contradictory correlations of paleoproterozoic glacial deposits local regional or global controls abstract there is little agreement among recently proposed correlation schemes for paleoproterozoic glaciogenic rocks scattered thinly around the globe correlations are hindered by the dearth of tight geochronological control on the ages of glacial deposits which are notoriously difficult to date most attempts at global correlation are based on comparison to the huronian supergroup in canada which contains three glaciogenic formations although the two lower glacial units were deposited in a rift setting and have a restricted distribution even in north america they have been used in attempts to effect international correlations the gowganda formation together with correlatives comprises the thickest and by far the most widespread paleoproterozoic glacial deposits in north america glacial deposits are also reported from south africa western australia and elsewhere where they appear to fall within the same time period as the huronian supergroup 2 45 2 2 ga but attempts to correlate individual diamictite bearing units to the three huronian glaciogenic formations have proved difficult none of the huronian glacial formations has been precisely dated until such geochronological data are available it is practical and prudent to recognize that during the early paleoproterozoic as in the cryogenian the earth was susceptible to glaciations that could be triggered by a variety of local events including uplift related to rifting and compressional orogeny within the 250 million years of the huronian glacial event 2 45 2 2 ga it is commonly assumed that ice sheet fluctuations were globally synchronous but this has not been demonstrated

earth s earliest global glaciation carbonate geochemistry and geochronology of the polisarka sedimentary formation kola peninsula russia atb and am were supported by nerc grant ne g00398x 1 to aef arp and djc vam is supported by nfr grant 191530 v30 projects 331000 and 802795 this is a contribution paper no 9 to icdp far deep

sedimentation in glacial environments and the identification of tills and tillites in ancient sedimentary sequences abstract tills and tillites are defined and subdivided into primary tills comprising lodgement tills melt out tills and sublimation tills and secondary tills comprising flow tills and deformation tills they are defined on the basis of their mode of deposition which endows them with their distinctive properties another category of till that of supra till is defined on the basis of its debris source derived supraglacially from valley sides and nunataks which gives these tills a coarse grained character the processes of till deposition and of outwash sedimentation are associated in several recurrent combinations which allow the definition of three principal glacial sediment associations and land systems which define the character of the sequences within which tills tend to occur the criteria which allow tills of different genesis to be identified and to be distinguished from non till diamictons are reviewed there are several features which are uniquely characteristic of till but unfortunately these are few it has been found generally necessary to employ an array of characteristics which though not individually unique to till when taken together can be strong evidence for particular till types the recognition of several typical and repetitive facies sequences which form in the major glacial sedimentary environments can also be important indicators of till genesis

a statistically significant long term characteristic time scale of test size variation of calcareous trochospiral benthic foraminifera ctbf during the past 120 m y 1 geo marine records are faithful recorders of the earth s evolutionary history and hence are most suitable for identifying long term cyclic patterns although a remarkable synchronism among calcareous trochospiral benthic foraminifera ctbf test size variation ocean productivity dissolved oxygen and global climate exists global geospectroscopy is essential for understanding the wider ramification of significant physical linkages of the various terrestrial and extra terrestrial processes earlier claims of a quasi periodicity of 26 36 m y in various geo bio ocean atmospheric records sparked a major controversy here we examine spectral characteristics of the latest available time series representing fluctuations in test size of ctbf for the past 120 m y using the multi taper method mtm and maximum entropy method mem of spectral analyses the analyses of test size variations of ctbf time series reveal hitherto unidentified a statistically significant and stable harmonic component of 35 5 m y against a robust red background noise further wavelet spectral analysis of this record exhibits a strong stationary power in the above frequency band suggesting temporal persistence of spectral peak through the entire evolutionary period this finding renders credible support to the concept of cyclic geo marine processes and will provide stimulus for further quantitative research into the coupling of marine geo bio chemical cycles

