the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

flux and size fractionation of 3he in interplanetary dust from antarctic ice core samples article i nfo accretion of extraterrestrial material to earth is of interest for a variety of reasons including as a possible driver of long or short term climate change and as a record of solar system events preserved in the geological record 3 he is highly enriched in extraterrestrial material and provides a useful tracer of its input into sedimentary archives previous work showed that polar ice could be a suitable archive for studying variations in extraterrestrial input additional measurements reported here confirm that the late quaternary 3 he flux derived from antarctic ice samples is similar to 3 he fluxes determined from marine sediments the mean flux from nine replicate 1 kg ice samples from the vostok ice core site 112 115 m depth age of 3800 years is 1 25 0 37 10 12 cm 3 stp cm 2 ka 1 mean 2se the large range for the 9 replicates is probably due to the small number of interplanetary dust particles idps present and illustrates that large ice samples are required for precise constraints on temporal variations in the 3 he flux size fraction experiments show that the majority of the 3 he flux is delivered by particles in the 5 10 m size range consistent with the hypothesis

search for the origin of exotic helium in deep sea sediments merrihue1 first observed helium with a very high 3he 4he isotope ratio 10 4 here termed exotic helium in a magnetic separate of a deep ocean sediment and attributed it to input of extraterrestrial material krylov et al 2 subsequently reported the existence of exotic he in several deep sea sediments while tilles3 measured argon isotope ratios in magnetic separates of two ocean sediments observing that the 3he 4he ratio is inversely correlated with the sedimentation rate ozima et al 4 concluded that the exotic he is carried by interplanetary dust particles idps we have now examined other deep sea sediments to characterize the exotic he and to identify its carrier and have confirmed that the exotic he resides mainly in a strongly magnetic fraction stepwise degassing experiments suggest that the exotic he consists of a single component the carrier of the he is likely to be chondritic idps the higher level of exotic he in magnetic fractions of sediments can be explained by secondarily produced magnetite in idps as a result of thermal shock during atmospheric entry

construction of the cambrian and ordovician seawater 87sr 86sr curve abstract the estimated path of seawater 87 sr 86 sr during the cambrian and ordovician is based on 172 analyses the sample suite is largely from locations in oklahoma new york tennessee and texas where rocks of this age are widely exposed and well studied the oldest reliable samples examined in this study are of middle cambrian age and yield an 87 sr 86 sr near 0 70888 sw 19 the estimated ratio rises to near 0 70915 sw 8 in the lower upper cambrian the highest value during the phanerozoic and probably for all geologic time the estimated path drifts lower to near 0 70903 sw 4 at the cambrian ordovician boundary this further declines to near 0 70875 sw 32 at the end of the ibexian and reaches a value near 0 70821 sw 86 at the whiterockian mohawkian boundary the decline in the seawater 87 sr 86 sr continues into later mohawkian time to flatten abruptly near 0 70782 sw 125 this ratio is essentially constant through the latest ordovician the curve rises near to a ordovician silurian boundary value poorly constrained from our data near 0 70784 sw 123 the early paleozoic seawater curve shows the alternating dominance of continental and oceanic sources of sr continental sr dominated seawater from the beginning of the cambrian into late cambrian from that time as the cambrian transgression proceeded and radiogenic crystalline rocks were covered oceanic sr began to dominate sr from low ratio oceanic sources dominated through early mohawkian time an abrupt balance is struck between oceanic and continental sources to end the decline in the late ordovician and maintained until near the end of the ordovician the general curve shape follows the pattern suggested by proposed sealevel changes the rise and fall of seawater 87 sr 86 sr during the cambrian and ordovician characterized seawater variation during all the paleozoic

hydrological factors in the interpretation of stable isotopic proxy data present and past a european perspective in the context of this paper stable isotopic proxies are those preserving directly or via known or calculable fractionation factors a record of 18o 16o and or 2h 1h in the water which contributed to their growth suitable proxies include amongst others tree ring cellulose bone phosphate lake sediment minerals and speleothem fluid inclusions a principal aim of studying isotopic proxies is to gain more information about past climates through a knowledge of how rainfall isotope compositions have changed over time however converting back from proxy to rainfall is not necessarily straightforward because however well the isotopic fractionation due to proxy growth or formation is understood a variety of modifying processes may have intervened between this and the actual rainfall events three main reservoirs of environmental water may contribute to the isotopic compositions of terrestrial proxy indicators surface waters soil moisture and groundwater each of these can in principle be isotopically modified compared to rainfall in the locality this review examines these effects and how they may change under different european climate types at the present day it is concluded that while groundwater isotopic compositions typically represent closely the bulk composition of rainfall under a variety of climatic regimes soil and particularly lake waters require detailed study if their isotope systematics are to be adequately understood the review also finds that to a large extent the present is the key to the past in terms of recharge processes during the late pleistocene though glaciation may have had profound isotopic consequences in certain areas while a more general aridity led to the apparent absence of recharge in most areas for much of the late devensian

nonmonotonic variation of seawater 87 sr 86 sr across the ivorian chadian boundary mississippian osagean evidence from marine cements within the irish waulsortian limestone abstract detailed analysis of compositionally unaltered marine fibrous cements mfc from a single core through the mississippian irish waulsortian limestone indicates that the variation of seawater 87sr 86sr is nonmonotonic across the ivorian chadian boundary this nonmonotonic variation has not been recognized by previous studies furthermore marine cements yielded 87sr 86sr ratios lower than previously reported values for the ivorian chadian osagean marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope c o composition comparable to previous estimates of mississippian marine calcite the isotope chemistry c o sr and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high mg calcite to low mg calcite microdolomite a conversion that probably took place in marine water during precipitation of zone 1 calcite cement the oldest non mfc cement high stratigraphic resolution was obtained by restricting the sample set to a single core 429 m long thereby eliminating chronostratigraphic correlation errors the mfc cements are pre arundian based on previous study and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones the core is estimated to represent about 9 8 million years of waulsortian limestone deposition 87sr 86sr ratios of pristine mfc samples range from a high of 0 707908 in the early ivorian to a low of 0 707650 in the late ivorian and mid chadian with an early chadian maximum at 0 707800 the maximum rate of change in seawater 87sr 86sr is 0 00012 ma comparable in magnitude to tertiary values our data document the presence of fine scale seawater 87sr 86sr modulations for the ivorian chadian in contrast to the previously published monotonic seawater 87sr 86sr curve for this interval and emphasize the importance of well characterized intraformational isotopic baselines

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

extraterrestrial 3he in seafloor sediments evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust orbital parameters and quaternary climate we have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the ontong java plateau western equatorial pacific ocean odp site 806 these results provide a two million year record of the burial flux of extraterrestrial 3he which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles the 3he burial flux prior to 700 ka was relatively low 0 5 pcc cm 2 kyr 1 but from 700 ka to the present the burial flux gradually increased to a value of 1 0 pcc cm 2 kyr 1 100 kyr periodicity in the 3he burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate with high 3he burial fluxes associated with interglacial periods this periodicity and phase are consistent with previous 3he measurements in north atlantic sediments although 100 kyr periodicity in 3he burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris the measurements and predictions differ by one half cycle in phase nevertheless our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and or inclination relative to the solar system invariable plane such control is a necessary but not sufficient condition for the hypothesis of muller and macdonald 1995 that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period we also identify several brief 25 kyr intervals of strongly enhanced 3he burial possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

fluid circulation and fault and fracture related diagenesis in mississippian syn rift carbonate rocks on the northeast margin of the metalliferous dublin basin ireland

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

persistent environmental change after the paleocene eocene thermal maximum in the eastern north atlantic the paleocene eocene thermal maximum petm 56 ma is associated with abrupt climate change carbon cycle perturbation ocean acidification as well as biogeographic shifts in marine and terrestrial biota that were largely reversed as the climatic transient waned we report a clear exception to the behavior of the petm as a reversing climatic transient in the eastern north atlantic deep sea drilling project site 401 bay of biscay where the petm initiates a greatly prolonged environmental change compared to other places on earth where records exist the observed environmental perturbation extended well past the 13 c recovery phase and up to 650 kyr after the petm onset according to our extraterrestrial 3he based age model we observe a strong decoupling of planktic foraminiferal 18 o and mg ca values during the petm 13 c recovery phase which in combination with results from helium isotopes and clay mineralogy suggests that the petm triggered a hydrologic change in western europe that increased freshwater flux and the delivery of weathering products to the eastern north atlantic this state change persisted long after the carbon cycle perturbation had stopped we hypothesize that either long lived continental drainage patterns were altered by enhanced hydrological cycling induced by the petm or alternatively that the climate system in the hinterland area of site 401 was forced into a new climate state that was not easily reversed in the aftermath of the petm

columnar calcites as testimony of diagenetic overprinting at the boundary between upper tournaisian dolomites and limestones belgium multiple origins for apparently similar features in topographic flat areas sedimentary settings may vary from one outcrop to another in these settings calcite precipitates may yield macroscopically similar columnar features although they are products of different sedimentary or diagenetic processes three columnar calcite crystal fabrics i e rosettes palisade crusts and macro columnar crystal fans have been differentiated near and at the contact between upper tournaisian dolomites and limestones along the southern margin of the brabant wales palaeohigh their petrographic characteristics and geochemical and fluid inclusion data provide information on the dia genetic processes involved rosettes composed of non luminescent columnar calcite crystal fans 1 5 cm in diameter developed on top of one another forming discrete horizons in repetitive sedimentary cycles the cycles consist of three horizons i a basal horizon with fragments from the underlying horizon ii a micrite microspar horizon with incipient glaebules iii an upper horizon consisting of calcite rosettes with desiccation features the petrographical features and 18o signatures of 10 0 to 5 5 and 13c values of 5 5 to 3 2 support either evaporative growth an evaporative pedogenic origin or overprinting of marine precipitates palisade crusts composed of a few to 10 mm long nonluminescent calcite crystals coat palaeokarst cavities successive palisade growth stages occur which are separated by thin laminae of micrite or detrital quartz displaying a geopetal arrangement palisade crusts are interpreted as intra mississippian speleothems this interpretation is supported by their petrographic characteristics and isotopic signature 18o 8 7 to 6 5 and 13 4 8 to 2 5 macro columnar crystals 1 50 cm long developed mainly perpendicular to cavity walls and dolomite clasts crystal growth stages in the macro columnar crystals are missing 18o values vary between 6 4 and 6 8 and 13c values between 5 2 and 0 9 these features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification 2005 international association of sedimentologist

chronostratigraphic significance of cathodoluminescence zoning in syntaxial cement mississippian lake valley formation new mexico abstract echinoderm syntax ial cement crystals have been collected at several stratigraphic levels within the lake valley formation which is overlain by a major pre pennsylvanian subaerial exposure surface the crystals were microsampled along growth bands and yielded high resolution elemental and isotopic information that record fluid evolution during their growth although cement crystals show little variation in cathodoluminescence character and bulk chemistry throughout the regional extent of the formation intracrystalline patterns in minor element and stable isotope chemistry allow for the identification of several settings of cementation including the marine phreatic marine meteoric mixing and meteoric phreatic zones when placed in a regional stratigraphic context crystal growth records enable reconstruction of the temporal and spatial extent of these diagenetic environments isotopic petrographic and stratigraphic constraints indicate that cementation was related to two temporally distinct meteoric systems most lake valley formation syntaxial cement formed in the marine phreatic and marine meteoric mixing zones during the earliest phase of cementation of this cement in excess of 60 formed in the marine phreatic zone and lowermost marine meteoric mixing zone smaller volumes precipitated from progressively fresher mixtures of marine and meteoric fluid during the gradual expansion of a freshwater lens that developed in response to pre pennsylvanian lowering of sea level meteoric phreatic fluids were relatively unimportant during this episode of cementation in contrast cement of entirely meteoric phreatic origin is associated with a later meteoric system but is found only in the northern part of the study area and at stratigraphic levels immediately adjacent to the pre pennsylvanian unconforrnity our results indicate that cements in the upper and lower parts of the lake valley formation are genetically and temporally unrelated thus the cement cathodoluminescence stratigraphy which has been previously applied to the lake valley can not be valid over the full extent of the formation this has wider implications our demonstration that diverse and temporally distinct diagenetic settings can produce identical cl characteristics shows that caution should be employed when using cement cl stratigraphy alone to extend diagenetic interpretations over large geographic regions or through substantial stratigraphic successions

quaternary time scale for the ontong java plateau milankovitch template for ocean drilling program site 806 a 2 m y oxygen isotope record of globigerinoides sacculifer from the ontong java plateau based on cores from ocean drilling program leg 130 is dated by matching variations to an orbital template the procedure allows us to present the most complete quaternary record available for the western equatorial pacific the template generating algorithm describes a balance between growth and melting of ice following basic milankovitch theory ice growth is taken as constant while melting is taken to depend on summer insolation current ice mass and average past ice mass template settings must be changed once between 1 and 1 2 ma to reflect a major shift in climate template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances one result of template dating is an exact age for the brunhes matuyama magnetic reversal boundary at 790 5 ka as well as several other precise dates 900 ka for the middle pleistocene climate shift 1070 1240 and 1450 ka for isotope stages 31 37 and 47 respectively sedi mentation rates fluctuate between 18 and 28 m m y a ca 400 ka cycle being the most prominent major anomalies arise within the transitional regime 1 2 to 1 ma the origin of the cycles is unknown we propose productivity variations in the western equatorial pacific

simultaneous presence of orbital inclination and eccentricity in proxy climate records from ocean drilling program site 806 ocean drilling program site 806 in the western pacific shows evidence of a remarkably constant average sedimentation rate this feature allows us to analyze ancient climate proxies without the need for orbital tuning a standard procedure in prior work but one that can lead to biased results spectral analysis of stable oxygen isotope ratios at this site a proxy for global ice volume shows a single narrow peak with a period 100 k y a result that supports our model which links glacial cycles to variations in the inclination of the earth s orbit in contrast spectral analysis of the coarse component fraction of the sediment primarily foraminifera shows a structure characteristic of standard milankovitch theory with a triplet of peaks with periods near those expected from the earth s eccentricity 95 125 and 400 k y bispectral analysis confirms these linkages but suggests that orbital inclination also plays some role in the coarse fraction from the clear presence of both signals in different proxies at the same site we conclude that although eccentricity affected the local climate it is orbital inclination that drove the variations in the global ice volume for the past million years

isotopic evidence for palaeowaters in the british isles before the 20th century groundwater circulation in the aquifers of the british isles had largely adjusted to the temperate maritime climate and sea levels established over the past 10 ka since the end of the pleistocene however in the last 100 a this natural regime has been disturbed by abstraction of water for public supply and industrial use and palaeowaters from earlier recharge episodes are now becoming a factor to be considered in water balance estimates this paper presents a synthesis of the existing palaeowater distribution in the british isles based on isotopic evidence 18o 2h and and14c as such it has relevance to palaeoclimatic studies in addition to the water resource implications the triassic basins of england and northern ireland contain saline waters beyond the range14c dating 40 ka stable isotopic ratios show enrichment in some basins and depletion in others without an overall pattern that would explain all the observed compositions the results for the wessex basin suggest recharge in pre quaternary times but for the other basins some flushing by pleistocene or holocene meteoric waters is indicated isolated occurrences of apparently long residence waters are found elsewhere throughout the british isles for example from carboniferous and lower palaeozoic strata in such cases environmental isotopes are more useful as constraints on hydrogeological models than as indicators of palaeoconditions major water supply aquifers are restricted almost entirely to england the two sandstone formations triassic sherwood sandstone and cretaceous lower greensand have a greater range of stable isotopic values between phreatic and confined conditions than the two carbonate formations jurassic lincolnshire limestone and cretaceous chalk this indicates that the sandstone aquifers are better archives of information on palaeoconditions than the carbonate aquifers they show that atmospheric circulation patterns over britain have probably remained the same since the late pleistocene however 14c data from all four of the major aquifers emphasise the hiatus in recharge during periglacial conditions which occurred between the late pleistocene and early holocene

formation and diagenesis of bedding cycles in uppermost cretaceous chalks of the dan field danish north sea metre scale lithologic cycles visible in core and on logs from maastrichtian chalks of the dan field were examined to determine their mechanisms of deposition and relation to hydrocarbon production the lower parts of cycles consist of porous cream coloured largely non stylolitic commonly laminated chalk with limited bioturbation mainly escape burrows cycles are capped by thinner intervals of white to grey hard stylolitic chalk with concentrations of bioclastic material intense burrowing and few preserved primary sedimentary structures the cycle caps contain nearly twice as much mg as compared to the more porous parts of cycles and also have slightly larger 18o values 4 1 for the caps 4 4 for porous zones there is a significant reduction of average cycle thickness as well as total thickness of the maastrichtian chalk section from sw to ne across the dan field the cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the ne side of the field these data indicate that episodic winnowing removed fine grained constituents from highstanding northeastern areas porous cycle bases were deposited at relatively high rates that precluded complete bioturbation preserved laminae coupled with escape burrows reflect episodic sediment influx in areas that flank the seafloor highs cycle tops apparently accumulated more slowly throughout the region but especially on seafloor highs perhaps because of reduced productivity of planktic organisms slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures and also led to incipient high magnesium calcite seafloor cementation sufficient to yield firmer sediment and enhanced burrow preservation but not to form true hardgrounds thus the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis rates of chalk deposition as inferred from physical and geochemical evidence appear to be a significant control on reservoir characteristics in north sea chalks the highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties

metal sources of the navan carbonate hosted base metal deposit ireland nd and sr isotope evidence for deep hydrothermal convection nd and sr isotope analyses are presented for gangue mineral samples from the giant carbonate hosted navan zn pb deposit ireland and for rocks from which navan metals may have been derived analysis of gangue minerals spanning the navan paragenetic sequence reveals systematic evolution in the composition of the mineralising fluid early fluid represented by replacive dolomite exhibits the lowest initial 87sr 86sr ratio 0 7083 0 7086 closest to that of the host limestone and to lower carboniferous seawater and the highest 143nd 144nd ratio 0 51161 0 51176 later generations of dolomite barite and calcite which encompass sulphide precipitation have higher initial 87sr 86sr ratios maximum 0 7105 and lower initial 143nd 144nd ratios minimum 0 51157 all samples have initial nd isotope ratios that are too low to have been acquired only from the host limestone drill core samples of presumed ordovician volcanic and sedimentary rocks from beneath the navan orebody have 143nd 144nd and 87sr 86sr ratios at the time of mineralisation of 0 51184 0 51217 and 0 7086 0 7138 respectively the data are interpreted to indicate mixing of sulphide rich limestone buffered brine with a metal bearing hydrothermal fluid which had passed through sub carboniferous rocks consistent with published fluid inclusion and s isotope data the 143nd 144nd ratio of this basement derived fluid is too low to have been imparted by flow through the devonian old red sandstone as required in models of regional fluid flow in response to hercynian uplift irrespective of whether such regional fluid flow occurred the hydrothermal nd must have been derived from sub devonian rocks these conclusions broadly support the hydrothermal convection cell model in which brines ultimately of surface origin penetrated to a depth of several kilometres leaching metals from the rocks through which they passed the data also support increasing depth of penetration of convection cells with time metals were subsequently precipitated in carbonate rocks at sites of mixing with cooler sulphide rich fluids however comparison of the navan hydrothermal gangue nd sr isotope data with data from lower palaeozoic rocks strongly suggests that the latter cannot alone account for the basement signature as the navan deposit lies immediately north of the iapetus suture this suggests that the laurentian margin includes precambrian basement

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

stable isotopic compositions of recent freshwater cyanobacterial carbonates from the british isles local and regional environmental controls recent 50 years old freshwater cyanobacterial carbonates from diverse environments streams lakes waterfalls throughout britain and ireland were analysed for their stable carbon and oxygen isotope compositions the mean 18o value of 5 9 pdb for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central britain 7 5 smow assuming a mean water temperature of 9 c the mean 18o of lake data 4 5 pdb is statistically different reflecting the effects of residence time and or variations in the oxygen isotopic composition of rainfall carbon isotopes have wide variations in both fluviatile and lake data sets 3 to 12 pdb these variations are principally controlled in the fluviatile samples by contribution of isotopically light soil zone carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution lake samples have the heaviest carbon isotope values reflecting a trend toward isotopic equilibrium between atmospheric co2 and aqueous hco 3 we infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information although the effects of stabilization and diagenesis on primary 18o values will need careful consideration primary carbon isotope compositions should be well preserved although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate

geochemical evidence for a comet shower in the late eocene analyses of pelagic limestones indicate that the flux of extraterrestrial helium 3 to earth was increased for a 2 5 million year my period in the late eocene the enhancement began 1 my before and ended 1 5 my after the major impact events that produced the large popigai and chesapeake bay craters 36 million years ago the correlation between increased concentrations of helium 3 a tracer of fine grained interplanetary dust and large impacts indicates that the abundance of earth crossing objects and dustiness in the inner solar system were simultaneously but only briefly enhanced these observations provide evidence for a comet shower triggered by an impulsive perturbation of the oort cloud

temporal variation of 3he 4he ratio recorded in deep sea sediment cores he isotopic composition was measured for 72 deep sea sediments from 16 sites in the pacific and atlantic oceans and the japan sea 3he content ranges from less than 10 12 cm3 stp g to about 6 10 11 cm3 stp g and 3he 4he from about 10 8 to 1 67 10 4 the he isotopic data lie fairly well on a mixing curve in a 3he 4he 3he diagram where the end members of the mixing curve are extraterrestrial he in interplanetary dust particles idp s and terrestrial he in ocean sediments this confirms the view that there are significant amounts of extraterrestrial he in ocean sediments from global fallout of idp s into deep sea sediments 3he flux estimated for two long cores gives a value of about 1 5 1 0 10 15 cm3 stp cm2 yr over a time span of 40 m y which corresponds to an annual fallout rate of idp s size 10 m of about 400 t in one of the cores we observe a 3he spike which is associated with anomalously high ir content 1 86 0 56 ppb this may be due to meteorite debris

isotope stratigraphy of the european carboniferous proxy signals for ocean chemistry climate and tectonics abstract carboniferous brachiopods from western europe and the former ussr have been utilized as carriers of isotopic proxy signals for paleoceanography and paleoclimatology during the assemblage of pangea this is of particular interest because this time interval coincides with the transition from greenhouse to icehouse conditions the 18 o values 3 ma running means show an overall 18 o increase from about 7 1 to 3 1 during the carboniferous the rise in the 18 o is stepwise with major shifts in the mid tournaisian and at the visean serpukhovian transition the comparable 13 c record shows a tournaisian peak at 5 a visean oscillation at about 2 5 1 a serpukhovian rise of more than 3 a plateau around 5 during the bashkirian and moscovian a 4 drop during the kasimovian and another rise of about 3 during the gzhelian the 87 sr 86 sr record shows an early carboniferous decline from 0 7083 to a mid visean minimum of 0 7077 followed by a rapid serpukhovian rise to a bashkirian and moscovian plateau at 0 7087 and a slight decline during the kasimovian and gzhelian the covariance of isotopic trends and the coincidence with geological phenomena indicates that the long term variations in ocean chemistry and climate may be coupled to the tectonic evolution of the earth system the namurian witnesses the onset of hercynian orogeny initiation of the main phase of carboniferous glaciation formation of the extensive coal deposits and the rise in 87 sr 86 sr 13 c and 18 o all this suggests that a tectonically controlled climate change resulting in cooling and increased carbon storage in the organic reservoir was the major triggering factor for all the above phenomena the late carboniferous glacial episode was interrupted by a warm interval in the stephanian the carbon and oxygen isotope data indicate that this episode may have been restricted to the early kasimovian a proposition at odds with the poorly dated record of glacial deposits that are interpreted as indicating a warm climate for the entire stephanian in addition the pattern of short lived peaks in 13 c and 18 o in the tournaisian and possibly visean may be interpreted as a reflection of short lived glacial episodes a proposition consistent with geologic records

radiogenic isotopes systematics and applications to earth surface processes and chemical stratigraphy radiogenic isotopes have wide application to chemical stratigraphy geochronology provenance studies and studies of temporal changes in earth surface processes this paper briefly reviews the principles of radiogenic isotope geochemistry and the distribution of a number of elements of interest in the environment and then uses this information to explore the range of applications to chemical stratigraphy and other fundamental subjects of sedimentary geology many of these applications center on the reconstruction of secular variations in paleoseawater isotope signals these seawater isotope curves can be used for stratigraphic correlation and to assess changes through earth history in processes such as tectonic uplift climate change biogeochemical cycling ocean circulation crustal growth and surface water evolution understanding the modern biogeochemical cycle of the element isotope system of interest is essential for gaining the maximum information regarding the operation of ancient cycles the main sources of uncertainty for reconstructing seawater isotope curves involve diagenetic alteration sample impurities and errors in age assignments rigorous sample selection criteria and pretreatment methods are keys to reconstructing the most accurate seawater isotope curves applications are discussed for the rb sr sm nd la ce lu hf re os u th pb k ar and cosmogenic isotope systems d 2003 elsevier b v all rights reserved

pre anglian fluvial and coastal deposits in eastern england lithostratigraphy and palaeoenvironments abstract clast lithology and derived microflora are used to provide a basis for correlating pre anglian fluvial and coastal deposits in eastern england and to provide a lithostratigraphic framework for these early and early middle pleistocene sediments three fluvial systems are recognised i the thames which drained an area from wales through midland england to the thames basin and southern east anglia the catchment was truncated over the period concerned and the western headwaters diverted to other river systems ii the bytham river which drained midland england and the southern pennines and reached the southern north sea delta in the region of north central east anglia this river extended its catchment over the period concerned iii the ancaster river which drained the southern pennines and received sediment from northeast england each of these catchments contributed sediment to the southern north sea delta estuary and contributed to the lithologies of the red norwich and wroxham crag formations the lithostratigraphic subdivisions are based on the fact that i the pliocene and early pleistocene rivers and the equivalent red and norwich crags are characterised by far travelled microflora which was transported in suspension and a locally derived clast assemblage transported as bedload and ii the later early and early middle pleistocene rivers and the equivalent wroxham crag are characterised by a far travelled microflora and a far travelled clast assemblage variations in the amount and composition of far travelled materials is determined by the relative importance of the three river systems this provides a basis for identifying different formations of the thames river sediments older sudbury formation and younger colchester formation and different members of the wroxham formation dobb s plantation member local material with minor components of thames and ancaster river sediments how hill member local material with major components of thames and ancaster river sediments mundesley member with major components of bytham river sediments these lithostratigraphic units are related to established north european biostratigraphy and oxygen isotope stratigraphy the factors forcing the changes in river energetics are explained in terms of the relative importance of geomorphic processes operating within the river catchments determined by the patterns of milankovitch orbital forcing

oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate because meteoric water 18 o values decrease with decreasing ambient temperature and increasing latitude 18 o values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures to test this we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from devonian to modern and in paleolatitude from 3 5 to 83 mean 18 o values for meteoric cements and speleothems both show the same negative correlation with paleolatitude the 18 o vs latitude trend for these carbonates is almost identical to that predicted for modern inland environments but differs from the trend for coastal environments this suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge if it is assumed that the modern meteoric water 18 o vs temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation then coastal and inland paleotemperatures can be calculated from the 18 o values of meteoric calcite 18 o mcl and seawater 18 o sw by using the equations t coastal 13 3 32 6 0 231 0 0613 18 o mcl 18 o sw and t inland 17 8 16 2 0 572 0 1233 18 o mcl 18 o sw where t is temperature in c calcite precipitated from coastal meteoric water at temperatures between 0 and 25 c will exhibit a narrow range in 18 o 6 to 4 where 18 o sw 0 the 18 o of calcite precipitated from inland meteoric water will be sensitive to paleotemperature ranging from 14 to 5 where 18 o sw 0 for temperatures of 0 to 25 c

helium and neon isotopes in deep pacific ocean sediments helium and neon concentration measurements along with isotope ratio determinations have been made for particles collected in the deep pacific with a magnetic sled and they are believed to be of extraterrestrial origin analyses were made for samples consisting of composites of many extremely fine particles and for several individual particles large enough to contain sufficient gas for analysis but small enough to escape melting in their passage through the atmosphere step heating was employed to extract the gas cosmic ray spallation products or solar wind helium and neon if present were not abundant enough to account for the isotopic compositions measured in the case of the samples of magnetic fines the low temperature extractions provided elemental and isotopic ratios in the general range found for the primordial gas in carbonaceous chondrites and gas rich meteorites the isotopic ratios found in the high temperature extractions suggest the presence of solar flare helium and neon

4he as a tracer of continental dust a 1 9 million year record of aeolian flux to the west equatorial pacific ocean the mass accumulation rates mars of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past the incorporation of other terrigenous components such as volcanic ashes in seafloor sediments however often obscures the aeolian dust record we describe a new approach which uses the delivery rate of crustal 4he to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components we have determined the flux of crustal 4he delivered to the seafloor of the ontong java plateau ojp in the western equatorial pacific over the last 1 9 myrs crustal 4he fluxes vary between 7 7 and 30 ncc cm 2 kyr 1 and show excellent correlation with global climate as recorded by oxygen isotopes with high crustal 4he fluxes associated with glacial periods over the entire interval studied furthermore the onset of strong 100 kyr glacial interglacial climate cycling is clearly seen in the 4he flux record about 700 kyrs ago these data record variations in the supply of asian dust in response to climate driven changes in the aridity of the asian dust sources consistent with earlier work on asian dust flux to the northern pacific ocean however in contrast to previous studies of sites in the central and eastern equatorial pacific ocean there is no evidence that the inter tropical convergence zone an effective rainfall barrier to the southward transport of northern hemisphere dust across the equator in the central and eastern pacific has influenced the delivery of asian dust to the ojp the most likely carrier phase for crustal helium in these sediments is zircon which can reasonably account for all the 4he observed in the samples as a first order estimate these results suggest that the mass accumulation rate of asian dust on the ojp over the last 1 9 myrs varied from about 4 to 15 mg cm 2 kyr 1 in contrast previous studies show that over the same interval the total mar of terrigenous dust i e asian dust plus local volcanics on ojp varied between about 34 and 90 mg cm 2 kyr 1

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

microbial crust with frutexites and iron staining in chalks albian cenomanian boundary hunstanton uk new petrographic observations and stable isotope data help reinterpret the complex sedimentology of the albian cenomanian boundary exposed in the famous red and white chalks of the cliff section at hunstanton in norfolk uk thin section analysis of a prominent crust at the top of the hunstanton red chalk formation reveals fossil microstructures attributable to the microbe frutexites these red chalk microstructures are less bushy that frutexites sensu stricto and poor preservation in part caused by later diagenetic iron migration means they are identified only tentatively as frutexites stable oxygen isotope values from the crust are similar to those from early diagenetic nodular chalks immediately below the crust and to partially altered chalks elsewhere in norfolk the 18o data are interpreted as albian seafloor depositional values albeit slightly altered by subsequent meteoric diagenesis the microbial affinities of frutexites are not yet proven thus the presence of frutexites alone is not diagnostic of either photic zone or deep water sedimentation however the presence of frutexites suggests that the red colour of the hunstanton red chalk is due at least in part to the mediation of iron fixing microbes in the accumulating chalk sediment at a dysoxic anoxic interface centimetre scale columnar and nodular structures above the frutexites crust that project upwards into the basal paradoxica bed of the overlying ferriby chalk formation are sites of localized syndepositional iron staining these nodules are not stromatolitic or microbial and are not evidence for deposition in shallow water or intertidal settings

geochemical constraints on the origin of enigmatic cemented chalks norfolk uk very hard cemented chalk stacks and crusts found locally in the upper part of the cretaceous chalk of north norfolk uk are related to solution features the solution features mainly pipes and caves formed after deposition of the overlying middle pleistocene wroxham crag probably by routing of sub glacial or glacial melt waters derived from late pleistocene glaciers new geochemical particularly stable isotope data shows that cementation of the chalks although related spatially to the solution features was not caused by glacier derived waters the carbon isotope composition of the chalk cements is typically around 9 5 indicative of biologically active soils moreover the oxygen isotope compositions of the cements around 5 are incompatible with water 18 o values much below 9 to 10 which probably precludes isotopically negative glacier derived water as resulting palaeo temperatures are below zero taken together the isotope data suggest chalk cementation occurred under interglacial conditions similar to the present dissolved calcium carbonate for cementation came from dissolution of reworked chalk in overlying mis 12 glacial tills

controls on stable isotope profiles in the chalk aquifer of north east norfolk uk with special reference to dissolved sulphate groundwater samples were collected at a site in n norfolk uk comprising a borehole penetrating upper chalk and piezometers open within the overlyinf glacial deposits and intervening weathered putty chalk the samples were analyzed for 18o water 34s and 18o so4 and 13c hco3 as well as major ions variations in solute concentration and isotopic composition with depth were found to be caused by limited groundwater circulation at the site and incomplete flushing of old groundwater the isotopic data prove the existence of a mineralized palaeowater at depth which has undergone so4 reduction and is being slowly modified by mixing with recent groundwater circulating at a shallow depth in a more transmissive chalk horizon one measured 18o h2o value of 8 25 vsmow represents the lightest value reported for the norfolk chalk aquifer and is evidence for a component of late pleistocene recharge trapped at depth the wide range of observed isotope ratios at the site e g 34s and 18o so4 between 7 8 to 27 8 cdt and 5 2 to 20 0 ovsmow respectively demonstrates the importance of vertical stratification of solutes and the implications this has for interpreting spatial hydrochemical surveys of groundwater

palaeo climate reconstruction from stable isotope variations in speleothems a review abstract speleothems are now regarded as valuable archives of climatic conditions on the continents offering a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores they are ideal materials for precise u series dating yielding ages in calendar years thereby circumventing the radiocarbon calibration problems associated with most other continental records stable isotope studies in speleothems have shifted away from attempting to provide palaeo temperature reconstructions to the attainable goal of providing precise estimates for the timing and duration of major o isotope defined climatic events characterised by high signal to noise ratios e g glacial interglacial transitions dansgaard oeschger oscillations the 8200 year event unlike the marine records speleothem data sets are not tuned and their independent chronology offers opportunities to critically assess leads and lags in the climate system that in turn can provide important insights into forcing and feedback mechanisms improved procedures for the extraction and measurement of stable isotope ratios in fluid inclusions trapped in speleothems are likely to provide in the near future a much enhanced basis for the quantitative interpretation of o isotope ratios in speleothem calcite the latter developments open up once again the tantalising prospect of palaeo temperature estimates but more importantly perhaps provide a direct test for a new generation of general circulation models whose hydrological cycles will incorporate the water isotopes the literature is reviewed briefly to provide for the reader a sense of the current state of the art and to provide some pointers for future research directions

environmental setting micro morphologies and stable c o isotope composition of cold climate carbonate precipitates a review and evaluation of their potential as paleoclimatic proxies abstract given the growing interest in carbonate deposits from polar regions as paleoclimatic proxies this review paper first provides a classification of the various types of cold climate carbonate precipitates followed by a summary of the 13 c and 18 o composition of the carbonate deposits and parent water from which the carbonates precipitated the cold climate carbonate precipitates were classified into three broad categories powders crusts and speleothem the carbonate powders include those that precipitated in relation to aufeis aggradation cryogenic aufeis calcite and in relation to the growth of various annual perennial ice formations in freezing caves cryptocrystalline calcite and calcite pearls the carbonate crusts can be further subdivided based on their lithic environment those that precipitated on the upper surface of bedrock clasts i e subglacially precipitated calcite and evaporative calcite crusts those that are located on the underside of clasts i e pedogenic carbonates and those that precipitated in rock outcrop fissures i e endostromatolites the cold climate carbonate precipitates have a highly variable isotopic composition with 18 o values ranging between 6 5 and 28 vsmow and 13 c values in the 10 20 vpdb range however each type of carbonate precipitates has a specific 13 c and 18 o range suggesting that their environmental setting and the mechanism by which they formed controls their 13 c and 18 o signature it was found that carbonate deposits that precipitated under equilibrium physico chemical conditions had a 13 c value that is in equilibrium with that of the parent water while its 18 o composition was more variable as it is in part controlled by the temperature of reaction and by the 18 o and calcite saturation state of the parent water by contrast the 18 o composition of biologically precipitated carbonate deposits endostromatolites reflect that of the parent water while its 13 c composition was enriched over that of the parent water due to bacterial methanogenesis in the case of kinetically precipitated carbonate deposits the 18 o and 13 c values are out of equilibrium relative to that of the parent water due to the faster rate of reaction

glacial cycles and astronomical forcing narrow spectral features in ocean sediment records offer strong evidence that the cycles of glaciation were driven by astronomical forces two million years ago the cycles match the 41 000 year period of earth s obliquity this supports the croll milankovitch theory which attributes the cycles to variations in insolation but for the past million years the spectrum is dominated by a single 100 000 year feature and is a poor match to the predictions of insolation models the spectrum can be accounted for by a theory that derives the cycles of glaciation from variations in the inclination of earth s orbital plane

age limits on middle pleistocene glacial sediments from osl dating north norfolk uk the lowland region of north norfolk contains some of the best preserved evidence for glacial deposition during the middle pleistocene in northwest europe despite the importance of these deposits there is limited chronological control and it is debated whether they belong to a single glaciation equated to the anglian glaciation marine isotope stage 12 or represent deposition over a number of middle pleistocene cold stages in order to develop an improved chronology for glaciation in this region we obtained 18 samples for optical stimulated luminescence osl dating from glacial outwash facies samples of coarse grained quartz sand were measured using the sar single aliquot regenerative dose protocol the low radioactive isotope concentrations in sediments of this region enable the traditionally accepted age limit of luminescence dating to be extended the form of the dose response curves pre heat plateaux tests and dose recovery experiments also indicate that laboratory doses can be reliably measured in these samples age overestimation due to partial bleaching is thought to be insignificant the osl ages suggest that the glacial sediments studied were deposited during mis 12 rather than in different post mis 12 stages

hydrochemical and stable isotope evidence for the extent and nature of the effective chalk aquifer of north norfolk uk in eastern england the chalk aquifer is covered by extensive pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer in this study the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions 18o and 2h are interpreted to reveal the extent and nature of the effective chalk aquifer of north norfolk it is found that the chalk aquifer can be divided into an upper region of fresh groundwater with a cl concentration of typically less than 100 mg l 1 and a lower region of increasingly saline water the transition between the two regions is approximately 50 m below sea level and results in an effective aquifer thickness of 50 60 m in the west of the area but less than 25 m where the eocene london clay boundary is met in the east of the area hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the chalk where the chalk is confined by low permeability chalky boulder clay isotopically depleted groundwater 18o less than 7 5 is present in contrast to those areas of unconfined chalk where glacial deposits are thin or absent 18o about 7 0 the isotopically depleted groundwater is evidence for groundwater recharge during the late pleistocene under conditions when mean surface air temperatures are estimated to have been 4 5 c cooler than at the present day and suggests long groundwater residence times in the confined aquifer elevated molar mg ca ratios of more than 0 2 resulting from progressive rock water interaction in the confined aquifer also indicate long residence times a conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater occupying areas where confined groundwater dates from the late pleistocene is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body

extraterrestrial accretion from the gisp2 ice core the rate of extraterrestrial accretion for particles in the size range 0 45 m to 20 m was determined from dust concentrates extracted from greenland ice sheet project 2 gisp2 ice core samples using instrumental neutron activation analysis inaa we determined the iridium ir content of the dust following a core specific correction for terrestrial ir and assuming a chondritic ir abundance of 500 ppb we measure an average accretion rate for 0 45 m to 20 m particles over the entire earth of 0 22 0 11 109 g yr kton yr for 317 years of ice through the interval 6 to 20 ka this is consistent with the interplanetary dust accretion rate of 0 17 0 08 x 109 g yr that we derive from published 3he data for the gisp2 core accounting for particles that are larger and smaller than those detected by or experiment our best estimate of the total accretion rate including particle sizes up to about 4 cm in diameter is 2 5 109 g yr the uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error based on monte carlo simulations we estimate the upper limit for total extraterrestrial accretion to earth of 6 25 109 g yr 95 confidence level this accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice estimates from ground based radar studies and with accretion estimates of 3he bearing interplanetary dust particles assuming that 3he is correlated with particle surface area it is however lower than estimates based on platinum group element studies of marine sediments the conflict may indicate systematic errors with either the marine or the non marine samples departures from the assumed particle spectrum of grun and coauthors or time variable accretion rates with the early holocene period being characterized by low levels of accretion

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

etude pedogenetique et isotopique des neoformations de calcite dans un sol sur craie caracteristiques et origines cryoturbated facies are found at the boundary between soil horizons and cretaceous chalk several types of secondary calcite appear in soil horizons orange coloured and rounded partially dissolved nodules deeply coloured angular aggregates transparent isolated rhombs and polycrystalline nodules needles the carbon and oxygen isotope compositions of these calcites are correlated 13c 4 9 18opdb 15 9 end members of this correlation are the orange rounded nodules 13c 8 18o 1 5 and the transparent angular polycrystalline nodules 13c 13 18o 6 partially dissolved nodules have formed under periglacial climatic conditions crystallisation would have occurred under the following equilibrium environmental conditions 18osmow soil solution 7 13c gaseousco2 5 2 t 2 c soil solution was enriched in 18o by evaporation and atmospheric co2 was enriched in 13c as compared to present day transparent polycrystalline nodules are compatible with present day environmental conditions 18o soil solution ranging from 9 to 4 and 13c soil co2 ranging from 24 5 to 23 these nodules crystallize between may and october at soil temperatures ranging from 10 to 25 c from evaporated soil solutions angular coloured aggregates may form under present day winter conditions for temperatures between 0 and 10 c however they may also result from present accretion of fragments of periglacial nodules all recent secondary calcite results from co2 degassing and evaporation of soil solutions degassing is controlled by the gradient of co2 partial pressure within the soil profile during winter this gradient is low and the resulting calcite precipitation is not significant during summer a large difference in pco2 appears between the root zone and deep soil horizons the degassing accounts for an increase of about 2 in 13c of the total dissolved inorganic carbon and of the related solid carbonate evaporation is the main driving force for secondary calcite precipitation

a holocene calcrete from north yorkshire england implications for interpreting palaeoclimates using calcretes glacial gravels of late devensian dimlington stadial age 26 000 13 000 years bp at west tanfield north yorkshire england have been cemented by carbonate rich solutions to produce a strongly indurated calcrete horizon the low mg cements occur as drusy spar needle fibres alveolar septal structures micrite and micropinnacles indicative of vadose zone cementation some complex pore partition structures attributed to precipitation along meniscus films also occur these partitions separate air dominated and water dominated microenvironments of the vadose zone the abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete in addition much of the carbonate appears to have been precipitated by biological mediation carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing as has been suggested for some arctic soil carbonates the pollen history of the area since the devensian suggests that this calcrete precipitated at low temperatures this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi arid regions and suggests that palaeo calcretes should not be used as absolute palaeoclimatic indicators the unusual occurrence albeit localized of a thick calcrete under a cool and wet climate probably reflects the well drained nature of the gravels the abundance of caco3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover which created a local environment favouring carbonate precipitation