monsoon related variations in zaire congo sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial atlantic during the last 200 000 years two sediment cores from the zaire fan and the angola margin have been investigated for their composition of terrigenous and biogenic constituents respectively for the late quaternary kaolinite feldspar ratios and variations of terrigenous element ratios of zr ti k rb and al reveal that the composition of zaire river sediment load has fluctuated in tune with precessional variations of boreal summer insolation in particular the correspondence of high kaolinite feldspar and al k ratios with low latitude insolation maxima strongly corroborates the assumption that west african monsoonal precipitation and chemical weathering was enhanced during periods of increased central african heating the most striking feauture observed is that opal accumulation has been 2 to 10 times higher in zaire fan sediments than outside in continental margin sediments off angola although biogenic ba and corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the zaire river plume and off angola from this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the zaire river plume than in the oceanic upwelling regime farther to the south off angola over the last 200 000 years the pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the zaire river to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold more arid glacial climate conditions we presume that paleoproductivity off the zaire was controlled by the following two sources of nutrients 1 marine nutrients including nitrate and phosphate as well as the uptake of ba on particulate corg caused by upwelling and 2 silica mainly delivered by the zaire river dissolved silicate hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical atlantic at a millenial timescale

evaluating zr rb ratio from xrf scanning as an indicator of grain size variations of glaciomarine sediments in the southern ocean the ln zr rb count ratio derived from x ray fluorescence xrf core scanning holds potential as a high resolution tracer for grain size variations of glaciomarine sediments to evaluate this appro

the dole effect over the last two glacial interglacial cycles detailed measurements of 18 o of atmospheric oxygen performed on air trapped in the vostok ice cores antarctica are used to extend the record of the dole effect over two climatic cycles back to 240 kyr b p except for glacial terminations i and ii and for an unexpected minimum occurring around 175 kyr the dole effect shows small variations adole within 0 5 these small variations however show a well marked 23 kyr precessional periodicity thus confirming the results obtained by bender et al 1994a for the first climatic cycle to explain the minimum value reached around 175 kyr we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions

a 300 kyr high resolution aridity record of the north african continent new oxygen isotope data and a high resolution carbonate record of a core from the northwest african margin are presented in combination with geochemical zr rb ratio al ba organic carbon and biogenic opal fluxes and sedimentological median grain size parameters to show variations in african continental aridity wind strength and upwelling productivity the carbonate record is diluted by the supply of aeolian dust from the arid saharan region of north africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines spectral analysis of the carbonate profile and comparison with specmap stacked oxygen isotope and etp solar radiation profiles shows that the precessional 23 kyr insolation induced african climate system is decoupled from global ice volume climate changes operating on a 100 kyr frequency detailed studies of glacial terminations i ii and iii confirm the presence of rapid short timescale climatic variations in aridity

tracing dust sources and transport patterns using sr nd and pb isotopes continental dust transported through the atmosphere can be a tracer of atmospheric circulation and transport as well as being a source of information about climate in the dust s source area this information about the provenance of dust transported from desert areas and deposited elsewhere on the oceans and continents can be evaluated using several observational techniques satellite imagery has been used to this end in recent decades and air mass back trajectories have been calculated to trace dust deposits back to their source area another method consists of analysis of the concentration and composition of certain tracers in the dust itself and comparing them to soils and sediments from possible source areas this method has the advantage of being useable for paleo atmospheric circulation and paleo dust transport studies whereas satellite imagery and air mass back trajectories are restricted to relatively recent times we review here different kinds of isotopic tracers that have been used to trace dust provenance but special attention is focused on the use of sr nd and pb isotopes we evaluate their use in studies of both spatially as well as temporally varying continental dust source and transport

rapid and quantitative major element analysis method for wet fine grained sediments using an xrf microscanner several types of x ray fluorescence xrf core scanners have been developed to fulfill the demands for high resolution rapid chemical analysis of wet sediment cores however their application to quantitative analysis has been hampered by the difficulty of evaluating the xrf absorption effect of water in a water film on the sediment surface and interstitial water within the sediments especially for lighter elements in this study we established a non destructive rapid high resolution and quantitative chemical analysis method for wet fine grained sediment samples using an xrf microscanner results of our experiments suggest that the degree of xrf absorption by the water film which develops on the wet sample surface covered with mylar film shows a clear positive correlation with the water content of the sediments under the measurement area a clear positive correlation also exists between the degree of xrf absorption by the interstitial water and the water content of the sediments under the measurement area we evaluated the xrf absorption by the thin water film using the relationship between the water content and thickness of the thin water film and the xrf absorption by the interstitial water using its relation with the water content the water content under the measurement area is estimated from the average transmitted x ray intensity that is measured on the same measurement area simultaneously with the measurement of xrf intensities the dry sample equivalent xrf intensities of major elements al si k ca ti and fe which are the xrf intensities of wet samples corrected for the xrf absorption by the thin water film and the interstitial water show better correlation with the major element concentrations determined by conventional xrf methods using this new method it is possible to measure the average water content and average concentrations of the 6 major elements simultaneously for a square area as small as 0 5 0 5 mm on the wet fine grained sediment within 6 min which is nearly ten times faster than the conventional xrf method

renyi dimensions of soil pore size distribution abstract the existence of fractal scaling for pore volume size distribution has been empirically shown and several fractal models have been constructed to replicate it deriving also other important physical properties one step forward may be given by using multifractal analysis which achieves a better characterization of a distribution by means of a singularity spectrum whenever suitable scaling properties are present renyi dimensions are a classical tool to characterize complex distributions that are closely related to the multifractal singularity spectrum but having an easier handling and interpretation in this article renyi dimensions of pore size distributions are computed from data obtained by 2 d image analysis of soil samples analyzed samples show suitable scaling properties the renyi dimensions d q appear defined with r 2 greater than 0 95 within a significative range of q s the variation of d q with respect to q and the shape of the renyi spectrum reveal that pore size distributions have properties close to multifractal self similar measures these results show that renyi dimensional analysis is an appropriate tool for characterizing soil pore size distribution and thus may be used as indicator of soil structure as well as for deriving important soil physical properties moreover this characterization opens the possibility of simulating the real distributions by adequate self similar models of fractal geometry as iterated function systems and related ones with the corresponding benefits for prediction purposes

formation of clay minerals and exhumation of lower crustal rocks at atlantis massif mid atlantic ridge 1 low temperature alteration products in gabbros from the ocean floor have significant implications for incipient processes of seawater rock interaction and exhumation tectonics of the lower crustal rocks in this paper we report mode of occurrence and mineralogical characteristics of clay minerals in gabbroic rocks recovered from integrated ocean drilling program iodp hole u1309d at an oceanic core complex atlantis massif mid atlantic ridge at 30 n the clay minerals were identified by optical microscope electron microprobe raman spectrometer and transmission electron microscope as mainly composed of mixed layer saponite talc saponite and vermiculite they are characteristically rich in iron that is significantly oxidized and distributed into the tetrahedral site suggesting a relatively high temperature condition for oxidation they are restricted to domains near the contacts between olivine and talc or form pseudomorphs after olivine near microcracks filled with zeolite or clay minerals these facts suggest the infiltration of oxidative seawater and reactions to variable fluid rock ratios at variable temperatures close association of vermiculite with microcracks radiated from serpentinized olivine suggests that the deep infiltration of seawater at an off axis region was caused by fracturing resulting from serpentinization and enhanced by relatively abundant olivine rich lithology at atlantis massif compared with gabbroic rocks of an oceanic core complex at ultraslow spreading ridge odp hole 735b those of atlantis massif substantially lack mixed layer smectite chlorite mixed layer smectite chlorite is a product of prehnite actinolite to greenschist facies alteration and looks to preserve a record of ambient thermal structure through which the massif passed on rising to a shallow level the absence of pervasive formation of mixed layer smectite chlorite under relatively reducing conditions suggests low permeability and or limited fluid rock reactions on the way to shallow levels from the observation and consideration of the characteristics of clay minerals sequence and distribution of static alteration related to fracturing original lithology and tectonic settings of the oceanic core complexes we conclude that atlantis massif was more rapidly exhumed to the oxidative subseafloor environment than atlantis bank the difference of exhumation rate possibly reflected either the disparity in spreading rate between the whole ridge systems or regional variation of exhumation tectonics between the two oceanic core complexes

atmospheric methane during the last four glacial interglacial cycles rapid changes and their link with antarctic temperature 1 atmospheric methane ch4 recorded in antarctic ice cores represents the closest ice proxy available for greenland temperature changes beyond times when greenland climate records are available the record over four climatic cycles from the vostok ice core offers the opportunity to study the phase relationship between greenland and antarctic climate changes through detailed ch4 profiles combining american and french analytical efforts we have improved the time resolution of the existing ch4 record from petit et al 1999 spectral analyses reveal high and low frequency variability including a strong precessional component the phase relationship between ch4 and the antarctic temperature proxy d shows a systematic lag of ch4 versus temperature by 1100 200 years on long timescales 50 400 kyr and a more complex behavior over shorter timescales i e 25 kyr suggesting that dansgaard oeschger type of climatic variability and associated interhemispheric linkage are robust features of late quaternary climate

the african humid period and the record of marine upwelling from excess 230th in ocean drilling program hole 658c using a high resolution 230 th normalized record of sediment flux we document the deglacial and holocene history of north african aridity and coastal upwelling at ocean drilling program hole 658c at both the end of the younger dryas and after the 8 2 ka event there are significant drops in terrigenous accumulation at our site indicating an increase in the monsoon moisture flux over africa at this time at 5 5 ka there is an abrupt end to the african humid period and a return to stronger upwelling conditions for carbonate and opal fluxes the 230 th normalization completely changes the shape of each record based on percentage variations alone this site is a clear example of how variations in one sediment component can obscure changes in the others and it demonstrates the need for radionuclide measurements more generally in paleoceanography by taking our new records and a large amount of previous data from this site we conclude that increases in african moisture are tightly coupled to decreases in coastal upwelling intensity

submarine metamorphism of gabbros from the mid cayman rise petrographic and mineralogic constraints on hydrothermal processes at slow spreading ridges the coarse grained submersible collected rocks from the mid cayman rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro petrographic and mineralogic studies showed that 1 some deformation is present in all rocks 2 the distribution of deformation is not uniform within a specimen and 3 alteration has taken place preferentially in the deformed zones the abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth the compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied na k and h2o to and removed ca from the rock in all about 15 of the original rock has been transformed to amphibolite in the 700 m vertical section our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur if deformation continues to take place during the spreading of seafloor alteration may also continue to occur along newly formed fractures and cracks

geochemical evidence for airborne dust additions to soils in channel islands national park california there is an increasing awareness that dust plays important roles in climate change biogeochemical cycles nutrient supply to ecosystems and soil formation in channel islands national park california soils are clay rich vertisols or alfi sols and mollisols with vertic properties the soils are overlain by silt rich mantles that contrast sharply with the underlying clay rich horizons silt mantles contain minerals that are rare or absent in the volcanic rocks that dominate these islands immobile trace elements sc th la and ta nd cr and rare earth elements show that the basalt and andesite on the islands have a composition intermediate between upper continental crust and oceanic crust in contrast the silt fractions and to a lesser extent clay fractions of the silt mantle have compositions closer to average upper continental crust and very similar to mojave desert dust island shelves exposed during the last glacial period could have provided a source of eolian sediment for the silt mantles but this is not supported by mineralogical data we hypothesize that a more likely source for the silt rich mantles is airborne dust from mainland california and baja california either from the mojave desert or from the continental shelf during glacial low stands of sea although average winds are from the northwest in coastal california easterly winds occur numerous times of the year when santa ana conditions prevail caused by a high pressure cell centered over the great basin the eolian silt mantles constitute an important medium of plant growth and provide evidence that abundant eolian silt and clay may be delivered to the eastern pacifi c ocean from inland desert sources

quantitative analysis of detrital modes statistically rigorous confidence regions in ternary diagrams and their use in sedimentary petrology abstract detrital modes are the basic type of quantitative data used in sedimentary petrology they represent estimates of the petrographic framework composition of sand or sandstone obtained by point counting of thin sections at present geological inferences from analysis of these numerical data are semi quantitative only because many of the data processing methods employed in sedimentary petrology lack a firm theoretical basis a review of existing methods reveals a number of fundamental statistical problems associated with the use of univariate statistics and the construction of so called hexagonal fields of compositional variation it is shown that these problems can be overcome by using multivariate methods that honour the non negativity and unit sum constraints on compositional data and incorporate the covariance structure spread in compositional data can be modelled on two levels 1 by regarding the grain as unit of observation and the total point count as a sample from a multinomial distribution 2 by regarding the total count as unit of observation and a set of counts as a sample from an additive logistic normal distribution numerical methods are presented for constructing statistically rigorous confidence regions in ternary diagrams by inversion of significance tests for these two distributions statistical analyses of the framework composition and heavy mineral assemblages of modern beach and river sands illustrate their use the case histories are followed by a brief overview of popular multivariate methods for reconstructing sediment provenance

sources of dissolved silica to the fjords of northern patagonia 44 48 s the importance of volcanic ash soil distribution and weathering dissolved silica dsi plays an important biogeochemical role in the fjords of northern chilean patagonia 44 48 s where it drives high biogenic productivity and promotes carbon burial it is generally believed that the dsi riverine input to lakes and coastal environments is controlled by a combination of factors including lithology climate topography vegetation and meltwater input in northern chilean patagonia several authors have proposed that the postglacial volcanic ash soils andosols may play a significant role in the high supply of dsi to the regional fjords to assess the influence of andosols on dsi concentrations in north patagonian rivers we mapped andosol thickness and compared our results with river chemistry the mineralogical and geochemical composition of three representative andosol profiles was also examined to evaluate the efficiency of weathering processes the andosol thickness map clearly demonstrates that volcanic ash was predominantly deposited on the eastern side of the regional volcanoes reflecting the influence of the prevailing westerly winds on the distribution of pyroclastic material mineralogical and geochemical results show that the andosol parent material has the typical andesitic basaltic signature of the regional volcanoes i e high amounts of amorphous material plagioclase k feldspar and pyroxene down profile variations in soil mineralogy and geochemistry indicate increased leaching of silica with depth resulting from weathering of the volcanic parent material for the five studied watersheds a highly positive correlation r2 0 98 was found between average andosol thickness and dsi concentrations suggesting that andosol thickness is the main parameter affecting dsi concentrations in north patagonian river systems on seasonal timescales increased precipitation winter and glacial meltwater summer input can significantly reduce dsi concentrations we argue that the weathering of andosols constitutes the most important source of dsi to the lakes and fjords of northern chilean patagonia explaining the particularly high regional rates of biogenic silica production copyright 2015 john wiley sons ltd

late holocene covariability of the southern westerlies and sea surface temperature in northern chilean patagonia abstract the climate of chilean patagonia is strongly influenced by the southern westerlies which control the amount and latitudinal distribution of precipitation in the southern andes in austral summer the southern westerly wind belt swwb is restricted to the high latitudes it expands northward in winter which results in a strong precipitation seasonality between 35 and 45 s here we present a new precipitation seasonality proxy record from quitralco fjord 46 s where relatively small latitudinal shifts of the swwb result in large changes in precipitation seasonality our 1400 yr record is based on sedimentological and geochemical data obtained on a sediment core collected in front of a small river that drains the patagonian andes which makes this site particularly sensitive to changes in river discharge our results indicate fe al and ti al values that are low between 600 and 1200 ce increasing at 1200 1500 ce and high between 1500 and 1950 ce increasing fe al and ti al values reflect a decrease in mean sediment grain size from 30 to 20 m which is interpreted as a decrease in seasonal floods resulting from an equatorward shift of the swwb our results suggest that compared to present day conditions the swwb was located in a more poleward position before 1200 ce it gradually shifted towards the equator in 1200 1500 ce where it remained in a sustained position until 1950 ce this pattern is consistent with most precipitation records from central and southern chile the comparison of our record with published regional sea surface temperature sst reconstructions for the late holocene shows that equatorward shifts of the swwb are systematically coeval with decreasing ssts and vice versa which resembles fluctuations over glacial interglacial timescales we argue that the synchronicity between sst and swwb changes during the last 1400 years represents the response of the swwb to temperature changes in the southern hemisphere

effects of checkerboard barriers on the distribution of aeolian sandy soil particles and soil organic carbon abstract sand barrier setup is an important measure to protect the topsoil against wind erosion salix psammophila checkerboard barriers are used widely in the mu us sandy land in order to ensure the safety of life and agricultural production to measure variations in soil structure and quality caused by barriers with different duration 7 years and 2 years the composition of aeolian sandy soil particles and the soil organic carbon soc distributions are studied the dunes with no barrier are used as controls ck analyzed by multi fractal parameters the topsoil samples are collected in the field and tested in the laboratory the predominant soil particles are fine sand medium coarse sand and clay and the contents of silt and very fine sand increase with extending barrier duration which shows a wider distribution range greater dispersion degree and larger soil particle heterogeneity the aeolian sandy particles particularly fine granular grains are deposited and accumulate at the bottom and top of dunes because of the barrier blocking effect the particle size distribution psd heterogeneity is the highest in 7 year barrier dunes and the lowest in the no barrier dunes the mean values of d0 capacity dimension of 7 year barrier dunes d1 entropy dimension of the no barrier dunes and d1 d0 of 2 year barrier dunes are maxima in addition the psd range and distribution increased with prolonged dune barrier duration as well as the changes of f shape of the multi fractal spectrum and width of multi fractal spectrum soc content increased with increasing barrier duration and decreasing d1 d0 the average soc contents of 7 year barrier dunes were the maximum and one of no barrier dunes had the minimum soc content and their difference was significant p correlations between the soc content and the psd multi fractal parameters of both 7 year and 2 year barrier dunes are significantly different p

circulation in the glacial north atlantic inferred from grain size measurements records of nutrient proxies in marine sediments indicate that the nutrient distribution and hence circulation of the glacial north atlantic ocean was markedly different from that of today1 2 but these tracers are influenced by several biogeochemical factors unrelated to ocean circulation3 6 and thus do not provide a direct measure of the vigour of circulation distributions of grain size in marine sediments can provide such information owing to the sorting effects of currents7 8 if the characteristics of the input sediment flux are known here we present a record inferred from grain size measurements of variations in the relative strength of deep and intermediate currents in the eastern north atlantic over the past 25 000 years we find that glacial intermediate water flowed rapidly at depths of between 1 100 and 2 000 m in contrast deep water circulation was sluggish during the last glaciation but increased in strength shortly after the glacial maximum this would have resulted in increased heat flux to high latitudes and may have triggered sudden deglaciation deep current strengths declined again at the start of termination stage ia and during the younger dryas perhaps as a result of iceberg discharges the so called heinrich events9 10 a remarkably similar record of grain size variations has been found in the western north atlantic11 indicating that these changes in circulation are ocean wide

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

the sedimentary record of glacial retreat marinelli fjord patagonia regional correlations and climate ties abstract the inland passage of chile comprises the most continuous latitudinal transect of ice fields ice caps and alpine glaciers in the southern hemisphere providing an excellent natural laboratory for the study of glacial response to climate change during the holocene here we report results from a marine geological investigation of marinelli fjord tierra del fuego 54 25 s 69 35 w and ainsworth fjord which merges with marinelli fjord data acquired during july 2005 onboard the rv ib nathaniel b palmer include high resolution seismic data multibeam swath bathymetry data and sediment cores post cruise analyses include detailed seismic facies and lithofacies analysis and radiocarbon dating the results indicate that a major transition in the glacimarine record from ice proximal facies to ice distal facies occurred in the outer basins of the fjord at 15 5 kyr to 12 5 kyr the ice proximal facies consists of pebbly mud with abundant laminations and is characterized by high average sedimentation rates 28 mm yr in contrast an ice distal facies is dominantly bioturbated mud with low average sedimentation rates 0 56 mm yr this transition marks the retreat of marinelli glacier from the outer basins with no evidence for glacial re advance subsequently during the mid to late holocene ainsworth glacier retreated and exposed ainsworth fjord our results are consistent with the climatic trends for southern south america where the final deglaciation of the region ended at 12 5 ka during the holocene stable ice conditions persisted until the mid 1960s when marinelli glacier began rapidly retreating to its modern day position

measurement of the sortable silt current speed proxy using the sedigraph 5100 and coulter multisizer iie precision and accuracy the weight percentage and mean size of the 10 63 m terrigenous silt fraction termed sortable silt of fine grained marine sediments have recently been argued to be proxies for near bottom palaeocurrent intensity this paper details the accuracy and precision of the sedigraph 5100 and the coulter multisizer iie in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep ocean sediments three sortable silt standards and a fine silt clay diluent were generated from natural deep sea sediment of the iceland basin these were used to produce silty clays with standard additions of sortable silt ranging from 1 to 20 of the total by weight accuracy was measured relative to the size of the pure spike measured by each instrument at sortable silt abundance 5 the sedigraph and the coulter multisizer estimate both weight percentage and mean grain size precisely and accurately while below 5 instrumental noise degrades their performance on the basis of the sedigraph results a series of percentage errors is defined which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near bottom current reconstruction it is suggested that ideally palaeocurrent reconstructions should be based on sedigraph determinations of the 0 63 m grain size distribution yielding both mean and abundance or on coulter multisizer determinations of the sortable silt grain size mean the coulter multisizer may prove especially useful in regions of very depleted 5 sortable silt component

compositional heterogeneity of olivine in thermally metamorphosed serpentinite from southwest japan compositionally heterogeneous crystals of olivine occur in thermally metamorphosed serpentinites from southwest japan they have a variation in forsterite fo content up to 10 mol within a specimen the chemical heterogeneity is the result of two causes the intermingling of metamorphic neoblasts with relict crystals of primary olivine and the compositional variation within the neoblasts themselves the metamorphic olivine in each specimen shows a bimodal distribution of fo content with a highly magnesian group fo 93 98 varying among specimens and a relatively ferroan one fo 85 94 textural relationships and the variation of nio and feo contents between the two groups of olivine suggest that the heterogeneity results from the local involvement of magnetite and awaruite in dehydration reactions of serpentine at the initial stages of metamorphism

a comment on titaniq under pressure the effect of pressure and temperature on the solubility of ti in quartz by jay b thomas e bruce watson frank s spear philip t shemella saroj k nayak and antonio lanzirotti trace concentrations of ti in quartz are used to indicate the pressure and temperature conditions of crystallization in the titaniq geothermobarometer of thomas et al contrib miner petrol 160 743 759 2010 it utilises the partitioning of ti into quartz as an indicator of the pressures and or temperatures of crystal growth for a given value of tio2 activity in the system if temperatures are inferred to 20 c pressure is constrained to 1 kbar and vice versa there are significant contrasts however between the conclusions from titaniq and those for natural quartz as well as other mineral phases in volcanic rocks application of the titaniq model to quartz from the 27 ka oruanui and 760 ka bishop high silica rhyolites where the values of t p and tio2 activity are constrained by other means fe ti oxide equilibria melt inclusion entrapment pressures in gas saturated melts melt and amphibole compositions yields inconsistent results if realistic values are given to any two of these three parameters then the value of the third is wholly unrealistic the model yields growth temperatures at or below the granite solidus pressures in the lower crust or upper mantle or tio2 activities inconsistent with the mineralogical and chemical compositions of the magmas cl imagery and measurements of ti and other elements in quartz are of great value in showing the growth histories and changes in conditions experienced by crystals but direct linkages to p t conditions during crystal growth cannot be achieved

an empirical method to predict sediment grain size from inorganic geochemical measurements grain size is one of the most fundamental properties of sediments it is frequently used in paleoclimate paleoceanographic and paleoenvironmental research as a proxy for river discharge current and wind strength and to identify mass flow deposits measuring grain size is however time consuming and destructive given the strong influence of grain size on sediment inorganic geochemistry single element variations measured by for example x ray fluorescence core scanning are increasingly used to estimate grain size variations at high resolution in sediment cores this approach is however limited to a narrow grain size range since individual elements only monotonically relate to grain size over a narrow size range here we present a simple code free multielement method based on partial least squares regression to predict sediment mean grain size from inorganic geochemical data over the range of sizes commonly encountered in sedimentary basins clay to sand the method was first tested using river sediment samples separated in 11 grain size fractions and it was later successfully applied to two sediment cores from the chilean fjords our method only requires measuring grain size on a limited number around 10 of selected training samples and it allows to predict mean grain size at x ray fluorescence core scanner resolution this method has the potential to be applied to any lake or marine sediment core provided sediment provenance weathering and diagenesis remain relatively stable through time and we anticipate that it will result in a significant increase in the resolution of sediment proxy records of climate and environmental change plain language summary sediment grain size is one of the best indicators of past transport conditions here we present a method to predict mean sediment grain size from inorganic geochemical measurements this method can be applied to geochemical measurements obtained by x ray fluorescence core scanning to generate grain size profiles quickly and at high resolution and we anticipate that it will result in much improved paleoclimate and paleoenvironmental reconstructions

a 550 ka record of aeolian activity near north west cape australia inferences from grain size distributions and bulk chemistry of se indian ocean deep sea sediments abstract the terrigenous fraction of sediments from a deep sea sediment core recovered from the northwestern western australian continental slope offshore north west cape se indian ocean reveals a history of western australian climate throughout the last 550 ka end member modelling of a data set of grain size distributions n 438 of the terrigenous sediment fraction allows to interpret the record in terms of aeolian and fluvial sediment deposition both related to palaeo environmental conditions in the north west cape area the data set can be best described by two aeolian end members and one fluvial one changes in the ratio of the two aeolian end members over the fluvial one are interpreted as aridity variations in northwestern western australia these grain size data are compared with bulk geochemical data obtained by xrf scans of the core log ratios of the elements zr fe and ti ca which indicate a terrigenous origin corroborate the grain size data we postulate that the mid to late quaternary near north west cape climate was relatively arid during the glacial and relatively humid during the interglacial stages owing to meridional shifts in the atmospheric circulation system opposite to published palaeo environmental records from the same latitude 20 s offshore chile and offshore namibia the australian aridity record does not show the typical southern hemisphere climate variability of humid glacials and dry interglacials which we interpret to be the result of the relatively large land mass of the australian continent which emphasises a strong monsoonal climatic system

titaniq a titanium in quartz geothermometer titanium is one of many trace elements to substitute for silicon in the mineral quartz here we describe the temperature dependence of that substitution in the form of a new geothermometer to calibrate the titaniq thermometer we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt at temperatures ranging from 600 to 1 000 c at 1 0 gpa the ti contents of quartz in ppm by weight from 13 experiments increase exponentially with reciprocal t as described by text log left x text qtz text ti right 5 69 pm 0 02 frac 3765 pm 24 t k application of this thermometer is straightforward typically requiring analysis of only one phase quartz this can be accomplished either by epma for crystallization temperatures above 600 c or by sims for temperatures down to at least 400 resulting temperature estimates are very precise usually better than 5 c potentially allowing detailed characterization of thermal histories within individual quartz grains although calibrated for quartz crystallized in the presence of rutile the thermometer can also be applied to rutile absent systems if tio2 activity is constrained

155 000 years of west african monsoon and ocean thermal evolution a detailed reconstruction of west african monsoon hydrology over the past 155 000 years suggests a close linkage to northern high latitude climate oscillations ba ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the gulf of guinea in the eastern equatorial atlantic eea reveal centennial scale variations of riverine freshwater input that are synchronous with northern high latitude stadials and interstadials of the penultimate interglacial and the last deglaciation eea mg ca based sea surface temperatures ssts were decoupled from northern high latitude millennial scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low latitude solar insolation the onset of enhanced monsoon precipitation lags behind the changes in eea ssts by up to 7000 years during glacial interglacial transitions this study demonstrates that the stadial interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the west african monsoon dynamics through an atmospheric linkage

the dole effect and its variations during the last 130 000 years as measured in the vostok ice core we review the current understanding of the dole effect the observed difference between the delta sup 18 o of atmospheric o sub 2 and that of seawater and its causes extended the record of variations in the dole effect back to 130 kyr before present using data on the delta sup 18 o of o sub 2 obtained from studying the vostok ice core and discuss the significance of temporal variations the dole effect reflects oxygen isotope fractionation during photosynthesis respiration and hydrologic processes evaporation precipitation and evapotranspiration our best prediction of the present day dole effect 20 8 per thousand is considerably lower than the observed value 23 5 per thousand and we discuss possible causes of this discrepancy during the past 130 kyr the dole effect has been 0 05 per thousand lower than the present value on average the standard deviation of the dole effect from the mean has been only per thousand and the dole effect is nearly unchanged between glacial maxima and interglacial periods the small variability in the dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant most periodic variability in the dole effect is in the precession band suggesting that changes in this global biogeochemical termmore reflects variations in low latitude land hydrology and productivity or possibly variability in low latitude oceanic productivity 65 refs 4 figs 3 tabs less

late quaternary aeolian dust input variability on the chinese loess plateau inferences from unmixing of loess grain size records abstract the grain size distributions gsds of loess paleosol sequences may provide valuable information on provenance transport and paleoclimate variability and consequently a wide range of methods to extract genetic information from loess gsds has been proposed here it is shown that a genetically meaningful decomposition of a series of late quaternary loess grain size records extending across the chinese loess plateau can be accomplished with the end member modelling algorithm emma the unmixing results in conjunction with loess accumulation rate estimates reveal that two contrasting dust supply patterns were active over the loess plateau during the last glacial interglacial cycle i a background sedimentation pattern that was dominant during interglacial periods especially over the central and southern parts of the loess plateau is reflected by the constant flux of the fine grained loess component ii an episodic highly variable dust input pattern that was dominant during glacial periods throughout the loess plateau and noticeable during interglacial periods mainly over the northern loess plateau and almost disappearing over the southern loess plateau is reflected in the admixture of two coarse grained loess components a genetic interpretation and the paleoclimatic significance of the mixing model are provided by comparison of the modelled end members with modern dust samples in terms of their gsd and flux rates

first evidence of a mid holocene earthquake triggered megaturbidite south of the chile triple junction abstract megaturbidites have been the focus of many paleoseismic and paleoenvironmental studies because they can provide evidence for catastrophic and or hazardous events with potentially major environmental implications during a recent research cruise in baker fjord chile 47 54 s 74 30 w a megaturbidite was described between the northern and southern patagonian icefields here we aim to determine the depositional processes of the megaturbidite and identify its origin based on the turbidite s location a possible origin was the early holocene drainage of paleo lake general carrera which was recently proposed in the literature as having produced a glacial lake outburst flood glof that drained through baker fjord due to the fjord s location in a subduction zone and close to the chile triple junction however seismic activity must also be considered as a potential triggering mechanism to achieve our goals we undertook a multi proxy analysis of sediment core md07 3121 including sedimentology grain size loss on ignition foraminifera counts magnetic properties bulk organic geochemistry and radiocarbon dating and we analysed bathymetric maps and sub bottom profiles our grain size results display a diagnostic fining upward trend and show evidence of seiching in the 733 cm thick megaturbidite the age of the event 5513 5211 cal yr bp contradicts the hypothesis of an early holocene glof origin bulk organic geochemical results indicate that the sediments that compose the turbidite are clearly of marine origin which further goes against a glof origin in addition the megaturbidite is underlain by a 1136 cm thick mass transport deposit mtd also composed of marine sediments according to the sub bottom profiles the mtd and the megaturbidite originate from the reworking of thick packages of sediment previously deposited on nearby sills and on the fjord s flanks furthermore similar coeval deposits are found in an adjacent sub basin we therefore interpret these deposits to be triggered by an earthquake during the late mid holocene while megathrust and intraslab earthquakes are possible in the region we argue that a crustal earthquake is the most likely seismic trigger in the study area this study reveals the first earthquake triggered megaturbidite south of the chile triple junction

multifractal analysis of discretized x ray ct images for the characterization of soil macropore structures a correct statistical model of soil pore structure can be critical for understanding flow and transport processes in soils and creating synthetic soil pore spaces for hypothetical and model testing and evaluating similarity of pore spaces of different soils advanced visualization techniques such as x ray computed tomography ct offer new opportunities of exploring heterogeneity of soil properties at horizon or aggregate scales simple fractal models such as fractional brownian motion that have been proposed to capture the complex behavior of soil spatial variation at field scale rarely simulate irregularity patterns displayed by spatial series of soil properties the objective of this work was to use ct data to test the hypothesis that soil pore structure at the horizon scale may be represented by multifractal models x ray ct scans of twelve water saturated 20 cm long soil columns with diameters of 7 5 cm were analyzed a reconstruction algorithm was applied to convert the x ray ct data into a stack of 1480 grayscale digital images with a voxel resolution of 110 microns and a cross sectional size of 690 690 pixels the images were binarized and the spatial series of the percentage of void space vs depth was analyzed to evaluate the applicability of the multifractal model the series of depth dependent macroporosity values exhibited a well defined multifractal structure that was revealed by singularity and renyi spectra the long range dependencies in these series were parameterized by the hurst exponent values of the hurst exponent close to one were observed indicating the strong persistence in variations of porosity with depth the multifractal modeling of soil macropore structure can be an efficient method for parameterizing and simulating the vertical spatial heterogeneity of soil pore space

an extended and revised lake suigetsu varve chronology from 50 to 10 ka bp based on detailed sediment micro facies analyses abstract lake suigetsu japan is a key site for radiocarbon 14c calibration and palaeo environmental reconstruction in east asia here we present a description of the sediment micro facies which in combination with a new approach to varve interpolation allows construction of a revised varve based chronology that extends the previous 2012 varve based chronology by 10 ka back to 50 ka bp challenges in varve counting and interpolation which were previously discussed in detail only for the last glacial interglacial transition are described here back to 50 ka bp furthermore the relative merits of varve counting by xrf scanning and by thin section microscopy are discussed facies analysis reveals four facies zones their transitions driven by both local and climatic controls the lamination quality of the sediment is highly variable and varve interpolation reveals that in the analysed time interval on average only 50 of the annual cycles are represented by seasonal layers in the remaining years seasonal layers are indistinguishable i e either did not form or were not preserved for varve interpolation an advanced version of the varve interpolation program was used which enabled the construction of the longest purely varve dated chronology published despite long intervals of poor lamination quality the calculated interpolation uncertainty is 8 9 and 4 6 which is well within expectations considering the high degree of interpolation and the length of the record

the avaatech xrf core scanner technical description and applications to ne atlantic sediments abstract x ray fluorescence xrf core scanning provides rapid high resolution down to 1 mm records of chemical composition on split sediment cores the measurements are non destructive and require very limited sample preparation the new avaatech xrf core scanner operational since 2002 covers the atomic mass range from al to u instrument parameters especially tube voltage can be adjusted to provide optimum settings for selected elements or sets thereof owing to the nature of the surface of split sediment cores particularly effects resulting from sample inhomogeneity and surface roughness results are semiquantitative yet provide reliable records of the relative variability in elemental composition downcore selected case studies from diverse sedimentary settings in the ne atlantic ocean illustrate a range of applications of xrf logging data these include preliminary stratigraphic interpretations glacial interglacial cycles provenance studies of the terrigenous sediment fraction lithological characterization early diagenetic processes and distinction between carbonate phases aragonite v calcite

the upwelling area off northwest africa a description of phenomena related to coastal upwelling abstract this paper presents a brief review of the oceanography along the upwelling coast of northwest africa the description contains information on seasonal variations of large scale winds ocean circulation and some gross of features of the hydrography characteristic meso scale patterns of currents during the upwelling season between 16 n and 26 n air and water temperature differences during upwelling season diurnal and semi diurnal variations of temperature atmospheric pressure wind and current regional peculiarities canyons and the banc d arguin

relation between low latitude insolation and δ18o change of atmospheric oxygen for the last 200 kyrs as revealed by mediterranean sapropels the isotopic ratio of atmospheric o2 18oatm deduced from ice cores displays large fluctuations during climatic cycles these are caused principally by changes in the oxygen isotopic ratio of sea water 18osw and changes in the biosphere and in the hydrological cycle as both 18oatm and 18osw coincide closely over the last 135 kyr bp it is generally believed that 18oatm is driven mainly by 18osw here we focus on the major discrepancy which arises between those two signals during the prior isotopic glacial stage 6 around 175 kyr bp discrepancy which calls into question the role of changes in 18osw as the driving mechanism for 18oatm we present arguments based on the occurrence and pollen content of mediterranean sapropels for another source of the 18oatm change insolation at low latitudes via hydrosphere biosphere activity

upwelling intensity and ocean productivity changes off cape blanc northwest africa during the last 70 000 years geochemical and micropalaeontological evidence abstract the accumulation of biogeochemical organic carbon calcium carbonates molybdenum and iodine micropalaeontological benthic foraminifera and terrigenous markers grain size si al zr al k al over the last 70 000 years in one core sedorqua 11k on the northwest african margin off cap blanc has been used to reconstruct past variations of local upwelling intensity and oceanic productivity the study demonstrates that productivity in this area increased during stage 3 particularly between 40 000 and 50 000 yr b p and during the last deglaciation between 6000 and 15 000 yr b p during most of isotopic stage 2 and particularly during the last glacial maximum productivity was much lower these variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity because of the establishment of upwelling cells over the shelf due to high sea level the conditions of sedimentation during the last 5 6 thousend years on the upper slope site 11k are largely dominated by advection from the shelf leading to strong sorting prior to deposition advection seems to have been minor during the other periods of enhanced productivity a conceptual model is proposed to link the productivity variations to atmospheric circulation in particular to the wind stress direction and seasonality

variations in tropical convection as an amplifier of global climate change at the millennial scale the global expression of millennial scale climatic change during the glacial period and the persistence of this signal in holocene records point to atmospheric teleconnections as the mechanism propagating rapid climate variations we suggest rearrangements in the tropical convection system globally affected the concentration and location of atmospheric water vapour and modulated terrestrial and marine emissions of ch4 and n2o providing a tropical mechanism of amplifying and perpetuating millennial scale climate changes a multi proxy reconstruction reflecting various aspects of the intensity of the arabian sea summer monsoon shows strong millennial scale variability over the past 90 kyr in which low intensity is associated with a southern shift of the intertropical convergence zone itcz and an eastward shift in the equatorial convergence zone the monsoon reconstruction which is based on new data from a somali margin sediment core is supported by previously reported tropical paleoclimatic records and suggests that global scale millennial climatic variability is in part driven by modulations in the tropical hydrological cycle and tropical emissions of the greenhouse gases ch4 and n2o

a synthetic pollen record of the eastern mediterranean sapropels of the last 1 ma implications for the time scale and formation of sapropels abstract the quaternary climate of southern europe south italy and greece is investigated by pollen analysis of the sapropels which were deposited in the deep eastern mediterranean sea during the last 1 million year ma the time scale of core kc01b in the ionian sea has been established by tuning its oxygen isotopic record to the ice volume model of imbrie and imbrie 1980 for the last 250 000 year 250 ka the previous pollen studies and astronomical tuning have been confirmed sapropels were deposited under a large range of mediterranean climates fully interglacial fully glacial and intermediary as revealed mainly by the balance between the respective pollen abundances of oak quercus and sage brush artemisia the high value of the oak reveals the warm and wet climate of an interglacial and the high value of the sage brush the dry and cold climate of a glacial whereas the mediterranean climate is directly related to the variation of the high latitude ice sheets the deposition of sapropels is not so in contrast with the wide climatic range sapropels were deposited only when summer insolation in the low latitudes reached its highest peaks however between 250 ka and 1 ma that stable pattern is not yet established only six sapropels are observed many expected ones do not appear even as ghosts signalled by peaks of barium abundance that remain after the post deposition oxidation of organic matter the pattern of sapropel formation in stable and direct relationship to highest insolation does not seem to apply for five of those sapropels neither climate extremes are observed they mainly formed during intermediary types of mediterranean climate in contrast one sapropel and one ghost relates to a relatively low peak of insolation and its climate is of a unique composite type not seen later this might suggest an unsuspected more complex pattern linking the formation of mediterranean sapropels to the astronomical configuration

calibration of xrf core scanners for quantitative geochemical logging of sediment cores theory and application abstract on line analysis of split sediment cores by xrf core scanners has become increasingly popular in the past decade because it allows nondestructive extraction of near continuous records of element intensities from sediment cores with a minimum of analytical effort a disadvantage of xrf core scanning relative to conventional geochemical analysis is the problematic conversion of core scanner output to element concentrations the main reason for this long standing problem is the poorly constrained measurement geometry attributable to inhomogeneity of the specimens e g variable water content and grain size distribution irregularities of the split core surface and in some setups spatial variations in thickness of an adhesive pore water film which forms directly below a protective foil covering the core surface we propose a log ratio calibration model for xrf core scanners derived from a combination of xrf spectrometry theory principles of compositional data analysis and empirical evidence the log ratio calibration model provides accurate and precise predictions of sediment composition element concentrations from xrf core scanner output with a limited number of parameters namely 2 d 1 where d equals the number of chemical elements whose concentrations are to be estimated the model can accommodate the inherent non linearity of the relation between relative intensities and concentrations which is apparent from the fact that it provides unbiased estimates an immediate corollary of our results is that log ratios of element intensities which are related to log ratios of element abundances by a simple linear transformation provide the most easily interpretable signals of relative changes in chemical composition consistent use of log ratios of element intensities or concentrations should minimise the risk of drawing erroneous conclusions from geochemical proxies the relative standard deviation precision of predicted element concentrations in core geob7920 is less than 2 stochastic simulations indicate that this level of precision can be attained with 40 randomly selected calibration specimens improved control over input errors and development of robust goodness of fit statistics allows xrf core scanning to be developed into a rigorous quantitative measurement technology the log ratio calibration equation derived in this study may be adapted to inter laboratory and inter instrument calibration as well

modelling the joint variability of grain size and chemical composition in sediments abstract the geochemical composition of siliciclastic sediments correlates strongly with grain size hence geochemical composition may serve as a grain size proxy in the absence of grain size variations geochemical data of siliciclastic sediments may be used to characterise size independent processes i e sediment provenance weathering mixing shape density sorting and diagenesis in general however geochemical data sets contain both types of information in order to formalise interpretation of geochemical data we propose a mathematical method to decompose the total geochemical variability of a series of genetically related specimens into a grain size dependent the shared signal and a grain size independent part the residual signal the former may serve as a proxy for grain size whereas the latter represents geochemical variability that would have been observed if all sediments would have had the same grain size distribution the two data sets are jointly decomposed by means of partial least squares pls and orthogonal projection subsequently the presence of significant grain size independent geochemical variability in the residual signal is determined in a statistically rigorous manner using a 2 test using a synthetic example we show that the residual record effectively reveals an imposed provenance signal which could not have been resolved from the geochemical or grain size data sets individually we analysed the relation between grain size and geochemical composition in three quaternary marine sediment cores located offshore west africa and south america geob7920 2 geob9508 5 and geob7139 2 both sites are characterised by biogenic sediment input in addition to fluvial and aeolian sediment input from the continent it was found that all cores show a strong but different correlation between the mean grain size and the bulk geochemical composition these results demonstrate that geochemical grain size proxies are empirical and site specific it was also found that the geochemical and grain size data in cores geob7920 2 and geob7139 2 do not contain unique information whereas in core geob9508 5 ti varies independently from the grain size this residual ti signal correlates with the transport mechanism as demonstrated by statistically different values of aeolian and fluvial dominated sediments however a unique interpretation of this residual signal in terms of the postulated grain size independent mechanisms could not be provided without additional information we conclude that the proposed model facilitates identification and validation of different element ratios as grain size proxies and more importantly as proxies for size independent processes for this reason the model paves the way for rigorous analysis of multi proxy data which are widely used in palaeoceanographic and palaeoclimatic research

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

correcting for the influence of ice rafted detritus on grain size based paleocurrent speed estimates the grain size of deep sea sediments provides an apparently simple proxy for current speed however grain size based proxies may be ambiguous when the size distribution reflects a combination of processes with current sorting only one of them in particular such sediment mixing hinders reconstruction of deep circulation changes associated with ice rafting events in the glacial north atlantic because variable ice rafted detritus ird input may falsely suggest current speed changes inverse modelling has been suggested as a way to overcome this problem however this approach requires high precision size measurements that register small changes in the size distribution here we show that such data can be obtained using electro sensing and laser diffraction techniques despite issues previously raised on the low precision of electro sensing methods and potential grain shape effects on laser diffraction down core size patterns obtained from a sediment core from the north atlantic are similar for both techniques reinforcing the conclusion that both techniques yield comparable results however ird input leads to a coarsening that spuriously suggests faster current speed we show that this ird influence can be accounted for using inverse modelling as long as wide size spectra are taken into account this yields current speed variations that are in agreement with other proxies our experiments thus show that for current speed reconstruction the choice of instrument is subordinate to a proper recognition of the various processes that determine the size distribution and that by using inverse modelling meaningful current speed reconstructions can be obtained from mixed sediments

rare earth elements in marine sediments and geochemical standards abstract a comparison was made of sediment dissolution methods used for analysis of rare earth elements ree and major elements fe al ti zr p dissolution by hf and lithium metaborate fusion were compared in seven geochemical standards and twenty marine sediments using hf dissolution of shelf and slope sediments this author previously observed that these marine sediments were significantly depleted in heavy ree hree relative to the crustal abundance i e shale this observation plus published ree compositions of river particles lead to the conclusion that the riverine input of particles to the oceans cannot be assumed to have the crustal abundance for ree comparisons of fusion total to hf dissolutions for marine sediments show that the latter method misses between 20 and 100 of the hree i e dy er yb lu and large percentages of zr in contrast fusion and hf dissolution yield similar concentrations for the light la ce nd and middle eu gd ree and other major elements the fusion data show that the marine sediments are more similar in ree composition to shales than previously reported by this author who underestimated the contribution of hf insoluble residues to the concentration of hree the heavy mineral zircon is the most likely cause of this observation zircons known to be enriched in hree were identified by x ray diffraction as a major component of the hf insoluble residues geochemical standards also have a significant but usually smaller fraction of their hree associated with hf insoluble phases these compositional differences have significance with respect to interpreting river inputs oceanic abundances and diagenesis of ree

na rich character of metasomatic metamorphic fluids inferred from preiswerkite in chromitite pods of the khoy ophiolite in iran role of chromitites as capsules of trapped fluids abstract we report a new occurrence of preiswerkite a rare na and al rich trioctahedral mica na analog of eastonite from a high al cr of chromite 0 4 chromitite pod of the khoy meta ophiolite in northwestern iran the preiswerkite is euhedral and associated with ca cr al garnet clinochlore titanite and calcite preiswerkite and associated minerals fill cavities in the chromitite pod and are absent in the surrounding peridotites the preiswerkite rich aggregate is possibly representative of aqueous fluid supplied to the mantle section of the khoy ophiolite peraluminous conditions which are indispensable for preiswerkite formation were established only in the pool of fluid trapped by the high al chromitite the fluid and preiswerkite precipitates were isolated from the peridotite by the chromitite capsule the na rich fluid was a precursor of the serpentinization fluid the fluid is an analog of the fluid released from the subducting slab which can precipitate jadeite under high pressure conditions

biotite in olivine gabbros from atlantis bank evidence for amphibolite facies metasomatic alteration of the lower oceanic crust abstract gabbroic rocks recovered from deep holes drilled in the ocean floor provide us with valuable information about in situ alteration processes of the lower oceanic crust we found that the occurrence of biotite is widespread in gabbroic rocks recently drilled from iodp hole u1473a at atlantis bank near the southwest indian ridge biotite is rare in oceanic gabbros thus we analyzed textural and compositional details of biotite and associated minerals to better understand the conditions governing their formation in olivine gabbros from hole u1473a and from nearby odp hole 735b biotite occurs mainly in coronitic aggregates mantling olivine it also forms monomineralic veins or occurs in biotite chlorite amphibole veins within plagioclase grains that contact the coronitic aggregates the coronitic aggregates typically have an outer biotite dominated zone and an inner zone mostly made up of al poor calcic amphibole the biotite and the calcic amphibole zones frequently include al rich calcic amphibole cummingtonite and talc respectively plagioclase in direct contact with al rich calcic amphibole has 65 90 mol anorthite the coronitic aggregates also frequently have an outermost zone composed of submicron scale biotite chlorite mixtures which show intermediate optical and chemical characteristics between biotite and chlorite and a composite pattern of raman shift spectra most biotite rich coronitic aggregates occur in proximity to felsic veins or to biotite or alkali feldspar microveins branched from felsic veins whereas most biotite chlorite coronas are connected to biotite chlorite or amphibole bearing microveins chlorite coronas around olivine though rare in atlantis bank gabbros occur in contact with chlorite bearing microveins and show no relationship with felsic veins based on the plagioclase amphibole equilibrium we evaluated temperature of 750 850 c for the formation of the biotite rich coronitic aggregates from the modes of occurrence compositions of minerals and thermodynamic modeling we conclude that the biotite coronas formed at higher temperatures and higher sio2 and or k h activities than chlorite coronas typically found in olivine gabbros from other mid ocean ridge localities the coronitic biotite chlorite mixtures formed in response to lower sio2 and or k h activities and possibly lower temperature than the biotite coronas such a difference in physical and chemical conditions for corona formation probably reflects the distance from felsic vein microvein or is related to the relative timing of reactions the high temperature metasomatic alteration of lower oceanic crustal gabbros shown in this study is most likely characteristic of oceanic core complexes from ultraslow spreading ridges

climatic conditions during marine oxygen isotope stage 6 in the eastern mediterranean region from the isotopic composition of speleothems of soreq cave israel at several times during marine oxygen isotope stage 6 the eastern mediterranean region was influenced by two extreme climatic systems the large ice sheet over northern europe and the wet tropics associated with african monsoons during this interval two major climatic events occurred in the region the sapropel s6 layer formed ca 176 ka in the eastern mediterranean basin owing to the increase in the african monsoon and another event although not large enough to form sapropel occurred ca 151 ka the isotopic composition of soreq cave speleothems seems to record these events as very low 18 o 13 c values dated as ca 178 and 152 ka the very low 18 o 13 c values of 6 and 11 to 12 respectively are typical of interglacial intervals but here they were recorded during a glacial interval such low peaks indicate that in this part of the eastern mediterranean region i e israel the rainfall amount increased dramatically moreover the isotopic record of the speleothems also shows that during the entire stage 6 although the climate was as cold as much of the last glacial the conditions were never as dry

partitioning of fluorine and chlorine between biotite and granitic melt experimental calibration at 200 mpa h 2 o experiments from 640 to 680 c 200 mpa h2o at fo2 nno employing a natural f rich vitrophyric rhyolite from spor mountain utah assessed the effect of variable mg 100mg mg mn fe on the partitioning of fluorine and chlorine between biotite bt and melt over this temperature interval bt fluorite quartz is the sole liquidus phase partition coefficients for fluorine between biotite and glass dfbt melt show a strong dependence on the mg of bt dfbt melt varies from 1 5 to 7 2 over the range of mg from 21 to 76 a strong linear correlation between dfbt melt and mg has a slope of 9 4 and extrapolates through the origin i e dfbt melt 0 at mg 0 an annite siderophyllite solid solution in these experiments dclbt melt values 1 to 6 in the same experiments vary inversely with mg the al content of biotite does not vary with the aluminum saturation index asi molar al2o3 alkali and alkaline earth oxides of melt but two exchange mechanisms involving al appear to operate in these micas 1 alvi aliv siiv mgiv and mgiv 2aliv 2siiv iv the effects of other components such as li or other intensive parameters including fo2 have yet to be evaluated systematically at comparable mg of bt however the spor mountain rhyolite yields higher dfbt melt values than an li rich strongly peraluminous melt previously investigated the results indicate that the mg of bt exerts the principal control on halogen partitioning with asi and t as second order variables the experimental partition coefficients compare well with other experimental results but not with most volcanic rocks magmatic bt from most rhyolites records higher dfbt melt due to reequilibration with degassed h2o depleted magma and perhaps with f2o 1 exchange that may accompany oxidation fe3 o fe2 oh 1 this behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near morococala bolivia bt is sharply zoned with f rich rims but bt core melt inclusion pairs fall on our experimental curve for dfbt melt these experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks for plutonic rocks the actual f content of melt not a relative activity ratio involving hf species can be reasonably estimated if the mica has not undergone subsolidus reequilibration this information is potentially useful for some shallow level ca poor magmas that are thought to be rich in f e g a and s type granites but do not conserve f well as rocks

crystallization of late stage morb under varying water activities and redox conditions implications for the formation of highly evolved lavas and oxide gabbro in the ocean crust abstract in order to understand late magmatic processes that occur in the deep oceanic crust we performed a phase equilibria study in a representative late stage system at a pressure of 200 mpa with a special focus on the role of water and oxygen fugacity the starting composition for the experiments was evaluated based on a statistical approach using evolved fresh morb glasses from the database petdb highest in feo and tio2 in average 17 92 wt and 3 73 wt respectively assumed to represent frozen liquids erupted at the seafloor generated by extensive differentiation of morb we conducted crystallization experiments in a range of temperatures from 850 to 1050 c with water activities from 0 1 to 1 and under redox conditions from fmq 1 1 to fmq 3 2 fmq fayalite magnetite quartz oxygen buffer the results show that in this fe and ti rich late stage system fe ti oxides are the liquidus phases followed by clinopyroxene apatite and plagioclase which is more stable at low water activity amphibole is stable at high ah2o and at temperatures lower than 900 c the evolution of the melt composition with decreasing temperature and ah2o follows in general the liquid lines of descent observed in other experiments in ferrobasaltic compositions orthopyroxene which occurs as late crystallizing phase in many oceanic gabbros was absent among the experimental mineral assemblages based on the evolution of coexisting clinopyroxene and plagioclase compositions our experiments define a trend similar to the trends from other suites of oceanic gabbros from various locations our experimental results shed new light on the formation of highly evolved lavas from fast and intermediate spreading mid ocean ridges implying a relatively simple two step differentiation model first primitive morbs differentiate along the 1 atm cotectic trend to typical ferrobasaltic compositions by fractionation of olivine plagioclase clinopyroxene this trend continues as long as fe ti oxides are not saturated which is strongly dependent on the prevailing oxygen fugacity when such ferrobasaltic magmas are cut off from replenishment by fresh morb the possibility for further differentiation to highly evolved melts is given by fractionation of oxides clinopyroxene plagioclase apatite and finally amphibole and apatite phase relations obtained in our experiments help to understand the formation of oxide gabbros from the ultra slow spreading southwest indian ridge iodp drill core hole 735b the experimental evaluated crystallization order with temperature is in accord with the record of crystallization of the hole 735b oxide gabbros

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

phase lag between intertropical convergence zone migration and subtropical monsoon onset over the northwestern indian ocean during marine isotopic substage 6 5 mis 6 5 high resolution faunal and isotopic analyses of foraminifera were performed on core md96 2073 10 94 n 52 62 e 3142 m depth located close to socotra island in the upwelling area of the somali basin nw indian ocean this work focuses on marine isotopic stage 6 5 in order to reconstruct paleo upwelling changes and their links with the arabian sea summer monsoon and the migration of the intertropical convergence zone itcz correspondence and cluster analyses of planktonic foraminiferal abundances partly controlled by temperature and water mass productivity together with an upwelling intensification index show the occurrence of a strong upwelling between 176 and 165 ka this upwelling intensification responds to a northward migration of the itcz an isotopic depletion in the planktonic foraminifera 18o records occurring between 180 and 167 ka is interpreted as proof of a large salinity decrease in the surface waters probably linked to a strong input of fresh rainfall waters induced by an intense monsoon activity the lag between the onset of upwelling intensification and the strong monsoonal impact over the same area suggests a decoupling between both phenomena the migration of the itcz is influenced by obliquity and precessional forcing while the arabian sea summer monsoon precipitation depends only on precessional forcing

multi fractal characteristics of the particle distribution of reconstructed soils and the relationship between soil properties and multi fractal parameters in an opencast coal mine dump in a loess area soil reconstruction is at the core of mine land reclamation reconstructed soil from an opencast mine dump in a loess area is a complex composite that was assembled by humans using multiple reconstruction technologies the soil is composed of varied soil particles with irregular shapes and has self similar structure to better quantify the characterization of reconstructed soil in this dump the soil particle size distribution from four modes of soil reconstruction in the shanxi pingshuo antaibao opencast coal mine dump was analyzed using multi fractal theory soil reconstruction modes included loess material containing gravel lg loess material containing coal gangue cg total loess material al and loess material containing ginger stone lgs soil particle composition gravel content field water capacity soil bulk density soil organic matter total nitrogen tn soil ph and electrical conductivity ec of different soil layers from different reconstruction modes were measured a generalized dimension spectrum d q multi fractal singularity exponent q and multi fractal spectrum function f q were calculated relationships between soil properties and fractal parameters were analyzed the results led to the following conclusions 1 the soil particle distribution of reconstructed soil had significant multi fractal characteristics in an opencast coal mine dump in a loess area d 0 d 1 d 1 d 0 and f can reflect the non uniform particle size distribution characteristics d 0 d 1 d 0 and f or d 0 d 1 d 0 and can be simplified as three parameters to quantify the multi fractal characterization of the reconstructed soil particle distribution 2 after reclamation the soil quality from different layers of different profiles in different reconstruction modes shows some improvement 3 there were strong correlations between multi fractal parameters and soil properties d 1 and increased and d 1 d 0 and f decreased with increasing gravel content soil bulk density organic matter tn and soil ec however it was opposite for ph and field water capacity gangue and gravel have a definite impact on the dispersion degree of soil particles in reconstruction this study provides a theoretical basis for land reclamation and quantitative expression of reconstructed soil particles distribution in opencast coal mine dumps in loess areas

cortex a shipboard xrf scanner for element analyses in split sediment cores abstract the cortex corescanner texel is a non destructive xrf system for the logging of split sediment cores it is a seaworthy instrument that remains active during heavy weather conditions the scanner produces reliable counts for k to sr highly correlated with concentrations measured with atomic absorption spectrometry aas the option to scan al si and elements heavier than sr is in preparation a core section of 1 m can be scanned at 2 cm intervals within an hour a maximum resolution of 1 mm can be achieved at present each region and sedimentary environment requires separate calibration in the nearby future the generator settings will be standardized which may result in the development of a standard calibration procedure because of the high resolution of its semi quantitative records the cortex is a useful tool for detailed time series analysis inter core correlation and shipboard analysis

quantitative models of sediment generation and provenance state of the art and future developments abstract an overview of quantitative approaches to analysis and modelling of sediment generation and provenance is presented with an emphasis on major framework components as determined by means of petrographic techniques conceptual models of sediment provenance are shown to be consistent with two classes of numerical statistical models i e linear mixing models and compositional linear models these cannot be placed within a common mathematical framework because the former requires that sediment composition is expressed in terms of proportions whereas the latter requires that sediment composition is expressed in terms of log ratios of proportions additivity of proportions a fundamental assumption in linear mixing models cannot be readily expressed in log ratio terms linear mixing models may be used to describe compositional variability in terms of physical and conceptual un mixing models of physical un mixing are appropriate for describing compositional variation within transport invariant subpopulations of grains as a result of varying rates of supply of detritus from multiple sources conceptual un mixing governs the relations among chemical mineralogical and petrographic characteristics of sediments which represent different descriptive levels within a compositional hierarchy compositional linear process models may be used to describe compositional and or textural evolution resulting from selective modifications induced by sediment transport as well as chemical and mechanical weathering current approaches to modelling of surface processes treat the coupled evolution of source areas and sedimentary basins in terms of bulk mass transfer only and do not take into account compositional and textural sediment properties moving from the inverse modelling approach embodied in provenance research to process based forward models of sediment generation which provide detailed predictions of sediment properties meets with considerable albeit not insurmountable problems strategies for developing more advanced parameterizations of sediment properties and for simulating their evolution are proposed and expansion of current data acquisition protocols is discussed in the light of data requirements for such advanced sediment generation models inclusion of sediment generation modules in models of landscape evolution and basin filling will mark a significant step towards full fledged source to sink simulations of earth surface processes

postglacial fluctuations of cordillera darwin glaciers southernmost patagonia reconstructed from almirantazgo fjord sediments abstract most outlet glaciers of the cordillera darwin icefield cdi patagonia 54 s are currently transitioning from calving to land based conditions whether this situation is unique to the modern climate or also occurred during the holocene is entirely unknown here we investigate the holocene fluctuations of outlet glaciers from the northern flank of the cdi using a multi proxy sedimentological and geochemical analysis of a 13 5 m long sediment core from almirantazgo fjord our results demonstrate that sedimentation in almirantazgo fjord started prior to 14 300 cal yr bp with glacier proximal deposits occurring until 13 500 cal yr bp after 12 300 cal yr bp most glaciers had retreated to land locked locations and by 9800 cal yr bp almirantazgo fjord was a predominantly marine fjord environment with oceanographic conditions resembling the present day setting our sediment record shows that during the first part of the holocene cdi glaciers were almost entirely land based with a possible re advance at 7300 5700 cal yr bp this is in clear contrast with the neoglaciation during which cdi glaciers rapidly re advanced and shrank back several times mostly in phase with the eastern outlet glaciers of the southern patagonian icefield spi two significant meltwater events indicative of rapid glacier retreat were identified at 3250 2700 and 2000 1200 cal yr bp based on an increase in grain size mode and related inorganic geochemical parameters this interpretation is additionally supported by concomitant decreases in organic carbon of marine origin and in cl counts salinity reflecting higher terrestrial input to the fjord and freshening of the fjord waters overall our record suggests that cdi outlet glaciers advanced in phase with eastern spi glaciers during the neoglaciation and retreated far enough into their valleys twice to form large outwash plains our results also highlight the potential of fjord sediments to reconstruct glacier variability at high resolution on multi millennial timescales

muddled or mixed inferring palaeoclimate from size distributions of deep sea clastics abstract one of the outstanding problems of palaeoclimate reconstruction from physico chemical properties of terrigenous deep sea sediments stems from the fact that most basin fills are mixtures of sediment populations derived from different sources and transported to the site of deposition by different mechanisms conventional approaches to palaeoclimate reconstruction from deep sea sediments do not distinguish between provenance and dispersal related variations and therefore often fail to recognise the true significance of variations in sediment properties we formulate a set of requirements that each proposed palaeo environmental indicator should fulfil and focus on the intrinsic coupling between grain size and chemical composition a critical review of past achievements in grain size analysis serves to introduce a conceptual model of spatio temporal grain size variation based on dynamic populations each dynamic population results from a characteristic combination of production and transport mechanisms that corresponds to a distinct subpopulation in the data analysed the mathematical statistical representation of the conceptual model can be obtained by means of the end member modelling algorithm emma applications of the model to several ocean basins are discussed as well as methods to examine the validity of grain size based palaeoclimate reconstructions palaeoclimate reconstructions of a high and low latitude basin illustrate the common degree of complexity of deep sea grain size records

alteration of the oceanic lower crust at a slow spreading axis insight from vein related zoned halos in olivine gabbro from atlantis massif mid atlantic ridge incipient stage alteration products in relatively fresh oceanic gabbros from deep boreholes provide critical information on hydration processes in the oceanic lower crust and their effect on lithosphere dynamics we present the results of a petrographic study on the alteration of olivine bearing gabbroic rocks recovered from the deeper parts of integrated ocean drilling program iodp hole u1309d in the atlantis massif near the mid atlantic ridge at 308n in these rocks alteration is localized in proximity to fluid infiltration veins or igneous contacts it is most conspicuous in halos surrounding amphibole chlorite veins or leucocratic veins in olivine bearing gabbros where coronitic fringes of tremolite chlorite and talc occur around discrete olivine grains many of the halos exhibit a zonal pattern with systematic changes in mineral assemblage generally consisting of three zones tremolite chlorite around relict olivine plagioclase contacts talc pseudomorphs after olivine and tremolite pseudomorphs after olivine the tremolite chlorite assemblage appears in increasing amounts and talc grows unevenly with increasing thickness toward the veins the alteration minerals have highly magnesian compositions reflecting the compositions of the precursor igneous phases within the zone closest to the veins green hornblende with a relatively high al content occurs showing textures suggestive of its later formation than the coronitic tremolite and chlorite considering the mode of occurrence and chemical composition of the minerals combined with thermodynamic calculations of silica and water activities in a simplified system we conclude that the zoned halos were caused by metasomatism owing to protracted or sequential infiltration of hydrothermal fluids at amphibolite facies conditions 450 7508c 1 5 2 kbar textural relationships clearly indicate that zoned halos formed earlier than serpentinization and clay mineral formation and suggest that the high temperature amphibolite facies alteration took place in a near axis region before the exhumation of the lower crustal rocks recent results of seafloor drilling have provided supporting evidence for the predominance of gabbroic rocks in oceanic core complexes the similarity in mineral association between zoned halos and schistose fault rocks suggests that preferential formation of talc and or chlorite rather than serpentine at contacts between gabbroic rocks and peridotite plays an essential role in detachment faulting and tectonic exhumation of oceanic core complexes from lower crustal levels

geochemistry of surface sediments from the fjords of northern chilean patagonia 44 47 s spatial variability and implications for paleoclimate reconstructions abstract the patagonian fjords have a clear potential to provide high resolution sedimentary and geochemical records of past climate and environmental change in the southern andes to improve our ability to interpret these proxy records we investigated the processes that control fjord sediment inorganic geochemistry through a geochemical mineralogical and sedimentological analysis of surface sediment samples from the fjords of northern chilean patagonia a simple terrestrial index based on measurements of salinity and fraction of terrestrial carbon was used to estimate the terrestrial input river discharge at each site our results demonstrate that under the cold climate conditions of patagonia chemical weathering is weak and the inorganic geochemical composition of the fjord sediments is primarily controlled by hydrodynamic mineralogical sorting i e the intensity of river discharge our results suggest that the distribution of fe ti and zr in surface sediments is controlled by their association with heavy and or coarse minerals whereas al is independent of hydrodynamic processes the elemental ratios fe al ti al and zr al are therefore well suited for estimating changes in the energy of terrestrial sediment supply into the fjords through time zr al is particularly sensitive in proximal environments while fe al is most useful in the outer fjords and on the continental margin in the most proximal environments however fe al is inversely related to hydrodynamic conditions caution should therefore be exercised when interpreting fe al ratios in terms of past river discharge the application of these proxies to long sediment cores from quitralco fjord and golfo elefantes validates our interpretations our results also emphasize the need to measure al based elemental ratios at high precision which can be achieved using simultaneous acquisition icp aes technology this study therefore constitutes a strong basis for the interpretation of sedimentary records from the chilean fjords

the heterogeneity of pore structure in lacustrine shales insights from multifractal analysis using n2 adsorption and mercury intrusion abstract a series of parameters from a variety of techniques such as pore size distribution surface area pore volume etc have been used to characterize the pore structure of unconventional shale reservoirs however the investigation on the quantification of pore structure heterogeneity is still scarce in this aspect a multifractal theory combined with n2 adsorption and mercury intrusion was used to obtain pore structure characteristics and quantify the heterogeneity of pore size distribution of nine lacustrine shale samples from the second member of kongdian formation ek2 in cangdong sag bohai bay basin china the results show that the ek2 shale contains complex pore network and pores with small pore size predominate in pore system both n2 adsorption branches and pore throat size distributions from mercury intrusion demonstrate multifractal nature the width of singularity spectra from multifractal analysis is used to quantitatively characterize the heterogeneity degree of pore size distribution and shows good consistency with pore size distributions derived from n2 adsorption and mercury intrusion the pore structure from mercury intrusion is much heterogeneous than that from n2 adsorption indicating that an increased number of large pores will increase the heterogeneity of pore size distributions compared with other rock compositions toc has an obvious impact on the heterogeneity of pore structure in small pore size range

sahel megadroughts triggered by glacial slowdowns of atlantic meridional overturning 1 the influence of the large scale ocean circulation on sahel rainfall is elusive because of the shortness of the observational record we reconstructed the history of eolian and fluvial sedimentation on the continental slope off senegal during the past 57 000 years our data show that abrupt onsets of arid conditions in the west african sahel were linked to cold north atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with heinrich stadials climate modeling suggests that this drying is induced by a southward shift of the west african monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric african easterly jet

african monsoon enhancement during the penultimate glacial period mis 6 5 170 ka and its atmospheric impact 1 a reconstruction of northwest african summer monsoon strength during the cold marine isotopic stage mis 6 indicates a link to the seasonal migration of the intertropical convergence zone itcz high resolution studies of eolian dust supply and sea surface temperature recorded in marine core md03 2705 on the mauritanian margin provide a better understanding about the penultimate glacial history of northwestern african aridity humidity and upwelling coastal activity today site md03 2705 experiences increased upwelling and dust flux during the winter months when the itcz is in a southerly position analyses of foraminifera isotopic composition suggest that during mis 6 5 180 168 ka the average position of the itcz migrated north marked by an increase in the strength of the summer monsoon which decreased eolian dust transport and the coastal upwelling activity the northward migration is in phase with a specific orbital combination of a low precessional index with a high obliquity signal high resolution analysis of stable isotopes 18o and 13c and microscale resolution geochemical ti al and quartz grain counts determinations reveal that the transition between monsoonal humid mis 6 5 and dry mis 6 4 conditions has occurred in less than 1 3 ka such rapid changes suggest a nonlinear link between the african monsoonal rainfall system and environmental changes over the continent this study provides new insights about the influence of vegetation and oceanic temperature feedbacks on the onset of african summer monsoon and demonstrates that during the penultimate glacial period changes in tropical dynamics had regional and global impacts

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids