nitrogen cycle in the late archean ferruginous ocean abstract the hamersley group comprises a late archean sedimentary succession which is thought to record the prelude to the atmospheric oxygenation in the paleoproterozoic the so called great oxidation event goe at 2 4 ga we studied drill core samples of sedimentary rocks from the upper mount mcrae shale and brockman iron formation deposited before the goe at 2 5 ga in order to characterize the environments and ecosystems prevailing during their deposition the rocks from the mount mcrae shale and brockman iron formation represent respectively proximal euxinic conditions and distal ferruginous depositional environments thus providing an opportunity to examine lateral variability in the open marine basin we analyzed the concentration and isotopic composition of carbon in carbonate and organic matter bulk nitrogen content and its isotopic composition as well as major element concentrations the 13 c carb values and carbonate content range from 3 2 to 10 7 and 0 1 to 58 wt respectively organic carbon content also varies over a large range from 0 05 to 4 6 wt with a near constant 13 c org value of 28 7 0 8 negative 13 c org excursions down to 31 are generally correlated with high organic matter content bulk nitrogen shows highly variable concentration between 1 3 and 785 ppm and 15 n values between 0 4 and 13 4 the 13 c carb values reflect a diagenetic carbonate origin with negative values typical for fe rich carbonates formed by organic matter mineralization with ferric oxyhydroxides in contrast 13 c org and 15 n values record primary isotope signatures derived from ancient living organisms the relatively constant 13 c org values at around 28 7 are interpreted as reflecting photoautotrophs utilizing a large pool of dissolved inorganic carbon inverse stratigraphic co variation between 15 n and 13 c carb values was observed for the brockman iron formation we propose that n and c biogeochemical cycles were coupled by fe redox cycling in the water column and in sediments of the late archean ocean several models for biogeochemical n cycling linked to the redox structure of the water column are considered under fully anoxic conditions the dominant n species available for assimilation by micro organisms in the photic zone could be ammonium nh 4 highly positive 15 n values would reflect the assimilation of nh 4 enriched in 15 n by partial oxidation to nitrite followed by quantitative removal of the produced nitrite by denitrification or anamox processes ammonium oxidation could have been driven by i o 2 produced locally via oxygenic photosynthesis or ii microbial oxidation utilizing fe iii oxyhydroxides formed in the water column under redox stratified conditions n assimilated by primary producers could have been in the form of no 3 based on modern and phanerozoic analogs the positive 15 n values would have resulted in this case from partial denitrification of no 3 coupled to anaerobic microbial oxidation of fe ii to fe iii we conclude that similar positive 15 n signatures may record very different n biogeochemical cycles under anoxic stratified and fully oxic conditions in the ocean interpretation of the n isotopes in terms of n biogeochemical cycle thus requires independent constraints on the redox structure of the ocean

nitrogen isotopic composition of ammoniated phyllosilicates case studies from precambrian metamorphosed sedimentary rocks ammonium concentrations in phyllosilicates from archean and proterozoic schists and carbonates were evaluated by fourier transform infrared microspectrometry aftir the y 15 nair values of structural nitrogen in these phases were subsequently measured by laser mass spectrometry a rapid estimate of the concentration of nh4 in biotite by aftir allows the preselection of the samples to be analyzed for their isotopic composition micas from 1 90 2 09 ga metasediments from finland and from 1 025 1 500 ga biotite schists from the moine succession in scotland have high concentrations of structural ammonium between 176 and 1549 ppm suggestive of a biological origin for the nitrogen the heavy nitrogen isotopic signatures y 15 n 4 9x to 19 7x of structural nh4 in these micas are considered consistent with biological denitrification in operation at the time of sedimentation evidence for denitrification in the mesoproterozoic is in agreement with geological and geochemical data for abundant free oxygen in the atmosphere facilitating the stability of marine no3 sequences of early archean ca 3 8 ga garnet mica schists from the isua supracrustal belt isb in southern west greenland contain biotite rich in nh4 between 233 and 512 ppm and nitrogen isotopic signatures y 15 n 1 9x to 5 9x which possibly point to an early evolution of ammonium assimilation and or nitrogen fixation this result appears to be consistent with molecular phylogenetic studies in the bacterial and archaeal domains which suggest that nitrogen fixation and ammonium assimilation were metabolic features of the last common ancestor lca of all organisms d 2004 elsevier b v all rights reserved

contemporary sand wedge development in seasonally frozen ground and paleoenvironmental implications abstract contemporary sand wedges and sand veins are active in seasonally frozen ground within the extensive discontinuous permafrost zone in northwest territories canada the region has a subarctic continental climate with 291 mm a 1 precipitation 4 1 c mean annual air temperature warm summers july mean 17 0 c and cold winters january mean 26 6 c five years of continuous observations indicate that interannual variation of the ground thermal regime is dominantly controlled by winter air temperature and snow cover conditions at sandy sites thin snow cover and high thermal conductivity promote rapid freezing high rates of ground cooling and low near surface ground temperatures 15 to 25 c resulting in thermal contraction cracking to depths of 1 2 m cracking potentials are high in sandy soils when air temperatures are

the sound and speed of ice wedge cracking arctic canada rifle like sounds produced by ice wedge cracking although seldom heard have been reported in the permafrost literature for a century the sounds similar to those produced by the rapid cracking of lake ice indicate a high speed of crack propagation the speed of ice wedge crack propagation has been measured at two field sites for the 1966 1989 period no evidence of rapid crack propagation has been detected at these sites the fracture patterns of ribs as seen on vertical crack faces and the numerous short interacting cracks as seen in plan view suggest slow rates of cracking enquiries made of many northern residents indicate that rifle like sounds are uncommon along the mainland coast from the alaska yukon boundary eastward to coppermine n w t but are heard more frequently in the arctic islands high speeds of crack propagation appear associated with sharp drops in temperature windblown sites and long straight ice wedges as a preliminary estimate crack propagation rates exceeding about 200

nitrogen isotope evidence for stepwise oxygenation of the ocean during the great oxidation event abstract the earth s oxygenation represents one of the most important environmental drivers of life s evolution with the first rise known as the great oxidation event goe corresponding to unpreceded accumulation of atmospheric o2 changes in the flux of marine nutrients and possibly global glaciations however the detailed evolution of the goe is still debated as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation here we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores t2 and t3 in the early paleoproterozoic turee creek group western australia to track the redox evolution of marine conditions during the goe t2 core samples of the kungarra formation which consists of clastic sedimentary rocks overlaid by the glaciogenic meteorite bore member were deposited 2 31 ga ago t3 core intercepts from bottom to top quartzite of the koolbye formation and shales and stromatolitic carbonates of the kazput formation which were deposited around 2 25 ga samples from t2 show minor variations of 13corg avg 34 5 1 7 n 30 with no significant difference between siliciclastic and glaciogenic sedimentary rocks in contrast t3 samples display an increase in 13corg from 32 0 to 24 8 n 54 from shales to carbonates in both t2 and t3 cores 13corg values are inversely correlated with al2o3 suggesting a strong petrological control on 13corg values inferred here as resulting from variable contributions of detrital organic matter bulk n contents are low from 13 5 to 56 7 ppm and 15 7 to 53 4 ppm in t2 and t3 samples respectively the 15n values show a bimodal distribution with one mode at 2 6 in t2 and another at 8 8 in t3 independent from lithological variations this 15n values shift between t2 and t3 is interpreted as reflecting a change from dominating n2 fixers to no3 assimilating organisms this implies an increase of no3 availability and thus of o2 concentration during the time interval separating the deposition of t2 and t3 sediments dissolved no3 and o2 concentrations of the turee creek marine basin are estimated from two models using n isotope data the dissolved no3 concentration has an upper limit ranging from 1 91 to 3 04 m about one order of magnitude below the average value of modern oceans the lower limit for dissolved oxygen concentration ranges from 1 8 to 4 4 m which is two orders of magnitude lower than modern oceans together with previous studies the present data place quantitative constraints on the redox changes associated with the great oxidation event and illustrate a stepwise increase of no3 bioavailability between 2 31 to 2 25 ga in relation with increasing o2 level

changes in marine nitrogen fixation and denitrification rates during the end devonian mass extinction abstract the end devonian was a critical interval in earth history recording the transition from the early paleozoic greenhouse climate to the late paleozoic icehouse climate a mass extinction at this time the hangenberg extinction eliminated 21 of genera and 16 of families of marine invertebrates although its causes remain poorly understood the marine nitrogen cycle is intimately related to the nutrient status of seawater microbial community composition and the redox condition of the oceans and thus it may provide insights into the mechanism s of the end devonian mass extinction and climate change here we analyzed high stratigraphic resolution bulk sediment nitrogen isotope variation 15 n bulk in three sections from south china 1 long an an isolated carbonate platform section 2 qilinzhai a shallow water carbonate platform section between the yangtze oldland and the youjiang trough and 3 malanbian a shallow water carbonate platform section between the yangtze and cathaysia oldlands and distant from deep water areas our 15 n bulk records show a major perturbation to the marine nitrogen cycle in the middle siphonodella praesulcata zone which is marked by a decline in 15 n bullk from 4 to 0 at long an and from 4 5 to 2 5 at malanbian this perturbation coincided with the main end devonian mass extinction as well as with a negative shift in carbonate carbon isotopes 13 c carb the shift to lower 15 n values suggests locally enhanced nitrogen fixation possibly as a consequence of increased denitrification in response to globally expanded marine anoxia thus these 15 n bulk data are consistent with the hypothesis that intense anoxia was the immediate cause of the end devonian mass extinction the 15 n bulk records generated in this study suggest that there was significant spatial and temporal variation in the nitrogen isotope composition of seawater nitrate in the yangtze marginal sea during the latest devonian this variation may have been related to the complex paleogeography of the south china craton which contained numerous isolated and continent attached platforms and deep water basins that influenced watermass circulation nutrient inventories and primary productivity levels and watermass ventilation and redox conditions

revisiting the depositional environment of the neoproterozoic callanna group south australia abstract the callanna group was deposited around 800 million years ago ma during an interval in earth s history that saw a transition towards a more oxygenated atmosphere increasing biodiversity among eukaryotic microfossils and climatic perturbations culminating in low latitude glaciations previous researchers proposed that the callanna basin was lacustrine and highly alkaline which could provide important new insights into environmental cause effect relationships at this time to further interrogate these records we examined standard biogeochemical proxies including organic carbon and nitrogen isotopes iron speciation metal abundances and carbonate associated sulfate much of the primary information has been lost because the rocks of the callanna group have experienced extensive metamorphism up to amphibolite facies and are altered by modern weathering however relics of these proxies combined with sedimentological features preserve evidence of redox stratification within this basin furthermore our observations in particular weakly fractionated nitrogen isotopes and abundant gypsum pseudomorphs are incompatible with the interpretation of high alkalinity the high salt content and occurrences of tidal indicators are most parsimoniously explained by frequent incursions of seawater thus the callanna group cannot speak straightforwardly to environmental conditions in non marine habitats at this time lastly the absence of a large carbon isotope anomaly indicates that these rocks do not correlate with the bitter springs formation

the contrasting biogeochemistry of iron and manganese in the pacific ocean abstract vertical and horizontal distributions of dissolved and suspended particulate fe and mn and vertical fluxes of these metals obtained with sediment traps were determined throughout the pacific ocean in general dissolved fe is low in surface and deep waters 0 1 to 0 7 nmol kg with maxima associated with the intermediate depth oxygen minimum zone 2 0 to 6 6 nmol kg vertical distributions of dissolved mn are similar to previous reports exhibiting a surface maximum a subsurface minimum a mn maximum layer coincident with the oxygen minimum zone and lowest values in deep waters near shore removal processes are more intense for dissolved fe than for dissolved mn dissolved mn in the surface mixed layer remains elevated much farther offshore than dissolved fe elevated near surface dissolved mn concentrations occur in the north pacific equatorial current suggesting transport from the eastern boundary near surface mixed layer dissolved mn concentrations are higher in the north pacific gyre reflecting enhanced northern hemisphere aeolian sources residence time estimates for the settling of refractory paniculate fe and mn from the upper water column are 62 220 days fe and 105 235 days mn in contrast residence times for the scavenging of dissolved fe and mn are 2 13 years fe and 3 74 years mn scavenging residence times for dissolved mn based on horizontal mixing in the surface mixed layer of the northeast pacific are 0 4 years nearshore to 19 years 1000 km offshore there is no evidence for in situ fe redox dissolution within sub oxic waters in the eastern tropical north pacific dissolved fe appeared to be controlled by dissolution from sub oxic sediments with oxidative scavenging in the water column or upper sediment layers however in situ mn dissolution within the oxygen minimum zone was evident

the first 20 years 1978 1979 to 1998 1999 of ice wedge growth at the illisarvik experimental drained lake site western arctic coast canada in august 1978 a large tundra lake was drained to study the aggradation of permafrost into newly exposed lake bottom sediments ice wedge growth which started in the first winter following drainage had ceased in most of the lake bottom within about twelve years the gradual cessation of thermal contraction cracking can be attributed to rapid vegetation growth snow entrapment an increase in winter ground temperatures and a decrease in the linear coefficient of thermal contraction associated with freeze thaw consolidation of the initially saturated lake bottom sediments the tilt and separation of markers in the active layer revealed gradual convergence towards the troughs even after ice wedge growth had ceased for the first few years the ice wedge growth rate was up to 3 cm a as determined by excavation drilling separation of the bottoms of benchmarks installed into permafrost and divergence of free floating inductance coils placed on the sides of ice wedges well below the bottom of the active la

n2 fixing cyanobacteria supplied nutrient n for cretaceous oceanic anoxic events the abundance of specific membrane lipids 2 methylhopanoids indicates that cyanobacteria played a key role in the seemingly global deposition of black shales during the early aptian ca 120 5 ma and late cenomanian ca 93 5 ma oceanic anoxic events organic matter rich sediments deposited during these events are characterized by a 15n content typical of newly fixed n2 indicating that cyanobacterial n2 fixation was the main source for nutrient n we propose that denitrification and anaerobic ammonium oxidation effectively cut off the return of nutrient n from the anoxic deep waters to the photic zone in the oceans giving n2 fixing cyanobacteria a competitive advantage over algae during these oceanic anoxic events

anaerobic nitrogen cycling on a neoarchaean ocean margin abstract a persistently aerobic marine nitrogen cycle featuring the biologically mediated oxidation of ammonium to nitrate has likely been in place since the great oxidation event goe some 2 3 billion years ago although nitrogen isotope data from some neoarchaean sediments suggests transient nitrate availability prior to the goe these data are open to other interpretations this is especially so as these data come from relatively deep water environments that were spatially divorced from shallow water settings that were the most likely sites for the accumulation of oxygen and the generation of nitrate here we present the first nitrogen isotope data from contemporaneous shallow water sediments to constrain the nitrogen cycle in shallow late archaean settings the bh 1 sacha core through the campbellrand malmani carbonate platform records a transition from a shallow siliciclastic carbonate ramp to a rimmed carbonate shelf with the potential for reduced communication with the open ocean in these settings nitrogen isotope 15 n data from sub to peri tidal and lagoonal settings are close to 0 indicating diazotrophy or the complete utilization of remineralised ammonium with an isotopic composition of near 0 our dataset also includes negative 15 n values that suggest the presence of an ammonium pool of concentrations sufficient to have allowed for non quantitative assimilation we suggest that this condition may have been the result of upwelling of phosphorus rich deep waters into the photic zone stimulating primary productivity and creating an enhanced flux of organic matter that was subsequently remineralised and persisted in the dominantly anoxic neoarchaean marine environment notably we find only limited evidence of coupled nitrification denitrification even in these shallow water environments calling into question previous suggestions that the late archaean nitrogen cycle was characterized by widespread aerobic nitrogen cycling rather aerobic nitrogen cycling was likely spatially heterogeneous and tied to loci of high oxygen production while zones of shallow water anoxia persisted

the direction of ice wedge cracking in permafrost downward or upward field studies have been carried out along the western arctic coast of canada in an attempt to determine whether all ice wedge cracks originate at the ground surface and therefore propagate downward or whether some cracks originate near the top of permafrost and then propagate both upward and downward the field studies have been concentrated upon 1 low and high centred tundra polygons a few thousand years old and 2 ice wedges growing for the first time on the bottom of a lake experimentally drained in 1978 the field instrumentation has included electronic crack direction indicators electronic elapsed timers and continuous temperature measurements the field studies reveal that many of the ice wedge cracks originated near the top of permafrost and then propagated upward to the ground surface as well as downward into ice wedge ice for the 1974 1982 period the field observations showed that about 57 of the ice wedges cracked from the ground surface downward and 43 cracked both upward and downw

nitrogen isotope and trace element composition characteristics of the lower cambrian niutitang formation shale in the upper middle yangtze region south china abstract the early cambrian is a key interval in the global development of biological evolution in which occurred many important environmental events and organic rich sedimentary layers were deposited which formed important hydrocarbon source rocks the lower cambrian niutitang formation is one of the most important marine sedimentary layers in the yangtze region south china and it records abundant important geological information that can be used for the reconstruction of the lower cambrian paleo climates and paleo environments nitrogen isotope compositions record and reflect variations in the original sedimentary environment and the redox conditions of marine ecosystems trace elements are also reliable indicators of sedimentary environments in this study we collected samples from the lower cambrian niutitang formation shale in the upper middle yangtze region of south china and measured the nitrogen isotope values of bulk sediments 15nbulk as well as their organic carbon isotope values 13corg total organic carbon toc contents total nitrogen tn contents and trace element concentrations the 15nbulk values of the niutitang shale range from 0 6 to 2 8 they show a certain positive excursion from bottom to top in the longitudinal direction and they exhibit a significant correlation with the longitudinal variations in trace elements these trace element characteristics suggest that the lower section of the niutitang formation existed in a strongly reduced sedimentary environment and was located in a hydrothermally active layer during the deposition of the lower section of black shale the nitrogen cycle was dominated by anaerobic nitrogen fixation thus its nitrogen isotope values are relatively lighter in contrast during the deposition of the upper section of the niutitang formation the redox conditions of water column were unstable the oxygen contents increased and oxygenated reactions were intermittent or localized thus producing relatively strong isotopic fractionation and causing the 15nbulk values in the upper section of black shale to exhibit a positive excursion we conclude that the sedimentary environment is the key factor affecting nitrogen isotopes and that the redox conditions of the water column exhibit a significant correlation with the nitrogen isotope values

the frasnian famennian mid late devonian boundary in the type section of the long rapids formation james bay lowlands northern ontario canada utilizing a combination of conodont based biostratigraphy and geochemistry the frasnian famennian f f mid late devonian boundary was located in the long rapids formation northern ontario it is present in a black shale sequence 21 46 m above the base of the long rapids formation just above a carbonate bed that has similarities to the upper kellwasser limestone recognized as a lithological marker for the f f boundary in germany belgium and france there is no evidence of an impact event and sedimentation across the boundary is continuous although there is a discontinuity layer just below the boundary and the early part of the linguiformis zone of the standard late devonian conodont zonation is missing strong positive 13 c organic and carbonate values were identified and are interpreted as evidence of mass mortality in anoxic waters at the f f extinction positive 18 o values suggest the extinction was also accompanied by a temperature drop negative 34 s values also indicate anoxia and negative 15 n values provide evidence of reduced surface water productivity no iridium anomaly was found at the boundary although higher than usual values were found about 85 cm below the f f boundary sedimentological and biological evidence in the long rapids formation indicates that the f f extinction was coincident with major sea level fluctuations in the late frasnian including a transgression beginning at the end of the linguiformis zone and continuing through the early triangularis zone and beyond marine oscillations were accompanied by anoxia which appears to be the killing mechanism for the conodont and other end frasnian extinctions

the linkage of nitrogen isotopic composition and depositional environment of black mudstones in the upper triassic yanchang formation ordos basin northern china abstract in this study we report new analyses of stable nitrogen isotopic compositions 15n of the black mudstones from the chang 7 member of the upper triassic yanchang formation in the ordos basin china to examine the possible linkage of such isotopic compositions with the depositional environment these mudstones were deposited mainly under suboxic bottom water column conditions the core descriptions and trace element concentrations differ between samples obtained from the chang 73 and chang 71 2 members situated at the bottom and top of the chang 7 member respectively such that the depositional condition of the former implies a more reduced water regime the bulk rock nitrogen isotope 15nbulk values of the samples obtained from the chang 73 member are higher than those of the chang 71 2 member no significant differences were identified in the organic carbon isotopes 13corg total organic carbon toc or total nitrogen tn values between the two units the results of 15nbulk cross plotting with the toc tn and carbon to nitrogen atomic ratio c n show an absence of significant modification of the primary nitrogen isotopic compositions from post depositional processes given the depositional history of these units we suggest that the sedimentary redox environment can have a significant influence on 15nbulk denitrification which occurs mainly in suboxic conditions plays an important role in such an environment resulting in 15n enrichment during the original deposition thus owing to its sensitivity to sedimentary redox environments the 15nbulk index can be an effective tool for evaluating depositional redox conditions particularly for further subdivision of suboxic environments

water column anoxia enhanced productivity and concomitant changes in δ13c and δ34s across the frasnian famennian boundary kowala holy cross mountains poland the investigation of the trace element and organic geochemistry of the frasnian famennian boundary section at kowala holy cross mountains poland shows that the lower water column was oxygen deficient during late frasnian and early famennian times the abundance and carbon isotopic composition of diaryl isoprenoids biomarkers indicative for green sulfur bacteria prove that euxinic waters reached into the photic zone at least episodically total organic carbon toc contents show two maxima that are time equivalent to the kellwasser horizons deposited in shallower water settings enhanced toc concentrations are explained by a higher primary productivity presumably as a consequence of an enhanced nutrient supply from the continent the increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to toc increased at the frasnian famennian transition the sulfur isotopic composition of pyritic and organically bound sulfur shows a 27 excursion across the boundary the observation that the 34s values of organic bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source likely early diagenetic sulfide a change in the 13c of total dissolved inorganic carbon as a consequence of an enhanced burial of 12c enriched organic carbon is indicated by a 3 excursion measured for toc as well as for individual n alkanes and isoprenoids the burial of large amounts of organic carbon is expected to result in a decrease in pco2 and should affect the photosynthetic carbon isotope fractionation ep the fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values as a consequence of high atmospheric and oceanic dissolved co2 concentrations during the devonian

a test of the nitrogen limitation hypothesis for retarded eukaryote radiation nitrogen isotopes across a mesoproterozoic basinal profile abstract nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late mesoproterozoic however evidence for how the nitrogen cycle was operating during the middle precambrian is so far rare more specifically it is unknown which steps in the biogeochemical nitrogen cycle e g nitrogen fixation nitrification or ammonification were rate limiting and thus controlling microbial community structures the mesoproterozoic belt supergroup in western montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue bulk 15 n values show a clear trend from 1 in the deepest part of the basin to 5 along basin margins which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling the total fractionation of carbon isotopes between carbonate and organic carbon 13 c increases from 20 to 32 in the same direction possibly indicating an ecological response to redox stratification and nitrogen speciation evidence from the belt supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean

nitrogen and argon isotopic signatures in graphite from the 3 8 ga old isua supracrustal belt southern west greenland abstract the problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early archean rocks have prompted the search for additional chemical and isotopic biomarkers here we report an attempt to identify the origin of carbonaceous matter in the 3 8 ga old isua supracrustal belt in southern west greenland by measuring the concentration and isotopic composition of a trapped nitrogen component stepped combustion pyrolysis mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen 1 nitrogen associated with a very small amount of reactive carbonaceous material 2 nitrogen intercalated in graphite correlated with intercalated radiogenic argon 3 nitrogen strongly retained at defects or chemically bound in the graphite structure the 15n of nitrogen associated with reactive carbonaceous matter ca 6 overlaps with that of average phanerozoic sedimentary organic matter and is believed to be part of nonindigenous postmetamorphic biologic material in situ raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material and this material is further referred to as graphite graphite interpreted as epigenetic associated with mg mn siderite in metacarbonates contains a very small strongly retained nitrogen component with a low 15n ratio 3 to 1 this range overlaps with values that are typically found in archean kerogens but also those of a metamorphically emplaced inorganic basaltic source geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this mg mn siderite derived graphite based on degree of graphite crystallinity amount of trapped radiogenic argon low nitrogen concentration and 15n signature we conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed isua supracrustal belt

nitrogen isotope fractionations during progressive metamorphism a case study from the paleozoic cooma metasedimentary complex southeastern australia abstract the well studied paleozoic cooma metamorphic complex in southeastern australia is characterized by a uniform siliciclastic protolith of uniform age with a continuous range of metamorphic grade from subgreenschist to upper amphibolite facies and migmatite grade in an annular pattern around the cooma granodiorite those conditions are optimal for investigating variations of n concentrations and 15 n values during progressive metamorphism nitrogen concentrations decrease and 15 n increases with increasing metamorphic grade sub chlorite zone 120 ppm n 15 n 2 3 chlorite zone 110 ppm n 15 n 3 0 biotite and andalusite zone 85 ppm n 15 n 3 8 sillimanite and migmatite zones 40 ppm n 15 n 10 7 covariation of k and n contents is consistent with n substituting for k as nh 4 in micas observed trends of increasing 15 n values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in 15 n with progressive metamorphism which causes an enrichment of 15 n in the residual nitrogen of the rock equilibrium models for rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by n 2 nh 4 exchange at temperatures of 300 600 c whereas observed large fractionations for the upper amphibolite facies and melt products in the migmatite grade samples may be interpreted as nh 3 nh 4 exchanges at temperature of 650 730 c lower values in the highest grade zones may also stem in part from input of 15 n depleted fluids from the granodiorite the magnitude of isotope fractionation of nitrogen is about 1 2 during progressive metamorphism of metasedimentary rocks from sub chlorite zone to biotite andalusite zone which is consistent with previous studies consequently the large spread of 15 n values in archean greenschist facies metasedimentary rocks of 6 to 30 can be accounted for by variable mixtures of mantle plume dominated volatiles with a 15 n of 5 and a 15 n enriched marine sedimentary kerogen component inherited from a ci chondrite veneer having 15 n of 30 to 42

basal icequakes recorded beneath an alpine glacier glacier d argentiere mont blanc france evidence for stick slip motion while basal icequakes associated with glacier motion have been detected under antarctica for several decades there remains very little evidence of stick slip motion for alpine glaciers here we analyzed 2357 basal icequakes that were recorded at glacier d argenti ere mont blanc massif between february and november of 2012 and that are likely to be associated with basal sliding these events have been classified into 18 multiplets based on their waveforms the strong similarity of the waveforms within each multiplet suggests an isolated repeating source despite this similarity the peak amplitude within each multiplet varies gradually in time by up to a factor of 18 the distribution of these events in time is relatively complex for long time scales we observe progressive variations in the amplitudes of events within each multiplet for intermediate time scales hours the events occur regularly in time with typical return times of several minutes up to several hours for short time scales from 0 01 to 100 s the largest multiplet shows clus tering in time with a power law distribution of the interevent times the location of these events and their focal mechanisms are not well constrained because most of these events were detected by a single seismometer nevertheless the locations can be estimated with an accuracy of a few tens of meters using a polarization analysis the estimated average depth of the basal events is 179 m which is in good agreement with the estimated glacier thickness the relative changes in distance between the source and the sensor can be measured accurately by correlating separately the p wave and s wave parts of the seismograms of each event with the template waveforms which are obtained by averaging the signals within each multiplet we observed small variations in the times between the p wave and the s wave of up to 0 6 ms over 50 days these variations cannot be explained by displacement of the sensor with respect to the glacier but might be due to small changes in the seismic wave velocities with time finally we found using numerical simulations that the observed signals are better explained by a horizontal shear fault with slip parallel to the glacier flow than by a tensile fault these results suggest that the basal events are associated with stick slip motion of the glacier over rough bedrock the rupture length and the slip are difficult to estimate nonetheless the rupture length is likely to be of the order of meters and the total seismic slip accumulated over one day might be as large as the glacier motion during the most active bursts

carbon and nitrogen isotope disturbances and an end norian late triassic extinction event major perturbations of organic carbon and nitrogen isotope ratios from a norian rhaetian late triassic boundary section in british columbia coincide with an extinction of the dominant deep water invertebrate fauna of the late triassic monotids and most ammonoids the carbon isotope excursion is attributed to the development of widespread oceanic stagnation that favored organic rich shale deposition the coincident nitrogen isotope excursion suggests that progressively more nitrate limited productivity forced a change to nitrogen fixing cyanobacteria populations as ocean stagnation created nutrient starved conditions the biotic crisis and geochemical events of the norian rhaetian boundary predate the latest rhaetian end triassic mass extinction thus the late triassic interval was marked by more than one extinction event

a euxinic southern north atlantic ocean during the cenomanian turonian oceanic anoxic event abstract during mid cretaceous times large amounts of organic carbon corg became sequestered in black shales possibly due to oceanic anoxic events characterized by the development of an extended oxygen minimum zone omz here we provide the first direct evidence for an open ocean omz in the cenomanian turonian c t southern north atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in c t black shales this together with evidence for bottom water anoxia and accumulation of redox sensitive trace metals and hydrogen rich organic matter indicates a continuously euxinic water column concurrent with the high corg accumulation rates which are 15 150 times greater in the southern than in the northern north atlantic and the low biological productivity this suggests that preservation controlled the accumulation of corg in c t black shales

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

pan arctic ice wedge degradation in warming permafrost and its influence on tundra hydrology ice wedges are common features of the subsurface in permafrost regions they develop by repeated frost cracking and ice vein growth over hundreds to thousands of years ice wedge formation causes the archetypal polygonal patterns seen in tundra across the arctic landscape here we use field and remote sensing observations to document polygon succession due to ice wedge degradation and trough development in ten arctic localities over sub decadal timescales initial thaw drains polygon centres and forms disconnected troughs that hold isolated ponds continued ice wedge melting leads to increased trough connectivity and an overall draining of the landscape we find that melting at the tops of ice wedges over recent decades and subsequent decimetre scale ground subsidence is a widespread arctic phenomenon although permafrost temperatures have been increasing gradually we find that ice wedge degradation is occurring on sub decadal timescales our hydrological model simulations show that advanced ice wedge degradation can significantly alter the water balance of lowland tundra by reducing inundation and increasing runoff in particular due to changes in snow distribution as troughs form we predict that ice wedge degradation and the hydrological changes associated with the resulting differential ground subsidence will expand and amplify in rapidly warming permafrost regions the polygonal patterns in permafrost regions are caused by the formation of ice wedges observations of polygon evolution reveal that rapid ice wedge melting has occurred across the arctic since 1950 altering tundra hydrology

redox conditions and nitrogen cycling during the triassic jurassic transition a new perspective from the mid panthalassa abstract we determined redox sensitive element concentrations along with 15ntn values from the triassic jurassic shales interbedded within deep sea cherts at the katsuyama section sw japan to clarify the redox and related nitrogen cycle conditions in mid panthalassa during the tjb biotic crisis focusing particularly on the linkages with camp volcanism slight enrichments of mn and mo across the tjb suggest a vertically expanded mid panthalassic omz above a well oxygenated deep ocean moreover high resolution 15ntn records during the triassic jurassic transition exhibit a negative nitrogen isotope excursion nnie just above the tjb relative to the previously reported negative carbon isotope excursion ncie across the tjb the nnie can be divided into two intervals negative interval 1 ni 1 during the ncie and ni 2 after the ncie the nnie can be explained as an interval of nitrate rich conditions reflecting an oxic panthalassic ocean during the triassic jurassic transition our geochemical dataset provides new insights into environmental perturbations in mid panthalassia during the tjb biotic crisis 1 a vertically expanded omz and decreased primary productivity owing to global warming caused by camp volcanism across the tjb resulted in an increased nitrate concentration during the ni 1 and 2 eutrophic nitrate rich conditions developed in mid panthalassa during the ni 2 due to the enhanced continental weathering and deep water upwelling significantly our results are the first to demonstrate that the omzs were expanded not only in the shallow marine regions but also in pelagic sites across the tjb suggesting that the globally expanded omzs can be regarded as a crucial driver for the tjb biotic crisis

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

interpretation of the nitrogen isotopic composition of precambrian sedimentary rocks assumptions and perspectives nitrogen isotope compositions in sedimentary rocks d 15 n sed are routinely used for reconstructing cenozoic n biogeochemical cycling and are also being increasingly applied to understanding the evolution of ancient environments here we review the existing knowledge and rationale behind the use of d 15 n sed as a proxy for the precambrian n biogeochemical cycle with the aims of i identifying the major uncertainties that affect analyses and interpretation of nitrogen isotopes in ancient sedimentary rocks ii developing a framework for interpreting the precambrian d 15 n sed record iii testing this framework against a database of precambrian d 15 n sed values compiled from the literature and iv identifying avenues of focused research that should increase confidence in interpreting precambrian d 15 n sed data this review highlights the intrinsic complexity of the d 15 n sed proxy and the significant effort that remains to realize its potential specifically it is crucial to gain a better understanding of how and when diagenesis and metamorphism affect the d 15 n of bulk and kerogen bound nitrogen ultimately more data are required to apply statistics to interpreting d 15 n sed variability within given geological time intervals finally numerical modeling of the d 15 n sed variability expected in different environments under varying redox scenarios is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record in spite of the challenges facing the application of this proxy to the precambrian the existing d 15 n sed record shows several features possibly related to the stepwise oxygenation of the surface environment underlining the potential for nitrogen isotopes to reveal clues about the evolution of early earth

recent warming of mountain permafrost in svalbard and scandinavia 1 three deep boreholes 100 m in mountain permafrost were recently drilled in svalbard janssonhaugen and in scandinavia tarfalaryggen and juvvasshoe for long term permafrost monitoring these holes form part of a latitudinal transect of boreholes in permafrost through europe established by the permafrost and climate in europe pace project six year thermal time series data collected from the three boreholes are presented these data provide the first opportunity for temporal trends in permafrost temperatures in svalbard and scandinavia to be analyzed results show that the permafrost has warmed considerably at all three sites significant warming is detectable down to at least 60 m depth and present decadal warming rates at the permafrost surface are on the order of 0 04 0 07 c yr 1 with greatest warming in svalbard and in northern scandinavia the present regional trend shows accelerated warming during the last decade

nitrate reduction sulfate reduction and sedimentary iron isotope evolution during the cenomanian turonian oceanic anoxic event onset of the oae in italy have unusually low bulk d 57 fe values unlike those found in the black shale livello bonarelli deposited during the oceanic anoxic event itself these latter conform to the phanerozoic norm for organic rich sediments pyrite formation in the pre oae black shales has apparently taken place via dissimilatory iron reduction dir within the sediment a suboxic process that causes an approximately 2 fractionation between a lithogenic fe iii oxide source and fe ii aq in contrast bacterial sulfate reduction bsr at least partly in the water column characterized the oae itself and was accompanied by only minor iron isotope fractionation this change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from 10 to 7 mm the gradual albeit irregular increase in fe isotope values during the oae as recorded in the italian section is taken to demonstrate limited isotopic evolution of the dissolved iron pool consequent upon ongoing water column precipitation of pyrite under euxinic conditions given that evidence exists for both nitrate and sulfate reduction during the oae it is evident that redox conditions in the water column were highly variable in both time and space

seawater as the source of minor elements in black shales phosphorites and other sedimentary rocks many of the minor elements in seawater today have a concentration depth profile similar to that of the biologically essential nutrients no 3 and po3 4 they show a relative depletion in the photic zone and enrichment in the deep ocean the difference between their surface and deep ocean values normalized to the change in po3 4 approaches the average of measured minor element p ratios in marine plankton although individual analyses of the latter show extreme scatter for a variety of reasons despite this scatter in the minor element analyses of plankton agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter with a composition approaching that of plankton this interpretation is further supported by sediment studies particularly of sediments which accumulate in shelf slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated the interelement relations observed for some of these sediments approach the average values of plankton these same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites rocks which have a high content of marine fractions e g organic matter apatite biogenic silica and carbonates many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity and or seawater with an elemental content different from that of the modern ocean was required to account for their minor element contents however the similarity in several of the minor element ratios in many of these formations to minor element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf slopes or in marginal seas of the ocean today the accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter settling from the photic zone and with a composition of average plankton a second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater the suite of elements in this fraction reflects redox conditions in the bottom water as determined by bacterial respiration for example high mn high cr v and high mo concentrations above those which can be attributed to the accumulation of planktonic matter characterize accumulation under bottom water oxidizing denitrifying and sulfate reducing conditions respectively

carbon and nitrogen isotope excursions in mid pleistocene sapropels from the tyrrhenian basin evidence for climate induced increases in microbial primary production the modern mediterranean sea is oligotrophic yet its sediment record contains layers of organic carbon rich sapropels at 21 ky precessional spacing that imply periods of elevated paleoproductivity that approached the high productivities of modern upwelling systems resolution to this paradox is provided by lines of evidence suggesting that the mode of primary productivity changed from one dominated by algae to one during times of sapropel deposition in which photosynthetic bacteria were important we have made a high resolution comparison of the organic carbon and nitrogen isotopic compositions of three sapropels and their background sediments in a 3 m sequence that corresponds to 1001 to 946 ka organic 13c values systematically increase from 26 to 21 and 15n values systematically decrease from 4 to 0 as organic carbon mass accumulation rates increase in the sapropel layers the increase in carbon isotope values mirrors the increases in primary productivity and associated organic matter export indicated by the increased mass accumulation rates the decrease in nitrogen isotope values implies major contributions of nitrogen fixing cyanobacteria to the total marine productivity the precessional minima with which sapropels coincide were times of wetter climate that stratified the surface mediterranean sea increased delivery of soil derived phosphorus and evidently amplified microbial primary production our high resolution study reveals several relatively rapid excursions into and out of the high productivity mode that suggest that sapropel deposition was a climate sensitive surface driven phenomenon that was not accompanied by basin wide stagnation

production of 15n depleted biomass during cyanobacterial n2 fixation at high fe concentrations 1 in this study we examine the effects of varying fe mo and p concentrations on 15 n fractionation during n 2 fixation in the cyanobacterium anabaena variabilis we show that when grown in fe enriched media fe 50 nm this organism produces biomass up to 3 o lower in 15 n than when grown in fe limited media fe 50 nm a compilation of our data with previous measurements of 15 n in n 2 fixing cyanobacteria reveals a general trend toward the production of more 15 n depleted biomass at higher fe concentrations we discuss our results in the context of negative 15 n values preserved in archean and some phanerozoic sediments generally attributed to the production of marine organic matter with low 15 n by n 2 fixation and potentially nh4 regeneration during periods of fluctuating nutrient dynamics we suggest that enhanced fe availability during periods of widespread ocean anoxia can further stimulate the production of 15 n depleted biomass by n 2 fixing organisms contributing to the isotopic record

evidence of frost cracking inferred from acoustic emissions in a high alpine rock wall ice formation within rock is known to be an important driver of near surface frost weathering as well as of rock damage at the depth of several meters which may play a crucial role for the slow preconditioning of rock fall in steep permafrost areas this letter reports results from an experiment where acoustic emission monitoring was used to investigate rock damage in a high alpine rock wall induced by natural thermal cycling and freezing thawing the analysis of the large catalog of events obtained shows i robust power law distributions in the time and energy domains a footprint of rock micro fracturing activity induced by stresses arising from thermal variations and associated freezing thawing of rock ii an increase in ae activity under sub zero rock temperatures suggesting the importance of freezing induced stresses ae activity further increases in locations of the rock wall that are prone to receiving melt water these results suggest that the framework of further modeling studies theoretical and numerical should include damage elastic interaction and poro mechanics in order to describe freezing related stresses

physical and temporal factors controlling the development of near surface ground ice at illisarvik western arctic coast canada near surface permafrost was sampled in summer 2010 at 26 sites in the illisarvik drained lake basin and nine sites in the surrounding tundra on richards island nwt to investigate the growth of segregated near surface ground ice permafrost and ground ice have developed in the lake basin since drainage in 1978 the lake bed soils are predominantly silts of varying moisture and organic matter contents with sandier soils near the lake margins excess ice contents in the basin were also variable and ice enrichment was observed to a maximum depth of 60 cm below the 2010 permafrost table shrub covered wet areas had the highest mean excess ice content in the top 50 cm of permafrost 10 while grassy dryer areas 4 and poorly vegetated marginal areas 1 were less enriched with ice site wetness was the most important variable associated with near surface excess ice content in the lake basin silt content was a secondary variable mean excess ice content in the top 50 cm of permafrost at tundra sites

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

elevated marine productivity triggered nitrogen limitation on the yangtze platform south china during the ordovician silurian transition abstract the ordovician silurian transition ost coincided with global environmental and biological changes including perturbations of the carbon and sulfur cycles oceanic redox changes and a mass extinction event but their effects on the contemporaneous marine nitrogen cycle remain poorly known in this study we measured bulk nitrogen isotopes in the datianba and shuanghe ost sections of south china which accumulated in the semi restricted inner yangtze sea the uniformly low 15nbulk mostly 1 to 1 of wufeng and longmaxi formation black shales representing the pre and post hirnantian intervals imply large losses of bioavailable nitrogen i e nitrate and ammonium via denitrification anammox as well as strong nitrogen fixation by diazotrophs persistence of low 15nbulk values within the guanyinqiao bed representing the hirnantian glacial interval indicates that denitrification anammox remained strong even during this major glacio eustatic fall and deep ocean ventilation event we hypothesize that elevated primary productivity caused by upwelling was a key factor producing a shallow redoxcline in the inner yangtze sea by comparing our nitrogen data with records from other cratons we suggest that oceanic anoxia was prevalent globally during the late katian to early rhuddanian and that ocean redox conditions were spatially heterogeneous during the hirnantian glaciation these considerations imply that bioavailable nitrogen was the limiting factor on marine primary productivity during the ost and that oceanic anoxia may have contributed to the late ordovician mass extinction especially on the yangtze platform

ocean redox structure across the late neoproterozoic oxygenation event a nitrogen isotope perspective abstract the end of the neoproterozoic era 1000 to 541 ma is widely believed to have seen the transition from a dominantly anoxic to an oxygenated deep ocean this purported redox transition appears to be closely linked temporally with metazoan radiation and extraordinary perturbations to the global carbon cycle however the geochemical record of this transition is not straightforward and individual data sets have been variably interpreted to indicate full oxygenation by the early ediacaran period 635 to 541 ma and deep ocean anoxia persevering as late as the early cambrian because any change in marine redox structure would have profoundly impacted nitrogen nutrient cycling in the global ocean the n isotope signature of sedimentary rocks 15 n sed should reflect the neoproterozoic deep ocean redox transition we present new n isotope data from amazonia northwest canada northeast svalbard and south china that span the cryogenian glaciations 750 to 580 ma these and previously published data reveal a n isotope distribution that closely resembles modern marine sediments with a mode in 15n close to 4 and range from 4 to 11 no apparent change is seen between the cryogenian and ediacarian data from earlier proterozoic samples show a similar distribution but shifted slightly towards more negative 15 n values and with a wider range the most parsimonious explanation for the similarity of these n isotope distribution is that as in the modern ocean nitrate and hence o2 was stable in most of the middle late neoproterozoic ocean and possibly much of proterozoic eon however nitrate would likely have been depleted in partially restricted basins and oxygen minimum zones omzs which may have been more widespread than in the modern ocean

nitrogen 15 enriched precambrian kerogen and hydrothermal systems 1 nitrogen isotope data are reported for kerogen from precambrian black shales at prehnite pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems nitrogen in micas was acquired from n bearing aqueous metamorphic hydrothermal fluids generated from breakdown of sedimentary kerogen and dehydration of k silicates at deeper crustal levels the 2 7 ga kerogens and hydrothermal k micas yield 15n values of 15 24 compared to existing data of 2 6 in phanerozoic counterparts paleoproterozoic equivalents have intermediate 15n of 7 12 implying a secular decrease in 15n of shale kerogen the 15n enriched nitrogen in archean shales and hydrothermal vein systems cannot be caused either by n isotopic shifts accompanying metamorphism or rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of 15n with n c n ratios or metamorphic grade the magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at 1 to greenschist and about 3 to amphibolite facies furthermore 15n enriched values cannot stem from long term preferential diffusional loss of 14n as samples were selected from terranes where 40ar 39ar ages are within a few million years of concordant u pb ages nitrogen is structurally bound in micas whereas ar is not it is possible that the 15n enriched values stem from a different n cycle in the archean with large biologically mediated fractionations yet the magnitude of the fractionations observed exceeds any presently known and chemoautotrophic communities tend to depleted values earlier results on archean cherts show a range from 6 to 30 given the temporal association of chert banded iron formation with mantle plumes the range is consistent with mixing between mantle n2 of 5 and the enriched marine reservoir identified in this study we attribute the secular trend from archean kerogen of 15 24 to phanerozoic kerogen averaging 3 4 to a secondary atmosphere derived from ci chondrite like material and comets with 15n of 30 42 shifts of 15n to its present values of 0 can be accounted for by a combination of sequestration of atmospheric n2 into sedimentary rocks and mantle degassing of 5 mantle n2

first evidence for the cenomanian turonian oceanic anoxic event oae2 bonarelli event from the ionian zone western continental greece integrated biostratigraphic planktonic foraminifera calcareous nannofossils chemostratigraphic bulk c and o isotopes and compound specific organic geochemical studies of a mid cretaceous pelagic carbonate black shale succession of the ionian zone western greece provide the first evidence for the cenomanian turonian oceanic anoxic event oae2 bonarelli event in mainland greece the event is manifested by the occurrence of a relatively thin 35 cm yet exceptionally organic carbon rich 44 5 wt toc carbonate free black shale near the cenomanian turonian boundary within the vigla limestone formation berriasian turonian compared to the bonarelli black shale interval from the type locality of oae2 in marche umbria italy this black shale exhibits greatly reduced stratigraphic thickness coupled with a considerable relative enrichment in toc isotopically enriched 13c values for both bulk organic matter 22 2 and specific organic compounds are up to 5 higher than those of underlying organic rich strata of the aptian lower albian vigla shale member and thus compare very well with similar values of cenomanian turonian black shale occurrences elsewhere the relative predominance of bacterial hopanoids in the saturated apolar lipid fraction of the oae2 black shale of the ionian zone supports recent findings suggesting the abundance of n2 fixing cyanobacteria in cretaceous oceans during the cenomanian turonian and early aptian oceanic anoxic events

15n 14n variations in cretaceous atlantic sedimentary sequences implication for past changes in marine nitrogen biogeochemistry at two locations in the atlantic ocean dsdp sites 367 and 530 early to middle cretaceous organic carbon rich beds black shales were found to have significantly lower 15n values lower15n 14n ratios than adjacent organic carbon poor beds white limestones or green claystones while these lithologies are of marine origin the black strata in particular have 15n values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools in contrast black organic carbon rich beds at a third site dsdp site 603 contain predominantly terrestrial organic matter and have c and n isotopic compositions similar to organic matter of modern terrestrial origin the recurring15n depletion in the marine derived cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry existing isotopic and other data indicate that the low15n relative abundance is the consequence of pelagic rather than post depositional processes reduced ocean circulation increased denitrification and hence reduced euphotic zone nitrate availability may have led to cretaceous phytoplankton assemblages that were periodically dominated by n2 fixing blue green algae a possible source of this sediment15n depletion lack of parallel isotopic shifts in cretaceous terrestrially derived nitrogen site 603 argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment

the tice event perturbation of carbon nitrogen cycles during the mid tournaisian early carboniferous greenhouse icehouse transition abstract carbonate carbon 13ccarb and bulk nitrogen 15n isotopic variation during the mid tournaisian was analyzed in the malanbian and long an sections of south china their c isotope profiles document a large positive excursion herein termed the mid tournaisian carbon isotope excursion tice during the siphonodella isosticha conodont zone although its magnitude differed between the two sections 6 at malanbian versus 3 at long an the tice event coincided with sedimentologic and oxygen isotopic evidence of climatic cooling and glaciation during the mid tournaisian it was probably triggered by an increase in organic carbon burial rates linked to changes in global ocean circulation the study sections also document a large positive shift in 15n from 1 7 to 4 2 at malanbian and from 1 5 to 3 8 at long an the n isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15n enriched marine nitrate that lasted for the duration of the late paleozoic ice age its initiation coincided with tice and thus may have been linked to ocean circulation changes that resulted in intensified upwelling and an increase in water column denitrification the continuation of the n isotope shift over millions of years may have been linked to glacio eustatic fall and a long term shift in the locus of denitrification from continental shelf sediments to continent margin oxygen minimum zones the tice event thus marks the onset of sustained continental glaciation during the late paleozoic ice age

nitrogen and organic carbon isotope stratigraphy of the yangtze platform during the ediacaran cambrian transition in south china abstract n and corg isotope results are presented from six sections along a west east transect in the south china basin scb covering both shallow and deeper domains in order to investigate biogeochemical cycling stratigraphic correlation and isotope systematics over the crucial ediacaran cambrian transition 15n bulk values range between 3 and 7 while 13corg values range between 21 and 39 similar isotopic trends have been identified for both these proxies across the basin although hiatuses differences in depositional setting and syn depositional bacterial fermentation may have caused some inconsistencies a trend towards negative n isotope values can be recognized above the pc c boundary in both shallow and deeper basin realms across the yangtze platform this negative 15n excursion is probably a response to photic zone anoxia and intense nitrogen fixation assimilation by diazotrophic cyanobacteria and green purple sulfur bacteria gsb and psb the xiaotan section and a composite section from the yangtze gorges area show meaningful similarities in their nitrogen isotope trends interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform using carbon isotope stratigraphy we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations increased bioturbation and food chain complexity across the ediacaran cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early cambrian reflecting the beginning of a new more biologically controlled era

perturbation of the marine nitrogen cycle during the late ordovician glaciation and mass extinction abstract the late ordovician was a critical interval in geologic history during which both the biosphere and marine environments underwent severe perturbations including one of the big five phanerozoic mass extinctions and the massive but short term 0 5 myr hirnantian glaciation the onset and termination of the hirnantian glaciation have been widely accepted as the triggers for the two extinction pulses that comprise the late ordovician biocrisis but the mechanisms that caused the hirnantian glaciation itself remain poorly known here we analyze the nitrogen isotope composition 15 n of two sections in south china wangjiawan and nanbazi in order to better understand nitrogen cycle perturbations in the late ordovician ocean and their relationship to contemporaneous climatic and biogeochemical changes low 15 n 1 in the upper katian and lower rhuddanian of both sections suggests intensive i e near quantitative denitrification and thus nitrogen fixation as the main source of biologically available nitrogen for primary producers a positive 15 n excursion in both sections during the hirnantian indicates weaker i e non quantitative denitrification possibly as a result of more vigorous thermohaline circulation and improved ocean ventilation weaker denitrification would have reduced the flux of n 2 o an intermediate product of denitrification to the atmosphere n 2 o is a potent greenhouse gas and a major decline in its production would have led to cooler climatic conditions and ultimately the hirnantian glaciation a global survey of published nitrogen isotope records suggests that similar processes operated broadly within the late ordovician global ocean

glacial interglacial variations in production and nitrogen fixation in the cariaco basin during the last 580 kyr the effect of sea level change on nutrient supply to the anoxic cariaco demonstrates the fundamental importance of nitrogen n2 fixation and phosphate to oceanic production as n2 fixation produces biomass of low 15n and has been reported to be an important component of the nitrogen cycle in the modern cariaco basin we propose that it contributes to the light interglacial 15n 2 3 values observed in the ocean drilling program odp site 1002 sediment record during the glacials the sediments are bioturbated oxic conditions with low total organic carbon toc contents and sedimentary 15n values of 5 suggesting that nitrogen n2 fixation contributed little to the n nutrition of cariaco surface waters the most plausible explanation for the inferred glacial interglacial changes in n2 fixation in the cariaco is that they have occurred in response to variations in the n p ratio of the nutrient supply driven by changes in denitrification

air temperature snow cover creep of frozen ground and the time of ice wedge cracking western arctic coast the time of ice wedge cracking is examined for several sites with young and old ice wedges along the western arctic coast the correlation between sharp air temperature drops and ice wedge cracking is highest where the snow cover is thin and least where the snow cover is thick the favoured duration and rate of a temperature drop that results in cracking is about 4 days at a rate of about 1 8 c d such temperature drops have a minimal effect in cooling the top of permafrost wherever there is an appreciable snow cover since short duration temperature drops often result in ice wedge cracking the thermal stresses that trigger cracking probably originate more within the frozen active layer than at greater depth in permafrost although most ice wedges tend to crack during periods of decreasing air temperatures about one third of those monitored have cracked during periods of increasing air temperatures long term measurements show that the active layer and top of permafrost move differentially all year in

spatio temporal evolution of ocean redox and nitrogen cycling in the early cambrian yangtze ocean abstract the early cambrian was a critical interval for the earth system during which a rise in oceanic and atmospheric oxygen levels coincided with the rapid diversification of metazoans a variety of contrasting models have been proposed for the spatiotemporal redox evolution of the early cambrian ocean these include the development of a well oxygenated deep ocean at the base of cambrian stage 3 commencing at 521 ma or alternatively persistent and widespread anoxic ferruginous conditions throughout the early cambrian ocean here we present redox sensitive trace element rste fe speciation and n and c isotope 15nsed and 13corg data for samples from a section zhongnancun of the early cambrian niutitang formation which was deposited on the outer shelf of the yangtze block south china the fe speciation and rste data provide evidence of a transition from euxinic through ferruginous to oxic conditions during deposition of the niutitang formation the combination of these new data with existing data from the inner shelf to basin environment implies regional redox stratification across the yangtze block during cambrian stages 2 and 3 with oxic shallow waters above ferruginous deep waters and spatial variability in the degree of mid depth euxinia oxygenation of deeper waters may have occurred by early cambrian stage 4 514 ma a compilation of 15n values from multiple early cambrian sections of the yangtze block indicate that n2 fixation dominated the nitrogen cycle during late cambrian stage 2 low 15n values

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

precambrian ammonium biogeochemistry a study of the moine metasediments scotland abstract the content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of precambrian age whole rock samples and mica separates from the moine succession proterozoic of scotland were analysed by a combination of capacitance manometry and static vacuum mass spectrometry prior to the analyses the presence of the ammonium ion was confirmed by obtaining infrared ir absorption spectra from the micas principally biotites whole rock ammonium contents are high for amphibolite facies metasediments 140 422 ppm the ammonium ion was not detected in the ir absorption spectra of biotites from one schist however this sample contains neither garnet nor white mica for most of the samples the 15n values are very positive between 12 4 and 16 6 with one sample having a lower value of 8 4 the major variations between samples can be interpreted in terms of original sedimentary features for the ammonium bearing samples muscovites have concentrations between 365 and 633 ppm biotites between 845 and 1739 ppm for individual samples nh4 mu nh4 bi varies between 0 31 and 0 43 a much narrower range than has been reported previously there are no significant or consistent differences in 15n between co existing biotite and white mica nor between the 15n values of the mineral separates and their respective whole rock values the latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar the only other potential host for ammonium in the rocks assuming that the ammonium has been derived from organic matter originally present at the time of deposition and by comparison with previous studies it can be inferred that the original 15n value of the moine sediments was somewhere between 5 and 10 similar to recent sediments this suggests that the nitrogen cycle in the proterozoic shelf sea from which the moine sediments were deposited was broadly similar to the present day overall the study underlines how under developed the n system is compared to other stable isotope systems notably c and s and we use the opportunity to highlight some areas that could be developed in the future however one aspect is clear future ammonium studies of metamorphic rocks will need to be performed at high resolution visible scale of heterogeneity with strong mineralogical petrological and field controls

paleoceanographic and paleoclimatic similarities between mediterranean sapropels and cretaceous black shales pliocene quaternary sapropels of the mediterranean sea and mid cretaceous black shales share many distinctive properties organic carbon concentrations that often exceed 5 in both types of sequences record episodes of exceptional accumulation of organic matter although both are rich in marine organic matter they have high c n ratios that mimic those of land plant organic matter which suggests less efficient organic matter recycling than in the modern ocean the coupled presence of isorenieratene derivatives and low 15n values that result from microbial primary productivity indicates that photic zone anoxia existed during their formation and implies strong stratification of the surface ocean enrichments of redox sensitive metals and sediment laminations indicate that anoxia existed deeper in the water column and at the seafloor multiple proxies indicate freshening of surface waters of the mediterranean and imply periods of wetter climate during times of sapropel deposition episodes of black shale deposition similarly coincide with times of wetter climate these comparisons suggest that the wetter climate created similar conditions of surface salinity stratification that enhanced microbial primary production and magnified organic matter export during deposition of both the mediterranean sapropels and mid cretaceous black shales

marine biogeochemical cycling during the early cambrian constrained by a nitrogen and organic carbon isotope study of the xiaotan section south china the precambrian cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet when changes in chemical and physical conditions together with key biological innovations helped to trigger a biodiversity explosion the yangtze platform south china mainly characterized by continuous and unaltered successions of this age is ideally suited to high resolution palaeomarine investigations of this event in this study 15n and 13corg records from xiaotan section yunnan sw china were investigated in order to provide insight into variations in primary productivity ecological developments and marine environment the ediacaran early cambrian xiaotan section is characterized by relatively high sedimentation rates and variable lithologies carbonates cherts phosphorites sandstones and siltstones that alternate through nearly 600 meters thickness organic carbon isotope values vary between 36 and 21 tightly following and helping to complete published 13ccarb trends the base of the cambrian is characterized in this section by a significant drop from 25 to 35 mirroring the 13ccarb fall observed at this and other sections worldwide higher in the section in the dahai member values increase to 20 again demonstrating communication between doc and dic pools during the early cambrian on 105 106 year time scales from the base of the overlying shiyantou formation 13corg values remain around 30 until the end of the section testifying to an interval of more modest change in dic 13c that would be consistent with biostratigraphic correlations nitrogen isotope values vary independently from carbon isotope trends exhibiting several major 15n cycles with values fluctuating between 9 and 1 robust trends in 15n within the dahai member testify to changes in the ratio between nitrogen fixation and denitrification possibly reflecting fluctuations in the water column chemocline as 13c values reached their global acme up section nitrogen isotope values show dampened cyclicity with values remaining between 0 and 5 indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles the overall variability in 15n closely resembles that of recent marine sediments while the observed cyclicity is defined by several samples in all cases these observations argue for good preservation of original isotopic signatures which bodes well for future n isotope studies of the precambrian cambrian boundary

the svalbard barents sea ice sheet historical current and future perspectives abstract the history of research on the late quaternary svalbard barents sea ice sheet mirrors the developments of ideas and the shifts of paradigms in glacial theory over the past 150 years since the onset of scientific research there in the early 19th century svalbard has been a natural laboratory where ideas and concepts have been tested and played an important but rarely acknowledged role in the break through of the ice age theory in the 1870 s the history of how the scientific perception of the svalbard barents sea ice sheet developed in the mid 20th century also tells a story of how a combination of fairly scattered and often contradictory observational data and through both deductive and inductive reasoning could outline a major ice sheet that had left but few tangible fingerprints since the 1980 s with increased terrestrial stratigraphical data ever more marine geological evidence and better chronological control of glacial events our perception of the svalbard barents sea ice sheet has changed the first reconstructions depicted it as a static concentric single domed ice sheet with ice flowing from an ice divide over the central northern barents sea that expanded and declined in response to large scale late quaternary climate fluctuations and which was more or less in tune with other major northern hemisphere ice sheets we now increasingly perceive it as a very dynamic multidomed ice sheet controlled by climate fluctuations relative sea level change as well as subglacial topography substrate properties and basal temperature in this respect the svalbard barents sea ice sheet will increasingly hold the key for understanding the dynamics and processes of how marine based ice sheets build up and decay

nitrogen and argon signatures in 3 8 to 2 8 ga metasediments clues on the chemical state of the archean ocean and the deep biosphere abstract n and ar elemental and isotopic analyses were conducted on archean metasediments of isukasia west greenland and pilbara craton western australia in order to investigate the n isotopic evolution during the first half of earth s history the selected samples are deep sea sediments and hydrothermal deposits having ages from 3 8 to 2 8 ga and affected by different degrees of metamorphism the release patterns of n and ar obtained by high resolution stepped combustion show the occurrence of at least two trapped components the first is released at 600 c and it is likely contained in fluid inclusions n is released together with primordial 36ar and shows a 15n value of 1 3 1 0 close to that of modern atmospheric n2 15n 0 this component is well preserved in hydrothermal vent silica deposits of north pole pilbara craton and nitrogen may represent ammonium salt dissolved in deep sea hydrothermal fluids the second n component released at temperatures higher than 1000 c is accompanied by radiogenic 40ar and shows a 15n value of 7 4 1 0 in a kerogen rich chert from north pole pilbara craton this n is likely biogenic and negative 15n values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic nh4 contained in hydrothermal fluids this 15n depleted biogenic component may occur in isukasia banded iron formation 15n 1 7 but further data are needed to confirm such a hypothesis in all other samples metamorphic induced rayleigh distillation has altered the pristine n isotopic signature

the nitrogen isotope composition of sediments from the proto north atlantic during oceanic anoxic event 2 sediment records of the stable isotopic composition of n 15n show light 15n values at several sites in the proto north atlantic during oceanic anoxic event 2 oae 2 at the cenomanian turonian transition 94 ma the low 15n during the event is generally attributed to an increase in n2 fixation and incomplete uptake of 4 for phytoplankton growth a compilation of all reliable data for the proto north atlantic during oae 2 demonstrates that the most pronounced negative shift in 15n from pre oae 2 to oae 2 occurs in the open ocean but with 15n never lower than 3 using a box model of n cycling for the proto north atlantic during oae 2 we show that n2 fixation is a major contributor to the 15n signal especially in the open ocean incomplete uptake of 4 for phytoplankton growth is important in regions dominated by downwelling with lateral transport of 4 acting as a major source in the southern proto north atlantic where bottom waters were euxinic the light 15n signature is largely explained by upwelling of 4 our study provides an overview of regional differences in 15n in the proto north atlantic and highlights the role of lateral exchange of water and nutrients in addition to local biogeochemical processes in determining 15n values of oae 2 sediments

nitrogen cycling during the cretaceous cenomanian turonian oceanic anoxic event ii low 15n values ranging from 1 2 to 3 9 and atomic carbon nitrogen c n ratios of 25 50 are characteristic of black shales deposited during the cenomanian turonian boundary interval and oceanic anoxic event ii oae ii an observed antithetic relationship between c n ratios and 15n could suggest a predominance of terrestrially derived organic matter or a diagenetic control on 15n variability shifting bulk 15n values lower however hydrogen indices hi generally 450 and a positive correlation of hi with c n mitigates against a significant terrestrial organic matter fraction high c n values are likely the result preferential degradation of labile n rich compounds during early diagenesis and loss of n as ammonium from sediments through time a hypothetical model that considers the degradation of a 15n enriched labile protein fraction yields only small 1 2 negative shifts in 15n however 15n depletion during diagenesis is contrary to normal isotope kinetics which should result in 15n enrichment of bulk organic matter therefore we conclude that the bulk 15n values in this study reflect primary changes in the nitrogen cycle the 15n data support the hypothesis of expanded nitrogen fixation driven by upwelling of nutrient nitrogen poor phosphorus replete waters during oae ii and from the mid cenomanian to santonian at demerara rise the low 15n values which are significantly lower than bulk 15n values in modern regions where nitrogen fixation is known to be important probably result from a more significant fraction of dissolved inorganic nitrogen being produced by nitrogen fixation during the peak of oae ii a marked shift to lower 15n values is observed this shift possibly reflects greater utilization of 15n depleted ammonium during a chemocline upward excursion cue dominance of low 15n values from other periods of more widespread marine anoxia is likely the result of similar processes

further evidence for the development of photic zone euxinic conditions during mesozoic oceanic anoxic events periods in earth history characterized by extensive organic carbon deposition so called oceanic anoxic events oaes are the subject of considerable scrutiny insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene a carotenoid found only in green sulphur bacteria and thus indicative of photic zone euxinia i e a water column containing free h 2 s in north atlantic deep sea sediments deposited during the cenomanian turonian boundary oae2 bonarelli event in this study we have used the distributions of chlorophyll and bacteriochlorophyll degradation products maleimides 1 h pyrrole 2 5 diones and high molecular weight porphyrins to examine further the cenomanian turonian boundary oae2 as well as other oaes of the early toarcian posidonienschiefer event and early aptian selli event in particular methyl isobutyl me i bu maleimide on structural grounds appears to be diagnostic of green sulphur bacteria this compound occurs in five of seven examined marls that record the early toarcian oae marche umbria italy further expanding the geographical range of toarcian sediments where evidence for photic zone euxinic conditions has been found me i bu maleimide occurs in three of six black shales spanning the livello selli marche umbria italy the type locality for the aptian oae1a providing the first evidence for photic zone euxinic conditions during this event with respect to the cenomanian turonian oae maleimide evidence for photic zone euxinic conditions was found in all the north atlantic sites investigated including those characterized by relatively organic lean sediments several tethyan sites and one site off western australia these data indicate that euxinic conditions were common in the water column during mesozoic oceanic anoxic events

a prominent isotopic fingerprint of nitrogen uptake by anaerobic methanotrophic archaea abstract marine sediments represent the largest reservoir of methane on earth anaerobic oxidation of methane aom mediated by anaerobic methanotrophic archaea anme and associated sulfate reducing bacteria is the main process controlling methane emission from marine sediments recent laboratory studies have revealed that anme or aom consortia can mediate nitrogen n fixation and ammonium assimilation these processes are known to preferentially utilize 14n rather than the 15n isotope however geochemical fingerprints of microbial nitrogen uptake in marine sediments are lacking limiting our understanding of its role in modern and ancient sedimentary environments here we report prominent negative excursions of bulk sediment 15n values up to 5 at sulfate methane transitions smts where anme show peak abundances in sediments at four sites of the haima seeps of the south china sea such 15n excursions are restricted to smts suggesting that 15n depletion results from anme mediated nitrogen uptake we posit that this process is ubiquitous at the smt a reaction front in coastal and continental margin sediments worldwide resulting in similar isotope patterns negative 15n excursions are consequently a promising proxy to track aom in the geological record and to better constrain carbon and nitrogen cycling in ancient sedimentary environments

rapid degradation of permafrost underneath waterbodies in tundra landscapes toward a representation of thermokarst in land surface models waterbodies such as lakes and ponds are abundant in vast arctic landscapes and strongly affect the thermal state of the surrounding permafrost in order to gain a better understanding of the impact of small and medium sized waterbodies on permafrost and the formation of thermokarst a land surface model was developed that can represent the vertical and lateral thermal interactions between waterbodies and permafrost the model was validated using temperature measurements from two typical waterbodies located within the lena river delta in northern siberia impact simulations were performed under current climate conditions as well as under a moderate and a strong climate warming scenario the performed simulations demonstrate that small waterbodies can rise the sediment surface temperature by more than 10 c and accelerate permafrost thaw by a factor of between 4 and 5 up to 70 of this additional heat flux into the ground was found to be dissipated into the surrounding permafrost by lateral ground heat flux in the case of small shallow and isolated waterbodies under moderate climate warming the lateral heat flux was found to reduce permafrost degradation underneath waterbodies by a factor of 2 under stronger climatic warming however the lateral heat flux was too small to prevent rapid permafrost degradation the lateral heat flux was also found to strongly impede the formation of thermokarst despite this stabilizing effect our simulations have demonstrated that underneath shallow waterbodies 1 m thermokarst initiation happens 30 to 40 years earlier than in simulations without preexisting waterbody

creep behaviour of undisturbed clay permafrost an in situ analysis of naturally occurring creep has been carried out at the proposed canadian arctic gas pipeline crossing of great bear river in the northwest territories this is the last of fou

nitrogen cycle dynamics in the late cretaceous greenhouse abstract great attention has been paid to the origin of anomalously low nitrogen isotope values during the late cretaceous nitrogen isotope values are often as low as 3 and are typically less than 2 even in relatively organic matter lean sediments here we evaluate nitrogen isotope variability during a relatively quiescent phase of the late cretaceous greenhouse between oceanic anoxic events 2 and 3 using the black shales of demerara rise dr selection of this site allows us to isolate some of the factors that control nitrogen cycle dynamics and contribute to low nitrogen isotope values new n isotope measurements from odp site 1259 reveal 15 n values that range from 0 2 to 3 5 and oscillate by 1 5 to 3 over 1 6 million years ma temporal calibration of our data using a new astronomical time scale reveals a strong 100 thousand year ka eccentricity cyclicity in 15 n we attribute this cyclicity to oscillations in the position of the intertropical convergence zone itcz over dr that modulate upwelling intensity chemocline depth and the degree of 15n depletion we also recognize a statistically significant correlation p 0 0022 between the tex86 indices and 15 n with the lowest 15 n corresponding to the highest tex86 indices this relationship suggests that the activity and ecology of ammonia oxidizing thaumarchaeota and the 15 n of dissolved inorganic nitrogen utilized by primary producers are linked we therefore interpret the observed variability in the 15 n data and tex86 indices as primarily reflecting fluctuation of upwelling intensity and chemocline depth and the significant inverse relationship between these data sets suggests that caution should be exercised when interpreting the tex86 in terms of temperature in similar paleoenvironmental settings

speciation and isotope ratios of nitrogen in fluid inclusions from seafloor hydrothermal deposits at 3 5 ga abstract we report here the results of our study on speciation and isotope ratios of n in fluid inclusions preserved in 3 5 ga hydrothermal deposits silica dikes and quartz veins from the north pole area of the pilbara craton western australia crush leach analysis and raman microspectrometry revealed that n within the fluid inclusions exists as n2 and nh4 a negative correlation between the so42 na and 40ar 36ar ratios of the fluid inclusions suggests mixing of two end members hydrothermal fluid with low so42 na and high 40ar 36ar ratios and 3 5 ga seawater with high so42 na and low 40ar 36ar ratios values of 15nn2 from the hydrothermal component vary over a considerable range 3 0 to 3 7 and those of the seawater component are well within this range i e 0 7 to 0 2 this suggests that the isotope ratio of n2 dissolved in the 3 5 ga seawater would have been 0 7 to 0 2 since isotope fractionation between n2 in the atmosphere and n2 dissolved in seawater is minimal the 15nn2 value of the 3 5 ga atmosphere would have been within the range 2 0 which is similar to the 15nn2 value of the present day atmosphere 15nn2 0 this study also suggests that the fluid inclusions contain nh4 that would have been derived from the seawater and or mantle at 3 5 ga therefore kerogens in archean cherts might have been partly derived from biological assimilation of nh4 in hydrothermal fluids

nitrogen and carbon cycle perturbations through the cenomanian turonian oceanic anoxic event 2 94 ma in the vocontian basin se france abstract oceanic anoxic event 2 spanning the cenomanian turonian boundary was an episode of major perturbations in global biogeochemical cycles characterized by the widespread burial of organic carbon in both oceans and epicontinental basins in the vocontian basin se france this event was expressed by the deposition of the niveau thomel a succession of interbedded dark shales grey marls and bioturbated carbonates as observed in the lambruisse section analyses of the total organic carbon toc content combined with the concentrations of redox sensitive trace elements revealed that conditions fluctuated from oxic to suboxic anoxic during the deposition of the niveau thomel transient periods of deoxygenation were related to perturbations in nutrient cycling reflected by near zero to negative 15n signatures indicative of increased diazotrophy the development of deoxygenated conditions in the vocontian basin was likely preconditioned by hydrographically restricted conditions as confirmed by mo vs u enrichments reducing bottom water ventilation associated with water column stratification intervals of reoxygenation were likely linked to climate induced changes in particular a phase of regional or global cooling associated with a drop in pco2 called the plenus cold event pce resulted in vigorous reoxygenation of the vocontian basin

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

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

high primary productivity and nitrogen cycling after the paleoproterozoic phosphogenic event in the aravalli supergroup india the causes of atmospheric oxygenation in the paleoproterozoic are multifaceted yet poorly understood an increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere a likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate which would have resulted in a fundamental change from the archean nitrogen cycle in this study we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the paleoproterozoic aravalli supergroup india the shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2 0 ga shales in the phosphate domain of the jhamarkotra formation from the lower aravalli group have distinct characteristics from coeval shales in the non phosphate domain carbonaceous shales of the phosphate domain contain relatively high v and u th typical of oxygen deficient environments these shales also have a high organic content of up to 14 wt generally homogeneous 13 corg values around 29 c nwr up to 360 and a large range of 15