an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

the last glacial maximum and late glacial environmental and climate dynamics in the baikal region inferred from an oxygen isotope record of lacustrine diatom silica abstract the last glacial maximum and late glacial environmental and climatic variability in the baikal region southern siberia russia has been studied in a sediment sequence from lake kotokel located 2 km east of lake baikal using the oxygen isotope composition of diatom silica 18 o diatom the purification of diatom frustules involved the process of trimethylsilylation which has been shown to be suitable for preparation of diatoms for oxygen isotope analysis the lake kotokel 18 o diatom record presented here spans intervals from about 24 6 to 22 9 ka bp further referred to as last glacial maximum and 16 7 11 5 ka bp further referred to as late glacial displaying variations in the oxygen isotope composition between 26 7 and 31 2 overall high 18 o diatom values of about 29 to 31 during the two investigated intervals characterize a strongly evaporative lake system in a dry environment and suggest a lower than present lake level due to enhanced evaporation the lake kotokel diatom isotope record is roughly in line with the 60 n summer solar insolation pointing to a linkage to broader scale climate change but displays weaker reaction to short term climatic oscillations i e bolling allerod or younger dryas the climate warming at 14 3 ka bp is marked by negative spikes in the 18 o diatom record due to isotopically low melt water input from the mountainous hinterland

diatom stable isotopes sea ice presence and sea surface temperature records of the past 640 ka in the atlantic sector of the southern ocean 1 high resolution 13c and 15n records of diatom bound organic matter 13cd 15nd from odp site 1094 south of the polar front are combined with reconstructions of summer sea surface temperature and winter sea ice to establish changes in surface water characteristics associated with the main climatic events of the past 640 ka the six glacial interglacial 13cd cycles show excellent agreement with specmap 18o and vostok co2 content suggesting that global physical processes rather than local biological factors were the main influences on 13cd by contrast there is weak correspondence between the vostok dust record and the 13cd and 15nd records the data do not support the hypothesis that glacial iron fertilization of the southern ocean was the main factor lowering atmospheric co2 if dust accumulation represents iron input the records indicate that sea ice cover water column stratification and iron addition from deep water mixing are more likely to explain our observations

biological forcing of hemipelagic sedimentary laminae evidence from odp site 893 santa barbara basin california core observation x radiography and scanning electron microanalysis of distinctly laminated indistinctly laminated and nonlaminated sediments from odp site 893 show that laminated as well as nonlaminated i e massive hemipelagic sediments can form in the absence of bioturbation this view contrasts with conventional sedimentological interpretations equating laminated hemipelagic sediments with anoxic water masses i e absence of bioturbation and nonlaminated sedimentary intervals exclusively with oxygenation episodes that permitted infaunal colonization we conclude that heterogeneities in the texture and or composition of sediment supply are necessary for the production of laminated sediments the absence of hydraulic and biological reworking permits their preservation laminae at site 893 have two prominent components biosilica and terrigenous detritus compositional contrasts between adjacent laminae in x radiographs reflect short term seasonal and subseasonal heterogeneities in sediment flux core intervals with large contrasts in density and or composition between adjacent laminae are termed as having high bimodality hb hb couplets produce distinctly laminated sediments that intergrade vertically with indistinctly laminated and nonlaminated sediments adjacent laminae in indistinctly laminated sediments possess minimal contrasts in bulk density and composition nonlaminated sediments are compositionally and texturally homogeneous many biosiliceous laminae responsible for distinctly laminated hb couplets record mass flocculation and sedimentation of ungrazed diatom frustules formed during discrete bloom events ecological interpretation of diatomite laminae suggests that many were effectively self sedimenting i e the production of gelatinous exudates by phytoplankton facilitated their aggregation and rapid sedimentation the results indicate that lamination style preserves meaningful ocean climate data concerning episodicity heterogeneity and export efficiency of biologically mediated sedimentary flux in upwelling dominated coastal ecosystems

geochemical investigation of soils developed in different lithologies in bhutan eastern himalayas within a project studying soil genesis on the southern slopes of the eastern himalayas a comprehensive set of major trace and rare earth element ree contents has been compiled for the first time six saprolites and their associated pedons were sampled on metamorphic migmatitic and leucogranitic parent materials at altitudes ranging from 1520 m to 3770 m metamorphic soils are largely illitic with si decreasing and al and fe increasing with decreasing degree of metamorphism the subsoil of a tourmaline bearing leucogranite is characterised by equally high si but higher contents of k and lower fe mg and ti a migmatitic profile in southern bhutan is dominated by hornblende and is quite distinct from the other lithologies with lower contents of si and k but higher fe mg ca and mn chondrite normalised ree patterns compare with post archaean materials with steep light rare earth lree and flat heavy rare earth hree patterns in comparison to generalised upper crustal composition ucc the bhutanese lithologies show a pronounced eu anomaly whereas the patterns appear rather homogenised in topsoils the influence of the parent materials causes deviations from the general trend with increasing depth the occurrence of marine tethyan sediments is confirmed by a strong positive ce anomaly in the central bhutanese phobjikha valley in the leucogranitic profile ree depletion is attributed to the presence of monazite as corroborated by trace element behaviour because of the hornblende ree patterns of the migmatitic profile are flat and without a eu anomaly chemical indices of alteration cia of between 71 and 92 indicate advanced weathering even at high altitudes correlation analyses show that ree are associated with the silt and clay fraction 63 m at elevations above approximately 3500 m physical weathering causes passive ree enrichment however no differentiation of ree concentrations or patterns within the profile is observed below 3500 m increased chemical weathering and leaching appears to be sufficient to release and translocate rees in this process hree are preferentially translocated whereas ce is more refractory than the other lanthanides this case study confirms that soil weathering and ree behaviour are closely related

