Palynology of marine sediment cores from the West African coast

Seven sediment cores from the cruises of the "Meteor" and "Valdivia" were examined palynologically. The cores were retrieved from the lower continental slope in the area of between 33.5° N and 8° N, off the West African coast. Most of the cores contain sediments from the last Glacial and Interglacial period. In some cases, the Holocene sediments are missing. Some individual cores contain sediments also from earlier Glacial and Interglacial periods. The main reason for making this palynological study was to find out the differences between the vegetation of Glacial and Interglacial periods in those parts of West Africa which at present belong to the Mediterranean zone, the Sahara and the zones of the savannas and tropical forests. In today's Mediterranean vegetation zone at core 33.5° N, forests and deciduous forests in particular, are missing during Glacial conditions. Semi-deserts are found instead of these. In the early isotope stage 1, there is a very significant development of forests which contain evergreen oaks; this is the Mediterranean type of vegestation development. The Sahara type of vegetation development is shown in four cores from between 27° N and 19° N. The differences between Glacial and Interglacial periods are very small. It must be assumed therefore that in this latitudes, both Glacial and Interglacial conditions gave rise to desert generally. The results are in favour of a slightly more arid climate during Glacial and more humid one during Interglacial periods. The southern boundary of the Sahara and the adjacent savannas with grassland and tropical woods were situated more to the south during the Glacial periods than they were during the Interglacial ones. In front of today's savanna belt, it can be seen from the palynological results that there are considerable differences between the vegetation of Glacial and Interglacial periods. The woods are more important in Interglacial periods. During the Glacial periods these are replaced from north to south decreasingly by grassland (savanna and rainforest type of vegetation development). The southern limit of the Sahara during stage 2 was somewhat between 12° N and 8° N which is between 1.5 and 5 degrees in latitude further south than it i s today. Not only do these differences in climate and vegetation apply to the maximum of the last Glacial and for the Holocene, but they apparently apply also to the older Glacial and Interglacial periods, where they have been found in the profiles. The North African deset belt can be said to have expanded during Glacial times both towards the north and towards the south. All the available evidence of this study indicates that the grass land or the semi-desert of the Southern Europe cam einto connection with those of the N Africa; there could not have been any forest zone between them. The present study was also a good opportunity for investigating some of the basic marine palynological problems. The very well known overrepresentation of pollen grains of the genus Pinus in marine sediments can be traced as fa as 21° N. The present southern limit for the genus Pinus is on the Canaries and on the African continent as approximately 31° N. Highest values of Ephedra pollen grains even occur south of the main area of the present distribution of that genus. These does not seem to be any satisfactory explanation for this. In general, it would appear that the transport of pollen grains from the north is more important than transport from the south. The results so far, indicate strongly that further palynological studies are necessary. These should concentrate particularly on cores from between 33° N and 27° N as well as between 17° N and 10° N. It would also be useful to have a more detailed examination of sediments from the last Intergalcial period (substage 5 e). Absolute pollen counts and more general examination of surface samples would be desirable. Surface samples should be taken from the shelf down to the bottom of the continental slope in different latitudes.

Composition of the sand fraction of sediment cores from the West-African continental margin

Seven cores from the West African continental margin in 12-18° N have been investigated by means of a coarse fraction analysis. Four of the seven cores contain allochthonous material: turbidites and debris flow deposits. The source of the allochthonous material is in about 300-600 m water depth. The age of the slide induced debris flow deposits is at the end of oxygen isotope stage 2. One debris flow deposit is covered by a turbidite (core GIK13211-1). The turbidites in the deep-sea core GIK13207-3 originate from river-influenced sediments from the West-African continental margin, whereas the autochthonous sequences are influenced by volcanic material from the Cape Verde Islands. Particle by particle supply from upper slope areas has been found in all four cores from the continental slope. Current sorting occurs on the submarine diapir (core GIK13289-3), whereas core GIK13291-1 on the NW-flanc, 200 m below core GIK13289-3, has no current sorting, except for stage 1 and parts of stage 5. The current sorting is reflected by parallel variations of median diameters of whole tests and of fragments of planktonic foraminifers, by higher median diameters of foraminifers on top of the diapir, by reduced accumulation rates and increased sand fraction percentages in core GIK13289-3 compared to core GIK13291-1. The Late Quarternary climatic history of the West-African near coastal area (12-18° N) has been redrawn: - in oxygen isotope stage 1 a humid climate is found in 12-18° N (This "humid impression" in 18° N, which is actually an arid area, is due to the poleward directed undercurrent, which transports Senegal river material to the north). - in oxygen isotope stage 2 an arid climate existed in 14-18° N, whereas in 12° N river discharfe persisted. But within stage 2 dune formation occured in 12° N on the (dry) shelf, additionally to fluviatile sediment input. - Older periods are preserved in autochthonous sediments of core GIK13289-3 and GIK13291-1, where oxygen stage 3,5 and 7 (the latter only in core GIK13289-3 present) show a humid climate (as well as in stage 5 of core GIK13255-3), interrupted by short arid intervals in core GIK12389-3, and stage 4 and 6 show an arid climate, interrupted by short humid periods The allochthonous stage 5 sediment in core GIK13211-1 also reflects a humid climate. The dissolution of planktonic foraminifers is strongest in th eLate Holocene and shows a minimum in the early Holocene, where also pteropods are preserved. The degree of carbonate dissolution is related mainly to the fine matter content (< 63 µm) whereas water depth is a less decisvive factor.

Geochemical analysis of bulk sediment and interstitial water of sediment cores from the North-West African continental margin

Molybdenum and vanadium were analysed in 9 scediment cores recovered from the continental slope and rise off NW Africa. Additionall chemical and sedimentological parameters as well as isotope stage boundaries were available for the same core profiles from other investigations. Molybdenum, ranging between <1 and 10 ppm, occurs in two associateions, either with organic carbon and sulphides in sediments with reducing conditions or with Mn oxides in oxidized near-surface core sections. Highest values (between 4 and 10 ppm Mo) are found in sulphide-rich core sections deposited during glacial times in a core from 200 m water depth. The possibility of anoxic near-bottom water conditions prevailing at thhis site during certain glacial intervals is discussed. In oxidized near-surface core sections, the diagenetic mobility of Mo becomes evident from strong Mo enrichment together with Mn oxides (values up to 4 ppm Mo). This enrichment is probably due to coprecipitation and/or adsorption of Mo from interstitial water to the diagenetically forming Mn oxides. The close relation between Mo and Corg results in strongly covarying sedimentation rates in both components reaching up to 10 times the rates in glacial compared to interglacial core sections. Vanadium (values between 20 and 100 ppm) does not show clear relations to climate and near-bottom or sediment milieu. It occurs mainly bound to the fine grained terrigenous fraction, associated with aluminium silicates (clay minerals) and iron oxides. Additionally positive covariation of vanadium with phosphorus in most core profiles suggest that some V may be bound to phosphates.

Ostracoda fauna of surface sediments from the Persian Gulf

During the "Meteor"-Expedition to the Persian Gulf in March-May 1965, approximately 300 samples were collected. Most of them have been already studied by various authors in sedimentological as well as micropaleontological respects. 49 samples were selected for ostracode studies. These samples are arranged to form a long-axis section ("Laengsprofil"), and 4 shorter cross-profiles, perpendicular to the long-axis profile in the Persian Gulf and Gulf of Oman. 52 species of ostracodes in this area were specifically determined; 39 of them are described under open nomenclature. 13 species are already known from surrounding sea areas: 2 species from the Red Sea; 2 species from the east coast of Africa; 1 species from the Mediterranean Sea; and others from the Indian and Pacific Oceans. 12 species show close relationships to species from the Indopacific Ocean. The ostracode species found in the area can be grouped after the method of BRAUN-BLANQUT into 2 bioassociations. Association 1 with the following 4 characteristic species : Cytherella cf. pulchra, Loxoconcha sp. A, Neomonoceratina sp. A, Alocopocythere reticulata. Association 2 with 1 characteristic species: Ruggieria (Ruggieria) sp. B. The association 1 is widespread in the entire studied area of the Persian Gulf, where it is considered to characterize the shallow water region down to 200 m. The association 2 is restricted to the deeper water below 200 m of the inner part of the Oman Gulf. Only a few species known from the shallow water association of the Persian Gulf are present. Within the two ostracode associations mentioned above 4 zones from the total studied area could be related to the water depth. The zones A-D are characterized more or less readily by the relative abundance of certain species: Zone A : 7-30 m depth, on substrates of poorly coarse-grained clayey marl; Zone B: 30-94 m depth, on substrates of richly coarse-grained calcareous marl; Zone C: 94-1961208 m depth, on substrates of richly coarse-grained calcareous marl; Zone D: 196/208-500 m depth, on substrates of calcareous clay, poor in benthos. The regional and bathymetric distribution of the ostracode fauna in the area studied was compared in relation to 10 environmental factors: water depth, temperature, salinity, water density, O2-concentration, phosphate-silica contents, pH-values, stratification of the water body, water currents and type of sediments. The major environmental factors which appear to control the ostracode distribution are water depth (as a complex factor), O2-concentration and the type of sediment. At 3 stations (GIK01058, GIK01074 and GIK01204) species of the shallow water association were found together with a few bathyal species. These stations are situated at the outer Biaban shelf, in an area where the bottom water of the Persian Gulf flows down the slope towards the Oman Gulf. Several samples of the Zone B in the major part of the Persian Gulf show also a high species diversity containing a high percentage of subfossil ostracode carapaces. It is probable that the recent biocoenosis has been mixed with a late quarternary thanatocoenosis.

Geochemical analysis of sediments and interstitial water in sediment cores from the North-West African continental margin and from the Central Pacific

A. Continental slope sediments off Spanish-Sahara and Senegal contain up to 4% organic carbon and up to 0.4% total nitrogen. The highest concentrations were found in sediments from water depths between 1000 and 2000 m. The regional and vertical distribution of organic matter differs significantly. Off Spanish-Sahara the organic matter content of sediment deposited during glacial times (Wuerm, Late Riss) is high whereas sediments deposited during interglacial times (Recent, Eem) are low in organic matter. Opposite distribution was found in sediments off Senegal. The sediments contain 30 to 130 ppm of fixed nitrogen. In most sediments this corresponds to 2-8 % of the total nitrogen. Only in sediments deposited during interglacial times off Spanish-Sahara up to 20 % of the total nitrogen is contained as inorganically bound nitrogen. Positive correlations of the fixed nitrogen concentrations to the amounts of clay, alumina, and potassium suggest that it is primarily fixed to illites. The amino acid nitrogen and hexosamine nitrogen account for 17 to 26 % and 1.3 to 2.4 %, respectively of the total nitrogen content of the sediments. The concentrations vary between 200 and 850 ppm amino acid nitrogen and 20 to 70 ppm hexosamine nitrogen, both parallel the fluctiations of organic matter in the sediment. Fulvic acids, humic acids, and the total organic matter of the sediments may be clearly differentiated from one another and their amino acid and hexosamine contents and their amino acid composition: a) Fulvic acids contain only half as much amino acids as humic acids b) The molar amino acid/hexosamine ratios of the fulvic acids are half those of the humic acids and the total organic matter of the sediment c) The amino acid spectra of fulvic acids are characterized by an enrichment of aspartic acid, alanine, and methionine sulfoxide and a depletion of glycine, valine, isoleucine, leucine, tyrosine, phenylalanine, lysine, and arginine compared to the spectra of the humic acids and those of the total organic matter fraction of the sediment. d) The amino acid spectra of the humic acids and those of the total organic matter fraction of the sediments are about the same with the exception that arginine is clearly enriched in the total organic matter. In general, as indicated by the amino compounds humic acids resemble closer the total organic matter composition than the low molecular fulvic acids do. This supports the general idea that during the course of diagenesis in reducing sediments organic matter stabilizes from a fulvic-like structure to humic-like structure and finally to kerogen. The decomposition rates of single aminio acids differ significantly from one another. Generally amino acids which are preferentially contained in humic acids and the total organic matter fraction show a smaller loss with time than those preferably well documented in case of the basic amino acids lysine and arginine which- although thermally unstable- are the most stable amino acids in the sediments. A favoured incorporation of these compounds into high molecular substances as well as into clay minerals may explain their relatively high "stability" in the sediment. The nitrogen loss from the sediments due to the activity of sulphate-reducing bacteria amounts to 20-40 % of the total organic nitrogen now present. At least 40 % of the organic nitrogen which is liberated by sulphate-reducing bacteria can be explained ny decomposition of amino acids alone. B. Deep-sea sediments from the Central Pacific The deep-seas sediments contain 1 to 2 orders of magnitude less organic matter than the continental slope sediments off NW Africa, i.e. 0.04 to 0.3 % organic carbon. The fixed nitrogen content of the deep-sea sediments ranges from 60 to 270 ppm or from 20 to 45 % of the total nitrogen content. While ammonia is the prevailing inorganic nitrogen compound in anoxic pore waters, nitrate predominates in the oxic environment of the deep-sea sediments. Near the sediment/water interface interstital nitrate concentrations of around 30 µg-at. N/l were recorded. These generally increase with sediment depth by 10 to 15 µg-at. NO3- N/l. This suggests the presence of free oxygen and the activity of nitrifying bacteria in the interstitial waters. The ammonia content of the interstitial water of the oxic deep-sea sediments ranges from 2 to 60 µg-at. N/l and thus is several orders of magnitude less than in anoxic sediments. In contrast to recorded nitrate gradients towards the sediments/water interface, there are no ammonia concentration gradients. However, ammonia concentrations appear to be characteristic for certain regional areas. It is suggested that this regional differentiation is caused by ion exchange reactions involving potassium and ammonium ions rather than by different decomposition rates of organic matter. C. C/N ratios All estimated C/N ratios of surface sediments vary between 3 and 9 in the deep-sea and the continental margin, respectively. Whereas the C/N ratios generally increase with depth in the sediment cores off NW Africa they decrease in the deep-sea cores. The lowest values of around 1.3 were found in the deeper sections of the deep-sea cores, the highest of around 10 in the sediments off NW Africa. The wide range of the C/N ratios as well as their opposite behaviour with increasing sediment depth in both the deep-sea and continental margin sediment cores, can be attributed mainly to the combination of the following three factors: 1. Inorganic and organic substances bound within the latticed of clay minerals tend to decrease the C/N ratios. 2. Organic matter not protected by absorption on the clay minerals tends to increase C/N ratios 3. Diagenetic alteration of organic matter by micro-organisms tends to increase C/N ratios through preferential loss of nitrogen The diagenetic changes of the microbially decomposable organic matter results in both oxic and anoxic environments in a preferential loss of nitrogen and hence in higher C/N ratios of the organic fraction. This holds true for most of the continental margin sediments off NW Africa which contain relatively high amounts of organic matter so that factors 2 and 3 predominate there. The relative low C/N ratios of the sediments deposited during interglacial times off Spanish-Sahara, which are low in organic carbon, show the increasing influence of factor 1 - the nitrogen-rich organic substances bound to clay minerals. In the deep-sea sediments from the Central Pacific this factor completely predominates so that the C/N rations of the sediments approach that of the substance absorbed to clay minerals with decreasing organic matter content. In the deeper core sections the unprotected organic matter has been completely destroyed so that the C/N ratios of the total sediments eventually fall into the same range as those of the pure clay mineral fraction.

Geologic-hydrologic setting and surface sedimentology in the Persian Gulf

1. Morphology and sedimentation The deepest parts of the Persian Gulf lie off the Iranian coast. Several swells separate the Persian Gulf into the Western Basin, the Central Basin and the Strait of Hormuz, which leads without noticeable morphological interruption onto the Biaban Shelf; the latter gradually drops off towards the continental slope, which itself has a strongly subdivided morphology. The sediment distribution in the Western Basin runs parallel to the basin's axis to a depth of 50 -60 m. This is caused by the shallow and uniform slope of the Iranian coast into the Western Basin, by clear exposure of the area to the Shamal-Winds and by tidal currents parallel to the basin's axis. Most other parameters also show isolines parallel to the coast line. Data from the sediment analyses show a net transport which extends out along the Central Swell: coarse fraction > 63 µ, total carbonate content, carbonate in fine fractions < 2 µ, 2-6 µ and 20-63 µ, calcite-aragonite ratios in the fine fractions 2-6 µ and 20-63 µ and quartz-dolomite ratios in fine fraction 2-6 µ. At least the uppermost 10-40 m of this sediment is late Holocene. This implies sedimentation rates of several meters per 1000 years. The slope from the Iranian coast into the Central Basin (max. depth 100 m) is generally steeper, with interspersed islands and flats. Both facts tend to disturb a sediment dustribition parallel to the basin's axis over extensive areas and may preclude any such trend from being detected by the methods and sample net used. The spatial distribution of the coarse fraction, however, seems to indicate sediment transport at greater water depths perpendicular to the basin's long axis and along the steepest gradients well into the Central Basin. The flats of the Central Basin have a sediment cover distinctly different from those of the deeper basin areas. Characteristic parameters are the extremely high percentages of coarse grained sediments, total content of carbonate CO2 over 40, low total organic carbon content, (however values are high if calculated on the basis of the < 63 µ fraction), low total N-content, and low C/N ratios. These characteristics probably result from the absence of any terrigenous material being brought in as well as from exposure to wave action. Finest terrigenous material is deposited in the innermost protected part of the Hormuz Bay. In the deep channel cut into the Biaban Shelf which carries the Persian Gulf out-flow water to the Indian Ocean, no fine grained sediment is deposited as shown by grain size data. 2. Geographic settings and sedimentation Flat lands border the Arabian coast of the Persian Gulf except for the Oman region. The high and steep Zagros Mountains form the Iranian coastline. Flat topography in combination with generally low precipitation precludes fluviatile sediment being added from the South. Inorganic and biogenic carbonates accumulating under low sedimentation rates are dominant on the shallow Arabic Shelf and the slopes into the Western and Central Basins. The fluviatile sediment brought in from the Iranian side, however decisively determine the composition of the Holocene sediment cover in the Persian Gulf and on the Biaban Shelf. Holocene sediments extend 20-30 km seaward into the Western Basin and about 25 km on to the Biaban Shelf. As mentioned before, sedimentation rates are of several meters/1000 years. The rocks exposed in the hinterland influence the sediments. According to our data the Redbeds of the Zagros Mountains determine the colour of the very fine grained sediments near the Iranian Coast of the Persian Gulf. To the West of Hormuz, addition of carbonate minerals is particularly high. Dolomite and protodolomite, deposited only in this area, as well as palygorskite, have proven to be excellent trace minerals. To the East of Hormuz, the supply of terrigenous carbonates is considerably lower. Clay minerals appear to bring in inorganically bound nitrogen thus lowering the C/N ratio in these sediments especially off river mouths. 3. Climate and sedimentation The Persian Gulf is located in a climatically arid region. This directly affects sedimentation through increased wind action and the infrequent but heavy rainfalls which cause flash floods. Such flash floods could be responsible for transporting sedheats into the Central Basin in a direction perpendicular to the Gulf's axis. Eolian influx is difficult to asses from our data; however, it probably is of minor importance from the Iranian side and may add, at the most, a few centimeters of fine sediment per 1000 years. 4. Hydrology and sedimentation High water temperatures favor inorganic carbonate precipitation in southern margin of the Gulf, and probably on the flats, as well as biogenic carbonate production in general. High evaporation plus low water inflow through rivers and precipitation cause a circulation pattern that is typical for epicontinental seas within the arid climate region. Surface water flows in from the adjoining ocean, in this case the Indian Ocean and sinks to the bottom of the Persian Gulf mainly in the northern part of the Western Basin, on the "Mesopotamischer Flachschelf" ard probably in the area of the "Arabischer Flachschelf". This sinking water continually rejuvenates the bottom out-flow water. The inflowing surface water from the Indian Ocean brings organic matter into the Persian Gulf, additional nutrients are added by the "fresh" upwelling waters of the Gulf of Oman. Both nutrients and organic matter diminish very rapidly as the water moves into the Persian Gulf. This depletion of nutrients and organic matter is the reasonfor generally low organic carbon contents of the Persian Gulf sediments. The Central Swell represents a distinct boundary, to the west of which the organic carbon content are lower than to the east when sediment samples of similar grain size distribution are compared. The outflow carries well oxygenated water over the bottom of the Persian Gulf and the resulting oxidation further decreases the content of organic matter. In the Masandam-Channel and in the Biaban-Shelf channel, the outflowing water prevents deposition of fine material and transports sediment particles well beyond the shelf margin. The outflowing water remains at a depth of 200-300 m depending on its density and releases ist suspending sediment load to the ocean floor, irrespectative of the bottom morphology. This is reflected in several parameters in which the sediments from beneath the outflow differ from nearby sediments not affected by the outflowing water. High carbonate content of total samples and of the individual size fraction as well as high aragonite and dolomite contents of individual size fractions characterize the sediment beneath the outflowing water. The tidal currents, which avt more or less parallel to the Gulf's axis, favor mixing of the water masses, they rework sediments at velocities reported here. This fact enlarges to a certain degree the extent of our interfaces which are based on only a few sample points (Persian Gulf and Biaban Shelf one sample per 620 km**2, continental slope one sample per 1000 km**2). The water on the continental slope shows and oxygen minimum at 200-1200 m which favors preservation of organically-bound carbon in the sediment. The low pH-values may even permit dissolution of carbonate minerals.

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.