evidence from diatom bound nitrogen isotopes for subarctic pacific stratification during the last ice age and a link to north pacific denitrification changes higher diatom bound d 15 n 70 wt lower opal content and 1200 ppm lower biogenic barium taken together and with constraints on sediment accumulation rate these results suggest a reduced supply of nitrate to the surface due to stronger stratification of the upper water column of the bering sea during glacial times with more complete nitrate consumption resulting from continued iron supply through atmospheric deposition this finding extends the body of evidence for a pervasive link between cold climates and polar ocean stratification in addition we hypothesize that more complete nutrient consumption in the glacial age subarctic pacific contributed to the previously observed ice age reduction in suboxia and denitrification in the eastern tropical north pacific by lowering the nutrient content of the intermediate depth water formed in the subpolar north pacific in the deglacial interval of the bering sea record two apparent peaks in export productivity are associated with maxima in diatom bound and bulk sediment d 15 n the high d 15 n in these intervals may have resulted from greater surface nutrient consumption during this period however the synchroneity of the deglacial peaks in the bering sea with similar bulk sediment d 15 n changes in the eastern pacific margin and the presence of sediment lamination within the bering sea during the deposition of the productivity peaks raise the possibility that both regional and local denitrification worked to raise the d 15 n of the nitrate feeding bering sea surface waters at these times

diatom bound 15n 14n new support for enhanced nutrient consumption in the ice age subantarctic 1 diatom bound 15n 14n was used to reconstruct the glacial nutrient status of the subantarctic zone in the southern ocean down core records from both the pacific and indian sectors show 15n of 5 to 6 during the last glacial maximum and a decrease coincident with the glacial termination to values as low as 2 the effect of either diatom assemblage or physiological change on the diatom bound 15n 14n is unknown and cannot yet be ruled out as a possible explanation for the observed change however the consistency between indian and pacific sector records and with other paleoceanographic data suggests that the glacial interglacial difference in diatom bound 15n 14n was driven by higher consumption of nitrate in the subantarctic surface during the last ice age such a change in nutrient consumption may have resulted from atmospheric iron fertilization and or decreased glacial mixed layer depths associated with sea ice melting enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of co2 in the ice age atmosphere it also would have reduced the preformed nutrient content of the low latitude thermocline leading to decreases in low latitude productivity suboxia and denitrification

sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization in two contrasting regions of the ocean the equatorial pacific and the southern ocean the 15n of core top sediments were strongly related to no3 in surface waters with distance from the equator in the equatorial pacific 15n increased from 7 to 16 as no3 decreased from 8 m to 0 1 m going from 60 to 30 s in the se indian ocean core top 15n increased from 5 to 11 as surface no3 decreased from 25 m to 0 1 m these results are strong evidence that sedimentary 15n in these regions is recording the increasing isotopic enrichment of near surface no3 with its depletion by phytoplankton in the case of the equatorial pacific 15n values for sinking particles collected at 150 m matched well the core top sediment values demonstrating little diagenetic alteration of the near surface generated isotopic signal these equatorial pacific data sets have variations with near surface no3 consistent with rayleigh fractionation kinetics for a fractionation factor u of 2 5 this value is substantially lower than previously found for temperate or polar regions perhaps as a result of differences in phytoplankton species assemblage or growth condition in the southern ocean south of the polar front comparison of 15n values for opal rich sediments south and sinking particles indicates an apparent 5 diagenetic enrichment relative to the surface generated signal that requires further investigation this exception aside our observations show that the surface water relationship of increasing 15n with increasing no3 depletion is generally transmitted to and preserved in the sediments an important requirement for further development and application of this important paleoceanographic tool

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

last glacial maximum to holocene sea surface conditions at umnak plateau bering sea as inferred from diatom alkenone and stable isotope records 1 the bering sea gateway between the pacific and arctic oceans impacts global climate when glacial interglacial shifts in shore line position and ice coverage change regional albedo previous work has shown that during the last glacial termination and into the holocene sea level rises and sea ice coverage diminishes from perennial to absent yet existing work has not quantified sea ice duration or sea surface temperatures sst during this transition here we combine diatom assemblages with the first alkenone record from the bering sea to provide a semiquantitative record of sea ice duration sst and productivity change since the last glacial maximum lgm during the lgm diatom assemblages indicate that sea ice covered the southeastern bering sea perennially at 15 1 cal ka b p the diatom assemblage shifts to one more characteristic of seasonal sea ice and alkenones occur in the sediments in low concentrations deglaciation is characterized by laminated intervals with highly productive and diverse diatom assemblages and inferred high coccolithophorid production at 11 3 cal ka b p the diatom assemblage shifts from one dominated by sea ice species to one dominated by a warmer water north pacific species simultaneously the sst increases by 3 c and the southeastern bering sea becomes ice free year round productivity and temperature proxies are positively correlated with independently dated records from elsewhere in the bering sea the sea of okhotsk and the north pacific indicating that productivity and sst changes are coeval across the region

poleward decrease in the isotope effect of nitrate assimilation across the southern ocean recent studies provide seasonally and spatially resolved information on the isotopic characteristics of nitrate supply and n cycling in southern ocean surface waters the new data improve our understanding of the nitrate supply to the antarctic surface and its isotopic characteristics especially with regard to the summertime subsurface minimum temperature t min layer in the antarctic we use these findings to update and compile estimates of the n isotope effect of nitrate assimilation e in the southern ocean near australia a poleward decrease in e emerges from 8 9 in the subantarctic zone saz 40 52 s to similar to 5 in the polar antarctic zone paz 66 s e is strongly correlated with mixed layer depth at the time of sampling we hypothesize that the correlation is driven by the physiological response of diatoms to light availability with light limitation leading to higher cellular efflux of nitrate and thus higher e

late pleistocene stratigraphy and palaeoceanographic implications in northern bering sea slope sediments evidence from the radiolarian speciescycladophora davisiana a high resolution abundance curve of the radiolarian species cycladophora davisiana was obtained from a late pleistocene sediment core pc 23a which was obtained from the northern slope of the bering sea the abundance pattern of this species was high during the glacial period and low during the holocene this general stratigraphic pattern was correlated with a previously reported pattern for the subarctic pacific ocean however planktonic foraminifera based accelerator mass spectrometry 14c dating revealed that a significant peak in the c davisiana curve during the glacial period correlates to 15 5 ka which is younger than that previously estimated as being the last glacial maximum the higher abundance of this species implies that there was well ventilated water with a large amount of organic matter in the intermediate water layer an unusually high propagation of c davisiana during the earliest holocene period bolling allerod b a and dansgaad oescheger interstadial event 12 was most likely related to a significant increase in the amount of organic matter released into the intermediate water depths by a brine pump system via ventilation in addition centennial scale variations in the abundance of c davisiana during the b a period suggest that the intermediate water ventilation was sensitively affected by brine rejection during sea ice formation as well as large scale atmospheric circulations such as the arctic oscillation copyright 2009 john wiley sons ltd

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

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

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

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

biogenic opal production changes during the mid pleistocene transition in the bering sea iodp expedition 323 site u1343 abstract biogenic opal content and mass accumulation rate mar at iodp expedition 323 site u1343 were found to fluctuate consistently generally being high under warm conditions and low under cold conditions during the last 2 4 ma continuous wavelet transform analysis of the normalized biogenic opal content indicates that export production in the bering sea varied predominantly at 41 ka periodicity before 1 25 ma and shifted to 100 ka periodicity at the onset of the mid pleistocene transition mpt 1 25 0 7 ma the 100 ka cycles dominated until the holocene export production in the bering sea decreased markedly in the bering sea two times during the mpt the first occurred at the beginning of the mpt 1 25 ma and the second in the middle of the mpt 0 9 ma these decreases coincided with both increases in the relative abundance of sea ice diatoms and decreases in the warm water diatom species neodenticula seminae indicating that reductions in export production in the bering sea during the mpt were associated with climate cooling decreases in export production in the bering sea during the mpt were most likely associated with the increased influence of polar arctic domains on the high latitude north pacific

global change over the last climatic cycle from the vostok ice core record antarctica abstract the study of ice cores provides the opportunity of access to numerous parameters which are related to climate or environmental changes many of these variables are of regional or even global geographical significance there are now several ice cores which penetrate into ice which has formed at the surface of polar ice sheets during the last ice age and the recently obtained vostok antarctica record provides detailed information over the last climatic cycle 160 ka in comparison with current holocene conditions the last glacial maximum 20 ka bp is characterized by much colder conditions up to 10 c and reduced precipitation 1 2 a large increase of continental and marine aerosols is explained by a more vigorous large scale atmospheric circulation associated with changing continental deserts and shelves as well as changes in sea ice extent the drastic climatic change occurring at the last glacial termination is correlated with a large increase of atmospheric co 2 from about 200 to 270 ppmv the isotopic temperature record from the vostok ice core depicts two drastic glacial interglacial terminations around 15 and 140 ka bp and a long glacial period 110 15 ka bp which includes two interstadials full glacial stages are characterized by larger aerosol loadings the co 2 record of global significance is well correlated with the isotope temperature profile with high 270 ppmv during interglacials and low 200 ppmv during full glacial conditions spectral analysis of the yostok temperature record supports the existence of a relationship between the pleistocene climate and orbital forcing on the other hand the existence of a co 2 climate correlation suggests that co 2 changes have had an important climatic role in amplifying the relatively weak orbital forcing

satellite and hydrographic observations of the bering sea green belt abstract green belt is the aptly named region of high productivity occurring principally along and above the shelf slope boundary in the bering sea topex altimeter measurements of sea surface topography seawifs imagery of chlorophyll a concentration and shipboard measurements of salinity and fluorescence are used to describe the surface structure of the green belt and its relationship to the bering slope current eddy field during the 2000 2001 and 2002 spring blooms during spring 2000 high surface chlorophyll a concentrations 10 mg m 3 were observed within a 200 km wide band adjacent to and seaward of the shelf break in the northwest bering sea this high concentration chlorophyll band was associated with an anticyclonic eddy group that propagated along isobaths above the continental slope and entrained chlorophyll from the shelf slope front during spring 2001 anticyclonic eddies in the northwest bering sea had propagated off slope prior to the onset of the spring bloom and were too far from the shelf slope front to entrain frontal chlorophyll during the bloom a second chlorophyll front associated with the leading edge of the off slope eddies was observed between these two fronts was a region of relatively low chlorophyll a concentration 1 mg m 3 the eddy field during the 2002 spring bloom was observed to propagate northwestward adjacent to the shelf break and entrain chlorophyll from the shelf slope region in a manner similar to what was observed during the 2000 spring bloom these observations suggest that eddies are important if not the principal agents that cause variability in the distribution of chlorophyll during the spring bloom in the central bering sea

influence of bering strait flow and north atlantic circulation on glacial sea level changes sea level fluctuations of about 20 30m occurred throughout the last glacial period these fluctuations seem to have been derived primarily from changes in the volume of northern hemisphere ice sheets 1 3 and cannot be attributed solely to ice melt caused by varying solar radiation 4 here we use a fully coupled climate model to show that the transport of relatively fresh pacific water into the north atlantic ocean was limited when lower sea level restricted or closed the bering strait resulting in saltier north atlantic surface waters this invigorated deep convection in the north atlantic ocean strengthening meridional overturning circulation and northward heat transport in our model which consequently promoted melting of ice sheets in north america and europe our simulations show that the associated sea level rise led to a reopening of the bering strait the flux of relatively fresh water into the north atlantic ocean muted meridional overturning circulation and led to cooling and ice sheet advance in the northern hemisphere we conclude that the repetition of this cycle could produce the sea level changes that have been observed throughout the last glacial cycle

a 20 15 ka high resolution paleomagnetic secular variation record from black sea sediments no evidence for the hilina pali excursion abstract a comprehensive magnetostratigraphic investigation on sixteen sediment cores from the southeastern black sea yielded a very detailed high quality paleosecular variation psv record spanning from 20 to 15 ka the age models are based on radiocarbon dating stratigraphic correlation and tephrochronology further age constraints were obtained by correlating four meltwater events described from the western black sea ranging in age from about 17 to 15 ka with maxima in k ti ratios obtained from x ray fluorescence xrf scanning and minima in s ratios reflecting increased hematite content in the studied cores since the sedimentation rates in the investigated time window are up to 50 cm ka 1 the obtained psvs records enabled a stacking using 50 yr bins a directional anomaly at 18 5 ka associated with pronounced swings in inclination and declination as well as a low in relative paleointensity rpi is probably contemporaneous with the hilina pali excursion originally reported from hawaiian lava flows however virtual geomagnetic poles vgps calculated from black sea sediments are not located at latitudes lower than 60 n which denotes normal though pronounced secular variations during the postulated hilina pali excursion the vgps calculated from black sea data migrated clockwise only along the coasts of the arctic ocean from ne canada 20 0 ka via alaska 18 6 ka and ne siberia 18 0 ka to svalbard 17 0 ka then looping clockwise through the eastern arctic ocean

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

glacial to interglacial changes in surface nitrate utilization in the indian sector of the southern ocean as recorded by sediment δ15n we present a new approach for paleoceanographic reconstruction of surface nutrient utilization in the southern ocean it has been observed in the contemporary ocean that due to preferential uptake of 14no3 during photosynthesis the 15n of planktonic organic matter increases with increasing nitrate depletion in surface waters our results demonstrate that the 15n signal produced in surface waters is reflected in the underlying surface sediments core top 15n is inversely correlated with surface nitrate concentration along a transect across the subtropical convergence and the polar front in the southeast indian ocean these results are consistent with a four box model showing that the nitrogen isotopic composition of sinking organic matter depends on percent nitrate utilization in the euphotic zone by comparing the 15n of surface sediments with that measured in the glacial sections of several cores we infer changes in the intensity and latitudinal distribution of nitrate uptake in this region during the last glacial maximum these preliminary results argue against increased biological uptake of nutrients in southern polar waters as a major mechanism for glacial lowering of atmospheric co2 they also suggest that subantarctic waters in the se indian sector became more nutrient depleted as they migrated northward increased nitrate depletion might have also occurred slightly south of the glacial polar front we use a six box model to explore the possible impact of this observation on atmospheric co2

changes in biogenic opal productivity with milankovitch cycles during the last 1 3 ma at iodp expedition 323 sites u1341 u1343 and u1345 in the bering sea abstract sediments from three sites u1341 u1343 and u1345 of iodp expedition 323 located in the bering sea were analyzed for variation of biogenic opal representing surface water paleoproductivity initial age models of three sites bottom age u1341 4 3 ma u1343 2 4 ma u1345 0 5 ma were established onboard by biostratigraphic and paleomagnetic investigations the initial age models were tuned by 18 o stratigraphy which was established on shore biogenic opal contents of sediments were analyzed for the interval of 1 3 ma to the present generally the biogenic opal values at site u1341 bowers ridge were significantly higher than those at the beringian slope sites u1343 and u1345 higher natural gamma radiation ngr counts and magnetic susceptibility ms at the beringian slope sites suggested a greater contribution of lithogenic matter than at site u1341 on bowers ridge the percentages of sea ice diatoms at the beringian slope sites are also higher than those at bowers ridge sites this is primarily attributed to the fact that the sediments from the beringian slope area were significantly diluted by lithogenic matter derived from the alaskan continent and were largely affected by sea ice cover the cyclicity of biogenic opal accumulation rates at site u1341 is determined to be about 160 40 and 30 ky whereas that at site u1343 is determined to be about 330 100 40 and 20 ky if these had been reflecting a global climate change the cyclicity of biogenic opal contents at all sites would be consistently the same however the results do not show such a consistency obliquity is observed at both sites of the bowers ridge and beringian slope a strong eccentricity was observed only at u1343 but no signature for eccentricity was detected at u1341 therefore the milankovitch cycle with 100 ky periodicity was only observed at the beringian slope area the results suggest that the biogenic opal at site u1341 indicated a local characteristic for biogenic productivity changes thus the absence of the 100 ky cycle of biogenic opal at site u1341 seems to be due to local phenomena such as water circulation although the changes in biogenic opal productivity at site u1341 are attributed to summer insolation

land sea linkage of holocene paleoclimate on the southern bering continental shelf detailed diatom records within surface and core sediments from the southern bering continental shelf sbcs reveal that the holocene evolution of sea ice distribution is associated with low pressure patterns holocene sea ice distribution over the sbcs was mainly controlled by the location of the aleutian low the corresponding paleoceanographic and paleoclimate conditions can be divided into three stages 1 the early holocene before 7000 cal yr bp was characterized by extensive sea ice distribution under two low pressure cells which covered the western bering sea and the gulf of alaska respectively 2 between 3000 and 7000 cal yr bp the low pressure system over the gulf of alaska became weak causing total sea ice mass over the sbcs to retreat 3 in the past 3000 years prevailing southwesterly winds over the sbcs due to the developing aleutian low have reduced further sea ice cover on the sbcs these paleoclimatic changes were probably a response to enso variation the timings of water mass exchanges on the sbcs coincided with sea level change along the alaskan peninsula as a result subsequent morphologic alterations have also influenced the paleoceanographic condition of the sbcs the effect of the surface coastal water and bottom marine water on the sbcs intensified about 6000 cal yr bp when sea level increased

micro and macronutrients in the southeastern bering sea insight into iron replete and iron depleted regimes abstract surface transects and vertical profiles of macronutrients dissolved iron d fe and dissolved manganese d mn were investigated during august 2003 in the southeastern bering sea we observed iron limited hnlc surface waters in the deep basin of the bering sea 15 20 mol kg nitrate 0 07 nmol kg d fe and 1 0 nmol kg d mn nitrate limited iron replete surface waters over the shelf

evidence for a major change in silicon cycling in the subarctic north pacific at 2 73 ma the initiation of northern hemisphere glaciation in the subarctic north pacific at 2 73 ma was marked by an abrupt cessation of high opaline accumulation considered to result from an increased stratification of the water column that should have led to higher utilization of nutrients in the surface ocean we present a new stable si isotope based record of si utilization that is hard to reconcile with this model a drop in 30si 28si by 0 4 at 2 73 ma is coincident with an increase in bulk n isotope composition the contrasting utilization records cannot have been both caused by a hydrographic change alone excluding a change in the si n export ratio these results either imply a relative increase in silicic acid supplied to the surface waters or a change in its si isotope composition while it is impossible to distinguish between these two possibilities both imply a regional or global change in the si biogeochemical cycle potentially caused by an enhanced storage of si in the underlying deep waters of the pacific

changes in the bering sea region atmosphere ice water system in the second half of the twentieth century abstract we examine the long range variability in the oceanographic regime of the bering sea during the second half of the twentieth century using the largest t s data set available containing about 58 000 stations we focus on changes in the ice regime and atmospheric processes over the bering sea and western arctic significant differences in the physical state of the bering sea were found between a group of anomalous cold 1955 1964 1971 1972 1975 1976 1977 1984 1988 years and warm 1958 1959 1962 1963 1967 1978 1981 1982 1987 years the time spatial eof analyses product of the first components show a general cooling in the bering sea in 1953 1977 and 1978 1993 and a strong warming in 1977 1978 likewise the spatial distribution of eof temperature 2 4 components indicates opposite thermal states in the western and eastern part of the bering sea if it becomes warmer in the eastern sector it becomes cooler in the western interaction between the aleutian low and siberian high determines the change over the bering sea region

late pliocene to early pleistocene 2 4 1 25 ma paleoproductivity changes in the bering sea iodp expedition 323 hole u1343e abstract late pliocene to early pleistocene paleoproductivity changes in the bering sea were reconstructed using geochemical concentrations and mass accumulation rates mars of caco 3 biogenic opal and total organic carbon toc and sedimentary nitrogen isotope ratios 15 n at iodp expedition 323 hole u1343e drilled in the northern slope area 1956 m deep of the bering sea caco 3 concentration is generally low but prominent caco 3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation regardless of glacial interglacial variations biogenic opal concentrations reflect orbital scale glacial interglacial variations however toc concentration did not show clear glacial interglacial variation probably due to poor preservation the sedimentary 15 n values vary synchronously with biogenic opal concentration on orbital timescales the co varying pattern of opal productivity and 15 n values at hole u1343e is a result of nutrient utilization controlled by diatom productivity in the bering slope area where fe is not a limiting factor biogenic opal and toc mars showed a temporal shift at around 1 9 ma from a high productivity period under nutrient enriched conditions to a low productivity period under relatively nutrient depleted conditions high diatom productivity with low 15 n values before 1 9 ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system this productivity decrease at about 1 9 ma was also found in the southern bering sea site u1341 and may be related to global opal reorganization

glacial interglacial changes in nutrient supply and stratification in the western subarctic north pacific since the penultimate glacial maximum abstract in piston cores from the open subarctic pacific and the okhotsk sea diatom bound 15n 15ndb biogenic opal calcium carbonate and barium were measured from coretop to the previous glacial maximum mis 6 glacial intervals are generally characterized by high 15ndb 8 and low productivity whereas interglacial intervals have a lower 15ndb 5 7 6 3 and indicate high biogenic productivity these data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima consistent with stronger upper water column stratification throughout the subarctic region during colder intervals an early deglacial decline in 15ndb of 2 at 17 5 ka previously observed in the bering sea is found here in the open subarctic pacific record and arguably also in the okhotsk and a case can be made that a similar decrease in 15ndb occurred in both regions at the previous deglaciation as well the early deglacial 15ndb decrease best explained by a decrease in surface nutrient utilization appears synchronous with southern hemisphere associated deglacial changes and with the heinrich 1 event in the north atlantic this 15ndb decrease may signal the initial deglacial weakening in subarctic north pacific stratification and or a deglacial increase in shallow subsurface nitrate concentration if the former it would be the north pacific analogue to the increase in vertical exchange inferred for the southern ocean at the time of heinrich event 1 in either case the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron to nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to bolling allerod warming

diatom silicoflagellate and ebridian biostratigraphy and paleoceanography in iodp 323 hole u1343e at the bering slope site abstract integrated ocean drilling program iodp expedition 323 was dedicated to reconstruct the details of the pliocene pleistocene paleoenvironmental conditions of the bering sea in this study fossil diatom assemblages from hole u1343e were investigated to define diatom biostratigraphy and reveal paleoceanography of the bering slope region throughout pleistocene as the results four diatom zones with neogene north pacific diatom zone codes npd were determined from the neodenticula seminae zone npd 12 to the n koizumii zone npd 9 diatom biohorizons defined in this study closely correspond with onboard data but the distinct difference is recognized at the top of the n koizumii additional four biostratigraphic zones estimated by three silicoflagellate and one ebridian datums suggest the core sediment age younger than 2 5 ma the fluctuations of several paleoenvironmental diatom indicators show that cold conditions with sea ice existed throughout the duration of the interval studied as well as sea ice expansion occurred at ca 1 9 ma and ca 0 9 ma moreover increases of neritic diatom species reveal a drop in sea level neodenticula seminae is the diatom tracer of the aleutian current and hence the decrease of this taxon suggests decrease in inflow of the north pacific water mass although the sea levels must have been dropped during the cold time interval the consistent occurrences of n seminae indicate that the surface water circulation in the bering sea was significantly influenced by the alaskan stream at ca 1 9 ma on the other hand during the other cooling event at ca 0 9 ma which corresponds to the mid pleistocene transition mpt event an increase of neritic diatom species and decrease of n seminae were observed this suggests that coastal environmental conditions accompanied the sea level drop and that the inflow from the north pacific was weakened the increases of sea ice and neritic diatom species suggest that the sea ice and coastal conditions were developed after the cooling at 0 9 ma the ages at remarkable changes of silicoflagellate assemblages correspond to the ages of global climate shifts at ca 1 4 ma 1 25 ma and 0 8 0 9 ma the unusual alternate dominances of distephanus medianoctisol in glacial and dictyocha species in interglacial periods during the 1 25 0 8 ma interval suggest the north south migration of water masses between the bering sea and the subarctic north pacific

nitrogen isotope and productivity variations along the northeast pacific margin over the last 120 kyr surface and subsurface paleoceanography 1 glacial interglacial changes in sedimentary 15n over the last 120 kyr display a remarkably similar pattern in timing and amplitude in core records extending from the denitrification zone in the eastern tropical north pacific etnp where subsurface denitrification is active to the oregon margin where no denitrification occurs today low 15n values 4 6 generally characterize glacial stages 2 and 4 and higher 15n values 7 10 are representative of the holocene millennial scale periods within stage 3 and stage 5 the inferred synchroneity of 15n variations along the entire margin implies that the nitrate isotopic signal produced in the oxygen poor subsurface waters in the etnp is rapidly advected northward and recorded at sites far beyond the boundaries of the modern denitrification zone similar to 15n primary production indicators percent corg ba al and percent opal show glacial interglacial as well as millennial scale variations along the ne pacific margin with higher primary production during warm periods however the relative phasing between 15n and paleoproduction tracers within individual records changes latitudinally whereas 15n and primary production vary approximately synchronously in the midlatitudes production lags 15n in the etnp by several kiloyears this lag calls for a new understanding of the processes driving denitrification in the etnp we suggest that oxygen input by the equatorial undercurrent as well as local organic matter flux controls denitrification rates in the etnp moreover the differences in relative timing point to a time transgressive development of upwelling favorable winds along the ne pacific margin after the last glaciation with those in the north developing several kiloyears earlier

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

variability in bering sea ice cover as affected by a regime shift in the north pacific in the period 1947 1996 in the late 1970s a regime shift or step occurred in the climate of the north pacific causing among many other effects a 5 reduction in the ice cover in the eastern bering sea as well as shifts in the position of the aleutian low analyses of the aleutian low from monthly mean northern hemisphere sea level pressure for winters december march for 1947 1996 are presented and compared with monthly mean ice cover from the bering sea for 1952 1996 the southern oscillation index and the western pacific oscillation before the regime shift below normal ice cover in the bering sea was typically associated with el nino conditions which caused the aleutian low to move eastward of normal carrying warm pacific air over the bering sea conversely above normal ice cover was associated with la nina conditions which caused the aleutian low to move westward of normal allowing higher pressure to move over the bering sea since the regime shift during el ninos the aleutian low has been moving even farther east causing winds to blow from the east and north off alaska and resulting in above normal ice in the bering sea before the regime shift the occurrence of el nino and la nina conditions was about even since the regime shift el nino conditions are about 3 times more prevalent in recent work e g mantua et al 1997 minobe 1997 there is evidence that this regime shift is the latest in a series of climate shifts going back to at least the late 1800s and may be due to a 50 to 70 year oscillation in a north pacific atmospheric ocean coupled system

climatically linked oscillations in arabian sea denitrification over the past 1 m y implications for the marine n cycle water column and core top 15n data show that arabian sea denitrification produces large nitrogen isotopic enrichments that are regionally recorded with fidelity in the sediments these results facilitate interpretation of a 1 m y 15n record for ocean drilling program site 722b on the owen ridge in terms of climatically linked oscillations in denitrification at the major orbital periods as at present denitrification was greatest during interglacial periods and apparently was not active during most glacial intervals cross spectral analysis of 15n with foraminiferal 18o global climate sea level index and lithogenic grain size monsoon strength index suggests forcings by changes in hydrography and productivity acting through the extent and intensity of the oxygen minimum zone the data suggest that denitrification may be an internal forcing mechanism for climate change during major glacial interglacial transitions through influence on marine n inventory and atmospheric co2 however compensation or amplification may occur from other sinks sediment denitrification or sources nitrogen fixation

chlorofluorocarbon evidence for recent ventilation of the deep bering sea the pattern of the global thermohaline circulation of today s oceans is controlled by deep water formation at the northern and southern limits of the atlantic the apparent lack of deep water formation in the north pacific on the other hand suggests that this ocean plays only a minor role in the global circulation but it is unclear whether this was also the case during periods of glaciation there is conflicting evidence in the sedimentary record for the existence of a deep water source in the north pacific during the last glacial maximum1 4 here we report the detection of anthropogenic chlorofluorocarbons in the bottom waters of the aleutian basin in the eastern bering sea which suggests that a small amount of bottom water has formed in this region during the past 40 years although the small volumes of water involved are unlikely to play a significant role in determining present day global circulation patterns the results lend credence to the possibility that this sea was an important source of deep water for the northwestern pacific ocean during the last glacial maximum