time dependent models of single and double layer mantle convection one outstanding problem of great geophysical importance is the vertical extent of convection in the earth s mantle1 steady state models of convection in the mantle have yet to produce a description of the mantle which is consistent with geochemical and geophysical observations here we present time dependent numerical models of two dimensional convection which simulate mantle convection starting from a fluid initially at rest with a purely conductive temperature profile we now find that for a rayleigh number ra of 107 the model experiences a transient period of double layer convection which lasts on the order of hundreds of millions of years when scaled to the earth s mantle it is suggested that transient periods of double layer convection in the earth s mantle may have existed in the past whether or not such a period exists today

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

subduction zone metamorphism calc alkaline magmatism and convergent margin crustal evolution abstract at the dawn of the age of plate tectonics akiho miyashiro published a seminal paper in which he drew attention to significant differences in regional metamorphic phase assemblages described contrasting geologic occurrences and inferred their characteristic ranges of physical conditions he advanced the paired metamorphic belt concept involving an oceanward narrow high p low t blueschist zone intimately intermixed with ophiolites and a landward broad low p high t realm associated with arc volcanic plutonic rocks more recent studies have illuminated the following 1 two main types of convergent plate junction can be distinguished pacific underflow of thousands of km of oceanic lithosphere and alpine closure of an intervening oceanic basin leading to continental collision 2 plate descent carries mafic felsic lithologies to depths of 35 120 km or more at relatively low temperatures producing high pressure hp pacific type and ultrahigh pressure uhp alpine type metamorphic terranes respectively 3 exhumed hp uhp complexes display low aggregate bulk densities reflecting chiefly buoyancy driven ascent of quartzofeldspathic allochthons 4 at a depth of 35 km in a warm lithosphere subduction zone sialic crust should melt if an aqueous fluid were abundant it does not hence the activity of h2o must be low at such depths 5 global observations indicate that volcanic plutonic arcs are sited 100 20 km above subducting oceanic plates devolatilization of sinking hydrated oceanic crust mantle promotes the generation of calc alkaline arc melts in this magmagenic zone 6 pacific type underflow of basaltic crust capped plates produces new and recycled continental crust whereas alpine type convergence reshuffles collided terranes but does not generate juvenile sialic crust

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

the first 800 million years of earth s history the earth grew initially from hot planetesimals of reduced material and began to differentiate early into a metal rich core a silicate rich mantle and a volcanic surface accretion ended with volatile rich oxidized material which did not interact with the core because of a mantle barrier quenching of magmas at the surface and foundering of cool material was effective in cooling the earth by 1600 j per gram of magma magmas transported radioactive elements and volatiles to the outer 50 km the lower mantle became stranded below the critical adiabat for melting the core may have been fully molten radiogenic and non radiogenic heat sources produced a surface heat flow that decreased from ca 1 3 x 10 14 w at 4 45 ga b p to the present value of 4 x 10 13 w in spite of uncertainties in the production and transport of heat it seems safe to conclude that intense volcanism occurred throughout the pre archaean era before survival of crust from phase equilibrium data for peridotites and calc alkaline rocks it is deduced that a range of ultrabasic to basic magmas was produced from a peridotitic upper mantle and that calc alkaline magmas were produced during foundering of crust an anorthositic crust was not formed from observations of lunar basins it is concluded that a minimum of ca 500 1000 impact basins were formed on earth in ca 100 200 ma before 3 95 ga b p plausible calculations suggest that ca 20 times as many basins were formed in the preceding ca 300 ma impact debris would be cooled by water in wet crustal rocks and oceans it is assumed that after 4 0 ga b p impacts would have only temporary local effects on the crust and emphasis is placed on upper mantle convection as the main driving force for geochemical transport sedimentation and metamorphism were important factors during foundering of crust oceanic9 and continental9 regions developed above up welling and down welling segments of convection cells and these regions resembled greenstone belts and high grade regions of the archaean era as heat flow declined polygonal tectonics was replaced by symmetrical linear tectonics during the archaean era and then by asymmetrical linear tectonics during the proterozoic era

on upper mantle seismic heterogeneities beneath fennoscandia abstract three dimensional seismic mapping of the upper mantle beneath fennoscandia baltic shield using an ach type of inversion technique in combination with p wave travel time residual observations from the local seismograph network gave the following results the central parts of the baltic shield are characterized by relatively high seismic velocities down to approximately 300 km those parts of the shield most affected by the caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval the lower part of the upper mantle 300 600 km does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern europe the seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements the general orientation of these anomalies coincides with that of the glacial uplift

transport models for crust and mantle evolution the exact solutions for the isotopic compositions and the concentrations of two models for mantle crust evolution are given for arbitrary rates of crustal growth and of backflow to the mantle in model i continents are derived by melt extraction over the history of the earth from undepleted mantle in model ii new additions to the continents are derived from a mantle reservoir which becomes increasingly depleted through time by repeated extraction of melts the key parameters are the chemical fractionation factors for crustal growth and refluxing and the integrated fractional mass removal rates from the crust and the mantle the relationships between the mean age of the mass of the continents and the isotopic parameters are given the models suggest that the pre archean era prior to 3 8 ae was dominated by rapid refluxing of crust to the mantle which during the early archean gave way to a regime where transport from the mantle to the crust substantially dominated over refluxing of crust to the mantle the mean age of the mass of the continents appears to be 1 8 ae even for small amounts 10 of refluxing model ii shows that highly incompatible elements have very short residence times in the mantle mass balance considerations implies that the continents were derived from only a small fraction 30 of the mantle

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

crustal recycling by slab failure models for crustal recycling are based upon estimates of the amount of material being returned to the mantle today thus a better understanding of those processes will help to constrain the models we suggest through the use of an early proterozoic example in the wopmay orogen that failure of the subducting slab during collision leads to large quantities of crustal material being recycled into the mantle that this process has been active since the early proterozoic and that it may explain the paucity of rift deposits in other orogens

earthquake prediction a critical review summary earthquake prediction research has been conducted for over 100 years with no obvious successes claims of breakthroughs have failed to withstand scrutiny extensive searches have failed to find reliable precursors theoretical work suggests that faulting is a non linear process which is highly sensitive to unmeasurably fine details of the state of the earth in a large volume not just in the immediate vicinity of the hypocentre any small earthquake thus has some probability of cascading into a large event reliable issuing of alarms of imminent large earthquakes appears to be effectively impossible key word earthquake prediction

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

why earthquake hazard maps often fail and what to do about it abstract the 2011 tohoku earthquake is another striking example after the 2008 wenchuan and 2010 haiti earthquakes of highly destructive earthquakes that occurred in areas predicted by earthquake hazard maps to be relatively safe here we examine what went wrong for tohoku and how this failure illustrates limitations of earthquake hazard mapping we use examples from several seismic regions to show that earthquake occurrence is typically more complicated than the models on which hazard maps are based and that the available history of seismicity is almost always too short to reliably establish the spatiotemporal pattern of large earthquake occurrence as a result key aspects of hazard maps often depend on poorly constrained parameters whose values are chosen based on the mapmakers preconceptions when these are incorrect maps do poorly this situation will improve at best slowly owing to our limited understanding of earthquake processes however because hazard mapping has become widely accepted and used to make major decisions we suggest two changes to improve current practices first the uncertainties in hazard map predictions should be assessed and clearly communicated to potential users recognizing the uncertainties would enable users to decide how much credence to place in the maps and make them more useful in formulating cost effective hazard mitigation policies second hazard maps should undergo rigorous and objective testing to compare their predictions to those of null hypotheses including ones based on uniform regional seismicity or hazard such testing which is common and useful in similar fields will show how well maps actually work and hopefully help produce measurable improvements there are likely however limits on how well hazard maps can ever be made because of the intrinsic variability of earthquake processes

characterizing potential offshore seismic sources using high resolution geophysical and seafloor sampling programs an example from cape egmont fault zone taranaki shelf new zealand standard marine geological techniques high resolution seismic reflection methods piston cores are used to investigate the neotectonics and paleoseismology of an active part of an offshore fault system the cape egmont fault zone taranaki continental shelf new zealand the main active structure within the zone the cape egmont fault is a reactivated pliocene to recent normal to oblique slip fault that is associated with shallow crustal seismicity and exhibits a 53 km long 1 to 5 m high low angle drape fold or scarp at the seafloor four possibly five fault segments varying in length from 7 51 km are delineated using geometric structural and stratigraphic information empirical estimates of maximum credible earthquake magnitudes based upon seismic moment calculations surface rupture lengths and vertical surface offsets suggest that the prehistoric earthquake s that formed the present seafloor surface trace probably had a magnitude of at least mw 6 4 mw moment magnitude and could have been as large as ms 7 3 7 4 ms surface wave magnitude tentative maximum recurrence intervals for such events inferred for each segment range from 8000 13 000 years with lower magnitude events having correspondingly shorter recurrence intervals of 500 900 years without careful study of offshore late quaternary faults and fault zones particularly where they are contiguous with areas of active onshore deformation existing regional seismic hazard predictions may be inaccurate

climate control of sulfate influx to lake hovsgol northwest mongolia during the last glacial postglacial transition constraints from sulfur geochemistry abstract changes in sulfur cycles in lake hovsgol northwest mongolia are discussed on the basis of total organic carbon toc total sulfur ts concentrations and stable sulfur isotope ratios 34 s in two sediment cores x104 and x106 covering the last glacial to the present correlation plots of toc and ts c s plots of the sediment cores indicate dissolved sulfate enrichment in sediment pore water and high sulfate reducing bacteria srb activity during the climate transition from the last glacial to postglacial period lg pg transition from ca 21 to 11 6 cal ka bp in addition positive shifts of 34 s values from 3 7 to 32 6 were observed during the lg pg transition these results suggest a large influx of 34 s rich dissolved sulfate into the lake during this period the 34 s rich dissolved sulfate may have accumulated on the shore wetlands and marshes of lake hovsgol during the last glacial period owing to the increase in atmospheric precipitation and lake level rise during the lg pg transition the 34 s rich dissolved sulfate may have been supplied from the shore to the lake increased sulfate influx during the lg pg transition has also been observed in a lake baikal sediment core our new sulfur geochemical data shows past environmental changes particularly moisture changes within continental asia during the climate transition

great salt lake and precursors utah the last 30 000 years sediment cores up to 6 5 m in length from the south arm of great salt lake utah have been correlated radiocarbon ages and volcanic tephra layers indicate a record of greater than 30 000 years a variety of approaches have been employed to collect data used in stratigraphic correlation and lake elevation interpretation these include acoustic stratigraphy sedimentologic analyses mineralogy geochemistry major element c o and s isotopes and organics paleontology and pollen the results indicate that prior to 32 000 year b p an ephemeral saline lake playa system was present in the basin the perennial lake which has occupied the basin since this time rose in a series of three major steps the freshest water conditions and presumably highest altitude was reached at about 17 000 year b p the lake remained fresh for a brief period followed by a rapid increase in salinity and sharp lowering in elevation to levels below that of the present great salt lake the lake remained at low elevations and divided at times into a north and south basin until about 8 000 year b p since that time with the exception of two short rises to about 1290 m the lake level has remained near the present elevation of 1280 m

nd and sr isotope systematics of clastic metasediments from isua west greenland identification of pre 3 8 ga differentiated crustal components recent field work on the isua metasediments has established that there are two lithological sequences a and b which are divided into a number of distinct formations each clastic metasedimentary unit has a relatively unique trace element signature the depleted mantle sm nd model ages of samples from all major units range from 3 59 to 3 98 ga whereas rb sr model ages cover a very wide range 1 9 4 6 ga the samples from the sequence a felsic gneiss formation a6 yield a relatively good rb sr isochron age of t 3 71 0 07 ga the sequence b samples do not yield a rb sr isochron however some of the samples plot close to the a6 rb sr isochron and sequences a and b appear to have essentially the same deposition age the sm nd data for both sequences yield initial end values in the range 1 to 3 for a deposition age tstrat 3 64 ga if tstrat 3 81 ga the range is 0 4 this range appears to reflect real variability of initial end values corresponding to the time of deposition data trends in end versus 1 nd diagrams suggest at least three distinct components in each of sequences a and b model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as 0 4 b y other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition the very high initial end values implied for early depleted mantle sources 2 to 4 suggest a very large pre 3 8 ga continental crust alternatively these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a 4 5 ga magma ocean

geochemical relationships of late paleozoic carbon rich shales of the midcontinent usa a compendium of results advocating changeable geochemical conditions abstract previous studies have shown that pennsylvanian black shales of the midcontinent us possess a more complex geochemical history than was once thought vine and tourtelot econ geol 65 1970 253 and coveney and glascock appl geochem 4 1989 347 note that the widespread nature of metalliferous pennsylvanian black shales and the diverse geological environments across the midwestern us pose the likelihood that metals contained in the black shales are of complex origins detailed geochemical and statistical representations using characteristics of numerous black shale members have shown that water depth may not have been uniformly deep as previously inferred by numerous workers earlier suggestions that these black shales were associated exclusively with anoxic depositional conditions have been challenged by a more realistic theory of multifaceted fluctuating environmental conditions a wide range of major and trace element concentrations combined with widely varying characteristics and diverse types of organic matter indicate the presence of fluctuating geochemical settings the presumption is that numerous associated chemical reactions occurred syngenetically one geochemical index degree of pyritization dop suggests a dysaerobic to aerobic setting for many of the shales whereas some other geochemical proxies i e v v ni indicate punctuated episodes of bottom water anoxia schieber abstr programs geol soc am 33 6 2001 a102 notes that in some black shales pyrite enrichment due to intermittent organic reworking explains the high dop values once thought to equate with extreme euxinic depositional conditions it seems likely therefore that even during apparent euxinic periods as indicated by bulk geochemistry oxygen deprived events were apparently short lived the seemingly euxinic fingerprint as shown by geochemical characteristics may result through time averaging abstr programs geol soc am 33 6 2001 a102 of intermittent euxinic events providing a misleading indication of dominant euxinic conditions thus detailed microstratigraphic sampling is necessary to delineate short punctuated occurrences of intermittent sub oxic episodes the midcontinent pennsylvanian shales may reflect a complex array of bottom water oxygenation variations and water depth fluctuations in response to climatic and localized events hatch j r and leventhal j s 1992 relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas usa in meyers p a pratt l m nagy b eds geochemistry of metalliferous black shales chem geol 99 65 82 complicated by post depositional events appl geochem 4 1989 347 this research examines the previous exclusively anoxic origin for midcontinent pennsylvanian black shales from a geochemical perspective and offers an alternative supported by detailed statistical presentations with the hypothesis that events of anoxia were punctuated by short lived dysaerobic or even near oxic events

hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single step microbial sulfate reduction coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380 m long sediment core show a large isotopic difference of up to 72 caused by in situ microbial sulfate reduction this is considerably larger than the assumed biological maximum of 46 derived from laboratory studies with pure cultures of sulfate reducing bacteria similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process involving sulfide reoxidation and disproportionation of sulfur intermediates our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single step fractionation this result indicates that the sulfate reducing communities and or their cellular metabolic activities in the deep biosphere may differ from those observed in near surface sediments or the water column

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr