vertical mixing in the ocean the thermohaline circulation of the ocean results primarily from downwelling at sites in the nordic and labrador seas and upwelling throughout the rest of the ocean the latter is often described as being due to breaking internal waves here we reconcile the difference between theoretical and observed estimates of vertical mixing in the deep ocean by presenting a revised view of the thermohaline circulation which allows for additional upwelling in the southern ocean and the separation of the north atlantic deep water cell from the antarctic bottom water cell the changes also mean that much less wind and tidal energy needs to be dissipated in the deep ocean than was originally thought

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

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

formation of forearc basins by collision between seamounts and accretionary wedges an example from the new hebrides subduction zone seabeam data reveal two deep subcircular reentrants in the lower are slope of the new hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin the malekula reentrant lies just south of the partly subducted bougainville seamount this proximity as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off japan suggests that the malekula reentrant formed by the collision of a seamount with the arc an arcuate fold thrust belt has formed across the mouth of the reentrant forming the toe of a new accretionary wedge the efate reentrant may show the next stage in basin development this reentrant lies landward of a lower slope ridge that may have begun to form as an arcuate fold thrust belt across the mouth of a reentrant this belt may have grown by continued accretion at the toe of the wedge by underplating beneath the reentrant and by trapping of sediment shed from the island arc these processes could result in a roughly circular forearc basin basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the aleutian trenches

why the itcz is mostly north of the equator abstract although the distribution of sunshine is symmetrical about the equator the earth s climate is not climatic asymmetries are prominent in the eastern tropical pacific and atlantic oceans where the regions of maximum sea surface temperature convective cloud cover and rainfall are north of the equator this is the result of two sets of factors interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located the ocean atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions the thermocline happens to shoal in the eastern equatorial pacific and atlantic but not in the eastern indian ocean because easterly trade winds prevail over the tropical atlantic and pacific wher

eddy saturation of equilibrated circumpolar currents abstractthis study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in southern ocean wind stress and global diapycnal mixing at eddy permitting and finer resolution the sensitivity of circumpolar transport to forcing magnitude is drastically reduced at sufficiently high resolution there is little or no sensitivity of circumpolar transport to wind stress even in the limit of no wind in contrast the meridional overturning circulation continues to vary with southern ocean wind stress but with reduced sensitivity in the limit of high wind stress both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions the circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution although sensitivity always remains in contrast the overturning circulation is more sensitive to change in diapycnal

a zonally averaged ocean model for the thermohaline circulation part i model development and flow dynamics abstract a two dimensional latitude depth ocean model is developed on the basis of the zonally averaged balance equations of mass momentum heat and salt its purpose is to investigate the dynamics and variability of the buoyancy forced thermohaline circulation for the time scales of interest an annually averaged model is selected and the momentum balance is taken to be diagnostic the east west pressure gradient which arises upon zonally averaging the momentum equations is parameterized in terms of the meridional pressure gradient the thermohaline circulation is driven by mixed surface boundary conditions i e temperatures are relaxed to prescribed values while the salt flux is held constant the dynamics of the flow is investigated in hemispheric and global geometries for both short and long time integrations the latter extending over many thousands of years as has been noted by previous investigators it is possible to perturb a steady state such that a diffusively dominated regime results

numerical modeling of the global semidiurnal tide in the present day and in the last glacial maximum 1 a hydrodynamic model incorporating a self consistent treatment of ocean self attraction and loading sal and a physically based parameterization of internal tide it drag is used to assess how accurately barotropic tides can be modeled without benefit of data and to explore tidal energetics in the last glacial maximum lgm m2 solutions computed at high resolution with present day bathymetry agree with estimates of elevations from satellite altimetry within 5 cm rms in the open ocean this accuracy and agreement with atlimetric estimates of energy dissipation are achieved only when sal and it drag are included in the model solutions are sensitive to perturbations to bathymetry and inaccuracies in available global databases probably account for much of the remaining error in modeled elevations the 100 m drop in sea level during the lgm results in significant changes in modeled m2 tides with some amplitudes in the north atlantic increasing by factors of 2 or more dissipation is also significantly changed by the drop in sea level if it drag estimated for the modern ocean is assumed dissipation increases by about 50 globally and almost triples in the deep ocean however it drag depends on ocean stratification which is poorly known for the lgm tests with modified it drag suggest that the tendency to a global increase in dissipation is a robust result but details are sensitive to stratification significant uncertainties about paleotides thus remain even in this comparatively simple case where bathymetry is well constrained

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

evolution of the eastern tropical pacific through plio pleistocene glaciation a tropical pacific climate state resembling that of a permanent el nino is hypothesized to have ended as a result of a reorganization of the ocean heat budget 3 million years ago a time when large ice sheets appeared in the high latitudes of the northern hemisphere we report a high resolution alkenone reconstruction of conditions in the heart of the eastern equatorial pacific eep cold tongue that reflects the combined influences of changes in the equatorial thermocline the properties of the thermocline s source waters atmospheric greenhouse gas content and orbital variations on sea surface temperature sst and biological productivity over the past 5 million years our data indicate that the intensification of northern hemisphere glaciation 3 million years ago did not interrupt an almost monotonic cooling of the eep during the plio pleistocene sst and productivity in the eastern tropical pacific varied in phase with global ice volume changes at a dominant 41 000 year obliquity frequency throughout this time changes in the southern hemisphere most likely modulated most of the changes observed

plio pleistocene climate evolution trends and transitions in glacial cycle dynamics abstract we describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5 3 myr in particular we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic 18 o when the exponential trend in 18 o variance is removed its spectral power exhibits strong proportional responses to amplitude modulations in orbital forcing over most of the plio pleistocene precession responses correlate with modulations in forcing for the last 5 myr but 41 kyr response is sensitive to obliquity modulation only before 1 4 myr where responses are sensitive to modulations in forcing we demonstrate that glacial cycles are orbitally forced rather than being self sustained or paced by orbital changes the shapes of glacial cycles have several nonlinear properties which may be indicative of glacial interglacial differences in climate sensitivity or response time the saw tooth asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation and the relative duration of interglacial stages decreases at 1 4 myr collectively trends in the shape of glacial cycles and the sensitivity of 18 o to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2 5 and 1 4 myr but show no significant change associated with the appearance of strong 100 kyr cycles during the mid pleistocene transition

meridional overturning circulation in the south atlantic at the last glacial maximum the geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin here we examine how the density difference across 30 s in the upper 2 km of the atlantic ocean and thus the magnitude of the shear associated with the overturning circulation has changed between the last glacial maximum and the present we use oxygen isotope measurements on benthic foraminifera to reconstruct density today the density in upper and intermediate waters along the eastern margin in the south atlantic is greater than along the western margin reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing north atlantic deep waters below the greater density along the eastern margin is reflected in the higher 18o values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km for the last glacial maximum the available data indicate that the eastern margin foraminifera had similar 18o to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher 18o values in the western margin benthic foraminifera above 1 km if this reversal in benthic foraminifera 18o gradient reflects a reversal in seawater density gradient these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the atlantic basin are balanced by the southward export of glacial north atlantic intermediate water

high temperature environments of human evolution in east africa based on bond ordering in paleosol carbonates many important hominid bearing fossil localities in east africa are in regions that are extremely hot and dry although humans are well adapted to such conditions it has been inferred that east african environments were cooler or more wooded during the pliocene and pleistocene when this region was a central stage of human evolution here we show that the turkana basin kenya today one of the hottest places on earth has been continually hot during the past 4 million years the distribution of 13c 18o bonds in paleosol carbonates indicates that soil temperatures during periods of carbonate formation were typically above 30 c and often in excess of 35 c similar soil temperatures are observed today in the turkana basin and reflect high air temperatures combined with solar heating of the soil surface these results are specific to periods of soil carbonate formation and we suggest that such periods composed a large fraction of integrated time in the turkana basin if correct this interpretation has implications for human thermophysiology and implies a long standing human association with marginal environments

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

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

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

climatic variations of the work done by the wind on the ocean s general circulation 1 the southern hemisphere westerlies exert an important influence on global climate supplying nearly half of the mechanical energy for the deep overturning circulation in a coarse resolution ocean model northward shifted winds increase the work done on surface geostrophic flows due to enhanced velocities associated with the antarctic circumpolar current acc alternatively energy supply is diminished by southward shifted winds primarily through reduced correspondence between wind stress and surface velocity in the southern ocean due to dynamical and topographic constraints on the acc when combined perturbations in latitude and magnitude of the westerlies are applied these results are reconciled with estimates of recent trends in wind work and volume transport in the southern ocean from observations and coupled climate models this indicates that the strength of the winds exerts a dominant effect that masks the opposing consequences of latitudinal migration in particular transport through drake passage shows a clear relationship with wind work and velocity when the winds move poleward leading to a reduction in all three quantities however under equatorward shifted winds stronger polar easterlies adjacent to the antarctic continent establishes a recirculation gyre leading to increased mechanical energy input and swifter currents but reduced transport significant o 25 changes in the mechanical energy supply from the winds may be possible on climatic time scales particularly associated with the spatial correlation between winds and the acc that does not depend critically on unresolved eddy processes in this model leading to a pathway for altering abyssal diapycnal mixing rates and stratification of the ocean interior

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

the reunion subchronozone at odp site 981 feni drift north atlantic abstract at ocean drilling program site 981 the reunion subchron is recorded from the marine isotopic stage mis 81 82 boundary to mis 79 as a single normal polarity zone spanning the 2 115 2 153 ma interval according to the age model produced by matching the site 981 18 o record to an orbitally tuned reference record the duration of the reunion subchron 38 kyr is almost four times that assumed in current geomagnetic polarity timescales high interval sedimentation rates 18 cm kyr during mis 81 at site 981 have resulted in an unusually detailed record of the subchron virtual geomagnetic poles during the onset and demise of the reunion subchron trace a series of mainly anticlockwise loops over eastern asia

tides and tidal torques of the world ocean since the last glacial maximum peltier s 1994 paleobathymetries are used to investigate tides and corresponding balances of torques of the world ocean since the last glacial maximum 21 000 years before present simulations with a numerical tidal model point out that variable ice loads at the earth s surface and rearrangements in the interior led to considerable modifications of the resonance behavior of the oceans associated evident changes of the tidal range were not confined to the vicinity of formerly ice covered areas in contrast to the tidal oscillation system that was subject to almost uniform changes since the beginning of ice retreat the dissipation rate of the m2 tide increased distinctly during the principal melting time from 15 to 8 kyr b p and reached a local extreme eight thousand years ago though surface and volume of the world ocean were already very similar to present conditions postglacial deformations during the following two thousand years caused the decrease of tidal dissipation from the maximum value to the present day level indicating the sensibility of tidal torques and dissipation rates to comparatively small variations of ocean s geometry since the vicinity of the tidal oscillation system to a resonance situation is more decisive with respect to total dissipation than the extension of shelves where most of the tidal dissipation is expected to take place states of extreme dissipation rates do not necessarily coincide with extreme bathymetric conditions

holocene productivity changes off adelie land east antarctica 1 this study presents the first high resolution multiproxy investigation of primary productivity pp during the holocene from the antarctic continental margins micropaleontological and geochemical data from the sediment core md03 2601 associated to sea ice model outputs give unprecedented insights into the biological pump of the antarctic coastal area off adelie land in response to climatic changes plurimillennial and millennial changes of pp are observed in the study area in response to changes in nutrient availability stratification and growing season duration which are linked to sea ice upwelling wind and glacier dynamics the precessional cycle seems to be responsible in the pp long term variations while forcing factors involved at the millennial timescale remain more enigmatic our results emphasize enhanced biological pump during warmer and windier holocene phases because of a longer growing season and greater nutrient input antarctic coastal and continental shelf zones may therefore represent a more intense carbon sink in the future

pacific plate subduction beneath the central mariana and izu bonin fore arcs new insights from an old margin 1 multichannel seismic mcs profiles and bathymetric data from the central mariana and izu bonin subduction systems image the subducting pacific plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc subducting oceanic crust varies along the mariana margin from 5 3 to 7 km thick and is covered by 0 5 2 km thick sediments and numerous seamounts oceanic crustal thickness east of the izu bonin trench is 6 km faulting resulting from flexure of the incoming pacific plate begins up to 100 km east of the trench axis near the 6 km depth contour the plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes 25 incoming crust breaks along new faults with a trench parallel strike the mariana trench axis is commonly a graben that accommodates an abrupt change within 8 beneath the outer fore arc we infer that the plate fails there rather than simply bends under the applied loads along portions of the mariana margin subducting seamounts displace the trench axis westward and uplift the toe of the slope surprisingly west of the toe there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction nor of significant subduction erosion or sediment underplating mcs profiles across the base of the mariana inner trench slope provide evidence for both complete subduction and small scale accretion of pacific plate sediments however we found no evidence for long term sediment accretion the subducting plate dips 9 12 beneath serpentinite seamounts on the izu bonin and mariana fore arcs along the mariana margin the majority of these seamounts are located 50 70 km west of the trench where the mantle wedge is 3 7 km thick between 8 10 km thick fore arc crust and the top of the subducting plate the apparent lack of significant deformation of the mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben

tectonic uplift and eastern africa aridification the history of eastern african hominids has been linked to a progressive increase of open grassland during the past 8 million years this trend was explained by global climatic processes which do not account for the massive uplift of eastern african topography that occurred during this period atmosphere and biosphere simulations quantify the role played by these tectonic events the reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation engendering the strong aridification and paleoenvironmental changes suggested by the data

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

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

deep water mass geometry in the glacial atlantic ocean a review of constraints from the paleonutrient proxy cd ca 1 paleonutrient proxies currently provide the strongest constraints on the past spatial distribution of deep water masses we review the state of knowledge derived from the trace metal proxy cd ca for the atlantic ocean during the last glacial maximum lgm we compile published benthic foraminiferal cd ca data supplemented with new data to reconstruct meridional cd sections through the holocene and lgm atlantic holocene cd ca reflects the modern water masses north atlantic deep water nadw antarctic bottom water aabw and antarctic intermediate water aaiw reasonably well except for anomalously high values in the intermediate to mid depth far north atlantic lgm cd ca clearly shows that nadw was replaced by the shallower glacial north atlantic intermediate water gnaiw with significant northward expansion of aabw the boundary level between gnaiw and aabw was very abrupt occurring between 2200 and 2500 m modern water depth combined cd ca and cibicidoides d 13 c data allow us to also calculate the air sea signature of d 13 c d 13 cas the lgm atlantic can be explained entirely by mixing between high d 13 cas gnaiwand low d 13 cas aabw negative d 13 cas values in glacial aabw were likely caused by poor ventilation during formation probably associated with extensive sea ice coverage overall our knowledge base would benefit from improved data coverage in the south atlantic and southern ocean and a better understanding of ancillary effects on the cd ca proxy itself

the 2010 mw 8 8 maule chile earthquake nucleation and rupture propagation controlled by a subducted topographic high knowledge of seismic properties in an earthquake rupture zone is essential for understanding the factors controlling rupture dynamics we use data from aftershocks following the maule earthquake to derive a three dimensional seismic velocity model of the central chile forearc at 36 s we find a high v p 7 0 km s and high v p v s 1 89 anomaly lying along the megathrust at 25 km depth which coincides with a strong forearc bouguer gravity signal we interpret this as a subducted topographic high possibly a former seamount on the nazca slab the maule earthquake nucleated at the anomaly s updip boundary yet high co seismic slip occurred where the megathrust is overlain by lower seismic velocities sparse aftershock seismicity occurs within this structure suggesting that it disrupts normal interface seismogenesis these findings imply that subducted structures can be conducive to the nucleation of large megathrust earthquakes even if they subsequently hinder co seismic slip and aftershock activity

origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north ecuador south colombia oceanic margin 1 splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone prestack depth migration of a multichannel seismic line across the north ecuador south colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 mw 7 7 tsunamogenic subduction earthquake the splay fault separates 5 6 6 km s velocity inner wedge basement rocks which belong to the accreted gorgona oceanic terrane from 4 to 5 km s velocity outer wedge rocks the outer wedge is dominated by basal tectonic erosion despite a 3 km thick trench fill subduction of 2 km high seamount prevented tectonic accretion and promotes basal tectonic erosion the low velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic decollement thrust subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks conversely duplexes are interpreted to form from and above subducting sediment at 14 to 15 km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic decollement coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget which comes out negative intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the gorgona terrane from cretaceous extension to likely eocene oblique compression the splay faults could have resulted from tectonic inversion of listric normal faults thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis

spatial correlation of interseismic coupling and coseismic rupture extent of the 2011 mw 9 0 tohoku oki earthquake 1 imaging the extent to which the rupture areas of great earthquakes coincide with regions of pre seismic interplate coupling is central to understanding patterns of strain accumulation and release through the earthquake cycle both geodetic and seismic estimates of the coseismic rupture extent for the march 11 2011 mw 8 9 9 0 earthquake tohoku oki earthquake may be spatially correlated 0 26 0 05 to 0 82 0 05 with a region estimated to be partially to fully coupled in the interseismic period preceding the earthquake though there is substantial variation in the estimated distribution and magnitude of coseismic slip the 400 km long region estimated to have slipped 4 m corresponds to an area of the subduction zone interface that was coupled at 30 of long term plate convergence rate with peak slip near a region coupled 80 the northern termination of rupture is collocated with a region of relatively low 20 interseismic coupling near the epicenter of the 1994 mw 7 6 sanriku oki earthquake and near a region of potential long term low coupling or ongoing slow slip slip on the subduction interface beneath the coastline 40 50 km depth is best constrained by the land based gps data and least constrained on the shallowest portion of the plate interface due to the 230 km distance between geodetic observations and the japan trench

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

location of potential energy sources and the export of dense water from the atlantic ocean the export of dense water from the atlantic and into the indo pacific oceans and heat transport in the opposite direction is traditionally attributed to the fact that dense water forms in the atlantic but not the pacific evidence from two models presented here suggests this is an incomplete explanation it is found that dense water export from the atlantic depends on a poten tial energy pe sink associated with dense water downwelling in the at lantic that is not balanced by a pe source within the atlantic therefore increasing the atlantic pe source reduces dense atlantic water export tide models suggest that the atlantic pe source may have been much higher at the last glacial maximum this could have significantly reduced dense wa ter export from the atlantic ocean even if atlantic overturning was stronger than it is today

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

north atlantic climate evolution through the plio pleistocene climate transitions during the plio pleistocene the earth witnessed the growth of large northern hemisphere ice sheets and profound changes in both north atlantic and global climate here we present a 3 2 myr long orbitally resolved alkenone sea surface temperature sst record from deep sea drilling project dsdp site 607 41 n 33 w water depth 3427 m in the north atlantic ocean we employ a multi proxy approach comparing these new observations with existing bottom water temperature bwt and stable isotope time series from the same site and sst time series from other sites shedding new light on plio pleistocene climate change north atlantic temperature records show a long term cooling with two major steps occurring during the late pliocene 3 1 to 2 4 ma and the mid pleistocene 1 5 to 0 8 ma closely timed with intervals of major change in northern hemisphere ice sheets existing evidence suggests that the late pliocene cooling may have been caused by a thresholded response to secular changes in atmospheric carbon dioxide co2 while an explanation for the mid pleistocene cooling may involve glacial interglacial changes in atmospheric co2 it seems to also require a change in the behavior of the ice sheets themselves north atlantic climate responses were closely phased with benthic oxygen isotope 18o changes during the 41 kyr world indicating a strong common northern hemisphere high latitude imprint on north atlantic climate signals after the mid pleistocene transition mpt north atlantic sst records and the site 607 benthic carbon isotope 13c record are more closely phased with 18o whereas bwt significantly leads 18o in the 100 kyr band suggesting a shift from a northern to a southern hemisphere influence on north atlantic bwt we propose that the expansion of the west antarctic ice sheet wais across the mpt increased the production and export of antarctic bottom water from the southern ocean and subsequently controlled its incursion into the north atlantic especially during glacial intervals it follows that the early 100 kyr response of bwt implies an early response of the wais relative to the northern hemisphere deglaciation thus in the 100 kyr world both northern hemisphere and southern hemisphere processes affect climate conditions in the north atlantic ocean

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

paleoenvironments of the earliest stone toolmakers gona ethiopia fluvio lacustrine sediments of the hadar and busidima formations along the northern awash river ethiopia archive almost three million years 3 4 to 0 6 ma of human evolution including the earliest documented record of stone toolmaking at 2 5 2 6 ma this paper brings together sedimentologic and isotopic evidence for the paleoenvironmental context of early hominids from both formations but with particular emphasis on the setting for the early toolmakers the pre 2 92 ma record hadar formation is characterized by low gradient fl uvial paludal and lacustrine deposition in an undissected topography most analogous to reaches of the modern middle awash river near gewane the gona area experienced repeated deep dissection and aggradation by the awash river starting between 2 92 and ca 2 7 ma and continuing through the top of the record at 0 6 ma busidima formation each aggradational succession is 10 20 m in thickness and fi nes upward from wellrounded conglomerates at the base to capping paleosols at the top during this period the ancestral awash represented by these fi ning upward sequences was dominantly meandering and fl owed northeast as it does today smaller channels tributary to the axial awash system are also extensively exposed in the busidima formation compared to the axial system conglomerates the tributary channels transported fi ner less mature volcanic clasts mixed with abundant carbonate nodules reworked from adjacent badlands stone artifacts oldowan 2 6 2 0 ma at the oldest archaeological sites are only associated with the axial awash system in the bedded silts or capping paleosols of the fi ning upward sequences the implements were made from rounded cobbles from the channels but manufacture and use of the tools was always away from the channel bars on the nearby sandy banks and silt dominated fl oodplains archaeological sites higher in the record acheulian 1 7 ma occur in similar axial river contexts as well as along tributary channels further removed from artifact raw material sources mature paleosols in the hadar and busidima formations are mostly pale to darkbrown vertisols typifi ed by abundant clay slickensides pseudo anticlinal and vertical fracturing and carbonate nodules such calcic vertisols are common in the region today demonstrating that the paleoclimate over the past 3 4 m y has been semi arid and strongly seasonal carbon isotopic results from pedogenic carbonates in the vertisols allow reconstruction of the proportion of c 3 plants trees and shrubs to c 4 plants grasses through time the 13

development of coarse grained facies in lacustrine rift basins examples from east africa detailed studies of high resolution and multifold seismic reflection data from the two largest east african rift lakes malawi and tanganyika reveal a complex suite of coarse grained depositional facies these facies occur within specific regions of the controlling half grabens that compose the rift lakes sand prone environments include subaqueous channels and small drowned fluvial complexes channel systems range from large erosional canyons to deep water turbidite channel levee systems lowstand and highstand deltas of axial and shoaling side rivers are volumetrically important coarse grained facies fan deltas develop along the base of major border faults during lake lowstands subaqueous talus fan deposits occur along the base of the border faults during lake highstands lowstand deltas are the best preserved progradational facies in these rift lakes in addition to simple tectonic control drastic tectonically or climatically induced lake level change significantly regulates the production of coarse grained lacustrine synrift deposits

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

the environment of the ethiopian highlands at the mid pleistocene transition fauna flora and hominins in the 850 700 ka sequence of gombore ii melka kunture abstract environment climatic change and human evolution have been debated over the last 50 years giving special attention to the plio pleistocene sites of the rift valley in this paper we discuss the environment and the limits of hominin adaptability based on evidence from melka kunture at 2000 m asl on the ethiopian highlands and specifically on the 850 ka to 700 ka sequence at sub site gombore ii human fossils and multiple acheulean occurrences as well as hippo remains and footprints combined with palynological analysis provide a highly detailed chronological resolution of the changing local environmental conditions during the last 150 ka of the mpt mid pleistocene transition including the sequence of events after a volcanic eruption layers containing footprints and fossils are evidence of near continuous occupation by hippos and their recolonization of the area after a disruptive volcanic eruption conversely acheulean implements and human fossils suggest that peopling by hominins occurred at a different and discontinuous pace even when the flora and fauna were re established and the environment was rather stable most notably the assembled evidence points to the limits of homo erectus s l adaptability apparently this hominin could no longer live at 2000 m asl when the climate deteriorated during glacial isotopic stage 20 becoming markedly colder than it is today but re colonized the area when the climate turned warmer again during isotopic stage 19

a reappraisal of the geomagnetic polarity time scale to 4 ma using data from the turkana basin east africa recalibration of the pliocene and early pleistocene geomagnetic time scale using the k ar dated fluvial sequence of the turkana basin in east africa agrees with calibrations based on astronomical calculations ages estimated here are olduvai subchron 1 78 1 96 ma reunion subchrons 2 11 2 15 ma and 2 19 2 27 ma matuyama gauss boundary 2 60 ma kaena subchron 3 02 3 09 ma mammoth subchron 3 21 3 29 ma gauss gilbert boundary 3 57 ma

high amplitude variations in north atlantic sea surface temperature during the early pliocene warm period 1 we provide the first continuous orbital resolution sea surface temperature sst record from the high latitude north atlantic a region critical to understanding the origin of the plio pleistocene ice ages and proximal to regions that became frequently glaciated after 2 7 ma we analyzed sediments from ocean drilling program site 982 over the last 4 ma for their alkenone unsaturation index and compared this surface water signal to a benthic 18o record obtained from the same section we find that while ocean surface temperatures were significantly warmer 6 c than modern temperatures during the early pliocene they were also as variable as those during the late pleistocene a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3 5 ma we propose two possible explanations for the high orbital scale sst variability observed either that a strong high latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the north atlantic drift the site was unusually sensitive to obliquity driven climate shifts on supraorbital time scales a strong sustained cooling of north atlantic ssts 4 5 c occurred from 3 5 to 2 5 ma and was followed by an interval of more modest cooling an additional 1 5 c from 2 5 ma to the present evolutionary orbital scale phase relationships between north atlantic sst and benthic 18o show that sst began to lead 18o significantly coincident with the onset of strong cooling at site 982 3 5 ma we speculate that these changes were related to the growth and subsequent persistence of a greenland ice sheet of approximately modern size through interglacial states

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

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

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

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

subducted oceanic relief locks the shallow megathrust in central ecuador whether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial here we use swath bathymetry pre stack depth migrated multichannel seismic reflection lines and wide angle seismic data collected across the central ecuador subduction segment to reveal a broad 55 km x 50 km 1 5 2 0 km high low height to width ratio multi peaked sediment bare shallow subducted oceanic relief owing to la plata island and the coastline being located respectively 35 km and 50 60 km from the trench gps measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow 80 km x 55 km locked interplate asperity within a dominantly creeping subduction segment the oceanic relief geometrical anomaly together with its highly jagged topography the absence of a subduction channel and a stiff erosive oceanic margin are found to be long term geological characteristics associated with the shallow locking of the megathrust although the size and level of locking observed at the subducted relief scale could produce a mw 7 event no large earthquake are known to have happened for several centuries on the contrary frequent slow slip events sse have been recorded since 2010 within the locked patch and regular seismic swarms have occurred in this area during the last 40 years these transient processes together with the rough subducted oceanic topography suggest that interplate friction might actually be heterogeneous within the locked patch additionally we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin which uplifted la plata island

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

two normal paleomagnetic polarity intervals in the lower matuyama chron recorded in the shungura formation omo valley southwest ethiopia abstract two hundred fifty meters of fluvio lacustrine deposits were sampled from the 760 meter thick shungura formation to study magnetostratigraphy within the lower matuyama chron two to three block samples at different positions were collected from 46 units in the section characteristic remanent magnetization chrm is isolated at temperatures between 390 c and 620 c or alternating fields between 25 and 100 mt the chrm is carried by detrital magnetite and maghemite directional analyses revealed three reversed and three normal polarity zones r3 n3 r2 n2 r1 n1 in the sequence the overall mean direction d s 359 1 i s 3 4 95 3 9 n 44 differs marginally from the expected mean dipole field direction d 1 0 i 8 8 95 2 3 n 26 besse j courtillot v 2003 apparent and true polar wander and the geometry of the geomagnetic field in the last 200 million years j geophys res 108 b10 2469 doi 10 1029 2003jb002684 using stratigraphic scaling between 40 ar 39 ar and k ar isotopic age data brown f h mcdougall i davies i maier r 1985 an integrated plio pleistocene chronology for the turkana basin in delson e ed ancestors the hard evidence alan r liss inc new york pp 82 90 mcdougall i brown f h 2006 precise 40 ar 39 ar geochronology for the upper koobi fora foramtions turkana basin northern kenya j geo soc lond 163 205 220 the normal zones n3 n2 and n1 are dated between 2 06 0 01 2 08 0 01 ma 2 15 0 01 2 20 0 01 ma and at 1 93 0 02 ma respectively these age estimates imply that n1 represents the bottom of the olduvai subchron while n2 and n3 might represent reunion subchrons if the sedimentation rates are assumed to be constant the duration for the older reunion i subchron is about four to seven times 57 15 kyr that currently adopted by the recent gpts cande s c kent d v 1995 revised calibration of the geomagnetic polarity timescale for the late cretaceous and cenozoic j geophys res 100 6093 6095 and the younger reunion ii subchron has a shorter duration of 18 25 kyr

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

environmental versus biological controls on mg ca variability in globigerinoides ruber white from core top and plankton tow samples in the southwest pacific ocean 1 laser ablation inductively coupled plasma mass spectrometry was used to analyze the individual chambers from tests of foraminiferal fossil and plankton tow globigerinoides ruber from the southwest pacific ocean from latitudes 3 s to 42 s the variability of mg ca between chambers of an individual intraindividual and individuals of the same population interindividual is such that when converted to temperature the extent of intra individual and interindividual variability appears to exceed that attributable to either calcification or seasonal temperature variability the pooled mean chamber mg ca from each core top and plankton tow site demonstrates a significant p 0 05 positive correlation with temperature we derive chamber specific calibrations where mg cach f 2 0 798 exp0 070 t mg cach f 1 0 891 exp0 067 t and mg cach f 0 590 exp0 072 t we do not observe any bias between the two morphotypes gs ruber ruber and gs ruber pyramidalis the chamber specific calibrations potentially offset mg ca based temperature reconstructions if used on bulk whole test mg ca or applied to misidentified chambers nevertheless these calibrations can be used to reliably estimate sea surface temperature although there is a general overriding temperature control on mg ca we show that removal of the effect of temperature at each site reveals a lognormal mg ca distribution this suggests that mg ca variability at each site is also affected by biological mechanism s that may control the distribution of interindividual mg ca in addition other te ca data al ca and mn ca from laser ablation trace element depth profiles can be used to identify detrital or diagenetic phases that may bias the trace element ca signal

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

intense interface seismicity triggered by a shallow slow slip event in the central ecuador subduction zone we document a 1 week long slow slip event sse with an equivalent moment magnitude of 6 0 6 3 which occurred in august 2010 below la plata island ecuador south of the rupture area of the 1906 mw 8 8 megathrust earthquake gps data reveal that the sse occurred at a depth of about 10 km within the downdip part of a shallow 15 km isolated locked patch along the subduction interface the availability of both broadband seismometer and continuous geodetic station located at the la plata island 10 km above the sse enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface during the slow slip sequence the seismic data show a sharp increase of the local seismicity with more than 650 earthquakes detected among which 50 have a moment magnitude between 1 8 and 4 1 however the cumulative moment released through earthquakes accounts at most for 0 2 of the total moment release estimated from gps displacements most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion while the earthquake sizes show a classical distribution gutenberg richter law with a b value close to 1 the space time occurrence presents a specific pattern first the largest earthquakes appear to occur randomly during the slow slip sequence which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip moreover the seismicity observed during the sse consists in individual events and families of repeating earthquakes these observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches this study offers an a posteriori interpretation of the seismogenesis in the central ecuador subduction zone where intense seismic swarms have been regularly observed 1977 1998 2002 and 2005 these swarms have likely been triggered by large magnitude slow slip events

geometrical effects of a subducted seamount on stopping megathrust ruptures 1 we have numerically simulated dynamic ruptures along a slip weakening megathrust fault with a subducted seamount of realistic geometry demonstrating that seamounts can act as a barrier to earthquake ruptures such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level for larger seamount height to width ratio and for shorter seamount to nucleation distance as the seamount height increases from 0 to 40 of its basal width the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as 20 we further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level these results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions including the case of the thrust fault being lubricated by seamount top fluid rich sediments as suggested from observations in the japan and sunda trenches

role of panama uplift on oceanic freshwater balance comparison between planktic foraminiferal oxygen isotope records from the caribbean sea ocean drilling program odp site 999 and the equatorial east pacific odp site 851 suggests an increase in caribbean surface water salinity between 4 7 and 4 2 ma the modern atlantic pacific salinity contrast of about 1 became fully established at 4 2 ma as reflected by a 0 5 planktic foraminifera 18o enrichment in the caribbean sea this is interpreted as the result of restricted surface water exchange between the tropical atlantic and pacific in response to the shoaling of the central american seaway as a consequence the atlantic and pacific surface ocean circulation regime changed as did the freshwater balance between the major ocean basins simultaneous shifts in benthic carbon isotope records in the caribbean sea suggest an intensification in north atlantic thermohaline circulation these results indicate that the panamanian isthmus formation caused several new ocean atmosphere feedback mechanisms that have affected climate since the early pliocene

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte