Southwest African climate independent of Atlantic sea surface temperatures during the Younger Dryas

To investigate land–sea interactions during deglaciation, we compared proxies for continental (pollen percentages and accumulation rates) and marine conditions (dinoflagellate cyst percentages and alkenone-derived sea surface temperatures). The proxies were from published data from an AMS-radiocarbon-dated sedimentary record of core GeoB 1023-5 encompassing the past 21,000 years. The site is located at ca. 2000 m water depth just north of the Walvis Ridge and in the vicinity of the Cunene River mouth. We infer that the parallelism between increasing sea surface temperatures and a southward shift of the savanna occurred only during the earliest part of the deglaciation. After the Antarctic Cold Reversal, southeast Atlantic sea surface temperatures no longer influenced the vegetation development in the Kalahari. Stronger trade winds during the Antarctic Cold Reversal and the Younger Dryas period probably caused increased upwelling off the coast of Angola. A southward shift of the Atlantic anti-cyclone could have resulted in both stronger trade winds and reduced impact of the Westerlies on the climate of southwestern Africa.     

Orbital scale vegetation change in Africa

Palynological records of Middle and Late Pleistocene marine sediments off African shores is reviewed in order to reveal long-term patterns of vegetation change during climate cycles. Whether the transport of pollen and spores from the source areas on the continent to the ocean floor is mainly by wind or predominantly by rivers depends on the region. Despite the differences in transportation, accumulation rates in the marine sediments decline exponentially with distance to the shore. The marine sediments provide well-dated records presenting the vegetation history of the main biomes of western and southern Africa. The extent of different biomes varied with the climate changes of the glacial interglacial cycle. The Mediterranean forest area expanded during interglacials, the northern Saharan desert during glacials, and the semi-desert area in between during the transitions. In the sub-Saharan mountains ericaceous scrubland spread mainly during glacials and the mountainous forest area often increased during intermediate periods. Savannahs extended or shifted to lower latitudes during glacials. While the representation of the tropical rain forest fluctuated with summer insolation and precession, that of the subtropical biomes showed more obliquity variability or followed the pattern of glacial and interglacials.     

Connection between South Mediterranean climate and North African atmospheric circulation during the last 50,000 yr BP North Atlantic cold events

High-resolution clay mineralogical analyses were performed on sediment deposited during the last 50,000 yr in the Alboran sea (ODP Site 976). The clay mineral record is compared with pollen assemblages and with annual precipitation (Pann) and mean temperatures of the coldest month (MTCO) reconstructed with the modern analog technique (MAT). Enhanced contribution of palygorskite, a typical wind-blown clay mineral, characterizes the North Atlantic cold climatic events. Coeval development of the semi-arid vegetation (Artemisia rich) associated with a drastic fall of reconstructed precipitations and temperatures, suggest cold and arid continental conditions in the West Mediterranean area during North Atlantic cold events. The clay mineral association, especially the palygorskite content and the illite-to-kaolinite ratio, indicate western Morocco as one of the major source of the clay-size fraction during the North Atlantic cold events. The maximum abundance of Artemisia associated with the presence of Argania pollen both indicate Morocco as the main origin for pollen during these cold periods. The comparison of these pollen and clay mineral-specific features allows us to pinpoint western Morocco as the dominant source of wind-blown particles during North Atlantic cold events. These specific mineralogical composition and palynological assemblages reveal enhanced aridity over North Africa and intensification of winds favouring dust erosion and transport from North Africa toward the Alboran Sea during the North Atlantic cold events. According to atmospheric models, such a meridian transport (1) likely results from the development of strong and stable anticyclonic conditions over the tropical Atlantic and North Africa, similar to today's summer meteorological configuration and (2) implies a northward position of the westerly winds during North Atlantic cold events. Finally the synoptic situation over the West Mediterranean during the North Atlantic cold events is compared with the North Atlantic Oscillation (NAO), suggesting that during the cold Atlantic events, weather regimes over Europe and North Africa may have been systematically shifted towards a positive NAO situation.     

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.     

Physical controls on sand composition and relative durability of detrital minerals during ultra‐long distance littoral and aeolian transport (Namibia and southern Angola)

Sediment in coastal Namibia to southern Angola is supplied dominantly from the Orange River with minor additional fluvial input and negligible modifications by chemical processes, which makes this a great test case for investigating physical controls on sand texture and composition. This study monitored textural, mineralogical and geochemical variability in beach and aeolian‐dune sands along a ca 1750 km stretch of the Atlantic coast of southern Africa by using an integrated set of techniques, including image analysis, laser granulometry, optical microscopy, Raman spectroscopy and bulk‐sediment geochemistry. These results contrast with previous reports that feldspars and volcanic detritus break down during transport, that sand grains are rounded rapidly in shallow‐marine environments, and that quartzose sands may be produced by physical processes. Mechanical wear is unable to modify the relative abundance of detrital components, including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly. The sole exceptions are poorly lithified or cleaved sedimentary/metasedimentary rock fragments, readily lost at the transition to the marine environment, and slow‐settling flaky micas, winnowed and deposited offshore. Coastal sediments tend to be depleted in relatively mobile amphibole, preferentially entrained offshore or re‐deposited in sheltered beaches, while less mobile garnet is retained onshore. No detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high‐energy littoral environments from the Orange mouth to south of the Namib Erg, but all minerals get rapidly rounded after passing into the dunefield. Pyroxene and opaques get rounded faster than harder quartz and garnet, but sand mineralogy remains unchanged. Excepting strong transient selective‐entrainment effects, physical processes are unable to modify sand composition significantly. Selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra‐long‐distance transport in high‐energy environments dominated by strong persistent winds and waves.     

Variability in terrigenous sedimentation processes off northwest Africa and its relation to climate changes: Inferences from grain-size distributions of a Holocene marine sediment record

Variations in deposition of terrigenous fine sediments and their grain-size distributions from a high-resolution marine sediment record offshore northwest Africa (30°51.0′N; 10°16.1′W) document climate changes on the African continent during the Holocene. End-member grain-size distributions of the terrigenous silt fraction, which are related to fluvial and aeolian dust transport, indicate millennial-scale variability in the dominant transport processes at the investigation site off northwest Africa as well as recurring periods of dry conditions in northwest Africa during the Holocene. The terrigenous record from the subtropical North Atlantic reflects generally humid conditions before the Younger Dryas, during the early to mid-Holocene, as well as after 1.3 kyr BP. By contrast, continental runoff was reduced and arid conditions were prevalent at the beginning of the Younger Dryas and during the mid- and late Holocene. A comparison with high- and low-latitude Holocene climate records reveals a strong link between northwest African climate and Northern Hemisphere atmospheric circulation throughout the Holocene. Due to its proximal position, close to an ephemeral river system draining the Atlas Mountains as well as the adjacent Saharan desert, this detailed marine sediment record, which has a temporal resolution between 15 and 120 years, is ideally suited to enhance our understanding of ocean-continent-atmosphere interactions in African climates and the hydrological cycle of northern Africa after the last deglaciation.     

Pollen distribution in the northeast Pacific Ocean

Distributional patterns of palynomorphs in core tops from the continental margin of the northeast Pacific Ocean (30°–60°N lat 118°–150°W long) reflect the effects of fluvial and marine sedimentation as well as the distribution of terrestrial vegetation. Maximum pollen concentration (grains/cm³ of marine sediment) occurs off the mouth of the Columbia River and off San Francisco Bay (the outlet of the San Joaquin and Sacramento Rivers) and appears to be coincident with areas of high terrigenous lutite deposition. The abundance of pollen and spores in shelf sediments is extremely variable with high concentrations typical only of the finest sediments. On the slope, rise and abyssal plain, pollen concentration shows a general decrease with distance from shore. This suggests that in the northeast Pacific pollen is transported into the marine environment primarily by rivers and that, in terms of sedimentation, pollen may be regarded as part of the organic component of fine-grained lutum.Pinus, the principal pollen component of marine sediment on the northeast Pacific margin, is concentrated adjacent to the major drainage systems of areas in which pine grows. Tsuga heterophylla, Picea, and Alnus, important components of the temperate conifer forest, are concentrated off the area of their optimal development, western Washington. Quercus, Sequoia, and Compositae concentrations are greater off the southern California coast where they are prominent in the vegetation. The relative (percent) abundance of most of these pollen taxa in marine sediments reflects a positive relationship to their distribution on land. Picea and Alnus are relatively more important north of 45°N, Tsuga heterophylla between 45°–50°N, and Quercus, Sequoia, and Compositae south of 40°N. Pine percentages increase seaward, from less than 10% of the pollen sum in shelf sediments to over 50% in sediments on the abyssal plain. This seems to indicate selective transport of pine pollen. Factor analysis of pollen data from the 61 core tops results in four pollen assemblages. Three of these assemblages (Quercus-Compositae-Sequoia, Tsuga heterophylla-Pinus, and Alnus-Picea) reflect the distribution of vegetation on the adjacent continent, one (Pinus) reflects primarily the effects of marine sedimentation.     

Pericontinental petroliferous basins of the South Atlantic Ocean

Sedimentary basins located at Atlantic margins of South America and Africa (Brazil, Congo, and Angola) are composed of thick Cretaceous-Pleistocene rocks (7–9 km). The principal oil-bearing complexes of both regions are dominated by terrigenous rocks. Their accumulation took place at the rifting stage, ended with precipitation of evaporites in the Aptian at the oceanic stage of evolution. Starting with Coniacian and Campanian, the accumulation of terrigenous clastic formations in the Santos and Campos basins was governed by main phases of orogenesis in the Peruvian Andes. In the Angola and Lower Congo basins, the sedimentation was governed by movements related to the development of the East African Rift System. The formation of carbonate and carbonate-clayey sediments prevailed here at early (Albian-Late Eocene) stages of the South Atlantic opening.     

Holocene Saharan dust deposition on the Cape Verde Islands: sedimentological and Nd-Sr isotopic evidence

Holocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north-eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin-suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.     

吐哈盆地及邻区早二叠世沉积特征与构造发育的耦合关系

通过对不同露头剖面和探井资料的分析,在吐哈盆地及邻区区分出三种不同类型沉积相：（1）裂谷型海相火山喷发沉积相;（2）裂谷型陆相火山喷发沉积相;（3）造山带磨拉石相;不同沉积相特征并结合构造分析认为,早二叠世在吐哈及邻区存在二种不同类型的盆地,即前陆盆地和裂谷盆地,同时,还探讨了大陆一大陆碰撞带上两种不同类型盆地的形成机理。     

The paleoclimatic interpretation of clay minerals in sediments — a review

The application of paleoclimatic interpretation to clay minerals from continental and marine sediments is reviewed and the advantages of this method are indicated. Levels relatively rich in chlorite, illite, palygorskite and quartz are interpreted as corresponding to relatively dry periods, while more humid periods lead to more intensive weathering and consequently to the dominance of clay minerals more advanced in the relative stability scale, such as kaolinite. Smectite is taken to indicate a climate with contrasting seasons and a pronounced dry season.Using this and similar schemes, the paleoclimates of areas adjoining the Mediterranean Basin, North Sea, North Atlantic, Southern Arctic, Equatorial and North-West Pacific, and North Phillipine Sea are reconstructed.Clay minerals in sediments, particularly marine sediments, can be useful indicators of paleoclimatic conditions. While they do not produce direct indications of climatic parameters, they provide integrated records of overall climatic impact. Occasionally, they may be superior to the more conventional paleoclimatic methods, such as pollen or oxygen-isotope analysis.The limitations imposed on the paleoclimatic interpretation of clay minerals in sediments are discussed on the basis of examples. The distinction between authigenic and detrital clay minerals is shown to be problematic, particularly with regard to smectite. The relationship between climatic parameters and clay-mineral formation is complicated by the intervention of extra-climatic factors such as topography, geomorphology, lithology and time. Post-depositional diagenetic changes may not be recognized. Differentiation during transport, due to size sorting or differential flocculation may overlap climate-induced differentiation. The association of clay-mineral assemblages with specific source areas presents difficulties due to dispersal pathways that are affected by type of transport agency, i.e., water, ice or wind. Transport agencies are not constant with time. By an interdisciplinary approach, with the paleoclimatic information derived from clay minerals being correlated with that obtained from the analysis of microfossils and oxygen isotope data, many of these limitations can be overcome.     

Utility of clay Minerals in the Determination of Sedimentary Transport Patterns in the Bay of Cadiz and the Adjoining Continental Shelf (SW-Spain)

The contents and the distribution of clay minerals in modern sediments of the Cadiz bay and the continental shelf have been studied aiming to establish the sedimentary exchange model and the pathways followed by the clay fraction between the bay and the adjoining continental shelf. The most abundant clay minerals in the muddy sediments consist mainly of illite, smectite, interstratified illite-smectite, kaolinite and chlorite. The application of factor analysis method (PCA) to clay minerals data are considered to be useful in the determination of sedimentary transport patterns. The data from clay mineral assemblages, and their distribution map, make possible to establish the fine sediments transport paths in the study area using clay mineral as natural tracers. This model of transport takes in consideration the facies distribution, the supplies sources and the way of fine sediments are incorporated to the marine environment.

Two flows paths have been established: the outflows coming from Cadiz bay and the littoral zones; and the inflows coming from external marine zones of the bay. These flows are controlled by tidal currents and the morphology of the coast. The action of surge and the marine currents, specially the Atlantic Surface Water flow, are also important in the transport of fine sediments coming from sources located to the north of the study zone.     

Climate variability during the Last Glacial Maximum in eastern Australia: evidence of two stadials?

A high‐resolution, multiproxy record encompassing the last glacial–interglacial transition is presented for Native Companion Lagoon, a coastal site in subtropical eastern Australia. Rates of aeolian sedimentation in the lake were established by trace element analyses of lacustrine sediments and used as a proxy for aridity. In conjunction with sediment moisture content, charcoal and pollen these provide a multi‐decadal record of palaeoenvironmental variability for the period 33–18 k cal. yr BP. Results indicate that the Last Glacial Maximum in eastern Australia spanned almost 10 k cal. yr, and was characterised by two distinct cold dry events at approximately 30.8 k cal. yr BP and 21.7 k cal. yr BP. Provenance of selected sediment samples by trace element geochemical fingerprinting shows that continental sourced aeolian sediments originated primarily from South Australia during these cold events and from sites in central Australia during the intervening time. Used in combination with a pollen record, the provenance of long‐travelled dust to mainland sites shows that the two cold events were characterised by frequent meridional dry southwesterly winds rather than zonal westerly airflow as previously believed. The intervening period was cool and humid, which we infer as being associated with more frequent southeasterly winds of maritime origin. These results lend support to previous research that indicates the Southern Hemisphere experienced a period of widespread climatic amelioration at the height of the last glacial known as the Antarctic Isotopic Maximum. Copyright © 2008 John Wiley & Sons, Ltd.     

Did Paleogene North Atlantic rift-related eruptions drive early Eocene climate cooling?

The delivery of volcanogenic sulphur into the upper atmosphere by explosive eruptions is known to cause significant temporary climate cooling. Therefore, phreatomagmatic and phreatoplinian eruptions occurring during the final rifting stages of active flood basalt provinces provide a potent mechanism for triggering climate change.

During the early Eocene, the northeast Atlantic margin was subjected to repeated ashfall for 0.5 m.y. This was the result of extensive phreatomagmatic activity along 3000 km of the opening northeast Atlantic rift. These widespread, predominantly basaltic ashes are now preserved in marine sediments of the Balder Formation and its equivalents, and occur over an area extending from the Faroe Islands to Denmark and southern England. These ash-bearing sediments also contain pollen and spore floras derived from low diversity forests that grew in cooler, drier climates than were experienced either before or after these highly explosive eruptions. In addition, coeval plant macrofossil evidence from the Bighorn Basin, Wyoming, USA, also shows a comparable pattern of vegetation change. The coincidence of the ashes and cooler climate pollen and spore floras in northwest Europe identifies volcanism as the primary cause of climate cooling. Estimates show that whilst relatively few phreatomagmatic eruptive centres along the 3000 km opening rift system could readily generate 0.5–1 °C cooling, on an annual basis, only persistent or repeated volcanic phases would have been able to achieve the long-term cooling effect observed in the floral record. We propose that the cumulative effect of repeated Balder Formation eruptions initiated a biodiversity crisis in the northeast Atlantic margin forests. Only the decline of this persistent volcanic activity, and the subsequent climatic warming at the start of the Eocene Thermal Maximum allowed the growth of subtropical forests to develop across the region.     

Lateglacial and Holocene terrestrial and marine proxies reflecting climate changes in the Malangen fjord area, Norway, northeast North Atlantic

A high-resolution Younger Dryas–late Holocene record of climate and environment from the Malangen fjord has been established on the basis of two marine sediment cores. Five pollen-spore assemblage zones have been defined covering the period c . 11 500 cal. yr BP (10 200 ¹⁴C yr BP) to c . 1600 cal. yr BP (1600 ¹⁴C yr BP) with a hiatus of c . 2000 cal. years between c . 10 200 and 8100 cal. yr BP (9000 and 7300 ¹⁴C yr BP). The Holocene vegetation development from pioneer vegetation to forest development, identified in the marine pollen record, correlates well with pollen records from terrestrial sections of northern Norway. The marine pollen record was also correlated directly with marine proxy records of the bottom water temperature investigated in the same sediment cores. Correlation between the marine and terrestrial proxies suggests that changes in the influx of warm Atlantic Water to the fjord led to an instant change in the vegetation of the surrounding land area. The results thus support a strong link between marine and atmospheric mean climatic states in the North Atlantic region throughout the Holocene.     

Tropical salt marsh succession as sea-level indicator during Heinrich events

Centennial–millennial dynamics of tropical salt marsh vegetation are documented in the pollen record from marine core MD03-2622, Cariaco Basin, Venezuela, which spans the glacial period between 63 and 29 ka. Five rapid and abrupt expansions of salt marsh vegetation are linked with North Atlantic Heinrich events (HEs). Within each event, a recurrent pattern – starting with species of Chenopodiaceae, followed by grasses, and subsequently by Cyperaceae species – suggests a successional process that is determined by the close relationship between sea-level and community dynamics. The salt tolerant Chenopodiaceae, at the base of each sequence, indicate hypersaline intertidal environments, which were most likely promoted by extremely dry atmospheric conditions. Rapid sea-level rise characterizes the onset of HE stadials, causing erosion of marsh sediments, and continued recruitment of pioneer species (Chenopodiaceae), which are the only ones capable of tolerating the rapid rate of disturbance. Once, as sea-level drops or as rise decelerates, marsh plants are able to trap and stabilize sediments, favouring the establishment of more competitive species (graminoids). The increment of marsh height as a result of autochthonous sediment accumulation reduces the extent of hypersaline environments, and allows the establishment of mesohaline species. These results add to the scarce knowledge on tropical salt marsh ecosystems, and provide independent paleoclimatic evidence on sea-level changes occurring simultaneously with Antarctica climate variations.     

安哥拉下刚果盆地吉拉索尔深水油田

吉拉索尔油田位于西非安哥拉17号深水区块,水深1200～1400m,距安哥拉海岸150km。1996年发现,2001年产油,是安哥拉投入开发的第一个深水油田。油田面积约140km2,地质储量约为15.5×108bbl,可采储量约7.25×108bbl,为轻质油。油田所在的下刚果盆地主要经历了裂谷期、过渡期和热沉降期三个阶段的构造演化,相应沉积了裂谷期陆相地层、过渡期蒸发岩层和热沉降期海相地层。主要烃源岩为下白垩统Bucomazi组湖相页岩,有机质丰度高,类型好,成熟度适中,为很好的烃源岩。盆地深水区主要储层为上渐新统—中新统河道充填浊积砂岩复合体,砂岩固结差,物性良好。吉拉索尔油田主要由几个河道浊积砂岩复合体叠置组成,其中夹几个薄层席状砂岩,复合体之间被海相泥页岩分隔。上覆第三系泥岩为其主要盖层。     

Carbonate aeolianites of western Saurashtra,India: experimental decipherment of the depositional mechanisms

Granular carbonate deposits of Late Pleistocene to Early Holocene age, commonly referred to as ‘miliolite limestone’, occur in a linear belt, parallel to the southern coast of Saurashtra, India. In the present study area these carbonate deposits are found in select valleys between ridges and mounds of pyroclastic material present in the Deccan trap plateau. Two different depositional histories have been proposed for these sediments. The presence of marine bioclasts led to the postulation of a marine origin for these deposits. The second school of thought propounded redeposition of the coastal sediments by aeolian processes. Although a few features could not be explained by the proposed aeolian model, critical comparison of these two views favoured the aeolian origin. The mode of occurrence, lithological and structural attributes, and microscopic evidence presented here, also support a possible aeolian origin for these deposits. Experimental observation indicates that these carbonate aeolianites represent backflow deposits, which accumulated because of the flow separation caused by the presence of topographic highs. The conspicuous concave‐up geometry of the deposit conformed to the shape of the separation bulb. In view of the inferred depositional mechanism, the disposition of the deposits and the signature of the palaeoflow direction suggest that the carbonate particles were derived from the north‐western coast of Saurashtra by strong south‐easterly winds. Massive granular carbonates with outsized basement clasts appear to be the product of avalanching of granular material from the higher contours because of oversteepening of the primary deposit.     

Rapid vegetation change during the early Holocene in the Faroe Islands detected in terrestrial and aquatic ecosystems

High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ¹⁸O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd.