preservation of primary lake signatures in alkaline earth carbonates of the eocene green river wilkins peak laney member transition zone abstract significant changes in carbonate mineralogy texture and stable isotope composition occur at the transition from the wilkins peak member to the laney member in the eocene green river formation bridger basin wyoming which reflect evolution of inflow waters lake waters and paleoenvironments the top of the wilkins peak member contains heterogeneous laminae of calcite and dolomite evaporites associated with these layers suggest deposition in hypersaline lakes diagenetic carbonate mineral textures include euhedral cement overgrowths and interlocking mosaics of calcite and dolomite crystals 20 70 m in size electron microprobe analyses indicate diagenetic overgrowth of fe rich dolomite on cloudy fe poor cores 18o values of carbonate laminae in the upper wilkins peak member vary by 6 with no depth dependent or mineralogic trends which also suggests diagenetic overprinting alternating organic rich and primary aragonite calcite and dolomite laminae were identified from the lower laney member primary lacustrine aragonite consists of well sorted prismatic crystals 5 10 m in length with micro lamination defined by crystal size variation primary precipitated calcite and dolomite laminae are monominerallic with well sorted polyhedral crystals 10 m in size primary mineralogy of the lower laney member changes from calcite to aragonite and dolomite stratigraphically upward along the same 15 m thick stratigraphic interval 18o values decrease upward by 3 all of which suggests 1 lake waters underwent evaporative concentration which together with calcite precipitation increased the lake water mg ca ratios and led to formation of aragonite and dolomite 2 source waters became lower in 18o possibly as inflow changed to higher altitude foreland rivers the results from this study show that understanding the primary lacustrine versus diagenetic origin of green river carbonate minerals is essential for paleoenvironmental and paleoclimate interpretations

interplate quasi static slip off sanriku ne japan estimated from repeating earthquakes 1 space time distribution of quasi static slip on the plate boundary off sanriku northeastern ne japan was estimated by using small repeating earthquakes we found 2509 repeating earthquakes occurring on the plate boundary for the last 17 years based on the interpretation that these events are caused by repeating slips of small asperities surrounded by stable sliding area we regarded these repeating events as creepmeters embedded on the plate boundary the results show that deeper portions of the plate boundary are sliding almost stably and earthquakes with magnitudes more than 6 are accompanied by quasi static slips in the surrounding areas

hydrothermal carbonate chimneys from a continental rift afar rift mineralogy geochemistry and mode of formation carbonate chimney like deposits up to 60 m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting lake abhe afar rift djibouti the chimneys formed sub aqueously in the lake water body at a higher water level than observed today alternating calcite and low mg calcite silica concentric layers compose the chimney structures mineralogical and geochemical investigations of the chimneys lake water and hot spring hydrothermal fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water in contrast to the hot spring fluid lake water is enriched in hree and possesses a pronounced positive ce anomaly features that are preserved in the carbonate chimney layers mixing calculations based on sr isotope and concentration data indicate a hydrothermal fluid contribution of 45 in the chimney interior which decreases to 4 in the external chimney layer sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks whereas the lake s sr budget is dominated by riverine input considering the fluid mixing ratios calculated by sr data the measured c and o isotope compositions indicate that chimney carbonates precipitated at temperatures between 14 c internal part and 22 c external part with 13c carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric co2 the low mg calcite layers including the outermost layer have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements ree and sr and ca isotope compositions ca isotope data reveal that internal chimney layers formed by non equilibrium calcite precipitation with a predominantly hydrothermal ca source the external low mg calcite layer received ca contributions from both hydrothermal fluid and lake water with the latter being the dominant ca source highly positive 44 40ca of lake water likely reflects non equilibrium ca carbonate precipitation during lake water evaporation with resulting 44ca enrichment of residual lake water the strong degree of 44ca enrichment may point towards multiple lake drying and ca reservoir depletion events coupled c o ca isotope data of the sampled carbonate chimney suggest late stage low temperature hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low mg calcite precipitation u th age dating suggests that the chimneys formed no earlier than 0 82 kyr bp 0 28 0 54

late holocene and recent hydroclimatic variability in the central kenya rift valley the sediment record of hypersaline lakes bogoria nakuru and elementeita abstract the sedimentology salt mineralogy and stable oxygen and carbon isotope signatures of a mid lake sediment sequence from hypersaline lake bogoria in kenya reveal marked climate driven changes in water column mixing regime and salinity over the past 1700 years combined with sedimentological data on short sediment sequences from nearby lakes nakuru and elementeita this results in a preliminary reconstruction of hydroclimatic variability in the central kenya rift valley since ca 300 ad stratigraphic analyses of bulk sediment composition texture and magnetic susceptibility of all three cores and smear slide xrd and stable isotope analyses on carbonate minerals in the bogoria sequence allowed to define a succession of sedimentary units corresponding to distinct phases in lake history in the lowermost unit four characteristic trona layers na3 hco3 co3 2h2o are attributed to predominantly dry conditions during the second half of the first millennium ad and the period equivalent to the medieval climate anomaly until the first half of the 12th century ad lake bogoria was probably polymictic at that time its surface level standing significantly lower than today the second unit displays uniform deposition of nahcolite nahco3 indicative of a strongly stratified lake where a layer of less saline surface water near permanently covers a hypersaline water mass with high pco2 we propose that this depositional phase reflects the rising lake level related to the transition from dry to more humid climate conditions at the onset of the little ice age as previously documented for other parts of equatorial east africa the third uppermost bogoria unit contains only sporadic depositions of magadiite na2si14o29 oh 3 11h2o and various sodium carbonate minerals this together with low magnetic susceptibility is interpreted to indicate humid little ice age conditions with highest lake level and reduced salinity around the late 15th to early 16th century ad from the late 18th to early 19th century and again in the 1870 s the region experienced two episodes of drought more severe than any recorded in historical time tentative evidence for these events in the bogoria record is not well constrained in time but dated desiccation surfaces in the nakuru and elementeita records confirm their widespread nature across eastern equatorial africa

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

multi basin depositional framework for moisture balance reconstruction during the last 1300 years at lake bogoria central kenya rift valley multi proxy analysis of sediment cores from five key locations in hypersaline alkaline lake bogoria central kenya rift valley has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years analyses including organic matter and carbonate content granulometry mineralogical composition charcoal counting and high resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them these palaeolimnological data were supplemented with information on present day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column a new age model based on 210pb 137cs and 14c dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake an extensive set of chronological tie points between the equivalent high resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins enabling inter basin comparison of sedimentation dynamics through time the resulting reconstruction demonstrates considerable moisture balance variability through time reflecting regional hydroclimate dynamics over the past 1300 years between ca 690 and 950 ad the central and southern basins of lake bogoria were reduced to shallow and separated brine pools in the former occasional near complete desiccation triggered massive trona precipitation between ca 950 and 1100 ad slightly higher water levels allowed the build up of high pco2 leading to precipitation of nahcolite under still strongly evaporative conditions lake bogoria experienced a pronounced highstand between ca 1100 and 1350 ad only to recede again afterwards for a substantial part of the time between ca 1350 and 1800 ad the northern basin was probably disconnected from the united central and southern basins throughout the last two centuries lake level has been relatively high compared to the rest of the past millennium evidence for increased terrestrial sediment supply in recent decades due to anthropogenic soil erosion in the wider bogoria catchment is a reason for concern about possible adverse impacts on the unique ecosystem of lake bogoria this article is protected by copyright all rights reserved

