Flamingos,salt lakes and volcanoes: hunting for evidence of past climate change on the high Altiplano of Bolivia

Hidden within the vast Bolivian Altiplano are archives of past climate change in the form of remarkable carbonate rocks surrounding lakes long since disappeared. Beyond the Salar de Uyuni, the largest salt flat in the world, lies a relatively untouched realm of volcanoes and salt lakes. Ancient shorelines from intervals in the Altiplano history, when large lakes were more abundant, may hold important information about a time when the climate in this region was punctuated by much wetter phases before present day aridity took a hold. Previous studies in this region have reconstructed robust chronological timelines for such events and highlight two large lake phases over the last 18 thousand years (the Tauca and Coipasa lake phases); however higher resolution climate data are scarce. Current studies on climate proxies from smaller lakes in southern Bolivia may shed light on some of these higher resolution climate events including El Niño–Southern Oscillation (ENSO) events. Laminated tufa found around the palaeoshorelines of the West Lípez Lakes is one such proxy, and can be analysed to investigate the potential roles of annual versus shorter‐term climatic variation in the evolving Altiplano climate at the time.     

Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.     

Late Pleistocene climate and water budget of the south American Altiplano

Geomorphic evidence from the Peruvian-Bolivian Altiplano indicates that during episodes prior to 28,000 and around 12,500-11,000 yr B.P., lakes covered an area about six and four times as large as at present, respectively. Within the constraints of the heat and water budget, model calculations are used to estimate the precipitation rate that would allow hydrologic equilibrium. On this basis it is suggested that rainfall on the Altiplano during the episodes of enlarged lakes was, respectively, some 300 and 200 mm annum⁻¹ larger than at present, representing increases of about 75 and 50%, respectively. Field evidence suggests that the episodes of enlarged lakes on the Altiplano may have preceded or coincided with periods of maximum glaciation in the neigh-boring Andes. In this region with high elevation of the ice equilibrium line, increased precipitation is particularly conducive to glaciation.     

Isotopic tracers of paleohydrologic change in large lakes of the Bolivian Altiplano

We have developed an ⁸⁷Sr/⁸⁶Sr, ²³⁴U/²³⁸U, and δ¹⁸O data set from carbonates associated with late Quaternary paleolake cycles on the southern Bolivian Altiplano as a tool for tracking and understanding the causes of lake-level fluctuations. Distinctive groupings of ⁸⁷Sr/⁸⁶Sr ratios are observed. Ratios are highest for the Ouki lake cycle (120-95 ka) at 0.70932, lowest for Coipasa lake cycle (12.8-11.4 ka) at 0.70853, and intermediate at 0.70881 to 0.70884 for the Salinas (95-80 ka), Inca Huasi (~ 45 ka), Sajsi (24-20.5 ka), and Tauca (18.1-14.1 ka) lake cycles. These Sr ratios reflect variable contributions from the eastern and western Cordilleras. The Laca hydrologic divide exerts a primary influence on modern and paleolake ⁸⁷Sr/⁸⁶Sr ratios; waters show higher ⁸⁷Sr/⁸⁶Sr ratios north of this divide. Most lake cycles were sustained by slightly more rainfall north of this divide but with minimal input from Lake Titicaca. The Coipasa lake cycle appears to have been sustained mainly by rainfall south of this divide. In contrast, the Ouki lake cycle was an expansive lake, deepest in the northern (Poópo) basin, and spilling southward. These results indicate that regional variability in central Andean wet events can be reconstructed using geochemical patterns from this lake system.     

Bathymetry and sediments on the carbonate platform off western India: Significance of Halimeda bioherms in carbonate sedimentation

Bathymetry across the carbonate platform off western India indicated small-size pinnacles and their lateral coalescence into 2 -6-m high mounds landward, and linear elongated carbonate ridges and troughs, mounds and banks up to a height of 20-m seaward of the platform. Seismic data indicated that these mounds were transparent with no rigid internal structure and can be defined as bioherms. The sediments were abundantly aragonite faecal pellets, Halimeda grains and ooids and their radiocarbon ages ranged from 11 to 7.5 ka BP. It appears that the growth of Halimeda bioherms on the platform was facilitated by intense upwelling during the early Holocene. The terrigenous sediments brought by rivers were deposited in the inner shelf and have not affected the growth of bioherms. It is estimated that the platform comprises at least 1.85 Gt of mass CaCO\(_3\) accumulated during the early Holocene and comparable to those on the Great Barrier Reef. Halimeda bioherms produce abundant carbonate sediments and their growth period represents a geological carbonate sink and release of high CO\(_2\) to the atmosphere. Detailed shallow seismic studies and sediment cores are needed to quantify the exact mass content of CaCO\(_3\) and model climate change during the early Holocene.     

Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia

The central trough of the Bolivian Altiplano is occupied by two wide salt crusts: the salar of Uyuni, which is probably the largest salt pan in the world (10,000 km²) and the salar of Coipasa (2,500 km²). Both crusts are essentially made of porous halite filled with an interstitial brine very rich in Li, K, Mg, B (up to 4.7 g/l Li, 4.3 g/l B, 30 g/l K and 75 g/l Mg). Lithium reserves are the highest known in the world, around 9 × 10⁶ tons. Potassium, magnesium and boron reserves in brines are also important (around 194 × 10⁶ tons K, 8 × 10⁶ tons B and 211 × 10⁶ tons Mg).

The crusts are the remnant of saline Lake Tauca (13,000–10,000 yr BP). Its salinity was estimated approximately at 80 g/l. Its paleochemistry was derived in two ways: (1) by dissolving the present amounts of all chemical components in the former lake volume, and (2) by simulating the evaporation of the major inflows to the basin. The resulting chemical compositions are quite different. The dissolution-derived one is 5 to 50 times less concentrated in Li, K, Mg, B than the evaporation-simulated ones. However all compositions present the same Na and Cl contents. This suggests either a removal of bittern salts or an enrichment of the former lake water in Na and Cl.

The most probable interpretation is that Lake Tauca redissolved a salt crust akin to that existing today. Several older lakes have been detected on the Altiplano. Nevertheless, such an explanation only pushes the problem back. It is likely that the anomaly was transferred from one lake to an other. Three hypotheses may be put forward: (1) bittern seepage through bottom sediments, (2) uptake of the missing components by minerals, and (3) leaching of ancient evaporites from the catchment area at the beginning of the lacustrine history of the basin. The excess halite could have been recycled from lake to lake. This latter process seems to be the most effective to explain the large excess of Na and Cl over the bittern solutes — Li, K, Mg and B. The occurrence of almost pure Na/1bCl saline springs flowing out from a gypsum diapir in the northern Altiplano gives substantial support to this hypothesis.     

Uranium in saline lakes of Northwestern Mongolia

Analysis of major- and trace-element compositions of water in hypersaline soda closed basin lakes of Northwestern Mongolia and Chuya basin (Gorny Altai) shows high enrichment in ²³⁸U (up to 1 mg/l). Proceeding from new data, uranium accumulation in water has been attributed to (i) location of the lakes and their watersheds in potential provinces of U-bearing rocks and (ii) uranium complexing with carbonate in presence of carbonate (bicarbonate) anions. Among the explored hypersaline soda lakes of the area, the greatest uranium resources are stored in Lake Hyargas Nuur (about 6000 ton).     

Late Pleistocene paleoclimatology of the central equatorial Pacific: A quantitative record of eolian and carbonate deposition

Detailed records of δ¹⁸O, δ¹³C, percentage and mass accumulation rate of CaCO₃, and eolian percentage, mass accumulation rate, and grainsize generated for core RC11-210 from the equatorial Pacific reveal the timing of paleoclimatic events over the past 950,000 yr. The CaCO₃ percentage record shows the standard Pacific correlation of high CaCO₃ content with glacial periods, but displays a marked change of character about 490,000 yr ago with older stages showing much less variability. The carbonate mass flux record, however, does not show such a noticeable change. Sedimentation rates vary from about 0.5 to 3.0 cm/1000 yr and, during the past 490,000 yr, sections with enhanced sedimentation rates correspond to periods of high CaCO₃ percentage. Eolian mass accumulation rates, an indication of the aridity of the source region, are usually higher during glacial times. Eolian grainsize, an indication of the intensity of atmospheric circulation, generally fluctuates at a higher frequency than the 100,000-yr glacial cycle. The mid-Brunhes climatic event centered at 300,000 yr ago appears as a 50,000-yr interval of low intensity and reduced variability of atmospheric circulation. Furthermore, the nature of this entire record changes then, with the younger portion indicating less variation in wind intensity than the older part of the record. The late Matuyama increase in amplitude of paleoclimatic signals begins 875,000 yr ago in the eolian record, 25,000 yr before the δ¹⁸O and CaCO₃ percentage amplitude increases about 850,000 yr ago.     

Evidence of Late Pleistocene ice-dammed lakes in West Siberia

The article discusses geological data on proglacial lakes and spillways in the West Siberian Plain, data on crucial features of the Late Pleistocene reorganization of the drainage pattern of northern Eurasia. The discussion focuses on Late Pleistocene sediments along the margin of the last ice sheet and south of it, including new data recently obtained by the Russian-Norwegian project PECHORA in Trans-Uralia. Based on these data, the margin of the last ice sheet in the western and central parts of West Siberia is localized well above the Arctic Circle, i.e. 150-250 km north of the previously suggested ice limit. The available geochronological evidence indicates that the last ice dam across West Siberia, which diverted the great Siberian rivers to the south, appeared at early stages of the last, Weichselian ice age. The normal, northbound, drainage was restored later, within the time-span accessible to radiocarbon dating, when two pre-Holocene river terraces with mammal fauna were formed. The Late Weichselian was the driest period with ubiquitous aeolian activity and an absence of large water bodies. Preceding ice-dammed lakes of West Siberia could only drain through the Turgai valley which leads southward into the Aral and Caspian seas. The sedimentary sequence of this passage consists of lacustrine clay, diamictic gravity flows and aeolian sediments younger than 29 kyr which infilled the former spillway mainly in the Late Weichselian. The basal sand and gravel mantling the bedrock floor, which descends from 55 m a.s.l. at 55°N to 30-40 m a.s.l. in the south, is the only signature of a southward drainage. This fluvial episode probably reflects overflow of a Siberian proglacial lake whose water level could reach 60 m a.s.l. prior to 29 kyr BP.     

Origin and diagenesis of lacustrine algal bioherms at the margin of the Ries crater, Upper Miocene, southern Germany

The marginal carbonate facies of the Miocene Ries meteorite crater lake in southern Germany contain bioherms up to 7 m high and 15 m across built by the green alga Cladophorites. The algae were externally encrusted during life by micrite, probably precipitated in response to photosynthetic uptake of CO₂, which produced tufts of fine (100 μm diameter), calcareous tubes. Coalescence of tufts, together with incorporation of peloidal and skeletal sand, created nodules and cones of algal tufa which in turn formed larger masses some of which are in the form of compound cones up to 2 m high. The bioherms are constructed by beds and groups of these cones and masses, and are surrounded by poorly cemented peloid, ostracod and gastropod sands. Five depositional and diagenetic stages of development can be distinguished: (1) growth and calcification (probably calcitic) of Cladophorites in shallow fresh- or slightly brackish water; (2) emergence due to a temporary fall in lake level and veneering of the algal tufa and adjacent sediments by laminated sinter; (3) resubmergence and deposition of peloidal and skeletal sands; (4) burial and partial phreatic dolomitization, together with dissolution of aragonite and penecontemporaneous deposition of thin isopachous rims of dolomite rhombs; (5) local vadose cementation by rhombs, spar and spikes of low-magnesian calcite. Stages 1–3 probably occurred several times. Most of the biohermal and surrounding sediments were produced by biological processes in the lake; subaerial sinter deposition and meteoric cementation have contributed relatively minor amounts of material and the majority of the sediments retain porosities of 10–30%.     

Pleistocene stratigraphy in the Dellen region, central Sweden

Robertsson, A.-M., Svedlund, J.-O., Andrén, T. & Sundh, M. 1997 (September): Pleistocene stratigraphy in the Dellen region, central Sweden. Boreas, Vol. 26, pp. 237–260. Oslo. ISSN 0300–9483. The Pleistocene stratigraphy in the Dellen region, central Sweden was studied using field observations made during mapping of Quaternary deposits and fabric analyses in excavated sections. The lithostratigraphy was also studied by seismic refraction measurements, analyses of grain-size distribution and organic carbon content. Biostratigraphical methods applied were pollen and diatom analyses. A general outline of the Pleistocene stratigraphy in the area is presented. Three different till beds are identified, the lowermost suggested to have been deposited during the Saalian glaciation and the other two during the Weichselian glaciation. According to the interpretation of the stratigraphy, it is questioned whether the first Weichselian ice sheet did in fact reach the Dellen area. A clayey sediment sequence at Norra Sannas accumulated during an interglacial, probably the Eemian. Most of the interglacial vegetation succession is reflected in the identified pollen flora. An initial phase with a light-demanding forest of Belula and Pinus was followed by immigration of Alnus, Picea and scattered occurrences of Corylus. A freshwater diatom flora was identified dominated by plankton taxa, e.g. Aulacoseira italica, A. distans and Cyclotella spp. In the lower part of the sequence a brackish-marine flora was registered, representing accumulation in a bay of the Eemian Sea. Fine-grained sediments at the Sundson and Vastansjd sites are interpreted as rebedded Eemian sediments according to the pollen flora. An (Early Weichselian) interstadial age is suggested for sediments found at Bjuraker. Dating by the ¹⁴C- and OSL methods was carried out on the interglacial and interstadial sediments, respectively. The ages range from approximately 19000 to 92000 BP. Correlation of interglacial vegetation history with central Finland and other areas is discussed.     

Pleistocene succession of the central interior United States

The Quaternary of the continental interior of the United States is characterized by deposits from glacial ice, with associated outwash and eolian deposits, and by alluvial deposits produced by the same climatic pulses. Erosional incision of valleys occurred early in the glacial pulse, outwash deposition during the waning phase of the pulse, and soil formation during times of relative stability between the glacial pulses. These features of deposition, erosion, and soil formation are presented in a series of curves. One way the marine record could be correlated with that of the continental interior is to compare and match the physical records of both environments.     

Magnetic polarity stratigraphy of the middle Pleistocene (Ensenadan) Tarija Formation of southern Bolivia

The Tarija Formation of southern Bolivia, which is well known for its classic vertebrate faunas, is of prime importance in understanding of the chronology of the Ensenadan Land Mammal Age. This formation consists of well-exposed and relatively fossiliferous sections of clays, clayey silts, sands, gravels, and tuffs which were deposited in a predominately fluviatile regime in a Pleistocene structural basin. Four stratigraphic sections, each measuring 110 m or less, were studied to establish a magnetic polarity stratigraphy. Paleomagnetic samples were collected from the finer-grained sediments at 100 sites spaced at stratigraphic intervals of 5 m or less. All paleomagnetic specimens were demagnetized in alternating fields of least 250 oersteds (oe). Some specimens were also thermally demagnetized at 200°C or more. Of the 100 sites, 77 were ultimately used to determine the magnetic polarity zonation. Based on the four sections sampled, the Tarija Formation spans a time interval from about 1 my to about 0.7 my B.P. or perhaps younger. The lower half of the composite section is of reversed polarity punctuated by a short normal event. This sequence probably represents the late Matuyama chron with the Jaramillo subchron. The upper part of the section is of normal polarity and represents early Brunhes time. A tuffaceous unit 43 m above the Brunhes-Matuyama boundary yielded a fission track (zircon) age of 0.7 ± 0.2 by B.P. These data indicate that the classic Tarija fauna is middle Pleistocene Ensendan in age.     

Solubility of plutonium(VI) carbonate in saline solutions

Among the plutonium oxidation states found to form in the environment, mobile plutonium(VI) can exist under oxidizing conditions and in waters with high chloride content due to radiolysis effects. We are investigating the solubility and speciation of plutonium(VI) carbonate under conditions relevant to natural waters and brines such as those found near some geologic radioactive waste repositories. The solid Pu(VI) phase PuO₂CO₃(s) was prepared and its solubility was measured in NaCl and NaClO₄ solutions in a CO₂ atmosphere as a function of pH and ionic strength (0.1-5.6 m). The concentration of soluble plutonium in solution was calculated from spectroscopic data and liquid scintillation counting. Spectroscopic measurements also revealed the plutonium oxidation state. The apparent solubility product of PuO₂CO₃(s) was determined at selected electrolyte concentrations to be, log K_s,0 = −13.95 ± 0.07 (0.1 m NaCl), log K_s,0 = −14.07 ± 0.13 (5.6 m NaCl), and log K_s,0 = −15.26 ± 0.11 (5.6 m NaClO₄). Specific ion interaction theory was used to calculate the solubility product at zero ionic strength, .     

Early Pleistocene magmatism in the central part of the Greater Caucasus

An isotope-geochronological study of young magmatism in the central part of the Greater Caucasus (Kazbek neovolcanic area) on the territory of Russia and Georgia has been carried out. It was proved for the first time that, in the Early Pleistocene, there was a separate impulse of magmatic activity in this area. The area of endogenic activity for the period identified was contoured on the basis of the integrated isotope-geochronological, petrological-geochemical, and geological data. It has been shown that the Early Pleistocene volcanism inherits the area of Neogene volcanism in the Kazbek region and, therefore, presents the final impulse of the second (Pliocene) stage of the Late Cenozoic magmatism. Thus, Early Pleistocene volcanism was not a precursor of Late Quaternary magmatism as the latter has other spatial patterns of the location of volcanic centers.     

Late Pliocene-early Pleistocene Athalassa Formation, north central Cyprus: carbonate sand bodies in a shallow seaway between two emerging landmasses

The late Pliocene-early Pleistocene Athalassa Formation accumulated within the Neogene Mesaoria basin of northern Cyprus, bordered by two subdued, but rising, landmasses, the Kyrenia Range to the north and the Troodos ophiolitic massif to the south. In the eastern part of the basin, the formation is dominated by cross-stratified bioclastic-rich sandstones interpreted as shallow-marine, carbonate sand bodies. The sediments are described in terms of six calcarenite facies and subfacies and one sandstone fades. Palaeocurrent analysis indicates that the carbonate sand bodies formed near the southern margin of the Kyrenia landmass and then migrated southwards across a shallow-marine platform. The driving force is believed to have been storms, generated by strong winds. Such winds during the late Pliocene-early Quaternary were probably stronger than today in this area. Other factors influencing deposition of the formation include minor tectonic instability, and probable eustatic sea-level changes. Study of the Athalassa Formation thus provides insights into late Pliocene-Pleistocene palaeoenvironments in the easternmost Mediterranean.     

High-resolution facies and porosity models of the upper Jurassic Arab-D carbonate reservoir using an outcrop analogue, central Saudi Arabia

Subsurface models of hydrocarbon reservoirs are coarse and of low resolution when compared with the actual geologic characteristics. Therefore, the understanding of the three-dimensional architecture of reservoir units is often incomplete. Outcrop analogues are commonly used to understand the spatial continuity of reservoir units. In this study, a Late Jurassic outcrop analogue for the Arab-D reservoir of central Saudi Arabia was used to build a high-resolution model that captures fine geologic details. Subsurface reservoir lithofacies were matched with those from the studied outcrop, and porosity values derived from published core and well log data from the Ain Dar, Uthmanyah, and Shudgum areas of the Ghawar Field, eastern Saudi Arabia, were then applied to the equivalent lithofacies in the outcrop. Maximum, minimum, and average subsurface porosity for each lithofacies were distributed in the facies model using a geostatistical algorithm to produce nine porosity models for the field data. Several realisations were run to visualise the variability in each model and to quantitatively measure the uncertainty associated with the models. The results indicated that potential reservoir zones were associated with grainstone, packstone, and some wackestone layers. Semivariogram analysis of the lithofacies showed good continuity in the N-S direction and less continuity in the E-W direction. The high-resolution lithofacies models detected permeability barriers and isolated low porosity bodies within the potential reservoir zones. This model revealed the porosity distribution in areas smaller than one cell in the subsurface model and highlighted the uncertainty associated with several aspects of the model.     

Factors and mechanisms of chemical composition formation of saline lakes in West Mongolia

正All the lakes are accumulative systems for the various chemical elements.However,the ratios of the elements in different lake types are very different.It is generally accepted that the composition of lake water determined by     

Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano)

Exposures of metamorphic basement in the Central Andes are scarce and reconstructions of the history of the Pacific margin of Gondwanaland must rely on a few isolated outcrops. We studied two areas of exposed basement in northernmost Chile (Belen) and westernmost Bolivia (Cerro Uyarani). The Belen metamorphic complex has been known for some time and consists of fault-bounded amphibolites, gneisses, schists, and minor quartzites overlain by folded Mesozoic to Cenozoic strata. The Cerro Uyarani is the only basement outcrop on the Bolivian Altiplano and has only recently been found and studied by geological reconnaissance. It consists of foliated mafic and felsic granulites, charnockites, and amphibolites. How do these basement occurrences compare and how do they relate to the other Precambrian crustal domains in the Central Andes? To answer these questions, we used geothermobarometers to reconstruct the P–T conditions of metamorphism, as well as geochemical analyses and petrological methods to study these rocks. The two basement blocks were found to have distinct geological histories and are probably separated by a major crustal domain boundary. Isotopic fingerprinting by Pb-isotopes clearly exclude Laurentian crustal components either in the protoliths or as reworked material. This signature is quite distinct from basement rocks farther south in Chile and northwestern Argentina.