controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

a basin redox transect at the dawn of animal life multiple eukaryotic clades make their first appearance in the fossil record between 810 and 715 ma molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation animals require oxygen to fuel their metabolism and low oxygen levels have been hypothesized to account for the temporal lag between metazoan origins and the cambrian radiation of large ecologically diverse animals here paleoredox conditions were investigated in the fifteenmile group ogilvie mountains yukon canada which hosts an 811 ma ash horizon and spans the temporal window that captures the inferred origin and early evolution of animals iron based redox proxies redox sensitive trace elements organic carbon percentages and pyrite sulfur isotopes were analyzed in seven stratigraphic sections along two parallel basin transects these data suggest that for this basin oxygenated shelf waters overlay generally anoxic deeper waters the anoxic water column was dominantly ferruginous but brief periods of euxinia likely occurred these oscillations coincide with changes in total organic carbon suggesting euxinia was primarily driven by increased organic carbon loading overall these data are consistent with proposed quantitative constraints on proterozoic atmospheric oxygen being greater than 1 of modern levels but less than present levels comparing these oxygen levels against the likely oxygen requirements of the earliest animals both theoretical considerations and the ecology of modern oxygen deficient settings suggest that the inferred oxygen levels in the mixed layer would not have been prohibitive to the presence of sponges eumetazoans or bilaterians thus the evolution of the earliest animals was probably not limited by the low absolute oxygen levels that may have characterized neoproterozoic oceans although these inferred levels would constrain animals to very small sizes and low metabolic rates

meteoric like fabrics forming in marine waters implications for the use of petrography to identify diagenetic environments petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments however on the basis of oxygen isotopic data we document similar fabrics forming in marine pore fluids in the shallow subsurface of great bahama bank therefore petrographic fabrics alone are not reliable indicators of diagenetic environments even for shallow water sediments in our study skeletal grainstones show two distinct diagenetic assemblages either dissolution of aragonitic grains and minimal cementation high permeability intervals or abundant blocky spar cement and neomorphism of aragonitic skeletal grains low permeability intervals these marine burial fabrics are present as shallow as 110 m below sea level well above the aragonite compensation depth a feature that must be considered for models of diagenesis in ancient carbonate sediments marine burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed

constraining pathways of microbial mediation for carbonate concretions of the miocene monterey formation using carbonate associated sulfate abstract carbonate concretions can form as a result of organic matter degradation within sediments however the ability to determine specific processes and timing relationships to particular concretions has remained elusive previously employed proxies e g carbon and oxygen isotopes cannot uniquely distinguish among diagenetic alkalinity sources generated by microbial oxidation of organic matter using oxygen nitrate metal oxides and sulfate as electron acceptors in addition to degradation by thermal decarboxylation here we employ concentrations of carbonate associated sulfate cas and 34scas along with more traditional approaches to determine the specific nature of concretion authigenesis within the miocene monterey formation integrated geochemical analyses reveal that at least three specific organo diagenetic reaction pathways can be tied to concretion formation and that these reactions are largely sample site specific one calcitic concretion from the phosphatic shale member at naples beach yields 34scas values near miocene seawater sulfate 22 vcdt abundant cas ca 1000 ppm depleted 13ccarb 11 vpdb and very low concentrations of fe ca 700 ppm and mn ca 15 ppm characteristics most consistent with shallow formation in association with organic matter degradation by nitrate iron oxides and or minor sulfate reduction cemented concretionary layers of the phosphatic shale member at shell beach display elevated 34scas up to 37 cas concentrations of 600 ppm mildly depleted 13ccarb 6 moderate amounts of mn ca 250 ppm and relatively low fe ca 1700 ppm indicative of formation in sediments dominated by sulfate reduction finally concretions within a siliceous host at montana de oro and naples beach show minimal cas concentrations positive 13c values and the highest concentrations of fe ca 11 300 ppm and mn ca 440 ppm consistent with formation in sediments experiencing methanogenesis in a highly reducing environment this study highlights the promise in combining cas analysis with more traditional techniques to differentiate among diagenetic reactions as preserved in the geologic record and shows potential for unraveling subsurface biospheric processes in ancient samples with a high degree of specificity

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

late holocene marine radiocarbon reservoir correction δr for the west coast of south africa in order to calibrate radiocarbon ages based on samples with a marine carbon component it is important to know the marine carbon reservoir correction or r value this study measured the r on both known age pre bomb marine shells and paired marine and terrestrial samples from two regions on the west coast of south africa the southwestern cape and namaqualand pooling the data by region produces r values that are similar enough to use a west coast weighted mean r of 146 85 14c years to correctly calibrate marine shell or mixed marine and terrestrial 14c ages there are however temporal differences in r throughout the holocene which we compare with proxy data for upwelling and sea surface temperatures

a shale hosted cr isotope record of low atmospheric oxygen during the proterozoic the emergence and expansion of animal life on earth represents a dramatic shift in the structure and complexity of the biosphere a lack of firm constraints on surface oxygen levels during the mid proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or instead simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger here we present chromium cr isotope data from marine black shales that provide evidence for minimal cr oxidation throughout the mid proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late neoproterozoic this observation requires very low background oxygen levels 1 of present atmospheric levels accepting previously proposed estimates of p o2 levels needed to induce cr isotope fractionation our data provide support for the persistence of an earth system in which baseline atmospheric p o2 would have been low enough to inhibit the diversification of animals until ca 800 ma more generally evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on earth

water worn shell and pebbles in shell middens as proxies of palaeoenvironmental reconstruction shellfish procurement and their transport a case study from the west coast of south africa abstract geoarchaeological studies today are a vital component of archaeological research the sedimentary environments of coastal settings in particular are highly dynamic and governed by a variety of factors among which changes in sea level play an important role the object of such studies often involves the study of sediments deposited by natural factors in overlapping geological and anthropogenic contexts archaeomalacological studies conducted on shell middens along the west coast of south africa in the last two decades have also identified the presence of additional natural components that became incorporated into archaeological sites through active but inadvertent human agency these sediments are relatively large particles 2 20 mm of water worn shells and water worn pebbles wwsp that became entangled among the byssus threads that rocky shore mussels use for attaching themselves to hard substrate prehistoric shellfish foraging and subsequent transport of rocky shore mussels along with their byssus contents to campsites ensured the inclusion of wwsp into archaeological middens this study shows that wwsp abundances and the proportion of their organic fraction water worn shell can be used as proxies for coastal palaeoenvironmental reconstruction and as a complement to archaeomalacological studies for inferring technologies involved in shellfish collection and transport conclusions presented here probably apply to other similarly configured shorelines and shell assemblages of comparable age from elsewhere in southern africa and beyond follow up studies from this sub region and beyond might confirm and or qualify the use of wwsp observations in other coastal settings

changes in shellfish species composition and mean shell size from a late holocene record of the west coast of southern africa abstract this paper presents and discusses the results of a systematic analysis of the shellfish assemblage recovered from pancho s kitchen midden a site located in the western cape coast of south africa several hypotheses are considered in an attempt to explain the observed changes in shellfish frequencies and shell size changes in species composition in this particular sequence are mostly attributable to changes in collecting practices shellfish gathering was generally undertaken within the mid and low intertidal zones throughout the late holocene with slightly more frequent collections within the upper subtidal in the last 600 years changes in water turbidity on the other hand seem to explain most of the variability in the mean sizes of black mussels nevertheless intense collection of large numbers of black mussels seems to have been an additional factor in bringing mussel sizes to a minimum around 3000 bp subsequent recovery of mean mussel sizes appears to be the result of several factors improvement in marine productivity and low water turbidity greater access to large individuals inhabiting the low intertidal and upper subtidal and or relatively low levels of shellfish exploitation by people changes in sea surface temperature of the magnitude reported here do not appear related to changes in species composition and shell size the comparison of this shellfish sequence with others from nearby sites and analysed in a similar fashion would be essential for the reconstruction of pre colonial shellfish gathering practices at the western cape

upper ocean oxygenation dynamics from i ca ratios during the cenomanian turonian oae 2 global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans leading to increases in organic production the degradation of which causes a further decrease in dissolved oxygen in extreme cases in the geological past this trajectory has led to catastrophic marine oxygen depletion during the so called oceanic anoxic events oaes how the water column oscillated between generally oxic conditions and local global anoxia remains a challenging question exacerbated by a lack of sensitive redox proxies especially for the suboxic window to address this problem we use bulk carbonate i ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the cretaceous oae 2 in general i ca ratios were relatively low preceding and during the oae interval indicating deep suboxic or anoxic waters exchanging directly with near surface waters however individual sites display a wide range of initial values and excursions in i ca through the oae interval reflecting the importance of local controls and suggesting a high spatial variability in redox state both i ca and an earth system model suggest that the northeast proto atlantic had notably higher oxygen levels in the upper water column than the rest of the north atlantic indicating that anoxia was not global during oae 2 and that important regional differences in redox conditions existed a lack of correlation with calcium lithium and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the i ca proxy during oae 2

carbon isotope excursions and the oxidant budget of the ediacaran atmosphere and ocean a possible global drop in marine carbon isotope values to as low as 12 peedee belemnite pdb recorded in the ediacaran shuram formation of oman has been attributed to the non steady state oxidation of oceanic dissolved organic carbon doc resulting from the rise in atmospheric oxygen to near modern values at the end of the precambrian geologic constraints indicate that the excursion lasted between 25 and 50 m y requiring a doc pool thousands of times to 10 000 times the modern inventory to conform with carbon isotope mass balance calculations for a 12 excursion at the consequent rates of doc oxidation oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of 800 k y oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the shuram excursion furthermore a doc driven excursion does not explain strong covariation between the carbon and oxygen isotope record these indicators show that negative isotope excursions recorded in the shuram and other ediacaran sections are unlikely to represent a global ocean signal

an iodine record of paleoproterozoic surface ocean oxygenation constraining oxygen levels in the early precambrian surface ocean has been a longstanding goal but efforts have been challenged by the availability of suitable proxies here we present a novel approach iodine geochemistry which broadens our perspective by providing constraints on shallow carbonate dominated marine settings iodate io 3 persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals allowing iodine to calcium ratios i ca in shallow carbonates to be used as a paleoredox indicator our data from a series of mesoarchean through paleoproterozoic carbonates deposited under shallow marine conditions reveal a progressive surface ocean oxygenation in the early paleoproterozoic these data seem to indicate that a largely anoxic surface ocean extended throughout the archean until the great oxidation event goe at ca 2 4 ga implying that previous inferences of pre goe oxygen production may reflect oxygen oases transient oxidation events or oxygen levels below those required for io 3 accumulation the data suggest formation and persistence of io 3 and consequently surface ocean oxygen concentrations of at least 1 m during the goe following the initial rise of oxygen carbonate associated iodine in globally extensive carbonate units deposited during the lomagundi positive carbon isotope excursion at ca 2 22 2 1 ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions

ca and mg isotope constraints on the origin of earth s deepest δ13c excursion abstract understanding the extreme carbon isotope excursions found in carbonate rocks of the ediacaran period 635 541 ma where 13 c of marine carbonates 13 c carb reach their minimum 12 for earth history is one of the most vexing problems in precambrian geology known colloquially as the shuram excursion the event has been interpreted by many as a product of a profoundly different ediacaran carbon cycle more recently diagenetic processes have been invoked with the very negative 13 c values of ediacaran carbonates explained via meteoric alteration late stage burial diagenesis or growth of authigenic carbonates in the sediment column thus challenging models which rely upon a dramatically changing redox state of the ediacaran oceans here we present 257 44 40 ca and 131 26 mg measurements along with mg mn and sr data from carbonates of the ediacaran aged wonoka formation fm of south australia to bring new isotope systems to bear on understanding the shuram excursion data from four measured sections spanning the basin reveal stratigraphically coherent trends with variability of 1 5 in 26 mg and 1 2 in 44 40 ca this ca isotope variability dwarfs the 0 2 0 3 change seen coeval with the permian triassic mass extinction the largest recorded in the rock record and is on par with putative changes in the 44 40 ca value of seawater seen over the phanerozoic eon changes in both isotopic systems are too large to explain with changes in the isotopic composition of ca and mg in global seawater given modern budgets and residence times and thus must be products of alternative processes relationships between 44 40 ca and sr and 26 mg and mg are consistent with mineralogical control e g aragonite vs calcite limestone vs dolostone on calcium and magnesium isotope variability the most pristine samples in the wonoka dataset preserving sr concentrations in the 1000s of ppm range and 44 40 ca values inherited from an originally aragonitic polymorph have 13 c carb of 8 to 7 thereby providing strong geochemical evidence that extremely negative 13 c carb values are primary products of the ediacaran surface environment

multiphase late quaternary aeolian sediment accumulation in western south africa timing and relationship to palaeoclimatic changes inferred from the marine record abstract extensive aeolian deposits exist along the western margin of south africa from cape town through to the namib sand sea and represent a significant palaeoenvironmental archive recording millennial scale patterns of regional and hemispheric environmental change to establish the timing of dune development this study applies optically stimulated luminescence osl dating techniques to the aeolian features of the west coast in total 35 samples were obtained from six dune cores along a north south transect extending from elands bay 32 26 s 18 14 e to kleinsee 29 14 s 16 59 e ages from these dunes exhibit five distinct peaks suggesting phases of activity deposition at 4 5 16 24 30 33 43 49 and 63 73 ka the spatial and temporal extent of the data have allowed for correlations to be made with evidence from marine cores from the southeast atlantic and a more coherent regional environmental history to be developed results suggest that 1 environmental changes in this region are strongly linked to variations in wind strength and atmospheric circulation systems during the late quaternary and 2 luminescence ages from aeolian deposits cannot necessarily be equated with periods of increased aridity with other factors such as wind strength and sediment supply being of critical importance

late quaternary dynamics of southern africa s winter rainfall zone abstract variations in the nature and extent of southern africa s winter rainfall zone wrz have the potential to provide important information concerning the nature of long term climate change at both regional and hemispheric scales positioned at the interface between tropical and temperate systems southern africa s climate is influenced by shifts in the intertropical convergence zone the westerlies and the development and position of continental and oceanic anticyclones over the last glacial interglacial cycle substantial changes in the amount and seasonality of precipitation across the subcontinent have been linked to the relative dominance of these systems central to this discussion has been the extent to which the region s glacial climates would have been affected by expansions of antarctic sea ice equatorward migrations of the westerlies more frequent intense winter storms and an expanded wrz this paper reviews the developing body of evidence pertaining to shifts in the wrz and the evolution of ideas that have been presented to explain the patterns observed dividing the region into three separate axes along the western and southern margins of the continent and across the interior into the karoo and the kalahari a range of evidence from both terrestrial sites and marine cores is considered and potential expansions of the wrz expansions are explored despite the limitations of many of the region s proxy records a coherent pattern has begun to develop of a significantly expanded wrz during phases of the last glacial period with the best documented being between 32 17 ka while more detailed inferences will require the recovery and analysis of longer and better dated records this synthesis provides a new baseline for further research in this key region

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

tracing crustal fluids applications of natural 129i and 36cl the two cosmogenic isotopes 129i and 36cl have half lives and geochemical characteristics that make their application of interest in the tracing of crustal fluids oil field brines and geothermal fluids the focus of this review is to compare 129i data from volcanic fluids with those from mud volcanoes and gas hydrate locations associated with the same subduction zone in order to demonstrate that fundamentally different hydrologic systems are present in active margins whereas 129i i ratios in volcanic fluids are site dependent and show a relation to the ages of subducting marine sediments ratios in fore arc fluids are similar in all sites investigated and are independent of the ages of the host sediments and the age of the subducting slab volcanic fluids contain iodine transported in sediments from the trench to the main volcanic zone whereas iodine in fore arc fluids is derived from organic material stored in the upper plates of subduction zones

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

west coast dune plumes climate driven contrasts in dunefield morphogenesis along the western and southern south african coasts abstract this study examines two major late quaternary coastal dune systems situated in the southwestern most extremity of africa the false bay and duinefontyn dune plumes formed in close proximity but under contrasting oceanographic regimes warm agulhas and cold benguela oceanic current systems respectively the false bay and duinefontyn dune plumes have hitherto lacked the objective numerical chronology required to realize their full potential in unraveling the complex palaeo oceanographic and palaeoclimatic history of the region here we present a dual amino acid racemizationn aar and optically stimulated luminescence osl chronological framework from the extensive three dimensional exposures in sea cliffs at false bay three generations of dune deposition corresponding with orbitally forced climatic sea level cycles mis 7 mis 5 and holocene were resolved whereas the duinefontyn plume is dominated by mis 5 and holocene sedimentation at false bay and duinefontyn glacial periods are marked by the development of mature palaeosols confirming the link between glacio eustacy and dune deposition determined previously on the west and southern coasts of south africa along the west coast dune plumes typify dunefield morphology whereas shoreline parallel dune cordons are commonly developed on the southern coast these regional variations in dune system morphogenesis are largely controlled by contrasting wind and rainfall patterns the attenuated dimensions of the mis 5 plumes at false bay and duinefontyn may indicate a weakening of the south atlantic anticyclone during the last interglacial at false bay stone artifacts illustrate a human presence in the region during mis 7

spatial and temporal trends in precambrian nitrogen cycling a mesoproterozoic offshore nitrate minimum abstract fixed nitrogen is an essential nutrient for eukaryotes as n 2 fixation and assimilation of nitrate are catalyzed by metalloenzymes it has been hypothesized that in mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal concentrations and high rates of denitrification in anoxic and episodically euxinic deep water masses restricting eukaryotes to near shore environments and limiting their evolutionary innovation to date this hypothesis has only been tested in the belt supergroup 1 4 ga with results that support an onshore offshore nitrate gradient as a potential control on eukaryote ecology here we present bulk nitrogen and organic carbon isotopic data from non isochronous cross basinal facies across the bangemall 1 5 ga and the roper 1 4 1 5 ga basins to better understand the extent and variability of onshore offshore nitrogen isotope gradients in the mesoproterozoic both basins show an average 1 2 enrichment in 15 n bulk from deep to shallow facies with a maximum range from 1 offshore to 7 5 onshore unlike the belt basin the bangemall and roper basins show some offshore 15 n bulk values that are enriched beyond the isotopic range associated with biological n 2 fixation alone this suggests a mixture of aerobic and anaerobic metabolisms offshore in shallow waters where 15 n bulk enrichment peaks an aerobic nitrogen cycle was evidently operating even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the bangemall and roper basins our data are consistent with a lateral gradient in nitrate availability within the photic zone with higher concentrations in near shore environments than offshore the variability in 15 n bulk values in each depositional environment and the consistently low 15 n bulk values from mesoproterozoic units compared to the paleoproterozoic and neoproterozoic suggest that nitrate concentrations in the global ocean were likely low this trend is now seen in all three mesoproterozoic basins so far examined and contrasts with the paleoproterozoic and neoproterozoic where nearly all 15 n bulk data plot above the n 2 fixation window thus we propose that the mesoproterozoic ocean was characterized by a nitrate minimum with the lowest concentrations in offshore environments this inference is consistent with a mesoproterozoic o 2 decline following a temporary paleoproterozoic o 2 peak and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near shore environments delaying their rise to ecological dominance until the neoproterozoic era

organically bound iodine as a bottom water redox proxy preliminary validation and application abstract carbonate associated iodine i ca has been used as a proxy of local upper ocean redox conditions and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of earth history to further explore the utility of iodine as a paleo environmental proxy we present here a new method of extracting organically bound iodine iorg from shale using volumes of samples on the order of tens of milligrams thus offering the potential for high resolution work across thin shale beds the ratio of iorg to total organic carbon i toc in modern surface and subsurface sediments decreases with decreasing bottom water oxygen which may be used to reconstruct paleo redox changes as a proof of concept we evaluate the i toc proxy in holocene sediments from the baltic sea landsort deep iodp 347 and discuss those data within a framework of additional independent redox proxies e g iron speciation and mo the results imply that i toc may be sensitive to hypoxic suboxic conditions complementary to proxies sensitive to more reducing anoxic euxinic conditions then we test the usage of i toc in sediments deposited during late cretaceous cenomanian turonian oceanic anoxic event oae 2 from 94 million years ago ma we generated i toc and iorg records from six oae 2 sections tarfaya morocco furlo central italy demerara rise western equatorial atlantic cape verde basin eastern equatorial atlantic south ferriby uk and kerguelen plateau southern indian ocean which provide a broad spatial coverage generally i toc decreases over the interval recorded by the positive carbon isotope excursion the global signature of oae 2 suggesting an expansion of more reducing bottom water conditions and consistent with independent constraints from iron speciation and redox sensitive trace metals e g mo relatively higher i toc values thus more oxic conditions are recorded at two high latitude sites for oae 2 supporting previous model simulations cgenie that indicated higher bottom water oxygen concentrations in these regions our results also indicate that organic rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory

late holocene coastal change in the southwestern cape south africa abstract radiocarbon dated elevated marine deposits and the record of anthropogenic and natural debris in archaeological deposits on the west coast of south africa indicate that significant coastal remodelling has taken place in the past 4300 years in response to sea level change geological and archaeological investigation of elevated deposits of sub fossil shell abandoned lagoons and washover bars has shown that many of these features relate to a mid holocene high stand of the sea of about 2 3 m above modern mean sea level the detailed course of local late holocene sea level change cannot be resolved from these elevated geological deposits but information from archaeological sites provides valuable indirect evidence of the effects of coastal change in response to minor fluctuations in relative sea level

an experimental approach for estimating seismic amplification effects at the top of a ridge and the implication for ground motion predictions the case of narni central italy abstract from march to september 2009 a velocimetric network was installed in narni central italy a village on the top of a limestone ridge the aim was to investigate local site effects due to the 220 m high ridge which is characterized by slopes ranging from 22 to 35 to investigate amplification without and with a reference site three stations were installed at the base of the hill and seven at the crest the network recorded 702 earthquakes many of them from the 2009 l aquila sequence to determine the dependence of amplification on the morphological features the spectra were computed for horizontal components rotated into a range of azimuths both the ratio of the horizontal to vertical component spectra and the ratio of the spectra at the ridge crest with respect to a reference station at the base of the ridge showed amplification by a factor of circa 4 5 for frequencies between 4 hz and 5 hz the highest amplifications were seen for the directions of the ground motion perpendicular to the main elongation of the ridge finally considering events with an epicentral distance less than 30 km empirical ground motion models were calibrated for maximum horizontal peak ground acceleration pga velocity and acceleration response spectra 5 damping up to 1 s to estimate the site corrective coefficients for topographic amplification the data show corrective coefficients between 0 35 and 0 48 log 10 scale amplification 2 2 3 0 for the spectral ordinates between 0 2 s and 0 3 s

pervasive aerobic nitrogen cycling in the surface ocean across the paleoproterozoic era abstract nitrogen isotope ratios in marine sedimentary rocks have become a widely used biogeochemical proxy that records information about nutrient cycling and redox conditions in earth s distant past while the past two decades have seen considerable progress in our understanding of the precambrian sedimentary nitrogen isotope record it is still compromised by substantial temporal gaps furthermore quantitative links between nitrogen isotope data marine redox conditions and nutrient availability are largely lacking in a precambrian context here we present new nitrogen isotope data from a suite of marine sedimentary rocks with ca 2 4 to 1 8 ga ages spanning the great oxidation event in the paleoproterozoic to better constrain the response of the nitrogen cycle to the first major redox transition in earth s history we further construct a simple box model to describe the major pathways that influenced the nitrogen isotope mass balance of the precambrian ocean and use this as a platform to evaluate the precambrian nitrogen isotope record within this framework we find that consistently positive nitrogen isotope values ranging from 1 1 to 7 7 across the early paleoproterozoic are strong evidence for an expansion of oxygenated surface waters since the isotopic signature of aerobic nitrogen cycling is recorded in the biomass of nitrate assimilating organisms this implicates widespread nitrate bioavailability in this time interval the decline in offshore nitrogen isotope ratios in the mesoproterozoic is consistent with the contraction of oxic waters which could have inhibited the expansion of nitrate fueled ecosystems to pelagic waters until the widespread oxygenation of the ocean in the latest neoproterozoic to early phanerozoic

coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification draken formation upper riphean svalbard the draken formation 120 250 m of northeast spitsbergen svalbard forms part of a thick upper proterozoic carbonate platform succession it consists predominantly of intraformational dolomitic conglomerates with excellent textural preservation six main lithofacies were recognized in the field quartz sandstones stromatolitic mats conglomerates with silicified intraclasts dolostone conglomerates with desiccated mudrocks oolitic pisolitic grainstones and fenestral dolostones a series of five main gradational biofacies were recognized from silicified and rare calcified microfossils biofacies 1 represents low energy subtidal benthos erect filaments and plankton acritarchs and vase shaped microfossils whereas biofacies 2 to 5 are microbial mat assemblages with filamentous mat builders and associated dwellers and washed in plankton ranging from basal intertidal to high intertidal supratidal colour values a measure of the lightness of the colour shade of sawn rock samples were quantified using a munsell chart and exhibit a pronounced variation means of major groups varying from 4 0 to 5 95 across the spectrum of subtidal to supratidal sediments as inferred from other criteria the lightening in progressively more exposed sediments is related to lowering of organic carbon contents probably mainly by oxidation six types of early cement have been recognized calcite microspar type 1 is common as a subtidal cement in many proterozoic formations whereas types 2 subtidal isopachous fringes 3 subtidal hardground dolomicrite and 4 intertidal meniscus dolomicrite are very similar to phanerozoic examples except for their dolomitic mineralogy types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena they characterize the fenestral lithofacies and compare with modern supratidal cements consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching dolomitization silicification were all significant syndepositional processes altering the original metastable carbonates the data set provides evidence for a spectrum of peritidal environments including ooid shoals protected subtidal tidal sandflats and protected carbonate mudflats different sections show a preponderance of particular facies the coastal lithofacies continuum was completely dolomitized unlike offshore to ooid shoal facies of adjacent formations dolomitization thus bears a relationship to depositional bathymetry although hydrodynamics clearly have a role the potential importance of whiting precipitation in raising mg ca in marginal marine environments is also stressed

shallow water redox conditions from the permian triassic boundary microbialite the rare earth element and iodine geochemistry of carbonates from turkey and south china redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments but their interpretation may not always be straightforward to evaluate the inherent complexities rare earths and yttrium rey iodine and major element concentrations are determined in two carbonate sections spanning the permian triassic p tr transition in demirtas turkey and cili south china we use major oxides to identify non seawater rey sources such as siliciclastics fe oxides phosphates and diagenetic fluids additionally we employ y ho ratio la anomaly and light rare earth element depletion to identify which samples preserve a seawater like reysn distribution in contrast to past interpretations we find that the p tr boundary microbialites in both sections contain rey signatures indicative of deposition in an oxic environment these boundary microbialites have their base at the extinction horizon and are widespread within the tethyan region in the cili section the underlying permian limestone also preserves a seawater like rey signature with a negative ce anomaly this indicates that the water column was oxygenated both before and after the extinction event the permian limestone in the demirtas section does not preserve a seawater like reysn distribution so the absence of a ce anomaly cannot be used to distinguish prevailing redox conditions during deposition however in these samples we find the presence of a diverse permian benthic community sufficient to identify deposition in an oxic environment the geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in south china the microbialite has been proposed as a disaster facies in part resulting from the exclusion of grazers although anoxia is one of the suggested mechanisms for the exclusion of grazers we find that it is not supported by geochemical and biotic evidence

extremely low oxygen concentration in mid proterozoic shallow seawaters abstract the mid proterozoic 1 8 0 8 ga witnessed the first appearance but unusually low diversification of eukaryotes the stagnant biotic evolution during this billion year long period commonly referred to as the boring billion was arguably ascribed to low oxygen levels in atmosphere and ocean however evidence supporting low oxygen in shallow marine environments where early eukaryotes first evolved is generally lacking or insufficient here we report rare earth element and yttrium ree y data particularly cerium ce anomalies from a suite of mid proterozoic sedimentary rocks of the north china platform the new data from north china in combination with available ce anomaly data from other proterozoic successions demonstrate that during mid proterozoic negative ce anomalies did not occur until 1 54 ga and after 1 54 ga only episodic negative ce anomalies were present in shallow water carbonates trace element enrichments u ef v ef and mo ef remained at the average continental crust level before 1 54 ga but showed a sudden increase at 1 54 ga the data suggest that oxygen concentration in shallow marine environments of the mid proterozoic ocean was extremely low probably

mg and ca isotope signatures of authigenic dolomite in siliceous deep sea sediments abstract authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the c 13 of the global dic dissolved inorganic carbon reservoir but their contributions to ancient sedimentary sections are difficult to assess in this study of authigenic dolomite from the miocene age monterey formation of offshore california mg and ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes the dolomite is a pre compaction authigenic phase that occurs as beds and nodules with c 13 ranging from 16 to 9 light c 13 values were likely acquired from the sedimentary zone of microbial sulfate reduction while heavy c 13 values were acquired from the zone of methanogenesis mg and ca isotopes are roughly anti correlated with intervals of negative c 13 associated with low mg 26 and higher ca 44 40 values the variability is observed over a wide range of length scales from 10 2 meters within individual authigenic beds nodules to 10 2 meters over the entire stratigraphic column and can be understood as the consequence of dolomite precipitation in pore fluids where mg supply is limited by diffusive transport the relationship of mg 26 and ca 44 40 to the more common stable isotope measurements of c 13 and o 18 represents a new diagenetically robust geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record

searching for an oxygenation event in the fossiliferous ediacaran of northwestern canada abstract late neoproterozoic ediacaran strata from northwestern canada provide a thick and rich sedimentological record preserving intercalated carbonates and shale extending from the 635 million year old marinoan glacial deposits up through the 541 million year old precambrian cambrian boundary this region also holds one of the classic localities for the study of early animal life with the ensuing suggestion that this temporal interval captures a gross change in the o 2 content of the earth s atmosphere to test this hypothesis and bring records of northwestern canada into line with other ediacaran fossil bearing basins we provide a detailed geochemical reconstruction from the wernecke mountains of the yukon where possible we also extend these records to the ogilvie mountains to the west and previously published data from the mackenzie mountains to the east our work in the wernecke mountains is set against a composite 13 c record for carbonate that preserves three distinct ediacaran isotope excursions the lowermost of which preserved in the gametrail formation is a putative shuram excursion equivalent what emerges from a multi proxy fe speciation sulfur isotopes major and trace element analyses reconstruction is a picture of a persistently anoxic and ferruginous ediacaran ocean notably absent is geochemical evidence for a prominent oxygenation event an expectation given the appearance of animals and large swings in 13 c the new insight gained through these data challenges the idea of an ediacaran jump in atmospheric oxygen which in turn muddles the link between animal evolution and local geochemical environments

iodine to calcium ratios in marine carbonate as a paleo redox proxy during oceanic anoxic events periods of globally distributed extreme oxygen depletion so called oceanic anoxic events oaes have been recognized in the mesozoic geological record and appear to be characteristic of episodes of extreme warmth here we explore the application of iodine calcium ratios i ca in marine carbonate as a new geochemical proxy to constrain seawater redox change and provide additional insights into the response of ocean chemistry to ancient climatic warming iodine has long been known as a redox sensitive and biophilic element mainly present as iodate and iodide in seawater iodate converting to iodide under anoxic conditions laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure likely substituting for the co 3 2 ion a fall in the i ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early toarcian and cenomanian turonian oaes is interpreted both as a response to a decrease in the iodate iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic matter burial the results suggest that i ca ratios in carbonates may be used to monitor seawater oxidation levels throughout earth history

weathering and paleosol formation in the 1 1 ga keweenawan rift abstract at 1 1 ga the north american craton began to rift resulting in flood style basaltic volcanism during the phase of active tectonism some of that basalt was weathered and redistributed by fluvial processes new observations near the northwestern shore of lake superior have also revealed the presence of seven paleosols formed on weathered immature sediments derived from the basalt that are described here for the first time only three other paleosols have previously been described from this part of the geologic record so these new paleosols represent a significant new discovery the paleosols are weakly developed and exhibit physical and chemical characteristics similar to phanerozoic entisols in contrast to many precambrian paleosols the keweenawen paleosols have been subject to minimal post burial alteration among the paleosols three distinct pedofacies are recognized 1 waterlogged gleyed fluvial proximal paleosols 2 dry fluvial distal paleosols and 3 cumulative fluvial proximal paleosols despite these facies differences using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt with one of the paleosols showing an additional extra basinal rhyolitic component by considering the mass balance behaviour of a variety of alkali and alkaline earth elements it is possible to rule out significant potassium metasomatism suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions and that they form an important new source of data about mesoproterozoic weathering

the hydrogeochemistry of early meteoric diagenesis in a holocene deposit of biogenic carbonates abstract this study presents the results of a quantitative hydrogeochemical investigation of early meteoric diagenesis in a deposit of biogenic mixed mineralic holocene carbonate sediments at ocean bight in the southeastern bahamas ground water samples were collected from a network of 27 wells arranged into 4 transects across the 1 5 km2 strand plain deposit four wells provided multi depth samples at 0 5 m intervals thus allowing for the construction of vertical chemical profiles analyses of the water consisted of temperature ph alkalinity ci ca2 mg2 sr2 so42 and 13c data derived from these analyses were used in a series of equilibri m and mass balance equations to calculate mineral saturation levels and magnitudes and rates of mineral transformation reactions interpretation of measured and calculated parameters provided a detailed picture of patterns and processes of meteoric diagenesis at ocean bight four features of meteoric diagenetic processes operating at ocean bight are distinct from processes documented at previously studied non biogenic and aragonite dominated holocene systems 1 lighter cementation with respect to magnitudes of calcite precipitation 2 diminished magnitudes of aragonite dissolution 3 elevated rates of mineral stabilization and 4 lower efficiencies of transformation reactions another distinction of this study which is not thought to be attributed solely to the presence of mixed biogenic carbonates is the isolation of transformation reactions to the vadose and upper phreatic zone this result contradicts the popularly held belief that metastable carbonate deposits undergo rapid equilibration throughout the freshwater phreatic zone this distribution of diagenetic reactions along with the analytical results characterizes a chemical environment dominated by carbonate mineral transformation reactions linked mainly to the oxidation of organic matter and not only to the relative solubility of the different carbonate minerals this finding may prove important in attempts to explain the preservation of metastable minerals in ancient geologic sequences despite repeated exposure to freshwater phreatic zones this finding also questions the assumption that the extent of paleo phreatic lenses is equal to the observed extent of freshwater phreatic alteration

local δ34s variability in 580ma carbonates of northwestern mexico and the neoproterozoic marine sulfate reservoir abstract many 34s records have been produced from carbonate associated sulfate cas in order to understand the oxidation state of the neoproterozoic oceans but interregional correlation is complicated by the absence of robust chronostratigraphic markers here a globally correlatable stratigraphic interval containing the wonoka shuram w s 13c excursion was analyzed to explore variability in the sulfur isotope record in the excursion containing units the local 34s record from multiple closely spaced sections in sonora mexico was examined to explore potential heterogeneities and then these were compared to more distant sections elsewhere in sonora the inception and isotopic minimum of the w s excursion is located in the clemente formation and coincides with an extensive oolite marker bed five sections containing this marker bed span 25 km of lateral distance exhibit significant variability in 34scas range 18 6 to 27 6 vcdt and high variability in cas concentration range 1200 ppm unlike the 34scas values cas concentrations show strong negative correlation with mn sr ratios and fecarb concentrations consistent with cas removal upon diagenetic recrystallization and an absence of an accompanying sulfur isotopic fractionation indeed samples containing low cas concentrations exhibit petrographic characteristics consistent with diagenetic recrystallization including fabric destructive neomorphism and relatively coarse crystalline textures coeval w s strata in death valley oman and the least altered sonora samples record a decrease in 34scas and an increase in cas concentration however the magnitude of the changes are distinct perhaps reflecting differential response to a transient oxidation event in contrast time equivalent facies of south china do not exhibit a decrease in 34scas nor a relative increase in cas concentrations the variability in geochemical characteristics likely developed from local and or regional marine 34ssulfate heterogeneity although neoproterozoic oceanic heterogeneity in 34ssulfate has been proposed it has not been reported on such close spatial scales in addition the existence of variability among the sonora sections in which strata were all deposited at similar depth indicates that heterogeneity could have occurred laterally finally these findings demonstrate that while cas concentrations may be drastically affected by diagenesis 34scas may retain primary signatures

the evolution of coastal barrier systems a case study of the middle late pleistocene wilderness barriers south africa abstract barrier systems contain lengthy but complex records of long term environmental fluctuations the wilderness embayment south africa contains a system of shore parallel barriers reaching up to 200 m above modern sea level this study reports the results of chronological topographical both on and off shore sedimentological and micromorphological analyses within the wilderness embayment sixty one new luminescence ages from sixteen sites in unconsolidated dunes and three separate barriers are presented which when combined with previously published luminescence ages from the area provide a high resolution chronological framework for the emplacement and evolution of the barrier system the preserved barriers have been constructed within at least the last two glacial interglacial cycles with notable phases between 241 221 ka 159 143 ka 130 120 ka 92 87 ka and post 6 ka multiple phases of barrier construction occurred during sea level highstands with sediment deposition on each individual barrier occurring over at least two interglacials holocene evolution of the system sheds light on earlier events with dune preservation occurring only during early regression from the mid holocene highstand tectonic stability at wilderness allowed glacio eustatically formed shorelines to occupy similar positions on multiple occasions this in conjunction with a relatively humid climate and a well vegetated landscape enabled deflated sediment from beaches to form dunes which stacked upon each other to form an extensive and complex vertical accretionary sequence repeated erosion and recycling of pre existing barriers as well as barrier construction on what is currently the off shore platform during still stands in sea level regressional cycles when sea levels dropped below ca 50 m from the present day has added to the complexity of the preserved terrestrial barrier record the wilderness barrier system contrasts with barriers developed elsewhere in the world where higher rates of crustal uplift have allowed preservation of a more complete and more widely spaced palaeorecord this research also shows the utility of integrating off shore topography as revealed by bathymetry with terrestrial topographic data for the better understanding of the evolution of palaeo coastlines and the preserved dune record found on present day coastal plains local variation in the topography of the continental shelf at wilderness has generated spatial and temporal complexity within the sedimentary records of individual barriers as well as having a significant influence on preservation

archaeological evidence for a possible first century ad earthquake in the necropolis of abakainon ne sicily abstract the greek necropolis of abakainon ne sicily southern italy was destroyed suddenly some time after the 2nd century bc in order to identify the destruction cause of the necropolis we analyzed geological geomorphological data and site stratigraphy evidence on the site suggests that the observed collapse and deformation may be consistent with a seismic event optically stimulated luminescence dating constrains the age of the collapse to the 1st century ad probably during the time span 14 37 ad when other nearby sites were also damaged by an earthquake as reported by historical sources this study contributes to a better characterization of earthquakes in the area during the first millennium ad and improves knowledge on historical seismicity in ne sicily

the origin of dolomites in tertiary sediments from the margin of great bahama bank abstract based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on great bahama bank we have determined three different environments of formation for the dolomites that are common throughout the pliocene and miocene parts of these cores the first environment of dolomitization occurs in association with development of nondepositional surfaces dolomite typically forms below each of these surfaces the concentration and extent of which is governed by the length of the period of nondeposition these dolomites are recognized by their association with the nondepositional surfaces characteristic positive 18o values indicative of formation from cold bottom waters and 18o and sr profiles with depth that suggest formation in the presence of diffusive temperature and sr gradients the second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes the dolomite forming here is termed background dolomite this is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space dolomitization by this mechanism uses a local source of mg2 and consequently the dolomite never constitutes more than between 5 and 10 of the sediment this type of dolomite is characterized by extremely high sr concentrations which reflect concentrations of sr2 in the pore fluids from which it formed the high concentrations of sr2 in the pore fluids arise through the continued recrystallization of metastable aragonite and high mg calcite to dolomite and low magnesium calcite driven by oxidation of organic material by sulfate not only does sulfate reduction provide an additional thermodynamic drive for recrystallization but because the absolute concentration of sr2 in the pore fluids is governed by the solubility of celestite srso4 removal of sulfate allows the sr2 ca2 ratio of the interstitial fluid to become much higher than normally encountered the final type of environment of dolomitization is associated with coarse grained reefal sediments the pervasive nature of the dolomitization and the relatively normal sr concentrations suggest the circulation of normal marine water in a relatively open system

marine organic matter cycling during the ediacaran shuram excursion ediacaran ca 635 541 ma marine carbonates capture a global 13 c carbon isotope excursion to extremely negative values 12 known as the shuram excursion se that cannot be explained by conventional mass balance scenarios furthermore the carbon isotopic variation of bulk organic matter om does not mirror that of carbonate through the excursion suggesting that the om reflects a mixture of different sources to evaluate this hypothesis we investigated thermally immature marine sedimentary rocks that record the se from the sultanate of oman compound specific carbon isotopic analyses of the extractable hydrocarbons reveal low 13 c values of long chain c 20 n alkanes and mid chain monomethyl alkanes as low as 40 such light signatures are rare in marine rocks of any age and provide evidence that the se reflects a primary carbon cycle perturbation the magnitude of the se recorded in these organic phases is smaller than observed in carbonate and implies that the primary perturbation to dissolved inorganic carbon dic was at least 5 7 and more likely 7 12 in magnitude when correcting for end member source mixing due to isotopic differences in stratigraphic patterns of the different organic compounds we propose that bulk organic carbon both bitumen and kerogen reflects source mixing between two distinct pools that previously masked the excursion in bulk 13 c org measurements om sources were derived both from autotrophs fixing 13 c depleted dic and from a less 13 c depleted heterotrophic microbial biomass feeding on a marine om pool sustained by petroleum expelled from older sedimentary om expulsion of these sedimentary fluids also helps explain both the duration and magnitude of the se

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

i ca evidence for upper ocean deoxygenation during the petm anthropogenic global warming affects marine ecosystems in complex ways and declining ocean oxygenation is a growing concern forecasting the geographical and bathymetric extent rate and intensity of future deoxygenation and its effects on oceanic biota however remains highly challenging because of the complex feedbacks in the earth ocean biota system information on past global warming events such as the paleocene eocene thermal maximum petm 55 5 ma a potential analog for present and future global warming may help in such forecasting documenting past ocean deoxygenation however is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column as yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones omzs we apply a novel proxy for paleoredox conditions the iodine to calcium ratio i ca in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans and compared our reconstruction with modeled oxygen levels the reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the atlantic indian oceans and possibly the pacific ocean as well during the petm due to vertical and potentially lateral expansion of omzs

dynamic changes in sulfate sulfur isotopes preceding the ediacaran shuram excursion large excursions in 13 c and 34 s are found in sedimentary rocks from the ediacaran period that may provide detailed mechanistic information about oxidation of earth s surface however poor stratigraphic resolution and diagenetic concerns have thus far limited the interpretation of these records here we present a high resolution record of carbon and sulfur isotopes from the khufai formation leading up to and including the onset of the shuram carbon isotope excursion we document large coherent excursions in the sulfur isotope composition and concentration of carbonate associated sulfate cas that occur both independently and synchronously with the carbon isotope excursion isotopic changes appear decoupled from major stratigraphic surfaces and facies changes suggesting regional or global processes rather than local controls our data suggest that very low marine sulfate concentrations are maintained at least through the middle khufai formation and require that the burial fraction of pyrite and the fractionation factor between sulfate and pyrite necessarily change through deposition reconciliation of simultaneous up section increases in marine sulfate concentration and 34 s cas requires the introduction of strongly 34 s enriched sulfate possibly from weathering of cryogenian and earlier ediacaran 34 s enriched pyrite our analysis of the onset of the shuram carbon isotope excursion observed in stratigraphic and lithologic context is not consistent with diagenetic or authigenic formation mechanisms instead we observe a contemporaneous negative excursion in sulfate 34 s suggesting linked primary perturbations to the carbon and sulfur isotope systems this work further constrains the size isotopic composition and potential input fluxes of the ediacaran marine sulfate reservoir placing mechanistic constraints on possible drivers of extreme isotopic perturbations during this critical period in earth history

patterns of local and global redox variability during the cenomanian turonian boundary event oceanic anoxic event 2 recorded in carbonates and shales from central italy careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic carbon deposition these comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events well documented examples are known from the mesozoic era which is characterized by episodes of widespread organic carbon deposition known as oceanic anoxic events this organic carbon burial typically leads to coeval positive carbon isotope excursions geochemical data are presented here for several palaeoredox proxies cr ti v mo zn mn fe speciation i ca and sulphur isotopes from a section exposed at furlo in the marche umbrian apennines of italy that spans the cenomanian turonian boundary here oceanic anoxic event 2 is represented by a ca 1 m thick radiolarian rich millimetre laminated organic rich shale known locally as the bonarelli level iron speciation data for thin organic rich intervals observed below the bonarelli level imply a local redox shift going into the oceanic anoxic event with ferruginous conditions i e anoxic with dissolved ferrous iron transiently developed prior to the event and euxinia i e anoxic and sulphidic bottom waters throughout the event itself pre oceanic anoxic event enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event however the greater global expanse of dysoxic to euxinic conditions during the oceanic anoxic event greatly reduced redox sensitive trace metal concentrations in seawater pyrite sulfur isotopes document a positive shift leading into the oae carbonate i ca ratios were generally low suggesting locally reduced bottom water oxygen conditions preceding the event and relatively increased o2 concentrations post event combined the furlo geochemical data suggest a redox stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event globally widespread euxinic bottom waters during the oceanic anoxic event followed by chemocline shallowing but sustained local redox stratification following the event this article is protected by copyright all rights reserved

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

the mid holocene sea level highstand at bogenfels pan on the southwest coast of namibia the radiocarbon ages of mollusc shells from the bogenfels pan on the hyper arid southern coast of namibia provide constraints on the holocene evolution of sea level and in particular the mid holocene highstand the bogenfels pan was flooded to depths of 3 m above mean sea level amsl to form a large subtidal lagoon from 7300 to 6500 calibrated radiocarbon years before present cal yr bp the mollusc assemblage of the wave sheltered lagoon includes nassarius plicatellus lutraria lutraria and the bivalves solen capensis and gastrana matadoa both of which no longer live along the wave dominated southern namibian coast the radiocarbon ages of mollusc shell from a gravely beach deposit exposed in a diamond exploration trench indicate that sea level fell to near or 1 m below its present day position between 6500 and 4900 cal yr bp the rapid emergence of the pan between 6500 and 4900 cal yr bp exceeds that predicted by glacio isostatic models and may indicate a 3 m eustatic lowering of sea level the beach deposits at bogenfels indicate that sea level rose to 1 m amsl between 4800 and 4600 cal yr bp and then fell briefly between 4600 and 4200 cal yr bp before returning to 1 m amsl since 4200 cal yr bp sea level has remained within one meter of the present day level and the beach at bogenfels has prograded seaward from the delayed arrival of sand by longshore drift from the orange river a 6200 cal yr bp coastal midden and a 600 cal yr bp midden 1 7 km from the coast indicate sporadic human utilization of the area the results of this study are consistent with previous studies and help to refine the holocene sea level record for southern africa

does the global stratigraphic reproducibility of δ13c in neoproterozoic carbonates require a marine origin a pliocene pleistocene comparison the large and systematic negative shifts in the 13 c values 12 of carbonate dominated rocks that preceded neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events these values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin distinct in magnitude and associated with recognizable stratigraphic markers such as glacial deposits in contrast diagenetic systems are commonly considered to have a more local and stochastic influence on 13 c values cores taken in quaternary carbonate platform sediments however reveal a curious similarity in magnitude thickness and core to core reproducibility where diagenetic alteration has occurred in response to sea level fall sea level changes produced similar 13 c and 18 o stratigraphic records at globally disparate locations which are unrelated to the global marine 13 c signal and bear no relation to the global carbon cycle by analogy with the pliocene pleistocene we propose that spatial reproducibility of 13 c in some neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle including diagenesis this observation does not negate the stratigraphic utility of the carbon isotopic values only the origin of the signal

recognition of earthquake related damage in archaeological sites examples from the dead sea fault zone abstract archaeological structures that exhibit seismogenic damage expand our knowledge of temporal and spatial distribution of earthquakes afford independent examination of historical accounts provide information on local earthquake intensities and enable the delineation of macroseismic zones they also illustrate what might happen in future earthquakes in order to recover this information we should be able to distinguish earthquake damage from anthropogenic damage and from other natural processes of wear and tear the present paper reviews several types of damage that can be attributed with high certainty to earthquakes and discusses associated caveats in the rare cases where faults intersect with archaeological sites offset structures enable precise determination of sense and size of slip and constrain its time among the characteristic off fault damage types i consider horizontal shifting of large building blocks downward sliding of one or several blocks from masonry arches collapse of heavy stably built walls chipping of corners of building blocks and aligned falling of walls and columns other damage features are less conclusive and require additional evidence e g fractures that cut across several structures leaning walls and columns warps and bulges in walls circumstantial evidence for catastrophic earthquake related destruction includes contemporaneous damage in many sites in the same area absence of weapons or other anthropogenic damage stratigraphic data on collapse of walls and ceilings onto floors and other living horizons and burial of valuable artifacts as well as associated geological palaeoseismic phenomena such as liquefaction land and rock slides and fault ruptures additional support may be found in reliable historical accounts special care must be taken in order to avoid circular reasoning by maintaining the independence of data acquisition methods

precambrian dolomites petrographic and isotopic evidence that they differ from phanerozoic dolomites dolomites are more common in precambrian than in phanerozoic rocks and from a petrographic isotopic study of the middle to upper proterozoic beck spring dolomite of eastern california it is suggested that precambrian dolomites are different from those of the phanerozoic petrographically all fabric details of precambrian dolomites studied are preserved just as if they were limestones and isotopically they show the same trend as limestones increasingly negative i8 o and to a lesser extent increasingly light i8 c from depositional components through early fibrous dolomite cements to later dolomite spar cements the data suggest that in precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis and this implies that precambrian seawater was different from that of the phanerozoic

holocene geoarchaeology of the sixteen mile beach barrier dunes in the western cape south africa holocene evolution and human occupation of the sixteen mile beach barrier dunes on the southwest coast of south africa between yzerfontein and saldanha bay are inferred from the radiocarbon ages of calcareous dune sand limpet shell patella spp manuports and gull dropped white mussel shells donax serra a series of coast parallel dunes have prograded seaward in response to an overall marine regression since the mid holocene with dated shell from relict foredunes indicating periods of shoreline progradation that correspond to drops in sea level at around 5900 4500 and 2400 calibrated years before the present cal yr b p however the active foredune extensively covered by a layer of gull dropped shell has migrated 500 m inland by the recycling of eroded dune sand in response to an approximate 1 m sea level rise over the last 700 yr manuported limpet shells from relict blowouts on landward vegetated dunes indicate human occupation of coastal dune sites at 6200 and 6000 cal yr b p and help to fill the mid holocene gap in the regional archaeological record coastal midden shells associated with small hearth sites exposed in blowouts on the active foredune are contemporaneous 1600 500 cal yr b p with large midden sites on the western margin of langebaan lagoon and suggest an increase in marine resource utilisation associated with the arrival of pastoralism in the western cape

the lycian sarcophagus of arttumpara pinara turkey testing seismogenic and anthropogenic damage scenarios abstract a lycian sarcophagus located in the ancient city of pinara southwest turkey shows a clockwise rotation of 6 37 with respect to its north south oriented foundation considering the seismotectonic potential of the area this rotation has been attributed to earthquake ground motion before we present a 3d model of the sarcophagus based on 11 5 million points from a 3d laser scan the sarcophagus shows a crater in the eastern side of the coffin which was most probably caused by the detonation of an explosive charge during looting as the direction of the rotation agrees with the sense of motion expected from a blast we will attempt to quantify whether the rotation has a natural seismogenic or anthropogenic cause with a rigid block model we studied the feasibility of an explosion or earthquake ground motion as the reason for the rotation of the coffin scaled recorded ground motions from local earthquakes and a strong motion record from the recent l aquila italy earthquake m w 6 3 were used to study the sarcophagus dynamic reactions the calculations show that the geometry of the structure requires large peak ground acceleration amplitudes to initiate rocking above 4 m s 2 this rocking in turn is necessary to produce rotation around the vertical axis by translational movements the size of the explosion is back calculated from the crater size and compared with duration and amplitude of an impulse necessary to rotate the coffin the small rotations resulting from all earthquake simulations and the plausible explosion size necessary to rotate the coffin by the observed amount make an explosion a much more probable cause for the rotation of the sarcophagus than an earthquake

quantitative archaeoseismological study of a roman mausoleum in pinara turkey testing seismogenic and rockfall damage scenarios abstract a roman mausoleum located in the ancient city of pinara southwest turkey shows traces of strong dynamic forces considering the seismotectonic potential of the area earthquake ground motions are a potential cause of damage on the other hand the mausoleum is located at the foot of an almost 100 m high steep cliff imposing rockfall hazard to the building we constructed a 3d model of the mausoleum based on 90 million points from nine laser scans from this model the shape and size of the construction was measured and the displacements of all visible block vertices have been derived based on the 3d model and examples of similar structures a fully reconstructed virtual discrete element model of the mausoleum with 180 individual blocks was constructed this model was exposed to earthquake ground motions and the impact of falling rocks the impact velocity of the latter was estimated with a site specific rockfall model the earthquake ground motion was estimated from local earthquake scenarios and quantified with 3d synthetic strong motion seismograms in addition analytic ground motion signals were used to study the general dynamic behavior of the mausoleum and the vulnerability of its columns comparison of the in situ measured block displacements and the damage pattern with the results of the rockfall and earthquake models clearly favor coseismic displacement as the cause of the damage a local earthquake with moment magnitude 6 3 is sufficient to produce the observed damage

holocene sea level fluctuations inferred from the evolution of depositional environments of the southern langebaan lagoon salt marsh south africa a holocene sea level curve is constructed from the facies distribution and radiocarbon ages of sediment recovered from the distal southern salt marsh of langebaan lagoon on the southwest coast of south africa calibrated radiocarbon analyses of an oyster rich bioclastic gravel indicate that the flandrian transgression flooded the lagoon to 0 3 m above present day levels by 6750 years ago 6 8 ka organic matter and shell material dated in distal lagoonal sediments indicate that sea level returned to present day levels by 4 9 ka and have since remained within 61 m of present day levels bleached shell and a hiatus in sedimentation suggest an approximate 1 m sea level lowstand between 2 5 and 1 8 ka changes in the macrobenthos assemblage since 1 5 ka that include the loss of the oysterostrea atherstonei razor clamsolen capensis brown musselarcuatula capensis and periwinkleoxystele variegata reflect loss of hard substrate decreased tidal flow velocities as reworked sands prograded into the southern la

heterogeneous redox conditions and a shallow chemocline in the mesoproterozoic ocean evidence from carbon sulfur iron relationships abstract oxygenation of earth s oceans in the proterozoic was a protracted process that began prior to the great oxidation event goe and culminated in deep ocean ventilation following neoproterozoic glacial episodes with both of these major thresholds accompanied by profound changes in the evolution of earth s biosphere our understanding of the redox structure of mesoproterozoic oceans however is considerably less certain limited proxy investigations of oceanic redox in the mesoproterozoic have focused largely on early mesoproterozoic shelf to basinal settings with comparatively little attention paid to epeiric sea environments that dominate the late mesoproterozoic sedimentary record here we report multi proxy shale redox data from epicratonic and pericratonic environments of the 1 1 ga atar and el mreiti groups taoudeni basin mauritania these strata were deposited across the west african craton during global sea level highstand and provide insight into the redox structure of mesoproterozoic epeiric seas iron speciation pyrite sulfur isotope and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments we suggest that a protracted increase in biospheric oxygenation in the late mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow nearshore areas contributing to the fragmentary pattern of early eukaryotic evolution and diversification

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

the effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies in the neoproterozoic johnnie formation death valley usa abstract carbonates within the rainstorm member in the terminal neoproterozoic johnnie formation of death valley california record a remarkable negative carbon isotope anomaly to a nadir of near 11 that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate 34 s values the carbonates including the laterally extensive johnnie oolite were deposited during marine flooding atop a sequence boundary best observed in cratonward sections the 12 c rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric o 2 which resulted in 1 the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides and 2 the ventilation of the oceans similar carbon isotope anomalies are recorded in broadly equivalent successions that post date known marinoan glacial deposits and pre date the precambrian cambrian boundary in oman india china australia and namibia supporting the view that the oxygenation event was global thus the biogeochemical anomaly post dates the last widely recognized neoproterozoic ice age but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin the radiation of a high diversity microbiota and the first appearance of large metazoan fossils ediacaran animals follow this carbon isotope anomaly suggesting that a critical threshold with respect to atmospheric o 2 had been crossed by this time a negative excursion of similar magnitude occurs in overlying strata at the precambrian cambrian boundary which likely reflects similar processes

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

investigating controls on boron isotope ratios in shallow marine carbonates abstract the boron isotope ph proxy has been widely used to reconstruct past ocean ph values in both planktic foraminifera and corals species specific calibrations are required in order to reconstruct absolute values of ph due to the prevalence of so called vital effects physiological modification of the primary environmental signals by the calcifying organisms shallow marine abiotic carbonate e g ooids and cements could conceivably avoid any such calibration requirement and therefore provide a potentially useful archive for reconstructions in deep pre cenozoic time however shallow marine abiotic carbonates could also be affected by local shifts in ph caused by microbial photosynthesis and respiration something that has up to now not been fully tested in this study we present boron isotope measurements from shallow modern marine carbonates from the bahama bank and belize to investigate the potential of using shallow water carbonates as ph archives and to explore the role of microbial processes in driving nominally abiogenic carbonate deposition for bahama bank samples our boron based ph estimates derived from a range of carbonate types i e ooids peloids hardground cements carbonate mud stromatolitic micrite and calcified filament micrite are higher than the estimated modern mean annual seawater ph values for this region furthermore the majority 73 of our marine carbonate based ph estimates fall out of the range of the estimated pre industrial seawater ph values for this region in shallow sediment cores we did not observe a correlation between measured pore water ph and boron derived ph estimates suggesting boron isotope variability is a depositional rather than early diagenetic signal for belize reef cements conversely the ph estimates are lower than likely in situ seawater ph at the time of cement formation this study indicates the potential for complications when using shallow marine non skeletal carbonates as marine ph archives in addition variability in 11b based ph estimates provides additional support for the idea that photosynthetic co2 uptake plays a significant role in driving carbonate precipitation in a wide range of shallow water carbonates

iodine records from the ediacaran doushantuo cap carbonates of the yangtze block south china abstract late neoproterozoic cap carbonates formed following the cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the snowball earth events large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post marinoan cap carbonates across the globe but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations in this study we present i ca mg profiles of four sections from a shallow to deep water transect of the basal doushantuo cap carbonates one of the most extensively studied ediacaran units in south china most samples have i ca mg values lower than 1 mol mol with a few exceptions in the shelf sections up to 2 5 mol mol all calcite samples with extremely low 13c have iodine signal below detection limit suggesting carbonate precipitation from anoxic seawater or diagenetic fluids among the four sections the slope section has the lowest i ca mg values suggesting that it may have experienced more extensive fluid buffered diagenesis and more severe loss of io3 in combination with available major trace rare earth element and isotope data the temporal i ca mg variations support freshwater mixing and deep water upwelling in proximal sites and transient oxidation during deposition of the doushantuo cap carbonates