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

the role of episodic bombardment in geophysics it is shown that cometary or asteroidal impacts strongly perturb the boundary conditions at the core outer mantle and atmosphere of the earth impact episodes occur following the capture or orbital disturbance of these bodies as the solarsystem passes through the spiral arms of the galaxy there is thus a tendency for magnetic field reversals principal plate movements ice ages and mass extinctions to be correlated and to recur on time scales dictated by galactic processes it follows on the basis of a purely astronomical theory that it may be possible to synthesise the major phenomena of geophysics knowledge of the continuous state of the galaxy then becomes a prerequisite for the understanding of terrestrial history

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

paleomagnetic and rock magnetic study of the mistastin lake impact structure labrador canada implications for geomagnetic perturbation and shock effects abstract we carried out an integrated rock magnetic and paleomagnetic study of the 36 ma mistastin lake labrador canada meteorite impact structure in order to investigate whether energy from the collision influenced the geodynamo and to assess the effects of shock on the magnetic properties of the target basement rocks stepwise demagnetization of 114 specimens isolates a well defined magnetization component throughout the crater whose overall mean deviates slightly from the expected direction for north america at the time of impact paleointensity results from seven samples meeting stringent selection criteria show no significant difference with a global compilation from 40 to 30 ma the combined results including those from a 80 m thick profile of an impact melt unit discovery hill lend no support that the impact caused an aberration of the geodynamo within a few centuries of a bolide collision that created the 28 km diameter crater both titanium rich and titanium poor titanomagnetite carry the magnetic remanence in the impact melt rocks their relative proportions compositions and domain states are cooling rate dependent magnetic hysteresis parameters of the magnetite bearing anorthositic basement rocks reveal systematic changes as a function of distance from the crater s center with an increasing prevalence of single domain like grains toward the center changes with radial distance are also found in the character of the verwey transition in magnetite basement rocks were thermally overprinted when lying less than a meter from the impact melt rocks mesoproterozoic basement rocks more than a meter below the impact melt rocks hold similar magnetization directions to those expected from a 1500 ma result for laurentia no evidence exists that shock heating of the basement rocks exceeded 200 c at distances of 6 7 km from the crater s center

palaeoproterozoic glaciomarine sedimentation in an extensional tectonic setting the honkala formation finland abstract the honkala formation of the honkajarvi group in the kainuu schist belt of northern finland central fennoscandian shield provides evidence of palaeoproterozoic glaciation on the fractured surface of the craton the diamictites and associated rocks of the honkala formation are some 190 m thick in the studied area they consist of a remnant of the glacigenic sequence deposited during an incipient stage of crustal extension the rocks rest unconformably on the late archaean basement and conformably on rift related volcanics the volcanics were erupted during a period of general fragmentation and rifting of the late archaean basement 2 4 2 3 ga ago at this time shallow intracratonic basins and depressions started to form on a continental scale subsequently many of these basins subsided below sea level the lower part of the honkala formation consists of turbiditic non glacial mudstones and sandstones up to 70 m thick glacial advance is indicated by ice rafted deposits forming a succession of some 10 to 50 m deposition took place in a glaciomarine rift basin extending towards the interior of the archaean craton waning glacial influence is indicated by some 70 m of sandstones the uppermost sandstones were influenced by intermittent chemical weathering indicating that these deposits were subjected to subaerial conditions and warmer climate correlation on a global scale is possible during the early palaeoproterozoic the fennoscandian and north american cratons were probably situated close together at a high latitude and both were affected by a major continental glaciation the fennoscandian craton then shifted to lower latitudes whereas the north american cratonic margin recorded further glaciations at high latitudes

correlation of paleoproterozoic glaciations based on u pb zircon ages for tuff beds in the transvaal and huronian supergroups abstract the rise in atmospheric oxygen between 2 45 and 2 2 ga has been linked to the demise of a methane rich atmosphere and the onset of multiple glaciations culminating in a possible snowball earth the glacial deposits represent possible global marker horizons however their correlation is uncertain because key sedimentary successions are undated hampering paleoenvironmental reconstructions three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern africa with those in north america however it has also been suggested that there were four glaciations and that the youngest was a snowball earth event recorded only in south african successions in situ u pb zircon ages for tuffs in southern african and north american successions establish the existence of four glaciations between 2 45 and 2 22 ga geochronological and stratigraphic data demonstrate that the three oldest glaciations predate 2 31 ga and that the final glaciation which is only recognized in south africa occurred between 2 26 and 2 22 ga the new age calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations at 2 31 ga the first marine sulfate evaporites were deposited contemporaneously with 13 c enriched carbonates indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen the appearance of red beds and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels which culminated with the fourth glaciation between 2 26 and 2 22 ga after which the atmosphere remained irreversibly oxygenated

paleoproterozoic positive δ13ccarb excursion in the northeastern sino korean craton evidence of the lomagundi event abstract the 2 33 2 06 ga positive 13 c carb excursion associated with environmental change and the breakup of the kenorland or superia supercontinent is called the lomagundi or jatulian event or great oxidation event and has been reported in many early precambrian cratons but not yet in the sino korean craton the guanmenshan formation of the liaohe group occurs in the northeastern part of the sino korean craton 13 c carb and 18 o values in 42 samples from this formation range from 3 5 5 9 v pdb and 15 4 24 8 v smow respectively showing a clear positive 13 c carb excursion that characterizes the lomagundi event thirty five of the 42 samples with less hydrothermal alteration have higher 13 c carb and 18 o carb values than the other 7 samples obviously affected by fluid flow confirming that it was fluid flow that reduced the 13 c carb and 18 o carb values this positive 13 c carb excursion places deposition of the guanmenshan formation within the age range of 2 33 2 06 ga

late jurassic age for the morokweng impact structure southern africa abstract a roughly 70 km diameter circular feature buried beneath the kalahari sands in south africa is revealed on regional aeromagnetic maps boreholes drilled into the centre of the structure intercept a 250 m thick sheet of quartz norite interpreted as an impact melt which overlies brecciated and shock metamorphosed basement granite zircons recovered from the quartz norite yield u pb ages of 145 0 8 ma and biotites provide ar ar ages of 144 4 ma these data provide strong evidence for the occurrence of a late jurassic impact crater the morokweng impact structure 100 m beneath the surface

depositional environments and an apparent age for the geci meta limestones constraints on the geological history of northern mozambique strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the mozambique orogenic belt in northern mozambique the latest depositional event was expressed by accumulation of marine sediments which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes the low grade metasedimentary rocks comprise the geci group which was mapped and studied petrographically geochemically and isotopically in order to provide constraints on the depositional environments age and latest history of the mozambique orogenic belt the group occurs as several large tectonically dissected intensively sheared folded and mylonitised sw ne trending lenses within unango complex granulite rocks in places primary depositional features are well preserved the dominant rocks are calcarenites dolarenites calcite matrix supported and dolostone clast supported carbonate breccias forming beds with erosional bases normal and reverse graded bedding and well developed bouma sequences dolomicritic microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation the geci meta carbonate rocks have low sio2 and al2o3 contents mgo cao ratios fluctuate between 0 05 and 0 70 averaging 0 15 0 18 1 n 111 acid soluble constituents have moderate concentrations of fe 777 310 ppm mn 131 85 ppm and sr 566 145 mn sr ratios are relatively low 0 26 0 19 13c and 18o values are invariably high with only moderate scatter 4 0 0 6 v pdb and 25 8 0 5 v smow respectively the least altered 87sr 86sr ratio of calcite is 0 70708 whereas dolomite is enriched in 87sr with the lowest ratio of 0 70730 the least altered 87sr 86sr and 13c ratios suggest an apparent depositional age of either 595 585 or 630 625 ma this provides a lower age limit for juxtaposition of the low grade geci group rocks and granulite facies rocks of the unango complex

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

rochechouart impact crater melt breccias record no geomagnetic field reversal abstract impact melt breccias from the rochechouart france meteorite crater possess paleomagnetic directions with both normal and reverse polarities raising the question whether shock from the collision initiated a geomagnetic field reversal stepwise thermal demagnetization together with a suite of rock magnetic experiments optical microscopy raman spectroscopy and electron microprobe analyses identify adjacent multiphase titanohematite in the samples containing normal and mixed polarities typical of lithologies bearing self reversal behavior stemming from magnetic exchange interaction melt breccias possessing mostly titanium free hematite as the magnetic remanence carrier have solely reverse polarity directions leading to the conclusion that the rochechouart meteorite impact did not spawn a geomagnetic event samples displaying paleomagnetic directions with normal or mixed polarity yield unreliable paleointensity data 30 samples with reverse polarity that pass stringent data selection criteria from thellier thellier paleointensity experiments with alteration tail and additivity checks define an average field value of 12 8 3 7 t this translates into a virtual dipole moment of 2 7 0 8 10 22 a m 2 which is relatively low but within uncertainty of other mesozoic paleointensity data

geological and biological consequences of giant impacts it seems inescapable that large impacts have been an ongoing phenomenon throughout geological time in the phanerozoic the estimated frequency of impact by bodies of 5 km diameter or more is on the order of 1 0 every 1 00 myr the 5 km lower limit is chosen arbitrarily but it is shown in this review that such a body is certainly large enough to cause very large terrestrial effects a small decrease in this size limit would lead to a large increase in the frequency of occurrence it may be claimed that bodies of a size likely to have drastic and sudden effects on niany physical and biological features of the earth have arrived with a frequency comparable to the average duration of a biostratigraphic stage nothing is said of the interval between arrivals which may be random i e stochastic currently there is not enough geological evidence to allow recognition of other than random spacing between a few positively identified events during the phanerozoic since impacts by extraterrestrial bodies have been clearly established it is reasonable to postulate that they must leave a record in the stratigraphic column why then do we currently have so few horizons that have been claimed to contain evidence of a global impact event since the effects we are considering are unquestionably global in this paper we bring together evidence from several disciplines that

formation and tectonic evolution of the khondalite series at the southern margin of the north china craton geochronological constraints from a 1 85 ga mo deposit in the xiong ershan area abstract the taihua complex is an important metamorphic rock suite at the southern margin of the trans north china orogen and hosts important mineral deposits its protolith and metamorphic ages are still controversial the 1 85 ga longmendian mo deposit in the xiong ershan is hosted by the khondalite series of the duangou formation taihua complex three kinds of rocks are collected for shrimp cameca u pb dating including the meta sedimentary rock that occurred as wallrock the mo mineralized meta gabbro that intruded the meta sedimentary rock and the leucosome that cut both the meta sedimentary rock and the meta gabbro zircon u pb analyses reveal that the precursor of the meta sedimentary rock was crystallized between 2200 4 and 2742 18 ma with most before 2500 ma whereas the protolith of the meta gabbro was formed at 2178 6 ma anatexis zircons in the leucosome yield a weighted mean age of 1943 8 ma that is indistinguishable with the metamorphic age of the meta gabbro 1952 4 ma in the intensively altered meta gabbro hydrothermal titanites were formed by at least two pulses of crystallization at 1940 7 ma and 1916 9 ma respectively the new data reveal that the khondalite series of the duangou formation were formed between 2200 ma and 2178 ma the results are inconsistent with previous subdivision of the taihua complex into the upper taihua and lower taihua subgroup instead they fit well with the tripartitite divison into the beizi group 3 0 2 55 ga the dangzehe group 2 55 2 3 ga and the shuidigou group 2 3 2 1 ga compiling zircon u pb ages indicate that in the trans north china orogen the 1 95 ga and 1 85 ga metamorphic events are widespread and prominent

spheroids at the cretaceous tertiary boundary are altered impact droplets of basaltic composition sand size spheroids of k feldspar in the cretaceous tertiary c t boundary clay at caravaca southern spain were interpreted by smit and klaver as having solidified from a melt resulting from the impact of a large extraterrestrial body sand size spheroids of k feldspar glauconite and magnetite quartz have been found in the c t boundary clay in northern italy and spheroids of k feldspar and pyrite were found in the boundary clay at deep sea drilling project site 465a in the central pacific these spheroids have textures similar to those of rapidly crystallized feldspar and mafic silicates they are interpreted as diagenetically altered microcrystalline spherules of basaltic composition produced by the impact of a large asteroid in an ocean basin at the end of the cretaceous they are analogous to the glassy microtektites produced by impacts on more siliceous target rocks 21 references 4 figures

miocene stable isotope record a detailed deep pacific ocean study and its paleoclimatic implications deep sea drilling project site 289 in the western equatorial pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the miocene deep ocean the isotopic record reflects major changes in paleoclimate and paleoceanography probably dominated by a major phase of antarctic ice cap growth the transition from a relatively unglaciated world to one similar to today occurred between 16 5 x 10 6 and 13 x 10 6 years before the present with the greatest change occurring between approximately 14 8 x 10 6 and 14 0 x 10 6 years before the present

the geology and geochemistry of the palaeoproterozoic makganyene diamictite abstract the palaeoproterozoic earth experienced a global glacial event at 2400 ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans the transvaal supergroup in the griqualand west basin south africa hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes the focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic makganyene formation in order to constrain its palaeoenvironmental settings the makganyene formation forms the base of the postmasburg group and has been regarded as resting on an erosive regional unconformity throughout the northern cape province systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal the makganyene formation is in fact conformable with underlying formations of the koegas subgroup in the deep southern prieska basin and rests on an unconformity only on the shallow ghaap platform to the north east the makganyene formation displays lateral facies changes that reflect the palaeogeography of the study area and the advance and retreat of ice sheets shelves geochemical investigations of glacial strata of the makganyene formation demonstrate that underlying banded iron formations of the transvaal supergroup acted as the main clastic source for the diamictite detritus geographic variations in bulk composition of the diamictites correlate well with field observations and show that sorting processes were controlled largely by the morphology of the palaeobasin carbon isotope results emphasize the transitional nature of the makganyene formation in terms of the environmental conditions that resulted in widespread global glaciation in the palaeoproterozoic on the basis of the above geological evidence it is proposed that the transvaal supergroup in the northern cape province represents a continuous depositional event that lasted approximately 250 ma and hence provides a unique opportunity for assessing the transitional changes experienced by the palaeoproterozoic earth

rb sr sm nd k ca o and h isotopic study of cretaceous tertiary boundary sediments caravaca spain evidence for an oceanic impact site isotopic ratios and trace element abundances were measured on samples of ir enriched clay at the cretaceous tertiary boundary and in carbonate and marl from 5 cm below and 3 cm above the boundary samples were leached with acetic acid to remove carbonate and with hydrochloric acid leachates and residues were measured the sr nd o and h isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials the data indicate that most of the clay material was derived from a terrestrial source with relatively low87sr 86sr and high143nd 144nd ratios the 18o data suggest that the detritus has been modified by submarine weathering k ca and rb sr systematics as well as o isotope ratios of k feldspar spherules within the boundary clay suggest that they are predominantly authigenic and may have formed after the time of deposition however sm nd and rb sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus the isotopic evidence for foreign terrestrial detritus in the boundary clay the low rare earth element concentrations and high ni concentration support the hypothesis of a terminal cretaceous asteroidal impact that produced a global layer of fallout the data are most easily explained if the impact site was on oceanic crust rather than continental crust and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere 3 km this would probably require a single large impactor

discovery of the huronian glaciation event in china evidence from glacigenic diamictites in the hutuo group in wutai shan abstract paleoproterozoic glacigenic diamictites have provided key evidence for the great oxidation event goe at 2 3 ga associated with the widespread huronian glaciation event hge here we report for the first time paleoproterozoic glacigenic diamictite from the sijizhuang formation of the hutuo group in wutai shan china formed during the late stage of hge the sijizhuang formation comprises diamictite sandstone and siltstone mudstone facies conforming to a glaciomarine association the diamictites show weak sorting and roundness and carry a few striated and faceted boulders typical dropstones and associated structures are present in the sandstone and siltstone mudstone facies diamictite facies which characterizes the lower portion of the sijizhuang formation displays an upward decrease with a concomitant increase in the proportions of sandstone and siltstone mudstone facies the lithofacies associations of diamictite with sandstone and siltstone mudstone indicate a frequently changing hydrodynamic environment the petrochemical compositions of different lithofacies show that the clasts were multi sourced and generated by limited weathering in keeping with a cold climate available isotope age data constrain the glacigenic deposition of the sijizhuang formation to between 2 40 and 2 13 ga and most likely from 2 2 to 2 1 ga this estimate compares well with the 2 4 2 1 ga age of the widespread hge but the sijizhuang diamictite appears to be younger than the last glacial unit gowganda formation in the type area of the hge in canada and suggests a locally protracted glacial event

sudden death at the end of the mesozoic abstract a paleoecological analysis of the fossil record before and after the cretaceous tertiary boundary indicates that the widespread extinctions and biological stresses around the boundary are best explained in terms of a sudden significant but short temperature rise l alvarez and co authors having found an enrichment in iridium at the same boundary postulated that it was associated with the impact of an extraterrestrial body if this body struck the ocean the water injected into the atmosphere may have led to a transient increase in the global surface temperature this temperature pulse may have been primarily responsible for the effects observed in the biosphere the pattern of extinction of higher plant species suggests that splash down occurred in the northern pacific bering sea area

the cretaceous tertiary boundary chronostratigraphic position and sequence of events the cretaceous tertiary boundary has always been considered to be exceptional and linked to special events and mechanisms where catastrophes have often been advocated the boundary stratotype stevns klint in denmark has been paleomagnetically analysed the c t boundary is now given an absolute age of 66 7 m y the boundary is shown to have occurred under stable geomagnetic conditions about 165 000 years before the polarity reversal of the base of anomaly 29 and about 0 7 1 0 m y before the extinction of dinosaurs in new mexico significant disturbances in the earth s rate of rotation and direction of spin axis at the passing from the cretaceous into the tertiary are strongly contradicted the cat boundary events cannot be explained by a single large catastrophe a sequence of different events and some gradualism need also to be included in present and future scenarios of the boundary events

impacts and glacio eustasy plate tectonic episodes geomagnetic reversals a concept to facilitate detection of impact events abstract it is suggested that the cretaceous tertiary c t impact destabilised and then profoundly changed the climatic structure of the earth for cenozoic times by establishing freezing conditions at the poles via the c t dust veil and the subsequent h 2 o greenhouse evaporation polar snow fields ice caps see level fall tp1 it is also suggested that the c t impact affected the earth s mantle shock induced phase transitions and via chain reactions also the core mantle boundary this mantle effect could possibly explain the laramian plate tectonic episode 65 ma and reversal t29 r n which occurred 150 000 years after the c t boundary as long term impact effects the combination of these and the multitude of associated events c f faunal breaks appear to correlate episodically in the geological past it is proposed that these repetitions can be used as a potential approach to narrow the search for impact traces also more examples of correlations between impact events and their possible effects are needed to ensure that the implications of the c t impact are generally valid

age petrogenesis and tectonic setting of the thessalon volcanic rocks huronian supergroup canada abstract the 2 5 ga huronian supergroup volcanic rocks in northern ontario represent one of the better preserved paleoproterozoic continental flood basalt sequences on earth the youngest age of huronian supergroup volcanism is refined to 2452 5 6 2 ma based on new u pb zircon analyses of the copper cliff rhyolite recent mapping and geochemical analyses of thessalon formation volcanic rocks define a well developed chemostratigraphy a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt andesite volcanic series these two mafic volcanic series are separated by felsic volcanic rocks rhyolite and dacite the mafic lavas are low ti tholeiites dominated by basalt and andesite and are characterized by enriched light rare earth element profiles and high th contents up to 12 ppm most basalt andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and in some cases limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle all magma batches display prominent negative nb ta and ti anomalies on primitive mantle normalized multi element diagrams interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism at least three distinct mantle source compositions are required to explain the range in the observed lava compositions the most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high th yb 10 and low zr nb 11 this reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward neoarchean subduction along the southern superior craton margin