errors in bathymetric retrievals using linear dispersion in 3 d fft analysis of marine radar ocean wave imagery the phenomenon of ocean wave shoaling and the associated reduction of ocean wave phase speed with decreased water depth provides useful information for inferring water depth d bathymetry in coastal environments one strategy for relating d to phase speed c and wave vector k of long wavelength ocean waves involves using the one dimensional 1 d linear gravity wave dispersion relationship c sup 2 g tanh kd k in principle this approach has limitations because the approach is based on a wkb approximation so it cannot be applied when d varies appreciably over the wavelength of a shoaling wave also the approach is restricted to waves that have small wave height the author uses a set of marine radar image sequences and applies this linear approximation using a 3 d fft analysis of 88 sets of image sequences spaced half an hour apart the author inverts the dispersion relation to solve for d depths between 3 6 and 5 8 m were tested for root mean square rms wave heights offshore between 8 and 3 m the author shows that for low to moderate wave heights the approach does generally retrieve the correct depth in water depths of 5 m and greater for moderate wave rms heights however an increase in the rms wave height from 1 m to 3 5 m produced a much poorer depth estimate proving the need for an application of a nonlinear wave model to the problem the errors also increase with shallower depths as expected as the error dependence on depth and wave height is determined

tectonic implications of a dense continuous gps velocity field at yucca mountain nevada 1 a dense continuous gps network was established in the yucca mountain area in 1999 to provide the most reliable measurements possible of geodetic strain patterns across the nation s only proposed permanent repository for high level radioactive waste the network lies astride a boundary between the geodetically stable central great basin and the active western great basin which at the latitude of yucca mountain is undergoing distributed right lateral shear at a rate of 60 nstrain yr monitoring from 1999 to 2003 3 75 years yields a velocity field characterized by nearly homogenous n20 w rightlateral shear of 20 2 nstrain yr net velocity contrast of 1 2 mm yr across a 60 km aperture in the vicinity of the proposed repository site comparison of time series of continuous results with earlier campaign surveys indicating 50 nstrain yr of westnorthwest extension from 1991 to 1997 suggests that the more rapid rates were in part transient motions associated with the 1992 ms 5 4 little skull mountain earthquake postseismic motions do not appear to affect the 1999 2003 velocity field in either campaign or continuous data the magnitude of the velocity contrast across the area the overall linearity of the gradient and the large area of undeforming crust to the east of yucca mountain are difficult to explain by elastic bending of the crust associated with the death valley fault zone a major right lateral strike slip fault about 50 km west of the repository site these observations along with apparent local variations in the velocity gradient suggest that significant right lateral strain accumulation with displacement rate in the 1 mm yr range may be associated with structures in the yucca mountain area the absence of structures in the area with equivalent late quaternary displacement rates underscores the problem of reconciling discrepancies between geologic and geodetic estimates of deformation rates index terms 1208 geodesy and gravity crustal movements intraplate 8110 1243 geodesy and gravity space geodetic surveys 8107 tectonophysics continental neotectonics 8109 tectonophysics continental tectonics extensional 0905 keywords geodesy tectonics yucca mountain

comparison of radar and video observations of shallow water breaking waves simultaneous microwave and video measurements of shallow water breaking waves are presented a comparison of the data from the two sensors shows that short duration spikes in the measured x band radar cross section are highly correlated with the presence of breaking waves in the video imagery in addition the radar backscatter from shallow water breaking events is responsible for 40 to 50 of the total cross section which is a much larger contribution than typically observed for deepwater breaking events based on estimates of the area of individual breaking regions determined from digitized video images the radar cross section per unit area of the turbulent breaking region is shown to be well approximated by a value of 1 9 db at 31 spl deg grazing finally there are some differences between the radar and video signals that suggest that microwave radar may be less sensitive than video techniques to relict foam not associated with active wave breaking in general the results indicate that radar is a very good detector of shallow water breaking waves and suggest that radar can be used for the measurement of the spatial and temporal variations of wave breaking

