Charcoal records of two sediment cores off SW Africa

We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo- fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote-sensing observations of month-by-month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Age models of sediment cores from the continental margin of northwest Africa

Diatom abundance and species composition were quantitatively studied in two latest Quaternary (~130 ka to the Present) sequences from the continental margin of northwest Africa. Off this region, coastal upwelling is well developed under the influence of the NE trade winds. Variations in diatom abundance in these cores are inferred to represent changes caused by varying degrees of the upwelling fertility. Times of high productivity are marked by high relative frequencies of Chaetoceros, while low productivity is marked by the dominance of Aulacoseira granulata. Upwelling increased during glacial episodes (isotopic stages 2-4 and 6) relative to isotopic stages 1 and 5. During the late Holocene, primary productivity levels are similar to those for Stage 5, but in the early Holocene upwelling intensities seem to have been weaker than today. The paleoproductivity reconstruction based on the diatom record is supported by paleoproductivity estimations based on the organic carbon content of the sediments (Sarnthein et al., 1987).

Hydrogen isotopic composition of sediment cores from a transect off western Africa

The hydrogen isotopic composition of plant leaf-wax n-alkanes (dDwax) is a novel proxy for estimating dD of past precipitation (dDp). However, vegetation life-form and relative humidity exert secondary effects on dDwax, preventing quantitative estimates of past dDp. Here, we present an approach for removing the effect of vegetation-type and relative humidity from dDwax and thus for directly estimating past dDp. We test this approach on modern day (late Holocene; 0-3 ka) sediments from a transect of 9 marine cores spanning 21°N-23°S off the western coast of Africa. We estimate vegetation type (C3 tree versus C4 grass) using d13C of leaf-wax n-alkanes and correct dDwax for vegetation-type with previously-derived apparent fractionation factors for each vegetation type. Late Holocene vegetation-corrected dDwax (dDvc) displays a good fit with modern-day dDp, suggesting that the effects of vegetation type and relative humidity have both been removed and thus that dDvc is a good estimate of dDp. We find that the magnitude of the effect of C3 tree - C4 grass changes on dDwax is small compared to dDp changes. We go on to estimate dDvc for the mid-Holocene (6-8 ka), the Last Glacial Maximum (LGM; 19-23 ka) and Heinrich Stadial 1 (HS1; 16-18.5 ka). In terms of past hydrological changes, our leaf-wax based estimates of dDp mostly reflect changes in wet season intensity, which is complementary to estimates of wet season length based on leaf-wax d13C.

Palynology of the Middle to Late Miocene from ODP Hole 175-1085A off the west coast of South Africa

Aim To test whether the radiation of the extremely rich Cape flora is correlated with marine-driven climate change. Location Middle to Late Miocene in the south-east Atlantic and the Benguela Upwelling System (BUS) off the west coast of South Africa. Methods We studied the palynology of the thoroughly dated Middle to Late Miocene sediments of Ocean Drilling Program (ODP) Site 1085 retrieved from the Atlantic off the mouth of the Orange River. Both marine upwelling and terrestrial input are recorded at this site, which allows a direct correlation between changes in the terrestrial flora and the marine BUS in the south-east Atlantic. Results Pollen types from plants of tropical affinity disappeared, and those from the Cape flora gradually increased, between 10 and 6 Ma. Our data corroborate the inferred dating of the diversification in Aizoaceae c. 8 Ma. Main conclusions Inferred vegetation changes for the Late Miocene south-western African coast are the disappearance of Podocarpus-dominated Afromontane forests, and a change in the vegetation of the coastal plain from tropical grassland and thicket to semi-arid succulent vegetation. These changes are indicative of an increased summer drought, and are in step with the development of the southern BUS. They pre-date the Pliocene uplift of the East African escarpment, suggesting that this did not play a role in stimulating vegetation change. Some Fynbos elements were present throughout the recorded period (from 11 Ma), suggesting that at least some elements of this vegetation were already in place during the onset of the BUS. This is consistent with a marine-driven climate change in south-western Africa triggering substantial radiation in the terrestrial flora, especially in the Aizoaceae.

Distribution and biomass of seston in the East Atlantic

Biomass of seston in the surface layers of coastal waters off Namibia reaches 1 g/m**2 and decreases with distance from the shore. Two regions of high seston biomass, one northern and one southern, are distinguished. A subsurface maximum of seston biomass, presumably coinciding with the stream of compensating countercurrent, is identified in the 200-500 m layer. Similar vertical distribution of plankton is known in upwelling areas of the eastern shores of the Atlantic and Pacific Oceans and in several other ocean areas, such as the area of the Kuril-Kamchatka Trench. This fact probably indicates that life cycles of pelagic animal forms of various taxonomic groups that inhabit them and phases of their ontogenic migrations are similar.

Age determination of sediment cores from the continental shelf and slope off North-West Africa

Three sediment cores from the continental shelf and slope off NW Africa (Banc d'Arguin; 52 m, 665 m and 973 m water depth) have been investigated by means of a coarse fraction analysis. The two shallower cores have been deposited during less than 10,000 years, the deeper one during the last 36,000 years. The Holocene sedimentation ( 4000 years) in the deeper part of core 79 the edge of the Banc d'Arguin is strongly influenced by reworking of Late Glacial dune sands and biogenic particles from shallower ware (<40 m), as well as eroding current influence. A decrease in grain size of silicate material and a decrease in lateral supply, correlated to a doubling of accumulation rates in the upper part of the core, indicates a more autochthonous sedimentation with less sorting influence in the youngest Holocene. The depth of provenance of the allochttonous material can be assumed in 100-300 m water depth as indicated by various biogenous particles. Small amounts of shallow water particles in the autochthonous layers indicate a supplay from shallow water, which probably occured b ythe mechanism of "particle by particle supply". None of the three cores indicates upwelling influence, although occanographers found intense upwelling in the area of the Banc d'Arguin. The Holocene climate in that area probably has been arid, small variations in terrigenous matter composition and grain size in the Early Holocene might be due to decreased wind strength or to an increase in rain fall. The Peak Glacial section (14,000-22,000 y. B.P.) of the deepest core 88 indicates a very much intensified eolian silt supply and an additional bottom supply of quartz sand In the interval 22,000-36,000 y. B.P. wind strength decreased, but probably no increase in humidity occurred. So this area in about 19° 40' N had an arid climate in the Late Holocene and in the Peak Glacial. The fragmentation of planktonic foraminifers and the abundance of aragonitic tests of pteropods in core 88 indicate an Early Holocene (8330 y. B.P.) preservation spike. Two minima in fragmentation correlated to maxima in pteropod content at about 15,700 and 21,000 y. B.P. are correlated to maxima in shallow water supply and thus do not reflect preservation conditions, but only lateral supply from the carbonate dissolution minimum zone in about 300 m water depth.

Stable isotope record of planktic foraminifera sampled by plankton haul and in surface sediments off North-West Africa

Foraminifera shells from modern sediments document the hydrography of the coastal upwelling region off Northwest-Africa (12-35° N) through the stable isotopic composition of their shells. Oxygen isotopes in planktonic foraminifers reflect sea surface temperatures (SST) during the main growing season of the differnt species: Globigerinoides ruber (pink and white) and G. sacculifer delineate the temperatures of the summer, Globorotalia inflata and Pulleniatina obliquiloculata those of the winter. Oxygen isotopes on Globigerina bulloides document temperature ranges of the upwelling seasons. d18O values in planktonic foraminifera from plankton hauls resemble those from the surface sediment samples, if the time of the plankton collection is identical with that of the main growing season of the species. The combined isotopic record of G. ruber (white) and G. inflata clearly reveals the latitudinal variations of the annual mean SST. The deviation of the d18O values from both species from their common mean is a scale for the seasonality, i.e. the maximum temperature range within one year. Thus in the summer upwelling region (north of 25° N) seasonality is relatively low, while it becomes high in the winter upwelling region south of 20° N. Furthermore, the winter upwelling region is characterized by relatively high d18O values - indicating low temperatures - in G. bulloides, the region of summer upwelling by relatively low d180 values compared with the constructed annual mean SST. Generally, carbon isotopes from the plankton hauls coincide with those from sediment surface samples. The enrichment of 13C isotopes in foraminifers from areas with high primary production can be caused by the removal of 12C from the total dissolved inorganic carbon during phytoplankton blooms. It is found that carbon isotopes from plankton hauls off Northwest-Africa are relatively enriched in 13C compared with samples from the western Atlantic Ocean. Also shells of G. ruber (pink and white) from upwelling regions are enriched in the heavier isotope compared with regions without upwelling. In the sediment, the enrichement of 13C due to high primary production can only be seen in G. bulloides from the high fertile upwelling region south of 20° N. North of this latitude values are relatively low. An enrichment of 12C is observed in shells of G. ruber (pink), G. inflata and P. obliquiloculata from summer-winter- and perennial upwelling regions respectively. Northern water masses can be distinguished from their southern counterparts by relatively high oxygen and carbon values in the "living" (=stained) benthic foraminifera Uvigerina sp. and Hoeglundina elegans. A tongue of the Mediterranean Outflow water can be identified far to the south (20° N) by 13C-enriched shells of these benthic foraminifera. A zone of erosion (15-25° N, 300-600 m) with a subrecent sediment surface can be mapped with the help of oxygen isotopes in "dead" benthic specimens. Comparison of d18O values in aragonitic and calcitic benthic foraminifers does not show a differential influence of temperature on the isotopic composition in the carbonate. However, carbon isotopes reflect slightly differences under the influence of temperature.

Benthic foraminifera of surface sediments from the continental slope off Sierra Leone, North-West Africa

On "Meteor" cruise 30 (1973) 22 piston-cores were collected off Sierra Leone from water-depths between about 5000 m (Sierra Leone Basin) and 500 m (upper continental slope) with the objective to study the sediment composition and age as well as processes of sedimentation on the continental slope in a tropical humid region. Granulometric analysis and determinations of the carbonate contents of the sediment samples were carried out, as well as qualitative and quantitative analysis of the components of the grain size fractions > 63 µm and of the planktonic and benthonic foraminifera > 160 µm. Presently, the cold Canary Current influences the composition of the planktonic foraminifera within the northwestern area of investigation (profile A), whereas the planktonic fauna of the eastern area (profile C) seems to be truly tropical. In all Quaternary sediments from the continental slope off Sierra Leone, species of Globorotalia are less abundant than in truly pelagic sediments. For that reason, the zonation of the Pleistocene sediments based on the presence or absence of Globorotalia cultrata does not always agree with the climatic changes reflected in the sediments. Concerning past climates better results can be obtained by using the changes in percentage abundances of Globigerina sp. sp. and Globigerinoides sp. sp. as indicators for cool and warm temperatures. The Tertiary sediments contain a pelagic foraminiferal assemblage. In the Holocene sediments the benthonic foraminifera do not only serve as good paleodepth indicators, but their communities are also restricted to defined water masses, which change their positions in accordance with climatic changes. Thus, Cassidulina carinata in the area of investigation is an excellent indicator for sediments deposited during times, which were cooler than today; this is true for all cores from the continental slope off Sierra Leone independent of water-depth although this species presently abounds at water-depths around 600 m. The cores from the continental rise and from the Sierra Leone Basin (M30-261, M30-146, M30-147) were deposited below the calcium carbonate compensation depth. Only small sections of the cores consist of the original carbonate-free sediments, whereas the main part of the sediment column is redeposited material, rich in foraminifera, which normally live on the upper continental slope, or even on the shelf. From these cores only M30-261 can be subdivided into biostratigraphic zones ranging from zone V to zone Y. In all cores from the middle and upper continental slope of the eastern area of investigation (profile C; KL 230, 209-204) and in cores KL 183 and KL 184 from the northwestern area (profile A) we observed an undisturbed succession of sediments from the biostratigraphic zones X (partly), Y and Z. All cores from the central area (M30-181, M30-182, M30-262 to 264) and M30-187 from the upper slope of profile A show variable hiatuses in the sedimentary record. Locally, high velocity bottom currents were probably responsible for erosion, nondeposition or minimal sedimentation rates. These currents might have been initiated partly by the somewhat exposed position of this part of the continental slope, where the shelf edge bends from a northwest towards an eastern direction, and partly by very young tectonic movements. Fracture zones with vertically displaced fault blocs are frequent at Sierra Leone continental margin. According to seismic measurements by McMaster et al. (1975) the sites of the central area are located on an uplifted fault bloc explaining the reduced sediment rates or erosion. Unlike the central area, the eastern area (profile C) is situated on a downfaulted bloc with high sediment rates. The sediments from the cores of profile B as well as the turbiditic deep-sea sediments were deposited under a higher flow regime; therefore they are coarser than the extremely fine-grained sediments of the cores from profile C. Since the sand fraction (> 63 µm) is mainly composed of foraminifera, besides pteropods and light-colored fecal pellets, the carbonate content increases with the increasing percentage of the coarse grain fraction. Higher concentrations of quartz were only observed in core sections with considerable carbonate dissolution (mainly in the X-Zone), and, in general, in all sediments from the eastern area with higher terrigenous input including larger concentration of mica. Especially during times transitional between glacials and interglacials (or interstadials) the bottom currents were intensified. The percentages of coarse fraction and carbonate increase with increasing current velocities. Calcium carbonate dissolution becomes important in water depths > 3500 m. During cooler times the lysokline is depressed. Light-colored fecal pellets were redeposited from Late Neogene sediments (M30-187, M30-181). In the area of investigation they occur in the Holocene and mainly the Pleistocene sediments of the cores from the northwestern and central area because only here Tertiary sediments have been eroded at the uppermost continental slope. In the central area there are at least two periods of non-sedimentation and/or erosion which can be confined as being (1) not older than middle Pliocene and not younger than zone V and (2) younger than zone W. The local character of the erosion is documented by the fact that a complete Late Quaternary section is present in the cores of the northwestern and eastern area, each within less than 100 km from incomplete cores from the central area.

Sedimentology and geochemistry of the sand fraction in surface sediments off West Africa

Surface sediments from 5 profiles between 30 and 3000 m water depth off W Africa (12-19° N) have been studied for their sand fraction composition and their total calcium carbonate and organic matter contents to evaluate the effect of climatic and hydrographic factors on actual sedimentation. On the shelf and upper slope (< 500 m), currents prevent the deposition of significant amounts of fine-grained material. The sediments forming here are characterized by high sand contents (> 60 %; in most samples > 89 %), low organic carbon contents (in most samples < 0.8 %), high median diameters of the sand fraction (120-500 µm), and by a predominance of quartz and biogenic relict shells (most abundant: molluscs and bryozoans) in the sand fraction. Median diameters of total sand fraction and of major biogenic sand fraction components (biogenic relict material, benthonic molluscs, benthonic and planktonic foraminifers) co-vary to some extent and show maximum values in 100-300 m water depth, reflectingthe sorting effect of currents (perhaps the northward flowing undercurrent). In this water depth, biogenic relict material is considerably enriched relative to wuartz, the second dominating sand fraction component on the shelf and upper slope, resulting in distinct calcium carbonate maxima of the bulk sediments. The influence of the undercurrent is also reflected in a northward transport of fine grained river load and perhaps in the distribution of the red stained, coarse silt and sand-size clay aggregates, which show maxima in 300-500 m water depth. They probably originate from tropical soils. Abundant coarse red-stained quartz on the shelf off Cape Roxo (12-130° N) suggests a southward extension of last glacial dune fields to this latitude. Below about 500 m water depth, current influence becomes negligible - as indicated by a strong decrease in sand content, a concomitant increase in sedimentary organic carbon contents (up to 2.5-3.5 %), and the occurence of high mica/quartz ratios in the sand fraction. Downslope transport, presumably due to the bioturbation mechanism, is indicated by the presence of coarse shelf-borne particles (glauconite, relict shells) down to about 1000 m water depth. The fine/coarse ratio (clay + silt/sand) of the sediments from water deoth > 500 m never exceed a value of 11 in northern latitudes (19° - 26° N), but shows distinct maxima, ranging from 50 to 120, at latitudes 18°, 17° 15°30', and 14° N in about 2000 m water depth. This distribution is attributed to the deposition of fine-grained river load at the continental slope between 18° and 14° N, brought into the sea by the Senegal and souther rivers and transported northward ny the undercurrent. Strong calcium carbonate dissolution is indicated by the complete disappearance of pteropodes (aragonite) and high fragmentation of the planktoic foraminifers (calcite) in sediments from water depth > 300-600 m. Fragmentation ratios of planktonic foraminifers were found to depend on the organic carbon/carbonate ratios of the sediment suggesting that calcite dissolution at the sea bottom may also be significant in shelf and continental slope water depths if the organic matter/carbonate ratio of the surface sediment is high and the test remain long enough within the oxidizing layer on the top of the sulfate reduction zone. The fact that in the region under study intensity and anual duration of upwelling decrease from north to south is neither reflected in the composition on the sand fraction (i.e. radiolarian and fish debris contents, radiolarian/planktonic foraminiferal ratios, benthos/plankton ratios of foraminifers), nor in the sedimentary organic carbon distribution. On the contrary, these parameters even show in comparable water depths a tendency for highest values in the south, partly because primary production rates remain high in the whole region, particularly on the shelf, due to the nutrient input by rivers in the south. In addition, several hydrographic, sedimentological and climatic factors severely affect their distribution - for example currents, dissolution, grain size composition, deposition of river load, and bulk sedimentation rats.