Heinrich event imprints in the Okinawa Trough: evidence from oxygen isotope and planktonic foraminifera

Planktonic foraminiferal faunas, oxygen isotope and modern analog technique sea surface temperature records were obtained in piston core DGKS9603 (28degrees08.869'N, 127degrees16.238'E, water depth 1100 in) collected from the middle Okinawa Trough. During the last glaciation, four cold events were identified and correlate Heinrich events (HE) H2-5 of the last 45 ka. During the last deglaciation, core DGKS9603 has begun to be influenced by the Kuroshio since about 16 cal ka BP. Three weakenings of this warm current occurred at about 2.8-5.3, 11.4 and 15.5 cal ka BP respectively. Among the three fluctuations, the oldest one is synchronous with HE1 and could be a response to the strong cooling observed in the North Atlantic Ocean. The fluctuation occurring at about 11.4 cal ka ago corresponds to the Younger Dryas within the age error bars. Our observations provide new evidence that the HEs documented from Greenland and the northern North Atlantic had a global climatic impact. Changes in the intensity of the East Asian monsoon could be the main mechanism responsible for the paleoccanographic variations observed in the Okinawa Trough. (C) 2001 Elsevier Science B.V. All rights reserved.

Evolution and variation of the Tsushima warm current during the late Quaternary: Evidence from planktonic foraminifera, oxygen and carbon isotopes

The evolution and variation history of the Tsushima warm current during the late Quaternary was reconstructed based on the quantitative census data of planktonic foraminiferal fauna, together with oxygen and carbon isotope records of mixed layer dweller a ruber and thermocline dweller N. dutertrei in piston core CSH1 and core DGKS9603 collected separately from the Tsushima warm current and the Kuroshio dominated area. The result showed that the Tsushima warm current vanished in the lowstand period during 40-24 cal ka BP, while the Kuroshio still flowed across the Okinawa Trough, arousing strong upwelling in the northern Trough. Meanwhile, the influence of freshwater greatly increased in the northern Okinawa Trough, as the broad East China Sea continental shelf emerged. The freshwater reached its maximum during the last glacial maximum (LGM), when the upwelling obviously weakened for the lowest sea-level and the depression of the Kuroshio. The modern Tsushima warm current began its development since 16 cal ka BP, and the impact of the Kuroshio increased in the middle and northern Okinawa Trough synchronously during the deglaciation and gradually evolved as the main water source of the Tsushima current. The modern Tsushima current finally formed at about 8.5 cal ka BP, since then the circulation structure has been relatively stable. The water of the modern Tsushima current primarily came from the Kuroshio axis. A short-term wiggle of the current occurred at about 3 cal ka BP, probably for the influences from the enhancement of the winter monsoon and the depression of the Kuroshio. The cold water masses greatly strengthened during the wiggle.

Depositional modes and lake-level variability at Lake Towuti, Indonesia, during the past ~29 kyr BP

Lake Towuti (2.5°S, 121.5°E) is a long-lived, tectonic lake located on the Island of Sulawesi, Indonesia, and in the center of the Indo-Pacific warm pool (IPWP). Lake Towuti is connected with upstream lakes Matano and Mahalona through the Mahalona River, which constitutes the largest inlet to the lake. The Mahalona River Delta is prograding into Lake Towuti’s deep northern basin thus exerting significant control on depositional processes in the basin. We combine high-resolution seismic reflection and sedimentological datasets from a 19.8-m-long sediment piston core from the distal edge of this delta to characterize fluctuations in deltaic sedimentation during the past ~29 kyr BP and their relation to climatic change. Our datasets reveal that, in the present, sedimentation is strongly influenced by deposition of laterally transported sediments sourced from the Mahalona River Delta. Variations in the amount of laterally transported sediments, as expressed by coarse fraction amounts in pelagic muds and turbidite recurrence rates and cumulative thicknesses, are primarily a function of lake-level induced delta slope instability and delta progradation into the basin. We infer lowest lake-levels between ~29 and 16, a gradual lake level rise between ~16 and 11, and high lake-levels between ~11 and 0 kyr BP. Periods of highest turbidite deposition, ~26 to 24 and ~18 to 16 kyr BP coincide with Heinrich events 2 and 1, respectively. Our lake-level reconstruction therefore supports previous observations based on geochemical hydroclimate proxies of a very dry last glacial and a wet Holocene in the region, and provides new evidence of millennial-scale variations in moisture balance in the IPWP.

Stable isotope record of planktonic foraminifera from sediment core MD96-2085

A high-resolution (~4-5cm/kyr) giant piston core record (MD962085) retrieved during an IMAGES II-NAUSICAA cruise from the continental slope of the southeast Atlantic Ocean reveals striking variations in planktonic foraminifer faunal abundances and sea-surface temperatures (SST) during the past 600 000 yr. The location and high-quality sedimentary record of the core provide a good opportunity to assess the variability of the Benguela Current system and associated important features of the ocean-climate system in the southeast Atlantic. The planktonic foraminifer faunal abundances of the core are dominated by three assemblages: (1) Neogloboquadrina pachyderma (right coiling) + Neogloboquadrina dutertrei, (2) Globigerina bulloides, and (3) Globorotalia inflata. The assemblage of N. pachyderma (right coiling) + N. dutertrei shows distinctive abundance changes which are nearly in-phase with glacial-interglacial variations. The high abundances of this assemblage are associated with major glacial conditions, possibly representing low SST/high nutrient level conditions in the southwestern Africa margin. In contrast, the G. bulloides and G. inflata assemblages show greater high-frequency abundance change patterns, which are not parallel to the glacial-interglacial changes. These patterns may indicate rapid oceanic frontal movements from the south, and a rapid change in the intensity of the Benguela upwelling system from the east. A single episode of maximum abundances of a polar water species N. pachyderma (left coiling) occurred in the beginning of stage 9 (~340-330 kyr). The event of the maximum occurrence of this species shown in this record may indicate instability in the Benguela coastal upwelling, or the Antarctic polar front zone position. A winter season SST estimate using transfer function techniques for this record shows primarily glacial-interglacial variations. The SST is maximal during the transitions from the major glacial to interglacial stages (Terminations I, II, IV, V), and is associated with the abundance maxima of a warm water species indicator Globigerinoides ruber. Cross-spectral analyses of the SST record and the SPECMAP stack reveal statistically significant concentrations of variance and coherencies in three major orbital frequency bands. The SST precedes changes in the global ice volume in all orbital frequency bands, indicating a dominant southern Hemispheric climate effect over the Benguela Current region in the southeast Atlantic.

Revised age models and foraminiferal records obtained for the Last Interglacial period in six marine sediment cores

The dataset contains the revised age models and foraminiferal records obtained for the Last Interglacial period in six marine sediment cores: - the Southern Ocean core MD02-2488 (age model, sea surface temperatures, benthic d18O and d13C for the period 136-108 ka), - the North Atlantic core MD95-2042 (age model, planktic d18O, benthic d18O and d13C for the period 135-110 ka), - the North Atlantic core ODP 980 (age model, planktic d18O, sea surface temperatures, seawater d18O, benthic d18O and d13C, ice-rafted detritus for the period 135-110 ka), - the North Atlantic core CH69-K09 (age model, planktic d18O, sea surface temperatures, seawater d18O, benthic d18O and d13C, ice-rafted detritus for the period 135-110 ka), - the Norwegian Sea core MD95-2010 (age model, percentage of Neogloboquadrina pachyderma sinistral, sea surface temperatures, benthic d18O, ice-rafted detritus for the period 134-110 ka), - the Labrador Sea core EW9302-JPC2 (age model, percentage of Neogloboquadrina pachyderma sinistral, sea surface temperatures, benthic d18O for the period 134-110 ka).

Element contents and isotope composition of sediments and basalts from DSDP Site 78-543

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb (206Pb/204Pb up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and 143Nd/144Nd increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas. Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of "normal" depleted MORB.

Inorganic geochemistry, epoch and water content in sediments at DSDP Site 91-596

Originally, we had planned to piston core at Site 595 in order to meet the sedimentologic and biostratigraphic objectives outlined in the introductory chapter. However, consultation with our colleagues, Thomas Jordan and John Orcutt on board Melville, indicated that coring near the ocean bottom seismometer (OBS) array around Hole 595B could alter the programmed signal to noise ratio above which teleseisms trigger recording in the OBSs. They requested that we core no closer than about 8 km from three OBSs nearest Hole 595B, and selected a target for us about that distance to the west. Since a new beacon was required at this distance, a new site number, 596, was designated. Briefly, we planned to obtain oriented hydraulic piston cores to the top of the cherts, then core through the cherts using the extended core barrel (XCB) to basement. With improved recovery, we hoped to reach the sediment/basalt contact, and thus obtain a reliable biostratigraphic determination of the basement age. We planned to obtain at least one core in basement, perhaps more, with time permitting. We planned no geophysical program for the hole.

Compositional and isotopic data on core MD84641

The organic carbon isotopic record of the sapropels(S1 and S3-S10) and intercalated marl oozes has been determined in a 12-m piston core from the eastern Mediterranean. The d13C_organic values are systematically lighter (mean=-21.0±0.82 per mil) in all sapropels and heavier (mean=-18.8±1.07 per mil) in the marl oozes. These differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the Mediterranean, there is no relationship between the C_organic/N ratios and the isotopic values, and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter. Temperature effects on isotope fractionation are also discounted because the change in d13C_organic values between glacial and interglacial horizons is in the opposite sense. Diagenesis, which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances, is unlikely to have caused the observed differences because this mechanism would cause an enrichmet in 12C, implying that all values were even heavier originally, and there is no secular trend in the d13C_organic record. The observed differences in d13C_organic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved CO2. First, freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the Mediterranean to become lighter because of the 13C deficiency in fresh waters. Hence photosynthesis would have produced isotopically lighter organic material. Second, changes in atmospheric pCO2 between glacial and interglacial periods, as shown by the Vostok ice core, caused marked changes in the concentration of free dissolved CO2 in the mixed layer; lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton, whereas levels were less limiting during the interglacials. Concentrations of dissolved CO2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated CO2 to the mixed layer by upwelling, and this could have further lightened the d13C_organic values in the sapropels themselves. Carbon isotope records may provide an alternative method for estimating atmospheric pCO2 levels over longer time periods than can be obtained from ice cores.

Abundance patterns and isotopic signals of morphotypes of Globigerinoides ruber

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

Late Pleistocene sediment record of the Maldives carbonate platform

A piston core from the Maldives carbonate platform was investigated for carbonate mineralogy, grain-size distributions, calcium carbonate content and organic carbon. The sedimentary record was linked to Late Pleistocene sea-level variations, using an age model based on oxygen isotopes obtained from planktonic foramanifera, nannofossil biostratigraphy and 14C age determinations. The correlation between the sedimentary record and Late Pleistocene sea-level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the Maldives carbonate platform. Platform flooding events were characterized by strongly increased deposition of aragonite and mud within the Inner Sea of the Maldives. Exposure events, in contrast, can be recognized by rapid decreases in the values of both proxy records. The results show that sediments on the Maldives carbonate platform contain a continuous record of Pleistocene sea-level variations. These sediments may, therefore, contribute to a better understanding of regional and even global sea-level changes, and yield new insights into the interplay between ocean currents and carbonate platform morphology.

Stable isotope analysis on planktic foraminifera in sediment cores from the eastern Mediterranean sapropel formation

During the late Pleistocene, sapropels (layers of organic-carbon rich sediment) formed throughout the entire Eastern Mediterranean Basin in close association with glacial/interglacial transitions. The current theory for the mechanism of sapropel formation involves a density stratification of the water column, due to the invasion of a large quantity of low-saline water, which resulted in oxygen depletion of the bottom waters. Most workers believe that this low-salinity water was glacial meltwater that entered the Mediterranean via the Black Sea and a series of interconnected glacial lakes, but the suggestion also has been made that the freshwater originated from the Nile River. In this study the oxygen isotope values of planktonic foraminifera,Globigerinoides ruber, have been examined in six gravity cores and one piston core from the southern Levantine Basin, and compared with the oxygen isotope records ofG. ruber from other areas of the Eastern Mediterranean. This study deals mainly with the latest sapropel which was deposited approximately 7000 to 9000 years ago. Results indicate that Nile discharge probably does reduce salinities somewhat in the immediate area surrounding the mouth of the Nile, but this water is rapidly mixed with the highly saline waters of the easternmost Mediterranean. Using a mixing equation and surface water salinity limitations, an approximate oxygen isotope balance of surface waters was calculated for the time of latest sapropel deposition. This calculation shows that neither Nile River discharge nor Black Sea input (nor both together) are large enough to account for the large-scale oxygen isotope depletion associated with latest sapropel deposition in the Eastern Mediterranean. This suggests that part of the isotopic change at Termination I is probably due to increased surface water salinities during the last glacial maximum. In addition, evidence from the timing of sapropel 1 deposition and the dissolved oxygen balance indicates that deposition of the latest sapropel is associated with increased surface water production of biogenic material, as much as three times higher than that of present day.

Accumulation rates and grain sizes of eolian dust in sediments of the Intertropical Convergence Zone in the northeast Pacific

A 328 cm-long piston core (KODOS 02-01-02) collected from the northeast equatorial Pacific at 16°12'N, 125°59'W was investigated for eolian mass fluxes and grain sizes to test these proxies as a tool for the paleo-position of the Intertropical Convergence Zone (ITCZ). The eolian mass fluxes of the lower interval below 250 cm (15.5-7.6 Ma) are very uniform at 5 +/- 1 mg/cm**2/kyr, while those of the upper interval above 250 cm (from 7.6 Ma) are over 2 times higher than the lower interval at 12 +/- 1 mg/cm**2/kyr. The median grain size of the eolian dusts in the lower interval increases from 8.4 Phi to 8.0 Phi downward, while that of the upper interval varies in a narrow range from 8.8 Phi to 8.6 Phi. The determined values compare well in magnitude to those of central Pacific sediments for the upper interval and equatorial and southeast Pacific sediments for the lower interval. This result suggests a possibility that the study site had been under the influence of southeast trade winds at its earlier depositional period due to the northerly position of the ITCZ, and subsequently of the northeast trade winds for a later period when the upper sediments were deposited. This interpretation is consistent with a mineralogical and geochemical study published elsewhere that assigned the provenance of the study core dust to Central/South America for the lower interval and to Asia for the upper interval. This study suggests that the distinct differences in eolian mass flux and grain size observed across the ITCZ can be used to trace the paleo-latitude of the ITCZ.

Age determination of sediment cores from the Skagerrak

Paleoceanographic and paleoenvironmental interpretations based on foraminifera, sedimentary data, radiocarbon dates, and stable isotope measurements were derived from two sections in the Skagerrak: a 115-m-thick Holocene marine section drilled onshore at Skagen near the northernmost tip of Jutland, Denmark, and a 9-m piston core from the Skagerrak, north of Skagen. The foraminiferal data show that arctic-subarctic environments in the deep Skagerrak-Kattegat area were succeeded by boreal conditions at 9.6 ka. This was a result of northward migration of the Atlantic polar front and inflow of warm Atlantic water into the area through the Norwegian Channel. A gradual warming of the water masses after 9.6 ka is indicated by the data. Rare foraminifera and high sedimentation rates are found between approximately 8.6 ka and 7.6 ka at both core locations. The modern foraminiferal assemblages of the area were fully established at 7.6 ka indicating that the modern circulation pattern in the Skagerrak-Kattegat after the opening of the English Channel and the Danish Straits was not established before this date. At 5.5 ka a sudden change to coarser sediments (higher-energy environments) and the appearance of the foraminifer Eoeponidella laesoeensis is recorded in the Skagen core. This indicates a rapid change in the hydrography reflecting altered meteorological and hydrographic conditions in the Skagerrak-Kattegat, including a strengthening of the Jutland Current and increased inflow of North Sea water into the Kattegat. The event is interpreted as a response to cooling at the end of the Holocene climatic optimum in late Atlantic time and possibly reflects a rapid cooling event of North Atlantic surface water masses.

Geochemistry of eolian dust from sediment core LL44-GPC-3

The large-diameter piston core LL44-GPC3 from the central North Pacific Ocean records continuous sedimentation of eolian dust since the Late Cretaceous. Two intervals resolved by Nd and Pb isotopic data relate to dust coming from America (prior to ~40 Ma) and dust coming from Asia (since ~40 Ma). The Intertropical Convergence Zone (ITCZ) separates these depositional regimes today and may have been at a paleolatitude of ~23°N prior to 40 Ma. Such a northerly location of the ITCZ is consistent with sluggish atmospheric circulation and warm climate for the Northern Hemisphere of the early to middle Eocene. Since ~40 Ma, correlations between Nd (~7.55 > epsilon-Nd(t) > ~10.81) and Pb (18.625 < 206/4Pb < 18.879; 15.624 < 207/4Pb < 15.666; 38.611 < 208/4Pb < 38.960; 0.8294 < 207/6Pb < 0.8389; 2.0539 < 208/6Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo-Tethys. Comparisons between the changes with time in the isotopically well-defined dust flux and Nd and Pb isotopic compositions of Pacific deep water allow one to draw two major conclusions: (1) dust-bound Nd became a resolvable contribution to Pacific seawater only after the one order of magnitude increase in dust flux starting at ~3.5 Ma. Therefore eolian Nd was unimportant for Pacific seawater Nd prior to 3.5 Ma. (2) The lack of a response of Pacific deep water Pb to this huge flux increase suggests that dust-bound Pb has never been important. Instead, mobile Pb associated with island arc volcanic exhalatives probably consists of a significant contribution to Pacific deep water Pb and possibly to seawater elsewhere far away from landmasses.

Description and geochemical composition of tephra layers from ODP Hole 186-1150A and Site 186-1151

The sediments collected at Sites 1150 and 1151 during Leg 186 included many tephra layers and volcaniclastic detritus. In order to identify these tephras, the major oxide compositions of individual glass shards were determined by electron probe microanalyzer. The uppermost four tephras in sediments from Hole 1150A are correlated with the Towada-Hachinohe tephra (To-H; Tohoku district), Shikotsu Daiichi (1st) tephra (Spfa-1; Hokkaido district), Narugo-Yanagisawa tephra (Nr-Y; Tohoku district), and Aso-4 tephra (Kyushu district), respectively. The uppermost tephra in Hole 1151C is correlated with To-H tephra. To-H, Spfa-1, and Aso-4 tephras are also present in piston core KH94-3, LM-8, collected between Sites 1150 and 1151. Eruptive ages of To-H and Spfa-1 estimated from the oxygen isotopic Stages of core KH94-3, LM-8 are between 14.9-15.3 and 39.5-40.1 ka.