mid to late holocene mediterranean climate variability contribution of multi proxy and multi sequence comparison using wavelet analysis in the northwestern mediterranean basin abstract forcings and mechanisms underlying holocene climate variability still remain poorly this work review already published paleoclimatic time series and proposes an alternative way to compare them using spectral analysis such an approach may emphasize joint features between different signals and lead us closer to the causes of climate changes ten paleoclimatic proxy records from 5 sequences from the gulf of lions and surrounding areas were compiled these paleoclimate time series were supplemented with proxies of the north atlantic oscillations nao el nino southern oscillations enso and the intertropical convergence zone itcz variability a comparison of their frequency content is proposed using wavelet analysis for unevenly sampled time series a new algorithm is used in order to propagate the age model errors within wavelet power spectra three main groups of shared features specific to the mid and late holocene after 7000 yrs cal bp can be defined on the basis of the results of these analyses an atlantic cyclic period solar cyclic periods and tropical cyclic periods the atlantic cyclic period is probably related to fluctuations of the atlantic thermohaline circulation which would induce changes in the storm track extension and position thereby impacting upon precipitation and storminess over a millennial scale the centennial scale solar variability might induce a nao like variability of the atmospheric circulation thereby influencing storminess in western europe and mediterranean finally tropical cyclic periods are possibly registered in of the gulf of lions and atlantic climate proxies potentially highlighting the influence of enso variability over the western mediterranean

detransformation of the british geomagnetic secular variation record for holocene times in this note we replot the british post glacial secular variation curve in terms of actual values of declination and inclination rather than the transformed declination and inclination system used in our original paper turner thompson 1981 this should facilitate the comparison of our record with contemporaneous european archaeomagnetic data our composite record was obtained by averaging the dated records from 10 cores of lake sediment the sequence of unit vectors describing the magnetic direction record of each core was rotated so as to place its mean direction the mean over the section of core dated between 0 and 7000 bp the time span of the shortest core at zero transformed declination and inclination dt o it 0 the transformed coordinate frame of each core is thus derived from its mean direction assumed to represent the mean geomagnetic field direction over the 7000 yr period rather than from the core tube and its orientation during coring hence the problem of not knowing the azimuthal orientation and inclination of the corer during core collection is avoided and the 10 records can be averaged in this coordinate system transformed coordinates then provide a convenient generalised method of plotting averaged sediment records however in the archaeomagnetic method sample orientation is straightforward and the results can readily be plotted as absolute declination and inclination values so for consistency when comparing the two we have attempted here to detransform our record the following procedures assumptions and approximations were taken to produce the detransformed magnetograms and bauer type plots of figs 1 and 2 1 amplitude correction although the combination of 10 data sets produces a good estimate of the signature of the geomagnetic variations the following factors contribute to an overall loss of amplitude in both declination and inclination a the quality of magnetic recording varies widely between different sediments depending on the porosity of the sediment the size and shape distributions and the nature of the magnetic carrier verosub 1979 the loch lomond sediments carry a good high amplitude pattern of remanent directions of which the most recent closely follow the london observatory records the much wetter and less compacted sediments of llyn geirionydd on the

interlaminated ice proximal glacimarine sediments in muir inlet alaska muir inlet in glacier bay alaska is a glacial fjord receiving a tremendous volume of sediment annually the rate of sediment accumulation is greatest proximal to muir glacier about 9 m yr 1 and decreases away from the glacier the primary sediment sources are meltwater streams discharging at subglacial and ice marginal positions to form overflows interflows and underflows continuous turbidity currents overflows and interflows interact with diurnal tidal currents and their volume and sediment concentration varies diurnally and annually with meltwater discharge these effects produce cyclic deposits of a thin fine grained sand or silt lamina that grades normally to a thicker poorly to very poorly sorted mud lamina this lamina couplet is termed a cyclopel underflows are suggested to occur in this glacimarine environment because of conditions unique to subglacial fluvial systems underflow deposits occur only in proximal positions 0 5 km from glacier face and are coarse grained reverse to normal graded and exhibit an increase in sorting and sand content up layer ice rafted debris identified as particles 177 m is ubiquitous though low 5 by weight and occurs as isolated particles frozen pellets or as lenses that in cores may have a lamina appearance proximally ice rafted debris is difficult to identify because proximal sediment is often as coarse grained deposited sediment may be reworked by tidal currents and sediment gravity flows depositional processes operating in muir inlet produce interlaminated sand silt clay that characterizes sediment proximal to a glacier and fines seaward to mud sediment is classified into one of three sediment types 1 1 type i sediment is very fine grained mean 8 65 7 17 o low in sand 0 1 11 2 and very poorly to poorly sorted it is the dominant sediment type in muir inlet and is transported by plumes and deposited by suspension settling 2 2 type ii sediment is fine to coarse grained mean 6 70 3 12 o low to high in sand 5 1 86 6 and very poorly to moderately sorted it represents reworked sediment proximal plume deposits or coarse grained laminae of cyclopels 3 3 type iii sediment is coarse grained mean 3 89 2 38 o high in sand 58 0 100 0 and poorly to well sorted it is deposited by sediment gravity flows or underflows

sedimentation by river induced turbidity currents field measurements and interpretation turbidity currents initiated from spring runoffs of an influent river were observed in the upper region of a reservoir in hokkaido japan by measuring water temperature velocity and suspended sediment concentration their profiles offer some physical parameters for the sedimentary conditions assuming the turbidity currents to be quasi uniform the bottom sediment deposited by the turbidity currents was then collected by a portable core sampler the bottom sediment consists of more than 90 silt and clay and thus offers a hydraulically smooth bed for shear flow a plane bed as a bed configuration was formed on the reservoir bed probably because of the low shear velocity and small grain size of sediment using a graphic method with log normal probability paper the bottom sediment is divided into several overlapping log normal subpopulations grain size analysis indicates that the bottom sediment may be regarded as cohesionless criteria for complete deposition of transported grains can then be incorporated into the extended shields diagram giving the minimum shear stress to erode bottom sediment applying the new diagram to the grain size distribution of the bottom sediment it is suggested that each of the log normal subpopulations was deposited in each of four different modes of deposition i e traction saltation or intermittent suspension suspension and suspension under equilibrium the last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity the mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of fd2 log10re fd is the densimetric froude number and re is the flow reynolds number correlates inversely with shear velocity which bears a linear relationship to mean velocity these results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy

submarine sedimentary features on a fjord delta front queen inlet glacier bay alaska side scan sonar images provide a view of an actively changing delta front in a marine outwash fjord in glacier bay alaska numerous interconnected gullies and chute like small channels form paths for the transport of sand and coarse silt from the braided glacial outwash streams on the delta plain to the sinuous turbidity current channels incised into the fjord floor these turbidity current channels carry coarse sediment through the fjord and into the adjoining glacial trunk valley several sedimentary processes affect the development of this delta front overflow plumes deposit fine sediment sediment gravity flows result from episodic delivery of large loads of coarse sediment and mass movement may be triggered by earthquakes and more regularly by spring tidal drawdown or hydraulic loading

active sand transport along a fjord bottom channel bute inlet british columbia an underwater channel system is incised in the holocene fjord basin sediments of bute inlet british columbia high resolution side scan sonar swaths and seismic profiles reveal two channels within a zone of extensive rotational sliding on the slopes of a fjord head delta the channels coalesce into a single sinuous channel that extends 14 km downfjord on a 0 9 slope where it splits into two distributaries which continue another 18 km ending within stacked depositional lobes piston cores show sands in the channels and lobes in contrast to deep water silty clays in the basin floor the sea floor features and sediment distributions result from active highly mobile slide generated sediment flows which transport sands via the channels from the delta to the fjord basin

the record of glacial lake champagne in kusawa lake southwestern yukon territory late pleistocene glaciers dammed glacial lake champagne in the valley of dezadeash river between a westward flowing glacier in the takhini valley and eastward flowing glaciers from the st elias complex modern kusawa lake lies in the southern extension of lake champagne geophysical and geomorphic evidence documents the deglaciation of the lake the presence of lake champagne and the postglacial sedimentary environment of the basin during the main phase of lake champagne the water level stood at 772 m in the northern part of kusawa lake and 756 m in northern dezadeash valley both probably controlled by a spillway floored at 756 m to the north into the nordenskiold river this indicates differential isostatic rebound of 0 2 m km from south to north at that time a trunk glacier occupied the southern portion of kusawa lake depositing a thick sequence of sediment in the basin a glacier in the primrose valley and the takhini trunk glacier built large deltas into lake champagne subsequently the level

unified structure in quaternary climate the quaternary climate record exhibits a structure of superimposed aperiodic oscillations starting at the 11 yr sunspot cycle and spaced by powers of 2 in period through the major 90 000 yr glacial cycle climate cycles that do not fall in this structure typically correspond to harmonics of the structure oscillations the inclusion of the known solar cycles and the presence of increased abundances of cosmogenic radionuclides at many structure periods suggest that the structure is related to long period solar variability

facies characteristics of a proglacial turbiditic sand lobe at svalbard abstract a shallow marine turbiditic lobe inferred to be associated with an ice contact delta was deposited during the last glaciation weichselian at svalbard the norwegian arctic the postglacially raised sandy lobe is exposed in a 200 m long river section and exhibits six units that reflect an early phase of glaciomarine mud deposition followed by three episodes dominated by deposition from sandy turbidity currents the lobe is characterized by a complex pattern of turbidite filled erosional features from thin scour fills reflecting a highly mobile flow pattern to stacked chute fill deposits showing confined flow the sandy turbidites show rhythmic deposition of massive to graded laminae deposited from suspension alternating with faint laminae or ripple cross lamination deposited from tractional transport semi continuous turbidity currents are thought to have been generated by retrogressive slumping of ice proximal well sorted sandy deposits on the delta front and slope the lateral and vertical changes of lithofacies within the sandy lobe are mainly attributed to normal mechanisms of turbiditic lobe growth with only minor effect from synsedimentary glacier terminus movement and minor changes in the meltwater outflow position this detailed mapping of facies changes allows a fairly accurate reconstruction of the axis of the former discharge outlet

power spectrum of sea level change over fifteen decades of frequency 1 the power spectrum of relative sea level change has been estimated over more than 15 orders of magnitude in frequency from a frequency of 1 591 ma to a frequency of 1 5 s although there are still regions of the spectrum where data sampling and duration do not allow the power to be calculated most notably between periods of 100 1000 years the general shape of the spectrum is that in which the power depends on the square of the reciprocal frequency apart from periods between 1 and 100 years where the power spectrum falls off less steeply with increasing frequency a spectrum in which power depends on reciprocal frequency squared is the same spectrum as that calculated from a random walk signal of finite length there are some causes that have defined frequencies such as those associated with tides and the milankovitch cycles of the ice ages but there is also a continuum of relative sea level change that requires other causes the implications of this are discussed in the light of global change and heating of the lithosphere from the bottom

turbidity current channels in queen inlet glacier bay alaska queen inlet is unique among glacier bay fjords because it alone has a branching channel system incised in the holocene sediment fill of the fjord floor bathymetry and seismic reflection profiles s

geomagnetic secular variation as recorded in british lake sediments and its application to archaeomagnetic studies abstract lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field as they potentially offer near continuous high resolution archives of magnetic information extending throughout the holocene to date lake sediment geomagnetic data has typically been excluded from british archaeomagnetic secular variation curves svc due to uncertainties with the acquisition of magnetisation by lake sediments this paper presents the argument that with regards to british datasets the real problem lies with poor chronological control and sets out to illustrate that with british archaeomagnetic data some progress has been made the results indicate that it is not currently possible to resolve secular variation on a time scale of 100 years from published british lake sediment data but more success has been made with data from archaeological sediments this level of detail has been considered necessary for the incorporation of lake sediment data into the british archaeomagnetic dataset as the ability to resolve short term geomagnetic changes is critical for the integration of any dataset into the british svc as the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes using currently available data this retrospective critique confirms that in archaeological sediments depositional and thermoremanent magnetisation can record the same direction over the same time interval

turbidity currents and submarine channel formation in rupert inlet british columbia 1 surge observations acoustic backscatter images obtained at 107 and 200 khz of surge type turbidity currents resulting from mine tailing discharge are presented excess densities in the head of the order of 100 kg m 3 are estimated from differences in acoustic backscatter intensity and in the amplitude of the bottom reflected signal these excess densities are comparable to estimates based on surge height and speed the speed being determined by assuming a universal head shape unlike gravity current surges from an impulsively started steady source the surge type turbidity flows consist of a high concentration slug of short duration followed by a longer lived wake in which sediment concentrations are lower by 2 3 orders of magnitude the profile of the high concentration slug resembles those for density slugs on small bottom slopes obtained in laboratory experiments discrete scatterers probably fish are observed to avoid the surges

sedimentary processes of canadian arctic fjords abstract conceptual models of circulation and sedimentation in arctic fjords are presented to illustrate the current understanding of these environments circulation is a vigorous process in these fjords and is enhanced by the loss of the freshwater cap during the long winter period of near zero inflow the process of brine rejection during the formation of sea ice and the role of glaciers and icebergs injecting freshwater at depth on the other hand the presence of sea ice blocks reduces the mixing action of wind which may otherwise represent the largest input of energy to fjord circulation it also provides a freshwater layer on melting and may in some circumstances damp the amplitude of tides during summer large loads of glacial sediment entering arctic fjords are separated into streams of overflowing suspended load and bed load in the presence of salt water as the concentrations of suspended load in the freshwater are normally much less than required to overcome the density of the salt water deposition occurs throughout the fjord by rapid settling of flocculated particles from the overflowing brackish water and from grain flows in the proximal environment created as the fluvial traction carpet moves over the delta foreset slope after sediment has been deposited it may be remobilized by slides which in turn may develop into fully turbulent flows as they move down the fjord floor a range of deposits from slump debris to minor turbidites are postulated to occur with the flocculated muds although not all have yet been examined in the arctic fjord stratigraphic record sea ice is an important sedimentological agent in arctic fjords it passively carries fluvial aeolian and colluvial material which is subsequently released from the ice during the stage of puddle draining at break up sea ice also actively incorporates fine and coarse sediments up to large boulders during freezing along the shores in winter and by wave and tidal current washing controlled by the presence of the ice during break up

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

sea level variations global sedimentation rates and the hypsographic curve sea level changes during the neozoic have been estimated by two different methods the first involves measuring the amount of present day land area which was flooded during the past and using the present day hypsographic curve to estimate the amount of sea level rise necessary to produce this flooding the second involves the estimation of the changing volume of mid oceanic ridges through time and estimating sea level changes after having allowed for isostatic adjustment a difference in sea level of 170 m is obtained from the two methods for the cretaceous 80 100 m y b p this is equivalent to a difference in continental flooding of 24 mm 2 using the present day hypsographic curve we attempt to explain this difference firstly by allowing for the fact that the present day ocean basins have more sediment in them than did the cretaceous ocean basins this produces a sea level change in the opposite direction to that produced by the reduction in mid ocean ridge volume since the cretaceous secondly we suggest another large factor in producing the difference is that the present day hypsographic curve is not the correct one to use when studying sea level stands in the cretaceous present day average continental heights are closely related to continental areas accepting this principle if continents are joined together in the past their average height must be greater and so their hypsographic curve must be steeper a given sea level rise would produce less continental flooding during times of continental aggregation than it would today

freshwater megaflood sedimentation what can we learn about generic processes abstract there is growing recognition that sedimentary deposits related to high energy large scale freshwater floods are widespread across the continents and in the main can be related to quaternary outbreak flows from ice dammed lakes such deposits may also occur in the neogene and earlier recognition of these megaflood deposits is hindered by the lack of well conditioned stratigraphic models of depositional successions however descriptions of successions from many locations often exhibit commonalities when compared carefully thus the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence so as to identify the key signatures of megaflood successions the deposits often are the only record of a flood and so the secondary purpose is to interpret the sedimentary sequences in order to recreate the behaviour of floods through single and multiple events finally some pointers are provided as to those areas of sedimentological research that might be profitably explored in more depth to improve understanding of megaflood dynamics a key characteristic of megaflood deposits is the apparent massive appearance of single units which on closer inspection usually are amalgamated layers of similar grain size distribution individual beds and sets of beds are commonly near horizontal planar and laterally extensive successions evidently are not channelized and extensivity is indicative of simultaneous and coherent deposition over large areas localised large scale clinoforms or smaller scale cross beds are common associated with bar front or dune progradation respectively but otherwise extensivity implies that cross cutting erosional contacts notably steeply inclined examples and small scale scours are not common except immediately around large isolated clasts such as dropstones and ice blocks a common vertical stack of sequences in any one succession consists of 1 basal thick coarse parallel bedded units 2 large scale clinoforms 3 horizontally bedded thin laminated units 4 ripple and dune cross beds 5 silt beds and 6 succession capping debris flow deposits in some cases such a succession usually is indicative of a single cycle of waxing and waning flood flow generally dominated by high concentration suspension deposition onto plane beds the high power allows the transport of fine gravel and coarse sand in suspension leading upon deposition to relatively coarse beds from suspension fall out clay and silt is generally sparse or absent being transported further down the system at the scale of the hydrograph the successions indicate that flows initially accelerate and then decelerate with significant shorter period flow pulses also evident in the sedimentary signatures in parallel bedded sequences primarily during waxing flows bedload transport often was ineffective in sorting sediment due to the high rates of deposition from the highly sediment charged waters resulting in diffuse accretion surfaces at each location similar sequence stacks are common leading to the impression of rhythmic deposition from pulsing flow including from repetitive series of floods although often composed of coarse grains many sequence stacks excluding any debris flows are not dissimilar to turbidity flow sequences the bouma sequence in particular although there is limited experimental evidence for turbidity currents associated with high energy freshwater flows theory does not preclude the deposition of similar sedimentary motifs from high concentration flows such that an analogy between some megaflood sedimentary beds and turbidites is not unreasonable despite considerable variation in megaflood sedimentary successions commonalities occur which are indicative of similar processes of deposition from high energy floods at geographically different locations the changes in style of deposition throughout a succession provide clues as to the nature of the flood flows and this information could be used to improve estimates of flood hydrograph behaviour however hypotheses linking deposit characteristics to flow dynamics are currently ill conditioned and often poorly tested the majority of published studies describe the coarser scale stratigraphy with only passing reference to the fine scale structure of individual beds closer attention to lamination styles particle orientation and syn depositional deformation structures for example should provide information on the flow mechanics at the time of bed deposition thus better consideration of the sedimentology of megaflood deposit beds will lead to better understanding of the controlling hydraulics and should result in better models of flood deposition

rainstorm induced event sedimentation at the tidewater front of a temperate glacier runoff from a late summer storm drained rapidly to a subglacial stream of mcbride glacier southeast alaska the stream transported large volumes of sediment that discharged from the glacier s tidewater front directly into fjord water at 40 m depth measurements of salinity temperature and suspended sediment concentration indicate that the sediment was transported by interflows within brackish fjord water sedimentation rates were exceptionally high up to five times normal laminated fine sand accumulated in proximal sediment traps and silty mud accumulated in distal traps the low frequency of large storms late in the melt season indicates that the sedimentary products would be rare but significant marker horizons in temperate glaciomarine sequences

visual stratigraphic dating of the gisp2 ice core basis reproducibility and application annual layers are visible in the greenland ice sheet project 2 ice core from central greenland allowing rapid dating of the core changes in bubble and grain structure caused by near surface primarily summertime formation of hoar complexes provide the main visible annual marker in the holocene and changes in cloudiness of the ice correlated with dustiness mark wisconsinan annual cycles both markers are evident and have been intercalibrated in early holocene ice layer counts are reproducible between different workers and for one worker at different times with 1 error over century length times in the holocene reproducibility is typically 5 in wisconsinan ice age ice and decreases with increasing age and depth cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50 000 years visible observations are not greatly degraded by brittle ice or many other core quality problems allowing construction of long consistently sampled time series high accuracy requires careful study of the core by dedicated observers

optimizing significance testing of astronomical forcing in cyclostratigraphy the recognition of astronomically forced milankovitch climate cycles in geological archives marked a major advance in earth science revealing a heartbeat within the climate system of general importance and key utility power spectral analysis is the primary tool used to facilitate identification of astronomical cycles in stratigraphic data but commonly employed methods for testing the statistical significance of relatively high narrow band variance of potential astronomical origin in spectra have been criticized for inadequately balancing the respective probabilities of type i false positive and type ii false negative errors this has led to suggestions that the importance of astronomical forcing in earth history is overstated it can be readily demonstrated however that the imperfect nature of the stratigraphic record and the quasiperiodicity of astronomical cycles sets an upper limit on the attainable significance of astronomical signals optimized significance testing is that which minimizes the combined probability of type i and type ii errors numerical simulations of stratigraphically preserved astronomical signals suggest that optimum significance levels at which to reject a null hypothesis of no astronomical forcing are between 0 01 and 0 001 i e 99 99 9 confidence level this is lower than commonly employed in the literature 90 99 confidence levels nevertheless in consonance with the emergent view from other scientific disciplines fixed value null hypothesis significance testing of power spectra is implicitly ill suited to demonstrating astronomical forcing and the use of spectral analysis remains a difficult and subjective endeavor in the absence of additional supporting evidence

eustasy and epeirogeny of continents on time scales between about 1 and 100 m y eustatic changes are believed to be the cause of the extensive flooding of the continents by shallow marine seas during the cretaceous and much of the paleozoic the possible causes of eustatic change which can be easily quantified over the past 100 m y have been estimated in order to arrive at a value for the freeboard decrease of the continents these causes in order of decreasing importance are ridge crest volume thermal uplift associated with excessive volcanism in the pacific during the cretaceous formation of continental ice sheets the collision of india with asia and the cooling of the oceans several effects act in the opposite direction and they can be grouped together because they are all due to there being larger quantities of sediments in the ocean basins today than there were 100 m y ago another effect causing a freeboard increase is the thinning of continental masses during continental breakup and the formation of new ocean basins the total change in sea level after having allowed for isostatic response is 202 m during the last 80 m y flooding of the continents can also be used to determine sea level changes with help from modern hypsographic curves when this is done it is found that the freeboard decrease is considerably less than that calculated from volume estimates although the inaccuracy of the various data would allow these estimates to be not significantly different it is likely that the estimate of freeboard decrease using flooded areas and modern hypsographic data is an underestimate due to the probability that the hypsographic curve has become less steep with time individual estimates of flooding from continents require considerably different amounts of freeboard change the common signal is usually assumed to be the eustatic signal whereas when this common signal is removed from the freeboard estimate for an individual continent the remaining signal is the epeirogenic signal epeirogenic movements can be as large as 150 m and can occur on time scales of a few tens of millions of years other examples of epeirogenic variations are also discussed we have developed a rough power spectrum for epeirogenic movements of the continents this appears to have 1 f variation we suggest that the spectrum could be produced by temperature changes at the bottom of the lithosphere

advances in archaeomagnetic dating in britain new data new approaches and a new calibration curve abstract archaeomagnetic dating offers a valuable chronological tool for archaeological investigations particularly for dating fired material the method depends on the establishment of a dated record of secular variation of the earth s magnetic field and this paper presents new and updated archaeomagnetic directional data from the uk and geomagnetic secular variation curves arising from them the data are taken from publications from the 1950 s to the present day 422 dated entries derived from existing archaeo and geomagnetic databases are re evaluated and 487 new directions added resulting in 909 entries with corresponding dates the largest collection of dated archaeomagnetic directions from a single country an approach to improving the largest source of uncertainty the independent dating is proposed and applied to the british iron age resulting in 145 directions from currently available databases being updated with revised ages and or uncertainties and a large scale reassessment of age assignments prior to inclusion into the magnetic moments of the past and geomagia50 databases from the significantly improved dataset a new archaeomagnetic dating curve for the uk is derived through the development of a temporally continuous geomagnetic field model and is compared with previous uk archaeomagnetic dating curves and global field models the new model arch uk 1 allows model predictions for any location in the uk with associated uncertainties it is shown to improve precision and accuracy in archaeomagnetic dating and to provide new insight into past geomagnetic field changes

estimation of spectral power laws in time uncertain series of data with application to the greenland ice sheet project 2 δ18o record 1 errors in the timing assigned to observations degrade estimates of the power spectrum in a complicated and nonlocal fashion it is clear that timing errors will smear concentrations of spectral energy across a wide band of frequencies leading to uncertainties in the analysis of spectral peaks less understood is the influence of timing errors upon the background continuum we find that power law distributions of spectral energy are largely insensitive to errors in timing at frequencies much smaller than the nyquist frequency though timing errors do increase the uncertainty associated with estimates of power law scaling exponents these results are illustrated analytically and through monte carlo simulation and are applied in the context of evaluating the power law behavior of oxygen isotopes obtained from greenland ice cores age errors in layer counted ice cores are modeled as a discrete and monotonic random walk that includes the possibility of biases toward under or overcounting the 18oice record from the greenland ice sheet project 2 is found to follow a power law of 1 40 0 19 for periods between 0 7 and 50 kyr and equivalent results are also obtained for other greenland ice cores

contrasting atmospheric and climate dynamics of the last glacial and holocene periods our present climate is relatively stable compared to that of the last glacial maximum about 20 000 years ago palaeoclimate records obtained from ice cores1 2 and deep sea sediment cores3 for the last glacial period show abrupt temperature changes on timescales of a few hundred years which have been attributed to cycles of ice build up and release associated with large ice sheets dansgaard oeschger cycles and heinrich events 3 and their coupling to ocean circulation4 5 but little is known about the dynamics of the atmosphere during the last glaciation ice sheets influence atmospheric circulation and studies using general circulation models have suggested stormier more variable atmospheric dynamics during the last glacial maximum than today6 9 here we report the results of an analysis of temporal trends over the past 91 000 years in the oxygen isotope signatures of a high resolution ice core record from greenland1 2 this analysis provides direct evidence that atmospheric circulation during the last glaciation was more turbulent than it is today