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

ocean wave dispersion surface measured with airborne ir imaging system image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system the images are geographically registered and 3d frequency wavenumber spectra are calculated and shown to have a strong 2d dispersion surface that is characteristic of wind waves and swell wave directions compare well with in situ measurements and their speeds are consistent with the water depth the authors conclude that temporal sequences of ir images acquired from long distance can provide information for extracting surface wave parameters

late pliocene greenland glaciation controlled by a decline in atmospheric co2 levels it is thought that the northern hemisphere experienced only ephemeral glaciations from the late eocene to the early pliocene epochs about 38 to 4 million years ago and that the onset of extensive glaciations did not occur until about 3 million years ago several hypotheses have been proposed to explain this increase in northern hemisphere glaciation during the late pliocene here we use a fully coupled atmosphere ocean general circulation model and an ice sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the greenland ice sheet in particular we find that greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the late pliocene by contrast our model results suggest that climatic shifts associated with the tectonically driven closure of the panama seaway with the termination of a permanent el nino state or with tectonic uplift are not large enough to contribute significantly to the growth of the greenland ice sheet moreover we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the earth s orbit

microwave radar cross sections and doppler velocities measured in the surf zone 1 the relationship between microwave imaging radar measurements of fluid velocities in the surf zone and shoaling breaking and broken waves is studied with field observations normalized radar cross section nrcs and doppler velocity are estimated from microwave measurements at near grazing angles and in situ fluid velocities are measured with acoustic doppler velocimeters advs joint histograms of radar cross section and doppler velocity cluster into identifiable distributions the nrcs values from pixels with large nrcs and high doppler velocities 2 m s decrease with decreasing bore height to the shoreline similar to scattering from a cylinder with decreasing radius the doppler velocities associated with these regions in the histograms agree well with theoretical wave phase velocities radar and adv measurements of fluid velocities between bore crests have similarly shaped energy density spectra for frequencies above about 0 1 hz but energy levels from the radar are an order of magnitude higher than those of the adv data instantaneous interbore doppler velocities are correlated with adv measured fluid velocities but are offset by 0 8 m s this offset may be due to bragg wave phase velocities wind drift range and azimuth sidelobes the finite spatial resolution of the radar and differences between mean flows measured at the surface with radar and flows measured below the surface with advs shoaling and breaking waves measured through radar grating lobes significantly affect both the doppler velocities near the edges of the images and the scattering from the rear faces of waves causing large doppler velocities to be observed in these regions

laboratory studies of radar sea spikes at low grazing angles we report on measurements of low grazing angle radar sea spikes in a laboratory wave tank using an x band radar with 10 cm range resolution and multiple range bin capability the sea spikes were observed to have a horizontal vertical polarization ratio much larger than unity this is similar to observations in earlier field experiments but is in contrast to a ratio of unity recently observed for moderate grazing angles jessup et al 1990 our experiment was designed to validate the low grazing field results and to establish design criteria for spatial temporal measurement instrumentation single point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features surface scattering features were generated using a paddle generated wave combined with wind waves for a series of wind speeds the surface feature responsible for the sea spike echo was visually identified as a short wave crest occurring on the windward side of the paddle wave driven to extreme steepness by the passage of the faster paddle wave crest long decorrelation times observed in the field are also seen in these studies of the order of 500 ms for horizontal polarization and less than 5 ms for vertical we suggest that rather than bragg scatter the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at the x band for low to moderate winds and perhaps at other frequencies as well

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

pleistocene deglaciation and the earth s rotation implications for mantle viscosity summary recent results from the analysis of postglacial rebound data suggest that the viscosity of the earth s mantle increases through the transition region models which fit both relative sea level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle in this paper we analyse the effect of deglaciation upon the earth s rotation and thereby show that the observed secular trend polar wander evident in the ils ipms pole path and measurements of the non tidal acceleration of the length of day are both consistent with the viscosity profile deduced from postglacial rebound the two analyses are therefore mutually reinforcing

rapid early cambrian rotation of gondwana based on the history of mesozoic cenozoic plate motions as well as simple dynamical considerations a speed limit for tectonic plates has been suggested at 20 cm yr previous paleomagnetic data from the early cambrian of gondwana are confl icting but generally imply rapid motions approaching that limit herein we describe results from a continuous paleomagnetic sampling of lower to middle cambrian strata from the amadeus basin central australia we fi characteristic remanence directions that show an 60 declination shift through the section assuming a tectonically assembled gondwana supercontinent by early cambrian time this large vertical axis rotation of its australian sector corresponds to an equally large translation across paleolatitudes for its brazilian and west african sectors analysis of all high quality paleomagnetic data from gondwana both confi rms and constrains the 60 rotation to have occurred toward the end of early cambrian time at rates exceeding 16 12 8 cm yr these observations suggest that either nonuniformitarian plate tectonics or an episode of rapid true polar wander occurred during the cambrian explosion of animal life

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

closing of the central american seaway and the ice age a critical review 1 what role did the closing of the central american seaway play in enabling continental ice sheets to wax and wane over north america and fennoscandia a summary of relevant evidence presented here permits a causal relationship between them but can be interpreted to show none the common denominator of such evidence is the approximate simultaneity of that closing with global cooling and the first major ice advance at the resolution of paleoclimate however geologic evidence from central america places only weak constraints on when interchange between pacific and caribbean water ceased the strongest evidence for when north and south america became connected by a continuous land bridge the great american exchange of vertebrates assigns climate change the causal role for an arid central american climate typical of glacial times seems necessary for animals that inhabited savannas to pass through panama paleoceanographic and geochemical studies of environments and water masses in the eastern pacific western caribbean and atlantic in general call for continual change since 6 ma or earlier if evidence concurrent with global change near 3 5 3 ma seems more widespread than for other times hypothesized connections between a closed seaway and glaciation commonly call for profoundly different north atlantic ocean circulation but simulations using general circulation models provide a spectrum of differences in circulation for open and closed seaways might much not all of the evidence implicating the closing of the seaway in global climate change in fact be a consequence of that change and blind to central america s tectonic development

ocean surface determination from x band radar image sequences 1 an empirical inversion method is presented for determination of time series of ocean surface elevation maps from nautical radar image sequences the method is based on the determination of the surface tilt angle in antenna look direction at each pixel of the radar images thereby in situ sensors are not required an external calibration is not necessary a conventional nautical x band radar operating at grazing incidence and horizontal polarization in transmit and receive is used as a sensor radar image sequences with their high spatial resolution and large coverage offer a unique opportunity to derive and study individual waves and wave fields in space and time and therefore allow the measurement of individual wave parameters and wave groups for validation of the inversion scheme the significant wave heights derived from the inverted radar data sets and from colocated wave records are compared it is shown that the accuracy of the radar retrieved significant wave height is within the accuracy of the in situ sensors furthermore a wave elevation time series is directly compared to a buoy record to show the capabilities of the proposed method

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

the application of a radar based depth inversion method to monitor near shore nourishments on an open sandy coast and an ebb tidal delta abstract coastal management in the netherlands has the aim to defend coastal zones by preventing flooding and mitigating erosion to that end large scale nourishments are placed in the nearshore which are supposed to dynamically preserve the coastal zone over a timescale of years to assess their effectiveness these nourishments are monitored over large areas and long durations as repetitive in situ measurements become too expensive remote sensing offers an attractive alternative mapping depth and near surface current fields via depth inversion algorithms dia however the information that can be derived from remotely sensed data is subject to improvement in this study a 3d fft based dia named xmfit x band matlab fitting is introduced which is robust accurate and fast enough for operational use focusing on depth estimates the algorithm was validated for two case studies in the netherlands 1 the sand engine a beach mega nourishment at a uniform open coast and 2 the tidal inlet of the dutch wadden sea island ameland characterizing a more complex coast considering both sites the algorithm performance was characterized by a spatially averaged depth bias of 0 9 m at the sand engine and a time varying bias of approximately 2 0 m at the ameland inlet when compared to in situ depth surveys the accuracy was lower but the time resolution higher depth estimates from the ameland tidal inlet were produced every 50 min by an operational system using a navigational x band radar to monitor the placement of a 5 million m3 ebb tidal delta nourishment a pilot measure for coastal management volumetric changes in the nourishment area over the year 2018 occurring at 7 km distance from the radar were estimated with an error of 7 depth errors statistically correlated with the direction and magnitude of simultaneous near surface current estimates additional experiments on sand engine data demonstrated that depth errors may be significantly reduced using an alternative spectral approach and or by using a kalman filter

the key role of global solid earth processes in preconditioning greenland s glaciation since the pliocene after 500 ma of absence major northern hemisphere glaciations appeared during the plio pleistocene with greenland leading other northern areas here we propose that three major solid earth processes underpinned build up of the greenland ice sheet first a mantle plume pulse responsible for the north atlantic igneous province at 60 ma regionally thinned the lithosphere younger plume pulses led to uplift which accelerated at 5 ma lifting the parts of the east greenland margin closest to iceland to elevations of more than 3 km above sea level second plate tectonic reconstruction shows a 6 northward component of greenland motion relative to the mantle since 60 ma third a concurrent northward rotation of the entire mantle and crust towards the pole dubbed true polar wander tpw contributed an additional 12 change in latitude these global geodynamic processes preconditioned greenland to sustain long term glaciation emphasizing the role of solid earth processes in driving long term global climatic transitions

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

cbathy a robust algorithm for estimating nearshore bathymetry 1 a three part algorithm is described and tested to provide robust bathymetry maps based solely on long time series observations of surface wave motions the first phase consists of frequency dependent characterization of the wave field in which dominant frequencies are estimated by fourier transform while corresponding wave numbers are derived from spatial gradients in cross spectral phase over analysis tiles that can be small allowing high spatial resolution coherent spatial structures at each frequency are extracted by frequency dependent empirical orthogonal function eof in phase two depths are found that best fit weighted sets of frequency wave number pairs these are subsequently smoothed in time in phase 3 using a kalman filter that fills gaps in coverage and objectively averages new estimates of variable quality with prior estimates objective confidence intervals are returned tests at duck nc using 16 surveys collected over 2 years showed a bias and root mean square rms error of 0 19 and 0 51 m respectively but were largest near the offshore limits of analysis roughly 500 m from the camera and near the steep shoreline where analysis tiles mix information from waves swash and static dry sand performance was excellent for small waves but degraded somewhat with increasing wave height sand bars and their small scale alongshore variability were well resolved a single ground truth survey from a dissipative low sloping beach agate beach or showed similar errors over a region that extended several kilometers from the camera and reached depths of 14 m vector wave number estimates can also be incorporated into data assimilation models of nearshore dynamics

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

ocean wind fields retrieved from radar image sequences 1 the dependency of radar backscatter on the surface wind field is investigated using a nautical x band radar operating at grazing incidence and horizontal polarization in transmit and receive this resulted in development of an algorithm for wind retrieval from nautical radar image sequences the algorithm consists of two parts in the first part wind directions are extracted from wind induced streaks that are in line with the mean surface wind direction these streaks are visible in the temporal integrated radar images at scales between 100 and 500 m the orientation of the streaks is determined from the local gradients which are derived from the radar images smoothed and reduced to the appropriate scales in the second part wind speeds are derived from the backscatter of the temporal integrated radar image sequence and the radar retrieved wind direction the dependency of the radar backscatter on the local surface wind and geometry of the radar is parameterized by training a neural network the algorithm is applied to radar image sequences acquired by a nautical x band radar mounted aboard an offshore platform in the north sea the radar derived winds are validated by comparison to in situ wind data measured at the platform the comparison of wind directions resulted in a correlation of 0 99 with a standard deviation of 14 2 for wind speeds the correlation is 0 97 with a standard deviation of 0 85 m s 1 in contrast to traditional offshore wind sensors the retrieval of the wind field from the backscatter of the ocean surface makes the system independent of the sensors motion and installation height and reduces the effects due to platform induced blockage and turbulence effects

high resolution bathymetry estimates via x band marine radar 2 effects of currents at tidal inlets abstract in this second of two papers the high spatial resolution cbathy depth estimate algorithm is extended to account for the existence of currents aligned with the wave directions the extended algorithm is applied using x band radar image time series collected at two hydrodynamically and morphologically complex environments an ebb tidal shoal new river inlet nc and an estuarine river mouth columbia river or wa even without accounting for doppler effects the depth estimates are capable of discerning the location of channels and shoals over domain sizes of 4 km2 at the ebb tidal shoal and 25 km2 at the estuary mouth however the tidal currents at these sites are shown to strongly modulate depth estimate accuracy when only the intrinsic linear wave dispersion relation neglecting doppler shift effects is inverted low pass filtering of the depth estimate time series via an uncertainty weighted tidal average explicitly attenuates the error oscillations beyond that of the uncertainty weighted kalman filter this post hoc method of compensating for the unmodeled doppler term achieves 0 35 m rmse and 0 02 m shallow bias at the shallow ebb tidal shoal as compared to a concurrent sounding survey but substantial bias remains at the estuary mouth due to asymmetric doppler effects for large following or opposing currents at large relative water depths simultaneous estimation of depth and currents via inversion of the dispersion relation including doppler shifts is more effective at the deep estuary mouth where currents more strongly affect the incident wave field in the estuary channel with locations as deep as 27 m kalman filtered depths achieve 0 92 m rmse and 0 38 m deep bias as compared to a concurrent sounding survey 95 of estimates are within 10 of those surveyed independent of the doppler compensation technique kalman filtered depths can be used to more tightly constrain high resolution current velocity estimates

did glacially induced tpw end the ice age a reanalysis previous studies of earth rotation perturbations due to ice age loading have predicted a slow secular drift of the rotation axis relative to the surface geography i e true polar wander tpw of order of several degrees over the plio pleistocene it has been argued that this drift and the change in the geographic distribution of solar insolation that it implies may have been responsible for important transitions in ice age climate including the termination of ice age cycles we use a revised rotational stability theory that incorporates a more accurate treatment of the earth s background ellipticity to reconsider this issue and demonstrate that the net displacement of the pole predicted in earlier studies disappears this more muted polar motion is due to two factors first the revised theory no longer predicts the permanent shift in the rotation axis or the so called unidirectional tpw that appears in the traditional stability theory and second the increased background ellipticity incorporated in the revised predictions acts to reduce the normal mode amplitudes governing the motion of the pole we conclude that ice age induced tpw was not responsible for the termination of the ice age this does not preclude the possibility that tpw induced by mantle convective flow may have played a role in major plio pleistocene climate transitions including the onset of northern hemisphere glaciation

accuracy of bathymetric assessment by locally analyzing radar ocean wave imagery february 2008 in this paper the error source in assessing the bathymetry by a recently presented method the dispersive surface classificator is discussed this method is based on the analysis of x band radar image sequences of sea surface waves to determine spatial maps of hydrographic parameters to implement this objective the radar deduced bathymetry is validating by multibeam echosounder data the accuracy of the method is high in homogeneous areas and reduced at the areas of bathymetric gradient and lower but comparable with multibeam echosounder data under the assumption of the spatial resolution the identification of systematic correlation of the absolute value of the error with the slope is significant and insignificant with the actual depth itself the spatial correlation of the error illustrates that the direction of the wave field influences the neighboring grid cell in the same direction the application of the method during crucial weather conditions is the main advantage and permits the accurate operational nearshore monitoring for several applications

depth inversion in shallow water based on nonlinear properties of shoaling periodic waves abstract two depth inversion algorithms dia applicable to coastal waters are developed calibrated and validated based on results of computations of periodic waves shoaling over mild slopes in a two dimensional numerical wave tank based on fully nonlinear potential flow fnpf theory in actual field situations these algorithms would be used to predict the cross shore depth variation h based on sets of values of wave celerity c and length l and either wave height h or left right asymmetry s2 s1 simultaneously measured at a number of locations in the direction of wave propagation e g using video or radar remote sensing techniques in these dias an empirical relationship calibrated for a series of computations in the numerical wave tank is used to express c as a function of relative depth koh and deep water steepness koho to carry out depth inversion wave period is first predicted as the mean of observed l c values and ho is then predicted either based on observed h or s2 s1 values the celerity relationship is finally inverted to predict depth h the algorithms are validated by applying them to results of computations for cases with more complex bottom topography and different incident waves than in the original calibration computations in all cases root mean square rms errors for the depth predictions are found to be less than a few percent whereas depth predictions based on the linear dispersion relationship which is still the basis for many state of the art dias have rms errors 5 to 10 times larger

effect of plate thickness on bending radius and energy dissipation at the subduction zone hinge 1 despite the thickness of subducting oceanic plates being identified as a control on slab and hinge behavior during subduction there have been few attempts to quantify its effect with fully dynamic models this paper presents a series of dynamic laboratory experiments of progressive subduction of a narrow plate into an upper mantle reservoir in which plate thickness is systematically varied from 4 mm scaling to 20 km up to 24 mm 120 km the experiments investigate its effect on bending radius subduction kinematics and the rate of energy dissipated at the hinge as a percentage of the total potential energy released through subduction the bending radius of the hinge is found to have a strong linear relationship with plate thickness where an increase in plate thickness corresponds to an increase in bending radius subduction velocity increases with plate thickness due to the increased negative buoyancy of thicker slabs which is not compensated for by any significant increase in mantle drag on the slab this increase is almost entirely accommodated by an increase in trench retreat rate with a fivefold increase over the whole thickness range while subducting plate velocity remains relatively constant the energy dissipation ratio be bu is low across all plate thicknesses with the plates falling within a range of 4 10 during steady state subduction the models show a moderate trend toward decreasing be bu with increasing plate thickness

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

simple energy balance model resolving the seasons and the continents application to the astronomical theory of the ice ages a one level seasonal energy balance climate model which has explicit two dimensional land sea geography is introduced and its properties analyzed the model distinguishes land from sea surface strictly by the local thermal inertia employed the transport is governed by a smooth latitude dependent diffusion mechanism the model s seasonal cycle is solved for with and without ice feedback in each case good agreement with real data is accomplished with minimal tuning of the adjustable parameters the model sensitivity for small solar constant changes is in agreement with that of other accepted models however when the solar constant is lower a few percent the ice cap grows discontinuously from covering greenland and vicinity to a size three times larger and mostly into northern canada a similar discontinuous growth occurs when the orbital elements are changed to favor cool summers in the northern hemisphere this discontinuous sensitivity is discussed in the context of the milankovitch theory of the ice ages and the associated branch structure is shown to be analogous to the so called small ice cap instability known from simpler models although the discontinuity in sensitivity is somewhat conjectural both linear and nonlinear models have their largest sensitivity to orbital parameter changes in the northern portions of the large continents in the western hemisphere this also corresponds to the ice sheet boundary suggesting that the land sea configuration is crucial to large glacier growth irrespective of moisture budgets other experiments and implications are also discussed the remarkable paper by hays et al 1976 hereafter his presented compelling empirical evidence that continental ice sheet volume changes and sea surface temperature variations in the last 700 000 years are correlated with the changes in the earth s orbital elements theoretical arguments for such a connection date back to the last century but the most detailed and convincing arguments are due to milankovitch 1941 and the theory usually is associated with his name if the hypothesis is correct the theory and observations provide the cleanest experiment to determine the sensitivity of the earth s climate to truly external influences such a measure of climate sensitivity is of obvious importance considering the recent interest in estimating the change in climate due to other forcings such as the enhanced greenhouse effect due to increased levels of co2 in the atmosphere e g ramanathan 1981 hansen et al 1981 manabe and stouffer 1980 in effect climate models might be tested or calibrated by this natural experiment climate modeling has progressed dramatically in the last decade with the construction and testing of several general circulation models gcm s although some interesting ice age experiments have been done with these models gates 1976 manabe and hahn 1977 they still include only crude parameterizations of the slower components such as the oceans these numerical models are the ultimate tool that can be brought to bear upon the climate problem however they are almost as complicated as the climate itself hence experiments must be carefully designed and the interpretation is often difficult due to the expense of obtaining definitively long runs in parallel with gcm construction has been the development of x permanently at applied research corporation landover maryland 20785

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

determination of bathymetric and current maps by the method disc based on the analysis of nautical x band radar image sequences of the sea surface november 2007 morphodynamic processes in coastal areas are affected by tidal currents and sea state the continuous observation of near coastal areas is important in order to monitor dangerous current regime and bathymetry changes therefore there is an urgent need for remote sensing techniques delivering the important hydrographic parameters with a high spatial resolution dispersive surface classificator disc is a newly developed method based on the analysis of nautical x band radar image sequences of sea surface waves to determine spatial maps of hydrographic parameters e g spatial maps of the bathymetry and the ocean current field the method disc is described and is illustrated by the presentation of results based on a dataset acquired with a ground based x band radar installation mounted on the island of sylt in the german bight the calculated bathymetric maps are verified by multibeam echo sounder observations

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

studies of dual polarized low grazing angle radar sea scatter in nearshore regions reports on experiments featuring horizontally hh and vertically vv polarized x band marine radar sea scatter imagery in the coastal environments of bermuda and la jolla under light to moderate winds and the absence of long gravity waves these conditions allowed the study of fundamental scattering mechanisms from small scale roughness and short waves of a few meters wavelength or less shorter than the radar pulse while a large fraction of radar data collected revealed the presence of slick bands the authors analyze the radar echo of ambient background outside of the slicks sea scatter data were digitally recorded for 360 spl deg azimuthal coverage for grazing angles between 1 3 5 spl deg and were converted to normalized radar cross section nrcs or spl sigma spl deg images the hh and vv polarizations show quite different spatial texture with hh exhibiting a discrete character and vv being more nearly spatially homogeneous grazing angle dependencies are different for hh and vv upwind downwind differences of mean nrcs show ratios of just 4 6 db for vv but are equal or greater than the 16 db noise imposed limit for hh for the low wind conditions hh nrcs grazing angle characteristics are shown to correlate with biological activity indicators of the coastal waters with a fourth power dependence in relatively unproductive waters to a nearly quadratic in productive waters arguments are presented that suggest different scattering mechanisms for the two polarizations evenly distributed bragg scatter patches for vv and scatter from small asymmetric bore features for hh a multipath illumination model for small bore features is outlined and scale sizes for the small scale breakers inferred from the experimental results presented are between 2 and 4 cm in height with crest widths between 24 and 48 cm

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

the approximation of a sea surface using a shore mounted x band radar with low grazing angle abstract the ability to measure real time surface elevation over a large spatial domain offers large benefits to multiple industries including offshore operations and marine renewable applications in combination with a real time short term wave prediction model safety related marine operations windows can be increased this focuses on quiescent period prediction during large sea states and the ability of pre emptive wave by wave tuning of marine energy converters leads to improved efficiencies this study presents a new method of converting x band radar sequence into approximate surface elevation in near real time using synthetic wave trough profile estimations forgoing the use of a linear stochastic model the artificial wave trough method applies a second order stokes wave to trough regions where radar shadows occur these shadows are caused as a result of high surround wave crest height where no surface information is gathered the calculated surface elevation is validated against in situ sensor data at two locations a correlation r of 0 69 is shown for the signature 1000 sensor location while a lower correlation of 0 56 is observed at the wave buoy location when the phase averaged wave parameters hm0 and tp derived from 20 min wave spectra for the entire duration of the measurement campaign are compared a reasonable agreement of 0 62 and 0 41 for the ad2cp and wave buoy location is achieved the presented method shows a good approximation of wave profile shape where the swell component is predominantly measured indicating the potential of x band radar surface elevation measurements in near real time for nearshore inhomogeneous seas

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

true polar wander during the middle paleozoic true polar wander would be recognized paleomagnetically as identical apparent polar wander paths for all surface elements the apparent polar wander paths for the late ordovician late devonian interval for laurentia baltica and gondwana have nearly identical looping shapes that can be brought into superposition the paths of these continents are well documented but even the less well known paths of south china and siberia reveal similar lengths the resulting reconstruction places the northern andean margin of south america opposite the appalachian margin of laurentia with baltica and laurentia adjoined in the fit of bullard and colleagues siberia and south china would be to the north of africa if their paleopoles are taken at face value for middle paleozoic time there is of course no information about oceanic domains butnit is interesting that all continental elements insofar as is known appear to have similar apparent polar wander tracks that can be plausibly superimposed without causing overlap in the positions of the continents albeit speculatively because of the lack of information about the oceanic elements it is suggested in this study that true polar wander may have occurred with a cumulative magnitude of about 75 during a 75 ma interval and may have been of greater magnitude than the apparent polar wander due to relative motions during the middle paleozoic this middle paleozoic rate of true polar wander appears to have been an order of magnitude greater than the average rate during the late mesozoic and tertiary

oceanographic and climatic evolution of the miocene mediterranean deduced from nd sr c and o isotope compositions of marine fossils and sediments isotopic compositions of marine sediments and fossils have been investigated from northern basins of the mediterranean to help constrain local oceanographic and climatic changes adjacent to the uplifting alps stable c and o isotope compositions of benthic and planktonic foraminifera from the umbria marche region umc have an offset characteristic for their habitats and the changes in composition mimic global changes suggesting that the regional conditions of climate and the carbon cycle were controlled by global changes the radiogenic isotope composition of these fossil assemblages allows recognition of three distinct periods in the first period from 25 to 19 ma high nd values and low 87sr 86sr of sediments and fossils support intense tectonism and volcanism related to the opening of the western mediterranean in the second period from 19 to 13 ma the 87sr 86sr ratio of mediterranean umc deviate from the global ocean which is compatible with rapid uplift of the hinterland and intense influx of sr from mesozoic carbonates of the western apennines this local control on the seawater was driven by a humid and warm climate and indicates restricted exchange of water with the global ocean generally the nd values of the fossils are very similar to those of indian ocean water with brief periods of a decrease in the nd values coinciding with volcanic events and maybe sea level variation at 15 2 ma in the third period from 13 to 10 ma the fossils have 87sr 86sr similar to those of miocene seawater while their nd values change considerably with time this indicates fluctuating influence of the atlantic versus the paratethys and or locally evolved seawater in the mediterranean driven by global sea level changes other investigated localities near the alps and from the odp 900 site are compatible with this oceanographic interpretation however in the late early miocene enhanced local control reflecting erosion of old crustal silicate rocks near the alps results in higher 87sr 86sr

estimation of wave phase speed and nearshore bathymetry from video imagery a new remote sensing technique based on video image processing has been developed for the estimation of nearshore bathymetry the shoreward propagation of waves is measured using pixel intensity time series collected at a cross shore array of locations using remotely operated video cameras the incident band is identified and the cross spectral matrix is calculated for this band the cross shore component of wavenumber is found as the gradient in phase of the first complex empirical orthogonal function of this matrix water depth is then inferred from linear wave theory s dispersion relationship full bathymetry maps may be measured by collecting data in a large array composed of both cross shore and longshore lines data are collected hourly throughout the day and a stable daily estimate of bathymetry is calculated from the median of the hourly estimates the technique was tested using 30 days of hourly data collected at the sandyduck experiment in duck north carolina in october 1997 errors calculated as the difference between estimated depth and ground truth data show a mean bias of 35 cm rms error 91 cm expressed as a fraction of the true water depth the mean percent error was 13 rms error 34 excluding the region of known wave nonlinearities over the bar crest the accuracy of the technique improved and the mean rms error was 20 cm 75 cm additionally under low amplitude swells wave height h 1 m the performance of the technique across the entire profile improved to 6 29 of the true water depth with a mean rms error of 12 cm 71 cm

changes of the earth s rotation axis owing to advection of mantle density heterogeneities polar wander the secular motion of the earth s rotation axis relative to its surface has been studied for many years dynamical arguments1 3 show that polar wander can arise from the redistribution of mass in a plastic deformable earth the rate depending on both the rate of mass redistribution and the rate at which the earth s rotational bulge can readjust to the changing rotation axis here we use a viscosity structure obtained through geoid modelling4 a mantle flow field consistent with tomographic anomalies5 and time dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree two geoid during the cenozoic era we show that the rotation axis will follow closely any imposed changes of the axis of maximum non hydrostatic moment of inertia the resulting path of the rotation axis agrees well with palaeomagnetic results7 with the model predicting a current rate of polar motion that explains 40 of that observed geodetically8

explanations of the tertiary global cooling trend abstract a series of general circulation model experiments are performed to examine the role of paleogeography as an explanation of the tertiary global cooling trend systematic long term variations in topography and land sea distribution are potential causes of long term climate change however climate model sensitivity experiments indicate a geographically related cooling during the last 20 million years but not a long term cenozoic trend coarse resolution changes in geography throughout the tertiary may not be the explanation of the tertiary global cooling trend either a series of events some geographically related or another forcing factor most probably atmospheric co2 concentration are offered as alternative explanations of the tertiary global cooling trend

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

plio pleistocene climate sensitivity evaluated using high resolution co2 records theory and climate modelling suggest that the sensitivity of earth s climate to changes in radiative forcing could depend on the background climate however palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction here we present atmospheric carbon dioxide co2 reconstructions based on multi site boron isotope records from the late pliocene epoch 3 3 to 2 3 million years ago we find that earth s climate sensitivity to co2 based radiative forcing earth system sensitivity was half as strong during the warm pliocene as during the cold late pleistocene epoch 0 8 to 0 01 million years ago we attribute this difference to the radiative impacts of continental ice volume changes the ice albedo feedback during the late pleistocene because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea level reconstructions we conclude that on a global scale no unexpected climate feedbacks operated during the warm pliocene and that predictions of equilibrium climate sensitivity excluding long term ice albedo feedbacks for our pliocene like future with co2 levels up to maximum pliocene levels of 450 parts per million are well described by the currently accepted range of an increase of 1 5 k to 4 5 k per doubling of co2

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

the influence of true polar wander on glacial inception in north america abstract the impact that long term changes in earth s rotation axis relative to the surface geography or true polar wander tpw and continental drift have had in driving cooling of high latitude north america since the eocene is explored recent reanalyses of paleomagnetic pole positions suggest a secular drift in earth s rotation axis of 8 over the last 40 myr in a direction that has brought north america to increasingly higher latitudes using modern temperature data in tandem with a simple model a reduction in the annual sum of positive degree days pdds driven by this polar and plate motion over the last 20 myr is quantified at sites in baffin island the tpw and continental drift driven decrease in insolation forcing over the last 20 myr rivals changes in insolation forcing caused by variations in earth s obliquity and precession using conservative pdd scaling factors and an annual snowfall equal to modern station observations the snowiest location in baffin island 20 myr ago had a mass balance deficit of 0 75 2 m yr 1 water equivalent thickness relative to its projected mass balance at 2 7 ma this mass balance deficit would have continued to increase as one goes back in time until 40 myr ago based on adopted paleopole locations tpw and continental drift that moved arctic north america poleward would have strongly promoted glacial inception in baffin island at 3 ma a location where the proto laurentide ice sheet is thought to have originated

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

paleolatitude of the hawaiian hot spot since 48 ma evidence for a mid cenozoic true polar stillstand followed by late cenozoic true polar wander coincident with northern hemisphere glaciation

sensitivity of mass balance of five swiss glaciers to temperature changes assessed by tuning a degree day model a degree day model is used to assess the sensitivity of the mass balance of five swiss glaciers to temperature changes the model uses temperature data extrapolated from nearby climate stations and is tuned by varying precipitation to make the model fit the observed distribution of mass balance with altitude once the model is tuned the effect of temperature change is simulated by recalculating the mass balance with the same parameters as before but with a temperature increase of 1 c throughout the year the largest mass balance changes involving increased ablation of 1 m w e a 1 c 1 occur at the snout with a progressively smaller increase with altitude the area averaged sensitivities for the five glaciers are 0 7 to 0 9 m w e a 1 c 1 if annual precipitation also increased by 20 it would partly offset the effect of the 1 c higher temperatures but could not compensate for it