slip distribution of the 1952 tokachi oki earthquake m 8 1 along the kuril trench deduced from tsunami waveform inversion 1 we inverted 13 tsunami waveforms recorded in japan to estimate the slip distribution of the 1952 tokachi oki earthquake m 8 1 which occurred southeast off hokkaido along the southern kuril subduction zone the previously estimated source area determined from tsunami travel times hatori 1973 did not coincide with the observed aftershock distribution our results show that a large amount of slip occurred in the aftershock area east of hatori s tsunami source area suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock we also found more than 5 m of slip along the deeper part of the seismogenic interface just below the central part of hatori s tsunami source area this region which also has the largest stress drop during the main shock had few aftershocks large tsunami heights on the eastern hokkaido coast are better explained by the heterogeneous slip model than previous uniform slip fault models the total seismic moment is estimated to be 1 87 1021 n m giving a moment magnitude of mw 8 1 the revised tsunami source area is estimated to be 25 2 103 km2 3 times larger than the previous tsunami source area out of four large earthquakes with m 7 that subsequently occurred in and around the rupture area of the 1952 event three were at the edges of regions with relatively small amount of slip we also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface

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

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

trace metal enrichments in lake tanganyika sediments controls on trace metal burial in lacustrine systems abstract we investigate the distributions of several key diagenetic reactants c s fe and redox sensitive trace metals mo cd re u in sediments from lake tanganyika east africa this study includes modern sediments from a chemocline transect which spans oxygenated shallow waters to sulfidic conditions at depth as well as ancient sediments from a longer core 2 m taken at 900 m water depth modern sediments from depths spanning 70 335 m are generally characterized by increasing enrichments of c s mo cd and u with increasing water depth but static fe distributions it appears that the sedimentary enrichments of these elements are to varying degrees influenced by a combination of organic carbon cycling and sulfur cycling these modern lake characteristics contrast with a period of high total organic carbon corg total sulfur stot and trace metal concentrations observed in the 900 m core a period which follows the most recent deglaciation 18 11 ky this interval is followed abruptly by an interval 11 6 ky that is characterized by lower c s u and mo consistent with other work we suspect that the low concentrations of s mo and u may indicate a period of intense lake mixing during which time the lake may have been less productive and less reducing as compared to the present an alternative but not mutually exclusive hypothesis is that changes in the lake s chemical inventory driven by significant hydrological changes could be influencing the distribution of sedimentary trace elements through time

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

spatiotemporal variation in b value within the subducting slab prior to the 2003 tokachi oki earthquake m 8 0 japan 1 the b value of an earthquake catalogue defined as the slope of the gutenberg richter frequency magnitude relationship logn a bm is typically close to unity in many tectonic settings however analysis of seismicity data for the subducting slab along the kuril trench reveals a zone of anomalously low b values near the hypocenter of the 2003 tokachi oki earthquake m 8 0 the zone of low b value 0 50 0 65 at depths of less than 80 km in the subducting slab changes in location with time in 1990 2003 and intensifies leading up to the 2003 earthquake this transient low b value zone corresponds roughly with the location of an asperity inferred from waveform inversion in recent research in the shallow zone corresponding to the immediate region 125 km along the subducting slab of the 2003 event the b value is shown to have decreased in the year prior to the main shock from initial values of 0 65 to values as low as 0 5 the occurrence of the 2003 earthquake a large thrust event on the subducting plate interface in this zone of anomalously low b values suggests that low b values in the subducting slab prior to the main shock and during the aftershock series are concordant with high stress concentrations associated with interseismic strain accumulation and redistribution in the vicinity of the rupture

isotopic hydrology and paleohydrology of the madagi kenya natron tanzania basin during the late quaternary abstract three generations of stromatolites mark the recent extension of paleolakes in the magadi natron basin on the border between tanzania and kenya the last two generations are observed some 50 m above the modern lakes level they are respectively attributed to the beginning of the last interglacial and to the pleistocene holocene transition analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10 14 m below the shoreline their relatively high 18o and 13c values reflect a long residence time for the paleolake water which would have been favorable to the establishment of an isotopic equilibrium between atmospheric co2 and the dissolved inorganic carbon despite deep carbon inputs in relation to carbonititic volcanism a study of the 18o 2h and 13c contents in the hydrothermal springs perennial rivers and lacustrine brines indicates that during the last period of significant renewal of the deep saline groundwater precipitation was depleted in heavy isotopes when compared to modern precipitation this difference corresponds to a drop of about 600 1000 m in altitudinal hydroclimatic zones finally a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin

some premonitory phenomena of the 1995 hyogo ken nanbu kobe earthquake seismicity b value and fractal dimension abstract this paper presents a detailed and complex analyses of the spatio temporal evolution of seismic activity between 1976 and 1998 in a broad area surrounding the epicentre of the 1995 hyogo ken nanbu kobe earthquake as well as in the source area various precursory changes in seismicity such as quiescence followed by increased seismic activity b value and fractal dimension correlation dimension d2 anomalies appear approximately two years before the occurrence of the major event an increased level of background seismicity is clearly observed after the occurrence of the kobe earthquake particularly in the northeast extension of the source area all these anomalous changes in seismic activity before and after the occurrence of the 1995 hyogo ken nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area

the 36c1 ages of the brines in the magadi natron basin east africa the depression in the east african rift which includes both lake magadi and lake natron forms a closed basin within which almost all the dissolved chloride originates in precipitation since there is no important source of very ancient sedimentary chloride this provides an ideal setting for the evaluation of the sup 36 cl methodology as a geochemical and hydrological tracer the main source of recent water as represented by the most dilute samples measured is characterized by a sup 36 cl cl ratio of 2 5 times 10 sup minus 14 in agreement with the calculated value expected in precipitation surface evaporation increases the chlorinity of the local freshwater inflow by about a factor of 110 without changing the isotopic ratio indicating that little chloride enters the system in the form of sediment leachate a second type of brine found in the basin occurs in a hot deep groundwater reservoir and is characterized by lower sup 36 cl cl ratios 1 2 times 10 sup minus 14 by comparing this value with the 2 5 times 10 sup minus 14 in recent recharge one obtains an approximate salt accumulation age of 760 ka which is consistent with thee time of the first appearance of the lake these older brines also have lower sup 18 omore and sup 2 h values which indicate that they were recharged during a climatically different era the sup 36 cl cl ratios in the inflowing waters and in the accumulated brine together with the known age of the lake magadi basin may be used to estimate the importance of the hypogene and epigene as opposed to the meteoric mode of sup 36 cl production such a calculation shows that the hypogene and epigene processes together contribute less than 6 of the total sup 36 cl present in the lake less

geochemistry of rare elements in waters and sediments of alkaline lakes in the sasykkul depression east pamirs abstract the saline lakes tuzkul chukurkul sasykkul lakes of the sasykkul depression located in the semidesert part of the pamirs provide a prominent example of the evaporative concentration of alkaline waters in a closed basin under arid climate conditions chemical evolution of the lake waters has resulted in the development of residual na cl co3 so4 and na cl co3 brines the contents of a number of rare elements were determined in waters and sediments of the sasykkul depression lakes speciation modelling suggests that the main aqueous species of ag are chloride complexes cu pb zn mn are carbonate complexes be bi nb are hydroxocomplexes mo w as v are oxyanions it was established that the salt lake sediments are significantly enriched in b ba mo w u th high contents of trace elements are considered to be due to chemical weathering of rocks within the catchment basin significant contribution from thermal springs derived from the residual fluids associated with acid igneous and volcanic rocks and following evapoconcentration in a closed basin the processes of evaporative concentration solution complexation chemical precipitation and water sediment interaction lead to the partitioning of trace metals in the salt sediments

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

biomineralization of iron phosphates in the water column of lake pavin massif central france the availabilities of iron and phosphorus have considerably impacted biological productivity in past and present natural aquatic environments and therefore have been key regulators of climate changes over geological time scales microbial organisms are known to play important roles in reactions that drive fe and p cycling at redox interfaces in earth s surface environments here we study the depth variations of fe and p speciation in lake pavin massif central france a deep bottom depth 92 m and permanently stratified lake with anoxic and ferruginous conditions in the water column below 60 m depth we particularly focus on the potential roles of microbes on fe and p transformations and traces left by these processes in the sedimenting particular matter bulk chemical analyses powder x ray diffraction and x ray absorption spectroscopy xas at the fe k edge were performed to characterize the mineralogy and fe oxidation state of solid particles at different depths in the water column and in sediments deposited at the bottom of the lake fe is mainly hosted by fe iii oxyhydr oxides and phyllosilicates in the shallower oxygenated water column of the lake 25 m the amount of fe in suspended matter increases with depth and an additional amorphous fe ii fe iii phosphate phase is detected close to the chemocline at 56 m depth while vivianite an fe ii phosphate with a formula of fe ii 3 po4 2 8 h2o becomes dominant in the deeper anoxic water 67 m and 86 m depths fe oxyhydr oxides are preserved down to these depths in the water column suggesting that fe reduction has little impact on the particulate fe budget over the monimolimnion these fe oxyhydr oxides undergo reductive dissolution at the surface of the sediments where vivianite is the main fe bearing phase these results are confirmed by imaging at the micrometer and nano scales using scanning electron microscopy sem transmission electron microscopy tem and synchrotron based scanning transmission x ray microscopy stxm as well as high spatial and energy resolution xanes spectra at the fe l2 3 edges and the c k edge moreover polyphosphate accumulating microorganisms were observed by microscopy and chemical imaging at all depths except in the deeper part of the lake 86 m and some fe was associated with these polyphosphate inclusions below 56 m finally the fe phosphates encrust microbial bodies suggesting that microbial activities might be involved in the formation of these phases based on comparisons with laboratory experiments we propose that fe oxidation in contrast to fe reduction may play a role in the precipitation of fe phosphates in the water column of lake pavin polyphosphate accumulating microorganisms could also be involved in fe phosphate formation in the lake for instance by increasing dissolved phosphate concentrations in the monimolimnion

record of seismotectonic events in siliceous cyanobacterial sediments magadi cherts lake magadi kenya the quaternary sediments of lake magadi in the kenya rift include large volumes of chert before their lithification these siliceous sediments underwent very strong deformation as recorded by diapirism with pillow chert mounds extrusion along dykes and fault ramps horizontal liquefaction slides with brecciation slumping as well as petee flow and shear structures eugster 1969 considered these structures to have resulted from desiccation of na silicate precipitates magadiite that were deposited over wide areas and were several metres thick magadiite can remain soft over long periods therefore these magadi type cherts are called the type examples of inorganic cherts however field observations and microbiological studies of the cherts show that real inorganic cherts are rare at the type locality of magadi type cherts most of the cherts are older than the high magadi beds and developed from flat topped calcareous bioherms of pleurocapsa gloecocapsa and other coccoid cyanobacteria thinly bedded filamentous microbial mats stromatolites bacterial slimes diatoms dascladiacea colonies and other organic matter silicification occurred from a silicasol via opal a to opal c with final recrystallisation to a chert of quartzine composition the metabolic processes of cyanobacteria controlled the ph and influenced the dissolution precipitation mechanism collapse liquefaction and extrusion of the pre lithified siliceous matrix was caused by seismotectonic rift activity which activated fault scarplets and large scale dyke systems it led to liquefaction and other earthquake induced structures along the fault ramps and on tilted blocks concentrated silicasols were generated by the interaction of alkaline waters with volcanic detritus coupled with biochemical processes after liquefaction and extrusion the material solidified by spontaneous crystallisation in an environment that was characterised by highly variable ph and salinity the lake magadi basin is a remarkable example of sedimentation in a continuously seismotectonically active basin this paper presents a first description of the micro organisms in magadi type cherts the silicification process and the deformation that occurred still in the putty like state before lithification

14 c and th u dating of pleistocene and holocene stromatolites from east african paleolakes during recent humid episodes stromatolites were built along paleolake margins some 60 m above the modern water level of lakes natron and magadi southern gregory rift valley three generations of stromatolites are observed the more recent ones frequently covering pebbles and boulders eroded from the older ones the youngest one yielded 14c ages ranging from approximately 12 000 to 10 000 yr b p their 13c values 2 6 suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric co2 thereby lending credence to the reliability of the 14c an initial 230th232th ratio in the detrital component was determined by thu measurements on the 14c dated stromatolites using this value a 230th234u chronology for the older stromatolites was calculated ages of 240 000 and 135 000 10 000 yr were obtained for the first and second generations respectively a humid episode apparently characterized eastern africa during each glacial interglacial transition 18o and 13c measurements on stromatolites when compared to values on modern waters and carbonates provide paleohydrological information long residence time of the paleolake waters and less seasonally contrasted regimes are inferred

chronostratigraphy and age modeling of pleistocene drill cores from the olduvai basin tanzania olduvai gorge coring project abstract the olduvai gorge coring project drilled a total of 611 72 m of core 575 48 m recovered of mostly fluvio lacustrine and fan delta volcaniclastic pleistocene strata at three sites in the olduvai basin tanzania in 2014 we have developed a chronostratigraphic framework for three of the cores based on 40ar 39ar dating of core and outcrop volcanic and volcaniclastic units core paleomagnetic stratigraphy and tephrochemical correlation between cores and from core to outcrop this framework is then used to constrain bayesian stratigraphic age models which permit age estimates for desired core levels with realistic confidence intervals the age models reveal that the deepest core level reached at 245 mbs is 2 24 ma 210 kyr older than the oldest strata exposed at olduvai gorge strata net accretion rates in this early phase of basin history were relatively rapid 57 69 cm kyr but decreased within 250 kyr to 15 cm kyr in lower bed i rates rebounded partially in upper bed i but subsequently declined to

u series disequilibria in early diagenetic minerals from lake magadi sediments kenya dating potential abstract at the southern end of the gregory rift valley lake magadi occupies the bottom of a relatively large drainage basin it is presently covered by a thick trona crust which overlies two silty clay units deposited during late pleistocene high lake stands these units consist of a mixture of detrital grains anorthoclase amphiboles quartz clays illite authigenic zeolites phosphates and sedimentary sodium silicates and cherts a late diagenetic calcite is occasionally observed the authigenic and or diagenetic mineralogical phases were extracted and analyzed for their uranium and thorium isotope contents all yielded highly correlated 234 u 232 th vs 238 u 232 th and 230 th 232 th vs 234 u 232 th isochrons defining two component mixing systems detrital and authigenic phases the detrital component is characterized by large excesses of 230th over 234u and by 230 th 232 th ratios carrying an imprint of the source rocks e g precambrian basement vs volcanics and indicating efficient uranium leaching processes during the previous pedologic cycle the slope of the isochrons defines the 234 u 238 u and 230 th 234 u ratios of the authigenic component i e the age of the uranium uptake episode zeolites yield an age of 10 4 0 6 ka this age is in agreement with the 14c chronology already established for the most recent high lake level episode in the basin 10 000 12 000 yr bp sodium silicates and cherts yield distinct ages of 98 5 20 and 6 3 ka respectively for the lower and upper lacustrine units these ages allow the conclusion that 1 sedimentary silicates are of late diagenetic origin and 2 that the lower lacustrine unit was deposited during a former high lake magadi level possibly during the lacustrine episode dated at 135 10 ka from uranium series measurements on littoral stromatolites finally the late diagenetic calcite which yields an age of about 5 ka indicates a significant change in the sediment interstitial water chemistry this is also shown by the occurrence of calcitic oncolites in flat deposits assigned to a mid holocene minor oscillation of lake magadi level on the basis of their 14c ages it is concluded that uranium series measurements in diagenetic minerals may give access to the age of their formation

modern and late holocene dolomite formation manito lake saskatchewan canada abstract major advances have occurred in our understanding of modern dolomite formation and penecontemporaneous dolomitization over the past several decades manito lake located in west central saskatchewan canada is a large 65 km 2 deep z max 22 m perennial saline 45 ppt tds lake in which modern and late holocene dolomite coexists with other endogenic and authigenic carbonate precipitates including aragonite monohydrocalcite calcite and mg calcite like many other lacustrine dolomites manito lake dolomite is microcrystalline less than 1 m to 5 m ca rich and poor to moderately ordered it occurs as relatively pure hardgrounds and as a component of nearshore microbialites it also forms isopachous cements in consolidated siliciclastic shoreline sediments manito lake dolomite is most likely forming by mainly biomediated precipitation at or near the sediment water interface i in pore spaces of coarse siliciclastic sediments i e beachrock ii as fine laminae associated with microbialites and iii as a major component of mudstone hardgrounds and pavements

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction

environmental significance of upper miocene phosphorites at hominid sites in the lukeino formation tugen hills kenya the lukeino formation contains an important sedimentary and fossiliferous record of the late miocene 6 09 5 68 ma which has in particular yielded the fossil remains of the oldest east african bipedal hominid called orrorin tugenensis this fluvio lacustrine sedimentary succession crops out in the kenyan part of the east african rift it is mainly composed of clay to sandy clay deposits intercalated with volcanic ash horizons and localized layers of carbonates and diatomites a detailed sedimentological and mineralogical study of the lukeino formation was conducted to throw light on the environmental conditions in which the hominids lived several centimetric relatively continuous and indurated phosphatic horizons of sedimentary origin were identified at two sites sunbarua and kapcheberek mineralogical xrd and geochemical analyses as well as observations by sem which was coupled with an energy dispersive spectroscopy eds microprobe indicate that the autochthonous phosphate layers are composed of a micritic matrix of francolite 38 93 with incorporation of silicates in variable proportions from one layer to another the phosphate matrix contains very well preserved and abundant diatom frustules in the basal phosphate layer these diatoms are identified as aulacoseira granulata implying a ph of 7 8 8 2 for freshwaters of the palaeolake lukeino calcitic tubular structures linked to a possible bacterial origin are also observed locally phosphate layers occur abruptly within a thick clay sandy series associated with an intense runoff phase during the deposition of this interval of the lukeino formation the massive and cyclic input of phosphorus to the lake promoted productivity to the stage where it caused a diatom bloom the establishment of several phosphate horizons testifies to successive phases of eutrophication of palaeolake lukeino the diatom cells provided some of the organic matter which was decomposed by bacterial activity at the bottom of the lake in suboxic conditions but in insufficient quantities to fully form the phosphatic materials the rest of the organic matter needed for phosphogenesis came from terrigenous supply plant debris suggesting the presence of dense vegetation in the catchment of the palaeolake lukeino during this well drained interval of deposition of the lukeino formation

fractal properties of seismicity in regions affected by large shallow earthquakes in western japan implications for fault formation processes based on a binary fractal fracture network model 1 the temporal variation in the b value which is defined as the slope of the gutenberg richter frequency n magnitude m relationship log n a bm and the multifractal dimension of the spatial distribution dq of seismic activity are analyzed using earthquake catalog data for regions affected by two recent large earthquakes in japan an anomalous decrease in dq and an increase in b value appear as a common seismic rupture response during the period of decreasing seismic energy release in the lead up to these large shallow earthquakes the change in fractal dimensions of faults inferred from sequential seismic events reflects the transition of fracture from a dispersive rupture process to an anomalous rupture process and shows that the growth of the fault network through fault linkage is abruptly restrained in the fault formation process leading up to the main shock the anomalies in the fractal and multifractal dimensions prior to large earthquakes indicate that the anomalous activation and clustering of smaller earthquakes represents criticality of the state around asperities along a fault

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

the role of excess co2 in the formation of trona deposits abstract the prevailing theory for the formation of trona na3 co3 hco3 2 h2o relies on evaporative concentration of na hco 3 water produced by silicate hydrolysis of volcanic rock or volcaniclastic sediments given the abundance of closed drainage basins dominated by volcanics it is puzzling that there are so few trona deposits and present day lakes that would yield dominantly na co3 minerals upon evaporation groundwater in the san bernardino basin southeastern arizona usa and northeastern sonora mexico would yield mainly na co3 minerals upon evaporation but waters in the surrounding basins would not analysis of the chemical evolution of this groundwater shows that the critical difference from the surrounding basins is not lithology but the injection of magmatic co2 many major deposits of trona and na co3 type lakes appear to have had excess co2 input either from magmatic sources or from the decay of organic matter it is proposed that along with the presence of volcanics addition of excess co2 is an important pre condition for the formation of trona deposits

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

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

acoustic emission microstructure and damage model of dry and wet sandstone stressed to failure twenty three uniaxial compression tests were performed on dry and wet flechtingen sandstone from germany compressive strength of wet core is 60 of the strength of dry core before fracture the transverse p wave speed drops by 13 and the pulse amplitude by 22 for wet and 37 for dry cores accumulated strain energy doubles for dry core acoustic emissions ae are detected with 10 sensors for 19 cores ae activity starts at 84 of the fracture strength of wet cores 55 mpa and at 91 of the strength of dry cores 87 mpa the ratio of located to recorded ae is 0 37 for dry and 0 13 for fully wet cores ae hypocenter patterns document the development of two opposite fracture cones the negative slope of cumulative ae amplitude frequency distribution drops by 50 before failure in dry cores the slope of the wet core drops and recovers energy discrimination of ae detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture using the analogy to visible light microfracturing events are separated into high energy short pulses blue ae and low energy pulses with long duration times red ae blue ae are explained by intragranular grain breakage red ae by multiple stick slip on crack planes or grain boundaries deformed cores show highly fractured calcite cement and mostly intact quartz grains the stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral calcite

quaternary history of the lake magadi basin southern kenya rift tectonic and climatic controls abstract sediments from the magadi basin south kenya rift preserve a one million year palaeoenvironmental record that reflects interactions between climatic volcanic and tectonic controls climate changes that impacted sedimentation include wet dry cycles on variable timescales and an overall progressive trend towards greater aridity volcanic influences involved inputs of tephra to the basin significant inflow of geothermal fluids and the effects of weathering erosion and transportation of clastics from trachyte and basalt terrains tectonic controls which were often step like reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions intensified aridity and evaporative concentration resulted in salinity and ph increasing with time which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water after 105 ka hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements rees the presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes calcrete formed in lake marginal settings during semi arid periods tectonic controls operated independently of climate but they interacted together to determine environmental conditions aquatic deposition was maintained during periods of increasing aridity because fault controlled ambient and geothermal springs continued to flow lakewards this recharge in turn limited pedogenesis palaeosols are common in other rift floor sequences trona formed when aridity and evapoconcentration increased but its precipitation also reflects increased magmatic co2 that ascended along faults basin fragmentation and north south fractures caused loss of cross rift east west drainage from rift marginal basalts resulting in reduced transition metals after 545 ka the magadi basin demonstrates how a careful reconstruction of these complex tectono climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings

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

the sanriku oki japan earthquake of december 28 1994 mw 7 7 rupture of a different asperity from a previous earthquake fault geometry depth and slip distribution of the sanriku oki earthquake of december 28 1994 ms 7 5 are estimated from seismic waveforms geodetic measurements and tsunami waveforms and compared with those of the 1968 tokachi oki earthquake mw 8 2 the most recent large earthquake in the epicentral region seismic wave inversions indicate a shallowly dipping thrust type mechanism and the focal depth of 22 28 km representing an underthrust event at the subduction interface the source time function has an initial stage of 20 s followed by a main pulse of 30 s duration a shape similar to that of the 1968 tokachi oki earthquake joint inversion of geodetic and tsunami data shows that the region of the largest slip 1 7 m corresponds to a region of relatively small slip for the 1968 tokachi oki earthquake furthermore the joint inversion shows that the region of the largest asperity of the 1968 earthquake had essentially zero slip during the 1994 sanriku oki earthquake these results indicate that the dominant asperities ruptured by the 1994 sanriku and 1968 tokachi oki earthquakes are different the result of the joint inversion also shows that the aseismic zone near the japan trench was not ruptured by the 1994 earthquake