stable isotopic investigation of clay minerals and pedogenesis in an interfluve paleosol from the cenomanian dunvegan formation n e british columbia canada abstract mineralogical and stable hydrogen and oxygen isotopic compositions have been obtained for clay minerals comprising an interfluve paleosol clayhurst ch 2 from the early to middle cenomanian dunvegan formation british columbia canada this paleosol provides a special opportunity to determine the stable isotopic composition of early late cretaceous paleoprecipitation on the western coast of the western interior seaway at a very northerly paleolatitude 65 detrital kaolinite and illite and complex dioctahedral mixed layer clay minerals of pedogenic origin are abundant throughout the paleosol in the d values of 93 to 81 and 18 o values of 12 9 to 11 6 were generally obtained for the ancient soil water and hence paleoprecipitation for one thin interval near the top of the paleosol higher values are indicated for the ancient soil water these results may indicate processes arising from a westward transgression of the western interior seaway

cycling of iron manganese silica phosphorus calcium and potassium in two stratified basins of shagawa lake minnesota geochemical studies of fe mn si p ca and k were made in two basins of shagawa lake minnesota during summers 1977 1979 within the metalimnion upward and laterally diffusing fe ii oxidizes and co precipitates a large fraction of soluble p nevertheless the soluble fep ratio decreases while progressing upward through the oxidation zone the mean paniculate fesi ratio is 3 0 0 5 in the thermocline but because of the stoichiometric excess of dissolved si over fe and preferential associations between fe and p the oxidation of fe has a negligible effect on the dissolved sio2 profiles the lake sediments are net sources of ca k and p during summer stratification these elements are transported upward through the thermocline and exported from the lake despite the very active sediment release of dissolved sio2 and upward transport through the thermocline the lake acts as a net sink for biogenic opal in summer the prominent soluble mn maxima in the thermocline of the east basin result from antecedent sediment release mn2 and vertical transport primarily in the west basin followed by dispersion and oxidation in the epilimnion and finally downward settling and dissolution within the east basin thermocline rhodochrosite solubility does not influence mn2 profile development very large percentage increases in epilimnetic fe mn si and p occur simultaneously during episodes of wind activated entrainment of metalimnion water these increases are in stoichiometric agreement with the entrained water mass provided account is also taken of the preformed slurries overlying the metalimnetic shelf sediments in mid summer the bathylimnetic soluble fep ratio remains low since the cultural inputs of p were terminated in early 1973 this geochemical stability is explained by the large reserves of potentially mobile p in the upper post cultural sediments phosphorus release from these sediments and transport phenomena account for the persistent eutrophic condition of the lake during mid late summer despite the 80 reduction in external p loading

chemical and isotopic investigations into the origin of clay minerals from the galapagos hydrothermal mounds field abstract a dark green authigenic nontronite is the major component of the galapagos hydrothermal mounds field sediments oxygen isotopic compositions of the chemically purified in situ mounds temperatures of up to 15 c assuming an authigenic origin the fe rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27 to 39 c compared with measured in situ temperatures of ca 3 5 to 6 5 c the higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the holocene pleistocene the apparent high isotopic temperature of the fe rich montmorillonite may reflect 1 formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents 2 a detrital origin of the mineral or 3 a mixture of authigenic fe montmorillonite and detrital al montmorillonite in this region

oxygen isotopic composition of ferric oxides from recent soil hydrologic and marine environments low temperature synthesis experiments on ferric oxide water systems have resulted in disparate oxygen isotope fractionation temperature t curves in this study recent ferric oxides mostly goethites of holocene age were collected and analyzed from a variety of modern soil stream and marine environments where formation temperature and the oxygen isotopic composition 18o of the water from which ferric oxides precipitated can be independently measured or estimated this allows comparison of experimental t relationships with data from natural systems selective dissolution methods were refined for the pretreatment of fine grained minerals in order to obtain reliable 18o values for pure and crystalline ferric oxides the difference 18o between the 18o value of goethite and that of local mean meteoric water ranges from 1 5 to 6 3 for soil goethites from new jersey indiana michigan iowa south dakota and taiwan we argue that these variations are largely the result of differences between the 18o of formation water and that of local mean meteoric water induced probably by 18o enrichment of soil waters by evaporation or other processes in soil horizons where ferric oxides are forming a marine goethite sample from scotland and a subaqueous bog iron sample from new jersey which can not be biased by evaporative processes provide crucial natural evidence that the difference in 18o between goethite and formation water is 1 5 at 10 c this result is consistent with our prior laboratory synthesis results bao and koch 1999 but in conflict with other experimental calibrations given the highly variable 18o value of soil or other surface water as well as the potential of initially formed ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases an understanding of formational and diagenetic conditions is absolutely essential when attempting to use the oxygen isotope composition of ferric oxides as a paleoclimatic proxy

environmental memory and a possible seasonal bias in the stable isotope composition of u th he dated goethite from the canadian arctic abstract goethite ax 2 from axel heiberg island 80 n on the margin of the arctic ocean is the dominant mineral in a sample of petrified eocene wood but u th and he measurements suggest that the goethite feooh crystallized in the latest miocene pliocene ca 5 5 to 2 8 ma measured d and 18 o values of ax 2 are 221 6 and 9 6 0 5 respectively the inferred d and 18 o values of the ancient water were about 139 and 18 6 respectively with a calculated temperature of crystallization of 3 5 c which compares with the modern summer j j a temperature of 3 c and contrasts with a modern mat of 19 c published results from various biological proxies on nearby ellesmere island indicate a pliocene 4 ma mat of either 6 or 0 4 c and corresponding seasonal amplitudes of about 18 or 13 c a conductive heat flow model suggests that a temperature of 3 c could represent goethite crystallization at depths of 100 200 cm for mat 6 c or 250 450 cm for mat 0 4 c over seasonally restricted intervals of time the 18 o value of the ax 2 water 18 6 is more positive than the modern j j a precipitation 22 in combination the paleotemperatures and 18 o values of ancient waters from ax 2 and published results from three eocene or pliocene proxy sites on axel heiberg and ellesmere islands are consistent with a warm season bias in those isotopic proxies the results are also consistent with higher proportions of j j a precipitation in the annual total if so this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates and suggests that the arctic hydrologic cycle as expressed in the seasonal distribution and isotopic composition of precipitation perhaps modified by a warmer arctic ocean differed from modern the 13 c value of the fe co 3 oh component in the ax 2 goethite is 6 6 which is much more positive than expected if crystallizing goethite incorporated co 2 derived primarily from oxidation of relict eocene wood with 13 c values of about 24 this apparent paradox may be resolved if the goethite is a product of oxidation of 13 c rich siderite which had previously replaced wood in an eocene methanogenic burial environment thus the goethite retains a carbon isotope memory of a diagenetic eocene event but a d and 18 o record of the latest miocene pliocene arctic climate

oxygen isotope analyses of fine silica grains using laser extraction technique comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation the laser fluorination technique reported here for analyzing the oxygen isotope composition d 18 o of fine quartz size fractions 50 20 20 10 10 5 5 2 2 1 and 1 lm has been validated by comparison with the ion microprobe technique it yields accurate d 18 o data with an external precision better than 0 15 this is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis both techniques were used to investigate d 18 o composition of a cretaceous quartzite and silcrete sequence from the southeast of france quartzite cements average 31 04 1 93 they formed from mid cretaceous seawater higher in the series silcretes cements average 26 66 1 36 they formed from upper or post upper cretaceous soil water and groundwater oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another including from an upper pedogenic silcrete to a lower groundwater silcrete occurred in a closed or weakly evaporating hydrological system

oxygen isotope evidence of climate change from pedogenic clay minerals in the himalayan molasse abstract in this study we examine the oxygen isotope ratios of smectite and kaolinite in a stratigraphic sequence of paleosols in the siwalik himalayan molasse on the potwar plateau northern pakistan there is a 3 4 o increase in the smectite 18 o values synchronous with a 3 5 o increase in 18 o values of pedogenic calcite at 8 5 6 5 ma previously observed by quade et al 1989 the covariation of the 18 o values of smectite with calcite gd 18 o values through time strongly suggests a pedogenic origin of smectite and that its 18 o values reflect the oxygen isotope ratio of the soil water present during pedogenesis these data show that the 18 o values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators kaolinite 18 o values are similar to smectite 18 o values as would be expected for pedogenic kaolinite but there is greater variation in these data which we attribute to incomplete mineral separation the measured permil oxygen isotope fractionation between calcite and smectite within individual paleosols is on average 3 o higher than the equilibrium fractionation this isotopic disequilibrium can be explained by the calcite forming predominantly during the dry winter season and the smectite forming predominantly during the wet summer season the synchronous 3 5 o increase in oxygen isotope ratios of smectite and calcite at 8 5 6 5 ma indicates that this 18 o 16 o increase is not due to diagenesis but rather results from a climate change the most likely causes of the 18 o 16 o increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation thus 18 o enrichment and or a change from more continental to more marine sourced precipitation the tibetan plateau which is suggested to have undergone rapid uplift around 8 ma e g harrison et al 1992 may have acted as an effective orographic barrier to low 18 o precipitation originating from central asia causing higher 18 o values of the precipitation falling on the potwar plateau currently it is impossible to distinguish between these two potential causes of the 18 o 16 o increase at 8 5 6 5 ma but these clay mineral 18 o values support that there was a significant climate change in this region

a stable isotope study of lateritic bauxites abstract stable isotope analyses of gibbsitic samples from a number of lateritic bauxites suggest that the accepted oxygen and hydrogen isotope fractionation factors for the mineral at surficial temperatures are incorrect a new ox of 1 0143 0 002 and hy of 0 998 0 015 is proposed for gibbsite and in addition an ox of 1 0175 0 001 and hy of 0 975 0 008 is proposed for boehmite at surficial temperatures application of these fractionation factors suggests that gibbsite from many of the equatorial bauxites analyzed formed in equilibrium with meteoric waters similar to those which commonly accompany monsoon type rainfall these waters are generally depleted in 18o and d compared to precipitation derived from tradewinds the isotopic composition of gibbsite from the mid tertiary weipa bauxite in north queensland indicates equilibrium with waters similar in isotopic composition to modern groundwaters in the region this suggests that a monsoon type climate similar to that of the present has prevailed in the region since at least the mid tertiary the oxygen and hydrogen isotope composition of gibbsite from the paragominas deposit in brazil is markedly out of isotopic equilibrium with modern tradewind derived regional rainfall but in equilibrium with waters comparable to those derived from monsoonal rains this suggests that since the formation of the deposit the climate has changed and the monsoons no longer operate in the region thus the link between the isotopic composition of bauxite minerals and the nature of the climate prevailing during their formation can be used to define tropical regions which have previously experienced a monsoonal climate providing valuable information on the nature of atmospheric circulation in the past

oxygen isotope systematics in a multiphase weathering system in haiti abstract physical and chemical partial dissolution techniques have been applied to a suite of young karst bauxite and laterite samples from the southern peninsula of haiti the 18o values have been obtained for nine mineral species and range from 1 0 for anatase rutile to 33 4 for authigenic quartz on one hand results for quartz calcite kaolinite gibbsite and boehmite compare favourably with 18o values predicted from accepted mineral water fractionation factors assuming modern temperatures 25 2 c and water 18o values 3 1 0 5 on the other hand the 18o values measured for anatase illite smectite chlorite smectite and some iron oxide samples do not compare favourably with predicted values departures from expected values for these minerals may be related to crystallization from water with a different 18o value than modern water an imprecise knowledge of some mineralwater fractionation factors under surficial conditions or nonattainment of isotopic equilibrium between mineral and water during formation partial dissolution techniques hold considerable promise for obtaining quantitative 18o values of individual minerals in single samples of complex fine grained regolith material

oxygen isotopes of phosphate and the origin of island apatite deposits abstract in an attempt to test the hypothesis that insular phosphorites on high islands e g nauru formed by chemical reactions between seabird guano and carbonate the oxygen isotope compositions of phosphate 18op in modern guano and in the apatite component of phosphorites of various ages and locations have been investigated guano from several species of seabird appears to have 18op values that are in equilibrium with the ambient seawater temperature at each site however recent apatites closely associated with modern guano can have 18op values that are as much as 2 higher than their inferred guano phosphate source this shift in 18op values could be the result of isotopic re equilibration with percolating meteoric waters which have been enriched by evaporative processes in the upper part of the soil profile bacterial degradation of the guano in the soil zone is a possible mechanism for resetting the 18op values in this environment ancient apatites on high islands such as nauru ocean island and makatea have 18op values that are significantly lower than the 18op values of recently formed apatites the results indicate that ancient apatite deposits could not have formed by processes similar to those forming recent apatites without unreasonably high palaeotemperatures and or equilibration with water with much lower 18o values such as meteoric water in a karstic terrain at the time of maximum infiltration if the 18op values in apatite are subject to modification during subaerial weathering and diagenesis perhaps aided by the metabolic activity of micro organisms the application of 18op signatures of insular phosphorites to palaeoenvironmental reconstructions is limited to post depositional history and provides few clues to original setting

hematite and calcite coatings on fossil vertebrates abstract hematite coatings are common on vertebrate fossils from paleocene eocene paleosol deposits in the bighorn basin wyoming in general hematite coatings are found only on fossils and are limited to soils exhibiting hydromorphic features and moderate maturity petrographic and isotopic evidence suggests that hematite and micritic calcite formed at nearly the same time in a pedogenic environment whereas sparry calcite formed later at greater burial depths the parent material of paleosols is rich in iron supplying an ample source of iron for hematite formation decomposition of animal tissues around bones may enhance the weathering of iron bearing minerals in soils surrounding carcasses while the bones might provide favorable sites for iron accumulation the predominance of discrete smectite together with regional geothermal history suggests that burial temperatures have not exceeded 70 c hematite coatings on fossils can serve as a substrate for geochemical analysis in continental paleoclimatic research owing to their pedogenic origin abundance and resistance to diagenetic alteration

geochemical and isotopic studies of bauxitization in the tatun volcanic area northern taiwan the tatun volcanic area in the northern tip of taiwan is composed of pleistocene andesite and andesitic tuff its high relief causes extraordinarily heavy rainfall 3000 mm yr 1 especially during the winter monsoon season bauxite deposits were formed on the windward northern slopes soil samples from two profiles in the bauxite deposits were studied by chemical analysis x ray diffraction differential thermal analysis and hydrogen and oxygen isotopic analyses the soil profiles can be divided into two zones by chemical characteristics in the upper zone zone a the chemical composition of the soil samples undergoes little change except depletion of na and ca due to leaching the major minerals are quartz chlorite and illite in the lower zone zone b all major elements suffer rapid dissolution except al2o3 gibbsite grows drastically at the expense of clays while quartz diminishes sharply fe2o3 and tio2 are enriched at the boundary between the two zones due to coprecipitation of fe and ti as hematite and goethite probably as a result of increasing oxygen fugacity mobilization of fe in zone b is attributed to its low eh thus zone a is inferred as a permeable layer and zone b as an aquifer the weathering rate in this area is estimated to be 12 cm ka 1 the most thoroughly leached soil samples contain up to 80 gibbsite the high gibbsite contents and the analyses of coexisting waters allow hydrogen and oxygen isotopic fractionation factors between gibbsite and water to be calculated as 0 992 0 002 and 1 016 0 001 respectively

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

oxygen isotope studies of diagenesis in the basal belly river sandstone pembina i pool alberta abstract the oxygen isotope compositions of authigenic clay and carbonate minerals from the pembina i pool basal belly river sandstone provide useful constraints on the diagenesis of this unit early diagenetic chlorite has 18o values 4 2 to 6 3 smow compatible with low temperature crystallization from brackish water the 18o values of diagenetic calcite 11 8 to 14 7 smow and kaolinite 10 4 to 12 8 smow are best explained by crystallization at temperatures of at least 45 to 70 c from pore fluids similar in 18o to the existing formation water 9 smow the implicatio of such temperatures is that the sandstone was much more deeply buried in the past than it is at present at least 1 300 1 400 m of overlying material may have been eroded since maximum burial

rusty relics of earth history iron iii oxides isotopes and surficial environments abstract the two most common low temperature iron iii oxides on earth are goethite feooh and hematite fe2o3 the 18o values of natural goethites range from 15 5 to 3 3 whereas 18o values of low temperature hematites range from 16 7 to 4 7 plots of d against 18o for continental goethites are approximately parallel to the meteoric water line of craig h craig 1961 science 133 1702 3 this suggests that goethite water fractionation factors are systematic over a wide range of surficial environments and may indicate that isotopic equilibrium is commonly attained or closely approached several experimental or calculated mineral water oxygen isotope fractionation curves have been determined for both goethite and hematite although there is not yet a consensus on which of these curves best approximates isotopic fractionation in natural samples oxygen isotope measurements of both goethite and hematite have provided evidence of significant continental climate change on time scales that

an assessment of isotopic equilibrium in goethites from a bog iron deposit and a lateritic regolith abstract hydrogen and oxygen isotope ratios have been measured in goethites from a lateritic regolith in the eastern amazon basin of brazil and a young bog iron deposit in new jersey usa the presence of exchangeable hydrogen and admixed minerals required the use of material balance calculations to determine relevant d and 18o values for the brazilian goethite d is 121 and 18o of 1 2 adjusted for al content these values yield a calculated temperature of formation of 24 3 c which is in good agreement with the modern average annual temperature in that locale goethite from the new jersey bog has a d of 134 and 18o of 0 9 its calculated temperature of formation 23 3 c resembles summer temperatures in southeastern new jersey bacterially mediated rates of precipitation of ferric hydroxide appear to be higher in the bog during the summer months crerar et al 1979 thus the temperature calculated for the new jersey goethite could indicate that the conversion of ferric hydroxide to goethite also occurs predominantly in the summer 13c values of the fe co3 oh component in goethite 21 7 for brazil and 19 3 for new jersey reflect the 13c values of the associated organic carbon 28 4 for brazil and 26 3 for the new jersey values of pco2 in the regolith 0 063 bar and bog 0 039 bar were calculated from the measured abundances of the fe co3 oh component the values of 13c and pco2 obtained from the goethites suggest that the ambient co2 in these systems originated primarily from oxidized organic matter in the low ph high pco2 waters characteristic of bogs and laterites isotopic equilibrium may be closely approached during the formation of goethite from ferric hydroxide ferrihydrite even if the initial precipitation of the ferric hydroxide precursor is bacterially mediated therefore meaningful paleotemperatures might commonly be calculated from stable isotope ratios of nonexchangeable oxygen and hydrogen in geothites from such environments

halloysite as a kinetically controlled end product of arid zone basalt weathering abstract mineralogical and isotope results paired with field observations suggest that halloysite is the favored albeit metastable aluminosilicate end product of arid zone basalt weathering on kohala hawai i and that the formation of smectite has been inhibited by kinetic factors soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance thus allowing us to interpret chemical and mineralogical changes as the result of a time dependent process the halloysite content of the soil increases with increasing soil age its growth is at the expense of allophane which in turn forms at the expense of primary minerals these mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil smectite the thermodynamically stable phyllosilicate phase predicted by soil solution composition is not found in these arid soils we determined that our soil system is controlled by kinetic factors and that therefore thermodynamic predictions do not reflect reality the main factors favoring kinetic control of halloysite formation are intense but short wet periods followed by prolonged extremely dry seasons and microenvironmental conditions leading to immediate uptake of released al by the halloysite precursor mineral allophane the 18o relationship between present day soil water and halloysite formed over the last 170 ka was used as a tracer of past soil conditions results from a reconstruction of paleo climate and soil conditions combined with 18o data observed mineral relationships along the 350 ka chronosequence and field evidence for long term aridity rule out the possibility that the arid side of kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and therefore altered mineralogical stabilities we conclude that pedogenic halloysite has been continually forming from the early stage of soil formation and that it has consistently formed under isotopic equilibrium conditions indicating that halloysite 18o compositions imply paleoclimatic conditions over the time of its formation that are similar to today s

the carbon isotope composition of organic matter occluded in iron nodules abstract this study presents 13 c and 14 c results for soil organic carbon and carbon occluded by iron nodules from a quaternary soil profile developed on basalt in western victoria australia the results suggest that the 13 c value of organic matter in the iron nodules is directly inherited from the surrounding soil profile without isotopic fractionation and that therefore the 13 c value of organic matter occluded by the iron nodules can be related to the vegetation present during nodule formation however 14 c results suggest that iron nodules are not closed systems with respect to organic carbon and that even chemically resistant immobile particulate carbon of probable microbial origin has been added to the nodule carbon pool since formation nevertheless the turnover time for chemically resistant particulate carbon in the iron nodules is at least three times that for the same fraction in the surrounding soil making iron nodules useful repositories of palaeoenvironmental information particularly where they have been removed from the active surface soil layer by burial

paleocene eocene climatic variation in western north america evidence from the δ18o of pedogenic hematite reconstruction of the pre pleistocene climate of continental interiors has been hampered by a lack of climatic proxies and limited temporal resolution we developed a new approach based on the oxygen isotope composition 18 o of hematite which occurs as a coating on fossil vertebrates and used this method to study climatic change across the paleocene eocene boundary in the bighorn basin wyoming hematite coatings form in soils or shallow sediments and therefore may be used to monitor near surface conditions hematite 18 o values are measured by combining selective leaching laser fluorination and mass balance calculation these values in conjunction with 18 o values for co occurring carbonates are used to assess the plausibility of published hematite water oxygen isotope fractionation relations though uncertainty remains as to the appropriate fractionation all are relatively insensitive to temperature variations thus differences in hematite 18 o values most likely reflect shifts in surface water 18 o values analysis of hematites spanning the paleocene eocene transition reveals a roughly 4 decrease in the 18 o of surface water in very early eocene time this episode probably reflects a cooling event perhaps as great as 6 c that started 0 7 m y after the terminal paleocene 13 c excursion which corresponds to a pulse of extreme marine warming following the cooling which lasted 0 6 m y temperatures rebounded to values close to those for late paleocene time the cooling episode coincides remarkably well with other indicators of environmental and climatic change from the basin such as mammalian turnover events and mean annual temperatures estimated from leaf physiognomy

methodological improvements for investigating silcrete formation petrography ft ir and oxygen isotope ratio of silcrete quartz cement lake eyre basin australia the oxygen isotopic composition 18o of the cement of a silcrete sample from the cordillo silcrete late eocene early oligocene of lake eyre basin australia was investigated the massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater in order to avoid errors inherent in estimates of the cement 18o value from isotopic analyses of bulk material and mass balance considerations methodological improvements are described the silcrete cement is physically separated from the detrital phases its purity is checked using cathodoluminescence cl spectroscopy optical microscopy x ray diffraction xrd analyses and fourier transform infrared ft ir spectroscopy show the cement is made of three phases i overgrowth ii crypto crystalline quartz and iii micro crystalline quartz thus its 18o value must be interpreted as that of a mixture although the mixture consists mainly of crypto crystalline quartz as the amount of si oh exchangeable oxygen in the cement would bias the results by an amount less than the analytical error 0 09 it is unnecessary to go through an equilibration procedure prior to isotopic analysis the 18o value of the cement sample is between 24 8 and 25 8 relative to v smow assuming an eocene oligocene temperature of 15 20 c in agreement with published regional temperature estimates the inferred 18o value of the quartz forming solution was between 6 9 and 12 2 this is lower than the minimum 18o value of 6 70 estimated for the modern groundwater near the sample site a plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one from shallow groundwater frequently recharged in a setting of slow tectonic uplift

unraveling climatic changes from intraprofile variation in oxygen and hydrogen isotopic composition of goethite and kaolinite in laterites an integrated study from yaou french guiana an integrated study of o and h isotopes in the lateritic profile of yaou french guiana was undertaken to investigate the usefulness of stable isotopes as tracers of climatic changes in continental environments the studied profile is composed of a 27 m thick saprolite mostly developed in the past under wet and dry tropical climate in association with a duricrust overlain by a 3 m thick yellow latosol formed more recently under present equatorial hot and humid climate 18o d values determined for weathering goethite pseudomorphs after pyrite and kaolinite microcrystalline clay groundmass throughout the 30 m deep profile reflect formation temperatures consistent with present 25 c and realistic past climatic temperatures 20 c 30 c indicating that weathering minerals formed in isotopic equilibrium with their genetic environment and were not subjected to significant isotope exchange after formation a distinct shift downward 2 for 18o 15 for d from low to high 18o d values occurs around 20 m depth in the saprolite it is interpreted as recording the change from the past tropical to the present equatorial climate goethite and kaolinite in the 5 10 m thick saprolite interval immediately above the active basement weathering front are in isotopic equilibrium with modern water and must have formed under present equatorial humid conditions in contrast goethite and kaolinite found higher up in the saprolite and in the duricrust formed in the past under tropical wet and dry climate from waters distinctly depleted in 18o and d relative to modern water the marked depletion of paleo meteoric water at yaou most likely reflects a more contrasted or monsoonal character of the ancient tropical climate the present study shows that ancient weathering minerals in lateritic profiles preserve their 18o d values and carry a time signal the time signal is best expressed in minerals formed rapidly at the weathering front and not subjected to post formational remobilization such as the goethite pseudomorphs after pyrite occurring at yaou groundmass kaolinite is more susceptible to partial remobilization through successive dissolution precipitation reactions which may obliterate the paleoclimatic signal unraveling the climatic record carried by weathering minerals in old soil systems is greatly enhanced by studying both oxygen and hydrogen isotopic compositions

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

a shorter 146sm half life measured and implications for 146sm 142nd chronology in the solar system radiometric dating relies on measuring the abundance of long lived radionuclides relative to the abundance of their radiogenic decay products a process determined by the original radionuclide s half life for primordial radionuclides that decay slowly such as 146sm decaying to 142nd this method provides the timing of some of the earliest processes in solar system history using accelerator mass spectrometry kinoshita et al p 1614 1 provide a revised estimate for the 146sm half life of 68 7 million years which is 30 shorter than the previously accepted value this shorter half life suggests that reductions need to be made in the estimated ages for differentiation of earth s mantle and the solidification of the moon s magma ocean and for other more recent processes 1 lookup doi 10 1126 science 1215510

paleoenvironment and the oxygen isotope geochemistry of ironstone of the upper ordovician neda formation wisconsin usa geological evidence suggests that the goethite dominated oolitic ironstone of the upper ordovician neda formation in the north central united states was subaerially weathered in the late ordovician the oxygen isotope ratios of the goethites in the open quotes hard close quotes and open quotes soft close quotes ores of this deposit are rather uniform with a delta sup 18 o value of about minus 1 0 per thousand this delta sup 18 o value indicates the presence of meteoric water at the time of goethite formation and supports the idea that the goethites in hard and soft ores were formed in the same chemical weathering environment 440 million years ago the delta sup 18 o value of accessory phosphate in some soft ore goethite rich ooids was measured and combined with the goethite delta sup 18 o value to calculate a temperature of formation of about 23 degrees c the corresponding delta sup 18 o value of the ancient water was calculated to have been minus 7 3 per thousand the relatively open quotes cool close quotes temperature calculated for the low altitude tropical site occupied by the neda ironstone in the late ordovician is consistent with published evidence from fossil marine invertebrates for global cooling at that time the delta sup 18 o value of minus 7 3 per thousand for the ancient tropical precipitation could indicate intense rainfall ofmore a seasonal perhaps monsoonal character 27 refs 7 figs 3 tabs less

d h of late miocene meteoric waters in western australia paleoenvironmental conditions inferred from the δd of u th he dated cid goethite abstract nineteen u th he ages were determined for eight samples from a core drilled in an ore grade channel iron deposit cid of the robe pisolite robe formation of mesa j in western australia with one exception uncorrected ages of the analyzed aliquots range from 6 7 0 4 ma to 30 2 3 1 ma while molar ratios of th u range from 0 42 to 5 06 the exception is an aliquot with an apparent age of 2 7 ma and th u of 5 70 a three component mixing model involving one generation of goethite and two generations of hematite suggests that the age of crystallization of the oolitic goethites is 7 1 ma if so the goethites have effectively been closed systems for 7 million years and should preserve a stable hydrogen isotope record of late miocene rainfall in the vicinity of mesa j cenozoic movement of the australian continent had placed mesa j and environs in the subtropics at a paleolatitude of about 29 s during the late miocene al adjusted d values of oolitic goethite in the eight cid samples range from 153 to 146 and imply that the d of the late miocene meteoric waters ranged from 61 to 53 with an average of 56 these relatively negative d values might indicate that near coastal late miocene rain was derived primarily from summer season tropical cyclones with storm tracks that extended into the subtropics of western australia the postulated late miocene tropical cyclones would have occurred more often and or exhibited greater intensity at a paleolatitude of 29 s than is the case for modern sites at approximately 30 s on the west coast of australia e g perth higher fluxes of meteoric water in the miocene summers would have facilitated dissolution and removal of bif sourced silica with concomitant enrichment in oxidized fe moreover wetter late miocene summers could have promoted multiple cycles of microbially mediated dissolution and recrystallization of fe iii oxides in the aerobic systems the oolitic textures may be indicative of such recycling however the oolitic goethites of mesa j were closed systems after 7 ma therefore the climate in the vicinity of mesa j seems to have changed in the late miocene to conditions that did not favor widespread recycling of fe iii oxides perhaps changing from seasonally wet to the modern dry climate

oxygen isotope geochemistry of kaolinite in laterite forming processes manaus amazonas brazil abstract mineralogic crystallographic and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the detrital sands and clays of the mid cretaceous to tertiary alter do chao formation central amazon basin brazil a detailed 18o profile of soil water from the upper 4 m of the same section was also measured kaolinite separates were prepared from different pedogenic facies of small hand specimens most of these kaolinite separates exhibit polymodal size distributions the separates were further divided into individual particle size modes for subsequent isotopic and crystallographic analysis in general hinckley index and 18o values of different size fractions of the same sample differ hinckley index values decrease strongly and fe content increases in the upward direction in the profile we conclude that 1 the kaolinite in the profile has progressively evolved in response to changing microenvironments as weathering fronts have moved progressively downward 2 different kaolinite fractions even within a single facies of a single sample a few cm3 in size typically formed under somewhat different conditions 3 the bulk of the fine grained kaolinite in the lateritic profile does not constitute a continuous inverted time record that is lower levels do not represent more recent kaolinite formation and higher levels do not represent older kaolinite formation and 4 isotopic compositions of kaolinites from lateritic paleosols should provide an indication of mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weathering environment we speculate that the different conditions under which the different kaolinite fractions have formed may be related at least in part to variations in soil microenvironments that are ultimately derived from seasonal and year to year variations in the chemistry and isotopic composition of infiltrating rainfall

oxygen isotope dating of the australian regolith australia has had a long and complex history of intense surficial weathering and this has led to the extensive development of laterites bauxites and deeply weathered profiles of a variety of ages across the continent1 2 a detailed knowledge of the age s of such weathering phenomena is of great importance for the reconstruction of australia s post palaeozoic geomorphic and palaeoclimatic history however in many instances absolute geo chronological information is not obtainable using conventional techniques here we describe and evaluate a new method that can provide an estimate of the age of intensely weathered profiles based on the oxygen isotope composition of minerals developed by deep weathering lateritization processes this method allows the definition of four ages of weathering in eastern australia from the late palaeozoic to the present and suggests that some profiles previously thought to be late cretaceous or tertiary in age may be of pre late mesozoic age the oxygen isotope data indicate that the minerals used in this study have retained their original isotopic composition for periods of up to 250 million years despite the continued presence of meteoric waters of progressively higher 18o content during this interval

geochemistry of waters associated with current karst bauxite formation southern peninsula of haiti abstract geological and petrographical observations have led to the hypothesis that there is current formation of bauxitic deposits in the southern peninsula of haiti the kaolinitic products of the lateritic alteration of the basalts are trapped in karst it is then proposed that they are transformed into gibbsite by the flushing out of dissolved silica the study of waters percolating the different outcrops show that the formation of gibbsite takes place during the rainy season during the dry season kailinite is not significantly altered the geochemistry of al si ca and co 2 is in agreement with the reaction kaolinite gibbsite dissolved silica the resulting gibbsite remains trapped in karst geochemical budgets show that the formation of such bauxitic deposits of 1 3 m thick can take place in only 100 000 a

hydrothermal alumina rich clays and boehmite on the gorda ridge abstract hydrothermal alteration of basalt on the northern gorda ridge produced an alteration crust up to 1mm thick that can be divided into two layers based on mineralogy an inner layer next to the basalt surface is an al rich interlayered smectite chlorite similar to high temperature al rich clays recovered from the east pacific rise 21 n with minor quartz zeolite phillipsite and relict plagioclase an exterior layer of boehmite alo oh and minor anatase results from severe leaching of the underlying clays the alteration crust is progressively enriched in al ti v and cr from interior to exterior by residual weathering the presence of boehmite and absence of a silica rich phase in the exterior of the crust indicates the hydrothermal fluids were silica undersaturated or silica was maintained in solution by extremely acidic fluids this constitutes the first reporting of boehmite as a major alteration mineral in a mid ocean ridge hydrothermal environment

stable isotope geochronology of the australian regolith australian regolith profiles can be assigned to one of three post palaeozoic age groups on the basis of the oxygen isotope composition of authigenic clay minerals developed in the profile systematic variations in the isotopic composition of meteoric waters and therefore of the authigenic regolith minerals that formed in equilibrium with them are a result of the continent s drift from high to low latitudes and changes in global climate residual clays collected in situ from regolith profiles of post mid tertiary age have 18o values between 17 5 and 21 3 with the exception of samples from northern australia north of approximately 20 s which have anomalously low values because meteoric waters in northern australia are largely derived from monsoonal rain the high 18o values of post mid tertiary residual clays are consistent with australia s northward drift to low warm latitudes during this time compared with the much higher latitudes it occupied for most of its post palaeozoic history late tertiary residual clays have 18o values that are indistinguishable from all other post mid tertiary clays indicating that further subdivision of the sample groups is not feasible on a continent wide scale comparison with hypothetical modern clays in equilibrium with modern meteoric waters confirms that there has been very little change in the isotopic composition of meteoric waters in australia since the mid tertiary pre mid tertiary clays have lower 18o values between 10 0 and 17 5 which are consistently lower than post mid tertiary clays 18o values of less than 14 to 15 are thought to reflect weathering events during much earlier geologic periods perhaps early or mid mesozoic when australia was at high latitude and the australian climate was humid and cool to cold several workers have documented the existence of very old regolith profiles and land surfaces in australia and support for the above hypothesis comes from three analyses of clays from profiles of stratigraphically demonstrable pre late mesozoic age which have 18o values between 10 2 and 12 3 regolith profiles composed of low 18o clays 15 are widespread in australia and it is possible that a much greater part of the modern landscape than previously recognized developed in the early or mid mesozoic the 18o values of transported i e sedimentary kaolinites reveals that the major source for many tertiary kaolinitic sediments was this postulated pre late mesozoic low 18o regolith the stableisotope evidence that much of the australian regolith formed in comparatively cold conditions suggests that contrary to some traditional interpretations lateritization and deep weathering phenomena are not solely the result of weathering in tropical or sub tropical climates the hydrogen isotope composition of the clays range from 115 to 50 however it is inferred that the majority of clays have undergone some post formational hydrogen isotope exchange which renders their d composition unsuitable for dating purposes

complex rupture mechanism and topography control symmetry of mass wasting pattern 2010 haiti earthquake the 12 january 2010 mw 7 0 haiti earthquake occurred in a complex deformation zone at the boundary between the north american and caribbean plates combined geodetic geological and seismological data posited that surface deformation was driven by rupture on the leog ne blind thrust fault while part of the rupture occurred as deep lateral slip on the enriquillo plantain garden fault epgf the earthquake triggered 4490 landslides mainly shallow disrupted rock falls debris soil falls and slides and a few lateral spreads over an area of 2150 km2 the regional distribution of these slope failures defies those of most similar earthquake triggered landslide episodes reported previously most of the coseismic landslides did not proliferate in the hanging wall of the main rupture but clustered instead at the junction of the blind leog ne and epgf ruptures where topographic relief and hillslope steepness are above average also low relief areas subjected to high coseismic uplift were prone to lesser hanging wall slope instability than previous studies would suggest we argue that a combined effect of complex rupture dynamics and topography primarily control this previously rarely documented landslide pattern compared to recent thrust fault earthquakes of similar magnitudes elsewhere we conclude that lower static stress drop mean fault displacement and blind ruptures of the 2010 haiti earthquake resulted in fewer smaller and more symmetrically distributed landslides than previous studies would suggest our findings caution against overly relying on across the board models of slope stability response to seismic ground shaking

reply to comment by c h chen k k liu and y n shieh on a stable isotope study of lateritic bauxites

oxygen and hydrogen isotopes in pedogenic minerals implications for paleoclimate evolution in amazonia during the cenozoic stable isotopes in gibbsite and kaolinite were analyzed to reconstruct the amazonia palaeoclimate during the cenozoic and to investigate the environmental conditions of the weathering process and the origin of the oxisol that cover the lateritic bauxite crust at least two types of kaolinite were identified in the oxisol near the surface the kaolinite is less fractionated and was developed at a higher temperature 35 c this is probably due to the effect of vegetation and or evapotranspiration or is a consequence of surface microenvironmental conditions or allochthonous inputs the other kaolinite samples ranged in formation temperatures from 21 c to 26 c and developed under conditions similar to the pleistocene and the modern climate of amazonia at least two types of gibbsite were found one type developed in the modern amazonia climate in a way similar to most kaolinite samples whereas the samples of the other type are more fractionated this is because they may have been formed in low recycling rainwater and or under a low amount of precipitation these two conditions indicate a seasonal climate for bauxite development that can be linked to the low exhumation of the andes cordillera and its geographical position relative to the trombetas region during the period ranging from the late oligocene to the early miocene at the end of the cenozoic the climate was less seasonal and wetter and the temperature was lower in addition there was the probable inland position of the trombetas region which allowed oxisol development the gibbsite developed earlier than the kaolinite of the oxisol but there is a partial inheritance or temporal equivalence among the bauxite horizon and the oxisol as well as multiple generations of gibbsite and kaolinite which is to be expected for materials developing over long periods in hard leaching conditions

oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites southern brazil and western senegal abstract the oxygen 18 o c and carbon 13 c c isotope compositions of the structural carbonate group co 3 in apatites from lateritic profiles were investigated the weathering profiles located in southern brazil and in western senegal are developed on three different types of apatite rich parent rock carbonatite metamorphosed marine phosphorite and sedimentary marine phosphorite the parent rock apatites are of magmatic hydrothermal metamorphic and sedimentary origins the in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically the overall range of measured 18 o c and 13 c c values of apatites of weathering origin 22 to 27 smow for 18 o c and 15 to 10 pdb for 13 c c is much smaller than the range of measured and or published isotope compositions of parent rock apatites 4 35 for 18 o c and 11 to 1 for 13 c c in any profile the apatites of weathering origin can exhibit lower similar or higher 18 o c values than parent rock apatites in contrast their 13 c c values are systematically and significantly lower than those of the parent rock apatites apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition assuming that apatite co 3 fractionates o in a way similar to calcite co 3 the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions the very low 13 c c values exhibited by these apatites indicate that the dominant sources of dissolved co 2 in the soil water are organic the isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies