a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

cosmogenic nuclide chronology of millennial scale glacial advances during o isotope stage 2 in patagonia the terrestrial glacial record reflects past snowline variability and atmospheric temperature changes when combined with secure chronologies these data can be used to test models of ice age climate we present new in situ cosmogenic 1 0 be 2 6 al and 3 he exposure ages supported by limiting 4 0 ar 3 9 ar and 1 4 c ages for seven of the youngest moraines east of lago buenos aires argentina 46 5 s that were deposited by a large outlet glacier of the patagonian ice cap following a major glaciation that deposited extensive moraines prior to 109 ka paired 1 0 be 2 6 al ages indicate that the next youngest complex of moraines was deposited from 23 0 1 2 to 15 6 1 1 ka 1 during the last glaciation ice was at its maximum extent prior to 22 ka and at least five moraines were deposited in less than 10 k y these data are in good agreement with three 1 4 c ages of ca 16 ka from varved sediment banked on top of the youngest of these five moraines and limiting 3 he ages which range from ca 33 to 19 ka the most extensive ice marginal deposits preserved within the last 109 k y were formed during marine oxygen isotope stage 2 no moraines dating to stage 4 were found for stage 2 the distribution of ages at lago buenos aires is similar to cosmogenic nuclide based glacial chronologies from western north america in fact the structure of the last mid latitude south american ice age specifically the overall timing a maximum ice extent prior to 22 ka and deglaciation after 16 ka is indistinguishable from that of the last major glaciation in the northern hemisphere despite a maximum in southern hemisphere insolation during this period the similar mid latitude glacial history in both hemispheres implies that a global climate forcing mechanism such as atmospheric cooling as opposed to oceanic redistribution of heat synchronizes the ice age climate on orbital time scales

origin and significance of calcium carbonate in soils of southwestern patagonia abstract a soil chronosequence spanning the last 0 76 1 0 ma shows a progressive increase in calcium carbonates based on 1 thickness of the bk bck horizons 2 maximum carbonate stage 3 profile accumulation of pedogenic carbonates and 4 mass flux of ca from a geochemical mass balance we favor a dust origin of the carbonates because 1 the till parent materials contain low amounts of caco3 2 there is minimal evidence for weathering of ca bearing minerals in the soil 3 the amount of caco3 in the profile is far in excess of what could be released by weathering 4 dust collected from the study area is enriched in caco3 and 5 a source area for the carbonates was identified although there likely were changes in dust deposition rates during the quaternary our calculations suggest that the current deposition rate of 0 18 g caco3 m 2 yr 1 is comparable to the long term profile accumulation rate of 0 16 g caco3 m 2 yr 1 230th u disequilibrium ages of carbonate pendants are consistent with cosmogenic surface exposure ages indicating that this method has potential for dating other geomorphic surfaces in patagonia cryogenic processes have affected many of the pedons particularly during the last glacial maximum

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

sedimentation at manop site h eastern equatorial pacific over the past 400 000 years climatically induced redox variations and their effects on transition metal cycling gravity cores recovered from manganese nodule project site h 6 33 n 92 49 w show marked downcore variations in the abundance of calcium carbonate organic carbon opal manganese and other components deposited over the past 400 000 years variations in the downcore abundance of organic carbon which ranges from 0 2 to 1 0 can be used to hindcast redox conditions in the surface sediments over this time the results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles downcore variations in solid phase mn ni and cu can be produced by such changes in redox conditions a model which predicts that solid phase mn can be trapped and buried when the mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and mn records and supports the reconstructed redox variations the history of redox variations at site h can be explained by changes with time in surface water productivity major productivity variations at the site occur over 100 kyr cycles with relatively higher productivity occurring during glacial stages thus quaternary climate changes influence surface water productivity redox conditions in sediments and the cycling of transition metals

periodicities of caco3cycles in the pliocene of sicily discrepancies with the quasi periods of the earth s orbital cycles a high resolution caco3 record in combination with a detailed magnetostratigraphy is presented for the rhythmically bedded marly sequence of the pliocene trubi formation on sicily italy magnetostratigraphic age calibration and subsequent spectral analysis of the variatrions in caco3 content yield a remarkably consistent discrepancy between the resulting periodicities and the corresponding quasi periods of the earth s orbital cycles this discrepancy can only be explained by assuming that ages assigned to polarity reversal boundaries in the geomagnetic polarity time scale lack sufficient precision a new polarity time scale is presented for the major part of the gilbert and gauss chron using the radiometrically dated gilbert gauss chron boundary at 3 40 ma as an age reference point and extrapolating an average quasi period of 21 7 kyr of the precession cycle as the periodicity of small scale colour annex caco3 cycles

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

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

early diagenetic reactions in interbedded pelagic and turbiditic sediments in the nares abyssal plain western north atlantic consequences for the composition of sediment and interstitial water abstract measured pore water concentrations of iron in interbedded pelagic and turbiditic sediments from the nares abyssal plain are in excellent agreement with sediment colour and measured redox potential the organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid phase concentration of constituents that are involved in early diagenetic reactions in the turbiditic sediments the concentration of no 3 generally goes to zero within a sediment depth of 1 m whereas at 8 m in a pelagic core from the same area the concentration of no 3 is still higher than it is in the bottom water the pore water concentration of mn 2 in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to mn ca co 3 the pore water of the turbiditic sediments is also saturated with respect to calcite the few diagenetic spikes in the pore water concentration of no 3 and mn 2 and the concentration depth profile of dissolved iron h 4 sio 4 and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments the simultaneous occurrence of peaks of dissolved iron silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals linked with these reactions is the irregular pore water concentration of phosphate which is shown to originate partly from the oxidation of organic matter but mainly from the desorption of phosphate from iron oxide potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations due to the unique combination of low porosity and relatively high sedimentation rates the sediments from the nares abyssal plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits

a re evaluation of the stratigraphy and evolution of the kohistan arc sequence pakistan himalaya implications for magmatic and tectonic arc building processes new field mapping and structural data combined with published geochemical data from the kohistan arc in the nw himalaya enable a re evaluation of the arc stratigraphy key lithological units and their relationships are more clearly defined permitting the construction of a revised magmatic tectonic history for the arc the oldest units are transitional oceanic type basalts which form the basement to the subduction related sequence arc type gabbroic sheets and plutons intrude the oceanic basalts together these form the kamila amphibolite belt metasediments and basaltic lavas were deposited within an extensional basin onto the kamila amphibolite belt basement this sequence exposed across the arc forms a distinct stratigraphic unit which is formally defined here as the jaglot group sediment charged turbidity currents transported material into the basin whilst submarine eruptions contributed the basaltic component this period of extension culminated in the eruption of high mg boninites of the chalt volcanic group which overlie the rocks of the jaglot group the earliest granitoids of the kohistan batholith predate suturing and intrude the jaglot and chalt sequences at c 100 ma kohistan sutured to asia suturing being accompanied by thickening of the arc with the development of major intra arc shear zones and a penetrative regionally developed steep cleavage at c 85 ma intra arc rifting permitted the emplacement into the arc of the voluminous gabbronorites of the chilas complex which clearly intrudes the kamila amphibolite belt to the south and the jaglot group to the north the chilas complex has been regarded as part of the pre suturing juvenile arc sequence field evidence summarized here show this to be not so heat advection associated with emplacement of the complex caused amphibolite facies regional metamorphism melting of the lower arc and plutonism some of the resultant granitoid plutons were unroofed and eroded during a compressional phase at between 80 and 55 ma before emplacement of further plutons and extrusion of basaltic through to rhyolitic volcanic rocks at between 55 and 40 ma at least three phases of extension and rifting each separated by short lived phases of compression charac terized arc evolution much of the magmatism is controlled by extensional tectonics within the overriding plate of the kind commonly associated with a retreating subduction zone

a pseudo thellier relative palaeointensity record and rock magnetic and geochemical parameters in relation to climate during the last 276 kyr in the azores region summary in the pseudo thellier method for relative palaeointensity determinations tauxe et al 1995 the slope of the nrm intensity left after af demagnetization versus arm intensity gained at the same peak field is used as a palaeointensity measure we tested this method on a marine core from the azores spanning the last 276 kyr we compared the pseudo thellier palaeointensity record with the conventional record obtained earlier by lehman et al 1996 who normalized nrm by sirm the two records show similar features intensity lows with deviating palaeomagnetic directions at 40 45 ka and at 180 190 ka the first interval is associated with the laschamps excursion while the 180 190 ka low represents the iceland basin excursion channell et al 1997 the pseudo thellier method in combination with a jackknife resampling scheme provides error estimates on the palaeointensity spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods particularly 23 kyr for climatic precession this suggests that palaeointensity is still slightly contaminated by climate fuzzy c means cluster analysis of rock magnetic and geochemical parameters yields a seven cluster model of predominantly calcareous clusters and detrital clusters the clusters show a strong correlation with climate for example samples from detrital clusters predominantly appear during rapid warming although both the pseudo thellier palaeointensity m a and fuzzy clusters show climatic influences we have not been able to find an unambiguous connection between the clusters and m a

reductive diagenesis magnetite dissolution greigite growth and paleomagnetic smoothing in marine sediments a new view abstract in many anoxic sedimentary environments the onset of sulfate reduction and pyritization of detrital iron bearing minerals leads to a precipitous decline in magnetic mineral concentration during early diagenesis the usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution detailed rock magnetic and electron microscope analyses of rapidly deposited 7 cm kyr latest pleistocene holocene sediments from the continental margins of oman 22 22 4 n 60 08 0 e and northern california 38 24 8 n 123 58 2 w demonstrate that pyritization during early diagenesis also leads to the progressive down core growth of the ferrimagnetic iron sulfide greigite greigite growth begins with nucleation of large concentrations of superparamagnetic sp nanoparticles at the inferred position of the sulfate methane transition which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of sp particles looping of hysteresis parameters on a day plot records the dissolution of single domain sd titano magnetite and the formation of sp greigite which then slowly and progressively grows through its sd blocking volume and acquires a stable paleomagnetic signal this looping trend is also evident in data from several published records oregon margin korea strait japan sea niger fan argentine margin and the ontong java plateau indicating that these processes may be widespread in reducing environments our observations have profound implications for paleomagnetic records from sulfate reducing environments the paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10 s of kyr and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10 s to 100 s of kyr we therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis

evidence of early holocene glacial advances in southern south america from cosmogenic surface exposure dating cosmogenic nuclide surface exposure dating reveals that glaciers in southern south america 46 s advanced at ca 8 5 and 6 2 ka likely as a result of a northward migration of the southern westerlies that caused an increase in precipitation and or a decrease in temperature at this latitude the older advance precedes the currently accepted initiation of holocene glacial activity in southern south america by 3000 yr both of these advances are temporally synchronous with holocene climate oscillations that occurred in greenland and the rest of the world if there are causal links between these events then rapid climate changes appear to be either externally forced e g solar variability or are rapidly propagated around the globe e g atmospheric processes

geochemical and paleomagnetic evidence for the occurrence of missing sapropels in eastern mediterranean sediments the cyclic occurrence of sapropels appears to be associated with maxima in the 65 n summer insolation target curve studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit geochemical and magnetic signals allow for the detection of such a missing sapropel in older sediments these signals are altered by ongoing diagenetic processes an important parameter to reveal missing sapropels is ba elevated concentrations of ba are always found in visible sapropels and even after oxidation of part of the sapropel the initial ba profile remains visible we use this tool to trace down missing sapropels in kc01b a 37 m kullenberg core from the calabrian ridge eastern mediterranean at least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel ba trace metal distributions and magnetic parameters are similar to those observed around visible sapropels

magnetostratigraphic results from the eastern arctic ocean ams 14c ages and relative palaeointensity data of the mono lake and laschamp geomagnetic reversal excursions summary a new high resolution magnetostratigraphic record from the eastern arctic ocean has yielded further evidence for the existence of the laschamp excursion 37 35 ka the mono lake excursion 27 25 5 ka and possibly another very short excursion 22 ka inferred from steep negative inclinations ages are based on nine ams accelerator mass spectrometry 14c dates oxygen isotope stratigraphy and correlation with odp site 983 estimates of relative palaeointensity variations for the time interval from 80 to 10 ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity the lowest values were obtained for the two excursions and the normal reversed n r and reversed normal r n transitions of the laschamp polarity excursion which itself is characterized by a slight increase of relative palaeointensity during its reversed state the results are in general agreement with palaeointensity studies from other regions indicating that these fluctuations could be global phenomena and that the geomagnetic field of the brunhes chron was very variable in amplitude as well as in geometry the new result is one of the rare records comprising large directional as well as large relative palaeointensity variations

magnetic mineral tracing of sediment provenance in the central bengal fan abstract we present a comprehensive magnetic mineral analysis of surface sediments from 110 sites in the central bengal fan to trace sediment provenance multiple magnetic parameters including magnetic mineralogy concentration and grain size were generated rock magnetic measurements indicate that the dominant magnetic minerals within the studied sediments are detrital magnetite and titanomagnetite which is further confirmed directly by scanning electron microscope sem and transmission electron microscope tem observations on magnetic extracts fuzzy c means clustering analysis of magnetic parameters indicates that the study area can be divided into three clusters considering the position of active valley we further divide cluster 1 into clusters 1a and 1b cluster 1a primarily covers the west side of the active valley with relatively high magnetic concentrations and dominantly coarse grained magnetic minerals which likely correspond to basaltic materials derived from the deccan plateau from the godavari river mouth to the inner bay of bengal magnetic concentration gradually decreases with a fining trend of magnetic grain size clusters 1b 2 and 3 are on the east side of the active valley where magnetic concentration and magnetic grain size decrease with increasing distance to the active valley combined with published data in the indian region and the upper bengal fan we suggest that the main provenances are the himalayan and the deccan plateau whose contributions to sediment distribution are quantitatively assessed our analysis suggests that the position of active valley plays a key role in sediment distribution in the central bengal fan while the effect of ocean currents is negligible

isotopic evolution of the tonga arc during lau basin rifting evidence from the volcaniclastic record new pb sr and nd isotope data from volcaniclastic sediments recovered from the tonga forearc and lau basin permit the isotopic evolution of a section of this arc system next to the modern island of ata to be traced through the backarc basin rifting process from 7 0 ma to the present the new data suggest that the isotopic character of the mantle wedge remains constant and of pacific mid ocean ridge basalt morb character during the early rifting phase the isotopic evidence supports trace element data in showing an increase in the sediment contribution to arc petrogenesis about 2 3 my after the start of lau basin rifting since 0 45 ma the sediment contribution decreased to pre rift values with the initiation of spreading in the adjacent backarc basin where the high sediment influence is not seen in the isotopes the isotopes show a relative increase in the volcaniclastic compared with pelagic sediment involvement during rifting the inferred peak in sediment subduction is probably the result of a decoupling of the two plates owing to roll back of the pacific lithosphere at the time of arc rifting

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

diagenetic magnetic enhancement of sapropels from the eastern mediterranean sea diagenetic dissolution of magnetic minerals has been widely observed in organic rich sediments from many environments organic rich sediments from the eastern mediterranean sea sapropels recovered during leg 160 of the ocean drilling program reveal a surprising catalogue of magnetic properties sapropels from all sites studied across the eastern mediterranean sea are strongly magnetic and the magnetization is directly proportional to the organic carbon content the magnetization of the sapropels is dominated by a low coercivity probably single domain magnetic mineral with an inverse magnetic fabric that exhibits a clear decay in magnetic properties when exposed to air during heating the magnetic particles irreversibly break down between 360 and 400 c the contrast between the magnetic properties of sapropels and surrounding sediments is marked with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments the contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non steady state diagenesis sulphate reducing conditions dominated during sapropel deposition while overlying sediments were deposited under oxic conditions the mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate reducing conditions that existed during times of sapropel formation attempts to identify this mineral have been unsuccessful but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase the influence of diagenesis on the magnetic properties of cyclically deposited eastern mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

sediment provenance in a tropical fluvial and marine context by magnetic fingerprinting of transportable sand fractions the sources and fluxes of sediment to the great barrier reef lagoon from northeastern australian rivers have been the subject of much concern and study with the large catchments of the burdekin and fitzroy rivers thought to be the key sources at present here the utility of newly developed magnetic fingerprinting methods for identifying sediment provenance both onshore and offshore and in association with individual large flood events is investigated within the burdekin catchment sediments are mobilized from different subcatchments by runoff generated by intense localized rainfall events magnetic measurements were made on untreated and acid treated samples of river channel sediments within the burdekin river subcatchments and from the estuarine and inner shelf depocentres of burdekin river sediments the acid treatment removes all discrete magnetic particles and coatings and leaves magnetic inclusions protected within host silicate grains as the basis of the measured magnetic signature of a sample the magnetic properties of the acid treated samples display statistically distinct sediment provenance groupings sand samples from the upper burdekin river appear magnetically distinct from samples from tributaries of the burdekin e g hann creek fanning river and also from nearby coastal rivers including the haughton suspended sand samples from a burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999 comparisons of the terrestrial acid treated sand fractions with the same acid treated sand size fractions from transects taken offshore suggest that the surface sediments in upstart bay and bowling green bay have different sources some of these sources are as yet unidentified but may represent the unsampled lower discharge south western burdekin subcatchments and or along shore drift of sand from the south perhaps even from the fitzroy river over millennial timescales of cyclone pumping the magnetic inclusion method precludes any obfuscation or confounding of sediment source which might arise from hydraulic sorting and or post depositional magnetic diagenesis or authigenesis

pulling apart the mid to late cenozoic magmatic record of the gulf of california is there a comondu arc the composition of the lithosphere can be fundamentally altered by long lived subduction processes such that subduction modified lithosphere can survive for 100 s myrs incorrect petrotectonic interpretations result when spatial temporal compositional trends of and source contributions to magmatism are not properly considered western mexico has had protracted cenozoic magmatism developed mostly in board of active oceanic plate subduction beneath western north america a broad range of igneous compositions from basalt to high silica rhyolite were erupted with intermediate to silicic compositions in particular showing calc alkaline and other typical subduction related geochemical signatures a major oligocene rhyolitic ignimbrite flare up 300 000 km3 switched to a bimodal volcanic phase in the early miocene 100 000 km3 associated with distributed extension and opening of numerous grabens extension became more focussed 18 ma resulting in localised volcanic activity along the future site of the gulf of california this localised volcanism known as the comondu arc was dominantly effusive and andesite dacite in composition past tectonic interpretations of comondu age volcanism may have been incorrect as these regional temporal compositional changes are alternatively interpreted as a result of increased mixing of mantle derived basaltic and crust derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting guadalupe plate

fuzzy c means cluster analysis of early diagenetic effects on natural remanent magnetisation acquisition in a 1 1 myr piston core from the central mediterranean abstract the influence of early diagenesis on the natural remanent magnetisation nrm in sediments from the calabrian ridge central mediterranean is analysed with the help of fuzzy c means fcm cluster analysis and non linear mapping nlm the sediments are variably coloured white beige purplish greenish and grey layers occur with occasionally intercalated sapropels the nrm acquired depends on both depositional conditions and diagenetic processes to describe these fcm was performed with in arm caco3 ba mn and s as variables an eight cluster model was derived with the clusters belonging to two categories one expressing mainly diagenetic processes i e dissolution and precipitation and the other expressing mainly depositional conditions the impact of diagenesis on nrm acquisition is profound and not restricted to the close vicinity of the anoxic sapropelitic layers as a consequence the influence of diagenetic processes on the nrm should be thoroughly assessed when selecting samples e g for the determination of the relative palaeointensity of the geomagnetic field application of multivariate classification techniques appears to be useful because it links rock magnetic parameters to the geochemical environment in the present piston core three short reversed geomagnetic events in the brunhes chron are preserved and indeed occur in clusters expressing no or minor diagenesis the recording of the blake event however has been prevented by later precipitation of magnetite in the corresponding interval

sediment chemistry and magnetic properties in an anomalously reducing core from the eastern mediterranean sea abstract in core kc19c 19 6 m long recovered in the abyssal plain between crete and cyprus in the eastern mediterranean a large number of organic rich layers sapropels occur which correlate to maxima in the insolation curve in contrast to other sites in the eastern mediterranean porewaters contain sulfide below a few meters below seafloor mbsf geochemical analyses were performed on the porewaters ammonium alkalinity sulfate manganese iron bisulfide chloride eh ph and sediments organic carbon barium total sulfur total iron manganese pyrite dithionite extractable iron in addition a series of magnetic parameters and ratios nrm arm irm in arm irm s ratio arm 20mt arm in were determined in the top of the sediments fe and mn hydr oxides occur at the oxic suboxic boundary just above the youngest sapropel si2 these hydr oxides appear to be more easily demagnetized by means of alternating fields than deeper hydr oxides which presumably are of detrital origin the transition from suboxic to anoxic sediments is located at 2 mbsf sulfide is produced possibly during sulfate reduction through anoxic methane oxidation at 17 5 mbsf from 17 5 mbsf sulfide migrates upward titrating reactive fe resulting in pyrite formation in the entire sediment column up to 2 mbsf at this depth the upward sulfide flux has been totally consumed by reaction with solid phase fe and dissolved fe ii diffusing downward from the suboxic zone above from 2 mbsf downward magnetic intensities are significantly reduced indicating reductive dissolution of fe oxide minerals and pyrite formation as a consequence no reliable nrm data can be obtained in the lower half of the core

middle pleistocene magnetostratigraphy and susceptibility stratigraphy data from a carbonate aeolian system mallorca western mediterranean abstract this study shows that successions of pleistocene carbonate aeolian deposits can be placed successfully in a geochronologic framework using magnetostratigraphic and susceptibility stratigraphic analysis supplemented by luminescence dating studies of wave cut platforms and biostratigraphic evidence the investigated aeolian system covers a significant part of southernmost mallorca and is exposed in impressive coastal cliff sections at the study site at els bancals the aeolian system has a maximum thickness of 16 m and is composed of alternating dark red colluvial deposits and greyish red aeolian dune and sand sheet deposits forming seven cyclostratigraphic units each cyclostratigraphic unit represents landscape stabilisation colluviation and soil formation followed by dunefield development when marine carbonate sand was transported far inland by westerly or north westerly winds the aeolian system is located on top of a wave cut marine platform 12 14 m a s l this platform probably formed during a sea level highstand in marine isotope stage mis 11 427 364 ka and renewed marine activity probably later in mis 11 is indicated by the formation of beach deposits two sections at els bancals were sampled for a paleomagnetic study additional samples were taken to detect variations in magnetic susceptibility ms the characteristic remanent magnetisation has been recovered for the most part of the succession in spite of diagenetic overprinting there is evidence for two probably three reversal polarity excursions possible connected to the levantine cr1 and cr0 biwa iii episodes if this correlation is correct the sampled succession represents a time interval in the middle pleistocene between ca 410 and ca 260 ka this age estimate is supported by the ms study and by luminescence dates of 333 70 ka aeolianite from lower part of the succession and 275 23 ka aeolianite from the top of the succession the nature of the succession suggests deposition during alternating warm and moist colluvial deposition soil formation and cold dry and windy conditions dunefield formation the susceptibility signal can be correlated with the insolation signal at 65 n suggesting that environmental variation on mallorca was linked to orbitally forced climate change and it seems that aeolian activity and dunefield formation were linked to glacial or stadial periods

paleomagnetic stratigraphy a correlation method abstract interdisciplinary applications of paleomagnetism have in general been restricted to the construction of chronostratigraphic time frames based on correlation with the magnetopolarity time scale for quaternary deposits beyond the range of the traditional 14c dating method paleomagnetic excursions may eventually prove useful as chronostratigraphic markers excursions are inferred from high amplitude paleomagnetic records of short duration and the reality extent and duration of most claimed excursions are still controversial these problems may be overcome by developing procedures to discriminate genuine records of excursions from distorted paleomagnetic signals one suggested approach is a more extensive application of magnetic fabric analysis in conjunction with systematic laboratory investigations of remanence acquisition processes in different sedimentary environments

long term cosmogenic 3 he production rates from 40 ar 39 ar and k ar dated patagonian lava flows at 47 s exceptionally well preserved basaltic lava flows on the cool arid eastern side of the southern andes near lago buenos aires lba at lat 47 s provide ideal sites for calibration of in situ produced cosmogenic 3he we report new 3he measurements and independent age determinations using unspiked k ar and 40ar 39ar incremental heating measurements from two basalt flows both age techniques give concordant ages with analytical uncertainties of 5 demonstrating their utility for dating surfaces older than 35 ka the limit of 14c dating the weighted mean age of the cerro volcan flow is 109 3 ka and of the rio pinturas flow is 68 3 ka scaled to sea level and high latitude d lal earth planet sci lett 104 1991 424 439 and standard atmospheric pressure j o stone j geophys res 105 2000 23 753 23 759 3he production rates on the cerro volcan flow are 130 4 atoms g 1 a 1 in clinopyroxene and 139 4 atoms g 1 a 1 in olivine 2 consistent with a small compositional dependence of the production rate the 3he production rate in olivine from the younger rio pinturas flow is 129 6 atoms g 1 a 1 which is indistinguishable from that obtained on cerro volcan the 3he production rate is at least 11 higher than previous determinations from low mid latitudes in the northern hemisphere corrections for magnetic field strength variability lower the scaled lba production rates by only 1 and do not account for the discrepancy we infer that the enhanced production rates are caused by lower atmospheric pressure in patagonia during glacial periods the southern westerlies which are associated with the steep circumpolar atmospheric pressure gradient are presently centered on lat 50 s but based on independent marine and terrestrial evidence are thought to have migrated north at least 5 during glacial periods the inferred paleobarometry provides independent evidence for northward shifts in the patterns of southern hemisphere atmospheric circulation

chemical remanent magnetization in synthetic magnetite as a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field its moment becomes blocked and it acquires a chemical remanent magnetization crm despite its importance to paleomagnetism the properties of crm and the controls on its behavior are still poorly understood we have modified techniques developed by taylor et al 1987 to synthesize magnetite at room temperature and atmospheric pressure whereby magnetite is produced by alteration of green rust under reducing conditions the period of synthesis is twenty four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mt the average direction of the crm parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field the intensity is linearly related to the applied field up to approximately 1 mt consistent with existing crm theory developed by analogy to the acquisition of thermal remanent magnetization trm above about 1 mt however the crm intensity approaches saturation at a rate substantially lower than that predicted by the trm analog theory furthermore the ratio of the crm to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field analogy to trm predicts this ratio to be independent of growth field both the principal eigenvector of the anisotropy of isothermal remanence air matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases the above observations suggest that the model of crm acquisition based on analogy to trm acquisition is incomplete we therefore propose a revised model which incorporates the increased alignment of easy axes with growth field finally the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish crm from trm in natural remanence

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

hydrogeochemistry of the shallow dutch groundwater interpretation of the national groundwater quality monitoring network since 1979 the dutch national institute of public health and environmental protection rivm has been developing the dutch groundwater quality monitoring network lmg this network presently consists of about 350 monitoring sites at each site well screens are placed at two depths 10 and 25 m below surface level samples are collected every year and are analyzed for all macrochemical parameters and some trace elements tritium contents were measured in the first sampling round the geochemistry of dutch groundwater is complex due to the different sources seawater surface water and rainwater complicated hydrogeology and human impact on flow systems and pollution structuring or data analysis is required for the interpretation of the large number of hydrogeochemical data from such a monitoring network an exploratory approach is to look within the data set for homogeneous groups each with a typical macro chemistry the selection criteria for the location of the monitoring sites of the lmg are mainly based on soil type and land use and to some extent on the hydrogeological situation however a classification based on the two most reliable criteria soil type and land use does not result in chemically distinguishable homogeneous groups or water types fuzzy c means clustering was successfully used to discern structure and natural groups in the lmg data for 1 year a seven cluster model was adopted the number of clusters was decided heuristically with the aid of nonlinear mapping on the basis of the geographic distribution the hydrogeochemical interpretability and the unimodality of the distribution of the parameters per cluster the consistency of the model is illustrated by the reproducibility of the clusters in different years the clusters are related to geochemical processes natural sources and anthropogenic input and are designated as follows 1 seawater in coastal areas 2 desalinization in organic rich holocene marine and peat deposits 3 surface water for downward seeping river water or surface water near the main rivers 4 carbonate reduction in peat areas or old groundwater in seepage zones 5 carbonate for carbonate dissolving precipitation 6 acid precipitation water in sandy topographic highs and 7 a polluted cluster characterized by agricultural contaminants although the influence of soil type and land use is noticeable in some of the clusters the geochemical controls which characterize the different clusters appear to outweigh their anticipated influence on the hydrogeochemistry on the scale of the national groundwater network consequently the homogeneous groups obtained through the cluster analysis present a better base for further statistical and hydrogeochemical evaluation than e g a stratification of the data based on soil type and land use

tectonic history of a jurassic backarc basin sequence the gran canon formation cedros island mexico based on compositional modes of tuffaceous deposits the jurassic gran canon formation cedros island baja california mexico constitutes an unusually well preserved and exposed example of ancient backarc basin fill petrofacies analysis conducted on tuff aceous sandstone and tuff samples from this formation complement and reinforce prior lithofacies interpretations but with some modification when temporal and spatial trends in petrographic data detrital modes are analyzed and compared to mod els based on data collected from deep sea drilling project and ocean drilling pro gram cores the trends indicate a second heretofore unrecognized phase of backarc rifting basalt lavas interstratified with da citic pyroclastic rocks of the primary vol canic lithofacies previously interpreted to record the eruption of differentiated mag mas at the climax of growth of the gran canon island arc are now as a result of this study considered to be the product of arc extension and rifting our method of modal analysis uniquely combines the quantification of textural at tributes of pyroclastic and epiclastic debris that reflect eruption style and magma com position as well as the effects of reworking and mixing in marine settings this study demonstrates that detailed petrographic analysis is useful in the interpretation of ancient volcaniclastic deposits suspected of having formed in backarc basin settings

pumice rafting and faunal dispersion during 2001 2002 in the southwest pacific record of a dacitic submarine explosive eruption from tonga a new influx of sea rafted pumice reached the eastern coast of australia in october 2002 approximately 1 year after a felsic shallow marine explosive eruption at a previously unknown volcano 0403 091 along the tofua volcanic arc tonga the eruption produced floating pumice rafts that first became stranded in fiji in november 2001 approximately i month after the eruption strandings of sea rafted pumice along shorelines have been the only record of products from this submarine explosive eruption at the remote submerged volcano computed drift trajectories of the sea rafted pumice using numerical models of southwest pacific surface wind fields and ocean currents indicate two cyclonic systems disturbed the drift of pumice to eastern australia as well as the importance of the combined wave and direct wind effect on pumice trajectory pumice became stranded along at least two thirds 2000 km of the coastline of eastern australia being deposited on beaches during a sustained period of fresh onshore winds typical amounts of pumice initially stranded on beaches were 500 4000 individual clasts per in and a minimum volume estimate of pumice that arrived to eastern australia is 1 25 x 10 5 m 3 pumice was beached below maximum tidal storm surge levels and was quickly reworked back into the ocean such that the concentration of beached pumice rapidly dissipated within weeks of the initial stranding and little record of this stranding event now exists most stranded pumice clasts ranged in size from 2 to 5 cm in diameter the largest measured clasts were 10 cm in australia and 20 cm in fiji the pumice has a low phenocryst content 3500 km and period of pumice floatation greater than or equal to1 year confirm the importance of sea rafted pumice as a long distance dispersal mechanism for marine organisms including marine pests and harmful invasive species billions of individual rafting pumice clasts can be generated in a single small volume eruption such as observed here and the geological implications for the transport of sessile taxa over large distances are significant an avenue for future research is to examine whether speciation events and volcanicity are linked the periodic development of globalism for some taxa e g corals gastropods bryozoa may correlate in time and or space with voluminous silicic igneous events capable of producing 10 6 km 3 of silicic pumice rich pyroclastic material and emplaced into ocean basins c 2004 elsevier b v all rights reserved

discharge of hydrothermal fluids through sediment at the escanaba trough gorda ridge odp leg 169 assessing the effects on the rock magnetic signal an integrated rock magnetic and multivariate statistical analysis of a suite of sediment samples recovered from odp sites 1037 and 1038 leg 169 escanaba trough gorda ridge spreading center ne pacific ocean has been carried out to assess the use of the sediment magnetic signal as a tracer of post depositional alteration related to the discharge of hydrothermal fluids through these sediments the uppermost above 40 mbsf section of three separate holes holes 1037 b 1038 i and 1038 g was sampled previous sedimentological and geochemical studies indicated that these holes represent a transect from unaltered to strongly hydrothermally influenced sediments analysis of nrm arm anisotropy of magnetic susceptibility ams hysteresis parameters and low temperature magnetic properties suggest increasing alteration of the magnetic minerals with increasing circulation of hydrothermal circulation of fluids fuzzy c means clustering combined with non linear mapping yields a more elaborate picture because similar alteration groupings at different holes can be compared at hole 1037 b a sedimentary reference site drilled away from the area of high heat flow the rock magnetic signal is a sensitive record of changes in detrital inputs magnetite and some monoclinic pyrrhotite are detected the detrital magnetic signal is still apparent at intermediate hole 1038 i but cluster analysis detects an increasing hydrothermal influence on the magnetomineralogy in the lower part of the hole strong hydrothermal alteration at hole 1038 g results in a complete loss of the detrital diagenetic magnetic signal which is replaced by a much weaker hydrothermal signal composed of monoclinic pyrrhotite and pyrite the alteration of the magnetic signal is not restricted to the uppermost few meters of the sediment column and the magnetic properties of the sediment can be used to trace even minor alterations of marine sediments related to the lateral and vertical advection of hydrothermal fluids which may easily be overlooked by more traditional methods fl 2000 elsevier science b v all rights reserved

relative paleointensity of the earth s magnetic field from marine sedimentary records a global perspective abstract to assess the potential of marine sediments as a source of relative paleointensity information we have undertaken an investigation of selected box cores stored in the scripps core repository we present here new results from two north atlantic box cores magnetic remanence in the sediments is carried by magnetite and is univectorial during alternating field and thermal demagnetization the median destructive field is 25 30 mt normalization by anhysteretic remanent magnetization yields relative paleointensity data that are highly concordant within and between cores age control provided by oxygen isotope data from both cores allows comparison of these relative paleointensity data with contemporaneous archeomagnetic data from czechoslavakia both data sets indicate a peak in paleofield intensity at around 8 10 ka this peak is less prominent in box core data from the western pacific and global averages of archeomagnetic and lava flow data however the peak is evident in data estimating changes in production of radiocarbon which is controlled in part by changes in the dipole moment of the geomagnetic field the excellent agreement between production variations predicted by the changes in dipole moment and those observed suggests that long term variations in atmospheric radiocarbon are dominated by geomagnetic field variations

the polygenetic kahouanne seamounts in the northern lesser antilles island arc evidence for large scale volcanic island subsidence abstract remotely operated vehicle rov explorations of the kahouanne seamounts located 25 km se of montserrat in the northern lesser antilles island arc have discovered the occurrence of honeycomb scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level mbsl these features combined with the flat topped morphology of the seamounts the occurrence of shallow water carbonates rhodoliths benthic foraminifera and the presence of oxidized highly vesicular volcanic fragments suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth the subsidence is likely to have been in response to a combination of 1 graben development southeast of the island of montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the lesser antilles volcanic arc and 2 regional subsidence inferred from studies of nearby carbonate platforms 40ar 39ar dating of basalt andesite lavas from the central seamount indicates active volcanism around 5 4 ma shallow water carbonates recovered from an apparent wave cut terrace at 600 mbsl in the same area were deposited approximately 3 ma based on sr isotope stratigraphy indicating significant subsidence of the complex since that time the total subsidence is in line with displacements in the kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of guadeloupe and martinique implying that intra arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc

the glacial geomorphology and pleistocene history of south america between 38 s and 56 s abstract this paper presents new mapping of the glacial geomorphology of southern south america between latitudes 38 s and 56 s approximately the area covered by the former patagonian ice sheets glacial geomorphological features including glacial lineations moraines meltwater channels trimlines sandur and cirques were mapped from remotely sensed images landsat 7 etm pan sharpened landsat 7 and aster the landform record indicates that the patagonian ice sheets consisted of 66 main outlet glaciers together with numerous local cirque glaciers and independent ice domes in the surrounding mountains in the northern part of the mapped area in the chilean lake district 38 42 s large piedmont glaciers developed on the western side of the andes and the maximum positions of these outlet glaciers are in general marked by arcuate terminal moraines to the east of the andes between 38 s and 42 s outlet glaciers were more restricted in extent and formed alpine style valley glaciers along the eastern flank of the andes south of 45 s a series of large fast flowing outlet glaciers drained the ice sheet the location of these outlet glaciers was topographically controlled and there was limited scope for interactions between individual lobes west of the andes at this latitude there is geomorphological evidence for an independent ice cap close to sea level on the taitao peninsula the age of this ice cap is unclear but it may represent evidence of glacier growth during the antarctic cold reversal and or younger dryas chronozone maximum glacier positions are difficult to determine along much of the western side of the andes south of 42 s because of the limited land there and it is assumed that most of these glaciers had marine termini in the south east of the mapped area in the fuegan andes cordillera darwin the landform record provides evidence of ice sheet initiation by adding published dates for glacier advances from the literature we present maps of pre last glacial maximum lgm glacier extent lgm extent and the positions of other large mapped moraines younger than lgm in age a number of large moraines occur within the known lgm limits the age of these moraines is unknown but since many of them lie well outside the established maximum neoglacial positions the possibility that they reflect a return to glacial climates during the younger dryas chronozone or antarctic cold reversal cannot be discounted

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

insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at odp iodp site 1256 from a cluster analysis of rock magnetic properties we analyze magnetic properties from ocean drilling program odp integrated odp iodp hole 1256d 6 44 1 n 91 56 1 w on the cocos plate in 15 2 ma oceanic crust generated by superfast seafloor spreading the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting fuzzy c means cluster analysis and nonlinear mapping are utilized to study down hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization the coercive force the ratio of the remanent coercive force and coercive force the low field magnetic susceptibility and the curie temperature to evaluate the effects of magmatic and hydrothermal processes on magnetic properties a statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas which differ from two distinct clusters mainly representing the dikes and gabbros extensive alteration can obliterate magnetic property differences between lavas dikes and gabbros the imprint of thermochemical alteration on the iron titanium oxides is only partially related to the porosity of the rocks thus the analysis complements interpretation based on electrofacies analysis all clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and or the presence of intergrowths the upper part of the extrusive sequence the granoblastic dikes and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

formation of the late palaeozoic early mesozoic karakaya complex and related ophiolites in nw turkey by palaeotethyan subduction accretion several pre jurassic tectonic units in nw turkey are crucial to the current debate regarding the timing and direction of subduction of palaeotethys a major ocean that separated eurasia and gondwana in late palaeozoic early mesozoic times the most critical unit is the karakaya complex a deformed low grade assemblage of oceanic origin which comprises a nw verging se dipping stack of tectonostratigraphic units here interpreted as a palaeotethyan accretionary complex the units display lithologies consistent with origins in seamount trench abyssal and rifted carbonate platform settings clastic basin sequences developed on top of the complex other relevant tectonostratigraphic units in the area include ultrabasic rocks of supra subduction zone affinity which tectonically overlie permian carbonate platform units with intervening metamorphic soles and melanges restored structural trends suggest the presence of a southward dipping palaeotethyan subduction zone bordering gondwana related units during the late permian triassic this was probably additional to more important regional northward subduction along the southern margin of eurasia from the eastern mediterranean to the himalayas

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

cosmogenic radionuclide chronology of pre last glacial cycle moraines in the western arthur range southwest tasmania cosmogenic 10be and 26al exposure ages of 12 boulders from two moraine complexes in the western arthur range of southwest tasmania which previously were considered to have been deposited during the last glacial maximum predate the last glacial cycle zero erosion minimum exposure ages range from 95 ka to 232 ka based on weighted mean 10be and 26al ages per boulder for a reasonable choice of erosion rates 10be boulder ages range from 105 to 326 ka respectively although a direct association of moraine construction to a specific marine isotope stage mis glaciation is not definitive erosion corrected exposure ages indicate glacial advances commensurate with mis 6 and 10 although the latter advance may be attributable to mis 8 if the erosion rate correction is an over estimate given the relatively close proximity of the dated moraines to their source cirques this new cosmogenic glacial chronology implies a very limited extent for any younger glacial advances that occurred in the western arthur range during the last glacial cycle post mis 5 including the global lgm

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

paleomagnetic record for the past 80 ka from the mahanadi basin bay of bengal abstract high resolution paleomagnetic investigations were performed on a 50 08 m long sediment core md161 20 from mahanadi basin bay of bengal core yielded reliable paleomagnetic results for top 20 m below seafloor mbsf which spans about 80 ka based on the analysis of rock magnetic data the core is subdivided into five distinct zones zone 1 and zone 2 cover top 20 mbsf and do not show any abrupt change in magnetic mineralogy concentration and grain size zones 3 and 5 show significant reduction in lf arm and sirm due to dissolution of magnetic minerals zone 4 shows moderate values of lf and sirm the low value of arm suggests that magnetic signal is mostly carried by magnetic grains in psd md state the paleomagnetic data for the top 20 mbsf show four prominent geomagnetic excursions at 9 mbsf 13 5 to 15 mbsf 16 3 mbsf and 18 to 18 2 mbsf the age depth relationship is established using stratigraphic correlation between well dated sedimentary core nghp 01 19b and the core md161 20 the ages of the observed excursions correspond to 18 to 20 ka 42 to 49 ka 54 to 57 ka and 69 to 70 ka the excursions at 42 to 49 ka 54 to 57 ka and 67 to 70 ka is similar to the known excursions the laschamp and the split norwegian greenland sea events ngs i and ngs ii the excursion at 18 20 ka is not observed globally and may be related to lithological sedimentological changes occurring during last glacial maxima lgm the virtual geomagnetic path vgp of laschamp excursion traces clockwise loop all excursions identified in present study fall in the periods of relatively low paleointensity

tectonic controls on sedimentation and diagenesis in the tonga trench and forearc southwest pacific sedimentation in the tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent tonga platform the shallowest flattest part of the forearc region the greatest sediment thicknesses accumulate in debris aprons close to the volcanic front collision of seamounts notably the capricorn seamount and the louisville ridge with the forearc radically shortens and steepens the adjacent modern trench slope allowing sediment to be redeposited deep into the trench rotation usually arcward of existing basins on the midslope during collision generates angular unconformities while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the tonga platform collision also reactivated major across strike fault zones on the forearc the zones are subsequently exploited by canyons depositing sediment into the trench collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins measuring approximately 5 km by 15 km in the midslope area this morphology is especially developed at 18 30 s to 20 s implying a 2 3 m y interval for their formation following louisville ridge collision trenchward of these depocenters sedimentation is slow resulting in manganese crust formation and localized mass wasting along fault scarps over longer periods of time 5 m y tectonic erosion reestablishes a wide gently sloping forearc into which canyons incise the shallow tonga platform by headwall erosion and collapse

magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern mediterranean sea summary magnetic properties irm arm xin s ratio at 0 3 t room temperature rt hysteresis and thermomagnetic curves and geochemical data fe s mn al ti organic c were studied in two eastern mediterranean boxcores abc26 and bc19 at a resolution of 3 5 mm the boxcores contain sapropel s1 9 6 kyr bp at a few decimetres below seafloor the magnetic fraction consists predominantly of single domain sd to pseudo singledomain psd magnetite in the entire cores the original input of magnetic grains comes from two sources aeolian dust both cores and volcanic ash from the minoan eruption of santorini core bc19 only non steady state diagenesis has changed the magnetic mineralogy considerably in these alternating organic rich organic poor sediments during deposition of sapropel s1 reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities coarser magnetic grain sizes and partial maghemitization in thermomagnetic curves two types of pyrite can be identified one oxidizes below 450 uc and the other above 450 uc the higher oxidation temperature is predominantly found below the sapropel this may be related to the microtexture of pyrite which is euhedral below sapropels and mainly framboidal within sapropels since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half c 5 cm of the sapropel the oxidized part of the sapropel is enriched in diagenetically formed fe oxides with relatively high coercivity and arm the maximum coercivity is found in a distinct layer between the present day mn and fe redox boundaries at the top of the unoxidized sapropel the freshly precipitated fe oxides in this centimetre thick layer contain a mixture of superparamagnetic sp grains and high coercivity sd magnetite higher in the oxidized zone the freshly precipitated fe oxides have aged into generally slightly lower coercivity sd grains with relatively high arm in addition to the diagenetic formation of fe oxides at the top of the sapropel formation of a ferrimagnetic fe monosulphide may have occurred within the sapropel during later stages of diagenesis which may have enhanced the arm signal in the organic rich interval in particular

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the california continental borderland the rock magnetic properties of marine sediments from the california continental borderland san pedro santa catalina and san nicolas basins have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals previous studies have noted that the natural remanent magnetization in these sediments primarily carried by detrital magnetite decays to 10 or less of its surface value soon after deposition this decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process the shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest grained magnetic material the intensity decreases which are linearly proportional to magnetite concentration decreases indicate that dissolution occurs with rate constants ranging from 0 3 to 1 6 kyr 1 the rate constants sulfide concentrations and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite

seismic stratigraphy of the central tonga ridge abstract seismic stratigraphic analyses of single and multichannel seismic reflection profiles from the central part of the tonga arc system sw pacific are used to interpret its geological evolution on the central tonga ridge a high standing part of the frontal arc as defined by karig 1970 and its flanks between 18 s and 22 s we identify and map six seismic sequences overlying inferred lower middle eocene volcanic acoustic basement available island sections and boreholes along with seismic facies suggest that the upper eocene to upper miocene stratigraphic succession on the central tonga ridge consists of volcaniclastic turbidites and hemipelagic sediments in contrast carbonate deposition predominates from the early pliocene to the present we interpret major hiatuses during the early oligocene early miocene and early pliocene based upon the available seismic data and previous results we infer that the extreme eastern tofua trough a partially filled basin that now separates the central tonga ridge from the tofua ridge the modern volcanic arc may be the site of latest eocene or oligocene extension which may be related to rifting and back arc spreading in the south fiji basin to the west another late miocene or early pliocene phase of rifting deformed strata deposited in the palaeogene basin and led to creation of at least part of the modern tofua trough available evidence suggests that the neogene extension is related to initiation of sea floor spreading in the lau basin widespread carbonate deposition beginning in the early pliocene suggests a decrease of volcanic activity in the vicinity of the central tonga ridge an apparent n s migration of miocene and younger depocentres may be related to sequential uplift of parts of the central tonga ridge in response to progressive southward subduction of the louisville ridge at the tonga trench

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

a paleomagnetic and geochemical record of the upper cochiti reversal and two subsequent precessional cycles from southern sicily italy abstract a detailed paleomagnetic and geochemical study of the upper cochiti n r reversal recorded in marine marls on southern sicily shows two consecutive and very rapid transitions r n and n r that coincide with distinct lithological boundaries a fe migration model is presented in which magnetite is formed under different diagenetic conditions during sulphate reduction originally formed primary magnetite is preserved this primary magnetite has recorded the pre transitional normal polarity after burial migration of ferrous iron occurs and subsequent oxidation produces secondary magnetite which records the post transitional reversed polarity and produces the two apparent transitions in addition low temperature lt and high temperature ht components record the reversal in a slightly different way the ht component being acquired with a delay it is probable that both components reside in magnetite but have a different origin and grain size distribution giving different blocking temperature spectra the actual upper cochiti reversal occurs at a level with an estimated astronomically calibrated age of 4 165 ma

middle pleistocene glaciation in patagonia dated by cosmogenic nuclide measurements on outwash gravels the well preserved glacial record in argentine patagonia offers a 1 ma archive of terrestrial climate extremes in southern south america these glacial deposits remain largely undated beyond the range of radiocarbon dating at ca 40 ka dating old glacial deposits several 105 a by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates here we show that cobbles on outwash terraces can reliably date old glacial deposits in the lago pueyrredon valley 47 5 s argentina favorable environmental conditions e g aridity and strong winds have enabled continuous surface exposure of cobbles and preservation of outwash terraces the data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit 10be concentrations in outwash cobbles reveal a major glacial advance at ca 260 ka concurrent with marine isotope stage 8 mis 8 and dust peaks in antarctic ice cores a 10be concentration depth profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca 0 5 mm ka 1 we compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by 100 ka thus the oldest boulders in this area do not date closely moraine deposition the 10be concentration in moraine cobbles help to constrain moraine degradation rates these data together with constraints from measured 26al 10be ratios suggest that all moraine boulders were likely exhumed after original deposition we determine the local last glacial maximum lgm occurred at 27 25 ka consistent with the maximum lgm in other parts of patagonia

magnetic mineral diagenesis in suboxic sediments at bettis site w n ne pacific ocean the effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from bettis site w n in the ne pacific ocean as part of the oxidative decomposition of organic matter ultrafine grained authigenic magnetites are produced immediately above the iron reduction zone this zone is commonly observed in deep sea sediments as a prominent color change from brown to greenish grey the magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties the magnetites are composed of relatively pure fe3o4 and fall within a restricted size range of 0 06 0 12 m which is nominally single domain some of these finest grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction leaving a relatively coarser population magnetite dissolution appears to continue with depth according to a first order surface area reaction process when at steady state iron reduction is independent of sulfide formation in these sediments because solid sulfur profiles show no increase at depth and most of the sulfur occurs as barite despite a well constrained carbonate stratigraphy comparisons of paleomagnetic directions between nearby cores show little agreement suggesting that the directions are composed of a complex mixture of remanences acquired at different depths subsurface magnetic peaks may represent paleoredox markers preserved by non steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

the blake magnetic polarity episode in cores from the mediterranean sea abstract palaeomagnetic and oxygen isotope studies on samples taken from five cores in the mediterranean area are described directions of the stable remanent magnetisation show the presence of an anomalous reversed zone during the transition between the oxygen isotope substages 5e and 5d this position corresponds to the age of about 117 kyr b p and thus to the blake polarity episode the duration of this episode is estimated to be about 6000 years the study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections the blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records

dissolution of titanomagnetite and sulphidization in sediments from lake kinneret israel summary magnetic extracts obtained from sediment cores recovered from lake kinneret israel were subjected to comprehensive scanning electron microscope and in line energy dispersive x ray spectroscopy analyses a variety of ferrimagnetic iron oxides and iron sulphide particles were identified along with apparently non magnetic iron sulphides the majority of the iron oxides are titanomagnetites that have undergone pervasive dissolution and in many cases were only preserved as skeletal 3 d lattices of ilmenite lamellae often the spaces between the submicron ilmenite lamellae are empty also sponge like dissolution features are observed in some residual titanomagnetite grains another group of titanomagnetite grains is characterized by superficial shrinkage cracks which indicate low temperature maghemitization in some cases these cracks appear to be where dissolution started most titanomagnetites contain impurities of al mg si and mn on the order of 1 per cent chromium magnetites with minor ti mg and al content on the order of a few percent but without mn were also found chromium magnetites mostly occur as octahedra in a well preserved state which indicates that they are less prone to dissolution magnetite spherules of 40 60 m in size which were mostly hollow were only found in material collected from the outer zone of the core section that was in contact with the steel core barrel these spherules were not found in material collected from the centre of the core they are therefore interpreted as contaminants that were produced during fabrication of the steel core barrel iron sulphides are present as pyrite either as larger aggregates of cubic crystals each of up to 20 m in size or as 5 10 m framboidal aggregates consisting mostly about 0 3 m idiomorphic octahedra the cubic pyrite crystals have heterogeneous compositions pure pyrite should be non magnetic but even aggregates of up to 150 m in size were magnetically separated they probably therefore contain a magnetic core of greigite or overgrown titanomagnetite the latter possibility is supported by observations of residual titanomagnetites with individual pyrite crystals residing directly on their surfaces or filling space between ilmenite lamellae irregular aggregates of flaky submicron iron sulphides were also found which are interpreted to be greigite the greigite has a low degree of visible crystallization pyrite and or greigite grains were found inside the aggregates or in agglomerates with organic remnants together visual observations and elemental analyses provide clear evidence of pervasive dissolution of primary titanomagnetite and concurrent sulphidization of sediments from lake kinneret

tephrochronology of the astronomically tuned kc01b deep sea core ionian sea insights into the explosive activity of the central mediterranean area during the last 200 ka abstract we present a major trace and rare earth element characterization of single glass shards from fifteen tephra layers found in the astronomically tuned kc01b deep sea core ionian sea central mediterranean 36 15 25 n 17 34 44 e 3642 m water depth kc01b is considered a reference core for the mediterranean area since it provides a new reliable astronomical tuned timescale for the last 1 112 my lourens 2004 the studied deposits punctuate the marine record in a time span ranging from ca 16 ka to ca 191 ka b p encompassing the climatic zones y x w and v they are related to volcanic activity of the campania plain aeolian islands mt etna and pantelleria island along with major main markers of the central mediterranean area such as y 1 y 5 and x 6 astronomically dated at 16 7 ka 39 1 ka and 110 5 ka respectively the succession contains a number of well preserved deposits which can represent useful inter archive instruments of correlation the campania plain in particular sourced at least nine compositionally homogeneous tephra layers prior to the campanian ignimbrite event the data obtained in this work provide a new detailed analytical reference database for ionian sea tephrochronology and for proximal distal correlation studies they may help to unravel from the marine record the history of poorly known or unknown explosive activity on land since the middle pleistocene along with chemical composition size and dispersal of the products

glacial dynamics in southernmost south america during marine isotope stage 5e to the younger dryas chron a brief review with a focus on cosmogenic nuclide measurements the timing of major glacial changes between marine isotope stage mis 5e and the younger dryas chron yd in southern south america is reviewed focus is placed on studies that use cosmogenic nuclide measurements however other dated records and modelling results are also discussed to provide the broader glacial geological context limits on the state of glacial geologic knowledge are examined and suggestions are made as to how some of these can be addressed in future studies studies show that on the east side of the andes at lago buenos aires 45 s there is evidence indicating that following a mis 6 glaciation ice was less extensive during mis 4 than mis 2 in contrast on the west side of the andes in the chilean lake district area mis 4 ice may have been more extensive than during mis 2 although patagonia experienced an mis 2 glaciation broadly in phase with that in other areas around the globe there are differences in glacial histories between areas on millennial timescales climate reversals are evident in many records between ca 17 and 10 ka the timing of such lateglacial activity which included less extensive advances than during the local last glacial maximum lgm varies somewhat between areas this could be due to the relative influence of middle to high latitude climatic regimes at present chronological uncertainties prevent confident assignment of cosmogenic ages for glacial events at the millennial timescale and defining associated climate variability between different study areas copyright 2008 john wiley sons ltd

central mediterranean mid pleistocene paleoclimatic variability and its association with global climate abstract planktonic foraminiferal assemblages were studied at high resolution in core kc01b from the ionian sea quantitative analysis allowed us to distinguish the main climatic features and associated paleoceanographic changes that occurred between marine isotopic stages mis 13 and 9 500 300 ka mis 12 and mis 10 are characterized by relatively temperate conditions and an oligotrophic oceanographic regime in the early part and by colder conditions and nutrient supply in the sub surface water masses in the upper part during these intervals small but distinct peaks of neogloboquadrina pachyderma sinistral sin are detected at times of extremely negative values of the planktonic foraminifera paleoclimatic curve their co occurrence with similar episodes in the atlantic suggests that the climate in the central mediterranean was associated with north atlantic millennial scale climate instability mis 11 and mis 9 are dominated by surficial warm water taxa the climate optimum is reached in the middle part of each of these stages as denoted by the presence of globigerinoides sacculifer and persists for approximately 20 and 6 ka during mis 11 and mis 9 respectively this warming is not constant but is characterized by three distinct intervals with elevated winter temperatures and or weak winter mixing distribution of globigerina bulloides turborotalita quinqueloba and n pachyderma dextral dex indicates that significant environmental changes occur across the transitions from glacial to interglacial mis 12 mis 11 termination v and mis 10 mis 9 termination iv the studied record documents a close linkage between mediterranean climate evolution and higher and lower latitude climate change throughout mis 13 9