neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

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

how partial melts of mafic lower crust affect ascending magmas at oceanic ridges we present experiments showing that the lower oceanic crust should melt efficiently and quickly when heated by hot ascending magmas average plagioclase olivine and plagioclase augite pairs from the lower crust at the southwest indian ridge have melt mineral saturation boundaries at 1 190 and 1 154 c respectively and melt rapidly 0 01 mm h at 50 c or more above these temperatures melting experiments performed on olivine plagioclase and augite plagioclase mineral pairs from actual oceanic lower crustal rock samples and under conditions applicable to a mor setting 1 220 1 330 c 1 atm quartz fayalite magnetite oxygen buffer 0 25 24 h indicate that the resulting disequilibrium melts are linear mixes of the mineral compositions the rates of melting are slower than the rate of heat diffusion into a sample and are approximated as 2 43 10 26 e0 004109 t degc m s1 2 for augite melted with plagioclase 3 47 10 15 e0 002041 t degc for plagioclase melted with augite and 1 79 10 21 e0 0032 t degc for plagioclase melted with olivine our results indicate that great care must be taken in backward models using basalt chemistry alone to explore mantle melting processes assuming only crystallization and fractionation during ascent as partial melts may mix with intruded hot magma

a transient heating event in the history of a highlands troctolite from apollo 12 soil 12033 a 1 5 mm clast of a highlands troctolite attached to a glassy alkali rich soil breccia was identified in a thin section of 1 2 mm particles from apollo 12 soil sample 12033 the troctolite consists of euhedral olivines fo89 in a groundmass of granulitic plagioclase an95 each olivine grain is rimmed by a thin selvage of olivine plagioclase glass containing vesicles and minute crystallites of spinel olivine and plagioclase this unique texture records a complex history beginning with the formation of the troctolite as a plutonic cumulate followed by subsolidus metamorphism partial remelting vesculation crystallite growth and quenching experimental runs on plagioclase olivine melting rates show that at temperatures above incipient melting the grain boundary selvages in the lunar troctolite would have formed very rapidly in less than about 10 min preservation of selvage glass required rapid cooling the troctolite was subject to a heat pulse most likely accompanying an impact event the same event or more probably a later one welded the troctolite to the soil breccia inasmuch as this particle was found on oceanus procellarum at a site crossed by a ray from copernicus and because olivine has been identified by remote sensing experiments as the dominant mafic mineral in the central peak of copernicus we suggest that the troctolite is a fragment of copernicus ejecta

origin of the giant eruption cloud of pinatubo june 15 1991 the series of eruptions of june 15 1991 at mt pinatubo philippines were observed hourly by satellite a giant discshaped cloud covering an area of 60 000 km2 appeared in the satellite images at 14 40 philippine time the cloud expanded radially against wind of 20 m s and spread to an area of more than 120 000 km2 within an hour according to eyewitness accounts there was heavy fine ash fall after 14 00 intermittent lapilli fall started at about 14 20 and heavy and continuous lapilli fall widely started at about 15 00 the occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall the air fall deposits of the major eruption are widely distributed including upwind from the vent they are composed of 3 units a silt size fine ash layer layer b a lapilli layer commonly including pumice grains of 1 cm in diameter layer c a lapilli bearing volcanic sand layer layer d judging from its wide distribution and depletion of coarse grains most of the fine ash of layer b is not distal deposits of a small eruption but is originated from a large co ignimbrite cloud it is suggested that the major eruption started with the generation of a pyroclastic flow which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall the results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg s

magmatic metasomatism and formation of the merensky reef bushveld complex the rare earth element ree contents of pyroxenes and other minerals from the merensky reef and stratigraphically adjacent rocks of the atok section bushveld complex have been determined with the ion microprobe merensky reef clinopyroxene and orthopyroxene contain much higher and more variable concentrations of the ree than their cumulus counterparts in rocks several meters below the reef chondrite normalized merensky clinopyroxene ce contents vary from 10 to 90 for ce and from 4 to 17 for yb they also possess deep negative eu anomalies the eu anomalics being deeper for crystals having high ree contents and relatively shallow for pyroxenes with low ree contents similar compositional characteristics are displayed by cl rich apatite which is an accessory phase in the rocks interstitial pyroxene in cumulates above and below the reef also tends to have elevated ree contents and in general is not in equilibrium with coexisting cumulus minerals the melt from which the cumulus minerals crystallized falls within the compositional range of continental basalts that from which merensky and postcumulus pyroxenes crystallized is inferred to be much more highly enriched in ree than any normal tholeiitic or alkalic basalt despite their highly evolved nature in terms of the ree the merensky reef pyroxenes are not evolved in terms of major elements the decoupling of incompatible trace and major elements is best explained by a metasomatic process it is speculated that metasomatism involved upward percolation of hydrated silicate melt through and its reaction with the crystalline cumulate pile the fact that the rocks enriched in the platinum group elements are also those that show evidence for metasomatism suggests that these elements were also metasomatically redistributed

late quaternary palaeoecology and palaeoclimatology from pollen records of the savannas of the llanos orientales in colombia abstract pollen analysis of sediment cores from laguna angel 4 28 n 70 34 w 200 m elevation and laguna sardinas 4 58 n 69 28 w 80 m a s l in the savannas of the llanos orientales of colombia are presented the pollen record of laguna angel includes 7 radiocarbon dates and shows the holocene vegetation and climatic history since 10 030 yr b p the record of laguna sardinas includes 7 radiocarbon dates and shows the late glacial and holocene history since 11 570 yr b p both pollen records show that the present day savanna vegetation dominated by grassland poaceae with only few woody savanna taxa e g curatella bryrsonima was present since the late glacial indicating dry climatic conditions with a marked dry season shrubs and trees of forest and or gallery forest e g mauritia mauritiella moraceae urticaceae melastomataceae alchornea and myrtaceae are well represented during the recorded period the proportions between forest and savanna were subject to change in the past but also the composition of the forest and savanna ecosystems changed laguna sardinas shows a greater abundance during the late glacial holocene transition than today for alchornea especially from 10 680 to 10 070 yr b p for this interval synchronous to the el abra stadial younger dryas equivalent in colombia more humid climatic conditions have been inferred which are primarily the result of higher moisture levels under colder conditions during the early and middle holocene from 9730 to 5260 yr b p in laguna angel and from 9390 to 6390 yr b p in laguna sardinas the maximum expansion of grassland occurred indicating very dry climatic conditions the following period from 5260 to 3890 yr b p in laguna angel and from 6390 to 3680 yr b p in laguna sardinas shows a decrease of savanna herbs and an increase of forest and gallery forests reflecting a wetter period probably with a shorter annual dry season in both lake records the period after ca 3500 yr b p is incomplete but is characterized by a high representation of the palm genera mauritiella in laguna angel and mauritia in laguna sardinas this marked change has been interpreted as evidence of strong a anthropogenic influence on the vegetation but also wetter climatic conditions than during the late glacial and early and middle holocene periods

palaeolimnological approaches to climate change with special regard to the biological record abstract lakes systems respond physically chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records this paper focuses principally on the biological record and describes recent research that aims to reconstruct i past variability in effective moisture and ii past variability in temperature changes in effective moisture are manifested especially by changes in lake water level in closed lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes in the composition of fossil assemblages especially diatoms in open freshwater lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of cladocera ostracods and diatoms the most promising biological method for reconstructing past temperature is chironomid analysis and a range of new chironomid temperature transfer functions are being developed however biological responses to changing temperature are usually indirect as a result of temperature effects on water column stratification nutrient cycling and alkalinity generation and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column a multi proxy whole lake approach is advocated as the best way forward not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems

emplacement and high temperature evolution of gabbros of the 16 5 n oceanic core complexes mid atlantic ridge insights into the compositional variability of the lower oceanic crust this study reports the composition of the oceanic crust from the 16 5 n region of the mid atlantic ridge a spreading ridge segment characterized by a complex detachment fault system and three main oceanic core complexes southern central and northern occs lithologies recovered from the core complexes include both greenschist facies and weathered pillow basalt diabase peridotite and gabbro while only weathered and fresh pillow basalt was dredged from the rift valley floor the gabbros are compositionally bimodal with the magmatic crust in the region formed by scattered intrusions of chemically primitive plutonic rocks i e dunites and troctolites associated with evolved oxide bearing gabbros we use thermodynamic models to infer that this distribution is expected in regions where small gabbroic bodies are intruded into mantle peridotites the occurrence of ephemeral magma chambers located in the lithospheric mantle enables large proportions of the melt to be erupted after relatively low degrees of fractionation a large proportion of the dredged gabbros reveal evidence for deformation at high temperature conditions in particular chemical changes in response to deformation and the occurrence of very high temperature ultramylonites 1000 c suggest that the deformation related to the oceanic detachment commenced at near solidus conditions this event was likely associated with the expulsion of interstitial evolved magmas from the crystal mush a mechanism that enhanced the formation of disconnected oxide gabbro seams or layers often associated with crystal plastic fabrics in the host gabbros this granulite grade event was soon followed by hydrothermal alteration revealed by the formation of amphibole rich veins at high temperature conditions 900 c

annual and inter annual variability of the present climate in northern south america and southern mesoamerica abstract present climate of northwestern south america and the southern isthmus is detailed in terms of major hydro climatic controls supported by evidence from station records reanalysis data and satellite information in this tropical region precipitation is the principal hydro climatological variable to display great variability the primary objective is to view the controls that operate at intra seasonal to inter decadal time scales this is a topographical complex region whose climate influences range in provenance from the south atlantic to the canadian prairies and from the north atlantic to the eastern pacific the situation is further complicated by interactions and feedbacks in time and space between these influences which are interconnected over various scales the greatest single control on the annual cycle is the meridional migration of the inter tropical convergence zone and its pattern of associated trade winds consideration of these alone and their interaction with the cordilleras of the andes and central america produce a variety of unimodal and bimodal regimes regionally two low level jet streams the westerly choco jet 5 n and the easterly san andres jet 12 14 n and their seasonal variability have tremendous significance as do mesoscale convective storms and mid latitude cold fronts from both the northern nortes and southern friagems hemispheres there are many examples of hydro climatological feedbacks within the region of these the most notable is the interaction between evaporation over the amazon precipitation onto the eastern andes and streamflow from the headwaters of the amazon this is further compounded by the high percentages of recycled precipitation over large areas of the tropics and the potential impacts of anthropogenic modification of the land surface the el nino southern oscillation phenomenon enso is the greatest single cause of interannual variability within the region yet its effects are not universal in their timing sign or magnitude a set of regional physical connections to enso are established and their varying local manifestations are viewed in the context of the dominant precipitation generating mechanisms and feedbacks at that location in addition some potential impacts of longer run variations within the ocean atmosphere system of the atlantic are examined independently and in conjunction with enso this review of the climatic controls and feedbacks in the region provides a spatial and temporal framework within which the highly complex set of factors and their interactions may be interpreted from the past

palaeoenvironmental changes during the last ca 8590 calibrated yr 7800 radiocarbon yr in the dry forest ecosystem of the patia valley southern colombian andes a multiproxy approach two sediment cores were recovered from two swamps in the dry forest lowland ecosystem of the patia valley colombia the 500 cm long core el patia 1 was analysed using diatoms pollen lithostratigraphy geochemistry charcoal and radiocarbon dates the 270 cm long core patia 2 was analysed using pollen lithostratigraphy and radiocarbon dates the environmental reconstruction includes the local development and history of the patia 1 swamp and the regional vegetation history of the two swamps the environmental reconstruction shows a fluvial system in the area before 7830 14c yr bp ca 8590 cal yr bp after this date the river gradually moved away leading to the formation of the swamp patia 1 open vegetation with patches of dry forest surrounded the swamp between 7670 and 6860 14c yr bp 8350 7690 cal yr bp the swamp become partially isolated from the river and was a permanent water body which was alkaline and mesotrophic during this period the surroundings were covered by dry forest from 6860 to 5870 14c yr bp 7690 6690 cal yr bp the swamp was totally isolated from the river and suffered from frequent periods of desiccation and erosion from 5870 to 3570 14c yr bp 6690 3890 cal yr bp the swamp turned into a marsh terrestrial environment and in the surrounding area the dry forest alternated with open vegetation it is possible that there were human impacts on the environment during this period between 3570 and 1990 14c yr bp 3890 2000 cal yr bp unstable conditions prevailed after 1990 14c yr bp 2000 cal yr bp swamp conditions of permanent water were re established open vegetation surrounded the swamp agriculture and human activities were present and have continued since then the climatic reconstruction suggests an increase in humidity at ca 7670 14c yr bp 8350 cal yr bp and a dry climate between 6860 and 5870 14c yr bp 7690 6690 cal yr bp

crustal structure and thickness along the yellowstone hot spot track evidence for lower crustal outflow from beneath the eastern snake river plain 1 receiver functions from seismic stations about the yellowstone hot spot track are migrated to depth using a vp vs map constructed from stacking of the direct and free surface moho reverberations i e h k analysis and a shear velocity tomogram constructed from surface wave measurements the thickest crust 48 54 km resides in the wyoming province beneath the sampled laramide age blocks and the thinnest crust 32 37 km resides in the montana basin and range province the eastern snake river plain esrp crust is thickest 47 km at its ne end beneath the young calderas and thinnest 40 km at its sw end beneath the older twin falls caldera two esrp crustal thickness domains are found 1 at the older twin falls and picabo calderas the mean esrp crust is 4 km thicker with respect to its margins and 2 adjacent to the heise caldera field the mean esrp crust is 4 km thicker with respect to its se margin crust but no thicker with respect to its nw margin crust this lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the heise caldera field confirmation of these crustal thickness variations is provided by inspection of common conversion point ccp stacks that delineate several secondary features the top of a thick high velocity 3 9 km s lower crust layer within the wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the yellowstone caldera this paired arrival would be consistent with a low velocity zone perhaps associated with magma staging beneath the caldera our most important finding is that the magmatic loads injected into the esrp crust over the last 4 12 myr in tandem with the esrp crustal viscosity structure have been sufficient to drive significant outflow of the esrp lower crust

continuous mixing of crystal mush and replenished magma in the ongoing unzen eruption zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing the dacite was formed by mixing of relatively high and low temperature t end member magmas the magnetite phenocrysts derived from the low t magma are reversely zoned by the mixing with high t magma a diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching for the mixed dacite erupted from may 1991 to may 1993 the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr this indicates that the mixing was continuous during the effusion the invariability of the other mixing signatures such as the thickness of reaction rims around biotite phenocrysts also supports the continuous mixing model low t end member magma is estimated by mass balance calculation to be a crystal rich mush of dacitic composition these observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion the proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes

drilling constraints on lithospheric accretion and evolution at atlantis massif mid atlantic ridge 30 n expeditions 304 and 305 of the integrated ocean drilling program cored and logged a 1 4 km section of the domal core of atlantis massif postdrilling research results summarized here constrain the structure and lithology of the central dome of this oceanic core complex the dominantly gabbroic sequence recovered contrasts with predrilling predictions application of the ground truth in subsequent geophysical processing has produced self consistent models for the central dome the presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100 220 kyr and not as a single magma pulse isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth below 800 m alteration is restricted to narrow zones surrounding faults veins igneous contacts and to an interval of locally intense serpentinization in olivine rich troctolite hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range deformation of the sequence was remarkably localized despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation presumably during unroofing via detachment faulting both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent southern ridge of the massif there the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70 of recovered samples we develop a working model of the evolution of atlantis massif over the past 2 myr outlining several stages that could explain the observed similarities and differences between the central dome and the southern ridge

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

cumulate recycling and crustal evolution in the bay of islands ophiolite the lower crustal section of the north arm mountain massif of the bay of islands ophiolite is composed of interlayered gabbroic and ultramafic cumulates despite pervasive ridge related deformation key contact relationships are preserved in low strain domains or late kinematic intrusions angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts olivine gabbro gabbro gabbro norite norite and fe oxide amphibole gabbros the ultramafic intrusions are composed of dunite wehrlite harzburgite lherzolite and olivine websterite clino to ortho pyroxenite reaction rims line many peridotitic gabbroic contacts the voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros fine grained harzburgitic sills crosscut the foliation of wehrlitic host rocks with orthopyroxenite reaction rims

silurian k bentonites of the dnestr basin podolia ukraine the dnestr basin of podolia ukraine is an epicratonic basin consisting of neritic carbonate and calcareous mudstone facies including a nearly complete silurian sequence ranging from late llandovery to late pridoli in age the silurian section has served as a standard for regional and interregional studies as a consequence of its well documented macro and microfaunal assemblages approximately 24 mid to late silurian k bentonites are present in this succession and their lateral persistence has aided in establishing regional correlations the k bentonites range from 1 to 40 cm in thickness and occur in the bagovitsa late wenlock malinovtsy ludlow and skala pridoli formations discrimination diagrams based on immobile trace elements together with rare earth element data suggest the k bentonites had a volcanic origin in a collision margin setting related to subduction thickness and stratigraphic distribution considerations are consistent with a source area in the rheic ocean

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

diatom community response to multiple scales of holocene climate variability in a small lake on victoria island nwt canada abstract a lake sediment core spanning 9900 years collected from a small lake on western victoria island nwt canada provides a high resolution record of diatom community dynamics over the holocene ten radiocarbon dates and 210 pb dating provided the core chronology loss on ignition loi magnetic susceptibility biogenic silica content and diatom concentrations provided information on changes in the sedimentary environment loi gradually increased over the holocene whereas magnetic susceptibility showed an inverse trend biogenic silica content showed three distinct peaks spaced approximately 3000 years apart major shifts in diatom assemblages occurred at 8100 8000 5800 5700 and 3800 3500 cal yr bp there is evidence of diatom community response to centennial scale variations such as the medieval warm period 1000 700 cal yr bp little ice age 800 150 cal yr bp and recent warming although recent changes in diatom community composition productivity and species richness are apparent they were surpassed at other periods throughout the holocene variations of the taxa within the genera staurosira pseudostaurosira fragilaria and staurosirella usually combined into one genus in arctic lake sediment studies suggest these taxa provide useful insight into paleonvironmental questions

pollen and diatom based environmental history since the last glacial maximum from the andean core fuquene 7 colombia the late pleistocene holocene ecological and limnological history of lake fuquene 2580 m a s l in the colombian andes is reconstructed on the basis of diatom pollen and sediment analyses of the upper 7 m of the core fuquene 7 time control is provided by 11 accelerator mass spectrometry ams 14c dates ranging from 19 670 240 to 6040 60 yr bp in this paper we present the evolution of the lake and its surroundings glacial times were cold and dry lake levels were low and the area was surrounded by paramo and subparamo vegetation late glacial conditions were warm and humid the el abra stadial a younger dryas equivalent is reflected by a gap in the sedimentary record a consequence of the cessation of deposition owing to a drop in lake level the early holocene was warm and humid at this time the lake reached its maximum extension and was surrounded by andean forest the onset of the drier climate prevailing today took place in the middle holocene a process that is reflected earlier in the diatom and sediment records than in the pollen records in the late holocene human activity reduced the forest and transformed the landscape climate patterns from the late glacial and throughout the holocene as represented in our record are similar to other records from colombia and northern south america the caribbean venezuela and panama and suggest that the changes in lake level were the result of precipitation variations driven by latitudinal shifts of the intertropical convergence zone copyright 2003 john wiley sons ltd

holocene hydrological reconstructions from stable isotopes and paleolimnology cordillera real bolivia multiproxy analyses of sediment cores from lago taypi chaka kkota ltck cordillera real bolivia provide a record of drier conditions following late pleistocene deglaciation culminating in pronounced aridity between 6 2 and 2 3 ka b p today ltck is a glacial fed lake that is relatively insensitive to changes in p e because it is largely bu ered from dry season draw down through the year round supply of glacial meltwater this was not the case during the middle to late holocene when glaciers were absent from the watershed lake water d18o values inferred from d18o analysis of sediment cellulose range from 12 9 to 5 3 and average 8 7 between 6 2 and 2 3 ka b p modern lake water d18o from ltck averages 14 8 which is compatible with the d18o 8 value of 14 3 for the surface sediment cellulose analyses of d18o from modern surface waters in 23 lakes that span the range from glacial fed to closed basin vary from 16 6 to 2 5 this approximates the magnitude of the down core shift in d18o 8 values in ltck during the middle to late holocene from 12 9 to 5 3 strong paleohydrologic change during the middle holocene is also evident in diatom assemblages that consist of shallow water non glacial periphytic taxa and bulk organic d13c and d15n that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions neoglaciation began after 2 3 ka b p as indicated by changes in the composition of the sediments lower d18o values and a return to diatom assemblages characteristic of the glacial sediments that formed during the late pleistocene collectively these data indicate that the past 2 3 ka b p have been the wettest interval during the holocene millennial scale shifts in the paleohydrologic record of ltck during the early to middle holocene conform to other regional paleoclimatic time series including lake titicaca and nevado sajama and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply in contrast an apparent 1200 year lag in the onset of wetter conditions at ltck 2 3 ka b p compared to lake titicaca 3 5 ka b p provides evidence for variable sub regional hydrologic response to climate change during the middle to late holocene 2000 elsevier science ltd all rights reserved

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

hydrogen peroxide treatment effects on the particle size distribution of alluvial and marsh sediments pretreatment of sediment with hydrogen peroxide to remove organic constituents and aid deflocculation is a common component of particle size analyses of terrestrial and marine sediments this study

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

geochemistry of siluro devonian tobique volcanic belt in northern and central new brunswick canada tectonic implications siluro devonian volcanic rocks of the northwestern mainland appalachians are found mainly in the tobique belt of new brunswick where they consist predominantly of bimodal mafic felsic suites erupted in a continental rift environment the axis of the tobique rift trends north northeast south southwest obliquely to the regional northeast southwest trend of the appalachians these geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the avalon terrene the basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper mantle source that was enriched in incompatible elements relative to the primordial mantle the mantle source was probably affected by the subduction processes

climatic change during the younger dryas chron in northern south america a test of the evidence abstract new ams and palynological data are presented from the colombian andes to assess vegetational and climatic change during the lateglacial holocene transition with special emphasis on the younger dryas yd chronozone the new evidence is compared and discussed with other colombian cores and with data from other countries in the region the lateglacial climatic reversal in colombia known as the el abra stadial has an estimated lower boundary of 11 200 conventional and 10 900 interpolated 14 c yr bp respectively although the el abra stadial is assumed to be the equivalent of the european yd the present data suggest that the el abra stadial is a biostratigraphical signal representing both the yd chronozone and the earliest holocene 11 000 9000 14 c yr bp on the basis of new ams dates and a re evaluation of the pollen zones we divided the colombian el abra biozone into two phases from ca 11 000 to ca 10 500 14 c yr bp there is a sharp increase of subparamo and paramo pollen reflecting a relatively cool phase during the yd chronozone zone y1 after ca 10 500 14 c yr bp a slight increase of arboreal pollen and the presence of cactaceae zone z1 point toward a relatively milder but drier phase extending to ca 9000 14 c yr bp in the earliest holocene our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern south america between 12 400 and 8800 14 c yr bp we propose an environmental drought during the el abra biozone with subsequent erosion of the sediments deposited during that period as a major factor explaining the poor presence of yd evidence in northern south america we conclude that at hydrologically sensitive sites without a clear lithological change around 11 000 9000 14 c yr bp a hiatus during the yd chronozone can only be detected if other cores are available with bracketed time control we state that the start of the temperature decline in central america and northern south america is related to the global yd cooling event however much work is still required to understand the duration and amplitude of the climatic reversal during the yd chronozone in this part of the globe

building of the deepest crust at a fossil slow spreading centre pineto gabbroic sequence alpine jurassic ophiolites this work presents new field and petrological data on a poorly known lower crustal section from the alpine jurassic ophiolites the pineto gabbroic sequence from corsica france the pineto gabbroic sequence is estimated to be 1 5 km thick and mainly consists of clinopyroxene rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gab bros in its deeper sector the sequence also encloses olivine rich troctolite and mantle peridotite bodies at dif ferent stratigraphic heights the composition and the lith ological variability of the pineto gabbroic sequence recall those of the lower crustal sections at slow and ultra slow spreading ridges the gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros which approximately constitute 2 3 of the section in particular the lower sector of the pineto gab broic sequence shows the existence of large scale frag ments of the deepest oceanic crust displaying a highly primitive bulk composition the mineral chemical varia tions document that the origin and the evolution of the pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation the clinopyroxenes from the olivine gabbros and the olivine rich troctolites also record the infiltration of olivine dissolving cr2o3 rich melts that presumably formed within the mantle into replacive dunite bodies cooling rates of the troctolites and the olivine gabbros were evaluated using the ca in olivine geospee dometer we obtained high and nearly constant values of 2 2 to 1 7 c year log units which were correlated with the building of the pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold litho spheric mantle

stratigraphic context geochemical and isotopic properties of magmatism in the siluro devonian inliers of northern maine implications for the acadian orogeny this paper reports detailed stratigraphic analysis whole rock geochemistry and nd isotopic ratios of basalts from the northern maine inliers these data place constraints on first order controversies about the tectonic conditions leading up to and during the early stages of the acadian orogeny in northern maine late silurian and early devonian stratigraphic sequences indicate a progressive change from shallow water and subaerial exposure ripogenus and the forks formations east branch group to a rapidly subsiding basin prior to the onset of acadian deformation subsidence was accompanied by mafic to intermediate volcanism of the west branch spider lake and fish pond volcanics and that continued with intrusion of the syn deformational greenville plutonic belt including the flagstaff lake igneous complex whose properties we report in this paper trace element geochemistry indicates that magmatism is transdiscriminant showing aspects of within plate volcanic arc and back arc affinities and nd isotopic ratios are moderately positive 2 3 to 3 8 indicating either an uncontaminated and moderately enriched mantle source or a depleted mantle source contaminated by continental crust four possible tectonic models of the acadian orogeny and the siluro devonian sedimentary volcanic sequences of northern maine are evaluated in the context of a subsiding basin associated with this magmatism developed on the amalgamated laurentian plate these include 1 slab detachment during southeast directed subduction of the laurentian continental margin 2 laramide style thrust basins above a shallow northwest dipping subduction zone 3 back arc extension followed by thin skinned shortening above a northwest dipping subduction zone and 4 moluccan style dual dipping subduction zones

diatom shifts as evidence for recent subarctic warming in a remote tundra lake nwt canada abstract changes in diatom assemblage composition were examined from the sediments of slipper lake an isolated tundra lake located 50 km north of current treeline in canada s northwest territories diatom shifts over the last ca 5600 years were related to the response of this ecosystem to climatic and environmental change during the late holocene with particular emphasis on the last few hundred years to date a detailed diatom analysis for this recent time frame has not been undertaken for the central canadian subarctic the first ca 5400 years of the sediment record was marked by modest shifts between benthic alkaliphilous diatom taxa e g fragilaria achnanthes and navicula species and heavily silicified tychoplanktonic aulacoseira species in the 19th century abrupt changes were delineated by a marked shift to a diatom assemblage characteristic of more planktonic habitats consisting of the cyclotella stelligera complex c stelligera c pseudostelligera which was absent in earlier sediments several possible mechanisms for these recent changes are examined including atmospheric deposition of acidifying compounds and anthropogenically derived nutrient enrichment however we conclude that climatically induced limnological changes associated with shorter duration of ice cover and a longer growing season can best explain the direction and magnitude of changes in our diatom record

holocene multidecadal to millennial scale hydrologic variability on the south american altiplano precipitation on the south american altiplano varies at a range of temporal scales a long term secular increase in moisture availability from the early mid holocene to the present driven by increasing summer insolation resulting from precessional changes in the earth s orbit has been documented in earlier studies however higher frequency holocene variability is not yet understood here we present high resolution diatom assemblage data from two small altiplano lakes lago lagunillas and lago umayo indicating changes in effective moisture in the southern tropical andes at decadal centennial and millennial timescales throughout the mid to late holocene a strong millennial scale component similar in pacing to periods of increased icerafted debris flux in the north atlantic is observed in both lake records which suggests that regional precipitation and north atlantic climate variability are coupled at these scales the interpretation of the higher frequency variability is hampered by the small numbe

holocene tropical south american hydroclimate revealed from a decadally resolved lake sediment δ 18 o record abstract oxygen isotope ratios of authigenic calcite 18 o cal measured at annual to decadal resolution from laguna pumacocha document andean precipitation variability during the last 11 200 years modern limnological data show that pumacocha 18 o cal reflects the average annual isotopic composition of the lake s surface waters 18 o lw and that 18 o lw tracks the isotopic composition of precipitation 18 o precip which is largely controlled by the intensity of the south american summer monsoon sasm based on these relationships we use down core 18 o cal measurements as a proxy for 18 o precip that varies with the intensity of sasm precipitation pumacocha 18 o cal increased rapidly between 11 200 and 10 300 yr b p from 14 5 to 10 5 reaching a maximum of 10 3 by 9800 yr b p after 9800 yr b p 18 o cal underwent a long term decrease that tracked increasing southern hemisphere summer insolation suggesting that enhanced sasm precipitation was linked to precessional forcing higher frequency trends did not follow insolation and therefore represent other variability in the climate system millennial scale trends from pumacocha strongly resemble those from lower resolution tropical andean ice and lake core isotopic records particularly the huascaran ice core and low elevation speleothems these relationships suggest that tropical andean isotopic records reflect variations in precipitation intensity related to precessional forcing rather than tropical temperatures they also demonstrate a coherent pattern of sasm variability although with differences between low elevation and andean records during the late glacial to holocene transition and the late holocene centennial and decadal sasm precipitation variability is also apparent reduced sasm rainfall occurred from 10 000 9200 7000 5000 1500 900 yr b p and during the last 100 years intensifications of the sasm occurred at 5000 2200 1500 and 550 130 yr b p with the amplitude of variability increasing after 2200 yr b p these periods may represent sasm responses to ocean atmosphere variability related to orbital and radiative forcing e g el nino southern oscillation and the intertropical convergence zone

volcanic expression of bimodal magmatism the cranberry island cadillac mountain complex coastal maine the cranberry island series is a thick succession of silurian volcanic breccias ash flow tuffs lava flows and hypabyssal intrusions comprising the southern margin of mount desert island and the cranberry islands of southeastern maine the rocks of the series preserve remarkable textures indicative of the simultaneous eruption of mafic and felsic magmas during both explosive and effusive events the cadillac mountain intrusive complex is an approximately 20 km wide composite layered mafic to felsic plutonic body that dominates mount desert island immediately north of the outcrop of the cranberry island series the rocks of the cranberry island series and those of the cadillac mountain intrusive complex are correlative in age and similar in composition the age of the cranberry island series based on new u pb geochronology of zircon is 424 ma pm 1 ma the cadillac mountain granite the somesville granite and the southwest harbor granite all parts of the cadillac mountain intrusive complex are

a late quaternary diatom record of tropical climatic history from lake titicaca peru and bolivia abstract a composite high resolution diatom stratigraphy from three piston cores and one box core in the deep sub basin of lake titicaca reveals large moisture variations during the past 30 kyr in the altiplano region diatom sequences indicate orbital and millennial scale variability in water level and salinity the pelagic freshwater diatom species cyclotella andina and cyclotella stelligera dominate glacial age sediments suggesting that the lake was above its present outlet generally wet conditions continued until 11 000 cal yr bp as indicated by high percentages of freshwater planktonic diatoms large pulses of benthic diatom species between about 11 000 and 10 000 cal yr bp suggest brief intervals of large amplitude declines in lake level during the early holocene 10 000 8500 cal yr bp a freshwater diatom assemblage suggests overflowing conditions pelagic freshwater diatoms are replaced ca 8500 cal yr bp by the salinity indifferent species cyclotella meneghiniana and by benthic taxa indicating the beginning of lake regression during the mid holocene 6000 3500 cal yr bp the abundance of the saline taxon chaetoceros muelleri coupled with high abundances of epiphytic and epipelic diatoms indicates maximum salinity and lowest lake levels in the entire 30 000 year record lake transgression began ca 4000 cal yr bp and the lake achieved modern levels by about 1500 cal yr bp these water level changes imply changes in effective moisture most likely resulting from large precipitation changes precipitation was high throughout the last glacial maximum 21 000 18 000 cal yr bp likely due to an enhanced south american summer monsoon during peak summer insolation in the southern hemisphere in contrast the mid holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

diatom responses to limnological and climatic changes at ribains maar french massif central during the eemian and early wurm abstract high resolution diatom analysis was carried out to assess the limnological and climatic changes that took place at ribains maar french massif central during the late pleistocene 131 105 ka bp with a focus on the eemian interglacial in particular numerical analyses were used to show that most of the variability in the fossil diatom assemblages was due to climate independently from the changes in the lake catchment vegetation as represented by pollen data diatom based quantitative reconstructions of the past limnological conditions as well as a comprehensive literature review on the auto ecological requirements for the principal diatom taxa were used to interpret the record an absolute time scale for the sequence was derived by matching the major pollen shifts with the radiometrically dated changes in oxygen isotopes observed in italian stalagmites this study shows that at ribains maar the transition from the riss saalian glacial to the eemian interglacial was marked by a gradual increase in the contribution of spring blooming diatom species indicating a longer growing season and milder winter spring conditions at that time a short cooling event interrupts this trend and may correspond to a stadial at the start of the eemian a peak in benthic taxa and the suppression of spring blooming flora probably reflects the effects of deglaciation on the catchment during the eemian interglacial itself three main phases were distinguished within the diatom record the first phase 8000 years in duration was dominated by stephanodiscus minutulus which suggests that intense mixing in the water column took place during spring the pollen record was simultaneously dominated by quercus and corylus that typify this phase as the climatic optimum of the eemian the second phase almost equal in duration to the first phase 7000 years is generally dominated by cyclotella taxa and suggests a less productive lake and much reduced period of spring mixing compared with the first phase in the pollen diagram this corresponds to an interval dominated by carpinus picea abies that indicates a cooler and wetter climate the third and last phase of the eemian 2000 year long saw the return to stephanodiscus dominated assemblages indicating a warming that may correspond to the dansgaard oeschger event 25 identified in the greenland ice core record in the early stage of the wurm glacial weichselian assemblages in the melisey i stadial 3000 year long were dominated by either aulacoseira subarctica or asterionella formosa which suggest colder spring conditions than during the late eemian but not as cold as the ones indicated by the pollen record stephanodiscus spp again dominate during the saint germain ia interstadial 5000 year long suggesting a return to the conditions that prevailed before the melisey stadial in agreement with the pollen record the record ends with the montaigu cold event which is characterised by a pinus peak in the pollen record and corresponds to a large abundance of a subarctica in the diatom sequence throughout the eemian the abundance of stephanodiscus spp which is thought to be driven by winter conditions show cyclic fluctuations that most likely match the cooling events identified in a pollen record from germany variation in insolation throughout the eemian may have been the driving factor behind the species succession observed in the diatom sequence while this study demonstrates that diatom analysis of lake sediment can provide very detailed information on long term climate change a review of the few other diatom investigations published on european eemian deposits shows that this technique has been so far seldom used to its full potential in this context in central and southern europe

emsian synorogenic paleogeography of the maine appalachians abstract the acadian deformation front in the northern appalachians of maine and new hampshire can now be closely located during the early emsian early devonian 408 406 ma tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408 ma time scale tie point for the early emsian the deformation front lay between a belt of lower devonian flysch and molasse that were deposited in an acadian foreland basin and had not yet been folded and a belt of early emsian plutons that intruded folded lower devonian rocks this plutonic belt includes the newly dated ore mountain gabbro u pb 406 ma which hosts magmatic sulfide mineralization along the deformation front a 407 ma pluton that locally truncates acadian folds katahdin was the feeder to volcanic rocks traveler rhyolite 406 407 ma that are part of the foreland basin succession involved in these same folds the emsian igneous rocks thus define a syncollisional magmatic province that straddled the d

calderas and ash flow tuffs of the mogollon mountains southwestern new mexico the mogollon mountains are a major volcanic source area in the southwestern mogollon datil volcanic field where about 2000 m of middle tertiary rocks are exposed the volcanic sequence includes eight major 100 km3 ash flow units believed to represent four caldera related compositionally zoned ash flow sequences each caldera related sequence shows normal compositional zonation from high silica rhyolite to rhyolite and dacite the mogollon caldera 34 ma is preserved only as a fragment in the wall of the bursum caldera the emory caldera 34 ma is about 50 km east of the mogollon mountains and only the high silica facies of its presumed outflow sheet fall canyon tuff reaches the mogollons the gila cliff dwellings caldera 30 ma earlier interpreted as the source of the bloodgood canyon tuff probably is a large remnant of a caldera or pair of calderas that were the source of the davis canyon and shelley peak tuffs the bloodgood canyon tuff was most likely erupted from the bursum caldera 29 28 ma but ponded in the gila cliff dwellings caldera the resurgent dome of the approximately 40 km diameter bursum caldera is the predominant topographic and structural feature of the mogollon mountains the caldera probably collapsed initially in response to eruption of the bloodgood canyon tuff then was largely filled with apache spring tuff prior to resurgent doming the cyclical eruption beginning about 34 ma of similar compositionally zoned ash flows from separate shallow magma chambers suggests derivation from a common parent magma of batholithic proportions a regional bouguer gravity low of about 40 mgals has been interpreted to reflect the presence of such a batholith major ash flow eruptions in the mogollon datil field were followed 26 25 ma by fundamentally basaltic volcanism characterized mainly by andesite about 22 21 ma high silica rhyolite tholeiitic basalt and alkali basalt were erupted as a bimodal suite roughly coincident with the beginning of basin range extensional faulting alkali olivine basalt as young as 5 5 ma is interlayered with basin fill gila conglomerate adjacent to the mogollon mountains

late glacial and holocene environmental and climatic changes from a limnological transect through colombia northern south america abstract we present a synthesis of the palaeolimnological and palaeoclimatic reconstructions of four sites in colombia the record from lake el caimito the westernmost site on the pacific coast dates from the late holocene and shows lacustrine sedimentation frequently interrupted by fluvial pulses these pulses probably reflect periods of increased precipitation related to la nina phases east of el caimito site is the patia swamp situated between the western and central andean cordilleras the patia records the dynamics of forest expansion reduction and changes in water levels although the climatic signal of the patia core is difficult to reconstruct there is a clear increase in humidity in the mid holocene the fuquene lake record on the eastern andean cordillera records dry and cold conditions during the late pleistocene very humid conditions for the early mid holocene and dry conditions during the mid late holocene las margaritas site on the eastern savannas records dry conditions during the early holocene and overall humid conditions for the mid and late holocene climate conditions from the fuquene and las margaritas sites seem to reflect the holocene movements of the intertropical convergence zone itcz the latter site being more affected by humidity coming from the amazon region

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

submillennium scale migrations of the rainforest savanna boundary in colombia 14c wiggle matching and pollen analysis of core las margaritas abstract we present the 11 150 cal yr long pollen record laguna las margaritas 3 23 n 73 26 w 290 m altitude located at a site sensitive to climatic change near the transition from the amazonian rainforest to the savanna of the llanos orientales in colombia in the 10 m long core nine ams 14 c bulk samples show ages from 9760 60 to 854 36 bp and provide initial time control thirty one additional ams 14 c samples of selected macrofossils provide time control from 6250 to 4050 cal bp for this interval precise time control was obtained by 14 c wiggle match dating from 11 150 to 9100 cal bp grass savanna dominated the landscape while gallery forest along the drainage system was poorly developed water availability was lower than today and the length of the dry season longer from 9100 to 7330 cal bp gallery forest expanded pointing to wetter conditions from 7330 to 5430 cal bp savanna was increasingly replaced by forest but with alternating abundance of both vegetation types from 5430 to 2500 cal bp forest and wooded savanna dominated the western llanos orientales suggesting high precipitation rates expansion of mauritia palm forest at 2500 cal bp reflects increasing water availability and stagnant water environments increasing savanna between 2500 and 1000 cal bp may represent a combined natural and anthropogenic signal development from open savanna to forest during the middle holocene is synchronous with a decreasing caloric seasonality and a southward migration of the intertropical convergence zone suggesting that the large scale climatic and vegetational change in the colombian savannas is precession forced high frequent migration of the savanna rainforest boundary started around 6000 cal bp and continued at least to 3000 cal bp precipitation regimes in northwest and northeast south america seem opposite dry conditions in the colombian savanna area seem to reflect a la nina setting of the climate system wet conditions reflect an el nino setting 14 c fluctuations reflect changes of solar activity and we tested the hypothesis that 14 c fluctuations correspond to climatic and vegetational change for the interval 6250 to 4050 cal bp we applied 14 c wiggle match dating wmd i e matching a series of radiocarbon ages in 14 c bp from the sediment with the dendrochronology based calibration curve and we also wiggle matched the pollen record vs the 14 c record in this first attempt in a neotropical ecosystem we could not find unambiguous support that changes in solar activity did trigger climatic and vegetational change in the savannas of the llanos orientales

subduction of young oceanic lithosphere and extensional orogeny in southwestern north america during mid tertiary time an extensional orogeny deformed the basin and range province probably beginning in the late eocene about 40 3 ma its characteristics include partial melting of the continental lithosphere during the ignimbrite flareup massive ductile extension including detachment faulting and rise of metamorphic core complexes the affected zone became about 1200 km wide possibly double its original width it rose an average of 1 2 km despite crustal thinning locally some of the highest mountains of north america up to 4 3 km high rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons the climax of extension occurred prior to the development of the present basin and range topography modeling of major and trace elements and sr and pb isotopes strongly suggests that mid tertiary volcanic magmas equilibrated and probably originated in the continental lithosphere components attributable to subducted oceanic lithosphere have not yet been identified the rocks seem to belong to two provinces separated by the quartz diorite boundary line of moore 1959 which also marks the western limit of north america at the end of the late paleozoic sonoman orogeny to the west low k rocks rest on a basement of predominantly oceanic accreted terranes to the east high k rocks rest on an autochthonous sialic basement within the high k province potassium variations can be correlated with crustal thickness there is no need to invoke a k h relationship conventional models of plate convergence and back arc extension which involve subduction of old rigid cool and dense oceanic lithosphere may not apply to the mid tertiary basin and range province the overridden farallon plate is more likely to have been young hot ductile buoyant and no denser than continental asthenosphere having been generated in a spreading center close to north america under these conditions motion of the subducting plate slows and slab pull is likely to approach zero even prior to ridge trench collision overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs subducted oceanic lithosphere no longer acted as a heat sink which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting extension and metamorphism had these processes not died down after ridge trench collision the western segment of the cordillera might have separated from north america to form a japanlike archipelago while the basin and range province foundered into an analog to the sea of japan instead of rupturing completely the basin and range province fractured into fault blocks

holocene paleolimnological reconstruction of a high altitude colombian tropical lake abstract aquatic environments in the high andean may be quite vulnerable to climatic conditions and hydrological processes precipitation evaporation that have the potential to alter water levels and chemistry this study reconstructs the paleohydrological history of lake tota colombia to provide a record of environmental changes in the eastern flank of the colombian andes during the late holocene a 54 cm core was collected at the margin of the lake at a depth of 1 5 m and the paleohydrological reconstructions were based on changes in diatom assemblages granulometry and bulk organic geochemical parameters the core was subdivided into 5 biozones by assessing the significance of the variance among the coniss clusters biozone t5 base 4000 calibrated years bp was dominated by the planktonic diatom discostella stelligera and was deposited in a period of high water level sedimentation during this period was rich in carbon due to phytoplanktonic production the granulometry data suggests that the lake was under high to moderate hydrological dynamic conditions indicating active deposition at the river delta front biozone t4 beginning at 3600 cal yrs bp was dominated by three species d stelligera cocconeis placentula var lineata and fragilaria sensu lato and this association could indicate that the lake was under the influence of large fluctuations in water level this interpretation is supported by granulometry and geochemical data particularly by the low levels of total organic carbon toc in biozone t3 corresponding to 3000 cal yrs bp we found a lowering of the water level suggested by low levels of toc poor granulometric sorting and higher carbon nitrogen c n ratios suggest a macrophyte influence on the sedimentation avoiding direct deltaic influence in this biozone the benthic diatom f sensu lato group was dominant in biozone t2 starting 2400 cal yrs bp the proxies indicate that the lake experienced increasing water levels and that stronger circulation patterns occurred in the water column than in the preceding periods as inferred by the presence of aulacoseira granulata a ticoplanktonic species the sediment in this biozone is well sorted and finer indicating a greater depth biozone t1 at the top of the core and since 2000 cal yrs bp to the present evidence for a recent reduction in water level of lake tota can be deduced by the presence of achnanthidium minutissimum var minutissimum

origin of oceanic ferrodiorites by injection of nelsonitic melts in gabbros at the vema lithospheric section mid atlantic ridge abstract oxide gabbros are a minor but diffuse component of the lower oceanic crust their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models here we report on a study of lower crust gabbros recovered from the vema lithospheric section at 11 n along the mid atlantic ridge where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusives and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in fe ti oxides and apatites their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of fe ti p saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing de compaction mineral resorption and chemical re equilibration melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment we propose a regime of lateral instead of vertical melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of fe ti p melt pockets in oceanic domains

late holocene environmental history of southern choco region pacific colombia sediment diatom and pollen analysis of core el caimito abstract we present a multi proxy study of pollen diatoms sediment characteristics and major elements of a 610 cm sediment core from lake el caimito located in the humid rain forest of southern choco pacific colombia we propose an integrated reconstruction of the local basin development and of the regional vegetation which is possibly related to the tectonic activity of an unstable coastal area time control is based on 7 ams 14 c dates that show that the record represents the last 3850 cal yr bp from 3850 to 2700 cal yr bp sandy deposits low carbon content absence of diatoms and low diversity of the pollen spectra indicate that the site was under the influence of the fluvial system erosive event s removed part of the sediment record and we observed a hiatus representing 700 years after that the basin became more isolated from the river drainage system from 2010 to 1430 cal yr bp mainly clay was deposited and repeatedly interrupted by river pulses that left sandy and silty horizons in the record benthic and littoral benthic diatom species indicate a shallow water body and a stable water chemistry mangrove forest was close to the lake apparently growing along the close by inlets regionally the main vegetation elements were palms and taxa in the families of moraceae urticaceae melastomataceae fabaceae and from a number of other families and genera characteristic of tropical lowland rain forest from 1430 to 810 cal yr bp the river impact gradually diminished each fluvial event that affected the local forest is shown in the pollen record by an expansion of cecropia dominated pioneer forest decreasing intensity of forest disturbance coincides with an increase in the diversity of fossil pollen taxa possibly reflecting an increasing plant diversity of the forest mangrove pollen declined indicating that the coastline moved seaward and suggesting tectonic uplift of the coastal area between 810 and 580 cal yr bp mangrove forest was closer to the lake again reflecting an inland migration of the coastline suggesting tectonic subsidence from 580 to 300 cal yr bp the last fluvial events were recorded diatom associations indicate oligotrophic and acidic water the mangrove belt moved seaward again suggesting tectonic uplift palms and cecropia became more abundant suggesting increased human impact in the near shore lowland forest during the last 300 years stable lacustrine conditions and lowland rain forest with the highest floral diversity is registered

late silurian early devonian transpressional rift origin of the quebec reentrant northern appalachians constraints from geochemistry of volcanic rocks silurian devonian rocks in the northwestern mainland appalachians form part of an overstep sequence deposited across the vestiges of iapetus after the collision of laurentia and gondwana they occur in three synclinoria gasl connecticut valley merrimack aroostook and fredericton synclinoria separated by two anticlinoria munsungun pennington boundary mountain bronson hill and miramichi silurian rocks in gasp6 connecticut valley and merrimack aroostook synclinoria consist of shelf and trough sediments derived mainly from laurentia to the northwest with some local contributions from an ephemeral landmass appalaclfia and from the miramiclfi highlands in contrast early devonian rocks in all three synclinoria are mainly flysch derived froxn the southeast in avalonia interbedded volcanic rocks are predominantly late silurian early devonian in age they are generally bimodal with the mafic rocks exhibiting a change from transitional alkalic tholeiitic to tholeiitic around the siluro devonian boundary broadly coincident with the salinic disturbance in the gaspe peninsula the geochemical characteristics of the basalts indicate that they were erupted in a continental intraplate environment with melting migrating upward through time across the garnet to spinel phase boundary as stretclfing increased thermal uplift during rifting is inferred to have led to erosion associated with the salinic disturbance the start of the volcanism appears to coincide with a switch from sinistral to dextral transpression along the orogen that may be related to the change from clockwise to anticlockwise rotation of laurentia relative to gondwana the effects of dextral transpression vary with the trend of the orogen intense deformation and metamorplfism occurred in the central mobile belt opposite the new york and st lawrence promontories whereas rifting developed in the quebec reentrant leading to tlfinning of the crest up welling of the asthenosphere melting and magmatism

orbitally driven east west antiphasing of south american precipitation the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales a comparison between a speleothem record of precipitation in northeast brazil and rainfall reconstructions from the rest of tropical south america shows that a similar antiphasing operated in the same hemisphere during the holocene the variations of tropical precipitation are antiphased between the hemispheres on orbital timescales this antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the hadley cells1 2 3 4 here we compare an oxygen isotopic record recovered from a speleothem from northeast brazil for the past 26 000 years with existing reconstructions of precipitation in tropical south america5 6 7 8 during the holocene we identify a similar but zonally oriented antiphasing of precipitation within the same hemisphere northeast brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high whereas the rest of southern tropical south america shows opposite characteristics simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation driven monsoon activity our results suggest that convective heating over tropical south america and associated adjustments in large scale subsidence over northeast brazil lead to a remote forcing of the south american monsoon which determines most of the precipitation changes in the region on orbital timescales

accumulation of organic carbon in five brazilian lakes during the holocene abstract a comparative study was made of organic carbon accumulation in five lacustrine environments in brazil large open water lake ponds with floating meadows and marshes the most representative cores were used to calculate the accumulation of total organic carbon toc in the sediment over the past 12 000 cal years bp carbon accumulation rates were determined using the toc concentration sediment in situ density 14 c calibrated ages and the interpolated sedimentation rate cm year 1 or sediment accumulation rate g m 2 year 1 the c n ratio and microscopic observations were used to characterise the sedimentary organic matter recent toc accumulation in the lakes was estimated at 7 41 g m 2 year 1 by averaging the last 500 years this variation roughly correlated with the present day ratio of watershed area vs lake area indicating that nutrient input is important for the modern accumulation of organic matter the accumulation rates were greatest in lakes with floating macrophytes the largest lake dom helvecio with a strong thermal stratification low primary production and low carbon accumulation had relatively well preserved organic matter when its low watershed area lake area ratio was considered the general increase in carbon accumulation during the holocene followed the expansion of lakes and the development of a wetter climate due to a growing penetration of the atlantic and amazonian wet air mass over brazil driven by insolation changes this change in carbon accumulation was not controlled by the higher water and nutrient inputs alone more organic matter was decomposed when lake levels were low at the beginning of the holocene some lakes were also influenced by higher inputs of allochthonous organic matter during the early and middle holocene these inputs included high concentrations of charcoal fragments lake morphology and its changes as the lake level rose as well as the nature of primary producers strongly influenced the carbon accumulation rates our understanding of how climate changes are linked to changes in the lacustrine environment should not be based on measuring organic matter alone but requires a good description of the organic matter and other limnological parameters

stacked gabbro units and intervening mantle a detailed look at a section of iodp leg 305 hole u1309d hole u1309d integrated ocean drilling program iodp legs 304 305 penetrated 1415 m into the seafloor of the atlantis massif an oceanic core complex at 30 n mid atlantic ridge more than 96 of all recovered rocks are gabbroic on the basis of a mineral chemical overview we suggest that between 800 and 1100 m below sea floor mbsf a magmatic unit occurs ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part below 1235 mbsf massive gabbronorites oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit the focus here is on the zone where both units interact and screens totaling 50 m of a microstructurally distinct olivine rich troctolite occur we argue that the olivine rich troctolite is a former mantle rock which was converted to a crust mantle transition zone dunite at the base of the upper magmatic unit later as melts derived from the lower magmatic unit percolated through it it was equilibrated to a more evolved chemistry and transformed to a fine grained olivine rich troctolite our main arguments against a possible cumulate nature of the olivine rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine rich troctolite its distinctly fine grained microstructure the high cr content of cpx and its ni rich olivine composition the high nio for a given mg mg fe in the olivine rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt evidence for the percolation of evolved melts through the olivine rich troctolites are ti rich interstitial pyroxenes and as inclusions in cr spinel highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter scale noritic veins the percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust mantle transition zone

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

integrated diatom pollen based holocene environmental reconstruction of lake las margaritas eastern savannas of colombia a multiproxy analysis that includes diatoms pollen element geochemistry carbon content analysis and radiocarbon dates was carried out on a core from las margaritas 3023 n 73026 w in order to

reactive flow as dominant evolution process in the lowermost oceanic crust evidence from olivine of the pineto ophiolite corsica the jurassic pineto ophiolite from corsica exposes a 1 km thick troctolite olivine gabbro sequence interpreted to represent a lowermost sector of the gabbroic oceanic crust from a ultra slow spreading system to constrain the petrogenesis of the olivine gabbros minor and trace element analyses of olivine forsterite 84 82 mol were carried out olivine from the olivine gabbros is depleted in incompatible trace elements sc v ti y zr and heavy rare earth elements with respect to olivines from associated troctolites depleted incompatible element compositions are also shown by olivine forsterite 86 mol from a clinopyroxene rich troctolite the incompatible element compositions of olivine argue against a petrogenetic process entirely driven by fractional crystallization we propose that melts migrating through an olivine plagioclase crystal mush chemically evolved by reaction with the existing minerals changing in composition as it flowed upward the melt residual from these interactions led to partial dissolution of preexisting olivine and to crystallization of clinopyroxene generating olivine gabbro bodies within a troctolite matrix reactive flow was the major evolution process active in the 1 km crustal transect exposed at the pineto ophiolite producing lithological variations classically attributed to fractional crystallization processes

basaltic pillow mounds in the vinalhaven intrusion maine the vinalhaven intrusion is a dominantly granitic pluton of probable devonian age located on vinalhaven island and adjacent islands maine it consists of four main units coarse grained granite fine grained granite a gabbro diorite unit consisting of interlayered mafic hybrid and granitic rocks and a heterogeneous granitic unit the gabbro diorite unit occurs along the south and east coast of the island as a sheet like body hundreds of meters to more than 1 km thick that dips beneath the central granitic units and rests on heterogeneous granitic rocks that form the base of the intrusion and are exposed on islands to the southeast load cast and pipe structures at the bases of mafic sheets indicate that the gabbro diorite unit represents a sequence of basaltic injections that ponded on crystal rich mush at the base of a silicic magma chamber and variably interacted with overlying crystal poor granitic magma the pluton therefore represents a fossilized silicic magma chamber that was periodically replenished by basaltic magma near the base of the gabbro diorite unit some basaltic injections produced large mounds up to more than 10 m high and 100 m wide of tightly packed meter scale chilled basaltic pillows tubes and sheets in a granitic matrix the mounds appear to represent flow fronts of basaltic injections that entered and ponded on the floor of a silicic magma chamber although physical conditions differ significantly these plutonic pillow mounds appear to share many characteristics with submarine pillow basalts and lava flows

a 24 700 yr paleolimnological history from the peruvian andes abstract a new paleolimnological dataset from lake pacucha 13 s 3095 m elevation in the peruvian andes provides evidence of changes in lake level over the past 24 700 yr a late glacial highstand in lake level gave way to an early holocene lowstand this transition appears to have paralleled precessional changes that would have reduced insolation during the wet season the occurrence of benthic salt tolerant diatoms and caco3 deposition suggest that the lake had lost much of its volume by c 10 000 cal yr bp pronounced holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c 8300 to 5000 cal yr bp while a polymictic lake formed at c 5000 cal yr bp these relatively wet conditions were interrupted by a series of drier events the most pronounced of which occurred at c 750 cal yr bp paleolimnological changes in the holocene were more rapid than those of either the last glacial maximum or the deglacial period

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction