paleosol carbonates from the omo group isotopic records of local and regional environmental change in east africa abstract pliocene and pleistocene sedimentary rocks from the omo turkana basin in east africa are well known for fossil and archeological evidence of human evolution and provide a unique opportunity to study four million years of environmental change in a rift basin this study uses carbon and oxygen isotope ratios of pedogenic carbonates to examine environmental variability across 5000 km 2 within the omo turkana basin an expanded isotopic dataset including the first isotopic data on pedogenic carbonates from the shungura formation and new data from the nachukui and koobi fora formations is compared to published isotopic records from both the omo turkana basin and the lower awash basin ethiopia regionally the carbon isotope record indicates a steady increase of c 4 vegetation in floodplain environments for the past 4 million years the oxygen isotopic record indicates that the isotopic composition of rainfall was depleted in 18 o relative to today s waters and that both basins likely received more rainfall in the pliocene than today a shift to higher 18 o values in paleosol carbonate after 2 ma in the omo turkana basin but not in the lower awash basin suggests that the ecology and hydrology in these two rift basins were influenced by different climatic regimes in addition to regional trends pedogenic carbonates sampled from different parts of the basin show that the distribution of c 4 vegetation and soil water 18 o values varied with proximity to the axial river system and specifically that c 3 vegetation was more dominant in soils of the shungura formation compared with coeval sediments downstream in the nachukui and koobi fora formations this large isotopic dataset from pedogenic carbonates provides the opportunity to examine how terrestrial systems responded to global climate change during the last 4 million years from both local and regional perspectives the isotopic data indicate that local basin and climate dynamics strongly influenced the impact of large scale environmental change in east african rift basins

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

tuffaceous marker horizons in the koobi fora region and the lower omo valley eight tuffs can now be correlated between the shungura formation in the lower omo valley ethiopia and the sediments in the koobi fora region kenya re evaluation of the stratigraphy shows that at least 475 m of sediment was deposited in the koobi fora region

stratigraphical notes on the okote tuff complex at koobi fora kenya the okote tuff complex of the koobi fora formation in northern kenya contains many archaeological sites1 and many hominid specimens derive from levels nearby2 the complex lies between the kbs tuff and the chari tuff both of which have been precisely dated3 4 and recently mcdougall in the accompanying manuscript 5 has measured the age of pumice clasts derived from the complex the pumices however differ in composition from the tuff in which they were deposited the age is still useful because a tuff of known stratigraphical position in the shungura formation is of the same composition here we discuss stratigraphical relations between the okote tuff complex and other related strata at koobi fora in the lower omo valley and west of lake turkana based on compositional study of ash layers this allows us to estimate an age of 1 60 0 05 myr for the homo erectus skeleton recovered at nariokotome in 1984 see accompanying article6

formation and spreading of subtropical mode water in the north pacific a time averaged description of the formation and spreading of north pacific subtropical mode water npstmw is presented the formation of npstmw is studied using transpac expendable bathythermograph data the data are transformed into a stream coordinate system and averaged the coordinate system uses the position of the kuroshio front as its origin maps are presented of the temperature structure and mixed layer distribution in winter relative to the kuroshio front the formation of npstmw occurs in two separate stages cooling and thickening as water flows at the surface in the kuroshio in winter the water is cooled by the intense ocean atmosphere heat flux taking place in the area when water leaves the kuroshio to the south the thermocline underneath becomes deeper convection is able to penetrate deep into the water column and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature the area south of the kuroshio where relatively homogeneous waters are found is the source region for npstmw at the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become npstmw the levitus 1982 data are used to study the way in which npstmw is spread away from its source region by the subtropical circulation the distribution of potential vorticity pv on four npstmw isopycnals is studied with low pv used as a tracer for npstmw the pv reveals a pattern of differential spreading whereby npstmw formed in the western part of the source region is advected to the west after its formation npstmw formed farther east is advected more to the south or east after formation differential spreading allows npstmw to be found in a large part of the northwestern subtropical gyre well away from its source region the distribution of npstmw described by masuzawa 1969 with npstmw getting colder from west to east is explained as a combination of the distribution of npstmw at its source and the way it is spread after it is formed

discovery of an anoxic basin within the strabo trench eastern mediterranean we report here the discovery of a basin which we call the tyro basin in the eastern mediterranean in which anoxic conditions exist and organic rich sediments are accumulating bottom water samples show extremely high salt concentrations a horizontal midwater sound reflector about 70 m above the basin floor was observed in a seismic reflection profile across the basin from shipboard observations we conclude that dissolution of messinian evaporites exposed by faulting in the sides of the basin causes stagnant bottom conditions this basin provides an opportunity to test some of the hypotheses concerning the formation of sapropels present in cores collected from the same area cores taken in an adjacent similar basin for which we propose the name kretheus basin indicate a recent change from anoxic to oxygenated conditions which was probably caused by tectonic activity associated with the strabo trench

rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations 1 the late quaternary a working hypothesis is proposed to account for the present accumulation of brines in isolated pockets of the ocean floor and for the formation of the underlying organic and metal rich sediments these are the tyro and bannock basins in the east mediterranean the red sea deeps and the orca basin in the northern gulf of mexico initiation of brine derived deposition in the red sea deeps and orca basin occurred between 12 000 and 8000 years b p this time bracket also encompasses the formation of the latest east mediterranean sapropel and the wettest global climate since the last glacial maximum this wet period first appeared in the tropics around 12 000 years b p then in the subtropical and middle latitudes during the same period the 23 000 year precession cycle brought the summer insolation of the northern hemisphere to its peak at 11 000 years b p with retreating northern hemisphere ice sheets the red sea deeps and the orca basin became anoxic during this humid period and metal rich sapropel deposition then began in contrast the tyro and bannock basins began accumulating a brine long before and persisted beyond this climatic stage the hypothesis involves two propositions 1 as in the eastern mediterranean sea marine anoxia was mainly the consequence of the large influx of continental runoff and local precipitation longer residence time of bottom waters so called stagnation in silled rimmed basins would have resulted from lower salinity at the sea surface in areas of deep water formation in the eastern mediterranean the red sea and the gulf of mexico and 2 miocene or older evaporites underlie these basins or outcrop on their flanks leaching from these evaporites was an ongoing process before the quasi stagnation phase but the initial leachate much less saline than the present brines was continuously flushed by bottom circulation the climate induced quiescence of bottom waters in these basins enabled the leachate to accumulate the feedback of stagnation by increased density progressively raised the salinity of entrapped bottom waters to the present brine concentration the high density has resisted brine removal by bottom circulation until present time long after cessation of the initiating wet period the brines therefore are stagnant fossil waters

chronostratigraphy and correlation of the plio pleistocene tephra layers of the konso formation southern main ethiopian rift ethiopia abstract the plio pleistocene konso formation is widely exposed in the konso area located at the southwestern end of the southern sector of the main ethiopian rift it is known for its rich acheulean archeological occurrences and abundant vertebrate fossils including those of a boisei and h erectus the formation also contains more than 30 tephra layers the lithologic characterization and major element compositions of discrete glass shards of these tephra combined with single crystal 40 ar 39 ar dating of seven tuffs have established the tephrostratigraphy and age range of the 200 m thick formation the succession at konso was deposited between approximately 1 9 and 1 4 ma and is divided into the sorobo turoha kayle and karat members in ascending stratigraphic order two of the konso tephra are correlative to the pliocene kbs and the early pleistocene chari tuffs of the omo turkana basin along the ethiopian and kenyan border this correlation enables precise time controlled comparisons of faunal and archeological remains between the two regions

geochemical sources for the terminal pleistocene early holocene obsidian artifacts of the site of beseka central ethiopia twenty obsidian artifacts from the terminal pleistocene early holocene phase of the site of beseka central ethiopia have been instrumentally characterized using edxrf and electron microprobe results show that most of the artifacts were derived from the silicic centers of fentale and kone a nearby source of abadir although with better quality raw material does not seem to have been utilized

distal tephras of the eastern lake victoria basin equatorial east africa correlations chronology and a context for early modern humans abstract the tephrostratigraphic framework for pliocene and early pleistocene paleoanthropological sites in east africa has been well established through nearly 50 years of research but a similarly comprehensive framework is lacking for the middle and particularly the late pleistocene we provide the first detailed regional record of late pleistocene tephra deposits associated with artifacts or fossils from the lake victoria basin of western kenya correlations of late pleistocene distal tephra deposits from the wasiriya beds on rusinga island the waware beds on mfangano island and deposits near karungu mainland kenya are based on field stratigraphy coupled with 916 electron microprobe analyses of eleven major and minor element oxides from 50 samples at least eight distinct distal tephra deposits are distinguished four of which are found at multiple localities spanning 60 km over an approximately north to south transect new optically stimulated luminescence dates help to constrain the late pleistocene depositional ages of these deposits our correlation and characterization of volcaniclastic deposits expand and refine the current stratigraphy of the eastern lake victoria basin this provides the basis for relating fossil and artifact bearing sediments and a framework for ongoing geological archaeological and paleontological studies of late pleistocene east africa a crucial time period for human evolution and dispersal within and out of africa

crustal growth along a non collisional cratonic margin a lu hf isotopic survey of the eastern cordilleran granitoids of peru abstract an extensive in situ lu hf isotopic study of zircon from the eastern cordilleran batholiths of peru using laser ablation multi collector inductively coupled mass spectrometry la mc icpms reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto andean margin of central western amazonia since 1 15 ga the hf isotope systematics are characterised by a range in initial 176hf 177hf compositions for a given intrusive event suggesting mixing of material derived from the paleoproterozoic crustal substrate and variable addition of juvenile sources from neoproterozoic to cenozoic time intrusives associated with phases of regional compressive tectonism correspond to mean initial ehf values of 6 73 2 43 1 57 for the ordovician famatinian carboniferous permian and late triassic respectively suggesting a minimum crustal contribution between 40 and 74 by mass the average initial hf systematics from granitoids associated with periods of regional extension such as the middle neoproterozoic permian triassic and cenozoic andean back arc plutonism are consistently shifted towards positive values mean initial ehf 0 7 to 8 0 indicating systematically larger inputs of juvenile magma 25 to 38 of ancient crust by mass in the absence of evidence for significant lateral accretion of exotic crust the time integrated hf record from the central proto andean margin of western amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile mantle derived melts during intervals of crustal extension

ages of tuff beds at east african early hominid sites and sediments in the gulf of aden the early hominids of east africa were dated by determining the ages of tuff beds at the sites despite much research using palaeomagnetic and k ar dating techniques some of those ages are still controversial1 2 to obtain independent age estimates for these tephra layers we have examined cores from dsdp sites 231 and 232 in the gulf of aden fig 1a which consist mainly of calcareous nannofossil ooze but also contain rare tephra horizons3 dated by interpolation from the established nannofossil stratigraphy fig 1b chemical analysis confirms that the identity and sequence of these horizons is the same as that at the east african sites we conclude that the age of the tulu bor tuff is 3 4 myr and hence that the hadar hominid specimens are also 3 4 myr old

sequence of tuffs between the kbs tuff and the chari tuff in the turkana basin kenya and ethiopia this paper describes a sequence of tuffs between the kbs and the chari tuff of omo group formations in kenya and ethiopia these tuffs have recently been shown to be 1 87 0 02 ma and 1 38 0 03 ma in age respectively the sequence of tuffs that is derived is consistent with 40 ar 39 ar ages reported separately and provides the stratigraphic framework for interpreting those ages further new correlations are established to the konso formation in southern ethiopia as drainage from the ethiopian rift to the omo turkana basin developed after deposition of the konso formation pumice clasts in tuffs of the omo turkana basin probably were transported there by the omo river the tuffs are divided into five groups on the basis of their stratigraphic position in relation to extensive ash layers the sequence of tuffs has import for the placement and age of archaeological sites in the koobi fora formation and for ages of mammalian faunas including hominids many tuffs were deposited during a 90 ka interval during which mediterranean sapropels are lacking suggesting that nile flow was reduced and that the level of a lake that occupied the omo turkana basin at the time was low thus the record of climatic influence on deposition in the omo turkana basin previously shown for the kibish formation 200 ka extends at least to early pleistocene time

vertical structure of mesoscale eddies in the eastern south pacific ocean a composite analysis from altimetry and argo profiling floats 1 the mean vertical structure of mesoscale eddies in the peru chile current system is investigated by combining the historical records of argo float profiles and satellite altimetry data a composite average of 420 526 profiles acquired by argo floats that surfaced into cyclonic anticyclonic mesoscale eddies allowed constructing the mean three dimensional eddy structure of the eastern south pacific ocean key differences in their thermohaline vertical structure were revealed the core of cyclonic eddies ces is centered at 150 m depth within the 25 2 26 0 kg m 3 potential density layer corresponding to the thermocline in contrast the core of the anticyclonic eddies aes is located below the thermocline at 400 m depth impacting the 26 0 26 8 kg m 3 density layer this difference was attributed to the mechanisms involved in the eddy formation while intrathermocline ces would be formed by instabilities of the surface equatorward coastal currents the subthermocline aes are likely to be shed by the subsurface poleward peru chile undercurrent in the eddy core maximum temperature and salinity anomalies are of 1 c and 0 1 with positive negative values for aes ces this study also provides new insight into the potential impact of mesoscale eddies for the cross shore transport of heat and salt in the eastern south pacific considering only the fraction of the water column associated with the fluid trapped within the eddies each ce and ae has a typical volume anomaly flux of 0 1 sv and yields to a heat and salt transport anomaly of 1 3 1011 w and 3 8 103 kg s 1 respectively

mesoscale eddy effects on the subduction of north pacific mode waters mesoscale eddy effects on the subduction of north pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved the eddy resolving models produce results closer to observations than the noneddy resolving model there are large discrepancies in subduction patterns between eddy resolving and noneddy resolving models in the noneddy resolving model subduction on a given isopycnal is limited to the cross point between the mixed layer depth mld front and the outcrop line whereas in eddy resolving models and observations subduction takes place in a broader zonally elongated band within the deep mixed layer region mesoscale eddies significantly enhance the total subduction rate helping create remarkable peaks in the volume histogram that correspond to north pacific subtropical mode water stmw and central mode water cmw eddy enhanced subduction preferentially occurs south of the winter mean outcrop with an anticyclonic eddy to the west and a cyclonic eddy to the east the outcrop line meanders south and the thermocline mld shoals eastward as eddies propagate westward the mld shoals shielding the water of low potential vorticity from the atmosphere the southward eddy flow then carries the subducted water mass into the thermocline the eddy subduction processes revealed here have important implications for designing field observations and improving models

preconditioning of winter mixed layer in the formation of north pacific eastern subtropical mode water 1 the preconditioning of winter mixed layer in the formation of the eastern subtropical mode water estmw in the north pacific ocean is investigated using an eddy permitting ocean general circulation model the result shows that convergence in the northward ekman transport of saltier water and weak summertime heat fluxes due to the presence of stratus cloud in the estmw formation region are both central to the initiation of significant mixed layer deepening that subsequently evolves into the estmw pycnostad in the main thermocline these two factors originate from air sea interactions inherent in the region of northeast pacific basin the approach described here has significant potential for determining the water mass distribution and may lead to an explanation of the sverdrup s eastern gyral in the subtropical north pacific

interdecadal thermocline variability in the north pacific for 1958 97 a gcm simulation an ocean general circulation model is forced with the ncep reanalysis wind stress for 1958 97 to understand mechanisms of ocean subsurface variability with relatively high horizontal 1 83 18 and vertical 41 levels resolutions the model produces mode waters on a range of density surfaces in the western central and eastern north pacific in qualitative agreement with observations these mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the kuroshio and its extension in the model subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad in contrast to what might be expected from the linear baroclinic rossby wave theory this variance maximum is associated with the longitudinal shift in the path of mode waters in particular deepened mixed layer and accelerated eastward currents in the kuroshio extension by wind changes in the mid 1970s act cooperatively to move the central mode waters toward the east causing large subsurface temperature anomalies besides the local maximum in the central north pacific subtropical gyre two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre respectively among these subsurface anomaly centers the one in the northwestern north pacific has a strong effect on the model sea surface temperature suggesting that the kuroshio and its extension are a key region to decadal interdecadal ocean atmosphere interaction finally possible effects of atmospheric thermal forcing are discussed

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

stratigraphic placement and age of modern humans from kibish ethiopia in 1967 the kibish formation in southern ethiopia yielded hominid cranial remains identified as early anatomically modern humans assigned to homo sapiens however the provenance and age of the fossils have been much debated here we confirm that the omo i and omo ii hominid fossils are from similar stratigraphic levels in member i of the kibish formation despite the view that omo i is more modern in appearance than omo ii 40ar 39ar ages on feldspar crystals from pumice clasts within a tuff in member i below the hominid levels place an older limit of 198 14 kyr weighted mean age 196 2 kyr on the hominids a younger age limit of 104 7 kyr is provided by feldspars from pumice clasts in a member iii tuff geological evidence indicates rapid deposition of each member of the kibish formation isotopic ages on the kibish formation correspond to ages of mediterranean sapropels which reflect increased flow of the nile river and necessarily increased flow of the omo river thus the 40ar 39ar age measurements together with the sapropel correlations indicate that the hominid fossils have an age close to the older limit our preferred estimate of the age of the kibish hominids is 195 5 kyr making them the earliest well dated anatomically modern humans yet described

chow bahir rift a failed rift in southern ethiopia the chow bahir rift system is a major graben in a 300 km broad rift zone recognized in southern ethiopia between the kenyan and ethiopian domes where the east african rift is not well defined an extinct failed rift discovered along the omo canyon to the north and on strike with the chow bahir rift ceased activity about 4 m y ago chow bahir is in a younger stage of abandonment as the main ethiopian rift propagates south into this region

scale in tephrostratigraphic correlation an example from turkish pleistocene archaeological sites issues of scale are persistent but rarely made explicit in tephrostratigraphy eruption magnitude the nature of available outcrop and the resolution and type of data available define the scale of any proposed correlation different scales require different approaches to demonstrate or more accurately to estimate the likelihood of correlation among two or more deposits this is emphasized through a probabilistic approach to the correlation of proximal and distal tephra deposits associated with lower and middle paleolithic archaeological sites from central turkey through the use of discriminant analyses and multiple analyses of variance

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

atlantic mediterranean water exchange during the late neocene the nature of water exchange between the mediterranean and the atlantic is examined for three time periods during the late neogene in order to assess the impact of flow patterns between these basins on bottom water conditions in the oceans the large volume of carbonates and evaporites deposited in the mediterranean during the messinian salinity crisis appears to have caused the open ocean to become more undersaturated in calcium carbonate resulting in increased dissolution of deep sea carbonates during the latest miocene during the earliest pliocene an estuarine type of flow existed between the atlantic and mediterranean waters entered the mediterranean at depth and flowed out at the surface benthic foraminiferal stable isotopic evidence from the north atlantic indicates that north atlantic deep water nadw was produced throughout the early pliocene and that the absence of saline mediterranean outflow did not have a significant effect on bottom water production eustatic lowering of sea level during the last glacial maximum 18 000 years b p reduced water exchange between the atlantic and the mediterranean as a result salinities in the mediterranean increased substantially but it seems unlikely that diminished nadw production during the last glacial episode can be attributed to a reduction in mediterranean outflow

provenance of the early mesoproterozoic radium creek group in the northern mount painter inlier correlating isotopic signatures to inform tectonic reconstructions abstract new in situ zircon la icpms geochronologic and hf isotope data from the radium creek group within the mount painter inlier provide important temporal constraints on the early mesoproterozoic palaeogeography of eastern proterozoic australia the entire radium creek group was deposited in a single basin forming phase and has a maximum depositional age of 1595 3 7 ma detrital zircon from these metasedimentary rocks have u pb age populations at ca 1595 ma 1660 1680 ma 1710 1780 ma ca 1850 ma and ca 2500 ma these grains are characterised by isotopically diverse and evolved sources and have crystallised within predominantly felsic igneous host rocks the relative age spectra and isotopic character has more similarity with the gawler craton than the arunta block curnamona province or the mount isa inlier these observations suggest that the mount painter province was adjacent to the gawler craton in the early mesoproterozoic our data supports a coherent south australian craton at ca 1595 ma and a contiguous continental mass that included the north and south australian cratons the mount painter inlier occupied a complex plate tectonic setting in the overriding plate of two convergent margins

the menengai tuff a 36 ka widespread tephra and its chronological relevance to late pleistocene human evolution in east africa abstract the east african rift preserves the world s richest middle and late pleistocene 780 12 ka geological archaeological and paleontological archives relevant to the emergence of homo sapiens this region also provides unparalleled chronological control for many important sites through tephrochronology the dating and correlation of volcanic ashes as widespread isochronous markers in the geological record there are many well characterized pliocene early pleistocene tephras that are widespread across east africa a comparable framework is lacking for the middle and late pleistocene a period characterized by spatially and temporally complex patterns of climate change as well as the emergence of modern homo sapiens and the dispersal of this species across and out of africa unraveling relationships among these spatial and temporally complex phenomena requires a precise chronology to this end we report the menengai tuff a widespread volcanic ash produced by the large scale caldera forming eruption in kenya and 40 ar 39 ar dated to 35 62 0 26 ka geochemical characterization of 565 glass shards from 36 samples by wavelength dispersive electron probe microanalysis show the menengai tuff was deposited over 115 000 km 2 and is found in the baringo chalbi elmenteita nakuru olorgesailie turkana and victoria basins all of which preserve rich late pleistocene paleoenvironmental and archaeological archives correlation and dating of the menengai tuff demonstrate that it is the most widespread tephra and largest eruption currently known from the late pleistocene of east africa as such it is a valuable marker in establishing a late pleistocene chronology for paleoclimatic archeological and paleontological records relevant to the study of human evolution

the role of the mixed layer in setting the potential vorticity of the main thermocline abstract a steady ventilation model is used to assess the effect of the mixed layer on the structure of the main thermocline the potential vorticity is found in a subtropical gyre after imposing the thickness and density of the mixed layer the ekman pumping and the hydrography on the eastern boundary the modeled potential vorticity becomes comparable in value to observations in the north atlantic if the mixed layer deepens poleward as is observed in winter the isopycnal gradients in potential vorticity are reduced on the denser ventilated surfaces if the mixed layer outcrops deviate from latitude circles and more realistically sweep southward along the eastern boundary the age of the subducted fluid is also in reasonable agreement with observations of the tritium helium age by jenkins this study suggests that ventilation may form much of the extensive region of nearly uniform potential vorticity observed on the 26 75 surface in the north atlantic with lateral mixing by eddies being required

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

subduction of north pacific central mode water associated with subsurface mesoscale eddy 1 during a shipboard high density hydrographic survey carried out in the western north pacific in fall 2008 we observed an anticyclonic eddy with a thickness of 150 dbar and a diameter of 40 km near 500 dbar depth at 27 5 n 145 e this subsurface mesoscale eddy contains the north pacific central mode water cmw which has anomalously low potential vorticity and high dissolved oxygen compared to the climatological cmw properties in the same region profiling float measurements detect similar cmw patches near and south of the kuroshio extension as well as southward cmw migration within the cmw formation region north of the kuroshio extension these observed facts suggest that cmw is subducted into the permanent pycnocline not only through large scale eastward advection near the northern edge of the subtropical gyre but also through southward cross frontal advection associated with the formation and migration of subsurface mesoscale eddies

a north pacific decadal variability in subduction rate 1 analysis of results from a global eddy resolving general circulation model has revealed the existence of a north pacific decadal variability in subduction rate this decadal variability corresponds well with the pacific decadal oscillation pdo the zero lag correlation between the two time series reaches 0 61 for the period of integration 1950 2003 and increases to as high as 0 80 after the climate shift in the mid 1970s much of the north pacific decadal variability in subduction rate is due to changes in winter mixed layer depth which in turn are closely related to changes in surface wind and heat flux

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

correlation of pliocene and pleistocene tephra layers between the turkana basin of east africa and the gulf of aden abstract electron microprobe analyses of glass shards from volcanic ash in pliocene and pleistocene deep sea sediments in the gulf of aden and the somali basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the turkana basin of northern kenya and southern ethiopia previous correlations are reviewed and new correlations proposed together these data provide correlations between the deep sea cores and to the land based sections at eight levels ranging in age from about 4 to 0 7 ma specifically we correlate the moiti tuff 4 1 ma with a tephra layer at 188 6 m depth in dsdp hole 231 and with a tephra layer at 150 m depth in dsdp hole 241 the wargolo tuff with a tephra layer at 179 7 m in dsdp hole 231 and with a tephra layer at 155 3 m depth in dsdp hole 232 the lomogol tuff defined here with a tephra layer at 165 m in dsdp hole 232a the lokochot tuff with a tephra layer at 140 1 m depth in dsdp hole 232 the tulu bor tuff with a tephra layer at 160 8 m depth in dsdp hole 231 the kokiselei tuff with a tephra layer at 120 m depth in dsdp hole 231 and with a tephra layer at 90 3 m depth in dsdp hole 232 the silbo tuff 0 74 ma with a tephra layer at 35 5 m depth in dsdp hole 231 and possibly with a tephra layer at 10 9 m depth in dsdp hole 241 we also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known the correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep sea sites such correlations may make it possible to interpret faunal evolution in the lake turkana basin and other sites in east africa within a broader regional or global paleoclimatic context

eddy activity in the four major upwelling systems from satellite altimetry 1992 2007 eddy activity in the four major eastern boundary upwelling systems ebus is investigated using 15 years of satellite altimetry data based on the analysis of more than 4000 long lived eddy trajectories in every ebus we show that mesoscale structures are mainly generated along the continental coasts and south of the main archipelagos and propagate westward with velocities increasing toward the equator these mesoscale eddies having radii of 70 160 km are then frequently observed along the coastal transition zones and frontal regions and some large oceanic areas are preferentially populated by cyclonic or anticyclonic eddies temporal variations of the number of newly formed eddies and the associated eddy activity index defined as the mean eddy energy density are finally examined at seasonal and interannual scales the strongest seasonal interannual respectively variations are observed in the california benguela upwelling systems the proposed indices also exhibit contrasted long term trends in each ebus which suggests that eddy activity might be sensitive to a warming climate

gypsum precipitation from cold brines in an anoxic basin in the eastern mediterranean in 1968 hot brines from which gypsum had precipitated were discovered in the red sea in a divergent plate boundary setting with hydrothermal activity1 we now report the discovery of cold brines in an entirely different geodynamic situation a convergent plate boundary in the eastern mediterranean where the heatflow is very low2 and no hydrothermal activity occurs decimetric pure gypsum crystals precipitate from the cold brines which occupy the bottom of a rimmed anoxic basin near the southern edge of the mediterranean ridge close to the sirte abyssal plain nucleation of the crystals does not occur at the surface but from the deep brines because the salinity of the water is normal down to the depth of 3 000 m we propose the name bacino bannock for this basin after the consiglio nazionale delle ricerche r v bannock from which the bottom water brines were surveyed and sampled

age of the okote tuff complex at koobi fora kenya the okote tuff complex crops out in the koobi fora region next to the north eastern shores of lake turkana northern kenya it occurs in the upper part of the 450 m thick essentially flat lying plio pleistocene sequence of fluvial deltaic and lacustrine sediments comprising the koobi fora formation1 3 many important archaeological sites and hominid fossils including the homo erectus finds discussed in the accompanying papers4 5 are stratigraphically close to the level of the okote tuff complex6 or its correlatives so that its precise numerical age is of great interest previously published isotopic ages mainly by the 40ar 39ar dating technique exhibited a very wide scatter7 here we report comprehensive k ar data showing that pumices within one of the tuffs of the okote tuff complex have an age of 1 64 0 03 myr which is in the earliest pleistocene8

organic carbon accumulation in bannock basin evaluating the role of productivity in the formation of eastern mediterranean sapropels abstract three sediment cores from bannock basin a hypersaline anoxic basin in the eastern mediterranean have been studied to evaluate the importance of productivity on organic carbon accumulation and sapropel formation in this region organic carbon accumulation rates were determined for two cores from within the anoxic hypersaline zone and for one core from the oxygenated portion of this basin organic carbon accumulation rates are higher by an order of magnitude in the surficial sediments of the anoxic zone due to better preservation an empirically derived preservation factor which allows calculation of paleoproductivity during the deposition of the most recent sapropel si indicates that productivity increased by an order of magnitude during the formation of si we conclude that increased productivity was coupled with a density stratification and a circulation reversal in the eastern mediterranean which together led to sapropel formation

the origin of rhythmic bedding in the pliocene trubi formation of sicily southern italy abstract foraminiferal palynological stable isotope geochemical clay mineralogical and granulometric data have been gathered for the successive layers of four rhythmic alterations of relatively carbonate poor and carbonate rich beds in the early pliocene trubi formation of sicily southern italy the marly rhythmites consist of a grey layer and a white more indurated layer in which an additional beige less indurated layer is intercalated they are therefore more complex than the well known marl limestone couples elsewhere these rhythmites probably originate from the alternate predominance of increased dilution of river supplied terrigenous material grey and decreased carbonate production in the surface waters beige the grey layers are considered to correspond to periods of increased local run off and higher surface water temperatures and productivity the beige layers signal more arid and cooler climatic conditions lower surface water productivity and stronger or more frequent dust loaded southern winds the white layers apparently reflect intermediate climatic and environmental conditions these periodical changes probably brought about by astronomical precession appear to have occurred not only during the pliocene but also during the late miocene

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

average circulation seasonal cycle and mesoscale dynamics of the peru current system a modeling approach h isolines the pccc appears to be directly forced by the cyclonic wind stress curl the model is able to produce the upwelling front the cold water tongue which extends toward the equator and the equatorial front as described in the literature model seasonal changes in sst and ssh are compared to measurements for the central pcs model eke is 10 to 30 lower than the observations the model eddy diameters follow a strong equatorward increase the injection length scales derived from the energy spectra strongly correlate to the rossby radius of deformation confirming the predominant role of baroclinic instability at 3 s the model solution appears to switch from a turbulent oceanic regime to an equatorial regime dominated by zonal currents

a role of eddies in formation and transport of north pacific subtropical mode water 1 hydrographic data acquired during 2001 by argo profiling floats deployed in the kuroshio recirculation region are used to verify the inference that mesoscale eddies prevailing in the recirculation region play an important role in the formation and transport of north pacific subtropical mode water npstmw that is the deeper winter mixed layer is formed preferentially where the thermocline is deeper in association with anticyclonic eddies in the succeeding seasons mesoscale eddies retain npstmw during their southwestward movement from the npstmw formation region so that anticylonic eddies contribute substantially to the npstmw transportation the spatial distributions of the mixed layer depth in winter and the npstmw thickness in the succeeding seasons based on the float data suggest that they are affected by mesoscale eddies the float data also provide the statistical relations of these quantities against the thermocline depth which are consistent with our inference

the occurrence and significance of pleistocene and upper pliocene sapropels in the tyrrhenian sea numerous sapropels and sapropelic strata from upper pliocene and pleistocene hemipelagic sediments of the tyrrhenian sea show that intermittent anoxia possibly related to strongly increased biological productivity was not restricted to the eastern mediterranean basins and may be a basin wide result of late pliocene pleistocene climatic variability even though the sapropel assemblage of the tyrrhenian sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic rich suspensates many pelagic sapropels have been recognized stratigraphic ages calculated for the organic rich strata recovered during odp leg 107 indicate that the frequency of sapropel formation increased from the lowermost pleistocene to the base of the jaramillo magnetic event coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the mediterranean a second very pronounced peak in sapropel formation occurred in the middle to late pleistocene 0 73 0 26 ma formainifers studied in three high resolution sample sets suggest that changes in surface water temperature may have been responsible for establishing anoxic conditions while salinity differences were not noted in the faunal assemblage however comparison of sapropel occurrence at site 653 with the oxygen isotopic record of planktonic foraminifers established by thunell et al proc odp sci results 107 1990 indicates that sapropel occurrences coincide with negative 18o excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in hole 653a a variant of the meltwater hypothesis accepted for sapropel formation in the late pleistocene eastern mediterranean may thus be the cause of several anoxic events in the tyrrhenian as well model calculations indicate that the amount of oxygen advection from western mediterranean deep water exerts the dominant control on the oxygen content in deep water of the tyrrhenian sea inhibition of deep water formation in the northern adriatic and the balearic basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the tyrrhenian sea

building a tephrostratigraphic framework for the paleolithic of central anatolia turkey the central anatolian volcanic province cavp in turkey preserves widespread deposits of quaternary tephra presently associated with a small but growing number of paleolithic archaeological sites we use multivariate analyses of the abundances of a suite of nine major and minor element oxides determined by electron probe microanalysis from these data we construct a classificatory model for correlating distal tephra to one of five volcanic edifices or eruptive phases within the cavp application of this model to distal deposits of primary tephra fall and reworked tuffaceous sediments from the archaeological sites of korkuyu and kaletepe deresi 3 indicates late to possibly early pleistocene ages for the artifact assemblages there and provides a fundamental tephrostratigraphic framework to examine spatial and temporal variation in hominin behavior comparable to that of other regions such as eastern africa

tracks made by swimming hippopotami an example from koobi fora turkana basin kenya here we report an ichnological surface close to koobi fora kenya in palaeontological collecting area 103 the surface is marked by hominin tracks as well as many traces from large animals a southern excavation of the surface some 70 m from the hominin tracks displays a diverse range of animal track typologies most of which appear to have been made by a four digit animal moving via punting or bottom walking in a shallow water body due to the track morphology and the associated fossil record the non hominin tracks are interpreted as being made by hippopotami potentially including pygmy species or juveniles the track typologies are explained using modern analogue observations of hippopotami sub aquatic locomotion this work provides important environmental context for adjacent hominin tracks and fossils as well as providing the first recorded description of fossilized swim tracks made by mammals the site has implications for the interpretation of swim tracks in the geological record particularly the widespread and controversial tracks made by sauropods and other dinosaurs

the influence of air sea interaction on the ventilated thermocline abstract air sea interaction influences the ventilated thermocline by forcing the mixed layer to deepen and cool poleward when there is flow from the mixed layer into the interior the mixed layer depth and density fields help to set the potential vorticity of the subducted fluid the importance of this process is assessed by incorporating a depth varying mixed layer in a ventilation model which is forced by ekman pumping and implied surface heating the formulation of the ventilation problem is simplified by only allowing density surfaces to outcrop along latitude circles and by assuming that there is no zonal inflow along the eastern boundary the surface heating enables a cross isopycnal flow within the mixed layer the volume of ventilated fluid within the subtropical gyre is increased by including the depth varying mixed layer and this fluid partly originates from the western boundary as well as from the ekman layer the depth varying mixed layer increases the depth at which isopycnals are subduc

pliocene and pleistocene hominid bearing sites from west of lake turkana kenya pliocene and pleistocene fossil localities near the western shoreline of lake turkana ranging in age between 1 million and 3 5 million years in age have produced important new hominid specimens including most of a homo erectus skeleton and a relatively complete early robust australopithecine cranium the lacustrine fluviatile and terrestrial strata are designated the nachukui formation which is subdivided into eight members the distribution of sedimentary facies within the nachukui formation suggests that as today the labur and murua rith ranges formed the western margin of the basin and were drained by eastward flowing rivers that fed into the forerunner of the present lake or a major river system there is also stratigraphic evidence for tectonic movement during the deposition of these sediments twenty three of the tuffs observed in the succession occur also in the koobi fora formation east of the lake and in the shungura formation of the lower omo valley and permit precise correlation among these three localities fortyseven fossiliferous sites from west turkana have yielded more than 1000 specimens of 93 mammalian species the mammalian fossils represent nine sequential assemblages that augment information about faunal and environmental change from elsewhere in the basin

african climate change and faunal evolution during the pliocene pleistocene abstract environmental theories of african faunal evolution state that important evolutionary changes during the pliocene pleistocene interval the last ca 5 3 million years were mediated by changes in african climate or shifts in climate variability marine sediment sequences demonstrate that subtropical african climate periodically oscillated between markedly wetter and drier conditions paced by earth orbital variations with evidence for step like 0 2 ma increases in african climate variability and aridity near 2 8 ma 1 7 ma and 1 0 ma coincident with the onset and intensification of high latitude glacial cycles analysis of the best dated and most complete african mammal fossil databases indicates african faunal assemblage and perhaps speciation changes during the pliocene pleistocene suggesting more varied and open habitats at 2 9 2 4 ma and after 1 8 ma these intervals correspond to key junctures in early hominid evolution including the emergence of our genus homo pliocene pleistocene shifts in african climate vegetation and faunal assemblages thus appear to be roughly contemporary although detailed comparisons are hampered by sampling gaps dating uncertainties and preservational biases in the fossil record further study of possible relations between african faunal and climatic change will benefit from the accelerating pace of important new fossil discoveries emerging molecular biomarker methods for reconstructing african paleovegetation changes tephra correlations between terrestrial and marine sequences as well as continuing collaborations between the paleoclimatic and paleoanthropological communities

late quaternary climatic change in france estimated from multivariate pollen time series abstract in regions like southern france where usual analytical techniques are unsatisfactory due to the heavy influence of human activity and the existence of a complex climate the quantitative reconstruction of climate from pollen is particularly difficult that is why an original method has been performed the first step of the method based on best analogs estimation and multiple regression is to calculate a relationship between climate monthly temperature and precipitation and modern pollen spectra for data from 182 sites the result is called the analog climate the second step of the method an original combination of canonical correlation and principal components analyses extracts the common information from several fossil pollen sequences to produce a so called paleobioclimate this step removes the effects of local differences in the vegetation among nearby sites and also reduces the effects of human disturbance in the third step the signals obtained from the first two steps are merged by kalman filter into a final reconstruction where the noise is reduced by around 37 the results presented with a 95 confidence level suggest that the pleni wurm was 7 to 13 c colder than present and had 20 to 60 less precipitation than today this large confidence takes into account the climatic diversity of the region at 13 000 yr b p the climate reached the modern level of precipitation but the temperature was just less than present the subsequent decrease of precipitation is less important than that of temperature this fact may explain the advance of alpine glaciers during the younger dryas preboreal warming was abrupt 3 to 4 c per 500 yr and precipitation increased more slowly

ventilation of the north atlantic and north pacific subduction versus obduction abstract ventilation in the north atlantic and north pacific is examined by analyzing the levitus climatological data and the hellerman and rosenstein wind stress data ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes subduction and obduction subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above veatilation in the north atlantic and north pacific can be clarified into four physically different regions the subductive region the obductive region the ambiductive region where both subduction and obduction take place and the insulated region where neither subduction nor obduction occurs although the total subduction rates in these two oceans are comparable the total obduction rates are considera

mesoscale eddies off peru in altimeter records identification algorithms and eddy spatio temporal patterns relatively little is known about coherent vortices in the eastern south pacific along the peruvian coast even with regard to basic facts about their frequency of occurrence longevity and structure this study addresses these issues with nearly 15 years of relatively high resolution satellite altimetry measurements we first compare two distinct automated methods for eddy identification the objective validation protocol shows that the rarely used geometrical or winding angle method based on the curvature of the streamline functions is more accurate than the commonly used okubo weiss algorithm which defines a vortex as a simple connected region with values of okubo weiss parameter weaker than a given threshold we then investigate vortices off peru using more than 20 000 mesoscale eddies identified by the winding angle method coherent eddies characterized by a high ratio of vorticity to deformation rate are typically formed along the coast and propagate westward at 3 6 cm s 1 the vortices have a mean radius of 80 km increasing northward and are most frequently observed off of chimbote 9s and south of san juan 15s the mean eddy lifetime is about 1 month but if eddies survive at least 2 months the probability for surviving an additional week or month is constant at 90 or 67 anticyclonic eddies tend to propagate northwestward whereas cyclonic vortices migrate southwestward in general cyclones and anticyclones are similar except for eddies surviving at least 6 months in this case after a similar 3 4 months of radius and amplitude growth amplitudes or sizes decay particularly rapidly for anticyclonic or cyclonic eddies in terms of intensity cyclonic eddies show a rapid decay during the first 3 months before arriving at a quasi constant value whereas anticyclones exhibit steady decline finally eddy temporal variations were examined at seasonal and interannual scales in the coastal region favorable to the formation of energetic mesoscale structures on seasonal scales eddy activity is maximal in fall and minimum in spring at interannual scales the eddy activity index was maximal during the strong el nino of 1997 1998 but another strong maximum of eddy activity also occurred late in 2004 these temporal variations are probably associated with the intensification of the upwelling thermal front and with the passage of coastal trapped waves which generate baroclinic instabilities further investigation of the mechanisms involved on the eddy genesis is needed

effects of mesoscale eddies on subduction and distribution of subtropical mode water in an eddy resolving ogcm of the western north pacific abstract the effects of mesoscale eddies on the subduction and distribution of the north pacific subtropical mode water stmw are investigated using an eddy resolving ocean general circulation model ogcm first the subduction rate is calculated and the contribution of eddies to the subduction of stmw is estimated it is found that eddy subduction significantly contributes to the total subduction of stmw second eddy thickness transport and diapycnal flux are directly diagnosed to investigate the large scale eddy induced transport process of stmw the large southward eddy thickness transport in the stmw core density is consistent with eddy subduction the eddy transport on the isopycnal surface of stmw is directed down the thickness gradient and traverses the mean flow the meridional eddy transport streamfunction indicates two eddy circulation cells south of 30 n associated with the circulation of stmw these cells flatten density surfaces similar to the effect of the gent and mcwilliams gm schem

effect of mesoscale eddies on subtropical mode water variability from the kuroshio extension system study kess abstract forty eight profiling floats have been deployed in the kuroshio extension ke region since may 2004 as part of the kuroshio extension system study kess project by combining the float temperature salinity measurements with satellite altimetry data this study investigates the role played by mesoscale eddies in controlling the property changes in north pacific subtropical mode water stmw following a 3 yr period of low eddy activity in 2002 04 the ke showed a transition to a high eddy kinetic energy state in 2005 this transition is the result of delayed oceanic response to the 2002 shift in the basin scale surface wind forcing in connection with the pacific decadal oscillation the high eddy kinetic energy state of the ke is characterized by successive shedding of strong cold core rings into the recirculation gyre resulting from the interaction of the ke jet with the shatsky rise or the preexisting cutoff rings by transporting northern origin high potential vorticity pv ke water into t

mesoscale eddies in the south china sea mean properties spatiotemporal variability and impact on thermohaline structure we investigated mean properties and the spatiotemporal variability of eddies in the south china sea scs by analyzing more than 7000 eddies corresponding to 827 eddy tracks identified using the winding angle method and 17 years of satellite altimetry data eddies are mainly generated in a northeast southwest direction and southwest of luzon strait there is no significant difference between the numbers of two types of eddies anticyclonic and cyclonic in most regions the mean radius and lifetime of eddies are 132 km and 8 8 weeks respectively both depending on where the eddies are formed anticyclonic and cyclonic eddies tend to deform during their lifetimes in different ways furthermore eddy propagation and evolution characteristics are examined in the northern scs eddies mainly propagate southwestward along the continental slope with velocities of 5 0 9 0 cm s 1 while in the central scs eddies tend to move with slight divergence but still in a quasi westward direction with velocities of 2 0 6 4 cm s 1 eddy propagation in the western basin to the east of vietnam is quite random with no uniform propagate direction investigation of 38 long lived eddies shows that eddies have a swift growing phase during the first 12 weeks and then a slow decaying phase that affects the eddy radii and eddy energy densities nevertheless vorticity has less variability in addition the effect of eddies on the thermocline and halocline is analyzed using 763 argo temperature profile data cyclonic eddies drive the thermocline shallower and thinner and significantly strengthen the thermocline intensity whereas anticyclonic eddies cause the thermocline to deepen and thicken and weaken the thermocline intensity to a certain degree the halocline impacted by cyclonic eddies is also shallower and thinner than that impacted by anticyclonic eddies finally eddy temporal variations are examined at seasonal and interannual scales eddy activity is sensitive to the wind stress curl and in the northern scs it is also related with the strength of the background flows

eastern north pacific subtropical mode water in a general circulation model formation mechanism and salinity effects the eastern north pacific subtropical mode water estmw is a water mass of low potential vorticity pv and appears as a weak pycnostad or thermostad distinct from other subtropical mode waters it forms in the absence of a deep winter mixed layer the formation mechanism of this estmw is investigated using an ocean general circulation model that is forced by monthly climatological temperature salinity and wind stress at the sea surface an equation based on the ventilated thermocline theory is used to diagnose the initial pv of a water mass right after its subduction in this equation three factors affect the initial pv the spacing of density outcrop lines the mixed layer depth gradient and the vertical velocity at the bottom of mixed layer among them the wide spacing between outcrop lines is the most important for estmw s low pv instead of the deep mixed layer which is most important for classical mode waters it is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low pv formation a low salinity tongue that extends southeastward off north america is responsible for the small surface density gradient in the eastern north pacific and contributes to the formation of the estmw an additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low salinity tongue

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

mediterranean quaternary sapropels an immediate response of the african monsoon to variation of insolation abstract in the subsurface of the east mediterranean sea eleven discrete basin wide organic rich black sapropels were deposited during the last 465 000 years they are dated by oxygen isotopic chronostratigraphy the most recent sapropel has been shown to be the consequence of heavy nile river summer floods due to increased monsoonal summer precipitation over ethiopia and not of scandinavian ice sheet meltwater influx into the east mediterranean sea as was previously believed while most sapropels were deposited during interglacials two thick sapropels formed during glacial periods as shown by their position in the oxygen isotopic stratigraphy and by their pollen record in core rc9 181 the new evidence presented here is that the sapropel sequence in this core including the two glacial sapropels correlates on a one to one basis with the highest values of an index of insolation on top of the atmosphere the concept of this index derives from an analysis of the african monsoon process the index variation is computed for the last 465 000 years from the caloric northern summer insolation in the tropical latitudes controlled by variations of the earth s orbital parameters it is conjectured that this index might be linked through the climate system to the monsoonal precipitation which in africa produces the nile river summer flood

three dimensional structure of the wind driven circulation in the subtropical north pacific abstract the subduction rate is calculated for the north pacific based on levitus climatology data and hellerman and rosenstein wind stress data because the period of effective subduction is rather short subduction rates calculated in eulerian and lagrangian coordinates are very close the subduction rate defined in the lagrangian sense consists of two parts the first part is due to the vertical pumping along the one year trajectory and the second part is due to the difference in the winter mixed layer depth over the one year trajectory since the mixed layer is relatively shallow in the north pacific the vertical pumping term is very close to the ekman pumping while the sloping mixed layer base enhances subduction especially near the kuroshio extension for most of the subtropical north pacific the subduction rate is no more than 75 m yr 1 slightly larger than the ekman pumping the water mass volume and total amount of ventilation integrated for each interval of 0 2 unit is computed the corre

oldowan reduction sequences methodological considerations abstract transport of resources is a major feature of oldowan hominin technological adaptations comparisons between different oldowan localities often employ measures of transport that are based on artefact attributes as proxies for the intensity of raw material utilization the technological flake category system toth n 1985 oldowan reassessed a close look at early stone artifacts journal of archaeological science 12 pp 101 120 has been used extensively to infer the relative intensity of lithic reduction within oldowan assemblages here we use a large experimental sample to test the relationship between a flake s stage in a reduction sequence and various quantitative attributes we demonstrate how many previously described attributes are affected by initial core size we then develop a multiple linear regression model that incorporates several variables to predict the placement of a flake within a generalized reduction sequence the model is then applied to oldowan assemblages in the koobi fora formation which explores the strengths and weaknesses of different methods of investigating reduction intensity on an assemblage level

subtropical countercurrent in an idealized ocean gcm abstract in the north pacific there is a shallow eastward current called the subtropical countercurrent which flows across the central subtropical gyre the present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model gcm with a simple geometry driven by surface wind stress and surface buoyancy forcing in the ocean gcm the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre the mixed layer front the narrow transition zone of the mixed layer depth slants from the western central subtropical gyre to the northeast and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line since the surface density is almost zonally uniform and the mixed layer front slants northeastward the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region while those on lighter isopycnals are form

subtropical mode water in the northeast pacific basin a new type of mode water in the upper thermocline of the eastern subtropical north pacific is identified and examined using data from world ocean circulation experiment high resolution repeat expendable bathythermograph xbt section px37 and archives of historical xbt data this water mass labeled eastern subtropical mode water is characterized by a subsurface potential vorticity minimum and is located east of hawaii northeast pacific basin in a density range of 24 25 4 it is a distinct water mass from the classical subtropical mode water stmw of the western pacific eastern stmw is formed as a relatively deep late winter mixed layer associated with the subtropical subpolar water mass boundary near 25 30 n 135 140 w and is capped and subducted into the permanent thermocline along a section between san francisco and honolulu eastern stmw production is seen in every year for which there is adequate data in this section the volume of eastern stmw formed each winter and the temperature of the potential vorticity minimum are similar during the periods 1970 1979 and 1991 1997

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels