production and destruction control of biogenous sedimentation in the tropical atlantic 0 300 000 years b p late pleistocene signals of calcium carbonate organic carbon and opaline silica concentration and accumulation are documented in a series of cores from a zonal meridional depth transect in the equatorial atlantic ocean to reconstruct the regional sedimentary history spectral analysis reveals that maxima and minima in biogenous sedimentation occur with glacial interglacial cyclicity as a function of both 1 primary production at the sea surface modulated by orbitally forced variation in trade wind zonality and 2 destruction at the seafloor by variation in the chemical character of advected intermediate and deep water from high latitudes modulated by high latitude ice volume from these results a pattern emerges in which the relative proportion of signal variance from the productivity signal centered on the precessional 23 kyr band decreases while that of the destruction signal centered on the obliquity 41 kyr and eccentricity 100 kyr periods increases below 3600 m ocean depth

can the aluminum and titanium contents of ocean sediments be used to determine the paleoproductivity of the oceans in the central equatorial pacific settling particles collected with sediment traps have al ti values greater than those of a typical crustal source the al ti values of the sediment trap material are not well correlated with either the flux of organic carbon or with total particle mass suggesting that this ratio cannot be used as a simple index of paleoproductivity however the al ti values of some trap samples are more than 3 times the ratio for typical crustal materials indicating a noncrustal source of excess al numerical analysis using linear programming indicates that the excess al is associated with opal and that this biological component carries as much as 1200 ppm of al because of the low particulate fluxes of lithogenic material in the equatorial pacific and the relatively high opal content of equatorial sediment trap material opal appears to be the predominant conveyor of al to the equatorial sediments our sediment trap al ti values are similar to ratios previously measured in equatorial sediments suggesting that measurements of this element pair in ocean sediments may be an indicator of opal rain to the seafloor the particle reactive nature of al and ti means their ratios in sediments may be relatively insensitive to opal preservation or other diagenetic processes however the al ti values of raining particles are highly sensitive to the relative proportion of opal and lithogenic material where opal lithogenic ratios are less than 20 the al ti values are indistinguishable from a crustal source ratio in addition the amount of excess al associated with opal in settling particles may be a function of the concentration of al in the water column and of the processes that fractionate dissolved al and ti these issues must be evaluated before the al ti values of ocean sediments can be used as a reliable proxy of opal flux to the seafloor

a 300 kyr high resolution aridity record of the north african continent new oxygen isotope data and a high resolution carbonate record of a core from the northwest african margin are presented in combination with geochemical zr rb ratio al ba organic carbon and biogenic opal fluxes and sedimentological median grain size parameters to show variations in african continental aridity wind strength and upwelling productivity the carbonate record is diluted by the supply of aeolian dust from the arid saharan region of north africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines spectral analysis of the carbonate profile and comparison with specmap stacked oxygen isotope and etp solar radiation profiles shows that the precessional 23 kyr insolation induced african climate system is decoupled from global ice volume climate changes operating on a 100 kyr frequency detailed studies of glacial terminations i ii and iii confirm the presence of rapid short timescale climatic variations in aridity

thorium 230 profiling in deep sea sediments high resolution records of flux and dissolution of carbonate in the equatorial atlantic during the last 24 000 years variations in carbonate flux and dissolution which occurred in the equatorial atlantic during the last 24 000 years have been estimated by a new approach that allows the point by point determination of paleofluxes to the seafloor an unprecedented time resolution can thus be obtained which allows sequencing of the relatively rapid events occurring during deglaciation the method is based on observations that the flux of unsupported 230th into deep sea sediments is nearly independent of the total mass flux and is close to the production rate thus excess 230th activity in sediments can be used as a reference against which fluxes of other sedimentary components can be estimated the study was conducted at two sites ceara rise western equatorial atlantic and sierra leone rise eastern equatorial atlantic in cores raised from three different depths at each site from measurements of 230th and caco3 changes in carbonate flux with time and depth were obtained a rapid increase in carbonate production starting at the onset of deglaciation was found in both areas this event may have important implications for the postglacial increase in atmospheric co2 by increasing the global carbonate carbon to organic carbon rain ratio and decreasing the alkalinity of surface waters and possibly the north atlantic deep water increased carbonate dissolution occurred in the two regions during deglaciation followed by a minimum during mid holocene and renewed intensification of dissolution in late holocene during the last 16 000 years carbonate dissolution was consistently more pronounced in the western than in the eastern basin reflecting the influence of antarctic bottom water in the west this trend was reversed during stage 2 possibly due to the accumulation of metabolic co2 below the level of the romanche fracture zone in the eastern basin

palaeoclimatological and chronological implications of the vostok core dust record the2 083 m vostok ice core recovered by the soviet antarctic expeditions has provided much information of climatic and environmental interest for a period covering a full glacial inter glacial cycle1 6 here we present and discuss the dust record obtained down to 2 202 m the final depth to which this core was extended in 1986 first we document the fact that major changes in aeolian deposits as recorded in the vostok core appear to be of global significance and confirm the existence of a link between high latitude aeolian deposits and the earth s orbital parameters7 8 second we propose atmospheric dust as a strati graphic marker to compare timing with other records of palaeocli mate and use the magnetic susceptibility profile measured along the rc11 120 indian ocean core9 for this purpose assuming that major dust events correspond to common aeolian inputs this approach indicates that the vostok and marine records were roughly in phase at the previous glacial interglacial transition

response of the tropical atlantic thermocline to late quaternary trade wind changes we applied the modern analog technique to foraminiferal counts from three equatorial atlantic gravity cores to estimate paleothermocline depths in the eastern and western tropical atlantic during the past 250 kyr the eastern equatorial core geob 1105 4 is characterized by variations in the 23 kyr cycle whereas the western sediment cores show a dominant periodicity at the 41 kyr band the phase relationships with respect to insolation and ice volume suggest that thermocline fluctuations in the east are primarily driven by the intensity of the monsoon due to variations in low latitude insolation in contrast changes of the thermocline depth in the western tropical atlantic which lead global ice volume at the 41 kyr band by 3 4 kyr seem to depend on the strength of the se trade winds in response to the meridional temperature gradients in the south atlantic

the early diagenesis of iron in pelagic sediments a multidisciplinary approach abstract biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial atlantic are investigated by means of pore water chemistry chemical leaching experiments and total elemental determinations color reflectance spectroscopy rock magnetic measurements and tem observations of the magnetic fraction results indicate that in the presence of nitrate ascorbate a weak reducing agent leachable iron decreased with depth from the sediment surface within this upper sediment region iron assimilation by bacteria is indicated as magnetosomes were found by tem observations throughout the whole core extractions with a strong reducing agent dithionite representing iron bound to iron oxides correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone the abrupt decrease of red color reflection intensity relative to the amount of iron oxides below the iron redox boundary is the result of a decrease in the specific surface area of iron oxyhydroxide oxides magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary arising from an increase in biogenic magnetite formed by magnetotactic assimilatory bacteria dissolution of very fine grained magnetite and the gradual decrease of coarser grained terrigenous titano magnetite with depth the early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content

late quaternary changes of western equatorial atlantic surface circulation and amazon lowland climate recorded in ceara rise deep sea sediments today the western tropical atlantic is the most important passage for cross equatorial transfer of heat in the form of warm surface water flowing from the south into the north atlantic circulation changes north of south america may thus have influenced the global thermohaline circulation system and high northern latitude climate here we reconstruct late quaternary variations of western equatorial atlantic surface circulation and amazon lowland climate obtained from a multiproxy sediment record from ceara rise variations in the illite smectite ratio suggest drier climatic conditions in the amazon basin during glacials relative to interglacials the 230thex normalized fluxes and the 13c 12c record of organic carbon indicate that sea level fluctuations shelf topography and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the ceara rise we attribute variations in thermocline depth reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species florisphaera profunda to changes in southeast trade wind intensity strong trade winds during ice volume maxima are associated with a deep western tropical atlantic thermocline strengthening of the north brazil current retroflection and more vigorous eastward flow of surface waters

glacial and interglacial wind regimes over the eastern subtropical atlantic and north west africa analysis of aeolo marine dust deposits in the subtropical eastern atlantic enables the strength of the major wind patterns during the late quaternary to be evaluated and gives an insight into the climate of north africa

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

late quaternary productivity changes in the western equatorial atlantic evidence from 230th normalized carbonate and organic carbon accumulation rates paleoproductivity changes in the western equatorial atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180 000 yr from a ceara rise sediment core geob 1523 1 accumulation rates were calculated by normalizing to excess 230th activity paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters results indicate minor productivity changes varying around 30 gc m 2 yr 1 for organic carbon derived estimations and around 45 gc m 2 yr 1 for carbonate derived estimations suggesting that the study area remained a low productivity region throughout the time period examined maxima in western atlantic paleoproductivity occurred during the warm substages of glacials and interglacials in contrast to the eastern tropical atlantic where maxima are recorded in the cold substages this contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east the carbonate preservation record was examined for the last 380 000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction only minor carbonate dissolution above 3300 m water depth is observed except for three main dissolution events at isotope stages 4 8 4 and 10 considerable carbonate loss during these intervals is attributed to a decreased production of north atlantic deep water which is associated with vertical expansion of southern source deep water

quaternary fluctuations in the northern hemisphere trade winds and westerlies the mass accumulation rate and grain size of the total colian component isolated from pelagic sediment in two north pacific cores piston core kk75 02 under the prevailing westerlies and deep sea drilling project site 503 beneath the trade winds have been used to evaluate changes in the intensity of atmospheric circulation and source area aridity over the past 700 000 yr the eolian grain size a direct indicator of wind intensity fluctuates at periodicities similar to those calculated for the earth s orbital parameters of precession obliquity and eccentricity both sites display greater variability in wind intensity prior to 250 000 yr ago eolian accumulation rates an indicator of source area aridity fluctuate at periodicities similar to those of the glacial interglacial cycles lower eolian accumulation rates during colder glacial times most likely reflect increased glacial age humidity at central asian and central american source areas

orinoco and amazon river sediment input to the eastern caribbean basin abstract carbonate and mineralogic analyses of piston cores from aves ridge eastern caribbean basin reveal climatically modulated relationship among quaternary carbonate deposition and terrigenous sediment influxes from the orinoco and amazon rivers low carbonate content and dark sediment color extending into the pleistocene suggest considerable dilution of pelagic components by terrigenous sediment on the western flank of aves ridge which is characterized by a southwest thickening sedimentary wedge contrary to typical trends observed in the more distal areas of the central basin west of aves ridge carbonate content is lowest during interglacial climates and highest during glacial climates a high clay mineral input correlates with interglacial climate however the orinoco tracer mineral pyrophyllite is low during interglacials and high during glacials the observed trends suggest that amazon sediment influx to the eastern caribbean is highest during interglacials or times of high sea level the amazon influx is promoted by deposition on the shelf and westward coastal transport of sediment during high sea level and reduced by shelf edge deposition and loss to the amazon canyon during low sea level the orinoco influx is similarly modulated but it undergoes a relative and possibly absolute increase during low sea level because of proximity to the depositional site the terrigenous dilution produces the reversal of carbonate trends however total sediment accumulation rates on southernmost aves ridge remain higher during glacials possibly as a result of increased carbonate production caused by upwelling

origin of increased terrigenous supply to the ne south american continental margin during heinrich stadial 1 and the younger dryas we investigate the redistribution of terrigenous materials in the northeastern ne south american continental margin during slowdown events of the atlantic meridional overturning circulation amoc the compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical atlantic shows an extreme rise in sedimentation rates off the parnaiba river mouth about 2 s during heinrich stadial 1 hs1 18 15 ka sediment core geob16206 1 raised offshore the parnaiba river mouth documents relatively constant 143nd 144nd values expressed as end 0 throughout the last 30 ka whereas the homogeneous end 0 data support the input of fluvial sediments by the parnaiba river from the same source area directly onshore the increases in fe ca al si and rb sr during hs1 indicate a marked intensification of fluvial erosion in the parnaiba river drainage basin in contrast the end 0 values from sediment core geob16224 1 collected off french guiana about 7 n suggest amazon sourced materials within the last 30 ka we attribute the extremely high volume of terrigenous sediments deposited offshore the parnaiba river mouth during hs1 to i an enhanced precipitation in the catchment region and ii a reduced north brazil current which are both associated with a weakened amoc

chemical accumulation variations under the peru current during the past 130 000 years the titanium to aluminum ratio in core v19 29 is correlated with aluminosilicate accumulation rates this correlation may be due to pleistocene eolian transport fluctuations which alter the mean grain size of sedimented eolian material the relation between aluminum accumulation rate and ti al established from accumulation rates integrated over 11 000 50 000 year intervals can be inverted to compute a high resolution record of aluminosilicate and calcium carbonate accumulation rates over the past 130 000 years carbonate accumulation rates are closely related to the oxygen isotope record in the core with a phase lag and damping constant that is compatible with the response time shown to be only 6000 years of calcium carbonate in the ocean carbonate sedimentation at this site responds to several processes independently correlated with climatic change the relative importance of these processes for carbonate sedimentation at this site can be constrained by the record in this core and other lines of evidence 15 of the increased carbonate deposition at this site during glacial periods may be due to diminished nadw north atlantic deep water formation 10 is due to carbonate productivity decreases in the north atlantic 25 may be due to a diminished shallow sea carbonate sink and the residual 50 must be due to a local productivity increase these assignments are consistent with observations on carbonate paleoceanography in the north atlantic aluminosilicate accumulation rate variations correlate with the record of eolian quartz deposition near northwest africa and in a general way with the climatic record but in detail the record differs substantially from the oxygen isotope record and may provide independent evidence on the nature of climate dynamics

significance of the sedimentary al ti ratio as an indicator for variations in the circulation patterns of the equatorial north atlantic this paper presents results of geochemical investigations conducted on three gravity cores recovered along an e w transect 3 5 n in the equatorial atlantic considering the high terrigenous content of the sediments approximately 12 74 wt al and ti concentrations are predominantly associated with the input of lithogenous phases the terrigenous fraction of western equatorial atlantic sediments is more influenced by material derived from the amazon river than by deposition of african dust particles as a possible route of transport the north equatorial countercurrent is considered fluctuations in al ti records generally reflect changes in the low latitude insolation cycle 19 23 kyr and thus indicate a close coupling to variations of the trade wind systems and continental climate changes however characteristics of the terrigenous signals in the eastern and western equatorial atlantic differ significantly while the precessional signal in the al ti record and low latitude portion in the 18o record are in phase in the eastern equatorial atlantic variations in the terrigenous fraction westward of the mid atlantic ridge precede the marine signal by 2 3 kyr which we ascribe to a shift between trade wind systems of the northern and southern hemispheres

scavenged excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial pacific ocean we present results from chemical analyses of al and ti in surface sediment sampled along two cross equator latitudinal transects at 135w and 140w in the central equatorial pacific ocean although traditionally both al and ti are considered to reside exclusively within terrigenous phases in marine sediment these sediments present extremely high al ti ratios that are several times that of average shale and other potential crustal sources a sharp maximum in al ti is observed slightly south of the equator where sedimentary bulk accumulation rate bar is also highest reflecting elevated productivity in the overlying water caused by surface water divergence bulk al ti decreases sharply away 2 latitude to near crustal values at 4 north and south the latitudinal profiles of al ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of al and ti these results indicate the presence of a significant scavenged component of al sourced directly from seawater during particle settling the data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135w transect calculations of alexcess indicate that the highest al ti ratios correspond to 50 of the total al being unsupported by the small amount of terrigenous phases present these results are consistent with previous and ongoing studies of biogenic sediment suspended particulate matter and sediment trap material quantitative use of al as an index of terrigenous material may therefore lead to an overestimation by a factor of two of the true terrigenous load in marine sediment sedimentary rock and settling particles because bulk al ti appears to respond to sedimentary bar which in biogenic regimes is linked to surface water productivity downcore records of al ti in biogenic sediment may track productivity changes through time such al ti proxy records may be applicable in sediment of all ages unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr

regional and global benthic δ18o stacks for the last glacial cycle although detailed age models exist for some marine sediment records of the last glacial cycle 0 150 ka age models for many cores rely on the stratigraphic correlation of benthic 18o which measures ice volume and deep ocean temperature change the large amount of data available for the last glacial cycle offers the opportunity to improve upon previous benthic 18o compilations such as the lr04 global stack not only are the age constraints for the lr04 stack now outdated but a single global alignment target neglects regional differences of several thousand years in the timing of benthic 18o change during glacial terminations here we present regional stacks that characterize mean benthic 18o change for 8 ocean regions and a volume weighted global stack of data from 263 cores age models for these stacks are based on radiocarbon data from 0 to 40 ka correlation to a layer counted greenland ice core from 40 to 56 ka and correlation to radiometrically dated speleothems from 56 to 150 ka the regional 18o stacks offer better stratigraphic alignment targets than the lr04 global stack and furthermore suggest that the lr04 stack is biased 1 2 kyr too young throughout the pleistocene finally we compare global and regional benthic 18o responses with sea level estimates for the last glacial cycle

eolian evidence for spatial variability of late quaternary climates in tropical africa abstract study of the eolian fraction of late quaternary sediments from the tropical atlantic reveals that two modes of long term climate variability have existed in tropical africa during the last 150 000 yr tropical northwest africa i e the southwestern sahara and sahel was driest during glaciations and stades but wetter than at present during interglaciations and interstades this may be a response to ice sheets at higher latitudes via equatorward displacement of the westerlies and the subtropical high in contrast central equatorial africa southeast of the sahara was most arid during interstades and times of ice growth and most humid during deglaciation wet periods in this area correspond to insolation maxima in northern hemisphere summer a 23 000 yr precessional rhythm is suggested supporting a direct link between african monsoon intensity and orbitally modulated insolation the late holocene is the only time observed when both areas are arid during an interglacial episode this may reflect in part anthropogenic disturbance of late holocene climates

tectonic history of the irtysh shear zone ne kazakhstan new constraints from zircon u pb dating apatite fission track dating and palaeostress analysis abstract the irtysh shear zone isz is an important structure in the framework of the central asian orogenic belt caob it represents the site of final collision of kazakhstan with siberia during hercynian times and records up to 1000 km of lateral displacement during subsequent reorganization in the caob edifice we present new zircon u pb apatite fission track and fault kinematic data along the isz and consequently derived its tectonic history with emphasis on its formation and reactivation episodes carboniferous 340 320 ma zircon u pb ages were obtained for the syn and post collisional kalba narym intrusives dating their emplacement in the framework of the siberia kazakhstan collision during this period the isz experienced an early brittle left lateral mainly transtensional stress regime late carboniferous early permian post collisional intrusives were emplaced and the stress regime changed to a late brittle regime characterized by more compressional conditions indicating rheological strengthening as a response to cessation of ductile shearing and cooling of the isz crust apatite fission track data and thermal history modeling reveal late cretaceous 100 70 ma cooling of the isz basement rocks as a response to denudation of a bordering late mesozoic altai orogen after this denudation event the tectonic activity ceased during the late mesozoic early cenozoic a final step of cooling from 25 ma exhibited by some of the thermal history models may reflect reactivation of the isz and initiation of cenozoic altai mountain building the late plio pleistocene phase of mountain building coincides with a new change in the palaeostress field characterized by minor transpressional right lateral shear conditions

planktonic foraminifera as recorders of past surface water stratification the 18 o difference between shallow and deep living planktonic foraminifera is a proxy for the stratification of surface waters in holocene sediments from the atlantic this difference increases from about 0 in subpolar regions to 3 in the tropics the 18 o values of the shallow dwellers globigerinoides sacculifer and neogloboquadrina pachyderma mainly reflect surface water conditions the calcification depth of the deep living species globorotalia truncatulinoides right coiling is affected by the stratification of the water column in the subtropics shell formation of g truncatulinoides right coiling begins in weakly stratified surface waters in the tropics the initial shell is secreted below the mixed layer this hydrographically induced difference in calcification depth allows the use of the isotopic difference between g truncatulinoides and the shallow species as a proxy for the stratification of surface waters the 18 o between g truncatulinoides and g sacculifer from tropical gravity core geob 1523 1 was significantly lower during isotope stage 2 this suggests that the glacial temperature stratification of western equatorial atlantic surface waters was significantly reduced relative to the holocene

modelling the joint variability of grain size and chemical composition in sediments abstract the geochemical composition of siliciclastic sediments correlates strongly with grain size hence geochemical composition may serve as a grain size proxy in the absence of grain size variations geochemical data of siliciclastic sediments may be used to characterise size independent processes i e sediment provenance weathering mixing shape density sorting and diagenesis in general however geochemical data sets contain both types of information in order to formalise interpretation of geochemical data we propose a mathematical method to decompose the total geochemical variability of a series of genetically related specimens into a grain size dependent the shared signal and a grain size independent part the residual signal the former may serve as a proxy for grain size whereas the latter represents geochemical variability that would have been observed if all sediments would have had the same grain size distribution the two data sets are jointly decomposed by means of partial least squares pls and orthogonal projection subsequently the presence of significant grain size independent geochemical variability in the residual signal is determined in a statistically rigorous manner using a 2 test using a synthetic example we show that the residual record effectively reveals an imposed provenance signal which could not have been resolved from the geochemical or grain size data sets individually we analysed the relation between grain size and geochemical composition in three quaternary marine sediment cores located offshore west africa and south america geob7920 2 geob9508 5 and geob7139 2 both sites are characterised by biogenic sediment input in addition to fluvial and aeolian sediment input from the continent it was found that all cores show a strong but different correlation between the mean grain size and the bulk geochemical composition these results demonstrate that geochemical grain size proxies are empirical and site specific it was also found that the geochemical and grain size data in cores geob7920 2 and geob7139 2 do not contain unique information whereas in core geob9508 5 ti varies independently from the grain size this residual ti signal correlates with the transport mechanism as demonstrated by statistically different values of aeolian and fluvial dominated sediments however a unique interpretation of this residual signal in terms of the postulated grain size independent mechanisms could not be provided without additional information we conclude that the proposed model facilitates identification and validation of different element ratios as grain size proxies and more importantly as proxies for size independent processes for this reason the model paves the way for rigorous analysis of multi proxy data which are widely used in palaeoceanographic and palaeoclimatic research

seasonal variations in the horizontal structure of the amazon plume inferred from historical hydrographic data near surface salinity observations from the national oceanographic data center hydrographic data archive for the region 4 s 10 n 60 40 w show that the amazon river discharge forms a plume of low salinity water that extends northwestward into the western tropical north atlantic the average near surface salinity within the plume increases away from the amazon river mouth consistent with an average entrainment of oceanic water of 2 105 m3 s every 1 of latitude there are two notable seasonal variations in the vicinity of the amazon river mouth 0 2 n the plume extends about 200 300 km eastward from june through december however from march to may the plume was often much wider 400 500 km in the vicinity of the river mouth possibly due to the combination of near maximum river discharge and winds with a southeastward component north of 5 n the amazon plume fans out over a broad region of the western tropical north atlantic during january through june low salinity amazon plume water north of 5 n is primarily found west of 52 w spreading northwestward toward the caribbean beginning typically in may some of the amazon plume water is deflected eastward in the north brazil current nbc retroflection from august through october surface salinities suggest 70 of the amazon plume water is carried eastward in the nbc retroflection while the remaining 30 flows northwestward toward the caribbean

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

systematics of variations in excess al and al ti in sediments from the central equatorial pacific the proposal that the accumulation rate ar of scavenged al in central equatorial pacific sediments is controlled by bulk particle flux and therefore can be used as a paleoproduction proxy is tested by examining the relationship between al and ti using data on core wec8803b gc51 published by murray et al 1993 the downcore profile of the excess al ar follows the bulk ar consistent with the al scavenging hypothesis and is due to a relatively constant abundance of excess al down core however there have been large changes in the aluminosilicate ar to this region suggesting that changes in the al ti value do not solely trace variability in the delivery rate of scavenged al assuming that the excess al in this core is not the result of an analytical artifact we present an alternative to the al scavenging hypothesis that involves authigenic clay mineral formation or some other process causing the occurrence of excess al at a relatively constant abundance of the sediment this process may well be independent of bulk particle flux or paleoproduction

chemistry of surface sediment along a north south transect across the equator in the central indian basin an assessment of biogenic and detrital influences on elemental burial on the seafloor nineteen surface sediment samples collected along a north south transect across the equator in the central indian basin cib were analysed for their major minor and rare earth element ree composition in absence of the element flux data we demonstrate here the association of various elements and their preferential sinks using inter elemental associations in sediment exhibiting latitudinal dissimilarity biogenic sedimentation north of the equator 1 to 4 n latitudes is dominated by carbonate ooze whereas south of the equator 4 to 14 s latitudes is dominated by siliceous ooze carbonate ooze deposition 60 caco3 above 4400 m and its drastic reduction to 80 of their respective total content in all the three sediment zones these observations suggest a close link between ba and a mn oxide phase on the other hand nearly overlapping latitudinal trends of alxs with carbonate in areas north of the equator and opal south of the equator strong positive correlation of alxs with opal r 0 8 similar ratios between carbonate free opal and alxs 16 in both biogenic sediments suggest a close link between productivity and alxs the alxs proportion in both biogenic sediment types is around 28 of the total al suggesting nearly the same level of primary productivity along the transect 4 n to 14 s which supports the estimates of berger berger w h 1989 global maps of the ocean productivity in w h berger v s smetacek g wefer eds productivity of the ocean present and past dahlem workshop reports wiley chichester pp 429 455 and kabanova kabanova j g 1968 primary production of the northern part of the indian ocean oceanology 8 214 255 i e 70 g cm 2 y 1 the rees exhibit mutual coherence in the sediment r 0 7 however they display an increasing affinity towards mn r increases from 0 6 to 0 8 and decreasing affinity towards ti r decreases from 0 8 to 0 3 with increasing atomic number such a subtle but systematic variation within ree group suggests preferential burial of heavy ree by mn oxide phase and light ree by detrital phase

mechanisms of pacific summer water variability in the arctic s central canada basin pacific water flows northward through bering strait and penetrates the arctic ocean halocline throughout the canadian basin sector of the arctic in summer pacific summer water psw is modified by surface buoyancy fluxes and mixing as it crosses the shallow chukchi sea before entering the deep ocean measurements from ice tethered profilers moorings and hydrographic surveys between 2003 and 2013 reveal spatial and temporal variability in the psw component of the halocline in the central canada basin with increasing trends in integrated heat and freshwater content a consequence of psw layer thickening as well as layer freshening and warming it is shown here how properties in the chukchi sea in summer control the temperature salinity properties of psw in the interior by subduction at isopycnals that outcrop in the chukchi sea results of an ocean model forced by idealized winds provide support to the mechanism of surface ocean ekman transport convergence maintaining psw ventilation of the halocline

variations in terrigenous input into the deep equatorial atlantic during the past 24 000 years estimates of terrigenous fluxes at three different water depths at two sites in the equatorial atlantic by normalization against excess 230 th flux indicate that the flux of terrigenous material to the seafloor was significantly higher during the last glacial period than it is today fluxes started to decrease during deglaciation and reached minimal values in the middle of the holocene from 15 000 to 5 000 years ago there was a substantial increase in flux with increasing water depth below 2 800 meters this increase may reflect resuspension and lateral transport of slope and rise sediment possibly because of intensification of deepwater circulation during that period

different precipitation patterns across tropical south america during heinrich and dansgaard oeschger stadials abstract detailed knowledge about tropical south american precipitation patterns during heinrich h and dansgaard oeschger do stadials provides relevant insights into the possible evolution of amazonian hydroclimate under future climate change sediment core geob16224 1 ca 7 n raised from a site in the continental slope off french guiana in western equatorial atlantic under the influence of the amazon river discharge documents the impacts of h and do stadials on both inorganic i e fe ca record and organic i e alkenone c37 concentration and c37 c38 ratio geochemistry between 41 and 13 ka bp our results show millennial scale covariations of increased fe ca values with decreased c37 concentration and c37 c38 ratios during h and do stadials comparing our high temporal resolution data with previously published records from ca 17 n to 4 s we are able to differentiate the influence of h and do stadials upon tropical south american precipitation we find that records under the influence of the south american summer monsoon e g western amazon and the northern edge of the intertropical convergence zone itcz e g northernmost south america exhibit strong climate shifts during both h and do stadials but regions under the influence of the southern edge of the itcz e g northeastern brazil experience a weaker reaction during do stadials than during h stadials

upper pleistocene amazon deep sea fan muds reflect intense chemical weathering of their mountainous source lands abstract upper pleistocene muds of the amazon deep sea fan contain an abundance of residual weathering products as indicated by their bulk chemical and mineralogical compositions the muds are chemically more mature than average shale average pelagic mud or the surrounding demerara abyssal sediments they are however very similar to the bulk chemical compositions of contemporary suspended sediments of both the amazon estuary and those carried by the major andean tributaries the modern sediment compositions reflect the rapid weathering now occurring in the andean source regions of the basin the major element geochemical signature of the upper pleistocene deep sea fan sediments suggests that intense chemical weathering in the mountainous source lands has controlled the chemistry of amazon river suspended sediments since at least the upper pleistocene minor and trace element signatures appear uniform throughout the fan and element concentrations are in general higher than in the intensely weathered amazon lowland soils but lower than in deep sea sediments

grain size distribution and depositional processes of the mineral component of abyssal sediments lessons from the north pacific grain size analyses of the mineral component of abyssal pacific surface sediments show two distinctly different size distributions that can be associated with sediments that are dominantly eolian and dominantly hemipelagic respectively in the range of 1 to 30 m eolian grains show a distinct size mode at about 2 m 9 and little material coarser than about 16 m 6 hemipelagic grains are characterized by a much flatter size distribution that has a broad size mode near 2 m 9 and important amounts of material coarser than 16 m 6 this ability to distinguish between hemipelagic and eolian grains will obviate some long standing concerns in the interpretation of deep sea sediments examples of the utility of this technique are given for the north pacific and south atlantic

late quaternary carbonate sedimentation at the sierra leone rise eastern equatorial atlantic ocean abstract in the eastern equatorial atlantic ocean changes in the concentration of carbonate in late quaternary sediments resulted from reduced production of carbonate in surface waters and increased dilution with non carbonate sediments during glacial maxima during glacial stages production of carbonate in surface water measured as its accumulation rate in shallow undissolved cores decreased by one half the glacial accumulation rate of non carbonate components increased 1 5 to 4 times over holocene values the greatest increases occurred in the deepest cores carbonate dissolution during stages 2 3 and 4 increased the proportion of foraminiferal fragments and decreased the accumulation rate of susceptible species in the deep sites in shallow sites slightly increased dissolution can be detected during stage 3 while greatly increased dissolution occurred during stage 4 bathymetric profiles of foraminiferal fragmentation and accumulation document a shoaling of the foraminiferal lysocline by 1000 m during glacial isotopic stages we present a mass balance model of sediment accumulation for carbonate and insoluble components and from this model we estimate the rate of downslope transport and dissolution of carbonate at the sierra leone rise our results show that during stage 4 the rate of carbonate loss to dissolution was greater than the rate observed today or during other interglacial stages the calculated rates of dissolution for stages 2 and 3 are not significantly different from those calculated for stage 1

diatom record of late quaternary climatic change in the eastern equatorial atlantic and tropical africa over the last 160 000 years diatom productivity in the equatorial atlantic has been high during glacial stages 2 4 and 6 and moderate during parts of substages 5b and 5d approximately 96 000 and 117 000 years b p productivity of diatoms is particularly low in substage 5e and in the middle of stage 1 higher productivity implies more vigorous trade winds during glacial intervals leading to stronger upwelling and enhanced lateral advection of surface waters the influence of the benguela current near the equator was strongest during stage 2 a separate regime of coastal upwelling may have existed in the gulf of guinea at this time upwelling and advection may not always respond to the same forcing as sea surface temperature results support the hypothesis that ethmodiscus rex oozes result from high diatom productivity runoff from the zaire and niger rivers was highest at the stage 2 1 transition and just prior to the stage 6 5 transition due to intensification of the african monsoon several other times of increased runoff are indicated periods of increased surface water circulation in the eastern equatorial atlantic appear to correspond to times of aridity in tropical africa conversely intervals of decreased circulation in the eastern equatorial atlantic coincide with times of humidity in tropical africa precessional forcing of continental climate is suggested however precessional forcing of surface ocean circulation can be neither confirmed nor refuted based on the records examined here influx of antarctic bottom water to the eastern equatorial atlantic has been insignificant throughout the last 160 000 years