the white stone band of the kimmeridge clay formation an integrated high resolution approach to understanding environmental change the kimmeridge clay is a jurassic mudrock succession that shows milankovitch band climatic cyclicity a key issue is to determine how the subtle changes that define this cyclicity result from climatic change using material from the natural environment research council rapid global geological events rgge kimmeridge drilling project boreholes the white stone band was investigated at the lamination scale using backscattered electron imagery and quantitative palynofacies fabric analysis shows the lamination to represent successive deposition of coccolith rich and organic matter rich layers individual laminae contain unsorted palynological debris with a consistent ratio of marine and terrestrial components such mixed organic matter input is interpreted as the result of storm transport linking water column processes to laminae deposition suggests seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria as the photic zone extended into the euxinic water column organic matter export to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon this was significantly increased by late season storm driven mixing of euxinic water into the photic zone increased frequency of storm systems would therefore dilute the coccolith input to give an oil shale hence climatically induced changes in storm frequency would progressively vary the organic content of the sediment and generate the climate cycle signal

porosity and hydrocarbon distribution in the north sea chalk reservoirs abstract since 1970 more than 25 billion barrels of hydrocarbons in place at reservoir conditions have been discovered in the north sea chalk reservoirs for accurate development drilling satisfactory reserve estimate and optimal recovery a precise description of the porosity and hydrocarbon distribution has proved necessary because such reservoirs were unknown it took some time to fully understand their local and regional variation based on a 12 year experience of the north sea chalks and the data from 180 wells a basic geological model was established and tested on the other fields in the area and is described in this paper thickness variations are due to differential diagenesis as well as depositional factors porosity generally decreases downsection and towards the flank as a result of the progressive invasion of rock by oil and subsequent inhibition of chemical compaction layering in porosity reflects differences in initial porosity hydrocarbon saturation varies with structural elevation and rock type i e depositional facies mineral content differential behaviour to early and late diagenesis pore and pore throat size and morphology tortuosity this report is divided into two parts development of the model and comments on the individual fields

origin of minor limestone shale cycles climatically induced or diagenetic the opinion is widely held that each limestone bed in minor limestone shale cyclic sequences however modified subsequently by diagenesis reflects an environmental event on the assumption that this is true attempts have recently been made to relate limestone shale cycles to climatic variations affected by milankovitch orbital cycles a test is proposed and illustrated mainly with reference to the blue lias formation of dorset england that appears to indicate that many limestones can be produced solely by rhythmic unmixing of caco 3 during diagenesis unless such limestones can be clearly distinguished from those that record genuine environmental signals orbital cycle analysis based on such sequences will give meaningless results

nano sized clay detected on chalk particle surfaces abstract chalk is a sedimentary rock consisting mostly of micrometer sized particles of biogenic calcite caco 3 it forms the reservoirs for oil in the danish part of the north sea and important drinking water aquifers in northern europe the interaction between organic molecules and the particle surfaces has a strong influence on the migration of oil in the reservoirs adhesion of oil components to chalk particle surfaces is thought to significantly reduce production and efforts are being made to determine the mechanisms behind enhanced improved oil recovery eor ior during water flooding aquifers beneath industrial areas can be heavily polluted with organic compounds and similar adsorption mechanisms could influence the geographical extent of the contaminant plume chalk as a sedimentary rock has been studied extensively for many years but the properties of the particle surfaces where adsorption takes place are largely unknown in this study we have used atomic force microscopy afm to show that the grain surfaces in offshore and onshore chalk are more heterogeneous than previously assumed the particles are not simply calcite surfaces but are partially covered by clay that is only 1 4 nm thick with chemical force mapping cfm we have probed the surface using a tip coated with organic molecules to represent a tiny mono functionalized oil droplet the functional groups of the self assembled monolayer on the tip were either ch 3 or coo from maps of adhesion it is evident that in calcite saturated water both the polar and the nonpolar functional groups adhere to the nano sized clay particles but not to calcite this is fundamentally important information for the development of conceptual and chemical models to explain wettability alterations in chalk reservoirs

early hettangian benthic planktonic coupling at doniford sw england palaeoenvironmental implications for the aftermath of the end triassic crisis abstract this paper presents a high resolution micropalaeontological study integrated with geochemical and sedimentological analyses of the triassic jurassic tr j transition at the doniford section sw england our analyses indicate the renewal of surface carbonate production in the blue lias formation early hettangian after the end triassic mass extinction based on the high abundance of surface carbonate producers schizosphaerellids which occurred in phase with development at sea floor of calcareous hyaline benthic foraminifers we deduce that a benthic pelagic coupling resulted in both the initial restoration of the biological carbonate pump and also a more efficient export production the presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end triassic maximal perturbation complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black shale sediments the hypothesis of a methane release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early jurassic is discounted based on evidence from the decoupled carbon isotope records of carbonate and organic matter apart from local control of a high amplitude sea level change across the tr j boundary the succession and nature of events at doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the central atlantic magmatic province camp volcanism a super greenhouse climate in the aftermath of the end triassic mass extinction appears to be the results of the combination of the long term co2 accumulation in the atmosphere system due to the camp volcanic activity as well as the inefficiency of the biological pump due to reduced export production

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

137cs and 210pb derived sediment accumulation rates and their role in the long term development of the mkuze river floodplain south africa abstract wetlands are well known to act as sinks for sediment and chemicals in the landscape within the mkuze river floodplain in northern kwazulu natal south africa deposition of clastic sediment occurs predominantly in close proximity to the river channel resulting in the formation of levees over time this leads to channel avulsion a process that may be accelerated by the activities of hippopotami whose trails create hydraulically favourable pathways sedimentation rates determined using the isotopes 210pb and 137cs indicate that the mkuze river floodplain is a relatively rapidly aggrading system that should experience frequent avulsion with average short term rates in the order of 0 25 to 0 50 cm y sediments on the floodplain are also an important sink for solutes which concentrate in the groundwater and precipitate out because of evapotranspiration over long timescales chemical sedimentation affects the landscape by influencing salinity vegetation distribution hydrological flows and local topography the mkuze river floodplain is an actively evolving system which continues to aggrade as a result of the combination of clastic and chemical sedimentation in a region characterised by a strong annual water deficit temporal patterns of clastic and chemical sedimentation are likely to exert influence on the long term development of wetland systems elsewhere in southern africa this study the first on sediment accretion rates using 210pb and 137cs dating for a southern african wetland demonstrates that radioisotopic methods are an important tool that can be applied toward fully understanding wetland formation evolution and functioning in the region

latest pleistocene and holocene geomagnetic paleointensity on hawaii geomagnetic paleointensity determinations from radiocarbon dated lava flows on the island of hawaii provide an estimate of broad trends in paleointensity for holocene time and offer a glimpse of intensity variations near the end of the last glacial period when the data from hawaii are compared with others worldwide the intensity of the geomagnetic field seems to have been reduced from the holocene average by about 35 percent between 45 000 and 10 000 years ago a long term reduction of this magnitude is compatible with reported increases in the production rate of cosmogenic nuclides during the same interval

137cs and 210pb dating of sediments from soft water lakes in new england u s a and scandinavia a failure of 137cs dating abstract deep water sediment cores from 32 scandinavian and 19 northern new england u s a lakes were analyzed for 137cs 210pb was analyzed in cores from 16 and 14 of these lakes respectively additional dating of the new england cores was based on chronostratigraphic pollen markers the profiles in sediment of 137cs deviate greatly from expectations based on fallout history the profiles have their maxima at the surface and 137cs occurs at depths dated by 210pb and pollen as many decades older than the beginning of 137cs fallout non exponential 210pb profiles in certain lakes most in new england and pollen dates suggest that bioturbation physical mixing changes in sedimentation rate or acidification may be occurring but this is insufficient to account for the deviant 137cs profiles in most of the lakes there is no evidence of mixing we argue that the occurrence of 137cs at prefallout sediment depths is due mostly to net downward molecular diffusion and adsorption and that surface concentration is due to net upward diffusion and adsorption and possibly also biological uptake recycling of sedimentary 137cs in the water column and delayed input from the watershed it is probable that much of the 137cs entering these lakes is carried by organic detritus mobility of 137cs in this sediment may be a function of release by organic decomposition and scarcity of clay minerals for adsorption correlation analyses of the sedimentary radionuclide activities vs physical and chemical characteristics of the lake water sediment and watersheds suggest that considerable trapping of 137cs and 210pb by terrestrial vegetation and organic soils is occurring the resultant delayed export to these lakes is contributing to the failure of 137cs as a dating tool but 210pb dating can be valid under these circumstances

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

210pb dating of scottish lake sediments afforestation and accelerated soil erosion changes in the rate of soil erosion in lake catchments can be identified from changes in the rate of sediment accumulation in lakes here we compare recently afforested sites with non afforested sites in the galloway area of southwest scotland we show that lakes with non afforested catchments have slow constant sediment accumulation rates whereas lakes with recently afforested catchments have changes in accumulation that parallel the known history of afforestation for loch grannoch the sediment accumulation rate increases from 0 1 cm yr 1 to over 2 cm yr 2 during the disturbance period data from l skerrow however suggest that the rate might decline to predisturbance levels after approximately 10 years as the forest canopy closes and drainage channels stabilize

the application of δ13c and c n ratios as indicators of organic carbon sources and paleoenvironmental change of the mangrove ecosystem from ba lat estuary red river vietnam in this study two sediment cores 70 cm were collected from separate mangrove forests straddling the ba lat estuary red river of northern vietnam to examine the origins of sedimentary organic carbon soc and reconstruct the paleoenvironment in addition mangrove leaves and particulate organic matter were collected and measured for 13c to trace the origins of soc the cores were analyzed by high resolution sections for 13c toc c n ratios sediment grain size water content and porosity with values of 13c toc and c n ratios ranging from 28 19 to 22 5 2 14 30 94 mg g and 10 29 18 32 respectively the 13c and toc relationship indicated that there were some small residual effects of diagenetic processes on toc and 13c values in mangrove sediments however the shifts of 13c and c n ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources with values of c n 12 and 13c 25 indicated terrestrial e g mangrove litter and marine phytoplankton sources respectively the covarying 13c c n ratios and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments subtidal tidal flat and intertidal mangrove general trends in 13c and c n followed a gradual increase in the c n ratio and a concomitant decrease in 13c from the subtidal through to tidal flat and to the intertidal mangrove 13c and c n ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem

improvements in procedure and paleointensity selection criteria picrit 03 for thellier and thellier determinations application to hawaiian basaltic long cores abstract in this paper we first describe the paleointensity furnaces and the procedures specifically designed to minimize oxidation of the samples during thellier and thellier paleointensity experiments routinely used at gif we also propose some improvements to the set of criteria commonly used to select the results of the thellier experiments in order to further increase the reliability of paleointensity determination finally to be included in databases the new set of paleointensity criteria picrit 03 paleointensity criteria in 2003 is discussed and the results obtained from two long hawaiian basaltic cores the scientific observation hole 4 soh 4 and the hawaiian scientific drilling project 1 hsdp1 are used to illustrate its effectiveness some of the results are re evaluated and a new composite record of absolute paleointensity and inclination spanning 420 kyr is presented the inclination values obtained from the two cores combined support the hypothesis of persistent shallow inclinations at hawaii in intensity the upper 100 kyr are covered by the data from soh 4 with 309 reliable determinations out of 766 examined obtained from 143 units which belong to the kilauea volcano at the top of this core the 341 reliable data covering the 100 420 kyr interval are from 149 units of core hsdp1 belonging to the mauna kea volcano the interval 320 to 420 kyr is particularly detailed because of high extrusion rate of the volcano the use of more stringent criteria decreases the number of determinations considered as reliable in previous publications but it improves their accuracy and robustness

orbital forcing of palynofacies in the jurassic of france and the united kingdom palynofacies analysis of marine jurassic sedimentary successions in france and the united kingdom demonstrates the pervasiveness of regular cycles in qualitative and quantitative aspects of the microscopic particulate organic content of the rocks especially that part which is ignored during routine palynology the cycles are attributed to precession obliquity and eccentricity forcing most of the palynofacies cycles are independent of lithology whether it be cyclic irregular or uniform terrestrially derived organic particles record the effects of orbital cyclicity on land which may be controlled by orbital cycles different from those controlling marine sediment deposition the majority of studies on orbital cyclicity in the sedimentary record concentrate on cyclicity in lithology this study demonstrates that sections lacking regular lithological cycles may still record evidence of orbital forcing of both the depositional environment and sediment source and transport

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

paleointensity high at 9000 years ago from volcanic rocks in japan six paleointensities were determined from young lava flows of fuji and oshima volcanoes whose ages are around 8000 to 15000 years b p two lava flows which have well defined ages at 8970 390 years b p and 9260 430 years b p give large paleointensities of 67 12 and 65 6 t respectively another lava flow of less accurate age also gives large paleointensity of 63 6 t the new paleointensity data are consistent with the trend of averaged paleointensity variation in japan for the last 9000 years they form a peak of paleointensity high at around 9000 years b p comparable to the well documented one at 2000 years b p this suggests that there exists a dominant component with a characteristic period of about 7000 years in paleointensity variation large paleointensity at about 8000 10000 years ago is also reported in france and iceland paleointensity variation averaged every 1000 years for the last 14000 years is quite similar in europe and japan the sine curve like variation of paleointensity observed in both europe and japan probably reflects the nature of the dipole moment if we consider much difference of longitude between the two regions

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

palaeoenvironmental significance of carbon and oxygen isotope stratigraphy of marine triassic jurassic boundary sections in sw britain abstract carbon isotope stratigraphy is a useful tool for stratigraphic correlation especially for strata deposited during major perturbations of the carbon cycle that affected the marine terrestrial and atmospheric reservoirs for the triassic jurassic boundary effectively defined by a first order mass extinction major fluctuations in carbon isotope values have been well documented but these datasets have generally been derived from bulk rock samples hence the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained here carbon and oxygen isotope data are presented from well preserved oyster shells liostrea comprising low magnesium calcite a mineral species relatively resistant to diagenetic alteration samples were obtained from lavernock point glamorgan wales a coastal section close to a candidate stratotype for the base of the jurassic at st audrie9s bay somerset england the carbon isotope signature from st audrie9s bay previously defined on the basis of analysis of bulk organic matter is confirmed by our new data major features are 1 the upper part of an initial9 negative isotope excursion in the lowest part of the section followed by 2 a pronounced positive excursion and 3 an extended main9 negative isotope excursion in the highest part of the section the data confirm that the carbon isotope stratigraphy previously documented from bulk organic matter in sw england records the chemical composition of the contemporaneous seawater bulk carbonates sampled over the same interval near lyme regis england show similar trends to those from oyster calcite in the lower part of the study section but there are more 13 c depleted values up section these lower values probably result from an admixture of primary and diagenetic carbonate palaeotemperatures calculated from oxygen isotope values from lavernock point oyster shells are relatively cool at the beginning of the positive carbon isotope excursion and increased by up to 10 c during the main negative carbon isotope excursion the new results are compatible with the view that positive carbon isotope excursions correspond to times of low atmospheric carbon dioxide content whereas negative carbon isotope excursions correspond to times of high atmospheric carbon dioxide content as is also found to be the case during the early jurassic toarcian oceanic anoxic event the mg ca and sr ca ratios and 18 o of investigated liostrea hisingeri show no correlation supporting data from modern bivalves that indicate that incorporation of mg and sr is controlled mainly by factors other than temperature

hemipelagic shelf sedimentation and climatic cycles the basal jurassic blue lias of south britain abstract the sedimentology cyclicity and hiatuses of the blue lias formation basal jurassic were investigated at five localities in south britain most carbonate mud which is the dominant carbonate component has been neomorphosed to microspar however carbonate mud was apparently supplied by coccoliths in zooplankton faecal pellets suggesting hemipelagic sedimentation clay is assumed to have been supplied by rivers the three sediment types involved reflect cyclic changes in bottom water oxygenation levels combined with changes in the clay to carbonate mud ratio walsh power spectra have been applied for the first time to measured sections in order to investigate the control of cyclicity this application is analogous to palaeoclimatological studies of pleistocene oxygen isotope ice volume records the spectra show that 1 or 2 sedimentary cycles of constant thickness occur in four out of five sections each of which represents several million years the maximum possible duration of these cycles is too short for their generation by sea level changes their regularity stability and order of duration tens of thousands of years are consistent with the hypothesis of sedimentary cyclicity controlled by climatic change itself forced by orbital changes in insolation milankovitch theory the duration of both cycles recognised is less than 93 000 years 93 ka and may record changes in orbital precession 21 ka and obliquity 41 ka assuming that one ammonite zone represents 1 ma comparison of the longest possible with the implied durations of the cycles suggests that the sections are at best 20 40 complete loss of sediment may have occurred during winter storms or hurricanes a model relating the sedimentary cycles to changes in the volume of runoff is proposed

some global features of palaeointensity in geological time summary a global palaeointensity data base was constructed from all published data from volcanic rocks in geological time older than 0 03 ma the data base contains a total of 1123 flow mean data retrieved from 83 original papers various features of the earth s dipole moment were examined from the data which are based on thellier and shaw methods long term variation of the earth s dipole moment seems to have existed in the past 300 ma with a broad minimum at 120 180 ma as suggested by prevot et al 1990 however due to limited site distribution we cannot regard this mesozoic dipole low as being completely established precambrian palaeointensity data are still insufficient to conclude any long term variation in this time range although the geodynamo processes of moderate magnitude definitely existed in the early time of the earth s history the 2 test was applied to the distribution of the virtual dipole moment for the past 5 ma in which the transitional data were excluded the results indicate that the distribution of virtual dipole moment is better represented by a log normal distribution rather than a normal distribution and this tendency seems to be true for the past 20 ma the relation of mean palaeointensity versus palaeomagnetic colatitude was examined for the past 10 ma for the data excluding transitions the relation is concordant with a theoretical curve from a geocentric axial dipole this reconfirms that the dipole field was dominant in the past geomagnetic field which is the dipole hypothesis in palaeomagnetism on the other hand the virtual dipole moment is much smaller for transitional palaeomagnetic fields and a virtual geomagnetic pole lower than 45 seems to be a reasonable criterion to be categorized as a transition the mean dipole moment for the pole latitude of 30 n 50 n band is larger than that for 30 s 50 s indicating that there might have been a persistent asymmetry of palaeointensity between normal and reversed states or some kind of geometrical asymmetry between the northern and southern hemispheres

nitrogen isotope record of a perturbed paleoecosystem in the aftermath of the end triassic crisis doniford section sw england 1 the triassic jurassic transition tj is characterized by successive perturbations of the carbon cycle during a time of biotic disruption as recorded by the carbon isotopic composition of organic matter 13corg the nitrogen isotopic composition of sedimentary organic matter 15norg constitutes a key parameter to explore the functioning of the ecosystem during carbon cycle perturbations and biological crises because it provide information on seawater redox conditions and or nutrient cycling here we report the first continuous 15norg record across the tj transition at the doniford bay section bristol channel basin uk combined with 13corg kerogen typology and carbon 13cmin and oxygen 18omin isotopic composition of bulk carbonates the end triassic is characterized by a major negative excursion both in 13corg and 13cmin very low toc total organic carbon wt and high 15norg values associated with a sea level lowstand a second 13corg negative excursion occurs during the lower hettangian this interval is characterized by phases of carbonate production increase alternated with phases of exceptional accumulations of type i organic matter up to 12 associated with lower 15norg and 13corg this alternation likely reflects a succession of nutrient input increase to the basin leading to enhanced productivity and eutrophication which promoted a primary production driven by organic walled prokaryotic organisms the following om export increase generates anaerobic conditions within the basin these events occur between periods of relatively good seawater column ventilation and nutrient recycling boosting the carbonate producer recovery ecosystems remain perturbed in the bristol channel basin during the aftermath of the end triassic crisis

immobility of 210pb in black sea sediments we measured 210pb in a core from the deep black sea which contained a turbidite of low 210pb activity surrounded above and below by pelagic sediments of much higher activity to evaluate in situ210pb mobility under the anoxic moderately sulfidic conditions by measuring at millimeter scales across the distinct laminae turbidite contacts and modeling the observed profile we estimated a lower limit of the kd of 107 deff 5 10 13 this result increases confidence in sediment chronologies from the black sea based on 210pb because it implies that profiles will not be altered significantly by diffusive transport of 210pb below the turbidite the 210pb inferred accumulation rate was 50 g m2 a which is lower than laminae based estimates by a factor of 3 4

palaeointensity study of quaternary east eifel phonolitic rocks germany summary compared to the other volcanic rocks of the quaternary east eifel volcanic field germany the phonolites stand out because of very low susceptibilities and intensities of natural remanent magnetization geomagnetic palaeointensities have been studied by means of the thellier method for seven of these rock units while five of them are lava flows or domes the other two have been deposited at temperatures of about 500 c as zeolitized tuffs results of rock magnetic investigations indicate that the magnetic carriers are ti poor titanomagnetites or magnetite heating above 200 300 c often leads to an alteration of the magnetic carriers of the rocks in the low temperature range well defined lineari segments in the succession of the thellier experiment data points are found for most of the lava samples as well as for the zeolitized tuffs because of inconsistencies of within unit results and the presence of viscous magnetization components the palaeointensities of only two rock units are regarded as reliable the obtained values are about 30pt as compared to the present day field of 48 pt

late pleistocene eifel eruptions insights from clinopyroxene and glass geochemistry of tephra layers from eifel laminated sediment archive sediment cores

the influence of radon diffusion on the 210pb distribution in sediments a mathematical model is presented which describes the distribution of radon 222 in sediments having a constant or variable depth distribution of radium 226 the model is extended to the distribution of lead 210 taking into account the mobility of radon the precursor of 210pb within the sediment column the 210pb model is compared at constant radium activity with the conventional approach which disregards the radon diffusion when estimating sedimentation rates by the 210pb method the ratio between apparent and real sedimentation rate s s expressed as a function of three dimensionless parameters demonstrates the importance of the radon diffusion effect this effect is particularly important for sediments with small initial excess 210pb activity small sedimentation rate large radon diffusivity or a combination of these factors applied to lake geneva the sedimentation is estimated to be larger by 30 50 than the original value by krishnaswami et al 1971 in sediments which are mixed at the surface physical mixing or bioturbation the 210pb activity in the mixed layer is diminished compared to that in the settling sediment material robbins et al 1977 and radon diffusion makes the activity difference even larger especially for low initial excess 210pb activity small sedimentation rate and large mixing intensity this result may be of importance for the balance of 210pb in an aquatic system if the calculations are based on activities measured in the sediment

stratigraphic correlation of presumed milankovitch cycles in the blue lias hettangian to earliest sinemurian england simple digital filtering methods allow the correlation of patterns of metre scale cycles between outcrop sections and well logs through the hettangian to earliest sinemurian part of the lower lias in southern england the matching of cyclic patterns is sufficiently confident to allow the identification of a significant break in the blue lias section at lyme regis dorset the standard ammonite zones of the hettangian stage are shown to be of quantifiably unequal duration

determination of geomagnetic palaeointensities from the quaternary west eifel volcanic field germany summary determination of geomagnetic palaeointensities has been attempted for 418 specimens from 51 volcanic units of the quaternary west eifel volcanic field this paper describes the measuring procedure using a modification of the thellier method the analysis of the derived data points the determination of linear segments and the quality of the individual palaeointensity values calculated from the corresponding slopes according to a series of acceptance and rejection criteria finally 312 reliable individual palaeointensity results have been obtained about 73 per cent of the accepted regression lines have correlation coefficients better than 0 990 and less than 10 per cent of the accepted thellier experiments have quality factors less than 1 0 the carriers of magnetization of the west eifel volcanic rocks are titanomagnetites with a broad range of titanium contents and oxidation states in a more or less unweathered mostly foiditic rock matrix accompanying rock magnetic experiments allowed a rough classification of the specimens corresponding to the oxidation state of the titanomagnetites specimens with highly oxidized titanomagnetites have been the most successful candidates for the thellier experiments however a prediction of the suitability of the specimens for the thellier experiments by sample selection according to this classification was not always appropriate instead for most volcanic units pilot specimens were first measured to determine the suitability of a specific sample type the temperature interval from which the evaluation of palaeointensity was possible and the best laboratory field intensity to keep the slope errors of the thellier experiments to a minimum palaeointensity determination was not possible for 14 volcanic units because only unsuitable specimens were available for each of the 37 remaining volcanic units at least five successful thellier experiments have been carried out leading to a final mean palaeointensity result these 37 values have relative standard errors of between 4 and 23 per cent mostly around 10 per cent specimens with differing rock magnetic properties from the same volcanic unit yielded consistent palaeointensity values therefore all west eifel palaeointensity results have to be regarded as reliable estimates of the earth s magnetic palaeofield strength mean palaeointensities range from 7 to 62 pt with an unexpectedly large number of palaeointensity values less than 30 pt about half of the low palaeointensity values are associated with intermediate virtual geomagnetic pole latitudes which may be due to a geomagnetic excursion

paleointensity of the earth s magnetic field recorded by two late quaternary volcanic sequences at the island of la reunion indian ocean a paleomagnetic study has been undertaken on two late quaternary volcanic sequences from the piton de la fournaise volcano island of la reunion in order to provide absolute paleointensities of the earth s magnetic field from a site in the southern hemisphere the pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time a detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the thellier method samples with minimum overprint composed by single domain or pseudo single domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during thellier s experiments values of the paleofield strength from 19 to 55 mt have been obtained on 23 flows among the 30 which were sampled few variations 7 5 9 9 1022 a m2 are observed in the virtual dipole moments vdms determined on the youngest sequence 5 12 ka those values are within the range of fluctuations of the worldwide variations of vdms for this period on the other hand data from the oldest sequence 82 98 ka are the first paleointensity values obtained in that time period and the observed vdms extend from 4 1 to 8 8 1022 a m2 these results suggest that for this time interval the field did not depart from its average behavior

magnetic fabric arising from paramagnetic phyllosilicate minerals in mudrocks the magnetic fabric of mudrock samples from the westbury member of the penarth group rhaetian the blue lias formation hettangian to sinemurian and the oxford clay callovian to oxfordian has been measured using a high field torque magnetometer the magnetic susceptibility anisotropy of these mudrocks originates from paramagnetic phyllosilicate minerals of which the most important is considered to be detrital chlorite the magnetic fabric preserves both a lineation and a foliation which are of a primary style this magnetic fabric is sensitive to the effects of recent weathering due to the ease with which chlorite is oxidized during weathering a model is proposed utilizing compaction to explain the acquisition of this magnetic fabric this model infers that the magnetic fabric represents in both degree and direction a measure of the preferred crystallographic orientation of the paramagnetic phyllosilicate minerals the magnetic fabric data suggest that the kmax direction corresponds with the quartz grain long axis preferred orientation and therefore may be used as a palaeocurrent indicator as in titanomagnetite bearing sediments evidence is presented to suggest that paramagnetic susceptibility anisotropy may be common in other mudrocks

palaeosecular variation recorded by 9 ka to 2 5 ma old lavas from martinique island new evidence for the la palma aborted reversal 617 ka ago

on the possibility of determining rates of removal of uranium from crystalline igneous rocks using u series disequilibria 1 a u leach model and its applicability to whole rock data the removal of u from granitic rocks is characterised by234u 238u 1 and 230th 234u 1 it is notable from weathered eye dashwa lakes canada granite samples and from the published data that230th 234u ratios seldom exceed a limiting value of about 2 a u leach model is discussed in detail which may account for these observations the model assumes that 230th is practically immobile and that the removal of u can then be measured against the decay rate of the unsupported230th a property of the model is that the isotope ratios reach constant values reflecting a kind of secular equilibrium and this implies that u is seldom removed faster than the decay of230th if the limited230th 234u ratios from the data correspond to secular equilibrium ratios then this allows simple calculation of u leach rates from any given subsurface sample a test for the model using235u 231pa ratios is suggested a comparison of leach rates derivable from u th pb systematics of colorado and wyoming granite samples with the corresponding estimates from the model shows that the latter are high by a factor of 10 50 data from rapidly weathered surface rinds suggests that the presence of labile u restricts the application of the model to subsurface low weathered zones

full vector geomagnetic field records from the east eifel germany abstract to create meaningful models of the geomagnetic field high quality directional and intensity input data are needed however while it is fairly straightforward to obtain directional data intensity data are much scarcer especially for periods before the holocene here we present data from twelve flows age range 200 to 470 ka in the east eifel volcanic field germany these sites had been previously studied and are resampled to further test the recently proposed multi method palaeointensity approach samples are first subjected to classic palaeomagnetic and rock magnetic analyses to optimise the subsequent palaeointensity experiments four different palaeointensity methods izzi thellier the multispecimen method calibrated pseudo thellier and microwave thellier are being used in the present study the latter should be considered as supportive because only one or two specimens per site could be processed palaeointensities obtained for ten sites pass our selection criteria two sites are successful with a single approach four sites with two approaches three more sites work with three approaches and one site with all four approaches site averaged intensity values typically range between 30 and 35 t no typically low palaeointensity values are found in line with paleodirectional results which are compatible with regular palaeosecular variation of the earth s magnetic field results from different methods are remarkably consistent and generally agree well with the values previously reported they appear to be below the average for the brunhes chron there are no indications for relatively higher palaeointensities for units younger than 300 ka however our young sites could be close in age and therefore may not represent the average intensity of the paleofield three of our sites are even considered coeval encouragingly these do yield the same palaeointensity within uncertainty bounds

eocene spheroidal dolomite from the western sirte basin libya scanning electron micrographs of crypto crystalline 4 m dolomites found in cores taken from the lower eocene facha member reservoir of the zella and aswad fields in libya show that they consist of tiny globular bodies mixed with anhedral crystals producing a chalky texture the sedimentary sequence indicates deposition in upper intertidal supratidal arid environments and in saline lagoons the crypto crystalline dolomite is facies controlled and probably formed as a primary precipitate similar to those reported from present day subaqueous environments in the coorong of south australia and holocene supratidal crusts off florida such chalky textured dolomites could form local fluid migration barriers within hydrocarbon reservoirs

a revised sea level curve for the early jurassic a comparison is undertaken of three different methods of eustatic sea level analysis 1 shallowing and deepening transgressive events inferred from the facies sequence 2 changes through time in the areal spread of epicontinental seas 3 seismic stratigraphy the results of these methods are compared mainly with reference to the lower jurassic new data are brought forward from several continents in a stage by stage study and it is concluded that thelias was characterized by interrupted sea level rise with no major regressions both the transgressions and regressions were relatively rapid with the major transgressions associated with widespread anoxic bottom waters estimates of the net amount of sea level rise through early jurassic time range from 75 m to 110 m

geomagnetic field strength during late miocene first paleointensity results from baja california 1 thellier paleointensity experiments were carried out on 73 selected samples from the late miocene baja california volcanic flows for the first time the selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves compatible with ti poor titanomagnetites judging from the ratios of hysteresis parameters it seems that the samples fall in the small pseudosingle domain grain size region thirty three samples from 10 individual flows yielded acceptable paleointensity estimates the natural remanent magnetization nrm fractions used for paleointensity determination range from 37 to 81 and the quality factor varies between 4 5 and 24 5 being normally 5 the mean paleointensity values per flow obtained in this study range from 18 1 3 1 to 45 6 4 3 t and the corresponding virtual dipole moments vdms range from 3 8 0 6 to 8 8 0 8 1022 a m2 these values correspond to mean value of 6 6 1 7 1022 a m2 which is 85 of the present geomagnetic axial dipole the average vdm value for late miocene according to all available selected data is 6 1 1 5 1022 a m2 which is higher than average paleofield for the period 5 160 ma 4 2 2 3 1022 a m2 after juarez and tauxe 2000 given the large dispersion and the very poor distribution of reliable absolute intensity data it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field more high quality determinations are strongly needed

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

paleointensity measurements of pyroclastic flow deposits co born with widespread tephras in kyushu island japan we report the results of paleointensity determination with welded pyroclastic flow deposits co born with widespread tephras derived from kikai k ah 7 3 ka aira at 28 ka and aso aso 1 270 ka aso 2 140 ka aso 3 120 ka aso 4 90 ka calderas in kyushu island japan since the widespread tephras are commonly used as good key layers their ages and oxygen isotope stages have been well established thermal demagnetization and several rock magnetic experiments showed that the pyroclastic flow samples had stable remanent magnetizations and are suitable for paleointensity measurements using thelliers method we obtained reliable paleointensities from six eruptive units and calculated corresponding virtual dipole moment vdm values ranging from 2 7 to 9 7 10 22 am 2 vdms for the youngest units k ah and at are consistent with the global trend of increasing dipole moment for the past 40 kyr aso 4 3 and 1 yield moderate vdms similar to the mean paleointensity for the past 1 myr and especially the results from aso 4 and 3 show good agreement with the vdm variation between 70 and 130 ka obtained from la reunion island j geophys res 96 1991 1981 earth planet sci lett 140 1996 173 a low vdm value and an abnormal vgp position obtained from aso 2 suggest a possibility that aso 2 records an excursion like event occurred around 140 ka between blake and jamaica events 2002 elsevier science b v all rights reserved

instantaneous sedimentation early microbial sediment strengthening and a lengthy record of chemical diagenesis preserved in lower jurassic ammonitiferous concretions from dorset ammonites from different localities and horizons within the lower lias of dorset record a complex diagenetic history preserved in carbonate cements within their body chambers the studied ammonites are commonly preserved uncrushed in thinly bedded 0 001 m peloidal sediments associated with small scale upward fining couplets the body chambers of the studied ammonites are partially infilled by uncompacted peloidal material with the remaining porosity both here and in the other chambers being occluded by a complex succession of carbonate cements these comprise early microdolomite pyrite inclusion rich radiaxial mn rich calcite cements which are enclosed by later inclusion free sparry cements the assemblages of ammonites found are life assemblages often preserving the remains of jaw structures and many rest oblique to the plane of sedimentation one likely explanation for these observations is that the ammonite assemblages were rapidly engulfed by a fine sediment drape resulting from event resuspension processes alternative explanations such as mass mortality due to bottom water anoxia events are more difficult to sustain following burial the package of sediment enclosing the ammonites rapidly became anoxic and early radiaxial calcite precipitated in response to microbial organic matter mineralisation mn fe and sulphate reduction reactions this early cement prevented compaction following precipitation of these early fringing cements cavities were progressively filled with zoned inclusion poor sparry carbonate cements these represent deeper slower equilibrium crystallization wherein pore water solute access was severely limited by grain boundary diffusion processes finally remaining porosity was infilled by a distinctive ferroan calcite the iron most probably sourced in deep thermally induced redox reactions

pu 137cs and excess 210pb in russian arctic sediments the activity ratios of pu and radiocesium isotopes have been used to delineate the major sources such as global and close in debris fallout nuclear fuel reprocessing and fabrication plant effluents in the environment we have measured 238pu 239 240pu 137cs and excess 210pb concentrations in 107 surficial sediments as well as in 5 sediment cores collected in the summer months of 1993 and 1994 from the ob and yenisey rivers russia and the kara sea a comparison of the sediment core inventories of 239 240pu and 137cs along with the 238pu 239 240pu activity ratios with those expected from global fallout allows us to estimate the relative amounts if any of reactor derived 238pu and 239 240pu from the dumped reactor sites in the study area in surficial sediment samples collected in 1993 and 1994 the 239 240pu concentrations varied between 4 2 and 856 mbq kg 1 with a mean of 239 mbq kg 1 in samples with a measurable 238pu the 238pu 239 240pu activity ratios varied between 0 010 and 0 069 with an average value of 0 035 0 014 this range can be compared to the average 238pu 239 240pu activity ratio of 0 030 for the year 1993 from nuclear weapons testing and snap fallout obtained from soil studies indicating very little 5 additional sources of 238pu to the sediments in the study area the inventories of pu in the 5 sediment cores from the study area varied between 2 67 0 67 and 24 5 2 2 bq m 2 with a mean value of 8 83 bq m 2 the 137cs concentrations in the upper 3 cm of the sediments varied between below detection limit to 71 4 bq kg 1 with a mean of 14 9 bq kg 1 the 137cs inventories in the 5 sediment cores varied between 156 7 28 3 and 1600 153 3 bq m 2 with a mean value of 583 3 bq m 2 the mean ratio of inventories of pu to that of 137cs 0 015 is comparable to the values in other places in the arctic region there is a significant correlation between total organic carbon and concentrations of 137cs 239 240pu and 210pb suggesting that organic matter enriched fine particles are a likely carrier phase for these nuclides the ratio of 137cs 239 240pu also suggests that most of these nuclides are derived from global fallout

absolute paleointensity between 60 and 400 ka from the kohala mountain hawaii abstract magnetic experiments including thermal demagnetization of the natural remanent magnetization nrm mineralogical studies and paleointensity measurements have been conducted on ten lava flows with ages between 60 ka and 400 ka from the kohala mountain hawaii of the samples 62 were subjected to double heating paleointensity experiments under vacuum in total 35 of the specimens did not exhibit significant magnetomineralogical changes during heating and met all the criteria for successful determinations of absolute paleointensity a technique of corrections 1 was attempted for samples that exhibited changes in their ability to acquire partial thermoremanent magnetization ptrm during heating but did not show acquisition of chemical remanence crm this procedure doubled the rate of success with consistent results between the uncorrected and the corrected data from within the same flows the successful paleointensity estimates obtained for 8 lava flows are found to be in good agreement with previous absolute paleointensities obtained from other areas the results are also consistent with the synthetic curve sint 200 of relative paleointensity obtained for the past 200 000 years from deep sea sediment cores 2 there is thus no reason to infer the presence of large non dipole fields in the vicinity of hawaii overall the geomagnetic intensity appears to have the same variability for at least the past 200 ka

geochronology of sediments in the sabine neches estuary texas u s a the accumulation rate of sediments in the sabine neches estuary near beaumont texas was studied in four sediment cores using radioisotopes 210pb and 239 240pu due to very low and variable activities of unsupported 210pb in the sediments the sedimentation rates could not be reliably established using the 210pb dating method however 239 240pu profiles showed distinct peaks corresponding to the maximum fallout in 1963 and using these pu peaks the average sedimentation rates were estimated at 4 5 mm yr 1 in the upper and lower estuary various assumptions that are commonly used in the 210pb dating method were tested it was found that the 226ra activities were in equilibrium with 238u and the possible loss of 222rn was judged to be insignificant the lack of an exponential profile of excess 210pb with depth did not appear to be a direct result of sediment mixing as the post depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas a significant positive correlation between the amount of 63 m fraction sediments and the total 210pb activity was observed in the surface sediments while the hydraulic residence time was relativelt short in this estuary 10 days the average total residence time of 210pb was 10 days between 50 and 94 of the total 210pb in the water column was found in the 0 5 m fraction resulting in low kd values the measured inventories of excess 210pb and 239 240pu in estuarine sediments were only 10 34 and 19 50 respectively of the total expected inventories it appears that the estuarine retention and sediment inventory of these radionuclides and by analogy other particle reactive metals are a function of the hydraulic residence time grain size and possibly complexation of these nuclides with doc

geomagnetic field intensity at hawaii for the last 420 kyr from the hawaii scientific drilling project core big island hawaii four hundred twenty five new paleointensity thellier thellier determinations out of 545 analyzed samples have been obtained from core hsdp which penetrates about 1000 meters 208 flows of the mauna loa and mauna kea volcanic series encompassing the last 420 kyr rock magnetic investigations identify pseudo single domain magnetite as the main magnetic mineral inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude the inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events the paleointensity record from the mauna loa sequence is not very detailed and does not allow precise comparison with other data in the 0 50 kyr interval the record from the mauna kea sequence on the contrary is very detailed and documents relatively short lived episodes of low and high field strength from 15 to 60 t the average virtual dipole moment 8 7 3 0 10 22 a m 2 is not significantly different from the value reported by kono and tanaka 1995 for the last 2 5 myr a comparison with other data from hawaii and other geographical regions is described in detail there are no drastic changes in paleointensity with the inclination anomaly in agreement with previous results from hawaii but in contrast with most published results which however consider data from polarity transition spectral analysis of a particularly detailed portion of the record between 420 and 326 kyr documents significant periodicities at 36 8 5 and 4 ka in the inclination record but not in the intensity record suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity

absolute paleointensities between 60 and 160 kyear bp from mount etna sicily abstract paleointensity measurements were conducted on 17 distinct lava flows from mount etna sicily in order to obtain records of absolute paleointensities for the period covering the last 60 160 kyear a complementary aspect of this study was to improve and extend the calibration of relative paleointensities obtained for the same interval from sedimentary sequences based on absolute values of virtual axial dipole moments seventy three samples have been subjected to double heating thellier coe experiments under vacuum and care was taken to perform additional studies dealing with the magnetic mineralogy and to scrutinize the effects of heating upon magnetic minerals potassium argon k ar datings provided precise ages in most cases absolute paleointensities were obtained with success for ten lavas flows the succession of the virtual axial geomagnetic dipoles vadm oscillate between extrema centered around a mean value of 6 3 2 6 10 22 a m 2 this result is in good agreement with the sedimentary record published for the same period and confirms that there has been an appreciable decrease in geomagnetic field intensity between 80 and 150 kyear

environmental controls on clay mineralogy of an early jurassic mudrock blue lias formation southern england we present a dataset including clay mineralogy gamma ray spectrometry organic matter content and magnetic susceptibility of the hettangian to lowest sinemurian successions of dorset and somerset southern uk blue lias formation bristol channel and wessex basins in both areas the clay assemblages comprise predominantly detrital illite kaolinite and illite smectite mixed layers clays probably originated from the erosion of the hercynian massifs the relative proportions of kaolinite and illite being modulated by arid humid climatic fluctuations the organic matter om content types ii to iv ranges up to 12 in both areas a clear stratigraphical trend in clay mineral assemblages is apparent in somerset whereas in dorset sharp contrasts between adjacent horizons and a greater dilution by carbonate mask the long term evolution correlations between both areas based on similar vertical trends in clay mineral abundance support the suggestion of a hiatus within the angulata zone of the dorset succession as expected the kaolinite illite ratio correlates with the th k ratio deduced from gamma ray spectrometry however significant departures from the correlation occur in om rich intervals suggesting that th may be partly adsorbed on to om particles surprisingly high magnetic susceptibility correlates with abundant kaolinite not with fe rich clays indicating either that kaolinite is accompanied by a soil inherited magnetisable phase possibly iron oxide or that illite rich rocks are more strongly diluted by carbonate than are kaolinite rich strata

absolute paleointensities recorded during the brunhes chron at la guadeloupe island abstract ten absolute paleointensity determinations within the brunhes chron have been obtained on andesitic lava flows from la guadeloupe island french west indies f w i the thellier and thellier method performed on 124 specimens under either ambient or argon atmosphere allows reliable determinations from high temperature steps a satisfactory within flow reproducibility has been observed and at least two samples for each flow have been used for calculation of the weighted mean paleointensity the average virtual axial dipole moment vadm 7 1 1 8 1022 a m2 for normal polarity flows from the brunhes chron recorded at la guadeloupe is compatible with the historical field the 10 vadm values obtained show a relatively good agreement with the deep sea composite record sint800 of relative paleointensity focusing on the 100 75 ka interval a 20 ka oscillation observed from a compilation of available volcanic data and present in high resolution sedimentary records has been tentatively related to the axial dipole moment variations finally a paleointensity decrease of a factor of 10 is observed for flows emitted during the matuyama brunhes m b transition as already observed in other records of this reversal because a large area of the globe around the caribbean islands was previously devoid of data the present dataset fills a gap towards construction of a global paleointensity database for the brunhes chron

combined sea level and climate controls on limestone formation hiatuses and ammonite preservation in the blue lias formation south britain uppermost triassic lower jurassic

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

the intensity of the time averaged geomagnetic field the last 5 myr the existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort despite the huge amount of work put into obtaining these data there remains a strong bias in the age and global distribution of the data toward very young results from a few locations also few of the data meet strict criteria for reliability and most are of unknown quality in order to improve the age and spatial distribution of the paleointensity database we have carried out paleointensity experiments on submarine basaltic glasses from a number of dsdp sites of particular interest are the sites that provide paleointensity data spanning the time period 0 3 5 ma a time of relatively few high quality published data points our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria and the combined data set yields an average dipole moment of 5 49 2 36u10 22 am 2 this average value is comparable to the average paleofield for the period 5 160 ma 4 2 2 3u10 22 am 2 t juarez l tauxe j s gee and t pick 1998 nature 394 878 881 and is substantially less than the value of approximately 8u10 22 am 2 often quoted for the last 5 myr e g mcfadden

regular terrestrially derived palynofacies cycles in irregular marine sedimentary cycles lower lias dorset uk the lower lias of dorset england comprises prominent limestone marl shale alternations considered to be obliquity orbitally controlled probably enhanced by diagenesis palynofacies analysis the study of microscopic particulate organic matter in a sediment identifies six superimposed palaeo environmental variations through 12 m of this sequence 1 90 cm cycles in abundance of terrestrially derived organic particles 2 45 cm cycles in abrasion of wood particles 3 1 2 1 8 m cycles in shape of wood particles 4 irregular cycles 20 cm to 5 2 m in limestone deposition 5 irregular alternations 50 cm to 2 5 m in shale marl deposition and 6 irregular variation in marine phytoplankton abundance these are interpreted as a obliquity forcing of marine currents bottom water oxygenation and organic preservation b precessional forcing of rainfall position of storm wave base and limestone deposition c eccentricity driven shallow marine energy changes preventing limestone deposition d irregular marine processes affecting shale marl deposition and marine plankton productivity thus three orbital cycles acted simultaneously with different effects on different environments palynofacies analysis is a sensitive tool for extracting palaeoenvironmental information particularly terrestrial variations recorded in marine sedimentary rocks in addition it is demonstrated that regular proxy climatic orbital cycles are preserved within irregular sedimentary sequences

a late quaternary full vector geomagnetic record from el golfo section el hierro canary islands regional full vector palaeosecular variation psv curves are essential to further our understanding of geodynamo operation such curves typically lack palaeointensity information with palaeodirectional being much more common particularly for periods before the holocene notably low palaeointensity values have been reported for significant portions of the late quaternary here we present full vector palaeomagnetic from the el golfo section on el hierro canary islands spain spanning the age range between 120 and 450 ka to substantiate these reported low palaeointensity values for the canary islands we sampled 28 flows mostly alkaline oceanic basalts of the shield building stage along the camino de jinama a few kilometres to the south of a section studied by szeremeta et al for psv in line with their results we observe an easterly declination deviation of 14 for the middle and lower part of our section we relate this to undersampling of the complete psv spectrum rotation of el hierro since 500 ka is deemed unlikely attempts to date the flows with the 40ar 39ar method on a multicollector instrument were unsuccessful due to the presence of copious amounts of methane in the extracted gases stratigraphic correlation to szeremeta et al s section lends support to ages technically of very low quality obtained for the sites reported on here importantly we add palaeointensity information in the present study for the first time on el hierro three different palaeointensity protocols were utilized izzi thellier multispecimen and pseudo thellier to provide an additional consistency check and to increase the success rate after applying strict selection criteria reliable palaeointensities could be obtained for 18 flows a success rate of 64 per cent if more than one protocol yielded results for the same flow the obtained intensities were often within error of each other testifying to their robustness flows with a common true mean direction tend to produce similar intensity values as well this testifies to the presumed under sampling of psv within the section obtained palaeointensity values typically range between 20 and 35 t distinctly lower than the present day field value of 39 t and lower than the average intensity during the brunhes chron the determinations are in line with other rather low values obtained elsewhere for this particular time span

210pb derived chronology in sediment cores evidencing the anthropogenic occupation history at corumbatai river basin brazil activity profiles of excess 210pb combined with chemical data determined in two sediment cores from corumbatai river basin sao paulo state brazil provided new insights into the reconstruction of historical inputs of anthropogenic constituents contributing to improving management strategies of the hydrological resources in the basin since most of the municipalities extensively utilize the waters of corumbatai river and tributaries for drinking purposes among other uses excellent significant relationships between loss on ignition loi and organic matter were found for sediments of both analyzed profiles silica was found to be inversely related to organic matter at both analyzed profiles its decrease accompanied an increase in the specific surface of the sediments this relationship was confirmed by a great number of inverse significant correlations among silica and oxides na2o k2o cao mgo al2o3 p2o5 fe2o3 mno and tio2 it was possible to identify the role of organic matter on adsorption of several oxides elements in the core sediments profiles apparent sediment mass accumulation rates corresponding to 224 and 802 mg cm 2 year 1 were obtained and are compatible with field evidence indicating a higher value associated with sand mining activities interfering with the natural normal sedimentation process due to modifications of the channel drainage

combined paleointensity and 40ar 39ar age spectrum data from volcanic rocks of the west eifel field germany evidence for an early brunhes geomagnetic excursion results of virtual geomagnetic pole position paleointensity and isotopic age determinations have been derived from lavas of the quaternary west eifel volcanic field germany the characteristic remanent magnetization carm directions and corresponding positions of virtual geomagnetic poles vgp have mainly been determined by bohnel et al they are exclusively of normal polarity the vgps form a strongly non fisherian distribution characterized by about 30 pole positions with latitudes below 60 n which are mainly confined to the longitude sector from 20 w to 120 e eight independent poles are clustered at 40 n and 45 e geomagnetic paleointensity values from 37 volcanic units of the west eifel have been determined paleointensities range from 7 to 62 t with an unexpectedly large number of paleointensity values below 30 t about half of the low paleointensity values are associated with intermediate vgp latitudes isotopic age determinations from west eifel volcanic rocks have been published fuhrmann and lippolt additional isotopic ages are presented here the west eifel volcanic field is usually devided into an older part in the nw and a younger one in the se based on petrological models and geological evidence all available isotopic ages of the west eifel volcanic rocks which are exclusively from the northwestern part of the field lie in the interval between 460 and 660 kyr for seven of the clustered low latitude vgps it was possible to determine paleointensities and 40ar 39ar ages within their error limits six poles have identical virtual dipole moments vdm at 3 5 1022a m2 furthermore the individual isotopic age intervals of the 40ar 39ar data overlap the assigned age of 510 30 kyr is considered to be the age of a geomagnetic excursion for higher latitude vgp positions 45 n vdm values range from 2 to 10 1022a m2 the mean vdm value about 7 5 1022a m2 of the younger volcanoes ages estimated to be below 100 kyr is not remarkably different from the present dipole moment of the earth in contrast the older volcanoes with higher latitude vgp positions yielded a mean value of only 5 3 1022a m2 and a bimodal frequency distribution of the vdm values isotopic ages determined for two of these volcanic units are close to the inferred excursion age suggesting a decrease of the geomagnetic dipole moment during a time interval which surpassed the duration of the excursion

mixing and cycling of uranium thorium and 210pb in puget sound sediments abstract activity profiles of excess 234 th excess 210 pb 232 th 230 th 234 u and 238 u and 228 232 th ratios determined in eight box cores of sediment from six sites in central puget sound provide new insights into the dynamic nature of solid phase mixing in surface sediments the exchange of 228 ra and other soluble species across the sediment water interface and the cycling of u th and 210 pb in this coastal zone comparison of excess 234 th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central puget sound water column surface sediment horizons with excess 234 th have no excess 228 th which might be used to ascertain sediment accumulation rates over the past decade instead deficiencies of 228 th due to loss of soluble 228 ra from pore water to the overlying water persist to 20 30 cm revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of puget sound solid phase u isotope concentrations tend to increase by up to a factor of two with depth in sediments as a result of dissolved u being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing 232 th and 230 th activities and 230 232 th ratios are constant with depth in sediments indicating constant detrital phase compositions and essentially no authigenic 230 th steady state 210 pb depositional activities in and fluxes to puget sound sediments average only about onehalf those for sediments of the open washington coast north of the columbia river mouth primarily because of a much lower supply of dissolved 210 pb in sea waters adverting into puget sound excess 234 th profiles in sediments reveal much more detail about the depth dependency dynamic nature and recent history of solid phase mixing processes than excess 210 pb profiles at least six of eight 234 th profiles show that mixing within the 210 pb defined surface mixed layer is depth dependent in three profiles 234 th derived mixing rates are fastest several centimeters below the sediment water interface indicating greater macro benthic activity at these depths depth dependent mixing coefficients derived from the best fit of a four layer advection diffusion decay model to the 234 th data are consistent with 210 pb profiles determined for the same sediments strongly suggesting that 234 th and 210 pb are mixed equivalently and in a multilayered manner

primary versus diagenetic origin of blue lias rhythms dorset uk evidence from sulphur geochemistry the question of a primary versus diagenetic origin for the limestone shale rhythms of the blue lias has been addressed through a study of pyrite abundance and isotopic composition pyrite is relatively abundant and isotopically light in the central portions of the bioturbated limestones as compared to adjacent less calcareous sediment the abundance of pyrite shows that the limestones were a focus for prolonged sulphate reduction and pyrite formation the isotopic data indicate that bioturbation oxidized some pyrite to produce isotopically light sulphate part of which was subsequently reduced back to pyrite before preservation by burial acidity generated by pyrite oxidation was buffered in the limestones by carbonate dissolution hence supersaturation of sulphides could be maintained by contrast in adjacent less calcareous sediments carbonate dissolution was unable to buffer acidity and bioturbational oxidation of pyrite formed iron rich pore solutions continued sulphate reduction in the limestones acted as a sink for iron from the adjacent sediments and with burial below the zone of bioturbation the alkalinity so generated caused cementation of the limestones diagenetic cementation would be enhanced during an hiatus in sedimentation an event which might be related to a milankovitch forcing mechanism but which would not be recorded in bioturbated less calcareous sediment thus leaving an imperfect record only cyclicity in pre diagenetic sedimentation patterns may be safely related to a milankovitch forcing mechanism as proposed by weedon

210pb derived chronology and the fluxes of 210pb and 137cs isotopes into continental shelf sediments east chukchi sea alaskan arctic abstract eleven sediment cores and eight sediment trap samples from the continental shelf of the eastern chukchi sea were analyzed for 210pb cs and 2226ra in selected samples the organic carbon content and stable carbon isotope ratios were also determined the excess 210pb 210pbxs concentrations in the 1 cm core tops are generally lower than those of surficial sediments of subarctic shelves and arctic lakes the sediment inventory derived depositional fluxes of 210ph vary between 0 09 and 0 38 dpm cm 2y 1 with a mean of 0 18 dpm cm 2 y 1 the apparent mass sediment accumulation rates which vary between 31 and 324 mg cm 2 y 1 with a mean value of 133 mg cm 2 y 1 are about an order of magnitude lower than the yearly particle flux obtained from the sediment trap experiments this difference between the relatively long term and short term yearly accumulation rates is likely due to the occasional resuspension of fine particles in the continental shelf areas and their advective transport to sites other than their initial deposition the sediment inventory of 137cs varied between 0 64 and 4 09 dpm cm 2 with a mean of 2 18 dpm cm 2 a comparison of this inventory with the expected fallout inventory of 137cs suggests that there had been no detectable input of 137cs to the study area from the former ussr nuclear dump sites

the base of the jurassic system new data on the first appearance of psiloceras planorbis in southwest britain recent collecting in the lower lias of localities at st audrie s bay north somerset sedbury cliff gloucester and lavernock point south glamorgan has yielded psiloceras planorbis j de c sowerby in beds considered by some to be of rhaetian triassic age these are the earliest records of this ammonite to date in southwest britain evidence is provided of a major facies change at the top of the pre planorbis beds followed by the diachronous first occurrence of psiloceras planorbis these new data have implications for the use of earliest occurrence of psiloceras planorbis as a chronostratigraphic marker in defining the base of the jurassic system and for the definition of the base of the hettangian stage in the recently proposed candidate global stratotype section at st audrie s bay

methane seeps on an early jurassic dysoxic seafloor abstract the rhythmically bedded limestone marl shale succession of the blue lias formation lias group early jurassic age of kilve in somerset sw england preserves a suite of large conical concretions that formed around methane seeps these are 1 2 m high and elliptical in plan axes 2 4 m with an outer limestone shell forming the flanks of the cone the cone flank is composed of micritic carbonate 20 30 cm thick which locally includes sheets and pods of intraclasts and bioclasts the cycle forming limestone beds of the host strata are composed of dark grey micrite with carbon isotope values 13c 0 6 to 0 8 consistent with carbon sourced from a mixture of seawater and by sulphate reduction and oxygen isotope values 18o 6 suggesting some degree of later diagenesis the pale grey micrite that forms the sides of the mounds includes three dimensional ammonites and intraclasts and thus cemented close to the sediment water interface prior to compaction the mound forming carbonate is markedly isotopically light with respect to carbon but not with respect to oxygen 13c 24 3 to 26 4 18o 2 to 3 5 the isotope signature indicates that cements were probably derived from a mixture of sources that included biogenic methane the intraclasts within the limestone suggest that syn depositional physical brecciation and mixing of cements had occurred and thus mixing of methane rich fluids with the overlying surface waters is likely also to have occurred the relatively heavy oxygen isotope values may be indicative of anaerobic oxidation of methane the mound bearing interval of the blue lias formation is benthos poor and comprises predominantly laminated black shales characteristic of poor bottom water oxygenation the largest of the mounds is however capped with fossiliferous breccias thus the mounds either formed benthic islands that elevated the biota into an oxic zone or alternatively they may have supported a chemotrophic community although cold seep deposits have been documented previously they are still comparatively rare this example is one of the oldest in europe and is unusual amongst described ancient seeps in preserving relief that extended above the ancient seafloor

surface coatings on ancient coccoliths coccoliths are rings of calcite plates often of intricate design produced by unicellular algae1 fig 1 still widespread in modern oceans these algae must have been extremely abundant in the cretaceous period and their skeletal remains form the main component of natural chalk both modern and ancient coccoliths are well preserved even in the corrosive environments of seawater and sediments and the surface properties of natural chalk are said to be different from those of synthetic calcite it has been suggested that this may be the result of a surface membrane the present study establishes the existence of a coating on ancient coccoliths and discusses its nature

geomagnetic paleointensities derived from volcanic rocks of the quaternary east eifel volcanic field germany geomagnetic paleointensities have been determined for 18 rock units of basanitic thephritic or nephelinitic composition from the quaternary east eifel volcanic field germany in general the rocks are well suited for paleointensity determination and gave consistent results of paleointensity for each rock unit at least five individual values have been measured with the thellier method compared with the present day field of 48 t the unweighted mean paleointensity values range from 29 to 62 t with relative standard errors mostly below 10 paleointensity values are concentrated at 30 and 50 t with a gap at 40 t the distribution is bimodal rather than gaussian although all paleopoles lie close to the geographic pole this cannot be explained by the very discontinuous distribution of volcanic activity in time which took place mainly around 200 and 400 ka in each age group both low and high paleointensity values are found however a further temporal separation within each age group cannot be excluded furthermore the paleointensity values of the west eifel volcanic field show a similar bimodal distribution which indicates that this case seems to be connected with a geomagnetic excursion recorded in those rocks therefore it might be considered that the east eifel rocks recorded a characteristic feature of the earth s magnetic field namely a quasi cyclic variation of paleointensity on the other hand the now available global data set of 128 virtual dipole moment values from the brunhes chron and older than 30 ka shows an almost gaussian distribution with a mean value of 7 4 3 4 1022 am2 if 14 values with transitional pole positions are excluded a quasi cyclic variation of the dipole moment therefore does not seem to be a persistent global characteristic of the earth s magnetic field

the effect on chalk recrystallization after treatment with oxidizing agents abstract chalk consists of the fossilized remains of the hard parts from some species of unicellular algae these algae use polysaccharides to control the morphology of their shields called coccoliths which are made of from 20 to 60 individual sub micrometer size calcite crystals this biogenic calcite has resisted recrystallization during the more than 65 million years of diagenesis in this study we examined the effect of oxidizing agents on the ability of the chalk to recrystallize samples from a north sea drill core that had never been exposed to oil and rordal chalk from the alborg portland quarry north jutland denmark were treated with oxidizing agents to remove organic material and the recrystallization rates were determined we treated the chalk with solutions of naocl and h2o2 with and without 0 1 m nacl and monitored recrystallization in caco3 saturated 0 1 m nacl solutions using h14co3 as a tracer x ray photoelectron spectroscopy xps showed that initially the north sea chalk had a higher surface concentration of organic carbon than the alborg sample but it decreased significantly after the oxidizing treatment loss of c o correlated with higher recrystallization rates rate increase was highest 50 after h2o2 treatment our results demonstrate that through geological time miniscule amounts of organic material remain associated with the chalk surface and act as an inhibitor for chalk recrystallization this implies that the polysaccharides originally made by the algae to control coccolith morphology have contributed to hindering chalk diagenesis to limestone keeping the biogenic particles much smaller even after 65 million years than inorganic calcite grown in the lab for a few days