glacial interglacial variablity in the benguela upwelling system spatial distribution and budgets of organic carbon accumulation 1 modern sedimentary total organic carbon toc content as a proxy for surface water export production was mapped on the shelf and on the upper continental slope of the benguela upwelling system using 137 core tops shelf maxima in toc can be correlated with maxima in surface water productivity on the slope high toc contents are observed offshore from sites of strong modern upwelling estimates of modern toc mass accumulation rates mar show that approximately 85 of the total is accumulating on the shelf toc mar were calculated mapped and budgeted for the holocene and for the last glacial maximum lgm using 19 sediment cores from the continental slope during the lgm centers of deposition and production have migrated offshore with respect to their holocene positions toc accumulation on the continental slope was approximately 84 higher during the lgm than during the holocene possibly reflecting enhanced productivity the toc distribution patterns and sediment echo sounding data suggest that undercurrents strongly influence the sedimentation off namibia winnowing and focusing result in great lateral heterogeneity of sedimentation rates and sediment properties individual cores therefore do not necessarily reflect general changes in export production these results highlight the need for detailed regional studies based on a large number of sediment cores for highly heterogeneous high productivity areas in order to derive general statements on total fluxes

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

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

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

thermal maturation and hydrocarbon migration within la popa basin northeastern mexico with implications for other salt structures some outcrops adjacent to the 25 km 15 mi long salt weld in the la popa basin northeastern mexico are visibly stained and contain enrichment of organic carbon their presence which is known only along the northwestern part of the weld and along the southwestern side of the weld provides evidence for hydrocarbon migration along the weld this distribution is presumed to be partially controlled by the restricted distribution of the source rock that generated the hydrocarbons within the salt withdrawal basin southwest of the weld but may have been in part controlled by a previously existing salt wall or the weld that formed as a result of salt evacuation acting as a barrier to lateral migration and serving as a vertical conduit for fluid flow upturned halokinetic strata adjacent to the weld provided a conduit for migration along bedding planes although no flowing seeps were seen during this study we documented discoloration and remineralization as well as the presence of waxy appearing soil that smelled like crude oil at some sites near the weld organic geochemical analyses confirm the presence of nonbiodegraded hydrocarbons along the salt sediment interface and within adjacent upturned lithologies up to 5 m 16 ft southwest of the weld the results document for the first time the definitive presence of actively migrating hydrocarbons associated with a subaerial salt weld an oil source rock correlation is inferred with the upper mudstone member of the potrerillos formation based on biomarker data thermal and subsidence modeling of the basin indicate that all of the strata in the basin are mature enough to have generated hydrocarbons the lower part of the stratigraphic section entered the gas generation window and much of the cretaceous section and all of the tertiary entered the oil window prior to uplift of the basin significant unroofing as much as 7 km 4 3 mi of sediment may have been removed occurred sometime after the eocene as suggested by previous studies

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

lateral transport controls distribution quality and burial of organic matter along continental slopes in high productivity areas in this study we demonstrate the relevance of lateral particle transport in nepheloid layers for organic carbon oc accumulation and burial across high productive continental margins we present geochemical data from surface sediments and suspended particles in the bottom nepheloid layer bnl from the most productive coastal upwelling area of the modern ocean the benguela upwelling system offshore southwest africa interpretation of depositional patterns and comparison of downslope trends in oc content organic matter composition and 14 c age between suspended particles and surface sediments indicate that lateral particle transport is the primary mechanism controlling supply and burial of oc we propose that effective seaward particle transport primarily along the bnl is a key process that promotes and maintains local high sedimentation rates ultimately causing high preservation of oc in a depocenter on the upper slope offshore namibia as lateral transport efficiently displaces areas of enhanced oc burial from maximum production at highly productive continental margins vertical particle flux models do not sufficiently explain the relationship between primary production and shallow marine oc burial on geologic time scales the widest distribution and strongest intensity of lateral particle transport is expected during periods of rapid sea level change at times in the geologic past widespread downslope lateral transport of oc thus may have been a primary driver of enhanced oc burial at deeper continental slopes and abyssal basins

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

origins and variability of the benguela current a subset of the benguela sources and transports best 1992 1993 data is analyzed to study the magnitude and variability of the large scale transports in the area the data consist of inverted echo sounder series and conductivity temperature depth stations the mean 16 month transport values for the upper 1000 m indicate that of the 13 sv northward transport within the benguela current 50 is derived from the central atlantic which from geometry may be chiefly south atlantic water 25 comes from the indian ocean which may be chiefly agulhas water and the remaining 25 may be a blend of agulhas and tropical atlantic water a simple schematic of the transport pattern with a somewhat restricted corridor for agulhas eddies translation is envisioned to the west of the eddy corridor flows the south atlantic source for the benguela current to the east is the agulhas indian ocean source the corridor is breached by south atlantic and indian water as the transient eddy field stirs these water masses

