Bulk 15N, and 13Corg ratios, and calculation of unutilized nitrate of surface samples off the southwest coast of Africa (Table 1)

Bulk delta15N values in surface sediment samples off the southwestern coast of Africa were measured to investigate the biogeochemical processes occurring in the water column. Nitrate concentrations and the degree of utilization of the nitrate pool are the predominant controls on sedimentary delta15N in the Benguela Current region. Denitrification does not appear to have had an important effect on the delta15N signal of these sediments and, based on delta15N and delta13C, there is little terrestrial input.

Model simulation output data (CCSM3) related to uplift of Africa and Himalaya/Tibet Plateau and the influence on tropical African vegetation

Hominid evolution in the late Miocene has long been hypothesized to be linked to the retreat of the tropical rainforest in Africa. One cause for the climatic and vegetation change often considered was uplift of Africa, but also uplift of the Himalaya and the Tibetan Plateau was suggested to have impacted rainfall distribution over Africa. Recent proxy data suggest that in East Africa open grassland habitats were available to the common ancestors of hominins and apes long before their divergence and do not find evidence for a closed rainforest in the late Miocene. We used the coupled global general circulation model CCSM3 including an interactively coupled dynamic vegetation module to investigate the impact of topography on African hydro-climate and vegetation. We performed sensitivity experiments altering elevations of the Himalaya and the Tibetan Plateau as well as of East and Southern Africa. The simulations confirm the dominant impact of African topography for climate and vegetation development of the African tropics. Only a weak influence of prescribed Asian uplift on African climate could be detected. The model simulations show that rainforest coverage of Central Africa is strongly determined by the presence of elevated African topography. In East Africa, despite wetter conditions with lowered African topography, the conditions were not favorable enough to maintain a closed rainforest. A discussion of the results with respect to other model studies indicates a minor importance of vegetation-atmosphere or ocean-atmosphere feedbacks and a large dependence of the simulated vegetation response on the land surface/vegetation model.

Sedimentology of surface sediments from the continental shelf and slope off West Africa (Table 1)

Samples from sutface sediments of the shell, continental slope, and adjacent deep sea regions off West Africa between 27° N and 15° N were investigated with respect to grain size distribution of the total samples, sand contents of the acid insoluble residues, carbonate content of the total samples, and the separate grain size fractions, specific surfaces, colours and mineralogical composition of the clay fractions. The grain size distributions of the total samples of the sediments of the shelf and the continental slope off Spanish-Sahara are controlled mainly by biogenic components. The supply of terrigeneous material in this area is very low. At deeper parts of the continental slope and in the deep sea areas, the relative amounts of carbonate minerals in the sediments are considerably reduced. The prevailing sand contents of the upper slope changes into clay dominance. On the shelf of Mauritania - represented by profiles extending down to 200 m water depth - the grain size is also controlled mainly by biogenic carbonates. Nevertheless, the admixture of fossil silicate components is important, too. The southern parts of the area is investigated are located in a region influenced by sediments of the Senegal River, which especially control the contents of silt and clay. The silicate sands, predominately of quartz, are fossil and form a mixed sediment with younger deposits. The carbonate contents of the different grain size fractions are formed either by sedimentation of carbonate and silicate particles of the respective grain size or by autochtonous disintegration of coarser sediment particles, as shown by the occurence of Mg-rich calcite and especially aragonite in the clay sized fraction. In the northern parts of the area investigated, which have very minute terrigeneous supply, the latter mechanism is the dominant factor, controlling the carbonate contents of the fine grain sized fractions. In the vicinity of the mouth of the Senegal the carbonate contents are influenced by extremely high dilution with terrigencous silicates. Mg-rich calcite and aragonite are produced preferentially in shallow slope and shelf areas up to 500 m of water depth. The specific surfaces of the carbonate-free clay fractions indicate that the clay fractions of the shelfareas with little terrigenous supply consits of relatively coarser particles. Very fine particles are removed and transported towards the deep sea. Lateral differentiation of this kind was not observed in the area off Senegal. The high surface areas, characterizing the clay fractions of this region, are thought to be due to high montmorillonite contents as was found for deep seas sediments. The mineralogical composition of the clay fraction from the southern parts of the area is characterized by high kaolinite and montmorillonite contents, while in the northern illite is predominating. At least two types of montmorillionites are present: in areas influenced by the Senegal mostly one type was found, which could swell to 17; on the shelves and slopes of the other regions the montmorillonite-group is represented by a montmorillonite-mica-type mixed-layer mineral. A "glauconite", found in the sand fraction, which had very similar properties to those of the montmorillonite-mica mixed-layer, is believed to be the source of this mixed-layer-type mineral. Palygorskite is present in all samples out of range of the Senegal supply. It may be an indicator of eolian transported material. The occurence of rich palygorskit deposits in the arid hinterlands emphasizes the terrestrial origin.

U-Pb (zircon) geochronologic analyses for rocks of the Transantarctic Mountains (Table 4.3-1)

The Byrd Glacier discontinuity us a major boundary crossing the Ross Orogen, with crystalline rocks to the north and primarily sedimentary rocks to the south. Most models for the tectonic development of the Ross Orogen in the central Transantarctic Mountains consits of two-dimensional transects across the belt, but do not adress the major longitudinal contrast at Byrd Glacier. This paper presents a tectonic model centering on the Byrd Glacier discontinuity. Rifting in the Neoproterozoic producede a crustal promontory in the craton margin to the north of Byrd Glacier. Oblique convergence of the terrane (Beardmore microcontinent) during the latest Neroproterozoic and Early Cambrian was accompanied by subduction along the craton margin of East Antarctica. New data presented herein in the support of this hypothesis are U-Pb dates of 545.7 ± 6.8 Ma and 531.0 ± 7.5 Ma on plutonic rocks from the Britannia Range, subduction stepped out, and Byrd Glacier. After docking of the terrane, subduction stepped out, and Byrd Group was deposited during the Atdabanian-Botomian across the inner margin of the terrane. Beginning in the upper Botomian, reactivation of the sutured boundaries of the terrane resulted in an outpouring of clastic sediment and folding and faulting of the Byrd Group.

(Table 1) Water column biogenic silica concentrations and Si isotopic composition during Marion Dufresne cruise MD166 off South Africa

Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial pCO2 variation. In this context, the natural silicon isotopic composition (d30Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratios in the surface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) which includes for the first time summer d30Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the subtropical zone up to the Weddell Gyre. In general, the isotopic composition of biogenic opal exported to depth was comparable to the opal leaving the mixed layer and did not seem to be affected by any diagenetic processes during settling, even if an effect of biogenic silica dissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh distillation) should satisfactorily describe the d30Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the d30Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production and export are low. This results from (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted, together with a large vertical silicic acid gradient, e.g. in the Polar Front Zone and at the Polar Front.

Path characteristics and colony arrival-departure statistics of king penguins (Aptenodytes patagonicus) on Possession and Kerguelen Island

Orientation based on visual cues can be extremely difficult in crowded bird colonies due to the presence of many individuals. We studied king penguins (Aptenodytes patagonicus) that live in dense colonies and are constantly faced with such problems. Our aims were to describe adult penguin homing paths on land and to test whether visual cues are important for their orientation in the colony. We also tested the hypothesis that older penguins should be better able to cope with limited visual cues due to their greater experience. We collected and examined GPS paths of homing penguins. In addition, we analyzed 8 months of penguin arrivals to and departures from the colony using data from an automatic identification system. We found that birds rearing chicks did not minimize their traveling time on land and did not proceed to their young (located in creches) along straight paths. Moreover, breeding birds' arrivals and departures were affected by the time of day and luminosity levels. Our data suggest that king penguins prefer to move in and out of the colony when visual cues are available. Still, they are capable of navigating even in complete darkness, and this ability seems to develop over the years, with older breeding birds more likely to move through the colony at nighttime luminosity levels. This study is the first step in unveiling the mysteries of king penguin orientation on land.

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.

Age model for ODP Leg 175 sites

Upwelling along the western coast of Africa south of the equator may be partitioned into three major areas, each having its own dynamics and history: (1) the eastern equatorial region, comprising the Congo Fan and the area of Mid-Angola; (2) the Namibia upwelling system, extending from the Walvis Ridge to Lüderitz; and (3) the Cape Province region, where upwelling is subdued. The highest nutrient contents in thermocline waters are in the northern region, the lowest in the southern one. Wind effects are at a maximum near the southern end of the Namibia upwelling system, and maximum productivity occurs near Walvis Bay, where the product between upwelling rate and nutrient content of upwelled waters is at a maximum. In the Congo/Angola region, opal tends to follow organic carbon quite closely in the Quaternary record. However, organic carbon has a strong precessional component, while opal does not. Despite relatively low opal content, sediments off Angola show the same patterns as those off the Congo; thus, they are part of the same regime. The spectrum shows nonlinear interference patterns between high- and low-latitude forcing, presumably tied to thermocline fertility and wind. On Walvis Ridge, as in the Congo-Angola region, the organic matter record behaves normally; that is, supply is high during glacial periods. In contrast, interglacial periods are favorable for opal deposition. The pattern suggests reduction in silicate content of the thermocline during glacial periods. The reversed phase (opal abundant during interglacials) persists during the entire Pleistocene and can be demonstrated deep into the Pliocene, not just on Walvis Ridge but all the way to the Oranje River and off the Cape Province. From comparison with other regions, it appears that silicate is diminished in the global thermocline, on average, whenever winds become strong enough to substantially shorten the residence time of silicate in upper waters (Walvis Hypothesis, solving the Walvis Paradox of reversed phase in opal deposition). The central discovery during Leg 175 was the documentation of a late Pliocene opal maximum for the entire Namibia upwelling system (early Matuyama Diatom Maximum [MDM]). The maximum is centered on the period between the end of the Gauss Chron and the beginning of the Olduvai Chron. A rather sharp increase in both organic matter deposition and opal deposition occurs near 3 Ma in the middle of the Gauss Chron, in association with a series of major cooling steps. As concerns organic matter, high production persists at least to 1 Ma, when there are large changes in variability, heralding subsequent pulsed production periods. From 3 to 2 Ma, organic matter and opal deposition run more or less parallel, but after 2 Ma opal goes out of phase with organic matter. Apparently, this is the point when silicate becomes limiting to opal production. Thus, the MDM conundrum is solved by linking planetary cooling to increased mixing and upwelling (ramping up to the MDM) and a general removal of silicate from the upper ocean through excess precipitation over global supply (ramping down from the MDM). The hypothesis concerning the origin of the Namibia opal acme or MDM is fundamentally the same as the Walvis Hypothesis, stating that glacial conditions result in removal of silicate from the thermocline (and quite likely from the ocean as a whole, given enough time). The Namibia opal acme, and other opal maxima in the latest Neogene in other regions of the ocean, marks the interval when a cooling ocean selectively removes the abundant silicate inherited from a warm ocean. When the excess silicate is removed, the process ceases. According to the data gathered during Leg 175, major upwelling started in the late part of the late Miocene. Presumably, this process contributed to the drawing down of carbon dioxide from the atmosphere, helping to prepare the way for Northern Hemisphere glaciation.

Geochemistry of recent sediments of the East African continental margin, Arabian Sea

Chemical analyses have been carried out on 40 samples from the sediment surface and 210 samples from cores that were taken from the edge of the African continental block at the Arabian Sea (coasts of Somalia and Kenya, from Cape Guardafui to Mombasa) on the occasion of the Indian Ocean Expedition of the German research vessel "Meteor" during the years 1964/65. The carbonate content shows its maximum on the northern part of the continental shelf of Africa, where fossil reef debris furnish the detritic portion of carbonate. In the southern part of the continental shelf of Africa the portion of carbonate is low, as it is heavily diluted by the non-carbonatic detritus. It is also in the deep-sea that a lower carbonate content is encountered below the calcite compensation depth. Trace elements in the carbonates: On the shelf and in its vicinity Sr and Mg are enriched. The enrichment has been brought about by the portion of reef debris, as this latter contains aragonite (enrichment of Sr) as well as high-magnesium calcite. The greatest part of the slope contains carbonates that are poor in trace elements and mainly made up of foraminifera (and of coccoliths). Below the carbonate compensation depth another enrichment of Mg takes place in the carbonates, which is probably due to a selective dissolution of calcite in comparison to dolomite. The iron and manganese contents of the carbonates are high (iron higher in coast proximity, manganese higher in the depth), but not genuine, as they come about in the course of the extraction of the carbonates as a result of the dissolution of authigenic Mn-Fe-minerals. Non-carbonatic portion of the sediments: In coast proximity an enrichment of quartz comes about. Within the quartz-rich zone it is the elements V, Cr, Fe, Ti, and B that have been enriched in the non-carbonatic components. This enrichment must be attributed to an elevated content of heavy minerals. In the case of Ti and Fe the preliminary enrichment brought about by processes of lateritisation on the continent plays a certain role. Toward the deep-sea an enrichment of the elements Mn Ni, Cu, and Zn takes place; these enrichments must be explained by authigenic Mn-Fe-minerals. Within the Mn-rich zone a belt running parallel to the coast stands out that shows an increased Mn-enrichment. However, this increase in enrichment does not apply to the elements Ni, Cu, and Zn. It is probable that this latter increased enrichment comes about as a result of the migration of manganese to the sediment surface. (Within the sediments there prevail reductive conditions, in the presence of which Mn is capable of migration, whereas at the sediment surface its precipitation comes about under oxidizing conditions). The quantity of organic matter mainly is dependent on grain size and on the rate of sedimentation. On the shelf an impoverishment of organic matter is to be encountered, as the sediments are coarse-grained. In the depth the impoverishment must be explained on the strength of a small rate of sedimentation. Between those two ranges organic substance is enriched. P and N show an enrichment in comparison to Corg with this applying all the more the smaller the absolute quantity of Corg is. In this particular case one has to do with an enrichment coming about during the diagenetic processes of organic matter. A comparison with the sediments from the Indian and Pakistani continental border in Arabian Sea shows as follows: on the African continental border the coarse detrital material has been transported farther out to deep-sea, which has something to do with the greater inclination of the surface of sedimentation. Carbonate is found in greater abundance on the African side. Its chemical composition is influenced by reef-debris which is missing by Indian-Pakistani side. The content of organic matter is lower on the African side. Contrary to that, the enrichments of N and P compared to organic matter are of an equal order of magnitude on both sides of the Arabian Sea.

Chlorophyll-a concentrations in phytoplankton fractions in waters of the Black Sea

Comparison of daily and diel variability of chlorophyll-a concentration at three long-term stations in meso- and eutrophic regions indicates that their values are similar. Daily patterns of deviation in chlorophyll concentration in small and large phytoplankton fraction from average daily values are presented. In conformity with a hypothesis of daily removal rhythms correlated with changes in diel light-dark periods, it was concluded that the mesotrophic region during the dark period is characterized by predominance of grazing on large phytoplankton in the upper layers and accumulation of detritus from cell fragments in the lower layer, while during the light period smaller phytoplankton predominantly grazed. The eutrophic region is characterized by predominance of grazing on small phytoplankton fraction in the upper layers during the dark period and settling out of fecal pellets containing chlorophyll into deeper depths; but during the light period, large phytoplankton predominantly grazed throughout the whole water layer.

Stable oxygen and carbon isotopes ratios of sediment core GeoB9528-3

The carbon isotopic composition of individual plant leaf waxes (a proxy for C3 vs. C4 vegetation) in a marine sediment core collected from beneath the plume of Sahara-derived dust in northwest Africa reveals three periods during the past 192,000 years when the central Sahara/Sahel contained C3 plants (likely trees), indicating substantially wetter conditions than at present. Our data suggest that variability in the strength of Atlantic meridional overturning circulation (AMOC) is a main control on vegetation distribution in central North Africa, and we note expansions of C3 vegetation during the African Humid Period (early Holocene) and within Marine Isotope Stage (MIS) 3 (approx. 50-45 ka) and MIS 5 (approx. 120-110 ka). The wet periods within MIS 3 and 5 coincide with major human migration events out of sub-Saharan Africa. Our results thus suggest that changes in AMOC influenced North African climate and, at times, contributed to amenable conditions in the central Sahara/Sahel, allowing humans to cross this otherwise inhospitable region.

Planktic foraminifera isotopes, temperature and d18O of seawater at the central Walvis Ridge for Termination II (MIS 6 to 5), sediment core 64PE-174P13

Salty and warm Indian Ocean waters enter the South Atlantic via the Agulhas leakage, south of Africa. Model simulations and proxy evidence of Agulhas leakage strengthening during glacial terminations led to the hypothesis that it was an important modulator of the Atlantic Ocean circulation. Yet, the fate of the leakage salinity and temperature anomalies remains undocumented beyond the southern tip of Africa. Downstream of the leakage, new paleoceanographic evidence from the central Walvis Ridge (southeast Atlantic) shows that salinity increased at the thermocline, and less so at the surface, during glacial termination II. Thermocline salinity change coincided with higher frequency of Agulhas rings passage at the core location and with salinity maxima in the Agulhas leakage area, suggesting that leakage waters were incorporated in the Atlantic circulation through the thermocline. Hydrographic changes at the Walvis Ridge and in the leakage area display a distinct two-step structure, with a reversal at ca. 134 ka. This matched a wet interlude within the East Asia weak monsoon interval of termination II, and a short-lived North Atlantic warming. Such concurrence points to a Bølling-Allerød-like recovery of the Atlantic circulation amidst termination II, with a northward shift of the Intertropical Convergence Zone and Southern Hemisphere westerlies, and attendant curtailment of the interocean connection south of Africa.