effect of organic carbon flux and dissolved oxygen on the benthic foraminiferal oxygen index bfoi variations in oceanic primary productivity flux of organic carbon to the sediments and dissolved oxygen levels in the water column are thought to be important in the control of benthic foraminiferal test size wall thickness morphology and species composition of assemblages by many foraminiferal paleontologists aspects of these processes should be reflected by the benthic foraminiferal oxygen index bfoi based on these foraminiferal characteristics however analyses indicate that the bfoi correlates most strongly with dissolved oxygen levels in overlying water r2 0 81 weakly with oceanic primary productivity r2 0 55 and weakly with organic carbon flux to the sediments r2 0 51 although both dissolved oxygen and organic carbon flux are main controlling factors for benthic foraminiferal assemblages the bfoi is a useful indicator extracted from benthic foraminiferal assemblages for estimating the condition of dissolved oxygen in cretaceous and cenozoic oceans

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

a reevaluation of crystal size distributions in chromite cumulates although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling it has not been demonstrated that crystal size distributions csds are consistent with such a process plots of crystal size fractions per unit volume vs crystal size for chromite grains from the stillwater complex show a log linear distribution with negative slope at larger sizes and a concave down distribution at smaller sizes the log linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas however the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions an interpretation consistent with textural and volume balance evidence the csd trends imply that cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains similarities between the stillwater data and csd trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

climatic and oceanic changes during the middle late ordovician transition in the tarim basin nw china and implications for the great ordovician biodiversification event abstract the middle late ordovician transition darriwilian to sandbian age witnessed a major pulse of the great ordovician biodiversification event gobe and distinctive oceanic geochemical fluctuations such as coeval negative c and sr isotope excursions in this study investigations into geochemical variations notably the hg abundance or hg toc have been carried upon the organic rich black shale of the middle upper ordovician saergan formation to unravel the causes of this pulse based on these data three phases were identified phase 1 0 to 3 m is characterized by rising hg toc up to 138 ppb wt and ti al values as well as high ciacorr corrected chemical index of alteration values 68 9 72 3 with negligible enrichment of redox sensitive elements rse and nutrient elements e g u 5 2 ppm v 153 ppm mo 1 8 ppm p2o5 0 2 suggesting intensified volcanism which could have emitted significant amounts of greenhouse gases thereby leading to climate warming in contrast phase 2 3 to 11 m is characterized by decreasing hg toc and ti al ratios relatively low though slightly fluctuating ciacorr values generally depleted in rse except moderately enriched u up to 14 6 ppm and increased p al and ba al ratios implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling phase 3 sandbian age 11 13 m witnessed continuous decrease in hg toc ratio an increase in ti al and ciacorr values fairly low values of rse enrichment and p al and ba al ratios indicating recurrent climate warming and the potential for slowed oceanic circulation and attenuated upwelling of nutrient rich deep waters onto the shallow shelf these changes could have diminished bioproductivity and organic output onto the seafloor this study offers insights into volcanic climatic oceanic interactions during a major pulse of the gobe around the middle late ordovician transition while black shales were extensively deposited

brittle star distribution patterns and population densities on the continental slope off central california echinodermata ophiuroidea abstract the ophiuroid communities on the continental slope off central california were examined using box cores and trawls box cores were taken between 550 and 3085 m at sites southwest of the farallon islands and otter and beam trawls were used below 2300 m at three sites between the farallon islands and point sur eighteen ophiuroid species from six families were identified eighty percent of the individuals collected with box cores belonged to two species ophiura leptoctenia and ophiacantha normani which were dominant between 1000 and 2000 m the largest ophiuroid faunal break occurred at around 2000 m and was associated with elevated dissolved oxygen levels decreasing sediment grain size and increasing sediment organic content a comparison of box core and trawl data from below 2300 m showed that box cores undersampled the ophiuroid community on the continental slope missing almost 50 of the species collected by trawls within the same area whereas trawls underestimated ophiuroid densities reporting on average 243 times fewer ophiuroids per m2 than did box cores there was a change in species relative abundance patterns between sampling locations ophiuroids exhibited patchy spatial distribution patterns on both a small scale of around 0 1 m2 and a large scale on the order of 100 1000 s of square meters

why did ancient chemosynthetic seep and vent assemblages occur in shallower water than they do today cold seep communities have relatively low diversity are dominated by one or two taxa present in high density and high biomass in comparison with the surrounding fauna and are restricted to aphotic habitats their associated heterotrophic fauna are usually distinctive from the fauna of their surroundings in contrast a more commonplace chemoautotrophically based community occurs in shallow photic habitats the associated heterotrophic fauna includes many of the species typical of the surrounding communities and typically dominates abundance whereas the species with chemoautotrophic symbionts dominate biomass all modern seep assemblages are deeper than 550 m many fossil seep assemblages occurred in water as shallow as the mid shelf 200 m in contrast communities where species with chemoautotrophic symbionts are biomass dominants but not numerical dominants are common in shallow waters at present but rarely reported in the geological record we suggest that the absence of cold seep communities on the continental shelf presently is due to a combination of predation and competitive exclusion by primary consumers limiting the presence of species dependent on chemoautotrophic symbionts we suggest that cold seep assemblages are more common at shelf depths in the fossil record for two reasons a the biases of preservation have accentuated their distribution by transforming communities where species with chemoautotrophic symbionts dominate by biomass but not numerically into cold seep appearing assemblages b the importance of predation pressure and oligotrophy has varied with decreased predation pressure accompanying increased oligotrophy favoring cold seep communities we suggest that the paucity of shallow water assemblages with species harboring chemoautotrophic symbionts as biomass dominants in the fossil record is based on the reliance of paleoecological analysis on numerical abundance data when energy flow analyses are required to identify these assemblages the distinctiveness of the fossil seep assemblage is intensified by taphonomic processes that bias the assemblage against small individuals and epifaunal species so that diversity declines the small heterotrophic component of the assemblage is significantly reduced and the epifaunal component is minimized the final assemblage is usually dominated by the better preserved large infaunal clams which perchance are also the species with chemoautotrophic symbionts in contrast preservation does not enhance the distinctiveness of these chemoautotroph harboring species in shallow water

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

late cretaceous upwelling system along the southern tethys margin israel interrelationship between productivity bottom water environments and organic matter preservation organic rich upper cretaceous sequences in israel were deposited in an extensive highly productive upwelling linked system which prevailed along the southern tethys margin and lasted for 19 m y santonian to late maastrichtian an understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in israel representing an outer shefela and an inner zin facies belt the nature of the upwelling system and its effect on the sedimentary record is related to two basic environmental parameters namely paleoproductivity intensity and oxygen levels at the bottom the assessment of these parameters and their interrelationship has been performed through the development of paleontological foraminiferal criteria which are independent of the organic matter content following the establishment of these criteria it is concluded that the productivity reached its maximum intensity during the late campanian which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin this distribution is expressed in varied lithology organic rich carbonates phosphorites and siliceous rocks at the core of upwelling and a uniform lithology organic rich carbonates at the margin of this system the uniform lithology of the maastrichtian in both basins composed of organic rich carbonates is ascribed to a gradual weakening of productivity the bottom conditions in the inner belt during the late campanian the time of maximum surface productivity were near anoxic changing to more aerated dysaerobic conditions during the early maastrichtian in the outer belt a more aerated bottom dysaerobic prevailed throughout the late campanian to late maastrichtian the elevated organic matter content in both basins reflects the overall environment of high productivity its actual variations however seem to be unrelated to changes in surface productivity and bottom oxygen levels

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

the role of viscous particle segregation in forming chromite layers from slumped crystal slurries insights from analogue experiments the question of how the mineral layering in layered intrusions forms has been extensively debated for many decades there are many types of layering and it is of course possible that a number of mechanisms are involved of particular interest is how chromite layers form because these may contain valuable metals such as platinum group elements in addition to cr one model for the formation of these layers is that they formed through slumping of semi consolidated cumulates from the margins of the intrusion into the magma chamber during this slumping the grains are sorted by density and or size differences this study examines the viability of this process using analogue modelling starting materials beads and glycerine were scaled to match the density and size of the minerals chromite orthopyroxene and plagioclase present in layered mafic ultramafic layered intrusions and to match the density and viscosity of the silicate magma a perspex flume tank divided by a removable partition at one end was fully filled with glycerine a homogenized mixture of the beads was placed in the smaller partition of the tank representing the margins of the magma chamber the tank was then inclined between 16 and 45 the partition was removed and the beads flowed into the main part of the box the experiments were recorded by video camera allowing us to follow the dynamics of the flow during each run segregation of the beads was observed in the final deposits the larger less dense beads representing plagioclase concentrated at the top of the flow with the intermediate sized and medium density beads representing orthopyroxene in the middle and the smaller denser beads representing chromite at the bottom thus mimicking natural examples in experiments where the angle of inclination was low long thin layers formed such as those found in the bushveld complex in experiments where the angle of inclination was high thick but short layers formed a dimensionless analysis allows better understanding of the dynamics of the flow at the macroscopic scale the flow regime is strongly influenced by the viscosity of the fluid and is considered macro viscous where the role of the interstitial liquid is non negligible

the origin of chromitites and related pge mineralization in the bushveld complex new mineralogical and petrological constraints this article reports a study of chromitites from the lg 1 to the ug 2 3 from the bushveld complex chromite from massive chromitite follows two compositional trends on the basis of cation ratios trend a decreasing mg mg fe with increasing cr cr al trend b decreasing mg mg fe with decreasing cr cr al the chromitites are divided into five stages on the basis of which trend they follow and the data of eales et al chemical geology 88 261 278 1990 on the behaviour of the mg fe ratio of the pyroxene and whole rock sr isotope composition of the environment in which they occur following eales et al chemical geology 88 261 278 1990 the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aal2o3 and in the case of stage 5 the appearance of cumulus plagioclase buffering the aal2o3 the similarity of pge profiles across the mg 3 and mg 4 chromitites that are separated laterally by up to 300 km and the variation in v in the ug 2 argue that the chromitites have largely developed in situ modelling using the programme melts shows that increase in pressure mixing of primitive and fractionated magma felsic contamination of primitive magma or addition of h2o do not promote crystallization of spinel before orthopyroxene in general they hinder it and that the cr2o3 content of the magma was of the order of 0 25 wt less than 20 of the chromite in the magma is removed before orthopyroxene joins chromite which implies a 13 km thickness of magma for the critical zone it is suggested that the large excess of magma has escaped up marginal structures such as the platreef the pge profile of chromitites depends on whether sulphide accumulated or not along with chromite modelling shows that contamination of critical zone magma with a felsic melt will induce sulphide immiscibility although not chromite precipitation the lg 1 to lg 4 chromitites developed without sulphide whilst those from the lg 5 upwards had associated liquid sulphide much of the sulphide originally in the lg 5 and above has been destroyed as a result of reaction with the chromite

palaeoecology and sedimentology of the dysaerobic bedford fauna late devonian ohio and kentucky usa abstract oxygen deficient biofacies models rely on lithologic and paleontologic attributes to identify distinctive biofacies interpreted to reflect levels of oxygenation in anaerobic dysaerobic and aerobic parts of a stratified water column this study of the bedford fauna from the bedford shale of ohio and kentucky and from adjacent black shale units reports faunal distributions different from those predicted by the accepted models this study suggests that although oxygenation was an important factor that determined the taxonomic makeup of the fauna bacterially mediated nutrient recycling and substrate characteristics were more important than oxygenation in determining faunal distribution in the dysaerobic zone

community benthic paleoecology from high resolution climate records mollusca and foraminifera in post glacial environments of the california margin abstract paleoecological reconstructions of past climate are often based on a single taxonomic group with a consistent presence less is known about the relationship between multi taxon community wide change and climate variability here we reconstruct paleoecological change in a late quaternary 16 1 3 4 ka sediment core from the california margin 418 m below sea level of santa barbara basin sbb usa using mollusca animalia and foraminifera rhizaria microfossils building upon previous investigations we use multivariate ordination and cluster analyses to interpret community scale changes in these distinctly different taxonomic groups across discrete climate episodes the strongest differences between seafloor biological communities occurred between glacial prior to termination ia 14 7 ka and interglacial climate episodes holocene communities were well partitioned indicating that sub millennial oceanographic variability was recorded by these microfossils we document strong evidence of chemosynthetic trophic webs and sulfidic environments from gastropod alia permodesta and bivalve lucinoma aequizonata which characterized restricted intervals previously interpreted as well oxygenated such as the pre bolling warming mollusc records indicate first order trophic energetic shifts between detrital and chemosynthetically fixed carbon molluscs associated with widely different physiological preferences occur here within single decadal intervals of sediment and as such mollusc assemblages may reflect significant inter decadal oceanographic variability foraminifera assemblages provide exceptional records of the sequential chronological progression of the deglacial climatic and oceanographic events whereas mollusc assemblages reflect non chronological similarities in reoccurring communities foraminifera taxa that drive community similarity here are also independently recognized as marker species for seafloor hypoxia regimes which provides support for the idea that oxygenation change is a principal driver of seafloor environmental variability

palaeoecology of the graptoloidea abstract graptoloids class graptolithina were colonial pterobranch hemichordates and formed a major component of the early paleozoic macrozooplankton by analogy with modern pterobranchs they filtered food particles from the water mass in which they lived by means of ciliated lophophores they are inferred to have been able to migrate vertically through the water column for feeding efficiency and predator avoidance as by modern euphausiids but their means of locomotion is still highly conjectural the rhabdosomes which housed the colonies grew by distal accretion and large examples are inferred to have taken up to several years to have reached full size the few studies of survivorship suggest a relatively constant mortality of individuals within the community tubular and blister like outgrowths of cortical tissue are seen rarely and are inferred to be a response to parasitism and rare faecal packages with crumpled rhabdosomes suggest ingestion by predators the function of the monospecific aggradations of conjoined colonies synrhabdosomes seen on rare bedding planes is uncertain but is unlikely to have been part of their normal life history and ecology monospecific assemblages of unconnected colonies on single bedding planes are not rare however and suggest localised populations of single species in the overlying water column in life other bedding planes have a range of species resulting from mass mortality from volcanic eruptions and suggest a diversity of species in the water column the onshore offshore and depth facies preference of some 200 ordovician species has led to an ecological model in which the graptoloid habitat is partitioned vertically and horizontally in waters of the neritic and pelagic realms one group of species group 1 is largely confined to a deep water biotope mesopelagic zone characterised by dysaerobic water where they are thought to have exploited high bioproductivity denitrification zones a second group group 2 occupies an epipelagic biotope and is confined to waters of the epipelagic zone aerobic water a third and less well defined group group 3 occupies the inner shelf waters of the epipelagic zone only the median duration of species in the deep water biotope is significantly shorter than that of species in the epipelagic biotope suggesting greater environmental stress and instability in the dysaerobic zone over geological time consistent with models of disruption of the density structure and circulation patterns of the oceans from climatic instability the diversity and abundance of species were enhanced in both the epipelagic and deep water biotopes in regions of marginal upwelling where nutrients were brought from the deep to the shallow water zones it was in these regions that the classical graptolitic shale developed

origin of anorthosite and magnetitite layers in the bushveld complex constrained by major element compositions of plagioclase the bushveld complex a layered mafic intrusion in south africa shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite in a continuous borehole core drilled through the uppermost 2 8k m of the intrusion typical rocks range upwards from troctolite through gabbronorite and ferrogabbronorite to ferrodiorite with extreme examples of anorthosite magnetitite and feldspathic pyroxenite the an content of plagioclase has previously been determined for 420 samples and decreases upward from an78 to an36 with six minor slow reversals variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole rocks forty five anorthosite layers have been identified ranging from 1 to 23 m thick none of these layers is associated with the abovementioned reversals in an content in plagioclase and nearly all have leucocratic rocks below and above with more than the likely cotectic proportions of plagioclase these observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation rhythmically pulsed crystallization possibly associated with pressure changes followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations twenty layers of magnetitite have been identified there is a significant reversal in an content in the overlying plagioclase compared with the underlying sample across only one such layer again this observation challenges hypotheses that such layers result from magma addition but is consistent with a pressure change hypothesis for triggering magnetite crystallization the upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions demonstrating that there is no downward crystallizing roof facies furthermore this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions hence we find no evidence supporting flotation or prolonged suspension of plagioclase

the chromite deposit of the ipueira medrado sill sao francisco craton bahia state brazil the ipueira medrado mafic ultramafic sill hosts the largest chromite deposit in brazil the sill is a 7 km long and 300 m thick elongated body consisting mainly of interlayered dunite and harzburgite but also containing minor mafic rocks the most distinctive feature of the ipueira medrado sill is a continuous 5 to 8 m thick massive chromitite layer that is currently being mined the anomalous thickness of this chromitite layer provides important constraints on current genetic models for the origin of massive chromitites the sill is divided into three zones marginal ultramafic and mafic the marginal zone 5 20 m thick consists of sheared gabbro and orthopyroxene rich harzburgite the ultramafic zone up to 250 m thick is subdivided into the lower ultramafic unit the main chromitite layer and the upper ultramafic unit the lower ultramafic unit consists of dunite with minor harzburgite and chain textured chromitite the main chromitite layer is composed of chain textured and massive chromitites and the upper ultramafic unit consists mainly of harzburgite but also minor chain textured chromitite and dunite pyroxene abundance increases toward the top in this zone and grades to an orthopyroxenite at the top the presence of magmatic intercumulus amphibole is significant up to 20 vol in this unit the mafic zone up to 40 m thick consists of leuconorites to melanorites that are partially metamorphosed under amphibolite facies conditions the variation of olivine and orthopyroxene compositions throughout the sill layers reveals two intervals of distinct magmatic evolution the interval located below the main chromitite layer is characterized by gradual upward evolution of mineral compositions toward more mg rich compositions fo93 and en94 the interval located above the main chromitite layer is characterized by rapid evolution toward more fe rich compositions fo85 and en84 with increasing stratigraphic height the interval below the main chromitite layer suggests crystallization in a dynamic magma chamber undergoing frequent replenishment with primitive magma the interval above the main chromitite layer suggests crystallization in a mainly closed fractionating magma chamber mass balance considerations suggest that an enormous amount of magma 10 000 m thick was associated with the formation of the main chromitite layer at the ipueira medrado sill such a volume of magma is obviously not represented in the stratigraphy of the ipueira medrado sill mineral chemistry data indicate that the composition of the resident magma was most primitive at the stratigraphic interval immediately below the main chromitite layer we suggest that the sill acted as a conduit through which a large volume of magma flowed under these conditions contamination of the parental magma with crustal material and fluids is a likely mechanism for explaining the origin of the chromitite abundant intercumulus amphibole in the upper ultramafic unit and high fe3 in chromites intercumulus orthopyroxene in main chromitite layer chromitites and sm nd and re os isotope data for silicate rocks and chromitites respectively indicate that the ipueira medrado sill formed from magmas contaminated by crustal material the host rocks of the ipueira medrado sill consist mainly of high grade gneiss and marbles that would have been appropriate source of fluids and sio2 and could have caused the increase in f o2

intensification of the oxygen minimum zone in the northeast pacific off vancouver island during the last deglaciation ventilation and or export production 1 the oxygen minimum zone omz off vancouver island was more oxygen depleted relative to modern conditions during the allerod 13 5 to 12 6 calendar kyr and again from 11 to 10 kyr the timing of omz intensification is similar to that seen throughout the north pacific although the onset appears to have been delayed by 1500 years off vancouver island radiocarbon dating of coeval benthic and planktonic foraminifera shows that between 16 0 and 12 6 kyr the age contrast between surface and intermediate waters 920 m depth off vancouver island was similar to or slightly less than that today there is no evidence of an increased age difference i e decreased ventilation during the deglaciation particularly during the allerod however sedimentary marine organic carbon concentration and mass accumulation rate increased substantially in the allerod suggesting that increased organic matter export was the principal cause of late pleistocene omz intensification off vancouver island

oxygen and organic matter thresholds for benthic faunal activity on the pakistan margin oxygen minimum zone 700 1100 m abstract a transition from fully laminated to highly bioturbated sediments on continental margins is thought to derive from increased animal activity associated with increasing bottom water oxygen concentration we examined faunal community responses to oxygen and organic matter gradients across the lower oxygen minimum zone omz on the bathyal pakistan margin where sediments grade from fully laminated sediment at 700 m 0 12 ml l 1 o 2 5 m to highly bioturbated sediment at 1100 m 0 23 ml l 1 o 2 10 m high resolution sampling of the seafloor every 50 m water depth was conducted along a single transect during inter and post monsoon periods in 2003 to address a the existence of oxygen thresholds regulating macrofaunal abundance composition diversity and lifestyles b the interactive effects of organic matter quantity and quality c associated community effects on sediment structure and d potential seasonality in these processes macrofaunal biomass and bioturbation depth were positively correlated with organic matter availability which peaked at 850 950 m 3 39 3 53 org c in contrast macrofaunal diversity h dominance r1d and burrow number exhibited threshold responses at oxygen concentrations of 0 12 0 20 ml l 1 5 9 m with few animals and highly laminated sediments present below this concentration and most taxa present in fully bioturbated sediments above it the highly mobile burrowing amphinomid polychaete linopherus sp exhibited almost complete dominance and high density at 750 850 m 0 12 0 14 ml l 1 o 2 5 6 m but despite its activity sediment laminae remained faintly visible formation of permanent burrows and detritivory were dominant macrofaunal lifestyles within the omz allowing laminae to persist at surprisingly high animal density and biomass results reflect a shift from organic matter to oxygen regulation of body size and biogenic structures following the monsoon this study suggests that for assemblages evolving under permanent severe hypoxia food availability remains a significant determinant of animal abundance and biogenic structure depth oxygen influences patterns of diversity and dominance and interacts with organic matter to generate abrupt faunal transitions on the pakistan margin

petrogenesis of the ultramafic zone of the stillwater complex in north america constraints from mineral chemistry and stable isotopes of li and o to investigate the petrogenesis of cyclic units in layered intrusions we examined chromitite dunite poikilitic harzburgite and bronzitite from the ultramafic zone of the stillwater complex and measured stable isotopes of li and o in their major minerals the li isotopes in olivine range from 4 to 26 in 7li with uniform li contents of 1 3 ppm whereas orthopyroxene and clinopyroxene have li contents of 0 5 5 ppm and 4 8 ppm and 7li ranges of 13 to 7 and 14 to 6 respectively the 18o values vary from 4 91 to 5 72 in olivine from 5 11 to 5 87 in orthopyroxene and from 4 64 to 5 86 in clinopyroxene for a given sample olivine displays more variable and higher 7li but lower 18o values than orthopyroxene indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene the general li and o isotopic compositions are interpreted as the result of re equilibration between interstitial liquids from which pyroxenes crystallized and cumulus minerals the inter mineral and inter sample isotopic variations correlate with mineral assemblages crystal sizes and major and trace element compositions revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids which collected on chromite surfaces and were later released from chromite seams played an additional critical medium of chemical exchange between minerals in the chromitites

response to the discussion by b robins and f chiodoni of geochemistry of cumulates from the bjerkreim sokndal intrusion s norway part i constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent abstract merging our data with those of robins and chiodoni robins b chiodoni f 2007 poles apart a discussion of the geochemistry of cumulates from the bjerkreim sokndal layered intrusion s norway part i constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent lithos doi 10 1016 j lithos 2007 03 007 we examine the major element compositions of 135 samples of so called phi c cumulates of the bjerkreim sokndal layered intrusion we show the limits of the tio 2 mgo ratio vs al 2 o 3 diagram in order to ascertain the proportions of cumulus minerals ilmenite and orthopyroxene in the ilmenite leuconorite mafic pole we also discard anomalous samples as we did in the previous paper the rest of the samples 124 phi c i e 92 of the whole population are then used to confirm the existence of plagioclase and mafic poles it is shown that the mafic pole is indeed a mixture of ilmenite and orthopyroxene not induced by spurious correlation

impact of oil based drill mud disposal on benthic foraminiferal assemblages on the continental margin off angola abstract in order to assess the possible environmental impact of oily cuttings discharged during oil exploration activities we studied the benthic foraminiferal faunas in a five station 4 km long sampling transect around a cuttings disposal site at about 670 m depth offshore angola w africa where drilling activities started 1 5 years before sampling living rose bengal stained and dead foraminiferal faunas were sampled in march 2006 the faunal patterns mirror the spatial distribution of hydrocarbons which are dispersed into a southeastern direction four different areas can be distinguished on the basis of the investigated faunal parameters density diversity and species composition of the living fauna and comparison with subrecent dead faunas the fauna at station s31 300 m se of the oil cuttings disposal site appears to be clearly impacted the faunal density and diversity are maximal but evenness is minimal taxa sensitive to organic enrichment such as uvigerina peregrina cancris auriculus and cribrostomoides subglobosus have largely disappeared whereas the low oxygen resistant taxon chilostomella oolina and opportunistic buliminids and bolivinids attain relatively high densities at station s32 500 m se of the disposal site environmental impact is still perceptible the faunal density is slightly increased and u peregrina apparently the most sensitive species is still almost absent the faunas found at 1 and 1 8 km se of the disposal site are apparently no longer impacted by the drill mud disposal faunal density and diversity are low and the faunal composition is typical for a mesotrophic to eutrophic upper slope environment finally station s35 2 km nw of the disposal site contains an intermediate fauna where both the low oxygen resistant c oolina and the more sensitive taxa u peregrina c auriculus and c subglobosus are present all taxa live close to the sediment water interface here indicating a reduced oxygen penetration into the sediment since the hydrocarbon concentration is low at this station it appears that the faunal characteristics are the consequence of a slightly different environmental setting and not due to a contamination with drill cuttings our data underline the large potential of benthic foraminifera as bio indicators of anthropogenic enrichment in open marine settings such as caused by the disposal of oily drill cuttings the foraminiferal faunas react essentially by a density increase of a number of tolerant and or opportunistic taxa and a progressive disappearance of more sensitive taxa in the most impacted area rather surprisingly large sized taxa appear to be more sensitive than small sized foraminiferal taxa

electrochemical processes in a crystal mush cyclic units in the upper critical zone of the bushveld complex south africa the upper critical zone ucz of the bushveld igneous complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order from bottom to top pyroxenite norite anorthosite electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the ug2 chromitite and the merensky reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral melt evolution the anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates here chromite compositions change abruptly from high and constant mg mg fe 2 and fe 2 fe 3 ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers furthermore the composition of disseminated chromites varies depending on the host silicate assemblage for example in ti v and zn contents importantly the abrupt change in chromite composition across the chromitite silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re equilibration with intercumulus liquid among those features is the decoupling of alkalis from other incompatible lithophile elements in comparison with cumulus plagioclase intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible li k and rb strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers to explain these features we propose a new model of post cumulus recrystallization which intensifies the modal layering in the crystal liquid mush producing the observed sequence of nearly monomineralic layers of chromitite pyroxenite and anorthosite that define the cyclic units the crucial element of this model is the establishment of redox potential gradients at contacts between chromite rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt because basaltic melts are ionic electrolytes with na as the main charge carrier the redox potential gradient induces electrochemical migration of na and other alkali ions selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions which may eventually result in the formation of sharply divided monomineralic layers the observed variations in ferric ferrous iron ratios in chromite from the cyclic units and fe distribution in plagioclase imply a redox gradient of

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

petrogenesis of the massive chromitite layer from the jacurici complex brazil evidence from inclusions in chromite the jacurici complex hosts the largest chromite deposit in brazil in an up to 8 m thick chromitite layer within a tectonically segmented 300 m thick intrusion the ore has been interpreted as the result of crustal contamination driven crystallization in a magma conduit this study addresses the stratigraphy mineralogical and textural relationships and mineral chemistry of the monte alegre sul segment focusing on chromite hosted inclusions from the main chromitite layer to understand the role of volatiles in the genesis of the massive chromitite silicate inclusions enstatite phlogopite magnesiohornblende diopside and olivine are commonly monomineralic and sub to euhedral and crystallized prior to or coeval with the chromite crystallization carbonate inclusions dolomite and magnesite are irregular or have negative crystal shapes suggesting entrapment as melt droplets sulfides pentlandite millerite heazlewoodite polydymite pyrite and chalcopyrite are often polymineralic irregular or hexagonal shaped indicating entrapment as sulfide melt and as monosulfide solid solution the inclusions indicate an h2o and s saturated resident magma with immiscible droplets of carbonate melt during chromite crystallization inclusion rich and inclusion free chromites that occur together have similar compositions and are considered to have formed from the same magma in response to variations in the degree of cr saturation hot primitive magma might have heated and mobilized co2 and probably water from devolatized and assimilated carbonate rich wall rocks increasing fo2 and triggering chromite crystallization we propose that the formation of the chromitite layer started as in situ crystallization with additional material added by slumping of locally remobilized chromite slurries facilitated by the presence of volatiles

paleoceanographic changes on the farallon escarpment off central california during the last 16 000 years abstract new benthic and planktic foraminiferal assemblage census data and benthic foraminiferal oxygen index bfoi values previously published marine climate proxy data stable isotopes and ca cd and unpublished results of total carbon organic carbon and calcium carbonate analyses of sediments recovered off central california on the farallon escarpment 1605 m water depth 37 13 4 n 123 14 6 w core f 8 90 g21 document paleoceanographic changes during the latest quaternary which reflect the intensity and source of north pacific intermediate water npiw and surface productivity accelerator mass spectrometry radiocarbon dates of both benthic and planktic species provide an excellent age depth model for the last 16 000 years covering the latest glacial bolling allerod younger dryas and early middle and late holocene intervals a q mode cluster analysis separated the benthic fauna into three clusters one pleistocene and two holocene whereas the planktic fauna was divided only into pleistocene and holocene clusters stable oxygen isotope values show an increase in water temperature of 1 c from the late glacial to late holocene and a change in faunal composition of the planktic assemblage implies surface waters warmed as well a general trend of decreasing dissolved oxygen concentration from the pleistocene high oxic 3 0 6 0 ml l o2 to the holocene low oxic 1 5 3 0 ml l o2 suggested by the bfoi and cd ca data reflect decreased ventilation as the source of the npiw shifted from the sea of okhotsk to the tropical east pacific at 11 000 cal bp the middle holocene cooling reported in other central and northern california margin studies is not apparent in f 8 90 g21 which compares more favorably with studies from southern california and british columbia total carbon and organic carbon values are highest in the bolling allerod early holocene and late holocene similarly calcium carbonate values are high in the bolling allerod and peak in the early holocene but decrease significantly in the latest middle and late holocene which coincides with a depauparate planktic foraminiferal fauna in the upper 60 cm 70000 0 cal bp of the core and poor preservation of the benthic foraminiferal fauna at 40 cm 3000 cal bp decoupling is evident between the planktic and benthic faunal response to changing climatic conditions with the surface dwelling assemblage often leading the bottom dwelling assemblage by several millennia

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

oxygen minimum zone edge effects evidence from the central california coastal upwelling system geologic and biological data from box cores gravity cores and bottom photographs across the modern coastal upwelling system of central california reveal that biogeochemical activity increases along the edges of the oxygen minimum zone omz 0 5 ml 1 o2 macrofauna and benthic foraminifers as well as relative concentrations of calcium carbonate glauconite and fecal pellets all display edge effects the major controls appear to be a combination of dissolved oxygen concentration of near bottom waters bottom currents and possibly bacterially mediated nutrient recycling as well as food supplies omz edge effects should be recognizable in the rock record and may prove to be powerful paleoenvironmental paleoecological indicators

foraminiferal record and environmental changes during the deposition of the early middle pleistocene sapropels in southern italy a foraminiferal investigation documents the palaeoenvironmental changes across the sapropel units associated with insolation cycles 90 and 86 in the early middle pleistocene im fosso 5 agosto section basilicata southern italy three benthic foraminiferal fossil associations associations a b and c identified by performing hierarchical cluster analysis on the quantitative data can be considered diagnostic of the palaeoenvironmental conditions before during and after the sapropel intervals they indicate a gradual decrease of bottom water oxygenation prior to the onset of sapropel deposition and a rapid return to stable environmental conditions at the end of the sapropel events the constant presence of benthic foraminifera throughout the sapropels implies that the bottom environment was not totally anoxic the high resolution distributional pattern of planktonic and benthic foraminifera within the sapropel associated with i cycle 90 documents a surface water cooling with consequent rapid re oxygenation of bottom waters that subdivides this interval in two phases both of them are characterized by an early subphase with warm climate conditions a decrease of salinity in surficial water layers and dominated by the low oxygen tolerant benthic species globobulimina affinis the late subphase shows thermal stratification during summer and a relatively deep and oxygenated mixed layer during winter in this subphase the contemporaneous replacement of g affinis the bless opportunistic but more resistant to low oxygen conditionsq species with bolivina alata bolivina dilatata and brizalina spathulata the bless resistant to low oxygen conditionsq species indicates the relative oxygen improvement in sediment pore water the frequent changes detected in the sapropel associated faunal composition underline the unstable nature of bottom water oxygen content d 2004 elsevier b v all rights reserved

macrobenthic zonation caused by the oxygen minimum zone on the shelf and slope off central chile abstract the relationship between macrobenthic 300 m zonation and the oxygen minimum zone omz o2

pressure fluctuations and the formation of the pge rich merensky and chromitite reefs bushveld complex the merensky reef and the underlying upper group 2 chromitite layer in the critical zone of the bushveld complex host much of the world s platinum group element pge mineralization the genesis is still debated a number of features of the merensky reef are not consistent with the hypotheses involving mixing of magmas uniform mixing between two magmas over an area of 150 by 300 km and a thickness of 3 30 km seems implausible the merensky reef occurs at the interval where main zone magma is added but the relative proportions of the pge in the merensky reef are comparable to those of the critical zone magma mineral and isotopic evidence in certain profiles through the merensky unit suggest either mixing of minerals not magmas and in one case the lack of any chemical evidence for the presence of the second magma the absence of cumulus sulphides immediately above the merensky reef is not predicted by this model an alternative model is proposed here that depends upon pressure changes not chemical processes to produce the mineralization in chromite rich and sulphide rich reefs magma was added at these levels but did not mix this addition caused a temporary increase in the pressure in the extant critical zone magma immiscible sulphide liquid and or chromite formed sinking sulphide liquid and or chromite scavenged pge as clusters nanoparticles or platinum group minerals from the magma and accumulated at the floor rupturing of the roof resulted in a pressure decrease and a return to sulphur undersaturation of the magma

thick chromitite of the jacurici complex ne craton sao francisco brazil cumulate chromite slurry in a conduit abstract the jacurici complex located in the ne part of the sao francisco craton hosts the largest chromite deposit in brazil the mineralized intrusion is considered to be a single n s elongated layered body disrupted into many segments by subsequent deformation the ore is hosted in a thick massive layer two segments ipueira and medrado have been previously studied we provide new geological information and chromite composition results from the monte alegre sul and varzea do macaco segments located farther north and integrate these with previous results the aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer all segments exhibit similar stratigraphic successions with an ultramafic zone 250 m thick hosting a 5 8 m thick main chromitite layer mcl and a mafic zone 40 m thick the chromite composition of the mcl mg numbers 0 48 0 72 and cr numbers 0 59 0 68 is similar to chromites from layered intrusions and other thick chromitites previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition up to fo 93 and orthopyroxene composition up to en 94 from harzburgite samples and that it originated from an old subcontinental lithospheric mantle we estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite or low siliceous high mg basalt with a higher fe mg ratio the petrologic evidence from the mafic ultramafic rocks suggests that a high volume of magma flowed through the sill which acted as a dynamic conduit crustal contamination has previously been considered as the trigger for the chromite crystallization based on isotope studies as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole up to 20 the abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization fluids may have played an important role in the chromite formation and or accumulation however the trigger for chromite crystallization remains debatable the anomalous thickness of the chromitite is a difficult feature to explain we suggest a combined model where chromite crystallized along the margins of the magma conduit producing a semi consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed subsequently the conduit was obstructed and the resident magma fractionated to produce a more evolved composition

layered intrusions and traffic jams a wide range of explanations has been proposed for the origin of repetitive layering in mafic ultramafic and in per alkaline intrusions here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions without the need to invoke extrinsic factors similar to traffic jams on a motorway small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals respectively these traffic jams separate layers of magma from the rest of the magma chamber the magma in the individual layers further evolves as a largely independent subsystem with gravitational sorting organizing the mineral distribution within each layer layering can develop in the intermediate range between full mineral separation in low viscosity or slowly cooling magma chambers and homogeneous crystallization in high viscosity or fast cooling chambers this self organization mechanism provides a novel explanation for the formation of rhythmic layering in low viscosity magmas for example in the ilimaussaq igneous complex in southwest greenland

the history of coastal upwelling off peru 11 s odp leg 112 site 680b over the past 650 000 years the coarse sediment fraction 63 m and benthic foraminiferal composition of quaternary sediment cores from odp leg 112 hole 680b 252 5 m water depth were sampled from the upwelling area off peru high resolution oxygen isotope stratigraphy based on analyses of the benthic foraminifera bolivina seminuda allows a detailed reconstruction of the upwelling history since oxygen isotope stage 16 variations in the relative abundances of the benthic foraminiferal species nonionella auris bolivina costata and bolivina seminuda document changes in fertility which were found to be enhanced during interglacial stages 1 3 5 7 9 and 11 generally bioturbated sediments are dominated by the genera cassidulina epistominella gyroidina trifarina and valvulineria these benthic foraminifera are members of the undercurrent fauna which is commonly associated with sediments that have a high terrigenous detrital component and a low organic carbon content 6 ml l o2 the entire core interval investigated in hole 680b appears to have been affected by the peruvian undercurrent generally undercurrent dominated sections were found to correspond with glacial oxygen isotope stages

comparison of seismic attenuation models using zero offset vertical seismic profiling vsp data for seismic frequencies it is common to use an empirical equation to model attenuation usually the attenuation coefficient is modeled with linear frequency dependence a model referred to as the kolsky futterman model other models have been suggested in the geophysical literature we compare eight of these models on a zero offset vertical seismic profiling vsp data set the kolsky futterman the power law the kjartansson the muller the azimi second the azimi third the cole cole and the standard linear solid sls models for three separate depth zones we estimate velocities and q values for all eight models a least squares model fitting algorithm gives almost the same normalized misfit for all models thus none of the models can be preferred or rejected based on the given data set slightly better overall results are obtained for the kolsky futterman model for one depth zone the sls model gave the best result

preservation of autochthonous shell beds by positive feedback between increased hardpart input rates and increased sedimentation rates abstract the preservation of nonrapidly buried autochthonous shell concentrations with noncementing epifaunal animals in life position presents a taphonomic dilemma if in fact an increase in shelliness is driven by a decrease in sedimentation rate a 150 cm thick densely packed shell bed with brachiopods from the lower jurassic of morocco shows lower levels of postmortem alteration than shell poor beds indicating that its formation is primarily governed by variations in hardpart input rates varying dominance and size structure of the main shell producer brachiopod zeilleria rehmanni indicate that its increased population density was the main trigger in the shell bed formation thinner and less common microbial crusts in the shell bed than in shell poor beds indicate that higher shelliness is not due to lack of sediment on the basis of actualistic data from modern mussel and oyster shell beds the suspension feeding of a high density population leads to high biodeposition rates through production of

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

effects of latitudinal changes in the oxygen minimum zone of the northeast pacific on the distribution of bathyal benthic decapod crustaceans abstract the presence of an oxygen minimum zone omz is one of the major characteristics of the eastern pacific the omz changes strongly adjacent to mexico in its thickness and intensity the ecological impacts of those changes were studied by examining the community structures of bathyal benthic and bentho pelagic decapod crustaceans and their oceanographic contexts on the mexican pacific slope along a wide latitudinal range 16 32 n decapod crustaceans were collected with a benthic sledge from 48 stations between 865 and 2165 m in three main areas offshore of northern baja california nbc off southern baja california sbc and in the southern mexican pacific smp physical chemical parameters were measured in the water column and sediment composition was analyzed for each station the narrowing and weakening of the omz north of ca 26 n was confirmed water with dissolved oxygen

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the