Rock magnetic, element, and color data of three sediment cores along the Senegalese continental margin

We present a suite of new high-resolution records (0-135 ka) representing pulses of aeolian, fluvial, and biogenic sedimentation along the Senegalese continental margin. A multiproxy approach based on rock magnetic, element, and color data was applied on three cores enclosing the present-day northern limit of the ITCZ. A strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13°N. These millennial-scale dust fluxes are synchronous with North Atlantic Heinrich stadials. Fluvial clay input driven by the West African monsoon predominates at 12°N and varies at Dansgaard-Oeschger time scales while marine productivity is strongly enhanced during the African humid periods and marine isotope stage 5. From latitudinal signal variations, we deduce that the last glacial ITCZ summer position was located between core positions at 12°26' and 13°40'N. Furthermore, this work also shows that submillennial periods of aridity over northwest Africa occurred more frequently and farther south than previously thought.

Age and clay mineralogy on sediment core GeoTü SL112 from the Mediterranean Sea

The present-day clay mineral distribution in the southeastern Levantine Sea and its borderlands reveals a complex pattern of different sources and distribution paths. Smectite dominates the suspended load of the Nile River and of rivers in the Near East. Illite sources are dust-bearing winds from the Sahara and southwestern Europe. Kaolinite is prevalent in rivers of the Sinai, in Egyptian wadis, and in Saharan dust. A high-resolution sediment core from the southeastern Levantine Sea spanning the last 27 ka shows that all these sources contributed during the late Quaternary and that the Nile River played a very important role in the supply of clay. Nile influence was reduced during the glacial period but was higher during the African Humid Period. In contrast to the sharp beginning and end of the African Humid Period recorded in West African records (15 and 5.5 ka), our data show a more transitional pattern and slightly lower Nile River discharge rates not starting until 4 ka. The similarity of the smectite concentrations with fluctuations in sea-surface temperatures of the tropical western Indian Ocean indicates a close relationship between the Indian Ocean climate system and the discharge of the Nile River.

Pollen distribution in marine sediment samples along the south-western African coast

The distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents. The wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories. Isobaric, 4-day backward trajectories are calculated using the modelled wind-field of ECHAM3, and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south-eastern Atlantic. Trajectories and clusters based on the modelled wind-field of the Last Glacial Maximum hardly differ from those of the present-day. Trajectory clusters show three regional, and two seasonal patterns, determining the pathways of aeolian pollen transport into the south-eastern Atlantic ocean. Mainly, transport out of the continent occurs during austral fall and winter, when easterly and south-easterly winds prevail. South of 25°S, winds blow mostly from the west and southwest, and aeolian terrestrial input is very low. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent. The northern Angola Basin receives pollen and spores from the Congolian and Zambezian forests mainly through river discharge. The Zambezian vegetation zone is the main source area for wind-blown pollen in sediments of the Angola Basin, while the semi-desert and desert areas are the main sources for pollen in sediments of the Walvis Basin and on the Walvis Ridge. A transect of six marine pollen records along the south-western African coast indicates considerable changes in the vegetation of southern Africa between glacial and interglacial periods. Important changes in the vegetation are the decline of forests in equatorial Africa and the north of southern Africa and a northward shift of winter rain vegetation along the western escarpment.

Terrestrial organic matter in marine sediments of the Central Equatorial Atlantic

Terrestrial organic matter (OM) in pelagic sediments is discussed with regard to depositional processes and land-sea interactions in the modern and past glacial/interglacial Equatorial Atlantic. Special emphasis is placed on a critical evaluation of different analytical approaches (C/N, Rock-Eval Pyrolysis, stable carbon isotopes, palynology, organic petrology, and selected biomarkers) which are currently used for the qualitative and quantitative assessment of terrigenous organic carbon. If binary mixing equations are used to calculate terrestrial and marine proportions of organic carbon, we consider the definition of endmember values to be most critical since these values may be biased by a great number of independent controls. A combination of geochemical methods including optical studies (organic petrology and palynology) is therefore suggested to evaluate each individual proxy. Organic geochemical analyses performed on sediments from the modern and Late Quaternary Equatorial Atlantic evidence fluctuations in eolian supply of terrigenous OM related to changes in intensity of the trade winds. Quantification of this organic fraction leads to differing proportions depending on the approach applied, i.e. the organic carbon isotopic composition or maceral analyses. Modern distribution of terrigenous OM reveals a decrease in supply towards the basin contributing less than a fifth of the total OM in pelagic areas. Organic geochemical data indicate that sedimentation in the modern northeastern Brasil Basin is affected by lateral advection of reworked OM probably from southern source areas. Glacial/interglacial deposits from the pelagic Equatorial Atlantic (ODP Site 663), covering isotopic stages 12 and 11, reveal that deposition of terrigenous OM was higher under past glacial conditions, in correspondence to generally enhanced dust fluxes. Proportions of terrigenous OM, however, never exceed 50% of the total OM according to maceral analyses. Other estimates, recently proposed by Verardo and Ruddiman (1996), are considered to be too high probably for analytical reasons. Palynological records in the Equatorial Atlantic parallel dust records. Increased portions of grass pollen suggest the admixture of C4-plant material under modern and past glacial conditions. It is therefore assumed, as one possible interpetation, that C4-plant debris has an effect on sedimentary d13Corg and might explain differences between isotopic and microscopic quantitative estimates. Using the difference between these two records, we calculate that maximum supply of C4-material remains below 20% of the total OM for the deep modern and past glacial/interglacial Equatorial Atlantic.

Alkenone variations, total organic carbon and stable oxygen isotope ratios of the western Arabian Sea

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.

Underway measurements of halocarbons (air) during POSEIDON cruise POS399 in June 2010

During the DRIVE (Diurnal and Regional Variability of Halogen Emissions) ship campaign we investigated the variability of the halogenated very short-lived substances (VSLS) bromoform (CHBr3), dibromomethane (CH2Br2) and methyl iodide (CH3I) in the marine atmospheric boundary layer in the eastern tropical and subtropical North Atlantic Ocean during May/June 2010. The highest VSLS mixing ratios were found near the Mauritanian coast and close to Lisbon (Portugal). With backward trajectories we identified predominantly air masses from the open North Atlantic with some coastal influence in the Mauritanian upwelling area, due to the prevailing NW winds. The maximum VSLS mixing ratios above the Mauritanian upwelling were 8.92 ppt for bromoform, 3.14 ppt for dibromomethane and 3.29 ppt for methyl iodide, with an observed maximum range of the daily mean up to 50% for bromoform, 26% for dibromomethane and 56% for methyl iodide. The influence of various meteorological parameters - such as wind, surface air pressure, surface air and surface water temperature, humidity and marine atmospheric boundary layer (MABL) height - on VSLS concentrations and fluxes was investigated. The strongest relationship was found between the MABL height and bromoform, dibromomethane and methyl iodide abundances. Lowest MABL heights above the Mauritanian upwelling area coincide with highest VSLS mixing ratios and vice versa above the open ocean. Significant high anti-correlations confirm this relationship for the whole cruise. We conclude that especially above oceanic upwelling systems, in addition to sea-air fluxes, MABL height variations can influence atmospheric VSLS mixing ratios, occasionally leading to elevated atmospheric abundances. This may add to the postulated missing VSLS sources in the Mauritanian upwelling region (Quack et al., 2007).

Geochemsitry of minerals and glass shards at DSDP Leg 67 Holes

During Leg 67, the Middle-America Trench transect off Guatemala was drilled across the convergent margin of southern Mexico and Central America south of the Tehuantepec Ridge. The data of Leg 66, north of the Tehuantepec Ridge, and that of Leg 67 provided the opportunity to establish a continuous chronology of airborne volcanic ashes intercalated within the sediments (Aubouin et al., 1979; von Huene et al., 1980). Sites of both expeditions are favorably located for obtaining a good record of the explosive volcanicity of these areas, given the proximity of the volcanic sources and the position of the sites under the prevailing winds.

Benthic foraminifera, stable isotope record and sedimentology of Holocene sediments in the Skagerrak

A high-resolution multi-proxy study of core MD99-2286 reveals a highly variable hydrographic environment in the Skagerrak from 9300 cal. yr BP to the present. The study includes foraminiferal faunas, stable isotopes and sedimentary parameters, as well as temperature and salinity reconstructions of a ca. 29 m long radiocarbon-dated core record. The multivariate technique fuzzy c-means was applied to the foraminiferal counts, and it was extremely valuable in defining subtle heterogeneities in the foraminiferal fauna data corresponding to hydrographic changes. The major mid-Holocene (Littorina) transgression, led to flooding of large former land areas in the North Sea, the opening of the English Channel and Danish straits and initiation of the modern circulation system. This is reflected by fluctuating C/N values and an explosive bloom of Hyalinea balthica. A slight indication of ameliorated conditions between 8000-5750 cal. yr BP is related to the Holocene Thermal Maximum. A subsequent increase in fresh water/Baltic water influence between 5750-4350 cal. yr BP is reflected by dominance of Bulimina marginata and depleted d18O-values. The Neoglacial cooling (after 4350 cal. yr BP) is seen in the Skagerrak as enhanced turbidity, increasing TOC-values and short-term changes in an overall Cassidulina laevigata dominated fauna suggesting a prevailing influence of Atlantic waters. This is in agreement with increased strength of westerly winds, as recorded for this period. The last 2000 years were also dominated by Atlantic Water conditions with generally abundant nutrient supply. However, during warm periods, particularly the Medieval Warm Period and the modern warming, the area was subject to a restriction in the supply of nutrients and/or the nutrient supply had a more refractory character.

Dinoflagellate cysts and pollen distribution of marine surface sediments off West Africa

An organic-walled dinoflagellate cyst analysis was carried out on 53 surface sediment samples from West Africa (17-6°N) to obtain insight in the relationship between their spatial distribution and hydrological conditions in the upper water column as well as marine productivity in the study area. Multivariate analysis of the dinoflagellate cyst relative abundances and environmental parameters of the water column shows that sea-surface temperature, salinity, marine productivity and bottom water oxygen are the factors that relate significantly to the distribution patterns of individual species in the region. The composition of cyst assemblages and dinoflagellate cyst concentrations allows the identification of four hydrographic regimes; 1) the northern regime between 17 and 14°N characterized by high productivity associated with seasonal coastal upwelling, 2) the southern regime between 12 and 6°N associated with high-nutrient waters influenced by river discharge 3) the intermediate regime between 14 and 12°N influenced mainly by seasonal coastal upwelling additionally associated with fluvial input of terrestrial nutrients and 4) the offshore regime characterized by low chlorophyll-a concentrations in upper waters and high bottom water oxygen concentrations. Our data show that cysts of Polykrikos kofoidii, Selenopemphix quanta, Dubridinium spp., Echinidinium species, cysts of Protoperidinium monospinum and Spiniferites pachydermus are the best proxies to reconstruct the boundary between the NE trade winds and the monsoon winds in the subtropical eastern Atlantic Ocean. The association of Bitectatodinium spongium, Lejeunecysta oliva, Quinquecuspis concreta, Selenopemphix nephroides, Trinovantedinium applanatum can be used to reconstruct past river outflow variations within this region.

Coastal erosion dynamics and storm characteristics of Bykovsky Peninsula

This study investigates the rate of erosion during the 1951-2006 period on the Bykovsky Peninsula, located north-east of the harbour town of Tiksi, north Siberia. Its coastline, which is characterized by the presence of ice-rich sediment (Ice Complex) and the vicinity of the Lena River Delta, retreated at a mean rate of 0.59 m/yr between 1951 and 2006. Total erosion ranged from 434 m of erosion to 92 m of accretion during these 56 years and exhibited large variability (sigma = 45.4). Ninety-seven percent of the rates observed were less than 2 m/yr and 81.6% were less than 1 m/yr. No significant trend in erosion could be recorded despite the study of five temporal subperiods within 1951-2006. Erosion modes and rates actually appear to be strongly dependant on the nature of the backshore material, erosion being stronger along low-lying coastal stretches affected by past or current thermokarst activity. The juxtaposition of wind records monitored at the town of Tiksi and erosion records yielded no significant relationship despite strong record amplitude for both data sets. We explain this poor relationship by the only rough incorporation of sea-ice cover in our storm extraction algorithm, the use of land-based wind records vs. offshore winds, the proximity of the peninsula to the Lena River Delta freshwater and sediment plume and the local topographical constraints on wave development.

Pollen analyses of sediment core GeoB9508-5

Millennial-scale dry events in the Northern Hemisphere monsoon regions during the last Glacial period are commonly attributed to southward shifts of the Intertropical Convergence Zone (ITCZ) associated with an intensification of the northeasterly (NE) trade wind system during intervals of reduced Atlantic meridional overturning circulation (AMOC). Through the use of high-resolution last deglaciation pollen records from the continental slope off Senegal, our data show that one of the longest and most extreme droughts in the western Sahel history, which occurred during the North Atlantic Heinrich Stadial 1 (HS1), displayed a succession of three major phases. These phases progressed from an interval of maximum pollen representation of Saharan elements between ~19 and 17.4 kyr BP indicating the onset of aridity and intensified NE trade winds, followed by a millennial interlude of reduced input of Saharan pollen and increased input of Sahelian pollen, to a final phase between ~16.2 and 15 kyr BP that was characterized by a second maximum of Saharan pollen abundances. This change in the pollen assemblage indicates a mid-HS1 interlude of NE trade wind relaxation, occurring between two distinct trade wind maxima, along with an intensified mid-tropospheric African Easterly Jet (AEJ) indicating a substantial change in West African atmospheric processes. The pollen data thus suggest that although the NE trades have weakened, the Sahel drought remained severe during this time interval. Therefore, a simple strengthening of trade winds and a southward shift of the West African monsoon trough alone cannot fully explain millennial-scale Sahel droughts during periods of AMOC weakening. Instead, we suggest that an intensification of the AEJ is needed to explain the persistence of the drought during HS1. Simulations with the Community Climate System Model indicate that an intensified AEJ during periods of reduced AMOC affected the North African climate by enhancing moisture divergence over the West African realm, thereby extending the Sahel drought for about 4000 years.