lithostratigraphy volcanism paleomagnetism and palynology of quaternary lacustrine deposits from barombi mbo west cameroon preliminary results we present preliminary results from the study of 23 50 m core from lake barombi mbo representing the last 25 000 years the lake is in an explosion crater formed during quaternary time the very laminated sediment is composed mostly of clay containing 5 10 organic carbon each couplet is commonly composed of a basal lamina rich in quartz plant debris muscovite and sponge spicules and of a more clayey upper lamina often with siderite a perturbed section near the base of the core before ca 21 000 yr b p could be the result of a violent release of gas such co2 comparable to the recent nyos gas eruption the paleomagnetic studies exhibit high frequency oscillations interpreted as paleosecular variations of the local geomagnetic field this first record obtained on the african continent can be closely compared to the type record obtained in western europe the pollen results demonstrate the presence of a forest refuge in west cameroon during the last great arid period ca 18 000 yr b p when equatorial forest was broken up elements of montane vegetation spread to the lowlands these phenomena resulted from a drying and cooling climate

e i corrected paleolatitudes for the sedimentary rocks of the baja british columbia hypothesis paleomagnetic inclinations from sediments of the western terranes of canada are consistently too shallow for their reconstructed paleogeographic positions two contradicting explanations for these discrepancies are 1 terranes have been displaced northward with respect to the stable american craton by several thousands of kilometres between the late cretaceous 75 ma and the eocene 50 ma and 2 sedimentary inclination error has caused a shallow bias in the paleomagnetic directions here we apply the elongation inclination e i method to paleomagnetic data sets from sedimentary rocks of supposedly allochtonous terranes of western north america to correct for inclination flattening our results indicate that the paleomagnetic directions from the continental silverquick sediments 95 92 ma of southern british colombia are not seriously affected by inclination error because the magnetic signal most likely concerns a chemical remanent magnetisation crm in contrast the marine sediments of the nanaimo group 84 72 ma of vancouver island region appear seriously affected by inclination flattening f 0 7 and the e i corrected mean inclinations are about 98 steeper than the original data we arrive at corrected inclinations paleolatitudes of i 578 k 388n for the silverquick and i 558 k 368n for the nanaimo sediments our corrected paleolatitudes indicate that the canadian terranes were indeed located adjacent to the baja californian margin during the late cretaceous thus supporting the baja bc hypothesis d 2005 elsevier b v all rights reserved

neogene tectonic rotation of the santa ynez range western transverse ranges california suggested by paleomagnetic investigation of the monterey formation a paleomagnetic investigation was conducted on dolomite beds in the middle and upper miocene monterey formation in order to determine the timing and amount of post early miocene tectonic rotation experienced by the santa ynez range part of the western transverse ranges province of california five stratigraphic sections of the monterey formation exposed along the coastline west of santa barbara were extensively sampled the sections were correlated using tuff beds and ages were assigned to each dolomite bed on the basis of magnetostratigraphy and diatom biostratigraphy the dolomite beds possess stable primary remanent magnetic directions which pass fold tests and reversal tests and show a consistent decrease in magnetic declination with decreasing age rocks 14 8 to 15 8 m y old have a mean magnetic declination of 92 7 whereas rocks 8 to 11 m y old have a mean magnetic declination of 36 7 the consistent upsection decrease in magnetic declination within these stratigraphic sections is attributed to a rapid 56 10 clockwise rotation of the entire santa ynez range between 15 2 0 1 and 10 1 m y b p the mean magnetic declination of 36 7 in rocks 8 to 11 m y old is probably due to continued clockwise rotation of the santa ynez range during the plio pleistocene an early miocene palinspastic reconstruction is proposed in which the area of the presently east west trending santa ynez range is restored to an initially north south orientation

paleomagnetism of coastal california and baja california alternatives to large scale northward transport paleomagnetic data from the santa lucia orocopia sloa and baja borderland bba allochthons of coastal california and baja california have been interpreted to indicate up to 2500 km of post mid cretaceous northward transport of these regions with respect to interior north america however with neogene strike slip offsets taken into account geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the san andreas and related fault systems we have examined paleomagnetic data from sloa and bba and conclude that apparent discordances can be explained without large scale pre neogene tectonic transport three major observations are fundamental to this analysis 1 paleolatitudes derived from volcanic rocks of the jurassic eugenia formation of bba and coast range ophiolite of sloa are concordant when compared to revised jurassic reference paleomagnetic poles from interior north america 2 isotopic and paleobarometric data from the peninsula ranges batholith in southern california indicate that the batholith has been tilted northeast side up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large scale northward transport 3 literal interpretation of the paleolatitudes determined from paleomagnetic directions in upper cretaceous and paleogene marine sedimentary rocks of sloa and bba requires north then south then north transport and a complex motion history between the two allochthons however concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of sloa and bba are biased towards low paleolatitudes by compaction shallowing

hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators abstract optical and geochemical techniques were applied to organic matter of late quaternary sediments from lakes victoria and rukwa east africa variations in total organic carbon toc and hydrogen content expressed as hydrogen index hi can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences toc and hi both decline as an exposure surface is approached mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation inert organic material of very low hi is typically concentrated at the overlying transgression surface the carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces although weathering may be partially responsible isotopic variations are mainly due to changes in the relative importance of material from plants using c4 photosynthesis such as grasses and sedges growing around the shore and on the exposed lake muds and of material from planktonic algae and terrestrial c3 plants

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

glacial interglacial variations in carbon cycling revealed by molecular and isotope stratigraphy of lake nkunga mt kenya east africa abstract the lipid content n alkanes n alkanols n alkanoic acids and the 13c values of these lipids were measured in nineteen sediment samples taken from a 21 18 m long core from lake nkunga a high altitude 1820 m a s l freshwater lake situated in the montane rain forest on mt kenya the record spans the early last glacial marine isotope stages 5a to 4 and the late holocene 1030 45 14c yr bp to the present day the intervening hiatus spans the last glacial maximum and the early to mid holocene the molecular biogeochemical results are interpreted in terms of past changes in climate vegetation and atmospheric co2 concentration three different sources of organic matter input were identified from the n alkyl lipids namely terrestrial plants aquatic plants and algae unusual distribution patterns of dominant mid chain length n alkanes c23 25 n alkanols c22 24 and n alkanoic acids c22 24 are attributed to aquatic macrophytes an 11 decrease in bulk carbon isotope values between the sediments of early glacial 17 and late holocene age 28 is also apparent in the individual homologues of the n alkyl lipids the observed isotopic shift in terrestrial higher plant biomarkers is similar to that observed in nearby sacred lake and is consistent with an expansion of c4 grasses or sedges during the early glacial the corresponding vegetation was a dry montane juniper olive forest with a grassy understorey representing cold dry conditions with lower co2

compaction corrected inclinations from southern california cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the peninsular ranges terrane paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of north america however the anomalously shallow paleomagnetic inclinations used to indicate large scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compaction of marine sediments to understand the magnitude of burial compaction effects on the post cretaceous motion history of the peninsular ranges baja borderlands terrane a rock magnetic compaction and paleomagnetic study of the ladd formation and the point loma formation from southern california was conducted the anhysteretic anisotropy of remanence of the characteristic remanence carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a dc magnetic field standard paleomagnetic studies of the units indicated that previous ladd formation results could be reproduced and a correction was made at the sample level we were unable to adequately reproduce earlier results of the point loma formation so the average remanence anisotropy was used to correct the previously reported mean direction the mean inclination of the ladd formation was conected from 46 95 8 to 58 95 4 and the mean inclination of the point loma formation was conected from 39 5 5 4 normal and 36 4 16 6 reversed to 56 0 5 1 normal and 53 0 16 7 reversed these results suggest that the peninsular ranges baja borderland terrane has been part of the western north america since the late cretaceous and that clay containing sedimentary rocks may typically experience from 10 to 15 of inclination shallowing due to burial compaction

solar semidiurnal tide in the dusty atmosphere of mars abstract vertical coupling due to the solar semidiurnal tide in mars s atmosphere and effects on zonal mean temperature and wind structures are investigated using a numerical model the model provides self consistent solutions to the coupled zonal mean and tidal equations from the surface to 250 km breaking convective instability of the semidiurnal tide is parameterized using a linear saturation scheme with associated eddy diffusivities thermal forcing in the model gives rise to surface pressure perturbations and middle atmosphere zonal mean winds and temperatures that are consistent with available measurements and general circulation models results presented here primarily focus on globally elevated dust levels during southern hemisphere summer conditions similar to those experienced by the viking 1 and viking 2 landers during the 1977 global dust storms semidiurnal temperature and wind amplitudes maximize in the winter hemisphere and exceed 50 k and 100 m s 1 above 150 km and are typically 10 2

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

vegetation change in equatorial west africa time slices for the last 150 ka abstract drawing on marine and terrestrial palynological data vegetation maps of equatorial west africa are presented for eleven time slices over the last 150 000 years long marine records are used as a basis for the regional picture and provide a chronology for the last glacial cycle much shorter terrestrial records help fill in the picture for the most recent periods and facilitate interpretation of regional patterns for the longer timescale temporal and spatial variation is revealed in relation to global patterns of climate change rain forest was widespread during oxygen isotope stages 1 and 5 but strongly reduced during stages 3 and 4 and especially during stages 2 and 6 when open grass rich vegetation prevailed glacial rain forest refuges are found in the southwest of the guinean mountains and along the eastern coast of the gulf of guinea miombo woodland of the zambezian vegetation zone expanded during stage 5 especially during the first half podocarpus forest had its largest expansion during substages 5d 115 to 105 ka and 5b 95 to 85 ka the last occurrence of podocarpus in the guinean mountains is during stage 5a the distribution of mangrove swamps was extremely reduced during glacial times during stage 6 savanna and open dry forest covered large areas along the northern coast of the gulf of guinea the southern saharan desert reached far to the south and the namib desert far to the north the area of rain forest was restricted mangrove swamps were strongly reduced and the area of podocarpus forest fluctuated during stage 5 large changes in the area of afromontane forest rain forest dry open forest and savanna occurred rain forest was widespread and mangroves were extensive along the coast during the last interglacial substage 5e podocarpus forest area strongly extended during substages 5d and 5b in substages 5c and 5a rain forest reclaimed areas it had lost in the previous substages 5d and 5b respectively mangrove swamps were less widespread in the later substages of stage 5 than during substage 5e during stage 4 the rain forest area was again strongly reduced and recovered only slightly during the following stage 3 also the mangrove swamp area was reduced except along the ivory coast and along the northwestern coast of the gulf of guinea podocarpus forest only occurred in angola and may be in congo during stage 4 again forest was much reduced during stage 2 and open vegetation types covered large parts of equatorial west africa mangrove swamps must have been rare at the beginning of the holocene mangrove swamps had recovered and reached their largest extension also the rain forests area increased in the early holocene and guinean and congolian rain forests were probably not separated by a savanna corridor now existing in togo and benin

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

late quaternary palaeoenvironments in the lake barombi mbo west cameroon deduced from pollen and carbon isotopes of organic matter abstract a sediment core 23 5 m long was recovered from a water depth of ca 110 m in lake barombi mbo a maar crater of the cameroon volcanic chain this paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis interpretation of this carbon isotopic curve is simplified 1 because of the weak role played by the phytoplankton in the balance of the organic particle flux and 2 because of the presumed absence of diagenetic alteration of the isotopic composition the carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the c4 biomass the major fluctuations of isotopic and pollen curves allow four main phases to be distinguished from ca 25 000 to 20 000 yr b p 13c values of 25 to 30 pdb are related to a forest environment associated with a montane floral element from 20 000 to 13 000 yr b p the 13c values are between 23 and 28 and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by cyperaceae and other aquatic plants from 13 000 to 10 000 yr b p the forest extended again reaching a maximum density during the period from 9500 until 3000 yr b p and with an average 13c value of 32 a new dry phase at around 2500 2000 yr b p with temporary openings in the forest marked by a 13c excursion above 30 but during the last 2 millennia the forest developed again with 13c values of 32

ionospheric storms on mars impact of the corotating interaction region measurements made by the aspera 3 and marsis experiments on mars express have shown for the first time that space weather effects related to the impact of a dense and high pressure solar wind corotating interaction region on mars cause strong perturbations in the martian induced magnetosphere and ionosphere the magnetic barrier formed by pile up of the draped interplanetary magnetic field ceases to be a shield for the incoming solar wind large blobs of solar wind plasma penetrate to the magnetosphere and sweep out dense plasma from the ionosphere the topside martian ionosphere becomes very fragmented consisting of intermittent cold low energy and energized plasmas the scavenging effect caused by the intrusions of solar wind plasma clouds enhances significantly by a factor of 10 the losses of volatile material from mars

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

anomalous behavior of the thermosphere during solar minimum observed by champ and grace 1 high resolution density observations inferred from accelerometer measurements on the challenging mini satellite payload champ and gravity recovery and climate experiment grace satellites for the period 11 17 december 2008 during the solar minimum are analyzed and compared to reference model predictions the density distribution as a function of altitude can be studied because the satellites were in the same 8 4 20 4 hour 0824 2024 local solar time plane champ at a mean altitude of 332 km and grace 144 km higher the mean observed and model density profiles at 476 km reveal considerable differences which except for a 25 bias is not the case at an altitude of 332 km these differences result from the fact that during this solar minimum the transition from predominantly o to he occurs at a much lower altitude in connection with the winter helium bulge than is predicted by the model when averaged over the 1 week period striking wave 3 structures are revealed with respect to longitude that are remarkably consistent between champ and grace we interpret these as the manifestations of nonmigrating tides propagating upward from the lower atmosphere comparisons are made with similar data from august 2008 which reveal a wave 4 structure however whereas previous results and conventional wisdom would suggest that these features are produced by the eastward propagating diurnal tides with zonal wave numbers s 2 de2 and s 3 de3 respectively the asymmetries and day night phase differences that are present suggest the predominance of semidiurnal tides se1 and se2 respectively along with the aforementioned diurnal tides as well as tides sw5 and sw6

high resolution pollen record from core kw31 gulf of guinea documents the history of the lowland forests of west equatorial africa since 40 000 yr ago abstract pollen data from core kw31 recovered off the mouth of the niger river 3 31 1n 05 34 1e 1181 m water depth provide an exceptional record of vegetation changes in the west african lowlands between 40 000 and 3500 cal yr b p the highly diverse microflora testify for the permanency of rain and secondary forests in the niger river catchment at least as gallery formations along rivers during the last glacial period when dry conditions occurred in relation to enhanced trade wind circulation the direct consequence of the post glacial warming and the correlative increase in monsoon fluxes over west africa was the increase in forest diversity and the expansion of rain and secondary forests on the nearby continent comparison between kw31 pollen record and continental pollen data from 5 s to 25 n allows the evaluation of migration rates of tropical forest populations throughout north west africa at the beginning of the holocene and the vegetation response to the shift toward aridity recorded widely at the end of the african humid period around 4000 cal yr b p

paleomagnetic evidence that the central block of salinia california is not a far traveled terrane new paleomagnetic results from late cretaceous 75 85 m y red beds on the central block of salinia indicate that salinia was located within 6 in latitude of its current cratonal north american position during the late cretaceous after correction for neogene san andreas fault transport the red beds formed as alluvial fan overbank deposits with hematite cement deposited directly on salinian granites in the la panza range paleomagnetic analysis shows two components of magnetization in the red beds a low blocking temperature present day overprint residing in goethite and a high blocking temperature 600 component residing in hematite the hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis the bedding corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions twenty three class i sites 95 20 have an average hematite direction with inclination 54 4 and declination 18 2 95 6 1 after structural correction these paleomagnetic data suggest that salinia resided at about 35 n latitude during the late cretaceous within 6 of its current location adjacent to cratonal north america by contrast a summary of paleomagnetic data from the peninsular ranges terrane and the sur obispo terrane which are currently outboard of salinia shows northward transport of these terranes of 12 to 22 relative to their current locations in north america since the cretaceous the offsets increase systematically away from the craton with the most outboard sur obispo terrane which is composed of accretionary prism and distal forearc material showing the largest degree of northward translation

bulk composition of sedimentary organic matter used in palaeoenvironmental reconstructions examples from the tropical belt of south america and africa abstract the organic matter of lake sediments contains information that allows for the reconstruction of past environmental and climatic changes the elemental composition and isotopic characteristics of sedimentary organic matter enable discrimination between allochthonous vegetal matter from within the catchment and autochthonous phytoplanktonic and fixed algae components this approach makes it possible to distinguish between the inputs of organic matter from c 3 vs c 4 type land plants sedimentary records from south america and africa provide examples of applications of organic bulk composition in palaeoenvironmental reconstructions the records from two african lakes show that during the last glacial maximum lgm the distribution of c org n org 13 c pairs indicates the dominance of organic material having a c 4 terrestrial origin these inferences are supported by pollen analysis which indicates opening up of the forest at both sites consistent with drier climatic conditions than present the record from siberia lake bolivia ca 3000 m altitude shows that during the lgm the sedimentary organic matter is dominated by the phytoplanktonic fraction as indicated by the c org n org 13 c pairs the petrographic analyses show that this material is not well preserved suggestive of drier conditions consistent with interpretations from the pollen record the beginning of the holocene is marked by a shift from a scattered c 4 signal to one with a greater terrestrial c 3 and aquatic phytoplanktonic influence these results indicate climatic amelioration towards more humid and warmer climatic conditions leading to forest development in the lake catchment together with increasing lake levels conducive to greater phytoplankton production these conditions persisted during the holocene as shown by the c org n org 13 c pairs in all lakes with a terrestrial c 3 signal predominating at the carajas and siberia lakes only dom helvecio lake shows a different pattern during the beginning of the holocene in the distribution of c org n org 13 c pairs this pattern indicates that the sedimentary organic material is dominated by c 4 terrestrial material coming from savanna expansion in the lake catchment during a period of relative aridity in central brazil

compaction corrected paleomagnetic paleolatitudes for late cretaceous rudists along the cretaceous california margin evidence for less than 1500 km of post late cretaceous offset for baja british columbia the paleolatitudinal distribution of bivalve rudists has important significance for the baja british columbia baja bc hypothesis that western canadian superterranes from british columbia have been displaced 3000 km since cretaceous time rudists are not observed in baja bc sedimentary rocks yet they are common in late cretaceous strata in california and baja california which have the same paleomagnetically determined paleolatitudes approximately 25 n as baja bc rocks of late cretaceous age in order to resolve this contradiction and to delimit more exactly the southern paleolatitudes of baja bc paleomagnetic inclinations corrected for the effects of burial compaction were used to determine the paleolatitudinal distribution of rudists along the california margin compaction corrected paleomagnetic data from the peninsular ranges and salinia terranes indicate that rudists were restricted to paleolatitudes between 34 and 40 n evidence of coastal upwelling in the latest cretaceous marca shale may explain the northern limit of the rudist distribution these data suggest that baja bc was no farther south than 40 n in the late cretaceous thus limiting its post cretaceous displacement to less than 1500 km and that burial compaction has also affected the paleomagnetism of nanaimo group sedimentary rocks from vancouver island this result also helps resolve the conflict between paleomagnetic results which show 1500 km of post late cretaceous offset between the insular coast plutonic complex superterrane and the intermontane superterrane and geologic observations which can allow only tens of kilometers of offset between these terranes in the methow tyaughton basin

exotic terranes of western california numerous distinct geological terranes compose the north american cordillera1 there may be as many as 50 terranes in california alone2 critical to deciphering the history of cordilleran tectonic assembly is an understanding of the displacement history of individual terranes it is therefore important to know 1 whether a terrane has undergone significant motion with respect to the stable craton that is whether it is allochthonous or exotic 2 if so when relative motion started and stopped 3 from where an individual terrane originated and 4 the nature of interterrane movements we consider here the problem of determining whether the now juxtaposed salinian and stanley mountain terranes of california became amalgamated at or near their present position with respect to cratonic north america or if they collided at a considerable distance from their present positions and were later accreted to north america as a composite package the palaeomagnetic data that we present indicate that the latter was the case

the nature and chronostratigraphy of quaternary pyroclastic accumulations from lake barombi mbo west cameroon abstract the k ar dating of a limburgitic flow indicates that the barombi mbo maar was active a million years ago the morphology of the maar and the unusual high k 2 o na 2 o ratio of the limburgite suggest either that this age is too old or that this flow is not the last activity of the maar twenty three meters of sediments were cored from the bottom core bm6 of the 110 m deep lake which currently fills the maar they are composed of clayey and sideritic laminae with interstratified ash layers in the lower half of the core these ashes composed of slightly vesiculated sideromelane with olivine and labradorite phenocrysts have been dated between 21 000 and 11 000 yr b p they show no alteration and correspond to aerial fallout of alkaline sodic basaltic affinity which is common in the area but are distinct from the mafic magmas which generated the maar the origin of the tilting and uplifting of a 1 5 m sedimentary column in the lower part of the core remains controversial holocene age pyroclastites are rare and consist of a mixture of granulitic xenocrysts and typically transitional tachylite similar to volcanics from the northeast manengouba magnetite microspherules occurring in holocene layers have no volcanic equivalent they could be related to biogenic or pedogenetic processes induced by more humid climatic conditions

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

solar flux variability of mars exosphere densities and temperatures 1 using densities derived from precise orbit determination of the mars global surveyor mgs spacecraft from 1999 to mid 2005 the response of mars exosphere to long term solar change is established and compared to that of earth and venus at mars exosphere temperatures weighted towards high latitude southern hemisphere daytime conditions change only 36 50 as much as those at earth as solar activity increases from solar minimum to solar maximum whereas the response at venus is one fifth that at mars general circulation models suggest that this difference may be strongly influenced by adiabatic cooling associated with the thermosphere general circulation however other processes such as differences in co2 cooling rates may also be playing a role

new paleomagnetic rock magnetic and petrographic results from the valle group baja california mexico exploring the causes of anomalously shallow paleomagnetic inclinations 1 a detailed paleomagnetic rock magnetic and sedimentologic study was conducted on cenomanian age strata of the valle group at el campito baja california to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of dec 342 6 inc 19 1 and 95 3 4 the inclination is 38 7 shallower than that predicted by the reference paleopole for north america laboratory experiments indicate that compaction could account for only 7 5 of the inclination shallowing fold tests at different scales yield both positive and negative results making a tight constraint on the remanence s age impossible a primary postdepositional remanence is supported by bedding parallel foliations in the anisotropy of anhysteretic remanence fabric although primary anomalously shallow inclinations could indicate significant southerly then northerly paleolatitudinal offset a more likely scenario is a late cenozoic low temperature remagnetization which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data the synthesis of the data indicates that the vizcaino terrane was contiguous with the cordillera margin during remagnetization and underwent a subsequent 13 5 5 4 clockwise rotation we tentatively interpret that the remagnetization resulted from burial and or miocene volcanism which caused an acquisition of thermochemical remanent magnetization tcrm this study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks

biochronology paleoenvironments and stratigraphic sequences of the late albian middle eocene fore arc vizcaino basin western baja california mexico the vizcaino fore arc basin accumulated approximately 4 km 13 000 ft of upper albian middle eocene siliciclastic marine sedimentary rocks derived from the peninsular ranges in baja california data from eight exploratory wells document the micropaleontological content and lithological characteristics of these rocks the strata studied represent mostly neritic upper bathyal marine environments and overlie a basement composed of cretaceous granitic rocks or aptian albian volcaniclastic sedimentary rocks correlative with the alisitos formation we recognize four major depositional sequences within the basin that are related to the regional geology the basal albian turonian sequence 1 represents the initiation of fore arc basin sedimentation contains continental conglomerates that change to bathyal shales and correlates with the lower part of the valle group of the vizcaino peninsula sequence 2 is coniacian paleocene includes basal conglomeratic sandstones grading into maastrichtian bathyal shales and usually overlies a coniacian santonian unconformity sequence 2 is represented at the surface by the rosario group in northwestern baja california and the upper part of the valle group in the vizcaino peninsula sequence 3 is paleocene middle eocene represents continuity of fore arc sedimentation in neritic upper bathyal conditions is capped by a major unconformity and correlates with the sepultura and bateque formations to the north and south of the basin respectively the uppermost miocene pliocene sequence 4 is composed of marine sandstone siltstone unconformably overlying sequence 3 and is correlative with the tortugas formation that represents sedimentation after the end of subduction of the farallon plate beneath the north america plate

magnetostratigraphy of displaced upper cretaceous strata in southern california a magnetostratigraphic study of upper cretaceous marine strata from the santa ana mountains in southern california has identified a campanian reversed magnetozone this reversed interval corresponding to marine magnetic anomaly 33 34 chron 33r of campanian age can be correlated with a campanian reversed magnetozone that has been reported from strata of the great valley sequence in central california the late cretaceous paleolatitude of the santa ana mountains is estimated from this study to be 26 6 n this is significantly different from the region9s expected cretaceous paleolatitude of 43 8 n as part of the north american stable craton and indicates that this region part of the peninsular ranges terrane was 1900 km farther south in cretaceous time relative to the stable craton

palaeomagnetic study of the holocene and upper pleistocene sediments from lake barombi mbo cameroun first results abstract the analyses of the stable deposition and post depositional remanent magnetization carried by the lacustrine sediments contained in a 24 m sequence collected from the bottom of the maar lake barombi mbo cameroun yield well defined declination and inclination oscillations which can be attributed to palaeosecular variations of the local geomagnetic field the transformation to a time scale based on the interpolation and extrapolation of 14 c ages allows comparison of this record with contemporaneous records obtained in other parts of the world the successive inclination peaks can be individually identified with those found in western europe in spite of small chronological discrepancies which are in the range of the standard deviation of the ages normalization of the nrms by anhysteretic magnetization provides a profile of relative palaeointensities which shows similarities with that reconstructed from the same time interval in a sequence from western europe although an important phase shift appears

sedimentation and palaeoenvironment in crater lake barombi mbo cameroon during the last 25 000 years lake barombi mbo is situated in a maar crater of the cameroon volcanic chain sediment cores were recovered with a livingstone corer this paper presents data on core bm6 23 5 m long from a water depth of 110 m the sediments are mainly organic matter rich clays 5 to 10 total organic carbon showing alternating brown and yellowish horizontal laminae on a cm scale the lower part of a couplet basal sublamina is rich in siliciclastic quartz mica grains sponge spicules and documents the deposition of flood detritus the upper unit upper sublamina is composed of grey to bluish clay which becomes green toward the top with frequently yellow layered concretions of siderite it corresponds to quiet conditions with settling of fine clayey particles from suspension the overall recurrence rate of these laminae is about 15 years this frequency is similar to present day large flood events in this intertropical zone from 14 000 to 20 000 yrs bp the sediment accumulation rate was quite low 15 30 g cm 2 10 3 yrs with less abundant precipitation pedogenic clay formed from mica family minerals from 12 000 yrs bp throughout the holocene the rate of accumulation increased 40 g cm 2 10 3 yrs and increasing precipitation favoured the pedogenic formation of kaolinites turbidity currents became less important to the overall sedimentation plankton production remained low documented by organic matter which is more of the exinite than the alginite type siderite concretions occur as early diagenetic products possibly related to alkalinity in anoxic waters at the sediment water interface which limited the formation of fe3 thin ash layers occur between 11 5 and 18 m depth in core bm6 bearing witness to several volcanic events during the late quaternary sediments are disturbed below 21 m with folded laminations and inverse ages these probably result from a slump glide of lower slope sediment but could also signal a possible gas event

palaeolatitude drift history of displaced terranes in southern and baja california the delineation of microplate tectonic terranes1 which make up western north america the timing of their accretions to western north america and the identification of their original geological settings are all topics of debate2 3 the most recent addition to this family of terranes is the santa lucia orocopia allochthon4 5 sloa fig 1 which is made up of salinia6 7 and several other smaller terranes8 and is located in central and southern california this composite terrane amalgamated in the cretaceous and accreted to southern california by the early palaeogene4 5 we describe here new palaeomagnetic results from localities outboard from the sloa in southern california usa and baja california mexico these results are used to define a newly identified allochthonous terrane in southern and baja california peninsular ranges terrane9 prt which is outboard from the sloa estimate the prt s palaeolatitude drift history for the past 160 myr and infer its original geological setting prt and sloa palaeolatitude drift histories are then combined to produce a new perspective on the tectonic development of southern and baja california over the past 160 myr

paleomagnetic and geologic data indicating 2500 km of northward displacement for the salinian and related terranes california results of a pair of paleomagnetic studies of sedimentary rocks in the salinian terrane of westernmost california indicate a northward displacement of about 2500 km since cretaceous time stratigraphic relations suggest that the salinian terrane became amalgamated with several others by the late cretaceous and all underwent the same northward translation this composite terrane allochthon became sutured to cratonal north america by early tertiary time one paleomagnetic study involved upper cretaceous turbidites of the pigeon point formation fine grained thin bedded turbidites yielded the most reliable results whereas medium to coarse grained sandstones are not always stably magnetized contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers thereby implying a postdepositional remanence process there is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization paleomagnetic results from four separate locations in the pigeon point formation gave positive fold tests and mean inclination values near 36 a mean geomagnetic latitude calculated from these results is 21 2 5 3 much more southerly than the value of 46 5 2 3 calculated for pigeon point using paleomagnetic pole data from late cretaceous rocks from cratonic north america declination values are not antipodal for normal and reversed polarity sites analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of neogene rotations of small crustal blocks adjacent to the san gregorio fault in a sequence of paleocene turbidites that crop out at point san pedro four sample localities yielded positive fold tests with mean inclination values near 40 declination values however were spread over a range between 230 and 320 comparison with depositional current directions from the same outcrops indicates clockwise rotations related to neogene right slip on the pilarcitos fault the mean geomagnetic latitude calculated for the point san pedro sediments is 24 5 4 0 and is more southerly than the 43 3 2 5 latitude expected for that location for the salinian terrane and the west coast of cratonic north america absolute rates of latitudinal motion were 8 5 cm yr northward and 3 cm yr southward respectively

late cretaceous intra arc thrusting in southern california thick zones of mylonitic rock are exposed in or between a number of the suspect crystalline terranes in the transverse ranges and eastern peninsular ranges of southern california after palinspastic reconstruction of strike slip displacements along neogene faults in the region the mylonite zones appear in a semicontinuous belt several hundred kilometers long mylonitic deformation in these areas is of a similar style and age lineation trends in the mylonitic rocks are closely aligned when palinspastic reconstruction takes into account the block rotations in the region and the sense of shear is consistently west directed the mylonite zones appear to have formed concurrently in a major midcrustal thrust system that juxtaposed the basement terranes during the late cretaceous a kinematic model is proposed in which a west directed ductile thrust zone along the eastern side of the peninsular ranges is offset by a sinistral tear fault or lateral thrust ramp along the southern margin of the san gabriel mountains from a ductile thrust underlying crystalline basement terranes to the north this model requires the suspect basement rocks of the peninsular and transverse ranges to be linked tectonically by the latest cretaceous this late cretaceous thrust system also appears to continue directly into autochthonous basement rocks in the san bernardino mountains the late cretaceous westward displacement of basement rocks in the transverse ranges relative to the northern end of the peninsular ranges batholith is mirrored by similar tectonic movement around the southern end of the sierra nevada batholith it is argued here that a formerly continuous batholithic belt was breached by ductile west directed thrusting during the late cretaceous between the two still recognizable batholiths the salinia terrane represents the westernmost portion of this westward escaping arc segment and would have been established as a crustal salient by the latest cretaceous this model implies that the basement terranes involved in this thrusting episode are parautochthonous fragments of a cretaceous continental arc and refutes suggestions that they have been accreted in the cenozoic

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

a compaction correction for the paleomagnetism of the cretaceous pigeon point formation of california the paleomagnetism of the cretaceous pigeon point formation turbidites was reexamined to determine whether the 25 of southerly paleolatitudinal offset originally observed champion et al 1984 for these rocks was all or in part due to compaction shallowing of their paleomagnetic inclination the study consisted of two parts 1 a standard paleomagnetic study including detailed thermal and alternating field demagnetization was conducted on oriented cores collected at pigeon point approximately 50 km south of san francisco california the results of this study were combined with the alternating field demagnetized results for samples provided by d champion from the initial pigeon point paleomagnetic study the combined data set has a mean direction for pigeon point i 41 6 d 346 9 similar to that originally obtained by champion et al 1984 2 material from the pigeon point formation was disaggregated given a laboratory analog of a postdepositional remanence and compacted to pressures as high as 0 13 mpa which caused volume losses up to 53 the laboratory compacted samples were alternating field demagnetized and their magnetic inclination and anisotropy of anhysteretic remanence were both measured these data were used to derive correction curves following jackson et al 1991 which describe the specific relationship between remanence anisotropy and inclination shallowing for the pigeon point formation two correction curves were determined one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation these two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification technically caused lineation and a burial compaction foliation the anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the pigeon point formation has been shallowed by burial compaction the compaction corrected pigeon point mean directions assuming either a prolate i 53 1 d 347 2 or an oblate i 49 8 d 346 8 deformation suggest only 13 to 16 of southerly paleolatitudinal offset for the pigeon point formation in the cretaceous not the 25 originally observed champion et al 1984 the resulting paleolatitude for the pigeon point formation could indicate that salinia served as a link between the cratonic sierra nevada arc to the north and the peninsula ranges baja borderlands allochthon to the south alternatively our results suggesting a 10 compaction inclination shallowing for the pigeon point turbidites may indicate that many of the paleomagnetic studies placing the peninsula ranges baja borderlands 15 to the south of north america in the cretaceous and tertiary may have suffered from a similar effect and that the allochthon has been nearly in place since the cretaceous

long term variability of mars exosphere based on precise orbital analysis of mars global surveyor and mars odyssey a long term perspective on mars exosphere variability at 405 km is provided by merging together density data derived from precise orbit determination of the mars global surveyor and mars odyssey mo satellites extending from 2001 to 2010 these data are heavily weighted toward afternoon local times at high latitudes in the southern hemisphere clear long term solar and annual variations are well captured by empirical formulas residuals from the empirical fit show evidence for relative depletions in exosphere density around mars closest approach to earth which would be consistent with a scavenging mechanism that is dependent on solar wind dynamic pressure superimposed on this variation with mars sun distance are positive density residuals during mars year my 25 my27 and my29 that are apparently due to elevated dust levels in mars middle atmosphere however during my24 my26 and my28 there are dust level increases without any corresponding increase in exosphere density we suspect that this inconsistency is related to a variable ability to sense the response to dust related effects imposed by the high latitude limitations of our measurements combined with interference between the mechanisms that translate middle atmosphere heating to an exosphere response evidence also supports the hypothesis that winter helium bulge effects contributed to the inferred interannual density variability during the 2007 2009 solar minimum period when the o he transition height likely resided near the 405 km orbit of mo

physiological significance of low atmospheric co2 for plant climate interactions abstract methods of palaeoclimate reconstruction from pollen are built upon the assumption that plant climate interactions remain the same through time or that these interactions are independent of changes in atmospheric co 2 the latter may be problematic because air trapped in polar ice caps indicates that atmospheric co 2 has fluctuated significantly over at least the past 400 000 yr and likely the last 1 6 million yr three other points indicate potential biases for vegetation based climate proxies first c 3 plant physiological research shows that the processes that determine growth optima in plants photosynthesis mitochondrial respiration photorespiration are all highly co 2 dependent and thus were likely affected by the lower co 2 levels of the last glacial maximum second the ratio of carbon assimilation per unit transpiration called water use efficiency is sensitive to changes in atmospheric co 2 through effects on stomatal conductance and may have altered c 3 plant responses to drought third leaf gas exchange experiments indicate that the response of plants to carbon depleting environmental stresses are strengthened under low co 2 relative to today this paper reviews the scope of research addressing the consequences of low atmospheric co 2 for plant and ecosystem processes and highlights why consideration of the physiological effects of low atmospheric co 2 on plant function is recommended for any future refinements to pollen based palaeoclimatic reconstructions

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

where are the missing faults in translated terranes abstract a central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers while the faults along which the terranes moved these great distances are not found relations long known from geology and from recent studies of modern transform plate boundaries suggest that preserving strike slip faults that record 1000 km scale translations is unlikely and therefore in many cases paleomagnetism may be our only reliable indicator of how far terranes translated four reasons are explored here for why large offset strike slip faults are likely to be missing in translated terranes 1 strike slip faults in continents commonly are partially or wholly destroyed or reactivated only 400 500 km of over 1000 km of offset on cretaceous early tertiary strike slip faults in northern british columbia are accounted for in southern british columbia the missing strike slip faults probably continued south into the hinterland of the rocky mountain thrust belt as oblique thrust faults much like the modern alpine marlborough fault system of new zealand the major strike slip faults in southeastern british columbia were obliterated by eocene extensional faulting uplift and erosion 2 offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale many of which are left behind and not found in the translated terrane the greater san andreas plate boundary has accumulated 1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern california there is a narrow terrane in the offshore borderland of california that has moved 1200 km relative to north america but no single fault has nearly that much offset 3 major faults in narrow oceans formed by oblique divergent plate boundaries have a low potential for preservation for example the deep dense oceanic crust of the gulf of california where the major transform faults of the oblique rift lie has a low potential for preservation in contrast baja california is currently a continental terrane that has translated 300 km relative to north america along the plate boundary in the gulf of california because of the relative buoyancy of continental crust baja california is likely to be preserved in any future accretion event 4 faults in the margins of oblique divergent plate boundaries are dominantly normal faults and not strike slip faults there is little strike slip faulting in central baja california even though the plate boundary has a rift angle of only 20 recent field data and modeling show that at 0 20 strike slip faults will dominate the secondary faults surprisingly at 20 to 35 there are both strike slip and normal faults and at 35 45 there are few or no strike slip faults even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion

equatorial origin for lower jurassic radiolarian chert in the franciscan complex san rafael mountains southern california lower jurassic radiolarian chert sampled at two localities in the san rafael mountains of southern california 20 km north of santa barbara contains four components of remanent magnetization components a b and b are inferred to represent uplift miocene volcanism and subduction accretion overprint magnetizations respectively the fourth component c isolated between 580 and 680 c shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during or soon after deposition both sequences are late pliensbachian to middle toarcian in age and an average paleolatitude calculated from all tilt corrected c components is 1 3 north or south this result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern california and mexico this result supports our model in which deep water franciscan type cherts were deposited on the farallon plate as it moved eastward beneath the equatorial productivity high were accreted to the continental margin at low paleolatitudes and were subsequently distributed northward by strike slip faulting associated with movements of the kula farallon and pacific plates upper cretaceous turbidites of the cachuma formation were sampled at agua caliente canyon to determine a constraining paleolatitude for accretion of the jurassic chert sequences these apparently unaltered rocks however were found to be completely overprinted by the a component of magnetization similar in situ directions and demagnetization behaviors observed in samples of other upper cretaceous turbidite sequences in southern and baja california imply that these rocks might also give unreliable results

correction of inclination shallowing and its tectonic implications the cretaceous perforada formation baja california abstract a paleomagnetic and magnetic fabric study of the aptian to albian perforada formation the oldest sedimentary unit of the cretaceous valle group was conducted to determine whether the perforada inclination had been shallowed by burial compaction and to correct the inclination for any shallowing the perforada formation is located on the vizcaino terrane of the vizcaino peninsula of west central baja california an accurate paleolatitude will indicate whether the vizcaino terrane is far travelled eighty eight specimens from 43 independently oriented hand samples and cores were collected from 11 sites a small 10 m fold was sampled in order to constrain the age of the magnetization with a fold test thermal and alternating field demagnetization was able to isolate characteristic magnetizations for most samples maximum unblocking temperatures near 580 c indicate that magnetite is the magnetic carrier the characteristic magnetization passes the fold test at the 99 confidence level anisotropy of anhysteretic remanence aar and anisotropy of magnetic susceptibility ams measurements show that five sites had well developed bedding parallel foliations while four sites had poorly developed aar fabrics but foliated ams fabrics the remaining two sites had poorly developed foliations for both ams and aar the mean inclination i 35 6 of the five sites with well developed aar foliations is 13 7 shallower than the mean inclination i 49 3 of the two sites with poorly developed aar and ams fabrics when the inclinations of the sites with well developed aar foliations are corrected using an individual particle anisotropy of 1 5 determined from magnetic extract epoxy samples dried in 30 50 mt dc magnetic fields the corrected mean inclination i 50 1 is indistinguishable from the mean inclination of the sites with poorly developed aar fabrics a corrected formation mean direction for the perforada formation d 311 8 i 47 0 95 9 3 indicates a 33 5 11 1 counterclockwise rotation and 5 6 7 7 of post cretaceous northward motion after 3 of neogene fault motion has been removed these results indicate that the vizcaino terrane has moved little with respect to the peninsular ranges terrane and cratonic north america since the cretaceous

the response of geomorphic systems to climatic and hydrological change during the late glacial and early holocene in the humid and sub humid tropics

mars high resolution gravity fields from mro mars seasonal gravity and other dynamical parameters abstract with 2 years of tracking data collection from the mro spacecraft there is noticeable improvement in the high frequency portion of the spherical harmonic mars gravity field the new jpl mars gravity fields mro110b and mro110b2 show resolution near degree 90 additional years of mgs and mars odyssey tracking data result in improvement for the seasonal j 3 gravity changes which compares well to global circulation models and odyssey neutron data and mars rotation and precession 7594 10 mas year once atmospheric dust is accounted for in the spacecraft solar pressure model solutions for mars solar tide are consistent between data sets and show slightly larger values k2 0 164 0 009 after correction for atmospheric tide compared to previous results further constraining core models an additional 4 years of mars range data improves the mars ephemeris determines 21 asteroid masses and bounds solar mass loss dgmsun dt

remagnetization of cretaceous forearc strata on santa margarita and magdalena islands baja california sur implications for northward transport along the california margin paleomagnetic data for two sections of cretaceous forearc strata with different structural attitudes on santa margarita and magdalena islands in baja california sur mexico indicate that these rocks have been remagnetized probably during the late cenozoic the in situ paleomagnetic directions however are similar to data from other cretaceous rocks on peninsular california with unexpectedly shallow inclinations and easterly declinations these data have been interpreted as indicating either northward tectonic transport 10 15 of latitude and clockwise rotation 20 or compaction shallowing of magnetic inclinations in sedimentary rocks combined with southwestward tilting of plutonic rocks the available paleomagnetic data for cretaceous forearc strata in southern and baja california can be divided into three groups 1 sections with normal polarity magnetizations that fail fold tests and are remagnetized 2 sections with normal polarity magnetizations with no or inconclusive fold tests that may or may not be remagnetized and 3 sections with both normal and reversed polarity intervals where pervasive remagnetization has not occurred other rocks of the mesozoic great valley group coast range ophiolite and franciscan complex in california also have secondary magnetizations with directions similar to younger geomagnetic field directions although these widespread remagnetizations could have variable local causes we propose regional burial and uplift related to changes in subduction parameters as a possible explanation two episodes of remagnetization are apparent one in the late cretaceous and a second in the late cenozoic on the other hand the unremagnetized and apparently reliable data from sedimentary and plutonic rocks on the baja peninsula consistently indicate northward translation 14 3 and clockwise rotation 29 8 with respect to north america since the late cretaceous

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

clockwise rotation and implications for northward drift of the western transverse ranges from paleomagnetism of the piuma member sespe formation near malibu california 1 new paleomagnetic results from mid tertiary sedimentary beds in the santa monica mountains reinforce the evidence for large scale rotation of the western transverse ranges and anisotropy measurements indicate that compaction induced inclination flattening may resolve a long standing controversy regarding the original paleolatitude of the rotated block previously published paleomagnetic data indicate that post oligocene rotation amounts to 70 110 clockwise affecting the channel islands santa monica mountains and santa ynez mountains the sespe formation near malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section the piuma member which consists of gray red sandstone and mudstone interbedded with minor tuff and limestone beds the piuma member has a paleomagnetic pole at 36 6 n 326 7 e a95min 5 0 a95max 9 6 obtained by thermal demagnetization of 34 oriented cores from oligocene and early miocene beds after correcting for plunge of the geologic structure the data are consistent with significant clockwise rotation 77 7 of the region relative to stable north america rotation of the western transverse ranges is generally viewed as a consequence of pacific north american plate interactions after 28 ma when east west subduction gave way to northwest transform motion in southern california inclinations from the piuma study indicate a paleolatitude anomaly of 11 7 and are consistent with a mean northward drift that exceeds generally accepted san andreas fault displacement by a factor of 3 however sedimentary inclination error may accentuate the anomaly anisotropy of isothermal remanent magnetization indicates inclination flattening of approximately 8 and correction for the effect reduces the paleolatitude anomaly to 5 3 5 8 compaction may explain the inclination flattening in these sedimentary rocks but the process does not adequately explain lower than expected inclinations found in previous studies of miocene volcanic rocks of the western transverse ranges

carbon sinks in small sahelian lakes as an unexpected effect of land use changes since the 1960s saga gorou and dallol bosso sw niger for several decades global change has resulted in an increase in runoff in the sahelian belt provoking major changes in the quality and quantity of sediments transported by drainage networks one of the astonishing consequences is the establishment of numerous permanent lakes the origins of particulate organic matter om preserved within lacustrine sediments of three lakes were investigated by coupling optical observations palynofacies and bulk geochemistry rock eval 6 pyrolysis an initial estimate of particulate organic carbon oc stored in these lacustrine sediments was assessed soil organic matter som was sampled from the surface 0 10 cm of various land use and land cover areas and was characterized and compared with sedimentary organic matter our results reveal that lake tankalawal is subjected to high autochthonous organic sedimentation toc ranges between 3 0 and 10 0 wt while lakes bangou kirey and bi are characterized by weak sedimentation of non indigenous om originating from the soil erosion and shore vegetation toc 3 0 wt in sediments the effects of early diagenesis on the om induce not only a loss of labile and aquatic om but also a significant loss of terrestrial om which is supposed to be more resistant than its aquatic counterpart both the preservation of om in top sediments and the relative preservation of terrestrial om impact the oc storage in lakes indeed oc storage in lacustrine sediments lake bangou kirey was two to seven times higher than the oc storage in sahelian soils where greater contributions of terrestrial om to sedimentary om are associated with lower oc storage in lacustrine sediments for lakes bangou kirey and bi oc accumulation rates were also assessed due to the identification of a sedimentary limit corresponding to the establishment of permanent lakes dated earlier 1960s high oc fluxes were estimated and ranged between 104 and 213 g oc m 2 yr 1 compared with other oc accumulation rates for various african lakes these high values are similar to those calculated for reservoirs and are related to anthropogenic pressure soil textures favoring erosion and proper physical and chemical conditions for om preservation in sediments accordingly in these sahelian environments that are generally viewed as non efficient in storing oc we claim that global change could promote a new oc sink if other similar studies reinforce our assertion then regional c budgets should be revisited