late pleistocene and holocene palaeogeography of the lower tagus valley portugal effects of relative sea level valley morphology and sediment supply abstract because of the deep glacial incision the lower tagus valley hosts a sedimentary record since 20 000 cal bp making this a unique site along the european atlantic margin with respect to palaeogeographic and sea level changes based on nine cross sections and 55 radiocarbon dates together with a newly created relative sea level curve we constructed five palaeogeographic maps of the infill of the lower tagus valley since 20 000 cal bp we illustrate that relative sea level rise and fluvial sediment supply were the prime forcing factors determining the depositional history and palaeogeographic changes around 20 000 cal bp a deeply incised braided river existed which was directly connected to the ocean across the narrow continental shelf after that 12 000 cal bp the gradually moister and warmer climate caused a change to a single channel river during the following period 12 000 7000 cal bp relative sea level rise resulted in a transgression in the lower tagus valley and the establishment of extensive tidal environments after relative sea level rise had ended 7000 cal bp the valley was progressively filled by a fluvial wedge and tidally influenced bayhead delta since 1000 cal bp the valley fill history was dominated by increased sediment input due to human induced degradation of catchment slopes generally climate was of subordinate importance during the entire studied period merely causing a single channel river resulting from the change from the cold heinrich event 1 to the temperate bolling allerod interstadial despite the tectonic activity in the region neotectonic uplift or subsidence was limited as supported by the horizontal relative sea level curve since 7000 cal bp neotectonics played a minor role due to the large distance from the fennoscandian ice sheet and the narrow continental shelf which prohibited strong glacio and hydro isostatic movements

holocene geochronology of a continentalshelf mudbelt off southwestern africa the holocene lithostratigraphy and geochronology of sediments on an inner continental shelf mud belt off the west coast of southern africa is presented based on a study of seven large gravity cores numerous grab samples and two vibracores two distinctive sections of the mudbelt have been identie ed the northern section is associated with the orange river prodelta and is dominated by laminated clay rich sediments while the southern section off namaqualand is associated with more homogeneous muds ams radiocarbon ages of 31 samples from these cores are problematic due to the absence of expected modern material at the sediment ocean interface and numerous age reversals and inconsistencies in some of the cores possible mechanisms to explain the apparently anomalously old surface and near surface samples are suggested most likely expla nations appear to lie in a combination of sediment retrieval problems the periodic loss of e ne particulate organic carbon from within the water column the sampling of possible relic sediments and most likely of all the incorporation into the sediments of 14 c depleted terrigenous organic matter the age reversals are especially evident in the laminated sediments and the entrainment of old carbon from the terrestrial source of these sediments is offered as the most parsimonious explanation stable carbon isotope analyses and a single lumi nescence age support the contention that radiocarbon date inconsistencies are a consequence of terrestrial inputs of carbon which is non contemporaneous with the actual time of sedimentation in studies of offshore accumulations of terrigenous material the marine derived organic fraction may therefore prove a more reliable measure of radiocarbon chronology

temporal variability of flooding events of guadiana river iberian peninsula during the middle to late holocene imprints in the shallow marine sediment record abstract sedimentological geochemical and benthic foraminiferal proxies were used to interpret changes of depositional environments in a mud entrapment inside the guadiana river paleo valley northern gulf of cadiz to understand the temporal variability of fluvial flooding events and to detect patterns of latitudinal climatic variability in western iberia the period between ca 5800 and ca 1250 cal yr bp was characterized by slowly accumulating coarse grained sediments high content of biogenic sand components and high abundances of shallow water benthic foraminiferal species after ca 1250 cal yr bp the sedimentary environment was dominated by fine grained sediments with high abundances of opportunistic benthic foraminiferal species and successful colonizers and high values of geochemical ratios indicative of enhanced terrigenous supply sedimentation rates increased drastically over the last ca 500 years the genesis of these environments was mainly driven by the variable frequency of fluvial flooding events driven by the regional climate variability low terrigenous sediment input under dry conditions prevailed in the older sedimentary unit flood frequency and terrigenous sediment supply increased during the younger unit the variability of the north atlantic oscillation at centennial scales highly influenced the sediment supply during the last ca 500 years with high terrigenous supply during negative nao conditions our results corroborate an n s gradient along the atlantic iberia during middle and late holocene with more humid conditions in the northwestern and drier conditions towards the southeast the gradient weakened over the past ca 500 years with increased rainfalls and flood events during the little ice age

post lgm upward shift of the mediterranean outflow water recorded in a contourite drift off nw spain abstract the nw iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times the sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts a combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper mediterranean outflow water mow core after the last glacial maximum lgm a coarser grained interval during deglacial and early holocene times 17 2 to 9 9 cal ka bp points to an increase in bottom current strength this increase in velocity was probably related to oceanic density fronts which migrated through the 300 m thick transition zone between the underlying labrador sea water and the overlying mow radiocarbon dates timed the current strengthening to 17 2 cal ka bp and a following weakening of the bottom current to 13 3 cal ka bp at 1965 m water depth and to 9 9 cal ka bp at 1885 m water depth the depth dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of mow or a vertical contraction of this water mass the upward movement happened over a time interval of approximately 3 4 thousand years in addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift these heterogeneities suggest a need of a detailed core coverage across current influenced deposits for palaeoceanographic studies to minimize misinterpretations

les crises du devonien superieur par l etude des faunes d ostracodes dans leur cadre paleogeographique abstract ostracod evolution studied at a global scale can contribute to an evaluation of the palaeobilogical crises during the late devonian the mostly pelagic entomozoidae are not affected data on benthic neritic ostracods come from the western canadian platform europe the russian platform north africa and the middle east species extinction rates vary according to provinces and areas extinction occur at the givetian frasnian lower upper frasnian and frasnian famennian boundaries only five families seem to disappear during the late devonian the appearance of new assemblages since the earlisst famennian is worldwide on platforms the thuringian ecotype ostracod assemblages underwent a major change since the early famennian it is characterized by many species with spinous carapaces and is known in the paleotethys during famennian dinantian times it is related to a glacial event in the north of the gondwana which generated cold deep water currents invading the paleotethysian basins the thuringian ecotype allows to propose a paleobathymetric model its species composition shows a double origin by evolution of early devonian roots and with lineages proceeding from circalittoral species causes of these late devonian events can be understood in a particular paleogeographic reconstruction where the paleotethys has no western faunal communication since the givetian frasnian boundary warming water associated with an oxygen deficiency eliminated several species at the givetian frasnian and lower upper frasnian boundaries a first cooling at the frasnian famennian time was sufficient to decimate species strongly adapted to the frasnian conditions the generalized regression at this time probably increased this last phenomenon

comparing oxygen isotope records of silurian calcite and phosphate δ18o compositions of brachiopods and conodonts oxygen isotope compositions of silurian calcitic brachiopod shells from gotland are compared with those of phosphatic inarticulate brachiopod shells and conodonts from identical stratigraphic horizons the 18ophosphate values of inarticulate brachiopod shells vary widely 13 0 to 17 5 v smow and are significantly lower than those of coeval conodont samples 17 5 to 19 5 v smow conodont samples from individual beds have homogenous 18ophosphate values with variations close to the analytical precision 0 2 together with a significant correlation of conodont 18ophosphate and articulate brachiopod shell 18ocarbonate values r 0 85 this observation suggests that primary marine oxygen isotope compositions are better preserved in conodont apatite than in inarticulate brachiopod shell apatite however with decreasing 18ocarbonate values the 18ophosphate values of conodonts are higher than expected for theoretical isotopic equilibrium with brachiopod calcite different habitats of conodonts and brachiopods cannot account for the observed offset in 18o because both organisms were thriving on a broad shallow marine carbonate platform whereas 18ophosphate values of silurian conodonts are close to 18ophosphate values of modern and mesozoic tropical biogenic apatites most silurian calcitic brachiopod shells from gotland are depleted in 18o when compared to modern or mesozoic brachiopods we therefore suggest that silurian conodonts from gotland record paleotemperature and 18o of silurian sea water more faithfully than coeval calcitic brachiopod shells

winter dynamics on the northern portuguese shelf part 2 bottom boundary layers and sediment dispersal abstract the northern portuguese shelf between 41 n and 42 n is characterised by the presence of a mid shelf mud deposit the douro mud patch dmp observations conducted between july 1996 and june 1999 under the framework of project omex ii ii are used to examine the impact of the dynamic processes in the lower water column particularly on the dmp the typical wave conditions observed during the winter maintain a highly energetic environment capable of promoting the erosion of fine sediments at mid shelf depths however the bottom nepheloid layers generated beneath these waves only extend a few meters above the bottom and are contained within rocky outcrops with similar heights that fringe the outer shelf each year there are about ten storms mostly associated with southerly winds that create downwelling conditions over the shelf the waves associated with these storms produce shear velocities over 3 cm s at mid shelf and bottom nepheloid layers which extend a few tens of meters above the bottom and spread offshore over the outer shelf and upper slope a rough estimate suggests that these events account for an offshore export of about 20 10 6 kg of fine sediments each year equivalent to 1 2 of sediments trapped at dmp

