broadband time domain modeling of earthquakes from friuli italy short period sp and long period lp seismograms written by the main shock and two principal aftershocks of the 1976 friuli italy earthquake sequence are modeled in the time domain using synthetic seismograms the main shock occurred on 6 may 1976 20h 00m m s 6 5 and both aftershocks on 15 september 1976 03h 15m m s 6 0 and 09h 21m m s 5 9 source models were determined initially by trial and error and then refined using a waveform inversion program two point sources of radiation are required to adequately model the aftershock short period records for the 09h 21m aftershock the model derived from short period records also produces good fits to the long period data the seismic moment of this earthquake is found to be 0 8 to 1 0 10 25 dyne cm the sp model for the 03h 15m aftershock on the other hand predicts long period synthetics which do not agree with the observations in particular the sp moment 0 37 10 25 dyne cm is about formula times smaller than the lp moment 1 10 25 dyne cm adding a long period component to the sp model considerably improves lp waveform and moment agreement in the case of the main shock a reasonable fit to the observed sp data is obtained using three point sources of radiation however lp synthetics computed using this model do not agree with the observations and the sp moment 0 65 10 25 dyne cm is a small fraction of the lp moment 3 to 5 10 25 dyne cm time function durations indicate that the individual events inferred from the sp records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars in comparison stress drops estimated from lp data are found to be 12 bars main shock and 24 bars 09h 21m aftershock these observations suggest that the short period instruments are sensitive to the high frequency radiation emitted from small high stress drop areas on the fault plane whereas the long period instruments record the overall motion during the earthquake

controls on mo isotope fractionations in a mn rich anoxic marine sediment gullmar fjord sweden abstract molybdenum isotopes have emerged as an important tool for determining the spatial extent of past ocean redox conditions however our understanding of mo cycling and associated isotopic fractionations in modern and ancient continental margin sediments is limited restricting attempts to fully utilize the potential of mo isotopes this is particularly the case with regard to the relative significance of mo isotope fractionations associated with biogeochemical mn and fe oxide cycling under conditions of low dissolved sulfide to rectify this we have investigated sediments from gullmar fjord sweden which contain high concentrations of mn oxides and exhibit a strong redox zonation between mn and fe reduction a combination of dissolved and particulate mo isotope measurements with porewater and solid phase mn and fe characterization reveals that the main influence on porewater 98 mo compositions is exerted by adsorption and desorption processes during reductive dissolution of both mn and fe oxyhydr oxide minerals in areas enriched in mn oxides distinctively low solid phase 98 mo values are observed 1 0 and 0 4 which occur due to the large mo isotope fractionation associated with mo adsorption to mn oxides however where mn oxide concentrations are lower authigenic 98 mo is largely controlled by fe oxyhydr oxide minerals resulting in more positive 98 mo isotopic compositions 0 5 and 2 our results suggest that for modern anoxic marine sediments where deposition occurs beneath an oxic to suboxic water column 98 mo is dominantly controlled by the relative availability of mn and fe oxides and the production of dissolved sulfide

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

the pleistocene glaciations in the dutch sector of the north sea c laban publisher university of amsterdam 1995 proefschrift phd thesis 194 pp isbn 90 9008587 4

a dislocation model of microplate boundary ruptures in the presence of a viscoelastic asthenosphere summary a microplate is modelled as an elastic plate with two long strike slip boundaries lying over a maxwell type viscoelastic asthenosphere the microplate is subjected to a constagt and uniform shear strain rate by the opposite motions of two adjoining larger plates after the occurrence of an earthquake at one of the microplate boundaries the time evolution of shear stress at the other boundary is studied it is found that stress build up at the second boundary is delayed due to stress diffusion governed by the asthenosphere relaxation earthquake occurrence at this latter boundary would be delayed depending upon both the microplate width and the ratio between the maxwell relaxation time of the asthenosphere and a characteristic time required for tectonic strain to recover rupture conditions it turns out that the parameters which determine the occurrence of seismic activity along the microplate boundaries are more strictly constrained in the presence of a viscoelastic asthenosphere than in the case of an elastic halfspace model

morphology and seismic stratigraphy of the inner continental shelf off nova scotia canada evidence for a 65 m lowstand between 11 650 and 11 250 c14 yr b p abstract nova scotia s position near the margin of large pleistocene ice sheets makes the scotian shelf region critical for evaluation of glaciation models and sea level change the sea floor of the inner scotian shelf was mapped using multibeam bathymetry a combination of conventional seismic and sonar techniques and sampling multibeam bathymetry provides an areal image of the sea floor combining this unique image with sub bottom seismic imaging the relationships between sea floor topography and the underlying strata were explored the inner continental shelf of nova scotia can be subdivided into five major terrain zones these are the 1 truncation zone 2 morainal zone 3 outcrop zone 4 basin zone and 5 scotian shelf end moraine complex the scotian shelf end moraine complex and basin zone are glacial depositional zones at the seaward edge of the inner shelf landward of these zones is a region of high relief bedrock with ridges and valleys largely devoid of surficial sediments extending from 80 to 120 m water depth outcrop zone this zone is interpreted as a relict bedrock surface preserved under frozen bed glacier conditions to the east of the outcrop zone in similar water depths are unmodified till ridges overlying bedrock morainal zone the truncation zone is a region of the inner shelf from 90 m to the present shoreline characterized by muted acoustic topography and planar erosional surfaces truncating bedrock and surficial sediments the truncation zone is subdivided into the valley subzone the transition subzone the platform subzone and the estuarine subzone the valley subzone is typified by sediment infilled valleys occurring in 75 to 90 m water depths these valleys contain seismic facies with a ponded style of deposition planed off at the sea floor or by erosional unconformities near the sea floor the transition subzone is marked by a relatively steep ramp and terraces the ramp at the type section extends from 75 to 65 m water depth the surface of the ramp can be either an erosional unconformity or a depositional surface formed by clinoform beds the 65 m former shoreline is interpreted as the top of the transition subzone ramp surface at one ramp locality clinoform beds form part of a progradational sequence called the sambro delta a mussel valve fragment mytilus edulis was obtained from a core in the foresets and radiocarbon dated an age of 11 650 110 yr b p was obtained adjusted for isotopic fractionation above 65 m the sea floor forms a low relief gently sloping erosional surface platform subzone estuarine deposits have been found above 50 m the new sea level curve constructed with the dated 65 m shoreline and recently published data is at variance with calculated rsl curves based on geophysical models primarily in the amplitudes and rates of rsl change reasons for the discrepancies may be late melting ice and regional variations in lithospheric strength and thickness

global ocean redox variations across the smithian spathian boundary linked to concurrent climatic and biotic changes abstract the smithian spathian boundary ssb was an interval characterized by a major global carbon cycle perturbation climatic cooling from a middle late smithian boundary hyperthermal condition and a major setback in the recovery of marine necto pelagic faunas from the end permian mass extinction although the ssb has been linked to changes in oceanic redox conditions key aspects of this redox variation e g duration extent and triggering mechanisms and its relationship to coeval climatic and biotic changes remain unresolved here we report a high resolution middle smithian to middle spathian u isotope 238u record based on marine carbonates of the zuodeng south china and jesmond british columbia sections to investigate the timing and global extent of ocean redox variation across the ssb our 238u record reveals values similar to or slightly heavier than modern seawater 0 39 during the middle smithian a rapid negative shift to highly negative values during the early late smithian a positive shift at the ssb and a rapid shift back to more negative values in the early middle spathian a simple u isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late smithian and the early middle spathian covering 11 of total seafloor area but that it contracted sharply during the ssb 2 the redox pattern documented by our 238u record shows a good first order correspondence to tropical sea surface temperature sst data for the smithian spathian in particular peak anoxia coincided with the middle late smithian boundary hyperthermal event and diminished anoxia with a pronounced decline in ssts at the ssb the temporal correlation between anoxia and low biodiversity levels of many marine clades e g conodonts and ammonoids during the late smithian indicates that oceanic anoxia may have played a role in the ssb biocrisis

the onset of oxidative weathering traced by uranium isotopes abstract the invention of photosynthesis was a key interval in earth s history initiating major changes in the evolution of the oceans and atmosphere many studies suggest that oxygen levels were already enhanced before the paleoproterozoic great oxidation event goe 2 45 to 2 32 ga however the timing of the onset of photosynthetic oxygenation as well as spatio temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated here we present a new and substantial uranium u isotope dataset comprising marine sediments e g black shales carbonates and iron rich sedimentary rocks from the hamersley basin western australia kaapvaal and zimbabwe cratons both southern africa with depositional ages ranging from 3 4 to 2 2 ga sub recent u mobilization was monitored by analyzing 234u along with 238u and detrital u contribution was monitored with al u or th u ratios samples with significant authigenic u enrichment efu 2 relative to post archean australian shale paas exhibit authigenic 238u ranging from 0 96 to 0 02 0 05 2 s d long term reproducibility which deviates markedly from typical 238u of the continental crust 0 40 to 0 20 remarkably we find increased 238u variability in the upper ghaap group 2 50 2 47 ga carbonates and black shales and the lower pretoria group 2 42 2 32 ga duitschland formation shales of the kaapvaal craton the predominantly light u isotope composition of those samples is likely explained by u isotope fractionation related to the onset of partial u mobilization during slow oxidative weathering of uraninite shortly before and during the goe the lack of low 238u values in the timeball hill formation overlying the duitschland formation indicate the onset of quantitative u mobilization during modern style oxidative weathering of uraninite and other u bearing minerals with limited u isotope fractionation these limited u isotope variations measured in the paleoproterozoic timeball hill formation further suggest that in contrast to modern oceans in the period following the goe u was largely quantitatively reduced and removed from the water column of the transvaal basin which remained mainly anoxic at depth

detection of a locked zone at depth on the parkfield california segment of the san andreas fault the parkfield california segment of the san andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked carrizo plain segment to the southeast the rate of shallow fault slip decreases from 25 30 mm yr northwest of the epicenter of the 1966 parkfield earthquake to zero at the southeastern end of the 1966 rupture zone data from a network of trilateration lines spanning the san andreas fault near parkfield and extending to the pacific coast near san luis obispo shed light on the rate of fault slip at depth since the 1966 earthquake in this study average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the parkfield fault segment the fault is taken to be a vertical surface with unknown distribution of strike slip displacement in an elastic half space a striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 parkfield earthquake the data require that the locked zone extend nearly as far north as the 1966 epicenter however the vertical extent of the locked zone is not well resolved over much of the parkfield segment the fault is slipping faster at the earth s surface than it is at seismogenic depths in order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the san andreas the inversion results suggest a spatially uniform normal strain of 0 06 strain yr the orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region the magnitude of the contraction is consistent with convergence rates inferred from global plate motion models

uranium isotope evidence for an expansion of marine anoxia during the end triassic extinction the end triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia suggesting that anoxia played a role in suppressing marine biodiversity however global changes in ocean anoxia are difficult to quantify using proxies for local anoxia uranium isotopes 238u in caco3 sediments deposited under locally well oxygenated bottom waters can passively track seawater 238u which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238u vi relative to 235u vi in anoxic marine sediments we measured 238u in shallow marine limestones from two stratigraphic sections in the lombardy basin northern italy spanning over 400 m we observe a 0 7 negative excursion in 238u beginning in the lowermost jurassic coeval with the onset of the initial negative 13c excursion and persisting for the duration of subsequent high 13c values in the lower middle hettangian stage the 238u excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium based on output from a forward model of the uranium cycle the excursion is consistent with a 40 100 fold increase in the extent of anoxic deposition occurring worldwide additionally relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering which is consistent with weathering induced eutrophication following the rapid increase in pco2 during emplacement of the central atlantic magmatic province the relative timing and duration of the excursion in 238u implies that anoxia could have delayed biotic recovery well into the hettangian stage

variations in u concentrations and isotope signatures in two canadian lakes impacted by u mining a combination of anthropogenic and biogeochemical processes abstract temporal and vertical variations in uranium u concentrations and u isotope 238u signatures were examined in sediment cores collected seven times over a one year period from two lakes in ontario canada which are contaminated with u by historical mining activities bow lake is holomictic experiencing seasonal anoxia while the sediments of meromictic bentley lake are permanently anoxic average annual peak concentrations of u in bow lake subsurface sediments were approximately 300 g l 1 and 600 g g 1 in porewater and bulk sediments respectively similar ranges of concentrations 900 g l 1 and 600 g g 1 respectively were observed in bentley lake sediments the exceedingly high levels of u observed in the porewaters of both lakes as well as the seasonal variability in u levels challenge the traditional paradigm regarding u chemistry i e that reduced u iv should be insoluble under anoxic conditions the average annual 238u values at the sediment water interface of both lakes were similar i e 0 47 0 09 and 0 50 0 16 relative to irmm 184 the deep sediments in both bentley lake and bow lake record u isotope composition with a typical fractionation of 0 6 relative to the surface water confirming authigenic u accumulation i e negligible contribution of particulate material from the tailings also the 238u values in porewater have an average offset of ca 0 1 relative to bulk sediments in anoxic zones and are reversed in the oxic sediment layer

authigenic sulfide and carbonate mineral formation in holocene sediments of the baltic sea formation of authigenic sulfides and carbonates in holocene 14c dated sediments is interpreted based on analyses of cations fes s fes2 s corg and sulfur isotopes mineral distribution was studied in thin sections and mineral morphology with sem eds the sediments were deposited in the gotland deep baltic sea during freshwater and brackish stages calcium rich rhodochrosite mnco3 occurs in concentrations up to 17 dw in brackish water sediments and are mainly confined to 0 02 0 5 mm thick laminae the frequency with which these laminae occur suggests that they formed with a periodicity of one to a few years and their purity suggests that formation took place in a few weeks we propose that these carbonate laminae resulted from oxic water inflow to the euxinic basin mn ii dissolved in the water column was rapidly oxidized and large amounts of mn oxides reached the sediment reduction of these mn oxides in a high alkalinity environment facilitated rhodochrosite precipitation at the sediment surface pyrite in the brackish water sediments formed at or above the sediment water interface in an iron limited environment due to alternating oxic and euxinic conditions fes was oxidized resulting in a high fes2 sfes s ratio median 170 formation of iron sulfides in the freshwater sediments occurred below the sediment water interface in a sulfate limited environment as a result of the rapid exhaustion of reactive iron in the brackish water sediments excess h2s diffused downwards to pyritize the uppermost part of the underlying freshwater sediments

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

a 400 year record of environmental change in an euxinic fjord as revealed by the sedimentary biomarker record a 400 year sedimentary record of the euxinic kyllaren fjord was investigated using organic biogeochemical geochemical and sedimentological approaches accumulation rates of a suite of biomarkers and in some cases their stable carbon isotopic composition were determined biomarkers were separated into three different groups terrestrial derived general aquatic marine derived and aquatic marine derived but related to high primary productivity variations in bulk sediment parameters and biomarker accumulation rates with depth were used to interpret the sedimentary record we infer that in the first half of the 19th century an environmental change occurred caused by a change in either the precipitation regime vegetation and or land use the data also show that in the last century and especially in the last decades a distinct increase in primary productivity occurred most likely this is caused by a natural eutrophication due to the transformation of the fjord from an open to a confined system by the building of a partially open dam in 1954 possibly combined with anthropogenic eutrophication a concurrent shoaling of the fjord chemocline may have had a mediating effect in the change towards a higher productivity by enhancing nutrient and carbon recycling enhanced carbon recycling is reflected by decreased 13c values of several biomarkers

the manganese and iron shuttle in a modern euxinic basin and implications for molybdenum cycling at euxinic ocean margins a meaningful application of mo as a paleo redox proxy requires an understanding of mo cycling in modern reducing environments stagnant euxinic basins such as the black sea are generally regarded as model systems for understanding euxinic systems during early earth history however drawing direct parallels between the black sea and open marine euxinic margins is somewhat complicated by differences in the seawater residence time between these two environments we report sediment and pore water mo u mn and fe data for a euxinic basin with a short seawater residence time the weakly restricted gotland deep in the baltic sea here prolonged periods of euxinia alternate with brief inflow events during which well oxygenated saline water penetrates into the basin during these inflow events dissolved mn and fe that has accumulated within the euxinic deep water can be oxidized and precipitated co variations of mo and u within the sediment suggest that these inflow and oxygenation events may favor mo accumulation in the sediment through adsorption to freshly oxidized mn and fe solid phases once mo is sequestered within the deeper euxinic water and sediments mo retention can be further facilitated by conversion to thiomolybdate species and interactions with organic matter and metal sulfides by comparing our data with those from previous studies where a mn and fe shuttle for mo has been demonstrated we identify two prerequisites for the occurrence of this mechanism first there must be a water column oxic anoxic redox boundary this provides a solubility contrast for mn and fe second the residence time of seawater in the system has to be short weeks to a few years the latter criterion can be met through regular inflow in weakly restricted basins or upwelling in oxygen minimum zones at open marine continental margins based on prior work we suggest that similar conditions to those currently represented by the gotland deep may have prevailed at euxinic ocean margins during the proterozoic a boundary between euxinic and oxic water masses overlying the continental shelf may have resulted in accelerated mo transport through the water column with mn and fe oxyhydr oxides we propose that this mechanism along with mo isotope fractionation during adsorption could contribute to the light mo isotope composition observed in open marine euxinic sediment facies of the proterozoic

transient and stepwise ocean oxygenation during the late ediacaran shuram excursion insights from carbonate δ238u of northwestern mexico abstract the ediacaran shuram excursion se was a key episode in the history of atmospheric and oceanic oxygenation that has also been linked to contemporaneous bioevolutionary events however key aspects of the redox state of ediacaran oceans during the se remain in doubt here the uranium isotope compositions 238u of marine carbonates from sonora northwestern mexico were measured to investigate ocean oxygenation during the se we found that the se in sonora is characterized by a shift toward higher 238u values from 0 7 to 0 3 consistent with a major oxygenation event that has now been documented in widely separated coeval upper ediacaran sections including the doushantuo formation in south china the bol shoy patom formation in siberia and the johnnie formation in california in the sonora succession this 238u shift exhibits a 0 8 myr lag relative to the 13ccarb excursion that defines the se to account for these observations we propose a two step oceanic oxygenation model for the se 1 initial oxidation of an aqueous reductant e g dissolved organic carbon that resulted only in a shift of 13ccarb from 5 to 3 and 2 subsequent oxidation of methane or particulate organic matter that caused a continuation of the negative shift in 13ccarb from 3 to 12 as well as a positive excursion of 238u due to progressive oceanic oxygenation this transient period of enhanced oceanic oxygenation of the late ediacaran ocean which may have promoted the evolution of early metazoans terminated after 5 0 to 6 5 myr

stress changes due to recent seismic events in the central apennines italy the central apennines italy are characterized by moderate seismic activity on normal faults oriented in directions parallel to the apenninic chain the subject of this study is the umbria marche apennines a segment approximately 200 km long where three main seismic events occurred in the last three decades the 1979 norcia earthquake was a mw 5 8 event taking place at the south end of the considered segment the 1984 gubbio earthquake was a mw 5 6 event which took place at the north end the 1997 1998 colfiorito sequence constituted 8 main shocks with magnitudes mw between 5 and 6 and epicenters located between the gubbio and the norcia earthquake areas a model made of an elastic half space is considered in which the seismic sources are represented by rectangular dislocations which have the appropriate values of source parameters and in which the static stress field produced by each event is calculated the analysis of the coulomb stress change c as a function of time shows that the coseismic stress transfer and fault interaction played an important role in the region during the past three decades 7 earthquakes of the 9 considered took place where c 0 such an interaction has been confirmed by the analysis of the aftershocks in the colfiorito zone post september 26 1997 about the 61 of the aftershocks considered took place where c 0 the comparison between the cs due to the coseismic stress transfer and the rate t due to the tectonic stress allows us to quantify the time advance of the earthquakes the cs pattern shows positive values in two areas that can be regarded as historical seismic gaps

global seawater redox trends during the late devonian mass extinction detected using u isotopes of marine limestones abstract the late devonian extinction ranks as one of the big five phanerozoic extinctions affecting up to 80 of marine species and is characterized by two major extinction events that are separated by 238 u across a 7 my interval of well dated upper devonian marine limestones from the devil s gate formation in nevada usa the measured 238 u curve shows no co variation with local sediment derived redox trends water depth dependent facies changes redox sensitive metals toc or diagnostic elemental ratios from these relationships we infer that the 238 u curve represents global seawater redox conditions two negative 238 u excursions indicating more reducing seawater are observed with durations of 3 8 my late frasnian and 1 1 my early famennian respectively steady state modeling of the observed 0 2 to 0 3 shifts in 238 u points to a 5 15 increase in the total area of anoxic seafloor during these excursions the late frasnian negative excursion is broadly coincident with the first extinction event late rhenana zone or lower kellwasser event whereas the early famennian negative excursion lower middle triangularis zones occurs after the most intense frasnian famennian boundary f f extinction event upper kellwasser event compilations of local sediment redox conditions from upper devonian marine deposits with conodont zone level age control indicates that the extinction events were coincident with widespread anoxic deposits accumulating in subtropical epeiric sea and some open ocean settings supporting previous interpretations that widespread marine anoxia had an important influence on the late devonian extinctions the temporal relationships between global ocean redox trends represented by the 238 u curve and the newly compiled subtropical marine redox sediment trends indicates that local epeiric seawaters carried a similar u isotopic composition as the open ocean for the majority of the studied interval except for a brief interval

the behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic lake cadagno switzerland abstract molybdenum mo isotope studies in black shales can provide information about the redox evolution of the earth s oceans provided the isotopic consequences of mo burial into its major sinks are well understood previous applications of the mo isotope paleo ocean redox proxy assumed quantitative scavenging of mo when buried into sulfidic sediments this paper contains the first complete suite of mo isotope fractionation observations in a sulfidic water column and sediment system the meromictic lake cadagno switzerland a small alpine lake with a pronounced oxygen sulfide transition reaching up to h 2 s 200 m in the bottom waters or about 300 m total sulfide s 2 h 2 s hs s 2 we find that mo behaves conservatively in the oxic zone and non conservatively in the sulfidic zone where dissolved mo concentrations decrease from 14 nm to 2 8 nm across this transition dissolved mo in the upper oxic waters has a 98 mo oxic 0 9 0 1 which matches that of the riverine input 98 mo river 0 9 0 1 in the deeper sulfidic waters a subaquatic source delivers mo at 1 55 0 1 but the dissolved mo is even heavier at 98 mo sulfidic 1 8 sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in mo with depth with 98 mo values significantly fractionated at 0 8 to 1 2 both near the chemocline and in the deepest trap suspended particulates in the sulfidic waters carry lighter mo than the ambient dissolved mo pool by 0 3 1 5 sedimentary mo concentrations correlate with total organic carbon and yield mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area solid phase mo in the sediment shows a slightly positive 98 mo trend with depth from 98 mo 1 2 to 1 4 while the pore waters show dramatic enrichments of mo 2000 nm with a relatively light isotope signature of 98 mo 0 9 1 0 these data are explained if mo is converted to particle reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles isotope fractionation is expressed in the water column despite the high sulfide concentrations because the rate of mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation however elemental and isotopic mass balances show that mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water this efficient mo drawdown is expected to occur in settings where h 2 s is very much in excess over mo or in a restricted setting where the water renewal rate is slow compared to the mo burial rate we present a model for the mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

a cenozoic seawater redox record derived from 238u 235u in ferromanganese crusts oceanic oxygen levels are projected to drop in certain areas due to warming climate but the net effect to the overall ocean redox state is difficult to predict here we measured the stable uranium isotope composition 238u 235u in globally representative hydrogenous ferromanganese crusts in order to reconstruct the redox evolution of the global ocean throughout the cenozoic samples averaging 3 myr intervals have analytically indistinguishable 238u 235u throughout the cenozoic combined with a u isotope mass balance model we suggest that the overall ocean redox state has remained remarkably stable on million year time scales throughout the cenozoic despite large surface temperature fluctuations during this time this suggests that stabilizing feedbacks for example nutrient limitation in low oxygen zones may have prevented dramatic large scale shifts in oxygen levels in the ocean however the fe mn crust record will be unlikely to reflect rapid perturbations in ocean redox state to investigate these events sediment archives with faster accumulation rates and redox proxies with faster response time must be explored

uranium and molybdenum isotope systematics in modern euxinic basins case studies from the central baltic sea and the kyllaren fjord norway abstract recent investigations have revealed significant fractionation of 238u 235u between organic rich sediments of anoxic marginal seas and seawater indicating redox dependent u isotope fractionation this study explores the conditions controlling u isotope fractionation in selected modern anoxic basins baltic sea landsort and gotland deeps and the kyllaren fjord in norway and compares u with mo isotope fractionation therefore the concentrations and isotope compositions of dissolved u and mo from the water column and organic rich sediments from three currently anoxic basins have been analysed the water column samples from the kyllaren fjord display a moderate depletion of u and a strong depletion of mo with increasing depth these variations are associated with a decrease in 238u and an increase in 98mo with depth from 0 35 to 0 70 and from 2 4 to 2 6 respectively from the u isotope composition of the deep euxinic water column of the kyllaren fjord a minimum value of 238ured 0 7 for the u isotope fractionation during reduction is inferred due to the high sedimentation rate in the kyllaren fjord surface sediment samples are only moderately enriched in u and mo 4 g g and 6 g g to 37 g g respectively and display 238u and 98mo averages of 0 26 0 10 and 2 18 0 21 respectively water column samples from the baltic sea display a minor decrease in the u and mo concentrations and only negligible u and mo isotope fractionation compared to open seawater the sediment layers from the baltic sea landsort deep which were deposited under mostly anoxic conditions are moderately enriched in u up to 8 g g and highly enriched in mo up to 222 g g however u and mo isotopic compositions are significantly lighter with 238u of 0 36 0 08 and 98mo of 0 04 0 11 than those of typical organic rich sediments from anoxic basins these findings reveal that compared to mo the u isotopic composition of sediments in restricted and strongly euxinic basins like the kyllaren fjord is significantly more dependent on 1 the extent of u removal from the water column which is lower than that of mo and on 2 the sedimentation rate i e the fraction of authigenic u relative to detrital u in the sediment which is also lower compared to that of mo in more open and only temporarily euxinic basins such as the investigated basins of the baltic sea strong mo is coupled with weak u isotope fractionation between water and sediment these signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light mo during frequently occurring flushing events with oxygen rich water from the open sea this implies that mo and u isotope signatures of sediments only record paleo water column redox conditions of restricted basins if the water column was permanently stratified

a multi proxy lithostratigraphic record of late glacial and holocene climate variability from piper lake nova scotia a multi proxy lithostratigraphic record from piper lake nova scotia reveals environmental variability during the late glacial and holocene piper lake is a small shallow 3 m closed dystrophic basin located in the eastern nova scotia highlands the site was deglaciated about 14 5 cal calibrated ka bp and elevated loss on ignition values and relatively low carbon nitrogen c n isotope ratios indicate the establishment of a productive aquatic environment consistent with allerod warming the late glacial lake record is punctuated by two thin very fine grained clay layers that are correlative to the killarney and younger dryas yd oscillations they were deposited when perennial ice covered the lake the post yd lithostratigraphy indicates the rapid establishment of an increasingly productive and stable landscape this trend is reversed three times during the holocene by minerogenic units a complex 25 cm thick diamicton unique to piper lake was deposited ca 10 8 10 3 cal ka bp by slumping that was associated with periglacial slope processes and or lake level changes a direct correlation to early holocene preboreal cooling appears unlikely two thin minerogenic units deposited at ca 8 1 and ca 4 9 cal ka bp were likely the result of regional cooling and are broadly correlative with events noted in the gisp2 greenland ice sheet project 2 ice core record the holocene lithostratigraphic record from piper lake may be a consequence of unique limnological factors alternatively the strong lithostratigraphic response may be the result of the absence of a strong and persistent regional climate mechanism north atlantic oscillation which if present might have obscured the impact of hemispheric or larger scale climate forcing

uranium isotope geochemistry in modern coastal sediments insights from toulon bay france abstract by assessing u geochemistry in marine sediments of toulon bay nw mediterranean sea authigenic u accumulation was found to be tightly linked to that of mo and v with slight differences in accumulation rate depending on sediment redox conditions in sediments collected on a transect along a river plume the authigenic accumulation of these redox sensitive elements appears to be linked to sediment grain size which probably drives the redox status of the sediments a typical u loss in re oxidized sediments was observed in sediments that had been previously disturbed however the dissolved u profiles do not show a simple and typical depletion trend but rather a depletion in the top 10 cm followed by release in deeper pore waters that could be linked to a potential reoxidation mobilization of authigenic u iv the released u could be further scavenged as a u p precipitate the u isotopic data 238u which are the first reported for the coastal hypoxic sediments of toulon bay average 0 12 0 12 in surface sediments however sediments situated in front of river mouths show higher values of 238u up to 0 75 these high values are reported for the first time in sediments below an oxic water column

forecast model for moderate earthquakes near parkfield california earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation in the forecast procedure proposed here repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters the early part of the model simulation corresponds to the available field data and the subsequent part constitutes an estimate of future faulting and ground deformation in particular the time location and size of unstable faulting are estimates of the pending earthquake parameters the forecast accuracy depends on the model realism and parameter resolution the forecast procedure is applied to fault creep and trilateration data measured near parkfield california where at least five magnitude 5 5 to 6 earthquakes have occurred regularly since 1881 the last in 1966 the quasi static model consists of a flat vertical plane embedded in an elastic half space spacially variable fault slip of strike slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle strain softening fault the field data are consistent with these approximate values of patch parameters radius of 3 km patch center 5 km deep and 8 km southeast of the 1966 epicenter and maximum brittle strength of 26 bars fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21 8 year recurrence interval however the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake as predicted by the model

untangling the diagenetic history of uranium isotopes in marine carbonates a case study tracing the δ238u composition of late silurian oceans using calcitic brachiopod shells abstract the u isotope composition of ancient marine carbonates has been used to determine the 238u of past oceans based on the premise that the 238u signatures of marine carbonates reflect the 238u of seawater at the time of deposition however recent work from the bahamas shows that diagenesis alters the 238u signatures of bulk carbonates by effectively creating a positive and variable offset from seawater therefore finding a sedimentary archive that reliably records and preserves the 238u signal from seawater becomes fundamental calcitic brachiopod shells may provide such an archive as their shell structure can endure the effects of diagenesis and thus their 238u signatures can record the u isotope composition of the seawater in which they lived to explore whether calcitic brachiopods can more robustly record seawater 238u than bulk carbonates we analyzed brachiopod shells and their host carbonate matrices from five different stratigraphic levels across the upper silurian lau carbon isotope excursion cie using the data from brachiopod shells our results suggest that seawater 238u values were relatively stable and lower than present day 238u 0 58 0 11 2sd n 17 indicative of a more anoxic ocean state across the positive cie we also find that carbonate matrices display a distinctive positive 238u offset from contemporaneous seawater that varies between beds some beds show a ca 0 30 offset similar to what is described in the bahamas whereas others display no offset from the inferred seawater u isotope composition we propose the variability of the observed 238u offset to be a function of the degree of u vi exchange between anoxic porewaters and seawater a diminished u vi exchange would define a closed system style of diagenesis and mute the net isotope offset expressed between sediments and contemporaneous seawater some of the factors affecting the degree of u vi exchange include organic matter fluxes into the sediments and diagenetic fluid flow rates which could vary due to relative sea level changes as a result the observed 238u variability of the analyzed carbonate matrix samples could be the result of the environmental changes caused by the sea level fluctuations described for the studied stratigraphic interval we conclude that calcitic brachiopod shells provide an important archive for evaluating the 238u of ancient oceans and the style of diagenesis of u in marine carbonates

molybdenum in the northeast pacific ocean1 the concentration of dissolved molybdenum has been determined in several profiles from the northeast pacific ocean the salinity normalized concentration is essentially constant in all open ocean samples 107 nm 1 2 5 nm the dissolved distribution is not significantly affected by biological cycling

tracking the onset of phanerozoic style redox sensitive trace metal enrichments new results from basal ediacaran post glacial strata in nw canada abstract a global rise in oxygen levels has been proposed to coincide with the ediacaran to cambrian radiation of animals yet the precise timing and nature of this change remains unresolved one hypothesis is that the ocean atmosphere system became temporarily well oxygenated in the earliest ediacaran directly following the marinoan snowball earth glaciation 635 ma the evidence for oxygenation is based on large enrichments of redox sensitive trace elements in black shale from south china presumably deposited under a euxinic water column these enrichments meet or exceed the bulk concentrations of redox sensitive trace elements found in phanerozoic shale deposited under euxinic water columns here we test the early ediacaran post snowball oxygenation hypothesis with new data from a high resolution multi proxy geochemical and sedimentological study of three stratigraphic sections in the earliest ediacaran sheepbed formation of the mackenzie and wernecke mountains nw canada iron speciation data from all sections suggest that the local water column was dominantly ferruginous with a notable exception of probable euxinic conditions recorded in part of one section redox sensitive elements show no appreciable enrichments in the basal sheepbed formation with maximum concentrations of molybdenum vanadium uranium and chromium only slightly above world average shale values the lack of substantial elemental enrichments within the sheepbed formation is consistent with results from coeval ferruginous strata in svalbard the dominance of local ferruginous conditions in nw canada cannot alone account for the muted redox sensitive element enrichments as v u and cr are thought to be enriched under these conditions and samples deposited under euxinic conditions also lack phanerozoic style enrichments these contrasting results from different localities highlight a need to further investigate the veracity of trace metal enrichments in all localities as representing global redox conditions as the results have important implications for the timing of oxygenation with respect to early animal evolution

highly dynamic marine redox state through the cambrian explosion highlighted by authigenic δ238u records abstract the history of oceanic oxygenation from the late neoproterozoic to the early cambrian is currently debated making it difficult to gauge whether and to what extent environmental triggers played a role in shaping the trajectory of metazoan diversification uranium isotope 238 u records from carbonates have recently been used to argue for significant swings in the global marine redox state from the late neoproterozoic to the early cambrian however geochemical signatures in carbonates the u isotope archive most commonly employed to argue for redox shifts are susceptible to diagenetic alteration and may have variable offsets from seawater values therefore there is an impetus to reconstruct seawater u isotopic evolution using another sedimentary archive in order to verify that these excursions can indeed be linked to global shifts in marine redox landscape here we report new u isotope data from two fine grained siliciclastic upper ediacaran to lower cambrian ca 551 515 ma successions in south china we find large 238 u swings between 0 63 and 0 39 for calculated values of authigenic u in the siliciclastic rocks consistent with correlative records from the carbonates the replication of these patterns in both carbonate and siliciclastic units provides confirmatory evidence that the early cambrian seawater was characterized by highly variable u isotope compositions these new 238 u data also provide higher resolution records of global oceanic redox conditions during cambrian age 3 coeval with a critical interval of the cambrian explosion these 238 u data bolster the case that the ediacaran cambrian transition experienced massive swings in marine redox state providing a dynamic environmental backdrop for and potentially even a key driver of the emergence and radiation of metazoans

the biogeochemistry of ferruginous lakes and past ferruginous oceans abstract anoxic and iron rich ferruginous conditions prevailed in the ocean under the low oxygen atmosphere that occurred through most of the archean eon while euxinic conditions i e anoxic and hydrogen sulfide rich waters became more common in the proterozoic ferruginous conditions persisted in deep waters ferruginous ocean regions would have been a major biosphere and earth surface reservoir through which elements passed through as part of their global biogeochemical cycles understanding key biological events such as the rise of oxygen in the atmosphere or even the transitions from ferruginous to euxinic or oxic conditions requires understanding the biogeochemical processes occurring within ferruginous oceans and their indicators in the rock record important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes their sediments commonly host siderite and ca carbonates which are important precambrian records of the carbon cycling lakes that were ferruginous in the past or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid depth oceanic waters during the proterozoic modern ferruginous meromictic lakes which host diverse anaerobic microbial communities are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans such lakes are believed to be rare but regional and geological factors indicate they may be more common than previously thought while physical mixing processes in lakes and oceans are notably different many chemical and biological processes are similar the diversity of sizes stratifications and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems near shore off shore silled basins or those dominated by terrestrial or hydrothermal element sources ferruginous systems both extant and extinct lacustrine and marine host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of earth s surface environment

uranium isotope evidence for limited euxinia in mid proterozoic oceans abstract reconstructing earth s oxygenation history is key to deciphering environmental controls on early biospheric evolution during the mid proterozoic eon low but potentially variable atmospheric po2 led to highly heterogeneous marine redox conditions with most studies indicating a relatively shallow depth of oxygen penetration the relative proportion of oxic anoxic and iron rich ferruginous and anoxic and sulfide rich euxinic conditions on the global seafloor is difficult to quantify however due to a general reliance on local redox proxies applied to the temporally discontinuous black shale record it is particularly important to constrain the global prevalence of euxinic bottom waters because sulfide toxicity has been implicated as a primary constraint on evolution in mid proterozoic oceans and limits the solubility of bioessential trace metals such as copper zinc and molybdenum here we present a suite of new uranium u isotope data from marine carbonate rocks that span the entire mid proterozoic interval 1 8 to 0 8 ga u isotopes in well preserved carbonate rocks represent a powerful new proxy that can be used to quantitatively constrain the global extent of marine euxinia the median 238 u value for mid proterozoic carbonate samples is 0 43 which is lower than the median value for modern post depositional carbonate sediments from the bahamas yet significantly higher than carbonate 238 u values recorded during other times of expanded anoxia in earth history such as during the end permian mass extinction when paired with a three sink isotope mass balance model our data indicate that no more than 7 of the global seafloor was euxinic during the mid proterozoic eon although transient pulses of expanded euxinia are recorded although these results challenge early expectations of a sulfide rich mid proterozoic ocean they are consistent with more recent inferences from the black shale trace metal record evidence therefore indicates that euxinic bottom waters were relatively limited in their spatial extent in the mid proterozoic oceans

uranium isotope variations in a dolomitized jurassic carbonate platform tithonian franconian alb southern germany abstract the uranium u isotopic composition 238u of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean however many limestones have undergone significant dolomitization during diagenesis to assess the potential impact of diagenetic changes on the u isotope composition of dolomitized rocks we examined the uranium isotopic composition of a jurassic carbonate platform of the franconian alb in southern germany this platform underwent dolomitization during shallow burial due to the supply of magnesium rich fluids in the form of slightly modified seawater this type of dolomitization is common in the geologic record the carbonate platform of the franconian alb can therefore serve as an example for many dolomitized carbonate platforms a positive correlation between the concentrations of redox sensitive elements e g re and v and 238u confirms that the authigenic uptake of u under reducing conditions preferentially incorporates u enriched in 238u this positive correlation between redox sensitive elements and 238u does not change within the dolomitized interval suggesting that the 238u is not altered during shallow burial dolomitization of limestones therefore our results indicate that dolostones can retain information about secular variations in seawater 238u the diagenetic uptake of u shifts 238u of bulk carbonate sediments to values heavier than contemporaneous seawater the magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set samples with high re and v concentrations and high re mo ratios are the most enriched in 238u this positive correlation could be used to constrain the amount of diagenetic offset of bulk 238u values from seawater this is particularly important in carbonate systems that had a primary calcitic mineralogy primary calcite precipitates have low u concentrations and so small amounts of 238u uptake during diagenesis can have a significant impact on the bulk composition the samples with the most depleted 238u values have high mo concentrations and low re mo ratios as carbonate sediments are generally mo poor the high mo concentrations suggests that the depleted 238u values might be linked to a manganese cycle that operated across the sediment water interface during the deposition of this jurassic carbonate platform

uranium removal in oceanic sediments and the oceanic u balance pore water uranium data from the nw atlantic ocean cariaco trench black sea and san clemente basin are used to calculate fluxes across the sediment water interface all samples were collected in situ and show depletions in pore water u relative to the overlying water concentration the uptake rate of u derived from pore water gradients reflects the redox environment with lowest fluxes 10 g cm 2 ky in oxic su ic sediments of the nw atlantic dominated by no 3 reduction and higher fluxes 25 70 g cm 2 ky in the anoxic sulfate reducing sediments of the cariaco trench black sea and san clemente basin the anoxic fluxes compare well with previous estimates based on solid phase data calculations indicate that the areally integrated removal of u into su ic hemipelagic sediments is of greater magnitude than that into anoxic sulfate reducing sediments these fluxes are discussed in the context of the oceanic u balance the removal of u in all oceanic sediments is estimated to account for about 65 of the riverine input and this sink plus removal of u in altered oceanic crust balances the most probable value for riverine input

aftershocks of the campania lucania italy earthquake of 23 november 1980 abstract the result of a study of 1130 aftershocks of the 1980 campania lucania earthquake are presented the data were collected primarily with a dense array of portable smoked drum seismographs deployed in the epicentral region a few days after the main shock tests performed on subsets of the data to establish the location accuracy are described in detail and epicentral maps and cross sections are presented and discussed one hundred and sixty well constrained fault plane solutions using upward traveling rays are presented the distribution of aftershocks forms an elongate cloud which does not define a plane this is not because the events are poorly located a subset of events located with epicentral errors of less than 1 km and depth errors less than 2 km forms a similar cloud to the complete aftershock set the aftershock mechanisms are very varied some are similar to the main event but many differ profoundly these include events on shallow or vertical planes and thrust events the latter include some events which require contraction in the direction in which the main event indicates extension the aftershocks of all mechanisms are intermixed in space in such a way that no simple system of faults can explain them a model to explain the aftershocks and the lack of surface ruptures in the main event is offered it is proposed that the main shock cuts between two aseismic detachment surfaces and that the aftershocks are a consequence of the complicated deformation that such a geometry demands whatever interpretation is adopted the results are a challenge to the commonly accepted view that aftershocks even approximately define simple fault planes or that aftershock mechanisms replicate the mechanism of the main shock defects in interpretations of the main shock and aftershocks process arise largely from a lack of information about the geometry of structures at depth in the epicentral region suitable deep reflection profiles could help to resolve this problem

proposed younger dryas glaciation of the eastern scotian shelf a refined synthesis of the stratigraphy of the scotian shelf is presented based on recently published borehole and seismic reflection data and detailed correlation along high resolution seismic profiles between the outer banks and the central basins one of the new stratigraphic units of the outer bank region is of glacigenic origin and represents the last glacial event near the margin of the shelf these deposits appear to be part of a morainal bank complex formed at a tidewater front a younger dryas age is proposed for the glacial deposits and the ice may have been widely distributed across the eastern scotian shelf evidence for the late glaciation is expressed in the central shelf area by an unconformity which on the bank areas is associated with subglacial meltwater channels 14c dates for the basin sediments of this region provide the most reliable chronology for the shelf stratigraphy the origin of sable island is closely related to the depositional history of the moraine and its surface has be

global enhancement of ocean anoxia during oceanic anoxic event 2 a quantitative approach using u isotopes during the mesozoic greenhouse world the oceans underwent several oceanic anoxic events oaes characterized by intervals during which organic rich black shales were deposited indicating strong oxygen depletion in the marine realm the cenomanian turonian oae2 ca 93 ma represents one of the most prominent events of the cretaceous with significant perturbations of the global carbon cycle although oae2 likely reached a global scale the spatial extent of seawater anoxia during this oae is poorly constrained here we demonstrate that variations in the 238u 235u isotope ratio 238u a newly developed paleoredox proxy can be used to quantify the extent of marine anoxia for black shales from the mid cretaceous oae2 we find a systematic shift toward lighter 238u and lower u concentrations as compared to modern equivalent organic rich sediments from the black sea this shift translates to a global increase of oceanic anoxia during oae2 by at least a factor of three as compared to the present day or to periods before and after oae2 the constant offset in u concentrations and isotope compositions of black shales throughout oae2 compared to modern black sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of oae2

century long record of mo isotopic composition in sediments of a seasonally anoxic estuary chesapeake bay abstract a double spike method was used to obtain mo isotope data for sediments and waters of the seasonally anoxic chesapeake bay and its primary tributary the susquehanna river the dissolved mo distribution in the estuary is non conservative reflecting minor mo loss to the sediments although removal of mo to the sediments does not have a large influence on the isotopic composition of the water column the 98mo of dissolved mo in most of the estuary is dominated by seawater six samples with salinity 15 have an average 98mo 2 17 0 12 which agrees well with a 98mo value for the cass 4 seawater standard of 2 23 a single sample of susquehanna river water has a 98mo of 1 02 consistent with recent findings of positive 98mo in rivers worldwide susquehanna river sediments in contrast have 98mo 0 1 the difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin the 98mo values for estuarine sediments are offset from values in the overlying water most samples deposited before 1925 have 98mo less than 0 similar to the susquehanna sediments subsequently there is an increase in the variability of 98mo with values ranging up to 0 8 the transition to increased variability coincides with the onset of authigenic mo deposition which was previously attributed to escalating summertime anoxia authigenic mo concentrations correlate poorly with 98mo in core samples suggesting that independent mechanisms influence the two parameters authigenic mo concentrations may be controlled by shifting pore water h2s levels while 98mo may be primarily affected by annual variations in mn refluxing

uranium isotope fractionation during coprecipitation with aragonite and calcite abstract natural variations in 238u 235u of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments in order to evaluate the reliability of this proxy we conducted aragonite and calcite coprecipitation experiments at ph 7 5 and 8 5 to study possible u isotope fractionation during incorporation into these minerals small but significant u isotope fractionation was observed in aragonite experiments at ph 8 5 with heavier u isotopes preferentially enriched in the solid phase 238u 235u of dissolved u in these experiments can be fit by rayleigh fractionation curves with fractionation factors of 1 00007 0 00002 0 00003 1 00005 0 00001 and 1 00003 0 00001 in contrast no resolvable u isotope fractionation was observed in an aragonite experiment at ph 7 5 or in calcite experiments at either ph equilibrium isotope fractionation among different aqueous u species is the most likely explanation for these findings certain charged u species are preferentially incorporated into calcium carbonate relative to the uncharged u species ca2uo2 co3 3 aq which we hypothesize has a lighter equilibrium u isotope composition than most of the charged species according to this hypothesis the magnitude of u isotope fractionation should scale with the fraction of dissolved u that is present as ca2uo2 co3 3 aq this expectation is confirmed by equilibrium speciation modeling of our experiments theoretical calculation of the u isotope fractionation factors between different u species could further test this hypothesis and our proposed fractionation mechanism these findings suggest that u isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater u particularly changes in seawater ph p co 2 ca2 or mg2 concentrations in general these effects are likely to be small

hydrogen sulphide release to surface waters at the precambrian cambrian boundary changes in environmental conditions at the precambrian cambrian transition around 542 million years ago have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the cambrian explosion the nature of the environmental changes is still a matter of debate however wille et al now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years with the help of a box model of the oceanic molybdenum cycle they find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal this suggests that the early cambrian animal radiation may have been triggered by a major change in ocean circulation following a long period during which the ocean was stratified with sulphidic deep water this paper reports molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 million years the findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water animal like multicellular fossils appeared towards the end of the precambrian followed by a rapid increase in the abundance and diversity of fossils during the early cambrian period an event also known as the cambrian explosion 1 2 3 changes in the environmental conditions at the precambrian cambrian transition about 542 myr ago have been suggested as a possible explanation for this event but are still a matter of debate1 2 3 here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 myr we find a transient molybdenum isotope signal immediately after the precambrian cambrian transition using a box model of the oceanic molybdenum cycle we find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal our findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water

uranium isotope fractionation suggests oxidative uranium mobilization at 2 50ga abstract geochemical data from late archean sedimentary rocks point to photosynthetic o 2 production and at least intermittent occurrences of locally mild oxidative weathering and surface ocean oxygenation oxygen oases prior to the early paleoproterozoic great oxidation event for example distinctive authigenic enrichments of mo and re in euxinic anoxic and sulfidic black shales are best explained by the oxidative mobilization of these metals from crustal sulfide minerals and their accumulation as oxyanions in seawater in contrast it is not clear if low u enrichments in the same shales reflect negligible oxidation of u from the upper crust or a very small oceanic u inventory that was derived from oxidative u mobilization here we report u isotope data for the 2 50 ga mt mcrae shale hamersley basin western australia which provides a more sensitive test for the presence or absence of authigenic u compared to u concentrations and enrichment factors normalized to average shale or upper crustal compositions that may not be representative of the local detrital composition we find instances where the u isotope composition in the upper mt mcrae shale 238 u 0 2 to 0 0 relative to standard srm950a is isotopically heavier than average upper crust 238 u 0 31 0 14 2sd based on granitoids and basalts the high 238 u values point to u isotope fractionation in the late archean marine environment and hence indicate the presence of a small amount of dissolved u in seawater and authigenic u in the mt mcrae shale volume dependent equilibrium isotope fractionation during the reduction of dissolved u vi to insoluble u iv like that observed in the modern black sea may explain the high 238 u signatures if u removal from bottom waters was not quantitative alternatively quantitative u removal would require that late archean seawater had high 238 u which could have arisen from the preferential sequestration of 235 u to fe oxyhydr oxide minerals elsewhere in the hamersley basin the supracrustal 238 u signatures are associated with some of the highest mo and re enrichments in the mt mcrae shale as well as distinctive mo s and n isotope signatures that are indicative of mild environmental oxygenation hence our findings suggest that small amounts of u were oxidatively mobilized from the upper crust at 2 50 ga unlike mo and re however u oxidation may not have occurred on land instead we hypothesize that oxidative u mobilization occurred by submarine weathering in an oxygen oasis

uranium isotope evidence for temporary ocean oxygenation in the aftermath of the sturtian snowball earth abstract the appearance and radiation of animals are commonly attributed to neoproterozoic oceanic oxygenation yet independent geochemical evidence for such an event remains equivocal strata deposited between the sturtian and marinoan snowball earth glaciations 660 to 640 ma contain the earliest animal biomarkers and possible body fossils to quantify the extent of seafloor oxygenation during this critical interval we present uranium isotope ratios 238u 235u denoted as 238 u from limestone of the taishir formation in mongolia through two stratigraphic sections that are separated by 75 km within the same depositional basin above the sturtian glacial deposits through 150 m of stratigraphy 238 u compositions have a mean value of 0 47 this interval is followed by a 0 3 decrease in 238 u coincident with the taishir negative carbon isotope excursion thereafter 238 u values remain relatively low until the erosional unconformity at the base of the marinoan glacial deposits using a box model we show that the best explanation for the higher 238 u values of the post sturtian limestones is extensive but temporary oxygenation of the seafloor and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post snowball weathering the decline in 238 u in overlying strata coincident with the taishir negative 13 c excursion indicates a subsequent decrease in seafloor oxygenation the u isotopic data combined with modeling results challenge the notion of a simple unidirectional oxygenation of neoproterozoic oceans

inverse correlation between the molybdenum and uranium isotope compositions of upper devonian black shales caused by changes in local depositional conditions rather than global ocean redox variations abstract coupled mo u isotope data from the upper devonian kettle point formation ontario canada provide a cautionary tale regarding interpretation of global ocean redox conditions using data from euxinic black shales in the gore of chatham core the kettle point black shales have high mo concentrations 48 473 g g and consistently high mo u ratios 3 times the mo u ratio of modern seawater suggesting a euxinic depositional environment these shales yield an inverse correlation r 0 89 p

marine anoxia and delayed earth system recovery after the end permian extinction delayed earth system recovery following the end permian mass extinction is often attributed to severe ocean anoxia however the extent and duration of early triassic anoxia remains poorly constrained here we use paired records of uranium concentrations u and 238u 235u isotopic compositions 238u of upper permian upper triassic marine limestones from china and turkey to quantify variations in global seafloor redox conditions we observe abrupt decreases in u and 238u across the end permian extinction horizon from 3 ppm and 0 15 to 0 3 ppm and 0 77 followed by a gradual return to preextinction values over the subsequent 5 million years these trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone omz that inhibited the recovery of benthic animal diversity and marine ecosystem function we hypothesize that in the early triassic oceans characterized by prolonged shallow anoxia that may have impinged onto continental shelves global biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the omz under this hypothesis the middle triassic decline in bottom water anoxia stabilization of biogeochemical cycles and diversification of marine animals together reflect the development of a deeper and less extensive omz which regulated earth system recovery following the end permian catastrophe

a new particulate mn fe p shuttle at the redoxcline of anoxic basins abstract pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters intense element cycling favoured by elevated microbial activity causes steep gradients of physico chemical parameters nutrients and redox sensitive trace metals this study presents a conceptual model for authigenic particle formation at pelagic redoxclines which is based on the tight coupling of mn fe and p cycles besides the well known occurrence of mn oxides textural sem edx and geochemical icp oes icp ms analyses of particles from the redoxclines of the black sea and the baltic sea gotland basin landsort deep evidence the existence of earlier postulated fe oxyhydroxo phosphates and emphasize mixed phases consisting of mn oxides and fe oxyhydroxo phosphates as a new solid species most of the analyzed particles are star shaped of about 5 m in size and occur as single particles or aggregates without any morphological differences between mn oxides fe oxyhydroxo phosphates and mixed phases throughout the redoxcline these minerals show a general succession with maximum abundance of mn oxides above the redoxcline followed by mixed phases and almost pure fe phosphates within and below the redoxcline respectively molar fe p ratios of single particles argue against the formation of known pure fe phosphates like vivianite or strengite at the lower end of the redox transition zone but are consistent with recent experimental findings for colloidal p bearing hydrous ferric oxides moreover morphological similarities suggest the formation of irregular fe oxyhydroxo coatings due to oxidation of upward diffusing fe 2 by oxygen and stepwise replacement of mn iv by fe iii on sinking mno x particles followed by immediate adsorption or even co precipitation of phosphate batch type experiments using biogenic mno x particles demonstrate the efficient potential of fe 2 oxidation by sinking mno x particles when entering sulfidic waters mno x particles are progressively reduced leading to an increasing relative abundance of fe and p rich particles in deeper parts of the water column these particles are also reductively dissolved thereby releasing fe 2 and phosphate to the water column this mn fe p shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales furthermore the particulate mn fe p shuttle must have played an important role for the cycling of p and certain trace metals in ancient ocean basins e g during certain periods of cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic anoxic interfaces of aquatic ecosystems

interaction between seismic and aseismic slip along a transcurrent plate boundary a model for seismic sequences abstract a model is proposed which may explain the multiple occurrence of earthquakes which is often observed in the same seismogenic region such earthquake sequences or swarms may consist of events several tens of kilometres apart in space and a few days to months apart in time the model considers a long vertical strike slip fault embedded in an elastic half space the fault is heterogeneous with respect to strength and to slip style seismic or aseismic and is subject to a slowly increasing ambient shear stress we assume that conditions for stable aseismic slip are present in a part of the fault plane together with unstable asperities the failure of an asperity producing one earthquake of the sequence is assumed to produce the onset or the acceleration of a seismic slip on a distant area of the fault in proximity to another asperity the conditions under which the latter asperity may fail producing another earthquake of similar magnitude are studied as functions of ambient stress rate asperity strength amount of aseismic slip and aseismic slip area it is found that the effect of aseismic slip can be remarkable in anticipating the occurrence of the second earthquake

vertical mixing in the baltic sea and consequences for eutrophication a review abstract in the transition area between the north sea and the baltic sea entrainment processes dominate the vertical mixing in the inflowing saline bottom water the hot spots of these processes are located at the darss sill and the bornholm channel in the western baltic sea in the central baltic sea the horizontal advection of saline water in deep layers below the permanent halocline dominate the temporal changes and associated transports this is accompanied by the turbulent vertical transport through the halocline into the surface layers the related vertical salt transport into the entire surface mixed layer estimated by various methods is slightly above 30 kg m 2 a during stagnation periods the residence time of the deep water in the eastern gotland basin increases roughly by a factor of five vertical mixing through the halocline is drastically reduced when inflows are lacking the potential processes of diapycnal mixing are discussed to the present knowledge the turbulent motion resulting from breaking internal waves is capable of turbulent transports through the halocline corresponding to the estimates of the salt transport into the surface mixed layer the actual knowledge about boundary mixing due to internal waves in the baltic sea is poor mesoscale eddies may contribute to the vertical mixing but it is not known whether they really do and which of the possible direct and indirect mixing mechanisms is most effective near bottom currents induced by inflow events likely enhance vertical mixing coastal upwelling certainly contributes to the vertical transport but the depth of its origin and the volume transport are hard to determine for large scale quantifications the short spatiotemporal scale of turbulent transports through the halocline resulting in a weakening of the halocline during summer together with the mixing of the entire surface layer down to the halocline in winter form a consistent description of the vertical salt transport it is hypothesised that the longer residence time of the deep water during stagnation periods results from the lack of energy imported by the inflows and directly or indirectly feeding the diapycnal mixing processes the vertical transport of nutrients such as the phosphate is quantitatively not sufficiently understood and needs further interdisciplinary research activities

modeling the contribution of m2 tidal amplification to the holocene rise of mean high water in the gulf of maine and the bay of fundy abstract increase in the m2 principal lunar semi diurnal component of the tidal range is an important contributor to the postglacial rise of the high tide level in the gulf of maine and the bay of fundy in addition to relative sea level rise the increase occurred as the shape of this semi enclosed embayment changed with sea level rise and its natural period approached resonance with the m2 tide a three dimensional tide model 3dens predicts the following m2 tidal ranges averaged for the gulf of maine bay of fundy as a percentage of the present range 54 59 at 7000 yrs b p 73 at 5000 yrs b p 78 at 4000 yrs b p 85 at 3000 yrs b p 94 at 2000 yrs b p and 98 at 1000 yrs b p with a predicted sea level rise of 0 15 0 90 m by the year 2100 m2 tidal range may increase 0 3 1 9

a model of interseismic fault slip in the presence of asperities summary a 2 d model which represents a slipping fault with non uniform coulomb friction is studied the fault plane is subject to a uniform ambient shear stress slowly increasing with time aseismic fault creep is assumed to start in a weak zone when the ambient stress reaches a strength threshold the solution for the resulting dislocation is worked out analytically using a technique based on chebyshev polynomials it is found that the dislocation partially propagates into the adjacent asperities concentrating stress onto them and preparing the conditions which will produce the asperity failure and the accompanying earthquake propagation is not self similar and occurs at increasing velocity a non linear slip hardening effect is reproduced the nearness to earthquake instability is measured by a dimensionless parameter which depends on coulomb friction and ambient shear stress and decreases to zero with time an upper boundary to the critical value of this parameter at which instability may occur is estimated and is found to depend on the ratio between the sizes of the asperity and the weak zone

authigenic molybdenum isotope signatures in marine sediments we present new mo isotope data from the mexican continen tal margin that in conjunction with previous data allow us to propose a mechanistic description of the mo isotope system in ma rine sediments we hypothesize that there are unique environmen tally dependent mo isotope signatures recorded in marine sedi ments that reflect the mechanisms responsible for authigenic mo accumulation open ocean anoxic sites defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water exhibit a 98 95 mo isotope signature of 1 6 we believe this val ue reflects mo sulfide formation via diagenetic processes within sediments quantitative formation of mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value 2 3 as typified by samples from the highly sulfidic black sea in contrast to these reducing settings manganese oxide rich sediments have measured mo isotope values that are more negative relative to seawater than any other sediment samples analyzed to date similar to fe mn crusts 0 7 most measured mo isotope compositions of marine sediments from open ocean settings appear to reflect a mix ture of lithogenic mo 0 0 and the mo signature of a specific authigenic mo accumulation mechanism we therefore suggest that mo isotopes may record unique signatures that reflect the domi nant chemical mechanism for mo sequestration into sediments

the effects of diagenesis on geochemical paleoredox proxies in sedimentary carbonates abstract metal and metal isotope records in carbonates have the potential to provide novel insights into ancient ocean atmosphere redox conditions paleoenvironmental conditions and biogeochemical cycling however trace element geochemical signatures in carbonates can record either diagenetic or depositional signatures here we explore the variability in uranium isotopes trace metal and rare earth element yttrium rey concentrations in carbonate successions that have undergone several common types of diagenetic alteration case studies include the cryogenian balcanoona reef australia marine dolomitization neomorphism the devonian canning basin reefs australia burial dolomitization karstification the paleozoic of the great basin usa high temperature and regional burial dolomitization and the carboniferous waulsortian limestone ireland pb zn mineralization in all of the examined cases there are significant heavier and or lighter shifts in u isotope values between the most petrographically pristine marine depositional components and altered or late stage diagenetic phases although we also found that rey patterns can be overprinted during diagenesis normalized rey profiles appear to be commonly retained through diagenetic recrystallization consistent with previous studies the direction and magnitude of the change in metal isotope systems during diagenesis cannot be generalized between case studies consistent with carbonate alteration being controlled by a wide range of factors including the composition and source of the alteration fluids in this light we build on framework developed from traditional isotope systems and support the view that future work on metal and metal isotope compositions of carbonates should include integrated sedimentological and petrographic analysis petrographic work not only represents a screening procedure for sedimentary geochemical work but can also significantly enhance the paleo environmental interpretation of metal isotope data

rapid expansion of oceanic anoxia immediately before the end permian mass extinction tive of an expansion of oceanic anoxia and higher th u ratios from 0 06 to 0 42 indicative of drawdown of u concentrations in seawater using a mass balance model we estimate that this isotopic shift represents a sixfold increase in the flux of u to anoxic facies implying a corresponding increase in the extent of oceanic anoxia the intensification of oceanic anoxia coincided with or slightly preceded the eh and persisted for an interval of at least 40 000 to 50 000 y following the eh these findings challenge previous hypotheses of an extended period of whole ocean anoxia prior to the end permian extinction

redox dependent changes in manganese speciation in baltic sea sediments from the holocene thermal maximum an exafs xanes and la icp ms study manganese mn enrichments in sediments of the deep basins of the baltic sea are believed to consist of ca mn carbonates that form from mn oxides following periodic inflows of oxygen rich north sea waters however a range of mn bearing mineral phases that besides mn carbonates e g ca rhodochrosite can include mn sulfides are known to be present in marine sediments with formation mechanisms that are sensitive to redox conditions in this study we use high resolution synchrotron exafs and xanes combined with la icp ms and micro xrf to investigate the nature of the mn enrichments in sediments from the holocene thermal maximum approx 8000 4000 cal yr bp at a site in the northern gotland basin analyses were performed on epoxy embedded sediment sequences we specifically address the role of changes in redox conditions in bottom waters as inferred from sediment molybdenum mo contents for mn sequestration we find that an up core increase in mo indicating a transition into more anoxic and sulfidic euxinic bottom water conditions is accompanied by a decline in total sediment mn contents while mn carbonates dominate the mn mineral fraction in the low mo interval in the more sulfidic high mo interval mn is associated with framboidal pyrite mn fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7 7 mol this exceeds ratios previously reported for sedimentary pyrite and exafs spectra indicate that sulfide bound mn is predominately tetrahedrally coordinated when mn fe ratios are high this suggests that mn is incorporated in other fe s phases such as mackinawite besides pyrite there is also evidence for the presence of a low concentration of mn associated with detrital aluminosilicates throughout the investigated intervals our results suggest that increased bottom water euxinia inhibited the formation of mn carbonate at this site during the holocene thermal maximum possibly due to a more rapid reduction of mn oxides in more sulfidic bottom waters c 2014 elsevier b v all rights reserved less

a heinrich like event h 0 dc 0 source s for detrital carbonate in the north atlantic during the younger dryas chronozone in the north atlantic we define h 0 as a heinrich like event which occurred during the younger dryas chron on the se baffin shelf prior to 11 ka surface water productivity was reasonably high as measured by the numbers of diatom and planktic foraminifera per gram but an abrupt increase in detrital carbonate dc 0 event from approximately 15 up to 50 carbonate by weight occurred at 11 14c ka and continued to circa 10 ka these deposits 2 6 m thick are dominated by detrital calcite and silt and clay sized sediments during this event dc 0 h 0 ice extended onto the inner shelf but did not reach the shelf break and probably originated from a center over labrador ungava as a consequence the pattern of ice rafted debris and sediment provenance shown by h o in the north atlantic is different from that during h 1 14 5 ka or h 2 20 ka when the ice sheet extended along the axis of hudson strait and may have reached the shelf break for example there is no concrete evidence for dc o is cores on the floor of the labrador sea due east of hudson strait hu75 55 56 but h o has been noted in cores off newfoundland and west of ireland a coeval carbonate event to dc 0 but this one dominated by dolomite occurs in hu82 su5 on the west side of davis strait with a source either from northern baffin bay or cumberland sound although other sources for north atlantic detrital carbonate cannot be totally excluded our evidence suggests that h 0 represents the expression of glaciological instability of the laurentide ice sheet within the general region of hudson strait and probably to the north cumberland sound and northernmost baffin bay there is one younger dc event dated circa 8 4 ka present in sediments along the labrador margin and in hudson strait which represents the final collapse of the ice sheet within hudson strait and hudson bay

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

uranium isotopes fingerprint biotic reduction knowledge of paleo redox conditions in the earth s history provides a window into events that shaped the evolution of life on our planet the role of microbial activity in paleo redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record the ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo history and to correlate specific biogeochemical events with the corresponding microbial metabolism here we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium u i e the accumulation of the heavy isotope in the u iv phase is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments thus isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought after tool to probe for biotic processes because uranium is a common element in the earth s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of u vi this tool is applicable to a multiplicity of geological epochs and terrestrial environments the findings of this study indicate that biological activity contributed to the formation of many authigenic u deposits including sandstone u deposits of various ages as well as modern cretaceous and archean black shales additionally engineered bioremediation activities also exhibit a biotic signature suggesting that although multiple pathways may be involved in the reduction direct enzymatic reduction contributes substantially to the immobilization of uranium

comparison of ediacaran platform and slope δ238u records in south china implications for global ocean oxygenation and the origin of the shuram excursion abstract the ediacaran shuram negative carbon isotope excursion se records major paleoceanographic changes during the late neoproterozoic possibly linked to a global oceanic oxygenation event yet its cause s remain uncertain earlier studies of the upper ediacaran doushantuo formation in south china based on local redox proxies have documented strong spatial redox heterogeneity along shelf to basin transects but variations of 238u a global redox proxy have not yet been examined in deep water se carbonates in this study we examined 238u variations through the se in the upper slope siduping section similar to platform se sections siduping exhibits a shift toward higher 238u values correlative with the peak of the se i e maximum negative 13ccarb confirming inferences of global ocean oxygenation during the se this raises an apparent paradox because a global negative carbon isotope excursion implies net oxidant consumption requiring an ocean based oxygenation mechanism we hypothesize that an increase in the efficiency of phosphorus burial due to a plankton driven shift from dominantly dissolved organic matter dom cycling to greater particulate organic matter pom export depleted the ocean of nutrient phosphorus by producing a steep redox gradient close to the sediment water interface we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light 13ccarb was precipitated in authigenic carbonate minerals this scenario can account for 238u differences between shallow water and deep water carbonates which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow water settings generating both a larger negative 13ccarb shift and a larger early diagenetic 238u offset

correlation of foreshocks and aftershocks and asperities a close correlation in spatial distribution of local seismic activity and energy release patterns before and after the 1979 petatlan mexico earthquake suggests heterogeneity within the fault plane of this major low angle thrust event associated with subduction along the middle america trench a simple two asperity model is proposed to account for the complexity foreshocks and aftershocks of the neighboring 1981 playa azul earthquake showed a similar pattern as both events occurred at the junction of the orozco fracture zone and the middle america trench we speculate that the observed complex fault plane is caused by subduction of the rugged ocean floor of the orozco fracture zone short term precursory seismicity prior to the petatlan earthquake can be explained by using the asperity model and migration of a slip front from the south east to the north west across the main shock source region

when do black shales tell molybdenum isotope tales molybdenum mo isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions organic rich black shales can be used to reconstruct ancient ocean mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater comparison of 98 95 mo in two devonian shale sequences of similar age the new york oatka creek and geneseo formations reveals that this assumption cannot be applied to all organic rich shales although both sequences contain laminated intervals elevated organic carbon and enrichments of redox sensitive metals the mean 98 95 mo differs systematically between the formations by 0 59 independent paleoredox indicators reveal that portions of the oatka creek formation were deposited under pervasively euxinic anoxic and sulfidic conditions whereas conditions during deposition of the geneseo formation were intermittently euxinic to suboxic oxygen deficient but not sulfidic in the water column we infer that reconstruction of ancient ocean 98 95 mo from organic rich shales requires independent verification of persistent local euxinia with these considerations in mind our data point to 98 95 mo in the devonian oceans 0 6 lighter than in today9s oceans consistent with expanded anoxia

diagenetic effects on uranium isotope fractionation in carbonate sediments from the bahamas abstract uranium isotope variations 238u recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time however the effects of diagenetic alteration on the u isotopic composition in carbonate sediments which are crucial to understand the accurate reconstruction of marine 238u are currently poorly constrained here we examine the effects of the aragonite to calcite transition in the pleistocene key largo limestone of south florida and assess the effects of vadose meteoric phreatic meteoric and phreatic marine diagenesis on u isotope fractionation in carbonate sediments from the bahamas transect including the well studied clino unda and odp site 1006 drill cores our results suggest that early diagenetic processes in bahamas carbonate sediments fractionate u isotopes by an average of 0 27 0 14 1 sd heavier than contemporaneous seawater downcore variations of 238u in slope and basin sediments display little if any correlation with u concentration and common geochemical indicators of diagenesis 13c 18o mn sr mg ca sr ca enrichments of redox sensitive elements or rare earth elements anomalies we propose two possible mechanisms to interpret the positive change in the 238u during carbonate diagenesis authigenic enrichment of isotopically positive u iv in carbonates and preferential incorporation of isotopically positive aqueous u vi species into carbonates these processes likely operate during early syndepositional diagenesis on the banktop further diagenesis during deeper burial is limited by the low solubility of u iv under reducing pore water conditions the early diagenetic behavior of u isotopes in bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past we suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater 238u from ancient carbonates furthermore early diagenesis induces significant statistical variability in sediment 238u values pointing to the need for large high resolution data sets in order to average out stochastic variations in individual bulk sediment samples

coupled molybdenum iron and uranium stable isotopes as oceanic paleoredox proxies during the paleoproterozoic shunga event the paleoproterozoic era was a time of remarkable importance in the redox evolution of earth s atmosphere and oceans here we present a multi proxy study of mo u and fe isotopes together with fe speciation of black shales and siltstones from the upper zaonega formation of the onega basin in karelia we attempt to better understand oceanic redox conditions during the 2 05 ga shunga event as the next step following the great oxidation event goe and the lomagundi carbon isotope excursion event a cautious examination of the fe speciation data shows that the studied section was deposited under dominantly euxinic conditions anoxic and sulfidic and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite during this episode the system was closed with respect to fe but not sulfur the mo and u isotopic compositions corrected for detrital input were not affected by the metamorphism and loss of s and are fairly uniform throughout the entire section the fe isotope compositions are exceptionally heavy in the lower part of the section up to 56 fe irmm 14 0 83 and become lighter towards the upper intervals which also show significant mo and u enrichments we suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column the upper part was deposited within a locally euxinic portion of the basin where h 2s availability was highest removal of mo and u was more efficient and precipitated pyrite captured relatively non fractionated dissolved fe in other words quantitative uptake of fe was favored in contrast the lower interval was deposited on the lower margin of a euxinic wedge where h 2s availability was lower and removal of mo and u was less efficient pyrite precipitation in this part of the water column reflected a more fractionated dissolved fe reservoir due to more protracted non quantitative fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of fe the u isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co precipitation into carbonates was the main process of u removal at this time we estimate the mo isotope composition of the contemporaneous ocean to be 98 mo sw 0 85 0 21 this is the lowest value yet reported for the proterozoic ocean suggesting that the oceanic mo cycle was dominated by euxinic and anoxic sinks with negligible mo removal into oxic environments recent studies have proposed a sharp increase in ocean atmosphere oxygen levels during the lomagundi event followed by a dramatic crash our results from black shales of the 2 05 ga shunga event are consistent with a post lomagundi decrease in biospheric oxygen levels

uranium isotopes as a proxy for primary depositional redox conditions in organic rich marine systems abstract in marine sediments authigenic uranium u enrichments and u isotope compositions are important tools for interpreting changes in redox conditions however their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted this study focuses on the u content and 238u 235u ratio of organic rich surface sediments from the namibian continental margin where high productivity results in an expanded oxygen minimum zone omz the investigated core sample sites are located on the shelf shelf break and slope where bottom water redox conditions vary from anoxic to suboxic to oxic respectively while all cores have relatively high total organic carbon toc contents up to 12 wt each location displays a unique u to toc relationship shelf sediment exhibit a fair correlation between u and toc while the shelf break and slope sediments show a pronounced decoupling of u and toc on the namibia continental margin particle rich nepheloid layers transport organic rich deposits from within the omz through oxic water to be redeposited on the slope due to the sensitivity of u to changes in redox conditions this lateral movement results in the release of the reduced u phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope oxidation of u during transport does not alter the average primary 238u 235u isotopic signature in redeposited sediment and the combination of high toc low u content and high 238 u values may become a useful tool for the identification of the boundaries of ancient omzs

integrated geochemical petrographic insights from component selective δ238u of cryogenian marine carbonates emerging geochemical proxies have improved our understanding of the broad scale history of earth s oxygenation however paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting this is particularly an issue with marine carbonates which although being potentially ideal ocean redox archives are commonly altered during diagenesis here we provide new insight into the robustness of uranium isotopes 238u 235u ratios 238u values as paleoredox tracers by determining texture specific 238u values from a well described cryogenian balcanoona reef complex in south australia we found high variability in 238u values between different carbonate components even within a single sample petrographically the best preserved components from the balcanoona reef are marine cements which have a mean 238u value of 0 23 essentially unfractionated from riverine inputs these values are interpreted as reflecting a marine system with widespread anoxic and iron rich settings less well preserved phases have 238u values spanning almost the entire extent of the documented isotopic range this integrated petrographic geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case by case basis in future carbonate metal isotope geochemistry

multiproxy evidence of an early holocene 8 2 kyr climate oscillation in central nova scotia canada an early holocene lake sediment record from central nova scotia contains a minerogenic oscillation that is closely correlative with the 8 2 kyr event ca 8200 cal yr bp an event that has not been reported elsewhere in atlantic canada a variety of biological and sedimentological indicators have been examined to characterise autochthonous and allochthonous changes that occurred during this time the minerogenic upper oscillation uo ca 8400 cal yr bp is marked by an increase in the chrysophyte diatom ratio following the oscillation the diatom community reflects a shift to more productive less acidic conditions the pollen record shows no major response to this short lived event lithostratigraphic analyses indicates that the uo is characterised by an increase in clastic content magnetic susceptibility and mean sediment grain size all indicators of changing environmental conditions most likely the result of regional cooling the taylor lake record adds to a growing body of evidence for a widespread hemispheric climate oscillation at 8 2 kyr copyright 2002 john wiley sons ltd

slip partitioning of the calaveras fault california and prospects for future earthquakes examination of main shock and microearthquake data from the calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8 9 km near the base of the zone of microearthquakes the spatial pattern of pre main shock microseismicity surrounding the coyote lake and morgan hill hypocenters is similar to the pattern of the post main shock microseismicity microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones estimates of the fault regions which slipped during the coyote lake and morgan hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic we propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes from the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the calaveras fault from an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next m 5 earthquakes the first zone is located near gilroy and was last ruptured by a m5 2 earthquake in 1949 the second zone is located south of calaveras reservoir and north of the 1988 m5 1 alum rock earthquake it has not slipped seismically since at least 1903 and the size of the aseismic region is sufficiently large to sustain a m5 5 earthquake

geochemical and multi isotopic 87sr 86sr 143nd 144nd 238u 235u perspectives of sediment sources depositional conditions and diagenesis of the marcellus shale appalachian basin usa abstract we investigate sediment sources depositional conditions and diagenetic processes affecting the middle devonian marcellus shale in the appalachian basin eastern usa a major target of natural gas exploration multiple proxies including trace metal contents rare earth elements ree the sm nd and rb sr isotope systems and u isotopes were applied to whole rock digestions and sequentially extracted fractions of the marcellus shale and adjacent units from two locations in the appalachian basin the narrow range of end values from 7 8 to 6 4 at 390 ma is consistent with derivation of the clastic sedimentary component of the marcellus shale from a well mixed source of fluvial and eolian material of the grenville orogenic belt and indicate minimal post depositional alteration of the sm nd system while silicate minerals host 80 of the ree in the shale data from sequentially extracted fractions reflect post depositional modifications at the mineralogical scale which is not observed in whole rock ree patterns limestone units thought to have formed under open ocean oxic conditions have 238u values and ree patterns consistent with modern seawater the 238u values in whole rock shale and authigenic phases are greater than those of modern seawater and the upper crust the 238u values of reduced phases the oxidizable fraction consisting of organics and sulfide minerals are 0 6 greater than that of modern seawater bulk shale and carbonate cement extracted from the shale have similar 238u values and are greater than 238u values of adjacent limestone units we suggest these trends are due to the accumulation of chemically and more likely biologically reduced u from anoxic to euxinic bottom water as well as the influence of diagenetic reactions between pore fluids and surrounding sediment and organic matter during diagenesis and catagenesis

trace elements at the intersection of marine biological and geochemical evolution life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes these requirements differ between organisms and have evolved over geological time likely guided in some part by environmental conditions until recently most of what was understood regarding trace element concentrations in the precambrian oceans was inferred by extrapolation geochemical modeling and or genomic studies however in the past decade the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new and potentially more direct insights into secular changes in seawater composition and ultimately the evolution of the marine biosphere compiled records of many bioessential trace elements including ni mo p zn co cr se and i provide new insight into how trace element abundance in earth s ancient oceans may have been linked to biological evolution several of these trace elements display redox sensitive behavior while others are redox sensitive but not bioessential e g cr u their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere ocean redox conditions that are linked to biological evolution for example the activity of oxygen producing photosynthetic cyanobacteria in this review we summarize available precambrian trace element proxy data and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life we also examine several biologically relevant and or redox sensitive trace elements that have yet to be fully examined in the sedimentary rock record e g cu cd w and suggest several directions for future studies

interaction between shallow and subcrustal dislocations on a normal fault abstract we propose a model which may explain seismic sequences which are often observed in seismogenic regions as for example in the apenninic chain italy in particular we consider a normal fault and earthquakes taking place at different depths a first shock in a shallower layer and a second in a deeper one the normal fault is embedded in a viscoelastic half space as a consequence of the rheology there are two different brittle layers a shallower and a deeper one where earthquakes can nucleate between these two layers the rheological behavior is ductile the thicknesses of the layers depend on the geothermal profile that is calculated taking into account the profile of the thermal and rheological parameters with depth the fault plane crossing layers with different rheological behavior is heterogeneous in respect to the slip style seismic in the brittle layers aseismic in the ductile layer dislocations in the shallower layer are assumed to produce aseismic slip in the area of the fault belonging to the ductile layer the stress concentrated by the seismic and aseismic dislocations on the fault plane section in the deeper brittle layer is evaluated it is compared with the tectonic stress rate in order to calculate how much earlier the second earthquake would occur compared to if just the bare tectonic sstress was acting it results that such an advance is comparable with typical recurrence times of earthquakes and so a mechanism of interaction between different seismic sources mediated by aseismic slip can be supposed the strains and displacements at the earth s surface due to seismic and aseismic slip are calculated they are large enough to be detected by present geodetic techniques

global ocean redox variation during the middle late permian through early triassic based on uranium isotope and th u trends of marine carbonates uranium isotopes 238u 235u in carbonates a proxy for global ocean redox conditions owing to their redox sensitivity and long residence time in seawater exhibit substantial variability in the daxiakou section of south china from the upper middle permian through the mid lower triassic 9 m y middle and late permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the 2 m y prior to the latest permian mass extinction lpme countering earlier interpretations of sustained or gradually expanding anoxia during this interval the lpme coincided with an abrupt negative shift of 0 5 in 238u that signifies a rapid expansion of oceanic anoxia intensely anoxic conditions persisted for at least 700 k y griesbachian lessening somewhat during the dienerian th u concentration ratios vary inversely with 238u during the early triassic with higher ratios reflecting reduced u concentrations in global seawater as a consequence of large scale removal to anoxic facies modeling suggests that 70 100 of marine u was removed to anoxic sinks during the early triassic resulting in seawater u concentrations of 5 that of the modern ocean rapid intensification of anoxia concurrent with the lpme implies that ocean redox changes played an important role in the largest mass extinction event in earth history

a conceptual model of the deglaciation of atlantic canada abstract we present a conceptual model for the deglaciation of atlantic canada in which a role is played by ice streams the ice margin lay close to the edge of the continental shelf at the last glacial maximum ice streams occupying shelf troughs comprised 1 a major ice stream in the laurentian channel 2 secondary streams in the bay of fundy gulf of maine trinity trough and notre dame channel and 3 lesser ice streams elsewhere early retreat in the northeast notre dame bay and in the southwest bay of fundy gulf of maine was mainly by calving along embayments by 18 ka radiocarbon years a calving embayment had opened on emerald basin scotian shelf retreat along channels marooned ice on intervening banks a major calving episode beginning just before 14 ka removed ice from the gulf of st lawrence triggering re advances down secondary troughs isolation of a newfoundland ice cap produced radial drainage via fiords and more extensive flow into the st george s bay lowlands on the west coast by 13 ka most ice was on land and was ablating by melting rather than calving shelf ice caps persisted until 11 ka

reconstruction of δ13c of chemocline co2 aq in past oceans and lakes using the δ13c of fossil isorenieratene abstract high abundances of the diaromatic carotenoid isorenieratene derived from photosynthetic green sulfur bacteria chlorobiaceae were found just below the chemocline in an anoxic fjord in norway throughout the annual cycle the stable carbon isotope composition of this carotenoid co varied with the 13c of co2 aq and is independent of the co2 and isorenieratene concentration this constant isotopic fractionation p of isorenieratene versus co2 4 1 was subsequently used in the reconstruction of 13c of co2 at the chemocline in ancient oceans and lakes these reconstructions indicate that 13c of co2 at the chemocline is often influenced by isotopically light co2 formed by remineralization of organic matter this process can depending on the depth and stability of the chemocline also effect the isotopic composition of the phytoplankton and thus isotopic records of sedimentary inorganic and organic carbon

uranium isotope evidence for an expansion of anoxia in terminal ediacaran oceans abstract anoxic and iron rich oceanic conditions prevailed throughout most of the archean and proterozoic 4000 to c 540 million years ago ma but the oceans are hypothesised to have become progressively oxygen rich during the ediacaran cambrian transition interval coincident with the rise of animal life we utilise the uranium isotope ratio of seawater 238u 235u reformulated as 238 u an effective tracer of oceanic redox conditions as a proxy for changes in the global proportion of anoxic seafloor we present a new 238 u dataset for carbonate rocks from the lower nama group namibia deposited in a shelf ramp succession during the terminal neoproterozoic 550 to 547 ma these data capture a transition from 238 u similar to the modern ocean towards persistently low 238 u average 0 81 0 06 such low 238 u are consistent with enhanced u drawdown from the water column under anoxic conditions and the preferential export of heavy 238u to sediments following u vi u iv reduction placing our results into a steady state ocean box model suggests at least a third of the global seafloor was covered by anoxic bottom waters compared with only 0 3 in today s oxygenated oceans comparison with 238 u from older sediments deposited in other basins further supports an expansion of anoxic bottom waters towards the end of the ediacaran our data are consistent with an emerging picture of a dominantly anoxic ediacaran ocean punctuated by brief ocean oxygenation events in the nama group the transition towards globally widespread anoxic conditions post dates the first appearance of both skeletal metazoans and soft bodied fauna of the nama assemblage this suggests that the global expansion of anoxia did not coincide with the decline of the ediacaran biota or drive the biotic turnover between the white sea and nama assemblages the impact of this global redox change on metazoan ecosystems is unclear since the expansion of anoxia if contained mainly within deeper waters may not have impinged significantly upon continental shelves that host the majority of biodiversity

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day