early carboniferous carbonate systems an alternative to the cainozoic paradigm abstract an alternative model for carbonate deposition is offered for the early carboniferous the major features of this interval are not explained using late cainozoic analogues early carboniferous low latitude shallow seas were apparently strongly influenced by upwelling and high nutrient levels these conditions favoured highly carbonate productive offshore systems but also resulted in the formation of extensive abiogenic carbonates as muds and ooids if correct the carboniferous serves to illustrate the dangers in extrapolating from present day systems to the geological record

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

relative precipitation rates of aragonite and mg calcite from seawater temperature or carbonate ion control the temperature and degree of carbonate mineral supersaturation co3 2 ion concentration of seawater are the two most likely controlling variables on the compositions of recent marine carbonate cements the relative importance of these variables is difficult to assess in nature because they have similar trends with depth 0 1500 m and latitude in modern oceans we carried out laboratory experiments to investigate the relative growth rates of calcite mg calcite and aragonite in seawater as functions of both temperature 5 25 and 37 c and of carbonate ion concentration 2 5 to 15 times supersaturated with respect to calcite the precipitation rates of aragonite relative to those of calcite increase strongly with increasing temperature and are not affected greatly by changes in saturation state at 5 c calcite precipitation rates are nearly equivalent to those of aragonite regardless of the degree of saturation at both 25 and 37 c aragonite precipitation rates are much more rapid than those of calcite up to a factor of 4 except at very low saturation states calcite compositions vary from less than 5 mol mgco3 at 5 c to 14 mol mgco3 at 37 c our results suggest that the well documented shift toward precipitation of lower mol mg calcite and the decrease in abundance of aragonite cements with increasing oceanic depth and latitude can be attributed largely to lower temperatures

holocene carbonate sedimentation on the west eucla shelf great australian bight a shaved shelf the southern continental margin of australia is a cool water carbonate sedimentary province located in a high energy swell dominated oceanographic setting a vibrocore transect of 14c dated sediments across the centre of the eucla shelf is the first record of holocene shelf deposition in the great australian bight much of the seafloor shallower than 70 m water depth the base of wave abrasion is bare cenozoic limestone in some places encrusted by late pleistocene coral rich limestone that is cemented by high magnesium calcite 12 mole mgco3 the areally extensive 100 km wide hard bored substrate supports an epibiota of coralline algae minor bryozoans and soft algae or is covered by patches of holocene sediment up to 1 5 m thick generally a basal bivalve lag 3 ka overlain by quartzose bioclastic palimpsest sand this pattern of active carbonate production but little accretion on the wave swept mid to inner shelf is similar to that on other parts of the southern australian continental margin the term shaved shelf is proposed for this style of carbonate platform formed by alternating periods of sediment accretion cementation and erosion the palimpsest sand is typically rich in bivalves coralline algae and locally detrital dolomite outer shelf holocene sediment below the base of wave abrasion but inboard of the shelf edge is a metre thick unit of fine microbioclastic muddy sand with minor delicate bryozoans overlying a 9 13 ka rhodolith gravel some of this outer shelf sediment appears to have been resedimented the shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production the holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers cf marginopora and depleted in bryozoans as compared to coeval deposits on the lacepede and otway shelves off southeastern australia these differences are interpreted to reflect warmer waters of the leeuwin current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east

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

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

oceanographic and climatic changes over the past 160 000 years at deep sea drilling project site 594 off southeastern new zealand southwest pacific ocean high resolution textural carbonate microfossil and oxygen 18o and carbon 13c stable isotopic analyses are presented for the late quaternary isotopic stages 1 to 6 interval of a core at deep sea drilling project site 594 situated just south of the present subtropical convergence in northernmost subantarctic surface waters on the southern flank of chatham rise in the southwest pacific downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes respectively interglacial oozes contain a northern subantarctic assemblage of planktonic foraminifera with rare cool subtropical species while glacial oozes are characterized by species typical of southern most subantarctic waters and include radiolaria with affinity for antarctic waters the planktonic 18o record for the site supports a 3 6 c temperature change in near surface waters between interglacial and glacial stages and indicates that during stage 5e the near surface waters were about 1 c warmer than at present a pronounced cooling during stage 5d matches that of the vostok ice core d record and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when 18o results suggest a temperature drop in glacial stage bottom waters of 2 4 c possibly a result of upward displacement of antarctic intermediate water by circumpolar deep water at the site the foraminiferal 13c records support a reduced influence of north atlantic deep water in the southwest pacific during glacial stages when nutrient enhancement occurred in both bottom and surface waters despite the pronounced changes between interglacial and glacial conditions inferred at site 594 contributed to by a substantial northward shift by at least 5 of latitude in the position of the antarctic convergence south of new zealand during glacial episodes we conclude that the subtropical convergence remained locked to chatham rise approximately 44 s throughout stages 1 to 6 major compression of the intervening belt of subantarctic water during glacial episodes and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest pacific account for the harsh frigid environment reported for on land southern new zealand at these times

thermal core mantle interactions and time averaged paleomagnetic field recent studies of historical geomagnetic secular variation and lower mantle seismic velocity suggest the earth s magnetic field is controlled by lateral variations in mantle temperature in this paper the average expected magnetic field pattern is compared with the time averaged field obtained from paleomagnetic pole positions from the last 5 m y the historical record suggests that the time averaged inclination anomalies are negative far sided in the atlantic region north of 30 s and the west pacific and positive in the south atlantic extreme north and south pacific and east pacific the pole positions give positive inclination anomalies in the south atlantic and extreme north pacific and negative anomalies elsewhere in agreement with the theory except for the east pacific this suggests that the historical record is not long enough to give an accurate picture of long term secular change in the eastern pacific but that elsewhere the time averaged field of paleomagnetism is predicted satisfactorily the preponderance of far sidedness in the paleomagnetic record is a consequence of the present distribution of temperature at the base of the mantle rather than a fundamental property of the geodynamo and it may therefore change on the long time scale of mantle convection in the historical record declinations average to negative values in the atlantic hemisphere and positive values in the pacific which if typical of longer term behavior could explain the preponderance of positive declinations for reversed polarity which are mainly in the atlantic hemisphere

geomagnetic field intensity and reversal asymmetry in late precambrian keweenawan rocks summary fifty four palaeointensity values measured by the thellier technique are used to study the cause of reversal asymmetry in late precambrian keweenawan rocks these data obtained mainly from diabase dykes carrying primary trm and their baked host rocks show that reversed palaeointensities are on average 40 per cent higher than their normal counterparts this result cannot be explained by any overall difference in remanence characteristics grain size or cooling rate dyke width between the rocks comprising the two polarity groups a significant difference in the local intensity of the geomagnetic field between times of normal and reversed polarity is thus indicated this conclusion is reinforced by positive baked contact tests for both normal and reversed thellier palaeointensities secondary component and single dipole offset models as explanations for keweenawan reversal asymmetry are in conflict with the palaeointensity results the difference in palaeointensity between normal and reversed rocks disappears when the data are reduced to the palaeoequator in accordance with a geocentric axial dipole field the palaeointensity results are therefore consistent with the idea that reversal asymmetry is caused by the north american plate occupying a lower palaeolatitude when the field had normal polarity acceptance of a geocentric axial dipole would imply that the keweenawan geomagnetic field intensity was about 45 per cent higher than at present however evidence at one locality of possibly three asymmetric reversals poses problems for a plate movement interpretation and forces consideration of a model whereby the asymmetry is caused by persistent non dipole field components rock magnetic studies show that reliable palaeointensity estimates can be obtained for rocks which have stable nrm with single domain or pseudo single domain grain sizes and with a high koenigsberger ratio baked contact rocks are the most reliable whereas hydrothermally altered lavas are unsuitable for thellier intensity determinations it is suggested that af demagnetization studies on companion specimens should be performed before thellier experiments in order to select specimens which have high values of median destructive field and which lack viscous or other overprints

inner core anisotropy anomalies in the time averaged paleomagnetic field and polarity transition paths abstract the diffusion of the dynamo generated magnetic field into the electrically conducting inner core of the earth may provide an explanation for several problematic aspects of long term geomagnetic field behavior we present a simple model which illustrates how an induced magnetization in the inner core which changes on diffusive timescales can provide a biasing field which could produce the observed anomalies in the time averaged field and polarity reversals the earth s inner core exhibits an anisotropy in seismic velocities which can be explained by a preferred orientation of a polycrystalline aggregate of hexagonal close packed hcp iron an elastically anisotropic phase room temperature analogs of hcp iron also exhibit a strong anisotropy of magnetic susceptibility ranging from 15 to 40 anisotropy at inner core conditions the magnetic susceptibility of hcp iron is estimated to be between 10 4 and 10 3 si we speculate here that the anisotropy in magnetic susceptibility in the inner core could produce the observed anomalies in the time averaged paleomagnetic field polarity asymmetry and recurring transitional virtual geomagnetic pole vgp positions

oxygen and carbon isotope fields for temperate shelf carbonates from tasmania and new zealand abstract temperate latitude bryomol carbonates mantle significant areas of the modern tasmanian and new zealand shelves and bounding basins the isotope fields for the temperate skeletons and whole sediments are distinctly separated from their warm shallow marine counterparts because of their heavier 18o and narrow range of 13c values however the temperae isotope fields overlap with the deep sea carbonate isotope field and the seafloor diagenesis isotope trend linepasses through these fields tasmanian whole sediments and skeletal isotope fields are similar and are closely clustered along the diagenesis trend line because of intragranular cementation the new zealand skeletal isotope field is broader and scateerd possibly due to lack of cementation and some biochemical fractionation but its whole sediment isotope field is tighter and contained within the tasmanian whole sediment field in contrast to warm water carbonates the enrichement of 18o is mall

marine cements in mid tertiary cool water shelf limestones of new zealand and southern australia large areas of southern australia and new zealand are covered by mid tertiary limestones formed in cool water shelf environments the generally destructive character of sea floor diagenesis in such settings precludes ubiquitous inorganic precipitation of carbonates yet these limestones include occasional units with marine cements 1 within rare in situ biomounds 2 within some stacked cross bedded sand bodies 3 at the top of metre scale subtidal carbonate cycles and 4 most commonly associated with certain unconformities the marine cements are dominated by isopachous rinds of fibrous to bladed spar interstitial homogeneous micrite and interstitial micropeloidal micrite often precipitated sequentially in that order internal sedimentation of microbioclastic micrite may occur at any stage the paradox of marine cemented limestone units in an overall destructive cool water diagenetic regime may be explained by the precipitation of cement as intermediate mg calcite from marine waters undersaturated with respect to aragonite in some of the marine cemented limestones aragonite biomoulds may include marine cement sediment internally suggesting that dissolution of aragonite can at times be wholly marine and not always involve meteoric influences we suggest that marine cementation occurred preferentially but not exclusively during periods of relatively lowered sea level probably glacio eustatically driven in the mid tertiary at times of reduced sea level there was a relative increase in both the temperature and the carbonate saturation state of the shelf waters and the locus of carbonate sedimentation shifted towards formerly deeper shelf sites which now experienced increased swell wave and or tidal energy levels fostering sediment abrasion and reworking reduced sedimentation rates and freer exchange of sediment pore waters energy levels were probably also enhanced by increased upwelling of cold deep waters onto the southern ocean margins of the australasian carbonate platforms where water mass mixing warming and loss of co2 locally maintained critical levels of carbonate saturation for sea floor cement precipitation and promoted the phosphate glauconite mineralization associated with some of the marine cemented limestone units

platform drowning leading to cool water carbonate deposition evolution of a late ordovician turinian chatfieldian mixed sediment platform within the taconic orogen long point group newfoundland appalachians late ordovician turinian chatfieldian drowning of a mixed carbonate siliciclastic platform within the taconic orogen newfoundland appalachians is recorded by net deepening of an initial warm shallow water platform succession lourdes formation culminating in a metre scale thick condensed interval that characterizes a drowning succession punctuated by storm deposits composition of transported material suggests that seaward drowning was coupled with back stepping of a high energy carbonate factory related to hinterland uplift and erosion that would eventually lead to drowning of the outer platform beneath marine transported siliciclastic sediments winterhouse formation in the new offshore shelf setting a sparse reciprocal stratigraphy of fine to very coarse grained phosphatic carbonate and mixed sediment is interpreted to document gravity flow deposition downgradient from either a sustained or episodically developed high energy cool water carbonate source along the inner shelf transported carbo

correlating magnetic field polarity changes with geologic phenomena two reported correlations are discussed in this paper the first claims a correlation between magnetic polarity and oxidation state of igneous rocks and the second claims that the hawaiian hot spot chain exhibits a significantly dominant normal polarity bias on the basis of mechanistic arguments and examination of the sample distribution the first correlation is shown to be due to a data artifact although the second correlation is supported by magnetic modeling of seamounts given in this paper it too may be due to a data artifact the most likely alternative explanation is that it originates from a bias imparted by viscous remanent magnetization vrm in the brunhes chron the role of vrm may be far more important in determining large scale magnetic anomalies than is generally believed

burial dominated cementation in non tropical carbonates of the oligocene te kuiti group new zealand abstract cementation of bryozoan echinoid benthic foraminiferal temperate shelf carbonates of the oligocene te kuiti group north island new zealand occurred mainly during subsurface burial the calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan given an iron supply and compared to the skeletal material with normal marine 18o values from 2 to 1 more depleted in 18o with depth of burial the 18o composition of bulk cement samples ranging from 1 to 7 these trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures 20 50 c respectively the 13c values 0 to 3 of the cements remain the same as the host marine shells suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution two gradational burial diagenetic environments influenced by marine derived porewaters are arbitrarily distinguished shallow burial phase and moderate burial phase during the shallow burial phase down to 500 600 m sub bottom depth the carbonates lost at least 25 of their original porosity by mechanical compaction and were selectively cemented by non ferroan or usually ferroan variably luminescent slightly 18o depleted sparry calcite cement 18o 2 to 4 mainly as syntaxial rims about echinoid grains these shallow burial cements form less than about 10 of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction during the moderate burial phase between about 600 and 1100 m sub bottom depth porosity loss continued typically to about 70 of its original value as a result of pressure solution of calcitic bioclasts associated with more advanced stages of chemical compaction this involved development of a wide variety of non sutured and microstylolitic solution seams including both single and composite wispy or continuous bedding parallel types and non parallel reticulate forms the released carbonate was precipitated as ferroan or non ferroan where iron supply was negligible dull luminescent strongly 18o depleted 18o 4 to 7 mainly equant calcite spar cement occluding available pore space in the limestones

the character of the field during geomagnetic reversals it is well established that the geomagnetic field which is predominantly dipolar at the earth s surface has alternated between normal and reverse polarity through geologic time figure 1 evidence of this behavior is preserved in the ancient magnetism of rocks the remanence of radio metrically dated lava flows for example shows that the field has reversed at least 23 times over the past 5 million years figure 2 reversals are not periodic it is apparent in figure 2 that lengths of recent polarity intervals range from lo4 to lo6 years and that the intervals follow a very irregular pattern this behavior contrasts sharply with that of the sun s field which switches polarity every 1 1 years paleomagnetic observations discussed in more detail below show that a geomagnetic reversal takes thousands of years to complete what happens to the geomagnetic field during the transitional period does the entire field just rotate from one orientation to the other does the field disappear

computer synthesis of geomagnetic palaeosecular variations computer models designed to synthesize palaeosecular variations of the geomagnetic field cast doubt on some widely accepted palaeomagnetic dogmas westward or eastward drift cannot be uniquely deduced from the sense of rotation of the geomagnetic vector the use of unit vectors in statistical analyses leads to systematic errors which render investigation of the global symmetry of the palaeofield more difficult than previously supposed at least some geomagnetic excursions may be regarded as large amplitude secular variations

early diagenesis of carbonates on a cool water carbonate shelf southern australia abstract the southern australian continental margin is an immense cool water carbonate factory the high energy open shelf is blanketed by heterozoan carbonates of both holocene and late pleistocene age three distinct grain types have been identified and radiocarbon dated these include 1 relict grains highly abraded iron stained intraclasts formed during the intermediate sea level stands of marine isotope stages 3 and 4 2 stranded grains gray and buff colored abraded biofragments and intraclasts marooned during the sea level rise associated with marine isotope stage 2 and 3 holocene grains unaltered biofragments formed during marine isotope stage 1 optical mineralogical and chemical characterization of grains of each age has revealed pathways of early diagenesis in this realm both calcitic and aragonite biogenic grains undergo dissolution while dissolution of calcitic components is minor and incomplete relict sediments are predominantly mg calcite aragonitic skeletons undergo significant dissolution within 30 000 years in contrast to early dissolution of aragonite micritic calcite cement with 12 mol mgco3 is a significant diagenetic phase in these sediments infilling many stranded and most relict skeletons similar cement precipitates between grains forming small numbers of cemented grain aggregates in most samples microprobe and icp ms analysis demonstrates fe oxides mn oxides and associated clay minerals discolor stranded and relict grains by coating grain surfaces and infesting micropores such coatings begin to form on holocene grains during or soon after formation the oxides scavenge trace elements including v ti cr and ree altering the elemental compositions of biogenic carbonate

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

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

diagenetic alterations in temperate shelf carbonates from southeastern australia abstract temperate shelf carbonates form in cool marine waters and have skeletal and mineralogical compositions which are different from their tropical counterparts they commonly lack non skeletal grains and are often composed of low and high magnesium calcite with subordinate aragonite many of the aragonitic components found in tropical carbonates such as corals ooids blue green algae and lime mud are absent temperate shelf carbonates undergo diagenesis in marine waters with lower carbonate saturation than do tropical carbonates and are exposed to cool climates with moderate to low rainfall marine cementation is rare because of low carbonate saturations in the surrounding waters however aragonite and high magnesium calcite cements have been reported forming under specialized conditions associated with biogenic precipitation submarine methane and sulphate reducing bacteria and more commonly in the intertidal environment where evaporation has increased carbonate concentrations in pleistocene and tertiary temperate shelf carbonates from southeastern australia evidence of marine diagenesis is rare to absent diagenetic stabilization of aragonite and high magnesium calcite has taken from 80 000 y to 1 my or longer during subaerial exposure this is slower than rates reported from tropical climates a general lack of aragonite in some facies within these temperate carbonates leads to a lack of secondary porosity and only sparse low magnesium calcite cement even after prolonged fresh water diagenesis however with lengthy exposure and under the right climatic conditions karstic solution and calcrete precipitation can occur in sequences containing siliciclastic clays pyrite and glauconite abundant iron is present in interstitial waters leading to the precipitation of ferroan calcite cements in the phreatic and shallow burial environments and to the substitution of iron for magnesium in stabilizing high magnesium calcite skeletal material a unique void filling micritic internal sediment occurs in discrete layers in many of the tertiary temperate shelf carbonate sequences in southeastern australia this internal sediment is localized as a pore filling material above permeability barriers such as fine grained sediments or volcanics and above paleo water tables which formed during periods of subaerial exposure it is a feature of the vadose zone and lithifies to form a dense micritic low magnesium calcite cement with characteristic pink brown coloration often associated with erosion surfaces and nodule beds dolomite is uncommon in the southeastern australian temperate shelf carbonates it forms associated with preferential fluid pathways or mixing zones ferroan dolomite forms in siliciclastic clay rich carbonates in the shallow burial environment the ubiquitous fine evaporite related dolomite so common in tropical carbonates is absent

submarine cementation and subaerial exposure in oligo miocene temperate carbonates torquay basin australia abstract at least four hardgrounds are present throughout the 27 m thick exposed oligo miocene cool water point addis limestone of the torquay basin in southeastern australia they range from thin a few centimeters thick incipient layers that have a nodular fabric to thick up to 1 m thick continuous hardgrounds the hardgrounds tend to be at the top of coarsening upward cycles and are typically overlain by low relief erosion surfaces the uppermost hardground is the best developed and most widespread in the point addis limestone and can be observed in all exposures of the succession at some localities it shows evidence of karstification such as dissolution pits clints and grikes and dissolution pinnacles whether karstified or not well developed hardgrounds are overlain by a lag c nglomerate that consists of hardground intraclasts with marine fe oxide crusts the hardgrounds are cemented by a first generation isopachous inclusion rich columnar and fibrous radiaxial calcite of marine origin with trace element compositions mg2 0 68 1 74 mole mgco3 fe2 800 4690 ppm sr2 0 260 ppm mn2 0 230 ppm and cathodoluminescence dull blotchy indicative of stabilized mg calcite the isopachous cements are invariably overlain by homogeneous peloidal or microbioclastic micrite having trace element compositions mg2 0 77 1 54 mole mgco3 fe2 760 10 030 ppm sr2 0 240 ppm mn2 60 340 ppm and cathodoluminescence dull blotchy again indicative of stabilized mg calcite this micrite is always closely associated wi h the isopachous cements and appears to be of marine origin perhaps being analogous to the micritic precipitates described from reefal settings clear calcite is the last cement generation in the hardgrounds and has attributes of meteoric cements nonluminescent mg2 0 12 0 87 mole mgco3 fe2 0 230 ppm sr2 and mn2 below detection limits the whole rock stable isotope compositions of the hardgrounds and host limestone indicate that the whole unit has been subjected to pervasive alteration by meteoric fluids the least altered carbonates analyzed from the point addis limestone are brachiopods d13c 1 5 to 2 2 pdb d18o 1 7 to 0 9 pdb and the unkarstified hardgrounds d13c 3 8 to 1 0 pdb d18o 0 6 to 0 6 pdb these hardground occurrences help promote the validity of a unique sea level driven model for the formation of hardgrounds in cool water settings we propose that the development of these hardgrounds was an entirely marine process produced by relative sea level drop and entry of the sea floor into the zone of wave reworking marine cementation may begin in the form of nodules at and below the seawater sediment interface and involves both slow sedimentation and shallowing of the cool waters the nodules can later merge to form continuous hardgrounds in such a high energy environment nondeposition and erosion are the dominant processes and marine cementation can occur further sea level drop would lead to subaerial exposure of the previously formed hardgrounds as found in one inst nce in the point addis limestone at coastal exposures

anatomy of a submarine channel system and related fan in a foramol rhodalgal carbonate sedimentary setting a case history from the miocene syn rift sardinia basin italy abstract during aquitanian burdigalian times thick mixed carbonate siliciclastic successions were deposited in basins located on the grabens and half grabens along the oligo miocene sardinia rift basin locally active tectonics sea level variations and ecological factors combined to control the development and distribution of foramol carbonate factories as well as the remobilisation and the redeposition of carbonate sediments into the adjacent deeper areas in the isili basin foramol rhodalgal carbonate factories developed on submerged structural highs which resulted from pre and syn sedimentary tectonics these carbonate factories were periodically shaved mainly during negative sea level oscillations and the sediments removed were funnelled towards the basin through a complex submarine channel network which included a tributary belt one main channel isili channel and the related fan the isili channel is up to 1 km wide 60 100 m deep and includes two stacked channel complexes each built up by several minor order channel units complex strata geometries characterise the isili channel and its related architectural elements e g overbank levee margin and channel thalweg which also include up to 15 m high bedforms individual channel complexes were temporally related to individual fan systems whose spatial distribution and internal geometry were strongly controlled by the type and rate of sediment accumulation and in turn by relative sea level oscillations facies associations include sandy to cobble sized gravity flow and bottom current deposits as well as megabreccias characterised by impressive displaced and tilted blocks which resulted from major channel margin collapses detailed analysis has led to the reconstruction of the internal geometry and depositional architecture of these carbonate bodies and to the determination of the main controlling factors the dimension and distribution of channel and channel related depositional bodies have been accurately determined this information provides a useful tool to analyse less extensively exposed analogues and to model foramol shelf to basin transitions and related channel and fan systems

high resolution 104 years deep sea foraminiferal stable isotope records of the eocene oligocene climate transition we have constructed high resolution 104 105 years benthic foraminiferal 13c and 18o records for the upper eocene through lower oligocene of two pelagic sequences deep sea drilling project dsdp site 522 in the angola basin south atlantic ocean and ocean drilling program odp site 744 in the southern indian ocean these records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 ma the oxygen isotope records indicate that the ubiquitous 18o increase oi 1 which marks the rapid expansion of continental ice sheets and a minimum of 3 to 4 c of cooling of bottom waters in the earliest oligocene 33 6 ma occurred in 350 kyr more than half the transition occurred over the final 40 50 kyr this period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr i e the duration of magnetochron c13n these records also indicate that this interval was characterized by at least two 100 kyr waxing and waning cycles oi 1a and oi 1b and possibly several higher frequency events the benthic foraminiferal 13c records show a positive 0 8 excursion that is nearly isochronous with the oi 1 oxygen isotope increase similar magnitude 13c increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle this excursion was followed by smaller amplitude 13c oscillations with periods of roughly 400 kyr we suspect that the ubiquitous oi 1 13c excursion resulted from a brief but substantial increase in export production and carbon burial

shallow paleomagnetic directions from historic lava flows hawaii a study designed to examine the basic theory of thermoremanent magnetization acquisition parallel to the earth s ambient magnetic field upon cooling and to evaluate intra flow variations of very young basalt flows has yielded some startling initial results two extensively sampled flows the 1950 flow of mauna loa and the 1972 flow from mauna ulu kilauea have been found to have paleomagnetic directions statistically different from the present geomagnetic field direction pfd in hawaii the 1950 flow shows very consistent but shallow directions throughout the flow with a mean inclination 6 shallower than the pfd directions from the 1972 flow vary from site to site both along the length of the unit and within vertical sections relative between site consistency is low with all sites but one having paleomagnetic directions distinct from the pfd however the mean inclination for this flow 33 4 is only about 3 shallower than expected the source of such anomalous behavior in the recording of ambient field directions is yet unknown although several possible causes are examined

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

cyclicity in pleistocene upper slope cool water carbonates unravelling sedimentary dynamics in deep water sediments great australian bight odp leg 182 site 1131a abstract ocean drilling program site 1131 located in the central great australian bight recovered an expanded series of pleistocene cool water carbonates three distinct facies are arranged in repeating sedimentary cycles omission intervals form the base of the cycles and are overlain by laminated deposits the upper part of the cycles consists of bioturbated sediments facies cyclicity parallels glacial interglacial sea level changes firmground hardground surfaces formed during latest stages of sea level falls laminated sediments were deposited during later sea level fall and or early phases of the sea level lowstand and the bioturbated intervals are related to interglacial sea level highstands bottom current activity is interpreted to play an important control on facies and cycle development during the late stages of sea level fall upwelling currents winnowed the slope sediments leading to hardground formation during latest sea level falls and early sea level lowstands a relative increase in the ramp sediment supply to the slope leading to the formation of the laminated facies was also a response to the continued action of the near bottom upwelling currents that reworked previously deposited sediments along the slope during the early sea level rises laminated upwelling influenced facies were progressively covered by the bioturbated facies the adopted approach to decipher sedimentary cyclicity involving the integrated use of wireline logs geochemical data fms images and sedimentological analysis of the lithofacies has allowed to recognise subtle facies changes in upper slope settings of a distally steepened carbonate ramp and their relationship with the sea level as well as to unravel the importance of the oceanic current regime in the deposition style on the ramp that otherwise would remain unsolved this integrated method could be applied to study more precisely apparently homogeneous fine grained calcareous sediment successions of distal ramp outcrops

production and accumulation of calcium carbonate in the ocean budget of a nonsteady state present day production of caco3 in tne world ocean is calculated to be about 5 billion tons bt per year of which about 3 bt accumulate in sediments the other 40 is dissolved nearly half of the carbonate sediment accumulates on reefs banks and tropical shelves and consists largely of metastable aragonite and magnesian calcite deep sea carbonates predominantly calcitic coccoliths and planktonic foraminifera have orders of magnitude lower productivity and accumulation rates than shallow water carbonates but they cover orders of magnitude larger basin area twice as much calcium is removed from the oceans by present day carbonate accumulation as is estimated to be brought in by rivers and hydrothermal activity 1 6 bt suggesting that outputs have been overestimated or inputs underestimated that one or more other inputs have not been identified and or that the oceans are not presently in steady state one missing calcium source might be groundwater although its present day input is probably much smaller than that of rivers if as seems likely caco3 accumulation presently exceeds terrestial and hydrothermal input this imbalance presumably is offset by decreased accumulation and increased input during lowered sea level shallow water accumulation decreases by an order of magnitude with a 100 m drop in sea level while groundwater influx increases because of heightened piezometric head and the diagenesis of metastable aragonite and magnesian calcite from subaerially exposed shallow water carbonates

the paleomagnetic field from equatorial deep sea sediments axial symmetry and polarity asymmetry paleomagnetic data from 89 equatorial deep sea sediment cores indicate that the configuration of the time averaged geomagnetic field depends strongly on polarity state but that it remains within 1 degree of axial symmery throughout the pliocene and pleistocene last 5 million years the relative magnitude of the nondipole field was greater by almost a factor of 2 during reverse than during normal polarity intervals these results thus support earlier suggestions that there may be a standing nonreversing component of the geomagnetic dynamo

paleomagnetic tomography of the core mantle boundary abstract analysis of updated global paleomagnetic data indicates that over the last 5 million years the normal and reverse polarity states are discernibly different this difference which can be characterized by a higher ratio of the axial quadrupole to dipole field for the reverse polarity relative to the normal polarity is attributed to lateral variations in temperature or composition at or near the core mantle boundary although several mechanisms are considered thermal electric effects associated with core mantle topography and or with chemical variations in d the seismic layer approximately 150 km thick at the base of the mantle are judged to be the most likely sources of the observed polarity asymmetry this asymmetry should not necessarily be expected to give rise to systematic trends in polarity transitions

carbonate cement fabrics displayed a traverse across the margin of the bahamas platform near lee stocking island in the exuma cays abstract consolidated to friable carbonate rocks found in the lee stocking island area in the exuma cays include 1 reef rock 2 channel stromatolites 3 shallow water hardgrounds 4 beachrock rimming the islands and 5 pleistocene bedrock the most common cement fabrics observed are aragonitic fibers which include acicular fan druse and square tipped coarse fibers cementing beachrock and stromatolites and an isopachous needle fiber rim cementing hardgrounds and stromatolites less common are high mg calcite bladed textures of the reef rock and stromatolites two types of blades are present the more common stubby variety of either high mg or low mg calcite and an elongated variety of high mg calcite which was found in only three beachrock samples aragonitic micrite envelopes usually surround grains in beachrock hardgrounds and stromatolites but only in association with fibrous cement an aragonitic crust cements the surfaces of lime mud beds of the tidal channel while a high mg calcite cryptocrystalline cement occurs in all the rock types calcified algal filaments of high mg calcite from the abundant green and blue green algae in the area are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock a low mg calcite equant spar cements the pleistocene samples and is associated with meteoric diagenesis and cementation of the pleistocene surface cement precipitation coincides with the path of the cool oceanic water from exuma sound as it warms and loses co 2 and moves up onto the bank near lee stocking with the incoming tide cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward fibrous cements become predominant as suggested by their crystal size and location on the shelf margin we think that the reef rock cryptocrystalline material are the fastest forming of the cements where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming the rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate on the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds but these may form at a slower rate than that of the margin though this rate is still quite high cementation gradients occur from the tidal channel to the intertidal zones of 1 west norman s pond cay where cement fabric suggests a reduced calcium carbonate availability and 2 west lee stocking island where a change in mineralogy suggests a change in water chemistry thus a sequence of cement fabrics and mineralogies can be traced micritic textures occur in a more seaward position fine fibrous aragonite fibers in a more lagoonal and levee position and coarser aragonite fibers and mg calcite cements in the intertidal and supratidal position this sequence is thought to track the evolution of the water mass

carbonate deposition during a time of mid latitude ocean cooling early permian subtropical sedimentation in the sverdrup basin arctic canada abstract carbonate rocks of the lower permian sakmarian to kungurian raanes and great bear cape formations accumulated in the sverdrup basin during a critical transition period when ocean waters cooled dramatically these rocks have many attributes similar to sediments deposited in modern subtropical zones the biota is dominated by a heterozoan assemblage of pelmatozoans bryozoans and brachiopods with the addition of photozoan elements such as fusulinids and colonial corals associated with the change in the sverdrup basin from a tropical to subtropical environment is the first occurrence of glauconite iron oxides and storm deposits the shift in depositional conditions is interpreted to reflect decreased seawater temperature that was brought about by a northward migration to a mid latitude position frequented by storms oceanographic change however was not unidirectional but was complicated by concomitant variations in sea level and oceanography recorded as three distinct phases determined by water depth and temperature 1 rocks of the raanes and lower great bear cape are deep water and heterozoan 2 the middle great bear cape records shallow water subtropical conditions with fusulinids and large colonial corals 3 the upper great bear cape is entirely heterozoan in all locations and by implication all depths all post raanes and great bear cape rocks of the sverdrup basin are entirely heterozoan taking caution for possible evolutionary changes subtropical carbonates can be recognized in rocks much older than previously believed the most important components for identifying subtropical carbonates of the past are large benthic foraminifers in an otherwise heterozoan assemblage and as such the ability to delineate this carbonate realm in the past is currently restricted to times when large benthic foraminifers grew in shallow marine environments

microstratigraphy of calcite cements in pliocene cool water limestones new zealand relationship to sea level burial and exhumation events

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

paleomagnetic study of late miocene through pleistocene igneous rocks from the southwestern usa results from the historic collections of the u s geological survey menlo park laboratory 1 seventy sites from the southwestern united states provide paleomagnetic results that meet certain minimum criteria and can be considered for the time averaged field initiative tafi the virtual geomagnetic poles for these 70 units are circularly distributed and their mean is nearly coincident with the rotational axis when other published data for the southwestern united states are included n 146 the virtual geomagnetic poles are again circularly distributed but their mean is significantly displaced from the rotational axis whichever of these data sets is used the mean poles for normal and reversed polarity data differ by 170 and are not antipodal at greater than 95 confidence when the data are separated into specific age groups the 95 confidence limits about the mean poles for the brunhes matuyama combined gauss gilbert and late miocene intervals all include the rotational axis angular dispersion about these four mean poles increases systematically with increasing age and is consistent with paleosecular variation model g

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

inclination anomalies from indian ocean sediments and the possibility of a standing nondipole field we report magnetic inclinations measured in deep sea sediments of the equatorial indian ocean which record the behavior of a nondipole component of the time averaged geomagnetic field during the plio pleistocene 0 5 ma the magnitude of the nondipole effect recorded in these sediments appears to depend on polarity state with inclinations showing departures from geocentric axial dipole directions which are small 2 o during normal polarity and larger 5 o during reverse polarity times the overall nondipole effect observed here is consistent with prior spherical harmonic estimates of the paleomagnetic field the polarity bias found agrees in both sense and magnitude with earlier reports of polarity asymmetry in the low degree zonal harmonic fields the presence of this asymmetry supports previous suggestions of the existence of a standing component of the nondipole field which does not invert during reversals of the main field we explore whether the standing field so indicated may have influenced paleomagnetic directions recorded during polarity transitions at other equatorial sites

cross stratified calcarenites from new zealand subaqueous dunes in a cool water oligo miocene seaway cross bedded cool water bioclastic limestones of the te kuiti group on the north island of new zealand are composed primarily of bryozoans echinoderms and benthic foraminifers their prominent large scale unidirectional cross stratification is interpreted as produced by migrating subaqueous dunes on the floor of a 50 100 km wide north east trending seaway in water depths of 40 60 m these dunes are thought to have developed in response to strong seaway parallel tidal currents combined with a north east directed set up or oceanic current cross stratification is organized into four hierarchical levels 1 cross lamination 2 first order sets 3 second order sets and 4 cross stratified successions the levels are based on increasing degrees of internal complexity distinct attributes such as internal organization cross set thickness foreset shape and lower bounding surface shape are used to describe and interpret the cross stratification all these attributes are here integrated in a new and expanded classification of unidirectional cross stratification that emphasizes flow and bedform dynamics rather than overall set shape individual cross stratified successions are interpreted to have formed by dunes with varying sinuosity superposition and flow history under conditions of different current strength but constant sediment production horizontally bedded successions are the result of robust active dune fields that grew during times of vigorous sediment transport formset successions were produced from large compound dunes and are the expression of languid and decaying dune fields that developed during times of decreasing sediment transport these decaying dunes were gradually smothered by continuously and locally produced bioclastic sediment formset cross stratified successions are most likely to develop in carbonates where the sediment is produced in place than in terrigenous clastics where the sediment is imported

the time averaged paleomagnetic field 0 5 ma persistent departures from the geocentric axial dipole field model of the time averaged paleomagnetic field over the past 5 myr have been analyzed using oceanic data from deep sea cores and continental data from igneous rocks and sediments the data set comprises the equivalent of 9490 spot readings of the field 5831 normal and 3659 reverse from 930 groups of data variations in declination anomalies between groups of data strongly suggest the existence of second order tectonic effects small rotations that make it difficult if not impossible to identify any nonzonal terms that might exist inclination anomalies have therefore been modeled in terms of low degree zonal harmonics the calculated inclination anomalies within 10 latitude bands are widely scattered for both oceanic and continental data and do not appear to have been drawn from distributions with common means we attribute this large scatter to previously undetected or uncorrected second order tectonic effects in the continental data or in the oceanic data to small departures from the vertical in deep sea cores if it is assumed that these effects are random between groups of data and without a systematic bias they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band this method also has the major advantage that it maximizes time averaging of the field when this is done an acceptable fit to the inclination anomaly data is found using a zonal harmonic model although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term the difference is not significant and is likely due to contamination effects this finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments this conclusion differs from all previous analyses the major differences occur in model estimates of the zonal octupole term the estimates varying widely depending on the particular data combination we show that false octupole terms can be introduced by several factors including inclination errors in sediments and lavas the use of unit vectors in the analyses and incomplete magnetic cleaning particularly of reversely magnetized rocks thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term we suggest that the best estimates for the zonal quadrupole g2 g20 g10 are obtained from the combined brunhes age igneous rocks and oceanic normal data 0 033 0 019 and for the combined matuyama age igneous rocks and oceanic reverse data 0 042 0 022 when these two sets are combined the overall best estimate of g2 is 0 038 0 012

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

late eocene early oligocene integrated isotope stratigraphy and biostratigraphy for paleoshelf sequences in southern australia paleoceanographic implications abstract a late eocene early oligocene planktonic foraminiferal oxygen isotope stratigraphy derived for two overlapping sections browns creek and castle cove through paleoshelf sequences in southern victoria australia contains a pronounced late eocene depletion warm peak 1 2 pdb centred at about 41 ma followed by a two stage enrichment cooling pattern the later one coinciding with the well known terminal eocene event 37 ma exhibiting 1 0 pdb erinchment the early oligocene isotope stratigraphy shows fluctuations of about 1 0 pdb with prominent depletion peak immediately following the terminal eocene 18 o enrichment the start of the terminal eocene 18 o enrichment coincides with the lad of the nannofossil indicator of the eo oligocene boundary discoaster saipanensis early np21 zone on this basis the traditional southern australia planktonic foraminiferal eocene oligocene boudary markers lad globigerapsis index lad subbotina linaperta subbotina linaperta locally survived into zone np22 well into the early oligocene sea surface paleotemperature estimates derived from the 18 o data assuming a w value of 1 2 pdb show that the late eocene ingression of the tropical planktonic foraminiferal genus hankenina coincided with peak surface water tempertures of 24 c the temperature decline in the southern australia shelf waters associated with the terminal eocene event may have been as much as 7 c reaching a minimum value of 13 c the later survival of g index and s linaperta coincides with a rapid recovery of water temperatures over the south australian shelf following the terminal eocene event this is attributed to the local warming of waters in shallow broad embayments where circulation patterns were decoupled from the developing circum antartic current to the south

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

long term accumulation of carbonate shells reflects a 100 fold drop in loss rate shells in modern seabeds can be thousands of years old far older than would be extrapolated from the rapid rates of shell loss detected in short term experiments an extensive shelldating program on the southern california usa shelf permits rigorous modeling of the dynamics of shell loss in the mixed layer discriminating the key rates of carbonate disintegration and sequestration for the first time we find that bivalve shells experience an initially high disintegration rate l 1 decadal half lives but shift abruptly within the first 500 yr postmortem to a 100 fold lower disintegration rate l 2 millennial half lives at sequestration rate t burial and or diagenetic stabilization this drop permits accrual of a long tail of very old shells even when sequestration is very slow and allows only a minority 1 of all shells to survive the first phase these high rates of disintegration and low rates of sequestration are consistent with independent measures of high carbonate loss and slow sedimentation on this shelf our two phase model thus reveals significant spatial and temporal partitioning of car bonate loss rates within the mixed layer and shows how shell age frequency distributions can yield rigorous and realistic estimates of carbonate recycling on geological time scales

dissolution behaviour of bryozoan sediments taphonomic implications for nontropical shelf carbonates abstract diagenesis of carbonate sediments on temperate marine shelves is commonly destructive in contrast to a dominantly constructive regime in tropical waters in some cases dissolution can be important in temperate shelf waters particularly affecting the metastable carbonate minerals if dissolution does occur it can be an important taphonomic process reducing skeletal remains available for fossilization and biasing fossil assemblages the dissolution behaviour of bryozoans has been investigated in an acid bath laboratory experiment using nontropical carbonate sediments from the new zealand shelf bryozoans of various shapes growth forms and compositions low to high mg calcite and aragonite were extracted and their weight loss over time was measured results show that variations in stability of carbonate mineralogies are insufficient to predict relative dissolution rates if shelf dissolution occurs even if only periodically one might anticipate that the less stable aragonite and high mg calcite skeletons would dissolve first in fact this experiments shows that small delicate skeletons with non compact morphologies dissolve most rapidly apparently with little regard for mineralogy fossil bryozoan assemblages from temperate marine shelf environments may therefore not only reflect the preferential loss of certain aragonitic species but also the loss of small delicate species new zealand s nontropical tertiary bryomol limestones often lack aragonitic skeletons they may also be relatively depleted in small delicately branched bryozoans in contrast tropical shelf carbonate sediments can retain more delicate skeletons through early cementation and lack of dissolution a difference in character between tropical and temperate carbonate shelf deposits may therefore be in the proportion of small delicate bryozoan colonies

maceration of calcareous skeletons in the december issue of sedimentology 1979 26 two papers on the disintegration of calcareous skeletons appeared in both articles the authors alexandersson 1979 fitzgerald parmenter milliman 1979 describe skeletal degradation producing fragments and even single calcium carbonate crystallites in the present marine environment at least part of this maceration alexandersson 1978 is attributed to the breakdown of the skeletal organic matrix on the sea bottom both studies were carried out on samples from the sediment surface of marine cold water regions there another destructive process is active etching and dissolution of the calcareous particles by the ambient carbonate under saturated water e g alexandersson 1975 1978 the possible coexistence of these biological and physicochemical destructive processes and the lack of information from cores have left open questions in the above mentioned two studies some of these questions were discussed by me lewy 1975 my observations and conclusions may now have a wider significance

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

the relative stabilities of the reverse and normal polarity states of the earth s magnetic field recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states the main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the cenozoic these additional events occur predominantly during reverse polarity time but it is unlikely that all of them represent true reversal events nevertheless the question of the relative stabilities of the polarity states is examined in detail both for the case when all 57 events are included in the reversal chronology and when they are all excluded it is found that there is not a discernible difference between the stabilities of the two polarity states in either case inclusion of these short events does however change the structure of the non stationarity in reversal rate but still allows a smooth non stationarity only 7 of the 57 short events are pre 38 ma but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation this could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the paleogene and late cretaceous overall it would appear that the present geomagnetic polarity timescale for 0 160 ma is probably a very good representation of the actual history and that different timescales and additional events now represent only changes in detail

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

tectonic architecture of the mountain front foreland basin transition south island new zealand assessed by fission track analysis oblique continental convergence across the alpine fault since the mid miocene has loaded the australia plate with the leading edge of the pacific plate and formed a foreland basin the basin occurs mostly offshore beneath the continental shelf but remnants of the marine basin fill overlie ordovician basement in a narrow coastal strip 10 20 km wide in westland between the tasman sea and the alpine fault the results of fission track analyses of apatite and zircon separates from this basement integrated with the regional structure and stratigraphy indicate that the foreland basin formerly extended across the whole of westland and that its succession increased in thickness eastward toward the alpine fault reaching a maximum of about 4 km continuing covergence across the main thrust alpine fault has caused its footwall to thicken by reverse fauting in a transitional zone 10 20 km wide immediately west of the mountain front southern alps consequently the inner margin of the foreland basin has been inverted and the succession almost completely eroded in the southern part of westland upper crustal shortening in the australia plate amounted to about 2 km associated uplift started about 5 m y ago and involved only one major fault bounded block in northern westland upper crustal shortening amounted to about 12 km two major fault bounded blocks were involved and uplift was underway by 10 ma assuming a preuplift geothermal gradient of 25 c km the long term rate of uplift ranged between 0 8 and 2 0 mm yr in southern westland and between 0 4 and 1 2 mm yr in different parts of northern westland

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

thermal core mantle interactions maps of the magnetic field at the core mantle boundary for 1715 1980 reveal static features in the field and an absence of westward drift from much of the core mantle boundary the static features suggest that flow in the core is coupled to the mantle we examine different coupling mechanisms and predict lateral inhomogeneity at the core mantle boundary comparison with models of the lower mantle and core mantle boundary derived seismically and from the geoid is encouraging though their resolution is currently inadequate for a full comparison to be made with the much better resolved magnetic field

evidence for dipolar fields during the cobb mountain geomagnetic polarity reversals recent palaeomagnetic studies1 4 of geomagnetic polarity reversals have presented conflicting interpretations of the behaviour of the earth s magnetic field during reversals some have argued that the grouping of transitional virtual geomagnetic poles vgps suggests the presence of large scale perhaps even dipolar symmetries during some reversals1 3 whereas others have claimed that the available data do not support the interpretation of simple transitional field geometries4 an earlier comparison5of two north atlantic sedimentary records of reversals bounding the cobb mountain subchron 1 1 myr ago with a volcanic record from tahiti6 revealed striking similarities in the sequences of transitional vgps suggesting the presence of large scale sym metries in these fields here i augment this comparison with two new records of the cobb mountain subchron from the western pacific7 which greatly extend the geographical and temporal coverage these records exhibit sequences of vgp positions that are very similar to those observed in the north atlantic and tahiti providing evidence of dipolar transitional fields during the cobb mountain reversals

internal precipitation of microcrystalline carbonate a fundamental problem for sedimentologists abstract recent studies of submarine lithification have shown that microcrystalline carbonate or carbonate mud traditionally thought to represent ooze deposited in quiet water commonly forms as an internal precipitate below the sediment water interface this internally precipitated micrite alters the mud grain ratio of pre existing sedimentary deposits and invalidates classifications that relate the amount of micrite matrix in limestones to the degree of water energy associated with the environment of deposition

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

effects of early sea floor processes on the taphonomy of temperate shelf skeletal carbonate deposits abstract cool water shelf carbonates differ from tropical carbonates in their sources modes and rates of deposition geochemistry and diagenesis inorganic precipitation marine cementation and sediment accumulation rates are absent or slow in cool waters so that temperate carbonates remain longer at or near the sea bed early sea floor processes occurring between biogenic calcification and ultimate deposition thus take on an important role and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool water carbonate deposits the physical breakdown processes of dissociation breakage and abrasion are mediated mainly by hydraulic regime and are always destructive impact damage reduces the size of grains removes structure and therefore information and ultimately may transform skeletal material into anonymous particles abrasion is highly selective amongst and within taxa their skeletal form and structure strongly influencing resistance to mechanical breakdown dissolution and precipitation are the end members of a two way chemical equilibrium operating in sea water in cool waters inorganic precipitation is rare there is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor but test maceration and early loss of aragonite in particular are reported dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter strong bioturbation and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits laboratory experiments demonstrate that surface to volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution biological processes on the sea floor include encrustation and bioerosion encrustation a constructive process may be periodic or seasonal and can be reversed it produces both information and material bioerosion in contrast is destructive and permanent in temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor on the order of hundreds to thousands of years overall processes on the temperate sea floor may combine to destroy more carbonate than they produce and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected where preservation does occur in such a destructive regime the effects of early sea floor processes will be key determinants of the deposit resulting in a taphofacies characteristic of temperate shelf carbonate sediments

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

shallow burial dolomitization and dedolomitization of mid cenozoic cool water calcitic deep shelf limestones southern australia abstract oligocene to mid miocene deep shelf bryozoan rich limestones across southern australia are variably altered to gray to orange ca rich medium crystalline sucrosic dolomite degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent the locale and timing of dolomitization are tightly constrained to shallow burial and mid to late miocene dolostone varies from friable to dense and well lithified dolomitization is fabric specific muddy sediments are preferentially replaced calcite bryozoans and brachiopods form biomolds geochemistry suggests that dolomitization was predominantly by seawater limestone interaction but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements sr isotopes confirm a mid miocene age for the dolomite if precipitated from seawater with no inherited limestone values dolomite crystals have undergone variable degrees of dissolution and meteoric calcite cement has locally filled the resultant rhombic voids dissolution began in the crystal cores implying some sort of metastability and expanded outward until in some ins ances the whole dolomite crystal was dissolved leaving dolomolds in otherwise unaffected limestone this dolomite occurrence well constrained by regional geology shows that widespread porous and permeable fabric destructive medium crystalline sucrosic dolostone can form in the shallow subsurface soon after sediment deposition such dolomite however may be metastable in the presence of variably mixed continental marine waters and newly formed crystals can undergo dedolomitization soon after precipitation

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable

the impact of t trec retrieved wind and radial velocity data assimilation using enkf and effects of assimilation window on the analysis and prediction of typhoon jangmi 2008