ALBERT

Description: We present late Quaternary records of aragonite preservation determined for sediment cores recovered on the Brazilian Continental Slope (1790-2585 m water depth) where North Atlantic Deep Water (NADW) dominates at present. We have used various indirect dissolution proxies (carbonate content, aragonite/calcite contents, and sand percentages) as well as gastropodal abundances and fragmentation of Limacina inflata to determine the state of aragonite preservation. In addition, microscopic investigations of the dissolution susceptibility of three Limacina species yielded the Limacina Dissolution Index which correlates well with most of the other proxies. Excellent preservation of aragonite was found in the Holocene section, whereas aragonite dissolution gradually increases downcore. This general pattern is attributed to an overall increase in aragonite corrosiveness of pore waters. Overprinted on this early diagenetic trend are high-frequency fluctuations of aragonite preservation, which may be related to climatically induced variations of intermediate water masses.

Description: Planktonic foraminiferal census counts are used to construct high-resolution sea surface temperature (SST) and subsurface (thermocline) temperature records at a core site in the Tobago Basin, Lesser Antilles. The record is used to document climatic variability at this tropical site in comparison to middle- and high-latitude sites and to test current concepts of cross-equatorial heat transports as a major player in interhemispheric climate variability. Temperatures are estimated using transfer function and modern analog techniques. Glacial - maximum cooling of 2.5°-3°C is indicated; maximum cooling by 4°C is inferred for isotope stage 3. The SST record displays millennial-scale variability with temperature jumps of up to 3°C and closely tracks the structure of ice-core Dansgaard/Oeschger cycles. SST variations in part of the record run opposite to the SST evolution at high northern latitude sites, pointing to thermohaline circulation and marine heat transport as an important factor driving SST in the tropical and high-latitude Atlantic, both on orbital and suborbital timescales.

Description: The scavenging of 230Th and 231Pa was investigated in the central Weddell Sea by combining results from a sediment trap and three sediment cores. Scavenging of both radionuclides is closely coupled with the annual cycle of particle fluxes. For 230Th the mean radionuclide flux measured in the trap is only 40% of its expected flux from production in the water column. This value is in excellent agreement with the long-term record in the sediment cores (33-43%). Similar results were obtained for 231Pa, although burial fluxes are generally higher. The data suggest that during the last 130 ka the Weddell Sea has been a net source for both radionuclides, with more than half of the 230Th and about half of the 231Pa being exported. As a consequence, 230Th normalized rain rates (assuming a constant flux equal to the production rate) overestimate the true rain rate in the Weddell Sea by 150%. The laterally transported 230Th and 231Pa activity exits the Weddell Sea to the north, where it is incorporated into the eastward flowing Antarctic Circumpolar Current (ACC). There it is scavenged in addition to local production. During its residence time in the S-Atlantic sector of the ACC about 3/4 of the dissolved 230Th imported from the Weddell Sea is transferred onto particles. Whether this particulate 230Th is entirely deposited in the S-Atlantic or is distributed over a larger area extending into the Indian and Pacific sectors of the ACC remains an open question. In the ACC, Th-normalization therefore leads to an underestimation of fluxes, but the effect is probably less than 50%. Interglacial-glacial shifts in the position of the productive belt are believed to cause temporal and regional variations in the depositon rate for 230Th.

Description: In order to evaluate bioturbation in abyssal Arabian-Sea sediments of the Indus fan profiles of 210Pb (half-life: 22.3 yr) and 234Th (half-life: 24.1 d) were measured in cores collected during September and October 1995 and April 1997, respectively. The density and composition of epibenthic megafauna and lebensspuren were determined in vertical seafloor photographs during April 1997. Mean eddy-diffusive mixing coefficients according to the distribution of excess 210Pb ( 210Pb-DB) were 0.072±0.028, 0.068±0.055, 0.373±0.119, 0.037±0.009 and 0.079±0.119 cm**2 yr**-1 in the northern, western, central, eastern and southern abyssal Arabian sea, respectively. Mean eddy-diffusive mixing coefficients according to the distribution of excess 234Th (234Th-DB) were 0.53, 1.64 and 0.47 cm**2 yr**-1 in the northern, western and central abyssal Arabian Sea, respectively. Mobile epibenthic megafauna at the western, northern, central and southern study sites were dominated by ophiuroids, holothurians, ophiuroids and natant decapods (the respective densities were 100, 82, 29 and 6 individuals 1000 m**-2). The northern study site was characterized by a high abundance of spoke traces and fecal casts. The central site showed spoke traces and many tracks. The southern site displayed the highest abundance of spoke traces, whereas at the western site hardly any lebensspuren were observed. There is evidence for at least two functional endmember communities in the Arabian Sea. In the northwestern Arabian Sea (WAST) vertical particle displacement seems to be dominated by macrofauna and primarily eddy-diffusive. In the southern Arabian Sea (SAST) non-local and 'incidental' mixing due to spoke-trace producers might become more important and superimpose reduced eddy-diffusive mixing. With respect to biological data CAST is an intermediate location. Given the biological data, average 210Pb-DB is higher and decimeter-scale variability of 210Pb-DB smaller at CAST than expected. These findings indicate that in a mixture of both endmember communities the organisms may interact in way that increases values of biodiffusivity, as reflected by 210Pb-DB, and reduces decimeter-scale 210Pb-DB heterogeneity in comparison to the simple sum of the isolated effects of the endmembers. For time scales 〈100 years there was no evidence for a relationship between food supply (POC flux) and bioturbation intensity, as reflected by 210Pb-DB and 234Th-DB. Bioturbation intensity should be controlled primarily by the composition of the benthic fauna, its specific adaptation to the environmental setting, and the abundance of each species of the benthic community. Food supply can have only an indirect influence on bioturbation intensity. In certain parts of the ocean the a priori overall positive relationship between POC flux and biodiffusivity might include restricted intervals displaying no or even negative relations.

Description: A detailed age model for core 17957-2 of the southern South China Sea was developed based on delta18O, coarse fraction, magnetostratigraphy, and biostratigraphy for the last 1500 kyr. The delta18O record has clear ~100-kyr cycles after the Mid-Pleistocene Revolution (MPR) at the entrance of marine isotopic stage (MIS) 22. Planktonic foraminifera responded to the MPR immediately, showing the increased sea surface temperature (SST) and dissolution after the MPR. Benthic foraminifera did not respond to it until the Brunhes/Matuyama boundary. Since the MPR, the depth of thermocline gradually became shallower until MISs 6-5. This major change within MISs 6-5 was also reflected in the decreased SSTs and increased productivity and Deep Water Mass. Thus two major Pleistocene paleoceanographic changes were found: One was around the MPR; the other occurred within MISs 6-5, which speculatively might be ascribed to the reorganization of surface and deep circulation, possibly induced by tectonic forces.

Description: In contrast to the wide range of studies carried out in temperate and high-latitude oceanic regions, only a few studies have focused on recent and Holocene organic-walled dinoflagellate cyst assemblages from the tropics. This information is, however, essential for fully understanding the ability of species to adapt to different oceanographic regimes, and ultimately their potential application to local and regional palaeoenvironmental and palaeoceanographic reconstructions. Surface sediment samples of the western equatorial Atlantic Ocean north of Brazil, an area greatly influenced by Amazon River discharge waters, were therefore analysed in detail for their organic-walled dinoflagellate cyst content. A diverse association of 43 taxa was identified, and large differences in cyst distribution were observed. The cyst thanatocoenosis in bottom sediments reflects the seasonal advection of Amazon River discharge water through the Guyana Current and the North Equatorial Countercurrent well into the North Atlantic. To establish potential links between cyst distribution and the environmental conditions of the upper water column, distribution patterns were compared with mean temperature, salinity, density and stratification gradients within the upper water column (0–100 m) over different times of the year, using correspondence analysis and canonical correspondence analysis. The analyses show that differences in these parameters only play a subordinate role in determining species distribution. Instead, nutrient availability, or related factors, dominates the distribution pattern. The only possible indicators of slightly reduced salinities are Trinovantedinium applanatum and Lingulodinium machaerophorum. Four assemblage groups of cyst taxa with similar environmental affinities related to specific water masses/currents can be distinguished and have potential for palaeoenvironmental reconstruction.

Description: [1] We used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical South Atlantic Ocean (10°N–37°S) to estimate annual mean sea surface temperatures (SSTs) and seasonality for the Last Glacial Maximum with a modified version of the Imbrie-Kipp transfer function method (IKTF) that takes into account the abundance of rare but temperature sensitive species. In contrast to CLIMAP Project Members [1981], the reconstructed SSTs indicate cooler glacial SSTs in the entire tropical/subtropical South Atlantic with strongest cooling in the upwelling region off Namibia (7–10°C) and smallest cooling (1–2°C) in the western subtropical gyre. In the western Atlantic, our data support recent temperature estimates from other proxies. In the upwelling regions in the eastern Atlantic, our data conflict with SST reconstructions from alkenones, which may be due to an environmental preference of the alkenone-producing algae or to an underestimation of foraminiferal SSTs due to anomalous high abundances of N. pachyderma (sinistral).

