regional late pliocene early pleistocene hiatuses of the southern ocean diatom evidence abstract the existence of regionally synchronous hiatuses in late neogene sediments of the southern ocean has been questioned on sedimentological grounds while their paleoclimatic interpretation has been questioned on theoretical grounds we evaluated the evidence used to identify these hiatuses in pliocene and lower pleistocene sediments by determining quantitatively the occurrence of diatom stratigraphic markers in eleven southern ocean piston cores three from the pacific sector and eight from the indian and atlantic sectors in the pacific sector some species have ranges similar to the published record but others show north south diachrony in the southeast indian sector some marker species have ranges at odds with the published magnetostratigraphy in the atlantic sector there is wide disparity between our data and published results in two of the cores our placement of the denticulopsis hustedtii datum differs with the published record by as much as 10 m a third core contained a number of barren intervals and could not be zoned while a fourth core previously zoned and dated with magneto and biostratigraphy contained too few diatoms for quantitative biostratigraphy we conclude that the question of regional hiatuses in southern ocean sediments while important cannot be addressed until a locally and regionally applicable biozonations are developed and b magnetostratigraphic data are properly evaluated resolution of these problems are essential before we can successfully use hiatus data to recover the paleoceanographic and paleoclimatic history of the southern ocean

late cenozoic glacial history of the ross embayment antarctica abstract knowledge of the cenozoic terrestrial and marine glacial history of antarctica has been developed through studies of on land surface outcrops and subsurface drillcores and through examination of drillcores and piston cores obtained from a variety of offshore marine geological settings these field and laboratory endeavors have been a major target of the antarctic earth science community for more than thirty years the ross sea sedimentary basins including the byrd basin west antarctica wilkes and pensacola basins east antarctica and the transantarctic mountains provide significant data on the cenozoic glacial record a record that has been traced back to at least 38 ma close to the eocene oligocene boundary this paper reviews the late cenozoic history of the transantarctic mountains and flanking sedimentary basins with emphasis on the debate between the stabilist view of antarctic ice sheet history see clapperton and sugden 1990 and our favored multi glacial dynamic ice sheet model

southern ocean pliocene paleotemperature variation from high resolution silicoflagellate biostratigraphy abstract applying the modern biogeographic distribution of silicoflagellate genera dictyocha and distephanus and calcareous nannoplankton as an analog the biostratigraphic record of these phytoplankton can be used qualitatively to examine past changes in surface temperature in the southern ocean the antarctic polar frontal zone pfz is a major biogeographic barrier limiting dictyocha and calcareous nannoplankton to areas north of this zone ocean drilling project odp holes 748b and 751a on the southern kerguelen plateau 900 km south of the present location of the pfz contain expanded pliocene sections through four time intervals 4 5 to 4 2 ma hole 751a 3 7 to 3 1 ma hole 748b 3 6 to 3 5 ma hole 751a and 3 2 to 3 0 ma hole 751a three distinct peaks in dictyocha and calcareous nannofossil abundance reflect surface water warming of at least 4 c through either southward migration of the pfz or a weakening of the thermal gradient across this zone peak warming in the intervals studied is centered near 4 3 ma with other brief yet significant warming events near 4 5 4 2 and 3 6 ma cooling as a result of northward migration of the pfz or intensified thermal gradient is indicated by the absence of dictyocha and calcareous nannofossils in other stratigraphic intervals these data support previous studies indicating pliocene oceanic and climatic variability in the southern high latitudes silicoflagellate abundance patterns from hole 751a identify taxonomic relationships between several morphotypes the simultaneous appearance and proportional occurrence of five sided and four sided dictyocha specimens suggests a close relationship between these two forms distephanus crux s l and dictyocha aspera var pygmaea also show a near mutually exclusive abundance pattern in hole 751a and are similar in basal ring size and morphology indicating they are conspecific they differ only in apical structure whereby distephanus crux s l may have formed an apical bar under fluctuating environmental conditions in the early pliocene these variations in silicoflagellate architecture appear to be related to paleoenvironmental changes indicating that silicoflagellate morphology may represent a more useful paleoceanographic tool that has been noted previously

geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

pronounced mid pleistocene southward shift of the polar front in the atlantic sector of the southern ocean abstract ocean drilling program odp site 1094 situated in the atlantic sector of the antarctic region below the polar front pf is dominated by silica rich sediments during the pleistocene but does contain occasional calcium carbonate rich intervals the analysis of the coccolithophore assemblages of one of these intervals in the mid pleistocene allows us to interpret a prominent southward displacement of the pf an age model constructed using paleomagnetic and biostratigraphic information places this event around 1 ma close to the jaramillo subchron in an interval of strong eccentricity and high amplitude in the insolation record we distinguish five intervals based on coccolithophore assemblage and abundances according to the age model used this calcium carbonate rich period should include marine isotope stage 31 the most significant interglacial period recorded at this time however this model cannot decipher if adjacent isotope stages are partially represented interval v the oldest is interpreted as an episode with progressive abundance of coccolithophores with a characteristic assemblage from the polar front zone pfz but with dominance of siliceous micro organisms interval iv contains the maximum abundance of coccolithophores with an assemblage characteristic of the subantarctic zone saz with peaks in abundance of calcidiscus leptoporus coccolithus pelagicus helicosphaera carteri and syracosphaera spp related with the proximity of the subantarctic front saf the southward displacement of the frontal system linked to an increase in sea surface temperature produced increases in diversity and in productivity conversely we consider that the presence of some coccolithophore taxa linked in other environments with anomalies in salinity could reflect a pulse in the melting of west antarctic ice sheet as suggested previously for this interval interval iii is characterized by a pfz assemblage reflecting again a northward shift of the pf interval ii is considered in this context as another southward pulse but less important than interval iv and with the absence of saf and or melting related taxa interval i is considered a transition from a situation similar to the present day pfz to the antarctic zone conditions as consequence of a northward displacement of pf and weddell gyre antarctic circumpolar current boundary

kinematics of the dadeldhura klippe shear zones w nepal implications for the foreland evolution of the himalayan metamorphic core mapping combined with structural analyses in the foreland edge of the metamorphic core of the himalayas in sw nepal highlights the existence of two north dipping shear zones with opposite sense of shear here the metamorphic core is mainly affected by non coaxial top to the south sense of shear at temperatures between 450 c and 550 c that switch to a top to the north sense of shear at the top of the metamorphic core we regionally correlate this upper shear zone with the south tibetan detachment system ar dating on white mica indicates that both shear zones operated between 23 ma and 17 ma restoration of the folded south tibetan detachment in far western nepal yields a minimum dip slip distance of 190 km compatible with predictions made by models of extrusion of a weak mid crustal channel our results support an orogenic model in which channel flow in the hinterland coexisted with thrust wedge mechanics in the foreland

late cenozoic glacial history of the ross embayment antarctica abstract the ross ice drainage system comprises about 25 per cent of the surface area of the antarctic ice sheet its fluctuations during the late quaternary have been determined in two ways from analyses of three deep ice cores extracted at byrd west antarctica and at vostok and dome circe east antarctica and from the distribution stratigraphy and chronology of glacial drifts in the transantarctic mountains the ice core studies give time series records for the last 160 ka that correspond well with the marine isotope record and clearly show glacial interglacial stadial and interstadial intervals they also suggest that during glaciation maxima the polar plateau was characterised by an accumulation rate 50 per cent lower and temperature 7 9 c lower than present values glacial geological studies reveal a hiatus in the drift record between older miocene early pleistocene glacial sediments and drifts of the last two glaciations isotope stages 6 and 2 there are two leading but contradictory interpretations of the older drift sirius formation one is based primarily on biological data and favours partial deglaciation of antarctica in the pliocene early pleistocene followed by regrowth of the east antarctic ice sheet to dimensions larger than now and then substantial uplift of the transantarctic mountains the other is founded on glacial geological observations and implies a larger than present ice sheet over antarctica during the miocene and only 200 m of uplift subsequently for the late quaternary reconstructed ice surface profiles based on detailed mapping and dating of drift sheets show that large outlet glaciers from the east antarctic polar plateau became about 1000 m thicker during glaciation maxima while the plateau ice barely thickened at all this is attributed to extensive northward migration of the ice grounding line in the ross embayment as global sea level fell dates obtained from the drift sheets confirm that glaciers in the ross embayment area reached maximal limits at 190 160 ka bp and at 23 14 ka bp recession from the last glaciation maximum was underway by 13 ka bp and was complete by ca 6 ka bp alpine glaciers terminating in dry valleys fluctuate asynchronously with the ross embayment glaciers they retreat during glacial maxima when they become starved of accumulation and readvance during warmer intervals because of their dependence on grounding line position glacier fluctuations in much of antarctica are primarily led by sea level changes controlled by ice sheets in the northern hemisphere

multiple miocene marine productivity events in west antarctica as recorded in upper miocene sediments beneath the ross ice shelf site j 9 abstract considerable debate surrounds the age and origin of sediments recovered from underneath the ross ice shelf antarctica at site j 9 although diatom biostratigraphy provides the best means of dating these sediments diatom fragmentation sediment reworking and microfossil assemblage mixing have led to conflicting interpretations regarding the age and depositional history of sediments collected as part of the ross ice shelf project r i s p our biostratigraphic approach differs from previous analyses of r i s p sediments by differentiating between microfossil assemblages recovered from the sediment matrix and from within abundant semi indurated sediment clasts diatomaceous clasts that were eroded and transported to site j 9 by glacier ice reveal the age and composition of unknown marine deposits in west antarctic interior basins in addition to rare reworked paleogene and cretaceous microfossils three distinct miocene diatom assemblages are recognized two from sediment clasts middle lower miocene and early middle miocene and a third restricted to matrix sediments middle upper miocene contrary to some published reports no microfossils unequivocally younger than middle late miocene are present other than modern subice shelf benthic foraminifera and ostracodes found in the uppermost sediments whether r i s p sediments are an in situ glacimarine deposit or a basal till is still uncertain however new data presented here support deposition by glacial marine processes during the middle late miocene an age indicated by foraminifera and the youngest diatom assemblage in r i s p pliocene and pleistocene marine sediments were probably stripped off by subsequent ice grounding events most likely in association with the ross sea unconformity reworked sedimentary clasts of diatomite suggest a highly productive open marine environment in west antarctica with intense productivity and limited glacial ice at sea level in the ross sea embayment during the middle early miocene

eastern mediterranean tectonics and tsunami hazard inferred from the ad 365 earthquake historical accounts describe an earthquake and tsunami on 21 july ad 365 that destroyed cities and drowned thousands of people in coastal regions from the nile delta to modern day dubrovnik the location and tectonic setting of this earthquake have been uncertain until now here we present evidence from radiocarbon data and field observations that western crete was lifted above sea level by up to 10 m synchronously with the ad 365 earthquake the distribution of uplift combined with observations of present day seismicity suggest that this earthquake occurred not on the subduction interface beneath crete but on a fault dipping at about 30 within the overriding plate calculations of tsunami propagation show that the uplift of the sea floor associated with such an earthquake would have generated a damaging tsunami through much of the eastern mediterranean measurement of the present rate of crustal shortening near crete yields an estimate of 5 000 yr for the repeat time of tsunamigenic events on this single fault in western crete but if the same process takes place along the entire hellenic subduction zone such events may occur approximately once every 800 yr in the year ad 365 an earthquake and tsunami destroyed much of the eastern mediterranean coastal regions the distribution of uplift at the time suggests that the earthquake occurred on a fault within the overriding plate at the subduction zone beneath crete and not on the subduction interface itself

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

no evidence for extreme long term warming in early pliocene sediments of the southern ocean abstract during the past 10 years there has been considerable debate with respect to the response of the antarctic ice sheets to early pliocene warmth interpretations generally fall into one of three categories 1 there was a major drawdown of the east antarctic ice sheet eais 2 drawdown of eais was relatively small 3 eais was largely unaffected by early pliocene warmth here this question is approached by using biogenic opal and carbonate in deep sea sediments as proxy for paleoceanographic change during the present day these measures are largely dictated by the position of the antarctic polar front apf with biogenic carbonate exhibiting an increase north of the apf and biogenic opal increasing to the south that the carbonate opal transition zone is a reliable feature in deep sea sediments is evidenced by the fact that it is recorded north of its present position during the last glacial maximum lgm and to the south of it during oxygen isotope stage 11 a warmer than present interglacial when these proxies are applied to southern ocean sediments high frequency glacial interglacial changes are seen in the pliocene and early pleistocene what appears to be a major glacial is recorded at about 4 5 myrs b p three short duration carbonate highs up to 18 suggesting a southward movement of apf occur at discrete intervals between about 4 6 4 8 myrs b p however these disappear or become reduced in amplitude toward the continent such data do not support any appreciable warming of surface waters near the antarctic continent neither does it support any claims of high circa 20 c sea level air temperatures over the eais it is concluded that there is no evidence for significant drawdown of eais during early pliocene warmth

cenozoic prograding sequences of the antarctic continental margin a record of glacio eustatic and tectonic events abstract sedimentary sections up to 6 14 km thick lie beneath many areas of the antarctic continental margin the upper parts of the sections contain up to 6 km of cenozoic glacial and possibly non glacial sequences that have prograded the continental shelf up to 85 km we describe the cenozoic sequences using two general categories based on their acoustic geometries type ia sequences which account for most prograding of the antarctic continental shelf have complex sigmoidal geometries and some acoustic characteristics atypical of low latitude margins such as troughs and mounds lying parallel and normal to the shelf edge and high velocities 2 0 2 6 km s for flat layers within 150 m of the seafloor type iia sequences which principally aggrade the paleoshelf lie beneath type ia sequences and have mostly simple geometries and gently dipping reflections the prograding sequences are commonly located near the seaward edges of major mesozoic and older margin structures relatively rapid cenozoic subsidence has occured due to the probable rifting in the ross sea thermal subsidence in the antarctic peninsula and isostatic crustal flexure in wilkes land in prydz bay and the weddell sea prograding sequences cover mesozoic basins that have undergone little apparent cenozoic tectonism grounded ice sheets are viewed by us and others as the principal mechanism for depositing the antarctic prograding sequences during the initial advance of grounded ice the continental shelf is flexurally overdeepened the inner shelf is heavily eroded and gently dipping glacial strata are deposited on the shelf i e type iia sequences the overdeepened shelf profile is preserved a during glacial times by grounded ice sheets episodically crossing the shelf eroding sediments from onshore and inner shelf areas and depositing sediments at the front of the ice sheet as outer shelf topset banks and continental slope foreset aprons i e type ia sequences and b during interglacial times like today by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge thereby creating troughmouth fans and sheet like prograding sequences i e type ia sequences numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the antarctic ice sheet the available drilling and seismic interpretations provide the following history 1 cenozoic ice sheets have existed in places near the continental shelf since middle to late eocene time 2 a grounded antarctic ice sheet first expanded to the continental shelf edge with probable overdeepening of the outer shelf in late eucene to early oligocene time in prydz bay possibly in early miocene time in the ross sea and at least by middle miocene time in the weddell sea 3 the relative amounts of shelf prograding and inferred ice volume variations and related sea level changes have increased since middle to late miocene time in the eastern ross sea prydz bay and possibly weddell sea our analysis is preliminary further acoustic surveys and scientific drilling are needed to resolve the proximal antarctic record of glacio eustatic climatic and tectonic events recorded by the prograding sequences

effect of cenozoic ice sheet fluctuations in antarctica on the stratigraphic signature of the neogene stratigraphic successions from the gulf of mexico offshore alabama northeast java indonesia ross sea antarctica and several other continental margins have been examined all are characterized by very similar neogene stratal geometries in seismic profiles and well log cross sections from these areas a large mid oligocene i e latest early oligocene earliest late oligocene basinward shift in coastal onlap is followed by a major early miocene transgression and aggradation which is in turn followed by early middle miocene transgressions and late middle and late miocene progradational episodes the succession culminates in plio pleistocene high frequency progradations and transgressions the interregional character of the neogene stratal signature and its similarity to the stratal geometry found in seismic data from the ross sea continental shelf antarctica suggest that the neogene stratal signature is a manifestation of glacioeustatic fluctuations a review of the literature and an analysis of recently acquired and published data indicate that the first major ice sheet grounding event in the ross sea occurred during middle to late oligoce time the ross sea is the repository for ice flowing from a major portion of the continental interior thus the glacial record of the ross sea should serve as a gage of ice volume changes on the continent that were large enough to influence global eustasy the ice advance onto the ross sea continental shelf during middle to late oligocene time may have been the result of a decrease in the rate of shelf subsidence as rifting in the ross sea slowed or ceased advance of the ice sheet resulted in widespread erosion of the continental shelf and shelf overdeepening it is hypothesized that metastable marine based ice sheets have waxed and waned on the antarctic continental shelf since at least the oligocene and that the these waxing and waning events were responsible for the development of a global neogene stratigraphic signature

quaternary climates and sea levels of the u s atlantic coastal plain uranium series dating of corals from marine deposits of the u s atlantic coastal plain coupled with paleoclimatic reconstructions based on ostracode marine and pollen continent data document at least five relatively warm intervals during the last 500 000 years on the basis of multiple paleoenvironmental criteria we determined relative sea level positions during the warm intervals relative to present mean sea level were 7 5 meters at 188 000 years ago 7 5 1 5 meters at 120 000 years ago 6 5 3 5 meters at 94 000 years ago and 7 3 meters at 72 000 years ago the composite sea level chronology for the atlantic coastal plain is inconsistent with independent estimates of eustatic sea level positions during interglacial intervals of the last 200 000 years hydroisostatic adjustment from glacial interglacial sea level fluctuations lithospheric flexure and isostatic uplift from sediment unloading due to erosion provide possible mechanisms to account for the discrepancies alternatively current eustatic sea level estimates for the middle and late quaternary may require revision

long continental records of climate an introduction abstract the symposium on long continental records presented new information on past climates on land it seems that throughout the last 2 5 million years the climate in the middle latitudes changed in the same complex fashion as during the better known late pleistocene and holocene times over 40 interglacial glacial shifts took place during this interval out of those 40 10 12 glacials were markedly stronger and or longer lasting than the others the apparently abrupt shifts from interglacials to glacials were probably gradual oscillatory coolings interrupted by warming episodes in that respect the detailed anatomy of the past climate changes does not differ substantially from the climate oscillations documented by instrumental weather records in the last two centuries the pronounced expansions of dry and cold steppes on land were accompanied by corresponding increases of the ice volume indicated by the oxygen isotope ratio the two exceptions occurred around 1 4 ma ago and 1 6 ma ago the reason for this discrepancy is not known by far the most expressed change of land environments occurred at about 2 4 ma ago

late pleistocene paleoceanography of the south atlantic sector of the southern ocean ocean drilling program hole 704a isotopic and sedimentologic data from ocean drilling program hole 704a suggest that isotopic stages 7 9 and 11 were marked by unusually strong interglacial conditions in surface waters of the southern ocean during interglacial stages 9 and 11 warm surface waters penetrated far poleward and may have led to destabilization of the west antarctic ice sheet in contrast the strongest glacial conditions in surface waters of the subantarctic south atlantic occurred during oxygen isotopic stage 12 comparisons of benthic carbon isotopic gradients between sites located in the north atlantic southern ocean and pacific indicate that the production of upper north atlantic deep water unadw was strongest during stages 7 9 and 11 and weakest during stage 12 these results suggest a possible link between the flux of unadw and paleoceanographic change in the southern ocean and support the traditional nadw antarctic connection whereby increased nadw leads to warming of the southern ocean

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

revision of the plio pleistocene cycles and their application to sequence stratigraphy and shelf and slope sediments in the gulf of mexico abstract the ages cycles and correlation of the pliocene and pleistocene have been discussed and argued for decades the revision of the pliocene pleistocene sequence chronostratigraphy presented in this paper is based on data obtained from more than 100 wells in the south additions and deepwater areas offshore louisiana and texas gulf of mexico the high resolution biostratigraphic data combined with well log sequence stratigraphic interpretations and correlations of the seismic record sections through a two way time log or synthetic seismogram permits the pliocene and pleistocene series to be divided into 14 fourth order sequences and seven third order sequences we have discussed and illustrated the most important events that have led up to this detailed revision of the plio pleistocene this documentation is very important because most of these papers have been published in journals that have not been readily available to most geologists and geophysicists