Arid Phase Research Articles

Source areas and transport mechanisms for freshwater and Brackish-water diatoms deposited in pelagic sediments of the equatorial atlantic

Distributions of freshwater and brackish-water diatoms from dust samples and modern sediments of the equatorial Atlantic demonstrate different transport mechanisms and source areas. Both Melosira spp. and Stephanodiscus spp. are transported via winds from the southern Sahara and Sahel in Northern Hemisphere winter. The core-top distribution of Cyclotella striata delineates the extent of the low-salinity plume formed by runoff from the Zaire River into the extreme eastern Atlantic. The transport mechanism and source areas for Melosira spp. during arid phases have not changed appreciably in the last 130,000 yr. There is no evidence for long-distance transport of freshwater diatoms by the southeast trade winds. This study confirms the validity of paleoclimatic research which inferred eolian transport of Melosira spp. in the winter dust plume from source regions north of the equator, although fluviatile input of Melosira valves into nearshore sediments cannot be ruled out.

Palaeoclimatic significance of changes in clay mineralogy across the Jurassic-Cretaceous boundary in England and France

Clay mineralogical changes across the Jurassic-Cretaceous boundary from the northern to the southern part of western Europe show similar patterns in: the mudrocks exposed on the Yorkshire Coast, the paralic sediments of the Dorset Wessex Basin, and the Jura and Vocontian Trough of S.E. France, This comprises a change from smectite-poor sediments in the Jurassic (with correspondingly abundant illite and kaolinite) to sediments with abundant smectite in the Jurassic-Cretaceous boundary beds, depleted in kaolinite. This change occurs in the Oxfordian in the south, to? latest Volgian in the north of the study area. Kaolinite reappears in abundance in the late Ryazanian or stratigraphic equivalent in all the locations studied. Comparison with other climatic indicators and other areas suggest that these variations can be ascribed to a phase of latest Jurassic-earliest Cretaceous aridity, the onset of which appears to be diachronous across W. Europe. Possible causes for the climatic change towards aridity are examined: the orographic effect of mountains or plateaus to the east of the study area is considered most likely.

The Barremian-Aptian arid phase in western Europe

Evidence is presented from nine areas in western Europe (Speeton, Yorkshire; southern North Sea; Channel and Weald Basins, southern England; Jeanne d'Arc, Lusitanian and Aquitanian Basins of the North Atlantic continental margins; Lower Saxony Basin, north Germany; the Paris Basin and southern France) for a phase of aridity which began during the Hauterivian-middle Barremian and continued to dominate the climate of the region until the mid-Aptian, although conditions during this period were not uniformly dry throughout. Criteria for recognising arid environments include the occurrence of evaporites, abundances of carbonate-rich sediments, red beds, firmgrounds, carbonate nodules and pyrite concretions, depletion of kaolinite, spore assemblages of limited morphological diversity, and the presence of numerous coniferalean and cheirolepidiacean pollen grains. The change from generally humid to dry climates during the Barremian is thought to have been linked to a lowering of sea-level. The arid phase clearly falls within the time span of the first Cretaceous Oceanic Anoxic Event.

Palyno-plankton stratigraphy and environmental changes during the Holocene in the Bengal Basin, India

Intertidal and freshwater biota have been recovered from four dated estuarine sediment sections, at Calcutta (CV type section), Dum Dum (DI), Barrackpore (BI) and Kolaghat (KI) in the Bengal Basin, India. Five regional palyno-plankton assemblage zones have been established, from four pollen diagrams. These zones are: 1. (I) Heritiera-Bruguiera-Acrostichum aureum-Fern- Ammonia, 2. (II) Avicennia- Sonneratia-Rhizophora-Ceriops-Bruguiera-Fern- Palaeocirrenalia-Ammonia, 3. (III) Ceriops-Bruguiera-Excoecaria-Avicennia-Sonneratia-Heritiera-Typha-Fern- Concentricystes rubinus (Type I), 4. (IV) Heritiera-Bruguiera-Cheno-Amaranthus-Poaceae- Typha-Fern- Concentricystes rubinus (Type I), 5. (V) Potamogeton-Heritiera-Typha-Poaceae-Fern- Gloeotrichia-Concentricystes rubinus (Type). Six environmental phases of deposition are revealed from the regional palyno-plankton assemblage zones. These are: 1. (1) Upland, arid phase characterised by “Kankar” nodule formation. 2. (2) Mixed brackish water and freshwater environment and inundation by seawater ( Ammonia transgression) from 7000 to 6500 years B.P. 3. (3) Mangrove swamp with regular inundation (6500 to 6400 years B.P.). 4. (4) Mangrove tidal flat with high precipitation (6400 to 6175 years B.P.). 5. (5) Mixed brackish water/freshwater Heritiera forest and colonisation by non-littoral species (6175 to 5000 years B.P.). 6. (6) Supratidal freshwater swamp from 5000 to 2000 years B.P. and continuing until the present. The significant environmental succession ,revealed through the palyno-plankton assemblage zones between 7000 to 2000 years B.P., indicated transgression of the sea between 7000 and 6175 years B.P., causing landward followed by seaward extension of the mangrove ecosystem in the depositional area.

Pollen analysis of late quaternary sediments: Evolution of upland savanna in Sandynallah (Nilgiris, South India)

The evolution of upland swampy savanna with luxuriant herbaceous cover in Sandynallah, in the Nilgiri hills of southern India, from the initial swampy or per-humid stage through humid, semi-humid and arid phases of dense, sparse to poor herbaceous vegetation respectively, is inferred from the pollen analytical data. The present study does not support the view that the swampy grass formations were a secondary development of degraded shola forests. The evidence presented in this paper supports a view that montane grasslands persisted in the area since ca. 30,000 yr B.P. It is probable that frost and soil factors have been the important determinants in maintaining and stabilizing the swampy upland savanna of Sandynallah since ca. 2500 yr B.P.

Eolian, fluvial and pan (playa) facies of the Tertiary Tsondab Sandstone Formation in the central Namib Desert, Namibia

The Pliocene to Recent sand sea in the central part of the 2000 km long Namib Desert is underlain widely by sediments comprising the Tsondab Sandstone Formation (TSF) of Early to Middle Tertiary age. This formation is the earliest record of terrestrial sedimentation in the central Namib Desert and formerly was thought to consist of only eolian dune and sand sheet deposits of a desert origin. However, recent field investigations indicate that the TSF also includes deposits of ephemeral watercourses and pans (playas). These fluvial and playa sediments are associated intimately with the eolian dune and sand sheet deposits, as is likewise the case in the present, extreme-arid Namib Desert. Sedimentary structures and trace fossils in the TSF eolianites resemble those found in the modern sand sea. Furthermore, these eolianites were deposited under a dominant southerly and a subordinate easterly quadrant wind regime which is similar to the present-day circulation pattern in the Namib. Since the cold Benguela Current was established only in the Late Tertiary, the TSF represents a major, ancestral arid phase in the Namib tract that was controlled by an early anticyclonic wind regime and a geographic location on the rainshadow side of southern Africa.

Fossil diatoms and the paleolimnology of Lake Rukwa, Tanzania

SUMMARY. 1. A 23.1 m sediment core from Lake Rukwa, southwest Tanzania, records 13,000 years of climatic change in five diatom assemblage zones. Zone E (13,000–12,7000 bp) reflects the transition from the waning late glacial arid phase to pluvial conditions, as indicated by the co‐occurrence of diatoms associated with fresh and relatively saline conditions. Zone D (12,700–4400 bp) represents a moist period with frequent stratification, indicated by the halophobic genera Melosira and Stephanodiscus, whieh also represent alternating silica‐rich and silica‐dcpleled waters. Beginning with Zone C (4400–3900 bp) the lake became much more saline with Thalassiosira predominating. This drying trend was reversed slightly in Zone B (3900–3300 bp). During the last three millennia (Zone A), lake salinity was further reduced, while transitory moist phases were occurring in more northerly lakes such as Victoria and Kivu.2. Lake Rukwa was relatively stable over a time scale of centuries, but on an annual basis is quite variable in salinity depending on the local rainy season.3. These results agree well with the generalized paleoclimatie chronology of East Africa and suggest that the Holocene climatic regime of East Africa extended at least to 8°S.

Holocene climatic changes over northwest India: An appraisal

Multidisciplinary studies of the climatic conditions of the present-day arid regions of the northwest Indian region indicate phases of distinct change during the recent post-glacial period. In view of the topical importance of the subject, and the sources of information being scattered throughout multidiscipli-nary research journals, an attempt has been made to summarize and present a coherent picture of the climate of the present-day desert regions of northwest India for the Holocene period. The present-day climatology and physiography of the region is presented as introductory material. The important studies which throw some light on the glacial arid phase, the wet phase, coinciding with the Harappan civilization, and the present-day arid phase are reviewed.

The Taxonomic Affinity of the New Guinean Magpie Gymnorhina Tibicen Papuana

SUMMARYBLACK, A. 1986. The taxonomic of the affinity of the New Guinean Magpie Gymnorhina tibicen papuana. Emu 86: 65–70.The New Guinean Magpie Gymnorhina tibicen papuana is distinctive in several ways, especially in the exceptionally large bill, which is both longer and deeper than in other forms. In dorsal plumage it is similar to the Western Magpie G.t. dorsalis but in other characters it resembles the long-billed form of the Black-backed Magpie G.t. longirostris of north-western Australia. Little is known of its behaviour, calls, nesting, eggs and social organization.The New Guinean and long-billed Magpies may have been derived from a common stock, which was isolated in a northern refuge during an arid phase of the Pleistocene.

The Origin and Geomorphological Significance of Closed Depressions in the Lubombo Mountains of Swaziland

In the Lubombo Mountains of Swaziland, south-east Africa, 53 closed depressions, from 50 m to 400 m in diameter and up to 5 m deep, have been identified. Frequently occurring in pairs or groups, the density of the pans reaches 0.3/km2. All of the depressions are located on remnants of the tilted African Plantation Surface. This is preserved in the region because the Jurassic rhyolites are less susceptible to weathering than are the surrounding basaltic and sedimentary rocks, which have been eroding since the Miocene. The closed depressions develop on the tuffaceous horizons within the acid volcanic sequence. They are probably the result of the local surface lowering, resulting from dissolution of plagioclase weathering products and possibly silica, in which case they are karstic. Though the presence of thick ferricretes suggests that the pans are of considerable age, possibly more than one million years old, there is no evidence of aeolian modification during the late Pleistocene arid phase.

Palaeosol development in Quaternary marine sediments and palaeoclimatic interpretations, Spencer Gulf, Australia

A series of palaeosols developed in the Quaternary marine sediments of northern Spencer Gulf when exposed during successive glacio-eustatic low sea levels over the last ca. 220,000 yrs. The palaeosols show different degrees of pedogenic development from which palaeoclimatic inferences are made. Those formed prior to the 125,000 yrs B.P. marine transgression are the most strongly developed and leached, exhibiting strong, well-structured, clay-rich B horizons, a feature not seen in later palaeosols. The younger palaeosols, formed after the late Pleistocene high sea-level phase, are not as strongly developed. Their most prominent features are carbonate accumulation horizons, even though they were, in places, exposed for as long as the pre-120,000 yrs B.P. palaeosols. Palaeoclimates inferred from the palaeosols suggest more humid conditions existed prior to ca. 125,000 yrs B.P. than have occurred since. Aeolian reworking of later palaeosols suggests arid phases occurred after ca. 125,000 yrs B.P.

Fluvial deposits of the middle Kuiseb valley, Namibia

A variety of fluvial deposits of various ages occur in the middle Kuiseb valley in the arid Namib. The characteristics, lithology, spatial and altitudinal relationships and possible chronology of these fluvial deposits are considered. A framework of events is suggested and these events are shown to have wider than local significance. It is postulated that only one major and general arid phase has occurred attendant on the establishment of the Benguela Current in the late Tertiary period. Present conditions are less arid. This conclusion is at variance with earlier chronologies: it postulates that major fluvial activity is probably mid Tertiary period at latest.

Rates of Herbage Ingestion and Turnover of Water and Sodium in Feral Swamp Buffalo, Bubalus Bubalis, in Relation to Primary Production in a Cyperaceous Swamp in Monsoonal Northern Australia.

During a monsoonal dry season, a free-living population of swamp buffalo inhabiting an ephemeral cyperaceous swamp on the floodplain of the South Alligator River had high rates of water turnover. The daily rate of consumption of Eleocharis sphacelata was c. 5.79 kg DM/animal or 49.2 plus or minus 2.77 g/l total body water-0.82. Annual herbage production in the 13.1 ha swamp was estimated at 80.43 t. Buffalo using the swamp varied from 42 in the mid-wet season and 20 in the late-wet season to c. 200 in the dry season. Herbaceous vegetation was grazed out by the end of the arid phase of the monsoonal cycle. The dependence of buffalo on water which confines them during the arid season and results in intense grazing and changes in floristic composition is discussed.

L'aridité saharienne(Saharan aridity)

The Saharan aridity results, as a whole, of the indication of very numerous parameters. The lack of one - or of some - parameters during the Quaternary, may have induced arid phases.

Late quaternary climatic changes in Australia and North Africa: A preliminary interpretation

Geological and biological data for the period 0–40,000 B.P. suggest that variations in precipitation and evaporation along the temperate and tropical margins of Australia and North Africa are closely related to variations in the position and strength of the subtropical anticyclones. Such changes in the subtropical anticyclones seem related to changes in the position and wave amplitude of the subtropical westerly jet stream. A dynamic interpretation of the inferred palaeoclimates suggests that important additional factors were the distribution of surface temperature anomalies over Siberia and the Sahara at 30,000 B.P.; the location of North Pacific and North American surface temperature anomalies at 24,000 B.P.; compression of the westerlies over Tasmania during 20,000–15,000 B.P.; breakdown of the summer monsoon during 17,000–12,000 B.P.; and a possible change in the slope and nature of the Intertropical Convergence during 11,000–7,000 B.P. Major environmental changes associated with the above influences were heavy rainfall, high lake levels, and increased fluvial activity in southern Australia and along both margins of the Sahara between 40,000 and 20,000 B.P.; low lake levels in the Afar at 30,000 B.P.; intertropical aridity and dune-building along the tropical margins of the Sahara and Australia, and desiccation in semi-arid New South Wales during the interval 17,000–12,000 B.P.; and very high lake levels and increased precipitation in the Sahel and in northern and southern Australia from 11,000 B.P. to 5,000 B.P. The past and present role of the desert anticyclones in controlling the start and the close of the arid phases is demonstrated, as is the correlation between Southern Hemisphere pressure systems and Saharan rainfall, and between Northern Hemisphere circulation changes and Australian rainfall. The subtropical anticyclones may act as buffers when the thermal balance between the two hemispheres is upset by an excess of surface ice in any one hemisphere, and this influences the climate well beyond the deserts.

Aridity in Australia: Age, origins and expression in aeolian landforms and sediments

The early evidence of aridity in Australia is provided by the contraction of large lakes and extensive faunal changes towards the end of Tertiary time. Although not well dated some indication of probable age is derived from the trends of sea-surface temperature in the Southern Ocean which played an important part in the evolution of the continental climatic pattern. Early changes probably date from the growth of the west Antarctic ice cap about 10–17 m.y. ago. By 2.5 m.y. a substantial cooling had taken place and Australia's climatic pattern probably had begun to resemble that of today. Whilst the events of Tertiary time prepared the way, the climatic oscillations of Quaternary age have been responsible for present expression and diversity of arid features recorded in the dunes, lakes and soils over large areas of the arid and semi-arid zone. Examination of events spanning the last 40 000 years provides a glimpse into the manner in which the Australian environment responded to major climatic changes of glacial and post-glacial age. Before 25 000 B.P. lake levels were generally high and desert dunes were relatively stable. By 25 000 an important change marked the onset of the last major arid phase over the southern part of the continent. Lake levels fell and increasing salinities assisted in the early construction of clay-rich dunes. The trend reached a peak in the interval 18 000–16 000 B.P. when gypsum and clay dunes were constructed on the eastern margins of lakes from Western Australia, South Australia, N.S.W. and Victoria, simultaneously with expansion of desert linear dunes. By 13 000 B.P. the stresses had relaxed; dunes became stabilized and the landscape had acquired many new features that have remained almost unchanged to the present day. The causes of major aridity are to be sought in greatly intensified atmospheric circulation aided by increased continental extent corresponding to glacial low sea levels and reduced seasonal precipitation. The correlation between glacial maxima and dune building episodes suggests the earliest arid landforms in southeastern Australia date from at least 0.3 m.y. Desert dunes were almost certainly present in Central Australia by that time and perhaps considerably earlier. The desert dust component in marine sequences, especially off the northwestern shelf may provide the best method of confirming and extending the chronology of arid depositional episodes back to and beyond the last low sea-level interval.

Pushkar: Prehistory and climatic change in Western India

Pushkar, near Ajmer, Rajasthan, lies in the dry zone to the east of the Indian desert. In the vicinity there is evidence for three arid phases, during the latter part of the Pleistocene, each repre...

Centres of dispersal and evolution in the neotropical region

Summary For the first time, centres of dispersal in the sense of De Lattin (1957) have been analysed for South and Central America. According to the results of this analysis, at least 40 centres have to be considered, the situation of which has been influenced by quaternary climatic fluctuations and vegetation fluctuations. The latest rate of differentiation of the faunal elements, which have to be associated with the centres (subspecific level) can be understood for a temporary indicator of the last function of the centres in the sense of centres of preservation of faunas and floras during regressive phases. The isolation of the montane forest centres occured before that of most of the lowland forest centres of the central and northern part of South America. During a postglacial arid phase, there has been a gene flow, via open migration roads, between numerous Amazonian Hylea‐Campo populations and populations of the costal savanna of Guiana, Venezuela and the Campo Cerrados. At the same time, these migra...

Evolution of the Indian Monsoon System and Himalayan-Tibetan Plateau uplift during the Neogene

DOI = 10.3126/hjs.v5i7.1260 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.58