identification of permian palaeowind direction from wave dominated lacustrine sediments lodeve basin france lacustrine environments are an excellent indicator of continental palaeoclimate in particular the sedimentary record of waves in lakes may be used to constrain atmospheric palaeocirculation wave ripples have been identified in a permian lacustrine basin the salagou formation 260 250 ma lodeve basin located in the southern french massif central part of the western european hercynian mountain chain wave ripple patterns are interpreted with regards to hydrodynamics and water palaeodepth it is shown that in the case of the salagou formation wave ripple orientations were controlled by the direction of the prevailing palaeowind the late permian wind blew from between north and 20 east of north possibly over several millions of years and certainly throughout the period of deposition of about 2000 m of strata in the lodeve basin permian lacustrine sedimentation is widespread and well preserved on the earth s surface and so wave ripple data may help constrain numerical modelling of the earth s past climates especially with regards to permian times outside of desert regions

sedimentary architecture and depositional controls of a pliocene river dominated delta in the semi isolated dacian basin black sea abstract sedimentological facies models for semi isolated basins are less well developed than those for marine environments but are critical for our understanding of both present day and ancient deltaic sediment records in restricted depositional environments this study considers an 835 m thick sedimentary succession of mid pliocene age which accumulated in the dacian basin a former embayment of the black sea detailed sedimentological and palaeontological analyses reveal a regression from distal prodelta deposits with brackish water faunas to delta top deposits with freshwater faunas sediments contain frequent hyperpycnal plumes and an enrichment in terrestrial organic material ichnofossils and in situ brackish and freshwater faunas deltaic progradation created thin sharply based sand bodies formed by multiple terminal distributary channels covering a wide depositional area the system experienced frequent delta lobe switching resulting in numerous thin parasequences parasequences are overlain by erosive reddish oxidized sand beds enriched in broken abraded brackish and freshwater shells these beds were formed after sediment starvation on top of abandoned delta lobes during each flooding event a robust magnetostratigraphic time frame allowed for comparison between the observed sedimentary cyclicity and the amplitude and frequency of astronomical forcing cycles our results indicate that parasequence frequencies are significantly higher than the number of time equivalent astronomical cycles this suggests that delta lobe switching was due to autogenic processes we consider the observed facies architecture typical for a delta prograding on a low gradient slope into a shallow brackish protected semi isolated basin furthermore in the absence of significant wave and tidal influence sediment progradation in such a protected depositional setting shaped a delta strongly river dominated

the response of lake margin sedimentary systems to climatically driven lake level fluctuations middle devonian orcadian basin scotland acknowledgements m j newman is gratefully acknowledged for his identification of fossil fish specimens from the easter ross coast which helped in confirmation of the position of the achanarras fish bed in this region the inverness field club and the carnegie trust are thanked by sda for financial support of fieldwork undertaken nigel trewin is acknowledged for inspiring a long lived interest in the devonian of northern scotland which has led to some fantastic journeys gary nichols and alberto saez are thanked for constructive reviews

on the true antiquity of eoarchean chemofossils assessing the claim for earth s oldest biogenic graphite in the saglek block of labrador abstract a recent claim to have found traces of earth s earliest life 3 95 ga utilising isotopically light carbon in graphite bearing metapelites from the saglek block of northern labrador canada is re evaluated applying rigorous geological and geochronological criteria the establishment of these criteria in previous evaluations of early life claims from southern west greenland and northern canada is reviewed in order to provide a backdrop to discussion of the saglek claim in particular we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims which are considerably easier to demonstrate from large relatively less tectonised supracrustal remnants like the isua greenstone belt than they are from smaller isolated enclaves of the kind found on akilia or the highly tectonised and imbricated unit that is found in the saglek block unambiguous field relationships between ca 3 9 ga tonalitic gneiss and the graphite bearing metasediments have not been demonstrated in the literature that the saglek claim relies upon and earlier u pb hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a mesoarchean to neoarchean time of deposition we conclude that irrespective of the validity of the carbon isotopic evidence field relationships and geochronological evidence fail to demonstrate an age of 3 95 ga for the potential traces of life

evidence in pre 2 2 ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota the loss of fe from some pre 2 2 ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2 2 ga i have examined the behavior of fe in both pre and post 2 2 ga paleosols from depth profiles of fe 3 ti fe 2 ti and fe ti ratios and fe 3 ti vs fe 2 ti plots this new approach reveals a previously unrecognized history of paleosols essentially all paleosols regardless of age retain some characteristics of soils formed under an oxic atmosphere such as increased fe 3 ti ratios from their parental rocks the minimum oxygen pressure p o 2 for the 3 0 2 2 ga atmosphere is calculated to be about 1 5 of the present atmospheric level which is the same as that for the post 1 9 ga atmosphere the loss of fe common in paleosol sections of all ages was not due to a reducing atmosphere but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere this reductive dissolution of ferric hydroxides occurred either 1 after soil formation by hydrothermal fluids or 2 during and or after soil formation by organic acids generated from the decay of terrestrial organic matter terrestrial biomass on the early continents may have been more extensive than previously recognized

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

life associated with a 2 76 ga ephemeral pond evidence from mount roe 2 paleosol dark sericitic material at and near the top of the 2 765 0 01 ga mount roe 2 paleosol in western australia contains 0 05 0 10 wt organic carbon with delta 13c values between 33 and 51 pdb peedee belemnite such negative isotopic values strongly indicate that methanotrophs once inhabited this material the textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds that these ponds became desiccated and that heavy rains transported the material to its present sites the discovery of methanotrophs associated with the mount roe 2 paleosol may extend their geologic record on land by at least 1 5 b y methanotrophy in this setting is consistent with the notion that atmospheric methane levels were or 20 mu atm during the late archean the radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the late archean

early life signatures in sulfur and carbon isotopes from isua barberton wabigoon steep rock and belingwe greenstone belts 3 8 to 2 7 ga

reassessing the evidence for the earliest traces of life the isotopic composition of graphite is commonly used as a biomarker in the oldest 3 5 gyr ago highly metamorphosed terrestrial rocks earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3 8 gyr old isua supracrustal belt isb in southern west greenland have suggested the presence of a vast microbial ecosystem in the early archean this interpretation however has to be approached with extreme care here we show that graphite occurs abundantly in secondary carbonate veins in the isb that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks metasomatism during these reactions graphite forms from the disproportionation of fe ii bearing carbonates at high temperature these metasomatic rocks which clearly lack biological relevance were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life the new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed early archaean rock record

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

metamorphic fluid flow in the northeastern part of the 3 8 3 7 ga isua greenstone belt sw greenland a re evalution of fluid inclusion evidence for early archean seafloor hydrothermal systems abstract fluid inclusions in quartz globules and quartz veins of a 3 8 3 7 ga old well preserved pillow lava breccia in the northeastern isua greenstone belt igb were studied using microthermometry raman spectrometry and sem cathodoluminescence imaging petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content the oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization these crystals contain a cogenetic immiscible assemblage of co2 rich h2o graphite and brine rich co2 halite carbonate inclusions the gas rich inclusions have molar volumes between 44 8 and 47 5 cm3 mol while the brine inclusions have a salinity of 33 eq wt nacl modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak metamorphic conditions of 460 c and 4 kbar carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions in most quartz crystals however recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have co2 rich assemblages with distinctly different molar volumes either between 43 7 and 47 5 cm3 mol or between 53 5 and 74 1 cm3 mol and immiscible halite saturated h2o co2 nacl other salt inclusions later intergranular trails have ch4 h2 x h 2 up to 0 3 inclusions of variable density ranging from 48 0 to 105 3 cm3 mol and metastable h2o nacl other salt brines 28 eq wt nacl finally the youngest fluid inclusion assemblages are found in non luminescent secondary quartz and contain low density ch4 molar volume 105 33 cm3 mol and low salinity h2o nacl 0 2 3 7 eq wt nacl these successive fluid inclusion assemblages record a retrograde p t evolution close to a geothermal gradient of 30 c km but also indicate fluid pressure variations and the introduction of highly reducing fluids at 200 300 c and 0 5 2 kbar the quartz globules in the pillow fragments only contain sporadic ch4 h2 and brine inclusions corresponding with the late generations present in the cementing and crosscutting veins we argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments only these relatively late fluid inclusions have been preserved and that they do not represent remnants of an early seafloor hydrothermal system as was previously proposed modeling the oxidation state of the fluids indicates a rock buffered system at peak metamorphic conditions but suggests a change towards fluid graphite disequilibrium and a log f h 2 f h 2 o above the quartz fayalite magnetite buffer during retrograde evolution most likely this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the igb finally this study shows that microscopic solid graphite in recrystallized metamorphic rocks from isua can be deposited inorganically from a fluid phase adding to the complexity of processes that formed reduced carbon in the oldest well preserved supracrustal rocks on earth

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

compositional heterogeneity of archean mantle estimated from sr and nd isotopic systematics of basaltic rocks from north pole australia and the isua supracrustal belt greenland abstract compositional variability found in modern mid ocean ridge basalt morb and ocean island basalt oib reflects differentiation processes associated with material recycling in the mantle crust system to investigate the timing at which this recycling system was established and how it transformed into the present day system we present geochemical analyses of the archean basalts from north pole np in the east pilbara craton western australia and the isua supracrustal belt isb southern west greenland these rocks represent archean accretionary complexes with ages of 3 5 ga and 3 7 3 8 ga respectively we analyzed the trace element contents including rare earth elements rees and sr and nd isotopic compositions of the basalts which may represent morbs and oibs from np and isb their trace element compositions are broadly similar but show distinct geochemical characteristics particularly with respect to rees that probably reflect differences in both the source mantle and degree of melting such differences are also evident in their initial nd isotopic compositions which were estimated based on equilibrium partitioning of rees and well defined isochron ages in contrast the sr isotopic compositions of the np and isb basalts are highly variable and their isochron ages are inconsistent with previous studies furthermore the partitioning of rb and sr in the np basalts indicates disequilibrium suggesting that the rb sr system has been disturbed by post igneous alteration and metamorphism based on these observations we propose the following model to explain the temporal variations in the geochemical composition of the archean mantle i 3800 ma recycling of plate material and melting occurred quite readily and therefore morbs and oibs were produced from differentiated mantle sources ii 3460 ma to 3800 ma mantle crust mixing occurred as the result of an extreme event such as mantle overturning reducing the compositional variation of the mantle and iii after 3460 ma mantle heterogeneity gradually developed in the material recycling system re establishing the compositional differences between morbs and oibs this model requires an extreme event to drive the homogenization during stage ii which may provide new insights into the evolution of the crust mantle system

shoreline processes and the age of the lake lahontan highstand in the jessup embayment nevada the well developed shoreline record of pluvial lake lahontan in the jessup embayment nevada is used to refine the history of late pleistocene lake level fluctuations and to assess controls on shoreline development and distribution controls on the strength and type of shorelines developed include local slope the amount and characteristics of sediment available for transport the availability of accommodation space and length of time the lake level resides at a particular shoreline elevation at the sehoo highstand and during the early part of the regression strong storm winds and waves from the south southeast set up a clockwise net shore drift pattern near the head of the embayment although significant differences in local slope geometry of the shoreline and wave energy existed in the embayment crestal heights of constructional shoreline features formed at the highstand vary 2 6 m in elevation and hence provide a relatively precise marker of the highstand elevation radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338 5 m in the embayment and receded from that elevation immediately prior to 13 070 60 yr b p similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m 13 280 110 yr b p and 1331 m 13 110 110 yr b p suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time perhaps only for years or decades the brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern pyramid lake which formed in less than seven months due to a rapid increase in lake level

young poorly crystalline graphite in the 3 8 gyr old nuvvuagittuq banded iron formation carbonaceous minerals in 3 8 billion year old rocks from west greenland have been used as evidence for early life on earth geochemical analyses of similar carbon minerals in 3 75 to 4 2 billion year old rocks from canada show that they can be derived from subsequent alteration more than a billion years after the initial formation

carbon isotopes and petrography of kerogens in 3 5 ga hydrothermal silica dikes in the north pole area western australia more than 600 specimens of 3 5 ga old hydrothermal silica dikes from the north pole area pilbara craton western australia have been studied petrographically the kerogens in 44 samples have been analyzed isotopically c and n and chemically c n and h the silica dikes are composed mainly of fine grained silica modal abundance 97 and are classified into two types by minor mineral assemblages b black type and g gray type the b type silica dikes contain kerogen 0 37 to 6 72 mgc g average 2 44 mgc g n 21 and disseminated sulfides dominantly pyrite and fe poor sphalerite in some cases carbonate and apatite are also present their silica dominated and sulfide poor mineral assemblages suggest precipitation from low temperature reducing hydrothermal fluid likely 100 200 c on the other hand the g type silica dikes are sulfide free and concentrations of kerogen are relatively low 0 05 to 0 41 mgc g average 0 17 mgc g n 13 they typically contain fe oxide mainly hematite which commonly replaces cubic pyrite and rhombic carbonate some g types occur along secondary quartz veins these textures indicate that the g type silica dikes were formed by postdepositional metasomatism oxidation of the b types and that the b types probably possess premetasomatic signatures the 13c values of kerogen in the b types are 38 1 to 33 1 average 35 9 n 21 which are 4 lower than those of the g types 34 5 to 30 0 average 32 2 n 19 and 6 lower than bedded chert 31 2 to 29 4 average 30 5 n 4 this indicates the preferential loss of 12c during the metasomatism estimated fractionation factor 0 9985 considering the metasomatic effect on carbon isotopes with probably minor diagenetic and metamorphic overprints we conclude that the original 13c values of the kerogen in the silica dikes would have been heterogeneous 5 and at least some material had initial 13c values of 38 the inferred 13c depletions of organic carbon could have been produced by anaerobic chemoautotrophs such as methanogen but not by aerobic photoautotrophs this is consistent with the estimated physical and chemical condition of the hydrothermal fluid which was probably habitable for anaerobic and thermophilic hyperthermophilic chemoautotrophs alternatively the organic matter may have been possibly produced by abiological reaction such as fischer tropsch type ftt synthesis under the hydrothermal condition however the estimated condition is inconsistent with the presence of the effective catalysts for the ftt reaction i e fe ni alloy magnetite and hematite these lines of evidence suggest the possible existence of biosphere in the 3 5 ga sub seafloor hydrothermal system

ion microprobe analysis of graphite from ca 3 8 ga metasediments isua supracrustal belt west greenland relationship between metamorphism and carbon isotopic composition in situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca 3 8 ga isua supracrustal belt west greenland the 13c values of micron scale graphite globules in the metasediments and the carbonate rocks vary from 18 to 2 and from 7 to 3 respectively the maximum 13c value of graphite globules in the metasediment rises from 14 to 5 as the metamorphic grade increases from epidote amphibolite to upper amphibolite facies in a single hand specimen the 13c values of graphite inclusions in garnet are 7 lower on average than those outside garnet similarly graphite armored by quartz apparently shows a few permil lower 13c values than those on grain boundaries between noncarbonate minerals the fact that early crystallized minerals include relatively 13c depleted graphite indicates that the regional metamorphism increased the 13c values of the isua graphite this is consistent with the regional trend of 13c enrichment accompanied by the increase of metamorphic grade the minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 c these observations indicate that isotopic exchange with isotopically heavy carbonate caused 13c enrichment of isua graphite the 13c values of graphite reported here 13c 18 were produced either as a metamorphic modification of organic carbon with initially much lower 13c values or as an abiological reaction such as decomposition of carbonate if the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13c enrichment of isua graphite previously reported large 13c depletion of graphite especially armored by apatite mojzsis et al 1996 was probably premetamorphic in origin this supports the existence of life at isua time ca 3 8 ga

devonian lake margin environments and processes se shetland scotland the devonian lake or lakes of se shetland were characterized by 4 environments 1 a marginal environment dominated by the high energies of the surf zone and just lakeward of the breaker zone 2 an additional marginal environment consisting of small shallow oxidizing lakes on a low gradient coastal flood plain over which waters from the main body of the lake fluctuated 3 an intermediate environment characterized by continuous to intermittent wave agitation in water depths of probably 5 10m the marginal environment was influenced by both wave and current processes a friction dominated outflow produced sedimentary structures that indicate a south easterly flow and depositional slope outflow competed with shoaling waves which advanced from e to w onto the lake shoreline where outflow was weakest wave processes dominated longshore drift was an additional component in the movement of the nearshore waters the devonian lake or lakes of se shetland possessed rather shallow shoaling margins

mineralogical and isotopic characterization of graphite deposits in the western part of the north qaidam orogen and east kunlun orogen northeast tibetan plateau china abstract medium to high grade metamorphic rocks comprising numerous graphite deposits are widely distributed in the periphery of the qaidam basin northern tibetan plateau china we present textural xrd and raman spectroscopy characteristics of graphite 13c results of graphite and cogenetic calcite and 18o results of calcite in three graphite occurrences located at the periphery of qaidam basin graphite displays hexagonal platy flaky irregular polygonal ovate morphologies with occasional spherulitic textures graphite morphologies combined with xrd and raman spectroscopy indicate that graphite has relatively high crystallinity raman microspectroscopy of graphite shows that peak metamorphic temperatures vary between 690 and 801 c graphite 13cv pdb values range between 26 3 and 21 6 indicating organic carbon as the major carbon source for graphite calcite 13cv pdb and calcite 18ov smow values range between 10 8 and 3 9 and between 6 6 and 29 5 respectively calcite 13cv pdb results show that organic carbon was mixed with inorganic carbon which resulted in an increase of 13cv pdb calcite 18ov smow values indicate that the graphite deposits experienced late stage water rock interaction the positive and negative anomalies of calcite 13cv pdb indicate that organic matter oxidation and recycling locally occurred during the final stage of the great oxidation event the wide distribution of regional metamorphic organogenic graphite deposits provides evidence for large scale paleoproterozoic biological activity in the periphery of the qaidam basin

holocene lake mega chad palaeoshorelines from space during the holocene a giant lake known as lake mega chad lmc extended over more than 350 000 km2 in southern sahara morphodynamic features of sedimentary systems outlining the lmc palaeoshorelines have been identified by the joint analyses of new topographic images digital elevation model acquired by radar interferometry and landsat thematic mapper images here we characterize for the first time at the scale of the chad basin a wave dominated sedimentary system including river deltas longshore sandridges beach ridges spits and a wave ravinement surface they provide new evidence of the environmental impact of quaternary climate changes in the sahelo saharan area continental trade winds controlled the longshore drift in the northern part of the palaeolake two distinct lmc episodes dated as lower and middle holocene are clearly identified contemporaneous with the two phases of wetter conditions usually recognized in central and northern africa

raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 ma akilia supracrustal rocks west greenland three dimensional molecular structural images of apatite grains and associated minerals embedded in a banded quartz pyroxene magnetite supracrustal rock from akilia southern west greenland were constructed by using raman confocal spectroscopy the rock sample is the same as that from which apatite hosted isotopically light graphitic inclusions were reported by mojzsis and colleagues in 1996 the results were challenged in 2005 by lepland and colleagues who failed to find carbon bearing inclusions in this and other akilia samples here we demonstrate that inclusions of graphite wholly contained within apatite occur in this rock the carbon isotopic composition of one such inclusion its graphitic composition established by raman spectroscopy was measured by secondary ion mass spectrometry to be isotopically light 13 c 29 4 in agreement with earlier analyses our results are thus consistent with the hypothesis that graphite containing apatite grains of the older than 3830 ma akilia metasediments may represent chemical fossils of early life

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

significance of graphite occurrences in the aracena metamorphic belt iberian massif the significance of syngenetic and epigenetic graphite occurrences from the variscan high temperature low pressure aracena metamorphic belt is discussed in the framework of the tectono thermal evolution of this southern zone of the iberian massif syngenetic graphite is associated with both low to medium grade metamorphic rocks la umbria series precambrian in age and high grade granulite facies rocks the precambrian fuente del oro series and a cambrian calc silicate series epigenetic fluid deposited occurrences correspond to overgrowths on existing metamorphic graphite grains and vein type mineralization two types of graphitized particles with remarkable differences in reflectance anisotropy and size can be distinguished in the precambrian metapelites of the la umbria series large 150 m length platy crystals with high reflectance and anisotropy are interpreted as detrital and are considered indirect evidence of an old orogenic cycle prior to the cadomian orogeny during which metamorphism exceeded greenschist facies the coexistence of two types of particles explains the scattering of values of the c parameter of graphite determined by xrd c 6 72 6 74 a and the anomalously high temperatures of the dta exothermic peak close to 600 c of graphite with respect to that inferred from mineral assemblages in these rocks the presence of graphite rich quartzites and gneisses within the fuente del oro series and the calc silicate series is evidence of sedimentation under reducing conditions in a continental shelf the characteristics of graphite reflect the high grade metamorphic conditions attained in the southern area of the aracena metamorphic belt pervasive flow of fluids related to a major variscan extensional event resulted in overgrowths on the pre existing graphite in the gneisses and quartzites of the calc silicate series as evidenced by the heterogeneous isotopic composition of graphite single crystals in these rocks a later stage of graphite precipitation is represented by scarce vein type occurrences in mafic granulites that document channelled flow of fluids

formation of meteorite hydrocarbons from thermal decomposition of siderite feco3 thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds siderite decomposition in the presence of water vapor at 300 c generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds the results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite alh84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on mars the results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early earth

variable style of transition between palaeogene fluvial fan and lacustrine systems southern pyrenean foreland ne spain two palaeogene fluvial fan systems linked to the south pyrenean margin are recognized in the eastern ebro basin the cardona su ria and solsona sanau ja fans these had radii of 40 and 35 km and were 800 and 600 km2 in area respectively during the priabonian to the middle rupelian the fluvial fans built into a hydrologically closed foreland basin and shallow lacustrine systems persisted in the basin centre in the studied area both fans are part of the same upward coarsening megasequence up to 800 m thick driven by hinterland drainage expansion and foreland propagation of pyrenean thrusts fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan channelized terminal lobe nonchannelized terminal lobe mudflat deltaic evaporitic playa lake and carbonate rich shallow lacustrine environments lateral correlations define two styles of alluvial lacustrine transition during low lake level stages terminal lobes developed whereas during lake highstands fluvial dominated deltas and interdistributary bays were formed terminal lobe deposits are characterized by extensive 100 600 m wide sheet like fine sandstone beds formed by sub aqueous quasi steady hyperpycnal turbidity currents sedimentary structures and trace fossils indicate rapid desiccation and subaerial exposure of the lobe deposits these deposits are arranged in coarsening fining sequences metres to tens of metres in thickness controlled by a combination of tectonics climatic oscillations and autocyclic sedimentary processes the presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake level falls deltaic deposits form progradational coarsening upward sequences several metres thick characterized by channel and friction dominated mouth bar facies overlying white grey offshore lacustrine facies deltaic bar deposits are less extensive 50 300 m wide than the terminal lobes and were also deposited by hyperpycnal currents although they lack evidence of emergence sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays whereas the terminal lobe sheets expand from a series of mid fan intersection points and coalesced to form a more continuous sandy fan fringe

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

evolution of the composition of seawater through geologic time and its influence on the evolution of life abstract the redox state of the surface environment of the early earth is still controversial and a detailed and quantitative estimate is still lacking we carried out in situ analyses of major trace and rare earth elements of carbonate minerals in rocks with primary sedimentary structures in shallow and deep sea deposits in order to eliminate secondary carbonate and contamination of detrital materials and to estimate the redox condition of seawater through time based on the ce content and anomalies of the carbonate minerals at given parameters of atmospheric co2 content pco2 and ca content of seawater we calculated the oxygen contents of shallow and deep seawater respectively the results show that the oxygen content of the deep sea was low and constant until at least 1 9 ga the oxygen content of shallow seawater increased after 2 7 ga but fluctuated it became quite high at 2 5 and 2 3 ga but eventually increased after the phanerozoic in addition the calculation of a high pco2 condition shows that seawater was more oxic even in the archean than at present suggesting a relatively low pco2 through geologic time our detailed calculations from compositions of carbonate minerals in three gorge area south china show a low oxygen content of seawater after the snowball earth until the late ediacaran an increase in the late ediacaran and a significant decrease around the precambrian cambrian and nemakit daldynian tommotian boundaries these variations were possibly caused by global regression and dissolution of methane hydrates

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

distinguishing biological from hydrothermal signatures via sulphur and carbon isotopes in archaean mineralizations at 3 8 and 2 7 ga

structural characterization of kerogen in 3 4 ga archaean cherts from the pilbara craton western australia hydrogen lean kerogen atomic h c 0 46 isolated from the 3 4 ga strelley pool chert in the north pole area pilbara craton western australia were studied by vibrational spectroscopy fourier transform infrared ftir spectroscopy and raman spectroscopy nuclear magnetic resonance spectroscopy solid state 13 c nmr spectroscopy catalytic hydropyrolysis followed by gas chromatography mass spectrometry hypy gc ms and isotope ratio mass spectrometry irms the kerogen occurs in sedimentary rocks as clasts and clots deposited together with other detrital materials that are finely disseminated throughout a chert matrix the bulk kerogen 13 c values range from 28 3 to 35 8 solid state 13 c nmr spectroscopy and ftir spectroscopy reveals that the kerogen is highly aromatic fa varying from 0 90 to 0 92 and contains only minor aliphatic carbon or carbon oxygenated c o functionalities the raman carbon first order spectra for the isolated kerogens are typical of spectra obtained from disordered sp 2 carbons with low 2 d ordering biperiodic structure the implications of the raman results show low 2 d ordering throughout the carbonaceous network indicate the incorrect usage of the term graphite in the literature to describe the kerogen or carbonaceous material in the warrawoona cherts hydropyrolysates contain aromatic compounds consisting of 1 ring to 7 ring polycyclic aromatic hydrocarbons which were covalently bound into the kerogen as well as alkanes linear branched and cyclic which were most probably trapped in the microporous network of the kerogen these pahs have mainly c1 and c2 alkylation while c3 substitued aromatics are low in abundance and do not show a high degree of branched alkylation for the first time we have shown a correlation between elemental analysis h c atomic ratios raman spectroscopic parameters id1 ig id1 id1 ig and la and the degree of alkylation of bound polyaromatic molecular constituents generated from hypy for archaean kerogens similarities in molecular

devonian lacustrine shore zone architecture giving perspective to cliff exposures with ground penetrating radar lake margin sedimentary systems have been the subject of only limited study the cyclic middle devonian lacustrine succession of northern scotland contains repeated developments of shore zone sandstones and thus provides an ideal location for the study of these units the cycles comprise deep lake shallow lake playa and shore zone facies detailed field observations are presented alongside ground penetrating radar data which has aided large scale and three dimensional characterization of the shore zone sand bodies loading and discrete channel forms are recognized in thin bedded sandstones within the lower portion of the lake shore zone successions up section the sandstone beds appear to become amalgamated forming subtle low angle accretionary bar complexes where imaged on the radar profiles the repeated development of shoreward migrating features succeeded by more shallow angled lakeward accreting surfaces is recognized these are ascribed to washover and swash backwash processes respectively the orientation of these features is similar to palaeocurrent measurements from oscillation ripples suggesting an alignment of the shore zone bars perpendicular to the prevailing wind direction further loaded sandstone beds and sand filled shallow channel features overlie the bar forms the context of the shore zone facies allows the controls on its formation to be examined the shore zone sandstones overlie playa facies which contain abundant desiccation horizons reflecting the most arid phase in the climatically controlled lacustrine cycle as climatic conditions ameliorated the rejuvenation of fluvial systems resulted in the transport of sand out into the basin initial deposition was limited to intermittent events where sediment was laid down on a water saturated substrate high resolution fluctuations in lake level resulted in periodic short lived reworking events along the lake margins which produced amalgamated sands forming low relief bars shore zone reworking is likely to have occurred over a wide area as the lake margin migrated back and forth and gradually transgressed

geology age and origin of supracrustal rocks at akilia west greenland new mapping geochronology and geochemistry of the supracrustal enclave and associated orthogneisses at akilia west greenland support a 3825 6 ma age for the sequence and a chemical sedimentary origin for the controversial fe rich quartz pyroxene gneisses lithologies of the enclave comprise laterally continuous mappable units of mafic amphibolite ultramafic rocks and two fe rich quartz pyroxene units a minor anthophyllite garnet rock with chemical characteristics suggesting a sedimentary protolith was identified the earliest foliation parallels lithologic contacts this foliation was isoclinally folded about a steep hinge surface and then tightly refolded about a steep ns hinge plane the latest folding produced limbs exhibiting contrasting strains the full deformation history is shared by all lithologies of the enclave two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low strain limb the supracrustal anthophyllite garnet rock experienced zircon growth at 3600 and 2700 ma in common with amphibolites these ages are interpreted to reflect metamorphism at granulite and amphibolite facies respectively consistent with independent regional evidence the crosscutting orthogneiss sheets preserve a complex history of zircon growth but only earliest cores indicate th u exchange equilibrium with the bulk rock at magmatic conditions weighted mean 207 pb 206 pb ages of cores of 3730 7 ma and 3825 6 ma date the crystallization ages of the sheets indicating that the akilia supracrustal rocks were deposited 3819 ma quartz pyroxene rocks on akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon isotope evidence for life at the time of their formation an alternative proposal is an origin by metasomatic alteration our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body metamorphic zircons give a minimum 207 pb 206 pb age of 3589 13 ma regardless of origin the two models of formation of quartz pyroxene rocks were evaluated using field petrologic and geochemical tests the distribution of the units the absence of field or petrologic indicators of metasomatism and geochemistry oxygen and nontraditional stable isotopes immobile elements and ree together support a sedimentary but not a metasomatic origin because their protolith was sedimentary the quartz pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age despite strong deformation and polymetamorphism the akilia supracrustal enclave contains information about terrestrial surficial processes at 3819 ma

preservation of carbon isotopes in kerogen from thermally altered mesoproterozoic lacustrine microbialites stable carbon isotope geochemistry is a well established and reliable tool for studying metabolisms of microbial communities in the precambrian record however the isotopic effects of high tempera

graphite and carbonates in the 3 8 ga old isua supracrustal belt southern west greenland we present a systematic study of abundance isotopic composition and petrographic associations of graphite in rocks from the ca 3 8 ga isua supracrustal belt isb in southern west greenland most of the graphite in the isb occurs in carbonate rich metasomatic rocks metacarbonates while sedimentary units including banded iron formations bifs and metacherts have exceedingly low graphite concentrations regardless of isotopic composition of graphite in metacarbonate rocks their secondary origin disqualifies them from providing evidence for traces of life stemming from 3 8 ga recognition of the secondary origin of isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3 8 ga biogenic graphite in these rocks thermal decomposition of siderite 6feco3 2fe3o4 5co2 c is the process seemingly responsible for the graphite formation the cation composition fe mg mn and ca of the carbonate minerals carbon isotope ratios of carbonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple pulses of metasomatism affected the isb causing the deposition of fe bearing carbonates and subsequent partial disproportionation to graphite and magnetite equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 c in agreement with other estimates of metamorphic temperature for the isb

contribution of siderite water interaction for the unconventional generation of hydrocarbon gases in the solimoes basin north west brazil abstract hydrocarbon gases with unconventional carbon isotopic signatures were observed in the solimoes sedimentary basin in north west brazil siderite contents measured with a new rock eval methodology in the drill cuttings samples of the famenian source rock were found to decrease with the increase of gas maturity and with the occurrence of the gas isotopic anomalies triassic diabase intrusions induced heating of the source rock which likely resulted in the gradual oxidative dissolution of siderite as suggested by the observation of etch pits on the siderite surfaces it is proposed that ferrous iron from the carbonate was involved in a redox reaction with water producing ferric iron and h 2 then reducing co 2 and yielding an inverse correlation between siderite content and gas maturity alternatively hydrogenation of highly mature kerogen by h 2 derived from siderite could explain the production of 13 c rich ch 4 mass balance considerations suggest that these mechanisms may account for a significant fraction of the hydrocarbon gases generated from the famenian source rock in the solimoes basin

carbonaceous cherts of the barberton greenstone belt south africa isotopic chemical and structural characteristics of individual microstructures abstract carbonaceous matter occurring in chert deposits of the 3 4 3 2 ga old barberton greenstone belt bgb south africa has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration here a detailed study is presented of in situ analysis of carbonaceous particles by lrs laser raman spectroscopy and sims secondary ion mass spectrometry reporting degree of structural disorder carbon isotope ratio and nitrogen to carbon ratio this combination of in situ analytical tools is used to interpret the 13 c values of only the best preserved carbonaceous remains enabling the rejection of non indigenous unmetamorphosed material as well as the exclusion of strongly hydrothermally altered carbonaceous particles raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub to lower greenschist facies metamorphic event although this identifies these organics as indigenous to the cherts it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re deposition of organics this suggest that morphological interpretation of these carbonaceous particles and in general of putative microfossils or microlaminae in hydrothermally altered early archean cherts should be made with caution a chert in the hooggenoeg formation which is older than and has been hydrothermally altered by a volcanic event 3445 ma ago contains strongly altered carbonaceous particles with a uniform n c ratio of 0 001 and a range of 13 c that is shifted from its original value cherts of the kromberg formation post date this volcanic event and contain carbonaceous particles with a n c ratio between 0 002 and 0 006 both the buck reef chert and the footbridge chert of the kromberg formation have retained fairly well preserved 13 c values with ranges from 34 to 24 and 40 to 32 respectively abiologic reactions associated with hydrothermal serpentinization of ultramafic crust such as fischer tropsch synthesis were an unlikely source for carbonaceous material in these cherts the carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material

geology of the eoarchean 3 95 ga nulliak supracrustal rocks in the saglek block northern labrador canada the oldest geological evidence for plate tectonics the earth is a unique planet which has been highly evolved diversified and complicated through geologic time and underwent many key events including giant impact magma ocean core formation large scale mantle differentiation and late heavy bombardment especially in its dawn but our knowledge of early earth is limited due to the lack of the hadean supracrustal rocks the supracrustal rocks with the eoarchean ages provide key evidence for the earth s early evolution but few supracrustal rocks have been comprehensively investigated therefore we mapped in seven areas of the saglek block northern labrador where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses early studies suggested that some of them have the mesoarchean ages because of the lack of the mesoarchean saglek dyke but we found the saglek dykes in the areas to recognize the eoarchean nulliak supracrustal rocks and uivak gneiss in all the areas recent reassessment of u pb dating and cathodoluminescence observation of zircons from the oldest suites of the uivak gneiss showed that the uivak gneiss has the eoarchean age 3 95 ga and forms the iqaluk uivak gneiss series because our geological survey clearly showed that the iqaluk uivak gneisses were intruded into the nulliak supracrustal belts the nulliak supracrustal rocks are the oldest supracrustal rock in the world the supracrustal belts consist of piles of fault bounded blocks which are composed of the ultramafic rocks mafic rocks and sedimentary rocks in ascending order similar to modern ocean plate stratigraphy ops in addition small scale duplex structures are found over the areas the presence of duplex structure and ops indicates that the 3 95 ga nulliak supracrustal belts originate from an accretionary complex the presence of the accretionary complex ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early earth

some perspectives on isotope biosignatures for early life abstract the idea of using stable isotope compositions of light elements particularly of carbon as a sign of biological activities biosignatures both present and past dates back to the dawn of stable isotope geochemistry in the 1940s in the wake of the discovery of large variations in 13 c 12 c ratios among various carbon bearing materials including plants and fossil fuels the contentious debate ensued between kalervo rankama and harmon craig in the early 1950s whether the origin of graphitic carbon in ancient rocks biogenic vs abiogenic can be uniquely identified by its isotopic compositions during the last half century great progress has been made in understanding biogeochemical processes in modern and ancient terrestrial environments rapid developments in the last decade of novel analytical techniques and the birth of new geoscience fields such as geomicrobiology and astrobiology has prompted resurgence in the application not only of conventional light stable isotopes but also of those of metal and other intermediate elements however recent debates on the origin of graphitic carbons from early archean rocks from australia and greenland are very much reminiscent of the rankama craig debate half a century ago here an attempt is made to review briefly the history of isotope biosignatures and to critically assess current criteria for early life a number of possible abiotic pathways exist that lead to the synthesis of various organic and reduced carbon compounds alkanes pah amino acids lipids graphitic carbon etc depleted in 13 c due to large kinetic isotope effects up to ca 60 it also seems very likely that the mantle contains indigenous carbons graphite diamond dissolved carbons carbide with low 13 c values 20 to 30 which have previously been considered recycled sedimentary organic carbons or surface contamination this analysis prompts us to reassess our current and future strategies for identifying early and extraterrestrial life using isotope biosignatures

evidence for biogenic graphite in early archaean isua metasedimentary rocks the archaean rocks of isua west greenland contain graphite but its origins are debated geochemical and microscopic analyses suggest that the graphite was formed from biologically formed carbon that was deposited at least 3 7 billion years ago

hydrodynamics in holocene lake mega chad holocene lake mega chad lmc was the largest late quaternary water body in africa the development of this giant paleo lake is related to a northward shift of the isohyetes interpreted as evidence for an enhanced monsoon african humid period numerous preserved coastal features have been described all around the lmc shore such features reveal the main paleo hydrodynamical tendencies in the context of a closed water body like lmc hydrodynamics are forced mainly by winds we use a three dimensional numerical model symphonie to simulate the mean hydrodynamics in lmc under both harmattan like northeasterly trade winds and monsoon like southwesterly winds forcings the northern part of lmc displays coastal features such as sand spits that are consistent with the simulations forced by harmattan like winds geomorphic features related to monsoon driven hydrodynamics are not clearly expressed they could have developed during the early stage of lmc but subsequently reworked at the time of sand spit building harmattan like driven hydrodynamics prevailed and related coastal features were preferentially preserved in the sedimentary record

carbon isotope thermometry in marbles of the adirondack mountains new york carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the adirondack mountains new york eighty nine calcite graphite pairs from the amphibolite grade nw adirondacks change systematically in temperature north westwards from 680 to 640 to 670 c over a 30 km distance reflecting transitions from amphibolite facies towards granulite facies to the north west and to the south east temperature contours based on calcite graphite thermometry in the nw adirondacks parallel mineral isograds with the orthopyroxene isograd falling above 675 c and indicate that regional metamorphic temperatures were up to 75 c higher than temperatures inferred from isotherms based on cation and solvus thermometry bohlen et al 1985 fifty five calcite graphite pairs from granulite grade marbles of the central adirondacks give regional metamorphic temperatures of 670 780 c in general agreement with cation and solvus thermometry data for amphibolite and granulite grade marbles show a 12 oo range in 13ccal and 13cgr a strong correlation between carbon isotopic composition and the abundance of graphite cgr crock indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism the trends seen in 13c vs cgr crock and 13ccal vs 13cgr could not have been preserved if significant amounts of co2 rich fluid had pervasively infiltrated the adirondacks at any time the close fit between natural data and calculated trends of 13c vs cgr crock indicates a biogenic origin for adirondack graphites even though low 13c values are not preserved in marble delamination of 17 graphite flakes perpendicular to the c axis reveals isotopic zonation with higher 13c cores these isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite samples located 2km from anorthosite show a decrease of 0 5 0 8 oo in the outer 100 of the grains while samples at distances over 8 km show smaller core to rim decreases of c 0 2 oo correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism core graphite compositions indicate contact metamorphic temperatures were 860 890 c within 1 km of the marcy anorthosite massif if samples with a significant contact metamorphic effect cal gr 3 2 oo are not included then the remaining 38 granulite facies samples define the relation 13c cal gr 3 56 106t 2 k

carbon isotopic composition of individual precambrian microfossils ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 ma bitter springs formation australia and the approximately 2100 ma gunflint formation canada the delta 13c pdb values from individual microfossils of the bitter springs formation ranged from 21 3 1 7 to 31 9 1 2 and the delta 13c pdb values from microfossils of the gunflint formation ranged from 32 4 0 7 to 45 4 1 2 with the exception of two highly 13c depleted gunflint microfossils the results generally yield values consistent with carbon fixation via either the calvin cycle or the acetyl coa pathway however the isotopic results are not consistent with the degree of fractionation expected from either the 3 hydroxypropionate cycle or the reductive tricarboxylic acid cycle suggesting that the microfossils studied did not use either of these pathways for carbon fixation the morphologies of the microfossils suggest an affinity to the cyanobacteria and our carbon isotopic data are consistent with this assignment

carbon isotopic thermometry calibrated by dolomite calcite solvus temperatures abstract the temperature dependence of carbon isotopic fractionations between calcite and graphite and between dolomite and graphite are calibrated by the calcite dolomite solvus geothermometry using marbles collected from the contact metamorphic aureole in the kasuga area central japan the carbon isotopic fractionations 13ccc gr and 13cdo gr systematically decrease with increasing metamorphic temperature the concordant relationships between the fractionations and solvus temperatures are approximately linear with t 2 over the temperature range 400 to 680 c 13ccc gr 5 6 106 t 2 k 2 4 13cdo gr 5 9 106 t 2 k 1 9 these systematic relationships between fractionation and temperature suggest that carbon isotopic equilibria between carbonates and graphite were attained in many cases the equation for the calcite graphite system has a slope steeper than bottinga s 1969 results it is however in good agreement with that of valley and o neil 1981 in the temperature range from 600 to 800 c because of the relatively high sensitivity to temperature these isotopic geothermometers are useful for determining the temperatures in moderate to high grade metamorphosed carbonate rocks

non biogenic graphite in 3 8 ga metamorphic rocks from the isua district greenland abstract a 3 8 ga old metamorphic rock that contains up to 5 5 wt reduced carbon as graphite was found in the isua district greenland this rock exhibits a strong schistosity comprises mostly talc contains high mgo 28 0 wt ni 553 ppm and cr 267 ppm and low al2o3 2 13 wt and bears a high fe 2 o 3 feo ratio 0 94 the original rock was probably a submarine ultramafic tuff the carbon content and the 13c value of the rock vary on a microscopic scale a few mm3 scale from 0 7 to 5 5 wt and from 15 to 12 respectively the second rock a quartzite contains 0 13 wt c as reduced carbon with a 13c value of 11 25 these 13c values fall within the range 27 to 10 reported by previous investigators for graphite from the isua district the 13c values of the isua graphite have been interpreted by previous investigators e g schidlowski and aharon 1992 as metamorphic modification of organic matter with initial 13c values of 30 an important implication of this model has been that the 3 8 ga ocean already supported some biological activity however an examination of the carbon content vs 13c relationship of the graphite bearing rocks from isua has revealed that the 13c value generally increases with increasing carbon contents the rocks with 13c values less than 20 contain extremely small amounts of carbon the mineralogical and elemental characteristics of the rocks of this study a thermodynamic consideration of the temperature and composition of metamorphic fluids and carbon isotope mass calculations suggest that the major component of graphite with 13c values around 12 was formed by an inorganic rather than by a biological process the graphite appeared to have formed at temperatures between 700 and 400 c by one or both of the following two processes 1 reactions between co2 and ch4 co2 ch4 2c 2h2o during cooling of fluids with a co 2 ch 4 molar ratio of 1 and or 2 reactions between the feo component in the ultramafic rock with co2 rich fluids 4 feo co2 2 fe2o3 c the 13c c values for the fluids for both i and 2 were probably between 12 and 5 suggesting that the carbon bearing fluids could have been derived from the mantle

microstructure specific carbon isotopic signatures of organic matter from 3 5 ga cherts of the pilbara craton support a biologic origin the 3 5 ga dresser formation from the north pole dome of the pilbara craton western australia contains some of the oldest evidence for life on earth here we present a detailed study of microstructure specific carbon isotopic composition of organic matter om preserved in dresser formation bedded cherts and hydrothermal chert vein using in situ secondary ion mass spectrometry sims the om in these rocks occurs mainly as clots that together with minor fine om layers and laminae are considered primary textures formed prior to host rock lithification other than rare om rich stylolites no evidence was found for later om migration beyond the micrometer scale average 13c om values in specific microstructural types range between 33 6 and 25 7 no correlation is seen between measured 13c values and h c ratios in the studied om microstructures this lack of correlation and the low metamorphic grade of the rocks studied argue against significant modification of om isotopic composition by later metamorphic alteration it is thus concluded that the range of 13c values found in the samples represents primary om isotopic variability within some individual samples variable 13c om values are correlated with specific microstructural types this observation is not consistent with solely abiotic om formation via fisher tropsch type reactions when compared with associated 13c ankerite values average 13c om values indicate c isotopic fractionation 13c ank om of 25 33 which translates to dissolved co2 om isotopic fractionation 13c co2 om of 20 30 this range of 13c co2 om is consistent with enzymatic c fixation via the calvin cycle utilized by photoautotrophs and the reductive acetyl coa pathway utilized by chemolithoautotrophs photosynthetic om formation is supported by the relatively shallow water depth inferred for the dresser environment and the restricted occurrence of stromatolites to shallow water deposits in this unit whereas chemolithosynthesis is supported by the abundance of om in sub seafloor hydrothermal chert veins the range of 13c om values observed in the samples may therefore represent the remains of different organisms utilizing different c fixation pathways other biologic effects such as the growth rate and density of microbial communities and further heterotrophic overprinting of the autotrophic biomass may have also contributed to the observed range of 13c om values

in situ iron isotope analysis of pyrites in 3 7ga sedimentary protoliths from the isua supracrustal belt southern west greenland abstract the timing of the emergence of life remains one of the principal unresolved questions in the earth sciences putative relicts of microorganisms in the eoarchean ca 3 6 3 85 ga high grade metamorphic terranes do not preserve morphological evidence for early life but some relicts can be identified by their geochemical signatures created by metabolic processes among the oldest rocks of sedimentary origin ca 3 8 ga occur in the isua supracrustal belt isb southern west greenland these have undergone metamorphism up to the amphibolite facies despite intense metamorphism the carbon isotope compositions of graphite clots from the isua metasedimentary rocks suggest biological carbon fixation and provide the oldest evidence for this biological process microbial dissimilatory iron reduction dir is considered to be an early form of metabolism the microbial dir produced fe 2 aq with a lower 56 fe value from a precursor fe 3 bearing iron mineral however 56 fe values lower than 1 are not found in sedimentary rocks prior to about 2 9 ga here we report in situ iron isotope analysis of pyrites in sedimentary rocks from the isb using a near infrared femtosecond laser ablation multicollector icp ms nir fs la mc icp ms a large variation of 56 fe values from 2 41 to 2 35 was documented from 190 points within pyrite grains from 11 rock specimens including those interpreted to be banded iron formations bifs chert amphibole rich chert quartz rich clastic sedimentary rocks mafic clastic sedimentary rocks carbonate rocks and conglomerates we found that the distribution of 56 fe values depends on the lithology whereas there is no correlation between their 56 fe values and the metamorphic grade the 56 fe values of pyrites in bifs range from 0 25 to 2 35 indicating partial oxidation in the deep ocean especially the high 56 fe values up to 2 35 suggest that the bif was formed through interaction of ferruginous seawater with a highly alkaline hydrothermal fluid under anoxic conditions pyrite grains in a conglomerate carbonate rocks mafic clastic sedimentary rocks and amphibole rich cherts show negative 56 fe values around 1 5 down to 2 41 pointing to microbial dir in the eoarchean shallow sea in addition the relatively low 56 fe values of pyrites in the shallow water sediments suggest anoxic anoxygenic photoautotrophic iron oxidation in the photic zone

the ground penetrating radar facies and architecture of a paleo spit from huangqihai lake north china implications for genesis and evolution abstract a paleo spit is identified from huangqihai lake in north china and proposed as a classic example for investigating the influence of lake level changes and storms on the construction of these bodies ground penetrating radar gpr data collected with a 400 mhz antenna are used together with sedimentologic observations from trenches to define the facies and sedimentary architecture of the spit six types of radar surface both depositional and erosional were identified radar facies bounded by radar surfaces were interpreted from reflection characteristics and termination patterns eight different radar facies were identified in the profiles and we assign these to three groups inclined horizontal and irregular the whole barrier spit system comprises spit bar and salt marsh units distinguished using gpr profiles linked radar and sedimentologic data are used to develop a model for spit bar evolution the spit was formed during a cycle of lake level change spanning 200 years the characteristics of the different building blocks washover lobes sheets and swash laminated sands within the spit bar unit and their depositional regimes are defined and interpreted based on the known lake evolution during the holocene a relative chronological framework is presented two storms in the 1880s and 1960s may have played an important role in building the spit bar

time sequence observations of wave formed sand ripples on an ocean shoreface analysis of an 18 day time lapse film record of shoreface ripple development with concurrent measurements of near bottom flow and surface waves provides new insight on equilibrium bedform conditions adjustment of ripple planform to variable hydrodynamics and ripple migration behaviour the study was conducted in approximately 10 m water depth 1 km off martinique beach on the atlantic coast of nova scotia canada under low energy summer wave conditions significant wave height and peak period during the study averaged 0 7 m and 8 s respectively with extremes up to 1 7 m and 11 s during passage of three weak weather disturbances six mutually exclusive ripple types have been defined 1 short wavelength regular ripples 2 variable bifurcated ripples 3 variable terminated ripples 4 short crested ripples 5 long wavelength regular ripples and 6 chaotic ripples ripple wavelength ranged from 0 07 m to 0 24 m and displayed a strong reynolds number dependence together with other published field data the results suggest a lower limit of 0 06 m for the wavelength of wave ripples in ocean shoreface environments ripple orientation ranged through 38 and responded rapidly to changes in wave approach direction but did not conform to the orientation of the adjacent shoreline ripples were observed to migrate both on and off shore with and against the wave advance direction at rates up to 0 1 m h 1 associated with net flows other than wave induced onshore asymmetry and mass transport migration mainly of ripple types 1 and 2 occurred during the peak of storm events but showed no obvious correlation with measured near bottom flow magnitude or direction ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight crested rippie types 1 and 5 are present disequilibrium in orientation or dimensions is expressed by increasing sinuosity bifurcation and crest termination in types 2 3 4 and 6

experimental study of igneous and sedimentary apatite dissolution control of ph distance from equilibrium and temperature on dissolution rates apatite dissolution experiments were conducted using both a fluidized bed and stirred tank reactor over a range of ph temperature solution saturation state and on non carbonated and carbonated apatite compositions igneous fluorapatite fap and sedimentary carbonate fluorapatite cfa respectively from 2 8 5 were not possible due to detection limits of the analytical techniques used in this study and the high insolubility of fap for the cfa compositions studied the dissolution rate decreased with increasing ph from 4 ph 7 during early stages of the dissolution reaction for both fap and cfa mineral components were released in non stoichiometric ratios with reacted solution ratios of dissolved ca p and ca f being greater than mineral stoichiometric ratios suggesting that ca was preferentially released compared to p and f from the mineral structure during the early stages of dissolution an increase in reacted solution ph accompanies this early elevated release of ca as the dissolution reaction proceeded to steady state dissolution became congruent when normalized to bet measured surface area fap dissolved faster from 4 ph 7 compared to cfa the apparent arrhenius activation energy ea of fap dissolution over the temperature range of 25 55 c at ph 3 0 i 0 1 and pco2 0 is 8 3 0 2 kcal mol 1 both the apparent exchange of solution h for solid bound ca at low ph in the early stage of dissolution and the ea of dissolution suggest a surface and not a diffusion controlled dissolution reaction for fap and cfa the degree of undersaturation of the solution gr with respect to fap was important in determining the dissolution rate at ph 3 0 i 0 1 and pco2 0 the dissolution rate of fap was 5 greater in the far from equilibrium region compared to the near equilibrium slope region a simple apatite weathering model incorporating the experimental results from this study was constructed and numerical calculations suggest that during the phanerozoic both the surface area of igneous rock available for weathering and the average global temperature were important factors in determining the p weathering flux from apatite dissolution it is possible that elevated global temperatures coupled with relatively high surface area of igneous rock during the early to mid paleozoic resulted in elevated p weathering fluxes which along with climatic evolutionary pressures of the neoproterozoic facilitated the radiation of multicellular organisms large scale phosphorite deposition and abundance of calcium phosphate shelled organisms during the early cambrian

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

preservation of 3 4 3 5 ga microbial biomarkers in pillow lavas and hyaloclastites from the barberton greenstone belt south africa exceptionally well preserved pillow lavas and inter pillow hyaloclastites from the barberton greenstone belt in south africa contain textural geochemical and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the archean the textures are micrometer scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures textures of similar size morphology and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites the tubes from the barberton greenstone belt were preserved by precipitation of fine grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration the presence of organic carbon along the margins of the tubes and low d 13 c values of bulk rock carbonate in formerly glassy samples support a biogenic origin for the tubes overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre metamorphic overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3 4 3 5 ga although the search for traces of early life on earth has recently intensified research has largely been confined to sedimentary rocks subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored archean biomass d 2005 elsevier b v all rights reserved

forced regressive deposits of a deglaciation sequence example from the late quaternary succession in the lake saint jean basin quebec canada differentiating between forced regressive deposits from deglacial periods in high latitude domains and forced regressive deposits from the onset of glacial periods in low latitude domains is fundamental for the accurate interpretation of glacial cycles within the geological record and then for the reconstruction of palaeogeography and palaeo climate a forced regressive deglacial sequence is documented from the lake saint jean basin quebec canada in this area the late pleistocene to holocene sediments have recorded the laurentide ice sheet retreat accompanied by the invasion of marine waters laflamme gulf from ca 129 cal kyr bp subsequently fluviodeltaic and coastal prograding wedges were deposited they followed the base level fall due to glacio isostatic rebound this succession representing a transition from glacial to post glacial periods within a previously glaciated area was investigated through recent mapping preserved landforms facies analysis and new optical stimulated luminescence and radiocarbon dates three basin scale geological sections share a common lower part made of isolated ice contact fan deposits overlying bedrock throughout the entire basin ice contact fans are capped by glacimarine muds above fluvial and coastal prograding systems were deposited and evolved through four steps i deltaic systems progressively increased in width ii coastal influence on sedimentation increased iii hydrographic drainage systems became more organised and iv deltas graded from steep gilbert delta to low angle foresets mouth bar delta deposited during the base level fall from glacioisostatic rebound the complete succession has been designated as a single falling stage system tract referred to as a deglacial falling stage system tract it is representative of a deglaciation sequence in areas previously covered by ice during glacial periods i e medium to high latitude domains diagnostic criteria are provided to identify such a deglacial falling stage system tract in the geological record which may aid identification of previously unknown glacial cycles

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

life on land in the precambrian microfossils have been discovered in cavity fill and replacement silica that occurs between chert breccia clasts in 1200 million year old paleokarst at the top of the mescal limestone central arizona and in 800 million year old paleokarst at the top of the beck spring dolomite southeastern california microbial communities on precambrian g550 million years ago land may have been extensive enough to affect weathering erosion sedimentation and geochemical processes

ripple marks indicate mid devonian paleo wind directions in the orcadian basin orkney isles scotland abstract few climate proxies provide information on paleo wind directions however fossilized bedform elements can provide insight into the direction of flow that formed them wave formed ripple marks for example which developed in shallow waters extend transversely to the wind direction in this study middle devonian paleo wind directions are reconstructed by measuring the orientation of 511 fossilized wave ripple marks in the rousay flagstone formation on the island of westray orkney scotland the orientation of ripple marks was measured in four different localities on the island a chi squared test demonstrates that these four ripple mark subsets show the same distribution of ripple mark orientation and thus indicates that the studied ripple marks display the same distribution of paleo winds two dominant ripple mark orientations were observed at all studied localities the most abundant ripple mark orientation suggests paleo winds from the present day north or south second most abundant are ripple marks that suggest paleo winds from the present day ene or wsw 70 or 250 when a 40 clockwise rotation of orkney since the middle devonian is taken into account the ripple marks suggest that the dominant wind directions were south southeast sse and north northeast nne in the tropical setting of the orcadian basin the sse wind direction is represented by the abundant ripple marks with a present day near n s orientation of the line perpendicular to the crests these ripple marks are also characterized by the largest wavelengths the abundance of the latter ripple marks with long wavelengths is the result of a lower water level and thus a larger wave exposed surface during the dry winter season from april to july which was mainly characterized by sse paleo trade winds

element mobility patterns record organic ligands in soils on early earth considerable mobilization of fe without mobilization of al in precambrian paleosols has been documented and attributed to either anoxic or ligand promoted dissolution to elucidate these mechanisms basalt was dissolved under oxic and anoxic conditions with and without citrate and the mobility of several elements was analyzed the extent of release of fe and p was minor in citrate free conditions or considerable with citrate regardless of oxygen pressure release of al was minor in all cases whereas release of cu was minor in anoxic conditions or considerable in oxic conditions release of cu was enhanced by citrate in comparison in the weathered surface of two of the oldest known basalt derived paleosols the mount roe 2 76 ga and the hekpoort 2 25 ga fe and p were considerably depleted and al retained consistent with the presence of organic ligands cu retained in the mount roe paleosol but considerably mobilized in the hekpoort paleosol documents formation under an anoxic atmosphere and an oxic atmosphere respectively as inferred by others on the basis of fe mobility the immobility of al in both paleosols is consistent with formation under conditions in which the annual volume of rainwater was lower than the topsoil pore volume mobilization of p in such paleosols developed under low rainfall conditions provides a new proxy for identification of ligands secreted by terrestrial organisms on early earth