human impact overwhelms long term climate control of weathering and erosion in southwest china during the holocene there has been a gradual increase in the influence of humans on earth systems high resolution sedimentary records can help us to assess how erosion and weathering have evolved in response to recent climatic and anthropogenic disturbances here we present data from a high resolution similar to 75 cm k y sedimentary archive from the south china sea provenance data indicate that the sediment was derived from the red river and can be used to reconstruct the erosion and or weathering history in this river basin accelerator mass spectrometry c 14 dating provides direct age control and reveals coherent variations in clay mineralogy geochemistry and terrigenous flux indicative of strong chemical weathering and physical erosion during the mid holocene warm period 6400 4000 cal calibrated yr b p followed by weakening from ca 4000 1800 cal yr b p and renewed intensification since 1800 cal yr b p comparison with climatic records from china indicates that precipitation and temperature controlled both physical erosion and chemical weathering intensity before 1800 cal yr b p however weathering proxies in the offshore sediment indicate recent increased soil erosion we suggest that enhanced human activity deforestation cultivation and mining since the end of the chinese han dynasty 220 ce has overwhelmed the natural climatic controls on erosion in the red river

worldwide initiation of holocene marine deltas by deceleration of sea level rise radiocarbon dated deltaic sequences of holocene age from different parts of the world began to accumulate within a restricted time range from about 8500 to 6500 years ago evaluation of major delta processes indicates that deceleration in sea level rise was the key factor in holocene delta formation within many deltas there is as much as a 2000 year age range between basal deposits in seaward and landward cores this age difference records the progressive landward migration of near mean sea level depositional environments during the lower to mid holocene establishment of a chronostratigraphic framework for holocene delta development provides a fundamental global baseline for distinguishing sea level change from vertical land motion by tectonism and isostasy and for evaluating rates of future marine incursion into low lying deltas

archaeology to refine holocene subsidence rates along the nile delta margin egypt archaeological material and sites in the northern nile delta record rates of land subsidence that are higher than those derived from radiocarbon dated subsurface sediments reassessment of subsidence rate calculations reveals that previous subsidence measurements of 1 5 mm yr for the delta are minimum rates because sediment reworking can result in radiocarbon dated core ages that are older than the ages of final burial integration of archaeological and geologic subsurface data is essential for accurate age determinations differentiation of sub sidence from sea level rise and more precise calculation of vertical earth movement application herein of archaeological data to geological problems helps to refine subsidence rate measurements between the flexure zone landward margin of the holocene nile delta depocenter and the coast

the timing and evolution of the post glacial transgression across the sea of marmara shelf south of istanbul high resolution reflection seismic profiles with core analyses of sedimentary sequence near the sea of marmara som entrance to the strait of istanbul soi bosphorus provide detailed record of sealevel changes since the late glacial maximum lgm the sequence is deposited over the lgm erosional surface at a maximum depth of 105 m it includes seven seismic units that can be confidently correlated with 14c dated sedimentary units in the cores unit 6 represents levee deposits of the paleo soi channel that was deposited by a black sea outflow during 11 2 10 6 kyr bp the reflection surfaces at base of unit 5 and 4 correspond to wave cut terraces at 71and 63 m respectively and are estimated to be 10 6 and 9 8 kyr bp the seismic unit 2 overlaying a mud drape unit 3 comprises deltaic sediments that was deposited during 6 400 3200 yr 14c bp isopach map and

deltas radiocarbon dating and measurements of sediment storage and subsidence in holocene marine deltaic sequences there is no simple relationship between radiocarbon age and depth of sampling the nile and ganges brahmaputra deltas show remarkably similar radiocarbon age depth relationships 1 most radiocarbon dates by conventional and accelerator mass spectrometric ams analyses are older some by 4000 yr than reasonably expected 2 dates are commonly inverted i e older upcore and 3 sample dates at delta surfaces commonly exceed 2000 yr b p these anomalous age depth phenomena are in part a response to reservoir effects introduction of old carbon and emphasized herein influence of sediment storage and remobilization that prevail in fluvial and delta plains in fluvial systems sediment generally is not transported in a single episode from its original source area to the sea rather material transported from upland reaches and river valley to coast is subject to a repeated series of depositional burial and erosional events induced by episodic flooding overbank sedimentation and channel migration because sediment is likely to be temporarily stored in the delta plain it is difficult to determine the time of a sediment9s final deposition and burial at a delta site a successful strategy requires use of several independent dating methods including ams analyses of in situ basal peats and fossils and where possible archaeological evaluation of associated artifacts that are in place accurate dates are needed for reliable measurement of rates of delta plain aggradation and land subsidence information to help implement effective coastal protection measures

a new interpolation method to measure delta evolution and sediment flux application to the late holocene coastal plain of the argens river in the western mediterranean abstract rapid environmental changes along the mediterranean coasts influenced the sedimentary dynamics shoreline position and human settlements in deltaic areas over the last millennia an innovative and multiproxy approach using geostatistical modelling was developed to estimate geomorphic evolution and sediment fluxes in deltaic areas while palaeoecological conditions and environmental changes were assessed from geochemistry and cluster analyses of molluscan fauna this method was applied to the coastal plain of the argens river in southern france depositional environments of prodelta delta front river channel floodplain marsh and abandoned channel were identified from the study of facies associations the late holocene sediment flux in the argens river bayhead delta ranged from 15 800 2300 to 52 000 8500 m3 yr 1 the highest rate of sediment deposition between 2500 and 2000 cal yr bp was associated with increased river flooding in southeastern france a general decrease in subaqueous sediment flux over the past 2500 years was primarily controlled by a decrease in accommodation space and delta slope from 500 to 0 cal yr bp a sharp increase in subaerial sediment flux probably due to hydroclimatic change was contemporaneous with a period of high flood frequency during the little ice age a southward shift of the north atlantic westerlies combined with either a negative phase of the east atlantic west russian pattern or a negative phase of the east atlantic pattern coupled to a northward migration of the intertropical convergence zone could have led to increased fluvial activity in the lower argens valley during the late holocene

floodplain processes in the bengal basin and the storage of ganges brahmaputra river sediment an accretion study using 137cs and 210pb geochronology floodplain deposits are recognized as potentially large sinks for fluvial sediments yet spatial and temporal patterns of accumulation are not known for many river systems in the tectonically active bengal basin the ganges and brahmaputra rivers coalesce and have formed a large floodplain delta complex where widespread overbank flooding may deposit a significant portion of the river s estimated sediment discharge to investigate the magnitude and distribution of this process accumulation rates were determined by 137cs and 210pb radioisotope geochronology at 60 sites located in three regions of the bengal basin differing in age physiography and river influence these geochronological methods have been applied to floodplain environments in only a few recent studies and the present research offers a revised approach for heterogeneous systems patterns of sediment deposition in the bengal basin reveal three dominant controls on floodplain accretion channel processes overbank flooding and surface runoff accretion is most rapid in the river braidbelt and adjacent floodplain decreasing rapidly with distance from the main channel accumulation rates increase again in low lying distal basins where several meters of annual precipitation and associated runoff rework surrounding floodplain surfaces and transport remobilized sediments to local catchments the extent and magnitude of this process indicate its comparable importance with overbank deposition for floodplain accumulation a sediment budget for the study area reveals that at least 15 of the estimated 1 billion tonnes of fluvial sediment discharge is stored annually and not reaching the oceans as previously assumed extrapolated to the remaining 60 000 km2 of floodplain not covered in this project the sequestered load could rise to 30 of annual sediment discharge

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

geochronology and paleoenvironment of pluvial harper lake mojave desert california usa abstract accurate reconstruction of the paleo mojave river and pluvial lake harper manix cronese and mojave system of southern california is critical to understanding paleoclimate and the north american polar jet stream position over the last 500 ka previous studies inferred a polar jet stream south of 35 n at 18 ka and at 40 n at 17 14 ka highstand sediments of harper lake the upstream most pluvial lake along the mojave river have yielded uncalibrated radiocarbon ages ranging from 24 000 to 30 000 14 c yr bp based on geologic mapping radiocarbon and optically stimulated luminescence dating we infer a 45 40 ka age for the harper lake highstand sediments combining the harper lake highstand with other great basin pluvial lake spring and marine climate records we infer that the north american polar jet stream was south of 35 n about 45 40 ka but shifted to 40 n by 35 ka ostracodes limnocythere ceriotuberosa from harper lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the mojave river thus confirming the lake was fed by the mojave river the 45 40 ka highstand at harper lake coincides with a shallowing interval at downstream lake manix

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

radiocarbon anomalies suggest late onset of agricultural intensification in the catchment of the southern part of the yangtze delta china abstract previously underused information from radiocarbon dates in well characterised sedimentary sequences in the southern yangtze delta china is here utilised in reconstruction of patterns of ancient erosion the southern yangtze delta was an important focus of food production in the early neolithic anomalous radiocarbon dates in holocene sediments from the southern part of the delta are unevenly distributed through time two clusters of dates in sediments ca 9000 7000 cal bp and ca 3000 1000 cal bp most likely indicate two periods of intensive erosion and redeposition of organic materials the older of the two clusters may have been associated with a marked episode of marine transgression whereas the younger cluster could reflect increasing levels of human disruption of soils and sediments in the catchment a relatively late increase in anthropogenic soil erosion supports recent archaeological evidence that following its early neolithic origins agricultural expansion and intensification occurred later on the southern yangtze delta than previously thought

evolution of the intra coastal tidal range in the rhine meuse delta and flevo lagoon 5700 3000 yrs cal b c abstract the evolution of the coastal and intra coastal tidal range in the western netherlands during the period 5700 3000 yrs cal b c 6875 4400 yrs b p is considered by comparing one coastal mean high water curve and two local mean high water curves mhw with respect to three alternative relative mean sea level msl curves 1 the existing 1982 msl curve for the western and northern netherlands 2 a msl curve based on the oldest part of the 1982 msl curve and the youngest part of a recently published msl curve based on data from the east central coastal plain and 3 a hypothetical msl curve for the east central coastal area assuming crustal subsidence of 0 10 m 1000 yr relative to the 1982 msl curve with regard to coastal tidal range the first and third msl curve yield a similar result a minor increase in coastal tidal range between 5200 yrs cal b c 6200 yrs b p and 4200 yrs cal b c 5350 yrs b p the second msl curve gives an increase in coastal tidal range from an assumed value of 2 m by 5200 yrs cal b c 6200 yrs b p to ca 3 m by 4100 yrs cal b c 5300 yrs b p conflicting with existing model results intra coastal tidal damping was a widespread phenomenon and played an important role in the evolution of the coastal plain for the first and second msl curves tidal attenuation in the rhine meuse delta and the east central coastal sector begins around 5250 yrs cal b c 6275 yrs b p with the third msl curve the reduction of the tidal range starts is 500 years later the first and third msl curves result in complete tidal damping in the rhine meuse delta and the east central coastal area with the second msl curve the tidal range in the rhine meuse delta remains ca 1 3 m whereas in the east central area tidal reduction is complete in spite of these different results it is clear that the period 5300 4300 yrs cal b c ca 6375 5400 yrs b p was very important for the subsequent evolution of the west netherlands coastal plain given that a the relative msl rise slowed down from about 0 8 m 100 yr to 0 23 m 100 yr b the coastal tidal range increased possibly by as much as 1 m resulting in higher sediment input into the intra coastal area while c further landward transportation of sediment was hampered due to intra coastal tidal damping which reduced rates of local mhw rise tidal current velocities and accommodation space

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

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

beryllium 10 terrestrial cosmogenic nuclide surface exposure dating of quaternary landforms in death valley quaternary alluvial fans and shorelines spits and beach bars were dated using 10 be terrestrial cosmogenic nuclide tcn surface exposure dating in death valley the 10 be tcn ages show considerable variance on individual surfaces samples collected in the active channels date from 6 ka to 93 ka showing that there is significant inheritance of 10 be tcn within cobbles and boulders that were dated which suggests that slopes erode and sediment is transferred very slowly in most regions within death valley comparisons of 10 be tcn ages on alluvial fan surfaces with chronostratigraphies based on soil development and optically stimulated luminescence dating show that minimum 10 be tcn ages within sample sets on individual surfaces most closely approximate to the age of landforms that are younger than 70 ka alluvial fan surfaces older than 70 ka have begun to undergo significant erosion so that the majority of 10 be tcn ages for data sets on individual surfaces underestimate the true age of the surface due to erosion and exhumation of fresh cobbles and boulders the spread of 10 be tcn ages for beach bars near beatty junction and shorelines 8 km south of furnace creek is large ranging from 119 ka to 385 ka and 109 ka to 465 ka respectively however new and previously published luminescence ages soil development and geomorphic context suggest that these landforms formed during high lake level stands in marine isotope stage 2 22 18 ka but these younger ages my reflect soil development and material elluviated into the bar deposit long after deposition this suggests that the cobbles and boulders dated with 10 be methods have inherited 10 be tcns which have been derived from older shorelines or associated landforms these results highlight the problems associated with using surface cobbles and boulders to date quaternary surfaces in death valley and other similar dryland settings moreover this study emphasizes the need to combine multiple different dating methods to accurately date landforms in death valley and similar settings

abrupt changes in north american climate during early holocene times recent studies of the greenland ice cores have offered many insights into holocene climatic dynamics at decadal to century timescales1 2 3 despite the abundance of continental records of holocene climate few have sufficient chronological control and sampling resolution to compare with the greenland findings4 butannually laminated sediments varves from lakes can provide high resolution continental palaeoclimate data with secure chronologies here we present analyses of varved sediments from deep lake in minnesota usa trends in the stable oxygen isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8 9 to 8 3 kyr before present probably characterized by increased outbreaks of polar air decreased precipitation temperatures and a higher fraction of the annual precipitation falling as snow the abrupt onset of this climate reversal over several decades was probably caused by a reorganization of atmospheric circulation and cooling of the arctic airmass in summer that resulted from the final collapse of the laurentide ice near hudson bay and the discharge of icebergs from the quebec and keewatin centres into the tyrell sea the timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8 200 years ago in greenland and other regions1 5 6 no shifts in the oxygen isotope composition of sediment carbonate occurred at 8 2 kyr before present at deep lake but varve thickness increased dramatically probably as a result of increased deposition of aeolian dust taken together our data suggest that two separate regional scale climate reversals occurred between 9 000 and 8 000 years ago and that they were driven by different mechanisms

melting and crystallization of andesite from chilungshan northern taiwan an experimental petrological study at atmospheric pressure abstract to investigate whether quartz can be crystallized from the magma at the chilungshan andesite in northern taiwan melting experiments involving 17 runs were performed at atmospheric pressure the liquidus temperature was determined to be 1285 c and the melting interval was estimated to be 193 c pagioclase crystallized first at 1285 c and iron titanium oxides subsequently crystallized at 1267 c quartz and orthopyroxene crystallized together at 1174 c finally clinopyroxene crystallized at 1166 c and orthopyroxene disappeared at 1155 c the crystallization of the chilungshan andesitic melt at atmospheric pressure was successively controlled by plagioclase iron titanium oxides quartz orthopyroxene and clinopyroxene the differentiation trend of the residual melt involved the depletion of aluminum and calcium but the residual melt became enriched in silicon and potassium in addition it became enriched in magnesium iron and titanium at the initial stages which were depleted at the final stages the study results suggest that quartz grains can be crystallized from the chilungshan andesitic melt at atmospheric pressure

balance of tectonically accreted and subducted sediment at the chile triple junction an active oceanic spreading ridge is being subducted beneath the south american continent at the chile triple junction mass balance estimations to characterize temporal and spatial variations in the frontal accretion or underplating and subduction of sediments since the late miocene were made using seismic and drill hole data at 200 km north of the triple junction almost 80 of the sediment on the downgoing nazca plate are subducted sediment subduction rate decreases towards the triple junction because of a low in sedimentation rates as the flank of the spreading ridge approaches the trench at the triple junction the forearc is almost completely destroyed by spreading ridge collision and subduction erosion less than 12 of the available sedimentary input is accreted south of the triple junction where the spreading ridge passed 6 ma ago a large fraction 60 of the sediment on the incoming antarctic plate has been scraped off and was frontally accreted to the chile forearc spreading ridge subduction leaves a distinctive geological fingerprint and has a large impact on the mass balance of the subduction zone however the high rates of change in the process may make this fingerprint hard to detect in fossil convergent orogens in the ridge collision zone the sediment supplied to the trench and the amount of sediment subducted show strong and distinctive variations on a 1 to 5 million year time scale on a 10 million year time scale sediment subduction to the earth s mantle is reduced by spreading ridge collision caused by the need of the overriding forearc to regain a low angle of taper by frontal accretion

reworking and discontinuities in holocene sedimentation in the nile delta documentation from amino acid racemization and stable isotopes in mollusk shells abstract the late holocene sedimentary history of the northwestern edge of the nile delta is reconstructed from amino acid racemization radiocarbon and stable isotope analysis of a series of bivalve shells from a core taken at the edge of lake maryut near alexandria amino acid racemization confirmed by radiocarbon and stable isotope analysis indicates that in many parts of the core mixed age mollusk assemblages are present dating of the time of sediment accumulation at various levels in the core is therefore based on the age of the youngest shells which are identified by amino acid racemization analysis lowest d l values ams radiocarbon analysis is used to determine the ages of the shells except for the most recent sediments last 100 yr for which aspartic acid racemization provides more precise ages amino acid racemization analysis of 59 shells from 21 levels in the 4 8 m holocene sequence in the core enabled identification of a large hiatus holocene sedimentation started at ca 2400 bc apparently corresponding to a westward shift of the course of the canopic nile distributary which lies to the east deposition continued at a rate of 1 4 mm per year ceasing at ca 550 bc at which time the canopic nile shifted eastward again and subsequently became defunct a marine connection during this period is indicated by stable isotope values of shells sediment accumulation began again only at the end of the 19th century when a series of irrigation canals was connected to the lake maryut basin stable isotope values indicate a strong freshwater influence deposition has continued to the present at a very high rate 19 mm per year due to supply of sediments by agricultural activity the relative ease with which racemization analyses can be carried out permits detailed analysis of core chronostratigraphy leading to more accurate reconstruction of core chronology hiatuses can be pinpointed and documented and age mixtures can be identified with appropriate individual shells being selected for ams radiocarbon analysis aspartic acid racemization analysis permits dating of samples from the last several hundred years a period for which radiocarbon analyses provide poor precision

exchanges of sediment between the flood plain and channel of the amazon river in brazil sediment transport through the brazilian sector of the amazon river valley a distance of 2010 km involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at obidos 1200 mt yr 1 the exchanges occur through bank erosion bar deposition settling from diffuse overbank flow and sedimentation in flood plain channels we estimated the magnitude of these exchanges for each of 10 reaches of the valley and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel the annual supply of sediment entering the channel from bank erosion was estimated to average 1570 mt yr 1 1 3 the obidos flux and the amount transferred from channel transport to the bars 380 mt yr 1 and the flood plain 460 mt yr 1 in channelized flow 1230 mt yr 1 in diffuse overbank flow totaled 2070 mt yr 1 1 7 the obidos flux thus deposition on the bars and flood plain exceeded bank erosion by 500 mt yr 1 over a 10 16 yr period sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 mt yr 1 over 16 yr crudely validating the process based calculations of the sediment budget which in turn illuminate the physical controls on each exchange process another 300 400 mt yr 1 are deposited in a delta plain downstream of obidos the components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain which in turn are influenced by tectonic features of the amazon structural trough

dating modern deltas progress problems and prognostics 1 abstract radiocarbon dating is the method most frequently used to date holocene deltaic sequences but less than one quarter of 14c dates are within 500 years of predicted age such dates tend to be unreliable in other words often too old and commonly inverted upsection and core sample dates obtained near deltaic plain surfaces may be as old as mid to late holocene stratigraphic irregularities result primarily from downslope reworking of upland alluvial sediment with displacement of old carbon in the sediment that accumulates in lower valleys and deltaic plains use of dates that are too old results in inaccurately calculated rates most often too low of relative sea level rise and or land subsidence more reliable timing of deltaic sediment requires a multiple method dating approach including where possible identification of associated archaeological material developing an accurate dating strategy is a critical step for implementing reliable coastal protection measures needed for the rapi

variation in radiocarbon ages of soil organic matter fractions from late quaternary buried soils radiocarbon dating of three organic matter fractions total humic acid and residue isolated from late quaternary buried soils of the central great plains reveals that there often are considerable differences among but no consistent order to the ages of fractions for late holocene soils the residue fraction or the total fraction generally produces the oldest age for late pleistocene soils however no fraction was consistently the oldest the absence of a consistent sequence of fraction ages is attributed to postburial contamination of soils when bulk samples from the same soil were split and sent to two laboratories different radiocarbon ages were usually obtained the variability in radiocarbon ages of soil organic matter confirms that caution should be taken when using radiocarbon ages obtained from different laboratories to make regional stratigraphic correlations

a revised holocene geochronology for the lower mississippi valley data from two locations 1 a hydrodam site at the old river diversion structure south of the latitude of natchez ms and 2 eight water wells from the latitude of baton rouge la are the basis for a revised holocene geochronology of the lower mississippi river floodplain based on 84 radiocarbon dates analyses of 64 borehole logs and 75 grain size analyses which together provide major insight into questions concerning the complexity of meander belt movement the influence of the younger dryas on the lower valley and on the pleistocene holocene interface following the younger dryas the early holocene was characterized by periodic flooding and deposition separated by times of little or no deposition

improving methodology for high resolution reconstruction of sea level rise and neotectonics by paleoecological analysis and ams 14c dating of basal peats abstract sea level research in several submerging coastal regions has traditionally been based on 14 c dating of basal peats that overlie a compaction free substratum and can be related to paleo ground water levels provided that an unequivocal relationship between ground water level and sea level can be assumed this approach contains two sources of uncertainty 1 the paleoenvironmental interpretation of samples is usually based on inherently inaccurate macroscopic descriptions in the field and 2 14 c ages of bulk peat samples may be erroneous as a result of contamination due to the uncertainties in both the altitude and the age the two crucial sources of evidence necessary to arrive at accurate sea level curves sea level index points are therefore represented by considerable but typically not quantified error boxes accelerator mass spectrometry ams opens new perspectives for this type of sea level research as illustrated by a paleoecological and ams 14 c study of basal peats from a small study area in the rhine meuse delta the netherlands where previous conventional work revealed highly problematic results a detailed macrofossil analysis has two purposes 1 an inferred paleoecological succession indicates a relatively accurate level of paludification of the site and hence rise of the ground water level 2 suitable macrofossils from that specific level are then selected for ams 14 c dating in spite of very small sample sizes our results are consistent and indicate that this approach can constitute a step forward in high resolution reconstruction of sea level rise the new results further enable a revision of holocene ground water gradient lines for the rhine meuse delta a knickpoint in these gradient lines can be related to the effect of faulting this approach therefore also has considerable potential to unravel and quantify neotectonic activity in submerging coastal settings

recent subsidence of the northern suez canal in contrast to a recent interpretation of delta coast stability1 we now show that the northeastern nile delta in egypt has been actively sinking relative to the sea during the recent holocene epoch the northern nile delta is only about 1 m above sea level making the northern suez canal and coastal cities of port said and port fuad combined population nearly 500 000 highly vulnerable subsidence and world sea level rises contribute to coastal erosion incursion of salt in the groundwater underlying the delta plain and silting problems in the canal entrances2 these processes must be considered when implementing protection measures for this area

neotectonic history of eastern maine evaluated from historic sea level data and 14c dates on salt marsh peats we constructed a local relative sea level curve for eastern maine from 14 c dates on salt marsh peats to evaluate potential holocene neotectonic activity this sea level chronology indicates that contrary to suggestions made previously low elevations of late glacial shoreline features in eastern maine are not related to neotectonic activity during the past 5 ka a comparison between seismicity and tide gauge records leads to the conclusion that there is no relation between seismic activity and contemporary subsidence of eastern coastal maine we propose that eastern maine has not been affected by anomalous subsidence during at least the past 5 ka

composition and trace element geochemistry of detrital clay and heavy mineral suites of the lowermost amazon river a provenance study abstract compositions of clays and heavy minerals as well as trace element geochemistry of the lowermost amazon sediments reflect source regions and the intense chemical weathering conditions illite and montmorillonite indicate mountainous sources under physical weathering conditions whereas kaolinite is related to the soil composition under humid tropical conditions where chemical weathering predominates the suite of unstable heavy minerals on the other hand reflects some sediment transport from the andes more supply from tributaries draining the basic and metamorphic rocks from the southeastern guiana shield and strongest influence of the upstream amazon and solimoes areas these source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain the brazilian shield contribution via the xingu river seems to be of minor importance although the alter do chao and the barreiras formations which are products of erosion of the shields supply suites of stable heavy minerals that reflect recycling rare earth and trace element data from the clay and heavy mineral fractions provide additional clues to the source areas in the downstream direction superimposed upon provenance of sediments from an undifferentiated arc the andes is provenance from strongly weathered continental crust the precambrian shields as well as sedimentary recycling small scale provenance can be delineated by the most immobile elements heavy rare earth elements in the heavy mineral fraction increase downstream toward estreitos to the para river

nile river sediment fluctuations over the past 7000 yr and their key role in sapropel development the provenance pattern of nile river sediments can be used as a proxy for paleoclimatic changes in east africa the 87sr 86sr ratios are particularly appropriate for such provenance investigations because the white nile drains predominantly crystalline basement rocks whereas the blue nile and atbara flow off the ethiopian highlands which consist of tertiary volcanic rocks a high resolution profile of 87sr 86sr and ti al ratios from a well dated core in the nile delta shows a close correspondence with known changes in nile flow over the past 7000 yr at times of higher river flow there was markedly decreased input of blue nile derived and total sediment this change was caused by northward movement of the inter tropical convergence zone resulting in increased vegetative cover in the ethiopian highlands due to higher rainfall and a longer wet season this inverse relationship between nile river flow and sediment flux may have had important implications in the development of agricultural technology in ancient egypt the marked minimum in 87sr 86sr at 4200 4500 yr b p is coincident with the end of the old kingdom in egypt and provides independent evidence that demise of the old kingdom might have been associated with an extended period of catastrophic low floods during the quaternary and late neogene there was periodic deposition of organic rich sediments sapropels in the eastern mediterranean that represent important indicators of major environmental change evidence from the ti al ratio suggests that the pattern of erosion and sediment supply from the nile catchment observed in this study also occurred throughout much of the neogene and quaternary the reduced inputs of blue nile sediment during times of sapropel formation contributed to the increased primary productivity by reducing the amount of phosphate removed on particles and to the observed change to n limitation in the eastern mediterranean which are important characteristics of sapropel deposition