oxygen isotope variations in phosphate of biogenic apatites ii phosphorite rocks abstract phosphorites from sedimentary sequences ranging in age from archaean to recent were analysed for 18 o in both the po 4 18 o p and co 3 18 o c in the apatite lattice the oxygen isotope record is considerably better preserved in phosphates than in either carbonates or cherts the use of the longinelli and nuti 8 temperature equation yields temperatures for recent phosphorites that are in good agreement with those measured in the field the 18 o p values of ancient phosphorites decrease with increasing age these changes with time are not likely to be due to post depositional exchange changes in 18 o values of seawater and variations of temperature with time can account for the 18 o p time trend but the latter explanation is preferred in ancient phosphorites 18 o c in structurally bound carbonate in apatite is not a reliable geochemical indicator

a 16 ma record of paleodiet using carbon and oxygen isotopes in fossil teeth from pakistan abstract the siwalik sequence of northern pakistan contains a 16 ma record of paleosol carbonate and fossil teeth from which a record of paleovegetation can potentially be reconstructed and compared the carbon isotopic composition of paleosol carbonate and organic matter from siwalik strata reflects a major paleoecological change on the floodplains of major rivers beginning7 3 ma ago by 6 ma c 3 dominated plant communities probably composed of mostly trees and shrubs were displaced by nearly continuous c 4 grassland we find that the carbon isotopic ratios in herbivore tooth enamel reflect this dramatic ecologic shift carbonate in enamel older than 7 ma averages 11 in 13 c pdb consistent with a largely c 3 diet enamel from the plio pieistocene averages 1 9 in 13 c similar to the value displayed by modern c 4 grazers analysis of post burial carbonate cements and the concordance with isotopic evidence from paleosols argues strongly against major isotopic alteration of the enamel while coexisting bone may have been altered early in burial this study confirms that enamel apatite is useful for paleodietary reconstruction much further back in the geologic record than was previously thought

long term and millennial scale climate variability in northwestern france during the last 8850 years vegetation and quantitative climate reconstructions from a northwestern france shelf core vk0358bis show orbital and suborbital climate variability for the last 8850 years in this region a long term cooling trend in summer temperatures marked by gradual temperate and humid forest decline parallels cooling in greenland and the decrease of mid latitude summer insolation reduction until at least 2000 cal yr bp at the long term scale the lowering in seasonal contrast revealed by vegetation changes follows the increase of precession corylus woodlands spread at the expense of deciduous quercus forest between 8740 and 8390 cal yr bp linked with the high seasonality conditions that counterbalancing the long term astronomical forcing trend were amplified by the north atlantic high latitudes winter sea ice expansion high seasonality conditions resulted from the agassiz and ojibway final outburst episodes and consequent gradual reduction of the meridional overturning circulation moc between 8390 8060 cal yr bp a sudden corylus woodland decline marks the 8 2 kyr cold event in northwestern france probably triggered by the severe moc reduction leading to the additional drop in winter temperature over europe and greenland nonetheless seasonality remains high during this interval the high seasonality conditions detected in vk03 58bis between 8740 and 8060 cal yr bp reflects the multicentennial scale climate cooling 8 6 8 0 kyr episode of the north atlantic following the agassiz and ojibway final outburst episodes climate became more stable however millennial scale climate cooling episodes are recorded in vk03 bis and are characterized by weak winter cooling and increases in precipitation furthermore dinocyst analysis and benthic gastropod turritella communis occurrences indicate regional changes such as the southward migration of the boreal biogeographical zone between 8740 and 8480 cal

geochemistry of modern brachiopods applications and implications for oceanography and paleoceanography abstract modern rhynchonellida terebratulida thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2 3940 m from the equator to high latitudes covering the atlantic pacific indian caribbean and mediterranean oceans seas well preserved fibers of the secondary layer combined with sr 450 1928 mg kg mn 1 199 mg kg and fe 1 610 mg kg contents confirm with few exceptions the excellent preservation of the modern brachiopod material evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations it revealed that oxygen isotope incorporation into shell calcite secondary layer of modern articulated brachiopods in most cases occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions thecideida as a group shallow water and low latitude incorporate oxygen isotopes 3 04 to 0 35 into shell calcite in equilibrium with their ambient water a large majority of genera and species of the terebratulida shallow water and low mid latitude also incorporate oxygen 2 30 to 3 48 into shell calcite in isotopic equilibrium with their surrounding water in contrast equilibrium incorporation of oxygen isotopes into shell calcite by the rhynchonellida is uncertain but a vital effect probably applies to the craniida the question of equilibrium disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time isotope compositions range up to 4 5 5 6 and 1 5 for 13 c and up to 5 5 5 9 and 0 3 for 18 o for shallow water low mid and high latitude brachiopods respectively strontium isotopes of modern brachiopods as expected show no significant variation with location latitude water depth temperature salinity and general water conditions once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered their isotopic elemental values should be valuable diagenetic indicators and important oceanographic chemostratigraphic proxies

surface and near surface sediments from the continental shelf off the russian river northern california abstract there are two basic types of detrital sediments in the study area modern and relict modern sediment consists mostly of sand on the inner shelf and silt and clay on the middle shelf the silt and clay overlie an older transgressive sand and extend northwest southeast with an average thickness of 10 m this unit pinches out toward the outer shelf where relict detrital sand dominates with some authigenic glauconite the detrital sand is much cleaner and darker than the inner shelf sand the important heavy minerals of the sand fraction for detecting sediment dispersal patterns are biotite hornblende glaucophane and glauconite biotite and hornblende are abundant in the southern parts of the study area close to point reyes beach and decrease in abundance northward and offshore glaucophane is abundant close to the russian river mouth and decreases in abundance southward glauconite is abundant on the outer shelf and decreases in abundance southward there are two sediment sources for the inner and middle shelf russian river discharge and coastal erosion particularly of beaches headlands and cliffs in the southern part of the area the sand sized sediment of the russian river is being deposited on the inner shelf between salt point to the north and bodega head to the south the russian river silt and clay are confined to the middle shelf some of this silt and clay is being transported into bodega submarine canyon the coastal material is eroded in the south and transported northward seaward and also into bodega submarine canyon

surface waters of the nw iberian margin upwelling on the shelf versus outwelling of upwelled waters from the rias baixas abstract a set of hydrographic surveys were carried out in the ria of vigo nw spain at 2 4 d intervals during four 2 3 week periods in 1997 covering contrasting seasons residual exchange fluxes with the adjacent shelf were estimated with a 2 d non steady state salinity temperature weighted box model exchange fluxes consist of a steady state term dependent on the variability of continental runoff and a non steady state term dependent on the time changes of density gradients in the embayment more than 95 of the short time scale variability of the exchange fluxes in the middle and outer ria can be explained by the non steady state term that in turns is correlated r 2 75 with the offshore ekman transport conversely 96 of the variability of exchange fluxes in the inner ria rely on the steady state term the outer and middle ria are under the direct influence of coastal upwelling which enhances the positive residual circulation pattern by an order of magnitude from 10 2 to 10 3 m 3 s 1 on the contrary downwelling provokes a reversal of the circulation in the outer ria the position of the downwelling front along the embayment depends on the relative importance of ekman transport q x m 3 s 1 km 1 and continental runoff r m 3 s 1 when q x r 7 2 the reversal of the circulation affects the middle ria our results are representative for the rias baixas four large coastal indentations in nw spain during the upwelling season spring and summer 60 of shelf surface waters off the rias baixas consist of fresh eastern north atlantic central water enacw upwelled in situ the remaining 40 consists of upwelled enacw that previously enters the rias and it is subsequently outwelled after thermohaline modification during the downwelling season autumn and winter 40 of the warm and salty oceanic subtropic surface water which piled on the shelf by the predominant southerly winds enters the rias

