diatom inferred salinity in palaeolakes an indirect tracer of climate change lakes in arid and semi arid regions respond to climatic change through shifts in lake water volume and ionic concentration because diatom distribution is highly correlated with lake hydrochemistry diatoms can be used to infer changes in salinity and brine composition and thus to infer past climates here we critically examine the use of diatom inferred salinity as a climate proxy with examples taken from both modern waterbodies and sedimentary profiles sediment records may contain assemblages mixed from periods or sites of differing hydrochemistries because of the high degree of spatial and temporal heterogeneity in saline systems dissolution and diagenesis in saline brines may further complicate interpretation of sedimentary assemblages furthermore threshold effects salinity regulation via groundwater seepage antecedent conditions and other aspects of local hydrology may modify the relationship between salinity change and climatic forcing these complexities necessitate critical examination of the ecology and taphonomy of sedimentary assemblages as well as evaluation of potential non linearities in the salinity climate relationship through comparison of diatom inferred salinity with other proxy records of hydrochemical change historical documents where they exist and with other regional lake systems

characteristics of the modelled meteoric freshwater budget of the western antarctic peninsula abstract rapid climatic changes in the western antarctic peninsula wap have led to considerable changes in the meteoric freshwater input into the surrounding ocean with implications for ocean circulation the marine ecosystem and sea level rise in this study we use the high resolution regional atmospheric climate model racmo2 3 coupled to a firn model to assess the various contributions to the meteoric freshwater budget of the wap for 1979 2014 precipitation snowfall and rainfall meltwater runoff to the ocean and glacial discharge snowfall is the largest component in the atmospheric contribution to the freshwater budget and exhibits large spatial and temporal variability the highest snowfall rates are orographically forced and occur over the coastal regions of the wap 2000 mm water equivalent w e y 1 and extend well onto the ocean up to the continental shelf break a minimum 500 mm w e y 1 is reached over the open ocean rainfall is an order of magnitude smaller and strongly depends on latitude and season being large in summer when sea ice extent is at its minimum for antarctic standards wap surface meltwater production is relatively large 50 mm w e y 1 but a large fraction refreezes in the snowpack limiting runoff only at a few more northerly locations is the meltwater predicted to run off into the ocean in summer we find a strong relationship of the freshwater fluxes with the southern annular mode sam index when sam is positive and occurs simultaneously with a la nina event there are anomalously strong westerly winds and enhanced snowfall rates over the wap mountains marguerite bay and the bellingshausen sea when sam coincides with an el nino event winds are more northerly reducing snowfall and increasing rainfall over the ocean and enhancing orographic snowfall over the wap mountains assuming balance between snow accumulation mass gain and glacial discharge mass loss the largest glacial discharge is found for the regions around adelaide island 10 gt y 1 anvers island 8 gt y 1 and southern palmer land 12 gt y 1 while a minimum 2 gt y 1 is found in marguerite bay and the northern wap glacial discharge is in the same order of magnitude as the direct freshwater input into the ocean from snowfall but there are some local differences the spatial patterns in the meteoric freshwater budget have consequences for local productivity and carbon drawdown in the coastal ocean

diatom based reconstruction of the subantarctic front migrations during the late miocene and pliocene abstract the antarctic circumpolar current acc is an influential component of the global climate system and therefore it is essential to clarify the history of the oceanic fronts associated with it however there have been only a few attempts to reconstruct their history during the neogene as most previous studies focused on quaternary ages this study uses the late miocene to pliocene ca 9 3 ma fossil records of diatoms from the atlantic sector of the southern ocean deep sea drilling project site 513 and ocean drilling program site 689 to reconstruct the paleo location of the subantarctic front saf which represents the northern limit of the acc the abundance of subtropical diatoms at site 513 shows significant fluctuations during ca 9 5 ma which strongly suggest that there have been repetitive north south migrations of the saf across site 513 during at least ca 9 5 ma in order to reconstruct the saf migration history in more detail changes in abundance of a typical subtropical diatom i e thalassionema nitzschioides var parva heiden emend moreno ruiz recorded at sites 513 and 689 are compared to abundance patterns of the same taxon at several drilling sites ocean drilling program sites 697 699 and 704 published in previous data reports results suggest several phases of saf migration involving the following shifts in saf position direction of shift is indicated in brackets ca 8 5 ma northward 8 0 ma northward 7 0 ma southward 6 1 ma northward 5 4 ma the direction differs region by region 4 9 ma northward 4 6 ma southward 3 8 ma northward the results also show that saf has experienced north south migrations with a latitudinal amplitude of ca 5 or more comparison between changes in fossil diatom assemblages at site 689 and the inferred saf shifts suggest that the northward southward shift of the saf could be induced by the cooling warming in the high latitude regions

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

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

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

post glacial arid episodes in ethiopia have implications for climate prediction several lines of geomorphological and stratigraphical evidence suggest that the widespread early post glacial wet phase in tropical africa between 12 500 and 5 000 yr bp refs 1 2 was interrupted by severe dry episodes during the intervals 11 000 10 000 8 500 6 500 and 6 200 5 800 yr bp refs 3 9 the magnitude of the second event has recently been questioned10 we present new stratigraphical and dating evidence from the ziway shala lake basin ethiopia indicating the severity and timing of dry phases during the interval 12 000 5 000 yr bp our findings have important implications for predictions of the climatic impact of increasing atmospheric levels of co2

late quaternary vegetation dynamics in a biodiversity hotspot the uluguru mountains of tanzania abstract late quaternary vegetation history and environmental changes in a biodiverse tropical ecosystem are inferred from pollen charcoal and carbon isotope evidence derived from a 48 000 yr sedimentary record from the uluguru mountains a component of the eastern arc mountains of kenya and tanzania results indicate that eastern arc forest composition has remained relatively stable during the past 48 000 yr long term environmental stability of the eastern arc forests has been proposed as a mechanism for the accumulation and persistence of species during glacial periods thus resulting in the diverse forests observed today the pollen and isotope data presented here indicate some marked changes in abundance but no significant loss in moist forest taxa through the last glacial maximum thereby providing support for the long term environmental stability of the eastern arc anthropogenic activities including burning and forest clearance were found to play a moderate role in shaping the mosaic of forest patches and high altitude grasslands that characterise the site today however this influence was tempered by the inaccessibility of the mountain

paleoclimatic significance of the stable isotopic composition and petrology of a late pleistocene stalagmite from botswana high resolution 18o and 13c analyses of a stalagmite from lobatse ii cave reveal late pleistocene environmental changes in botswana large shifts in 18o and 13c are observed between two main periods of deposition the first period between 51 000 and 43 000 yr b p was warm and humid and may have been associated with some c3 vegetation the second period between 27 000 and 21 000 yr b p had temperatures approximately 2 c lower and vegetation dominated by drought adapted c4 plants the intervening period between 43 000 and 27 000 yr b p bounded by two major hiatuses in stalagmite growth produced discontinuous speleothem formation probably under dry conditions new 230 th 234u mass spectrometry age determinations for the stalagmite generally agree with previously measured 230 th 234u ages

palaeoecological evidence for holocene environmental change from the virunga volcanoes in the albertine rift central africa abstract this study presents two new well dated sedimentary pollen and charcoal records from high altitude crater sites in the virunga volcanoes located in the albertine rift central africa currently part of one of the world s most biodiverse areas here we argue that holocene vegetation changes in a ca 8000 year palaeoenvironmental record from a crater swamp at an altitude of 3474 m and a ca 2800 year record from a crater lake at an altitude of 4127 m are linked to variations in both climate and human activity climatic changes during the mid to late holocene are reflected in the high altitude sites and more widely in adjacent parts of the albertine rift vegetation changes comprising a decline in ericaceous vegetation at ca 5000 cal yrs bp and subsequent expansion of afroalpine vegetation together with a later increase in taxa associated with lower montane forest particularly podocarpus reflect increasing aridity during the mid to late holocene human induced environmental change in the virunga volcanoes is apparent only within the last millennium despite the long history of human occupation of the area both study sites record significant forest clearance at ca 900 cal yrs bp involving a reduction in lower montane forest taxa and increases in disturbance indicators changes in the composition of upper montane forest and particularly the expansion of hagenia are possibly linked to anthropogenic induced changes in the fire regime and are apparent from ca 900 cal yrs bp human induced environmental modification from the early part of the last millennium likely associated with onset of the late iron age appears to have extended to high altitudes the importance of natural long term climate change as a major cause of environmental change in the albertine rift has been eclipsed within the last millennium by human induced environmental effects

the antarctic dipole and its predictability this study investigates the nature of interan nualvariability ofantarcticseaiceanditsrelationshipwith the tropical climate we nd that the dominant interan nual variance structure in the sea ice edge and surface air temperature elds is organized as a quasi stationary wave which we call the antarctic dipole adp it is charac terized by an out of phase relationship between the ice and temperature anomalies in the central eastern pacic and atlantic sectors of the antarctic the dipole consists of a strong standing mode and a weaker propagating motion within each basin s ice eld it has the same wavelength as the antarctic circumpolar wave acw and dominates the acw variance the dipole is clearly associated with tropical enso events it can be predicted with moderate skill using linear regression involving surface temperature two to four months ahead the prediction performs better in extreme warm cold years and best in la ni na years

antarctic sea ice and palaeoproductivity variation over the last 156 000 years in the indian sector of southern ocean abstract antarctic sea ice plays a vital role in global climate via its impact on ocean circulation biological productivity and co2 partitioning between the ocean and the atmosphere however very little is known about its past history especially in the southwestern indian sector of the southern ocean so we here provide new quantitative records of winter sea ice concentration wsic and duration wsid sea surface temperatures sst and productivity in sediment core sk 200 33 55 s 45 e from the permanently open ocean zone over the last 156 000 years the new records combined with regional records indicate that the hydrological structures migrated northward by a few degrees of latitude during all glacial periods with the southern antarctic circumpolar current front reaching the core site the antarctic polar front located at 46 s and the winter sea ice wsi probably extending to 49 s in contrast hydrological fronts and wsi edge migrated poleward by a couple of degrees of latitude during the early holocene and last interglacial comparison to sst and wsi records from different sectors of the so suggests higher amplitude variations in wsi in the atlantic sector as compared to the indian and western pacific sectors over the last glacial interglacial cycle which we attribute to the presence of the weddell gyre transporting far to the north the sea ice produced in the weddell sea the new records also suggest a drop in productivity in the pooz during glacial periods probably related to greater wsi extent and reduced growing season

southern ocean sea ice and frontal changes during the late quaternary and their linkages to asian summer monsoon abstract the present study documents the interactions between southern hemisphere high latitude antarctica southern ocean southern indian ocean subtropics agulhas leakage and asian summer monsoon the study uses sst and sea ice reconstructions along with diatom absolute abundances and diatom biometry from two sediment cores located at the subantarctic front saf and antarctic polar front apf in the southwest indian sector of the southern ocean sea ice records suggest the presence of the mean winter sea ice limit at around the modern apf location during mis 2 and mis 4 and episodic and unconsolidated winter sea ice far north as 43 s during lgm when the ssts were lowest higher diatom productivity and larger mean sizes of f kerguelensis and t lentiginosa recorded at the northern core site during the glacial stages suggest a northward shift of the apf a decrease in diatom productivity and sizes at the southern core site highlights stratified permanent open ocean zone pooz surface waters in response to longer sea ice presence during the glacial stages the comparative study between the records of southern hemisphere high latitude and asian summer monsoon climate variability revealed that the asian summer monsoon variability could have been more likely forced by low latitude insolation gradient changes and supported by antarctic climate changes via meridional shifts of the fronts and sea ice the past changes in the intensity of asian summer monsoon along with the southern ocean frontal variation might have influenced the southern indian ocean surface circulation by changing the agulhas leakage intensity

african hydroclimatic variability during the last 2000 years abstract the african continent is characterised by a wide range of hydroclimate regimes ranging from humid equatorial west africa to the arid deserts in the northern and southern subtropics the livelihoods of much of its population are also vulnerable to future climate change mainly through variability in rainfall affecting water resource availability a growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of africa s natural environment this paper co authored by members of the pages africa 2k working group presents an extensive assessment and discussion of proxy historical and instrumental evidence for hydroclimatic variability across the african continent spanning the last two millennia while the african palaeoenvironmental record is characterised by spatially disjunctive datasets with often less than optimal temporal resolution and chronological control the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time in this study we focus sequentially on data for six major time windows the first millennium ce the medieval climate anomaly 900 1250 ce the little ice age 1250 1750 ce the end of the lia 1750 1850 ce the early modern period 1850 1950 and the period of recent warming 1950 onwards this results in a continent wide synthesis of regional moisture balance trends through history allowing consideration of possible driving mechanisms and suggestions for future research

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

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

timing of the younger dryas event in east africa from lake level changes the last deglaciation was interrupted by an abrupt cooling event the younger dryas at 11 000 ndash 10 000 yr bp uncalibrated radiocarbon timescale 1 originally recognized in climate records from northwest europe the younger dryas has now been identified in marine and ice core records worldwide2 6 in the tropics a broadly contemporaneous change in climate is recorded by decreases in water levels and increased salinity of lakes7 9 14 indicating a period of arid climate caused by a reduction in ocean to land moisture flux the exact timing of these changes in relation to the younger dryas event in high latitude records has remained unclear however here we present climate records based on analyses of diatom assemblages geochemistry and magnetic mineralogy of radiocarbon dated sequences of laminated lake sediments from lake magadi in the east african rift these records provide a detailed record of climate change in lowland equatorial africa throughout the last deglaciation 12 800 ndash 10 000 14c yr bp we find that lake level and humidity maxima coincide with the most rapid phases of ice melting in the northern hemisphere and that the climate changes including the younger dryas event were synchronous at low and high latitudes thus the effects of abrupt climate change appear to be felt at both high and low latitudes without a significant time lag

sedimentation in lake malawi east africa during the past 10 000 years a continuous paleoclimatic record from the southern tropics a suite of piston cores recovered from lake malawi 9 14 s 34 35 e east africa in 1986 has been analyzed for major and minor elements organic c and n calcium carbonate and diatoms an internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera stephanodiscus and melosira with age control obtained primarily from 14c dating of the carbonate differences with time in fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth the depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr b p indicating less seasonality carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column the carbonate which precipitates from surface waters is most abundant during the interval from about 10 000 to 6000 yr b p this micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced sedimentary evidence suggests that lake levels were 100 150 m lower than present during this period this record is different from climatic trends in northern intertropical africa but appears to also be related to changes in insolation and monsoon circulation this is the northernmost basin in africa reported to exhibit a southern hemisphere response to the early holocene northern hemisphere summer insolation maximum the climatic hingeline north of lake malawi 9 s implied by our results is significantly south of that indicated by general circulation model simulations however the cores show evidence for periods of abrupt climate change during the interval of generally arid climate

late quaternary highstands at lake chilwa malawi frequency timing and possible forcing mechanisms in the last 44 ka the extensive shoreline deposits of lake chilwa southern malawi a shallow water body today covering 600 km2 of a basin of 7500 km2 are investigated for their record of late quaternary highstands osl dating applied to 36 samples from five sediment cores from the northern and western marginal sand ridges reveal a highstand record spanning 44 ka using two different grouping methods highstand phases are identified at 43 7 33 3 ka 26 2 21 0 ka and 17 9 12 0 ka total error method or 38 4 35 5 ka 24 3 22 3 ka 16 2 15 1 ka and 13 5 12 7 ka finite mixture model age components with two further discrete events recorded at 11 01 0 76 ka and 8 52 0 56 ka highstands are comparable to the timing of wet phases from other basins in east and southern africa demonstrating wet conditions in the region before the lgm which was dry and a wet lateglacial which commenced earlier in the southern compared to northern hemisphere in east africa we find no evidence that wet phases are insolation driven but analysis of the dataset and gcm modelling experiments suggest that heinrich events may be associated with enhanced monsoon activity in east africa in both timing and as a possible causal mechanism

a revised 30 000 year paleoclimatic and paleohydrologic history of lake albert east africa abstract new geochemical and palynological data and a sedimentological reinterpretation of two cores from lake albert allow a revision of the paleoclimatic and paleohydrologic history of northwest uganda over the last 30 000 14 c yr bp our findings document one and possibly three periods of significantly reduced inflow to the nile river system from the white nile based on sediment fabric and mineralogy shifts in percentages of total organic carbon toc and total organic nitrogen ton hydrogen index hi and 13 c and 15 n as well as poor preservation of sporopollenin and siliceous microfossils we recognize two mature paleosols that formed between 18 000 and 12 500 14 c yr bp at the surface of exposed lacustrine muds a thin fossiliferous zone between the two soils documents a brief period of flooding lake albert was below the level of the coring site and thus a closed basin for several thousand years during the late pleistocene around 12 500 14 c yr bp the younger vertisol was flooded by a major rise in lake level relatively deep water conditions were established soon after this transgression but an apparent change in sedimentation rate sometime between 8 000 and 3 400 14 c yr bp may reflect another briefer period of low water which resulted in interruption of sedimentation at the coring site palynological data document a rapid and abrupt shift to drier conditions around lake albert from 11 400 to 9 900 14 c yr bp coincident with the cool younger dryas event of the northern hemisphere geochemical evidence suggests that throughout the terminal pleistocene and early holocene and again in the late holocene the water column was relatively well mixed with little tendency to persistent anoxia

