surface area control of shallow cratonic to deep marine carbonate accumulation sites of calcium magnesium carbonate accumulation have been shifting from shallow shelf to deep marine settings since the late cretaceous during this time total area of flooded epicratonic shelf has decreased about 50 and areas of shelf carbonate deposition have declined from 27 106 km to present areas of 1 4 106 km while the maximum extent of platform carbonate accumulation has decreased from about 50 to 28 north latitude in terms of cratonic carbonate this decrease in tropical shelf area has resulted in a sixfold reduction in shoal water limestone accumulation from a cretaceous rate of 0 6 km m y to a present value of about 0 1 106 km m y over the same time period the global carbonate compensation depth has deepened by about 1 5 km attendant with a threefold areal increase of calcite ooze depositionn 40 to 130 106 km this expansion has resulted in an eightfold increase in pelagic limestone accumulation from a cretaceous rate of 0 05 106 km m y to a present rate of about 0 4 106 km m y transfer of accumulation from shallow cratonic to deep marine depositional settings records the central role of sedimentary environment surface area in predicating the differential partitioning of finite carbonate fluxes to phanerozoic global oceans

normal faulting and the topographic roughness of mid ocean ridge flanks the ocean floor on the flanks of mid ocean ridges is covered by abyssal hills topographic features elongated perpendicularly to the direction of relative plate motion these topographic features are interpreted as normal fault blocks and or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading the purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults given a fault population and a mechanical model for the response of the lithosphere to faulting we obtain expressions for the variation in root mean square roughness with profile length and for the power spectral density of a profile given three parameters a fault density number of faults per unit length crossed by the profile an average squared fault scarp height and a characteristic length of flexure to keep matters simple we make a number of assumptions and approximations namely that the lithosphere behaves as an elastic plate that faults have an infinite length and a vertical dip that the response of topography to a number of faults is simply the sum of the responses to each fault and that faults have random locations and scarp heights independently chosen from some statistical distribution the theory predicts that the roughness length relationship power spectral density should follow power laws for scales wavelengths less than a characteristic scale proportional to the length scale of flexure we compare the predictions of the theory with actual measurements of mid ocean ridge flank roughness and find good first order agreement in particular we use independent estimates of fault densities average squared fault scarp heights and flexural length scales to predict the topographic roughness of the east pacific rise and we find that normal faulting can explain all the observed topographic roughness nevertheless there are some differences between predictions and observations these differences are likely to be due to processes other than faulting that create topographic relief e g volcanism and to spatial correlations of fault scarp heights despite these shortcomings the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief

ordovician faunas of burgess shale type the renowned soft bodied faunas of the cambrian period which include the burgess shale disappear from the fossil record in the late middle cambrian after which the palaeozoic fauna dominates the disappearance of faunas of burgess shale type curtails the stratigraphic record of a number of iconic cambrian taxa one possible explanation for this loss is a major extinction but more probably it reflects the absence of preservation of similar soft bodied faunas in later periods here we report the discovery of numerous diverse soft bodied assemblages in the lower and upper fezouata formations lower ordovician of morocco which include a range of remarkable stem group morphologies normally considered characteristic of the cambrian it is clear that biotas of burgess shale type persisted after the cambrian and are preserved where suitable facies occur the fezouata biota provides a link between the burgess shale communities and the early stages of the great ordovician biodiversification event

the isostatic compensation of the azores plateau a 3d admittance and coherence analysis the compensation of the azores plateau is re examined by means of a 3d admittance study involving computations of the admittance between the bathymetry and 1 the geoid 2 the free air gravity and 3 the mantle bouguer anomalies the geoid to bathymetry relationship rather than being analysed in the space domain is studied in the spectral domain in this way the information related to the flexural response of the lithosphere is retained in the geoid to bathymetry signal and can be compared to that of the gravity to bathymetry we find that the anomalously shallow depths of the azores plateau are not due to dynamic forces sustained by mantle upwelling the plateau is supported by a thickened crust which mainly results from large volumes of accreted extrusives and consequent deflection of the underlying elastic plate both the free air gravity and the mantle bouguer admittance point to a flexural isostatic model with a moho depth of 12 km and an elastic thickness in the range of 3 6 km however the analysis of the coherency between bathymetry and the mantle bouguer anomaly indicates that buoyant material at the base of the crust partially accounts for the uplift of the plateau in our best fitting model the average elastic thickness is 4 km and the amplitude of the buoyant load at the moho is 1 3 of that of the surface volcanic load buoyancy forces at the moho may arise from underplated material resulting from subcrustal plutonism

isostatic response of the alula fartak and owen transforms in the eastern gulf of aden and the adjoining arabian sea summary the alula fartak and owen transforms are the active parts of major fracture zones with a distinct topographic expression in the eastern gulf of aden and the arabian sea respectively while the alula fartak transform offsets the sheba ridge by about 180 km and is associated with a broad steep sided valley with a relief of nearly 3 5 km the owen transform offsets the carlsberg ridge by nearly 300 km and is associated with a broad step like valley surrounded by deeper water depths the gravity and topography data along several profiles selected across these two transforms have been analysed using cross spectral analysis in order to investigate their isostatic compensation the observed admittance estimates have been compared with three theoretical isostatic compensation models two local compensation models airy i and ii and one regional compensation plate model comparing the longer wavelength admittance estimates suggests that the regional compensation model gives the best fit for both the alula fartak transform and the owen transform with effective elastic thickness te of 5 km and slightly less than 10 km respectively for the alula fartak transform the airy ii model might also be acceptable though with large scatter in the observed values it suggests a mean value of 9 km for the mantle layer with a 6 km thick crust for the owen transform on the other hand the two local compensation models failed the difference in te estimate between the two transforms could be ascribed to differences in thermal structure arising from their varied tectonic history a comparison with the isostatic response estimates of transform fracture zones along the slow spreading mid atlantic ridge suggests that the regional compensation model is generally applicable for transform fracture zone topography along such mid ocean ridges

global chemical erosion during the last glacial maximum and the present sensitivity to changes in lithology and hydrology geographically based calculations for 18 000 years ago 18 ka and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial interglacial timescales runoff fields were derived from general circulation model predictions of precipitation minus evaporation then global lithologic maps were prepared so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates assuming that significant chemical erosion does not occur beneath ice sheets we find that chemical erosion in ice free areas at 18 ka was only slightly greater than today this result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff the small 20 increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates which weather faster than average data on glacial interglacial movements of the calcite compensation depth ccd are inconclusive but seem to indicate little change whereas even a 20 increase in the riverine bicarbonate flux should cause an observable deepening if the ccd did not indeed change then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means such as increased carbonate productivity about half of the observed decrease in atmospheric pco2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice free areas consideration of the maximum possible effect of meltwater at ice margins leads to an 80 increase in the global chemical erosion rate at 18 ka such an increase would lead to a larger and faster drop in atmospheric pco2 but also to an excessive deepening of the global ccd this scenario seems unrealistic our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of quaternary marine sr isotopic ratios therefore if these records are indeed globally representative and entirely due to increased silicate chemical erosion as opposed to changes in source areas or hydrothermal inputs then other factors such as a shift in the global weathering regime which we do not explicitly consider here may be involved sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non spatially resolved calculation

beyond beecher s trilobite bed widespread pyritization of soft tissues in the late ordovician taconic foreland basin pyritization of soft tissues is extremely rare pyritized fossils have been discovered at six new localities spanning 54 km of outcrop of the ordovician lorraine group of new york state suggesting that soft tissue pyritization is widespread in the taconic basin notable new taxa with soft tissue preservation include ostracods and other arthropods such fossils are rare and occur within 4 9 cm thick mudstones representing single rapid depositional events high ratios of reactive iron to total iron and high values of 34 s together with a near absence of disarticulated and fragmented skeletal material suggest that organisms in these pyritic horizons were buried rapidly and underwent bacterial sulfate reduction in porewaters rich in highly reactive iron and low in organic carbon these conditions facilitated iron sulfi de precipitation within and on decaying carcasses such conditions occur repeatedly in some fi ne grained distal turbiditic facies of the taconic foreland basin pyritized soft bodied fossils await discovery elsewhere in the lorraine group

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

the fezouata fossils of morocco an extraordinary record of marine life in the early ordovician the discovery of the fezouata biota in the latest tremadocian of southeastern morocco has significantly changed our understanding of the early phanerozoic radiation the shelly fossil record shows a well recognized pattern of macroevolutionary stasis between the cambrian explosion and the great ordovician biodiversification event but the rich soft bodied fezouata biota paints a different evolutionary picture the fezouata assemblage includes a considerable component of cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the early ordovician study of the fezouata biota is in its early stages and future discoveries will continue to enrich our view of the dynamics of the early phanerozoic radiation and of the nature of the fossil record supplementary material a complete faunal list is available at http www geolsoc org uk sup18843

exceptional fossil record distribution of soft tissue preservation through the phanerozoic preservation of soft bodied fossil biotas konservat lagerstaten that preserve traces of volatile nonmineralized tissues readily degraded by bacteria are not evenly spaced through geologic time when compared to outcrop area exceptional faunas appear to be over represented in the cambrian and jurassic these concentrations in time correspond to particular environments indicating that controls on the distribution of exceptional faunas may have operated on a global scale the reduction in the number of exceptional faunas after the cambrian may reflect the evolution and diversification of deep bio turbators specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks

post cambrian closure of the deep water slope basin taphonomic window exceptional faunas konservat lagerstatten that preserve traces of volatile nonmineralized tissues are statistically overabundant in the cambrian period almost all examples preserved in continental slope and shelf basin environments are of this age the hypothesis that an increase in the amount and complexity of bioturbation was an important agent in the elimination of this deep water slope basin taphonomic window is supported post cambrian ichnofaunal assemblages contain a higher proportion of pascichnia and agrichnia ethologies produced by a mobile infauna they also illustrate the lateral partitioning of organisms into different environmental niches both opportunistic and equilibrium infaunas occur in low oxygen environments in which the preservation of nonbiomineralized tissues was favored direct consumption of carcasses by bioturbating organisms was less important than changes to sediment properties as a result of bioturbation notably enhanced microbial degradation of reactive organic matter increased permeability and the disruption of geochemical gradients necessary for mineral authigenesis

a note on the isostatic compensation and origin of king s trough summary the spectral analysis of a 550 km long ship board gravity and topography profile across the central part of king s trough in the north eastern atlantic ocean shows that the topography is regionally compensated on lithosphere older than 20myr this feature with its enigmatic trend oblique to that of the mid atlantic ridge therefore dates from no earlier than the late oligocene and was formed by intra plate activity such a constraint to its age agrees with the recent analysis of geological samples taken on the northern flank of the trough close to the position of the profile it is not consistent with previous suggestions that king s trough was created either by the rotations of the iberian plate to form the pyrenees or by a readjustment in the strike of the mid atlantic ridge furthermore this analysis shows no evidence for either thin crust or high density mantle under the trough there is no structural evidence to suggest that it formed as part of a compressional boundary

a new lagerstatte from the middle ordovician st peter formation in northeast iowa usa a new fossil fauna has been discovered from a recently recognized shale unit within the middle ordovician st peter formation in northeast iowa it contains a variety of invertebrates and vertebrates including soft body tissues impressions and 3 dimensionalpreservations the exceptional preservation reveals a new konservat lagerstatte the winneshiek lagerstatte and opens a unique window into the community that inhabited the margins of the laurentian cratonic seaway during middle ordovician transgression among the fossils several conodont assemblages including the apparatus of enigmatic coleodontids are preserved some conodont assemblages associated with soft body tissues are particularly noteworthy

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

the fezouata shale lower ordovician anti atlas morocco a historical review abstract exceptionally preserved fossils yield crucial information about the evolution of life on earth the fezouata biota from the lower ordovician of morocco is a konservat lagerstatte of major importance and it is today considered as an ordovician burgess shale this biota was discovered only some 15 years ago but geological studies of the area date back to the beginning of the 20th century pioneering geological investigations lead to the discovery of ordovician strata in the anti atlas 1929 and ultimately resulted in their formal subdivision into four main stratigraphic units 1942 in the agdz area the presence of fossiliferous tremadocian lower ordovician strata was suspected as early as 1939 but only definitively confirmed in 1955 in the 1960s 1990s jacques destombes provided the first detailed biostratigraphic framework for the lower ordovician of the anti atlas and collected thousands of fossils that were subsequently described in a series of monographs in the early 2000s exceptionally preserved fossils were discovered in the fezouata shale tremadocian late floian in the central anti atlas by mohamed ou said ben moula this biota now known as the fezouata biota is of utmost importance for it demonstrates the extent in the fossil record of non biomineralising animals typical of the cambrian explosion into the ordovician during the great ordovician biodiversification although most components are still in need of formal descriptions a fairly good picture of the composition and organisation of this biota and how it contributes to our understanding of the early evolution of metazoan communities can now be depicted moreover recent studies have substantially clarified the biostratigraphical and palaeoenvironmental context of the fezouata shale and are now being followed up by detailed investigations of the taphonomy geochemistry and micropalaeontology of this unique konservat lagerstatte these efforts will soon greatly benefit from the recovery of fresh unweathered samples from drill cores

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

geochemical and climatic effects of increased marine organic carbon burial at the cenomanian turonian boundary perhaps the most significant event in the cretaceous record of the carbon isotope composition of carbonate1 2 other than the 1 2 5 negative shift in the carbon isotope composition of calcareous plankton at the cretaceous tertiary boundary3 is the rapid global positive excursion of 2 13c enrichment which took place between 91 5 myr and 90 3 myr late cenomanian to earliest turonian c t boundary event 1 4 5 this excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon tdc reservoir resulting from an increase in rate of burial of 13c depleted organic carbon which coincided with a major global rise in sea level5 during the so called c t oceanic anoxic event oae 6 here we present new data from nine localities which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the c t boundary7 8 is nearly synchronous with that for carbonate and is widespread throughout the tethys and atlantic basins fig 1 as well as in more high latitude epicontinental seas the postulated increase in the rate of burial of organic carbon may have had a significant effect on co2 and o2 concentrations in the oceans and atmosphere and consequent effects on global climate and sedimentary facies

the late ordovician early silurian glacial period abstract late ordovician to early silurian tillites and evidence of extensive glacial erosion were first discovered in the western sahara in the late 1950 s although the concept of a glacial period at this time had been around since the late 19th century further evidence of glaciation is now available from west africa south africa the arabian peninsula eastern north america west and central southe america and europe several glacial episodes can be distinguished the earliest evidence local mountain glaciation appears to be of caradoc age the major episode may have begun in caradoc time and lasted much of ashgill time when a major ice sheet covered at least north central and west africa which was then centered near the south pole however independent palaeontological and sea level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 ma this conflict has yet to be resolved the latest evidence of glaciation is wenlock in age but is confined to south america which by then was replacing africa over the south polar region however this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice sheet dimensions the lack of land in the north polar region probably explains the absence of tillites from the then northern hemisphere land masses the total area affected by glaciation in early palaeozoic time in north africa alone is at least 6 106 km2 combined with other glaciated areas glaciation may have been on the scale of that of present day antarctica

gravity study of the pitcairn easter hotline shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the pitcairn easter hotline in the south pacific the anomalies are associated with one of the positive geoid undulations observed in the area from satellite data several smaller topographic features volcano tectonic ridges oriented n65 e are superimposed on the topographic high admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere 4 to 8 km below the crust the volcano tectonic ridges are locally compensated in a classical airy sense the swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the easter island hotspot and of the easter microplate accretion centers

burgess shale type biotas were not entirely burrowed away burgess shale type biotas occur globally in the cambrian record and offer unparalleled insight into the cambrian explosion the initial phanerozoic radiation of the metazoa deposits bearing except

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

a late surviving xenopod arthropoda from the ordovican period wales the middle ordovician llanfallteg formation has yielded remains of soft bodied organisms previously known only from cambrian burgess shale type deposits a new arthropod etania howellsorum gen et sp nov is described here characterized by a semi circular cephalon clusters of spinose endites on the endopod and exopods with ovoid distal lobes these characters are consistent with xenopod affinities a clade otherwise known exclusively from the cambrian period the discovery of e howellsorum demonstrates that a number of burgess shale type taxa including xenopods survived past the cambrian period albeit within a restricted environment and may have been outcompeted during the great ordovician biodiversification event gobe

oxygen and carbon isotopes and event stratigraphy near the ordovician silurian boundary anticosti island quebec the growth and destruction of continental ice sheets in north africa and adjacent parts of gondwana during the hirnantian stage had profound influence on the sedimentary style of mixed carbonate siliciclastic sequences on anticosti island quebec canada these strata deposited in a tropical mid shelf setting show direct evidence of local sea level changes of up to 40 m after correction for tectonic and load induced subsidence this indicates maximum global glacio eustatic changes of 32 to 36 m this implies maximum ice volume of about 12 to 14 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0 5 to 0 6 given that the oxygen isotope ratios in syn glacial samples from the anticosti sequence range from 8 9 to 2 8 18o it is not possible to clearly distinguish the effects of ice cap development from temperature and diagenetic controlled variation the strong negative shift in 13c values at the top of the ellis bay formation may indicate a pronounced increase in atmospheric co2 coincident with destruction of the late ordovician ice caps

the crustal structure of the madagascar ridge summary the madagascar ridge is an elongated aseismic plateau in the southwest indian ocean which separates two ocean basins of mid to late cretaceous age from interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the cretaceous as thickened oceanic crust at a mid ocean ridge which possibly overlay a mantle hot spot the seismic data consist of record sections from five recording sonobuoys forty one shots were fired along a 210 km long nne ssw line across the central part of the ridge simple horizontal layer analysis of first arrivals reveals a lower crustal refractor with vp 7 kms 1 at a depth of 8 8 5 km and the moho at an apparent depth of 18 km but fails to characterize the crustal structure as either oceanic or continental we obtain a definitively oceanic structure by using synthetic seismogram techniques together with tau p analysis and second arrival correlations the model which best fits the data consists of 2 2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms 1 beneath this at a depth of 7 5 km is a 9 5 km thick layer with vp 6 9 7 1 kms 1 a 7 5 kms 1 layer not observed in the first arrival analysis is found at the base of the crust the moho is a transition zone 1 km thick and occurs at 25 km depth the gravity data consist of a continuous ne sw profile across the ridge and on to the flanking basins two dimensional gravity modelling allows a structure consistent with the synthetic seismogram results and supports the existence of a high density basal layer in the crust we use in addition a time series analysis approach to estimate the depth of compensation assuming an airy model and lithospheric thickness assuming an elastic plate model convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 5 km and a plate thickness of less than 2 5 km

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

survival of burgess shale type animals in a middle ordovician deep water setting exceptional preservation of non biomineralized arthropods sponges and vermiform taxa occurs in the darriwilian middle ordovician llanfallteg formation of south wales uk the assemblage contains elements typical of ordovician communities juxtaposed with those more commonly associated with cambrian konservat lagerstatten this assemblage is preserved in rocks of a deep marine succession dominated by fine grained siliciclastic and volcaniclastic density current deposits non biomineralized taxa of cambrian aspect are preserved as pyritized carbonaceous compressions on the spectrum of burgess shale type preservation trilobites with phosphatized digestive structures have also been recovered the assemblage of the llanfallteg formation konservat lagerstatte demonstrates that some burgess shale type faunal elements survived into the middle ordovician within cool deep water refugia in the welsh basin offshore from communities dominated by typical ordovician taxa supplementary material assemblage composition data and sedimentological images can be found at https doi org 10 6084 m9 figshare c 2182218

exceptionally preserved late ordovician biotas from manitoba canada there are few body fossil biotas known from early paleozoic accretionary shorelines and very few examples of ordovician soft bodied assemblages this study documents two recently discovered biotas from separate sedimentary basins in manitoba canada that provide unique information about tropical shoreline communities shortly before the late ordovician extinction event each site represents a distinct depositional environment but they share biotic elements including eurypterids xiphosurids and large problematic tubes the william lake biota representing more restricted conditions includes jellyfish that are among the best hydromedusan body fossils known rocks at the airport cove site deposited under more open circulation contain scolecodonts and noncalcified algae these biotas have some parallels with the recently described middle ordovician winneshiek lagerstatte but are also similar to some late silurian assemblages considered together early paleozoic marginal marine deposits are a rich but as yet poorly known source of paleobiodiversity data

an application of the response function technique to profiles of bathymetry and gravity in the gulf of aden summary the relationship between bathymetry and gravity in the eastern and the western gulf of aden is analysed by computing response functions the form of the response functions indicates that the short wavelength topography is isostatically compensated the most appropriate mechanism of compensation is one in which the topography is not locally compensated but regionally compensated by flexure of the lithosphere though the sets of profiles from the eastern and the western gulf of aden are separated by only 500 750 km the observed response functions are very different the lithosphere in the eastern gulf of aden can be modelled by an elastic plate 15 3 km thick whereas an elastic plate 3 2km thick is probably an appropriate model for the lithosphere in the western gulf of aden a different mechanism of isostasy the pratt operates in the compensation of the long wavelength topography of the mid ocean ridge in the gulf of aden a larger negative broad bouguer gravity anomaly across the western gulf of aden than across the east indicates that upper mantle temperatures increase from east to west the long wavelength shallowing of the gulf of aden and increasing elevation of the adjacent continents from east to west are therefore probably mainly the result of thermal uplift the subdued nature of the median valley and consequently other short wave length topography in the west can be explained by hot low viscosity asthenosphere and the inferred thin lithosphere in the west also suggests high temperature mantle finally a magnetic anomaly superimposed on the axial anomaly which is probably related to the depth of the curie temperature is more common in the western gulf of aden than in the east and this is also consistent with upper mantle temperatures increasing to the west there are a number of similarities between the mid ocean ridge in the western gulf of aden and the reykjanes ridge it is suggested that the western gulf of aden is anomalous because of its proximity to the afar hot spot in the same way that the reykjanes ridge is often considered to be anomalous because of its proximity to the iceland hot spot

flexural models of continental lithosphere based on the long term erosional decay of topography summary the most easily observed geophysical characteristic of continents is their surface topography topographic power in the spectral range 1000 100 km has been computed for north american geological provinces and is found to decrease systematically as the tectonic age of the underlying crust increases these data have been quantitatively modelled in terms of topographic erosion and stress relaxation within the continental lithosphere through geological time spans the lithosphere is treated as a thin maxwell viscoelastic plate which deforms flexurally in response to the vertical surface loads of topography and its erosion effective rheological model parameters are flexural rigidity d a measure of the lithosphere s elastic strength and viscous relaxation time constant a measure of its viscosity erosion is assumed to occur at a rate linearly proportional to the height of topography at any given time the proportionality being determined by the erosion time constant the models are constructed in space and time frequency domains using simple linear systems theory two loading schemes are considered model 1 assumes that a flexurally competent lithosphere is suddenly loaded by topography which subsequently erodes whereas model 2 ignores the emplacement and erosion of topography during orogenesis when the lithosphere is thermally activated and considers only the load effected by erosion beginning some time later when the lithosphere has cooled and strengthened both models predict that the remnant topography of very old regions such as the canadian shield can be maintained in fact is favoured by a viscoelastic lithosphere with as small as 1 10ma the continued existence of such topography does not necessarily imply significant long term lithospheric strength model 1 results indicate low values of the flexural rigidity d 1021 1022nm which are interpreted to characterize not so much presentday d but that which was effective when topography was formed model 2 more successfully reproduces the data and constrains the product d to values 1024 1025nm ma which compare very favourably to those found from studies of individual loads on continental lithosphere the model results show that the erosion time constant is probably wavenumber dependent and falls in the range 200 400ma for the topographic wavelengths under consideration

negative gravity anomaly over spreading rift valleys mid atlantic ridge at 26 n abstract a pronounced negative free air gravity anomaly commonly occurs over the median valley of slow spreading ocean ridges previous results using wiener filtering and cross spectral analysis techniques for the mid atlantic ridge obtained estimates of the elastic plate thickness in the range of 7 13 km and the existence of a residual negative gravity anomaly over the median rift valley suggesting that the rift valley has a response function different than the remainder of the spreading ridge in this paper we have improved the derivation of the topography gravity admittance function for spreading ocean crust by carefully avoiding several sources of spectral splattering when processing the data 1 selecting data from a cruise that followed a flowline of central north atlantic relative plate motion and hence is least corrupted by fracture zones and 2 accounting for the difference in distance between the gravity meter and the regional variation in elevation as the ridge crest is traversed improvements of lesser importance include the use of cubic splines to interpolate to equally spaced data rather than linear interpolation and correction of the free air anomaly values for long wavelength variations of the indirect effect comparison of the resulting admittance function to elastic flexure response functions suggests an elastic plate thickness of about 8 km the improved admittance function when convolved with the ridge topography provides a predicted gravity profile that accounts very well for the negative anomaly over the rift valley therefore the isostatic response function for the rift valley is similar to that for the topography away from the rift valley

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

crustal structure of atlantic fracture zones i the charlie gibbs fracture zone summary a seismic refraction experiment was carried out in 1982 over and in the vicinity of the southern transform valley of the charlie gibbs double fracture zone to measure the rate of crustal thinning normal to the southern transform valley axis three reversed lines were shot with obs two were east west one parallel to the fault on normal crust and one along the fault the third line was north south normal to and intersecting the first two lines travel times were modelled by ray tracing thin 4 km thick anomalously low velocity crust was detected in the fracture zone along the n s line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge transform intersection the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment a gravity profile along the n s line was also analysed the bouguer anomaly was modelled subject to the constraints of the seismic ray tracing after applying corrections for the three dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone the gravity models supported the seismic interpretation and also indicated low densities within the fracture zones two alternative models were obtained for the structure under hecate bank an east west ridge which lies within the crust generated at the short central spreading segment of the cgfz one model features dense material within the crust the other normal but uplifted crust the evidence favours the latter mode but the cause of the uplift is uncertain the uplift may have

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change

a comparison of the isostatic response of bathymetric features in the north pacific ocean and philippine sea summary this paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western pacific ocean the purpose is to separate and describe possible differences in isostatic compensation between several type regions of oceanic crust a mid ocean ridge juan de fuca a mid plate seamount chain hawaiian ridge fracture zone topography on old crust north of hawaii and a marginal basin philippine sea results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions these differences are set in the perspective of three simple compensation mechanisms two of these consider local airy models in which raised topography is compensated at depth either by crustal roots or low density mantle a third considers the effects of an elastic plate of variable thickness supporting crustal variations conclusions are that a a thick plate possibly in excess of 30 km supports the hawaiian ridge b a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed c the juan de fuca ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub crustal low densities at depths of 15 to 20 km and d the philippine sea shows no evidence for regional support ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger juan de fuca ridge the major difference between admittance functions for the philippine sea and comparably aged regions of the north pacific ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins