influence of diagenetic processes in thau lagoon on cadmium behavior and benthic fluxes dgt diffusive gradient in thin films and det diffusive equilibration in thin films combined probes were used in thau lagoon sediments to describe variations of dissolved concentrations of metals such as cadmium manganese and iron through the sediment water interface two contrasted stations regarding organic carbon fluxes were studied from december 2001 to may 2003 during four field campaigns station c4 in the middle of the lagoon and station c5 in a shellfish farming zone laboratory experiments and field deployments in such environment showed that dgt sampled pore water labile cadmium whereas iron and manganese concentrations were underestimated these results suggest that no steady state in the flux of metals onto the gel was established for fe and mn kinetics of metal sulfide dissolution precipitation may control metal fluxes onto the gel probe in marine sedimentary environments analysis of sediment and water column samples showed cadmium concentrations above natural background 3 3 and 7 6 nmol kg 1 for station c4 and station c5 sediments respectively between 40 and 800 pmol l 1 for the water column suggesting contamination spatial and temporal patterns of cadmium behavior were observed the sediment at station c4 was generally a source of cadmium whereas at station c5 it was a sink the vertical extension of the diagenetic series was more important at station c4 with deeper oxygen penetration and lack of dissolved sulfide whereas station c5 showed steep h2s gradients at the same depths the data suggested that cadmium source was more likely organic matter cadmium mobility was probably controlled by aerobic mineralization at station c4 and by dissolution precipitation of sulfides at station c5 seasonal variations were observed in the depth of oxygen penetration and sulfide diffusion generating important remobilization of cadmium during december 2001 conversely in may 2003 at station c5 bottom water suboxic conditions i e o2 60 enhanced reductive conditions in the sediment favoring uptake of cadmium by the sediment from the water column

enhanced short term sediment deposition within the nazare canyon north east atlantic abstract in january and may 1999 interface sediments were sampled within the nazare canyon a large submarine canyon that deeply indents the western iberian continental margin ne atlantic at around 39 35n excess 234th profiles were established with high resolution to calculate inventories and bioturbation coefficients a northern transect at 42 n is briefly presented as a reference for the western iberian margin synthesis of these parameters is used to examine short term deposition within the nazare canyon 234thxs was always detected with significant activities on surface in most of the samples along the reference transect and in the adjacent area surface 234th excesses are about 10 20 dpm g 1 in contrast 234thxs reaches very high levels up to 170 dpm g 1 in the canyon there is a large range of variability in inventories and bioturbation rates in relation with depth and location these results show a preferential deposition in the canyon and focus in the upper and middle part of the canyon on a monthly time scale the nazare canyon appears as a significant locus of sedimentation for the western iberian margin

bioturbation short lived radioisotopes and the tracer dependence of biodiffusion coefficients bioturbation refers to the mixing of sediment particles resulting from benthic faunal activity it is the dominant particle mixing process in most marine sediments and exerts an important control on diagenetic processes in models bioturbation is usually treated as a diffusive process where the biodiffusion coefficient db characterizes the biological mixing intensity biodiffusion coefficients are classically computed by fitting a diffusive model to vertical profiles of particle bound radioisotopes one peculiar observation is tracer dependence db values from short lived tracers tend to be larger than those obtained from long lived tracers from the same site recent theoretical work based on random walk theory and lattice automaton bioturbation simulations labs has suggested that this tracer dependence is simply a model artifact and has concluded that the biodiffusion model is not applicable to the short observational time scales associated with short lived radioisotopes here we have compiled a global dataset of db values obtained from different radiotracers to assess tracer dependence from a data perspective tracer dependence is significant in low mixing environments like slope and deep sea sediments but is not present in intensely mixed coastal areas tracer dependence is absent when the number of mixing events is larger than 20 or the potential length scale is greater than 0 5 cm roughly this comes down to tracer derived db values greater than 2 cm2 yr 1 this condition is met for 68 50 and 8 of published db values obtained from coastal continental slope and abyssal environments respectively these results show that short lived radioisotopes are suitable to quantify biodiffusion mixing in sedimentary environments featuring intense bioturbation

the biogeochemistry of mercury at the sediment water interface in the thau lagoon 2 evaluation of mercury methylation potential in both surface sediment and the water column abstract methylation rates of mercury have been determined in both surface sediments and the water column of a shallow coastal lagoon thau france using in situ incubation experiments the experiments were conducted in order to evidence and evaluate the significance of such pathways on the fate of methylmercury mehg as influenced by both benthic and pelagic dynamics isotopically labelled hg species have been used as chemical tracers allowing the direct determination of specific methylation and demethylation yields each experimental method cores experiments and water experiments has been carefully evaluated in terms of sensitivity and reproducibility of the transformation rates and has been demonstrated as a powerful method to investigate transformation processes although mercury methylation in surface sediments is a major process significant mehg formation in the water column has been measured for the first time in a coastal environment in spring conditions methylation yields are found to be higher in the water column 6 3 than in sediments 0 8 1 3 area integrated rates for the experimental site demonstrate however that mehg is mostly produced in surface sediments with a formation rate of 12 nmol m 2 d 1 compared to 1 8 nmol m 2 d 1 in the water column biological characteristics of the incubated samples indicate that sediment and plankton microorganisms are involved in hg methylation which is thus directly associated to the pelagic benthic turnover occurring in the lagoon

recent sedimentation and sedimentary budgets on the western iberian shelf abstract thus study is based on 31 box core samples collected mainly from the mud patches of the galician and north portuguese shelf each of these cores some decimetres in length have been described radiographied and sampled each centimetre for grain size analysis and measurement of the particulate organic carbon corg content 210pb excess profiles have been measured using gamma spectrometry which combined with the analysis of the vertical variations of the sedimentary facies have allowed estimates of sedimentation rates these rates were found to vary between 0 05 and 0 40 cm yr 1 considering the latitudinal variation of sedimentation rates around the 100m contour we note that the maximum rates occurred off the douro estuary in the southern sector and also near the galician rias to the north the organic content corg of sediments varied between 0 4 and 2 latitudinally corg showed a decrease from the south to the north these results coupled with the sedimentary map see dias gonzalez garcia diaz del rio 2001 have permitted us to establish the detrital mass budget and an inventory of corg burial

sedimentary processes in the thau lagoon france from seasonal to century time scales as a part of the microbent programme an investigation of the sedimentation framework was carried out at the water sediment interface in the thau lagoon french mediterranean coast two main sites c4 in the middle of the lagoon and c5 near oyster farms were visited six times between december 2001 and may 2003 interface sediments were studied using classical sedimentology parameters radiography rx grain size distribution and analysis of selected radionuclides 234th 7be 210pb 226ra on a century time scale excess 210pb 210pbxs presents classical profiles with an upper mixed layer followed by an exponential decrease of activities to undetectable levels below 20 30 cm at the central site c4 cores seem to register episodic changes in mean grain size presenting recurrently peaks the upper 10 cm of 210pbxs profiles at site c5 exhibit a mixed layer associated with coarser sediments this could be related to biological activity sedimentation rates derived from 210pbxs varied from 0 15 cm y 1 at the edge of the basin to 0 25 cm y 1 at the central site on a seasonal time scale 234th and 7be both show significant variations in activities and in penetration within the sediment bioturbation rates derived from both radionuclides agree well and range between 1 10 cm2 y 1 at site c4 and 1 31 cm2 y 1 at site c5 234th and 7be fluxes at the water sediment interface show too seasonal variations more pronounced for site c5 this latter site presents especially a higher variability that is well marked with season probably in relation with its position near oyster farms

geochemistry and deposition of 7be in river estuarine and coastal waters the atmospheric flux of cosmogenic 7be 53 3 day half life and the mode of 7be deposition in river estuarine and coastal environments have been examined the atmospheric flux of 7be commonly supports inventories ranging from 1 0 to 2 0 pci cm2 1 pci 0 037 bq beryllium 7 concentrations in water phase samples collected across salinity gradients in several estuaries along the eastern coastline of the united states range from 0 03 to 0 53 pci l and primarily reflect variations in 7be supply and sorption kinetics the major process controlling the concentration of 7be on estuarine suspended particles appears to be the length of time that these particles remain in the water column field particle to water distribution coefficients for 7be have a median value of about 4 104 but range over an order of magnitude reflecting short term variations in 7be input particle dynamics and particulate iron content rather than equilibrium sorption desorption responses to changes in water salinity or particle type residence times of 7be in the water column range from a few days in estuarine areas of rapid fine particle deposition to several weeks in high energy environments where pronounced sediment resuspension reintroduces deposited 7be back into the water column inventories of 7be in sediments range from nondetectable to 3 3 pci cm2 with the highest inventories in areas where fine particles are accumulating rapidly such sites are also major repositories for other particle reactive substances a 7be budget for the james estuary indicates that less than 5 of the expected 7be input is in the water column and that the short term estuarine trapping efficiency for atmospherically derived 7be is somewhere between 50 and 100

the polymodal frequency magnitude relationship of earthquakes earthquake frequency magnitude relationships can be well described as a polymodal composite of normal distributions a pattern consistent with varying causal mechanisms in different tectonic settings and or magnitude ranges but a pattern also in conflict with the assumption of self similarity or the fractal nature of earthquakes from the giardini catalog 4660 deep 350 km worldwide earthquakes presumably homogeneous with respect to causal mechanism are well described as a single normally distributed population analysis of 10 341 earthquakes ranging in magnitude from 2 0 to 8 2 from the 1989 to 1991 national earthquake information center quick epicenter determination catalogue yields three populations each of whose magnitude frequency is expressed in terms of probability some nonsimilar frequency magnitude distributions for low magnitudes are also explicable as the result of one or more normally distributed populations in our model b is a measure of the dispersion of the population

phosphorus diagenesis in sediment of the thau lagoon we describe the depth distribution of phosphorus in relation to the distribution of major redox species dissolved o2 n o 3 n h 4 mn and fe and particulate s organic c reactive mn and fe oxides in modern sediments of two stations in the thau lagoon sediments close to the oyster bank zone are enriched in organic carbon and are highly bioturbated while those outside the bank are less bioturbated and organic carbon levels are lower in all sites early diagenesis follows the well accepted depth sequence of redox reactions of organic matter mineralization the upper sediments of the station enriched in organic carbon contain high amounts of reactive particulate organic phosphorus that arrives at the sediment surface through biodeposition only a part of this phosphorus is released to the bottom water after mineralization since more than 50 of total p is buried as an authigenic phosphate mineral in the middle of the lagoon outside the oyster bank zone organic matter seems to be much more refractory but the distribution of the major redox compounds indicates that this organic matter is partially mineralized a portion of the phosphate and ammonium released during mineralization does not escape the sediment since the concentration gradient is close to zero between 1 and 25 cm depth below the sedimentewater interface pore water n and p are likely fixed by biological uptake the characterisation and magnitude of this process require further study 2006 elsevier ltd all rights reserved

pb isotopes in mussels as tracers of metal sources and water movements in a lagoon thau basin s france abstract metal concentrations cu zn cd pb and lead isotopic compositions have been determined in mussel s fleshes from the thau lagoon s france mussel spat from sea was introduced at different locations in the lagoon in june 1995 and subsequently sampled in october 1995 and january may and july 1996 each location was chosen for its particular environment 1 an industrial harbour at sea 2 water treatment plant effluents 3 nearby city with no seawater entries and 4 the middle of the lagoon a reference point was also kept at sea variations in heavy metal concentrations generally correlate with weight of the animals but some could be linked to particular local and temporal events such as outflows or tourist activity the pb isotopic compositions of the soft parts allow 1 to identify the predominant sources of pb and their seasonal variations 2 to estimate their respective impact and 3 to present a dynamic vision of the water movements in the lagoon comparing with climatic observations wind and their mixing with seawater entries the possible use of isotopic tools in understanding the capture and the storage of metal in different organs in the mussels is also discussed

magnitude frequency relation for small earthquakes a clue to the origin of ƒmax of large earthquakes in order to test the hyothesis that the max of large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3 we constructed the frequency magnitude relation for earthquakes with magnitude range from 1 3 to 4 recorded at a borehole seismograph station operated by the university of southern california at baldwin hills in the middle of the newport inglewood fault a procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time the cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit for ts p for each magnitude we found a clear depature of the observed log frequency magnitude relation for m 3 the observed frequency of earthquakes with m 0 5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3 we conclude that the observed frequency magnitude relation departs from self similarity for earthquakes with magnitude smaller than about 3 this departure coincides with the widely observed departure of the moment corner frequency relation from self similarity these observations are consistent with the interpretation of max of large earthquakes in terms of the slip weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers

benthic response to shellfish farming in thau lagoon pore water signature vertical distributions of dissolved species across the sediment water interface swi including major cations sodium potassium magnesium calcium minor cations lithium strontium barium redox sensitive species dissolved manganese iron sulfate sulfide ammonium and other chemical parameters ph alkalinity soluble reactive phosphorous dissolved silica were studied in a mediterranean lagoon used for intensive shellfish farming in order to quantify the impact of this activity on diagenetic processes and the influence of seasonal changes two stations contrasted with respect to organic carbon fluxes were sampled in thau lagoon from march 2001 to august 2002 during four field campaigns in winter spring summer and fall well defined layers enriched with redox sensitive species were observed following the conventional sequence of early diagenetic reactions however differences were observed between both stations in depths and thickness layers concentration gradients extended down to more than 92 cm depth at the central position of the lagoon station c4 8 m depth and down to 40 cm depth inside shellfish farming zones station c5 9 m depth station c4 showed an unusual diagenetic signature sharp dissolved oxygen iron nitrate and manganese gradients existed at the swi but gradients of dissolved sulfide and alkalinity as well as other parameters dissolved silica ba etc were recorded only from 25 to 30 cm depth downward seasonal changes were observed in pore water composition as deep as 30 50 cm in station c4 only 15 cm in station c5 the center of the lagoon is not directly subjected to biodeposits deriving from shellfish activity isotopic and bioturbation data allowed to rule out a reworking of the sediment deeper than a few centimeters in addition to organic content of the sediment physical parameters were likely to induce the 10 20 cm gap between dissolved iron and sulfide profile as well as the higher vertical extent of diagenetic sequence observed at station c4 conversely to station c5 station c4 underwent stronger currents and wave effect probably generating advective transport of water through the sediment but no permeability data were available to confirm this hypothesis during summer climatic conditions generated vertical stratification of the water column and transient suboxic conditions at the bottom such conditions drove the upward shift of redox fronts compacting the diagenetic sequence these effects were reinforced at station c5 by shellfish and its farm structures mainly attenuation of current and increased heat absorption

sedimentary records of bulk organic matter and lipid biomarkers in the bering sea a centennial perspective of sea ice variability and phytoplankton community abstract in this study organic geochemical analyses of two sediment cores bl16 and lv63 23 recovered from the western bering sea were carried out to examine the sea ice variability and its relationship to phytoplankton community evolution over the past century bulk stable organic carbon isotopic composition 13ctoc showed pronounced depletion on the northern shelf since the late 1970s indicating greater terrigenous organic matter om under warming during recent decades variation in sedimentary om in the southward core was closely associated with marine primary productivity and regional deposition processes arctic sea ice proxy ip25 throughout the two cores with different temporal profile patterns demonstrated sea ice presence with the spatio temporal variability across the study area over the past century the phytoplankton marker ip25 index pip25 a proxy for estimating semi quantitatively sea ice concentrations reflected a decreased sea ice cover with more distinct interannual fluctuations between 0 7 and 0 2 especially in core bl16 after the late 1970s coinciding with the recent warming scenario increased concentrations of phytoplankton biomarkers brassicasterol and dinosterol and their ratios as well as the pip25 record in core bl16 indicated a synchronous variability of reduced sea ice cover with the enhancement of phytoplankton productivity since the late 1970s these results suggested a coupled interaction of the sea ice condition and planktonic ecosystem in the north bering shelf our results also revealed recent since the 2000s spatial heterogeneity in sea ice coverage between the northern and southern parts of the bering sea

reconstruction of paleostorm events in a coastal lagoon herault south of france article i nfo finding records of past catastrophic storm events is essential to evaluate the long term climatic evolution in a given coastal area this question has been addressed by the study of sediment cores sampled in a small coastal lagoon of the french mediterranean coast pierre blanche lagoon two cores were studied in detail and revealed the presence of three main storm events the sedimentation rates calculated using the cfcs 210 pb model in agreement with 137 cs data are 3 0 4 and 4 2 0 7m m y 1 near the border and in the center of the lagoon respectively this suggests that the 0 6 m deep lagoon could be filled with sediments over the next 150 yr our study shows that storm events can be characterized in sedimentary sequences identified by facies grain size and faunal assemblages lagoonal and marine species comparison of 210 pb 14 c chronology and historical accounts suggest that the three identified

non similar frequency magnitude distribution for m 1 seismicity a series of 1503 induced seismic events ranging in magnitude from 2 to 1 as computed based on a locally calibrated seismic moment scale were located within a well defined volume at depth the seismicity analysis was carried out over 2 months and included events recorded before during and following the after shock period of a mn 2 6 rockburst a non similar pattern was clearly apparent for both cumulative and non cumulative frequency magnitude distributions showing a relative enhancement of seismicity between magnitudes 0 5 and 0 this supports the existence of non similar scaling at these magnitudes the time variation of the b value as computed for a limited self similar magnitude domain 1 1 to 0 4 did not appear to be related to the rockburst instead it is suggested that the rockburst perturbed the local stress field and consequently the distribution of the very small events no correlations were found between b and space clustering

a biomarker based reconstruction of sea ice conditions for the barents sea in recent centuries variations in sea ice occurrence for the barents sea since c ad 1700 have been determined by analysis of the abundance of the sea ice biomarker ip25 in three marine sediment box cores obtained from locations in the north southeast and southwest parts of the region depth age models for each core were established using excess 210pb activity profiles comparisons between these depth age models with those reported previously for the barents sea suggest that the proxy sea ice record may alternatively extend back to c ad 1500 1600 sedimentation accumulation rates and bulk densities were combined with ip 25 concentrations resulting in temporal changes to ip25 fluxes which have been interpreted in terms of sea ice variability the ip25 sea ice proxy data are also compared with other bulk organic geochemical parameters total organic carbon and c n ratios and with sediment particle size distributions the data indicate ice free conditions for the southwest barents sea for the past c 300 yr consistent

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

a characteristic earthquake model of the seismicity preceding the eruption of mount st helens on 18 may 1980 abstract the two classes of earthquakes which preceded the explosive eruption of mount st helens washington on 18 may 1980 can be related to two different forms of energy release this is manifest both in the predominant periods of the two classes of seismic energy release at 1 0 and 0 55 s and in the ratio of their source dimensions in the range 2 1 to 6 1 inferred from characteristic magnitudes of the two possible classes of earthquakes apparent in the discrete frequency magnitude distribution the observed increase in the ratio of long to short period amplitudes of surface waves recorded on mso a local wwssn station can be numerically related to the growth of a magma chamber or conduit within which are generated the transient oscillations thought to be responsible for volcanic tremor these calculations predict an increase in the chamber s diameter at a rate of 2 7 m day 1 at depth in the 2 months preceding the eruption and this is consistent with an observed growth rate of 2 m day 1 of the precursory bulge which appeared at the surface on the mountain s north flank at the time the frequency magnitude distribution is analysed in two parts corresponding to a power law at low magnitudes and a gaussian distribution of typical fault lengths at high magnitudes an extension of the characteristic earthquake model first developed for individual fault zones the power law can in general have a non integer exponent which can be equated to a fractal self similarity dimension d temporal changes in d which is proportional to the seismic b value are proposed as an alternative and complementary description of the way seismic energy is released to the more commonly used viewpoints of changes in stress and heterogeneity despite a reassessment of the completeness of the magnitude catalogue no clear seismic precursor to the explosive eruption is observed in the b value although an earlier phase of phreatic eruptions was strongly associated with a b value anomaly

earthquake magnitude recent research and current trends abstract the magnitude recommendations adopted by the iaspei assembly at zurich in 1967 have had both a stabilizing and a stimulating effect on magnitude determinations and related research from 1967 onwards one magnitude research paper has appeared on the average almost every week thus making this parameter the most studied one in seismology new facilities and more accurate methods e g concerning instrumental equipment and interpretation techniques have made it possible to improve earlier achievements the application of magnitude scales has been extended in all respects e g with regard to epicentral distances focal depths wave types and wave periods magnitude frequency relations have become the most investigated equations within seismology observationally as well as theoretically they have wide applications e g for estimating the maximum magnitudes of future earthquakes an important item in earthquake prediction the magnitudes provide significant information on other source parameters such as wave energy fault length seismic moment relations between different types of magnitude yield valuable information on source properties for instance relations between magnitudes based on body and surface waves are used for efficient discrimination between earthquakes and underground explosions it is our purpose to review the magnitude development in the post zurich period 1967 1980 partly for geologists tectonophysicists and engineers who need an overview partly for seismologists who need an introduction to an overwhelmingly comprehensive literature

obliquity forcing with 8 12 times preindustrial levels of atmospheric pco2 during the late ordovician glaciation results from coupled ice sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the late ordovician were very sensitive to changes in atmospheric p co 2 and orbital forcing at the obliquity time scale 30 40 k y without orbital forcing ice sheets can grow with p co 2 level as high as 10 times preindustrial atmospheric level pal however with orbital forcing ice sheets can grow only with p co 2 levels of 8 times pal or lower these results indicate that the threshold of p co 2 for the initiation of glaciation is on the lower end of previously published estimates of 8 20 times pal the ice sheet model results further indicate that during exceptionally long periods of low summer insolation and low p co 2 levels 8 10 times pal large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher p co 2 values this finding suggests that in order to end the late ordovician glaciation with a rise in p co 2 atmospheric p co 2 must have risen to at least 12 times pal ice sheets therefore introduce nonlinearities and hysteresis effects to the ordovician climate system these nonlinearities might have also played a role in the initiation and termination of other glaciations in earth history

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

frequency magnitude distribution of earthquakes in vrancea relevance for a discrete model a high resolution analysis of a homogeneous and complete data set of intermediate depth microearthquakes h 60 km extending over an interval of 9 3 years emphasizes a significant deviation at low magnitudes ml 4 in a linear frequency magnitude distribution it appears as a distinct change in the slope of the cumulative distribution and as a seismicity deficit followed by a relative enhancement in the corresponding noncumulative curve this is in agreement with the presence of two characteristic mechanisms and allows the identification of both the magnitude threshold of asperity like earthquakes and the transition zone ml 3 3 3 9 from crack like earthquakes background seismicity to asperity like events these features are better pointed out on well individualized active zones the background seismicity shows a tremendeous decrease in its b slope from 1 03 to 0 57 during a 6 year interval before the occurrence of a major earthquake in 1986 mw 7 3 followed by a fast recovery to 1 03 in 2 years after this event such a behavior could be correlated to the continuous growth of the shear stress free surface on the fault as in a percolation process followed by its sudden diminution due to the locking of the fault the above results provide relevance for the discrete character of faulting and enable a coherent modeling of the earthquake generation process from microearthquake to major event scale lengths

striking similarities in temporal changes to spring sea ice occurrence across the central canadian arctic archipelago over the last 7000 years abstract a 7000 year spring sea ice record for victoria strait arc 4 and dease strait arc 5 in the canadian arctic archipelago caa has been determined by quantification of the sea ice diatom derived biomarker ip 25 in two marine sediment piston cores obtained in 2005 the chronologies of the arc 4 and arc 5 cores were determined using a combination of 14 c ams dates obtained from macrobenthic fossils and magnetic susceptibility measurements the ages of the tops of the piston cores were estimated by matching chemical and physical parameters with those obtained from corresponding box cores these analyses revealed that while the top of the arc 4 piston core was estimated to be essentially modern ca 60 cal yr bp a few hundred years of sediment appeared to be absent from the arc 5 piston core downcore changes to ip 25 fluxes for both cores were interpreted in terms of variations in spring sea ice occurrence and correlations between the individual ip 25 flux profiles for victoria strait dease strait and barrow strait reported previously were shown to be statistically significant at both 50 and 100 year resolutions the ip 25 data indicate lower spring sea ice occurrences during the early part of the record ca 7 0 3 0 cal kyr bp and for parts of the late holocene ca 1 5 0 8 cal kyr bp especially for the two lower latitude study locations in contrast higher spring sea ice occurrences existed during ca 3 0 1 5 cal kyr bp and after ca 800 cal yr bp the observation of consecutively lower and higher spring sea ice occurrence during two periods of the late holocene coincides broadly with the medieval warm period and little ice age epochs respectively the ip 25 data are complemented by particle size and mineralogical data although these may alternatively reflect changes in sea level at the study sites the ip 25 data are also compared to previous proxy based determinations of palaeo sea ice and palaeoclimate for the caa including those based on bowhead whale remains and dinocyst assemblages the spatial consistency in the proxy data which most notably indicates an increase in spring sea ice occurrence around 3 cal kyr bp provides a potentially useful benchmark for the termination of the holocene thermal maximum for the central caa

frequency distribution and quantification of deep earthquakes the frequency distribution of deep earthquakes d 350 km is analyzed on a regional scale using magnitude data from 4660 events reported in the international seismological center bulletin 1964 1982 and 391 centroid moment tensor solutions 1977 1987 we use the moment magnitude mw to define b values comparable with studies of shallow seismicity scaling laws relating mw mb and mo are derived mw 1 6mb 3 2 mb log mo 4 3 2 4 which compare very favorably with published laws uncertainties and biases of the mb and mo data sets are considered the slope of the frequency distribution is evaluated over a restricted magnitude range the results are very robust and independent from the magnitude range used from the mb magnitude data we obtain regional b values in the range 0 4 b 1 22 with a worldwide average value b 0 87 these results are confirmed by the analysis of the seismic moment data the steepest frequency distribution is observed in the tonga islands region b 1 22 although 60 of the worldwide deep seismicity occurs here only few events since 1977 had seismic moment exceeding 1019 n m the high fractal dimension d 2 4 implies a volume partially filled with fault planes in agreement with the observations on the seismic morphology of the region at the other extreme south america is characterized by the largest recorded deep events and an almost complete absence of small ones b 0 4