J H Fred Jansen

A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic

Abstract The terrigenous fraction of sediments recovered from Walvis Ridge, SE Atlantic Ocean, reveals a history of southwestern African climate of the last 300 kyr. End-member modelling of a data set of grain-size distributions (n=428) results in three end members. The two coarsest end members are interpreted as eolian dust, the third end member as hemipelagic mud. The ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the SE trade winds. Trade winds were intensified during glacials compared to interglacials. Changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern African aridity. Late Quaternary southwestern African climate was relatively arid during the interglacial stages and relatively humid during the glacial stages, owing to meridional shifts in the atmospheric circulation system. During glacials the polar front shifted equatorward, resulting in a northward displacement of the zone of westerlies, causing increased rainfall in southwestern Africa. The equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient, leading to enhanced atmospheric circulation and increased trade-wind intensity.

Carbon isotope analyses of n-alkanes in dust from the lower atmosphere over the central eastern Atlantic

Atmospheric dust samples collected along a transect off the West African coast have been investigated for their lipid content and compound-specific stable carbon isotope compositions. The saturated hydrocarbon fractions of the organic solvent extracts consist mainly of long-chain n-alkanes derived from epicuticular wax coatings of terrestrial plants. Backward trajectories for each sampling day and location were calculated using a global atmospheric circulation model. The main atmospheric transport took place in the low-level trade-wind layer, except in the southern region, where long-range transport in the mid-troposphere occurred. Changes in the chain length distributions of the n-alkane homologous series are probably related to aridity, rather than temperature or vegetation type. The carbon preference of the leaf-wax n-alkanes shows significant variation, attributed to a variable contribution of fossil fuel- or marine-derived lipids. The effect of this nonwax contribution on the 13 C values of the two dominant n-alkanes in the aerosols, n-C29 and n-C31 alkane, is, however, insignificant. Their 13 C values were translated into a percentage of C4 vs. C3 plant type contribution, using a two-component mixing equation with isotopic end-member values from the literature. The data indicate that only regions with a predominant C4 type vegetation, i.e. the Sahara, the Sahel, and Gabon, supply C4 plant-derived lipids to dust organic matter. The stable carbon isotopic compositions of leaf-wax lipids in aerosols mainly reflect the modern vegetation type along their transport pathway. Wind abrasion of wax particles from leaf surfaces, enhanced by a sandblasting effect, is most probably the dominant process of terrigenous lipid contribution to aerosols. Copyright

African vegetation controlled by tropical sea surface temperatures in the mid-Pleistocene period

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants—adapted to warm, dry conditions and low atmospheric CO2 concentrations—have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature and CO2 concentration are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content—driven by tropical sea surface temperature changes and the strength of the African monsoon—controlled aridity on the African continent, and hence large-scale vegetation changes.

Aluminium-rich opal: an intermediate in the preservation of biogenic silica in the Zaire (Congo) deep-sea fan

Diatom frustules, skeletons of radiolarians and silicoflagellates, and also phytoliths in the sediments of the Zaire deep-sea fan have a crumbly appearance, very spongy ultrastructures, and have very high atomic Al/Si ratios between 0.13 in the inner parts and 0.16 in the outer parts of the fan. Also small amounts of Ca and K are always found. The high Al content is related to a shift of the amorphous Silica bulge in XRD spectra from 0.41 to 0.36–0.37 nm, which shows that Al has substituted Si throughout the entire test of the great majority of the siliceous microfossils. This substitution is related to a low apparent solubility of about 350 mmol m−3 of H4SiO4, and to a low Silica release-rate constant of 1–2 × 10−12 cm s−12. The high content of Al in biogenic Silica is caused by a combination of selective dissolution of Silica and precipitation of Al dissolved from minerals in the sediment. The Zaire deep-sea fan combines several environmental factors favourable for the formation of Al-rich opal, viz. a large production of diatoms in offshore waters, a high concentration of dissolved Al in plume waters, and the presence of kaolinite and gibbsite (both minerals which easily release Al) in the sediment supplied by the Zaire River.

Taraxerol and Rhizophora pollen as proxies for tracking past mangrove ecosystems

Angola Basin and Cape Basin (southeast Atlantic) surface sediments and sediment cores show that maxima in the abundance of taraxerol (relative to other land-derived lipids) covary with maxima in the relative abundance of pollen from the mangrove tree genus Rhizophora and that in the surface sediments offshore maxima in the relative abundance of taraxerol occur at latitudes with abundant coastal mangrove forests. Together with the observation that Rhizophora mangle and Rhizophora racemosa leaves are extraordinarily rich in taraxerol, this strongly indicates that taraxerol can be used as a lipid biomarker for mangrove input to the SE Atlantic. The proxy-environment relations for taraxerol and Rhizophora pollen down-core show that increased taraxerol and Rhizophora pollen abundances occur during transgressions and periods with a humid climate. These environmental changes modify the coastal erosion and sedimentation patterns, enhancing the extent of the mangrove ecosystem and/or the transport of mangrove organic matter offshore. Analyses of mid-Pleistocene sediments show that interruption of the pattern of taraxerol maxima during precession minima occurs almost only during periods of low obliquity. This demonstrates the complex environmental response of the interaction between precession-related humidity cycles and obliquity-related sea-level changes on mangrove input.

LIVING PLANKTONIC FORAMINIFERA IN THE EASTERN SOUTH ATLANTIC DURING SPRING: INDICATORS OF WATER MASSES, UPWELLING AND THE CONGO (ZAIRE) RIVER PLUME

Abstract Planktonic foraminifera were collected from pumped surface-water samples and net tows over the upper 150 m of the water column in the eastern South Atlantic Ocean during October and November 1989. Of the 24 recorded species, 12 occur abundantly and form 6 foraminiferal assemblages. The assemblages correlate with the physico-chemical properties of the 6 vapor surface-water masses. The correlation of species with low salinity is due to properties co-variant with salinity. At several locations, high concentrations of planktonic foraminifera were observed caused by frontal mixing or reproduction. The G. sacculifer-G. ruber white assemblage marks the South Equatorial Under Current. minor contribution of G. menardii and N. dutertrei to this assemblage reflects the subsurface Equatorial Under Current. The N. dutertrei-G. siphonifera assemblage reflects the South Equatorial Counter Current and Equatorial Under Current. The N. pachyderma-G. bulloides assemblage reflects the coastal upwelling over the shelf and the filaments of mixed oceanic and upwelled water in the coastal of the Benguela Current (BC). The Angola-Benguela Front, at the interface of the warm currents and the BC, is reflected by the G. inflata-G. bulloides assemblage. An unusual coincidence of G. sacculifer-G. bulloides in one assemblage results from the interaction of the subtropical gyre and the oceanic branch of the BC. The G. rubber pink-N. dutertrei assemblage is attributed to the warm low-saline plume in front of the Congo (Zaire) River.

Accumulation and dissolution of opal in Quaternary sediments of the Zaire deep-sea fan (northeastern Angola Basin)

Abstract Opal concentrations in a representative core were measured with X-ray powder diffraction. In the last 225 ka there was an average opal accumulation of 640 mg cm −2 ka −1 , and a long-term loss to other silica phases or to the water column of 140 mg cm −2 ka −1 which is equal to about 50% of the actual diffusive silica flux out of the sediment surface layer. Eventually, 500 mg cm −2 ka −1 was preserved in the sediment as opal. The variability of opal concentrations is mainly due to climate-controlled variations in the opal accumulation rate. There is a significant positive relation between the original concentrations and the percentages of Cycladophora davisiana , in this region a marker species for increased fertility. This indicates that the opal record primarily reflects variations of the biogenic opal production in the water column. The effect was modified by short-term dissolution processes.

Fast reconnaissance of carbonate dissolution based on the size distribution of calcareous ooze on Walvis Ridge, SE Atlantic Ocean

We present a new index of carbonate fragmentation based on the size distribution of bulk sediments in core MD962094 from Walvis Ridge (SE Atlantic Ocean). The carbonate fragmentation index is constructed by taking a ratio of the two coarsest fractions in the grain size distributions of the bulk calcareous ooze. The coarsest two fractions (25�90 small mu, Greekm and >90 small mu, Greekm) of the bulk sediments consist primarily of complete shells and fragments of adult foraminifera shells, and juvenile foraminifera shells and fragments, respectively. The ratio of the proportions of the two fractions is interpreted as a measure of fragmentation of the foraminifera shells caused by carbonate dissolution. Downcore changes in our carbonate fragmentation index compare very well with those in the coarse-carbonate fragmentation index in sediments from a nearby core on Walvis Ridge. The latter commonly used fragmentation index is defined as a ratio of foraminifera fragments over whole foraminifera in the >150-small mu, Greekm fraction as seen with a light microscope. Fragmentation is relatively high during glacial stages and relatively low during interglacial stages during the last 300 kyr, caused by the combined effect of wind-driven upwelling of corrosive water and increased production of organic matter, decreasing the preservation potential of carbonates both during and after deposition. The carbonate fragmentation index we present here provides a precise and fast method to establish a downcore fragmentation record. It can be applied to bulk sediments that are carbonate-rich (CaCO3>68%) and to all other deep-marine sediments of which the grain size distribution of the carbonate-free fraction is available.

Changements climatiques et paléocéanographiques en Afrique centrale atlantique au cours de la dernière déglaciation : contribution palynologique

Abstract Palynological analyses of two marine cores recovered from the Congo fan provide a high-resolution record of palaeoclimatic changes that have taken place on the adjacent continent and the margin since the Late Pleistocene. Between 27 and 14 14C kyr BP, the basin was characterized by well-developed grasslands, and the coastal area was under the influence of seasonal coastal upwellings. From 14-13 14C kyr BP, significant changes in the rainforest and coastal vegetation are recorded, characterized by an increase of the rainforest until 5-4 14C kyr BP, followed by the development of herbaceous vegetation. Sea level fluctuations are documented by Rhizophora frequencies.

Two Pleistocene silicoflagellate biohorizons from the Angola Basin (SE Atlantic)

Abstract The silicoflagellate Dictyocha perlaevis perlaevis Frenguelli is found to define a marker for the 140 ka BP and 250 ka BP time stratigraphic horizons in the Angola Basin. It makes up 25 to 35% of the total silicoflagellate assemblage within this zone, compared to 5 to 10% in sediment below and above.