petrology and geochronology of paleoproterozoic garnet bearing amphibolites from the dunhuang block eastern tarim craton abstract the dunhuang block located in the eastern margin of the tarim craton possesses a wealth of information about the geological evolution of the tarim craton in this study we conducted detailed petrological geochemical and geochronological investigations on garnet bearing amphibolites as well as granites in the dunhuang block which record important metamorphic and magmatic events that are poorly constrained based on field occurrence geochronological and geochemical characteristics the garnet bearing amphibolites can be divided into two groups group i garnet bearing amphibolite displays typical features of island arc basalts which may formed at 1 83 ga in a collisional setting during the assembly of the columbia supercontinent whereas group ii has geochemical characteristics of ocean island basalt which may formed at 1 61 ga in an ocean island setting the ca 1 83 ga amphibolite has positive hf t values 4 4 to 8 9 with tdm1 model ages of 2 02 1 83 ga indicating an important addition of juvenile mantle derived materials during the formation of its protolith the ca 1 61 ga amphibolite has hf t values ranging from 0 6 to 5 6 and corresponding tdm1 model ages varying from 2 03 ga to 1 84 ga which suggest that juvenile mantle derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials the u pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca 400 ma which is coeval with the widespread magmatism in the beishan orogen we thus propose that these two groups of garnet bearing amphibolites were possibly not related to each other during paleoproterozoic time and a late paleozoic orogenic event brought them together during which both of them underwent metamorphism

a 4 ma record of thermal evolution in the tropical western pacific and its implications on climate change article i nfo orbital resolution thermal histories over the last 4 ma at odp site 1143 in the tropical western pacific are reconstructed using alkenone paleothermometry the temperature profile is characterized by a steady state of 29 c with fluctuations b1 c before 2 7 ma and by a strong oscillating state from 2 7 ma largely due to cooling by up to 4 c from 29 c in interglacial to 26 c in glacial intervals this implies a relative warm and stable surface hydrography during the early and mid pliocene in this region influenced by the warm pool before temperature decreases in responding to global cooling and the formation of the distinct glacial stages since the late pliocene therefore the smaller sst gradient between tropical eastern and western pacific and between southern and northern south china sea before the late pliocene indicates a super sized pliocene pacific warm pool while the larger sst gradient since then marks progressively intensification of the zonal walker circulation and meridional hadley circulation representing the monsoon circulations in the region the intensification of the walker and hadley circulations over the tropical pacific may also have helped on the onset of glaciations and subsequent deglaciations during the late pliocene and pleistocene 2011 published by elsevier b v

stratigraphic and tectonic settings of early paleolithic of north west armenia the quaternary stratigraphy and tectonic development of the southern javakheti upland and the adjacent upper akhurian and lori uplifted basins in nw armenia lesser caucasus and geological position of the early and middle acheulian lithic industries were studied using a multi method approach studies of sedimentary sections petrological and geochemical correlation of lavas and tuffs k ar dating of volcanic rocks and sims 238u 206pb dating of tuffs examination of obtained fossils and pollen analysis and determination of remanent magnetic polarity of volcanic rocks and clastic sediments have been used to compile the stratigraphic scheme and to estimate the age of units the low mountain topography was differentiated to ridges and basins to the beginning of quaternary the eruptions of basalts and basaltic andesites evened topography of the upper akhurian and lori basins in the gelasian the lava flows spread along big river valleys at tens of kilometers at the late gelasian the eruptions of basaltic trachyandesites trachyandesites and dacites replaced the basaltic eruptions the latter dammed the akhurian river flowing to the south and the upper akhurian river found the flowing to the east via the valley like depression of the karakhach pass to the dzoraghet debed valley the course grained tuffaceous clastic karakhach unit was deposited during the olduvai subchron not earlier than 1 9 1 85 ma and the earliest calabrian the water transit between the upper akhurian and lori basins was interrupted later because of rise of the karakhach pass volcanic activity renewed for a short time in the early calabrian 1 7 and 1 5 1 4 ma the end calabrian and earliest middle pleistocene sedimentation 1 0 5 ma occurred in stagnant water partly lacustrine conditions this was expressed by formation of the relatively fine grained kurtan unit during the last 0 5 ma the region underwent flexure fault deformation and tectonic uplift at 350 800 m the epoch of formation of the karakhach unit was characterized by middle mountain topography and humid climate not later than 1 85 ma the region was occupied by the earliest hominines producing lithic industries of the early acheulian aspect they contained crude hand axes and other macro tools made of local dacite and basalt sites of karakhach muradovo and agvorik early appearance of these industries might be caused by natural parting of dacite and basalt to tabulated fragments that gave a possibility to make such macro tools the middle acheulian artifacts were found in the kurtan i section of the kurtan unit

tegula atra lesson 1830 mollusca gastropoda in the marine quaternary of patagonia argentina sw atlantic biostratigraphical tool and palaeoclimate palaeoceanographical signal results of the systematic review and regional palaeobiogeographical context of tegula atra including morphometric multivariate and cladistic analyses show that it is a keystone species in the marine quaternary of argentina that can be used as pleistocene biostratigraphical tool and paleoclimate palaeoceanographical signal while it was absent in warmer than present high sea level episodes during the miocene entrerriense transgression ca 10 ma and pleistocene mis11 it exhibits an excellent and abundant fossil record within dominantly cool coastal settings exclusively during the late pleistocene mis9 7 and 5 between rio negro and southern santa cruz provinces patagonia it first appeared in the sep during the late pliocene cooling trend dispersed during the late pleistocene into the swa presumably by rafting on macroalgae along the cabo de hornos and malvinas falkland currents but became extinct in the mar argentino magellan malacological province during the holocene amelioration trend its absence at present represents a climate change driven range shift and independent evidence of palaeoceanographical changes after the lgm and at the pleistocene holocene transition changes in sst ca 2 c higher wind velocities less light less nutrient availability less extension and intensity of cold less and warm increased shallow water currents altering water masses and biogeographical boundaries altogether these changes and the holocene scenario were disadvantageous causing direct effects on its physiology and survival in turn preventing the occurrence of the associated macroalgae durvillaea antarctica and its successful dispersal in the swa or retraction to the cold humboldt system waters this study reinforces the importance of dispersalist models to explain the origin of key taxa adding for a better understanding of molluscan taxonomic differences along the swa and sep margins of south america with implications for future coastal scenarios the distribution of t atra across time is a new example of the strong linkage between earth history climatic cycles atmospheric and oceanic circulation and the late quaternary biotic responses showing a possible consequence of future climate change on nearshore communities

late pliocene greenland glaciation controlled by a decline in atmospheric co2 levels it is thought that the northern hemisphere experienced only ephemeral glaciations from the late eocene to the early pliocene epochs about 38 to 4 million years ago and that the onset of extensive glaciations did not occur until about 3 million years ago several hypotheses have been proposed to explain this increase in northern hemisphere glaciation during the late pliocene here we use a fully coupled atmosphere ocean general circulation model and an ice sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the greenland ice sheet in particular we find that greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the late pliocene by contrast our model results suggest that climatic shifts associated with the tectonically driven closure of the panama seaway with the termination of a permanent el nino state or with tectonic uplift are not large enough to contribute significantly to the growth of the greenland ice sheet moreover we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the earth s orbit

two stage mid brunhes climate transition and mid pleistocene human diversification abstract global climate shifted to markedly warmer interglacial conditions across the mid brunhes transition mbt 400 ka however a global mbt synthesis that spans marine and terrestrial evidence remains elusive which limits our understanding of the role of the mbt in mid pleistocene human evolution we synthesize asian precipitation reconstructions within a context of global palaeoclimatic records and find that the mbt occurred in two stages first stronger warming of northern hemisphere continents weaker southern hemisphere warming and related more extensive northward displacement of the intertropical convergence zone itcz during interglacial marine isotope stage mis 13 intensified and expanded precipitation in asian monsoon regions and in other widespread northern hemisphere regions with accompanying carbon reservoir changes featuring globally high marine benthic 13c values because of vegetation expansion at 500 ka subdued southern hemisphere warming and northward itcz displacement decreased southern hemisphere precipitation simultaneously during mis 13 second a shift to globally warmer interglacials at 400 ka with elevated atmospheric co2 concentrations smaller ice volume and higher sea level resulted in sustained high interglacial precipitation in east asia from mis 11 onward and sustained high marine benthic 13c values during mis 11 we also synthesize palaeoanthropological data and find that the climate and ecosystem changes across the mbt coincided with the timing of human lineage diversification including the emergence of neanderthals and denisovans in eurasia and homo sapiens in africa and their potential coexistence with h heidelbergensis h erectus h floresiensis h naledi and other homo archaics the timing of the mbt also coincided with novel hominin behavioural developments including fire control and the transition from handaxe industries to more versatile levallois techniques combined with environmental theories of human evolution this chronological coincidence suggests a potential link between mid pleistocene environmental changes and human evolution

stable isotopic composition of fossil mammal teeth and environmental change in southwestern south africa during the pliocene and pleistocene abstract the past 5 million years mark a global change from the warmer more stable climate of the pliocene to the initiation of glacial interglacial cycles during the pleistocene marine core sediment records located off the coast of southwestern africa indicate aridification and intensified upwelling in the benguela current over the pliocene and pleistocene however few terrestrial records document environmental change in southwestern africa over this time interval here we synthesize new and published carbon and oxygen isotope data of the teeth from large mammals 6 kg at langebaanweg 5 million years ago ma elandsfontein 1 0 0 6 ma and hoedjiespunt 0 35 0 20 ma to evaluate environmental change in southwestern africa between the pliocene and pleistocene the majority of browsing and grazing herbivores from these sites yield enamel 13c values within the range expected for animals with a pure c3 diet however some taxa have enamel 13c values that suggest the presence of small amounts c4 grasses at times during the pleistocene considering that significant amounts of c4 grasses require a warm growing season these results indicate that the winter rainfall zone characteristic of the region today could have been in place for the past 5 million years the average 18o value of the herbivore teeth increases 4 4 between langebaanweg and elandsfontein for all taxa except suids this increase may solely be a function of a change in hydrology between the fluvial system at langebaanweg and the spring fed environments at elandsfontein or a combination of factors that include depositional context regional circulation and global climate however an increase in regional aridity or global cooling between the early pliocene and mid pleistocene cannot explain the entire increase in enamel 18o values spring fed environments like those at elandsfontein may have provided critical resources for mammalian fauna in the mid pleistocene within an increasingly arid southwestern africa ecosystem

transient nature of the earth s climate and the implications for the interpretation of benthic δ18 o records abstract from the marine benthic 18 o records it is known that the earth s climate has experienced significant variability over the past 40 million years in general a number of assumptions are often needed to disentangle the benthic 18 o data into its temperature and ice volume contributions in this study a transient 1 d ice sheet model is used which overcomes these shortcomings by relating temperature to the benthic 18 o data leading to a self consistent and continuous record of 18 o temperature and sea level the contribution of land ice to benthic 18 o is examined with a set of sensitivity experiments varying the mean 18 o of the ice for different ice sheets it is shown that the scaling factor of sea water 18 o w to sea level is not constant over the long time scales however our sensitivity experiments do show that over the long time scale the general assumed 1 0 1 1 per 100 m of sea level is in reasonable agreement with our model results moreover the sea level ice volume response to temperature s t is shown to vary through time with the largest response found when variations in antarctic ice volume dominates the sea level variations during the oligocene to early miocene furthermore this response is quite large compared to the individual response of ice sheets during the plio pleistocene whereas the combined s t in this period is comparable to that during the oligocene accordingly the transient behaviour is very important for the interpretation of data records with respect to climate sensitivity and climate change during the past 40 million years

evolution of the eastern tropical pacific through plio pleistocene glaciation a tropical pacific climate state resembling that of a permanent el nino is hypothesized to have ended as a result of a reorganization of the ocean heat budget 3 million years ago a time when large ice sheets appeared in the high latitudes of the northern hemisphere we report a high resolution alkenone reconstruction of conditions in the heart of the eastern equatorial pacific eep cold tongue that reflects the combined influences of changes in the equatorial thermocline the properties of the thermocline s source waters atmospheric greenhouse gas content and orbital variations on sea surface temperature sst and biological productivity over the past 5 million years our data indicate that the intensification of northern hemisphere glaciation 3 million years ago did not interrupt an almost monotonic cooling of the eep during the plio pleistocene sst and productivity in the eastern tropical pacific varied in phase with global ice volume changes at a dominant 41 000 year obliquity frequency throughout this time changes in the southern hemisphere most likely modulated most of the changes observed

the fall dump a new perspective on the role of a shade flora in the annual cycle of diatom production and export flux investigations of diatom fluxes recorded in laminated sediments using scanning electron microscope techniques together with evidence from sediment trap studies have contributed to a reappraisal of the annual cycle of diatom production and export flux we propose that where there is a strong seasonal thermocline and nutricline a number of diatoms hitherto regarded as typical sparse summer flora characteristic of oligotrophic waters are able to generate substantial production at depth these species including rhizosolenia spp stephanopyxis palmeriana thalassiothrix spp and some coscinodiscus spp may represent a shade flora that have adapted to grow in low light conditions and or to regulate their buoyancy to move between a deep nutrient source and the euphotic zone although rates of growth and primary production are substantially lower than species characteristic of spring bloom or upwelling conditions the total primary production integrated over the several month period of summer stratification may be as significant as the spring bloom or greater the term fall or autumn bloom as a counterpart of the spring bloom is therefore a misnomer whereas the spring bloom involves a rapid burst of reproduction and sedimentation the fall dump is the sedimentation of a long lived episode of production lasting the duration of the seasonal thermocline and triggered by the fall winter mixing that breaks down stratification the fall dump may produce as much or in some cases more export production than the spring bloom the results of this study suggest that a reorientation of thinking on diatom ecology and palaeoecology may be required

opal sedimentation shifts in the world ocean over the last 15 myr biogenic silica opal accumulation records were used to trace mechanisms consequence and geographic pattern of shifts in the main locus of opal deposition of the world ocean over the last 15 myr over this time interval the main opal sink seems to have moved from the north atlantic to the pacific equatorial pacific eastern equatorial pacific eastern boundary current upwelling systems california namibia peru and finally to the southern ocean the interplay between opal deposition and a series of climatic tectonic oceanographic and biologic events has been analyzed and discussed these events include the cenozoic global cooling trend intensified glaciation in antarctica late miocene early pliocene biogenic bloom development of northern hemisphere glaciation nhg closing of the panama seaway transition of the climate system from a monopolarto a bipolar glaciated world mid pleistocene revolution mpr nutrient availability evolution of diatoms and c4 plants and changes in continental weathering rates while the observed shifts are mostly traceable to oceanic reorganizations and global climatic evolution conditions favorable to opal deposition involve the above mentioned complex mix of processes for this reason the interpretation of opal deposition records might not always be straightforward we however believe that it can still provide clear indications of large scale oceanographic reorganizations in the geological past d 2004 elsevier b v all rights reserved

expansion of pelagic denitrification during early pleistocene cooling bioavailable nitrogen is removed from the oceans in oxygen deficient benthic and pelagic environments by denitrification future warming is predicted to reduce ocean oxygenation and to cause hypoxic regions to expand potentially accelerating denitrification a compilation of high resolution sedimentary nitrogen isotope delta n 15 records from the eastern tropical pacific north pacific and the arabian sea and a global multi site survey are presented as evidence for weak pelagic denitrification at the end of the pliocene warm period mean delta n 15 values increased in the major oxygen minimum zones omzs between 2 1 and 1 5 ma pelagic denitrification strengthened during a period of long term global cooling despite solubility driven increases in initial oxygen contents of antarctic intermediate and subantarctic mode waters ventilating the omzs this trend is opposite to the predicted mean trend for a cooling ocean as well as to the observed glacial interglacial variation several alternatives to explain the shift are proposed including a rise in net respiration a progressive increase in the ventilation age of the deep ocean associated with million year scale secular cooling and a shoaling of the remotely ventilated thermocline to shallow depths corresponding to the zone of peak subsurface respiration given no evidence for a net increase in production we assert that large scale climate driven changes in ocean circulation regulate long timescale variations in the extent of pelagic denitrification additional data and modeling are required to fully explain the observations c 2013 elsevier b v all rights reserved

the early matuyama diatom maximum off sw africa benguela current system odp leg 175 abstract several sites of ocean drilling program leg 175 between 20 and 30 s 1081 1082 1083 1084 and 1085 in the benguela current system show a distinct opal and diatom maximum within the late pliocene and early quaternary spanning the lower half of the matuyama reversed polarity chron mdm matuyama diatom maximum this maximum is centered around 2 0 2 6 ma and follows a rapid increase of diatom deposition near 3 1 ma within the mdm we recognized intervals dominated by the antarctic subantarctic species thalassiothrix antarctica which have the aspect of diatom mats the mdm and the mats developed during a time of sustained southern ocean influence 3 2 1 6 ma probably reflecting a period of seasonally pulsed continuous advection of subantarctic waters into the benguela oceanic current the onset and cessation of the mdm raises questions about modes in the operation of the silica cycle and the relation of these to major oceanographic events a depositional model for the late pliocene is proposed which involves positional changes in a longshore system of fronts substantial equatorward excursions of subantarctic waters during mdm time and subsurface waters greatly enriched in silicate

hydroclimate change in subtropical south africa during the mid piacenzian warm period abstract the mid piacenzian warm period mpwp 3 264 3 025 ma of the pliocene epoch has been proposed as an analog for future climate scenarios disagreement between the paleoenvironmental reconstruction and model simulations of the climate in subtropical regions for this period suggests that more investigation of the subtropical climate variability of the mpwp is needed this study presents pollen microcharcoal and benthic foraminifera oxygen isotope records generated from marine sediment cores of international ocean discovery program iodp exp 361 site u1479 from the cape basin offshore of south africa for the period between 3 337 and 2 875 ma with an average sample resolution of 3 ka this record represents the highest resolution record of mpwp vegetation change from the region our results indicate that the vegetation during the mpwp was dominated by fynbos species rich heathy vegetation in the cape floristic region with variable proportions of ericaceae moreover the development of the afrotemperate forest tall multilayered indigenous forests in south africa reflects shifts in the amounts of precipitation between winter and summer in the year round rainfall zone the vegetation variation is probably influenced by the latitudinal insolation gradient in response to precession forcing several glacials depicted by the benthic foraminifera oxygen isotope record were characterized by lower percentage values of restionaceae higher percentage values of ericoid fynbos and afrotemperate forest these events correspond well with cooler se atlantic sea surface temperatures driven by interactions of both atmospheric and oceanographic processes the cooler sea surface temperatures attributed to antarctic ice sheet expansion reduced agulhas leakage heat and salt transfer from the indian ocean to the atlantic ocean and or intensified southern benguela upwelling resulted in less precipitation in the winter rainfall zone of south africa

increased sensitivity of the plio pleistocene northwest pacific to obliquity forcing abstract insolation driven changes in poleward heat transport and changes in greenhouse gas concentrations can explain aspects of the rise of obliquity paced climate variability during the pliocene pleistocene transition based on an alkenone based sea surface temperature sst reconstruction of the kuroshio current extension kce we propose here that emergence of this region as the primary locus of ocean atmosphere heat transfer in the pacific ocean promoted northern hemisphere glaciation nhg our record shows that with intensification of nhg at 2 7 ma the kce cooled 2 4 c during glacial intervals likely in nh winter spring these high amplitude 41 kyr sst cycles slightly lead o 18 variations ruling out global ice volume changes as the primary cause the lead of sst over o 18 cycles matches the phasing between these two proxies observed in the tropics supporting changes in co2 concentrations as a unifying mechanism of ocean surface temperature change however the amplitude of the kce sst cycle is twice that of the tropical records pointing to an additional regional process we infer that cooling of the north pacific sea surface by the east asian winter monsoon and associated westerlies intensified during glacial intervals this transfer of heat and moisture from the ocean to the atmosphere potentially furthered glacial formation by accelerating snow fall in north america therefore these results might also support a role for tropical extratropical heat balance in enhancing glacial growth via the obliquity pacing

pliocene pleistocene evolution of sea surface and intermediate water temperatures from the southwest pacific over the last 5 million years the global climate system has evolved toward a colder mean state marked by large amplitude oscillations in continental ice volume equatorward expansion of polar waters and strengthening temperature gradients have been detected however the response of the mid latitudes and high latitudes of the southern hemisphere is not well documented despite the potential importance for climate feedbacks including sea ice distribution and low high latitude heat transport here we reconstruct the pliocene pleistocene history of both sea surface and antarctic intermediate water aaiw temperatures on orbital time scales from deep sea drilling project site 593 in the tasman sea southwest pacific we confirm overall pliocene pleistocene cooling trends in both the surface ocean and aaiw although the patterns are complex the pliocene is warmer than modern but our data suggest an equatorward displacement of the subtropical front relative to present and a poleward displacement of the subantarctic front of the antarctic circumpolar current acc two main intervals of cooling from 3 ma and 1 5 ma are coeval with cooling and ice sheet expansion noted elsewhere and suggest that equatorward expansion of polar water masses also characterized the southwest pacific through the pliocene pleistocene however the observed trends in sea surface temperature and aaiw temperature are not identical despite an underlying link to the acc and intervals of unusual surface ocean warmth 2 ma and large amplitude variability in aaiw temperatures from 1 ma highlight complex interactions between equatorward displacements of fronts associated with the acc and or varying poleward heat transport from the subtropics

biotic and geochemical evidence for a global latitudinal shift in ocean biogeochemistry and export productivity during the late pliocene during the late pliocene 3 to 2 5 ma oceanic records of opal and c37 alkenone accumulation from around the world show a secular shift towards lower values in the high latitudes and higher values in the low and mid latitudes these shifts are broadly coincident with the intensification of northern hemisphere glaciation and are suggestive of changes in export productivity with potential implications for pliocene atmospheric carbon dioxide concentrations the interpretation of a global latitudinal shift in productivity however requires testing because of the potential uncertainties associated with site to site comparisons of records that can be influenced by highly nonlinear processes associated with production export and preservation here we assess the inferred pliocene latitudinal productivity shift interpretation by presenting new records of c37 alkenone accumulation from ocean drilling program odp site 982 in the north atlantic and biotic assemblages calcareous nannoplankton from this site and odp site 846 in the eastern tropical pacific our results corroborate the interpretation of c37 alkenone accumulation as a proxy for gross export productivity at these sites indicating that large scale productivity decreases at high latitudes and increases at tropical sites are recorded robustly we conclude that the intensification of northern hemisphere glaciation during the late pliocene was associated with a profound reorganisation of ocean biogeochemistry

the prism pliocene palaeoclimate reconstruction time for a paradigm shift global palaeoclimate reconstructions have been invaluable to our understanding of the causes and effects of climate change but single temperature representations of the oceanic mixed layer for dat

late pliocene upwelling in the southern benguela region the late pliocene has been proposed as a possible analogue for understanding future climate change and for testing climate models previous work has shown that during the pliocene the major upwelling systems were relatively warm and that this meant they were either inactive contracted or were upwelling warmer waters than present here we examine evidence from a site located on the margins of the modern benguela upwelling system to test whether the upwelling cells had migrated or contracted relative to present during the pliocene we applied several organic geochemistry proxies and foraminiferal analyses to reconstruct the pliocene history of odp site 1087 31 28 s 15 19 e 1374 m water depth including the uk37 and tex86 indices for reconstructing sea surface temperatures phytoplankton biomarker concentrations and stable isotope ratios for estimating export primary productivity and for oxygen isotope statigraphy and planktonic foraminifera assemblage abundances for inferring water mass changes these proxies show that between 3 5 and 3 0 ma the southern benguela region was cooler than the northern benguela region by 5 c the latter being where the main upwelling cells are found today from the multiproxy data obtained we also infer that more extensive upwelling was present in the southern benguela region during the pliocene than at present and that the benguela upwelling cells shifted northwards after the pliocene epoch as a result of changes in the local wind field we also find evidence that the benguela upwelling was sensitive to the pronounced cooling during the m2 and km2 glacial stages potentially associated with the expansion of sea ice and cooling in antarctica in the late pliocene

north pacific seasonality and the glaciation of north america 2 7 million years ago in the context of gradual cenozoic cooling the timing of the onset of significant northern hemisphere glaciation 2 7 million years ago is consistent with milankovitch s orbital theory which posited that ice sheets grow when polar summertime insolation and temperature are low however the role of moisture supply in the initiation of large northern hemisphere ice sheets has remained unclear the subarctic pacific ocean represents a significant source of water vapour to boreal north america but it has been largely overlooked in efforts to explain northern hemisphere glaciation here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic pacific ocean indicating that 2 7 million years ago late summer sea surface temperatures in this ocean region rose in response to an increase in stratification at the same time winter sea surface temperatures cooled winter floating ice became more abundant and global climate descended into glacial conditions we suggest that the observed summer warming extended into the autumn providing water vapour to northern north america where it precipitated and accumulated as snow and thus allowed the initiation of northern hemisphere glaciation

palaeoclimate global warmth with little extra co 2 most climate models consider only short term processes such as cloud and sea ice formation when assessing earth s sensitivity to greenhouse gas forcing mounting evidence indicates that the response could be stronger if boundary conditions change drastically

accumulation of organic and inorganic carbon in pliocene pleistocene sediments along the sw african margin the ocean drilling program leg 175 recovered a unique series of stratigraphically continuous sedimentary sections along the sw african margin an area which is presently affected by active coastal upwelling the accumulation rates of organic and inorganic carbon are a major component of this record four leg 175 sites 1082 1084 1085 1087 are chosen as part of a latitudinal transect from the present northern to southern boundaries of the benguela current upwelling system to decipher the pliocene pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate the pattern of caco3 and corg mass accumulation rates mars over 0 25 myr intervals indicates that the evolution of carbon burial is highly variable between the northern and the southern benguela regions as well as between sites that have similar hydrological conditions this as well as the presence over most locations of high amplitude rapid changes of carbon burial reflect the partitioning of biogenic production and patterns of sedimentation into local compartments over the benguela margin the combined mapping of caco3 and corg mars at the study locations suggests four distinct evolutionary periods which are essentially linked with major steps in global climate change the early pliocene the mid pliocene warm event a late pliocene intensification of northern hemisphere glaciation and the pleistocene the early pliocene spatially heterogeneous patterns of carbon burial are thought to reflect the occurrence of mass gravitational movements over the benguela slope which resulted in disruption of the recorded biogenic production this was followed 3 5 3 ma by an episode of peak carbonate accumulation over the whole margin and subsequently by the onset of benguela provincialism into a northern and a southern sedimentary regime near 2 ma this mid and late pliocene evolution is interpreted as a direct response to changes in the ventilation of bottom and intermediate waters as well as to dynamics of the subtropical gyral circulation and associated wind stress

role of tropics in changing the response to milankovich forcing some three million years ago appeared in regions that today have intense oceanic upwelling the eastern equatorial pacific and the coastal zones of southwestern africa and california there was furthermore a significant change in the earth s response to milankovich forcing obliquity signals became large but those associated with precession and eccentricity remained the same the latter change in the earth s response can be explained by hypothesizing that the global cooling during the cenozoic affected the thermal structure of the ocean it caused a gradual shoaling of the thermocline around 3 ma the thermocline was sufficiently shallow for the winds to bring cold water from below the thermocline to the surface in certain upwelling regions this brought into play feedbacks involving

north atlantic climate evolution through the plio pleistocene climate transitions during the plio pleistocene the earth witnessed the growth of large northern hemisphere ice sheets and profound changes in both north atlantic and global climate here we present a 3 2 myr long orbitally resolved alkenone sea surface temperature sst record from deep sea drilling project dsdp site 607 41 n 33 w water depth 3427 m in the north atlantic ocean we employ a multi proxy approach comparing these new observations with existing bottom water temperature bwt and stable isotope time series from the same site and sst time series from other sites shedding new light on plio pleistocene climate change north atlantic temperature records show a long term cooling with two major steps occurring during the late pliocene 3 1 to 2 4 ma and the mid pleistocene 1 5 to 0 8 ma closely timed with intervals of major change in northern hemisphere ice sheets existing evidence suggests that the late pliocene cooling may have been caused by a thresholded response to secular changes in atmospheric carbon dioxide co2 while an explanation for the mid pleistocene cooling may involve glacial interglacial changes in atmospheric co2 it seems to also require a change in the behavior of the ice sheets themselves north atlantic climate responses were closely phased with benthic oxygen isotope 18o changes during the 41 kyr world indicating a strong common northern hemisphere high latitude imprint on north atlantic climate signals after the mid pleistocene transition mpt north atlantic sst records and the site 607 benthic carbon isotope 13c record are more closely phased with 18o whereas bwt significantly leads 18o in the 100 kyr band suggesting a shift from a northern to a southern hemisphere influence on north atlantic bwt we propose that the expansion of the west antarctic ice sheet wais across the mpt increased the production and export of antarctic bottom water from the southern ocean and subsequently controlled its incursion into the north atlantic especially during glacial intervals it follows that the early 100 kyr response of bwt implies an early response of the wais relative to the northern hemisphere deglaciation thus in the 100 kyr world both northern hemisphere and southern hemisphere processes affect climate conditions in the north atlantic ocean

plio pleistocene denitrification in the eastern tropical north pacific intensification at 2 1 ma 1 global climate has changed substantially over the past 4 million years ma toward on average colder conditions and higher amplitude oscillation between glacial and interglacial periods corresponding changes in marine biogeochemistry could have linked oceanic circulation changes to the global carbon cycle but remain poorly understood we report a high resolution 4 ma long 15n record from ocean drilling program site 1012 on the california margin which monitors denitrification intensity in one of the principal oxygen minimum zones omzs of the open ocean the 15n record demonstrates that eastern tropical north pacific etnp denitrification was weak for much of the late pliocene though there was no apparent change associated with northern hemisphere glaciation nhg at 2 7 ma denitrification strengthened substantially by 2 in 15n at 2 1 ma since 2 1 ma intense denitrification in the etnp occurred during interglacial periods and was generally very climate sensitive we infer that two oceanic changes the shoaling of the thermocline in the eastern pacific and thus the strengthened walker circulation and the development of a modern like frontal system in high latitude southern ocean around 2 1 ma provided favorable conditions for intensification of etnp denitrification that was also likely mirrored south of the equator intermediate water circulation changes likely created the poorly ventilated zones susceptible to the suboxic condition required by denitrification 2 1 ma thus marks a critical point for the reorganization of the ocean s nitrogen cycle in which open ocean denitrification became a significant loss term during interglacial phases of climate given linkage between the ocean s n and c cycle etnp denitrification changes probably contributed to plio pleistocene climatic changes

a deep eastern equatorial pacific thermocline during the early pliocene warm period abstract during the early pliocene warm period 4 6 4 2 ma in the eastern equatorial pacific upwelling region sea surface temperatures were warm in comparison to modern conditions warm upwelling regions have global effects on the heat budget and atmospheric circulation and are argued to have contributed to pliocene warmth though warm upwelling regions could be explained by weak winds and or a deep thermocline the temporal and spatial evolution of the equatorial thermocline is poorly understood here we reconstruct temporal and spatial changes in subsurface temperature to monitor thermocline depth and show the thermocline was deeper during the early pliocene warm period than it is today we measured subsurface temperature records from eastern equatorial pacific odp transect sites 848 849 and 853 using mg ca records from globorotalia tumida which has a depth habitat of 50 100 m in the early pliocene subsurface temperatures were 4 5 c warmer than modern temperatures indicating the thermocline was relatively deep subsurface temperatures steeply cooled 2 3 c from 4 8 to 4 0 ma and continued to cool an additional 2 3 c from 4 0 ma to present compared to records from other regions the data suggests the pronounced subsurface cooling between 4 8 and 4 0 ma was a regional signal related to restriction of the isthmus of panama while continued cooling from 4 0 ma to present was likely related to global processes that changed global thermocline structure additionally the spatial evolution of the equatorial thermocline along a n s transect across odp sites 853 849 and 848 suggests an intensification of the southeast trades from the pliocene to present large scale atmospheric and oceanographic circulation processes link high and low latitude climate through their influence on equatorial thermocline source water regions and consequently the equatorial thermocline through these low latitude high latitude linkages changes in the equatorial thermocline and thermocline source water played an important role in the transition from the warm pliocene to the cold pleistocene

late pliocene vegetation and climate in namibia southern africa derived from palynology of odp site 1082 the present day condition of bipolar glaciation characterized by rapid and large climate fluctuations began at the end of the pliocene with the intensification of the northern hemisphere continental glaciations the global cooling steps of the late pliocene have been documented in numerous studies of ocean drilling program odp sites from the northern hemisphere however the interactions between oceans and between land and ocean during these cooling steps are poorly known in particular data from the southern hemisphere are lacking therefore i investigated the pollen of odp site 1082 in the southeast atlantic ocean in order to obtain a high resolution record of vegetation change in namibia between 3 4 and 1 8 ma four phases of vegetation development are inferred that are connected to global climate change 1 before 3 ma extensive rather open grass rich savannahs with mopane trees existed in namibia but the extension of desert and semidesert vegetation was still restricted 2 increase of winter rainfall dependent renosterveld like vegetation occurred between 3 1 and 2 2 ma connected to strong advection of polar waters along the namibian coast and a northward shift of the polar front zone in the southern ocean 3 climatically induced fluctuations became stronger between 2 7 and 2 2 ma and semiarid areas extended during glacial periods probably as the result of an increased pole equator thermal gradient and consequently globally enhanced atmospheric circulation 4 aridification and climatic variability further increased after 2 2 ma when the polar front zone migrated southward and the influence of atlantic moisture brought by the westerlies to southern africa declined it is concluded that the positions of the frontal systems in the southern ocean which determine the locations of the high pressure cells over the south atlantic and the southern indian ocean have a strong influence on the climate of southern africa in contrast to the climate of northwest and central africa which is dominated by the saharan low pressure cell

evolution of antarctic intermediate water during the plio pleistocene and implications for global climate evidence from the south atlantic abstract formed in the southern ocean antarctic intermediate water aaiw plays a fundamental role in the modern climate system as an important sink for atmospheric co2 and as a mid depth conduit supplying nutrient rich waters to subtropical and tropical upwelling regions during the transition from warm pliocene greenhouse conditions to pleistocene icehouse conditions around 3 ma model simulations suggest dramatic circulation changes in the southern ocean aaiw formation region due to cooling and expanded sea ice cover around antarctica however the history and properties of aaiw over this time period are still poorly understood here we trace aaiw source water provenance temperature and salinity changes over the last 4 myr using nd isotopes of fe mn oxyhydroxide encrusted foraminifera benthic foraminiferal mg ca and stable isotopes from southwest atlantic dsdp site 516 our results show that modern aaiw properties emerged gradually over the last 3 myr as evidenced by gradual cooling 6 c and freshening of intermediate waters over this same interval end values decrease and diverge away from those of pacific sourced waters and converge on values of present day circumpolar deep water cdw these observations are in accordance with model simulations indicating increased deep vertical mixing at the polar front associated with sea ice extension these modified aaiw source waters penetrated increasingly northward into the atlantic at intermediate depths enhancing nutrient supply and contributing to surface cooling along subtropical coastal upwelling sites these circulation changes may have facilitated ocean co2 storage providing a positive feedback amplifying global cooling and intensification of northern hemisphere glaciation

the early middle pleistocene transition of asian summer monsoon abstract the global climate cyclicity transferred from an early pleistocene mode dominated by the obliquity periodicity to a new middle pleistocene state dominated by the eccentricity periodicity in the absence of any significant changes in orbital forcing known as the early middle pleistocene transition empt is an unresolved issue of milankovitch orbital theory here we contribute new insights into the orbital characteristic and dynamics of the asian summer monsoon precipitation changes across the empt spectral analyses of precipitation over the past 1 6 myr reconstructed from a new loess magnetic susceptibility stack suggest that the empt of the summer monsoon precipitation characterized by a shift from obliquity to eccentricity rhythm occurred over a 640 kyr long term period from 1 24 to 0 6 ma rather than quasi instantaneously on a much shorter duration the eccentricity cyclicity initiated as early as 1 24 ma subsequently intensified and became the dominated pacing from 1 1 ma while obliquity weakened at 1 1 ma and significantly weakened at 0 7 ma these empt characteristics of the summer monsoon precipitation reflect complex impacts from global climate systems and regional climate changes in the northern and southern hemispheres and in both low and high latitude regions

warm upwelling regions in the pliocene warm period 1 given the importance of upwelling processes to coastal productivity and regional climate it is critical to study the role of upwelling regions within the context of global climate change we generated sea surface temperature sst records for the last 5 million years in three important upwelling regions the eastern equatorial pacific the california margin and the peru margin prior to 3 0 ma ssts at all sites were significantly warmer than today by 3 9 c indicating that cold upwelling regions that characterize the modern pacific ocean did not exist in the early pliocene warm period 4 6 to 3 1 ma earth s most recent period of sustained global warmth alkenone phosphorus and organic carbon mass accumulation rate records indicate that changes in productivity and sst were decoupled and that upwelling of nutrient enriched water occurred even when ssts were warm during the early pliocene thus the long term trends in sst are likely explained by changes in the temperature of upwelled water rather than in the strength of upwelling favorable winds alone the fact that gradual cooling of upwelling regions began before the onset of significant northern hemisphere glaciation provides further evidence that the growth of ice sheets and their influence on atmospheric winds alone can not explain the cooling of upwelling regions our results suggest that the long term average ssts of upwelling regions are influenced by global changes in the depth and or temperature of the ventilated thermocline

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

pleistocene sea surface temperature evolution early cooling delayed glacial intensification and implications for the mid pleistocene climate transition the mid pleistocene climate transition mpt is defined by the emergence of high amplitude quasi 100 ka glacial interglacial cycles from a prior regime of more subtle 41 ka cycles this change in periodicity and amplitude cannot be explained by a change in external astronomical forcing here we review and integrate published records of sea surface temperatures ssts to assess whether a common global expression of the mpt in the surface ocean can be recognized and examine our findings in light of mechanisms proposed to explain climate system reorganization across the mpt we show that glacial interglacial variability in ssts is superimposed upon a longer term cooling trend in oceanographic systems spanning the low to high latitudes regional variability exists in the timing of the onset and magnitude of cooling but in most cases a long term cooling trend begins or intensifies from 1 2 ma marine isotope stage mis 35 34 the sst cooling accompanies a long term trend towards higher global ice volume as recorded in benthic foraminifera 18o but predates a step like increase in 18o at 0 9 ma mis 24 22 that is argued to reflect expansion of continental ice sheets the strongest expression of pleistocene cooling is found during glacial stages whereas minor or negligible trends in interglacial temperatures are identified however pronounced cooling during both glacial and interglacial maxima is evident at 0 9 ma alongside the long term sst cooling trends quasi 100 ka cycles begin to emerge in both the sst and 18o records at 1 2 ma and become dominant with the expansion of the ice sheets at 0 9 ma we show that the intensified glacial stage cooling is accompanied by evolving pco2 abyssal ocean ventilation atmospheric circulation and or dust inputs to the southern ocean these changes in diverse environmental parameters suggest that glacial climate boundary conditions evolved across the mpt in turn these modified boundary conditions may have altered climate sensitivity to orbital forcing by placing pre existing ice sheets closer to some threshold of climate ice sheet response

the plio pleistocene development of atlantic deep water circulation and its influence on climate trends abstract using benthic stable isotope records from 10 sites in the atlantic ocean including two new records from walvis ridge in the southeast atlantic sites 1264 and 1267 we review changes in atlantic deep water circulation in the context of plio pleistocene climate overall we find non linear responses of atlantic deep water circulation to a cooling climate with differently evolving glacial and interglacial states our main conclusion is that peak north atlantic deep water nadw production was reached between 2 0 and 1 5 ma most prominently seen by a maximum in ventilated high 13 c conditions in the mid depth southeast atlantic site 1264 we infer that a major source of nadw at this time was the export of dense overflow water from the nordic seas into the abyssal east atlantic sea surface temperature records from the north and south atlantic support this notion and indicate that the peak nadw production between 2 0 and 1 5 ma was compensated by a stronger warm surface water return flow i e atlantic meridional overturning circulation amoc was enhanced causing long term 10 5 year heat piracy from the south to the north atlantic in the wider picture of plio pleistocene climate evolution we find that a long term enhancement in the average state of amoc 2 4 1 3 ma coincides with the 41 kyr world hence we speculate that the transitory negative feedback response of enhanced amoc to a cooling climate supplied heat to key areas of ice sheet growth acting to limit their size and maintain the 41 kyr world once a threshold in global cooling was reached the strength of amoc lessened providing a positive feedback for the early middle pleistocene transition and the associated build up of northern hemisphere ice sheets

oceanographic and climatic evolution of the southeastern subtropical atlantic over the last 3 5 ma abstract the southeast atlantic ocean is dominated by two major oceanic systems the benguela upwelling system one of the world s most productive coastal upwelling cells and the agulhas leakage which is important for transferring warm salty water from the indian ocean to the atlantic ocean here we present a multi proxy record of marine sediments from odp site 1087 we reconstruct sea surface temperatures u 37 k and tex86 indices marine primary productivity total chlorin and alkenone mass accumulation rates and terrestrial inputs derived from southern africa ti al and ca ti via xrf scanning to understand the evolution of the southeast atlantic ocean since the late pliocene in the late pliocene and early pleistocene odp site 1087 was situated within the benguela upwelling system which was displaced southwards relative to present we recognize a series of events in the proxy records at 3 3 3 0 2 2 1 5 0 9 and 0 6 ma which are interpreted to reflect a combination of changes in the location of major global wind and oceanic systems and local variations in the strength and or position of the winds which influence nutrient availability although there is a temporary sst cooling observed around the initiation of northern hemisphere glaciation inhg proxy records from odp site 1087 show no clear climatic transition around 2 7 ma but instead most of the changes occur before this time this observation is significant because it has been previously suggested that there should be a change in the location and or strength of upwelling associated with this climate transition rather the main shifts at odp site 1087 occur at ca 0 9 ma and 0 6 ma associated with the early mid pleistocene transition empt with a clear loss of the previous upwelling dominated regime this observation raises the possibility that reorganisation of southeast atlantic ocean circulation towards modern conditions was tightly linked to the empt but not to earlier climate transitions

the relationship between environmental change and the extinction of the nannoplankton discoaster in the early pleistocene the long ranging nannoplankton genus discoaster went extinct in the early pleistocene one of the most significant events in the cenozoic nannofossil record the causal factors of this extinction are poorly constrained at least partially because the ecology of discoaster is not well understood we investigate the relationship between the extinction and environmental changes by comparing a suite of high resolution nannofossil assemblage data at four ocean drilling program sites to published environmental proxy records through the use of multivariate analysis we determine discoaster shared environmental preferences with the extant species florisphaera profunda which thrives in the lower photic zone near the nutricline in locations where the water column is stratified we show that the last occurrence of discoaster was globally diachronous rather than synchronous as previously thought and likely took place in the equatorial atlantic ocean we propose that the demise of discoaster was a two stage process a decrease in sea surface temperatures and increase in climate variability led to a global decline in discoaster abundance the extinction event itself correlates to shoaling of the nutricline as signified by a decrease in the carbon isotopic gradient between surface and thermocline foraminifera we hypothesize that the combination of global cooling and a shoaling of the nutricline greatly reduced discoaster s deepwater niche and contributed to its extinction this may suggest that ocean stratification and the depth of the nutricline are important contributors to overall nannoplankton diversity

the late pliocene benguela upwelling status revisited by means of multiple temperature proxies as compared to the late pleistocene alkenone based sea surface temperature sst in the benguela region revealed relatively warm and stable sst recorded between approximate to 3 5 and 2 0 ma and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal we assess how the hydrological patterns recorded in sst proxies are embedded in the geological record by performing a proxy proxy comparison we used laser ablation inductively coupled plasma mass spectrometry to measure the mg ca on the planktonic foraminifera species globigerina bulloides allowing in situ measurements of mg ca on individual foraminiferal tests mg ca derived temperatures provide much colder temperatures than alkenone derived sst by up to 10 degrees c we build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle namely alkenones preferentially capturing sst when the surface ocean is warmer than the mean annual average sst and g bulloides capturing sst when upwelling intensifies multichamber analysis also suggests that g bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis allowing the extraction of both surface and subsurface temperature from mg ca measured on different chambers the range of temperatures recorded between our multiple sst proxies is supported by the range of temperatures simulated with a general circulation model when different seasons different water depth and different orbital configurations occurring during the late pliocene are considered a greater seasonal cycle in sst during the pliocene can account for alkenone and mg ca derived temperature contrast pointing to a radically different mode of upwelling activity in the benguela region compared to today

nutrient leakage from the north pacific to the bering sea iodp site u1341 following the onset of northern hemispheric glaciation 1 intergrated ocean drilling program expedition 323 recovered a sediment record covering the last 4 3 ma from the bering sea integrated ocean drilling program site u1341 bowers ridge 2177 m water depth to resolve pliocene pleistocene paleoenvironmental changes in this marginal basin 190 sediment samples were analyzed for their bulk element composition aluminium contents in bowers ridge sediments are variable but overall higher toward younger sediments probably related to the intensification of the northern hemispheric glaciation and increasing sea ice transport in the bering sea the gradual increase of terrigenous input is mirrored by decreasing sio2 and excess si sixs contents but the overall si enrichment of the deposits reflects continuous opal deposition since the pliocene at bowers ridge unlike in the north pacific the sixs record at site u1341 does not support a dramatic decrease in opal export following the onset of the northern hemispheric glaciation around 2 7 ma but sio2xs have higher accumulation rates up to 8 g cm2 ka between 2 6 and 1 8 ma bp during this period the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas we here discuss how the onset of north pacific stratification at 2 7 ma bp may have caused leakage of nutrient rich deep intermediate north pacific water into the bering sea via the deep kamtchatka strait leading to increased opal deposition and likely reactive organic matter export at bowers ridge as a result magnetic and geochemical records were overprinted by intensified diagenesis significantly affecting their applications as paleoceanographic proxies

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

intensification of the walker and hadley atmospheric circulations during the pliocene pleistocene climate transition when comparing new sea surface temperature sst records between the western and eastern equatorial pacific spanning the last 3 2 ma we found that the zonal temperature gradient over the entire tropical pacific irreversibly increased by 3 to 4 degrees c from 2 2 to 2 0 ma here we suggest a pronounced increase in atmospheric circulation from a weak to a strong zonal walker circulation wc during the early pleistocene evidence from other oceanic areas also suggests a strengthening in the meridional hadley circulation hc during the same time period therefore we also suggest that the invigoration of both atmospheric circulation patterns was intimately coupled during the plio pleistocene transition and likely linked to a shrinkage in the zonal extension of the tropical to subtropical warm sphere associated with a prominent increase in the pole to equator temperature gradient our conclusion refutes assumptions that the intensification of atmospheric circulation in the tropics and subtropics significantly contributed to the initiation of continental ice sheet formation at high latitudes since the onset of extensive northern hemisphere glaciation nhg occurred similar to 2 75 ma ago in the late pliocene instead the development of a stronger atmospheric circulation similar to 2 2 2 0 ma ago could have significantly contributed to the plio pleistocene climate cooling c 2010 elsevier b v all rights reserved

pliocene pleistocene variability of upwelling activity productivity and nutrient cycling in the benguela region in this study we present combined high resolution records of sea surface temperature sst phytoplankton productivity and nutrient cycling in the benguela upwelling system bus for the past 3 5 ma the sst record provided evidence that upwelling activity off namibia mainly intensified ca 2 4 2 0 ma ago in response to the cooling of the southern ocean and the resultant strengthening of trade winds as revealed by productivity related proxies bus intensification led to a major transition in regional biological productivity when considering the termination of the matuyama diatom maximum a diatom high production event major oceanic reorganization in the benguela was accompanied by nutrient source changes as indicated by a new nitrogen isotopic delta n 15 record that revealed a stepwise increase at ca 2 4 and ca 1 5 ma ago the change in source region likely resulted from significant changes in intermediate water formation tied to the reorganization of oceanic conditions in the southern ocean which may have in turn mainly controlled the global ocean n cycle and therefore the n isotopic composition of nutrients since 3 5 ma ago

nitrogen cycling on the namibian shelf and slope over the last two climatic cycles local and global forcings 1 in the light of new surface sediment 15n data collected over the namibian shelf and slope we examined glacial interglacial variations of n inventory in the area of luderitz 25 6s by deciphering 15n signals of three cores distributed from the upper to the lower continental slope the lower slope cores display low 15n during cold periods and high 15n during climatic optima akin to many other records from the world ocean whereas the upper slope core displays a high frequency low amplitude 15n signal without obvious glacial interglacial variability this dissimilarity results from the segregation of the upwelling structure in two cells decoupling nutrient dynamics of the shelf from those beyond the shelf edge the 15n signal of the coastal cell is relatively constant irrespective of wind strength variations and shows that nitrate was never depleted in the surface water for the deeper cores comparisons between n isotopic signals and indicators of paleoproductivity total organic carbon and upwelling intensity sea surface temperature and dust grain size reveal that over milankovitch cycles nitrate delivery to the photic zone was driven by the nutrient richness of the south atlantic central water depending in turn on aghulas water inflow and denitrification at a global scale rather than by atmospheric forcing we propose that the 15n signals of the deeper cores do not only mirror changes in relative nitrate utilization as it seems the case over annual timescales but are arguably influenced by global ocean changes in middepth nitrate 15n

persistent warmth across the benguela upwelling system during the pliocene epoch a feature of pliocene climate is the occurrence of permanent el nino like or el padre conditions in the pacific ocean from the analysis of sediment cores in the modern northern benguela upwelling we show that the mean oceanographic state off southwest africa during the warm pliocene epoch was also analogous to that of a persistent benguela el nino at present these events occur when massive southward flows of warm and nutrient poor waters extend along the coasts of angola and namibia with dramatic effects on regional marine ecosystems and rainfall we propose that the persistent warmth across the pliocene in the benguela upwelling ended synchronously with the narrowing of the indonesian seaway and the early intensification of the northern hemisphere glaciations around 3 0 3 5 ma the emergence of obliquity related cycles in the benguela sea surface temperatures sst after 3 ma highlights the development of strengthened links to high latitude orbital forcing the subsequent evolution of the benguela upwelling system was characterized by the progressive intensification of the meridional sst gradients and the emergence of the 100 ky cycle until the modern mean conditions were set at the end of the mid pleistocene transition around 0 6 ma these findings support the notion that the interplay of changes in the depth of the global thermocline atmospheric circulation and tectonics preconditioned the climate system for the end of the warm pliocene epoch and the subsequent intensification of the ice ages

plio pleistocene climate sensitivity evaluated using high resolution co2 records theory and climate modelling suggest that the sensitivity of earth s climate to changes in radiative forcing could depend on the background climate however palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction here we present atmospheric carbon dioxide co2 reconstructions based on multi site boron isotope records from the late pliocene epoch 3 3 to 2 3 million years ago we find that earth s climate sensitivity to co2 based radiative forcing earth system sensitivity was half as strong during the warm pliocene as during the cold late pleistocene epoch 0 8 to 0 01 million years ago we attribute this difference to the radiative impacts of continental ice volume changes the ice albedo feedback during the late pleistocene because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea level reconstructions we conclude that on a global scale no unexpected climate feedbacks operated during the warm pliocene and that predictions of equilibrium climate sensitivity excluding long term ice albedo feedbacks for our pliocene like future with co2 levels up to maximum pliocene levels of 450 parts per million are well described by the currently accepted range of an increase of 1 5 k to 4 5 k per doubling of co2

the response of the bering sea gateway during the mid pleistocene transition abstract the mid pleistocene transition and the response of sea ice in the subarctic north pacific are examined using a diatom record from the north eastern bering sea slope iodp core u1344a high percentages of diatom sea ice species in the assemblage foster the development of groupings specific to differing ice conditions thalassiosira antarctica var borealis emerges as a key diatom species that delineates the development of thick multiannual sea ice and marks a series of advancements from 0 9 to1 2 ma after which it dominates the assemblage until 0 6 ma the decline of neodenticula seminae indicative of alaskan stream water inflow is in agreement with neighbouring core u1343e confirming the reduction of bering sea circulation after 0 9 ma we propose that during the mpt a stepped advancement of sea ice occurs in the bering sea as a response to increased moisture delivery to the high latitudes an extended period of diatom productivity between 0 75 and 0 65 ma marks major deglacial conditions towards the end of the mpt by 0 6 0 55 ma the work highlights the important role played by sea ice and supports the contention of a colder mpt in the subarctic pacific

a 5 million year comparison of mg ca and alkenone paleothermometers geochemical sea surface temperature sst proxies such as the magnesium to calcium ratio mg ca in foraminifera and the alkenone unsaturation index uk 37 are becoming widely used in pre pleistocene climate records this study quantitatively compares previously published mg ca and uk 37 data from ocean drilling program odp site 847 in the eastern equatorial pacific to assess the utility of these proxies to reconstruct tropical sst over the last 5 ma foraminiferal mg ca sst calibrations that include a dissolution correction are most appropriate at this location because they provide sst estimates for the youngest sample that are close to modern mean annual sst the long term trends in the two records are remarkably similar and confirm a 3 5 c cooling trend from the early pliocene warm period to the late pleistocene noted in previous work absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate sst from mg ca are taken into account comparing the two sst records at odp site 847 to other records in the region shows that the eastern equatorial pacific was 2 4 c warmer during the early pliocene compared to today

the formation of the namib sand sea inferred from the spatial pattern of magnetic rock fragments the formation of the namib sand sea inferred from the spatial pattern of magnetic rock fragments

biogeochemical changes within the benguela current upwelling system during the matuyama diatom maximum nitrogen isotope evidence from ocean drilling program sites 1082 and 1084 1 the matuyama diatom maximum mdm is a time of peak opal accumulation from 2 6 to 2 0 ma within the benguela current upwelling system that was initiated by increased influence of southern ocean water on the eastern south atlantic we measured opal total organic carbon toc and caco3 fluxes and c and n stable isotopes in sediments deposited from 2 4 to 1 95 ma at sites 1082 and 1084 to explore the biogeochemical dynamics within the benguela region the infusion of southern ocean water delivered dissolved nutrients and southern ocean flora and fauna resulting in local opal accumulation increasing up to 8 g cm2ky and the production of diatom mats some 15n measurements of diatom bound organic matter indicate that the mats grew within the benguela region the bulk sediment 15n records are taken to reflect changes in the 15n of nitrate in the incoming water where lower values at 2 4 ma reflect less nitrate utilization in the antarctic a long term increase in relative nitrate uptake in the southern ocean is evidenced by the gradual increase in 15n toward 1 9 ma