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

retreating glacier fronts on the antarctic peninsula over the past half century the continued retreat of ice shelves on the antarctic peninsula has been widely attributed to recent atmospheric warming but there is little published work describing changes in glacier margin positions we present trends in 244 marine glacier fronts on the peninsula and associated islands over the past 61 years of these glaciers 87 have retreated and a clear boundary between mean advance and retreat has migrated progressively southward the pattern is broadly compatible with retreat driven by atmospheric warming but the rapidity of the migration suggests that this may not be the sole driver of glacier retreat in this region

antarctic circumpolar current s role in the antarctic ice system an overview abstract the antarctic circumpolar current acc provides fundamental control on the antarctic ice system the tilt of the isopycnals of the acc in response to strong westerlies serves to thermally isolate the antarctic continent from directly receiving the overwhelming subtropical ocean surface heat this same tilt provides the northern boundary of the polar seas as such it contains the statically stable cold fresh surface polar waters required for sea ice formation in this manner it effectively sets the northern limit for seasonal sea ice formation the isopycnal tilt also allows warm deep water to upwell to the surface near the continental margin in western antarctica where the acc skirts the continental shelf leading to excessive ocean heat flux to the atmosphere in winter and providing heat to melt the underside of the glacial ice

environmental changes at lake cheshi zambia since 40 000 years b p abstract sediment and microfossil analyses of a 7 5 m core from lake cheshi suggest that south central africa experienced late quaternary climate changes similar to those in east africa the lake formed around 34 000 yr b p after a prelacustrine phase of at least 6000 yr from climatic or tectonic causes ratios of precipitation to evaporation were probably similar to those of today until a decline about 15 000 13 000 yr b p when the lake shrank and became chemically concentrated maximal lake levels occurred between 8000 and 4000 yr b p and were followed by a low stand under presumably arid conditions about 3500 yr b p encroachment of sudd vegetation contributed to shallowing during the last 3000 yr a phase of microfossil dilution may reflect human activity in the basin or climatic or hydrological changes melosira valve morphology seems to reflect mixing regimes sponge and testate amoeba remains were most numerous relative to diatoms during low water phases

a 22 kyr bp sedimentological record of lake rukwa 8 s sw tanzania environmental chronostratigraphic and climatic implications abstract the sedimentological study of a 12 8 m long core r96 i from lake rukwa tanzania 8 s provides a new record of past lake level fluctuations that took place in response to changes of the regional climate since the last glacial period from 21 to 15 cal kyr bp nearshore and swamp marsh environments are evidenced from clastic deposition macrophytes debris and ca mg carbonate enrichments a transgressive sequence starting around 15 cal kyr bp lead to the humid lacustrine optimum between 15 and 7 cal kyr bp from 12 to 10 cal kyr bp anoxic lake bottom environments were favored by the concomitant effects of high primary productivity increased subsidence and inputs of volcanic ash in the lake the middle holocene 7 3 cal kyr bp is characterized by high concentrations of silt carbonates and low organic content which indicate the occurrence of relatively oxic shallow and saline depositional environments especially around 7 and 3 4 cal kyr bp although grainsize and toc profiles suggest that shallow environments likely persisted in the upper holocene low mg concentration values for the last 3 kyr may indicate a trend toward relatively more dilute environments the two main lake rukwa low stand periods at 21 15 and 7 3 cal kyr bp correspond remarkably well with the insolation maximum at the equator which occurred at 17 kyr bp for the spring equinox and at 6 kyr bp for the autumn equinox respectively we suggest that during these periods of minimum inter hemispheric insolation gradients monsoon circulation weakened in south equatorial regions and the itcz was located north of 8 s in east africa

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

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

on the ice and heat balance in fram strait the sea ice margin northwest of spitsbergen is schematically represented by a stationary wedge maintained by opposing ice and warm atlantic water advection ice melt and cooling of the surface water generate a mixed layer with properties approximating those found elsewhere in the arctic basin the heat energy required to melt the advancing ice is roughly comparable to the rate of heat loss of the west spitsbergen current and suggests that direct heat exchange with the atmosphere is less important only a small fraction of the nearly fresh water from ice melt exits through eastern fram strait and most of it appears to return to the arctic basin whether or not it contributes to the formation and maintenance of the arctic mixed layer depends on the largely unknown circulation pattern north of fram strait an example of the large interannual variation of the mean wind stress field is shown to illustrate the likelihood of an equally large variance of the ice and surface water circulation

climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania the interactions between climatic and volcanic forcing on diatom communities contained in a 50 000 year sedimentary sequence from lake massoko tanzania were examined at the century scale 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels whereas at the millennial scale diatom inferred shifts in precipitation evaporation based on conductivity optima and diatom life form ratios were broadly similar to lake level reconstructions from lake rukwa lake malawi and others in the region some fluctuations of lake massoko are consistent with the precession driven changes in insolation but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone sea surface temperatures are strongly correlated to multimillennial scale climate patterns over this region of africa

first records of winter sea ice concentration in the southwest pacific sector of the southern ocean we use a generalized additive model gam to provide the first winter sea ice concentration record from two cores located within the southwest pacific sector of the southern ocean to compliment the application of gam a time series analysis on satellite records of sea ice concentration data was used to extend the standard 13 25 year time series used for paleoceanography after comparing gam sea ice estimates with previously published paleo sea ice data we then focus on a new paleo winter sea ice record for marine sediment core e27 23 59 37 1 s 155 14 3 e allowing us to provide a more comprehensive view of winter sea ice dynamics for the southwest pacific ocean the paleo winter sea ice concentration estimates provide the first suggestion that winter sea ice within the southwestern pacific might have expanded during the antarctic cold reversal throughout the holocene core e27 23 documents millennial scale variability in paleo winter sea ice coverage within the southwest pacific holocene winter sea ice expansion may have resulted from the laurentide ice sheet deglaciation increased intensity of the westerly winds as well as a northern migration of the subtropical and or sub antarctic fronts brief consideration is given to the development of a paleo summer sea ice proxy we conclude that there is no evidence that summer sea ice ever existed at core sites so136 111 and e27 23 over the last 220 and 52 000 years respectively

sea ice winter convection and the temperature minimum layer in the southern ocean the structure of the near surface waters in the southern ocean poleward of the antarctic polar front but away from continental margins is investigated with a three dimensional time dependent numerical model which resolves the annual sea ice cycle the growth and decay of the ice field is predicted using one of semtner s 1976 thermodynamic ice models in terms of specified atmospheric data and computed thermohaline characteristics of the ocean layers the ice field is found to be sensitive to the lateral advection of heat by the oceanic circulation as well as vertical heat transports due to deep winter convection the model treats the temperature minimum layer in the southern ocean as the remnant of a deep winter mixed layer which becomes capped by surface heating and precipitation in summer the predicted thermohaline characteristics of the temperature minimum layer and the surface mixed layer are in good agreement with observations finally the annual air sea heat exchange predicted by the model is discussed the model s predicted area averaged heat loss experienced by the ocean south of the atlantic polar front is much smaller than was previously estimated this is attributed to errors in the southern region of the model domain it is suggestedmore that models which include the interactions between sea ice and surface deep and bottom waters along the continental margins are needed to investigate this region less

macronutrient supply uptake and recycling in the coastal ocean of the west antarctic peninsula nutrient supply uptake and cycling underpin high primary productivity over the continental shelf of the west antarctic peninsula wap here we use a suite of biogeochemical and isotopic data collected over five years in northern marguerite bay to examine these macronutrient dynamics and their controlling biological and physical processes in the wap coastal ocean we show pronounced nutrient drawdown over the summer months by primary production which drives a net seasonal nitrate uptake of 1 83 mol n m 2 yr 1 equivalent to net carbon uptake of 146 g c m 2 yr 1 high primary production fuelled primarily by deep sourced macronutrients is diatom dominated but non siliceous phytoplankton also play a role strong nutrient drawdown in the uppermost surface ocean has the potential to cause transient nitrogen limitation before nutrient resupply and or regeneration interannual variability in nutrient utilisation corresponds to winter sea ice duration and the degree of upper ocean mixing implying susceptibility to physical climate change the nitrogen isotope composition of nitrate 15nno3 shows a utilisation signal during the growing seasons with a community level net isotope effect of 4 19 0 29 we also observe significant deviation of our data from modelled and observed utilisation trends and argue that this is driven primarily by water column nitrification and meltwater dilution of surface nitrate this study is important because it provides a detailed description of the nutrient biogeochemistry underlying high primary productivity at the wap and shows that surface ocean nutrient inventories in the antarctic sea ice zone can be affected by intense recycling in the water column meltwater dilution and sea ice processes in addition to utilisation in the upper ocean

paleoevaporation and paleoprecipitation in the tanganyika basin at 18 000 years b p inferred from hydrologic and vegetation proxies abstract paleo hydrologic and vegetation proxy data from the tanganyika basin are integrated in energy and water balance equations to infer past evaporation and precipitation during the last glacial maximum lgm our approach is first validated on the modern system large variations are assigned to input variables to simulate the interannual precipitation variability equations are then applied to the lgm we first change those input parameters inferred from proxies basin and lake surfaces temperature and land albedo our lgm simulation suggests in percent of modern mean values decreases in evaporation from the lake el 5 between 13 and 3 and land ec 8 19 5 bodies in precipitation p 11 21 0 and p ec 42 44 40 decreases in p and e are amplified el 8 16 0 ec 14 24 2 p 17 26 6 when including empirical changes in atmospheric transmission coefficient and bowen ratio sensitivity runs suggest that even large changes in cloud cover and air humidity should not modify these trends the results suggest that the earth s glacial interglacial boundary conditions play a significant role on climate of subequatorial southern africa

quaternary ostracodes and molluscs from the rukwa basin tanzania and their evolutionary and paleobiogeographic implications abstract much of the spectacular biodiversity of the african great lakes is endemic to single lake basins so that the margins of these basins or their lakes coincide with biogeographic boundaries longstanding debate surrounds the evolution of these endemic species the stability of bioprovinces and the exchange of faunas between them over geologic time as the rift developed because these debates are currently unsettled we are uncertain of how much existing distribution patterns are determined by modern hydrological barriers versus reflecting past history this study reports on late quaternary fossils from the rukwa basin and integrates geological and paleoecological data to explore faunal exchange between freshwater bioprovinces in particular with lake tanganyika lake rukwa s water level showed large fluctuations over the last 25 ky and for most of this period the lake contained large habitat diversity with different species assemblages and taphonomic controls along its northern and southern shores comparison of fossil and modern invertebrate assemblages suggests faunal persistence through the last glacial maximum but with an extirpation event that occurred in the last 5 ky some of the molluscs and ostracodes studied here are closely related to taxa or part of clades that are currently endemic to lake tanganyika but others testify to wider and perhaps older faunal exchanges between the rukwa bioprovince and those of lake malawi and the upper congo in particular lake mweru the rukwa basin has a long history of rifting and lacustrine conditions and at least temporarily its ecosystems appear to have functioned as satellites to lake tanganyika in which intralacustrine speciation occurred paleontological studies of the rukwa faunas are particularly relevant because of the basin s important role in the late cenozoic biogeography of tropical africa and because many of the molecular traces potentially revealing this history would have been erased in the late holocene extirpation

diatom based transfer functions for inferring past hydrochemical characteristics of african lakes abstract a new dataset of 282 modern diatom samples and associated environmental information has been created by merging existing regional datasets from north and east africa and niger the relationships between diatom species distributions and hydrochemistry are examined using canonical correspondence analysis cca and partial cca variables reflecting water conductivity ph and cation and anion composition account for significant and independent components of the total variation in the diatom data predictive models transfer functions are developed using the method of weighted averaging for conductivity r 2 0 87 ph r 2 0 77 and ratios between alkali and alkaline earth metals r 2 0 81 and carbonate bicarbonate and sulphate chloride ions r 2 0 82 prediction errors are estimated using the computer intensive method of jackknifing these transfer functions enlarge the potential domain for reconstruction of past hydrochemistry from fossil diatoms preserved in lake sediments

interplay of structural climatic and volcanic controls on late quaternary lacustrine deltaic sedimentation patterns in the western branch of the east african rift system rukwa rift basin tanzania abstract this paper presents the first detailed outcrop based sedimentologic investigation of the pleistocene holocene upper lake beds succession in the rukwa rift basin located in the western branch of the east african rift system southwestern tanzania the goal of this investigation is to examine the sedimentary facies and reconstruct the depositional environments of this important archive of quaternary climate and environmental change eleven diagnostic facies associations comprising 24 facies were identified and provide the basis for recognition of three key deposystems 1 alluvial to fluvial channel system 2 lake delta system and 3 profundal lacustrine system analysis of paleocurrent indicators and sandstone provenance indicate widely dispersed source regions and drainage patterns that were strongly influenced by major border fault systems and episodic volcanism six stratigraphic sequences a f ranging from 2 to 17 m thick were identified based on stratal stacking patterns and the development of sequence bounding unconformities and lacustrine flooding surfaces sedimentation processes facies architecture and stratigraphic packaging record a complex interplay between quaternary climate fluctuations and intense episodic volcanism in the nearby rungwe volcanic province set against large scale tectonic controls associated with synchronous development of the east african rift system sequence stratigraphic analysis reveals that the rukwa rift basin episodically shifted between a balanced fill lake basin and an overfilled lake basin deep water basin wide lake expansion occurred at different times during the late quaternary the final depositional sequence preserved in the basin a fluvial underfilled lake basin initiated 7 9 ka and has persisted to the present day high frequency climate change played the key role in sequence development in the upper lake beds however voluminous rift related volcanism and erosion of abundant labile volcanic materials from the rungwe volcanic province as well as syntectonic evolution of the rift led to high sedimentation rates and transformation of flash floods and debris flows in the hinterlands rift margin to hyperpycnal flows towards the basin depocenter

the freshwater system west of the antarctic peninsula spatial and temporal changes climate change west of the antarctic peninsula is the most rapid of anywhere in the southern hemisphere with associated changes in the rates and distributions of freshwater inputs to the ocean here results from the firstcomprehensivesurveyofoxygenisotopesinseawaterinthisregionareusedtoquantifyspatialpatternsof meteoric water glacial discharge and precipitation separately from sea ice melt high levels of meteoric water are found close to the coast due to orographic effects on precipitation and strong glacial discharge concentrations decrease offshore driving significant southward geostrophic flows up to 30 cm s 21 these produce high meteoric water concentrations at the southern end of the sampling grid where collapse of the wilkins ice shelf may also have contributed sea ice melt concentrations are lower than meteoric water and patchier because of the mobile nature of the sea ice itself nonetheless net sea ice production in the northern part of the sampling grid is inferred combined with net sea ice melt in the south this indicates an overall southwardicemotion thesurveyiscontextualizedtemporallyusingadecade longseriesofisotopedatafrom a coastal antarctic peninsula site this shows a temporal decline in meteoric water in the upper ocean contrary to expectations based on increasing precipitation and accelerating deglaciation this is driven by the increasingoccurrenceofdeeperwintermixedlayersandhaspotentialimplications forconcentrations oftrace metals supplied to the euphotic zone by glacial discharge as the regional freshwater system evolves the continuing isotope monitoring described here will elucidate the ongoing impacts on climate and the ecosystem

regions of rapid sea ice change an inter hemispheric seasonal comparison this bi polar analysis resolves ice edge changes on space time scales relevant for investigating seasonal iceocean feedbacks and focuses on spatio temporal changes in the timing of annual sea ice retreat and advance over 1979 80 to 2010 11 where arctic sea ice decrease is fastest the sea ice retreat is now nearly 2 months earlier and subsequent advance more than 1 month later compared to 1979 80 resulting in a 3 month longer summer ice free season in the antarctic peninsula and bellingshausen sea region sea ice retreat is more than 1 month earlier and advance 2 months later resulting in a more than 3 month longer summer icefree season in contrast in the western ross sea antarctica region sea ice retreat and advance are more than 1 month later and earlier respectively resulting in a more than 2 month shorter summer ice free season regardless of trend magnitude or direction and at latitudes mostly poleward of 70 n s there is strong correspondence between anomalies in the timings of sea ice retreat and subsequent advance but little correspondence between advance and subsequent retreat these results support a strong ocean thermal feedback in autumn in response to changes in spring sea ice retreat further model calculations suggest different net ocean heat changes in the arctic versus antarctic where autumn sea ice advance is 1 versus 2 months later ocean atmosphere changes particularly in boreal spring and austral autumn i e during march may are discussed and compared as well as possible inter hemispheric climate connections

degradation of a footwall fault block with hanging wall fault propagation in a continental lacustrine setting how a new structural model impacted field development plans the sirikit field thailand the sirikit field is a large field developed in a complex tilted fault block the relatively simple stratigraphy on the tilted fault block flank becomes difficult to trace onto the tilted fault block crest a recent reinterpretation of the reprocessed three dimensional seismic reflection data revealed that the difficulties of correlating reservoirs toward the crest are caused by features associated with degradation of the footwall particularly 1 the prevalence of low angle detachment faulting and 2 the development of discordant stratigraphic sections resulting from multiple episodes of erosion at the tilted fault block crest followed by the eastward onlap of the section onto the eroded footwall surface the main characteristic of this structure is the preservation of the onlapping sections through periodic propagation of the main bounding fault toward the hanging wall which resulted in the accretion of former hanging wall rocks to the footwall this propagation sequence appears to be atypical for most degraded footwalls described in the literature the degraded footwall model replaced earlier splaying bounding fault models used for development planning implementing this new structural model opened up new appraisal opportunities as the compartmentalization and reservoir distribution pictures changed appraisal of the reservoir now present above the hydrocarbon water contacts following the new structural model led to a 15 increase in recoverable hydrocarbon volumes

inland thinning of the amundsen sea sector west antarctica 1 together with the pine island glacier pig the thwaites tg and smith sg glaciers are the principal drainage systems of the amundsen sea as sector of western antarctica here we use satellite radar altimetry and interferometry to show that a rapid thinning of ice has occurred within the fastest flowing sections of all as outlet glaciers the pattern of thinning extends to distances greater than 150 km inland between 1991 and 2001 the tg and sg thinned by more than 25 and 45 m at their grounding lines and a total of 154 16 km 3 of ice or 0 43 mm of eustatic sea level rise was lost from the as sector glaciers to the ocean we show that the thickness changes may have caused the pig tg and sg to retreat inland by over 8 4 and 7 km respectively in line with independent estimates of grounding line migration

post cretaceous intraplate volcanism in the central indian ocean basin abstract intraplate volcanism in the central indian ocean basin ciob has been of much interest since this tectonically young and structurally complex basin holds signatures of at least one major plate reorganisation at 65 5 ma and two major plate boundary structural deformations occurring at 54 5 3 ma collision of india with eurasia and at 7 ma intraplate deformation in the indo australian plate to ascertain the extent and intensity of intraplate eruptions an area close to 345 000 km 2 was surveyed recently by swath multibeam system and extensively sampled morphology density off axis production rate and petrology of a host of large seamounts and abundant smaller volcanic edifices suggest their non uniform growth in the past the assessment points to the possibility that paleogene tectonic reactivation in the basinal areas could be related to intraplate volcanism which although of significance may be limited in occurrence

13c palynological evidence of differential residence times of organic carbon prior to its sedimentation in east african rift lakes and peat bogs abstract most terrestrial plants producing large amounts of organic matter in the east african rift follow the calvin c3 photosynthetic pathway their end products have 13c values of ca 27 2 vs pdb on the contrary most cyperaceae notably cyperus papyrus and c latifolius are characterized by higher 13c contents 13 c 10 5 1 in relation to their hatch and slack c4 photosynthetic cycle in consequence 13c values in total organic matter tom from peat bog or lake cores essentially responded to the proportion of detritus from c4 cyperaceae immediate evidence of the development or disappearance of cyperaceae around lake margins or in peat bogs can be found in pollen assemblages lag times between pollen signals and correlative 13c shifts in tom from cores are therefore indicative of the residence time of organic matter prior to its sedimentation delayed sedimentation of tom will result in 14c anomalies which depend on several parameters most of them being site specific as shown by examples from a peat bog in burundi and from southern lake tanganyika an independent assessment of the chronology by high resolution paleomagnetic correlations indicates a ca 1 5 ka apparent 14c age of tom in lake tanganyika at the pleistocene holocene transition

new evidence for a reduced water balance in east africa during the last glacial maximum implication for model data comparison some syntheses of lake level data for the last glacial maximum lgm in east africa 10 n and 30 s east of 25 e show apparently wetter conditions than present for some basins whereas palaeovegetation reconstruction indicates a generally dry climate pmip gcm simulations for the lgm support both scenarios for this region when run under different boundary conditions here we compare three new records from lakes in the data poor southern part of east africa lake malawi lake massoko and lake rukwa we also re assess previously published lake level data and apply a salinity transfer function to the diatom record from lake manyara our results show that in contrast to previous interpretations these lakes were at least as low as today at the lgm and are thus in agreement with the palaeovegetation data relative drought across east africa is best simulated by gcms that use computed ssts rather than the higher climap values lower ssts and the presence of the northern hemisphere ice sheets must have been dominant over any monsoon precipitation rise caused by astronomically induced summer insolation enhancement in the southern african tropics

a 3 million year index for north african humidity aridity and the implication of potential pan african humid periods abstract mediterranean sediments are valuable archives of both african monsoon variability and higher latitude climate processes and can also be used to provide an environmental context for early human migrations and settlements however the long history of mediterranean palaeoclimate studies largely pre dates the advent of widespread x ray fluorescence xrf core scanning so there are few continuous and high resolution geochemical records from this key region that extend beyond the last glacial cycle here we present xrf core scanning results for odp site 967 eastern mediterranean that have been fully calibrated into element concentrations spanning the last 3 million years my comparison with independent geochemical data from conventional xrf highlights disparities for certain element element ratios thus suggesting the need for caution when taking ratios of scanning xrf data principal component analysis of the calibrated xrf dataset reveals two dominant components detrital inputs pc1 and a sapropel monsoon run off signal pc2 which we use to establish a new orbitally tuned chronology we observe inverse covariation between pc2 and a previously published aeolian dust record from odp site 967 larrasoana et al 2003 and combine these records to produce a composite index of humidity and aridity for the wider north african region over the past 3 my we propose that by combining run off and dust signals in a single metric our index captures the effects of both strengthening northward migration increased run off and weakening southward retreat increased dust of the north african monsoon comparison of the index with published records of northwest and east african palaeohumidity suggests that it tracks the timing of green sahara periods throughout the plio pleistocene and that at least 30 of these intervals coincided with increased humidity across east africa we tentatively suggest that these specific episodes may be termed pan african humid periods as a means to highlight large scale climate trends and to provide an environmental framework for palaeo anthropological research

late holocene sea level changes and paleoclimate recorded in lake lungue southern mozambique abstract we report on late holocene paleoenvironmental and sea level changes in southern mozambique based on analysis of diatom stratigraphy mineral magnetic susceptibility saturation isothermal remnant magnetization sirm and organic carbon content in a sediment core from lake lungue located ca 30 km north of xai xai city in the limpopo river floodplain eleven radiocarbon dates performed on terrestrial shells allowed establishment of an age depth model high content of brackish marine taxa especially diploneis suborbicularis and navicula yarrensis suggests that the lake lungue basin was part of the indian ocean coastal zone between ca 740 and 910 ce suggesting higher relative sea level during this phase a similar diatom distribution in older parts of the sequence of unknown age indicates that the site was connected to the indian ocean also at some stage prior to 740 ce between ca 910 and 1130 ce the basin was under a combined effect of sporadic marine water influx and limpopo river actions a freshwater lake was established sometime between 1130 and 1360 ce when the basin became less affected by marine influences due to lower sea levels instead the lake system was more influenced by limpopo river meandering dynamics and flooding events and eventually isolated into a more or less independent lake ecosystem where diatom diversity most likely responded to lake levels fluctuations driven by shifts in relative humidity and rainfall amounts within the floodplain and also from upstream sources during this phase high abundance of freshwater planktonic species i e aulacoseira granulata and a ambigua indicates high lake levels and wetter conditions dated to ca 1360 1560 ce from 1560 ce until present the lake has been subject to drier conditions and higher evaporation as indicated by lower lake levels reported by a decline in freshwater planktonic taxa and increase in brackish taxa e g amphora robusta

centennial to millennial changes in maar lake deposition during the last 45 000 years in tropical southern africa lake masoko tanzania abstract the masoko maar southern tanzania provides one of the most continuous late quaternary lacustrine sedimentary records from africa a detailed chronostratigraphic framework coupled with sedimentological and magnetic measurements allows us to construct a 30 year resolution continuous sedimentary sequence covering the last 45 000 years and to address local depositional environment and climate variability in the tropical southern africa based on present day observations and measurements our results indicate that the low field magnetic susceptibility of the sediment is highly controlled by climate driven processes wind stress and or lake level amplitude changes acting on the titanomagnetite rich shoreline reservoir the tephra and turbidite free magnetic susceptibility record is strongly modulated by a persistent multi decadal to centennial variability 80 to 200 years probably linked to the gleissberg and suess cycles of solar activity at lower frequency the variability of deposition is controlled by the precessional cycle and its harmonics suggesting a dominant multi millennial forcing of low latitude insolation on climatic changes in tropical southern africa inferred wetter conditions during the last glacial maximum and the younger dryas at masoko 9 s indicate southward shifts of the intertropical convergence zone associated with the north atlantic glacial dynamics and or contrasted hydrological changes in the rungwe highlands compared to the neighbouring areas finally former regional transfer function between diatom assemblages and water chemistry suggested drier conditions during the last glacial maximum at lake masoko barker p williamson d gasse f gibert e 2003 climatic and volcanic forcing revealed in a 50 000 year diatom record from lake massoko tanzania quaternary research 60 368 376 in this context further climate proxy data such as pollen and hydrobiological studies in small deep lakes are needed to support our alternative interpretation of the masoko record

evolution of the southern ocean winter mixed layer and sea ice open ocean deepwater formation and ventilation an analytical model has been developed to describe the nature of the winter ocean sea ice interaction in the southern ocean to determine the principal processes responsible for maintaining stability in this marginally stable region and to predict the response to changes in the external forcing and initial conditions the model accurately describes the sparse winter observations and the general temporal evolution of the ice mixed layer depth and salinity for the period following the elimination of the seasonal pycnocline by rapid fall ice growth a scale analysis and sensitivity study reveal the system parameter dependence this indicates that the magnitude of entrainment is proportional to fst h0 s where fs is the external salt flux h0 the depth of the winter mixed layer and s the salinity gradient through the pycnocline thermodynamic ice growth reflects predominantly the competition between the oceanic diffusive heat gain across the base of the mixed layer and the loss to the atmosphere through the ice and leads a strong thermal gradient across the pycnocline leads to a significant diffusive flux which balances most of the heat lost to the atmosphere this limits the amount of ice growth possible and plays the principal role in maintaining the system stability since the ice growth is required for destabilization through haline rejection during the growth process the system is also sensitive to the large ratio of heat to salt in the pycnocline this introduces a negative feedback where increasing ice growth induces entrainment by the associated salt flux which in turn drives an entrainment heat flux which compensates for heat lost to the atmosphere this moderates the ability to grow ice and drive further entrainment the absolute strength of the salinity gradient on the other hand dictates the effectiveness which a same magnitude external salt flux will have in driving entrainment the effectiveness with which changes in these parameters as well as several other external parameters can influence the southern ocean stability is explicitly evaluated through the sensitivity study and analysis

freshening of antarctic bottom water in the indian ocean sector of southern ocean abstract recent observations reveal a rapid reduction in salinity and density of antarctic bottom water aabw in the indian ocean sector of southern ocean previous studies have shown fast freshening of aabw since 1995 but an increased rate of freshening from 2006 to 2010 was exhibited in the present study by both ecmwf oras4 reanalysis data as well as in situ data during the same period the degrees of change observed in the aabw were more predominant in the in situ data becoming warmer 0 05 c fresher 0 01 and lighter 0 01 kg m 3 between 2006 and 2010 the height of fresh water added per unit area for 500 m thickness 4000 4500 m was 2 2 cm per annum cm a 1 the aabw observed in the present study could have originated from weddell sea or cape darnley polynya the decadal changes indicate an enhanced sea ice formation due to increased positive phases of southern annular mode sam which may have led to the increase in fresh water input resulting in freshening of aabw