myrmekitic intergrowth of tourmaline and quartz in eclogite hosting gneisses of the tso morari ultrahigh pressure metamorphic terrane eastern ladakh india a possible record of high pressure conditions

iron and manganese diagenesis in deep sea volcanogenic sediments and the origins of pore water colloids abstract volcanogenic sediments are typically rich in fe and mn bearing minerals that undergo substantial alteration during early marine diagenesis however their impact on the global biogeochemical cycling of fe and mn has not been widely addressed this study compares the near surface 0 20 cm below sea floor cmbsf aqueous no 3 and solid phase reactive fe distributions between two volcanogenic sediment settings 1 a deep sea tephra rich deposit neighbouring the volcanically active island of montserrat and 2 mixed biosiliceous volcanogenic sediments from abyssal depths near the volcanically inactive crozet islands archipelago shallow penetration of o2 aq into montserrat sediments was observed 5 cmbsf and largely controlled by the oxidation of organic matter dissolved fe and mn distributions in montserrat pore waters were lowest in the surface oxic layer 0 3 m fe 32 m mn with maxima 20 m fe 200 m mn in the upper 1 15 cmbsf unlike montserrat fe and mn in crozet pore waters were ubiquitously partitioned between 0 2 m and 0 02 m filtrations indicating that the pore water distributions of fe and mn in the traditionally termed dissolved size fraction are dominated by colloids with respective mean abundances of 80 and 61 plausible mechanisms for the origin and composition of pore water colloids are discussed and include prolonged exposure of crozet surface sediments to early diagenesis compared to montserrat favouring nano particulate goethite formation and the elevated dissolved si concentrations which are shown to encourage fine grained smectite formation in addition organic matter may stabilise authigenic fe and mn in the crozet pore waters we conclude that volcanogenic sediment diagenesis leads to a flux of colloidal material to the overlying bottom water which may impact significantly on deep ocean biogeochemistry diffusive flux estimates from montserrat suggest that diagenesis within tephra deposits of active island volcanism may also be an important source of dissolved mn to the bottom waters and therefore a source for the widespread hydrogenous mnox deposits found in the caribbean region

sediment geochemical evidence for an early middle gilbert early pliocene productivity peak in the north pacific red clay province abstract current attempts to understand climatic variability during the early to middle pliocene require paleoceanographic information from the pacific and indian oceans that may serve to test and or constrain future circulation models ocean drilling program odp sites 885 886 are located in the central subarctic north pacific at water depths exceeding 5700 m recent studies of rock magnetic properties suggest that the fine grained fe oxide component in sediment at sites 885 886 experienced reductive dissolution during the early middle gilbert because such an interval in the north pacific red clay province suggests a maximum in the sedimentary flux of organic carbon and or a minimum in bottom water dissolved o 2 concentrations and hence a peak change in north pacific oceanographic conditions a geochemical investigation was conducted to test the hypothesis quaternary sediment at hole 886b was subjected to an oxyhydroxide removal procedure and chemical analyses indicate that bulk sediment concentrations of fe and the fe sc ratio decrease significantly upon reductive dissolution downcore chemical analyses of untreated sediment at hole 886b demonstrate that similar depletions also occur across the proposed interval of reduced sediment downcore chemical analyses also indicate that a pronounced increase in the ba sc ratio occurs across the interval these results are consistent with an interpretation that abyssal sediment of the north pacific experienced a decrease in redox conditions during the early middle gilbert and that this change in oxidation state was related to a peak in paleoproductivity if the zenith of late miocene to middle pliocene enhanced productivity observed at other indo pacific divergence regions similarly can be constrained to the early middle gilbert there exists an oceanographic boundary condition in which to test future models concerning pliocene warmth

geochemistry and isotope systematics of small to medium volume neogene quaternary ignimbrites in the southern central andes evidence for derivation from andesitic magma sources the geochemical and isotopic compositions of small to medium volume 10 km3 50 km3 neogene quaternary ignimbrite sheets of the southern central andes 25 00 26 45 s 67 30 69 30 w have been analysed in order to assess the petrogenesis of this type of calc alkaline magma the ignimbrites are derived from the active volcanic zone around salar de la isla sis in chile and from behind the active arc in the area of the salar de antofalla saf argentina the ignimbrites in both areas are characterized by 87sr 86sr t ratios 0 7066 0 7090 that are lower and end t values 2 4 to 5 1 that are higher than in the large volume caldera sourced ignimbrites of cerro galan 100 km further east and those of ignimbrites from other megacalderas of the central andes this difference reflects smaller amounts of crustal material in the sis and saf ignimbrites rather than a change in basement composition as indicated by pb provinciality the isotope signature of the sis and saf ignimbrites is similar to that of late to post miocene composite calc alkalic andesite dacite stratovolcanoes of the frontal arc in this region this finding combined with major and trace element modelling lends support to a petrogenetic model for the sis and saf ignimbrites involving fractional crystallization from a lil depleted andesitic parental magma which was beforehand modified by crustal contamination

dust transport from patagonia to antarctica a new stratigraphic approach from the scotia sea and its implications for the last glacial cycle abstract we studied two deep sea cores from the scotia sea to reconstruct past atmospheric circulation in the southern hemisphere and to resolve a long standing debate on the interpretation of magnetic susceptibility ms records in southern ocean so sediment high sedimentation sites md07 3134 0 2 1 2 m kyr and md07 3133 0 3 2 1 m kyr cover the last 92 5 kyr and 36 kyr respectively both exhibit a one to one coupling of the ms and ca2 signal to the non sea salt nss ca2 signal of the edml ice core clearly identifying atmospheric circulation as means of distribution comparison of additional proxies also excludes major influence by volcanic sources sea ice icebergs or oceanic current transport the close resemblance of the dust proxies over the last glacial cycle in turn allows for the establishment of an age model of unprecedented resolution and precision for so deep sea sediment because atmospheric transport involves no major leads or lags this is of particular importance because ms is routinely measured on deep sea cores in the so but the sediments usually lack biogenic carbonate and therefore had only limited stratigraphic control so far southern south america ssa is the likely source of eolian material because site md07 3133 located closer to the continent has slightly higher ms values than site md07 3134 and also the ms record of patagonian site salsa shows comparable variability patagonia was the dust source for both the scotia sea and east antarctica dust fluxes were several times higher during glacial times when atmospheric circulation was either stronger or shifted in latitude sea level was lowered shelf surfaces were exposed and environmental conditions in ssa were dominated by glaciers and extended outwash plains hence ms records of so deep sea sediment are reliable tracers of atmospheric circulation allowing for chronologically constrained reconstructions of the circum antarctic paleoclimate history

magnetobiostratigraphic chronology and palaeoenvironmental history of cenozoic sequences from odp sites 1165 and 1166 prydz bay antarctica a transect of three sites was drilled during leg 188 of the ocean drilling program odp proximal to the east antarctic ice sheet eais across the prydz bay continental shelf site 1166 slope site 1167 and rise site 1165 we present results of a palaeomagnetic and rock magnetic study of sediments recovered at sites 1165 and 1166 magnetostratigraphic interpretations are presented for both holes and are mainly constrained by diatom and radiolarian biostratigraphies interpreted in the light of recent refinements to southern ocean zonal schemes and datum calibrations for these microfossil groups site 1165 records a history of sedimentation on the continental rise extending back to earliest miocene times about 22 ma several long term changes characterise this record including an overall trend of decreasing sedimentation rates from the bottom to the top of the hole there is a progressive decrease in the sedimentation rate above about 308 mbsf meters below sea floor which is marked by a transition from dark grey fissile claystones to greenish grey diatom bearing clays at this transition ice rafted debris sand grains and total clay content also increase the chronology presented here indicates a middle miocene age 14 3 ma for the lithological transition correlation to odp hole 747a from the kerguelen plateau suggests that this lithological transition coincides with the base of the mi 3 3a 18o event which suggests palaeoclimatic control on middle miocene sedimentation changes at site 1165 core recovery was poor at site 1166 consequently the magnetostratigraphic data are of limited value the deepest cores recovered at site 1166 record brief intervals in the early history of the eais for the prydz bay region extending back through the early stage of glaciation to pre glacial times an early cretaceous fluvio lacustrine unit lagoonal deposits and sandy fluvio deltaic units of mid late eocene age contain a sporadic record of the transition from humid and mild conditions to cool temperate conditions

paleomagnetism of lake van sediments chronology and paleoenvironment since 350 ka abstract in the framework of the paleovan project a high resolution paleomagnetic and rock magnetic study was carried out on a 149 m sedimentary sequence recovered from ahlat ridge in the deepest part of the lake van eastern turkey icdp site 5034 2 high sedimentation rates average 30 cm ka allow high resolution measurements of natural remanent magnetization nrm magnetic susceptibility anhysteretic remanence arm and hence of anhysteretic susceptibility arm over the last 350 ka the carrier of the remanence is detrital titanomagnetite largely from volcanic sources based on the similarity of magnetite grain size of tephra layers and the other lacustrine lithologies observed in a plot of the versus arm bulk magnetic parameters often covary with paleoclimatic signals in the lake van sediments a correlation exists between variations of and arm intensity and glacial interglacial marine isotopic stages as well as dust flux and temperatures observed in ice cores from greenland and antarctica the quality of the paleomagnetic record is compromised by weak nrm intensities as well as by the presence of tephra and turbidites throughout the sequence nonetheless a correlation is observed between the relative paleointensity rpi record based on nrm arm and the calibrated piso rpi stack that supports the independently derived age model for the site

large subduction earthquakes along the fossil moho in alpine corsica subduction earthquakes release vast amounts of energy to crust and mantle lithosphere the products of such drastic events are rarely observed in the field because they are mostly lost by subduction we present new observations of deformation products formed by a few very large and numerous small intermediate depth alpine subduction earthquakes that are preserved along the exhumed gabbro mantle peridotite contact of the piemont liguria oceanic basin in corsica the abrupt release of energy resulted in shear heating events that completely melted both gabbro and peridotite the large volumes of melt that were generated can be studied in the fault and injection vein breccia complex along the fault zone the energy required for wholesale melting of a large volume of peridotite along the fault combined with previous estimates of stress drops show that very large earthquakes took place along the moho of the subducting plate because these fault rocks formed by intraplate seismicity we suggest by analogy with present day subduction that they represent a proxy for the lower seismogenic zone

magnetization carriers and remagnetization of bedded chert abstract article history received 22 october 2010received in revised form 11 january 2011accepted 25 february 2011editor p demenocalkeywords bedded chertremanencebiogenic magnetitehematite bedded chert is the only sediment type representative of the paleozoic to early mesozoic pelagic marineenvironment becauseoftheirassociationwithophiolitesandislandarcrocks presenceofdatablemicrofossilsandpaleohorizontalreferenceprovidedbywell developedbeddingsurfaces beddedchertsequencesareoftentargeted for paleomagnetic tectonic studies however processes of magnetization acquisition in biosiliceoussediments and consequently the signi cance of their magnetic record are not well understood our rockmagnetic study of a triassic jurassic radiolarian chert sequence the mino terrane central japan shows thatthe ferrimagnetic assemblage of the gray chert units is of detrital origin while the red chert s assemblage isdominated by authigenic phases pigmentary hematite and biogenic magnetite which contribute to thenatural remanent magnetization the presence of magnetofossils places red oxic chert in the category ofprospective environmental archives magnetite producing magnetotactic bacteria were apparently able totolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore waters during the deposition of red chert layers a strong uniaxial anisotropy due to chain alignment of thebiogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscousmagnetization by chert even at low metamorphic temperatures 2011 elsevier b v all rights reserved

integrated stratigraphy and palaeoecology of the lower and middle miocene of the porcupine basin a high resolution palynological analysis and a detailed palaeomagnetic study of a marine sequence recovered during iodp expedition leg 307 in the porcupine basin southwest of ireland provide new insights into the regional depositional history and palaeoenvironmental evolution during early neogene times the hole 1318b studied was drilled on the upper slope of the continental margin in a water depth of 409 m upslope from a province of carbonate mounds the belgica mound province the diverse and well preserved dinoflagellate cyst associations consist typically of deep neritic and oceanic species mixed with a neritic component transported from the shelf reflecting the deep depositional setting at the continental margin the palaeomagnetic record together with the ranges of key dinoflagellate cyst species constrain the age of the studied sequence between 16 7 ma and 12 01 ma that is between the late burdigalian and late serravallian the distinct unconformity terminating the miocene sequence correlates to the global sequence boundary ser4 tor1 dated at 10 5 ma and represents according to previous extensive seismic studies a basin wide erosional event the overlying sediments are of middle pleistocene or younger age downslope from iodp site 1318 carbonate mounds root on the erosional surface the dinoflagellate cyst associations from the porcupine basin distinctly mirror the global cooling phase following the middle miocene climatic optimum cooling phase mi3 a short lived glaciation is particularly well expressed and here dated at 13 6 ma the palynomorph record furthermore indicates a reduction of the productivity and an increase of oceanic oligotrophic species after 14 ma suggesting a reduction or perhaps even a shutdown of the upwelling

shallow and deep crustal control on differentiation of calc alkaline and tholeiitic magma article u 230 th isotopes 87 sr 86 sr lower crust assimilation the role of changing crustal interaction and plumbing geometry in modulating calc alkaline vs tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath volcan parinacota changes in petrologic affinity correlate with periods of cone building sector collapse and rebuilding of the volcano over the last 52 ka and are well explained by changes in magma recharge regime with increasing recharge and magma output lavas transition from low fe strongly calc alkaline phenocryst rich silicic compositions to medium fe near tholeiitic mafic and aphanitic characteristics strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously these are characterized by distinctive 87 sr 86 sr ratios which suggest an initially compartmentalized system relatively high 230 th 232 th activity ratios of 0 72 in early erupted calc alkaline lavas are consistent with interaction with high u upper crust low 230 th 232 th activity ratios of 0 55 and up to 33 th excess in younger near tholeiitic lavas correlate with steep ree patterns indicating lower crustal interaction thorium excesses at the time of eruption approach the maximum that can be generated via small degree garnet residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short on the order of b2 10 4 yr in contrast lavas with greatest calc alkalinity are also at or near secular

age assignment of a diatomaceous ooze deposited in the western amundsen sea embayment after the last glacial maximum reliable dating of glaciomarine sediments deposited on the antarctic shelf since the last glacial maximum lgm is challenging because of the rarity of calcareous micro fossils and the recycling of fossil organic matter consequently radiocarbon 14 c ages of the acid insoluble organic fraction aio of the sediments bear uncertainties that are difficult to quantify here we present the results of three different methods to date a sedimentary unit consisting of diatomaceous ooze and diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner shelf in the westernamundsen sea west antarctica in three coresconventional 14 c dating of the aio in bulk samples yielded age reversals down core but at all sites the aio 14 c ages obtained from diatomaceous ooze within the diatom rich unit yielded similar uncorrected 14 c ages between 13517 56 and 11543 47 years before present a bp correction of these ages by subtracting the core top ages which probably reflect present day deposition as indicated by 210 pb dating of the sediment surface at one core site yielded ages between ca 10500 and 8400 cal a bp correction of the aio ages of the diatomaceous ooze by only subtracting the marine reservoir effect mre of 1300 a indicated deposition of the diatom rich sediments between 14100 and 11900 cal a bp most of these ages are consistent with age constraints between 13 0 and 8 0ka for the diatom rich unit which we obtained by correlating the relative palaeomagnetic intensity rpi records of three of the sediment cores with global and regional reference curves as a third dating technique we applied conventional radiocarbon dating of the aio included in acid cleaned diatom hard parts extracted from the diatomaceous ooze this method yielded uncorrected 14 c ages of only 5111 38 and 5106 38 a bp respectively we reject these young ages because they are likely to be overprinted by the adsorption of modern atmospheric carbon dioxide onto the surfaces of the diatom hard parts prior to sample graphitisation and combustion for 14 c dating the deposition of the diatom rich unit in the western amundsen sea suggests deglaciation of the inner shelf before ca 13ka bp the deposition of diatomaceous oozes elsewhere on the antarctic shelf around the same time however seems to be coincidental rather than directly related copyright 2009 john wiley sons ltd

sediment chemistry and magnetic properties in an anomalously reducing core from the eastern mediterranean sea abstract in core kc19c 19 6 m long recovered in the abyssal plain between crete and cyprus in the eastern mediterranean a large number of organic rich layers sapropels occur which correlate to maxima in the insolation curve in contrast to other sites in the eastern mediterranean porewaters contain sulfide below a few meters below seafloor mbsf geochemical analyses were performed on the porewaters ammonium alkalinity sulfate manganese iron bisulfide chloride eh ph and sediments organic carbon barium total sulfur total iron manganese pyrite dithionite extractable iron in addition a series of magnetic parameters and ratios nrm arm irm in arm irm s ratio arm 20mt arm in were determined in the top of the sediments fe and mn hydr oxides occur at the oxic suboxic boundary just above the youngest sapropel si2 these hydr oxides appear to be more easily demagnetized by means of alternating fields than deeper hydr oxides which presumably are of detrital origin the transition from suboxic to anoxic sediments is located at 2 mbsf sulfide is produced possibly during sulfate reduction through anoxic methane oxidation at 17 5 mbsf from 17 5 mbsf sulfide migrates upward titrating reactive fe resulting in pyrite formation in the entire sediment column up to 2 mbsf at this depth the upward sulfide flux has been totally consumed by reaction with solid phase fe and dissolved fe ii diffusing downward from the suboxic zone above from 2 mbsf downward magnetic intensities are significantly reduced indicating reductive dissolution of fe oxide minerals and pyrite formation as a consequence no reliable nrm data can be obtained in the lower half of the core

rock magnetic signature of a miocene playa cycle in central asia and environmental implications the increased aridification of central asia during the miocene coincides in time with lake formations and the evolution of playa environments in the region however miocene continental climate dynamics and the forcing of aridification are still not well constrained neogene lacustrine mudflat deposits in the ili basin in southeast kazakhstan provide a well exposed paleoclimate archive here we present a detailed rock magnetic study of a middle miocene playa cycle deposited in a closed basin we use high resolution rock magnetic parameters lithological studies and geochemistry to reconstruct the playa environment and the depositional conditions the rock magnetic mineralogy of the playa cycle is controlled by hematite and two fine grained magnetite phases increased magnetic concentrations occur during dry mudflat conditions with a lower groundwater table and increased aridity the underlying processes controlling the observed variation in magnetic concentrations are a complex interplay of diagenetic processes during and after deposition the data support an authigenic origin of both magnetite phases one formed before and the other after sediment consolidation early diagenetic formation of fine grained magnetite by microbial activity is followed by post depositional formation of a secondary fine grained magnetite phase the rock magnetic results such as magnetic concentration dependent parameters arm sirm and s ratio indicate a sensitive record of ground water availability and aridity changes in the ili basin we suggest that they can serve as an effective proxy for detailed paleo environment reconstruction of playa evolution not only in the middle miocene ili basin but also in comparable floodplain playa lake settings

the curved magallanes fold and thrust belt tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study the magallanes fold and thrust belt ftb presents a large scale curvature from n s oriented structures north of 52 s to nearly e w in tierra del fuego island we present a paleomagnetic and anisotropy of magnetic susceptibility ams study from 85 sites sampled in cretaceous to miocene marine sediments magnetic susceptibility is lower than 0 0005 si for 76 sites and mainly controlled by paramagnetic minerals ams results indicate that the sedimentary fabric is preserved in the undeformed areas of tierra del fuego and the more external thrust sheets units where an incipient lineation due to layer parallel shortening is recorded prolate ams ellipsoids indicating a significant tectonic imprint in the ams fabric are observed in the internal units of the belt ams results show a good correlation between the orientation of the magnetic lineation and the fold axes however in peninsula brunswick the ams lineations are at 20 counterclockwise to the strike of the fold axes pretectonic stable characteristic remanent magnetizations chrm were determined in seven sites a counterclockwise rotation 21 2 9 2 is documented by chrm data from four sites near the hinge of the belt in peninsula brunswick and near canal whiteside while there is no evidence of rotation near the nearly e w oriented vicuna thrust within tierra del fuego the curved shape of the cenozoic magallanes ftb is not related to vertical axis rotation and thus the magallanes ftb can be considered as a primary arc

a late miocene early pliocene antarctic deepwater record of repeated iron reduction events in this study we present a late miocene early pliocene record of sixty four zones with prominent losses in the magnetic susceptibility signal taken on a sediment drift odp site 1095 on the pacific continental rise of the west antarctic peninsula the zones are comparable in shape and magnitude and occur commonly at glacial to interglacial transitions high resolution records of organic matter magnetic susceptibility and clay mineral composition from early pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses instead reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation we propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments it is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the world

does lithology influence relative paleointensity records a statistical analysis on south atlantic pelagic sediments abstract the relative paleointensity rpi method assumes that the intensity of post depositional remanent magnetization pdrm depends exclusively on the magnetic field strength and the concentration of the magnetic carriers sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces just how strong these mechanical and electrostatic forces are depends on many petrophysical factors related to mineralogy particle size and shape of the matrix constituents we therefore test the hypothesis that variations in sediment lithology modulate rpi records for 90 selected late quaternary sediment samples from the subtropical and subantarctic south atlantic ocean a combined paleomagnetic and sedimentological dataset was established misleading alterations of the magnetic mineral fraction were detected by a routine fe test funk j von dobeneck t reitz a 2004 integrated rock magnetic and geochemical quantification of redoxomorphic iron mineral diagenesis in late quaternary sediments from the equatorial atlantic in wefer g mulitza s ratmeyer v eds the south atlantic in the late quaternary reconstruction of material budgets and current systems springer verlag berlin heidelberg new york tokyo pp 239 262 samples with any indication of suboxic magnetite dissolution were excluded from the dataset the parameters under study include carbonate opal and terrigenous content grain size distribution and clay mineral composition their bi and multivariate correlations with the rpi signal were statistically investigated using standard techniques and criteria while several of the parameters did not yield significant results clay grain size and chlorite correlate weakly and opal illite and kaolinite correlate moderately to the nrm arm signal used here as a rpi measure the most influential single sedimentological factor is the kaolinite illite ratio with a pearson s coefficient of 0 51 and 99 9 significance a three member regression model suggests that matrix effects can make up over 50 of the observed rpi dynamics

detection of silica mediated dissolution of magnetic grains in sediments using forc diagrams 1 recently silica mediated dissolution has been recognized as a potentially important factor influencing magnetic studies of marine and lacustrine sediments although direct evidence for the dissolution of magnetic particles in silica rich environments is lacking the process is expected to produce changes in the magnetic grain size distribution a hypothesis that is tested in this study on sediments from lake tanganyika east africa using first order reversal curves forcs results from different magnetic intensity zones within the studied samples clearly show changes in the grain size distribution of magnetic minerals in particular zones with high biogenic silica content bsi correlated with depletion in fine grained magnetic material whereas zones with lower bsi showed no depletion these results are consistent with the idea that silica mediated dissolution results in the preferential removal of fine grained magnetic material and indicate that forc diagrams are effective in characterizing silica mediated dissolution in sediments

a quantitative model of magnetic enhancement in loessic soils we present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils the model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores the rate at which this occurs depends on the frequency of drying wetting cycles and on the average moisture of the soil both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production which is proportional to a new parameter called magnetite enhancement proxy mep our model is tested by comparing mep calculated for known present day climates with the magnetic enhancement of modern soils the magnetic enhancement factor defined as the ratio between a given magnetic enhancement parameter and mep is expected to be a site independent constant we show that magnetic enhancement differences between soils from the chinese loess plateau and from midwestern u s are explained by our model which yields similar magnetic enhancement factors for the two regions our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

magnetostratigraphic calibration of eocene oligocene dinoflagellate cyst biostratigraphy from the norwegian greenland sea the presence of abundant age diagnostic dinoflagellate cysts in ocean drilling program odp hole 913b leg 151 deep sea drilling project hole 338 leg 38 and odp hole 643a leg 104 has enabled the development of a new biostratigraphy for the eocene oligocene interval in the norwegian greenland sea this development is important because the calcareous microfossils usually used for biostratigraphy in this age interval are generally absent in high latitude sediments as a result of dissolution in parallel with this biostratigraphic analysis we developed a magnetic reversal stratigraphy for these norwegian greenland sea sequences this has allowed independent age determination and has enabled the dinocyst biostratigraphy to be firmly tied into the global geomagnetic polarity timescale gpts the relatively high resolution of this study has enabled identification of dinoflagellate cyst assemblages that have affinities with those from the north sea and the north atlantic which allows regional correlation correlation of each site with the gpts has also allowed comparison of the stratigraphic record preserved in each drill hole hole 913b is the most complete and is the best preserved record of the eocene and oligocene in the northern hemisphere high latitudes and can serve as a reference section for palaeoenvironmental reconstructions of this age interval

magnetite dissolution in siliceous sediments 1 magnetite dissolution and consequent loss of magnetization is widely observed in reducing sedimentary environments where the decrease in eh ph values with depth is driven by bacterially mediated degradation of organic carbon we have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and or silicic volcanic ash these depletions in magnetization are greater than can be accounted for by dilution with magnetite poor sediments and suggest that postdepositional destruction of magnetite has occurred biosiliceous sediments usually also contain elevated concentrations of organic carbon which makes it difficult to separate organic carbon related magnetite dissolution from other possible mechanisms for magnetite dissolution however the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox state of the environment this suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations and appropriate eh ph conditions and predict that magnetite will break down to produce iron bearing smectite a survey of magnetic susceptibility and pore water geochemical data from widely distributed ocean drilling program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities this observed link also holds for environments with low biogenic silica productivity and low organic carbon content but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments

pb isotopes define basement domains of the altiplano central andes detailed pb isotopic maps of the central andes based on 345 163 previously published 182 new analyses of ores volcanic rocks and their host rocks elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region the data clearly show that most of the pb in central andean igneous and ore samples is derived from the local basement including pb in ore deposits of the bolivian tin belt some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern as well as to metallogenic boundaries such as the western edge of the bolivian tin belt

magnetite diagenesis in marine sediments from the oregon continental margin the magnetochemistry of sediments from the oregon continental margin is examined to determine the effects of iron sulfur diagenesis on the paleomagnetic record magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites these processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities systematic changes in other rock magnetic properties and increases in solid phase sulfur concentrations with depth hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size magnetite abundances decrease downcore from initial values of about 0 1 and nominal grain diameters lie within a narrow pseudosingle domain range of 0 08 to 0 6 m a first order surface area reaction model da dt ka is proposed to explain the magnetite dissolution mechanism where a is the total magnetite surface area and k is the rate constant the solution of this equation predicts that the surface area and concentration decrease exponentially and the concentration in addition depends on grain size application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area despite extensive magnetite destruction magnetic directions in such sediments appear to reliably record long wavelength trends of the geomagnetic field

localized pluton deformation and linked focused flow of low volume fraction residual melt in deforming plagioclase cumulates shear hosted sub cm to 50 m wide fe ti zr ree ftzr rich melt bands occur exclusively at folded margins of the bolangir anorthosite pluton eastern india transverse to margin parallel foliation in anorthosite the n to nnw trending ftzr melt bands are oriented at a low angle 90 fractionation of anorthosites from crustally contaminated high al gabbro hag parent melt at the high degree of crystal fractionation crystal content higher than the particle locking threshold the ftzr melts during last stages of crystallization were locked within the plagioclase dominated cumulus pile the occurrence of ftzr melt bands exclusively at folded as opposed to straight segments of pluton margin is attributed to melt enhanced strain localization in the pluton induced by the ftzr melts locked within anorthosite further melt induced straining led to focusing of the ftzr melts along dilatant shear zones formed synchronously with the east west shortening deformation the variable widths of the melt bands correspond to the varying sizes of actively deforming magmatic plagioclase in anorthosite the extraction and transport of locked in low volume fraction melt documented here suggest long duration far field stresses may segregate and transport large volumes of low volume fraction melts in planetary interior over considerable distance

composition and structural control of crustal domains in the central andes 1 present day ratios of pb isotopes 324 published samples 435 new and nd sr isotopes 150 published 180 new on proterozoic to holocene igneous metamorphic and sedimentary rocks define at high spatial resolution distinct isotopic domains of the crust in the central andes these domains correlate with the internal compositional structure of the crust as revealed by a three dimensional density model pb nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect proterozoic mafic dominated and paleozoic felsic dominated crustal lithologies age and composition mafic versus felsic of these domains have controlled the rheology of the andean crust have influenced crustal deformation patterns and correlate with the central andean plateau segmentation

orbitally forced paleoenvironmental and paleoclimate changes in the late postevaporitic messinian of the central mediterranean basin paleoenvironmental and paleoclimate changes that occurred during the late postevaporitic stage of the mediterranean basin in the messinian foreland domain of the adriatic region offer a new perspective on the relationship between orbital forcing and climate response the magnetic susceptibility record of the fonte dei pulcini a section maiella mountains italy allows us to orbitally tune the record between 5 394 and 5 336 ma and to temporally constrain the paleoenvironmental and paleoclimate changes evidenced by quantitative paleontological palynomorphs ostracods and calcareous nannofossils stable isotope 18 o and 13 c and x ray diffraction xrd analyses the base of the fonte dei pulcini a section is characterized by paratethyan ostracods and dinocysts which point to the late messinian lago mare biofacies loxocorniculina djafarovi zone of the mediterranean messinian stratigraphy from paleontological and geochemical 18 o analyses there is no evidence of a marine incursion in the fonte dei pulcini a section the major changes in terms of paleodepth paleosalinity evaporation versus precipitation aridity versus humidity and reworking processes occurred in the upper part of the fonte dei pulcini a section during the last messinian insolation cycle i cycle 511 512 which is characterized by high amplitude oscillations in contrast the lower part of the fonte dei pulcini a section which was deposited during relatively low amplitude insolation cycles is characterized by more stable environmental conditions comparing summer insolation with the paleoenvironmental changes at the fonte dei pulcini a section we identify delays of several thousands of years between orbital forcing and climate response

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

low temperature magnetic properties of pelagic sediments ocean drilling program site 805c tracers of maghemitization and magnetic mineral reduction magnetic properties of pelagic sediments from the western equatorial pacific ocean drilling program odp site 805c have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth a saturation isothermal remanent magnetization mrs was given to the samples by applying a strong magnetic field 2 5 t at 20 k after cooling both in the absence zero field cooling zfc and presence of a 2 5 t magnetic field field cooling fc the thermal demagnetization of both mrszfc and mrsfc was measured from 20 to 300 k no pronounced verwey transition was observed in the data from samples shallower than the iron redox boundary below the boundary the verwey transition is visible for all samples studied we interpret this behavior as reflecting low temperature oxidation maghemitization of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary the low temperature oxidation may occur both at the water sediment interface and deeper in the suboxic zone of sediments as a result of bacteria mediated processes including the reduction of nitrate compounds mrszfc and mrsfc thermal demagnetization curves of all samples diverge for the entire temperature range measured the divergence may be caused by the low temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system together maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low temperature properties of the sediment studied as a result low temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments the changes in effective oxidation state below the iron redox boundary detected by low temperature methods may be useful in the study of some paleoenvironmental processes

pliocene glacial cyclicity in a deep sea sediment drift antarctic peninsula pacific margin abstract giant deep sea sediment drifts are widespread features along the continental rise of the west antarctic peninsula representing the most proximal continuous sedimentary recorders for west antarctic ice events and glacial interglacial cyclicity sediment physical geochemical records and x ray images derived from odp leg 178 site 1095 sediment drift 7 show characteristics in glacial interglacial cyclicity and associated sedimentary processes during early pliocene two boundary types dividing half cycles have been recognized 1 interglacial to glacial transitions that are distinct and characterized by a sharp boundary and abrupt change in lithology 2 glacial to interglacial transitions that are diffuse and characterized by a gradual decline of sediment physical and geochemical values with a marked reduction in sedimentation rates the boundaries defined in conjunction with magneto and biostratigraphic tie points form the framework for the calculation of relative sedimentation and accumulation rates used for reconstructing early pliocene paleoproductivity and terrigenous flux to the drift warm early pliocene climate conditions for west antarctic peninsula region are indicated by open ocean conditions in glacials short residence times of the grounded ice sheet at the shelf edge with sporadic fluctuations as well as periodic collapses of the ice sheet front we propose a 5 phase drift sedimentation model for the early pliocene and postulate a highly dynamic ice sheet likely reduced in thickness resulting in oscillations which lie closer to the shelf edge than was the case during the late quaternary

rock magnetism in two loess paleosol sequences in cordoba argentina this work presents new rock magnetic results along two loess paleosol profiles in nearby locations in argentina the main objective of this study is to compare the magnetic signals and mineral content of two profiles and determine if the climate during marine isotope stage 5 mis 5 and mis 3 were similar to the present one the two profiles are located in two different geomorphological settings with effects on the water saturation characteristics and seasonality selected samples taken at these profiles were analyzed using laboratory procedures and environmental magnetism parameters to determine the climatic influence during the late pleistocene despite their proximity there are several differences between both profiles such as their depth and geomorphological positions among others the results of these analyses led to the following conclusions climate conditions during mis 5 were very similar to those of the present conditions the hypothesis for this area suggests a slight increase in the magnetic signal associated with the generation of small amounts of magnetite and preservation of detrital magnetite and titano magnetite the results in this paper show a slight gain in the buried soils of cordoba that would confirm the hypothesis

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

lead isotopic evidence for synextensional lithospheric ductile flow in the colorado river extensional corridor western united states this is the published version copyright 1998 american geophysical union all rights reserved

petrogenesis and tectonic setting of the huhetaoergai granitic pluton in the northern ya gan fault zone northern alxa china constraints from whole rock geochemistry zircon u pb ages and hf isotope compositions this study presents a new data set that contains zircon u pb ages whole rock geochemistry and hf isotopes of the huhetaoergai granitic pluton in the northern ya gan fault zone northern alxa chi

sources of lead in the san cristobal pulacayo and potosi mining districts bolivia and a reevaluation of regional ore lead isotope provinces new lead isotope data on ores crustal rocks and leachates of crustal rocks combined with data in the literature provide important new constraints on the sources of ore metals in southwest to south central bolivia including the very large recently discovered silver zinc deposit at san cristobal the pulacayo polymetallic district and the giant potosi silver tin base metal deposit lead isotope ratios of ores and igneous rocks from the san cristobal deposit and from paleozoic and cretaceous sedimentary rocks are compared with published data on high grade middle proterozoic metamorphic basement rocks these data constrain the major source of lead and by inference of other ore metals at san cristobal to be the metamorphic basement rocks leaching experiments on samples of paleozoic and cretaceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than the whole rock compositions however lead isotope ratios of both whole rocks and leachates of these upper crustal rocks are too radiogenic for them to be major sources of ore lead at san cristobal lead isotope ratios of ores from pulacayo and potosi are similar to each other and lie within the range of paleozoic and cretaceous sedimentary whole rock compositions leaching of pb from the sedimentary rocks cannot explain the isotopic compositions of the pulacayo and potosi ores and the isotopic homogeneity of the potosi ores also argues against mixing of lead from diverse sources in the hydrothermal system lead from the sedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metals from the resulting magma lead isotope ratios of san cristobal ores are different from those of pulacayo potosi and other deposits to the east but resemble the compositions of ores and volcanic rocks in western bolivia on this basis we identify a new ore lead isotope province extending from san cristobal northward across the eastern altiplano and into southern peru this province is coincident with but smaller than the extent of the proposed arequipa antofalla metamorphic basement craton the degree of incorporation of ore metals from the metamorphic basement appears to depend on the timing and or location of the mineralizing event ore deposits in the northern part of province iv formed before the thickening of andean crust beginning around 20 ma and incorporated minor amounts of metals from the metamorphic basement younger deposits farther to the south contain major to dominant components of basement lead

sr isotope zoning in plagioclase from parinacota volcano northern chile quantifying magma mixing and crustal contamination we present analyses of sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from parinacota volcano central volcanic zone northern chile which were analysed for major and minor element zoning in a previous study although the isotopic range of the bulk rock samples is small at this volcano 0 7067 0 7070 except for one flow of mafic andesite at 0 7061 significant variations are seen 0 70649 0 70700 within and between plagioclase crystals a general negative correlation is observed between sr isotope composition and sr concentration in the liquid in equilibrium with each plagioclase zone as calculated from chemical zoning data and partition coefficients additional scatter is superimposed on this general trend indicating a decoupling between isotopic and chemical variations for sr in one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event we apply energy constrained recharge assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal results suggest 20 assimilation of the local wall rock gneiss at high initial temperatures the isotopic data confirm the involvement of two contrasting mafic magmas which are sampled at flank cinder cone vents one lower ajata has a low sr content with high 87sr 86sr the other upper ajata has a high sr content with lower 87sr 86sr in some samples from parinacota the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87sr 86sr mafic magma in others where chemical zoning profiles suggest that recharge was from the low 87sr 86sr magma the 87sr 86sr of the groundmass and crystal rims is higher than expected this indicates either additional parent magmas to the two previously identified or further crustal assimilation either at lower crustal depths before crystallization of plagioclase or just after the last recharge our results illustrate the complexity of magma crust interaction beneath parinacota which is likely to be representative of many other central andean volcanoes formed on thick crust such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase in a textural petrographic context despite a small whole rock isotopic range the distinct contamination patterns of various samples suggest an important role for the geometry location and evolution of the magma plumbing system and in general variations of the thermal and compositional structure of the crust underneath the volcano

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

preservation efficiencies and accumulation rates for biogenic silica and organic c n and p in high latitude sediments the ross sea rates of biogenic sediment accumulation biogenic silica and organic c n and p and pore water flux have been established for a variety of depositional environments in the ross sea on the basis of 14c measurements in kasten cores sediment accumulation rates ranged from 250 cm kyr 1 in the coastal basin of granite harbor to 1 2 cm kyr 1 in the shelf and slope environments of the northern and eastern ross sea burial of biogenic material was most rapid in the southwestern ross sea where biogenic silica accumulation rates ranged from 2 to 31 g cm 2 kyr 1 and organic carbon accumulation rates ranged from 0 05 to 1 4 g cm 2 kyr 1 in the northern and eastern ross sea biogenic silica and organic carbon accumulation rates typically equaled 0 02 0 03 g sio2 cm 2 kyr 1 and 0 002 0 004 g c cm 2 kyr 1 flux core measurements were used to estimate seabed regeneration rates for biogenic silica organic carbon and phosphorus pore water fluxes in general showed much less variability across the ross sea than did the biogenic accumulation rates pore water silicate fluxes in the study area ranged from 0 6 to 5 3 g sio2 cm 2 kyr 1 whereas carbon fluxes ranged from 0 1 to 1 2 g c cm 2 kyr 1 and phosphate fluxes varied from 0 006 to 0 012 g p cm 2 kyr 1 seabed preservation efficiencies were calculated for biogenic silica organic carbon and phosphorus by combining the solid phase and pore water data the seabed preservation efficiencies for biogenic silica 1 86 were greater than for organic carbon 1 71 at all nine stations examined the preferential preservation of biogenic silica relative to organic carbon also was apparent in ross sea sediments because the biogenic silica organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment water interface sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data p preservation efficiencies remained relatively high 24 65 even when the accumulation rates were low 1 2 cm kyr 1 because of the near zero phosphate fluxes out of the seabed of the total p in the seabed 0 04 0 09 wt approximately 25 existed in the form of organic p the total amount of biogenic silica accumulating on the ross sea shelf is 2 3 1012 g sio2 yr 1 which is approximately an order of magnitude less than the ledford hoffman et al 1986 estimate that was based on 210pb chronologies biogenic silica accumulation rates in the southern central and western ross sea increased during the mid and late holocene reaching their maximum values during the past 500 to 1000 years

diagenetic formation of bedded chert implications from a rock magnetic study of siliceous precursor sediments abstract bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz rich chert beds and clay rich interbeds the origin of the lithologic variations is not well understood a rock magnetic study of a sequence of alternating bands of radiolarian rich and clay rich bands recovered by odp leg 129 which is considered to be the best known analog for the bedded chert precursor sediments shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types with only a minor 1 5 times on average increase in bulk concentration in clay rich bands comparison with magnetic properties of the diagenetically mature chert shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification a simple silica transfer model with dissolution of silica in proto shale donor beds and deposition in proto chert receiver beds indicate that the known features of the bedded chert sequences such as the difference in thickness cosmic dust content between chert and shale can be reproduced in case of donor beds having initially lower clay higher biogenic silica content compared to receiver beds while contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays an association of proto shale beds with high productivity episodes is supported by an independent paleoproductivity proxy their higher al ti values and is in full agreement with the known silica diagenesis trends particularly with the retarding influence of detrital minerals on silica transformations the impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession silica dissolution starts in clay poor intervals when diffusing silica encounters clay rich beds adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz early deposition of quartz cement prevents significant compaction of chert layers while the loss of silica in donor beds leads to their compaction and concomitant increase in non reactive clay and cosmic dust particles severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies

early to middle eocene magneto biochronology of the southwest pacific ocean and climate influence on sedimentation insights from the mead stream section new zealand the mead stream section south island new zealand consists of a 650 m thick series of continuous well exposed strata deposited on a south pacific continental slope from the late cretaceous to the middle eocene we examined the uppermost paleocene middle eocene part of the section which consists of 360 m of limestone and marl for detailed magnetic polarity stratigraphy and calcareous nannofossil and foraminifera biostratigraphy magneto biostratigraphic data indicate that the section straddles magnetic polarity chrons from c24r to c18n calcareous nannofossil zones from np9a to np17 cnp11 cne15 following a recently revised paleogene zonation and from the waipawan to the bortonian new zealand stages i e from the base of the ypresian to the bartonian international stages the mead stream section thus encompasses 17 m y 56 39 ma of southwest pacific ocean history the ages of calcareous nannofossil biohorizons are consistent with low to midlatitude data from the literature indicating that during the early middle eocene the low to midlatitude calcareous nannofossil domain extended at least to 50 s 55 s in the south pacific correlation of the magnetic polarity stratigraphy from the mead stream section with the geomagnetic polarity time scale allows us to derive sediment accumulation rates sar which range between 8 and 44 m m y comparing the sar with paleotemperature proxy records we found that two intervals of increased sar occurred during the early eocene climatic optimum 52 50 ma and during the transient warming event peaking with the middle eocene climatic optimum 40 5 ma this correlation indicates that at mead stream the climate evolution of the early middle eocene is recorded in a sedimentation pattern whereby on a million year time scale warmer climate promoted continental weathering transportation and accumulation of terrigenous sediments

a high resolution record of early miocene antarctic glacial history from odp site 1165 prydz bay 1 odp site 1165 located 400 km northwest of prydz bay contains a high resolution early miocene record of pulses of ice rafted debris ird originating from the ancestral lambert glacier system and the antarctic coast to the east the 520 m of early miocene sediments consist of dark gray claystone with silt laminae contourites alternating with decimeter scale layers of greenish gray bioturbated claystone that commonly contain ice rafted debris downhole logs also record the alternations between the two facies the ird bearing greenish grey claystone corresponds to high resistivity and density values because of increased cementation by silica and corresponds to lower natural gamma radiation values because of diluted clay content the downhole logs thus allow a continuous and detailed stratigraphic record of the ird bearing layers to be obtained the ird bearing layers represent deglaciations and interglacials when a high flux of icebergs with incorporated material were melting out over the site the contourite rich layers represent glacials when the polar current over the site was relatively strong the sequence is dated by magnetostratigraphy and the timing of the major ird pulses is paced by orbital eccentricity indicating that the volume of the east antarctic ice sheet also fluctuates on this timescale after 19 7 ma minor precessional and subprecessional ird layers appear in the record indicating that the ice sheet becomes more prone to deglaciation through this interval perhaps associated with the gradual warming trend through the early miocene

global geologic maps are tectonic speedometers rates of rock cycling from area age frequencies relations among ages and present areas of exposure of volcanic sedimentary plutonic and metamorphic rock units lithosomes record a complex interplay between depths and rates of formation rates of subsequent tectonic subsidence and burial and or rates of uplift and erosion thus they potentially serve as efficient deep time geologic speedometers providing quantitative insight into rates of material transfer among the principal rock reservoirs processes central to the rock cycle areal extents of lithosomes exposed on all continents from two map sources geological survey of canada gsc and the food and agricultural organization fao of the united nations educational scientific and cultural organization unesco indicate that volcanic sedimentary plutonic and metamorphic rocks occupy 8 73 7 and 12 of global exposures respectively plots of area versus age of all mapped rock types display a power law relation where 6 5 of continental area is resurfaced with younger 10 volcanic 90 sedimentary units every million years and where areas of rock exposure decrease by 0 86 for each 1 increase in outcrop age r 2 0 90 area age relations for volcanic and sedimentary lithosomes are similar to the power law distribution defined by all rock units because 81 of mapped area consists of these two lithologies and reflect progressive decrease in amount of exposure with increasing age over the long term continental surfaces are blanketed by new volcanic rocks and sediments at rates of 1 5 and 12 1 10 6 km 2 ma respectively in contrast to power law distributed volcanic and sedimentary rocks that form at the earth9s surface age frequency distributions for plutonic and metamorphic rocks exhibit lognormal relations with modes at ca 154 and 697 ma respectively a dearth of younger exposures of plutonic and metamorphic rocks reflects the fact that these rock types form at depth and some duration of tectonism is therefore required for their exposure increasing modal ages from quaternary for volcanic and sedimentary successions to early mesozoic for intrusive rocks to neoproterozoic for metamorphic rocks demonstrate that greater amounts of geologic time are required for uplift to bring more deeply formed rocks to the earth9s surface the two different age frequency distributions observed for these major rock types a general power law age distribution for volcanic and sedimentary rocks and a lognormal distribution for plutonic and metamorphic rock ages reflect the interplay between depths of formation and mean rates of vertical tectonic displacement age frequency distributions for each of the major rock types are closely replicated by a model that presumes that individual crustal elements behave as a large population of random walks in geologic time and crustal depth and where the processes of surficial erosion associated with tectonic uplift serve to impose an absorbing boundary on this random walk space comparisons between model predicted age frequencies and those apparent in global map data suggest that mean rates of crustal subsidence and uplift are approximately equal in magnitude with mean rates of vertical tectonic diffusion of lithosomes from crustal depths of formation of about half a kilometer per million years rates of uplift and subsidence are strongly dependent on durations of tectonic dispersion lithosome ages however mean rates on the order of hundreds of meters per million years suggested by map age frequencies are the same as would be anticipated on the basis of hundreds of published rates of erosional uplift and exhumation determined by more conventional geochronometers this agreement suggests that geologic maps serve as effective deep time speedometers for the geologic rock cycle

authigenic smectite on diatom frustules in bolivian saline lakes abstract observation under the electron microscope of diatom frustules from bolivian altiplano saline lakes shows that many of these are coated with particles occurring as tiny sheets the frustules can be found to be almost completely replaced by these sheets isolated sheet aggregates seem to have resulted from completely transformed frustules section observations of altered frustules bear out that the sheets have grown from biogenic silica through replacement selected area diffraction dark field observation microdiffraction and elemental microanalysis show that the particles on the diatom frustules consist of a poorly crystallized mg smectite the unambiguous localisation of this authigenesis allows us to reconstruct its hydrochemical and sedimentological environment observation of the most recent lake sediments has pointed out that at least two main conditions are required for this authigenesis at 5 c saturation with respect to amorphous silica and a ph above 8 2 variations in the mg concentration have no significant effect

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

nutrient leakage from the north pacific to the bering sea iodp site u1341 following the onset of northern hemispheric glaciation 1 intergrated ocean drilling program expedition 323 recovered a sediment record covering the last 4 3 ma from the bering sea integrated ocean drilling program site u1341 bowers ridge 2177 m water depth to resolve pliocene pleistocene paleoenvironmental changes in this marginal basin 190 sediment samples were analyzed for their bulk element composition aluminium contents in bowers ridge sediments are variable but overall higher toward younger sediments probably related to the intensification of the northern hemispheric glaciation and increasing sea ice transport in the bering sea the gradual increase of terrigenous input is mirrored by decreasing sio2 and excess si sixs contents but the overall si enrichment of the deposits reflects continuous opal deposition since the pliocene at bowers ridge unlike in the north pacific the sixs record at site u1341 does not support a dramatic decrease in opal export following the onset of the northern hemispheric glaciation around 2 7 ma but sio2xs have higher accumulation rates up to 8 g cm2 ka between 2 6 and 1 8 ma bp during this period the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas we here discuss how the onset of north pacific stratification at 2 7 ma bp may have caused leakage of nutrient rich deep intermediate north pacific water into the bering sea via the deep kamtchatka strait leading to increased opal deposition and likely reactive organic matter export at bowers ridge as a result magnetic and geochemical records were overprinted by intensified diagenesis significantly affecting their applications as paleoceanographic proxies

rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the california continental borderland the rock magnetic properties of marine sediments from the california continental borderland san pedro santa catalina and san nicolas basins have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals previous studies have noted that the natural remanent magnetization in these sediments primarily carried by detrital magnetite decays to 10 or less of its surface value soon after deposition this decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process the shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest grained magnetic material the intensity decreases which are linearly proportional to magnetite concentration decreases indicate that dissolution occurs with rate constants ranging from 0 3 to 1 6 kyr 1 the rate constants sulfide concentrations and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite

paleomagnetic and geochemical record from cores from the sea of marmara turkey age constraints and implications of sapropelic deposition on early diagenesis abstract we present results of a multi proxy analysis of two sediment cores from the marmara sea the cores were analyzed using paleomagnetic and geochemical measurements two sapropels are documented in the last 11 kyr and are recorded in several locations across the marmara sea these two sapropels have contrasting magnetic properties the magnetic record is affected by intense early diagenesis the most recent upper sapropelic layer has low remanence and susceptibility values a record of paleomagnetic inclinations could still be isolated above the diagenesis front and is compared with secular variation models the lower sapropel is identified in the deep part of the oldest studied core klg07 and has distinct magnetic properties characterized by high remanence and susceptibility values using the magnetic properties it is possible to constrain bottom water ventilation and reconnection episodes between the marmara sea and the black sea following the sea level rise during the last glacial to inter glacial transition

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

dynamics and physical parameters of the lastarria debris avalanche central andes abstract volcanic debris avalanches are extremely destructive phenomena with the potential to travel many kilometers from their source region either as rockslides or as mass flows given that they may even be triggered at inactive volcanoes their hazard is often underestimated understanding the dynamics of such mass movements is essential for evaluating and mitigating hazards a number of case studies have been carried out around the world but there is still a need for further studies of flow dominated avalanches which remain poorly constrained these studies would have high educational value providing striking examples to teach decision makers and at risk populations about the hazard in this study we investigate the 7500 cal year b p lastarria debris avalanche it is a 7 km long deposit with exceptional preservation of both the flow structures and the collapse scar detailed fieldwork morphometric mapping of over 600 surface features and numerical modelling was carried out to constrain the avalanche s trigger and flow parameters numerical models and field scaling relationships are in good agreement suggesting maximum velocities of 210 to 270 km h 1 negligible basal friction low cohesion 50 kpa and an intermediate friction coefficient structures are dominantly oriented parallel to transport direction suggesting minimal influence from a smooth paleotopography lastarria provides an example of a shallow flank failure initiated along stratigraphic planes that allowed low strength pyroclastic strata to disaggregate rapidly and then quickly accelerate to flow as a granular material at high velocity beyond the base of the volcano overall lastarria provides excellent constraints on granular avalanche initiation and flow which are valuable for hazard assessments and for the study of less well preserved flow deposits elsewhere the question of which precursory signs may warn of such a flank failure remains open and is important to address in future studies

cycling of organic carbon and biogenic silica in the southern ocean estimates of water column and sedimentary fluxes on the ross sea continental shelf we examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the ross sea seeking to understand the processes leading to the formation of opal rich organic poor sediments over much of the southern ocean between january 1990 and december 1994 we conducted three cruises performing tracer incubation studies 14c 15n 30si 32si to measure rates of primary production nitrate based new production biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the ross sea shelf in spring mid summer and late summer we deployed sediment traps from january 1990 to early march 1992 to measure the mid water 250 m and near bottom gravitational fluxes of particulate organic carbon nitrogen and biogenic silica year round at three sites and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf at 9 of those sites we also measured nutrient efflux from the sediments enabling us to calculate benthic recycling fluxes of organic matter and opal these data permit estimates of the annual production near surface recycling vertical sinking flux delivery to the seabed benthic regeneration and long term burial of both organic and siliceous material integrated over a 3 3 105 km2 area that covers 75 80 of the ross sea shelf the resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column as well as in the upper seabed selective preservation of silica within the upper 50 m is not indicated and both organic matter and silica are transported from 250 m to the sea floor with virtually 100 efficiency the sio2 c mass ratios for surface layer production 250 m sinking flux delivery to the seabed and long term burial are approximately 0 85 6 1 6 2 and 27 respectively this progressive enrichment in silica results in long term burial of 5 8 of the biogenic silica and 0 17 of the organic carbon produced by phytoplankton in the surface layer a factor of 30 greater preservation efficiency for silica than for carbon nevertheless the ratio of opal burial to opal production in the ross sea is only about twice the apparent global average of 3 and 1 3 of the estimated burial production ratio for the southern ocean as a whole it thus appears that both silica preservation and the decoupling between the cycles of silica and carbon must be even more effective in the waters overlying abyssal southern ocean sediments than they are over the ross sea shelf

magmatism in the huarina belt bolivia and its geotectonic implications abstract the huarina belt is a tectono stratigraphic unit extending along the western side of the eastern cordillera and southern altiplano it includes the well known bolivian tin belt both geophysical and isotopic evidence suggest that the huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the proterozoic most of the phanerozoic backarc magmatism in the bolivian andes occurred along this belt local ordovician basalts and andesites and clasts of devonian granitoids occurring in late devonian early carboniferous sedimentary units are testimonies of a scarce palaeozoic igneous activity during the mesozoic rift related magmatism resulted in peraluminous to metaluminous high k calcalkaline triassic granitoids as well as discrete cretaceous centers of alkaline and carbonatitic rocks in the cenozoic huge high k generally peraluminous ignimbrite sheets along with numerous volcanic and intrusive centers were generated along the belt shoshonitic alkaline and ultrapotassic rocks have been locally described on the basis of both geochemical and geophysical evidence a general evolution model is proposed for the huarina belt during paleozoic and mesozoic times the predominant extensional regime of the continental crust led to lithospheric thinning basin generation and production of within plate magmas during the cenozoic prevailing compressional conditions led to a gradual thickening of the lithosphere inducing delamination of its denser and less viscous root the mantle generated magmas interplayed widely with crustal melts giving rise to the dominantly peraluminous magmas

progressive dissolution of titanomagnetites at odp site 653 tyrrhenian sea abstract magnetic properties microscopy and ferrous ferric iron ratio document increasing down core alteration of titanomagnetite to iron sulfide at odp site 653 leg 107 tyrrhenian sea changes in magnetic properties are interpreted to indicate a grain size increase in titanomagnetite to 40 m below seafloor bsf and a grain size decrease from this level to about 150 m bsf as dissolution affects first the fine grained and then the coarser grained magnetite the availability of dissolved sulfate from underlying messinian evaporites may account for the sulfide supply at depth and hence for the prolonged dissolution process dissolution at leg 107 sites adjacent to site 653 is less pronounced possibly by the availability of organic matter that can be metabolised by sulfate reducing bacteria