evolution of the indian summer monsoon and terrestrial vegetation in the bengal region during the past 18 ka the indian summer monsoon ism is a major global climatic phenomenon long term precipitation proxy records of the ism however are often fragmented and discontinuous impeding an estimation of the magnitude of precipitation variability from the last glacial to the present to improve our understanding of past ism variability we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower ganges brahmaputra meghna catchment and the indo burman ranges over the last 18 000 years 18 ka the records derive from a marine sediment core from the northern bay of bengal nbob and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the bengal region the isotopic composition of terrestrial plant waxes dd and d13c of n alkanes are compared to results from an isotope enabled general atmospheric circulation model isocam for selected time slices pre industrial mid holocene and heinrich stadial 1 comparison of proxy and model results indicate that past changes in the dd of precipitation and plant waxes were mainly driven by the amount effect and strongly influenced by ism rainfall maximum precipitation is detected for the early holocene climatic optimum ehco 10 5 6 ka bp whereas minimum precipitation occurred during the heinrich stadial 1 hs1 16 9 15 4 ka bp the isocam model results support the hypothesis of a constant moisture source i e the nbob throughout the study period relative to the pre industrial period the model reconstructions show 20 more rain during the mid holocene 6 ka bp and 20 less rain during the heinrich stadial 1 hs1 respectively a shift from c4 plant dominated ecosystems during the glacial to subsequent c3 c4 mixed ones during the interglacial took place vegetation changes were predominantly driven by precipitation variability as evidenced by the significant correlation between the dd and d13c alkane records when compared to other records across the ism domain precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ism from the last glacial to the present

formation and effects of alternate bars major river regulation works have often resulted in alternate bar formation experimental work is described which provides more information on this phenomenon alternate bar formation is closely associated with meandering inferring that alternate bar formation is caused by flow induced periodic disturbances of the horizontal velocity profile based on these considerations a criterion has been developed which defines the upper limit of alternate bar formation and this has been satisfactorily compared to other formation criteria and experimental data the lower limit is given by the initiation of bedload transport a simple relation between slope channel width and grain size is then deduced to indicate whether for a given channel alternate bars can develop for any discharge this relation takes into account armoring effects another equation is proposed to predict the scour depth resulting from alternate bar formation such scour often induces costly river maintenance works finally some interesting results on flow resistance hav been found leading to a tentative equation for the intermediate roughness range

isotope analyses of molecular and total organic carbon from miocene sediments carbon isotope compositions of n alkanes pristane and phytane and total organic carbon were measured and compared against isotopic trends of coeval alkadienones from miocene sediments containing very low organic carbon contents compound specific isotope analysis ofn alkanes and isoprenoid lipids in conjunction with abundance distributions of n alkanes reveal the influence of terrestrially derived organic carbon at all sites analyzed in general n alkanes are derived from allochthonous sources with the exception of n c37 from site 516 which appears genetically related to coeval alkadienones further pristane and phytane from pelagic sites 608 and 516 apparently derive from terrestrial sources as well although a marine origin cannot be excluded dtoc values lack a coherent relationship to toc and d 13 c37 2 differential alteration and mixing of diverse isotopic signals most likely contribute to temporal variation and spatial differences in dtoc therefore when working with sediments from oligotrophic settings we do not recommend dtoc as an indicator of phytoplankton d 13 c values copyright 1999 elsevier science ltd

efficient sequestration of terrigenous organic carbon in the new britain trench abstract the fate of terrigenous organic carbon octerr in the ocean remains an enigma for four decades hadal trenches the deepest ocean realm 6 11 km deep were recently proposed to be oc depocenters but whether and how much octerr was sequestrated there remain elusive here we conducted comprehensive analyses for four sediment cores from the new britain trench nbt close to papua new guinea to assess source translocation and burial of oc the bulk and molecular radiocarbon data suggest that the nbt landward slope and axis sediments mainly receive young and biogenic rather than petrogenic oc the three endmember mixing model based on 14c 13c and oc contents reveals that sediments of the nbt axis 8225 m comprise relatively high oc contents 0 66 0 08 of which biogenic octerr accounts for 62 10 the high proportion of biogenic octerr was attributed to the selective translocation of octerr enriched coarse particles and rapid delivery of sediments supported by unique v shape feature of the trench in contrast the sediment oc at the oceanward slope is primarily of a marine origin suggesting that octerr was efficiently trapped in the trench bottom it is estimated that the burial rate is 2 75 0 32 g c m 2 yr 1 for oc and 1 69 0 41 g c m 2 yr 1 for octerr in the nbt given a fact that many trenches are close to the landmasses we propose that the hadal trenches may contribute significantly to the burial of octerr in the ocean

pre aged plant waxes in tropical lake sediments and their influence on the chronology of molecular paleoclimate proxy records sedimentary records of plant wax hydrogen ddwax and carbon d 13 cwax stable isotopes are increasingly applied to infer past climate change compound specific radiocarbon analyses however indicate that long time lags can occur between the synthesis of plant waxes and their subsequent deposition in marginal marine sediments the influence of these time lags on interpretations of plant wax stable isotope records is presently unconstrained and it is unclear whether such time lags also affect lacustrine sediments we present compound specific radiocarbon 14 cwax data for n alkanoic acid plant waxes n c26 to n c32 from 1 a sediment core from lake chichancanab yucatan peninsula mexico 2 soils in the lake chichancanab catchment and 3 surface sediments from three other lakes in southeastern mexico and northern guatemala 14 cwax ages in the surface sediments are consistently older than modern and may be negatively correlated with mean annual precipitation and positively correlated with lake catchment area 14 cwax ages in soils surrounding lake chichancanab increase with soil depth consistent with deep subsoil horizons being the primary source of lacustrine aged plant waxes which are likely delivered to lake sediments through subsurface transport plant waxes in the lake chichancanab core are 350 1200 years older than corresponding ages of bulk sediment deposition determined by 14 c dates on terrestrial plant macrofossils in the core a ddwax time series is in closer agreement with other regional proxy hydroclimate records when a plant wax 14 c age model is applied as opposed to the macrofossil based core chronology inverse modeling of plant wax age distribution parameters suggests that plant waxes in the lake chichancanab sediment core derive predominantly from millennial age soil carbon pools that exhibit relatively little age variance 200 years our findings demonstrate that high temporal resolution climate records inferred from stable isotope measures on plant waxes in lacustrine sediments may suffer from possible chronologic distortions as a consequence of long residence times of plant waxes in soils they also underscore the importance of direct radiocarbon dating of these organic molecules

mapping of c4 plant input from north west africa into north east atlantic sediments abstract mapping the abundance of 13c in leaf wax components in surface sediments recovered from the seafloor off northwest africa 0 35 n reveals a clear pattern of 13c distribution indicating systematic changes in the proportions of terrestrial c3 and c4 plant input at 20 n latitude we find that isotopically enriched products characteristic of c4 plants account for more than 50 of the terrigenous inputs this signal extends westward beneath the path of the dust laden sahara air layer sal high c4 contributions apparently carried by january trade winds also extend far into the gulf of guinea similar distributions are obtained if summed pollen counts for the chenopodiaceae amaranthaceae and the poaceae are used as an independent c4 proxy we conclude that the specificity of the latitudinal distribution of vegetation in north west africa and the pathways of the wind systems trade winds and sal are responsible for the observed isotopic patterns observed in the surface sediments molecular isotopic maps on the marine sedimentary time horizons e g during the last glacial maximum are thus a robust tool for assessing the phytogeographic changes on the tropical and sub tropical continents which have important implications for the changes in climatic and atmospheric conditions

recycling of graphite during himalayan erosion a geological stabilization of carbon in the crust at geological time scales the role of continental erosion in the organic carbon oc cycle is determined by the balance between recent oc burial and petrogenic oc oxidation evaluating its net effect on the concentration of carbon dioxide and dioxygen in the atmosphere requires the fate of petrogenic oc to be assessed here we report a multiscale nanometer to micrometer structural characterization of petrogenic oc in the himalayan system we show that graphitic carbon is preserved and buried in marine sediments while the less graphitized forms are oxidized during fluvial transport radiocarbon dating indicates that 30 to 50 of the carbon initially present in the himalayan rocks is conserved during the erosion cycle graphitization during metamorphism thus stabilizes carbon in the crust over geological time scales

estimating the hydrogen isotopic composition of past precipitation using leaf waxes from western africa abstract the hydrogen isotopic composition of plant leaf wax n alkanes d wax is a novel proxy for estimating d of past precipitation d p however vegetation life form and relative humidity exert secondary effects on d wax preventing quantitative estimates of past d p here we present an approach for removing the effect of vegetation type and relative humidity from d wax and thus for directly estimating past d p we test this approach on modern day late holocene 0 3 ka sediments from a transect of 9 marine cores spanning 21 n 23 s off the western coast of africa we estimate vegetation type c 3 tree versus c 4 grass using 13 c of leaf wax n alkanes and correct d wax for vegetation type with previously derived apparent fractionation factors for each vegetation type late holocene vegetation corrected d wax d vc displays a good fit with modern day d p suggesting that the effects of vegetation type and relative humidity have both been removed and thus that d vc is a good estimate of d p we find that the magnitude of the effect of c 3 tree c 4 grass changes on d wax is small compared to d p changes we go on to estimate d vc for the mid holocene 6 8 ka the last glacial maximum lgm 19 23 ka and heinrich stadial 1 hs1 16 18 5 ka in terms of past hydrological changes our leaf wax based estimates of d p mostly reflect changes in wet season intensity which is complementary to estimates of wet season length based on leaf wax 13 c

higher erosion rates in the himalaya geochemical constraints on riverine fluxes the modern erosion rate of continental scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains these two parameters are very difficult to measure directly in continental scale basins in this paper we examine the chemical composition of the suspended load bedload and dissolved load of the ganga and brahmaputra rivers and compare them with the average composition of himalayan source rocks a mass balance equation of erosion fluxes shows that a si rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock it corresponds to bedload sediment and flood plain deposits which are enriched in quartz by mineral sorting during transport the combined budget of si al and fe in the river system allows us to estimate this si rich flux by this method the total himalayan erosion is estimated to be twice the measured flux of suspended load the comparison between the brahmaputra and the ganga shows that the eastern himalaya has a higher erosion rate 2 9 mm yr than the western himalaya 2 1 mm yr this is likely the result of the higher runoff in the brahmaputra basin the intensity of the monsoon acts as a first order control of the erosion rate in the range

paleohydrological changes during the last deglaciation in northern brazil abstract we here report a reconstruction of hydrological balance variations in northern brazil for the last 20 ka deduced from the d values of aquatic and land plant molecules extracted from the sediment infill of lake caco our reconstructed precipitation lake water isotope ratio and evaporation evapotranspiration isotope effect allows us to obtain an estimate of moisture balance and to a lesser extent precipitation amount and seasonality changes during the end of the last glacial maximum lgm between ca 20 and 17 3 ka high d values and smaller fractionation of leaf waxes indicate an arid to semi arid climate with a long lasting dry season an abrupt change towards much wetter conditions occurred within ca 500 years from 17 3 to 16 8 ka as shown by a 50 decrease in d h ratios and a marked increase in h isotopic fractionation of leaf waxes this abrupt isotopic change coincides with a major transformation from savanna dominated vegetation to humid rain forest around the lake based on pollen data comparisons with other paleo precipitation records from south american sites indicate that lateglacial humid conditions were controlled by intensification of the itcz and or a southward shift of its mean position across our study site our isotope data show only a small rise in aridity during younger dryas event 13 11 5 ka although the holocene was not screened in details d h ratios of terrestrial and aquatic compounds show near constant offsets suggesting stable and relatively humid climate conditions during this period

post glacial climate forcing of surface processes in the ganges brahmaputra river basin and implications for carbon sequestration abstract climate has been proposed to control both the rate of terrestrial silicate weathering and the export rate of associated sediments and terrestrial organic carbon to river dominated margins and thus the rate of sequestration of atmospheric co2 in the coastal ocean over glacial interglacial timescales focused on the ganges brahmaputra rivers this study presents records of post glacial changes in basin scale indian summer monsoon intensity and vegetation composition based on stable hydrogen d and carbon 13 c isotopic compositions of terrestrial plant wax compounds preserved in the channel levee system of the bengal fan it then explores the role of these changes in controlling the provenance and degree of chemical weathering of sediments exported by these rivers and the potential climate feedbacks through organic carbon burial in the bengal fan an observed 40 shift in d and a 3 4 shift in both bulk organic carbon and plant wax 13 c values between the late glacial and mid holocene followed by a return to more intermediate values during the late holocene correlates well with regional post glacial paleoclimate records sediment provenance proxies sr nd isotopic compositions reveal that these changes likely coincided with a subtle focusing of erosion on the southern flank of the himalayan range during periods of greater monsoon strength and enhanced sediment discharge however grain size normalized organic carbon concentrations in the bengal fan remained constant through time despite order of magnitude level changes in catchment scale monsoon precipitation and enhanced chemical weathering recorded as a gradual increase in k si and detrital carbonate content and decrease in h2o si proxies throughout the study period these findings demonstrate a partial decoupling of climate change and silicate weathering during the holocene and that marine organic carbon sequestration rates primary reflect rates of physical erosion and sediment export as modulated by climatic changes together these results reveal the magnitude of climate changes within the ganges brahmaputra basin following deglaciation and a closer coupling of monsoon strength with oc burial than with silicate weathering on millennial timescales

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

molecular and isotopic records of c4 grassland expansion in the late miocene abstract siwalik paleosol and bengal fan sediment samples were analyzed for the abundance and isotopic composition ofn alkanes in order to test for molecular evidence of the expansion of c4 grasslands on the indian subcontinent the carbon isotopic compositions of high molecular weight alkanes in both the ancient soils and sediments record a shift from low 13c values ca 30 to higher values ca 22 prior to 6 ma this shift is similar in magnitude to that recorded by paleosol carbonate and fossil teeth and is consistent with a relatively rapid transition from dominantly c3 vegetation to an ecosystem dominated by c4 plants typical of semi arid grasslands the n alkane values from our paleosol samples indicate that the isotopic change began as early as 9 ma reflecting either a growing contribution of c4 plants to a dominantly c3 biomass or a decrease in water availability to c3 plants molecular and isotopic analyses of other compounds including n alcohols and low molecular weightn alkanes indicate paleosol organic matter contains contributions from a mixture of sources including vascular plants algae and or cyanobacteria and microorganisms a range of inputs is likewise reflected in the isotopic composition of the total organic carbon from these samples in addition then alkanes from two samples show little evidence for pedegenic inputs and we suggest the compounds were derived instead from the paleosol s parent materials we suggest the record of vegetation in ancient terrestrial ecosystems is better reconstructed using isotopic signatures of molecular markers rather than bulk organic carbon this approach provides a means of expanding the spatial and temporal records of c4 plant biomass which will help to resolve possible tectonic climatic or biological controls on the rise of this important component of the terrestrial biosphere

centers of organic carbon burial and oxidation at the land ocean interface abstract continental margin systems collectively receive and store vast amounts of organic carbon oc derived from primary productivity both on land and in the ocean thereby playing a central role in the global carbon cycle the land ocean interface is however extremely heterogeneous in terms of terrigenous input marine primary productivity sediment transport processes and depositional conditions e g such as bottom water oxygen level continental margins are also highly dynamic with processes occurring over a broad range of spatial and temporal scales the rates of oc burial and oxidation are consequently variable over both space and time hindering our ability to derive a global picture of oc cycling at the land ocean interface here we review the processes controlling the fate of organic matter in continental margin sediments with a special emphasis on hot spots and hot moments of oc burial and oxidation we present a compilation of compositional data from a set of illustrative settings including fjords small mountainous river margins large deltaic systems and upwelling areas bulk oc stable isotope and radiocarbon compositions reveal the diversity and complexity characteristic of oc buried in marginal seas this primarily relates to differences in marine and terrestrial inputs the composition of the terrestrial component e g vascular plant oc soil and petrogenic oc inputs and processes modulating the fate of oc within the marine environment e g priming this widely contrasting behavior of oc among these systems illustrates that the reactivity of oc is a product of its chemical composition and regional conditions interpreted in the context of bulk compositional data as well as that obtained on specific molecular markers e g lignin derived phenols the possibility exists to tease apart complex mixtures of terrestrial and marine inputs and to shed light on the role of the myriad of depositional and post depositional processes finally we discuss a set of hot topics that warrant further investigation such as the role of photochemistry fungi halogenation and reactive iron in dictating the fate of oc in the changing coastal ocean

loading and fate of particulate organic carbon from the himalaya to the ganga brahmaputra delta abstract we use the evolution of river sediment characteristics and sedimentary corg from the himalayan range to the delta to study the transport of corg in the ganga brahmaputra system and especially its fate during floodplain transit a detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers we study the relationships between sediment characteristics mineralogy grain size specific area and corg content in order to evaluate the controls on corg loading contributions of c3 and c4 plants are estimated from corg stable isotopic composition 13corg we use the evolution of 13corg values from the himalayan range to the delta in order to study the fate of corg during floodplain transit ganga and brahmaputra sediments define two distinct linear relations with specific area in spite of 4 5 times higher specific area ganga sediments have similar corg content grain size and mineralogy as brahmaputra sediments indicating that specific area does not exert a primary control on corg loading the general correlation between the total corg content and al si ratio indicates that corg loading is mainly related to 1 segregation of organic particles under hydrodynamic forces in the river and 2 the ability of mineral particles to form organo mineral aggregates bed and suspended sediments have distinct 13corg values in bed sediments 13corg values are compatible with a dominant proportion of fossil corg derived from himalayan rocks erosion suspended sediments from himalayan tributaries at the outflow of the range have low 13corg values 24 8 average indicating a dominant proportion of c3 plant inputs in the brahmaputra basin 13corg values of suspended sediments are constant along the river course in the plain on the contrary suspended sediments of the ganga in bangladesh have higher 13corg values 22 4 to 20 0 consistent with a significant contribution of c4 plant derived from the floodplain our data indicate that during the plain transit more than 50 of the recent biogenic corg coming from the himalaya is oxidised and replaced by floodplain corg this renewal process likely occurs during successive deposition erosion cycles and river course avulsions in the plain

the potential of leaf wax biomarkers from fluvial soil sediment sequences for paleovegetation reconstructions upper alazani river central southern greater caucasus georgia abstract leaf wax derived long chain n alkanes are increasingly used for paleovegetation reconstructions but have hardly been applied to fluvial sediments so far however such analyses could significantly enhance knowledge of the former vegetation in many regions devoid of non fluvial sediment archives therefore this exemplary study investigates potentials and challenges of n alkane biomarker analyses in a fluvial sediment paleosol sequence fsps along the upper alazani river in the central southern greater caucasus generally fsps contain two different biomarker signals a catchment signal in fluvial sediments and a local in situ signal in intercalated paleosols this allows paleovegetation reconstructions for both catchment area and sampling site however several archive related challenges exist concerning the catchment signal from fluvial sediments i this could be biased by different vegetation zones land use histories in subcatchments ii long term pedogenesis on the slopes or intermediate sediment storage in the catchment could have caused a time lag between biomarker formation and sedimentation iii taphonomic processes could have biased the preserved n alkane pattern iv there could be petrogenically inherited or in river produced n alkanes v the signal could be overprinted by post sedimentary processes concerning the local paleosol signal this could be biased by inherited catchment derived biomarkers or by taphonomic processes these factors are thoroughly discussed for the investigated fsps with respect to paleovegetation patterns our investigations gave the following results i leaf waxes in fluvial sediments deposited prior to around the pleistocene holocene transition and between ca 5 and 1 7 cal ka bp are indicative of more deciduous trees shrubs in the catchment compared with the other periods that were dominated by grasses herbs considering a possible time lag of ca 1 4 ka between biomarker formation and deposition this indicates delayed holocene reforestation in the catchment compared with western and central europe in agreement with regional pollen studies the subsequent decline of deciduous trees shrubs could be linked to human activity in the catchment during the late holocene ii leaf waxes in floodplain paleosols indicate local grass herb dominance throughout the holocene which is possibly caused by long lasting local human activity

c4 plants decline in the himalayan basin since the last glacial maximum abstract the bengal fan turbiditic system supplied by the ganga brahmaputra river system provides an integrated record of himalayan system erosion thanks to minor inputs of marine organic matter and almost complete preservation of riverine organic carbon organic matter buried in bengal fan sediments is a proxy of himalayan basin paleo vegetation the active channel levee system of the middle fan documents the last 19 ka and allows the reconstruction of vegetation change in the himalayan basin since the last glacial maximum lgm we measured 13 c of organic carbon c org in order to track changes in the proportions of c3 and c4 plants in the himalayan basin from lgm to mid holocene 13 c of bulk c org shifts of 3 4 towards more negative values relative abundance of individual n alkanes reveals that terrestrial higher plant inputs represent a dominant fraction of c org buried in bengal fan sediments 13 c of higher plants biomarkers mimic that of bulk c org showing that the later can be used as continental paleo vegetation proxy 13 c negative shift from lgm to mid holocene mostly indicates the transition from a dominant input of c4 plants to a dominant input of c3 plants and therefore reveals that c4 plants were more abundant in the basin under glacial conditions vegetation repartition in the basin simulated using the caraib dynamic vegetation model is consistent with c org data the model indicates a dominance of c4 plants in the gangetic plain during the lgm while eastern part of the basin remains dominated by c3 plants the comparison between our data and proxies of regional paleo climate suggests that the large decline of c4 plants after the lgm was due to combined increase of atmospheric co 2 and humidity levels integrated record of himalayan basin paleo vegetation suggests more arid conditions during the lgm than during the mid holocene and agrees with reconstructions of the monsoon indicating stronger sw monsoon during interglacial and stronger ne monsoon during glacial periods

sources transport and deposition of terrestrial organic material a case study from southwestern africa abstract southwestern africa s coastal marine mudbelt a prominent holocene sediment package provides a valuable archive for reconstructing terrestrial palaeoclimates on the adjacent continent while the origin of terrestrial inorganic material has been intensively studied the sources of terrigenous organic material deposited in the mudbelt are yet unclear in this study plant wax derived n alkanes and their compound specific 13c in soils flood deposits and suspension loads from regional fluvial systems and marine sediments are analysed to characterize the origin of terrestrial organic material in the southwest african mudbelt soils from different biomes in the catchments of the orange river and small west coast rivers show on average distinct n alkane distributions and compound specific 13c values reflecting biome specific vegetation types most notably the winter rainfall associated fynbos biome of the southwestern cape in the fluvial sediment samples from the orange river changes in the n alkane distributions and compound specific 13c compositions reveal an overprint by local vegetation along the river s course the smaller west coast rivers show distinct signals reflecting their small catchment areas and particular vegetation communities marine surface sediments spanning a transect from the northern mudbelt 29 s to st helena bay 33 s reveal subtle but spatially coherent changes in n alkane distributions and compound specific 13c indicating the influence of orange river sediments in the northern mudbelt the increasing importance of terrigenous input from the adjacent western coastal biomes in the central mudbelt and contributions from the fynbos biome to the southern mudbelt these findings indicate the different sources of terrestrial organic material deposited in the mudbelt and highlight the potential the mudbelt has to preserve evidence of environmental change from the adjacent continent

temperature and leaf wax δ2h records demonstrate seasonal and regional controls on asian monsoon proxies orbital scale precipitation isotope records can elucidate climate forcing mechanisms and provide benchmarks for climate model validation the ability to differentiate the influence of temperature seasonality and vapor transport history on precipitation isotope proxies is critical to both objectives we present a 300 k y leaf wax hydrogen isotope record from the south china sea with the effects of local condensation temperature removed 2 h wax t 2 h wax t reflects annually integrated precipitation 2 h in the pearl river catchment of southeast china depleted 2 h wax t lags minimum precession p min by 1 0 0 7 k y reflecting the influence of maximum summer insolation and minimum winter insolation with a minor influence of global ice volume which lags p min by 3 3 0 4 k y in contrast annually integrated cave 18 o minima in central china 1000 km north of our site lag p min by 2 7 0 3 k y in phase with ice volume minima this phase indicates that precipitation 18 o in central china is more strongly influenced by ice volume forcing at the precession band with a lesser influence of northern hemisphere insolation our new 2 h wax t data demonstrate that precipitation isotopes in asia have strong regional variability interpreting water isotope records within the context of regionally varying temperature seasonality and sensitivity to changing glacial boundary conditions is imperative to understanding asian hydroclimatic change

large scale hydrological change drove the late miocene c4 plant expansion in the himalayan foreland and arabian peninsula carbon isotope changes in paleosols from siwalik pakistan and marine sediments from the bengal fan indicate a major c 4 plant expansion in the himalayan foreland during the late miocene however the timing and mechanisms behind the c 4 plant expansion remain enigmatic here we present high resolution 60 k y biomarker and compound specific isotope data spanning the past 11 m y from ocean drilling program site 722 in the arabian sea an 5 6 increase in leaf wax 13 c values indicates a marked rise of c 4 plants from 10 to 5 5 ma with accelerated expansion from 7 9 to 5 5 ma a concurrent 50 rise in leaf wax d values is attributed to a combined effect of changes in precipitation amount and evaporation indicating that source regions for the plant waxes became progressively drier from 10 to 5 5 ma in contrast to earlier reports our isotope records biomarker abundances alkenone u k 37 and globigerina bulloides abundance data do not suggest enhanced summer monsoon circulation during this time interval rather our results suggest that large scale hydrological changes drove the late miocene expansion of c 4 plants in the himalayan foreland and arabian peninsula

can sedimentary leaf waxes record d h ratios of continental precipitation field model and experimental assessments abstract d h ratios of leaf waxes d wax derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology ideally d wax can be used to reconstruct precipitation d h ratios d p which is a well established paleoclimate proxy however many other factors such as vegetation and relative humidity rh also affect d wax variation how the combination of these factors affects sedimentary d wax is unclear here we use a transect of 32 lake surface sediments across large gradients of precipitation relative humidity and vegetation composition in the southwestern united states to study the natural factors affecting sedimentary d wax d values of c 28 n alkanoic acids show significant correlation with d p values r 2 0 76 with an apparent isotopic enrichment of 99 8 indicating that sedimentary d wax values track overall d p variation along the entire transect leaf waxes produced by plants grown under controlled conditions rh 80 60 40 show a small increase in d h ratios as rh decreases consistent with prediction from the craig gordon model however the isotopic effect of rh on d wax along the natural transect is partially countered by the opposing influence of vegetation changes the correlation between d wax and d p values is significantly higher r 2 0 84 in the drier portions of the transect than in the wetter regions r 2 0 64 this study suggests that d h ratios of sedimentary leaf waxes can be used as a proxy for precipitation d variations with particularly high fidelity in dry regions although more studies in other regions will be important to further test this proxy

experimental constraints on bar migration through bends implications for meander wavelength selection the bed of an initially straight channel often deforms into a series of migrating alternate bars that can cause bank erosion and the development of periodic planform curvature which is often expressed as a meandering channel trace the planform curvature in turn leads to topography similar to alternate bars but stationary with respect to the meander the migration of alternate bars over this steady topography may lead to interactions influencing meander initiation and wavelength selection we studied these interactions in flume channels with bends of various lengths and angles and having various width to depth ratios in channels whose wavelength was equal to the length of bars in straight channels nearly eight channel widths bars migrated through sine generated channels with angles up to 10 the critical bend angle was smaller for channels of longer wavelength and smaller width to depth ratio migration was nonuniform and bars temporarily stalled when in phase with the curvature induced topography leading to amplification of the topography flow against an opposing bank can lead to such a strong boundary shear stress divergence that migration of the bars is prevented because sediment that otherwise would have led to bar migration is scoured from the bar front we propose that the amplification of bar pool topography in bends where free alternate bars are stalled enhances scour of the bank which leads to selection of a meander wavelength equal to that of the alternate bars

controls on the age of vascular plant biomarkers in black sea sediments abstract transfer of organic carbon oc from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport before transport however terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils using compound specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated here we show compound specific radiocarbon 14c ages of terrigenous biomarkers and bulk 14c ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the black sea 14c ages of long chain n alkanes long chain n fatty acids and total organic carbon toc are highest in front of the river mouths correlating well with bit branched and isoprenoid tetraether indices which indicates contribution of pre aged soil derived terrigenous organic matter the radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter average terrestrial residence times of vascular plant biomarkers deduced from n c29 31 alkanes and n c28 30 fatty acids ages from stations directly in front of the river mouths range from 900 70 years to 4400 170 years these average residence times correlate with size and topography in climatically similar catchments whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the black sea catchment along transect data imply petrogenic contribution of n c29 31 alkanes and input via different terrigenous biomarker transport modes i e riverine and aeolian resulting in aged biomarkers at offshore core locations because n c29 31 alkanes show contributions from petrogenic sources n c28 30 fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers moreover sedimentary n c28 and n c30 fatty acids appear clearly much less influenced by autochthonous sources than n c24 and n c26 fatty acids as indicated by increasing radiocarbon ages with increasing chain length and are thus more representative as vascular plant biomarkers

fluxes and sources of particulate organic carbon in the ganga brahmaputra river system 1 sources and fluxes of particulate organic carbon oc in the ganga brahmaputra river system were estimated from 13c 12c ratios of bedrocks soils bank and suspended river sediments from a himalayan watershed narayani nepal and the bangladesh floodplain in the watershed of the narayani oc 13c values for bank and suspended sediments have similar values and narrow ranges with a mean of about 24 2 at the base of the himalayas on the bangladesh floodplain bank and suspended sediments are enriched in 13c 13c about 22 5 relative to those collected at the base of the himalayas the oc exported at the base of the himalayan range is estimated to include about 15 from the 13c enriched c4 biomass while the c4 contribution is about 25 of the oc exported by the ganga and brahmaputra a calculated total oc flux of 0 65 1012 molc yr 1 is exported to the ocean or trapped in the plain in sediments of the ganga and brahmaputra the clay size fraction is depleted in 13c 2 6 relative to the bulk sediment possible explanations are that either the organic matter associated with clays is largely inherited from the himalayan watersheds or and considered more likely the clays and coarser sediments sample different oc pools possibly with different ages the growth of c4 crops has dramatically increased since the mid twentieth century on the floodplain the oc budget of the actual ganga brahmaputra system is broadly comparable to that derived from the quaternary sediments of the bengal fan that represents about 10 of the global oc contribution to the continental margins

origin transport and deposition of leaf wax biomarkers in the amazon basin and the adjacent atlantic paleoenvironmental studies based on terrigenous biomarker proxies from sediment cores collected close to the mouth of large river systems rely on a proper understanding of the processes controlling origin transport and deposition of biomarkers here we contribute to the understanding of these processes by analyzing long chain n alkanes from the amazon river system we use the dd composition of long chain n alkanes from river bed sediments from the amazon river and its major tributaries as well as marine core top samples collected off northeastern south america as tracers for different source areas the d13c composition of the same compounds is used to differentiate between long chain n alkanes from modern forest vegetation and petrogenic organic matter our d13c results show depleted d13c values 33 to 36 in most samples indicating a modern forest source for most of the samples enriched values 31 to 33 are only found in a few samples poor in organic carbon indicating minor contributions from a fossil petrogenic source long chain n alkane dd analyses show more depleted values for the western tributaries the madeira and solimoes rivers 152 to 168 while n alkanes from the lowland tributaries the negro xingu and tocantins rivers 142 to 154 yield more enriched values the n alkane dd values thus reflect the mean annual isotopic composition of precipitation which is most deuterium depleted in the western amazon basin and more enriched in the eastern sector of the basin samples from the amazon estuary show a mixed long chain n alkane dd signal from both eastern lowland and western tributaries marine core top samples underlying the amazon freshwater plume yield dd values similar to those from the amazon estuary while core top samples from outside the plume showed more enriched values although the variability in the river bed data precludes quantitative assessment of relative contributions our results indicate that long chain n alkanes from the amazon estuary and plume represent an integrated signal of different regions of the onshore basin our results also imply that n alkanes are not extensively remineralized during transport and that the signal at the amazon estuary and plume includes refractory compounds derived from the western sector of the basin these findings will aid in the interpretation of plant wax based records of marine sediment cores collected from the adjacent ocean

multimolecular tracers of terrestrial carbon transfer across the pan arctic 14c characteristics of sedimentary carbon components and their environmental controls distinguishing the sources ages and fate of various terrestrial organic carbon oc pools mobilized from heterogeneous arctic landscapes is key to assessing climatic impacts on the fluvial release of carbon from permafrost through molecular 14c measurements including novel analyses of suberin and or cutin derived diacids das and hydroxy fatty acids fas we compared the radiocarbon characteristics of a comprehensive suite of terrestrial markers including plant wax lipids cutin suberin lignin and hydroxy phenols in the sedimentary particles from nine major arctic and subarctic rivers in order to establish a benchmark assessment of the mobilization patterns of terrestrial oc pools across the pan arctic terrestrial lipids including suberin derived longer chain das c24 26 28 plant wax fas c24 26 28 and n alkanes c27 29 31 incorporated significant inputs of aged carbon presumably from deeper soil horizons mobilization and translocation of these old terrestrial carbon components was dependent on nonlinear processes associated with permafrost distributions by contrast shorter chain c16 18 das and lignin phenols as well as hydroxy phenols in rivers outside eastern eurasian arctic were much more enriched in 14c suggesting incorporation of relatively young carbon supplied by runoff processes from recent vegetation debris and surface layers furthermore the radiocarbon content of terrestrial markers is heavily influenced by specific oc sources and degradation status overall multitracer molecular 14c analysis sheds new light on the mobilization of terrestrial oc from arctic watersheds our findings of distinct ages for various terrestrial carbon components may aid in elucidating fate of different terrestrial oc pools in the face of increasing arctic permafrost thaw

the provenance of vegetation and environmental signatures encoded in vascular plant biomarkers carried by the ganges brahmaputra rivers organic matter carried by rivers and deposited in continental and marine sediments contains valuable information on past environmental conditions and their impact on the terrestrial biosphere in order to use sedimentary records to reconstruct past environmental conditions on the continents such as vegetation cover and type or precipitation intensity we need to understand the provenance of the organic signatures and how they are transferred and potentially modified by fluvial transport to the sedimentary reservoir in particular we need to understand how environmental conditions are imprinted in the composition of riverine particulate organic carbon here we investigate the stable carbon and hydrogen isotopic signatures of vascular plant leaf wax biomarkers in the modern day ganges and brahmaputra river complex one of the largest fluvial systems on earth the distribution of n alkanes and n alkanoic acids in ganges brahmaputra river sediments suggests that vascular plant inputs are consistently a significant component of the organic carbon pool molecular 13c measurements reveal that c3 vegetation inputs delivered by the himalayan rivers are partly degraded and replaced by mixed c3 c4 vegetation input in the floodplain the hydrogen isotopic composition of long chain c24 n alkanoic acids in combination with their stable c isotopic composition provides constraints on the isotopic composition of the environmental water used by higher plants within the drainage basin calculated compositions compare well with the depleted d isotopic ratios of the river water during the summer monsoon suggesting that h isotopic compositions of long chain alkanoic acids are a valuable proxy for the composition of summer monsoon moisture in the ganges brahmaputra basin detailed studies of the isotopic anatomy of modern river systems are critical for informed interpretation of marine sedimentary records in river influenced continental margins

sustained wood burial in the bengal fan over the last 19 my the ganges brahmaputra g b river system transports over a billion tons of sediment every year from the himalayan mountains to the bay of bengal and has built the world s largest active sedimentary deposit the bengal fan high sedimentation rates drive exceptional organic matter preservation that represents a long term sink for atmospheric co2 while much attention has been paid to organic rich fine sediments coarse sediments have generally been overlooked as a locus of organic carbon oc burial however international ocean discovery program expedition 354 recently discovered abundant woody debris millimeter to centimeter sized fragments preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the bengal fan 8 n 3 700 m water depth with recovery spanning 19 my analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically in the last 5 my export included c4 plants implying that coarse woody lowland export continued after c4 grassland expansion albeit in reduced amounts substantial export of coarse woody debris in the last 1 my included one wood rich deposit 0 05 ma that encompassed coniferous wood transported from the headwaters in coarse layers we found on average 0 16 weight oc which is half the typical biospheric oc content of sediments exported by the modern g b rivers wood burial estimates are hampered by poor drilling recovery of sands however high magnitude low frequency wood export events are shown to be a key mechanism for c burial in turbidites

temporal variability in composition and fluxes of yellow river particulate organic matter this study examines temporal variations of the abundance and carbon isotopic characteristics of particulate organic carbon poc and specific source compounds in the context of hydrological variability in the yellow river the content and bulk carbon isotopic characteristics 13c and 14c of poc were relatively uniform over the hydrologic seasonal cycle we attribute these temporally invariant geochemical characteristics to the dominant contribution of loess material to the suspended particulate matter spm in contrast molecular level signals revealed that hydrologic conditions exert a significant influence on the proportional contributions of petrogenic and especially fresh plant derived oc while pre aged soil oc is mobilized via deeper erosion processes e g gully erosion mudslides and is independent of hydrodynamics and surface runoff a coupled biomarker isotope mixing model was applied to estimate the time varying supply of contemporary modern biomass pre aged soil and fossil oc components to chinese marginal seas from the yellow river we found that natural e g precipitation and human induced e g water and sediment regulation variations in hydrological regime strongly influence the flux with the magnitude of the corresponding annual fluxes of poc ranging between 0 343 0 122 mt yr 1 and 0 581 0 213 mt yr 1 but less strongly infleunce proportions of the different oc constituents inter annual differences in pre aged soil and fossil oc fluxes imply that extreme climate events e g floods modulate the exhumation and export of old carbon to the ocean but the oc homogeneity in the pre aged mineral soil dominated watersheds facilitates robust predictions in terms of oc transport dynamics in the past sediment cores and in the future

postglacial climate reconstruction based on compound specific d h ratios of fatty acids from blood pond new england we determined hydrogen isotope ratios of individual fatty acids in a sediment core from blood pond massachusetts usa in order to reconstruct climate changes during the past 15 kyr in addition to palmitic acid c16n acid which has been shown to record lake water d h ratios our surface sediments and down core data indicate that behenic acid c22n acid produced mainly by aquatic macrophytes is also effective for capturing past environmental change calibration using surface sediments from two transects across eastern north america indicates that behenic acid records d variation of lake water down core variations in d values of behenic acid and pollen taxa are consistent with the known climate change history of new england by evaluating the hypothesis that d h fractionations of long chain even numbered fatty acids c24 c32n acids relative to lake water provide independent estimates of relative humidity during the growing season we find that differences between lake level records and isotopically inferred humidity estimates may provide useful insight into seasonal aspects of the hydrologic cycle combined analyses of d h of short and long chain fatty acids from lake sediment cores thus allow reconstructions of both past temperature and growing season relative humidity comparison of d records from two lakes in new england provides critical information on regional climate variation and abrupt climate change such as the 8 2 ka event

leaf wax biomarkers in transit record river catchment composition rivers carry organic molecules derived from terrestrial vegetation to sedimentary deposits in lakes and oceans storing information about past climate and erosion as well as representing a component of the carbon cycle it is anticipated that sourcing of organic matter may not be uniform across catchments with substantial environmental variability in topography vegetation zones and climate here we analyze plant leaf wax biomarkers in transit in the madre de dios river peru which drains a forested catchment across 4 5 km of elevation from the tropical montane forests of the andes down into the rainforests of amazonia we find that the hydrogen isotopic composition of leaf wax molecules specifically the c28 n alkanoic acid carried by this tropical mountain river largely records the elevation gradient defined by the isotopic composition of precipitation and this supports the general interpretation of these biomarkers as proxy recorders of catchment conditions however we also find that leaf wax isotopic composition varies with river flow regime over storm and seasonal timescales which could in some cases be quantitatively significant relative to changes in the isotopic composition of precipitation in the past our results inform on the sourcing and transport of material by a major tributary of the amazon river and contribute to the spatial interpretation of sedimentary records of past climate using the leaf wax proxy

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

a new look at old carbon in active margin sediments recent studies suggest that as much as half of the organic carbon oc undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering these estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils freshwater sediments and river deltas and thus only minimally contributes to the extraneously old 14c ages of total organic matter often observed on adjacent shelves here we test this paradigm by measuring the 14c and 13c values of individual higher plant wax fatty acids as well as the 13c values of extractable alkanes isolated from the eel river margin california the isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition a coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated with pre aged terrestrial material instead composing a considerable proportion of all organic matter if these findings are characteristic of other continental margins proximal to small mountainous rivers then the importance of petrogenic oc burial in marine sediments may need to be reevaluated 2009 the geological society of america

sources and abundances of leaf waxes in aerosols in central europe abstract atmospheric transport is an understudied mechanism for leaf wax hydrogen isotope applications that contributes to mobilizing and depositing these compounds on the surface of the earth while previous efforts have identified the importance of atmospheric leaf wax deposition in remote marine locations the processes are not well constrained on land in temperate latitudes where lakes are common and sedimentary leaf wax hydrogen isotope values are an attractive tool for understanding past precipitation changes this work presents results from a field study that was conducted in 2010 and 2011 at hainich national park germany in order to evaluate the quantity and sources of leaf waxes in the atmosphere aerosols were sampled at approximately weekly intervals inside the forest canopy and n alkane distributions and hydrogen isotope values were compared with those from major tree species surrounding the sampling site despite sampling in what was expected to be a major production center the distribution and hydrogen isotope values of atmospheric n alkanes bore little resemblance to those of the local vegetation comparison with local meteorological data and to 10 day and 36 h back air mass trajectories indicated shifting effects of winds and temperature and that mesoscale transport processes were more important than long range mechanisms back trajectories also highlighted source effects with easterly winds coinciding with relatively lower leaf wax hydrogen isotope values from more continental regions these results suggest that leaf wax aerosols average over spatial scales that exceed typical surface catchment areas for small lake systems even in forested areas yet that the area over which these compounds are derived is still relatively regional depositional fluxes were also estimated in order to assess the potential importance of atmospheric transport to sedimentary archives although difficult to constrain these estimates suggest that atmospheric deposition may be non negligible for lake systems in cases where inputs from rivers or surface runoff are limited together these observations provide new insights on how leaf waxes from different sources are integrated during aeolian transport and the spatial scales over which these processes occur

mean flow and turbulence around a laboratory spur dike the three dimensional turbulent flow field around a spur dike in a plane fixed bed laboratory open channel was studied experimentally using a microacoustic doppler velocimeter mean and turbulence characteristics in all three spatial directions were evaluated at upstream and downstream cross sections near the dike results showed that the primary flow separated in both lateral and vertical directions two counter rotating flow circulations consisting of the lateral and vertical velocity components originated at the dike section downstream of the dike the circulation in the flow separation zone is stronger than the one in the contracted primary flow zone the maximum bed shear stresses estimated using reynolds stresses is about three times as large as the mean bed shear stress of incoming flow

δ13c of organic carbon in the bengal fan source evolution and transport of c3 and c4 plant carbon to marine sediments carbon isotopic measurements on organic carbon oc in sediment cores from the bengal fan odp leg 116 show a dramatic 10 increase beginning ca 7 ma ago and a rapid decrease after 0 9 ma these shifts reflect changes in the mixing ratio of terrigenous carbon derived from c3 and c4 plants the rapid increase in 13c of bengal fan oc at 7 ma shows that late miocene expansion of c4 plants already documented in the siwaliks was widespread over all the himalayan foreland after 7 ma relations of 13c with sedimentological parameters show that c4 plants dominate in the foreland whereas c3 plants remain abundant in the mountain range variations in the source of the sediments and of the oc appear to be sensitive to climate hydrologic conditions in the basin major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass

terrestrial vegetation change inferred from n alkane σ13c analysis in the marine environment abstract we report gas chromatography isotope ratio monitoring mass spectrometry gc irm ms measurements of the 13c values of individual biomarker compounds n alkanes extracted from a 3 m marine sediment core taken near the mouth of the johnstone river north queensland australia the technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine derived carbon the results indicate that there has been a 2 increase in the 13c values of terrestrially derived n alkanes c29 c33 since clearing of the forested johnstone river drainage basin for sugarcane and pasture began in the late 19th century a much slower 1 increase in 13c values after 6 000 years bp and prior to european settlement may be related to a decrease in rainfall in the basin or to an increase in the abundance of c4 plants as a result of increased aboriginal burning the results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin and demonstrate that the 13c values of terrestrially derived n alkanes in the marine environment can be used to assess basin wide vegetation changes in adjacent river catchments on geological timescales

hydrologic controls on seasonal and inter annual variability of congo river particulate organic matter source and reservoir age we present dissolved organic carbon doc concentrations particulate organic matter pom composition 13c 15n 14c n c and particulate glycerol dialkyl glycerol tetraether gdgt distributions from a 34 month time series near the mouth of the congo river an end member mixing model using 13c and n c indicates that exported pom is consistently dominated by c3 rainforest soil sources with increasing contribution from c3 vegetation and decreasing contribution from phytoplankton at high discharge large c4 inputs are never observed despite covering 13 of the catchment low and variable 14c values during 2011 annual mean 148 82 when discharge from left bank tributaries located in the southern hemisphere reached record lows likely reflect a bias toward pre aged pom derived from the cuvette congolaise swamp forest in contrast 14c values were stable near 50 between january and june 2013 when left bank discharge was highest we suggest that headwater pom is replaced and or diluted by c3 vegetation and pre aged soils during transit through the cuvette congolaise whereas left bank tributaries export significantly less pre aged material gdgt distributions provide further evidence for seasonal and inter annual variability in soil provenance the cyclization of branched tetraethers and the gdgt 0 to crenarchaeol ratio are positively correlated with discharge r 0 70 p value 4 3 10 5 due to the incorporation of swamp forest soils when discharge from right bank tributaries located in the northern hemisphere is high both metrics reach record lows during 2013 supporting our interpretation of increased left bank contribution at this time we conclude that hydrologic variability is a major control of pom provenance in the congo river basin and that tropical wetlands can be a significant pom source despite their small geographic coverage

large and rapid climate variability during the messinian salinity crisis evidence from deuterium concentrations of individual biomarkers during the messinian 6 m y ago massive sea level fall and widespread deposition of evaporites occurred in the mediterranean sea when it became isolated from the world oceans here we present the first hydrogen isotope data from individual sedimentary biomarkers n alkanes and isoprenoids that tracked climatically driven hydrographic changes in response to extreme evaporation during the messinian salinity crisis the stable hydrogen and carbon isotope compositions of these biomarkers show a range of 160 in d values and 14 in 13 c values and roughly covary this indicates that the source waters of the biomarkers were therefore in some cases extremely enriched in deuterium having average d as great as 66 vsmow vienna standard mean ocean water such values are only known from desert climates today because the offset between the d values of n alkanes and isoprenoids preserved in the miocene sedimentary rocks is similar to the offset found in modern biological samples we conclude that diagenesis did not significantly affect the primary deuterium concentrations

climatic and geomorphic drivers of plant organic matter transport in the arun river e nepal fixation of atmospheric co2 in terrestrial vegetation and subsequent export and deposition of terrestrial plant organic matter in marine sediments is an important component of the global carbon cycle yet it is difficult to quantify this is partly due to the lack of understanding of relevant processes and mechanisms responsible for organic matter transport throughout a landscape here we present a new approach to identify terrestrial plant organic matter source areas quantify contributions and ascertain the role of ecologic climatic and geomorphic controls on plant wax export in the arun river catchment spanning the world s largest elevation gradient from 205 to 8848 m asl in eastern nepal our approach takes advantage of the distinct stable hydrogen isotopic composition expressed as d d values of plant wax n alkanes produced along this gradient transported in river waters and deposited in flood deposits alongside the arun river and its tributaries in mainstem flood deposits we found that plant wax n alkanes were mostly derived from the lower elevations constituting only a small fraction 15 of the catchment informed by remote sensing data we tested four differently weighted isotopic mixing models that quantify sourcing of tributary plant derived organic matter along the arun and compare it to our field observations the weighting parameters included catchment area net primary productivity npp and annual rainfall amount as well as catchment relief as erosion proxy when weighted by catchment area the isotopic mixing model could not explain field observations on plant wax d d values along the arun which is not surprising because the large arid tibetan plateau is not expected to be a major source weighting areal contributions by annual rainfall and npp captured field observations within model prediction errors suggesting that plant productivity may influence source strength however weighting by a combination of rainfall and catchment relief also captured the observed d d value pattern suggesting dominantly erosive control we conclude that tributaries at the southern himalayan front with high rainfall high productivity high relief and high erosion rates dominate plant wax exports from the catchment

source to sink evolution of lignin composition in the madre de dios river system with connection to the amazon basin and offshore while lignin geochemistry has been extensively investigated in the amazon river little is known about lignin distribution and dynamics within deep stratified river channels or its transformations within soils prior to delivery to rivers we characterized lignin phenols in soils river particulate organic matter pom and dissolved organic matter dom across a 4 km elevation gradient in the madre de dios river system peru as well as in marine sediments to investigate the source to sink evolution of lignin in soils we found more oxidized lignin in organic horizons relative to mineral horizons the oxidized lignin signature was maintained during transfer into rivers and lignin was a relatively constant fraction of bulk organic carbon in soils and riverine pom lignin in dom became increasingly oxidized downstream indicating active transformation of dissolved lignin during transport especially in the dry season in contrast pom accumulated undegraded lignin downstream during the wet season suggesting that terrestrial input exceeded in river degradation we discovered high concentrations of relatively undegraded lignin in pom at depth in the lower madre de dios river in both seasons revealing a woody undercurrent for its transfer within these deep rivers our study of lignin evolution in the soil river ocean continuum highlights important seasonal and depth variations of river carbon components and their connection to soil carbon pools providing new insights into fluvial carbon dynamics associated with the transfer of lignin biomarkers from source to sink

local scour at sloped wall spur dike like structures in alluvial rivers the effect of side slope on the prediction of the maximum local scour depth around sloped wall spur dike like structures has been investigated using two existing prediction methods where slope factors were considered the first one was developed from experimental data and is empirical in nature while the second one was developed using an analytical treatment by considering the flow concentration in the restricted region of scour hole both of the above approaches are applicable for clear water scour with the aid of the analytical model the slope factors for the empirical formulas have been extrapolated for milder side slopes the results of the above two methods together with the maximum measured local scour depth around a spur dike like structure in an alluvial river have been compared and discussed with particular emphasis on the effect of side slope on estimates of the maximum local scour depth

molecular proxies for paleoclimatology we summarize the applications of molecular proxies in paleoclimatology marine molecular records especially are proving to be of value but certain environmentally persistent compounds can also be measured in lake sediments loess deposits and ice cores the fundamentals of this approach are the molecular parameters the compound abundances and carbon hydrogen nitrogen and oxygen isotopic contents which can be derived by the analysis of sediment extracts these afford proxy measures which can be interpreted in terms of the conditions which control climate and also reflect its operation we discuss two types of proxy those of terrigenous and those of aquatic origin and exemplify their application in the study of marine sediments through the medium of ten case studies based in the atlantic mediterranean and pacific oceans and in antarctica the studies are mainly for periods in the present the holocene and particularly the last glacial interglacial but they also include one study from the cretaceous the terrigenous proxies which are measures of continental vegetation are based on higher plant leaf wax compounds i e long chain circa c30 hydrocarbons alcohols and acids they register the relative contributions of c3 vs c4 type plants to the vegetation in the source areas the two marine proxies are measures of sea surface temperatures sst the longer established one u37k is based on the relative abundances of c37 alkenones photosynthesized by unicellular algae members of the haptophyta the newest proxy tex86 is based on c86 glycerol tetraethers gdgts synthesized in the water column by some of the archaeal microbiota the crenarchaeota

holocene changes in biome size and tropical cyclone activity around the northern south china sea abstract the south china sea scs characterized by a large continental shelf is located at the edge of the asian monsoon domain in this study two marine sediment cores from the northern scs nscs continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the 13c and d values of plant wax n alkanes throughout the holocene last 11 200 years i e 11 2 ka the composite 13cwax record indicates an overall predominance of c3 vegetation over the last 11 2 ka before 8 ka bp higher 13cwax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf after the inundation of the shelf by eustatic sea level rise until ca 8 ka bp grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the 13cwax record the composite temperature corrected dwax t record suggests that moisture source variability drove precipitation isotopic composition changes during the holocene lower dwax t values before 8 3 ka bp are interpreted as a larger moisture contribution by pacific ocean tropical cyclones whereas higher dwax t values after 8 5 ka bp are interpreted as a larger moisture contribution from the indian ocean summer monsoon higher incidence of tropical cyclones in the nscs during the early holocene was related to a temporary westward shift of the western pacific warm pool and enhanced insolation over the northern hemisphere both external and internal forcing mechanisms regulated moisture source changes in east asia during the holocene

the origin of present day antarctic precipitation from surface snow deuterium excess data the deuterium excess d is defined as a linear combination of the d h and 18 o 16 o ratios in natural waters we present an interpretation of deuterium excess data in surface antarctic snow using an isotopic model to derive information on the origin of present day antarctic precipitation the data come from near the coast d47 adelie land and from inland south pole 1000 km from the sea the one dimensional isotopic model belongs to the rayleigh family but fully accounts for mixed cloud processes between 0 c and 30 c modeling d in polar snowfalls meets the problem of a large sensitivity to the saturation conditions prevailing at snow formation therefore we decided to tune the saturation in the model in order to yield realistic mean d values but we use the seasonal phase of d versus which is less sensitive to the saturation conditions as an independent validation of the model predictions both inland and near the coast our model can simulate the observed phase between d and in snow with moisture of subtropical origin 40 20 s although we do not treat specifically storm precipitation near the coast this study indicates that the observed phase between d and brings up a new validation of isotopic models it supports a distant vapor source for large scale precipitation delivered to the antarctic continent

morpho tectonic control on the distribution of c3 c4 plants in the central himalayan siwaliks during late plio pleistocene abstract the siwalik deposits of the himalayan foreland basin hfb preserve the late miocene pleistocene record of the himalayan tectonics monsoonal variation and the expansion of c4 plants previous vegetation reconstructions emphasized the late miocene expansion of c4 plants with an implicit assumption that the vegetation thrived in the floodplain of lowland rivers the coarsening upward sequence of the siwalik group suggests deposition in an alluvial fan setting in which the lower and middle siwaliks are deposits of distal region whereas the upper siwaliks represents a proximal region of the fan the modern alluvial fans forming in the himalayan foothills show a significant difference in elevation ca 300 m and vegetational composition between their proximal and distal regions in the hfb the elevation difference is expected to be more pronounced due to the surface uplift of siwalik deposits along the himalayan frontal thrust hft the increased elevation would have affected the vegetation distribution in the upper siwaliks which implies that the vegetation in the proximal part of the fan might not represent lowland floodplain however the vegetation composition is less understood from the upper siwaliks region due to the scarcity of conventional proxies in these younger foreland deposits in this context 13 c values of bulk soil organic matter som n alkane and n alkanoic acid in the paleosols were measured from late mio pleistocene siwaliks at surai khola nepal to comprehend the impact of elevation on vegetation distribution in the hfb the results suggest most commonly observed expansion of the c4 plants at ca 7 ma and a unique second phase of expansion of c3 plants post 3 ma the higher d values of n alkane and n alkanoic acid suggest that the climate was drier in the last 4 myr most likely driven by the onset of the northern hemisphere glaciation nhg the growth of c3 plants was favored due to cool climatic conditions induced by a higher elevation in the proximal part of the fan water bearing conglomerates in the upper siwaliks may have helped the c3 plants to thrive in a relatively drier climate this multi proxy paleovegetation reconstruction demonstrates a morpho tectonic control on the abundance of c3 c4 plants in the siwaliks with the possible influence of nhg

fluctuation in leaf wax d h ratio from a southern california lake records significant variability in isotopes in precipitation during the late holocene abstract the hydrogen isotopic composition of plant molecular markers in modern vegetation and marine sediments in southern california have been intensively studied here we report a late holocene reconstruction from zaca lake in coastal southern california together with modern hydrological and vegetation studies in the catchment the small catchment and decadal sampling resolution throughout the 9 m 3000 yr sedimentary record provided a high resolution terrigenous counterpart of nearby marine records from the santa barbara basin today d values of precipitation average 51 5 18 1 n 7 modern plant leaf wax d values for the c 28 n alkanoic acid averaged 141 11 1 n 10 for quercus agrifolia with a calculated fractionation relative to precipitation e wax precip of 94 22 1 n 10 in contrast there was negligible production of the c 28 acid by co dominant pinus coulteri downcore the c 28 acid d values ranged between 101 and 177 mean 150 8 1 n 490 abundance distributions suggested that the sedimentary c 28 acid was dominated by quercus implying that paleoprecipitation varied between extremes of 8 and 92 mean 63 14 compound 1 n 490 the 3000 yr leaf wax d h record from zaca lake revealed substantial temporal variability greater than observed in a speleothem reconstruction of similar resolution we suggest that the plant based proxy may magnify the variability by sampling spring precipitation preferentially centennial duration positive isotopic excursions were associated with more sub tropical moisture sources and drier conditions including during the medieval climatic anomaly and negative excursions were associated with n pacific sources and wetter conditions including during the little ice age

multiple plant wax compounds record differential sources and ecosystem structure in large river catchments abstract the concentrations distributions and stable carbon isotopes 13c of plant waxes carried by fluvial suspended sediments contain valuable information about terrestrial ecosystem characteristics to properly interpret past changes recorded in sedimentary archives it is crucial to understand the sources and variability of exported plant waxes in modern systems on seasonal to inter annual timescales to determine such variability we present concentrations and 13c compositions of three compound classes n alkanes n alcohols n alkanoic acids in a 34 month time series of suspended sediments from the outflow of the congo river we show that exported plant dominated n alkanes c25 c35 represent a mixture of c3 and c4 end members each with distinct molecular distributions as evidenced by an 8 1 0 7 1 standard deviation spread in 13c values across chain lengths and weak correlations between individual homologue concentrations r 0 52 0 94 in contrast plant dominated n alcohols c26 c36 and n alkanoic acids c26 c36 exhibit stronger positive correlations r 0 70 0 99 between homologue concentrations and depleted 13c values individual homologues average 31 3 and 30 8 respectively with lower 13c variability across chain lengths 2 6 0 6 and 2 0 1 1 respectively all individual plant wax lipids show little temporal 13c variability throughout the time series 1 0 9 indicating that their stable carbon isotopes are not a sensitive tracer for temporal changes in plant wax source in the congo basin on seasonal to inter annual timescales carbon normalized concentrations and relative abundances of n alcohols 19 58 of total plant wax lipids and n alkanoic acids 26 76 respond rapidly to seasonal changes in runoff indicating that they are mostly derived from a recently entrained local source in contrast a lack of correlation with discharge and low stable relative abundances 5 16 indicate that n alkanes better represent a catchment integrated signal with minimal response to discharge seasonality comparison to published data on other large watersheds indicates that this phenomenon is not limited to the congo river and that analysis of multiple plant wax lipid classes and chain lengths can be used to better resolve local vs distal ecosystem structure in river catchments