global ice sheet system interlocked by sea level abstract denton and hughes 1983 quaternary research 20 125 144 postulated that sea level linked a global ice sheet system with both terrestrial and grounded marine components during late quaternary ice ages summer temperature changes near northern hemisphere melting margins initiated sea level fluctuations that controlled marine components in both polar hemispheres it was further proposed that variations of this ice sheet system amplified and transmitted milankovitch summer half year insolation changes between 45 and 75 n into global climatic changes new tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the antarctic ice sheet thus fitting the concept of a globally interlocked ice sheet system but recent atmospheric modeling results manabe and broccoli 1985 journal of geophysical research 90 2167 2190 suggest that factors other than areal changes of the grounded antarctic ice sheet strongly influenced southern hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres atmospheric carbon dioxide linked to high latitude oceans is the most likely candidate shackleton and pisias 1985 atmospheric carbon dioxide orbital forcing and climate in the carbon cycle and atmospheric co2 natural variations archean to present e t sundquest and w s broecker eds pp 303 318 geophysical monograph 32 american geophysical union washington d c but another potential influence was high frequency climatic oscillations 2500 yr it is postulated that variations in atmospheric carbon dioxide acted through an antarctic ice shelf linked to the grounded ice sheet to produce and terminate southern hemisphere ice age climate it is further postulated that milankovitch summer insolation combined with a warm high frequency oscillation caused marked recession of northern hemisphere ice sheet melting margins and the north atlantic polar front about 14 000 14c yr b p this permitted renewed formation of north atlantic deep water which could well have controlled atmospheric carbon dioxide w s broecker d m peteet and d rind 1985 nature london 315 21 26 combined melting and consequent sea level rise from the three warming factors initiated irreversible collapse of the interlocked global ice sheet system which was at its largest but most vulnerable configuration

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

10 be in ice at vostok antarctica during the last climatic cycle we have previously reported accelerator mass spectrometer measurements of the cosmogenic isotope 10be half life 1 5 myr in an 906 m ice core recovered at dome c antarctica1 in addition to an enhanced concentration during a period of reduced solar activity maunder minimum ad 1645 1715 2 we found substantially increased concentrations during that part of the core corresponding to the last ice age both effects have been confirmed and extended in greenland ice cores3 4 the recovery of a 2 083 m ice core at vostok antarctica5 together with an extended oxygen isotope chronology reported elsewhere in this issue6 permits an extension of 10be studies over the whole of the last climatic cycle 125 kyr we report here such measurements which show an excellent correlation with the oxygen isotope and thus presumably climate record over the whole of this period the results imply that precipitation rates in the antarctic during the last interglacial were similar to those of the holocene but were roughly halved during the last glaciation

theoretical analysis of wing dike impact on river flood stages the question of whether wing dikes bank perpendicular river training structures or groynes cause higher flood levels has been debated in the united states for many years some researchers point to empirical data that show large stage increases which are associated with wing dike construction whereas other researchers have suggested that such increases are contrary to engineering theory in a recent report the u s government accountability office usgao presented this question as a priority to be resolved by engineers and scientists as a first step to better understand the connection between navigation structures and flood levels on the middle mississippi river mmr a simplified theoretical analysis is presented to test the assertion made in the usgao report and elsewhere that such increases are contrary to hydraulic theory this analysis predicts that wing dike construction may lead to water level lowering for in bank flows and to water level increases for out of bank flood flows this confirms that in principle wing dikes may have contributed to the observed flood water level trends in the mmr more detailed follow up studies are required to accurately quantify the impact of wing dikes on flood levels doi 10 1061 asce hy 1943 7900 0000698 2013 american society of civil engineers

stable isotope stratigraphy from the antarctic continental margin during the last one million years abstract a stable isotope record from the eastern weddell sea from 69 s is presented for the first time a 250 000 yr record from the southern ocean can be correlated in detail to the global isotope stratigraphy together with magnetostratigraphic sedimentological and micropalaeontological data the stratigraphic control of this record can be extended back to 910 000 yrs b p a time scale is constructed by linear interpolation between confirmed stratigraphic data points the benthic 18o record epistominella exigua reflects global continental ice volume changes during the brunhes and late matuyama chrons whereas the planktonic isotopic record neogloboquadrina pachyderma may be influenced by a meltwater lid caused by the nearby antarctic ice shelf and icebergs the worldwide climatic improvement during deglaciations is documented in the eastern weddell sea by an increase in production of siliceous plankton followed with a time lag of approximately 10 000 yrs by planktonic foraminifera production peak values in the difference between planktonic and benthic 13c records which are 0 5 higher during warm climatic periods than during times with expanded continental ice sheets also suggest increased surface productivity during interglacials in the southern ocean

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

quaternary glaciation in africa key chronologies and climatic implications multiple episodes of quaternary glaciation are evidenced on 10 distinct mountain localities throughout africa with the best dated sites from kilimanjaro and mt kenya in equatorial east africa a general paucity of radiogenic dates constrains the glacial chronology and regional sequences have largely been based on correlations by relative weathering of features excellent glacial moraine preservation and other features of erosion put limits on the spatial extent of palaeoglaciers and have facilitated advances in palaeo equilibrium line altitude ela analyses all radiocarbon dates n 30 provide minimum ages of deglaciation and are consistent with a widespread synchronous advance during the last glacial maximum lgm 21 2 ka k ar dates on kilimanjaro lava flows initially recognised up to three glaciations prior to the lgm over 100 cosmogenic radionuclide crn 36 cl exposure ages on glacial erratics and bedrock from kilimanjaro and mt kenya date 16 landforms confirming maximum glacial extent prior to marine isotope stage mis 10 no glacial geological features have been dated to mis 6 while on kilimanjaro the crn ages confirm a clear lgm mis 2 maximum mt kenya lacks definitive lgm moraines lateglacial advances or stillstands on both mountains are suggested by low relief moraines dating to 14 6 1 2 ka two boulders on mt kenya and 16 3 1 9 ka and 15 8 2 5 ka on kilimanjaro multiple moraines upvalley have early holocene ages ranging from 11 2 0 8 to 8 6 0 2 ka isolated and equivocal dates on both mountains also provide speculative evidence that the maximum extent since mis 5e may have occurred close to 30 ka similar to sites in south america new palaeoclimatic understanding from lake and ocean proxies suggests hypotheses for the source of low latitude climate changes causing the advances that can be tested with improved dates a high degree of intra regional variance in ela is observed with differences by aspect reflecting precipitation gradients future work should include more crn dates improved mapping and inverse climate mass balance modelling copyright 2008 john wiley sons ltd

late quaternary relative humidity changes from mt kilimanjaro based on a coupled 2 h 18 o biomarker paleohygrometer approach abstract our understanding of african paleoclimate hydrological history is mainly based on lake level and lake sediment studies it improved during the last decade thanks to emerging stable isotope techniques such as compound specific deuterium analysis of sedimentary leaf wax biomarkers 2hleaf wax here we present the results from a multi proxy biomarker study carried out on a 100 ka loess like paleosol sequence preserved in the maundi crater at 2780 m a s l on the southeastern slopes of mt kilimanjaro in equatorial east africa the maundi stable isotope records established for hemicellulose derived sugars lignin and pectin derived methoxyl groups leaf wax derived fatty acid and n alkane biomarkers 18osugars 2hmethoxyl groups 2hfatty acids and 2hn alkanes respectively reveal similar patterns but also some distinct differences are obvious the periods from 70 to 60 ka the last glacial maximum lgm and the younger dryas yd are characterized by more positive values whereas during the holocene and around 30 39 and 56 ka bp more negative values are determined the application of a coupled 2hn alkane 18osugar paleohygrometer approach allows us to derive information about late quaternary changes of air relative humidity at the maundi study site reconstructed changes of mean day time relative humidity rhd are in good agreement with pollen results from the study area apart from the overall regional moisture availability the intensification versus weakening of the trade wind inversion which affects the diurnal montane atmospheric circulation on the slopes of mt kilimanjaro is suggested as a local factor which may contribute to the observed variability of rhd at maundi study site the combined usage of 2hn alkanes and 18osugars allowed us to reconstruct 2h 18o of source water utilized by plants in the study area which is directly linked to local precipitation the results of this reconstruction caution against a straightforward interpretation of 2hleaf wax and 18osugars records as proxies for isotopic composition of local precipitation because variable and primarily rh dependent isotopic evaporative enrichment of leaf water can mask changes of 2hprec 18oprec in the past the biomarker based 2h 18osource water records derived for the maundi site revealed a discernible link with the reconstructed rhd record lower rhd values were generally observed during periods characterized by more negative 2h 18osource water values indicating a reverse relationship with the expected precipitation amount this indicates that the empirical relationship between amount of rainfall and its isotopic composition observed nowadays on monthly timescale in the east african region might not be valid on millennial time scale

