a deep subaqueous fan depositional model for the palaeoarchaean 3 46 ga marble bar cherts warrawoona group western australia the 3 46 ga marble bar chert member of the east pilbara craton western australia is one of the earliest and best preserved sedimentary successions on earth here we interpret the finely laminated thin bedded cherts mixed conglomeratic beds chert breccia beds and chert folded beds of the marble bar chert member as the product of low density turbidity currents high density turbidity currents mass transport complexes and slumps respectively integrated into a channel levee depositional model the marble bar chert member constitutes the oldest documented deep sea fan on earth with thin bedded cherts breccia beds and slumps composing the outer levee facies tracts and scours and conglomeratic beds representing the channel systems

advancing santorini s tephrostratigraphy new glass geochemical data and improved marine terrestrial tephra correlations for the past 360 kyrs abstract the island of santorini in the aegean sea is one of the world s most violent active volcanoes santorini has produced numerous highly explosive eruptions over at least the past 360 kyrs that are documented by the island s unique proximal tephra record however the lack of precise eruption ages and comprehensive glass geochemical datasets for proximal tephras has long hindered the development of a detailed distal tephrostratigraphy for santorini eruptions in light of these requirements this study develops a distal tephrostratigraphy for santorini covering the past 360 kyrs which represents a major step forward towards the establishment of a tephrostratigraphic framework for the eastern mediterranean region we present new epma glass geochemical data of proximal tephra deposits from twelve plinian and numerous inter plinian santorini eruptions and use this dataset to establish assignments of 28 distal marine tephras from three aegean sea cores kl49 kl51 and lc21 to specific volcanic events based on interpolation of sapropel core chronologies we provide new eruption age estimates for correlated santorini tephras including dates for major plinian eruptions upper scoriae 1 80 8 2 9 ka vourvoulos 126 5 2 9 ka middle pumice 141 0 2 6 ka cape thera 156 9 2 3 ka lower pumice 2 176 7 0 6 ka lower pumice 1 185 7 0 7 ka and cape therma 3 200 2 0 9 ka but also for 17 inter plinian events older plinian and inter plinian activity between 310 ka and 370 ka documented in the distal terrestrial setting of tenaghi philippon ne greece is independently dated by palynostratigraphy and complements the distal santorini tephrostratigraphic record

mechanisms of iron oxide transformations in hydrothermal systems abstract coexistence of magnetite and hematite in hydrothermal systems has often been used to constrain the redox potential of fluids assuming that the redox equilibrium is attained among all minerals and aqueous species however as temperature decreases disequilibrium mineral assemblages may occur due to the slow kinetics of reaction involving the minerals and fluids in this study we conducted a series of experiments in which hematite or magnetite was reacted with an acidic solution under h 2 rich hydrothermal conditions t 100 250 c p h 2 0 05 5 mpa to investigate the kinetics of redox and non redox transformations between hematite and magnetite and the mechanisms of iron oxide transformation under hydrothermal conditions the formation of euhedral crystals of hematite in 150 and 200 c experiments in which magnetite was used as the starting material indicates that non redox transformation of magnetite to hematite occurred within 24 h the chemical composition of the experimental solutions was controlled by the non redox transformation between magnetite and hematite throughout the experiments while solution compositions were controlled by the non redox transformation in the first 3 days in a 250 c experiment reductive dissolution of magnetite became important after 5 days and affected the solution chemistry at 100 c the presence of maghemite was indicated in the first 7 days based on these results equilibrium constants of non redox transformation between magnetite and hematite and those of non redox transformation between magnetite and maghemite were calculated our results suggest that the redox transformation of hematite to magnetite occurs in the following steps 1 reductive dissolution of hematite to fe aq 2 and 2 non redox transformation of hematite and fe aq 2 to magnetite

frost rings in trees as records of major volcanic eruptions new data about climatically effective volcanic eruptions during the past several thousand years may be contained in frost damage zones in the annual rings of trees there is good agreement in the timing of frost events and recent eruptions and the damage can be plausibly linked to climatic effects of stratospheric aerosol veils on hemispheric and global scales the cataclysmic proto historic eruption of santorini thera in the aegean is tentatively dated to 1628 26 bc from frost ring evidence

first evidence of a distal early holocene ash layer in eastern mediterranean deep sea sediments derived from the anatolian volcanic province article i nfo a hitherto unknown distal volcanic ash layer has been detected in a sediment core recovered from the southeastern levantine sea eastern mediterranean sea radiometric stratigraphic and sedimentological data show that the tephra here termed as s1 tephra was deposited between 8970 and 8690 cal yr bp the high silica rhyolitic composition excludes an origin from any known eruptions of the italian aegean or arabian volcanic provinces but suggests a prevailing central anatolian provenance we compare the s1 tephra with proximal to medial distal tephra deposits from well known mediterranean ash layers and ash fall deposits from the central anatolian volcanic field using electron probe microanalyses on volcanic glass shards and morphological analyses on ash particles we postulate a correlation with the early holocene dikkartin dome eruption of erciyes dag volcano cappadocia turkey so far no tephra of the central anatolian volcanic province has been detected in marine sediment archives in the eastern mediterranean region the occurrence of the s1 tephra in the south eastern part of the levantine sea indicates a wide dispersal of pyroclastic material from erciyes dag more than 600 km to the south and is therefore an important tephrostratigraphical marker in sediments of the easternmost mediterranean sea and the adjacent hinterland

cellularly preserved microbial fossils from 3 4 ga deposits of south africa a testimony of early appearance of oxygenic life abstract fossil record of earliest earth s life is scant and restricted to simple kerogenous filaments spheres and lenticular microfossils which origin and taxonomic affiliation are still ambiguous here we report clusters of cell like bodies found in massive and weakly laminated black cherts of the 3 4 ga kromberg formation onverwacht group barberton greenstone belt south africa known earlier for benthic microbial mats and microfossils morphological traits and mineralization of the kromberg microfossils match those known from modern and fossil cyanobacteria micro raman and sem eds analyses showed that the cell like bodies fossilized mostly due to early mineralization with al silicates enclosing dispersed carbonaceous kerogenous matter derived from their thermally altered cell remains although in widespread opinion the early archean life is predominantly represented by benthic microbial mats the random non laminated distribution of the studied cyanobacteria like microfossils in the kromberg cherts is suggestive for probably benthic planktonic life cycle of these microbiota the paper also discusses how the morphological similarity of the kromberg fm cell like microfossils to geologically much younger and extant coccoidal cyanobacteria may influence the debate concerning the time of origin of earth s oxic atmosphere

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

u rich archaean sea floor sediments from greenland indications of 3700 ma oxygenic photosynthesis abstract 3700 ma metamorphosed pelagic shale from west greenland contains up to 0 4 wt reduced carbon with 13c values down to 25 6 pdb peedee belemnite the isotopic signature and mode of occurrence suggest that the carbon derived from planktonic organisms the pb isotopic composition shows that the shale had high primary u th this indicates that organic debris produced a local reducing environment which precipitated u transported to the site of sedimentation by oxidized ocean water the existence of highly productive plankton that fractionated c isotopes strongly and set up oxidation contrast in the environment suggests that oxygenic photosynthesis evolved before 3700 ma

stromatolite reef from the early archaean era of australia the 3 430 million year old strelley pool chert spc pilbara craton australia is a sedimentary rock formation containing laminated structures of probable biological origin stromatolites determining the biogenicity of such ancient fossils is the subject of ongoing debate however many obstacles to interpretation of the fossils are overcome in the spc because of the broad extent excellent preservation and morphological variety of its stromatolitic outcrops which provide comprehensive palaeontological information on a scale exceeding other rocks of such age here we present a multi kilometre scale palaeontological and palaeoenvironmental study of the spc in which we identify seven stromatolite morphotypes many previously undiscovered in different parts of a peritidal carbonate platform we undertake the first morphotype specific analysis of the structures within their palaeoenvironment and refute contemporary abiogenic hypotheses for their formation finally we argue that the diversity complexity and environmental associations of the stromatolites describe patterns that in similar settings throughout earth s history reflect the presence of organisms

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

reconstructing the tephra dispersal pattern from the bronze age eruption of santorini using an advection diffusion model the advection diffusion model tephra2 has been used in conjunction with the downhill simplex method dsm and one at a time oat inversion methods to reconstruct the eruption conditions and seasonality consistent with the deposit patterns from the bronze age minoan eruption of santorini we investigated three datasets representing different depositional environments proximal terrestrial distal terrestrial and deep sea core in order to determine source conditions such as plume height erupted mass and grain size and recreate the tephra fall deposit from the plinian co ignimbrite and combined eruptive phases the results of the dsm and oat method agreed adequately well with each other for erupted mass plume height and grain size distribution both approaches were able to successfully recreate the plinian deposit but estimating conditions that created the co ignimbrite and deep sea core dataset were less successful the reduced agreement is the result of the low quantity 6 to 28 deposit points and quality inconsistent deposit depths at localities adjacent to each other of the datasets and the different dynamics between co ignimbrite and plinian columns with the former not well represented in the model different sampling methods between archaeological and volcanological disciplines and post depositional processes which have acted on the tephra deposits since the bronze age can explain the discrepancy between these computed and observed deposits the seasonality of the minoan eruption was investigated by using seasonal wind profiles for winter spring summer and autumn we find that the bronze age eruption of santorini is likely to have during the spring and summer months with a main dispersal axis aligned east crete would have received very little ash fall and the eruption would not have caused much disruption to the life of the inhabitants of the island

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

marine tephra from the cape riva eruption 22 ka of santorini in the sea of marmara abstract a discrete tephra layer has been discovered in three marine sediment cores from the sea of marmara eastern mediterranean the rhyodacitic glass chemistry and the stratigraphical position suggest a santorini provenance and in particular a correlation with the marine y 2 tephra that is known from the southern aegean sea and eastern levantine basin this tephra represents the distal facies of the cape riva eruption of santorini which has been dated by 14c on land at 21 950 cal yr bp hitherto the y 2 tephra has been detected only in marine sediment cores recovered south to southeast of its volcanic source the new occurrence in the sea of marmara approximately 530 km nne of the santorini eruptive centre suggests a more north easterly dispersal of fallout products of the cape riva eruption than previously supposed

primary haematite formation in an oxygenated sea 3 46 billion years ago the timing of the origin of photosynthesis remains controversial the discovery of ancient haematite crystals that formed in a jasper formation in australia which was created in a marine setting suggests that oxygen was being produced at least locally by photosynthesis as early as 3 46 billion years ago

origin and global tectonic significance of early archean cherts from the marble bar greenstone belt pilbara craton western australia abstract five sections of bedded chert in mafic ultramafic rocks of the archean warrawoona group in the marble bar greenstone belt pilbara craton were analyzed in order to understand their depositional environment and to provide some constraints on early archean tectonics the sections are divisible into two types based on their field occurrence mineralogy and geochemistry thicker ones a and b that overlie fe rich low k tholeiites and thinner ones c1 c2 and c3 overlying komatiitic basalts the thickest ferruginous section a 45 m thick is the marble bar chert of the towers formation and is interpreted to have been an in situ precipitate derived from a high t hydrothermal solution emanating from a mid oceanic ridge mor the following geochemical features are similar to those of modern hydrothermal iron rich sediments at a mor i p v zn and y are positively correlated with fe ii a positive eu anomaly normalized to chondrite decreases from 6 6 to 1 3 and the magnitude of a negative ce anomaly decreases from 0 6 to 1 0 as ree and lree hree increase the 13 m thick b section in the apex basalt dominated by sio2 and containing significant amounts of ba up to 4330 ppm originated from a low t mor hydrothermal solution this section is characterized by an association with massive black gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center geochemical evidence from the greenstones underlying the b section indicates that they are of morb origin in contrast to a and b c1 c2 and c3 sections are intercalated conformably with komatiitic basalts of the apex basalt and are 3 6 m thick the geochemical signatures of these three sections suggest that they were most likely formed by low t weak hydrothermal activity that may have been associated with hot spot volcanism they show strong enrichment of cr and ni reflecting a significant contribution of komatiitic basaltic detritus during sedimentation the presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity among the three sections the uppermost c3 exhibits greater enrichment in zr nb hf and th and higher th sc and la yb n relative to the lower c1 and c2 sections this could mean that the depositional environment of c3 was relatively closer to a continent or to island arcs composed of granitoids and or their volcanic equivalents siliceous mudstones and sandstones of the uppermost clastic rocks t section panorama formation have geochemical signatures analogous to those of felsic plutonic volcanic rocks high th sc negative eu anomalies and high ree in some siliceous mudstones from this unit imply that differentiated granitoids or volcanic equivalents were already exposed erupted at least locally in the pilbara at this time this study shows these warrawoona group cherts were deposited in a variety of environments ranging from a mid oceanic spreading center to a convergent plate boundary via a hot spot this variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the early archean

trace element composition of single glass shards in distal minoan tephra from sw turkey volcanic glass from the seventeenth century bc c 3600 bp minoan eruption of santorini thera has been recovered from a 4 cm thick tephra deposit at golhisar golu a small lake in sw turkey analysis of single grains of this glass by laser ablation inductively coupled plasma mass spectrometry la icp ms provides for the first time an accurate determination of the trace element composition of the juvenile glass component of this eruption free from contaminants such as phenocrysts or detrital material individual glass shards 150 m show a large degree of variation in their trace element composition indicative of substantial small scale compositional heterogeneity in the magma this is a result of the proximity of crystallizing phases and provides insights into the chemical evolution of the minoan magma the range of elemental compositions displayed by individual glass shards combined with the consistency of many trace element ratios are characteristics of the minoan glass these features provide a trace element signature for the minoan tephra which will be of value in stratigraphic correlation across the eastern mediterranean

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

u th pb isotope data indicate phanerozoic age for oxidation of the 3 4 ga apex basalt abstract the occurrence of ferric oxides in archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient earth recognizing that post archean weathering may also produce oxide minerals drill cores have been used to obtain samples beneath the depth of phanerozoic weathering the first core of the archean biosphere drilling project abdp 1 documented hematite as alteration products in 3 4 ga basalts from the marble bar area of the pilbara craton nw australia and this has been used to infer the presence of an o2 bearing atmosphere in the archean it is possible however that despite recovery of samples from 100 m depth oxidation of the basalts occurred much younger than the depositional age in this study the age of oxidation of the apex basalt from the abdp 1 drill core at marble bar is constrained by u th pb geochronology lead and u concentrations of the basalts from the abdp 1 drill core vary greatly between

dendrochemical analysis of a tree ring growth anomaly associated with the late bronze age eruption of thera the most marked tree ring growth anomaly in the aegean dendrochronological record over the last 9000 years occurs in the mid 17th century bc and has been speculatively correlated with the impact of the late bronze age eruption of thera santorini if such a connection could be proved it would be of major interdisciplinary significance it would open up the possibility of a precise date for a key archaeological geological and environmental marker horizon and offer a direct tie between tree ring and ice core records some 3600 years ago a volcanic explanation for the anomaly is highly plausible yet in the absence of a scientifically proven causal connection the value of the proposed correlation is limited in order to test the hypothesis dendrochemical analysis via synchrotron radiation scanning x ray fluorescence microscopy sxfm inductively coupled plasma atomic emission spectroscopy icp aes and inductively coupled plasma mass spectroscopy icp ms was carried out on growth ring series from four trees displaying the anomaly increases of sulfur calcium and rare earth elements following the onset of altered growth plus concentration spikes of zinc and hafnium in the first affected growth ring provide promising new evidence in support of a volcanic causal factor although a volcanic association is implied the new data are not sufficient to prove a link to the exact eruption source

the environmental impact of the minoan eruption of santorini thera statistical analysis of palaeoecological data from golhisar southwest turkey eastwood w j tibby j roberts n birks h j b lamb h f 2002 the environmental impact of the minoan eruption of santorini thera statistical analysis of palaeoecological data from golhisar southwest turkey holocene 12 4 431 444

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

new and improved determinations of velocity of ice streams b and c west antarctica measurements of velocity have been made on and next to ice streams b and c west antarctica the results are more precise than previous work and constitute a 93 increase in the number of values these velocities are used to describe the confluence of flow into the ice streams and the development of fast ice stream flow the onset of fast streaming flow occurs in many separate tributaries that coalesce down glacier into the major ice streams for those inter stream ridges that have been studied the flow is consistent with steady state along ice stream b gradients in longitudinal stress offer little resistance to the ice flow the transition from basal drag control to ice shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag velocities in the trunk of ice stream c are nearly zero but those at the up glacial head are similar to those at the head of ice stream b

recognition of the minoan tephra in the acigol basin western turkey implications for inter archive correlations and fine ash dispersal here we report the recognition of the minoan tephra layer in the acigol basin western turkey which represents the most eastward occurrence of this tephra on land and the first in continental deposits out of lacustrine sediments in mainland turkey the correlation is supported by glass and mineral geochemistry which includes solution and laser ablation icp ms data for trace and rare earth elements thickness and grain size data allow the extension of already described occurrences to an area of about 359 000 km 2 from the eastern mediterranean to black sea throughout central western turkey coupling dispersal area and grain size data suggests the joint influence of high and low atmosphere dynamics in dispersal of fine ash during volcanic eruptions the thickness and lateral continuity of the minoan tephra in the acigol basin testifies to the widespread impact on the late bronze age environment of mainland turkey of this important ash deposit copyright 2013 john wiley sons ltd

human environment interaction in the hinterland of ephesos as deduced from an in depth study of lake belevi west anatolia abstract lacustrine sediments are important archives for high resolution palaeoenvironmental reconstructions of the holocene despite the density of ancient cities and settlements along the western coast of turkey the archives from coastal lakes in this area have until now not been recognized to their fullest potential and are therefore only poorly studied the exceptional geo bio archive of lake belevi is located close to the ancient city of ephesos in western turkey two sediment cores have been analysed using geochemical sedimentological microfaunal palynological and archaeoparasitological methods the in depth study of these holocene deposits is supported by a robust age depth model that used 33 radiocarbon dates and tephrochronology the results reveal the existence of a freshwater lake in the early holocene which turned brackish when the rising sea level connected it with the sea the delta evolution of the kucuk menderes led to the re establishment of a freshwater lake the natural vegetation was represented by open oak woodlands there are hints for first agricultural activities in the environs of belevi as early as 8000 cal yr bp intensive cultivation of olea is proven since 3000 cal yr bp starting during the 3rd millennium bp the human impact with enhanced deforestation activities and correlative high sedimentation rates is attested for sites such as belevi ephesos elaia pergamon and miletos for the first time tephra from the eruption of minoan santorini has been identified in the environs of ephesos this ash covered the vegetation by a thick layer wherefore low growing plants were strongly affected the comparison between the results from the quasi natural area of lake belevi and the area around the city of ephesos gives insights into the development and use of the landscapes the environmental changes as well as the duration and intensity of the human impact

influence of weathering and hydrothermal alteration on the ree and δ56fe composition of iron formation caue formation iron quadrangle brazil abstract continental margin superior type iron formation of the caue formation iron quadrangle brazil is composed of a laminated magnetite rich chemical meta mudstone that was altered during hydrothermal circulation of basinal fluids greenschist to amphibolite facies metamorphism and subsequent supergene enrichment of fe the absence of traction current deposits and storm generated structures suggests that deposition occurred on the distal shelf possibly as deep as the continental slope paragenesis based lithogeochemical analyses of two drill cores with contrasting alteration styles highlight the importance of hydrothermal metamorphic and supergene processes on the rare earth element plus yttrium ree y and 56fe compositions of iron formation the first drill core penetrates to a depth of 540 m in the ouro fino syncline ofs where lithofacies have experienced intense hydrothermal and supergene alteration the second core is from the gandarela syncline gs and contains 450 m of metamorphosed iron formation that has undergone later supergene enrichment of fe petrographically euhedral to subhedral magnetite in the ofs and gs is fabric destructive and depending on the style and degree of alteration can co occur with dolomite siderite goethite and or martite synsedimentary magnetite and other primary fe oxide phases are absent the effect of supergene enrichment and weathering is best captured along the depth profile of each core by the inverse relationship between sio2 and fe2o3 sio2 values are lowest and fe2o3 values highest near the surface the presence of goethite to 500 m records the maximum depth of these processes in the subsurface the ree y and y anomalies from the deepest and least altered lithofacies in both cores are typical of deposition in a redox stratified water column the weathered lithofacies show several changes in the ree y patterns ce anomalies and absence of y anomalies caused mainly by fluid percolation the change in fe isotopic composition with depth is more complex in drill core from the ofs negative 56fe values shift from 0 91 at depth to less negative values of 0 6 near the surface the most altered samples do not lie on this trend because they are characterized by positive values of 0 30 and 1 33 the 56fe values from the gs increase linearly from 0 11 to 1 23 towards the surface with the most negative values 0 7 and 1 16 near the top the most negative values from the ofs could reflect synsedimentary microbial dissimilatory fe reduction and the positive values from the gs may be related to partial fe2 oxidation between the hydrothermal fe source and fe oxyhydr oxides however when interpreted together with mineralogical and ree data these data likely record the various alteration processes that affected the caue iron formation such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and fe ore forming processes

tephrostratigraphy and tephrochronology in the philippi peat basin macedonia northern hellas greece abstract three tephra layers have been identified in the upper 15 m of a 190 m section of peat beneath the philippi fen they provide significant lithological and chronological markers throughout the fen and the aegean region the upper tephra pht1 consists of shards of a transparent calcalkaline felsic glass and fragments of plagioclase augite hypersthene and rare hornblende magnetite apatite and quartz peat directly beneath pht1 gave a radiocarbon approximate age of 13 000 yr the middle tephra layer pht2 which resembles pht1 in chemistry and petrography but contains more crystals and lithic fragments rests on peat dated ca 18 000 14 c yr bp the lower tephra pht3 has colorless to brown glass shards with a trachytic chemistry and a mineral assemblage of sanidine sodic plagioclase biotite aegirine augite hornblende titanite and apatite bracketing radiocarbon ages imply that pht3 accumulated about 30 000 14 c yr bp the likely ages of pht1 and pht2 together with their mineralogical and chemical characteristics suggest that these tephras came from the volcanic field of thera in the hellenic arc pht2 particularly was derived from a major known explosive eruption ca 18 000 yr bp the cape riva eruption correlative to the y 2 tephra layer evidence for pht3 suggests derivation from the campanian province of italy and correlation with the campanian ignimbrite and the y 5 ash beneath the mediterranean sea

outcrop versus core and geophysical log interpretation of mid cretaceous paleosols from the dakota formation of kansas abstract paleosols of the mid cretaceous late albian to early cenomanian dakota formation in kansas and nebraska are well exposed in road river and clay pit exposures but show systematic overprinting by modern weathering examination of deep drill cores reveals that modern weathering effects include 1 oxidation of pyrite to jarosite 2 reaction of jarosite and calcite to clear selenite and 3 oxidation of sphaerosiderite and siderite nodules to hematite surface oxidation in this region extends no deeper than 2 m from the surface hematite nodules and coatings of leaves are found in core only at one stratigraphic horizon which was not as appears in outcrop a laterite transformed matrix density and photoelectric absorption logs of cores reveal that kaolinite clays are most abundant within deeply weathered paleosols sayela pedotype at this stratigraphic level near the albian cenomanian boundary when angiosperm forests were especially diverse and lived in a subtropical humid climate other paleosols of the early cenomanian were less deeply weathered and fossil plants of this age included temperate humid angiosperm mangal and swamp woodland but conifer dominated floodplain forests this short lived mid cretaceous warm wet spike has also been recognized in other paleosols of minnesota utah nevada and new mexico and coincides with marine black shale of the breitstroffer event midcontinental marine transgression a sequence stratigraphic boundary and a marked increase in early angiosperm diversity

tephra from the minoan eruption of santorini in sediments of the black sea the explosive eruption of santorini volcano in the aegean sea about 3 300 years ago is of considerable archaeological and volcanological significance1 5 here we report the discovery of tephra from this minoan event in laminated sediments of the black sea this finding provides constraints on the distribution of debris from the eruption we estimate a minimum fallout area of 2 l06 km2 extending from the black sea in the north to the southeastern mediterranean sea the main dispersal axis trends through southern turkey in agreement with other studies of minoan tephra6 7 the tephra deposits should provide a useful reference horizon for assessing the chronology of black sea sediments which has been much debated8 15

distribution and chemical composition of tephra layers from eastern mediterranean abyssal sediments abstract most cores of late quaternary sediments from the eastern mediterranean sea contain many ash layers glass shards from discrete layers have been analysed chemically for major elements the layers from different cores have been correlated and unambiguously related to known volcanic events on the basis of the chemical data most of the layers are derived from explosive eruptions of the santorini volcanic complex other layers have their source in other volcanoes of the hellenic arc yali nisyros kos one layer originated from the campanian area italy

geochemistry of marmara lake sediments implications for holocene environmental changes in western turkey abstract beginning from the early holocene marked environmental changes have been revealed by a multi proxy study of two radiocarbon dated sediment cores from lake marmara both cores are composed of fine clastic sediments deposited over the last 1845 years the core taken from the western depocentre supplementary consists of a 20 cm thick interval which represents a time period of 10 28 to 8 28ka yrbp a desiccation event caused 6435 year hiatus in the sedimentary record the lake was established again at 1 85ka yrbp after a transitional humid phase at the beginning of the late holocene overall arid conditions were continuously effective especially the difference between the geochemical composition of early and late holocene sediments indicates distinctive environmental conditions respective humid phases took place in between 10 28 and 8 28ka yrbp early holocene and 1 85 to 1 72ka yrbp late holocene the early holocene phase is characterized by higher fluvial activity controlled by the humid climate and active tectonics right after the middle holocene droughts a short termed tendency to relatively fresh and oxic conditions prevailed however the small surface area of the lake and its shallow conditions caused fast variations of hydrology oxidation level salinity and organic productivity during the last 1850 years we suggest that a combination of the instability of the eastern mediterranean climate and local conditions is the main control on the changing paleo environment during the study period

biogeochemistry deepening the early oxygen debate the timing of the earliest production of oxygen by photosynthesis is hotly debated haematite crystals from pilbara australia may provide evidence for a deep ocean that was at least occasionally oxygenated by photosynthetic microbes 3 46 billion years ago

geochemical constraints on the genesis of the ekou banded iron formation shanxi province north china the ekou banded iron formation bif in the wutai area is hosted within the late archean baizhiyan formation the mineral assemblage is used to identify oxide and silicate facies the oxide facies is composed of magnetite and quartz and the silicate facies is characterized by the presence of silicate minerals a geochemical analysis shows that the major elemental compositions are dominantly sio2 and fe2o3t with very little al2o3 and tio2 and minor abundances of incompatible elements and transition elements these results indicate that negligible terrigenous materials were involved in the bif deposition the rare earth elements rees normalized by post archean australian shale exhibit the characteristics of light ree lree depletion heavy ree hree enrichment and positive la y and eu anomalies the y ho ratios are superchondritic these results indicate that the material that formed the ekou bif originated from the mixing of seawater and submarine hydrothermal fluids a ce anomaly deficiency and heavy fe isotope enrichment indicate that the ekou bif formed in an anoxic marine environment the 30sinbs 28 values for quartz in the ekou bif are similar to those of other bifs with distribution ranges of modern sinters and black smokers the 18ov smow distribution is similar to that of hydrothermal sedimentary siliceous rocks these results suggest that the formation of the ekou bif was closely connected to submarine volcanic exhalation activity

caldera development during the minoan eruption thira cyclades greece the well known caldera of thira santorini greece was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a ne trending line of vents before the minoan eruption of 1400 b c thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half andesitic lavas formed the overlapping shields of the north and were contemporaneous with and in many places interbedded with the southern tuff deposits although there appears to be little difference between the composition of magmas erupted differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial producing lava flows whereas in the south eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs many of these tuffs are plastered onto the walls of what appears to have been an older caldera most probably associated with an eruption of rhyodiacitic tephra 100 000 years ago the minoan eruption of about 1400 b c had four distinct phases each reflecting a different vent geometry and eruption mechanism the minoan activity was preceded by minormore eruptions of fine ash 1 the eruption began with a plinian phase from subaerial vent s located on the easternmost of the lave shields 2 vent s grew toward the sw into the flooded depression subsequent activity deposited large scale base surge deposits during vent widening by phreatomagnetic activity 3 the third eruptive phase was also phreatomagmatic and produced 60 of the volume of the minoan tuff this activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding less

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

nonredox transformations of magnetite hematite in hydrothermal systems the transformation of magnetite to hematite or hematite to magnetite in nature has generally been considered a redox reaction and linked to a specific redox state of fluid however a nonredox reaction fe2o3 hm fe2 h2o fe3o4 mt 2h may have been the principal mechanism for the transformations of iron oxides in nature especially in hydrothermal environments for example the transformation of goethite and or hematite primary precipitates to magnetite in banded iron formations bifs probably occurred through nonredox reactions with fe2 bearing hydrothermal fluids during the accumulation of a bif sequence rather than through redox reactions involving organic matter during and or after the bif deposition the proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite rich secondary ores extending the target for orebodies to much deeper zones below the paleosurface another important implication of the proposed mechanism is that the presence or absence of magnetite and or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid

hematite replacement of iron bearing precursor sediments in the 3 46 b y old marble bar chert pilbara craton australia the history of atmospheric oxygen prior to the great oxidation event 2 45 2 2 ga is not well understood hematite in the marble bar chert from a nasa funded drill hole abdp1 in the pilbara craton australia has been cited as evidence for an oxygenated ocean 3 46 b y ago however isotopic data from the same drill hole have been used to argue for an anoxic ocean it is generally agreed that the hematite is primary representing either a direct hydrothermal precipitate or a dehydration product of iron oxyhydroxides that formed during anoxygenic photosynthesis here we present new petrographic evidence from the marble bar chert in drill hole abdp1 that shows that hematite in jasper bands formed via mineral replacement reactions the hematite mostly occurs as sub micron sized inclusions within chert so called dusty hematite that are typically arranged into polygonal clusters surrounded by a rim of clear quartz resembling shrinkage structures the lateral transition from laminated chert enclosing minute inclusions of greenalite siderite and magnetite to chert dominated by dusty hematite provides evidence for in situ replacement of iron bearing minerals the presence of hematite rich bands containing octahedral crystals with residual cores of magnetite indicates that some of the hematite was derived from the replacement of magnetite this interpretation is supported by the widespread occurrence of magnetite in jasper displaying progressive stages of replacement from unaltered octahedral inclusions in quartz to hematite pseudomorphs along quartz grain boundaries the occurrence of dusty hematite in fractures sedimentary laminae and the outer margins of polygonal clusters containing greenalite is consistent with fluid mediated oxidation of iron rich precursor minerals the presence of syn sedimentary chert breccias comprising rotated fragments of laminated chert indicates that the precursor sediment was silicified shortly after deposition the abundance of dusty greenalite inclusions which are texturally the earliest components of the laminated chert suggests that the precursor sediment contained an iron rich clay mineral our results show that hematite has replaced ferrous rich minerals after deposition and provide a mechanism to explain the origin of hematite in the marble bar chert which is consistent with the origin of hematite in adjacent basalts a secondary origin for hematite invalidates arguments for an oxygen bearing ocean 3 46 b y ago and provides a viable explanation for the formation of archean jasper bands our findings show that misinterpretations about the origin of hematite in early precambrian cherts could lead to false conclusions about the chemistry of the ancient ocean and atmosphere

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

tephrostratigraphy chronology and climatic events of the mediterranean basin during the holocene an overview the identification and characterisation of high frequency climatic changes during the holocene requires natural archives with precise and accurate chronological control which is usually difficult to achieve using only 14c chronologies the presence of time spaced tephra beds in quaternary mediterranean successions represents an additional independent tool for dating and correlating different sedimentary archives these tephra layers are potentially useful for resolving long standing issues in paleoclimatology and can help towards correlating terrestrial and marine paleoclimate archives known major tephras of regional extent derive from central and southern italy the hellenic arc and from anatolia a striking feature of major holocene tephra deposition events in the mediterranean is that they are clustered rather than randomly distributed in time several tephra layers occurred at the time of the s1 sapropel formation between c 8 4 and 9 0 ka bp mercato gabellotto fiumebianco e1 cappadocia and ot

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

episodic negative anomalies of cerium at the depositional onset of the 2 65 ga itabira iron formation quadrilatero ferrifero of minas gerais brazil abstract magnetite bands in sericitic phyllite of the batatal formation mark the transition to the overlying 2 65 ga itabira iron formation or caue itabirite in the quadrilatero ferrifero of minas gerais brazil the magnetite bands have low non crustal th u ratios suprachondritic y ho ratios and sporadic anomalies of ce that are characterised as truly negative the data point to mild oxygenation of sea water at the depositional onset of the itabira iron formation some 200 ma before the great oxidation event

the use of chromium reduction in the analysis of organic carbon and inorganic sulfur isotope compositions in archean rocks one of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well preserved samples although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred alteration due to weathering can be avoided by working on drill core samples this implies however that the amount of sample is limited which may in turn restrain the number of possible chemical and isotopic analyses that can be performed in order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction for later sulfur isotope analyses is also suitable to decarbonate samples for later organic carbon isotope analyses in the case of carbonated rocks both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time in addition it allows both organic carbon toc and 13c and sulfur isotope composition measurements to be performed on the exact same powder which is essential when trying to understand couplings between s and c cycles in heterogeneous samples we thus tested the efficiency of the acidic chromium solution commonly used to extract sulfur from sulfide for sample decarbonation on various archean rocks our results show that no significant carbon isotope fractionation is caused by this new decarbonation protocol even for the samples with low organic carbon content the chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record at least when the rock samples have experienced low greenschist facies metamorphism further tests will be needed to verify if this protocol can also be used for less mature organic matter

the discovery of santorini minoan tephra in western turkey dispersal of volcanic ash from the violent bronze age minoan eruption of the santorini volcano in the southern aegean has been the subject of much research1 6 a sediment core taken from a small mountain lake in western turkey contains a layer of tephra from the minoan eruption the santorini origin is established by refractive index major glass chemistry and stratigraphic position the lake is located north of the recognized dispersal area for the tephra this deposit provides evidence for dispersal of the minoan tephra well to the northeast of santorini and implies that the west coast of asia minor must have experienced heavy tephra fallout from the bronze age eruption the minoan tephra has not previously been identified from sites in turkey

discovery of minoan tsunami deposits the hellenic arc is a terrane of extensive quaternary volcanism one of the main centers of explosive eruptions is located on thera santorini and the eruption of the thera volcano in late minoan time 1600 1300 b c is considered to have been the most significant aegean explosive volcanism during the late holocene the last eruptive phase of thera resulted in an enormous submarine caldera which is believed to have produced tsunamis on a large scale evidence suggesting seawater inundation was found previously at some archaeological sites on the coast of crete however the cause of the tsunami and its effects on the area have not been well understood on the aegean sea coast of western turkey didim and fethye and crete gouves we have found traces of tsunami deposits related to the thera eruption the sedimentological consequences and the hydraulics of a thera caused tsunami indicate that the eruption of thera volcano was earlier than the previous estimates and the tsunami did not have disruptive influence on minoan civilization

fe isotopes of a 2 4 ga hematite rich if constrain marine redox conditions around the goe abstract the hematite and manganese rich hotazel iron formation griqualand west basin south africa was deposited at a key moment in time close to the goe between 2 4 and 2 3 ga it stratigraphically overlies the ongeluk formation comprising thick flood basalts which in turn interfinger with and cover the makganyene formation diamictites the inferred remnants of the first paleoproterozoic snowball earth interval no extensive research has been conducted to date on the basal part of the hotazel formation due to poor exposure though it constitutes an important link between a period of large scale ice cover extensive volcanism and the onset of atmospheric oxygenation here we present a detailed petrographic geochemical and fe isotope study of a roughly 3 metre long drill core exposing the ongeluk to hotazel contact our results show that after the cessation of ongeluk volcanism primary precipitation of fe iii oxyhydroxides from the photic surface zone of the original basin became the dominant sedimentation mechanism negative 56fe values between 0 26 and 0 50 in micro drilled hematite rich chert indicate that surface water 56fe compositions at the time of deposition were depleted yet 56fe and bulk rock fe mn values are still substantially higher 1 2 than those reported higher up in the mn rich layers of the hotazel sequence suggesting that redox potentials were still comparatively limited during the earliest stages of the hotazel depositional environment the base of the hotazel formation thus forms a transitional interval between precipitation from essentially ferruginous seawater as recorded in the ghaap group bifs and from isotopically and chemically highly evolved surface waters as demonstrated by the hotazel mn rich layers in the absence of fe ii as a strong reducing agent large volumes of photosynthetic oxygen may have eventually escaped into the atmosphere leading to the onset of atmospheric oxygenation our results thus contradict previous models that place the onset of the goe before the hotazel formation concurrent with and mechanistically linked to the makganyene and ongeluk events instead we show that the hotazel basal sediments are still pre goe consistent with their circa 2 4 ga age and the continuation of mif s higher in the stratigraphic record

early earth microbes and the rise of oxygen reconstructions of atmospheric chemistry and microbial life early in the earth s history have been contentious observations increasingly point to the evolution of complex and variable environments earlier in time

irish tree rings santorini and volcanic dust veils there has recently been renewed interest in the dating of the violent eruption of the aegean island of santorini in the second millennium bc both by its possible effects on tree ring growth in the united states1 suggesting a date of 1628 1626 bc and by acidity peaks in ice cores from south greenland2 suggesting 1645 bc we now show that oak trees growing on bogs in northern ireland produce significant concentrations of extremely narrow rings within a few periods less than 20 years long and that these periods correspond to the dates suggested by other methods for major volcanic eruptions in particular one of them corresponding to a short period beginning in 1628 bc was probably caused by santorini this date is qualitatively better than those derived from carbon 14 or ice cores because it is based on an absolute tree ring chronology

experimental evidence for non redox transformations between magnetite and hematite under h2 rich hydrothermal conditions abstract transformations of magnetite fe ii fe 2 iii o 4 to hematite fe 2 iii o 3 and vice versa have been thought by many scientists and engineers to require molecular o 2 and or h 2 thus the presence of magnetite and or hematite in rocks has been linked to a specific oxidation environment however the availability of reductants or oxidants in many geologic and industrial environments appears to have been too low to account for the transformations of iron oxides through redox reactions here we report the results of hydrothermal experiments in mildly acidic and h 2 rich aqueous solutions at 150 c which demonstrate that transformations of magnetite to hematite and hematite to magnetite occur rapidly without involving molecular o 2 or h 2 fe 3 o 4 mt 2h aq fe 2 o 3 hm fe aq 2 h 2 o the transformation products are chemically and structurally homogeneous and typically occur as euhedral single crystals much larger than the precursor minerals this suggests that in addition to the expected release of aqueous ferrous species to solution the transformations involve release of aqueous ferric species from the precursor oxides to the solution which reprecipitate without being reduced by h 2 these redox independent transformations may have been responsible for the formation of some iron oxides in natural systems such as high grade hematite ores that developed from banded iron formations bifs hematite rich deposits formed on mars corrosion products in power plants and other industrial systems

identification and dating of tephra layers from quaternary sedimentary sequences of inner anatolia turkey abstract results presented here are part of a research programme to investigate the environmental evolution in central anatolia during the pleistocene in this study tephra are used as chronostratigraphical markers between lacustrine sequences studied in cores or in sections surrounding eruptive centres of the central anatolian province at different locations in the konya plain and cappadocia tephra have been identified characterized and linked to eruptions of the nearby volcanoes seven different tephra have been clearly identified in lacustrine sequences of the konya plain 14 c dates on shells or organic matter as well as 230 th 234 u ages on gypsum give a chronological framework to the tephra layers in addition the study of the sediment fill of the eski acigol maar near nevsehir has allowed the recognition of several eruptions during late glacial and holocene in the vicinity of acigol the results show the first tephra to be identified in lacustrine sequences of anatolia they also show the cappadocian volcanism to be very young 35 ka this young volcanism is often of a basaltic type and does not include thick differenciated pyroclastic deposits

changes in the west antarctic ice sheet the portion of the west antarctic ice sheet that flows into the ross sea is thinning in some places and thickening in others these changes are not caused by any current climatic change but by the combination of a delayed response to the end of the last global glacial cycle and an internal instability the near future impact of the ice sheet on global sea level is largely due to processes internal to the movement of the ice sheet and not so much to the threat of a possible greenhouse warming thus the near term future of the ice sheet is already determined however too little of the ice sheet has been surveyed to predict its overall future behavior

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

biological fe oxidation controlled deposition of banded iron formation in the ca 3770 ma isua supracrustal belt west greenland the redox balance of the archean atmosphere ocean system is among the most significant uncertainties in our understanding of the earliest history of earth s surface zone most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca 2 45 ga after the great oxidation event but there is less agreement on when o2 production began and how this may have been consumed by reduced species such as fe ii in the oceans the fe redox cycle through time has been traced using banded iron formations bifs and fe isotopes are increasingly used to constrain the conditions of earth s paleoenvironments including the pathways of formation of bifs iron isotope analyses of bifs from the 3 7 to 3 8 ga isua supracrustal belt isb obtained by micro sampling of magnetite rich layers and conventional analysis as well as by in situ femtosecond laser ablation fs la icp ms indicate a consistently narrow range of non zero 56fe values analysis of magnetite by fs la icp ms allows for precise and accurate micron scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry sims analyses magnetite 56fe values range from 0 4 to 1 1 among different bands but within individual layers magnetite grains are mostly homogeneous although these bifs have been metamorphosed to amphibolite facies the metamorphism can neither explain the range in fe isotope compositions across bands nor that between hand samples the isotopic compositions therefore reflect primary low temperature sedimentary values the positive 56fe values measured from the isb magnetites are best explained by deposition of fe iii oxides produced by partial oxidation of fe ii rich ocean water a dispersion reaction model which accounts for rates of hydrothermal fe ii aq input rates of oxidation and rates of fe oh 3 settling suggests exceptionally low o2 contents 0 001 of modern o2 contents in the photic zone such low levels suggest an anoxygenic pathway is more likely and the data can be well modeled by anoxygenic photosynthetic fe ii oxidation comparison of the fe isotope data from the isua bifs with those from the 2 5 ga bifs from the hamersley and transvaal basins australia and south africa respectively suggests a striking difference in fe sources and pathways the 2 5 ga magnetite facies bifs of australia and south africa have 56fe values that range from 1 2 to 1 2 over small scales and are on average close to 0 which is significantly lower than those reported here from the isua bifs the wide range in fe isotope compositions for the hamersley and transvaal bifs in concert with c and o isotope data have been interpreted to reflect bacterial dissimilatory fe iii reduction dir the absence of low 56fe values in the isua bifs as well as the lack of fine scale isotopic heterogeneity may indicate formation prior to widespread dir

post depositional ree mobility in a paleoarchean banded iron formation revealed by la ce geochronology a cautionary tale for signals of ancient oxygenation abstract precambrian banded iron formations bif are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater here we investigated trace element signatures in bif of the 3 22 ga moodies group barberton greenstone belt south africa which are interbedded with near shore siliciclastic sedimentary rocks and represent one of the oldest known shallow water occurrences of bif unusual rare earth element ree signatures notably with pronounced negative ce anomalies in shale normalized spectra have been previously reported for chemical sediments of the moodies group which we confirm here through an expanded dataset for moodies bif spanning three different localities we find negative ce anomalies as low as 0 2 ce ce that are associated with unusual enrichment of lree relative to hree in the sample set while total ree abundances and certain ree features appear strongly related to the concentration of detrital indicators e g zr and are likely primary other features notably lree enrichment cannot be explained as a primary feature of the sediment this is better explained by later addition of ree from a lree enriched but ce depleted fluid that generated the significant negative ce anomalies observed in surface samples of moodies group bif this ree addition event influenced both sm nd and la ce isotope systematics the latter yielding an isochron of 60 32 ma thus constraining the timing of emplacement of the negative ce anomalies to the past 100 ma possibly upon surface exposure of the barberton greenstone belt to wetter conditions during the cenozoic our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post depositional modification than previously thought and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of earth surface oxygenation

hematite formation by oxygenated groundwater more than 2 76 billion years ago geoscientific drilling in the marble bar area of the pilbara craton western australia resulted in the discovery of locally abundant hematite in archean basalts 200 m below the present land surface the hematized basalts occurring along a bedding parallel shear zone are cross cut by pyrite veinlets 3 mm in width and contain euhedral pyrite grains 10 500 m in diameter with sharp crystal edges indicating that the hematite formed before the pyrite we have dated the pyrite in the veinlets at 2 763 0 016 ga using the re os method therefore the hematite formed prior to 2 763 ga the basalts containing the hematite belong to the apex basalt of the warrawoona group and were erupted onto the archean seafloor at 3 46 ga due to 2 9 ga orogenic deformation and subsequent deep erosion the apex basalt was exposed at the surface of a continental landmass prior to 2 77 ga sometime in the period between 2 9 ga and 2 77 ga the basalt section we describe was less than 200 m below the late archean land surface and within range of groundwater percolation through the shear zone in the basalts geological mineralogical and geochemical lines of evidence strongly suggest that the infiltration of o2 rich groundwater through the bedding parallel shear in the basalts formed hematite prior to 2 76 ga and hence oxygenated surface environments at least localized and or short lived emerged more than 300 million years before the widely accepted great oxidation event during 2 45 and 2 32 ga

evidence for free oxygen in the neoarchean ocean based on coupled iron molybdenum isotope fractionation abstract most geochemical proxies and models of atmospheric evolution suggest that the amount of free o2 in earth s atmosphere stayed below 10 5 present atmospheric level pal until the great oxidation event goe that occurred between 2 2 and 2 4 ga at which time free o2 in the atmosphere increased to approximately 10 1 to 10 2 pal although photosynthetically produced o2 oases have been proposed for the photic zone of the oceans prior to the goe it has been difficult to constrain absolute o2 concentrations and fluxes in such paleoenvironments here we constrain free o2 levels in the photic zone of a late archean marine basin by the combined use of fe and mo isotope systematics of ca mg carbonates and shales from the 2 68 to 2 50 ga campbellrand malmani carbonate platform of the kaapvaal craton in south africa correlated fe and mo isotope compositions require a key role for fe oxide precipitation via oxidation of aqueous fe ii by photosynthetically derived o2 followed by sorption of aqueous mo to the newly formed fe oxides a dispersion reaction model illustrates the effects of fe oxide production and mo sorption to fe oxides and suggests that a few to a few tens of m free o2 was available in the photic zone of the late archean marine basin consistent with some previous estimates the coupling of fe and mo isotope systematics provides a unique view into the processes that occurred in the ancient shallow ocean after production of free o2 began but prior to oxygenation of the deep ocean or significant accumulation of free o2 in the atmosphere these results require oxygenic photosynthesis to have evolved by at least 2 7 ga and suggest that the neoarchean ocean may have had a different oxygenation history than that of the atmosphere the data also suggest that the extensive iron formation deposition that occurred during this time was unlikely to have been produced by anoxygenic photosynthetic fe ii oxidation finally these data indicate that the ocean had significant amounts of o2 at least 150 myr prior to previously proposed whiffs of o2 at the archean to proterozoic transition

high altitude plio quaternary fluvial deposits and their implication on the tilt of a horst western anatolia turkey abstract this study investigates the origin and regional tectonic implications of high altitude plio quaternary fluvial deposits developed over the bozdag horst which is an important structural element within the horst graben system of western anatolia turkey a total of 23 deposits occur near the modern drainage divide comprising fluvial to occasionally lacustrine deposits the deposits are all elongated in n s direction with a width length ratio of 1 10 the largest of them is of 13 km in length with a maximum observable thickness of about 100 110 m morphological lithological deformational characteristics of these deposits and the drainage system of the area all suggest that the deposits were formed due to uplift and southward tilting of the bozdag horst this tilting which is estimated as 1 2 to 2 2 caused accumulation of the stream load along channels flowing from south to north all the deposits were later dissected by the same streams with the exception of one deposit which still preserves its original lake form these deposits are of quaternary age which corresponds to the latest n s directed extensional tectonic phase in the region