monsoon related variations in zaire congo sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial atlantic during the last 200 000 years two sediment cores from the zaire fan and the angola margin have been investigated for their composition of terrigenous and biogenic constituents respectively for the late quaternary kaolinite feldspar ratios and variations of terrigenous element ratios of zr ti k rb and al reveal that the composition of zaire river sediment load has fluctuated in tune with precessional variations of boreal summer insolation in particular the correspondence of high kaolinite feldspar and al k ratios with low latitude insolation maxima strongly corroborates the assumption that west african monsoonal precipitation and chemical weathering was enhanced during periods of increased central african heating the most striking feauture observed is that opal accumulation has been 2 to 10 times higher in zaire fan sediments than outside in continental margin sediments off angola although biogenic ba and corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the zaire river plume and off angola from this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the zaire river plume than in the oceanic upwelling regime farther to the south off angola over the last 200 000 years the pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the zaire river to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold more arid glacial climate conditions we presume that paleoproductivity off the zaire was controlled by the following two sources of nutrients 1 marine nutrients including nitrate and phosphate as well as the uptake of ba on particulate corg caused by upwelling and 2 silica mainly delivered by the zaire river dissolved silicate hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical atlantic at a millenial timescale

glacial valve size variation of the southern ocean diatom fragilariopsis kerguelensis preserved in the benguela upwelling system southeastern atlantic abstract the valve size variations of the southern ocean so diatom fragilariopsis kerguelensis from the sediments of antarctic polar front and subantarctic front were shown to be strongly correlated with iron availability at the glacial interglacial timescale however recent evidence from the subtropical region of the so has suggested sea surface temperature to be a key controlling factor by mediating silica uptake other than the iron availability we present here the first record of f kerguelensis size variation in core geob3606 1 collected in the benguela upwelling system bus se atlantic 25 28 s 13 05 e where f kerguelensis valves were preserved during the glacial period 68 30 ka as a consequence of silicic acid leakage from polar to mid latitudes our study presents the first record of diatom morphology variations of f kerguelensis from a mid latitude core location to test the previous hypotheses proposed for so our results demonstrate that f kerguelensis valves at geob3606 1 were almost half of the mean size of specimens compared to the so downcore records further f kerguelensis valves were smaller and more abundant during early marine isotope stage mis 3 and conversely larger and less abundant during mis4 and late mis3 however such patterns are not supported by the linear regression analyses suggesting that iron availability could have mediated the size changes of f kerguelensis rather than the species productivity during 68 30 ka these findings agree with the recently gained knowledge on the life cycle patterns of f kerguelensis related to reproduction strategies cell size range and formation of initial cells

a corollary to the silicic acid leakage hypothesis 1 the silicic acid leakage hypothesis salh attempts to explain part of the large and regular atmospheric co2 changes over the last glacial interglacial cycles it calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low latitude surface waters driven by a leakage of high si n waters from the southern ocean recent studies that present low opal accumulation rates from the glacial eastern equatorial pacific have challenged salh in a corollary to salh we argue that the key to salh is the dominance of diatoms over coccolithophorids and this does not depend on the magnitude of diatom production per se in support of our claim we show in a numerical model that atmospheric co2 can be lowered with even a reduced absolute flux of silicic acid leakage provided that si n in the leakage is elevated and that the excess si can be used by diatoms to shift the floral composition in their favor

the benguela current and associated upwelling on the southwest african margin a synthesis of the neogene quaternary sedimentary record at dsdp sites 362 and 532

late quaternary productivity changes from offshore southeastern australia a biomarker approach reconstructions of primary productivity at low latitudes have been the focus of several studies to better understand how the export of nutrient rich intermediate southern ocean so waters influences productivity at these latitudes this was triggered by the general observation of minima in the planktonic foraminiferal delta c 13 values during deglaciations which was interpreted as an isotopic signal of intermediate so waters together with a concomitant increase in diatom productivity at some equatorial sites however the impact of these so waters on productivity at higher latitudes is not well constrained here we compare a high resolution planktonic foraminiferal delta c 13 record with total organic carbon and biomarker records for proboscia diatoms and haptophytes in marine sediments from offshore southeastern australia the planktonic foraminiferal delta c 13 record shows distinct minima during deglaciations and the marine isotope stage 4 3 transition tentatively suggesting that c 13 depleted so waters reached the coast of southeastern australia however it did not result in increased productivity during these periods instead the highest primary productivity period as indicated by total organic carbon and alkenone accumulation rates occurred during the last glacial maximum while proboscia diatoms mainly proliferated during interglacials and marine isotope stage 3 matching periods of increased diatom productivity in some sites of the eastern equatorial pacific our study suggests that increased primary productivity offshore southeastern australia was mainly due to stronger westerly winds during glacial periods while proboscia diatom productivity was probably controlled by the transport of silicic acid to this area

reduced north atlantic deep water flux to the glacial southern ocean inferred from neodymium isotope ratios the global circulation of the oceans and the atmosphere transports heat around the earth broecker and denton1 suggested that changes in the global ocean circulation might have triggered or enhanced the glacial interglacial cycles but proxy data for past circulation taken from sediment cores in the south atlantic ocean have yielded conflicting interpretations of ocean circulation in glacial times 13c variations in benthic foraminifera2 3 4 5 6 support the idea of a glacial weakening or shutdown of north atlantic deep water production whereas other proxies such as cd ca ba ca and 231pa 230th ratios show little change from the last glacial maximum to the holocene epoch7 8 9 here we report neodymium isotope ratios from the dispersed fe mn oxide component of two southeast atlantic sediment cores both cores show variations that tend towards north atlantic signatures during the warm marine isotope stages 1 and 3 whereas for the full glacial stages 2 and 4 they are closer to pacific ocean signatures we conclude that the export of north atlantic deep water to the southern ocean has resembled present day conditions during the warm climate intervals but was reduced during the cold stages an increase in biological productivity may explain the various proxy data during the times of reduced north atlantic deep water export

the effect of rayleigh taylor instabilities on the thickness of undifferentiated crust on kuiper belt objects abstract previous calculations of the internal structure and thermal evolution of kuiper belt objects kbos by desch et al desch s j cook j c doggett t c porter s b 2009 icarus 202 694 714 have predicted that kbos should only partially differentiate with rock and ice separating into a rocky core and icy mantle below an undifferentiated crust of ice and rock this crust is thermally insulating and enhances the ability of subsurface liquid to persist within kbos a dense rock ice layer resting on an icy mantle is gravitationally unstable and prone to rayleigh taylor rt instabilities and may potentially overturn here we calculate the ability of rt instabilities to act in kbos and determine the thickness of undifferentiated crusts we have used previously calculated growth rates of the rt instability to determine the critical viscosity of ice needed for the rt instability to operate we calculate the viscosity of ice at the cold temperatures and long timescales relevant to kbos we find that crustal overturn is only possible where the temperature exceeds about 150 k and that rt instabilities cannot act on geological timescales within about 60 km of the surfaces of a kbo like charon although this crustal thickness is less than the 85 km previously calculated by desch et al desch s j cook j c doggett t c porter s b 2009 icarus 202 694 714 it is still significant representing 25 of the mass of the kbo we conclude that while rt instabilities may act in kbos they do not completely overturn their crusts we calculate that saturn s moon rhea should only partially differentiate resulting in a moment of inertia c mr 2 0 38

submillennial to millennial variability of diatom production off mauritania nw africa during the last glacial cycle 1 the low latitude upwelling regime off the mauritanian coast in the subtropical ne atlantic accounts for a significant part of global export production although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity productivity dynamics off mauritania are less straightforward because of the complex atmospheric and hydrographic setting here we integrate micropaleontological diatoms and geochemical bulk biogenic sediment components x ray fluorescence and alkenones proxies to examine on submillennial to millennial changes in diatom production that occurred off mauritania nw africa for the last 25 ka during the last glacial maximum lgm 19 0 23 0 ka b p moderate silicate content of upwelled waters coupled with weakened ne trade winds determined moderate diatom productivity no significant cooling is observed during the lgm suggesting that our alkenone based sst reconstruction represents a local upwelling related signal rather than a global insolation related one extraordinary increases in diatom and opal concentrations during heinrich event 1 h1 15 5 18 0 ka b p and the younger dryas yd 13 5 11 5 ka b p are attributed to enhanced upwelling of silica rich waters and an enlarged upwelling filament due to more intense ne trade winds the synchronous increase of caco3 and k intensity and the decreased opal and diatoms values mark the occurrence of the bolling allerod ba 13 5 15 5 ka b p due to weakened eolian input and more humid conditions on land although the high export of diatoms is inextricably linked to upwelling intensity off mauritania variability in the nutrient content of the thermocline also plays a decisive role

rapid palaeoceanographic changes in the benguela upwelling system for the last 160 000 years as indicated by abundances of planktonic foraminifera abstract two sediment cores retrieved from the continental slope in the benguela upwelling system geob 1706 19 33 7 s 11 10 5 e and geob 1711 23 18 9 s 12 22 6 e reveal striking variations in planktonic foraminiferal abundances during the last 160 000 years these fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off namibia four species make up over 95 of the variation within the core and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages the fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment the cold water planktonic foraminifer neogloboquadrina pachyderma sinistral n pachyderma s dominates the modern day coastal upwelling centres and neogloboquadrina pachyderma dextral and globigerina bulloides characterize the fringes of the upwelling cells globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional reduced nutrient levels of the subtropical waters in the fossil record episodes of high n pachyderma s abundances are interpreted as evidence of increased upwelling intensity and the associated increase in nutrients the n pachyderma s record suggests temporal shifts in the intensity of upwelling and corresponding trophic domains that do not follow the typical glacial interglacial pattern periods of high n pachyderma s abundance describe rapid discrete events dominating isotope stages 3 and 2 the timing of these events correlates to the temporal shifts of the angola benguela front jansen et al 1997 situated to the north of the walvis ridge absence of high abundances of n pachyderma s from the continental slope of the southern cape basin indicates that southern ocean surface water advection has not exerted a major influence on the benguela current system the coincidence of increased upwelling intensity with the movement of the angola benguela front can be interpreted mainly by changes in strength and zonality of the trade wind system

fossil hyrax dung and evidence of late pleistocene and holocene vegetation types in the namib desert abstract pollen was derived from fossil dung of herbivorous hyraxes deposited in a rock shelteron the highest mountain in namibia da ures or brandberg situated on the namib desert margin radiocarbon dating ranging in age between modern times and 30000yrbp showed it representsthe rst empirical pollen evidence of continental palaeovegetation during the late pleistocene alongthe western escarpment of southern africa the initial results indicate last glacial maximum vegeta tion differed totally from the current pattern as vegetation types were dominated by small asteraceaeshrubs in contrast to those of the holocene and modern times which show more succulents grassandwoody elements arboreal pollen the resultssuggest that cape oral communities didnot reachinto thetropics along thewestern escarpment ofafrica despite such pollentypes occurring inmarinecores copyright 2004 john wiley sons ltd keywords palynology namibia restionaceae late pleistocene last glacial maximum

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

200 000 yr diatom records from atlantic upwelling sites reveal maximum productivity during lgm and a shift in phytoplankton community structure at 185 000 yr abstract time series of diatom accumulation rates dar and assemblage composition for the last 200 000 yr from the atlantic major coastal upwelling loci of the eastern boundary currents and eastern equator are compared together with independent proxies of export production to upper ocean productivity and to nutrient concentration and utilization fluctuations in dar are concomitant at all sites and in good correspondence to the other independent productivity proxies indicating glacial periods as the most productive times of the past 200 000 yr in both hemispheres of the eastern atlantic maximum productivity about one order of magnitude higher than at present occurred at the last glacial maximum lgm despite the atlantic similarities in both coastal and equatorial upwelling types important regional and site specific variations emerge in the southern hemisphere site a contradiction of proxies diatoms and corg is noted at the base of stage 6 185 000 yr the good correlation found between the corg and the benthic foraminifera fluxes is interpreted as a reflection of a shift in the phytoplankton community structure related to either a change in the n p si ratio or similar nutrient conditions associated with increased water column stratification

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

silicic acid leakage from the southern ocean a possible explanation for glacial atmospheric pco2 1 using a simple box model we investigate the effects of a reduced si n uptake ratio by antarctic phytoplankton on the marine silica cycle and atmospheric pco2 recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added hutchins and bruland 1998 takeda 1998 franck et al 2000 the southern ocean may have supported diatoms with reduced si n uptake ratios compared to today during the dustier glacial times petit et al 1999 a similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as phaeocystis our model shows that reduced si n export ratios in the southern ocean create excess silicic acid which may then be leaked out to lower latitudes any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes caco3 production and therefore diminishes the carbonate pump in our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric co2 from the interglacial 277 to 230 242 ppm depending on where the excess silicic acid is consumed by comparison the atmospheric pco2 sensitivity of general circulation models to carbonate pump forcing is 3 5 fold greater which combined with carbonate compensation can account for peak glacial atmospheric pco2 we discuss the importance of the initial rain ratio of caco3 to organic carbon on atmospheric pco2 and relevant sedimentary records that support and constrain this silicic acid leakage scenario

antarctic sea ice and the control of pleistocene climate instability a hypothesis is presented for the origin of pleistocene climate instability based on expansion of antarctic sea ice and associated changes in the oceans salinity structure the hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of toggweiler and samuels 1993b in which deepwater upwelling is restricted to high southern latitudes the reconfigured conveyor is shown to be potentially stabilized in an on mode by precipitation at high southern latitudes and potentially destabilized into on and off modes by the counteracting influence of antarctic sea ice the mechanism is clarified by the use of a hydraulic analogue we hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between pleistocene warm and cold periods the hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods including aspects of interhemispheric timing

on the reconstruction of upwelling history namibia upwelling in context abstract eastern boundary upwelling is an important aspect of the modern ocean despite the relatively small area involved consequences of increased upwelling during the neogene as a result of intensifying trade winds include increased heat transport by the ocean increased mixing increased fractionation of phosphate from silicate and opportunities for evolution of new species coastal upwelling has been studied since the first decade of the 20th century the processes involved are complex and differ depending on geographic setting off namibia upwelling history has been studied by a number of drilling expeditions the last of these was odp leg 175 which occupied seven sites on the continental slope between walvis ridge and cape town during september and october of 1997 productivity proxies all along southwestern africa suggest the presence of increased upwelling and organic matter supply to the sea floor during glacial periods at the same time there is a decrease in the supply of diatoms and other siliceous plankton remains this is the walvis opal paradox established through the contrasting results of studies by diester haass 1985 and oberhansli 1991 we propose that the walvis opal paradox is fundamental to the understanding of glacial interglacial productivity fluctuations on a global scale furthermore a central feature of the history of late neogene upwelling off namibia is the matuyama opal maximum centered between gauss and olduvai magnetic chrons wefer et al proc odp 175 1998 it is due to the fact that diatom supply first increases during the gauss when the planet cools and then decreases again during additional cooling on entering the quaternary on a 400 000 year scale peak productions are coincident with or slightly lag maximum seasonal contrast potential in the high latitude insolation curve we suggest that this is further evidence that the nutrient content of thermocline waters was diminished during glacial periods the reasons why this should be so remain to be discovered

diatom distribution in southern ocean surface sediments atlantic sector implications for paleoenvironmental reconstructions abstract a study of 230 surface sediment samples collected in the atlantic sector of the southern ocean between the southernmost weddell sea and the subtropical zone documents the modern distribution of diatoms revealing patterns of paleoenvironmental significance estimations of diatom valves per gram dry sediment display numbers of 50 200 106 in the zone of high opal burial located between the mean position of the winter sea ice edge and the polar front and maximum values of greater than 200 106 in the near shore sedimentary basins off the antarctic peninsula lowest diatom concentrations and assemblages strongly affected by dissolution were encountered in the weddell basin despite alteration of the diatom assemblages prior to their incorporation into the sediment record the biogeographic distribution and the abundance pattern of most of the 35 studied diatom species shows a close relationship with the surface hydrography water temperature these relationships can be used to estimate past surface water temperatures based on statistical treatments of the assemblages or on simple relations of species occurrences in the geological record another close link occurs between the distribution of sea ice and sea ice related diatoms

the biogeography of major diatom taxa in southern ocean sediments 2 open ocean related species diatoms from 228 southern ocean core top sediment samples were examined to determine the geographic distributions of 32 major diatom species taxa preserved in the sediments of three zonally distinct regions sea ice open ocean and the tropical subtropical in the first of three papers 14 species taxa occurring in the region where sea ice covers the ocean surface on an annual basis are geographically documented comparisons are drawn between the diatom abundances on the sea floor sea ice parameters annual duration and concentration in february and september and february sea surface temperature such parameters are commonly used in reconstructing past oceanographic conditions in the sea ice and open ocean zones analysis of the geographic patterns and sea surface parameter correlations reveals species specific distributions regulated primarily by sea ice coverage and sea surface temperature which support the use of diatom remains for the estimation of these past seasurface environmental parameters comparison with reliable accounts of the 14 species from the sediments or plankton also provides the first glimpses into species specific ecology and habitat linkages d 2005 elsevier b v all rights reserved

testing the silica leakage hypothesis with sedimentary opal records from the eastern equatorial pacific over the last 150 kyrs 1 we have measured 230th normalized opal fluxes in several cores from the eastern equatorial pacific eep to test the validity of the silica leakage hypothesis which purports that redistribution of silicic acid from the southern ocean to the low latitudes was responsible for a significant portion of the reduction in atmospheric carbon dioxide co2 during the last glacial period the silica leakage hypothesis predicts higher opal fluxes in the eep and lower opal fluxes in the southern ocean during periods of low atmospheric co2 these predictions are not borne by the sedimentary record during glacial oxygen isotope stage 2 ois 2 13 27 kyrs b p however we find a prominent opal flux maximum in the eep in the middle of ois 3 ca 40 60 kyrs bp coinciding with low opal fluxes in several cores from the subantarctic zone this observation is consistent with silica leakage from the southern ocean to the equatorial upwelling region during ois 3 when both low dust flux and extended sea ice could have contributed to limiting diatom productivity in the southern ocean since this event is not associated with a clear minimum in the vostok ice record of co2 its impact on atmospheric co2 appears to be small

millennial scale instability of the antarctic ice sheet during the last glaciation records of ice rafted detritus ird concentration in deep sea cores from the southeast atlantic ocean reveal millennial scale pulses of ird delivery between 20 000 and 74 000 years ago prominent ird layers correlate across the polar frontal zone suggesting episodes of antarctic ice sheet instability carbon isotopes 13 c of benthic foraminifers a proxy of deepwater circulation reveal that south atlantic ird events coincided with strong increases in north atlantic deep water nadw production and inferred warming interstadials in the high latitude north atlantic sea level rise or increased nadw production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the weddell sea region of west antarctica

miocene history of the benguela current and antarctic ice volumes evidence from rhythmic sedimentation and current growth across the walvis ridge deep sea drilling project sites 362 and 532 the history of the northward growth and of the fluctuations of the benguela current from the middle miocene ca 14 ma to the uppermost late miocene ca 5 ma has been reconstructed from analyses of the coarse and clay fractions and geochemical analyses of the organic matter in sediments from deep sea drilling project sites 362 and 532 in the middle miocene the benguela current had not yet reached the walvis ridge and consequently no local upwelling occurred carbonate preservation was good during this period of low productivity rhythmic changes related to antarctic ice volumes were weak leading to slightly higher organic matter concentrations and higher montmorillonite illite m i ratios in sediments deposited during times of greater continental ice montmorillonite was carried north from the orange river by the benguela current whereas illite originated from the nearby namib desert in the early late miocene the benguela current reached the walvis ridge in colder periods and led to well developed rhythmic increases in opal which was transported from near coastal upwelling areas organic matter concentration carbonate dissolution and m i ratios were also enhanced during periods of greater ice volume during the late miocene farther northward migration of the benguela current led to a change in the uppermost miocene sediments opal contents and m i ratios were high during times of low ice volume rather than in colder times during the cold periods local winds from the namib desert increased the illite supply to the walvis ridge and depressed the m i ratio in these periods the benguela current flowed farther to the north reaching the angola basin as it did in quaternary glacial periods throughout these changes sediment organic carbon content was always enhanced in high ice volume periods from near bottom downslope transport of organic matter from shelf regions during regressions and carbonate dissolution in the sediments was increased as a result of oxidation of this organic matter

the early matuyama diatom maximum off sw africa a conceptual model abstract an important discovery during ocean drilling program leg 175 when investigating the record of upwelling off namibia was the finding of a distinct late pliocene diatom maximum spanning the lower half of the matuyama reversed polarity chron mdm matuyama diatom maximum and centered around 2 6 2 0 ma this maximum was observed at all sites off southwestern africa between 20 s and 30 s and is most strongly represented in sediments of site 1084 off luderitz namibia the mdm is characterized by high biogenic opal content high numbers of diatom valves and a diatom flora rich in southern ocean representatives with thalassiothrix antarctica forming diatom mats as well as coastal upwelling components before mdm time diatoms are rare until ca 3 6 ma after the mdm in the pleistocene the composition of the diatom flora points to increased importance of coastal upwelling toward the present but is accompanied by a general decrease in opal and diatom deposition here we present a simple conceptual model as a first step in formalizing a possible forcing mechanism responsible for the record of opal deposition in the upwelling system off namibia the model takes into account southern ocean oceanography and a link with deepwater circulation and deepwater nutrient chemistry which in turn are coupled to the evolution of north atlantic deep water nadw the model proposes that between the mdm and the mid pleistocene climate revolution opal deposition off namibia is not directly tied to glacial interglacial fluctuations as seen in the global 18o record but that instead a strong deepwater link exists with increased nadw production as seen in the deepwater 13c record accounting for higher supply of silicate to the thermocline waters that feed the upwelling process the opal record of site 1084 shows affinity to eccentricity on the 400 kyr scale but not for the 100 kyr scale this points toward long term geologic processes for delivery of silica to the ocean

a 350 ky radiolarian record off luderitz namibia evidence for changes in the upwelling regime abstract the study of radiolarian assemblages from core md 962086 provides new information on the variability in the upwelling intensity and origin of upwelled water masses over the past 350 ky in one of the major filamentous regions of the benguela upwelling system bus located off luderitz namibia the use of key radiolarian species to trace the source of upwelled waters and the use of a radiolarian based upwelling index uri to reconstruct the upwelling intensity represent the first use of radiolarians for paleoceanographic reconstructions in the bus these radiolarian based proxies indicate strongest upwelling during marine isotope stages mis 3 5 and 8 which compares well with other studies while during mis 3 and 8 the radiolarian based proxies indicate the influx of waters of southern ocean origin they also point to the increased influence of tropical waters during the lower portion of mis 5 during mis 2 4 and 6 the radiolarian assemblages indicate generally lower upwelling intensities although this signal is complicated by the increased occurrence of organic carbon in the sediments during these intervals during mis 2 there appears to be less of an input of southern ocean waters to the bus although during the also glacial mis 4 and 6 there is evidence for an increased influence of cold antarctic waters the comparison of the results from core md 962086 with other studies in the bus area indicates a non uniform pattern of upwelling intensity and advection of cold southern waters into this system during mis 2 weaker upwelling signaled by the radiolarian based proxy in mis 4 is in contrast to other studies that indicate higher productivity during this time period in general the data show that there is a strong spatiotemporal complexity in upwelling intensity in the bus and that the advection of water into it is not strongly tied to glacial interglacial variations in climate

elemental and organic carbon measurements with a two step heating gas chromatography system in snow samples from the tibetan plateau in this study an optimized two step heating gas chromatography system is used to measure elemental carbon ec and organic carbon oc content in snow and ice with the ability to quantify the elemental and organic carbon species in a snow or ice sample of 60 80 g in this system oc and ec are transformed into co2 in a stream of oxygen at 3408c and 6508c respectively the resulting co2 is accumulated in two molecular sieve traps and then put into a gas chromatograph equipped with a flame ionization detector by heating the traps to 2008c in a helium stream background contamination mainly caused by impurities in the oxygen stream and accuracy are dominated by the variability of the blank loads on the pre cleaned filters which are 0 50 0 04 1 mgc for oc and 0 38 0 04 1 mgc for ec the system is suitable for snow and ice sample measurements with the same precision as shown for the blank tests ec and oc concentrations have been measured in snow samples collected from different glaciers on the tibetan plateau the results allow quantification for the first time of the different carbonaceous particle contents on the tibetan plateau and other regions the concentrations of ec and oc particles in snow show a clearly decreasing trend from east to west and from north to south on the plateau excluding the pamirs region the highest mean ec content 79 2 ng g 1 was found in the northeast region and the lowest 4 3 ng g 1 was found in the western himalaya we note that even slight surface melting results in fresh snow getting dirtier especially in regions with higher pollution such as seen on a glacier in the qilian shan here the ec and oc concentrations in the fresh snow average 6 6 and 87 5 ng g 1 but after 2 days of surface melting they increased to 52 6 and 195 5 ng g 1 this suggests that surface snow melting can reduce snow albedo due to the accumulation of carbonaceous particles

oceanographic control of biogenic opal and diatoms in surface sediments of the southwestern atlantic based on 66 surface sediment samples collected in the sw atlantic ocean between 27 and 50 s this study presents an overview of the spatial distribution of biogenic opal and diatom concentrations and diatom assemblages biogenic opal has highest values in the deepest pelagic stations and decreases toward the slope diatoms closely follow the spatial trend of opal diatom assemblages reflect the present day dominant hydrographical features antarctic diatoms are the main contributors to the preserved diatom community in core top sediments with coastal planktonic and tropical subtropical diatoms as secondary components dominance of antarctic diatoms between 35 and 50 s in the pelagic realm mirrors the northward displacement of antarctic source water masses characterized by high nutrient content and low salinity northward of ca 35 s the highest contribution of tropical subtropical pelagic diatoms typical for nutrient poor and high salinity waters matches the main southward path of the brazil current mixing of antarctic and tropical waters down up to 45 s is clearly illustrated by the diatom assemblage concentrations of biogenic opal and diatoms rather reflect the path of predominant water masses but are less correlated with surface water productivity in the sw atlantic

changes in style and intensity of production in the southeastern atlantic over the last 70 000 yr abstract accomplishing reliable paleo reconstructions of productivity and upwelling conditions in eastern boundary current systems requires the use of cores collected in a basin wide spatial pattern based on diatom assemblage analysis and the concentration and the bulk biogenic components of three gravity cores recovered from the benguela upwelling system bus between 19 and 25 s i describe rapid paleoceanographic changes that occurred during the last 70 ka b p in the southeastern atlantic the pattern of biogenic production and accumulation differs to varying degrees among the three core sites along the sw african coast the highest sedimentation and accumulation rates at 25 s off luderitz conform with the present day well known pattern of highest productivity and most intense coastal upwelling highest diatom values at 25 s during mis3 points to more intense upwelling due to the combination of strong seaward extending upwelling filaments shoaling of the upwelled water and the influence of silicate rich waters of antarctic origin productivity decreased along the central bus throughout mis2 when the siliceous calcareous productivity regime shifted toward a system dominated by calcareous producers although intensity and strength of winds created adequate conditions for upwelling during mis2 diatom production decreased the complete replacement of the upwelling associated diatom flora by a non upwelling related diatom community during mis1 reflects weakened upwelling weakened seaward extension of the upwelling filaments and dominance of warmer surface waters combining changes in the composition of the diatom assemblage and variations of the bulk biogenic components allows for reliable reconstruction of paleoproductivity and upwelling changes for the se atlantic during the last 70 ka b p

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

oscillations of the siliceous imprint in the central benguela upwelling system from mis 3 through to the early holocene the influence of the southern ocean based on a high resolution analysis of the diatom signal and biogenic bulk compo nents at site geob3606 1 25 s off namibia we describe rapid palaeoceanographic changes in the benguela upwelling system bus from early mis 3 through to the early holocene 55 000 to 7 000 14 c yr bp coastal upwelling strongly varied at 25 s from mis 3 through to mis 2 the abrupt decrease in the accumulation rate of biogenic silica and diatoms from mis 3 into mis 2 records rapid oceanographic changes in the bus off namibia during mis 3 leakage of excess h4sio4 acid from the southern ocean into low latitude surface waters as indicated by the occurrence of antarctic dia toms enhanced the production of spores of chaetoceros at the expense of calcareous phytoplankton furthermore shallower antarctic intermediate water aaiw would have enriched the thermocline off namibia with silicate transported from the southern ocean the strong decrease of the siliceous signal throughout mis 2 represents a decrease in the nutrient input to the bus even though the dia tom assemblage is still dominated by spores of the upwelling associated diatom genus chaetoceros depletion of silicate in the thermocline from the onset of mis 2 through to the early holocene reflects the shutdown of aaiw injection from the southern ocean into the bus causing upwelled waters to become reduced in silicate hence less favourable for diatom production the deglaciation and early holocene are characterised by the replacement of the upwelling associated flora by a non upwel ling related diatom community reflecting weakened upwelling retraction of the seaward extension of the chlorophyll filament off luderitz and dominance of warmer waters copyright 2003 john wiley sons ltd

glacial interglacial variations in marine phosphorus cycling implications for ocean productivity 1 using a box model of organic carbon c and phosphorus p cycling in the global ocean we assess the effects of changes in the continental supply of reactive p oceanic mixing and sea level on the marine p cycle on glacial interglacial timescales our model results suggest that mixing is a dominant forcing during early glaciation causing retention of p in the deep ocean thereby lowering primary production and associated organic c and p burial sea level fall is the dominant forcing during late glaciation when reduced coastal trapping of reactive p enhances its transfer to the open ocean restoring primary production and p burial during postglacial periods changes in circulation and weathering dominate open ocean processes and oceanic primary production peaks as primary production is reduced upon glaciation changes in the marine p cycle are unlikely to drive enhanced primary production and co2 drawdown during glaciation

the biogeography of major diatom taxa in southern ocean surface sediments 3 tropical subtropical species abstract this paper gives a modern circumscription of tropical subtropical diatoms regarding their relationship with sea surface temperatures sst and sea ice cover diatoms from 228 core top sediment samples collected from the southern ocean were studied to determine the geographic distribution of eight major diatom species taxa preserved in surface sediments generally located north of the subantarctic front the comparison of the relative contribution of diatom species with modern february sst and sea ice cover reveals species specific sedimentary distributions regulated both by water temperatures and sea ice conditions although selective preservation might have played some role their presence in surface and downcore sediments from the southern ocean are reliable indicators of high sst and poleward transport of waters from the tropical subtropical atlantic our work supports the use of diatom remains to reconstruct past variations of these environmental parameters via qualitative and transfer function approaches

differences between mono generic and mixed diatom silicon isotope compositions trace present and past nutrient utilisation off peru in this study we combine for the first time silicon si isotope compositions of small mixed diatom species 30sibsio2 and of large handpicked mono generic i e genus coscinodiscus diatom samples 30sicoscino with diatom assemblages extracted from marine sediments in the peruvian upwelling region in order to constrain present and past silicate utilisation the extension of a previous core top data set from the peruvian shelf demonstrates that 30sicoscino values record near complete si utilisation as these are similar to the isotopic composition of the subsurface source waters feeding the upwelling in contrast the 30sibsio2 of small mixed diatom species increase southward along the shelf as well as towards the shore we attribute highest 30sibsio2 values partly to transient iron limitation but primarily to the gradual increase of si isotope fractionation within the seasonal diatom succession which are mainly recorded by small diatom species during intense bloom events in contrast lower 30sibsio2 values are related to initial si isotope utilisation during periods of weak upwelling when low si oh 4 concentrations do not permit intense blooms and small diatom species record substantially lower 30si signatures as such we propose that the intensity of the upwelling can be deduced from the offset between 30sibsio2 and 30sicoscino 30sicoscino bsio2 which is low for strong upwelling conditions and high for prevailing weak upwelling we apply the information extracted from surface sediments to generate a record of the present day main upwelling region covering the past 17 700 years and find that this location has also been characterized by a persistent offset 30sicoscino bsio2 by comparison with the diatom assemblages we show that the coastal upwelling system changed markedly between weak and strong upwelling conditions in addition our model calculations to quantify species specific si isotope fractionation effects based on the diatom assemblages indicate an overall minor influence that cannot explain the high amplitude in the measured 30sibsio2 record

a record of past ocean atmosphere radiocarbon differences from the northeast pacific a record of the 14c age differences between atmosphere and the mixed layer of the northeast pacific ocean was obtained from 58 radiocarbon measurements on shells and associated wood from coastal sediments in british columbia this series of mixed layer reservoir ages spans the holocene and extends into the period of the younger dryas cold event in the north atlantic and surrounding regions the data reflect the extent of upwelling in the northeast pacific and thus constrain models of carbon cycling and ocean circulation most of the reservoir ages are only slightly less than than the present preindustrial value of about 800 years and may be identical when probable sampling errors are taken into account either the north pacific reached a state close to its present mode of circulation at or before the end of the younger dryas or a different circulation fortuitously produced a similar reservoir age our results support the hypothesis that slow changes in atmospheric radiocarbon concentrations over the holocene arose from 14c production rate changes data corresponding to a radiocarbon age of around 6400 years b p give a reservoir age of 1200 years if this result is confirmed it indicates a much greater variability for the holocene ocean circulation than was previously believed possible

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

the reconstruction of late quaternary antarctic sea ice distribution the use of diatoms as a proxy for sea ice the study of annual diatom transfer from the southern ocean surface to the sea floor using time series sediment traps combined with the mapping of diatom assemblages from surface sediments results in the definition of a proxy for past variations of sea ice extent the application and robustness of the proposed proxy are demonstrated by the study of late pleistocene sediment cores recovered from different environmental regimes in the southern ocean relative amounts of the sea ice indicator diatoms fragilariopsis cylindrus and fragilariopsis curta of 3 in diatom assemblages preserved in sediments are representative of the presence of seasonal winter sea ice summer sea ice occurrence is signalled by a distinct drop in biogenic sedimentation rate as a result of strongly reduced biogenic export in the presence 3 of diatom assemblage of the deep temperature diatom fragilariopsis obliquecostata the spatial and temporal distribution pattern of the sea ice proxy indicates significant sea ice variations during marine isotope stages 1 and 5 in the present antarctic zone of the southern ocean during glacial maxima the antarctic winter sea ice was extended into the present polar front zone while the antarctic summer sea ice was most probably expanded into the area of the present winter sea ice edge during the short climatic optima at the onset of interglacial periods the winter sea ice extent was distinctly reduced