modelled atmospheric temperatures and global sea levels over the past million years the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles marine records of sediment oxygen isotope compositions show that the earth s climate has gone through a succession of glacial and interglacial periods during the past million years but the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep water temperature affect the recorded isotopic composition1 2 3 4 5 separating these two effects would require long records of either sea level or deep ocean temperature which are currently not available here we use a coupled model of the northern hemisphere ice sheets6 and ocean temperatures forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores to quantify both contributions simultaneously we find that the ice sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima suggesting that strong ocean cooling preceded slow ice sheet build up the model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 n ice volume and global sea level we find that during extreme glacial stages air temperatures were 17 1 8 c lower than present with a 120 10 m sea level equivalent of continental ice present

evidence for a late wisconsin glaciation of the weddell sea the huge ice shelves in west antarctica the ross and filch ner ronne ice shelves have probably extended out on the continental shelf during the late wisconsin1 previous discussions which have focused on the ross sea have suggested 1 that the ice extended across the whole continental shelf2 3 or 2 that there was only a minor expansion4 here we present sedimentological data from the weddell sea fig 1 which suggest that a late wisconsin grounded ice sheet extended to the shelf edge the evidence includes a recent thicker ice in ellsworth mountains at the head of the filchner ronne ice shelf5 this thickening would lead to an expansion of the inland ice sheet over the continental shelf filling up the weddell sea embayment

coupled thermal and hydrological evolution of tropical africa over the last deglaciation we analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the congo river outflow and the results enabled us to reconstruct large scale continental temperature changes in tropical africa that span the past 25 000 years tropical african temperatures gradually increased from 21 to 25 c over the last deglaciation which is a larger warming than estimated for the tropical atlantic ocean a direct comparison with sea surface temperature estimates from the same core revealed that the land sea temperature difference was through the thermal pressure gradient an important control on central african precipitation patterns

late quaternary glacial history of george vi sound area west antarctica during the last glacial maximum in west antarctica separate ice caps developed on alexander island and on palmer land became confluent in george vi sound and discharged northward from latitude 72 s radiocarbon 32 000 yr and amino acid approximately 120 000 yr age determinations on shell fragments hiatella solida found in basal till suggest a wisconsin age for the glaciation that incorporated them the pattern of ice flow differed from that deduced for this area in the climap reconstruction following the maximum stage there was a stadial event when outlet valley glaciers flowed from smaller ice caps into george vi sound more widespread recession permitted the george vi ice shelf to deposit palmer land erratics on eastern alexander island before isostatic recovery raised them to final elevations of about 82 m the ice shelf may have been absent at about 6500 yr b p when large barnacles bathylasma corolliforme were living in the sound small glaciers readvanced to form at least two terminal moraines before the ice shelf re formed and incorporated the barnacle shells into its moraine on alexander island the shells gave a 14c age corrected for antarctic conditions of about 6500 yr b p and an amino acid ratio consistent with a holocene age valley glaciers readvanced over the ice shelf moraine before oscillations of both valley glaciers and the ice shelf led to the formation of the present sequence of contiguous ice cored moraines probably during the little ice age such oscillations may represent a climatic control not yet observed in the dry valleys of victoria land the only other part of antarctica studied in detail for glacier fluctuations

forcing of wet phases in southeast africa over the past 17 000 years a continuous record of hydrologic variability for the past 17 000 years at the mouth of the zambezi river shows that hydrologic conditions in southeast africa were controlled by variations in local insolation and migrations of the intertropical convergence zone rather than by indian ocean temperature records of hydroclimatological variability in southern africa since the last glacial maximum have produced a conflicting picture of the relative influences of events in the remote northern hemisphere and the more local indian ocean this paper presents a continuous record of hydrological variability for the past 17 000 years from marine sediment core drilled off the mouth of the zambezi river the record shows that hydrological conditions in the region were influenced by variations in local insolation and migrations of the intertropical convergence zone which were probably driven by events in the northern hemisphere rather than the indian ocean intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast africa since the last glacial maximum about 20 000 years ago in particular the relative importance of atmospheric and oceanic processes is not firmly established1 2 3 4 5 southward shifts of the intertropical convergence zone itcz driven by high latitude climate changes have been suggested as a primary forcing2 3 whereas other studies infer a predominant influence of indian ocean sea surface temperatures on regional rainfall changes4 5 to address this question a continuous record representing an integrated signal of regional climate variability is required but has until now been missing here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro climatology in southeast africa during rapid climate changes over the past 17 000 years our results are based on a reconstruction of precipitation and river discharge changes as recorded in a marine sediment core off the mouth of the zambezi river near the southern boundary of the modern seasonal itcz migration indian ocean sea surface temperatures did not exert a primary control over southeast african hydrologic variability instead phases of high precipitation and terrestrial discharge occurred when the itcz was forced southwards during northern hemisphere cold events such as heinrich stadial 1 around 16 000 years ago and the younger dryas around 12 000 years ago or when local summer insolation was high in the late holocene that is during the past 4 000 years

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

orbital forcing of climate over south africa a 200 000 year rainfall record from the pretoria saltpan late pleistocene variations in rainfall in subtropical southern african are estimated from sediments preserved in the pretoria saltpan a 200000 year old closed basin crater lake on the interior plateau of south africa we show that south african summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession as predicted by orbital precession geometry berger 1978 this south african record is out of phase with north african palaeomonsoon indices street and grove 1979 rossignol strick 1983 mcintyre et al 1989 the amplitude of the rainfall response to insolation forcing agrees with climate model estimates prell and kutzbach 1987 these results document the importance of direct orbital insolation forcing on both subtropical north and south african climate as well as the predicted antiphase sensitivity to precessional insolation forcing

d h ratios of methoxyl groups of the sedimentary organic matter of lake holzmaar eifel germany a potential palaeoclimate hydrology proxy stable hydrogen isotope ratios dd values of methoxyl groups in lignin and pectin from a variety of plant types from different locations have been shown to mirror dd values of precipitation with a mean uniform isotopic fractionation since dd in precipitation is mainly influenced by climatic conditions including temperature evaporation and precipitation amount methoxyl groups of organic matter have been proposed as a potential palaeoclimate proxy here we measured the dd values of the methoxyl groups of sedimentary organic matter derived from sediment core segments of lake holzmaar eifel germany the sediment core covers the entire holocene and part of the late glacial until 13 650 years bp since the sedimentary organic matter was indicated by organic source proxies of predominantly phytoplanktonic origin we considered the dd values of the methoxyl groups as a proxy for the dd value of the lake water presently and probably during most of the holocene the dd value of precipitation was primarily controlled by temperature 2 4 5 c as well as in isotopic equilibrium with the dd value of the lake water we used these modern temperature sensitivities of the eifel region together with the measured dd values of the methoxyl groups of the sedimentary organic matter to evaluate temperature changes recorded for the holocene the resulting mean temperature variance was found to be consistently less than 1 8 4 c and is broadly in line with that reported from the ngrip ice core for the same period the transition from the holocene to the late glacial period was accompanied with more negative dd values of the methoxyl groups 13 for the younger dryas besides the association with temperature we assumed a higher susceptibility of the dd value of lake holzmaar to evaporative enrichment during the younger dryas which could serve as an explanation for discrepancies reported for existing palaeoclimate hydrology records of the eifel region our findings indicate that the dd values of methoxyl groups have great potential for assisting with palaeoclimate and palaeohydrology particularly when the organic matter is well characterized and of uniform origin 2014 elsevier ltd all rights reserved

late quaternary changes in carbon cycling on mt kenya east africa an overview of the δ13c record in lacustrine organic matter abstract the 13 c signature of sedimentary organic matter acts as a tracer for past changes in the terrestrial and aquatic carbon cycles compound specific 13 c analysis enables these two signals to be disentangled we compare the molecular isotopic records from a transect of five lakes between 1820 and 4595 m a s l on mt kenya east africa spanning the last 100 000 years with multiproxy palaeoenvironmental data in order to determine the mechanisms underlying late quaternary variations in carbon cycling at different altitudes the amplitude of down core variations in the 13 c values of total organic carbon varied from 4 8 to 17 4 the 13 c signals in compounds of terrestrial aquatic macrophyte and algal origin were tightly coupled reflecting strong atmospheric control during the early glacial and the last glacial maximum the effects of sub ambient co 2 and enhanced moisture stress reinforced each other resulting in carbon starvation and very high 13 c values in contrast smaller millennial scale shifts in 13 c during the late glacial and holocene were driven primarily by fluctuations in monsoon precipitation however the impact of late quaternary atmospheric changes were complex involving variations in biomass burning vegetation cover soil erosion lake levels and water chemistry in addition to direct physiological responses by higher plants and algae

paleolimnological evidence for the onset and termination of glacial aridity from lake tanganyika tropical east africa geochemical and sedimentological data in a continuous 60 000 year sediment core record from the kalya horst region of central lake tanganyika provide a detailed history of paleoclimate mediated weathering and overflow events from upstream lake kivu univariate elemental profiles bivariate elemental ratios and multivariate analyses of chemical trends show variations between the dry late pleistocene 32 18 ka cal yr bp and the wetter conditions that both preceded and post date that interval this record places important new constraints on the timing of aridity in east africa during the high latitude last glacial maximum lgm based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica elemental indicatorsintheoldestportionofthesedimentaryrecord 60 50kacalyrbp characterizethisintervalasacomparatively wetperiod similar to modern conditions our record demonstrates that the ensuing transition toward arid conditions in tropical africa during high latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50 32 ka cal yr bp followed by intense aridity from 32 18 ka cal yr bp the initiation of inflow from upstream lake kivu into lake tanganyika is evidenced at 10 6 ka cal yr bp through its influence on both elemental profiles mg ca and through its effect on 87 sr 86 sr increases in elemental mg ca sr concentrations coincide with the timing of the lake kivu overflow metal geochemistry suggests that the overflow from lake kivu into lake tanganyika may have ceased between 8 and 6 ka cal yr bp suggesting a period of middle holocene aridity in east africa 2007 elsevier b v all rights reserved

half precessional dynamics of monsoon rainfall near the east african equator extensive records exist with which to assess the relationship between external climate forcings such as changes in insolation and climate variability for middle and high latitudes but records from equatorial regions are relatively few especially from regions experiencing the passage of the intertropical convergence zone a continuous and well resolved climate proxy record of hydrological variability during the past 25 000 years from equatorial east africa is now presented and analysed

antarctic ice volume and contribution to sea level fall at 20 000 yr bp from raised beaches the contribution of the antarctic ice sheets to global sea level fall at the last glacial maximum lgm depends largely on how the extent and thickness of peripheral ice changed model studies1 3 suggest that there was widespread thickening from 500 m to more than 1 000 m of the ice sheet margins sufficient to induce a drop in sea level of at least 25 m geological evidence4 5 on the other hand indicates only limited ice expansion and a sea level fall of just 8 m here we use recent data on the altitudes and ages of raised beaches from the ross embayment and east antarctica to investigate the timing and extent of antarctic deglaciation these indicate that the ice margin during the lgm was thinner and less extensive than has been formerly thought and that its contribution to the drop in sea level was only 0 5 2 5 m deglaciation was well advanced by 10 kyr bp and was complete by 6 kyr bp these findings imply either that sea level during the lgm fell less than present estimates suggest or that an ice volume considerably greater than currently accepted must have been present in the northern hemisphere

glacial marine sedimentation and quaternary glacial history of marguerite bay antarctic peninsula abstract marguerite bay situated between the subpolar glacial regime of the northern antarctic peninsula and the polar glacial regime of west antarctica is ideally located to test various models of glacial and glacial marine sedimentation and specific scenarios of late wisconsin ice sheet expansion piston cores and single channel seismic reflection data were collected during the deep freeze 85 and 86 expeditions to determine the late quaternary history of the area seismic data in the bay show a rugged seafloor with numerous deep troughs and a very thin layer of sediment over crystalline basement or older sediments glacial erosion is important in modifying existing features although the ultimate repository of the eroded material is not known it is not found within the bay the piston cores are topped by diatomaceous muds which are underlain by terrigenous muds and muddy gravels that imply deposition beneath an ice shelf basal tills were penetrated at three sites reflecting deposition by a grounded marine ice sheet a reconstruction of the glacial history of marguerite bay since the last glacial maximum shows grounded ice filling the bay in late wisconsin time rising sea level caused an uncoupling of the ice sheet and slow retreat of an ice shelf throughout the holocene

recent glacial history and rapid ice stream retreat in the amundsen sea microfossil analyses of core top samples from 20 sites in the eastern and southern amundsen sea delimit three biotic provinces which are related in part to the activity of a large ice stream and modern sea ice distribution in recent sediments on the outer shelf planktonic and calcareous benthic foraminifera are abundant but diatoms are rare here low diatom abundances in sediments despite high abundances in the overlying sea ice are attributed to nondeposition caused by ocean currents pine island bay sediments are almost barren of microfossils even though diatoms are abundant in overlying surface waters we attribute this discrepancy to deposition of exclusively non organic sediments beneath a former extension of the floating terminus of pine island glacier which probably retreated from this area during the last century the polynya which occupies this area during present day summers is probably so recent a feature that little or no biogenic sediment has accumulated recent sediments in the eastern margin province between pine island bay and the outer shelf contain abundant diatoms and arenaceous foraminifera high diatom abundances here probably record the presence of the amundsen sea polynya during the last few millennia the presence of a compact diamicton beneath recent sediments on the outer shelf and in the eastern margin province suggests that grounded ice formerly occupied the amundsen sea shelf with ice streams in deep elongated troughs these ice streams may have retreated up to 200 km during the holocene

pore fluid constraints on the temperature and oxygen isotopic composition of the glacial ocean pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical atlantic indicate that the oxygen isotopic composition 18 o of seawater at this site during the last glacial maximum was 0 8 0 1 per mil higher than it is today combined with the 18 o change in benthic foraminifera from this region the elevated ratio indicates that the temperature of deep water in the tropical atlantic ocean was 4 c colder during the last glacial maximum extrapolation from this site to a global average suggests that the ice volume contribution to the change in 18 o of foraminifera is 1 0 per mil which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from barbados corals and terrestrial climate proxies

the effects of solar irradiation changes on the migration of the congo air boundary and water levels of paleo lake suguta northern kenya rift during the african humid period 15 5 ka bp abstract the water level record from the 300 m deep paleo lake suguta northern kenya rift during the african humid period ahp 15 5 ka bp helps to explain decadal to centennial intensity variations in the west african monsoon wam and the indian summer monsoon ism this water level record was derived from three different sources 1 grain size variations in radiocarbon dated and reservoir corrected lacustrine sediments 2 the altitudes and ages of paleo shorelines within the basin and 3 the results of hydro balance modeling providing important insights into the character of water level variations abrupt or gradual in the amplifier paleo lake suguta the results of these comprehensive analyses suggest that the ahp highstand in the suguta valley was the direct consequence of a northeastwards shift in the congo air boundary cab which was in turn caused by an enhanced atmospheric pressure gradient between east africa and india during a northern hemisphere insolation maximum rapidly decreasing water levels of up to 90 m over less than a hundred years are best explained by changes in solar irradiation either reducing the east african indian atmospheric pressure gradient and preventing the cab from reaching the study area or reducing the overall humidity in the atmosphere or a combination of both these effects in contrast although not well documented in our record we hypothesize a gradual end of the ahp despite an abrupt change in the source of precipitation when a decreasing pressure gradient between asia and africa prevented the cab from reaching the suguta valley the abruptness was probably buffered by a contemporaneous change in precession producing an insolation maximum at the equator during october whether or not this is the case the water level record from the suguta valley demonstrates the importance of both orbitally controlled insolation variations and short term changes in solar irradiation as factors affecting the significant water level variations in east african rift lakes

vegetation climate and fire dynamics in east africa inferred from the maundi crater pollen record from mt kilimanjaro during the last glacial interglacial cycle abstract the pollen charcoal and sedimentological record from the maundi crater located at 2780 m elevation on the south eastern slope of mt kilimanjaro is one of the longest terrestrial records in equatorial east africa giving an interesting insight into the vegetation and climate dynamics back to the early last glacial period our sediment record has a reliable chronology until 42 ka bp an extrapolation of the age depth model as well as matching with other palaeo records from tropical east africa suggest a total age of about 90 ka bp at the bottom of the record during the last glacial the distribution as well as the composition of the vegetation belts classified as colline savanna submontane woodland montane forest ericaceous belt and alpine vegetation changed the early last glacial is characterized by high amounts of poaceae and asteraceae pollen suggesting a climatically dry but stable phase based on the absence of pollen grains in samples deposited around 70 ka bp we assume the occurrence of distinct drought periods during the pre lgm last glacial maximum a higher taxa diversity of the ericaceous and montane zone is recorded and suggests a spread of forest and shrub vegetation thus indicating a more humid period the taxa diversity increases steadily during the recorded time span the decent of vegetation zones indicate dry and cold conditions during the lgm and seem to have been detrimental for many taxa especially those of the forest vegetation however the early last glacial seems to have been markedly drier than the lgm the reappearance of most of the taxa most importantly alchemilla araliaceae dodonea hagenia ilex myrsine moraceae piperaceae during the deglacial and holocene period suggest a shift into humid conditions an increase in ferns and the decrease in grasses during the holocene also indicate increasing humidity fire played an important role in controlling the development and elevation of the ericaceous zone and the tree line during the holocene no increased anthropogenic impact around the maundi crater can be observed since neither higher fire activity nor a spread of hemerophilic plants is recorded this pollen archive reveals shifts in the upper vegetation zones ericaceous zone and montane forest zone of at least 1100 m but underlines the role of mt kilimanjaro as a glacial refuge for montane forest species similar to that of the eastern arc mountains

flow details near river groynes experimental investigation experiments have been carried out in a fixed bed flume for a schematized straight river reach with groynes on one side to study the dynamics of the flow near groynes the flume had a geometrical scale of 1 40 based on typical dimensions of the dutch river waal both emergent and submerged groynes were studied the measurements demonstrate the differences in the nature of the turbulence between submerged and emerged groynes stages and provide insight into the flow pattern in the vicinity of groynes the shape and the extent of the mixing layer at different flow stages and the dynamic behavior of the velocity along the mixing layer between the main channel and the groyne fields a parameterization of the turbulence characteristics of the flow near groynes is presented large scale velocity fluctuations are found in all test cases with timescales that vary with the flow stage the large scale u and v velocity fluctuations are in phase in the center of the mixing layer and out of phase for the points on the boundaries of the mixing layer

wet and arid phases in the southeast african tropics since the last glacial maximum plant leaf wax carbon isotopes provide a record of c 3 versus c 4 vegetation a sensitive indicator of aridity from the southeast african tropics since the last glacial maximum wet and arid phases in southeast africa were in phase with conditions in the global tropics from 23 to 11 ka but at the start of the holocene these relationships ended and an antiphase relationship prevailed the abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the intertropical convergence zone itcz during the last glacial southward displacements of the itcz are also linked to arid conditions in southeast africa during the younger dryas and the little ice age

the seismic stratigraphic record of lake level fluctuations in lake challa hydrological stability and change in equatorial east africa over the last 140 kyr abstract seismic reflection data from crater lake challa mt kilimanjaro equatorial east africa reveal a 210 m thick sedimentary infill containing distinct seismic stratigraphic signatures of late quaternary lake level fluctuations extrapolation of a well constrained age model on the cored upper part of the sequence suggests that these lake level fluctuations represent a detailed and continuous record of moisture balance variation in equatorial east africa over the last 140 kyr this record indicates that the most severe aridity occurred during peak penultimate glaciation immediately before 128 kyr bp coeval with heinrich event 11 and during a last interglacial megadrought period between 114 and 97 kyr bp in comparison last glacial maximum lgm aridity was modest it was preceded by 75 000 years of relatively stable and moist climate conditions interrupted by eleven short lived dry spells five of which match the timing of heinrich events 2 to 6 climate history near the east african equator reflects variation in the precessional forcing of monsoon rainfall modulated by orbital eccentricity but precession driven moisture fluctuations were less extreme than those observed in northern and southern tropical africa the near continuous moist climate from 97 to 20 5 kyr bp recorded in the lake challa record contrasts with the trend towards greater aridity after 70 kyr bp documented in equatorial west africa this long period of moist glacial climate and a short relatively modest lgm drought can be attributed to greater independence of western indian ocean monsoon dynamics from northern high latitude glaciation than those in the tropical atlantic ocean this rather persistent moist glacial climate regime may have helped maintain high biodiversity in the tropical forest ecosystems of the eastern arc mountains in tanzania

atmospheric circulation patterns during late pleistocene climate changes at lake malawi africa abstract the climate of tropical africa transitioned from an interval of pronounced orbitally paced megadroughts to more humid and stable conditions approximately 70 000 years ago scholz et al 2007 the regional atmospheric circulation patterns that accompanied these climatic changes however are unclear due to a paucity of continental paleoclimate records from tropical africa extending into the last interglacial we present a new 140 kyr record of the deuterium hydrogen isotopic ratio of terrestrial leaf waxes d wax from drill cores from lake malawi southeast africa that spans this important climatic transition d wax shifts from highly variable and relatively d depleted to more stable and d enriched around 56 ka contemporary with the onset of more humid conditions in the region moisture source and transport history dominate the d wax signal at lake malawi with local rainfall amount playing a secondary role for much of the paleorecord analysis of modern moisture sources for lake malawi suggests that d depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier d depleted air mass currently located in central southern africa to the lake malawi catchment this d depleted air mass is associated with the descending limb of the hadley cell which implies significant changes in the hadley circulation during the megadroughts and related changes in the position of the intertropical convergence zone over africa these findings demonstrate the ability of d wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance and elucidate potential mechanisms for pronounced hydrological change in southeast africa during the late pleistocene

nature and classification of waterlain glaciogenic sediments exemplified by pleistocene late paleozoic and late precambrian deposits this study of waterlain glaciogenic sediments is designed to present both a review and new information on glaciogenic subaquatic deposits of differing age in a number of localities in north and south america and south africa the late paleozoic glaciogenic deposits of the parana basin in brazil and the karoo basin of south africa are singled out for special attention as they show a reasonably complete lateral sequence of terrestrial to off shore glaciogenic sedimentation although the environment of subaquatic glaciogenic sedimentation varies from one area to the next certain common elements are found which can be used to develop a generalized model for both glaciomarine and glaciolacustrine sedimentation for descriptive purposes the model is divided into two broad categories a shelf facies and a basinal facies the shelf facies is marked by massive diamicton ite which may be 200 m or more in thickness and which is frequently overlain by a complex of clastic sediments consisting primarily of gravity and fluid flows the basinal facies is marked by products of subaquatic slumps and more distal turbidites and glaciomarine sediments new terminology is introduced the massive diamicton ite which is diagnostic of the shelf facies probably represents deposition from the base of active ice in a subaquatic environment and is termed undermelt diamicton ite the gravity and fluid flows which are usually found overlying undermelt diamicton ite and in the basinal facies are subdivided into six categories glaciogenic subaquatic outwash glaciogenic suspension flow glaciogenic chaotic debris flow glaciogenic subaquatic debris flow glaciogenic slurry flow and glaciogenic turbidity flow the relative abundance of undermelt diamicton ite and the various types of gravity and fluid flows can be used to define inner shelf outer shelf inner basin and outer basin facies of glaciomarine sedimentation

calibration and application of the branched gdgt temperature proxy on east african lake sediments branched glycerol dialkyl glycerol tetraethers brgdgts are a novel proxy for mean annual air temperature maat and have the potential to be broadly applicable to climate reconstruction using lacustrine sediments several calibrations have been put forth relating brgdgt distributions to maat using a variety of linear regressions including the methylation mbt and cyclization cbt indices of brgdgts the relative abundances of the major non cyclized brgdgts mbrgdgts and best subsets regression bsr of the fractional abundances of the nine most common brgdgts however these calibrations have rarely been applied to lake sediment cores to reconstruct temperatures and test the applicability of this proxy as a paleothermometer we present an expanded east african lakes surface sediment brgdgt dataset based upon 111 lakes and examine three methods of calibrating brgdgts to maat these methods include recalculations of the east african lake mbt cbt calibration and mbrgdgts calibrations as well as a new stepwise forward selection sfs calibration that uses the four combined brgdgts that explain the most variance in temperature in our calibration set we apply these new calibrations as well as five previously published lacustrine brgdgt calibrations to the brgdgt distributions of our surface sediment dataset and a 48 kyr sediment core from sacred lake mt kenya producing the first brgdgt temperature reconstruction available from a small tropical lake we compare the reconstructed temperatures to previously published paleotemperature records from east africa to help us assess the performance of the brgdgt calibrations we find that the sfs calibration has a consistently low root mean squared error of prediction rmsep over the entire range of maat while the mbt cbt and mbrgdgt calibrations have relatively large rmseps particularly between lakes with similar temperatures but variable ph this suggests that these techniques do not properly deconvolve the temperature and ph signals recorded in the distributions of the brgdgts we further find that only the sfs calibration produces a credible reconstructed temperature history from sacred lake when compared to other last glacial maximum paleotemperature estimates from east africa thus we advocate for the use of the sfs calibration when reconstructing paleotemperatures from brgdgts in east africa

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

late pleistocene temperature history of southeast africa a tex86 temperature record from lake malawi we present a tex86 derived surface water temperature record for lake malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past similar to 74 kyr with millennial scale resolution average temperature during marine isotope stage mis 5a was 26 5 degrees c with a range from 25 7 to 27 3 degrees c comparable to holocene temperatures mis 4 was a relatively cold period with temperatures generally decreasing from 25 5 degrees c at 68 ka to a minimum of 20 degrees cat similar to 60 ka 1 5 2 degrees c colder than the last glacial maximum lgm termination of mis 4 is characterized by a rapid increase of 3 4 degrees c in only similar to 0 5 kyr temperatures were relatively stable throughout mis 3 at the resolution of this study with an average of 23 8 degrees c and a range from 25 1 to 22 9 degrees c the lack of millennial scale temperature variability during mis 3 suggests that lake malawi s documented response to the bipolar seesaw brown et al 2007 is not reflected in its thermal history our temperature estimates for the lgm and holocene are consistent with a previously published tex86 record from lake malawi with a temperature of similar to 22 6 degrees c for the lgm similar to 25 26 degrees c in the mid holocene and similar to 25 28 for the late holocene in general the present extended tex86 record indicates that temperature variability in tropical east africa during late mis 5 and mis 4 was as great as that associated with the deglaciation and holocene a decrease in southern hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during mis 4 but after 60 ka other factors such as the extent of the polar ice sheets or atmospheric co2 may have forced temperature in tropical africa to a greater extent than local summer insolation

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

the oxygen isotopic composition of seawater during the last glacial maximum high resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep sea sediments from sites in the north and south atlantic the data provide direct measurements of changes in the isotopic composition of bottom waters during the last glacial maximum lgm results from ocean drilling program odp site 981 in the north atlantic currently bathed in north atlantic deep water nadw reproduces previous results from the ceara and bermuda rises constraining the glacial interglacial change in 18 o of the deep atlantic to be 0 7 0 8 results from site 984 which is located north of site 981 and at a shallower water depth yield a similar value 0 8 providing insight into the properties of glacial north atlantic intermediate water gnaiw sites from odp leg 177 in the south atlantic span the modern boundary between northern and southern sources of deep water data from the northern site 1088 yield a similar result to sites in the tropical and north atlantic 0 7 at the southern site 1093 located south of the polar front the change is substantially larger 1 1 representing the change in 18 o of southern source waters since the lgm these results confirm previous estimates that the global average change in 18 o of seawater is 1 0 0 1 hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations giving further support to these results at all sites studied the temperature of the deep ocean during the lgm calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera was within 1 c of the freezing point of seawater

retro modeling the middle mississippi river summary a one dimensional 1 d unsteady flow retro model was developed using historic c 1900 hydrologic and geospatial data and implemented using hec ras the objective of this investigation was to create a 1 d unsteady flow model for the middle mississippi river for the beginning of the 20th century in order to assess the magnitude and types of changes in flood stages associated with 20th century river engineering the retro model was constructed from survey data dating to 1888 1889 and hydrologic data from 1900 to 1904 the late 19th century survey data was supplemented by a modern high resolution dem used to fill gaps in the historic data land cover data recorded during this historic survey was used to establish floodplain roughness values based on published manning s n for the various land cover types and these roughness values were then adjusted to calibrate the model comparison of the retro model results with the 2004 upper mississippi river system flow frequency study umrsffs flood stages showed increases in flood stages of 2 3 4 7 m for large events 50 year recurrence interval these results confirm previous research results showing large scale reductions in flood conveyance on the middle mississippi during the 20th century increased roughness of the floodway coupled with reduction in channel and floodplain area due to wing dike and levee construction are the likely explanation for the observed increases in flood stages between 1889 and 1998 channel widths through the study reach decreased 40 and floodplain area for the 100 year flood decreased by 60 in addition manning s n values in the retro model were lower than the values used in the umrsfss suggesting that 1 the modern floodway is rougher than the historic floodway 2 this increased roughness is not a result of explicit changes in land cover but rather 3 the increase is a result of implicit roughness changes such as wing dike construction the retro model developed in this investigation provides a framework for modeling hydrodynamic and ecological responses to altered hydrologic regimes during more than a hundred years of channel modification

late quaternary behavior of the east african monsoon and the importance of the congo air boundary both atlantic and indian ocean climate dynamics exert influence over tropical african hydroclimate producing complex patterns of convergence and precipitation to isolate the indian ocean influence on african paleohydrology we analyzed the deuterium hydrogen ratio of higher plant leaf waxes delta d wax in a 25 000 year sediment record from lake challa 3 degrees s 38 degrees e in the easternmost area of the african tropics whereas both the seismic record of inferred lake level fluctuations and the branched and isoprenoidal tetraether bit index proxy record changes in hydrology within the challa basin delta d wax as a proxy for the isotopic composition of precipitation delta d p is interpreted as a tracer of large scale atmospheric circulation that integrates the history of the moisture transported to the lake challa area specifically based on modern day isotope rainfall relationships we argue that lake challa delta d wax reflects the intensity of east african monsoon circulation the three hydrological proxy records show generally similar trends for the last 25 000 years but there are important differences between them primarily during the middle holocene we interpret this deviation of delta d wax from local hydrological history as a decoupling of east african monsoon intensity which heavily influences the isotopes of precipitation in east africa today from rainfall amount in the challa basin in combination the hydrological proxy data from lake challa singularly highlight zonal gradients in tropical african climate that occur over a variety of timescales suggesting that the congo air boundary plays a fundamental role in controlling hydroclimate in the african tropics

benthic foraminiferal assemblages from the eastern weddell sea between 68 and 73 s distribution ecology and fossilization potential surface sediment samples taken with a vented box corer from the eastern weddell sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content the live fauna was differentiated from empty tests comprising the foraminiferal death assemblage based on the dead assemblages potential fossil assemblages were calculated to facilitate the analogy with late neogene core material five distinct live assemblages inhabit the continental margin today six dead assemblages and five potential fossil assemblages respectively correspond to these biocoenoses a predominantly calcareous live fauna dominated by trifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope below this on the upper slope down to 2000 m water depth the predominantly calcareous bulimina aculeata assemblage coincides with the core of warm 0 c weddell deep water and with fine and more organic rich sediments these calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment which depends largely on the grain size distribution of the sediment and the fluxes of organic matter therefore a still calcareous t angulosa dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated by martinottiella nodulosa the characteristic arenaceous fossil remnant of the former predominantly calcareous live b aculeata fauna on a continental terrace between 2500 and 3500 m water depth cribrostomoides subglobosus dominates the live fauna but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized by oridorsalis umbonatus and epistominella exigua comprises the dead assemblage and the potential fossil assemblage respectively on the lower continental slope between the carbonate lysocline 3500 m and the carbonate compensation depth 4000 m tests of nuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna which is associated with the lower part of the antarctic bottom water aabw this corroborates earlier investigations suggesting a relationship between the carbonate corrosiveness of water masses and the distribution of n umbonifer this is important for inferring paleo routes and estimates of paleo production rates of aabw during the neogene

flood enhancement through flood control flood stages for constant discharge have increased 2 4 m over the past century at numerous locations in the mississippi river basin however no increases are observed on rivers such as the meramec and the upper missouri which have been spared extensive river engineering projects flood stage increases on the middle mississippi river and lower missouri river are mostly attributable to channelization

east african climate change and orbital forcing during the last 175 kyr bp abstract variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long term fluctuations in african lake levels the understanding of such relationships is essential for designing climate prediction models for the tropics our assessment of the nature and timing of east african climate change is based on lake level fluctuations of lake naivasha in the central kenya rift 0 55 s 36 20 e inferred from sediment characteristics diatoms authigenic mineral assemblages and 17 single crystal 40ar 39ar age determinations assuming that these fluctuations reflect climate changes the lake naivasha record demonstrates that periods of increased humidity in east africa mainly followed maximum equatorial solar radiation in march or september interestingly the most dramatic change in the naivasha basin occurred as early as 146 kyr bp and the highest lake level was recorded at about 139 133 kyr bp this is consistent with other well dated low latitude climate records but does not correspond to peaks in northern hemisphere summer insolation as the trigger for the ice age cycles the naivasha record therefore provides evidence for low latitude forcing of the ice age climate cycles

coherent changes of southeastern equatorial and northern african rainfall during the last deglaciation during the last deglaciation wetter conditions developed abruptly 14 700 years ago in southeastern equatorial and northern africa and continued into the holocene explaining the abrupt onset and hemispheric coherence of this early african humid period is challenging due to opposing seasonal insolation patterns in this work we use a transient simulation with a climate model that provides a mechanistic understanding of deglacial tropical african precipitation changes our results show that meltwater induced reduction in the atlantic meridional overturning circulation amoc during the early deglaciation suppressed precipitation in both regions once the amoc reestablished wetter conditions developed north of the equator in response to high summer insolation and increasing greenhouse gas ghg concentrations whereas wetter conditions south of the equator were a response primarily to the ghg increase

late quaternary record of sea level changes in the antarctic the late quaternary sediment sequence of the continental margin in the eastern weddell sea is well suited for palaeoenvironmental reconstructions twocores rom the upper slope which contain the sedimentary record of the last 300 ky have been sedimentologically investigated age models are based onlithostratigraphy and are correlated with the stable isotope record as a result of a detailed analysis of the clay mineral composition grain size distributionsand structures this sedimentary record provides the first marine evidence that the antarctic ice sheet extended to the shelf edge during the last glacial the variations in volume and size of the ice sheet were also simulated in numerical models changes in accumulation rate and ice temperature are of someimportance but the model revealed that fluctuations are primarily driven by changes in eustatic sea level and that the ice edge extended to the shelf edgeduring the last glacial maximum this causal relationship implies that the maximum ice extension strongly depends on the magnitude and duration of thesea level depression during a glacial period the results of sedimentological investigations and of the numerical models show that the antarctic ice sheetfollows glacial events in the northern hemisphere by teleconnections of sea level

acoustic bed velocity and bed load dynamics in a large sand bed river 1 development of a practical technology for rapid quantification of bed load transport in large rivers would represent a revolutionary advance for sediment monitoring and the investigation of fluvial dynamics measurement of bed load motion with acoustic doppler current profilers adcps has emerged as a promising approach for evaluating bed load transport however a better understanding of how adcp data relate to conditions near the stream bed is necessary to make the method practical for quantitative applications in this paper we discuss the response of adcp bed velocity measurements defined as the near bed sediment velocity detected by the instrument s bottom tracking feature to changing sediment transporting conditions in the lower missouri river bed velocity represents a weighted average of backscatter from moving bed load particles and spectral reflections from the immobile bed the ratio of bed velocity to mean bed load particle velocity depends on the concentration of the particles moving in the bed load layer the bed load layer thickness and the backscatter strength from a unit area of moving particles relative to the echo strength from a unit area of unobstructed bed a model based on existing bed load transport theory predicted measured bed velocities from hydraulic and grain size measurements with reasonable success bed velocities become more variable and increase more rapidly with shear stress when the transport stage defined as the ratio of skin friction to the critical shear stress for particle entrainment exceeds a threshold of about 17 this transition in bed velocity response appears to be associated with the appearance of longer flatter bed forms at high transport stages

a 220ka terrestrial δ18o and deuterium excess biomarker record from an eolian permafrost paleosol sequence ne siberia the natural abundances of stable oxygen 18o 16o and hydrogen isotopes 2h 1h are valuable proxies of climate changes in the past yet to date no continuous 18o and only few 2h records are available from loess paleosol sequences taking advantage of a recently developed method based on compound specific 18o analyses of hemicellulose sugar biomarkers in soils zech and glaser 2009 rcm 23 3522 3532 we here present a first terrestrial 18o biomarker record from an eolian permafrost paleosol sequence in ne siberia that covers the last 220 ka the 18o values of the hemicellulose biomarkers arabinose and xylose range from 22 5 to 32 8 and from 21 3 to 31 9 respectively and reveal systematic glacial interglacial shifts the modern topsoil and the interglacial paleosols exhibit more positive 18o values whereas the glacial paleosols are characterized by more negative 18o values this is in agreement with the 2h record obtained for sedimentary n alkane leaf wax biomarkers we present a conceptual model for interpreting the combined 18o and 2h biomarker record based on this model we suggest that both our 18o and the 2h records primarily reflect the temperature controlled isotopic composition of paleoprecipitation modified by evaporative isotope enrichment of leaf water during transpiration considering fractionation factors during sugar and n alkane biomarker biosynthesis allows reconstructing the leaf water isotopic composition and the deuterium excess of the leaf water the deuterium excess may serve as proxy for evaporative enrichment and allows calculating relative humidity using a craig gordon model accordingly relative humidity in ne siberia was higher during marine isotope stage mis 6 compared to mis 2 4 and 5d and thus could help explaining the much larger extent of the late saalian glaciation compared to the weichselian glaciations using the craig gordon model we also calculated 18o of the plant source water 18osource water which can be assumed to primarily reflect 18o of paleoprecipitation our 220 ka 18osource water record is well in agreement with the 18odiatom record from crater lake el gygytgyn in ne siberia and enables a regional paleoclimate reconstruction and interpretation accordingly summer temperature was periodically warmer than at present during the weichselian glacial period and there is a strong july insolation forcing of the summer temperature in the extremely continental study area overall our study highlights the great potential of the novel hemicellulose biomarker 18o method for paleoclimate reconstructions especially when combined with 2h analyses of n alkane lipid biomarkers

δd values of individual n alkanes from terrestrial plants along a climatic gradient implications for the sedimentary biomarker record abstract the compound specific hydrogen isotope composition d of sedimentary n alkanes is increasingly used as a palaeoclimate proxy to explore the origin of the hydrogen isotope signal of n alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n alkane composition and d values of 31 plant biomass samples birch betula beech fagus oak quercus alder alnus hornbeam carpinus and myrtle myrtus as well as non stomata containing sphagnum cladonia and moss from 14 sites along a climatic gradient from northern finland to southern italy eleven of the sites were previously investigated for the n alkane d values in adjacent lake sediments and are compared to the leaf wax n alkane d values the distribution of terrestrial n alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake evidently n alkane distributions within the same species at different sites were found to be variable these findings complicate the use of certain n alkane distributions in sediments as a palaeoenvironmental indicator d values of plant biomass n alkanes show a significant positive correlation with the precipitation d values biomass n alkane d values are in general less negative than the associated sedimentary n alkanes of terrestrial origin possibly due to the stronger evapotranspiration during the summer when sampling took place this suggests that n alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn when most leaves are incorporated into the sediment the variability of the d values from non stomata containing plants is much higher than for broadleaf trees since stomata can regulate water loss through evapotranspiration the average chain length acl of n alkanes from deciduous tree leaves increases from north to south with increasing acl the hydrogen isotopic fractionation e between source water and n alkanes also increases our results prove that terrestrial n alkane d values record the d value of precipitation modified by the amount of isotope enrichment in the leaf water due to plant anatomy stomata vs no stomata and site specific meteorological conditions such as evapotranspiration relative humidity and soil moisture therefore terrestrial n alkanes in lake sediments can deliver information on changes in these parameters over geological timescales in lake sediments where aquatic n alkanes such as n c 17 record the lake water d value the isotopic difference between terrestrial and aquatic n alkanes could therefore be used to reconstruct changes in evapotranspiration

the deuterium excess records of epica dome c and dronning maud land ice cores east antarctica new high resolution deuterium excess d data from the two epica ice cores drilled in dronning maud land edml and dome c edc are presented here the main moisture sources for precipitation at edc and edml are located in the indian ocean and atlantic ocean respectively the more southward moisture origin for edml is reflected in a lower present day d value compared to edc the edml and edc isotopic records 18o and d show the main climate features common to the east antarctic plateau and similar millennial scale climate variability during the last glacial period however quite large 18o and d differences are observed during mis5 5 and the glacial inception with a long term behaviour a possibility for this long term difference could be related to uncertainties in past accumulation rate which are used in the glaciological models regional climate anomalies between the two sites during mis5 5 could also be consistent with the observed edml edc 18o and d gradient anomalies simulations performed with the general circulation model echam4 for different time slices provide a temporal temperature isotope slope for the edml region in fair agreement to the modern spatial slope tsite and tsource records are extracted from both ice cores using a modelling approach after corrections for past 18o seawater and elevation changes a limited impact of d on antarctic temperature reconstruction at both edml and edc has been found with a higher impact only at glacial inception the aim antarctic isotope maximum events in both ice cores are visible also after the source correction suggesting that these are real climate features of the glacial period the different shape of the aim events between edc and edml as well as some climate features in the early holocene points to a slightly different climate evolution at regional scale a comparison of our temperature reconstruction profiles with the aerosol fluxes show a strong coupling of the nssca fluxes with antarctic temperatures during glacial period and a tighter coupling of 18o and tsite with ssna flux at edml compared to edc during the glacial period and mis5 5

a stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized greenland ice core records refining and extending the intimate event stratigraphy due to their outstanding resolution and well constrained chronologies greenland ice core records provide a master record of past climatic changes throughout the last interglacial glacial cycle in the north atlantic region as part of the intimate integration of ice core marine and terrestrial records project protocols have been proposed to ensure consistent and robust correlation between different records of past climate a key element of these protocols has been the formal definition and ordinal numbering of the sequence of greenland stadials gs and greenland interstadials gi within the most recent glacial period the gs and gi periods are the greenland expressions of the characteristic dansgaard oeschger events that represent cold and warm phases of the north atlantic region respectively we present here a more detailed and extended gs gi template for the whole of the last glacial period it is based on a synchronization of the ngrip grip and gisp2 ice core records that allows the parallel analysis of all three records on a common time scale the boundaries of the gs and gi periods are defined based on a combination of stable oxygen isotope ratios of the ice 18o reflecting mainly local temperature and calcium ion concentrations reflecting mainly atmospheric dust loading measured in the ice the data not only resolve the well known sequence of dansgaard oeschger events that were first defined and numbered in the ice core records more than two decades ago but also better resolve a number of short lived climatic oscillations some defined here for the first time using this revised scheme we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the last glacial period that are represented in the greenland ice records the final product constitutes an extended and better resolved greenland stratotype sequence against which other proxy records can be compared and correlated it also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations

branched glycerol dialkyl glycerol tetraethers in pleistocene loess paleosol sequences three case studies abstract branched glycerol dialkyl glycerol tetraethers branched gdgts are membrane lipids presumably derived from anaerobic heterotrophic soil bacteria empirical studies have shown that the composition of branched gdgts in topsoils varies in degree of cyclization and methylation expressed as cbt and mbt i e cyclization and methylation index of branched tetraethers depending on soil ph and mean annual air temperature mat here we present results from three case studies namely three well studied loess paleosol sequences lpss in which we measured the branched gdgts in order to test the applicability of these biomarkers for paleoclimate reconstruction the results show that major disagreements exist between expectations based on available stratigraphic pedological and geochemical data on the one hand and gdgt derived reconstructions on the other hand caution is therefore advised when interpreting the respective indices in lps studies we discuss several potential directions for future research i some of the target peaks in the hplc chromatograms consist of multiple unidentified compounds this needs to be addressed by way of improved component separation identification and new calibration studies ii factors other than mat and soil ph that could affect membrane composition should be further investigated e g nutrient availability and redox conditions as well as seasonality and changes in bacterial communities iii gdgts may be produced in non negligible amounts at depth and thus overprint the original in situ signal a concern that could be termed growth depth effect iv gdgts should also be analyzed in dust source regions and contemporaneous eolian sediments in order to address a potential redeposition effect

chronology of quaternary glaciations in east africa abstract a new glacial chronology for equatorial east africa is developed using in situ cosmogenic 36 cl measured in 122 boulders from moraines on mount kenya and kilimanjaro the oldest deposits sampled on kilimanjaro yield a limiting 36 cl age of 360 calendar kyr all 36 cl ages are in calendar years cal kyr or cal yr on mount kenya the oldest moraines give ages of 355 420 kyr liki i and 255 285 kyr teleki given the uncertainty in our 36 cl ages the liki i moraine may correspond to either marine isotope stage 10 or 12 whereas the teleki moraine correlates with stage 8 there is no evidence for stage 6 on either mountain the liki ii moraines on mt kenya and moraines of the fourth glaciation on kilimanjaro give ages of 28 3 kyr and 20 1 kyr respectively they represent the last glacial maximum lgm and correlate with stage 2 of the marine isotope record a series of smaller moraines above the lgm deposits record several readvances that occurred during the late glacial on mt kenya these deposits date to 14 6 1 2 kyr liki iia 10 2 0 5 kyr liki iii 8 6 0 2 kyr liki iiia and 200 yr lewis the corresponding deposits on kilimanjaro have mean ages of 17 3 2 9 kyr fourth glaciation saddle 15 8 2 5 kyr little glaciation saddle and 13 8 2 3 kyr fourth glaciation kibo these data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle

3 d flow and scour near a submerged wing dike adcp measurements on the missouri river 1 detailed mapping of bathymetry and three dimensional water velocities using a boat mounted single beam sonar and acoustic doppler current profiler adcp was carried out in the vicinity of two submerged wing dikes located in the lower missouri river near columbia missouri during high spring flows the wing dikes become submerged creating a unique combination of vertical flow separation and overtopping plunging flow conditions causing large scale three dimensional turbulent flow structures to form on three different days and for a range of discharges sampling transects at 5 and 20 m spacing were completed covering the area adjacent to and upstream and downstream from two different wing dikes the objectives of this research are to evaluate whether an adcp can identify and measure large scale flow features such as recirculating flow and vortex shedding that develop in the vicinity of a submerged wing dike and whether or not moving boat single transect data are sufficient for resolving complex three dimensional flow fields results indicate that spatial averaging from multiple nearby single transects may be more representative of an inherently complex temporally and spatially variable three dimensional flow field than repeated single transects results also indicate a correspondence between the location of calculated vortex cores resolved from the interpolated three dimensional flow field and the nearby scour holes providing new insight into the connections between vertically oriented coherent structures and local scour with the unique perspective of flow and morphology in a large river

climatic patterns in equatorial and southern africa from 30 000 to 10 000 years ago reconstructed from terrestrial and near shore proxy data abstract as part of a wider study of last glacial and deglacial climates in the southern hemisphere continents we here review terrestrial and near shore marine records from equatorial and southern africa between 30 000 and 10 000 years ago 30 10 ka this time interval covers the lead up to the last glacial maximum lgm 21 2 ka the lgm proper and the ensuing deglacial records selected for review needed to meet three requirements continuity or near continuity over the period a well established chronology and at least one but preferably several unambiguous proxy ies we aim to show how regional climates of the sub continent have responded to orbital forcing as opposed to other global glacial interglacial boundary conditions and how they are related to high latitude climates sea and land surface conditions positions of the intertropical convergence zone itcz and of the westerly belt evidence of past climates derived from many independent proxies is given from west to southwest africa moisture from the atlantic ocean then from equatorial east africa to the southern subtropical summer rainfall domain moisture mainly from the indian ocean the lgm was cooler than today and generally drier in the tropics north of 8 9 s glacial to holocene increase in monsoonal precipitation primarily related to orbitally induced summer insolation in the northern hemisphere occurred by steps of increasing amplitude 17 16 14 5 11 5 ka major wet dry spells coincide with abrupt warm cold events in high northern latitudes and related itcz migrations in the southern tropics the main post glacial increase in tropical rainfall generally appears more gradual and in phase with antarctic warming data suggest a restricted northward migration of the itcz and concentration of tropical rainfall well south of the equator during the lgm and the younger dryas drier glacial conditions prevailed in southeastern africa while parts of southwestern africa point to enhanced humidity during the lgm suggesting that the winter westerly belt was either stronger than today or displaced further north possibly as a result of more extensive antarctic sea ice inferred african climatic fluctuations show the competing influences of tropical and high latitude climates of both hemispheres and suggest changes in both meridional and zonal circulation modes this review also reveals major geographical and methodological gaps and a number of unresolved issues providing pointers for future research

reply to the comment of sternberg on zech et al 2014 oxygen isotope ratios 18 o 16 o of hemicellulose derived sugar biomarkers in plants soils and sediments as paleoclimate proxy i insight from a climate chamber experiment gca 126 614 623

late wisconsin and early holocene glacial history inner ross embayment antarctica abstract lateral drift sheets of outlet glaciers that pass through the transantarctic mountains constrain past changes of the huge ross ice drainage system of the antarctic ice sheet drift stratigraphy suggests correlation of reedy iii reedy glacier beardmore beardmore glacier britannia hatherton darwin glaciers ross sea mcmurdo sound and younger terra nova bay drifts radiocarbon dates place the outer limits of ross sea drift in late wisconsin time at 24 000 13 000 yr b p outlet glacier profiles from these drifts constrain late wisconsin ice sheet surface elevations within these constraints we give two extreme late wisconsin reconstructions of the ross ice drainage system both show little elevation change of the polar plateau coincident with extensive ice shelf grounding along the inner ross embayment however in the central ross embayment one reconstruction shows floating shelf ice whereas the other shows a grounded ice sheet massive late wisconsin holocene recession of grounded ice from the western ross embayment which was underway at 13 040 yr b p and completed by 6600 6020 yr b p was accompanied by little change in plateau ice levels inland of the transantarctic mountains sea level and basal melting probably controlled the extent of grounded ice in the ross embayment the interplay between the precipitation low late wisconsin and high holocene values and the influence of grounding on outlet glaciers late wisconsin thickening and late wisconsin holocene thinning with effects dying out inland probably controlled minor elevation changes of the polar plateau

lake level and paleoenvironmental history of lake tanganyika africa as inferred from late holocene and modern stromatolites fossil and living stromatolites are abundant around the margins of lake tanganyika africa and provide a wealth of paleolimnologic and paleoclimatic information for the late holocene six lines of evidence show that stromatolites and cements are precipitating in the lake today 1 carbonate saturation state calculations 2 documentation of living stromatolites and their depth distribution 3 new stable isotope data showing the lake s present mixing state and ancient evaporation and inflow balance 4 new radiocarbon data and a reevaluation of apparent 14 c ages derived from lake tanganyika carbonates 5 the presence of modern mg calcite cements derived from lake waters and 6 the presence of modern biologically mediated mg calcite precipitates in the lake lake tanganyika s lake levels have been remarkably stable over the past 2800 yr fluctuating around the marginally open to marginally closed level through most of this time period lake lowstands and high 18 o values from the ninth century b c to the early fifth century a d indicate that the lake basin was comparatively dry during this time however the period prior to the most recent opening of lake kivu into the lake tanganyika basin ca a d 550 was not marked by major lake lowstands nor was this opening accompanied by a dramatic lakelevel rise the kivu opening was roughly coincident with a significant shift toward isotopically lighter 18 o and 13 c lake water which persists today the lake remained close to its outlet level between the sixth and thirteenth centuries a d lake levels rose between the fourteenth and sixteenth centuries at some time between the late sixteenth and early nineteenth centuries lake level fell to perhaps its lowest level in the past 2800 yr by the early nineteenth century lake level had begun to rise to the overflow level apparently the result of a regional increase in precipitation evaporation ratios weak 18 o 13 c covariance for late holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level with only occasional periods of closure however there is no simple relationship between solute input from lake kivu isotope input from lake kivu and lake levels in lake tanganyika lake kivu waters are the primary source of major ions in lake tanganyika but are much less important in controlling the 18 o and the lake level of lake tanganyika because the ruzizi river s discharge into lake tanganyika is largely derived from sources other than lake kivu the overflow events in the two lakes have been uncoupled during the late holocene

quantitative estimates of full glacial temperatures in equatorial africa from palynological data this paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial africa 3 28 s 29 34 e 2240 m it has been achieved with an extended modern african reference data set by adding spectra from 228 new sites and using another selection for pollen taxa the purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues the results are in good agreement with those previously published reinforcing the validity of the method the mean standard error is reduced by 0 3 c the mean temperature for the holocene appears 1 4 c warmer than the present and the last glacial maximum 25 18 kyrs bp cooling is better specified at 3 1 9 c a conservative value more consistent with reconstructed sea surface temperature in the equatorial ocean

the antarctic palaeo record and its role in improving predictions of future antarctic ice sheet change this paper reviews the ways in which the palaeo record of antarctic ice sheet change can be used to improve understanding of contemporary ice sheet behaviour and thus enhance predictions of future sea level change the main areas where the palaeo ice sheet record can contribute are understanding long term ice sheet trajectory providing data against which ice sheet models can be tested to identify and understand the range and types of natural ice sheet behaviour to balance the global water budget to correct contemporary glaciological measurements of mass change and to understand the relationship between polar ecosystems and the ice sheet i review each in turn and argue that research priorities include understanding past west antarctic ice sheet collapse and its timing a focus on the palaeo record of rapid retreat events and how these unfolded in the geological past improving the number and range of ice sheet reconstructions particularly through the holocene continuing to investigate the potential for using sediments and landforms to parameterise basal conditions in ice sheet models and understanding past east antarctic ice sheet dynamics particularly the evidence for partial deglaciation copyright 2009 john wiley sons ltd

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr