hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

the stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems iron fe binding phases in two hydrothermal plumes in the southern ocean were studied using a novel voltammetric technique this approach reverse titration competitive ligand exchange adsorptive cathodic stripping voltammetry showed that on average 30 21 of dissolved fe in the hydrothermal plumes was stabilised by chemically labile binding to ligands the conditional stability constant log k fel of the observed complexes was 20 61 0 54 mean 1 sd for the two vent sites intermediate between previous measurements of deep ocean ligands 21 4 23 kondo et al 2012 and dissolved weak estuarine ligands 20 gerringa et al 2007 our results indicate that approximately 7 5 of all hydrothermal fe was stabilised by complexation with ligands furthermore 47 26 of the dissolved fe in the plume existed in the colloidal size range 0 02 0 2 m our data suggests that a portion 7 5 of hydrothermal fe is sufficiently stabilised in the dissolved size fraction 0 2 m to make an important impact on deep ocean fe distributions lateral deep ocean currents transport this hydrothermal fe as lenses of enhanced fe concentrations away from mid ocean ridge spreading centres and back arc basins

the distribution of fe in the antarctic circumpolar current abstract the large scale distributions of dissolved and total fe in surface and deep waters of the antarctic circumpolar current exhibit strong relationships with hydrography and biological processes the mean dissolved fe concentrations are low in surface waters of the antarctic circumpolar current 0 31 0 49 nm with a minimum of 0 17 nm and higher averaging 1 11 9 nm in the polar frontal region enhanced dissolved surface water concentrations in the polar frontal region are attributed to input from the continental shelf and coincide with phytoplankton spring blooms of large diatoms the effects of sea ice melting and iceberg melting on the fe concentrations were relatively small dissolved deep water concentrations 400 m in the antarctic circumpolar current ranged from 0 4 to 2 8 nm circumpolar deep water has relatively high dissolved fe concentrations in the polar frontal region 0 4 2 8 nm compared with deep waters further to the south 0 6 1 1 nm similarly total dissolvable unfiltered fe concentrations in the upper circumpolar deep water tend to decrease southward from the polar frontal region in the lower circumpolar deep water total dissolvable fe concentrations are higher than in the upper circumpolar deep water due to the existing nepheloid layer and sources on the mid atlantic ridge dissolved and total dissolvable fe concentrations in the antarctic bottom water are higher than those of other water masses in the antarctic circumpolar current consistent with the nepheloid layer as well as diagenetic input from shelf sediments the high nutrient low chlorophyll areas of the antarctic ocean and northeast pacific ocean have different major fe input sources of similar magnitude in the antarctic circumpolar current upward transport of fe is the main input source whereas in the north pacific ocean aerosols are the dominant source

continuous sampling of hydrothermal fluids from loihi seamount after the 1996 event for at least 9 years prior to july 1996 hydrothermal fluids flowed from pele s vents on loihi seamount hawaii in july august 1996 a tectonic volcanic event occurred that destroyed pele s vents creating a pit crater pele s pit and several sites with hydrothermal venting in october 1996 we deployed two new continuous water samplers osmosamplers at two of these hydrothermal sites and collected fluids using traditional sampling techniques to monitor the evolution of crustal and hydrothermal conditions after the event the samplers were recovered in september 1997 and additional discrete vent fluid samples were collected the osmosampler located along the south rift at naha vents captured a change in composition from a low chlorinity high k fluid relative to bottom seawater to a high chlorinity low k fluid these changes are consistent with the fluid cooling during ascent and being derived from several different sources which include high 330 c and low 330 c into which magmatic volatiles were added during the deployment thermal and fluid fluxes decreased at naha the transport of heat and chemicals was decoupled the chemical and thermal evolution of hydrothermal fluids after the event on loihi is consistent with previous models based on events that have occurred along mid ocean ridges the event at loihi clearly had an effect on the local hydrography however the integrated effect of chemical fluxes to global budgets from similar events is uncertain chemical fluxes from similar events may have a global impact if ratios of chemical e g co2 fe mn mg sulfate and k to thermal anomalies greatly exceed or are in the opposite direction to fluxes from mid ocean ridge hydrothermal systems

dissolved and particulate fe in a hydrothermal plume at 9 45 n east pacific rise slow fe ii oxidation kinetics in pacific plumes production of fe iii particles in hydrothermal plumes is of fundamental importance to the long term effect of hydrothermal circulation on seawater composition to elucidate the fundamental controls on fe redox kinetics and solution particle partitioning in neutrally buoyant plumes we sampled near field 3 km plume particles at 9 45 n on the east pacific rise in 1996 returning in 1997 to sample both particulate and dissolved phases 0 40 m filter concentrations of dissolved fe varied from 320 to 20 nm in proximal 0 3 km from vent site to distal samples 1 3 km downfield constituting 85 50 of total fe respectively based on vent fluid dilution factors calculated from dissolved mn a mass balance for vent fluid fe at this site indicates that 65 of fe is lost to particulate sulfide settling in the buoyant plume and that particulate fe in distal 1 3 km samples is twice as concentrated as predicted from dilution of particles in proximal plume water these observations are consistent with a calculated fe ii oxidation half time of 3 3 h long enough that fe iii colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions preventing generation of high particulate fe concentrations 11 56 nm observed a general investigation of fe ii oxidation rates in plumes worldwide gives fe ii oxidation half lives as short as 17 min at some atlantic sites and as long as 6 h at some pacific sites the calculations indicate that the distribution of fe particles in plumes depends chiefly on inter basin differences in ambient deep water chemistry primarily ph and dissolved o2 and on local currents driving plume dilution and to a much lesser extent on variations in primary vent fluid composition long term changes in thermohaline circulation or ocean biogeochemistry may therefore alter fe dynamics and minor element fluxes associated with global hydrothermal activity independent of variations in crustal production rates

isotopic signature of dissolved iron delivered to the southern ocean from hydrothermal vents in the east scotia sea it has recently been demonstrated that hydrothermal vents are an important source of dissolved fe dfe to the southern ocean the isotopic composition 56fe of dfe in vent fluids appears to be distinct from other sources of dfe to the deep ocean but the evolution of 56fe during mixing between vent fluids and seawater is poorly constrained here we present the evolution of 56fe for dfe in hydrothermal fluids and dispersing plumes from two sites in the east scotia sea we show that 56fe values in the buoyant plume are distinctly lower as low as 1 19 than the hydrothermal fluids 0 29 attributed to i precipitation of fe sulfides in the early stages of mixing and ii partial oxidation of fe ii to fe iii 55 of which subsequently precipitates as fe oxyhydroxides by contrast the 56fe signature of stabilized dfe in the neutrally buoyant plume is 0 3 to 0 5 this cannot be explained by continued dilution of the buoyant plume with background seawater rather we suggest that isotope fractionation of dfe occurs during plume dilution due to fe ligand complexation and exchange with labile particulate fe the 56fe signature of stabilized hydrothermal dfe in the east scotia sea is distinct from background seawater and may be used to quantify the hydrothermal dfe input to the ocean interior

magnetization of 0 26 5 ma seafloor at the ultraslow spreading southwest indian ridge 61 67 e 1 we investigate the magnetic signature of volcanic and nonvolcanic seafloor areas along the southwest indian ridge between 61 e and 67 e and analyze their relationship with crustal thickness variations and past to present ridge segmentation this part of the southwest indian ridge is an end member for the global ridge system in terms of low melt supply thin crust and ultraslow spreading rates it is characterized by large expanses of seafloor that show no evidence for a volcanic upper crustal layer we find that variations of intrinsic magnetization and thickness of the basaltic extrusive layer where it is present dominate the present day along axis crustal magnetization off axis the magnetization contrast is on average higher for volcanic seafloor than for smooth nonvolcanic topography indicating that the contribution of the basaltic upper crustal layer to the production of magnetic anomalies remains important however magnetic anomalies that record past magnetic polarity events are found almost everywhere in the survey area even over domains that lack a volcanic upper crustal layer arguing thus for the contribution of other sources we propose that both gabbros and serpentinized peridotites contribute to these anomalies although not systematic and weak over most parts of the survey area an induced component of magnetization is clearly present in some nonvolcanic seafloor domains serpentinized peridotites are the likely carriers of this induced magnetization component

an authoritative global database for active submarine hydrothermal vent fields 1 the interridge vents database is available online as the authoritative reference for locations of active submarine hydrothermal vent fields here we describe the revision of the database to an open source content management system and conduct a meta analysis of the global distribution of known active vent fields the number of known active vent fields has almost doubled in the past decade 521 as of year 2009 with about half visually confirmed and others inferred active from physical and chemical clues although previously known mainly from mid ocean ridges mors active vent fields at mors now comprise only half of the total known with about a quarter each now known at volcanic arcs and back arc spreading centers discoveries in arc and back arc settings resulted in an increase in known vent fields within exclusive economic zones consequently reducing the proportion known in high seas to one third the increase in known vent fields reflects a number of factors including increased national and commercial interests in seafloor hydrothermal deposits as mineral resources the purpose of the database now extends beyond academic research and education and into marine policy and management with at least 18 of known vent fields in areas granted or pending applications for mineral prospecting and 8 in marine protected areas

hydrothermal vents as a kinetically stable source of iron sulphide bearing nanoparticles to the ocean hydrothermal vents emit metals such as iron to the ocean a chemical analysis of hydrothermal fluids suggests that up to 10 of the iron is present as stable iron bearing nanoparticles

middepth circulation of the eastern tropical south pacific and its link to the oxygen minimum zone there is an incomplete description of the mid depth circulation and its link to the oxygen minimum zone omz in the eastern tropical south pacific subsurface currents of the omz in the eastern tropical south pacific are investigated with a focus at 400 m depth close to the core of the omz using several acoustic doppler current profiler sections recorded in january and february 2009 five profiling floats with oxygen sensors were deployed along 85 50 w in february 2009 with a drift depth at 400 m their spreading paths are compared with the model flow field from a 1 10 tropical pacific model tropac01 and the simple ocean data assimilation soda model overall the mean currents in the eastern tropical south pacific are weak so that eddy variability influences the flow and ultimately feed oxygen poor water to the omz the center of the omz is a stagnant area so that floats stay much longer in this region and can even reverse direction in one case of one float deployed at 8 s returned to the same location after 15 month on the northern side of the omz in the equatorial current system floats move rapidly to the west most current bands reported for the near surface layer exist also in the depth range of the omz a schematic circulation flow field for the omz core depth is derived which shows the northern part of the south pacific subtropical gyre south of the omz and the complicated zonal equatorial flow field north of the omz

hydrothermal contribution to the oceanic dissolved iron inventory mineral dust and marine sediment resuspension are generally considered the primary sources of the nutrient iron to the oceans numerical model results suggest that iron released by hydrothermal activity is also an important source of dissolved iron particularly in the southern ocean iron limits phytoplankton growth and hence the biological carbon pump in the southern ocean1 models assessing the impacts of iron on the global carbon cycle generally rely on dust input and sediment resuspension as the predominant sources2 3 although it was previously thought that most iron from deep ocean hydrothermal activity was inaccessible to phytoplankton because of the formation of particulates4 it has been suggested that iron from hydrothermal activity5 6 7 may be an important source of oceanic dissolved iron8 9 10 11 12 13 here we use a global ocean model to assess the impacts of an annual dissolved iron flux of approximately 9 108 mol as estimated from regional observations of hydrothermal activity11 12 on the dissolved iron inventory of the world s oceans we find the response to the input of hydrothermal dissolved iron is greatest in the southern hemisphere oceans in particular observations of the distribution of dissolved iron in the southern ocean3 chever et al manuscript in preparation bowie et al manuscript in preparation can be replicated in our simulations only when our estimated iron flux from hydrothermal sources is included as the hydrothermal flux of iron is relatively constant over millennial timescales14 we propose that hydrothermal activity can buffer the oceanic dissolved iron inventory against shorter term fluctuations in dust deposition

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

an overview of dissolved fe and mn distributions during the 2010 2011 u s geotraces north atlantic cruises geotraces ga03 abstract high resolution dissolved fe dfe and dissolved mn dmn distributions were obtained using a trace metal clean rosette during the geotraces ga03 zonal transect cruises usgt10 and usgt11 across the north atlantic ocean this manuscript provides a general overview of the dfe as well as dmn and dissolved al dal distributions that reveal several fe inputs at varying depths across the study region elevated dfe concentrations correlate with elevated dal concentrations in the surface waters of the subtropical gyre indicating a significant atmospheric source of fe in contrast there is no apparent significant dust source for mn in the subsurface waters dfe maxima are a result of the remineralization process as revealed by their correspondence with dissolved oxygen minima within the oxygen minimum the ratio of dfe to apparent oxygen utilization aou is lower than would be expected from the measured fe content of surface water phytoplankton suggesting that a significant amount of dfe that is remineralized at depth 63 90 is subsequently scavenged from the water column the rate of remineralization which is based on the slope of dfe aou plot is similar across a wide area of the north atlantic in addition to the remineralization process sedimentary inputs are apparent from elevated dmn signals in the eastern basin particularly near the african coast in the western basin sedimentary input is also occurring along the advective flow path of the upper labrador sea water ulsw as ulsw transits along the north american continental shelf region the largest dfe anomaly 68 nm which also corresponds to a dmn anomaly up to 33 nm is seen in the neutrally buoyant hydrothermal plume sampled over the mid atlantic ridge and that signal is visible for 500 km to the west of the ridge

deformation in the diffuse india capricorn somalia triple junction from a multibeam and magnetic survey of the northern central indian ridge 3 s 10 s 1 we use new multibeam and magnetic observations from 3 s 10 s along the central indian ridge to describe morphotectonic features and spreading fabrics within the study area and quantify deformation across the diffuse india capricorn somalia triple junction a megamullion adjacent to the vityaz transform fault is the first reported for the northern central indian ridge the principal transform displacement zone ptdz of the vema transform fault consists of several closely spaced faults that can be traced for more than 150 km serpentinite or volcanic intrusions that are offset by the vema pdtz have inferred ages of 30 000 45 000 years testifying to the youth of active features on the valley floor abyssal hill orientations throughout the study area have remained constant since at least 4 ma implying little or no change in the opening direction during this period abyssal hills from opposite sides of the ridge have the same orientations within our 1 3 errors implying that slip along reactivated fracture zones in the diffuse triple junction neither significantly rotates nor shears the intervening seafloor transform fault azimuths in the survey area agree with the nuvel 1 africa australia slip direction but are 5 9 clockwise from the africa india direction in contrast seafloor spreading rates in the study area agree better with the nuvel 1 africa india predictions these seemingly conflicting observations are reconciled by a model in which seafloor within the diffuse triple junction rotates about the india capricorn pole at angular rotation rates that increase monotonically from zero to the full rotation rate across the deforming zone

the effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

geochemical evidence for iron mediated anaerobic oxidation of methane anaerobic oxidation of methane aom by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere we show geochemical evidence for aom in deep lake sediments and demonstrate that aom is likely driven by iron fe reduction pore water profiles from lake kinneret sea of galilee israel show that this sink for methane is located below the 20 cm depth in the sediment which is well below the depths at which nitrate and sulfate are completely exhausted as well as below the zone of methanogenesis iron dependant aom was verified by fe iii amended mesocosm studies using intact sediment cores and native iron oxides were detectable throughout the sediments because anaerobic fe iii respiration is thermodynamically more favorable than both sulfate dependent methanotrophy and methanogenesis its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited similar conditions are likely to prevail in other incompletely pyritized aquatic sediments indicating that aom with fe iii is an important global sink for methane

the cycling of iron zinc and cadmium in the north east pacific ocean insights from stable isotopes abstract dissolved stable isotope ratios of the transition metals provide useful information both for understanding the cycling of these bioactive trace elements through the oceans and tracing their marine sources and sinks here we present seawater dissolved fe zn and cd concentration and stable isotope ratio 56fe 66zn and 114cd profiles from two stations in the pacific ocean the safe station 30 n 140 w in the subtropical north east pacific from the geotraces ic2 cruise and the marginal san pedro basin 33 8 n 118 4 w within the south california bight these data represent to our knowledge the first full water column profiles for 66zn and 56fe from the open ocean north pacific and the first observations of dissolved 66zn and 114cd in a low oxygen marginal basin at the safe station 56fe is isotopically lighter throughout the water column 0 6 to 0 1 relative to irrm 014 compared to the north atlantic suggesting significant differences in fe sources or fe cycling between these two ocean basins a broad minimum in 56fe associated with the north pacific oxygen minimum zone omz

fluxes and distribution of dissolved iron in the eastern sub tropical north atlantic ocean aeolian dust transport from the saharan sahel desert regions is considered the dominant external input of iron fe to the surface waters of the eastern sub tropical north atlantic ocean to test this hypothesis we investigated the sources of dissolved fe dfe and quantified dfe fluxes to the surface ocean in this region in winter 2008 surface water dfe concentrations varied between 1 5 nm correlated positively with apparent oxygen utilization aou and showed the importance of organic matter remineralization as an dfe source as a consequence vertical diffusive mixing formed an important fe flux to the surface ocean in this region even surpassing that of a major dust event

total dissolvable and dissolved iron isotopes in the water column of the peru upwelling regime vertical distributions of iron fe concentrations and isotopes were determined in the total dissolvable and dissolved pools in the water column at three coastal stations located along the peruvian margin in the core of the oxygen minimum zone omz the shallowest station 121 161 m total water depth was characterized by lithogenic input from the continental plateau yielding concentrations as high as 456 nm in the total dissolvable pool at the 2 other stations stations 122 and 123 fe concentrations of dissolved and total dissolvable pools exhibited maxima in both surface and deep layers fe isotopic composition 56fe showed a fractionation toward lighter values for both physical pools throughout the water column for all stations with minimum values observed for the surface layer between 0 64 and 0 97 at 10 20 m depth and deep layer between 0 03 and 1 25 at 160 300 m depth an fe isotope budget was established to determine the isotopic composition of the particulate pool we observed a range of 56fe values for particulate fe from 0 02 to 0 87 with lightest values obtained at water depth above 50 m such light values in the both particulate and dissolved pools suggest sources other than atmospheric dust deposition in the surface ocean including lateral transport of isotopically light fe samples collected at station 122 closest to the sediment show the lightest isotope composition in the dissolved and the particulate pools 1 25 and 0 53 respectively and high fe ii concentrations 14 2 2 1 nm consistent with a major reductive benthic fe sources that is transferred to the ocean water column a simple isotopic model is proposed to link the extent of fe ii oxidation and the fe isotope composition of both particulate and dissolved fe pools this study demonstrates that fe isotopic composition in omz regions is not only affected by the relative contribution of reductive and non reductive shelf sediment input but also by seawater column processes during the transport and oxidation of fe from the source region to open seawater

iron isotope fractionation in a sulfide bearing subterranean estuary and its potential influence on oceanic fe isotope flux we trace pathways of fe reactions in the indian river lagoon florida usa subterranean estuary using fe isotopes to provide new constraints on fe isotopic fractionation in a sulfide bearing subterranean estuary porewater 56fe values increase from 1 16 at 115cm depth to 0 2 at 7cm depth due to isotope fractionation in three distinct lithostratigraphic zones the deepest zone contains orange sands with elevated fe oxide contents 0 2wt that dissolve through diagenetic fe oxide reduction and elevate fe concentrations in porewaters 100 to 300 m l this reaction causes porewater 56fe values to be 1 lighter than the sediment 56fe values an intermediate zone contains white fe poor sands with fe oxide contents 0 1wt and dissolved fe concentrations 20 m l this zone is a sink for dissolved fe through adsorption of isotopically heavy dissolved fe ii onto mineral surfaces this adsorption results in porewater 56fe values that are as much as 1 8 lighter than sediment 56fe values the uppermost zone contains organic carbon and fe sulfide rich black sediments with low dissolved fe 1 m l and elevated porewater sulfide up to 600 m l concentrations precipitation of isotopically light fe sulfides increases the porewater 56fe values as much as 0 68 more than corresponding sediment 56fe values the near surface fe sulfide precipitation delivers to the lagoon dissolved fe with slightly positive 56fe values averaging about 0 24 via submarine groundwater discharge sgd iron sulfide precipitation in sulfide containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy fe to the coastal oceans

integrated fe and s isotope study of seafloor hydrothermal vents at east pacific rise 9 10 n abstract in this study we report on coupled fe and s isotope systematics of hydrothermal fluids and sulfide deposits from the east pacific rise at 9 10 n to better constrain processes affecting fe isotope fractionation in hydrothermal environments we aim to address three fundamental questions 1 is there significant fe isotope fractionation during sulfide precipitation 2 is there significant variability of fe isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments 3 are there any systematics between fe and s isotopes in sulfide minerals the results show that chalcopyrite precipitating in the interior wall of a hydrothermal chimney displays a limited range of 56fe values and 34s values between 0 11 to 0 33 and 2 2 to 2 6 respectively the 56fe values are on average slightly higher by 0 14 relative to coeval vent fluid composition while 34s values suggest significant s isotope fractionation 0 6 0 2 during chalcopyrite precipitation in contrast systematically lower 56fe and 34s values relative to hydrothermal fluids by up to 0 91 and 2 0 respectively are observed in pyrite and marcasite precipitating in the interior of active chimneys these results suggest isotope disequilibrium in both fe and s isotopes due to s isotopic exchange between hydrothermal h2s and seawater so42 followed by rapid formation of pyrite from fes precursors thus preserving the effects of a strong kinetic fe isotope fractionation during fes precipitation in contrast 56fe and 34s values of pyrite from inactive massive sulfides which show evidence of extensive late stage reworking are essentially similar to the hydrothermal fluids multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal fe isotope fractionation similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between 56fe values in both high temperature fe rich black smokers and lower temperature fe depleted vents

dissolved iron and iron isotopes in the southeastern pacific ocean the southeast pacific ocean is a severely understudied yet dynamic region for trace metals such as iron since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters in this study we report the dissolved iron dfe distribution from seven stations and fe isotope ratios 56fe from three of these stations across a near zonal transect from 20 to 27 s we found elevated dfe concentrations associated with the oxygen deficient zone odz with light 56fe implicating porewater fluxes of reduced fe however temporal dfe variability and rapid 56fe shifts with depth suggest gradients in odz fe source and or redox processes vary over short depth spatial scales the dfe concentrations decreased rapidly offshore and in the upper ocean dfe was controlled by biological processes resulting in an fe c ratio of 4 2 mol mol calculated vertical diffusive fe fluxes were greater than published dust inputs to surface waters but both were orders of magnitude lower than horizontal diffusive fluxes which dominate dfe delivery to the gyre the 56fe data in the deep sea showed evidence for a 0 2 antarctic intermediate water end member and a heavy 56fe of 0 55 for distally transported hydrothermal dissolved fe from the east pacific rise these heavy 56fe values were contrasted with the near crustal 56fe recorded in the hydrothermal plume reaching station aloha in the north pacific the heavy hydrothermal 56fe precludes a nanopyrite composition of hydrothermal dfe and instead suggests the presence of oxides or more likely binding of hydrothermal dfe by organic ligands in the distal plume

water masses in the humboldt current system properties distribution and the nitrate deficit as a chemical water mass tracer for equatorial subsurface water off chile abstract three sections are used to analyze the physical and chemical characteristics of the water masses in the eastern south pacific and their distributions oceanographic data were taken from the scorpio may june 1967 piquero may june 1969 and krill june 1974 cruises vertical sections of temperature salinity dissolved oxygen nitrate nitrite phosphate and silicate were used to analyze the water column structure five water masses were identified in the zone through t s diagrams subantarctic water subtropical water equatorial subsurface water antarctic intermediate water and pacific deep water their proportions in the sea water mixture are calculated using the mixing triangle method vertical sections were used to describe the geographical distributions of the water mass cores in the upper 1500 m several characteristic oceanographic features in the study area were analyzed the shallow salinity minimum displacement towards the equator the equatorial subsurface salinity maximum associated with a dissolved oxygen minimum zone and a high nutrient content displacement towards the south and the equatorward intermediate antarctic salinity minimum associated with a dissolved oxygen maximum the nitrate deficit generated in the denitrification area off peru and northern chile is proposed as a conservative chemical tracer for the equatorial subsurface waters off the coast of chile south of 25 s

water mass analysis for the u s geotraces ga03 north atlantic sections we present the distributions of hydrographic properties potential temperature salinity dissolved oxygen and micromolar level inorganic macronutrients along two sections occupied in the subtropical north atlantic as part of the first u s geotraces ga03 survey during 2010 and 2011 the purpose of this work is to place subsequent papers in this special issue in a general context and to provide a framework in which the observed distributions of trace elements and isotopes can be interpreted using these hydrographic properties we use a modified optimum multiparameter water mass analysis method to diagnose the relative contributions of various water types along the sections and rationalize their distributions the water mass compositions appear largely consistent with what is understood from previous studies about the large scale circulation and ventilation of the north atlantic with perhaps one exception we found that the north atlantic deep water both east and west of the mid atlantic ridge is more strongly influenced by iceland scotland overflow water relative to denmark straits overflow water about 3 1 than inferred from other tracer studies typically 2 1 it remains unclear whether this is an artifact of our calculation or a real change in deep water composition in the decades between the determinations

least squares multiple tracer analysis of water mass composition we present an inverse method that uses measurements of the concentration of several tracer variables to find the mixture of source water types that best describes in a weighted least squares sense the composition of the water sample the solution includes two physically realistic constraints first that all sources together must sum to 100 and second that no source is present in negative amounts these constraints improve the statistical stability of the solution the method is particularly valuable in regions involving strong mixing among a relatively large number of source water types in contrast to conventional ts analysis more water types can be considered the assumption of mixing along isopycnals is unnecessary and sensitivity to errors in individual tracer measurements is reduced by averaging the influence of a larger number of tracers we demonstrate the method using data from two oceanographic regions analysis of summer data from the continental margin off vancouver island british columbia canada confirms and quantifies the importance of deep upwelling of california undercurrent water onto the continental shelf it also identifies localized outer shelf filaments of low salinity water originating from the vancouver island coastal current finally we reanalyze the north african data of tomczak and confirm his interpretation of diapycnal mixing at the frontal boundary between north atlantic central water and south atlantic central water masses

iron and macronutrients in california coastal upwelling regimes implications for diatom blooms the supply of iron relative to that of the macronutrients nitrate phosphate and silicic acid plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes the presence or absence of a broad continental shelf influences the supply of iron the iron input to central california upwelling waters varies spatially and can be characterized by two end member regimes one end member which includes monterey bay and extending north to pt reyes is an iron replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron 10 nm entrained together with high concentrations of nitrate and silicic acid in these iron replete regions extensive blooms of large diatoms deplete macronutrient concentrations which results in correspondingly high chlorophyll a concentrations the other end member located to the south of monterey bay off the big sur coast is an iron deplete regime where upwelling is focused offshore of a narrow continental shelf upwelled waters in the big sur region are characterized by low dissolved and particulate iron concentrations 1 nm together with high concentrations of nitrate and silicic acid extremely low iron concentrations unused nitrate and silicic acid and a low abundance of large diatoms characterize surface waters in these iron deplete regions and thus represent coastal upwelling high nutrient low chlorophyll systems limited by the micronutrient iron

beyond the black sea paradigm the sedimentary fingerprint of an open marine iron shuttle we present iron fe concentration and fe isotope data for a sediment core transect across the peru upwelling area which hosts one of the ocean s most pronounced oxygen minimum zones omzs the lateral progression of total fe to aluminum ratios fet al across the continental margin indicates that sediments within the omz are depleted in fe whereas sediments below the omz are enriched in fe relative to the lithogenic background rates of fe loss within the omz as inferred from fet al ratios and sedimentation rates are in agreement with benthic flux data that were calculated from pore water concentration gradients the mass of fe lost from sediments within the omz is within the same order of magnitude as the mass of fe accumulating below the omz taken together our data are in agreement with a shuttle scenario where fe is reductively remobilized from sediments within the omz laterally transported within the anoxic water column and re precipitated within the more oxic water below the omz sediments within the omz have increased 56fe 54fe isotope ratios relative to the lithogenic background which is consistent with the general notion of benthic release of dissolved fe with a relatively low 56fe 54fe isotope ratio the fe isotope ratios increase across the margin and the highest values coincide with the greatest fe enrichment in sediments below the omz the apparent mismatch in isotope composition between the fe that is released within the omz and fe that is re precipitated below the omz implies that only a fraction of the sediment derived fe is retained near shore whereas another fraction is transported further offshore we suggest that a similar open marine shuttle is likely to operate along many ocean margins the observed sedimentary fingerprint of the open marine fe shuttle differs from a related transport mechanism in isolated euxinic basins e g the black sea where the laterally supplied reactive fe is quantitatively captured within the basin sediments we suggest that our findings are useful to identify omz type fe cycling in the geological record

dissolved iron in the tropical and subtropical atlantic ocean 1 detailed depth profiles and surface transects of dissolved iron dfe defined by 0 4 m filtration were investigated on three cruises in the subtropical and tropical atlantic ocean where dust deposition varied by 3 orders of magnitude surface dfe and dissolved mn concentrations reflected dust deposition trends but are not proportional to the estimated inputs using estimates of the atmospheric flux of dfe surface dfe residence times were calculated to be on the order of 1 to 5 months deepwater dfe concentrations varied with water masses depending on their source age and transit pathways at a site located on the edge of the equatorial system 10 n 45 w high dfe 1 nmol kg was associated with an oxygen minimum zone at depths of 130 to 1100 m dfe concentrations in north atlantic deep water nadw decreased by 30 from sites in the north atlantic to a site in the south atlantic 24 5 s 37 w and dfe was lower in the antarctic derived water masses 0 4 nmol kg than in nadw at the south atlantic site an estimate of deepwater scavenging residence time for dfe was 270 140 years based on decreasing dfe along the nadw flow path

impact of anthropogenic combustion emissions on the fractional solubility of aerosol iron evidence from the sargasso sea we report empirical estimates of the fractional solubility of aerosol iron over the sargasso sea during periods characterized by high concentrations of saharan dust summer 2003 and by low concentrations of aerosols in north american maritime north atlantic air masses spring 2004 and early summer 2004 we observed a strong inverse relationship between the operational solubility of aerosol iron defined using a flow through deionized water leaching protocol and the total concentration of aerosol iron whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low this relationship is consistent with source dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model wherein bulk aerosols represent a conservative mixture of two air mass end members that carry different aerosol types saharan air which contains a relatively high loading of aerosol iron 27 8 nmol fe m 3 that has a low fractional solubility 0 44 and north american air which contains a relatively low concentration of aerosol iron 0 5 nmol fe m 3 that has a high fractional solubility 19 historical data for aerosols collected on bermuda indicate that the low iron loadings associated with north american air masses are typically accompanied by elevated v al fe al and v mn mass ratios in the bulk aerosol relative to saharan dust which are indicative of anthropogenic fuel combustion products the identification of similar compositional trends in our sargasso sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with north american air masses reflects the presence of anthropogenic combustion products which contain iron that is readily soluble relative to iron in saharan soil dust we thus propose that the source dependent composition of aerosol particles specifically the relative proportion of anthropogenic combustion products is a primary determinant for the fractional solubility of aerosol iron over the sargasso sea this hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean

geochemical and iron isotopic insights into hydrothermal iron oxyhydroxide deposit formation at loihi seamount abstract low temperature hydrothermal vents such as those encountered at loihi seamount harbor abundant microbial communities and provide ideal systems to test hypotheses on biotic versus abiotic formation of hydrous ferric oxide feox deposits at the seafloor hydrothermal activity at loihi seamount produces abundant microbial mats associated with rust colored feox deposits and variably encrusted with mn oxyhydroxides here we applied fe isotope systematics together with major and trace element geochemistry to study the formation mechanisms and preservation of such mineralized microbial mats iron isotope composition of warm 2 s depleted hydrothermal fluids yielded 56 fe values near 0 1 indistinguishable from basalt values suspended particles in the vent fluids and feox deposits recovered nearby active vents yielded systematically positive 56 fe values the enrichment in heavy fe isotopes between 1 05 and 1 43 relative to fe ii in vent fluids suggest partial oxidation of fe ii during mixing of the hydrothermal fluid with seawater by comparing the results with experimentally determined fe isotope fractionation factors we determined that less than 20 of fe ii is oxidized within active microbial mats although this number may reach 80 in aged or less active deposits these results are consistent with fe ii oxidation mediated by microbial processes considering the expected slow kinetics of abiotic fe oxidation in low oxygen bottom water at loihi seamount in contrast feox deposits recovered at extinct sites have distinctly negative fe isotope values down to 1 77 together with significant enrichment in mn and occurrence of negative ce anomalies these results are best explained by the near complete oxidation of an isotopically light fe ii source produced during the waning stage of hydrothermal activity under more oxidizing conditions light fe isotope values of feox are therefore generated by subsurface precipitation of isotopically heavy fe oxides rather than by the activity of dissimilatory fe reduction in the subsurface overall fe isotope compositions of microbial mats at loihi seamount display a remarkable range between 1 2 and 1 6 which indicate that fe isotope compositions of hydrothermal fe oxide precipitates are particularly sensitive to local environmental conditions where they form and are less sensitive to abiotic versus biotic origins it follows that feox deposits at loihi seamount provides important modern analogues for ancient seafloor fe rich deposits allowing for testing hypotheses about the biogeochemical cycling of fe isotopes on early earth

slow spreading submarine ridges in the south atlantic as a significant oceanic iron source low levels of the micronutrient iron limit primary production and nitrogen fixation in large areas of the global ocean measurements in the south atlantic suggest that slow spreading submarine ridges serve as a significant oceanic iron source in these waters

oxygen minimum zones in the eastern tropical atlantic and pacific oceans within the eastern tropical oceans of the atlantic and pacific basin vast oxygen minimum zones omz exist in the depth range between 100 and 900 m minimum oxygen values are reached at 300 500 m depth which in the eastern pacific become suboxic dissolved oxygen content 4 5 mol kg 1 with dissolved oxygen concentration of less than 1 mol kg 1 the omz of the eastern atlantic is not suboxic and has relatively high oxygen minimum values of about 17 mol kg 1 in the south atlantic and more than 40 mol kg 1 in the north atlantic about 20 40 of the north pacific volume is occupied by an omz when using 45 mol kg 1 or 90 mol kg 1 respectively as an upper bound for omz oxygen concentration for ocean densities lighter than 27 2 kg m 3 the relative volumes reduce to less than half for the south pacific 7 and 13 respectively the abundance of omzs are considerably smaller 1 and 7 for the south atlantic and only 0 and 5 for the north atlantic thermal domes characterized by upward displacements of isotherms located in the northeastern pacific and atlantic and in the southeastern atlantic are co located with the centres of the omzs they seem not to be directly involved in the generation of the omzs omzs are a consequence of a combination of weak ocean ventilation which supplies oxygen and respiration which consumes oxygen oxygen consumption can be approximated by the apparent oxygen utilization aou however aou scaled with an appropriate consumption rate aour gives a time the oxygen age here we derive oxygen ages using climatological aou data and an empirical estimate of aour averaging oxygen ages for main thermocline isopycnals of the atlantic and pacific ocean exhibit an exponential increase with density without an obvious signature of the omzs oxygen supply originates from a surface outcrop area and can also be approximated by the turn over time the ratio of ocean volume to ventilating flux the turn over time corresponds well to the average oxygen ages for the well ventilated waters however in the density ranges of the suboxic omzs the turn over time substantially increases this indicates that reduced ventilation in the outcrop is directly related to the existence of suboxic omzs but they are not obviously related to enhanced consumption indicated by the oxygen ages the turn over time suggests that the lower thermocline of the north atlantic would be suboxic but at present this is compensated by the import of water from the well ventilated south atlantic the turn over time approach itself is independent of details of ocean transport pathways instead the geographical location of the omz is to first order determined by i the patterns of upwelling either through ekman or equatorial divergence ii the regions of general sluggish horizontal transport at the eastern boundaries and iii to a lesser extent to regions with high productivity as indicated through ocean colour data

processes affecting the isotopic composition of dissolved iron in hydrothermal plumes a case study from the vanuatu back arc abstract hydrothermal discharge is one of multiple fe sources contributing to the marine dissolved fe budget but its relative importance remains controversial and largely unconstrained isotopic characterization of fe sources and mass balance calculations are considered as a potential tool to unravel the complex interplay of various fe fluxes but require a more profound knowledge of the processes that fractionate fe isotopes in hydrothermal plumes we present dissolved fe isotope data from two buoyant plumes and profiles traversing the non buoyant plume above the nifonea vent field in the vanuatu back arc our findings support opposing controls from precipitation of fe sulfides and fe oxyhydroxides that either enrich the heavy or the light isotopes within the residual dissolved fe fraction but also suggest significant influence from size partitioning of oxidized fe following precipitation as well as potential effects from organic complexation at low aggregation of precipitated fe oxyhydroxides or stabilization of significant fe iii aq dissolved 56 fe is only marginally affected by fe ii oxidation which strengthens the role of fe sulfide precipitation during early plume rise in determining the isotope signature of hydrothermal fe exported to the open ocean dissolved 56 fe range from 0 73 0 05 to 0 16 0 05 and suggest that sulfide rich basalt hosted hydrothermal systems and slow oxidation kinetics in the pacific favor export of stabilized dissolved fe with heavier 56 fe values than the true vent fluids 56 fe of 0 5

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

early diagenesis of redox sensitive trace metals in the peru upwelling area response to enso related oxygen fluctuations in the water column pore water and solid phase data for redox sensitive metals mn fe v mo and u were collected on a transect across the peru upwelling area 11 s at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from 0 to 93 m by comparing authigenic mass accumulation rates and diffusive benthic fluxes we evaluate the respective mechanisms of trace metal accumulation retention and remobilization across the oxygen minimum zone omz and with respect to oxygen fluctuations in the water column related to the el nino southern oscillation enso sediments within the permanent omz are characterized by diffusive uptake and authigenic fixation of u v and mo as well as diffusive loss of mn and fe across the benthic boundary some of the dissolved mn and fe in the water column re precipitate at the oxycline and shuttle particle reactive trace metals to the sediment surface at the lower and upper boundary of the omz at the lower boundary pore waters are not sufficiently sulfidic as to enable an efficient authigenic v and mo fixation as a consequence sediments below the omz are preferentially enriched in u which is delivered via both in situ pre cipitation and lateral supply of u rich phosphorites from further upslope trace metal cycling on the peruvian shelf is strongly affected by enso related oxygen fluctuations in bottom water during periods of shelf oxygenation surface sediments receive particulate v and mo with metal oxyhydr oxides that derive from both terrigenous sources and precipitation at the retreating oxycline after the recurrence of anoxic conditions metal oxyhydr oxides are reductively dissolved and the hereby liberated v and mo are authigenically removed this alternation between supply of particle reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of v and mo compared to u on the peruvian shelf the decoupling of v mo and u accumulation is further accentuated by the varying susceptibility to re oxidation of the different authigenic metal phases while authigenic u and v are readily re oxidized and recycled during periods of shelf oxygenation the sequestration of mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions our findings reveal that redox sensitive trace metals respond in specific manner to short term oxygen fluctuations in the water column the relative enrichment patterns identified might be useful for the reconstruction of past omz extension and large scale redox oscillations in the geological record

hydrothermal venting along earth s fastest spreading center east pacific rise 27 5 32 3 s 1 during march april 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of earth s fasting spreading mid ocean ridge 27 5 32 3 s on the east pacific rise we compared the distribution and chemistry of hydrothermal plumes to geological indicators of long term spreading rate and moderate term ridge inflation variations in magmatic budget in this large offset propagating rift setting these geological indices span virtually the entire range found along fast spreading ridges worldwide hydrothermal plumes overlaid 60 of the length of superfast 130 km myr spreading axis surveyed and defined at least 14 separate vent fields we observed no plumes over the slower spreading propagating segments finer scale variations in the magmatic budget also correlated with hydrothermal activity as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation along the entire ridge crest the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years plume samples with high volatile metal ratios generally indicating recent seafloor volcanism were scarce along axis trends in both volatile 3he ch4 ph a proxy for co2 and particulate s and nonvolatile fe mn species showed a first order agreement with the trend of ridge inflation nevertheless a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments

iron isotope fractionation in a buoyant hydrothermal plume 5 s mid atlantic ridge fe isotopes are a potential tool for tracing the biogeochemical redox cycle of fe in the ocean specifically it is hypothesized that fe isotopes could enable estimation of the contributions from multiple fe sources to the dissolved fe budget an issue that has received much attention in recent years the first priority however is to understand any fe isotope fractionation processes that may occur as fe enters the ocean resulting in modification of original source compositions in this study we have investigated the fe inputs from a basalt hosted deep sea hydrothermal system and the fractionation processes that occur as the hot chemically reduced and acidic vent fluids mix with cold oxygen rich seawater the samples collected were both end member vent fluids taken from hydrothermal chimneys and rising buoyant plume samples collected directly above the same vents at 5 s mid atlantic ridge our analyzes of these samples reveal that for the particulate fe species within the buoyant plume 25 of the fe is precipitated as fe sulfides the isotope fractionation caused by the formation of these fe sulfides is fe ii fes 0 60 0 12 the source isotope composition for the buoyant plume samples collected above the red lion vents is calculated to be 0 29 0 05 this is identical to the value measured in end member vent fluids collected from the underlying tannenbaum chimney the resulting isotope compositions of the fe sulfide and fe oxyhydroxide species in this buoyant plume are 0 89 0 11 and 0 19 0 09 respectively from mass balance calculations we have been able to calculate the isotope composition of the dissolved fe fraction and hypothesize that the isotope composition of any stabilised dissolved fe species exported to the surrounding ocean may be heavier than the original vent fluid such species would be expected to travel some distance from areas of hydrothermal venting and hence contribute to not only the dissolved fe budget of the deep ocean but also it s dissolved fe isotope signature

hydrothermal metal supplies enhance the benthic methane filter in oceans an example from the okinawa trough abstract anaerobic oxidation of methane aom is an important process that regulates methane budget in the global carbon cycle sulfate is traditionally regarded as the most important electron acceptor for aom however recent studies reveal that reactive metal reduction driven aom metal aom may also present on natural environments partitioning the methane c sink from anaerobic oxidation between sulfate reduction driven aom sulfate aom and metal aom is thus becoming a scientific challenge to be addressed this study aimed to show that metal aom has a potential to complement sulfate aom especially when methane seeps met sediments enriched in iron and manganese oxides supplied by hydrothermal plumes samples of authigenic carbonate were collected from the cold seeps on the western slope of the okinawa trough ot a semi enclosed back arc basin with cold seeps and hydrothermal activities coexist spatially petrologic and mineralogical observations provided faithful evidence of a close genetic relationship between goethite and authigenic carbonates likely indicating the presence of metal aom although some contributions of organoclastic fe reduction cannot be ruled out the most possible place of metal aom was between the lower part of the sulfate methane transition zone smtz and the immediate upper part of the methanogenic zone the characteristics of 13ccarb 53 7 to 3 3 vienna peedee belemnite vpdb also point to our carbonate samples as the products of aom these authigenic carbonates was further identified as three possible types of carbonate of different sources sulfate aom metal aom and hydrothermal carbonates depending on the correlations among concentrations of carbonate associated sulfate cas 13ccarb and 18ocarb based on these findings we tentatively suggested that the reduction of reactive metals carried by hydrothermal plumes might drive the methane oxidation in the adjacent cold seep areas resulting in the precipitation of unique carbonates and enhancement the efficiency of the benthic methane filter this study reported the coupling or interaction between the two extreme submarine environments by metal aom for the first time which might help in understanding and improving the global carbon and metal cycles now and in the past

iron isotopes in the seawater of the equatorial pacific ocean new constraints for the oceanic iron cycle abstract this study presents the isotopic compositions and concentrations of dissolved and particulate iron from two seawater profiles of the western and central equatorial pacific ocean sampled during the eucfe cruise most of the 56 fe values are positive relative to irmm 14 from 0 01 to 0 58 in the dissolved fraction dfe and from 0 02 to 0 46 in the particulate fraction pfe the mean measurement uncertainty of 0 08 2sd allows the observation of significant variations most of the isotope variations occur in the vertical and not in the horizontal direction implying that each isotope signature is preserved over long distances within a water mass the thermocline waters of the papua new guinea png area mostly influenced by sedimentary inputs display a mean 56 dfe value of 0 37 0 15 2sd this isotopic signature suggests that the process releasing dissolved iron into the seawater in this area is a non reductive dissolution of sediments discharged by local rivers and likely re suspended by strong boundary currents rather than dissimilatory iron reduction dir within the sediment characterized by negative 56 dfe these positive 56 dfe values seem to be the result of a mean isotopic fractionation of 56 fe dfe pfe 0 20 0 11 2sd produced by the non reductive dissolution at 0 n 180 e the fe isotope signature of the equatorial undercurrent euc waters is identical to that of the png station within the range of the uncertainty this suggests that the dissolved iron feeding the euc and ultimately the eastern pacific high nutrient low chlorophyll area is of png origin likely released by a non reductive dissolution of terrigenous sediments significant fe removals are observed within the thermocline and the intermediate waters between the png and the open ocean stations the corresponding isotopic fractionations appear to be small with 56 fe removed sw fe values of 0 30 0 31 to 0 18 0 12 2sd for dfe removal and of 0 10 0 04 to 0 05 0 31 2sd for pfe removal in the chlorophyll maximum of the open ocean station the isotopic fractionation associated with biological uptake is estimated at 56 fe phyto dfe 0 25 0 10 to 0 13 0 11 2sd although these fractionations are based on a limited dataset and need to be further constrained they appear to be small and to limit the transformations of the iron source signatures within the ocean

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

both soluble and colloidal iron phases control dissolved iron variability in the tropical north atlantic ocean the size partitioning of dissolved iron dfe 0 4 m into soluble sfe 0 02 m and colloidal 0 02 m cfe 0 4 m phases was investigated at seven stations in the tropical north atlantic ocean and the results are compared to the dfe size fractionation study of bergquist et al 2007 in the same region downwind of the north african dust plumes cfe comprised 80 7 of the surface dfe pool at six stations supporting the hypothesis that atmospherically derived fe is maintained in the colloidal size fraction at the deep chlorophyll maximum colloidal fe had minimum concentrations or was completely absent suggesting that cfe was either preferentially taken up by microbes and or scavenged aggregated at these depths at remineralization depths sfe was the dominant fraction both in the subtropical gyre like stations 76 sfe sfe 0 42 0 03 nmol kg and in the oxygen minimum zone 56 sfe sfe 0 65 0 03 nmol kg only at remineralization depths of stations with intermediate oxygen concentrations 100 110 mol kg did colloidal fe dominate contributing 58 of dfe indicating that cfe may be serving as a conduit of dfe loss during mixing of high fe omz and low fe gyre waters north atlantic deep water nadw had a typical sfe concentration of 0 34 0 05 nmol kg in the deepest samples composed of a nadw antarctic bottom water mixture where the bottom water may have attained a 0 1 nmol kg hydrothermal fe input during transit past the mid atlantic ridge sfe did not increase coincidentally with dfe indicating that any potential hydrothermal fe contribution was colloidal in general the results of this study counter the previous hypothesis of bergquist et al 2007 that the colloidal fe fraction predominately controls dfe variability instead suggesting that both soluble and colloidal fe are variable and both contribute to the observed dfe variability throughout the north atlantic the nearly 50 50 dfe partitioning into soluble and colloidal phases below the dcm suggest one of two partitioning mechanisms persists 1 soluble and colloidal fe exchange rates reach a steady state over which regional uniquely partitioned fe sources can be overlain or 2 the partitioning of fe binding ligands between the two size fractions is variable in the open ocean and directly controls dfe partitioning

vertical and horizontal extension of the oxygen minimum zone in the eastern south pacific ocean recent hydrographic measurements within the eastern south pacific 1999 2001 were combined with vertically high resolution data from the world ocean circulation experiment high resolution profiles and bottle casts from the world ocean database 2001 and the world ocean atlas 2001 in order to evaluate the vertical and horizontal extension of the oxygen minimum zone 600 m off peru between 5 and 13 s and to about 1000 km offshore its upper boundary is shallowest 150 m off peru shoaling towards the coast and extending well into the euphotic zone in some places offshore the thickness and meridional extent of the oxygen minimum zone decrease until it finally vanishes at 140 w between 2 and 8 s moving southward along the coast of south america the zonal extension of the oxygen minimum zone gradually diminishes from 3000 km 15 s to 1200 km 20 s and then to 25 km 30 s only a thin band is detected at 37 s off concepcion chile simultaneously the oxygen minimum zone s maximum thickness decreases from 300 m 20 s to less than 50 m south of 30 s the spatial distribution of ekman suction velocity and oxygen minimum zone thickness correlate well especially in the core off chile the eastern south pacific intermediate water mass introduces increased vertical stability into the upper water column complicating ventilation of the oxygen minimum zone from above in addition oxygen enriched antarctic intermediate water clashes with the oxygen minimum zone at around 30 s causing a pronounced sub surface oxygen front the new estimates of vertical and horizontal oxygen minimum zone distribution in the eastern south pacific complement the global quantification of naturally hypoxic continental margins by helly and levin 2004 global distribution of naturally occurring marine hypoxia on continental margins deep sea research i 51 1159 1168 and provide new baseline data useful for studies on the role of oxygen in the degradation of organic matter in the water column and the related implications for biogeochemical cycles coastal upwelling zones along the eastern pacific combine with general circulation to provide a mechanism that allows renewal of upper pacific deep water the most oxygen poor and oldest water mass of the world oceans

iron sources and dissolved particulate interactions in the seawater of the western equatorial pacific iron isotope perspectives this work presents iron isotope data in the western equatorial pacific marine aerosols and top core margin sediments display a slightly heavy fe isotopic composition 56 fe of 0 33 0 11 2sd and 0 14 0 07 respectively samples reflecting the influence of papua new guinea runoff sepik river and rabaul volcano water are characterized by crustal values in seawater fe is mainly supplied in the particulate form and is found with a 56 fe between a0 49 and 0 34 0 07 the particulate fe seems to be brought mainly by runoff and transported across continental shelves and slopes aerosols are suspected to enrich the surface vitiaz strait waters while hydrothermal activity likely enriched new ireland waters dissolved fe isotopic ratios are found between a0 03 and 0 53 0 07 they are almost systematically heavier than the corresponding particulate fe and the difference between the signature of both phases is similar for most samples with 56 fe dfe pfe 0 27 0 25 2sd this is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases the dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved fe this process seems to be locally significantly more intense than fe reductive dissolution documented along reducing margins it may therefore constitute a very significant iron source to the ocean thereby influencing the actual estimation of the iron residence time and sinks the underlying processes could also apply to other elements

nanoparticulate pyrite and other nanoparticles are a widespread component of hydrothermal vent black smoker emissions abstract the presence of nanoparticulate pyrite is reported in hydrothermal emissions from rainbow tag and snakepit on the mid atlantic ridge mar when coupled with previously collected data from east pacific rise 9 n epr and lau basin these data demonstrate that pyrite nanoparticles are a widespread component of black smoker emissions from hydrothermal vents and are found in significant concentrations at a fast spreading mid ocean ridge epr a back arc basin lau basin and a slow spreading mid ocean ridge mar the maximum percentage of filtered iron emitted as nanoparticulate pyrite was found to be as high as 25 10 and 5 respectively as a widespread component of hydrothermal vent emissions these nanoparticles may be an important source of iron to the world s oceans metals such as cu and zn are detected in pyrite containing aggregates at all sites and chalcopyrite was a component of nanoparticle aggregates at mar iron containing silicate nanoparticles are also identified and indicate that nanoparticles other than sulfides should also be considered when determining transport implications of hydrothermal vent emissions the varied morphologies and the presence of different minerals within these nanoparticles provide insight into their formation and stability

radiocarbon decay and oxygen utilization in the deep atlantic ocean a parameter based on the sum of the concentrations of po4 and o2 divided by the redfield coefficient o2 po4 is used to separate the contributions of the northern and southern components to deep waters in the atlantic this separation allows the amount of radiocarbon lost by radiodecay and the amount of oxygen lost to respiration during residence in the deep atlantic to be calculated maps of these quantities reveal strong west to east gradients and weak north to south gradients consistent with ventilation along the western boundary from both ends of the ocean coupled with mixing outward from the boundary the o2 and 14c deficiences are highly correlated suggesting an o2 utilization rate of 12 m kg per century the apparent mean isolation time of water in the deep atlantic is about 200 years

chaotic topography mantle flow and mantle migration in the australian antarctic discordance oceanic crust formed over the past 30 million years at the australian antarctic discordance aad is characterized by chaotic sea floor topography reflecting a weak magma supply from an unusually cold underlying mantle during the past 3 4 million years however a source of increased magma supply coinciding with the known indian pacific mantle isotopic boundary has propagated into the eastern aad displacing the chaotic terrain and replacing it with normal sea floor pacific mantle reached the eastern boundary of the aad at least 7 million years ago but it was not until 3 4 million years ago that lavas derived from pacific mantle were first erupted within the aad this long hiatus combined with the ridge transform geometry across the aad boundary constrains the locus of mantle migration to a narrow relatively shallow region directly beneath the spreading axis of the southeast indian ridge

iron speciation in the eastern tropical south pacific oxygen minimum zone off peru vertical profiles of total dissolved iron dfe fe ii and hydrographic parameters were obtained along three transects across the continental shelf off peru in october 2005 fe ii and dfe concentrations were determined using luminol chemiluminescence and isotope dilution inductively coupled plasma mass spectrometry icp ms respectively the objective was to study the relationships among redox chemistry nitrogen cycle processes bottom topography and the distribution and chemistry of iron in the region the results are the most thorough survey to date of fe geochemistry in this region exceedingly high dfe 50 75 nmol l 1 was measured over the wide northern continental shelf with most of the dissolved fe present as fe ii below the oxycline in southern peru the shelf is narrower and dissolved fe concentrations were 10 fold lower moreover a smaller fraction of the dissolved fe was present as fe ii in the south even below the oxycline at the western ends of the transects fe ii maxima were coincident with deep i e secondary nitrite maxima this suggests a relationship between nitrate reduction and fe ii accumulation in the water column however over the shelf fe ii was also influenced by benthic processes large lateral gradients in dissolved fe across the shelf slope break reflect fe removal by oxidative scavenging and it seems plausible that much of the fe is trapped by redox cycling on the shelf nevertheless the maintenance of dissolved fe ii filaments within the secondary nitrite maxima constitutes an important mechanism for fe transport offshore

the flux of iron and iron isotopes from san pedro basin sediments iron is an important nutrient in the ocean but the different sources and sinks of iron are not well constrained here we use measurements of fe concentration and fe stable isotope ratios to evaluate the importance of reducing continental margins as a source of fe to the open ocean dissolved iron concentration fe and iron stable isotope ratios 56 fe were measured in the san pedro and santa barbara basins dissolved 56 fe ranges from 1 82 to 0 00 in the san pedro basin and from 3 45 to 0 29 in the santa barbara basin and in both basins the lowest 56 fe values and highest fe concentrations are found at the bottom of the basin reflecting the input of isotopically light fe from reducing sediment porewaters in the san pedro basin we are also able to fingerprint an advective source of fe from shallow continental shelves next to the basin and the atmospheric deposition of fe into surface waters a one dimensional model of the fe isotope cycle has been constructed for the deep silled san pedro basin by fitting model output to data values of several important iron cycle parameters are predicted including a flux of fe from sediment porewaters into the water column of 0 32 1 14 mol m 2 d 1 a first order dissolved fe precipitation rate constant of 0 0018 0 0053 d 1 a flux 56fe of 2 4 and an isotope effect for fe precipitation of 56 fe particulate dissolved 0 8 applying our model predicted fe cycle parameters to the global ocean suggests that continental margins contribute 4 12 of world ocean dissolved fe and make the ocean s fe lighter by 0 08 to 0 26 the dramatically negative 56 fe signature seen in the water column of the san pedro and santa barbara basins demonstrate the utility of fe isotopes as a tracer for continental margin fe input from reducing sediments to the oceans while the isotopic fractionation observed during loss of fe from the dissolved phase suggests that this signature will be modified by subsequent reactions our modeling provides an initial framework for testing how these signals are transmitted into the open ocean

the vertical distribution of iron stable isotopes in the north atlantic near bermuda seawater dissolved iron isotope ratios 56 fe have been measured in the north atlantic near bermuda in a full depth profile seawater dissolved 56 fe is isotopically heavy compared to crustal values throughout the water column 56 fe irmm 014 0 30 to 0 71 iron isotope ratios are relatively homogenous in the upper water column between 0 30 to 0 45 above 1500 m and 56 fe increases below this to a maximum of 0 71 at 2500 m decreasing again to 0 35 at 4200 m the 56 fe profile is very different from the iron concentration profile in the upper water column fe is variable while 56 fe is relatively constant and in the deeper water column 56 fe varies while fe remains relatively constant the 56 fe profile is also not well correlated with other hydrographic tracers in the north atlantic such as temperature salinity or the concentrations of oxygen phosphate silica and cfc 11 the dissimilarity between 56 fe profiles and profiles of fe and other hydrographic tracers shows that fe isotope ratios provide a unique sort of information about ocean chemistry and they suggest that fe isotopes may therefore be a valuable new tool for tracing the global sources sinks and biogeochemical cycling of fe

pore fluid fe isotopes reflect the extent of benthic fe redox recycling evidence from continental shelf and deep sea sediments pore fluid fe isotopes may be a unique tracer of sediment respiration by dissimilatory fe reducing bacteria but to date pore fluid fe isotope measurements have been restricted to continental shelf settings here we present 56fe values of pore fluids from two distinct sedimentary settings 1 a riverine dominated site on the northern california margin eel river shelf 120 m water depth and 2 biogenic opal rich volcaniclastic deep sea sediments from the southern ocean north and south of the crozet plateau 3000 4000 m water depth the fe isotope compositions of crozet region pore fluids are significantly less fractionated 56fe 0 12 to 0 01 than the eel river shelf 56fe 0 65 to 3 40 and previous studies of pore fluid fe isotopes relative to average igneous rocks our data represent the first measurements of fe isotope compositions in pore fluids from deep sea sediments a comparison of pore fluid 56fe with the relative abundance of highly labile fe in the reactive sedimentary fe pool demonstrates that the composition of fe isotopes in the pore fluids reflects the different extent of sedimentary fe redox recycling between these sites

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

a zonal picture of the water column distribution of dissolved iron ii during the u s geotraces north atlantic transect cruise geotraces ga03 we report measurements of the transient species iron ii in filtered water column samples collected during the u s geotraces north atlantic transect cruise geotraces ga03 which was comprised of legs from lisbon to cape verde in october november 2010 and from woods hole to cape verde in november december 2011 dissolved iron ii dfe ii was determined at sea in 0 2 m filtered samples as soon as possible after collection and filtration using flow injection analysis with mean detection limits of 0 06 nm and 0 01 nm during the 2010 and 2011 cruise legs respectively water column concentrations along the cruise transects were generally low 0 2 nm with the exception of deep water samples collected over the trans atlantic geotraverse hydrothermal field in which dfe ii was as high as 70 nm and accounted for more than 80 of the dissolved iron pool in the near field hydrothermal plume smaller local concentration maxima were observed near 1000 m depth in the low oxygen iron rich waters to the east of cape verde where dfe ii is correlated with apparent oxygen utilization and in the upper water column at several stations in the subtropical gyre where dfe ii can account for greater than 50 of the dissolved iron pool in the lower euphotic zone elevated dfe ii concentrations were also observed over much of the water column on the bermuda platform although the source of this enrichment remains uncertain in the absence of data between woods hole and bermuda

partitioning of dissolved iron and iron isotopes into soluble and colloidal phases along the ga03 geotraces north atlantic transect abstract the size partitioning of dissolved fe dfe 56 fe of 1 3 to 1 5 relative to dfe 56 fe 0 5 in the surface ocean where sfe and cfe cycle independently and identical sfe and dfe isotope ratios below 250 m where sfe and cfe are constantly partitioned and exchanged

on the iron isotope homogeneity level of the continental crust abstract assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer in recent studies we concluded that most igneous rocks from the continental crust were homogeneous to 71 wt sio2 that we concluded were significantly heavier than less silicic rocks from the crust on the basis of fe isotopic analyses of seven bulk rock samples from different geological settings beard and johnson beard b l johnson c m 2006 comment on heavy iron isotope composition of granites determined by high resolution mc icp ms by f poitrasson and r freydier chemical geology volume 222 pages 132 147 chem geol this issue challenge this finding on the basis of previously published data it is shown here that this controversy stems from the confusion between whole rock and mineral iron isotope analyses since this isotopic tracer like many others is more heterogeneous at the mineral scale than the bulk rock scale thus minerals cannot be used in place of bulk rock fe isotope compositions the controversy also arises from different levels of analytical uncertainties if only the best fe isotope probes of planetary reservoirs bulk rocks are considered then the conclusion that igneous rocks with sio2 71 wt are significantly heavier than the mean 57fe of the continental crust poitrasson f freydier r 2005 heavy iron isotope composition of granites determined by high resolution mc icp ms chem geol 222 132 147 remains valid nonetheless given that granites represent only a minor fraction of the continental crust this finding does not change the bulk igneous fe isotope composition estimate previously defined for the earth

the effect of plume processes on the fe isotope composition of hydrothermally derived fe in the deep ocean as inferred from the rainbow vent site mid atlantic ridge 36 14 n abstract the rainbow hydrothermal vent site which is the largest known point source for dissolved fe delivered to the deep north atlantic ocean has remained invariant in its fe isotope composition over at least the past 16 000 years based on analysis of metalliferous sediments beneath the plume because of the conservative behavior of fe in the rainbow plume 56 fe values of particles in the neutrally buoyant plume 0 18 0 05 and underlying sediments 0 19 0 05 are indistinguishable from the 56 fe values of the high temperature fluid sources 0 23 0 04 particles from the near vent buoyant stage of the plume however have higher 56 fe values 0 15 to 1 20 relative to the original vent fluid consistent with fractionation during oxidation of fe ii aq to fe iii aq isotope compositions become invariant in the plume once all fe ii aq is fully oxidized preserving the original composition of the vent fluid the constant fe isotope compositions of the vent fluids over time implies that changes in seawater fe isotope composition of the north atlantic ocean as they are recorded in fe mn crusts requires changes in the relative fluxes of fe to the ocean

iron isotope fractionation in subterranean estuaries dissolved fe concentrations in subterranean estuaries like their river seawater counterparts are strongly controlled by non conservative behavior during mixing of groundwater and seawater in coastal aquifers previous studies at a subterranean estuary of waquoit bay on cape cod usa demonstrate extensive precipitation of groundwater borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands waquoit bay is thus an excellent natural laboratory to assess the mechanisms of fe isotope fractionation in redox stratified environments and determine potential fe isotope signatures of groundwater sources to coastal seawater here we report fe isotope compositions of iron coated sands and porewaters beneath the intertidal zone of waquoit bay the distribution of pore water fe shows two distinct sources of fe one residing in the upward rising plume of fe rich groundwater and the second in the salt wedge zone of pore water the groundwater source has high fe ii concentration consistent with anoxic conditions and yield 56fe values between 0 3 and 1 3 in contrast sediment porewaters occurring in the mixing zone of the subterranean estuary have very low 56fe values down to 5 these low 56fe values reflect fe redox cycling and result from the preferential retention of heavy fe isotopes onto newly formed fe oxyhydroxides analysis of fe oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong 56fe and fe concentration gradients over less than 2m yielding an overall range of 56fe values between 2 and 1 5 the relationship between fe concentration and 56fe of fe rich sands can be modeled by the progressive precipitation of fe oxides along fluid flow through the subterranean estuary these results demonstrate that large scale fe isotope fractionation up to 5 can occur in subterranean estuaries which could lead to coastal seawater characterized by very low 56fe values relative to river values