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

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

phase difference between sea level and magnetic reversal rate in this letter i report a negative correlation and a phase difference of 10 myr between recent data on long term eustatic sea level change1 and geomagnetic reversal rate for the past 150 myr the phase difference is detectable both by visual inspection and through a correlation analysis i also show that the same phase difference but with a positive correlation could theoretically have existed between this eustatic sea level data and sea floor creation rate in light of the ridge volume hypothesis of long term eustasy the analysis suggests the view that sea floor creation rate and or subduction rate and geomagnetic reversal rate are closely associated and perhaps synchronized in their changes

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

pulsation tectonics as the control of long term stratigraphic cycles a new theory of pulsation tectonics involving the cyclic eruption of thermal plumes from the d layer in the lowermost mantle explains the long term cycles of marine stratigraphy heating of the mantle changes conditions at the core mantle boundary that result in the quiescence of the earth s magnetic field the rising plumes after a phase lag later induce faster plate spreading and widespread intraplate volcanism subsequently colder rock returning to the lower mantle reestablishes the d thermal boundary layer and sets up conditions for more turbulent core convection and frequent magnetic reversals meanwhile lower mantle plumes cease erupting and the plates spread more slowly the cycles of fast and slow plate spreading and quiet and reversing magnetic field have a duration of 60 100 m y global changes in spreading rate are one of the main causes of large scale 100 300 m eustatic sea level changes and variations in output of volcanic gases consequential changes in albedo global temperature humidity forestation atmospheric and oceanic co2 oceanic acidity and oxygenation lead to variations in the preservation of organic carbon in the oceans and epicontinental seas and in the level of the calcite compensation depth during rising sea levels there is an increase in organic carbon preservation and relatively little calcite is preserved with falling sea levels less organic carbon and more calcite are preserved in the deep sea the long term cycles in carbon and calcite bearing sediments correspond generally to the pulsations of grabau 1934 the long term rhythms which nearly equal the lengths of geologic time periods

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

increased production of calcite and slower growth for the major sediment producing alga halimeda as the mg ca ratio of seawater is lowered to a calcite sea level abstract laboratory experiments were conducted to subject the calcareous alga halimeda a prolific producer of aragonite in the modern ocean to a variety of ambient mg ca ratios these experiments were based on three predictions 1 that halimeda would calcify at a lower rate when the ambient mg ca ratio was reduced from its high level in modern seawater 5 2 2 that this alga would grow more slowly under these conditions because its calcification yields as a byproduct co2 which is available for photosynthesis and 3 that the calcite content of halimeda s skeleton would increase as a percentage of total caco3 as predicted reduction of the mg ca ratio of ambient artificial seawater from its modern level to estimated early oligocene and early eocene levels 2 5 and 1 5 respectively resulted in progressively lower rates of calcification production of organic matter and linear growth for halimeda incrassata colonies the ambient mg ca ratio exerted a stronger influence over the rate of calcification than ca2 which in the geologic past has risen in seawater whenever the ambient mg ca ratio has declined thus although elevation of ca2 at such times would have promoted the calcification and growth of halimeda the concomitant decline of the mg ca ratio would have had a stronger negative effect in addition the percentage of skeletal material consisting of calcite rose from 8 in modern day seawater to 16 in artificial early oligocene seawater and 46 in artificial early eocene seawater magnesium was incorporated into this calcite as a function of the ambient mg ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite halimeda exerts less biological control over its biomineralization than the related genera penicillus and udotea which in previous experiments produced only 22 25 of skeletal caco3 as calcite in seawater having a very low mg ca ratio 1 0 nonetheless it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low mg caco3 as calcite in ancient seas such as those of the cretaceous whose mg ca ratios were close to unity

reply to the comment by s r gaffin on the correlation of geomagnetic reversals and mean sea level in the last 150 m y by w marzocchi et al

mechanism for geomagnetic polarity reversals recent studies of old magnetic observations1 and the seismic velocity of the lower mantle2 suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3 in particular a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern africa it has intensified throughout the twentieth century by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle and is drifting westwards a similar patch lies beneath south america here i show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal the almost linear increase in the frequency of polarity reversals that has occurred since the cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern africa

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

global reconstructions of cenozoic seafloor ages implications for bathymetry and sea level abstract although accurate estimates of cenozoic seafloor ages will serve to further our understanding of the relationship between mantle dynamics plate tectonics and a variety of surficial geological processes it is difficult to estimate ages of subducted seafloor however given the near constancy of surface velocities within a tectonic stage we can estimate cenozoic plate ages even for subducted lithosphere we reconstruct seafloor ages based on the cenozoic plate reconstructions and absolute rotation poles of gordon and jurdy r g gordon and d m jurdy cenozoic global plate motions j geophys res 91 1986 12389 12406 for the western pacific we explore alternative models based on the reconstructions of hall r hall cenozoic geological and plate tectonic evolution of se asia and the sw pacific computer based reconstructions model and animations j asian earth sci 20 2002 353 434 both reconstructions indicate an increase in average seafloor age since the early cenozoic resulting in an increase in the volume of ocean basins and a decreased sea level since the early cenozoic these trends are more pronounced for the gordon and jurdy r g gordon and d m jurdy cenozoic global plate motions j geophys res 91 1986 12 389 12 406 reconstruction because the hall r hall cenozoic geological and plate tectonic evolution of se asia and the sw pacific computer based reconstructions model and animations j asian earth sci 20 2002 353 434 reconstruction retains older seafloor in the western pacific which approximately halves the predicted sea level decrease since the early cenozoic 250 vs 125 m compared to geologic estimates of 150 m these changes in sea level occur despite decreases in oceanic lithosphere production rates of only about 20 in both models thus the changing distribution of seafloor age has a larger effect on sea level than changes in spreading rates or ridge lengths these reconstructions can also be used to estimate past heat flow the volume of subducted buoyancy and changes in the bathymetry of the cenozoic ocean basins

coupling of the core dynamo and mantle thermal or topographic static features in the earth s magnetic field and fixed sources of secular variation suggest the dynamo is anchored to the lowermost mantle via either thermal or topographic coupling here we show a correspondence with dynamic core topography which is related to mantle temperature and suggest further effects due to topography associated with subducting slabs the two coupling mechanisms may occur on different time scales providing a potential means for discriminating between them

phanerozoic shallow water diversity driven by changes in sea level shallow water 50 m marine diversity curves for the phanerozoic based on area changes due to sea level variation and continental movements are presented it is shown that a change in the degree of endemism can alter diversity more than the effect of sea level change or change in the number of faunal provinces the pattern produced for a constant degree of endemism shows some rapid but not periodic falls in diversity these occur in the middle cambrian at the end of the ordovician devonian permian cretaceous and eocene and in the messinian there is no recurring single cause for these falls

a deep seismic reflection profile over a caledonian granite in central england in 1980 a short deep reflection profile undertaken jointly by the university of leicester and the institute of geological sciences crossed a buried caledonian granite to the south west of melton mowbray in leicestershire little structure is seen in the section between the top of the granite at 0 25 s two way time stwt and a zone of very strong sub horizontal reflections at 6 s twt a further reflection zone is identified at 7 2s twt below which there are a series of strong but impersistent reflections to the full reording time of 12 s twt the data indicate that the granite must be relatively homogeneous and underlain by several marked sub horizontal boundaries at mid to lower crustal levels this suggests that during emplacement either the granite must have had little effect on the lower crust sampled by the profile or the lower crust was mobile enough to have developed layering since the granite was emplaced during the caledonian orogeny

hydrologic variability and the onset of modern el nino southern oscillation a 19 250 year record from lake elsinore southern california there are very few terrestrial palaeoclimate archives spanning the last glacial maximum through the holocene from coastal southern california yet knowledge of past climate dynamics is critical for assessing present and future constraints on the region s dwindling freshwater resources we present initial results from two drill cores extracted from the present day edge of lake elsinore southern california s largest natural lake using a multiproxy approach including lithologic description mass magnetic susceptibility loi 550 c total organic matter and loi 950 c total carbonate we infer first order long term climate change over the past 19 250 calendar years furthermore we suggest possible first order forcing mechanisms that drive this change which include presence absence of continental ice sheets insolation and complex ocean atmosphere interactions our results indicate four distinct millennial scale climate modes over the past 19 250 calendar years these modes include a wet last glacial maximum 19 250 17 120 cal yr bp a relatively dry late glacial holocene transition 17 120 9450 cal yr bp a wet early holocene 9450 7670 cal yr bp and a highly variable mid to late holocene climate i e alternating wet dry cycles 7670 cal yr bp present we attribute the mid to late holocene climate interval to the onset of el nino southern oscillation ca 7000 cal yr bp and a more vigorous hydrologic system these results are supported by a variety of regional terrestrial and marine palaeoclimate archives copyright 2005 john wiley sons ltd

anomalous cyclicity in climate and stratigraphy and modeling nonlinear oscillations for geological periods lacking evidence of extensive glaciation such as the cretaceous the presence of anomalously large globally synchronous cycles in coastal onlap stratigraphy haq et al 1987 may be a new example of a nonlinear geophysical response to weak external forcing a number of analogies can be drawn between this situation and the anomalously large 100 kyr late pleistocene ice volume cycle vis a vis weak 100 kyr milankovitch insolation changes apropos of these analogies we discuss formal models for evolving free sedimentary or glacial surfaces subject to lateral mass transport and subsidence and observe that nonlinear diffusivity obtains in each case in particular the systems both involve retarded transport we then define equilibrium surfaces and show them to be closely related to either the subsidence profile or the ablation profile in the absence of mass supply feedback these equilibrium surfaces should be unconditionally stable and amplified reponses to external forcing are unlikely we then consider feedbacks on mass supply and show first how to construct a three variable glacial wedge that freely oscillates glacial models of this type suggest that a fundamental role is played in the oscillation by isostatic relaxation times this effect should be considered for sedimentary systems as well we finish with a speculation about a simple sediment supply feedback that might be associated with relative sea level changes and illustrate graphically how the multiple equilibrium points introduced by this feedback could lead to large coastal onlap shifts from relatively small perturbations to the system much more work will be needed however to determine the parameter values required to achieve such behavior and if such values are physically realistic

the correlation of geomagnetic reversals and mean sea level in the last 150 m y abstract through an appropriate statistical method the correlation between the rate of occurrence of geomagnetic reversals and the mean sea level according to the best available data is analyzed a highly negative significant correlation significance level

the 13 million year cenozoic pulse of the earth abstract the geomagnetic polarity reversal rate changes radically from very low to extremely high such process indicates fundamental changes in the earth s core reorganization and core mantle boundary heat flow fluctuations however we still do not know how critical such changes are to surface geology and climate processes our analysis of the geomagnetic reversal frequency oxygen isotope record and tectonic plate subduction rate which are indicators of the changes in the heat flux at the core mantle boundary climate and plate tectonic activity shows that all these changes indicate similar rhythms on million years timescale in the cenozoic era occurring with the common fundamental periodicity of 13 myr during most of the time the periodicity is disrupted only during the last 20 myr such periodic behavior suggests that large scale climate and tectonic changes at the earth s surface are closely connected with the million year timescale cyclical reorganization of the earth s interior