Description: A palaeoceanographic reconstruction of the Late Quaternary tropical Atlantic Ocean has been made on the basis of dinoflagellate cyst associations of two sediment cores: the first core was recovered from below the highly productive waters of the equatorial divergence and the second from the oligotrophic western tropical Atlantic Ocean. Palaeoenvironmental indicators for productivity, sea surface temperature (SST) and salinity (SSS) based on selected organic-walled dinoflagellate cyst species have been established. On the basis of these palaeoenvironmental indicators, a strengthened intensity of the equatorial divergence in the eastern region during glacials and cold periods of interglacials has been reconstructed. The highest SST probably occurred around substage 5.5 and might refer to weakest upwelling intensity. In comparison, SST and SSS appear to have been generally higher in the western tropical Atlantic Ocean, with probably enhanced values during glacial intervals. Pronounced differences in accumulation rates and relative abundances of cysts formed by congruentidiacean dinoflagellates and relative abundances of oligotrophic cyst species between the eastern and the western region can be related to differences in palaeoproductivity, suggesting much higher values in the eastern area. The coherence between variation in frequency of the indicators for productivity and the boreal summer insolation and monsoon intensity in the eastern tropical Atlantic Ocean suggests an oceanographic reflection of regional intertropical, rather than boreal, dynamics.

Description: Only very few studies focus on recent calcareous dinoflagellate cyst diversity, geographic distribution and ecology, so that information on the distribution patterns and environmental affinities of individual cyst species is extremely limited. This information is, however, essential if we want to use calcareous dinoflagellate cysts for palaeoenvironmental reconstruction. Surface sediment samples from the generally oligotrophic western equatorial Atlantic Ocean, offshore northeast Brazil, were therefore quantitatively analysed for their calcareous dinoflagellate cyst content, including the calcareous vegetative coccoid Thoracosphaera heimii. Seven calcareous dinoflagellate cyst species/morphotypes and T. heimii were encountered in high concentrations throughout the area. Substantial differences in the distribution patterns were observed. The highest concentrations of cysts are found in sediments of the more oligotrophic, oceanic regions, beyond the influence of Amazon River discharge waters. Dinoflagellates producing calcareous cysts thus appear to be capable of surviving low nutrient concentrations and produce large numbers of cysts in relatively stable and predictable environments affected by minimal seasonality. To test for the environmental affinities of individual species, distribution patterns in surface sediments were compared with temperature, salinity, density and stratification gradients within the upper water column (0-100 m) over different times of the year, using principal components analysis and redundancy analysis. T. heimii and four of the seven encountered cyst species (Sphaerodinella? albatrosiana, two morphotypes of Sphaerodinella? tuberosa and Scrippsiella regalis) relate to these parameters significantly and the variations in the cyst associations appear to be associated with the different surface water currents characterising the area. The results imply that calcareous dinoflagellate cyst distributions can potentially be used to distinguish between different open oceanic environments and they could, therefore, be useful in tracing water mass movements throughout the late Quaternary.

Description: During the large-scale deep-sea programme BIGSET in situ measurements of sediment community oxygen consumption (SCOC) were carried out during three cruises between 1995-1998 at five abyssal sites (3190-4450 m water depth) in the deep Arabian Sea in order to elucidate the regional and temporal variation of benthic carbon remineralisation. SCOC ranged from 0.9-6.3 mmol O2 m**-2 d**-1, with highest values in the western and northern Arabian Sea and lowest values in the southern Arabian Sea. For the central Arabian Sea intermediate oxygen uptake rates were detected. This regional pattern mirrors the overall regional pattern of primary productivity in surface waters and vertical particulate organic carbon (POC) flux at 1000 mab. Primary productivity in Arabian Sea surface waters and particulate flux into the deep-sea are controlled by the monsoon system and the flux maxima during the SW and NE monsoon are among the highest particle fluxes recorded in the deep open ocean. Highest flux rates were recorded in the western and northern Arabian Sea and decreased towards the central and southern Arabian Sea. SCOC at our western, northern and eastern Arabian Sea stations WAST, NAST and EAST were considerably higher than so far detected in other abyssal areas of the global oceans, and vertical POC flux can account for only 20-50% of benthic carbon remineralisation (BCR). Possible explanations for the high rates of BCR at these stations that are situated close to the continental margins are discussed: the accelerated deposition of very labile organic matter due to eolian dust particles, enhanced rain efficiencies, and lateral advection. A significant temporal variability in SCOC only could be detected at the eastern and western Arabian Sea stations WAST and EAST.