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

late quaternary sea ice history in the indian sector of the southern ocean as recorded by diatom assemblages abstract a modern analog technique mat5201 31 has been applied to fossil diatom assemblages to provide down core estimates of february sea surface temperatures ssts and of sea ice duration over the past 220 000 years at 56 40 s 160 14 e at the core location sea ice progression lagged the sst drop by 1 ka at interglacial glacial transitions and sea ice retreat was almost synchronous to the sst increase at glacial interglacial terminations sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times although ssts were almost constant during glacials this indicates that ssts are the major parameter determining the advance and retreat of sea ice at transitions but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo air temperature and meridional wind stress the strong correlation r 0 75 between sea ice duration at the core location and the vostok co2 record argues for a control of antarctic sea ice extent on atmospheric co2 concentration via the modification of the ocean to atmosphere gas balance

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

accumulation of biogenic and lithogenic material in the pacific sector of the southern ocean during the past 40 000 years the 230th normalization technique was applied to surface sediments and down core records from the pacific sector of the southern ocean between 120 w and 170 w in order to reconstruct vertical rain rates corrected for lateral redistribution of sediment preserved rain rates of opal organic carbon and biogenic ba all indicate that biological productivity south of the antarctic polar front apf was much lower than today during the last glacial maximum lgm probably in response to increased ice cover however by analogy with modern opal fluxes south of the acc we show that year round ice cover is not a prerequisite for very low rates of opal accumulation available records from north of the apf indicate little to no change in opal export in contrast to records from the indian and atlantic sectors which show enhanced opal export north of the apf during the lgm glacial records from north of 66 s in the pacific all show lithogenic fluxes that were greater than today but significantly lower than lithogenic fluxes in the glacial atlantic and indian sectors together these new records from the pacific sector show that during the lgm the three basins of the southern ocean were significantly more differentiated than today the observed inter basin distribution of lithogenic fluxes and opal burial and the presumed geographical distribution of increased dust input to the southern ocean during the lgm are consistent with an atlantic indian pacific gradient in the degree of iron stimulated diatom productivity within the zone lying between the apf and the subantarctic front previous studies which focused primarily on the atlantic sector may have overestimated the glacial increase in the strength of the biological pump in the southern ocean

using silicon isotopes to understand the role of the southern ocean in modern and ancient biogeochemistry and climate abstract the growth of siliceous phytoplankton mainly diatoms in the southern ocean influences the preformed nutrient inventory in the ocean on a global scale silicic acid use by diatoms and deep circulation combine to trap dissolved si in the southern ocean resulting in high levels of silica production and expansive diatom oozes in southern ocean sediments the analysis of the silicon isotope composition of biogenic silica or opal and dissolved silicic acid provide insight into the operation of the global marine silicon cycle and the role played by the southern ocean in nutrient supply and carbon drawdown both in the modern and in the past silicon isotope studies of diatoms have provided insight into the history of silica production in surface waters while the analysis of spicules from deep sea sponges has defined both the spatial and the temporal variability of silicic acid concentrations in the water column together these and other proxies reveal variations in the northward flow of southern ocean intermediate and mode waters and how changes in southern ocean productivity altered their preformed nutrient content we present a new hypothesis the silicic acid ventilation hypothesis savh to explain the geographical variation of opal based proxy records in particular the contrasting patterns of opal burial change found in the low and high latitudes by understanding the silicon isotope systematics of opal and silicic acid in the modern we will be able to use opal based proxies to reconstruct past changes in the southern ocean and so investigate its role in global carbon cycling and climate

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

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

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

a displaced neritic diatom delphineis karstenii in pelagic sediments of the southeast atlantic this paper documents the occurrence ofdelphineis karstenii a neritic diatom in pelagic sediments of the northern cape and southern angola basins southeast atlantic ocean maximum abundances are found in the southern angola basin although the source area appears to be the diatomaceous sediments which blanket much of the namibian continental shelf the observed pattern implies large scale displacement ofd karstenii valves first northward along the continental margin and then westward into deep waters transport evidently occurs in the surface waters of the benguela current which flow northward along the namibian coast and then turn westward away from the coast of angola and out into the angola basin these observations suggest thatdelphineis valves and perhaps other shallow water diatom valves may be displaced downslope on other continental margins failure to recognize such displacement could lead to incorrect ecological and paleoecological interpretations

the interplay between the surface and bottom water environment within the benguela upwelling system over the last 70 ka the benguela upwelling system bus located between 30 and 20 s is one of the fundamental high productivity systems of the world ocean the bus has previously been studied in terms of primary productivity and ecology over glacial interglacial timescales however the response and coupling with the benthic environment have received little attention here for the first time we present a high resolution reconstruction of the bus highlighting the link between surface and benthic productivity and their response to climatic and oceanographic changes over the last 70 ka the study is based on benthic foraminiferal faunal analysis together with analyses of diatom assemblages grain size of the terrigenous fraction and stable o and c isotopic and bulk biogenic components of core geob3606 1 we reveal significant shifts in benthic foraminiferal assemblage composition tight coupling existed between the surface and bottom water environment especially throughout marine isotope stages 4 and 3 mis4 and mis3 due to the high export production the site has essentially experienced continuous low oxygen conditions however there are time periods where the hypoxic conditions were even more notable two of these severe hypoxic periods were during parts of mis4 and mis3 where we find an inverse relationship between diatom and benthic foraminifera accumulation meaning that during times of extremely high phytodetritus export we note strongly suppressed benthic productivity we also stress the importance of food source for the benthos throughout the record shifts in export productivity are attributed not only to upwelling intensity and filament front position but also regional global climatic and oceanographic changes had significant impact on the bus dynamics

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

silicic acid leakage from the southern ocean opposing effects of nutrient uptake and oceanic circulation 1 studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of southern ocean intermediate waters comparison of 30si records from the southern ocean to records of biogenic opal accumulation from low latitude upwelling systems indicates a strong negative correlation between the silicic acid content of southern ocean surface waters and opal deposition at low latitudes during the last 80 kyr bp this implies that si leakage was somehow rendered ineffective when the silicic acid content of southern ocean waters was high comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods this indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of antarctic surface waters in determining the magnitude of silicic acid leakage from the southern ocean

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

diatom response to oceanographic and climatic changes in the congo fan area equatorial atlantic ocean during the last 190ka bp changes in siliceous productivity in the eastern equatorial atlantic ocean off the western african margin over the last several glacial cycles have been either related to global to regional oceanographic changes upwelling intensity or climate changes precipitation and river discharge based on diatom assemblages in core kzai 02 located to the south of the mouth of the congo river integrated with a selection of geochemical proxies we show that siliceous productivity in the southeastern angola basin responded to non linear interactions between both oceanographic and climate changes over the last 190 000 years high diatom accumulation rates were recorded in the middle part of mis 6 in cold substage mis 5d and in mis 3 2 during these intervals high diatom productivity was sustained essentially by nutrients including dissolved silica injected by the congo river into the ocean the highest productivity was observed during mis 3 when nutrients were sourced both from the river and regional upwelling low diatom accumulation rates were recorded during early and late mis 6 mis 5e early mis 4 and during the holocene these resulted either from low river discharge and overall low nutrient stocks in the angola basin despite evidence for upwelling or from extremely high river discharge in the case of the latter the terrigenous load drastically increased the turbidity of the surface waters in the southeastern angola basin and lowered phytoplankton productivity despite the presence of sufficient dissolved silica

biogenic opal in southern ocean sediments over the last 450 000 years implications for surface water chemistry and circulation we present records of biogenic opal percentage and burial rate in 12 piston cores from the atlantic and indian sectors of the southern ocean these records provide a detailed quantitative description of changing patterns of opal deposition over the last 450 kyr the striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the southern ocean under different global climate conditions three broad depositional patterns can be distinguished northernmost records 39 42 s latitude are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity records from cores just north of the present antarctic polar front 46 49 s show even larger increases in opal burial rate during glacial intervals but have variance concentrated in the 100 and 23 kyr bands southernmost records 51 55 s are completely out of phase with those to the north with greatly reduced opal burial rates during glacial periods taken as a whole the opal records show no evidence for the increased total antarctic productivity predicted by recent geochemical models of atmospheric co2 variability the areal expansion of southern ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial interglacial opal patterns the excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region however local circulation changes may act to modify this basic signal possibly accounting for the observed differences in the opal variance spectra

asynchronous alkenone and foraminifera records from the benguela upwelling system abstract radiocarbon stratigraphy is an essential tool for high resolution paleoceanographic studies age models based on radiocarbon ages of foraminifera are commonly applied to a wide range of geochemical studies including the investigation of temporal leads and lags the critical assumption is that temporal coupling between foraminifera and other sediment constituents including specific molecular organic compounds biomarkers of marine phytoplankton e g alkenones is maintained in the sediments to test this critical assumption in the benguela upwelling area we have determined radiocarbon ages of total c37 c39 alkenones in 20 samples from two gravity cores and three multicorer cores the cores were retrieved from the continental shelf and slope off namibia and samples were taken from holocene deglacial and last glacial maximum core sections the alkenone radiocarbon ages were compared to those of planktic foraminifera total organic carbon fatty acids and fine grained carbonates from the same samples interestingly the ages of alkenones were 1000 to 4500 yr older than those of foraminifera in all samples such age differences may be the result of different processes bioturbation associated with grain size effects lateral advection of recycled material and redeposition of sediment on upper continental slopes due to currents or tidal movement are examples for such processes based on the results of this study the age offsets between foraminifera and alkenones in sediments from the upper continental slope off namibia most probably do not result from particle selective bioturbation processes resuspension of organic particles in response to tidal movement of bottom waters with velocities up to 25 cm s recorded near the core sites is the more likely explanation our results imply that age control established using radiocarbon measurements of foraminifera may be inadequate for the interpretation of alkenone based proxy data observed temporal leads and lags between foraminifera based data and data derived from alkenone measurements may therefore be secondary signals i e the result of processes associated with particle settling and biological activity

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

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