magnetic properties of coastal loess on the midao islands northern china implications for provenance and weathering intensity abstract loess deposits are widely distributed over the islands in the bohai gulf and neighboring regions of northern china the coastal loess located to the east of the chinese loess plateau clp is believed to have a local source from exposed marine sediments and a distant source from inner arid regions in asia in this study magnetic geochemical and diffuse reflectance spectroscopic analyses were conducted on the loess to explore its relationship with the loess on the clp and its paleoclimatic implications we find that the coastal loess shows similar geochemical composition background susceptibility and magnetic enhancement to loess on the clp magnetic minerals characteristic of pedogenesis are more important in the paleosol layer with fine ferrimagnetic minerals abundant in paleosol layer sb1 and hematite in paleosol layer sb2 geochemical evidence suggests that weathering intensity in paleosol sb2 is similar to or even stronger than in paleosol sb1 pedogenic enhancement of magnetic susceptibility in the study area is not so strong which may be due to enhanced hematite formation as opposed to ferrimagnetic mineral production and preservation

origin of maghaemite in chinese loess deposits aeolian or pedogenic maghaemite and magnetite common ferrimagnetic minerals in surficial soils have physical properties which are similar except for the thermal instability of the former utilising this exception a study of thermomagnetic properties has been made to systematically separate these two magnetic components in loess and palaeosol samples from the upper parts of the luochuan section of the central chinese loess plateau this investigation has shown that although maghaemite thermally unstable component occurs in greater amounts in palaeosols ms 0 0187 am2 kg 1 1 1 than in loess units ms 0 0156 am2 kg 1 1 1 in relative terms its contribution to the ferrimagnetic component of magnetisation is higher in the loess units ms 32 2 2 0 than the soils ms 23 5 3 7 such an observation suggests that much of the maghaemite component originates in the aeolian source area with some addition of pedogenic maghaemite occurring in the palaeosols

paleoclimatic significance of magnetic properties on the red clay underlying the loess and paleosols in china abstract the red clay unit of northern china is a widespread miocene pliocene 7 2 2 5 ma aeolian deposit that underlies much of the quaternary loess paleosol sequence in the central loess plateau it comprises a sequence of highly developed soils and interbedded layers of less weathered loess like material here referred to as reddish loess in common with the loess paleosol alternations of the overlying quaternary deposits the soil reddish loess alternations are interpreted to represent climatic fluctuations in this case between warm humid and relatively dry cool conditions however while magnetic susceptibility variations in the quaternary deposits provide a good proxy for pedogenic intensity in the red clay the relationship is less clear and magnetic susceptibility data are in conflict with other measures of pedogenesis in an attempt to resolve this issue we have investigated a red clay section and overlying loess paleosol sequence at xifeng and have supplemented these data with samples from the red clay at lingtai our study indicates that the red clay and loess paleosol sequence have a common magnetic mineralogy comprising magnetite maghemite and hematite and possibly goethite and that both sequences show a good correlation between magnetic susceptibility and the magnitude of the superparamagnetic sp component this implies that susceptibility enhancement in the red clay is strongly related to the magnitude of the sp content a component that is widely recognised as having a pedogenic origin in quaternary deposits further support for a correlation between pedogenesis and magnetic susceptibility in the red clay is provided by a good correlation between magnetic susceptibility and the rb sr ratio an independent weathering index however differences in the magnetic mineralogy of the two units are also evident the minimum magnetic susceptibility of the red clay is less than half the value of the overlying quaternary deposits at locations such as xifeng and lingtai while the maximum sp content is relatively larger than for the quaternary deposits while we cannot discount the possibility that the former difference is a reflection of different parent materials for the two units climate may also be a contributory factor in explaining these differences precipitation is a major control on magnetic enhancement in paleosols maher and thompson 1995 quat res 44 383 391 liu et al 1995 palaeogeogr palaeoclimatol palaeoecol 113 243 248 but soil anoxia and iron oxide dissolution occur if moisture exceeds some critical level soil moisture is therefore a critical factor in determining whether the correlation between pedogenesis and magnetic susceptibility in loess deposits is positive central loess plateau china magnetic enhancement negative alaska siberia magnetic destruction or uncertain pakistan argentina new zealand cycles of enhancement and destruction while our results indicate a generally positive correlation between pedogenesis and magnetic susceptibility in the red clay they also imply that a more thorough paleoclimatic interpretation can only be achieved using complementary but independent techniques more work is required to deconvolve the climatic record of the red clay but the reward may be the extension of the record of paleomonsoon evolution back into the pliocene and late miocene

a new late holocene sea level record from the mississippi delta evidence for a climate sea level connection abstract a detailed relative sea level rsl record was constructed for the time interval 600 1600 ad using basal peat to track sea level and containing 16 sea level index points that capture 60 cm of rsl rise the study area is in the mississippi delta where the spring tidal range is 0 47 m the impact of ocean currents on sea surface topography is limited and crustal motions are well constrained age control was obtained by ams 14 c dating and most ages represent weighted means of two subsamples sample elevations were determined by combining differential gps measurements with optical surveying all index points were plotted as error boxes using 2 confidence intervals for the ages plus all vertical errors involved in sampling and surveying as well as the indicative range of the samples a striking clustering of sea level index points between 1000 and 1200 ad suggests a possible acceleration in the rate of rsl rise removal of the long term trend 0 60 mm yr 1 allows for the possibility of a sea level oscillation with a maximum amplitude of 55 cm however given the size of the error boxes the possibility that oscillations did not occur cannot be entirely ruled out comparison of the new rsl record with various proxy climate records suggests that sea level in this area may have responded to hemispheric temperature changes including the medieval warm period and the little ice age however given the error margins associated with this reconstruction it is stressed that this causal mechanism is tentative and requires corroboration by high resolution sea level reconstructions elsewhere

mineral magnetic studies of the vermiculated red soils in southeast china and their paleoclimatic significance abstract vermiculated red soil the main pedostratigraphical unit of the red soil sequences widespread in subtropical and tropical china has experienced a higher degree of pedogenesis than loess paleosols and the underlying red clays in the loess plateau of northern china previous studies suggested that magnetic properties of the vermiculated units of the red soils are significantly degraded compared to those of the non vermiculated counterparts the mechanism for this magnetic decrease however has not been well determined in order to resolve this problem this study has investigated two red soil sequences of different origin in southeast china by integrating environmental and mineral magnetic research the results show that the magnetic depletion in the vermiculated units was probably caused by both dissolution of magnetic minerals in the white veins and transformation of highly magnetic maghemite to weakly magnetic hematite it appears that stronger pedogenesis favors the transformation of more maghemite to hematite and magnetic properties associated with the relative concentration of maghemite to hematite can be used as climate indicator in the vermiculated red soils the variation trends of the magnetic parameters of the vermiculated red soils prior to 0 85 0 9 ma suggest a strengthening of the east asian summer monsoon the reverse trend of these parameters after 0 85 0 9 ma may correlate with a significant strengthening of the east asian winter monsoon since 0 85 ma

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

the record of changing hematite and goethite accumulation over the past 22 myr on the chinese loess plateau from magnetic measurements and diffuse reflectance spectroscopy 1 high field isothermal remanence hirm and diffuse reflectance spectroscopy drs wereusedto characterize hematite and goethite in a suite of bulkand particle sized fractioned samples from the chinese loess plateau representing the main changes in magnetic mineralogy over the past 22 myr the record of drs defined hematite largely reflects variations in the pedogenic fraction 8 mm makes the main contribution to goethite concentrations in the bulk samples pedogenic goethite concentrations generally vary inversely with those of hematite the lack of consistent correlations between hirm inferred mineral content and drs measurements casts doubt on the validity of using hirm to characterize the concentrations and relative proportions of hematite and goethite in sediments representing a long time span or a wide grain size distribution the progressive decline in the coercivity of the antiferromagnetic minerals in bulk samples throughout the whole period is mainly an expression of changes in the detrital fraction reflecting changes in the source region possibly involving increased aluminum substitution the good correlation between drs defined hematite and frequency dependent magnetic susceptibility in several chronologically defined subgroups of samples is consistent with the hypothesis that maghemite forms a transitional phase in a weathering sequence from ferrihydrite to hematite the results shed new light on using hirm to characterize hematite and goethite in sediments and on the history of weathering and climate change over the past 22 myr in the dust source regions and on the loess plateau

rock magnetism and geochemistry of two plio pleistocene chinese loess palaeosol sequences implications for quantitative palaeoprecipitation reconstruction abstract stratigraphic and spatial variations in the magnetic properties of chinese loess are widely accepted as reflecting changes in the intensity of soil forming processes over time and space recently several workers have used statistical correlations between the magnetic susceptibility and related parameters of modern loess plateau soils and modern climate to generate transfer functions to reconstruct past temporal and spatial trends in precipitation here we try to assess the viability of this approach by conducting magnetic and geochemical measurements of two long 2 5 my loess palaeosol sequences from different locations on the modern precipitation gradient across the loess plateau one site duanjiapo is located on the southern humid margin of the plateau and the other site luochuan is located in a drier modern environment some 160 km further north our results indicate discrepancies between magnetic and geochemical indicators of weathering and soil forming intensity and that any attempt to predict palaeoprecipitation at duanjiapo using published approaches would be significantly in error overall our results indicate a complex response of magnetic mineral transformations to climate including the production of both ferrimagnetic and antiferromagnetic minerals within the same climatic interval together with the destruction of magnetic material within some of the palaeosols therefore we suggest considerable caution should be exercised in generating quantitative climatic reconstructions from a single magnetic parameter

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

the dynamics of the west african monsoon part v the detection and role of the dominant modes of convectively coupled equatorial rossby waves abstract this study is the last in a series of papers addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the signals of convectively coupled equatorial rossby er waves within the summer african monsoon have been investigated after filtering noaa outgoing longwave radiation olr data within a box delineated by the dispersion curves of the theoretical er waves two families of waves have been detected in the 10 100 day periodicity band by performing a singular spectrum analysis on a regional index of er filtered olr for each family the first eof mode has been retained to focus on the main convective variability signal within the periodicity band of 30 100 days an er wave pattern with an approximate wavelength of 13 500 km has been depicted this er wave links the mjo mode in the indian monsoon sector with the main mode of convective variability over west and central africa this confirms the investigations carried out in previous studies wi

convection over tropical africa and the atlantic ocean during northern summer part ii modulation by easterly waves abstract using meteosat data and european centre for medium range weather forecast ecmwf analyses we examine easterly waves and their relation with the cloudiness over west africa and the tropical atlantic ocean for three summers june july august and september 1983 85 spectral analysis of the low level meridional wind in the 2 8 5 1 day band reveals maximum wave amplitude near the west african coast at 20 n during august and september the wave amplitude is larger than during june and july and a secondary maximum appears around 7 5 n composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ecmwf analyses the phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions this phase varies however grossly in four large land and ocean regions centered at 7 5 and 17 5 n respectively for which we derive vertical cro

east asian summer monsoon intensity inferred from iron oxide mineralogy in the xiashu loess in southern china abstract the xiashu loess in comparison to the well studied loess sequences in the chinese loess plateau clp provides a good opportunity for studying east asian monsoon variations from a southern china perspective here we present a study of the iron oxide mineralogy of the xiashu loess using integrated geochemical and diffuse reflectance spectroscopy drs measurements as well as magnetic data our results show that the free iron oxide fe d to total iron fe t ratio fe d fe t hematite hm to goethite gt ratio hm gt and saturation isothermal remanent magnetization sirm to magnetic susceptibility ratio sirm all indicate particularly strong summer monsoons during the formation of paleosols ps5 and ps4 equivalent to marine isotope stage 13 and 11 respectively however magnetic susceptibility and fe d fe t are not consistently reliable indicators of summer monsoon intensity for the whole section our results indicate that a multi proxy approach can give a more reliable summer monsoon intensity reconstruction the summer monsoon shows a cooling trend and a declining of precipitation from 0 5 to 0 3 ma after which it becomes warmer and wetter towards the top of paleosol ps1 equivalent to mis 5 however ps1 was formed under a relatively cooler temperature and wetter soil conditions in comparison to ps5 and ps4 such supra orbital variations in the east asian summer monsoon superimposed on the effects of glacial interglacial cycles in southern china are also reflected in the 0 4 0 5 ma cycle of marine carbon isotopes in the global ocean possibly indicating a strong link between terrestrial weathering and the global carbon cycle

multi scale description of a sahelian synoptic weather system representative of the west african monsoon a reference case of a sahelian weather system observed during the hydrological atmospheric pilot experiment hapex sahel in august 1992 is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints it is shown that the case study is representative of the climatology at all these scales and presents many interacting scales and physical processes at intraseasonal scale the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the african easterly jet aej and convection at the month and day scales the convective activity occurs in an apparent zonal break of the tropical easterly jet the month of august 1992 exhibits intense synoptic activity the vorticity field is characterized by northerly dry and southerly wet components located at 850 hpa on each side of the aej their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity around 21 august the 700 hpa vorticity field features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation convective activity occurs mainly ahead of the 700 hpa vorticity maximum with the formation of a squall line on air mountains propagating south westward at 15 m s 1 the convective system propagates about twice as fast as the vortex core in contrast with the convection in the european centre for medium range weather forecasts re analysis which stays in phase with the vorticity the squall line corresponds to the largest contributor to the systems passing in august 1992 over the hapex sahel region its environmental conditions and its effects on the atmosphere including the surface parameters are presented copyright 2002 royal meteorological society

magnetic enhancement is linked to and precedes hematite formation in aerobic soil 1 soil formation usually increases magnetic susceptibility most often by increasing the concentrations of magnetite and maghemite which are two ferrimagnetic iron oxides here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite which is later transformed into hematite the iron oxide that gives red color to soil we show that the maghemite hematite ratio is influenced by the particular environment and the degree of soil development so it constitutes an effective tool for paleoenvironmental and planetary studies

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

linking african easterly wave activity with equatorial waves and the influence of rossby waves from the southern hemisphere abstracta connection is found between african easterly waves aews equatorial westward moving mixed rossby gravity wmrg waves and equivalent barotropic rossby waves rws from the southern hemisphere sh the amplitude and phase of equatorial waves is calculated by projection of broadband filtered era interim data onto a horizontal structure basis obtained from equatorial wave theory mechanisms enabling interaction between the wave types are identified aews are dominated by a vorticity wave that tilts eastward below the african easterly jet and westward above the tilt necessary for baroclinic wave growth however a strong relationship is identified between amplifying vorticity centers within aews and equatorial wmrg waves although the waves do not phase lock positive vorticity centers amplify whenever the cross equatorial motion of the wmrg wave lies at the same longitude in the upper troposphere southward flow and east of this in the lower troposphere northward flow two mechanisms could

identifying goethite and hematite from rock magnetic measurements of soils and sediments we describe a sequence of experiments involving the acquisition of isothermal remanent magnetization irm in high 2 t fields followed by orthogonal demagnetization and the cooling and heating of remanent magnetization mr over the temperature range 196 c to 680 c the main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples which often contain complex mineral mixtures the procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite each characterized by x ray diffraction xrd spectra and second to controlled mixtures of the two minerals the results confirm the mineral diagnostic nature of the experiments used they are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments goethite can be unambiguously identified often accompanied by hematite in the majority of the samples studied despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable the rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are xrd analyses the results suggest that goethite s contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated

numerical study of a sahelian synoptic weather system initiation and mature stages of convection and its interactions with the large scale dynamics summary a 24 h multi scale simulation of the life cycle of a sahelian squall line is presented the initial and coupling e elds have been taken from the european centre for medium range weather forecasts re analysis era 15 after humidity corrections to avoid spurious absolute and conditional convective instabilities comparisons of modelsynthetic and meteosat infra red radiances indicate that the simulated scenario of convection development and structure are realistic though differences are found concerning the size of the squall line with a delay of 2 3 h the simulated arc shaped structure and surface signature e t well with radar and ground station observations respectively the mature stage of the simulated squall line presents a cross line structure similar to conceptual models an intense rear to front e ow is generated below the trailing stratiform part an acceleration of the african easterly jet aej is simulated behind the system occurring across its full width this leads to the generation of a dipole of counter rotating vortices the resulting potential vorticity signature leads to local reinforcement of the barotropic instability on the north e ank of the aej this simulation is used to analyse the impact of the convection at synoptic scales and to compare it with era 15 it suggests that the convection contributes to reinforce the monsoon at least up to 500 km the monsoon ine ow moves eastward and westward with time scales shorter and longer than 24 h respectively the tropical easterly jet intensie es to the south west of the convection area up to 700 km away the aej is strongly intensie ed ahead of the trough where active convection occurs organized convection developing in the vicinity of the aej core induces weakening of the aej ahead of the squall line and strengthening behind it a common result in the simulation and in era 15 is that the aej core moves westward faster due to the convective activity

the influence of the mjo on upstream precursors to african easterly waves abstractthe madden julian oscillation mjo produces alternating periods of increased and reduced precipitation and african easterly wave aew activity in west africa this study documents the influence of the mjo on the west african monsoon system during boreal summer using reanalysis and brightness temperature fields mjo related west african convective anomalies are likely induced by equatorial kelvin and rossby waves generated in the indian ocean and west pacific by the mjo which is consistent with previous studies the initial modulation of tropical african convection occurs upstream of west africa near the entrance of the african easterly jet aej previous studies have hypothesized that an area to the east of lake chad is an initiation region for aews called the trigger region in this study this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry mjo phases in the west african monsoon region in the trigger region cold tropospheric tempera

environmental magnetism of pleistocene sediments in the north pacific and ontong java plateau temporal variations of detrital and biogenic components 1 an environmental magnetic study was conducted to investigate temporal variations of the sources of magnetic minerals in the pacific ocean using sediment cores of pleistocene age the proportion of interacting i to noninteracting n i single domain components was estimated from first order reversal curve diagrams and the proportion of middle m to low l coercivity components was determined from decomposition of isothermal remanent magnetization irm acquisition curves the i and m components are interpreted to be carried by terrigenous maghemite while the n i and l components represent biogenic magnetite an inverse correlation between the ratio of anhysteretic remanent magnetization susceptibility arm to saturation irm sirm and the i n i ratio was confirmed for sediments across wide expanses in the pacific ocean this implies that arm sirm can be used to estimate the relative abundances of biogenic and terrigenous components in sediments in the north pacific the relative abundance of the i and m components increases in glacial periods variations of arm sirm and s ratio s 0 1t resemble each other and decrease in the same time intervals these variations reflect increases of terrigenous input in glacial periods as eolian dust on the ontong java plateau in the western equatorial pacific on the other hand minima in arm sirm and s 0 1t occur at glacial to interglacial transitions and the relative abundance of the i and m components increases in these periods terrigenous material in this region is considered to be transported mainly from the new guinea and solomon islands by the equatorial undercurrent the higher proportion of the terrigenous component at glacial to interglacial transitions is coeval with carbonate preservation maxima reported from the ontong java plateau suggesting a linkage between the two

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

the magnetic properties of loess and paleosol samples from the chinese loess plateau spanning the last 22 million years abstract magnetic measurements susceptibility isothermal and anhysteretic remanence and quotients derived from these are presented for a suite of loess and paleosol samples from sites on the chinese loess plateau the three sites xifeng dongwan and qinan qa i together span most of the last 22 million years the results provide the first multi parameter magnetic measurements from pre pleistocene loess paleosol sequences that began accumulation in the early miocene mass specific measurements have been corrected for carbonate concentrations which peak in the loess layers and range up to 57 all the magnetic properties recorded indicate variability on both orbital and supra orbital timescales the changing magnetic properties allow the record to be divided into a sequence of eight sample groups that broadly parallel the main supra orbital changes in magnetic susceptibility the pre pleistocene parts of the record include intervals mainly late miocene and pliocene during which loess as well as paleosol layers are characterized by high values for properties such as frequency dependent susceptibility fd usually associated with strong weathering most of the miocene record prior to 8 5 ma with the exception of a brief interval from ca 14 4 to 15 9 ma shows reduced values for indicators of pedogenic ferrimagnetic concentrations in both the loess and paleosol layers as well as a low amplitude of variability between loess and paleosol values the palaeoenvironmental implications of this part of the sequence are unclear since they hinge on the type of model of pedogenic magnetic mineral formation adopted high field remanence measurements indicate that the coercivity of the antiferromagnetic minerals represented declined progressively throughout the whole period in both the loess and the paleosol samples though there is no unambiguous evidence for long term diagenetic alteration of the magnetic properties on timescales beyond the orbital ones that drive the loess paleosol alternations the present results suggest caution in applying existing climofunctions based on magnetic susceptibility to deposits spanning the whole of the last 22 ma

lack of correlation between paleoprecipitation and magnetic susceptibility of chinese loess paleosol sequences we have conducted a detailed mineral magnetic study of loess unit 8 l8 and paleosol unit 8 s8 from three localities jingbian yichuan and duanjiapo along a n s transect in the chinese loess plateau as expected the low field magnetic susceptibility has higher values in s8 and lower values in l8 similarly superparamagnetic particle concentrations increase in s8 with increasingly humid cli mates along the n s transect which suggests that pedogenic magnetic enhancement is related to climate however in s8 at duanjiapo is so low that there is no correlation between and the degree of pedogenesis in this loess paleosol cycle it therefore appears that is not consistently the most suit able indicator of paleoclimate in chinese loess paleosol se quences our results indicate that there are complexities in the mineral magnetic response to pedogenesis which could invalidate interpretation of the record of chinese loess paleosol sequences in terms of paleoprecipitation

rock magnetic properties related to thermal treatment of siderite behavior and interpretation detailed analyses of rock magnetic experiments were conducted on the oxidation products of high purity natural crystalline siderite that were thermally treated in air atmosphere susceptibilities increase sharply between 400 and 530 c indicative of some new ferrimagnetic mineral phase generation both a drop between 540 and 590 c on the heating cycle and a dramatic increase from 590 c to 520 c on the cooling cycle occurred and are well consistent with the characteristic of magnetite a distinct hopkinson type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700 c over and over it has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite bearing specimens is changed to a normal magnetic fabric during incremental heating over 410 490 c this is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals a strong chemical viscous remanent magnetization could be produced during siderite oxidation in an external field rock magnetic experimental results show that magnetite maghemite and hematite are the transformation products of high temperature oxidation of siderite in air maghemite was not completely inverted to hematite even at temperature as high as 690 c during incremental thermal treatments the mineral transformation processes were confirmed by conventional optical microscopic observation x ray diffractometry and mossbauer spectroscopic analyses these results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks we conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments furthermore it is clear that great care should be exercised in thermal demagnetization of siderite bearing rocks in paleomagnetic magnetic anisotropy and rock magnetic studies

three dimensional structure and dynamics of african easterly waves part i observations the mean structure of african easterly waves aews over west africa and the adjacent atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered satellite derived outgoing longwave radiation these results are compared with previous studies and an idealized modeling study in a companion paper which provides evidence that the waves bear a close structural resemblance to the fastest growing linear normal mode of the summertime basic state flow over africa there is a significant evolution in the three dimensional structure of aews as they propagate along 10 n across west africa at this latitude convection occurs in northerly flow to the east of the greenwich meridian then shifts into the wave trough and finally into southerly flow as the waves propagate offshore into the atlantic itcz in contrast to the north of the african easterly jet along 15 n convection remains in southerly flow throughout the waves trajectory along 10 n over west africa the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction as in the companion paper offshore and along 15 n the relationship between the convection and dynamics is more complex and not as easily explained in terms of dynamical forcing alone

rainfall dependent transformations of iron oxides in a tropical saprolite transect of hainan island south china spectral and magnetic measurements 1 the iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall the correlation seems to be reversed under extreme climates with high rainfall here we present a transect of saprolitic soils from hainan island south china with high rainfall 1440 2020 mm yr little temperature variation 23 24 c and extreme weathering along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall whereas goethite concentration displays a large increase however there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes these pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite magnetic susceptibility and rainfall control of an inflection point this study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range a conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point

sahel droughts and enso dynamics correlations between summer sahel rainfall and southern oscillation index has increased during the last thirty years at high frequency time scale periods lower than 8 years an intertropical atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature sst anomalies between the eastern equatorial parts of pacific and atlantic this zonal connection worked well during most of the e1 nino southern oscillation enso events occurring after 1970 positive negative sst anomalies in the eastern pacific atlantic led to rainfall deficits over the whole west africa at low frequency time scale periods greater than 8 years positive sst anomalies in the indian ocean and in equatorial pacific existing after 1970 have been associated with decreasing rainfall intensity over west africa through another zonal divergent circulation these different time scales remote sst forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between sahel drought and enso dynamics after 1970

interannual variation of the tropical easterly jet abstract the 200 mb tropical wind fields analyzed at florida state university for 1965 74 and the 200 and 700 mb tropical wind fields from the national meteorological center for 1979 82 were used to explore the mechanism for the interannual variation of the tropical easterly jet this jet is generally weaker during the summers of warm events dry summers in the southern oscillation when anomalously warm surface water appears over the eastern and central equatorial pacific and drought occurs over the indian subcontinent it is observed that divergence convergence exists on the upstream downstream side of the jet the tropical divergent circulations i e the east west walker and the local hadley circulations during such summers are weakened and shifted eastward therefore divergence anomalies appear in the upper troposphere over equatorial africa or the cast coast of africa whole convergence anomalies exist over the indian subcontinent or the arabian sea these changes of the tropical divergent ci

mesoscale convective systems in relation to african and tropical easterly jets abstractthis paper documents the interaction processes between mesoscale convective systems mcs the tropical easterly jet tej and the african easterly jet aej over west africa during the monsoon peak of 2006 observed during the african monsoon multidisciplinary analyses amma project the results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle a systematical study reveals that intense and long lived mcss correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of tej and in exit regions of aej a case study on a particularly well documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence favoring the persistence of mcss

wave activity on the tropical easterly jet abstract this article examines the question of the existence of waves on the tropical easterly jet tej over west africa the tej is a well known feature of the asian monsoon and waves on the jet have been implicated in various weather phenomena its role in west african meteorology has received little attention a model simulation of wet and dry years over west africa predicted wave development on the tej ncep reanalysis data confirmed the existence of these waves using case studies in the dry year 1983 and the wet year 1950 both the simulated and observed waves are of planetary scale with a period of 5 6 days potential vorticity pv theory suggests that they develop via interactions between the surface and the tej overall the results suggest that interactions between the tej and african easterly jet play an important role in the development of wave disturbances over west africa

horizontal structure and seasonality of large scale circulations associated with submonthly tropical convection the relationship between deep tropical convection and large scale atmospheric circulation in the 6 30 day period range is examined regression relationships between filtered outgoing longwave radiation at various locations in the tropics and 200 and 850 mb circulation are mapped for the standard seasons and the spatial structure and seasonal dependence of the results are interpreted in view of the basic state circulation in regions where the convection is embedded in upper level easterlies anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal along with patterns of meridional outflow into subtropical anticyclonic perturbations lagged relationships suggest that the convection is forcing the circulation in many of these cases the outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons with more symmetric signals about the equator seen in the equinoctial seasons the longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection there tends to be a first baroclinic mode vertical structure to these circulations such that equatorial westerlies are present at 850 mb within the convection with closed circulations on either side of the equator resembling equatorial rossby modes especially common over the atlantic and pacific sectors as a contrast in regions located within upper level westerlies or along the margin of influence of upper westerly disturbances convection appears to be forced by upper level wave energy propagating into the deep tropics with the heating located in the upward motion region ahead of upper level troughs this occurs over the atlantic and eastern pacific sectors during northern winter and spring and over australia the south pacific and south america during southern summer when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection the results confirm theoretical and modeling ideas that suggest that rossby wave energy is able to propagate into the deep tropics in regions where upper level westerlies exist in the tropics

depositional and post depositional controls on magnetic signals from saltmarshes on the north west coast of ireland magnetic measurements were carried out on samples from two saltmarshes deposited during the last century on the north west coast of ireland based on profiles from cliffed saltmarsh edge mid marsh and supratidal locations the processes that affect the generation and persistence of magnetic properties in environments were investigated this was done to help in understanding the geochemical processes affecting north west irish coast saltmarshes and to determine the frequency and provenance of sediment laden flooding events hence disentangling the importance of clastic and organic contributions to saltmarsh growth magnetic properties indicate depositional and post depositional effects varying on both temporal and spatial scales the interpretations presented here suggest that both biogenic and diagenetic processes have a strong effect on the magnetic properties recorded in the former case the growth of magnetotactic bacteria is the main process in the latter case it is reductive diagenesis associated with organic matter decomposition that is important the biogenic and diagenetic effects overprint and degrade the detrital magnetic signatures respectively this suggests that magnetic measurements within the context of north west irish coast saltmarshes can provide limited information on clastic sediment input to these environments comparisons between sites on the north west coast of ireland show similar magnetic properties despite differences in deposition rates implying that significant post depositional alteration of magnetic properties occurs within the first 20 40 years after deposition magnetic properties of north west coast irish saltmarshes are put into a regional context by comparison with magnetic profiles from similar sites

analysis of convection and its association with african easterly waves abstract the association between convection and african easterly wave aew activity over tropical africa and the tropical atlantic during the boreal summer is examined using satellite brightness temperature tb and ecmwf reanalysis datasets spectral analysis using 18 yr of tb data shows significant variance in the 2 6 day range across most of the region within the regions of deep convection this time scale accounts for about 25 35 of the total variance the 2 6 day convective variance has similar amplitudes over western and eastern africa while dynamic measures of aew activity show stronger amplitudes in the west this study suggests that weak aew activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern africa near darfur western sudan and ethiopia has a role in initiating aews westward the subsequent development and growth of aews in west africa is associated with stronger cohe

magnetic discrimination between al substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance received 22 june 2011 revised 20 december 2011 accepted 24 december 2011 published 23 february 2012 1 hematite a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas is weakly magnetic however it carries a stable natural remanent magnetization and thus can reflect paleoenvironment changes to quantify the influence of al content in hematite on its magnetic properties two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension hfh series and by thermal dehydration of goethite hg series crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively more specifically the hfh series samples display oblate plate like morphologies while the hg series samples are prolate highly acicular hfh series samples display higher saturation magnetization but lower magnetic coercivity than that of the hg series it is tenable that a better lattice ordering of al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite resulting in weaker saturation magnetization for hg series samples the origin of single domain sd hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity a positive correlation indicates the presence of pure al free sd hematite while a negative correlation indicates a chemical origin of sd al substituted hematite these results bear new information on decoding the complex magnetic properties of sd al hematite in nature environments and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by al hematite

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

origin of magnetic mineral concentration variation in the southern ocean 1 in the southern ocean magnetic mineral concentration increases in glacial periods the variation pattern closely resembles eolian dust flux records from antarctic ice cores but the cause of the linkage remains unclear as the dust flux is too small for the source of terrigenous materials in the southern ocean we have conducted an environmental magnetic study of late pleistocene sediments from the south indian ocean to investigate the origin of the magnetic concentration changes biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution reflecting occurrence of single domain magnetites in a chain we interpret that a non interacting component on first order reversal curve diagrams and low coercivity components with small dispersion from isothermal remanent magnetization irm component analyses represent biogenic magnetites and that the interacting and middle coercivity components represent terrigenous maghemites the ratio of anhysteretic remanent magnetization susceptibility to saturation irm reflects relative abundance of the biogenic and terrigenous components it was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals in glacials the abundance of both biogenic and terrigenous components increased with increased proportions of the latter increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies indicative of less oxic conditions and increased sedimentation rates these observations suggest that the increased magnetic concentration in glacials in the southern ocean may be explained by iron fertilization the production of biogenic magnetites was enhanced associated with increased ocean productivity which was fueled by increased eolian dust flux

convectively coupled equatorial waves 1 convectively coupled equatorial waves ccews control a substantial fraction of tropical rainfall variability their horizontal structures and dispersion characteristics correspond to matsuno s 1966 solutions of the shallow water equations on an equatorial beta plane namely kelvin equatorial rossby mixed rossby gravity and inertio gravity waves because of moist processes the tilted vertical structures of ccews are complex and their scales do not correspond to that expected from the linear theory of dry waves the dynamical structures and cloud morphology of ccews display a large degree of self similarity over a surprisingly wide range of scales with shallow convection at their leading edge followed by deep convection and then stratiform precipitation mirroring that of individual mesoscale convective complexes ccews have broad impacts within the tropics and their simulation in general circulation models is still problematic although progress has been made using simpler models a complete understanding of ccews remains a challenge in tropical meteorology

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments iron oxides affect many elemental cycles in aquatic sediments via numerous redox reactions and their large sorption capacities for phosphate and trace elements the reactive ferric oxides and oxyhydroxides are usually quantified by operationally defined selective chemical extractions that are not mineral specific we have used cryogenic 57fe mossbauer spectroscopy to show that the reactive iron oxyhydroxide phase in a large variety of lacustrine and marine environments is nanophase goethite feooh rather than the assumed surface complex stabilized two line ferrihydrite and accompanying mixture of clay and oxyhydroxide fe bearing phases this result implies that the kinetic and stability parameters of the type of nanogoethite that we observe to be present in sediments should be first determined and then used in models of early diagenesis the identity and characteristics of the reactive phase will also set constraints on the mechanisms of its authigenesis

temperature dependence of magnetic susceptibility in an argon environment implications for pedogenesis of chinese loess palaeosols summary temperature dependence of magnetic susceptibility t has been widely used to determine changes in mineralogy of natural samples during heat treatment we carried out integrated rock magnetic experiments to interpret the t curves of the chinese loess palaeosols in argon we used both raw materials and heated samples in addition we also investigated the magnetic properties of magnetic extracts and residues to quantify contributions from each fraction to the bulk magnetic properties for the heating curves the susceptibility loss 30 per cent between 300 400 c is caused by the inversion from pedogenic fine grained maghemite to haematite suggesting that the susceptibility loss can be used as a new concentration index of the pedogenic fine grained superparamagnetic sp particles in the chinese loess palaeosols unlike the warming curves the cooling curves are dominated by newly formed fine grained magnetites with a dominant size of 35 nm the onset for the new production of these fine grained magnetic particles occurs at 400 c it is interesting that the room temperature magnetic susceptibility ph of the samples heated after a 700 c run is independent of the degree of pedogenesis and saturates at approximately 33 35 10 7 m3 kg 1 indicating that the susceptibility enhancement is controlled only by the reduction of fe bearing aeolian minerals during heating it appears that the 700 c thermal treatment in argon could be in some sense an analogue to the pedogenic processes thus we predict that 33 35 10 7 m3 kg 1 is the maximum susceptibility that pedogenesis can generate for the last interglacial palaeosol unit s1 in practice ph would be useful to quantify the aeolian inputs to the chinese loess plateau

iron geochemical zonation in a tidally inundated acid sulfate soil wetland abstract tidal inundation is a new technique for remediating coastal acid sulfate soils cass here we examine the effects of this technique on the geochemical zonation and cycling of fe across a tidally inundated cass toposequence by investigating toposequence hydrology in situ porewater geochemistry solid phase fe fractions and fe mineralogy interactions between topography and tides exerted a fundamental hydrological control on the geochemical zonation redistribution and subsequent mineralogical transformations of fe within the landscape reductive dissolution of fe iii minerals including jarosite kfe3 so4 2 oh 6 resulted in elevated concentrations of porewater fe2 30 mmol l 1 in former sulfuric horizons in the upper intertidal zone tidal forcing generated oscillating hydraulic gradients driving upward advection of this fe2 enriched porewater along the intertidal slope subsequent oxidation of fe2 led to substantial accumulation of reactive fe iii fractions up to 8000 mol g 1 in redox interfacial tidal zone sediments these fe iii precipitates were poorly crystalline and displayed a distinct mineralisation sequence related to tidal zonation schwertmannite fe8o8 oh 6so4 was the dominant fe mineral phase in the upper intertidal zone at mainly low ph 3 4 this was followed by increasing lepidocrocite feooh and goethite feooh at circumneutral ph within lower intertidal and subtidal zones relationships were evident between fe fractions and topography there was increasing precipitation of fe sulfide minerals and non sulfidic solid phase fe ii in the lower intertidal and subtidal zones precipitation of fe sulfide minerals was spatially co incident with decreases in porewater fe2 a conceptual model is presented to explain the observed landscape scale patterns of fe mineralisation and hydro geochemical zonation this study provides valuable insights into the hydro geochemical processes caused by saline tidal inundation of low lying cass landscapes regardless of whether inundation is an intentional strategy or due to sea level rise

sulfur biogeochemical cycling and novel fe s mineralization pathways in a tidally re flooded wetland abstract sulfur biogeochemical cycling and associated fe s mineralization processes exert a major influence over acidity dynamics electron flow and contaminant mobility in wetlands benthic sediments and groundwater systems while s biogeochemical cycling has been studied intensively in many environmental settings relatively little direct information exists on s cycling in formerly drained wetlands that have been remediated via tidal re flooding this study focuses on a tidal wetland that was drained in the 1970s causing severe soil and water acidification and subsequently remediated by controlled re flooding in 2002 we examine so 4 2 reduction rates and fe s mineralization at the tidal fringe 7 years after the commencement of re flooding the initial drainage of the wetland examined here caused in situ pyrite fes2 oxidation resulting in the drained soil layers being highly acidic and rich in so 4 2 bearing fe iii minerals including jarosite kfe3 so4 2 oh 6 tidal re flooding has neutralized much of the previous acidity with the pore water ph now mostly spanning ph 5 7 the fastest rates of in situ so 4 2 reduction up to 300 nmol cm 3 day 1 occur within the inter tidal zone in the near surface soil layers to 60 cm below ground surface the so 4 2 reduction rates correlate with pore water dissolved organic c concentrations thereby suggesting that electron donor supply was the predominant rate determining factor elemental s was a major short term product of so 4 2 reduction comprising up to 69 of reduced inorganic s in the near surface soil layers this enrichment in elemental s can be partly attributed to interactions between biogenic h2s and jarosite a process that also contributed to enrichment in pore water fe2 up to 55 mm and so 4 2 up to 50 mm the iron sulfide thiospinel greigite fe3s4 was abundant in near surface soil layers within the inter to sub tidal zone where tidal water level fluctuations created oscillatory redox conditions there was evidence for relatively rapid pyrite re formation within the re flooded soil layers however the results indicate that pyrite re formation has occurred mainly in the lower formerly drained soil layers whereas the accumulation of elemental s and greigite has been confined towards the soil surface the discovery that pyrite formation was spatially decoupled from that of elemental s and greigite challenges the concept that greigite is an essential precursor required for sedimentary pyrite formation in fact the results suggest that greigite and pyrite may represent distinct end points of divergent fe s mineralization pathways overall this study highlights novel aspects of fe s mineralization within tidal wetlands that have been drained and re flooded in contrast to normal undisturbed tidal wetlands as such the long term biogeochemical trajectory of drained and acidified wetlands that are remediated by tidal re flooding cannot be predicted from the well studied behaviour of normal tidal wetlands

magnetic analyses of soils from the wind river range wyoming constrain rates and pathways of magnetic enhancement for soils from semiarid climates in order to constrain the rate of magnetic enhancement in soils we investigated modern soils from five fluvial terraces in the eastern wind river range wyoming profiles up to 1 2 m deep were sampled in 5 cm intervals from hand dug pits or natural riverbank exposures soils formed in fluvial terraces correlated to the sacajawea ridge 730 610 ka bp bull lake 130 100 ka bp and pinedale age 20 ka bp glacial advances one soil profile formed in holocene age sediment abundance mineralogy and grain size of magnetic minerals were estimated through magnetic measurements magnetic enhancement of the a horizon as well as an increase in fine grained magnetic minerals occurred mostly in bull lake profiles but was absent from the older profile such low rates of magnetic enhancement may limit the temporal resolution of paleosol based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols a loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis absolute and relative hematite abundance increase with age making both useful proxies for soil age and the dating of regional glacial deposits all coercivity proxies are consistent with each other which suggests that observed changes in hirm and s ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions even though the modified l ratio varies widely

sahel rainfall tropical easterly jet relationship on synoptic to intraseasonal time scales abstractthe tropical easterly jet tej is a characteristic upper level feature of the west african monsoon wam circulation moreover the tej over west africa is significantly correlated with su

iron geochemistry of loess and red clay deposits in the chinese loess plateau and implications for long term asian monsoon evolution in the last 7 0 ma abstract recently some groups of authors have demonstrated that the tertiary red clay deposits underlying the pleistocene loess paleosol sequence in the chinese loess plateau are wind blown in origin thus extending the eolian record in the plateau from 2 6 ma back to about 7 0 ma and providing a good opportunity to reconstruct long term monsoon changes as magnetic susceptibility the widely used paleomonsoon proxy of loess deposits is problematic in reconstructing the monsoon history recorded in the red clay the development of other proxies is urgently needed in this study we analyzed the ratios of citrate bicarbonate dithionite cbd extractable fe2o3 to total fe2o3 concentrations in four loess sections along a north south transect of the loess plateau and in the lingtai loess red clay sequence of 305 m thick the fe2o3 ratio in the loess sections clearly shows higher values in soil units than in loess horizons and a southward increase in this ratio is also remarkable along the transect these are in good agreement with the temporal spatial variations in the east asia summer monsoon intensity identified by other monsoon proxies and soil development observations the fe2o3 ratio thus attests to being a good proxy indicator of the summer monsoon evolution the lingtai fe2o3 ratio record shows high values over three time intervals 4 8 4 1 ma 3 4 2 6 ma and during the interglacial periods of the past 0 8 ma the increase in summer monsoon intensity over the three intervals also coincides with the well developed soil characteristics it is therefore concluded that the east asia summer monsoon has experienced a non linear evolution since the late miocene the occurrence of a monsoon prime at about 4 8 4 1 ma implies that the tibetan plateau one of the most important boundary conditions in maintaining the monsoon circulation must have been uplifted to a critical height at that time

climatic interpretation of the luochuan and lingtai loess sections china based on changing iron oxide mineralogy and magnetic susceptibility abstract hematite and goethite estimated from diffuse reflectance spectra and magnetic susceptibility ms have been determined from both the luochuan and lingtai loess sections of the loess plateau of central china changing concentrations of these iron oxides and ms were used to determine paleoclimatic change based on a model that relates pedogenic production of hematite and magnetite to precipitation we propose two precipitation phases on the loess plateau during the last 2 6 ma from the wucheng red clay boundary up to paleosol s5 mis 13 precipitation on the loess plateau was moderate probably not frequently exceeding 450 mm yr and highly seasonal from s5 to the present precipitation during interglacial times has gradually increased to the present levels of 600 650 mm yr for the localities examined throughout the pleistocene spanning both glacial and interglacial conditions hematite to goethite ratios display a consistent decrease this decrease suggests that precipitation on the loess plateau has been getting less seasonal and or has been occurring during times when temperature was lower in addition to these long term changes it also appears that iron oxide records from the loess plateau exhibit 400 500 ka cycles from the wucheng red clay boundary to s5 these 400 500 ka cycles may be analogous to similar length cycles recorded in 13 c marine records in the south china sea and may be linked through weathering and silica production as a result of the asian monsoon

coercivity analysis of magnetic phases in sapropel s1 related to variations in redox conditions including an investigation of the s ratio 1 abstract the most recent sapropel s1 in the eastern mediterranean has been extensively investigated with geochemical and rock magnetic techniques different redox conditions prevailed in different zones of the sediment through time the oxidized sapropel zone is particularly interesting because earlier studies indicated that new magnetic material was formed including possible magnetosomes here we utilize component analyses of isothermal remanent magnetization irm acquisition curves and the analysis of first order reversal curves forc to further investigate the magnetic mineralogy in the entire box core abc26 the original input of eolian dust consisted of both magnetite and hematite in the oxidized sapropel and in the active oxidation zone an additional magnetite component is present this magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion suggesting a narrow grain size distribution this is a strong indication that magnetosomes which are formed in the active oxidation zone are the magnetic carriers of this coercivity fraction forc diagrams support these findings the s ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core s ratio behavior close to 1 in the top sediment and in the oxidized sapropel a drop in values in the active oxidation zone and again close to 1 in the syn sapropel the observed s ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered irm components whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component their s ratios are different because of the differences in coercivity characteristics coercivity and dispersion of the two magnetites in these zones thus in these zones the s ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages also in the rest of the core the s ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high versus low coercivity minerals the s ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy especially when more than two components are present therefore the classical interpretation of the s ratio should be treated with caution

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near shore marine sediments the magnetic properties of low field susceptibility frequency dependent susceptibility fd and susceptibility of anhysteretic remanent magnetisation arm are used to define sets of natural soil and sediment samples within which by the criteria of king et al 1982 and maher 1988 the modal ferrimagnetic grain diameter is less than 0 07 0 1 m within this sample set two distinct groups with low and high values respectively for the quotients arm and arm fd are defined the first group includes sediment samples from sites where published studies propose a detrital origin for the fine grained ferrimagnetic content where catchment samples are available for comparison they fall within the same envelope as do clays from palaeosol samples within chinese loess this envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within maher s 1988 new mt series the high quotient envelope of values includes sediments from the adriatic sea and clays from saltmarsh and shallow water marine sediments in nw britain in these cases no catchment source is postulated for the fine grained magnetite the size range of magnetite in this set appears to be almost exclusively stable single domain ssd and a bacterial origin seems likely sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which in the case of the potomac are ordered by distance down river although at this stage magnetic measurements alone cannot discriminate between bacterial and fine grained detrital ferrimagnets in sediments they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

airborne radar imaging of subaqueous channel evolution in wax lake delta louisiana usa shallow coastal regions are among the fastest evolving landscapes but are notoriously difficult to measure with high spatiotemporal resolution using uninhabited aerial vehicle synthetic aperture radar uavsar data we demonstrate that high signal to noise l band synthetic aperture radar sar can reveal subaqueous channel networks at the distal ends of river deltas using 27 uavsar images collected between 2009 and 2015 from the wax lake delta in coastal louisiana usa we show that under normal tidal conditions planform geometry of the distributary channel network is frequently resolved in the uavsar images including 700 m of seaward network extension over 5 years for one channel uavsar also reveals regions of subaerial and subaqueous vegetation streaklines of biogenic surfactants and what appear to be small distributary channels aliased by the survey grid all illustrating the value of fine resolution low noise l band sar for mapping the nearshore subaqueous delta channel network

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals