geochronology of the pre kbs tuff sequence omo group turkana basin the omo turkana basin of northern kenya and southern ethiopia developed in the northern kenya rift about 4 3 ma ago in the early pliocene nearly 800 m of sediments included within the omo group crop out in the basin numerous rhyolitic tuffs in the sequence not only have facilitated secure correlations between the formations of the omo group but also have provided material for precise 40 ar 39 ar age measurements here we report ages on alkali feldspar crystals from pumice clasts within tuffs in the lower part of the omo group up to the level of the kbs tuff which has previously been dated at 1 87 0 02 ma the results from 17 stratigraphic levels encompassing the 2 4 ma time interval from the base of the group to the kbs tuff provide a numerical time framework for the geological history of the lower part of the pliocene sequence the new ages which have a precision of the order of 1 are all consistent with the stratigraphic order providing confidence that they accurately record the ages of the volcanic eruptions with deposition of the tuffs and pumices occurring shortly thereafter

early human settlements in northern africa paleomagnetic evidence from the ain hanech formation northeastern algeria abstract the question of the earliest hominid settlements in northern africa has been under debate for a number of years due to the lack of precise chronologies here we present new paleomagnetic data that supports an olduvai subchron age for the archaeological sites at ain hanech and el kherba in northern algeria our study is based on a 22 m thick magnetostratigraphy of the ain hanech formation which includes contextualized oldowan and acheulian lithic tools characteristic remanent magnetization directions were obtained from both thermal and alternating field demagnetization procedures of specimens from twenty five sampled horizons our results reveal the presence of the olduvai subchron in the upper part of the stratigraphic section constraining the age of these important archaeological sites in northern africa

environmental and climatic control on seasonal stable isotope variation of freshwater molluscan bivalves in the turkana basin kenya abstract we present growth incremental stable isotope records 18o and 13c of modern and 2 ma fossil bivalve shells from the semi arid turkana basin n kenya s ethiopia these data suggest that seasonal cyclicity in 18o and 13c recorded by modern and fossil shells is driven by wet dry seasonal changes in host water chemistry forced by monsoonal rainfall over the ethiopian highlands fully lacustrine shells show lower amplitude or even absent seasonal cyclicity in comparison with deltaic or riverine shells because of the buffering effect of the large water volume in the lake setting riverine shells arguably have the clearest relation to seasonally variable climate parameters riverine molluscs thus provide a potentially valuable proxy for varying rainfall 18o values in the turkana basin catchment cross plots of molluscan 18o and 13c data reveal dominant environmental control on molluscan isotope values with remarkably large isotopic differences between lacustrine deltaic and riverine environments we interpret this isotope pattern to directly result from the different mixing proportions of omo river source water with evaporated lake water in these environments the interpretation of fossil molluscan 18o and 13c data in a palaeoclimatological context is not straightforward since the potential influence of temporal changes in lake water temperature surface evaporation or river discharge on the 18o budget of the lake is smaller than the isotopic shifts caused by shifting facies patterns in the sedimentary record even though it is clear from the rich molluscan faunas that the 2 ma palaeo lake lorenyang must have been significantly less alkaline than modern lake turkana and likely provided good drinking water and abundant availability of food for the different species of hominins inhabiting the region at that time stable isotope values of molluscan bivalves are not suitable to record the difference in alkalinity between these two settings

recurrent spring fed rivers in a middle to late pleistocene semi arid grassland implications for environments of early humans in the lake victoria basin kenya the effect of changing palaeoclimate and palaeoenvironment on human evolution during the pleistocene is debated but hampered by few east african records directly associated with archaeological sites prior to the last glacial maximum middle to late pleistocene deposits on the shoreline of eastern lake victoria preserve abundant vertebrate fossils and middle stone age arte facts associated with riverine tufas at the base of the deposits which are ideal for palaeoenvironmental reconstructions new data from tufas identified on rusinga island and on the mainland near karungu kenya are provided from outcrop thin sections mineralogical stable isotopic and u series dating analyses tufa is identified in four sites nyamita 94 0 3 3 and 111 4 4 2 ka kisaaka aringo 455 45 ka and obware the age ranges of these tufa deposits demonstrate that spring fed rivers were a recurrent variably preserved feature on the pleistocene landscape for ca 360 kyr poor sorting of clastic facies from all sites indicates flashy ephemeral discharge but these facies are commonly associated with barrage tufas paludal environments with 13c values of ca 10 indicative of c3 plants and fossil hippopotamus all of which indicate a perennial water source other tufa deposits from nyamita obware and aringo have a mixed c3 c4 signature consistent with a semi arid c4 grassland surrounding these spring fed rivers the 18o values of tufa from nyamita are on average ca 1 more negative than calcite precipitated from modern rainfall in the region suggesting greater contribution of depleted monsoonal input similar to the last glacial maximum microdebitage and surface collected artefacts indicate that early modern humans were utilizing these spring fed rivers the presence of spring fed rivers would have afforded animals a reliable water source sustaining a diverse plant and animal community in an otherwise arid environment

past and recent changes in the large scale tropospheric cycles of lead and other heavy metals as documented in antarctic and greenland snow and ice a review abstract the investigation of the occurrence of lead and other heavy metals in antarctic and greenland ancient ice and recent snow is of great interest in reconstructing the past natural tropospheric cycles of these metals and determining to what extent these cycles have now been altered by man because concentrations to be measured are exceedingly low down to the sub pg g level reliable data can however be obtained only if full control of contamination is achieved from field sampling to laboratory analysis the available data show that the past natural concentrations of pb cd zn cu and hg in antarctic ancient ice are highly dependent upon climatic conditions the highest values occurring during the coldest periods of the ice ages especially during the last glacial maximum about 18 000 years ago human activity has led to an increase in lead concentrations during recent centuries both in antarctica tenfold increase and in greenland two hundredfold increase this last increase was followed by a 7 5 fold decrease during the past two decades mainly as a consequence of the rapid fall in the use of lead alkyl additives in the usa as confirmed by recent isotopic data

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

environmental change and human occupation of southern ethiopia and northern kenya during the last 20 000 years abstract our understanding of the impact of climate driven environmental change on prehistoric human populations is hampered by the scarcity of continuous paleoenvironmental records in the vicinity of archaeological sites here we compare a continuous paleoclimatic record of the last 20 ka before present from the chew bahir basin southwest ethiopia with the available archaeological record of human presence in the region the correlation of this record with orbitally driven insolation variations suggests a complex nonlinear response of the environment to climate forcing reflected in several long term and short term transitions between wet and dry conditions resulting in abrupt changes between favorable and unfavorable living conditions for humans correlating the archaeological record in the surrounding region of the chew bahir basin presumably including montane and lake marginal refugia for human populations with our climate record suggests a complex interplay between humans and their environment during the last 20 ka the result may contribute to our understanding of how a dynamic environment may have impacted the adaptation and dispersal of early humans in eastern africa

cadmium diagenesis in laurentian trough sediments the depth distributions of cd mn and fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the laurentian trough the results reveal a surface layer in which the content of solid phase cd first decreases but then increases sharply with depth in contrast mn decreases regularly over the same depth interval in the oxygenated zone of this layer cd probably associated with organic matter is released to the pore water resulting in concentrations that are more than an order of magnitude higher than in the overlying water column some of the dissolved cd is returned to the water column and some migrates downward and is precipitated at depth this part of the cd cycle is virtually complete within the surface layer the base of which corresponds to the base of the zone of mn enrichment in the subsurface layer solid phase cd and mn show little or no concentration change however dissolved cd which reaches often non detectable concentrations 0 05 nm in this layer increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer the presence of dissolved cd complexes revealed by 1 5 to 8 fold increases in electrochemically active cd in pore water following uv treatment as well as inconsistent distributions of dissolved and solid phase cd indicate complexities in diagenesis that merit further investigation

thermodynamic influences on the marine carbon isotope record air sea exchange represents an important controlling factor for the distribution of carbon isotopes in the modern ocean even below the surface mixed layer here we show that this thermodynamic influence varies between water masses with a magnitude which is large relative to the amplitude of typical deep sea sediment 13c records thus changes in air sea exchange processes must be regarded as a significant source of variability for any marine 13c signal

δ13c records from fish fossils as paleo indicators of ecosystem responses to lake levels in the plio pleistocene lakes of tugen hills kenya abstract the carbon isotopic ratios of organic matter in fish fossils from diatomites and other lake beds in the hspdp drill core from tugen hills kenya 2 56 3 29 ma reflect trophic resource uses and can indicate the dietary habitats of fish in the paleolake this information offers insight into how fish communities responded to lake level fluctuations during the plio pleistocene in the east african rift valley we have compared this record with fish fossil isotopes from both a previously published study of a lake malawi drill core 139 ka present and core top modern ca 1978 samples collected at the water sediment boundary from lake turkana kenya of known environmental provenance both the lake malawi drill core fossils 7 2 to 27 5 vpdb and modern lake turkana samples 16 to 24 6 vpdb have 13c values indicating a mix of near shore and deep water pelagic species in contrast the 13c values for the tugen hills core fossils vary only between 20 and 27 vpdb the absence of 13c values greater than 19 suggests none of these fossils are derived from near shore benthic habitats the lack of shallow water benthic lacustrine fish fossils through the tugen hills lake cycles may indicate that the rate of change from low lake stands to deeper lake phases was very rapid and shallow water communities were not established for long enough to leave a fish fossil record at the core site these results strongly suggest that lake level responses to climate variability in the baringo basin of the east african rift were very abrupt during the plio pleistocene transition

lake level changes and hominin occupations in the arid turkana basin during volcanic closure of the omo river outflows to the indian ocean in the east african rift the western margin of lake turkana northern kenya exposes mio plio pleistocene lake sediments with dated volcanic horizons constraining basin dynamics at the astronomical time scale since the late pliocene coastal archaeological sites have formed within the lacustrine dynamics here lake levels are reconstructed from 2 4 to 1 7 ma using sedimentary facies and water depth controlled sediment association the lacustrine stratigraphy is measured with a total station and cyclostratigraphy is derived from tephrochronology the water depths are evaluated from paleochemical properties of lake sediments analyzed by inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry our reconstruction highlights that lake turkana rose during 100 ka insolation eccentricity maxima periods in response to higher monsoonal inputs of the omo river however lake turkana also expanded through an insolation minimum at 2 17 1 95 ma this asynchronous lake phase coincides with volcanic closure of the omo river and lake turkana outflow sill to the east and the indian ocean an archaeological hiatus occurs during this endorheic lake phase and alkalinity increases at the beginning of the hiatus the lake rose again during insolation eccentricity maxima at 1 9 1 7 ma and a new outflow sill opened to the west and the nile basin hominin coastal occupations return during this exorheic freshwater lake phase

evidence for the blake event recorded at the eemian archaeological site of caours france a palaeomagnetic study of the last interglacial calcareous tufa sequence at the archaeological site of caours northern france identified a geomagnetic excursion that we interpret as the blake event earlier palaeontological molluscs mammals and geochemical proxy studies at this site allowed recognition of full interglacial conditions prevailing during the deposition of the tufa sequence the tufa sequence and associated palaeolithic levels have been dated to the eemian interglacial by a set of tims u th measurements on calcitic concretions average 123 3 ka by previous correlations of the blake event with the eemian sensu stricto as defined in the netherlands pollenzones at neumark nord 2 germany and rutten the netherlands it has been shown that the continental eemian starts after the peak of marine isotope stage mis 5e the identification of the blake event at caours implies a post mis 5e peak age for all four levels of the palaeolithic occupation in perspective of the time lag between the mis 5e peak and the beginning of the eemian identified in other studies it is very likely that during the main occupation of caours a significant barrier was in place between north western france and great britain in the form of the english channel it is possible that the chronological position of the last interglacial environments in northwestern europe in relation to sea level change is a key factor behind the apparent absence of last interglacial palaeolithic sites in great britain

porewater cadmium geochemistry and the porewater cadmium δ13c relationship results from two continental margin cores collected off pt sur california make possible a direct determination of the relationship between dissolved cadmium and the carbon isotopic composition of dissolved inorganic carbon in the interstitial water of suboxic marine sediments the 13c values of dissolved inorganic carbon decrease nearly 1 in the top 0 5 cm of the sediments and display a more gradual decrease through the top 20 cm of the sediments dissolved cadmium concentrations increase by 0 6 to 1 5 nmol kg in the top 0 5 cm of the sediments and then rapidly decrease to values of approximately 0 12 nmol kg between 5 and 10 cm into the sediments apparently as a result of scavenging onto iron oxide surfaces this pattern contrasts with porewater cadmium profiles previously reported for pelagic sediments which show no evidence of cadmium scavenging by the sediments however porewater cadmium concentrations are poorly correlated with porewater 13c values in both oxic and suboxic sediments these results suggest that the cadmiumcalcium ratios and 13c values of infaunal benthic foraminifera need not be tightly linked and that the relationship between these two aspects of foraminiferal shell chemistry may vary in response to changes in sediment geochemistry

on the body force term in internal tide generation abstract the generation of internal tides can be ascribed to the action of a buoyancy force caused by the flow of the barotropic tide over topographic features it is commonly assumed that the barotropic flow can be taken as hydrostatic but it is shown here that this leads to a linearized governing equation for the baroclinic tide that is only valid if the baroclinic tide is also hydrostatic a governing equation for the baroclinic tide valid for any situation is derived here and is shown to be exactly equivalent to a simple transformation of the governing equation for the combined barotropic and baroclinic tides

new magnetostratigraphy for the olduvai subchron in the koobi fora formation northwest kenya with implications for early homo article i nfo a problematic magnetostratigraphy for the koobi fora formation has contributed to debates on the evolutionary implications for early hominin fossils to address this 50 independent samples distributed over a nearly 63 m thick interval were collected from the lower middle kbs member type section in fossil collection area 102 northeast turkana basin characteristic directions obtained by thermal demagnetization define a coherent magnetostratigraphy that is supported by alternating field studies on 28 sister specimens and the prior tephrochronological framework two long polarity intervals were recognized each 30 40 m in thickness and interpreted as the upper part of the normal polarity olduvai subchron and the overlying reverse polarity matuyama chron the end olduvai consists of a normal reverse normal polarity sequence occurring over a thickness of at least 1 m but perhaps up to 5 m suggesting that this subchron has a short reverse interval in its uppermost part such a fine scale structure also has been reported from several other sites like the pliocene pleistocene boundary and point stratotype section at vrica italy which serves as a basis for formally delimiting three temporally discrete polarity subintervals for the olduvai subchron these paleomagnetic results that place the upper boundary of the olduvai at 48 m above the base of the kbs member coupled with published radioisotopic dates firmly secure the age of partial cranium knm er 3733 in the interval 1 78 1 48 ma with an interpolated age of 1 7 ma giving this fossil the most unambiguous numerical age constraints as compared to the oldest homo cranial remains from europe and asia nonetheless assured placement of the top of the olduvai subchron in the kbs member is not sufficient in the face of other uncertainties to influence conventional interpretations of the timing and direction for the global dispersal of early homo erectus

human evolution in a variable environment the amplifier lakes of eastern africa abstract the development of the cenozoic east african rift system ears profoundly re shaped the landscape and significantly increased the amplitude of short term environmental response to climate variation in particular the development of amplifier lakes in rift basins after three million years ago significantly contributed to the exceptional sensitivity of east africa to climate change compared to elsewhere on the african continent amplifier lakes are characterized by tectonically formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area such amplifier lakes respond rapidly to moderate precessional forced climate shifts and as they do so apply dramatic environmental pressure to the biosphere rift basins when either extremely dry or lake filled form important barriers for migration mixing and competition of different populations of animals and hominins amplifier lakes link long term high amplitude tectonic processes and short term environmental fluctuations east africa may have become the place where early humans evolved as a consequence of this strong link between different time scales

trace metal solubility in an anoxic fjord abstract intermittent anoxia in the saanich inlet water column provides an easily accessible marine o2 h2s interface to study the response of metals to both a steep redox gradient and the availability of reactive reduced sulfur species our study indicates a strong anoxic zone sink for copper and cadmium and the characteristically enhanced solubility of manganese and iron thiosulfate and sulfite are below detection limits 1 m and 0 1 m respectively and thus not important in metal complexation elemental sulfur concentrations are high at the oxic anoxic interface and throughout the anoxic zone indicating the potential for metal complexation by polysulfides a thermodynamic approach employing metal sulfide formation and class specific sulfidic ligand complexation to generate equilibrium profiles adequately describes the solubility of iron copper and cadmium the extension of this scheme to other transition and class b metals in other marine environments with redox fronts is suggested

paired carbon isotope and cadmium data from benthic foraminifera implications for changes in oceanic phosphorus oceanic circulation and atmospheric carbon dioxide abstract the carbon isotope and cadmium content of foraminiferal shells from oceanic sediments provides information on past chemical distributions in the ocean starting from reasonable assumptions it can be shown that paired data on both tracers provides more information than does either separately the slope of paired isochronous 13c and cd ca data from fossil benthic foraminifera depends on oceanic p cd and gd 13c p ratios the zero cd ca intercept of this line depends on mean oceanic 13c p and the 13c p ratio given data sufficient to define oceanic mean 13c and cd contents it is possible to place quantitative constraints on the product of the c p redfield ratio and mean oceanic phosphorus and to estimate a circulation parameter which is independent of changes in ocean chemical inventories with the limited available data no significant changes in oceanic p cd can be detected between the holocene and most recent glacial maximum if this data is taken to be representative of the whole ocean then the p and cd contents of the ocean were about 17 higher during the last glacial maximum than at present ocean box models indicate that this change in oceanic p and a 2 c deep ocean cooling during glacials would cause a 47 ppmv reduction in atmospheric pco2 during glacial intervals

fluxes of uranium and thorium series isotopes in the santa barbara basin abstract samples from the manop santa barbara basin sediment trap intercomparison were analyzed for the isotopes of uranium thorium radium lead and polonium all of the traps showed approximately the same compositions and isotopic ratios indicating that they trapped similar materials the234th flux via falling particles was very close to the flux predicted from the production and scavenging rates of234th from the water column the210pb content of the trapped particles and the surface sediments were the same however the measured flux of210pb was seven times greater than the predicted flux predicted and measured fluxes of228th and210po were similarly out of balance to explain this apparent inconsistency we suggest as others have done that the santa barbara basin is an area where scavenging from the water column is intensified and where sediments deposited initially on the margins may be physically remobilized on a short time scale these two effects increase the apparent area from which the basin derives the longer lived isotopes but does not increase significantly the supply of the short lived234th

paleoenvironmental conditions at madigou mdg a newly discovered early paleolithic site in the nihewan basin north china abstract paleoenvironmental reconstructions provide the context for human evolution and behavior however it is difficult to resolve relative proportions of specific habitats at a given place and time how these may have changed over time and the explicit nature of particular habitats for human adaptation this paper examines the paleoenvironmental context of early pleistocene archaeological occurrences at madigou hebei province north china but with particular emphasis on the setting for early stone tool makers madigou mdg is one of several early pleistocene paleolithic sites in the east part of the nihewan basin north china sedimentological features of the excavated section indicate that the site was formed near the margin of a lake paleomagnetic results suggest that hominins occupied the site ca 1 2 ma based on well constructed pollen sediment grain size magnetic susceptibility isotopic 13 c and 18 o and iron oxide analysis of the section excavated in 2011 and 2012 five stages of environmental change in the nihewan beds are identified in the earliest phase the environment and climate changed from 1 cool and semi humid with open grassland to 2 cold and dry with open sparse steppe usually temperate herbaceous not just grass and then 3 warm and humid climate with lightly wooded grassland followed by 4 cold and dry climate with sparse steppe and finally 5 temperate and semi humid with open grassland this evidence suggests that hominins occupied the site from stages 2 4 in an open habitat varying from lightly wooded grassland to an ecosystem dominated by sparse steppe

aquatic biomarkers record pleistocene environmental changes at paleolake olduvai tanzania abstract several hypotheses invoke climatic variability as a driving force for hominin evolution thus high resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses sedimentary sequences recovered by the olduvai gorge coring project ogcp help evaluate climatic and environmental changes at olduvai gorge tanzania through the analyses of various biogeochemical proxies the stratigraphic sequence of ogcp core 2a can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons such as the bed i basalt and tuff ib the lacustrine interval from 76 6 to 86 9 m depth is ideally suited for high resolution analyses of biogeochemical proxies as it is rich in organic carbon 1 toc the hydrogen isotopic composition of nc31 in this interval of ogcp core 2a records the effects of precession driven wet dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems comparable to those documented by previous investigations of outcrop analogues at olduvai here we examine stratigraphic variations in the abundance of biomarkers c28 steradienes c17 and c23 n alkanes c27 and c28 a norsteranes fern 8 ene and chromans derived from aquatic organisms algae cyanobacteria sponges macrophytes etc to determine the response of paleolake olduvai to climate variability in general these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession driven wet dry cycles recorded by terrestrial biogeochemical signatures however they also provide evidence of abrupt

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

new single crystal 40ar 39ar ages improve time scale for deposition of the omo group omo turkana basin east africa six tuffaceous beds within the omo group of the omo turkana basin have been dated using the 40 ar 39 ar single crystal total fusion method on anorthoclase yielding eruption ages the omo group constitutes up to 800 m of subaerially exposed sediments surrounding lake turkana within the east african rift system in northern kenya and southern ethiopia rhyolitic explosive eruptions produced tuffs and pumice clasts that are considered to have been deposited shortly after eruption the new age data on feldspars from the pumice clasts range from 4 02 0 04 ma for the naibar tuff of the koobi fora formation to 1 53 0 02 ma for tuff k of the shungura formation the orange tuff in the kbs member of the koobi fora formation was dated at 1 76 0 03 ma providing good control in this part of the sequence where formerly there was a 200 ka gap data are consistent with earlier measurements and significantly improve age resolution within the omo group which has yielded many vertebrate fossils including hominin fossils comprising a number of species we suggest new age estimates for a limited number of hominin specimens supplementary material eleven tables and nine figures are available at www geolsoc org uk sup18506

sub milankovitch paleoclimatic and paleoenvironmental variability in east africa recorded by pleistocene lacustrine sediments from olduvai gorge tanzania abstract over the past century numerous discoveries throughout east africa have advanced our understanding of hominin evolution and provided substantive evidence that climatic and environmental variability played a critical role in evolutionary developments stratigraphic records with high temporal resolution aid in testing evolutionary hypotheses that invoke changes in climate and environment at various timescales as drivers of hominin evolution olduvai gorge tanzania has a rich history of hominin fossil discoveries from its 2 0 ma sedimentary sequence which includes multiple deep lake intervals in 2014 the olduvai gorge coring project ogcp recovered a sequence of sediment cores that provide an extensive record of the pleistocene sedimentary history including paleolacustrine systems in this area with 94 core recovery these cores are ideally suited for high resolution analyses of organic geochemical proxies and provide an exceptional opportunity to build on previously published outcrop based paleoenvironmental data from olduvai the ogcp core 2a section between 76 6 and 86 9 m depth is considered as a time stratigraphic equivalent of strata that host the hominin fossils oh24 and oh56 at the olduvai dk archaeological site this depositional interval age bracketed between 1 88 ma bed i basalt and 1 85 ma tuff ib was sampled for organic geochemical analyses the carbon isotopic composition of organic matter 13ctoc from this section varies between values representative of more forested and open grassland ecosystems over 21 kyr this observation is consistent with the milankovitch precession cycle driving the availability of water which was previously observed in lower resolution studies of outcrop samples complementary organic geochemical proxies provide further evidence of these shifts in environmental conditions and record sub milankovitch scale changes superimposed on the precession cycle this suggests the occurrence of short term fluctuations in the environments inhabited by hominins which opens new lines of investigation on how environmental scenarios may affect particular evolutionary mechanisms that drove various evolutionary responses

the flushing of two deep sea basins southern california borderland a time series of water temperature salinity and nutrient measurements has been made in san pedro and san nicolas basins of the southern california borderland between i977 and 1988 these basins exhibit temporally coincident changes in temperature and nutrient content defining a period of basin flushing 1982 1984 and a period of stagnation 1984 1987 during the flushing period cooler oxygenand no rich water replaces warmer oxygenand no depleted bottom water flushing produces a decrease of 0 2 c in the average temperature of san pedro basin bottom water 790 900 m and an increase in the average oxygen content by 12 pm these values are 0 i and 3 for san nicolas basin 1 400 l 800 m the residence time of water in san pedro basin during the flushing period is 12 6 months and j t2 months in san nicolas basin a box model approach constrained by the time series data for the stagnation period was used to determine values of the effective vertical eddy diffusivities for the 790 m horizon in san pedro basin and 1 400 m horizon in san nicolas basin 1 3 0 4 and 9 9k3 4 cm sk respectively

variability of nonlinear internal waves in the south china sea affected by the kuroshio and mesoscale eddies long term observations of nonlinear internal waves in the south china sea reveal seasonal to interannual variability during two selected segments of inverted echo sounder observations tidal forcing in luzon strait is almost identical but the observed amplitudes of nonlinear internal waves in the south china sea are very different the effects of the kuroshio and mesoscale eddies reproduced by hybrid cooridnate ocean model hycom reanalysis simulation are then investigated the kuroshio can enhance the zonal tilt of the thermocline and induce intruding flow in luzon strait during the two selected segments different thermocline slopes did not significantly change the internal tide generation but the intruding flow may result in a 11 difference in the amplitude of generated m2 internal tides during the two selected segments mesoscale eddies appeared on the path of internal wave propagation a cold eddy in one case and a warm one in the other the eddies changed local stratification and induced additional background currents thus affecting the nonlinear evolution of internal tides in addition wave front steering due to the mesoscale eddies dramatically affected the observed amplitude changes of the nonlinear internal waves the edge rather than the center of the nonlinear internal wave front passed through the observational stations resulting in reduced amplitude in the observations

obscuring of long eccentricity cyclicity in pleistocene oceanic carbon isotope records abstract long eccentricity 400 kyr cycles in carbon isotope records from the pacific and atlantic oceans and the mediterranean sea of the past 5 0 ma are compared all records show maximum 13c values 13cmax at eccentricity minima during the pliocene but this relationship obscured in the pleistocene after 1 6 ma in particular for the open ocean deep water 13c records since a clear anti phase relationship was set up between oceanic 18o and 13c in the 100 kyr band from this time we attribute the obscured 400 kyr signal to a major change in the oceanic carbon reservoir probably associated with restructure of the southern ocean a similar change occurred in the miocene at 13 9 ma when the 400 kyr cyclicity in 13c records flattened out together with a drastic cooling and antarctic ice sheet expansion a remarkable exception is the mediterranean surface water 13c record which remained paced by the long term eccentricity cycle throughout the pliocene and pleistocene suggesting a low latitude climatic origin of the 400 kyr signal that is independent of glacial interglacial forcing since the earth is currently passing through an eccentricity minimum it is crucial to understand the nature of the 13cmax events

orbital rhythms monsoons and playa lake response olduvai basin equatorial east africa ca 1 85 1 74 ma the lithostratigraphy at the pliocene pleistocene boundary exposed in olduvai gorge east africa was used to determine the duration and cause of climate variations wet dry periods the area has rich paleontological and archaeological records and the time interval is a critical period in human evolution five episodes of lake expansion and contraction can be interpreted from a stratigraphic interval defined by tuffs between ca 1 85 and 1 74 ma suggesting precession controlled cyclicity the geochronometry of the sequence was determined using the paleomagnetic record top of subchron cn2 1 785 ma the ages of two tuffs tuff ib and tuff if and sedimentation rates insolation w m 2 variations calculated as a function of eccentricity and precession predict five cycles of varying insolation 10 between 2 0 and 1 5 ma rainfall would have increased by one third between dry and wet portions of each 21 000 k y cycle this study represents the first dated terrestrial record at the pliocene pleistocene boundary with sufficient resolution to link lake cycles a climate proxy to astronomic forcing the wet dry climate pendulum may have been a factor in the natural selection processes of hominin evolution and the first wave of hominin migrations out of africa 1 8 ma

precise 40ar 39ar geochronology for the upper koobi fora formation turkana basin northern kenya the plio pleistocene koobi fora formation about 560 m thick crops out east of lake turkana and is part of the much larger depositional system of the omo turkana basin the upper half of the koobi fora formation from just below the kbs tuff to above the chari tuff is particularly notable for its wealth of hominid fossils and archaeological sites silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation pumice clasts within the tuffs contain anorthoclase phenocrysts ideal for 40ar 39ar single crystal dating feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the kbs tuff to the silbo tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the kbs tuff 1 869 0 021 ma and the chari tuff 1 383 0 028 ma and to yet higher in the sequence to the silbotuff 0 751 0 022 ma these results provide a precise and accurate time scale for the upper part of the sequence in the whole of the omo turkana basin a number of these tuffs are recognized elsewhere in east africa thus ages determined at koobi fora also apply to the wider region

diatomaceous sediments and environmental change in the pleistocene olorgesailie formation southern kenya rift valley abstract the olorgesailie formation is comprised of lacustrine volcaniclastic and alluvial sediments that formed in the southern kenya rift between about 1 2 million and 490 000 years ago diatoms are common in much of the sequence and preserve a record of environmental change within the basin a high resolution diatom stratigraphy has been developed for these deposits the data document the presence of freshwater and saline lakes as well as wetlands transfer functions indicate that these water bodies ranged in conductivity between about 200 20 000 s cm 1 with ph varying between about 7 5 and 10 3 pedogenesis affected multiple horizons within the succession reflecting periods of emergence that encompass much of the time represented by the olorgesailie formation a variety of other sedimentological indicators carbonates rhizoliths mudcracks erosional channels also record periods when the sampled portions of the basin were dominated by terrestrial conditions stone tools are common at several levels indicating the use of the area by hominins lakes and wetlands were potentially usable as sources of potable water by hominins for part of the basin history but at other times were undrinkable other water sources springs rivers would have been necessary during these periods paleoenvironments in the basin were complex and changed frequently with time such shifts in resources and habitat distribution during olorgesailie formation time seem likely to have influenced the behavior and evolution of local plant and animal populations including homo

an astronomically tuned climate framework for hominins in the turkana basin understanding the influence of orbital climate cycles on hominin evolution remains a key challenge in paleoanthropology the two major unresolved issues are the absence of a climate proxy yielding highresolution b20 kyr terrestrial climate records and the lack of age control on hominin fossil occurrences at sufficiently high resolution here we present a novel climate proxy strontium isotope ratios 87sr 86sr of lacustrine fish fossils from the turkana basin that solves these issues by recording orbitally forced variation in summer monsoon intensity over the ethiopian highlands we successfully applied the climate proxy to a 150 kyr time interval of 2 million year old paleolake deposits containing hominin fossils existing age control of the studied interval was improved by a new magnetostratigraphic record precisely locating the base of the olduvai chron c2n near the bottom of the sequence spectral analysis demonstrates that 87sr 86sr variability is primarily determined by precession which enables us to place hominin fossils in an astronomically tuned climate framework the sr climate proxy is potentially applicable to all homininbearing lake deposits in the turkana basin ranging in age from 4 2 to 0 8 million years ago ma our results demonstrate that between 2 and 1 85 ma the turkana basin remained well watered and inhabited by hominins even during periods of precession maxima when summer monsoon intensity was lowest this is in contrast to other basins in the east african rift system ears that were impacted heavily by precessionforced droughts we hypothesize that during lake phases the turkana basin was an aridity refugium for permanent water dependent fauna including hominins over the precessional climate cycles

strontium isotope evidence for late pleistocene reestablishment of an integrated nile drainage network evidence of late pleistocene closed basin conditions in equatorial african lakes has far reaching implications for the history of the nile river there has been widespread acceptance that overflow of lakes victoria and albert into the nile occurred some time during the terminal pleistocene but recent suggestions that lake victoria remained a closed basin until ca 7 2 ka imply a much longer period of greatly reduced white nile discharge this hypothesis has major consequences for current ideas about the river9s paleohydrology early human exploitation of riverine resources nile delta sedimentation and late quaternary genesis of sapropels in the eastern mediterranean here we resolve the timing of lake victoria9s overflow by using strontium isotopes as a tracer for water from the principal lakes and rivers of the upper nile our data demonstrate that overflow of lakes victoria and albert into the nile drainage network occurred no later than 11 5 14 c ka

radium 228 distribution and mixing in san nicolas and san pedro basins southern california borderland sediment profiles of 228ra and 232th and water column profiles of 228ra are presented for san nicolas and san pedro basins located in the southern california borderland these data are used to estimate fluxes of 228ra from the sediments the vertical eddy diffusivity kz in the water column and the upwelling velocity wz the fluxes of 228ra from sediments estimated by integrating the downcore deficiency of 228ra with respect to its parent nuclide 232th are 335 115 atoms m2s for san nicolas basin and 166 50 atoms m2s for san pedro basin san nicolas basin sediments have an excess of 228ra in the upper 4 cm but irrigation and diffusion create a 228ra deficiency from 4 to 30 cm san pedro basin sediments are anoxic and laminated have a 228ra deficiency above 6 12 cm depth and have no surface excess of 228ra basin waters appear to be well mixed horizontally below sill depth and in steady state with respect to 228ra concentrations of 228ra increase from sill depth to the bottom ranging from approximately 1 0 dpm 100 kg to 2 4 dpm 100 kg in san nicolas basin and from 1 6 dpm 100 kg to 3 1 dpm 100 kg in san pedro basin estimates of kz and wz at several horizons in these basins are made by evaluating two steady state mass balance equations one for 228ra and one for heat this approach is hindered by an inability to accurately define the temperature in waters flowing over the sills and the apparent failure of topography to completely restrict horizontal exchange in the upper part of the water column below sill depth in each basin upper limits for kz are 23 cm2 s at 1700 m in san nicolas and 2 8 cm2 s at 850 m in san pedro basin our best estimates for kz and wz for the deeper part of the basins are kz 5 17 cm2 s and wz 50 150 m y in san nicolas basin and kz 1 3 cm2 s and wz 20 50 m y in san pedro basin the differences in kz between basins reflect differences in the buoyancy gradients upwelling velocities increase from the bottom to the sill of each basin

geochemical evidence for anoxic deep water in the arabian sea during the last glaciation various paleoceanographic studies have indicated that the deep ocean was probably depleted in dissolved oxygen during the last glacial period approximately 18 kyr b p delta sup 18 o stage 2 compared to present time however direct evidence of low oxygen content in the deep waters has been lacking here the authors report geochemical evidence of near anoxic conditions in the deep arabian sea during the entire last glacial cycle delta sup 18 o stages 2 3 and 4 anoxia is inferred from the concomitant enrichment of organic carbon and authigenic uranium in the glacial sections of a core from the deep eastern arabian sea the anoxic conditions during the last glacial period probably caused by a change in deep water circulation evidently enhanced preservation of organic matter and simultaneous removal of uranium from seawater 57 refs 3 figs 2 tabs

isotopic records of climate seasonality in equid teeth abstract we investigate how oxygen isotopes in equid teeth can be used as a record of seasonality first we use in situ laser ablation and conventional microsampling techniques to understand time averaging of environmental signals in intra tooth isotope profiles in modern feral horse teeth n 5 from mongolia where there is a large seasonal gradient in the oxygen isotopic composition of precipitation we demonstrate that laser ablation can be used to sample inner middle and outer enamel layers in large mammalian herbivore teeth the inner enamel layer records less attenuated isotopic signals than other layers as predicted by the mineralization patterns but intra tooth signal amplitude is similar for laser and conventional sampling methods second we use modern zebra teeth n 21 collected in eastern africa to evaluate how intra tooth oxygen isotope variation relates to rainfall patterns in the tropics we show that the intra tooth isotopic range increases with intra annual precipitation isotopic range which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole finally we address isotopic seasonality during the early pleistocene in eastern africa using oxygen isotopes in fossil equid teeth from southwestern kenya n 11 and northern tanzania n 5 we find variable isotopic seasonality in the past similar to present day eastern africa consistent with the notion that hominins and other mammals were able to accommodate environmental variability on intra annual scales in addition to well documented orbital cycles

particulate iron and manganese in the santa barbara basin california abstract particulate fe and mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes a study has been made of suspended particulate composition in the santa barbara basin a shallow near shore basin off southern california with sub oxic conditions below sill depth observations have revealed several interesting phenomena relating to the geochemistry of fe and mn most striking is a profound enrichment of particulate fe in samples from the bottom two hundred meters these particulates have a constant fe p mole ratio of about three and may originate at the sediment water interface or may be transported to the basin from local marshes for particulate mn enrichments are observed both in the sub sill waters and near the base of the euphotic zone a consideration of particle removal rates suggests that the sub photic zone enrichment has a biogenic origin in the sub sill waters enrichment in mn is apparently due to the precipitation of dissolved mn diffusing from the anoxic basin sediments a simple mass balance suggests that most of the mn lost from the sediments is transported from the santa barbara basin in dissolved form

hominin evolution in settings of strong environmental variability abstract investigations into how climate change shaped human evolution have begun to focus on environmental dynamics i e the nature and tempo of climate and landscape variability an approach that de emphasizes static reconstructions of early hominin habitats the interaction among insolation cycles is especially apparent in the paleoenvironmental records of the east african rift system where the longest records of human evolution are preserved however environmental indicators such as deep sea oxygen isotopes terrestrial dust flux paleosol carbon isotopes and lake sediments do not point consistently to any simple trend or climate driver of evolutionary change comparison of environmental indicators cautions against an exclusive focus on any given end member of environmental fluctuation driest or wettest warmest or coolest and argues for the impact of the entire range of variability in shaping evolutionary change a model of alternating high and low climate variability for tropical africa further implies that specific environmental indicators reflect different aspects of east african environmental dynamics the model may thus help reconcile some of the conflicting interpretations about the environmental drivers of hominin evolution first and last appearances of hominin lineages benchmark biogeographic events and the emergence of key adaptations and capacities to alter the surroundings are consistently concentrated in the predicted longest intervals of high climate variability the view that emerges is that important changes in stone technology sociality and other aspects of hominin behavior can now be understood as adaptive responses to heightened habitat instability

topographies lacking tidal conversion the consensus is that in a stratified sea a classical model of tidal flow over irregular but smooth topography necessarily leads to the generation of internal tides regardless of the shape of the topography this is referred to as tidal conversion here it is shown however that there exists a large class of topographies for which there is neither tidal conversion nor any scattering of incident internal waves this result is obtained in a uniformly stratified rigid lid sea using a barotropic tide that owing to its large horizontal scale is supposed to be simply a mass conserving periodic back and forth flow the baroclinic response at the tidal frequency is upon non dimensionalizing and stretching of coordinates determined by a standard hyperbolic boundary value problem bvp we here solve this hyperbolic bvp by mapping a domain of complicated yet a priori unknown shape onto a uniform depth channel for which the same hyperbolic problem is known to display neither conversion of the barotropic tide nor scattering of internal wave modes the map achieving this is required to satisfy hyperbolic cauchy riemann equations defined as analogues of the cauchy riemann equations that are used in solving elliptic problems mapping the rigid lid surface in the original cartesian frame onto a rigid lid surface in the transformed frame this map is solved in terms of one arbitrary function each particular function defines a new topographic shape that can be computed a posteriori the map is unique provided the jacobian of transformation does not vanish which is guaranteed for subcritical bottom topography whose slope is everywhere less than that of the characteristics for topographies that can thus be mapped onto a channel tidal conversion and scattering are absent examples discussed include the classical wedge a near gaussian ridge a continental slope and near sinusoidal topographies

magnetostratigraphy of the hominin sites and paleolakes drilling project hspdp baringo tugen hills barsemoi core kenya abstract the principal objective of the hominin sites and paleolakes drilling project hspdp is to study the relationship between climate and environmental change and the implications on human evolution in eastern africa for this purpose hspdp has recovered a 228 m core in the chemeron formation of the baringo basin kenya the chemeron formation spans approximately 3 7 myr from around 1 6 to 5 3 ma and has yielded many vertebrate fossils including fossil hominins the magnetostratigraphy of the baringo core contributes to the chronological framework a total of 567 individual paleomagnetic samples were collected from 543 levels at regular intervals throughout the core and 264 were processed using thermal and alternative field stepwise demagnetizations in most samples distinct low temperature lt 20 150 c and high temperature ht 150 550 c characteristic remanent magnetization chrm could be determined typical demagnetization behaviors and some rock magnetic experiments suggest titanomagnetite acts as the main carrier of the ht chrm with pervasive secondary overprints in normal polarity expressed by the lt component normal and reversed polarities were identified based on the secondary overprints lt chrm directions either parallel or antiparallel to the ht chrm directions respectively our study identified four paleomagnetic reversals interpreted as the matuyama gauss gauss kaena kaena gauss and the gauss mammoth transitions these boundaries provide chronostratigraphic tie points that can be combined with those derived from 40ar 39ar dating of tuffs deino et al this issue and together indicate that the hspdp baringo core has an age range of 3 3 ma to 2 6 ma the consistent paleomagnetic and radioisotopic age constraints are incorporated into a bayesian age model of the core deino et al this issue

a deliberate tracer experiment in santa monica basin the mixing of heat and chemical constituents across density strata in the ocean plays a major role in ocean circulation and in the response of the climate to thermal and chemical perturbations we are developing a tracer technique which in conjunction with fluid dynamical measurements promises to improve our understanding and our estimates of such diapycnal mixing in a prototype tracer experiment in santa monica basin 50km west of los angeles two tracers sulphur hexafluoride sf6 and per fluorodecalin pfd have been injected on an isopycnal surface near the centre of the basin after 50 days the tracers had mixed along the isopycnal surface to nearly every part of the basin although relatively little had penetrated to the basin walls the diapycnal spreading of the tracer distributions during this first stage of the experiment yielded an apparent eddy diffusivity of 0 33 0 08 cm2s 1 at the ambient density gradient of 4 0 0 5 10 9g cm 4

early pleistocene lake formation and hominin origins in the turkana omo rift abstract prior research has correlated the formation of plio pleistocene lakes in east africa to global regional climate changes and interpreted the lacustrine basins as significant settings of hominin evolution paleo lake lorenyang from the turkana omo rift is important to these issues as its marginal deposits contain some of if not the earliest currently known specimens of acheulian stone tools and african homo erectus magnetostratigraphic and sedimentological evidence indicates that the oldest preserved paleo lake lorenyang deposits are dated at 2 148 2 128 ma and derive from the nw turkana basin predating those from the omo basin by 100 kyr and the ne turkana basin by 190 kyr apparently the lake expanded asynchronously in the rift potentially due to a volcano tectonic influence on the location of drainage networks depositional slopes or topographic elevation differences within and between the basins at the time of flooding the onset of the lake temporally coincides with the eruption of basalt lava flows dated to 2 2 2 0 ma that blocked the southeast outlet of the turkana basin this provides a plausible mechanism for hydrologic closure and lacustrine basin formation through volcano tectonic impounding it also points to a non climatic cause for the initial formation of paleo lake lorenyang at 2 14 ma first appearances for african h erectus 1 87 ma and acheulian tools 1 76 ma in the turkana omo rift postdate the lake s initial formation by about 270 kyr and 380 kyr respectively such timing differences contrast with studies that correlate all three to the 400 kyr eccentricity maximum at 1 8 ma although the turkana omo rift is just one example it does provide alternative insights to views that link climate hominin evolution and lake formation in east africa

chronostratigraphic correlation of lacustrine deposits using 87sr 86sr ratios eocene green river formation wyoming u s a abstract the reconstruction of detailed basin scale depositional histories from sedimentary rocks fundamentally depends on the availability of reliable time markers unlike marine strata lacustrine strata typically lack rapidly evolving cosmopolitan fauna or flora that might serve this purpose depending on their geologic context lacustrine strata may also lack tephras that could provide isochronous markers or radioisotopic age variations in 87 sr 86 sr ratios could potentially provide an alternative means of chronostratigraphic correlation for carbonate rich lake deposits based on the hypothesis that sr isotopes are well mixed in a lake and do not experience significant fractionation to test this hypothesis we measured 87 sr 86 sr ratios in 114 samples from two drill cores of the upper wilkins peak member from the green river formation that are located 23 km apart these cores can be independently correlated using distinctive tephras and organic carbon rich mudstone horizons measured 87 sr 86 sr ratios range from 0 71154 to 0 71504 and vary inversely with lake water depth as interpreted by sedimentary lithofacies characteristics lower ratios of 87 sr 86 sr are found in lithofacies deposited during lake highstands which are marked by laminated dark gray mudstone and elevated organic carbon enrichment as measured by fischer assay analysis higher 87 sr 86 sr ratios occur in lithofacies deposited during lake lowstands which are marked by organic lean gray green mudstone 87 sr 86 sr in approximately time equivalent samples from the two cores show a strong positive correlation r 0 68 despite the likely presence of small temporal mismatches between approximately correlative samples we conclude that lake water was consistently well mixed with respect to sr across distances of at least 23 km these results suggest that 87 sr 86 sr can serve as a powerful tool to aid high resolution chronostratigraphic correlation of lake deposits

a leaf wax biomarker record of early pleistocene hydroclimate from west turkana kenya abstract climate is thought to play a critical role in human evolution however this hypothesis is difficult to test due to a lack of long high quality paleoclimate records from key hominin fossil locales to address this issue we analyzed organic geochemical indicators of climate in a drill core from west turkana kenya that spans 1 9 1 4 ma an interval that includes several important hominin evolutionary transitions we analyzed the hydrogen isotopic composition of terrestrial plant waxes dwax to reconstruct orbital timescale changes in regional hydrology and their relationship with global climate forcings and the hominin fossil record our data indicate little change in the long term mean hydroclimate during this interval in contrast to inferred changes in the level of lake turkana suggesting that lake level may be responding dominantly to deltaic progradation or tectonically driven changes in basin configuration as opposed to hydroclimate time series spectral analyses of the isotopic data reveal strong precession band 21 kyr periodicity indicating that regional hydroclimate was strongly affected by changes in insolation we observe an interval of particularly high amplitude hydrologic variation at 1 7 ma which occurs during a time of high orbital eccentricity hence large changes in processionally driven insolation amplitude this interval overlaps with multiple hominin species turnovers the appearance of new stone tool technology and hominin dispersal out of africa supporting the notion that climate variability played an important role in hominin evolution

southern california inner basin sediment trap calibration the deployment of particle interceptor traps pits in the three inner basins of the southern california bight santa barbara santa monica and san pedro where preserved laminated bottom sediments occur provides a natural calibration between the parameters determined with the pits and those derived from the historical sedimentary deposits the accumulation rates and chemical composition of the pit materials compare favorably with these recently deposited bottom sediments the ratio of the measured particle collection rate to the sediment accumulation rate for these three basins averaged0 93 0 20 radionuclide 210pb 228th 232th and trace element fe cu zn cd pb compositions of the pit materials and surface sediments agree within generally 30 in each basin an application of pits as a sampling system in an outer basin san nicolas where the sedimentary record has been obscured by bioturbation is also presented to further demonstrate the utility of pits for spatial and temporal studies of various aspects of particle transport and sediment deposition based on the calibration of the pits in the three inner basins we conclude from the comparison of the radionuclide and trace element compositions between the pit materials and surface sediments from the san nicolas basin that recently deposited sedimentary material has been mixed by bioturbation with older material from the last several hundred years excess210pb and the228th 232th activity ratio in the pit material are 6 and 15 times higher than in the surface sediment total pb and1n hno3 leachable pb are 4 and 8 fold higher in the pit material

paleogeography of miocene western amazonia isotopic composition of molluscan shells constrains the influence of marine incursions strontium oxygen and carbon isotope compositions of well preserved mollusks bivalves indicate a dominantly freshwater depositional setting for the lower miocene upper miocene pebas formation in western amazonia molluscan 87sr 86sr ratios identify different freshwater sources andean runoff was the dominant water source in miocene western amazonia though there was occasional influx of waters from cratonic catchments at only one stratigraphic level isotope signals indicate increased mesohaline aquatic salinities in concert with a clearly more saline molluscan faunal assemblage strontium isotope based salinity estimates are surprisingly low when compared to other paleosalinity estimates based on the interpretation of ichno faunal assemblages and sedimentological structures we propose that these seemingly contrasting observations can be unified if miocene western amazonia was occupied by a long lived lacustrine wetland system with a restricted connection via the los llanos basin to the caribbean sea abundant runoff supplied fresh water to this system which effectively blocked the influx of saline waters through the restricted marine connection to the north much like modern lake maracaibo such a system could have been the site of microtidal currents and thus could have hosted brackish water fauna in a dominantly freshwater depositional system

cadmium and delta13c paleochemical ocean distributions during the stage 2 glacial maximum changes in heat and salt transport by ocean currents have been implicated in ice age climate cycles weyl 1 968 newell 1 974 broecker et al 1 990b in addition to the direct physical effects of the ocean on climate the ocean carbon system controls atmospheric co2 and the co2 fluctuations documented in ice core bubbles barnola et a1 1 987 neftel et a1 1 988 are thought to be a significant factor in climate oscillations hansen et al 1 984 dramatic changes in oceanic circulation patterns and carbon system chemistry have occurred during the late quaternary glacial interglacial global climate cycles boyle keigwin 1 982 curry lohmann 1 983 shackleton pisias 1 984 boyle keigwin 1 987 duplessy et al 1 988 curry et al 1 988 broecker et al i 990a these changes in the ocean system may be significant driving forces in the progression of ice age co2 cycles toggweiler sarmiento 1 985 ennever mcelroy 1 985 wenk siegenthaler 1 985 boyle 1 988a b broecker peng 1 990 the oceanic distributions of c513c cd and other carbon system linked properties are controlled by the interaction between biological uptake and decomposition with the general circulation of the ocean because records of past oceanic cd and c5 1 3c are preserved in shells of planktonic and benthic foraminifera

environment and climate of early human evolution evaluating the relationships between climate the environment and human traits is a key part of human origins research because changes in earth s atmosphere oceans landscapes and ecosystems over the past 10 myr shaped the selection pressures experienced by early humans in africa these relationships have been influenced by a combination of high latitude ice distributions sea surface temperatures and low latitude orbital forcing that resulted in large oscillations in vegetation and moisture availability that were modulated by local basin dynamics the importance of both climate and tectonics in shaping african landscapes means that integrated views of the ecological environmental and tectonic histories of a region are necessary in order to understand the relationships between climate and human evolution

vegetation change in the baringo basin east africa across the onset of northern hemisphere glaciation 3 3 2 6 ma abstract vegetation in east africa is generally thought to have shifted from forests to more open grasslands and savannas as global climate cooled and high latitude ice sheets expanded during the plio pleistocene such a shift would have greatly influenced landscape resources and potentially hominin evolution as well existing records of african vegetation spanning these time scales are generally derived from offshore marine records that record continental scale changes or paleosol carbonate records that record very local vegetation changes during the short time intervals of soil carbonate formation here we present a new record of basin scale vegetation change from the late pliocene 3 3 2 6 ma derived from a drill core from the chemeron formation located in the baringo basin tugen hills region of the kenya rift valley specifically we present a new record of the relative abundance of c4 grasses and c3 vegetation based on the carbon isotopic composition of leaf wax biomarkers 13cwax which captures a signal of regional vegetation change these data demonstrate that vegetation in the baringo basin varied greatly between c3 forests and c4 grasslands and that vegetation exhibits both long term secular trends and orbital scale variations the contribution of c3 plants was lower than estimates based on low resolution carbon isotope data from paleosol carbonates and organic matter in the basin c3 plants averaged 53 of the vegetation during the late pliocene from 3 3 to 3 04 ma after which time 13cwax indicates more open vegetation and 41 c3 plants this transition may have been driven by changes in basin geomorphology but also possibly occurred as part of larger scale drying and expansion of c4 vegetation in east africa in addition to this secular change we observe high amplitude variability in the 13cwax record including oscillations between 80 and 0 c3 plants these vegetation changes are correlated with changes in precipitation inferred from 2hwax and lake level oscillations inferred from sedimentary facies implying that high amplitude orbital scale variations in precipitation drove significant changes in vegetation resources during the late pliocene in the baringo basin these variations have important implications for changes in terrestrial resources in light of the evolutionary innovations in the hominin fossil record related to changes in foraging strategies

climatic change recorded in the sediments of the chew bahir basin southern ethiopia during the last 45 000 years abstract east african paleoenvironments are highly variable marked by extreme fluctuations in moisture availability which has far reaching implications for the origin evolution and dispersal of homo sapiens in and beyond the region this paper presents results from a pilot core from the chew bahir basin in southern ethiopia that records the climatic history of the past 45 ka with emphasis on the african humid period ahp 15 5 ka calbp geochemical physical and biological indicators show that chew bahir responded to climatic fluctuations on millennial to centennial timescales and to the precessional cycle since the last glacial maximum potassium content of the sediment appears to be a reliable proxy for aridity showing that chew bahir reacted to the insolation controlled humidity increase of the ahp with a remarkably abrupt onset and a gradual termination framing a sharply defined arid phase 12 8 11 6 ka calbp corresponding to the younger dryas chronozone the chew bahir record correlates well with low and high latitude paleoclimate records demonstrating that the site responded to regional and global climate changes

diurnal and semidiurnal internal tide energy flux at a continental slope in the south china sea 1 barotropic surface and baroclinic internal tides were measured at four mooring sites during a field investigation of acoustic propagation characteristics and physical oceanography in the northern south china sea the mooring positions were in a line moving up the shallow portion of a continental slope at water depths between 350 and 85 m using time series of temperature and velocity at several depths 1 month series of semidiurnal and diurnal species internal tidal energy flux vectors were computed for three sites with a 14 day series computed for the fourth shallow site the internal tides had a temporal signature that was not in complete accord with the barotropic tides showing an enhancement of diurnal internal tides with respect to semidiurnal bathymetric slope barotropic tidal fluid particle trajectories and scale of generation site versus internal tide wavelength are investigated as possible causes of the differing response of the species

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

magnetostratigraphy of the koobi fora formation lake turkana kenya the koobi fora formation a pliocene and pleistocene sequence of sedimentary deposits northeast of lake turkana has yielded numerous fossils and stone artifacts of early hominids stratigraphic correlation of the hominid bearing deposits throughout the turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies we have reinterpreted previously published magnetostratigraphy from the upper part of the koobi fora formation because the original stratigraphy and dating of tuffs have been revised in our reinterpretation we include previously unpublished data from the uppermost part of the formation the upper magnetozones correlate with parts of the brunhes normal polarity chron and matuyama reversed polarity chron about 0 6 0 85 ma and are separated from the magnetozones of the upper part of the matuyama 2 0 1 25 ma by a disconformity the olduvai normal polarity subchron is represented within the matuyama but the lower part of the matuyama 2 4 2 0 ma is missing due to an erosional disconformity we have also determined magnetozones in the lower part of the koobi fora formation which had not been sampled for paleomagnetism during the earlier studies our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of koobi fora tuffs with dated tuffs in the shungura formation of southern ethiopia the tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the koobi fora and shungura formations the lower part of the koobi fora spans the interval from about 4 ma to 2 4 ma within the gilbert reversed polarity and gauss normal polarity chrons rock magnetic studies indicate that detrital magnetite carries most of the stable remanence although hematite contributes to the remanence as indicated by thermal demagnetization the hematite which presumably formed by postdepositional oxidation of original iron oxide grains carries chemical remanent magnetization crm that diminishes the sharpness of polarity zone boundaries the crm accumulated continuously within the uppermost 10 m of sediment below the land surface so that the crm reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70 000 years in sections where the sedimentation rate was approximately 15 cm 1000 years

miocene benthic foraminiferal cd ca records south atlantic and western equatorial pacific cadmium to calcium records in benthic foraminifera interpreted in conjunction with carbon isotope records reflect oceanic circulation patterns and oceanic inventories of cd carbon 13 carbon 12 and indirectly phosphorus cd ca ratios have been determined in benthic foraminifera cibicidoides spp for the late middle and early miocene from a site in the south atlantic deep sea drilling project dsdp site 525 leg 74 2500 m walvis ridge and from a site in the western equatorial pacific dsdp site 289 leg 30 2200 m ontong java plateau both sites have well established carbon isotope records for benthic foraminifera typical of the global record dominated by inventory changes cd ca ratios and carbon isotope values were averaged for the miocene in time intervals substantially longer than estimated oceanic residence times of cd c or p oceanic mean cd ca and carbon isotopes were estimated as weighted means of the values from both sites using weighting factors based on replicating modern oceanic means this may tend to underestimate oceanic inventories earlier than the late miocene because of changing water mass dominance at the south atlantic site from these means the oceanic cd inventory appears relatively stable during the miocene while there are large changes in carbon isotope inventories miocene cd inventory estimates are at least 20 lower than published estimates for quaternary stages 1 and 2 increasing oceanic cd inventories over the past 10 m y are consistent with hypotheses of increased river fluxes in the recent geologic past cd ca differences between sites may be indicative of the extent of basin basin fractionation from bottom water circulation although the validity of this comparison depends on how representative each of these two sites is through time for its respective ocean basin the pacific atlantic cd ca difference based on these two sites was larger in the late miocene compared to the middle and early miocene the relationship between cd ca and carbon isotope inventories and intersite gradients for these sites suggests that either oceanic phosphorus inventories or parameters related to marine organic matter composition must have been different in the miocene than at present large changes in the carbon isotope balance reflected as both positive and negative excursions in the carbon isotope inventories occurred with little effect on cd inventories changes in phosphorus fluxes associated with these carbon isotope flux changes may have been compensated for by changing deposition fluxes to other phosphorus sinks phosphorus inventories may have increased from the middle to late miocene with increasingly negative oceanic carbon isotope inventories however confirmation of these suggestions depend on collecting more cd ca and 13c data throughout the miocene oceans and on developing the application of other constraints for these interpretations

sensitivity of the phosphate cadmium carbon isotope relation in the ocean to cadmium removal by suboxic sediments reconstructions of past deep ocean nutrient concentrations rely largely on the carbon isotopic composition and cadmium content of shells of benthic foraminifera when comparing records of these two paleonutrient tracers a relation similar to the phosphate cadmium carbon isotope relation in the water column today is generally assumed we present new water column sediment and pore water data from the california continental margin which demonstrate that cd accumulation is enhanced in suboxic sediments these results suggest that the oceanic cd inventory is sensitive to changes in the areal extent and redox intensity of suboxic sediments and thus that the phosphate cadmium relation in the ocean could have been different in the past the magnitude of this effect and its evolution through time is presently hard to quantify because the oceanic cd budget is poorly constrained

plio pleistocene sedimentation in west turkana turkana depression kenya east african rift system paleolake fluctuations paleolandscapes and controlling factors abstract pliocene and pleistocene sediments from west turkana kenya east african rift system form emblematic syn rift successions for understanding the evolution of extensional basin and continental rifting they also constitute world renowned fossil bearing strata from which 500 hominin fossils were discovered over the past decades with 100 inventoried archeological sites however associated sedimentary dynamics and architectures as well as paleoenvironments are only partially reconstructed and the relative contribution of climate and tectonism to paleoenvironmental change over time remains unclear here through the interpretation of sedimentary facies the delineation of sequences and the analysis of 13c in soil carbonates we provide the first exhaustive reconstruction from 4 00 to 0 75 ma of i fluctuations of the paleolake that occupied the turkana depression ii the successive sedimentary dynamics and related paleolandscapes that characterized the west turkana area and iii respective roles of climate and tectonism in the sedimentation we show evidence for seven major transgression regression t r cycles between 4 00 and 0 75 ma superimposed locally by lower amplitude t r cycles comparing the sedimentological interpretations and the 13c values in soil carbonates literature data we reveal that fluctuations of rainfall over the ethiopian dome which hosts the drainage basin of the omo river the main tributary of lake turkana controlled high amplitude lake level fluctuations during the plio pleistocene period we also demonstrate that vegetation and tree cover evolved differently in the omo valley and west turkana furthermore we outline that two different sedimentary systems reflecting two distinct modes of sedimentation alternated through times in the west turkana area as a response to the variations in sediment supply coming from the western rift shoulder i e lapurr range that alternatively generated wave or river dominated sedimentary systems in conclusion we reveal that climate regulated water input paleolake water level fluctuations and vegetation tectonism determined sediment supply to the basin controlled in west turkana by pulses of increased activity of the main border fault i e the murua rith lapurr fault

the geochemical cycle of rhenium a reconnaissance rhenium re is one of a suite of elements including uranium and molybdenum that display conservative behavior in seawater and are enriched in anoxic sediments the decay of187re to187os provides a geochronometer in ancient sedimentary rocks and gives rise to os isotopic variations in nature in order to better characterize its sources to seawater re was measured in three major rivers amazon orinoco ganges brahmaputra and some of their tributaries re concentrations span four orders of magnitude from 0 02 to 400 pmol kg with the highest concentrations found in rivers draining black shales in the venezuelan andes mainstream re levels in the three rivers are between 1 and 10 pmol kg with a flux weighted average of 2 3 pmol kg the residence time for re in the oceans is estimated to be 750 000 yr with respect to river inputs re profiles from the atlantic and pacific oceans confirm that re behaves conservatively in seawater with no significant uptake onto particles and or recycling within the water column this is also true in the anoxic water column of the black sea re removal into anoxic sediments occurs at or below the sediment water interface as demonstrated in sediment pore waters from chesapeake bay in oxic sediments re is not cycled with manganese oxides and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component or in manganese nodules burial of re in anoxic sediments which accumulate on 0 3 of the ocean floor removes approximately 50 of the riverine re flux to the oceans hence oceanic re concentrations may be very sensitive to changes in the area of anoxic sedimentation

lacustrine 87 sr 86 sr as a tracer to reconstruct milankovitch forcing of the eocene hydrologic cycle abstract the green river formation grf provides one of the premier paleoclimate archives of the early eocene climatic optimum 50 ma representing the apex of the early cenozoic greenhouse climate rhythmic lake level variability expressed in the grf has inspired numerous hypotheses for the behavior of the eocene hydrologic cycle including its linkage to astronomical forcing solar variability and the el nino southern oscillation enso however the lack of sufficient proxy data to document atmospheric water mass transport and the geographic pattern of evaporation precipitation runoff has made it difficult to discriminate between different models for astronomical forcing variable 87sr 86sr ratios of bedrock that encompass the grf provide an opportunity to reconstruct the spatial expression of the eocene hydrologic cycle and its linkage to lake level here sr isotope data from the wilkins peak member a rhythmic succession that has been demonstrated to record milankovitch forcing of lake levels indicate that high lake levels reflect an increased proportion of runoff from less radiogenic rocks west of the basin eliminating a number of the existing astronomical forcing hypotheses the 87sr 86sr variability is consistent with a change in mean enso state which is predicted by climate models to be linked to orbital insolation thus the 87sr 86sr data reveal a coupling of high frequency enso and low frequency astronomical climate variability and also predict the existence of sizable astronomically forced alpine snowpack during the last greenhouse climate more broadly this study demonstrates the utility of 87sr 86sr as a powerful tool for reconstructing the deep time hydrologic cycle

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity