the concentration of uranium in freshwater vadose and phreatic cements in a holocene ooid cay a method of identifying ancient water tables abstract the concentration of uranium has been measured using the fission track technique in vadose and freshwater phreatic cements from south joulters cay in the bahamas these analyses reveal dramatic differences in the u concentration of vadose 0 28 0 11 ppm and freshwater phreatic 1 80 75 ppm cements this distinctive change in concentration is proposed to result from 1 variations in the co2 of the waters of the phreatic and vadose environments and 2 selective exclusion of u during the precipitation of calcite the pco2 is important because it controls the ph and hence the concentration of co32 ions in the pore fluids as well as the amount of dissolution of the u rich aragonitic precursors for example an increas in pco2 will lower the ph and decrease the concentration of co32 present in the porewaters as the u present as the uranyl ion uo22 is mainly complexed with co32 a decrease in the concentration of co32 will not only alter the speciation of u but also increase the uo2 co3 22 co32 ratio of the fluid and hence the concentration of u in the precipitated carbonate cements the second mechanism for explaining the increase of u in phreatic cements relies on the low distribution coefficient for the incorporation of u into low mg calcite lmc the precipitation of lmc therefore leads to an increase in the concentration of u in the residual solution as fluids do not remain in the vadose zone but rather accumulate in the freshwater phreatic portion of the lens phreatic waters become relatively enriched in u compared to vadose fluids the clear difference in the u concentrations of cements in these two zones may enable the u concentration of cements to be used as an indicator of paleo water tables in ancient rocks even where other geochemical and petrographic evidences are equivocal

strong biological controls on sr ca ratios in aragonitic marine bivalve shells 1 it is well known that skeletal remains of carbonate secreting organisms can provide a wealth of information about past environments sr ca ratios have been successfully used as a temperature proxy in corals and sclerosponges previous work on aragonitic bivalve shells has not been conclusive but suggests a major control of growth rate on sr ca ratios as many studies have used bivalve growth rates to determine temperature we tested if sr ca ratios could predict temperature through its relationship with growth rate shells from the two species of clams from the same family veneroidea studied here saxidomus giganteus and mercenaria mercenaria show vastly different seasonal sr ca profiles a strong relationship between average annual sr ca ratios and annual growth rate was found in s giganteus shells from both washington r2 0 87 and alaska r2 0 64 usa but not in m mercenaria shells from north carolina usa furthermore the sr ca growth rate relationship was also evident upon a more detailed inspection of subannual growth rates in s giganteus r2 0 73 although there were significant positive correlations between sr ca ratios and temperature in s giganteus shells the correlations were weak 0 09 r2 0 27 and thus sr ca ratios cannot be used as a reliable temperature proxy in these species of aragonitic bivalves it is clear from this study that sr ca ratios are not under thermodynamic control in either clam species since thermodynamics predict a negative correlation between sr ca ratios and temperature in aragonite this points toward dominance of biological processes in the regulation of sr2 this is also reflected by the largely differing sr ca partition coefficients dsr in these shells dsr 0 25 when compared to inorganic coral and sclerosponge studies dsr 1 all of which show a negative dependence of sr ca on temperature we suggest that caution be taken when using sr ca in any biogenic aragonite as a temperature proxy when the dsr greatly deviates from one as this indicates the dominance of biological controls on sr ca ratios

trace and minor element records in aragonitic bivalve shells as environmental proxies abstract investigation of the geochemical composition of bivalve shells can provide information on changes in the marine environment occurring during the lifespan of an organism three species locally abundant in the adriatic sea were chosen in this study namely glycymeris pilosa callista chione and venus verrucosa of these g pilosa has the longest lifespan exceeding 50 years and therefore presents a potential archive of decadal climate variability the other two species c chione and v verrucosa are commercially important samples were collected alive by scuba diving in the north adriatic sea near barbariga istria major growth increments in these shells form on an annual basis and are clearly visible in shell cross sections enabling correct temporal positioning of geochemical data obtained from different shell parts in this study we applied laser ablation inductively coupled plasma mass spectrometry la icp ms in line scan mode to determine the trace and minor element composition along the major growth axis in the hinge area of the shells objectives of this study were to compare signals within and between species and to relate results to available environmental parameters high resolution time series of na cashell mg cashell and sr cashell display characteristic cyclic variations na cashell minima and mg cashell and sr cashell maxima occur near shell annual growth lines in all three studied species results indicate that seawater temperature is not a main driver for incorporation of these elements ba cashell data vary between years and species and have noncyclic sharp peaks callista chione and v verrucosa specimens show higher peaks and background ba cashell values than g pilosa most pronounced peaks in ba cashell occurred at the beginning of the growing season corresponding to the end of year 2011 and beginning of year 2012 and coincided with unusual dense water formation and subsequent changes in the plankton communities in the adriatic sea implications of our study suggest that synchronous peaks in studied specimens are caused by external forcing which could occur on larger regional scales encouraging future studies covering wider area in the adriatic sea and other parts of the mediterranean

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

uranium in aragonitic marine bivalve shells abstract uranium concentrations in foraminiferal and coral skeletons track oceanic co 3 2 and can be useful as a proxy of ocean acidification but this ph proxy is yet to be investigated in bivalves two saxidomus giganteus shells from the industrialized puget sound wa usa and one from the more pristine kodiak island ak usa were sampled through ontogeny for u ca using laser ablation inductively coupled mass spectrometry all three shells show a similar pattern of seasonal u ca cycles during the first six years of life followed by a sharp decrease to below the detection limit for the remainder of the clams life 10 20 years consistent with a biological or ontogenic forcing vital effect however analyses along a growth line carbonate formed at the same time show a decrease in u ca from the outside of the shell toward the inside consistent with diagenesis clearly u ca is not under environmental control in these aragonite shells but the cause of the variability is not currently clear

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

trace element heterogeneity along isochronous growth layers in bivalve shell consequences for environmental reconstruction to precisely reconstruct environmental changes from high resolution bivalve shell geochemistry the shell layers must represent environmental conditions at the time of deposition therefore calcium carbonate along growth lines that formed at the same time should show the same geochemical signature we test this assumption by evaluating the geochemical spatial homogeneity along growth layers i e shell secreted at a given time on shell cross sections of modern shells of the bivalve protothaca thaca for ontogenitically young and old specimens analyses were carried out using laser ablation icp ms and solution nebulization icp ms scanning electron microscope images showed signs of early diagenesis of old shell sections close to the umbo compared with recent close to the ventral margin the spatial distribution of sr ca mg ca and ba ca along growth layers differed consistently between old and recent shell sections elemental distribution at the ventral margin of young specimens was similar for the two specimens analyzed and similar to the element distribution in the older specimens at a similar ontogenic stage closer to the umbo if a balancing of the shell composition with the surrounding seawater between the old and recent part of the shell supported by diagenesis cannot be excluded we assume that trace element incorporation process at a given time varied with the age of the specimen in connection with changes of mantle characteristics whereas ba ca and sr ca were almost constant for shell secreted at a given time close to the current ventral margin all individuals showed significant mg ca heterogeneities with reproducible patterns we assumed that this would be due to a zonation of the outer epithelium of the mantle that would favor or inhibit the transport of mg into the extrapallial fluid and subsequently into the shell time series obtained crossing successive growth layers using laser ablation icp ms and solution nebulization icp ms on a same shell growth section were similar for sr ca and ba ca ratios and not for mg ca which reflected both the different spatial resolutions of the analytical techniques and the heterogeneous distribution of mg ca in the p thaca shells sr and ba seemed to be most promising as potential high resolution environmental proxies our study showed that the location of high resolution analyses points in shell cross section must be i precisely defined and ii consistently followed along the shell section to provide accurate environmental records i e without bias related to shell layer heterogeneities

non marine radiaxial fibrous calcites examples of speleothems proved by electron backscatter diffraction abstract the occurrence of radiaxial fibrous calcites rfc in speleothems is documented for the first time using the new electron backscatter diffraction ebsd method up to now in speleothems these precipitates have only been observed as mg containing calcites in close association with aragonite in caves of the temperate to cool central european regions with mean annual temperatures between 4 and 16 c as the four studied occurrences are located in caves in dolomitic and or ankeritic host rocks magnesium is considered to be of particular importance for triggering the formation of radiaxial fibrous mg calcite

reconstructing holocene conditions under the mcmurdo ice shelf using antarctic barnacle shells this study evaluates the potential of barnacles for paleoceanographic reconstruction and in particular of the antarctic species bathylasma corolliforme to reconstruct past conditions under ice shelves like other barnacle species this antarctic barnacle secretes a robust low mg calcite shell with distinct growth increments on the external surface indicating growth over a number of years 30 50 in samples studied here the bathylasma samples used in this study grew in the ross sea and became entrained at the grounding line of a coastal ice shelf in mcmurdo sound offering potential as an archive of changing conditions in this difficult to access environment under the mcmurdo ice shelf nine barnacle shells were subsampled at high resolution 60 m for 18o and 13c analysis these samples were dated with 14c and u th techniques although the later did not yield useful age information due to open system behaviour of barnacle calcite measured 18o values indicate that bathylasma calcifies close to equilibrium with ambient seawater one older sample 200 ka has similar 18o and 13c values as the eight holocene samples suggesting that barnacle calcite is not prone to significant diagenesis in this setting apparent isotope equilibrium and lack of diagenesis make barnacles a promising archive for reconstruction of past ocean conditions cycles of 18o observed within each sample sometimes correlate with the external growth ridges but not always and have amplitudes of typically 0 6 this magnitude of variation suggests shell growth during a significant portion of the year although it remains unclear whether growth continues throughout the year however the prominent growth bands suggest at least a signficant seasonal slowing in growth rate variations in barnacle 18o within each plate and between plates are too large to be caused by temperature and instead reflect changes in 18o of the seawater allowing some assessment of seawater salinity under the ice shelf salinities are lowered by addition of meltwaters but the barnacle 18o data suggest values do not go below 33 these data indicate that bathylasma has potential as an archive of paleoclimate information for shallow intermediate water depths and in particular for difficult to reach environments such as below ice shelves of antarctica

decoding nonlinear growth rates in biogenic environmental archives 1 the record of an environmental proxy along a growth axis in biogenic carbonates can reflect changing environmental conditions experienced during the lifetime of the organism when a chronology of the growth axis is not available a method based on anchor points is commonly used to introduce a time grid between these anchor points a constant growth rate is generally assumed despite the fact that growth rates change during life in most organisms here we present a method which refines the constant growth rate assumption at least in situations where the environmental proxy has a periodic component in its signal a nonlinear growth rate can then be estimated enabling the construction of a more realistic time base the anchor point method and the method proposed here are first compared on a synthetic record it is shown that the choice of the anchor points which is always a subjective one is largely influenced by the stochastic noise on the data record this results in biased growth rate profiles and biased time grids next we apply the method on the mg record in the bivalve isognomon ephippium on three samples of the clam saxidomus giganteus from washington state and on the vanuatu coral stable isotope record the method developed is applicable to a wide range of proxy records which are measured along an indirect time grid and are complicated by nonlinear accretion rates furthermore this method which takes variable growth rates into account opens perspectives on quantitative modeling of the relationship between environmental proxies recorded by accretive archives and the instrumental record like temperature or salinity

the combined effects of ocean acidification mixing and respiration on ph and carbonate saturation in an urbanized estuary puget sound is a large estuary complex in the u s pacific northwest that is home to a diverse and economically important ecosystem threatened by anthropogenic impacts associated with climate change urbanization and ocean acidification while ocean acidification has been studied in oceanic waters little is known regarding its status in estuaries anthropogenically acidified coastal waters upwelling along the western north american continental margin can enter puget sound through the strait of juan de fuca in order to study the combined effects of ocean acidification and other natural and anthropogenic processes on puget sound waters we made the first inorganic carbon measurements in this estuary on two survey cruises in february and august of 2008 observed ph and aragonite saturation state values in surface and subsurface waters were substantially lower in parts of puget sound than would be expected from anthropogenic carbon dioxide co2 uptake alone we estimate that ocean acidification can account for 24e49 of the ph decrease in the deep waters of the hood canal sub basin of puget sound relative to estimated pre industrial values the remaining change in ph between when seawater enters the sound and when it reaches this deep basin results from remineralization of organic matter due to natural or anthropogenically stimulated respiration processes within puget sound over time however the relative impact of ocean acidification could increase significantly accounting for 49e82 of the ph decrease in subsurface waters for a doubling of atmospheric co2 these changes may have profound impacts on the puget sound ecosystem over the next several decades these estimates suggest that the role ocean acidification will play in estuaries may be different from the open ocean published by elsevier ltd

assessing elemental ratios as a paleotemperature proxy in the calcite shells of patelloid limpets abstract archaeological shell and fish middens are rich sources of paleoenvironmental proxy data patelloid limpet shells are common constituents in archaeological middens found along european african and south american coastlines paleotemperature reconstructions using oxygen isotope ratios of limpet shells depend on the ability to constrain the oxygen isotope ratio of seawater therefore alternative proxies are necessary for coastal localities where this is not possible the study evaluates whether mg ca sr ca li ca li mg and sr li ratios are reliable proxies of sea surface temperature sst in the calcite layer of shells of the patelloid limpets patella vulgata and nacella deaurata we compare mg ca sr ca li ca li mg and sr li ratios to the seasonal variations in contemporaneous 18oshell values which primarily record seasonal changes in sst elemental ratios mg ca sr ca li ca li mg and sr li show no significant correlations with reconstructed sst in p vulgata and n deaurata shells nor do they show sinusoidal cycles expected from a sst proxy in addition shell 13c values show no significant ontogenetic trends suggesting that these limpets exhibit little change in metabolic carbon incorporation into the shell with increasing ontogenetic age although shell growth rate exhibits a logarithmic decrease with age based on calculated linear extension rates growth rate does not correlate with elemental profiles in these limpets overall elemental ratios are not reliable recorders of paleotemperature in patelloid limpets

effects of seawater carbonate ion concentration and temperature on shell u mg and sr in cultured planktonic foraminifera we investigate the sensitivity of u ca mg ca and sr ca to changes in seawater co3 and temperature in calcite produced by the two planktonic foraminifera species orbulina universa and globige rina bulloides in laboratory culture experiments our results demonstrate that at constant temperature u ca in o universa decreases by 25 7 per 100 mol co3 k g 1 as seawater co3 increases from 110 to 470 mol kg 1 results from g bulloides suggest a similar relationship but u ca is consistently offset by 40 at the same environmental co3 in o universa u ca is insensitive to temperature between 15 c and 25 c applying the o universa relationship to three u ca records from a related species globigerinoides sacculifer we estimate that caribbean and tropical atlantic co3 was 110 70 mol kg 1 and 80 40 mol kg 1 higher respectively during the last glacial period relative to the holocene this result is consistent with estimates of the glacial interglacial change in surface water co3 based on both modeling and on boron isotope ph estimates in settings where the addition of u by diagenetic processes is not a factor down core records of foraminiferal u ca have potential to provide information about changes in the ocean s carbonate concentration below ambient ph ph 8 2 mg ca decreased by 7 5 o universa t o 16 6 g bulloides per 0 1 unit increase in ph above ambient ph the change in mg ca was not significant for either species this result suggests that mg ca based paleotemperature estimates for the quaternary during which surface ocean ph has been at or above modern levels have not been biased by variations in surface water ph sr ca increased linearly by 1 6 0 4 per 0 1 unit increase in ph shell mg ca increased exponentially with temperature in o universa where mg ca 0 85 exp 0 096 t whereas the change in sr ca with temperature was within the reproducibility of replicate measurements copyright 2004 elsevier ltd

calcite fabrics growth mechanisms and environments of formation in speleothems from the italian alps and southwestern ireland five fabrics were identified in alpine and irish caves on the basis of morphological and microstructural characteristics and re lated to growth mechanisms and growth environment columnar and fibrous fabrics grow when speleothems are continuously wet and from fluids at near equilibrium conditions low supersaturation sicc 0 35 through the screw dislocation mechanism the highly defective microcrystalline fabrics form at the same supersaturation range as co lumnar fabric but under variable discharge and the presence of growth inhibitors dendritic fabrics which have the highest density of crystal defects develop in disequilibrium conditions high supersaturation un der periodic very low flow regime periods that result in prolonged out gassing cave calcareous tufa forms in disequilibrium conditions only the calcite crystals of fabrics formed at low supersaturation seem to precipitate near isotopic equilibrium conditions

determination of aragonite trace element distribution coefficients from speleothem calcite aragonite transitions the processes that govern the incorporation of trace elements into speleothems can often be linked to environmental changes although element incorporation into speleothem calcite is now reasonably well understood current knowledge regarding trace element variability in speleothem aragonite is very limited of particular interest is whether trace element distribution coefficients are above or below one in order to assess the extent to which prior aragonite precipitation has affected speleothem aragonite trace element records this study uses nine calcite to aragonite transitions in seven speleothems from diverse environmental settings to derive the first quantitative estimates of the distribution coefficients for several elements in speleothem aragonite dmg ar 9 7e 5 9 01e 5 dba ar 0 91 0 88 dsr ar 1 38 0 53 and du ar 6 26 4 54 1 sd for one speleothem from western germany the distribution coefficients are generally higher which is potentially related to the very low growth rates 11 m year of this sample in particular dsr ar appears to show a negative correlation with growth rate when growth rate is below 20 m year in summary our results demonstrate that speleothem aragonite dmg ar is below one du ar is considerably above one and dsr ar is above one or close to unity for dba ar reaching a similar conclusion is difficult due to the relatively high uncertainty enhanced prior aragonite precipitation will thus result in lower u and higher mg concentrations in speleothem aragonite although in many cases mg in speleothem aragonite is most likely dominated by other processes this result suggests that u concentrations in aragonitic stalagmites could serve as a very effective proxy for palaeo rainfall

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

oxygen and carbon isotopic systematics of aragonite speleothems and water in furong cave chongqing china abstract to understand oxygen and carbon stable isotopic characteristics of aragonite stalagmites and evaluate their applicability to paleoclimate the isotopic compositions of active and fossil aragonite speleothems and water samples from an in situ multi year october 2005 july 2010 monitoring program in furong cave located in chongqing of china have been examined the observations during october 2005 june 2007 show that the meteoric water is well mixed in the overlying 300 500 m bedrock aquifer reflected by relatively constant 18o 0 11 0 14 1 of drip waters in the cave which represents the annual status of rainfall water active cave aragonite speleothems are at oxygen isotopic equilibrium with drip water and their 18o values capture the surface water oxygen isotopic signal aragonite to calcite transformation since the last glaciation is not noticeable in furong stalagmites our multi year field experiment approves that aragonite stalagmite 18o records in this cave are suitable for paleoclimate reconstruction with high u 0 5 7 2 ppm and low th 20 1270 ppt the furong aragonite stalagmites provide very precise chronology as good as 20s yrs 2 of the climatic variations since the last deglaciation the synchroneity of chinese stalagmite 18o records at the transition into the bolling allerod t ba and the younger dryas from furong hulu and dongge caves supports the fidelity of the reconstructed east asian monsoon evolution however the furong record shows that the cold older dryas od occurred at 14 0 thousand years ago agreeing with greenland ice core 18o records but 200 yrs younger than that in the hulu record the od age discrepancy between chinese caves can be attributable to different regionally climatic environmental conditions or chronological uncertainty of stalagmite proxy records which is limited by changes in growth rate and subsampling intervals in absolute dating seasonal dissolved inorganic carbon 13c variations of 2 3 in the drip water and 5 7 in the pool and spring waters are likely attributed to variable degrees of co2 degassing in winter and summer the variable 13c values of active deposits from 11 to 0 could be caused by kinetically mediated co2 degassing processes the complicated nature of pre deposition kinetic isotopic fractionation processes for carbon isotopes in speleothems at furong cave require further study before they can be interpreted in a paleoclimatic or paleoenvironmental context

radiaxial fibrous calcites a new look at an old problem abstract marine radiaxial fibrous and fascicular optic calcites are very common but poorly understood pore filling cements in paleozoic and mesozoic marine neritic and upper bathyal limestones the main diagnostic feature of these cements is their converging or diverging crystal c axis respectively the reasons for this anomalous texture are at present unknown another controversy is due to the relative lack of occurrences in cenozoic strata and their apparent absence in quaternary marine limestones despite these uncertainties marine fibrous cements are thought by some to be amongst the best proxies for the geochemistry of ancient oceans because of the absence of metabolic effects during their precipitation applying electron backscatter diffraction analysis we here for the first time document radiaxial fibrous and subordinate fascicular optic fibrous biominerals from well preserved jurassic and cretaceous low mg calcite belemnite rostra the finding of fibrous biogenic calcites in combination with the recent description of holocene and modern fibrous calcites in stalagmites represents in the view of the authors a significant advance in carbonate research here these findings are placed in their wider processes oriented context and the significance of biogenic and speloan fibrous calcite for their marine counterparts is assessed comparing the physico chemical and organomineralic properties of different precipitation sites indicates that the diagnostic variations in the crystal c axis orientation are not related to a specific nucleation environment or substratum in contrast preliminary crystallographic analyses suggest that kinetic factors during nucleation or subtle gradients in the statistical replacements of ca 2 by mg 2 in the crystal structure may lead to local strain resulting in converging or diverging crystal c axes

from tabular to rhombohedral dolomite crystals in zechstein 2 dolostones from scharzfeld sw harz germany a case study with combined cl and ebsd investigations abstract tabular dolomite crystals found within dolomite rhombs have been investigated by cathodoluminescence cl microscopy and spectroscopy combined with electron backscatter diffraction ebsd for the first time the dolomites formed in the upper permian stassfurt carbonate ca2 at the southern margin of the german polish zechstein basin cathodoluminescence petrography of the dolostone succession revealed that the dolomites developed in four phases electron backscatter diffraction analysis reveals tabular crystal growth during the two first generations while the last two generations are characterized by rhombohedral crystal shapes the tabular dolomite cement crystals and their microcrystalline equivalents in matrix and components have a stoichiometric composition with good to very good lattice ordering manganese and iron contents of the tabular crystals are low and their carbon and oxygen isotope composition confirms an early diagenetic dolomite formation under marine evaporitic conditions from precursor carbonates of upper permian age cl spectroscopy reveals that the tabular dolomite generation 1 has a very high percentage of mn2 on the ca lattice position which results in a visually yellowish green cl emission although relatively increased mn2 contents at the ca lattice position appear to be rather common in evaporitic dolomites the combination of a tabular crystal shape and a preferred input of mn2 at the ca lattice position is a remarkable phenomenon as tabular dolomite crystals so far are exclusively reported from evaporitic diagenetic settings they could be the result of a high mg ca ratio which blocks c axis orientated growth of dolomite crystal the occurrence of well ordered dolomite of which the geochemical zoning can be studied in such detail is rare for the earliest synsedimentary stages of dolomite formation in marine environments because these early stages commonly consist of not or badly ordered ca dolomites a primary geochemical zoning of such dolomite usually gets lost during stabilisation and transformation to better ordering and stoichiometry

and ratios in skeletal calcite of mytilus trossulus covariation with metabolic rate salinity and carbon isotopic composition of seawater minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity although molluscan calcite is secreted at or near oxygen isotope equilibrium the influence of metabolic activity i e vital effects on skeletal srca ratios and 13c values is not well known we present measurements of skeletal chemistry from a consecutive samples milled in chronological order from the organism s final year of growth and b adjacent samples within three separate growth bands of the marine mussel mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell seawater temperature and salinity were monitored for one year at squirrel cove british columbia at the end of the year a young rapidly growing mussel mussel a and an old slowly growing mussel mussel b were harvested from this site growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry results from consecutive samples show 1 a significantly higher average srca ratio in mussel a than that in mussel b and 2 13c values that correlate well with salinity in mussel a but not in mussel b results from time equivalent adjacent samples show 1 higher srca and 13c values near the ventral margin than at time equivalent regions on lateral margins of the shell and 2 little or no variation in 18o values these observations suggest that skeletal chemistry srca and 13c is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell hence the chemistry of calcite secreted at the ventral margin is precipitated in near equilibrium with seawater and allows accurate estimation of seawater srca ratios and carbon isotopic composition in contrast shell precipitation along lateral margins is dominantly controlled by metabolic activity and calcite chemistry is not in equilibrium with ambient seawater in this context we present a biomineralization model where variations in skeletal srca and 13c values are explained in the context of mantle metabolic activity and seawater salinity

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

minor and trace elements in sclerosponge ceratoporella nicholsoni biogenic aragonite near the inorganic endmember abstract the trace and minor element composition have been investigated in the accretionary skeletons of two specimens of the caribbean sclerosponge species ceratoporella nicholsoni one from tongue of the ocean bahamas toto and the other from pear tree bottom jamaica ptb both sclerosponges were analyzed at high resolution using laser ablation icp ms la icp ms and verified using wet chemical methods icp oes the shallow ptb sclerosponge 25 m was analyzed for a short temperature calibrated record of trace and minor elements while the toto sclerosponge 146 m was analyzed for a century long record from subsurface waters the resulting ba ca mg ca pb ca and u ca ratios were compared with sr ca ratios which have been shown to reliably record seasonal scale temperature change in sclerosponges close to equilibrium precipitation of inorganic aragonite sclerosponge aragonite incorporates minor and trace elements according to general mineralogic constraints with less vital effects than other calcium carbonate secreting organisms this is perhaps most obvious in the behavior of mg ca ratios which show an inverse correlation to temperature unlike any other calcite or aragonite organism despite having mg concentrations 1 mmol mol within published estimates of equilibrium aragonite ratios of mg ca and ba ca show a strong positive correlation despite large differences in ionic radii and u ca values 3 15 mol mol are 2 5 higher in sclerosponge aragonite than in aragonite from corals supporting earlier work ratios of u ca show an insignificant correlation with temperature dependent sr ca ratios r 2 0 05 supporting existing theories that temperature signals recorded by u ca ratios can be convolved with ph co 3 2 and phosphate signals differences between sclerosponge aragonite and other calcium carbonate arising from siliceous spicule contamination are unlikely because of the small mole percent of silicon in the aragonite small differences between the two sclerosponges were noticed in pb ca and sr ca because of the average ages of the sampled material and average temperatures of the ambient environment respectively anomalies between la icp ms and icp oes were noticed in measurements of ba ca and pb ca due to standardization differences for la icp ms ba ca and measurements close to the instrument detection limits of pb icp oes

uranium concentration and isotope ratio profiles within mercenaria shells geochronological implications shells of nineteen specimens of mercenaria from unconsolidated sediments of the southeastern usa coast ranging in age from alive to pliocene were each separated into three to six sections parallel to the shell surface the uranium concentration and isotope ratio of each subsample was determined the results confirm those of previous studies that whole fossil shells have about 8 times as much uranium 300 vs 40 ppb and considerably higher 234u 238u ratios than do whole live collected shells however it was also found that uranium concentrations fall very steeply by factors of 300 and 15 in live and fossil shells respectively from the exterior of the shell to about two thirds of the shell thickness between this depth and the interior surface the uranium concentrations of live shells increase by about a factor of 5 while fossils have gradients which vary from moderately positive to moderately negative the most surprising finding of the study is that the interior portions of the shells become closed chemical systems by no more than about 15 ka after their death this is known from the observation that these portions of late pleistocene shells still have oceanic uranium isotope ratios while the exterior portions of the same shells have ratios which reflect an uptake of uranium from continental groundwaters the water the shells have been in contact with for the vast majority of their existence thus the u series ages of the interior portions of mercenaria shells may be valid after a small correction though shells older than about 200 ka seem less promising firm conclusions cannot be drawn regarding them because of the great scatter in the data

annual cycles of u ca in coral skeletons and u ca thermometry abstract we have discovered annual variations in the 238 u 40 ca ratio of porites coral skeletons measurements were made using thermal ionization mass spectrometric techniques yielding precisions of 2 2 for 238 u 40 ca and l 2 for 88 sr 40 ca coralline aragonite subsamples weighed 2 mg enabling submillimeter sampling resolution corresponding to monthly temporal resolution the annual nature of the cycles was confirmed by comparison with annual banding observable in x radiographs for two modern and one fossil sample the amplitude of the u ca variation ranges from 6 to 23 well outside of analytical error as annual u ca cycles appear to be a general feature of primary coralline aragonite the preservation of such features will be important in identifying unaltered coral for u series dating studies u ca variations mimic and are in phase with annual variations in 88 sr 40 ca for a given fractional shift in sr ca the fractional shift in u ca is about 6 times larger for the two modern corals 238 u 40 ca is strongly anticorrelated with measured temperature suggesting that 238 u 40 ca has potential as a paleothermometer if temperature is the only significant control on coralline 238 u 40 ca we reach the following conclusions from analyses of fossil samples a vanuatu sample which grew halfway through the last deglaciation gives u ca temperatures 4 to 5 c below modern values u ca thermometry applied to published data for barbados corals indicates that 1 temperature generally correlates with sea level 2 glacial temperatures stages 2 and 4 were 4 to 6 c lower than interglacial temperatures stages 7a 5e and 1 and 3 temperatures rose from glacial to interglacial values early in the last deglaciation thermometry applied to papua new guinea corals indicates that 1 temperatures were 5 to 6 c lower than interglacial temperatures from 13 to 10 ky bp then rose to present values 9 ky bp 2 the temperature depression between 13 and 10 ky bp is consistent with low temperatures observed in vanuatu during the same time interval and 3 the papua new guinea deglacial temperature history differs from that of barbados the results generally support estimates of tropical temperatures obtained from sr ca thermometry and snow line elevation data but disagree with those based on foram transfer functions a thermodynamic model suggests that coralline 238 u 40 ca may also be sensitive to marine carbonate ion concentration raising the possibility that some of the observed glacial interglacial 238 u 40 ca variation may result from glacial interglacial carbonate ion changes however the key experiments that might establish a coralline 238 u 40 ca carbonate ion relationship have yet to be performed

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

first description of phanerozoic radiaxial fibrous dolomite abstract the petrographic analysis and crystallographic analysis of concretionary carbonate cements coal balls from carboniferous paralic swamp deposits reveal the presence of length fast radiaxial fibrous dolomite rfd a fabric not previously reported from the phanerozoic this finding is of significance as earlier reports of phanerozoic radiaxial fibrous carbonates are exclusively of calcite mineralogy dolomite concretions described here formed beneath marine transgressive intervals within palustrine coal seams this is of significance as seawater was arguably the main source of mg2 ions for dolomite formation here data from optical microscopy cathodoluminescence electron backscattered diffraction x ray diffraction and geochemical analyses are presented to characterize three paragenetic dolomite phases and one calcite phase in these concretions the main focus is on the earliest diagenetic non stoichiometric degree of order 0 41 0 46 phase i characterized by botryoidal dolomite constructed of fibres up to 110 m wide with a systematic undulatory extinction and converging crystal axes petrographic and crystallographic evidence clearly qualifies phase i dolomite as radiaxial fibrous conversely fascicular optical fabrics were not found carbon isotope ratios 13c are depleted between 11 8 and 22 1 as expected for carbonate precipitation from marine pore fluids in organic matter rich paralic sediment oxygen isotope 18o ratios range between 1 3 and 6 0 the earliest diagenetic nature of these cements is documented by the presence of ubiquitous non compacted fossil plant remains encased in phase i dolomite as well as by the complex zoned luminescence patterns in the crystals and is supported by crystallographic and thermodynamic considerations it is argued that organic matter and specifically carboxyl groups reduced thermodynamic barriers for dolomite formation and facilitated mg caco3 precipitation the data shown here reveal a hitherto unknown level of complexity with respect to radiaxial fibrous carbonates and are of importance for those concerned with dolomite and carbonate petrography in general

assessing the reproducibility and reliability of estuarine bivalve shells saxidomus giganteus for sea surface temperature reconstruction implications for paleoclimate studies studies using oxygen isotopes y 18 o of mollusk shells to determine paleotemperature need to assume water y 18 o values which could severely influence calculated temperatures we analyzed aragonitic shells of the butter clam saxidomus giganteus deshayes 1839 to determine the reproducibility of the isotopic signal between individuals and to assess how precisely temperature could be calculated given known salinity and temperature furthermore carbon isotopes are also investigated as an environmental proxy the abundance of well preserved s giganteus shells in archeological and geological deposits in northwestern north america makes them a particularly suitable species for paleoclimate studies seasonally resolved stable oxygen isotope profiles in three s giganteus shells collected from the same site in puget sound washington usa were well correlated 0 77br 2 b0 87 although there were differences up to 0 58x in high resolution y 18 o profiles of the three shells the difference between the average y 18 o of each shell was less than half of this 0 19x and half of what has been reported for between colony coral variability y 13 c profiles on the other hand were more complex with shell y 13 c being about 2 5x lower than expected equilibrium values however this roughly conforms to the idea that about 10 of the shell carbon originates from metabolic co2 both y 18 o and y 13 c indicate that s giganteus do not grow during periods of reduced salinity despite the excellent reproducibility of y 18 o between shells and the fact that salinity effects were duly considered calculated temperature still differed from instrumental temperature applying different salinity y 18 o water relationships to average shell y 18 o and considering salinity from the shell collection site and a nearby offshore station resulted in calculated average water temperatures ranging from 1 7 to 6 4 8c warmer than measured although we could not determine if s giganteus precipitate their shells in isotopic equilibrium we believe that the difficulty in predicting temperature arose from not being able to accurately determine

the reliability of uranium series mollusc dates from the western mediterranean basin abstract fossil mollusc shells from western mediterranean raised beach deposits have been dated using u series techniques sub sampling has allowed intra shell isotopic variability to be assessed although most of the calculated ages appear plausible and some internal consistency exists closer scrutiny of the data reveals that the shells have behaved as geochemically open systems and all the ages therefore must be considered suspect some previously published u series mollusc data from the same region may also contain evidence of open system behaviour and are considered equally unreliable simple modelling suggests considerable variation in isotopic uptake behaviour both within and between shells

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

controls on magnesium manganese strontium and barium concentrations recorded in freshwater mussel shells from ohio abstract freshwater bivalve shells may record inter annual fluctuations in water chemistry which in turn may archive variations in solute load due to pollution or discharge events here using weekly surface water chemistry collected at two locations and shells of lampsilis cardium grown in the surface water we investigate if the mg ca mn ca sr ca and ba ca ratios measured at high resolution in the shells on an intra annual time scale consistently reflect the chemistry of the surface water two relationships appear highly correlated sr cacarb and sr cawater and mn cacarb and temperature ba cacarb correlated strongly with ba cawater in one shell but temperature in another shell yielding inconclusive results mn cacarb and mg cacarb were not strongly correlated with elemental ratios in the water but mn cacarb did show variable weak to moderate correlation with ph we found consistent partition coefficient dme values between individual shell samples grown in the same water with mean dmg 0 0006 dmn 17 dba 0 13 and dsr 0 176 sr cacarb and sr cawater exhibited the strongest most consistent correlation with partitioning coefficients that were consistent with published values suggesting sr cacarb can be used as a proxy for sr cawater in freshwater mussels

strontium magnesium and barium incorporation in aragonitic shells of juvenile arctica islandica insights from temperature controlled experiments abstract in order to constrain spatial and temporal temperatures and environmental conditions in the north atlantic ocean during the holocene high resolution seasonal to annual marine proxies with excellent chronological constraints are needed the long lived ocean quahog arctica islandica which has the potential to provide a precise annually dated record via crossdating techniques is a fairly well developed and tested marine proxy archive in particular oxygen isotopes derived from a islandica shell carbonate have provided a wealth of information on marine climate and ocean circulation dynamics however shell derived oxygen isotopes are influenced by both the isotopic source water signature covarying with salinity and seawater temperature if seawater isotopic signature is not known temperature reconstructions become challenging thus an independent technique to estimate past seawater temperatures is highly desired however based on previous studies on adult and juvenile clams the utility of elemental ratios in a islandica shell material as environmental proxies remains questionable to further evaluate the influence of seawater temperature on elemental and isotopic incorporation during biomineralization a islandica shells were grown at constant temperatures under two regimes during a 16 week period from march 27 to july 21 2011 at the darling marine center in walpole maine individual juvenile clams were stained with calcein and cultured at 10 30 0 30 c for six weeks after this the clams were again stained with calcein and cultured at 15 00 0 40 c for an additional 9 5 weeks average salinity values were 30 20 0 70 and 30 70 0 70 in the first and second phases of the experiment respectively continuous sampling within and across the temperature conditions from 10 30 c to 15 00 c coupled with the calcein markings provided the ability to place each sample into a precise temporal framework and to establish exact average growth rates for the shells sampled after accounting for changes in the isotopic composition of seawater oxygen isotopes from one sampled shell effectively recorded seawater temperatures during the study and also gave confidence to the temporal fit of the data elemental ratios sr ca mg ca ba ca from five aragonitic shells were determined via laser ablation inductively coupled plasma mass spectrometry sr ca and mg ca data showed little coherence with temperature during the culture experiment including the rapid 5 c increase in seawater temperature however ba ca ratios showed an inverse relationship with seawater temperatures although this relationship was noisy additionally salinity interactions were present during the 15 00 c treatment further highlighting complex incorporation of elements during biomineralization incorporation of sr mg and ba were strongly and variably impacted by growth rates combined the results from these culture experiments demonstrate that sr ca mg ca ba ca ratios in juvenile a islandica shell material are dominated by physiological processes and thus not reliable as environmental proxies

time series δ26mg analysis in speleothem calcite kinetic versus equilibrium fractionation comparison with other proxies and implications for palaeoclimate research abstract magnesium isotope time series mc icp ms analyses from a pleistocene speleothem collected in a limestone cave in nw africa morocco are reported and discussed in a process oriented context in addition high resolution c o and sr isotope data and mg and sr element abundances were compiled from the same stalagmite sub samples were collected along the stalagmite growth axis and a second data set was drilled perpendicularly within one growth interval hendy test the analytical results show clearly co variant systematic and cyclical fluctuations for all proxies collected along the growth axis and with respect to the analytical error invariant data within one growth increment magnesium isotope ratios 26 mg fluctuate between 4 39 0 02 2 and 4 17 0 05 2 and are within the range of published results of speleothem calcite from limestone caves the difference of 0 22 is significantly beyond the error of the external reproducibility of 0 03 2 for 26 mg considering the analytical error neither a purely kinetic nor an equilibrium fractionation process explains the observed isotope pattern for the time being it is suggested that two external factors drive the speleothem mg isotope cyclicity 1 climate driven arid versus humid variances in the precipitation rate of a carbonate phase from meteoric water within the karstic system prior to entering the cave system and 2 changing rates of silicate aeolian material versus carbonate weathering both of these processes fractionate the mg isotopic composition of runoff seepage water there is evidence that the magnesium isotope system applied to speleothem archives bears significant information concerning continental climatic variability in arid zones and deserves further research

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

climate and cave control on pleistocene holocene calcite to aragonite transitions in speleothems from morocco elemental and isotopic evidence the occurrence of aragonite in speleothems has commonly been related to high dripwater mg ca ratios because mg is known to be a growth inhibitor for calcite laboratory aragonite precipitation experiments however suggested a more complex array of controlling factors here we present data from pleistocene to holocene speleothems collected from both a dolostone and a limestone cave in northern morocco these stalagmites exhibit both lateral and stratigraphic calcite to aragonite transitions aragonite fabrics are well preserved and represent primary features in order to shed light on the factors that control alternating calcite and aragonite precipitation elemental mg sr ba u p y pb al ti and th abundances were measured using la icp ms and analysed with principal component analysis samples were analyzed at 100 200 m resolution across stratigraphic and lateral transitions carbon and oxygen isotope ratios were analysed at 100 m resolution covering stratigraphic calcite to aragonite transitions results show that the precipitation of aragonite was driven by a decrease in effective rainfall which enhanced prior calcite precipitation different geochemical patterns are observed between calcite and aragonite when comparing data from the grotte de piste and grotte prison de chien this may be explained by the increased dripwater mg ca ratio and enhanced prior aragonite precipitation in the dolostone cave versus lower dripwater mg ca ratio and prior calcite precipitation in the limestone cave a full understanding for the presence of lateral calcite to aragonite transitions is not reached trace elemental analysis however does suggest that different crystallographic parameters ionic radius amount of crystal defect sites adsorption potential may have a direct effect on the incorporation of sr mg ba al ti th and possibly y and p

partitioning of mg sr ba and u into a subaqueous calcite speleothem abstract the trace element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site e g mg sr ba and u an important step in interpreting trace element abundances is understanding the underlying processes of their incorporation this includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients where the partitioning d of element x dx is the molar ratio x ca in the calcite divided by the molar ratio x ca in the parent water and evaluating the degree of spatial variability across time constant speleothem layers previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings or have used synthetic solutions under cave analogue laboratory conditions to produce similar dripstones however dripstones are not the only speleothem types capable of yielding useful palaeoclimate information in this study we investigate the incorporation of mg sr ba and u into a subaqueous calcite speleothem cd3 growing in a natural cave pool in italy pool water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting enabling the calculation of precise solution x ca we determine the trace element variability of modern subaqueous calcite from a drill core taken through cd3 to derive dmg dsr dba and du then compare these with published cave cave analogue and seawater analogue studies the dmg for cd3 is anomalously high 0 042 0 002 compared to previous estimates at similar temperatures 8 c the dsr 0 100 0 007 is similar to previously reported values but data from this study as well as those from tremaine and froelich 2013 and day and henderson 2013 suggest that na sr might play an important role in sr incorporation through the potential for na to outcompete sr for calcite non lattice sites dba in cd3 0 086 0 008 is similar to values derived by day and henderson 2013 under cave analogue conditions whilst du 0 013 0 002 is almost an order of magnitude lower possibly due to the unusually slow speleothem growth rates

isotopic evolution of the mantle a model we investigate a model of mantle evolution which acknowledges the importances in early earth history of large degrees of partial melting and crystal fractionation rather than as in recent models small degrees of partial melting operating on primary mantle the major source regions are in the upper mantle and evolve from a primitive mantle melt by crystal fractionation and the formation of chemically distinct cumulate layers the rb sr and sm nd ratios in melts are relatively unfractionated in processes involving large 15 degrees of partial melting the ratios are even less fractionated in orthocumulates forming from these melts small degrees of partial melting or removal of residual fluids after nearly complete crystalization of cumulate layers are the processes that lead to significant fractionation the importance of these processes increases as the upper mantle cools the observation of nearly primitive isotopic ratios does not necessarily imply the presence of ancient primitive undifferential reservoirs the transfer of intercumulus fluids from a crystalizing cumulate layer gives time dependent rb sr and sm nd ratios in both the donor and receiving reservoirs since the late stage fluids become more enriched as crystalization proceeds enrichment in such a system is progressive with time time integrated nd sm and rb sr ratios therefore need not depart much from primitive ratios even though current values for lile concentration rb sr nd sm etc may be large the time constant for lile enrichment appears to be 2 gyr about the same as required for the u pb system and the same as the characteristics time associated with mantle cooling the mantle reservoirs may be much more ancient than generally assumed

an evaluation of mg ca sr ca and ba ca ratios as environmental proxies in aragonite bivalve shells the influence of salinity and water chemistry on mg ca sr ca and ba ca ratios in the aragonitic shells of the manila clam was investigated clams were reared at constant temperature 20 c under different controlled conditions of salinity commonly encountered in their natural habitat clams were held in three tanks with a constant salinity of 35 for the first 35days and then two tanks were changed to lower salinities 20 and 28 for the next 29days individual shell mg ca sr ca and ba ca ratios were studied through time despite stable conditions temperature salinity and mg cawater for clams reared at salinity 35 during the experiment mg ca shell ratios increased through the time moreover the salinity decrease at t35 slowed the increase of mg ca shell ratios at salinity 28 and resulted in an mg cashell decrease at salinity 20 despite similar mg cawater ratios in the different salinity treatments microprobe analyses illustrate that mg varies along contemporaneous growth lines the variable shell mg ca ratios suggest that incorporation of magnesium into shell carbonate is strongly regulated by the organism and not by environmental conditions interestingly microprobe analyses illustrated that mg was not associated with shell sulfur as other studies have suggested sr ca shell ratios of clams reared at salinity 35 and under constant conditions were also not as constant as expected if sr ca ratios were an environmental proxy there was an inverse correlation between shell sr ca and salinity despite a slight positive correlation between salinity and sr ca ratios of the water indicating that sr ca ratios do not reflect environmental conditions a strong inverse correlation between salinity and ba ca shell ratios and a positive correlation between ba ca shell and ba ca water was observed therefore ba ca shell ratios seem to be a promising proxy of high resolution 1day salinity variations in estuarine waters via the relationship between ba ca water and salinity this study clearly illustrates that both mg ca and sr ca ratios in aragonite shells are not under environmental control and that ba ca ratios are with the later tracking high resolution water ba ca ratios and hence estuarine salinity variations

unraveling environmental histories from skeletal diaries advances in sclerochronology high resolution proxy archives from aquatic settings are essential to better understand processes and mechanisms of global change during the last decade it has become increasingly evident that calcified tissues of bivalve mollusks and cold water corals in particular can significantly increase our knowledge of seasonal to multi decadal paleoclimate and paleoenvironmental variability in the extratropical oceans and coastal marine settings daily tidal fortnightly and annual growth patterns of periodically formed skeletal hard parts provide a means to place the proxy record in a precise temporal context their extreme longevity coupled with the running similarity between growth increment time series of contemporaneous bivalves opens the possibility of constructing stacked chronologies covering centuries to millennia which provide information on decadal climate and environmental variability and magnitudes of climate variability through time a current major research focus is on how to translate geochemical properties of the skeletons into quantifiable environmental proxy data and how to eliminate the adverse influence of vital effects on the proxy records this special issue presents the most recent advances in the field of sclerochronology specifically calcified tissues of bivalve mollusks cold water corals and tropical shallow water corals the majority of the contributions deal with the potential reconstruction of temperature ph and productivity through the analysis of variable growth rates sr ca mg ca ba ca u ca li ca 13c 18o and 44 40ca four papers present composite chronologies of two long lived bivalve species and discuss inherent decadal climate variability and one study demonstrates that sclerochronology can shed light on socioeconomic patterns of past human populations as well as resource use and resource management by people

influence of organic matrix on the post mortem destruction of molluscan shells to examine the role of organic constituents in the destruction of calcium carbonate skeletons we aged fresh shells of the bivalves nucula sulcata organic rich nacreous aragonites with low crystallite surface areas and cerastoderma edule organic poor porcellaneous aragonites high crystallite surface areas under both sterile and non sterile microbial conditions in aragonite undersaturated saturated and supersaturated seawaters for periods up to 11 months deterioration was tracked by sem and weight loss and compared to damage produced by reagents of specific effect the same qualitative sequence of damage was observed in all tanks for both species but rates of deterioration were geq 2 times higher in microbial than in sterile tanks at a given saturation state and were as high or higher in the microbial saturated and supersaturated tanks than in the sterile undersaturated tank damage to shell surfaces was limited almost entirely to loss of organic matrix which eventually exposed and

strong kinetic effects on sr ca ratios in the calcitic bivalve pecten maximus although sr ca ratios in abiogenic calcite are strongly controlled by precipitation rates such a kinetic effect has never been demonstrated in calcitic bivalve shells therefore we report sr ca ratios together with daily growth rates in the calcitic shells of four individuals of the bivalve pecten maximus age class i ratios of sr ca were found to be variable among individuals that grew at the same location illustrating that vital effects dominate over environmental controls although daily growth rate was correlated with shell sr ca ratios it explained only half of the sr ca variations however daily shell surface area increment an estimation of the total quantity of carbonate precipitated for a given time explained 74 of the sr ca variability in the shells of p maximus this proves for the first time in a calcitic bivalve that shell sr ca partitioning is mainly controlled by kinetic effects the sr ca ratio should therefore be tested as a potential proxy of calcification rate in modern or fossil calcitic biocarbonates

tonalites in crustal evolution tonalites including trondhjemite as a variety played three roles through geological time in the generation of earth s crust before about 2 9 ga ago they were produced largely by simple partial melting of metabasalt to give the dominant part of archaean grey gneiss terranes these terranes are notably bimodal andesitic rocks are rare tonalites played a crucial role in the generation of this protocontinental and oldest crust 3 7 2 9 ga ago in that they were the only low density high sio 2 rocks produced directly from basaltic crust in the enormous event giving the greenstone granite terranes mostly 2 8 2 6 ga ago tonalites formed in lesser but still important proportions by partial melting of metabasalt in the lower regions of down buckled greenstone belts and by remobilization of older grey gneisses tectonism in the archaean 3 9 2 5 ga ago perhaps was controlled by small cell convection mckenzie weiss 1975 little or no ophiolite or eclogite formed and only minor andesite plate tectonics of modern type involving large rigid plates commenced in the early proterozoic uniformitarianism thus goes back one half of the age of the earth tonalites compose about 5 10 of crust generated in proterozoic and phanerozoic time at convergent oceanic continental margins they occur here as minor to prominent members of the compositionally continuous continental margin batholiths a simple model of generation of these batholiths is offered mantle derived mafic magma pools in the lower crust above a subduction zone reacts with and incorporates wall rock components bowen 1922 and breaches its roof rocks as an initial diapir this mantle magma also develops a gradient of partial melting in its wall rocks this wall rock melt accretes in the collapsed chamber and moves up the conduit broached by the initial diapir the higher less siliceous fractions of melting first the lower more siliceous and further removed fractions of melting last the process gives in the optimum case a mafic to siliceous sequence of diorite or quartz diorite through tonalite or quartz monzodiorite to granodiorite and granite the model implies that great masses of cumulate phases and refractory wall rock form the roots of continentalmargin batholiths and that migmatites overlie that residuum and underlie the batholiths

archean mantle fractionation nd isotopic data require that most archean igneous rocks are derived from depleted or undepleted mantle sources while geochemical model stuides necessitate enriched sources for most archean tonalites this apparent contradiction can be reconciled if the enriched sources are produced by short lived 150 m y enrichment of depleted sources extensive depleted mantle reservoirs were probably produced during early degassing of the upper mantle 4 4 b y ago and complementary enriched reservoirs collected at the base of the lithosphere sources for tonalites may be produced from short lived enrichment of basalt above convective downcurrents by metasomatic and magmatic components derived chiefly from depleted mantle and by degassing of sinking short lived komatiitic lithosphere

strontium 86 labeling experiments show spatially heterogeneous skeletal formation in the scleractinian coral porites porites this paper presents the results of an effort to label calcium carbonates formed by marine organisms with stable isotopes to obtain information about the biomineralization processes the growing skeleton of the scleractinian coral porites porites was labeled three times with enhanced abundances of sr 86 the distribution of sr 86 in the skeleton was imaged with the nanosims ion microprobe with a spatial resolution of similar to 200 nm and combined with images of the skeletal ultra structure importantly the distribution of the sr 86 label in the p porites skeleton was found to be strongly heterogeneous this constrains the physical dimensions of the hypothetical extracellular calcifying fluid ecf reservoir at the surface of the growing skeleton which is implicit in most geochemical models for coral biomineralization these new experimental capabilities allow for a much more detailed view of the growth dynamics for a wide range of marine organisms that biomineralize carbonate structures citation houlbreque f a meibom j p cuif j stolarski y marrocchi c ferrier pages i domart coulon and r b dunbar 2009 strontium 86 labeling experiments show spatially heterogeneous skeletal formation in the scleractinian coral porites porites geophys res lett 36 l04604 doi 10 1029 2008gl036782

link between archaean continent formation and anomalous sub continental mantle the petrology and geochemistry of the subcontrnental mantle from 70 to 160 km depth under archaean cratons as sampled by kimberlites differs from the mantle sources of mid ocean ridge basalts morb and hotspot volcanics here i propose that both the formation of this anomalous mantle and the origin of large masses of tonalitic granodioritic crust in the early archaean can be explained by a single open system fractional crystallization model if an extensive partially molten layer existed at shallow levels in the pre archaean due to the production of large amounts of morb type magma above sites of mantle upwelling the magma at loci of fractional crystallization would reach a steady state in major and trace element compositions which would be similar to that of archaean granodiorites and tonalites while this fractional crystallization process operated the cumulate would chemically resemble a hydrated tholeiite pressure induced breakdown of amphibole could cause partial re melting of this cumulate and this could lead to depletion resulting in a sub continental mantle keel with chemical characteristics similar to those observed

petrology and geochemistry of annually laminated stalagmites from an alpine cave obir austria seasonal cave physiology calcite stalagmites from obir an alpine 1100 m altitude perennially wet cave were studied using optical and electron backscatter diffraction petrology bulk icp ms analysis and microanalysis by ion microprobe and micro x ray fluorescence using synchrotron radiation drip water penetrates 70 m through triassic limestones with some pb zn mineralization to the chamber saulenhalle where the stalagmites were collected and with which we compared drip water compositions however the cave is also characterized by strong seasonal ventilation leading to low pco2 in winter all samples display autumnal event lamination defined by a narrow optically visible zone with trace element enrichments within which synchrotron studies have resolved micron scale enrichments of pb and zn crystallite scale 10 m lateral trace element variations reflect alternate smooth faces and rough crystal edges induced by high zn content seasonal falls in sulphate s and increase in 13c can be associated with the winter ventilation the elemental covariations support the transport of pb zn p f br and i by organic colloids but final depositions from aerosols rather than dripwater should be considered the chemical variations are powerful expressions of seasonal cave physiology but the study points to important gaps in our understanding of colloidal element transport in ventilated caves

selective blackening of bioclasts via mixing zone aragonite neomorphism in late triassic limestone zlatibor mountains serbia a peculiar facies of the norian rhaetian dachstein type platform carbonates which contains large amounts of blackened bioclasts and dissolutional cavities filled by cements and internal sediments occurs in the zlatibor mountains serbia microfacies investigations revealed that the blackened bioclasts are predominantly solenoporaceae with a finely crystalline originally aragonite skeleton of fine cellular structure blackening of other bioclasts also occurs subordinately solenoporacean dominated reefs developed behind the platform margin patch reef tract were the main source of sand sized detritus the blackened and other non blackened bioclasts are incorporated in automicrite cement radiaxial fibrous calcite cements in the dissolutional cavities are also black dark grey or white reworked black pebbles were reported from many occurrences of peritidal deposits in those cases the blackening took place under pedogenic meteoric diagenetic conditions in contrast in the inner platform deposits of the ilidz ya limestone the blackening of bioclasts occurred in a marine meteoric mixing zone as indicated by petrographic features and geochemical data of the skeleton replacing calcite crystals attributes of mixingzone pore waters were controlled by mixing corrosion different solubility of carbonate minerals and microbial decomposition of organic matter in the moderate energy inner platform environment large amounts of microbial organic tissue were accumulated and subsequently decomposed triggering selective blackening in the course of early shallow burial diagenesis the d 18 o and d 13 c values of the mixing zone precipitates and replacive calcite do not produce a linear mixing trend variation mainly resulted from microbial decomposition of organic matter that occurred under mixing zone conditions the paragenetic sequence implies cyclic diagenetic conditions that were determined by marine meteoric and mixing zone pore fluids the diagenetic cycles were controlled by sea level fluctuations of moderate amplitude under a semi arid to semi humid climate

seawater nutrient and carbonate ion concentrations recorded as p ca ba ca and u ca in the deep sea coral desmophyllum dianthus as paleoceanographic archives deep sea coral skeletons offer the potential for high temporal resolution and precise absolute dating but have not been fully investigated for geochemical reconstructions of past ocean conditions here we assess the utility of skeletal p ca ba ca and u ca in the deep sea coral d dianthus as proxies of dissolved phosphate remineralized at shallow depths dissolved barium trace element with silicate type distribution and carbonate ion concentrations respectively measurements of these proxies in globally distributed d dianthus specimens show clear dependence on corresponding seawater properties linear regression fits of mean coral element ca ratios against seawater properties yield the equations p cacoral lmol mol 0 6 0 1 p casw lmol mol 23 18 r 2 0 6 n 16 and ba cacoral lmol mol 1 4 0 3 ba casw lmol mol 0 2 r 2 0 6 n 17 no significant relationship is observed between the residuals of each regression and seawater temperature salinity pressure ph or carbonate ion concentrations suggesting that these variables were not significant secondary dependencies of these proxies four d dianthus specimens growing at locations withoarag 6 0 6 displayed markedly depleted p ca compared to the regression based on the remaining samples a behavior attributed to an undersaturation effect these corals were excluded from the calibration coral u ca correlates with seawater carbonate ion u cacoral lmol mol 0 016 0 003 co 2 3 lmol kg 3 2 0 3 r 2 0 6 n 17 the residuals of the u ca calibration are not significantly related to temperature salinity or pressure scatter about the linear calibration lines is attributed to imperfect spatialtemporal matches between the selected globally distributed specimens and available water column chemical data and potentially to unresolved additional effects the uncertainties of these initial proxy calibration regressions predict that dissolved phosphate could be reconstructed to 0 4 lmol kg for 1 3 1 9 lmol kg phosphate and dissolved ba to 19 nmol kg for 41 82 nmol kg basw carbonate ion concentration derived from u ca has an uncertainty of 31lmol kg for 60 120 lmol kg co 2 3

high resolution synchrotron x ray fluorescence investigation of calcite coralloid speleothems elemental incorporation and their potential as environmental archives synchrotron high resolution and micro x ray fluorescence elemental mapping of two coeval coralloid speleothems from lamalunga cave italy are complemented with petrographic morphological and microstratigraphic studies the importance of these speleothems relies on their direct and indirect association with a complete neanderthal skeleton altamura man found inside the cave the coralloids grew discontinuously between 64 6 ka and the holocene and reveal exceptionally high concentrations of mg sr and si particularly on convex surfaces where evaporation is more intense the incorporation of trace elements depends on several factors including location shape and geometrical evolution during their growth as well as climate and environmental parameters this resulted in calcite precipitation with sr compositions from 100 to 1200 ppm and an average concentration of 7000 ppm mg an unusually high si content up to 16 is possibly derived from volcanic ash transported as particulate and in solution inside the cave the most common fabrics observed consist of non fluorescent elongated columnar calcite forming clean isopachous bands and fluorescent fibre like crystals associated with laminated lenticular bands high in sr mg and si variability in sr mg and si concentrations appears to induce fabric changes in the coralloids elongation and lattice distortion of the crystals was found to coincide with high mg concentrations the transition from compact elongated to open to fibre like is here interpreted as due to high concentrations of si and sr which are preferentially incorporated in the speleothem at crystal boundaries and intra laminae it is here inferred that coralloid fabric changes and their elemental content potentially record local rainfall variations through time with the clean compact calcite marking high infiltration and open fibre like and micrite fabrics recording dry periods

comparative geochemistry of cadmium rhenium uranium and molybdenum in continental margin sediments the concentrations of authigenic phases of cd re u and mo increase with depth in four 45 cm long sediment box cores collected along the axis of the laurentian trough gulf of st lawrence average authigenic accumulation rates estimated from element inventories are similar to rates in other continental margin environments strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of cd and re this suggests that slow precipitation kinetics controls the accumulation of cd and re in these sediments the accumulation rate of authigenic u is more variable it may be tied to the kinetics of microbially mediated u reduction and be controlled by the availability of reactive organic matter authigenic mo is distinguished by a sharp subsurface concentration minimum above which mo cycles with manganese mo released to pore water upon reduction of mn oxides diffuses downward and enriches the subsurface sediment mo accumulates most rapidly in the sediment with the highest sulfide content slow conversion of molybdate to thiomolybdate may explain the much slower mo accumulation rate in the less sulfidic sediments a component of authigenic mo accumulates with pyrite in an approximately constant mo fe ratio the accumulation rate of pyrite and associated mo is insensitive to avs abundance pyrite formation may be limited by the reactivity of iron oxide minerals

stromatolites at 3 500 myr and a greenstone granite unconformity in the zimbabwean archaean two controversial areas of geological endeavour are the establishment of the antiquity of life and the tectonic setting of greenstone sequences we record here the recent discoveries in the fort victoria greenstone belt of stromatolites in limestones assigned to 3 500 myr minimum age sebakwian group rocks of the rhodesian archaean craton within zimbabwe and a nearby outcrop of a thin sedimentary formation basal to a thick 2 700 myr volcanic pile resting with definite unconformity on 3 500 myr mushandike granite

an open system fractional crystallization model for very early continental crust formation abstract a model is discussed which seeks to explain the origin of archaean tonalitic trondhjemitic granodioritic ttg crust by an open system fractional crystallization mechanism following the assumption that in the pre and early archaean the melt which formed above sites of large scale mantle upwelling was too voluminous and too hot to solidify an almost global near surface partially molten zone could form the magma in this layer would converge above mantle sinks and undergo fractional crystallization recharged fractionating reservoirs would form at sites of convergence the major element composition of the magma in such reservoirs would evolve towards one resembling that of ttg rocks provided that amphibole was a significant fractionating phase from such reservoirs their incompatible as well as compatible trace element abundances would also become similar to those of ttg terrains it is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs it is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes so that model ages cannot be greater than the age of their consolidation consideration of the main aspects of greenstone belts and archaean high grade terrains indicates no contradictions with the model the intrusion of late archaean adamellites and granites is interpreted as an indirect effect of pressure induced breakdown of amphibole in the cumulate underlying the ttg crust which in turn resulted from crustal shortening cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere

geochemical gradients within modern and fossil shells of concholepas concholepas from northern chile an insight into u th systematics and diagenetic authigenic isotopic imprints in mollusk shells abstract seriate geochemical measurements through shells of one modern one holocene and two sangamonian concholepas concholepas from marine terraces of northern chile were performed to document diagenetic vs authigenic geochemical signatures and to better interpret u series ages on such material subsamples were recovered by drilling from the outer calcitic layer to the inner aragonitic layer of each of the studied shells unfortunately this sampling procedure induces artifacts notably the convertion of up to 20 of calcite into aragonite and of up to 6 of aragonite into calcite as well as in the epimerization of a few percent of isoleucine into d alloisoleucine l isoleucine negligible sampling artifacts were noticed for stable isotope and total amino acid contents diagenetic effects on the geochemical properties of the shells are particularly pronounced in the inner aragonitic layer and more discrete in the outer calcitic layer the time dependent decay of the organic matrix of the shell is illustrated by a one order of magnitude lower total amino acid content in the sangamonian specimens by comparison with the modern shell conversely the sangamonian shells u contents increase by a similar factor and 13c 18o enrichments as high as 2 to 3 seem also to occur through the same time interval possibly due to partial replacement of aragonite by gypsum the decay of the organic matrix of the aragonitic layer of the shell is thought to play a major role with respect to u uptake processes and stable isotope shifts nevertheless asymptotic 230th ages 100 ka in the inner u rich layers of the sangamonian shells and 234u 238u ratios compatible with a marine origin for u suggest u uptake within a short diagenetic interval when marine waters were still bathing the embedding sediment thus u series ages on fossil mollusks from such a hyper arid environment should not differ much from the age of the corresponding marine unit deposition however the diagenetic enrichments in stable isotopes raise concerns about their use for paleoenvironmental reconstructions under such climate conditions

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

radiaxial calcite in lower miocene strata subsurface enewetak atoll abstract radiaxial calcite is abundant in lower miocene strata 377 819 m deep on enewetak atoll these radiaxial crystals have equant bladed and fibrous morphologies and occur as isopachous crusts 0 05 2 0 mm thick radiaxial terminations consist of fibrous subcrystals bundled together into larger acute obtuse or rounded shapes enewetak radiaxial calcite ranges from clear to very inclusion rich most inclusions are microcavities caused by organic infestations videtich 1985 alternating bands of inclusion rich and inclusion poor calcite mimic existing termination patterns radiaxial calcites fill intergranular pores sponge borings fractures and aragonite molds aragonite has been pervasively dissolved from the lower miocene strata containing radiaxial calcite banded radiaxial calcit can be traced from intergranular pores into aragonite molds radiaxial calcites have a mean magnesium concentration of 3 2 mole mgco3 with a range from 1 6 to 11 1 mole mgco3 inclusion rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations enewetak radiaxial calcite has 13c values of 1 3 to 2 5 pdb and 18o values of 1 8 to 0 4 pdb petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition probably in a burial environment dissolution of aragonite by deep marine water is suggested by 1 abundant marine cement associated with aragonite dissolution 2 a lack of meteoric cements associated with aragonite dissolution and 3 no evidence of subaerial exposure in lower miocene strata between 400 and 819 m modern pacific ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed 375 850 m tidal fluctuations and temperature profiles indicate that lower miocene strata containing radiaxial calcite are in open communication with modern pacific ocean water therefore aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater undersaturated with respect to aragonite circulating through the atoll variations in magnesium concentration in enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater the first alternative is preferred precipitation of radiaxial calcite at different rates at different degrees of saturation could cause variable magnesium concentrations more deeply buried radiaxial calcite may have precipitated more slowly in deeper less supersaturated water resulting in lower magnesium concentrations likewise slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion rich bands enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric 1 precipitation at fluctuati g rates and 2 formation in waters undersaturated with respect to aragonite