metallogenesis at paleo spreading centers lead isotopes in sulfides rocks and sediments from the troodos ophiolite cyprus abstract new pb isotopic data for metalliferous deposits and pelagic sediments from the troodos ophiolite complex cyprus are presented we use these data to re evaluate the genetic models for these deposits in the light of recent results obtained for different types of hydrothermal sites presently active on spreading ridges in contrast to previous conclusions of earlier studies we find that the troodos hydrothermal deposits sulfides as well as umbers and mn crusts have isotopic compositions very similar to that of most of the rocks from this ophiolite complex they display a 207 pb 204 pb ratio higher than present day typical ridges this result has been interpreted as evidence for a subduction related origin of the troodos ophiolite this different geodynamic setting of the ophiolite is also the reason for the isotopic difference between troodos and semail oman sulfides the latter being typical of a normal oceanic crust when examined in more detail the troodos sulfide results are very homogeneous within one site but show significant differences from one site to another at least three different isotopic end members are required to interpret these variations one of these end members is found in the pelagic sediments data analyzed here which have a 207 pb 204 pb typical of continental crust signature higher than that of the rocks and sulfides on present ridges the involvement of such a component has been similarly described for the guaymas basin and for the gorda rise deposits however some of the isotopic variations observed between sulfides cannot be interpreted only by this end member one possibility is that a second sedimentary component as yet unidentified has been involved in the hydrothermal circulation as recently demonstrated for red sea brines finally the determination of the exact proportion of pb leached out of the ophiolite basaltic crust itself would require a more complete knowledge of the initial isotopic heterogeneities within the massif indeed several rock results are significantly less radiogenic than those of the sulfides which however show themselves initial compositions this could result either from a sediment contribution to the sulfide pb or from a biased sampling of the massif heterogeneities this question remains open in the present state of the data

isotopic composition of carbonate bound organic nitrogen in deep sea scleractinian corals a new window into past biogeochemical change over the last two decades the skeletal remains of deep sea corals have arisen as a geochemical archive of pleistocene oceanographic change here we report the exploration of the isotopic composition of the carbonate bound organic nitrogen hereafter cb 15 n in the deep sea scleractinian coral desmophyllum dianthus as a possible tool for reconstructing past changes in the ocean nitrogen cycle the measurement protocol is adapted from a high sensitivity method for foraminifera shell bound 15 n we explored the variability of cb 15 n within specimens among corals collected at different depths in a given ocean region and among different ocean regions modern d dianthus cb 15 n is strongly correlated with the 15 n of n export as estimated from sediment traps shallow subsurface nitrate and surface sediments suggesting that cb 15 n is a reliable proxy for 15 n of n export d dianthus cb 15 n is consistently 8 9 8 9 higher than 15 n of n export indicating that d dianthus acquires its nutrition primarily from suspended particulate organic matter pom that derives from sinking pom not directly from sinking pom

evidence from diatom bound nitrogen isotopes for subarctic pacific stratification during the last ice age and a link to north pacific denitrification changes higher diatom bound d 15 n 70 wt lower opal content and 1200 ppm lower biogenic barium taken together and with constraints on sediment accumulation rate these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the bering sea during glacial times with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition this finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification in addition we hypothesize that more complete nutrient consumption in the glacial age subarctic pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical north pacific by lowering the nutrient content of the intermediate depth water formed in the subpolar north pacific in the deglacial interval of the bering sea record two apparent peaks in export productivity are associated with maxima in diatom bound and bulk sediment d 15 n the high d 15 n in these intervals may have resulted from greater surface nutrient consumption during this period however the synchroneity of the deglacial peaks in the bering sea with similar bulk sediment d 15 n changes in the eastern pacific margin and the presence of sediment lamination within the bering sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the d 15 n of the nitrate feeding bering sea surface waters at these times

porphyry copper mineralisation in the central srednogorie zone bulgaria the porphyry copper systems in the central part of the srednogorie zone bulgaria are represented by three major deposits elatsite medet and assarel and several smaller deposits and occurrences all of them within the panagyurishte ore district the hydrothermal systems are related to late cretaceous calc alkaline igneous complexes ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks several of the porphyry systems are spatially associated with shallow level intermediate and high sulphidation volcanic hosted epithermal deposits of economic importance such as the major gold copper mine at chelopech located 10 km from the elatsite porphyry copper deposit mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks k silicate alteration is characteristic for pre ore hydrothermal activity in all of them and it is located mostly in their central parts propylitic alteration is prominent in the medet and assarel deposits the assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre ore alterations including propylitic sericitic and advanced argillic assemblages the initial stages of the hydrothermal systems are characterised by high temperatures 550 500 c and highly saline 50 20 wt nacl equiv and vapour rich fluids of likely magmatic origin the composition of the fluids gradually changes from h2o nacl fecl2 to h2o nacl kcl and h2o nacl dominated as the fluids cool react with wall rocks and may become diluted with meteoric water fe ti oxide mineral associations were formed early in all deposits later followed in the elatsite deposit by an assemblage of bornite chalcopyrite platinum group element pge phases co ni thiospinels ag and bi tellurides and selenides the main ore stage in all deposits is dominated by chalcopyrite pyrite associated with propylitic alteration at medet k silcate sericite alteration at elatsite and chlorite sericite alteration at assarel specific geochemical features of the ore systems are marked by precipitation of numerous fine grained mineral inclusions in chalcopyrite and pyrite palladoarsenide pd rammelsbergite cu ni sulpharsenides in elatsite co and ni pyrite carrolite vaesite sulvanite and colusite in medet enargite as sulvanite colusite goldfieldite wittichenite calaverite and aikinite in assarel native gold and electrum are observed as small grains except in elatsite where larger aggregates up to several millimetres are found molybdenite is present as small flakes within chalcopyrite or forms felted aggregates within thin quartz veinlets in the elatsite and medet deposits and very rarely at assarel thin veinlets of quartz galena sphalerite are found at the upper parts of the deposits supergene alteration of the primary sulphide minerals is developed in the assarel deposit where a thick blanket of secondary copper mineralisation chalcocite covellite has formed in the other two deposits secondary mineral associations cu and fe oxides and hydroxides are limited to near the surface differences in the deposits studied suggest that elatsite and medet are formed probably at deeper levels of the porphyry systems whereas assarel is developed at rather shallower and lower temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure

unchanged nitrogen isotopic composition of organic matter in the south china sea during the last climatic cycle global implications the 15n of surface and down core sediments spanning the last 20 200 kyr from the entire south china sea scs ranges only from 3 0 to 6 5 with no correlation with discernible paleoclimatic oceanographic changes detailed profiles of the uppermost sediment column including fluff samples indicate a minor diagenetic overprint of 0 3 1 2 at the sediment water interface the absence of any correlation with reconstructed glacial interglacial changes in primary production terrigenous input and or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140 000 years this in turn implies that the 15n of the nitrate used during primary production remained approximately constant during the last climatic cycle the proposed scenario infers an unchanged nitrogen isotopic composition of the western pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic n inventory

compound specific δ 15 n and chlorin preservation in surface sediments of the peru margin with implications for ancient bulk δ 15 n records abstract understanding the processes that control the preservation of paleoceanographic proxies is of clear importance surface sediments from the peru margin oxygen minimum zone are subject to lateral and downslope transport by bottom currents that decrease organic matter om quality indicators of bulk om quality pyrolysis hydrogen index pyrolysis s1 s2 and c n demonstrate significant degradation between 150 and 400 m water depth within the oxygen minimum zone concentrations of the three most abundant chlorins chlorophyllone pheophytin and pyropheophytin decrease from 750 to 150 nmol g toc 1 from 150 to 400 m water depth though the relative abundances of the chlorins in an individual sample do not change this suggests that the three chlorins have similar reactivity over the ambient conditions values for 15n of bulk sediments 15nbulk decrease by 3 from the inner shelf to the upper slope 1000 m but co occurring compound specific 15n values 15nchlorin do not decrease downslope the low variability of 15nchlorin values supports a single source for the chlorins and demonstrates the recalcitrance of 15nchlorin values despite degradation this set of observation raises questions about which type of om fraction best records primary signatures we assess two possible models to guide our interpretation of these disparate datasets 1 that decreasing 15nbulk values are the result of degradation of a 15n enriched fraction during downslope transport and that 15nchlorin values reflect primary values 2 that 15nbulk values are primary and that chlorins are derived from material transported from upslope these data reaffirm that in active sedimentary environments such as the eastern tropical pacific transport of om can significantly alter bulk geochemical parameters of om integrity but the impacts on the 15n record of bulk sediments and chlorins are less clear and require more study to be thoroughly understood

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

diatom bound 15n 14n new support for enhanced nutrient consumption in the ice age subantarctic 1 diatom bound 15n 14n was used to reconstruct the glacial nutrient status of the subantarctic zone in the southern ocean down core records from both the pacific and indian sectors show 15n of 5 to 6 during the last glacial maximum and a decrease coincident with the glacial termination to values as low as 2 the effect of either diatom assemblage or physiological change on the diatom bound 15n 14n is unknown and cannot yet be ruled out as a possible explanation for the observed change however the consistency between indian and pacific sector records and with other paleoceanographic data suggests that the glacial interglacial difference in diatom bound 15n 14n was driven by higher consumption of nitrate in the subantarctic surface during the last ice age such a change in nutrient consumption may have resulted from atmospheric iron fertilization and or decreased glacial mixed layer depths associated with sea ice melting enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of co2 in the ice age atmosphere it also would have reduced the preformed nutrient content of the low latitude thermocline leading to decreases in low latitude productivity suboxia and denitrification

complementary constraints from carbon 13c and nitrogen 15n isotopes on the glacial ocean s soft tissue biological pump a three dimensional process based model of the ocean s carbon and nitrogen cycles including 13c and 15n isotopes is used to explore effects of idealized changes in the soft tissue biological pump results are presented from one preindustrial control run pictrl and six simulations of the last glacial maximum lgm with increasing values of the spatially constant maximum phytoplankton growth rate max which accelerates biological nutrient utilization mimicking iron fertilization the default lgm simulation without increasing max and with a shallower and weaker atlantic meridional overturning circulation and increased sea ice cover leads to 280 pg more respired organic carbon corg storage in the deep ocean with respect to pictrl dissolved oxygen concentrations in the colder glacial thermocline increase which reduces water column denitrification and with delay nitrogen fixation thus increasing the ocean s fixed nitrogen inventory and decreasing 15nno3 almost everywhere this simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well but it overestimates deep ocean 13cdic and underestimates 15nno3 at high latitudes increasing max enhances corg and lowers deep ocean 13cdic improving the agreement with sediment data in the model s antarctic and north pacific oceans modest increases in max result in higher 15nno3 due to enhanced local nutrient utilization improving the agreement with reconstructions there models with moderately increased max fit both isotope data best whereas large increases in nutrient utilization are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well the best fitting models reproduce major features of the glacial 13cdic 15n and oxygen reconstructions while simulating increased corg by 510 670 pg compared with the preindustrial ocean these results are consistent with the idea that the soft tissue pump was more efficient during the lgm both circulation and biological nutrient utilization could contribute however these conclusions are preliminary given our idealized experiments which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes the analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes whereas carbon isotopes are sensitive to circulation changes and indicate well the three dimensional corg distribution nitrogen isotopes are more sensitive to biological nutrient utilization

metallogenesis at spreading centers lead isotope systematics for sulfides manganese rich crusts basalts and sediments from the cyamex and alvin areas east pacific rise

epithermal alteration and mineralization in the comstock district nevada the precious and base metal deposits of the world renowned comstock district of western nevada constitute one of several superimposed miocene hydrothermal systems in the district mesozoic metasedimentary and igneous rocks are overlain by oligocene to miocene ash flow tuffs a thick sequence of middle miocene andesitic volcanic rocks and intrusions host the bulk of the hydrothermal alteration and ore deposits some of the magmatic events were directly associated with hydrothermal activity secondary biotite alteration locally affects preore dioritic intrusions preore barren quartz alunite pyrophyllite and kandite alteration high sulfidation style is zoned outward from mostly discontinuous crudely radial fractures associated with andesitic intrusions blanket like cristobalite alunite and kaolinite alteration exposed at the periphery of the district may be linked to preore alteration near the paleowater table later large volumes of subore grade massive quartz veins and quartz adularia stockworks were deposited in steeply to moderately east dipping sets of north and northeast striking faults that also localized the major precious metal bearing lodes quartz chlorite illite and very localized muscovite alteration deep low sulfidation developed coeval with vein deposition epidote bearing propylitic alteration forms halos around the higher temperature parts of the lodes adularia and rare bladed calcite indicate boiling during deep low sulfidation hydrothermal activity but little is apparently associated with ore deposition the main stage au ag cu zn pb ore was deposited late in the deep low sulfidation system in irregularly spaced lenticular dilatant zones or structural intersections massive nonbladed calcite was deposited just below and in the lower parts of orebodies in the comstock lode but it was deposited in the upper parts of orebodies or was barren elsewhere in the district pliocene to holocene reactivation of faults has disrupted the district redistributing the relative positions of many of the orebodies and alteration assemblages

millennial scale oscillation of denitrification intensity in the arabian sea during the late quaternary and its potential influence on atmospheric n2o and global climate the intensity of denitrification in the arabian sea during the last 65 kyr is reconstructed using high resolution 15 n records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity the results reveal a close link to the greenland ice core record with low or absent water column denitrification during the last glacial maximum the stadials and at the time of the heinrich events including the younger dryas in contrast denitrification was high during the holocene and the interstadials the intensification of denitrification is related to stronger sw monsoonal upwelling which enhances organic matter flux and degradation resulting in a strengthening of the midwater oxygen deficiency such a combination of enhanced upwelling and denitrification has also been implied for the eastern tropical north pacific etnp where these events occur during the holocene and to some extent during the interstadials too today the arabian sea and the etnp together contribute substantially to the global marine water column denitrification and a significant fraction of the ocean atmosphere n 2 o flux originates from these areas changes in n 2 o emissions from these areas could thus have effected the recently described stadial interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores moreover denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory which in turn influences global primary productivity and co 2 sequestration by the biological pump short term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate

benthic nitrogen fluxes and fractionation of nitrate in the mauritanian oxygen minimum zone eastern tropical north atlantic we present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen din no3 no2 nh4 and oxygen o2 from 7 sites with variable sand content along 18 n offshore mauritania nw africa bottom water o2 concentrations at the shallowest station were hypoxic 42 m and increased to 125 m at the deepest site 1113 m total oxygen uptake rates were highest on the shelf 10 3 mmol o2 m 2 d 1 and decreased quasi exponentially with water depth to 3 2 mmol o2 m 2 d 1 average denitrification rates estimated from a flux balance decreased with water depth from 2 2 to 0 2 mmol n m 2 d 1 overall the sediments acted as net sink for din observed increases in 15nno3 and 18ono3 in the benthic chamber deployed on the shelf characterized by muddy sand were used to calculate apparent benthic nitrate fractionation factors of 8 0 15eapp and 14 1 18eapp measurements of 15nno2 further demonstrated that the sediments acted as a source of 15n depleted no2 these observations were analyzed using an isotope box model that considered denitrification and nitrification of nh4 and no2 the principal findings were that i net benthic 14n 15n fractionation eden was 12 9 1 7 ii inverse fractionation during nitrite oxidation leads to an efflux of isotopically light no2 22 1 9 and iii direct coupling between nitrification and denitrification in the sediment is negligible previously reported eden for fine grained sediments are much lower 4 8 we speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater seawater exchange in permeable sediments and the hypoxic high productivity environment although not without uncertainties the results presented could have important implications for understanding the current state of the marine n cycle

increased glacial age ventilation of the chilean margin by antarctic intermediate water antarctic intermediate water is an oxygen rich water mass that spreads throughout the southern hemisphere oceans marine sediment chemistry indicates that the water mass was produced at higher rates and had higher oxygen concentrations during the last glacial maximum antarctic intermediate water is at present a water mass that brings oxygen to intermediate depths throughout the southern hemisphere oceans models have suggested that intermediate waters had higher concentrations of oxygen during the last glacial period1 2 consistent with globally reduced denitrification3 and increased production of antarctic intermediate water4 however some palaeoceanographic reconstructions5 6 have indicated that production decreased in the southeast pacific ocean at this time here we analyse the concentrations of re and mn the sedimentary concentrations of which are controlled by the amount of dissolved oxygen at the sea floor from three sediment cores located along the chilean margin for the past 30 000 years our results from the cores which bracket the present day water column extent of antarctic intermediate water show that the depth range of well oxygenated antarctic intermediate water increased off chile during the last glacial maximum dissolved oxygen content began to decrease approximately 17 000 years ago coincident with rapid antarctic warming and a poleward shift of the southern westerly winds7 our estimates of productivity from accumulation rates of organic carbon and opal do not co vary with the seafloor oxygen variations ruling out local control of seafloor oxygenation we conclude that the data are best explained by a combination of increased oxygenation and increased flux of antarctic intermediate water during the last glacial maximum

refining the planktic foraminiferal i ca proxy results from the southeast atlantic ocean abstract profound changes in upper ocean oxygenation have taken place in recent decades and are expected to continue in the future but the complexity of the processes driving these changes has yet to be fully unraveled planktic foraminiferal i ca is a promising tool to reconstruct the extent of past upper ocean oxygenation but a thorough assessment is necessary to evaluate both its potential and its limitations we used foraminifers from holocene core tops southeast atlantic ocean to document planktic i ca across a range of oceanographic conditions subsurface o2 concentrations may be the dominant control on planktic foraminiferal i ca and planktic i ca decreases rapidly at low o2 conditions o2

oxygen minimum zone expansion in the eastern tropical north pacific during deglaciation concentrations lamination preservation and d 15 n values from cores located between 540 750 m water depth in the gulf of tehuantepec southern mexico these results differ from similar proxies found even 5 n of the site where there is evidence for omz changes related to rapid climate change in the northern hemisphere instead omz expansion in the gulf was contemporaneous with changes in sea ice extent and zonal wind shifts around antarctica pointing to subantarctic mode water and antarctic intermediate water as likely sources of oxygen depleted water these observations reinforce the importance of the southern ocean as a primary modulator of northern hemisphere ocean climate as far as 15 n in the etnp citation hendy i l and t f pedersen 2006 oxygen minimum zone expansion in the eastern tropical north pacific during deglaciation geophys res lett 33 l20602 doi 10 1029 2006gl025975

lead isotopic analysis within a multiproxy approach to trace pottery sources the example of white slip ii sherds from late bronze age sites in cyprus and syria lead isotope analyses were carried out on fragments of white slip ii ware a late bronze age cypriote pottery ware and on raw materials possibly used for their production sherds originate from three late bronze age sites hala sultan tekke and sanidha in cyprus and minet el beida in syria and clays come from the surroundings of sanidha a production site for white slip ware x ray fluorescence xrf and a principal component analysis pca are combined with pb isotope analyses to further investigate the effectiveness of the latter method within a multiproxy approach for pottery provenance study the pottery sherds from the three sites are compared between themselves and with potential raw material additional x ray diffraction xrd and analyses using a scanning electron microscope sem equipped with an energy dispersive x ray detection edx facility were performed on selected sherds and clays this work confirms that the clay source used for pottery production in sanidha derives from local weathered gabbro it also shows that different origins can be proposed for white slip ii ware sherds from hala sultan tekke and minet el beida and that clays were prepared prior to white slip ii ware production it finally confirms the effectiveness of pb isotopes in tracing pottery provenance not only by comparing sherd assemblages but also by comparing sherds to potential raw materials

pb isotopic composition of ophiolitic volcanogenic sulphide deposits troodos complex cyprus the volcanogenic sulphide ore deposits in the ophiolitic rocks of the troodos massif cyprus are thought to be 80 myr old1 analogues of the metal sulphide accumulations actively being deposited from 350 c water at a depth of 2 6 km on the east pacific rise 21 n 2 4 in both cases mineralization is thought to be a consequence of heat transfer by convective seawater circulation within the oceanic crust5 6 evidence includes fluid inclusions in quartz from ore deposits in cyprus with modal trapping temperatures of 298 370 c which have a bulk composition of 3 2 0 5 equiv wt nacl 1 n 273 this salinity is identical to seawater with 3 5 wt total dissolved solids4 7 we have examined the isotopic composition of pb as a geochemical tracer to clarify the origin of one of the metals contained in the sulphide concentrations in the troodos complex it was anticipated that the bulk of the lead would have been derived by leaching of the mafic ophiolitic rocks and indeed such a component is found to be present however the isotope ratios suggest that the lead is actually a mixture and also contains an important component derived from sea water

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 relationship between epithermal and porphyry hydrothermal systems within the tavua caldera fiji the shoshonitic tavua caldera in viti levu fiji defines the volcanic center of the tavua volcano the caldera occurs at the intersection of the northeast trending viti levu lineament and a northwest trending lineament locally manifested by the nasivi shear zone fault related subsidence which contributed to the evolution of the tavua caldera the central inner caldera and the structural preparation for mineralization occurred on these two regional fault systems and local concentric normal faults a continuum of mineralization and alteration styles is evident along the nasivi shear zone caused by a northwesterly evolution and migration of intrusion and hydrothermal activity porphyry style subeconomic copper gold mineralization and associated alteration occurred at depth in the nasivi shear zone at the center of the caldera approximately contemporaneously with the development of the inner caldera acid sulfate style epithermal alteration and minor gold mineralization immediately postdated and were superimposed on this porphyry style alteration farther northwest along the nasivi shear zone transitional ac d sulfate to adularia sericite epithermal alteration and minor gold mineralization occur variation in hydrothermal style is considered to reflect an increasing distance from magmatic source combined with an increasing proportion of meteoric water mixing with the hydrothermal fluid northwest of this system at the intersection of the nasivi shear zone and the viti levu lineament the 4 4 m oz adularia sericite style epithermal emperor gold mine occurs high level epithermal mineralization and alteration occur 4 km northeast of the emperor gold mine in an area which has a demonstrable present day hydraulic connection with the mine variations in fluid inclusion homogenization temperatures from the mine indicate that fluids entered the structurally prepared mine area via the nasivi shear zone emperor epithermal mineralization postdated magmatism by approximately 0 4 ma stable isotope data the presence of albitic alteration beneath mineralization and high salinities within the mine indicate that the emperor northern prospects hydrothermal system is related to an intrusion into the nasivi shear zone beneath the mine this is consistent with a demonstrable northwesterly migration of intrusions with time along the nasivi shear zone albitic phyllic and propylitic assemblages comprise the main porphyry style alteration types in the caldera center calculated temperatures of formation of chlorite in these assemblages indicate that these three alteration facies formed in the temperature range 270 degrees to 340 degrees c in good agreement with available fluid inclusion data and the stability ranges of the mineral assemblages kubler indices of illite likewise imply that these assemblages formed at relatively high temperature epithermal lode alteration mineralization at emperor is shown by fluid inclusion data to have formed around 200 degrees c kubler indices of illite confirm that this alteration formed at significantly lower temperatures than did porphyry alteration and that chlorites in the emperor mine have calculated temperatures of formation mostly below 200 degrees c high level epithermal alteration occurring northeast of emperor is inferred to have formed at even lower temperatures

subepithermal au pd mineralization associated with an alkalic porphyry cu au deposit mount milligan quesnel terrane british columbia canada at the mount milligan cu au porphyry deposit quesnel terrane british columbia canada barren and weakly mineralized late stage hydrothermal veins occur in volcanic rocks adjacent to zones of cu au porphyry mineralization and have overprinted the porphyry stage veins the earliest of the late stage hydrothermal veins are barren and consist of quartz pyrite carbonate chlorite tourmaline these veins are similar to transitional to late stage hydrothermal veins in other alkaline porphyry cu au deposits and we consider these to be the equivalent of transitional post porphyry pre epithermal quartz sericite pyrite veins in calc alkaline porphyry environments a later generation of volumetrically minor mineralized veins are composed of pyrite hg and as bearing quartz carbonate chlorite and contain early electrum arsenopyrite tetra hedrite tennantite platinum group element pge tellurides galena sphalerite barite and chalcopyrite as inclusions in pyrite and a later assemblage of electrum pge tellurides arsenides and antimonides galena sphalerite chalcopyrite and various au ag te bi minerals in annealed fractures and open space infillings in quartz and pyrite metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite primary fluid inclusions in quartz in the barren and weakly mineralized veins are two phase l v homogenize to liquid over a narrow range in t 170 270 c n 96 12 veins and show a wide range in salinity 4 2 wt nacl equiv to 28 7 wt cacl2 equiv when all samples are considered however individual veins show narrow ranges in salinity and homogenization temperature la icp ms analyses indicate that the fluids were highly enriched in as to 2 260 ppm sb to 230 ppm b to 5 400 ppm au 1 2 ppm and pd 0 5 1 ppm but depleted in cu 80 ppm compared to typical porphyry stage fluids metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins the inclusions are interpreted to contain a contracted magmatic vapor produced by boiling that lost cu during the formation of porphyry stage veins at depth fluids show decreasing b as sb and increasing sr ca and salinity with time stable c o and h isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition rather metal precipitation was possibly the result of mixing of the magmatic derived fluid with a heated saline groundwater the precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low to intermediate sulfidation epithermal systems fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between 200 and 1 500 bars and 240 and 280 c after the formation of the mineralized veins circulation of low salinity metal depleted fluids occurred these latest stage fluids may have formed by mixing of the saline magmatic fluid groundwater hybrid with meteoric water the results of this study suggest a genetic link between porphyry stage events and the deposition of au and pge in late stage veins in an alkalic igneous environment recognition of hydrothermal processes involving the transport of au pge as sb bi te b rich fluids in the subepithermal regimes implies that low sulfidation epithermal au deposits may have been present in the shallower parts of the magmatic hydrothermal complex and that there is potential for the discovery of pge rich epithermal veins in less deeply exhumed terranes on the other hand the formation of high grade low sulfidation epithermal au pge deposits may be prohibited if porphyry epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur

a 6000 year geochemical record of human activities from alexandria egypt abstract recent multidisciplinary investigations of sediment cores from the ancient marine bay of alexandria egypt have documented local human activities during the iron age circa 900 1000 b c prior to alexander the great s arrival in 331 b c goiran 2001 veron et al 2006 stanley et al 2007 2010 corroborating the existence of the so called rakhotis as evoked in previous archaeological literature jondet 1916 weill 1919 chauveau 1999 baines 2003 lead pb isotopic analyses lia from alexandria bay indicate a possible anthropogenic imprint as early as circa 2300 2650 200 b c and to a lesser extent 3500 3800 170 b c veron et al 2006 here we demonstrate that lia in sediments from the nearby maryut lagoon display isotopic anomalies resulting from the release of contaminant pb into the lagoon during the egyptian early dynastic at 2897 187 b c and predynastic at 3520 145 b c periods that corroborate geochemical data from alexandria bay pb concentrations in maryut sediments show enrichments that mirror isotopic findings the absence of contaminant pb imprints within the sediments from the nearby nile canopic branch confirms that isotopic anomalies found in alexandria bay and maryut lagoon sediments are local and do not originate from long distance transport of contaminant pb associated with nile suspended material and or wind derived aerosols lia in alexandria sediment cores correspond to commonly mined pb and cu copper ores from turkey black sea region cyprus crete and the oman gulf lia substantiate the use of copper based relics in the alexandria region during the pre and early dynastic periods and provide insights into metal trading within the eastern mediterranean during the chalcolithic early bronze age transition 6000 years ago

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

a special issue devoted to porphyry and epithermal deposits of the southwest pacific an introduction the southwest pacific is host to a number of mineralization styles including numerous world class porphyry cu au and epithermal au ag deposits cooke et al 2005 garwin et al 2005 the majority of the giant porphyry and epithermal au and cu deposits were formed within a series of middle to late cenozoic 25 1 ma arcs garwin et al 2005 maryono et al 2018 these arcs form a complex boundary between the eurasian pacific and indian australian tectonic plates stretching from japan in the northeast and myanmar in the northwest through the philippines indonesia new guinea and the volcanic island chains of the solomon islands new hebrides vanuatu and fiji fig 1

a near field sea level record of east antarctic ice sheet instability from 32 to 27 myr 1 fossil facies and isotope analyses of an early high paleolatitude 55 s section suggests a highly unstable east antarctic ice sheet from 32 to 27 myr the waxing and waning of this ice sheet from 140 to 40 of its present volume caused sea level changes of 25 m ranging from 30 to 50 m related to periodic glacial 100 000 to 200 000 years and shorter interglacial events the near field gippsland sea level gsl curve shares many similarities to the far field new jersey sea level njsl estimates however there are possible resolution errors due to biochronology taphonomy and paleodepth estimates and the relative lack of lowstand deposits in njsl that prevent detailed correlations with gsl nevertheless the lateral variations in sea level between the gsl section and njsl record that suggest ocean siphoning and antisiphoning may have propagated synchronous yet variable sea levels

circulation and oxygenation of the glacial south china sea abstract degree of oxygenation in intermediate water modulates the downward transferring efficiency of primary productivity pp from surface water to deep water for carbon sequestration consequently the storage of nutrients versus the delivery and sedimentary burial fluxes of organic matter and associated biomarkers to better decipher the pp history of the south china sea scs appreciation about the glacial interglacial variation of the luzon strait ls throughflow which determines the mean residence time and oxygenation of water mass in the scs interior is required based on a well established physical model we conducted a 3 d modeling exercise to quantify the effects of sea level drop and monsoon wind intensity on glacial circulation pattern thus to evaluate effects of productivity and circulation induced oxygenation on the burial of organic matter under modern climatology wind conditions a 135 m sea level drop results in a greater basin closeness and a 24 of reduction in the ls intermediate westward throughflow consequently an increase in the mean water residence time from 19 0 to 23 0 years however when the wind intensity was doubled during glacial low sea level conditon the throughflow restored largely to reach a similar residence time 18 4 years as today regardless its closeness comparing with present day scs surface circulation pattern in glacial model exhibits 1 stronger upwelling at the west off luzon island and 2 an intensified southwestward jet current along the western boundary of the scs basin superimposed hypothetically by stronger monsoon wind the glacial scs conditions facilitate greater primary productivity in the northern part manganese a redox sensitive indicator in images core md972142 at southeastern scs revealed a relatively reducing environment in glacial periods considering the similarity in the mean water residence time between modern and glacial cases the reducing environment of the glacial southeastern scs was thus ascribed to a productivity induced rather than ventilation induced consequence

origin and evolution of ore forming fluids and genesis of the duhuangling and jiusangou gold deposits in eastern yanbian northeast china the duhuangling and jiusangou gold deposits are located east of the yanbian area northeast china we present the results of systematic studies of the ore geology fluid inclusion fi features and isotopic geochemistry of the two gold deposits four mineralization stages have been defined in each gold deposit pyrite sericite quartz stage i quartz pyrite chalcopyrite arsenopyrite stage ii quartz native gold polymetallic sulfide stage iii and quartz calcite stage iv fis from the two gold deposits are similar and contain five types pure vapor phase v type vapor rich phase rv type liquid rich phase rl type daughter mineral bearing polyphase s type and pure liquid phase l type stage i fis homogenize at temperatures of 350 450 c yielding salinities of 5 40 48 67 wt fis from both stages ii and iii samples homogenize at 160 350 c yielding salinities of 5 71 39 75 wt fis in pyrite from the stage ii have 3he 4he ratios of 0 0069 0 0148 ra and 40ar 36ar ratios o

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

a review of nitrogen isotopic alteration in marine sediments key points use of sedimentary nitrogen isotopes is examined on average sediment 15n 14n increases approx 2 per mil during early burial isotopic alteration scales with water depth abstract nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record however bulk sedimentary nitrogen isotope ratios which can be determined routinely and at minimal cost may be altered during burial and early sedimentary diagenesis particularly outside of continental margin settings the causes and detailed mechanisms of isotopic alteration are still under investigation case studies of the mediterranean and south china seas underscore the complexities of investigating isotopic alteration in an effort to evaluate the evidence for alteration of the sedimentary n isotopic signal and try to quantify the net effect we have compiled and compared data demonstrating alteration from the published literature a 100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that at sites located off of the continental margins an increase in sediment 15n 14n occurs during early burial likely at the seafloor the extent of isotopic alteration appears to be a function of water depth depth related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of n isotopic alteration moreover the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global n cycle

combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic pacific record we present an sif4 separation line coupled to a laser fluorination system which allows for an efficient combined silica d18o and d30si analysis 50 min per sample the required sample weight of 1 5 2 0mg allows for high resolution isotope studies on biogenic opal besides analytical tests the new instrumentation set up was used to analyse two marine diatom fractions 63 mm 10 20mm with different diatom species compositions extracted from a bolling allerod holocene core section md01 2416 north west nw pacific to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species related isotope effects vital and environmental effects while d30si offsets between the two fractions were not discernible supporting the absence of species related silicon isotope effects systematic offsets occur between the d18o records although small these offsets point to species related isotope effects as bias by contamination can be discarded the new records strengthen the palaeoceanographic history during the last deglaciation in the nw pacific characterized by a sequence of events with varying surface water structure and biological productivity with such palaeoceanographic evolution it becomes unlikely that the observed systematic d18o offsets signal seasonal temperature variability this calls for reconsideration of vital effects generally excluded to affect d18o measurements

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

a cool nutrient enriched eastern equatorial pacific during the mid pleistocene transition this work was supported in part by nsf mgg 1060779 and usssac postcruisesupportforleg202tor s r we thank p rumford and the iodp core repository for providing samples and assistance with xrf core scanning k kelley for analytical assistance and z kerrigan j freedman and a stahl for the help with sample preparation new data will be archived at the ncei paleoceanography data repository

geological controls on the stable tellurium isotope variation in tellurides and native tellurium from epithermal and orogenic gold deposits application to the emperor gold telluride deposit fiji abstract natural variability of stable te isotope compositions in terrestrial settings including hydrothermal ore deposits and the geochemical conditions that control their compositions are poorly understood previously published te isotope data on tellurides sulfides and native tellurium are limited in number and derived from samples that were geologically unconstrained with regards to their location within an ore deposit here we have obtained a range of values of 130 125te of 0 47 0 13 to 0 66 0 13 n 13 relative to an in house gravimetrically prepared te standard of 99 9999 te metal for located samples of sylvanite auag 2te4 krennerite au ag te2 and native tellurium from the alkaline igneous rock related emperor gold telluride deposit fiji based on similarities in the geologic setting of the alkaline igneous rock related gold silver telluride deposits in colorado cripple creek boulder county and la plata with emperor and the narrow range of 130 125te values for tellurides and native tellurium from emperor and previously published unlocated samples from colorado deposits cripple creek 130 125te 0 53 to 0 44 n 13 boulder county 130 125te 0 25 to 0 16 n 7 la plata 130 125te 0 78 to 0 0 17 n 11 it is likely that the main factor controlling the range of isotope values in these epithermal systems is the te isotope composition of the genetically related alkaline igneous source rocks in comparison previously published unlocated samples of tellurides from the giant orogenic golden mile au deposit kalgoorlie australia show an isotopic range of 130 125te 1 00 to 0 51 n 6 and appear to have been more affected by the metamorphic nature of the ore fluids and or the higher degree of water rock interaction during hydrothermal fluid transport it is speculated that the zonation from isotopically heavy to light te isotopic compositions with distance from the likely causative intrusion within the emperor deposit is primarily controlled by rayleigh fractionation of isotopes within the ore fluid

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

some metallogenic features of gold and copper deposits related to alkaline rocks and consequences for exploration porphyry copper gold skarn copper gold sediment hosted carlin style gold breccia pipe low sulphidation epithermal gold pluton related mesothermal or orogenic gold vein and volcanogenic massive sulphide deposits associated with alkaline rocks are commonly broadly similar to those hosted by their calc alkaline counterparts in contrast porphyry molybdenum gold deposits are confined to alkaline igneous centres alkaline suites are notably deficient in high sulphidation epithermal gold deposits and even in the advanced argillic lithocaps which host them this is surprising given that the required oxidised sulphur species are seemingly more abundant than in calc alkaline igneous centres highly efficient buffering of acidic fluid by metasomatised alkaline rocks may offer a viable explanation all types of intrusion related zinc deposits also appear to be poorly developed in alkaline provinces the characteristics of several gold and copper deposits associated with alkaline rocks including the giant porgera cripple creek ladolam olympic dam and phalaborwa examples are judged to diverge appreciably from their most closely related deposit types rendering them arguably unique most of these aberrant mineralisation styles may be due to variations in magmatic fluid compositional and liberation characteristics consequent upon the extreme diversity of ore related alkaline magmas most types of gold and copper deposits developed in calc alkaline provinces also constitute exploration targets in and around alkaline igneous centres although porphyry copper gold and low sulphidation epithermal gold deposits are considered to possess the greatest potential perhaps of even greater interest however is the possibility of encountering unconventional giant gold and copper deposits which lack closely analogous examples if already defined aberrant deposits are truly unique then exploration designed specifically to detect additional examples is pointless furthermore exploration for new unique deposits is difficult because their defining geological parameters are unknown alkaline rocks within or behind calc alkaline arcs at convergent plate boundaries probably offer the greatest exploration potential although anorogenic intracontinental extensional settings should not be ignored

pb isotope systematics of a fossil hydrothermal system from the troodos ophiolite cyprus evidence for a polyphased alteration history pb isotopic ratios have been determined on massive sulphides from the troodos ophiolite and their inferred source rocks epidosites and greenschist altered dikes the latter have distinctly higher pb isotope ratios than the sulphides the high 207 pb 204 pb ratios for the whole rocks indicate the involvement of an old sedimentary pb component extensive leaching of three epidosite samples shows that the radiogenic pb can be removed and the resulting trend for the leached samples is consistent with a pb pb age of 95 ma for these rocks inferred initial pb isotope ratios are similar to those of the sulphides this study and fresh glasses from the akaki canyon in cyprus therefore the pb incorporated in the massive sulphides may originate from these greenschist facies hydrothermally altered rocks the isotopic composition of the leachate pb points toward a sedimentary origin calculation of the pb isotopic ratios for 90 ma old average troodos sediment shows that the foreign pb in the epidosites may very well be derived from this source this pb probably does not reside within the mineral lattices of these metamorphic rocks but rather is adsorbed on mineral surfaces or is present in microscopic particulate matter that may have been filtered from the circulating seawater unmodified seawater is considered an unlikely pb source because of the extremely small dissolved pb contents of seawater furthermore no seawater or sedimentary signature is evident in the sulphides several models of pb transport from different sources into these rocks may be envisaged such as dissolution of miocene evaporites and brine formation equilibration of pore waters to the pb isotope ratios of host sediments and direct infiltration of deep sea particulate matter from the nepheloid layer with the limited amount of data presently available it is impossible to critically determine the most likely processes of pb transport copyright 2000 elsevier science ltd high temperature convectively driven submarine hydrother mal systems at sea floor spreading centres are believed to be of major importance to the heat transport from the newly formed oceanic crust these seawater fed hydrothermal systems are also believed to have major impacts on the chemical mineral ogical and physical properties of the oceanic lithosphere one of the most eminent processes is the formation of sulphide deposits by precipitation of sulphides from upwelling hot hy drothermal fluids upon mixing with cold circulating seawater in the more permeable portions in the upper parts of the oceanic lithosphere present day knowledge of these submarine hydrothermal systems comes partly from their most accessible component the hot springs that emanate hydrothermal fluids at or near the ridge axis but also from seawater basalt and seawater diabase interaction experiments and petrologic and geochemical studies on both in situ oceanic crust and ophiolites because the in ferred source rocks from these hydrothermal systems are lo

melting of the patagonian ice sheet and deglacial perturbations of the nitrogen cycle in the eastern south pacific local 230 th normalized biogenic vertical fluxes from the chilean continental margin they document in detail the sharp transition from relatively low wcd rates during the last glacial maximum lgm to high ones during the deglaciation and holocene but give no evidence that the glacial interglacial difference of wcd could have been caused by changes in local primary productivity furthermore we found no evidence that changes in ventilation due to samw formation could explain the nitrogen isotope record we present evidence for an alternative mechanism related to the melting of the patagonian ice sheet pis which is consistent with recent published proxy data and the regional physical oceanography

evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the eastern mediterranean sea abstract nitrogen isotopes of chlorins degradation products of chlorophyll reflect the isotopic composition of nutrient n utilized by marine phytoplankton communities here we show that in sediments of the eastern mediterranean pleistocene and holocene values of 15n for chlorins and total nitrogen vary in concert with a consistent offset of 5 reflecting the fractionation imparted during chlorophyll biosynthesis samples from the integrated ocean drilling program sites 964 and 969 were analyzed at a sampling resolution of 4 10 cm clustered around sapropel events 2 3 4 and 5 100 170 ka in low organic content sediments chlorin values of 0 coincident with total nitrogen values of 5 indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment in sapropel horizons the chlorin and total nitrogen values are 5 more negative 5 and 0 respectively resembling previously reported modern day water column particulates 0 we suggest that nutrient conditions in the eastern mediterranean correspond to three scenarios and that the similarity between sapropel and modern day bulk 15n is coincidental organic poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of atlantic sourced nitrate sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine p fluxes to surface waters present day conditions are dominated by anthropogenic n sources these scenarios agree with a model of sapropel formation in which stratification caused by increased fresh water inputs led to n fixation due to p n nutrient imbalance enhanced production combined with stratification promoted and maintained anoxic deep waters consequently increasing organic matter preservation such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans

influence of diagenesis on sedimentary delta n 15 in the arabian sea over the last 130 kyr abstract sedimentary 15n records are valuable archives of ocean history but they are often modified during early diagenesis here we quantify the effect of early diagenetic enrichment on sedimentary n isotope composition in order to obtain the pristine signal of reactive n assimilated in the euphotic zone this is possible by using paired data of 15n and amino acid composition of sediment samples which can be applied to estimate the degree of organic matter degradation we determined 15n and amino acid composition in coeval sediments from ocean drilling program odp hole 772 b in the central arabian sea and from hole 724 c situated on the oman margin in the western arabian sea coastal upwelling area the records span the last 130 kyr and include two glacial interglacial cycles these new data are used in conjunction with data available for surface sediments that cover a wide range of organic matter degradation states and with other cores from the northern and eastern arabian sea to explore spatial variations in the isotopic signal in order to reconstruct pristine n values we apply the relationship between organic matter degradation and 15n enrichment in surface sediments to correct the core records for early diagenetic enrichment reconstructed 15n values suggest a significant role of n2 fixation during glacial stages an evaluation of two preservation indices based on amino acid composition reactivity index ri jennerjahn and ittekkot 1997 and the degradation index di dauwe et al 1999 in both recent sediments and core samples suggests that the ri is more suitable than the di in correcting arabian sea 15n records for early diagenetic enrichment

from phytoplankton to oil shale reservoirs a 19 million year record of the late cretaceous tethyan upwelling regime in the levant basin abstract the complexity of sedimentary organic matter formation its source to burial and influence on bottom water communities are addressed in this study of the 19 million years long upper cretaceous tethyan upwelling regime in the levant a multi proxy approach is amalgamated into a model through which the paleoceanographic complexity of an intense and long lasting upwelling system is better understood a shift in primary producer assemblages and reorganization of the bottom water bacterial consortium followed the campanian maastrichtian boundary enforcing conspicuous sedimentological and faunal changes the siliceous and phosphatic bearing sediments of the campanian gave way to maastrichtian uniform and regionally distributed organic rich carbonates accompanied by an increase in sedimentation rates from 1 to 6 cm kyr this has facilitated both the increase in organic matter production and its excellent preservation and through diagenetic pathways affected its elemental composition these paleoceanographic modifications have also affected foraminiferal assemblages from massive blooms of triserial buliminid benthic foraminifera during the campanian which have sequestered diatom chloroplasts to diverse trochospiral forms during the maastrichtian using nitrate instead of oxygen for their respiratory pathways we propose that shifts in the type of primary producers promote a change in the amount and type of preserved organic matter in bottom water communities on the lithological composition of the rock mass and ultimately on the hydrocarbon generation potential of the studied source rock the continued rise of southern tethyan sea level and global highstands during the upper maastrichtian culminating into arabian plate wide maximum flooding surfaces cause the migration of upwelling activity away from the study area terminating the high productivity sequence in the levant

shift in limiting nutrients in the late ediacaran early cambrian marine systems of south china abstract the ediacaran to cambrian periods are characterized by the diversification and extinction of metazoans carbon cycle perturbation and ocean oxygenation ocean nutrient cycles play a major role in the primary production and productivity driven shallow water anoxia that was responsible for mass extinction in some phanerozoic intervals however the relationships between these cycles and primary production and biological events during the ediacaran to cambrian periods have not previously been well explored this study presents stratigraphic profiles of nitrogen and organic carbon isotopes for sections belonging to the shelf and deep basin regions of the yangtze platform south china the nitrogen isotope profiles for these two regions share a similar trend showing 1 relatively stable 15n values of 5 6 in the early middle ediacaran 635 ma ca 560 ma and 2 a shift towards lower 15n values 3 4 since the shuram 13ccarb excursion the 15n values in the shelf section experienced a two stage decrease from the late ediacaran to cambrian stage 2 of the terreneuvian series down to 1 and then an increase from 1 to 4 across the cambrian stage 2 3 boundary i e the terreneuvian cambrian series 2 boundary at 521 ma these data in combination with previously reported nutrient profiles 15n p and mo allow the following new conclusions to be drawn 1 the early middle ediacaran ocean was characterized by n mo colimitation of primary production on a geological timescale that is different from the p limitation on long term primary production in the modern ocean 2 a shift in the ultimate limiting nutrient from n and mo to p likely occurred in late ediacaran early cambrian marine systems this shift may have been a prerequisite for the extinction of small shelly fossils in response to the development of photic zone anoxia for several million years in cambrian stage 2 which was followed by the third and most prominent phase of metazoan diversification

pinwarian 1 50 ga volcanism and hydrothermal activity at the eastern margin of the wakeham group grenville province quebec volcanic belts developed along the southeastern continental margin of laurentia between 1 70 and 1 30 ga and subsequently metamorphosed at high grade are today largely concealed among gneiss complexes of the grenville province at the eastern end of the wakeham group and in the la romaine supracrustal belt to the east four 1 50 ga volcanic centres were found among gneissic synvolcanic intrusions typical of the 1 52 1 46 ga pinwarian continental magmatic arc upper amphibolite to granulite facies rhyolitic to dacitic lavas and coarse lapillistone overlie or are intimately associated with arenites typical of the wakeham group garnetite ironstone carbonate rock calc silicate rock and sillimanite bearing nodules veins and gneiss locally preserving lapilli are also present the distribution paragenesis and modes of most of these latter units differ from those of normal metasediments but are diagnostic of metamorphosed exhalites and hydrothermal alteration zones in the la romaine supracrustal belt

nitrogen isotope variations in santa barbara basin sediments implications for denitrification in the eastern tropical north pacific during the last 50 000 years nitrogen isotope variations preserved in santa barbara basin sediments are used to evaluate changes in denitrification in the eastern tropical north pacific etnp during the last 50 000 years a significant component of the subsurface waters 100 400 m that presently fill the santa barbara basin is derived from the low oxygen denitrifying zone in the etnp and the nitrate in these waters has a 15n value of 8 9 during the last glacial the 15n values of santa barbara basin sediments were typically 6 7 indicating decreased denitrification in the etnp and a better oxygenated intermediate water mass in the santa barbara basin at this time this reduced denitrification during the last glacial would have increased the pool of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period at the onset of deglaciation sediment 15n values increase by more than 2 indicating increased denitrification in the etnp during younger dryas time 15n values decreased by 3 and record a brief return to better ventilated conditions in the subsurface waters of the etnp this is followed by an increase in 15n to over 9 at 10 000 years ago indicating intense denitrification at the beginning of the holocene

impact of glacial interglacial sea level change on the ocean nitrogen cycle the continental shelves are the most biologically dynamic regions of the ocean and they are extensive worldwide especially in the western north pacific their area has varied dramatically over the glacial interglacial cycles of the last million years but the effects of this variation on ocean biological and chemical processes remain poorly understood conversion of nitrate to n2 by denitrification in sediments accounts for half or more of the removal of biologically available nitrogen fixed n from the ocean the emergence of continental shelves during ice ages and their flooding during interglacials have been hypothesized to drive changes in sedimentary denitrification denitrification leads to the occurrence of phosphorus bearing n depleted surface waters which encourages n2 fixation the dominant n input to the ocean an 860 000 y record of foraminifera shell bound n isotopes from the south china sea indicates that n2 fixation covaried with sea level the n2 fixation changes are best explained as a response to changes in regional excess phosphorus supply due to sea level driven variations in shallow sediment denitrification associated with the cyclic drowning and emergence of the continental shelves this hypothesis is consistent with a glacial ocean that hosted globally lower rates of fixed n input and loss and a longer residence time for oceanic fixed n a sluggish ocean n budget during ice ages in addition this work provides a clear sign of sea level driven glacial interglacial oscillations in biogeochemical fluxes at and near the ocean margins with implications for coastal organisms and ecosystems

isotopic composition and speciation of sedimentary nitrogen and carbon in the okinawa trough over the past 30 ka total organic carbon to total nitrogen ratios c n and their isotopic compositions 13ctoc and 15ntn are oft applied proxies to discern terrigenous from marine sourced organics and to unravel ancient environmental information in high depositional asian marginal seas the inclusion of n bearing minerals in the bulk sediment dilutes or masks signals sought after in 15n and 13c analyses thus leading to incorrect and even contradictory interpretations we used koh kobr to separate operationally defined total organic matter into oxidizable labile and residual fractions for content and isotope measurements in a sediment core in the okinawa trough significant amounts of carbon and nitrogen existed in the residual phase in which the c n ratio was 9 resembling most documented sedimentary bulk c n ratios in the china marginal seas the residual carbon which was included in operationally defined total organic carbon toc displayed a 13c range 22 7 to 18 9 mean 20 7 similar to the oft used marine end member after removing the interference from the residual fraction we found that the temporal pattern of 13cloc loc denotes labile organic carbon was more variable the residual nitrogen content was associated with illite suggesting a terrestrial origin additionally 15n in the residual fraction likely reflected the climatic controls e g precipitation on lithogenic source materials further studies are required to explore the controlling factors for carbon and nitrogen isotopic speciation and to retrieve the information locked in the residual fraction

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

the acceleration of oceanic denitrification during deglacial warming the marine nitrogen cycle was altered during the transition from glacial to interglacial conditions an analysis of 15n records throughout the world s oceans suggests that rates of denitrification in the water column accelerated during the last deglaciation

particulate δ15n in laminated marine sediments as a proxy for mixing between the california undercurrent and the california current a proof of concept the measurements of particulate 15n in coastal marine laminated sediments provide a high resolution proxy for fluctuations in the intensity of denitrification in the water column in the eastern tropical north pacific oxygen minimum zone this denitrification signal is transported northward by the california undercurrent thus serving as a tracer of ocean circulation this is verified through comparisons between salinity in the thermocline off southern california santa monica basin and the difference between 15nsed within age equivalent sediments from a southern pescadero slope and northern santa monica basin site trends in this parameter 15nsed relate to pacific decadal oscillation pdo phase changes between 1900 and 1990 we hypothesize that the decline in 15nsed during warm pdo phases is due to a strengthening of the california undercurrent transporting 15n enriched nitrate from the eastern tropical north pacific northward the deviation from this trend after 1990 suggests recent changes in circulation and or california current water nutrient biogeochemistry

denitrification in continental shelf sediments has major impact on the oceanic nitrogen budget denitrification rates in sediments within the oxygen deficient waters off mexico and from the gulf of maine were investigated on the basis of interstitial nutrient profiles nitrate fluxes into the sediments were calculated from gradients across the sediment water interface and vertical molecular diffusion coefficients and averaged 0 151 mexico and 0 0920 gulf of maine pmol no 3 cm 2 s 1 these are minimum values since these gradients may have been underestimated in the gulf of maine bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate in addition only 15 22 of the expected ammonium is present in gulf of maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification this large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water the global denitrification rate in continental shelf sediments was reassessed to be 50 tg n yr 1 1 tg 1012 g demonstrating that sediments are an important sink for oceanic nitrogen globally current nitrogen losses from the oceans may exceed inputs by 60 90 tg n yr 1 over the glacial interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area an oscillating oceanic nitrogen budget over these time scales could occur given the sequence of 1 scouring and dumping of terrestrial nitrogen into the oceans during glacial advance 2 removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat and 3 reincorporation of this n into terrestrial biomass during the interglacial period

isotopic constraints on glacial interglacial changes in the oceanic nitrogen budget early deglacial maximum in nitrate 15 n 14 n in suboxic zones and no significant glacialto late holocene change in global ocean nitrate 15 n 14 n consistent with the work of brandes and devol 2002 we find that the steady state 15 n 14 n of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column therefore a deglacial peak in the ratio of water column to sediment denitrification caused by either a strong feedback between water column denitrification and the n reservoir or by an increase in sediment denitrification due to sea level rise can explain the observed deglacial 15 n 14 n maximum in sediments underlying water column denitrification zones the total denitrification rate and the mean ocean nitrate concentration are also important determinants of steady state nitrate 15 n 14 n for this reason modeling a realistic deglacial 15 n 14 n maximum further requires that the combined negative feedbacks from n2 fixation and denitrification are relatively strong and n losses are relatively small our results suggest that the glacial oceanic n inventory was at most 30 greater than today s and probably less than 10 greater index terms 4267 oceanography general paleoceanography 4805 oceanography biological and chemical biogeochemical cycles 1615 4845 oceanography biological and chemical nutrients and nutrient cycling 4870 oceanography biological and chemical stable isotopes keywords feedback nitrogen isotopes paleoceanography

the influence of rapid millennial scale climate change on nitrogen isotope dynamics of the cariaco basin during marine isotope stage 3 understanding changes to the marine nitrogen cycle on millennial and shorter time scales can help determine the influence of rapid climate change on the fixed n pool and its sources and sinks rapid changes in denitrification have been observed in the eastern tropical north pacific etnp and arabian sea however millennial scale 15n records in regions influenced by n2 fixation are sparse we present a sedimentary 15n record from the cariaco basin during marine isotope stage mis 3 35 55 ka the 15n record displays a pattern of millennial scale variability that tracks the greenland ice core dansgaard oeschger cycles with higher values observed during interstadial periods lower values during stadial periods and abrupt transitions in between conditions during interstadials are similar to those at present in the cariaco basin with the sedimentary 15n signal reflecting a combination of local processes and an imported regional signal if interpreted to reflect regional processes the interstadial 15n values average 5 1 support the argument that n2 fixation did not increase in the tropical north atlantic during the last glacial the lower 15n values during stadials when lower sea level resulted in increased physical isolation of the basin can be explained primarily by local processes in spite of the importance of local processes striking similarity is observed between the cariaco record and millennial scale 15n records from the etnp and arabian sea the apparent synchronicity of changes observed in all three regions suggests an atmospheric teleconnection between the three sites and high latitude climate forcing during mis 3

southern ocean control on the extent of denitrification in the southeast pacific over the last 70 ka abstract temporal changes in oceanic denitrification the bacterial reduction of nitrate under suboxic conditions highlight the potential importance of n inventory changes and the production of n 2 o on the climate system at the same time the cause of the globally observed variation in denitrification remains unclear high resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from odp site 1234 on the chile margin record integrated denitrification changes within the peru chile upwelling system over the last 70 ka denitrification changes in the southeast pacific are coherent with antarctic climate changes recorded by the byrd ice core 18 o record and lead northern hemisphere climate events the southern hemisphere character of the chile margin 15 n record suggests that episodes of reduced denitrification in the se pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of subantarctic mode water samw which forms in the subantarctic zone of the southern ocean and feeds into the low latitude thermocline

deep sea coral evidence for lower southern ocean surface nitrate concentrations during the last ice age the southern ocean regulates the ocean s biological sequestration of co 2 and is widely suspected to underpin much of the ice age decline in atmospheric co 2 concentration but the specific changes in the region are debated although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core based measurements the use of different proxies in different regions has precluded a unified view of southern ocean biogeochemical change here we report measurements of the 15 n 14 n of fossil bound organic matter in the stony deep sea coral desmophyllum dianthus a tool for reconstructing surface ocean nutrient conditions the central robust observation is of higher 15 n 14 n across the southern ocean during the last glacial maximum lgm 18 25 thousand years ago these data suggest a reduced summer surface nitrate concentration in both the antarctic and subantarctic zones during the lgm with little surface nitrate transport between them after the ice age the increase in antarctic surface nitrate occurred through the deglaciation and continued in the holocene the rise in subantarctic surface nitrate appears to have had both early deglacial and late deglacial holocene components preliminarily attributed to the end of subantarctic iron fertilization and increasing nitrate input from the surface antarctic zone respectively

a combined fluid inclusion and s pb isotope study of the neoproterozoic pingshui volcanogenic massive sulfide cu zn deposit southeast china abstract the pingshui cu zn deposit is located in the jiangshan shaoxing fault zone which marks the neoproterozoic suture zone between the yangtze block and cathaysia block in south china it contains 0 45 million tons of proven ore reserves with grades of 1 03 wt cu and 1 83 wt zn this deposit is composed of stratiform massive sulfide ore bodies which contain more than 60 vol sulfide minerals these ore bodies are hosted in altered mafic and felsic rocks spilites and keratophyres of the bimodal volcanic suite that makes up the neoproterozoic pingshui formation metallic minerals include pyrite chalcopyrite sphalerite tennantite tetrahedrite and magnetite with minor galena gangue minerals are quartz sericite chlorite calcite gypsum barite and jasper three distinct mineralogical zones are recognized in these massive sulfide ore bodies a distal zone composed of sphalerite pyrite barite zone i an intermediate zone characterized by a pyrite sphalerite chalcopyrite assemblages zone ii and a proximal zone containing chalcopyrite pyrite magnetite zone iii a thin layer of exhalative jaspilite overlies the sulfide ore bodies except in the proximal zone the volcanic rocks of the pingshui formation are all highly altered spilites and keratophyres but their trace element geochemistry suggests that they were generated by partial melting of the depleted mantle in an island arc setting homogenization temperatures of the primary fluid inclusions in quartz from massive sulfide ores are between 217 and 328 c and their salinities range from 3 2 to 5 7 wt nacl equivalent raman spectroscopy of the fluid inclusions showed that water is the dominant component with no other volatile components fluid inclusion data suggest that the ore forming fluids were derived from circulating seawater the 34 s values of pyrite from the massive sulfide ores range from 3 6 to 3 4 indicating that the sulfur was primarily leached from the arc volcanic rocks of the pingshui formation both pyrite from the massive sulfide ores and plagioclase from the spilites have similar lead isotope compositions implying that the lead was also derived from the pingshui formation the low lead contents of the massive sulfide ores and the geochemistry of their host rocks are similar to many vms cu zn deposits in canada e g noranda and thus can be classified as belonging to the bimodal mafic subtype the presence of magnetite and the absence of jaspilite and barite at the 505 m level in the pingshui deposit suggest that this level is most likely the central zone of the original lateral massive sulfide ore bodies if this interpretation is correct the deep part of the pingshui cu zn deposit may have significant exploration potential

pb and sr isotopic systematics of some basalts and sulfides from the east pacific rise at 21 n project rita abstract tholeiitic basalts and sulfide deposits from the cyana and alvin diving programs rita project on the east pacific rise were analyzed for pb and sr isotopes the basalt data plot within the field defined previously by other east pacific rise basalts 206 pb 204 pb 18 35 18 58 207 pb 204 pb 15 48 15 53 208 pb 204 pb 37 8 38 1 87 sr 86 sr 0 7022 0 7025 pb u and sr contents 0 5 0 05 and 110 ppm respectively and values 6 are typical of morb whereas th u ratios 3 5 are significantly higher the pb isotopic ratios of the sulfide samples are very homogeneous 206 pb 204 pb 18 47 207 pb 204 pb 15 49 208 pb 204 pb 37 90 and plot in the middle of the basalt field this indicates that 1 the sulfide pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments and 2 the solutions from which the sulfides were deposited had uniform pb isotopic composition the pb contents of three sulfide samples is relatively high 170 1310 ppm the sr contents of five sulfide samples are widely scattered from 12 to 210 ppm with 87 sr 86 sr ratios intermediate between basaltic and seawater values 0 70554 0 00005 to0 70795 0 00011 leaching experiments show that both basalt derived sr and seawater sr were present in the solutions which deposited the sulfides in some cases sr was also adsorbed from seawater onto the sulfides following their deposition basalt derived sr and seawater sr are also present in associated non sulfide phases

the dynamics of the marine nitrogen cycle across the last deglaciation 1 we use a geochemical box model to investigate the changes in marine n fixation and denitrification required to match the observed sedimentary 15n changes between 30 kyr b p and the late holocene this is achieved by optimizing a set of seven parameters that describe the strengths of three ocean internal n feedbacks and the response of the oceanic n cycle to four external forcings scenarios that best match the 15n constraints indicate a strong transient decrease in n fixation in the early deglacial in response to the decrease in iron input by dust around 15 kyr b p n fixation rebounds primarily in response to an abrupt increase in water column denitrification caused by an expansion of anoxia benthic denitrification is not well constrained by our model but tends to increase in sync with water column denitrification as a result of the transient imbalance between n fixation and denitrification we infer a glacial to interglacial decrease in the marine n inventory of between 15 and 50 the model diagnoses this reduction in order to simultaneously fit the data from all ocean basins requiring it to reduce the degree by which water column denitrification in the oxygen minimum zones is influencing the 15n of nitrate of the whole ocean dilution effect our optimal solution suggests a glacial n cycle that operated at nearly the same rates as that in pre industrial times but sensitivity cases with substantially lower rates fit the data only marginally worse an important caveat of our study is the assumption of an unchanging ocean circulation an initial sensitivity experiment shows that this affects primarily the magnitude of the change in the n inventory while the diagnosed deglacial dynamics with global marine n fixation taking a dip before the onset of denitrification remains a robust result

centennial to millennial scale changes in oxygenation and productivity in the eastern tropical south pacific during the last 25 000 years oxygen minimum zones omz have expanded in all tropical oceans during the last 50 years resulting in habitat contraction and considerable changes in marine biogeochemistry however for a better understanding of the omz dynamics under the current climate change two questions are relevant 1 how do the magnitude and temporal changes in oceanic dissolved oxygen of the last few decades compare to the natural variability on longer timescales and 2 what were the local and remote factors driving omz changes in the past in the present study we use a stacked record covering the last 25 kyr from the eastern tropical south pacific etsp omz to reconstruct changes in oxygenation and productivity we use a suite of proxies including the presence of laminations redox sensitive metals u mo re ni and cu total organic carbon and 15 n measurements water column denitrification and sediment redox conditions show pronounced centennial to millennial scale variability during the last 25 kyr with oxygenation levels as low as at present global cold periods at different timescales such as the last glacial maximum 23 19 kyr bp and the little ice age 1500 1850 ad were associated with a weak omz and low export production while warm intervals such as the deglaciation part of the medieval climate anomaly and the last 100 years are associated with a stronger omz and high export production water column denitrification and sediment redox conditions were strongly coupled during the last 25 kyr bp apart from one remarkable exception during the antarctic cold reversal sediments were less reducing but the water column denitrification was high resulting in a strong but shallow omz this may have been produced by an enhanced antarctic intermediate water flow contrary to our expectations and modeling predictions for the next few decades we observe a weak etsp omz during the warm mid holocene which may have been the result of a stronger walker circulation that brought oxygen rich waters to intermediate depths off peru via equatorial undercurrents in combination with other paleoceanographic reconstructions our results show that oxygenation variability in the etsp omz was influenced by ocean circulation changes in the tropical pacific high latitude oceanographic and climatic changes and local productivity

large climate driven changes of oceanic oxygen concentrations during the last deglaciation during the last glacial termination the solubility of gases in the ocean decreased as ocean temperatures rose however marine sediments have not unanimously recorded ocean deoxygenation throughout this time some records show increasing oxygenation since the last glacial maximum particularly in the deep sea while many document abrupt oxygenation changes often associated with apparent changes in the formation rate of north atlantic deep water here we present a global compilation of marine sediment proxy records that reveals remarkable coherency between regional oxygenation changes throughout deglaciation the upper ocean generally became less oxygenated but this general trend included pauses and even reversals reflecting changes in nutrient supply respiration rates and ventilation the most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation associated with a reinvigoration of north atlantic deep water formation at this time the upper indo pacific ocean was less oxygenated than today meanwhile the bulk of the deep ocean became more oxygenated over the deglaciation reflecting a transfer of respired carbon to the atmosphere the observed divergence from a simple solubility control emphasizes the degree to which oxygen consumption patterns can be altered by changes in ocean circulation and marine ecosystems

deglacial weakening of the oceanic soft tissue pump global constraints from sedimentary nitrogen isotopes and oxygenation proxies abstract among the many potential explanations for the 80 ppm rise of atmospheric co2 concentrations at the end of the last ice age most involve a weakening of the oceanic soft tissue pump here we use global data compilations of sedimentary nitrogen isotopes and benthic oxygenation proxies to provide a synoptic global perspective on the deglacial soft tissue pump weakening the net change between the last glacial maximum and holocene shows a removal of respired carbon everywhere that proxy data is available with the exception of the upper 1 5 km of the north pacific while excess nitrate accumulated in polar oceans these observations could be explained by intensifying iron limitation a shoaling of organic matter remineralization and or a change in ocean circulation but are inconsistent with a change in the magnitude or position of the southern mid latitude westerlies the net soft tissue pump weakening was front loaded in the early deglaciation 17 5 14 ka when atmospheric 13c and 14c were changing rapidly and appears to have contributed little net change thereafter superimposed on the overall deglacial trend were pronounced transient changes that coincided with variability in the atlantic meridional overturning circulation amoc and bipolar seesaw the seesaw variability is most clearly expressed as anti phased oxygenation changes between the upper 1 5 km of the north pacific and the deep north atlantic consistent with an atlantic pacific ventilation seesaw but it dominated transient variability in records throughout the world although the soft tissue pump seesaw made little contribution to the contrast between the glacial and interglacial states it could have contributed to deglacial variability in atmospheric co2 and might have catalyzed the deglaciation

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

the chromium isotope composition of reducing and oxic marine sediments the chromium cr isotope composition of marine sediments has the potential to provide new insights into the evolution of earth surface redox conditions there are significant but poorly constrained isotope fractionations associated with oxidative subaerial weathering and riverine transport the major source of seawater cr and with partial cr reduction during burial in marine sediments the major sink for seawater cr a more comprehensive understanding of these processes is needed to establish global cr isotope mass balance and to gauge the utility of cr isotopes as a paleoredox proxy for these purposes we investigated the cr isotope composition of reducing sediments from the upwelling zone of the peru margin and the deep cariaco basin chromium is present in marine sediments in both detrital and authigenic phases and to estimate the isotopic composition of the authigenic fraction we measured 53cr on a weakly acid leached fraction in addition to the bulk sediment in an effort to examine potential variability in the cr isotope composition of the detrital fraction we also measured 53cr on a variety of oxic marine sediments that contain minimal authigenic cr the average 53cr value of the oxic sediments examined here is 0 05 0 10 2 n 25 which is within the range of 53cr values characteristic of the bulk silicate earth this implies that uncertainty in estimates of authigenic 53cr values based on bulk sediment analyses is mainly linked to estimation of the ratio of cr in detrital versus authigenic phases rather than to the cr isotopic composition of the detrital pool leaches of cariaco basin sediments have an average 53cr value of 0 38 0 10 2 n 7 which shows no dependency on sample location within the basin and is close to that of atlantic deepwater cr 0 5 this suggests that authigenic cr in anoxic sediments may reliably reflect the first order cr isotope composition of deepwaters for peru margin samples the average 53cr values of bulk sediments 0 59 0 06 2 and leach fractions 0 61 0 06 2 are also comparable with those of cariaco basin samples and modern deepwater values finally we found that the 53cr of peru margin samples correlates with 15n on glacial interglacial timescales which we attribute to secular variation in basinal or global ocean redox conditions thus the 53cr stratigraphic record of peru margin sediments indicates that cr isotopes may be suited to tracking geologically short term changes in ocean oxygenation

the lead and strontium isotope geochemistry of metalliferous sediments associated with upper cretaceous ophiolitic rocks in cyprus syria and the sultanate of oman metalliferous sediments consisting dominantly of fine grained iron and manganese oxides and hydroxyoxides have been widely recorded from the crests of sea floor spreading ridges and as a basal faci

mechanisms controlling export production at the lgm effects of changes in oceanic physical fields and atmospheric dust deposition 1 using a biogeochemical ocean model that includes the iron cycle we carry out preindustrial control ctl and glacial last glacial maximum lgm climate simulations focusing on changes in export production ep the model successfully reproduces general trends of a paleoclimate reconstruction of ep at the lgm except over the atlantic ocean by conducting a series of sensitivity simulations we investigate the mechanism controlling ep at the lgm in each basin in the southern ocean the model successfully reproduces the dipole pattern of the paleoreconstruction the higher latitude decrease and lower latitude increase of ep it is found that the lower latitude increase of ep comes from iron fertilization effects by enhanced dust deposition while the higher latitude decrease of ep is caused by the reduction of surface shortwave due to spreading of sea ice there we also find that increased dust input in other basins remotely affects ep in the southern ocean in the pacific ocean the model suggests that iron fertilization effects are dominant in open ocean regions in the atlantic ocean the model simulates overall reduction of ep whereas the paleoreconstruction suggests the increase in some regions we propose that the atlantic response is strongly affected by distribution of iron limitation in a control climate in our ctl simulation the biological production is limited not by iron but by phosphate in the atlantic ocean which leads to the decrease of ep in spite of the significant increase of dust deposition there it is implied that the accurate evaluation of iron limitation in the present ocean is critical for evaluating changes in ep and associated reduction of atmospheric co2 concentration at the lgm

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

nitrogen isotopic variations in the gulf of california since the last deglaciation response to global climate change high resolution records of the nitrogen isotopic composition of organic matter 15norg opal content and opal accumulation rates from the central gulf of california reveal large and abrupt variations during deglaciation and gradual holocene changes coincident with climatic changes recorded in the north atlantic homogenous sediments with relatively low 15norg values and low opal content were deposited at the end of the last glacial period during the younger dryas event and during the middle to late holocene in contrast laminated sediments deposited in the two deglacial stages are characterized by very high 15norg values 14 and opal accumulation rates 29 41 mg cm 2 yr 1 abrupt shifts in 15norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling vertical mixing and or the latitudinal position of the intertropical convergence zone

a global marine fixed nitrogen isotopic budget implications for holocene nitrogen cycling 1 a nitrogen stable isotopic model was constructed in order to constrain the holocene marine fixed nitrogen budget the primary sources and sinks considered were riverine and atmospheric sources nitrogen fixation sedimentary and water column denitrification and sediment burial the source budget was found to be insensitive to changes in nitrogen fixation rates and thus could not be used to constrain this term however the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification if the isotopic value of marine fixed nitrogen has not changed during the holocene as supported by sedimentary records then in order to balance the isotopic value of sinks and sources approximately 280 tg n yr 1 of sedimentary denitrification is required if such a high rate of denitrification has been sustained throughout the holocene it implies that present day estimates of marine nitrogen fixation are grossly underestimated it also implies that the marine nitrogen budget has a residence time of less than 2000 years