regional warming pliocene 3 ma paleoclimate of europe and the mediterranean the paleoclimate of the middle pliocene ca 3 ma was generally warmer than present particularly at middle to high latitudes it has been suggested that this period may represent an analogue for future climate change mechanisms that have been proposed to account for this warming are enhanced thermohaline circulation and or greater concentrations of co 2 in the atmosphere we present new climate model simulations supported by geological data that suggest that in the european and mediterranean region the climate was warmer by 5 c wetter by 400 1000 mm yr and less seasonal than present modeling results suggest that an intensification of the icelandic low pressure system and the azores high pressure system occurred during the middle pliocene as a direct result of higher annual sea surface temperatures and reduced ice cover in the northern hemisphere this change increased the surface pressure gradient over the region and strengthened annual westerly wind velocity by 4 m s 1 associated increases in wind stress by 20 n m 2 over the north atlantic ocean may have enhanced the flow of surface currents such as the gulf stream and north atlantic current which in turn would have sustained the higher sea surface temperatures these changes to the regional climate system would have driven greater atmospheric and oceanic transport of heat from equatorial regions to the north atlantic ocean particularly during winter such conditions are proposed as a likely cause of the warming indicated for europe and the mediterranean ca 3 ma

enhanced airborne dispersal of silicic tephras during the onset of northern hemisphere glaciations from 6 to 0 ma records of explosive volcanism and climate change in the subpolar north atlantic a 6 m y long composite marine record of explosive silicic volcanism from five ocean drilling program sites in the subpolar north atlantic was compared with several marine records of global and local paleoclimate proxies benthic d 18 o and ice rafted debris records coarsening and high frequency of occurrence of icelandic tephras were recorded in 3 6 3 ma sediments suggesting that these tephras were dispersed farther from the source by enhanced westerly winds over the subpolar north atlantic the 40 ar 39 ar ages were determined by laser probe on k feldspar and biotite phenocrysts of tephras that were erupted from the jan mayen volcanic system compared to the tuned paleomagnetic age model the 40 ar 39 ar dating 0 618 6 0 007 ma to 4 90 6 0 05 ma yields a new age model that postdates by 155 k y the inception of ice rafting on the iceland plateau during the cold marine isotope stage m2 i e 3 3 3 14 ma

onset of cenozoic antarctic glaciation abstract this paper considers the wide range of evidence both direct and indirect for the onset of cenozoic antarctic glaciation it distinguishes two useful phases of antarctic glacial onset an initial phase of mountain glaciation from which ice streams occasionally and in isolated locations reached sea level and a subsequent phase of full glaciation with an ice sheet as large as today s extending everywhere to sea level according to direct evidence generally proximal from the continent itself or surrounding southern ocean the first of these occurred probably during the late eocene while the second developed at the eocene oligocene boundary indirect evidence mainly involving proxy measurements from dsdp and odp sites remote from the southern ocean suggests that middle and late eocene glaciations may have been full also ice sheets possibly even larger than today s but short lived and that the e o boundary onset differed from these mainly in producing a stable ice sheet in pursuing the notion of glacial onset we examined the direct record separately for east antarctica west antarctica and the antarctic peninsula the different sub ice topography and geographic positions of which suggest that their glacial histories could have been different the direct record for an initial middle or late eocene phase is very sparse only the rare occurrence of ird at southern ocean dsdp and odp sites suggests the possibility of early ice and all three regions include mountains that could have hosted such ice although the indirect record and climate modelling in combination suggest that full glaciation of each region was probably synchronous we find differences in the available direct evidence there is abundant evidence that east antarctica became fully glaciated in the earliest oligocene but certain evidence of glaciation of a similar age extending to sea level is sparse for the antarctic peninsula and is not found until the late oligocene for west antarctica high resolution direct evidence is required to resolve uncertainties in glacial history

growth of an intermittent ice sheet in iceland during the late pliocene and early pleistocene abstract deep sea paleoclimatic records show that the quaternary climate around iceland was and probably still is very sensitive to rapid shifts in north atlantic oceanic circulation studies of several key sections in iceland indicate that similar oscillations are reflected in the late pliocene and early pleistocene terrestrial stratigraphy correlations between six rock sequences in western northern eastern and southern iceland show a fairly distinct trend during the pliocene pleistocene transition indicating the progressive growth of an ice sheet from southeast toward the north and west the correlations are based on k ar dates and paleomagnetic studies a total of 11 glacial horizons are recorded in a stratigraphic column from eastern iceland extending back 6 5 myr in western iceland 7 glacial horizons are preserved in a rock section dated from 7 0 to 1 8 myr and in northern iceland 14 glacial horizons are identified in a section that extends back to 9 0 myr well over 20 glacial horizons have been identified in the stratigraphic column in iceland full scale glacial interglacial cyclicity with regional ice cover is indicated at approximately 2 6 myr a further amplification leading to islandwide glaciations is identified at 2 2 2 1 myr these results show that changes in the ice cover in iceland correlate with the deep sea oxygen isotope records from benthic and planktonic foraminifera as well as ird studies from the north atlantic

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

ocean circulation beneath the ronne ice shelf the intimate thermal contact between the base of antarctic ice shelves and the underlying ocean enables changes in climate to have a rapid impact on the outflow of ice from the interior of antarctica1 2 furthermore water modified by passage under ice shelves particularly in the weddell sea is believed to be an important constituent of antarctic bottom water3 a water mass that can be observed as far north as 50 n in the deep oceans4 antarctic bottom water is both cold and oxygen rich and plays an important part in the cooling and ventilation of the world s oceans because of the difficulty in gaining access the oceano graphic regime beneath ice shelves is very poorly sampled5 by successfully drilling through the ice however we were able to obtain oceanographic data from beneath the largest antarctic ice shelf the ronne filchner ice shelf in the southern weddell sea we find that our data agree well with the predictions of a relatively simple oceanographic plume model of sub ice shelf circulation6 this model can therefore be used with some confidence to investigate the links between climate changes ice shelf melting and bottom water production

evolution of iceberg melting biological productivity and the record of icelandic volcanism in the irminger basin since 630 ka abstract planktic 18 o and 13 c records and point count records of biogenic volcanic and nonvolcanic terrigenous ice rafted debris ird sediment components from hole 919a in the irminger basin northern north atlantic provide a comprehensive dataset from which a paleoceanographic reconstruction for the last 630 kyr has been developed the paleoceanographic evolution of the irminger basin during this time contains both long term patterns and significant developmental steps one long term pattern observed is the persistent deposition of hematite stained ice rafted debris this record suggests that the modern and late pleistocene discharges of icebergs from northern redbed regions to the irminger sea lie in the low end of the range observed over the last 630 kyr in addition arctic front fluctuations appear to have been the main controlling factor on the long term accumulation patterns of ird and planktic biogenic groups the hole 919a sediment record also contains a long term association between felsic volcanic ash abundances and light 18 o excursions in both interglacial and glacial stages which suggests a causal link between deglaciations and explosive icelandic eruptions a significant developmental step in the paleoceanographic reconstruction based on benthic evidence was for diminished supply of denmark strait overflow water dsow beginning at 380 ka possibly initiated by the influx of meltwater from broad scale iceberg discharges along the east greenland coast there is also planktic evidence of a two step cooling of sea surface conditions in the irminger basin first at 338 309 ka and later at 211 190 ka after which both glacials and interglacials were colder as the arctic front migrated southeast of site 919 in addition to offering these findings this reconstruction provides a longer term geologic context for the interpretation of more recent paleoceanographic events and patterns of deposition from this region

final closure of panama and the onset of northern hemisphere glaciation abstract the greenland ice sheet is accepted as a key factor controlling the quaternary glacial scenario however the origin and mechanisms of major arctic glaciation starting at 3 15 ma and culminating at 2 74 ma are still controversial for this phase of intense cooling ravelo et al 1 a c ravelo d h andreasen m lyle a o lyle m w wara regional climate shifts caused by gradual global cooling in the pliocene epoch nature 429 2004 263 267 proposed a complex gradual forcing mechanism in contrast our new submillennial scale paleoceanographic records from the pliocene north atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation nhg since it was linked to a 2 3 c surface water warming during warm stages from 2 95 to 2 82 ma these records support previous models g h haug r tiedemann effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation nature 393 1998 673 676 2 claiming that the final closure of the panama isthmus 3 0 2 5 ma j groeneveld s steph r tiedemann d nurnberg d garbe schonberg the final closure of the central american seaway geology in prep 3 induced an increased poleward salt and heat transport associated strengthening of north atlantic thermohaline circulation and in turn an intensified moisture supply to northern high latitudes resulted in the build up of nhg finally culminating in the great irreversible climate crash at marine isotope stage g6 2 74 ma in summary there was a two step threshold mechanism that marked the onset of nhg with glacial to interglacial cycles quasi persistent until today

pliocene pleistocene ice rafting history and cyclicity in the nordic seas during the last 3 5 myr a continuous 3 5 myr ird record was produced from ocean drilling program odp site 907 a timescale based on magnetic polarity chrons oxygen isotope stratigraphy for the last 1myr and orbital tuning was developed the record documents a stepwise inception of large scale glacial cycles in the nordic seas region the first being a marked expansion of the greenland ice sheet at 3 3 ma a second step occurred at 2 74 ma by an expansion of large scale ice sheets in the northern hemisphere ice sheet variability around the nordic seas was tightly coupled to global ice volume over the past 3 3 myr between 3 and 1 ma most of the variance of the ird signal is in the 41 kyr band whereas the last 1 myr is characterized by stronger 100 kyr variance the gamma ray porosity evaluator grape density record is closely linked with ird variations and documents sub orbital variability resembling the late quaternary heinrich bond cycles

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

regime of the filchner ronne ice shelves antarctica seismic radio echo twerle balloon and oceanographic data of the filchner ronne ice shelves are analysed a large thin area in the central ronne ice shelf is found to differ from the morphology of the ross and filchner ice shelves this area is partly filled by basal saline ice which can be attributed to regional regelation caused by oceanic circulation

north atlantic deep sea sedimentation of late quaternary tephra from the iceland hotspot piston cores recovered from the north atlantic were used to study the sedimentation of holocene and pleistocene volcanic ash in the irminger and iceland basins ash zones 1 11 100 yr b p 2 55 000 yr b p and 3 305 000 yr b p were identified from their major element glass composition the silicic and alkalic ash zones 1 and 2 originate from the southeastern volcanic zone of iceland where they correlate with the solheimar ignimbrite from katla volcano and the thorsmork ignimbrite from tindfjallajokull volcano respectively the low alkali composition of silicic ash zone 3 indicates a source from one of the silicic centers in the active rift system ash zones 2 and 3 occur in the irminger basin as dispersed glass shards over a depth interval of several tens of centimeters their compositional and granulometric characteristics reflect an initial fallout on pack ice north of iceland followed by ice rafting sedimentation in the denmark strait prior to bioturbation 18o stratigraphy of foraminifera in the cores indicates that the ash zones were deposited during a cold interval at the time when seas north of iceland were ice covered sedimentary features indicate that turbidity currents were also involved in the dispersal of ash zones 1 and 2 south of iceland the initiation of these gravity currents from the shelf can be attributed to either glacier bursts jokulhlaups carrying tephra or the entrance of pyroclastic flows into the ocean

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

rapid onset of glacial conditions in the subarctic north pacific region at 2 67 ma clues to causality north pacific deep sea sediments recovered by ocean drilling leg 145 constrain the timing of the late pliocene major intensification of northern hemisphere continental glaciation a change in sediment character from nonglacial to glacial specifically abrupt increases in the amount of terrigenous clastic sediment and ice rafted debris occurred all across the north pacific at about 2 67 ma this change occurred very rapidly perhaps within 2000 yr too quickly to be a direct response to either tectonic or orbital forcing as the number and thickness of volcanic ash layers increased 10 fold at this same time we suggest that rapid intensification of glaciation may be associated with the widespread episode of explosive volcanism that began then

explosive silicic volcanism in iceland and the jan mayen area during the last 6 ma sources and timing of major eruptions abstract fifty three major explosive eruptions on iceland and jan mayen island were identified in 0 6 ma old sediments of the north atlantic and arctic oceans by the age and the chemical composition of silicic tephra the depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 ma and paleoclimatic proxies 18 o ice rafted debris for the last 1 ma major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off rift volcanic zones in iceland and to the jan mayen volcanic system the tholeiitic central volcanoes along the iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4 9 ma they were the source of at least 26 eruptions of dominant rhyolitic magma composition including the late pleistocene explosive eruption of krafla volcano of the eastern rift zone at about 201 ka the central volcanoes along the off rift volcanic zones in iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition with three distinct episodes recorded at 4 6 5 3 3 5 3 6 and 0 1 8 ma the longest and last episode recorded 11 pleistocene major events including the two explosive eruptions of tindfjallajokull volcano thorsmork ca 54 5 ka and katla volcano solheimar ca 11 9 ka of the southeastern transgressive zone eight major explosive eruptions from the jan mayen volcanic system are recorded in terms of the distinctive grain size mineralogy and chemistry of the tephra the tephra contain k rich glasses k 2 o sio 2 0 06 ranging from trachytic to alkali rhyolitic composition their normative trends ab q or and their depleted concentrations of ba eu and heavy ree reflect fractional crystallisation of k feldspar biotite and hornblende in contrast their enrichment in highly incompatible and water mobile trace elements such as rb th nb and ta most likely reflect crustal contamination one late pleistocene tephra from jan mayen was recorded in the marine sequence its age estimated between 617 and 620 ka and its composition support a common source with the borga pumice formation at sor jan in the south of the island

new oceanographic data from beneath ronne ice shelf antarctica oceanographic data have been obtained via an access hole made through ronne ice shelf the site which is the third in a series of similar studies lies 17 km west of korff ice rise where 825 m of ice overlies a 485 m deep water column measurements included conductivity and temperature profiles and an instrument mooring was deployed for long term measurements of currents temperature and conductivity at the sea floor there was a 150 m layer of well mixed water with a potential temperature and salinity of 1 97 c and 34 72 the water cooled and freshened towards the ice shelf base ultimately reaching 2 41 c and 34 51 the hydrographic and water current data imply a flow into the deepest part of the sub ice shelf cavity of about 200 000 m s 1 of the deeper relatively warm water which would be able to power an average basal melt rate of 0 2 m a 1 for the western portion of ronne ice shelf

cryptotephras the revolution in correlation and precision dating from its icelandic origins in the study of visible tephra horizons tephrochronology took a remarkable step in the late 1980 s with the discovery of a ca 4300 year old microscopic ash layer in a scottish peat bog since then the search for these cryptotephra deposits in distal areas has gone from strength to strength indeed a recent discovery demonstrates how a few fine grained glass shards from an alaskan eruption have been dispersed more than 7000 km to northern europe instantaneous deposition of geochemically distinct volcanic ash over such large geographical areas gives rise to a powerful correlation tool with considerable potential for addressing a range of scientific questions a prerequisite of this work is the establishment of regional tephrochronological frameworks that include well constrained age estimates and robust geochemical signatures for each deposit with distal sites revealing a complex record of previously unknown volcanic events frameworks are regularly revised and it has become apparent that some closely timed eruptions have similar geochemical signatures the search for unique and robust geochemical fingerprints thus hinges on rigorous analysis by electron microprobe and laser ablation inductively coupled plasma mass spectrometry historical developments and significant breakthroughs are presented to chart the revolution in correlation and precision dating over the last 50 years using tephrochronology and cryptotephrochronology

identification of a mis 6 age c 180 ka icelandic tephra within ne atlantic sediments a new potential chronostratigraphic marker the incidence of volcanic ash tephra within marine sediments serves as a useful stratigraphic marker and tool for correlation in addition where an independent age estimate exists tephra layers can provide a means of dating the sediments themselves here we present a geochemically characterized size sorted tephra layer within marine isotope stage mis 6 most likely resulting from primary air fall from an icelandic volcanic source this tephra layer is tentatively correlated to the kerlingarfjoll volcanic system using major element geochemistry the ash layer has an interpolated age of 181 6 ka based on the age model for md04 2822 we briefly review the occurrence of silicic tephra in the north atlantic region from mis 7 to mis 5e inclusive and find potential correlatives to the md04 2822 mis 6 ash layer in the norway basin and irminger sea

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

temperature variability beneath ronne ice shelf antarctica from thermistor cables thermistor cables have been deployed at two sites beneath ronne ice shelf antarctica one site is to the east of a submarine ridge that delineates the eastern boundary of the ronne depression and the other is 100 km to the north above the eastern slope of the depression long records from the cables up to 22 months indicate a large difference in the temperature variability at the two sites being an order of magnitude greater in the ronne depression site 2 although the records appear otherwise similar there is no significant correlation between them the high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime winter freezing in the open water north of the ice front generates western shelf water wsw a type of high salinity shelf water which travels southwest beneath the ice shelf appearing at site 2 as a slope trapped current at the bottom of the water column baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days the disturbances cause a periodic east west advection of water masses across the ronne depression site 2 is on the eastern slope of the depression where the wave induced eastward motion forces ice shelf water to rise resulting in periodic ice platelet formation in the water column as surmised from conductivity temperature depth measurements at the site the depth of the wsw layer decreases by 40 to 60 m during a 100 day period starting some 4 months after the beginning of the summer assuming an absence of significant wsw production during the summer the 4 month delay implies a minimum average speed of wsw flow of about 0 02 m s 1 the wsw flux into the ronne depression is estimated at 3 105 m3s 1

seasonal sea ice cover during the warm pliocene evidence from the iceland sea odp site 907 abstract sea ice is a critical component in the arctic and global climate system yet little is known about its extent and variability during past warm intervals such as the pliocene 5 33 2 58 ma here we present the first multi proxy ip25 sterols alkenones palynology sea ice reconstructions for the late pliocene iceland sea odp site 907 our interpretation of a seasonal sea ice cover with occasional ice free intervals between 3 50 3 00 ma is supported by reconstructed alkenone based summer sea surface temperatures as evidenced from brassicasterol and dinosterol primary productivity was low between 3 50 and 3 00 ma and the site experienced generally oligotrophic conditions the east greenland current and east icelandic current may have transported sea ice into the iceland sea and or brought cooler and fresher waters favoring local sea ice formation between 3 00 and 2 40 ma the iceland sea is mainly sea ice free but seasonal sea ice occurred between 2 81 and 2 74 ma sea ice extending into the iceland sea at this time may have acted as a positive feedback for the build up of the greenland ice sheet gis which underwent a major expansion 2 75 ma thereafter most likely a stable sea ice edge developed close to greenland possibly changing together with the expansion and retreat of the gis and affecting the productivity in the iceland sea

late pliocene millennial scale climate variability in the northern north atlantic prior to and after the onset of northern hemisphere glaciation 1 sediments recovered at ocean drilling program site 984 on the reykjanes ridge provided multicentennial scale records of late pliocene climate change over the onset of northern hemisphere glaciation nhg 2 95 2 82 ma short term climate variations prior to and after the onset of continent wide glaciation were compared to test the hypothesis of whether dansgaard oeschger do cycles may have been triggered by continental ice breakouts during two selected interglacial stages prior to and after nhg g15 and g1 climate variability resembled that found in the holocene and the mid pliocene warm period in contrast do like periodicities of 1470 2900 and 4400 years indeed only occurred in glacial stages after the onset of nhg g14 g6 and 104 but not in stage g20 prior to the onset these results suggest a causal link between do cycles and the late pliocene onset of major nhg and ice breakouts in the north atlantic

notes on southern ocean hydrography sea ice and bottom water formation abstract the southern ocean circulation and sea ice distribution is briefly described the formation of extremely cold bottom water in the weddell sea and its relation to the floating ronne filchner ice shelves is discussed it is shown that a concentrated swift eroding bottom current with anomalous low 18 o ratio transports the cold and dense ice shelf water from the shelf towards large depths comments are made on possible implications of this process for the large scale deep water circulation and for the interpretation of sediment cores