geochemical evolution of groundwater in the cambrian vendian aquifer system of the baltic basin abstract the shallowly buried marginal part of the cambrian vendian confined aquifer system of the baltic basin is characterised by fresh and low 18 o composition water whereas the deeply settled parts of the aquifer are characterized by typical na ca cl basinal brines spatial variation in water geochemistry and stable isotope composition suggests mixing origin of the diluted water of three end members glacial melt water of the weichselian ice age 115 000 10 000 bp na ca cl composition basin brine and modern meteoric water the mixing has occurred in two stages first the intrusion and mixing of isotopically depleted glacial waters with basinal brines occurred during the pleistocene glacial periods when the subglacial melt water with high hydraulic gradient penetrated into the aquifer the second stage of mixing takes place nowadays by intrusion of meteoric waters the freshened water at the northern margin of the basin has acquired a partial equilibrium with the weakly cemented rock matrix of the aquifer

diploptene in antarctic sediments abstract the presence of diploptene 17 h 21 h hop 22 29 ene in sediment cores from bransfield strait and mcmurdo sound antarctica is reported the organic geochemical data for these sediments provide evidence for the authigenic marine origin of this hopene

low temperature hydrothermal maturation of organic matter in sediments from the atlantis ii deep red sea abstract hydrocarbons and bulk organic matter of two sediment cores no 84 and 126 chain 61 cruise located within the atlantis ii deep have been analyzed although the brines overlying the coring areas were reported to be sterile microbial inputs and minor terrestrial sources represent the major sedimentary organic material this input is derived from the upper water column above the brines both steroid and triterpenoid hydrocarbons show that extensive acid catalyzed reactions are occurring in the sediments in comparison with other hydrothermal guaymas basin or intrusive systems cape verde rise the atlantis ii deep exhibits a lower degree of thermal maturation this is easily deduced from the elemental composition of the kerogens and the absence of polynuclear aromatic hydrocarbons of a pyrolytic origin in the bitumen the lack of carbon number preference among the n alkanes suggests especially in the case of the long chain homologs that the organic matter of atlantis ii deep sediments has undergone some degree of catagenesis however the yields of hydrocarbons are much lower than those observed in other hydrothermal areas the effect of lower temperature and poor source rock characteristics appear to be responsible for the differences

distribution and transport of hydrocarbons in surface sediments of the alaskan outer continental shelf abstract surface sediments from the outer continental shelf of alaska were analyzed for hydrocarbons as part of an environmental survey sponsored by noaa blm sediments were collected from the proposed oil lease areas of beaufort sea southeastern bering sea norton sound navarin basin gulf of alaska kodiak shelf and lower cook inlet data on normal and branched alkanes and di and triterpenoids from capillary gas chromatography gc and gas chromatography mass spectrometry gc ms indicate that the area displays little evidence of petroleum hydrocarbons i e seeps except at a few isolated stations beaufort sea sediments have the highest hydrocarbon budget whereas kodiak shelf sediments have the lowest the molecular markers of the hydrocarbons are of a mixed marine autochthonous and terrestrial allochthonous origin norton sound and cook inlet sediments contain the highest levels of terrigenous lipids and kodiak shelf the lowest the abundance of alkenes implies that relatively low oxidizing conditions exist in some of the depositional areas studied suggesting that extraneous petroleum hydrocarbons introduced into these sediments would be expected to have a relatively long residence time the distribution of polynuclear aromatic hydrocarbons pah is complex and shows a pyrolytic origin the data on perylene strongly favor the hypothesis that it is performed from terrigenous precursors and transported into the marine environment with no apparent in situ generation polynuclear aromatic hydrocarbons of diagenetic origin are probably dominant in gulf of alaska and kodiak shelf sediments two stations north of kalgin island in lower cook inlet and one in southeastern bering sea show typical weathered petroleum distribution of n alkanes and triterpenoids probable pathways of transport and fate of petroleum hydrocarbons in case of an oil spill are also briefly discussed

geochemical evidence for possible natural migration of marcellus formation brine to shallow aquifers in pennsylvania the debate surrounding the safety of shale gas development in the appalachian basin has generated increased awareness of drinking water quality in rural communities concerns include the potential for migration of stray gas metal rich formation brines and hydraulic fracturing and or flowback fluids to drinking water aquifers a critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers we present geochemical evidence from northeastern pennsylvania showing that pathways unrelated to recent drilling activities exist in some locations between deep underlying formations and shallow drinking water aquifers integration of chemical data br cl na ba sr and li and isotopic ratios 87sr 86sr 2h h 18o 16o and 228ra 226ra from this and previous studies in 426 shallow groundwater samples and 83 northern appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations the strong geochemical fingerprint in the salinized cl 20 mg l groundwater sampled from the alluvium catskill and lock haven aquifers suggests possible migration of marcellus brine through naturally occurring pathways the occurrences of saline water do not correlate with the location of shale gas wells and are consistent with reported data before rapid shale gas development in the region however the presence of these fluids suggests conductive pathways and specific geostructural and or hydrodynamic regimes in northeastern pennsylvania that are at increased risk for contamination of shallow drinking water resources particularly by fugitive gases because of natural hydraulic connections to deeper formations

laser ablation coupled with icp ms applied to u pb zircon geochronology a review of recent advances abstract improvements in the technology of laser ablation and icp ms instruments make la mc icpms a rapid precise and accurate method for u pb zircon geochronology in this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time some key points have been progressively improved the crater size has been reduced to achieve real in situ measurements the laser wavelength has been reduced as well as the duration of each pulse in order to lower inter element fractionation the blank from the gas has to be lowered as far as possible double focusing instruments and magnetic field sectors allow flat topped peaks required for precise isotope ratio measurement to be obtained the use of a multi ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam combining the use of a 213 nm uv laser and a mc icpms equipped with a multi ion counting system operating in static mode the common precisions achieved for the key ratios 207 pb 206 pb and 206 pb 238 u are better than 1 and 3 2 respectively including error propagation associated with standard normalization until now the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age a strategy for common pb correction is proposed according to the age of the zircon and according to the th u ratio of the grains after recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1 and even significantly better for proterozoic samples in order to show the performance achieved by modern la mc icpms geochronology we tested four zircon samples covering a wide age range from 290 to 2440 ma these new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods id tims and shrimp further developments in the technology of ion counters equipping modern mc icpms and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to sims for in situ age determination

analysis of modern and pleistocene hydrologic exchange between saginaw bay lake huron and the saginaw lowlands area two numerical models one simulating present groundwater flow conditions and one simulating ice induced hydraulic loading from the port huron ice advance were used to characterize both modern and pleistocene groundwater exchange between the michigan basin and near surface water systems of saginaw bay lake huron and the surrounding saginaw lowlands area these models were further used to constrain the origin of saline isotopically light groundwater and porewater from the study area output from the groundwater flow model indicates that at present conditions head in the marshall aquifer beneath saginaw bay exceeds the modern lake elevation by as much as 21 m despite this potential for flow simulated groundwater discharge through the saginaw bay floor constitutes only 0 028 m 3 s 1 cfs bedrock lithology appears to regulate the rate of groundwater discharge as the portion of the saginaw bay floor underlain by the michigan confining unit exhibits an order of magnitude lower flux than the portion underlain by the saginaw aquifer the calculated shoreline discharge of groundwater to saginaw bay is also relatively small 1 13 m 3 s 1 or 40 cfs because of low gradients across the saginaw lowlands area and the low hydraulic conductivities of lodgement tills and glacial lake clays surrounding the bay in contrast to the present groundwater flow conditions the port huron ice induced hydraulic loading model generates a groundwater flow reversal that is localized to the region of a pleistocene ice sheet and proglacial lake this area of reversed vertical gradient is largely commensurate with the distribution of isotopically light groundwater presently found in the study area mixing scenarios constrained by chloride concentrations and 1 8 o values in porewater samples demonstrate that a mixing event involving subglacial recharge could have produced the groundwater chemistry currently observed in the saginaw lowlands area the combination of models and mixing scenarios indicates that structural control is a major influence on both the present and pleistocene flow systems

origin distribution and hydrogeochemical controls on methane occurrences in shallow aquifers southwestern ontario canada abstract natural gas reservoirs in organic rich shales in the appalachian and michigan basins in the united states are currently being produced via hydraulic fracturing stratigraphically equivalent shales occur in the canadian portion of the basins in southwestern ontario with anecdotal evidence of gas shows yet there has been no commercial shale gas production to date to provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production such as leakage of natural gas saline water and or hydraulic fracturing fluids and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern ontario an extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow domestic wells in bedrock and overburden aquifers throughout southwestern ontario was utilized in addition select wells n 36 were resampled for detailed dissolved gas composition 13 c of ch 4 c 2 c 3 and co 2 and d of ch 4 dissolved gases in groundwater from bedrock and overburden wells were composed primarily of ch 4 29 7 98 6 mol of total gas volume n 2 0 8 66 2 mol ar o 2 0 2 3 4 mol and co 2 0 1 2 mol ethane was detected but only in low concentrations 100 in situ saturation were found in bedrock wells completed in the upper devonian kettle point formation middle devonian hamilton group and dundee formation and in surficial aquifers overlying these organic rich shale bearing formations indicating that bedrock geology is the primary control on methane occurrences a few n 40 samples showed na cl br evidence of brine mixing with dilute groundwater however only one of these samples contained high 60 in situ saturation ch 4 the relatively low 13 c values of ch 4 89 9 to 57 3 covariance of d values of ch 4 and h 2 o positive correlation between 13 c values of ch 4 and co 2 and lack of higher chain hydrocarbons c 3 in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin the presence or absence of alternative electron acceptors e g dissolved oxygen fe no 3 so 4 in addition to organic substrates controls the occurrence of microbial ch 4 in shallow aquifers microbial methane has likely been accumulating in the study area since at least the late pleistocene to the present as indicated by the co variance and range of d values of ch 4 314 to 263 and associated groundwater 19 to 6 d h 2 o

the diagenetic fate of taraxer 14 ene and oleanene isomers abstract the extractable organic matter in holocene to early miocene deep sea sediments from site 645 odp leg 105 in baffin bay contains almost exclusively biological markers of terrigenous origin pentacyclic triterpenoids of the ursene and amyrin oleanene type often occur as the most abundant compounds in the aliphatic hydrocarbon ketone and alcohol fractions of the sediment extract a specific diagenetic reaction involves the conversion of taraxer 14 ene into olean 12 ene in the upper 700 metres of the sedimentary sequence the taraxerene to oleanene conversion is complete before the onset of isomerisation of diasterenes at c 20 in the sediment as well as in laboratory simulation experiments olean 12 ene further isomerises to olean 13 18 ene and olean 18 ene the latter of which may be the direct precursor of 18 h and 18 h oleanane found in sediments containing more mature organic matter and in crude oils the subsurface interconversion of these triterpenoid skeletons indicate that oleanane does not necessarily start life as an oleanoid

hydrocarbons in phosphatized and non phosphatized sediments from the phosphate basin of gafsa abstract the sedimentary layers in the phosphate basin of gafsa show interlayering of phosphatized and less phosphatized beds shales marls limestone and cherts 1 1 the n alkane distribution of the chloroform extracts and the pyrochromatograms obtained from the related kerogens are very similar in all the strata suggesting that the organic matter which accumulated in these strata had a similar origin marine planktonic 2 2 the occurrence of some significant fossil molecules in poorly phosphatized sediments chert including c ring monoaromatic steroids and benzohopanoids confirms the autochthonous marine origin and the immature character of the organic matter and reflects intensive bacterial anaerobic activity the lack of these fossil molecules in phosphorites is essentially due to the more oxidative conditions occurring during phosphatogenesis in contrast the presence of these fossil molecules in humic acid fractions from phosphorites reflects a specific property of humic substances to incorporate hydrocarbons from petrological studies humic acids and incorporated fossil molecules are thought to be preserved because they were trapped inside phosphorite pellets or ooids at an early stage of diagenesis 3 3 the chromatograms of hydrocarbons and pyrochromatograms for the humic compounds which can be extracted from each of the strata are very similar to those of the non humic organic matter of the same samples this suggests that hydrocarbons have been incorporated by the humic acids at an early stage of diagenesis the amount of such humic associated hydrocarbons can reach 20 of the humic acid weight 4 4 the amount of hydrocarbons in the bulk sediment confirms that the conditions favourable or not favourable to phosphate concentrations are a result of variation in eh during deposition and not variation in organic matter type

geochemistry and isotope chemistry of michigan basin brines devonian formations detailed chemical and isotope analysis of 87 formation waters collected from six devonian aged units in the michigan basin are presented and discussed in terms of the origin of the dissolved components and the water total dissolved solids in these waters range from 200 000 to 400 000mg 1 upper devonian formations produce dominantly nacacl brine while deeper formations produce canacl water ratios of cl br and na br along with divalent cation content mcl2 indicate that these brines are derived from evapo concentrated seawater other ion concentrations appear to be extensively modified from seawater values by water rock reactions the most important reactions are dolomitization which explains the ca content of the brines and reactions involving aluminosilicate minerals stable isotope 18o and d compositions indicate that water molecules in the deeper formations are derived from primary concentrated seawater isotope enrichment by exchange with carbonates and perhaps gypsum cannot be discounted isotope values indicate water in the upper devonian formations is a mixture of seawater brine diluted with meteoric derived water dilution has predominantly occurred in basin margins two scenarios are presented for the origin of the brines in the devonian formations 1 they originated when the devonian sediments and evaporites were first deposited or 2 they are residual brine liberated from the deeper devonian and possibly silurian salt deposits

an outline of early proterozoic crustal evolution in the sao francisco craton brazil a review abstract the early proterozoic evolution of the sao francisco craton sfc occurred between 1 8 2 5 ga and includes the transamazonian cycle 1 9 2 2 ga one of the main periods of crustal growth affecting the south american continent this synthesis of the available data is largely concerned with the early proterozoic terranes of the sfc and presents a modeled crustal evolution integrated with that of the west congo craton interpretation of radiometric ages geochemical and structural data metamorphism p t conditions and lithostratigraphical correlations leads to the characterization of three early proterozoic belts in the sfc named the correntina guanambi itabuna and mineiro belts these belts are here described following a geographic scenario as given by their representatives medium to high grade sequences low grade supracrustal sequences and intrusive igneous rocks the anatomy and regional structures of these belts indicate that they developed along the margin of a large archean protocontinent although characteristic older mantled gneiss domes are also found within the itabuna and mineiro belts available isotopic evidence suggests an episodic growth of the continental crust during the late archean 2 6 3 2 ga with the formation of microcontinents that gradually coalesced around 2 6 ga ago after the archean proterozoic transition the tectonic evolution of the proterozoic mobile belts evolved by mantle accretion differentiation and crustal reworking the main early proterozoic magmatic arc of the sfc is represented by the itabuna mobile belt whereas the minciro belt is chiefly ensialic the nature of the salvador correntina belt is still poorly constrained by geochronological and geochemical data by the end of the transamazonian cycle around 1 8 1 9 ga ago the sfc was fully cratonized

genetic and temporal relations between formation waters and biogenic methane upper devonian antrim shale michigan basin usa abstract controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane natural gas in most conventional deposits has migrated from a source rock to a reservoir rarely remaining associated with the original or cogenetic formation waters we investigated an unusual gas reservoir the late devonian antrim shale in which large volumes of variably saline water are coproduced with gas the antrim shale is organic rich of relatively low thermal maturity extensively fractured and is both source and reservoir for methane that is generated dominantly by microbial activity this hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas providing a unique opportunity to characterize methane generating mechanisms the well developed fracture network provides a conduit for gas and water mass transport within the antrim shale and allows invasion of meteoric water from overlying aquifers in the glacial drift steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases dilute waters grade into dense brines 300 000 ppm over lateral distances of less than 30 km radiogenic 14c and 3h and stable isotope 18o and d analyses of shallow antrim shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20 000 yr bp carbon isotope compositions of methane from antrim shale wells are typical of the established range for thermogenic or mixed gas 13c 47 to 56 however the unusually high 13c values of co2 coproduced with methane 22 and dissolved inorganic carbon dic in formation waters 28 require bacterial mediation the d values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis methane ethane propane ratios and 13c values for ethane indicate 1 the presence of a thermogenic gas component that increases basinward and 2 progressive bacterial oxidation of ethane as the antrim shale subcrop is approached multiple episodes of pleistocene glaciation over northern michigan appear critical to the development of these gas deposits loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water the physical erosion cycle of repeated glacial advances and retreats exhumed the antrim shale around the northern margin of the michigan basin subjecting it to near surface physiochemical and biochemical processes the chemical and hydrologic relations demonstrated in the antrim shale reservoir suggest a dynamic connection between pleistocene glacial history of the midcontinent region and development of recoverable microbially generated natural gas reserves

hydrocarbons in the sediments of port valdez alaska consequences of five years permitted discharge abstract hydrocarbons have been investigated in the sediments of port valdez alaska after three to five years of oil terminal operation with a routine daily discharge of 170 kg of petroleum residue in an otherwise undeveloped area surficial benthic sediments 0 5 cm and core segments down to 30 cm have been analyzed information about total hydrocarbons unresolved complex mixture normal alkanes pristane phytane hopanes and polycyclic aromatic hydrocarbons indicates that petroleum biogenic hydrocarbons and combustion derived aromatic hydrocarbons are present in sediments around the terminal at concentrations up to 218 g g 1 dry sediment the vertical horizontal and temporal distributions of anthropogenic hydrocarbons indicate that most of the sedimentary hydrocarbon accumulation has occurred within 1 km of the terminal a simple calculation suggests that less than 3 of the total oil discharged during routine operations of the terminal has entered the sediments of port valdez for comparison sediment hydrocarbon accumulation associated with a nearby small boat harbor was also examined

isotopic and trace element constraints on the origin and evolution of saline groundwaters from central missouri na ca cl groundwaters with salinities of 1 to 30 discharge from natural springs and artesian wells in mississippian carbonates and ordovician sandstones and carbonates in central missouri carbonate saturation and quartz supersaturation are maintained throughout the salinity range major and trace element and isotopic variations in the waters are used to place constraints on models for rock water interaction and regional hydrology the groundwaters have d values that range from 108 to 45 and 18 o values that range from 14 7 to 6 5 smow these data lie approximately along the meteoric water line ranging from values similar to local recharge in missouri to significantly lower values that are similar to 18 o and d for meteoric recharge in high altitude and high latitude regions of north america the more saline samples have the lowest 18 o and d contents a trend that is opposite to that observed in most other studies of saline waters the h o isotopic correlation and the range in salinity and several major and trace element concentrations in central missouri groundwaters are readily explained by a saline dilute water mixing model 87sr 86sr ratios for the groundwaters range from 0 7155 to 0 7161 these values are significantly higher than previously published values for 1 estimates of 87 sr 86 sr in phanerozoic seawater 0 7068 0 7092 and 2 carbonates of the mississippian burlington keokuk fm 0 7075 0 7105 through which some of the waters migrate as they pass to the surface nd 0 values in the groundwaters range from 10 9 to 8 1 and 147 sm 144 nd ratios range from 0 108 to 0 128 these values are similar to or lower than previously published analyses of carbonates and other authigenic phases from the burlington keokuk fm mean nd 0 7 8 0 8 1 mean 147 sm 144 nd 0 141 the h o sr and nd isotopic data and the results of model calculations preclude 1 models involving the modification of ancient seawater for the origin of the saline waters and 2 extensive interaction between the groundwaters and their host carbonates the waters apparently acquired their 18 o and d values as meteoric recharge and their 87sr 86sr and rare earth element signatures from extraformational crustal sources high 87 sr 86 sr low nd 0 low 147sm 144nd and largely preserved these isotopic signatures during subsequent migration through the carbonates chemical exchange via rock water interaction is required for isotopic exchange to occur therefore the present state of chemical equilibrium between the groundwaters and carbonate aquifer minerals may serve to limit rock water interaction and hence preserve the extraformational isotopic signatures in the waters the integration of geochemical isotopic and hydrologic data on a local and regional scale suggests a history for the central missouri groundwaters involving 1 meteoric recharge in the front range of colorado 2 dissolution of permian halite in the subsurface of kansas 3 interaction with predominantly silicate mineral assemblages in paleozoic strata and possibly precambrian basement with aquisition of crustal sr and ree signatures 4 dilution and migration to shallow aquifer levels in central missouri and 5 mixing with local meteoric recharge through mississippian carbonates with no significant change of the isotopic signatures acquired in stage 3

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

latitudinal change of normal paraffin composition in the northwest pacific sediments abstract we analyzed normal saturated paraffins n paraffins in 70 surface sediments in the northwest pacific to examine the spatial distribution of organic materials all sediments show bimodal n paraffin abundance patterns against carbon number with maxima at c 16 and c 31 indicating that n paraffins in marine sediments are a mixture of long chain n paraffins from land based higher plant waxes and short ones from marine organisms we observed in this study a significant change of carbon preference index cpi from 2 5 in low latitudes to 4 5 in mid high latitudes around 19 25 n the following influences are discussed as the cause of cpi offset between low and mid high latitudes 1 meteorological conditions 2 n paraffin composition of higher land plants 3 contribution of petroleum components and 4 mixing ratio of higher land plants and marine organisms however none of them can be the possible cause of the cpi offset because n paraffin content and composition vary significantly according to the species of a bacterium difference of the species of bacterial communities between low and mid high latitudes may be a possible reason for the cpi offset found in sediment from the northwest pacific

hydrocarbon geochemistry in surface sediments of alaskan outer continental shelf part 1 c15 hydrocarbons the c15 hydrocarbons in 220 surface sediments from the outer continental shelf of alaska were analyzed as part of an environmental survey supervised by national oceanic and atmospheric administration and bureau of land management gas chromatography and gas chromatography mass spectrometry were employed for the identification of organic compounds in the bitumen extracts surface sediments from one station in southeastern bering sea and two stations in lower cook inlet display n alkanes and triterpenoid distribution typical of weathered petroleum major faulting in the vicinity of the southeastern bering sea station may account for the leakage of petroleum from subsurface reservoir rocks oil production in upper cook inlet or a hitherto unrecorded seep in the rea could contribute petroleum hydrocarbons to the sediments from the two stations in lower cook inlet apart from these stations there is little evidence of near surface petroleum seeps in the sites investigated

hydrocarbon geochemistry of the puget sound region ii sedimentary diterpenoid steroid and triterpenoid hydrocarbons cyclic components of the aliphatic hydrocarbon mixtures extracted from puget sound sediment cores include a suite of c19 and c20 diterpenoid hydrocarbons of which fichtelite sandaracopimaradiene and isopimaradiene have been identified although apparently also derived from vascular plants these diterpenoid hydrocarbons have relative abundances distinctly different from the co existing plant wax n alkane suite five c27 c28 and c29 diasteranes and four c29 c30 and c31 17 h 21 h hopanes occur in relatively constant proportion as components of a highly weathered fossil hydrocarbon assemblage these chromatographically resolved cycloalkanes along with the strongly covarying unresolved complex mixture have been introduced to puget sound sediments from adjacent urban centres at increasing levels over the last 100 yr in the absence of any major oil spill naturally occurring triterpenoid hydrocarbons including hop 22 29 ene diploptene are also present a new group of c30 polyenes has been detected which contains compounds apparently structurally related to a co existing bicyclic c25 diene and to c20 and c25 acyclic multibranched hydrocarbons described in a previous paper barrick et al 1980

alkane and pah biomarkers as tracers of terrigenous organic carbon in arctic ocean sediments abstract we present the first sedimentary biomarker study encompassing the entire arctic ocean a large data set of organic markers for terrigenous petroleum and combustion inputs alkanes hopanes and steranes parent and alkyl polycyclic aromatic hydrocarbons pahs is examined for patterns in space and time using principal components analysis pca and partial least squares pls biomarker patterns reveal the central arctic ocean basin sediments to be compositionally distinct from those of the mackenzie river beaufort sea and barents sea but similar to those of the laptev sea pah distributions reflected in pah ratios and pca projections demonstrate that arctic ocean sediment is dominated by natural inputs to the extent that anthropogenic combustion pahs are not significant we find only modest changes between the glacial and post glacial sediments for atmospherically transported hydrocarbon biomarkers while particle associated biomarkers were captured strongly at basin edges during the glacial period and much more evenly transported across basins during the post glacial period the orders of magnitude decreases in particle associated petrogenic alkanes and pahs in central basins during glacial times coupled with the uniformity of most petrogenic biomarker parameters for most basin and shelf locations reflect a massive reduction in ice transport that makes the margins the most likely source of petrogenic material for the pleistocene holocene central arctic basins the proximity of large coal deposits of various maturity levels along the lena river the overlap in pah and biomarker composition of the laptev sea and surficial sediments from the central arctic ocean and the location of the laptev sea at the origin of the main transpolar drift all point to eroded coals from the lena river laptev sea as the likely source of petrogenic hydrocarbons to the central arctic ocean the ubiquitous presence of allochthonous coal in arctic ocean surface sediments provides a major constraint on the use of petrogenic biomarkers to infer the presence of subsurface petroleum reserves

comparative study of sterols in shelf and slope sediments off northeastern taiwan abstract bottom sediments and suspended matter from the shelf and slope areas off northeastern taiwan were analyzed for sterol n alkanol and n alkane compositions the algal sterols cholesterol ratios mean 1 were 1 00 0 31 n 7 for the shelf sediments 3 29 0 61 n 7 for the slope sediments and 1 54 0 19 n 3 for the suspended matter the much higher proportion of cholesterol in the shelf sediments is most likely derived from mollusks in the relict sediments an additional sterol source for the shelf sediments probably is suspended matter in the water column although the shelf has no apparent sedimentation results from phytol and n alkanols suggest recent inputs of lipids from the water column to the shelf the stanol stenol ratios are lower for the older shelf sediments than for the younger slope sediments

organic geochemistry of marine sediments in antarctic region marine lipids in mcmurdo sound abstract the compositions of resolvable lipid components from four sediment cores 0 90 cm of mcmurdo sound ross sea have been examined the various lipid components occur in the following order of abundance sterols fatty acids n alcohols n alkanes pah the data indicate that the organic matter is mainly derived from recycled kerogen mixed with modern marine input the distribution of lipids and lignin analyses indicate that there is little recognizable higher plant debris in the sediments the sediments contain unaltered biogenic triterpenoids and there is no evidence for natural and or anthropogenic petroleum influx in the region aeolian transport of organic carbon from the continents appears to be negligible the dominance of labile alkenes c 25 compounds and hopenes and the presence of unsaturated fatty acids down to a depth of 90 cm reflect a very early diagenetic stage the persistent cold climate has probably helped in the better preservation of these labile lipids in the water column and in the young sediments diploptene 17 h 21 h hop 22 29 ene appeats to originate from autochthonous marine productivity in mcmurdo sound

normal paraffins in shales as an indicator of depositional environment abstract normal paraffin compositions were determined for marine sediments from coastal and pelagic environments and for shales in the jurassic cretaceous bedded chert sequence from the franciscan terrane western north america the pelagic sediments and shales from a lower horizon of the bedded chert sequence are characterized by short chained n paraffins the coastal sediments and shales from the upper horizon show high abundance of n paraffins especially for long chained ones and a strong odd carbon predominence the comparison of n paraffin compositions of shales with those of marine sediments suggests that the bedded chert sequence had deposited in continuous environments from pelagic to coastal this inference is in harmony with that based on the major minor and rare earth element compositions the result suggests that shales in the franciscan bedded chert sequence preserve well the normal paraffin signals acquired at the deposition and demonstrates that the normal paraffin composition can be a useful indicator of sedimentary environment

hydrocarbon geochemistry in surface sediments of alaskan outer continental shelf part 2 distribution of hydrocarbon gases the distribution of c1 to c4 hydrocarbon gases in surficial sediments from the alaskan continental shelf was determined as part of a regional environmental survey sponsored by national oceanic and atmospheric administration and bureau of land management noaa blm methane ethane ethene propane propene n butane and iso butane were observed in low concentrations 0 1 10 ppb as weight gas to dry weight sediment in nearly all of the 190 sediment samples examined with methane exceeding c2 to c4 hydrocarbons in abundance by about a factor of 10 no gasoline range c5 to c8 hydrocarbons were detected above blank levels ethene was found generally to be more abundant than ethane while propane was slightl more abundant than propene the correlation between these hydrocarbons suggests a common possibly biogenic origin sediments from norton sound have anomalously high methane concentrations probably as a result of biological methanogenesis in subsurface organic rich peat layers most of the regions sampled showed median values of the c1 c2 c3 ratio of 20 the generally low c1 concentrations probably resulted from the selective loss of c1 by biologic oxidation or ebullition from the surface sediments no major c2 to c4 hydrocarbon anomalies were discovered however at some stations in kachemak bay kodiak shelf and norton sound anomalously high c3 c3 1 ratios indicated the poss ble presence of thermogenic gases at depth

geochemistry and fluxes of hydrocarbons to the beaufort sea shelf a multivariate comparison of fluvial inputs and coastal erosion of peat using principal components analysis abstract the allochthonous inputs of hydrocarbon to the canadian beaufort shelf were studied by applying principal components analysis pca to well validated and rigorously blank corrected samples incorporation of a wide range of perdeuterated n alkanes and pah into the analysis scheme ensured that only reliably quantified variables were used to interpret the hydrocarbon geochemistry application of pca to mackenzie river samples demonstrated a homogeneous system from which we infer coupling or equilibrium between the river particulate hydrocarbons and the dissolved fraction particulate particle size 0 7 m hydrocarbon flux from the mackenzie river is by far the most important terrestrially derived source of hydrocarbons to the beaufort sea the mackenzie river particulates have a distinct n alkane signature which can be used to identify the riverine influence on the hydrocarbon geochemistry of the beaufort sea shelf based on one year s data the flux of total alkanes is 440 94 tonne a and pah is 49 8 tonne a uncertainties are one standard deviation of the sampling and analytical variation the particulate flux exceeds the accompanying dissolved hydrocarbon flux by two orders of magnitude and has a strong seasonal cycle winter contributes less than an estimated 0 6 of total annual flux deltaic silt from the western mackenzie delta and the smaller amounts of detritus from coastal erosion of peat are minor hydrocarbon sources and contribute in total less than 10 to the budget for most alkanes an important exception with regard to shelf geochemistry is the significant quantity of peat derived higher plant n alkanes

lignin geochemistry of north american coastal and continental shelf sediments abstract the concentrations and composition of lignin in coastal and shelf sediments at five locations along the north american continent georges bank north atlantic shelf northwestern gulf of mexico southern california norton sound and nearshore beaufort sea have been characterized using alkaline cuo oxidation lignin concentrations are generally highest at close proximity to the mouth of major rivers indicating that deposition of river borne vascular plant debris is the principal source of lignin to the offshore sediments arctic sediments contain the highest concentrations presumably as a result of an abundant supply of terrestrial organic material and the prevalence of frigid temperatures and surface ice cover during much of the year concentrations are lowest in the georges bank north atlantic sediments where a dynamic physical environment persists over much of the year and the supply of terrestrial organic matter to the offshore in the form of major rivers is lacking stable carbon isotopic compositions show that sediments containing the highest lignin concentrations also contain isotopically lighter organic matter consistent with a terrestrial source input vanillyl syringyl and cinnamyl phenol distributions in these offshore sediments are consistent with primary contributions from gymnosperm predominant and angiosperm woods and secondary contributions from nonwoody tissues these estimates may be subject to some uncertainty due to diagenetic alteration of individual lignin phenols in contrast p hydroxyl phenol yields are not strongly correlated with those of the other lignin derived phenols suggesting generation from both lignin and non lignin sources elevated acid aldehyde ratios relative to undegraded plant tissues suggest that the lignin transported to these offshore sediments has undergone some biodegradative oxidation

origins of hydrocarbons in bering sea sediments i aliphatic hydrocarbons and fluorescence abstract sixty three samples from thirteen gravity cores were analyzed by gc fid qualitative gc ms and fluorescence spectroscopy to determine the concentration distribution and sources of hydrocarbons in bering sea sediments aliphatic hydrocarbons and fluorescence associated with aromatic compounds were detected in sediments at all locations the sediments contain variable amounts of marine biological debris terrigenous plant biowaxes recycled continental material petroleum and combustion derived hydrocarbons hydrocarbon concentrations were low and similar to previous reports from the alaskan outer continental shelf average ucm and total n alkanes varied from 10 7 to 73 3 and 2 6 to 4 6 g g respectively olefins of a presumed biological origin were widely distributed throughout bering sea sediments the presence of a complete suite of normal alkanes isoprenoids an unresolved complex mixture alkylated pahs and mature biological markers suggest that locations in the western and southern bering sea are contaminated with petroleum this petroleum may be derived from natural seepage recycled mature continental material autochthonous exposed immature mature sediments and or deposition of water column tarballs the petroleum at one southern bering sea location is typical of a condensate and is significantly different from that observed at western bering sea locations vertical distributions and the presence of significant amounts of volatile hydrocarbons suggest that petroleum seepage is a likely source of contamination at these locations

saturated and olefinic hydrocarbons in recent sediments from the baltic sea abstract saturated and olefinic hydrocarbons in the top sections of contemporary sediments from four stations in the baltic sea teili 1 f 81 eb 1 and xv 1 were analysed by capillary gas chromatography and gc ms techniques the 0 2 2 4 4 5 and 10 11 cm sections showed a small unresolved complex mixture ucm however in all these sections a strong predominance of odd over even carbon numbered compounds exists indicating an insignificant petroleum hydrocarbon input in the areas studied the resolved hydrocarbons are of mixed marine autochthonous and terrigenous origin the 0 5 cm sections contained the highest proportions of short chain and branched chain hydrocarbons derived mostly from algae and bacteria in general an increase was observed in the ratios of saturated to unsaturated and of longer chain to short chain hydrocarbons with increasing depth because of the facile diagenesis of unsaturated components and faster biodegradation of short chain hydrocarbons in addition between one and seven hydrocarbons the exact structure of which could not be assigned were detected in the samples most of these components were cyclic and unsaturated and a number were triterpenes

sedimentary provenance in the central ribeira belt based on laser ablation icpms207pb 206pb zircon ages abstract 207 pb 206 pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post 1 8 ga metasedimentary cover from the occidental and oriental terranes from the neoproterozoic central ribeira belt in eastern brazil are discussed ages from the occidental terrane show that proterozoic ages predominate with a modal class at 2 1 2 2 ga and that archean ages are less common and lie within the 2 5 3 6 ga range a wider dispersion on proterozoic ages down to 1 1 ga and youngest neoproterozoic ages 550 700 ma were also observed and could be related to the brasiliano panafrican metamorphic events the data show that detrital sediments of the occidental terrane were derived predominantly from erosion of a 1 8 2 2 ga paleoproterozoic terrane with a small proportion of a 2 6 3 6 ga archean crust ages from the oriental terrane are greatly dispersed but show a significant proportion of ages between 1 7 and 2 1 ga although the great majority are between 0 5 and 1 6 ga only 3 out of 144 grains yield archean ages between 2 5 and 2 9 ga based on the greatest errors observed on the data from the oriental terrane 100 300 ma and a continuum of obtained ages between 2072 ma paleoproterozoic and 517 ma cambrian the more plausible interpretation is that the dispersion of ages results from partial resetting of the u pb system by brasiliano metamorphism in proterozoic archean zircons although a mesoproterozoic source could not be discarded

a study of the organic matter in tunisian phosphates series relevance to phosphorite genesis in the gafsa basin tunisia abstract in the gafsa basin tunisia peloidal fracolite is interbedded with shales limestones and rocks of intermediate lithology the rocks were deposited in an epicontinental sea and have been buried no more than 500 m the organic matter they contain is therefore thermally immature and in all units derived mainly from marine plankton particularly dinoflagellates the organic matter in the phosphorite units is composed predominantly of humic acids ha which are essentially located within the pellets these pellets are surrounded by an outer matrix in which the organic matter is similar to the kerogen comprised in the shally strata alternating with the phosphorite and limestone beds during weathering the content of the organic matter assocciated with the matrix decreases more rapidly than that of the organic matter entrapped within the pellets geochemical and palynological study of the organic matter in interbedded rocks of different lithology shale limestone phosphorite suggest that the shales were deposited in a reducing environment the limestones in one that was oxic whilst the phosphorites were formed in a redox environment inermediate between the two the interpretation of the data presented in this paper is that the genesis of shales phosphate and some carbonate beds excluding coquinas did not result from the deposition of initially sharply differentiated sediments but resulted from differences in diagenesis of sediments initially containing planktonic organic matter associated with clay and carbonate minerals

early diagenesis of lipid biomarker compounds in north atlantic sediments near surface sediments 0 45 mm depth interval from the iceland basin 59 n 21 w 3070 m water depth and biscay abyssal plain 48 n 17 w 4105 m water depth were sectioned at millimeter scale resolution to assess alterations in key lipid biomarkers during early diagenesis inventories g cm 2 in the topmost 45 mm of biomarkers from presumed terrestrial sources c20 linear n alkanes n alkanoic acids n alkanols and from phytoplankton sources phytosterols long chain alkenones and alkenoates were 1 5 4 times more abundant in the iceland basin core than in the biscay abyssal plain core indicating greater sedimentary inputs and or preservation at the iceland basin site biomarker concentrations varied significantly over millimeter scale depth intervals pronounced subsurface concentration maxima were present at both sites at the biscay abyssal plain site steep concentration gradients in the uppermost few mm of the sediment column indicate extensive diagenetic losses at or near the sediment water interface strong covariance was seen in the concentrations of various compounds within each class e g long chain n alkanes n alkanols n alkanoic acids and long chain alkenones and alkyl alkenoates indicating that individual compounds within each class are associated with the same organic matrix and have similar degradation and mixing rates thus biomarker indices for assessment of land derived inputs cpi c29 c31 acl and sea surface temperature ipt remain largely unaltered by early diagenetic processes in oxic abyssal sediments in spite of extensive bioturbation and degradative losses within the uppermost few centimeters of the sediment column

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

chemical evidence of differential particle dispersal in the southern washington coastal environment aliphatic hydrocarbons cupric oxide oxidation products of lignin and polycyclic aromatic hydrocarbons pah were analyzed by capillary gas chromatography in sediments from the southern washington continental shelf and slope the concentration of diploptene relative to plantwax n alkanes increased systematically in surface sediments with distance offshore along east west transects of the study area and remained constant in surface sediments along the midshelf silt deposit analogous trends were also observed for the concentration of cinnamyl phenols relative to vanillyl phenols and total methylphenanthrenes relative to phenanthrene these changes in sedimentary composition are evidence that diploptene from some terrestrial source lignin characteristic of non woody vascular plant tissue and a fossil organic material contained within weathered rock debris disperse across the washington continental shelf and slope in geographic patterns distinct from that for other river derived chemically related materials the compositional variations are explained by the particulate associations of the land derived chemicals and differential hydraulic dispersion of their respective carrier particles after discharge at the mouth of the columbia river

overview of the recent sediment hydrocarbon geochemistry of atlantic and gulf coast outer continental shelf environments abstract an overview of the hydrocarbon geochemistry of recent marine sediments from the u s atlantic and gulf of mexico outer continental shelf ocs regions is presented hydrocarbon levels along the atlantic ocs are fairly uniform ranging between in the north atlantic eastern gulf south texas gulf and coastal louisiana sediments a linear relationship exists between total hydrocarbon and total organic carbon concentrations indicating that each area consists of a geochemical province defined by a source input or depositional regime specific to the region the use of trace parameter bulk parameter ratios such as total hydrocarbon or individual pah concentrations total organic carbon content in defining such provinces and thus in serving as a basis for evaluating variations in source inputs as part of future surveys is demonstrated this approach views the hydrocarbon content and composition of atlantic and gulf coast ocs sediments as points on a spectrum defined by input types sources and deposition patterns sedimentation and erosion which conceptually link all the regions into a unified system

an evaluation of petrogenic hydrocarbons in northern gulf of alaska continental shelf sediments the role of coastal oil seep inputs abstract we compared hydrocarbons in water suspended particulate matter spm and riparian sediment collected from coastal watersheds along the yakataga foreland with corresponding hydrocarbons in gulf of alaska benthic sediments this comparison allows an evaluation of hydrocarbon contributions to marine sediments from natural oil seeps coal and organic matter e g kerogen associated with eroding siliciclastic rocks the samples from oil seeps show extensive loss of low molecular weight n alkanes n c 14 and the presence of large bimodal unresolved complex mixtures ucms in contrast to the hydrocarbon fingerprints on the spm and riparian sediment samples collected upstream from the oil seeps after entering the fluvial systems hydrocarbons from seep oils are rapidly diluted and associate with the spm phase as oil mineral aggregates oma johnston creek the watershed containing the most prolific seep conveys detectable seep derived hydrocarbons to the gulf of alaska but overall seep inputs are largely attenuated by the non seep petrogenic hydrocarbon content of the high spm loads in contrast to the geochemical signature of seep oil gulf of alaska benthic sediments are characterized by abundant alkylated naphthalene homologues relatively smooth n alkane envelopes n c 9 through n c 34 but with elevated levels of n c 27 n c 29 and n c 31 and small ucms further hydrocarbons in benthic sediments are highly intercorrelated taken together these characteristics indicate that seep oil is a negligible petrogenic hydrocarbon source to the gulf of alaska continental shelf coaly material separated from the benthic sediment samples using a dense liquid 2 00 g cm 3 also accounted for a minor portion of the total pah 1 6 and total n alkanes 0 4 2 in the benthic samples most of the hydrocarbon burden in the sediments is found in the denser sediment fraction and likely derives from organic matter contributed by denudation of siliciclastic formations in the yakutat terrane we therefore conclude that previous investigations relying on source allocation models have considerably overestimated oil seeps as a hydrocarbon source to the gulf of alaska

constraining timing and p t conditions of continental collision and late overprinting in the southern brasilia orogen se brazil u pb zircon ages and geothermobarometry of the andrelândia nappe system abstract the andrel ndia nappe system ans is an important component of the neoproterozoic southern brasilia orogen sbo in se brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the sao francisco paleocontinent and paranapanema block during the brasiliano orogeny the ans is mainly composed of a pile of deformed meta sedimentary rocks that record an inverted metamorphic pattern ranging respectively from amphibolite facies at the bottom to high p granulite conditions at the top the ans is here simplified into the lower middle and upper nappes four samples from different tectonic settings within the ans were collected for zircon isotopic investigation in addition geothermobarometry modelling were performed on seven samples from the middle nappe retro eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high p conditions of 12 16 kbar with temperatures of 700 800 c during the continental subduction dated here by u pb shrimp in zircon at 625 6 ma a period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet muscovite staurolite kyanite schist with leucosome parallel to the main foliation dated by u pb shrimp in zircon at 618 5 ma re equilibrium at pressures of 4 7 kbar with temperatures of 500 700 c is ascribed to a second heat pulse under lower pressure conditions the period of melting associated to this event in the middle nappe occurred at 586 9 ma and is marked by an anatectic tourmaline garnet muscovite bearing s type granite this granite body is part of a ne sw belt of granites with ages between 605 and 563 ma and covering both the central ribeira orogen cro and the sbo the age and the alignment of this granite belt justify the interpretation that it is genetically related to the cro thus the new data provide evidence that tectonothermal events related to the collisional front of the cro extended up to 200 km overlapping the consolidated sbo at ca 580 ma

u pb la icp ms detrital zircon ages of the sao joao del rei and carandai basins new evidence of intermittent proterozoic rifting in the sao francisco paleocontinent abstract the tiradentes formation sao joao del rei basin is a shallow marine quartzite succession resting on the paleoproterozoic crust of the southern part of the sao francisco craton a regional angular unconformity separates the tiradentes strata from the younger carandai barroso and prados formations carandai basin the carandai formation is constituted of fault bounded diamictites containing lithoclasts derived from basement rocks and the tiradentes formation the barroso formation is a sag basin limestone succession whereas the prados formation is a pelitic succession that fills a ne sw trending aulacogen weak deformation and metamorphism 600 590 ma overprinted the sao joao del rei and carandai sequences due to the brasiliano orogen and likewise the overlying andrel ndia megasequence

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

decoupling of solutes and water in regional groundwater systems the murray basin australia documenting the origins residence times and movement of groundwater and the solutes that it contains is critical to understanding hydrogeological systems this study uses cl mass balance to determine the cl accession time i e the time required for cl to accumulate and cl to estimate the residence times of cl in the victorian portion of the murray basin southeast australia much of the murray basin contains saline groundwater with total dissolved solids tds concentrations commonly 14 000 mg l and locally up to 300 000 mg l the total mass of cl stored in the victorian portion of the basin is estimated as between 12 400 and 47 100 mt using present day rainfall totals and cl concentrations in rainfall the cl accession time is 170 to 650 ka aquifer thicknesses and groundwater salinity both increase westwards in this part of the murray basin consequently the cl accession times increase westward from 0 1 0 6 ka to 286 1080 ka by contrast c activities of the majority of the groundwater are 2 pmc and commonly much higher notwithstanding the difficulty in correcting c residence times the widespread occurrence of groundwater with above background c activities implies that groundwater residence times are generally 30 ka which is substantially shorter than the cl accession times rcl the ratio of cl to total cl 10 values of the groundwater are between 20 and 230 and are uncorrelated with cl concentrations while it is difficult to determine precise cl residence times the observation that the rcl values are significantly higher than those that represent secular equilibrium with the aquifer matrix rcl of 5 to 10 indicates that they are up to a few hundred thousand years and similar to the cl accession times the rcl values together with the geology of the aquifers stable isotope ratios and major ion geochemistry precludes halite dissolution incorporation of connate water or the long term diffusion of cl from clays as mechanisms of producing the elevated cl concentrations rather it is most likely that the high cl concentrations result from recycling of solutes in saline lakes and playas or repeated cycles of evapotranspiration in the unsaturated zone

geochemistry of acid brines lake tyrrell victoria australia abstract pore water groundwater and sediments were collected from lake tyrrell to study the evolution of brines in an acidic environment lake tyrrell is a 150 km2 inland salt playa acid saline groundwater enters the playa through springs along the lake margin and is evapo concentrated past halite precipitation the brines were found to contain sea salts but some solutes do not follow the seawater evaporation curve processes that occur in neutral alkaline systems e g gypsum formation occur here but precipitation of alunite kal3 so4 2 oh 6 jarosite kfe3 so4 2 oh 6 and fe oxides also affect solute behavior processes that occur in the recharge area and along the groundwater flow path k uptake calcrete and gypcrete formation affect the brine s chemical signature regardless of the chemical variability found among the samples solutes have evolved from a common source sea salt acidification which appears to occur early in the evolution of the groundwater is related to pyrite oxidation and may be enhanced by precipitation of fe oxides alunite and jarosite and evaporation acidity is maintained in the system because of the lack of buffering capacity of aquifer minerals we suggest that this acid brine from australia is another end member in the eugster jones hardie model for the evolution of water by evaporation in closed basins

the neoproterozoic southern passive margin of the sao francisco craton insights on the pre amalgamation of west gondwana from u pb and hf nd isotopes abstract sedimentary basins are in many cases the only preserved record of the geological processes active through geological time these deposits preserve age isotopic and geochemical signatures of the source rocks providing insights into tectonic setting in this context we investigate the lithostratigraphy provenance and tectonic environment of the carrancas group a meta sedimentary sequence characterised by a basal psammitic unit lower unit passing up into interbedded psammo pelites intermediate unit which are overlain by a further psammitic unit upper unit this succession along with the canastra paranoa and vazante groups brasilia orogen comprises a mature passive margin developed at the western southern margin of the sao francisco craton during the neoproterozoic detrital zircon u pb ages and nd hf isotope data enable revised stratigraphic correlations across the basin highlighting variations in the source of detritus deposition began after 0 92 ga with the sedimentation of the lower unit with input of juvenile 2 1 ga detritus endt 0 1 to 3 2 derived from the sao francisco craton a marine transgression characterises the transition to the intermediate unit which gradually covered over juvenile 2 1 ga terrains and rendered them unavailable as possible source rocks for detrital zircons evolved archaean palaeoproterozoic basement rocks the espinhaco supergroup and evolved neoproterozoic granitoids all belonging to the sao francisco craton acted as sources for the intermediate and upper units u pb 2 7 ga to 0 9 ga endt 10 4 to 7 1 the goias massif might also have acted as a source providing juvenile mesoproterozoic detritus the sediments of the upper unit register a significant shift in source with incoming of juvenile mesoproterozoic and crustal 1 9 1 7 ga detritus this change in the ages of the detrital material may be related to exhumation of the espinhaco supergroup during the evolution of the passive continental margin basin the whole rock nd tdm model ages become progressively younger from the base to the top of the sequence nd tdm 2 6 1 8 ga the collisional events between amazonian and sao francisco cratons and the paranapanema block set the minimum age of deposition along this passive continental margin at around 0 67 ga

organic molecular evidence of seafloor hydrocarbon seepage in sedimentary intervals down a core in the northern south china sea abstract the hydrocarbons in sedimentary organic matter om at site 4b in the pearl river mouth basin prmb northern south china sea scs were analyzed the odd even predominance oep of long chain n alkanes n c24 with cpi carbon preference index values from 1 62 to 3 80 and n c29 n c31 being the two most abundant in most of the samples are strongly indicative of a terrestrial higher plant source and the even odd predominance eop distribution of mid chain n alkanes n c16 22 suggests a marine bacterial input probably chemical autotrophic bacteria however the biomarker distributions at the 65 70 cm 80 85 cm and 85 90 cm intervals show a similar to those of crude oils which were characterized by a dominance of an unresolved complex mixture ucm in the n c20 region low cpi values 9 of total tricyclic terpanes c30 hopane these features are typical characteristics of mature om that has experienced catagenesis and metagenesis suggesting that outside hydrocarbons migrated into these three intervals in terms of petroleum systems developed in the prmb hydrocarbon seepage from deep source rocks via a diaper structure and fault system is proposed to be responsible for the occurrence of this mature om

extensive microbial modification of formation water geochemistry case study from a midcontinent sedimentary basin united states the upper devonian antrim shale in the michigan basin is an economically significant source of microbially produced methane along the basin margins where meteoric recharge has been focused oxygen and hydrogen stable isotope compositions of antrim formation waters show that fresh waters recharged from pleistocene glaciation and modern precipitation suppressed basinal brine salinity to great depths and enhanced methanogenesis this paper presents results of integrated elemental and isotope analyses of antrim shale formation waters from the margins and center of the michigan basin focusing on solute sources and geochemical modifications associated with regionally extensive microbial methanogenesis cl br na systematics reveal that salinity is controlled not only by mixing between variable amounts of basinal brine and meteoric water but also by halite dissolution where fluids recharged through underlying devonian carbonate aquifers with localized evaporite deposits divalent cations carbonate system parameters and carbon isotope compositions of dissolved inorganic carbon have been systematically and profoundly altered by microbial methanogenesis large decreases in formation water ca mg and ca sr ratios accompany increasing carbonate alkalinity values in areas with high rates of microbial gas production thermodynamic and reaction path modeling show that these changes are consistent with calcite precipitation during progressive microbial methanogenesis similar variations in fluid chemistry are evident in databases from other sedimentary basins containing black shales and coal beds associated with microbial gas microbial methanogenesis may play an important role in the geochemical evolution of divalent cation relationships in crustal fluids and should be considered in models of formation water origin and evolution

organic matter sources and early diagenetic alterations in arctic surface sediments lena river delta and laptev sea eastern siberia ii molecular and isotopic studies of hydrocarbons abstract the hydrocarbons extracted from three surface sediment samples from the delta of the lena river and adjacent areas of the laptev sea were examined to derive information on their source s natural vs anthropogenic terrestrial vs marine the abundance nature distribution and individual stable carbon isotope ratios of these hydrocarbons were determined it thus appeared that the laptev samples are characterized by i a negligible level of oil pollution due to anthropogenic and or natural petrogenic inputs ii an almost exclusive occurrence of long chain predominantly odd carbon numbered n alkanes and iii some differences in the level of bacterial alteration with a relatively higher degradation of the n alkanes in the seaward sediment a non terrestrial origin is proposed for these n alkanes based on the present results and on previous observations on carboxylic acid moieties and the pyrolysis products of the three surface sediments such hydrocarbons are probably not derived from the epicuticular waxes of higher plants but chiefly correspond to autochthonous products likely of algal origin these observations added to recent results on samples from other types of marine environments stress that an abundance of long chain predominantly odd n alkanes in extracts should not be systematically considered alone as evidence for a major contribution of higher plant waxes the relatively low contribution of terrestrial n alkanes in the laptev sediments should result from the combination of i a major dilution of the terrestrial input due to high primary productivity in summer promoted both by a large influx of nutrients provided by the lena river and by the specific conditions in such an arctic environment and ii the high level of degradation of the organic material transported by the lena river thus the latter probably provided a low input of lipids from terrestrial sources to the laptev samples

regional controls on the geochemical evolution of saline groundwaters in the edwards aquifer central texas geochemical and isotopic parameters are used to evaluate models for the evolution of saline groundwaters in the edwards aquifer which lies on the northwestern margin of the gulf of mexico sedimentary basin saline groundwaters termed badwaters range in salinity from 1000 to 12 500 mg l 1 total dissolved solids models for badwater evolution must account for complexities owing to 1 the range in compositions of saline basinal fluids that may migrate into the edwards aquifer from its down dip section and from underlying hydrostratigraphic units 2 the range of depositional diagenetic and mineralogic variations in the hist aquifer rocks 3 volcanic and intrusive igneous rocks that locally crosscut the aquifer 4 extensive faulting which in places juxtaposes edwards aquifer units with adjacent hydrostratigraphic units calcium mg na so4 cl and hco3 variations define six badwater hydrochemical facies strontium isotope values of freshwaters 87sr86sr 0 7077 0 7086 badwaters 0 7076 0 7094 and brines from aquifer rocks down dip 0 7078 0 7097 are nearly all higher than the range of most edwards aquifer rocks 0 7074 0 7077 and volcanic and intrusive igneous rocks that crosscut the aquifer 0 7032 0 7055 argillaceous carbonate rocks that underlie the edwards aquifer are likely sources of sr with high 87sr86sr values transitions between the hydrochemical facies in some locations correspond to changes in depositional environments in aquifer rocks and to changes in the intensity of faulting through the aquifer major element mineral saturation state and srca 87sr86sr variations in badwaters of the westernmost aquifer region indicate that incongruent dissolution of gypsum and recrystallization of calcite control the geochemical evolution of these badwaters srca na cl 87sr86sr variations in badwaters of extensively faulted areas to the east of the westernmost aquifer region are consistent with fluid mixing processes involving at least five endmember fluids including 1 saline groundwaters from two underlying hydrostratigraphic units 2 two endmember brines from down dip edwadrs aquifer equivalent units and 3 freashwaters that have interacted extensively with aquifer rocks the compositions of badwaters from the northeasternmost region reflect fluid mixing between freshwaters and saline groundwaters from underlying hydrostratigraphic units

c25 and c30 biogenic alkenes in sediments and detritus of a new england salt marsh as part of a geochemical study of c25 and c30 biogenic alkenes in estuarine environments distributions of these compounds in detritus and sediments collected from a new england salt marsh round swamp on conanicut island in narragansett bay rhode island have been determined the alkene assemblages detected consisting primarily of four acyclic c25 dienes and trienes and a c30 bicyclic diene qualitatively resemble those previously reported for other sediments in which anoxic conditions were prevalent these similarities exist despite significant differences in the principal sources of sedimentary organic matter suggesting that the occurrence of these specific alkenes is more likely associated with an in situ process common to anoxic environments than with a direct input from a specific source size fractionation 840 m and 840 m to 1 2 m of marsh detritus revealed that the larger size fraction consisting primarily of decaying spartina debris contains significant amounts of alkenes this result together with alkene subsurface profiles which show high surface concentrations decreasing to near background levels by 20 cm suggest that anaerobic bacteria are mediating in situ production of these compounds previous studies of bacterial hydrocarbons have not reported the presence of these c25 and c30 alkenes although similar compounds have been isolated from several species of methanogenic bacteria however attempts to induce alkene synthesis by decomposing spartina anaerobically in the laboratory were unsuccessful in light of this result the exact source of alkenes in marsh sediments remains uncertain the absence from marsh sediments of other c25 alkenes whose sedimentary distributions had been previously correlated with the presence of marine planktonic organic matter implies the existence of different origins for structurally related constituents of this hydrocarbon series

stable isotope geochemistry of saline near surface ground water east central michigan basin near surface 100 m ground water in east central michigan basin is characterized by high total dissolved solids 100 to 53 000 mg l the source of the salinity and the cause of its distribution are unclear the hypothesis investigated was that the source of the salinity is brines advecting or diffusing upward into near surface meteoric water and the cause of the salinity distribution is the slow flushing of water in the argillaceous sediments by recent meteoric water 18o d 13c 34s 3h and selected geochemical variables were studied in the glacial drift and bedrock aquifers composing the ground water system the following isotopic ranges were found 18o 6 93 to 18 46 d 47 3 to 137 4 13c 6 26 to 20 65 and 34s 3 56 to 67 44 18o and d plot along the meteoric water line and most are extremely light 13c is skewed to lighter values and 34s becomes heavier with decreasing dissolved so4 concentration the results are interpreted to indicate that the saline ground water is a mix of modern day meteoric water and water that recharged the aquifers at a time when the climate was much cooler the cause of the salinity distribution is the slow flushing of the aquifers by present day recharge water the source of the salinity could be the upward diffusion or advection of michigan basin formation water but this has not been confirmed microbial reduction of so4 is occurring affecting the concentrations of so4 and hco3 and the values of 13c and 34s

paleowaters in silurian devonian carbonate aquifers geochemical evolution of groundwater in the great lakes region since the late pleistocene abstract changes in the climatic conditions during the late quaternary and holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the great lakes region increased hydraulic gradients from melting of kilometer thick pleistocene ice sheets reorganized regional scale groundwater flow in paleozoic aquifers in underlying intracratonic basins here we present new elemental and isotopic analyses of 134 groundwaters from silurian devonian carbonate and overlying glacial drift aquifers along the margins of the illinois and michigan basins to evaluate the paleohydrology age distribution and geochemical evolution of confined aquifer systems this study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the midcontinent region and extends into deeper portions of the illinois and michigan basins focused on the freshwater saline water mixing zones in addition the hydrogeochemical data from silurian devonian aquifers were integrated with deeper basinal fluids and brines in upper devonian black shales and underlying cambrian ordovician aquifers to reveal a regionally extensive recharge system of pleistocene age waters in glaciated sedimentary basins elemental and isotope geochemistry of confined groundwaters in silurian devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals dissolution of halite and anhydrite cation exchange microbial processes and mixing with basinal brines carbon isotope values of dissolved inorganic carbon dic range from 10 to 2 87sr 86sr ratios range from 0 7080 to 0 7090 and 34 s so 4 values range from 10 to 30 a few waters have elevated 13cdic values 15 from microbial methanogenesis in adjacent organic rich upper devonian shales radiocarbon ages and 18o and d values of confined groundwaters indicate they originated as subglacial recharge beneath the laurentide ice sheet 14 50 ka bp 15 to 13 18o these paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake bed clays and or shales the presence of isotopically depleted waters in paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional scale groundwater flow modern groundwater flow in the great lakes region is primarily restricted to shallow unconfined glacial drift aquifers recharge waters in silurian devonian and unconfined drift aquifers have 18o values within the range of holocene precipitation 11 to 8 and 7 to 4 5 for northern michigan and northern indiana ohio respectively carbon and sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions 13cdic 14 7 to 11 1 and 87sr 86sr 0 7080 0 7103 the distinct elemental and isotope geochemistry of pleistocene versus holocene age waters further confirms that surficial flow systems are out of contact with the deeper basinal scale flow systems these results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes which are important for determining the sustainability of present day water resources and stability of saline fluids in sedimentary basins

u pb geochronology of archean magmatism and proterozoic metamorphism in the quadrilatero ferrifero southern sao francisco craton brazil the quadrilatero ferrifero is a metallogenic district au fe mn located in the southernmost sao francisco craton in eastern brazil it is composed of archean granitoids gneisses and a greenstone belt sequence rio das velhas supergroup overlain by a proterozoic clastic chemical sedimentary sequence hosting a thick lake superior type banded iron formation minas supergroup a u pb study of the main units leads to the following conclusions granitoid emplacement was dated at 2776 7 6 ma and 2721 3 ma two felsic volcanic rocks from the rio das velhas supergroup yielded ages of 2776 23 10 ma and 2772 6 ma these data indicate that greenstone belt volcanism was coeval with granitoid emplacement a common feature of greenstone belt terrains but so far undocumented in the southern sao francisco shield the younger granitoid intrusion marks the stabilization of the shield in late archean time zircon cores older than 2776 ma together with other data indicate the presence of continental crust 2 88 3 1 ga old in the quadrilatero ferrifero a detrital zircon from the uppermost formation in the minas supergroup sabara formation is concordant at 2125 4 ma and provides the first age constraint on the deposition of the supergroup monazite from two pegmatites and titanite from an amphibolite enclave in gneisses from the bacao complex in central quadrilatero ferrifero yielded ages between 2060 ma and 2030 ma these data may constrain the minimum age for the deposition of the minas supergroup in addition they indicate that the early proterozoic transamazomian orogeny is represented by amphibolite facies metamorphism and partial melting of archean crust

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

ages of detrital zircon from siliciclastic successions south of the sao francisco craton brazil implications for the evolution of proterozoic basins abstract in this work we report 207pb 206pb la icpms ages of 152 detrital zircons from lower greenschist facies quartzites from proterozoic basin successions of the southern border of the sao francisco craton southern minas gerais state brazil these are the intracratonic sao joao del rei basin the intraplate continental margin andrel ndia basin and the serra do ouro grosso sequence developed on a crystalline basement older than 1 8 ga and deformed and metamorphosed during the brasiliano orogeny ca 0 59 0 50 ga the data constrain both the ages of the sources and the interval of sedimentation the detrital zircons of the serra do ouro grosso sequence were derived predominantly from the erosion of a neoarchean crust 2 5 2 8 ga old with only one grain showing a paleoproterozoic age 2 245 83 ma older than the transamazonian event zircons extracted from a shelf quartzite of the lowermost sequence of the sao joao del rei basin indicate derivation from the 1 8 2 2 ga transamazonian crust with subordinate contribution from the 2 5 2 9 ga archean crust the 1 809 41 ma age is interpreted as the maximum limit for sedimentation in this basin the results confirm the regional correlation with the espinhaco rift successions the zircons extracted from an autochthonous quartzite of the andrel ndia sequence yielded ages in the 1 0 2 2 ga range with a modal class at 1 2 1 3 ga only two of the forty analyzed zircons yield archean ages the youngest zircon yields 1 086 85 ma the zircons from the allochthonous quartzite yield ages between 1 0 2 7 ga with a modal class at 2 1 2 2 ga only five of 45 analyzed grains yield archean ages the youngest zircon has an age of 1 047 77 ma the results indicate that the detrital sediments deposited during the second marine flooding event of the andrel ndia sedimentation were mainly derived from the erosion of mesoproterozoic and paleoproterozic rocks the 1 047 77 ma age is interpreted as the maximum depositional age for the described association

volumetrically significant recharge of pleistocene glacial meltwaters into epicratonic basins constraints imposed by solute mass balances melting of kilometer thick continental ice sheets during pleistocene glaciation profoundly altered regional scale groundwater flow in the low lying stable interior of the north american craton in this paper we show that large volumes of glacial meltwater penetrated to great depths in underlying sedimentary basins through regionally extensive silurian devonian carbonate aquifers disrupting relatively stagnant saline fluids and creating a strong disequilibrium pattern in fluid salinity these dilute isotopically light meteoric waters migrated into overlying fractured organic rich upper devonian shales and significantly enhanced microbial methanogenesis generating a unique class of natural gas deposits along the shallow basin margins new data on formation water chemistry of upper devonian antrim shale gas wells along the northern and western margins of the michigan basin were integrated with previously published shale data and brines in subjacent silurian devonian carbonates this comprehensive database provides important constraints on fluid and solute transport and makes a compelling case for the reorganization of drinking water resources and salinity structures along the shallow basin margins by pleistocene glaciation na ca cl br relations and mass balance calculations were used to determine the relative volume of meteoric waters and sources of salinity in antrim shale fluids the majority of shale formation waters contain greater than 50 meteoric water with most containing over 80 meteoric water up to 300 m beneath the shale subcrop despite the presence of a strong salinity gradient meteoric water recharge dissolved variable amounts of halite from evaporite bearing silurian devonian carbonates along the flow path into the shale and displaced highly saline nacacl remnant marine brines the majority of antrim shale fluids owe greater than 60 of their salinity to halite dissolution while less than 40 of cl is from mixing with brines in silurian devonian strata oxygen and hydrogen isotope chemistry and carbon 14 age dating indicate these nacl brines were likely generated since the late pleistocene our conceptual model of fluid flow along the michigan basin margins and its role in generation of microbial gas were extrapolated to the larger glaciated midcontinent region where the silurian devonian carbonate subcrop is continuous along the illinois michigan and appalachian basin margins and is overlain by upper devonian black shales major differences in the hydrostratigraphy and fluid salinity of the epicratonic basins controlled the extent of meteoric water invasion and generation of microbial methane increased hydraulic gradients from melting of the continental ice sheets greatly enhanced recharge of dilute waters into deep silurian devonian aquifers and overlying fractured shales reversing regional scale groundwater flow and altering the major ion composition these fluid migration events occurred over relatively short time scales thousands of years compared to well documented basinal scale fluid migration events driven by tectonics and sediment compaction over millions of years understanding the hydrogeochemistry of the saline meteoric water mixing zones and the sources of salinity in sedimentary basins are both important for constraining fluid and solute transport chemical evolution of basinal fluids and physical stability of brines during meteoric water invasion