true amplitude elastic gaussian beam imaging of multicomponent walkaway vertical seismic profiling data we develop the true amplitude prestack migration of multicomponent data based on the use of elastic gaussian beams for walkaway vertical seismic profile vsp acquisition systems it consists in a weighted summation of multishot data with specific weights computed by tracing elastic gaussian beams from each imaging point of the target area towards the sources and receivers each pair of beams may be connected with either a pair of p rays pp image or the p ray towards sources and the s ray to receivers ps image and is uniquely determined by dip the angle of the bisector between the rays and the vertical direction and opening the angle between the rays angles shooting from the bottom towards the acquisition system helps to avoid well known troubles in particular multipathing for the imaging conditions in complex velocity models the ability to fix the dip angle and implement summation over opening angles leads to the so called selective images that contain mostly interfaces with desired slopes on the other hand a set of images computed for a range of opening angles by summation over all available dip angles is used as input of an avo like inversion procedure for the recovery of elastic parameters the feasibility of this imaging procedure is verified by synthetic data for 2d realistic elastic models

biogenic silica record of the lake baikal response to climatic forcing during the brunhes abstract this work presents a detailed orbitally tuned biogenic silica record of continental paleoclimate change during the brunhes chron the brunhes matuyama boundary lies within the warm isotopic stage 19 in baikal and the boundaries between eight lithological cycles correspond to terminations in the marine oxygen isotope record the high amplitude and resolution of climatically driven changes in biosi content in lake baikal sediments permits tuning of almost every precessional cycle during the brunhes and reveals the structure of interglacial stages for example the last three interglacial stages mis 5 7 and 9 clearly consist of five substages a b c d e corresponding to precessional insolation peaks abrupt and intense regional glaciations in siberia during substages 5d and 7d were driven by extreme insolation minima during substage 9d cooling was more gradual in response to more moderate forcing the impact of strong glaciation is also observed in the middle of stage 15 where full glacial conditions appear to have lasted for over 30 000 yr during substages 15d 15c and 15b marine oxygen isotopic stage 11 appears to be the warmest period during the brunhes in the lake baikal record with at least three substages a new hypothesis is presented regarding the response of the lake baikal biosi record to insolation forcing based on the mechanism controlling modern diatom blooms biogenic silica production is hypothesized to be dependent on changes in the heat balance of the lake and consequently on changes in the thermal structure of the water column this mechanism is also sensitive to short term sub milankovich cooling events such as the mid eemian cooling the montaigu event during substage 5c and a cooling which appears to be analogous to the montaigu event during substage 9c the continuity of the lake baikal paleoclimate record its sensitivity to orbital forcing and its high resolution make it an excellent candidate for a new paleoclimatic stratotype section for continental asia

plio pleistocene megacycles record of climate and tectonics abstract paleoclimatic records from the deep sea floor show that in the last 3 m y more than 100 oscillations of mild and cool global climate took place with a larger amplitude than any experienced during the holocene variations of dry and wet climates in the chinese loess plateau and in the loess belt of central europe have similar frequency it has been convincingly proved that the periodicity of the past gross climate changes agrees with that of the earth s orbital perturbations however while the amplitude of the orbital cycles and of the oceanic oxygen isotope variations is relatively uniform several of the cold climate episodes recorded on land appear more severe than the remainder they correspond to chinese loess units l1 l2 l5 l6 and l9 correlating with central european loess of cycles b c f h and j these units correspond to oxygen isotope stages 2 6 12 16 and 22 and to the weichselian saalian and elsterian ice advances in addition chinese loess layers l13 l15 wl1 wl2 wl3 and wl4 appear to represent exceptionally dry and cold episodes apparently correlative with the inter bavelian cooling and with menapian eburonian and praetiglian glacials of the rhine delta based on this observation we propose to distinguish within the last 2 5 m y ten megacycles describe them in the chinese loess sequences and name them in the order of increasing age mc1 mc10 each megacycle starts with an unusually thick loess unit interpreted as a deposit of an exceptionally severe glaciation and terminates with a cluster of well developed soils indicative of warm humid climates interrupted by relatively mild glacials the megacycles of the last 2 5 m y lasted between 200 and 340 millennia each with an average duration of 255 thousand years when correlated with the stratigraphic record of the chinese loess plateau the wurm terrace as well as the saale glacier advances the eem marine transgression the weichsel ice advance and the holocene correspond to a still incomplete megacycle mc1 the riss terrace the elster ice advance holstein transgression and subsequent mild and cool intervals ending with the schoningen interglacial sensu urban 1992 belong to the megacycle mc2 recognition of four megacycles within brunhes resolves the long standing discrepancy between the limited number of classical continental glacial and interglacial units and the frequent climate oscillations recorded in the deep sea sediments and the loess sequences only in the holsteinian and eemian interglacials was the crustal rebound delayed enough to permit oceanic transgressions reaching deep inland there is no apparent astronomic reason for the exceptional intensity and deep southward penetration of continental ice sheets during only some of the cold intervals since the differences are much more pronounced on land than in the ocean and since the major ice advances are closely linked with terraces it seems that the megacycles were preconditioned by tectonic movements intermittent pulses of rapid uplift of mountain ranges could have impacted climate worldwide by deflecting atmospheric circulation in the sense of ruddiman and raymo 1988 it is also probable that the tectonically changing shape of the sea floor sills reoriented the oceanic circulation which then could have affected plio pleistocene climates world wide the loess covered terraces in the lanzhou area demonstrate that the tectonic movements in the northern part of the tibet plateau correlate closely with the advances of continental ice in europe suggesting that the uplift of the tibet plateau significantly contributed to the gradual climate deterioration of the last million years