carbon isotope geochemistry of the frasnian famennian transition abstract carbon isotope records for inorganic and total organic carbon toc as well as for individual organic biomarkers show two positive excursions in the late frasnian with comparable shape and amplitude in 13c the positive shifts in 13c can be correlated with the deposition of the kellwasser horizons the inorganic carbon isotope excursions are explained by an enhanced organic carbon burial that is expected to lower the concentration of oceanic dissolved co2 toc 13c values do not exhibit a larger shift than that recorded by inorganic carbon 13c values this is unexpected since a decrease in the dissolved co2 concentration should result in lower carbon isotope fractionation during photosynthesis ep 13c values of presumed algal biomarkers low molecular weight n alkanes pristane phytane although offset by 2 5 exhibit essentially identical records as toc and confirm this unexpected result it is proposed that high atmospheric and oceanic co2 concentrations during the devonian resulted in maximum photosynthetic fractionation any change in the co2 concentration would thus not affect ep as such 13c of primary organic carbon and 13ccarb will exhibit parallel excursions the data imply that carbon isotopes and the relationship between ep and co2 will not be effective to study changes in pco2 levels during the devonian since co2 concentrations were too high

recent sedimentation and sedimentary budgets on the western iberian shelf abstract thus study is based on 31 box core samples collected mainly from the mud patches of the galician and north portuguese shelf each of these cores some decimetres in length have been described radiographied and sampled each centimetre for grain size analysis and measurement of the particulate organic carbon corg content 210pb excess profiles have been measured using gamma spectrometry which combined with the analysis of the vertical variations of the sedimentary facies have allowed estimates of sedimentation rates these rates were found to vary between 0 05 and 0 40 cm yr 1 considering the latitudinal variation of sedimentation rates around the 100m contour we note that the maximum rates occurred off the douro estuary in the southern sector and also near the galician rias to the north the organic content corg of sediments varied between 0 4 and 2 latitudinally corg showed a decrease from the south to the north these results coupled with the sedimentary map see dias gonzalez garcia diaz del rio 2001 have permitted us to establish the detrital mass budget and an inventory of corg burial

storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters bottom sediment transport on the nw iberian shelf was monitored during a downwelling storm in september 2014 collected data was analyzed and fed into a 3d coastal ocean model to understand storm driven sediment transport on the shelf and its impact on mid shelf mud depocenters mdcs a significantly enhanced level of bottom sediment resuspension nearly two orders of magnitude higher than that in the pre storm period was recorded at the mooring site field data analysis reveals that it was induced by a short lasting strong bottom current in combination with enhanced wave current interaction simulation results indicate that this strong current was part of a coastal jet resulted from downwelling an across shelf horizontal density gradient as high as 0 32 g m4 occurred at the interface between the downwelling and the bottom waters forming a remarkable front due to buoyancy effect the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region resulting in a northward directed coastal jet simulation results suggest that during the storm local near bottom sediment suspensions with concentrations on the order of 10 kg m3 would be triggered by wave current interaction and flow convergence associated with the front direct impact on the development of mdcs by transport and deposition of concentrated sediment suspensions is indicated by model results the seaward limit of the front coincided with the shoreward edge of the mdc nucleus suggesting the front as a primary control on the deposition of fine grained sediment this article is protected by copyright all rights reserved

recurrent decadal scale dust events over holocene western africa and their control on canyon turbidite activity mauritania abstract sediment records from continental shelves and slopes might provide paleoenvironmental information in the highest temporal resolution but are often hampered due to strong erosional and reworking processes here we present a holocene sedimentary record from an exceptional shelf mud belt depocenter off northern mauritania compared to a second sediment core located inside a large canyon system at the adjacent continental slope both records are of outstandingly continuous and highest temporal resolution 9 a cm and are investigated by sedimentological and geochemical methods a series of sharply defined recurrent dust peaks is preserved in the shelf archive each event has lasted for a single decade only and seems to coincide with an individual turbidite bed in the canyon a joint mechanism should thus be responsible for both of these deposits and we suggest a regional atmospheric trigger only short lasting trade wind strengthening would cause such pronounced aridity over western saharan africa the effect would be massive dust export to shelf and slope recently developed high resolution aridity and humidity records from western africa support the existence of these events over the holocene and identify them as being controlled by the atlantic system as far south as 19 n

