discrimination of biogenic and detrital magnetite through a double verwey transition temperature magnetite occurs widely in natural environments in both inorganic and biogenic forms discrimination of the origin of magnetite has important implications from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings in this study we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of oman low temperature magnetic data reveal two distinct verwey transition tv temperatures that are associated with the presence of biogenic and inorganic magnetite this interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses our study justifies the use of two distinct tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite simple low temperature magnetic measurements therefore provide a tool to recognize rapidly the origin of magnetite within natural samples in addition our analyses reveal progressive down core dissolution of detrital and biogenic magnetite but with preservation of significant amounts of fine grained magnetite within sediments that have been subjected to severe diagenetic alteration we demonstrate that preservation of magnetite in such environments is due to protection of fine grained magnetite inclusions within silicate hosts our results therefore also provide new insights into diagenetic processes in marine sediments

vegetation zones in nw africa during the brunhes chron reconstructed from marine palynological data abstract six east atlantic sites between 9 and 35 n allow the reconstruction of the latitudinal position and extension of the nw african vegetation zones during the last 0 7 ma the response of the vegetation to alternating glacial dry and interglacial humid climatic conditions has been recorded the vegetation response to a climatic amelioration starts with the northward expansion of the tropical rain forest followed by the northward shift of the saharan sahelian boundary and ends with the southward shift of the saharan mediterranean transitional steppes and the expansion of the mediterranean oak forests most sensitive is the reaction of the saharan sahelian boundary to climatic change large shifts of the saharan sahelian boundary seem to occur faster than small ones annual precipitation estimates for the latitude of dakar 15 n range from 100 to 1000 mm at 24 n yearly precipitation probably ranged from 0 to 150 mm smallest extent of the guinean tropical rain forest probably occurred during the arid conditions of isotope stages 2 and 6 reducing the carbon fixation of these forests to a minimum large extent of the guinean tropical rain forest occurred during most of the odd isotope stages more humid interglacials

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

sea surface temperatures of the western arabian sea during the last deglaciation 1 in this study we present a sea surface temperature sst record from the western arabian sea for the last 20 000 years we produced centennial scale d 18 o and mg ca sst time series of core niop929 with focus on the glacial interglacial transition the western arabian sea is influenced by the seasonal ne and sw monsoon wind systems lowest ssts occur during the sw monsoon season because of upwelling of cold water and highest ssts can be found in the low productivity intermonsoon season the mg ca based temperature record reflects the integrated sst of the sw and ne monsoon seasons the results show a glacial interglacial sst difference of 2c which is corroborated by findings from other arabian sea cores at 19 ka b p a yet undescribed warm event of several hundred years duration is found which is also reflected in the d 18 o record a second centennial scale high sst low d 18 o event is observed at 17 ka b p this event forms the onset of the stepwise yet persistent trend toward holocene temperatures highest mg ca derived ssts in the niop929 record occurred between 13 and 10 ka b p interglacial sst is 24c indicating influence of upwelling the onset of arabian sea warming occurs when the north atlantic is experiencing minimum temperatures the rapid temperature variations at 19 17 and 13 ka b p are difficult to explain with monsoon changes alone and are most likely also linked to regional hydrographic changes such as trade wind induced variations in warm water advection

monsoon fluctuations over the past 350 kyr high resolution evidence from northeast asia northwest pacific climate proxies marine pollen and radiolarians abstract high resolution analyses of pollen and radiolarians from northwest pacific cores v28 304 and rc14 99 provide continuous directly linked evidence of oceanic atmospheric climate processes through the past three pleistocene glacial interglacial cycles systematic fluctuations in marine pollen assemblages which reflect changes in the forest communities of japan are compatible with fluctuations in faunal assemblages and inferred oceanographic properties offshore each interglacial has distinguishable characteristics which imply that the sequence timing and magnitude of climatic change were not identical during the three most recent interglacial intervals most interglacials contain at least one 10 15 kyr period of exceptionally high percentages 20 of cryptomeria which we interpret as reflecting increased levels of precipitation in response to intensification of the summer monsoon with the exception of oxygen isotope substage 5e in rc14 99 interglacial summer sea surface temperatures were similar to or slightly warmer than today glacial environments in central japan and in the northern subtropical gyre were similar to those which now support cold temperate and boreal forests on northernmost japan and a dominant subpolar fauna and associated low sea surface temperatures offshore climatic change was apparently not as severe in southern japan where temperate vegetation largely replaced warm temperate flora or in the southern subtropical gyre where only winter ssts were cooler than present we attribute these changes in vegetation and surface water conditions to shifts in the atmospheric polar and oceanographic subarctic fronts in response to the seasonal variations in the siberian high maxima in cryptomeria systematically lag maxima in solar insolation at 30 n this correlation between our summer monsoon indicator and summer insolation substantiates the view that the strength of the asian monsoon is linked to variations in northern hemisphere solar radiation

a 25 ka indian ocean monsoon variability record from the andaman sea recent paleoclimatic work on terrestrial and marine deposits from asia and the indian ocean has indicated abrupt changes in the strength of the asian monsoon during the last deglaciation comparison of marine paleoclimate records that track salinity changes from asian rivers can help evaluate the coherence of the indian ocean monsoon iom with the larger asian monsoon here we present paired mg ca and d 18 o data on the planktic foraminifer globigerinoides ruber white from andaman sea core rc12 344 that provide records of sea surface temperature sst and d 18 o of seawater d 18 osw over the past 25 000 years ka before present bp age control is based on nine accelerator mass spectrometry ams dates on mixed planktic foraminifera mg ca sst data indicate that sst was 31c cooler during the last glacial maximum lgm than the late holocene andaman sea d 18 osw exhibited higher than present values during the lateglacial interval ca 19 15 ka bp and briefly during the younger dryas ca 12 ka bp lower than present d 18 osw values during the bolling allerod ca 14 5 12 6 ka bp and during the early holocene ca 10 8 5 5 ka bp are interpreted to indicate lower salinity reflect some combination of decreased evaporation precipitation e p over the andaman sea and increased irrawaddy river outflow our results are consistent with the suggestion that iom intensity was stronger than present during the bolling allerod and early holocene and weaker during the late glaciation younger dryas and the late holocene these findings support the hypothesis that rapid climate change during the last deglaciation and holocene included substantial hydrologic changes in the iom system that were coherent with the larger asian monsoon r 2007 elsevier ltd all rights reserved

formation and preservation of pedogenic carbonates in south india links with paleo monsoon and pedological conditions clues from sr isotopes u th series and rees the influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the western ghats karnataka plateau south west india has been studied along the climosequence the current mean annual rainfall mar varies within a 80 km transect from 6000 mm at the edge of the plateau to 500 mm inland pedogenic carbonates occur in the mar range of 500 1200 mm in the semi arid zone mar 500 900 mm carbonates occur i as rhick hardpan calcretes on pediment slopes and ii as nodular horizons in polygenic black soils i e vertisols in the sub humid zone mar 900 1500 mm pedogenic carbonates are disseminated in the black soil matrices either as loose irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size they also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate cemented lumps of centimetric to decimetric size chemical and isotopic sr 87 sr 86 compositions of the carbonate fraction were determined after leaching with 0 25 n hcl the corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed the trend defined by the sr 87 sr 86 signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition considering the residual fraction of each sample as the most likely lithogenic source of ca in carbonates it is estimated that from 24 to 82 55 on average of ca is derived from local bedrock weathering leading to a consumption of an equivalent proportion of atmospheric co2 these values indicate that climatic conditions were humid enough to allow silicate weathering mar at the time of carbonate formation likely ranged from 400 to 700 mm which is 2 to 3 fold less than the current mar at these locations the sr u and mg contents and the u 234 u 238 activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation the relatively high concentrations of sr u and mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite possibly due to a better confinement of the pore waters which is supported by their high u 234 u 238 signatures and or to higher content of dissolved carbonates in the pore waters the occurrence of ce mn and fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation the carbonate ages determined by the u th method range from 1 33 0 84 kyr to 7 5 2 7 kyr and to a cluster of five ages around 20 kyr i e the last glacial maximum period the young occurrences are only located in the black soils which therefore constitute sensitive environments for trapping and retaining atmospheric co2 even on short time scales the maximum age of carbonates depends on their location in the climatic gradient from about 20 kyr for centimetric nodules at mule hole mar 1100 mm yr to 200 kyr for the calcrete at gundlupet mar 700 mm yr durand et al 2007 the intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils c 2010 elsevier ltd all rights reserved

magnetic anisotropy magnetostatic interactions and identification of magnetofossils 1 single domain magnetite particles produced by magnetotactic bacteria mtb and aligned in chains called magnetosomes are potentially important recorders of paleomagnetic paleoenvironmental and paleolife signals rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes magnetofossils preserved in geological materials however ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post depositional diagenesis in this paper magnetic properties of three sets of samples containing extracted magnetosomes of the culturedmagnetospirillum magneticumstrain amb 1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties intact mtb and well dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction this results in a change of the magnetic signature toward properties typical of interacting single domain particles i e a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization decreasing in the crossing point of the wohlfarth cisowski test and in the delta ratio between losses of field and zero field cooled remanent magnetization across the verwey transition as well as vertical broadening of the first order reversal curve distribution we propose a new diagram that summarizes the verwey transition properties with diagnostic limits for intact and collapsed chains of magnetosomes this diagram can be used in conjunction with other parameters to identify unoxidized magnetofossils in sediments and rocks

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

a solar irradiance trigger for millennial scale abrupt changes in the southwest monsoon 1 marine sedimentary records of the last glacial from tropical monsoon latitudes indicate climate fluctuations comparable to rapid changes in 18o recorded in greenland synchronizing two high resolution sedimentary records from the oman and pakistan margins we resolve millennial scale reversals in sea surface temperature sst gradient sst across the arabian sea which directly correspond with the majority of dansgaard oeschger d o events for the last 65 kyr the relative amplitude of individual monsoon and d o events appears comparable suggesting coupled and at least hemispheric forcing to explore this quasi cyclic forcing we compare alkenone based geologic data with modern satellite derived sst estimates between sites interstadial conditions fall within the range of monsoons during the holocene but stadial conditions have no analogs following published associations between eurasian winter snow cover ewsc and monsoon rainfall and el nino southern oscillation enso events and anomalous ewsc we find a good correlation between the southern oscillation index soi and arabian sea sst throughout the modern data set an apparent 11 year cyclicity in the soi reveals an association between the monsoon soi and solar output variability the soi primarily tracks solar total irradiance but the soi monsoon linkage becomes nonlinear during excursions of the soi associated with el nino events strong el ninos coincide precisely with minimum solar irradiance during the solar cycle which we attribute to threshold behavior in tropical pacific ssts and associated trade wind strength we propose that both short term interannual decadal and long term centennial millennial changes in solar output are consistent with records of enso variability monsoon intensity and d o event timing

the role of the indian summer monsoon and the mid latitude westerlies in himalayan glaciation review and speculative discussion new dates for late quaternary glaciations in the himalayas show that during the last glacial cycle glaciations were not synchronous throughout the region rather in some areas glaciers reached their maxima at the global glacial maximum of c 18 20 ka bp whereas in others glaciers were most extensive at c 60 30 ka bp comparison of these data with palaeoclimatic records from adjacent regions suggest that on millennial timescales himalayan glacier fluctuations are controlled by variations in both the south asian monsoon and the mid latitude westerlies

earth s precession cycle and quaternary climatic change in tropical africa high abundances of the freshwater diatom melosira spp reflecting eolian input from dry lake beds in tropical africa occur in deep sea sediments of the equatorial atlantic at discrete times during the late quaternary1 2 we report here an approximately 260 kyr record of melosira accumulation in an equatorial atlantic sediment core and demonstrate significant influence of the earth s precessional cycle on the climate of tropical africa

planktonic foraminifera of the northern indian ocean distribution and preservation in surface sediments in the northern indian ocean planktonic foraminiferal tests accumulate in a wide variety of surface water environments and depositional settings this variability enables us to isolate the effects that surface water ecology and differential dissolution have on the distribution of planktonic foraminifera from 251 geographically widespread surface sediment samples foraminiferal abundance varies from 0 to 104 whole foraminifera in the greater than 150 m fraction per gram dry sediment values 10 characterize the three deep basins of the equatorial indian ocean and the western bay of bengal foraminiferal tests are most abundant on carbonate covered ninety east and carlsberg ridges absolute abundance patterns are mainly controlled by non ecological processes variations in dissolution resistant species rsp with water depth reveal that the foraminiferal lysocline fl varies regionally the fl is deepest 3 800 m in the equatorial region rises abruptly to 3 300 m in the arabian sea and varies from 2 600 m to near 2 000 m moving northward in the bay of bengal deep samples with anomalously low rsp 30 suggest redeposition systematic geographic and depth related variation is observed for the 17 most abundant foraminiferal species dissolution resistant species g menardii g tumida g dutertrei p obliquiloculata generally exhibit a rapid and continuous increase in relative abundance at and below the fl susceptible species g ruber g bulloides g glutinata for example exhibit a rapid and continuous decrease in relative abundance at and below the fl moderately susceptible species g conglobatus g aequilateralis g conglomerata for example rapidly increase in abundance at the fl and systematically decrease with depth below the fl principal components analysis pca of faunal data from minimally dissolved 30 rsp samples reveals important ecologically related species intercorrelations the major biogeographic gradient is the negative covarying relationship between ag bulloides g glutinata species pair and a grouping ofg sacculifer g conglobatus g aequilateralis andg ruber pca of all samples demonstrates how differential dissolution alters this and other species relationships species groupings that incorporateg ruber g menardii andg dutertrei are particularly affected by dissolution comparison of average faunal data from minimally dissolved samples in the northern indian ocean with similar samples from other tropical regions suggests varying environmental factors produce distinct faunas within the tropical ocean for example g bulloides g falconensis andg hexagona are significantly more abundant in northern indian ocean surface sediments while such species asg ruber g sacculifer g dutertrei andp obliquiloculata dominate in other tropical regions

the quaternary glacial history of the zanskar range north west indian himalaya abstract three glacial stages and a minor advance are recognised from the zanskar range of the north western indian himalaya a glaciated palaeosurface 280 m above river level a r l with associated erratics represents the oldest and most extensive glaciation the chandra stage this ice covered a landscape of broad gentle valleys probably as an ice cap with an erratic distribution indicating that the ice shed lay to the south over the high himalaya a change in valley form from broad glacial troughs to narrow v shaped gorges along with large subdued moraine ridges delimits a separate later extensive valley glaciation the batal stage with its maximum at 78 0 12 3 ka bp distinct sets of moraine ridges represent a less extensive younger glaciation the kulti stage which dates close to the global last glacial maximum a minor advance the sonapani is represented by sharp crested moraine ridges within 2 km of current ice bodies possibly belonging to the little ice age glacier elevation indexes geis calculated for this and adjacent areas show a rise in elevation from the south west to the north east but dip again to the indus valley for both the batal and kulti stages reflecting attenuation of the south westerly monsoon and possible channelling of westerly depressions along the broad upper indus valley gei depression are 500 and 300 m for the batal and kulti stages respectively the change in extent and style from the chandra stage to the later glaciations may be related to uplift of more southerly ranges blocking monsoon precipitation and incision of the landscape such that ice reached lower altitudes over shorter horizontal distances why ice extent was greater during oxygen isotope stage ois 4 than 2 is not clear although recent high resolution plaeomonsoon records show complex peaks in monsoon intensity and it is possible that there was a more favourable combination of precipitation and temperature earlier rather than later during the last glaciation

properties of intracellular magnetite crystals produced by desulfovibrio magneticus strain rs 1 desuffiovibrio magneticus strain rs 1 is an anaerobic sulfate reducing bacterium cells form intracellular nanocrystals of magnetite but are only weakly magnetotactic in order to understand the unusual magnetic response of this strain we studied magnetite crystals within cells grown with fumarate and sulfate many cells grown under either condition did not form magnetic crystals while others contained only i to 18 small similar to 40 nm magnetite containing magnetosomes bulk magnetic measurements of whole cells showed a superparamagnetic like behavior indicating that many of the magnetite crystals are too small to have a permanent magnetic moment at ambient temperature the temperature of the verwey transition is lower similar to 86 k than of magnetite from other magnetotactic strains likely indicating partial oxidation of magnetite into maghemite as a result of the small size and small number of magnetite magnetosomes the magnetic moments of most cells grown anaerobically with fumarate or sulfate are insufficient for magnetotaxis in addition to intracellular magnetite in some cultures another iron oxide hematite formed on the surfaces of cells the hematite grains are embedded in an extracellular polymeric material indicating that the crystals likely resulted from a biologically induced mineralization process although the hematite particles appear to consist of aggregations of many small 5 to 10 nm grains the grains have a consensus orientation and thus the whole particle diffracts as a single crystal the aligned arrangement of nanoparticles within larger clusters may reflect either a templated nucleation of hematite crystallites in an extracellular organic matrix or result from a self assembling process during the crystallization of hematite from ferric gels or ferrihydrite

luminescence chronology of river adjustment and incision of quaternary sediments in the alluvial plain of the sabarmati river north gujarat india river adjustment and incision in the sabarmati basin gujarat india have been examined at a site near mahudi towards this the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated the upper fluvial sequence along with the overlying aeolian sand and riverbed scroll plains provide clues to the evolution of the present sabarmati river sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase luminescence chronology bracketed this sequence to between 54 and 30 ka which corresponds to oxygen isotope stage 3 during which the sw monsoon was enhanced the overlying aeolian sand has been dated to 12 ka indicating that dune accretion occurred simultaneously with the strengthening of the sw monsoon during the early holocene adjustment of the sabarmati along a n s transect is placed around 12 ka and the incision is bracketed between 12 and 4 5 ka river adjustment could have been tectonic however the incision was facilitated by the availability of continuous flow in the river caused by the sw monsoon the basin experienced two tectonic events at about 3 and 0 3 ka as demonstrated by the morphology of the scroll plains

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

three hundred eighty thousand year long stable isotope and faunal records from the red sea influence of global sea level change on hydrography stable isotope and faunal records from the central red sea show high amplitude oscillations for the past 380 000 years positive 18o anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the arabian sea was reduced due to a reduced geometry of the bab el mandeb strait salinities as high as 53 and 55 are inferred from pteropod and benthic foraminifera 18o respectively for the last glacial maximum during this period all planktonic foraminifera vanished from this part of the red sea environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level the foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka spectral analysis of the planktonic 18o record documents highest variance in the orbital eccentricity obliquity and precession bands indicating a dominant influence of climatically driven sea level change on environmental conditions in the red sea variance in the precession band is enhanced compared to the global mean marine climate record specmap suggesting an additional influence of the indian monsoon system on red sea climates

sea surface temperature and productivity records for the past 240 kyr in the arabian sea abstract deep sea sediments of two cores from the western ty93 929 p and the southeastern md900963 arabian sea were used to study the variations of the indian monsoon during previous climatic cycles core ty93 929 p was located between the sw monsoon driven upwelling centres off somalia and oman which are characterized by large seasonal sea surface temperature sst and particle flux changes by contrast core md900963 was situated near the maldives platform an equatorial ocean site with a rather small sst seasonality less than 2 c for both cores we have reconstructed sst variations by means of the unsaturation ratio of c37 alkenones which is compared with the 18o records established on planktonic foraminifera in general the sst records follow the 18o variations with an sst maximum during oxygen isotope stage 5 5 the last interglacial at about 120 130 kyr and a broad sst minimum during isotope stage 4 and 3 3 approximately 40 50 kyr the sst difference between the holocene and the last glacial maximum lgm is of the order of 2 c in both cores the ssts during isotope stage 6 are distinctly higher by 1 2 c than the cold sst minima during the last glacial cycle lgm and stage 3 to reconstruct qualitatively the past productivity variations for the two cores we used the concentrations and fluxes of alkenones and organic carbon together with a productivity index based on coccolith species florisphaera profunda relative abundance within each core there is a general agreement between the different palaeoproductivity proxies in the southeastern arabian sea core md900963 glacial stages correspond to relatively high productivity whereas warm interstadials coincide with low productivity all time series of productivity proxies are dominated by a cyclicity of about 21 23 kyr which corresponds to the insolation precessional cycle a hypothesis could be that the ne monsoon winds were stronger during the glacial stages which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone by contrast the records are more complicated in the upwelling region of the western arabian sea core ty93 929 p this is partly due to large changes in the sedimentation rates which were higher during specific periods isotope stages 6 5 4 5 2 3 and 2 unlike core md900963 no simple relationship emerges from the comparison between the 18o stratigraphy and productivity records the greater complexity observed for core ty93 929 p could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons the sw monsoon being enhanced during interglacial periods whereas the ne monsoon was increased during glacial intervals a similar line of reasoning also could help explain the sst records by the superimposition of variations of three components global atmospheric temperature and sw and ne monsoon dynamics

seasonal rainfall in the sinai desert during the late quaternary inferred from fluorescent bands in fossil corals skeletal bands of alternating high and low density in massive coral species have been used to record their growth history1 2 in the red sea living colonies of the genus porites deposit low density skeletal bands during summer and high density bands during winter3 additionally yellow green fluorescence can sometimes be seen in these massive corals imparted to them by the incorporation of humic material carried by coastal runoff4 annual banding of fluorescent sequences in living scleractinian corals has proved to be useful in the study of terrestrial runoff in the near shore environment5 6 here we report the finding of similar yellow green fluorescent bands in fossil porites from late quaternary reef terraces in southern sinai which are absent from living porites in the nearby fringing reefs the periodic sequences of the fluorescing humics were found to be superimposed on the low density sub bands of the fossil corals we interpret our observations as evidence that during the late quaternary reef forming peaks the climate was wetter than the extreme desert conditions now prevailing with a possible summer rainfall regime

wind variability over the northern indian ocean during the past 4 million years insights from coarse aeolian dust iodp exp 359 site u1467 maldives abstract the lithogenic fraction of carbonate drift sediments from iodp exp 359 site u1467 maldives provides a unique record of atmospheric dust transport over the northern indian ocean during the past 4 myr grain size data provide proxies for dust flux controlled by source area aridity as well as wind transport capacity wind speed entrainment and long range transport of dust in the medium to coarse silt size range is linked to the strength of the arabian shamal winds and the occurrence of convective storms which prolong dust transport dust flux and the size of dust particles increased between 4 0 and 3 3 ma corresponding to the closure of the indonesian seaway and the intensification of the south asian monsoon there is no clear trend in dust flux between 3 3 and 1 6 ma whereas wind transport capacity decreased between 1 6 ma and the recent dust flux increased and shows higher variability especially during the last 500 kyr transport capacity increased between 1 2 and 0 5 ma and slightly decreased since then frequency analysis shows that dust transport varies on orbital timescales with eccentricity control being the most prominent 400 kyr throughout the record 100 kyr between 2 0 and 1 3 ma and since 1 0 ma higher frequency cycles obliquity and precession are more pronounced in wind transport capacity than in the amount of dust this indicates that the amount of coarse dust in sediments from the maldives as a far field site is more prone to changes in transport mechanisms than to changes in dust source area aridity

geochemistry of calcretes calcic palaeosols and hardpan coimbatore southern india formation and paleoenvironment abstract there is no major chemical variation in the late quaternary calcic nodules formed within vertisols in the foot hill regions and hardpan calcretes greater than 200 ka formed over the precambrian substrate that occur as duricrust horizons in the plains around coimbatore region tamil nadu india the bulk chemistry of calcic nodules and the hardpan calcretes show very little variability in cao sio2 al2o3 and fe2o3 content the trend of trace metal content in calcic nodules and in the hardpan calcrete is mn zn ni cr pb co cu and zn mn ni co pb cu cr respectively paas normalised ree data of the calcretes calcic nodules and the hardpan demonstrate a positive euanomaly this could be attributed to feldspar apatite mineral alteration and soil digenetic processes differential weathering leaving behind plagioclase phenocrysts and apatite grains which are enriched in eu the stable isotope values of the calcic nodules do not exhibit a wide range 18o 3 39 to 5 84 and 13c 3 01 to 6 64 compared to the hardpan calcretes 18o 2 91 to 12 98 and 13c 0 05 to 7 4 the palaeoenvironment during the formation of the calcretes nodules was dominated by the c4 plants with sparse vegetation cover and that the parent material bedrock supported a thin veneer of soil column with sparse vegetation cover as present day calcic nodule accumulations in the soils has resulted from differential weathering and chemical histories within the soil profiles while the hardpan calcretes formed by complex pedogenic and chemical groundwater processes in arid semi arid conditions over along geological period during the late mis3 to the lgm period the south west monsoon was weak with mean annual rainfall mar between 300 and 500 mm yr the sea level was low and the rainfall at the time of carbonate formation would have remained damp enough to allow silicate weathering of the soil sediments

orbital forcing of climate over south africa a 200 000 year rainfall record from the pretoria saltpan late pleistocene variations in rainfall in subtropical southern african are estimated from sediments preserved in the pretoria saltpan a 200000 year old closed basin crater lake on the interior plateau of south africa we show that south african summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession as predicted by orbital precession geometry berger 1978 this south african record is out of phase with north african palaeomonsoon indices street and grove 1979 rossignol strick 1983 mcintyre et al 1989 the amplitude of the rainfall response to insolation forcing agrees with climate model estimates prell and kutzbach 1987 these results document the importance of direct orbital insolation forcing on both subtropical north and south african climate as well as the predicted antiphase sensitivity to precessional insolation forcing

west african paleoclimates during the last climatic cycle inferred from an atlantic deep sea pollen record abstract pollen studies of marine sediments from atlantic deep sea core v22 196 13 50 n 18 57 w 3728 m water depth are used to reconstruct paleoclimates of north tropical west africa since 140 000 yr b p high concentration of pollen mainly from saharan and arid littoral zones suggests a strengthened tradewind circulation since ca 60 000 yr b p that became even stronger after 40 000 yr b p and reached a maximum 17 000 yr b p periods of eolian transport of pollen to the atlantic occurred about 94 000 and 105 000 yr b p and during termination ii conversely high relative values of pollen from sudanian and guinean zones and from mangrove and freshwater vegetation coincident with maxima of dinocyst concentration are interpreted as reflecting both strong fluvial inputs and the expansion of humid vegetation on the adjacent continent during wet periods dated between 140 000 and 118 000 105 000 and 96 000 92 000 and 73 000 52 000 and 44 000 yr b p and 12 000 yr b p and the holocene the strong correlation of these results from the eastern tropical atlantic with those from the arabian sea corroborates fluctuations of monsoon climate

ferromagnetic resonance and low temperature magnetic tests for biogenic magnetite magnetite is both a common inorganic rock forming mineral and a biogenic product formed by a diversity of organisms magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity magnetosomes arranged in linear chains of crystals magnetosomes and their fossils magnetofossils have been identified using transmission electron microscopy tem in sediments dating back to 510 570 ma and possibly in 4 ga carbonates in martian meteorite alh84001 we present the results from two rock magnetic analyses the low temperature moskowitz test and ferromagnetic resonance fmr applied to dozens of samples of magnetite and other materials the magnetites in these samples are of diverse composition size shape and origin biologically induced extracellular biologically controlled magnetosomes and chiton teeth magnetofossil synthetic and natural inorganic we confirm that the moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure we also demonstrate that the fmr spectra of four different strains of magnetotactic bacteria and a magnetofossil bearing carbonate have a form distinct from all other samples measured in this study we suggest that this signature also results from the magnetosomes unique arrangement in chains because fmr can rapidly identify samples with large fractions of intact isolated magnetosome chains it could be a powerful tool for identifying magnetofossils in sediments

a strong angular dependence of magnetic properties of magnetosome chains implications for rock magnetism and paleomagnetism 1 single domain magnetite particles produced by magnetotactic bacteria magnetosomes and aligned in chains are of great interest in the biosciences and geosciences here we investigated angular variation of magnetic properties of aligned magnetospirillum magneticum amb 1 cells each of which contains one single fragmental chain of magnetosomes with measurements at increasing angles from the chain direction we observed that i the hysteresis loop gradually changes from nearly rectangular to a ramp like shape e g bc and remanence decrease ii the acquisition and demagnetization curves of irm shift toward higher fields e g bcr increases and iii the forc diagram shifts toward higher coercivity fields e g bc forc increases for low temperature results compared to unoriented samples the samples containing aligned chains have a much lower remanence loss of field cooled fc and zero field cooled zfc remanence upon warming through the verwey transition higher ratio fc zfc for the measurement parallel to the chain direction and lower ratio larger fc and zfc values for the perpendicular measurement micromagnetic simulations confirm the experimental observations and reveal that the magnetization reversal of magnetosome chain appears to be noncoherent at low angles and coherent at high angles the simulations also demonstrate that the angular dependence of magnetic properties is related to the dispersion degree of individual chains indicating that effects of anisotropy need to be accounted for when using rock magnetism to identify magnetosomes or magnetofossils once they have been preserved in aligned chains additionally this study experimentally demonstrates an empirical correspondence of the parameter bc forc to bcr rather than bc at least for magnetite chains with strong shape anisotropy this suggests forc analysis is a good discriminant of magnetofossils in sediments and rocks

quaternary history of calcium carbonate fluctuations in the western equatorial indian ocean somali basin abstract a study of the quaternary history of calcium carbonate fluctuations in the equatorial indian ocean based on the swedish deep sea expedition core 154 demonstrates that the calcite compensation depth ccd was shallower during interglacial periods than during glacials higher percentages of calcium carbonate lower percentages of dissolution resistant planktonic foraminifera and lower values of the berger dissolution index suggesting better calcium carbonate preservation are recorded during glacial periods and or at glacial interglacial transitions in contrast interglacial periods are marked by stronger calcium carbonate dissolution the data indicate that dissolution is the main controlling factor of calcium carbonate in the western equatorial indian ocean higher dissolution during interglacial periods and enriched 13 c are due to higher noncarbonate productivity and enhanced antarctic bottom water aabw ventilation in the equatorial indian ocean

glacial to interglacial contrasts in the calcium carbonate content and influence of indus discharge in two eastern arabian sea cores abstract two cores collected in the eastern arabian sea were studied for coarse fraction calcium carbonate content and globorotalia menardii complex abundance based on the fluctuations of calcium carbonate and g menardii complex abundance the holocene pleistocene boundary is placed at 50 cm level from the top of the core and the last glacial maximum lgm at 80 cm depth for the northern arabian sea core while for the southern arabian sea core the boundaries of holocene pleistocene and lgm are placed at 40 cm and 65 cm respectively the carbonate content and g menardii complex abundance variations in these cores reveal that carbonate maxima with less abundance of g menardii complex reflects glacials cold periods while carbonate minima with high abundance of g menardii complex reflects interglacials warm periods high percentage of coarse fraction and caco 3 during glacials attributed to high productivity low percentage of coarse fraction and caco 3 is due to less productivity during interglacials productivity variations during glacial and interglacial periods in the arabian sea is discussed in relation to sw and ne monsoon intensities northern arabian sea core shows less caco 3 during holocene period and high during periods suggesting that the dilution by terrigenous flux derived from the indus river is significantly lower during glacial periods than in the holocene period

climatology of the eastern arabian sea during the last glacial cycle reconstructed from paired measurement of foraminiferal δ18o and mg ca abstract paired measurements of mg ca and 18 o of globigerenoides sacculifer from an eastern arabian sea eas sediment core indicate that sea surface temperature sst varied within 2 c and sea surface salinity within 2 psu during the last 100 ka sst was coldest 27 c during marine isotope stage mis 4 and 2 sea surface salinity was highest 37 5 psu during most of the last glacial period 60 18 ka concurrent with increased 18 o g sacculifer and c n ratios of organic matter and indicative of sustained intense winter monsoons sst time series are influenced by both greenland and antarctic climates however the sea surface salinity time series and the deglacial warming in the sst record beginning at 18 ka compare well with the lr04 benthic 18 o stack and antarctic temperatures this suggests a teleconnection between the climate in the southern hemisphere and the eas therefore the last 100 ka variability in eas climatology appears to have evolved in response to a combination of global climatic forcings and regional monsoons the most intense summer monsoons within the holocene occurred at 8 ka and are marked by sst cooling of 1 c sea surface salinity decrease of 0 5 psu and 18 o g sacculifer decrease of 0 2

a 300 kyr record of upwelling off oman during the late quaternary evidence of the asian southwest monsoon in the northwest arabian sea upwelling occurs each summer driven by the strong sw monsoon winds upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off oman that are preserved in the sediments along the oman continental margin creating a geologic record of monsoon driven upwelling sediments recovered from the oman continental margin during ocean drilling program leg 117 provide an opportunity to examine how upwelling has varied during the late quaternary spanning a longer interval than piston cores recovered prior to the odp cruise variations in foraminifer shell accumulation and in the relative abundance of globigerina bulloides indicate dominant cycles of variation at 1 100 kyr the dominant frequency of glacial interglacial variations and at 1 23 kyr the frequency of precessionally driven cycles in seasonal insolation the strongest monsoon winds indicated by increased upwelling occurred during interglacial times when perihelion was aligned with the summer solstice an orbital change that increased the insolation received during summer in the northern hemisphere during glacial times upwelling was reduced and although the precessional cycles were still present their amplitude was smaller at both frequencies the upwelling cycles are in phase with minimum ice volume evidence that glacial interglacial climate changes also include changes to the climate system that influence the low latitude monsoon we attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over asia and or to changes in the areal extent and seasonal cycle in asian snow cover that decrease the summer land sea temperature contrast other mechanisms may also be involved these new upwelling time series differ substantially from previous results however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late quaternary

dune activity as a record of late quaternary aridity in the northern kalahari new evidence from northern namibia interpreted in the context of regional arid and humid chronologies abstract sediments from presently vegetated linear dune ridges in the caprivi strip of northern namibia are optically dated to assess the timing of dune building phases and aridity in the late quaternary the dunes have been constructed from sediments likely to have been initially transported into the area by fluvial processes dune sediments do not preserve obvious evidence of hiatuses in deposition in the dated period 48 121 ka but continuous deposition and therefore continuous aridity is considered unlikely given differences in dated phases compared with chronologies from western zimbabwe to the southeast and western zambia to the north that include ages derived from the same equivalent sampling depths laboratory experiments provide evidence that dune sediments have not been bioturbated since deposition such that sample palaeodoses determined from different sampling depths are a true reflection of the duration of sediment burial independent chronologies from cave sediments provide evidence that humid episodes have occurred in the region within the dated range of dune construction it is concluded that dune construction in the late quaternary has been punctuated that the preserved record reflects the attributes of the aeolian processes that affect linear dune development and that the late quaternary record of dune building in the middle and northern kalahari was spatially complex

the resolving power of coseismic surface displacement data for fault slip distribution at depth 1 the spatial slip distribution of the august 17 1999 izmit turkey earthquake has previously been determined from both geodetic and seismological data though the models derived agree on a broad scale they are significantly different in detail these differences could be due to the inversion techniques applied and or additional constraints implemented and or be due to poor resolution it is also conceivable that the relative agreement at larger scales results in part from a resolution artifact common to all models to investigate this we perform a resolution analysis of fault slip models based on the inversion of surface displacement data assuming the theory of elastic dislocations in a half space and adopting a parameterization for which model predictions are capable of fitting the spatial variations of the data the inverse problem proves to be intrinsically ill conditioned

magnetic properties of marine magnetotactic bacteria in a seasonally stratified coastal pond salt pond ma usa summary magnetic properties of suspended material in the water columns of freshwater and marine environments provide snapshots of magnetic biomineralization that have yet to be affected by the eventual time integration and early diagenetic effects that occur after sediment deposition here we report on the magnetism geochemistry and geobiology of uncultured magnetite and greigite producing magnetotactic bacteria mb and magnetically responsive protists mrp in salt pond falmouth ma usa a small coastal marine basin 5 m deep that becomes chemically stratified during the summer months at this time strong inverse o2 and h2s concentration gradients form in the water column and a well defined oxic anoxic interface oai is established at a water depth of about 3 5 m at least four morphological types of mb both magnetite and greigite producers and several species of magnetically responsive protists are found associated with the oai and the lower sulphidic hypolimnion magnetic properties of filtered water were determined through the water column across the oai and were consistent with the occurrence of magnetite and greigite producing mb at different depths sharp peaks in anhysteretic remanent magnetization arm and saturation isothermal remanent magnetization sirm and single domain sd values of arm sirm occur within the oai corresponding to high concentrations of mb and mrp with magnetically derived cell densities of 10 4 10 6 ml 1 low temperature 1 per cent is present within magnetite magnetosomes produced either

early holocene monsoonal fluctuations in the garhwal higher himalaya as inferred from multi proxy data from the malari paleolake abstract a 4 9 m thick lake sequence formed due to the landslide damming of a stream in the semiarid garhwal himalaya was studied to understand past monsoonal variations in the region the optically stimulated luminescence osl chronology indicates that the lake existed between 12 and 7 ka ago chronologically constrained trends of sand percent organic phosphorus op apatite inorganic phosphorus aip and parameters of environmental magnetism were measured in the paleolake profile measured proxies indicate that the indian summer monsoon ameliorated in the early holocene after 12 ka cooling and it appears that all the proxies from the lake have captured this globally recognized early holocene warming four phases of wet conditions intensified monsoon are recognized at 11 5 ka 11 10 5 ka 10 9 ka and 8 7 ka with maximum uncertainties of 1000 years the wet phases are characterized by high magnetic susceptibility increased op and reduced aip in an attempt to understand the primary forcing of the sharp fluctuations in monsoonal activity in the region we show that changes in magnetic susceptibility match variations of residual atmospheric 14c suggesting a role for solar variability as an explanation of climatic variability

a 90 kyr upwelling record from the northwestern indian ocean using a novel long chain diol index abstract presently upwelling is of major importance for driving primary productivity in the arabian sea but its intensity in the past is not well constrained here we used long chain 1 14 alkane diols specific lipids of diatoms of the genus proboscia as new proxies to reconstruct upwelling conditions in the arabian sea variations in the seasonal lipid fluxes were determined using sediment traps in the somalia upwelling system deployed 80 km off the coast on the somali continental slope niop 905 10 45 444 n 51 56 655 e at 1265 m water depth 268 m above the sea floor and 270 km off the coast in the deep somali basin south of socotra niop 915 10 43 068 n 53 34 422 e at 3047 m depth 1000 m above the sea floor highest fluxes of c 28 and c 30 1 14 diols up to almost 600 g m 2 day 1 were only observed during nutricline shoaling at the onset of the southwest monsoon swm prior to massive upwelling by contrast fluxes of c 30 1 15 diols derived from as yet undefined biological sources only increased marginally during the swm and also during the northeast monsoon nem when instead of upwelling enhanced vertical mixing led to a second productivity pulse sediment core niop 905 taken at the continental slope site showed strong fluctuations in relative concentrations of long chain 1 14 and 1 15 diols with time which we quantified as the summed concentrations of c 28 and c 30 1 14 diols divided by the summed concentrations of c 28 and c 30 1 14 diols and c 30 1 15 diols this diol index follows the same trend as other upwelling intensity records from the arabian sea that are based on sea surface temperature reconstructions organic carbon content barium aluminium ratios and abundance and stable isotope composition of specific foraminiferal species the diol index was relatively high during the holocene ca 0 7 but much lower during the late glacial maximum ca 0 2 it was generally low during the last glacial but elevated values were found during the first half of marine isotope stage 3 between 60 and 45 ka and at the end of marine isotope stage 5 1 approximately 80 ka suggesting intensified glacial upwelling our data shows that long chain diols are suitable proxies to reconstruct past upwelling intensities in the arabian sea

characterizing the superparamagnetic grain distribution f v hk by thermal fluctuation tomography 1 in 1965 d j dunlop showed that the joint distribution of particle volumes and microcoercivities f v hk0 can be determined for magnetically monomineralic thermally stable single domain ssd ensembles by taking advantage of the joint temperature and field dependence of relaxation time we have developed a procedure that follows dunlop s strategy to obtain f v hk0 for ensembles containing both superparamagnetic and ssd grains based on backfield remanence curves measured over a range of temperatures each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the neel plot a suitable set of such line integral samples can be used to reconstruct the f v hk0 distribution using the methods of tomographic imaging samples of the basal tiva canyon tuff have narrow size distributions of elongate ti poor titanomagnetite tomographic inversion of the low temperature backfield spectra yield sharply peaked f v hk0 distributions from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes because our simplified forward calculations incompletely account for the effects of orientation distribution the width of the coercivity distribution at each temperature is underestimated and consequently the inverse calculations yield grain distributions that are overly broad frequency and temperature dependent susceptibilities calculated for the inverted f v hk0 distributions accord fairly well with measured susceptibilities for the weakly interacting tiva canyon samples less well for a moderately interacting paleosol specimen and poorly for a strongly interacting ferrofluid

glacial to interglacial contrasts in the northern indian ocean differences between the 18o 16o ratios of planktonic foraminifera deposited during the last glacial maximum about 18 000 yr ago and those deposited during the holocene show that in the indian ocean the south west monsoon was weaker than today but that the north east monsoon was stronger the upwelling observed in modern conditions along the southern coast of arabia had disappeared because of the low speed of the southwestern winds during glacial summers the reduction in rainfall and runoff over the continent caused a reduction of the salinity gradient in the bay of bengal and along the western coast of india increased precipitation fell on the sea south of 10 n while strong evaporation over the northernmost arabian sea produced an enhanced salinity gradient in that area

the palaeoenvironmental significance of dune size hierarchies the size of a dune is a function of many factors one of which is the length of the period of consistent wind regime under which it grew ephemeral dunes are defined as those that are adjusted to diurnal wind regimes meso dunes to annual wind regimes and mega dunes to wind regimes that persist for longer periods these longer periods could be major climatic cycles but there seem to be no dunes whose size reaches that allowed by the length of some of these periods there may therefore be a maximum size beyond which mega dunes cannot grow dictated by sand supply aerodynamic processes such as speed up or by the scale of the atmospheric boundary layer these concepts are tested on dunes in the wahiba sands in oman analysed from a landsat image it is hypothesised that a large set of megadunes here formed in the lgm that a smaller set of mega dunes may be of younger dryas age and that there is a meso dune series of holocene age

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

upwelling intensity and ocean productivity changes off cape blanc northwest africa during the last 70 000 years geochemical and micropalaeontological evidence abstract the accumulation of biogeochemical organic carbon calcium carbonates molybdenum and iodine micropalaeontological benthic foraminifera and terrigenous markers grain size si al zr al k al over the last 70 000 years in one core sedorqua 11k on the northwest african margin off cap blanc has been used to reconstruct past variations of local upwelling intensity and oceanic productivity the study demonstrates that productivity in this area increased during stage 3 particularly between 40 000 and 50 000 yr b p and during the last deglaciation between 6000 and 15 000 yr b p during most of isotopic stage 2 and particularly during the last glacial maximum productivity was much lower these variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity because of the establishment of upwelling cells over the shelf due to high sea level the conditions of sedimentation during the last 5 6 thousend years on the upper slope site 11k are largely dominated by advection from the shelf leading to strong sorting prior to deposition advection seems to have been minor during the other periods of enhanced productivity a conceptual model is proposed to link the productivity variations to atmospheric circulation in particular to the wind stress direction and seasonality

oxygen isotope evidence for a stronger winter monsoon current during the last glaciation a variety of palaeoclimate data from the northern indian ocean has shown that the asian summer monsoon was weaker during the last glacial maximum 18 000 years ago than it is today1 5 it has also been inferred from these data that the winter monsoon was stronger at that time5 here we present evidence that the transport of low salinity water from the bay of bengal to the arabian sea caused by the winter monsoon6 was higher during the last glacial maximum this inference is based on a negative excursion up to about 1 in magnitude in the oxygen isotope ratios of three planktonic foraminiferal species detected in a sediment core from the arabian sea by high resolution 1 2 cm sampling our results place an upper limit of about 4 000 years on the length of time during which the winter monsoon was stronger than today

toward a new 250 c pyrrhotite magnetite geothermometer for claystones abstract we investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation immature burial temperature 40 c of bure callovo oxfordian claystones in the basin of paris france early mature burial temperature 85 c of opalinus lower dogger claystones from the mont terri anticline in front of the jura fold belt switzerland and mature to overmature burial temperature 7 s 8 the p transition is also detected in early mature to mature opalinus and chartreuse claystones we conduct additional experimental heating of natural opalinus claystones one set of experiments is referred to as short term heating 1 h from 100 c to 200 c it is dedicated to an investigation of the effect of short lived heating processes in geology a second set of heating experiments is designed to approach burial conditions using a gold capsule in burial like experiments we heated opalinus claystones from 150 c to 250 c for several weeks under a pressure of 100 mpa in both experiments we observe a correlative diminution of the pyrrhotite signature at 35 k with increasing temperature we interpret this trend as the appearance of magnetite we derive a parameter pm from the warming curve of a saturated isothermal remanent magnetization acquired at 10 k zfc we report on a consistent evolution of pm with temperature in the range of 40 c to 250 c including natural samples heated samples at 95 c and burial like heated samples pm first increases between 40 c up to 85 c implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature for temperatures above 85 c pm decreases up to 250 c implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite pm values obtained from mature to overmature claystones from the chartreuse are lower than the pm values obtained from the burial like heated opalinus claystones suggesting that the formation of magnetite is driven by kinetics the continuous trend of the pm parameter suggests that the magnetic properties of pyrrhotite magnetite claystones can be used to infer paleo temperatures and we propose to name this geothermometer mageval

change in the intensity of low salinity water inflow from the bay of bengal into the eastern arabian sea from the last glacial maximum to the holocene implications for monsoon variations a 100 400 km wide region of the coastal eastern arabian sea eas off the west coast of india is characterized by a low salinity tongue formed by the inflow of low salinity surface water from the bay of bengal bob this low salinity tongue is largely driven by the sea level higher in bob than in the arabian sea and by alongshore pressure gradient between southern and northern eas and is expected to respond to summer monsoon freshwater flux to the bay here we report past variation in the relative intensity of summer and winter monsoons based on changes in the north south salinity gradient within this low salinity tongue the salinity gradient is estimated from paired measurement of 18 o and mg ca in globigerinoides sacculifer extracted from sediment cores collected at northern high salinity and southern low salinity locations within this tongue the last glacial maximum lgm to peak holocene 18 o and sea surface temperature gradients at both locations are 2 and 2 o c respectively while the sea surface salinity gradient at northern eas is 0 5 psu higher than in the southern eas suggesting distinctly different sss structure in the lgm eas the north south surface salinity gradient was also larger by 0 5 psu during the lgm 1 2 psu as compared to the gradient during the holocene 0 7 psu increased north south surface salinity gradient during the lgm suggests diminished flow of low salinity water into the coastal eas caused by combined effect of decreased freshening of the bob and reduced seasonal mountain rivers discharge into the eas such surface hydrographic conditions in the coastal eas clearly indicate significantly weakened summer monsoons and strengthened winter monsoons during the lgm

a 400 000 year record of vegetational and climatic changes from a volcanic basin leizhou peninsula southern china abstract the tianyang lake in leizhou peninsula is an enclosed crater formed during the early to middle pleistocene over 200 m of diatomaceous mud and clay were deposited in the last 400 000 years providing a continuous middle and late quaternary lacustrine sequence for paleoenvironmental studies the pollen record spans the last four glacial interglacial cycles equivalent to oxygen isotope stages ois 10 to 1 during most of the glacial phases fagaceous monsoon forest was replaced by altitudinal vegetation indicating a regional landscape transformation on the peninsula the montane forest reached down to the lowland by at least 600 m during the ois 6 and 8 in a cool and relatively wet condition with temperature estimated to be more than 4 c lower than present a further lowering of the montane forest belts by about 800 1000 m occurred in the ois 4 2 as implied by the appearance of many temperate elements during the last glacial maximum this mixed forest subsequently gave way to the poaceae grassland a savanna vegetation type favored by drier and cooler climatic conditions this drastic change implies a more significant drop in temperature and precipitation than the previous three glacial periods ois 10 8 and 6 it is thus inferred that the weakening of summer monsoon was more significant towards the end of the late pleistocene particularly during the last glacial maximum lgm

understanding the arabian sea reflections on the 1994 1996 arabian sea expedition abstract the arabian sea expedition now five years past its field observations is at a stage when some of its dominant themes can be summarized of the large range of possible topics five are considered here 1 is the arabian sea a source or sink for carbon dioxide 2 is the arabian sea mother nature s iron experiment 3 do grazing zooplankton control carbon flux to the seabed 4 does the paleoceanographic record help us predict the ocean s response to climate change and 5 what are the predominant physical processes of the arabian sea a short history of each issue and results from the field work of 1994 1996 are presented

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

a quantitative assessment of the arabian sea sediment record using gcm simulated upwelling abstract a general circulation model gcm of the ocean is used to study the effects of changes in the monsoon winds of present day and 9000 yr b p on the upwelling in the arabian sea the vertical transport from the ocean gcm is used as a measure of upwelling strength to assess the relative abundance of globigerina bulloides at present and 9000 yr b p during the climatic optimum the high abundance of g bulloides at 9000 yr b p is probably a result of a doubling of the vertical transport at that time

evidence for abundant isolated magnetic nanoparticles at the paleocene eocene boundary new rock magnetic results thermal fluctuation tomography high resolution first order reversal curves and low temperature measurements for samples from the paleocene eocene thermal maximum and carbon isotope excursion in cored sections at ancora and wilson lake on the atlantic coastal plain of new jersey indicate the presence of predominantly isolated near equidimensional single domain magnetic particles rather than the chain patterns observed in a cultured magnetotactic bacteria sample or magnetofossils in extracts the various published results can be reconciled with the recognition that chain magnetosomes tend to be preferentially extracted in the magnetic separation process but as we show may represent only a small fraction of the overall magnetic assemblage that accounts for the greatly enhanced magnetization of the carbon isotope excursion sediment but whose origin is thus unclear

do benthic foraminifer records represent a productivity index in oxygen minimum zone areas an evaluation from the oman margin arabian sea abstract benthic foraminifer accumulation rates bfar and benthic to planktic foraminifer ratios b p have been suggested to represent an index of paleoproductivity in the world oceans it is shown here however that bfar and b p do not record the surface water productivity signal in the intense upwelling and oxygen minimum zone omz along the oman margin greater bfar and b p values are noticed during the last glacial period when the monsoon and upwelling strength were weaker and the surface water productivity was reduced this is in contrast to the suggestion that bfar and b p values are an indication of enhanced productivity on the other hand bfar and b p values were lower during the holocene as a consequence of intensified monsoon and upwelling and resulting enhanced productivity in the arabian sea this appears to suggest that bfar and b p do not reflect the productivity variations in the omz of the arabian sea we speculate that dissolved oxygen concentrations might instead be controlling the benthic foraminifer abundance

multiple episodes of aggradation and calcrete formation in late quaternary aeolian sands central thar desert rajasthan india abstract a 12 m thick section in a dune sandy plain terrain of the central thar in rajasthan has provided a near continuous record of environmental change for the past 160 ka the site presently receives 280 mm rainfall almost entirely from the summer monsoon the base of this section comprises a gravel bed of an ephemeral stream and the overlying six litho units each with discrete boundaries comprise a succession of aeolian sands luminescence dating provided an estimate of the timing of the sand aggradation periods to 160 90 60 27 and 17 14 ka and helped constrain the timing of calcrete formation periods in each aggradation unit discrete nodular calcretes formed by the leaching of carbonate from the overlying solum this is analogous to present day conditions in sandy plains during periods of increased rainfall and landscape stability several of these calcretes are however devoid of their donor solum suggesting solum removal during a subsequent period of decreased rainfall and resultant surface instability this is supported by the presence of reworked nodules on the surfaces of some calcretes a prominent phase of calcrete development followed the aeolian sand aggradation at 60 ka suggesting climate amelioration that also caused the formation of groundwater related calcrete and mottling the study suggests that stage ii calcrete nodules form in a time frame of 10 20 ka and confirms limited data on the duration and stage of calcrete development in the literature the 13c values of calcrete carbonate lie in a narrow range 0 5 to 1 1 suggesting formation under soils with c4 vegetation this implies that even during phases of climatic amelioration the high temperatures and increased seasonality of rainfall did not permit significant development of c3 plants in the central thar

reconstructing the phytoplankton community of the cariaco basin during the younger dryas cold event using chlorin steryl esters 1 a record of the downcore distribution of chlorin steryl esters cses through the younger dryas was produced from cariaco basin sediments in order to assess the potential use of cses as recorders of the structure of phytoplankton communities through time using an improved high performance liquid chromatography method for the separation of cses we find significant changes in the distribution of cses during the younger dryas in the cariaco basin during the younger dryas enhanced upwelling in the cariaco basin caused an increase in the diatom population and therefore an increase in the relative abundance of cses derived from diatoms in contrast the dinoflagellate population and therefore cses derived from dinoflagellates decreased in response to the climate change during the younger dryas these community shifts agree well with the shifts observed in the present day on a seasonal basis that result from the north south migration of the intertropical convergence zone over the cariaco basin we also identify changes in the abundance of several cses that seem to reflect rapid warming and cooling events this study suggests that cses are useful proxies for reconstructing phytoplankton communities and paleoenvironments index terms 1055 geochemistry organic geochemistry 4267 oceanography general paleoceanography 4855 oceanography biological and chemical plankton keywords younger dryas cariaco basin chlorin steryl esters

impact of productivity events on the benthic foraminiferal fauna in the arabian sea over the last 150 000 years we have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest arabian sea covering the last 150 000 years and compared the results with the stable isotopic record from both benthic uvigerina peregrina and planktonic globigerinoides sacculifer foraminifera the 18o record of the planktonic foraminiferal species shows excursions towards heavier 18o values when compared to the benthic record and the specmap stack particularly in stage 3 between approximately 30 and 45 50 kyr we call this the stage 3 event species diversity and relative abundance data on benthic foraminifera reveal that some species e g cibicidoides wuellerstorfi oridorsalis umbonatus pullenia bulloides pyrgo spp and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event whereas other e g astrononion novozealandicum cibicidoides bradyi eggerella bradyi gyroidina altiformis hoeglundina elegans pullenia subcarinata and sigmoilina edwardsii decreased using correspondence analysis we determined that approximately 50 of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor the relative abundance of some species appears to vary on a precessional cycle perhaps responding to monsoon driven upwelling and associated increased food supply whilst the abundances of other species are related to the 100 kyr ice volume cycle and may be responding to changes in intermediate and deep water chemistry geochemical paleoproductivity tracers such as the ba content of the sediment have a very similar distribution to specific foraminiferal factors in both time and frequency domains certain planktonic foraminifera e g globigerina bulloides also display obvious maxima in the stage 3 event the stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index we conclude that there was a period 15 to 20 kyr duration during stage 3 with cooler than normal surface waters enhanced productivity and rapid settling of biogenic matter

combined use of magnetometry and spectroscopy for identifying magnetofossils in sediments identification of the mineral remains of magnetotactic bacteria mtb known as magnetofossils is of particular interest because their occurrence can be used for environmental and climatic reconstructions single domain magnetite particles which are biomineralized in the cell body of mtb have characteristic properties that can be used to detect their fossil remains acquisition of anhysteretic and isothermal remanent magnetization arm and irm first order reversal curve forc diagrams and ferromagnetic resonance fmr spectra were used to detect the magnetic mineral inventory in holocene lake sediments a comparative analysis in terms of the discriminatory power of these methods is presented the forc diagrams contain two distinct features a sharp horizontal ridge centered on the horizontal axis bc and a feature with symmetric spread along the vertical bb axis the coercivity spectra derived from the central ridge coincides with that derived from arm and irm acquisition curves and is compatible with the presence of noninteracting linear chains of single domain magnetite the second feature on forc diagrams is indicative of interacting particles in clusters in the fmr spectra from bulk sediment two populations are separated empirically based on the forc information an asymmetric signal is taken to describe the population which contains single domain particles in clusters empirical spectral separation of this contribution results in fmr spectra that are similar to those of intact mtb which strongly suggests that a fraction of linear magnetosome chains is present combination of fmr and forc results demonstrates the strong potential of these methods for identifying magnetofossils based on alignment and interaction patterns of magnetic particles

a glimpse of the quaternary monsoon history from india and adjoining seas abstract the evolution of the monsoon is briefly summarized taking into account previous studies conducted on both terrestrial and marine records recovered from the indian subcontinent and the adjoining seas while the initiation of the monsoon is debated it seems clear that a major intensification of the summer monsoon occurred at 8 2 ma a seasonal monsoon circulation with distinct summer and winter monsoon phases was established at 2 8 ma in the late quaternary the summer monsoon weakens during glacial periods as compared to interglacial periods the centennial to sub centennial scale changes in the monsoon during both glacial and interglacial periods suggest strong links between high latitude processes and the tropical monsoon and its role in modulating northern hemispheric climate the glacial terminations are marked by weak monsoon activity the summer monsoon weakened during the middle holocene after the early holocene optimum a change in the monsoon intensity though alters the salinity of the seas surrounding india the quantitative paleosalinity reconstructions still have large associated uncertainties and an effort should be made to lower them

monsoon climate of the early holocene climate experiment with the earth s orbital parameters for 9000 years ago values for the precession and obliquity of the earth 9000 years ago indicate that the global average solar radiation for july 9000 years ago was 7 percent greater than at present when the estimated solar radiation values are used in a low resulation climate model the model simulates an intensified continent scale monsoon circulation this result agrees with paleoclimatic evidence from africa arabia and india that monsoon rains were stronger between 10 000 and 5000 years ago than they are today

the southwest indian monsoon over the last 18 000 years previously published results suggest that the strength of the sw indian monsoon can vary significantly on century to millenium time scales an observation that has important implications for assessments of future climate and hydrologic change over densely populated portions of asia we present new well dated multi proxy records of past monsoon variation from three separate arabian sea sediment cores that span the last glacial maximum to late holocene to a large extent these records confirm earlier published suggestions that the monsoon strengthened in a series of abrupt events over the last deglaciation however our data provide a somewhat refined picture of when these events took place and suggest the primacy of two abrupt increases in monsoon intensity one between 13 and 12 5 ka and the other between 10 and 9 5 ka this conclusion is supported by the comparisons between our new marine data and published paleoclimatic records throughout the african asian monsoon region the comparison of data sets further supports the assertion that maximum monsoon intensity lagged peak insolation forcing by about 3000 years and extended from about 9 5 to 5 5 ka the episodes of rapid monsoon intensification coincided with major shifts in north atlantic european surface temperatures and ice sheet extent this coincidence coupled with new climate model experiments suggests that the large land sea thermal gradient needed to drive strong monsoons developed only after glacial conditions upstream of and on the tibetan plateau receded cold north atlantic sea surface temperatures european ice sheets and extensive asian snow cover it is likely that abrupt changes in seasonal soil hydrology were as important to past monsoon forcing as were abrupt snow related changes in regional albedo our analysis suggests that the monsoon responded more linearly to insolation forcing after the disappearance of glacial boundary conditions decreasing gradually after about 6 ka our data also support the possibility that significant century scale decreases in monsoon intensity took place during the early to mid holocene period of enhanced monsoon strength further highlighting the need to understand paleomonsoon dynamics before accurate assessments of future monsoon strength can be made

punctuated aridity in southern africa during the last glacial cycle the chronology of linear dune construction in the northeastern kalahari abstract the mega kalahari of central southern africa is one of the most extensive quaternary desert basins on a regional scale present day aeolian activity is restricted to episodic dune crest reactivation in the most arid southwestern desert core there is however abundant evidence of former periods of both more arid and more humid conditions many of which have little or no chronological control we have employed optical dating of quartz sand grains to develop a chronology of arid intervals as recorded by phases of linear dune construction in the northeastern sector of the mega kalahari we identify repeated phases of aeolian deposition during the last interglacial glacial cycle at ca 95 115 41 46 20 26 and post 20 ka which are separated by depositional hiatuses that we infer to correspond to more humid periods these aeolian depositional events correlate with and are inferred to relate to millennial scale cold sea surface temperature events in the southeast atlantic which have been linked to sub milankovitch climate changes recognised in northern hemisphere oceanic and cryospheric environmental archives covering the same time period while the present landscape is the product of either post 20 ka hwange national park dune field or 20 30 ka victoria falls dune field aeolian activity and subsequent erosion and reworking much of the vertical expression of the larger dune forms corresponds to the earlier periods of activity the linear ridges of the area are rich archives of late quaternary terrestrial climate change

low temperature remanence in stable single domain magnetite 1 a numerical model is developed in order to simulate the loss of low temperature remanence while warming through the verwey transition we concentrate on the amount of remanence lost fc and zfc during field cooled and zero field cooled low temperature demagnetization curves for stable single domain ssd particles of 1 inorganic magnetite and 2 biogenic magnetite in the form of magnetosome chains the model predicts that delta ratios fc zfc for inorganic ssd magnetite increase from 1 to 1 6 with increasing aspect ratio and that magnetosome chains have elevated delta ratios of at least 2 0 disaggregated magnetosomes are predicted to behave similarly to inorganic acicular ssd magnetite the numerical results agree with experimental results

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

a 225 kyr record of dust supply paleoproductivity and the oxygen minimum zone from the murray ridge northern arabian sea abstract analysis of a 15 m long sediment core recovered from the murray ridge northern arabian sea reveals a 225 000 year record of climate induced variations in surface water productivity mid water ventilation and dust input in the northern arabian sea productivity and dust input records closely resemble those from the owen ridge and oman margin also the phase lag of 10 kyr between maximum productivity and precession minimum is in phase with the summer monsoon upwelling record from the owen ridge and the oman margin this implies that the productivity variations in the northern arabian sea during the last 225 kyr are primarily controlled by changes in the indian ocean summer monsoon the winter monsoon obviously played a minor role in the productivity history of the northern arabian sea another result from this study is that the mg al record from the murray ridge nicely documents the post glacial flooding of the persian gulf which is the main glacial source for dolomite dust in this region finally it is concluded that the intensity of the omz co varies with surface water productivity being weak during periods of reduced summer monsoon especially during glacial periods

first evidence for episodic flooding events in the arid interior of central saudi arabia over the last 60 ka although evidence for quaternary environmental changes in the arabian peninsula is now growing research has mostly been conducted in the united arab emirates uae and in the sultanate of oman there have been virtually no recent studies in saudi arabia especially in the central region such as around al quwaiayh in this area there are a series of outwash plains developed along the eastern edge of the arabian shield that formed in the late quaternary four sedimentary sections which are representative of the deposits that have accumulated have been studied and five luminescence ages obtained these are the first luminescence ages acquired from quaternary sediments in central saudi arabia the preserved fluvial deposits in the study area have formed during humid events at ca 54 ka ca 39 ka and ca 0 8 ka in more recent times aeolian sands have been encroaching on to the distal parts of the outwash plains copyright 2008 john wiley sons ltd

evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub oxic tasman sea sediments abstract sediments from the tasman basin in the southwest pacific ocean show a cyclic pattern of variation in mineral magnetic properties including susceptibility and remanence over the brunhes chron the magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by tem investigation of the magnetic fraction the intervals of low bacterial magnetite concentration occurring in glacial periods are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage there is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped either as the result of variation in bottom water oxygen concentration or sedimentation rate the variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration the bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick

sensitivity of the african and asian monsoons to mid holocene insolation and data inferred surface changes abstract orbital forcing alone is not sufficient to explain the massive northward penetration of monsoon rains in africa shown by data during the mid holocene 6000 yr ago feedbacks associated with changes in ssts and land surface cover may be necessary to produce a sufficient increase in the monsoon a step toward a better understanding of the respective role of oceans and land surfaces is to design sensitivity studies with prescribed forcings inferred from observations in the first study ssts are lowered in the upwelling regions offshore of west africa and somalia and increased in the bay of bengal and south china sea in the second simulation the modern sahara desert is replaced by a combination of xerophytic woods scrub and grassland in both cases the amount of water vapor advected from oceanic sources is increased north of 10 n in africa in response to the increased land sea temperature contrast thereby enhancing rainfall but the magnitude of the simulated changes is much larger when land s

high resolution stratigraphic correlation of benthic oxygen isotopic records spanning the last 300 000 years abstract in order to compare the response of different oceanographic regions to global climate change very detailed stratigraphic techniques are required the global signal of ice volume changes recorded in the oxygen isotopic composition of foraminifera can provide the tool for developing the necessary high resolution stratigraphy in order to evaluate the resolution of a stratigraphy based on detailed isotopic records two techniques are used to correlate a set of benthic oxygen isotope records from seven piston cores taken in the north and south atlantic the indian and the equatorial and north pacific oceans the first technique was modified from the graphic correlation procedure of shaw 1964 this procedure requires the identification of isotopic events that are correlated from core to core detailed correlations for intervals of the cores between events are provided by a series of straight line segments connecting all common events the second technique developed by martinson et al 1982 uses inverse procedures to define a continuous non linear mapping function that correlates the isotopic records the mapping function maximizes the correlation coefficient between data sets being compared the techniques are independent in that they rely on different criteria for correlating the data series stratigraphic correlations obtained by these procedures are in excellent agreement the mean difference between the correlations is on the order of the sampling intervals of each core and when corrected for sedimentation rates suggests that benthic isotope records from the suite of seven cores can be correlated to a resolution of 2000 to 4000 yrs

low temperature cycling of isothermal and anhysteretic remanence microcoercivity and magnetic memory abstract this paper reports low temperature cycling ltc through the verwey transition of anhysteretic remanence arm partial arms and partially demagnetised saturation isothermal remanence sirm induced at room temperature in pseudo single domain and multidomain md magnetite the remanences were cooled in zero field to 50 k and then heated back to room temperature by inducing partial arms over different field ranges and by partially alternating field demagnetising sirms it was possible to isolate both low coercive force and high coercive force fractions of remanence on cooling through the verwey transition a sharp increase in the remanence was observed the relative size of the jump increased as the high coercive force fraction was increasingly isolated this behaviour is interpreted as being due to both an increase in the single domain multidomain threshold size on cooling through the verwey transition and to the reduction or elimination of closure domains in the low temperature phase in addition the memory ratio i e the fraction of remanence remaining after ltc divided by the initial remanence was found to be higher for the high coercive force fraction than the low coercive force fraction in our interpretation the high coercivity fraction behaviour is associated with reversible domain re organisation effects whilst the low coercive force fraction s behaviour is associated with irreversible domain re organisation and de nucleation processes due to the decrease in magnetocrystalline anisotropy on cooling to the verwey transition the high coercive force fraction is likely to be magnetoelastically controlled thus a rock displaying high coercive force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures in addition ltc analysis can be used to identify the domain state dominating the natural remanence in magnetite bearing rocks

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

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

comment on new late pennsylvanian paleomagnetic results from parana basin southern brazil is the recent giant gaussian process model valid for the kiaman superchron by brandt et al

variability in upwelling fields in the northwestern indian ocean 1 model experiments for the past 18 000 years a nonlinear reduced gravity ocean model is used to assess the effects of changes in the monsoon winds during glacial and interglacial conditions on the seasonal circulation in the northwestern indian ocean winds from the national center for atmospheric research community climate model simulations for 18 kyr b p the most recent glacial maximum and a period of weaker monsoon winds 9 kyr b p near the beginning of the present interglacial and a period of stronger monsoon winds and present day 0 kyr b p conditions are used to drive the model to a steady seasonal cycle strength of upwelling fields are inferred in each case by integrating upward displacements of the model pycnocline over the primary upwelling season of mid april to mid august in both the 0 kyr b p and the 9 kyr b p cases a broad band of upwelling and decreased model upper layer thickness extends along the coast of the arabian peninsula out 350 500 km offshore during the southwest monsoon this upwelling is driven by the strongly positive wind stress curl beneath the cyclonic side of the atmospheric findlater jet located to the north and west of the jet axis farther offshore in the anticyclonic region to the south and east of the jet axis the negative wind stress curl drives downwelling and increased upper layer thickness in the 9 kyr b p case the atmospheric jet is much stronger and narrower than in the 0 kyr b p case with stronger values of both positive and negative curl to the northwest and southeast respectively of the jet axis this drives much stronger upwelling to the northwest of the jet axis and downwelling to the southeast in the 18 kyr b p case the findlater jet is very weak and upwelling patterns are uniformly weak across the basin the spatial distribution and temporal variation of the upwelling fauna in the sediment record is consistent with the model upwelling fields in all three cases the ocean model thus provides the link between the climate model and the sedimentation data verifying the hypothesis that variations in upwelling driven by variations in the strength of the monsoon jet are responsible for the observed variations in the sediment record

low temperature magnetic properties of pelagic carbonates oxidation of biogenic magnetite and identification of magnetosome chains 1 pelagic marine carbonates provide important records of past environmental change we carried out detailed low temperature magnetic measurements on biogenic magnetite bearing sediments from the southern ocean ocean drilling program odp holes 738b 738c 689d and 690c and on samples containing whole magnetotactic bacteria cells we document a range of low temperature magnetic properties including reversible humped low temperature cycling ltc curves different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of ltc curves a dipole spring mechanism in magnetosome chains is introduced to explain reversible ltc curves this dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite similar to published results for uniaxial stable single domain sd particles the dipole spring mechanism reversibly restores the remanence during warming in ltc measurements this supports a previous idea that remanence of magnetosome chains is completely reversible during ltc experiments we suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains although the presence of isolated uniaxial stable sd particles and magnetically interacting particles can complicate this test magnetic measurements through the eocene section of odp hole 738b reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic hard component co occurrence of these two magnetic fingerprints during the late eocene in the southern ocean indicates less oxic conditions probably due to increased oceanic primary productivity and organic carbon burial

can sediment geochemistry record changes in coastal upwelling palaeoproductivity evidence from northwest africa and the arabian sea abstract the causes and consequences of organic carbon c org burial in sediments underlying regions of coastal upwelling are of profound importance to studies of palaeoceanography and palaeoclimatology alike the primary control on coastal upwelling wind stress and resulting ekman transport is linked intimately to variation in the earth s insolation record via a variety of feedback mechanisms in comparison the nutrients required to sustain productivity and the factors affecting the c org content of the sediment are related to variation in global glacial boundary conditions using a variety of geochemical indicators it is apparent that the record of upwelling off northwest africa driven by the northeast trade winds is characterized by arid glacial conditions and high wind stress sustaining elevated euphotic zone productivity the resulting c org accumulation record reflects higher palaeoproductivity during glacial episodes in contrast the arabian sea upwelling is seasonal faunal and geochemical ba al u th ratios indicators suggest strong interglacial 23 ka cyclicity in the southwest monsoon the c org record is spatially and temporally consistent but in disagreement with other palaeoproductivity indicators these results point to the decoupling of surface ocean productivity in areas of coastal upwelling from deep ocean circulation and nutrient supply this result has important consequences for driving atmospheric p co 2 changes on timescales less than that of oceanic mixing further applications of geochemical tracers to areas of coastal upwelling such as northwest africa and peru are needed urgently

coherent response of arabian sea upwelling and pollen transport to late quaternary monsoonal winds the indian summer monsoon causes large seasonal changes in the environments of both eastern africa and the arabian sea here we compare time series 140 kyr long of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation during interglacial intervals the pollen and upwelling records of the western arabian sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the earth s rotational axis 23 kyr we conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records these data indicate that two distinct modes of monsoon variability may exist an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records and a glacial mode with both weaker monsoons and decreased linkage between arabian sea pollen and upwelling records

early holocene to present palaeoenvironmental shifts and short climate events from the tropical wetland and lake sediments kukkal lake southern india geochemistry and palynology the kukkal basin tamil nadu india receives most of its rain from the southwest monsoon swm a sediment core from kukkal lake preserves a continuous sediment record from the early holocene to p

a methane based time scale for vostok ice abstract tuning the vostok methane signal to mid july 30 n insolation yields a new ice core gas time scale this exercise has two rationales 1 evidence supporting kutzbach s theory that low latitude summer insolation in the northern hemisphere controls the strength of tropical monsoons and 2 interhemispheric ch 4 gradients showing that the main control of orbital scale ch 4 variations is tropical monsoonal sources the immediate basis for tuning ch 4 to mid july insolation is the coincident timing of the most recent pre anthropogenic ch 4 maximum at 11 000 10 500 calendar years ago and the most recent july 30 n insolation maximum all ages in this paper are in calendar years unless specified as 14 c years the resulting ch 4 gas time scale diverges by as much as 15 000 years from the gt4 gas time scale petit et al nature 399 1999 429 prior to 250 000 years ago but it matches fairly closely a time scale derived by tuning ice core 18 o atm to a lagged insolation signal shackleton science 289 2000 1897 most offsets between the ch 4 and 18 o atm time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the 18 o atm signal the ch 4 time scale provides an estimate of the timing of the vostok co 2 signal against specmap marine 18 o often used as an index of global ice volume on the ch 4 time scale all co 2 responses are highly coherent with specmap 18 o at the orbital periods co 2 leads 18 o by 5000 years at 100 000 years eccentricity but the two signals are nearly in phase at 41 000 years obliquity and 23 000 years precession the actual phasing between co 2 and ice volume is difficult to infer because of likely sst overprints on the specmap 18 o signal co 2 could lead or be in phase with ice volume but is unlikely to lag behind the ice response