the origin of deep buried channels of elsterian age in northwest germany buried channels cut to a depth of over 100m into the underlying substratum of north germany are almost exclusively of elsterian age evaluation of geophysical well logs in the hamburg area for the first time allows the construction of detailed longitudinal and transverse profiles it can be shown that an apparently chaotic infill known from borehole records in some cases is actually a regular sedimentary sequence fining from proximal to distal channel formation seems to have been the result of catastrophic meltwater release in the marginal parts of the ice sheet the channel infill reflects these episodic outbursts of meltwater during deglaciation the channel remnants became infilled with fine sediments the channels ceased to exist as morphologically detectable forms at the end of the holsteinian stage

response of lake baikal ecosystem to climate forcing and pco2 change over the last glacial interglacial transition abstract the 13 c record of total organic carbon in the lake baikal core bdp 93 2 over the last glacial interglacial transition shows remarkable correlation with atmospheric co 2 fluctuations and the negative isotopic shift of 3 5 4 in response to the global increase in paleo p co 2 level with rapid evolution of the lake baikal carbon cycle from full glacial to full interglacial modes however the p co 2 signal in the baikal 13 c sedimentary organic matter is inundated by the basin wide productivity signal in the holocene the baikal diatom abundance record as a paleotemperature proxy lags behind the gisp2 ice core 18 o temperature proxy record by ca 1 ka possibly reflecting the response time of the baikal ecosystem to major climatic transitions there is no lag associated with the pronounced younger dryas cooling however suggesting a different forcing mechanism with a short response time for this abrupt climatic event in continental interior asia the phase difference observed between the diatom abundance and 13 c proxies in lake baikal core bdp 93 2 suggests their independent responses to different forcing mechanisms further justifying their importance for paleoclimate reconstructions from lake baikal records

new geomagnetic paleosecular variation results from the old crow basin yukon territory and their use in stratigraphic correlation a total of 626 fully orientated samples were collected from two laterally equivalent vertical sections each approximately 20 m thick in pleistocene sediments exposed in riverbank outcrops on the old crow river yukon territory the paleomagnetic records recovered reveal a sequence of three features correlatable with results obtained from previously sampled sections some 5 km upstream this correlation which is confirmed by magnetic susceptibility measurements allows the stratigraphic position of the old crow tephra to be inferred even though it does not actually occur at this locality it also provides strong support for the reality of these paleosecular variation signals the most prominent of these take the form of repeating linear perturbations which are here interpreted as evidence of stationary sources of reversely directed flux in the outer core such reverse flux patches exist elsewhere in the modern field but a simple magnetostatic model suggests that the paleosources were apparently an ord

evidence from lake baikal for siberian glaciation during oxygen isotope substage 5d abstract the paleoclimatic record from bottom sediments of lake baikal eastern siberia reveals new evidence for an abrupt and intense glaciation during the initial part of the last interglacial period isotope substage 5d this glaciation lasted about 12 000 yr from 117 000 to 105 000 yr b p according to correlation with the specmap isotope chronology lithological and biogeochemical evidence of glaciation from lake baikal agrees with evidence for the advance of ice sheet in northwestern siberia during this time period and also with cryogenic features within the strata of kazantzevo soils in southern siberia the severe 5d glaciation in siberia was caused by dramatic cooling due to the decrease in solar insolation as predicted by the model of insolation changes for northern asia according to milankovich theory coupled with western atmospheric transport of moisture from the open areas of northern atlantic and arctic seas which became ice free due to the intense warming during preceeding isotope substage 5e other marine and continental records show evidence for cooling during 5d but not for intense glaciation late pleistocene glaciations in the northern hemisphere may have begun in northwestern siberia

paleomagnetic evidence for long term 104 year memory and periodic behavior in the earth s core dynamo process a new paleomagnetic secular variation psv record from the late quaternary wilson creek beds of mono lake california contains a distinctive periodic vector waveform that follows and is almost certainly related to the mono lake excursion a comparison of all published psv results from the wilson creek beds suggests that the magnetic field at mono lake went through an interval 36 000 28 000 ybp of very low amplitude psv followed by the mono lake excursion 28 000 27 000 ybp and four subsequent recurrences 27 000 12 500 ybp of the excursion waveform with relatively diminished amplitudes lower than the excursion amplitude but higher than psv amplitudes at mono lake since 12 500 ybp this pattern clearly suggests that the mono lake excursion is related to typical psv it also suggests that the core dynamo process responsible for psv is capable of near impulse onset persistence over a time scale of 15 000 years and quasi periodic behavior that is non wave length dispersive in time

the middle to late pleistocene geomagnetic field recorded in fine grained sediments from summer lake oregon and double hot springs nevada u s a abstract a 400 000 year record of the paleomagnetic field has been acquired from 22 meters of middle to late pleistocene fine grained sediments from summer lake in south central oregon and double hot springs in northwestern nevada the stratigraphy is based on 55 tephra layers nine of which have been correlated with tephra layers from other localities on the basis of their distinct major and trace element geochemistry and their distinct petrography the paleomagnetic samples carry a strong and stable magnetization that does not appear to have been affected by the inclination error commonly associated with the magnetization of sediments the samples have accurately recorded the declination and inclination of the geomagnetic field at or near the time of deposition except for errors arising from rotations of discrete blocks of sediment predominantly about vertical axes errors introduced by this type of rotation were corrected by using paleomagnetic directions associated with correlated tephra layers the summer lake paleomagnetic record suggests that secular variations occurred throughout the middle and late pleistocene often maintaining the same waveform through several oscillations the amplitudes of these variations were similar to those of holocene variations and the periods ranged from 15 000 years to greater than 100 000 years

the use of paleomagnetic declination to test correlations of late wisconsinan glaciolacustrine sediments in central new york detrital remanent magnetization drm recorded in laminated glaciolacustrine silt and clay of the western mohawk valley of central new york was used to construct a secular variation record of geomagnetic declination for parts of the late wisconsinan in the interval 12 5 18 ka b p remanent declination has been used for time stratigraphic testing of correlations between laminated fine grained glaciolacustrine sediments at different exposures in the western mohawk valley a discontinuity in the remanent declination record supports the interpretation that an observed lithostratigraphic discontinuity in the region represents an unconformity diamicton units composed of till and proximal proglacial diamictons are overlain by laminated clay and silt units that have time transgressive lower contacts the clay and silt units exhibit stratigraphic onlap in the direction of ice recession a characteristic that is recognizable in their paleo magnetic records the use of a secular variation record of remanent declination has an advantage over lithostratigraphic and morphostratigraphic correlation techniques in that it is time dependent for a part of the late pleistocene it provides a powerful tool for correlating physically disjunct stratigraphic assemblages in new york and new england where no other dating technique has provided interregional correlations with comparable resolution sampling of different exposures of contemporaneous sediment was used to demonstrate the fidelity of remanent declination as a recorder of geomagnetic declination the remanent inclination record was not useful for testing correlations because inclinations in strata known to be contemporaneous were different as compared to remanent inclinations from other contemporaneous lacustrine sequences in the eastern united states the average remanent inclination of the western mohawk valley sediments is at least 11 degrees too shallow shallow inclinations may be the result of errors acquired at the time of deposition or compaction resulting from drying of the sediment consolidation by overriding ice or lithostatic load of overlying sediment subglacial deformation dewatering and some outcrop conditions especially ground water seepage were recognized as processes that may cause or facilitate postdepositional disturbance of remanent magnetization

the link between tectonic and paleoclimatic events at 2 8 2 5mabp in the lake baikal region abstract the records of diatom abundance clay fraction content and magnetic susceptibility from lake baikal drill core bdp 96 1 reflect the climate driven plio pleistocene evolution of the lake baikal sedimentation system in addition to the dramatic variations in the proxy records in response to the northern hemisphere ice ages the baikal record also indicates climatic deterioration from 2 8 to 2 5 ma bp with evidence for an early siberian glaciation around the matuyama gauss paleomagnetic reversal boundary the drill core data also allow correlation of this early glacial interval with the basin wide seismic sequence boundary b10 which marks the unconformity produced by the active neotectonic phase in the baikal rift zone at the bdp 96 drill site however the strong b10 acoustic reflection was produced not by an erosional boundary but instead by deposition of glacial clay layers the paleomagnetic age scale of bdp 96 1 constrains the upper age of the neobaikalian uplift subsidence phase in the baikal sayan region at ca 2 5 ma bp the coincident timing of the paleoclimatic and tectonic events recorded in lake baikal sediments suggests a close causal link between regional tectonics and climate and indicate that the late pliocene uplift contributed to the initiation of the early glaciation in southeast siberia between 2 8 and 2 6 ma bp by creating an elevated terrain with lower snowline favorable for alpine glaciation and by changing the heat balance of the region

paleomagnetism of two late pleistocene lake basins in colorado an evaluation of detrital remanent magnetization as a recorder of the geomagnetic field paleomagnetic studies of magnetite bearing lacustrine sediments from two late pleistocene sections age range about 12 000 23 000 14c years one at devlins park in the front range the other at triangle park in the northern end of the sawatch range colorado indicate that detrital remanent magnetization of fine grained silty clay sediments at both locations was capable of recording changes in the magnetic field over periods as short as 100 200 years whereas coarser grained silts in the triangle park section seem to be less efficient recorders of the geomagnetic field the remanence of the fine grained sediments contains an apparent inclination error of about 9 at both localities in addition locally operative depositional and or postdepositional processes have produced both intrabasinal and interbasinal systematic differences in the remanence records of up to about 30 in declination and 10 in inclination comparison of the directional data from devlins park with those from four other broadly contemporaneous north american lake basins reveals that although there are abundant minor and several major discrepancies there are general similarities in the longer wavelength features particularly of declination the differences in the records are attributable to depositional and postdepositional factors variable and secondary overprinting local field differences sampling techniques and incorrect stratigraphic correlations the overall similarity in the records probably reflects the influence of the geomagnetic field the greatest difficulty in determining the significance of the similarities and differences between the records from the various lakes is the present lack of precise age and stratigraphic control

age and correlation of a paleomagnetic episode in the western united states by 40ar 39ar dating and tephrochronology the jamaica blake or a new polarity episode high resolution paleomagnetic records from two sites near pringle falls oregon are compared with similar records from summer lake oregon 170 km to the southeast paoha island in mono lake 660 km to the southeast and benton crossing in long valley approximately 700 km to the southeast in east central california the sequences at pringle falls contain a distinctive coarse pumice lapilli tephra layer which we have dated as 218 10 ka by 40ar 39ar step heating of plagioclase feldspar stratigraphically this tephra is closely associated with a suite of several other tephra layers that bracket the interval studied paleomagnetically each tephra layer is distinguished by the unique chemical composition of its volcanic glass shards the pumice layer dated at pringle falls is correlated with layers at three of the other localities using all the tephra layers we can correlate the lake stratigraphic sequences and associated paleomagnetic records among the four distant localities additional age control is obtained from a fifth locality at tulelake in northern california where the stratigraphic interval of interest is bracketed between 171 43 and approximately 140 ka characteristics of the paleomagnetic records indicate virtually identical paleofield variation particularly the geometry of a normal to normal n n geomagnetic polarity episode the observed paleofield behavior resembles the blake geomagnetic polarity episode but is significantly older than the generally accepted age of the blake episode either the age of the blake episode is significantly underestimated or the polarity episode documented here is older perhaps the jamaica episode or is an as yet unreported episode a corollary of the latter option is that paleomagnetic polarity episodes of different ages may have similar transition polar paths a conclusion implying that a common mechanism is involved

a 20 m middle pleistocene sea level highstand bermuda and the bahamas due to partial collapse of antarctic ice marine deposits at 20 3 m on the tectonically stable coastlines of bermuda and the bahamas support the hypothesis of a partial collapse of the antarctic ice sheet during the middle pleistocene beach sediments fill a sea cave at 22 m in bermuda and horizontal fenestrae filled beds crop out on platforms at two sites as high as 21 m in eleuthera bahamas carbonate beach sands are bound by an early generation of isopachous fibrous cement that is characteristic of a phreatic marine environment amino acid racemization and tims thermal ionization mass spectrometry dates constrain the age of the deposits to between 390 and 550 ka while proxy evidence supports a correlation with oxygen isotope stage 11 this direct geologic evidence of a 20 decrease in polar ice during the middle pleistocene has important implications for the stability of ice sheets during warm interglaciations

lake levels and sedimentary environments during deposition of the trego hot springs and wono tephras in the lake lahontan basin nevada usa abstract the wono and trego hot springs ths tephras are widespread in the lahontan basin and have been identified in a variety of sedimentary environments at different elevations davis 1983 reported lake level to be at about 1256 m when the ths tephra was deposited an interpretation questioned by benson et al 1997 who interpreted lake level to be 1177 m at that time this is a significant difference in lake size with important implications for interpreting the climate that prevailed at that time based on new interpretations of depositional settings of the ths bed at multiple sites the larger lake size is correct additional sites containing the wono tephra indicate that it was deposited when lake level was at about 1217 m in the western subbasins and at about 1205 m in the carson sink sedimentary features associated with progressively deeper paleowater depths follow a predictable pattern that is modulated by proximity to sediment sources and local slope fine to coarse sands with wave formed features are commonly associated with relatively shallow water silty clay or clay dominates in paleowater depths 25 m with thin laminae of sand and ostracods at sites located adjacent to or downslope from steep mountain fronts

a geomagnetic secular variation study 31000 19 500 bp in western canada summary palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern british columbia are reported radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp no evidence for any large geomagnetic excursions such as the so called mono lake excursion is found but a distinctive pattern of normal secular variation is observed with declination and inclination swings of 45 and 25 peak to peak amplitude respectively for the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude but three relatively large perturbations occur at approximately 4000 yr intervals these perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non isotropic secular variation model proposed by cox the bias involved is about 6 in declination and 3 in inclination overall mean d 5 8 e i 64 2 95 0 9 n 212 horizons which leads to a pole which is both far sided and right handed if the horizons involved in the three major perturbations are eliminated the mean direction d 1 2 e i 67 2 95 0 8 n 125 horizons does not differ from that of a geocentric axial dipole despite the small cone of confidence

paleomagnetic investigation of the bonneville alloformation lake bonneville utah abstract paleomagnetic secular variation in a portion of the bonneville alloformation is compared with secular variation in lacustrine sediments in the mono basin california and with secular variation in lake lahontan sediments in the northwestern great basin the comparison places an age of about 18 000 yr b p and a span of 1000 to 3000 yr on part of a transgressive stage of lake bonneville near delta utah that is coeval with a wet period in the lahontan basin

paleomagnetic investigation of lake lahontan sediments and its application for dating pluvial events in the northwestern great basin abstract a comparison of paleomagnetic secular variation in sediment of pleistocene lake lahontan in the northwestern great basin with secular variation in lake sediment in the mono basin california indicates that lake lahontan was in the valley of the truckee river between pyramid lake and wadsworth nevada from about 19 000 to 13 000 yr b p the secular variation in older lake lahontan sediment in the truckee river valley has the general features of secular variation in middle pleistocene lacustrine sediments near rye patch dam nevada 125 km to the east on the basis of field mapping and tephrochronology the sections of older lacustrine sediments are not coeval the apparent but erroneous correlation of those sediments emphasizes the need for multiple dating methods when paleomagnetic secular variation is used to date stratigraphy

dating late pleistocene pluvial events and tephras by correlating paleomagnetic secular variation records from the western great basin abstract paleomagnetic records are used to correlate sedimentary sequences from pluvial lakes chewaucan and russell in the western great basin this correlation is the basis for age control in the relatively poorly dated sequence from lake chewaucan the resulting chronology supports a lack of sedimentation in lake chewaucan during the interval 27 400 to 23 200 yr b p an assertion supported by the presence of a lag deposit at the corresponding stratigraphic horizon because the lake chewaucan outcrop near summer lake oregon is near the bottom of the lake basin we conclude that lake chewaucan was at a lowstand during this time interval the chewaucan lowstand is coeval with the lowstand accompanying the wizard s beach recession isotope stage 3 previously seen in the geologic record from nearby pluvial lake lahontan the ages of six tephra layers including the trego hot springs tephra were also estimated using the paleomagnetic correlation together the new age of the trego hot springs tephra 21 800 yr b p and the lake surface level prehistory of lake chewaucan imply a revised model for the lake surface level prehistory of lake lahontan the revised model includes a longer duration for the wizard s beach recession and the occurrence of a younger lowstand of short duration soon after the lowstand corresponding to the wizard s beach recession

repeating waveform initiated by a 180 190 ka geomagnetic excursion in western north america implications for field behavior during polarity transitions and subsequent secular variation new paleomagnetic lithologic and stratigraphic data are presented from the sediments of lake chewaucan in the summer lake basin oregon the new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality a complex yet distinct waveform is observed in all three components of the paleomagnetic vector the waveform begins as the 180 190 ka pringle falls long valley summer lake ii geomagnetic excursion and continues for two cycles after the excursion until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6 5e boundary the waveform s directional morphology in virtual geomagnetic pole vgp space is defined by two clockwise loops followed by a distinctive counterclockwise clockwise counterclockwise looping sequence the vgp paths of the two cycles after the excursion are rotated 180 about earth s spin axis with respect to the vgp paths of the excursion cycle the waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion the average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion this observation indicates that excursion initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions because of the similarities noted previously between this excursion and full polarity transitions trie et al 1991 our new observations constrain models for a wide range of field behavior including polarity transitions excursions and secular variation

a rock magnetic record from lake baikal siberia evidence for late quaternary climate change rock magnetic measurements of sediment cores from the academician ridge region of lake baikal siberia show variations related to late quaternary climate change based upon the well dated last glacial interglacial transition variations in magnetic concentration and mineralogy are related to glacial interglacial cycles using a conceptual model interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals the variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods we also infer an increased contribution of eolian sediment during the colder windier and more arid glacial conditions when extensive loess deposits were formed throughout europe and asia eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals variations in magnetic concentration and mineralogy of lake baikal sediment correlate to the specmap marine oxygen isotope record the high degree of correlation between baikal magnetic concentration mineralogy and the specmap oxygen isotope record indicates that lake baikal sediment preserves a history of climate change in central asia for the last 250 ka this correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method future work must include providing better age control and additional climate proxy data thereby strengthening the correlation of continental and marine climate records

insolation regime in siberia as a major factor controlling diatom production in lake baikal during the past 800 000 years abstract a unique diatom biostratigraphy of the brunhes chron in lake baikal revealed by detailed studies of pelagic diatom paleo assemblages indicates that dramatic changes in insolation during the past 800 ka bp produced surprisingly rapid diatom speciation and extinctions the orbitally tuned age model allows the diatom assemblages to be compared with individual marine isotopic stages and substages during the brunhes most of the interglacial periods are characterized by explosive speciation and appearances of new pelagic planktonic diatom species in contrast most of the glacial periods are characterized by spectacular extinctions three broad periods characterized by different degrees of stability in the pelagic diatom assemblages are distinguished during the brunhes chron in the lake baikal sedimentary record mis 19 15e mis 15a 11 and mis 9 5 these periods are apparently associated with the long term changes in the amplitude of precessional insolation variations examination of individual interglacial periods further reveals the sensitivity of the baikal ecosystem to the regional energy balance virtually every precessional insolation peak during the early brunhes is characterized by a unique and peculiar paleo assemblage for the first time the siliceous microfossil record has demonstrated such a high degree of sensitivity to insolation forcing of regional climate despite the dramatic changes in the pelagic plankton driven by extreme climatic changes diatoms never went fully extinct during any glacial period although greatly or completely renewed during each subsequent interglacial the pelagic diatom assemblage still managed to return dominated by the new baikal endemic species the diatom assemblage observed in modern lake baikal despite being mostly endemic has formed during the past 50 100 ka and is therefore rather young these results demonstrate the remarkable capacity of this large freshwater system to sustain the dynamic stability on the large time scales

research note mono lake excursion in the lahontan basin nevada summary in the lahontan basin of north western nevada the mono lake excursion is recorded in a wave cut bank at pelican point on the west side of pyramid lake the record is almost identical to one across the lake near pyramid island 10 km away and in the carson sink 60 km further to the east at each locality the older half of the excursion is present only as a reduction in the relative field intensity while the younger half shows the expected easterly declination and steep positive inclination that result in a small counterclockwise loop of the virtual geomagnetic poles the younger half occurs during high relative field intensity and the entire intensity pattern is similar to the one for the excursion in the mono basin california 250 km to the south the presence of only part of the mono lake excursion in the lahontan basin might have resulted from the slow sedimentation rate in pyramid lake estimated to be about 12 cm kyr 1 with the field behaviour being more extensively integrated in the subsamples used in an alternate explanation the absence of the older part of the excursion can be attributed to overprinting when magnetic grains that were free to rotate in unconsolidated sediment were realigned during the high relative field intensity in the younger part of the excursion

vegetation response in the southern lake baikal region to abrupt climate events over the past 33calkyr abstract we reconstructed vegetation changes and fire events in the southern lake baikal region over the past 33 cal kyr using a sediment core from the buguldeika saddle southern lake baikal to examine vegetation response to past short term climate variations herbs such as artemisia and asteraceae were dominant just before the last glacial maximum lgm low pollen accumulation rates pars with little herb pollen indicate sparse herbaceous vegetation cover during the lgm picea expanded after the bolling interval suggesting expansion of riparian areas and shrub alnus rapidly increased during the following allerod interval abies and pinus subgen haploxylon began to increase at the beginning of the holocene and pinus subgen diploxylon has been highly dominant since the middle holocene the vegetation change after the lgm is similar to that from other sites in the lake baikal region but the responses to short term climate events are pronounced fluctuations in herb elements and vegetation cover as indicated by the par correspond to the younger dryas yd cooling event the decline in the par and increase in herb taxa were remarkable at the beginning of the yd but larix and picea trees were distributed in river valleys near lake baikal in the latter half the rapid expansion of betula and artemisia following frequent fires in the early holocene may have been in response to the 8 2 cal ka bp cooling event from these findings the southern lake baikal region including the selenga river basin may possibly have been more sensitive than other parts of the region to abrupt climate changes

level of lake lahontan during deposition of the trego hot springs tephra about 23 400 years ago abstract the trego hot springs tephra bed is a silicic tephra about 23 400 yr old found at several localities in pluvial lake sediments in northern nevada southern oregon and northeastern california it has been characterized petrographically by the major and minor element chemistry of its glass and by its stratigraphic position with respect to other tephra layers at a newly described locality on squaw creek northwest of gerlach nevada at the north end of the smoke creek desert trego hot springs tephra has been found in sediments of the sehoo and indian lakes formations the depositional environments of these sediments show that when the tephra fell pluvial lake lahontan stood between 1256 and 1260 m and that immediately thereafter the lake rose to at least 1275 m these data corroborate earlier findings by benson quaternary research 9 300 318 from radiometric dating of calcareous tufa however the lake lahontan area has been affected by isostatic subsidence and rebound in response to changing water loads so that caution is required in the use of lakeshore elevations in correlation

fracture detection by gaussian beam imaging of seismic data and image spectrum analysis localization of fractured areas is of primary interest in the study of oil and gas geology in carbonate environments hydrocarbon reservoirs in these environments are embedded within an impenetrable rock matrix but possess a rich system of various microheterogeneities i e cavities cracks and fractures cavities accumulate oil but its flow is governed by a system of fractures a distinctive feature of wave propagation in such media is the excitation of the scattered diffracted waves by the microheterogeneities this scattering could be a reliable attribute for characterization of the fine structure of reservoirs but it has extremely low energy and any standard data processing renders them practically invisible in comparison with images produced by specular reflections therefore any attempts to use these waves for image congestion of microheterogeneities should first have a preliminary separation of the scattering and specular reflections in this paper the approach to performing this separation is based on the asymmetric summation it is implemented by double focusing of gaussian beams to do this the special weights are computed by propagating gaussian beams from the target area towards the acquisition system separately for sources and receivers the different mutual positioning of beams in each pair introduces a variety of selective images that are destined to represent some selected singular primitives of the target objects such as fractures cavities and edges in this way one can construct various wave images of a target reservoir particularly in scattered diffracted waves additional removal of remnants of specular reflections is done by means of spectral analysis of the scattered diffracted waves images to recognize and cancel extended lineaments numerical experiments with sigsbee 2a synthetic seismic data and some typical structures of the yurubcheno tokhomskoye oil field in east siberia are presented and discussed

middle late pleistocene relative palaeointensity of the geomagnetic field from lacustrine sediments lake chewaucan western united states summary detailed palaeomagnetic and rock magnetic studies of a 15 m succession of middlellate pleistocene lacustrine sediments from ancient lake chewaucan southern oregon western united states indicate that the remanence bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies we have used arm sirm and x for normalization of the nrm all three parameters give essentially identical results in their relative stratigraphic variations which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration patterns in grain size variation as indicated by small scale quasi cyclic fluctuations in hysteresis parameters may be due to environmental changes such as lake level variation however these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record we therefore conclude that the large scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations parts of the normalized remanence record where firm chronological constraints exist may correlate with features of relative palaeointensity records from deep sea sediments our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time scale which would make possible the direct correlation of palaeoclimate records from deep sea and continental environments

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

filtering of milankovitch cycles by earth s geography abstract earth s land sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation we examine this modification in the frequency domain by generating 800 000 yr time series of maximum summer temperature in selected regions with a linear two dimensional seasonal energy balance climate model previous studies have demonstrated that this model has a sensitivity comparable to general circulation models for the seasonal temperature response to orbital forcing on land although the observed response in the geologic record is sometimes significantly different than modeled here differences attributable to model limitations and feedbacks involving the ocean atmosphere cryosphere system there are several results of significance 1 in mid latitude land areas the orbital signal is translated linearly into a large 10 c seasonal temperature response 2 although the modeled seasonal response to orbital forcing on antarctica is 6 c the annual mean temperature effect

continental climate response to orbital forcing from biogenic silica records in lake baikal changes in insolation caused by periodic changes in the earth s orbital parameters provide the primary forcing for global ice ages1 6 but it is not clear to what extent the climates in continental interiors are controlled directly by regional variations in insolation and to what extent they are driven instead by the highly nonlinear response of the oceans and ice sheets here we investigate this question using the record of biogenic silica in lake baikal as a proxy for climate change in this high latitude mid continental region we find a good correlation between this record and that of marine oxygen isotopes4 over the past 250 kyr the baikal record exhibits both a strongly nonlinear component manifested in a 100 kyr periodicity and weaker direct insolation components manifested in the 41 kyr obliquity and 23 and 19 kyr precession orbital cycles these results show that even though extreme continental climates such as this are influenced directly by insolation variations they are dominated by the nonlinear rhythm of the oceans and ice sheets

record of a mid pleistocene depositional anomaly in west antarctic continental margin sediments an indicator for ice sheet collapse abstract modern global warming is likely to cause future melting of earth s polar ice sheets that may result in dramatic sea level rise a possible collapse of the west antarctic ice sheet wais alone which is considered highly vulnerable as it is mainly based below sea level may raise global sea level by up to 5 6 m despite the importance of the wais for changes in global sea level its response to the glacial interglacial cycles of the quaternary is poorly constrained moreover the geological evidence for the disintegration of the wais at some time within the last ca 750 kyr possibly during marine isotope stage mis 11 424 374 ka is ambiguous here we present physical properties palaeomagnetic geochemical and clay mineralogical data from a glaciomarine sedimentary sequence that was recovered from the west antarctic continental margin in the amundsen sea and spans more than the last 1 myr within the sedimentary sequence proxies for biological productivity such as biogenic opal and the barium aluminum ratio and the supply of lithogenic detritus from the west antarctic hinterland such as ice rafted debris and clay minerals exhibit cyclic fluctuations in accordance with the glacial interglacial cycles of the quaternary a prominent depositional anomaly spans mis 15 mis 13 621 478 ka the proxies for biological productivity and lithogenic sediment supply indicate that this interval has the characteristics of a single prolonged interglacial period even though no proxy suggests environmental conditions much different from today we conclude that if the wais collapsed during the last 800 kyr then mis 15 mis 13 was the most likely time period apparently the duration rather than the strength of interglacial conditions was the crucial factor for the wais drawdown a comparison with various marine and terrestrial climate archives from around the world corroborates that unusual environmental conditions prevailed throughout mis 15 mis 13 some of these anomalies are observed in the pelagic southern ocean and the south atlantic and might originate in major ice sheet drawdown in antarctica but further research is required to test this hypothesis

correlation of late quaternary tephra layers in a long pluvial sequence near summer lake oregon abstract near summer lake in southern oregon 54 tephra beds of late quaternary age are exposed in pluvial lake sediments of lake chewaucan seven of the tephra beds near the top can be correlated with tephra deposits younger than 117 000 yr at mount st helens washington at crater lake oregon and in northwestern nevada in the deposits of pluvial lake lahontan however most of the section at summer lake lies below the correlated units and contains 39 tephra beds older than 117 000 yr major element chemistry of tephra glasses was determined by electron microprobe analysis petrography supports the correlations made from chemical evidence compositions correlated range from 70 to 76 sio 2 the least silicic summer lake glass contained 57 extrapolation of depositional rate suggests that most of the sediments at summer lake are younger than about 335 000 yr but older lake beds containing tephra layers occur at one place the long lacustrine record suggests that lake chewaucan persisted through the last interpluvial stage and that the lake may have dried up at the end of the pleistocene due to diversion of the chewaucan river by relict shore features

muted climate variations in continental siberia during the mid pleistocene epoch the large difference in carbon and oxygen isotope data from the marine record between marine oxygen isotope stage 12 mis 12 and mis 11 spanning the interval between about 480 and 380 kyr ago has been interpreted as a transition between an extremely cold glacial period and an unusually warm interglacial period with consequences for global ice volume sea level and the global carbon cycle1 2 3 4 the extent of the change is intriguing because orbital forcing is predicted to have been relatively weak at that time5 here we analyse a continuous sediment record from lake baikal siberia which reveals a virtually continuous interglacial diatom assemblage a stable littoral benthic diatom assemblage and lithogenic sediments with interglacial characteristics for the period from mis 15a to mis 11 from about 580 to 380 kyr ago from these data we infer significantly weaker climate contrasts between mis 12 and 11 than during more recent glacial interglacial transitions in the late pleistocene epoch about 130 to 10 kyr ago for the period from mis 15a to mis 11 we also infer an apparent lack of extensive mountain glaciation

mono lake geomagnetic excursion found at summer lake oregon a 25 cm zone of anomalous paleomagnetic directions from lacustrine sediments near summer lake oregon has the same age and the same paleomagnetic signature as the younger part of the proposed mono lake geomagnetic excursion both field observations and rock magnetic studies support the hypothesis that the anomalous directions represent actual geomagnetic field behavior thereby confirming the existence of the mono lake excursion at the oregon site although confirmation of its existence at this site does not make the mono lake excursion any more suitable as a chronostratigraphic horizon this excursion may now be used as an additional constraint on models of geomagnetic field behavior including those that predict the morphology of transitional geomagnetic fields

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

the new bdp 98 600 m drill core from lake baikal a key late cenozoic sedimentary section in continental asia abstract the new 600 m drill core bdp 98 from the academician ridge of lake baikal recovered a continuous sedimentary record of the past 10 ma the entire section is represented by lacustrine sediments which gradually change from distal deltaic facies at the bottom of the section to fine undisturbed hemipelagic sediments of the upper 300 m interval the entire 10 ma lacustrine section contains abundant diatoms thus allowing extension of plio pleistocene diatom and biogenic silica records into the miocene above the matuyama gauss paleomagnetic reversal boundary the bdp 98 record contains clearly delineated glacial interglacial lithologic cycles below this boundary the diatom signal is quite different average diatom contents are higher and variations are of lower amplitude although most likely paleoclimatic in origin these variation presumably reflect past changes in the moisture regime of southeast siberia under conditions of warm subtropical climate during the miocene and early middle pliocene the continuous bdp 98 drill core which covers the hiati present in the composite continental sections of the baikal region is a key section for reconstructing the neogene quaternary climatic evolution of continental asia the bdp 98 section also places several important time constraints on the rifting history of lake baikal by providing reliable correlation of lithological and physical properties of the drill core sediments with calculated positions of the acoustic reflection boundaries interpreted from multichannel seismic studies the lithologic composition indicates that on the stable block of academician ridge where the bdp 96 and bdp 98 drill sites are located acoustic reflection boundaries are not associated with major erosional events but instead result from changes in sediment density and composition several lithologic indices further suggest that significant changes have occurred in the physics and chemistry of lake baikal waters affecting the carbonate equilibrium and oxygen regime of baikal

gigantic seismogenic landslides of summer lake basin south central oregon subsidence of summer lake basin situated in the northwestern basin and range province in south central oregon has exposed a kilometer thick neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding winter ridge slide mountain escarpment subsidence occurs along the active 40 km long winter ridge slide mountain fault which is capable of producing mw 7 earthquakes with near field maximum horizontal acceleration approaching 1 g gigantic landslides cubic kilometers in volume scallop the southwestern part of the escarpment and the deposits run out several kilometers characteristic of rock avalanches geotechnical rock mass characterization and slope stability analyses confirm observations that these landslides were initiated within the weak tuffaceous sedimentary rocks along shallow east dipping planar failure surfaces are insensitive to groundwater fluctuations and are stable under static conditions pseudostatic analyses reveal that strong shaking is required to trigger landsliding the bennett flat landslide failed as a single event between 900 270 ka and 16 8 ka the landslide has a depletion zone volume of 1 1 km3 but only 0 3 km3 of debris is discernible in the accumulation zone the foster creek landslide also appears to have failed as a single event between 180 and 16 8 ka the landslide has a depletion zone volume of 2 9 km3 but only 0 3 km3 of debris is discernible in the accumulation zone the punchbowl landslide evidences multiple events the most recent of which failed in the holocene and had a volume of 0 4 km3 0 5 km3 of debris is found in the accumulation zone

radiocarbon ages and environments of deposition of the wono and trego hot springs tephra layers in the pyramid lake subbasin nevada abstract uncalibrated radiocarbon data from core plc92b taken from wizards cove in the pyramid lake subbasin indicate that the trego hot springs and wono tephra layers were deposited 23 200 300 and 27 300 300 14 c yr b p uncorrected for reservoir effect sedimentological data from sites in the pyramid lake and smoke creek black rock desert subbasins indicate that the trego hot springs tephra layer was deposited during a relatively dry period when pyramid lake was at or below its spill point 1177 m to the winnemucca lake subbasin the wono tephra layer was deposited when lake depth was controlled by spill across emerson pass sill 1207 m to the smoke creek black rock desert subbasin 18 o data from core plc92b also support the concept that the trego hot springs tephra fell into a relatively shallow pyramid lake and that the wono tephra fell into a deeper spilling lake

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

sedimentation processes and new age constraints on rifting stages in lake baikal results of deep water drilling with this paper we present a first attempt to combine the direct results on lithology composition and age dating in the boreholes bdp 93 bdp 96 and bdp 97 with geological and seismic data from the areas where those sections were drilled the sedimentary environments represented by the bdp boreholes are markedly different and possess characteristic lithological features the results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the lake baikal rifting dynamics neither the basin wide unconformity interpreted from seismic data nor the interpreted change from shallow water to deep water facies at the boundary of the seismic stratigraphic complexes were found in the bdp 96 boreholes on academician ridge also lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the pliocene at academician ridge the continuous deep water hemipelagic sedimentation at academician ridge has existed for the past 5 ma the beginning of an intense rifting phase of the neobaikalian sub stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin wide unconformity b10 different age estimates for this boundary ranged from late pliocene 3 5 ma to plio pleistocene boundary as shown by bdp 96 borehole b10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact the new age for this boundary in bdp 96 is approximately 2 5 ma this new age constraint suggests that the upper sedimentary strata of northern baikal 1 5 1 7 km thick have formed during the past 2 5 ma with average sedimentation rates of 60 70 cm ka the bdp 93 boreholes at buguldeika suggest that uplift in primorsky range took place prior to 1 07 1 31 ma a date which exceeds the age of previous geological models