, northwest Pacific Ocean

Oxygen isotope analysis of planktonic and benthic foraminifera in piston core S-2 collected from the Shatsky Rise (33°21.75N, 159°07.70E; water depth 3107 m) provides a paleoceanographic record for the last 540 000 years in the northwestern Pacific Ocean. Although peaks in the abundance of sinistral Neogloboquadrina pachyderma occur during Marine Isotope Stage 2, and particularly 6 and 12, the southward shifting of the Subarctic front did not reach the core site during these glacial periods. However, mass accumulation rates of total organic carbon, biogenic opal, and terrigenous matter contents indicate that surface productivity increased during cold periods. In addition, the C/N ratio analyzed in organic matter reached values of up to 10 during glacial periods. These results imply that delivery of eolian dust to this site was enhanced by strengthened westerly winds during glacial periods. Down-core fluctuations in δ¹³C values of Globigerinoides ruber and Globorotalia inflata nearly overlap, particularly during the period from 540 to 260 ka. This latter trend suggest that the subtropical surface water mass prevailed at the core site throughout that period, based upon the very small vertical δ¹³C gradient through water column in modern Kuroshio Current water.     

Sensitivity of south‐east Atlantic planktonic foraminifera to mid‐Pleistocene climate change

Abstract: The last one million years are important in terms of climate development during the so‐called Mid‐Pleistocene Transition when amplification of the glacial–interglacial cycles occurred. This study describes abundance changes in fossil planktonic foraminifera in sediments from Core T89‐40, retrieved from the Walvis Ridge in the south‐east Atlantic, across this time period. Cycles between upwelling and subtropical planktonic foraminiferal assemblages are shown to mirror changes between glacial and interglacial periods, respectively. During interglacial marine isotopic stages (MIS) 9, 11 and 31, however, anomalously high abundances of the polar left‐coiled Neogloboquadrina pachyderma occur, presumably linked to unusual seasonal upwelling waters. The planktonic foraminiferal abundance record shows 41‐ky cyclic variations in the regional oceanography linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity). These orbitally induced oscillations in oceanographic change occurred throughout the entire record. The most conspicuous feature of the planktonic foraminiferal record is the near absence of left‐coiled Globorotalia truncatulinoides between 960 and 610 ka (MIS 26‐15). The abrupt disappearance of this species is synchronous with the onset of the Mid‐Pleistocene Transition in MIS 26.     

Foraminifera and coccolithophorid assemblage changes in the Panama Basin during the last deglaciation: Response to sea-surface productivity induced by a transient climate change

The responses of community assemblages of planktonic and benthonic foraminifera and coccolithophorids to transient climate change are explored for the uppermost 2 m of cores ODP677B (1.2°N; 83.74°W, 3461 m) and TR163-38 (1.34°S; 81.58°W, 2200 m), for the last ∼ 40 ka. Results suggest that the deglaciation interval was a time of increased productivity and a major reorganization of planktonic trophic webs. The succession in dominance between the planktonic foraminifera species Globorotalia inflata, Globigerina bulloides, and Neogloboquadrina pachyderma denote four periods of oceanographic change: (1) advection (24-20 ka), (2) strong upwelling (20-15 ka), (3) weak upwelling (14-8 ka) and (4) oligotrophy (8 ka to present). Strong upwelling for the deglaciation interval is supported by the low Florisphaera profunda/other coccolithophorids ratio and the high percentage abundance of Gephyrocapsa oceanica. Benthonic foraminifera assemblage changes are different in both cores and suggest significant regional variations in surface productivity and/or oxygen content at the seafloor, and a decoupling between surface productivity and export production to the seafloor. This decoupling is evidenced by the inverse relationship between the percentage abundance of infaunal benthonic foraminifera and the percentage abundance of N. pachyderma. The terrigenous input of the Colombian Pacific rivers, particularly the San Juan River, is suggested as a possible mechanism. Finally, the Globorotalia cultrata/Neogloboquadrina dutertrei ratio is used to reconstruct the past influence of the Costa Rica Dome-Panama Bight and cold tongue upwelling systems in the Panama Basin. A northern influence is suggested for the late Holocene (after 5 ka) and the last glacial (before 20 ka), whereas a southern influence is suggested for the 20-5 ka interval. There is a correspondence between our reconstructed northern and southern influences and previously proposed positions of the Intertropical Convergence Zone (ITCZ) after the Last Glacial Maximum (LGM).     

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.     

Uvigerina proboscidea abundances and paleoceanography of the northern Indian Ocean DSDP Site 214 during the Late Neogene

This study attempts to understand the significance of Uvigerina proboscidea in paleoceanographic reconstructions at the northern (tropical) Indian Ocean DSDP Site 214 from the Late Miocene through the Pleistocene. In this interval at this site, U. proboscidea is the most abundant species of the benthic assemblage and shows abrupt frequency changes (about 1–74%). Based on relative percentages of U. proboscidea calibrated with oxygen and carbon isotope record and the sediment accumulation rates, the modern distribution of the species in the Indian Ocean, and other evidence, the peaks of abundance of U. proboscidea are inferred to represent times of high-surface productivity. This productivity is related to intensified trade winds during strong southwest (SW) Indian monsoons, causing widespread upwelling along equatorial divergence in the Indian Ocean. The sudden increase of U. proboscidea abundance at approximately 8.5–7.5 Ma reflects significant upwelling at the equatorial divergence. This event corresponds to the permanent build-up of West Antarctic ice sheets, and a major increase in SW Indian monsoons related upwelling in the northwestern Indian Ocean. The Chron-6 carbon shift at approximately 6.2 Ma is marked by another peak of abundance, reflecting widespread ocean fertility. The highest abundances of U. proboscidea and highest sediment accumulation rates occur between 5.8 and 5.1 Ma, which coincides with the greatest development of Antarctic ice sheets and strong southwest monsoons. The higher percentages at 3.2–3.1 Ma, approximately 2.4 Ma, and 1.6 Ma all represent phases of high productivity at the equatorial divergence.     

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

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

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.     

Spatial and temporal variations in calanoid copepod distribution in the Straits of Malacca

The distribution and abundance of planktonic calanoid copepods were studied from samples collected at 13–20 stations during four oceanographic cruises (pre- and post- monsoons, and during northeast (NE) and southwest (SW) monsoons) performed between 1998 and 2000 in the Straits of Malacca. Space and time variations of calanoid copepods were described using univariate (number of species, diversity indices, abundance) as well as multivariate (MDS, ANOSIM, SIMPER) techniques from the Plymouth Routines in Multivariate Ecological Research (PRIMER) package. There were significant differences in abundance between the cruises. k-Dominance curves also revealed significant differences in the relative species abundance distributions among the monsoon periods, and a decrease in diversity from northern to southern parts of the Straits during each cruise. Multi-dimensional scaling revealed four groups of abundances with differences in species composition. Evidence from analysis of similarity (ANOSIM) suggested that the differences in communities among monsoon periods were significant, although spatial differences among samples in geographic locations in the northern, central and southern parts of the Straits were insignificant. These differences resulted from an overall change in the balance of relative abundance of few dominant species, rather from changes of many species. Similarity percentage analyses (SIMPER) indicated that the major species contributing to the average dissimilarity between monsoons varied temporally.     

High frequency palaeoceanographic changes during the past 140?000 yr recorded by the organic matter in sediments of the Iberian Margin

Biogenic records of the marine palaeoproductivity (carbonates, organic carbon, and C₃₇ alkenones) and the molecular stratigraphy of past sea surface temperatures (SSTs; U₃₇^K′) were studied at high resolution in two cores of the Iberian Margin. The comparison of these records indicates that the oceanographic conditions switched abruptly during the past 160 kyr between three kinds of regimes. A first regime with high (17-22°C) SST and low productivity typifies the interglacial periods, marine isotopic stages (MIS) 5 and 1. Several periods during MIS 6, 2, and the terminations II and I are characterised by about 4-5°C colder SST and a higher organic matter accumulation, both of which define the second regime. This anticorrelation between SST and marine productivity suggests that these variations are related to the intensity of the coastal upwelling. By contrast with this upwelling behaviour, extremely low biological productivity and very cold SST (6-12°C) occurred during short phases of glacial MIS 6, 4, and 2, and as abrupt events (≈1 kyr or less) during MIS 3. The three oceanographic regimes are consistent with micropalaeontological changes in the same cores based on foraminifera and diatoms.The general trend of these hydrologic changes follows the long-term glacial/interglacial cycle, but the millennium scale variability is clearly related to Heinrich events and Dansgaard-Oeschger cycles. Strengthening of the upwelling corresponds probably to an intensification of the subtropical atmospheric circulation over the North Atlantic which was influenced by the presence of continental ice sheets. However, extreme glacial conditions due to massive discharges of icebergs interrupted the upwelling. Interestingly, both terminations II and I coincided with strong but transient intensification of the upwelling.     

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.     

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.     

Coccolith evidence for Quaternary nutricline variations in the southern South China Sea

Nutricline variations during the last 1560 ka in the southern South China Sea are reconstructed using the relative abundance of the coccolithophore Florisphaera profunda in ODP Site 1143. Nutricline depth shows both long-term large magnitude variation and high frequency glacial–interglacial variation. On the long-term scale, the nutricline experienced four significant changes, which occurred at 900, 480, 250, and 50 ka respectively. According to these changes, five stages could be recognized during the last 1560 ka. From 1560 to 900 ka, the nutricline depth increased gradually. Around 900 ka, the nutricline abruptly shallowed and then remained stable until 480 ka. The nutricline was shallow during the time interval between 480 and 250 ka. At 250 ka, it deepened again and increased gradually until 50 ka. After 50 ka, the nutricline depth decreased gradually to modern values. On the glacial–interglacial scale, the variations in nutricline depth show different patterns before and after 900 ka. Before 900 ka, the nutricline was deep during glacial periods and shallow during interglacials. However, after 900 ka, the nutricline was deep during interglacials and shallow during glacials. Spectral analysis of the relative abundance of F. profunda shows a similar trend. In addition to the eccentricity (113, 76 ka), obliquity (55, 39 ka), and precession (24, 19 ka), we also find a 431 ka cycle. The former three periods reflect glacial–interglacial variations in nutricline, and the period of 431 ka reflects long-term variations in nutricline. We suggested that the variations in nutricline in the southern South China Sea were caused by global and regional climate changes. Glacial–interglacial variations in nutricline are mainly controlled by the East Asian monsoon, and the long-term variations might be related to the global climatic events, such as the mid-Pleistocene Revolution and the mid-Brunhes event.     

Late Quaternary millennial-scale variability in pelagic aragonite preservation off Somalia

In order to better understand Late Quaternary pelagic aragonite preservation in the western Arabian Sea we have investigated a high-resolution sediment core 905 off Somalia. Pteropod preservation is enhanced in times of reduced monsoon-driven productivity, indicated by low amounts of C_org and low barium to aluminium (Ba/Al) ratios. All periods corresponding to Heinrich events in the North Atlantic are represented by maxima in shell preservation of the common pteropod Limacina inflata (LDX values < 2, except for H5-equivalent with a poorer shell preservation, LDX > 2.66). Good shell preservation is also found during stadials at 52.1–53.2, 36, 33.2, and 31.9 ka. Relative abundance of pteropods and their fragments in the coarse fraction reaches maxima during Marine Isotope Stage (MIS) 5.2, during time-equivalents of Heinrich events 4–6 and in stadials at  53,  42.5, and 41.4 ka.On longer time scales, the pteropod abundance corresponds to the ‘Indo-Pacific carbonate preservation type’ with poor preservation during interglacials and better preservation during glacials. Late MIS 5 to early MIS 4 sections (84.1–64.8 ka) and the Late Holocene interval (6.5–0 ka) of core 905 contain only traces of pteropods. The early Holocene (9.2–6.5 ka) part is characterized by low pteropod amounts. Between 64.8 and 43.4 ka strong fluctuations occur and an intermediate average relative pteropod abundance is revealed. Between 43.4 and 9.2 ka the highest amounts in relative pteropod abundance in core 905 are observed. Besides the regional monsoonal influence on deepwater chemistry, changes in deepwater circulation occurring on glacial/interglacial and stadial/interstadial time scales might have affected pteropod preservation. However, it remains elusive whether 1) deep water formation in the Arabian Sea, 2) inflow of Glacial North Atlantic Intermediate Water or 3) change in water mass properties of the Circumpolar Deep Water (which is the water mass currently bathing this site) contributed to the observed pteropod preservation pattern.     

Pleistocene fluctuations in the Agulhas Current Retroflection based on the calcareous plankton record

Piston core (PS2487-6), recovered south of Cape Town, and sediment surface samples, recovered in the area of the Agulhas Current retroflection, were used to construct paleoceanographic scenarios for the late Quaternary in a region with an important role in global water mass transfer. Coccolithophore (calcareous nannofossil) and planktonic foraminifera assemblages and oxygen isotope data were collected. Stratigraphic control is based on calibration of the δ¹⁸O stratigraphic signals with calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Study of the surface sediments permits the characterisation of the Agulhas Current, Subtropical Convergence and Subantarctic coccolithophore assemblages. The Agulhas Current assemblage has relatively high proportions of Florisphaera profunda, Gephyrocapsa oceanica and Umbilicosphaera spp. These species are absent or present in low proportions in subantarctic waters. The abundance of coccolithophores during isotope stages 1, 5 and 7 is characteristic of relatively warm, stratified surface waters, with a deep nutricline and chlorophyll maximum, which strongly suggests that the area was under the influence of the Agulhas Current retroflection. The incursion of Globigerinoides ruber, abundant today in the Agulhas Current, also supports this interpretation. Conversely, during glacial stages 2–4 and 6, a strong reduction in warm and stratified water indicators can be observed, together with an increase in cold-eutrophic species.The interval from isotope stages 8 to 12 displays an assemblage dominated by Gephyrocapsa caribbeanica, the highest values being seen in the so-called Mid-Brunhes event, accompanied by a clear reduction in subtropical Holocene species. During the glacial interval from isotope stages 10–12, G. caribbeanica dominated the assemblage. The ecological interpretation of this species is controversial due to a clear evolutionary overprint. Isotope stage 12 is here interpreted as having been the coldest one in the period studied. It is characterised by a strong increase in Neogloboquadrina pachyderma (dextral and sinistral) and a remarkable decrease in tropical and subtropical planktonic foraminifera. From isotope stage 13 to 18, a hiatus is interpreted. Below this hiatus the stratigraphic resolution is poorer, although the glacial–interglacial cyclicity is well defined in the Agulhas Current Retroflection area (Core PS2487-6) for the last 25 isotope stages. The glacial–interglacial cyclicity is thought to be due to a fluctuation in the Subtropical Convergence Zone, probably linked to the eastward and westward displacement of the Agulhas Current retroflection. In any case, the core studied was always in a subtropical environment, under the influence of the Agulhas Current, which was enhanced during interglacial periods. For the whole of the interval studied, increases in Calcidiscus leptoporus, Umbilicosphaera spp., and Syracosphaera spp. among the calcareous nannofossils, and in G. ruber and Globigerinoides sacculifer within the planktonic foraminifera, clearly respond to interglacial pulses, reaching maximum values during short periods close to the major deglaciations.     

A 200-ka pollen and oxygen-isotopic record from two sediment cores from the eastern Arabian Sea

We present here a pollen record from the northern Indian Ocean that goes back to 200 ka, the boundary between marine isotopic stages (MIS) 7 and 6. Pollen, oxygen-isotopic composition and organic carbon have been examined for two sediment cores from the eastern Arabian Sea (15°02′N and 71°41′E, 13°16′N and 71°00′E), to reconstruct the long-term palaeoclimate and palaeovegetation of the Indian subcontinent. Oxygen-isotope data suggest that glacial periods (MIS 2, 4 and 6) are characterised by low precipitation because of a weak southwest monsoon and a strong northeast monsoon. In contrast, interglacial periods (MIS 1, 3 and 5) are marked by high fresh water input resulting from a strong southwest monsoon. During the last glacial-interglacial cycle, sea-surface temperature and surface salinity changed as a result of variations in the evaporation-precipitation (E-P) balance. Throughout the core, the dominant pollen types are Poaceae and Chenopodiaceae and/or Amaranthaceae (>50%). Their dominance during glacial periods (MIS 2, 4 and 6) suggests that the climate was cold and dry. This dominance is also suggestive of salinity-tolerant vegetation colonizing large areas near seashore due to lower sea level or high E-P conditions. During interglacial periods (MIS 1, 3 and 5) when there was high precipitation, the arid taxa were sparse, whereas Poaceae and Piperaceae were abundant. Hence, Chenopodiaceae and/or Amaranthaceae and Artemisia are suggestive of cold and arid/semi-arid climate, and Poaceae and Piperaceae of warm and wet conditions. Mangrove pollen is not well represented in the cores.     

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.     

The seasonality of monsoonal primary productivity in Sri Lanka

The relationship between phytoplankton primary production and seasonality of physico-chemical parameters were examined for five man-made lakes in the dry-zone of Sri Lanka. Sri Lanka experiences two monsoons dividing the year into four meteorological seasons: — the North East (October–December) and South West (April–June) monsoons and the two inter-monsoons. A significant log linear relationship was found between Secchi disc depth and the depth of the euphotic zone which was lowest during the NE monsoon. Maximum mean photosynthetic rate ranged from 0.935 ± 0.067 SE to 0.479 mg O₂l^–1 h^–1 ± 0.115. Gross primary productivity which ranged from 0.378 g O₂ m^–2 h^–1 in the NE monsoon to 0.980 g O₂ m^–2 h^–1 in the SW monsoon showed significant season variation. This is shown to be determined either directly or indirectly by the light regime.     

Organic geochemical changes in Pliocene sediments of ODP Site 1083 (Benguela Upwelling System)

The intensification of the Northern Hemisphere Glaciation (INHG) was a major event in the development of the current climate state, and as one of the most productive regions in the world's oceans, the behaviour of the Benguela Upwelling System (BUS) following the INHG is of wide interest. To investigate post-INHG changes in productivity and organic matter accumulation, total organic carbon and biomarker accumulation rates were determined for sediments from ODP Site 1083 and compared to alkenone-derived sea surface temperatures and nitrogen isotopic compositions. These data indicate that the interval between 2.6 and 2.4 Ma was characterized by dramatic changes in upwelling intensity and organic carbon export on the northern edge of the modern BUS. The upwelling is reflected by significant changes in alkenone-derived SST estimates between glacial and interglacial intervals, with a total variability of 16 °C. The studied interval is also characterized by large changes in organic matter export as reflected by changes in TOC and biomarker accumulation rates, which show maxima during OIS 98 and during the transition from OIS 97 to 96. Intervals of elevated TOC are also characterized by elevated concentrations of sedimentary microbial biomarkers and lower %CaCO₃, suggesting that enhanced delivery of labile organic matter to the seafloor resulted in enhanced remineralisation with released CO₂ being consumed by CaCO₃ dissolution. However, in apparent contrast to recent Pleistocene sediments at the same site, organic matter export after the INHG was not solely driven by upwelling intensity. Of the three Pliocene glacial–interglacial cycles examined (OIS 101 to 96), each is unique with respect to the timing and magnitude of changes in organic matter accumulation. Each is also characterized by different algal assemblages as inferred from biomarker distributions, with OIS 97 and 96 particularly dominated by diatoms. We suggest that these differences reflect the important but evolving role of Southern Ocean waters in the Pliocene BUS: nutrient depletion of SO waters occurred during parts of Pliocene glacial intervals such that even intense upwelling did not persistently result in enhanced organic matter accumulation rates.     

Late pleistocene paleoclimatology: Planktonic foraminiferal analyses of sediment cores from the central North Atlantic

Planktonic foraminiferal analyses of six deep-sea sediment cores from the central North Atlantic east of the Azores Islands between 37°N and 40°N show distinct oscillations in planktonic foramineferal assemblages during the last 300,000 years. A paleoclimatic curve has been constructed using “Total Fauna Analysis” that reveals three glacial and four interglacial episodes. Relatively minor climatic oscillations are superimposed upon the major glacial-interglacial episodes. The paleoclimatic curve is similar to previous paleoclimatic curves from the Atlantic and adjacent areas. Minor paleoclimatic fluctuations are more distinct in paleoclimatic curves from high latitudes of the Atlantic.The faunal assemblages are transitional between subarctic and subtropical assemblages. During portions of the interglacial episodes, the assemblage is dominated by Globorotalia inflata. Neogloboquadrina pachyderma (dextral-coiling) or Globigerina bulloides dominate during the remainder of the interglacial episodes and during the glacial episodes. Glacial episodes are also marked by particularly high frequencies of Globigerina quinqueloba and Globorotalia scitula. Interglacial episodes are also marked by increases in Globorotalia truncatulinoides, Globigerinoides ruber, and Globigerinella aequilateralis.The planktonic foraminiferal faunal oscillations in the cores are complex and cannot be entirely explained by temperature variation. Other parameters such as salinity, nutrients and biological competitin must influence the faunal oscillations.The faunas suggest no major planktonic foraminiferal faunal boundaries migrated across the area between 37°N and 40°N in the central North Atlantic during the last 300,000 years.