flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

stable isotope paleoecology of white river ungulates during the eocene oligocene climate transition in northwestern nebraska abstract we use the carbon and oxygen stable isotope content of mammalian tooth enamel to evaluate ancient biomes habitat partitioning and water use among 12 ungulate taxa from the white river group in nebraska the white river group spans an interval of climate change from the upper eocene to the lower oligocene when global climate cooled in a heterogeneous fashion we test whether changes in biome structure or mammalian ecology occurred during this interval we find that mean 13c values for both eocene and oligocene faunas are consistent with dry open biomes e g woodland savanna or woody scrubland a significant increase in mean faunal 13c values 0 4 occurs in the lower oligocene primarily from the loss of individuals feeding in wetter denser parts of the ecosystem in the upper eocene megacerops sp brontothere and perchoerus probus peccary have low mean 13c values consistent with feeding in wetter denser habitats presumably riparian whereas significantly higher mean values in the cursorial horse mesohippus spp and the clawed oreodont agriochoerus antiquus are consistent with feeding in more open drier habitats the extinctions of megacerops sp and trigonias sp a rhinocerotid with relatively low 13c values along with a decrease in the abundance of p probus and other taxa suggests reduction in the extent of wet habitats in the oligocene the rare tapir colodon sp is the only taxon sampled with a mean 13c value low enough to indicate consistent feeding in a wet habitat in the oligocene however range through taxa occupy the same relative positions in isotopic space in both the eocene and oligocene faunas suggesting stability in drier habitats a significant increase also occurs in mean faunal 18o values 1 0 in the oligocene primarily from the loss or reduction in abundance of water dependent species variance in 18o values increases significantly in the oligocene which is consistent with greater water stress in plants from drier conditions oxygen values increase in a antiquus 0 9 the most aridity sensitive species in both faunas as expected for drier conditions but values in another aridity sensitive taxon mesohippus spp are virtually unchanged together 13c and 18o enamel values generally suggest a shift to drier conditions in the early oligocene although 13c values yielded more consistent results the lack of a large negative shift in 18o values expected with a decrease in mean annual temperature suggests that any temperature changes in the early oligocene were relatively small in the continental interior of north america

paleoecology of aphelops and teleoceras rhinocerotidae through an interval of changing climate and vegetation in the neogene of the great plains central united states abstract the rhinocerotidae was one of most widespread and abundant mammalian groups in north america during the oligocene and miocene but were extinct by the early pliocene potential drivers of extinction include the expansion of c4 grasslands cooling climate and increased seasonality here we examine potential changes in the relative abundance of two common genera aphelops and teleoceras from the middle miocene to the pliocene of the great plains we also use stable isotopes in tooth enamel to test whether great plains rhinoceroses were able to incorporate c4 vegetation into their diet after the expansion of c4 grasslands relative abundance of rhinoceroses fluctuates during the middle to late miocene but then rapidly declines at the end of the miocene around 6 5 ma coinciding with the expansion of c4 grasslands on the great plains stable carbon isotopes indicate that both aphelops and teleoceras were predominantly c3 feeders for most of their history during c4 expansion some individuals were able to adapt to modest c4 consumption but were not as successful as contemporaneous horses our results suggest that the extinction of rhinoceroses was associated with environmental stress related to diminishing food resources following the expansion of c4 vegetation possibly coupled with a shorter growing season as climate cooled in addition tooth enamel 18o values in rhinoceroses do not support the interpretation that teleoceras a hippo ecomorph had a semi aquatic lifestyle tooth enamel 18o values reflect a strong latitudinal temperature gradient suggesting that rhinoceroses effectively track 18o values in surface waters and are potentially useful for paleoclimatic reconstructions

how well do precambrian paleomagnetic data agree with the phanerozoic apparent polar wander path a baltica case study abstract precambrian paleomagnetic data are crucial for various tectonic and geophysical applications the reconstructions of fusion and fission of supercontinents in particular due to many objective reasons the variety of stability tests for these data are abridged in comparison to phanerozoic results and is further hindered by the rather common refusal to consider the resemblance between precambrian and younger poles in this paper we selected precambrian paleomagnetic data with ages of

oxygen and hydrogen isotopes in the hydrologic cycle changes of the isotopic composition of water within the water cycle provide a recognizable signature relating such water to the different phases of the cycle the isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes as a result meteoric waters are depleted in the heavy isotopic species of h and o relative to ocean waters whereas waters in evaporative systems such as lakes plants and soilwaters are relatively enriched during the passage through the aquifers the isotope composition of water is essentially a conservative property at ambient temperatures but at elevated temperatures interaction with the rock matrix may perturb the isotope composition these changes of the isotope composition in atmospheric waters surface water soil and groundwaters as well as in the biosphere are applied in the characterization of hydrological system as well as indicators of paleo climatological conditions in proxy materials in climatic archives such as ice lake sediments or organic materials

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

the central scandinavian dolerite group protracted hotspot activity or back arc magmatism constraints from u pb baddeleyite geochronology and hf isotopic data the central scandinavian dolerite group csdg occurs in five separate complexes in central sweden and sw finland u pb baddeleyite ages of dolerite dikes and sills fall into three age intervals 1

genetic interpretation of lead isotopic data from the columbia river basalt group oregon washington and idaho lead isotopic signatures of basalts from the columbia river basalt group of central oregon washington and western idaho and of the high alumina olivine basalts from southeastern oregon plot mostly in the field of pb isotopic data from volcanic rocks of the cascade range in contrast to the mid ocean ridge basalts morb the pb isotopic data for several of the formations form small tight clusters and the nd and sr isotopic data show discrete variation between these basalt groups pseudoisochrons are not observed a single model will account for the observed isotopic and trace element data from samples from most of the columbia river basalt group this model calls for partial melting of the convecting oceanic type mantle and mixing with or contamination by fluids derived from continental derived sediments it requires that crustal components are not only subducted along the trench but that they are also transported in the low velocity zone of the mantle at least 400 km behind the active volcanic arc into the back arc environment represented by the columbia plateaus province lead isotopes in the saddle mountains basalt the youngest formation suggest that these magmas were formed by partial melting of a 2 6 b y old subcontinental lithosphere that extends as far west as south central washington near ice harbor dam the source material for these flows is characterized by high th u th pb rb sr and nd sm ratios and light ree enrichment an additional component presumably granitic crust is necessary to explain the pb isotopic data of the umatilla and perhaps the pomona members of the saddle mountains basalt this contamination model is supported by published 18o data and petrological interpretation published studies of basalts from the deccan traps the tasmanian dolerites the snake river group and the volcanics of the yellowstone plateau province also suggest that melting of old subcontinental lithosphere beneath the continental crust may be an important process in basalt genesis on a global scale

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

correlation between the oxygen isotope ratio of north american bison teeth and local waters implication for paleoclimatic reconstructions abstract the oxygen isotope ratios of tooth enamel carbonate from 64 north american bison bison bison from eleven locations were measured the mean enamel oxygen isotope ratios for bison populations ranged from 28 3 to 15 9 smow and correlated well with the annual mean oxygen isotope ratios of local surface waters and precipitation the standard deviation of oxygen isotope values among different individuals within each bison population averaged 1 0 and ranged from 0 7 to 1 4 the variability of enamel oxygen isotope ratios was not significantly different among different populations and did not correlate with changes in temperature precipitation or relative humidity these results demonstrate that the average oxygen isotope values of bison tooth enamel can be used for use as a quantitative proxy for reconstructing the values of surface waters and therefore may provide valuable paleoclimatic information this study provides a baseline comparison for analyses of the oxygen isotope ratios of bison and other large herbivores from across north america

magnetic fabric and its significance in the florianopolis dyke swarm southern brazil summary magnetic fabric was determined by applying the anisotropy from the low field magnetic susceptibility ams technique in 62 mafic dykes from the mesozoic florianopolis santa catarina island dyke swarm southern brazil these dykes cut the crystalline basement rocks which are mainly proterozoic they are vertical or subvertical in dip and trend mainly ne although nw trending dykes are also found dykes are tholeiitic in composition and are geochemically similar to those from the ponta grossa swarm thicknesses vary from 0 3 to 60 m polished sections show that titanomagnetites carry the ams in these dykes hysteresis parameters show that the magnetic minerals fall in the psd range two types of magnetic fabric are recognized type i is characterized by k1 k2 parallel to the dyke wall representing magma flow within the dykes type ii with k1 k3 parallel to the dyke wall was found in four dykes type i is found in 94 per cent of the dykes and approximately 20 per cent of these have k1 inclinations of less than 30 suggesting horizontal or subhorizontal flow about 80 per cent have k1 inclinations of greater than 30 due to inclined to vertical flow the comparison of ams studies from both the florianopolis and the ponta grossa dykes suggests a source position closer to santa catarina island than the ponta grossa arch

characterising the middle miocene mi events in the eastern north atlantic realm a first high resolution marine palynological record from the porcupine basin abstract the warm climate of the miocene peaked during the middle miocene climatic optimum mmco 17 14 5 ma after the mmco global climate went through several short lived cooling events the mi events miocene isotope events one of the more severe mi events is mi 3 associated with east antarctic ice sheet growth species turnover in terrestrial and marine realms northern hemisphere and mid latitude aridification and antarctic sea surface temperature cooling co 2 reconstructions as well as the aforementioned observations suggest that a drawdown of co 2 and or changes in ocean circulation led to the changes surrounding mi 3 a combination of eccentricity and obliquity amplitude modulation minima favourable conditions for ice growth has also been suggested as a possible triggering mechanism however an exact cause cannot be pinpointed yet high resolution records necessary to investigate the exact order of events surrounding mi 3 and the possible role of orbital forcing a very likely trigger are sparse integrated ocean drilling program iodp leg 307 recovered such a high resolution record from the middle miocene at the porcupine basin offshore south western ireland well preserved palynomorphs mainly organic walled dinoflagellate cysts acritarchs and some pollen were extracted from site u1318 and relative and absolute abundance changes were determined using dinocysts and calcareous nannoplankton the age model for the record was improved based on the palynology the mi 3a mi 3b and mi 4 events were successfully identified and concomitant palaeoenvironmental change was observed these events although different in magnitude can be associated with a decrease in sea surface temperature as well as with a likely fall in sea level furthermore possible palaeoenvironmental preferences of 5 dinocyst taxa were determined based on observations from the record and multivariate statistics

postdepositional tilt of the miocene pliocene ogallala group on the western great plains evidence of late cenozoic uplift of the rocky mountains the rocky mountains and adjacent western great plains share a common history of late cenozoic stream incision both epeirogenic uplift and climate change with no associated uplift have been proposed as the cause of thissubcontinental scale erosional episode however the lack of a well defined cenozoic paleoelevation history for the region has hampered our ability to distinguish between the two a tilt analysis of the cheyenne tablelands in the western great plains of wyoming and nebraska provides us with a datum from which postdepositional changes in slope can be determined miocene to earliest pliocene gravels of the ogallala group 17 5 5 ma cap the tablelands which currently tilt down to the east at slopes as great as 10 2 however paleohydraulic analysis of the ogallala gravels indicates depositional slopes of 10 3 to 10 4 implying a postdepositional increase in tilt if a hinge point at the eastern edge of the cheyenne tablelands is assumed this tilt translates into differential uplift of 680 m 815 410 m at the western edge of the great plains flexural isostatic rebound of the tablelands due to known young erosion in surrounding basins only produces a few hundred meters of uplift therefore even if all of the recent erosion in the region can be attributed to climate change the resulting rebound is insufficient to account for the observed uplift of the tablelands thus the tilting of the cheyenne tablelands is most consistent with broad wavelength tectonic uplift centered under the rocky mountains initiated during ogallala deposition and continuing since deposition ceased

the palaeomagnetism of mesoproterozoic eriksfjord group red beds south greenland multiphase remagnetization during the gardar and grenville episodes the eriksfjord group comprises 3000 m of lavas and sediments rapidly deposited in a rift which developed within an andean type batholith in juxtaposition to the southern margin of the laurentian shield in south greenland at ca 1300 ma the lavas have been shown to preserve a detailed record of the geomagnetic field at the time of eruption incorporating normal reversed and transitional directions this study has examined the magnetic properties of the intervening red sediments they are found to possess a diagenetic remanence imparted by mediating fluids at later times the impact of diagenesis is stratigraphically controlled the base of the rift infill has magnetizations partially resident in magnetite which are either unstable to thermal cleaning or record a single polarity b magnetization d i 284 67 31 samples 95 5 5 palaeopole at 244 1 e 47 5 n dp dm 7 5 9 1 this corresponds in polarity and closely in direction to remanence observed in mid gardar gabbro giant dykes and dyke swarms emplaced along the axis of the rift system at ca 1160 ma the causative diagenetic magnetite appears to have grown from hydrothermal systems motivated by this magmatism in a sealed reservoir setting within the lower part of the rift infill the ilimaussaq alkaline igneous complex was emplaced into the southern extension of the rift at ca 1130 ma and possesses a dual polarity magnetization d i 327 81 95 6 4 10 sites eriksfjord lavas within the thermal aureole are overprinted to varying degrees by comparable magnetizations with steep inclinations the mean pole position 283 e 71 n dp dm 12 12 lies near the apex of an apparent polar wander loop incorporating the gardar track ca 1300 1140 ma and the keweenawan track ca 1115 1050 ma magnetizations in the eriksfjord sedimentary succession have not been significantly reset by emplacement of the ilimaussaq complex but higher levels of the rift infill are dominated by an a magnetization d i 305 34 95 4 3 57 samples palaeopole at 202 1 e 32 4 n dp dm 2 8 4 9 resident in haematite the pole position does not correspond with any part of the gardar track but does correlate with the return keweenawan track at ca 1090 ma close to the time of grenville orogenesis along the bordering southeastern margin of the laurentian shield this remanence is attributed to diagenesis during extensional tectonism linked to the collapse of the grenville orogen formerly sited 100 200 km to the south

the three dimensional upb method generalized models and implications for upb two stage systematics four forms of the three dimensional u pb method established so far are physically generalized within the framework of a u pb two stage evolution such a generalization provides an explicit insight into the principle of conventional u pb discordia dating its implications for both u pb geochronology and pb isotopic geology are illustrated for whole rock systems when applying the three dimensional data processing method to the experimentally determined results on the u and pb concentrations and pb isotopic composition of a given suite of cogenetic samples one cannot only obtain a unique solution to timing of two episodic events i e ages t1 and t2 directly but also shed some light on the feature of the source region of initial pb incorporated at time t1 and on the nature of u pb differentiation at the two episodic events common pb for correction in the discordia dating is clarified to be responsible for the initial pb at t1 rather than t2 such a clarification would have an impact on the conventional u pb geochronology that makes assumptions about the isotopic composition of common pb in calculating the discordias the three dimensional u pb method has advantages over the two dimensional ones u pb discordias or pb pb isochron for treating the results of u pb isotopic measurements on a suite of cogenetic samples because it takes into account three variables instead of two variables in data processing and thus enables straightforward calculation of ages responsible for two episodic events without a priori assumptions this is a particular advantage for treating the u pb isotopic data of whole rock samples and therefore for discovering possible low temperature u pb differentiation responsible for terrane uplift

neogene forests from the appalachians of tennessee usa geochemical evidence from fossil mammal teeth abstract neogene land mammal localities are very rare in the northeastern u s therefore the late miocene early pliocene gray fossil site in eastern tennessee can clarify paleoecological dynamics during a time of dramatic global change in particular the identification of ancient forests and past climate regimes will better our understanding of the environmental context of mammalian evolution during the late cenozoic stable isotope analyses of bulk and serial samples of fossil tooth enamel from all ungulates present at the gray site elucidate paleoecological reconstructions the herbivorous megafauna include taxa of likely north american and eurasian ancestry including the tapir tapirus polkensis rhino teleoceras cf t hicksi camel cf megatylopus sp peccary tayassuidae and proboscidean gomphotheriidae the tapir rhino camel and peccary yield mean stable carbon isotope 13c tooth enamel values of 13 0 13 3 13 8 and 13 1 respectively suggesting forest dwelling browsers this range of 13c values indicates the presence of a c3 dominated ancient local flora because 13c values decline with increasing canopy density the ancient temperate forests from the gray site were moderately dense the lack of significant c4 plant consumption i e tooth enamel 13c values

palaeomagnetism of middle proterozoic basic intrusives in central sweden and the fennoscandian apparent polar wander path abstract palaeomagnetic results are presented from three sets of post svecokarelian intrusions in central sweden the first is a system of e w trending doleritic dykes including the large breven and hallefors dykes and with a probable emplacement age of c 1530 ma their original nrm is disturbed by later events but two magnetic directions group ew1 with decl 359 5 incl 6 2 95 8 2 n 35 and group ew 2 with decl 11 1 incl 51 3 95 10 1 n 14 have been isolated and give palaeopoles at 27 7 n 196 8 e and 61 7 n 176 1 e respectively the group ew1 direction is considered to represent the original emplacement nrm this is based on comparison with contemporary palaeomagnetic data from elsewhere in fennoscandia the second set comprises dolerites and quartzporphyries the tuna dykes age 1371 ma they present a stable magnetization with decl 14 8 incl 18 1 95 9 6 and a palaeopole at 21 1 n 180 4 e the third set studied are dolerites belonging to the dala and jamtland complexes of the central scandinavian dolerite group csdg age c 1250 1200 ma post jotnian they present well defined magnetic directions that give palaeopoles close to the equator around 155 e in agreement with other published palaeomagnetic csdg data however there are deviating results obtain from dolerites belonging to the dala complex of the csdg those deviating have a mean decl of 12 9 incl 1 4 95 18 1 n 5 which gives a palaeopole at 27 0 n 179 5 e situated within the area of the 1600 1500 ma poles results from three probably sweconorwegian c 1000 850 ma doleritic dykes are also presented the results obtained are compared with other contemporary palaeomagnetic poles from fennoscandia and a revised apparent polar wander path is presented and discussed

fossil horses and carbon isotopes new evidence for cenozoic dietary habitat and ecosystem changes in north america abstract the transformation from low crowned to high crowned horse teeth during the miocene has traditionally been interpreted as an adaptive response to the spread of savanna grasslands during the middle miocene by about 15 m y ago carbon isotope data from 50 north american horse teeth spanning eocene to pleistocene age indicate that savanna grasslands presumably dominated by c 4 grasses as they are today first became widespread much later than do horses with high crowned teeth prior to 7 m y horses had a c 3 based diet and after 7 m y horses started eating c 4 grasses this change in diet occurred when the major drop in the diversity of horses occurs during the late miocene the change in vegetation reflected in horses diet may be related to a significant reduction in atmospheric co 2 level toward the end of the miocene which provided the c 4 grasses with an adaptive advantage and led to their expansion at the expense of c 3 plants

oxygen isotope variations in phosphate of biogenic apatites iv mammal teeth and bones groups of rats grown from birth to death in identical conditions but with different 18o of drinking water w were studied for variations of 18o of their body water bw and bone phosphate p there is a high linear correlation r 0 99 between bw and between p and w the regression lines have similar slope coefficients 0 53 and 0 49 values of p of different teeth and bones are the same within the precision of the method 0 5 the isotopic fractionation coefficient between phosphate and body water is 1 0178 and close to the estimated value of 1 0173 derived from the phosphate paleotemperature equation deviations from constant fractionation between bone phosphate and environmental drinking water depend primarily on rates of drinking and metabolism environment conditions have relatively little effect on oxygen isotope fractionation between water sources and bone phosphate

diversity and evolutionary trends of the family rhinocerotidae perissodactyla abstract diversity and evolutionary trends of the family rhinocerotidae are presented herein the sample has a temporal range from the middle eocene to recent including 41 genera and 142 species from europe asia north america and africa diversity and the first and last records have been established for each area along a temporal scale the relative body size and weight and the gracility index of the mciii have been also analysed the results show that the highest values in diversity and first and last records are reached in asia eurasia and africa share many taxa while north america is more independent at generic and specific levels very small and very slender taxa are present in eurasia and north america absent in africa they coexist with brachypodial and generally large forms in eurasia whereas in north america they are replaced by large more robust forms from the middle miocene onward middle large sized species with intermediate gracility predominate often together with the brachypodial ones the latter type disappears by the late miocene with just one genus remaining in asia during the pliocene at this time a general decline of the rhinocerotidae occurs related to a major climatic change becoming extinct in north america by the end of the pleistocene rhinocerotids disappear from europe and wide areas of asia but they remain in southeast asia and in africa south of the sahara

tertiary history of c4 biomass in the great plains usa we use the carbon isotope composition of paleosols to reconstruct the history of c4 biomass on the great plains from ca 23 to 1 ma the proportion of c4 biomass was uniform and moderate 12 34 throughout the miocene increased between 6 4 and 4 0 ma and reached modern levels by 2 5 ma ecological changes in great plains ungulates preceded the increase in c4 biomass the contrasts in the paleosol and ungulate records may indicate initial development of c3 grasslands after the middle miocene or a greater role for ecological interactions within communities in structuring ungulate faunas contrasts in paleosol records from different continents point to regional rather than global controls on the evolution of c4 grasslands

carbon isotope ratio of cenozoic co2 a comparative evaluation of available geochemical proxies 1 the carbon isotope ratio 13c of plant material is commonly used to reconstruct the relative distribution of c3 and c4 plants in ancient ecosystems however such estimates depend on the 13c of atmospheric co2 13cco2 at the time which likely varied throughout earth history for this study we use benthic and planktonic 13c and 18o records to reconstruct a long term record of cenozoic 13cco2 confidence intervals for 13cco2 values are assigned after careful consideration of equilibrium and non equilibrium isotope effects and processes as well as resolution of the data we find that benthic foraminifera better constrain 13cco2 compared to planktonic foraminiferal records which are influenced by photosymbiotes depth of production seasonal variability and preservation furthermore sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera 13c and 18o values indicate that these factors act to offset one another our reconstruction suggests that cenozoic 13cco2 averaged 6 1 0 6 1 while only 11 2 million of the last 65 5 million years correspond to the pre industrial value of 6 5 with 90 confidence here 13cco2 also displays significant variations throughout the record at times departing from the pre industrial value by more than 2 thus the observed variability in 13cco2 should be considered in isotopic reconstructions of ancient terrestrial plant ecosystems especially during the late and middle miocene times of presumed c4 grassland expansion

isotopic reconstruction of past continental environments abstract vertebrate fossils and continental sediments provide a rich record of variations in the isotopic composition of surface environments to interpret these records a greater understanding of isotopic sources as well as fractionations associated with animal physiology soil geochemistry and diagenesis has been essential tooth enamel and fish otoliths yield subannual records of surface environments whereas soil minerals may integrate signals over many thousands of years carbon isotope variations in fossil vertebrates and soils record changes in the structure of vegetation and the isotope composition and concentration of atmospheric co2 oxygen isotope variations may be indirectly related to climate through reconstruction of the oxygen isotope composition of meteoric water or directly related to temperature through application of oxygen isotope paleothermometry to soil minerals or otoliths in africa nitrogen isotope variations show promise as a proxy for rainfall abundance though the gen

a new stable isotope record of neogene paleoenvironments and mammalian paleoecologies in the western great plains during the expansion of c4 grasslands abstract stable isotopes in horse teeth from the north american great plains show that equids began to incorporate c4 plants in their diets about 6 5 million years ago as c4 grasslands expanded however the ecological response of many other ungulates to this expansion is poorly documented we use stable carbon isotopes in mammalian tooth enamel to test whether other ungulates adapted by incorporating c4 vegetation in their diets the openness of habitats before the expansion of c4 grasslands is uncertain with plant phytolith studies suggesting a patchy environment with open habitat grasses and forest and pollen and macrofloral studies suggesting more forested habitats to address this problem we use a model that predicts carbon isotope values for tooth enamel for a variety of environments based on values in modern plants carbon and oxygen isotopes were analyzed in medium to large herbivores from three late miocene and three pliocene local faunas comprising a total of 13 and 7 genera respectively and 59 and 42 individuals results indicate that before the expansion of c4 grasslands taxa with high crowned teeth were consuming predominantly c3 vegetation in contrast by the late pliocene most taxa studied were consuming a component of c4 vegetation and only the peccary platygonus sp had a pure c3 diet c4 consumption increased in the late pliocene 3 0 ma big springs local fauna probably in response to increased c4 biomass most landscapes in the late miocene of nebraska were open such as woodland savanna or c3 grassland although low carbon values from the pratt slide local fauna suggest a denser presumably forested area this general pattern suggests an expansion of open habitats no later than 12 ma early clarendonian through the miocene pliocene transition there was an overall shift to lower 18o enamel values which parallels the long term decrease in global mean annual temperature inferred from the marine record our results indicate that major changes in the diets of medium and large herbivores broadly corresponded with increased c4 biomass and cooling climate from the latest miocene to the late pliocene

a revised magnetostratigraphy for the mid proterozoic gardar lava succession south greenland abstract new palaeomagnetic results from the ca 1300 ma gardar lava succession within the eriksfjord formation of south greenland are presented this study incorporates stepwise thermal demagnetisation of 256 samples from 66 lava flows from all three lava groups in this formation accompanied by principal component analysis the results identify two complete reversals n r n in the upper lava group and two r n and n r in the lower group with a possible palaeoexcursion in the lower group a minimum of two further reversals are predicted in the time interval represented by the sandstone units intercalated with the lava flows both complete reversals in the upper group exhibit inclination asymmetries of 30 this degree of asymmetry is satisfied by a geomagnetic field model equivalent to a dipole axially offset by 570 km however a more complex source is not precluded by the present evidence four new pole positions are calculated for the gardar lavas two for the upper lava group and one each for the middle and lower groups the middle lava group shows an anomalous mean direction and pole position which is unrepresentative of the remainder of the lava succession and is thought to record a virtual geomagnetic pole representative of a transitional field direction the presence of a non axial geocentric dipole field means that the use of the laurentian apparent polar wander path for dating the new poles is not valid

magnetic anisotropy produced by magma flow theoretical model and experimental data from ferrar dolerite sills antarctica summary volcanic rocks forming sills dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement we model a sill as a steadystate flow of a bingham fluid driven by a pressure gradient in a horizontal conduit the magma velocity as a function of depth is calculated from the motion and constitutive equations vorticity and strain rate are determined for a reference system moving with the fluid the angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations it is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction and that the magnetic foliation plane changes periodically from horizontal to vertical the results are compared with the magnetic fabric of ferrar dolerite sills victoria land east antarctica derived from low field susceptibility measurements

cenozoic paleoclimate on land in north america abstract paleotemperature and paleoprecipitation over the past 40 m yr can be inferred from the degree of chemical weathering and depth of carbonate nodules in paleosols of oregon montana and nebraska paleosol records show that late eocene 35 ma middle miocene 16 ma late miocene 7 ma and early pliocene 4 ma warm climatic episodes were also times of a wet climate in oregon montana and nebraska oregon and nebraska were humid during warm wet times but montana was no wetter than subhumid within the rain shadow of intermontane basins global warm wet paleoclimatic spikes steepened rather than flattened geographic gradients of rocky mountain rain shadows long lived mountain barriers created dusty dry basins with sedimentation rates high enough to preserve milankovitch scale 100 41 kyr global paleoclimatic variation in some sequences of paleosols greenhouse warm wet climates indicated by paleosols were also peaks of diversity for north american plants and animals and coincided with advance

cenozoic expansion of grasslands and climatic cooling abstract mountains ocean currents forests and swamps have played an important role in regulating global climate for hundreds of millions of years but the truly novel event of the cenozoic was the evolution and expansion of grasslands with their uniquely coevolved grasses and grazers neogene expansion of the climatic and geographic range of grasslands at the expense of woodlands is now revealed by recent studies of paleosols fossils and their stable isotopic compositions grasslands and their soils can be considered sinks for atmospheric co2 ch4 and water vapor and their cenozoic evolution a contribution to long term global climatic cooling grassland soils are richer in organic matter than are woodland and desert soils of comparable climates and when eroded their crumb clods form sediment unusually rich in organic matter grasslands also promote export of bicarbonate and nutrient cations to lakes and to the oceans where they stimulate productivity and c burial this increased productivity and

magma differentiation and crystallization in basaltic conduits by two competing petrogenetic processes abstract magma differentiation processes that take place in conduits delivering basaltic magmas to the earth s surface are still poorly understood to get new insights into this problem we have studied in detail the whole rock geochemistry of a section across a dolerite dyke 21 m thick and three sections across its narrow apophysis 69 cm 29 cm and 17 cm thick from the russian fennoscandian shield the dyke is fresh aphyric and shows no evidence of in situ contamination by crustal rocks chemical zonation in the dyke changes systematically with the thickness of the sections in particular rocks become more primitive inwards mgo mg number and normative an increase and p 2 o 5 decreases in the main dyke while rocks become more evolved inwards in the apophysis in addition the chemical zonation of the apophysis is anomalous since compatible and incompatible elements behave in a manner inconsistent with fractional crystallization of basaltic magma to explain the chemical zonation we introduce a novel concept that attributes compositional trends in mafic dykes to competition between two processes a the filling of dykes with magmas that become increasingly more evolved with time as a result of fractionation in the deeper parts of conduits and b in situ crystal growth on dyke walls accompanied by effective removal of evolved melt by through flowing magma the processes have opposite effects on rock geochemistry with the first making dykes more evolved and the second more primitive inwards the key idea is that dykes become more evolved inwards when quenching of inflowing magmas controls the distribution of all components and become more primitive inwards when in situ crystal growth governs the distribution of all components anomalous compositional trends develop when these two processes control the distribution of different groups of chemical components the chemical zonation in the studied dyke can be attributed to a change from rocks mainly produced by quenching of progressively more evolved magmas in the apophysis to those predominantly formed by in situ crystal growth in the main dyke this happens in response to a decreasing degree of undercooling as sections of the magma conduit become thicker we believe that the competitive operation of these two petrogenetic processes is likely a general feature of magma differentiation and crystallization in basaltic conduits

paleomagnetic study of neoarchean paleoproterozoic dykes in the kaapvaal craton 431 oriented samples were collected from 27 dolerite dykes at 17 sites belonging to 2 95 2 65 and 1 90 ga swarms that trend se e and ne respectively from the bushveld igneous complex into the eastern kaapvaal craton ages determined by olsson et al 2010 olsson in soderlund et al this volume samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility ams for the 2 95 ga se trending dykes high temperature coercivity p component has unblocking temperatures up to 590 degrees c and coercivity 40 90 mt and demonstrate ssw declination and intermediate positive inclination based on positive contact and conglomerate tests we argue for a primary origin of this component the paleopole bad calculated from component does not correspond to any of the previously obtained archean paleoproterozoic paleopoles for the kaapvaal craton and represents a new key pole for 2 95 ga the high coercivity h component for the 2 65 ga old e trending dykes has a ssw declination and steep positive inclination paleomagnetic pole ryk recalculated from this component is close to the paleopoles obtained by wingate 1998 and strik et al 2007 for 2 78 ga ventersdorp volcanics the third group ne trending dykes of the 1 90 ga black hill swarm demonstrate an m component with dual polarity high coercivity component with sse declination and negative intermediate inclination the paleopole bhd calculated from this component is close to the 1 87 ga pole of the kaapvaal craton obtained by hanson et al 2004 overprint directions include a very well developed thermo chemical overprint dec 329 degrees inc 36 degrees which is believed to be associated with a similar to 0 18 ga regional karoo thermal event c 2010 elsevier b v all rights reserved less

environmental change in the great plains an isotopic record from fossil horses abstract carbon and oxygen isotope ratios of fossil horse tooth enamel from nebraska and texas show evidence for late neogene environmental changes in the great plains the earliest unambiguous c4 dietary signal among texas equids coincides with the age of the classic late hemphillian age coffee ranch fauna which we suggest is 6 6 ma based on volcanic ash correlations c4 vegetation was present in the diets of a small fraction of late hemphillian equids in nebraska and was thereafter ordinary in the diets of both nebraska and texas equids there is no unequivocal evidence for abundant c4 vegetation in the diets of pre late hemphillian equids and we suggest that the ensuing dietary change reflects c4 biomass expansion in the latest miocene carbon isotope ratios of post hemphillian horses in nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co occur with horse remains indicating that the two proxies carbon isotopes and presence absence of tort

proterozoic basic dykes in the ukrainian shield a palaeomagnetic geochronologic and geochemical study the accretion of the ukrainian shield to fennoscandia a palaeomagnetic geochronologic and geochemical study of basic dykes in the ukrainian shield has been undertaken with the purpose of testing a hypothesis for timing of accretion of the ukrainian shield with fennoscandia on the basis of geological data it has been suggested that the two shields amalgamated at ca 1900 1800 ma with this study of 11 basic dykes the palaeomagnetic database for the ukrainian shield is extended the palaeomagnetic data indicate three generations of dykes this is supported by the age data and the difference in geochemical compositions ages of two ca 25 m wide dykes susly in the north western block of the shield have been determined one for which a positive palaeomagnetic field test is demonstrated the lower of the two ar 40 ar 39 hornblende ages of this dyke 2061 20 ma is similar to the u pb zircon age of the granitic host rock ca 2067 ma u pb zircon which suggests that the dyke intruded shortly after crystallization of the granite combined with the magnetization of the baked contact and that of another dyke a new ca 2060 ma pole is presented plat 15 7 degrees plon 182 9 degrees a 95 13 7 degrees which may be regarded as a vgp an u pb age of 1722 12 ma of baddeleyite from the khmelnik dyke which yields a well defined vgp is similar to the age of nearby anorthosite complexes a new 1740 1770 ma key pole for the ukranian shield is also presented plat 26 5 degrees plon 169 1 degrees a 95 3 9 degrees on the basis of palaeomagnetic and geochronological data a 2060 1720 ma apparent polar wander for the ukrainian shield is defined this apparent polar wander is significantly different from that of fennoscandia during the same time interval and indicates that the two shields were not in their present relative positions at ca 1770 ma a scenario is here proposed where the ukrainian shield and sarmatia collided with fennoscandia at ca 1900 1800 ma and after that event at ca 1720 1660 ma it rotated approximately 43 degrees into its present position c 2010 elsevier b v all rights reserved less

ungulate diets reveal patterns of grassland evolution in north america abstract equids from the late miocene hemphillian north american land mammal age of texas evolved c4 dominated diets earlier than their nebraska counterparts leading researchers to hypothesize a northward expansion of c4 grasses between 8 and 6 ma however competing hypotheses from multiple paleoecological proxies support one of three hypotheses that northward c4 expansion occurred at the expense of c3 woodlands c3 grasslands or was not occurring during the hemphillian we test these alternative hypotheses by comparing the ecosystems of hemphillian texas and nebraska 6 3 7 5 ma using hoofed mammal dietary reconstruction hypsodonty and digestive strategy and by incorporating published tooth enamel isotopes paleosol isotopes and phytolith assemblage data we also use hypsodonty mesowear and microwear in a detailed comparison of two localities that are roughly contemporary cambridge nebraska at 7 ma and coffee ranch texas at 6 6 ma to rule out time averaging as an explanatory variable through analogy with modern african communities we reconstruct texas and nebraska ecosystems as bushland and woodland respectively using the proportions of hypsodont taxa we estimate mean annual precipitation values of 1217 mm year for nebraska and 1368 mm year for texas using rarefaction we also do not find differences in the richness of brachydont hypsodont and hindgut fermenting taxa we therefore conclude that both ecosystems were largely similar in hoofed mammal faunal structure during the late miocene when compared to published paleosol and phytolith data which show no differences in c3 c4 biomass among latitudes our results allow us to reject hypotheses of northward c4 expansion during the late miocene we therefore suggest that the equid enamel isotopes may have been biased by two potential factors the primary reliance on a single locality of appropriate age and possible behavioural lag in c4 feeding among nebraska equids these results suggest that the formation of a modern c3 c4 latitudinal gradient occurred later than is previously suggested by equid enamel carbon isotopes

palaeomagnetism of the mesoproterozoic gronnedal ika alkaline complex southwest greenland abstract the gronnedal ika complex 1299 17 ma rb sr isochron is the earliest dated episode within the gardar igneous province of south greenland it was intruded by ne trending dyke swarms and dislocated by faulting later in the history of the province a shallow a magnetic remanence component present at most sites and recorded by both polarities mean d i 269 21 95 5 1 53 samples is interpreted as a primary cooling related magnetisation it is slightly steeper than remanence in similar plutons 10 ma younger in the east of the province and extends definition of a motion of the laurentian shield towards the palaeoequator recognised from the earliest phases of gardar magmatism 1310 1290 ma the a remanence is sporadically retained in amphibolite bodies near the contact of the complex although overprinting by the single polarity ne trending dyke emplacement episode between 1220 and 1160 ma mean remanence d i 283 60 95 6 1 22 samples is more prominently recorded in the country rocks other steep single polarity components were probably overprinted during more than one episode in late gardar and mesozoic recent times the palaeopole linked to the emplacement age of the gronnedal ika complex 228 e 9 n dp dm 2 8 5 3 is moved to 212 e 13 n following adjustment for phanerozoic drift between greenland and north america a westerly apw motion through 25 is defined by plutons emplaced early in the gardar episode 1300 1290 ma and it continues with a southerly motion towards pole positions from the mackenzie igneous episode 193 e 3 s dated 1267 ma

magnetic seismic and mechanical properties of porphyritic meta rhyolites the effect of microscopic foliation on rock strength and p wave velocity in drill cores abstract seismic velocity and rock strength in drill cores have previously been shown to vary depending on the angle between the measuring direction and the macroscopic foliation plane but very little research has been performed on samples with a microscopic foliation in this study we use the anisotropy of magnetic susceptibility ams to identify a foliation in what has been assumed to be isotropic rhyolites we then use scatterplots and simple linear regression analysis to identify and quantify the relationships between ams diametrical p wave velocity and point load pl strength in the ferromagnetic samples our study suggests that the pl strength increases with 56 when the angle between the foliation plane and the measuring direction is increased from 14 close to the foliation plane to 65 such a high change in rock strength with direction in rocks with no macroscopic foliation could have serious implication e g in rock mechanical modeling and ams can thus be used as a tool predict fracturing

carbon and oxygen isotopic variability in neogene paleosol carbonates constraints on the evolution of the c4 grasslands of the great plains usa abstract understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas we focus on the central and southern portions of the great plains which is the largest continuous grassland in north america and has a well documented mammalian fossil record modern biomass in this region is dominated by c4 grasses but paleontological evidence indicates the region was forested prior to the middle miocene implying the region was formerly dominated by c3 plants we examine long and short term variations in the abundance of c4 grasses and climatic conditions in the central and southern great plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region the carbon isotope composition 13c of paleosol carbonate reflects the proportion of c3 and c4 plants that grew in an ancient soil the long term pattern of 13c values indicates that the percentage of c4 grasses in the great plains was moderate ca 20 throughout most of the miocene increased from 6 4 to 4 0 ma and reached modern levels by 2 5 ma based on the range in 13c values within sections and the mean 13c values for different carbonate morphologies that reflect different intervals of carbonate accumulation the abundance of c4 biomass did not vary substantially on short time scales the variability in c4 biomass on short time scales does not correlate with age of section latitude longitude or the mean percentage of c4 biomass the oxygen isotope composition 18o of soil carbonate is controlled by soil temperature and the 18o value of soil water great plains paleosol 18o values vary considerably in relation to time and latitude removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of neogene climate change quantitative interpretation of the 18o record is difficult without independent control on temperature or the oxygen isotope composition of soil water both of which have probably varied with long term climate change comparisons of measured 18o values for each section with predicted values based on modern temperature data and estimated modern meteoric water 18o values suggest that 1 most miocene sections have 18o values consistent with warmer temperatures and more positively shifted soil water 18o values than today 2 three miocene sections in western nebraska probably had soil water compositions that were more negative than similar aged localities in the region and 3 the plio pleistocene sections have 18o values consistent with cooler temperatures and more negative soil water 18o values than today the paleosol 13c values do not correlate strongly with the pattern of climate change recorded by the 18o values suggesting that long term change in mean temperature is not the main control on the abundance of 13c biomass in the great plains we propose two testable hypotheses to account for our data and other published data that bear on the evolution of the great plains ecosystem

magnetic fabric in the umvimeela dyke satellite of the great dyke zimbabwe abstract anisotropy of magnetic susceptibility ams has been determined for 22 locations from the 500 km long gabbroic umvimeela dyke of zimbabwe ams is controlled by the presence of multi domain magnetite grains which appear to have preserved a variety of magnetic fabrics related to magma movement at the time of dike emplacement three locations along the umvimeela dyke have been identified as feeder points at which magma flowed into the dike from below lateral flow away from these points supplied the rest of the dike system the presence of several foci of igneous activity along the umvimeela dyke parallels the development of the neighbouring contemporaneous great dyke layered complex which consists of a line of contiguous subchambers individual feeder points to the umvimeela dyke may be directly related to activity in adjacent subchambers

mesoproterozoic dyke swarms in foreland and nappes of the central scandinavian caledonides structure magnetic fabric and geochemistry as an example of microstructural and magnetic fabric evolution and geochemistry of mafic dykes during a subsequent orogenic overprint a major mesoproterozoic dyke complex in scandinavia the vasterbotten complex of the central scandinavian dolerite group is traced westwards into the crystalline nappes of the early phanerozoic caledonian orogen using geophysical field microscopic magnetic and geochemical information dykes and sills are characterized and their overprint during caledonian orogeny documented the vasterbotten complex is composed of sets of dykes trending ne sw nw se and wnw ese respectively similar dykes are exposed in allochthonous positions lower and middle allochthons in the caledonian fold and thrust belt the autochthonous dykes are generally undeformed and retain both their primary texture and mineralogy chilled margins are well preserved in the caledonian lower and middle allochthons similar dykes in crystalline basement rocks are progressively faulted and sheared when proceeding from the marginal to the interior parts of the orogen dyke margins are more likely to be sheared than the interior parts of dykes in the lower allochthon under very low and low grade metamorphic conditions dykes are distinctly less competent than granitic host rocks thick dykes are more competent than gneisses thin dykes do not show such competence contrasts in the middle allochthon metre scale dykes with patches of altered plagioclase phenocrysts can still be discerned in low strain domains highly sheared dykes are drawn out to thin layers of centimetre thickness dykes are deformed together with the crystalline country rocks under greenschist grade metamorphic conditions without major competence contrasts magnetic fabrics show an evolution similar to the silicate mineral fabrics the magnetic fabrics in the dykes are transformed successively from ferromagnetic magmatic in the autochthon to ferromagnetic deformational in the lower allochthon and finally paramagnetic deformational in the middle allochthon as a consequence the magnetic susceptibility decreases for several orders of magnitude geochemically the dykes are dominantly sub alkaline basalts typical for continental tholeiites and can be distinguished from the neoproterozoic dykes in the sarv nappe equivalents highest part of the middle allochthon which show a more morb like e morb magmatic signature preliminary age information from a dyke in the lower allochthon of the borgefjell area and the middle allochthon is consistent with the assumption that these dykes are time equivalent with the central scandinavian dolerite group therefore the studied dykes may represent an extension of the vasterbotten complex or a new complex of the central scandinavian dolerite group according to section restorations the caledonian allochthons were situated further wnw relative to their present position and originally the mafic dykes cut across all of the fennoscandian lithosphere at least to the present atlantic margin and the earlier passive margin of the baltica terrane as a consequence these dykes may provide a link for pre caledonian and pre grenvillian plate reconstructions

relationship between δ18o values for skeletal apatite from reindeer and foxes and yearly mean δ18o values of environmental water abstract the oxygen isotope composition of bone and tooth phosphate of 50 fox specimens and 30 reindeer specimens from various locations with different climatic and environmental conditions was measured the existing relationship between these values and the mean oxygen isotope composition of local meteoric water has been calculated in the case of foxes specimens belonging to two genera vulpes and alopex and three different species were measured the samples fit a straight line whose equation can be used for paleoclimatological studies either in arctic or in temperate regions for reindeer rangifer a relatively large range of isotopic values was obtained from each location suggesting imperfect equilibrium conditions with environmental water the calculated equation can be used for semi quantitative information on local paleowaters at high latitudes only

flat latitudinal gradient in paleocene mammal richness suggests decoupling of climate and biodiversity mammal faunas from western north america exhibit no significant change in species richness with latitude during the torrejonian ca 63 60 ma and tiffanian ca 60 58 ma north american land mammal ages in contrast to a strong richness gradient in modern mammalian faunas of the same region today the latitudinal gradient in oxygen isotope composition of mammalian bioapatite from the paleocene faunas is similar to that of modern meteoric and surface waters suggesting that the temperature gradient in the paleocene was similar to the modern one the flat richness gradient in the middle paleocene indicates either different responses to climatic gradients of faunas dominated by extinct clades of placental mammals or distinct ecological processes during the paleocene diversification of mammals following the end cretaceous mass extinction

uplift and cooling magnetisation record in the bamble and telemark terranes sveconorwegian orogenic belt se norway and the grenville sveconorwegian loop article i nfo the 1100 ma sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against and onto the south western margin of the fennoscandian shield to resolve the magnetic signature acquired during post orogenic uplift and cooling and evaluate wider correlations with the contemporaneous grenville belt of north america this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium high metamorphic grade bamble terrane 48 sites and 390 cores and the adjoining medium low grade telemark terrane 33 and 240 cores juxtaposed by an orogen parallel porsgrunn kristiansand fault zone with major vertical displacement magnetite and ilmeno hematites are magnetic carriers with the latter more significant in the higher metamorphic grades magnetic intensities are stronger in the higher grade terrane presumably due to the growth of metamorphic ferromagnets but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust anisotropy of magnetic susceptibility ams largely reflects the ne sw tectonic grain of the last stage of sveconorwegian ductile deformation the magnetisation record is filtered by excluding magnetisations possibly acquired during regional mesozoic dyke emplacement development of the permo carboniferous oslo rift and late proterozoic magmatism the remaining record is a dual polarity signature summarised by mean poles at 31 9 n 50 9 e n 191 components in the bamble terrane and at 34 2 n 58 9 e n 151 components in the telemark terrane however these means are non fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best defined parts

post jotnian basic intrusions in the fennoscandian shield and the break up of baltica from laurentia a palaeomagnetic and ams study abstract a palaeomagnetic and anisotropy of magnetic susceptibility ams study has been performed on dolerite sills of the central scandinavian dolerite group csdg in the fennoscandian shield the dolerites occur in four previously known complexes in central sweden and finland and from the results of this palaeomagnetic study another complex has been identified in northern sweden these complexes cover an area of at least 100 000 km2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites the measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca nw or se directed magma flows the dolerites of the csdg are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings in a palaeomagnetic reconstruction of baltica versus laurentia at ca 1 27 ga the two continents were joined with ne greenland attached to nw baltica ams data from a few dolerites and a basalt in ne greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in sweden this may suggest a common magma source located at the reconstructed contact between baltica and laurentia both the dolerites in greenland and those in sweden are of tholeitic composition indicating an intraplate origin which supports the interpretation of joined continents at that time the tensional regime that is reflected by the huge sill complexes is in our interpretation related to the break up of baltica from laurentia at ca 1 27 ga ago

using la icpms mapping and sector zonation to understand growth and trace element partitioning in sector zoned clinopyroxene oikocrysts from the norra ulvo gabbro sweden petrographic analysis and geochemical mapping are used to characterize sector zoned clinopyroxene oikocrysts from the norra ulvo gabbro sweden the sector zones are distinguished optically and by differences intio2 al2o3 and incompatible trace elements the oikocrysts grew with a branching morphology as evidenced by numerous triple points in a single oikocryst oikocrysts grew from the center of domains outward and the growth within individual domains did not completely fill the space until the oikocryst had reached its full size in some domains sector zones are overgrown by growth zones that lack sectors the last crystallization infilled channels that allowed late stage liquids to move through the oikocryst at the contacts with chadacrysts calculation of partitioning behavior of trace elements between adjacent sectors shows it to be regular and consistent with experimental results the data show that the catial2o6 component greatly enhances the solubility of most incompatible elements in clinopyroxene

abrupt landscape change post 6 ma on the central great plains usa the principal control on landscape evolution in the central great plains of the united states over the past 10 m y is a contentious subject new sedimentary data collected from late miocene ogallala group and pliocene broadwater formation of the nebraskan great plains demonstrates a twofold increase in the median grain size from 20 mm to 40 mm exported from the rocky mountains across the miocene pliocene boundary paleoslope reconstructions derived from these data support the tilting of the miocene ogallala group after 6 ma but demonstrate that the transport slope of the lower part of the unconformably overlying pliocene succession is identical to the present day slope these data allow us to constrain the timing of differential uplift in the great plains to between 6 and 3 7 m y the wavelength and short duration of this tilting are best explained by the initiation of localized dynamic topography our results also suggest a threefold to fourfold increase in specific stream power at this time meaning that pliocene rivers draining the central rockies were considerably more competent than their miocene predecessors incision during this period was not continuous a significant episode of aggradation from 3 7 to 2 5 ma is best explained by high rates of sediment supply relating to the warm wet mid pliocene climate optimum the modern pattern of incision on the great plains occurred from 2 5 ma and not from the end of the miocene as is sometimes supposed reflecting the onset of major northern hemisphere glaciation

the palaeomagnetism of middle proterozoic dyke swarms of the gardar province and mesozoic dykes in sw greenland summary the precambrian metamorphic basement in sw greenland was subjected to widespread emplacement of dolerite dykes between circa 1290 and 1160 ma during this period the regional stress field rotated in a counter clockwise sense a regional palaeomagnetic study of 59 of these dykes and contacts is reported whilst contact tests generally indicate primary magnetizations two and three component structures in some dykes record partial overprinting at later stages of the igneous cycle component directions are distributed from shallow negative westerly to steep nw positive over most of the outcrop the former magnetizations are found in the earlist e w trending dykes steepening and rotation of magnetization directions correlates with rotation of dyke trends towards later ne sw trends the exception occurs where dykes are deflected in a sigmoidal way through an older alkaline complex the quasi continuous nature of dyke emplacement records migration of the palaeofield direction between circa 1290 and 1160 ma and representative sequential pole positions are a1 215 3de 3 1dn 21 dykes dp dm 4 8 9 6d a2 220 4de 7 4dn 5 dykes dp dm 4 3 8 5d and a3 222 3de 33 5dn 30 dykes dp dm 4 5 7 0d palaeomagnetic poles from the gardar igneous province define the major part of a double apw loop anchored by dated poles from igneous complexes and executed between 1300 and 1140 ma this interval appears to have been dominated by one field polarity palaeomagnetic results are also reported for five dykes belonging to the mesozoic coast parallel swarm and palaeomagnetic results from these dykes in south and south west greenland are reassessed they define a dual polarity axis of intermediate inclination d i 336 51d 13 sites palaeomagnetic pole td1 at 168 3de 55 6dn dp dm 4 7 6 9d plus a steeper single polarity group mean direction d i 329 69d 14 sites palaeomagnetic pole td2 at 207 7de 68 6dn dp dm 6 6 7 8d the td1 pole corresponds to north american apparent polar wander at circa 165 ma compatible with the 168 138 ma age constraint whilst the td2 pole correlates with near static apw between 129 and 88 ma it appears to record an episode of dyke emplacement across south greenland during the cretaceous normal superchron shortly before commencement of sea floor spreading between greenland and labrador

the correlation between 18o 16o ratios of meteoric water and surface temperature its use in investigating terrestrial climate change over geologic time abstract correlations between mean annual temperature mat and the weighted average oxygen isotope composition of yearly precipitation 18opt are well known but the utility of modern relations to make reliable estimates of temperature change over geological time is uncertain this question has been addressed by using seasonal subsets of the global data base of temperature and isotopic measurements to represent two different climate modes a comparison of middle to high latitude 18opt temperature relations for each climate mode reveals 1 a significant offset between them and 2 a difference in the strength of their correlations the offset in relations is due to differences in temperature and water vapor budget in the tropics and can lead to serious underestimates of temperature change differences in the strength of correlations arise from the influence of climate mode specific non temperature factors on 18opt the overall result is that no single relation can be used in all cases to make unambiguous temperature estimates using a temporal record of 18opt values one way to overcome these problems is to reconstruct 18opt temperature relations for the time periods being investigated if an appropriate proxy for 18opt is available it may also be possible to estimate temperature without relying on 18opt temperature relations a promising alternative to these options is to use records of 18opt to test predictions of global climate models an approach that may allow a reliable and more complete reconstruction to be made of climate change over geologic time

determination of magnetic properties of rocks by analysis of demagnetization curves hematite ilmenite bearing rocks from sw sweden granulites from the high grade sveconorwegian corresponding to grenvillian gneiss terrane of sw sweden were investigated in order to relate magnetic properties to the granulite facies mineralogy and the amphibolite facies retrograde mineralogy we used standard paleomagnetic techniques susceptibility measurements electron and optical microscopy and a new way to analyze demagnetization curves based on vector difference sums the new technique allowed quantitation of the relative contributions of partial natural remanent magnetization nrm components as well as numerical extraction of thermal and coercive force alternating field af unblocking data two major components were identified one stable which is carried by hematite ilmenite and one less stable carried by multidomain md magnetite the age of the hematite ilmenite component is circa 930 ma and it has steep negative inclination coercivities in excess of 150 mt and unblocking temperatures between 540 degrees c and 640 degrees c two corresponding poles are reported 1 paleolatitude paleolongitude plat plong 218 degrees 24 3 with dp dm 9 4 degrees 9 9 degrees paleopole and 2 plat plong 203 degrees 51 degrees with d p d m 4 4 degrees 5 6 degrees vgp the md magnetite component is a recent or cretaceous paleogene steeply positive inclined nrm component associated with strong induced magnetization plat plong 2 degrees 74 degrees and d p d m 11 degrees 14 degrees vgp the md magnetite was unblocked at conditions of 6 40 mt and 60 400 degrees c our new analysis method was successful in showing that retrograde amphibole formation strongly decreases the importance of the hematite ilmenite component on behalf of the md magnetite it has a diminishing effect on a negative aeromagnetic anomaly in the area the 930 ma components constitute 80 0 of the vector difference sums depending on rock composition however as this fraction goes down to 60 the induced magnetization seems to outweigh the hematite ilmenite influence on aeromagnetic anomalies due to its opposite direction these rocks still have a strong and stable hematite ilmenite dominated nrm less

palaeomagnetism of middle proterozoic c 1 25 ga dykes from central north greenland summary from a nunatak in central north greenland 81 son 44 7 w nine sites of middle proterozoic basic dykes cutting archaean basement were palaeomagnetically investigated after af and thermal cleaning the nine dyke sites and three adjacently baked gneiss sites give a stable characteristic remanent mean direction of d 2654 i 21 5 n 12 ags 5 6 the direction being confirmed by a detaded and positive baked contact test the polarity of the dykes in the nunatak area is opposite to that of the zig zag dal basalts and the midsommers dolerites in eastern north greenland some 200 300 km away the volcanics of which are assumed to be of similar age about 1 25 ga the remanent directions of the two sets of data are antiparallel within the 95 per cent significance level of confidence when rotating greenland 18 clockwise back to north america by the bullard fit the pole of the central north greenland dolerites ndl falls at 14 3 n 144 3 w the reversed pole 14 3 s 35 7 e fits well on to the loop between 1 2 and 1 4 ma on the apparent polar wander swath of berger york for cratonic north america the palaeomagnetic results from the middle proterozoic basic dykes from central north greenland thus strengthen previous palaeomagnetic results from the midsommersa dolerites and zig zag dal basalts from the peary land region in eastern north greenland suggesting that greenland was part of the north american craton at least for the period between c 1 3 and 1 ma and probably up to the end of cretaceous time the major geographical meridian of greenland was orientated approximately e w and the palaeolatitude of greenland was about 10 150

the expansion of c4 grasses and global change in the late miocene stable isotope evidence from the americas j3c values in paleosols and fossil teeth have been used to document the expansion of c plants in south asia africa and north america during the late miocene however the exact timing and rate of expansion of c vegetation is unclear outside the old world because of a lack of high resolution records we present a high resolution record from northwest argentina in which the s13c values of soil carbonate rise above a threshold of 8 0 suggesting the presence of c plants starting at 7 3 6 7 ma sr3c values of fossil teeth from well dated sections in south and north america display a concomitant increase of c plants in the diets of large herbivores these results show that the late miocene expansion of c plants was global but occurred at different rates in each region while it is has been suggested that declining pc0 levels during the late neogene caused c plant expansion climate change such as an increase in summer dominated rainfall regimes globally is an alternative explanation the 6 0 soil carbonate records from south asia east africa and now also northwest argentina all show an increase of at least 3 4 in the late neogene either the result of climate change or of greater evaporation in average grassland soils

the palaeomagnetism of major middle proterozoic igneous complexes south greenland and the gardar apparent polar wander track abstract the gardar province comprises a major rift related igneous suite emplaced in three pulses at ca 1300 1280 ma ca 1220 ma and 1160 1125 ma this study reports a palaeomagnetic study of the northeastern sector of the present outcrop and incorporates igneous units belonging to the first and last episodes early lavas from the basal part of the rift infill record a reversal of magnetisation and define a palaeomagnetic pole at 207 e 10 n 5 sites dp dm 7 14 this primary magnetisation record has been partially overprinted in the aureoles of large alkaline complexes the latter plutons include the north qoroq intrusion 1291 61 ma yielding a pole position at 203 e 13 n 65 samples from 12 sites dp dm 6 12 the motzfeldt intrusion 1291 31 ma yielding a pole position at 212 e 10 n 25 samples dp dm 6 12 and the narssarssuaq stock yielding a pole position at 207 e 9 n 23 samples dp dm 4 7 the younger south qoroq complex 1160 8 ma yields a pole position at 216 e 42 n 67 samples from 9 sites dp dm 11 16 positive contact tests indicate primary cooling related magnetisations these bodies are predominantly of single polarity equivalent to w directions of magnetisations although reversals are recorded the majority of a suite of ne sw trending dykes emplaced at ca 1154 1143 ma are magnetised in a similar direction d i sol3 51 pole position at 231 e 33 n 18 dykes dp dm 5 7 to the slightly older south qoroq complex together with earlier results from the gardar province these data define a motion of the region from shallow to equatorial and then back to intermediate palaeolatitudes between 1290 and 1140 ma pole positions trace out an apparent polar wander path comprising a small loop followed by a long track this track is the outward path of the great logan palaeomagnetic loop for which data from the keweenawan province of north america ca 1100 1050 ma define the return path the loop was executed between ca 1200 and 1050 ma following a near static interval between ca 1290 and 1200 ma this long static interval was also characterised by constant field polarity asymmetry of geomagnetic field reversals is recognised in early gardar events it is not apparent in younger gardar data although it recurs in keweenawan times

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical