sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

a large california flood and correlative global climatic events 400 years ago a gray silt layer 1 2 cm thick in the central santa barbara basin dated by varve counts to a d 1605 5 yr implies an intensity of precipitation flooding of regional rivers and transport of terrigenous detritus unmatched in the last 1000 yr the inferred flood may correlate with the reported rare occurrence of a perennial lake 14c dated to 390 90 b p in california s mojave desert 300 km east of the area draining into the santa barbara basin the dating of the a d 1605 5 yr flood event is consistent with tree ring evidence for a wet and cold paleoclimate elsewhere in the region regional and global climate evidence indicates that much of the world also experienced rapid intense cooling around a d 1605 this cooling was probably accompanied by an equatorward shift of prevailing wind patterns and associated storm tracks

early diagenesis in differing depositional environments the response of transition metals in pore water the cycling of fe mn ni co cu cr v and mo during early diagenesis was investigated in sediments from five different depositional environments in the california borderland dissolved o2 no3 no2 and so4 were also measured at each site to establish the position of redox boundaries pertinent to this study sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals bottom water o2 concentration sediment redox conditions cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux at 10 m oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments differing only in scale under these conditions the recycling of mn oxides resulted in an enriched layer near the interface at bottom water 02 concentrations 5 m mn oxides are not recycled under these conditions fe oxides and so42 are important oxidants the depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides trapping and recycling of ni and co with mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments scavenging of these metals is favored by a deep oxic zone in the sediments in contrast cr v and mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation the source of the reduced species is assumed to be biogenic material the accumulation of these metals is favored by reducing conditions in the sediments cu is enriched in the sediments by transport with detrital biogenic material followed by adsorption onto sediment solids cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site at the base of the slope the magnitude of cu released in the slowly accumulating pelagic sediment exceeds the cu binding capacity of the solids resulting in pore water cu concentrations in excess of 100 nm in general metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study

contemporaneous early diagenetic formation of organic and inorganic sulfur in estuarine sediments from st andrew bay florida usa estuarine sediment samples were collected from nine stations in st andrew bay near panama city florida usa contrasting grain size and varying concentration of organic carbon allowed a comparative study of the relationships among organic degradation processes sulfur cycling and diagenetic removal of dissolved sulfide produced by bacterial sulfate reduction abundance and sulfur isotopic composition were determined for dissolved sulfide dissolved sulfate pyrite humic acid sulfur fulvic acid sulfur and elemental sulfur presence of free dissolved sulfide and enrichment in 34s of porewater sulfate in samples from organic rich muddy sediments of st andrew bay indicate high rates of sulfate reduction and anoxic conditions below 5 mm depth such conditions are not reflected in samples from organic poor sandy near shore sediments collected from highly productive areas occupied by seagrass beds in this estuarine system pyrite and fulvic acid sulfur are the largest sinks for sulfide produced by bacterial sulfate reduction strong correlation between abundances of pyrite and fulvic acid sulfur suggests coincident formation in the uppermost 10 cm of sediment large differences in isotopic composition between these two species and consistent enrichment in 34s of the fulvic acid fraction however indicate two different reaction pathways abundance and sulfur isotopic composition of humic acid and fulvic acid sulfur suggest that organic sulfur is derived from mixing between 1 34s depleted sulfur with an unknown oxidation state that was recycled from bacterial hydrogen sulfide and 2 34s enriched sulfur interpreted as primary biosynthetic sulfur originally assimilated from dissolved sulfate samples taken from sandy sediment localities where aquatic vascular plants are rooted in the anoxic zone show a strong depletion in 34s in the fulvic acid sulfur fraction suggesting recycled hydrogen sulfide as the dominant source of organic sulfur

climatically controlled marker layers in santa barbara basin sediments and fine scale core to core correlation varved sediments with annual layering can provide a detailed history of environmental conditions of marginal ocean basins and of lakes to reconstruct this history requires fine scale correlation and correct age assignment in the santa barbara basin a combination of x radiography water content profiles and microfossil stratigraphy yields correlations of the desired resolution x radiography permits identification of individual layers 1 2 mm thick that differ in porosity or pore water content and hence x ray absorption microfossils and sedimentological parameters provide the clues to the presence for el nino years e g in a d 1984 1959 and 1942 varves allowing the tie in to absolute years within this century el nino layers once identified are readily recognized on the radiographs and within the porosity profiles counting of laminae in x radiographs and core to core correlation via water content then allow establishing age scales for other cores a prominent shell layer in the first half of the 19th century is dated at a d 1838 this event macoma i is of supraregional significance

extreme climatic conditions recorded in santa barbara basin laminated sediments the 1835 1840macoma event abstract the santa barbara basin sbb record of laminated sediment well known for its excellent time resolution shows the presence of an unusual once in a millenium event in the period 1835 1840 at a depth of 55 57 cm below the surface of the seafloor this macoma event is the brief flourishing of a macrofaunal community in the deep center of the sbb from which large organisms are normally excluded by the lack of oxygen shells of the pelecypodmacoma leptonoidea dall and associated fossils which congregate in a thin layer dated at 1840 provide evidence for a sudden local extinction of macrobenthos by suffocation various stratigraphic evidence suggests thatm leptonoidea colonized the basin floor for a few years prior to 1840 as a result of temporarily decreased productivity and increased oxygen content in the water spilling over the sill into the basin historical eyewitness reports indicate unusually severe winter storms from southeasterly directions during the 1830s in the sbb we suggest that coastal upwelling was severely reduced during 1835 1839 with the effect of reducing the intensity of the oxygen minimum layer themacoma event apparently is one manifestation of large scale climatic anomalies felt throughout north america and which were possibly linked to the major volcanic eruption of cosiguina nicaragua in 1835

variscan intra orogenic extensional tectonics in the ossa morena zone evora aracena lora del rio metamorphic belt sw iberian massif shrimp zircon u th pb geochronology

mantle wedge hydration in nicaragua from local earthquake tomography the continental margin of nicaragua and costa rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab a decreasing magma production and a shift in the volcanic front to investigate this transition a joint on and offshore local earthquake tomography was performed low p wave velocities and high vp vs ratios indicative for hydration were found in the upper mantle and lowermost crust beneath the sandino basin the mantle wedge hydration can be estimated to 2 5 wt per cent beneath south nicaragua in contrast the mantle wedge beneath north costa rica is weakly or not hydrated the hydration leads to a local gap in the seismicity in nicaragua the lateral transition between the hydrated and non hydrated areas occurs within a distance of about 10 km this transition coincides with a change in the crustal thickness in the order of 5 10 km thickening to the south and in the tectonic regimes the change in the tectonic regimes towards a stronger extension along the margin of nicaragua could be the key for understanding the observations the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low vp high vp vs and low seismicity

stability limits of hydrous minerals in sediment and mid ocean ridge basalt compositions implications for water transport in subduction zones phase relationships in hydrated natural sediment and mid ocean ridge basalt morb compositions were investigated experimentally from 6 to 15 gpa and 700 c to 1400 c in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle phengite and hydrous aluminum silicates topaz oh and phase egg are stable in the sediment composition at depths greater than 150 km and constitute h2o reservoirs similarly lawsonite is an h2o reservoir in morb composition at depths greater than 150 km according to mass balance calculations based on experimental run products the weight proportions of phengite and h2o at 6 gpa in the sediment are about 50 and 2 respectively at pressures from 8 to 12 gpa topaz oh al2sio4 oh 2 appears as the hydrous mineral and the sediment retains about 0 5 0 7 wt h2o at higher pressures phase egg alsio3 oh appears as the hydrous phase with a large stability field the h2o content in the sediment is about 0 5 wt at pressures greater than 12 gpa in natural morb compositions lawsonite is the hydrous phase at pressures lower than 10 gpa about 1 wt h2o in the morb composition is retained by lawsonite at 6 gpa and 800 c above 6 gpa a lawsonite out reaction is controlled by compositional changes of garnet and clinopyroxene if the oceanic crust descends with an overlying sediment layer water can be carried down into the transition zone by the sediment the amount of transported water is controlled by the chemical compositions of the subducting sediments water in the morb layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle causing the generation of a peridotite region including hydrous phases chondrodite clinohumite phase a and phase e if this hydrous peridotite is dragged down by the descending oceanic crust water can be transported into the transition zone by the subduction process the amount of transported water is controlled by the thermal state of the slab and the chemical composition of morb

evolution of subduction dip angles and seismic stress patterns during arc continent collision modeling mindoro island abstract here we dynamically model the temporal development of arc continent collision with particular attention to the evolution of slab dip angles and stress fields during approach to collisional locking suturing our modeling is based on a simplified representation of mindoro island and the southern manila trench which provide a natural laboratory in that convergence is ongoing to the north but collision is complete in the south so that distance of 2d slices along the arc may serve as a reasonable proxy for time we consider in detail the effects of the negative petrological buoyancy imparted to the slab upon encountering the thermally uplifted 410 km wadsleyite forming phase transition as well as the effects of initial separation distance between the incoming palawan continental fragment and the overriding philippine mobile belt assemblage of arc continent terranes despite simplifications in representing this tectonically complex region our model reproduces important seismic observations including the progressive steepening of slab dip and the growth of down dip extensional stresses during the progress of collision it also reveals that a significant contribution of order 100 mpa to the maximum attainable down dip extensional stress arises from the negative petrological buoyancy of the uplifted 410 km transition and it illuminates how the initial separation of arc continent fragments controls the temporal offset between collisional locking and the onset of negative petrological buoyancy finally associated calculations of maximum shear stress may offer a partial explanation for the aseismic nature of tomographically proposed slab extensions below the wadati benioff zone

early diagenetic alteration of organic matter by sulfate reduction in quaternary sediments from the northeastern arabian sea abstract hemipelagic sediments from the northeastern arabian sea off pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter om on the basis of elemental analysis of total organic carbon toc and total sulfur ts the decay of om by sulfate reducing bacteria were calculated concentrations of total iron reactive iron total nitrogen tn total phosphorus tp and hydrogen indices hi were used to assess organic matter reactivity and redox conditions stable sulfur isotopic compositions provided further information about pathway timing and mechanisms of sulfate reduction microscopical observations indicate that throughout the study area om is amorphous 75 95 in almost all samples with only a small contribution of terrigenous om bacterially mediated sulfate reduction leads to continuously decreasing toc ts ratios from 0 4 m to a burial depth of 2 3 m at this depth the anaerobic oxidation of particulate organic matter ceases sulfate reduction in this narrow interval accounts for the decay of up to 70 of the om which has primarily entered the anoxic domain during the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified the regeneration of up to 60 of tn and more than 50 of tp can lead to a feedback of nutrients to the seawater this recycling process may contribute to the high productivity in the northeastern arabian sea

imaging of subducted lithosphere beneath south america tomographic images are produced for the deep structure of the andean subduction zone beneath western south america the data used in the imaging are the delay times of p pp and pwp phases from relocated teleseismic earthquakes in the region regionally structural features larger than about 150 km are resolved by the data presentations of layer anomaly maps and cross sections reveal 1 the nazca slab is probably continuous laterally and at depth over most regions 2 the offset between the north and south deep earthquake zones containing the 1994 deep bolivia main shock and its aftershocks can be modelled by a northwest striking and steeply northeast dipping slab structure and 3 the nazca slab clearly penetrates the lower mantle beneath central south america but is partly deflected in the southern deep zone

seasonal dynamics of elemental sulfur in two coastal sediments a spectrophotometric method for elemental sulfur s0 analysis without interference from other reduced sulfur compounds was adapted for the use in reducing sediments the s0 distribution in two coastal sediments was studied regularly from summer to winter and compared to factors regulating the s0 accumulation such as redox potentials the rate of bacterial sulfide production and the general sulfur chemistry dense coatings of sulfur bacteria developed on the sediment surface of a sulfuretum which had an s0 concentration of up to 41 mol s cm 3 the 2 5 mm thick bacterial coating contained 40 of all s0 in the sediment a more typical marine sediment with a few cm thick oxidized surface layer had an s0 maximum of 1 3 mol s cm 3 at 2 4 cm depth the s0 maximum in both sediments increased from summer to winter as the sediments gradually became more oxidized the deeper layers maintained a low s0 concentration most of the s0 in the upper few mm of a laboratory sulfuretum was present inside sulfur bacteria and actively migrated up and down with the bacteria depending upon the changing light and oxygen conditions

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

variations in slab dip along the subducting nazca plate as related to stress patterns and moment release of intermediate depth seismicity and to surface volcanism 1 the subducting nazca plate shows a high degree of along strike heterogeneity in terms of intermediate depth seismicity 70 300 km orientations of slab stress and volcanism we compile the intermediate depth earthquakes of south america from the harvard centroid moment tensor cmt catalogue to determine along strike dip variations and we explore the variable level of correlation between these observations primary results are three fold 1 consistency among orientation of tension axes highest level seismicity and occurrence of volcanism is associated with steeply dipping regions 20 30 2 volcanism and intermediate depth seismicity exhibit no direct correlation suggesting that earthquake faulting is not a necessary condition for transport of dehydrated fluids out to the mantle wedge for initiation of melting 3 comparison of slab thermal structures between regions with and without volcanism suggests that for certain wedge widths the maximum temperature in the mantle wedge is higher for the former than for the latter

genesis of a variscan batholith field petrological and mineralogical evidence from the corsica sardinia batholith abstract the genesis of the corsica sardinia batholith csb can be explained in a continental collision setting during the time of construction of the batholith which is dated at about 70 ma three magmatic associations were emplaced mg k calc alkaline u1 composite mafic silicic calc alkaline u2 and post collisional alkaline u3 in addition the level of intrusion became progressively shallower from 5 6 kbar for u1 to less than ikbar for u3 in a large scale uplift associations u1 and u2 exhibit non cogenetic mafic and felsic components the processes and products of interaction between mantle and crustal derived melts are however different and may be related to the thickness of the crust which controls both the parameters t pco2 ph2o andptot and the time of mingling of the products of anatexis and basaltic melts both u1 and u2 granitoids were formed by crystallization of a magma derived by fusion of a source of greywacke composition the melting occurred successively under granulitic u1 and amphibolite facies conditions u2 in association u1 basaltic melts displaying potassic lamprophyre like composition up to 8 k2o are of uncertain affinity furthermore crustal interaction with the basic magma precludes precise definition of the source in the u2 composite association the primary tholeiitic character of the mantle derived basic rocks is constrained by both mineralogical and geochemical data dry and hydrated subunits are distinguished among the basic rocks associated with the granitoids no hybridization processes have been identified in the core of the mafic complexes of the dry subunits but they are recognized at the level of emplacement at the margins at the contact with the granitoids contamination in the mafic hydrated subunit is due to the increase in water content of crustal origin in the magma during the ascent and emplacement of the basic complexes leading to the development of abundant green amphibole extensive hybridization is restricted to the base of the crust where it led to incomplete mixing yielding granodiorites and enclaves the genesis and emplacement of the composite calc alkaline u2 association took place in an extensional setting in an uplifting basement postdating a collisional event during the devonian an eastern austro alpine block was thrust onto the western ebro balearic continent the u1 mg k granitoids resulted from anatexis under conditions of pco2 ph2o this took place at the base of the austro alpine crust which was undergoing fusion due to its adiabatic ascent the u2 calc alkaline granitoid rocks were derived in a hydrated environment by fusion of the same austro alpine crust in corsica and of the ebro balearic crust on sardinia and in the pyrenees the conditions of interaction between mafic and felsic magmas may serve as a geodynamic marker providing information on the thickness of the crust in time and space and on the genesis of continental crust the persistence during the palaeozoic of dominantly tholeiitic mafic magmatism suggests that hot spot activity occurred in this area of southwestern europe during pre variscan and variscan times up to the time of opening of the liguro piedmontese ocean birthplace of the alpine orogen

pyritization of iron in tropical coastal sediments implications for the development of iron sulfur and carbon diagenetic properties saint lucia lesser antilles abstract pore water chemistries and rates of pyritization in tropical coastal sediments were investigated in two semi protected bays praslin bay and savannes bay along the eastern side of the caribbean island of saint lucia sediments were collected from deltaic nearshore non deltaic and offshore environments and 210pb derived sedimentation rates ranged between 0 1 and 1 7 g cm2 yr sediments were mixed carbonate siliciclastic 2 44 caco3 and had higher average fehr fet ratios 0 4 0 6 than either global averaged continental margin sediments 0 28 or local bedrock and soils 0 11 0 15 organic carbon concentrations c n ratios and 13coc measurements demonstrated that the organic carbon in offshore savannes bay sediments was dominantly marine in origin while the nearshore savannes bay and offshore praslin bay sediments contained organic carbon from both marine and terrestrial sources organic matter in praslin bay deltaic sediments and in organic rich storm deposits was mainly 61 100 terrestrial in origin in most sediments spy concentrations and dop values increased from zero to 0 4 1 5 wt and 0 18 0 56 respectively as fed fet and c s ratios decreased with depth despite limited net sulfate reduction as marked by lower pore water so4 cl ratios and higher alkalinity values relative to overlying seawater in the upper 15 cm of sediment average dgt diffusive gradients in thin films sulfide fluxes were similar for all environments 5 8 nmol cm2 d except for savannes bay nearshore sediments which had a much higher average flux 76 nmol cm2 d average dgt iron fluxes were 5 13 times average dgt sulfide fluxes except in savannes bay nearshore sediments where the rates were comparable savannes bay nearshore sediments were also notable for possessing the greatest organic carbon 22 mg cm2 yr and pyrite sulfur 9 7 mg cm2 yr burial rates and represented the only environment where fedgt and sdgt based pore water compositions reached saturation with amorphous fes or mackinawite in all other environments sulfide production and likely iron sulfide formation preferentially occurred in microniche environments most 34spy values 22 and 34 vcdt indicated that the supply of sulfate was not limited during sulfate reduction though much higher values up to 7 8 identified pyrite formed in relatively closed system conditions sediment burial time vs ln fed fet and c s values showed that half lives of fed sulfidation were 71 159 years and that a normal marine c s ratio of 2 8 was attained after 22 79 years a comparison of net rates of pyritization with dgt iron fluxes demonstrated that iron was recycled less than 1 9 times before final burial overall pyrite sulfur burial rates correlated linearly with organic carbon burial rates providing further evidence that pyrite formation was primarily controlled by the deposition of organic carbon

an alkenone u 37 k quasi annual sea surface temperature record a d 1440 to 1940 using varved sediments from the santa barbara basin abstract analyses for c37 alkenones u37k and total organic carbon toc in a varved core from the depocenter of the santa barbara basin 34 13 5 n 120 01 w 590 m water depth have been made on an approximately 1 2 year basis from a d 1440 to 1940 the u37k based sea surface temperature sst record oscillates around 15 5 c with an amplitude of less than 3 c at approximately interannual decadal and centennial frequencies historical el nino southern oscillation enso events are matched only partially by warmer u37k ssts three records our sst a published pacific coral 18o and a published tree ring temperature anomaly from the california valleys show no evidence of colder temperatures during the little ice age ca 1500 to 1840 a d significantly there is a strong centennial scale cyclicity in the u37k sst record which approaches 1 c in amplitude toc burial and alkenone fluxes are largely controlled by sedimentation rate as expressed in varve thickness

sulfur enrichment of humic substances in a delaware salt marsh sediment core abstract humic sulfur operationally defined as the sulfur extracted with humic substances in 0 l n naoh solution comprises up to 51 of the total sulfur inventory in a sediment core taken from a delaware spartina alterniflora marsh pyrite sulfur is the next largest fraction except at the near surface sediments where zerovalent sulfur concentrations are significant x ray photoelectron spectroscopy indicates that the humic sulfur consists of sulfoxides or sulfones and in a more reduced state organic sulfides and or organic polysulfides a subsurface decrease in the humic acid c s atomic ratio to 56 2 suggests that the upper 4 cm of marsh sediment is the locus for humic sulfur formation s alterniflora detritus and microbial biomass cannot fully account for observed sulfur enrichment of humic c s atomic ratios therefore the enrichment of humic substances by sulfur is probably via reaction of reduced sulfur compounds with organic matter a humic sulfur formation rate of 10 6 mol s cm 3 a 1 is calculated for the surface sediments and leads to an areal production of 18 mol s cm 2 a 1 of humic sulfur humic sulfur formation and preservation is enhanced by the limited availability of iron for the rapid precipitation of iron sulfide minerals and the apparent resistance of organic sulfur compounds towards reoxidation to sulfate especially in the upper 9 cm of marsh sediment where inorganic sulfur compounds are rapidly oxidized

the ca 350 ma beja igneous complex a record of transcurrent slab break off in the southern iberia variscan belt abstract we report the results of an isotopic study of the large gabbro dioritic beja igneous complex bic in the boundary between the highly contrasting ossa morena om and south portuguese sp zones of the southern iberian variscan orogen this boundary is interpreted as a major suture zone materialized by discontinuous scattered strips of mafic ultramafic rocks the so called beja acebuches ophiolite complex baoc and by melange deposits of middle to late devonian age in the pulo do lobo accretionary prism plap the beja gabbro was interpreted either as part of the ophiolite like units or as a broadly arc related massif reflecting the northward subduction of oceanic lithosphere u pb zircon id tims dating of two diorites and a granodiorite points to igneous emplacement ages of 350 2 ma serpa 352 2 ma torrao and 353 4 ma sao pedro respectively whereas a felsic dyke yields a slightly younger age of 345 2 ma these results show that published ar ar dates do not represent igneous crystallization ages but merely reflect regional cooling below ca 500 c at least 10 ma after the major intrusive event probably as a result of uplift of the omz side of the suture zone relative to the subsiding spz 87sr 86sr350 and end350 display a large range of values from 0 7041 to 0 7093 and from 4 0 to 6 1 respectively which documents a rather complex petrogenetic history with an important role played by crustal contamination processes the more primitive sr and nd isotope signatures are measured in the mafic cumulates while radiogenic sr and unradiogenic nd isotope compositions occur in the more evolved rock types the broad trend of decreasing end350 with decreasing sm nd and increasing sio2 concentration is reminiscent of crustal assimilation combined with fractional assimilation afc end values of flasergabbros and associated cumulates ascribed to the ophiolite like unit in the guadiana valley are close to zero or even slightly negative irrespective of the age chosen for the correction for in situ radioactive decay of 147sm in the range 350 500 ma assumed to encompass the true geological age this observation conflicts with a truly oceanic derivation of these rocks which could rather represent early stage deformed and metamorphosed equivalents of the beja gabbro most metabasalts of the same area have positive albeit variable end consistent with mildly depleted mantle sources and or significant crustal assimilation only a few samples reach highly positive values end350 500 ca 8 similar to those typical for n morbs as do the acebuches amphibolites near aracena end350 500 ca 9 these data suggest that the baoc is not only disrupted but also composite since the gabbroic rocks are not cogenetic with the basalts based on the late devonian age of the collision between the omz and spz the emplacement of the beja gabbro cannot be related to an active subduction of oceanic lithosphere instead the generation in the mantle and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant lp ht metamorphism is tentatively ascribed to a process of transcurrent slab break off that followed oblique subduction and collision

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

an insight into the bucaramanga nest abstract we used the local seismicity for the period of 1993 to 2001 in the northeast of colombia to show the existence of two slabs in the north and south of the bucaramanga nest the northern slab has a dip angle of about 25 and the southern slab has a 50 dip angle while the dip in the bucaramanga nest is about 29 in order to explain the nature of the bucaramanga nest we proposed the scenario of collision between these two slabs using a 3d finite element model fem we show that collision can concentrate modify and perturb the stress field the active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the bucaramanga nest the perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro earthquakes and the complexity in the source of the moderate size earthquakes in the bucaramanga nest

geochemistry of inorganic and organic sulphur in organic rich sediments from the peru margin abstract we have determined 1 the abundance and isotopic composition of pyrite monosulphide elemental sulphur organically bound sulphur and dissolved sulphide 2 the partition of ferric and ferrous iron 3 the organic carbon contents of sediments recovered at two sites drilled on the peru margin during leg 112 of the ocean drilling program sediments at both sites are characterised by high levels of organically bound sulphur obs obs comprises up to 50 of total sedimentary sulphur and up to 1 of bulk sediment the weight ratio of s to c in organic matter varies from 0 03 to 0 15 mean 0 10 such ratios are like those measured in lithologically similar but more deeply buried petroleum source rocks of the monterey and sisquoc formations in california the sulphur content of organic matter is not limited by the availability of porewater sulphide isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface at least partly by reaction with polysulphides most inorganic sulphur occurs as pyrite pyrite formation occurred within surface sediments and was limited by the availability of reactive iron but despite highly reducing sulphidic conditions only 35 65 of the total iron was converted to sulphide 10 30 of the total iron still occurs as fe iii in surface sediments the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide either pyrite like monosulphide formed by direct reaction between dissolved sulphide and detrital iron and or the sulphur species responsible for converting fes to fes2 is isotopically similar to dissolved sulphide likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting fes2 fes ratio of 1 1 where pyrite dominates the pool of iron sulphides at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and or polysulphide elemental sulphur s0 is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment water interface in surface sediments s0 is isotopically heavier than dissolved sulphide fes and fes2 and is unlikely to have been involved in the conversion of fes to fes2 polysulphides are thus implicated as the link between fes and fes2

u pb geochronology and petrology of the late paleozoic gil marquez pluton magmatism in the variscan suture zone southern iberia during continental collision and the amalgamation of pangea the origin of plutonic complexes that stitch suture zones developed during collision is not well understood in southern iberia the pulo du lobo suture zone pdlz is intruded by the syn to postcollisional gil marquez pluton gmp thought to be part of the sierra norte batholith u pb la icpms zircon data on various phases of the gmp yield from oldest to youngest 1 a 354 4 7 6 ma unfoliated gabbro 2 a 345 6 2 5 ma foliated intermediate phase 3 a 346 5 5 4 ma unfoliated porphyritic granite 4 a 335 1 2 8 ma unfoliated biotite granite this sequence is consistent with cross cutting relationships observed in the field the range in ages is consistent with interpretations that the gmp is part of the composite ca 350 308 ma snb inherited ages preserved in the gmp intermediate and felsic phases indicate that its magmas traversed through south portuguese zone and pdlz crust during emplacement the ca 345 ma emplacement of the late kinematic foliated intermediate phase constrains the age of late stage strike slip deformation within the pdlz and the lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement nor during its subsequent history the unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision while the ca 335 ma biotite granite intrudes the santa ira flysch thereby providing a tight constraint for the latest stage of deformation in the pdlz

subtropical hydroclimate during termination v 430 422 ka annual records of extreme precipitation drought and interannual variability from santa barbara basin abstract hydroclimate extremes are expected to become more frequent and intense with anthropogenic climate forcing but future el nino southern oscillation enso behavior is unclear understanding of extreme hydroclimate variability and magnitude prior to human influences is limited by short instrumental records however continuous sedimentary archives of past hydroclimate variability can complement and extend these records laminated santa barbara basin sbb sediment cores mv0508 33jpc 21jpc and 29jpc preserve annually resolved multi centennial scale precipitation records from termination v t v the transition from glacial marine isotope stage mis 12 to interglacial mis 11 at 424 ka these records provide insights into the subtropical hydroclimate response to warming foraminiferal 18 o variations indicate rapid warming and or salinity changes and were used to verify the t v age assignment a paleoprecipitation proxy was developed using the first principal component of scanning xrf elemental counts pc1 which has high loadings for siliciclastic sediment associated elements k ti and si sedimentary laminae couplets identified in pc1 were annually tuned to investigate t v paleoprecipitation variability as modern sbb laminae represent annual deposits extreme flooding and decadal to centennial droughts were identified with magnitudes exceeding modern observations enso like 2 7 year paleoprecipitation periodicities coincide with wetter intervals but enso variability is reduced during droughts a 1500 year arid interval may be related to poleward shifting of general atmospheric circulation as ice sheets melted such that the subtropical dry zone intersected california southern california paleoprecipitation reconstructions from past warm climates provide insight into precipitation variability and magnitude on interannual timescales and can reduce uncertainties in predictions of interannual climate including enso

origin and diagenetic transformations of c25 and c30 highly branched isoprenoid sulphur compounds further evidence for the formation of organically bound sulphur during early diagenesis abstract a number of c25 and c30 highly branched isoprenoid hbi sulphur compounds e g thiolanes 1 oxo thiolanes thiophenes and benzo b thiophenes with 2 6 10 14 tetramethyl 7 3 methylpentyl pentadecane and 2 6 10 14 18 pentamethyl 7 3 methylpentyl nonadecane carbon skeletons were identified in sediments ranging from holocene to upper cretaceous these identifications are based on mass spectral characterisation desulphurisation and in some cases by comparison of mass spectral and relative retention time data with those of authentic standards the presence of unsaturated c25 and c30 hbi thiolanes in a recent sediment from the black sea age 3 6 10 3 a strongly supports their formation during early diagenesis the co occurrence of hbi polyenes c25 and c30 and unsaturated hbi thiolanes c25 and c30 possessing two double bonds less than the corresponding hbi polyenes in this recent sediment testifies to the formation of unsaturated hbi thiolanes by a reaction of inorganic sulphur species with double bonds of the hbi polyenes furthermore a diagenetic scheme for hbi sulphur compounds is proposed based on the identification of hbi sulphur compounds in sediment samples with different maturity levels

paleoproductivity ventilation and organic carbon burial in the santa barbara basin odp site 893 off california since the last glacial biogenic burial rates and coccolith assemblage data were generated for the past 16 ka at odp site 893 in the santa barbara basin off california to determine if dysoxic events in the basin were related to changes in marine productivity coccolith abundance data show that changes in surface water conditions did indeed change in concert with oxygen levels at the sea floor however organic carbon burial rates varied independently indicating that oxygenation of the water column is related mainly to intermediate water ventilation a strong correlation of organic carbon burial rates with lithogenic and to a lesser extent with carbonate and opal accumulation rates confirms recent conclusions that the export of organic carbon from the sea surface is largely controlled by the presence of ballast minerals

chronology petrogenesis and heat sources for successive carboniferous magmatic events in the southern central variscan vosges mts ne france plutonic bodies of the central and southern vosges mts can be assigned to two major early carboniferous magmatic events a visean mg k event c 345 and 340 336 ma and a younger s type event 329 322 ma new petrological geochemical and sr nd isotopic data highlight the existence of two groups of mg k intrusions that might be related to the nature of their primary magma sources that is chur like and enriched mantle which interacted with juvenile and mature crustal material respectively the differences between these two groups are explained by a geodynamic scenario involving deep subduction and relamination of the saxothuringian continental crust under the moldanubian continent the relaminated radiogenic saxothuringian material is thought to have been responsible for dehydration melting of both subducted crust and underlying metasomatized mantle thereby generating the mg k magma subsequently emplaced at middle crustal depth during their ascent the mafic magmas interacted with crustally derived felsic melts significantly later c 10 15 myr a widespread mid crustal anatexis occurred generating voluminous granite intrusions from mixed crustal sources paragneisses and or immature felsic intermediate metaigneous rocks mixed with mg k plutons the principal heat source for such a major melting event is related to the presence of mg k plutons rich in heat producing elements which were responsible after the time lag specified for a temperature increase at mid crustal levels by in situ radiogenic heat production the current study underlines the importance of deep continental crust subduction and relamination for the magmatism and development of collisional orogens supplementary material analytical methods and data and supplementary figures are available at http www geolsoc org uk sup18795

early variscan visean granites in the core of central pyrenean gneiss domes implications from laser ablation u pb and th pb studies abstract laser ablation u pb and th pb single zircon and monazite geochronology was applied to seven samples from two variscan gneiss domes in the central pyrenean axial zone detrital zircon ages obtained from a paragneiss in the western aston dome range from neoarchean ca 2 57 ga to late neoproterozoic ca 560 ma suggesting that late ediacaran sediments were deposited along the northern margin of gondwana cadomian magmatism is recorded for the first time in the central pyrenees from a biotite metagranite ca 545 ma in the central aston dome a second sample of an orthogneiss from the aston dome yields a middle ordovician protolith age ca 467 ma similar in age to other dated orthogneisses of the axial zone three samples from granite stocks situated in the core of the aston and bossost domes have crystallization ages of ca 339 337 ma providing the first evidence of visean igneous activity in the axial zone in contrast to the larger and significantly younger ca 312 305 ma granite plutons along the margins of the gneiss domes visean magmatism caused lp ht contact metamorphism m 2 migmatization throughout the aston dome though less well constrained by overgrowth ages on zircon ca 331 ma and monazite ages ca 340 320 ma is likely to be related to visean intrusions the main s 2 schistosity in the aston and bossost domes is coeval with the visean intrusions inclusion trails in porphyroblasts nucleating in the contact aureoles document the continuous development of s 2 related to local extension crystal plastic deformation of the granites indicates that the d 2 deformation event continued after their crystallization visean magmatism played a crucial role in early stages of dome formation by thermally weakening the metasedimentary mantle and to a certain degree the orthogneiss as well enhancing the development of orogen parallel shear zones in an overall n s compressive setting

organic matter preservation and sulfur uptake in sediments from the continental margin off pakistan abstract the bulk and molecular geochemical characteristics of a suite of kerogens isolated from organic matter rich sediments from the pakistan margin has been characterised by elemental analysis rock eval pyrolysis and pyrolysis gas chromatography py gc special attention was given to the geochemistry of alkylthiophenes which are thought to reflect the incorporation of sulfur into organic matter om in the context of early diagenetic sulfate reduction organically bound sulfur comprises up to 60 of total sedimentary sulfur the weight ratios for organic sulfur s org over organic carbon c org varies from 0 01 in some of the recently deposited sediments to 0 11 in older samples the results show that sulfur incorporation into organic matter occurs primarily within the upper few meters of the sediments the amount of sulfur which can be incorporated into the organic matter clearly depends on the quality of om the hydrogen index values hi which characterise the total amount of generated hydrocarbon equivalents of the pyrolysate in mg hc equiv g c org are related to the extent of reworking of organic matter either in the water column or at the sediment water interface or to remineralization processes by sulfate reduction itself which is strongly controlled by the availability of reactive iron high quality organic matter characterised by elevated hi values is capable of sequestering higher amounts of sulfur as expressed by the thiophene content which represent the organically bound sulfur of the pyrolysate in mg g c org as indicated by the ratio of thiophenes versus hi which characterises the proportion of thiophenes in the total pyrolysate sulfurisation of organic matter seems to be terminated at a ratio of 0 01

a fast and easy method to derive highest resolution time series datasets from drillcores and rock samples abstract we present a fast method to obtain highest resolution greyvalue datasets from digital core images these datasets are processed using a software package developed by us artificially introduced trends in the datasets are corrected disturbances i e cracks voids are eliminated to reduce the high level noise in data we have developed a vector filter that effectively removes such noise application of the method to odp drillcores from the santa barbara basin allowed exact correlation of two drillcores from the same basin the influence of the termination of the last glacial cold period on the flux rates in the basin are reflected in the greyvalue datasets we found a transitional period of about 3000 yr between the end of the last glacial cold period and the onset of post glacial climatic oscillation in the basin fourier analyses show that the high resolution greyvalue time series record climatic cycles with frequencies as high as 1 cycle yr varves

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

planktonic foraminiferal response to the 1997 1998 el nino a sediment trap record from the santa barbara basin here we report results on a sediment trap study from the santa barbara basin in which we examine the response of planktonic foraminifera to el nino conditions planktonic foraminifera census counts were performed on biweekly sediment trap samples collected over the interval spanning late summer 1995 through early spring 1998 a period that captured the transition from normal conditions in 1996 to full el nino conditions during early 1997 through early 1998 intra annual variations in the foraminifera assemblage reflect changes in seasonal climate variability significant differences occurring in both foraminiferal relative abundance and flux values between non el nino and el nino years warmer waters in the basin during late 1997 were reflected by a significantly increased presence of warmer water species generally absent under normal conditions surprisingly despite greatly reduced upwelling conditions foraminiferal flux values during the spring 1997 upwelling season were more than four times greater than the year before the contrast between normal and el nino foraminifer populations has important implications for reconstructing the past history of both el nino events and more general climate histories from the santa barbara basin varved sediment record

diagenesis and iron paleo redox proxies new perspectives from magnetic and iron speciation analyses in the santa barbara basin abstract sedimentary redox proxies are usually employed to reconstruct the paleo redox conditions of bottom water environments assuming that porewater and bottom water dissolved oxygen concentrations are similar using a combination of geochemical and magnetic techniques we investigate the relationship between iron speciation and mineralogy in recent 1760 2009 ce sediments retrieved from the santa barbara basin sbb a modern silled basin with low oxygen dissolved o2

evolution of a kyanite bearing belt within a ht lp orogen the case of nw variscan iberia kyanite replaces andalusite in a belt of ordovician and silurian pelitic rocks that form a narrow synform pinched between high grade antiforms in nw variscan iberia kyanite occurs across the belt in al rich black pelites in assemblages i kyanite chloritoid chlorite muscovite and ii kyanite staurolite chlorite muscovite in i kyanite occurs in the matrix and in kyanite muscovite aggregates that pseudomorph earlier andalusite porphyroblasts the aggregates are found across the belt and can still be recognized in assemblage ii and even in iii andalusite staurolite biotite muscovite this latter being a hornfelsic silurian schist where kyanite is relic and staurolite occurs in the matrix and is resorbed inside new massive pleochroic andalusite kfmash and mnkfmash pseudosections have been constructed using thermocalc for al rich and al poorer compositions from the belt chloritoid zoning in al rich rocks containing assemblage i plus chloritoid chlorite thermometry complemented with garnet chlorite thermometry in al poorer lithologies mean that the path is one of increasing pressure and temperature conditions prior to assemblage i with earlier andalusite stable are those of the andalusite chloritoid chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims the passage from assemblage i to ii implies a prograde path within the kyanite field assemblage iii represents peak conditions indicating a prograde staurolite consuming reaction across a kfmash field leading eventually to a locally found andalusite biotite muscovite hornfels the lowest pressure stages are recorded by cordierite biotite in al poor pelites garnet bearing mnkfmash assemblages in al poorer pelites record conditions similar to assemblages ii and iii the replacement of andalusite by kyanite in assemblage i is attributed to downdragging of andalusite bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the vivero fault

evidence of a slab of subducted lithosphere beneath central taiwan from seismic waveforms and travel times abstract knowing whether there is a slab of subducted lithosphere beneath central taiwan is important for our understanding of the regional tectonic evolution in this study we aim at resolving this issue by investigating the seismic effects of a potential near receiver slab using data from a local broadband array rays from deep and intermediate depth earthquakes in the tonga kermadec subduction zone to central taiwan stations exhibit reduced amplitudes and travel times relative to kmnb in the 0 1 0 4 hz band with a correlation between the degree of reduction and earthquake latitudes the overall amplitude reduction observed at central taiwan stations is not observed for earthquakes from the hindu kush our analysis suggests that the observations are not caused by crustal structure but by receiver side heterogeneous mantle or more specifically an eastern dipping aseismic slab beneath central taiwan the extent of the slab is determined by jointly modeling amplitude and travel time observations at sslb and tpub using a 2 d pseudospectral method

petrology of garnet cordierite sillimanite gneisses from the el tormes thermal dome iberian hercynian foldbelt w spain the core of the el tormes thermal dome situated in the central part of one of the main metamorphic belts of the iberian peninsula is formed by garnet cordierite biotite sillimanite pelitic gneisses these rocks that very often are cut by minor intrusions of al rich s type granites are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite plagioclase biotite the garnet mantled and corroded mainly by cordierite has never been found to occur in contact with the prismatic sillimanite of the matrix thus indicating that the continuous reaction gr sill q cd has taken place the presence of corroded biotite inside the garnet rimming cordierite of the aggregates as well as inside the cordierite of the matrix which usually includes remmants of sillimanite indicates that the continuous reaction bi sill q cd fk h2o has occurred too therefore a realistic net reaction for these aggretates should be represented by the univariant at a given x text h text 2 text o equilibrium biotite sillimanite garnet quartz cordierite k feldspar h2o 1

zircon thermometry and u pb ion microprobe dating of the gabbros and associated migmatites of the variscan toledo anatectic complex central iberia in the central iberian zone there are several large thermal domes in which small bodies of ultramafic mafic and intermediate rocks appear intimately associated with crustal granites and migmatites the closest spatial association between the ultramafic mafic and intermediate rocks and migmatites is in the toledo anatectic complex where field relationships suggest that these rocks are coeval and have an age close to 340 ma this and the recent discovery in the neighbouring ossa morena zone of a large mid crustal seismic reflector interpreted as a 335 350 ma mafic sill reinforce the hypothesis that heat for crustal melting was supplied from early variscan mantle magmas emplaced in the middle crust however precise ion microprobe u pb zircon dating and ti in zircon thermometry in toledo do not support this idea whereas the mean age of four mafic bodies is 307 2 ma the migmatites are c 25 ma older the migmatites hosting ultramafic mafic and intermediate bodies have the same age and ti in zircon temperatures as migmatites far from any mafic intrusion these data reveal that ultramafic mafic and intermediate magmas are late variscan they were emplaced in already cooling anatectic zones once the extensional collapse was initiated and their thermal impact on the mid crustal variscan anatexis of central iberia was negligible

diachronous post orogenic magmatism within a developing orocline in iberia european variscides 1 u pb zircon crystallization ages of 52 late variscan granitoid intrusions from nw iberia 19 from new data 33 from previous studies constrain the lithospheric evolution of this realm of the variscan belt of western europe and allow assessment of the relationship between oroclinal development and magmatism in late carboniferous early permian times the u pb ages in conjunction with a range of geological observations are consistent with the following sequence of events i oroclinal bending starts at 310 305 ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts ii between 305 and 300 ma melting continues under the outer arc of the orocline central iberian zone of the iberian variscan belt and mid crustal melting is initiated coevally the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure iii between 300 and 292 ma foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle iv cooling of the lithosphere between 292 and 286 ma resulting in a drastic attenuation of lower crustal high temperature melting by 285 ma the thermal engine generated by orocline driven lithospheric thinning delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts the model proposed explains the genesis of voluminous amounts of granitoid magmas in post orogenic conditions and suggests that oroclines and similar post orogenic granitoids common constituents of numerous orogenic belts may be similarly related elsewhere

evolutionary changes over the last 1000 years of reduced sulfur phases and organic carbon in varved sediments of the santa barbara basin california abstract sediments from the santa barbara basin sbb of the last 1000 years were analyzed for elemental sulfur se mineral sulfide sulfur sm lipid based organically bound sulfur sx residual organically bound sulfur so total organic carbon toc and total nitrogen tn the approximate annual resolution of the time series data permitted us to evaluate the influence of oceanic variables on the benthic environment and the accumulating varved sediment in the sbb a bacterial mat community at or near the sediment surface is influencing sediment porosity and the production and distribution of se early diagenesis dampens the amplitude of environmental signals in the geochemical time series the concentrations of sm and so increase with burial depth at the expense of decreasing concentrations of sx and se at depths greater than 5 5 cm corresponding to an age of 7 years sm becomes the largest pool of reduced sulfur it requires approximately 500 years of primarily bacterially mediated diagenesis equivalent to a burial depth of 1 4 m for sbb sediments to gradually approach and finally enter the region assumed to represent typical normal marine sediments on a plot of weight percent toc vs sm

particulate iron and manganese in the santa barbara basin california abstract particulate fe and mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes a study has been made of suspended particulate composition in the santa barbara basin a shallow near shore basin off southern california with sub oxic conditions below sill depth observations have revealed several interesting phenomena relating to the geochemistry of fe and mn most striking is a profound enrichment of particulate fe in samples from the bottom two hundred meters these particulates have a constant fe p mole ratio of about three and may originate at the sediment water interface or may be transported to the basin from local marshes for particulate mn enrichments are observed both in the sub sill waters and near the base of the euphotic zone a consideration of particle removal rates suggests that the sub photic zone enrichment has a biogenic origin in the sub sill waters enrichment in mn is apparently due to the precipitation of dissolved mn diffusing from the anoxic basin sediments a simple mass balance suggests that most of the mn lost from the sediments is transported from the santa barbara basin in dissolved form

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

stress fields associated with metastable phase transitions in descending slabs and deep focus earthquakes inside subducting slabs the interaction of different metastable phase transformations olivine spinel spinel spinel spinel perovskite magnesiowustite associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates we have described the thermal field within the downgoing slab in a lagrangian framework by a self consistent high resolution two dimensional thermokinetic coupling model which also incorporates the effects of nucleation site saturation using an adaptive method the threshold for the nucleation site saturation is found to be less than 0 1 transformation degree in the cold slab interior this can significantly shift the metastable wedge of olivine deeper as suggested from previous thermokinetic slab models however this effect is compensated by the latent heat release during the olivine spinel and spinel spinel phase transitions latent heat release produce very sharp phase boundaries in the slab for fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior however the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field we have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up to date temperature field and physical properties determined from recent high p t experiments we find that the slab is denser up to 100 kg m 3 than the surrounding mantle remarkable denser portions up to 260 kg m 3 are located just below the uplifted and equilibrium phase transitions while lighter portions up to 210 kg m 3 can be found just above the depressed spinel to perovskite magnesiowustite transition and for the metastable wedge of olivine these density differences effectively act as a body force and produce a significant stress field which we calculated with a two dimensional finite element code the results show that the buoyancy induced forces produce maximum shear stress up to 23 mpa along the metastable wedge and deeper portion just above the depressed last phase transition involving spinel for the latter the dominant state of stress is down dip compression the calculated p t dependent state of stress is very similar to the depth distribution of deep focus

from extreme rainfall to drought 250 years of annually resolved sediment deposition in santa barbara basin california abstract the mediterranean climate of southern california is marked by droughts and extreme precipitation events here we use elemental variations generated by scanning x ray fluorescence xrf to identify droughts and floods in recently deposited 1755 2008 sediments of santa barbara basin sbb from box core spr0901 04bc the first principal component pc1 of the scanning xrf elements has high loadings for elements associated with the lithogenic component of sbb laminae couplets while the second pc2 is associated with biogenic components we interpret pc1 as a proxy for river runoff and pc2 as a proxy for marine productivity high values of pc1 are associated with el nino events and positive warm phases of the pacific decadal oscillation pdo while low values of pc2 are associated with el nino events and negative cool phases of the pdo droughts such as the 1934 40 1949 56 and 1989 91 events coincide with low pc1 values in addition to distinguishing interannual and decadal variability in the elemental composition of sbb sediments several historic floods can be recognized including a gray flood layer associated with the 1861 62 flood and a peak in pc1 associated with the flood following the st francis dam disaster in 1928

earthquakes and strain in subhorizontal slabs 1 using an extensive database of fault plane solutions and precise locations of hypocenters we show that the classic patterns of downdip extension dde or downdip compression ddc in subduction zones deteriorate when the dip of the slab is less than about 20 this result is depth independent demonstrated by both intermediate focus depths from 70 to 300 km and deep focus depths greater than 300 km earthquakes the absence of pattern in seismic strain in subhorizontal slabs also occurs locally over scales of about 10 km as evident from a detailed analysis of a large mw 7 1 earthquake sequence beneath fiji following the paradigm that a uniform strain of dde ddc results from sinking of the cold dense slab as it encounters resistance from the highly viscous mantle at depth breakdown of dde ddc in subhorizontal slabs reflects waning negative buoyancy slab pull in the downdip direction our results place a constraint on the magnitude of slab pull that is required to dominate over localized sources of stress and to align seismic strain release in dipping slabs under the condition of a vanishing slab pull eliminating the only obvious source of regional stress the abundance of earthquakes in subhorizontal slabs indicates that a locally variable source of stress is both necessary and sufficient to sustain the accumulation of elastic strain required to generate intermediate and deep focus seismicity evidence is growing that the process of seismogenesis under high pressures including localized sources of stress is tied to the presence of petrologic anomalies

h type hybrid granitoids a proposed revision of the granite type classification and nomenclature abstract recent studies on calc alkaline plutonic rocks reveal field and petrographic relationships with strong implications for the processes involved in their genesis the presence of magmatic inclusions and magma mingling zones in these rocks supports an origin by magma mixing compositional variations and isotopic anomalies are a good test for such petrogenetic models most of these calc alkaline plutonic rocks which characterize both collision related and active plate margin environments have been classically identified as i igneous type granitoids following chappell and white s 1974 classification and then interpreted as derived from partial melting of older igneous rocks restite model if the magma mixing origin of a type of granitoids can be demonstrated from field and petrographic evidence and supported by chemical and isotopic variations the nomenclature must be changed in this sense the new category of h hybrid type is tentatively introduced it includes most of the so called i type granitoids and some of the s type as deduced from field relationships of magma mingling zones a mixing model is suggested that can operate on a large scale to explain the petrogenesis of the hercynian calc alkaline granodiorites of iberia finally a modified classification of common plutonic rocks in orogenic environments is proposed

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

phase transition buoyancy contributions to stresses in subducting lithosphere the sequence of phase transitions undergone by minerals with increasing depth in earth s mantle is perturbed within subducting lithospheric slabs by their thermal structure such perturbation of equilibrium phase relations gives rise to relative buoyancy contrasts between slab and mantle that contribute to the state of stress within the slab while other factors contribute to overall slab stresses thermal and phase transition effects largely control the structure of the stress field within the slab the resulting maximum in down dip compressive stress within the slab corresponds to the observed peak in depth distribution of deep seismicity furthermore metastable persistence of lower pressure phases within the cold slab should give rise to localized shear stresses whose distribution corresponds to observed features of subduction zone seismicity these observations are independent of the variety of failure mechanisms proposed for deep seismogenesis

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

the relationship between wadati benioff zone geometry and p t and b axes of intermediate and deep focus earthquakes for 865 earthquakes with focal depths exceeding 90 km this paper presents a concise global summary of the distributions of p t and b axes of their focal mechanisms with respect to the downdip along strike and normal axes of wadati benioff zones the focal mechanisms are best double couple solutions for the centroid moment tensors reported by the harvard group with the p t and b axes corresponding to the principal axes of the individual moment tensors we find as did previous investigators that events below 300 km have predominantly downdip p axes with 50 of all p axes of deep focus events lying within 28 of downdip for events between 90 and 300 km depth there are regional differences but most regions possess events with nearly downdip p axes or events with nearly downdip t axes in almost all regions having downdip p or t axes bingham statistics show that the greatest concentration of p or t axes lies somewhat below the plane of the wadati benioff zone for events of all focal depths in nearly all regions b axes cluster along the strike direction with 50 occurring within 38 of the strike from these regional results we define a typical focal mechanism as one having the b axis within 30 of the along strike and having either the t or p axis within 30 of the downdip such a focal mechanism might occur in response to a stress tensor having its greatest or least principal stresses channeled approximately along the downdip direction of the subducting slab with the intermediate principal stress lying horizontally within the slab controlled possibly by slab bending however by this criterion only 29 of the focal mechanisms for deep and intermediate events are typical thus the majority of the individual events cannot be explained simply as occurring in response to a downdip and slab normal greatest and least principal stress system with a horizontal along strike intermediate principal stress a few regions such as java and the marianas are clearly anomalous in that they possess significantly fewer typical focal mechanisms than do most other regions

porphyry copper deposit density estimating numbers of undiscovered mineral deposits has been a source of unease among economic geologists yet is a fundamental task in considering future supplies of resources estimates can be based on frequencies of deposits per unit of permissive area in control areas around the world in the same way that grade and tonnage frequencies are models of sizes and qualities of undiscovered deposits to prevent biased estimates it is critical that for a particular deposit type these deposit density models be internally consistent with descriptive and grade and tonnage models of the same type in this analysis only deposits and prospects that are likely to be included in future grade and tonnage models are employed and deposits that have mineralization or alteration separated by less than an arbitrary but consistent distance 2 km for porphyry copper deposits are combined into one deposit only 286 deposits and prospects that have more than half of the deposit not covered by postmineral rocks sediments or ice were counted nineteen control areas were selected and outlined along borders of hosting magmatic arc terranes based on three main features 1 extensive exploration for porphyry copper deposits 2 definable geologic settings of the porphyry copper deposits in island and continental volcanic arc subduction boundary zones and 3 diversity of epochs of porphyry copper deposit formation porphyry copper deposit densities vary from 2 to 128 deposits per 100 000 km2 of exposed permissive rock and the density histogram is skewed to high values ninety percent of the control areas have densities of four or more deposits 50 percent have densities of 15 or more deposits and 10 percent have densities of 35 or more deposits per 100 000 km2 deposit density is not related to age or depth of emplacement porphyry copper deposit density is inversely related to the exposed area of permissive rock the linear regression line and confidence limits constructed with the 19 control areas can be used to estimate the number of undiscovered deposits given the size of a permissive area in an example of the use of the equations we estimate a 90 percent chance of at least four a 50 percent chance of at least 11 and a 10 percent chance of at least 34 undiscovered porphyry copper deposits in the exposed parts of the andean belt of antarctica which has no known deposits in a permissive area of about 76 000 km2 measures of densities of deposits presented here allow rather simple yet robust estimation of the number of undiscovered porphyry copper deposits in exposed or covered permissive terranes

a 9000 year flood history for southern california a revised stratigraphy of varved sediments in santa barbara basin abstract the center of santa barbara basin sbb preserves annual laminations through most of the holocene providing an important locality for high resolution late quaternary paleoclimate and paleoceanography reconstructions an accurate chronology is necessary for these sediment based records and enables comparisons with more distal data time series of similar quality here we present an improved high resolution radiocarbon chronology for the last 9000 yrs based on 89 accelerator mass spectrometric ams 14c dates of mixed planktonic foraminiferal carbonate from three sediment cores collected in sbb mv0811 14jc spr0901 06kc and odp hole 893a accurate core to core correlation is demonstrated using prominent gray flood and olive turbidite layers identified in sbb and dated in multiple cores gray layer deposits were found more frequently in wetter intervals as determined by multi proxy hydroclimate records in southern california e g lake deposits and tree rings mass accumulation rates mars calculated for the sbb depocenter using the improved radiocarbon age model indicate high mars are associated with these gray layers that have been associated with floods thus sediment accumulation in sbb is largely controlled by sediment delivery via river runoff following precipitation events although mars may also be influenced by other factors that enhance erosion in river catchments folded sediment is observed within the basal core section of mv0811 14jc which stratigraphically correlates with 14c age reversals in odp hole 893a we associate this sediment slump feature with the large goleta submarine slide complex in santa barbara channel and estimate the submarine slide event occurred at 9000 200 yrs bp furthermore small olive turbidite layers identified and dated in sbb can be temporally linked with large earthquakes along san andreas fault and can therefore potentially reconstruct earthquake history in southern california

genesis and evolution of a syn orogenic basin in transpression insights from petrography geochemistry and sm nd systematics in the variscan pedroches basin mississippian sw iberia abstract the pedroches basin is an outstanding syn orogenic variscan mississippian depocenter located in the vicinity of and overstepping onto the neoproterozoic cadomian suture between the ossa morena and central iberian zones of the sw iberian massif its formation appears to be related to transtensional processes at a major releasing bend in the reactivated suture during sinistral escape of the ossa morena zone from the zone of frontal variscan collision between northern gondwana and laurussia subsequent basin inversion resulted in compartmentalization and internal deformation and was probably related to transpression at a restraining bend along the reactivated suture badajoz cordoba shear zone basin fill consists of both igneous and metasedimentary rocks generally deposited in a shallow marine storm dominated platform environment by using new petrographic geochemical and sm nd isotopic data we attempt to characterize 1 the sources and geological evolution of the pedroches basin fill and 2 the processes involved in basin development and sediment supply and dispersal sampling was carried out in both sedimentary and igneous rocks belonging to three major structural units within the central part of the basin guadiato unit guadalbarbo unit and south pedroches valley unit the combined nd isotopic and geochemical data from the pedroches mississippian metasedimentary rocks indicate an upper continental crust provenance and a high degree of sedimentary recycling of these materials throughout the basin i e they are derived from old recycled upper crust without any significant juvenile component despite the presence of interbedded basaltic rocks the igneous rocks in turn correspond to submarine basalts and trachy andesitic basalts plus some hybrid rocks including crust derived components concerning the mafic rocks major element compositions reveal a tholeiitic and transitional tholeiitic character for the guadalbarbo unit rocks morb type and an alkaline affinity for those of the guadiato unit trace element analyses along with sm nd isotopic ratios suggest two kinds of provenance sources for the pedroches basin syn sedimentary igneous rocks 1 an important crust contaminated and or old enriched mantle source for the guadiato unit and the pedroches batholith and 2 a time integrated depleted mantle source for the guadalbarbo rocks in which no significant interaction with continental crust components is discernible these latter rocks suggest that a strong attenuation or even almost complete tectonic and or thermal erosion of the continental lithosphere was reached at least locally in this region

continental margin deformation along the andean subduction zone thermo mechanical models the chilean andes extend north south for about 3000 km over the subducting nazca plate and show evidence of local rheological controls on first order tectonic features here rheological parameters are tested with numerical models of a subduction zone incorporating slab pull and upper plate convergence which calculate the development of stress and strain over a typical period of 4 myr the models test the effects of subduction interface strength arc and fore arc crust rheology and arc temperature on the development of superficial near surface faulting as well as viscous shear zones in the mantle deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities upper plate shortening and trench advance are both correlated and favored to a first order by upper plate weakness and to a second order by interface strength in cases of a strong interface a weak forearc crust is dragged downward by tectonic erosion a scenario for which indications are found along the northern chilean margin in contrast for a resistant forearc the slab pull force transmits to the surface and produces topographic subsidence this process may explain present day subsidence of the salar de atacama basin and or the persistence of a central depression specific conditions for northern chile produce a shear zone that propagates from the subduction zone in the mantle through the altiplano lower crust into the subandean crust as proposed by previous studies models with a weak interface in turn allow buoyant subducted material to rise into the continental arc in case of cessation of the slab pull this buoyant material may rise enough to change the stress state in the continental crust and lead to back arc opening in a case of young and hydrated oceanic plate forced by the slab pull to subduct under a resistant continent this plate is deviated and indented by the continental mantle and stretches horizontally at 100 km depth this situation might explain the flat wadati benioff zone of central chile

distribution and sources of polycyclic aromatic hydrocarbons pahs in laminated santa barbara basin sediments abstract the santa barbara basin sbb preserves a high resolution sediment record that is varved down to ca 1700 ce and laminated for at least 2500 years the distributions of 25 polycyclic aromatic hydrocarbons pahs were investigated in a suite of samples from sediment cores representing different depositional scenarios varves laminae terrigenous flood layers and marine turbidites the concentrations of these pahs were determined using gas chromatography mass spectrometry in selected ion mode and evaluated in terms of their distributions diagnostic ratios and loadings recognized by principal component analysis pca two biogenic pahs perylene and retene provided the greatest variation in their downcore concentration values perylene concentrations increased downcore exhibiting markedly higher abundance in sediments older than 300 ce consistent with the interpretation that perylene is formed diagenetically in reducing sediments a modest increase in perylene concentrations in sediments from the late 1960s may be associated with flood derived transport of higher amounts of degraded terrestrial organic matter from fire denudated landscapes into the coastal environment the markedly higher concentrations of retene in flood layers and turbidites relative to varves laminae suggest that these lithologies are more effective in transporting pyrogenic pahs and can be interpreted as a proxy for wildfires in sbb changes in key diagnostic ratios reveal the shift from a dominance of petrogenic pahs in older sediments to a strong pyrogenic signal beginning in the latter half of the 20th century statistical analysis using pca of the pah distributions in the samples revealed that modern sediments showed a strong positive relationship with pc1 which represents the greatest variation among the samples whereas flood layers showed a strong relationship with the second major variable pc2 thus statistical approaches provide the capability to elucidate modern anthropogenic signatures within the complex pah distributions of sbb and also establish a pah profile that characterizes the lithological signature of flood layers

the role of hydraulic fractures and intermediate depth earthquakes in generating subduction zone magmatism the presence of magmatism and intermediate depth 70 300 km deep seismicity at subduction zones is at first sight surprising magmatism is unexpected because the subduction of cool oceanic lithosphere makes these regions the coldest in the mantle the current model for subduction zone magmatism is that water released from the subducting slab enters the relatively warm mantle wedge leading to a reduction in melting temperature and magmatism1 2 3 4 but there is a problem with this scheme because it is thought that water cannot leave the slab by porous flow to enter the wedge the occurrence of intermediate depth earthquakes is surprising because of the inhibitory effect of the very high frictional stress on faults expected from the high pressure at these depths one proposal put forward to explain intermediate depth seismicity is that high pore pressure might facilitate faulting by decreasing the friction5 6 7 the hypothesis presented here is that non percolating water provides the high pore pressure that the consequent faulting temporarily interconnects the water pores and when a sufficient vertical height of water is interconnected a hydrofracture is produced which transports the water out into the mantle wedge thereby generating subduction zone magmatism

a new view for the geodynamics of ecuador implication in seismogenic source definition and seismic hazard assessment a new view of ecuador s complex geodynamics has been developed in the course of modeling seismic source zones for probabilistic seismic hazard analysis this study focuses on two aspects of the plates interaction at a continental scale a age related differences in rheology between farallon and nazca plates marked by the grijalva rifted margin and its inland projection as they subduct underneath central ecuador and b the rapidly changing convergence obliquity resulting from the convex shape of the south american northwestern continental margin both conditions satisfactorily explain several characteristics of the observed seismicity and of the interseismic coupling intermediate depth seismicity reveals a severe flexure in the farallon slab as it dips and contorts at depth originating the el puyo seismic cluster the two slabs position and geometry below continental ecuador also correlate with surface expressions observable in the local and regional geology and tectonics the interseismic coupling is weak and shallow south of the grijalva rifted margin and increases northward with a heterogeneous pattern locally associated to the carnegie ridge subduction high convergence obliquity is responsible for the north andean block northeastward movement along localized fault systems the cosanga and pallatanga fault segments of the north andean block south american boundary concentrate most of the seismic moment release in continental ecuador other inner block faults located along the western border of the inter andean depression also show a high rate of moderate size earthquake production finally a total of 19 seismic source zones were modeled in accordance with the proposed geodynamic and neotectonic scheme

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

phytane from chemolytic analysis of modern marine sediments a product of desulfurization or not a set of modern marine sediments was analyzed to evaluate the role of sulfurization as a mechanism for biomarker preservation during early diagenesis the set consisted of sediments accumulating within various oxic to anoxic depositional environments raney nickel treatment of the polar fraction of total extractable lipids ptel from each sample yielded 3 7 11 15 tetramethylhexadecane phytane as the dominant product accompanied in several cases by minor levels of c27 c28 and c29 5 h steranes n c31 alkane and carotane although others have ascribed such products to a desulfurization reaction our work reveals significant production of phytane from the action of raney nickel on the esterified side chain of chlorophyll results suggest that catalytic dehydration hydrogenation of chlorophyll accounts for 50 and 4 of the raney nickel phytane rnphy yield measured in suboxic sediments from the washington shelf and anoxic sediments from saanich inlet respectively if 8 20 of the pheopigment detected in the suboxic sediments and all of the pheopigment detected in the anoxic sediment exists as pheophytin the catalytic dehydration hydrogenation process would account for 100 and 50 respectively of the observed rnphy yield the collective findings from our work underscore the necessity to exercise extreme caution when interpreting the geochemical significance of hydrocarbon products from raney nickel treatment of ptel from sediments despite the organic chemical fact that raney nickel is a well known desulfurizing agent desulfurization cannot be assigned necessarily as the source of hydrocarbons generated by the action of this reagent on complex lipid extracts from sediments particularly those from modern environments

natural vs human induced facies changes in recent shallow fjord sediments of the store lungegardsvannet in bergen western norway a basin wide change from an older homogeneous bioturbated to a younger laminated facies occurs in sediments from the 26 m deep permanently anoxic store lungegardsvannet fjord the younger facies consists of alternating packages of light and dark layers each containing millimetre scale laminae the composition of the dark layers differs from that of the light layers in the occurrence of predominantly marine benthic diatoms larger grain size lower loss on ignition loi and lower trace metal cu pb zn concentrations total trace metal concentrations correlate with the redox sensitive trace elements mo and v r 0 8 and with loi r 0 6 the correlation of the minerogenic matter and the grain size related elements al and li is poor r 0 2 the shoaling of the sill depth and reduction of the fjord area resulting in the reduction of bottom water exchange accounted for the shift from bioturbated to laminated sediments during the end of the last century the alternating changes within the laminated facies are the result of alternating wind forces during years of calm wind conditions 1908 1919 and 1945 1961 undisturbed sedimentation of organic matter from primary production and an associated accumulation of trace metals prevail years of high wind force 1919 1945 and 1961 1991 trigger shore erosion and an increased influx of minerogenic matter

a global survey of stress orientations in subducting slabs as revealed by intermediate depth earthquakes summary we examine a variety of mechanisms that have been proposed as contributors to the stress fields expressed as intermediate depth seismicity in subducting slabs to this end we study principal stress orientations for a global data set of 1900 intermediate depth focal solutions determining the patterns of events characterized primarily by downdip compression downdip tension or neither in regions dominated by downdip principal stresses we find that conjugate stress axes exhibit preferential slab normal orientations furthermore we observe a clear trend of thermal control in which colder slabs exhibit greater components of downdip compression while warmer slabs display greater downdip tension in those regions not dominated by downdip principal stresses a significant number of events exhibit lateral stresses in the form of subhorizontal principal axes in the plane of the slab we conclude that the evidently complementary roles played by lithospheric age and subduction rate in constraining stress regimes support thermomechanical and petrological buoyancy models for control of intermediate depth stresses moreover observed lateral stresses support the traditional model of a squeezed ping pong ball and stress patterns overall are consistent with some influence by reactivated fossil faults

el nino in santa barbara basin diatom radiolarian and foraminiferan responses to the 1983 el nino event abstract santa barbara basin box cores collected in august of 1985 were sub sampled and processed in search of microfossil evidence of the 1983 el nino microplankton and microbenthos indicate the presence of that el nino bounded by non el nino conditions specifically the event is recognized by an increase in diatom diversity and the presence of several warm water taxa e g coscinodiscus plicatoides hemidiscus cuneiformis nitzschia kolaczekii pseudoeunotia doliolus thalassiosira lineata thalassiosira nodulolineata an increase in polycystine radiolarian flux and the presence of the eastern tropical pacific species spongaster pentas an increase in the diversity of planktonic foraminifera and the occurrence of the warm water species globoturborotalita rubescens there is also evidence for an oxygenation of the basin during the 1983 el nino as suggested by the increase in the abundance of the benthic foraminifera bolivina vaughani and by the decrease in the preservation of phaeodarian radiolarian spines which are best preserved under anoxic conditions a new onboard procedure for sampling of box cores and freezing of subcores is described

separation and recovery of various sulphur species in sedimentary rocks for stable sulphur isotopic determination abstract existing methods to extract particular forms of sulphur in rocks and sediments are reviewed and evaluated for application in a chemical scheme to recover sulphur as either ag2s or baso4 precipitates for isotopic analysis the s species distinguished include acid soluble sulphides and sulphates elemental s pyrite barite and various categories of organically bound s there is no evidence of cross contamination or isotopic fractionation during the separation as applied to mixtures synthesized from internal control samples the precision obtained on repetitive extractions is equivalent to that of the measurement technique itself typically 0 2 in the 34s value the chemical separation scheme is demonstrated by application to sections of peat collected from the howards pass xy deposit in the yukon territory canada

can earth s rotation and tidal despinning drive plate tectonics abstract we re evaluate the possibility that earth s rotation contributes to plate tectonics on the basis of the following observations 1 plates move along a westerly polarized flow that forms an angle relative to the equator close to the revolution plane of the moon 2 plate boundaries are asymmetric being their geographic polarity the first order controlling parameter unlike recent analysis the slab dip is confirmed to be steeper along w directed subduction zones 3 the global seismicity depends on latitude and correlates with the decadal oscillations of the excess length of day lod 4 the earth s deceleration supplies energy to plate tectonics comparable to the computed budget dissipated by the deformation processes 5 the gutenberg richter law supports that the whole lithosphere is a self organized system in critical state i e a force is acting contemporaneously on all the plates and distributes the energy over the whole lithospheric shell a condition that can be satisfied by a force acting at the astronomical scale assuming an ultra low viscosity layer in the upper asthenosphere the horizontal component of the tidal oscillation and torque would be able to slowly shift the lithosphere relative to the mantle

electrical conductivity of the pampean shallow subduction region of argentina near 33 s evidence for a slab window 1 we present a three dimensional 3 d interpretation of 117 long period 20 4096 s magnetotelluric mt sites between 31 s and 35 s in western argentina they cover the most horizontal part of the pampean shallow angle subduction of the nazca plate and extend south into the more steeply dipping region sixty two 3 d inversions using various smoothing parameters and data misfit goals were done with a nonlinear conjugate gradient nlcg algorithm a dominant feature of the mantle structure east of the horizontal slab is a conductive plume rising from near the top of the mantle transition zone at 410 km to the probable base of the lithosphere at 100 km depth the subducted slab is known to descend to 190 km just west of the plume but the wadati benioff zone cannot be traced deeper if the slab is extrapolated downdip it slices through the plume at 250 km depth removal of portions of the plume or blocking vertical current flow at 250 km depth significantly changes the predicted responses this argues that the plume is not an artifact and that it is continuous the simplest explanation is that there is a wedge shaped slab window that has torn laterally and opens down to the east with its apex at the plume location stress within the slab and seismic tomography support this shape its northern edge likely explains why there is no deep seismicity south of 29 s

a study of sulphur enrichment in the humic fraction of marine sediments during early diagenesis abstract the evolution of the sulphur content of humic acids extracted from a near shore sediment core was investigated special attention was taken to avoid s contamination of the humic materials during extraction the s c ratios increased continuously with depth to values which strongly suggest s addition to the humic matrix during early diagenesis by reactions between organic matter and h2s or its oxidation products their light isotopic composition supports this view however subsequent isotopic exchange has obscured the mechanism initially involved since a large fraction of the enrichment occurred above the sulphidic zone redox boundaries such as the interface of anoxic microniches within more oxidized zones or the sulphidic suboxic boundary of the sediment column must have been important sites for s addition bioturbation by increasing the contact between sedimentary zones of a different redox regime will likely enhance such processes

nitrogen cycling in the sediments of santa barbara basin and eastern subtropical north pacific nitrogen isotopes diagenesis and possible chemosymbiosis between two lithotrophs thioploca and anammox riding on a glider abstract this article presents data on the isotopic composition of pore water ammonium and n org in the anoxic sediments of the santa barbara basin and the eastern subtropical north pacific esnp region ammonium 15 n behaves differently in the two regions reflecting variability in bacterial processes of sedimentary nitrogen cycling in santa barbara basin stoichiometry of tco 2 and nh 4 production defined by pore water chemistry indicates that the only reaction influencing pore water ammonium concentration in these sediments appears to be organic matter decomposition pore water ammonium was found to be 1 3 heavier than decomposing organic matter in contrast the upper 15 30 cm of esnp pore waters have ammonium with 15 n that is more than 10 heavier than decomposing organic matter at greater depth the 15 n of deeper pore waters approaches that of decomposing organic matter mass balance calculations indicate that the observed isotopic enrichment is not due to the preferential loss of isotopically enriched organic matter pore water profiles show that the zone with heavy isotopes is characterized by a tco 2 nh 4 inputs ratio of 4 or less we attribute this to the presence of thioploca bacteria who transport no 3 to depth and reduce it to nh 4 while oxidizing sulfide based on pore water stoichiometry and reaction diffusion modeling of pore water isotopic composition we propose that the 15 n enriched ammonium is produced as the result of chemosymbiosis between thioploca and anammox like bacteria in these sediments our work provides the new insights into benthic microbial ecology and the role of coupled individual metabolic pathways in sedimentary nitrogen cycling in santa barbara basin factors which contribute to the isotopic fractionation of 1 3 between pore water ammonium and solid n org were also considered in this sediments 23 of the rain of organic nitrogen is lost to diagenesis therefore the isotopic effect of degradation on the residual n org is estimated to be small 0 7 the most likely cause of fractionation is preferential degradation of an isotopically heavier more labile marine fraction relative to an isotopically lighter more refractory terrestrial component of the organic matter but other possibilities cannot be ruled out when corrected for the observed fractionation it appears that the 15 n of these sediments may represent non steady state conditions

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

sulfur diagenesis and partitioning in holocene peru shelf and upper slope sediments abstract sediments of the peru margin were sampled in two transects across the shelf and upper slope 75 to 1000 m in october of 1992 this region is characterized by high surface water productivity and consequent high organic carbon fluxes low terrigenous inputs as well as an intense oxygen minimum zone omz impinging on the slope at depths between 75 and 550 m sediments analyzed in this study were collected from one ship deployed box core and five submersible deployed push cores organic carbon toc concentrations in surface sediments range from 1 to 18 on a dry weight basis formation of pyrite begins at the sediment water interface swi maximum concentrations of pyrite sulfur in the cores from this study range from 0 5 to 1 8 wt concentrations of pyrite are influenced by availability of reactive iron and an oxidant capable of converting avs to pyrite highly reactive iron is stripped from terrigenous detritus in the core of the omz some of this iron may contribute to glauconite formation in sediments near the base of the omz where it is concentrated into a glauconitic green sand as a result of reworking and winnowing in a shallow water site 109 m at the upper edge of the omz high concentrations of pyrite total iron and iron aluminum ratios indicate that easily reduced iron species are available for sulfidation at the swi pyrite formation at this site is nearly complete in the upper 4 cm of the sediment in contrast pyrite concentrations are slightly lower at the surface of two of the deeper water sites at 309 m in the center of the omz and to a lesser extent at 671 m at the bottom edge of the omz and appear to be influenced in part by availability of reactive iron concentrations of acid volatile sulfides avs are low in all sediments 0 04 wt but are slightly elevated in sediments from the center of the omz the ratio of organic sulfur s org to organic carbon toc at the sediment surface of the cores examined here is remarkably constant 0 012 0 002 and within the range observed in marine biota 0 01 to 0 02 suggesting that organic sulfur at the swi is dominantly biogenic although s org toc ratios increase downcore there is no clear correlation between concentrations of organic sulfur and organic carbon below the swi there is also no clear relation between concentrations of organic sulfur and total iron organic sulfur concentrations vary from site to site 0 06 to 0 33 wt but are relatively constant downcore at any given site maximum concentrations are in cores from the center of the omz

historical oceanographic events reflected in13c 12c ratio of total organic carbon in laminated santa barbara basin sediment an 1844 1987 time series of carbon stable isotope ratios from dated sedimentary total organic carbon toc from the center of the santa barbara basin sbb is compared with historical climate and oceanographic records four isotopically distinct biogeochemical sources of toc are important phytoplankton derived marine biomass macroalgal biomass from kelp forests terrigenous biomass mainly flushed into the sbb via river discharge and redeposited fossil organic carbon the significance of the latter two sources is largely limited to a few unusual flood and oil spill events whereas the combination of 13c depleted phytoplankton and 13c enriched macroalgal biomass appears to be responsible for most of the isotopic variance of the marine coastal biomass as recorded in sedimentary toc the isotopic response of marine organic carbon in sediments records strong el nino southern oscillation enso and the frequently associated severe storm and wave events in sbb varved sediments the plausible major isotopic mechanisms are 1 increased physical liberation of 13c enriched kelp carbon from locally abundant giant kelp macrocystis spp forests during times of physical and environmental stress and 2 decreased productivity of 13c depleted phytoplankton during enso events

distinct sources for syntectonic variscan granitoids insights from the aguiar da beira region central portugal abstract the variscan syntectonic granitoid plutons from the aguiar da beira region central portugal were emplaced into metasediments of late proterozoic early cambrian age during the last variscan ductile tectonic event d 3 which is related to dextral and sinistral shearing the older intrusion with a u pb id tims zircon age of 321 8 2 0 ma consists of porphyritic biotite granodiorite granite with transitional i s type geochemical signature relatively low 87 sr 86 sr 322 ratios 0 7070 0 7074 end 322 values of 3 9 to 4 6 and whole rock and zircon 18 o values of 10 6 and 8 0 respectively by contrast the younger intrusion is an s type muscovite biotite leucogranite emplaced at 317 0 1 1 ma showing more radiogenic 87 sr 86 sr 317 0 7104 0 7146 lower end 317 values of 7 7 to 8 7 and higher 18 o wr 11 3 and 18 o zr 9 5 the combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite granite involving the anatexis of lower crustal metaigneous protoliths and possible hybridization with mantle derived magmas a shallower origin at mid crustal levels from pure crustal derivation through moderate degrees of partial melting of proterozoic cambrian metasediments is proposed instead for the muscovite biotite leucogranite

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs