several distinct wet periods since 420 ka in the namib desert inferred from u series dates of speleothems abstract the scarcity of numerical dates of the arid areas in southern africa is a challenge for reconstructing paleoclimate this paper presents a chronological reconstruction in the central part of the namib desert namibia for the last 420 000 yr it is based on 230th u dates tims from a large stalagmite and a thick flowstone layer in a small cave located in the hyper arid central namib desert the results provide for the first time evidence of three or possibly four succeeding wet periods of decreasing intensity since 420 ka through which speleothem deposited at approximately 420 385 ka 230 207 ka and 120 117 ka following the 100 ka milankovitch cycle speleothem growth was not recorded for the holocene these wet periods interrupted the predominantly dry climate of the namib desert and coincided with wet phases in deserts of the northern hemisphere in the murzuq basin sahara the negev israel the nafud desert saudi arabia and the arid northern oman arabian peninsula

ch 4 consuming microorganisms and the formation of carbonate crusts at cold seeps to understand the role played by microorganisms in the formation of cold seep carbonates we conducted an integrated microbial mineralogical and organic geochemical study of methane related authigenic carbonate crusts formed on eastern mediterranean mud volcanoes we show that supersaturation with respect to carbonate minerals is induced by microbial anaerobic oxidation of methane combined lipid biomarker analysis and 16s rrna gene surveys identified a highly diversified methane consuming archaeal community possibly comprising novel species implying that the anaerobic oxidation of methane is phylogenetically widespread and directly implicating these organisms in the process of crust precipitation moreover pore water sulphate gradients produced by co occurring methane based sulphate reduction exert the main control on aragonite versus magnesian calcite precipitation we propose that this may be the dominant mode of carbonate crust formation at cold seeps world wide in agreement with aquatic chemistry predictions and explaining carbonate mineralogy

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

archaeal polar lipids in subseafloor sediments from the nankai trough implications for the distribution of methanogens in the deep marine subsurface abstract although the methane in marine methane hydrates is mainly of microbial origin information about the distribution of methanogens in subseafloor sediments is limited to address this issue we analyzed sediment core samples from two sites in the nankai trough off the pacific coast of central japan including those bearing methane hydrates from depths 100 m below the seafloor mbsf for isopranyl ether linked polar lipids i e with polar head groups of phosphate sugar or both as biomarkers of archaea including methanogens in most samples including the deepest 381 mbsf archaeol and sn 2 and sn 3 hydroxyarchaeols were detected as their hydrolyzed derivatives concentrations of these three archaeal lipids correlated strongly with each other suggesting a common biological source the 13 c values of phytane derived from the phytanyl groups in the archaeal lipids were distinctly higher than those of methane indicating that methanogens rather than anaerobic methanotrophic archaea were the major biological source depth profiles of polar sn 2 hydroxyarchaeol concentration were consistent with those of the potential methane production activity previously estimated from incubation of core sediments from the same sites this observation together with results of previous studies showing the presence of sn 2 hydroxyarchaeol mainly in shallow young sediments strongly suggests that this polar lipid is a valid biomarker for in situ methanogens in sediments there was a strong correlation between the concentration of polar sn 2 hydroxyarchaeol and that of total organic carbon suggesting that bulk organic matter concentration is a primary control on the distribution of methanogens in sediments

a 780 year annually resolved record of indian ocean monsoon precipitation from a speleothem from south oman 1 meteorological records of monsoon rainfall in the indian ocean are generally less than 100 years long the relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales to determine what factors influence the intensity of rainfall on these timescales or to place possible changes in the twentieth century into a broader historical context development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward we have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern arabia our results show that monsoon variation over the past century is not outside of the range of the past 800 years decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the indian ocean spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales decadal to interannual cycles in the record appear to originate in the tropical pacific ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 3354 meteorology and atmospheric dynamics precipitation 1854 1620 global change climate dynamics 3309 1699 global change general or miscellaneous keywords monsoon paleoclimate rainfall indian ocean speleothem oxygen isotopes

vegetation change in equatorial west africa time slices for the last 150 ka abstract drawing on marine and terrestrial palynological data vegetation maps of equatorial west africa are presented for eleven time slices over the last 150 000 years long marine records are used as a basis for the regional picture and provide a chronology for the last glacial cycle much shorter terrestrial records help fill in the picture for the most recent periods and facilitate interpretation of regional patterns for the longer timescale temporal and spatial variation is revealed in relation to global patterns of climate change rain forest was widespread during oxygen isotope stages 1 and 5 but strongly reduced during stages 3 and 4 and especially during stages 2 and 6 when open grass rich vegetation prevailed glacial rain forest refuges are found in the southwest of the guinean mountains and along the eastern coast of the gulf of guinea miombo woodland of the zambezian vegetation zone expanded during stage 5 especially during the first half podocarpus forest had its largest expansion during substages 5d 115 to 105 ka and 5b 95 to 85 ka the last occurrence of podocarpus in the guinean mountains is during stage 5a the distribution of mangrove swamps was extremely reduced during glacial times during stage 6 savanna and open dry forest covered large areas along the northern coast of the gulf of guinea the southern saharan desert reached far to the south and the namib desert far to the north the area of rain forest was restricted mangrove swamps were strongly reduced and the area of podocarpus forest fluctuated during stage 5 large changes in the area of afromontane forest rain forest dry open forest and savanna occurred rain forest was widespread and mangroves were extensive along the coast during the last interglacial substage 5e podocarpus forest area strongly extended during substages 5d and 5b in substages 5c and 5a rain forest reclaimed areas it had lost in the previous substages 5d and 5b respectively mangrove swamps were less widespread in the later substages of stage 5 than during substage 5e during stage 4 the rain forest area was again strongly reduced and recovered only slightly during the following stage 3 also the mangrove swamp area was reduced except along the ivory coast and along the northwestern coast of the gulf of guinea podocarpus forest only occurred in angola and may be in congo during stage 4 again forest was much reduced during stage 2 and open vegetation types covered large parts of equatorial west africa mangrove swamps must have been rare at the beginning of the holocene mangrove swamps had recovered and reached their largest extension also the rain forests area increased in the early holocene and guinean and congolian rain forests were probably not separated by a savanna corridor now existing in togo and benin

palaeoclimate in the saharan and arabian deserts during the middle palaeolithic and the potential for hominin dispersals to disperse out of sub saharan africa it was necessary for hominins to cross the deserts of either the sahara and or arabia thus understanding the palaeoclimate of the saharo arabian region is central to determining the role these deserts played in the peopling of the planet when did they act as barriers and when were they more humid opening dispersal routes across them to address these questions we have conducted a temporal and spatial evaluation of dated sites from 20 to 350 ka using combined probability density function pdf and geographical information system gis analyses of all sites dated using uranium thorium u th or optically stimulated luminescence osl methods radiocarbon dates were not considered because of contamination problems in this time range the results show that during mis 2 there is little evidence for humidity in arabia as would be expected during the height of the last glacial maximum however the sahara shows a sharp rise in probability at the beginning of mis 2 peaking near the boundary with mis 3 at 29 ka there appear to be brief periods of humidity in mis 3 and 6 though at different times in the sahara ca 37 44 138 154 and 180 ka and arabia ca 40 54 and 163 ka during mis 5 both regions show much evidence for humidity with pdf peaks corresponding to insolation maxima though not all maxima are represented in either the saharan or arabian record this situation can be explained by eccentricity modulated precession when eccentricity is strong insolation is enhanced but also more variable and the desert climate is generally more humid particularly at times of high insolation the opposite happens when eccentricity is low and deserts tend to be more arid but local factors exert more of an influence on climate affecting the timing and strength of the brief humid periods experienced so that they no longer coincide with insolation maxima the spatial distribution of humid sites is compatible with a number of different modern human dispersal theories southern arabia experienced humid periods centred on 54 ka and 125 ka and this could have facilitated dispersal from east africa to southern arabia and beyond via the bab el mandab the sahara shows considerable evidence for humidity during mis 5 and may have had dispersal across its expanse at this time

speleothem u th age constraints for the last glacial conditions in the apuan alps northwestern italy abstract during the quaternary several glaciations occurred in the mountain regions around the mediterranean and in recent years new ages have better constrained their timing however this is not the case for the apuan alps a high rainfall mountain chain adjacent to the mediterranean sea here in spite of the widespread evidence for glaciers the complete lack of geochronological information hinders our understanding of glaciation history in this paper we utilize speleothem ages to better constrain the timing of these glacial features we re examine 293 uranium thorium ages from 19 speleothems collected in five caves at different elevations after a period of very low growth between 160 and 132 ka the analysed speleothems grew almost continuously to 75 ka this period was followed by intermittent growth with lower deposition rate and presence of hiatuses until 12 5 12 ka this is consistent with an ice coverage persisting over the apuan alps inhibiting or interrupting the growth of speleothems via the limited availability of groundwater and the scarcity absence of soils this interval is much greater than the time interval that has previously been attributed to the existence of glaciers on the apuan alps which has been assumed to be restricted to marine isotope stage mis 2 instead ice cover probably also appeared in the apuan alps during mis 4 the phase of restarting of growth which may implies the definitive or substantial glacier melts seem to predate the holocene

holocene and eemian climatic optima in the korean peninsula based on textural and carbon isotopic records from the stalagmite of the daeya cave south korea abstract textural and stable isotopic records from the absolute dated stalagmite of the daeya cave dy 1 provide new insights into the climatic evolution of the korean peninsula during the holocene and eemian climatic optima the stalagmite yielded ages of 8572 227 to 5907 158 and 1 23 456 535 to 1 19 837 1089 years which coincide with the holocene and eemian climatic optima respectively the stalagmite s 13 c record closely resembles previously reported chinese speleothem 18 o data thus it can be suggested that textural and geochemical results of the dy 1 reflect east asian monsoon intensity which is forced by summer insolation patterns in the northern hemisphere lighter carbon isotopic compositions well developed fibrous calcite crystals and their relatively faster growth rate in the stalagmite sample are interpreted to reflect the warmest and wettest climate conditions of the holocene and eemian interglacials both climatic optima took place when insolation was decreasing from its maximum level temperature in greenland was highest and sea level approached its maximum level these climatic optima also coincide with decreasing antarctic temperatures compared the dy 1 data to other proxies it is suggested that the holocene and eemian climatic optima developed through a balance among boreal insolation monsoon intensity and sea level also continental ice volume which are the main climatic forcing factors in the northern hemisphere these trends also follow the bi polar seesaw mechanism as previously described

a high resolution record of holocene climate change in speleothem calcite from cold water cave northeast iowa high precision uranium thorium mass spectrometric chronology and 18o 13c isotopic analysis of speleothem calcite from cold water cave in northeast iowa have been used to chart mid holocene climate change significant shifts in 18o and 13c isotopic values coincide with well documented holocene vegetation changes temperature estimates based on 18o 16o ratios suggest that the climate warmed rapidly by about 3 c at 5900 years before present and then cooled by 4 c at 3600 years before present initiation of a gradual increase in 13c at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present

distribution and isotopic signatures of archaeal lipid biomarkers associated with gas hydrate occurrences on the northern cascadia margin abstract we have investigated the distributions and carbon isotopic compositions of archaeal membrane lipids in gas hydrate bearing sediments collected from the northern cascadia margin offshore from vancouver island sites u1327 and u1328 by the r v joides resolution during iodp expedition 311 archaeal lipid biomarkers including glycerol dialkyl glycerol tetraethers gdgts tend to become abundant below 100 mbsf meters below sea floor tricyclic biphytane bp 3 which is a robust biomarker derived from gdgt crenarchaeol and other bps exhibit 13c values of ca 20 and become abundant between 130 and 230 mbsf at site u1328 in this depth range concentrations of ammonium and phosphate in interstitial waters also increase suggesting that a larger population and higher activity of heterotrophic community consisting of crenarchaeota and other archaea decompose the sedimentary organic matter thereby liberating ammonium and phosphate such crenarchaeotic activity can produce other metabolic products such as molecular hydrogen by fermentation of organic matter during diagenesis furthermore near the organic matter decomposition zone 130 to 230 mbsf a probable methanogen biomarker 13c depleted bp 1 with 13c values as low as 48 8 becomes abundant indicating that methanogens utilize these diagenetic products the molecular and isotopic distributions of archaeal lipid biomarkers indicate that the archaeal community plays an important role in the biogeochemical cycles of deep sea sediments including both methanogenesis and nutrient recycling

late pleistocene variability of arabian sea summer monsoon winds and continental aridity eolian records from the lithogenic component of deep sea sediments the modern indian ocean summer monsoon is driven by differential heating between the asian continent and the indian ocean to the south this differential heating produces a strong pressure gradient which drives southwest monsoon winds during june july and august satellite and meteorological observations aerosol measurements sediment trap studies and mineralogical studies indicate an atmospheric mode of transport for modern lithogenic sediments in the northwest arabian sea analyses of lithogenic grain size and mass accumulation rate mar records from the owen ridge indicate that eolian transport has been the primary mode of transport for the past 370 kyr visual inspection shows that the mar record is positively correlated with global ice volume as indicated by the marine 18o record in contrast the grain size record varies at a much higher frequency showing little correlation to either the mar or the 18o records spectral analyses confirm these relationships indicating that the lithogenic grain size and mar records are coherent only over the precession band whereby the grain size leads the mar by 124 8 kyr we conclude that an eolian transport mechanism is the only mechanism that allows for this phase difference and at the same time is supported by comparison of the grain size and mar with independent eolian records we use lithogenic grain size as a paleoclimatic indicator of summer monsoon wind strength and lithogenic mar as a paleoclimatic indicator of source area aridity these interpretations are supported by comparison of the lithogenic records to independent indicators of wind strength globigerina bulloides upwelling record and aridity a loess record from central china such comparisons indicate high coherence and zero phase relationships our work supports the findings of previous studies which have documented the link between monsoon strength and the earth s axial precession cycles both the lithogenic mar and the grain size records have high coherency with precessional insolation maximum lithogenic mar source area aridity is in phase with 18o global ice volume and leads maximum precessional insolation by 88 6 kyr we attribute this lead to the influence of glacial conditions on the aridity and therefore the deflation potential of the source areas maximum lithogenic grain size summer monsoon wind strength lags maximum precession by 148 9 kyr we attribute this lag both to the influence of global and or local ice volume and to the availability of latent heat from the southern hemisphere indian ocean the two of which combine to determine the strength of the indian ocean monsoon

late quaternary humidity and aridity dynamics in the northeast rub al khali united arab emirates implications for early human dispersal and occupation of eastern arabia the major driver of palaeoclimatic dynamics over the southeast arabian region is the varying latitudinal interface between the westerly driven shamaal winds and a northward branch of the southwesterly indian ocean summer monsoon iom at present this latter system is restricted in extent to the southern coastal regions of yemen and oman monsoonal winds are widely believed to have increased in strength and penetrated much further north across southeast arabia at various intervals during the quaternary resulting in a series of humid phases which early studies suggested focussed within mis 3 and mis 1 recent speleothem studies from oman and uae have indicated humidity within mis 5 which with current debates surrounding early human dispersal and occupation could be significant for affirming the regional potential for early occupation we investigate two new geoproxy suites from al ain and tawi asmar northern uae in the presently hyper arid rub al khali sand sea that preserve evidence of multiple episodes of aeolian deposition and humidity in the form of fluvial gravels sands and silts and dune sands 35 osl ages demonstrate humid episodes occurred in mis 5e 5a and the early holocene with a major dry dune building phase in late mis 3 and the onset of dry conditions again from 6 ka the fluvial sedimentary evidence from mis 5e and 5a suggest that the present hyper arid desert landscape of the northeastern rub al khali was at those times represented by an environment experiencing significant seasonal wadi flow this provides an environmental context that supports recent studies indicating archaeological visibility in arabia dating from mis 5 and the presence of anatomically modern humans in arabia at this time

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

timing and hydrological conditions of sapropel events in the eastern mediterranean as evident from speleothems soreq cave israel this paper explores the connection between the timing of sapropel events in the eastern mediterranean sea and the time of low 18o events in speleothems of the soreq cave israel a new well dated 18o and 13c profile of the soreq cave speleothems for the last 140 kyr reveals five prominent low 18o events dated at 124 to 119 kyr with the main peak at 122 kyr event v 108 to 100 kyr with peaks at 107 and 102 kyr event iv 85 to 79 kyr with peak at 80 kyr event iii 55 to 52 kyr with a peak at 54 kyr event ii and 8 5 to 7 kyr with peaks at 8 5 and 7 kyr event i these events are characteristic of period of enhanced rainfall in the eastern mediterranean area particularly so for events v and i where the marked decrease in 18o and the correspondingly marked increase in 13c indicate very wet conditions the chronology of the low 18o events v iv iii and i match the ages estimated for the formation of sapropels s5 s4 s3 and s1 which are also associated with high hydrological activity thus it is suggested that the low 18o events in the soreq cave speleothems constrain the maximum duration of the sapropel formation pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that sapropels s5 and s1 were deposited during very wet periods when there were major decrease in the sea surface salinity coupled with frost free winters and drought free summers sapropels s4 and s3 were formed during periods of increased precipitation but ones in which the general climate was mediterranean semi arid

warm period growth of travertine during the last interglaciation in southern germany abstract late quaternary travertine at two sites near stuttgart formed entirely during interglacial periods the travertine contains structures from growth induced by bacteria and such structures have been dated by 230th u mass spectrometry the resulting ages from both sites imply growth episodes of short duration with growth rates up to 5 mm yr 1 at 99 800 1300 yr b p 2 n 8 and 105 900 1300 yr b p 2 n 7 these episodes were likely part of marine isotope stage mis 5 3 deposition of silt interrupted travertine growth at one of the sites 105 000 yr b p likely correlatives of this silt are the st germain i b stade recorded in the grand pile peat bog and a cold episode 1000 yr long recorded by 18o values in the grip ice core travertine also formed during stage 5 5 115 000 yr and during the early holocene we found no evidence for travertine accumulation in stages 2 3 4 and 5 1 at both sites the sr u ratio and the initial 234u 238u activity ratio resemble those of modern spring water however the sites differ in the chemical composition of spring water and in stratigraphic sequence of travertine accumulation

glacier advances during the last 400 ka as evidenced in st beatus caves be switzerland abstract while stalagmites within caves are commonly used to obtain information about past climate change little attention has so far been given to the cave its morphology and many other types of deposits contained in it the discontinuous nature of cave sediments and the possibility that caves are re shaped by more recent water circulations has discouraged many researchers despite the fact that it is just this intimate relationship between the cave and surface conditions that makes caves such valuable archives this article presents a method of determining a relative chronology which can then be dated numerically information from st beatus cave and other nearby caves provides evidence of glacial advances that occurred within the following time windows 350 235 180 160 135 114 99 76 54 and 30 16 ka it also describes the following dated periods of valley deepening 805 760 m a s l 350 ka 760 700 m a s l 235 180 ka 700 660 m a s l 160 135 ka and 660 560 m a s l 30 16 ka these dates are new for this part of the alpine arc

temporal variability in the northern arabian sea oxygen minimum zone omz during the last 225 000 years the northern arabian sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen the intensity of this oxygen minimum zone omz has been reconstructed over the past 225 000 years using proxies for surface water productivity water column denitrification winter mixing and the aragonite compensation depth acd changes in omz intensity occurred on orbital and suborbital timescales lowest o2 levels correlate with productivity maxima and shallow winter mixing precession related productivity maxima lag early summer insolation maxima by 6 kyr which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer periods with a weakened or even non existent omz are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds the timing of deep winter mixing events corresponds with that of periods of climatic cooling in the north atlantic region

potential influence of temperature changes in the southern hemisphere on the evolution of the asian summer monsoon during the last glacial period abstract a precisely 230th dated 18o record of a stalagmite from yangkou cave chongqing southwest china is proposed to reconstruct the evolution of the east asian summer monsoon easm during the last glacial period lgp from 37 8 to 48 7 ka bp the 18o record reveals dansgaard oeschger d o 8 12 and heinrich h4 h5 climatic events the inferred d o events at easm territory do not exhibit the asymmetrical feature of rapid warming slow cooling recorded in greenland ice cores the yangkou record is consistent with other records of hulu and xiaobailong caves in the chinese monsoon realm changes in the period from d o 12 to d o 9 and h4 events in the stalagmites is similar to the temperature changes recorded in the european project for ice coring in antarctica epica dronning maud land ice core edml this similarity implies that southern hemisphere thermal condition could affect the easm the location of the intertropical convergence zone itcz and also the hadley circulation changes in d o 12 and the end of the h5 in chinese records are different from those in socotra island located in the indian summer monsoon ism region which are concurrent with ngrip records during the h5 period in the southern hemisphere temperature increased gradually to a maximum and the hadley circulation intensity in the southern hemisphere decreased simultaneously the southeast trade winds in the southern hemisphere gradually weakened resulting in the weakening of the ism and eventually the weakest ism at 47 2 ka bp our records support that the evolution of the summer monsoon in the low latitude asian zones was not only affected by summer insolation and climatic changes in the high latitudes of the northern hemisphere but also by temperature changes in the southern hemisphere

middle and late pleistocene humid periods recorded in palaeolake deposits of the nafud desert saudi arabia present climate in the nafud desert of northern saudi arabia is hyper arid and moisture brought by north westerly winds scarcely reaches the region the existence of abundant palaeolake sediments provides evidence for a considerably wetter climate in the past however the existing chronological framework of these deposits is solely based on radiocarbon dating of questionable reliability due to potential post depositional contamination with younger 14c by using luminescence dating we show that the lake deposits were not formed between 40 and 20 ka as suggested previously but approximately ca 410 ka 320 ka 200 ka 125 ka and 100 ka ago all of these humid phases are in good agreement with those recorded in lake sediments and speleothems from southern arabia surprisingly no holocene lake deposits were identified geological characteristics of the deposits and diatom analysis suggest that a single perennial lake covered the entire south western nafud ca 320 ka ago in contrast lakes of the 200 ka 125 ka and 100 ka humid intervals were smaller and restricted to interdune depressions of a pre existing dune relief the concurrent occurrence of humid phases in the nafud southern arabia and the eastern mediterranean suggests that moisture in northern arabia originated either from the mediterranean due to more frequent frontal depression systems or from stronger indian monsoon circulation respectively however based on previously published climate model simulations and palaecolimate evidence from central arabia and the negev desert we argue that humid climate conditions in the nafud were probably caused by a stronger african monsoon and a distinct change in zonal atmospheric circulation

speleothem evidence from oman for continental pluvial events during interglacial periods growth periods and stable isotope analyses of speleothems from hoti cave in northern oman provide a record of continental pluvial periods extending back over the past four of earth s glacial interglacial cycles rapid speleothem growth occurred during the early to middle holocene 6 10 5 ka b p 78 82 ka b p 120 135 ka b p 180 200 ka b p and 300 325 ka b p the speleothem calcite deposited during each of these episodes is highly depleted in 18 o compared to modern speleothems the d 18 o values for calcite deposited within pluvial periods generally fall in the range of 24 to 28 relative to the vienna peedee belemnite standard whereas modern speleothems range from 21 to 23 the growth and isotopic records indicate that during peak interglacial periods the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record the association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions and not changes in solar radiation are the primary control on continental wetness on glacial interglacial time scales

long term variation of the principal mode of boreal spring hadley circulation linked to sst over the indo pacific warm pool abstractthe variability of the boreal spring march may mam hadley circulation hc is investigated focusing on the long term variation of the first principal mode for 1951 2008 which is an equatorially asymmetric mode am with the rising branch located around 10 s this mode explains about 70 of the variance of the mam hc and shows an obvious upward trend and thus contributes to the strengthening of the mam hc the robust warming trends of sea surface temperature sst over the indo pacific warm pool ipwp play an essential role in the variations of the mam hc when sst over the ipwp is warm anomalous meridional circulation is induced with descending branches located in regions 30 20 s and 5 15 n and rising motion located near 10 s the anomalous rising south of the equator is due to the inhomogeneous warming of sst over the ipwp sst within the ipwp in the southern hemisphere shows a larger warming trend than that in the northern hemisphere the position of the anomalous convergence associate

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

high resolution records of rainfall events from clay bands in tufa individual rainfall events are recorded as thin clay bands in annually laminated freshwater carbonate tufa runoff from rainfall events entrains clay that then adheres to the tufa surface a specimen collected from southwest japan exhibits 100 clay bands formed during a 15 yr period from 1988 to 2002 the chronology of the clay bands was determined by using the temperature dependent cycle of oxygen isotope ratios in the carbonate and the clay content was quantified by electron probe microanalysis si x ray strength the obtained si x ray strength curve and actual rainfall record follow a nearly identical pattern the clay content of each band correlates with rainfall amount rainfall of 50 mm week produced sufficient amounts of suspended cave clay to produce clay bands in the tufa because the clay bands survive postdepositional alteration this new method can be applied to ancient tufa deposits and has great potential as a high resolution rainfall record for use in climate studies

structural diversity and fate of intact polar lipids in marine sediments abstract marine sediments harbor an enormous quantity of microorganisms including a multitude of novel species the habitable zone of the marine sediment column begins at the sediment water interface and probably extends to depths of several thousands of meters studies of the microbial diversity in this ecosystem have mostly relied on molecular biological techniques we used a complementary method analysis of intact polar membrane lipids to characterize the in situ microbial community in sediments covering a wide range of environmental conditions from peru margin equatorial pacific hydrate ridge and juan de fuca ridge bacterial and eukaryotic phospholipids were only detected in surface sediments from the peru margin in contrast deeply buried sediments independent of their geographic location were dominated by archaeal diether and tetraether lipids with various polar head groups and core lipids we compared ring distributions of archaeal tetraether lipids derived from polar glycosidic precursors with those that are present as core lipids the distributions of these related compound pools were distinct suggestive of different archaeal sources i e the polar compounds derive from sedimentary communities and the core lipids are fossil remnants from planktonic communities with possible admixtures of decayed sedimentary archaea this in situ production of distinct archaeal lipid populations potentially affects applications of the tex86 paleotemperature proxy as demonstrated by offsets in reconstructed temperatures between both pools we evaluated how varying cell and lipid stabilities will influence the sedimentary pool by using a box model the results are consistent with i a requirement of continuous inputs of freshly synthesized lipids in subsurface sediments for explaining the observed distribution of intact polar lipids and ii decreasing lipid inputs with increasing burial depth

a 160 000 year record of dune development and atmospheric circulation in southern arabia aeolian deposits in the wahiba sands sultanate of oman reveal patterns of atmospheric circulation over the past 160 000 years luminescence dating indicates a correlation of dune activity with periods of low global sea level and decreased monsoon intensity evidence from dune orientation and sedimentary structures shows exclusively northbound transport of sand during times of high latitude glaciation these results are in contrast to the current paleocirculation model that assumes an increase of northwesterly winds because of a southward shift of the intertropical convergence zone our results indicate that the circulation pattern during glacial times was comparable to that of the present

the 1994 sefidabeh eastern iran earthquakes revisited new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault in 1994 three shallow earthquakes of mw 6 occurred close together on blind thrusts near sefidabeh in eastern iran in an earlier study of the teleseismic waveforms the geomorphology and the faulting in the epicentral region it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system whose activity could be detected by its effect on the surface drainage in this study we present a sar interferogram that precisely determines the location and amount of coseismic surface displacements showing that the earthquakes in the sefidabeh sequence probably occurred on en echelon fault segments associated with three stepping ridges we also present u th dates of 100 ka for lake deposits uplifted by the growing ridge from the cumulative dated uplift and knowledge of the surface displacements due to an earthquake sequence we estimate that 120 such events have occurred in the past 100 ka with an average recurrence interval of 830 yr and an average convergence rate of 1 5 mm yr 1 on the sefidabeh thrust each estimate has an uncertainty of a factor of two either way we argue that the sefidabeh fault originally formed by coalescence of many small fault segments and has grown in length at about 2 cm yr 1 in the past 100 ka though the coseismic surface deformation observed in the sar interferogram closely resembles folding the overall topography does not because of inherited topography associated with earlier geological deformation in spite of this the activity of the buried thrust fault can easily be detected by its effect on the surface drainage a significant lesson when interpreting landscapes that are not entirely due to the present day deformation

late pliocene and pleistocene climatic oscillations and monsoon upwelling recorded in sediments from the owen ridge northwestern arabian sea

biomarker indicators for anaerobic oxidizers of methane in brackish marine sediments with diffusive methane fluxes the anaerobic oxidation of methane aom is a major methane sink in marine sediments and plays a crucial role in mitigating methane fluxes to the overlying water column we investigated biomarker distributions and compound specific isotopic signatures in sediments from sites on the northern european continental margin that are characterized by a diffusive flux of methane at all sites the organic matter om is predominantly derived from terrestrial higher plants with subordinate abundances of algal biomarkers but biomarkers for archaea and bacteria are also present the co occurrence of the archaeal lipids archaeol sn 2 hydroxyarchaeol and 2 6 10 15 19 pentamethylicosane pmi with non isoprenoidal glycerol diethers inferred to derive from sulfate reducing bacteria srb is similar to microbial biomarker assemblages observed at cold seeps the archaeal and inferred srb biomarker concentrations typically reach maxima close to the sulfate methane transition zone smtz where archaeal biomarkers are depleted in 13c the 16s rrna gene sequences from the smtz of the aarhus bay sediment core indicate the occurrence of anme 1 archaea consistent with inferences derived from biomarker distributions the observations suggest that aom in these diffusive settings is mediated by consortia of archaea and bacteria similar to those found at many seep and methane hydrate sites around the world

bacterial populations and processes in sediments containing gas hydrates odp leg 146 cascadia margin abstract bacterial populations and activity were quantified at three sites in the cascadia margin accretionary wedge off the west canadian american coast odp leg 146 at two sites sediments contained gas hydrates site 889 890 had a discrete zone of hydrate approximately 10 m above a bottom simulating reflector bsr at 225 m below sea floor mbsf and site 892 had disseminated hydrate in the upper 20 mbsf and a bsr at 74 mbsf site 888 was a control site without gas hydrates the control site 888 and top approximately 90 mbsf of site 889 890 had bacterial distributions similar to previous pacific ocean sites in the upper approximately 30 m of site 892 however bacterial populations were much lower suggesting inhibition by the high concentrations of h 2 s within the hydrate zone below this depth bacterial populations rose to concentrations consistent with other sites the control site was dominated by so 4 reduction and rates of ch 4 oxidation in the top 90 m were low 0 002 0 033 nmol cm 3 d 1 at site 889 890 bacterial populations and activity were stimulated in the discrete hydrate zone ch 4 oxidation rates increased in the middle of this zone to 134 5 nmol cm 3 d 1 ca 9 times the average rate at other depths resulting in a significant 10 increase in the total bacterial population the anaerobic process es responsible for this oxidation remain unclear despite so 4 reducing bacteria previously associated with ch 4 oxidation also being stimulated in this zone fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these relatively high rates of ch 4 oxidation this first microbiological study of gas hydrates indicates that bacterial processes are influenced by gas and fluid venting and they play a major role in geochemical changes within these deep 200 mbsf sediments

insolation cycles as a major control of equatorial indian ocean primary production analysis of a continuous sedimentary record taken in the maldives indicates that strong primary production fluctuations 70 to 390 grams of carbon per square meter per year have occurred in the equatorial indian ocean during the past 910 000 years the record of primary production is coherent and in phase with the february equatorial insolation whereas it shows diverse phase behavior with 18o depending on the orbital frequency eccentricity obliquity or precession examined these observations imply a direct control of productivity in the equatorial oceanic system by insolation in the equatorial indian ocean productivity is driven by the wind intensity of westerlies which is related to the southern oscillation therefore it is suggested that a precession forcing on the southern oscillation is responsible for the observed paleoproductivity dynamics

multiple environmental and ecological controls on archaeal ether lipid distributions in saline ponds abstract the distribution of archaeal ether lipids is thought to be governed by salinity in hypersaline and marine environments due to differences in their biological sources however little is known about their source and relationship with salinity in lacustrine environments to explore this we analyzed the distribution of specific archaeal lipids isoprenoid gdgts and diethers of 11 ponds along a salinity gradient 0 8 to 279 in inner mongolia ne china functional gene mcra and compound specific stable carbon isotope data were used to explore the biological source s of archaeal diether and tetraether lipids both lipid and dna based methods showed methanogens to be abundant in ponds which is consistent with the dominance of gdgt 0 relative to other isoprenoid gdgts as well as the occurrence of archaeol c20 c20 dgd hydroxyarchaeol and the mcra gene however the occurrence of the extended archaeol c20 c25 dgd also suggests the presence of halobacteriales non halophilic methanogens and halophilic methanogens this means that the biological sources of archaeal diethers and tetraethers are complicated we subsequently evaluated the applicability of tex86 temperature and ace salinity proxies in these settings however due to the mixed sources of archaeal diethers and tetraethers these proxies are not applicable in these ponds and it appears that salinity is not the dominant control on archaeal ether lipid distributions in these saline ponds

holocene variations of monsoon rainfall in rajasthan two reconstructed histories of the monsoon rainfall in rajasthan show that the monsoon was weak or absent in latest glacial time with the advent of holocene climatic patterns fresh water lakes formed in dune fields and the pollen rain preserved in these reservoirs provides a basis for the reconstruction of the monsoon history the two reconstructions separated by only 150 km have some features in common and some striking differences both show maximum monsoon amounts in the early holocene with a roughly two thirds decrease to the present both show salinization in sub boreal time both show long intervals of near complete desiccation in the last four millennia the shorter term variations however are not closely parallel these shorter term variations may be explained in terms of the behavior of the present day interannual variability

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

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

late quaternary chronology of major dune ridge development in the northeast rub al khali united arab emirates the northeastern sector of the rub al khali desert in the eastern united arab emirates uae is dominated by large ne sw trending dune ridges orientated perpendicular to the currently prevailing northwesterly wind regime in this study extensive use has been made of artificially exposed sections through these major dune ridges that reveal internal sedimentary structures and allow an intensive high resolution sampling programme to be carried out here we present the optical dating results for samples from 7 sections the results indicate that dune activity and preservation occurred within the periods 7 3 ka 16 10 ka and 22 20 ka with evidence of earlier preservation during marine oxygen isotope stages mis 3 and 5 with net accumulation rates in the range 2 2 25 m ka 1 in several instances hiatuses in the preserved record of dune accumulation coincide with stratigraphic bounding surfaces visible in the exposed section profiles with associated truncation of internal sedimentary structures caution must be exercised when interpreting such gaps in the recorded accumulation chronologies of these dunes since these may simply constitute phases of low preservation potential rather than phases of low aeolian activity other factors such as sediment supply and availability in relation to sea level dynamics may be significant and are also considered

molecular and stable isotope compositions of natural gas hydrates a revised global dataset and basic interpretations in the context of geological settings abstract a global dataset of molecular and stable isotope compositions of gases released from 209 different specimens of natural gas hydrate is presented and discussed the 26 hydrate bearing areas from 21 geographic regions are grouped into high gas flux hgf settings low gas flux lgf settings and hydrated gas accumulations hga methane ch4 is the most abundant hydrate bound gas while co2 and c2 hydrocarbon gases are frequently present in small amounts non hydrocarbon gases such as h2s are uncommon the stable isotope composition of hydrate bound gases varies significantly e g 13c of ch4 from 74 7 to 39 6 suggesting that gases of both microbial and thermogenic origin form gas hydrates hydrate bound gases are derived from autochthonous located predominantly within the gas hydrate stability zone ghsz and allochthonous located in deep sediments sources the occurrence and concentration of gas hydrates in sediments are controlled not by the origin of gases but rather by their sources which in turn strongly depend on geological setting allochthonous gases microbial and or thermogenic dominate in hgf and hga settings where they are focused in the shallow ghsz along faults within mud volcanoes in permeable carrier beds and other geological features from underlying petroleum systems relatively high concentrations of gas hydrate average 5 15 of pores and locally up to 100 of volume occurring over small areas are typical of hgf and hga settings in contrast autochthonous and diffuse allochthonous gases mostly microbial occur in stratigraphically and structurally simple lgf settings and result in relatively low concentrations of gas hydrate average 2 of pores in the ghsz spread over large areas the major implication of this finding is that successful prediction of resource and geohazard potential of gas hydrates is possible only if regional petroleum systems extending well below the ghsz are properly evaluated in addition to the diagnosis of origins and sources of hydrate bound gases molecular and isotopic data help to better identify hydrate bearing intervals and provide valuable insights into the dynamics of hydrate bearing sites

diphytanyl glycerol ether distributions in sediments of the orca basin abstract archaebacterially produced diphytanyl glycerol ether dpge was examined in core sediments from the orca basin an anoxic hypersaline basin in the northwestern gulf of mexico to observe its spatial variability and potential origin a differential extraction protocol was employed to quantify the isopranyl glycerol ethers associated with unbound intermediate bound and kerogen bound lipid fractions archaebacterial lipids were evident at all depths for the unbound and intermediate bound fractions concentrations of dpge ranged from 0 51 to 2 91 g g dry sediment at the surface and showed secondary maxima deeper in basin sediments intermediate bound dpge concentrations exhibited an inverse relationship to unbound dpge concentrations kerogen bound dpge concentrations were normally below detection limits earlier studies describing the general homogeneity of lipid components within the overlying brine and at the brine seawater interface suggest that the large scale sedimentary dpge variations observed in this study result from spatial and temporal variations in in situ production by methanogenic or extremely halophilic archaebacteria

fossilization and degradation of intact polar lipids in deep subsurface sediments a theoretical approach intact polar membrane lipids ipls are frequently used as markers for living microbial cells in sedimentary environments the assumption with these studies is that ipls are rapidly degraded upon cell lysis and therefore ipls present in sediments are derived from in situ microbial production we used a theoretical approach to assess whether ipls in surface sediments can potentially represent fossilized ipls derived from the upper part of the water column and whether ipls can be preserved during sediment burial previous studies which examined the degradation kinetics of ipls show that phospholipids i e ester linked lipids with a phosphor containing head group degrade more rapidly than glycosidic ether lipids i e ether linked lipids with a glycosidically bound sugar moiety based on these studies we calculate that only a minor fraction of phospholipids but a major fraction of glycosidic ether lipids biosynthesized in the upper part of the water column can potentially reach deep sea surface sediments using a simple model and power law kinetic degradation parameters reported in the literature we also evaluated the degradation of ipls during sediment burial our model predicts a log log relationship between ipl concentrations and depth consistent with what has been observed in studies of ipls in subsurface sediments although our results do not exclude production of ipls in subsurface sediment they do suggest that ipls present in the deep biosphere may contain a substantial fossil component potentially masking in situ ipl production

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

speleothem based paleoclimate record from northern oman u th age dating and stable isotope measurements of speleothems from hoti cave in northern oman yield paleoclimate information from the region extending to 125 ka the results 1 provide further confirmation of an early holocene wet period in southern arabia extending from some time prior to 9 7 ka and ending at 6 2 ka 2 demonstrate a second period of wetness closely coinciding with the last interglacial period marine isotope stage mis 5e 3 indicate that during mis 5e southern arabia was considerably wetter than during the early holocene and 4 demonstrate that periods of increased monsoon wind strength based on data from marine sediments do not always coincide with evidence of greatly increased precipitation even from nearby continental areas

a 20 ka sediment record from the hajar mountain range in n oman and its implication for detecting arid humid periods on the southeastern arabian peninsula abstract playa like sediments from the hajar mountain range in northern oman 22 83 n 59 00 e 1050 m asl document variations of the paleoenvironmental and paleoclimate conditions over the last 20 ka based on high resolution sediment sampling and their osl dating sedimentation rates were calculated and used as a proxy for paleorainfall the results show that the glacial to lateglacial was characterized by arid conditions with a following transitional period of even less rainfall at 10 5 ka sedimentation rates increases abruptly indicating the onset of the early holocene humid period ehhp rainfall reaches its maximum at 9 8 ka ehhp 2 and a decreasing sedimentation rate after 8 ka characterizes the arid period of the middle to late holocene variations of the hydrological regime are associated with the intensity of the boreal summer indian monsoon and its related position of the itcz for the onset of the ehhp a northerly shift of the itcz is postulated thus confirming earlier results from the southern arabian peninsula

palaeoclimatic records of the loess palaeosol sequences of the chinese loess plateau whether during past climate stages or into a progressively warming world changes in precipitation constitute a key component of climatic change quantitative proxies for palaeo precipitation are relatively rare the magnetic properties of the windblown loess units and interbedded palaeosols of the famous chinese loess plateau provide key palaeo precipitation data for this populous monsoon dominated region the loess palaeosol sediments record rainfall totals directly complementing the oxygen isotope records of chinese speleothems these isotopic records predominantly reflect moisture source and hence large scale atmospheric circulation changes the two major asian monsoon systems appear to display antiphase behaviour dominance of the indian summer monsoon system seems associated with minimum precession maximum northern hemisphere summer heating dominance of the east asian summer monsoons with maximum precession at 2 8 ma more intense development of the east asian winter monsoon initiated major increases in dust deposition rates and formation of relatively unweathered loess layers glacial stage loess units then interleaved with interglacial interstadial stage palaeosols throughout the quaternary period decoupling of the loess palaeosol rainfall records from the chinese cave records of moisture source shows that the indian and east asian monsoon winds were continuously driven by precessional forcing while summer monsoonal rainfall was greatly suppressed during cool glacial stages the timing of these east asian climatic transitions the subsequent intensification of northern hemisphere glaciations and the association between monsoon circulation changes and north atlantic temperatures indicates a possibly leading global role for these monsoonal changes via alterations in the poleward distribution of heat and moisture

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

textural elemental and isotopic characteristics of pleistocene phreatic cave deposits jabal madar oman abstract two main types of karst formation are commonly known the surficial meteoric one and the subsurface hypogenic karst which can be related to both carbonic h2co3 and sulfuric h2s acids this paper documents evidence for a third co2 regime related type of karst that is less commonly described petrographic and geochemical properties of exhumed pleistocene phreatic cave deposits from the diapiric jabal madar dome in northern oman are documented and discussed in a process oriented context these calcites form at the interface between two fundamentally different diagenetic and hydrogeological domains the deep seated hydrothermal and the near surficial meteoric vadose one four calcite phases are recognized i acicular ii blocky to stubby elongated iii proto palisade and iv macro columnar calcites the macro columnar calcites forming the last stage of precipitation are conspicuous due to their cyclical red zonation and they form the main geochemical focus of this study fluid inclusion data point to fluid temperatures of between 30 to 50 c monophase liquid inclusions and elevated salinities 1 6 to 7 3 wt nacl equivalent low carbon isotope data 8 to 9 are in agreement with the influx of soil zone co2 whereas decreasing 18o 15 values might point to mixing of saline hydrothermal and 18o depleted meteoric freshwater i e two fluid sources trace element and stable isotope data shift between the different cement phases and vary cyclically across the red zoning in macro columnar calcites with respect to the intra crystal variability these patterns are perhaps best explained in the context of redox potential two interpretations are presented the one favored here suggests that the cyclical red zoning in macro columnar calcites is controlled by pleistocene monsoonal climate patterns

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

hydrologic and climatic implications of stable isotope and minor element analyses of authigenic calcite silts and gastropod shells from a mid pleistocene pluvial lake western desert egypt abstract authigenic calcite silts at wadi midauwara in kharga oasis egypt indicate the prolonged presence of surface water during the marine isotope stage 5e pluvial phase recognized across north africa exposed over an area of 4 25 km 2 these silts record the ponding of water derived from springs along the libyan plateau escarpment and from surface drainage the 18 o values of these lacustrine carbonates 11 3 to 8 0 pdb are too high to reflect equilibrium precipitation with nubian aquifer water or water of an exclusively atlantic origin mg ca and sr ca of the silts have a modest negative covariance with silt 18 o values suggesting that the water may have experienced the shortest residence time in local aquifers when the water 18 o values were highest furthermore intra shell 18 o sr ca and ba ca analyses of the freshwater gastropod melanoides tuberculata are consistent with a perennially fresh water source suggesting that strong evaporative effects expected in a monsoonal climate did not occur or that dry season spring flow was of sufficient magnitude to mute the effects of evaporation the input of a second isotopically heavier water source to aquifers possibly indian ocean monsoonal rain could explain the observed trends in 18 o and minor element ratios

molecular and isotopic analysis of anaerobic methane oxidizing communities in marine sediments convergent lines of molecular carbon isotopic and phylogenetic evidence have previously indicated hinrichs k u hayes j m sylva s p brewer p g delong e f 1999 methane consuming archaebacteria in marine sediments nature 398 802 805 that archaea are involved in the anaerobic oxidation of methane in sediments from the eel river basin offshore northern california now further studies of those same sediments and of sediments from a methane seep in the santa barbara basin have confirmed and extended those results mass spectrometric and chromatographic analyses of an authentic standard of sn 2 hydroxyarchaeol hydroxylated at c 3 in the sn 2 phytanyl moiety have confirmed our previous tentative identification of this compound but shown that the previously examined product was the mono tms rather than di tms derivative further analyses of 13 c depleted lipids appreciably more abundant in samples from the santa barbara basin have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13 c these are additional glycerol ethers and fatty acids the alkyl substituents in the ethers mostly monoethers with some diethers are non isoprenoidal the carbon number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar suggesting strongly that they are produced by the same organisms their structures n alkyl and methyl branched n alkyl require a bacterial rather than archaeal source the non isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic sulfate and nitrate reducing organisms which lie near the base of the rrna based phylogenetic tree based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate it appears likely that the non archaeal 13 c depleted lipids are products of one or more previously unknown sulfate reducing bacteria which grow syntrophically with the methane utilizing archaea their products account for 50 of the fatty acids in the sample from the santa barbara basin at all methane seep sites examined the preservation of aquatic products is apparently enhanced because the methane oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column

a high resolution stalagmite record of the holocene east asian monsoon from mt shennongjia central china high resolution oxygen isotope 18o profiles of six stalagmites from sanbao cave in hubei province central china established with 1413 oxygen isotope data and 65 230th ages provide a continuous history of east asian summer monsoon easm intensity for the period from 13 0 2 thousand years before present ky bp relative to ad 1950 the 18o record includes four distinct stages in the evolution of the easm 1 an abrupt transition 11 5 ky bp into the holocene 2 a period of gradual increase in monsoon intensity 11 5 9 5 ky bp 3 the maximum humid period 9 5 6 5 ky bp and 4 a period of gradual decline in monsoon intensity 6 5 0 2 ky bp comparison of sanbao with regional records of comparable resolution reveals that the timing of the beginning and end of the holocene optimum as defined by the minimum in 18 o was similar in the indian and east asian monsoon systems this supports the idea that shifts in the monsoon tied to shifts in the mean position of the inter tropical convergen

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