INTEGRATED LANDSCAPE ANALYSES OF CHANGE OF MIOMBO WOODLAND IN TANZANIA AND ITS IMPLICATION FOR ENVIRONMENT AND HUMAN LIVELIHOOD

Landscapes bear witness to past and present natural and societal processes influencing the environment and human livelihoods. By analysing landscape change at different spatial scales over time the effects on the environment and human livelihoods of various external and internal driving forces of change can be studied. This paper presents such an analysis of miombo woodland surrounding the Mkata plains in central Tanzania. The rich natural landscape diversity of the study area in combination with its historical and political development makes it an ideal observation ground for this kind of study. The paper focuses on long‐term physical and biological changes, mainly based on satellite information but also on field studies and a review of documents and literature. The miombo woodlands are highly dynamic semi‐arid ecosystems found on a number of nutrient‐poor soil groups. Most of the woodlands are related to an old, low‐relief geomorphology of erosion surfaces with relatively deep and leached soils, or to a lesser extent also on escarpments and steep Inselberg slopes with poor soils. Each period in the past has cast its footprints on the landscape development and its potential for a sustainable future use. On a regional level there has been a continual decrease in forest area over time. Expansion of agriculture around planned villages, implemented during the 1970s, in some cases equals the loss of forest area (Mikumi‐Ulaya), whilst in other areas (Kitulangalo), the pre‐independence loss of woodland was small; the agricultural area was almost the same during the period 1975–1999, despite the fact that forests have been lost at an almost constant rate over the same period. Illegal logging and charcoal production are likely causes because of the proximity to the main highway running through the area. Contrasting to the general regional pattern are the conditions in a traditional village (Ihombwe), with low immigration of people and a maintained knowledge of the resource potential of the forest with regards to edible plants and animals. In this area the local community has control of the forest resources in a Forest Reserve, within which the woody vegetation has increased in spite of an expansion of agriculture on other types of village land. The mapping procedure has shown that factors such as access to transport and lack of local control have caused greater deforestation of certain areas than during the colonial period. Planned villages have furthermore continued to expand over forest areas well after their implementation, rapidly increasing the landscape fragmentation. One possible way to maintain landscape and biodiversity values is by the sustainable use of traditional resources, based on local knowledge of their management as illustrated by the little change observed in the traditionally used area.     

Oncolites from lacustrine sediments in the Cretaceous of north-eastern Spain

Compact micritic oncolites up to 8 cm in maximum diameter occur within Maestrichtian (Upper Cretaceous) Garumniense continental marls of Sierra del Montsec (Lerida Province) of north-eastern Spain. Synsedimentary development is documented by patches of terrigenous quartz that occur among oncolitic protuberances. Soluble nuclei (limestone fragments and bivalves) further suggest an origin through accretion, rather than that of soil pisolite. Similarities between the petrography and isotopic compositions of the oncolites and those of interbedded Garumniense limestones suggest similar sedimentary origins. However, these lacustrine oncolites, like modern counterparts described by others, probably developed through in situ metabolic precipitation of calcium carbonate. Evidence of this origin is their high degree of concentricity, which is unlikely to have developed through sedimentary accretion, inasmuch as the oncolites ‘float’ in quiet-water marls.     

Glacial deposits and Late Weichselian ice-sheet dynamics in the northeastern Baltic Sea

Glacial deposits and landforms, interpreted from the continuous seismic reflection data, have been used to reconstruct the Late Weichselian ice-sheet dynamics and the sedimentary environments in the northeastern Baltic Sea. The bedrock geology and topography played an important role in the glacial dynamics and subglacial meltwater drainage in the area. Drumlins suggest a south-southeasterly flow direction of the last ice sheet on the Ordovician Plateau. Eskers demonstrate that subglacial meltwater flow was focused mostly within bedrock valleys. The eskers have locally been overlain by a thin layer of till. Thick proximal outwash deposits occupy elongated depressions in the substratum, which often occur along the sides of esker ridges. Ice-marginal grounding-line deposit in the southern part of the area has a continuation on the adjacent Island of Saaremaa. Therefore, we assume that its formation took place during Palivere Stadial of the last deglaciation, whereas the moraine bank extending southwestward from the Serve Peninsula is tentatively correlated with the Pandivere Stadial. The wedge-shaped ice-marginal grounding-line deposit was locally fed by subglacial meltwater streams during a standstill or slight readvance of the ice margin. The thickness of the glacier at the grounding-line was estimated to reach approximately 180 m. In the western part of the area, terrace-like morphology of the ice-marginal deposit and series of small retreat moraines 10–20 km north of it suggest stepwise retreat of the ice margin. Therefore, a rather thin and mobile ice stream was probably covering the northeastern Baltic Sea during the last deglaciation.     

TEMPERATURE PROXY RECORDS COVERING THE LAST TWO MILLENNIA: A TABULAR AND VISUAL OVERVIEW

Proxy data are our only source of knowledge of temperature variability in the period prior to instrumental temperature measurements. Until recently, very few quantitative palaeotemperature records extended back a millennium or more, but the number is now increasing. Here, the first systematic survey is presented, with graphic representations, of most quantitative temperature proxy data records covering the last two millennia that have been published in the peer-reviewed literature. In total, 71 series are presented together with basic essential information on each record. This overview will hopefully assist future palaeoclimatic research by facilitating an orientation among available palaeotemperature records and thus reduce the risk of missing less well-known proxy series. The records show an amplitude between maximum and minimum temperatures during the past two millennia on centennial timescales ranging from c. 0.5 to 4°C and averaging c. 1.5–2°C for both high and low latitudes, although these variations are not always occurring synchronous. Both the Medieval Warm Period, the Little Ice Age and the 20th century warming are clearly visible in most records, whereas the Roman Warm Period and the Dark Age Cold Period are less clearly discernible.     

STORURDI: A LATE HOLOCENE ROCK-SLOPE FAILURE (STURZSTROM) IN THE JOTUNHEIMEN, SOUTHERN NORWAY

An extensive accumulation of rock debris at the mouth of Urdadalen in the Jotunheimen, southern Norway, is interpreted as the deposit of a rock-slope failure (sturzstrom). This is the first large-scale rock-slope failure to be recognised in the Jotunheimen but is unlikely to be the only one. The debris descended to the valley floor, passed across the valley axis and moved 40 m up the opposite side of the valley. Some debris passed over the lip of Urdadalen and now forms a boulder tongue that extends for c. 800 m into Utledalen. A maximum velocity of 80 m s⁻¹ for debris movement from crest of hillside to toe of boulder tongue has been estimated, and a minimum velocity of 28 m s⁻¹ was required for the debris to run up the opposite side of Urdadalen. The degree of boulder hardness/weathering as determined by Schmidt hammer suggests the sturzstrom occurred in the Late Holocene (1.825 ± 0.76 cal. ka bp ), during a period of climatic deterioration. The localised geomorphological impact of the failure event has been to dam the valley and create a small lake, to cause valley widening, to produce a north-facing hillside embayment at 1200–1400 m above sea level with the potential to collect and retain snow that might, in future, lead to niche glacier development and cirque initiation, and to provide a large volume of rock debris for entrainment and onward transport in future glacial cycles.     

NORTH ATLANTIC OSCILLATION SIGNATURES IN WESTERN IBERIAN TREE-RINGS

Exactly dated ring-width chronologies derived from Pyrenean oak and sweet chestnut trees growing in northern Extremadura, Spain, were evaluated for their potential as proxies for regional precipitation and North Atlantic Oscillation variability. The relationships among tree-rings, instrumental climatic records, and three versions of the NAO index were computed for different time subperiods over the last century. The results indicate that tree-ring records reflect, with variable intensity, both short-term and long-term variations in climate. Multiple correlation and regression analysis revealed that summer precipitation appeared to be the major factor affecting tree growth at inter-annual timescales. Moreover, since fluctuations in accumulated variability in annual rainfall over southwest Iberia are controlled by winter precipitation, the accumulated rainfall (August of the year n -1 to July of year n ) and winter NAO indices are also strongly correlated with tree-ring records at interdecadal timescales. This relationship appears to be especially strong during the second half of the 20th century, which is consistent with an increase of the NAO signal in the annual precipitation during the later part of the century. These results indicate that tree-rings from western Iberia are potential proxies of the NAO variability, useful to be included in palaeoclimatic model studies.     

GLACIATION OF MT ALLEN, STEWART ISLAND (RAKIURA): THE SOUTHERN MARGIN OF LGM GLACIATION IN NEW ZEALAND

The origin of two ridges on the eastern slopes of Mt Allen, southern Stewart Island, has remained equivocal, with differences of opinion over the exact process-mechanisms of formation. A variety of approaches was used to test a number of possible hypotheses about the origin of the ridges. These include topographic and spatial positioning, geomorphology, sedimentology and palaeoclimatological extrapolations to reconstruct two small former cirque glaciers with equilibrium line altitudes (ELAs) of c. 600 m. It would appear the two ridges reflect a glacial origin, the glaciers interpreted as forming during the Last Glacial Maximum (LGM) in New Zealand. Whilst glaciation during this time (18–19 ka) was extensive in the Southern Alps, the restricted nature of glaciation on Mt Allen suggests the low altitude restricted glaciation to niche sites on the lee side of upland areas.     

Fluid flow in sedimentary basins: model of pore water flow in a vertical fracture

Open fractures provide high-permeability pathways for fluid flow in sedimentary basins. The potential for flow along permeable or open fractures and faults depends on the continuity of flow all the way to the surface except in the case of convective flow. Upward flowing fluid cools and may cause cementation due to the prograde solubility of quartz, but in the case of carbonates such flow may cause dissolution. The rate and duration of these processes depend on the mechanisms for sustaining fluid flow into the fracture, the geometries of fracture and sedimentary beds intersected, permeability, pressure and temperature gradients. Heat loss to the adjacent sediments causes sloping isotherms which can induce non-Rayleigh convection. To analyse these problems we have used a simple model in which a single fracture acts as a pathway for vertically moving fluid and there is no fluid transport across the walls of the fracture except near its inlet and outlet. Four mechanisms for fluid flow into the lower part of the fracture are considered: decompression of pore water; compaction of intersected overpressared sediments; focusing of compaction water derived from sediments beneath the fracture; and finally focusing of pore water moving through an aquifer. Water derived from the basement is not considered here. We find that sustained flow is unlikely to have velocities much higher than 1–100 m/yr, and the flow is laminar. The temperature of the fluid expelled at the top of the fracture increases by less than 1% and the vertical temperature gradient in the fracture remains close to the geothermal gradient. Where hot water is introduced from basement fractures (hydrothermal water) during tectonic deformation, much higher velocities may be sustained in the overlying sediments, but here also this depends on the permeability near the surface. Most of the cooling of water with (ore) mineral precipitation will then occur near the surface. In most cases, pore water decompression and sediment compaction will yield only very limited pore water flux with no significant potential for cementation or heating of the sediments adjacent to the fracture. Focusing of compaction water from sediments beneath the fracture or from an intersected aquifer can yield fluxes high enough to cement an open fracture significantly but the flow must be sustained for a very long time. For velocities of 1–100 m/yr, it takes typically 0.3–30 Myr to cement a fracture by 50%. The highest velocities may be obtained when a fracture extends all the way to the surface or sea floor. When a fracture does not reach the sediment surface, the flow velocity is reduced by the displacement of water in the sediments near the top of the fracture. The flow into the fracture from the sediments may often be rate limiting rather than the flow on the fracture. Sedimentary rocks only a few metres from the fracture will receive a much lower flux than the fracture. The fracture will therefore close due to cementation before significant amounts of silica can be introduced into adjacent sandstones. The isotherm slope in the adjacent sediments will in most cases be less than 10–20°. Non-Rayleigh convection velocities in the sediments adjacent to the fracture are too small to cause any significant diagenetic reactions such as quartz cementation. These quantifications of fluid flow in fractures in sedimentary basins are important in terms of constraining models for diagenesis, heat transport and formation of ore minerals in a compaction-driven system.     

太平洋高压移动规律性的探讨

夏半年太平洋高压的平均变化,带著突变性质的增强和减弱,突然的增强发生6月到7月,而4月至6月更有逐渐减弱的趋向,到9月中旬以後则突然的减退.夏季西太平洋高压脊的东西进退和整个西风环流变化及西风环流的情况密切关连,同时太平洋高压本身也是西风环流及其变化的重要因素之一.当西风环流强时,太平洋高压比较强大,这时太平洋高压脊的变化很少,在西风环流小波动东移的过程中,太平洋高压脊只有微小的摆动.但西风带波动较大时,随着西风槽和高压脊的东移,太平洋高压脊便有了比较长时期的和明显的东退和西伸,它的周期一般都在5、6天左右.如果西风环流起了大型的变化,即西风环流由强转弱,再由弱转强,太平洋高压脊随之不断的东退和西伸,进退的范围可达30-40经距.它的周期较长,一般都在半个月的光景,最长的可达一个月,10天以内的很少.太平洋高压脊除了和西风带的环流,极地高压和低压槽有关外,也和它南边的气压系统,如台风和热带风暴有关.     

海陆分布对于大气环流的地形与热力的影响

关於海陆分布对大气环流的影响,有两派意见.一派著重於热力的影响,另一派则着重於动力的影响.对海洋而言,在冬季大陆可以看作冷源,在夏季则好似一热源.由於海洋上与大陆上受热不同的结果,冬季在西伯利亚可长期维持着一强大高压.关於冬夏季风的产生,一般也都认为是由於海陆受热不同的效果.