hierarchical segmentation of the malawi rift the influence of inherited lithospheric heterogeneity and kinematics in the evolution of continental rifts we used detailed analysis of shuttle radar topography mission digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts we focused on the cenozoic malawi rift which represents the southern extension of the western branch of the east african rift system this north trending rift traverses precambrian and paleozoic mesozoic structures of different orientations we found that the rift can be hierarchically divided into first order and second order segments in the first order segmentation we divided the rift into northern central and southern sections in its northern section the rift follows paleoproterozoic and neoproterozoic terrains with structural grain that favored the localization of extension within well developed border faults the central section occurs within mesoproterozoic neoproterozoic terrain with regional structures oblique to the rift extent we propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults in the southern section mesoproterozoic neoproterozoic rocks were reactivated and developed the border faults in the second order segmentation only observed in the northern section we divided the section into five segments that approximate four half grabens asymmetrical grabens with alternating polarities the change of polarity coincides with flip over full grabens occurring within overlap zones associated with 150 km long alternating border faults segments the inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the malawi rift during its opening resulting from extension

major pathways of atlantic water in the northern north atlantic and nordic seas toward arctic 1 the major pathways of near surface atlantic water in the northern north atlantic and nordic seas are identified as current speeds above 30 cm s using 1014 lagrangian drifters combined with previously published hydrography the inflow over the scotland greenland ridge and establishment of the two branch norwegian atlantic current nwac are described in light of the circulation in the northern north atlantic the western branch of the nwac appears as a jet in the polar front topographically guided from the iceland faroe front through the nordic seas toward fram strait the eastern branch starts as a shelf edge current above the irish scottish continental shelf and after passing through the faroe shetland channel it continues northward along the norwegian shelf edge toward the arctic with a branch bifurcating into the barents sea the nwac appears to maintain its two branch structure throughout the nordic seas with the atlantic water confined to a 200 600 km wide wedge

the neogene to recent rallier du baty nested ring complex kerguelen archipelago taaf indian ocean stratigraphy revisited implications for cauldron subsidence mechanisms 1 ups cnrs fre 2566 orsayterre department des sciences de la terre universite de paris sud f 91405 orsay cedex france bbonin geol u psud fr 2 cnrs umr 6524 magmas et volcans department de geologie petrologie geochimie universite jean monnet f 42023 saint etienne cedex 2 france 3 cnrs ep 125 department des sciences de la terre universite de nice sophia antipolis f 06108 nice cedex 2 france 4 department of geology university college dublin belfield dublin 4 eire 5 gemoc macquarie university sydney australia

evolution of the broadly rifted zone in southern ethiopia through gravitational collapse and extension of dynamic topography abstract the broadly rifted zone brz is a 315 km wide zone of extension in southern ethiopia it is located between the southern main ethiopian rift and the eastern branch of the east african rift system ears represented by the kenya turkana rift the brz is characterized by ne trending ridges and valleys superimposed on regionally uplifted 2 km average elevation terrain previous studies proposed that the brz is an overlap zone resulted from northward propagation of the kenya turkana rift and southward propagation of the southern main ethiopian rift to understand the relationship between the brz s extensional style and its crustal and upper mantle structures this work first estimated the moho depth using the two dimensional 2d radially averaged power spectral analysis of the world gravity map verification of these results was accomplished through lithospheric scale 2d forward gravity models along e w profiles this work found that the moho topography beneath the brz depicts a dome like shape with a minimum depth of 27 km in the center of the dome this work proposes that the moho doming crustal arching underlying the brz and associated topographic uplift are the result of asthenospheric mantle upwelling beneath the brz this upwelling changed to a ne directed lateral mantle flow at shallower depth this is supported by seismic tomography imaging which shows slow s wave velocity anomaly at lithospheric depth of 75 km to 150 km stretching in a ne sw direction from beneath the brz to the afar depression this work proposes that the asthenospheric upwelling created gravitationally unstable dynamic topography that triggered extensional gravitational collapse leading to the formation of the brz as a wide rift within the narrow rift segments of the ears

a regional gravity study of the west african rift system in nigeria and cameroon and its tectonic interpretation abstract this study brings together existing and new gravity data to investigate the nature and cause of the bouguer gravity field associated with the cretaceous west african rift system in nigeria and cameroon the new gravity measurements include data collected over the basement area between the benue trough and the cameroon border and fill an important gap in the gravity coverage after removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly associated with the rift system is interpreted in terms of two and three dimensional crustal models these models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the yola rift the results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle benue is approximately 20 km thick while beneath the gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching then the maximum possible crustal extensions of 95 65 and 55 km have taken place perpendicular to the benue gongola and yola rifts respectively these crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the benue and yola rifts have been subjected to varying amounts of shear displacement during the cretaceous and early tertiary times crustal extension estimates across the west african rift system could be as much as four times greater than the published extension estimates across the east african rift system these rift systems have the same plate tectonic setting being the failed third arms of successful triple junctions and are the result of extensional and shear tectonics the differences in crustal extension and the resulting isostatic response of the lithosphere beneath these rifts can explain why the west african rift system has been associated with subsidence processes throughout its development whereas the east african rift system in northeast africa has been strongly affected by uplift

late weichselian and holocene relative sea level history of broggerhalvoya spitsbergen abstract radiocarbon dated whalebones from raised beaches record a relative sea level history for broggerhalvoya western spitsbergen that suggest a two step deglaciation on svalbard at the end of the late weichselian glaciation the late weichselian marine limit was reached at about 13 000 yr b p and was followed by relatively slow emergence until about 10 000 yr b p either in response to ice unloading in the barents sea initial retreat of local fjord glaciers or some combination of the two rare whale skeletons dating between 13 000 and 10 000 yr b p indicate that the norwegian sea was at least seasonally ice free during that interval deglaciation of spitsbergen is recorded by the rapid emergence of broggerhalvoya after 10 000 yr b p this was followed by a transgression during the mid holocene here named the talavera transgression and another in modern times raised beach morphologies suggest striking differences in nearshore depositional processes before and after 10 000 yr b p that are probably related to changes in the rate of uplift and in sea ice conditions

climate records from a bivalved methuselah arctica islandica mollusca iceland we measured annual shell growth rates of a 374 year old radiometrically confirmed bivalve mollusk specimen of arctica islandica linnaeus this bivalve represents the oldest ever reported individual animal in addition we analyzed 1051 individual powder samples from ontogenetic years two to 40 80 of the entire shell height for isotope analyses oxygen and carbon isotopes exhibit clear intra annual cycles the highest measured carbon isotope values on average 2 73x occurred during summer i e during maximum primary productivity the d 18 oaragonite derived temperature time series cover the period of ad 1496 1533 temperatures calculated from oxygen isotopes ranged from 4 5 to 9 3 8c and exhibit a mean value of 6 2 8c the latter coincides well with the 1854 2003 mean value of 6 81 8c for sea surface temperature between february and september growing season of a islandica neither oxygen nor carbon isotopes exhibit age related unidirectional trends however d 13 caragonite and d 18 oaragonite values fluctuated at decadal periods of four six and eight to nine years nao type periods as well as 12 14 years which may represent teleconnections to cycles in the tropical atlantic annual shell growth is positively correlated to intra annual d 18 oaragonite minima i e warm summer temperatures r 2 0 34 and to intra annual d 13 caragonite minima higher food supply r 2 0 42 using a linear multiregression model 65 of the variation in annual shell growth can be explained by summer temperature and food supply the formation of extremely narrow annual increments coincides with major volcanic eruptions e g tambora 1815 a period of extremely variable growth occurred during the culmination of the little ice age in iceland between ca ad 1550 and 1620 shell growth during ad 1765 1780 however was characterized by very little year to year variability probably as the result of extremely mild climate near the end of the little ice age this study demonstrates that shells of a islandica provide

tilt depth method a simple depth estimation method using first order magnetic derivatives aeromagnetic data are routinely presented as contour or color shaded maps of the total magnetic intensity tmi an interpreter s task is to identify features anomalies contained within the map and qualitatively and or quantitatively interpret them into geologic structures at depth if the map contains anomalies that have large magnetic intensities the bodies might be considered to have large magnetizations or to be at shallow depths small amplitude anomalies superimposed on these anomalies could be masked or even missed by an interpreter thus the task of the interpreter is to use the spectral content of the anomalies to try and resolve these ambiguities part of this process is also to obtain estimates of the depth and shape of the body causing the anomalies

lithospheric structure of the malawi rift implications for magma poor rifting processes this project was supported by the national science foundation continental dynamics nsf cd grant ear 1255233 and office of international and integrative activities oiia grant iia 1358150 this project is also supported by the nsf earthcube integration grant 1740627 we thank the computational infrastructure for geodynamics geodynamics org which is funded by the national science foundation under award ear 0949446 and ear 1550901 for supporting the development of advanced solver for problems in earth s convection

a tectonic model for the location of palaeozoic ring complexes in air niger west africa the fir region in niger is one of the largest peralkaline granite provinces in the world in addition to the granites some plutons are characterized by an abundance of plagioclase rich cumulates gabbros troctolites and anorthosites with subordinate metaluminous granites and quartz syenites these anorogenic ring complexes were intruded along a 400 km zone near the 9 e meridian the province formed during a major magmatic event newly dated at around 407 8 ma and comprises 28 plutons which range from 0 8 to 65 km in diameter and show no correlation between distribution and age remote sensing over the whole fir massif demonstrates the existence of two main trends of faults or lineaments n50 e n90 e and n120 e n150 e autocorrelation analysis of the ring complexes reveals three large parallel high density strips orientated n20 e based on the structural and geological setting coupled with map analysis a new tectonic model for the location of the complexes is presented we propose a dextral n5 e shear zone model where the emplacement of the ring structures is controlled by n20 e dextral riedel shear r with a secondary n50 e tensional gash t this tectonic model implies a control by lithospheric structures for the emplacement of the ring complexes as well as a relationship between a transtensional tectonic regime and intra plate alkaline magmatism the new geochronological data on the ayir massif allow us to derive a silurian devonian palaeomagnetic pole representative for gondwana which fits data for the other continents better than previous estimates

marine 14c reservoir ages for 19th century whales and molluscs from the north atlantic abstract in order to compare radiocarbon dates on marine and terrestrial samples the former have to be corrected for a reservoir age we present reservoir ages from dating 21 whales collected 1860 1901 and recalculating dates of 23 molluscs collected 1857 1926 most of the whales were caught along the coast of norway but one is from france and one from iceland we assume the former mainly lived in the north and equatorial atlantic and in the norwegian sea whales feed only on pelagic organisms and will provide the reservoir age for the open ocean surface water however they travel long distances and will integrate the reservoir ages of the different water masses along their way molluscs dated from norway spitsbergen and arctic canada are stationary and monitor the sea water passing their dwelling site but some also take up carbon from particulate food or sediment pore water coastal water also often contains some continental carbon we present two different views on how to analyze and interpret the data mangerud recommends to use reservoir ages based on a combination of the whale and mollusc dates i e 380 30 and 360 30 yr relative to intcal04 and british oak respectively and a r value of 20 30 for the surface water in the n atlantic and norwegian sea bondevik and gulliksen maintain that the reservoir age and r along the norwegian coast is latitude dependant with r values increasing from 3 22 in the south to 105 24 at spitsbergen whales reflecting north atlantic open ocean surface water have lower r 7 11 than most molluscs

late weichselian local ice dome configuration and chronology in northwestern svalbard early thinning late retreat the chronology and configuration of the svalbard barents sea ice sheet sbsis during the late weichselian lw are based on few and geographically scattered data thus the timing and configuration of the sbsis has been a subject of extensive debate we present provenance data of erratic boulders and cosmogenic 10be ages of bedrock and boulders from northwest spitsbergen nws svalbard to determine the thickness configuration and chronology during the lw we sampled bedrock and boulders of mountain summits and summit slopes along with erratic boulders from coastal locations around nws we suggest that a local ice dome over central nws during lw drained radially in all directions provenance data from erratic boulders from northern coastal lowland reinsdyrflya suggest northeastward ice flow through liefdefjorden 10be ages of high elevation erratic boulders in central nws 687 836 m above sea level ranging from 18 3 1 3 ka to 21 7 1 4 ka indicate that the centre of a local ice dome was at least 300 m thicker than at present 10be ages of all high elevation erratics 400 m above sea level central and coastal locations indicate the onset of ice dome thinning at 25 20 ka 10be ages from erratic boulders on reinsdyrflya ranging from 11 1 0 8 ka to 21 4 1 7 ka indicate an ice cover over the entire reinsdyrflya during lw and a complete deglaciation prior to the holocene but apparently later than the thinning in the mountains lack of moraine deposits but the preservation of beach terraces suggest that the ice covering this peninsula possibly was cold based and that reinsdyrflya was part of an inter ice stream area covered by slow flowing ice as opposed to the adjacent fjord which possibly was filled by a fast flowing ice stream despite the early thinning of the ice sheet 25 20 ka we find a later timing of deglaciation of the fjords and the distal lowlands several bedrock samples 10be from vertical transects in the central mountains of nws pre date the lw and suggest either ice free or pervasive cold based ice conditions our reconstruction is aligned with the previously suggested hypothesis that a complex multi dome ice sheet configuration occupied svalbard and the barents sea during lw with numerous drainage basins feeding fast ice streams separated by slow flowing possibly cold based inter ice stream areas

and ratios in skeletal calcite of mytilus trossulus covariation with metabolic rate salinity and carbon isotopic composition of seawater minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity although molluscan calcite is secreted at or near oxygen isotope equilibrium the influence of metabolic activity i e vital effects on skeletal srca ratios and 13c values is not well known we present measurements of skeletal chemistry from a consecutive samples milled in chronological order from the organism s final year of growth and b adjacent samples within three separate growth bands of the marine mussel mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell seawater temperature and salinity were monitored for one year at squirrel cove british columbia at the end of the year a young rapidly growing mussel mussel a and an old slowly growing mussel mussel b were harvested from this site growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry results from consecutive samples show 1 a significantly higher average srca ratio in mussel a than that in mussel b and 2 13c values that correlate well with salinity in mussel a but not in mussel b results from time equivalent adjacent samples show 1 higher srca and 13c values near the ventral margin than at time equivalent regions on lateral margins of the shell and 2 little or no variation in 18o values these observations suggest that skeletal chemistry srca and 13c is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell hence the chemistry of calcite secreted at the ventral margin is precipitated in near equilibrium with seawater and allows accurate estimation of seawater srca ratios and carbon isotopic composition in contrast shell precipitation along lateral margins is dominantly controlled by metabolic activity and calcite chemistry is not in equilibrium with ambient seawater in this context we present a biomineralization model where variations in skeletal srca and 13c values are explained in the context of mantle metabolic activity and seawater salinity

changes in north atlantic radiocarbon reservoir ages during the allerod and younger dryas estimates of the radiocarbon age of seawater are required in correlations between marine and terrestrial records of the late quaternary climate we radiocarbon dated marine shells and terrestrial plant remains deposited in two bays on norway s west coast between 11 000 and 14 000 years ago a time of large and abrupt climatic changes that included the younger dryas yd cold episode the radiocarbon age difference between the shells and the plants showed that sea surface reservoir ages increased from 400 to 600 years in the early yd stabilized for 900 years and dropped by 300 years within a century across the yd holocene transition

late weichselian marine 14c reservoir ages at the western coast of norway abstract a shallow marine late weichselian deposit on the outer coast of western norway contains both terrestrial plant material and articulated marine shells we have 14 c dated both types of material from eight different stratigraphic levels covering the time interval 12 300 to 11 100 14 c yr b p the difference between 14 c dated terrestrial plant material and marine shell material the marine reservoir age ranges from 200 to 525 yr with a weighted average of 380 32 yr this is almost identical to the present reservoir age of 379 20 yr for southern norway in the mid younger dryas yd interval the reservoir age in the north atlantic 55 n 65 n was 700 800 yr considerably greater than the present reservoir age and the age we have measured for the bolling allerod interval the reason for this increase during the yd is probably a combination of reduced inflow of surface waters to the north atlantic and more extensive sea ice cover evidence from marine cores show that the southeastern norwegian sea experienced rapid fluctuations in the inflow of warm atlantic surface water during the period 12 300 to 11 000 yr b p however the reservoir age apparently did not increase during these colder periods older dryas i and ii the reason is probably that in contrast to the yd these colder periods did not last long enough and or were of too limited extent to alter the reservoir age of the ocean a comparison of the obtained 14 c dates with the varve 14 c chronology from lake suigetsu indicates an age of 12 770 cal yr b p for the al yd boundary

temporal variation in the interhemispheric 14c offset contemporaneous tree ring dated wood from trees in the northern and southern hemispheres gives different 14c dates previous studies vogel et al 1986 1993 using wood from south africa and the netherlands have shown depletion s of 4 56 0 85 0 and 5 12 0 62 0 respectively this trans lates to age differences of 36 7 and 41 5 years yrs with the southern hemisphere giving the older dates more recently stuiver and braziunas 1998 have shown that an offset of 23 4 yrs exists between combined 19th century wood measure ments from tasmania and chile in the southern hemisphere and the west coast of the u s washington in the northern hemisphere in this study measurements on contemporaneous decadal samples of oak from the british isles and cedar from new zealand over the period 1725 to 1885 ad show a deple tion of 3 4 0 589 00 27 2 4 7 yrs however data after 1895 ad has a mean offset of 0 66 1 06 o 5 3 8 5 yrs with increased variance compared to 19th century data this we believe is attributable to anthropogenic fossil fuel which due to its long residence time in the earth has long since lost any 14c component and when burned preferentially depletes the northern hemisphere atmosphere of 14c

radiocarbon content of pre bomb marine mollusks and variations in the 14c reservoir age for coastal areas of the barents and kara seas russia fourteen mollusks collected alive between 1900 and 1945 from the russian barents and kara seas were analyzed by ams 14c dating to evaluate variations in the 14c marine reservoir for arctic coastal sites which is important for correcting ages in paleoenvironmental time series and advancing understanding of the exchange of carbon the 14c ages on the mollusks reveal a range of marine reservoir values r t from 159 14c yr to 764 14c yr the oldest r t values of 764 to 620 14c yr are for the bivalve portlandia arctica which often inhabit cold and low salinity waters and muddy substrates the depleted 14c content for this bivalve reflects possibly the incorporation of old carbon from freshwater inputs and or the consumption of old organic matter from the underlying sediments and pore waters other mollusks with sessile habitats and pelagic food sources gave significantly lower r t values between 159 and 344 14c yr the youngest r t values indicate enrichment in 14c and may partially reflect enhanced transfer of 14c enriched co2 from the atmosphere to the ocean surface with wind generated wave agitation this study underscores that a variety of processes can lead to variable 14c depletion and enrichment of surface waters yielding a ca 600 year age span for contemporaneous arctic mollusks there may be added uncertainty in the 14c reservoir correction for deposit feeder species such as portlandia sp and perhaps for certain benthic foraminifera e g nonion labradoricum because these taxa often incorporate old organic matter from the substrate a reservoir correction of 700 years may be more appropriate for infaunal deposit eater species particularly in glacier dominated environments mollusks and foraminifera with sessile habits and pelagic food sources should be selected preferentially for 14c dating because their shells may more closely reflect the 14c content of the global ocean mixed layer

the shell of the queen scallop aequipecten opercularis l as a promising tool for palaeoenvironmental reconstruction evidence and reasons for equilibrium stable isotope incorporation abstract the queen scallop aequipecten opercularis has been an important part of the marine fauna of the northeast atlantic region since the miocene and is a suitable candidate for palaeoenvironmental studies using stable isotopes modern shells were investigated to determine if carbonate is precipitated in isotopic equilibrium in this species and to confirm that growth continues through the year specimens of a opercularis were cultured under monitored semi natural conditions over an autumn winter period and the analyses of these specimens were supplemented with isotope data from preserved indigenous north sea specimens the results indicated that the cultured specimens grew in relatively low temperatures that oxygen isotopes of shell exterior carbonate were precipitated in isotopic equilibrium and that a large proportion of the ambient temperature range was recorded in shell carbonate the indigenous north sea specimens exhibited a good correspondence to predicted oxygen isotope values for summer the isotopic record related to winter was however somewhat truncated due to growth decelerations in the specimens carbon isotopes of shell exterior carbonate were calculated as being derived primarily from external sources and showed comparatively little influence from metabolically derived carbon this suggests that carbon isotopes were at least close to equilibrium with ambient waters the limited incorporation of metabolic products suggests that a opercularis would be a good monitor of changing carbon isotope ratios in seawater dissolved inorganic carbon the occurrence of equilibrium shell precipitation is explained using a pteriomorph shell calcification model which proposes that transport of ions through the periostracum caused the composition of the extrapallial fluid to be effectively the same as seawater carbonate secretion is therefore not influenced significantly by biological vital effects

the last glacial maximum on spitsbergen svalbard most scientists have concluded previously that the west coast of spitsbergen svalbard remained ice free during the late weichselian between 25 000 and 10 000 yr b p we conclude that the glaciation was more extensive terraces that were postulated to have been ice free are covered by a thin late weichselian till sudden drop in the marine limit and basal radiocarbon dates of raised glaciomarine sediments demonstrates that the glaciers in the main fjords isfjorden and van mijenfjorden terminated west outside of the fjord mouths basal radiocarbon dates from glaciomarine clay above till in cores from the continental shelf west of spitsbergen yielded ages of about 12 500 yr b p from which we conclude that the ice extended to the shelf edge based on the extent of amino acid diagenesis in radiocarbon dated molluscs the duration of the maximum extension of the late weichselian glaciation was short certainly less than 10 000 years during the ice free period preceding that glaciation at least back to 40 000 yr b p the glaciers on svalbard were not significantly larger than at present as shown by marine deposits close to the glacier snouts many radiocarbon dates place deglaciation of the outer coast at about 12 500 yr b p at about 10 000 yr b p the rest of the archipelago rapidly became ice free

crustal density structure in the spanish central system derived from gravity data analysis central spain abstract shallow and deep sources generate a gravity low in the central iberian peninsula long wavelength shallow sources are two continental sedimentary basins the duero and the tajo basins separated by a narrow mountainous chain called the spanish central system to investigate the crustal density structure a multitaper spectral analysis of gravity data was applied to minimise biases due to misleading shallow and deep anomaly sources of similar wavelength first an estimation of gravity anomaly due to cenozoic sedimentary infill was made power spectral analysis indicates two crustal discontinuities at mean depths of 31 1 3 6 and 11 6 0 2 km respectively comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the moho discontinuity a 3d depth model for the moho was obtained by inverse modelling of regional gravity anomalies in the fourier domain the moho depth varies between a mean depth of 31 km and 34 km maximum depth is located in a nw se trough gravity modelling points to lateral density variations in the upper crust the central system structure is described as a crustal block uplifted by ne sw reverse faults the formation of the system involves displacement along an intracrustal detachment in the middle crust this detachment would split into several high angle reverse faults verging both nw and se the direction of transport is northwards the detachment probably being rooted at the moho

new insights into magnetic derivatives for structural mapping magnetic and gravity derivatives can be likened to seismic attributes in that they can help define estimate the physical properties of the source structure causing the anomaly this contribution looks at the tilt derivative first reported in 1994 and more recently used to derive the local wavenumber 1997 we will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives we provide the simple theory behind the derivatives use a range of simple 2d models to illustrate their response and apply them to mapping a mineral target in namibia

the north atlantic atmosphere sea surface 14c gradient during the younger dryas climatic event abstract we attempt to quantify the 14c difference between the atmosphere and the north atlantic surface during a prominent climatic period of the last deglaciation the younger dryas event yd our working hypothesis is that the north atlantic may have experienced a measurable change in 14c reservoir age due to large changes of the polar front position and variations in the mode and rate of north atlantic deep water nadw production we dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere sea surface 14c gradient we selected terrestrial vegetal macrofossils and planktonic foraminifera neogloboquadrina pachyderma left coiling mixed with the same volcanic tephra the vedde ash bed which occurred during the yd and which can be recognized in north european lake sediments and north atlantic deep sea sediments based on ams ages from two norwegian sites we obtained about 10 300 yr bp for the atmospheric 14c age of the volcanic eruption foraminifera from four north atlantic deep sea cores selected for their high sedimentation rates 10 cm kyr 1 were dated by ams 21 samples for each core the raw 14c ages assigned to the ash layer peak is significantly older than the 14c age obtained on land part of this discrepancy is due to bioturbation which is shown by numerical modelling nevertheless after correction of a bioturbation bias the mean 14c age obtained on the planktonic foraminifera is still about 11 000 11 100 yr bp the atmosphere sea surface 14c difference was roughly 700 800 yr during the yd whereas today it is 400 500 yr a reduced advection of surface waters to the north atlantic and the presence of sea ice are identified as potential causes of the high 14c reservoir age during the yd

crustal structure from gravity signatures in the iberian peninsula through two dimensional fi ltering and spectral analysis of gravity data we infer the density structure of the iberia peninsula s lithosphere western europe the gravity anomaly map of the iberian peninsula displays long wavelength anomaly minima related to alpine ranges these anomalies are primarily linked to a greater crustal thickness low pass filtering of the anomaly map using a cutoff wavelength of 150 km was adequate for effective separation of shallow and deep sources used for computing the three dimensional 3 d moho interface tsuboi s technique identical to the equivalent stratum theorem is used here to map the 3 d moho interface by selecting mean source depths from the results of the spectral analysis and assuming a homogeneous density contrast of 350 kg m3 the most characteristic feature of the 3 d moho geometry was the presence of several lows associated with mountain ranges created by alpine tectonics in those areas the gravity derived moho reaches a depth of up to 45 km in the pyrenees and cantabrian mountains and close to 40 km under the betics central system and iberian chain under the iberian massif the western part of the iberian peninsula composed of a variscan basement the moho was located at a depth of 30 36 km these results are consistent with existing seismic data and indicate that gravity based techniques can provide very good estimates of lithosphere structure

marine reservoir corrections for bowhead whale radiocarbon age determinations twenty two pairs of radiocarbon dates on driftwood and bowhead whale balaena mysticetus bones from raised beaches dates on whale bone and terrestrial plant detritus from a stratigraphie section and 25 additional dates on whale bones from the lowest 1 ka bp raised beaches in the eastern canadian arctic suggest that a marine reservoir correction of about 200 years is appropriate for normalized age determinations on bone collagen from the bowhead whale in this region this is less than the correction 400 years normally applied to carbonate shells of marine molluscs from this region the carbon in bowhead collagen appears to be derived from the whales zooplankton food rather than from marine bicarbonate

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

tectonic controls on rift basin morphology evolution of the northern malawi nyasa rift radiometric k ar and ar 40 ar 39 age determinations of volcanic and volcaniclastic rocks combined with structural gravity and seismic reflection data are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern malawi nyasa rift and to characterize changes in basin and flank morphologies with time faulting and volcanism within the tukuyu karonga basin began at approximately 8 6 ma when sediments were deposited in abroad initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief extensions primarily by a slip along the border fault and subsequent regional isostatic compensation led to the development of a 5 km deep basin bounded by broad uplifted flanks along the low relief basin margin opposite border fault younger stratigraphic sequences commonly onlap older wedge shaped sequences although their internal geometry is often progradational intrabasinal faulting flankuplift and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2 5 ma when cross rift transfer faults developed to link the tukuyu karonga basin to the rukwa basin local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin s depocenter sequence boundaries are commonly erosional along this low relief hanging wall margin and conformable in the deep lake basin the geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake irrespective of paleoclimatic fluctuations