multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

nd and sr isotopic ratios and trace element geochemistry of epidote from the swiss molasse basin as provenance indicators implications for the reconstruction of the exhumation history of the central alps abstract epidote is a common heavy mineral in sandstones its use as a provenance indicator however is limited because it appears in a wide range of lithologies and geological settings in order to discriminate between mantle and crust derived source rocks we measured isotopic and trace element compositions of detrital epidote from sandstones of the swiss molasse basin the compositions of the detrital epidotes were compared to epidotes from penninic metabasites and austroalpine metagranites from the present day exposures and to epidote from ophiolitic and metagranitic pebbles from the molasse basin we found that nd isotopic ratios of epidote allow a good discrimination between different types of source rocks with end values ranging between 6 1 and 8 6 for metabasic lithologies and between 9 7 and 2 8 for metagranitic lithologies sr isotope ratios reveal similar trends but do not allow such a clear discrimination ba u rb nb and th contents of epidote correlate well with the end values yielding the same provenance signal according to isotopic and trace element compositions three types of epidote bearing sandstones can be distinguished one received epidote from lower austroalpine metagranites another from penninic ophiolites and a third type of sandstone received epidote from both sources the provenance of the detrital epidote has important bearing on the exhumation history of the central alps elucidating the transition between the erosion of lower austroalpine felsic lithologies and the onset of erosion of the penninic lower plate according to our data the penninic lower plate became exposed simultaneously in the hinterland of all molasse fans at 21 20 ma at the same time the relief collapsed as reflected by a drastic decrease of sediment accumulation rates in the foreland basin the combination of both events fits well to the geodynamic scenario of large scale lateral extension processes affecting the central alps in post collisional times

initial pb sr nd isotopic heterogeneity in a single allanite epidote crystal implications of reaction history for the dating of minerals with low parent to daughter ratios we analyzed 17 fragments from a zoned allanite epidote crystal ca 2 2 mm 4 0 mm which had formed during different prograde and retrograde stages of ultra high pressure uhp and amphibolite facies metamorphism 240 230 ma sulu belt e china for the isotopic composition of pb nd and sr and contents of pb u and th sr and rb and nd and sm since most fragments had 238u 204pb and 232th 204pb values less than 1 corrections for in situ pb growth are small and uncertainties in the recalculation of the pb isotopic compositions to 240 ma are insignificant the recalculated pb falls on a linear trend in the 206pb 204pb vs 207pb 204pb diagram with the allanite defining the low 206pb 204pb end 17 07 of this trend and the epidote defining its high 206pb 204pb end 17 56 the recalculated data scatter in the 206pb 204pb vs 208pb 204pb diagram which implies that the initial pb isotopic variation reflects the involvement of at least three different pb sources the low 87rb 86sr values account for a change in 87sr 86sr by in situ 87sr growth of less than 0 0007 which implies that the isotopic heterogeneity of 87sr 86sr 0 70601 0 7200 is a primary feature the pb and sr isotope data unequivocally demonstrate that contributions from different precursor minerals result in initial isotopic heterogeneity in the metamorphic reaction product it is likely that such an initial isotopic heterogeneity also exists for nd but it could not be resolved in the present study initially heterogeneous pb and sr isotope compositions imply that age differences between core and rim of large crystals may result in the determination of highly arbitrary geological rates especially for minerals with relatively low parent to daughter ratios

variation of magnetic properties in sediments from lake towuti indonesia and its paleoclimatic significance abstract we investigated the mineral magnetic behavior of sediments from lake towuti located in the eastern sulawesi ophiolite belt indonesia rock magnetic analysis supplemented by x ray diffraction and scanning electron microscopy analysis were performed on sediment core tow10 9b from the north basin of lake towuti to give insights on the environmental and sedimentary processes controlling the magnetic properties of the sediment and its paleoclimatic significance the results show that the core has three distinct zones of varying magnetic properties careful examination demonstrates that these zones correspond to varying levels of iron oxide dissolution and magnetite precipitation that are climatically and environmentally dependent the magnetically strongest zone is characterized by weak iron oxide dissolution and intense magnetite precipitation likely driven by changes in the stratification and or water level of the lake during dry conditions in marine isotope stage 2 mis2 period whereas the two magnetically weaker zones are characterized by signs of dissolution and correspond to relatively wet conditions respectively during marine isotope stage 3 mis3 and the holocene although our data show that major changes in concentration dependent parameters such as magnetic susceptibility and saturation isothermal remanent magnetization sirm in lake towuti sediment correlate with changes in regional rainfall many of the concentration changes are more strongly affected by in situ chemical processes than by changes in erosion and terrestrial sediment supply these findings urge caution in the interpretation of magnetic mineral concentration profiles as indicators of clastic sediment inputs

neodymium isotope disequilibria in subducted sediments and potential consequences for subduction zone recycling neodymium isotope geochemistry is pivotal to understanding magma genesis and crustal recycling processes at convergent plate margins here we present nd isotope compositions of rare earth element ree rich minerals from blueschist to eclogite facies rocks that represent previously subducted continental sediments we find nd isotope heterogeneity in all of our samples apatite preserves diverse detrital isotopic compositions despite metamorphism up to at least 600 c and 3 0 gpa whereas lawsonite appears to be isotopically homogenous epidote and monazite are major ree hosts under eclogite facies conditions in our samples these phases also retain nd isotope heterogeneity even up to ultrahigh pressure conditions of 700 c and 4 0 gpa these findings contest the assumption of nd isotope equilibration during subduction zone metamorphism so caution is needed when using nd isotopes to calculate subducted sediment contributions to arc magmas or for determining rates of crustal recycling through subduction zones

palaeoenvironmental developments in the lake tondano area n sulawesi indonesia since 33 000 yr b p abstract geo morpho logical sedimentological and ecological developments in the lake tondano area since 33 000 yr b p and limnological changes in the moderately sized lake situated at 680 m asl in the northern part of sulawesi indonesia are discussed first the environmental setting of the lake is presented study of the regional distribution of lacustrine sediments and a detailed analysis of their sedimentary facies texture organic matter content suggest major changes in size and depth of the lake and in past sedimentation insight into the long term development of the lake sedimentation processes is highly relevant for sustainable use of the present lake analysis of diatom assemblages provides further detail of the changing aquatic settings of the lake lake levels rise quickly around 33 000 and 12 000 yr b p fall dramatically between 30 000 and 13 000 yr b p and are lowered gradually since approximately 6000 yr b p following early holocene high lake levels drainage of the lake is affected by both volcanic depositional events and regional climatic events palynological analysis is indicative of local palaeoecological settings in the lake area and regional climatic change a distinct late pleistocene phase with lower precipitation and lowered mean temperatures is inferred furthermore progressive deforestation of the tondano uplands is evident as well as diffuse anthropogenic volcanic vegetation disturbance from the early mid holocene onwards information from sedimentary facies diatom assemblages and local palaeoecology pollen are integrated to reconstruct palaeoenvironmental settings and processes in the lake area this record of environmental change as well as the pollen based record of regional vegetation and climate change corroborates other palaeoenvironmental data derived from the few terrestrial sites in the region the data attribute a greater magnitude of temperature and precipitation change in the region than is commonly deduced in studies based on marine faunal and sedimentary records the lake tondano sedimentary record is highly suitable for further analysis aimed at determining the exact timing and amplitudes of environmental change in the se asian equatorial region

neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of ree rich accessory minerals radiogenic isotopes are widely used to investigate crustal evolutionary processes however recent claims of nd and sr isotope disequilibrium during anatexis question the reliability of such information we have conducted an in situ sm nd isotope study of apatite allanite titanite xenotime and monazite in metasedimentary rocks of different metamorphic grade to test nd isotope equilibrium during metamorphism our results show that apatite retains an original probably detrital highly variable nd isotopic signature until at least 500 c before being isotopically homogenised irrespective of textural context within the rock once equilibrated apatite retains its nd isotope signature throughout anatexis in contrast allanite and titanite are equilibrated at temperatures as low as 350 400 c ree rich accessory minerals in high grade rocks 600 c show very similar initial end values at the time of metamorphism we conclude that under these metamorphic conditions nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely intra grain nd isotope zoning of monazite indicates that partial melting was open system involving the injection of externally derived melt into migmatites this process likely to be common in anatectic terranes but not always obvious at hand specimen scale or from bulk rock geochemical data can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith

high resolution analysis of early diagenetic effects on magnetic minerals in post middle holocene continental shelf sediments from the korea strait 1 two studied sediment cores from the korea strait contain mud sequences 14 m and 32 62 m in thickness that were deposited during the last 6 000 years the sediments have uniform lithology and geochemical properties however marked down core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties an expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep sea sediments the studied sediments are divided into four descending intervals based on magnetic property variations interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite and the lowest solid phase sulfur contents interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations which suggest active reductive dissolution of detrital magnetic minerals interval 3 is marked by a progressive loss of hematite with depth and coincides with the minimum magnetite concentration interval 4 has an increasing down core enhancement of authigenic greigite which apparently formed at depths of 3 30 m below the sediment water interface due to arrested pyritization reactions these results indicate delays of thousands of years for acquisition of magnetizations carried by greigite which suggests that studies of geomagnetic field behavior from greigite bearing continental shelf sediments should be conducted with care also virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn depositional environmental processes

authigenic mineralogy depositional environments and evolution of fault bounded lakes of the yunnan plateau south western china the dianchi erhai and fuxian lakes lie in faulted basins in a subtropical humid region of the yunnan plateau china three groups of authigenic minerals have been recognized in their recent sediments carbonate minerals fe bearing minerals and silica minerals the main authigenic minerals are goethite calcite aragonite siderite and quartzine goethite is chemically precipitated from a colloidal suspension calcite is a widespread chemical precipitate that is present in deep parts of the lakes and in shallow areas associated with aquatic macrophytes aragonite is mainly biochemical in origin and commonly associated with shallow benthos siderite forms in reducing environments associated with pore waters with a high pco2 that resulted from microbial degradation of organic matter it forms mainly in deep water environments quartzine which occurs mainly in delta front and prodeltaic sites forms from diatom dissolution and dissolved silica introduced by streams six authigenic mineral associations are recognized each of which can be related to depositional setting within the lake and the stage of lake development the same associations can also be recognized in a 480 m long core recovered from dianchi lake strong reducing environments and migrating pore fluids with high pco2 have led to the early diagenetic alteration of some of the initial authigenic minerals using the mineral associations from the modern lakes the pliocene to recent history of dianchi lake has been interpreted and is in general agreement with palaeoenvironmental reconstructions based upon palaeontological and other evidence

long term infiltration and tracer transport in fractured rocks field observations and model analyses summary this paper presents modeling analyses of long term infiltration and tracer tests in fractured tuffs at yucca mountain nv usa the experiments were conducted on a 20 m thick tuff section in a flyover formed by two exploratory tunnels the infiltration test last for 870 days both measured infiltration and seepage show significant temporal and spatial variations the tracer test used inorganic tracers i br f and organic tracers fluorobenzoic acids released 559 days after the infiltration test started leaching from dry salts from fracture walls was found to have affected tracer breakthroughs the unsaturated flow was evaluated by optimizing 45 parameter values in a three dimensional model which accounts for fracture matrix interaction and heterogeneous hydraulic properties in a column based scheme the field data are valuable asset to evaluate the modeling approaches for fractured rocks and the relative importance of the matrix diffusion process results show that matrix diffusion is an important process for transport and that effective matrix diffusion coefficients at the field scale are larger than those at the laboratory scale for the solutes

climate variations in europe over the past 140 kyr deduced from rock magnetism rapid shifts in climate during the last glacial are now well documented particularly from the oxygen isotope records of the two greenland ice cores grip1 2 and gisp23 in the grip record1 2 these climate events are also seen during the preceding eemian interglacial which may be an analogue for the future climate warmed by the greenhouse effect but these shifts are not found in the eemian section of the gisp2 core3 casting doubt on whether the rapid shifts in the grip oxygen isotope record really do represent a climate signal here we present magnetic susceptibility pollen and organic carbon records from maar lake deposits in the massif central france these data provide an independent record of past climate and we find that they correlate well with the ice core records during the last glacial during the eemian two rapid cooling events seen in our record also correlate with those seen in the grip ice core supporting the idea that rapid climate change did occur in the eemian interglacial and demonstrating that it extended to continental europe

nd isotope re equilibration during high temperature metamorphism across an orogenic belt evidence from monazite and garnet abstract monazite is a powerful u th pb geochronometer commonly used to determine the tempo of orogenic processes as a light rare earth element lree enriched mineral monazite also records key information for understanding the behavior of sm nd isotope systematics during tectonothermal events however sm nd isotope studies of monazite have received far less attention than its use as a u pb geochronometer here we investigate coupled u pb ages and sm nd isotopic compositions of monazite from partially melted metasedimentary rocks in the jiao liao ji orogenic belt jljb north china craton to provide insight into sm nd isotope behavior during high temperature metamorphism and crustal melting we utilize several complementary dating systems in the same sample including garnet lu hf and sm nd zircon u pb and monazite u pb geochronology to gain precise time constraints on the p t evolution of the jljb garnet lu hf isochron dates of 1 97 1 94 ga are interpreted to represent prograde garnet growth consistent with the prograde metamorphic zircon dates of 1 96 1 90 ga garnet sm nd dates agree with retrograde metamorphic zircon dates and all monazite u pb dates define a relatively narrow age range of 1 86 1 83 ga interpreted to represent retrograde cooling the lack of older monazite grains matching zircon or garnet dates suggests the dissolution of pre existing monazites into partial melt monazite in all samples irrespective of occurring as garnet inclusions or in the matrix yields homogenous nd isotope compositions end 5 at 1 85 ga which are consistent with the garnet and bulk rock values thus nd isotope re equilibration between bulk rock and minerals occurs synchronously across the orogenic belt during high t metamorphism 750 c where melting was involved with no evidence of a pre metamorphic isotope signature this suggests mobility in the sm nd isotope system during high grade metamorphism and melting where the nd isotope signature of monazite likely represents the most recent tectonothermal event

mineral magnetic proxies of erosion oxidation cycles in tropical maar lake sediments lake tritrivakely madagascar paleoenvironmental implications abstract mineral magnetic measurements were performed on the upper 16 m of five piston cored sedimentary sequences from lake tritrivakely madagascar ams 14 c dating and correlation of the five magnetic susceptibility records allowed establishment of a composite sedimentary record of the last 46 kyr our data suggest that mineral magnetic changes result from changes in concentration of strongly ferrimagnetic terrigenous minerals and from preservation dissolution cycles of detrital iron oxides primarily high coercivity fe iii phases such as hematite in alternately oxic and anoxic depositional environments high coercivity iron oxide phases are restricted to stratigraphic intervals that are interpreted to represent dry periods with oxic conditions comparison with other paleoenvironmental proxies pollen total organic carbon lithology suggests that the sediments record relatively humid conditions before 22 kyr bp and relatively dry conditions during the last glacial maximum lgm and the early holocene such results contrast with the previous observations of relatively dry glacial conditions and humid mid holocene conditions in the summer rainfall area of east and se africa

provenance of jurassic tethyan sediments in the hp uhp zermatt saas ophiolite western alps rubidium sr and sm nd isotope data and rare earth element ree concentrations of the metasedimentary rocks within the zermatt saas zs ophiolite complex of the western alps are used to investigate element mobility and to determine the provenance of the metasediments in order to place constraints on the precollisional paleogeography of the piemont ligurian portion of the neo tethys ocean present day 8 7 sr 8 6 sr variations for the zs metasediments scatter about an early tertiary alpine metamorphic age whereas local nappes that have been interpreted to reflect african apulian and european basements scatter about a variscan like age this suggests 8 7 sr 8 6 sr isotope systematics were nearly completely homogenized for most of the zs metasediments during early tertiary metamorphism probably because they were relatively wet prior to metamorphism in contrast to the sr isotope data ree data and nd isotope compositions of the zs metasediments overlap those of average upper continental crust average shale and the local nappes and nd model ages of the zs metasediments overlap with those of variscan age rocks these relations suggest that the rees of the zs metasediments were not disturbed during high to ultra high pressure alpine metamorphism based on ree data nd isotope compositions and mixing models the zs metasediments comprise two groups that require distinct source terranes one group group i seems to be mixing of an old crustal component such as the paragneissic basement nappe samples with metasediments similar to the second group group ii the source material for group ii is dominated by homogenization of the variscan like orthogneissic basement nappe samples the provenance of group i samples is interpreted to be local where source nappes must have been proximal to the piemont ligurian basin prior to alpine convergence the similarity in dispersion of nd isotope compositions of the metasediments and likely source terranes suggests that the metasediments reflect deposition in small isolated basins early in the formation of the piemont ligurian ocean

environmental changes since 13 000 cal bp reflected in magnetic and sedimentological properties of sediments from lake holzmaar germany abstract late and postglacial sediments from a german maar lake holzmaar in the west eifel volcanic field were investigated with rockmagnetic and sedimentological methods the concentration of magnetic minerals is very variable and zones exist where magnetic properties are dominated by paramagnetic minerals remanence carrying ferromagnetic minerals are dominated by a low coercivity fraction identified by its curie temperature and by scanning electron microscopy as titanomagnetite with a grain size range between a few microns and 25 m an additional high coercivity phase probably hematite is present throughout the entire record in low concentrations rockmagnetic results indicate the possible postdepositional dissolution of titano magnetite and the authigenic growth of greigite in one confined section a positive correlation between initial magnetic susceptibility and sediment accumulation rate and between and the non arboreal pollen percentage is observed while and organic carbon content show a negative correlation these coeval variations are due to a nearly exclusively detrital sedimentary input the variations show a broad dependence on climatic conditions for times prior to significant human activities in the region and the impact of the human settlement history afterwards early holocene climatic variations indicating two cooling events agree with ice rafted debris events of the north atlantic

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

magnetism of the huguangyan maar lake sediments southeast china and its paleoenvironmental implications abstract the huguangyan hgy lake a typical maar lake in the southeast of china is sensitive to the evolution of the asian monsoon system including the east asian summer monsoon easm and the indian summer monsoon ism so far the mechanism of how magnetic properties of the hgy lake sediments respond to climate changes has not been well determined in this study we performed a detailed mineral magnetic analysis on the high resolution hgy sediments results revealed that finer grained magnetite superparamagnetic and single domain magnetite mixed with a small amount of goethite and hematite is the dominant magnetic mineral which is consistent with the magnetic properties of the red weathered soils collected from the catchments but differs greatly from that of the aeolian inputs therefore magnetic minerals of the hgy sediments were originated mainly from the lake catchment rather than the wind blown dust inputs considering the high tephra wall relative elevation of 10 20 m and great precipitation surface runoff by rainfall is interpreted as the main process to transport red soils into the lake the magnetic records of hgy sediments revealed that the summer monsoon was enhanced at the early holocene and then declined abruptly at the mid late holocene 5 5 kyr cal bp this pattern strongly mimics the evolution pattern of ism but is inconsistent with the easm thus we suggest that the magnetic records from hgy have the great potential to investigate the evolution of ism

origin of the sedimentary magnetic record at ocean drilling program sites on the owen ridge western arabian sea the 3 2 m y whole core magnetic susceptibility record obtained during ocean drilling program leg 117 from the owen ridge in the western arabian sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information the salient features of this record are that 1 it correlates strongly with variations in the concentration and flux of eolian dust 2 it is driven strongly at earth orbital periodicities and 3 there is a significant change in the spectral character at 2 4 m y which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas in view of its potential paleoclimatic significance we have examined the origin of the rock magnetic signal from the owen ridge in more detail we find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis we ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process the first occurs at about 1 5 m depth and is characterized by a loss of fine grained ferrimagnetic material the second stepshift in magnetic properties occurs at a depth of 6 8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity magnetite and high coercivity hematite and or goethite magnetic mineral components scanning electron microscope and energy dispersive xray analyses show that the bulk of the coarse grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the fe ti ratio of these grains across the lower diagenetic front this suggests that titanomagnetite grains with higher ti contents may be more resistant to the process of reductive diagenesis and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front

the detection of bacterial magnetite in recent sediments of lake chiemsee southern germany the sediments of lake chiemsee located in the alpine foreland in southern germany host a variety of magnetotactic bacteria mtb which contain intracellular crystals of magnetite arranged in linear chains to detect bacterial magnetite in the carbonate dominated surface sediments and further quantify its contribution to the magnetic signal of the sediments we conducted detailed rock magnetic measurements as well as complimentary non magnetic analyses electron microscopy powder x ray diffraction and sediment pore water analysis our results demonstrate that biogenic single domain magnetite characterized by bullet and truncated hexagonal prismatic shapes is the dominant ferrimagnetic component in the topmost few centimetres of the sediment the changes of magnetic properties with depth are due to the occurrence of live mtb and the downward increasing dissolution of biogenic magnetite moreover the ratios of remanence loss on warming through the verwey transition after field cooling and zero field cooling of saturation isothermal remanence dfc dzfc were determined as 1 47 and 1 25 for freeze dried and air dried sediment samples containing mtb respectively these low ratios suggest that the bacterial magnetite chains were disrupted to a large extent and or that the bacterial magnetosomes might have undergone partial

rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake lago verde los tuxtlas east central mexico magnetic and non magnetic mineral analyses were conducted on a lacustrine sequence from lago verde in the tropical coast along the gulf of mexico that covers the last 2000 years the site witnessed the transformation of the environment since the early olmec societies until forest clearance in the last century through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical america volcanic activity has played a major influence on sediment magnetic properties as a purveyor of ti magnetites ti maghemites and as a factor of instability in the environment anoxic reductive conditions are evident in most of lago verde s sedimentary record direct observations of magnetic minerals and ratios of geochemical fe ti and ferrimagnetic f and paramagnetic p susceptibility data are used as parameters for magnetite dissolution p fe f and precipitation f ti of magnetic minerals intense volcanic activity and anoxia are recorded before a d 20 leading to the formation of framboidal pyrite increased erosion higher evaporation rates lower lake levels anoxia and reductive diagenesis in non sulphidic conditions are inferred for laminated sediments between a d 20 850 this deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the maya civilization dissolution of magnetite a high organic content and framboidal pyrite point to anoxic sulphidic conditions and higher lake levels after a d 850 higher lake levels in lago verde broadly coincide with the increased precipitation documented during the medieval warm period a d 950 1350 in the northern tropical and subtropical regions of the american continent for the little ice age a d 1400 1800 the relatively moist conditions inferred are in concordance with the glacial advances recorded in central mexico higher erosion rates reflect destruction of the rainforest over the last 40 years

chlorine 36 dating of very old groundwater 2 milk river aquifer alberta canada the milk river aquifer in southern alberta canada consists of sandstone interbedded between thick shale units the groundwater is confined and discharges by both upward and downward leakage through the shales the concentration of cl increases by about 2 orders of magnitude downgradient through the aquifer this cl increase complicates 36cl dating of the system climatic changes flushing of connate water introduction of cl from older water below the aquifer and ion filtration have been proposed as possible explanations for the increase the 36cl data show a consistent decrease of the 36cl cl ratio downgradient but an increase in the 36cl concentration downgradient in certain parts of the aquifer dates calculated from the 36cl cl ratio show a reasonable distribution in light of the hydraulic controls on the system whereas dates calculated from the 36cl concentration give negative ages this suggests that ion filtration is responsible for the cl increase and that the 36cl cl dates are to be preferred we estimate water ages to be more than 2 m y near the distal end of the aquifer

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

diagenetic sensitivity of paleoenvironmental proxies a rock magnetic study of australian continental margin sediments a rock magnetic study of the upper 40 m of the ocean drilling program site 133 820a recovered at the outer edge of the northeastern australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level we identified intervals enriched in single domain sd magnetite in the studied section unlike previous studies that postulated a detrital source we show that the bulk of the sd fraction is biologically produced and is likely to be authigenic the abundance of sd magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms the biogenic magnetite was preserved during the high sedimentation rate periods likely due to a short residence time in the corrosive zone of active iron reduction the presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution more advanced dissolution during the periods of slow sedimentation coincident with sea level highstands resulted in significant changes in the composition of the detrital assemblage particularly in relative abundances of different mineral phases the overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite magnetite goethite due to the higher stability of hematite reductive diagenesis in general will lead to the lowering of the goethite hematite g h ratio which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy the sensitivity of the g h proxy to diagenesis should be taken into account in paleoenvironmental studies

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

environmental change in the late pleistocene and later holocene at wanda site soroako south sulawesi indonesia abstract this paper reports the pollen analysis of an 8 m core from a mire at 445 m altitude and 2 s latitude on ultramafic soils near the western shore of lake matano south sulawesi the area is almost undisturbed by humans and the record is believed to cover about 50 75 000 yr b p with some breaks in the record a section from 17 000 to around 5500 yr b p and the last 1000 years of the record may be missing or disturbed the results show that montane forest grew around the site continuously through the late pleistocene with a maximum increase in higher altitude taxa from 35 000 to after 17 000 yr b p the time of glacial maxima elsewhere also in this period increases in grass and casuarina indicate possible drier conditions the holocene sections record lower altitude forest with increased taxonomic diversity including secondary species the results show that vegetation in the region has been sensitive to climatic change the pleistocene ecology being consistent with a temperature change of about 2 5 3 5 c with a phase up to 50 drier than present the site climate was possibly affected by shelf exposure in the gulf of bone due to lower sea levels however the tropical forest demonstrates overall long term stability in which changes in dominance may reflect minor shifts in disturbance and tree longevity a distinctive record of fine charcoal occurs throughout the deposit this shows that the forest has never been continuously fire free for a very long time the cause of the fires may be natural as no evidence for human occupation was discerned

late holocene paleoclimatic changes in western central africa inferred from mineral abundance in dated sediments from lake ossa southwest cameroon abstract minerals derived from catchment soils were determined using ftir spectroscopy in the well dated core ow4 from lake ossa a lowland rainforest area in cameroon this quantification provides a hydrologic record indicating that the magnitude of runoff events and by inference rainfall pattern has varied during the late holocene the comparison between minerogenic inputs and vegetation changes improves the understanding of the inferred climate dynamics since at least 5400 cal yr b p the paleomonsoon rainfall intensity decreased as shown by a general decrease in mineral fluxes this observation is consistent with a gradual weakening of the boreal summer insolation in tropical latitudes however the major vegetational change lags behind the onset of the decrease in mineral fluxes from 2800 to ca 1000 cal yr b p the forest receded the amount of rainforest taxa decreased and is replaced by pioneer trees and poaceae when the mineral fluxes attained their lowest values this episode of maximum dryness is attributed to an abrupt climatic event of global significance which is superimposed onto the paleomonsoon variability it is related to a cold event which in turn produced a change in the lower atmospheric circulation that was characterized by a strengthening of northern trade winds probably correlated with sea surface temperature variations in the eastern tropical atlantic area

holocene environmental changes as recorded by mineral magnetism of sediments from anguli nuur lake southeastern inner mongolia plateau china abstract two cores one 1141 cm long an s and the other 885 cm long an a were retrieved from anguli nuur lake 41 18 24 n 114 20 27 e 1315 masl one of the largest lakes in the transition zone between a semi humid and semi arid climate parallel to the present limit of the southeast monsoon along the southeastern inner mongolia plateau in north china mineral magnetic parameters lf arm irm300mt sirm and irm 300mt were measured on an s and two additional parameters arm and hirm and four inter parametric ratios arm sirm irm300mt sirm irm 300mt sirm and sirm lf were calculated potential sources of these lake sediments catchment soils and dune materials close to the lake and in a distant sand plain were sampled and the magnetic properties of the surface material specimens were measured a chronological model was developed for an s by comparing and combining ams14c dates of an s with 137cs 210pb and ams14c dates of an a with the help of surface material magnetism the magnetic data of an s in combination with particle size toc and c n and pollen analyses indicate the environmental changes during the last 10 000 years around this lake conditions began to ameliorate at 10 900 cal yr bp 9600 14c yr bp and thus relatively wet and warm environments prevailed during 10 900 8900 cal yr bp 9600 8000 14c yr bp the holocene optimum or the wettest and warmest conditions was during 8900 7400 cal yr bp 8000 6500 14c yr bp the environment began to deteriorate from 7400 cal yr bp 6500 14c yr bp and the driest and coolest conditions occurred during 2200 480 cal yr bp there may have been a minor amelioration after 480 cal yr bp the inferred changes in palaeoenvironmental conditions around anguli nuur lake are broadly in agreement with those around most other sites on the inner mongolia plateau

evidence for lake formation as a response to an inferred holocene climatic transition in brazil abstract geochemical mineralogical and palynological records from a holocene sediment core 12 7 m long from lake silvana southeastern brazil served to identify the source of detrital sediments within catchment soils the lake basin was first flooded in the early holocene 9400 14 c yr b p when a gibbsite rich b soil horizon started to accumulate four distinct hydrologic phases are consistent with vegetational changes indicated by pollen data phases of slope instability colluviation and low lake level correspond to pollen free intervals and point to a severe retreat of the vegetation cover the rapid sedimentation of slope wash sediments alternating with alluvial sediments appears to have formed a tributary fan 7 8 m thick that dammed the lake silvana basin 8500 14 c yr b p probably as a response to a drastic increase in precipitation rates

sm nd disequilibrium in high pressure low temperature himalayan and alpine rocks in order to decipher the causes of sm nd isotopic disequilibrium in high pressure low temperature rocks sm nd isotopic analyses were carried out on minerals from four himalayan tso morari unit and four alpine dora maira monte viso sesia lanzo eclogitic rocks of different lithologies and different intensities of post eclogitic metamorphism in most of these

superparamagnetism and reduction diagenesis in pelagic sediments enhancement or depletion superparamagnetic grains ultra fine d 0 05 m particles unable to retain a remanence at room temperature have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks in marine sediments fine grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction on the basis of new magnetic hysteresis low temperature and geochemical studies of pelagic sediments from the western equatorial pacific ocean it is proposed that reduction processes increase rather than deplete the ultra fine magnetic grain population at the modern fe redox boundary enhanced superparamagnetism coincides with a coarsening of remanence carrying grains superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically driven fluctuations in organic carbon corg supply together with hysteresis characteristics changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady state magnetic mineral reduction

sequential growth of early diagenetic freshwater siderites in the holocene coastal deposits korea spherulitic siderite grains ranging from 100 to 250 am in diameter occur abundantly in a sediment layer which is almost 8600 years old in namyang bay on the west coast of korea these large siderite grains consist predominantly of feco3 average 65 and mnco3 average 22 and have low mg ca and mg fe ratios compositional scatter plots together with the high mn and low mg contents indicate that meteoric pore water was mainly involved in the siderite precipitation examination of the siderite grains with an electron microprobe reveals a systematic micron scale center to margin compositional variation within individual grains three zones are recognized on the basis of the compositional variations zone i center in which mn decreases and fe increases zone ii intermediate in which mn increases and fe decreases and zone iii outer in which both mn and fe decrease and mg increases this zoning is interpreted as a result of depth related early diagenetic reactions with different fe mn and mg concentrations in the sediment pore waters meanwhile compositional mapping of spherulitic grains implies that a siderite grain consists of numerous tiny siderite precipitates such dust like tiny siderite precipitates appear to be adhered to the large spherulitic siderite grain in some cases large siderite grains are adjoined together and occurred as a siderite aggregate we suggest that a spherulitic siderite grain is growing sequentially by adhering of numerous tiny siderite precipitates formed in each diagenetic zone onto the siderite grain like the formation of a snowball d 2004 elsevier b v all rights reserved

coupling of anatectic reactions and dissolution of accessory phases and the sr and nd isotope systematics of anatectic melts from a metasedimentary source advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the rb sr and sm nd isotope systems in anatectic melts from a muscovite bearing metasedimentary source modeling results show that 1 the sm nd ratios and nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt and 2 the relative proportion of micas muscovite and biotite and feldspars plagioclase and k feldspar that enter the melt is a key parameter determining the rb sr and 87 sr 86 sr ratios of the melt furthermore these two factors are not in practice independent in general nonmodal partial melting of a pelitic source results in melts following one of two paths in e nd 87 sr 86 sr ratio space a higher temperature fluid absent path path 1 represents those partial melting reactions in which muscovite biotite dehydration and apatite but not monazite dissolution play a significant role the melt will have elevated rb sr 87 sr 86 sr sm nd and e nd values in contrast a lower temperature fluid fluxed path path 2 represents those partial melting reactions in which muscovite biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue the melt will have lower rb sr 87 sr 86 sr sm nd and e nd values than its source the master variables controlling both accessory phase dissolution and hence the sm nd system and melting reaction and hence the rb sr systematics are temperature and water content the complexity in sr nd isotope systematics in metasediment derived melts as suggested in this study will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component

increasing australian indonesian monsoon rainfall linked to early holocene sea level rise the australian indonesian summer monsoon affects rainfall variability across the indo pacific region reconstructions of monsoon strength from stalagmites show that precipitation increased from 11 000 to 7 000 years ago as rising global sea level caused the flooding of the indonesian continental shelf the australian indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical indo pacific region it has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation1 2 3 but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult owing to the lack of high resolution records of past monsoon behaviour here we present a precisely dated reconstruction of monsoon rainfall over the past 12 000 years based on oxygen isotope measurements from two stalagmites collected in southeast indonesia we show that the summer monsoon precipitation increased during the younger dryas cooling event when atlantic meridional overturning circulation was relatively weak4 monsoon precipitation intensified even more rapidly from 11 000 to 7 000 years ago when the indonesian continental shelf was flooded by global sea level rise5 6 7 we suggest that the intensification during the younger dryas cooling was caused by enhanced winter monsoon outflow from asia and a related southward migration of the intertropical convergence zone8 however the early holocene intensification of monsoon precipitation was driven by sea level rise which increased the supply of moisture to the indonesian archipelago

magnetic mineral dissolution in pleistocene fluvio lacustrine sediments nihewan basin north china abstract the nihewan fluvio lacustrine sequence north china has recorded late pliocene pleistocene climatic and environmental changes and contains valuable information on early human evolution in high latitude east asia we carried out a combined mineral magnetic and geochemical investigation on a sequence from the xiantai section eastern nihewan basin results suggest that large amplitude magnetic property variations between so called high magnetic and low magnetic units mainly result from preservation dissolution cycles of detrital magnetic minerals in alternating oxic and anoxic depositional environments in our preservation dissolution model the high and low magnetic units represent glacial and interglacial deposits respectively this contributes to a better understanding of the link between magnetic properties and climate in the xiantai fluvio lacustrine sequence based upon this relationship early humans may have occupied the xiantai and xiaochangliang sites during an interglacial period during both interglacial and glacial periods at the donggutuo site and during a glacial period at the maliang site our work provides a paleoenvironmental context for early human adaptation and occupation in mainland east asia during the early pleistocene

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

sedimentation on the inner shelf of the east china sea magnetic properties diagenesis and paleoclimate implications abstract high resolution magnetic analysis was performed on the 19 22 m core md06 3040 27 43 3663 n 121 46 8822 e 46 m water depth from the east china sea ecs inner continental shelf which was recovered during the images xiv md155 marco polo 2 cruise of the r v marion dufresne ipev magnetic grains are mainly transported to the inner shelf from erosion on mainland china delivered by the yangtze river nine 14 c ams dates indicate that the core spans about the last 10 4 ka with an average sedimentation rate of nearly 2 mm yr the primary magnetic mineral assemblage of the uppermost sediments contains magnetite with some contributions from hematite and greigite two main stepwise changes are observed in the magnetic mineral content due to post depositional reductive diagenesis the first occurs from the top of the core to 3 5 m and is characterized by reduction of both the magnetite and hematite contents the second occurs at 8 45 9 m and is characterized by a loss of fine grained ferrimagnetic material variations in the magnetic mineral content of this core are related to geochemical and redox environment changes in the ecs through the oxic suboxic sulfidic methanic zones from top to bottom despite significant reduction of a large amount of the ferrimagnetic mineral content due to diagenesis intervals characterized by lower magnetic mineral concentrations and higher coercivities are still identified at 0 45 0 6 1 5 1 7 2 9 3 1 4 4 4 7 5 3 5 7 5 9 6 1 and 8 2 8 4 ka these intervals are in phase within dating errors these intervals are in phase with weak summer monsoon events which indicate that a relationship between magnetic results and paleoclimatic events is partially preserved

combining nd isotopes in monazite and hf isotopes in zircon to understand complex open system processes in granitic magmas mapping the age and trace element and sm nd isotope compositions of monazite grains from a peraluminous cretaceous granite using laser ablation split stream analysis reveals a wide range in nd isotope and rare earth element ree compositions within and between single grains these data corroborate isotopic variability indicated by hf isotope analysis of zircon in the same granite sample the ree variations indicate that monazite grew during fractional crystallization hf and nd isotopes indicate that the granitic magma was generated from at least two distinct proterozoic sources of approximately the same age one component that had highly radiogenic initial 176hf 177hf and 143nd 144nd and a second component that was notably less radiogenic this study highlights the utility of in situ ree and sm nd isotope data in monazite in magmatic systems further it refines the zircon based constraints on magmatic processes because of sensitivity of light rees to fractional crystallization lower probability of complications owing to inheritance and smaller analytical volumes required