hydrocarbon and fatty acid distributions in rostherne lake sediment england abstract hydrocarbon and carboxylic acid distributions in the top 30 cm section of a contemporary lacustrine sediment rostherne mere cheshire england of age 0 150 yr present variations which can be correlated with changes in input of organic material as well as the effects of early diagenesis thus the higher proportions of short chain and branched lipids in the top 0 7 cm sediment section are as expected on the basis of the relatively greater contribution of algal and other microbial matter to the surface sediment layer similarly the increase in the ratio of saturated to unsaturated straight chain acids with depth is in accord with the more facile diagenesis of the unsaturated components acid hydrolysis of solvent extracted sediment afforded lipid fractions featuring a more significant microbial contribution than pre hydrolysis extracts as denoted by the greater abundance of c15 and c17iso and anteiso branched acids also no trend with depth in the ratio of saturated to unsaturated acids was observed for post hydrolysis extracts indicating that bound compounds may be somewhat protected relative to soluble lipids a contribution from petroleum derived hydrocarbons to the sediment apparent from the complex branched cyclic b c alkane distributions decreases with depth and is virtually absent from the 18 30 cm sediment section

dissolved and particulate fe in a hydrothermal plume at 9 45 n east pacific rise slow fe ii oxidation kinetics in pacific plumes production of fe iii particles in hydrothermal plumes is of fundamental importance to the long term effect of hydrothermal circulation on seawater composition to elucidate the fundamental controls on fe redox kinetics and solution particle partitioning in neutrally buoyant plumes we sampled near field 3 km plume particles at 9 45 n on the east pacific rise in 1996 returning in 1997 to sample both particulate and dissolved phases 0 40 m filter concentrations of dissolved fe varied from 320 to 20 nm in proximal 0 3 km from vent site to distal samples 1 3 km downfield constituting 85 50 of total fe respectively based on vent fluid dilution factors calculated from dissolved mn a mass balance for vent fluid fe at this site indicates that 65 of fe is lost to particulate sulfide settling in the buoyant plume and that particulate fe in distal 1 3 km samples is twice as concentrated as predicted from dilution of particles in proximal plume water these observations are consistent with a calculated fe ii oxidation half time of 3 3 h long enough that fe iii colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions preventing generation of high particulate fe concentrations 11 56 nm observed a general investigation of fe ii oxidation rates in plumes worldwide gives fe ii oxidation half lives as short as 17 min at some atlantic sites and as long as 6 h at some pacific sites the calculations indicate that the distribution of fe particles in plumes depends chiefly on inter basin differences in ambient deep water chemistry primarily ph and dissolved o2 and on local currents driving plume dilution and to a much lesser extent on variations in primary vent fluid composition long term changes in thermohaline circulation or ocean biogeochemistry may therefore alter fe dynamics and minor element fluxes associated with global hydrothermal activity independent of variations in crustal production rates

aromatized arborane fernane hydrocarbons as molecular indicators of floral changes in upper carboniferous lower permian strata of the saar nahe basin southwestern germany abstract thirty seven coal samples of upper carboniferous and lower permian age from three boreholes in the saar nahe basin germany have been studied by organic geochemical and coal petrological methods the investigations were aimed at the recognition of floral changes in the upper carboniferous and lower permian strata the results show that compositional changes in the extracts are only partly caused by variations in coalification specific aromatic hydrocarbons appear in upper westphalian d coal seams and increase in concentration up to the rotliegendes the dominant compound has been identified by mass spectrometry and nmr spectroscopy as 5 methyl 10 4 methylpentyl des a 25 norarbora ferna 5 7 9 triene math and always occurs associated with 25 norarbora ferna 5 7 9 triene both compounds are thought to originate from isoarborinol fernene 3 ol or fernenes the strongly acidic conditions during deposition of the coals might have induced the 4 5 cleavage combined with a methyl shift in an arborane fernane type pentacyclic precursor yielding the math based on petrological investigations palynomorphs related to early gymnospermopsida such as pteridospermales and coniferophytes cordaitales and coniferales increased in abundance in the strata beginning with the upper westphalian d concomitant with the above mentioned biomarkers the results suggest the arborane fernane derivatives originate from the plant communities producing these palynomorphs

glacial sediments and erosional pavements near the cambrian precambrian boundary in western henan province china exceptionally well preserved glaciated pavements and a variety of glacigenic sediments comprising the luoquan formation in henan province central china are believed to have formed close to the cambrian precambrian boundary they may therefore be younger than the better known sinian tillites representing the changan and nantuo ice ages they perhaps belong to a distinct glacial epoch that is recorded elsewhere in china and in other continents but for which there is less evidence than for earlier late proterozoic glacial periods lithofacies represented in the luoquan formation are massive diamictite signifying deposition as lodgement or melt out till bedded diamictite waterlain till or glaciomarine glaciolacustrine sediments with dropstones rhythmites with dropstones glaciolacustrine or fjord deposits possibly varves tabular conglomerates and sandstones outwash deltaic or sub aqueous mass flow deposits lens shaped breccia deposits sub aqueous mass flow deposits and gravel sand wedges frost contraction crack infillings the glaciated pavements reveal well preserved striations grooves often intersecting 9plastically moulded9 forms crescentic gouges and steps on a regional scale the linear features are of variable orientation but they indicate a dominant ice flow direction towards the southeast all this evidence points to upland glaciation with erosion followed by deposition in a terrestrial glacial and glaciolacustrine environment

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

hydrothermal scavenging at the mid atlantic ridge radionuclide distributions we have carried out a study of u series radionuclide distributions in the neutrally buoyant plume overlying the tag hydrothermal vent field at 26 n mid atlantic ridge limited uptake of u accompanies the formation of fe oxyhydroxide particulate material within the hydrothermal plume the particulate u concentrations are too small for measurable dissolved u anomalies to be produced within the water column at plume height however such uptake followed by deposition in the underlying sediments could account for 10 of the u enrichment that has been reported previously for metalliferous ridge crest sediments particulate 234th 230th and 228th activities in the tag hydrothermal plume represent 65 of total th activities typical of the deep ocean low dissolved th activities coincide with the high particulate th activities indicating that hydrothermal scavenging strongly affects th isotope distributions in the water column close to hydrothermal vents much of the scavenging occurs during the initial formation of fe rich hydrothermal precipitates but some further scavenging continues as the material is dispersed away from its source distributions of particulate 210pb and 231pa activities in the neutrally buoyant plume are also extremely high 7 12 dpm 102 l and 30 160 dpm 106 l respectively like th 210pb and231pa show evidence for scavenging within the hydrothermal plume total th activities at plume height are 50 higher than background this focusing of th into the plume is consistent with the entrainment and re emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume

messel oil shale western germany assessment of depositional palaeoenvironment from the content of biological marker compounds abstract the extractable lipids of two outcrop samples of messel oil shale have been analysed by gc and gc ms both samples contain as major lipids similar distributions of 4 methylsteroids hopanoids and long chain c 28 c 36 alkanediols and 1 hydroxyalkanones compared with other reported distributions of sedimentary alkanediols and 1 hydroxyalkanones those reported herein extend to higher carbon numbers greater variety of mid chain positional isomers and are the oldest the major sterol has been shown to be 4 methyl 24 ethyl 5 h cholestan 3 ol whereas 4 23 24 trimethyl 5 h cholest 22 en 3 ol dinosterol is more abundant than the 24 ethyl structural isomer two novel c 31 4 methylsterols have been recognised and are proposed to have a 23 methyl 24 ethyl 28 side chain desmethylsteroids are of low abundance compared with 4 methylsteroids the two outcrop samples differ principally in their distributions of hydrocarbons whereas sample 1 contains negligible steroidal hydrocarbons sample 2 exhibits abundant desmethylsterenes and 4 methylsterenes such differences illustrate the inhomogeneous nature of the messel deposit in regard to its content of biological marker compounds alkaline hydrolysis of the kerogen of sample 1 releases mainly hydroxy compounds similar in composition to the corresponding extractable lipids together with trace amounts of ketones and hydrocarbons similarly lithium aluminium hydride treatment of the kerogen of sample 1 yields hydroxy compounds but with a different composition to the alkaline hydrolysis products overall the content of biological marker compounds suggests that the major sources of the sedimentary organic matter were dinoflagellates and bacteria especially cyanobacteria the algal assemblage was probably dominated by dinoflagellates of low species diversity with other algal classes in relatively low abundance deposition probably took place into an anoxic hypolimnion supporting a significant population of chlorobium type bacteria

an assessment of the effect of chemical scavenging within submarine hydrothermal plumes upon ocean geochemistry profiles of 210pb over the endeavour and north cleft segments of the juan de fuca ridge are used to model a time scale for the scavenging by hydrothermal plumes of reactive elements in seawater the hydrothermal plumes above these ridge segments are sites of intense scavenging removal of 210pb at endeavour the total 210pb activities within the plume are as low as 8 dpm 100 l and dissolved activities are as low as 3 dpm 100 l at the north cleft which is characterized by higher particulate fe concentrations the total 210pb activities are 4 5 dpm 100 l the dissolved activities are 1 2 dpm 100 l and the 210pb activities are deficient with respect to the activity of the 210po daughter these are perhaps the lowest 210pb activities ever measured in the deep sea the large gradient of 210pb between the plume and surrounding deep water suggests that scavenging is focused into the plumes through horizontal transport the implication therefore is that this process might impact the ocean on a scale larger than that local to the ridge crest by coupling published measurements of particle flux from endeavour with 210pb activities on particles trapped at that site the total volume of seawater stripped of 210pb per year for that site was calculated to be 7 4 1012 l y globally the extrapolated volume flux of seawater stripped of reactive constituents is 5 7 1015 l y such that the entire ocean is processed in this manner in 2 4 105 y the geochemical cycle of elements with ocean residence times much shorter than this e g pb and th will not be greatly affected by hydrothermal scavenging on the other hand this process holds significance for the geochemistry of other elements scavenged by hydrothermal plumes such as p and v whose ocean residence times are 104 y

the origin and fate of 4 methyl steroids ii dehydration of stanols and occurrence of c30 4 methyl steranes abstract certain features of the distributions of sedimentary 4 methyl steroid hydrocarbons have been investigated i competitive dehydration experiments with stanols adsorbed on a shale suggests a possible explanation for the low abundance of 4 methyl sterenes relative to their desmethyl counterparts in immature sediments ii a 4 methyl spirostene which is a minor product of the backbone rearrangement of 4 methylcholest 4 ene in the laboratory has been tentatively assigned its presence in a toarcian black shale with a second isomer provides further evidence that the sedimentary backbone rearrangement of 4 methyl sterenes is analogous to that of their desmethyl counterparts and iii the major c30 4 methyl steranes in semecourt shale toarcian paris basin differ from those in an eocene lacustrine shale from china this indicates the potential complexity of c30 steroidal alkanes both 4 methyl and desmethyl which can occur in sedimentary organic matter and shows that care must be taken in using m z 217 mass chromatograms alone for routine input and maturity assessment using steroidal alkanes

hydrothermal deposition on the juan de fuca ridge over multiple glacial interglacial cycles abstract hydrothermal systems play an important role in modern marine chemistry but little is known about how they may have varied on 100 000 year timescales here we present high resolution records of non lithogenic metal fluxes within sediment cores covering the last 500 000 years of hydrothermal deposition on the flanks of the juan de fuca ridge six adjacent gridded cores were analyzed by x ray fluorescence for fe mn and cu concentrations corrected for lithogenic inputs with ti and normalized to excess initial 230 th to generate non lithogenic metal flux records that provide the longest orbitally resolved reconstructions of hydrothermal activity currently available fe fluxes vary with global sea level over the last two glacial cycles suggesting higher hydrothermal deposition during interglacial periods the observed negative relationship between fe and mn indicates variable sediment redox conditions and diagenetic remobilization of sedimentary mn over time thus mn fluxes may not be a reliable indicator for hydrothermal activity in the juan de fuca ridge sediment cores cu fluxes show substantial high frequency variability that may be linked to changes in vent temperature related to increased magmatic production during glacial periods deglacial hydrothermal peaks on the juan de fuca ridge are consistent with previously published records from the mid atlantic ridge and the east pacific rise moreover on the juan de fuca ridge the deglacial peaks in hydrothermal activity are followed by relatively high hydrothermal fluxes throughout the ensuing interglacial periods relative to the previous glacial period

the changing vision of marine minerals abstract non fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings the theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time prior to the advent of plate tectonics we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land this view adequately explained marine placer deposits heavy minerals and gems aggregates sand and gravel and precipitates phosphorites and manganese nodules although numerous sites of placer mineral deposits are known on continental shelves worldwide current activity pertains to diamond mining off southwestern africa tin mining off southeastern asia and intermittent gold mining off northwestern north america which are all surpassed economically by worldwide recovery of marine sand and gravel in turn dwarfed by offshore oil and gas with the advent of plate tectonics plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower temperature deposits hosted in oceanic crust mafic at ocean ridges and felsic at volcanic island arcs and of magmatic ni cu sulfide chromite and pge deposits inferred to be present in the oceanic upper mantle lower crust based on their occurrence in ophiolites some 300 sites of hydrothermal active and relict mineralization most of them minor are known at this early stage of seafloor exploration on ocean ridges in fore arc volcanoes at back arc spreading axes and in arc rifts deposits formed at spreading axes and transported off axis by spreading are present in oceanic lithosphere but are virtually unknown the tag trans atlantic geotraverse hydrothermal field in the axial valley of the mid atlantic ridge latitude 26 n is considered to exemplify a major volcanogenic massive sulfide vms deposit forming at a spreading axis the most prospective of these occurrences lie within the 200 nautical mile 370 km wide exclusive economic zone eez of the nations of the volcanic island arcs of the western pacific where metal content of massive sulfides ag au ba cu pb sb zn exceeds that at ocean ridges plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization with the potential to elucidate the occurrence of individual deposits e g eocene carlin type gold deposits investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence consideration of questions posed by brian skinner 1997 of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals

variations in the content and composition of organic matter in sediments underlying active upwelling regimes a study from odp legs 108 112 and 117

dinoflagellate origin for sedimentary 4α methylsteroids and 5α h stanols biological marker compounds provide useful tools for evaluating the depositional environments of recent and ancient sediments and petroleums1 largely through established relationships between the organic matter of sediments and source organisms1 2 such relationships are best tested and explored by investigating the lipid components of natural populations of autochthonous biota that can be clearly recognized as contributors to underlying bottom sediments in this context we have studied the lipids of the freshwater dinoflagellate peridinlum lomnickii woloszynska order peridiniales class dinophyceae collected from the waters of priest pot3 a eutrophic lake in the english lake district its distributions of both 4 methylsterols and 4 methylstanones closely resemble those of the underlying bottom sediments demonstrating that dinoflagellates are important contributors of these sedimentary compounds the 4 methylsteroidal hydrocarbons found in ancient sediments and petroleums4 6 are presumably diagenetic products of such 4 methylsteroids and therefore reflect dino flagellate inputs to the original depositional environments furthermore the prominence of 5 h cholestan 3 ol in p lomnickii suggests that dinoflagellates may be the long sought direct biological source of sedimentary 5 h stanols

late quaternary mediterranean sapropels ii organic geochemistry and palynology of s1 sapropels and associated sediments six s1 sapropel samples from the eastern mediterranean were investigated organic geochemically to study the lateral variation within this organic rich layer five of these samples were also analyzed palynologically in addition some sediment samples from below and overlying the sapropel intervals were investigated for their organic geochemical characteristics organic matter of s1 sapropels is of a mixed marine terrigenous and bacterial origin a trend of relatively increasing amounts of continentally derived organic matter towards more seaward and deeper realms can be observed from both palynological and organic geochemical data this trend is supported to some extent by 13c values of the organic matter the sapropelic intervals deposited on the nile cone are characterized by expanded thicknesses and a diluted organic carbon content due to a higher sedimentation rate the environmental conditions in terms of preservation during sapropel formation over the eastern mediterranean were probably not uniform at site 29 the conditions were favourable for the deposition of sapropel with a higher organic carbon content than at the other locations this might have been caused by better preservation conditions an increasing discharge from the river nile seems to be the driving force for formation of the s1 sapropels based on this assumption a model for sapropel formation is proposed although not all the data could be explained properly by this model most phenomena can be explained

the use of rbsr and kar dating methods as a stratigraphic tool applied to sedimentary rocks and minerals abstract rb sr and k ar methods can be used for direct numerical chronostratigraphy of sediments using the clay silicates they contain the interpretation of the ages calculated depends on the satisfaction of the following requirements 1 1 the series studied must include diagenetic clay sediments coarsely detrital or alightly metamorphic formations must be excluded studies in progress concerning decarbonatation may lead to the application of direct dating of sediments being extended to carbonate sediments 2 2 careful mineralogical study of the clay minerals must prove early diagenetic recrystallization or new formation this part of the study must precede any isotopic work 3 3 the isotopic study must involve the use of both rb sr and k ar methods the extreme sensibility of the k ar isotopic system to late diagenesis helps to choose between early or late diagenesis a younger k ar age may generally be regarded as that of the late diagenesis whereas the rb sr age may be that of the early diagenesis 4 4 the initial 87sr 86sr isotopic ratio is not always a reliable criterion owing to the effects of the dilution of the radiogenic 87sr by the great amounts of primary sr contained in carbonates 5 5 the regularly decreasing succession of isotopic ages must follow the stratigraphic succession when this condition is satisfied there is a high probability that the calculated ages are indeed those of the deposition dates

diagenesis of free and bound lipids in terrestrial detritus deposited in a lacustrine sediment recent sediments from an oligotrophic lake loch clair having uniform organic input for 2000 yr show changes in lipid abundance and composition with increasing depth attributed to diagenesis stability of free lipids decreases in the order n alkanes alkan 2 ones sterols n alkanoic acids n alkanols n alkenoic acids diagenetic loss of shorter chain homologues is complete within 400 yr stabilisation of bound relative to free lipids increases the proportion of the former with sediment age and reduces loss of shorter chain bound homologues in an eutrophic lake cross mere shorter chain free and bound sedimentary lipids show increased abundance compared with loch clair the relative importance of higher input of shorter chain lipids derived from aquatic detritus and slower initial diagenesis due to the anoxic hypolimnion as causative factors for this difference between lake types cannot yet be assessed

hydrothermal scavenging at the mid atlantic ridge modification of trace element dissolved fluxes abstract we have carried out an icp ms study of the distributions of trace elements fe ca mg al mn cu zn v as cr co pb y mo ag sn cd and be in particulate material collected from the tag hydrothermal plume 26 n mid atlantic ridge to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent fluids are balanced by the precipitation of associated particulate phases comparison of particulate trace element distributions with particulate fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes i co precipitation from vent fluids ii scavenging from seawater and iii preferential settling from the plume and oxidative dissolution within the water column for y and be continuous scavenging from seawater appears to be important whereas for v as and cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume from a comparison of trace element fe ratios in the particles and associated mid atlantic ridge vent fluids it appears that for many elements e g cu co ag pb al cd be and zn the flux of dissolved material from vent fluids is matched closely by the precipitation of hydrothermal particulate phases of the various elements studied here hydrothermal activity only appears to act as a significant source for dissolved ca and dissolved mn

radiochronological age and correlation of proterozoic sediments in brazil abstract a review of available rb sr and k ar datings obtained on sedimentary sequences metamorphosed or not interbedded volcanics and cross cutting intrusives of the precambrian of brazil yields the following conclusions 1 1 the roraima and rio fresco formations resting upon the amazonian craton have been affected by the trans amazonian orogeny and are of lower proterozoic age 2 2 the beneficiente group in the same region seems to be of middle proterozoic lower riphean age 3 3 upon the sao francisco craton and upon the lower proterozoic rio dos remedios complex the paraguacu group might be of middle riphean age and the glacial sequences macaubas group and bebedouro formation date back to between 950 and 1000 ma 4 4 the age of the bambui and una groups in the same region remains undetermined it might be either upper riphean or upper riphean and vendian 5 5 the molassic series associated with the brazilian orogeny are dated back to between the proterozoic phanerozoic boundary and the ordovician

correlation of proterozoic sediments of western and central africa and south america based upon radiochronological and palaeontological data abstract nearly 70 new rb sr isochron ages and many k ar conventional ages have been determined between 1975 and 1980 on proterozoic sedimentary or metasedimentary sequences in western and central africa and south america some stratigraphic results have been established 1 1 five formations have been dated of the lower proterozoic 2 2 a long sedimentation gap occurs mainly in western african and in some regions of central africa and south america between nearly 1600 and 1100 ma 3 3 the upper riphean assemblages of stromatolites have been dated and compared to those of the eurasian craton 4 4 two main glacial events have been dated the first one placed at ca 950 ma the second one during the vendian at ca 650 620 ma 5 5 it can be stated that when applied to precambrian sequences all stratigraphic methods must be used together

scavenging of 234th and phosphorus removal from the hydrothermal effluent plume over the north cleft segment of the juan de fuca ridge dissolved and total 234th t1 2 24 1 days profiles were measured over the hydrothermal field located within the north cleft segment of the juan de fuca ridge above the hydrothermal effluent plume which rose approximately 200 m from the seafloor 90 of the 234th was dissolved and the total 234th activity was in equilibrium with its parent uranium this is typical of deep water for which the residence time of dissolved 234th with respect to particle uptake is greater than 200 days within the effluent plume however as little as 30 of the 234th was in the dissolved state and the residence time of dissolved 234th with respect to particle uptake was then of the order of only a few weeks the flux jth of dissolved 234th to the particle phase was about 300 disintegrations per minute dpm cm 2 yr 1 the total 234th was still approximately in equilibrium with u indicating that the residence time of the 234th bearing particles with respect to sinking was greater than 100 days particulate p v and as while insignificant above the plume attained concentrations as high as 60 0 65 and 0 45 nmol l within the plume the particulate p 234th ratio and the 234th flux jth are used to calculate a removal flux of p resulting from hydrothermal scavenging of 50 g cm 2 yr 1 or approximately 5 0 106 g p yr 1 per kilometer of active ridge crest similarly 1 0 105 g km 1 of v and as are removed per year from the water column by hydrothermal scavenging over this region extrapolation to a global scale suggests that ridge crest scavenging removes 10 of these elements from the ocean

composition and sedimentation of hydrothermal plume particles from north cleft segment juan de fuca ridge in 1990 and 1991 particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the north cleft segment of the juan de fuca ridge were sampled to study their changing composition and fluxes away from the vent field in the rising buoyant plume 75 of the p v cr and as scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent cu and zn are most enriched in buoyant plume particles collected from the first few meters above the vent however the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense cu and zn rich sulfide phases using the plume data coupled with the results of our analysis of sediment trap samples we estimated that more than 99 and 99 9 respectively of the total hydrothermal fe and mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean

geology mineralogy and lithogeochemistry of metalliferous mudstones associated with the lemarchant volcanogenic massive sulfide deposit tally pond belt central newfoundland the cambrian lemarchant zn pb cu au ag bimodal felsic volcanogenic massive sulfide vms deposit tally pond group central newfoundland canada contains metalliferous mudstones that occur either immediately on top or laterally along strike from massive sulfide mineralization or as interflow mudstones within hanging wall basaltic rocks the mudstones are brown to black graphite rich and locally have intercalations of siliciclastic volcaniclastic and or amorphous chert layers and in some cases fine lamina of organic matter the main sulfide phases are pyrite framboidal and euhedral and pyrrhotite with minor chalcopyrite sphalerite arsenopyrite and galena barite is a common sulfate the metalliferous mudstones occur at various stratigraphic levels and have variable inputs of hydrothermal high fe al and base metal values and detrital components lower fe al and base metal values with positive shale normalized eu anomalies eu eu 1 negative ce anomalies ce ce 1 and an average y ho ratio of 28 3 these signatures suggest precipitation from reduced high temperature hydrothermal vent fluids in an oxygenated water column coupled with the hydrothermal particles having a short residence time within the hydrothermal plume i e a vent proximal setting deposition from an oxygenated water column is also supported by the presence of abundant barite in both the lemarchant massive sulfides and the metalliferous mudstones as well as locally preserved marcasite in the mudstones redox sensitive trace elements as u v mo cr ni and co were scavenged as oxyanions onto fe oxyhydroxides during mudstone formation and these elevated levels of scavenged redox sensitive trace elements cause apparent anoxic signatures immobile element systematics la th zr 10 and th la sc indicate that the detrital components of the lemarchant mudstones were mixtures of both continental and mafic sources most likely deposited within a pericontinental rifted arc environment our research illustrates that the lithogeochemistry of metalliferous mudstones can identify vent proximity and provides insights into the physiochemical conditions of the deposit formation and the ambient environment these results have implications for similar precious metal bearing vms deposits globally

geochronological correlation of the late precambrian sequences on and around the stable zones of equatorial africa abstract very few radiometric age determinations have a bearing on the late precambrian sequences of the stable zones of equatorial africa a geochronological correlation of these sequences must therefore rest upon a compilation of ages from these stable zones and the corresponding mobile zones about 15 ages of varying precision more or less well tied up with the stratigraphical sequences together with results of the study of stromatolite assemblages allow a provisional correlation to be made one conclusion of this correlation is that most mixtites surrounding the congo basin are in a 950 50 ma age bracket and are presumably to be correlated

particle recycling in the tag hydrothermal plume abstract analysis of th isotope distributions in the tag hydrothermal plume yields evidence for pronounced 234 th 230 th isotope fractionation this fractionation can be attributed to a combination of four processes 1 particle ageing 2 differential scavenging of 234 th and 230 th 3 differential focusing of 234 th and 230 th and 4 recycling of old particles we present arguments for the importance of the last of these processes particle recycling particles in the tag hydrothermal plume exhibit grain sizes which fall in the range 1 10 m applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases stokes law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse grained 5 m radius particles and 0 2 0 3 m d 1 for fine grained particles 1 m radius these velocities have been used in a physical model of re entrainment and recycling in neutrally buoyant plumes our results indicate that particle recycling and re entrainment in hydrothermal plumes may be important over length scales of 1 10 km the model predicts that the proportion of particles re entrained is independent of particle size but increases with plume strength for plumes with volumetric fluxes greater than 10 3 m 3 s 1 the proportion of particles re entrained exceeds 50 in agreement with previous calculations based upon 230 th distributions sampling strategies in the past have typically concentrated upon processes operating within 1 km of active vent fields and will need to be revised accordingly in the future

a forced baroclinic vortex around a hydrothermal plume a steady frictional flow around a hydrothermal plume is described which is forced by the entrainment of water into the buoyant part of the plume as it rises above its source the circulation is anticyclonic clockwise in the northern hemisphere near the equilibrium density level or spreading level of the plume and cyclonic below this level the strength of this flow is adequate to trap anomalous properties from the hydrothermal fluid near the source in the presence of weak background flow some observational support for such a flow is presented

biogeochemistry of the miocene lacustrine deposit at clarkia northern idaho u s a abstract the lipid preservation observed in the sediments from clarkia reflects an extremely immature thermal history many biomarkers have still retained functionality even after 17 20 million years the soluble components are predominantly higher plant and algal derived lipids and the original sources for these compounds are still preserved in the sediment thus biogeochemical data are in agreement with the associated palaeontological evidence chemical changes noted in the lipids preserved in these sediments are similar to those observed in modern fresh water mesotropic or eutrophic lacustrine sediments therefore it can be inferred that the processes that operate in modern lacustrine sediments are similar to those that took place within the sediments at clarkia 17 20 million years ago

lipids of aquatic organisms as potential contributors to lacustrine sediments ii abstract the relationship between the lipid composition of organisms in the water column of an eutrophic lake and the lipid composition of underlying sediments previously examined for n alkanols and steroids is now reported for hydrocarbons ketones and carboxylic acids the n c7 alkane and alkenoic acids from two primary sources are rapidly metabolized in the water column and surficial sediment bacterial biomarkers including hopenes and i ai branched fatty acids were detected in the photosynthetic bacterial layer occurring just above the sediment water interface within the sediment the apparent conversion of free n alkanes alkan 2 ones and hydroxy acids to the corresponding bound form is noted microbiological oxidation of n alkanes to alkan 2 ones is supported by the detection of the intermediate alkan 2 ols with a distribution similar to that of the ketones the geochemistry of sediment deposited c 1900 prior to biological study of the site was interpreted from stable biomarkers and the diagenetic changes recognised in the study of contemporary deposition a qualitative difference in algal input to the older sediment is inferred from the low 7 sterol content and presence of 2 6 10 trimethyl 7 3 methylbutyl dodecane however there was still significant dinoflagellate input as indicated by the presence of 4 methylsterols a difference in higher plant input to the older sediment indicated from the n alkane alkene and triterpenoid ketone distributions is consistent with the recent development of tree cover

provenance of exhalites associated with the lemarchant volcanogenic massive sulphide vms deposit central newfoundland canada insights from nd isotopes and lithogeochemistry neodymium isotope data on exhalites and tuffs from the cambrian lemarchant volcanogenic massive sulphide vms deposit provide insights into the tectonic environment of the tally pond group canada new data from exhalites from the lemarchant area show evolved values of end 513 6 0 to 1 8 whereas the associated volcanic rocks have end 513 of 0 4 to 1 4 the lemarchant exhalite end compositions overlap the underlying ganderian neoproterozoic sandy brook group end t 6 5 to 1 9 and crippleback intrusive suite end t 5 9 to 5 2 the evolved nd isotopic signatures suggest that the volcanic rocks of the tally pond group were formed upon ganderian arc basement which itself was possibly built upon or proximal to the gondwanan amazonian margin erosion of older crustal material and tally pond group volcanic rocks together with coeval eruption of the volcanic rocks released nd rich detritus into the water column uptake of eroded detrital and scavenged nd resulted in mixed nd sources juvenile and evolved which are archived in the exhalites the results of this study are not only of significance for occurrences of exhalites within the tally pond group but also have exploration implications for vms districts globally

a geochemical study of long chain n aldehydes in washington coastal sediments a series of n aldehydes c20 to c32 with strong even to odd carbon preference was identified in the mixture of solvent extractable lipids isolated from various washington coastal sediments a limited survey of the lipid composition of surface waxes of foliage and cuticular waxes of pollen from several major plant species indigenous to the pacific northwest revealed that similar series of n aldehydes are intrinsic to regional vegetation debris from such plants discharged at the mouth of the columbia river represents a preformed source of the n aldehyde series accumulating in sediments from the washington coastal region although the evidence suggests n aldehydes are largely introduced erosionally to this region as a chemical component of land derived debris and are not formed in situ in the coastal sediments further study is warranted to establish the postdepositional stability of this series of compounds relative to other lipids of higher plantwax origin despite but few literature reports of the occurrence of these geochemicals long chain n aldehydes are very likely common in many sediments from environments that receive an input of detrital organic matter from higher plants

lipid geochemistry of lake kinneret lake kinneret a relict lake from the neogene is characterised by the dominance among its phytoplankton of the dinoflagellate peridinium cinctum the lipid geochemistry of lake kinneret is discussed herein in terms of the biology chemistry and hydrology of the lake lipids isolated from two sediment sections surface and 15 cm deep obtained from the deepest point of lake kinneret include 1 4 methyl 5 h stanols and related derivatives characteristic of p cinctum the novel sterol 4a methylgorgosterol and peridinosterol and 4 methylgorgostanol not previously reported to occur in lacustrine sediments 2 c30 and c32 alkane 1 15 diols not previously reported to occur in contemporary lacustrine deposits and 3 products of early diagenesis many similarities were observed with the more widely studied marine dinoflagellates and marine sediments with dinoflagellate input

geology and lithogeochemistry of hydrothermal mudstones from the upper block near the duck pond volcanogenic massive sulfide vms deposit newfoundland canada evidence for low temperature venting into oxygenated mid cambrian seawater pyrite and pyrrhotite rich mudstones are spatially associated with cambrian 512 509 ma volcanogenic massive sulfide vms deposits throughout the tally pond group central newfoundland canada at the duck pond mine sulfide rich mudstones are hosted within a weakly mineralized upper block that structurally overlies the deposit but is older 513 versus 509 ma the mudstones are laminated 10 30 cm thick and pyrite and pyrrhotite rich and occur along pillow lava selvages or in between pillow lavas rhyolite flows and volcaniclastic rocks the mudstones are laterally extensive and proximal to the mudstone host rocks are hydrothermally altered to epidote quartz chlorite basalt host and sericite quartz rhyolite host lithogeochemical data for the sulfide rich mudstones reflect the varying contributions of elements from sedimentary detritus hydrothermal discharge and hydrogenous scavenging from middle cambrian seawater the mudstones have minor detrital element abundances and significant hydrothermal element enrichments i e elevated fe2o3 s pb zn cu and ba concentrations high fe al ratios the hydrothermal mudstones are also enriched in oxyanions i e p2o5 u v cr ni co and hg interpreted to have been enriched via oxidative scavenging from seawater by fe oxide oxyhydroxide particles the mudstones also have ree y signatures similar to modern oxygenated seawater with high y ho and negative ce anomalies ce ce 0 40 0 86 average 0 58 which correlate with adsorbed oxyanion concentrations the low eu eu 1 02 1 86 average 1 22 in the mudstones suggest that they were deposited from low temperature 250 c fe rich hydrothermal fluids that likely formed a buoyant plume into an oxygenated water column the ree y oxyanion signatures suggest that the particles within the hydrothermal plume had sufficient residence time to scavenge oxyanions from seawater and inherit a middle cambrian seawater signature the predominant seawater ree y oxyanion signature in the duck pond upper block sulfide rich mudstones suggests that they are distal hydrothermal sedimentary rocks that could have formed up to 10 km from their original vent sources correspondingly to utilize hydrothermal mudstones as vectors to mineralization in the tally pond belt and similar belts globally it is critical to identify vent proximal samples that have hydrothermal signatures i e high fe al base metals ba s with subdued seawater and adsorption signatures i e chondritic y ho low p2o5 ni u co cr v and hg indicating minimal residence time in the water column and deposition proximal to the vent

structure of hydrothermal plumes at the logatchev vent field 14 45 n mid atlantic ridge evidence from geochemical and geophysical data abstract in the seventh cruise of r v professor logatchev anomalies of natural electric field ef eh and ps were discovered using a towed instrument package rift at 14 45 n on the mar logatchev hydrothermal field the anomalous zone az is situated close 10 35 m to two low temperature venting areas of degrading sulphides and a black smoker irina microsmoke forming a distinct buoyant plume over or close to the main area of high temperature venting situated to the south east from the az no ef or eh anomalies were observed according to the results of mir dives the highly mineralised solutions from smoking craters at the main mound mostly form non buoyant plumes reverse plumes the buoyant plume structure shows the differentiation of the electrical and eh fields within the plume maxima of the ef eh and e h 2 s anomalies were revealed in the lower part 15 m of the plume the negative redox potential plume coupled with a sulphide anomaly is more localized in comparison with the ef this observation indicates a distinct change in the composition of buoyant plume water which may be due to the formation and fallout of early formed fe sulphide particles soon after venting

near field variability of the tag non buoyant plume 26 n mid atlantic ridge we report the initial results of a nine hour monitoring of the non buoyant plume above the tag hydrothermal vent field at 26 n on the mid atlantic ridge a ctd conductivity temperature and pressure and nephelometer particle sensor were continuously raised and lowered through the non buoyant plume for nine hours at a site situated 100 m south of the hydrothermal mound this method permits a detailed record of the variability of the non buoyant plume close to the vent field to be made our results show that the particulate maximum of the non buoyant plume appears 150 m lower in the water column at the start and end of the record compared with profiles recorded towards the middle of the record a corresponding structure also appears in the temperature and salinity records with the result that the plume particle maximum resides on the 27 8035 isopycnal throughout modelling of the rise height of an axisymmetric plume can only account for 95 m of the variability in the final level of neutral buoyancy plume bending due to currents is a likely explanation for the recorded temporal variability

a model for the formation of hydrothermal manganese crusts from the pitcairn island hotspot the distribution of elements in the hydrothermal mn crusts from the pitcairn island hotspot can be explained by considering processes occurring within the hotspot volcano these include the role of boiling of the ascending hydrothermal fluid which leads to sulphide deposition and seawater intrusion into the upper part of the volcano this in turn results in the development of an oxycline above which feooh can precipitate the high mn fe ratios and low contents of ni and zn in the massive crusts are thought to reflect the rapid incorporation of the ore elements into sulphide minerals such that only a small proportion of these elements are available for incorporation in the crusts the small positive eu anomaly in the crusts on a nasc normalized basis reflects the lower temperatures in the hydrothermal fluids at the pitcairn island hotspot 25dgc compared to those at mid ocean ridges c 350 c the oxyanions cro42 vo43 aso43 and po43 the rare earth elements ree and th are scavenged by feooh particles within the upper part of the seamount and can therefore be incorporated into the crusts associated with the fe rich phase molybdenum and uranium are scavenged in only negligible amounts by the feooh molybdenum is therefore most probably scavenged from seawater by manganese oxides during the actual formation of the mn crusts the alkali metals li rb and cs remain in solution in the hydrothermal fluids and are selectively incorporated into the buserite structure of the mn crusts in the absence of strong competition from the divalent transition metal ions the compositional characteristics of the hydrothermal mn crusts can therefore be explained in terms of a simple model of element scavenging and uptake occurring mainly within the seamount a laser ablation icpms profile in one of the hydrothermal crusts ds 63 4 revealed varying ree concentrations and patterns in the different layers of the crust these data indicate that the hydrothermal component was variable during the formation of the crust it was calculated to be almost 100 in the upper 18 0 mm of this crust apart from the samples at 6 0 8 0 mm and about 80 in the lower layers

palaeoproterozoic gold mineralization of the eteke archaean greenstone belt gabon its relation to the eburnean orogeny abstract the gold deposits of the eteke district gabon are hosted by metasediments and intermediate to ultramafic metavolcanites of the eteke archaean unit 2940 6 ma a greenstone belt within the folded and metamorphosed ogooue domain that was formed by the successive transport and stacking of nappes during palaeoproterozoic eburnean d1 d2 collisional tectonism this lithotectonic pile in the east of the eteke district rests on the chaillu archaean basement and palaeoproterozoic francevillian foreland basin deposits the gold is contained in quartz carbonate stockworks hydrothermalite lenses and mineralized quartz veins that indicate mineralization during a single d2 hydrothermal event extending from an early d2 stage stockwork to a late d2 stage veins the mineralization began during the early d2 stage following a major phase of tangential tectonism d1 when the eteke unit still in a shallow position brittle deformation in the outer foreland zone of the orogenic belt would have received the repercussions of crustal thickening taking place in the inner zone of the belt ogooue series s s these repercussions are reflected by the development of stockworks and hydrothermalite lenses related to reverse faults that favoured the emplacement of tonalitic dykes and sills and channelled the first hydrothermal fluids at the peak d2 stage the eteke unit which by this time formed part of the outer zone of the orogenic belt was caught up in nappe tectonism that imparted a major s2 foliation with microfolding and foliation of the early stockworks the early d2 reverse faults evolved to synmetamorphic thrusts epi to mesozonal conditions along which syntectonic granites and rare mineralized veinlets were emplaced reflecting a dormancy of the hydrothermal event the nappe tectonism ended with transport of the peak d2 lithotectonic pile onto the chaillu archaean terrain and nearby francevillian foreland basins accompanied by a late d2 emplacement of gold bearing quartz veins cutting the s2 foliation and marking the end of the mineralizing hydrothermal event

geochronology of sedimentary and metasedimentary precambrian rocks of the west african craton abstract this contribution summarizes current knowledge of the geochronology of the upper proterozoic sedimentary rocks covering the west african craton this was done by using direct dating methods correlations between the northern edge of the tindouf basin and the northern and southern part of the taoudeni basin as well as the volta basin are proposed an important depositional lack over almost the entire craton is confirmed between 1 7 ga which represents the time of the latest eburnean activities or cooling effects and about 1 0 ga which is the start of the general upper proterozoic sedimentation this lack is remarkable as the other cratons from gondwana continent are covered by middle proterozoic sediments the upper proterozoic sedimentation on the west african craton may be divided into a basal mainly sandstone period ca 1000 900 ma a middle mainly limestone period ca 900 750 ma and an upper mainly siltstone period ca 750 650 ma tectonic volcanic and thermal activities in connection with the pan african orogeny are recorded only around the craton they induced either sedimentation lacks in morocco or sedimentation excesses in hoggar unsolved problems such as the precise stratigraphic position of the uppermost proterozoic tillitic episode and the correlation within the moroccan anti atlas are also raised

sedimentation from gravity currents generated by turbulent plumes sedimentation from radially spreading gravity currents generated at the top of ascending sediment laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current for radial spreading this model predicts a gaussian distribution of sediment accumulation away from the source with an exponential constant b which depends on flow rate q and particle settling velocity v b nv q in the experiments described sedimentation occurs from gravity currents generated by ascent of buoyant particle laden plumes of fresh water in a tank of salty water the sediment accumulation shows close agreement with the theoretical model and the gaussian decay constant b can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line in s s0 br2 where r is the radial distance s is the sediment mass flux per unit area and s0 is the value of s at r 0 data from the dispersal of volcanic ejecta from a large c 24 km high plinian eruption column in the azores also show good agreement with the theory confirming that it is general and independent of scale and the nature of the fluid the experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity sedimentation of coarse grain sizes between the source and the corner occurs from the inclined plume margins and does not behave as predicted by the theoretical model

late quaternary mediterranean sapropels i on the origin of organic matter in sapropel s7 abstract at least twelve organic rich layers so called sapropels s1 s12 occur in late quaternary sediments from the eastern mediterranean organic matter in sapropel s7 organic carbon up to 11 is characterized in detail by gas chromatography gc and gas chromatography mass spectrometry gcms lipids were thus separated to obtain information about their mode of occurrence this procedure separates the lipids into three classes 1 free lipids extracted by soxhlet extraction 2 residual bound lipids extracted after saponification of the soxhlet extracted residue and 3 residual bound lipids extracted after acid treatment of the saponified residue in addition an extract was obtained after saponification of the free lipids the organic matter investigated is of a mixed marine e g dinoflagellates coccolithophorids terrigenous e g cuticles and bacterial origin with the former predominating the formation of sapropel s7 is probably caused by a combination of stagnation and high productivity both induced by a large input of fresh riverine water carrying nutrients and terrigenous organic material

geodynamic evolution of the bangweulu block northern zambia abstract the bangweulu block is a cratonic unit underlying most of northern zambia and adjacent parts of tanzania and zaire its geodynamic evolution is shown to be entirely proterozoic except for cainozoic rifting and related sedimentation the block has a crystalline basement of schists acid metavolcanics and granitoids which formed during a deformational and magmatic event of eburnian age between 2000 and 1800 ma ago the metavolcanics have affinities with high k calc alkaline suites from modern subduction controllled tectonic regimes suggesting an eburnian plate margin to the northwest of the block the polyphase sedimentary cover of the bangweulu block comprises the mporokoso group a 5000 m thick sequence of fluvial aeolian and lacustrine sediments deposited in the intracratonic mporokoso basin in response to the final phase of the eburnian tectogenic cycle the kasama formation a sequence of fluvial sediments which is 100 300 m thick on the bangweulu block but increases in thickness to nearly 400 m in the downfaulted pericratonic irumide basin formed in the initial phase of the kibaran tectogenic cycle the luitikila beds a 3000 m thick unit of post orogenic molasse of the kibaran irumide belt which encroaches with reduced thickness on the bangweulu block the luapula beds deposited on the margins of the bangweulu block and representing parts of the kundelungu sedimentary sequence accumulated in the pan african pericratonic basin faults and shear zones bordering the bangweulu block developed in sequence during the eburnian kibaran and pan african orogenies the older ones are invariably reactivated during the younger tectogenetic events resulting in substantial lateral and rotational movement of the bangweulu block with respect to adjacent structural terranes

role of metalliferous mudstones and detrital shales in the localization genesis and paleoenvironment of volcanogenic massive sulphide deposits of the tally pond volcanic belt central newfoundland canada the cambrian tally pond volcanic belt in central newfoundland contains numerous volcanogenic massive sulphide vms deposits and prospects associated with exhalative metalliferous mudstones deposits in the belt are bimodal felsic vms deposits that are both base metal bearing e g duck pond boundary and base metal and precious metal bearing lemarchant at the lemarchant deposit metalliferous mudstones are stratigraphically and genetically associated with mineralization in the remainder of the tally pond belt detrital shales occur predominantly in the northeastern part of the belt mostly as unrelated mid ordovician structural blocks in the upper sections of the cambrian volcanic stratigraphy but locally also are intercalated with metalliferous mudstones their relationships to massive sulphides are less obvious with many spatially but not necessarily genetically related to mineralization upper cambrian to lower ordovician black shales from bell island which represent pelagic sedimentation not associated with hydrothermal activity and volcanism are compared with the tally pond belt mudstones and shales exhalative mudstones like those at lemarchant have elevated fe al and base metal values and have shale normalized negative ce and positive eu anomalies indicative of deposition from high temperature 250 c hydrothermal fluids within an oxygenated water column mudstones and shales sampled from other tally pond prospects have more variable signatures ranging from hydrothermal to nonhydrothermal black shales no positive eu anomalies flat rare earth element patterns low fe al and base metal contents to those that have mixed signatures accordingly mudstones from areas with a lemarchant like hydrothermal and vent proximal character are more attractive exploration targets than mudstones and shales with predominantly detrital signatures

real time observation of dispersed hydrothermal plumes using nephelometry examples from the mid atlantic ridge abstract as part of the 1984 1985 noaa vents program on the mid atlantic ridge nephelometry was used to provide real time detection and tracking of dispersed hydrothermal plumes at all nine 1984 study sites hydrothermal activity was detected by in situ real time nephelometer measurements and later confirmed by dissolved mn and particulate fe measurements these same techniques were employed in a site specific survey of the trans atlantic geotraverse tag area in 1985 where large water column anomalies in turbidity and in dissolved mn helped lead to the discovery of high temperature black smokers the optical response of the nephelometer was to hydrothermally derived particulate matter thus strong correlations existed between the nephelometer readings and total suspended matter r 0 98 n 34 and particulate fe r 0 88 n 32 in addition digital nephelometer data correlated well with dissolved mn r 0 88 n 78 throughout a large concentration range 0 2 31 0 nmol kg these data provide good evidence for the utility of in situ nephelometer measurements for locating and surveying plumes from hydrothermal vents it also appears possible within limits to predict concentrations of in situ total suspended matter of particulate fe and of dissolved mn

the origin and fate of 4 methyl steroid hydrocarbons i diagenesis of 4 methyl sterenes abstract treatment of 4 methylcholest 4 ene under mild acid conditions at low temperatures gives chemical evidence for certain features seen in the distributions of sedimentary 4 methyl steroid hydrocarbons and further indicates that many low temperature diagenetic reactions of steroids are explicable in terms of acid catalysed rearrangements specifically the results provide i indirect evidence that the 4 ene skeleton is a key intermediate in the dehydration of 4 methyl stanols in sediments ii an explanation for the distribution of 4 methyl sterenes and a nor sterenes in the lacustrine messel shale eocene iii an explanation for the presence of 4 methyl steranes in relatively immature sedimentary rocks despite the precursor stanols having the 4 methyl configuration with increasing maturity in the paris basin shales lower toarcian the less stable 4 methyl steranes decrease gradually in abundance relative to their 4 methyl counterparts at a rate fairly similar to the change in pristane stereochemistry