depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

postglacial vegetation history of the mariana lake region alberta a fossil pollen record extending from 11 300 bp to the present was recovered from mariana lake alberta 55 57 n 112 01 w initially the region had a sparse vegetation dominated by forbs and graminoids which at approximately 10 500 bp succeeded to picea glauca forest picea mariana sphagnum peatlands began to develop between 10 000 and 9500 bp after 9000 bp sphagnum dominated sites decreased p glauca declined while betula increased from 7500 to 5500 bp populus reached its maximum holocene representation in the vegetation sphagnum dominated peatlands almost disappeared the changes following 9000 bp were likely caused by increased aridity however the presence of p mariana abies and larix in the pollen record suggests that mesic sites remained beginning at 6500 6000 bp the extent of sphagnum peatlands began to increase abies declined in importance following 5000 bp possibly due to paludification of mesic sites the expansion of parkland into central alberta during the mid holocene did not

the climate of north america and adjacent ocean waters ca 6 ka the climate of north america and the adjacent ocean at 6000 bp was estimated using five independent approaches using pollen data the terrestrial climate was estimated by the movement of ecozone boundaries and by the method of modern analogues both analyses indicate warmer temperatures in the western great lakes area and the northern great plains a model of sphagnum dominated peatland initiation when forced by canadian climate model 6 ka output projected a cooler and or wetter climate for continental western north america contrary to this a reconstruction of the distribution of sphagnum dominated peatlands in western canada indicates that they were located north of their modern distribution suggesting warmer and or drier conditions at 6000 bp this interpretation is strengthened by observations of lower lake levels at 6000 bp in western canada this drier climate may have been associated with warmer conditions as indicated by the quantitative climate reconstructions in general eastern north a

crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts abstract accessory minerals commonly occur attached to or included in the major crystalline phases of felsic and some intermediate igneous rocks apatite is particularly common as inclusions but fe ti oxides pyrrhotite zircon monazite chevkinite and xenotime are also known from silicic rocks accessories may nucleate near the host crystal liquid interface as a result of local saturation owing to formation of a differentiated chemical boundary layer in which accessory mineral solubility would be lower than in the surrounding liquid differentiation of this boundary layer would be greatest adjacent to ferromagnesian phenocrysts especially fe ti oxides it is with oxides that accessories are most commonly associated in rocks a boundary layer may develop if the crystal grows more rapidly than diffusion can transport incorporated and rejected elements to and from the phenocryst diffusion must dominate over convection as a mode of mass transfer near the advancing crystal liquid interface in order for a boundary layer to exist accumulation of essential structural constituent elements of accessory minerals owing to their slow diffusion in evolved silicate melt also may force local saturation but this is not a process that applies to all cases local saturation is an attractive mechanism for enhancing fractionation during crystallization differentiation if accessory minerals attached to or included in phenocrysts formed because of local saturation their host phenocrysts must have grown rapidly when accessories nucleated in comparison to lifetimes of magma reservoirs some inconsistencies remain in a local saturation origin for accessory phases that cannot be evaluated without additional information

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

coexisting monazite and allanite in peraluminous granitoids of the tribec mountains western carpathians monazite a typical light rare earth element lree mineral of s type granitoids in the western carpathians was found in the peraluminous biotite granodiorite tonalite in the tribec mountains commonly containing polymineralic inclusions these inclusions are dominated by anhedral allanite although allanite also occurs rarely as discrete grains not enclosed by monazite the monazite studied here is relatively homogeneous and characterized by high th contents with proportions of huttonite thsio4 and brabantite cath po4 2 up to 14 6 and 9 3 respectively the discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type of host rock however allanite included in monazite is extremely variable in lree al fe and mg contents this variation is interpreted to result from entrapment of allanite melt in monazite before local equilibrium was attained the change from allanite to monazite as the stable lree rich phase is related to an overall decrease in ca concentration caused by the onset of plagioclase crystallization the early precipitation of allanite was possible because of the high lree concentrations in the melt the crystallization temperature of allanite must have been higher than monazite saturation 856 845 c and 798 790 c for two analyzed samples the zr saturation temperature based on zircon solubility and ree thermometry based on monazite solubility reflect an increase in temperature from the edge to the center of the pluton which coincides with an increase in the huttonite content in monazite the primary lree assemblage is accompanied by small grains of late huttonite replacing monazite and brabantite replacing allanite

a mid holocene record of vegetative change in central alberta sediments from three lakes in central alberta smallboy lake 53 35 n 114 8 w e i pond 53 38 n 112 51 w and hastings lake 53 25 n 112 53 w have been analyzed for their pollen content charcoal remains and in two lakes pyrite spherule concentration the earliest record radiometrically dated at 7400 years bp indicates the existence of mixed wood parkland vegetation by 5000 years bp the regional vegetation had a considerably more open structure than now and was subject to frequent fires presumably a response to the warm dry hypsithermal climate of this time the termination of the hypsithermal interval 4000 years bp is recorded in all three lakes by a marked increase in precipitation the onset of a cooler moister climatic regime stimulated forest closure and reduced regional fire activity although the local vegetation of each of the three study sites responded in a unique way to the changing climate by 3000 years bp the vegetation resembled the modern vegetation little change is re

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

water contents of silicic to intermediate magmas silicic to intermediate magmas have water contents that range from 1 wt to around 7 wt depending on the character and depth of the magma source type of melting reaction and the degree of differentiation the condition of water saturation is rare implying that crustal magma genesis generally takes place under fluid absent conditions or that any fluid present must contain volatile species other than h2o the source and quantity of water present during partial melting profoundly affect the types of melting reactions residues and magmas as well as subsequent magma evolution for s type granitic magmas melting reactions involving muscovite breakdown in pelites yield low temperature wet melts that form veins pockets and small plutons in regional metamorphic terranes biotite breakdown in pelites near the amphibolite granulite facies transition yields hotter magmas with variable water contents these are emplaced at medium to high levels under granulite facies conditions biotite breakdown in quartzofeld spathic rocks yields high temperature dry magmas emplaced as subvolcanic batholiths and volcanics

postglacial palaeoecology of the subalpine forest grassland ecotone of southwestern alberta new insights on vegetation and climate change in the canadian rocky mountains and adjacent foothills postglacial vegetation change at the subalpine forest grassland ecotone in the foothills of southwestern alberta is reconstructed from fossil pollen plant macrofossils and fossil charcoal the fossils were obtained from the sediments of a small lake chronological control for the record is provided by tephra analysis and the 14c dating of terrestrial plant macrofossils the fossil record is compared with the results from similar studies at sites in the rocky mountains and foothills of alberta and british columbia the comparison of vegetation change at the upper and lower elevational limits of the subalpine forest provides a record of changes in temperature and aridity over the holocene at 10 000 yr b p the study site supported a shrub and herb dominated vegetation picea pinus abies forests were present at mid to high elevations in the mountains and in the northern foothills by this time the climate in the southern foothills was likely arid but not severely cold a relatively open picea and pinus forest was established in the study area between 9400 yr b p and 8000 yr b p fire activity in the region was at a postglacial maximum during this period the transition to forest suggests moister conditions by 8000 yr b p the importance of members of the chenopodiaceae amaranthaceae and typha latifolia increased suggesting an onset of arid conditions characterised by low and fluctuating water levels in lakes between 8000 yr b p and 5000 yr b p the picea and pinus contorta forest at the study site was largely replaced by grassland and pinus flexilis at high elevation sites treeline was located above its modern position during this period little vegetation change is recorded at sites in the northern foothills and at mid elevations at this time the vegetation change in the southern foothills and at high elevations suggests both an elevational and latitudinal displacement of vegetation zones during the mid holocene warm and arid conditions were prevalent in southwestern alberta picea glauca pinus contorta populus tremuloides forest has occupied the site since 5000 yr b p when cooler and moister conditions were established

holocene paleohydrology of central alberta testing the general circulation model climate simulations paleoecological investigations of 28 lakes and bogs provide the basis for a holocene paleohydrological record for central alberta shallow basins empty during the early holocene began flooding shortly after 8000 years ago most filled from 6500 to 4500 years ago and none filled later than 3000 years ago pollen of a hypersaline indicator genus ruppia was found in cores of lakes where the plant does not presently grow indicating lower lake levels evaporation stress and increased salinity from 8000 to 3000 years ago moore lake 54 30 n 100 30 w may have dropped 15 m from 9200 to 5900 years ago and lofty lake 54 44 n 112 29 w largely dried up between 8700 and 6300 years ago lower water levels are suggested for several lakes by shifts in fossil diatom populations from planktonic to benthic particularly epipelic species including pioneering taxa these shallow warm lakes were highly productive and sedimentary pigment levels reached maximum values between about 9000 and 4000 years ago the pale

pedogenic zones in postglacial sediment of lake manitoba canada the postglacial sediment record of lake manitoba is composed dominantly of silty clay to clayey silt with little variation in most physical chemical and mineralogical properties distinct in the sedimentary sequence of this large lake however are four zones that have a low moisture content blocky to pelletal structure and gleyed colour all of these zones formed during the warmest and driest postglacial period in the region 9500 to 4500 years ago although several possible hypotheses can be formulated regarding the origin of these zones including changes in sedimentation rate clay mineralogy lake chemistry and depth of winter freezing the most likely explanation is that they represent pedogenic horizons formation by pedogenesis during dry or extremely low water conditions is not mutually exclusive of the other hypotheses and several are favoured by low water conditions

the late quaternary palaeoenvironmental history of a presently deep freshwater lake in east central alberta canada and palaeoclimate implications abstract the palaeoenvironmental history of moore lake alberta canada 54 30 n 110 30 w has been investigated through analysis of pollen diatoms total chrysophyte stomatocysts and sedimentary pigments including the blue green algal xanthophylls oscillaxanthin and myxoxanthophyll the initial lake was shallow vegetation of the region was treeless and no doubt reflected the unique late glacial pioneering environment planktonic diatoms characteristic of a deep eutrophic lake were dominant by ca 11400 yr b p when a birch spruce dominated boreal forest developed however in response to early holocene warmth and aridity the lake had become saline by ca 10 000 yr b p when chaetoceros spp and cyclotella caspia became dominant while an open parkland grassland vegetation had developed subject to increased fire frequency freshwater taxa reappeared between ca 9100 and ca 8400 yr b p providing evidence of a wetter period however after ca 8400 yr b p planktonic saline taxa returned as dominants by ca 6200 yr b p birch spruce forests were once again developed in the area in response to more effective precipitation between ca 5800 and 4000 yr b p oscillations between fresh and saline diatom taxa occurred suggesting some aridity reversals during the last ca 4000 years planktonic eutrophic diatom taxa dominated diatom numbers were initially low but increased ca 11 400 yr b p and remained high until ca 10 000 yr b p they were minimal during the saline interval afterwards as the lake freshened numbers of diatoms increased and several large peaks occurred perhaps in response to internal nutrient loading coupled with increased runoff and erosion oscillaxanthin concentrations were initially high but during the saline interval they became undetectable except during the freshwater interlude and then as the lake became fresh large maxima occurred in contrast myxoxanthophyll concentrations were high during this saline interval large myxoxanthophyll peaks occurred simultaneously with those of oscillaxanthin as the lake freshened overall lake palaeoproduction levels during the holocene appear to have altered little the moore lake record demonstrates short lived palaeoclimatic changes of unknown cause superimposed upon the broad trends and the value of the palaeolimnological record which can be much more sensitive than fossil pollen vegetation reconstruction in demonstrating climate history results are compared with others for western canada and the great plains

beryllium 10 dating of the foothills erratics train in alberta canada indicates detachment of the laurentide ice sheet from the rocky mountains at 15 ka the foothills erratics train consists of large quartzite blocks of rocky mountains origin deposited on the eastern slopes of the rocky mountain foothills in alberta between 53 5 n and 49 n the blocks were deposited in their present locations when the western margin of the laurentide ice sheet lis detached from the local ice masses of the rocky mountains which initiated the opening of the southern end of the ice free corridor between the cordilleran ice sheet and the lis we use 10 be exposure dating to constrain the beginning of this decoupling based on a group of 12 samples well clustered in time we date the detachment of the western lis margin from the rocky mountain front to 14 9 0 9 ka this is 1000 years later than previously assumed but a lack of a latitudinal trend in the ages over a distance of 500 km is consistent with the rapid opening of a long wedge of unglaciated terrain portrayed in existing ice retreat reconstructions a later separation of the western lis margin from the mountain front implies higher ice margin retreat rates in order to meet the younger dryas ice margin position near the boundary of the canadian shield 2000 years later

chlorophyll production degradation and sedimentation implications for paleolimnology1 chlorophyll a production degradation and sedimentation were studied simultaneously during summer stratification in three lakes with contrasting plankton communities pigment budgets showed that chlorophyll production and pigment resuspension were both major sources of water column pigments photodegradation rates were rapid and indicated that detritus particles that remained in the epilimnion for periods longer than about 3 days lost nearly all detectable pigments therefore only rapidly sinking detrital particles or those produced in deep layers at low light intensity could make appreciable contributions to sedimentary chlorophyll degradation products pheophorbide a a grazing indicator was the dominant chlorophyll a degradation product found in sediment traps pigment sedimentation increased significantly with mean size of cladocerans and omnivorous copepods in contrast sedimentation rates of chlorophyll degradation products did not increase with primary production in these lakes the deposition of chlorophyll degradation products in sediments depended primarily on the size and biomass of grazers

evidence from north west canada for an early holocene milankovitch thermal maximum according to the milankovitch theory of global climatic change maximum summer solar radiation at high latitudes of the northern hemisphere occurred at 10 000 yr bp refs 1 2 in particular it predicts summer solstice radiation greater by 9 10 preliminary climate simulation experiments with these increased values of radiation confirm that high latitude land surfaces received maximal insolation at 10 000 yr refs 3 4 paradoxically however the large volume of fossil pollen and other evidence from north america indicates a maximum of holocene warmth at 7 000 6 000 yr ref 5 and a recent review of the evidence from new england suggests that the warming began at 9 000 and ended at 5 000 yr but also stresses the difficulties of interpretation in terms of climate change6 we summarize here data from sites in the north west corner of mainland canada fig 1 that directly support the milankovitch hypothesis

chrysophycean microfossils in paleolimnological studies abstract the chrysophyceae represent a diverse group of algae with approximately 1000 described species most of which are planktonic and found in oligotrophic lakes a characteristic feature of this class is the endogenous formation of a siliceous resting stage known as a statospore vegetative cells in the family mallomonadaceae are covered by overlapping siliceous scales that are often sculptured and ornamented both the statospores and scales are species specific and because they are siliceous they are usually well preserved in lake sediments this paper reviews the applicability of these microfossils in the interpretation of past lake conditions

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

underwater faunal assemblages radiocarbon dates and late quaternary vertebrates from cold lake alberta and saskatchewan canada late quaternary faunal remains from three underwater settings in cold lake alberta and saskatchewan canada include at least 13 vertebrate taxa consistent with assemblages that post date the last glacial maximum lgm seven new radiocarbon dates range from 10350 40 yr bp to 161 23 yr bp and provide insight into the post lgm biotic history of east central alberta and west central saskatchewan the presence of an essentially modern large mammal biota is suggested for the mid holocene and possibly earlier if the absence of extinct or extirpated taxa in association with late pleistocene bison at the ab sk site is meaningful taphonomically some of the remains suggest deposition in open environments during the holocene possibly when lake levels were lower the recovery of late quaternary faunal remains from a present day lacustrine setting is novel and suggests that similar records may occur in other lakes in western canada including those in areas with scarce quaternary vertebrate records

early holocene limnological and climatic variability in the northern great plains information on the timing and direction of climatic and environmental change on a millennial scale exists for many regions of north america whereas little is known about decadal to centennial scale variability here we present a high resolution analysis of diatom inferred salinity from a site in the northern great plains to reconstruct multidecadal and centennial scale climatic patterns during the early holocene the diatominferred salinity indicates a transition from fresh to highly saline conditions between c 13 400 and 7700 cal yr bp which suggests a major shift in climate from wet to dry conditions the overall trend toward increasing salinity is interrupted by several freshwater intervals between c 9800 and 7950 cal yr bp which may be the result of an increase in the frequency of monsoonal flow from the gulf of mexico the early holocene is considered to be a time of rapid change in climate and vegetation within the holocene although rates of change in the moon lake diatom assemblages were hi

a multi proxy limnologic record of rapid early holocene hydrologic change on the northern great plains southwestern saskatchewan canada clearwater lake saskatchewan is a relatively shallow topographically closed perennial lake cen trally located in the palliser triangle region of western canada despite its present day small size clearwater lake is one of only a few lacustrine basins in this vast 400 000 km2 region of the northern great plains whose stratigraphic record extends to the early holocene multiple proxy indicators are used to interpret the early holocene limnologic and hydrologic conditions in the basin changes in solute chemistry and concentration are reflected by siliceous microfossils and the endogenic and authigenic mineralogy of the sediment the diatoms also provide evidence of fluctuations in water depth and water column stratification plant macrofossil assemblages sediment texture and bedding features and detrital mineralogy help to reconstruct drainage basin conditions and water level changes stable o nd c isotopic composition of inorganically precipitated endo genic carbonate minerals provide insight into

7000 year record of lake level change on the northern great plains a high resolution proxy of past climate mineralogical and paleobotanical characteristics of the sedimentary fill in a shallow saline lake in southeastern alberta indicate that drought intervals are aperiodic and that climatic extremes unprecedented in historic time occurred earlier during the holocene when century long intervals of repeated intense droughts alternated with long periods when droughts were rare low water intervals signifying periods of drought are recorded by carbonate rich laminae containing abundant plant fossils indicative of hypersalinity in contrast highstands of relatively fresh water outlining moist periods are represented by massive silicate rich sediment with lower numbers of halophytic indicators

late quaternary palaeolakes rivers and wetlands on the bolivian altiplano and their palaeoclimatic implications drill cores of sediments from the rio desaguadero valley bolivia provide new information about the climate of tropical south america over the past 50 000 years the modern rio desaguadero is fed by lake titicaca overflow and by local tributaries in the wetter northern altiplano and discharges into lake poopo in the more arid central altiplano during the late quaternary the rio desaguadero valley was the site of several generations of palaeolakes and wetlands that formed during periods of increased precipitation and local runoff augmented by increased overflow from lake titicaca sediments recovered by drilling in eight localities along the 390 km long valley of the rio desaguadero yield a regional history of lacustrine sedimentation and effective precipitation lacustrine strata in the drill cores record 12 distinct wet periods in the past 50 000 years four of these wet periods resulted in the formation of major palaeolakes in the rio desaguadero valley during the last glacial maximum from before 20 000 to 16 000 cal yr bp during the late glacial from about 14 000 to 12 000 cal yr bp in the early holocene from about 10 000 to 7900 cal yr bp and in the late holocene from 4500 cal yr bp to present the period that appears to have been most arid was between 7900 and 4500 cal yr bp the altiplano wet periods were generally synchronous with north atlantic cold events respectively the last glacial maximum the younger dryas the 8200 cal yr bp event and the neoglacial implying a relationship between past precipitation variability in tropical south america and north atlantic sea surface temperature copyright 2005 john wiley sons ltd

paleohydrological indicators in playas and salt lakes with examples from canada australia and africa abstract the sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region these records are comprised of siliciclastic sediments and chemically precipitated minerals that typically vary temporally and spatially in some cases the interpretation is straightforward with mineralogical sequences related to changing brine concentrations and hydrology as well as to climate in other cases it is difficult to identify the controlling factors which include climate groundwater geological setting and basin morphology in addition diagenesis commonly affects the sedimentary record models of lake level fluctuations are given for basins in the canadian prairies and southeastern australia the lake manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper water sediment during low water stages smaller canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators such as specific carbonate and salt mineralogy nature of bedding grain size and secondary alterations decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates to salts and then back to clastics as groundwater levels fall below the lake floor similar parameters are used in australian lakes the hydrological model for the lake tyrrell playa in victoria whose present ionic composition is similar to seawater shows clastics being deposited when average lake levels are high followed by deposition of primary carbonates gypsum and halite once seasonal drying on the lake floor begins secondary gypsum grows within already deposited sediment and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes continued water level decline leads to pedogenesis subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle and a complete cycle in the sedimentary record appears to be uncommon