the old alluvium of singapore and the extinct drainage system to the south china sea in both sand quarry faces and boreholes the old alluvium of singapore is a matrix supported pebbly sand with minor beds of better sorted sands and clays the beds can be grouped into four textural classes pebbles coarse sand with fine pebbles medium to coarse sand clay and silt each with characteristic sedimentary structures the deposit appears to be the proximal facies of an ancient braided river alluvium of possible pleistocene age this widespread but scattered alluvium is found both on land and offshore on the sunda shelf in southeast asia it is believed to have been deposited during low sea levels in singapore the mineralogy of the grains forming the old alluvium suggests a mixed provenance of granitic and low grade metamorphic origin the volume and freshness of the deposited material indicate an environment of considerable relief seasonal rainfall and extremely active erosional processes conditions considerably different from the current ones the old alluvium was probably deposited by seasonal rivers which experienced periodic large floods such regional conditions could have prevailed over much of southeast asia at the time of deposition of the old alluvium

whole layer convection in a heterogeneous planetary mantle 1 geochemical observations demonstrate that earth s mantle is heterogeneous but the sizes forms and characteristics of its reservoirs are not constrained using laboratory experiments we have systematically studied a simple nonhomogeneous system where two layers of miscible fluids with different densities depths and viscosities are subjected to a destabilizing temperature contrast when the buoyancy number b i e the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly is lower than 0 3 0 5 the whole layer regime develops with a deformed interface and convective patterns over the whole tank depth the system systematically evolves toward one fluid rayleigh benard convection because of stirring however the two isolated fluids can persist for very long time compared to the characteristic timescale of thermal convection and give rise to numerous transient behaviors of particular interest for planetary mantles are the pulsatory dynamics where the interface between the two layers deforms in large domes moving up and down quasiperiodically according to the scaling laws derived from the experiments such a mechanism could indeed provide a simple and single physical explanation for the superswells observed at present on earth and more generally for the long term episodicity in planetary interiors observed in geological records

continental tectonics and continental kinetics abstract we present a model of continental growth which combines the results of geochemical studies and tectonic ideas about the evolution of continents through geological time the process of continental growth is mainly controlled by surface phenomena continental material is extracted from the mantle along subduction zones at the periphery of oceans and is destroyed in collision zones where it is remobilized and made available for subduction we derive an equation for s the portion of the earth s surface occupied by continents which reads as follows ds dt a 1 s b s coefficients a and b depend on the geometry of plates on their number and on their velocities we assume that they decrease exponentially with time with the same time scale this model satisfies both geochemical and tectonic constraints and allows the integration of several current observations in a single framework

a thermo chemical regime in the upper mantle in the early earth inferred from a numerical model of magma migration in a convecting upper mantle abstract a numerical model of mantle magmatism in a convecting upper mantle has been developed to study the thermo chemical evolution of the upper mantle of the early earth the solid state convection in the upper mantle is modeled by a convection of a binary eutectic material with a newtonian temperature dependent rheology in a two dimensional rectangular box placed on a heat bath as a model of the lower mantle the density depends on the chemical composition and melt content as well as temperature of the material the material contains heat producing elements that are incompatible and exponentially decay with time mantle magmatism is modeled by a permeable flow of melt generated by a pressure release melting induced by the solid state convection through matrix the permeable flow is driven by a buoyancy due to the density difference between the melt and the matrix the thermo chemical evolution in the box occurs in two stages if the deeper part of the box is not so strongly depleted in heat producing elements in spite of the upward migration and concentration of heat producing elements into a crustal layer along the top surface boundary due to magmatism in the earlier stage active magmatism occurs because of a strong internal heating due to the heat producing elements a chemically stratified structure develops well in the box with dense magmatic products in the deeper part and less dense residual materials in the shallower part and the temperature distribution becomes strongly superadiabatic over the entire box the temperature at the base of the box becomes as high as the solidus temperature the chemically stratified structure is however suddenly destroyed by convective mixing and the temperature in the deeper part of the box suddenly drops by several hundred degrees when the internal heat source becomes too weak owing to the decay of heat producing elements which sustain the active magmatism and hence keep the effect of chemical differentiation due to the magmatism stronger than the effect of convective mixing in the later stage of the evolution the box becomes chemically homogeneous and magmatism occurs only mildly if heat producing elements are efficiently transported into the crustal layer and the deeper part of the box becomes strongly depleted in heat producing elements owing to the magmatism only a mild magmatism occurs even at the beginning a chemically stratified structure does not develop well and the temperature in the box rapidly decreases to a stationary value the regime of hot and chemically stratified upper mantle suggested from the earlier stage of the case with mild depletion of heat producing elements at depth fits in with many observations of the archean continental crust

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

mapping of c4 plant input from north west africa into north east atlantic sediments abstract mapping the abundance of 13c in leaf wax components in surface sediments recovered from the seafloor off northwest africa 0 35 n reveals a clear pattern of 13c distribution indicating systematic changes in the proportions of terrestrial c3 and c4 plant input at 20 n latitude we find that isotopically enriched products characteristic of c4 plants account for more than 50 of the terrigenous inputs this signal extends westward beneath the path of the dust laden sahara air layer sal high c4 contributions apparently carried by january trade winds also extend far into the gulf of guinea similar distributions are obtained if summed pollen counts for the chenopodiaceae amaranthaceae and the poaceae are used as an independent c4 proxy we conclude that the specificity of the latitudinal distribution of vegetation in north west africa and the pathways of the wind systems trade winds and sal are responsible for the observed isotopic patterns observed in the surface sediments molecular isotopic maps on the marine sedimentary time horizons e g during the last glacial maximum are thus a robust tool for assessing the phytogeographic changes on the tropical and sub tropical continents which have important implications for the changes in climatic and atmospheric conditions

on the sensitivity of parameterized convection to the rate of decay of internal heat sources thermal histories for the earth based on parameterized mantle convection models have previously indicated that the earth departs significantly from a steady thermal state the non steady state is manifest by a present day excess of heat loss over heat production we investigate the dependence of the heat production heat loss ratio and mantle viscosity on heat source decay rate by varying the relative and absolute concentrations of the heat producing isotopes of k u and th in the mantle we examine three models with progressively increasing effective half lives respectively characterized by k u ratios of 6 104 chondritic 1 104 terrestrial and 0 25 104 low ka u each of these models can be constrained to yield a common present day heat flow mantle temperature and mantle viscosity thus demonstrating the inability of those present day constraints to differentiate between widely varying blends of the earth s nuclear fuel the present day heat production heat loss ratio ranges from about 73 for the chondritic model to about 86 for the low k u model other possible criteria diagnostic of the earth s radioisotope mix are the present day isotopic abundances the low k u model requires u and th greater by a factor of 2 5 over the chondritic model and k less by a factor of 10 however measurements of concentrations in candidate mantle rocks reveal a range that far exceeds the relatively small variations that distinguish the different compositional models paleothermal conditions also may be indicative of the earth s radioisotope blend the thermal state in the archean calculated for the low k u earth model is characterized by a mean mantle temperature about 100 c hotter and heat flow about 2 times greater than the present day whereas a chondritic earth model yields a mantle temperature 300 c hotter and heat flow 4 times greater however current estimates of paleo heat flow and mantle melting temperatures are burdened with uncertainties sufficiently large to preclude their use as criteria diagnostic of the earth s radioisotope endowment

no aridity in sunda land during the last glaciation evidence from molecular isotopic stratigraphy of long chain n alkanes molecular distribution and stable carbon isotopic composition n 13 c of n alkanes determined in a sediment core 17962 from the southern south china sea scs show that no obvious change has taken place in vegetation on sunda land from the last glacial period to the beginning of the holocene a shift in the accumulation rate of longchain n alkanes c27 to c33 was recorded at a depth of about 620 cm probably indicative of precipitation enhancement related to winter monsoon the n 13 c values are within the range of 333 7 to 327 1x for c29 n alkanes and 333 9 to 328 4 for c31 n alkanes in the whole core sequence such an isotopic composition is characteristic of n alkanes biosynthesized mainly by plants utilizing the c3 photosynthetic pathway continuous development of c3 plants on sunda land and its surroundings evidences that the climate during the last glacial period in the southern scs was not drier than what it is today 2003 elsevier b v all rights reserved

contrast in surface water δ18o distributions between the last glacial maximum and the holocene in the southern south china sea abstract published core data of planktonic foraminiferal 18o and alkenone sea surface temperature sst are used to investigate seawater 18o w distributions in the south china sea scs during the last glacial maximum lgm and the holocene the holocene w distributions show a pattern correlated to modern salinity distributions displaying an increasing trend from the south to the north however the lgm w distributions show a different pattern displaying increasing values from the central to the southern scs and the lgm holocene differences in w w corrected by ice volume effect though negative in values display decreasing amplitude toward the south both of which could suggest increasing salinity toward the south by conventional approach this is somewhat counter intuitive because enhanced freshening could be expected for the southern scs due to extra discharge from the river systems on the emerged sunda land during the lgm here we propose an alternative explanation to reconcile the lgm w and w distributions with the above expectation in the proposal changes in w salinity relationship i e lowering the w salinity slope due to significant runoff contributions to freshwater input and less 18o depleted precipitation during the lgm are deeply discussed to reasonably explain the abnormity in lgm w distributions

models for the archean thermal regime a variety of different regionalized thermal evolution models are presented all models include an oceanic style of heat loss in which mantle convection penetrates to the surface in the form of spreading and subducting plates in continental regions the lithosphere is assumed to act as a conductive lid on mantle convection that can be either layered or mantle wide the continental lithosphere is defined in different ways it can be of fixed thickness insensitive to temperature or extend to some fixed cutoff temperature a thermally controlled rheological lithosphere when a fixed temperature is used widespread melting occurs in the archean and one has the further option of allowing these melts to erupt onto the surface thus providing a new heat transfer mechanism and also a general subsidence of the continental lithosphere due to the continuous volcanic loading the models are first used to determine the concentration of heat producing elements required to have a present day mantle potential temperature of 1350 c starting from a high temperature 2000 c 4 5 billion years ago the results depend primarily on whether the mantle is layered if layered 15 30 ppb uranium normalized to the silicate portion of the earth and withk u 104 th u 3 8 are needed while mantle wide cases need uranium concentrations in excess of 35 ppb the models are also used to calculate the thermal structure of continental lithosphere through time only lithospheres of fixed thickness 175 km can explain the recent discovery of diamonds with model ages of 3 2 3 4 billion years for a thick lithosphere to have existed in the archean it would seem most likely that its thickness be the result of some chemical distinction rather than simply its rheological properties such chemically distinct lithosphere would have greater likelihood of surviving to the present which raises the issue of biased preservation

response of western pacific marginal seas to glacial cycles paleoceanographic and sedimentological features abstract this paper reviews the recent progress in late quaternary studies in five northwestern pacific marginal seas especially the south china sea as an example a series of marginal seas separate asia from the pacific ocean and significantly modify the material and energy flux linkage between land and sea during glacial cycles the sea level induced environmental signal was amplified in the marginal seas giving rise to drastic changes in areas and configurations of these seas and to reorganization of sea water circulation in basins since most of the western pacific marginal seas are influenced by monsoon circulation and some of these are located within the western pacific warm pool the glacial geographic changes have produced a profound impact on regional and global climate for example the decrease of sea area and sea surface temperature sst in the marginal seas was one of the factors responsible for the enhanced aridity of inland china during the glaciation glacial intensification of the winter monsoon and increased seasonality of sst in marginal seas might explain at least partly the apparent discrepancy between the tropical paleotemperature estimations based on terrestrial and open ocean records in this region as the western pacific marginal seas trap terrigenous material supplied by east asia the deep water sedimentation rates there can be one to two orders of magnitude higher than in the open ocean down slope sediment transport is more active when the sea level changes particularly during the deglaciation at least four types of carbonate cycles have been recognized in the western pacific marginal seas and each of those contains environmental signals from the surface and deep sea water as well as from the drainage basin

a kinematic thermal history of the earth s mantle kinematic convection models of the earth s thermal history are computed assuming that subducted slabs along with some entrained material sink all the way to the core mantle boundary the temperatures within the lithosphere and within the mantle away from sinking slabs are computed assuming that the return flow from the core mantle boundary to the surface can be considered as slow vertical advection in the models a drop in asthenospheric temperature occurs near the end of the archean when subducted material first returns to the surface this drop is about 200 c if the subducted material does not mix thermally with the mantle on its way down and if only a thin thermal layer exists at the core mantle boundary we consider the possibility that this temperature drop is an explanation for the transition between archean and proterozoic tectonic styles an implication of the kinematic scheme is that the asthenospheric temperature increases gradually during the archean until the cooler material reaches the surface after the archean the thermal evolution is similar to a well mixed model a difficulty with the convection scheme as well as with any temperature profile in the mantle which is basically adiabatic is that excessively low temperatures in the core are implied if a 100 km thick thermal boundary layer at the core mantle boundary is included then higher core temperatures occur but there is a smaller drop in asthenospheric temperature at the end of the archean

marine and terrestrial biomarker records for the last 35 000 years at odp site 658c off nw africa several high resolution proxy environmental records have been obtained for the last 35 kyr from odp hole 658c a well studied site ca 200 km oa cap blanc nw africa the collective assessment based on the marine proxies u k 0 37 sst contents of toc and chlorins upwelling radiolarian index and the percentage of florisphaera profunda surprisingly indicates that the last glacial maximum lgm was characterized by warmer sea surface temperature sst weaker upwelling and lower marine productivity compared with the preceding older glacial and subsequent deglaciation periods of the terrigenous proxies the mean grain size of the non carbonate fraction and the terrigenous alkane content indicate that wind strength and aridity were high the weaker upwelling at the 658 site during the lgm may have resulted from changes in the strength and direction of the wind systems and or shifts in the position and geometry of the upwelling cell 2000 elsevier science ltd all rights reserved

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

double diffusive convection in this paper we present a rather personal view of the important developments in double diffusive convection a subject whose evolution has been the result of a close interaction between theoreticians laboratory experimenters and sea going oceano graphers more recently applications in astrophysics engineering and geology have become apparent in the final section we attempt to draw some general conclusions and suggest that further progress will again depend on a close collaboration between fluid dynamicists and other scientists

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

molecular and isotopic records of c4 grassland expansion in the late miocene abstract siwalik paleosol and bengal fan sediment samples were analyzed for the abundance and isotopic composition ofn alkanes in order to test for molecular evidence of the expansion of c4 grasslands on the indian subcontinent the carbon isotopic compositions of high molecular weight alkanes in both the ancient soils and sediments record a shift from low 13c values ca 30 to higher values ca 22 prior to 6 ma this shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth and is consistent with a relatively rapid transition from dominantly c3 vegetation to an ecosystem dominated by c4 plants typical of semi arid grasslands the n alkane values from our paleosol samples indicate that the isotopic change began as early as 9 ma reflecting either a growing contribution of c4 plants to a dominantly c3 biomass or a decrease in water availability to c3 plants molecular and isotopic analyses of other compounds including n alcohols and low molecular weightn alkanes indicate paleosol organic matter contains contributions from a mixture of sources including vascular plants algae and or cyanobacteria and microorganisms a range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples in addition then alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol s parent materials we suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers rather than bulk organic carbon this approach provides a means of expanding the spatial and temporal records of c4 plant biomass which will help to resolve possible tectonic climatic or biological controls on the rise of this important component of the terrestrial biosphere

regionalized models for the thermal evolution of the earth abstract traditional models for the heat loss in oceanic and continental regions are combined into a regionalized model for the thermal evolution of the earth the need for regionalization is obvious when one considers that the mantle loses 3 to 4 times as much heat per unit area in oceanic regions than in continental areas the present day rate of heat loss together with a geochemical estimate of the concentration of heat producing elements in the earth fixes the response time of the thermally convecting mantle the response time in turn can be used to select the most reasonable representation for mantle convection in terms of the sensitivity of viscosity on temperature and layering versus mantle wide circulation present geochemical estimates of the bulk composition of the earth are most easily reconciled with the observed heat flow if the mantle is layered and its rheology is slightly less temperature dependent than generally assumed the layered system can produce sufficiently high temperatures to explain the high magnesian komatiites of the archean one difficulty with the models is that they predict widespread melting at shallow depth in the early stages of earth history but do not address how such melting affects and alters the heat transfer mechanisms

a pollen record of the last 37 ka in deep sea core 17940 from the northern slope of the south china sea this paper presents the palynological record of deep sea core 17940 from the south china sea 20 07 n 117 23 e water depth 1727 m this core is about 13 30 m long and covers the last 37 ka a total of 103 samples were counted at 10 cm intervals with a time resolution of ca 360 yr three pollen zones are recognized zone p1 14c age ca 37 0 15 0 ka b p 13 06 8 70 m is characterized by the expansion of montane conifers tsuga picea and abies and colonization by temperate grassland mainly artemisia on the exposed northern continental shelf as well as by frequent natural fires indicated by high values of charcoal concentrations this implies that the climate of this period was cool and dry frequent alternations of montane conifers and grassland dominated by artemisia indicate that oscillations of comparatively cool and humid conditions with temperate and dry conditions occurred according to the oxygen isotope data the period corresponds to stage 3 and the last glacial maximum climatic warming p2 a 8 70 7 23 m 15 0 11 3 ka b p associated with a rapid rise in sea level from ca 14 ka b p is inferred by expansion of tropical subtropical broad leaved forest with a replacement of grassland by mangroves mainly rhizophora and sonneratia this time interval may be assigned to the bolling allerod warming stage subsequently climatic cooling reflected by expansion of both montane conifers and upper montane rain forests mainly podocarpus and dacrydium during the period of ca 11 3 10 0 ka b p p2 b 7 23 6 6 m may be correlated with the younger dryas cold spell the similarity of pollen assemblages during the holocene and in the surface sediment from the northern south china sea implies that the vegetation and climate during the last 10 000 years was close to those of the present during the last 1400 years human activities are reflected in the sharp increase of dicranopteris fern spores

unusual normal alkane distributions in offshore new zealand sediments the occurrence of unique even carbon dominated normal alkanes in sediment extracts was confirmed in sediments from the eastern continental shelf offshore of north island new zealand in hawke bay the range in carbon chain lengths was from n c16 to least n c36 extending distributions well beyond the previously reported 24 carbons stable isotope analysis of individual alkanes suggest an independent origin for the odd and even n alkanes it is suggested that sedimentary even number n alkane predominances are more widespread than previously thought but go unrecognized due to substantial overprinting by terrestrial derived odd carbon number n alkanes

fundamental issues in the geology and geophysics of venus a number of important and currently unresolved issues in the global geology and geophysics of venus will be addressable with the radar imaging altimetry and gravity measurements now forthcoming from the magellan mission among these are the global volcanic flux and the rate of formation of new crust the global heat flux and its regional variations the relative importance of localized hot spots and linear centers of crustal spreading to crustal formation and tectonics and the planform of mantle convection on venus and the nature of the interactions among interior convective flow near surface deformation and magmatism

a δ 13 c record of late quaternary climate change from tropical peats in southern india stable isotope ratios of carbon in soils or lake sediments1 3 and of oxygen and hydrogen in peats4 5 have been found to reflect past moisture variations and hence to provide valuable palaeoclimate records previous applications of the technique to peat have been restricted to temperate regions largely because tropical climate variations are less pronounced making them harder to resolve here we present a 13c record spanning the past 20 kyr from peats in the nilgiri hills southern india because the site is at high altitude 2 000 m above sea level it is possible to resolve a clear climate signal we observe the key climate shifts that are already known to have occurred during the last glacial maximum 18 kyr ago and the subsequent deglaciation in addition we observe an arid phase from 6 to 3 5 kyr ago and a short wet phase about 600 years ago the latter appears to correspond to the mediaeval warm period which previously was believed to be confined to europe and north america6 7 our results therefore suggest that this event may have extended over the entire northern hemisphere

the thermal regime of venus models for the thermal evolution of venus are constructed numerically the mantle is proposed to consist of two independently convecting layers upper and lower mantles the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200 c and the core may solidify at temperatures below the liquidus previously these models were calculated with the usual parameterization of convection based mostly on investigations of constant viscosity convection now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology the thermal evolution of venus is assumed to have begun 4 6 billion years b y ago it can be divided into three periods 1 adjustment of the upper mantle to the thermal regime of the lower mantle 0 5 b y 2 transition of the entire mantle to the asymptotic regime 3 4 b y 3 asymptotic regime parameters of the convecting planet in the asymptotic regime do not depend on initial conditions the planet forgets its initial state and are found analytically the present day heat flux is equal to 50 erg cm 2s 1 the results are close to those obtained previously comparison of the present thermal models of venus and earth with the recent studies of melting of the fe fes system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the venusian core at present the position of the iron triple point e liquid makes this conclusion very probable the thermal regime of the venusian crust is also discussed convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition the prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust

low frequency variability of the scottish coastal current induced by along shore pressure gradients abstract mean currents over much of the continental shelf west of scotland have a northward component low frequency currents measured close to the west coast of scotland in 1983 and 1984 show a mean northward flow but indicate that it is highly variable over time scales corresponding to the passage of weather systems it is shown that coastal along shore pressure gradients are an essential part of the description of the along shore dynamics northward pressure gradient and current pulses are associated with depressions passing to the north of britain whilst less frequent southward pulses are the response to smaller depressions tracking further south across the british isles part of the observed northward mean flow in the region may thus result from a pumping effect arising from the passage of successive depressions to the north of britain

molecular isotopic stratigraphy of long chain n alkanes in lake baikal holocene and glacial age sediments the molecular distribution and the carbon isotopic composition d 13 c of n alkanes extracted from a lake baikal core spanning the last 20 kyr of sediment accumulation have been investigated a terrestrial origin has been inferred for the odd carbon numbered long chain c27 n alkanes on the basis of the observed high cpi27 33 values range 8 7 10 8 typical of n alkanes derived from leaf waxes of higher plants a shift in the abundance of n c27 alkane relative to n c31 homologue is observed across the late pleistocene glacial holocene interglacial climate change perhaps indicative of the climate induced vegetational change previously deduced from palynological analyses compound specific isotope analyses indicate remarkably uniform d 13 c values in the range of 31 0 to 33 5 for the leaf wax c27 c33 nalkanes in the entire cored sequence such an isotopic compositional range is characteristic for n alkanes biosynthesized by plants utilizing the c3 photosynthetic pathway our data suggest that the observed 13 c enrichment in the bulk organic matter in the glacial age sediments relative to d 13 c values of total organic carbon in the holocene section is therefore unlikely to be attributed to an expansion of c4 type vegetation in the baikal watershed during the late pleistocene glacial interval 2000 elsevier science ltd all rights reserved

new evidence from the south china sea for an abrupt termination of the last glacial period abrupt changes in climatic conditions have been seen at high latitudes in the north atlantic1 and the antarctic2 3 at 13 kyr bp it is important to determine whether this abrupt change was confined to high latitude regions or whether it was global here we present results demonstrating an abrupt change in the rate and character of sedimentation in the south china sea at the close of the last glacial period radiocarbon dating and its position in the oxygen isotope shift suggest that this change may be coincident with the changes found at high latitudes

molecular and isotopic biogeochemistry of the miocene clarkia formation hydrocarbons and alcohols the miocene lacustrine deposit at clarkia northern idaho u s a is well known for its rich and exceptionally well preserved higher plant leaf fossils in the present study the distributions abundance and isotopic compositions of aliphatic hydrocarbons and alcohols isolated from two sediment samples and from two associated fossils magnolia latahensis and platanus dissecta are investigated the general similarity of these characteristics in the sediments suggests a relatively uniform depositional environment and consistent terrestrial input the sediment hydrocarbons are comprised mainly of compounds derived from higher plants and bacteria whereas the alcohols reflect contributions from both higher plants and algae a notable feature of the higher plant derived terpenoid hydrocarbons is the prevalence of des a triterpenes largely in the absence of their pentacyclic precursors indicating the dominance of a ring degradation for these pentacyclic triterpenoids the des a triterpenes include several novel compounds namely 19 h and 19 h 28 nor des a lup 17 ene and des a oleana 9 13 18 diene the sediment samples contain high abundances of c24 c32 long chain alkanediols consisting of a series of diols 1 diols 1 15 diols and an unusual series of 1 3 diols both fossil leaves show clear differences in their compound distributions and concentrations from the associated sediments which suggests that they retain a composition distinct from the sediment groundmass the 13c values of c25 c33 n alkanes and c24 c32 n alcohols in sediments and fossils are consistent with an origin from terrestrial c3 higher plants algal lipids in the two sediments are represented by dinostanol 24 6 and 25 9 inherited from dinoflagellates and long chain 1 15 alkanediols 26 4 to 29 7 derived from microalgae the long chain 1 and 1 3 alkanediols are more enriched in 13c 21 8 to 26 4 suggesting an origin from sources other than higher plants probably aquatic photoautotrophic organisms the 13c depletion of some hopanoids 35 9 to 61 0 attests to contributions from methanotrophic bacteria and suggests an active methane cycle at the time of deposition

late quaternary cycles of mangrove development and decline on the north australian continental shelf mangrove communities in the australian tropics presently occur as narrow belts of vegetation in estuaries and on sheltered muddy coasts palynological data from continental shelf and deep sea cores indicate a long term cyclical component of mangrove development and decline at a regional scale which can be linked to specific phases of late quaternary sealevel change extensive mangrove development relative to today occurs during periods of marine transgression whereas very diminished mangrove occurs during marine regressions and during rarer periods of relative sea level stability episodes of flourishing mangrove cannot be linked to phases of humid climate as has been suggested in studies elsewhere rather the cycle of expansion and decline of mangrove communities on a grand scale is explained in terms of contrasting physiographic settings characteristic of continental shelf coasts during transgressive and regressive phases in particular by the existence or lack of well developed tidal estuaries copyright c 1999 john wiley sons ltd

convection in a two layer mantle with a strongly temperature dependent viscosity recent geochemical and isotopic data support the hypothesis that the earth s mantle convects in two separate layers with an interface at the 650 km seismic discontinuity this paper studies the implications of this hypothesis with regard to the thermal boundary layers that are expected to develop adjacent to the interface between the convecting layers we are particularly concerned with a possible decrease in the mantle viscosity across this interface in order to study the structure of the thermal boundary layers in a fluid with a strongly temperature dependent viscosity we utilize an approximate method developed by howard 1966 if the upper and lower mantle have the same viscosity law we find that stratified convection requires a lower mantle viscosity which is several orders of magnitude lower than the upper mantle viscosity this is not consistent with studies of postglacial rebound it is possible to obtain a viscosity law for the lower mantle that gives a viscosity nearly equal to that of the upper mantle and reasonable temperatures however the required law is not consistent with the currently available experimental data on lower mantle materials therefore we conclude that thermal arguments favor whole mantle convection

numerical models of thermally and mechanically coupled two layer convection of highly viscous fluids summary numerical solutions for thermal convection of two superposed viscous fluid layers with the same thicknesses as the earth s upper and lower mantles and separated by a fixed horizontal interface are obtained at the onset of convection and in the nonlinear regime several heating configurations and thermal boundary conditions are considered including an isothermal bottom boundary a prescribed heat flux bottom boundary and internal heating with an insulated lower boundary at the onset of convection and with the same material properties in each layer the lower layer is more unstable than the upper layer and long horizontal wavelengths proportional to the lower layer thickness predominate for layers with sufficiently different material properties the upper layer can be more unstable and short horizontal wavelengths proportional to the upper layer thickness can predominate at the onset of convection when the material properties are chosen to make the two layers equally unstable there are two stability curve rayleigh number ra versus wavelength minima with the lower minimum at long wavelength linear stability results typify weakly nonlinear convection except when both layers are equally unstable when the layers have unequal stability the stable layer has a conductive temperature field and its flow is viscously driven by the convective flow in the unstable layer when the two layers are equally unstable both layers are separately convecting even at rayleigh numbers of only twice the critical value for convection onset at large enough supercritical ra the lower layer convects with a long wavelength cell even when linear stability theory predicts short wavelength cells the upper layer always begins convecting with short wavelength cells however the upper layer cell over the lower layer downwelling plume increases in size with increasing ra thus horizontal wavelength plate dimension for example cannot be used to discriminate between whole mantle and layered mantle convection mechanical coupling of the layers downwelling plume over upwelling plume occurs for all cases with the same material properties in each layer and for the case with the same ra in each layer when the material properties of the layers are sufficiently different to produce short wavelength cells at the onset of convection the upper layer has two thermally coupled cells downwelling plume over downwelling plume and a shear zone at the interface over both lower layer plumes a large 50 per cent of total temperature difference across the interface occurs in all cases examined implying that two layer mantle convection should also have a large temperature change across the 670 km discontinuity however temperature dependent viscosity may reduce the magnitude of this temperature difference interpretations of geoid anomalies over subduction zones and seismic traveltime anomalies of slabs should account for possible thermal coupling of separate upper and lower mantle convective systems

timing and duration of supergene mineralization at the xinrong manganese deposit western guangdong province south china cryptomelane 40 ar 39 ar dating supergene mn oxide deposits are widely distributed in guangxi guangdong yunnan and hunan provinces south china accounting for 18 of the total mn reserves in the country direct dating of supergene mn enrichment however is lacking in this paper we present high resolution 40ar 39ar ages of mn oxides from the xinrong mn deposit western guangdong to place numerical constraints on the timing and duration of supergene mn enrichment a total of ten cryptomelane samples spanning a vertical extent of 67 m were dated using the 40ar 39ar laser incremental heating technique with seven samples yielding well defined plateau or pseudo plateau ages ranging from 23 48 0 91 to 2 06 0 05 ma 2 one sample yields a staircase spectrum that does not reach a plateau the spectrum however indicates the presence of two or more generations of mn oxides in the sample whose ages are best estimated at 22 34 0 31 and 10 2 0 86 ma respectively the remaining two samples gave meaningless or uninterpretable results due to significant 39ar recoil and contamination by old phases the 40ar 39ar data thus reveal a protracted history of weathering and supergene mn enrichment that started at least in the end of the oligocene or beginning of miocene and extending into the latest pliocene staircase apparent age spectra resulting from banded or botryoidal samples yield an average growth rate of mn oxides at 0 6 0 7 10 3 mm kyr 1 the values indicate that a 1 mm grain of mn oxides may host minerals precipitated during a time span of ca 1 5 m y and accumulation of mn oxides to form economic deposits under weathering environments may take millions of years the distribution of weathering ages shows that the oldest mn oxides occur on the top of the profile whereas the youngest minerals are found at the bottom suggesting downward propagation of weathering fronts however two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations such a complexity of age distribution is interpreted in terms of preferential penetration of mn rich weathering solutions along more permeable fault zones or as a result of multi stages of dissolution and re precipitation of mn oxides a synthesis of geochronological and geological data suggests that formation of the xinrong deposit was a consequence of a combination of favorable lithological climatic and structural conditions because the climatic and structural conditions are similar among the provinces of south china during the cenozoic the geochronological results obtained at xinrong may also have implications for the timing of supergene mn enrichment throughout south china

secular trends in the thermal evolution of metamorphic terrains the thermal evolution of convergent orogens is sensitive to the deformation history of the mantle part of the lithosphere if the dense mantle lithosphere thickens as a consequence of convergent deformation and remains attached during subsequent denudation of the thickened crust then high pressure low to intermediate temperature glaucophane schist assemblages will form during both thickening and denudation the actual form of the p t paths for a given deformation depending primarily on the initial thermal state of the lithosphere and particularly the ratio of the thickness of the crust and mantle lithosphere however if mantle lithosphere is thinned during convergence by processes such as thermal boundary layer detachment then any early formed high pressure low to intermediate temperature glaucophane schist assemblages will be obliterated during subsequent high temperature overprints since the stability of the thermal boundary layer during convergent deformations is controlled by the rate of convergence and the convective vigour of the subjacent mantle any change in the convective vigour of the mantle through geologic time in response to for example changing global heat production may therefore be reflected in the thermal evolution of metamorphic terrains formed during convergent deformations models of the thermal evolution of deforming lithosphere appropriate to an earth in which the effect of declining global heat production and convective vigour is mediated by the process of thermal boundary layer detachment suggest that the preservation of high pressure low to intermediate temperature glaucophane schist assemblages in convergent orogens has become increasingly probable through geological time a suggestion in accord with the observed secular trend in the character of metamorphic terrains preserved in the geological record

double diffusive convection in groundwater wells 1 double diffusive convection caused by variations in solute concentration and temperature within a fluid body has previously been studied in connection with oceanic mixing processes astrophysics metallurgy geology and vadose zone hydrology but has not previously been studied in connection with thermohaline transport in a groundwater well this paper considers whether double diffusive convection induced by salinity and or temperature variations is a plausible mechanism for heat and solute transport in a groundwater well this is examined theoretically using rayleigh number stability onset criteria for fluid in a cylinder that considers the cylindrical well geometry and geothermal solute boundary conditions theoretical results suggest that both monotonic and oscillatory double diffusive convection are plausible transport phenomena in groundwater wells these analyses have important implications for hydrogeology because they provide another theoretically plausible mechanism by which heat and solute may mix in a well this previously unaccounted for thermohaline mixing phenomenon may therefore be critical in the interpretation of well hydrogeologic and hydrochemical data

low latitude hydro climatic changes during the plio pleistocene evidence from high resolution alkane records in the southern south china sea abstract high resolution records of long chain n alkane biomarkers from the southern south china sea reveal tightly response of n alkane distribution to hydro climate changes over the past 5 ma with increasing longer chain n c31 alkanes indicating a correlation with drier conditions and increasing shorter chain n c27 alkanes with wetter conditions the variations of the c31 c27 ratio or the alkane chain length index imply humid conditions before 2 9 ma progressively reduced moisture since then and to bigger fluctuations between wet and dry conditions since 1 2 ma this long term hydro climate trend is superimposed by glacial dry and interglacial wet patterns over the plio pleistocene glacial cycles combined with other proxy records our results indicate that precipitation over the tropical asia pacific strengthened before the onset of the northern hemisphere glaciation and the mid pleistocene climate transition at about 1 2 ma these dramatic humidity changes over major climate transitions imply a crucial role of tropical hydrology dynamics on global climate change in the late cenozoic

burial of different types of organic carbon in core 17962 from south china sea since the last glacial period abstract total organic carbon toc black carbon bc and land derived organic carbon loc extracted from marine sediment core 17962 from the southern south china sea and analyzed by pyrolysis gas chromatography elemental analyzers and mass spectroscopy can be interpreted in terms of regional land biomass burning climatic conditions vegetation cover and ocean paleoproductivity carbon from terrestrial sources including bc and loc appears to be a significant component of toc 3 1 39 3 range 16 5 average for bc 4 4 56 6 range 14 6 average for loc the production of bc and loc was only slightly higher during the last maximum glaciation lgm than during the holocene therefore dramatic changes in climate and vegetation cover do not seem to have occurred at the pleistocene holocene boundary paleoproductivity data calculated from marine organic carbon proved to be more accurate than those calculated from toc and exhibit the same pattern of temporal variation however the glacial interglacial contrast in the refined palaeoproductivity is less evident than that calculated from toc

the effect of a shallow low viscosity zone on the mantle flow the geoid anomalies and the geoid and depth age relationships at fracture zones summary using a 2 d finite element method we studied the flow that is driven by the horizontal temperature gradient at a fracture zone and calculated the resulting geoid and topography anomalies assuming a three layered viscosity structure for the upper mantle consisting of a conducting lid overlying a low viscosity channel which in turn overlies a unit viscosity layer extending to the base of the upper mantle we studied the effect of varying 1 the viscosity contrast between the fluid layers 2 the rayleigh number based on the viscosity of the bottom layer and 3 the thickness of the low viscosity channel the flow at first downwells immediately adjacent to the fracture zone on the older side its time scale and characteristic wavelength depend on the viscosity and depth of the top layer which is nearest the largest temperature gradient in the fluid after the initiation of the driven flow additional flows are generated away from the fracture zone through shear and thermal coupling and boundarylayer instabilities eventually however the flow extends throughout the fluid so that the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers when the rayleigh number that is based on the viscosity of the top layer and the depth of both fluid layers is less than lo6 the geoid anomalies are dominated by the convective signal when this rayleigh number exceeds lo6 the geoid anomalies retain a step across the fracture zone out to large ages we compared our results with geoid anomalies over the udintsev fracture zone and found that the predicted geoid anomalies are in good agreement with the observed anomalies when the viscosity of at least the top layer is greater than one order of magnitude less than post glacial rebound values 102 pas however we also compared the calculated topographic steps with those predicted by the average depth age relationships observed in the oceans and found that a viscosity contrast of at least one order of magnitude at a depth in the uppermost mantle is required to produce realistic topographic steps

estimates of martian crustal thickness from viscous relaxation of topography isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients which may cause the lower crust to flow and reduce the relief areas of thicker crust are generally associated with more rapid relaxation of topography on mars topographic features such as impact basins and the hemispheric dichotomy have survived for 4 gyr we use a finite difference representation of depth dependent non newtonian lower crustal flow to investigate how topography decays with time for a dry diabase rheology total radiogenic concentrations 80 of terrestrial values and crustal radiogenic concentrations similar to terrestrial basalts we find that an upper bound on the mean planetary crustal thickness is 100 km in the probably unrealistic case where all the radiogenic elements are in the crust this maximum crustal thickness can be increased to 115 km the main uncertainty in these results is the total radiogenic abundances on mars comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow both hellas and the dichotomy are isostatically compensated if the mechanism is airy isostasy then the lower bound on mean crustal thickness is 30 km crustal thicknesses of 30 100 km on mars can be produced by mid ocean ridge spreading at potential temperatures of 1350 1600 c however for such crustal thicknesses the lithosphere is likely to be positively buoyant making subduction difficult

late miocene atmospheric co2 concentrations and the expansion of c4 grasses the global expansion of c4 grasslands in the late miocene has been attributed to a large scale decrease in atmospheric carbon dioxide co2 concentrations this triggering mechanism is controversial in part because of a lack of direct evidence for change in the partial pressure of co2 pco2 and because other factors are also important determinants in controlling plant type distributions alkenone based pco2 estimates for the late miocene indicate that pco2 increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago the estimates presented here provide no evidence for major changes in pco2 during the late miocene thus c4 plant expansion was likely driven by additional factors possibly a tectonically related episode of enhanced low latitude aridity or changes in seasonal precipitation patterns on a global scale or both

stable regions in the earth s liquid core summary slow cooling of the whole earth can be responsible for the convection in the core that is required to generate the magnetic field previous studies have assumed the cooling rate to be high enough for the whole core to convect here we study the effects of a low rate of cooling by assuming the temperature at the base of the mantle to remain constant with an initially entirely molten adiabatic core we argue that in such a situation convection would stop at the top of the core and calculate the consequent thermal evolution a stable density stratified layer grows downwards from the core mantle boundary reaching a thickness of 100 1000 km in a few thousands of millions of years there is some geomagnetic evidence to support belief in the existence of such a stable layer

internally heated mantle convection and the thermal and degassing history of the earth a parameterized internally heated convection model for the earth has been developed the mantle viscosity is temperature and volatile content dependent a heat flow rayleigh number relationship appropriate for an internally heated mantle is assumed for each model an initial homologous temperature and bulk water content are assumed and the initial radiogenic heat production is constrained to give a present day heat flow of 0 07 w m 2 the model is run for 4 6 gyr and temperature heat flow degassing and regassing rates stress and viscosity are calculated a nominal case is established which shows good agreement with accepted mantle values the effects of changing various parameters have also been tested all cases show rapid cooling early in the planet s history and strong self regulation of viscosity due to the temperature and volatile content dependence the values chosen for initial temperature and volatile content have negligible effects on the results while assumptions about present day heat flow viscosity and mantle volatile content can have important consequences the effects of weakly stress dependent viscosity are examined within the bounds of this model and are found to be small mantle water is typically outgassed rapidly to reach an equilibrium concentration on a time scale of less than 200 m y for almost all models the main exception being for models which start out with temperatures well below the melting temperature for some cases the degassing efficiency is allowed to decrease with time as the mantle cools which combined with the rapid early loss of volatiles leads to a net regassing of the mantle to the present day and loss of water from the surface

parameterized thermal convection in a layered region and the thermal history of the earth parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply and show that layering considerably increases the response time the same arguments are applied to a planet consisting of spherical shells they show that the response of the earth s heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface if it does so the surface heat flux is only about 50 greater than the heat generation rate however if upper and lower mantle convection occurs in two separate shells as seismological and geochemical arguments suggest the response time is increased the present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed the best estimate of the present ratio of the heat loss to heat generation is about two and this value is consistent with the behavior of an earth model consisting of a two layered mantle overlying a core

molecular biomarker record of sea surface temperature and climatic change in the south china sea during the last 140 000 years the d 18 oa ndd 18 c isotopic composition of globigerinoides ruber the concentration of c37 alkenones n nonacosane and n hexacosan 1 ol and the populations of coccolith species in core 17961 2 have been used to characterize the climatic changes which occurred in the south china sea scs during the last climatic cycle the relative composition of diand triunsaturated c37 alkenones the u k index has been used to estimate the variation in sea surface temperatures the concentrations of the terrigenous markers n nonacosane and n hexacosan 1 ol have allowed to infer changes in continental water dynamics during the glacial and interglacial times the stratigraphic record of these compounds has shown that the influence of continental waters i e the molengraaff river and restricted water circulation in scs gave rise to a marginal system of higher but slower response to climatic change both in terminations i and ii the change from glacial to interglacial conditions involves a considerable reduction of continental water input into this semi enclosed system the high resolution study of the biomarker proxies in termination i has shown that the sst increase lagged about 2 3 ky the d 18 o isotopic decrease of cibicidoides wuellerstorfi probably the sst increase corresponds to the period of the sunda shelf inundation onset 1999 elsevier science b v all rights reserved

the origin of terrestrial heat flow summary a theory of the origin of the earth s heat which agrees with all observations is presented the radiogenic heat production from the decay of u and th was computed from their abundances in ordinary chondrites from which the earth accreted and from the abundance of potassium in the mantle plus crust the total radiogenic heat production is 1 14 1013 w because u and th are incompatible in the silicates they have migrated upward into the continental rocks the sediments eroded from the continents are returned to the continents on subducting oceanic slabs therefore about 90 per cent of u and th are concentrated in the continents this is confirmed by a worldwide survey of helium in the oceans which found only 5 per cent of the helium that should have been there had the heat come from radioactive decay subtracting the radiogenic heat production of 1 14 1013 w from the total heat loss of 4 2x1013 w yields 3 06 1013 w to be supplied by the non radioactive sources in the mantle it is proposed that non radiogenic heat originates from secular cooling and from the release of gravitational potential energy by the absorption of feo from the mantle into the core this reaction has been produced in the laser heated diamond anvil cell at temperatures and pressures of the core mantle boundary layer as the molten iron extracts most of the feo out of the perovskite of the mantle a light residue consisting of an iron depleted perovskite and stishovite a form of silica rises in the form of plumes and megaliths generating heat by viscous friction if the oxygen now present in the core got there by this process 1027 g of feo have descended into the core generating energy at an average rate of twice the present rate of loss of non radiogenic heat adding the contribution of secular cooling an average rate of non radiogenic heat production of 1014 w could have been sustained for the last 4 6 billion yr

archaean plate tectonics revisited 2 paleo sea level changes continental area oceanic heat loss and the area age distribution of the ocean basins in a previous paper we derived plate tectonic models for continental accretion from the early archaean 3800 m y b p until the present the models are dependent upon the number of continental masses the seafloor creation rate and the continental surface area the models can be tested by examining their predictions for three key geological indicators sea level changes stable isotopic evolution e g continental surface area and oceanic heat loss models of paleo sea level changes produced by the accretion of the continents reproduce the following features of earth history 1 greater continental emergence lower sea level during the archaean than the proterozoic 2 maximum continental emergence about 3000 m y b p and 3 maximum continental submergence high sea level from 30 to 125 m y b p the high sea level stand between 380 525 m y b p is only weakly reproduced probably due to the simplified nature of the model changes in the number of continental masses can result in tectonic erosion or accretion of the continents with resulting changes in sea level the two major transgressions in the phanerozoic although still requiring some increase in the total terrestrial heat loss can be sucessfully explained by a combination of increases in continental surface area and in seafloor creation rate changes in the total heat loss of the ocean basins predicted by our plate tectonic models closely parallel the changes in terrestrial heat production predicted by wasserburg et al 1964 this result is consistent with thermal history models which assume whole mantle convection the history of changes in continental surface area predicted by our best continental accretion models lies within the ranges of estimated continental surface area derived from independent geochemical models of isotope evolution

late quaternary paleoceanography of the south china sea glacial interglacial contrasts in an enclosed basin the sea surface paleotemperature of the south china sea scs is estimated quantitatively based on the quantitative analysis of planktonic foraminiferal faunas recovered from piston cores v36 06 3 v36 06 5 and so49 kl8 on the northern continental slope the results lead to a reconstruction of the history of paleocirculation patterns during the last 200 000 years the postglacial and interglacial transbasinal circulation pattern was replaced by a semienclosed basin flow pattern during glacial episodes lowered sea level transformed the scs into a semienclosed basin with the only passageway to the open ocean in its northeastern corner the east asian monsoon which governs circulation patterns in the scs both today and during the late quaternary created both clockwise summer and counterclockwise winter surface gyres during glacials the inflow of tropical surface water masses from both the pacific and indian oceans decreased or was completely cut off from this marginal sea causing a greater decrease in glacial sea surface temperature than in the open ocean of the same latitude inferred increased influence of pacific temperate waters and decreased influence of tropical subtropical waters are confirmed by changes in the planktonic foraminiferal assemblages glacial interglacial fluctuation in sea surface temperature sst 6 8 9 3 c for winter and 2 3 c for summer as well as the seasonality 4 6 c for postglacial and interglacials and 9 10 c for glacials are much greater than the adjacent open western pacific reflecting the amplification of the climatic signal in the marginal sea

a hydroclimate regime shift around 270 ka in the western tropical pacific inferred from a late quaternary n alkane chain length record abstract the inert leaf wax deposited in deep sea sediment preserves clues of the past climate change in particular the n alkane distribution patterns characterized by the n alkane average chain length index acl provide useful information on the hydrological changes as longer chain alkanes are more frequently biosynthesized under relatively arid condition to prevent water loss our data from the western south china sea reveal the superimposition of a systematic regime shift in the regional mean precipitation at ca 420 270 ka on the glacial interglacial changes in the quaternary the dominance of shorter n alkane chain length between 420 ka and 270 ka is interpreted as overall wetter conditions during this period compared to the older or younger intervals a similar timing of hydrological regime shifts has also been observed in other paleoclimate records including major changes in productivity in the eastern pacific and pollen records from the western pacific we suggest that the changes in the regional mean hydroclimate states near 420 ka and 270 ka are most likely related to a regime change in the frequency and or intensity of the el nino southern oscillation with the overall wetter hydroclimate prior to 270 ka resulting in a more prevalent la nina like condition in the pacific

mantle dynamics and basalt petrogenesis abstract differentiation at mid ocean ridges generates a layered lithosphere consisting of a basaltic crust immediately underlain by harzburgite and further underlain by pyrolite which has experienced depletion only of highly incompatible elements the body forces driving subduction are concentrated mainly in the upper half of the lithosphere which is relatively cool and brittle during subduction the lower layer of relatively ductile slightly depleted pyrolite is stripped off and resorbed into the upper mantle thereby providing a future source region for morb magmas the slab which sinks to 600 km is comprised mainly of differentiated former basalt and harzburgite which undergo a different series of phase transformations to those experienced by mantle pyrolite in consequence the former basaltic crust remains denser than surrounding mantle whereas former harzburgite becomes relatively buoyant below the 650 km seismic discontinuity the resulting non uniformity in stress distribution causes the slab to buckle at this depth and accrete to form a large relatively cool ovoid megalith of mixed former harzburgite and basaltic crust heating of the megalith occurs over 1 2 b y leading to partial melting of the former basaltic crust the resultant liquids contaminate regions of former harzburgite rendering them fertile in the sense of future capacity to produce basaltic magmas after thermal equilibration the newly fertile former harzburgite becomes buoyant leading to the separation of diapirs which rise into the upper mantle such diapirs rising beneath sub oceanic lithosphere experience small degrees of partial melting to produce ocean island basalts mainly of the alkaline suite diapirs of fertile former harzburgite rising beneath continents become incorporated into the sub continental lithosphere this is a cumulative process and is ultimately responsible for the development of the chemical physical and isotopic characteristics of the sub continental lithosphere including its capacity to produce alkaline magmas when subjected to small degrees of partial melting

the topographic and gravitational expression of density anomalies due to melt extraction in the uppermost oceanic mantle abstract evidence suggests that basaltic extrusion can decrease the density of the depleted source region by up to 2 for 20 partial melting this hypothesis is tested by calculating the effect of the density anomaly between the depleted and undepleted mantle on the topography gravity and geoid fields and comparing the anomalies to the observed topography gravity and geoid anomalies over volcanic features in the oceans if the source region lies at the base of the lithosphere we find that the gravity and geoid anomalies are proportionally more sensitive to the density contrast than the topography anomaly this effect is most pronounced for wavelengths less than 1000 km since mid ocean ridge volcanism the largest source of volcanism in the oceanic plates is fairly uniform along the ridge the depleted material in the oceanic plates and the corresponding topography and gravity anomalies should also be somewhat uniform in extent any long wavelength anomalies will be obscured in the topography and geoid fields by other anomalies at similar wavelengths such as slab and deep mantle anomalies however at mid plate swells and seamounts if the density contrast is greater than 0 4 the topography gravity and geoid anomalies are measurable to be consistent with the wavelengths represented in the geoid and topography data at hawaii the depleted material must be spread into a thin layer by convective flow at mid plate swells even in the thin layer model the topography and geoid anomalies due to the depleted material would still be as large as 200 m and 2 m respectively after the convective flow producing the bulk of the swell is removed the anomaly due to the depleted material would remain and would be apparent in the geoid and topography fields at seamounts if the source region is near the base of the plate then the presence of depleted material should be distinguishable in the geoid field but be somewhat obscured by flexural effects therefore although a topographic or geoid anomaly due to depletion has not been observed the density anomaly cannot be discounted because only careful scrutiny and modelling of mid plate swells and seamounts would distinguish its effect

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

terrestrial vegetation change inferred from n alkane σ13c analysis in the marine environment abstract we report gas chromatography isotope ratio monitoring mass spectrometry gc irm ms measurements of the 13c values of individual biomarker compounds n alkanes extracted from a 3 m marine sediment core taken near the mouth of the johnstone river north queensland australia the technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine derived carbon the results indicate that there has been a 2 increase in the 13c values of terrestrially derived n alkanes c29 c33 since clearing of the forested johnstone river drainage basin for sugarcane and pasture began in the late 19th century a much slower 1 increase in 13c values after 6 000 years bp and prior to european settlement may be related to a decrease in rainfall in the basin or to an increase in the abundance of c4 plants as a result of increased aboriginal burning the results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin and demonstrate that the 13c values of terrestrially derived n alkanes in the marine environment can be used to assess basin wide vegetation changes in adjacent river catchments on geological timescales

speculations on the thermal and tectonic history of the earth summary the connection between the earth s thermal history and convection in the mantle is exploited to elucidate the early evolution of the earth it appears probable that convection extending over almost all of the mantle has dominated vertical heat transport throughout the whole of the earth s history only in boundary layers at the surface and at a depth of 650 700 km is conduction likely to be important the resulting evolution appears to be consistent with geological observations on early precambrian rocks various arguments are put forward in favour of two horizontal scales of convective flow in the mantle at depths less than 650 km the large scale flow is related to the motion of major plates and must be ordered over distances of more than 5000 km its evolution and energetics are discussed and there are no obvious problems in maintaining the proposed convective motions small scale flow with an extent of the order of 500 km appears necessary both to explain the heat flow through older parts of the earth s surface and to reconcile the geophysical observations with the results of numerical experiments though the existence of the small scale flow is at present speculative various tests of its presence are proposed

the flooding of sundaland during the last deglaciation imprints in hemipelagic sediments from the southern south china sea abstract during the last 30 ka the south china sea scs experienced pronounced palaeogeographic changes associated with the postglacial sea level rise which significantly modified the hydrography of this marginal sea the most crucial effects in the southern part of the basin were the submergence of sundaland and the opening of the southern channels connecting the scs to the tropical indo pacific isotopic sedimentological and organic geochemical parameters determined in two sediment cores from the southern scs one in the open sea and the other close to the continental shelf sites 17961 and 17964 respectively show that the main hydrographical changes during this period were related to critical thresholds in sea level rise the main changes occurred at about 15 13 5 ky bp coincident with meltwater pulse mwp ia when sea surface temperatures ssts at both sites experienced a rapid 1 5 c rise and the clay content and n nonacosane concentrations dropped significantly both trends reflect a rapid retreat of the coastline and an initial flooding of sundaland at that time a second important change starting with the beginning of mwp ib at about 11 5 ky bp and culminating at 10 ky bp involved the establishment of modern hydrographic conditions this is evident from the rapid convergence of the foraminiferal oxygen isotope records and the establishment of holocene sst values these results highlight the need to include the flooding emergence of sundaland as an important boundary condition in future modelling studies of asian palaeomonsoons

thermal state and thermal regime of the earth s interior abstract a simplified version of the thermal state and thermal regime of the earth s mantle is given it is supposed that the large scale convection in the mantle is double layered convection in the upper mantle is determined by heat flux from below and in the lower mantle by internal heating it is estimated that the thermal relaxation time of the upper mantle is 10 8 y and the time of the thermal relaxation of the lower mantle lm is 1 5 10 9 y the thermal relaxation of the earth is determined by lm in the convective mantle the superadiabatic temperature drops are formed in the thermal boundary layers in the upper mantle t 1 is 600 k at the top of the lower mantle t 2 is 650 k at the bottom of the lower mantle t 3 is 650 k the adiabatic temperature at the core mantle boundary is 2900 k and the real temperature must be 3500 4500 k the thermal boundary layers in the mantle are the super heated zones and must coincide with low q zones q is the mechanical quality for s waves the first zone of low q is at a depth of 100 250 km the second one is at 670 850 km and the third one is at 2700 2900 km the source zone for hot spots may be in the second thermal boundary layer it is interesting that the horizontal velocity of the layer which is 1 cm y 1 must determine the limits of precision for absolute velocities in plate tectonics the boundary between the upper and lower mantle must be the weak chemical boundary because the upper mantle is the source for the earth s crust the thermal regime and instability of the third thermal boundary layer at a depth of 2600 2900 km may play a very important role in the magnetohydrodynamics of the earth s core and particularly for the change in polarity of the geomagnetic dipole and for understanding the geomagnetic time scale

recognizing oib and morb in accretionary complexes a new approach based on ocean plate stratigraphy petrology and geochemistry abstract we present a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy ops and on the petrology and geochemistry of basalts from mid oceanic ridges morb and oceanic islands oib using examples from four accretionary complexes ac in sw japan akiyoshi mino tamba chichibu and shimanto the key to the problem is the model of ops which includes an association of igneous and sedimentary rocks that form on an oceanic plate during its travel from a mid oceanic ridge to a subduction zone we propose the reconstruction of the tectonic settings of basalts according to their relationships with associated ops sediments their petrogenesis and their geochemical features five types of ops are recognized in the accretionary complexes of sw japan 1 sandstone shale 2 sandstone shale and chert 3 sandstone shale chert and morb 4 sandstone shale chert morb and gabbro peridotite 5 seamount ops including oib cap carbonates slope clastics and basal shale chert the alkaline tholeiitic or calc alkaline composition of basaltic melts which are typical of oceanic islands mid oceanic ridges and island arcs respectively can be identified by the sequence in crystallization of their major phenocrysts i e olivine ol clinopyroxene cpx and plagioclase pl and by their compositions alkaline and calc alkaline mafic lavas are characterized by an ol cpx pl succession whereas tholeiitic melts by their ol pl cpx succession titanium rich minerals e g ti augite kaersutite ti biotite are typical of alkaline lavas the application of geochemistry based tectonic discrimination diagrams is also a powerful tool if not supported by geological and petrological data may result in confusion due to magma contamination post magmatic alteration and secular change of mantle thermal conditions we propose that a direct comparison of normalized multi element patterns and key binary plots from older volcanic rocks with their modern analogues provides a more viable and reliable method of basalt discrimination our ops petrology geochemistry method allows us to confirm the above conclusions that the lavas of the akiyoshi mino tamba and southern chichibu ac formed in oceanic islands because they are associated with seamount ops sediments crystallized from ol to cpx and pl contain ti augite and kaersutite and are enriched in tio2 lree and nb in this paper we present geochemical data from the inuyama basalts of the mino tamba ac and from the toba complex in the huge mikabu greenstone belt of the chichibu ac the inuyama basalts are in contact with jurassic pelagic cherts but their geochemical features are confusing they contain phenocrysts of ol ti augite and kaersutite and therefore probably formed in seamounts the toba volcanic rocks are a part of the huge ophiolite belt they have flat to slightly lree enriched ree patterns are characterized by an ol cpx succession of phenocrysts and they plot in the oib field in binary plots suggesting they formed in an oceanic plateau

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

modes of mantle convection and the removal of heat from the earth s interior earth thermal histories based on two layer and whole mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle two layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection consequently two layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models whole mantle convection removes 3 times more primordial heat from the earth s interior and 6 times more heat from the core in 4 5 billion years than does two layer convection the average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium depleted chondrite thus no potassium is necessary in the core to balance the observed present day surface heat flux however most of the radioactives are concentrated in the upper mantle for two layer convection model geotherms for whole mantle convection compare well with independent estimates of temperature at various depths including the core mantle boundary while the lower mantle is too hot for two layer convection models thermal histories of the upper mantle in two layer models are similar to those of whole mantle models thermal histories for the lower mantle in two layer models are similar to those of the core in whole mantle convection models this is because the lower mantle is thermally insulated by the upper mantle if it convects separately just as the core is thermally insulated by the whole mantle the differences between whole and two layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle which is much larger in volume a whole mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented

growth of the continental crust a planetary mantle perspective abstract this paper addresses the net growth through time of the total continental crust cc not to be confused with sporadic growth of individual continental masses e g by accretion of island arcs a process that does not necessarily require net growth geochemists tend to emphasize differences between cc and oceanic crust oc but from a whole planet perspective cc and oc are remarkably similar in composition and density the cc oc distinction is canonically defined in terms of elevation however the earth s bimodal elevation pattern clearly reflects a genetic distinction the oc participates directly in mantle convection whereas the cc is the remaining relatively stable portion of the crust the earth s advective cooling mechanism requires massive subduction of lithosphere which due to the inherent buoyancy of crust tends to require a low crust total lithosphere thickness ratio given relatively constant lithosphere thickness except near thermally buoyant mid ocean ridges the earth responds by heaping most of its crust into a few thick continents allowing subduction to freely operate over a large area of relatively thin crust this crustal thickness bimodality engenders the bimodal elevation pattern prevailing wisdom tends to hold that the rate of cc growth has been slow and ongoing typical models show cc volume roughly proportional to 1 h where h is the total heat generation by the earth s k th and u however such models are generally vague as to the mechanism that supposedly holds back growth even after 4 5 ga the strong t dependence of mantle rheology probably ensured that the mantle took only a few million years to cool within a few hundred degrees of its modern t gradual compositional evolution of the cc from a plausible initial composition e g basaltic anorthositic might have slightly lowered the mean cc density but this effect is of second order importance in comparison to the general increase in crust total lithosphere thickness ratio for any given crustal thickness engendered by temporal cooling of the mantle intense cratering occurring coevally with early intense crust mantle recycling probably reconstituted virtually all of the cc extant prior to 3 9 ga but cc was probably not recycled into the mantle any faster than it was regenerated lunar samples and meteorites provide strong circumstantial evidence that in primordial times the outer few hundred kilometers of all terrestrial planets were largely molten outcomes of primordial magmatism must have been strongly dependent on the pressure regimes within the planetary interiors in general the far greater dt dp of silicate melting curves vs adiabats probably tended to foster a layered magmasphere with a partially molten or paracumulate layer constrained by dt dp relationships to a p range of roughly 3 gpa surmounted by fully molten magma ocean layer also limited in thickness by efficient convective cooling in the earth p stabilized garnet and al rich pyroxene were key factors limiting ultimate yield of crustal feldspar this effect has been examined with quantitative models of high p fractional crystallization including in some models periodic replenishment of the magma ocean from below results indicate that a plausibly deep 300 400 km magma ocean would produce a crust comparable in both composition and thickness to that of the modern earth mildly worrisome discrepancies involve cao for which high p pyroxene compositions are poorly constrained and sio 2 the crust probably evolved to its present sio 2 rich composition through differentiation catalyzed by h 2 o which presumably was not as abundant in the magma ocean as in the crust soon afterward nominal model results for mean crustal thickness are typically 1 2 1 3 the modern global mean but for this parameter the model is designed to give conservatively high results so the discrepancy is probably not significant after the chaotic first few million years of solar system history i suggest that planetary size and composition for more than time govern crustal volume

secular cooling of the earth as a source of intraplate stress abstract as a result of secular cooling and contraction of the earth an increasingly extensional horizontal stress is imposed on the central portions of the plates while the rate of increase of this stress is small in most situations it is enhanced for large plates and during any episodes of accelerated global cooling this source of stress may have contributed to the rifting and breakup of long lived supercontinental plates

a simple parameterized model of earth s thermal history with the transition from layered to whole mantle convection abstract recent studies of mantle convection suggest a possible scenario for earth s history in which the convection has changed its style from layered to whole mantle because of the existence of an endothermic phase boundary to understand the possible thermal consequence of this transition we shall re examine the thermal history of earth with different types of convection mode i e whole mantle convection layered mantle convection and convection with a transition using a simple parameterized scheme for the whole mantle convection model we shall confirm the results presented by previous workers i e a small of 0 2 with a high urey ratio 0 8 this is almost identical to the whole mantle case the implication is that because whole mantle convection is powerful enough earth may loose the memory of early layering that may have existed more than 1 ga b p the change in flow velocity associated with the transition is expected to be a factor of five or more the temperature of the upper mantle before the transition took place may be lower than the present mantle temperature

the vegetation and climate at the last glaciation on the emerged continental shelf of the south china sea studies on the dispersal mechanism and source areas of pollen from hemipelagic sediments recovered from the continental slopes of the south china sea scs reveal that vegetation existed on the exposed shelves at the last glacial maximum lgm and the latter part of the marine isotope stage 3 at the low sea level stand artemisiadominated grassland covered the northern continental shelf and tropical lowland rainforest and mangroves grew on the southern shelf sunda land consequently the climate in the northern scs must have been much colder and drier during the last glacial period compared to the present sunda land experienced only a marginally lower temperature but was not drier than today the enhanced contrast between the northern and southern parts of the scs in vegetation and climate during the lgm may be ascribed at least partly to the strengthening of winter monsoon during the last glacial period 2000 elsevier science b v all rights reserved

effect of a shallow low viscosity zone on small scale instabilities under the cooling oceanic plates we have studied the role of a low viscosity zone in the uppermost mantle on convective instabilities in the cooling oceanic plates the upper mantle is represented by three layers in a finite element model a conducting lid overlies a low viscosity zone which in turn overlies a layer of constant viscosity extending to the base of the upper mantle at 675 km in depth the viscosity contrast the rayleigh number the relative fluid layer thicknesses and the conducting lid thickness are varied if the rayleigh number of the low viscosity zone exceeds a critical value then convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the rayleigh number however the scale of the convection will eventually grow and extend throughout the upper mantle we compare our model to the small scale geoid and topography anomalies observed in the central pacific which are 50 80 cm in geoid and 250 m in topography 10 40 m y after plate formation and whose wavelength 150 500 km increases with age the magnitude early onset time and lifetime of the anomalies suggest a viscosity contrast of greater than 2 orders of magnitude the increase in wavelength with time also suggests a high rayleigh number 106 and the initial 150 250 km wavelength indicates a low viscosity zone thickness of 75 125 km small scale convection does not alter the slope of the depth age curve but does elevate the depth age curve by 250 m it is not until the onset of long wavelength convection that the depth age curve departs from age predicted by a cooling half space these results agree with the depth age behavior observed in the pacific where the depth age curve is elevated in the region of small scale anomalies and departs from age at 70 m y the models also predict heat flow values on old ocean floor which are higher than the plate model 50 55 mw m2 but which are in agreement with heat flow measurements in the atlantic and pacific