Lake Desiccation Research Articles

The sources and evolution of sulfur in the hypersaline Lake Lisan (paleo-Dead Sea)

δ 34S values in gypsum are used to evaluate the fate of sulfur in the hypersaline Lake Lisan, the late Pleistocene precursor of the Dead Sea (70–14 ka BP), and applied as a paleo-limnological tracer. The Ca-chloride Lake Lisan evolved through meromictic periods characterized by precipitation of authigenic aragonite and holomictic episodes characterized by enhanced gypsum precipitation. The lake deposited two major gypsum units: the “Lower Gypsum unit” (deposited at ∼56 ka) showing δ 34S values of 18–20‰, and the “Upper Gypsum unit” (deposited at 17 ka) displaying significantly higher δ 34S values of 26–28‰. Laminated and disseminated gypsum, residing within the aragonite, exhibit δ 34S values in the range of − 26‰ to 1‰. The isotopic composition of the gypsum was dictated by freshwater sulfate input that replenished the upper layer of the lake (the mixolimnion), bacterial sulfate reduction (BSR) that occurred under the anoxic conditions of the lower brine (the monimolimnion), and mixing between these two layers. During meromictic periods, the sulfate reservoir in the lower brine was replenished by precipitation of gypsum from the upper layer, and its subsequent dissolution due to sulfate deficiency induced by BSR activity. This process describes a “sulfur pump” mechanism and its effect on δ 34S in the water can be modeled by a modified Rayleigh distillation equation. Steady state δ 34S values (∼40‰) were reached in the lower brine after long meromictic periods. Following overturn episodes, induced by diminishing freshwater input and lake level decline, large quantities of δ 34S enriched gypsum precipitated. The negative δ 34S values in laminated and disseminated gypsum provide evidence for BSR activity in the lower brine that removed isotopically depleted sulfides from the water column, causing significant isotopic enrichment of remaining sulfate. Following the lake desiccation, the sediments were exposed and the latter sulfides oxidized and re-crystallized as gypsum.

Genetic variation in the Desert Springsnail (Tryonia porrecta): implications for reproductive mode and dispersal

Allozymes and mitochondrial cytochrome c oxidase subunit I (mtCOI) sequences were analysed to determine whether populations of the western North American gastropod Tryonia porrecta (from California, Nevada, Utah, and northwest Mexico) are strongly differentiated in accordance with traditional interpretation of regional fauna as ancient relicts inhabiting isolated fragments of late Tertiary palaeodrainages. These data were also used to assess whether this species, for which males have not been recorded, is a rare example of a molluscan parthenogen. Both data sets strongly supported monophyly of T. porrecta populations. Five of the nine sampled populations consisted of a single monoallelic allozyme genotype while the others contained two to 10 distinct genotypes. Allozymic data for genetically diverse Utah populations provided evidence of clonal and sexual reproduction. mtCOI haplotypes of T. porrecta formed two subgroups which differed by 1.99-2.60%. The common haplotype was found in seven populations with rare haplotypes observed in single populations. Based on these results and an available mtCOI molecular clock for related hydrobiid snails, T. porrecta is interpreted as a primarily parthenogenetic species that undergoes occasional sexual reproduction and has accumulated substantial diversity following its mid-Pliocene to mid-Pleistocene origin. Our results also suggest that the distribution of present-day populations of these gill-breathing snails did not result from fragmentation of an ancient, well-integrated drainage but instead reflects overland colonization of habitats which only recently became available following desiccation of late Quaternary pluvial lakes.

Contributions of different land cover types in Otindag Sandy Land and Bashang area of Hebei Province to the material source of sand stormy weather in Beijing

Different land cover types in Otindag Sandy Land and Bashang area of Hebei Province are linked to a material source of sand stormy weather in Beijing based on results of field vegetation and soil survey and laboratory works. Results of grain size analysis show that dust release potential in per unit area of moving sandy land is small, while lowland meadow and meadow steppe in stony hills have high potential of releasing dusts in per unit area during sand stormy weather occurrence. Further considering the effects of vegetation, it is inferred that the moving sandy land served as a material source of past dust storm and the possibility of dust release in per unit area is low in current time. Typical steppe in stony hills is undergoing desertification and its dust release possibility in per unit area is high. Farmland has strong potential of release dusts when they are ploughed in spring, but a large amount of therophytes grow and thus prevent dusts from release when cultivation was terminated. Potentials of dust release in per unit area in fixed sand dunes, stony mountain meadow steppe and lowland meadow are low due to high cover of perennials. Sand dune reactivation and desiccation of lakes and lowlands under estimated future climatic change will make them serve as a future material source of sand stormy weather.

A 340,000 year continental climate record from tropical Africa – news from opal phytoliths from the equatorial Atlantic

Information on the tropical African continental climate and vegetation during the last 340 kyr is derived from a detailed study of opal phytoliths at the eastern equatorial Atlantic site M16772 (1°21′S, 11°58′W). Total phytoliths as well as C 3 and C 4 grass phytoliths are compared to the distribution pattern of other continental climate proxies (freshwater and limnobiontic diatoms). Total phytolith accumulation rate is dominated by C 4 grasses and their variability confirms the cold stages and interstadials as times of arid conditions in the southern Sahara and Sahel regions, and indicates an increase in aridness from Termination II to Termination I. The good agreement between the total phytoliths and freshwater diatom records observed for most of the 340 kyr is interpreted as indicative of atmospheric dust as the main source of both sediment constituents to this core site. Exceptionally higher land rainfall and river input to the eastern equatorial Atlantic are suggested by the excess of freshwater diatoms over total phytoliths and the limnobiontic diatoms observed during ice growth phases of oxygen isotopic interglacial stages 9, 7 and the 5/4 transition. Major alterations in continental aridity and/or wind strength conditions over north Africa, as reflected by the total and arid phytolith frequency spectrum, appear clearly determined by global ice volume and the 100 kyr cycle characteristic for high-latitude climate change. Limnobiontic diatoms, which reflect lake desiccation, respond also to the 41 kyr obliquity cycle. Freshwater diatoms, on the other hand, contain significant variance at 23 kyr periodicities, reflecting the precessional forcing of the African monsoons which determine the precipitation level over the south Saharan and Sahelian source regions. Rapid hydrological fluctuations, as depicted by the changes in the relative contribution of C 3 and C 4 grasses to the total phytoliths, appear to respond to higher than 19 kyr frequencies, which are observable in all but the aridity index spectra.

A mid-Holocene drought interval as evidenced by lake desiccation in the Alashan Plateau, Inner Mongolia, China

The mid-Holocene in China is traditionally thought to be a warm and humid period with a strong summer monsoon, and is often termed the Holocene Climatic Optimum or Megathermal Period. Here we present lake geomorphologic and lithological evidence from the Alashan Plateau, part of the Mongolian Plateau, that indicates strong lake desiccation during the mid-Holocene. High resolution pollen data from Zhuyeze Lake, at the present summer monsoon margin, is also presented. These data show that present lakes and wetlands in the Juyanze Lake basin west of the Badain Jaran desert, in the Zhuyeze Lake basin between the Badain Jaran and Tengger deserts, and in lakes in the eastern Tengger desert, dried or experienced low lake levels in the mid-Holocene around 5000–7000 cal yr BP. Pollen data further indicate that the vegetation cover declined in both the local areas and in the Qilian Mountains, suggesting the climate was drier than that associated with the present Asian summer monsoon. This mid-Holocene drought interval was present throughout a quite large region of the south Inner Mongolian Plateau. The period was also probably colder, at least in the high Asian plateaus and mountains.

Early Permian silt-bed fluvial sedimentation in the Orogrande basin of the Ancestral Rocky Mountains, New Mexico, USA

The Lower Permian (Wolfcampian) Abo Formation of south-central New Mexico was deposited by a silt-dominated fluvial system along the western half of the Orogrande basin a few degrees north of the equator in western Pangaea. Fluvial channel deposits consist primarily of: (1) inclined siltstone stratasets up to 4.5 m thick and 25 m wide interpreted as point bar deposits, and (2) symmetrically infilled siltstone stratasets up to 2.4 m thick and 14 m long that may represent avulsion crevasse channels. Both types of channels are dominated by climbing ripple cross-laminae and plane bed laminae, but trough cross-beds are also present, as are several types of soft-sediment deformation structures and desiccation cracks. Red silty mudstones interpreted as floodplain deposits comprise up to 70% of the formation and are interbedded with thin (<2 m) tabular siltstone beds of crevasse-splay/levee origin, as well as rare lacustrine carbonates (∼0.5 m thick) and structureless siltstones (<1 m thick) interpreted as loessites. Common pedogenic features in floodplain strata and on the tops of some fluvial channel beds include root traces, peds, and calcic nodules/tubules, whereas gypsum, gley colour mottling, and translocated clay and/or iron oxides are rare. Also present is an ichnofauna dominated by opportunistic arthropods that colonized the moist upper surfaces of recently deposited crevasse-splay/levee beds and the uppermost surfaces of fluvial channels following avulsion. The relative abundance of channel and crevasse-splay/levee deposits increases eastward in response to moderate asymmetrical subsidence of the basin, whereas calcic paleosols are more numerous on the more slowly subsiding western margin of the basin. A semi-arid to sub-humid palaeoclimate with seasonal precipitation favoured relatively deep rivers whose point bars were seasonally exposed, the formation of calcic and vertic soils, a sparse ichnofauna and flora, periodic desiccation of small carbonate lakes, and perhaps reworking of loess from the floodplain and/or upstream catchments.

Patterns and processes of Late Quaternary environmental change in a montane region of southwestern Europe

This paper examines the Late Quaternary (c. 20,300–<505 cal yr BP) environmental history of Siles, a lake situated at 1320 m in the Segura mountains of southern Spain, with the goal of establishing the mechanisms exerting control on vegetation change. Palaeoecological indicators include pollen, microcharcoal, spores of terrestrial plants, fungi, and non-siliceous algae, and other microfossils. The Siles sequence is shown to be sensitive to climatic change, although the control exerted by climate on vegetation is ultimately shaped by disturbances and species interactions, determining the occurrence of century-scale lags and threshold responses. Biotically induced changes of vegetation are also shown at the intrazonal level of variation. The new sequence is placed in the context of two previous records to postulate a picture of Holocene environmental change for the Segura region. The existence of mid-elevation glacial refugia for a number of temperate and Mediterranean trees is shown. A mid-Holocene phase (c. 7500–5200 cal yr BP) emerges regionally as the time of maximum forest development and highest lake levels. The early Holocene occurs as a generally dry, pyrophytic period of pine forests, with grassland scrub in high altitudes, and the late Holocene as a period of protracted vegetation sensitivity, with return to development of pine forests, spread of xerophytic communities, and increased fire activity, under the context of dry spells, localized anthropogenic disturbance, and shallowing and desiccation of lakes. Several events described here correlate with established times of abrupt transitions in the climates of northern Europe, the Mediterranean basin, north Africa, and the Sahel.

A multi-proxy record of Holocene climatic change in southwestern Spain: the Laguna de Medina, Cádiz

Palaeolimnological data (diatoms, ostracods, foraminifera, molluscs, aquatic pollen and lithology) from a radiocarbon dated sediment core from a saline lake, the Laguna de Medina, provide the first complete record of Holocene lake-level change for southwest Spain for the last c. 9000 cal. years. The lake has always been relatively shallow but has exhibited marked fluctuations in salinity and water depth, especially in the earlier record when oscillations culminate in maximum lake levels from c. 6960-6680 cal. BP ( c. 6070-5830 BP), indicating enhanced early to mid-Holocene humidity and a mid-Holocene humidity maximum. Prolonged shallowing thereafter reflects in part increased aridity in the later Holocene. Lake desiccation followed by a c. 800-yr phase (zone 2) of major limnological change commencing at c. 8000 cal. BP ( c. 7200 BP), and a number of other abrupt desiccation events, are also noteworthy. The mid-Holocene maximum is consistent with widespread evidence for high lake levels around 6000 BP, but the underlying climatic mechanisms are uncertain; there is some evidence it may apply predominantly to the westerly (Atlantic) Peninsula, withi earlier maxima in the east. Phases of abrupt limnological change show affinities with African data; as in African lakes, the ′zone 2′ phase appears to be a response to global change centred on c. 8.1-8.2 cal. BP. Other correlations made are tenuous, due partly to the lack of preservation in the upper record of some of the proxies used. Other desiccation events currently appear to be of more local significance, reflecting high decadal- to century-scale climatic variability throughout the Holocene.

Late Oligocene lacustrine deposition of the Sodmin formation, Abu Hammad Basin, Red Sea, Egypt: sedimentology and factors controlling palustrine carbonates

The Late Oligocene lacustrine sequence of the Sodmin Formation is exposed in the Abu Hammad Basin, 27 km west of Hamrawein. The Sodmin Formation comprises two main lithostratigraphical units that are interpreted to represent the infilling of a closed lake located within a dextral shear setting that developed in association with the earliest stage of rifting of the Red Sea. The lower unit consists of grey lime-mudstone, marl and micritic limestone intercalated towards the top with fine siliciclastics, which reflect two stages of palagolake evolution. Initially, the lake was relatively deep with steep margins, experienced occasional periodic emergence, and had lacustrine carbonates deposited. During the second stage, the lake was generally shallower, had gently sloping margins and had become a palustrine setting in which fluctuations of the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, and modification was very intense in the marginal facies. The upper unit includes dolostone and cherts, which are indicative of lake desiccation and evaporitic conditions. Evidence from geochemical, petrographical and mineralogical analyses clearly indicates a gradual shift from open, freshwater lake conditions during deposition of the lower unit to closed, saline and alkaline waters during deposition of the upper unit. Climate and topography were the main controls on vertical facies variations of the palustrine carbonates. Climate influenced episodic lake water level fluctuations, whereas topography of the basin, which changed through time in response to tectonic uplift of the lake margins, influenced water run-off as well as the frequency and length of subaerial exposure.

Environmental changes documented by sedimentation of Lake Yiema in arid China since the late Glaciation

In this study, a 6 m long core (16,000 BP) at the center of the dry Lake Yiema, a closed lake of Shiyang River drainage in Minqin Basin of the arid northwestern China, was retrieved to recover the history of climate changes and lake evolution in the area. Five radiocarbon dates on organic matter were obtained. A chronological sequence is established based on these five dates and other dates from nearby sites. Magnetic susceptibility, particle size and chemical composition were analysized for climate proxies. The proxies indicate that a drier climate prevailed in the Shiyang River drainage during the last glacial. Lake Yiema was dry and eolian sand covered most part of the lake basin. During the early and middle Holocene, a moister climate prevailed in the drainage. Climate became dry stepwise with an abrupt transition from one stage to another during the entire Holocene and became driest since about 4,200 BP. Maximum dry climate spells occurred at about 12,000-10,000 BP and after about 4,200 BP. A dry climate event also existed at about 7,600 BP. Periodical sand storms with about 400-yr cycle happened during the middle Holocene. Desiccation processes of the lake started at 4,200 BP, and were accelerated since the last 2,500 yrs by the inflow water diversion for agriculture irrigation. During the past 2,500 yrs, the lake size has been closed associated with the human population, implying that the human impact has been accelerating the lake desiccation superimposed on the natural climate deterioration.

Stratigraphic response to saline lake-level fluctuations and the origin of cyclic nonmarine evaporite deposits: The Pleistocene Blanca Lila Formation, northwest Argentina

Research Article| February 01, 1997 Stratigraphic response to saline lake-level fluctuations and the origin of cyclic nonmarine evaporite deposits: The Pleistocene Blanca Lila Formation, northwest Argentina Dirk S. Vandervoort Dirk S. Vandervoort 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar Author and Article Information Dirk S. Vandervoort 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1997) 109 (2): 210–224. https://doi.org/10.1130/0016-7606(1997)109<0210:SRTSLL>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dirk S. Vandervoort; Stratigraphic response to saline lake-level fluctuations and the origin of cyclic nonmarine evaporite deposits: The Pleistocene Blanca Lila Formation, northwest Argentina. GSA Bulletin 1997;; 109 (2): 210–224. doi: https://doi.org/10.1130/0016-7606(1997)109<0210:SRTSLL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Stratigraphic analysis of Pleistocene nonmarine bedded evaporites in northwest Argentina elucidates the effect that episodic saline lake expansion and contraction have on the stratigraphic completeness of units deposited and preserved in a continental saline environment. Stratigraphic successions in these rocks contain features that relate uniquely to fluctuations in saline lake level. Bounding the top of each succession is a surface with evidence for an abrupt increase in water depth. Because sediment accumulation and stratal preservation vary spatially as a function of lake level, the degree of stratigraphic completeness varies among locations. These results have implications for attempts to extract the record of continental climate change from saline lake strata.Bedded evaporites in these strata accumulated in a saline lake characterized by a marginal zone of gypsum and ulexite with clastic interbeds and an accumulation of halite in the saline lake center. Four internally conformable successions of genetically related lithofacies assemblages are recognized. Each succession consists of a basal sandstone that is the initial accumulation of the lake-level rise and lake highstand. Overlying this sandstone are cyclic evaporite and fine-grained clastic deposits of the lake-level fall. These units are modified by desiccation-related processes resulting from lake contraction and desiccation around the lake margins. Subsequent lake-level rise resulted in subaqueous mechanical erosion of clastics and dissolution of interbedded evaporites. In more basinward regions, dissolution of evaporites occurred prior to accumulation of lake highstand units. In the saline lake center, the lake never dried up entirely, and evidence for partial lake desiccation is equivocal. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Eolian sediments generated by anthropogenic disturbance of playas: human impacts on the geomorphic system and geomorphic impacts on the human system

In many of the Earth's arid and semiarid lands, saline lakes, playas, and similar landforms are disturbed as a result of human activity. Diversion and/or consumptive use of surface or groundwaters has created the effect of a climate change in numerous drainage basins, resulting in the desiccation of lakes and reactivation of eolian processes at many locations. Playas are natural sites for extensive eolian activity because of the deposition of clastic and chemical sediments in basins by surface water (via fluvial transport) and groundwater (via efflorescence). Wind erosion and deposition of playa sediments has had a major role in the development of landforms and sedimentary units in the present (lunette fields worldwide; Simpson Desert, Australia) and geological past, from the Triassic (Mercia Mudstone, England) to the Quaternary (Lahontan Basin and Cima Volcanic Field, USA). Anthropogenic disturbance or desiccation of playa systems has resulted in the eolian transport of sand (e.g. Lop Nor, China; Konya Basin, Turkey; Rajasthan, India; Kappakoola, Australia; several sites in West Africa) and/or dust (e.g. Aral Sea, Kazakhstan/Uzbekistan; Old Wives Lake, Canada; Kara Bogaz Gol, (ex-)USSR; Lake Texcoco, Mexico; Owens (dry) Lake, Mono Lake and other playas, USA). Typically, this is accomplished by abstraction of water and/or removal of vegetation from terminal lake basins. An extensive review of the literature documents many examples and/or potential examples of such phenomena in numerous nations. The reactivation of eolian processes from closed basins produces air pollution in the form of fugitive dust (naturally occurring compounds released into the atmosphere by human actions), and has significant environmental and economic impacts on human activities in the surrounding areas. Restoration or mitigation of degraded land on or surrounding playas has been accomplished at Lake Texcoco, Kara Bogaz Gol and the Konya Basin, and is being actively implemented at Mono Lake, Owens (dry) Lake and the Aral Sea.

Salt ramps: Wind‐induced depositional features on Tunisian playas

AbstractSemicircular and crescent‐shaped accumulations of salt crystals developed on salt crusts were measured on three Tunisian playas in September 1990. These features have been termed salt ramps. Their morphology and chemistry suggest that they are formed in the late stages of shallow ephemeral lake desiccation in playa basins. They form by salt precipitation from shallow brine lakes that are blown across salt‐encrusted playa surfaces by the wind. Moreover, they appear to be short‐lived features and their degradation is related to the flooding of playas with less saline water, and possibly rainfall and deflation.

Late Quaternary environments in Ruby Valley, Nevada

Palynological data from sediment cores from the Ruby Marshes provide a record of environmental and climatic changes over the last 40,000 yr. The modern marsh waters are fresh, but no deeper than ∼3 m. A shallow saline lake occupied this basin during the middle Wisconsin, followed by fresh and perhaps deep waters by 18,000 to 15,000 yr B.P. No sediments were recovered for the period between 15,000 and 11,000 yr B.P., possibly due to lake desiccation. By 10,800 yr B.P. a fresh-water lake was again present, and deeper-than-modern conditions lasted until 6800 yr B.P. The middle Holocene was characterized by very shallow water, and perhaps complete desiccation. The marsh system deepened after 4700 yr B.P., and fresh-water conditions persisted until modern times. Vegetation changes in Ruby Valley were more gradual than those seen in the paleolimno-logical record. Sagebrush steppe was more widespread than at present through the late Pleistocene and early Holocene, giving way somewhat to expanded shadscale vegetation between 8500 and 6800 yr B.P. Shadscale steppe contracted by 4000 yr B.P., but had greater than modern coverage until 1000 to 500 yr ago. Pinyon-juniper woodland was established in the southern Ruby Mountains by 4700 yr B.P.

Lake-level variation in the Lahontan basin for the past 50,000 years

Selected radiocarbon data on surficial materials from the Lahontan basin, Nevada and California, provide a chronology of lake-level variation for the past 50,000 yr. A moderate-sized lake connected three western Lahontan subbasins (the Smoke Creek-Black Rock Desert subbasin, the Pyramid Lake subbasin, and the Winnemucca Dry Lake subbasin) from about 45,000 to 16,500 yr B.P. Between 50,000 and 45,000 yr B.P., Walker Lake rose to its sill level in Adrian Valley and spilled to the Carson Desert subbasin. By 20,000 yr B.P., lake level in the western Lahontan subbasins had risen to about 1265 m above sea level, where it remained for 3500 yr. By 16,000 yr B.P., lake level in the western Lahontan subbasins had fallen to 1240 m. This recession appears synchronous with a desiccation of Walker Lake; however, whether the Walker Lake desiccation resulted from climate change or from diversion of the Walker River is not known. From about 15,000 to 13,500 yr B.P., lake level rapidly rose, so that Lake Lahontan was a single body of water by 14,000 yr B.P. The lake appears to have reached a maximum highstand altitude of 1330 m by 13,500 yr B.P., a condition that persisted until about 12,500 yr B.P., at which time lake level fell ≧100 m. No data exist that indicate the level of lakes in the various subbasins between 12,000 and 10,000 yr B.P. During the Holocene, the Lahontan basin was the site of shallow lakes, with many subbasins being the site of one or more periods of desiccation. The shape of the lake-level curve for the three western subbasins indicates that past changes in the hydrologic balance (and hence climate) of the Lahontan basin were large in magnitude and took place in a rapid step-like manner. The rapid changes in lake level are hypothesized to have resulted from changes in the mean position of the jet stream, as it was forced north or south by the changing size and shape of the continental ice sheet.

Influences of quaternary climatic changes on processes of soil development on desert loess deposits of the Cima volcanic field, California

Summary Soils formed in loess are evidence of both relict and buried landscapes developed on Pliocene-to-latest Pleistocene basalt flows of the Cima volcanic field in the eastern Mojave Desert, California. The characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow. Four distinct phases of soil development are recognized: phase 1 - weakly developed soils on flows less than 0.18 M.y. old; phase 2 - strongly developed soils with thick argillic horizons on 0.18 – 0.7 M.y. old flows; phase 3 - strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0.7 to 1.1 M.y. old flows; and phase 4 - degraded soils with petrocalcic rubble on Pliocene flows. A critical aspect of the development of stage 1 soils is the evolution of a vesicular A horizon which profoundly affects the infiltration characteristics of the loess parent materials. Laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development. Slight reddening of the interiors of peds from vesicular-A horizons of phase 1 soils and presence of weakly developed B horizons indicates a slight degree of in situ chemical alteration. However, clay and Fe oxide contents of these soils show that these constituents, as well as carbonates and soluble salts, are incorporated as eolian dust. In contrast to phase 1 soils, chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in-situ chemical alteration. These data, the abundant clay films, and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist. Flow-age data and soil-stratigraphic evidence also indicate that several major loess-deposition events occurred during the past ∼ 1.0 M.y. Loess events are attributed to past changes in climate, such as the Pleistocene-to-Holocene climatic change, that periodically caused regional desiccation of pluvial lakes, reduction of vegetational density, and exposure of loose, unconsolidated fine materials. During times of warmer interglacial climates, precipitation infiltrates to shallower depths than during glacial periods. Extensive, saline playas which developed in the Mojave Desert during the Holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous, nongypsiferous argillic horizons of phase 2 and 3 soils.

Effects of Holocene climatic change on some tropical geomorphic processes

Man's interference with the landscape process places him in the role of a “paraglacial” agent: He tends to duplicate ice-age stresses such as deforestation, accelerated erosion, and climate alteration. Against a background climatic control dictated by the Milankovich mechanisms, the 10,000-year history of the Holocene has seen very large secondary modulations that must be better understood so that they may be distinguished from anthropogenic effects. If they are exogenetically controlled, as it seems, then they are probably predictable from astronomic data. Four geomorphic type areas are selected for demonstrating Holocene changes in tropical regions, because they have been somewhat neglected within the framework of Quaternary science and because they include some of the most fragile and easily disrupted environments: lakes, semiarid desert margins, coastlines, and coral reefs. In a nutshell, the tropical Holocene has seen three major changes: (a) the evolution from the hyperaridity of the last pleniglacial stage to the “postglacial pluvial”; (b) the “climatic optimum,” which was highly diachronous and strongly retarded as it shifted from low latitudes to high; (c) the postoptimum “deterioration” that has involved desiccation of lakes, readvances of the deserts, fall of sea level, and truncation of coral reefs. While this deterioration is predictable in terms of an interglacial-glacial climatic transition, strong natural climatic oscillations, not yet well understood, together with man's activities, make the future a cause for concern.

Lake Formation and Desiccation in Africa

Lake Formation and Desiccation in Africa