redox dynamics in the chesapeake bay the effect on sediment water uranium exchange the effect of seasonal variations in productivity and redox dynamics on the sediment water exchange of uranium was investigated on a twelve cruise time series in the chesapeake bay the deep waters of the bay undergo seasonal anoxia in response to high primary productivity and water column stratification from late spring to early fall dissolved oxygen was used to monitor water column redox conditions and dissolved iron manganese and sulfide were measured to monitor sediment redox conditions dissolved 238u was measured in the water column and sediment porewaters to monitor water column sediment exchange uranium incorporation in bay sediments results from two distinct processes productivity dependent scavenging from the water column and redox dependent cycling of uranium between sediments and bottomwater uranium is removed from surface waters of the bay by scavenging with biodetritus during periods of high primary productivity bottomwater and sediment redox conditions determine whether this particle bound uranium is buried or released to overlying water particulate uranium is released to bottomwaters and porewaters during the degradation of biodetritus and oxidation of authigenic uranium low oxygen in bottomwaters in the summer results in minimal exchange of uranium between the sediments and bottomwater due to the stability of reduced u iv high bottomwater oxygen concentrations associated with bay turnover in the fall results in release of authigenic uranium by oxidation to the soluble vi form enrichment of uranium in fall bottomwater suggests that authigenic uranium is very labile when exposed to oxic environmental conditions this process is enhanced by physical mixing when anoxic sediments are resuspended into the oxic bottomwaters

trace element signatures of chilean upwelling sediments at 36 s abstract with this contribution we continue the discussion on trace element te cycling in chilean upwelling sediments from within and below the oxygen minimum zone omz boning et al 2005 marine geology 220 1 21 low authigenic i e non lithogenic contents of te ag cd mo re and se were interpreted to result from the distinct seasonality in productivity induced organic flux bottom water oxygenation as well as dilution by terrigenous matter while all sediments were reducing enough to become depleted in mn highest enrichments in mo cd and re occur in sediments in contact with the omz here for a meaningful discussion total contents of as bi co cr cu ni pb sb tl v u and zn were corrected for the lithogenic background by determining te al ratios from local river samples the latter showed significantly lower te al ratios than average shale crust or andesite which are classically used for background correction hence the precise assessment of the lithogenic te contribution for the calculation of authigenic te contents allows to examine the source sink relationship of individual authigenic te for chilean sediments four te groups may be distinguished i mo re u and sb likely originate from sea water and accumulate via diffusive supply across the swi ii cd cr cu ni and zn require biologically induced pre concentration within biogenic material iii ag as se tl and v show an intermediate behaviour and iv bi and pb are contributed by anthropogenic activity highest enrichments in ni cu u as and v occur in sediments in contact with the omz the distinct depletion in co along with mn indicates the overall reducing but non sulfidic character of the chilean omz the degree of sedimentary sulfate reduction seems to be the most important control on chilean and peruvian near coastal enrichment of cd mo and u

insights on geochemical cycling of u re and mo from seasonal sampling in boston harbor massachusetts usa this study examined the removal of u mo and re from seawater by sedimentary processes at a shallow water site with near saturation bottom water o2 levels 240 380 mol o2 l very high organic matter oxidation rates annually averaged rate is 880 mol c cm2 y and shallow oxygen penetration depths 4 mm or less throughout the year under these conditions u mo and re were removed rapidly to asymptotic pore water concentrations of 2 2 3 3 nmol kg u 7 13 nmol kg mo and 11 14 pmol kg re the depth order in which the three metals were removed determined by fitting a diffusion reaction model to measured profiles was re u mo model fits also suggest that the mo profiles clearly showed the presence of a near interface layer in which mo was added to pore waters by remineralization of a solid phase the importance of this solid phase source of pore water mo increased from january to october as the organic matter oxidation rate increased bottom water o2 decreased and the o2 penetration depth decreased experiments with in situ benthic flux chambers generally showed fluxes of u and mo into the sediments however when the overlying water o2 concentration in the chambers was allowed to drop to very low levels mn and fe were released to the overlying water along with the simultaneous release of mo and u these experiments suggest that remineralization of mn and or fe oxides may be a source of mo and perhaps u to pore waters and may complicate the accumulation of u and mo in bioturbated sediments with high organic matter oxidation rates and shallow o2 penetration depths benthic chamber experiments including the nonreactive solute tracer br indicated that sediment irrigation was very important to solute exchange at the study site the enhancement of sediment seawater exchange due to irrigation was determined for the nonreactive tracer br tco2 nh4 u and mo the comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the mo flux but less important for u the effect of these reactions on mo exchange was highly variable enhancing mo and to a lesser extent u uptake at times of relatively modest irrigation but inhibiting exchange when irrigation rates were faster these results reinforce the observation that mo can be released to and removed from pore waters via sedimentary reactions the removal rate of u and mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic u and mo in the solid phase the fluxes of u and mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date these results confirm that removal of redox sensitive metals from continental margin sediments underlying oxic bottom water is important and suggest that continental margin sediments play a key role in the marine budgets of these metals

holocene rainfall variability in southern chile a marine record of latitudinal shifts of the southern westerlies geochemical and clay mineral parameters of a high accumulation marine sediment core from the chilean continental slope 41 s provide a 7700 yr record of rainfall variability in southern chile related to the position of the southern westerlies we especially use the iron content measured with a time resolution of ca 10 yr on average of 14 caccelerator mass spectrometry dated marine sediments as a proxy for the relative input of iron poor coastal range and iron rich andean source rocks variations in this input are most likely induced by rainfall changes in the continental hinterland of the core position based on these interpretations we find a pronounced rainfall variability on multicentennial to millennial time scales superimposed on generally more arid conditions during the middle holocene 7700 to 4000 cal yr b p compared to the late holocene 4000 to present this variability and thus changes in the position of the southern westerlies are first compared to regional terrestrial paleoclimate data sets from central and southern chile in order to derive possible wider implications and forcing mechanisms of the holocene latitudinal shifts of the southern westerlies we then compare our data to ice core records from both tropical south america and coastal antarctica these records show similar bands of variability centered at ca 900 and 1500 yr comparisons of band pass filters suggest a close connection of shifts of the southern westerlies to changes within the tropical climate system the correlation to climate conditions in coastal antarctica shows a more complicated picture with a phase shift at the beginning of the late holocene coinciding with the onset of the modern state of el nin o southern oscillation system the presented data provide further evidence that the well known millennial scale climate variability during the last glacial continued throughout the holocene fl 2001 elsevier science b v all rights reserved

glacial interglacial variability in denitrification in the world s oceans causes and consequences the late quaternary history of water column denitrifcation of the eastern pacific margins and the arabian sea is reconstructed using sedimentary 15n measurements the 15n values in six piston cores raised from these regions show remarkably similar cyclic variations being heavy 9 10 5 during the interglacials and 2 3 lighter during the glacials this implies that denitrification in these regions decreased substantially during the glacial periods the glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone since water column denitrification in these areas accounts for about half of the fixed nitrogen loss in the modern ocean the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods because nitrate is a limiting nutrient oceanic productivity and attendant changes in co2 may therefore have been modulated on glacial interglacial timescales by variations in the oceanic no3 content

biogeochemistry of surface sediments off concepcion 36 s chile el nino vs non el nino conditions abstract we compared the signals of several water column properties upwelling intensity sea level anomaly temperature nutrients dissolved oxygen chlorophyll a and surface sediments of the continental shelf off concepcion 36 s during the 1997 1998 el nino with those of a normal year 2002 2003 we found that the primary hydrographic effect of el nino 1997 1998 was a reduction in the input of nutrient rich oxygen poor equatorial subsurface water over the shelf this affected the biology of the water column as evidenced by the reduced phytoplankton biomass surface sediment properties biogenic opal organic carbon bulk 15n observed during el nino 1997 1998 reflected a reduced export production and the sediments failed to show the water column seasonality that occurs under normal conditions in addition weakened denitrification and or upper water column fertilization could be inferred from the sedimentary 15n although diminished export production was preserved in the surface sediments revealing less degraded organic matter in the upwelling period of the el nino year than in the normal year we suggest that the fresher organic material on the seafloor was probably associated with a severe reduction in the polychaete parapronospio pinnata which is considered to be the most important metazoan remineralizer of organic carbon at the sediment water interface in the study area

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

petrology and geochemistry of lavas from antuco volcano a basaltic volcano of the southern andes 37 25 s sierra velluda plio pleistocene cerro condor pleistocene and antuco holocene volcanoes form a volcanic group located in the southern andes at latitude 37 25 s lavas from these volcanoes become less silicic with decreasing age antuco volcano consists of two units pre and post caldera collapse antuco i and antuco ii and a parasitic cone antuco lavas are basalts and those from antuco i and ii are relatively uniform in mineral and chemical composition common phenocrysts are plagioclase an65 an81 18 33 vol olivine fo 78 3 18 vol and clinopyroxene 0 5 3 vol the same phenocrysts occur in the parasitic cone lavas but plagioclase is much more abundant key compositional features of antuco lavas are 52 sio2 17 al2o3 3 3 4 2 na2o 1 2 tio2 0 9 k2o and mg mg fe ratios 0 60 these major element features characterize many circumpacific basalts in island arc and continental margin environments the antuco i and ii lavas have light ree abundances equal to 26 33 chondrites and heavy ree abundances equal to 8 9 chondrites some samples have positive 10 eu anomalies but no negative eu anomalies occur in these lavas trace element abundances in the antuco lavas are similar to those in theoretical liquids derived by 5 10 partial melting of garnet free peridotite followed by 10 fractionation of a clinopyroxene 40 and olivine 60 cumulate thus a plausible model is that the lavas were derived from the mantle by partial melting followed by mafic mineral fractionation

deglacial pattern of circulation and marine productivity in the upwelling region off central south chile abstract a high resolution sea surface temperature and paleoproductivity reconstruction on a sedimentary record collected at 36 s off central south chile geob 7165 1 36 33 s 73 40 w 797 m water depth core length 750 cm indicates that paleoceanographic conditions changed abruptly between 18 and 17 ka comparative analysis of several cores along the chilean continental margin 30 41 s suggests that the onset and the pattern of deglacial warming was not uniform off central south chile due to the progressive southward migration of the southern westerlies and local variations in upwelling marine productivity augmented rather abruptly at 13 14 ka well after the oceanographic changes we suggest that the late deglacial increase in paleoproductivity off central south chile reflects the onset of an active upwelling system bringing nutrient rich oxygen poor equatorial subsurface water to the euphotic zone and a relatively higher nutrient load of the antarctic circumpolar current during the last glacial maximum when the southern westerlies were located further north productivity off central south chile in contrast to off northern chile was reduced due to direct onshore blowing winds that prevented coastal upwelling and export production

what do we learn from peaks of uranium and of manganese in deep sea sediments abstract uranium and manganese are two redox sensitive trace elements with complementary behavior under anoxic conditions u is immobile while reduced mn is dissolved in pore water of sediments uranium diffuses from sea water into reducing sediments and is enriched there the depth of the reducing layers and the sediment accumulation rate determine the enrichment of authigenic uranium on the contrary mn is dissolved in the reducing sediment sections diffuses in pore water and forms peaks of mn oxide as soon as oxygen is available thus the profiles of both mn and authigenic u record the depth of the redoxcline during periods of oxygen limitation in the sediments estimates of the depth of incorporation of uranium during the last glacial were derived for several cores at localities in the atlantic numerical modeling of the redoxcline as a function of the oxygen content of the water column and of the flux of organic matter to the sediments constrains these values during periods of extreme shallow redoxcline depths burn down of organic matter and deepening of the redoxcline during the holocene may have significantly altered the organic matter and uranium signals of glacial sediments at present day conditions sediments accumulating at rates smaller than 2 cm ky 1 may have lost the u signals peaks of manganese are then the only reliable record for a less ventilated sediment column in the past

climatic changes vs catastrophic events in lacustrine systems a geochemical approach abstract a geochemical study of oligotrophic lake sanabria nw iberian peninsula allows us to distinguish the geochemical response of a lacustrine system to catastrophic events and climatic events the main major elements analysed were k na mg ca ti al fe mn p and s the organic matter content in the sediment of lake sanabria is determined by the rainfall regime of the region periods when sediments were enriched in ti mg k and al are indicative of relatively wet paleoenvironmental conditions whereas intervals when sediments were enriched in s and p associated with organic matter are indicative of dry conditions a factor analysis of the geochemical data shows a progressive temporal displacements from organic rich conditions relatively high values of s and p to inorganic rich conditions relatively high values of ti mg k al and na maximal inorganic content occurred at the beginning of two discrete sedimentary levels the lol2 cal 1160 1230 yr ad and the lol1 cal 1510 1680 yr ad horizons the lol2 and lol1 horizons constitute two discrete events related to climatic changes during the beginning and end of the little ice age respectively in contrast the geochemical analysis provides evidence that the samples associated with the 1959 catastrophic event that occurred in the catchment area of lake sanabria do not follow the long term geochemical evolution paths of the climate driven system

preservation of particulate non lithogenic uranium in marine sediments particulate non lithogenic uranium pnu excess u above detrital background levels found in marine particulate matter is formed in surface waters throughout the ocean previous studies have shown that pnu is regenerated completely prior to burial of particles in sediments within well oxygenated open ocean regions however the fate of pnu has never been examined in ocean margin regions or in anoxic basins here we evaluate the preservation of pnu in ocean margin sediments and within semi enclosed basins using samples from sediment traps deployed at multiple depths and surface sediments organic carbon fluxes at the sediment trap locations ranged from 0 1 to 4 3 g cm 2 kyr while the dissolved oxygen concentration in the water column ranged from 3 mt o 270 m preservation of pnu increases with decreasing dissolved oxygen concentration approaching 100 preservation at oxygen concentration 25 m pnu contributes as much as 40 to 70 of the total authigenic u in sediments in the santa barbara basin and seasonally anoxic saanich inlet and some 10 to 50 of the total authigenic u in sediments off the central california margin copyright 2002 elsevier science ltd

diagenesis of oxyanions v u re and mo in pore waters and sediments from a continental margin this research tests the hypothesis that trace metals respond to the extent of reducing conditions in a predictable way we describe pore water and sediment measurements of iron fe manganese mn vanadium v uranium u rhenium re and molybdenum mo along a transect off washington state usa sediments become less reducing away from the continent and the stations have a range of oxygen penetration depths depth to unmeasurable o2 concentration varying from a few millimeters to five centi meters when oxygen penetrates 1 cm or less fe is reduced in the pore waters but reoxidized near the sediment water interface preventing a flux of fe 2 to overlying waters whereas mn oxides are reduced and mn 2 diffuses to overlying waters both re and u authigenically accumulate in sediments only at the most reducing location where the oxygen penetrates 0 3 cm below the sediment water interface does the surface 30 cm of sediments become reducing enough to authigenically accumulate mo stations in close proximity to the juan de fuca ridge crest are enriched in mn and fe from hydrothermal plume processes both v and mo clearly associate with mn cycling whereas u may be associating with either mn oxides and or fe oxyhydroxides rhenium is uncomplicated by adsorption to mn oxides and or fe oxyhydroxides and re accumulation in sediments appears to be due solely to the extent of reducing conditions therefore authigenic sediment re enrichment appears to be the best indicator for intermediate reducing conditions where oxygen penetrates less than 1 cm below the sediment water interface when coupled with negligible authigenic mo enrichment copyright 2005 elsevier ltd 1 introduction oceanic and atmospheric carbon cycles are strongly coupled with the ocean reservoir of dissolved inorganic carbon being 60 times larger than that of the atmosphere broecker and peng 1987 models that couple both ocean and atmosphere have been used to investigate why atmospheric carbon dioxide pco2 atm was reduced during the last glacial age and thus may be useful in predicting the environmental impact of future changes in pco2 atm some of these models require a large increase in organic carbon export through the more efficient use of nutrients to predict the lower pco2 atm observed during

geochemical characteristics of chilean upwelling sediments at 36 s abstract we examined the cycling of c s fe and the trace elements te ag cd mo mn re and se in chilean upwelling sediments from water depths of 127 2744 m at 36 s the sediments are clay rich with moderate total organic carbon toc and low biogenic caco3 contents for the shallow site within oxygen minimum zone omz which shows some thioploca biomass highest observed toc contents correspond to highest productivity induced toc flux and or enhanced preservation under o2 deficient conditions pyrite formation may be restricted by the concurrent presence of elemental s in thioploca and or the availability of reactive iron in the other sediments 370 m water depth pyrite formation is strongly restricted due to the low availability of metabolizable toc for sulfate reduction calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than toc was remineralized by sulfate reduction than expected from redfield c n relationship manganese is significantly depleted due to reductive dissolution of mn hydr oxides within all sediments mo is only significantly enriched vs the lithogenic background in the shallow core implying anoxic conditions contents of re and mo as well as re mo ratios indicate suboxic conditions for the other cores cd and re contents decrease with increasing water depth reflecting decreasing reducing conditions in contrast authigenic ag contents increase with water depth suggesting enrichment controlled by seawater availability and or opaline input and not redox conditions elevated se contents in all cores may be either due to bio detrital input or diffusion from seawater accumulation rates of toc ts and te off chile are in the same order as those from the gulf of california and lower than those off peru distinct seasonality in upwelling and dilution of chilean sediments with terrigenous material lead to rather low metabolizable toc contents and lower te trapping capacity

evaluation of sediment trace metal records as paleoproductivity and paleoxygenation proxies in the upwelling center off concepcion chile 36 s a b s t r a c t this study analyzes the records of several trace metals sensitive to redox conditions in continental shelf sediments off concepcion chile 36 s the continental margin off concepcion 36 s 73 w lies beneath an important upwelling center characterized by high primary production rates and consequently high fluxes of organic matter in spring and summer this material settles to the seafloor where it decays pro ducing periods of very low oxygen content in bottom waters 1 ml l y 1 the goals of this study were to determine the input of trace metals to the sediment and to decipher how this information can be used to reveal variations in primary productivity or bottom oxygenation gravity cores collected at two stations vg06 2 over the mid shelf station 88 m water depth upper boundary of the oxygen minimum zone and vg06 3 over the outer shelf 120 m water depth within the oxygen minimum zone were sampled for high resolution profiles 1 cm of trace metals biogenic opal stable isotopes and total organic carbon the results suggest that the variability in the trace metal distribution on the continental shelf off concepcion is determined by redox conditions and the organic carbon flux to the bottom some sections of the sediment cores from the outer shelf showed appreciable authigenic enrichment of u cd and mo ef 5 10 2 5 and 10 16 respectively along with heavier values of d 15

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

surface sediment distribution along the chilean continental slope related to upwelling and productivity abstract the coastal upwelling system of the peru chile current belongs to the most productive regions in the world oceans in spite of this remarkable fact only very little is known about the sediment distribution in its southern part off the coast of chile to increase the knowledge about this region a multiparameter study of the surface sediment distribution along the chilean continental slope between 27 and 42 s has been carried out detailed analyses of sedimentary data toc and carbonate contents 13corg 15n of the species composition of planktic foraminifera and of the stable isotope composition of the two planktic foraminifera species neogloboquadrina pachyderma dex and globigerina bulloides reveal a close relation to environmental conditions in the region the pattern of a southward increase of surface water productivity known from satellite data is closely reflected in the organic carbon and 15n data information about the water mass distribution especially about varying influences of upwelling waters and warmer subtropical surface waters can be derived from the species composition of the planktic foraminifera while the stable isotope data provide additional clues about water mass characteristics high productivity along the chilean margin north of 40 s is sustained by coastal upwelling while further to the south advection of nutrients with the antarctic circumpolar current and a significant riverine input fuel the highest productivity found in the study area this data set will provide the base for future paleoclimatic and paleoceanographic reconstructions of the southeast pacific region

upwelling and the distribution of chlorophyll a within the bay of concepcion chile abstract the influence of upwelling on the distribution of chlorophyll a within the bay of concepcion chile is discussed in light of continuous measurements of surface in vivo chlorophyll fluorescence and temperature taken simultaneously along horizontal transects and hydrographic stations data results suggest significant temporal variability both in the distribution of in vivo fluorescence temperature and salinity within the bay and in the characteristics of the exchange between the bay and the adjacent shelf waters induced by variable upwelling upwelling is produced by the predominant south westerly winds during the summer significant variations in the wind direction occur with periods from two to seven days during active upwelling exchange is characterized by a surface outflow through the mouth of the bay and an inflow at depth low chlorophyll fluorescence is confined to the upwelling areas on the eastern shore either within or outside the bay high chlorophyll fluorescence is confined to the central and western bay density data suggest a three layered circulation pattern at the mouth of the bay during the upwelling relaxation involving an inflow both at the surface and bottom and outflow at mid depth associated with this exchange is an active high chlorophyll transport from the bay to the adjacent coastal waters at mid depth and inflow of low chlorophyll water from the adjacent shelf at the surface and near the bottom

burial of redox sensitive metals and organic matter in the equatorial indian ocean linked to precession authigenic metals uranium cadmium and molybdenum organic carbon oc and total c37 alkenone totc37 concentrations were measured for the last 350 kyr in core md900963 located in the eastern equatorial arabian sea authigenic metal concentrations on a carbonate free basis range between 1 and 17 ppm 0 5 and 6 ppm and 0 5 and 4 ppm for u cd and mo respectively the profiles are characterized by well defined 23 kyr cycles between oxic and mildly suboxic conditions the redox sensitive metal profiles also follow variations in the concentrations of oc 0 2 0 9 and alkenones 0 2 6 7 ppm the coupled variations in inorganic and organic constituents are attributed to a 23 kyr cycle in primary production above site md900963 as suggested by clear correlations with independent micropaleontologic proxies primary productivity indices based on foraminifera and coccoliths and fragmentation of foraminiferal shells the 23 kyr cycles do appear to be primarily driven by productivity rather than changes in bottom water oxygen comparison with other records indicates that if this interpretation is correct productivity variations across much of the indian ocean have been dominated by precessional forcing with high productivity in phase with low summer insolation in the northern hemisphere this interpretation contrasts with the traditional attribu tion of enhanced productivity in the indian ocean with periods of high summer insolation copyright 2002 elsevier science ltd

pitfalls of normalization of marine geochemical data using a common divisor abstract normalization of trace element contents of sediment through division by the content of an immobile element usually aluminum is common practice in marine geochemical studies this might appear to be a simple way of correcting for dilution by sedimentary phases barren of a particular trace element for instance carbonates or silica and for comparison with its content in a standard clay or shale the purpose of this paper is to revive the awareness of the pitfalls of this practice by giving numerical examples of the often unexpected results of normalization taking a statistical perspective it is first shown that uncorrelated variables acquire spurious correlations when normalized but normalization can also increase decrease change sign of or even blur the correlations between unmodified variables next a number of realistic scenarios are worked out to show that the correlations between normalized element contents still suffer from the closure effect only in a few simple cases it is possible to extract from the normalized data realistic estimates of trace element contents in distinct sedimentary phases such as organic matter this is ascertained from data on copper contents in sediment from the black sea and arabian sea when the coefficient of variation i e standard deviation divided by the mean of the aluminum data is relatively low and much lower than for the original values of the trace elements comparisons of correlations can equally well or even better be made on the basis of the unmodified values then the comparison of trace element data with their values for standard shales is discussed the inherent problem is that the composition of the commonly used standard shale and consequently the reference values of normalized elements are not necessarily representative of the local or regional sediments in the study area also this method falls short of identifying the processes responsible for enrichment or diminution in trace element contents finally consideration is given to some aspects of a proper use of al normalization

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

patterns of organic carbon and nitrogen isotopic compositions of latest pliocene sapropels from six locations across the mediterranean sea abstract inter layered in the organic poor sediments that comprise most of the sedimentary record of the mediterranean sea are organic rich sapropels that represent periods of wetter climate increased primary productivity and suboxic to anoxic conditions at depth in order to explore processes important in the spatial and temporal variability of sapropel forming conditions we selected sedimentary sequences containing two latest pliocene sapropels at odp sites 974 and 975 in the western basin the vrica plio pleistocene type section and odp sites 964 967 and 969 in the eastern basin our main tools are nitrogen and organic carbon isotopes which are diagnostic of changes in nutrient origins and availability and in primary productivity extremely low 15n values to 5 1 indicate high rates of nitrogen fixation during sapropel formation stimulated by nearly complete consumption of nitrate in the water column due to exceptionally high rates of denitrification interruptions in sapropels represent brief intervals of low productivity and diminished nitrogen fixation the geographical distribution of the interruptions and the presence of spatial patterns in toc contents corg ntot ratios and 15n values suggest a central role of the eastern basin in initiating sapropel formation the eastern mediterranean evidently experienced higher rates of primary productivity during sapropel deposition and probably more intense dysoxic conditions in the water column organic carbon isotopes present an intriguing geographic trend across the eastern mediterranean that is probably related to non biological processes including variations in water temperature salinity and amounts of local fluvial runoff

on the influence of topography in the induction of coastal upwelling along the chilean coast the contribution of the winds and the topography in the induction of upwelling along the chilean coastal zone is calculated the topographic induced upwelling is estimated taking into account the effect of the meridional changes of the coastline orientation the wind induced upwelling was calculated for the austral summer using nscat scatterometer data resulting a total amount of 2 1 106 m s 1 of water transported offshore for the whole chilean upwelling zone it is shown that the topographic effect dominates the wind induced upwelling in several parts of the chilean coast particularly at 23 s antofagasta 25 5 s 30 s coquimbo 33 s valparaiso and 37 s concepcion antofagasta coquimbo valparaiso and concepcion are well known zones of intense upwelling the results show that the topography plays a key role in the upwelling of the chilean coast specially in its northern part

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

upwelling and productivity along the peru chile current derived from faunal and isotopic compositions of planktic foraminifera in surface sediments abstract we report on the spatial distribution of isotopic compositions of the two planktic foraminifera species globigerina bulloides and neogloboquadrina pachyderma dex and the faunal assemblages of planktic foraminifera in 91 surface sediment samples along the chilean continental slope between 23 s and 44 s both 13c and 18o data of n pachyderma dex show little variability in the study area north of 39 s the isotopic values of n pachyderma dex are heavier than those of g bulloides whereas south of 39 s this relation inverses this is indicative for a change from a well mixed deep thermocline caused by coastal upwelling north of 39 s to well stratified water masses in a non upwelling environment south of 39 s in addition the faunal composition of planktic foraminifera marks this change by transition from an upwelling assemblage north of 39 s to a high nutrient non upwelling assemblage south of 39 s which is characterized by decreased contributions of upwelling indicators such as g bulloides n pachyderma sin and globigerinita glutinata in general we can conclude that food and light rather than temperature are the primary control of the planktic foraminiferal assemblage between 23 s and 44 s off chile our data point to higher marine productivity at upwelling centers north of 25 s and at 30 33 s south of 39 s significant supply of nutrients by fluvial input most likely boosts the productivity

settling barium fluxes in the arabian sea critical evaluation of relationship with export production abstract time series measurement of biogenic and barium fluxes was made using nine sediment traps deployed in the western central and eastern part of the arabian sea with the objective of evaluating barium as a proxy for surface ocean productivity our observations show a strong linear correlation between baexcess fluxes and biogenic opal and organic carbon corg fluxes indicating a biogenic origin of particulate ba however the correlation between biogenic and baexcess fluxes is remarkably strong in areas of uniform productivity like the central arabian sea when compared to regions of episodic productivity the processes that precipitate particulate ba appear to be less active during periods of high biogenic flux a large discrepancy is documented between the export flux calculated from the barium based algorithm francois r honjo s manganini s j ravizza g e 1995 biogenic barium fluxes to the deep sea implications for paleoproductivity reconstruction global biogeochemical cycles 9 289 303 and from corg fluxes sarnthein m winn k duplessy j c fontugne m r 1988 global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years paleoceanography 3 361 399 at different depths however 35 increase in ba fluxes and a concomitant increase in ba corg ratios are documented between 919 and 2002 m water depth in the western arabian sea this increase may be due either to barite formation within the fecal pellets of mesozooplankton thriving at deeper levels or to scavenging of ba by mn oxyhydroxides manganese oxyhydroxides seem to act as a prominent scavenging phase for particulate ba when the mnexcess content in the settling particles exceeds 200 ppm especially in the western and central arabian sea where in situ precipitation of mn oxides is reported the estimated preservation efficiency of ba in the arabian sea sediments ranges between 50 and 61 this is two fold higher than the global average suggesting that ba may be a promising proxy for paleoproductivity estimation

temporal variability of 210pb fluxes and bioturbation in shelf sediments beneath the high primary production area off concepcion central southern chile 36 s abstract we estimated monthly fluxes of 210 pb in shelf sediments beneath a high productivity area off central southern chile 36 s during 1 year september 2002 august 2003 sediment cores were obtained using a multiple corer and were analyzed mainly for 210 pb total pigments and macrofauna abundance the 210 pb inventories and fluxes were estimated for surface sediments 0 5 cm and bioturbation coefficients were inferred using chlorophyll a reactive profiles in general 210 pb content was inversely correlated with phytodetritus fluxes high photosynthetic pigment contents in surface sediments were consistently associated with lower 210 pb contents macrofaunal activity responded to oxygen and organic matter supplies at the sediment surface generally concentrated in the first centimeters but particularly so during months of high organic matter fluxes and deficient bottom water oxygen conditions at this study site several processes involved in the 210 pb surface distribution make it difficult to accurately estimate ages at the surface we postulate that the organic fluxes promote changes in the faunal activity which in combination with sediment resuspension and water circulation over the shelf produce seasonal variations in the 210 pb inventories

sedimentary trace element records over the last 200 kyr from within and below the northern arabian sea oxygen minimum zone two piston cores pcs containing two full glacial cycles were retrieved at murray ridge one core pc463 contains sediment deposited at 920 mbsl within the other pc464 has sediment deposited at 1470 mbsl below the current oxygen minimum zone omz o2 2 m variations in the organic carbon oc content correlate well between the two cores high oc contents correspond with periods of high insolation and are accompanied by a stable and intensive omz the lithogenic fractions are similar and have a rather constant composition the v mn co cu se zn and sb contents in these fractions are in fair agreement with the values in standard shales ss the nicontent of the lithogenic fraction ishigherthanin ssdue to its high content in the regional eolian input andthe mo re andu contents are higher as a result ofsyngenetic and or diagenetic overprints in both cores mn and cohavea negative the other elements a positive correlation with oc four processes contributing to sedimentary enrichment in trace elements can be envisaged

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

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

late quaternary variability of sedimentary nitrogen isotopes in the eastern south pacific ocean 1 we present high resolution bulk sedimentary 15n data from the southern edge of the present day oxygen minimum zone of the eastern south pacific the record is interpreted as representing changes in water column nitrogen removal during the last 70 000 years we found significant fluctuations in the isotopic signal that suggest major reorganizations of the oxygen minimum zone at millennial timescales these fluctuations were not related to other millennial scale changes like the northern hemisphere s dansgaard oeschger climate swings or local changes in primary productivity so appear to be dictated by the southern hemisphere s climate rhythm this is preliminarily corroborated by an overall agreement between our 15n data and the sedimentary proxy of ice sheet dynamics in patagonia which is in turn correlated with surface water properties at the midlatitude subduction region of the eastern south pacific intermediate waters finally potential implications on late quaternary changes in atmospheric co2 concentrations are discussed

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

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

contrasting behavior of tungsten and molybdenum in the okinawa trough the east china sea and the yellow sea by using catalytic current polarography and high resolution inductively coupled plasma mass spectrometry w and mo in seawater were determined in the okinawa trough a backarc rift and in the east china sea and the yellow sea mo was distributed conservatively throughout the study area and its salinity normalized concentration was 104 6 nm n 105 w was also uniformly distributed south of the kuroshio current 56 7 pm n 51 anomalous high concentrations of w maximum 254 pm were found in the iheya graben in the middle okinawa trough gt 1 000 m depth which were probably supplied by hydrothermal activity the concentrations of mo and w in the east china sea and the yellow sea showed linear correlation with salinity 26 lt s lt 35 the mo data can be explained by mixing of seawater and river water of the changjiang mo 10 nm qu et al 1993 however the values of w extrapolated to s 0 were largely different between two cruises 1 200 pm in may june 1987 and 540 pm in june 1994 and much higher than the reported concentrations of 160 pm for world rivers by turekian 1969 and 30 pm for unpolluted japanesemore rivers by sohrin et al 1989 moreover significantly high w was observed in the bottom water at stations near the changjiang river estuary and the western yellow sea while these data may suggest that w is released from the anoxic sediments of the continental shelf more data are needed to elucidate the mechanism controlling the distribution of w less

uppermost campanian maestrichtian strontium isotopic biostratigraphic and sequence stratigraphic framework of the new jersey coastal plain firm stratigraphic correlations are needed to evaluate the global significance of unconformity bounded units sequences we correlate the well developed uppermost campanian and maestrichtian sequences of the new jersey coastal plain to the geomagnetic polarity time scale gpts by integrating sr isotopic stratigraphy and biostratigraphy to do this we developed a maestrichtian ca 73 65 ma sr isotopic reference section at deep sea drilling project hole 525a in the southeastern atlantic ocean maestrichtian strata can then be dated by measuring their 87 sr 86 sr composition calibrating to the gpts of s c cande and d v kent 1993 personal commun and using the equation age ma 37 326 894 52 639 89 87 sr 86 sr sr stratigraphic resolution for the maestrichtian is estimated as 1 2 to 2 m y at least two unconformity bounded units comprise the uppermost campanian to maestrichtian strata in new jersey the lower one the marshalltown sequence is assigned to calcareous nannofossil zones cc20 21 nc19 and cc22b nc20 it ranges in age from 74 1 to 69 9 ma based on sr isotope age estimates the overlying navesink sequence is assigned to calcareous nannoplankton zones cc25 26 nc21 23 it ranges in age from 69 3 to 65 ma based on sr isotope age estimates the upper part of this sequence the tinton formation has no calcareous planktonic control sr isotopes provide an age estimate of 66 1 2 ma latest maestrichtian sequence boundaries at the base and the top of the marshalltown sequence match boundaries elsewhere in the atlantic coastal plain owens and gohn 1985 and the inferred global sea level record of haq et al 1987 they support eustatic changes as the mechanism controlling depositional history of this sequence however the latest maestrichtian record in new jersey does not agree with haq et al 1987 we attribute this to correlation and time scale differences near the cretaceous paleogene boundary high sedimentation rates in the latest maestrichtian of new jersey shrewsbury member of the red bank formation and the tinton formation suggest tectonic uplift and or rapid progradation during deposition of the highstand systems tract

a model for radionuclides in sediments influenced by mixing and compaction a model for the activity of radionuclides in sediments has been set up based on the one dimensional advection diffusion equation the model considers sedimentation mixing and compaction observed distributions of deposit feeding organisms and the nature of physical disturbances indicate that a gaussian diffusion coefficient may be appropriate two layer models with constant and gaussian mixing respectively in the upper layer are compared with measured pb 210 profiles for two cores from green bay lake michigan and it is found that the maximal diffusion coefficient d0 in the gaussian model is considerably greater than the constant d value e g d0 4 versus d 0 2 cm2 year if compaction effects are neglected the solution for the gaussian model is based on an approximate power series method compaction can be taken into account by using an exponential sedimentation rate or bulk sediment density by comparing with measured data for a lake ontario core it is seen that compaction alone can give an appreciable flattening of the activity curves near the top layers

terrigenous sediment supply along the chilean continental margin modern regional patterns of texture and composition the regional patterns of texture and composition of modern continental slope and pelagic sediments off chile between 25 s and 43 s reflect the latitudinal segmentation of geological morphological and climatic features of the continental hinterland grain size characteristics are controlled by the grain size of source rocks the weathering regime and mode of sediment input eolian off northern chile vs fluvial further south bulk mineral assemblages reveal a low grade of maturity regional variations are governed by the source rock composition of the different geological terranes and the relative source rock contribution of the coastal range and andes as controlled by the continental hydrology the relative abundance of clay minerals is also predominantly influenced by the source rock composition and partly by continental smectite neoformation latitudinal variations of illite crystallinities along the chilean continental slope and west of the peru chile trench clearly reflect modifications of the weathering regime which correspond to the strong climatic zonation of chile

comparative geochemistry of cadmium rhenium uranium and molybdenum in continental margin sediments the concentrations of authigenic phases of cd re u and mo increase with depth in four 45 cm long sediment box cores collected along the axis of the laurentian trough gulf of st lawrence average authigenic accumulation rates estimated from element inventories are similar to rates in other continental margin environments strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of cd and re this suggests that slow precipitation kinetics controls the accumulation of cd and re in these sediments the accumulation rate of authigenic u is more variable it may be tied to the kinetics of microbially mediated u reduction and be controlled by the availability of reactive organic matter authigenic mo is distinguished by a sharp subsurface concentration minimum above which mo cycles with manganese mo released to pore water upon reduction of mn oxides diffuses downward and enriches the subsurface sediment mo accumulates most rapidly in the sediment with the highest sulfide content slow conversion of molybdate to thiomolybdate may explain the much slower mo accumulation rate in the less sulfidic sediments a component of authigenic mo accumulates with pyrite in an approximately constant mo fe ratio the accumulation rate of pyrite and associated mo is insensitive to avs abundance pyrite formation may be limited by the reactivity of iron oxide minerals

sulfate reduction and methanogenesis in a thioploca dominated sediment off the coast of chile continental shelf sediments of the central chile upwelling area are dominated by the presence of dense mats of the filamentous sulfur depositing bacterium thioploca spp we examined rates and pathways of s and methane cycling in these sediments along a transect from the bay of concepcion to the continental slope sulfate reduction rates 170 4670 nmol cm 3 d 1 were equal to or exceeded rates reported for other subtidal marine sediments elemental s and pyrite were the dominant end products of sulfate reduction in thioploca mats on the continental shelf whereas in the highly reducing beggiatoa dominated sediments of the nearby bay of concepcion acid volatile s was the principal end product dissolved organic c values were lowest at the stations with the highest sulfate reduction rates and increased offshore sediment porewater methane concentrations in all surface sediments were low 12 nmol cm 3 and methane production rates at the station most dominated by thioploca were extremely low 0 5 nmol cm 3 d 1 low methane production rates and concentrations were matched by low methane oxidation rates 0 1 nmol cm 3 d 1 radio tracer studies showed that methane production was almost exclusively from methylamines substrates which are noncompetitive with sulfate reduction rather than from acetate or co2 h2 bacterial mpn most probable number counts also indicated the presence of a methylotrophic population of methanogens surprisingly high numbers of autotrophic acetogenic bacteria were found suggesting that the bacterial population involved in anaerobic doc degradation is more complex than expected in spite of the high sulfate reduction rates sulfide concentrations in the shelf and slope were low or undetectable 0 5 m and sulfate concentrations were never depleted below bottom water levels down to depths of 25 30 cm calculations suggest that thioploca were oxidizing a maximum of 35 of sulfide production not enough to prevent sulfate depletion either other sulfide oxidizers were also present or transient hydrodynamic conditions coupled with bioturbation resulted in oxidation of the sediments