A 31,000-year high-resolution record from two sediment cores off northwest Africa

Benthic foraminiferal assemblage compositions and sedimentary geochemical parameters were analyzed in two radiocarbon dated sediment cores from the upwelling area off NW Africa at 12°N, to reconstruct productivity changes during the last 31 kyr. High-latitude cold events and variations in low-latitude summer insolation influenced humidity, wind systems, and the position of the tropical rain belt over this time period. This in turn caused changes in intensity and seasonality of primary productivity off the southern Northwest African continental margin. High accumulation rates of benthic foraminifera, carbonate, and organic carbon during times of north Atlantic melt water events Heinrich 2 (25.4 to 24.3 kyr BP) and 1 (16.8 to 15.8 kyr BP) indicate high productivity. Dominance of infaunal benthic foraminiferal species and high numbers of deep infaunal specimens during that time indicate a strong and sustained supply of refractory organic matter reworked from the upper slope and shelf. A more southerly position of the tropical rainbelt and the Northeast trade wind belt during Heinrich 2 and 1 may have enhanced wind intensity and almost permanent upwelling, driving this scenario. A phytodetritus-related benthic fauna indicates seasonally pulsed input of labile organic matter but generally low year-round productivity during the Last Glacial Maximum (23 to 18 kyr BP). The tropical rainbelt is more expanded to the North than during Heinrich Events, and relatively weak NE trade winds resulted in seasonal and weak upwelling, thus lower productivity. High productivity characterized by a seasonally high input of labile organic matter, is indicated for times of orbital forced warming, such as the African Humid Period (9.8 to 7 kyr BP). An intensified African monsoon during boreal summer and the northernmost position of the tropical rainbelt within the last 31 kyr resulted in enhanced river discharge from the northward-extended drainage area (or river basin) initiating intense phytoplankton blooms. In the late Holocene (4 to 0 kyr BP) strong carbonate dissolution may have been caused by even more enhanced organic matter fluxes to the sea floor. Increasing aridity on the continent and stronger NE trade winds induced intensive, seasonal coastal upwelling.

Age determination of sediment cores from the Shelf Edge off Mauritania, Northwest Africa

The impact of late glacial changes on the sedimentary record was investigated in two long vibracores, collected from the shelf edge off Mauritania, northwest Africa. Lithology and radiocarbon dates indicate that the sedimentary sequences were mainly controlled by sea-level changes on the shelf. The upper Pleistocene sequence is characterized by deposition in coastal environments, while the Holocene sequence represents deposition in shelf environments. During low sea level, much sediment was supplied to the present outer shelf, and the data imply an average accumulation rate of up to 43.0 cm/1000 yrs during the late Pleistocene, which is substantially higher than the Holocene rate. Shelf sediments were continuously reworked and redistributed on a regional scale during falling and rising sea level. The presence of reworked material results in radiocarbon ages which are too old. The mollusc. Venus striatula, which presently is found north of, but not along, the Mauritanian coast, occurs in the upper Pleistocene sequence, suggesting cooler water conditions in the shelf during late glacial times. This species probably migrated to the south during late glacial times, following the southward extension of the cold Canary Current. Radiocarbon dates of the shells broadly coincide with a lowstand of sea level over this part of the continental shelf.

SHRIMP U-Pb zircon data from Gjelsvikfjella, Dronning Maud Land, East Antarctica

The Maud Belt in Dronning Maud Land (western East Antarctic Craton) preserves a high-grade polyphase tectono-thermal history with two orogenic episodes of Mesoproterozoic (1.2-1.0 Ga) and Neoproterozoic (0.6-0.5 Ga) age. New SHRIMP U-Pb zircon data from southern Gjelsvikfjella in the northeastern part of the belt make it possible to differentiate between a series of magmatic and metamorphic events. The oldest event recorded is the formation of an extensive 1140-1130 Ma volcanic arc. This was followed by 1104 ± 8 Ma granitoids that might represent, together with so far undated mafic dykes, part of a decompression melting-related bimodal suite that reflects the sub-continental Umkondo igneous event. The first high-grade metamorphism is constrained at 1070 Ma. The metamorphic age data are similar to those obtained from other parts of the Maud Belt, but also from the Namaqua-Natal Belt in South Africa, but the preceding arc formation was diachronous in the two belts. This indicates that the two belts did not form a continuous volcanic arc unit as suggested in previous models, but became connected only at the end of the Mesoproterozoic. Intense reworking during the Neoproterozoic, probably as a result of continent-continent collision between components of Gondwana, is indicated by ductile refliation, further high-grade metamorphic recrystallisation and metamorphic zircon overgrowths at approximately 530 Ma. This was followed by late- to post-tectonic magmatism, reflected by 500 Ma granite bodies and 490 Ma aplite dykes as well as a 480 Ma gabbro body.

Abundance of microzooplankton in surface waters of the East Atlantic and Mediterranean in August-September 1970

Abundance of microzooplankton was studied from August to October 1970 in a ship laboratory using the method of concentration of water samples by filtration and then counting living organisms under a microscope. The main groups (in order of decreasing abundance) were as follows: infusorians, nauplii, copepodids, radiolarians, appendicularians, and some others (rotifers, worm and mollusk larvae). Concentration of infusorians rarely exceeded 100 #/l, possibly an underestimate. Nauplii often numbered 20 to 30 #/l. Study of vertical distribution of microzooplankton showed that peak concentrations in the Mediterranean Sea were at depth of 20-30 m regardless of day time. There were 2 peaks in the Atlantic Ocean, one in the 10- to 20-m layer, the other in the 50- to 75-m layer.

Ladderane lipid analysis and pore water and sediment chemistry of sediment cores from the continental shelf and slope off northwest Africa

The presence and abundance of anaerobic ammonium-oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300-3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q-PCR) analysis of anammox-specific 16S rRNA genes and anammox-specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g**-1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]-ladderane monoether-PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox-specific 16S rRNA gene copy numbers and the concentration of the C20 [3]-ladderane monoether-PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 x 10**8 copies g**-1 sediment and 30 pg g**-1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification.

Pollen analysis of surface sediment off northwest Africa

Over 100 samples of recent surface sediments from the bottomn of the Atlantic Ocean offshore NW Africa between 34° and 6° N have been analysed palynologically. The objective of this study was to reveal the relation between source areas, transport systems, and resulting distribution patterns of pollen and spores in marine sediments off NW Africa, in order to lay a sound foundation for the interpretation of pollen records of marine cores from this area. The clear zonation of the NW-African vegetation (due to the distinct climatic gradient) is helpful in determining main source areas, and the presence of some major wind belts facilitates the registration of the average course of wind trajectories. The present circulation pattern is driven by the intertropical front (ITCZ) which shifts over the continent between c. 22° N (summer position) and c. 4° N (winter position) in the course of the year. Determination of the period of main pollen release and the average atmospheric circulation pattern effective at that time of the years is of prime importance. The distribution patterns in recent marine sediments of pollen of a series of genera and families appear to record climatological/ecological variables, such as the trajectory of the NE trade, January trades, African Easterly Jet (Saharan Air Layer), the northernmost and southernmost position of the intertropical convergence zone, and the extent and latitudinal situation of the NW-African vegetation belt. Pollen analysis of a series of dated deep-sea cores taken between c. 35° and the equator off NW African enable the construction of paleo-distribution maps for time slices of the past, forming a register of paleoclimatological/paleoecological information.