alternating field characteristics of pseudo single domain 2 14 μm and multidomain magnetite abstract we report normalized af demagnetization curves of anhysteretic remanences arm s produced by 1 10 and 40 oe steady fields and of saturation isothermal remanence irm s in a suite of dispersed unannealed magnetite powders with median sizes of 2 4 6 10 and 14 m pseudo single domain or psd size range and 100 m multidomain or md size interpreted in the light of the domain structure test first proposed by lowrie and fuller 12 the relative stability trend of curves for the 2 m sample is of single domain sd type the 1 oe arm being most resistant to demagnetization followed by the 10 oe and 40 oe arm s and irm s for the 100 m sample the trend is exactly reversed and is of md type in the 4 14 m samples hitherto undescribed transitional trends between sd type and md type occur at 6 m 1 oe 10 oe and 40 oe arm s preserve an sd type trend but for all af s 75 oe irm s is more resistant than any of these remanences at 10 m this trend is unmistakable and only at 14 m do the 1 oe 10 oe and 40 oe arm curves merge we conclude 1 that the lowrie fuller test distinguishes between small md grains enhanced by psd remanence and large md grains lacking psd remanence rather than between sd and md structures per se and 2 that in the psd transition region from 6 to 14 m in magnetite irm s changes over to md type relative stability around 6 m whereas 10 oe and 40 oe arm s achieve an md type trend around 14 m in accord with the predicted field dependence of the psd threshold size our theoretical interpretation assumes that the intrinsic internal field coercive force spectra of weak field and strong field remanences are identical but that the observed external field spectrum is shifted to lower fields as a result of the internal demagnetizing field nj r of the remanence j r the effect is slight for weak field j r s but substantial for irm s since all coercivities high as well as low are shifted the result of the lowrie fuller test is determined simply by the shape of the intrinsic coercivity spectrum or the corresponding af demagnetization curve depending on the model of self demagnetization used either subexponential or sublinear af decay curves of weak field remanence will automatically lead to an md type trend whereas by either model the decay curves that characterize sd and psd remanences decaying slowly initially and then more rapidly will always produce and sd type trend

anhysteretic remanent magnetization of fine magnetic particles 1 various magnetic parameters are in common use for estimating the grain size of magnetic particles among these the ratio of the intensity of anhysteretic remanent magnetization arm to that of isothermal remanent magnetization as well as their alternating field af demagnetization curves are used as an indicator of the domain state of the particles several models have been proposed to describe physically the acquisition of arm in a biased af field jaep 1969 first developed a semiquantitative theory based entirely on the thermal fluctuation analysis developed by neel 1949 1954 1955 significant discrepancies were found between his model and experimental results on magnetite a new general theory of arm based on the work of jaep is presented here with particular regard to the influence of various parameters like grain size coercivity and mineralogy on arm intensity an analytical expression for arm intensity in the special case of very fine particles was derived from this theory and a good agreement with experimental results and data from the literature was found a new estimation of the atomic reorganization time was obtained from arm measurements on a sample of the yucca mountain tuff which has well known mineralogy and grain size distribution the results are in agreement with the value proposed by mcnab et al 1968 for magnetite some authors considered magnetic interactions as the key to understand the arm in fine particles and this is certainly true for strongly interacting samples in this case arm would be useless for the characterization of magnetic grains however many sediments have a very low concentration of well distributed magnetic grains for these samples the explanation of an arm in terms of intrinsic properties of the grains as qualitatively proposed by other authors is more suitable

mediterranean moisture source for an early holocene humid period in the northern red sea paleosalinity and terrigenous sediment input changes reconstructed on two sediment cores from the northernmost red sea were used to infer hydrological changes at the southern margin of the mediterranean climate zone during the holocene between approximately 9 25 and 7 25 thousand years ago about 3 per thousand reduced surface water salinities and enhanced fluvial sediment input suggest substantially higher rainfall and freshwater runoff which thereafter decreased to modern values the northern red sea humid interval is best explained by enhancement and southward extension of rainfall from mediterranean sources possibly involving strengthened early holocene arctic oscillation patterns and a regional monsoon type circulation induced by increased land sea temperature contrasts we conclude that afro asian monsoonal rains did not cross the subtropical desert zone during the early to mid holocene

coercive forces and coercivity spectra of submicron magnetites abstract remanent coercivity spectra have been determined for four dispersions of magnetite with mean particle sizes of 0 04 0 22 m using four different methods 1 acquisition of isothermal remanent magnetization irm in continuous fields 2 stepwise reversal of saturation irm by opposite polarity continuous fields 3 stepwise alternating field af demagnetization of saturation irm and 4 af demagnetization of weak field thermoremanent magnetization trm alternating field methods gave narrower distributions with lower median coercivities than continuous field cf methods the two cf spectra are offset the remanent coercive force hr which is the median coercivity determined by cf reversal of saturation remanence is always less than the median coercivity h r determined from irm acquisition the median destructive field of saturation irm h 1 2 the median coercivity determined by af demagnetization is less than eitherhr orh r the relation reported by dankers 1 h r h 1 2 2hr is obeyed for these samples and for a companion suite of single domain magnetites and maghemites ordinary coercive force hc is substantially affected by reversible magnetization and is a poor indicator of the median coercivity of remanence hc is much smaller thanh 1 2 hr orh r and varies in a different way with particle size

hydroclimate in africa during the medieval climate anomaly the medieval climate anomaly mca is a recognized period of distinct pre industrial climate change with a core period of 1000 1200 ce the field of palaeoclimatology has made major progress over the past 15 years during which a great number of high and medium resolution case studies were published reconstructing climate change of the past millennia in many parts of the world regional data coverage has now reached a point which allows compiling palaeoclimate maps for well defined time intervals here we present hydroclimatic trend maps for the mca in africa based on 99 published study locations key hydroclimatic proxy curves are visualized and compared in a series of 16 correlation panels proxy types are described and possible issues discussed based on the combined mca dataset temporal and spatial trends are interpreted and mapped out three areas have been identified in africa in which rainfall seems to have increased during the mca namely tunisia western sahel and the majority of southern africa at the same time a reduction in precipitation occurred in the rest of africa comprising of nw and ne africa west africa eastern africa and the winter rainfall zone of south africa mca hydroclimate change in africa appears to have been associated with characteristic phases of ocean cycles as also supported by modern climate observations aridity in morocco typically coincides with the positive phase of the north atlantic oscillation nao whilst increased rainfall in the western sahel is often coupled to the positive phase of the atlantic multidecadal oscillation amo reduction in rainfall in the region gulf of aden southern red sea to eastern africa could be linked to a negative indian ocean dipole iod or a derived long term equivalent indian ocean cycle parameter the intertropical convergence zone itcz appears to have been shifted pole wards during the mca for both the january and july positions mca hydroclimate mapping revealed major data gaps in the sahara south sudan somalia central african republic democratic republic of congo angola northern mozambique zambia and zimbabwe special efforts are needed to fill these gaps e g through a dedicated structured research program in which new multiproxy datasets are created based on the learnings from previous african mca studies

late quaternary glacial interglacial cyclicity models of the red sea the benthic foraminiferal faunas from samples of two piston cores retrieved along a north south transect in the red sea were studied the northern core was collected during meteor cruise m 31 2 while the southern one was collected during the sonne cruise 121 benthic foraminiferal faunas from both sites exhibit large variability with respect to density diversity species composition and assemblages combined with stable oxygen and carbon isotope records of planktic and benthic foraminifera one hundered thirty benthic foraminiferal species were identified in the investigated cores the faunal data set of the northern core was reduced to five assemblages factors while the southern one was reduced to four assemblages all assemblages were ranked according to their ecological significance besides relative abundance of major benthic foraminiferal suborders textulariina agglutinating foraminifera miliolina and rotaliina in addition to infaunal epifaunal relative abundance were used as paleoenvironmental proxies allowing the reconstruction of past changes in deep water salinity ventilation and organic carbon fluxes at the sea floor four distinct glacial interglacial cycle during the past 380 kyr have been recognized in the red sea the identified four cycles reveal deviation in deep sea ecosystem between the northern and southern red sea in the northern red sea salinity fluctuations productivity and deep water ventilation and formation had the major impact on benthic foraminiferal pattern corresponding to glacial interglacial cycles and glacio eustatic sea level changes coupled with the impact of mediterranean climate regime on the otherhand in the southern red sea region the oscillation trend of benthic foraminiferal pattern within the glacials and interglacials stages indicates a high frequency environmental alternation this alternation is consistent with the extent of ne monsoonal wind that controls the intensity and extension of the productivity which in turn determine organic matter fluxes and oxygen level at the sea floor

paleoceanographic record of the last glacial interglacial cycle in the red sea and gulf of aden abstract hydrographic conditions in the red sea are strongly controlled by regional climatic conditions and the restricted water exchange with the gulf of aden indian ocean in turn intermediate and deep water masses in the gulf of aden reflect the nature of red sea outflow planktonic and benthic foraminiferal faunas and the lithology and mineralogy of red sea and gulf of aden sediments were studied in order to evaluate changes in water mass conditions in the two basins during the last 18 000 years the results indicate that changes in the local climate and circulation regime during this period resulted in significant hydrographic changes in surface and bottom waters in the red sea and intermediate waters in the gulf of aden during the last glacial maximum high aridity and glacio eustatic sea level lowering resulted in surface and bottom water salinities in the red sea which were considerably higher than at present in the gulf of aden glacial surface salinities and temperatures remained similar to holocene values while surface water productivity may have been lower surface salinities in the red sea decreased after 18 000 years b p as the connection between the red sea and gulf of aden was reestablished however bottom water salinities remained high until at least 11 000 years b p pluvial conditions during deglaciation enhanced the density difference between surface and bottom waters in the red sea and resulted in a decrease in the oxygen content of the bottom waters hydrographic conditions in the western gulf of aden reflect a change in the properties of outflowing red sea water during deglaciation

thermoremanence anhysteretic remanence and susceptibility of submicron magnetites nonlinear field dependence and variation with grain size we have measured initial susceptibility 0 and the dependence of anhysteretic remanent magnetization arm and thermoremanent magnetization trm on applied field for seven samples of magnetite with mean grain sizes from 40 to 540 nm trm acquisition is nonlinear in geomagnetically relevant weak fields contrary to the usual assumption made in paleointensity determination over the 40 540 nm range 0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores arm is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples arm and trm have similar variations as d 1 for d 1 m suggesting a common source for this pseudo single domain psd dependence on grain size however trm is 10 20 times more intense than arm in grains around 0 2 m in size the trm microstate seems to be a two domain structure whereas the arm microstate may be a vortex structure which has never before been convincingly demonstrated by magnetic measurements independent evidence comes from theoretical fits to trm and arm field dependence data in moderate and strong fields trm in 215 540 nm magnetites is explained by two domain theory but arm is not in smaller grains d 0 1 m a psd theory predicts that trm is carried by single domain sd moments of entire grains but arm resides in moments 1 per grain 20 40 times weaker these remanence levels match those of metastable sd and vortex states respectively we therefore propose that magnetite grains in the 0 1 0 5 m size range can remain in metastable sd or two domain states following acquisition of trm but revert to a vortex ground state when field cycled for example in arm acquisition or alternating field demagnetization

rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations 1 the late quaternary a working hypothesis is proposed to account for the present accumulation of brines in isolated pockets of the ocean floor and for the formation of the underlying organic and metal rich sediments these are the tyro and bannock basins in the east mediterranean the red sea deeps and the orca basin in the northern gulf of mexico initiation of brine derived deposition in the red sea deeps and orca basin occurred between 12 000 and 8000 years b p this time bracket also encompasses the formation of the latest east mediterranean sapropel and the wettest global climate since the last glacial maximum this wet period first appeared in the tropics around 12 000 years b p then in the subtropical and middle latitudes during the same period the 23 000 year precession cycle brought the summer insolation of the northern hemisphere to its peak at 11 000 years b p with retreating northern hemisphere ice sheets the red sea deeps and the orca basin became anoxic during this humid period and metal rich sapropel deposition then began in contrast the tyro and bannock basins began accumulating a brine long before and persisted beyond this climatic stage the hypothesis involves two propositions 1 as in the eastern mediterranean sea marine anoxia was mainly the consequence of the large influx of continental runoff and local precipitation longer residence time of bottom waters so called stagnation in silled rimmed basins would have resulted from lower salinity at the sea surface in areas of deep water formation in the eastern mediterranean the red sea and the gulf of mexico and 2 miocene or older evaporites underlie these basins or outcrop on their flanks leaching from these evaporites was an ongoing process before the quasi stagnation phase but the initial leachate much less saline than the present brines was continuously flushed by bottom circulation the climate induced quiescence of bottom waters in these basins enabled the leachate to accumulate the feedback of stagnation by increased density progressively raised the salinity of entrapped bottom waters to the present brine concentration the high density has resisted brine removal by bottom circulation until present time long after cessation of the initiating wet period the brines therefore are stagnant fossil waters

millennial scale variability in red sea circulation in response to holocene insolation forcing in order to assess how insolation driven climate change superimposed on sea level rise and millennial events influenced the red sea during the holocene we present new paleoceanographic records from two sediment cores to develop a comprehensive reconstruction of holocene circulation dynamics in the basin we show that the recovery of the planktonic foraminiferal fauna after the younger dryas was completed earlier in the northern than in the central red sea implying significant changes in the hydrological balance of the northern red sea region during the deglaciation in the early part of the holocene the environment of the red sea closely followed the development of the indian summer monsoon and was dominated by a circulation mode similar to the current summer circulation with low productivity throughout the central and northern red sea the climatic signal during the late holocene is dominated by a faunal transient event centered around 2 4 ka bp its timing corresponds to that of north atlantic bond event 2 and to a widespread regionally recorded dry period this faunal transient is characterized by a more productive foraminiferal fauna and can be explained by an intensification of the winter circulation mode and high evaporation the modern distribution pattern of planktonic foraminifera reflecting the prevailing circulation system was established after 1 7 ka bp

renewal of deep water in the red sea during 1982 1987 between october 1982 and may 1983 the deep water in the northern red sea was ventilated through slope convection the deep water became cooler fresher and more oxygenated dense bottom water formed in the gulf of suez during winter cooling sank down the continental slope thereby entraining near surface and intermediate depth waters from the application of a one dimensional plume model a vertical transport of 0 58 106 m3 s 1 over a period of 7 months was estimated long term time series of atmospheric heat fluxes show that such a major convection event can occur every 4 7 years the corresponding renewal time of the red sea deep water is thus of the order of 40 90 years in agreement with previous estimates

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

revisiting the lowrie fuller test alternating field demagnetization characteristics of single domain through multidomain glass ceramic magnetite abstract this paper reports the alternating field demagnetization characteristics of glass ceramic magnetite assemblages carrying weak field thermoremanent magnetization trm weak field anhysteretic remanent magnetization arm and saturation remanence j rs average grain sizes vary from less than 0 1 m to approximately 100 m and hysteresis parameters indicate that these assemblages encompass single domain sd through truly multidomain md behavior in all assemblages weak field trm and weak field arm are more stable to alternating field demagnetization than is j rs this response is especially remarkable in the 100 m assemblage which otherwise displays truly md behavior although the sd samples pass the lowrie fuller test for sd behavior calculations presented here show that populations of noninteracting uniaxial sd grains should behave in just the opposite sense to that reported originally by lowrie and fuller this discrepancy could indicate that sd glass ceramic magnetite populations are more affected by magnetic interactions than would be expected for magnetite crystals that nucleated individually from a silicate matrix this interpretation is supported by the sd assemblages failing the cisowski test that is the curves for acquisition and af demagnetization of j rs intersect well below the 50 mark however a second and intriguing explanation of the sd like results obtained from all samples is that alternating field demagnetization characteristics reflect a strong dependence of local energy minimum domain state and its associated stability on the state of magnetization

carbonate preservation and climatic changes in the central red sea during the last 380 kyr as recorded by pteropods abstract numerical abundances of pteropods and planktic foraminifera and the mode of pteropod preservation were determined in core kl 11 taken from the central part of the red sea the abundance of pteropods shells g dry sediment was compared to that of planktic foraminifera pt pf pt a technique that permits detection of changes in carbonate preservation for the last 380 kyr the numerical abundance of pteropods is influenced by the properties of the water column and preservation is influenced by the bottom water during the last 200 kyr except during isotope stage 5 5 carbonates are in general well preserved during this period the abundance pattern of the pteropods and planktic foraminifera is very similar and follows the climatic signal of the red sea with high numbers during the interglacial stages changing to very low numbers during glacial maximum conditions the similar abundance trends of the two groups and between them and the 18o curve indicates abundance is strongly linked to salinity from isotope stage 6 to the bottom of the core the pteropods occur in low numbers unlike the planktic foraminifera which continue to display the high amplitude glacial interglacial cyclicity the deviations mainly during interglacial stages between the abundance pattern of the two groups and the low pt pf pt values indicate a significant change in carbonate preservation distinctive carbonate dissolution intervals are recognized in the red sea correlating to large scale deep water dissolution events of the mid brunhes dissolution cycle in the indian ocean the anti estuarine circulation pattern of the red sea prevents a direct connection between the deep water masses of the two oceans and rules out the likelihood that changes in the deep water circulation caused these carbonate dissolution events the numerical variations between nonmigratory epipelagic and migratory mesopelagic pteropods were used to evaluate changes in the structure of the water column abundance maxima of mesopelagic pteropods as in the recent red sea indicate an aerated water column with 0 5 ml o2 l oxygen concentrations at the minimum zone mesopelagic abundance maxima coincide commonly with negative monsoonal index values indicating a more aerated water column connected to increasing aridity in the red sea region abundance maxima of epipelagic pteropods indicate a strongly stratified water column at times causing severe depletion in oxygen at intermediate water depths epipelagic peak events coincide often with positive monsoon index values implying an overall milder and more humid climate in the red sea probably associated with enhanced precessional controlled southwest monsoon activity

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

grain size distribution of pedogenic magnetic particles in chinese loess paleosols 1 variations in magnetic parameters of the long chinese loess paleosol sequences reflect paleoclimatic fluctuations however there remain ambiguities whether the magnetic proxies are controlled by the concentration or by the distribution of magnetic grain sizes we systematically investigated the frequency dependence of low field magnetic susceptibility fd and the decay rate dr dependence of anhysteretic remanence magnetization arm susceptibility arm dr for a set of representative loess paleosol samples from the yuanbao section western chinese loess plateau after removing the effect of aeolian coarse grained magnetites both fd and arm dr are nearly independent of the degree of pedogenesis indicating a presence of fine grained magnetic particles ranging from superparamagnetic to single domain with an almost constant grain size distribution therefore the magnetic enhancement of magnetic susceptibility or arm is caused dominantly by changes in the concentration of these fine grained pedogenic grains

modes of subsurface intermediate and deep water renewal in the red sea a linear box model of the red sea is presented the model which inverts hydrographic and 3he data for the main features of the width integrated circulation is able to describe a large part of the seasonal and annual mean circulation by combining and reconciling major elements of current theories of the red sea circulation available flow measurements and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the red sea the model forms deep water in the extreme north in two modes as first suggested by cember 1988 the convective mode renews the bottom water at an annual mean rate of 0 04 sv in the winter the isopycnal mode water renews the uppermost part of the deep water mass it flows to the south along the bottom of the pycnocline at an annual mean rate of 0 02 0 04 sv a new feature in the model s near surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of 0 04 sv also from the extreme north there is also a middepth return flow to the north at an annual mean rate of 0 07 sv this flow has been suggested but never rigorously quantified before

multicentennial scale hydrological changes in the black sea and northern red sea during the holocene and the arctic north atlantic oscillation 1 paleoenvironmental proxy data for ocean properties eolian sediment input and continental rainfall based on high resolution analyses of sediment cores from the southwestern black sea and the northernmost gulf of aqaba were used to infer hydroclimatic changes in northern anatolia and the northern red sea region during the last 7500 years pronounced and coherent multicentennial variations in these records reveal patterns that strongly resemble modern temperature and rainfall anomalies related to the arctic oscillation north atlantic oscillation ao nao these patterns suggest a prominent role of ao nao like atmospheric variability during the holocene beyond interannual to interdecadal timescales most likely originating from solar output changes

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

on the sources formation and circulation of red sea deep water red sea deep water is renewed from sources in the far north of the red sea upwelling of deep water throughout the basin maintains the sharp pycnocline with its base near the depth of the sill at bab el mandeb before now deep water renewal was thought to occur mainly by means of convective deep water formation in the north which renews the bottom water and gives rise to a southward flowing bottom current carbon 14 and 3he data from the geosecs indian ocean expedition lead to the identification of an additional volumetrically larger mode of northern deep water formation in which new deep water is injected immediately beneath the pycnocline this mode of deep water formation to be called the isopycnal mode also gives rise to a southward flowing current in this case at the top of the deep water mass geosecs data also demonstrate the existence at intermediate depths in the deep water of a return flow to the north the surface history in the nuclear era of the 14c in the deep water source region was reconstructed by direct radiocarbon measurements on annual growth bands of corals calculation then shows that new deep water forms in the convective mode at a rate of 0 05 0 01 sverdrup sv expressed as if the flow were continuous and is due principally to processes occurring in the mouth of the gulf of suez the injection beneath the pycnocline of new deep water formed in the isopycnal mode is 0 11 0 02 sv this new deep water appears to originate mainly in the extreme open northern red sea the gulf of aqaba makes a modest but significant contribution to both modes the total deep water renewal rate is about 0 16 sv yielding a bulk deep water residence time of about 36 years oxygen consumption in the deep water shows approximately exponential decrease of consumption rate with depth as expected on geological and geophysical grounds hydrographically significant fluxes of heat and salt from the sea floor into the red sea deep water are discernible in the hydrography helium 3 flux from the sea floor into the deep water is about 69 1 stp year

magnetic fingerprint of southern portuguese speleothems and implications for paleomagnetism and environmental magnetism environmental magnetism of speleothems is still in its early stage of development here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems and what are the factors that control its preservation and reliability to address these issues we used a multidisciplinary approach including rock magnetism petrography scanning electron microscopy stable carbon and oxygen isotope compositions and major and trace element concentrations we applied this to a set of samples from different stages of speleothem evolution present day dripwater glass plates a weathered stalactite a fresh stalagmite cave sediments and terra rossa soils these samples come from the penico and excentricas caves located in two distinct aquifers of the algarve region south portugal our results show that the main magnetic carriers of the speleothems under study are primary detrital and consist of maghemite and magnetite similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater hence they represent a regional environmental signature interestingly a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities we propose that these low intensities can be the result from i different remanence acquisition mechanisms between stalagmite and stalactite and or ii iron dissolution by fungal activity we also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock soil atmosphere while oxygen isotope composition and alkaline earth element concentrations inform about calcite water interaction processes these results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo environmental signal

stratigraphic and paleoceanographic significance of late quaternary pteropods from deep sea cores in the gulf of aqaba elat and northernmost red sea abstract the quantitative distribution of 16 thecosomatous pteropod species from four deep sea cores recovered from the gulf of aqaba elat and northernmost red sea is summarized the distribution facilitates a relatively high resolution stratigraphic subdivision of the 6 8 m long core sequences and their correlation the sequences of pteropod assemblages can be related to oxygen isotope curves and thus dated beyond the radiocarbon range the oldest strata penetrated are dated at about 150 000 yr b p considering the present day distribution of the species recognized the core assemblages are of a tropical subtropical character throughout the late quaternary on the basis of the composition of the living pteropod populations in the hypersaline 41 moderately oligotrophic and desert enclosed gulf of aqaba the pteropod core assemblages associated foraminiferal assemblages and the oxygen isotope values determined on both groups a sequence of paleoceanographic changes during the late quaternary is derived these changes are primarily influenced by productivity of the waters and pertain less to temperature and salinity the glacial periods isotope stages 2 3 4 and upper 6 were characterized by lower temperature than at present higher salinity and greater fertility of the waters during the glacial maxima monospecific creseis acicula assemblages characterize periods of metahaline more than 50 salinity conditions in highly productive waters although not identical in detail the last interglacial stage 5 assemblages are similar to the holocene warm water oligotrophic ones between 8000 and 3000 yr b p a short period of slightly higher productivity is indicated

sensitivity of red sea circulation to monsoonal variability during the holocene an integrated data and modeling study received 20 october 2009 revised 6 march 2010 accepted 30 june 2010 published 4 november 2010 1 we used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the red sea in response to reduced sea level and modified atmospheric conditions during the holocene with holocene sea level close to the modern level the red sea was sensitive to changes in atmospheric conditions and it only shows a relatively mild response to sea level change changes in the monsoon system influence the exchange flow through the strait of bab el mandab the meridional overturning circulation of the red sea and its hydrography forced by humid conditions the modeled red sea temperature increased by 1 5 c while when arid conditions were imposed the temperature decreased by 2 5 c similar heating and cooling events during the early and late holocene are seen in a sea surface temperature record from the northern red sea derived from the temperature sensitive tex86 molecular biomarker which suggests that humid conditions prevailed during the early holocene and more arid conditions prevailed during the late holocene the gradual decline in red sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength this monsoon trend and the resulting changes in the red sea circulation are supported by the distribution of crenarchaea fossil lipids in red sea sediments from this period monsoon driven changes in the exchange flow through the strait of bab el mandab affected the crenarchaea population structure and therefore their molecular fossil distribution in the sediments of the red sea potentially provides an index for the summer monsoon strength during the holocene

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

climatic changes in the northern red sea during the last 22 000 years as recorded by calcareous nannofossils 1 we present a high resolution record of calcareous nannofossils from the northern red sea for the last 22 kyr extreme conditions with enhanced salinities during the last glacial maximum are characterized by high values of gephyrocapsa ericsonii the dominance of emiliania huxleyi in heinrich event 1 indicates a climatic cooling favoring the bloom of opportunistic species the calcareous nannofossils record a two step onset of the postglacial humid climate punctuated by the younger dryas both steps show an early oligotrophic phase dominated by florisphaera profunda and gladiolithus flabellatus and a subsequent fertile phase characterized by e huxleyi the younger dryas is described by high values of gephyrocapsa oceanica indicating an increased mixing of the water column in the late holocene repetitive increases in abundance of f profunda and g flabellatus reflect restricted oligotrophic conditions caused by the high aridity following the holocene humid period

holocene climate variability of the western mediterranean surface water dynamics inferred from calcareous plankton assemblages a high resolution study centennial scale has been performed on the calcareous plankton assemblage of the holocene portion of the ocean drilling program site 976 alboran sea with the aim to iden

micropaleontological and taphonomic characteristics of mass transport deposits in the northern gulf of eilat aqaba red sea abstract submarine mass transport deposits mtds are a well known phenomenon in tectonically active regions evidence for such deposits is commonly found in the continental slope sedimentary records as distinct units with coarser grain size compared to the usual and continuous pelagic sedimentation the gulf of eilat aqaba is located between the southernmost end of the dead sea transform and the spreading center of the red sea and is considered as an active tectonic region in this study an innovative approach using symbiont bearing larger benthic foraminifera lbf to identify mtds in the gulf of eilat aqaba gea sedimentary record is presented the abundance size and preservation state of lbf shells were analyzed in two radiocarbon dated sediment cores collected at different deposition environments at water depth of 532 m and 316 m the microfaunal and taphonomic results show that the coarse units are characterized by a generally higher numerical abundance of lbf dominated by operculina ammonoides amphistegina papillosa and amphistegina bicirculata these benthic assemblages are found in deeper depths than their original habitat ranging between 50 and 120 m in accordance with their symbionts light requirements in the coarse units lbf 1 mm appear in high frequency up to 161 specimens per g sediment and poorly preserved shells are also abundant containing up to 247 specimens per g sediment in addition these units also contain high numbers of yellowish and blackish colored lbf shells as opposed to null in the non disturbed units and unlike their natural white color the large shell size indicates that high energy is involved in the displacement of the sediments the poor state of preservation also suggests a turbulent flow during transportation which requires a high energy triggering mechanism the color alteration is probably associated with a diagenetic process related to increasing burial time depth also supported by the stratigraphic older ages of the mtds suggesting a long burial before the sediments were displaced in addition according to the dating of the record some units correlate with historical and pre historical earthquakes reinforcing lbf species as a reliable proxy for mass transport events

a comparative study of the arm and isr of some natural magnetites of md and psd grain size summary arm and isr stability with respect to alternating field and low temperature treatment were measured for a set of artificial rock specimens each of the specimens contained a known amount of a natural magnetic mineral of a known grain size the following minerals were used a homogeneous natural magnetite a natural titanomagnetite with submicroscopic fe rich inclusions and a natural titanomagnetite and a natural titano maghemite that both show exsolution phenomena the grain size of the fractions varied from 5 to 250 m it is found that the stability of arm and irs of multi domain grains with respect to alternating fields and low temperature treatment is similar whereas for the smallest grains studied 5 m arm is more stable than isr multi domain arm data from this study are in agreement with current multi domain arm theory low temperature behaviour of arm and isr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains

climate variability during mis 20 18 as recorded by alkenone sst and calcareous plankton in the ionian basin central mediterranean abstract this study shows the first mediterranean high resolution record of alkenone derived sea surface temperature sst in the marine sediments outcropping at the ideale section is southern italy central mediterranean from late marine isotope stage mis 20 through early mis 18 the sst pattern evidences glacial interglacial up to submillennial scale temperature variation with lower values 13 c in late mis 20 and substage 19b and higher values up to 21 c in mis 19c and in the interstadials of mis 19a the sst data are combined with the new calcareous plankton analysis and the available chronologically well constrained carbon and oxygen isotope records in the is the multi proxy approach together with the location of the is near the italian coasts the lower circalittoral upper bathyal depositional setting and high sedimentation rate allow to document long and short term paleoenvironmental modifications sea level rainfall inorganic organic fresh water input to the basin as a response to regional and global climate changes the combined proxies reveal the occurrence of a terminal stadial event in late mis 20 here med htix and warm cold episodes here med batix and med ydtix during termination ix tix which recall those that occurred through the last termination ti during these periods and the following ghost sapropel layer insolation cycle 74 784 ka in the early mis 19 high frequency internal changes are synchronously recorded by all proxies the substage mis 19c is warm but quite unstable with several episodes of paleoenvironmental changes associated with fluctuating tropical subtropical water inflow through the gibraltar strait variations of the cyclonic regime in the ionian basin and the southward shift of westerly winds and winter precipitation over southern europe and mediterranean basin three high amplitude millennial scale oscillations in the patterns of sst and calcareous plankton key taxa during mis 19a are interpreted as linked to changes in temperature as well as in salinity due to periodical water column stratification and mixing the main processes involved in the climate variability include changes in oceanographic exchanges through the gibraltar strait during modulations of atlantic meridional overturning circulation and or variations in atmospheric dynamics related to the influence of westerly and polar winds acting in the paleo ionian basin a strong climate teleconnection between the north atlantic and mediterranean is discussed and a prominent role of atmospheric processes in the central mediterranean is evidenced by comparing data sets at the is with italian and extra mediterranean marine and terrestrial records

holocene atmosphere ocean interactions records from greenland and the aegean sea we compare paleoclimate proxy records from central greenland and the aegean sea to offer new insights into the causes timing and mechanisms of holocene atmosphere ocean interactions a direct atmospheric link is revealed between aegean sea surface temperature sst and high latitude climate the major holocene events in our proxies of aegean sst and winter spring intensity of the siberian high gisp2 k record follow an 2300 year spacing recognised also in the 14c record and in worldwide holocene glacier advance phases suggesting a solar modulation of climate we argue that the primary atmospheric response involved decadal centennial fluctuations in the meridional pressure gradient driving aegean sst events via changes in the strength duration and or frequency of northerly polar continental air outbreaks over the basin the observed natural variability should be accounted for in predictions of future climate change and our timeframe for the aegean climate events in addition provides an independent chronostratigraphic argument to middle eastern archaeological studies

calcareous plankton and the mid brunhes climate variability in the alboran sea odp site 977 abstract quantitative calcareous plankton analyses have been performed at the ocean drilling program site 977 in the alboran sea through the mid brunhes interval 300 540 ka the results evidence orbital suborbital and millennial scale climate variability which primary reflects temperature change the patterns of increase decrease of coccolithophore and planktonic foraminifera warm water taxa reproduce the stage and substage climate variability in the planktonic 18o in good agreement with other planktonic isotope records from mediterranean sea and atlantic ocean variable conditions characterized by alternating cool nutrient rich or warm and stable sea surface waters occurred during interglacial marine isotope stage mis 13 in response to high amplitude oscillations of insolation during obliquity maximum and to north africa monsoon strength influencing distribution of taxa mis 13a resulted warmer than mis 13c based on the higher abundances of warm water taxa they increase just after insolation maxima suggesting a primary response of calcareous plankton to precessional orbital control following short term warm mis 11e and more humid mis 11d phases a climate optimum lasting about 16 kyr is recognized during mis 11c when major tropical subtropical water arrival into the alboran sea occurred as evidenced by the presence of trilobatus sacculifer mis 11c records unstable climate condition that persisted up to the late mis 11 indicating multiple pulses of southward migrations of subpolar front following the full interglacial mis 11c warm and oligotrophic conditions occurred during mis 9 as during mis 11 especially in mis 9e although without a climate optimum reduced abundance of warm water taxa and decrease of globorotalia inflata highlight colder surface water conditions and decreased atlantic mediterranean water exchange during glacials especially during the prominent sea level low stand of mis 12 short term prominent peaks of the polar subpolar neogloboquadrina pachyderma and coccolithus pelagicus ssp pelagicus and additional key taxa preferring fresher and turbid water together with minima in total nannofossil accumulation rate and enhanced coccolith reworking sustain a correlation with the north atlantic heinrich type ht events ht1 ht1a ht3 ht4 6 thus documenting the sensitiveness of the alboran sea in recording european and laurentide ice sheet dynamics the comparable chronology of most of these cold spells in the available balearic and ionian calcareous plankton records highlights that atlantic melt water entering alboran basin through gibraltar strait may have reached the central mediterranean preserving some of its original polar signature

palaeoenvironment palaeoclimate and sea level changes in the shuaiba lagoon during the late holocene last 3 6 ka eastern red sea coast saudi arabia multi proxy investigations of sediment lithology geochemistry benthic foraminifera and 18o and 13c of elphidium striatopunctatum were carried out on a sediment core shua3 2 2 m long spanning

holocene itcz and indian monsoon dynamics recorded in stalagmites from oman and yemen socotra abstract high resolution oxygen isotope 18o profiles of holocene stalagmites from four caves in northern and southern oman and yemen socotra provide detailed information on fluctuations in precipitation along a latitudinal transect from 12 n to 23 n 18o values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the itcz and dynamics of the indian summer monsoon ism during the early holocene rapidly decreasing 18o values indicate a rapid northward displacement in the mean latitudinal position of the summer itcz and the associated ism rainfall belt with decadal to centennial scale changes in monsoon precipitation correlating well with high latitude temperature variations recorded in greenland ice cores during the middle to late holocene the summer itcz continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation a trend which is also recorded in other monsoon records from the indian and east asian monsoon domains importantly there is no evidence for an abrupt middle holocene weakening in monsoon precipitation although abrupt monsoon events are apparent in all monsoon records they are short lived and clearly superimposed on the long term trend of decreasing monsoon precipitation for the late holocene there is an anti correlation between ism precipitation in oman and inter monsoon spring autumn precipitation on socotra revealing a possible long term change in the duration of the summer monsoon season since at least 4 5 ka bp together with the progressive shortening of the ism season gradual southward retreat of the mean summer itcz and weakening of the ism the total amount of precipitation decreased in those areas located at the northern fringe of the indian and asian monsoon domains but increased in areas closer to the equator

interannual variability of planktic foraminiferal populations and test flux in the eastern north atlantic ocean jgofs planktic foraminiferal assemblages vary in response to seasonal fluctuations of hydrographic properties between water masses and after periodical changes and episodic events e g reproduction storms distinct annual variability of the planktic foraminiferal flux is also known from sediment trap data in this paper we discuss the short term impacts on interannual flux rates based on data from opening closing net hauls obtained between the ocean surface and 500 m water depth data were recorded during april may june and august at around 47 n 20 w biotrans in 1988 1989 1990 1992 1993 and during may 1989 and 1992 at 57 n 20 22 w species assemblages closely resemble each other when comparing the mixed layer fauna with the fauna of the upper 100 m and the upper 500 m of the water column in addition species assemblages 100 m are almost indistinguishable from assemblages that are 125 m in test size the standing stock of planktic foraminifers at biotrans can vary by more than one order of magnitude over different years however species assemblages may be similar when comparing corresponding seasons early summer assemblages june are distinctly different from late summer assemblages august significant variations in the species composition during spring april may are independent of the mixed layer depth spring assemblages are characterized by high numbers of globigerinita glutinata in particular day to day variations of the number of specimens and in species composition may have the same order of magnitude as interannual variations this appears to be independent of the reproduction cycle species assemblages at 47 n and 57 n are similar during spring although surface water temperatures and salinities differ by up to 10 c and 0 7 psu we suggest that the main factors controlling the planktic foraminiferal fauna are the trophic properties in the upper ocean productive layer planktic foraminiferal carbonate flux as calculated from assemblages reveals large seasonal variations a quasi annual periodicity in flux levels and substantial differences in timing and magnitude of peak fluxes at the biotrans station the average annual planktic foraminiferal caco 3 fluxes at 100 and 500 m depth are estimated to be 22 4 and 10 0 g m 2 yr 1 respectively

cautionary note on magnetic grain size estimation using the ratio of arm to magnetic susceptibility the ratio of anhysteretic remanent magnetization arm to magnetic susceptibility is widely used for rapid estimation of magnetic grain size variations on interpreting arm variations no significant influence of interactions among magnetic grains on arm acquisition is usually assumed implicitly although some previous experiments using artificial sediments and biogenic magnetites opposed this assumption our rock magnetic study of deep sea surface sediments from the pacific ocean showed that arm depends strongly on the concentration of magnetic minerals and on the r value of cisowski 1981 an index of the strength of magnetic interactions this result demonstrates that arm can be influenced significantly by magnetic interactions in natural sediments we corrected for the influence of the magnetic interactions using the observed concentration dependence of arm regional variations of arm values before the correction were geologically unnatural in terms of magnetic grain size variations previous interpretations of arm variations on magnetic grain size without consideration of magnetic interactions may not be valid

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

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

magnetic domain state and coercivity predictions for biogenic greigite fe3s4 a comparison of theory with magnetosome observations the discovery of bacteria that precipitate greigite within intracellular organelles magnetosomes offers new evidence about the origin of greigite in natural environments unlike magnetite only scarce information is available about the magnetic characteristics of greigite for this reason and the present inability to grow these microorganisms in pure culture it is not known whether or not the magnetosomes in the newly discovered greigite precipitating bacteria are of single domain sd size as are the magnetosomes from magnetite precipitating bacteria the hypothesis of natural selection for magnetotactic behavior predicts that the greigite bearing magnetosomes should also be single magnetic domains using previously reported magnetic properties and crystallographic features for greigite we have calculated the size and shape boundaries expected for sd and superparamagnetic spm behavior in this mineral for further characterization of the greigite crystals we analyzed the domain state at various length width ratios assuming crystal shapes of parallelepipeds and prolate spheroids magnetite was used as control for the current theories supporting these calculations we also present a simple algorithm to calculate the upper size limit of single domain grains our results show that the crystals of bacterial greigite characterized so far are located in the region close to the single domain superparamagnetic boundary and should have relatively low coercivity if these crystals contribute to the magnetization of sediments remanence produced by bacterial greigite could be mistaken for large multidomain magnetite in alternating field demagnetization studies

diversity and peculiarities of soil formation in eolian landscapes insights from the mineral magnetic records abstract magnetic properties of loess paleosol sequences are widely utilized as sensitive easily obtained and reliable proxy archives of past climate change in continental settings however soil formation in aeolian landscapes may occur under different and changing conditions of dust sedimentation which will affect the growth and development of the pedogenic magnetic component this phenomenon is explored in our study by analyzing depth variations of the following magnetic characteristics frequency dependent magnetic susceptibility anhysteretic magnetic susceptibility and the ratio between anhysteretic susceptibility and isothermal remanent magnetization acquired in a 100 mt back field arm irm 100 mt along three loess paleosol profiles from ne bulgaria a conceptual model for the time evolution of the grain size of the pedogenic magnetic fraction with soil depth is proposed which is build upon the mechanism of soil formation accretional or in stable land surfaces or a combination of the two another magnetic ratio isothermal remanence acquired in a 300 mt back field divided by low field magnetic susceptibility irm 300 mt is utilized as a proxy for the changes in the ratio of silt to clay fractions indicative of changes in dust sedimentation regimes during glacial interglacial transitions according to the proposed conceptual model verified by the magnetic signature of three holocene soils from ne bulgaria discrimination between accretional soils and soils developed without dust additions during soil forming period can be done accretional soils are characterized by parallel changes in grain size sensitive magnetic proxies soils developed at stable landscape conditions show gradation of the depths at which maximum enhancement of various proxies occurs with deepest occurrence of the maximum in fd followed by depth of maximum arm and arm irm 100 mt another implication of the new conceptual model is that the mean coercivity of the pedogenic component of accretional soils will be higher than that of soils developed without eolian input at equal temperature conditions because of the soils thermal gradient and different depths at which pedogenic minerals form in the two settings the results revealed that the holocene soils so has developed under different soil forming conditions than paleosols from the same sites

the superparamagnetism of yucca mountain tuff yucca mountain tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range schlinger et al 1988 magnetic measurements on three samples comprising hysteresis loops at low and ambient temperatures acquisition and demagnetization of isothermal remanent magnetization thermal demagnetization of the saturation remanence and of a low temperature thermoremanence and frequency and temperature dependent susceptibilities allow evaluation of the magnetic properties in terms of neel s 1949 single domain theory precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results in contrast an attempt to derive mean particle volumes by fitting a langevin curve to the room temperature magnetization curves fails probably because the precondition for the langevin function kv kt 1 is not met it is only for the sample with the smallest grains and in weak fields 20 mt that a langevin fit provides a reasonable volume estimate there is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility thus verifying that neel s theory is sufficient for the magnetic description of single domain assemblages spanning the superparamagnetic stable single domain boundary however some deviations between modeled and measured susceptibilities exist and the physical causes may include size dependent anisotropy nonuniform magnetizations and also an uncertain preexponential time constant 0 while 0 10 11 s gives the best fit for the sample with the largest grains 0 10 9 s is more reasonable for the others thus 0 may indeed be size and temperature dependent as predicted by brown 1959 the commonly cited parameter fd frequency dependence of susceptibility reaches 30 at room temperature rt for one sample with a blocking temperatures just below rt while fd 0 at rt for a superparamagnetic sample with smaller grains these results thus exemplify that fd is not limited to 15 as a number of studies suggest and that fd 0 must not be taken to imply the absence of superparamagnetic grains

paleoenvironmental events during the last 13 000 years in the central red sea as recorded by pteropoda a high resolution record was obtained by investigating a sedimentary sequence from two cores taken in the central red sea r v meteor cruise 5 leg 2 the numerical variations between nonmigratory and migratory pteropods and their stable isotopic record were studied together with variations in the carbonate and total organic carbon content in order to estimate changes in the structure of the water column during the last 13 kyr the results indicate the existence of a highly stratified water column during deglaciation and early holocene 13 8 5 ka during this period the depth of the mixed layer varied between less than 50 100 m at most and productivity was markedly reduced in comparison to the recent situation the intermediate and deep water were constantly highly depleted in oxygen a pronounced younger dryas event was recognized at 10 4 ka which coincides with an arid phase on the border land the oxygenation of the intermediate water improved remarkably after 7 5 ka and peaked during the period between 4 6 and 2 0 ka during this period the mixed layer reached its present depth and the productivity which was maximal during middle holocene attained present level during the last 4 6 kyr

contrasting tectonothermal domains and faulting in the potomac terrane virginia maryland discrimination by 40ar 39ar and fission track thermochronology new 4 0 ar 3 9 ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the potomac terrane in northern virginia thus confirming previous interpretations that it is a composite terrane the rocks of the potomac terrane were examined along the potomac river where it has been previously subdivided into three units the mather gorge sykesville and laurel formations in the mather gorge formation at least two metamorphic thermal domains were identified the blockhouse point and bear island domains separated by a fault active in the late devonian early ordovician ca 475 ma cooling ages of amphibole in the bear island domain reflect cooling from taconic metamorphism whereas the blockhouse point domain was first metamorphosed in the devonian the 4 0 ar 3 9 ar data from muscovites in a third eastern domain within the mather gorge formation the stubblefield falls domain record thrusting of the sykesville formation over the mather gorge formation on the plummers island fault in the devonian the existence of two distinctly different thermal domains separated by a tectonic boundary within the mather gorge argues against its status as a formation hornblende cooling ages in the sykesville formation are early devonian ca 400 ma reflecting cooling from taconic and acadian metamorphism the ages of retrograde and overprinting muscovite in phyllonites from domain bounding faults are late devonian acadian and late pennsylvanian alleghanian marking the time of assembly of these domains and subsequent movement on the plummers island fault our data indicate that net vertical motion between the bear island domain of the mather gorge complex and the sykesville formation across the plummers island fault is east side up zircon fission track cooling ages demonstrate thermal equilibrium across the potomac terrane in the early permian and apatite fission track cooling ages record tilting of the potomac terrane in the cretaceous or later with the west side up at least 1 km

geomorphological changes in the coastal area of farasan al kabir island saudi arabia since mid holocene based on a multi proxy approach abstract the geomorphological evolution of the southeastern coastal area of farasan al kabir island saudi arabia is revealed by the mapping of modern landforms and a multi proxy and high spatial resolution study including grain size particulate organic carbon mineralogy element geochemistry benthic foraminifera analysis and radiocarbon dating of a 3 3 m long sediment core the modern geomorphological features comprise a variety of arid landforms such as plateau cliffs and pediments of pleistocene coral limestones playa depressions located on plateau surfaces alluvial fans butte and sandy beaches the mid holocene evolution of the borehole area is resulted from the detailed analysis of five sedimentary units detected along the core matar 1 and includes three distinct stages a from 5253 223 y cal bp to 3138 223 y cal bp carbonate coarse grained material consisting of coral fragments molluscs calcareous algae and benthic foraminifera are deposited on a shallow marine fringing reefal platform which becomes progressively a nearshore backreef around 3675 215 y cal bp and later around 3138 223 y cal bp a reef ramp b since 3040 220 y cal bp the borehole area obtains the characteristics of a high energy beach that receives increasing inputs of terrigenous material c subsequently a supratidal backshore setting is established influenced mostly by terrestrial processes and occasionally by marine processes as it is indicated by the decreasing and sometimes sporadic presence of benthic foraminifera and recently a sedimentary veneer consisting of terrigenous carbonate and evaporite material is formed by terrestrial mainly wadi and aeolian processes

a preliminary magnetic study of soil samples from west central minnesota abstract various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west central minnesota there is a marked change in magnetic properties as a function of depth in the core x ray analysis and curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil the intensity of anhysteretic remanent magnetization arm decreases sharply as the depth increases in contrast the stability of arm was found to be higher for the subsoil the surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization vrm the vrm acquisition coefficient s a of the subsoil s a 3 18 10 6 emu g 1 3 18 10 6 a m 2 kg 1 was about ten times weaker than that of the top soil sample s a 3 868 10 7emu g 1 3 868 10 7 a m 2 kg 1 the magnetic domain state indicator the ratio of coercivity of remanence to coercive force h cr h c was 1 5 and 3 85 for the top soil and subsoil respectively it appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size we conclude that the magnetic grains in surface soil samples were more single domain sd like whereas the magnetite grains in the subsoil samples were more likely in pseudo single domain psd or small multidomain md range the observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs

magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment first experimental results and theory 1 the widespread occurrence of magnetotactic bacteria mtb in several types of marine and freshwater sediment and the role of fossil magnetosomes magnetofossils as main remanent magnetization carriers therein has important paleomagnetic and paleoenvironmental implications despite numerous studies on mtb biology and on magnetofossil preservation in geological records no detailed information is yet available on how magnetotaxis i e the ability to navigate along magnetic field lines is performed in sedimentary environments and on how magnetofossils possibly record the earth magnetic field we provide for the first time experimental evidence for these processes mtb living in sediment are poorly aligned with the geomagnetic field contrary to what is observed in water this can explain the seemingly excessive magnetic moment of most mtb the observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients experiments with magnetofossil rich sediment suggest that a natural remanent magnetization nrm is acquired by magnetofossils in the so called benthic mixed layer where natural mtb populations usually occur the acquired nrm is proportional to the applied field at least up to 160 t and its intensity is compatible with values observed in nature for same sediment types therefore if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes they can provide useful relative paleointensities we propose a preliminary model to explain early stages of magnetofossil nrm acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing

palaeoceanographic evolution of the central red sea during the late holocene three multicores were studied in the central red sea off port sudan at a w e transect in order to reconstruct palaeoceanographic conditions of the past 6000 years downcore fluctuations in the rel

aplanktonic zones in the red sea abstract we use micropalaeontological and stable isotope results for a series of cores from north to south through the red sea to assess temporal and spatial patterns of change in planktonic foraminiferal faunas leading up to the remarkable full glacial red sea aplanktonic zones aplanktonic zones reflect salinities in the red sea in excess of the lethal 49 p s u limit caused by reduced exchange transport through the strait of bab el mandab due to glacial sea level lowering concerning the last glacial cycle aplanktonic conditions began at 39 ka bp in the north where salinities eventually reached 55 57 p s u paradoxically planktonic faunas are reported to have survived the last glacial maximum in the northern gulf of aqaba suggesting a poorly understood freshwater dilution at that time aplanktonic conditions s 49 p s u reached the south central red sea by 22 ka bp while planktonic foraminiferal faunas continued albeit in very low abundances and limited diversity s 45 p s u in the southernmost red sea during marine isotope stage mis 6 the 49 p s u isohaline appears to have resided in the central red sea between cores kl11 and md921017 we observe a systematic sequence of species disappearances before all glacial maxima of the last 500 kyr absence of a logical relationship with sea levels suggests that the disappearance sequences are not related to a general salinity increase instead we argue that the sequences were driven by complex reorganisations in hydrography stratification productivity food availability and subsurface oxygenation reproduction habitats the onset is marked by dramatic basin wide expansion of conditions that today are restricted to only the southern red sea suggesting an expansion of the dominance of ne monsoonal circulation over the entire red sea this expansion occurred 15 or more kyr before the aplanktonic zones of mis 12 6 and 2 and also before mis 10 and 8 which never reach the aplanktonic stage regarding the last glacial cycle this event occurred as early as 75 ka bp mis 4 5 boundary after this major climatic reorganisation we reconstruct progressive intensification of the new conditions especially marked by northward expansion and intensification of the subsurface oxygen minimum zone omz during the last glacial cycle a shallow and very distinct omz affected the central red sea as early as 68 ka bp and the north as late as 55 ka bp the omz expansion intensification appears to have been interrupted by episodes of increased ventilation

a pronounced dry event recorded around 4 2 ka in brine sediments from the northern red sea partly laminated sediments were sampled from the brine filled anoxic shaban deep basin in the northern red sea at about 4200 cal yr bp more than two millennia of anoxic sedimentation is replaced by a sub oxic facies strongly suggesting the episodic absence of the brine at the same time stable oxygen isotopes from surface dwelling foraminifera show a sharp increase within less than 100 yr pointing to a strong positive salinity anomaly at the sea surface this major evaporation event significantly enhanced the renewal of deep water and the subsequent ventilation of the small shaban deep basin the timing and strength of the reconstructed environmental changes around 4200 cal yr bp suggest that this event is the regional expression of a major drought event which is widely observed in the neighboring regions and which strongly affected middle east agricultural civilizations

late quaternary biostratigraphy and paleotemperatures of the red sea and the gulf of aden based on planktonic foraminifera and pteropods abstract the quantitative distributions of planktonic foraminifera and pteropods were analyzed in seven red sea cores and two deep sea cores from the gulf of aden and off sokotra island biostratigraphic intervals were distin guished corresponding to oxygen isotope stages 1 5 in the red sea and stages 1 3 in the gulf of aden the faunal assemblages demonstrate that in the red sea hydrological conditions changed significantly during the late quaternary especially at the last glacial maximum when water exchange at the bab el mandeb straits was at a minimum and at the pleistocene holocene boundary at the end of deglaciation water exchange renewal and pluvial conditions resulted in surface salinity decrease and stable stratification which was followed by a rise in planktonic fauna abundance and stagnation of bottom waters the more constant composition of rather cold water subtropical faunal assemblages in the gulf of aden during the last 40 000 50 000 yrs points to more stable hydrological parameters and to local upwelling or influx of arabian upwelling waters during the last stadial mean annual sea surface temperatures fell by at least 5 c compared to the present in the central part of the red sea and by 3 5 c in the western gulf of aden