the seawater sr ca ratio in the past 50 myr from bulk carbonate sediments corrected for diagenesis abstract the strontium calcium sr ca ratio in paleoseawater is important for evaluating the global fluxes of continental weathering products and carbonate deposition in the oceans existing reconstructions of the paleoseawater sr ca ratio using different methods give conflicting results we use a modeling approach to estimate paleo sr ca for the past 50 million years that uses pore fluid chemistry and bulk sediment measurements of carbonate ooze and chalk from the ontong java plateau as input the effects of diagenesis in modifying the sr ca ratio recorded in bulk carbonate sediments are estimated using a reactive transport model and those effects are accounted for in determining the initial sr ca ratio of the biogenic carbonate sediments the sr ca of the paleo oceans is calculated based on an effective partition coefficient k sr bio 0 198 inferred from sr ca in modern sediments the model simulates deposition of sediments on the seafloor reactions between pore fluid and solid dissolution and precipitation of calcite and diffusion within the pore fluid measured pore fluid sr ca and sulfate concentrations provide input for the sr model this approach applied to four ocean drilling program odp sites 803 805 806 807 that have long cenozoic records of carbonate deposition indicates that the sr ca ratio of seawater has increased slowly over the past 50 million years from a value of about 8 mmol mol to the modern value of 9 4 with fluctuations of about 1 over 10 myr periods this result is consistent with previous estimates based on foraminifera and coral aragonite but differs from recent estimates obtained with other approaches the near constant seawater sr ca over the past 50 million years is consistent with likely weathering sources to the oceans the fluctuations suggest that there were modest shifts in the aragonite calcite ratio of marine carbonate

reconstructing past seawater mg ca and sr ca from mid ocean ridge flank calcium carbonate veins proxies for past seawater chemistry such as mg ca and sr ca ratios provide a record of the dynamic exchanges of elements between the solid earth the atmosphere and the hydrosphere and the evolving influence of life we estimated past oceanic mg ca and sr ca ratios from suites of 1 6 to 170 million year old calcium carbonate veins that had precipitated from seawater derived fluids in ocean ridge flank basalts our data indicate that before the neogene oceanic mg ca and sr ca ratios were lower than in the modern ocean decreased ocean spreading since the cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early tertiary may explain the recent rise in these ratios

fluvial hydraulics of deltas and alluvial fans delta formation and its associated steam channel hydraulics have been studied using data obtained in a large laboratory flume the findings for deltas are generally applicable to alluvial fans delta or fan shape is directly related to the pattern of stream flow its loci and mode of deposition in response to a change in water discharge sediment feeding rate or base level stream channel adjustments on the delta are characterized by the formation of a single channel or multiple distributary channels the changes in channel pattern are associated with large changes in channel width and stream power expenditure stream channel changes are related to the stream s tendency to seek equal power expenditure along the channel they also provide delicate adjustments in velocity and sediment load to favor restoration of channel s equilibrium

sulfur cycling at the mid atlantic ridge a multiple sulfur isotope approach abstract the role of sulfur in two hydrothermal vent systems the logatchev hydrothermal field at 14 45 n 44 58 w and several different vent sites along the southern mid atlantic ridge smar between 4 48 s and 9 33 s and between 12 22 w and 13 12 w is examined by utilizing multiple sulfur isotope and sulfur concentration data isotope compositions for sulfide minerals and vent h 2 s from different smar sites range from 1 5 to 8 9 in 34 s and from 0 001 to 0 051 in 33 s these data indicate mixing of mantle sulfur with sulfur from seawater sulfate combined 34 s and 33 s systematics reveal that vent sulfide from smar is characterized by a sulfur contribution from seawater sulfate between 25 and 33 this higher contribution compared with epr sulfide indicates increased seawater sulfate reduction at mar because of a deeper seated magma chamber and longer fluid upflow path length and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid ocean ridges additionally isotope data uncover non equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 c 34 s values between 0 2 to 8 8 for dissolved and precipitated sulfide from logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at smar 34 s values between 24 5 and 6 5 and 33 s values between 0 001 and 0 125 for sulfide bearing sediments and mafic ultramafic host rocks from drill cores taken in the region of logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction basalts and basaltic glass from smar sites with 33 s 0 008 reveal lower 33 s lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials we conclude that the combined use of both 34 s and 33 s provides a more detailed picture of the sulfur cycling in hydrothermal systems at the mid atlantic ridge and uncovers systematic differences to hydrothermal sites at different mid ocean ridge sites multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow furthermore based on 33 s we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when 34 s were identical

cretaceous seawater and hydrothermal fluid compositions recorded in abiogenic carbonates from the troodos ophiolite cyprus abstract we evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates precipitated from fluids within the oceanic crust of the 91 ma troodos ophiolite cyprus calculated variations in fluid mg ca sr ca and 87sr 86sr with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust and yield literature values for late cretaceous seawater mg ca sr ca and 87sr 86sr this indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in situ oceanic crust whereas most carbonates recovered from in situ oceanic crust were precipitated at temperatures 44 c gradually transform cretaceous seawater into a troodos hydrothermal fluid comparison of the troodos data with a global dataset of abiogenic carbonates from in situ oceanic crust shows that the chemical pathways of low temperature fluid evolution are similar for all cretaceous sites these different sites represent varied geotectonic settings mid ocean ridge vs suprasubduction zone with different basement composition basalt basaltic andesite boninite and situated in different ocean basins atlantic pacific mediterranean tethys the similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures however abiogenic carbonates from younger oceanic crust differ from the cretaceous trends and follow different fluid evolution pathways this indicates that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures a thermodynamic model of fluid crust interaction in which modern and cretaceous seawater are heated to 200 c while an average troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the cretaceous than they are today and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures for example twice the amount of carbonate formed during alteration of the oceanic crust in the cretaceous compared to modern times indicating that the flux of co2 from the hydrosphere atmosphere system into the oceanic crust was greater in the cretaceous than it is nowadays and that it probably varied throughout geologic time

relationship between the sr and o isotope compositions of hydrothermal fluids and the spreading and magma supply rates at oceanic spreading centers we find a first order correlation between the sr and o isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers the isotope signatures of hydrothermal fluids at slow spreading ridges are more rock dominated than those at fast spreading ridges implying that they have undergone a greater extent of sr and o exchange with the crust the local spatial and temporal variability in sr and o isotope signatures within individual hydrothermal sites can be attributed to short term variations in the hydrothermal system in response to magmatic or tectonic events phase separation at depth and mixing with seawater in the upwelling zone however these processes cannot explain the systematic global relationship of an increasing rock signature in hydrothermal fluids with decreasing spreading rate a greater extent of sr and o exchange between rocks and the circulating fluids can be achieved by 1 a longer fluid flow pathway through the oceanic lithosphere 2 more fractured rock that exposes a greater reaction area 3 an increased residence time of the fluid due to slower hydrothermal circulation or 4 a combination of all three our preferred interpretation is that fluid flow paths are longer and depths of penetration are likely greater at slow spreading ridges

effect of paleoseawater composition on hydrothermal exchange in midocean ridges variations in the mg ca sr and so4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges mor we present a model for evaluating the nature and magnitude of these previously unappreciated effects using available estimates of paleoseawater composition over phanerozoic time as inputs and 87sr 86sr of ophiolite epidosites and epidote quartz veins as constraints the results suggest that modern hydrothermal fluids are not typical due to low ca and sr relative to mg and so4 in modern seawater at other times during the last 500 million years particularly during the cretaceous and ordovician hydrothermal fluids had more seawater derived sr and ca a prediction that is supported by sr isotope data the predicted 87sr 86sr of vent fluids varies cyclically in concert with ocean chemistry with some values much higher than the modern value of 0 7037 the seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic ca and sr to seawater in hydrothermal systems which varies by a factor of 1 6 over the phanerozoic with minima when seawater mg and so4 are low this effect provides a modest negative feedback on seawater composition and 87sr 86sr changes for the mid cretaceous the low 87sr 86sr of seawater requires either exceptionally large amounts of low temperature exchange with oceanic crust or that the weathering flux of continentally derived sr was especially small the model also has implications for mor hydrothermal systems in the precambrian when low seawater so4 could help explain low seawater 87sr 86sr

oceanic faulting and fault controlled subseafloor hydrothermal alteration in the sheeted dike complex of the josephine ophiolite based upon detailed mapping 1 10 and 1 100 of a large water polished outcrop of the sheeted dike gabbro transition zone in the josephine ophiolite of nw california and sw oregon the following history of alternating episodes of magmatic structural and hydrothermal events has been documented using crosscutting relationships petrography geochemistry and strontium and oxygen isotopic data 1 crystallization of gabbro and later subvertical mafic dike injection 2 amphibolite facies metamorphism 3 extensional faulting and tilting of dikes 4 continued faulting tilting and dike injection associated with retrograde metamorphism at greenschist facies conditions 5 continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid rock ratios and 6 subvertical injection of a highly fractionated dike which truncates all previous features features 1 5 trace element geochemistry indicates the highly fractionated dike feature 6 is genetically related to the other dikes and thus was intruded at or near the paleospreading axis hence all previous events features 1 5 can be constrained to have occurred at the rift axis including large scale tilting 50 of the sheeted dikes and extensional faulting the 87sr 86sr initial ratios from recrystallized whole rocks and from hornblende epidote and prehnite separates from veins display a systematic increase with relative age from 0 7033 for altered gabbro screens and mafic dikes to 0 7049 for prehnite in the youngest oceanic fault rock calculated oxygen isotope fluid compositions for the same suite of samples range from 18ofluid 5 to 1 with time indicating a change to a seawater dominated hydrothermal system with time consistent with observed increased permeability due to seafloor extensional faulting the crosscutting relationships alteration mineral assemblages and isotopic data suggest 1 an early stage of high temperature 450 c hydrothermal alteration with low permeability i e grain scale flow followed by 2 a decrease in temperature 350to 200 c and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis the consistency of these crosscutting relationships at similar pseudo stratigraphic levels at different localities in the josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading slow to intermediate spreading centers such as the modern mid atlantic ridge

calcium carbonate veins in ocean crust record a threefold increase of seawater mg ca in the past 30 million years chemical sr mg and isotopic 18o 87sr 86sr compositions of calcium carbonate veins ccv in the oceanic basement were determined to reconstruct changes in sr ca and mg ca of seawater in the cenozoic we examined ccv from 10 basement drill sites in the atlantic and pacific ranging in age between 165 and 2 3 ma six of these sites are from cold ridge flanks in basement 46 ma which provide direct information about seawater composition ccv of these young sites were dated using the sr isotopic evolution of seawater for the other sites temperature corrections were applied to correct for seawater basement exchange processes the combined data show that a period of constant low sr ca 4 46 6 22 mmol mol and mg ca 1 12 2 03 mol mol between 165 and 30 ma was followed by a steady increase in mg ca ratios by a factor of three to modern ocean composition mg ca sr ca relations suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes however additional forcing may be involved in explaining the timing and magnitude of changes a plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering which in turn is a result of subduction zone recycling of co2 from pelagic carbonate formed after the cretaceous slow down in ocean crust production rate

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

the relationship between organoclastic sulfate reduction and carbonate precipitation dissolution in marine sediments abstract pore fluid sulfate calcium and magnesium concentrations in ocean drilling sites 981 984 925 929 and 803 807 are modeled to estimate the rates of sulfate reduction and carbonate precipitation dissolution these sites represent scenarios where the sulfate reduction rate is high moderate and low results show that the rate of carbonate precipitation decreases with decreasing sulfate reduction rate when sulfate reduction rate is sufficiently low carbonate minerals dissolve sulfate reduction produces h and bicarbonate which compete in driving carbonate minerals to dissolve or precipitate the results suggest that the carbonate reactions are mainly driven by the produced h when sulfate reduction rate is low and by the produced bicarbonate when the sulfate reduction rate is high

a tidal influenced hydrothermal system temporarily cooled by a tropical storm abstract the lutao hydrothermal field is an intertidal arc volcanic system located offshore southeast taiwan hosting a zhudanqu zdq vent and a huwaichi hwc spring with strongly contrasting fluid chemistry low mg moderately enriched cl and h with respect to seawater indicate that the zdq endmember was derived from the brine phase that was formed during low degree subcritical phase separation in contrast the endmember for the hwc vent fluids is related to the vapor phase temperature and pressure of the phase separation were estimated as 150 c and 7 bar respectively the water rock ratio was roughly calculated as about 2 the lutao hydrothermal system was slightly affected by semi diurnal tides by some combination of tidal loading and tidal currents the time delay between tides and the response of the hydrothermal system was about 3 h while freshwater was almost absent in the hwc vent fluids at normal conditions the typhoon fung wong on sep 21st 2014 led to intrusions of freshwater into the vent fluids with a percentage of 16 both the zdq and the hwc endmember compositions showed some changes after the typhoon event suggesting a cooling of the reaction zone after the typhoon passed by the hydrothermal system began to recover evidenced by increasing percentages of the hwc endmember and decreasing freshwater contributions the flux of the hwc endmember was estimated as 460 560 l h 1 based on these observations this study for the first time reports a shallow depth tidal influenced hydrothermal system that was temporarily cooled by a tropical storm

development and geometric similarity of alluvial deltas movable bed experiments from a flume into a basin were conducted to study the development of alluvial deltas the experimental setup was aimed at the bed load mode results showed that the development of the delta can be divided into three stages in the first stage the delta progressed mainly in its length and a successfully derived equation described the shape of the delta in this stage in the second stage the delta developed mainly in its width the length width front thickness and central thickness of the delta were used to scale the geometric similarity the plane geometry of deltas can be described using gaussian functions transverse and longitudinal profiles fit the hyperbolic and linear function respectively the length width and thickness of the delta deposition at the end of the second stage have their own maximum values rc bm and zf respectively for each case the total volume v of sediment deposition could be simply related to these maximum values by v arc bmzf 1 b with shape parameters a and b around 0 178 and 174 the third stage was characterized by the interactions between the delta development and stream channel variations the ratios of the delta slope to the channel slope were closely related to the occurrence condition of the stream channel finally the temporal variations of stream channels were investigated qualitatively

recharge flux to ocean ridge black smoker systems a geochemical estimate from odp hole 504b we use the sr isotopic composition of rock and anhydrite as a monitor of fluid composition in dsdp odp hole 504b to calculate the recharge flux to the axial high temperature hydrothermal circulation the fluid and rock sr isotope profiles are well fit by a tracer transport mass balance model that approximates fluid solid exchange by linear kinetics the calculated time integrated flux of 1 7 0 2 x 106 kg m 2 is significantly less than the 5 x 106 kg m 2 calculated from thermal models that assume all magma is intruded into a high level magma chamber at the base of the sheeted dykes our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in hole 504b and the intrusion of magma as lenses distributed through the lower oceanic crust it leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balance

strontium and oxygen isotopic constraints on fluid mixing alteration and mineralization in the tag hydrothermal deposit abstract strontium and oxygen isotopic measurements of samples recovered from the trans atlantic geotraverse tag hydrothermal mound during leg 158 of the ocean drilling program provide important constraints on the nature of fluid rock interactions during basalt alteration and mineralization within an active hydrothermal deposit fresh mid ocean ridge basalt morb with a 87 sr 86 sr of 0 7026 from the basement beneath the tag mound was altered at both low and high temperatures by seawater and altered at high temperature by near end member black smoker fluids pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater 200 c the development of a silicified sulfide mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz pyrite paragonite stockwork cut by quartz pyrite veins initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a mg bearing mixture of upwelling high temperature 300 c black smoker type fluid with a minor 300 c interaction between the wallrock and these mg bearing fluids results in the complete recrystallization of the wallrock to chlorite quartz pyrite the quartz pyrite paragonite assemblage replaces the chloritized basalts and developed by reaction at 250 360 c with end member hydrothermal fluids having 87 sr 86 sr 0 7038 similar to present day vent fluids the uniformity of the 87 sr 86 sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87 sr 86 sr ratio of end member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork precipitation of anhydrite in breccias and as late stage veins throughout most of the mound and stockwork down to at least 125 mbsf records extensive entrainment of seawater into the hydrothermal deposit 87 sr 86 sr ratios indicate that most of the anhydrite formed from 2 1 mixture of seawater and black smoker fluids 65 15 seawater oxygen isotopic compositions imply that anhydrite precipitated at temperatures between 147 c and 270 c and require that seawater was conductively heated to between 100 c and 180 c before mixing and precipitation occurred anhydrite from the tag mound has a sr ca partition coefficient kd 0 60 0 28 2 this value is in agreement with the range of experimentally determined partition coefficients kd 0 27 0 73 and is similar to those calculated for anhydrite from active black smoker chimneys from 21 n on the east pacific rise the 18 o so 4 of tag anhydrite brackets the value of seawater sulfate oxygen 9 5 dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid ocean ridges

reconstructing the oxygen isotope composition of late cambrian and cretaceous hydrothermal vent fluid oxygen isotope analyses 18o of 16 quartz epidote pairs from late cambrian betts cove and mings bight newfoundland ordovician thetford mines quebec canada and cretaceous troodos cyprus ophiolites are used to calculate the 18o of the hydrothermal fluids from which they crystallized we combine these with 3 quartz fluid inclusion measurements and 3 quartz magnetite measurements from the cambrian ophiolites to explore how the range in the 18o of submarine hydrothermal vent fluid has varied between the late cambrian cretaceous and today the range of calculated 18o values of vent fluid 4 to 7 4 is larger than that of modern seafloor hydrothermal vent fluid 0 to 4 we employ two numerical models to ascertain whether this range is most consistent with changes in paleo seawater 18o or with changes in the reactive flow path in ancient hydrothermal systems a static calculation of the vent fluid oxygen isotope composition as a function of the water rock ratio suggests that in an ocean with a lower 18o than today the range of vent fluid 18o should be larger our data however show little evidence that the 18o of the ocean was much lower than the global ice free value of 1 2 a dual porosity model for reactive flow through fractured and porous media is used to model the relative evolution of the 87sr 86sr and 18o of vent fluid in contact with rock our 87sr 86sr and 18o for cretaceous epidotes suggest the strontium concentration of the cretaceous oceans may have been much higher than at present the 87sr 86sr and 18o data from cambrian epidotes are strikingly different from the younger samples and are difficult to model unless fluid rock interaction in the cambrian hydrothermal systems was substantially different it is also possible that some of the quartz epidote veins have been reset by obduction related metamorphism our data suggest that the high calcium to sulfate ratio in early and cretaceous seawater may have affected the degree of strontium isotope exchange causing hydrothermal fluids to have 87sr 86sr closer to that of seawater than in modern systems

origin of fluids and anhydrite precipitation in the sediment hosted grimsey hydrothermal field north of iceland the sediment hosted grimsey hydrothermal field is situated in the tjornes fracture zone tfz which represents the transition from northern iceland to the southern kolbeinsey ridge the tfz is characterized by a ridge jump of 75 km causing widespread extension of the oceanic crust in this area hydrothermal activity occurs in the grimsey field in a 300 m 1000 m large area at a water depth of 400 m active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical for this field clear metal depleted up to 250 c hydrothermal fluids are venting from the active chimneys anhydrite samples collected from the grimsey field average 21 6 wt ca 1475 ppm sr and 3 47 wt mg the average molar sr ca ratio is 3 3 10 3 sulfur isotopes of anhydrite have typical seawater values of 22 0 7 34s indicating a seawater source for so42 strontium isotopic ratios average 0 70662 0 00033 suggesting the precipitation of anhydrite from a hydrothermal fluid seawater mixture the endmember of the venting hydrothermal fluids calculated on a mg zero basis contains 59 8 mol kg sr 13 2 mmol kg ca and a 87sr 86sr ratio of 0 70634 the average sr ca partition coefficient between the hydrothermal fluids and anhydrite of about 0 67 implies precipitation from a non evolved fluid a model for fluid evolution in the grimsey hydrothermal field suggests mixing of upwelling hydrothermal fluids with shallowly circulating seawater before and during mixing seawater is heated to 200 250 c which causes anhydrite precipitation and probably the formation of an anhydrite rich zone beneath the seafloor

plagioclase and epidote buffering of cation ratios in mid ocean ridge hydrothermal fluids experimental results in and near the supercritical region abstract experiments have been performed with na ca k cl fluids of seawater chlorinity and diabase basalt and plagioclase bearing mineral mixtures at 350 425 c and 250 400 bars to help constrain hydrothermal alteration processes at mid ocean ridges dissolved ca na and ph for all experiments responded systematically to differences in dissolved sio2 concentrations and the compositions of plagioclase reactants diabase alteration at low fluid rock mass ratios 0 5 to 1 produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a ph consistent with plagioclase alteration to epidote trace element data from vent fluids specifically b and sr together with major element chemistry ca na sio2 ph provides evidence that the reaction zone for black smoker fluids at mid ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote nearly fresh plagioclase and clinopyroxene and partially to completely hydrated olivine and orthopyroxene fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums using equilibrium between plagioclase the dominant reactant and epidote the dominant reaction product in experiments we estimate that temperatures in reaction zones are in excess of 375 c for most vent systems these temperatures are higher than measured vent temperatures suggesting that hotspring fluids commonly loose heat during ascent to the seafloor

multiple sulfur isotope constraints on the modern sulfur cycle we present 28 multiple sulfur isotope measurements of seawater sulfate 34sso4 34sso4 and 33sso4 33sso4 from the modern ocean over a range of water depths and sites along the eastern margin of the pacific ocean the average measured 34sso4 34sso4 is 21 24 0 88 2 0 88 2 with a calculated 33sso4 33sso4 of 0 050 0 050 0 014 2 0 014 2 with these values we use a box model to place constraints on the gross fraction of pyrite burial in modern sediments this model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of 34spyrite 34spyrite which is poorly constrained but instead uses new information about the relationship between 34s 34s and 33s 33s in global marine sulfate our calculations indicate that the pyrite burial flux from the modern ocean is between 10 and 45 of the total sulfur lost from the oceans with a more probable range between 20 and 35

environmental controls on mid ocean ridge hydrothermal fluxes abstract high temperature hydrothermal fluxes at mid ocean ridges are thought to be an important component of oceanic biogeochemical cycles however little consideration has been given to how these fluxes vary as a consequence of changing environmental conditions over earth history here we consider how changes in sea level and ocean chemistry are likely to have impacted on axis high temperature hydrothermal fluxes focusing on phanerozoic conditions changes in sea level lead to changes in hydrostatic pressure near the base of hydrothermal systems where both peak fluid rock reaction temperatures and phase separation occur in general higher global sea level will lead to higher peak temperatures of fluid rock reaction additionally phase separation at higher pressure tends to lead to formation of a more cl rich vapor that constitutes a larger mass fraction of the system these combined factors may serve to significantly modify hydrothermal fluxes even for sea level changes on the scale of 100 m changes in ocean chemistry can also affect axial hydrothermal fluxes in several ways seawater sulfate contents control the amount of anhydrite that forms which has both physical porosity filling and chemical effects the most important aspect of ocean chemistry in controlling the composition of high temperature vent fluids may be ocean salinity if evaporite formation and dissolution has changed ocean salinity substantially over the phanerozoic hydrothermal fluxes could have been greatly modified ocean chemistry also plays a large role in controlling processes operating in hydrothermal plumes and hence the net flux of elements into and out of the ocean associated with hydrothermal systems we conclude that there is a need for substantial further work to quantify the effects of sea level and ocean chemistry on high temperature hydrothermal fluxes including the development of more robust models that integrate field laboratory and theoretical observations

an evaporite based high resolution sulfur isotope record of late permian and triassic seawater sulfate abstract variations in the sulfur isotope composition of dissolved marine sulfate through time reflect changes in the global sulfur cycle and are intimately related to changes in the carbon and oxygen cycles a large shift in the sulfur isotope composition of sulfate at the permian triassic boundary has been recognized for long time and a number of studies were carried out to understand the causes and significance of this shift however data for the middle and late triassic are very sparse and the stratigraphic evolution of the sulfur isotope composition of seawater is poorly constrained due to the small number of samples analyzed and or due to the limited stratigraphic intervals studied moreover in the last few years the triassic timescale has significantly changed due to a wealth of new radiometric and stratigraphic data in this study we show that for the late permian and the triassic it is possible to obtain a precise reconstruction of the evolution of the sulfur cycle for parts of it at sub million year resolution by analyzing exclusively gypsum and anhydrite deposits we base our reconstruction on new data from the middle and late triassic evaporites of northern switzerland and literature data from evaporites from germany austria italy and the middle east we propose a revised correlation between the well dated marine tethyan sections in northern italy and the evaporites from northern switzerland and from the germanic basin calibrated to the newest radiometric absolute age scale this new correlation allows for a precise dating of the evaporites and constructing a composite sulfur isotope evolution of seawater sulfate from the latest permian lopingian epoch to the norian we show that a rapid positive shift of approximately 24 at the permian triassic boundary can be used to constrain seawater sulfate concentrations in the range of 2 6 mm thus higher than previous estimates but with less rapid changes finally we discuss two possible evolution scenarios for the middle triassic negative shift to values of 15 that subsequently remain constant during the late triassic

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

high temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems abstract calcium isotope ratios in epidote from epidosites in ophiolites of varying phanerozoic ages have 44ca 40ca ratios that are lower by 0 1 to 0 6 relative to typical mid ocean ridge hydrothermal fluids epidosites are inferred to form in high temperature parts of seafloor hydrothermal systems at temperatures above 300 c and where fluid fluxes are high so the ca isotopic composition of the epidote is likely to reflect fractionation during growth of crystals from aqueous solution available ca isotope data from mor hydrothermal vent fluids and mantle peridotites constrain the 44 ca of likely modern hydrothermal fluids to a narrow range at 44 ca 0 05 0 1 a reactive transport model is used to evaluate whether the 44 ca of hydrothermal fluids might have been higher during the cretaceous and late cambrian the ages of the troodos oman and betts cove ophiolites from which we have data for these calculations we use the epidosite 87sr 86sr as a guide to the extent of ca isotopic exchange that affected the ancient hydrothermal fluids which were derived from seawater with higher ca and sr and lower sulfate concentration than modern seawater the calculations suggest that the ancient hydrothermal fluid 44 ca values were not much different from modern values with the possible exception of the late cambrian example we infer that the epidote fluid ca isotope fractionation averaging 44 ca 0 2 to 0 6 is most likely due to kinetic isotope fractionation during mineral precipitation there is evidence from the literature that hydrothermal epidote may commonly form from oversaturated solutions which makes the kinetic isotope interpretation plausible the equilibrium epidote fluid ca isotope fractionation is estimated to be 44 ca eq 0 based on recently reported dft calculations our results suggest that kinetic calcium isotope fractionation can affect hydrothermal silicate minerals and may be only slightly smaller in magnitude than the effects observed in ca bearing minerals at low temperature kinetic isotope effects during mineral growth could provide new insights into the formation mechanisms of hydrothermal silicate minerals

40k 40ca isotopic constraints on the oceanic calcium cycle the contributions of crustal silicates to the oceanic calcium cycle are investigated using high precision 40 k 40 ca measurements in archean and proterozoic carbonates river waters and a series of terrestrial and extraterrestrial mafic ultramafic samples using a multidynamic data collection scheme with strict controls on instrumental mass fractionation we show that a reproducibility of 0 35 e units 2 is obtained for the 40 ca 44 ca ratio using the thermofinnigan triton this represents an improvement by a factor of 3 compared with previous generations of data well defined excesses of 40 ca from 40 k decay were found in river waters draining the himalayas but not in the mississippi and columbia rivers all marine carbonate samples ranging in age from archean to recent show no discernable effects of 40 k decay to within the limit of 0 35 e units 2 in 40 ca 44 ca our results therefore indicate that the ca isotopic composition of seawater has remained constant and indistinguishable from that of the mantle for the past 3 5 ga despite the influx of radiogenic calcium delivered by weathering of the high k ca components of the continental crust thus over most of geologic time the contributions of the high k ca sources from the continents are far below the contributions of hydrothermal sources mass balance constraints indicate that unless the present day contribution of continental silicates is much less than 8 of the river ca flux the total hydrothermal flux of calcium must exceed the input from high temperature vents at ridge axes by at least one order of magnitude the present day oceanic mass balance requires a high input of calcium 4 15 10 12 mol ca yr at low temperature hydrothermal sites

hydrothermal alteration processes at midocean ridges experimental and theoretical constraints from ca and sr exchange reactions and sr isotopic ratios diabase and basalt were reacted with na ca k cl fluids of seawater chlorinity at 375 425 c 375 400 bars and fluid rock mass ratios of 0 5 1 0 to assess the role of basalt chemistry and texture on sr and ca mobility during high temperature hydrothermal alteration an additional experiment utilizing an 84sr spiked fluid was performed to help quantify reaction rates of processes affecting sr mobility the experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges alteration of basalt and diabase is characterized by formation of tremolite actinolite smectite chlorite and clinozoisitic epidote smectite chlorite respectively diabase alteration produced dissolved sr ca ratios similar to those observed for ridge crest hot spring fluids whereas alteration of a mostly crystalline basalt produced significantly lower ratios this observation supports the premise that the sr ca ratios observed in vent fluids may be produced during deep seated reaction of the hydrothermal fluids with diabase dikes and or gabbro at relatively low fluid rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures 400 c leads to formation of a mineral with low dsr ca possibly hydrothermal plagioclase results of the 84sr spiked experiment indicated that only 4 of the sr in basalt was mobilized after 800 hours of reaction despite the fact that b and li were nearly quantitatively leached it is thus suggested that b and li are good indicators of the amount of fresh rock encountered by fluids while sr concentration and isotopic data can be used to estimate the degree of alteration that the sr bearing primary minerals have undergone the degree of alteration of subsurface reservoirs may be estimated by dividing the fluid rock ratio obtained for highly mobile elements by the fluid rock ratio obtained from path integrated sr isotope and concentration data based on this method for example it can be estimated that only 5 of the primary sr bearing minerals encountered by fluids in the subsurface at epr 21 n are actually converted to secondary phases

strontium oxygen and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in dsdp hole 504b costa rica rift dsdp hole 504b was drilled into 6 ma crust about 200 km south of the costa rica rift galapagos spreading center penetrating 1 35 km into a section that can be divided into four zones zone i oxic submarine weathering zone ii anoxic alteration zones iii and iv hydrothermal alteration to greenschist facies in zone iii there is intense veining of pillow basalts zone iv consists of altered sheeted dikes isotopic geochemical signatures in relation to the alteration zones are recorded in hole 504b as follows zone depth m average87sr 86sr average 18o o average d o i 275 550 0 7032 7 3 63 ii 550 890 0 7029 6 5 45 iii 890 1050 0 7035 5 6 31 iv 1050 1350 0 7032 5 5 36 full size table table options view in workspace download as csv alteration temperatures are as low as 10 c in zones i and ii based on oxygen isotope fractionation strontium isotopic data indicate that a circulation of seawater is much more restricted in zone ii than in zone i fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 20 c in zones iii and iv which is consistent with secondary mineral assemblages the strontium oxygen and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in zones iii and iv are estimated to be 0 7037 2 and 3 respectively strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water rock ratios in weight of about 1 6 this value is close to that of the end member hydrothermal fluids issuing at 21 n epr basement rocks are not completely hydrothermally altered about 32 of the greenstones in zones iii and iv have escaped alteration thus 1 g of fresh basalt including the 32 unaltered portion are required in order to make 1 g of end member solution from fresh seawater in water rock reactions

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

morphodynamics of subaqueous levee formation insights into river mouth morphologies arising from experiments 1 levee formation by deposition from sediment laden flows debouching into open waters has been conceptually and analytically linked to jet hydrodynamics since the middle of the 20th century despite this long standing association between jets and levees the morphodynamics controlling subaqueous levee development remain largely unquantified and poorly understood here we report the results of physical experiments on subaqueous levee development modeled on floodplain tie channel processes in the laboratory we created subaqueous levees from a sediment laden jet entering a basin of still water levee formation occurred where the local shear velocity of the jet declined below the critical shear velocity for entrainment of sediment into suspension the rate of levee deposition depended on the rate of lateral sediment transfer to the jet margins and the settling velocity of suspended sediment in our friction dominated flows lateral sediment flux to the levees was primarily due to dispersive lateral transport driven by turbulence high rates of sediment transfer from the jet core to the margins led to levee growth that outpaced deposition along the flow centerline under hypopycnal conditions we failed to produce levees we observed distinct differences in levee morphology due to changes in the size and density of suspended sediment under identical hydrodynamic conditions our experimental results and review of published data on hypopycnal river mouths suggest that suspended sediment characteristics relative magnitude of bed friction and channel outlet aspect ratios may have a more significant influence on river mouth morphology than the buoyancy of discharging waters as envisioned in existing conceptual models

calcium isotopes in evaporites record variations in phanerozoic seawater so4 and ca reconstructing variations in the major element chemistry of seawater provides constraints on how the global carbon and sulfur cycles have changed over geological time but archives for this information are rare this work generates a new independent record of ancient seawater composition in particular the relative abundance of ca and so 4 through ca isotope measurements in marine evaporites evaporite sequences that reach halite saturation record large ca isotope variability if so 4 ca whereas a small range in ca isotope ratios is observed if ca so 4 analyses of geological evaporites indicate so 4 rich seawater in the neogene and permian and ca rich seawater in the cretaceous and silurian these results agree with previous reconstructions using fluid inclusions in halite and demonstrate a new approach for extending our understanding of oceanic and geochemical evolution

history of seawater carbonate chemistry atmospheric co 2 and ocean acidification humans are continuing to add vast amounts of carbon dioxide co2 to the atmosphere through fossil fuel burning and other activities a large fraction of the co2 is taken up by the oceans in a process that lowers ocean ph and carbonate mineral saturation state this effect has potentially serious consequences for marine life which are however difficult to predict one approach to address the issue is to study the geologic record which may provide clues about what the future holds for ocean chemistry and marine organisms this article reviews basic controls on ocean carbonate chemistry on different timescales and examines past ocean chemistry changes and ocean acidification events during various geologic eras the results allow evaluation of the current anthropogenic perturbation in the context of earth s history it appears that the ocean acidification event that humans are expected to cause is unprecedented in the geologic past for which sufficiently well preserved records are available

a cenozoic seawater sr ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes a cenozoic multi species record of benthic foraminiferal calcite sr ca has been produced and is corrected for inter specific offsets typically less than 0 3 mmol mol and for the linear relationship between decreasing benthic foraminiferal sr ca and increasing water depth the water depth correction determined from holocene late glacial maximum and eocene paleowater depth transects is not vert similar0 1 mmol mol km the corrected cenozoic benthic foraminiferal sr ca record ranges from 1 2 to 2 0 mmol mol and has been interpreted in terms of long term changes in seawater sr ca enabling issues related to higher resolution variability in sr ca to be ignored we estimate that seawater sr ca was not vert similar1 5 times modern values in the late cretaceous but declined rapidly into the paleogene following a minimum in the eocene seawater sr ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late miocene by assuming scenarios for changing seawater calcium concentration and using published carbonate accumulation rate data combined with suitable values for sr partition coefficients into carbonates the seawater sr ca record is used to estimate global average river sr fluxes these fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity tectonic outgassing to calculate changes in global average river 87sr 86sr through the cenozoic the absolute magnitude of sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties nevertheless our results suggest that river sr flux increased from 35 ma to the present day roughly two fold accompanied by an overall increase in 87sr 86sr by not vert similar0 to 0 001 between 75 and 35 ma river 87sr 86sr also increased by not vert similar0 001 to 0 002 but was accompanied by a decrease two to three fold in river sr flux

ophiolites as faithful records of the oxygen isotope ratio of ancient seawater the solund stavfjord ophiolite complex as a late ordovician example

hydrothermal circulation through mid ocean ridge flanks fluxes of heat and magnesium thermally driven convection of seawater occurs through oceanic crust of all ages at the seafloor spreading axis on mid ocean ridge flanks and in the ocean basins at the ridge axis and on the flanks circulating seawater produces a major chemical exchange between the oceans and the crust based on heat budget constraints and the composition of hot springs between 10 and 40 of the river flux of mg can be taken up during high temperature alteration of the basaltic crust along the ridge axis most of the hydrothermal heat loss however occurs on the mid ocean ridge flanks where the temperatures are lower and the seawater flux correspondingly larger the estimated heat loss on the flanks is so large that upwelling must occur over a large fraction 5 30 of the seafloor less than 65 ma in age if temperatures are 20 c and seepage velocities are on the order of 10 to 100 cm y the circulating seawater needs to lose on average less than 1 2 of its mg content in order to solve the mg mass balance for the oceans chemical fluxes through mid ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date the most critical parameter for characterizing crustal conditions is temperature in basement which is a function of crustal age basement topography and sediment thickness and permeability basement temperature which can usually be inferred from heat flow and seismic reflection surveys largely determines the change in the composition of seawater circulating through basement this change can be inferred in turn from profiles of sediment porewater chemistry all sites studied to date with temperatures at the sediment basement interface 25 c have a large component of advective heat loss and show only a small 80 mg loss both types of sites may be important for chemical fluxes whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust only small changes are needed whether the warmer sites are important depends on how much heat is lost by advection in this type of setting the warmer sites could produce significant chemical fluxes even if they are scarce they could account for the entire river input of mg to the oceans even if they represent only 8 20 of the total advective heat loss on ridge flanks

magmatic 87sr 86sr relicts in hydrothermally altered quartz diorites brabant massif belgium and the role of epidote as a sr filter the porphyritic quartz diorites of the caledonian brabant massif have been totally altered ca rb sr zr ce y measurements and sr nd isotopic analyses were performed on the quenast plug and the lessines sill in an attempt to study the relative mobility of sr and evaluate the extent direction and magnitude of the 87sr 86sr alterations sr electron microprobe analyses of epidote were also carried out to assess its role in the sr distribution

alluvial fans formed by channelized fluvial and sheet flow i theory alluvial fans and fan deltas are of three basic types those built up primarily by the action of constantly avulsing river and stream channels those constructed by sheet flows and those resulting from the successive deposition of debris flows the present analysis is directed toward the first two types a mechanistic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean i e averaged over many avulsions bed slope and elevation in an axially symmetric fan an example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel in each example the case of channelized flow is compared to the case of sheet flow the model is applied to the tailings basin of a mine in the companion paper

the pulse of calm fan deltas abstract at the heart of interpreting the history of earth surface evolution preserved in the rock record is distinguishing environmental allogenic forcing from internally generated autogenic noise allogenic deposits classically have been recognized by their cyclic nature which apparently results from periodic changes in base level sediment supply or tectonics autogenic deposits which are quite variable in their origin and scale are caused by the nonlinearity of sediment transport and might be expected to have a random or scale free fractal signature here we describe a robust mechanism that generates cyclic deposits by an autogenic process in experimental fan deltas sheet flow over the fan surface induces deposition and an increase in fluvial slope and curvature to a point where the surface geometry is susceptible to a channelization instability similar to channel initiation on hillslopes channelized flow results in incision and degrading of the fan surface to a lower slope releasing a

calcium isotope δ 44 40 ca fractionation along hydrothermal pathways logatchev field mid atlantic ridge 14 45 n we investigate the logatchev hydrothermal field at the mid atlantic ridge 14 degrees 45 n to constrain the calcium isotope hydrothermal flux into the ocean during the transformation of seawater to a hydrothermal solution the ca concentration of pristine seawater ca sw increases from about 10 mm to about 32 mm in the hydrothermal fluid endmember ca hydend and thereby adopts a delta ca 44 40 hydend of 0 95 0 07 parts per thousand relative to seawater sw and a sr 87 sr 86 isotope ratio of 0 7034 4 we demonstrate that delta ca 44 40 hydend is higher than that of the bedrock at the logatchev field from mass balance calculations we deduce a delta ca 44 40 of 1 17 0 04 parts per thousand sw for the host rocks in the reaction zone and 1 45 0 057 parts per thousand sw for the isotopic composition of the entire hydrothermal cell of the logatchev field the values are isotopically lighter than the currently assumed delta ca 44 40 for bulk earth of 0 92 0 187 parts per thousand sw skulan j depaolo d j and owens t l 1997 biological control of calcium isotopic abundances in the global calcium cycle geochim cosmochim acta 61 12 2505 2510 and challenge previous assumptions of no ca isotope fractionation between hydrothermal fluid and the oceanic crust zhu p and macdougall j d 1998 calcium isotopes in the marine environment and the oceanic calcium cycle geochim cosmochim acta 62 10 1691 1698 schmitt a d chabeaux f and stille p 2003 the calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance earth planet sci lett 6731 1 16 here we propose that ca isotope fractionation along the fluid flow pathway of the logatchev field occurs during the precipitation of anhydrite two anhydrite samples from the logatchev hydrothermal field show an average fractionation of about delta 44 4c ca 0 57 parts per thousand relative to their assumed parental solutions ca isotope ratios in aragonites from carbonate veins from odp drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average delta ca 44 40 of 1 54 0 08 parts per thousand sw the relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine ca isotope budget in particular with regard to ocean crust alteration c 2008 elsevier ltd all rights reserved

fluctuations of outlet and valley glaciers in the southern andes argentina during the past 13 000 years abstract in the southern argentine andes ten advances of valley glaciers were used to reconstruct the late glacial and holocene glacier history the accumulation areas of these glaciers lie in the precordillera and are thus independent of fluctuations of the south patagonian icefield like the viedma outlet glacier the valley glaciers advanced three times during late glacial time 14 000 10 000 yr b p the youngest advance correlates with the younger dryas stade based on two minimum ams 14 c dates of 9588 and 9482 yr b p the second oldest advance occurred before 11 800 yr b p during the first half of the holocene ca 10 000 5000 yr b p advances culminated about 8500 8000 7500 and 5800 5500 yr b p during the second half of the holocene advances occurred between ca 4500 and 4200 yr b p as well as between 3600 and 3300 yr b p in the rio condor valley three subsequent advances have been identified