precisely dated ams 14c marine cores reveal the complexity of millennial scale asian monsoon variability in the northern south china sea md972146 md972148 abstract high sedimentation rate sr cores retrieved from the south china sea scs form the basis for studying the marine components of millennial scale asian monsoon am variability and for comparison with the am reconstructions from cave records on land e g dongge and hulu however carefully correlating the scs sedimentary records to the cave records that are precisely dated by u th methods with resolution of decadal scale has not been completed such a correlation is essential when comparing am influences expressed over land and sea but requires a construction of marine ams 14c age models that are precise enough to be compared to the cave u th age models for the purpose of establishing such a correlation this study presents new data from intensive ams 14c dated marine cores retrieved from the northern scs md972146 md972148 the discrepancy of marine and cave 18o record for the interval of 18 30 ka might be due to the change of marine 14c reservoir age in scs surface water during the glacial period and to the change in interhemispheric dominance of the am systems with the new ams 14c dating on md972146 and md972148 we examined the millennial scale records of planktonic foraminifer 18o and carbonate contents of md972146 md97248 and sonne 17940 2 and compared those records with dongge hulu 18o record of the past 30 ka our results show that in the intervals corresponding to the high latitude northern hemisphere nh younger dryas yd and heinrich i event h1 the ams 14c dated millennial scale oscillations show relatively heavy 18o and low carbonate contents but h2 and h3 are in the opposite direction our results indicate the complexity for the marine cores which were used in interpreting the millennial scale am variability

sedimentary facies and radiocarbon dates of the nam dinh 1 core from the song hong red river delta vietnam abstract the song hong red river delta which is located on the western coast of the gulf of bac bo gulf of tonkin in the south china sea is characterized by three morphological systems that are clearly distinguished spatially fluvial tidal and wave dominated we obtained a 70 m long core of sediments the nam dinh 1 core from the central part of the delta in 1999 this paper presents the first detailed description of sedimentary facies with high resolution radiocarbon dating for the latest pleistocene holocene sediments of the song hong delta the core sediments were divided into three units on the basis of the sediment facies in ascending order unit 1 fluvial sediments consisting of fining upward gravels to clay unit 2 coarsening upward marine clay to shelly sand and unit 3 coarsening upward marine clay and laminated sand twenty one radiocarbon dates showed that the core succession was deposited after the last glacial maximum lgm the ages were 15 11 cal kyr bp for unit 1 11 9 cal kyr bp for unit 2 and 8 0 cal kyr bp for unit 3 these data indicate that sediments were deposited in transgressive fluvial estuarine to regressive deltaic environments formed in the central delta plain of the song hong after the lgm in response to sea level changes

abrupt change of sediment records in the southern south china sea during the last glacial period and its environment significance abstract chemical compositions of sediments at core ns07 25 6 39 945 n 113 32 936 e water depth 2006 m are analyzed to discuss abrupt change of sediment records in the southern south china sea scs during the last glacial period and its environmental implications major and trace elements experienced three significant changes which occurred at 31 9 26 1 and 18 5 ka bp respectively at 31 9 ka bp when sea surface temperature remained relatively low terrigenous sediment flux increased but biogenic activities weakened greatly at 26 1 ka bp both terrigenous sediment flux and biogenic fractions number were steady however some elemental ratios changed noticeably at that time before 26 1 ka bp core sediments with low nb ta but high zr hf ratios were mainly derived from the paleo mekong and or molengraaff rivers exposed on the sunda shelf at that time after 26 1 ka bp sediments with high nb ta but low zr hf ratios probably relate to strengthened impact of deep water current since 18 5 ka bp sea level ascended rapidly from 110 m below the present terrigenous sediment flux decreased and biogenic activities strengthened continuously probably related with the east asian monsoon evolution

understanding sedimentation in the song hong yinggehai basin south china sea the cenozoic song hong yinggehai basin in the south china sea contains a large volume of sediment that has been used in previous studies together with regional geomorphology to argue for the existence of a large palaeodrainage system that connected eastern tibet with the south china sea to test this and to understand the significance of sediment volumes deposited in the song hong yinggehai basin this study compared erosion histories of source regions with sediment volumes deposited during the two main stages in basin evolution spanning active rifting and subsidence 30 15 5 ma and postrift sedimentation 15 5 ma to present the study of basin provenance by detrital zircon u pb dating revealed hainan was an important and continuous source of sediment and a bedrock thermochronological study quantified its overall contribution to basin sedimentation comparison between the accumulated mass of basin sediment and volumes of eroded bedrock calculated from apatite thermochronometry across the modern red river drainage in northern vietnam as well as hainan island accounted for the bulk of sediment deposited since 30 ma consequently if an expanded paleodrainage ever existed it must have predated the oligocene

sedimentological and geochemical imprint of environmental changes in late pleistocene palaeodelta hosting deposits southwest of the hainan island south china sea abstract the study was aimed at elucidating relationships between lithological and geochemical characteristics of sediments as indicators of sea level changes during the late quaternary on the eastern slope of the yinggehai song hong basin the focus was on sediments of the late pleistocene paleodelta formed on the shelf off the hainan island examination of two cores revealed a significant lithological and geochemical variability of the sediments particularly in the lower segments of the delta deposits associated with the delta front core ldw and with the distal part of the prodelta core zbw the upper segment of the deltaic sediments in core ldw most likely represents a delta plain formed under the prevalent trend of increasing sea level on other hand the upper segment of the deltaic sediments in core zbw should be regarded as associated with sediments of the proximal part of the prodelta formed when the delta front was advancing and the sea level was first rising and then stabilised attention was paid to the relationship between lithological and geochemical features of the sediments and the seismic profiling records particularly with respect to the paleodelta sediment series an attempt was also made to link and correlate the sediment series with sea level changes

nonstationary synchronization of equatorial qbo with sao in observations and a model it has often been suggested that the period of the quasi biennial oscillation qbo has a tendency to synchronize with the semiannual oscillation sao apparently the synchronization is better the higher up the observation extends using 45 yr of the 40 yr european centre for medium range weather forecasts ecmwf re analysis era 40 data of the equatorial stratosphere up to the stratopause the authors confirm that this synchronization is not just a tendency but a robust phenomenonin the upper stratosphere a qbo period starts when a westerly sao w sao descends from the stratopause to 7 hpa and initiates the westerly phase of the qbo w qbo below it ends when another w sao a few sao periods later descends again to 7 hpa to initiate the next w qbo the fact that it is the westerly but not the easterly sao e sao that initiates the qbo is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the e sao does not create a zero wind line unlike the w sao the currently observed average qbo period of 28 months which is not an integer multiple of sao periods is a result of intermittent jumps of the qbo period from four sao to five sao periods the same behavior is also found in the two and a half dimensional interactive isentropic research thinair model it is found that the nonstationary behavior in both the observation and model is caused not by the 11 yr solar cycle forcing but by the incompatibility of the qbo s natural period determined by its wave forcing and the quantized period determined by the sao the wave forcing parameter for the qbo period in the current climate probably lies between four sao and five sao periods if the wave forcing for the qbo is tuned so that its natural period is compatible with the sao period above e g at 24 or 30 months nonstationary behavior disappears

stress distribution due to olivine spinel phase transition in descending plate and deep focus earthquakes the stress field due to olivine spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant clapeyron s slope and for a nonequilibrium transition taking a kinetic effect into consideration the material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures the calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km the magnitude of principal stress being greater than 0 5 gpa however for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces and the central part is compressional in the depth range from 300 to 600 km unlike the case of equilibrium transition the maximum principal stress in the latter case is greater than 2 gpa which is much more than that for the equilibrium transition the principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases comparison of the calculated results to the actually observed seismicity depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations

effective use of cerium anomalies as a redox proxy in carbonate dominated marine settings abstract rare earth elements and yttrium rey have a distinct distribution pattern in seawater and this pattern may be faithfully preserved in carbonate sediments and rocks anomalous concentrations of redox sensitive cerium ce compared with neighbouring rey originate in oxic water column conditions and as such ce anomalies can provide a potentially useful redox proxy in carbonate dominated marine settings the methods used to extract rey from carbonates vary widely and may suffer from widespread leaching of rey from accessory non carbonate minerals and organic matter limiting the application of ce anomalies for palaeo redox reconstruction we have systematically compared different methods on 195 carbonate samples with varying purity carbonate from both modern and ancient environments we used sequential leaching experiments in nitric acid to identify the most seawater like portion of the carbonate sample where contributions from non carbonate minerals and organic matter are minimised we also compared the results of sample dissolution in different types and strengths of acid our results confirm that rey concentrations can be inadvertently contaminated by partial leaching of clays and fe oxyhydr oxides during a single step digestion and we suggest a pre leach of 20 of the sample followed by a partial leach of 40 of the sample to selectively dissolve carbonate we suggest that rey studies are optimised in carbonates with 85 caco 3 and show that dolomites behave differently during the leaching process and must be treated separately we present rey patterns for modern carbonate rich sediments from a range of environments and show that seawater rey are faithfully preserved in some non skeletal carbonate but modified leaching procedures are necessary for impure unlithified or organic rich carbonate sediments we combine rey with fe speciation data to identify how fe oxides and clays contribute to the rey signal and explore how the two proxies can be used together to provide a complex and high resolution redox reconstruction in carbonate dominated marine environments

sedimentary responses to the east asian monsoon and sea level variations recorded in the northern south china sea over the past 36 kyr abstract sediment grain size is an important proxy to detect sedimentary process and record paleoenvironmental variations in continental slopes grain size measurements were performed for 298 samples of the 5 96 m long core pc338 spanning the past 36 kyr recovered from the northern south china sea scs results show that sea level and climatic influences on grain size distributions are crucial a turbidite formed during the last glacial maximum lgm is ascribed to the favorable topography close to the red river estuary induced by the low sea level four end member em components for the normal sediment layers were unmixed using a lognormal parametric end member modeling emm with the sea level decreasing more em4 15 100 m and less em1 0 2 m supplied the scs slope because em2 2 10 m consisted of fluvial suspension is dominated by the east asian summer monsoon easm precipitation it has the potential to substitute for the easm intensity em2 varied in phase with many south china paleoclimate records and the 30 n july insolation over the past 36 kyr and cooling climate events including the lgm heinrich events h1 and h3 younger dryas 8 2 ka and 10 3 ka lowered the em2 content our results further confirm the synchronous easm evolution in the south china continent mainly forced by the northern hemisphere summer insolation

the sedimentary and tectonic evolution of the yinggehai song hong basin and the southern hainan margin south china sea implications for tibetan uplift and monsoon intensification 1 the yinggehai song hong basin is one of the world s largest pull apart basins lying along the trace of the red river fault zone in the south china sea south of hainan island this basin crosscuts the rifted margin of the northern south china sea in this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins the basins started to open after similar to 45 ma especially after similar to 34 ma the yinggehai basin was folded and inverted in the middle miocene after 21 ma in the north and 14 ma in the south before rapidly subsiding again after similar to 5 ma because of continued tectonism this subsidence has caused shale diapirism especially driven by associated sedimentation in the late pliocene 2 6 2 0 ma extension along the adjacent south hainan margin shows preferential lower crustal extension suggestive of lower crustal flow increasing toward the continent ocean transition during breakup sediment supply is reconstructed to peak in the middle miocene then falls between 14 and 10 3 ma to reach a low in the late miocene however rates rose again in the pliocene pleistocene the red river sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 ma or with large scale river capture away from the red river after similar to 10 ma both lines of evidence point to major uplift in the red river drainage being middle miocene or older the recent preindustrial red river carried much more sediment than the average pleistocene accumulation rate indicating modest sediment buffering onshore at least in recent geologic time

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

the descent rates of the shear zones of the equatorial qbo abstract the influence of vertical advection on the descent rate of the zero wind line in both phases of the equatorial quasi biennial oscillation qbo is investigated with the help of the thin air stratosphere two and a half dimensional model the model qbo is forced by two symmetric easterly and westerly waves and yet the model reproduces qualitatively the observed asymmetry in the descent rates of the two shear zones due to the enhanced heating during easterly descent combined with the equatorial heating induced by the extratropical planetary waves observations show that the maximum easterly accelerations occur predominantly from may until july which is when the modeled equatorial planetary wave induced heating rates are weakest hence model results are consistent with the theory that vertical advection induced by extratropical planetary waves slows significantly the descent of the easterly shear zone the model also shows the observed increase in vertical wind shear during stalling of the easte

an unexpected disruption of the atmospheric quasi biennial oscillation one of the most repeatable phenomena seen in the atmosphere the quasi biennial oscillation qbo between prevailing eastward and westward wind jets in the equatorial stratosphere approximately 16 to 50 kilometers altitude was unexpectedly disrupted in february 2016 an unprecedented westward jet formed within the eastward phase in the lower stratosphere and cannot be accounted for by the standard qbo paradigm based on vertical momentum transport instead the primary cause was waves transporting momentum from the northern hemisphere seasonal forecasts did not predict the disruption but analogous qbo disruptions are seen very occasionally in some climate simulations a return to more typical qbo behavior within the next year is forecast although the possibility of more frequent occurrences of similar disruptions is projected for a warming climate

the effect of strain energy on growth rates during the olivine spinel transformation and implications for olivine metastability in subducting slabs we have studied growth kinetics during the transformation of mg1 8fe0 2sio4 san carlos olivine to its high pressure polymorphs wadsleyite phase and ringwoodite phase at 800 1200 c and nominal pressures of 16 20 gpa in experiments in which a large 500 600 m olivine single crystal contained in a matrix of either fine grained olivine or nacl was transformed reaction rims of wadsleyite ringwoodite form on the margins of the single crystal by incoherent grain boundary nucleation and interface controlled growth contrary to theoretical expectations the growth rate of these reaction rims decreases sharply as a function of time for instance at 1100 c and 18 gpa growth ceases on an experimental timescale after the rim width reaches 20 25 m in order to explain this observation we develop an elastic model based on the theory of a misfitting inclusion comparing the results of this model with the experimental data suggests that elastic strain energy which develops because of the large volume decrease associated with the transformation is responsible for the decreasing growth rates on the other hand experimental and theoretical results suggest that elastic strain energy is relatively unimportant when grains of the product phase are randomly dispersed and distinct reaction rims do not form this is the case when the nucleation rate is low the growth rate is fast and the reactant olivine is fine grained on a geological timescale in subducting lithosphere where the grain size of the olivine is large the growth rates of grain boundary nucleated reaction rims are likely to be controlled by viscoelastic relaxation therefore current kinetic models of olivine metastability in subducting slabs which are based on simple extrapolations of experimental data and on the assumption that the growth rate is constant at fixed temperature and pressure need to be reevaluated

new interpretation for the main sediment source of the rapidly deposited sediment drifts on the northern slope of the south china sea abstract the slope of the northern south china sea scs is one of the world s most active areas of modern marine sedimentary processes a rapidly deposited sediment drift rdsd was identified on the slope near dongsha island the sources of this body have been studied and discussed by several authors but no consensus has been achieved based on multi channel seismic profiles interpretation and data analysis of long term observation of bottom currents we identified that although the amounts of sediments input from the pearl and hanjing rivers and from kao ping and tseng wen rivers are large considering the current situation of northern scs none of the pearl river the hanjiang river the kao ping river and the tseng wen river can maintain the deposition pattern of the northern scs margin as the single source especially to maintain the supper high sedimentation rate at the rdsd area the miocene drape formation mdf calmly draped over an unconformity after rifting stage resulting in a relatively constant thickness of about 2000 m throughout the overall northern scs continental margin and followed by a standard configuration of a clinoform the pliocene formation and another clinoform of about another 700 m in total thickness due to dongsha uplift since 3 ma roughly 1 2 of mdf and the n2 formation in the dongsha area measures 150 km by 200 km were denudated resulting in an average annual denudation rate of 60 106 t y if these sediments were deposited on a slope region within an area measuring 100 km by 100 km the sedimentation rate could reach 250 cm ky this is clearly an impressive new sediment source that has never been discussed before moreover the rdsd is near the dongsha uplift and on the adjacent slope we believe that this new sediment source should be the main contributor for the rdsd development

phase alignment of the tropical stratospheric qbo in the annual cycle abstract this paper investigates the occurrence of phase alignment of the tropical stratospheric quasi biennial oscillation qbo with the annual cycle first updating previous studies observational results are shown for ncep reanalysis data and singapore radiosondes both datasets show strong phase alignment of the qbo at 24 5 km phase alignment is investigated in a 3d mechanistic stratospheric model including explicit large scale planetary waves forced by a lower boundary geopotential anomaly and a simple equatorial wave parameterization the model simulates a qbo like oscillation with the period depending on the lower boundary momentum flux of the parameterized waves phase alignment is manifested in two different ways first simulated oscillations of both integer and noninteger year periods are shown to lock on to a certain phase of the annual cycle second when the magnitude of the lower boundary momentum flux is varied about a range implying oscillation period close to 2 yr the period of th

quasi biennial oscillation in vertical velocity inferred from trace gas data in the equatorial lower stratosphere vertical velocity variations associated with the quasi biennial oscillation qbo in the equatorial lower stratosphere are investigated with the halogen occultation experiment haloe data from 1993 to 1998 the vertical velocity is inferred from ascent rates of an annual cycle in total water h2o 2 ch4 profiles around the 20 60 hpa layer over the equator the zonally averaged ascent rates of total water anomalies exhibit qbo related variations with anomalies of 0 10 0 15 mm s 1 between 20 and 40 hpa at the equator whereas there scarcely exists an annual cycle in the ascent rates the qbo variation of the ascent rates shows that its positive anomalies precede negative anomalies of temperature and ozone variations by about 2 3 months at the equator and 30 60 hpa this phase relationship is in conflict with former results from several two dimensional model studies which have reported that vertical velocity variations are almost out of phase with temperature anomalies preceding ozone anomalies by a few months it is supposed that these differences could be caused by the observed large tendency of temperature anomalies which is related to the observed rapid acceleration of the qbo westerly an estimate of vertical advection of zonal momentum expected from the ascent rates suggests that the asymmetric acceleration of the zonal wind qbo can be explained by the qbo induced vertical velocity being asymmetric between the easterly and the westerly shear zones

isolated deep earthquakes and the fate of subduction in the mantle the locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere the occurrence over the last decade of isolated large deep earthquakes outside the main wadati benioff zones in the kurile izu bonin and chile deep seismic areas in addition to the observed flat distributions of deep seismicity beneath the banda sea and fiji plateau areas and the large number of deep earthquakes located away from the main slab in the tonga arc supports recent tomographic and other seismological studies leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle the focal geometries of these isolated earthquakes which often exhibit near vertical compressional stresses are distinct from adjacent events in the main wadati benioff zone they are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide these isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle conversely the deep seismicity in the adjacent wadati benioff zone is associated with the deflection of the subducting lithosphere

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

interaction of the quasi biennial oscillation and stratopause semiannual oscillation analysis of rawinsonde and rocketsonde data at ascension island 7 6 s 14 4 w and kwajalein 8 7 n 167 e in 1962 1991 suggests that the quasi biennial oscillation qbo in the middle stratosphere is synchronized with the seasonal cycle and that descending westerly phases of the stratopause semiannual oscillation sao are strongly influenced by the underlying qbo the effect of the seasonal cycle on the qbo in the middle stratosphere is revealed in two perhaps unrelated observations first a tendency for deseasonalized qbo westerly maxima to occur in local winter or to avoid local summer second a smooth uninterrupted connection between descending sao westerly shear zones and the formation of a new qbo westerly shear zone aloft the timing of deseasonalized qbo westerly maxima in the middle stratosphere allows a simple composite of 2 and 3 year cycles to be constructed from the data illustrating the effect of the qbo on descending westerly phases of the stratopause sao

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

pleistocene paleoceanography of the south china sea progress over the past 20 years article i nfo the south china sea scs has become one of the foci in paleoceanographic research and hydrocarbon explora tions over the last 20 years over the period at least 16 international cruises including odp ocean drilling pro gram leg 184 have investigated the scs and more than 2000 exploratory wells have been drilled the present paper attempts to synthesize progress in pleistocene paleoceanography because of the high depositional rates and favorable carbonate preservation the scs keeps valuable paleoceanographic archives for the low latitude pacific region which have helped to uncover potential ties between the low latitude oceanic biogeochemical evolution and step wise development of the boreal ice sheet it has been found that the 1 6 ma event marked a reorganization of the oceanic carbon reservoir associated with the restructuring of the southern and the tropical pacific ocean the mpt mid pleistocene transition and mbe mid brunhes event centered at 0 9 ma and 0 4 ma respectively were preluded by carbon isotope maxima 13 cmax iii and 13 cmax ii suggesting that major steps in the ice sheet development were preceded by significant changes in oceanic biogeochemistry the late pleistocene records have demonstrated the amplifying role of the scs in responding to glacial cycles as the amplitudes of sst and 18 ov ariations have been significantly amplified in the scs as compared with the open ocean at the same latitudes the amplification is ascribed to its semi enclosed morphology and the prevail ing monsoon climate the closure of the southern margin during glacial times led to stratification of the upper waters and specific faunal changes in the southern scs the seasonally reversing monsoon winds not only drive the surface circulation but also the biological production the n s gradient in oceanography and the sed iment transportdynamics such asthe asymmetric growth of the mekong river delta becauseof its distinguished position in the low latitudes between the largest continent and the largest ocean the scs provides important insight into the mechanism of environmental evolution with regards to ocean continent or high low latitude interactions

kelvin waves and the quasi biennial oscillation an observational analysis a method is derived for estimating the kelvin wave contribution to the vertical flux of westerly momentum in the equatorial stratosphere which is based on temperature and geopotential perturbations this method is used to estimate the momentum transfer due to kelvin wave activity as derived from the cryogenic limb array etalon spectrophotometer claes temperature data set for the onset of the westerly phase of the qbo during 1992 and the first few months of 1993 that is during the first part of the upper atmosphere research satellite uars mission the results are compared with the zonal winds as observed by the high resolution doppler imager hrdi also flown on board uars and the united kingdom meteorological office ukmo data assimilation model product a correlative data set to the uars mission the analysis shows that although the kelvin wave momentum flux convergence is occasionally sufficient to account for the observed qbo westerly acceleration the observed flux is sporadic in nature and virtually disappears during the second half of the sample when the westerly vertical shear zone approaches the 100 hpa level an estimate of the total westerly momentum flux necessary to produce the observed descent of the westerly phase of the qbo is made using the transformed eulerian mean tem formalism the results suggest that kelvin waves are not sufficient to force the descent of the westerly phase of the qbo there appears to be a need for enhanced westerly forcing throughout the descent of the westerly phase of the qbo this enhanced forcing is most likely provided by gravity waves that are unresolved by the satellite observations these results are in agreement with the results derived from general circulation models

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

rapid coupling between ice volume and polar temperature over the past 150 000 years a new approach to dating a continuous sea level record using speleothem u th ages shows that past variations in global ice volume occurred within centuries of polar climate change with rates of sea level rise reaching at least 1 metre per century

a 400 000 year record of vegetational and climatic changes from a volcanic basin leizhou peninsula southern china abstract the tianyang lake in leizhou peninsula is an enclosed crater formed during the early to middle pleistocene over 200 m of diatomaceous mud and clay were deposited in the last 400 000 years providing a continuous middle and late quaternary lacustrine sequence for paleoenvironmental studies the pollen record spans the last four glacial interglacial cycles equivalent to oxygen isotope stages ois 10 to 1 during most of the glacial phases fagaceous monsoon forest was replaced by altitudinal vegetation indicating a regional landscape transformation on the peninsula the montane forest reached down to the lowland by at least 600 m during the ois 6 and 8 in a cool and relatively wet condition with temperature estimated to be more than 4 c lower than present a further lowering of the montane forest belts by about 800 1000 m occurred in the ois 4 2 as implied by the appearance of many temperate elements during the last glacial maximum this mixed forest subsequently gave way to the poaceae grassland a savanna vegetation type favored by drier and cooler climatic conditions this drastic change implies a more significant drop in temperature and precipitation than the previous three glacial periods ois 10 8 and 6 it is thus inferred that the weakening of summer monsoon was more significant towards the end of the late pleistocene particularly during the last glacial maximum lgm

relationship of deep seismicity to the thermal structure of subducted lithosphere recent experimental work on silicate olivine polymorphs1 has confirmed earlier observations of mechanical failure in analogous compounds2 3 as a result of phase transformations when non hydrostatic stresses were applied to a metastable phase this transformational faulting 4 mechanism is considered to be a leading candidate5 among others involving olivine transformation6 7 for the cause of deep earthquakes evidence consistent with this mechanism comes from the observation2 4 that worldwide earthquakes become more numerous with increasing depth after a seismicity minimum at about 350 km depth8 this depth lies within the range anticipated for mineralogical transformations in the subducted lithosphere9 but individual subduction zones differ in their thermal structures so if transformational faulting indeed contributes to deep seismicity this should be reflected in the location of the seismicity minimum for each zone the depth at which the minimum occurs should depend on temperature in the same way as do the polymorphic transformations of olivine and should always lie deeper than the depth at which the equilibrium transformation takes place here we test these predictions for eight subduction zones worldwide we find that with one exception the north japan zone the depth of the seismicity minimum decreases linearly with increasing thermal age of the slab a measure of its temperature profile these results support the proposal that deep earthquakes are a consequence of phase transformations in olivine

nonlinear representation of the quasi biennial oscillation a nonlinear principal component analysis nlpca is applied to monthly mean zonal wind observations from january 1956 through december 2007 taken at seven pressure levels between 10 and 70 hpa in the stratosphere near the equator to represent the well known quasi biennial oscillation qbo and investigate its variability and structure the nlpca is conducted using a simplified two hidden layer feed forward neural network that alleviates the problems of nonuniqueness of solutions and data overfitting that plague nonlinear techniques of principal component analysis the qbo is used as a test bed for the new compact model of nlpca the two nonlinear principal components of the dataset of the equatorial stratospheric zonal winds determined by the compact nlpca offer a clear picture of the qbo in particular their structure shows that the qbo phase consists of a predominant 28 3 month cycle that is modulated by an 11 yr cycle as well as by longer cycles the differences in wind variability between westerly and easterly regimes and between northern hemisphere winter and summer seasons and the tendency for a seasonal synchronization of the qbo phases are well captured

seismological evidence for the existence of anisotropic zone in the metastable wedge inside the subducting izu bonin slab the mechanism of deep earthquakes has been a puzzle since their discovery almost 70 years ago experimental observations at high pressure have reported faulting caused by spinel filled anti cracks in metastable olivine and suggesting that the anticrack mechanism is responsible for deep earthquakes we compared the shear wave splitting of earthquakes from the upper and lower planes of the double seismic zone to detect anisotropy caused by the faultings inside the metastable wedge we detected the anisotropic zone inside the metastable wedge at the depth of about 340 390 km which corresponds to the depth range of a second peak on the histogram of earthquakes the observed polarization direction east west is almost parallel to the direction of subduction which can be explained by a view that the shear wave splitting is caused by the en echelon arrays of spinel filled anti cracks and anticrack associated faultings

hydrographic variability in the northern south china sea over the past 45 000 years new insights based on temperature reconstructions by uk 37 and texh86 proxies from a marine sediment core md972146 abstract millennial scale climate variability was persistent feature for greenland ice core and north atlantic marine records during the marine isotope stage mis 3 studying high quality climatic archives outside of the greenland and north atlantic with precise constraint of absolute dating and high resolution sampling is a prerequisite to understand how the millennial scale changes well expressed in north hemisphere nh high latitudes had been propagated out to the other regions here we generated the surface and subsurface hydrography u k 37 and tex h 86 temperatures and terrestrial material input n alkanes records with precise ams 14 c dated control for the 45 ka from a core md972146 retrieved from the northern south china sea scs then we compared our records to the oxygen isotope record from a greenland ngrip ice core and the mean grain size record from a loess sequence at gulang china we found that the millennial scale oscillations of hydrolography and terrestrial material inputs records during the time since the last deglaciation and of 33 45 ka share similar patterns in timing and amplitude of the nh high latitude climate however we found that a non nh pattern of millennial scale oscillations for the late mis 3 20 33 ka of our hydrography and terrestrial material inputs records our studies imply that the changes of obliquity also may be important in determining the timing and amplitude of millennial scale east asian monsoon eam variability expressed in the northern scs hydrography and terrestrial material inputs

global ice volume during mis 3 inferred from a sea level analysis of sedimentary core records in the yellow river delta abstract estimates of global ice volume during the glacial phase of the most recent ice age cycle are characterized by significant uncertainty reflecting the relative paucity of geological constraints on sea level relevant to this time interval for example during the middle stages of marine isotope stage 3 published estimates of peak global mean sea level gmsl relative to the present range from 25 m to 87 m the large uncertainty in gmsl at mis 3 has significant implications for estimates of the rate of ice growth in the period leading to the last glacial maximum 26 ka we refine estimates of global ice volume during mis 3 by employing sediment cores in the bohai and yellow sea that record a migration of the paleoshoreline at 50 37 ka through a transition from marine to brackish conditions in particular we correct relative sea level at these sites for contamination due to glacial isostatic adjustment using a sea level calculation that includes a gravitationally self consistent treatment of sediment redistribution and compaction and estimate a peak global mean sea level of 38 7 m during the interval 50 37 ka with suitable sedimentary core records the approach described herein can be extended to refine existing constraints on global ice volume across the entire glacial period

transformation induced faulting requires an exothermic reaction and explains the cessation of earthquakes at the base of the mantle transition zone abstract in many subduction zones earthquakes occur continuously to the bottom of the mantle transition zone and stop abruptly at the endothermic phase change marking the top of the lower mantle suggesting cause and effect new experiments on polycrystals undergoing the ilmenite perovskite transformation show that transformation induced faulting occurs only in the exothermic direction of the reaction the close structural and thermodynamic analogies between the transformation studied here in cdtio 3 and those expected in the silicates of the earth s mantle argue strongly that these results are relevant to transformations occurring in subducting lithosphere moreover consideration of the thermodynamics of this asymmetry show that this result should be a general phenomenon if so it follows that faulting by this mechanism cannot occur in the lower mantle providing a natural explanation for the termination of earthquakes just before 700 km depth if a wedge containing metastable olivine exists at depth in subduction zones and is necessary for deep earthquakes our results remove the constraint that this wedge must taper to zero where the deepest earthquakes occur the characteristics of the large deep earthquakes of 1994 and their aftershocks show that deep earthquakes can propagate beyond the zone of previous hypocenters and trigger aftershocks there providing important new information about the mechanism of deep earthquakes and pointing to the need for additional experiments

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

the quasi biennial oscillation analysis using era 40 data 1 the era 40 data set is used to examine the equatorial quasi biennial oscillation qbo the data set extends from the ground to 0 1 hpa 65 km and covers a 44 year period january 1958 to december 2001 including 18 5 qbo cycles analysis of this data set of unprecedented spatial and temporal coverage has revealed a threefold structure in height in the qbo zonal wind anomalies at the equator in addition to the well known twofold structure in the lower and middle stratosphere that is easterlies overlying westerlies or vice versa there is a third anomaly in the upper stratosphere the sign of this upper stratospheric anomaly is the same as the lower stratospheric anomaly thus forming anomalies of alternating sign throughout the depth of the equatorial stratosphere the amplitude of this upper stratospheric anomaly is 10 m s 1 approximately one third of the amplitude of the lower stratospheric signal the frequency and descent rates of the east and west qbo phases are analyzed in detail with particular attention to any 11 year solar cycle influence in addition to the observed solar modulation of the duration of the qbo west phase the analysis shows a solar modulation of the mean descent rate of the easterly shear zone the mean time required for the easterly shear zone to descend from 20 to 44 hpa is 2 months less under solar maximum conditions than under solar minimum conditions 7 4 months versus 9 7 months this rapid descent of the easterly shear zone cuts short the west phase of the qbo in the lower stratosphere during solar maximum periods

formation and fate of sedimentary depocentres on southeast asia s sunda shelf over the past sea level cycle and biogeographic implications abstract sea level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late quaternary times fluctuations on millennial time scales produce locally complex deposits in coasts and on shelves associated with short term influence on species development this article reviews the sedimentary and biogeographic history of the tropical siliciclastic sunda shelf as an end member of continental shelves regarding extreme width an enormous sediment supply and highest biodiversity in response to rapid sea level fluctuations we describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data field observations and calculation of hydro isostatic adjustment effects on changing relative sea level these segments are characterized by individual sedimentary processes and deposits and by a specific potential for material storage and re mobilization long term regressive intervals led to overall sigmoidal promoting extremely thick and wide succeeding units in contrast rapid lateral shifts of defined depocentres over long distances took place in response to short term sea level fluctuations fully isolated small scale sediment bodies formed when sea level changed at exceptionally high rates as a result of the high availability of organic rich sediments mangrove and freshwater peats formed frequently over late quaternary times the appearance of thick massive and widespread peats is mainly linked to time intervals of a sea level rise at slow rates whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level the preservation potential of the regressive units is generally high due to highest initial sediment supply stabilizing soil formation during exposure and rapid subsidence preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change besides complex channel incision an overall lowering of the sediment surface related to erosion as deep as 20 m or more over wide areas took place mainly during sea level lowering the final export of shelf material is documented by enormous mass wasting packages on the associated continental slope from a palaeogeographic perspective the rapid formation or disappearance of special habitat zones such as mangrove fringes and extended mud flats led to species establishment or truncation in distribution in addition the opening or closure of ocean passages as narrow bridges allowing limited species crossing or as fully colonized corridors had severe impact on eco fragmentation and the expansion or contraction of species independent of such particular conditions sea level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast related ecosystems

formation of a metastable olivine wedge in a descending slab we present a thermal model of a descending slab in which the transformation of olivine to spinel is controlled by pressure and temperature dependent reaction kinetics two different formulations of the kinetics are considered with the main discriminant being the temperature range over which olivine converts to spinel at pressures of about 15 gpa about 500 515 c and 560 650 c we use a finite element method to solve the coupled heat conduction perpendicular and parallel to the dip of the slab and kinetics equations and we include the latent heat of the phase transformation latent heat release together with heat conduction parallel to the dip of the slab reduces significantly the length of the metastable olivine wedge and results in a very thin 5 km two phase region we employ the thermal parameter sin is the velocity of the descending slab is the age of the slab and is the dip of the slab to the horizontal to interpret the results for the length of the metastable wedge for values of the thermal parameter smaller than about 4000 and 7000 km depending on the model of the kinetics no metastable olivine wedge exists the critical value of 7000 km for the thermal parameter corresponds to the kinetics model with the lowest transition temperature range the length of the metastable olivine wedge is also found to be very sensitive to the model of the kinetics and to the effects of adiabatic heating if the occurrence of deep earthquakes is related to the transformation of metastable olivine to spinel then data on earthquake depth versus thermal parameter require that the onset of the reaction takes place at temperatures of about 550 575 c in this case the slab thermal parameter must be larger than 10 000 km for the metastable olivine wedge to extend down to 660 km depth but deep earthquakes occur near 660 km depth in slabs with thermal parameter as small as about 5000 km south america for example either some deep earthquakes are unrelated to olivine metastability or our knowledge of olivine spinel reaction kinetics is incomplete

investigating the history of east asian monsoon and climate during the last glacial interglacial period 0 140 000 years mineralogy and geochemistry of odp sites 1143 and 1144 south china sea abstract monsoon climate is an important component of the global climatic system a comprehensive understanding of its variability over glacial interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate continental weathering and productivity a detailed multiproxy study including bulk and clay mineralogy grain size analysis phosphorus geochemistry sedex extraction organic matter characterization and nitrogen stable isotopes was carried out on samples from odp sites 1143 and 1144 leg 184 south china sea covering the past 140 000 years we tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial interglacial cycle when sea level variations linked to the growth and melting of ice caps interact with monsoon variability during interglacial periods of high sea level summer monsoon was strong and humid and warm climate characterized the adjacent continent and islands clay minerals bear signals of chemical weathering during these intervals high calcite and reactive phosphorus mass accumulation rates mars indicate high productivity especially in the southern region of the basin during glacial intervals strong winter monsoon provided enhanced detrital input from the continent as indicated by high detrital mar glacial low sea level resulted in erosion of sediments from the exposed sunda shelf to the south and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas enhanced supply of nutrients from the continent both by river and eolian input maintained high primary productivity reduced circulation during these periods possibly induced active remobilization of nutrients such as phosphorus from the sediments intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in china and marine climatic records in the north atlantic confirming a teleconnection between low and high latitude climate variability

structure and sediment budget of yinggehai song hong basin south china sea implications for cenozoic tectonics and river basin reorganization in southeast asia the temporal link between offshore stratigraphy and onshore topography is of key importance for understanding the long term surface evolution of continental margins here we present a grid of regional high quality reflection seismic and well data to characterize the basin structure we identify fast subsidence of the basin basement and a lack of brittle faulting of the offshore red river fault in the yinggehai song hong basin since 5 5 ma despite dextral strike slip movement on the onshore red river fault we calculate the upper crustal whole crustal and whole lithospheric stretching factors for the yinggehai song hong basin which show that the overall extension observed in the upper crust is substantially less than that observed for the whole crust or whole lithosphere we suggest that fast basement subsidence after 5 5 ma may arise from crustal to lithospheric stretching by the regional dynamic lower crustal mantle flow originated by collision between india eurasia and indian oceanic subduction below the eurasian margin in addition we present a basin wide sediment budget in the yinggehai song hong basin to reconstruct the sedimentary flux from the red river drainage constrained by high resolution age and seismic stratigraphic data the sediment accumulation rates show a sharp increase at 5 5 ma which suggests enhanced onshore erosion rates despite a slowing of tectonic processes this high sediment supply filled the accommodation space produced by the fast subsidence since 5 5 ma our data further highlight two prominent sharp decreases of the sediment accumulation at 23 3 ma and 12 5 ma which could reflect a loss of drainage area following headwater capture from the paleo red river however the low accumulation rate at 12 5 ma also correlates with drier and therefore less erosive climatic conditions

the effects of phase transition kinetics on subducting slabs the effects of kinetics on the non equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically a one dimensional model consisting of the kinetic equations and the heat diffusion equation with latent heat release has been constructed numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume for slow slab velocities less than 6 cm yr near equilibrium conditions are found finger like structures emanating from the phase boundaries are obtained in this regime these phase boundary prolusions may cause earthquakes for slab velocities of around 10 cm yr the metastable olivine region may be pushed down to a depth of about 600 km with a sharp phase boundary a direct transition from olivine to perovskite may be feasible

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

the influence of olivine metastability on deep subduction of oceanic lithosphere abstract the dynamics of subduction of oceanic lithosphere is mainly controlled by thermal buoyancy forces and mineralogy in self consistent models the influence of metastable olivine mo on the dynamics and evolution of subducting oceanic lithosphere is investigated we consider both phase boundaries olivine ol to spinel sp 410 km and sp to perovskite pv magnesiowustite mw 660 km treating the kinetics of the phase transition in a simplified way while models do not allow the formation of mo penetrate straight into the lower mantle slabs with a sufficient amount of mo flatten out at the 660 km boundary before penetrating at a later stage furthermore our models show that the effect of mo to slow down subduction velocities within the transition zone is rather moderate

effects of temperature dependent thermal diffusivity on shear instability in a viscoelastic zone implications for faster ductile faulting and earthquakes in the spinel stability field the introduction of a new model of thermal diffusivity has motivated us to reinvestigate a one dimensional viscoelastic shear zone model with realistic rheology temperature dependent thermal diffusivity t and viscous dissipation although thermal diffusivity in the shear zone is spatially heterogeneous with t and viscous heating the spatial distribution of t does not affect shear zone evolution for the mesh resolution used in the model as temperatures increase above room temperature thermal diffusivity decreases the lower thermal diffusivity causes a slight spatial thinning of the shear zone and an acceleration of the onset of instability relative to cases using a room temperature value of thermal diffusivity increasing the nonlinearity of t enhances shear zone thinning and speed up of instability the amount of enhancement depends on temperature mineralogy and the rate of shear heating the rheology of spinel creates a more unstable situation for the shear zone than that of olivine but the boundary separating instability and stability is sensitive to changes in material properties a decrease in the grain size does not influence the timescale of instability unless grain size reduction causes diffusion creep to be the dominant deformation mechanism viscoelastic thermal mechanical instabilities occur on timescales ranging from a few hundred to several thousand years in most slabs no instability is found to occur in spinel regions at temperatures above 1200 k likewise shear instability in olivine at upper mantle depths will not occur at temperatures greater than 1100 k