Planktonic Foraminiferal Oxygen Research Articles

Estimating Plio‐Pleistocene North African Monsoon Runoff Into the Mediterranean Sea and Temperature Impacts

AbstractSapropels are dark, organic‐rich layers found in Mediterranean sediments that formed during periods of bottom water anoxia. While various mechanisms have been proposed to have caused anoxic conditions, a primary factor is considered to be water column stratification induced by freshwater runoff related to intensified North African monsoon precipitation during precession minima. Monsoon intensification also induced Green Sahara Periods that may have impacted North African hominin dispersal. In this study, we present a novel regression‐based deconvolution of a high‐resolution planktonic foraminiferal oxygen isotope record to estimate the combination of freshwater runoff reaching the eastern Mediterranean and associated surface warming of the water column over the past 5 million years. Sapropels are known to occur in clusters associated with periods of high orbital eccentricity. Our analysis reveals a consistent influence of orbital eccentricity in modulating the North African monsoon, and a possible shift in runoff source area induced by the initiation of Northern Hemisphere ice sheets. Our findings provide important insights into the role of the North African monsoon in shaping Mediterranean environmental changes over the past 5 million years.

Middle Miocene-Pleistocene Magneto-Cyclostratigraphy from IODP Site U1501 in the Northern South China Sea

Site U1501 of International Ocean Discovery Program (IODP) Expedition 368 locates on a broad regional basement high in the northern margin of the South China Sea (SCS). This study refines the chronostratigraphy of the upper 160 m sedimentary succession from Hole U1501C using paleomagnetic measurements and cyclostratigraphic analysis on the Natural Gamma Radiation (NGR) data. Rock magnetic analysis displays that the magnetic signal of the sediments is mainly carried by single-domain (SD) and multi-domain (MD) magnetite. A total of 12 geomagnetic reversals are identified and correlated to the geomagnetic polarity time scale (GPTS) in Geologic Time Scale 2020 (GTS 2020), combining biostratigraphic data and planktonic foraminiferal oxygen records. The Milankovitch cycles of 405-kyr long orbital eccentricity, ∼100-kyr short orbital eccentricity, and obliquity cycles are identified in the NGR profile. A 15.54 Myr astronomical time scale is constructed by tuning the short eccentricity cycles filtered from the NGR profiles to the La2010 astronomical solution with the constraints of the magnetostratigraphic results, biostratigraphic age datum and planktonic foraminiferal oxygen records. This new high-resolution age model provides a new temporal constraint on the tectonic and paleoenvironmental evolution in the South China Sea.

Calcareous nannofossil biostratigraphy of the Lower\u2013Middle Pleistocene boundary of the GSSP, Chiba composite section in the Kokumoto Formation, Kazusa Group, central Japan, and implications for sea-surface environmental changes

The Chiba composite section (CbCS), in the middle of the Boso Peninsula in central Japan, was ratified as the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary, accompanied by the Matuyama–Brunhes (M–B) paleomagnetic polarity boundary in January 2020. This study examined the calcareous nannofossil biostratigraphy of the CbCS to describe potential nannofossil events and discuss sea-surface environments around the M–B paleomagnetic polarity boundary. There are no clear biohorizons at the M–B paleomagnetic polarity boundary, although a temporary disappearance of Gephyrocapsa specimens (≥ 5 μm in diameter), an important calcareous nannofossil genus in the Pleistocene, occurs just above the Lower–Middle Pleistocene boundary. Although this is a characteristic event around the M–B paleomagnetic polarity boundary, it is unclear whether the event is globally traceable.Changes in the environmental proxy taxa of calcareous nannofossils in the CbCS revealed that sea-surface environments were driven by glacial-interglacial and millennial-scale climate forces. The time-transgressive change of the Tn value, a calcareous nannofossil temperature index, is mostly concordant with the planktonic foraminiferal oxygen isotope fluctuation. Abundant occurrences of a warm-water species, Umbilicosphaera spp., indicate that the Kuroshio Current was strong after ~ 783 ka. Even the strong influence of the Kuroshio Current, cooling events related to southward movements of the Kuroshio Front occurred every 10,000 years based on the presence of a cold-water taxon, Coccolithus pelagicus. Additionally, the inflow of coastal waters strengthened after ~ 778 ka because of abundant occurrences of Helicosphaera spp. Millennial-scale sea-surface changes were also inferred from detected floral fluctuations of less than 10,000 years.Graphical abstract

High resolution paleo-environmental changes during the Sapropel 1 in the North Ionian Sea, central Mediterranean

High-resolution paleoceanographic reconstruction of surface water properties during the most recent Sapropel event (S1) has been carried out by means of quantitative analyses of planktonic foraminiferal assemblages, planktonic foraminiferal oxygen isotopes (δ18O) and XRF elemental data from a 655 m depth core recovered in the North Ionian Sea. The results show that the S1 interval presents two distinctive warm phases (S1a and S1b), separated by a cold interruption event (S1i). High resolution faunal and geochemical analyses allow to identify two sub-phases within S1a interval, the oldest one has similar characteristics to S1b interval while the youngest sub-phase has less stratified surface waters with relatively lower nutrient content. The high abundance of Globigerinoides ruber white variety opposite to the low percentages of Neogloboquadrina pachyderma during the pre-S1 phase suggests that the onset of surface waters stratification occurred prior to the beginning of Sapropel deposition, acting as a pre-conditioning phase. Paleo-productivity proxies indicate that the deposition of S1 initiated after an increase in nutrient content, potentially related to increased fluvial inputs. Based on the integrated ecological interpretation of our records we argue that S1a and S1b are characterized as warm, stratified and nutrient rich surface waters in the Ionian Sea, while proxies related to oxygen content indicate dysoxic deep waters linked to a combination of the high nutrient content and stratified water column. The S1 interruption phase is characterized by the entrance of colder waters that caused mixing of the stratified water column and re-ventilation of the deep dysoxic waters.

Variations in the western Pacific warm pool across the mid-Pleistocene: Evidence from oxygen isotopes and coccoliths in the West Philippine Sea

We present planktonic foraminifera oxygen isotope and Florisphaera profunda abundance data from the International Marine Global Change Study Program (IMAGES) Core MD06-3050, which was collected in the West Philippine Sea on the margin of the Western Pacific Warm Pool (WPWP). Our records reveal marked changes in the upper water structure in the Kuroshio source region as early as 1.5Ma, continuing through the mid-Pleistocene climate transition (MPT). These changes are evidenced by reconstructions of the thermocline (based on planktonic foraminiferal oxygen isotope differences) and the nutricline (based on the relative abundance of the coccolithophore species Florisphaera profunda) in the Kuroshio source region. The evolution of the thermocline/nutricline featured three prominent phases: 1) phase I (1.5–1.1Ma), which was characterized by low-amplitude variations in a shallow thermocline; 2) phase II (1.1–0.8Ma), which was characterized by high-amplitude variations in a relatively shallow thermocline/nutricline; and 3) phase III (0.8–0.5Ma), which was characterized by low-amplitude variations in a deep thermocline/nutricline. Combined with the zonal sea surface temperature gradient in the tropical Pacific, the evolution of the upper water structure can be linked to tropical climatic changes across the MPT, including the intensification of atmospheric circulation and a gradual cessation of the long-term El Niño-like state. The upper water structure of the WPWP was asymmetric before 0.8Ma, with a shallow thermocline/nutricline along the northern margin and a deep thermocline/nutricline in the center of the WPWP and South China Sea (SCS). The modern WPWP was established by approximately 0.8Ma, and the upper water structure has changed synchronously as a whole since that time. A comparison of the δ18O values derived from Globigerinoides ruber tests from the MD06–3050 core with those from other cores in the western Pacific suggests that the lower glacial/interglacial G. ruber δ18O values in MD06-3050 before 0.8Ma could be related to the enhancement of the WPWP and to changes in atmospheric circulation patterns.

Late Quaternary chronostratigraphy of the Aegean Sea sediments: special reference to the ages of sapropels S1–S5

Four sapropel layers (S1, S3, S4, and S5) are identified in five 6-10-m-long piston cores collected from the Aegean Sea basins. A chronostratigraphic framework is established for the last ~130 ka using benthic and planktonic foraminiferal oxygen isotope curves, total organic carbon contents, volcanic ash layers, and limited radiocarbon dates. These data show that the onsets of sapropels S3, S4, and S5 in the Aegean Sea basins were not synchronous, highlighting the heterogeneity of the Aegean Sea basins in terms of rapid versus lagged responses to changing ocean-climate boundary conditions. In all cases, however, the development of sapropels S3, S4, and S5 in the Aegean Sea predate their counterparts in the eastern Mediterranean by several hundred to several thousand years. The onsets of sapropel deposition were abrupt, but sapropel terminations were more gradual, controlled both by the amplitude of paleoclimatic changes and the physiography/location of the basins.

Initiation of East Asia monsoon failure at the climate transition from the Medieval Climate Anomaly to the Little Ice Age

We have reconstructed decadally-resolved continuous sea surface temperature and seawater δ18O (hence salinity) records over the last 1300yr from alkenone and planktonic foraminiferal oxygen isotope ratio analyses of the East Sea/Japan Sea marine sediments to investigate East Asia monsoon variability. Comparisons of the records with other paleoclimate records indicate a possible connection between changes in the mid-latitude East Asia monsoon and Pacific Decadal Oscillation (PDO) over this period. The results show that during the Medieval Climate Anomaly (MCA) when the PDO index was negative, East Asia was characterized by surface warming with a strengthened summer monsoon. Summer monsoon-related precipitation increased and pluvials possibly dominated in the region at that time. Onset of Asia monsoon failure and severe drought occurred at the end of the MCA and extended to the Little Ice Age (LIA) when the PDO became positive.

Holocene shifts of the southern westerlies across the South Atlantic

AbstractThe southern westerly winds (SWW) exert a crucial influence over the world ocean and climate. Nevertheless, a comprehensive understanding of the Holocene temporal and spatial evolution of the SWW remains a significant challenge due to the sparsity of high‐resolution marine archives and appropriate SWW proxies. Here we present a north‐south transect of high‐resolution planktonic foraminiferal oxygen isotope records from the western South Atlantic. Our proxy records reveal Holocene migrations of the Brazil‐Malvinas Confluence (BMC), a highly sensitive feature for changes in the position and strength of the northern portion of the SWW. Through the tight coupling of the BMC position to the large‐scale wind field, the records allow a quantitative reconstruction of Holocene latitudinal displacements of the SWW across the South Atlantic. Our data reveal a gradual poleward movement of the SWW by about 1–1.5° from the early to the mid‐Holocene. Afterward, variability in the SWW is dominated by millennial scale displacements on the order of 1° in latitude with no recognizable longer‐term trend. These findings are confronted with results from a state‐of‐the‐art transient Holocene climate simulation using a comprehensive coupled atmosphere‐ocean general circulation model. Proxy‐inferred and modeled SWW shifts compare qualitatively, but the model underestimates both orbitally forced multimillennial and internal millennial SWW variability by almost an order of magnitude. The underestimated natural variability implies a substantial uncertainty in model projections of future SWW shifts.

Holocene climate variability in the Labrador Sea

Formation of Labrador Sea Water proper commenced about 7000 years ago during the Holocene interglacial. To test whether fresher surface water conditions may have inhibited Labrador Sea Water convection during the early Holocene we measured planktonic foraminiferal ( Globigerina bulloides ) oxygen isotopes (δ 18 O) and Mg/Ca ratios at Orphan Knoll (cores HU91-045-093 and MD95-2024, 3488 m) in the Labrador Sea to reconstruct shallow subsurface summer conditions (temperature and seawater δ 18 O). Lighter foraminiferal δ 18 O values are recorded during the early Holocene between 11000 and 7000 years ago. Part of these lighter foraminiferal δ 18 O values can be explained by increased calcification temperatures. Reconstructed seawater δ 18 O values were, however, still on average 0.5‰ lighter compared with those of recent times, confirming that fresher surface waters in the Labrador Sea were probably a limiting factor in Labrador Sea Water formation during the early Holocene.

Reconstruction of paleohydrological and paleoenvironmental changes using organic carbon and biomarker analyses of sediments from the northern East China Sea

Paleohydrological and paleoenvironmental changes were reconstructed using sedimentary records from the northern East China Sea, representing the past 15,000 years (from the last glacial period to the Holocene). The n-alkane distributions show many factors, from the changing origins (plant types) of organic matter input to marine sediments. Two climatic conditions were distinguished (warm/humid and cold/dry) by comparing the organic carbon isotope ratio (δ13Corg) with the reported planktonic foraminiferal oxygen isotope ratio (δ18O) of Globigerinoides ruber. A shift in the n-alkane-specific δ13C values reveals that the hydrological changes in the northern East China Sea were related to the carbon sources for aquatic macrophytes. The alkenone sea surface temperatures [SSTs (UK′37)] record of the 07YSPC12 shows a range of 20.3 °C–26.5 °C (average 22.9 °C). Lower temperature SSTs were recorded during the Younger Dryas, with an average of 21.7 °C. The variation in the alkenone SSTs is interpreted to indicate the environmental conditions of the growth of alkenone synthesized organisms.

Dynamic boundary-monsoon intensity hypothesis: evidence from the deglacial Amazon River discharge record

Glacioeustatic- and temperature-corrected planktonic foraminiferal oxygen isotope (∆δ 18O) records from ODP Site 942 on the Amazon Fan provide a means of monitoring past changes in the outflow of the Amazon River. This study focuses on the last deglaciation and reveals that during this period there were significant variations in the outflow, which implies large changes in moisture availability in the Amazon Basin. Aridity in the Amazon Basin seems to occur between 20.5 ka (calendar) to 17.0 ka and 13.6 ka to 11 ka. The second arid period correlates with the start of the Antarctic Cold Reversal and aridity continues throughout the Younger Dryas period. We find that the large-scale trends in Amazon River outflow are dissimilar to high-latitude variability in either hemisphere. Instead high-resolution variations correlate with the δ 18O difference between Greenland and Antarctica ice core temperature records. This suggests a link between Hemispheric temperature gradients and moisture availability over the Amazon. Based on our results and previously published work we present a new testable ‘dynamic boundary-monsoon intensity hypothesis’, which suggests that tropical moisture is not a simple belt that moves north or south. Rather, the northern and southern boundaries of the South American Summer Monsoon (SASM) are independently dynamic and driven by temperature gradients within their individual hemispheres. The intensity of rainfall within the SASM, however, is driven by precessionally modulated insolation and the resultant convection strength. Combining these two influences produces the dynamic heterogenic changes in the moisture availability observed over tropical South America since the Last Glacial Maximum.

Variations of the Nile suspended discharges during the last 1.75 Myr

Planktonic foraminiferal oxygen isotopes, carbonate and organic carbon contents, as well as XRF core scanning Ti, V and Ba intensities have been analyzed on sediments of a Mediterranean marine core (Core MD90-964) to reconstruct changes in the Nile discharges during the last 1.75 Ma. Chronology of Core MD90-964 is established by correlating the planktonic foraminiferal δ 18O record to the Mediterranean Globigerinoides ruber δ 18O stack. A total number of 42 dark layers are observed under visual observation, among which 21 of them are identified as sapropels by organic carbon contents (> 1 wt.%). It is observed that oxidation of sapropels is very active during 1000 and 240 kyr, indicating that the local climate is much colder and dryer during this interval. Our results provide new evidences that oxidation of organic material play a major role in determining the organic carbon contents in sapropels. Carbonate contents and normalized titanium and vanadium contents in bulk sediments are suggested to reflect mostly changes in the Nile discharge. It is found that variation of the African monsoon intensities can strongly affect changes in the Nile suspended discharge via both runoff and drainage precipitation. Elevated Nile discharges are generally observed during African monsoon maxima (deposition of sapropelic layers). Our results suggest that oscillations in the Nile suspended discharges are more the result of river transport capability than that of erosion potential in source areas. It is also observed that cyclicity of the variation of the Nile suspended discharge is generally paced by a 78-kyr cycle, the physical meaning of which remains hitherto unclear. It is probably a bundling of 2–3 obliquity cycles, as a result of nonlinear responses of Atlantic SST to orbital forcing.

Planktonic foraminiferal oxygen isotope analysis by ion microprobe technique suggests warm tropical sea surface temperatures during the Early Paleogene

Cool tropical sea surface temperatures (SSTs) are reported for warm Paleogene greenhouse climates based on the δ18O of planktonic foraminiferal tests. These results are difficult to reconcile with models of greenhouse gas–forced climate. It has been suggested that this “cool tropics paradox” arises from postdepositional alteration of foraminiferal calcite, yielding erroneously high δ18O values. Recrystallization of foraminiferal tests is cryptic and difficult to quantify, and the compilation of robust δ18O records from moderately altered material remains challenging. Scanning electron microscopy of planktonic foraminiferal chamber‐wall cross sections reveals that the basal area of muricae, pustular outgrowths on the chamber walls of species belonging to the genus Morozovella, contain no mural pores and may be less susceptible to postdepositional alteration. We analyzed the δ18O in muricae bases of morozovellids from the central Pacific (Ocean Drilling Program Site 865) by ion microprobe using 10 μm pits with an analytical reproducibility of ±0.34‰ (2 standard deviations). In situ measurements of δ18O in these domains yield consistently lower values than those published for conventional multispecimen analyses. Assuming that the original δ18O is largely preserved in the basal areas of muricae, this new δ18O record indicates Early Paleogene (∼49–56 Ma) tropical SSTs in the central Pacific were 4°–8°C higher than inferred from the previously published δ18O record and that SSTs reached at least ∼33°C during the Paleocene‐Eocene thermal maximum. This study demonstrates the utility of ion microprobe analysis for generating more reliable paleoclimate records from moderately altered foraminiferal tests preserved in deep‐sea sediments.

Paleoenvironmental records from the northern South China Sea since the Last Glacial Maximum

Core ZHS-176 contains the paleoenvironmental records from the northern South China Sea (NSCS) since the Last Glacial Maximum (LGM). A coupled approach based on clay mineral assemblages, planktonic foraminiferal oxygen and carbon isotopes, and calcium carbonate content is used to trace the sources of the fine-grained sediment and to investigate the paleoenviornmental evolution in this area. Clay mineral assemblages are dominated by illite (average about 39%) and chlorite (about 27%), which comes mainly from Taiwan and the East China Sea. Kaolinite, which accounts for about 13%, comes mainly from the Zhujiang (Pearl) River, and Luzon Island is the main source for smectite (about 21%). The planktonic foraminiferal oxygen isotopic oscillations during the last glacial period are coeval with climate variations recorded in the Greenland ice core and Western Pacific sediment. These variations include the LGM, Heinrich event 1, Bϕlling-Allerϕd (B/A), and Younger Dryas. For the Holocene, three periods of strong precipitation (S1–S3) and three periods of weak precipitation (W1–W3) are identified. The oxygen isotopic record exhibits correlation with climate records from distant regions, including the high-latitude Northern Hemisphere, providing evidence for global tele-connection among regional climate. A brief, negative planktonic foraminiferal carbon isotopic excursion during B/A reflects increased methane released from marine gas hydrate due to the rapid warming of the water. By comparing calcium carbonate content curves of the core ZHS-176 with these of other five boreholes lying above the lysocline, a remarkable low calcium carbonate event is found during the early Holocene in NSCS.

Sea Ice Formation Rates Recorded in Planktonic Foraminiferal Oxygen and Carbon Isotopes in the Western Arctic Ocean during the Late Quaternary

An integrated study was carried out on stable isotopes of planktonic foraminifera Neogloboquadrina pachyderma(sin.)(Nps),planktonic foraminiferal abundance and ice-rafted detritus(IRD) in the western Arctic Ocean to investigate sea ice formation during the late Quaternary.Our results show that light excursions in Nps δ18O and δ13C happened seven times,likely indicating the production and sinking of isotopically light brines caused by enhanced rate of the sea ice formation.Because concomitant decrease occurred in planktonic foraminiferal abundance and IRD,these light Nps δ18O and δ13C values unlikely reflect inputs of freshwater and warm Atlantic waters.They more likely suggest reduction of Atlantic waters into the Arctic Ocean and substantial decrease of Chukchi Sea shelf waters.On the contrary,heavy δ18O values would indicate reduction of freshwater and Pacific waters into the Arctic Ocean and heavy δ13C values be caused by well ventilated surface and halocline waters transported from the shelf into the Arctic Ocean.

Post-depositional remanent magnetization lock-in and the location of the Matuyama–Brunhes geomagnetic reversal boundary in marine and Chinese loess sequences

Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama–Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤ 20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences.

Signatures of rapid climatic changes in organic matter records in the western Mediterranean Sea during the last glacial period

AbstractRecords of calcium carbonate, organic matter (organic carbon content, palynology) and planktonic foraminiferal oxygen isotopes from ODP Site 976, Alboran Sea, are used to reconstruct the evolution of continental climates and oceanic productivity in the westernmost Mediterranean over the past 50,000 years. The records mimic the Greenland ice core records in that they display the Heinrich events and the rapid Dansgaard/Oeschger-type stadial-interstadial oscillations. Warm interstadials correlate with an expansion of deciduous forests on the adjacent continents and enhanced river runoff and organic matter flux from terrestrial sources. In contrast, cold stadials are characterized by an expansion of semi-desert vegetation, reduced river runoff and limited terrestrial organic matter flux. The organic carbon record displays a longer-term cyclicity that correlates with the North Atlantic ’Bond’ cooling cycles. The records enable the documentation of rapid changes of continental climates in the western Mediterranean borderlands that caused changes in river runoff and in oceanic productivity that were driven by rapid fluctuations of the Atlantic inflow into the Mediterranean Sea. The combined terrestrial-marine patterns demonstrate the close linking of western Mediterranean climate with the climatic evolution in the North Atlantic region.

Benthic foraminifera and bottom water evolution in the middle-southern Okinawa Trough during the last 18 ka

A piston sediment core E017 from the middle-southern Okinawa Trough was investigated. A preliminary study of the deep-water evolution since 18 cal. ka BP was performed based on the quantitative census data of benthic foraminiferal fauna, together with planktonic foraminiferal oxygen and carbon isotope, AMS(14)C dating, and the previous results achieved in the southern Okinawa Trough. The result shows that the benthic fauna was dominated by Bulimina aculeata (d'Orbigny), Uvigerina peregrina (Cushman), Hispid Uvigerina and Uvigerina dirupta (Todd) during the glaciation-deglaciation before 9.2 cal. ka BP, while Epistominella exigua (Brady), Pullenia bulloides (d'Orbigny), Cibicidoides hyalina (Hofker), Sphaeroidina bulloides (d'Orbigny) and Globocassidulina subglobosa (Brady) predominated the fauna in the post-glacial period after 9.2 cal. ka BP. The benthic foraminifera accumulation rate (BFAR), paleoproductivity estimates and benthic foraminiferal assemblage conformably indicate that surface water paleoproductivity and organic matter flux during the glaciation-deglaciation were higher than those of the post-glacial period in the middle-southern Okinawa Trough, and gradually enhanced from the southern to the central Okinawa Trough during the glaciation-deglaciation, which could be caused by the discrepancy of the terrigenous nutrients supply. High abundances of E exigua, an indicator of pulsed organic matter input, after 9.2 cal. ka. BP may indicate that the intensity of seasonally riverine pulsed flux during the post-glacial period was stronger than that of the glaciation-deglaciation period, and the seasonal influx in the central trough might be stronger than in the south. The temporal distributions of the typical species indicating bottom water oxygen content and ventilation condition show that the ventilation of the bottom water during the post-glacial period is more active than the glaciation-deglaciation, which reflects that the evolution of the intermediate and deep waters of the northwestern Pacific during the last glaciation has no evident influence on the deep-water of the middle-southern Okinawa Trough. Additionally, the variations in agglutinated benthic foraminiferal abundance and other carbonate dissolution proxies indicate that carbonate dissolution gradually increased since the last 18 ka in the Okinawa Trough and rapidly enhanced at 9.2 cal. ka BP. The modern shallow carbonate lysocline could form at 3 cal. ka BP.

Erosional history of the eastern Tibetan Plateau since 190 kyr ago: clay mineralogical and geochemical investigations from the southwestern South China Sea

A high-resolution study of clay mineralogy and major element geochemistry has been conducted on high sedimentation rate cores (MD01-2393 and MD97-2150) collected off the Mekong River mouth in the southwestern South China Sea in order to reconstruct the erosional and weathering history of the Mekong Basin. The chronology is based upon planktonic foraminiferal oxygen isotope records combined with carbonate stratigraphy. Clay minerals and major element results suggest that the Mekong River is the major sedimentary source over the past 190 kyr for both cores. Illite and chlorite were derived mainly from the eastern Tibetan Plateau. Kaolinite was derived mainly from active erosion of inherited clays from reworked sediments in the middle part of the Mekong Basin. Smectites originated mainly through chemical weathering of parent aluminosilicate and ferromagnesian silicate under warm and humid conditions in the middle to lower parts of the Mekong Basin. Smectites/(illite+chlorite) and smectites/kaolinite ratios coupled to K 2O/SiO 2 and Al 2O 3/SiO 2 ratios allow us to reconstruct a history of chemical weathering versus physical erosion. Good correlations between those ratios suggest monsoon-controlled weathering and erosion over the eastern Tibetan Plateau and the Mekong Basin. Variations with time scales of 0.3–0.6 kyr for the last 70 kyr and 1–2 kyr for 70–190 kyr ago in the chemical weathering/erosional history are strongly related to the East Asian monsoon evolution, which is itself related to the summer solar radiation calculated for the Northern Hemisphere.

Changes in Northeast Atlantic temperature and carbon flux during the Cenomanian/Turonian paleoceanographic event: the Goban Spur stable isotope record

Stable isotopes of bulk sediment and well preserved tests of planktonic and benthic foraminifera from mid-latitude NE Atlantic DSDP Site 551 (Goban Spur) provide the first estimates of carbon isotope gradients within the water column at a lower bathyal site during the Cenomanian/Turonian boundary interval (CTBI). The CTBI carbon isotope excursion is prominent (up to 2‰ shift in δ 13C) in the bulk (coccolith) signal, but less pronounced (approximately 0.5‰ shift in δ 13C) in planktonic and benthic foraminifera. This difference indicates a very steep 13C gradient in the upper water column and a very efficient biological pump during the CTBI carbon isotope excursion. We suggest significantly increased seasonal primary production in the uppermost water column with an enhanced shallow water chlorophyll maximum as a cause for this steep carbon isotope gradient. Deep-water and surface-water temperature changes during the CTBI are estimated using benthic and planktonic foraminiferal oxygen isotopes. Warm deep-water masses (13–16°C) and a low temperature gradient within the water column prevailed in the late Cenomanian. Additional warming (approximately 2°C for both surface and deep water) occurred in the latest Cenomanian prior to CTBI black shale deposition. This pattern of CTBI black shale deposition during a temperature maximum is also evident at two low latitude locations (ODP Site 1050, Blake Nose and Tarfaya, southern Morocco).