the essence of time fertile skarn formation in the variscan orogenic belt abstract extending from northern america to central china the variscan belt is a paleozoic orogen exceptionally well endowed in magmatic hydrothermal ore deposits including skarn deposits yet the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained here we present innovative u pb laser ablation inductively coupled plasma mass spectrometry geochronology of garnet from different skarns in the erzgebirge a classic metallogenic province in central europe garnet ages obtained not only constrain the timing of fertile skarn formation and associated sn w fe zn cu and in mineralization but also clearly distinguish these from barren skarn bodies we show that barren skarns formed during times of peak regional metamorphism at 340 ma whereas mineralized skarns are temporally associated with late orogenic magmatism at 325 313 ma as well as post orogenic magmatism at 308 295 ma the recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic hydrothermal systems in ancient collisional orogens on a regional scale this has important implications to direct future mineral exploration

geochemical and mineralogical control of different genetic types of deep sea nodules from the pacific ocean deep sea nodules from the northeast pacific nodule belt and the southeast pacific sonne basin being formed in areas bordering the equatorial zone of high biological productivity accumulate by two basically different growth processes a early diagenetic growth by supply from pore water and b hydrogenetic growth by supply from near bottom sea water these growth processes lead to different genetic types of nodules early diagenetic type a hydrogenetic type b and mixed type ab a further type ac very rich in mn is being formed by increasing influence of early diagenesis these types can clearly be distinguished by their shapes surface textures mineral constituents of oxide fraction internal microstructures and geochemistry a genetical classification is being proposed on the basis of statistically computed interelement relationships todorokite very poor in fe is the main mn phase in the early diagenetic substance mno2 intimately intergrown with feooh xh2o is the main phase in the hydrogenetic substance consequently an important difference can be pointed out the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of me2 e g mn2 ni2 cu2 zn2 whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles mobilization of mn2 and fractionation from fe is controlled by the amount of decomposing organic matter in the peneliquid layer of the sediments the main factor controlling the intensity of early diagenesis is the biological productivity in surface waters the crucial point of reversal at a mn fe ratio of about 5 obtained by hyperbolical regression of the analyses of nodules from the southeast pacific represents best concentrations in ni and cu mn fe quotients greater than 5 cause a decrease of ni and cu content nodules from the northeast pacific nodule belt generally contain higher concentrations in cu than nodules from the southeast pacific this can be explained by an additional supply of cu transported below ccd by siliceous plankton

metal diagenesis in oxic marine sediments the metal nutrient relationships observed for nickel and cadmium in the deep ocean are continued at the interface between seawater and oxidizing pore water this continuum results in pore water concentrations of these metals which are only slightly greater than near bottom seawater levels manganese concentrations in these oxidizing pore waters are also extremely low less than three times bottom water in contrast release in the boundary layer produces a maximum of dissolved copper which is 10 40 times ambient seawater assuming these pore waters are at steady state flux estimates based on these measurements suggest that the manganese in todorokite rich nodules of the central equatorial pacific was not supplied by upward diffusion through pore waters below the interface most nodular nickel is precipitated with manganese while nodular copper is supplied by diffusion

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

chronostratigraphy of odp 181 site 1121 sediment core southwest pacific ocean using 10be 9be dating of entrapped ferromanganese nodules a 10be 9be based chronostratigraphy has been determined for odp 181 site 1121 sediment core recovered from the foot of the campbell plateau southwest pacific ocean this core was drilled through the campbell skin drift in ca 4500 m water depth on the mid western margin of the extensive campbell nodule field beneath the flow of the major cold water deep western boundary current dwbc in the absence of detailed biostratigraphy beryllium isotopes have provided essential time information to allow palaeo environmental interpretation to be undertaken on the upper 7 m of the core measured 10be 9be ratios of sediment and of ferromanganese nodules entrapped in the sediment decrease systematically with depth in the core in accordance with radioactive decay however the 10be 9be data diverge from ca 3 m below the seafloor mbsf to the top of the core giving rise to several possible geochronological models the preferred model assumes that the measured 10be 9be ratios of the nodule rims reflect initial 10be 9be ratios equivalent to contemporary seawater and that these can be used to derive the true age of the sediment where the nodules occur the nodule rim ages can be then used to interpret the sediment 10be 9be data which indicate an overall age to ca 7 mbsf of ca 17 5 ma the derived chronology is consistent with diatom biostratigraphy which indicates an age of 2 2 3 6 ma at 1 mbsf calculated sedimentation rates range from 8 to 95 cm m y 1 with an overall rate to 7 mbsf of ca 39 cm m y 1 the lowest rates generally coincide with the occurrence of entrapped nodules and reflect periods of increased bottom current flow causing net sediment loss growth rates of individual nodules decrease towards the top of the sediment core similar to the observed decrease in growth rate from core to rim of seafloor nodules from the campbell nodule field this may be related to an overall increase in the vigour of the dwbc from ca 10 ma to the present

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

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

the influence of the carbonate dissolution rate on the growth and composition of co rich ferromanganese crusts from central pacific seamount areas abstract the metal composition of oceanic ferromanganese deposits occurring in seamount regions line islands chain and mid pacific mountains varies with water depth and age the results of metal determinations of carbonate plankton samples suggest that carbonate dissolution in the water column might have an important influence on the accretion and composition of hydrogenetic precipitates two ferromanganese crust generations of different age have been observed the precipitation of the older crust took probably place during early oligocene the younger crust began to form during middle miocene between the two crust generations periods of carbonate sedimentation and of phosphorite deposition occur the hydrogenetic formation of the crusts is controlled by the metal supply from the water column according to the laws of colloidal surface chemistry dissolution experiments with carbonate plankton samples show that the main fe source for the hydrogenetic crust formation are colloidal fe hydroxide particles being released in the water column from the dissolution of carbonate plankton skeletons in the case of mn maximum dissolved mn occurs in the oxygen minimum zone as the result of in situ break down of organic matter and the in situ reduction of mn bearing solid phases closely beneath the oxygen minimum zone a fe supply mobilized within the oxygen minimum zone has also to be taken into account in the water column below the oxygen minimum zone a mixture of colloidal particles of mn fe oxyhydroxide and colloidal al fe silicate precipitate together on the surface of substratum rocks the mixing ratio of these colloidal phases controlling the metal composition of the ferromanganese precipitates is depth dependent and shows also temporal variations in general mn fe ratio ni and co contents decrease with depth down to the calcite compensation depth the most probable mechanism for the ultimate removal of co and ni from the water column might be a surface reaction mno2 is specifically able to absorb hydrous co2 and ni2 ions because of the surface enrichment of co and the strong electrical field of mn iv a subsequent oxidation of co2 to co3 takes place leading to higher enrichment of co in comparison to ni the most important factor governing the high co enrichment in the ferromanganese crusts is the growth rate the lower the growth rate the higher the co content maximum values of up to 2 co occurring in samples from water depths between 1500 and 1100 m 1 are related to lower carbonate dissolution rates and corresponding lower fe supply the metal supply from the water column is strongly related to distinct environmental factors such as bio productivity range of lysocline and calcite compensation depth rate of carbonate dissolution and activity of the antarctic bottom water thus our model shows that the growth periods and the metal composition of hydrogenetic seamount crusts are controlled by changes in the paleoceanography and reflect distinct environmental conditions

trace metals in metalliferous sediments manop site m interfacial pore water profiles we present data showing enrichments of metals mn cu v cr cd and ni and dissolved organic matter dom in surficial pore waters depths 1 cm at a metalliferous sediment site in the pacific we also present a model of metal release during particulate organic carbon poc oxidation and opal dissolution at the sediment seawater interface a comparison of measured interfacial metal concentrations with those predicted from the model suggest that cd ni and cr are released during oxidation of poc at the sediment seawater interface however for v cu and mn additional source s and or processes are required to account for the measured interfacial concentrations for cu recycling via interfacial remineralization and release to bottom waters with subsequent scavenging out of bottom waters by particulates probably provides the major additional source for v we suggest that in addition to some contribution from a benthic flux bottom water scavenging scenario complexation with dom at the interface serves as a trap to accumulate v released from particulates in the zone of oxygen reduction for manganese we suggest that most of the interfacial pore water content is produced from reduction of sedimentary mn iv by reactions with dom at the sediment seawater interface the calculated benthic flux of manganese as a result of this reaction may account for a large fraction of the particulate manganese rain that is apparently not preserved in the sediment at this site

goethite aging explains ni depletion in upper units of ultramafic lateritic ores from new caledonia abstract an upward loss of ni is commonly reported in the oxide rich unit of ni laterite deposits developed over ultramafic rocks in tropical regions especially in freely drained and deeply weathered regoliths because goethite is the major mineral constituent of such oxide rich units this ni loss has been linked to compositional changes in goethite in the present study we have investigated possible correlations between ni contents in the bulk laterite and the evolution of goethite in terms of composition and crystallinity in two ni rich and one ni poor lateritic profiles from new caledonia ni k edge extended x ray absorption fine structure exafs spectroscopy indicates that goethite hosts the main fraction of ni in the three profiles investigated asbolane lithiophorite identified as accessory minerals by x ray diffraction xrd have little effect on the vertical variations in bulk ni content in spite of the fact that the ni contents of these mn oxides can be significant at certain depths the gradual decrease in ni content from the bottom to the top of the three ni lateritic profiles correlates with a decrease in the ni content of goethite as determined by electron probe micro analysis in addition xrd data show that these compositional trends are linked to an increase of the mean coherent domain size of goethite these observations support the hypothesis that ni is expelled from goethite as it ages through successive dissolution and recrystallization cycles during the lateritization process comparison of laterites having different degrees of weathering suggests that this aging process could also play a significant role in the regional variability of ni content in ni laterite deposits

on the isotope composition of reactive iron in marine sediments redox shuttle versus early diagenesis the isotope composition of reactive iron fe in marine sediments and sedimentary rocks is a promising tool for identifying fe sources and sinks across ocean basins in addition to cross basinal fe redistribution which can modify fe isotope signatures fe minerals also undergo diagenetic redistribution during burial the isotope fractionation associated with this redistribution does not affect the bulk isotope composition but complicates the identification of mineral specific isotope signatures here we present new fe isotope data for peru margin sediments and revisit previously published data for sediments from the california margin to unravel the impact of early diagenesis on fe isotope compositions of individual fe pools sediments from oxic california margin sites are dominated by terrigenous fe supply with fe release from sediments having a negligible influence on the solid phase fe isotope composition the highly reactive fe pool sum of fe bound to oxyhydr oxide carbonate monosulfide and pyrite of these sediments has a light isotope composition relative to the bulk crust which is consistent with earlier studies showing that continental weathering shifts the isotope composition of fe oxyhydr oxides to lighter values ferruginous sedimentswithin the peruvian oxygen minimumzone are depleted in fe relative to the lithogenic background which we attribute to extensive fe release to the water column the remaining highly reactive fe pool has a heavier isotope composition compared to california margin sediments this observation is in agreement with the general notion of an isotopically light benthic fe efflux most of the reactive fe delivered and retained in the sediment is transferred into authigenic mineral phases within the topmost 10 to 20 cm of the sediments we observe a first order relationship between the extent of pyritization of fe monosulfide and the isotope composition of authigenic pyrite with increasing pyritization the isotope composition of authigenic pyrite approaches the isotope composition of the highly reactive fe pool we argue that the isotope composition of authigenic pyrite or other fe minerals that may undergo pyritization may only be used to trace water column sources or sinks if the extent of pyritization is separately evaluated and either close to 100 or 0 alternatively one may calculate the isotope composition of the highly reactive fe pool thereby avoiding isotope effects due to internal diagenetic redistribution in depositional settings with high fe but lowsulfide concentrations source and sink signatures in the isotope composition of the highly reactive fe pool may be compromised by sequestration of fe within authigenic silicate minerals authigenic silicate minerals appear to be an important burial phase for reactive fe below the peruvian oxygen minimum zone

characteristics of manganese nodules from the central indian basin relationship with the sedimentary environment the hydrogenetic and diagenetic manganese nodules from the cib show an unequivocal genetic relationship with neogene sedimentological patterns hydrogenetic nodules were observed at the top of the sedimentary column and interstratified in thin sequences of biosiliceous sediments and underlying miocene red clays in cores from topographic highs early to late pliocene radiolarian fauna were found at the base of the biosiliceous deposits these condensed biosiliceous sediments are less than 4 0 m thick and periods of non deposition and or erosion may have occurred in the central deep area 11 15 s significant biogenic deposition began during the early to late pliocene as a result of the development of a high biogenic productivity belt to the south producing the favourable conditions for diagenetic accretionary processes for mn nodules this indicates growth of hydrogenetic nodules prior to the late pliocene and the subsequent inception of diagenetic accretion formation of the most recent diagenetic nodules is associated with higher rates of sedimentation in the deep sea troughs where the siliceous sediments are much thicker this should promote the diagenetic mobilization of mn and its precipitation under oxic and suboxic conditions nevertheless diagenetic nodules are restricted to the upper 40 cm of sediments younger than 400 000 years suggesting fast growing diagenetic nodules and the dissolution of buried nodules below the first 50 cm of siliceous oozes hydrogenetic nodules are formed by dense layers of mno2 fe oh 3 and amorphous silica and they show smooth texture and botryoidal shapes analysis of bulk and selected portions of layered crust or rind shows very low mnfe ratios 2 and relatively high co and ti contents in most cases the cores of hydrogenetic nodules are fragments of altered ash layers from the indonesian volcanic arc which is located more than 2500 km to the east the ash layers are also interstratified with miocene red clays diagenetic nodules show a granular texture and comprise concentric layers with dendritic or columnar structures mainly of 10 a manganate higher mn ni and cu contents are observed in this phase late diagenetic enrichment of these nodules is continuous within the upper 40 cm of recent ni that enhances the stability of early diagenetic 10 a manganate buserite and c the formation of todorokite some nodules referred to as mixed in the literature show hydrogenetic internal layers which grade outward to a diagenetic crust this transitional character suggests slow changes in the sedimentary environment as growth advances the nodules of the cib and the east equatorial pacific eep are similar in terms of both metallic composition and accretionary patterns however the nodules of the cib are considerably younger and present a continuous accretionary history the history of the manganese nodules from the cib is related to the post late miocene sedimentary and geodynamic evolution of this basin

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

trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments interstitial water samples from the guatemala basin and the coast of baja california have been analyzed for manganese iron copper nickel and nitrate the data provide a systematic look at changes in trace metal diagenesis proceeding from red clay to highly reducing nearshore sediments in red clay sediments the nitrate concentrations suggest that only aerobic respiration is occurring manganese and iron are below detection nickel concentrations remain the same as in bottom seawater but copper shows a pronounced maximum just at the sediment water interface proceeding to hemipelagic sediments denitrification becomes increasingly important and manganese and iron remobilization occur in the sediments the linear manganese and nitrate profiles suggest regions of production or consumption separated by zones of diffusion this differs from the conventional picture of a continuous series of reactions within the sediments manganese reduction always occurs before iron reduction the pore water nickel correlates well with manganese in these sediments suggesting that nickel is associated with mno2 in the solid phase the pore water flux ratio of manganese and nickel agrees well with the ratio in solid phase authigenic oxides copper still displays a core top concentration maximum as well as a second maximum associated with the remobilized manganese the calculated ratio of the cu c flux ratios support the argument for copper remobilization during organic carbon oxidation comparison of the upward and downward diffusive fluxes with the rate of copper buried by sedimentation shows that at least half of the copper buried must be of diagenetic origin and less than 25 of the copper reaching the sediments is buried

the global ocean flux of particulate organic carbon areal distribution and magnitude the magnitude and distribution of the particulate organic carbon poc rain rate to the seafloor in the atlantic pacific and indian ocean basins between 61 n and 61 s has been estimated from benthic oxygen flux estimates for water depths 1000 m only the calculation utilizes the extensive data sets of sedimentary organic carbon caco3 and accumulation rate to extrapolate between individual benthic flux measurement sites using an empirically derived correlation between the seafloor oxygen flux and these parameters the poc flux through the 1000 m depth horizon was then estimated from published correlations between sediment trap determined fluxes and water depth total oxygen utilization in the deep ocean is estimated to be 1 2 1014 mol o2 yr 1 a value that agrees well with previous estimates which were based on surface water primary productivity sediment trap and depth relationships and with deep water respiration rates estimated from apparant oxygen utilization aou 14c relationships on the basis of the derived global ocean flux distribution it is concluded that 1 dissolved organic carbon doc inputs are not required to account for estimated deep water respiration rates 2 the majority of the poc input to the deep ocean occurs within 30 of the equator 3 the proportion of primary production that reaches the deep sea does not vary greatly with latitude 4 gyre and continental margin regions contribute roughly equally to the deep poc flux with a relatively minor contribution from the equatorial divergence region 5 of the estimated 7 2 1013 mol c yr 1 of poc that sinks below the 1000 m depth horizon 45 3 3 1013 mol c yr 1 reaches the seafloor where it is oxidized 6 when normalized to basin area average deep flux rates in the atlantic and pacific are similar while highest rates are observed in the indian ocean and 7 the results can be fully reconciled only if the benthic flux of doc is significantly less than the benthic o2 flux

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

petrogenesis of ferromanganese nodules from east of the chagos archipelago central indian basin indian ocean abstract deep sea ferromanganese nodules occur over a large area and on many different sediment types of the central indian basin indian ocean selected samples were studied to determine their chemical and mineralogical compositions and microstructural features repeated laminations of variable thickness alternately dominated by todorokite and vernadite are characteristic of these nodules these laminae show on electron microprobe line scans corresponding interlaminar partitioning of mn cu ni and fe co the bulk chemical compositions of these nodules plot in both the hydrogenetic and early diagenetic fields on the fe mn ni cu co 10 ternary diagram the binary diagram depicting the covariation of mn ni cu against fe co shows two distinct parallel regression lines one delineated by nodules from terrigenous siliceous ooze and siliceous ooze terrigenous sediments and the other by nodules from red clay siliceous ooze red clay and calcareous ooze red clay an increasing diagenetic influence in the nodules with the nature of the host sediment types was observed in the sequence terrigenous siliceous ooze and red clay siliceous calcareous ooze red clay a negative correlation between mn fe ratio and co and a positive correlation between mn fe ratio and ni cu was established the nodules show dendritic laminated and globular microstructures formed by primary growth of fe mn oxide laminae depositional hiatuses in the primary microstructures indicate that the growth of these nodules was episodic the oxide laminations show extremely complex growth patterns scattered biogenic remains and mineral grains acted as accessory seeds for growth of oxide layers in addition to the main nuclei none of the primary microstructures can be uniquely linked to a particular growth process or growth rate radial cracks cutting across primary microstructures are often filled by todorokite of a later generation post depositional modifications of the nodules were largely controlled by accreted biogenic remains as indicated by their progressive dissolution with increasing depth from nodule surfaces their pseudomorphic replacement by todorokite and the later growth of phillipsite and todorokite in the microfossil molds the growth patterns of the in filled oxides are often controlled entirely by the cavity walls and are discordant with the primary growth fabric primary todorokite was recrystallized to coarser grains of different chemical composition later generation veins of todorokite cut across and chaotically disrupted primary laminae

a comparative analysis of compositional variations in and between marine ferromanganese nodules and crusts in the south pacific and their environmental controls abstract regional variation of mn fe co ni and cu in ferromanganese oxides nodules and crusts in the central south pacific is related to primary productivity oxygen minimum layer and calcium carbonate compensation depth the largely latitudinal influence of these environmental parameters on nodule and crust composition reflects their predominantly latitudinal variation primary productivity is the principal regional environmental control influencing diagenetic enrichment of these elements in nodules through its effect mediated by the ccd on supply and concentration of labile organic matter vs carbonate remains to the sediments it influences hydrogenetic enrichment of these elements in nodules and crusts through its effect mediated by the oxygen minimum layer mainly in the case of crusts on their export from surface waters the elements varying susceptibility to being scavenged or organically bound influences the contrasting composition of diagenetic vs hydrogenetic ferromanganese oxides which is further influenced by depth hydrogenesis is the fundamental process governing nodule and crust formation superimposed on which is diagenesis under specific circumstances both are subject to intermittent interruption diminution and augmentation by changes in environmental parameters application of regionally operative environmental controls locally explains local compositional variations and helps refine exploration criteria for economically viable nodules and crusts

ferromanganese nodules from manop sites h s and r control of mineralogical and chemical composition by multiple accretionary processes abstract the chemical composition of ferromanganese nodules from the three nodule bearing manop sites in the pacific can be accounted for in a qualitative way by variable contributions of distinct accretionary processes these accretionary modes are 1 1 hydrogenous i e direct precipitation or accumulation of colloidal metal oxides in seawater 2 2 oxic diagenesis which refers to a variety of ferromanganese accretion processes occurring in oxic sediments and 3 3 suboxic diagenesis which results from reduction of mn 4 by oxidation of organic matter in the sediments geochemical evidence suggests processes 1 and 2 occur at all three manop nodule bearing sites and process 3 occurs only at the hemipelagic site h which underlies the relatively productive waters of the eastern tropical pacific a normative model quantitatively accounts for the variability observed in nearly all elements zn and na however are not well explained by the three end member model and we suggest that an additional accretionary process results in greater variability in the abundances of these elements variable contributions from the three accretionary processes result in distinct top bottom compositional differences at the three sites nodule tops from h are enriched in ni cu and zn instead of the more typical enrichments of these elements in nodule bottoms in addition elemental correlations typical of most pelagic nodules are reversed at site h the three accretionary processes result in distinct mineralogies hydrogenous precipitation produces mno2 oxic diagenesis however produces cu ni rich todorokite and suboxic diagenesis results in an unstable todorokite which transforms to a 7 a phase birnessite upon dehydration the presence of cu and ni as charge balancing cations influence the stability of the todorokite structure in the bottoms of h nodules which accrete dominantly by suboxic diagenesis na and possibly mn 2 provide much of the charge balance for the todorokite structure limited growth rate data for h nodules suggest suboxic accretion is the fastest of the three processes with rates at least 200 mm 106 yr oxic accretion is probably 10 times slower and hydrogenous 100 times slower since these rates predict more suboxic component in bulk nodules than is calculated by the normative analysis we propose that suboxic accretion is a non steady state process variations in surface water productivity cause pulses of particulate flux to the sea floor which result in transient mn reduction in the surface sediments and reprecipitation on nodule surfaces

effects of phosphatization on the geochemical and mineralogical composition of marine ferromanganese crusts most hydrogenetic ferromanganese crusts in the central pacific consist of two growth generations a phosphatized older growth generation that is impregnated with carbonate fluorapatite cfa and a younger non phosphatized growth generation geochemical and mineralogical investigations of non phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by cfa certain elements are depleted in old crusts compared to young crusts in the order si fe al ti co mn pb others like ni zn cu y and rees rare earth elements are mostly enriched suboxic conditions in the phosphate rich water of the oxygen minimum zone that infiltrated the crusts caused a partial redissolution of crust phases resulting in mobilization of associated elements a partial recrystallization of more stable phases and a secondary input of nutrient type elements the frequent occurrence of todorokite in phosphatized crust layers indicates that mn mobilized from the vernadite crust phase was partially recrystallized as todorokite which is more stable under suboxic conditions incorporating ni cu and zn into the crystal lattice but rejecting co other metals like pb y and rees probably form stable phosphate phases in phosphatized crusts our investigations emphasize that under the influence of phosphatization diagenetic remobilization and reorganization in fe mn crusts took place in contrast to non phosphatized crusts the older phosphatized crust generation no longer represents a complete primary crust precipitate therefore young non phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition mineralogy and evolution we also suggest that most methods for age determination like the 87sr86sr isotopic dating method or co chronometry cannot be used for phosphatized crusts without caution

mid cretaceous tectonic evolution of the tongareva triple junction in the southwestern pacific basin the trace of the ridge ridge ridge triple junction that connected the pacific farallon and phoenix plates during mid cretaceous time originates at the northeast corner of the manihiki plateau near the tongareva atoll for which the structure is named the triple junction trace extends 3250 km south southeast to and beyond a magnetic anomaly 34 bight it is identified by the intersection of nearly orthogonal abyssal hill fabrics which mark the former intersections of the pacific phoenix and pacific farallon ridges a distinct trough is commonly present at the intersection a volcanic episode from 125 to 120 ma created the manihiki plateau with at least twice its present volume and displaced the triple junction southeast from the nova canton trough to the newly formed manihiki plateau almost simultaneously the plateau was rifted by the new triple junction system and large fragments of the plateau were rafted away to the south and east the tongareva triple junction originated ca 119 ma when carbonate sedimentation began atop the manihiki plateau subsequent spreading rates on the pacific phoenix and pacific farallon ridges averaged 18 20 cm yr until 84 ma

origin of the oligocene manganese deposit at obrochishte bulgaria insights from c o fe sr nd and pb isotopes abstract the large manganese mn deposit at obrochishte ne bulgaria is part of a cluster of similar early oligocene deposits located around present day black sea they collectively constitute the earth s second largest endowment of mn after the kalahari manganese field in africa we have employed a battery of isotopic techniques c o fe sr nd pb to help understand the genesis of this deposit carbon isotope data indicates that some sections of the mn ore layer have diagenetic mnco3 mineralization formed by reaction of mn oxides with organic carbon corg whereas other sections have mnco3 precipitated directly from the seawater column oxygen isotopes show that the high grade mn mineralization had seawater as the fluid source whereas some lower grade sections had a mix of ground water and seawater as fluid sources sr and nd isotope values of ore leachates also indicate that the mn deposition occurred in normal early oligocene seawater nd and pb isotope values suggest that the clastic host sediments were sourced from continental bedrock rather than younger arc volcanic rocks to the west iron isotope composition of the mn ore implies deposition in a redox stratified basin similar to the modern black sea with much of the fe sequestered in deep anoxic euxinic water as sulfides similar to the modern black sea most of the detrital fe was transferred from shallow oxic sediments into deep anoxic euxinic water by an iron shuttle and remobilized mn sequestered in the upper suboxic water layer however during the oligocene the iron shuttle operated intermittently due to the chemocline falling mostly below the shelf break thereby limiting the efficiency of the shuttle mechanism we propose a model for the lower oligocene strata in which intense weathering during the eocene weathering phase produced a thick lateritic crust on the southern european continent the drastic sea level drop at the end of the eocene initiated downcutting of streams through this weathered material transferring fe and mn oxides to the redox stratified western black sea here these oxides were partly or entirely dissolved in the suboxic mn oxides partly fe oxyhydroxides entirely dissolved and anoxic euxinic mn oxides entirely dissolved dissolved fe2 re precipitated water layers eventually fe was re precipitated as sulfide in the deep anoxic euxinic water while mn accumulated in the suboxic water layer transgression in the early oligocene brought this mn rich water onto the shallow shelf where it precipitated as mn oxide then converted to mn carbonates during early diagenesis some mn was also contributed by submarine groundwater discharge further transgression brought lower oxygen water onto the shelf and mn carbonate precipitated directly from the water column the findings from this work provide insights about the unique oligocene geochemical event in the region that lead to the formation of the 2nd largest cluster of mn deposits in the world

geochemical composition and inferred accretion rates of sediments and managanese nodules from a submarine hill in the madeira abyssal plain eastern north atlantic abstract the porewater and sediment composition of two boxcores and of a small gravity core taken on a manganese nodule covered hill and in the madeira abyssal plain proper respectively are compared the pore water study of the two boxcores indicates that oxic conditions prevail in both cores in addition it indicates that no detectable fluxes of mn or fe occur from the porewater to the ocean bottom water variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate which acts as a diluting agent a clear carbonate minimum is observed at 20 22 cm depth in the two cores this minimum is likely to be associated with the last glacial period 10 20 kyr b p this association is supported by the sediment accumulation rate of 15 mm kyr as found by extrapolation from the rate for pelagic sediments in the madeira abyssal plain the bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of atlantic nodules the trace metal and rare earth elements composition indicate a hydrogenous origin for the manganese nodules of this study on the basis of the chemical composition and that of nodules relative to that of the adjacent sediments an average nodule accretian rate of 2 8 3 3 mm myr has been calculated although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition internal structures of the nodules reveal great inhomogeneity both visually and chemically these fluctuations may be related to variations in the fluxes of mn and fe which in turn could be climate related

secular variation of iron isotopes in north atlantic deep water a high precision iron isotope time series for a ferromanganese crust demonstrates that the iron isotope composition in north atlantic deep water has changed substantially over the past 6 million years and that iron isotope variations in the crust are closely correlated to those of lead isotopes the close correlation between the two isotope series indicates that the observed iron isotope variations predominantly reflect those of iron input from terrigenous sources and provides no evidence for biologically induced mass fractionation within north atlantic deep water

long term stability of global erosion rates and weathering during late cenozoic cooling reports that the geological record of erosion indicates a fourfold increase in global sedimentation rates during the past 5 million years merited a global explanation explanations offered include an increased rate of mountain formation and global cooling now jane willenbring and friedhelm von blanckenburg reanalyse original data sets and use ocean records of beryllium isotope ratios to obtain an indirect measure of past erosion rates and rock weathering they find no clear evidence either for an increase in global erosion or for a pulse of weathered material in the oceans during the past 12 million years they suggest that cenozoic global cooling must have some other cause and the mountain building that did occur had little effect on erosion or weathering flux globally accelerated uplift and global cooling have been suggested as possible causes for a fourfold increase in global sedimentation rates and by inference in erosion rates or weathering fluxes during the past 5 million years here proxy evidence is provided that indicates stable weathering fluxes in the late cenozoic era it is proposed that processes different from an increase in denudation caused cenozoic global cooling over geologic timescales co2 is emitted from the earth s interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial this balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions1 changes in this balance have been attributed to changes in topographic relief where varying rates of continental rock weathering and erosion1 2 are superimposed on fluctuations in organic carbon burial3 geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation1 2 4 widespread observations of a recent 0 5 myr fourfold increase in global sedimentation rates require a global mechanism to explain them4 5 6 accelerated uplift and global cooling have been given as possible causes2 4 6 7 but because of the links between rates of erosion and the correlated rate of weathering8 9 an increase in the drawdown of co2 that is predicted to follow may be the cause of global climate change instead2 however globally rates of uplift cannot increase everywhere in the way that apparent sedimentation rates do4 10 moreover proxy records of past atmospheric co2 provide no evidence for this large reduction in recent co2 concentrations11 12 here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record13 as evidence we recast the ocean dissolved 10be 9be isotope system as a weathering proxy spanning the past 12 myr ref 14 this proxy indicates stable weathering fluxes during the late cenozoic era the sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean we conclude that processes different from an increase in denudation caused cenozoic global cooling and that global cooling had no profound effect on spatially and temporally averaged weathering rates

biogenic iron oxyhydroxide formation at mid ocean ridge hydrothermal vents juan de fuca ridge here we examine fe speciation within fe encrusted biofilms formed during 2 month seafloor incubations of sulfide mineral assemblages at the main endeavor segment of the juan de fuca ridge the biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic fe oxidizing microorganisms our objectives were to determine the form of biofilm associated fe and identify the sulfide minerals associated with microbial growth we used micro focused synchrotron radiation x ray fluorescence mapping mu xrf x ray absorption spectroscopy mu exafs and x ray diffraction mu xrd in conjunction with focused ion beam fib sectioning and highresolution transmission electron microscopy hrtem the chemical and mineralogical composition of an fe encrusted biofilm was queried at different spatial scales and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved the fe encrusted biofilms formed preferentially at pyrrhotite rich fe1 xs 0 x 0 2 regions of the incubated chimney sulfide at the nanometer spatial scale particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2 line ferrihydrite were identified infrequently at the micron spatial scale fe mu exafs spectroscopy and mu xrd measurements indicate that the dominant form of biofilm fe is a short range ordered fe oxyhydroxide characterized by pervasive edge sharing fe o6 octahedral linkages double corner sharing fe o6 linkages which are common to fe oxyhydroxide mineral structures of 2 line ferrihydrite 6 line ferrihydrite and goethite were not detected in the biogenic iron oxyhydroxide bio the suspended development of the bio mineral structure is consistent with fe iii hydrolysis and polymerization in the presence of high concentrations of fe complexing ligands we hypothesize that microbiologically produced fe complexing ligands may play critical roles in both the delivery of fe ii to oxidases and the limited fe iii oxyhydroxide crystallinity observed within the biofilm our research provides insight into the structure and formation of naturally occurring microbiologically produced fe oxyhydroxide minerals in the deep sea we describe the initiation of microbial seafloor weathering and the morphological and mineralogical signals that result from that process our observations provide a starting point from which progressively older and more extensively weathered seafloor sulfide minerals may be examined with the ultimate goal of improved interpretation of ancient microbial processes and associated biological signatures

deep ocean mineral deposits as a source of critical metals for high and green technology applications comparison with land based resources abstract ferromanganese fe mn crusts are strongly enriched relative to the earth s lithosphere in many rare and critical metals including co te mo bi pt w zr nb y and rare earth elements rees fe mn nodules are strongly enriched in ni cu co mo zr li y and rees compared to fe mn crusts nodules are more enriched in ni cu and li with subequal amounts of mo and crusts are more enriched in the other metals the metal ions and complexes in seawater are sorbed onto the two major host phases feo oh with a positively charged surface and mno 2 with a negatively charged surface metals are also derived from diagenetically modified sediment pore fluids and incorporated into most nodules seafloor massive sulfides sms especially those in arc and back arc settings can also be enriched in rare metals and metalloids such as cd ga ge in as sb and se metal grades for the elements of economic interest in sms cu zn au ag are much greater than those in land based volcanogenic massive sulfides however their tonnage throughout the global ocean is poorly known and grade tonnage comparisons with land based deposits would be premature the clarion clipperton fe mn nodule zone ccz in the ne pacific and the prime fe mn crust zone pcz in the central pacific are the areas of greatest economic interest for nodules and crusts and grades and tonnages for those areas are moderately well known we compare the grades and tonnages of nodules and crusts in those two areas with the global terrestrial reserves and resources nodules in the ccz have more tl 6000 times mn te ni co and y than the entire global terrestrial reserve base for those metals the ccz nodules also contain significant amounts of cu mo w li nb and rare earth oxides reo compared to the global land based reserves fe mn crusts in the pcz have significantly more tl 1700 times te 10 times more co and y than the entire terrestrial reserve base other metals of significance in the pcz crusts relative to the total global land based reserves are bi reo nb and w ccz nodules and pcz crusts are also compared with the two largest existing land based ree mines bayan obo in china and mountain pass in the usa the land based deposits are higher grade but lower tonnage deposits notably both land based deposits have

nickel partitioning in biogenic and abiogenic ferrihydrite the influence of silica and implications for ancient environments fe iii oxyhydr oxides are ubiquitous in modern soils and sediments and their large surface area leads to scavenging of trace elements experimental trace element partitioning between fe iii oxyhydr oxides and aqueous solutions have been used to elucidate the geochemical composition of the precambrian oceans based on the trace element concentrations in precambrian banded iron formations bifs however previous partitioning experiments did not consider the potential influence of microbially derived organic material even though it is widely believed that bacterial phytoplankton was involved in fe ii oxidation and the deposition of bif primary minerals therefore the present study focuses on sorption of ni to and co precipitation of ni with both biogenic ferrihydrite precipitated by the freshwater photoferrotroph rhodobacter ferrooxidans sw2 and the marine photoferrotroph rhodovulum iodosum as well as chemically synthesized ferrihydrite we considered the influence of cellular organic material medium composition and the availability of dissolved silica our results show a preferential association of ni with ferrihydrite and not with the microbial cells or extracellular organic substances we found that the addition of silica 2 mm did not influence ni partitioning but led to the encrustation of some cells with ferrihydrite and amorphous silica the two to threefold lower ni fe ratio in biogenic as compared to abiogenic ferrihydrite is probably due to a competition between ni and organic matter for sorption sites on the mineral surface additionally the competition of ions present at high concentrations in marine medium for sorption sites led to decreased ni sorption or co precipitation based on our data we conclude that if the fe iii minerals deposited in bifs were at least to some extent biological then the ni concentrations in the early ocean would have been higher than previously suggested this study shows the importance of considering the presence of microbial biomass and seawater ions in paleomarine reconstructions

the temporal record of magmatism at cerro uturuncu bolivian altiplano twenty six new 40ar 39ar plateau ages for 23 lavas and domes from the uturuncu volcano in the altiplano of sw bolivia reveal a protracted eruptive history from 1050 5 to 250 5 ka eruptions have been exclusively effusive producing some 50 km3 of high k dacites and silicic andesites bimodal mineral compositions complex mineral textures the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at uturuncu post 458 ka distinct high and low mgo cr magmas are resolved these magmas erupt during similar times suggesting that eruptions are tapping different parts of the magma system albeit from the same vent system volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath uturuncu and calculated buoyancy forces are sufficient to drive effusive eruptions eruptive activity is episodic with six eruptive periods separated by hiatuses of 50 kyr cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka the episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying altiplano puno magma body

a new method for the determination of ceiii ceiv ratios in geological materials application for weathering sedimentary and diagenetic processes abstract relative abundances of rare earth elements ree in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans one of the ree cerium ce can exist in either trivalent or tetravalent forms depending on the redox condition thus knowledge of the oxidation state of ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments however the use of ce for this purpose has been hampered by an inability to measure its oxidation state directly here we present a new method employing the x ray absorption near edge structure for making such determinations in samples with ce concentrations as low as 15 ppm by analyzing a range of diverse geological materials granites manganese nodules and cherts we showed that the method could identify significant differences in ce speciation between materials formed under widely differing redox conditions used together with the degrees of ce anomalies measured in ree patterns this information has promise for refining our knowledge of various geochemical processes such as weathering sedimentary and diagenetic processes that control the ree behavior such a refinement should in turn improve geological interpretations based upon ree data

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

pollen based summer temperature reconstructions for the eastern canadian boreal forest subarctic and arctic pollen based paleoclimate reconstructions using response surface and modern analog methods reveal an 8000 year record of july temperature fluctuations for 25 eastern canadian lake sites located from the forest tundra to the high canadian arctic postglacial conditions characterized by warmer than present summer temperatures prevailed in baffin island and ne labrador beginning about 7500 and 7000 14c yr bp respectively resulting in warmer than present conditions throughout the region by 6000 14c yr bp 0 5 c to 1 c further south in quebec and w labrador july temperatures were 1 2 c colder than present until after 6000 14c yr bp and only reached modern values after all residual laurentide ice had melted increased summertime insolation and the final disappearance of laurentide ice during the middle holocene probably caused july temperatures throughout eastern canada to peak between approximately 5000 and 3500 14c yr bp mid holocene warming relative to today was more pronounced in baffin island and ne labrador 1 c to 2 c compared to the boreal and subarctic regions of quebec and w labrador 1 c over the past 4000 years decreasing summertime insolation and colder sea surface temperatures in the davis strait and labrador sea contributed to a decline in july temperatures of 1 2 c throughout baffin island and the tundra regions of n quebec and labrador the absence of similar cooling in the records from the boreal forest may support the notion that the mean position of the summertime polar front blocked the colder arctic air during the late holocene

mineralogy and crystal chemistry of mn fe co ni and cu in a deep sea pacific polymetallic nodule minor element concentrations in marine ferromanganese nodules are primarily controlled by the mineralogy which itself depends on redox conditions at the sediment water interface results are presented for the first in depth x ray microstructural and microspectroscopic investigation of a mixed hydrogenetic diagenetic nodule which is representative of ferromanganese deposits on abyssal plains the measurements were conducted by micro x ray diffraction and x ray absorption spectroscopy both xanes and exafs on hydrogeneous and diagenetic regions of the nodule the hydrogenetic diagenetic interface was imaged by x ray microfluorescence after which regions of interest were chosen to represent mineralogical and chemical transformations that occurred at the early stage of suboxic diagenesis in the hydrogenetic nodule oxic environment mn is speciated as fe vernadite a nanocomposite material composed of intergrown feroxyhite feooh and monodispersed phyllomanganate layers having no interlayer mn vernadite in the diagenetic nodule suboxic environment mn is speciated dominantly as mg rich 10 a vernadite which consists of a random intergrowth of vernadite and its transformation product todorokite the authigenic 10 a vernadite precipitated from the components of vernadite in fe vernadite that were dissolved in suboxic microenvironments of the sediment direct evidence supporting a redox driven dissolution reaction is provided by the valence composition of mn as measured by micro xanes which is 0 69mn4 0 24mn3 0 07mn2 average 3 62 0 04 v u for fe vernadite and 0 61mn4 0 23mn3 0 16mn2 average 3 28 0 04 v u for 10 a vernadite ni and cu derived mainly from dissolved vernadite and oxidized organic matter replace structural mn3 4 in both the mno2 layer and todorokite domains of 10 a vernadite pure todorokite in highly diagenetic regions of the nodule has an average formula of mg2 0 167 mn4 0 783 mn3 0 099co3 0 002ni2 0 076cu2 0 040 o2 n h2o with an atomic ratio of cu ni co mn 0 13 which is slightly lower than 0 167 1 6 the maximum metal uptake capacity reported for marine nodules by analogy with synthetic todorokites we infer that mg2 which has a hydrated diameter close to that of the 3 3 tunnel size of todorokite and mn3 and cu2 which prefer jahn teller distorted octahedra play a crucial role in templating the topotactic transformation of 10 a vernadite to todorokite and stabilizing todorokite in suboxic marine sediments

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

iron mineral structure reactivity and isotopic composition in a south pacific gyre ferromanganese nodule over 4ma abstract deep sea ferromanganese nodules accumulate trace elements from seawater and underlying sediment porewaters during the growth of concentric mineral layers over millions of years these trace elements have the potential to record past ocean geochemical conditions the goal of this study was to determine whether fe mineral alteration occurs and how the speciation of trace elements responds to alteration over 3 7 ma of marine ferromanganese nodule mfn formation a timeline constrained by estimates from 9be 10be concentrations in the nodule material we determined fe bearing phases and fe isotope composition in a south pacific gyre spg nodule specifically the distribution patterns and speciation of trace element uptake by these fe phases were investigated the time interval covered by the growth of our sample of the nodule was derived from 9be 10be accelerator mass spectrometry ams the composition and distribution of major and trace elements were mapped at various spatial scales using micro x ray fluorescence xrf electron microprobe analysis empa and inductively coupled plasma mass spectrometry icp ms fe phases were characterized by micro extended x ray absorption fine structure exafs spectroscopy and micro x ray diffraction xrd speciation of ti and v associated with fe was measured using micro x ray absorption near edge structure xanes spectroscopy iron isotope composition 56 54fe in subsamples of 1 3 mm increments along the radius of the nodule was determined with multiple collector icp ms mc icp ms the spg nodule formed through primarily hydrogeneous inputs at a rate of 4 0 0 4 mm ma the nodule exhibited a high diversity of fe mineral phases feroxyhite feooh goethite feooh lepidocrocite feooh and poorly ordered ferrihydrite like phases these findings provide evidence that fe oxyhydroxides within the nodule undergo alteration to more stable phases over millions of years trace ti and v were spatially correlated with fe and found to be adsorbed to fe bearing minerals ti fe and v fe ratios and ti and v speciation did not vary along the nodule radius the 56 54fe values when averaged over sample increments representing 0 25 0 75 ma were homogeneous within uncertainty along the nodule radius at 0 12 0 07 2sd n 10 our results indicate that the fe isotope composition of the nodule remained constant during nodule growth and that mineral alteration did not affect the primary fe isotope composition of the nodule furthermore the average 56 54fe value of 0 12 we find is consistent with fe sourced from continental eolian particles dust despite mineral alteration the trace element partitioning of ti and v and fe isotope composition do not appear to change within the sensitivity of our measurements these findings suggest that fe oxyhydroxides within hydrogenetic ferromanganese nodules are out of geochemical contact with seawater once they are covered by subsequent concentric mineral layers even though fe bearing minerals are altered trace element ratios speciation and fe isotope composition are preserved within the nodule

high resolution la icp ms mapping of deep sea polymetallic micronodules and its implications on element mobility abstract enrichments of rey rare earth yttrium and other trace metals co and ni in deep sea ferromanganese fe mn micronodules have received increasing attention in both deep sea research and mineral exploration due to the presence of multiple easily crushed and poorly crystallized phases in micronodules the genesis of micronodules and their adsorption of various trace elements are poorly understood to address this gap we examined the spatial distributions of elements in cross sections of micronodules from the western tropical north pacific ocean using high resolution hr la icp ms raster mapping coupled with laser raman and x ray photoelectron spectroscopy xps the ferromanganese micronodules we studied are dominated by fe and mn oxides with minor carbonate minerals such as siderite rhodochrosite and calcite la icp ms maps show that these micronodules consist of a mn rich core and a fe rich rim the fe enriched rim is enriched in as and surrounds a mg mn cu co and ni concreted core laser raman maps show that the micronodule core contains more birnessite an important scavenger of trace metals in deep sea sediments than the rim the birnessite filled core of these micronodules does not have elevated rey indeed birnessite line channels may feed metal rich fluid containing rey to adjacent minerals including well crystallized bio apatite and zeolite as high ce and y levels are spatially correlated with these minerals the observed element profiles and xps observations showing the coexistence of multiple oxidation states of mn 2 3 and 4 fe 2 and 3 and ce 3 4 demonstrate that the fe mn phases of these micronodules are of a diagenetic origin and that they are sites of redox driven metal enrichment in deep sea sediment

the relationship between manganese minerals and metallic elements in deep sea manganese nodules abstract the relationship between manganese minerals and the concentration of fe cu ni zn and al in mn rich crystalline areas in deep sea manganese nodules were investigated using microdiffraction and electron microprobe analysis thirty three microsamples were obtained from eight polished sections of five pacific ocean manganese nodules birnessite and todorokite are the principal manganese minerals in the x ray diffraction powder patterns on polished sections kept in a vacuum only birnessite was identified in sixteen microsamples only todorokite was identified in one microsample another sixteen microsamples were mixtures of birnessite and todorokite microsamples in which only birnessite was identified contain cu ni and zn in the same concentrations as the microsamples in which both birnessite and todorokite were identified it appears that birnessite as well as todorokite can incorporate cu ni and zn with the exception that no todorokite was observed in microsamples with very small amounts of fe cu and ni there is no systematic correlation between the manganese mineralogy and the concentration of fe cu ni and zn in the microsamples the microsamples examined in this study contain varying amounts of al 0 01 2 57 wt 0 92 wt on average todorokite appeared in the microsamples which contained more than 0 7 wt al mineralogical considerations suggest that at least part of the al detected may be incorporated in todorokite

rare earth element zonation in pacific ferromanganese nodules abstract the lower surfaces of ferromanganese nodules from the north equatorial pacific ocean which are enriched in mn cu and ni and the upper surfaces which are enriched in fe p and co have been analyzed for la ce nd sm eu gd dy er and yb the ree contents are lower and the ce anomaly is smaller in the lower surfaces than in the upper surfaces the magnitude of the ce anomaly increases with decreasing mn fe ratio indicative of a seawater origin the zonal distribution of the other ree supports the conclusion derived previously from inter nodule and nodule sediment relationships that diagenetic fixation of rare earths in sediments affects their enrichment by nodular iron oxyhydroxides

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

ferromanganese oxide deposits from the central pacific ocean ii nodules and associated sediments bulk chemical mineralogical and selective leach analyses have been made on a suite of abyssal ferromanganese nodules and associated sediments from the s w equatorial pacific ocean compositional relations between nodules sediment oxyhydroxides and nearby ferromanganese encrustations are drawn assuming that the crusts represent purely hydrogenetic ferromanganese material crusts mno2 rich nodules and sediment oxyhydroxides are compositionally similar and distinct from diagenetic todorokitebearing nodules compared to fe mn crusts sediment oxyhydroxides are however slightly enriched relative to mn and ni in fe cu zn ti and al and depleted in co and pb reflecting processes of non hydrogenous element supply and diagenesis mno2 nodules exhibit compositions intermediate between fe mn crusts and sediment oxyhydroxides and thus are considered to accrete oxides from both the water column and associated sediments deep ocean vertical element fluxes associated with large organic aggregates biogenic calcite silica and soft parts have been calculated for the study area fluxes associated with organic aggregates are one to three orders of magnitude greater than those associated with the other phases considered are in good agreement with element accumulation rates in sediments and are up to four orders of magnitude greater than element accumulation rates in nodules metal release from labile biogenic material in surface sediments can qualitatively explain the differences between the composition of fe mn crusts and sediment oxyhydroxides todorokite rich diagenetic nodules are confined to an eastwards widening equatorial wedge it is proposed that todorokite precipitates directly from interstitial waters since the transition metal chemistry of interstitial waters is controlled dominantly by reactions involving the breakdown of organic carbon the supply and degradation rate of organic material is a critical factor in the formation of diagenetic nodules the wide range of trace metal mn ratios observed in marine todorokite reflects a balance between the release of trace metals from labile biogenic phases and the reductive remobilisation of mn oxide both of which are related to the breakdown of organic carbon

meteoric cosmogenic beryllium 10 adsorbed to river sediment and soil applications for earth surface dynamics abstract rainfall scavenges meteoric cosmogenic 10be from the atmosphere 10be falls to the earth s surface where it binds tightly to sediment particles in non acidic soils over the life span of those soils as such meteoric 10be has the potential to be an excellent geochemical tracer of erosion and stability of surfaces in a diverse range of natural settings meteoric 10be has great potential as a recorder of first order erosion rates and soil residence times even though this tracer was first developed in the late 1980s and showed great promise as a geomorphic tool it was sidelined in the past two decades with the rise of the sister nuclide in situ 10be which is produced at a known rate inside quartz minerals since these early days substantial progress has been made in several areas that now shed new light on the applicability of the meteoric variety of this cosmogenic nuclide here we revisit the potential of this tracer and we summarize the progress 1 the atmospheric production and fallout is now described by numeric models and agrees with present day measurements and paleo archives such as from rain and ice cores 2 short term fluctuations in solar modulation of cosmic rays or in the delivery of 10be are averaged out over the time scale soils accumulate 3 in many cases the delivery of 10be is not dependent on the amount of precipitation 4 we explore where 10be is retained in soils and sediment 5 we suggest a law to account for the strong grain size dependence that controls adsorption and the measured nuclide concentrations and 6 we present a set of algebraic expressions that allows calculation of both soil or sediment ages and erosion rates from the inventory of meteoric 10be distributed through a vertical soil column the mathematical description is greatly simplified if the accumulation of 10be is at a steady state with its export through erosion in this case a surface sample allows for the calculation of an erosion rate explored further this approach allows calculation of catchment wide erosion rates from river sediment similar to the approach using 10be produced in situ in contrast to the in situ 10be approach however these analyses can be performed on any sample of fine grained material even where no quartz minerals are present therefore this technique may serve as a tool to date sediment where no other chronometer is available to track particle sources and to measure earth surface process rates in soil suspended river sediment and fine grained sedimentary deposits

using 234u 238u to assess diffusion rates of isotope tracers in ferromanganese crusts abstract ferromanganese crusts record seawater isotope history for several elements useful for paleoceanography interpreting crust isotope profiles however requires an assessment of the rate of diffusion of each element within the crust we address this issue using u and th isotope profiles coupled with concentration measurements of some trace elements whose isotopes are often measured in crusts a depth profile of 230thxs 234u for a north atlantic crust alv539 yields a growth rate of 3 05 mm myr consistent with an existing 10be 9be record but the growth rate implied by a 234u 238u profile in the same crust is 6 times faster due to diffusion of u in the crust the difference between the observed 234u 238u profile and that expected from 230thxs 234u enables the effective diffusivity of u to be calculated as 1 2 10 6 cm2 year recalculated literature data indicate that other crusts have diffusivities which range down to 4 7 10 8 cm2 yr this variation is to be expected given the range of porosity observed for ferromanganese crusts concentration measurements of various elements in the surface layer of the crust provide relative distribution coefficients for each element between seawater and crust these distribution coefficients enable the effective diffusivity of each element to be calculated by comparison with the diffusivity of u such diffusivities are assessed for crust alv539 and give a first indication of the expected preservation of isotope signals in ferromanganese crusts li os and sr have diffusivities of 2 10 4 3 10 5 and 2 10 5 cm2 year respectively and diffusion is expected to be sufficiently fast that crusts will not accurately preserve past seawater compositions th nd pb and be have diffusivities less than 10 9 cm2 year and are highly immobile in crusts a reassuring result for th and be crust chronologies and for nd and pb isotope tracing of past water masses using crusts hf has a diffusivity of 3 10 8 in alv539 fast enough to perturb but not destroy its isotope history for studies of hf in this and other crusts diffusion should therefore be considered measuring u isotope profiles provides a potential way of assessing such diffusion for hf or for other elements with relatively low distribution coefficients into ferromanganese crusts