latest quaternary oceanographic changes in the okhotsk sea based on diatom records abstract the okhotsk sea is a locus of the north pacific intermediate water formation today the formation of the okhotsk intermediate water is linked with seasonal sea ice cover diatom records from the okhotsk sea show the following 1 during the last glacial maximum between 21 000 yr b p and 18 000 yr b p sea ice covered the western side of the sea throughout the whole year but the eastern part was open this permitted more efficient heat exchange with the atmosphere than waters and hence ventilation of the glacial okhotsk sea waters which in turn caused a greater extent of intermediate water formation through the bussol strait 2 the perennial sea ice cover extended northward and sea ice was present throughout the whole year over a great part of the sea except for the eastern part between 18 000 yr b p and 17 000 yr b p 3 the ice free area extended greatly to the south during the interval between 17 000 yr b p and 11 000 yr b p 4 there was no perennial ice cover over the whole sea and the ice free area that appeared as the seasonal ice area began to extend to the western part between 11 000 yr b p and 8 000 yr b p 5 after 8000 yr b p the ice free waters contracted to the present extent

diatom δ18o evidence for the development of the modern halocline system in the subarctic northwest pacific at the onset of major northern hemisphere glaciation establishing a time frame for the development of the modern halocline and stratified water column in the subarctic north pacific has significant paleoclimatic implications here we present a d18o diatom record consisting of only two species that represents autumn winter conditions in the region across the onset of major northern hemisphere glaciation boundary at circa 2 73 ma a decrease in d18o diatom of 4 6 occurs whereas previously published d18o foram results show a 2 6 increase the d18o diatom and uk37 sea surface reconstructions indicate both a significant freshening of 2 4 practical salinity units and an increase in surface temperatures in the summer to early winter period from circa 2 73 ma onward in contrast the concomitant increase in d18o foram is likely to be reflective of conditions beneath the mesothermal structure and or spring conditions when warmer sea surface temperatures are not present in the region these results are consistent with the development of the modern halocline system at 2 73 ma with year round stratification of the water column and a strengthened seasonal thermocline during the summer to early winter period resulting in one of the largest summer to winter temperature gradients in the open ocean the onset of stratification would also have led to a warm pool of surface water from circa 2 73 ma which may have provided a potential source of extra moisture needed to supply the growing north american ice sheets at this time

north pacific halocline and cold climate induced by panamanian gateway closure in a coupled ocean atmosphere gcm 1 the influence of the closure of the panamanian gateway during the late cenozoic on climate in and around the north pacific is investigated by using a coupled ocean atmosphere general circulation model with an open and closed gateway in the case of an open closed gateway deep convection is present absent in the north pacific the deep convection is associated with surface saline water transported from the subtropical atlantic through the open gateway to the north pacific on the other hand with the closed gateway the lack of saline water transport from the atlantic induces halocline formation over the subarctic pacific with cold climate the deep convection in the north pacific leads to a vigorous thermohaline circulation with larger meridional heat transport and causes warmer climate in and around the north pacific these results are generally consistent with paleoceanographic and paleoclimatic estimates related to the closure of the panamanian gateway

length of 3 d mixing controlled plumes for a fully penetrating contaminant source with finite width 1 an analytical 3 d model is developed to estimate steady state plume lengths in contaminated aquifers for the case of an instantaneous bimolecular reaction the developed model generalizes the previous vertically oriented 2 d model of liedl et al 2005 by considering a source that has finite width 2w and is extended over the entire depth m of the aquifer it is shown that the model of liedl et al 2005 overestimates steady state plume lengths if the source width is below a certain maximum relevant source width 2wrel results also suggest that the ratio of the bulk transverse dispersivities in the horizontal and vertical directions is critical in defining the size of the plume and that the maximum plume length is observed when 2w and m are approximately equal for any ratio above 10

origin and evolution of cycladophora davisiana ehrenberg radiolaria in dsdp site 192 northwest pacific this paper documents the evolutionary history of cycladophora davisiana ehrenberg from an uppermost miocene to pleistocene sedimentary record in the high latitude northwest pacific it apparently evolved from c sakaii motoyama through a series of intermediates c sakaii has a relatively large shell with an external spongy layer the evolutionary transition is characterized by a relatively rapid decrease in thorax size with a reduction of the spongy appendage this change occurred during about 0 4 m y from 2 8 to 2 4 ma without cladogenesis following this interval a decrease in thorax size continued gradually up to the recent resulting in a very small morphology although the population of c davisiana first appeared at about 2 5 ma some morphotypic specimens may occur in earlier periods as indistinguishable very small endmembers in the c sakaii populations timing of the first appearance events both of morphotypic specimens and of a population of c davisiana in site 192 and previously reported cores does not disprove the idea that c davisiana evolved first in the northwest pacific region and later migrated into other regions of the world ocean biometrics clearly indicate no direct phylogenetic relationships between c davisiana and c cornutoides kling in the studied core thus the latter species which was originally described as a variation and later elevated to a subspecies of the former species is separated from the former species and raised to the species rank

biogeography of neogene calcareous nannofossils in the caribbean and the eastern equatorial pacific floral response to the emergence of the isthmus of panama abstract principal components analysis of nannofossil assemblages in five odp sites in the caribbean and the eastern equatorial pacific was used to monitor neogene surface water circulation changes in 15 83 10 71 ma floral distributions clearly show the existence of the circum tropical current between the caribbean and the eastern equatorial pacific and no surface water communication between the northern and southern caribbean in 10 71 9 36 ma the circum tropical current weakened and the northward intra caribbean current had been initiated northern and southern caribbean surface waters again became separated in 8 35 3 65 ma and the circum tropical current was regenerated after 2 76 ma the northward intra caribbean current has completely been established and the circum tropical current disappeared this suggests that a barrier to the circum tropical current had formed indicating the final closure of the isthmus of panama

pliocene pleistocene evolution of the north pacific ocean atmosphere system interpreted from fossil diatoms the relative abundance of diatom taxa in 216 core tops has been quantified and compared using cluster analysis with assemblages in six cores located beneath different water masses of the north pacific ocean all but one sequence span at least 1 5 m y early pleistocene to holocene and one extends to 2 6 ma the quantitative abundance of each taxon and the space time occurrence of each cluster are interpreted as tracers of oceanographic conditions in addition to high frequency glacial interglacial oscillations the records display rapid steplike changes at 2 4 ma 1 6 1 5 ma 1 0 ma 0 8 0 7 ma and 0 35 ma two secondary transitions recorded only in the northwestern cores occur at 2 0 ma and 1 1 ma we conclude that both the high frequency oscillations and the stepwise transitions reflect variations in extent of subarctic water masses under the influence of the siberian high pressure and aleutian low pressure atmospheric cells we further conclude that 1 the subarctic water mass has developed progressively and attained its modern structure only in the late pleistocene 2 some stages of this evolution may have occurred earlier in the eastern subarctic 3 the holocene is an atypical warm extreme so that the present is not a very good key to the past 4 there were several glaciations between 2 4 and 2 0 ma and 5 the subtropical gyre is more conservative than the subarctic gyre and major changes in the region have been due to forcing from the north we propose a conceptual model of oscillating metastable climatic modes maintained by internal feedback and suggest that steplike changes may occur when one set of boundary conditions is temporarily exceeded

enhanced stratification and seasonality in the subarctic pacific upon northern hemisphere glaciation new evidence from diatom bound nitrogen isotopes alkenones and archaeal tetraethers abstract coincident with the intensification of northern hemisphere glaciation nhg around 2 73 million years ma ago sediment cores from both the open subarctic north pacific and the antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed representing one of the most abrupt and dramatic changes in the marine sediment record associated with the development of pleistocene glacial cycles in the north pacific bulk sediment nitrogen isotope data and alkenone derived sea surface temperature sst estimates suggest that the productivity decline was driven by reduced exchange between surface and deep water due to weaker wind driven upwelling and or a strengthening of the halocline i e stratification in this study of the 2 73 ma transition at ocean drilling program odp site 882 in the western subarctic north pacific diatom bound nitrogen isotopes 15 n db alkenone mass accumulation rate and alkenone and archaeal tetraether based sst reconstructions support the stratification hypothesis indicating perennially lower export production generally higher nitrate consumption and greater inter seasonal variation in sst after the 2 73 ma transition in addition the 15 n db of large and small size fractions of coscinodiscus spp suggest that these diatoms grew mostly during the spring bloom during the late pliocene switching to their current fall to winter growth period at the 2 73 ma transition this view is consistent with their decline in dominance and provides further evidence for increased stratification reduced vertical exchange in the north pacific after 2 73 ma the 15 n db data indicate that over the 100 kyr period after the 2 73 ma transition studied here nitrate consumption did not reach late pleistocene ice age levels and that nitrate consumption in post 2 73 ma warm stages was similar to that before the transition even though productivity was greatly reduced we tentatively attribute this to relatively weak dust borne iron inputs in the early post 2 73 ma period

pliocene paleoclimatic interpretation of dsdp site 580 nw pacific using diatoms abstract high resolution quantitative diatom data are tabulated for the early part of the late pliocene 3 25 to 2 08 ma at dsdp site 580 in the northwestern pacific sample spacing averages 11 k y between 3 1 and 2 8 ma but increases to 14 to 19 k y prior to 3 1 ma and after 2 8 ma q mode factor analysis of the middle pliocene assemblage reveals four factors which explain 92 4 of the total variance of the 47 samples studied between 3 25 and 2 55 ma three of the factors are closely related to modern subarctic transitional and subtropical elements while the fourth factor which is dominated bycoscinodiscus marginatus and the extinct pliocene speciesneodenticula kamtschatica appears to correspond to a middle pliocene precursor of the subarctic water mass knowledge of the modern and generalized pliocene paleoclimatic relationships of various diatom taxa is used to generate a paleoclimate curve twt based on the ratio of warm water subtropical to cold water diatoms with warm water transitional taxa thalassionema nitzschioides thalassiosira oestrupii andcoscinodiscus radiatus factored into the equation at an intermediate 0 5 value the twt ratios at more southerly dsdp sites 579 and 578 are consistently higher warmer than those at site 580 throughout the pliocene suggesting the validity of the ratio as a paleoclimatic index diatom paleoclimatic data reveal a middle pliocene 3 1 to 3 0 ma warm interval at site 580 during which paleotemperatures may have exceeded maximum holocene values by 3 5 5 c at least three times this middle pliocene warm interval is also recognized by planktic foraminifers in the north atlantic and it appears to correspond with generalized depleted oxygen isotope values suggesting polar warming the diatom twt curve for site 580 compares fairly well with radiolarian and silicoflagellate paleoclimatic curves for site 580 planktic foraminiferal sea surface temperature estimates for the north atlantic and benthic oxygen isotope curves for late pliocene although higher resolution studies on paired samples are required to test the correspondence of these various paleoclimatic indices

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

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

paleoproductivity and paleoceanography of the last 4 3 myrs at iodp expedition 323 site u1341 in the bering sea based on biogenic opal content abstract site u1341 in the southern bering sea was drilled and cored down to 600 meters below sea floor mbsf during integrated ocean drilling program iodp expedition 323 covering a nearly complete record of the last 4 3 million years myrs analyses of the biogenic opal content of sediments at the site provide detailed and useful information on past biological productivity and paleoceanographic changes that occurred in the region including shifts in the oceanographic condition during the intensification of the northern hemisphere glaciation nhg and the mid pleistocene transition mpt an overall decreasing trend in the biogenic opal record combined with evidence from microfossil assemblages indicates a gradual shift in environmental conditions during the last 4 3 myrs from warm and nutrient rich conditions to cool conditions with sea ice on the other hand biogenic opal mass accumulation rates mar were high during 2 6 2 1 ma after the intensification of the nhg unlike in the western north pacific high biological productivity during this specific interval is consistent with the results of previous studies in the other marginal seas possibly suggesting that iron leakage from the bering continental shelf occurred after the mpt the data suggest that there was sea ice expansion and discharge of lithogenic matter during glacial periods and high productivity during interglacial periods

middle pliocene sea surface temperatures a global reconstruction identification and analyses of pliocene marine microfossils from 64 globally distributed stratigraphic sequences have been used to produce a middle pliocene sea surface temperature reconstruction of the earth this reconstruction shows little or no change from current conditions in low latitude regions and significant warming of the ocean surface at mid and higher latitudes of both hemispheres this pattern of warming is consistent with terrestrial records and suggests a combination of enhanced meridional ocean heat transport and enhanced greenhouse effect were responsible for the middle pliocene warmth

changes in atmospheric rivers and moisture transport over the northeast pacific and western north america in response to enso diversity the year to year changes in atmospheric rivers ars and moisture transport over the northeast pacific and western north america are investigated during december to february djf from 1979 80 to 2015 16 changes in ar frequency intensity and landfall characteristics are compared between three enso phases central pacific el nino cpen eastern pacific el nino epen and la nina nina during epen events the subtropical jet extends to the south and east with an anomalous cyclonic flow around a deeper aleutian low more moisture is transported towards north america and ar frequency is increased over western north america in cpen events the aleutian low shifts further southward relative to its position in epen resulting in an increase in the frequency and intensity of landfalling ars over the southwestern us in nina events the landfalling ar frequency is reduced associated with anomalous anticyclonic circulation over the eastern north pacific we diagnose the contribution of multiple factors to the seasonal mean moisture transport using moisture budgets during the three enso phases the change in low frequency circulation dynamical process is the leading contributor to the seasonal mean moisture flux divergence while the contributions of the synoptic anomalies and the change in moisture anomaly thermodynamic process are not significant along the west coast of north america

drastic change in the late pliocene subarctic pacific diatom community associated with the onset of the northern hemisphere glaciation abstract to date there is scarcely clear understanding of the detailed responses of the subarctic pacific phytoplankton communities to the significant climate transition across the initiation of the northern hemisphere glaciation nhg approximately 2 7 million years ago we present more highly resolved evidence of the rapid and drastic change in the phytoplankton biosphere using diatom as a tracer of environmental changes and based on quantitative assemblage analysis of two marine cores from oceanic regions of the north pacific subarctic gyre in the late pliocene subarctic pacific coscinodiscus marginatus and neodenticula kamtschatica were widely dominant but the flora rapidly and drastically shifted to one rich in neodenticula complex n koizumii n seminae at the onset of nhg perhaps due to intense changes in paleoceanographic conditions such as cooling and or a collapse of the previously nutrient regime in the region we also suggest that vertical pumping of biogenic matter by diatoms may have been more efficient in this region earlier than the onset of nhg after the nhg intensification the major primary productivity role in the subarctic pacific shifted from the c marginatus rich flora to the neodenticula complex the drastic decrease of biogenic opaline accumulation reportedly associated with the initiation of nhg would have resulted from not only a decrease of primary production but also from the change in diatom assemblages and consequent effects on their principal role in export flux

enhanced circulation during a warm period the early pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today to characterize thermohaline circulation during this warm period 4 4 3 1 ma we combined new benthic foraminiferal isotopic data from the southeast atlantic with published data and reconstructed pacific and atlantic ocean nutrient distributions the data indicate enhanced ventilation of the deep atlantic and intermediate pacific during the early pliocene enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of north pacific and atlantic subtropical gyres was probably also enhanced future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation

effects of late neogene climatic cooling on north pacific radiolarian assemblages and oceanographic conditions abstract radiolarian census and abundance data were collected from three deep sea cores drilled by the ocean drilling program sites 884 887 and 1151 to investigate patterns of ecologic changes in space and time during the last 16 million years for the mid latitude to subarctic north pacific high concentrations of radiolarians occurred between 9 0 and 2 7 ma radiolarian species richness was highest in the early middle miocene at each site and gradually decreased up to about 7 ma coinciding with a well established global cooling trend a degree of overlap index calculated for radiolarian assemblages revealed 11 faunal change events of which 8 corresponded to global cooling events and expansions of polar ice sheets three of the faunal change events were observed within the peak of radiolarian accumulation rate and were ascribed to changes in primary productivity in the north pacific rather than global climatic changes our assemblage analyses revealed that north south differentiation in radiolarian assemblages in the northwestern pacific has existed since 16 ma and became more distinct via major steps at 6 8 ma and 2 7 ma coinciding with major glaciation events and that east west faunal contrasts in the subarctic region became obvious beginning at 11 7 ma and changed to a different mode around 6 8 ma the observed east west faunal differences possibly reflect east to west climate differences that were characterized by cooler temperatures in the east than the west during the late miocene 11 7 6 8 ma and then by the opposite temperature trend 6 8 ma recent a severe glaciation at 2 7 ma played a large role particularly in temporal changes in radiolarian accumulation rate and assemblage composition

paleoecological significance of laminated diatomaceous oozes during the middle to late pleistocene north atlantic ocean iodp site u1304 abstract we examined diatom assemblages in a series of remarkable laminated diatomaceous ooze ldo horizons in the marine sediments from integrated ocean drilling program iodp site u1304 to reconstruct the middle to late pleistocene paleoceanographic evolution of the northern north atlantic ocean four confirmed diatom biohorizons combined with calcareous nannofossil and paleomagnetic stratigraphies established the chronological framework for the material the planktonic araphid needle like species thalassiothrix longissima was the greatest contributor to the ldo facies from the results of a principal component analysis using the percent abundances of 65 significant p 5 diatom taxa except for tx longissima which was extremely dominant in almost all horizons observed we identified two principal component pc axes taxa probably associated with the stratigraphic distribution of the major zonal marker neodenticula seminae ranging from 1 26 to 0 84 ma in this ocean loaded on pc1 with a high value pc2 was related to the ocean surface temperature the stratigraphic variability of the pc2 score indicated that switching between warm and cold water assemblages occurred concurrently with ldo deposition or extreme tx longissima dominance episodes in several horizons particularly after 0 84 ma suggesting that the subarctic convergence sac oceanic front passed over site u1304 during pleistocene glacial interglacial cycles our floral evidence supports the model of nearly monospecific ldo formation caused by the enhanced physical accumulation of particular diatoms such as tx longissima on the other hand nd seminae which probably contributes to spring phytoplankton blooms in the modern ocean was present only between 1 26 and 0 84 ma in this area thus we infer that the main contributor of export flux in the regional annual primary production cycle would have shifted drastically from one of a spring phytoplankton bloom leader nd seminae to minor but mass dump assemblages tx longissima etc in the mid pleistocene

impact of distinct origin locations on the life cycles of landfalling atmospheric rivers over the u s west coast author s zhou y kim h abstract 2019 american geophysical union all rights reserved an atmospheric river ar event represents strong poleward moisture transport and is defined as a series of spatiotemporally connected instantaneous ar objects utilizing an ar tracking algorithm with a depth first search a widely used algorithm in computer science we examine the life cycle characteristics of ar events that make landfall over the u s west coast by their distinct origin locations landfalling ar events from the northwest pacific 120 e 170 w wlar events temporally last longer 5 3 versus 3 6 days on average and have stronger intensity of integrated vapor transport 508 versus 388 kg m 1 s 1 on average than those originating from the northeast pacific 125 w 170 w elar events a persistent tripole geopotential height anomaly pattern over the north pacific modulates the origin locations and propagation of landfalling ar events wlar events are associated with anomalous highs over northeastern asia and the northeast pacific and an anomalous low over the central north pacific this pattern provides favorable conditions for wlar events to start propagate northeastward and make landfall in the northwestern west coast wlar events contribute approximately 25 of the total winter precipitation over washington and british columbia elar events are associated with a similar tripole pattern to the wlar events with an eastward shift the anomalous low over the northeast pacific helps elar events to start propagate northeastward and make landfall in the southwestern west coast precipitation induced by elar events contributes up to 30 of total winter precipitation over california

the inland penetration of atmospheric rivers over western north america a lagrangian analysis abstractalthough atmospheric rivers ars typically weaken following landfall those that penetrate inland can contribute to heavy precipitation and high impact weather within the interior of western north america in this paper the authors examine the evolution of ars over western north america using trajectories released at 950 and 700 hpa within cool season ars along the pacific coast these trajectories are classified as coastal decaying inland penetrating or interior penetrating based on whether they remain within an ar upon reaching selected transects over western north america interior penetrating ar trajectories most frequently make landfall along the oregon coast but the greatest fraction of landfalling ar trajectories that eventually penetrate into the interior within an ar is found along the baja peninsula in contrast interior penetrating ar trajectories rarely traverse the southern high sierra at landfall interior penetrating ar trajectories are associated with a more amplified flow

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

mid pliocene warmth stronger greenhouse and stronger conveyor abstract three million years ago prior to the onset of northern hemisphere glaciation global mean temperatures may have been as much as 3 5 c warmer than at present we present evidence based on the carbon isotopic composition of marine organic matter that atmospheric co2 levels at this time were on average only about 35 higher than the preindustrial value of 280 ppm we also present carbon isotopic evidence for stronger thermohaline circulation in the atlantic ocean during the warmest intervals and propose that the north atlantic conveyor belt may act as a positive feedback to global warming by enhancing sea ice retreat and decreasing high latitude albedo based on our results it seems unlikely that the mid pliocene warm period was a doubled co2 world

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

impact of rossby wave breaking on u s west coast winter precipitation during enso events abstractthis study demonstrates that water vapor transport and precipitation are largely modulated by the intensity of the subtropical jet transient eddies and the location of wave breaking events during the different phases of enso clear differences are found in the potential vorticity pv meteorological fields and trajectory pathways between the two different phases rossby wave breaking events have cyclonic and anticyclonic regimes with associated differences in the frequency of occurrence and the dynamic response during la nina there is a relatively weak subtropical jet allowing pv to intrude into lower latitudes over the western united states this induces a large amount of moisture transport inland ahead of the pv intrusions as well as northward transport to the west of a surface anticyclone during el nino the subtropical jet is relatively strong and is associated with an enhanced cyclonic wave breaking this is accompanied by a time mean surface cyclone which brings zonal moisture tran

the contribution of orbital forcing to the progressive intensification of northern hemisphere glaciation in this study we reconstruct the timing of the onset of northern hemisphere glaciation this began in the late miocene with a significant build up of ice on southern greenland however progressive intensification of glaciation did not begin until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia were glaciated at approximately 2 74 ma 40 ka before the glaciation of alaska 2 70 ma and about 200 ka before significant glaciation of the north east american continent 2 54 ma we also review the suggested causes of northern hemisphere glaciation tectonic changes such as the uplift of the himalayan and tibetan plateau the deepening of the bering strait and the emergence of the panama isthmus are too gradual to account entirely for the speed of northern hemisphere glaciation we therefore postulate that tectonic changes may have brought global climate to a critical threshold but the relatively rapid variations in the earth s orbital parameters and thus insolation triggered the intensification of northern hemisphere glaciation this theory is supported by computer simulations which despite the relative simplicity of the model and the approximation of some factors e g using a linear carbon dioxide scenario neglecting the geographical difference between the pliocene and the present suggest that it is possible to build up northern hemisphere ice sheets between 2 75 and 2 55 ma by varying only the insolation controlled by the orbital parameters c 1998 elsevier science ltd all rights reserved

late neogene changes in diatom sedimentation in the north pacific abstract during the late neogene north pacific diatom sedimentation underwent major changes in response to high latitude cooling and changes in surface and deep water circulation at 9 ma diatom mass accumulation rates mars increased in the nw pacific and off northeast japan possibly due to shoaling of the isthmus of panama which lead to an enrichment of nutrients in north pacific deep waters during the latest miocene diatom mars increased progressively off southem california 6 5 ma at high latitudes of the north pacific 6 2 ma and off northeastern japan 5 5 ma presumably in response to high latitude cooling at about 4 5 ma diatom sedimentation abruptly increased in the nw pacific but declined off japan and california coincident with the onset of a prolonged period of high latitude warmth enhanced upwelling of nutrient rich deep waters in the nw pacific probably stimulated diatom production there a major step in high latitude cooling at 2 7 ma caused a reversal of these mid pliocene diatom sedimentation patterns upwelling of deep nutrient rich waters waned at higher latitudes leading to a decline in diatom productivity while wind driven coastal upwelling increased off southern california and stimulated diatom growth

pliocene and pleistocene paleoceanography in the western subarctic pacific based on diatom analyses of odp leg 145 hole 884b and iodp expedition 323 holes u1341b and u1343e abstract late pliocene pleistocene fossil diatom assemblages from ocean drilling program odp leg 145 hole 884b in the western subarctic pacific were investigated to improve the diatom biostratigraphy moreover the paleoceanographic records at hole 884b were compared with those in integrated ocean drilling program iodp expedition 323 holes u1341b and u1343e in the bering sea for the time interval of 2 5 0 ma in order to clarify the differences in the diatom assemblage fluctuations and the history of past ocean circulations around the subarctic pacific in hole 884b five diatom zones from the neodenticula koizumii neodenticula kamtschatica zone to the neodenticula seminae zone were identified the decreases in the relative abundance of temperate water species were recognized at ca 1 9 ma in hole u1343e ca 2 1 ma in holes 884b and u1341b the decreases of the temperate water species at these sites indicate that a widespread cooling event must have occurred over the subarctic pacific at 2 ma in addition the continuous low occurrences of the sea ice related species from around 2 ma onward which indicate the presence of sea ice were observed at sites 884 and u1341 we also point out that the age differences in decrease of temperate water species at the three sites i e delay in response of the temperate water species at site u1343 may be related to changes of seawater inflow from the north pacific to sites u1341 and u1343 and of the east kamchatka current from the bering sea to site 884

diatom mats in gulf of california sediments implications for the paleoenvironmental interpretation of laminated sediments and silica burial preservation of laminations in hemipelagic and pelagic sediments is routinely attributed to reduced oxygen conditions that inhibited benthic activity as such the degree of preservation of laminae is used commonly as a paleo oxygenation indicator recent evidence from deep sea pelagic sediments in the eastern equatorial pacific and north atlantic however demonstrates that preservation of laminae may occur by rapid deposition of ungrazed thalassiothrix longissima diatom mats that overwhelm the benthos in otherwise oxic bottom waters here we report that similar diatom mat laminae are common in hemipelagic sediments of the guaymas basin in the gulf of california have aided preservation of the laminated sediment sequence and have also enhanced opal burial examples of diatom mats influencing sediment preservation are also described from the northern california margin and from the miocene monterey formation southern california thus even in classical settings of laminated sediment occurrence caution needs to be taken in correlating degree of lamina preservation with paleo oxygen levels such studies must be underpinned by detailed microfabric and micropaleontological analyses

pliocene and pleistocene diatom datum levels related with paleoceanography in the northwest pacific abstract thick sections of pliocene and pleistocene biosiliceous clay and ooze were recovered by the hydraulic piston corer hpc at three northwest pacific sites dsdp sites 578 579 and 580 they contain a well preserved paleomagnetic record which made it possible to evaluate diatom events used in low and high latitudes in the transitional region of the northwest pacific equatorial pacific events are usually isochronous between the equatorial and subarctic regions however species which have short ranges in low latitudes tend to have diachronous first and last appearances in higher latitudes all subarctic north pacific datum species are present in the sediments at three sites which lie north and south across the subarctic front but their ranges become shorter in southern regions they do not penetrate into the equatorial region spatial distributions of these events are influenced by the paleo position of the subarctic front the migration of species from their home area outwards in the form of the first appearance is related to the fluctuations of the subarctic front the last appearance of species is a response to the change of the surface water temperature that is beyond the limit of tolerance of the species or an unstable oceanic environment due to major change of climate

a rapidly deposited pennate diatom ooze in upper miocene lower pliocene sediment beneath the north pacific polar front abstract rapidly deposited thalassionema thalassiothrix pennate diatom oozes previously have been described in upper miocene lower pliocene sediment beneath the frontal boundary of the eastern equatorial pacific here we document a new occurrence of thalassionema thalassiothrix ooze in upper miocene lower pliocene sediment beneath the frontal boundary of the subarctic north pacific the ooze is a 6 m interval of siliceous sediment at ocean drilling program odp sites 885 886 that was rapidly deposited between approximately 5 0 and 5 9 ma bulk sediment in this interval may contain greater than 85 pennate diatom tests there are also abundant laminae and pockets that are composed entirely of thalassionema and thalassiothrix diatoms the presence of a rapidly deposited ooze dominated by pennate diatoms indicates unusual past conditions in the overlying surface waters time coincident deposition of such oozes at two distinct frontal boundary locations of the pacific suggests that the unusual surface water conditions were causally linked to large scale oceanographic change this same oceanographic change most likely involved 1 addition of nutrients to the ocean or 2 redistribution of nutrients within the ocean the occurrence and origin of pennate diatom oozes may be a key component to an integrative understanding of late neogene paleoceanography and biogeochemical cycling

impact of shifting patterns of pacific ocean warming on north atlantic tropical cyclones two distinctly different forms of tropical pacific ocean warming are shown to have substantially different impacts on the frequency and tracks of north atlantic tropical cyclones the eastern pacific warming epw is identical to that of the conventional el nino whereas the central pacific warming cpw has maximum temperature anomalies located near the dateline in contrast to epw events cpw episodes are associated with a greater than average frequency and increasing landfall potential along the gulf of mexico coast and central america differences are shown to be associated with the modulation of vertical wind shear in the main development region forced by differential teleconnection patterns emanating from the pacific the cpw is more predictable than the epw potentially increasing the predictability of cyclones on seasonal time scales

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

drastic change in the geographical distribution of the cold water nannofossil coccolithus pelagicus wallich schiller at 2 74 ma in the late pliocene with special reference to glaciation in the arctic ocean abstract we reconstruct the late pliocene to quaternary paleoceanography of the pacific and atlantic oceans based on the geographical distribution of coccolithus pelagicus wallich schiller which has been regarded as a typical cold water species at 2 74 ma the distribution of c pelagicus defines a biogeographical boundary in the marginal seas of mid to high latitude regions of the pacific such as near the japanese islands kuril islands kamchatca peninsula aleutian islands alaska and the northeastern pacific off canada the relative abundance of c pelagicus is greater than 80 in these marginal sea areas in the north pacific and japan sea on the basis of characteristics of the nannofossil assemblages and recent nannoplankton biogeography in the norwegian greenland sea the change in the relative abundance of c pelagicus at 2 74 ma is thought to be the result of the southward migration of most species except c pelagicus with the onset of heavy glaciation the results also indicate that the geographical distribution of c pelagicus changed in three steps at 2 75 1 65 and 1 2 ma and it became rare to barren in low latitude regions

seasonal variation in skeletal silicification of neodenticula seminae a marine planktonic diatom sediment trap experiments in the nw pacific ocean 1997 2001 abstract neodenticula seminae is an important component of pelagic diatom floras in the subarctic north pacific and its adjacent seas we conducted an sem image analysis of four series of samples collected by sediment traps in the nw pacific in order to investigate the relations between its morphology evolution and environment seasonal variation in the skeletal morphology was common lightly silicified morphs were dominant during spring blooms and in the summer under oligotrophic conditions owing to surface water stratification and progressive nutrient limitation whereas heavily silicified morphs became abundant in the fall and winter subsequent to the onset of intensive vertical mixing these observations suggest that the morphology of n seminae is influenced by environmental factors including the nutrient availability as well as by its reproductive strategy

surface ocean cooling in the subarctic north pacific during the late pliocene suggests an atmospheric reorganization prior to extensive northern hemisphere glaciation abstract this paper presents records of tex 86 l and u 37 k derived sea surface temperatures ssts from ocean drilling program site 882 in the subarctic north pacific over the last 5 8 million years the tex 86 l derived sst record shows a cooling trend from 17 c at 3 6 ma to 8 c at 2 3 ma this cooling preceded the onset of extensive northern hemisphere glaciation nhg at 2 7 ma coinciding with the intensification of the east asian winter monsoon surface cooling in the north atlantic and the gradual development of nhg the results of this study suggest that the reorganization of atmospheric circulations in the northern high latitudes from 3 6 to 2 3 ma was associated with the early stage development of the ice sheets and sea ice in the arctic and subarctic regions inducing the cooling of the north atlantic and the subarctic north pacific and the intensification of the east asian winter monsoon

biogeochemical cycling in the bering sea over the onset of major northern hemisphere glaciation the bering sea is one of the most biologically productive regions in the marine system and plays a key role in regulating the flow of waters to the arctic ocean and into the subarctic north pacific ocean cores from iodp expedition 323 to the bering sea provide the first opportunity to obtain reconstructions from the region that extend back to the pliocene previous research at bowers ridge south bering sea has revealed stable levels of siliceous productivity over the onset of major northern hemisphere glaciation nhg c 2 85 2 73 ma however diatom silica isotope records of oxygen 18odiatom and silicon 30sidiatom presented here demonstrate that this interval was associated with a progressive increase in the supply of silicic acid to the region superimposed on shift to a more dynamic environment characterized by colder temperatures and increased sea ice this concluded at 2 58 ma with a sharp increase in diatom productivity further increases in photic zone nutrient availability and a permanent shift to colder sea surface conditions these transitions are suggested to reflect a gradually more intense nutrient leakage from the subarctic northwest pacific ocean with increases in productivity further aided by increased sea ice and wind driven mixing in the bering sea in suggesting a linkage in biogeochemical cycling between the south bering sea and subarctic northwest pacific ocean mainly via the kamchatka strait this work highlights the need to consider the inter connectivity of these two systems when future reconstructions are carried out in the region

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

role of panama uplift on oceanic freshwater balance comparison between planktic foraminiferal oxygen isotope records from the caribbean sea ocean drilling program odp site 999 and the equatorial east pacific odp site 851 suggests an increase in caribbean surface water salinity between 4 7 and 4 2 ma the modern atlantic pacific salinity contrast of about 1 became fully established at 4 2 ma as reflected by a 0 5 planktic foraminifera 18o enrichment in the caribbean sea this is interpreted as the result of restricted surface water exchange between the tropical atlantic and pacific in response to the shoaling of the central american seaway as a consequence the atlantic and pacific surface ocean circulation regime changed as did the freshwater balance between the major ocean basins simultaneous shifts in benthic carbon isotope records in the caribbean sea suggest an intensification in north atlantic thermohaline circulation these results indicate that the panamanian isthmus formation caused several new ocean atmosphere feedback mechanisms that have affected climate since the early pliocene