Dry Climatic Periods Research Articles

Holocene climatic variations and postglacial sea‐level rise geochemically recorded in the sediments of the brackish karst lake An Loch Mor, western Ireland

Palaeohydrology of the brackish karst lake An Loch Mór (Inis Oirr) was reconstructed based on its 11 600 year sedimentation record. Low calcareous early Holocene sediments, characterized by high TOC contents and deposited at generally low accumulation rates, show a gradual decrease upwards in the concentration of the siliciclastic sediment until c. 7000 cal. yr BP. From c. 9000 cal. yr BP, sediment accumulation became increasingly dominated by the deposition of autochthonous calcite and organic matter. The deposition of autochthonous calcite and its chemical composition (Ca/Sr, Ca/Mg) are determined by the subsurface inflow of freshwater from the catchment of the lake and by the balance between freshwater and seawater influx. During the early Holocene, the lake mainly received input of sea salt by sea‐spray. The sediments document the further development from seasonal towards the onset of diurnal infiltration of seawater at c. 5100 cal. yr BP. At that time, the relative sea level must have risen nearly to its present level in Galway Bay. Diurnal seawater infiltration during high tides was controlled by freshwater input. Freshwater inflow from the catchment gradually increased with human impact on local vegetation (EVPT decrease). Permanent diurnal seawater infiltration in concert with this general freshwater increase made the geochemical sedimentation record highly sensitive to changes in annual precipitation. We use information from the palynological record to interpret geochemically inferred freshwater inflow variations in terms of dry and wet climatic periods and document distinct century‐scale successions between wet and dry climatic episodes for western Ireland.

The significance of climate fluctuations for lake level changes and shifts in subsistence economy during the late Neolithic (4300–2400 b.c.) in central Europe

In the last decades, data on the economy and environment of the Neolithic period of lake dwellings (4300–2400 b.c.) in central Europe has increased considerably and also palaeoecological data on lake level fluctuations has been thoroughly elaborated. Lake shores were mainly settled during warm and rather dry climate periods which caused a fall in the lake levels. Nevertheless, there were strong and partly very short-term shifts in the economy during the lake-dwelling period. These can be recognised only because the settlement layers can be very precisely dated by dendrochronology. In this article we discuss in an interdisciplinary way the possible interrelations between climatic and economic changes. To explain the latter, we assume crop failures as the main reason, which caused intensified hunting and gathering. There are three different possibilities which might explain this: cold and wet summers, severe droughts during spring and summer, or local over-exploitation of soils in densely settled areas.

Do river profiles record along‐stream variations of low uplift rate?

Spatial variations of gradients in landscapes may be used to identify and quantify recent deformation. The problem with doing this is to determine whether tectonic or climatic forcing is responsible for these variations, especially for low uplift rate environments (≪1 mm yr−1) where climate changes may have erased tectonic features. We evaluate the respective contribution of low uplift rate (∼0.1 mm yr−1) and Pleistocene climate oscillations on gradient variations of two comparable river profiles crossing different uplift zones in the southern Upper Rhine Graben. We compare the observed points of discontinuity in river profile (knickpoints) and convex portions (knickzones) with those predicted by a detachment‐limited model that includes stochastic short‐term and cyclic long‐term variations in climate, a bedrock detachment threshold and rock uplift. The detachment‐limited model is chosen as it predicts the development of persistent knickpoints. Differing values of the shear stress exponent, erosion threshold, climate variability and uplift pattern have been checked. Our modeling suggests that climate changes had no significant effects on profiles and that anomalies are more likely due to anticline growth. This surprising result arises from the combination of a very low regional uplift rate and the detachment‐limited assumption. The detachment‐limited model implies an upstream propagation of knickpoints and knickzones generated by uplift at the outlet during dry climate periods of low erosion. The greater the uplift rate, the larger the variations in river bed elevation. Thus, for high uplift rate, knickpoints and knickzones generated by climate oscillations are more likely to hide tectonic features. This result seems counterintuitive because it suggests that tectonic knickzones will be better preserved in low uplift rate environments, provided that the lithology is homogeneous.

Reconstructing the recent dynamics of mires using a multitechnique approach

Summary 1 The recent vegetation dynamics of 16 ombrotrophic mires in southern Quebec (Canada) was studied through palaeoecological (dendrochronology, plant macrofossil and pollen analyses) and historical (aerial photograph analyses) techniques. 2 Plant macrofossil analyses show that prior to European colonization ( c . 1800), mires were predominately open environments dominated by Sphagnum mosses. Many sites subsequently became forested with little or no Sphagnum . 3 Aerial photograph analyses indicate that widespread forest expansion occurred in all mires between 1948 and 1995. The percentage of the total mire area occupied by forest increased from 22.5 to 56.5%, an overall gain of 137 ha of forest habitat. 4 A dry climatic period during the first part of the 20th century, drainage resulting from human activities, and fire events, seem to be the main causal factors of vegetation changes. 5 Forest expansion is expected to cause profound changes in bog ecosystems, such as altering hydrological conditions and organic accumulation processes, and to be detrimental to regional biodiversity and the carbon balance. 6 Overall, this study highlights the need to incorporate multiple approaches in reconstructing the vegetation dynamics of an ecosystem, and in determining causal factors of vegetation change.

Climatic change and contemporaneous land-use phases north and south of the Alps 2300 BC to 800 AD

Fluctuations in the Δ 14C curve and subsequent gaps of archaeological findings at 800–650 and 400–100 BC in western and central Europe may indicate major climate-driven land-abandonment phases. To address this hypothesis radiocarbon-dated sediments from four lakes in Switzerland were studied palynologically. Pollen analysis indicates contemporaneous phases of forest clearances and of intensified land-use at 1450–1250 BC, 650–450 BC, 50 BC–100 AD and around 700 AD. These land-use expansions coincided with periods of warm climate as recorded by the Alpine dendroclimatic and Greenland oxygen isotope records. Our results suggest that harvest yields would have increased synchronously over wide areas of central and southern Europe during periods of warm and dry climate. Combined interpretation of palaeoecological and archaeological findings suggests that higher food production led to increased human populations. Positive long-term trends in pollen values of Cerealia and Plantago lanceolata indicate that technical innovations during the Bronze and Iron Age (e.g. metal ploughs, scythes, hay production, fertilising methods) gradually increased agricultural productivity. The successful adoption of yield-increasing advances cannot be explained by climatic determinism alone. Combined with archaeological evidence, our results suggest that despite considerable cycles of spatial and demographic reorganisation (repeated land abandonments and expansions, as well as large-scale migrations and population decreases), human societies were able to shift to lower subsistence levels without dramatic ruptures in material culture. However, our data imply that human societies were not able to compensate rapidly for harvest failures when climate deteriorated. Agriculture in marginal areas was abandoned, and spontaneous reforestations took place on abandoned land south and north of the Alps. Only when the climate changed again to drier and warmer conditions did a new wide-spread phase of forest clearances and field extensions occur, allowing the reoccupation of previously abandoned areas. Spatial distribution of cereal cultivation and growth requirements of Cerealia species suggest that increases in precipitation were far more decisive in driving crop failures over central and southern Europe than temperature decreases.

Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA

Palaeoclimate records from late-Holocene sediments in Chesapeake Bay, the largest estuary in the USA, provide evidence that both decadal to centennial climate variability and European colonization had severe impacts on the watershed and estuary. Using pollen and dinoflagellate cysts as proxies for mid-Atlantic regional precipitation, estuarine salinity and dissolved oxygen (DO) during the last 2300 years, we identified four dry intervals, centred on ad 50 (P1/D1), ad 1000 (P2/D2), ad 1400 (P3) and ad 1600 (P4). Two centennial-scale events, P1/D1 and P2/D2, altered forest composition and led to increased salinity and DO levels in the estuary. Intervals P3 and P4 lasted several decades, leading to decreased production of pine pollen. Periods of dry mid Atlantic climate correspond to ‘megadroughts’ identified from tree-ring records in the southeastern and central USA. The observed mid-Atlantic climate variability may be explained by changes in atmospheric circulation resulting in longer-term, perhaps amplified, intervals of meridional flow. After European colonization in the early seventeenth century, forest clearance for agriculture, timber and urbanization altered estuarine water quality, with dinoflagellate assemblages indicating reduced DO and increased turbidity.

Changes of treelines and alpine vegetation in relation to post‐glacial climate dynamics in northern Fennoscandia based on pollen and chironomid records

AbstractPalaeoclimatic records derived from a variety of independent proxies provide evidence of post‐glacial changes of temperature and soil moisture in northern Fennoscandia. We use pollen percentage, pollen influx, stomatal and chironomid records from Toskaljavri, a high‐altitude lake in northern Finland, to assess how treelines and alpine vegetation there have responded to these climate changes. The evidence suggests that the cool, moist climate of the early Holocene supported birch forest in the area 9600 cal. yr BP onwards and that a rise of temperature triggered the immigration of pine at 8300 cal. yr BP. At 6100–4000 cal. yr BP altitudinal treeline in the area was formed by pine, in contrast to the modern situation where mountain birch reaches a higher elevation. Alpine vegetation also demonstrates clear changes. Plant communities typical of dry, oligotrophic heaths of northern Fennoscandia expanded during the dry climatic period at 7000–4000 cal. yr BP and decreased in response to cooler and moister conditions after 4000 cal. yr BP. Alpine plant communities favouring moist sites show an inverse pattern, expanding after a change towards moister climate after 4000 cal. yr BP. In a redundancy analysis (RDA), a statistically significant proportion of the variability in the total chironomid assemblages was captured by changes in the pollen types reflecting alpine vegetation typical of moist sites. Although chironomid community changes appeared to follow the major patterns in the alpine vegetation succession, the present study does not support a direct link between the changing treeline position and chironomid stratigraphy. Rather, the data indicate that the terrestrial and aquatic environments have each responded directly to the same ultimate cause, namely changing Holocene climate. Copyright © 2002 John Wiley & Sons, Ltd.

Episodic Holocene loess deposition in central Nebraska

Abstract The Mirdan Canal section in central Nebraska, USA, provides evidence for multiple episodes of Holocene loess deposition in the central Great Plains. This irrigation canal cut exposes more than 4 m of Holocene sediment that overlies the late Wisconsinan Peoria Loess. The oldest Holocene sediment is local slopewash that fills gullies, but overlying beds are interpreted as Holocene loess (Bignell Loess). Three buried soil profiles occur within the Holocene sediment, and have yielded uncalibrated 14 C ages of 3010 yr BP (uppermost buried soil) and 8790 yr BP (third buried soil from top). A fourth buried soil yielded an age of 9330 yr BP, and is correlated with the Brady Soil. The Bignell Loess has a darker color, slightly more organic matter, more clay, more coarse silt and sand, less smectite, and less volcanic glass than the underlying Peoria Loess. The Bignell Loess is most plausibly interpreted as dust from local flood plains, containing a mixture of reworked Peoria Loess and older silts with contrasting mineralogy. Episodes of loess deposition occurred in the early, middle and late Holocene, probably during periods of dry climate but possibly also as a consequence of large erosive floods that made wind-erodible sediment available on nearby flood plains.

A paleoclimate reconstruction for southwestern Texas using oxalate residue from lichen as a paleoclimate proxy

A calcium oxalate rock coating is ubiquitous on limestone surfaces inside dry rock shelters and under rock overhangs within the canyons of the southwestern Edwards Plateau in southwestern Texas. The oxalate was likely produced by epilithic lichens that #ourished in these niches during dry climate regimes. During wet climate conditions the productivity of the lichen would be severely reduced due to physiological response to moisture regime. Thus, lichen productivity and the production of calcium oxalate may have changed through time in response to wet}dry climate#uctuations. Twenty-ve AMS radiocarbon ages of rock crust samples collected from 14 sites demonstrate that oxalate was produced episodically during the middle and late Holocene. The occurrence of oxalate is correlated with periods of dry climate, whereas gaps in the record of oxalate deposition coincide with more mesic climate intervals. The results of this study demonstrate the potential for obtaining paleoclimate data from biogenic residues on rock surfaces. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.

Stable isotopic studies of dinosaur eggshells from the Nanxiong Basin, South China

Stable carbon and oxygen isotopes have been observed in dinosaur eggshell samples, identified asMacroolithus yaotunensis, collected in two sections of the Nanxiong Basin, Guangdong Province, South China. Multiple positive δ18O perturbations that occurred during the K/T transition of about 150 ka suggest that there may have been at least three periods of the extreme dry climate with a mean annual air temperature over 27δC. δ13C ranges from -8.37% to -11.60% of the dinosaur eggshells, added to the metabolic enrichment of 16%, may indicate that the dinosaurs represented by this type of eggshells probably consumed the plant species of C3 type with the δ13C values of -24.3%— -27.6‰ Moreover, the trend toward more negative δ13C values of the eggshells during the K/T transition could be an indirect consequence of a lasting increase in atmospheric CO2 concentration.

Timing of weathering-induced alteration of manganese deposits in Western Australia; evidence from K/Ar and 40 Ar/ 39 Ar dating

Under particular climatic conditions, tetravalent K-Mn oxides can be formed during terrestrial chemical weathering of Mn ores. In this study the results of K/Ar and 40 Ar/ 39 Ar dating of weathering-related K-Mn oxides (mainly cryptomelane (K.Ba)Mn 8 O 16 .xH 2 O) from supergene Mn deposits in the Pilbara and Peak Hill Mn provinces (NW Australia) and from scattered occurrences of K-Mn oxides found in deeply weathered rocks in Western Australia are presented. In ideal circumstances, K/Ar and 40 Ar/ 39 Ar ages will reflect the time since the formation of K-Mn oxides. However, 40 Ar/ 39 Ar and K/Ar dating of K-Mn oxides which formed as a result of chemical weathering may be complicated by admixture of mica inherited from bedrock and by mixing of different generations of weathering-related K-Mn oxides. To correct for admixture of inherited mica a two-stage K/Ar dating technique was applied. Some samples of dated cryptomelane had finely banded textures, and therefore might represent several generations. The age differences between these generations were investigated using microsampling and laser microprobe 40 Ar/ 39 Ar dating. Total fusion age determinations of several groups of growth bands of a late Oligocene densely layered cryptomelane from the Woodie Woodie Mn deposit (NW Australia) suggest an averaged growth rate of 0.8+ or -0.3 mm/m.y. However, the growth rate calculated for much of the band sequence is 1.5 mm/m.y., which is five times greater than for the remaining youngest group of bands (0.3 mm/m.y.), thus suggesting uneven growth rates. The low growth rate might be the result of very slow growth of cryptomelane and/or the presence of hiatuses in cryptomelane precipitation. Total fusion ages measured on void-filling banded and colloform cryptomelane in samples from a vertical profile through a mesalike deposit near Horseshoe (NW Australia) show peaks at 52 to 48, 44 to 43, 40 to 39, 35, 30 to 29, and 7 Ma. These results are interpreted as reflecting formation of weathering products in the deposit over a remarkably extended period, possibly in an episodic manner related to alternation of more humid and drier climatic periods during the Tertiary. The results from this study show some evidence of episodic formation of weathering-related manganese oxides within the deposits studied, although modification of age patterns by local hydrological and geomorphological factors allows only a limited degree of correlation of these periods on a regional scale.

Pedogenic calcite as evidence for an early Holocene dry period in the San Francisco Bay area, California

Rainfall at the site of Union City, California, during early Holocene time appears to have been about half that of today, 470 mm/yr. We base this conclusion on detailed descriptions and particle-size analyses of 12 soil profiles and 1:20 scale logs of the fluvial stratigraphy in two 100-m-long, 5-m-deep excavations dug perpendicular to the axis of an alluvial fan along the Hayward fault. Subsidence and right-lateral movement along the fault allowed an offset stream to produce a nearly continuous alluvial record documented by 35 14 C ages on detrital charcoal. Bk (calcitic) horizons in paleosols developed in the fan suggest that a relatively dry climatic period occurred from 10 to 7 ka (calendar-corrected ages). The pedogenic calcite exists primarily as vertically oriented filaments and fine, cavernous nodules formed at ped intersections. Soils and paleosols formed before 10 ka or since 7 ka did not have Bk horizons. Bk horizons that were buried suddenly at 7 ka were overlain by leached zones averaging 41 ± 3 cm thick—about half the current depth of leaching.

The Pollen Record of Empetrum Nigrum in Southern Pennine Peats: Implications for Erosion and Climate Change

1 Records of Empetrum nigrum pollen from blanket peat profiles at 18 sites in the southern Pennines, covering a variety of topographic situations, were collated. Inferences were drawn from the pollen database about climatic change over the last 3000 years and the time of onset of gully erosion of the peats. 2 Interpretations of the pollen record were based on observations of the present-day distribution of E. nigrum in British blanket mires, and on studies by surface-sample analysis of its pollen production and dispersal at a North Wales bog. Empetrum favours better-drained situations such as hummocks and gully sides, and its pollen is dispersed only over short distances. Pollen was found in quantity only at one hummock site. 3 Two episodes of high Empetrum pollen were found consistently in the peat profiles: prior to about 860 BC, and between c. AD I 100 and 1250. These episodes were interpreted as the product of extended periods of drier climate. At sites exhibiting severe gully erosion at the present day, high Empetrum pollen values have been maintained subsequent to AD 1250; sites along the middle and upper reaches of erosion gullies, however, show high values only within the last 200-250 years. 4 Because of the sensitivity of the southern Pennine bog vegetation to climatic change, the stratigraphic record of Sphagnum and Racomitrium macrofossils, in combination with the Empetrum pollen record, can be used to provide a framework of climatic change in the region over the last 3000 years.

Lacustrine and palustrine facies in the Bembridge Limestone (late Eocene, Hampshire Basin) of the Isle of Wight, southern England

The carbonate lake system represented by the late Eocene Bembridge Limestone Formation (Isle of Wight, southern England) comprises two main facies: lacustrine and palustrine . These facies are arranged vertically in transgressive/regressive cycles, corresponding to a model of carbonate lakes of low gradient and fluctuating margins with extensive palustrine fringes. These lakes developed in a coastal plain of limited relief, where two paleogeographical domains predominate. A lacustrine domain is represented in northern and eastern sections in the Isle of Wight by cycles representing upward passage from central lacustrine sub-facies (marl/marly limestone) through marginal lacustrine sub-facies (biomicrite) to sub-facies indicative of exposure (brecciated-nodular limestone). A palustrine domain, predominant in west Wight, is represented by a predominantly pedogenic palustrine facies with clotted-peloidal to ooidal textures and some intercalated laminated (laminar calcrete) horizons. Within the pedogenic facies, textures have evolved from those of unaltered biomicrites through clotted-peloidal to dense ooidal to open ooidal textures. A third domain, developed in close geographical proximity to more saline waters, is represented by gypsiferous lake-margin facies found in north Wight in which microlenticular gypsum developed post-depositionally as a result of evaporation following subaerial sediment exposure. In the lacustrine domain, there appears to be an association between cyclicity and changes in base (sea) level reflecting a likelihood that the lakes were part of a paralic, coastal plain complex with distal marine connections. Alternatively, the cyclicity might have developed as a result of the alternation of wetter and drier climatic periods.

Alternative models of vertebrate speciation in Amazonia: an overview

The main hypotheses proposed to explain barrier formation separating populations and causing the differentiation of vertebrate species in Amazonia are based on different (mostly historical) factors, as follows. (1) Changes in the distribution of land and sea or in the landscape due to tectonic movements or sea-level fluctuations (Paleogeography hypothesis). (2) The barrier effect of Amazonian rivers (River hypothesis). (3) A combination of the barrier effect of broad rivers and vegetational changes in Northern and Southern Amazonia (River-refuge hypothesis). (4) The isolation of forest blocks near areas of surface relief in the periphery of Amazonia during dry climatic periods of the Tertiary and Quaternary (Refuge theory). (5) Competitive species interactions and local species isolations in peripheral regions of Amazonia due to invasion and counterinvasion during cold/warm periods of the Pleistocene (Disturbance-vicariance hypothesis). (6) Parapatric speciation across steep environmental gradients without separation of the representative populations (Gradient hypothesis). Several of these hypotheses are probably relevant to a different degree for the speciation processes in different faunal groups or during different geological periods. The paleogeography hypothesis refers mainly to faunal differentiation during the Tertiary and in combination with the Refuge hypothesis; Milankovitch cycles leading to global climatic-vegetational changes affected the biomes of the world not only during the Pleistocene but also during the Tertiary and earlier geological periods. New geoscientific evidence for the effect of dry climatic periods in Amazonia supports the predictions of the Refuge theory.

High-resolution sequence stratigraphy of the Lower Triassic ‘Buntsandstein’ in the Netherlands and northwestern Germany

Detailed log correlations of the largely fluvio-lacustrine Lower Triassic ‘Buntsandstein’ (Late Permian-Early Anisian), carried out on 80 wells in the Dutch onshore and offshore areas, can be linked to northwest-German high-resolution sequence stratigraphy. The correlations show that cyclic sedimentation occurred in large parts of the basin. Seven 1st-order sequences are recognised, namely the Main Claystone, Rogenstein, Volpriehausen, Detfurth, Hardegsen, Solling and Lower Röt Sequences. They are overlain by the lower part of the Upper Röt–Lower Muschelkalk Sequence. Distinct sequence boundaries have been identified at the bases of four sequences: Volpriehausen, Detfurth, Solling and Upper Röt. The higher-order sequences consist of fining-upwards cycles with a thickness of up to tens of metres. The sequences are laterally persistent and have a characteristic expression on gamma-ray and sonic logs. In the Lower Buntsandstein, they display a uniform character throughout most of the area, with only minor differences in thickness or lithology. NNE-oriented lows and swells were formed during deposition of the Volpriehausen, Detfurth and Hardegsen Sequences. Uplift prior to the deposition of the Solling Sequence caused deep erosion on the swells in the basin and minor erosion in the lows. The high-resolution sequences probably represent alternating, relatively wet and dry climatic periods, with a periodicity of about 100 000 years. An analysis of the sequences suggests that their reduced thickness on the swells is mainly the effect of erosion. This is supported by analyses of the accumulation patterns and rates.

Origin of the Whewellite-Rich Rock Crust in the Lower Pecos Region of Southwest Texas and Its Significance to Paleoclimate Reconstructions

A calcium oxalate (whewellite)-rich crust occurs on exposed limestone surfaces in dry rock and open air shelters in the Lower Pecos region of southwest Texas. The crust, which also contains gypsum and clay, formed over silica-rich limestone during the Holocene. SEM and optical photomicrographs reveal similarities between whewellite microstructures and the lichenAspicilia calcarea.This desert lichen is known to produce calcium oxalate, and has been found in several sites in the region. The ubiquity of the whewellite-rich crust in the Lower Pecos shelters suggests that the lichen flourished in the past. SinceA. calcareais a desert species, the virulence of the organism likely peaked during xeric climate episodes then waned during mesic periods. Thus, radiocarbon ages of whewellite would correspond to dry climate periods experienced in the region, while periods with few or no14C data would indicate wet climate episodes. A preliminary paleoclimate reconstruction based on fourteen AMS14C dates indicates the Lower Pecos experienced dry to wet climate fluctuations during the late Holocene. This reconstruction generally agrees with other models established for Texas.

Tertiary paleoweatherings and silcretes in the southern Paris Basin

The southern border of the Paris Basin has undergone successive weathering events during the Tertiary. Fluviatile and lacustrine deposits are scattered, being mainly confined to several North-South trending grabens. These are generally devoid of fossils which would allow them to he dated. A discontinuous blanket of weathering products is the main record of this long period of continental evolution. These weathered materials result mainly from alteration of the Cretaceous flint-bearing chalk and are composed of corroded flints enclosed in brown and red mottled kaolinitic clays. In places, the succession of Cretaceous layers has been preserved in the paleoprofiles, testifying of in situ paleoprofiles, but most profiles are the result of reworking as shown by the mixture of materials coming from different stratigraphic horizons. These paleoweathered materials led to various residual and sedimentary deposits, from flint breccia along the graben scarps to flint gravels, clay deposits and lacustrine limestones in the grabens. These weathering products correspond to warm and humid climatic periods. Their reworking results from a renewal of tectonic activities combined with a period of drier climate. A later silicification process has affected the weathering cover and the Tertiary deposits of the graben as well. Extensive pedogenic silcrete crusts, with typical columnar structures, developed on the plateaux. This period of silicification corresponds to a climate with a marked dry season which led to the development of glacis. Massive silicification, preserving the primary sedimentary structures, developed in the grabens. This was sub-contemporaneous with the pedogenic silcretes which lie over them and which were developed at their expense. The significance of the massive silicification is not yet clear, nor the geochemical mechanism of their formation. In the process, primary clay minerals are destroyed and silica must have been fixed while aluminium was leached out of the profile. The silcretes are covered by lacustrine limestones and calcretes which accumulated in the lowest parts of the paleolandscape and toward the centre of the basin. They seal the silcrete development and are dated from Upper Eocene, a period of very dry climates as shown by thick gypsum deposits in the center of the basin. No remnants from Oligocene-Miocene times are preserved in the region studied. During Late Miocene, the disruption of these landscapes took place because a very low sea level led to the renewal of erosion. As a result residual landforms with siliceous crusts now cap the higher elevations.

Cyclical patterns in the Pleistocene human occupation of Southwest Tasmania

AbstractOne hundred Pleistocene radiocarbon determinations from southwest Tasmanian archaeological sites are used to construct a moving sum of determinations every 500 years within periods spanning 1000 years. The resulting plot shows a cyclical pattern with an increasing amplitude through time. The cyclical pattern can be correlated with climatic indicators from Pulbeena Swamp (northwest Tasmania) and allows us to suggest that the archaeological sites were subject to relatively low levels of occupation during periods of cold and dry climate.

Mesozoic-Tertiary weathering and soil formation on slates of the Rhenish Massif, Germany

Weathering of Paleozoic slates and sandstones of the Rhenish Massif in western Germany, which occurred during the Upper Mesozoic and Tertiary under warm, humid tropical to subtropical climatic conditions, led to the formation of a regolith up to 150 metres thick. In flat upland areas of the Hunsru¨ck and Eifel, where the tectonic uplift was relatively weak, remnants of this weathering mantle, more or less truncated by Upper Tertiary and Pleistocene erosion, are still preserved. On overlapping sections and drill cores of an autochthonous paleosoil and the saprolite underneath, the morphological, geochemical and mineralogical zonation of a complete profile of the weathering mantle has been reconstructed. A fossil pre-Oligocene Plinthitic Acrisol, covered by Oligocene fluvial sediments, displays characteristics formed under a high groundwater table. On the other hand the underlying saprolite can be subdivided into a lower reduction horizon and an upper oxidation horizon, each of them more than 40 meters thick. This indicates a polygenesis of the weathering mantle during the Tertiary, due to descending groundwater tables caused by the beginning tectonic uplift of the Rhenish Massif and drier climatic periods during the Middle Tertiary. During the Upper Oligocene the deposition of kaolinitic clays derived from removed horizons of the kaolinitic soils started in fault-block depressions. Periglacial processes during the Pleistocene led to a further removal of the weathering mantle and to the deposition of superficial layers on top of the autochthonous remnants of the saprolite. Presently the kaolinitic sediments as well as the saprolite are exploited as kaolin deposits.