environmental change during the early cretaceous in the purbeck type durlston bay section dorset southern england a biomarker approach abstract the purbeck type section durlston bay dorset uk exhibits littoral lagoonal to lacustrine facies it shows a gradual climatic environmental change from semi arid conditions associated with evaporites at the jurassic cretaceous transition to a more humid climate at the end of the berriasian though generally organic poor total organic carbon toc the distribution of alkanes hopanes and steranes indicates that the origin of the om is mainly algal bacterial and only changed moderately with the climatic evolution the saline and anoxic bottom water conditions indicated for most of the samples point to the recurrence of salinity stratification within the basin a high contribution of odd numbered c 23 c 31 n alkanes from algaenan containing freshwater algae in particular botryococcus is noted for all the samples despite the low abundance of these organisms in the corresponding palynofacies this prominence probably results from the high resistance to degradation and the selective preservation of botryococcus related lipids the subtle balance of aridity and freshwater inputs favoured both an abundance of botryococcus and the development of anoxia leading to the enhanced preservation of om during the intermediate climatic episode while the conditions were less favourable during the semi arid and more humid episodes

branched aliphatic alkanes with quaternary substituted carbon atoms in modern and ancient geologic samples a pseudohomologous series of branched aliphatic alkanes with a quaternary substituted carbon atom baqcs specifically 2 2 dimethylalkanes and 3 3 and 5 5 diethylalkanes were identified in warm 65 c deep sea hydrothermal waters and late cretaceous black shales 5 5 diethylalkanes were also observed in modern and holocene marine shelf sediments and in shales spanning the last 800 million years of the geological record the carbon number distribution of baqcs indicates a biological origin these compounds were observed but not identified in previous studies of 2 0 billion to 2 2 billion year old metasediments and were commonly misidentified in other sediment samples indicating that baqcs are widespread in the geological record the source organisms of baqcs are unknown but their paleobiogeographic distribution suggests that they have an affinity for sulfides and might be nonphotosynthetic sulfide oxidizers

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

conjunctive use of spectral gamma ray logs and clay mineralogy in defining late jurassic early cretaceous palaeoclimate change dorset u k abstract detrital clay mineralogy is controlled by weathered source rock climate transport and deposition that in turn influence the spectral gamma ray sgr response of resultant sediments whilst a palaeoclimate signal in clay mineralogy has been established in some ancient successions the sgr response remains contentious largely because the data sets have yet to be collected at the same or appropriate vertical scales to allow comparison in addition the influence of organic matter on sgr is not always considered here we present clay mineralogical total organic carbon toc and sgr analyses from the late jurassic and early cretaceous of the wessex basin a period of previously documented palaeoclimate change the aim of this paper is to estimate the sensitivity of sgr as palaeoclimatic tool sgr and clay mineral data having been collected at the same sample points making this one of the most rigorous comparison of clay mineral and sgr to date overall the correlation between high thorium potassium or thorium uranium and kaolinite associated with a well established palaeoclimate change shows that elevated thorium may be used as a proxy for humid palaeoweathering as suggested by few previous studies

sediment resuspension mechanisms in old tampa bay florida the mechanisms that resuspend bottom sediments in old tampa bay a shallow microtidal subtropical estuary in west central florida were determined by analysing data collected during several periods from 1988 to 1990 hydrodynamic and suspended solids concentration data were collected at a relatively deep 4 m site where a permanent platform was built and at a relatively shallow 1 5 m site where a submersible instrument package was deployed bottom sediments were non cohesive silts and fine sands the primary sediment resuspension mechanism at both sites was wind waves which were generated by strong and sustained winds associated with winter storms and tropical storms at the platform waves were depth transitional and estimated bottom shear stresses were most sensitive to wave period and water depth concentrations of suspended solids at this site corresponded well with wave motion and non linear wave current interaction was small at the shallow water site concentrations of suspended solids were elevated during periods of strong north easterly winds and large bottom orbital velocities at both sites wind direction was an important factor in determining the occurrence and magnitude of sediment resuspension resuspended sediments settled within several hours as storm intensity diminished winds and waves generated by thunderstorms were more transient than those generated by winter storms and tropical storms based on the data collected during this study thunderstorms are less likely to resuspend bottom sediment than winter storms and tropical storms maximum tidal currents at the study sites are usually less than 15 cm s 1and did not increase observed concentrations of suspended solids

geochemical characterization of mangrove sediments of the zuari estuarine system west coast of india abstract the grain size clay mineralogy and geochemistry were studied in the sediment cores collected from the mangrove environments of the zuari estuary to understand sources and factors affecting accumulation mobility bioavailability and toxicity of metals finer sediments organic matter and metals were higher in the middle estuary and canal sediments while coarser sediments with fewer metal concentrations were seen in the lower estuary kaolinite smectite illite and traces of chlorite constituted the clay mineral assemblage and had a minor influence on metal distributions in the study area the hydrodynamic conditions changed from lower estuary towards the upstream regions owing to mixing of riverine and sea water that led to finer sediment deposition in the middle estuary the variations in metal abundance were attributed to a difference in hydrodynamic conditions regulated by the tide freshwater flow and geomorphology of the zuari estuary the results revealed that the estuary received material from natural weathering of rocks as well as from anthropogenic sources such as mining and industrial domestic discharges enrichment factor and geo accumulation index showed that fe mn and cr were enriched in the mangrove sediments whereas fractionation of metals revealed that concentrations of bioavailable mn pose a considerable risk to biota increased accumulation of fe and mn in the upper middle estuary and canal sediments trap trace metals that may considerably affect sediment quality and dredging of these sediments can cause re suspension and mobilize metals from loosely bound sedimentary forms to the water column

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

evidence for gloeocapsomorpha prisca in late devonian source rocks from southern alberta canada abstract organic rich intervals within the late devonian camrose member of southern alberta canada show optical and chemical characteristics very similar to those of ordovician samples whose organic matter is dominated by gloeocapsomorpha prisca derived alginite this includes saturate fraction gas chromatograms dominated by n alkanes with a pronounced odd carbon number up to c19 and much lower abundances of c19 n alkanes acyclic isoprenoids and biomarkers microscopy reveals these samples contain g prisca like alginite with a morphology that previously has been suggested for the growth of this organism under higher salinity conditions which is in agreement with the depositional environment of the camrose member one unusual feature of the g prisca rich samples is the very high concentration of c21 and c23n alkylnaphthalenes and n alkylbenzenes in the aromatic fractions of their extracts the origin of these compounds is not known but their occurrence could be related to the high concentrations of c21 and c23n alkyl phenols that have been noted in pyrolysates of estonian kukersites by previous workers other camrose member extracts show very different distributions that are more typical for carbonates deposited under anoxic higher salinity conditions many oils that occur in nisku formation reservoirs in the enchant hays field in southern alberta show saturate fraction gas chromatograms and other evidence that indicate an important contribution to these oils from the g prisca rich intervals these are thought to have a contribution from camrose member source rocks hence caution should be used in only attributing g prisca geochemical characteristics to cambro ordovician oils as on rare occasions they can also occur in younger source rocks and their derived oils

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

composition and origin of hydrothermal petroleum and associated lipids in the sulfide deposits of the rainbow field mid atlantic ridge at 36 n the lipid components in hydrothermal sulfide deposits from the rainbow vent field mid atlantic ridge at 36 n were studied by gas chromatography mass spectrometry the rainbow vent field is one of two known active hydrothermal systems related to abyssal circulation where high temperature fluids are formed during serpentinization of ultrabasic crustal rocks the major amount of the extractable organic matter from the sulfides consists of normal and branched alkanes ucm pahs terpenoids and fatty acids the branched alkanes are comprised of unique gem diethylalkane series possibly from sulfide oxidizing bacteria and biphytanes from archaea the characteristic lipid and biomarker compounds found in the hydrothermal samples support a predominantly biological origin of the bitumens from the thermal transformation of the biomass of microorganisms bacteria and archea and minor macrofauna of this vent field a search for molecular evidence for abiogenic thermocatalytic synthesis of organic compounds was negative however methane in the hydrothermal fluids and possibly a minor amount of the alkanes in the sulfides may be of an abiogenic origin in the rainbow vent field

origin and early diagenesis of arborinone isoarborinol in sediments of a highly productive freshwater lake abstract this paper reports on the presence of arborinone isoarborinol and some of their early diagenetic degradation products in a tropial lake isoarborinol and arborinne two 3 oxygenated pentacyclic triterpenes whose origin was initially ascribed to higher plants were found to be highly abundant in lake valencia venezuela surface sediments the primarily autochthonous origin of the organic matter in sediments from this hypereutrophic lake seems to indicate a microbial source for these compounds as previously suggested by other authors the structures of two novel tetracyclic aliphatic alkene degradation products of arborinone isoarborinol are also proposed and sediment core profiles for both parent and degradation products are discussed

distribution and origin of ethyl branched alkanes in a cenomanian transgressive shale of the western interior seaway usa abstract monoethylalkanes mea c16 c22 with an even number of carbon atoms are relatively abundant in the saturated hydrocarbon fraction of the basal graneros shale cenomanian western interior seaway usa on the basis of isomer distribution and isotopic composition it is proposed that mea derive from eubacterial organisms possibly cyanobacteria living in fresh to brackish water environments

heavy metal accumulation in a flow restricted tropical estuary abstract levels of heavy metals cd co cr cu fe mn ni pb and zn organic carbon content and textural characteristics in the surficial sediments of cochin estuary sw coast of india and adjacent coast are presented anthropogenic inputs from industries have given rise to a gradient in concentration of metals in estuarine sediments metal accumulation initiated by precipitation of iron complexes is probably responsible for the enrichment of cd cu pb and zn in the estuary whereas the coastal sediments did not accumulate them to the same degree statistical analyses of the coastal and estuarine geochemical data indicated a different association of elements in the two provinces the present enrichment factors for zn 25 and cd 10 place the region among the impacted estuaries in the world accumulation of metals in this estuarine system is aggravated by the weak flushing these pressures are deemed inevitable with growth initiated in the harbour and associated with mega city development in the offing

geochemical and sedimentological characteristics of surface sediments from ashtamudi estuary southern india implications for provenance and modern sedimentary dynamics geochemical and end member mixing analyses emma of the grain size distribution were conducted on recent surface sediments from ashtamudi estuary southern india to investigate the hydrodynamic factors that influence the depositional processes in the region the complex interplay between the natural fluvial and marine and anthropogenic influences on the ashtamudi estuary have been delineated based on the inter relationships between geochemical elements and the end members em derived from the grain size parameters si al and fe are the major elements in the sediment of ashtamudi estuary constituting 26 47 9 21 and 8 14 respectively four grain size end members were identified with dominant modes of 7 em1 6 em2 4 5 em3 and 3 3 em4 corresponding to fine silt medium silt coarse silt and fine sand respectively the high contributions of al fe cr and ni combined with the em1 and em2 in the upstream of the estuary indicated that the fine grained sediments were derived from the fluvial inputs into the basin the marine tidal influenced lower estuary is characterised by high concentrations of si ti ca sr and coarse fraction end member em3 the elemental concentration of cu zn and co along with em4 shows higher values along the shore region suggesting human interventions in the basin this integrated geochemical analysis and emma provide advancement in the knowledge of the transportation mechanisms and regional sediment dynamics from the estuarine system in southern india

occurrence and origin of mono di and trimethylalkanes in modern and holocene cyanobacterial mats from abu dhabi united arab emirates n alkanes highly branched isoprenoids monomethylalkanes mmas dimethylalkanes dmas and trimethylalkanes tmas are the most abundant components in the hydrocarbon fractions of extracts of four modern and two holocene cyanobacterial mats t1500 and 5110 170 y bp collected in abu dhabi united arab emirates the homologous families of mmas dmas and tmas were identified by comparison of mass spectral and relative retention time data with published data dmas were also identified by synthesis of authentic standards 3 9 dimethyltricosane 5 9 dimethyltricosane and 11 15 dimethylheptacosane mmas dmas and tmas of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures mmas and dmas in the c16 c22 range are characterized by methyl substituents mainly located at c 6 or 6 and c 7 or 7 and are derived from cyanobacteria they are relatively abundant components in the modern cyanobacterial mats but with increasing age of the mats they become much less abundant on the contrary mmas dmas and tmas in the c24 c45 range are exclusively found in the holocene cyanobacterial mats their longest chains mainly contain an odd number of carbon atoms and they always carry the methyl substituents at odd numbered carbon atoms the similarity in composition of this very specific group of branched alkanes with that encountered in insect epicuticular waxes suggests that these sedimentary hydrocarbons originate from insects which probably grazed on the cyanobacterial mats

novel triterpene derived hydrocarbons of the arborane fernane series in sediments part ii two ring b aromatized triterpenes related to the arborane fernane series were isolated from an italian triassic black shale 4 and 7 and a french jurassic laminated bituminous limestone 7 they were identified by ms and nmr spectroscopy these compounds 25 norferna 5 7 9 triene 7 c29 and 22 25 29 30 tetranor 18 ferna 5 7 9 triene 4 c26 in the case of the italian sediment belong to a family which extends to at least c32 as shown by gc ms the presence of higher homologs and ring a methylated structures in the italian triassic shale is analogous with the hopane series which together with carbon isotopic data obtained by gas chromatography isotope ratio mass spectrometry gc irms strongly support a bacterial origin for most of these compounds this suggests that 4 and 7 may result from a yet unrecognized rearrangement of sedimentary hopenes which have subsequently undergone ring b aromatization as an alternative the c29 compound 7 could arise from c30 precursors not necessarily of bacterial origin fernenes this hypothesis would apply particularly to the case of the jurassic limestone which does not show any trace of 4 nor of compounds with an extended side chain or a methyl group on ring a related structures also bearing an isopropyl group could derive by microbiological transformations from the corresponding fernen 3 ols or isoarborinol as can be deduced from the occurrence of ring a degraded monoaromatic hydrocarbons 3 and 5

product distributions from chemical degradation of kerogens from a marl from a miocene evaporitic sequence vena del gesso n italy abstract lithium in ethylamine li etnh 2 treatment has been carried out on sulphur rich kerogens from 10 sub samples of the cycle iv marl from the vena del gesso evaporitic sediments upper miocene italy the treatment releases after chromatography of the resulting solvent extracts up to 30 of the kerogen total organic carbon the biomarkers released are dominated by components of algal origin the concentrations of the aliphatic hydrocarbons obtained are of the same order of magnitude as the bound alkanes released upon desulphurization of polar fractions from organic extracts and show similar distributions the presence of diaromatic isoprenoids derived from chlorobiaceae indicates that photic zone anoxia occurred during the deposition of the cycle iv marl such conditions being more prevalent at the middle of the section polar material makes up by far most of the organic material released and contains in particular alcohols and amides which may result from the cleavage of ester bonds present in these immature kerogens the amides being secondary products formed by reaction with the etnh2 in the case of the alcohols however these components could also arise from cleavage of sulphide bound moieties

application of biological markers in the recognition of palaeohypersaline environments the hydrocarbon fractions of a marl sample from a messinian basin in the northern appenines and four oils rozel point oil utah and three seep oils from sicily were studied

higher plant biomarkers reflect palaeovegetation changes during jurassic times abstract the relative abundances of three higher plant derived biomarkers retene cadalene and ip ihmn have been measured in marine sedimentary rocks from the northwest margin of australia it is thought that each biomarker represents input from a different plant type the distributions of these three compounds form a fingerprint representing higher plant input hpf variations in hpf in oxfordian sediments were nearly identical in all three locations with retene becoming very abundant relative to the other two compounds with decreasing age of the sediment this finding strongly suggests that the composition of terrestrial input during deposition largely determines hpf and that the possible effects of diagenesis and catagenesis on the distribution of the three biomarkers are relatively unimportant the marked increase in the abundance of retene relative to that of cadalene during the oxfordian is interpreted to reflect an increase in the contribution of plants that produced precursors for retene i e conifers brought about by a significant change in climate this was exemplified by measuring the distributions of retene and cadalene expressed in the higher plant parameter hpp for a suite of sediments from the carnarvon basin western australia covering the complete jurassic period the hpp profile displays three major cycles each covering a period of at least 10 million years this profile not only compared well with published palaeoclimate data but also showed a remarkable similarity with second order cycles in the global sea level curve thus strongly supporting the proposal that variations in hpf and hpp are indications of changes in palaeoclimate the relation with global sea level further suggests that global factors e g the atmospheric carbon dioxide concentration may play a major role in determining the observed variations in the distributions of these higher plant derived biomarkers

multihazard simulation and cyberinfrastructure the integration of multihazard simulations and remotely sensed observations is providing enormous benefits to earthquake and tsunami research integrating data and models through cyberinfrastructure is enabling understanding of earthquake and tsunami generation mechanisms in the asia pacific region and improving assessment strategies for mitigating risk earthquake rupture processes occur on all scales from microns to global and from sub seconds to millions of years earthquakes cause damage but also generate tsunamis which create additional damage remotely sensed observations coupled with geologic field measurements and simulations contribute to our understanding of earthquake processes which is necessary for mitigating loss of life and property from these damaging events remotely sensed observations play a unique role in the mitigation of natural hazards measurements of surface motions can be used to infer strain accumulation and transfer within interacting fault systems as well as the mechanisms of earthquakes which is an important input to tsunami generation models and disaster response remotely sensed geodetic imaging data include interferometric synthetic aperture radar insar the global positioning system gps and other techniques such as lidar or optical imaging geodetic imaging can be used to understand aspects of earthquakes including the tectonic plate motions that drive earthquakes gps regional and local crustal deformation associated with faults or other sources gps insar lidar optical and detailed motions associated with earthquakes at a resolution of 1 hz gps the united states hosted the 8th international symposium of the apec cooperation of earthquake simulation aces october 22 26 2012 in maui hawaii the workshop focused on assimilation of remotely sensed observations to advance multihazards simulation the apec cooperation for earthquake simulation aces is a multilateral grand challenge science research cooperation of apec the asia pacific economic cooperation aces aims to develop realistic simulation models for the complete earthquake generation process and to assimilate observations into such models this capability provides a powerful virtual laboratory to probe earthquake behavior and the earthquake cycle hence it offers a new opportunity to gain understanding of the earthquake nucleation process precursory phenomena and space time seismicity patterns needed for breakthrough advances in earthquake forecasting and hazard quantification the project represents a grand scientific challenge because of the complexity of phenomena and range of scales from microscopic to global involved in the earthquake generation process aces symposia provide unique opportunities for the apec economies to work together on the topics important for the hazard mitigation of natural disasters this volume follows earlier pageoph topical volumes based on aces symposia see references below this topical volume reflects the 4 day workshop which consisted of plenary talks technical sessions working sessions and poster sessions it brought together an interdisciplinary set of researchers it brings together remote sensing experts modelers and computer scientists the symposium and this

mid chain branched mono and dimethyl alkanes in hot spring cyanobacterial mats a direct biogenic source for branched alkanes in ancient sediments hot spring cyanobacterial mats from yellowstone national park u s a and the orakei korako thermal area new zealand have been found to contain suites of monomethyl alkanes compounds frequently reported in cultured and natural populations of cyanobacteria the complete range of possible structural isomers for mid chain branching for c17 c18 and c16 c18 alkanes occurs in the two mats respectively several c19 dimethyl and c20 multibranched alkanes were also observed as major hydrocarbons of the orakei korako mat the recognition of series of mid chain branched alkanes in modern cyanobacterial mats suggests that their occurrence in ancient sediments may reflect direct biogenic contributions rather than formation by diagenetic processes

chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae origin of ultralaminae abstract new structures termed ultralaminae were recently observed by transmission electron microscopy usually in high amounts in a number of kerogens from oil shales and source rocks morphological similarities were noted between ultralaminae and the thin ca 15 nm resistant outer walls composed of non hydrolyzable macromolecules algaenans commonly occurring in extant chlorophyceae especially in the cosmopolitan genus scenedesmus identification of the pyrolysis products of s quadricauda algaenan showed i a highly aliphatic structure based on a macromolecular network of long up to c32 polymethylenic chains probably cross linked by ether bridges and ii a close correlation based on the formation of n alkylnitriles between this algaenan and two ultralaminar kerogens the rundle oil shale mainly composed of ultralamina accumulations and the green river shale ultralaminae dispersed within an amorphous matrix these fossil ultralaminae therefore likely originated from the selective preservation of the thin algaenan containing outer walls of scenedesmus and or of other chlorophyceae containing outer walls of a similar morphology and composition relative distributions of n alkylnitriles and of n alkanoic acids in the pyrolysates of s quadricauda algaenan and of the rundle oil shale indicated that nitriles are not derived from secondary reactions of carboxylic acids but originate from preexisting nitrogen functions likely amides previous evidence of kerogen formation via selective preservation of algaenans was restricted to rather uncommon kerogens the present results added to ultralamina common occurrence and abundance point to a wide involvement and to a large contribution of the selective preservation of algaenan containing thin outer walls of chlorophyceae in the formation of kerogens in a number of lacustrine source rocks and oil shales all the available information suggest that the three way correlation selective preservation of algaenans from thin resistant chlorophycean outer walls formation of fossil ultralaminae presence of n alkylnitriles with a typical distribution in the pyrolysates of ultralaminar kerogens observed in this study might be of a general character

grain size geochemistry magnetic susceptibility proxies in identifying sources and factors controlling distribution of metals in a tropical estuary india the aim of this study is to understand the various sources and factors controlling the abundance and distribution of the metals through the analyses of selected metals iron manganese chromium copper zinc and cobalt sand silt clay organic carbon and magnetic susceptibility of the surface sediments of zuari estuary in three different seasons total suspended matter tsm concentration and salinity of near surface and bottom waters were also measured the study undertaken for three seasons based on 18 selected stations all along the estuary indicated that the concentration of most of the metals were comparatively higher during pre monsoon than in monsoon and post monsoon seasons further along the length of the estuary high concentrations were observed at lower middle and lower estuary and also at few stations at the upstream end salinity distribution of tsm size of the sediment organic matter geomorphological setup fresh water input from land and release of industrial waste within the estuary played a role in distribution and concentration of metals magnetic measurements indicated the dominance of haematite like minerals and helped in understanding the source and depositional processes

source and depositional controls on heavy metal distribution in marine sediments of the gulf of iskenderun eastern mediterranean the concentrations of major al mg fe and mn and trace zn cr ni co cu and pb elements as well as carbonate organic carbon and grain size distributions have been determined in surface sediments collected at 73 stations from the gulf of iskenderun the shelf sediments of the gulf of iskenderun are largely muds but sediments with higher contents of sand and gravel also occur due to the presence of high biogenic caco3 contents up to 80 in the samples the abundance and regional distributions of elements in the surface sediments of the gulf are largely controlled by differences in grain size and in the level of dilution by skeletal shell remains and by the variation in composition of source materials mg ni cr co and to a lesser extent zn and pb are significantly enriched relative to their average crustal abundances these metal enrichments are found especially in the eastern part along the sw ne trending areas of the gulf and are mostly consistent with the chemical and mineralogical compositions of the major basic and ultrabasic source rocks present on the adjacent coast and hinterland there is strong evidence that suggest that aluminosilicates fe and mn compounds possibly oxyhydroxides and organic matter act as the most effective carriers for transfer of available metals zn cr ni co and cu to the sediments

molecular indicators of palaeosalinity and depositional environment of small scale basins within carbonate platforms the late triassic hauptdolomite wiestalstausee section near hallein northern calcareous alps austria abstract microfacies and biomarker analyses are used to determine the environmental conditions during deposition of marly dolomicrites from the hauptdolomite wiestalstausee section near hallein salzburg austria part of the late triassic hauptdolomite dachstein carbonate platform of the northern calcareous alps phytoplankton and photosynthetic bacteria are considered to be the major primary producers of the organic matter in these immature 0 5 equivalent vitrinite reflectance carbonate rich rocks contributions from vascular plants are suggested from the n alkane distribution pattern and high content of c 29 steranes enhanced microbial activity in the depositional environment is proposed on the basis of the hopanoid content the occurrence of aryl isoprenoids probably derived from carotenoids of the photosynthetic green sulfur bacteria chlorobiaceae indicates the presence of free h 2 s in the bottom water mesohaline conditions in the upper part of the water column are deduced from the presence of dimethylated and trimethylated 2 methyl 2 tridecylchromans mttcs co variation in the distribution of mttcs and pristane phytane ratio and the concentration of a c 16 aryl isoprenoid argue for photic zone anoxia due to salinity stratification the biomarker composition points to a high productivity marine environment with limited water exchange and a stratified water column confirmed by microfacies analysis a small scale basin formed within the restricted inner lagoonal area of the hauptdolomite dachstein carbonate platform through erosion by currents followed by a sea level rise is considered the most appropriate depositional environment of the dolomicrites of the wiestalstausee section

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

paleoclimatic significance of the temperature dependent susceptibility of holocene loess along a nw se transect in the chinese loess plateau we have conducted mineral magnetic investigations of samples of modern dune sand pristine loess and present day loess along a nw se transect from the northern to the central loess plateau in china the methods used include the temperature dependence of susceptibility tds before and after citrate bicarbonate dithionite cbd treatment as well as x ray diffraction xrd analysis the xrd analyses demonstrate that magnetite and hematite both exist in the chinese loess paleosol sequence and its modem source area but the tds measurements show that magnetite is the predominant contributor to magnetic susceptibility maghemite is present in the pristine loess and the present day loess due to pedogenesis the pedogenic processes that produce the maghemite are closely linked to paleoclimate and for this transect precipitation appears to be the most important climatic variable we suggest that tds can be used as a useful method for the analysis of pedogenesis and climatic change

iron species determination to investigate early diagenetic reactivity in marine sediments iron speciation was determined in hemiplegic sediments from a high productivity area to investigate systematically the early diagenetic reactivity of fe a combination of various leaching agents 1 m hci dithionite buffered in citrate acetic acid hf h2so4 acetic cr ii was applied to sediment and extracted more than 80 of total fe subsequent fe species determination defined specific mineral fractions that are available for fe reduction and fractions formed as products of fe diagenesis to determine the fe speciation of sheet silicates we explored an extraction procedure hf h2so4 and verified the procedure by application to standard rocks variations of fe speciation of sheet silicates reflect the possible formation of fe bearing silicates in near surface sediments the same fraction indicates a change in the primary input at greater depth which is supported by other parameters the fe ii fe iii ratio of total sediment determined by extractions was compared with mossbauer spectroscopy at room temperature and showed agreement within 10 m6ssbauer spectroscopy indicates the occurrence of siderite in the presence of free sulfide and pyrite supporting the importance of microenvironments during mineral formation the occurrence of other fe ii bearing minerals such as ankerite ca fe mg carbonate can be presumed but remains speculative

magnetic properties of sediments of the red river effect of sorting on the source to sink pathway and its implications for environmental reconstruction we conducted a mineral magnetic study of river bank and subaqueous delta sediments from the red river in order to examine the role of sedimentary sorting on the variation of sedimentary magnetic properties from source to sink the magnetic mineralogy mainly consists of magnetite and hematite bulk sediment particle size variations have a strong influence on magnetic properties with the frequently used magnetic parameters fd arm arm and arm sirm exhibiting positive correlations with the 5 m water depth part of the subaqueous delta have lower and sirm values a finer ferrimagnetic grain size and higher proportions of hematite consistent with selective loss of coarse ferrimagnetic grains on the source to sink pathway we suggest that variations in magnetic properties in response to particle size compositions and therefore depositional environment changes should be carefully addressed when magnetic proxies such as arm sirm are used in the study of coastal and marine environmental changes e g sea level change in such cases the combined use of magnetic properties and geochemical indicators such as al ti ratio may provide better results for paleoenvironmental reconstruction

isoarborinol through geological times evidence for its presence in the permian and triassic abstract optical rotation measurements and hplc chiral separations using a cyclodextrin phase performed on aromatic hydrocarbons isolated from diverse geological sources and belonging to the arborane or fernane triterpenoid series have shown that isoarborinol one of the possible biological precursors was abundantly present at the time of deposition of permian and triassic sediments this fact considerably reinforces the hypothesis that arborane derivatives in sediments often found in abundance in lacustrine or lagoonal environments must originate from microorganisms such as aerobic bacteria or algae rather than from angiosperms a group of higher plants whose evolution dates from the cretaceous and currently believed to be the principal source of isoarborinol

characterization of estonian kukersite by spectroscopy and pyrolysis evidence for abundant alkyl phenolic moieties in an ordovician marine type ii i kerogen abstract the kerogen of a sample of estonian kukersite ordovician was examined by spectroscopic solid state 13c nmr ftir and pyrolytic off line flash methods this revealed an important contribution of long linear alkyl chains in kukersite kerogen the hydrocarbons formed upon pyrolysis are dominated by n alkanes and n alk 1 enes and probably reflect a major contribution of selectivity preserved highly aliphatic resistant biomacromolecules from the outer cell walls of gloeocapsomorpha prisca this is consistent with the abundant presence of this fossilized organism in kukersite kerogen in addition high amounts of phenolic compounds were identified in the pyrolysates series of non methylated mono di and trimethylated 3 n alkylphenols 5 n alkyl 1 3 benzenediols and n alkylhydroxybenzofurans were identified all series of phenolic compounds contain long up to c19 linear alkyl side chains kukersite kerogen is therefore an aliphatic type ii i kerogen despite the abundance of free phenolic moieties this study shows that phenol derived moieties are not necessarily associated with higher plant derived organic matter the flash pyrolysate of kukersite kerogen was also compared with that of the kerogen of the guttenberg oil rock ordovician which is also composed of accumulations of fossilized g prisca similarities in the distributions of hydrocarbons and sulphur compounds were noted especially for the c1 c6 alkylbenzene and alkylthiophene distributions however no phenolic compounds were detected in the flash pyrolysate of the guttenberg kerogen possible explanations for the observed similarities and differences are discussed

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

rearranged des e hopanoid hydrocarbons in sediments and petroleum abstract two tetracyclic triterpene derived hydrocarbons des e d c friedo 25 norhopa 5 7 9 triene 1 and des e d c friedohop 8 ene 8 have been isolated from an italian triassic black shale and identified by ms and nmr spectroscopy their structure the occurrence in the sediment of their ring a methylated counterparts as well as carbon isotopic data indicate that these compounds originate from bacterial hopanoids their formation most probably involves a sedimentary rearrangement of a functionalised des e hopane intermediate this hypothesis was further substantiated by experiments in which 8 was formed from 18 h des e hopan 17 ol 10 by treatment with k 10 montmorillonite clay in addition some minor unsaturated hydrocarbons formed as by products are also present in sediments

chemical molecular and isotopic differentiation of organic facies in the tertiary lacustrine duaringa oil shale deposit queensland australia abstract total organic matter extractable organic matter bitumen and pyrolysates released on low temperature sealed tube pyrolysis of kerogen were analysed chemically and isotopically based on bulk bitumen and biomarker parameters the sediments are immature to a depth of approx 1300 m within the terrestrial and lacustrine depositional environments of the tertiary duaringa basin various organic facies have been classified according to inorganic input and maceral abundances of organic matter derived from land plant and algal sources the latter derived mainly from pediastrum and with a varying botryococcus content the lacustrine upper lamosite is chemically and isotopically distinct from the lower lamosite due to a changing algal community relative contributions from allochthonous land plants and for the lower lamosite a slightly more saline and restricted lake environment the lower lamosite is associated with the highest concentrations of c 28 c 30 4 methylsteranes and 13 c depletion in toc in the upper lamosite there is a strong positive correlation between botryococcus content and 13 c enrichment the organically lean claystones have sources nearly identical to the lamosites but the organic matter is diluted due to rapid influx of siliciclastics the terrestrial facies the cannel coal and carbonaceous shales are mainly distinguished from the lacustrine facies by higher relative proportions of c 29 steranes c 30 4 methylsteranes and 13 c values for the pyrolysed hydrocarbons intermediate between the two lamosite facies the carbonaceous oil shale has a composite geochemical signature intermediate between the two end members lamosite and cannel coal oleanane and ring a degraded triterpanes derived from lupane arborane and oleanane skeletons are distributed throughout all organic facies representing both autochthonous and allochthonous higher plant inputs higher concentrations of oleanane and des a lupane are associated with the terrestrial facies hopanes dominate over steranes in all organic facies and their concentration ranges are independent of facies for the lamosites the aliphatic biopolymer algaenan is implicated as the dominant source for n alkanes from analysis of their yields homologue abundances and individual 13 c values in the terrestrial facies resinites are paraffinic and are a major source for the n alkanes released on pyrolysis compound specific analysis of extractable and pyrolysate hydrocarbons allowed deconvolution of different source inputs to free lipids and kerogen for the lamosites low molecular weight n alkanes in the bitumen are derived mainly from a pediastrum source while botryococcus is associated with waxy n alkanes however the situation is reversed in the kerogen pediastrum algeanan has a longer average alkyl chain length than botryococcus algaenan the magnitude of the isotopic differences between single n alkanes released at different temperatures is useful in determining multiplicity of sources

structure and distribution of branched aliphatic alkanes with quaternary carbon atoms in cenomanian and turonian black shales of pasquia hills saskatchewan canada abstract twelve pseudohomologous series of branched alkanes with one or two quaternary carbon atoms baqcs c14 c33 range are present in the extractable organic matter of cenomanian and turonian black shales of pasquia hills saskatchewan canada five of these pseudohomologous series are also observed in a holocene sample from the russian arctic three of the pseudohomologous series were identified by kenig et al pnas 100 2003 12554 12558 as 3 3 and 5 5 diethylalkanes deas and 2 2 dimethylalkanes dmas the nine other pseudohomologous series of baqcs are tentatively identified on the basis of their characteristic mass spectral fragmentation and retention indices as 3 3 dimethylalkanes dmas 3 ethyl 3 methyl and 5 ethyl 5 methylalkanes 9 9 7 7 and 6 6 diethylalkanes deas 3 3 3 3 and 5 5 5 5 tetraethylalkanes and 5 butyl 5 ethylalkanes pseudohomologous series of branched alkanes with tertiary carbon atoms such as monomethyl and monoethylalkanes are also present in the same carbon number range all these pseudohomologous families share the particularity of including isomers with exclusively odd or exclusively even carbon numbers clearly indicating a biosynthetic origin baqcs such as 5 5 deas 3 3 deas and 3 ethyl 3 methylalkanes seem to be widespread in sediments and sedimentary rocks ranging from modern to 2 1 billion years a xenobiotic source for baqcs can be excluded at this point baqcs seem to have a very low biodegradability in part explaining their widespread occurrence in ancient sediments

biological marker compounds as indicators of the depositions history of the maoming oil shale abstract the eocene maoming oil shale from guangdong province occurs as a laterally uniform stratigraphic section typically 20 25 m thick from which the aliphatic hydrocarbon constituents of six representative samples were investigated using gc and c gc ms the sediments evaluated included the basal lignite a vitrinite lens from the overlying claystone and four intervals from the massive oil shale bed as expected the lignite and vitrinite differ markedly from the oil shales the lignite is dominated by bacterial hopanoids and components of higher plant origin including c 29 steroids and triterpenoids such as oleanenes visually the oil shale samples show corroded and degraded phytoclasts spores wispy particles of fluorescent organic material attributable to dinoflagellates and especially in the uppermost sample colonial algal bodies the distributions of biological markers in the oil shales show many features in common notably a dominance of dinoflagellate derived 4 methylsteroids and a significant proportion of higher plant derived n alkanes with marked odd over even carbon number predominance overall they exhibit several features that resemble characteristics of the messel shale the hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales the aliphatic hydrocarbons of the uppermost shale are dominated by a number of c 31 and c 33 botryococcane homologues and other unusual branched alkanes possibly derived from green algae all of the samples are immature overall molecular and microscopic examination of the stratigraphic succession of the maoming oil shale suggests a shallow lacustrine environment within which peats were deposited this lake subsequently deepened to support abundant algal populations especially dinoflagellates culminating in a dominance of botryococcoid algae

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

vascular plant biomarkers as proxies for palaeoflora and palaeoclimatic changes at the dogger malm transition of the paris basin france abstract vascular plant biomarkers are used here as a chemostratigraphic tool for tracing both palaeoflora and palaeoclimatic changes during the callovian and oxfordian in the paris basin the retene cadalene ratio shows low values in claystones from the middle callovian to the end of the lower oxfordian then increases drastically from the end of the lower oxfordian cordatum zone synchronously with the progressive development of the oxfordian carbonate platform the relative proportion of retene a conifer biomarker is strongly correlated with the abundance of some of its diagenetic precursors and with saturated diterpanes some of these are very common and abundant in the pinaceae family while specific markers for non pinaceae conifers are not detected thus the increase in the retene cadalene ratio around the cordatum zone reflects the rising proportion of pinaceae on the london brabant massif which was the nearest land from the locations studied as pinaceae show many morphological adaptations to dry climate compared to the other plants reported in the jurassic taphofloras of the paris basin their increasing proportion indicates an increasing aridity this climatic change is also supported by published geochemical palaeobiogeographical palynological palaeobotanical and mineralogical data a comparable increase in the relative proportion of retene was also described for the oxfordian of western australia which could be an argument for a worldwide increase in aridity at the end of the lower oxfordian

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

changing environmental conditions in recent past reading through the study of geochemical characteristics magnetic parameters and sedimentation rate of mudflats central west coast of india abstract geochemical sediment components major and trace metals magnetic related parameters concentration mineralogy and grain size and radionuclide 210 pb analyses were carried out on sediment cores collected from mudflat regions of central west coast of india viz kolamb creek malvan mandovi estuary panaji and tadri creek gokarn in order to study the variations in sediment characteristics with time the changes in spatial and temporal depth distributions of sediment composition were observed to be mainly controlled by the catchment area geology geomorphology and anthropogenic activities the sedimentation rate in the study area varied from 0 13 to 2 cm yr the rate was extremely low at the lower half of the cores 0 13 0 31 cm yr before 1980 as compared to the near surface 1 21 2 cm yr after 1980 with two clear phases of sedimentation rates the higher sedimentation rate in the upper portions of the cores was found to correspond to increased deposition of finer sediment components metals and magnetic minerals which in turn were found to be mainly controlled by geology and or human activities the results of the present study reveal variations in sediment properties with time and support the use of geochemical in addition to magnetic susceptibility parameters on mudflats to understand changing environmental conditions of recent past

a review of mesozoic climates there is overwhelming evidence based on the distribution of distinctive sediments and fossils and oxygen isotope data that the climate of the mesozoic world was appreciably more equable than that of today with no polar ice caps but precise quantitative data are not available except for an episode of late cretaceous cooling there is no good documentation of any significant change in global temperature distributions through the era the distribution of coals and evaporites together with other criteria indicates a pattern of humid arid and zones appreciably different in important respects from that of today during the triassic and jurassic western pangaea in low to middle latitudes was largely arid but in the early cretaceous the lands on the margin of the newly opening central atlantic and western tethys experienced a humid climate by late cretaceous times arid zones had become very restricted in extent because of insufficient suitable data attempts at climatic modelling have had only modest success and only to a limited extent can the major long term changes in climate between the permian and the present be explained in terms of changing geography the most probable explanation of mesozoic equability is an increased atmospheric co 2 content a number of enigmas remain such as the existence of flourishing forests in polar palaeolatitudes whereas for the late cenozoic short term climatic changes can be related successfully to variations in the geometry and mechanics of the earth sun system there is a long way to go before comparable success can be claimed for the mesozoic

abundance and geochemical significance of c2n dialkylalkanes and highly branched c3n alkanes in diverse meso and neoproterozoic sediments several series of branched alkanes with quaternary carbon centers have been detected by gc ms in the saturated hydrocarbon fractions of neoproterozoic sedimentary rocks associated with fossilised microbial mats officer basin south australia and of shallow water carbonates spitsbergen east greenland and baffin island canada a series of 5 5 diethylalkanes was predominant in most of these sediments and was typically accompanied by related series in lower abundance which based on mass spectral characteristics are tentatively assigned as 6 6 diethylalkanes 5 butyl 5 ethylalkanes and 6 butyl 6 ethylalkanes each series displays strong odd or even carbon number preferences several of the sediments contained additional series of structurally undefined c3n alkanes notable for very negative retention time factors which become more negative with increasing molecular weight indicating that branching increases with each c3 addition the strongly sequential carbon preference of the respective c2n and c3n branched alkane series and a predominantly light 13c signature of the 5 5 diethylalkanes cf co occurring n alkanes are consistent with an origin from organisms with a distinct physiology and or carbon source the relatively wide occurrence and high abundance of these branched alkanes in proterozoic sediments containing benthic microbial mats suggests a relationship with specific paleoenvironmental conditions for example strong water column redox gradients that may have been more prevalent than in the phanerozoic