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

turbidite variability and architecture of sand prone deep water slopes eocene clinoforms in the central basin spitsbergen abstract the architecture and turbidite variability within six wedge shaped downslope thinning sand prone slope accumulations are documented from eocene shelf margins on spitsbergen the central basin formed as a small foreland or piggy back basin and the studied turbidites accumulated mainly on the slope portion of sand prone clinoforms that developed during depocenter migration and infilling of the basin the shelf margin clinoforms have amplitudes minimum water depths from 100 to 350 meters and their shelf slope and basin floor segments are well imaged and can be walked out along many of the mountainsides only a small percentage of the clinoforms are sand prone and these developed when sea level occasionally fell to or below the shelf edge of the sand prone clinoforms some had their sand budget partitioned mainly out onto the basin floor basin floor fans but most trapped the sand on the slope only the latter are now visible as downslope thinning wedges some 2 5 3 5 km in downdip extent the turbidites within this type of clinoform have been examined and classified the lower slope to base of slope segment of the studied clinoform complexes are dominated by lobes consisting of broad shallow channels and sheet like turbidites becoming heterolithic and muddy out on the basin floor beds on the lower slope vary from thick up to 4 5 meters ungraded or laminated sandstones to thinner ungraded sandstones with coarse cappings the middle slope segment of clinoform complexes is dominated by narrow channels chutes that feed downslope to progradational chute mouth lobes chutes contain ungraded and laminated sandstone beds up to 3 m thick whereas the chute mouth lobes show alternations of thinner ungraded to laminated or rippled sandstones these lobes become more heterolithic and muddy downslope the shelf edge to upper slope segment of clinoform complexes is dominated by upward coarsening and thickening sheetsands of steep fronted shelf edge deltas the sandsheets of the delta front can be traced updip into mouth bar and distributary channel sandstones it is argued that shelf margin accretion represented by the sand prone slope wedges was achieved mainly by sand laden currents that flooded from the shelf edge as hyperpycnal flows this hypothesis is supported by 1 the direct connection between channel and mouth bar systems at the shelf edge and the turbidites of the slope lobes 2 the systematic progradational character of the slope lobes 3 the absence of large scale slump scars gullies or canyons on the slope and 4 the dominance of a type of turbidite that implies deposition from sustained flow detailed examination of the architecture of one of the slope wedges shows that there are unconformities developed within the stratigraphy below the shelf edge and that these erosional terraces beheaded the deltas perched on the uppermost slope the erosion surfaces indicate fall of sea level to this position despite the magnitude of this fall up to 80 meters the lack of canyons on the slope prevented the construction of basin floor fans such falls of sea level on non canyoned slopes simply promote sand prone accretion of the shelf margin

a method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating summary spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present day field a method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation for an archaeomagnetic region the size of the british isles 900 km radius the maximum error in relocating vectors to a central location is predicted to be of the order of 1 2

physical and structural properties of the greenland ice sheet project 2 ice core a review substantial data sets have been collected on the relaxation characteristics density grain size c axis fabrics and ultrasonic velocities of the greenland ice sheet project 2 gisp2 core to its contact with bedrock at 3053 4 m changes in all these properties paralleled closely those found in cores from byrd station antarctica and dye 3 greenland density increased progressively with depth to a maximum of 0 921 mg m3 at about 1400 m at which depth the ice became bubble free below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures since drilling much of the ice core has undergone significant volume expansion relaxation due to microcracking and the exsolving of enclathratized gases especially in the brittle ice zone between 650 and 1400 m grain size increased linearly to about 1000 m thereafter remaining fairly constant until the younger dryas event at 1678 m where a twofold to threefold decrease in grain size occurred these grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical including a small increase in c axis concentration across the younger dryas holocene boundary increased dust levels in the wisconsin ice have contributed to the maintenance of a fine grained texture which with its strong vertical c axis fabric persisted to nearly 3000 m however beginning at about 2800 m layers of coarse grained ice intermixed with the much finer grained matrix ice are observed below 3000 m the ice became very coarse grained this change attributed to annealing recrystallization at elevated temperatures in the ice sheet was accompanied by a dispersed or ring like redistribution of the c axes about the vertical ultrasonic measurements of vertical and horizontal p wave velocities made at 10 m intervals along the entire length of the gisp2 core fully confirmed the results of the crystallo optical observations a return to fine grained ice coincided with the first appearance of brown silty ice 13 m above bedrock bedrock material consisted of 48 cm of till including boulders and cobbles overlying gray biotite granite comprising the true bedrock there is evidence that disturbed structure in the gisp2 cores begins little more than 70 of the way through the ice sheet this disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10 of the ice sheet

deltaic infill of a deglaciated arctic fjord east greenland sedimentary facies and sequence stratigraphy abstract the deltaic fill of a deglaciated side entry fjord in western jameson land east greenland shows the record of an interplay between regional and local controls including climate and relative sea level change onshore and offshore topography and sediment yield a four phase model is suggested for the delta development and fjord filling at the deglaciation stage phase 1 the newly flooded fjord floor primarily accumulated sediment from reworked glacial material these deposits were relatively thin and draped the uneven sea floor topography the subsequent onset of paraglacial delta sedimentation phase 2 was characterized by a high meltwater runoff and extreme sediment yield and corresponded to the highstand and gradual fall of relative sea level a gilbert type delta with a short distributary plain formed at the head of the tributary fjord which received a very high amount of fine grained sediment the stage of enhanced delta progradation phase 3 was characterized by decreased sediment yield and runoff and a rapid fall of relative sea level delta progradation was fast because of rapidly decreasing accommodation space and a solitary river channel deeply incised the delta the equilibrium stage phase 4 spanned most of holocene time and involved a lowered rate of relative sea level fall seasonal runoff supplied limited amounts of sediment to the river mouth and the delta front became increasingly reworked by wave action and longshore drift the side entry fjord a small deglaciated valley invaded by the sea was filled chiefly by highstand and regressive systems tracts whereas relatively little sea floor aggradation was associated with the transgressive systems tract the stratigraphic pattern documented in this paper contrasts with the pattern usually reported from nonglaciated incised valley systems which are filled mainly by the transgressive systems tract typically involving a retreating estuarine system the pattern also differs from some large gradually deglaciated fjord valleys containing thick transgressive glaciomarine deposits accumulated during glacier retreat the study demonstrates the large variability of valley fills and illustrates the importance of topography sediment supply and climate as well as the style of deglaciation on the sediment filling of a fjord valley

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

reconciling discrepancies between uk37 and mg ca reconstructions of holocene marine temperature variability significant discrepancies exist between the detrended variability of late holocene marine temperatures inferred from mg ca and uk37 proxies with the former showing substantially more centennial scale variation than the latter discrepancies exceed that attributable to differences in location and persist across various calibrations indicating that they are intrinsic to the proxy measurement we demonstrate that these discrepancies can be reconciled using a statistical model that accounts for the effects of bioturbation sampling and measurement noise and aliasing of seasonal variability the smaller number of individual samples incorporated into mg ca measurements relative to uk37 measurements leads to greater aliasing and generally accounts for the differences in the magnitude and distribution of variability an inverse application of the statistical model is also developed and applied in order to estimate the spectrum of marine temperature variability after correcting for proxy distortions the correction method is tested on surrogate data and shown to reliably estimate the spectrum of temperature variance when using high resolution records applying this inverse method to the actual mg ca and uk37 data results in estimates of the spectrum of temperature variance that are consistent this approach provides a basis by which to accurately estimate the distribution of intrinsic marine temperature variability from marine proxy records

occurrence of hyperpycnal flows and hybrid event beds related to glacial outburst events in a late ordovician proglacial delta murzuq basin sw libya abstract a late ordovician glacially related delta is documented based on architecture and facies from outcrops in the western murzuq basin libya sandstone megabeds highlight clinoform geometries of a proglacial low angle shallow water delta the progradational wedge extends over 25 km from the sse to the nnw and is about 100 m thick two types of clinothems have been distinguished and eleven facies sequences are characterized in relation with their along slope positions sand prone facies sequences in type 1 clinothems relate to high magnitude low frequency glacial outburst floods generating a freshwater highly concentrated plunging hyperpycnal flow at the river mouth during the waxing stage the slope adjustment of an out of grade profile causes significant erosion the resulting additional sediment charge involves along foresets a downslope flow transformation into a co genetic sandy debris flow and high density turbidity flow hybrid event beds including linked debrites and turbidites are the resulting deposits in the lower slope decreasing sediment concentration of the high density turbidity flow leads to sediment lofting and to a second flow transformation resulting in a lower density turbidity flow and a concomitant rising sediment plume deposition results in crude ripple cross stratification debrite deposition is temporally restricted to an early segment of the rising limb of the flood hydrograph during the ensuing waxing stage uninterrupted sustained flow conditions maintain up to the peak flow permitting the aggradation of thick sandstone successions with crude ripple cross stratification during the waning stage backstepping of the whole depositional system leads to prevailing sedimentation in the delta plain setting mouth bar development and channel plugs are characterized by climbing dune cross stratification cdcs shale prone sedimentation in type 2 clinothems essentially corresponds to background ablation related sedimentation but also includes minor debrite bearing outburst related event beds this study documents the links between glacial outbursts hyperpycnal flows and hybrid beds in low angle delta settings and illustrates a type of proximal sand dominated hyperpycnite dam failure of subglacial lakes is inferred for the outburst generation comparison with modern and pleistocene examples may suggest that outburst events were days to several years in duration type 1 clinothem with a 1 40 ky recurrence period type 2 clinothem

rhythmically bedded glaciomarine sediments on whidbey island washington abstract several facies of late pleistocene glaciomarine sediments on whidbey island washington are rhythmically bedded the first consists of stratified silty sand which fines upward transitionally into pebbly mud several successions each approximately 0 6 m thick comprise a proximal meltwater facies thought to have been deposited in a shallow 10 m submarine fan environment both the positive lobe shape morphology of the exposure and the inferred depositional processes support this view underflow currents either high concentration turbidity currents or grain flows are thought to have deposited the silty sand while flocculation settling from turbid overflow and or interflow plumes and ice rafting were responsible for pebbly mud deposition such inferences are based on text ral analysis and observations of sedimentary structures the rhythmic interbedding is explained by invoking seasonal fluctuations of meltwater discharge this caused a variation of depositional process in areas proximal to meltwater input into the basin rhythmic interbedding is also observed within a channelized sequence of gravel sand and mud which comprise a fining upwards succession that includes turbidites laminations of sand and silt found in the upper portion of this section closely resemble the type of varved bedding commonly reported from glaciolacustrine sequences

time variations of the earth s magnetic field from daily to secular variations in time of the earth s natural magnetic field have a frightfully wide spectrum ranging over more than 20 orders of magnitude they extend from well over 103 hz where they merge into the radioelectric noise spectrum to probably more than 100 m y in rough summary the higher frequencies stem from external sources in the ionosphere and above into the magnetosphere whereas the longer periods arise from internal sources primarily fluid motions in the core at depths in excess of 2900 km methods for separating external and internal signals have been available since the time of gauss 1 838 but the accuracy the space distribution and the time duration of available data are often not sufficient to unambiguously solve the problem for all types of variations although periods shorter than 1 yr should clearly be assigned to external sources the extent to which the spectra of the external and internal parts overlap is the subject of recent debate and progress currie 1966 1 968 proposed that there was little overlap and a cutoff of 3 7 yr a long debated l l yr contribution due to the solar cycle was clearly demonstrated by currie 1973a courtillot le mouei 1976a b and alldredge 1976 and the cutoff period was thus rejected to at least 20 yr the situation seemed to be confused again when courtillot et al 1978a provided evidence for sharp impulses of 1 yr duration i n the internal so called secular variation the debate that followed over the reality of brief internal events is one of the topics addressed in this review we first provide a short historical account of successive discoveries

the record of density induced underflows in a glacial lake as part of an overall study of sedimentation processes in a proglacial lake an effort was made to compare field results with some of the general equations for density flows the results suggest that in relatively small glacial lakes the occurrence of underflows with lower sediment loads involves a complex interplay between thermal and sediment effects which is extremely sensitive to varying hydrologic and climatic conditions in terms of actual transport mechanics the results i indicate that a higher value of 0 6 or 0 7 gives a closer agreement between the measured velocity values and the established equations on moderately shallow slopes ii provide field support for the experimentally derived relationship of britter linden 1980 for the velocity of underflows and suggest the equation may be applicable in situations below 5 slopes and iii support the relationship between velocity of the front and body of a continuous underflow for moderate slope situations suggested by middleton 1966b finally the velocity values measured by electromagnetic current meters stationed in the lake the grain size data obtained from mapping core data and the application of other criteria support the concept that in this environment the underflows are capable of erosion

to tune or not to tune detecting orbital variability in oligo miocene climate records article i nfo article history we address the problem of detecting quasi periodic variability at orbital frequencies within pre pleistocene climate records using depth derived and orbitally tuned chronologies many studies describing orbital vari ability in pre pleistocene sediment hosted isotope records employ climatic records that are set on orbitally tuned chronologies without accounting for the bias in spectral power estimates introduced by orbital tuning in this study we develop a method to quantify the effects of tuning upon spectral estimates and in particular to more properly determine the statistical significance of spectral peaks associated with orbital frequencies we apply this method to two marine sediment 18 o records spanning the oligo miocene from odp cores 1090 and 1218 we find that using linear age depth relationships reveals statistically significant spectral peaks matching eccentricity in core 1090 and obliquity and precession in core 1218 where the last appears most significant tuning the chronologies to the orbital solutions of laskar et al 2004 increases the statis tical significance of the precession peak whereas the obliquity and eccentricity peaks become indistinguish able from those expected from tuning noise this result can be understood in that tuning records with high signal to noise ratios tends to lead to more significant spectral peaks whereas a linear age depth relationship is better suited for detecting peaks when signal to noise ratios are low we also demonstrate this concept using synthetic records

glaciomarine facies deposited by retreating tidewater glaciers an example from the late pleistocene of northern ireland lithofacies variability lithofacies relationships and sedimentary structures indicate that well exposed late pleistocene glaciogenic sequences in south county down northern ireland formed in subaqueous environments regional considerations of ice flow patterns and isostatic depression indicate that the sediments accumulated in a glaciomarine basin four major lithofacies associations are identified the first consists of muds diamictic mud and stratified sand gravel beds deposited by dispersed and concentrated meltwater sources the second occurs as a thick apron of massive and stratified diamictons formed by rainout from debris rich icebergs and resedimentation seaward of tidewater grounding zones the third association is preserved as fills in large subaqueous channels eroded into the diamicton apron these ills are characterized by slumping and debris flow interbedded sand gravel diamicton and mud from channel flanks interactions between underflows and overflows rhythmically bedded mud along channel axes and traction current activity prograded sand aprons at the top of the sequence the fourth comprises a late glacial emergent beach facies sand and gravel that truncates the earlier deposits along the entire exposure abrupt variations in dominant clast type erratics and matrix carbonate content within the sediment pile reflect indirect glacial input from three distinct geological provenances into the glaciomarine basin variability of this type should not be used for stratigraphic correlations in place of direct glacial stratigraphy this type of situation may be more common than hitherto realized in the late pleistocene mid latitude forelands characterized by complexly interbedded diamicton sequences which formed when isostatic depression exceeded sea level fall

a depth derived pleistocene age model uncertainty estimates sedimentation variability and nonlinear climate change 1 a new chronology of glaciation spanning the last 780 000 years is estimated from 21 marine sediment cores using depth as a proxy for time to avoid biasing this depth derived age estimate the depth scale is first corrected for the effects of sediment compaction to provide age uncertainty estimates the spatial and temporal variability of marine sediment accumulation rates are estimated and modeled as an autocorrelated stochastic process depth derived ages are estimated to be accurate to within 9000 years and within this uncertainty are consistent with the orbitally tuned age estimates nonetheless the remaining differences between the depth and orbitally tuned chronologies produce important differences in the spectral domain from the 18o record using the depth derived ages we infer that there are weak nonlinearities involving the 100 kyr and obliquity frequency bands which generate interaction bands at sum and difference frequencies if an orbitally tuned age model is instead applied these interactions are suppressed with the system appearing more nearly linear

anisotropy of magnetic susceptibility sedimentological igneous and structural tectonic applications as was noted by graham 1954 anisotropy of magnetic susceptibility ams data has many applications to the study of geological processes this type of anisotropy expresses directional variation in the magnetization induced in a rock most notably in its iron oxides and especially in its magnetite and hematite the ams is commonly expressed by an ellipsoid of which the principal axes are k1 k2 and k3 from greatest to least this ellipsoid is generally interpreted in terms of the distribution and shape of grains of magnetic minerals in a rock although the ellipsoid determined depends also on magnetocrystalline anisotropy and on factors of instrumental technique ellwood et al in press 1987 the challenge for the geoscientist in analyzing ams data is first to identify the origin of the ams geometry then second to relate that geometry to the natural processes which produced the inferred or measured distribution of magnetic materials

sea level higher than present 3500 years ago on the northern main hawaiian islands new data from an emerged coastal bench and associated fossil beach on kapapa island oahu hawaii preserve a detailed history of middle to late holocene sea level these include 29 new calibrated radiocarbon ages and elevations indicating mean sea level reached a maximum position of 2 00 0 35 m ca 3500 yr b p these results correlate with additional evidence from hawaii and other pacific islands and provide constraints on oahu s long term uplift rate 0 03 0 07 mm yr previously based solely on pleistocene age shorelines our sea level reconstruction is consistent with geophysical model predictions of earth s geoid response to the last deglaciation and with observations of increased antarctic ice volume during the late holocene

delta slope processes and turbidity currents in prodeltaic submarine channels queen inlet glacier bay alaska large submarine channels lead away from the coarse grained fjord head delta in queen inlet glacier bay alaska coarse sediment loads from carroll glacier are transported down the delta foreslope and through the submarine channels and finer material is dispersed throughout the fjord from overflow plumes slides and slumps occur intermittently on the delta foreslope a recording current meter positioned in a submarine channel 1 m above the bottom and 2 7 km seaward from the delta brink indicated during july 1989 predominantly quasi continuous downfjord currents although several small surges were also observed flows occurred throughout the daily tidal cycle in both neap and spring periods for intervals of up to several hours often beginning and ending rapidly current velocities ranged from 0 to 120 cm s 1 averaging 12 cm s 1 over one 56 h period and 16 cm s 1 over a 13 h period suspended sediment concentrations 1 m above bottom commonly exceeded 2 g l 1 and attained 8 2 g l 1 the density added to

flat meridional temperature gradient in the early eocene in the subsurface rather than surface ocean sea surface temperature estimates from the early eocene indicate an unusually flat meridional temperature gradient a re evaluation of the proxy used to derive these temperatures argues against this interpretation

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

three dimensional spherical models of layered and whole mantle convection we present numerical calculations of three dimensional spherical shell thermal convection for constant viscosity and stratified viscosity models of whole layer and two layer mantle convection these four examples are intended to provide theoretical guidance for determining the style of convection that is occurring in earth s mantle an impermeable interface between the upper and lower converting shells in the two layer solutions is placed at a depth of 670 km to coincide with the mantle seismic discontinuity that divides the upper and lower mantle the interface results in an internal thermal boundary layer that raises the mean temperature in the lower shell by about 1400 k compared to the whole layer solutions the patterns of convection in the upper part of the whole layer solutions are dominated by narrow arcuate sheetlike downflows in a background of weak upflow in contrast the upper shells of the two layer solutions have complicated networks of convective rolls with the upflows and downflows having very similar structure the structure of convection in the lower shells is similar to that in the lower part of the whole layer solutions an increase decrease in the viscosity in the lower upper shell decreases increases the convective velocity in the lower upper shell and increases decreases the horizontal scale in the lower upper shell the upper and lower shells are viscously coupled at the 670 km interface when viscosity is the same in the two shells however when viscosity in the lower shell is 30 times greater than that in the upper shell the coupling is partially viscous and partially thermal based on the horizontal structure of subduction zones on earth s surface and on tomographic images of temperature variations in earth s mantle we conclude that the style of convection in earth s mantle is more like that of the whole mantle models

sedimentation in proglacial sunwapta lake alberta the hydrologic and limnologic conditions of sunwapta lake a small proglacial lake in the canadian rocky mountains were investigated with special reference to the sedimentology of this lake discharge and suspended sediment concentrations of the inflowing streams were measured to give estimates of sediment input distribution of large loads of fine glacial sediment is largely by an inflow and wind controlled circulation which commonly fluctuates diurnally the spatial distribution of sedimentation was determined by use of sediment collecting pans placed on the lake bed turbidity currents are rare with only one event monitored in 2 years this event was caused by a burst of highly sediment charged inflow water the rarity of turbidity currents is attributed to the high sediment concentration in the lake water which results from the small size of the lake in relation to the inflow wind generated water movement at the lake floor ranges from 0 0 0 03 m s while the maximum recorded velocity of the turbid

late pleistocene holocene sediments on the spanish continental shelves model for very high resolution sequence stratigraphy abstract sequence stratigraphy analysis of high resolution seismic profiles geopulse uniboom and 3 5 khz of late pleistocene holocene sediments has been carried out on five sectors of the spanish continental margin four types of depositional settings are distinguished in these sectors 1 low subsident ramps alboran margin cadiz gulf 2 high subsident 2m kyr ramps alicante valencia 3 ria type morphology on the atlantic passive margin ria de muros and 4 fault scarp morphology systems with subsidence balearic margin a type 1 sequence is interpreted in all these sectors being composed of lowstand systems tract transgressive systems tract and highstand systems tract this conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle characteristic shelf features such as terraces terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub sequences in all the systems tracts which must be related to very short period sea level stillstands and fall we propose a very high resolution sequence stratigraphy model in which the last sea level hemicycle is punctuated by p cycles 4500 years which give rise to the neo glacial events h cycles 2200 950 years and c cycles 500 50 years these cycles interact with each other thus establishing the placing of the high and low sea levels this attention to detail 1 explains sedimentary evolution on both the shelf and upper slope during late pleistocene holocene time 2 illustrates some departures from the classical sequence stratigraphy model and 3 also demonstrates that the late pleistocene holocene eustatic curve is not one simple transgression but is modulated by three differing period cycle groups below the milankovitch band our model is delimited by fluctuating sea level during pleistocene holocene times such features should be identifiable on any continental margin however localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle

forced regressive deposits of a deglaciation sequence example from the late quaternary succession in the lake saint jean basin quebec canada differentiating between forced regressive deposits from deglacial periods in high latitude domains and forced regressive deposits from the onset of glacial periods in low latitude domains is fundamental for the accurate interpretation of glacial cycles within the geological record and then for the reconstruction of palaeogeography and palaeo climate a forced regressive deglacial sequence is documented from the lake saint jean basin quebec canada in this area the late pleistocene to holocene sediments have recorded the laurentide ice sheet retreat accompanied by the invasion of marine waters laflamme gulf from ca 129 cal kyr bp subsequently fluviodeltaic and coastal prograding wedges were deposited they followed the base level fall due to glacio isostatic rebound this succession representing a transition from glacial to post glacial periods within a previously glaciated area was investigated through recent mapping preserved landforms facies analysis and new optical stimulated luminescence and radiocarbon dates three basin scale geological sections share a common lower part made of isolated ice contact fan deposits overlying bedrock throughout the entire basin ice contact fans are capped by glacimarine muds above fluvial and coastal prograding systems were deposited and evolved through four steps i deltaic systems progressively increased in width ii coastal influence on sedimentation increased iii hydrographic drainage systems became more organised and iv deltas graded from steep gilbert delta to low angle foresets mouth bar delta deposited during the base level fall from glacioisostatic rebound the complete succession has been designated as a single falling stage system tract referred to as a deglacial falling stage system tract it is representative of a deglaciation sequence in areas previously covered by ice during glacial periods i e medium to high latitude domains diagnostic criteria are provided to identify such a deglacial falling stage system tract in the geological record which may aid identification of previously unknown glacial cycles

experimental quasi steady density currents abstract hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths their deposits are probably very common in the rock record although they are not frequently recorded in the literature these currents may last longer and be steadier than surge like turbidity flows and consequently their deposits are likely to be significantly different in this paper simple laboratory experiments are used to investigate how flows and deposits may vary the thickness of the experimental flows near the flow front varies with distance from the source depending on slope effluent discharge grain size and sorting runout distance varies with the proportion of fine grained sediment rather than with mean grain size flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting the variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area approximating to bed thickness is inversely dependent on slope angle variation in initial suspended particle concentration results in deposits of similar shape but different volume although the total mass of the deposit depends on the discharge the maximum mass per unit area approximating to maximum bed thickness does not the distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges perhaps relating to more efficient turbulent suspension at higher re more complete energy cascade in addition to longer horizontal component of settling trajectory at higher velocity the amplitude of mass variation depends on the grain size as with laboratory surges gladstone et al sedimentology 1998 833 844 the finer the sediment the more efficient is the sediment transport