soil variability in the area of an archaeological site near sparta greece results are analysed from a late roman site one of 20 studied in the laconia rural sites project the aim was to consider the extent to which effects of human activity at the time the site was inhabited were still evident in the soil and the exent to which these could be distinguished from natural soil variations the site has not been excavated patterns of soil properties of artefacts on the ground surface and of those encountered in the topsoil whilst sampling are analysed for a 5 m interval grid soil samples from 20 cm depth were analysed for total p pb cu zn fe mn ni ca mg k and organic c mass specific and volume magnetic susceptibility secondary crystalline and amorphous fe ph caco3 particle size distribution and colour all variables are mapped and data are further analysed using cluster regression and principal components analyses relatively high values for soil p cu pb ca organic c magnetic susceptibility and clay content and low ph and darker soil colour are associated with areas of the grid where artefact counts were highest the distinctive character of the soil in these areas reinforced by cluster and principal components analyses is attributed to a combination of inherent soil and anthropogenic effects

late quaternary variations of elemental ratios c si and n si in diatom bound organic matter from the southern ocean le cycle du carbone de l ocean austral est certainement un des facteurs majeurs controlant les concentrations atmospheriques passees en co 2 cependant les taux d accumulation de l opale biogene ne sont pas lineairement lies aux taux d accumulation du carbone organique et ne peuvent donc pas etre directement utilises en tant que traceur de la paleoproductivite nous montrons ici qu il est possible de mesurer les teneurs en carbone et azote dans la matiere organique intrinseque des diatomees c diat and n diat respectivement et que les signaux fossiles ne sont pas le resultat d artefacts analytiques l analyse de deux carottes provenant des secteurs atlantique et indien de l ocean austral indique que les c diat and n diat ont respectivement varie de 30 40 et de 120 175 au cours des cycles glaciaires interglaciaires les rapports c n ont en consequence varie de 3 a 7 au cours des cycles glaciaires interglaciaires si les variations enregistrees dans la matiere organique intrinseque sont representatives des changements du bulk des diatomees nous aurons un nouvel outil pour documenter le cycle oceanique du carbone et pour determiner son role dans les variations passees de pco 2 atmospheriques des experiences en laboratoire sur des cultures de diatomees sont cependant necessaires afin de valider l outil c diat and n diat comme traceur des conditions physiologiques des diatomees et des flux de carbone exportes depuis la couche euphotique

climatically linked carbon isotope variation during the past 430 000 years in southern ocean sediments we use the isotopic composition of carbon from organic matter enclosed within diatom frustules as a proxy for paleoproductivity and paleo dissolved carbon dioxide concentrations in ocean surface waters a southern ocean record from south of the antarctic polar front and spanning 430 000 years of carbon isotopic variation in diatomaceous organic matter is presented for the first time the most refractory diatomaceous organic matter fraction was extracted and analyzed to avoid problems associated with diagenesis the results clearly indicate cyclic changes in organic carbon isotopic ratios with 13c depleted values associated with all of the last five glacial periods reflecting changes in surface water properties and primary productivity changes in dissolved carbon dioxide concentrations are the most probable cause of these cycles but the possible effect of seawater ph changes cannot be excluded

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

climatic and lacustrine morphometric controls of diatom paleoproductivity in a tropical andean lake abstract the coupling of lake dynamics with catchment biogeochemistry is considered the key element controlling primary production in mountain lakes at time scales of a few decades to millennia yet little is known on the impacts of the morphometry of lakes throughout their ontogeny as lake chungara central andean altiplano northern chile experienced long term lake level fluctuations that strongly modified its area volume ratio it is an ideal system for exploring the relative roles that long term climatic shifts and lake morphometry play on biosiliceous lacustrine productivity in this paper we review previous data on the percent contents of total organic carbon total inorganic carbon total nitrogen total biogenic silica isotopic composition of organic matter carbonates and diatom frustules as well as data on the abundance of the chlorophycean botryococcus braunii in this lake for the period 12 400 1300 cal yr bp we also include new data on organic carbon and biogenic silica mass accumulation rates and the diatom assemblage composition of an offshore core dated using 14 c and u th biosiliceous productivity in lake chungara was influenced by shifts in allochthonous nutrient inputs related to variability in precipitation humid phases dated at approx 12 400 to 10 000 and 9600 to 7400 cal yr bp coincide with periods of elevated productivity whereas decreases in productivity were recorded during arid phases dated at approx 10 000 to 9600 and 7400 to 3550 cal yr bp andean mid holocene aridity period however morphometry related in lake controls led to a lack of a linear response of productivity to precipitation variability during the late glacial to early holocene lowstands facilitated complete water column mixing prompting episodic massive blooms of a large centric diatom cyclostephanos cf andinus thus moderate productivity could be maintained regardless of aridity by this phenomenon of morphometric eutrophy during the early history of the lake the subsequent net increase in lake level introduced modifications in the area of the epilimnion sediments versus the total volume of the epilimnion preventing complete overturn surpassing a certain depth threshold at approx 8300 cal yr bp caused cessation of the morphometric eutrophy conditions associated with cyclostephanos cf andinus superblooms after 7300 cal yr bp the lake experienced a decrease in biosiliceous productivity and a change of state that involved a stronger dependence on precipitation variability with a lesser contribution of diatoms to the total primary productivity our results show that the interpretation of lacustrine paleoproductivity records as paleoclimatic archives needs to take into account the effects of changes in the epilimnion sediment area to epilimnion volume ratio in association with lake ontogeny

the palaeohydrological evolution of lago chungara andean altiplano northern chile during the lateglacial and early holocene using oxygen isotopes in diatom silica oxygen isotopes of diatom silica and petrographical characterisation of diatomaceous laminated sediments of lago chungara northern chilean altiplano have allowed us to establish its palaeohydrological evolution during the lateglacial early holocene ca 12 000 9400 cal yr bp these laminated sediments are composed of light and dark pluriannual couplets of diatomaceous ooze formed by different processes light sediment laminae accumulated during short term diatom blooms whereas dark sediment laminae represent the baseline limnological conditions during several years of deposition oxygen isotope analysis of the dark diatom laminae show a general 18o enrichment trend during the studied period comparison of these 18odiatom values with the previously published lake level evolution suggests a correlation between 18odiatom and the precipitation evaporation ratio but also with the evolution of other local hydrological factors as changes in the groundwater outflow as well as shifts in the surface volume ratio of lago chungara the lake expanded probably increasing this ratio during the rising lake level trend due to changes in its morphology enhancing evaporation furthermore the lake s hydrology was probably modified as the groundwater outflow became sealed by sediments increasing lake water residence time and potential evaporation both factors could cause isotope enrichment natural environment research council nerc copyright 2008 reproduced with the permission of nerc published by john wiley sons ltd

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

a 14kyr record of the tropical andes the lago chungara sequence 18 s northern chilean altiplano abstract high resolution geochemical analyses obtained using an x ray fluorescence xrf core scanner as well as mineralogical data from the lago chungara sedimentary sequence in the northern andean chilean altiplano 18 s provided a detailed reconstruction of the lacustrine sedimentary evolution during the last 14 000 cal yr bp the high resolution analyses attained in this study allowed to distinguish abrupt periods identify the complex structures of the early and mid holocene arid intervals and to compare their timing with titicaca lake and sajama ice records three main components in the lake sediments have been identified a biogenic component mainly from diatoms b volcanics ash layers from the nearby parinacota volcano and c endogenic carbonates the correlation between volcanic input in lago chungara and the total particles deposited in the nevado sajama ice core suggests the parinacota volcano as the common source the geochemical record of lago chungara indicates an increase in siliceous productivity during the early holocene lagging behind the rise in temperatures inferred from the nevado sajama ice core the regional mid holocene aridity crisis can be characterized as a number of short events with calcite and aragonite precipitation in the offshore lake zones

palaeoclimate interpretation of stable isotope data from lake sediment archives the isotope composition of authigenic and biogenic carbonates and diatom silica are commonly used as palaeoclimate proxies from lake sediments this article reviews the controls on the isotope composition of lacustrine skeletal and non skeletal deposits and illustrates how stable isotope studies contribute to an understanding of changes in temperature precipitation patterns evaporation and the carbon cycle it highlights the differences in the palaeoclimate potential of a wide range of lakes ranging from open to closed lake basins a large number of the case histories but not all are drawn from studies of temperate lakes from europe large closed lake systems in the tropics and elsewhere lose water predominantly through evaporation and contain sediments with variable and generally high 18o values fluctuations in the isotope composition of authigenic or biogenic minerals are mainly a function of long term changes in the precipitation evaporation ratio in contrast small open lakes which have a degree of through flow typically contain sediments with 18o values that vary by no more than a few these variations are generally ascribed to variations in temperature or the isotope composition of precipitation p from which either an annual or seasonally specific signal can be gained these types of lakes are common in northern europe and at high altitudes the interpretation of isotope data from a lacustrine succession requires a knowledge of the local processes that might control and modify the signal their effects need to be quantified and a robust calibration using the modern lake system is necessary to establish the relationship between the measured signal the isotopic composition of the host waters and climate

an anthropic soil transformation fingerprinted by rey patterns abstract this focus article deals intentionally with modern soil disturbance in situ this is of interest to archaeologists as after disturbances both short and long term pedogenesis re starts obliterating previous signs soil modifications induced by human activity may be linked to pedogenetic evidence for disturbance with archaeological evidence for the cultural activities we contrasted two 750 m 3 soil pedons an anthrosol and a kastanozem from which the anthrosol is derived using 77 descriptors of soil properties which have been utilized in archaeological studies pedo morphological routine laboratory biochemical metals and rare earth elements plus yttrium rey with the aim of identifying a group of descriptors able to sort the occurrence of human interventions but which one is more promising our findings indicate that by the use of rare earth spatial patterns it is possible to classify the occurrence of human interventions in the case of emplacement of new parent material in respect to bulk soil disturbance in situ

holocene oxygen isotope record of diatoms from lake kotokel southern siberia russia and its palaeoclimatic implications the oxygen isotope composition of diatom silica d 18 odiatom from marine and lake sediments is helpful for the interpretation of the past climate and environments especially when complemented by other proxy records this paper presents a holocene oxygen isotope record of diatoms from lake kotokel located 2 km east of lake baikal in southern siberia russia the isotope record displays variations in d 18 odiatom from 23 7 to 30 3 from about 11 5 ka bp until today comparing the isotope composition of recent lake kotokel water mean d 18 o 12 to that of the most recent diatom sample d 18 o 27 5 an isotope fractionation in the right order of magnitude was calculated the kotokel d 18 o diatom record is controlled by changes in the isotopic composition of the lake water rather than by

carbon cycling within an east african lake revealed by the carbon isotope composition of diatom silica a 25 ka record from lake challa mt kilimanjaro the carbon cycle of a lake is a balance between supply from the atmosphere and catchment and the net demand exerted by primary producers minus losses back to the atmosphere and to sediment storage evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi proxy approach one promising technique is to explore the carbon isotope composition 13cdiatom of organic matter incorporated within the silica frustules of diatom algae here we present a 25 000 year record of 13cdiatom from the sediments of crater lake challa on the eastern flank of mt kilimanjaro and along with other proxy data we make inferences about the three major phases in the history of the lake s carbon cycle from 25 ka to 15 8 ka years bp 13cdiatom is positively correlated with the 13c of bulk sediment organic matter 13cbulk indicating that high diatom productivity as recorded by high biogenic silica at this time was preferentially removing 12c and enriching the 13c of lake water dissolved inorganic carbon from 15 8 to 5 5 ka the correlation between 13cdiatom and 13cbulk breaks down suggesting carbon supply to the lake satisfied or exceeded the demand from productivity from 5 5 ka bp the positive correlation resumes indicating an increase in the internal demand for carbon relative to external supply diatom frustule bound carbon isotopes offer an original tool in examining long term fluctuations in a lake s carbon budget and how the balance between supply and demand has changed through time

soil fertility and slope processes in the western cumberland escarpment of kentucky influences on the development of horticulture in the eastern woodlands the archaeological record of dry rockshelters in the red river valley of eastern kentucky contains botanical remains of several weedy annual seed bearing plants that point to the independent development of agriculture during the late archaic and early woodland period the objective of this study is to gather quantitative data through the chemical and physical analysis of soils collected within a valley transect to gain insights into landscape processes and soil fertility that may have played a role in the early development of plant domestication and the location of garden plots in relation to rock shelter sites in the upper cumberland plateau region of eastern kentucky soils on upland benches formed in limestones have soil fertility values that are comparable to fertile floodplain soils upslope outcrops of calcareous bedrock are contributing basic cations to these bench soils additionally in comparison to other landscape positions these upland soils have high levels of organic carbon available p and greater c n ratios soil morphology radiocarbon dates and archaeological evidence indicate that many of the numerous earthflows and debris flows within the region are early to middle holocene in age these slides were likely generated by mid holocene climatic shifts in which the frequency and intensity of summer precipitation increased across the southeastern united states these slope failures may have contributed to the development of plant domestication by creating forest canopy gaps and disturbed soils in which weedy plants thrived

a comparison of the palaeoclimate signals from diatom oxygen isotope ratios and carbonate oxygen isotope ratios from a low latitude crater lake the analysis of oxygen isotope ratios 18o from authigenic lake carbonates has become a well established palaeoclimate technique less common is the use of 18o in biogenic silica e g diatoms sponge spicules phytoliths but the technique is being increasingly utilised for non carbonate lakes this paper aims to compare 18odiatom with 18ocarbonate in a low latitude closed basin lake due to the influence of ph rarely are both carbonate and diatoms preserved in lake sediments in concentrations high enough to measure oxygen isotopes the 18o composition of both carbonate and silica should reflect the isotope composition and temperature of the lake water at the time of precipitation and hence theoretically they should be equivalent only one comparative study of 18odiatom and 18ocarbonate currently exists and that found differences between their 18o compositions probably because they precipitated in different seasons unless there is evidence to suggest that silica and carbonate precipitate at the same time their 18o records are likely to be different but complementary in this study we show that 18odiatom and 18ocalcite from lake tilo ethiopia show some similar broad climate trends however the 18odiatom curve is generally more variable and does not record two regional arid events picked up by the 18ocalcite data several possible reasons for this are discussed the precision of the 18odiatom extraction technique contamination of the diatom samples from tephra volcanic glass vital effects in the diatom samples differences in their respective equilibrium isotope fractionation rates and the possibility that the dominant diatom aulacoseira granulata and calcite precipitated in different seasons during this time currently the problems associated with the cleaning and extraction of 18odiatom suggests that details found in 18ocalcite records may be lost in 18odiatom records where contaminants such as tephra and clay minerals are difficult to remove

century to millennial scale climatic variability in lake malawi revealed by isotope records diatom based oxygen isotope data 18odiatom from lake malawi show multi centennial scale wet dry intervals spaced approximately every 2 3 ka throughout a 25 ka sequence the 18odiatom record is supported by a lower resolution deuterium dpa isotope curve derived from palmitic acid we interpret these isotope data in terms of major shifts in precipitation and evaporation moderated by seasonal controls on the host organisms dry periods marked by relatively positive isotope values represent the extension of abrupt holocene events noted from northern and equatorial africa to 10 15 s these events in lake malawi correspond to cool episodes in greenland thereby demonstrating teleconnections generated by meridional temperature gradients sea surface temperatures are likely to be the primary transmitter of deglacial climate changes although trade wind strength and direction is critical in controlling precipitation patterns in tropical regions conversely the global hydrological cycle driven by low latitude regions represents an important positive feedback amplifying deglacial processes

a review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction diatom silica frustules are a form of biogenic opal and contain oxygen isotopes that are being increasingly used in palaeoclimate studies frustules are comprised of an inner tetrahedrally bonded silica skeleton si o si with an outer hydrous layer the hydrous layer is freely exchangeable and must be removed prior to oxygen isotope measurement using oxidising reagents and or high temperatures analysis of the oxygen isotope composition of diatom silica requires samples that are almost pure diatomite since extraction techniques will liberate oxygen from all the components in the sediment there is a generally acceptable protocol involving chemistry sieving and settling techniques and more recently laminar flow separation recent studies of lacustrine diatoms have shown that even a small proportion of contaminant can have a significant influence on the oxygen isotope value all lake sediments require their own specific procedure and every sample must be scrutinised by microscopy to check for the level of contamination prior to analysis where sediment cannot be purified sufficiently a semi quantitative assessment of the diatom content can facilitate mass balance techniques when robust preparation methods are used diatom silica offers an important palaeoclimate proxy providing an additional complementary rather than an alternative host of oxygen isotopes in carbonates in lacustrine isotope studies it is often assumed but rarely proven that diatom silica form in isotope equilibrium and hence empirically derived palaeotemperature equations usually made in vitro can be used most studies lack the detailed investigation of contemporaneous materials from natural waters of the region under study to test these equations further investigations need to be conducted in order to understand the systematic relationship between temperature water isotope composition and diatom isotope composition to enable quantitative interpretation of the sediment record experiments to test for vital effects and inter specific differences have so far revealed little variation it is useful to know when and where the diatom silica is formed in the modern environment so details of diatom ecology are important in the interpretation of isotope data here we show that valuable palaeoclimate data can be gained from the oxygen isotope composition of diatom silica 18odiatom especially since diatoms are abundant in many lakes sensitive to climate variation where other hosts e g carbonates are absent to date the most successful studies have been conducted in areas where the 18odiatom registers changes in the oxygen isotope composition of the lake water rather than temperature which is then related to other aspects of climate in these studies the range of values obtained is often greater than the error introduced by contaminating materials

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

rare earth elements in limestone marlstone couplets from the albian cenomanian cismon section venetian region northern italy assessing ree sensitivity to environmental changes abstract we have studied the ree distribution through the albian cenomanian interval of a stratigraphic succession cismon section scaglia variegata formation venetian prealps consisting of rhythmically alternating carbonate rich and carbonate poor strata which are the sedimentary expression of systematic variations of surface fertility and bottom water redox conditions in the depositional basin limestone semicouplets show seawater like shale normalized patterns hree enriched with negative ce anomalies and high er nd ratios indicating a clear dependence on the ree chemistry of surface seawaters marlstones and shales exhibit flatter patterns more moderate ce anomalies and low er nd good correlations between er nd ce ce and mn suggest that superimposed on ree signatures imparted by detrital and biogenic phases the ree composition shows evidence of their sensitivity to palaeoredox conditions resulting in ree fractionation and ce cycling ree data are consistent with results from previous stratigraphic and geochemical major and trace elements investigations in reflecting palaeoenvironmental conditions rhythmically varying from efficiently mixed seawater during the limestone deposition to oxygen depleted bottom waters at the time of marlstone deposition

patterns of regional hydrological variability in central southern altiplano 18 26 s lakes during the last 500 years abstract paleohydrological reconstructions based on sedimentological geochemical and isotopic records from a lake transect in the central southern altiplano 18 26 s indicate abrupt moisture fluctuations during the last 500 years a change to modern conditions occurred in the late 19th century in all the records from northern chile lago chungara 18 15 s and the atacama laguna miscanti 23 45 s to the southern tip of the altiplano laguna el peinado nw argentina 26 30 s a previous drier period shows different patterns of timing duration and intensity in chungara the arid period was shorter and occurred during the late 19th and early 20th centuries while in miscanti it occurred earlier and ended at the beginning of the 20th century in el peinado conditions were wetter during the 17 19th centuries and the arid period occurred prior to the 17th century other records from the region show abrupt paleohydrological and paleoclimatic changes synchronous with the termination of the little ice age despite local differences and dating uncertainties the little ice age stands out as a significant though complex climatic event in the andean altiplano the discrepancies between the northern and southern altiplano records during the last few centuries may reflect contrasting responses to external forcing in two areas with different climatic regimes

climate catchment runoff and limnological drivers of carbon and oxygen isotope composition of diatom frustules from the central andean altiplano during the lateglacial and early holocene diatom based carbon and oxygen isotope analyses 13cdiatom and 18odiatom were performed on diatom rich laminated sediments of lake chungara andean altiplano northern chile deposited during the lateglacial and early holocene 12 400 8300 cal years bp to reconstruct climate change and environmental response across this major climate transition the 13cdiatom data show both centennial to millennial scale changes related to fluctuations in lake productivity and co2 concentration in the lake water and variations in carbon sources to the lake through time the 18odiatom data reflect changes in lake hydrology and climate the combination of 13cdiatom and 18odiatom data reveals interactions between the internal lake processes and its catchment runoff during wet periods low 18odiatom values 13cdiatom indicates an enhanced contribution of allochthonous carbon whereas during dry periods high 18odiatom 13cdiatom values suggest more autochthonous carbon production and recycling these decadal to centennial oscillations are not recognized after 10 000 cal years bp possibly as a result of enso like phenomenon weakening humid conditions during the lateglacial early holocene transition 12 400 10 100 cal years bp were possibly due to the establishment of la nina like conditions in the tropical south pacific whereas dry conditions in the early holocene 10 100 9100 cal years bp may be caused by the northward migration of the itcz due to both enso like weakening and an insolation minimum finally a return to humid conditions at the end of the early holocene 9100 8300 cal years bp is coincident with an increase in summer insolation

lacustrine sedimentation in active volcanic settings the late quaternary depositional evolution of lake chungara northern chile lake chungara 18 15 s 69 09 w 4520 m above sea level is the largest 22ae5 km2 and deepest 40 m lacustrine ecosystem in the chilean altiplano and its location in an active volcanic setting provides an opportunity to evaluate environmental volcanic vs climatic controls on lacustrine sedimentation the late quaternary depositional history of the lake is reconstructed by means of a multiproxy study of 15 kullenberg cores and seismic data the chronological framework is supported by 10 14c ams dates and one 230th 234u dates lake chungara was formed prior to 12ae8 cal kyr bp as a result of the partial collapse of the parinacota volcano that impounded the lauca river the sedimentary architecture of the lacustrine succession has been controlled by i the strong inherited palaeo relief and ii changes in the accommodation space caused by lake level fluctuations and tectonic subsidence the first factor determined the location of the depocentre in the nw of the central plain the second factor caused the area of deposition to extend towards the eastern and southern basin margins with accumulation of high stand sediments on the elevated marginal platforms synsedimentary normal faulting also increased accommodation and increased the rate of sedimentation in the northern part of the basin six sedimentary units were identified and correlated in the basin mainly using tephra keybeds unit 1 late pleistocene early holocene is made up of laminated diatomite with some carbonate rich calcite and aragonite laminae unit 2 mid holocene recent is composed of massive to bedded diatomite with abundant tephra lapilli and ash layers some carbonate rich layers calcite and aragonite occur unit 3 consists of macrophyte rich diatomite deposited in nearshore environments unit 4 is composed of littoral sediments dominated by alternating charophyterich and other aquatic macrophyte rich facies littoral carbonate productivity peaked when suitable shallow platforms were available for charophyte colonization clastic deposits in the lake are restricted to lake margins units 5 and 6 diatom productivity peaked during a lowstand period unit 1 and subunit 2a and was probably favoured by photic conditions affecting larger areas of the lake bottom offshore carbonate precipitation reached its maximum during the early to mid holocene ca 7ae8 and 6ae4 cal kyr bp this may have been favoured by increases in lake solute concentrations resulting from evaporation and calcium input because of the compositional changes in pyroclastic supply diatom and pollen data from offshore cores suggest a number of lake level fluctuations a late pleistocene deepening episode ca 12ae6 cal kyr bp four shallowing episodes during the early to mid holocene ca 10ae5 9ae8 7ae8 and 6ae7 cal kyr bp and higher lake levels since the mid holocene ca 5ae7 cal kyr bp until the present explosive activity at parinacota volcano was very limited between c 12ae8 and 7ae8 cal kyr bp mafic rich explosive eruptions from the ajata satellite cones increased after ca 5ae7 cal kyr bp until the present

characterization of trace elements and carbon isotopes across the ediacaran cambrian boundary in anhui province south china implications for stratigraphy and paleoenvironment reconstruction abstract characteristics of redox sensitive elements and organic carbon isotopes of a drilling section covering the ediacaran cambrian e c boundary at anhui province south china were studied total organic carbon toc organic carbon isotope values of organic compounds abundances of mo v u and ni and ratios of v v ni and th u co vary across the section suggesting that the redox condition is the main factor controlling carbon isotope values of organic matter and sequestration of trace elements the e c boundary is characterized by the lowest organic carbon isotope and the highest ratios of mo toc v toc and u toc in the section which are correlated with reported carbon isotope excursions in other regions this is the first time that signatures of mo v and u in the hetang formation at the lower yangtze platform are used for stratigraphic correlations the results highlight abnormally high ratios of mo toc v toc and u toc near the e c boundary petrological and geochemical characteristics in this section suggests that intense upwelling activities during the e c transition may be the main contributor to 13 c depleted co 2 and enrichment of trace metals the co existing pervasive transgression led to ocean anoxia and facilitated the preservation of 13 c depleted organic matter and trace metal sequestration furthermore oxidation of the bottom ocean water may have completed during the early cambrian a combination of these factors may be among the reasons for significant evolutionary changes during the e c transition

geology of mankind for the past three centuries the effects of humans on the global environment have escalated because of these anthro pogenic emissions of carbon dioxide global climate may depart significantly from natural behaviour for many millennia to come it seems appropriate to assign the term anthropocene to the present in many ways human dominated geological epoch supplementing the holocene the warm period of the past 10 12 millennia

variation in holocene el nino frequencies climate records and cultural consequences in ancient peru analysis of mollusks from archaeological sites on the north and central coasts of peru indicates that between ca 5800 and 3200 2800 cal yr b p el nino events were less frequent than today with modern rapid recurrence intervals achieved only after that time for several millennia prior to 5 8 ka el nino events had been absent or very different from today the phenomena called el nino have had severe consequences for the modern and colonial historically recorded inhabitants of peru and el nino events also influenced prehistoric cultural development the onset of el nino events at 5 8 ka correlates temporally with the beginning of monumental temple construction on the peruvian coast and the increase in el nino frequency after 3 2 2 8 ka correlates with the abandonment of monumental temples in the same region

multidecadal to century scale arid episodes on the northern altiplano during the middle holocene abstract we present reconstructions of multidecadal to centennial scale shifts in the hydrologic balance of the northern altiplano during the middle holocene using stable isotopic geochemical and sedimentologic evidence from lake titicaca sediments large downcore changes in the 13corg of bulk organic matter previously shown to be a site specific lake level proxy indicate that extended periods of lake level regression occurred in response to sustained reductions in mean annual precipitation and or increases in air temperature supporting evidence from a floating interval of finely laminated sediments indicates that salinity and stratification were enhanced at lake titicaca during the middle holocene the age model used in this study is based on 7 ams 14c age dates estimated from bulk decalcified sediment from a 60 cm sequence spanning 7200 5600 14c cal yr bp simulations from a hydrological energy balance model of the titicaca basin indicate that the 38 m drop in lake level that preceded the maximum lowstand at 6200 yr bp would require a significant decrease in mean annual precipitation of up to 50 and or a significant increase in mean annual temperature of up to 8 c today the altiplano is often affected by drought during periods of enhanced summertime westerly wind flow which reduces the delivery of moisture from the east this investigation supports the idea of sustained periods of enhanced westerly air flow during the mid holocene and provides additional submillennial scale information about the timing and magnitude of regional drought results of our frequency domain analysis of the floating lamination interval indicate significant power at a decadal periodicity 10 12 yr associated with the schwabe cycle of solar activity frequency domain analysis also reveals power in a 75 year time series of modern lake level we suspect that solar influence on large scale atmospheric circulation features results in changes to the moisture balance in the titicaca basin over decadal and possibly century scale time periods

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

diatom δ13c δ15n and c n since the last glacial maximum in the southern ocean potential impact of species composition measurements of delta c 13 delta n 15 and c n on diatom bound organic matter were made over the holocene and last glacial maximum lgm from three sediment cores in the southern ocean one each from the atlantic indian and pacific sectors the site in the scotia sea atlantic sector differed considerably from the other two sites by having markedly lower delta c 13 more variable delta n 15 and c n ratios and a sedimentary diatom assemblage that was never dominated by fragilariopsis kerguelensis although environmental parameters certainly have a strong impact on the isotope ratios delta c 13 is also correlated to the proportion of f kerguelensis in the three cores investigated here r 2 0 8 extreme values of delta c 13 delta n 15 and c n at the last glacial maximum were also related to the abundance of winter stages of eucampia antarctica these results suggest that diatom specific isotope records should be interpreted in conjunction with information on the species composition of the samples

evolution of the lake titicaca basin and its diatom flora over the last 370 000 years abstract in recent years deep drilling undertaken as part of the international continental drilling program has generated multiple long lacustrine sedimentary records to reconstruct continental paleoclimate in many cases the tectonic and geomorphic history of these basins is under constrained and poorly known which affects the interpretation of climate history from geophysical geochemical and paleobiotic proxies in the sedimentary record in addition non analog biotic assemblages that reflect evolutionary processes may constrain the reconstruction of past environments in the drill core record of lake titicaca spanning the last 370 ka the diatom stratigraphy reflects both the influence of climate and the long term evolution of the lake basin and its biota in the upper part of the drill core sequence glacial intervals were deep and dominated by freshwater planktic taxa and peak interglacial intervals were shallow and dominated by benthic species some with saline affinities in the basal sections of the drill core record benthic diatoms are dominant in both glacial and interglacial units with freshwater taxa dominating the glacial strata this suggests that the ancient lake basin was shallower during intervals of both wet and dry climate and that the modern deep lake may result from a progressive subsidence and deepening of the basin over time in addition morphological evolution in one of the major lineages of planktic diatoms cyclostephanos indicates substantial change in the limnological environment that affected species morphology and may have driven speciation

reconciling down core anticorrelation of diatom carbon and nitrogen isotopic ratios from the southern ocean 1 oceanic nutrient cycling in the southern ocean is supposed to have an important impact on glacial interglacial atmospheric co2 changes and global climate in order to characterize such nutrient cycling over the last two climatic cycles we investigated carbon and nitrogen isotopic ratios of diatom bound organic matter 13cdiat and 15ndiat respectively in two cores retrieved form the atlantic and indian sectors of the antarctic ocean the two cores show the same isotopic patterns the 13cdiat values are depleted during glacial periods and enriched during interglacial periods indicating lower productivity during cold times the 15ndiat values are enriched during glacial periods and depleted during interglacial periods arguing for greater nitrate utilization during cold times taken at face value this apparent contradiction leads to opposite conclusions on the role of the southern ocean biological pump on the atmospheric co2 changes however the two sets of data can be reconciled by a sea ice plus mixing rate scenario that calls upon a balance between the effect of cutting off gas transfer at the ocean atmosphere boundary and the effect of reducing vertical transport of nutrients through the pycnocline

self sedimentation of phytoplankton blooms in the geologic record abstract understanding the formation of laminated organic rich sediments is an essential topic for researchers interested in fossil fuels biogeochemical cycles earth s environmental history and global change biologists have very recently demonstrated that some marine phytoplankton blooms actively govern their own sedimentation by the formation of sticky transparent gels that facilitate rapid aggregation accelerated sinking and efficient export flux here we present fossil evidence of unfragmented low diversity phytoplankton assemblages preserved as sedimentary laminae and irregular flocs that are attributable to a similar phytoplankton driven sedimentary mechanism we term self sedimentation the geological evidence suggests that self sedimentation precludes significant heterotrophic grazing propels the formation of some conspicuous hemipelagic sedimentary laminae and results in efficient carbon and opal flux to the sediments we suggest that the self sedimentation phenomenon may have broad implications for the geological history of biogeochemical cycling oceanic ecological dynamics and abrupt atmospheric environmental change broader recognition of the self sedimentation phenomenon as explicitly defined here is a prerequisite to testing these unconventional hypotheses

biogeochemical processes controlling oxygen and carbon isotopes of diatom silica in late glacial to holocene lacustrine rhythmites biogeochemical cycles and sedimentary records in lakes are related to climate controls on hydrology and catchment processes changes in the isotopic composition of the diatom frustules 18odiatom and 13cdiatom in lacustrine sediments can be used to reconstruct palaeoclimatic and palaeoenvironmental changes the lago chungara andean altiplano 18 15 s 69 10 w 4520 masl diatomaceous laminated sediments are made up of white and green multiannual rhythmites white laminae were formed during short term diatom super blooms and are composed almost exclusively of large sized cyclostephanos andinus these diatoms bloom during mixing events when recycled nutrients from the bottom waters are brought to the surface and or when nutrients are introduced from the catchment during periods of strong runoff conversely the green laminae are thought to have been deposited over several years and are composed of a mixture of diatoms mainly smaller valves of c andinus and discostella stelligera and organic matter these green laminae reflect the lake s hydrological recovery from a status favouring the diatom super blooms white laminae towards baseline conditions 18odiatom and 13cdiatom from 11 990 to 11 530 cal years bp allow us to reconstruct shifts in the precipitation evaporation ratio and changes in the lake water dissolved carbon concentration respectively 18odiatom values indicate that white laminae formation occurred mainly during low lake level stages whereas green laminae formation generally occurred during high lake level stages the isotope and chronostratigraphical data together suggest that white laminae deposition is caused by extraordinary environmental events el nino southern oscillation and changes in solar activity are the most likely climate forcing mechanisms that could trigger such events favouring hydrological changes at interannual to decadal scale this study demonstrates the potential for laminated lake sediments to document extreme pluriannual events

evaluation of lacustrine organic δ13c as a lake level indicator a case study of lake qinghai and the satellite lakes on the tibetan plateau abstract in recent years the sedimentary 13corg values have been recognized to be indicative of lake level changes in lakes on the tibetan plateau tp however the sensitivity of sedimentary 13corg to lake level changes and the potential relationship between lake level variations and climatic forces are still unknown in this particular region here we systematically evaluate sedimentary 13corg as a proxy of lake level changes in lakes with different sizes on the tp consistent with previous studies the total organic matter of sediments was mainly contributed by aquatic plants in lakes on the tp in the studied lakes sedimentary 13c values mainly reflect lake level changes as the species of aquatic plants with different 13corg values strongly respond to lake level however sedimentary 13c values are much more sensitive to lake level changes in a moderate depth lake 10 m than those in a deep lake 27 m or a shallow lake