resolving a late oligocene conundrum deep sea warming and antarctic glaciation abstract changes in ice volume and resulting changes in sea level were determined for the late oligocene 26 23 ma astronomical timescale ats by applying 18 o to sea level calibrations to deep sea 18 o records from odp sites 689 690 929 1090 and 1218 our results show that maximum global ice volume occurred during two late oligocene 18 o events oi2c 24 4 ma and mi1 23 0 ma inferred glacioeustatic lowering with volumes up to 25 greater than the present day east antarctic ice sheet eais ice volume during glacial minima was on the order of about 50 of the present day eais this is supported by late oligocene stratigraphic records from antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the antarctic continental shelf in the ross sea and prydz bay in contrast composite deep sea 18 o records show a significant decrease 1 between 26 7 and 23 5 ma which have long been interpreted as bottom water warming combined with deglaciation of antarctica however a close examination of individual 18 o records indicates a clear divergence after 26 8 ma between records from southern ocean locations i e ocean drilling program sites 689 690 744 and those of other ocean basins high 18 o values 2 9 3 3 from these southern ocean 18 o records are consistent with an ice sheet on the east antarctic continent equivalent to present day values and cold bottom water temperatures 2 0 c these differences suggest a reduction in deep water produced near the antarctic continent i e proto antarctic bottom water proto aabw which were quickly entrained and mixed with warmer and presumably more saline bottom water originating from lower latitudes expansion of a warmer deep water mass and the weakening of the proto aabw may explain the large intra basinal isotopic gradients that developed among late oligocene benthic 18 o records these conclusions are also supported by ocean modeling suggesting a reduction of deep water formed in the southern ocean strengthening of deep water from the northern hemisphere and decreasing temperatures in high southern latitudes occurred as the drake passage opened to deep water low 18 o values reported from deep sea locations other than the southern ocean are shown to bias estimates of antarctic ice volume calling for a re evaluation of the notion that antarctic ice volume was significantly reduced during the late oligocene

late oligocene and early miocene glacimarine sedimentation in the sw ross sea antarctica the record from offshore drilling hambrey m j barret p j powell r d 2002 late oligocene and early miocene glacimarine sedimentation in the sw ross sea antarctica the record from offshore drilling glacier influenced sedimentation on high latitude continental margins 203 105 128 geological society special publication meeting on glacier influenced sedimentation on high latitude continental margins location university of bristol

zarqa megafacies widespread subglacial deformation in the sarah formation of saudi arabia and implications for the sequence stratigraphy of the hirnantian glaciation

stratigraphic architecture of the cenozoic succession in the mcmurdo sound region antarctica an archive of polar palaeoenvironmental change in a failed rift setting the victoria land basin forms part of the failed west antarctic rift and preserves a cenozoic succession up to 4 km thick that records the onset of cenozoic glaciation and the history of antarctic glaciation over the past 34 myr this succession is relevant both to investigations of modern climate change and to studies of long term palaeoclimate change in general this study provides a sedimentological and stratigraphic review of the victoria land basin succession based on analysis of several continuous drillcores acquired since the 1970s and supported by seismic stratigraphic analysis of a large array of seismic reflection data an array of fifteen lithofacies is recognized within the victoria land basin cenozoic succession including fossiliferous and diversely bioturbated mudrocks and diatomites texturally mature sandstones and conglomerates mixed mudstones and sandstones with dispersed gravel with restricted bioturbation and diamictites and associated lithologies these facies record a variety of marine glaciomarine proglacial and subglacial environments locally volcanic and volcaniclastic deposits are interbedded in the succession lithofacies are arranged in repetitive vertical stacking patterns depositional sequences that record glacial advance retreat cycles with attendant relative sea level changes seven varieties of depositional sequences stratigraphic motifs are recognized within the succession as a whole and interpreted to record a range of depositional settings from rifts unaffected by glacial ice motif 7 through varying degrees of glacial influence with abundant meltwater contributions motifs 6 to 3 to cold polar glaciated environments such as that of today motifs 2 and 1 overall there is a gradual trend upward through the succession from motif 7 at the base towards motif 1 at the top but the trend is not monotonic a significant conclusion of this work is that a record of dynamic climate and glacial conditions is preserved through the entire 34 myr period of the cenozoic icehouse at least in the victoria land basin intervals characterized by consistent stratigraphic style motifs are recognized throughout the victoria land basin succession these intervals are of 1 to 6 myr duration each containing numerous depositional sequences they are one to two orders of magnitude longer than glacial interglacial cycles and record periods during which environmental conditions varied in an internally consistent manner these intervals are considered to reflect convolutions of orbital parameters that remained stable for periods of 106 a and then switched to alternative configurations such intervals are directly analogous to 1 to 8 myr intervals characterized by glaciogenic strata that are preserved within the late palaeozoic of eastern australia among other areas and may be a recurring stratigraphic response to icehouse climate regimes through geological time

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

tectonic and climatic signals from apatite detrital fission track analysis of the cape roberts project core records south victoria land antarctica abstract the cenozoic tectonic reorganization of the west antarctic rift system in the ross sea region occurred concurrently with a major change in the global climate system and a global reorganization of plate motions this region thus provides the opportunity to study in detail a range of geological issues dealing with tectonic and climate feedbacks during the late eocene early oligocene greenhouse icehouse transition at high latitudes with the aim to decipher tectonic vs climatic forcing in the ice sheet drainage evolution a detrital apatite fission track study was carried out throughout the cenozoic drill cored stratigraphic succession of the cape roberts project mcmurdo sound victoria land basin apatite fission track ages of detrital samples with depositional ages between 34 ma and 17 ma were decomposed into statistically significant age populations three age peaks were detected reflecting different bedrock provenance areas i a young peak p1 between 34 ma and 26 ma recording the signal of a source area exhumed with a constant denudation rate but absent in the proximal continental area and ii two peaks older than 40 ma that are instead compatible with thermochronological data available from the onshore bedrock p1 peak testifies a late oligocene early miocene denudation event and traces a southern provenance of a fraction of the sediments filling the cape roberts basin we propose that this denudation event might have been triggered by renewed n s transtensional shearing along the western shoulder of victoria land basin controlling glacial flow pattern and the associated sediment transport

calibration between eustatic estimates from backstripping and oxygen isotopic records for the oligocene eustatic estimates from the backstripping of oligocene sections are compared quantitatively with d 18 o data each of the nine oligocene d 18 o events maxima identified in previous studies correlates with a stratigraphically determined sea level lowstand oxygen isotopic records from planktonic foraminifers from western equatorial atlantic ocean drilling program odp site 929 indicate an isotopic increase of 0 16 per 10 m decrease in the depth of the ocean apparent sea level asl amplitudes of asl change also correlate with moderate and high resolution benthic foraminiferal d 18 o records from odp sites 803 western tropical pacific and 929 and from deep sea drilling project dsdp site 522 south atlantic ocean with an isotopic change of 0 22 per 10 m of asl change r 2 5 0 807 and 0 960 respectively and with records from odp site 689 southern ocean 0 13 per 10 m of asl change r 2 5 0 704 this correlation suggests that southern ocean deep water temperature changes were smaller than tropical sea surface temperature changes between million year scale glacials and interglacials it also suggests that the deep sea southern ocean records may provide the best means to calibrate sea level to oxygen isotopes

constraining the amplitude of late oligocene bathymetric changes in western ross sea during orbitally induced oscillations in the east antarctic ice sheet 1 implications for glacimarine sequence stratigraphic models abstract late oligocene shallow glacimarine sequences recovered from western ross sea antarctica by the cape roberts drilling project display orbitally influenced cycles of advance and retreat of a laterally extensive ice sheet across the continental shelf in concert with changes in contemporary water depth during interglacial periods when the glacier terminated on land the coastline was largely ice free and wave influenced and sediments accumulated in hydrodynamic equilibrium with the contemporary wave climate here we present estimates of paleobathymetry from intervals of three milankovitch duration glacimarine sequences 9 10 and 11 that accumulated in open ocean conditions we utilise an approach where the percentage of mud

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

extended orbitally forced palaeoclimatic records from the equatorial atlantic ceara rise abstract we extend existing high resolution oligocene miocene proxy records from ocean drilling program odp leg 154 the extended record spans the time interval from 17 86 to 26 5 ma the data are age calibrated against a new astronomical solution that affords a re evaluation of the intricate interaction between orbital milankovitch forcing of the climate and ocean system and the fidelity with which this forcing is recorded in oxygen and carbon stable isotope measurements from benthic foraminifera and associated lithological proxy records of magnetic susceptibility colour reflectance and the measured sand fraction our records show a very strong continual imprint of the earth s obliquity cycle modulate in amplitude every 41 ka a very strong eccentricity signal in the carbon isotope records and a strong but probably local imprint of climatic precession on the coarse fraction and magnetic susceptibility records our data allowed us to evaluate how the interaction of long multi million year beats in the earth s eccentricity and obliquity are implicated in the waxing and waning of ice sheets presumably on antarctica our refined age model confirms the revised age of the oligocene miocene boundary previously established by analysis of the lithological data and allows a strong correlation with the geomagnetic time scale by comparison with data from odp site 1090 southern ocean

pleistocene variability of antarctic ice sheet extent in the ross embayment cores acquired by the andrill mcmurdo ice shelf project and 1b provide the basis for a new sedimentation model for glacimarine depositional sequences that reflect cyclic glacial interglacial fluctuations of a marine based ice sheet in the western ross embayment over the past 2 0 ma notwithstanding periodic erosion during advances of the ice sheet uncertainties inherent to the sedimentological interpretation and a limited number of chronological datums it is clear that subglacial to grounding zone sedimentation was dominant at the and 1b site during the late pleistocene with interglacials being represented only by thin intervals of ice shelf sediment each sequence is characterised by subglacial massive diamictite that pass upwards into glacimarine diamictites and mudstones this provides the first direct evidence that the marine based antarctic ice sheet has oscillated between a grounded and floating state at least 7 times in the ross embayment over the last 780ka implying a milankovitch orbital influence an unconformity in and 1b that spans most 200 kyr of the mid pleistocene transition is inferred to represent widespread expansion of a marine based ice sheet in the ross embayment at 0 8 ma prior to 1 0 ma interglacial periods are characterised by open water conditions at the drill site with high abundances of volcanoclastic deposits and occasional diatomaceous sediments these may have responded to precession 20 kyr or obliquity 40 kyr orbital control the occurrence of 6 7 m of phonolitic glass reworked from mt erebus in interglacial deposits beneath last glacial maximum till requires open ocean or ice shelf conditions in the western ross sea around the drill site within the past 250 ka and implies a ross ice shelf similar to or less extensive than today during marine isotope stage 7 or 5

oligocene eustasy from two dimensional sequence stratigraphic backstripping estimates of the magnitudes of changes in third order 0 522 m y eustasy were obtained for oligocene sequences defined by a suite of largely onshore boreholes of the new jersey coastal plain benthic foraminiferal biofacies and multiple age constraints in a sequence stratigraphic framework formed the database for this study the geometry of the margin through time was determined using two dimensional backstripping the depth ranges of benthic foraminiferal biofacies were determined from a combination of standard factor analysis techniques and the backstripped geometries benthic foraminiferal biofacies were then used to determine the depths of the oligocene margin profiles obtained from backstripping the water depths of 16 of these horizons were confirmed by independent benthic biofacies determinations from at least two wells this internal consistency indicates that the data and the twodimensional backstripping approach were robust where benthic biofacies require a vertical shift of the horizons generated by two dimensional backstripping eustatic changes were required and were readily calculated results indicate a major eustatic fall from the end of the eocene to the first record of oligocene deposition subsequent long term shoaling of sea level through the oligocene was 30 m in 10 m y superimposed on this long term trend were higher frequency third order variations in eustasy with amplitudes of 40 m or less

high resolution 104 years deep sea foraminiferal stable isotope records of the eocene oligocene climate transition we have constructed high resolution 104 105 years benthic foraminiferal 13c and 18o records for the upper eocene through lower oligocene of two pelagic sequences deep sea drilling project dsdp site 522 in the angola basin south atlantic ocean and ocean drilling program odp site 744 in the southern indian ocean these records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 ma the oxygen isotope records indicate that the ubiquitous 18o increase oi 1 which marks the rapid expansion of continental ice sheets and a minimum of 3 to 4 c of cooling of bottom waters in the earliest oligocene 33 6 ma occurred in 350 kyr more than half the transition occurred over the final 40 50 kyr this period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr i e the duration of magnetochron c13n these records also indicate that this interval was characterized by at least two 100 kyr waxing and waning cycles oi 1a and oi 1b and possibly several higher frequency events the benthic foraminiferal 13c records show a positive 0 8 excursion that is nearly isochronous with the oi 1 oxygen isotope increase similar magnitude 13c increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle this excursion was followed by smaller amplitude 13c oscillations with periods of roughly 400 kyr we suspect that the ubiquitous oi 1 13c excursion resulted from a brief but substantial increase in export production and carbon burial

reservoir potential of sands formed in glaciomarine environments an analogue study based on cenozoic examples from mcmurdo sound antarctica this paper provides documentation of unexpectedly high reservoir quality glaciomarine sands found in the cenozoic succession beneath mcmurdo sound antarctica as an analogue study for evaluations of hydrocarbon prospectivity in basins elsewhere the oligocene to lower miocene succession of the victoria land basin an extant portion of the west antarctic rift system comprises diamictites mudrocks and sandstones with minor conglomerates these lithologies are arranged in repetitive stacking patterns cycles interpreted to record repeated advance and retreat of glaciers into and out of the basin with attendant eustatic and isostatic effects phases of ice retreat within the cycles comprise an array of mudrocks sandy mudrocks and sandstones deposited mainly during relative sea level highstands clean wellsorted unconsolidated and porous sands 25 m thick from such intervals which are interpreted to be mainly deltaic in origin were encountered some of these sands which have visible porosity as high as 41 flowed into the well bore together with significant volumes of cold formation water diagenetic modification of sands in these intervals is minimal which can be attributed to the low temperature nature of the subsurface environment accordingly glaciomarine sands in near field glaciogenic successions should be considered as potential reservoir facies in prospectivity assessments

cold glaciers erode and deposit evidence from allan hills antarctica here we report previously undescribed features of erosion and deposition by a cold polar glacier a recent study challenged the assumption that cold glaciers neither slide nor abrade their beds but no geological evidence was offered the features we describe include abrasion marks subglacial deposits glaciotectonically deformed substrate isolated blocks ice cored debris mounds and boulder trains all products of a recent cold ice advance and retreat mapping these features elsewhere in antarctica will document recent shifts in the east antarctic ice sheet margin providing new insight on regional mass balance changes

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

astronomical calibration age for the oligocene miocene boundary the stratotype section for the base of the miocene is at a reversed below to normal above magnetic transition that is claimed to represent magnetic chron c6cn 2n o deep sea drilling project dsdp site 522 is the only location we are aware of that unambiguously records the three normal events of c6cn we have quantitatively determined the range of the short lived nannofossil sphenolithus delphix and the lower limit of s disbelemnos in dsdp holes 522 and 522a in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site astronomical tuning of ocean drilling program odp sites 926 928 and 929 shows that s disbelemnos appears at 22 67 ma and that the entire range of s delphix is from about 22 98 ma to 23 24 ma using these ages linear interpolation in dsdp site 522 suggests that the age of c6cn 2n o and of the oligocene miocene boundary is 22 92 0 04 ma our value conservatively expressed as 22 9 0 1 ma is 0 9 m y younger than the currently accepted age of the oligocene miocene boundary and of c6cn 2n o which was assigned an age of 23 8 ma based on an estimate of 23 8 1 ma for the oligocene miocene boundary the bulk sediment carbon isotope data from dsdp site 522 is correlated to the record from benthic foraminifera at odp site 929 to refine the calibration of magnetic reversals from c6cn 1n o to c7n 2n o at dsdp site 522 on the astronomical time scale

steeply dipping extension fractures in the newark basin new jersey abstract late triassic and early jurassic bedrock in the newark basin is pervasively fractured as a result of mesozoic rifting of the east central north american continental margin tectonic rifting imparted systematic sets of steeply dipping en echelon mode i extension fractures in basin strata including ordinary joints and veins these fractures are arranged in transitional tensional arrays resembling normal dip slip shear zones they contributed to crustal stretching sagging and eventual faulting of basin rift deposits extension fractures display progressive linkage and spatial clustering that probably controlled incipient fault growth they cluster into three prominent strike groups correlated to early intermediate and late stage tectonic events reflecting about 50 60 of counterclockwise rotation of incremental stretching directions finite strain analyses show that extension fractures allowed the stretching of basin strata by a few percent and these fractures impart stratigraphic dips up to a few degrees in directions opposing fracture dips fracture groups display three dimensional spatial variability but consistent geometric relations younger fractures locally cut across and terminate against older fractures having more complex vein cement morphologies and bed normal folds from stratigraphic compaction a fourth youngest group of extension fractures occur sporadically and strike about e w in obliquely inverted crustal blocks a geometric analysis of overlapping fracture sets shows how fracture groups result from incremental rotation of an extending tectonic plate and that old fractures can reactivate with oblique slip components in the contemporary compressive stress regime

paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and 18o 16o measurements on benthic foraminifera 1 we explore the applicability of paired mg ca and 18o 16o measurements on benthic foraminifera from southern ocean site 747 to paleoceanographic reconstructions on pre pleistocene timescales we focus on the late oligocene through pleistocene 27 0 ma history of paleotemperatures and the evolution of the 18o values of seawater 18osw at a temporal resolution of 100 200 kyr absolute paleotemperature estimates depend on assumptions of how mg ca ratios of seawater have changed over the past 27 myr but relative changes that occur on geologically brief timescales are robust results indicate that at the oligocene to miocene boundary 23 8 ma temperatures lag the increase in global ice volume deduced from benthic foraminiferal 18o values but the smaller scale miocene glaciations are accompanied by ocean cooling of 1 c during the mid miocene phase of antarctic ice sheet growth 15 13 ma water temperatures cool by 3 c unlike the benthic foraminiferal 18o values which remain relatively constant thereafter temperatures vary by 3 c and reach maxima at 12 and 8 5 ma the onset of significant northern hemisphere glaciation during the late pliocene is synchronous with an 4 c cooling at site 747 a comparison of our 18osw curve to the haq et al 1987 sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18o 16o ratios at 12 11 ma in particular when benthic foraminiferal 18o values do not support a further increase in ice volume the 18osw curve comes to a maximum that corresponds to a major mid miocene sea level regression the agreement between the character of our mg ca based 18osw curve and sequence stratigraphy demonstrates that benthic foramaniferal mg ca ratios can be used to trace the 18osw on pre pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater mg ca ratios the mg ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes which can lead to misinterpretations of the 18o record

geologic constraints on the chaotic diffusion of the solar system the correlation of earth s orbital parameters with climatic variations has been used to generate astronomically calibrated geologic time scales of high accuracy however because of the chaotic behavior of the solar system two initially close calculations of earth s orbit diverge exponentially and have a large uncertainty beyond several million years in the past this chaotic behavior is related to a combination of angles in the precession motion of the orbits of earth and mars which currently is in resonance how long stays in libration critically depends on the dynamical model and initial conditions for the solar system here we show that geologic data can differentiate between astronomical solutions that do and do not exhibit a transition in since 40 ma and that sediments can thus provide a history for the evolution of we find that the chaotic transition of from libration to circulation did not occur after ca 30 ma we can thus constrain the chaotic diffusion of the solar system in the past and our results provide new and challenging constraints for astronomical models

local wave climate and long term bed shear stress characteristics in monterey bay ca abstract five and a half years of wave measurements at two stations in monterey bay ca show that the local wave climate at the south marina station is markedly different from that at the north santa cruz harbor station measured significant wave heights at the marina station are profoundly greater than at the santa cruz harbor station especially during winter during summer southerly or southwesterly pacific ocean swells can be dominant and therefore the peak wave periods at the marina station protected from these swells by the monterey peninsula headland are much shorter than at the santa cruz station this disparity of wave characteristics at the two stations are the direct causes of the difference in the long term probability of sediment suspension and transport sheet flow conditions under which significant sand transport events presumably take place occur about 20 to 40 times more often at the marina station than at the santa cruz harbor station

constraining the amplitude of late oligocene bathymetric changes in western ross sea during orbitally induced oscillations in the east antarctic ice sheet 2 implications for global sea level changes abstract calibrations of deep sea 18 o ice volume records imply glacial interglacial g i eustatic sea level fluctuations of 10 30 m during the late oligocene but it has hitherto not been possible to evaluate these inferred changes from direct evidence for coeval oscillations of sea level and changes in the volume of the antarctic ice sheets here we utilise a grainsize derived paleobathymetry curve dunbar g b naish t r powell r p barrett p j this volume constraining the amplitude of late oligocene bathymetric changes in western ross sea during orbitally induced oscillations in the east antarctic ice sheet 1 implications for cycle stratigraphic models palaeogeography palaeoclimatology palaeoecology to determine the amplitude of eustatic sea level fluctuations from three well dated late oligocene shallow glacimarine sedimentary cycles cored by the cape roberts project in western ross sea our approach estimates the eustatic sea level contribution to the paleobathymetry curve by placing constraints on total subsidence decompacted sediment accumulation and glacio isostasy these sedimentary cycles link the extent of the east antarctic ice sheet and local paleobathymetry changes directly to orbital cycles by using a new chronostratigraphy that enables the three sedimentary cycles to be correlated with individual cycles on the new astronomically tuned 18 o timescale our new eustatic estimates are consistent with the late oligocene 18 o to sea level calibrations of pekar et al pekar s f deconto r m harwood d m 2006 resolving a late oligocene conundrum deep sea warming and antarctic glaciation palaeogeography palaeoclimatology and palaeoecology 231 29 40 and show that eustatic sea level fluctuated between 10 m and 40 m and represented ice volume fluctuations involving 15 to 60 of the present day antarctic ice sheet moreover our eustatic estimates imply a 18 o calibration for the mi 1 excursion that supports a significant expansion of ice on antarctica perhaps equivalent to 120 of the present day eais assuming no northern hemisphere ice with an attendant fall in global sea level of 50 m

estimating palaeobathymetry of wave graded continental shelves from sediment texture the concept of the wave graded continental shelf with sea floor sediment coarsening from offshore mud to shoreface sand has been well known from the time of johnson 1919 although most of the modern shelf shows textures unrelated to water depth on account of relict features or sediment starvation the geological record is more likely to preserve sites where sediment is being fed to a subsiding inner shelf these consistently show the landward coarsening pattern of the wave graded shelf recording past water depth history in accumulated sea floor sediment the landward coarsening pattern is driven primarily by wave induced bed shear stress which increases shoreward exponentially although it also varies from place to place with wave climate and can be influenced by sediment concentration and currents in this study the relationship between bed shear stress sediment texture and water depth has been investigated by comparing per cent mud and wave climate data from shore normal transects of three modern wave graded coastal settings wellington harbour low energy and the manawatu coast moderate energy in new zealand and monterey bay in california moderate high energy samples from all three locations show a progressive change from poorly sorted mud offshore to well sorted fine sand nearshore with the sand mud transition ranging from 3 m low energy to 50 m moderate high energy reflecting differences in average bed shear repeat measurements of per cent mud on seasonal annual and decadal time scales along a manawatu coastal transect showed no measurable change demonstrating equilibrium between sediment supply wave energy water depth and sediment texture a simple model based on the relationship between wave climate shear stress and per cent mud and using data and conditions from the modern manawatu coast is applied to two mid pliocene cyclothems exposed 50 km inland giving results comparable with estimates from foraminifera and the deep sea isotope record per cent mud offers more detail for palaeobathymetric trends in shallow water shelf strata than other proxy depth recorders although it is limited to depths above wave base and requires an independent estimate of wave climate if depths are to be quantified

sequence stratigraphy of sixth order 41 k y pliocene pleistocene cyclothems wanganui basin new zealand a case for the regressive systems tract this study is based on a late pliocene and early pleistocene approximately 2 6 1 7 ma succession about 1 km thick of 20 sixth order 41 k y duration cyclothems of shelf origin exposed in the rangitikei river valley in the eastern part of wanganui basin the cyclothems correlate with 18 o isotope stages 100 58 and each 41 k y glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive highstand and regressive systems tracts unlike most examples inferred from the stratigraphic record these systems tracts were deposited during phases of known sea level cycles indicated by the contemporary oxygen isotope ice volume curve because of the high rate of subsidence of wanganui basin glacioeustatic sea level falls during most cycles were not of sufficient magnitude to expose the outer shelf thus the rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break simple one dimensional modeling shows that moderate to high rates of basin subsidence 1 2 mm yr and low rates of sedimentation 0 2 mm yr during transgressions combined to produce an accommodation surplus at the relative highstand this surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational highstand systems tract shelf siltstone and chiefly by strongly progradational shoreface sediments of the regressive systems tract rangitikei regressive systems tracts are distinguished from forced regressive systems tracts sensu hunt and tucker 1992 by their different stratal geometry by definition forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and or slope in contrast regressive systems tracts exhibit a gradational lower contact above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary cyclothems display two types of motif termed rangitikei dt depositional transgression and rangitikei nt nondepositional transgression which include the following architectural elements in ascending stratigraphic order 1 a basal sequence boundary that is coincident with either the transgressive surface of erosion which displays small scale up to 50 cm erosional relief and may be penetrated by the ichnofossil ophiomorpha or its deeper water correlative conformity 2 either a thick 5 30 m transgressive systems tract comprising a deepening upward nearshore to inner shelf mixed carbonate siliciclastic lithofacies succession depositional transgression or a thin 2 m transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment starved offshore shelf nondepositional transgression 3 a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract 4 a highstand systems tract comprising a 10 20 m thick interval of aggradational shelf siltstone and 5 a thick up to 45 m progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract condensed shell beds are associated with intrasequence and sequence bounding discontinuities and together with the sedimentological and stratal characteristics of the sequences are important indicators of stratigraphic architecture four types of shell bed are associated with surfaces formed by four different types of stratal termination onlap backlap downlap and flooding surface shell beds cf kidwell 1991 are associated respectively with the transgressive surface of erosion apparent truncation at the top of the transgressive systems tract the downlap surface and local marine flooding surfaces a fifth shell bed type termed a compound shell bed forms in offshore environments where the downlap surface converges with the sequence boundary and elements of both the downlap and the backlap shell beds become mixed or superposed the shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale

high resolution sequence stratigraphy of clastic shelves iv high latitude settings abstract high frequency clastic shelf sequences deposited in high latitude settings display marked differences in term of facies and stratigraphic architecture with respect to their lower latitude counterparts this is due to the presence of ice which 1 leads to the accumulation of glacigenic and glacimarine deposits 2 provides an additional control on accommodation and 3 determines the position of the shoreline transgressions and regressions in glaciated settings are controlled respectively by the retreat and advance of the ice shoreline i e the water ice contact irrespective of relative sea level changes once the ice retreats across the land the traditional land shoreline is exposed and the control on sequence architecture is exerted by the interplay between relative sea level changes and sediment supply as in low and middle latitude settings a general model that includes both glacial and non glacial climatic regimes is provided by this paper in this frame the classic sequence stratigraphic model represents one ice free end member which is opposed to an ice permanent end member between these end members sequences may accumulate in part under ice free conditions and in part under conditions dominated by ice on the shelf the main implication of this is that the classic sequence stratigraphic model may be viewed as only a possible scenario in the stratigraphic record rather than the rule

climate response to orbital forcing across the oligocene miocene boundary spectral analyses of an uninterrupted 5 5 million year my long chronology of late oligocene early miocene climate and ocean carbon chemistry from two deep sea cores recovered in the western equatorial atlantic reveal variance concentrated at all milankovitch frequencies exceptional spectral power in climate is recorded at the 406 thousand year ky period eccentricity band over a 3 4 million year period 20 to 23 4 my ago ma as well as in the 125 and 95 ky bands over a 1 3 million year period 21 7 to 23 0 ma of suspected low greenhouse gas levels moreover a major transient glaciation at the epoch boundary approximately 23 ma mi 1 corresponds with a rare orbital congruence involving obliquity and eccentricity the anomaly which consists of low amplitude variance in obliquity a node and a minimum in eccentricity results in an extended period approximately 200 ky of low seasonality orbits favorable to ice sheet expansion on antarctica

early oligocene ice sheet expansion on antarctica stable isotope and sedimentological evidence from kerguelen plateau southern indian ocean sedimentological and stable isotope data from a pelagic sequence recovered from the southern indian ocean provide the most convincing evidence to date for short term expansion of a large ice sheet on antarctica during the earliest oligocene 36 ma terrigenous debris identified as ice rafted in origin on the basis of textural compositional and size criteria is present at the same stratigraphic level as the ubiquitous early oligocene oxygen isotope shift the highest benthic foraminiferal 18o values 3 0 of the paleogene occur in samples from within the ice rafted debris interval these values are similar to those recorded by holocene benthic foraminifera implying that the ice sheet may have attained a volume similar to that of the present day ice sheet on antarctica the stratigraphic distribution of ice rafted debris and high oxygen isotope values indicate that these conditions persisted for roughly 100 ka

sea ice feedback and cenozoic evolution of antarctic climate and ice sheets 1 the extent and thickness of antarctic sea ice have important climatic effects on radiation balance energy transfer between the atmosphere and ocean and moisture availability this paper explores the role of sea ice and related feedbacks in the cenozoic evolution of antarctic climate and ice sheets using a numerical climate model with explicit dynamical representations of sea ice and continental ice sheets in a scenario of decreasing cenozoic greenhouse gas concentrations our model initiates continental glaciation before any significant sea ice forms around the continent once variable ice sheets are established seasonal sea ice distribution is highly sensitive to orbital forcing and ice sheet geometry via the ice sheet s control on regional temperature and low level winds although the expansion of sea ice has significant climatic effects near the coast it has only minimal effects in the continental interior and on the size of the ice sheet therefore the cenozoic appearance of antarctic sea ice was primarily a response to the growth of grounded ice sheets and was not a critical factor in episodes of paleogene and neogene glaciation the influence of the east antarctic ice sheet on sea ice southern ocean surface temperatures and winds has important implications for ocean circulation the marine carbon cycle and the development of the west antarctic ice sheet the sensitivity of sea ice to grounded ice sheets implies reconstructions of sea ice based on marine diatoms are good indicators of glacial conditions in the continental interior and may provide insight into the long term stability of antarctic ice sheets

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

reconstructing the transport history of glacigenic sediments a new approach based on the co variance of clast form indices abstract two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts the c40 index the percentage of clasts with c a ratios 0 4 and the ra index the percentage of very angular and angular clasts in a sample analysis of the between and within sample co variance of these indices provides a powerful tool for discriminating actively and passively transported clasts in glacigenic deposits use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at storbreen jotunheimen these fall between those of actively transported clasts in basal till low c40 and ra values and those of unmodified frost weathered clasts high c40 and ra values both indices decline down moraine within sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts these analyses also indicate that actively transported clasts form the dominant component of the moraines particularly near the former glacier terminus further applications of the approach developed here are suggested

basal inflection controlled shelf edge wedges off new jersey track sea level fall a key marker for examining the latest pleistocene history of the new jersey margin is the high amplitude long recognized r horizon reflector this time transgressive surface was formed 47 33 ka and represents the integrated topographic and bathymetric surface developed during the complicated sea level oscillation associated with the regression that preceded the last glacial maximum lgm from landward to seaward across the outer shelf the r reflector changes from subhorizontal in most locations 9 m beneath the modern seafloor to seaward dipping forming the base of two offlapping sediment wedges the previously described outer shelf wedge and a deep shelf wedge seaward of it this transition occurs across two inflection zones where the dip of the r reflector steepens seaward that can be traced for tens of kilometers along strike and mark the landward limits of these wedges these inflection zones are possibly former wave dominated shorefaces these represent the primary topographic elements present during the last regression we speculate that these inflections dictated both the locations for deposition of the two prograding offlapping wedges that developed during the complex sea level fall prior to the lgm and their successive erosion before and after the lgm we suggest that such inflection zones and their associated wedges are important markers of regression in clastic dominated outer shelf settings along passive margins

evidence of a full west antarctic ice sheet back to the early oligocene insight from double dating of detrital apatites in ross sea sediments the west antarctic ice sheet is the most unstable component of the antarctic cryosphere its fluctuations are well documented since the pliocene but its behaviour over the last 35 ma is more controversial particularly during periods of past high global pco2 values similar to those predicted in future global climate scenarios here we present new u pb dating of detrital apatite grains previously dated by the fission track method from cape roberts project oligocene to pliocene marine sediments in the ross sea two past ice flow patterns were identified one formed by outlet glaciers sourcing short travelled apatites and one northerly directed bringing far travelled apatite grains the latter provides the first robust physical evidence for the presence and repeated expansion of an oligocene west antarctic ice sheet

high resolution ice volume estimates for the early miocene evidence for a dynamic ice sheet in antarctica ice volume estimates for the early miocene 23 16 ma ats were determined by applying d 18 o to sea level calibrations to high resolution d 18 o records from odp sites 1090 and 1218 these calibrated records indicate that ice volume ranged between 50 and 125 of the present day east antarctic ice sheet eais during most of the early miocene 23 17 ma maximum icevolume occurred at each of the early miocene isotopic events i e mi events concomitant with bottom water temperatures generally between 1 and 2 8c rapid bb 1 myr and high amplitude ice volume changes also occurred intermittently during this period with some fluctuations ranging from a fully glaciated east antarctic continent to a partial collapse of the ice sheet equivalent to a 50 70 reduction in the eais these large scale ice volume changes often occurred at b100 kyr time scales suggesting an orbitally driven dynamic eais existed during the early miocene in contrast the calibrated d 18 o record from site 1090 indicates significantly less ice volume 25 70 of the present day eais was present between 17 and 16 ma these results are supported by numerical climate ice sheet modeling studies that show increased orbitally driven ice volume variability with elevated levels of atmospheric co2 the distribution of bottom water masses during the miocene was likely significantly different from today owing to variability in the production of deep water near the antarctic continent i e proto antarctic bottom water proto aabw weakening of proto aabw is suggested to have occurred during the early miocene especially during glacial minima resulting in it becoming more spatially restricted to the southern ocean and becoming entrained into a warmer deep water mass increased production of a warmer bottom water mass is also considered to have been important in the lighter isotopic values observed in the atlantic and indian oceans these changes are supported by evidence of a large dynamic eais based on strata cored on the antarctic margin and require a reevaluation of the view that antarctic ice volume was significantly reduced for most of the early miocene previous estimates have been constrained by the lack of deep water isotopic records proximal to antarctica and unduly influenced by the expansion of warmer deep water into most of the ocean basins at this time d 2005 elsevier b v all rights reserved

the middle pleistocene merced 2 and 3 sequences from ocean beach san francisco the merced formation comprises a 2 km thick shallow marine and non marine succession that was deposited in a small transtensional basin along the san andreas fault during the late pliocene to middle pleistocene the sediments dip between 10 and 80 to the northeast and are locally disrupted by small faults during tilting the beds have been rotated into subparallelism with the san andreas fault zone splays of which bound the outcrop belt of merced sediments to both the southwest and northeast the merced formation contains more than 20 transgressive regressive sedimentary rhythms cyclothems or sequences that are generally between 40 and 120 m thick and which were deposited mostly during interglacial time under the influence of rising highstand and early falling sea levels sequences merced 2 units m1 n of soc econ paleontol mineral field guidebook 3 1984 1 and merced 3 units o p though in fault contact comprise typical plio pleistocene shallow water cyclothems the merced 2 sequence is 22 m thick and comprises a sandy and shelly transgressive systems tract including a basal type a shellbed an in situ type b mid cycle shellbed and a highstand systems tract of massive siltstone the merced 3 sequence is 47 m thick and comprises a basal compound shellbed a thin highstand systems tract siltstone and a sand rich regressive systems tract the rst comprises distal shoreface sands with an abundant in situ molluscan fauna and upper shoreface and back beach trough cross bedded sands and pebbly sands the top of the merced 3 cycle comprises a beach sand capped by a palaeosol and lignite the beetle bed which together mark the subaerial exposure of the site during the ensuing glacial sea level lowstand analysis of 10be across the merced 3 sequence shows major peaks indicative of sedimentation starvation in the basal transgressive systems tract shellbed and in the capping lignite of the beetle bed smaller 10be peaks are associated with a shellbed that is inferred to represent winnowing at the foot of the shoreface and with a minor exposure surface that delimits a small paracycle in the top of the sequence otherwise 10be abundances decline regularly across the merced 3 sequence consistent with an increasing sedimentation rate as shoreface progradation and regression progressed the character of cycles merced 2 and merced 3 respectively resembles the seafield and rangitikei sequence motifs described from similar plio pleistocene sediments in new zealand the cycles are of mid pleistocene age and were probably deposited during interglacial oxygen isotope stages 21 and 19 respectively

astronomic calibration of the late oligocene through early miocene geomagnetic polarity time scale at ocean drilling program odp site 1090 subantarctic south atlantic benthic foraminiferal stable isotope data from cibicidoides and oridorsalis span the late oligocene through early miocene f24 16 ma at a temporal resolution of f5 ky over the same interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale gpts thereby providing direct correlation of the isotope record to the gpts in an initial age model we use the newly derived age of the oligocene miocene o m boundary of 23 0 ma of shackleton et al geology 28 2000 447 revised to the new astronomical calculation la2003 of laskar et al icarus in press to recalculate the spline ages of cande and kent j geophys res 100 1995 6093 we then tune the site 1090 y 18 o record to obliquity using la2003 in this manner we are able to refine the ages of polarity chrons c7n through c5cn 1n the new age model is consistent within one obliquity cycle with previously tuned ages for polarity chrons c7n through c6bn from shackleton et al geology 28 447 450 2000 when rescaled to la2003 the results from site 1090 provide independent evidence for the revised age of the oligocene miocene boundary of 23 0 ma for early miocene polarity chrons c6aar through c5cn our obliquity scale age model is the first to allow a direct calibration to the gpts the new ages are generally within one obliquity cycle of those obtained by rescaling the cande and kent j geophys res 100 1995 6093 interpolation using the new age of the o m boundary 23 0 ma and the