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

reappraisal of hydrocarbon biomarkers in archean rocks hopanes and steranes found in archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes but the syngeneity of these hydrocarbon biomarkers is controversial to resolve this debate we performed a multilaboratory study of new cores from the pilbara craton australia that were drilled and sampled using unprecedented hydrocarbon clean protocols hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit but when they were detectable they had total hopane 37 9 pg per gram of rock and total sterane 32 9 pg per gram of rock concentrations comparable to those measured in blanks and negative control samples in contrast hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389 5 pg per gram of rock and 1 039 pg per gram of rock respectively polycyclic aromatic hydrocarbons and diamondoids which exceed blank concentrations exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1 000 ng per gram of rock in hydropyrolysates from the ultraclean cores these results demonstrate that previously studied archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons therefore existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by 2 7 billion years ago although suitable proterozoic rocks exist no currently known archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation so future exploration for archean biomarkers should screen for rocks with milder thermal histories

diazotrophic cyanobacteria as the major photoautotrophs during mid cretaceous oceanic anoxic events nitrogen and carbon isotopic evidence from sedimentary porphyrin abstract we determined both the nitrogen and carbon isotopic compositions of nickel chelated deoxophylloerythroetioporphyrin ni dpep a major sedimentary porphyrin extracted from the livello selli and livello bonarelli black shales deposited in the western tethys sea during mid cretaceous oceanic anoxic events oaes based on empirical isotopic relationships between the tetrapyrrole nuclei of chlorophylls and photoautotroph cells we estimate that the mean nitrogen isotopic composition of the entire photoautotrophic communities during these periods ranged from 2 to 1 this result strongly suggests that n2 fixation was an important primary process in photoautotrophic production during these oaes the estimated carbon isotopic composition of the photoautotrophs was elevated between 20 and 22 relative to typical cretaceous examples indicating as much as a 5 reduction in the magnitude of carbon isotopic fractionation associated with photosynthesis during oaes in the western tethys sea this anomaly can be well explained if cyanobacteria were the dominant producers because they commonly conduct carboxylation and or active transport of carbon substrates resulting in reduced carbon isotopic fractionation we therefore conclude that diazotrophic cyanobacteria were the dominant components of primary production during oae 1a and oae 2 in the western tethys sea

potassium distribution and metasomatism in pelites and schists how and when relation to postdepositional events a large global database for over 9000 archean to holocene pelites in numerous basins has been developed to evaluate diagenetically unaltered and altered compositions the mineralogy and geochemistry of quaternary muds and ancient mudstones provide first order controls on diagenetic potassium metasomatism in pelites clay mineralogy of quaternary muds is dominated by smectite and kaolinite with minor detrital mica and illite thus k2o is 4 k al is 0 25 and k rb is 220 in most quaternary muds with diagenetic alteration of clays to illite smectite and illite k2o content increases to 4 8 and k al rises to 0 2 0 5 or more in extreme potassium metasomatism authigenic k feldspar develops sources of potassium are internal to shale sandstone units wherein detrital k feldspar is dissolved during diagenesis providing k for clay illitization in closed system isochemical alteration as in some gulf coast and north sea basin cases potassium in amounts greater than that available from closed system alteration is provided from extraformational circulating brines in open system diagenesis k rb with diagenesis varies according to whether the metasomatizing fluid was relatively poor in rb seawater derived brines or richer in rb as in many hydrothermal waters diagenetic fluids may originate from seawater or meteoric water enriched in cations by water rock interactions along their subsurface migration paths and or by dissolution of evaporites potassium metasomatizing fluids penetrate low permeability pelites through faults fractures and microfractures and by diffusion in the mineral matrix typically k metasomatism possibly with pb zn cu ba and f mineralization occurs long after sediment deposition and burial at temperatures of 50 to 150 c related to tectonic activity and fluid movement different modes of metasomatism are recognized and correlated with geological setting in the passive margin gulf coast tertiary basin open system k metasomatism where present is limited to areas adjacent to steeply dipping growth faults which act as conduits for episodic pulses of altering fluids migrated upward from deeper basin sources in contrast widespread pervasive open system k metasomatism produced by continuous long term gravity advective and diffusive flow of brine from the late paleozoic appalachian ouachita wichita mountains highland to altered earlier paleozoic shales plus sandstones bentonites and carbonates in the illinois and anadarko ardmore foreland basins this mode of k alteration following major mountain building events is characteristic of many precambrian and paleozoic collision orogens and associated foreland basins another mode is in rift basins with no external drainage in which indigenous waters are evaporated to brines these brines cause albitization of k feldspar to yield potassium for illitization of detrital clays

15 n depleted nitrogen in early archean kerogens clues on ancient marine chemosynthetic based ecosystems a comment to beaumont v robert f 1999 precambrian res 96 62 82

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

storage and mobility of nitrogen in the continental crust evidence from partially melted metasedimentary rocks mt stafford australia abstract the mt stafford region central australia provides well exposed metasedimentary rocks whose protoliths were deposited at ca 1 84 1 81 ga and that were metamorphosed at greenschist to granulite facies pressure temperature conditions at 1 8 ga the higher grade rocks show evidence of low pressure partial melting 2 4 kbars over a temperature range of 650 to 850 c these exposures afford evaluation of the extent to which n is retained through devolatilization and partial melting reactions experienced in the shallow to mid continental crust in the mt stafford metapelitic metapsammitic suite whole rock 15 n air values of the lowest grade greenschist facies rocks zone 1 range from 2 3 to 6 2 mean 3 6 1 1 5 n 7 and values for the highest grade granulite facies rocks zone 4 range from 2 1 to 8 5 mean 5 2 1 2 1 n 20 six of the twenty samples at the highest grade have values higher than those of the greenschist rocks regarded as likely protoliths all but a few of the zone 4 rocks have n concentrations lower than the mean n concentration for the lowest grade rocks ranges of 107 346 ppm mean 254 ppm for zone 1 rocks and 43 361 ppm mean 185 ppm for zone 4 rocks for the higher grade rocks which experienced multiple partial melting reactions varying retention of n as nh 4 in k feldspar or as n 2 in cordierite may indicate that n is held in peritectic products of partial melting rather than in the melt although assessments of element loss in the mt stafford suite are greatly complicated by the large degree of protolith related heterogeneity some elements do show modest decline in concentration with increasing grade consistent with their loss during devolatilization and partial melting nitrogen appears most similar in behavior to rb the lile most similar in ionic radius to nh 4 but of these two elements n shows somewhat greater evidence for whole rock loss k 2 o shows only very subtle hints of whole rock loss in some samples and cs shows considerably greater loss relative to n and rb resulting for example in increases in rb cs at the higher grades increased whole rock ba rb for some higher grade samples relative to that for lower grade equivalents likely reflects the formation of a major ba reservoir in k feldspar at the expense of biotite resulting in no obvious whole rock ba loss with rb liberated from biotite breakdown partitioning into the melts that at least partly left the system reduced whole rock b concentrations at the higher grades resulted from the breakdown of tourmaline during the melting reactions these varying degrees of reduction in whole rock n rb cs and b and possibly k 2 o concentration are accompanied by decreased concentrations in h 2 o as indicated by loi li and u the loss of u resulting in increased th u in the highest grade rocks apparent differential loss of these elements based on the whole rock data could reflect the amounts of melt produced and the extents to which the melts were removed at the scales sampled by the hand specimens that were analyzed six of the twenty zone 4 samples showing greater shift in 15 n to values of 7 4 to 8 4 tend to be among the samples showing the largest changes in other major and trace element concentrations and ratios believed to reflect melt removal e g k 2 o cs tio 2 b al 2 o 3 th u and ratios among rb sr ba and cs the metasedimentary rocks at mount stafford also contain 217 to 1186 ppm c likely as reduced c graphite but at the higher grades probably also as co 2 in the channels of cordierite at greenschist grade whole rock 13 c v pdb values are relatively uniform with organic compositions mean 25 1 1 1 6 n 7 but at the highest grade zone 4 values are as high as 17 7 mean 22 2 1 2 3 n 8 overall c and n apparently largely of organic origin show surprising retention in rocks having experienced peak temperatures of 800 c demonstrating their compatibility in mineral phases in fluid and melt residues for n as nh 4 in k feldspar and as n 2 in cordierite for c as graphite and as co 2 in cordierite these data have implications for deep earth n cycling bearing on the size of the n reservoir in the mid continental crust and thus long term storage of n previously residing in the atmosphere and biosphere and for the use of the 15 n of upper crustal intrusive rocks as a tracer of their deep crustal sources

nitrogen cycle perturbations linked to metazoan diversification during the early cambrian abstract the rapid diversification of metazoans during the late ediacaran early cambrian is one of the most remarkable evolutionary events in earth history however the triggering mechanism remains highly contentious particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling in this study we report new bulk nitrogen isotope 15n and organic carbon isotope 13corg data from the lower cambrian niutitang formation in two deep water sections in south china to provide a better understanding of the relationship of the nitrogen cycle ocean redox and biological evolution during this key period the 15n data from two study sections are characterized by consistently low values 1 to 2 throughout the niutitang formation despite large variations in 13corg we interpret these low 15n values to have resulted from assimilation of recycled ammonium nh4 and n2 fixation compilation of available 15n data reveals large temporal and spatial 15n variations from 1 to 7 in time equivalent strata in south china such 15n variations imply the co existence of aerobic and anaerobic n cycles and highly heterogeneous ocean redox conditions during the early cambrian in south china which is consistent with inferences from other geochemical proxies temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early cambrian

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

facies and origin of shales from the mid proterozoic newland formation belt basin montana usa shales constitute more than 60 of the world s sediments yet while facies models for sandstones and carbonates are at a high level of sophistication the study of shales has clearly lagged behind in the mid proterozoic newland formation six major shale facies types deposited in nearshore to basinal environments are distinguished on the basis of bedding characteristics textural features and the proportions of silt clay and carbonate textural features of these shale types are related to sedimentary environments as deduced from associated lithologies the shales are undisturbed by bioturbation and their textural and sedimentary characteristics reflect subaqueous growth of microbial mats erosion and deposition by storms deposition of flocculated vs dispersed clays continuous slow background sedimentation winnowing by waves or currents and subaerial exposure

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

the cycling and redox state of nitrogen in the archaean ocean organisms that produce oxygen through photosynthesis existed during the late archaean eon about 2 500 million years ago but controversial evidence suggests that they may have evolved several hundred million years earlier oxygen is expected to react with oceanic nitrogen altering its redox state the reaction leaves a signature in the isotopic composition of the nitrogen bound in organic matter here we present a record of the nitrogen isotopic composition of kerogen extracted from minimally altered shales from the campbellrand malmani platform in south africa between the palaeo archaean and about 2 670 million years ago the 15n values of the kerogen rose by about 2 we interpret this increase as an indication of the onset of coupled nitrification and denitrification or anammox reactions in the surface oceans which require the presence of free oxygen a second increase in nitrogen isotopic composition around 2 520 million years ago implies instability of the n cycle with loss of fixed n this evidence for available oxygen in the oceans occurs at least 200 million years before geochemical indications of the presence of significant levels of atmospheric oxygen we suggest that coupled nitrification and denitrification drove the loss of fixed inorganic nitrogen leading to nitrogen limitation and conclude that the low levels of biologically available nitrogen limited the growth of oxygen producing plankton delaying the accumulation of oxygen in the atmosphere the initial production of oxygen in early earth s oceans altered the redox chemistry and cycling of nitrogen a record of nitrogen isotopes from preserved organic matter indicates nitrogen cycling in the presence of free oxygen 2 67 billion years ago about 200 million years before the first geochemical evidence for atmospheric free oxygen

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

selenium isotope ratios redox changes and biological productivity across the end permian mass extinction abstract the causes of the greatest mass extinction in earth s history in the latest permian remain actively debated here we use se isotopes and abundances in marine sediments from an outer shelf environment to test one of the most common hypotheses for the collapse of the biosphere i e widespread euxinia in the open ocean our data show a small positive excursion in 82 78 se prior to the extinction consistent with local euxinia however this is followed by a significant negative excursion with a minimum of 1 8 relative to nist srm 3149 immediately preceding the principal extinction horizon a net fractionation of this magnitude likely resulted from partial reduction of se oxyanions dissolved in the water column due to their low abundance se oxyanions are rapidly scavenged in anoxic basins or regions of high biological productivity with little net isotopic fractionation we therefore interpret the uniquely negative fractionations in this section as an indicator for relatively oxygenated conditions in this marine basin at the time when biological productivity declined the offset between the peak excursion and the major extinction horizon possibly reflects a slow down in ocean circulation leading to nutrient limitation which may thus have prohibited a rapid recovery of the local biosphere in the early triassic although we are unable to extrapolate to the global ocean due to the short residence time of se in seawater our data are consistent with the newly emerging view that euxinia developed along ocean margins and in oxygen minimum zones before the extinction but was probably replaced by sub oxic conditions during the 1 kyr peak productivity decline and was thus not solely responsible for the extinction event

stability of the nitrogen cycle during development of sulfidic water in the redox stratified late paleoproterozoic ocean nitrogen cycling has been evaluated across a depth transect in the late paleoproterozoic animikie basin north america spanning the end of earth s final period of global iron precipitation and a major transition to euxinic conditions in areas of high productivity sediments from near shore where productivity was highest have 15n compositions up to 3 higher than at more distal sites this suggests that as nh4 mixed vertically upward into the oxic photic zone from the anoxic ocean interior it was either assimilated by organisms or oxidized subsequent enhanced production of n2 by denitrification or anammox anaerobic ammonium oxidation led to the observed increase in 15n close to shore any deficit in biologically available n was overcome by n2 fixing organisms but the input of n with low 15n from this process did not overwhelm the increase in 15n from denitrification because there is no evidence for conditions of severe n stress arising from trace metal limitation particularly mo of n fixation during the transition to euxinic conditions losses of n were either very small potentially because low o2 levels limited nh4 oxidation or alternative pathways that retained n were important the fact that mo appears to have remained bioavailable for n fixation either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester mo on a global scale or that rivers directly delivered mo to surface waters on the inner shelf the effects of n2 fixation on 15n increased to more distal parts of the shelf consistent with models invoked for modern upwelling zones over broad continental margins

nitrogen isotope evidence for alkaline lakes on late archean continents abstract nitrogen isotope ratios in ancient sedimentary rocks are generally interpreted as a proxy for metabolic nitrogen pathways and the redox state of the water column fractionation processes occurring under anoxic alkaline conditions during the dissociation of nh 4 to h and volatile nh3 are frequently overlooked although this mechanism imparts large isotopic fractionations here we propose that nh3 volatilization is largely responsible for 15n values of up to 50 at high c n ratios in the late archean tumbiana formation this sequence of sedimentary rocks represents a system of lakes that formed on subaerial flood basalts and were partly filled by basaltic volcanic ash aqueous alteration of volcanic glass followed by evaporative concentration of ions should have led to the development of high alkalinity with a ph of 9 or higher as in modern analogues in this sedimentologically unusual setting nitrogen isotope ratios thus provide indirect evidence for the oldest alkaline lake system in the rock record these very heavy lacustrine 15n values contrast markedly with those of archean marine sedimentary rocks making a precambrian soda ocean unlikely today alkaline lakes are among the most productive ecosystems on earth some nutrients in particular molybdenum are more soluble at high ph and certain prebiotic reactions would likely have been favored under alkaline conditions in similar settings earlier in earth s history hence alkaline lakes in the archean could have been significant for the origin and early evolution of life

distribution of ammonium in minerals of metamorphic and granitic rocks abstract sixty nine analyses are given for nh4 in minerals of metamorphic and granitic rocks mostly from the ryoke belt japan the distribution of nh4 in coexisting minerals is quite systematic suggesting that nh4 is one of the stable geochemical components in high temperature processes biotite has the highest content of nh4 followed by muscovite k feldspar and plagioclase pure quartz is almost free from nh4 calcic plagioclase contains less nh4 than does sodic plagioclase the partition coefficients d pl bi d kf bi and d kf bi are on the average 0 11 0 38 and 0 43 respectively the fractionation of nh4 in these minerals is quite similar to that of rb but much smaller than that of cs distribution of nh4 as well as those of rb and cs appears to be explained by its ionic radius and the shortest cation o distances in alkali positions of minerals

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

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

differential metamorphic effects on nitrogen isotopes in kerogen extracts and bulk rocks abstract the last decade has seen a steady rise in the number of publications on nitrogen isotopes in sedimentary rocks which have become an established tool for investigating the evolution of life and environmental conditions nitrogen is contained in sedimentary rocks in two different phases bound to kerogen or substituted in potassic minerals mostly k bearing phyllosilicates and feldspars isotopic measurements and interpretations typically focus either on kerogen extracts alone or on bulk rocks that include both phases the community is split about which sample type more accurately captures the original composition of biomass to address this question we combined nitrogen isotopes and carbon to nitrogen ratios with carbon to hydrogen ratios which act as an independent proxy for metamorphic alteration our results reveal that metamorphism drives kerogen bound nitrogen isotopically lighter while silicate bound nitrogen becomes heavier for rocks up to greenschist facies the isotopic effect of this internal partitioning up to 3 4 is larger than the isotopic effect of metamorphic nitrogen loss from the system up to 1 2 the opposite may be true for higher metamorphic grades we conclude that for low grade sedimentary rocks with more than 60 of their total nitrogen residing in the silicate phase the primary isotopic composition of the biomass is best approximated by bulk rock measurements whereas for high grade rocks the kerogen extract may be the more accurate proxy the isotopic difference between nitrogen phases can serve as a rough indicator of the degree of metamorphic alteration

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

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

characteristics of molecular nitrogen generation from overmature black shales in south china preliminary implications from pyrolysis experiments abstract high content of molecular nitrogen n2 is one of the natural gas exploration risks in petroliferous basins where black shales act as source rocks of either hydrocarbon gas or molecular nitrogen in this study two overmature and one low maturity shale samples and their kerogens were investigated to determine the generation characteristics of molecular nitrogen as well as methane and the differences in the release processes of inorganic nitrogen fixed in ammonium bearing minerals and organic nitrogen bound in kerogen the results illustrate that with increasing pyrolysis temperature the yield of methane first increases and then decreases with an inflection temperature of 650 c eqvro 3 4 whereas the yield of molecular nitrogen shows a continuous increase throughout the pyrolysis experiment the molecular nitrogen during the stage of methane generation i e eqvro 3 4 the results also reveal that at the experimental maxima of thermal maturity eqvro 4 9 the generation potential of inorganic molecular nitrogen mg g ninorg is much lower than that of organic molecular nitrogen mg g norg indicating that in our experiments the organic nitrogen in kerogen is more easily to be converted into molecular nitrogen than the inorganic nitrogen in ammonium bearing minerals all these results indicate that molecular nitrogen content in shale gas may change dramatically during thermal evolution and source rocks with exceptionally high maturity and high abundance of organic nitrogen likely lead to a high molecular nitrogen risk particularly in regions of poor preservation conditions of shale gas which may be the main reasons for the high molecular nitrogen content in the lower cambrian shale gas in south china

the azores current system from a meridional section at 24 5 w 1 high spatial resolution hydrographic data including lowered acoustic doppler current profiler ladcp measurements were acquired along a meridional section at 24 5 w in october 2009 the data are analyzed in detail with the purpose of definitively defining and quantifying the zonal azores current system the azores current and azores countercurrent are delimited each extending meridionally for 110 km the azores current is located between 33 5 n and 34 5 n flanked to the north by the azores countercurrent 35 25 36 25 n vertically both currents reach the n 27 975 kg m 3 level 2000 m depth their mass transports ranging across thermocline as well as intermediate layers the azores current transports 13 9 sv 1 sv 106 m3 s 1 109 kg s 1 eastward with its maximum associated with the azores front 33 75 n the azores countercurrent flows below the surface transporting 5 5 sv westward this contributes to a net eastward flow of 8 4 sv across the section at intermediate layers the azores countercurrent transports mixed mediterranean water to the west and the azores current transports mixed sub arctic intermediate water to the east shipboard adcp and satellite derived geostrophic velocity are used to confirm the transports revealed by the hydrographic data

nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the karoo basin south africa abstract we have investigated the n geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes hvc in the karoo basin in south africa the hvc formed during phreatic eruptions associated with rapid devolatilization and pressure build up in contact aureoles around early jurassic sill intrusions by combining outcrop data from a hvc and core data from contact aureoles we investigate the relationship between light element release during metamorphism and vertical fluid migration sandstone and breccia from the hvc contain early diagenetic ammonium nh 4 bearing feldspar buddingtonite and illite ammonium occupies up to 95 of the a site in feldspar corresponding to concentrations up to 5 2 wt n bulk rock n isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups background samples have 15nair between 1 5 and 4 9 whereas minerals from the vent complex have 15n in the range 7 5 to 10 6 the n geochemistry of contact metamorphic shale from the lower stratigraphic units of the karoo basin shows that the vitrinite reflectance and 15n values are positively correlated shale with reflectivity values 4 ro are enriched in 15n with 15n values between 6 and 14 implying the release of isotopically light n into metamorphic fluids probably as n2 we suggest that the relatively high 15n values of the early diagenetic buddingtonite in the hvc reflect exchange of buddingtonite with n bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration we present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation focusing n bearing metamorphic fluids sourced in deeper levels of the basin the release of organic n from sediments at depth in volcanic basins could play a role in the geochemical cycle of n becoming particularly important during periods of intense volcanic activity

isotopic evidence for massive oxidation of organic matter following the great oxidation event the stable isotope record of marine carbon indicates that the proterozoic eon began and ended with extreme fluctuations in the carbon cycle in both the paleoproterozoic 2500 to 1600 million years ago ma and neoproterozoic 1000 to 542 ma extended intervals of anomalously high carbon isotope ratios 13c indicate high rates of organic matter burial and release of oxygen to the atmosphere in the neoproterozoic the high 13c interval was punctuated by abrupt swings to low 13c indicating massive oxidation of organic matter we report a paleoproterozoic negative 13c excursion that is similar in magnitude and apparent duration to the neoproterozoic anomaly this shunga francevillian anomaly may reflect intense oxidative weathering of rocks as the result of the initial establishment of an oxygen rich atmosphere

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

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

ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western maine usa abstract ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic inorganic interactions during diagenesis as well as of the devolatilization history and fluid rock interaction during metamorphism in this study a collection of geochemically well characterized biotite separates from a series of graphite bearing paleozoic greenschist to upper amphibolite facies metapelites western maine usa were analyzed for ammonium nitrogen nh 4 n contents and isotopic composition 15nnh4 using the hf digestion distillation technique followed by the ea irms technique biotite separates sampled from 9 individual metamorphic zones contain 3000 to 100 ppm nh 4 n with a wide range in 15n from 1 6 to 9 1 average nh 4 n contents in biotite show a distinct decrease from about 2750 ppm for the lowest metamorphic grade 500 c down to 218 ppm for the highest metamorphic grade 685 c decreasing abundances in nh 4 are inversely correlated in a linear fashion with increasing k in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect despite expected increasing 15nnh4 values in biotite with nitrogen loss a significant decrease from the garnet zones to the staurolite zones was found followed by an increase to the sillimanite zones this pattern for 15nnh4 values in biotite inversely correlates with mg mg fe ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between nh 4 nh 3 or nh 4 n 2 reflecting devolatilization history and redox conditions during metamorphism

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

ammonium concentration and nitrogen isotope composition in metasedimentary rocks from different tectonometamorphic units of the european variscan belt the content and isotopic composition of ammonium nh4 were measured by using kjeldahl distillation for 63 samples of predominantly metasedimentary rocks from two different geological settings of the european variscides i e the erzgebirge and the zone of erbendorf vohenstrauss in germany the studied erzgebirge schists represent an early paleozoic passive margin sequence that reappears in different deep subducted metamorphic units this allows an efficient examination of nitrogen loss and isotope fractionation during prograde metamorphism ammonium is found to be progressively depleted accompanied by a shift in 15n from the low grade units 2 kbar 300 c with 638 124 ppm and 15n 2 2 0 6 to the garnet phyllite unit 9 kbar 470 c with 621 190 ppm and 15n 3 5 0 9 the mica schist eclogite unit 12 kbar 550 c with 394 113 ppm and 15n 3 9 0 8 and to the gneiss eclogite unit 12 kbar 730 c with 99 32 ppm and 15n 7 7 2 0 using equilibrium models for rayleigh distillation and batch volatilization suggest that the nitrogen depletion took place by ammonia release only for the mica schists this isotope fractionation can be explained by the loss of molecular nitrogen in comparison with the early paleozoic schists the proterozoic ortho and paragneisses of the erzgebirge gneiss unit 6 8 kbar 650 c contain significantly lower amounts of ammonium 70 ppm and reach relatively low 15n values 2 5 to 3 6 the paragneisses from the zone of erbendorf vohenstrauss zev sampled from the ktb pilot hole represents a separate tectonometamorphic unit that has undergone polyphase metamorphic conditions with the major imprint being amphibolite facies 7 kbar 650 700 c the metagraywackes have nh4 contents of 80 ppm and 15n of approximately 6 gneisses in the vicinity of a late cataclastic shear zone at 2000 m depth are characterized by a shift in nh4 250 ppm and 15n up to 12 which suggest late fluid rock interactions with a nitrogen and 15n enriched fluid

cessation of a productive coastal upwelling system in the panthalassic ocean at the permian triassic boundary abstract while the end permian extinction in the marine realm is well known from the tethys ocean it remains little studied in the vast panthalassic ocean opal creek alberta canada is a biostratigraphically continuous permian triassic boundary ptb section that is interpreted to have been deposited in a deep outer shelf setting along the panthalassic western margin of pangaea significant organic carbon and nitrogen isotope excursions precede the extinction of the dominant late permian benthic organisms siliceous sponges geochemical comparison with underlying guadalupian age rocks and analysis of the conodont fauna suggest that the latest permian at opal creek records the shutdown of a productive cold water upwelling ecosystem with a mid water column oxygen minimum followed by the transition to a warm and less vigorously circulating system with a bottom water oxygen minimum the transition from a low diversity cold water conodont fauna in the middle permian to a more diverse latest permian fauna containing equatorial species coincides with a transition from elevated 15n values characteristic of coastal zones with denitrification in a mid water oxygen minimum to more normal marine values a spike in the 13c of organic carbon following a latest permian marine transgression along western pangaea is here attributed to transient increased productivity in the photic zone preceding the main extinction likely related to the synergistic effects of warming increased nutrient runoff and residual upwelling this is followed by an equilibration to lower 13corg values characteristic of a lower productivity regime in the early triassic

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

nitrogen isotope record of fluid rock interactions in the skiddaw aureole and granite english lake district the skiddaw granite and its contact metamorphic aureole in the english lake district provide an excellent opportunity to test the capability of the n isotope system to trace devolatilization and large scale transfer of crustal fluids in the aureole skiddaw group metasedimentary rocks with relatively uniform lithology and major element compositions show a dramatic decrease in n content toward the granite contact from or 800 ppm at distances 2 5 km from the contact to 1 5 km these rocks have extremely uniform delta 15 n air near 3 7 per mil whereas closer to the contact or 1 km delta 15 n is shifted to higher values up to 8 7 per mil the coupled decreases in n content and increases in delta 15 n are compatible with the removal of n having low delta 15 n in fluids during continuous prograde devolatilization reactions involving the break down of white mica and the stabilization of biotite cordierite and andalusite bearing assemblages in the same metasedimentary rocks the lack of obvious trends in major element concentrations including sio 2 tio 2 sio 2 al 2 o 3 and the ratios of other major oxides to tio 2 and al 2 o 3 with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism ratios of whole rock n b rb and ba concentrations to whole rock k 2 o content are believed to reflect the differing fluid mica partitioning and involving varying relative proportions of white mica and biotite of these trace elements during devolatilization reactions greisenized skiddaw granite from a borehole is enriched in n range of 17 225 ppm for whole rocks and white mica separates relative to the unaltered granite whole rock 30 ppm and has delta 15 n of 1 0 to 4 8 per mil the n concentrations and delta 15 n of the wall rocks and greisenized granites combined with c isotopic data carbonate and carbonaceous matter for the same rocks are consistent with the mobilization of fluids having low delta 15 n and delta 13 c pdb values from the devolatilized aureole into the cooling intrusive body such transport is consistent with the predictions of recent theoretical models of late stage hydrothermal evolution in cooling intrusive systems

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

giant quartz vein systems in accretionary orogenic belts the evidence for a metamorphic fluid origin from δ 15 n and δ 13 c studies abstract nitrogen isotope compositions of hydrothermal micas in giant structurally hosted quartz vein systems place constraints on the origin of the hydrothermal fluids the vein systems are from neoarchean terranes in the superior province of canada and the norseman terrane western australia the four quartz vein systems studied in different terranes formed at metamorphic grades ranging from lower greenschist to lower amphibolite facies coevally with metamorphism of the host terrane nitrogen resides predominantly as nh 4 in hydrothermal mica for lower to upper greenschist facies quartz veins n contents and 15 n values of micas are between 40 and 200 ppm and 15 and 21 respectively whereas in quartz veins formed at the greenschist to amphibolite transition and lower amphibolite facies micas have n contents of 20 70 ppm and 15 n of 11 24 in contrast micas and k feldspars from granitoids in the neoarchean abitibi and red lake greenstone belts are characterized by systematically lower 15 n of 5 to 5 and generally lower n contents of 20 50 ppm comparable to other granitoids carbon and oxygen isotope compositions of hydrothermal ferroan dolomite and calcite show systematic depletions with increasing metamorphic grade the mean values range from 2 2 to 3 7 for 13 c and 13 8 to 12 3 for 18 o in veins formed at lower to upper greenschist facies but from 5 8 to 7 1 for 13 c and 11 4 to 9 8 for 18 o at higher metamorphic grades nitrogen isotope compositions rule out mantle 15 n mean 5 magmatic 15 n 5 to 15 n mean 4 4 2 0 for the quartz veins accordingly the results are consistent with fluids derived from metamorphic dehydration of oceanic crust and sediments within accretionary orogenic belts

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