Description: We analysed the alkenone unsaturation ratio (UK'37) in 87 surface sediment samples from the western South Atlantic (5°N–50°S) in order to evaluate its applicability as a paleotemperature tool for this part of the ocean. The measured UK'37 ratios were converted into temperature using the global core-top calibration of Müller et al. (1998, doi:10.1016/S0016-7037(98)00097-0) and compared with annual mean atlas sea-surface temperatures (SSTs) of overlying surface waters. The results reveal a close correspondence (〈1.5°C) between atlas and alkenone temperatures for the Western Tropical Atlantic and the Brazil Current region north of 32°S, but deviating low alkenone temperatures by -2° to -6°C are found in the regions of the Brazil–Malvinas Confluence (35–39°S) and the Malvinas Current (41–48°S). From the oceanographic evidence these low UK'37 values cannot be explained by preferential alkenone production below the mixed layer or during the cold season. Higher nutrient availability and algal growth rates are also unlikely causes. Instead, our results imply that lateral displacement of suspended particles and sediments, caused by strong surface and bottom currents, benthic storms, and downslope processes is responsible for the deviating UK'37 temperatures. In this way, particles and sediments carrying a cold water UK'37 signal of coastal or southern origin are transported northward and offshore into areas with warmer surface waters. In the northern Argentine Basin the depth between displaced and unaffected sediments appears to coincide with the boundary between the northward flowing Lower Circumpolar Deep Water (LCDW) and the southward flowing North Atlantic Deep Water (NADW) at about 4000 m.

Description: The increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable Sunda Shelf in Southeast Asia. The time from 21 to 14 thousand calendar years before the present has been poorly covered in other records. The record generally confirms sea-level reconstructions from coral reefs. The rise of sea level during meltwater pulse 1A was as much as 16 meters within 300 years (14.6 to 14.3 thousand years ago).

Description: Dinoflagellate cysts were analysed from IMAGES core MD952042 (37°48?N; 10°01?W) retrieved from the Tagus Abyssal Plain. Previous results of stable isotope and magnetic susceptibility measurements as well as of planktonic foraminiferal temperature reconstruction from this core, suggest the occurrence of “Heinrich-like events” (i.e. large ice-sheet decay) during Marine Isotopic Stage 5 (MIS 5). Dinoflagellate assemblages of this time period have revealed six dinocyst events that are characterised by peaks in Bitectatodinium tepikiense percentages. These events occur synchronously with “Heinrich-like events” previously identified. They are coeval with major retreats of the forest on land, indicating, therefore, drastic changes in the regional climate. However, results from the Ice-Rafted Detritus (IRD) analysis of the 〉150 ?m lithic fraction shows that MIS 5 of MD952042 has only recorded one significant input of iceberg discharge, located at the MIS 6/MIS 5 transition. It seems therefore that it is the only event that could be called a “true Heinrich event”.

Description: As part of the German JGOFS Indian Ocean Programme, a cruise occurred in the Arabian Sea in May 1997 at the onset of the southwest monsoon. Data on gelbstoff, chlorophylla and tryptophan-like fluorescence as well as gelbstoff absorption were obtained. They indicate, that these optical parameters support the interpretation of hydrographic and biogeochemical conditions. Tryptophan-like fluorescence seems to be a useful indicator of changes in the constitution of the biomass. A comparison of gelbstoff absorption and fluorescence data from the upper 100 m reveals only a weak covariance. Special emphasis is given to the fit procedure used for retrieving the exponential slopeS of the spectral absorption coefficient.S values with a mean of 0.016 nm-1 are found in the 350 to 480 nm wavelength range. A mean slopeS set to the frequently reported value of 0.014 nm-1 in the exponential description of gelbstoff absorption might lead to a systematic over/underestimation, and thus to systematic errors, if single-wavelength absorption values are extrapolated to other spectral regions on the basis of this parameter.

Description: A core recovered on the Iberian margin off southern Portugal can be correlated with Greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over Greenland. Our age model identifies the base of every interstadial between 64,000 and 24,000 years ago and uses the Greenland Ice Core Project (GRIP) timescale. The oxygen isotope signal in benthic foraminifera (on this GRIP-based timescale) is quite different from the planktonic record and resembles the temperature record over Antarctica when this is synchronized with Greenland using the record of methane in the atmospheric air in the polar ice cores. We interpret the benthic record as indicating significant fluctuations in ice volume during millennial events, and we suggest that Antarctic temperature changed as a function of ice volume.

Description: We here present records of total organic carbon (TOC) and C37 alkenones, used as indicators for past primary productivity, from the western (WAS) and eastern Arabian Sea (EAS). New data from an open ocean site of the WAS upwelling area are compared with similar records from Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the EAS. These records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing. On the basis of their phase relationship to boreal summer insolation they can be divided into three groups: in the WAS differences between monsoonal proxies (1) and productivity (2) document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds, whereas in the EAS phasing indicates maximum productivity (3) at times of stronger NE monsoon winds associated with precession-related maxima in ice volume.

Description: As part of the large-scale, interdisciplinary deep-sea study "BIGSET", the relationship between the monsoon-induced regional and temporal variability of POC deposition and the small-sized benthic community was investigated at several sites 2316-4420 m deep in the Arabian Sea during four cruises between 1995 and 1998. Vertical and horizontal distribution patterns of chloroplastic pigments (a measure of phytodetritus deposition), readily soluble protein and activity, and biomass parameters of the small-sized benthic community (Electron Transport System Activity (ETSA); bacterial ectoenzymatic activity (FDA turnover) and DNA concentrations) were measured concurrently with the vertical fluxes of POC and chloroplastic pigments. Sediment chlorophyll a (chl. a) profiles were used to calculate chl. a flux rates and to estimate POC flux across the sediment water interface using two different transport reaction models. These estimates were compared with corresponding flux rates determined in sediment traps. Regional variability of primary productivity and POC deposition at the deep-sea floor creates a trophic gradient in the Arabian Basin from the NW to the SE, which is primarily related to the activity of monsoon winds and processes associated with the topography of the Arabian Basin and the vicinity of land masses. Inventories of sediment chloroplastic pigments closely corresponded to this trophic gradient. For ETSA, FDA and DNA, however, no clear coupling was found, although stations WAST (western Arabian Sea) and NAST (northern Arabian Sea) were characterised by high concentrations and activities. These parameters exhibited high spatial and temporal variability, making it impossible to recognise clear mechanisms controlling temporal and spatial community patterns of the small-sized benthic biota. Nevertheless, the entire Arabian Basin was recognised as being affected by monsoonal activity. Comparison of two different transport reaction models indicates that labile chl. a buried in deeper sediment layers may escape rapid degradation in Arabian deep-sea sediments.

Description: The Mid-Pleistocene transition (MPT) of the global climate system, initiated by a shift towards much larger northern hemisphere ice shields at around 920 ka and ending with predominance of 100 kyr ice age cyclicity since about 640 ka, is one of the fundamental enigmas in Quaternary climate evolution. Climate proxy records not exclusively linked to global ice volume are necessary to advance understanding of the MPT. Here we present a high-resolution Pleistocene magnetic susceptibility time series of 12 sediment cores from the subtropical South Atlantic essentially reflecting dissolution driven variations in carbonate accumulation controlled by changes in deep water circulation. In addition to characteristics known from delta18O records, the data sets reveal three remarkable features intimately related to the MPT: (1) an all-Pleistocene minimum of carbonate accumulation in the South Atlantic at 920 ka, (2) a MPT interim state of reduced carbonate deposition, indicating that the MPT period may have been a discrete state of the Pleistocene deep water circulation and climate system and (3) a terminal MPT event at around 540-530 ka documented in several peculiarities such as thick laminated layers of the giant diatom Ethmodiscus rex.

Description: Geochemical and mineralogical compositions of modern and Late Quaternary marine sediments from the Scotia Sea trace sources and transport paths of terrigenous sediment. We discuss downcore variations of compositional data of two sediment cores from the northern and southern Scotia Sea that correlate with fluctuations in magnetic susceptibility. Sediments were derived from very different sources at both localities, as revealed by contrasting clay-mineral assemblages. However, a common feature is the input of more basic and undifferentiated crustal material with the potential of high magnetic susceptibility during glacial periods, indicated by variable quartz/feldspar ratios and major, trace and rare earth elements. Terrigenous sediments mainly originate from nearby terrestrial sources or are introduced through interbasinal sediment transfer from adjacent seas. The observed temporal compositional variations have to be attributed to changes in the relative detrital contributions from the diverse source areas. Ice-mass extensions in southern Patagonia, on the Antarctic Peninsula and adjacent islands likely control the supply of glaciogenic detritus to the open ocean during times of glacial expansion, diluting the sediment input of interbasinal origin. Current transport is mainly responsible for sediment dispersal to the pelagic Scotia Sea and may amplify the glaciological source signals during glacial climate periods, because of a stronger wind forcing of the Antarctic Circumpolar Current.

Description: Benthic and selected planktic foraminifera and stable isotope records were determined in a piston core from the Gulf of Aden, NW Arabian Sea that spans the last 530 ka. The benthic foraminifera were grouped into four principal assemblages using Q-mode Principal Component Analyses. Comparison of each of these assemblages with the fauna of the nearby regions enabled us to identify their specific environmental requirements as a function of variability in food supply and strength of the oxygen minimum zone and by that to use them as indicators of surface water productivity. The benthic foraminiferal productivity indicators coupled with the record of Globigerina bulloides, a planktic foraminifer known to be sensitive to productivity changes in the region, all indicate higher productivity during glacial intervals and productivity similar to present or even reduced during interglacial stages. This trend is opposite to the productivity pattern related to the SW summer monsoon of the Arabian Sea and indicates the role of the NE winter monsoon on the productivity of the Gulf of Aden. A period of exceptionally enhanced productivity is recognized in the Gulf of Aden region between ~60 and 13 kyr indicating the intensification of the NE winter monsoon to its maximal activity. Contemporaneous indication of increased productivity in other parts of the Arabian Sea, unexplained so far by the SW summer monsoon variability, might be related to the intensification of the NE winter monsoon. Another prominent event of high productivity, second in its extent to the last glacial productivity event is recognized between 430 and 460 kyr. These two events seem to correspond to periods of similar orbital positioning of rather low precession (and eccentricity) amplitude for a relatively long period. Glacial boundary conditions seem to control to a large extent the NE winter monsoon variability as also indicated by the dominance of the 100 ka cycle in the investigated time series. Secondary in their importance are the 23 and 41 ka cycles which seem also to contribute to the NE monsoonal variability. Following the identification of productivity events related to the NE winter monsoon in the Gulf of Aden, it is possible now to extend this observation to other parts of the Arabian Sea and consider the contribution of this monsoonal system to the productivity fluctuations preserved in the sedimentary records.

Description: Two cores from the southern South China Sea contain discrete ash layers that mainly consist of rhyolithic glass shards. On the basis of the SPECMAP time scale, the ash layers were dated to ca. 74 ka, the age of the youngest Toba eruption in northern Sumatra. This link is supported by the chemical composition of the glass, which is distinct from volcanic glass supplied from the Philippines and the northern South China Sea, but is almost identical with the chemistry of the Toba ash. The youngest Toba ash layers in the South China Sea expand the previously known ash-fall zone over more than 1800 km to the east. The dispersal of ashes from Sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that (1) the Toba eruption probably happened during the Southeast Asian summer monsoon season, and (2) the volume of erupted magma was larger than previously interpreted.

Description: We present time series of export productivity proxy data including 230Thex-normalized deposition rates (rain rates) of 10Be, dissolution-corrected biogenic Ba, and biogenic opal as well as authigenic U concentrations which are complemented by rain rates of total (detrital) Fe and sea ice indicating diatom abundances from five sediment cores across the Atlantic sector of the Southern Ocean covering the past 150,000 years. The results suggest that 10Be rain rates and authigenic U concentration cannot serve as quantitative paleoproductivity proxies because they have also been influenced by detrital particle fluxes in the case of 10Be and bulk sedimentation rates (sediment focussing) and deep water oxygenation in the case of U. The combined results of the remaining productivity proxies of this study (rain rates of biogenic opal and biogenic Ba in those sections without authigenic U) and other previously published proxy data from the Southern Ocean (231Pa/230Th and nitrogen isotopes) suggest that a combination of sea ice cover, shallow remineralization depth, and stratification of the glacial water column south of the present position of the Antarctic Polar Front and possibly Fe fertilization north of it have been the main controlling factors of export paleoproductivity in the Southern Ocean over the last 150,000 years. An overall glacial increase of export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Ocean are unlikely to have contributed to the major glacial atmospheric CO2 drawdown observed in ice cores.

Description: Glaciomarine deposits at the Antarctic continental margin represent an important paleoglaciological and paleoceanographical record of environmental changes in both Antarctica and the adjacent Southern Ocean. In order to reconstruct the climatic-controlled sedimentation history in the Amundsen and Bellingshausen Seas during Quaternary glacial cycles, sedimentological, geochemical and clay mineralogical investigations were carried out on 34 surface sediment samples and On 7 sediment cores. The study of the surface sediment samples allows one to decipher the recent sediment supply and transportation processes. The input of glacial debris is highest in the eastern Bellingshausen Sea because of the subpolar climatic conditions prevailing over the adjacent Antarctic Peninsula. Fine sediment pathways of transport in the Amundsen and Bellingshausen Seas were recon- structed by analysing the clay mineralogical composition of the surface sediments and connecting it with specific source rocks in the West Antarctic hinterland. Grain size data show a complex dispersal Pattern of ice-rafted debris (IRD). According to these data, redeposition by currents plays a major role near the shelf break both in marginal seas and on the continental rise in the eastern Bellingshausen Sea. ff

Description: Despite the increasing interest in the South Atlantic Ocean as a key area of the heat exchange between the southern and the northern hemisphere, information about its palaeoceanographic conditions during transitions from glacial to interglacial stages, the so-called Terminations, are not well understood. Herein we attempt to increase this information by studying the calcareous dinoflagellate cysts and the shells of Thoracosphaera heimii (calcareous cysts) of five Late Quaternary South Atlantic Ocean cores. Extremely high accumulation rates of calcareous cysts at the Terminations might be due to a combined effect of increased cyst production and better preservation as result of calm, oligotrophic conditions in the upper water layers. Low relative abundance of Sphaerodinella albatrosiana compared with Sphaerodinella tuberosa in the Cape Basin may be the result of the relatively colder environmental conditions in this region compared with the equatorial Atlantic Ocean with high relative abundance of S. albatrosiana. Furthermore, the predominance of S. tuberosa during glacials and interglacials at the observed site of the western Atlantic Ocean reflects decreased salinity in the upper water layer.

Description: Several cores recovered from the northern belt of the Southern Ocean were analysed to study the Pleistocene calcareous nannofossil records. Calcareous nannofossil events previously described in medium and low latitudes were identified and calibrated with the oxygen isotope and geomagnetic time scales. Although sedimentation rates, hiatuses and degree of calcareous nannofossil preservation sometimes prevent the identification and/or accurate calibration of some of these events, a useful stratigraphic framework was obtained. The possibility of using these calibrated events from high to low latitudes facilitates correlations and should facilitate isotope event identification in a region with low temperature, where calcareous plankton stratigraphies are in general restricted. In general, Pleistocene southern high latitude calcareous nannofossil events show synchronism with those observed in warm and temperate surficial waters. Small discrepancies in the assigned ages are sometimes related to low sampling resolution due to low sedimentation rates. The first occurrence (FO) of Emiliania huxleyi and the last occurrence (LO) of Pseudoemiliania lacunosa are observed in Marine Isotope Stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed close to the MIS 4/5 boundary. MIS 6 is characterised by an increase in G. muellerae and MIS 7 features a dramatic decrease in the proportion of Gephyrocapsa caribbeanica. This latter species began to increase its proportions from the MIS 13/14 boundary to MIS13, showing diachronism between the different sites. The LO of Reticulofenestra asanoi is observed at MIS 22, confirming this event as a global synchronous reference datum. By contrast, the FO of R. asanoi occurs at MIS 35 and is diachronous with the existing data from other oceanic regions. A re-entry of medium sized Gephyrocapsa (3-5 mm maximum diameters) can be identified in some cores close to MIS 25; although the low abundance of this taxon prevents an accurate calibration, it may be concluded that this event is diachronous as compared with the existing low-latitude data. The LO of large morphotypes of Gephyrocapsa is well correlated with MIS 37, showing synchronism with other oceanic regions, whereas the FO of this species is not well calibrated due to the absence of age-control points.

Description: Mapping the abundance of 13C in leaf-wax components in surface sediments recovered from the seafloor off northwest Africa (0–35°N) reveals a clear pattern of delta13C distribution, indicating systematic changes in the proportions of terrestrial C3 and C4 plant input. At 20°N latitude, we find that isotopically enriched products characteristic of C4 plants account for more than 50% of the terrigenous inputs. This signal extends westward beneath the path of the dust-laden Sahara Air Layer (SAL). High C4 contributions, apparently carried by January trade winds, also extend far into the Gulf of Guinea. Similar distributions are obtained if summed pollen counts for the Chenopodiaceae-Amaranthaceae and the Poaceae are used as an independent C4 proxy. We conclude that the specificity of the latitudinal distribution of vegetation in North West Africa and the pathways of the wind systems (trade winds and SAL) are responsible for the observed isotopic patterns observed in the surface sediments. Molecular-isotopic maps on the marine-sedimentary time horizons (e.g., during the last glacial maximum) are thus a robust tool for assessing the phytogeographic changes on the tropical and sub-tropical continents, which have important implications for the changes in climatic and atmospheric conditions.

Description: Seventeen surface sediment samples from the North Atlantic Ocean off NE-Greenland between 76° and 81°N, and nine samples from the South Atlantic Ocean close to Bouvet Island between 48° and 55°S were taken with the aid of a Multiple Corer and investigated for their live (Rose Bengal stained) benthic foraminiferal content within the upper 15 cm of sediment. Preferentially endobenthic Melonis barleeanum, Melonis zaandami, and Bulimina aculeata as well as preferentially epibenthic Lobatula lobatula were counted from 1-cm-thick sediment slices each and analyzed for stable carbon and oxygen isotopic compositions of their calcareous tests. Live and dead specimens were counted and measured separately. The carbon isotopic composition of the foraminifera was compared to that of the dissolved inorganic carbon (DIC) of simultaneously sampled bottom water. During a period of one month, one station off NE-Greenland was replicately sampled once every week and samples were processed as above. Live specimens of Lobatula lobatula are confined to the uppermost two centimeters of sediment. Live specimens of Melonis spp. are found down to 8 cm within the sediment but with a distinct sub-surface maximum between 2 and 5 cm. The down-core distribution of live Bulimina aculeata shows a distinct surface maximum in the top centimeter and constant but low numbers down to 11-cm subbottom depth. The average stable carbon isotopic composition (d13C versus per mil PDB) of live Lobatula lobatula off NE-Greenland is by 0.4±0.1 per mil higher than the d13CDIC of the ambient bottom water at the time of sampling. There is evidence that this species calcify before the ice-free season, when bottom water d13CDIC is supposed to be higher. This would reconfirm the one-to-one relationship between d13C of ambient water DIC and cibicids, widely used by paleoceanographers. Live Melonis barleeanum show a negative offset from bottom water DIC of -1.7±0.6 per mil in the uppermost sediment and of -2.2±0.5 per mil in 3-4-cm subbottom depth. All d13C values of live Melonis spp. decrease within the upper four centimeters, regardless of the time of sampling and site investigated. The offset of live Bulimina aculeata from bottom water d13CDIC values of 8 stations rather constantly amounts to -0.6±0.1 per mil, no matter what subbottom depth the specimens are from. At one station however, where is strong indication of elevated organic carbon flux, the negative offset averaged over all sub-bottom depths increases to -1.5±0.2 per mil. Buliminids actively move within the sediment and by this either record an average isotope signal of the pore water or the signal of one specific calcification depth. The recorded signal, however, depends on the organic carbon flux and reflects general but site-specific pore water d13CDIC values. If compared with epibenthic d13C values from the same site, not influenced by pore water and related phytodetritus layer effects, Buliminad13C values bear some potential as a paleoproductivity proxy. Specimens of Melonis spp. seem to prefer a more static way of life and calcify at different but individually fix depths within the sediment. Although live specimens thus record a stratified pore water d13C signal, there is no means yet to correct for bioturbational and early diagenetic effects in fossil faunas.

Description: Fluxes of organic carbon normalised to a depth of 1000 m from 18 sites in the Atlantic and the Southern Ocean are presented, comprising nine biogeochemical provinces as defined by Longhurst et al. (1995. Journal of Plankton Research 17, 1245-1271). For comparison with primary production, we used a recent compilation of primary production values derived from CZCS data (Antoine et al., 1996. Global Biogeochemical Cycles 10, 57-69). In most cases, the seasonal patterns stood reasonably well in accordance with the carbon fluxes. Particularly, organic carbon flux records from two coastal sites off northwest and southwest Africa displayed a more distinct correlation to the primary production in sectors (1 x 1°) which are situated closer to the coastal environments. This was primarily caused by large upwelling filaments streaming far offshore, resulting in a cross-shelf carbon transport. With respect to primary production, organic carbon export to a water depth of 1000 m, and the fraction of primary production exported to a depth of 1000 m (export fraction=EF1000), we were able to distinguish between: (1) the coastal environments with highest values (EF1000=1.75-2.0%), (2) the eastern equatorial upwelling area with moderately high values (EF1000=0.8-1.1%), (3) and the subtropical oligotrophic gyres that yielded lowest values (EF1000=0.6%). Carbon export in the Southern Ocean was low to moderate, and the EF1000 value seems to be quite low in general. Annual organic carbon fluxes were proportional to primary production, and the export fraction EF1000 increased with primary production up to 350 gCm**-2 yr**-1. Latitudinal variations in primary production were reflected in the carbon flux pattern. A high temporal variability of primary production rates and a pronounced seasonality of carbon export were observed in the polar environments, in particular in coastal domains, although primary production (according to Antoine et al., 1996. Global Biogeochemical Cycles 10, 57-69), carbon fluxes, and the export fraction remained at low.