late holocene sediment accumulation on the northern california shelf oceanic fluvial and anthropogenic influences the late holocene sedimentary record of the northern california continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux and provides perspective on the cumulative impacts of major floods and land use change in the drainage basin during historical time piston cores collected on the shelf 50 150 m water depths off the eel and klamath rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k y and dated using 137cs and 14c methods to resolve spatial patterns in sediment accumulation rates sediment accumulation rates averaged over the past several millennia display along and across shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography the highest rates 3 6 mm yr 0 4 0 8 g cm2 yr are associated with structural lows on the middle shelf whereas the lowest rates 0 2 0 6 mm yr 0 03 0 05 g cm2 yr occur at structural highs and at the shelf edge this accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf and with depocenters that form on shorter time scales in response to oceanic flood events the sediment column exhibits an upward fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the holocene transgression sedimentary event beds indicative of major river floods including two probable hyperpycnal discharges of the eel river within the past 1 k y are preserved stratigraphic evidence of accelerated sediment delivery to the shelf during historical time in the form of increased burial of fluvial mud relative to fine grained sand is superimposed on this natural record this change is concordant with the regional history of forest clearing and related land use practices in the watershed after the early 1800s and appears to record the combined impacts of timber harvesting and several extreme floods after ca a d 1950

a multiproxy approach of the holocene evolution of shelf slope circulation on the nw iberian continental shelf abstract textural mineralogical geochemical and microfaunal data benthic foraminifera were studied along the omex core ksgx 40 recovered in the galicia mud deposit of the nw iberian outer continental shelf off the ria de vigo north of spain this core included the records of the last ca 4 8 ka cal bp and consists from the base to the top of a sedimentary sequence exhibiting gradual upward decrease in grain size sediments of this core are mainly siliciclastic largely composed of quartz k feldspars plagioclases and phyllosilicates mica illite kaolinite and chlorite showing a great continental influence in this zone two periods of deposition of finer sediments are registered between 2 2 1 2 ka cal bp and 0 5 0 ka cal bp since the last 2 2 ka bp but mainly during both muddy intervals the galicia mud deposit was nourished with a lower and finer supply of detrital minerals compensated by higher amounts of organic matter as it is suggested by a benthic foraminifera high productivity bfhp proxy processes involved in organic matter degradation by aerobic organisms led to depressed levels of oxygen in the sediments as shown by a benthic foraminiferal oxygen index bfoi peaks of redox sensitive elements like mn fe cu ni cr co zn ni and pb as well as the presence of diagenetic minerals such as pyrite suggest the development of anoxic conditions beneath the sedimentary surface and early diagenetic changes due to high organic matter flux in both muddy intervals two different hydrodynamic regimes were inferred through the analysis of the different proxies textural mineralogical geochemical and benthic foraminifera 1 a strong hydrodynamic regime between 4 8 and 2 2 ka cal bp characterized by the prevalence of winter storms which gave rise to a deep mixed layer on the shelf 2 weak hydrodynamic regime between 2 2 1 2 ka cal bp and 0 5 0 ka cal bp with a high predominance of upwelling and an increase in oceanic stratification

evolution status of the distal mud deposit associated with the pearl river northern south china sea continental shelf abstract the sedimentary characteristics of distal mud deposits a product of along shore transport of river discharged suspended sediment contain information on their different evolutionary stages in the present study the distal mud associated with the pearl river southern china is investigated using the data sets obtained from seabed sediment sample analysis shallow geophysical survey and 210pb dating the results indicate that although the mud deposit in consideration occupies an area of more than 8000 km2 the thickness is small with a young age i e

sediment distribution patterns on the galicia minho continental shelf abstract a sedimentological and bathymetric study of the minho galicia shelf shows a strong contrast between a southern shelf region with a thin partially relict cover of sands and gravel and a northern region where fine grained sediments predominate this contrast is explained through differences in the sediment supply the oceanographic environment storms and ocean currents and the morphology of the shelf which results from its underlying tectonic framework most sediment is supplied to the galicia minho shelf by river discharges onto the minho shelf particularly that from the douro river in the northern part of the shelf the galician rias act as sediment traps rather than sediment suppliers the bulk of the sediment washes out of the rivers during episodic storm events while most of the coarse sediments remain deposited close to the coast the fine grained material is exported to the outer areas of the shelf subsequently coarse sediments close to the coast are transported southwards by the littoral drift whereas the fine grained material is frequently resuspended through the action of the large swells who influence reaches deep into the water this frequent resuspension has a long term sorting effect on the sediments furthermore resuspended sediments on the middle and outer regions of the shelf are transported northwards by a poleward flowing bottom current as a consequence of the differential transport of coarse sediments to the south and of the fine grained sediments to the north the outer reaches of the minho shelf are relatively poor in recent sediments in many areas relict sediments as well as features associated with ancient coastlines and river mouths still appear as seabed features in contrast the northern regions of the shelf are covered by a thin veneer of fine grained material that smooth other most of these fossil features the fine grained sediment fractions mostly very fine sands to coarse silts are deposited in two large mud patches the douro and the galicia mud patches which are situated at water depths of around 100 120 m these two mud patches are both controlled by the local hydrodynamics and morphology the beiral de viana to the west of the douro mud patch is a plateau up to 20 m high lying parallel to the shelf break and is a morphological expression of an underlying horst system this plateau acts as a barrier that prevents the drift of some of the fine grained material to the west out over the shelf break and the continental slope the galicia mud patch is situated on the eastern part of the galician shelf to the north of the douro mud patch it is situated near the extension of the porto tomar fault which results in the shelf being usually steep in this region down to a depth of about 100 m west of this area the slope is much more gentle northward transport of the sediment is strongly reduced by the e w trending outcrops of plutonic and metamorphic rocks

oxygen isotope analyses of co existing carbonate and phosphate in biogenic apatite a way to monitor diagenetic alteration of bone phosphate abstract the oxygen isotopic composition of phosphate 18 op and structural carbonate 18 oc of hydroxylapatite was determined in 31 bone and tooth samples of modern mammals from different countries these two variables are highly correlate r 2 0 98 and the calculated best fit of linear regression is very similar to the equation calculated from the phosphate and carbonate palaeotemperature equations 1 2 according to previous measurements 3 6 on fossils of different ages from different areas it seems quite improbable to find isotopically altered skeletal remains showing a good correlation between 18 op and 18 oc as is the case with modern samples it therefore seems possible at least in some cases to use these measurements for monitoring fossil bone and tooth diagenetic alteration when a set of points lie on the equilibrium line or close to it the 18 o values could be considered close to the original values in contrast when the points lie to the left or to the right of this line this probably means that the values are diagenetically modified due to interaction with meteoric water or 18 o enriched water respectively

modelling of the oxygen isotope evolution of seawater implications for the climate interpretation of the δ18o of marine sediments it is often argued that the 18o value of oceanic water was maintained close to 0 for hundreds of millions of years as a consequence of oxygen isotope exchange between oceanic crust and seawater however for several decades the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because with increasing age a general trend of decreasing 18o values about 6 is observed in most carbonates cherts and phosphates especially for the paleozoic and early mesozoic we developed a dynamical model of seawater crust interaction that computes the 18o value in these two reservoirs as function of time this model takes into account the continuous production of crust at oceanic ridges its expansion rate the permeability profile with space and time the mineralogical mode of the crust and the kinetics of oxygen isotope exchange between rock forming minerals and seawater the model indicates that the 18o value of seawater may vary by 2 with a time response ranging from 5 to 50 ma for expansion rates of 1 to 10 cm a 1 the variation of 2 is fixed by both integrated water rock ratio and closure time of the seawater crust system by sediments variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low 18o values of pre jurassic marine sediments according to our model marine paleotemperatures could be up to 10 c lower than those expected when applying the classical hypothesis of an ice free ocean with a 18o value of 1

deciphering kinetic metabolic and environmental controls on stable isotope fractionations between seawater and the shell of terebratalia transversa brachiopoda this study presents carbon and oxygen stable isotope data obtained from high resolution sampling 500 m of the brachiopod terebratalia transversa from the san juan islands washington state usa the aim of this study is to unravel the respective effects of physiological and environmental controls on the stable isotopic composition of brachiopod shell calcite based on an sem study of the shell a three dimensional sampling was performed in order to investigate carbon and oxygen isotope variations along isochrons and ontogenetic transects the primary as well as the outer part of the secondary shell layer display large and variable isotopic offsets as high as 7 for 13c and 6 for 18o relative to expected equilibrium values the significant positive correlations between 18o and 13c values indicate that the isotopic compositions of these shell domains are mainly controlled by kinetic isotope fractionation effects the ontogenetic 13c and 18o variations can be used to establish a growth curve of the studied specimen whose age is estimated close to 8 years the extent of apparent o 18 disequilibrium observed in this study is about as large as any yet measured in any organism which makes this brachiopod an interesting and important case the carbon and oxygen isotope compositions increase from the outer towards the inner part of the secondary layer where they reach values of 0 0 8 and 0 8 to 0 2 respectively corresponding calculated temperatures are between 12 and 13 8 c and fall in the range of seawater temperatures recorded off the san juan islands 7 13 c these results suggest that the calcite fibres were secreted more and more slowly during the thickening of the shell throughout the animal s life finally approximating isotopic equilibrium this study illustrates that the stable isotopic composition of the modern brachiopod t transversa is predominantly influenced by kinetic fractionation and to a minor degree by metabolic effects except for the innermost part of the shell the measured isotope ratios do not reflect environmental conditions during shell precipitation it remains to be tested by comparable studies whether other modern brachiopod species or even fossil brachiopods reflect comparable fractionation effects these results point out the need for re examination of paleozoic oceanographic conditions

oxygen isotope variations in phosphate of biogenic apatites i fish bone apatite rechecking the rules of the game abstract the major advantage of the oxygen in phosphate isotope paleothermometry is that it is a system which records temperatures with great sensitivity while bone and teeth building organisms are alive and the record is nearly perfectly preserved after death fish from seven water bodies of different temperatures 3 23 c and different 18 o values 16 to 3 of the water were analysed the 18 o values of the analysed po 4 vary from 6 to 25 the system passed the following tests a the temperatures deduced from isotopic analyses of the sequence of fish from lake baikal are in good agreement with the temperatures measured in the thermally stratified lake b the isotopic composition of fish bone phosphate is not influenced by the isotopic composition of the phosphate which is fed to the fish but only by temperature and water composition isotopic analysis of fossil fish in combination with analysis of mammal bones should be a useful tool in deciphering continental paleoclimates

mud depocentres on the continental shelf a neglected sink for anthropogenic contaminants from the coastal zone in this study published and unpublished data from the santos estuarine complex and bay and the adjacent continental shelf sao paulo state brazil were gathered in order to evaluate the entrapment of anthropogenic chemical contaminants hydrocarbons heavy metals in a mid shelf mud depocentre results show that these contaminants produced by industrial activities in the adjacent coastal zone and released into the bay waters are distributed far over the shelf since they are found in the mid shelf mudbelt in locally significant concentrations two main aspects are highlighted by this study the first underlines the fact that the material stored in the mudbelt is not related to a specific fluvial source discharging to the shelf instead the contaminants used as tracers stem from multiple injection sources along the heavily used coastline of the santos industrial zone the second finding suggests that the anthropogenic compounds are not only accumulating in the surface sediments of fine grained shelf depocentres rather these substances are also already found several centimetres below the modern seabed they can thus be easily reinjected into the water column by storms benthic activity and human disturbances such as seabed dredging and bottom trawling

late pleistocene holocene seismic stratigraphy of nha trang shelf central vietnam the late pleistocene holocene stratigraphic architecture on the steep and narrow shelf off nha trang central vietnam has been explored by high resolution seismic profiles integrated with sediment core data sequence stratigraphic results reveal five major seismic units and three bounding surfaces which are composed of two distinctive sequences those sequences are bounded by two regional unconformities sb1 sb2 which have been formed in respond to different sea level regimes the revealed relict beach ridge deposits at water depth of about similar to 130 m below the present water depth indicate that the last glacial lowstand lgm sea level in this area was lower than in neighboring areas and it probably resulted from subsidence due to high sedimentation rate and or neotectonic movements of the east vietnam fault system the late pleistocene high amplitude of sea level change during a long fourthorder and superimposed by shorter fifth order cycle is the principal factor in reorganizing the formation of the nha trang continental shelf sequence other local controlling factors as fluctuations in sediment supply morphological variations of the lgm surface subsidence rate and hydrodynamic conditions provided the distinctive features of the nha trang shelf sequence stratigraphic model in comparison with neighboring other areas c 2014 elsevier ltd all rights reserved

inversion of marine multifrequency electromagnetic profiling data a new approach to resolve surficial sediment stratification s u m m a r y electromagnetic induction emi methods are widely used on land tomap electric conductivity and or magnetic susceptibility distributions of surficial sediments in contrast the application of these methods in marine environments is relatively novel based on the recently developed electromagnetic benthic profiler marum neridis iii we investigate the potential of concentric loop emi methods to recover conductivity and susceptibility of layered marine sediments sensitivity analyses based on a data and model normalized jacobian matrix were performed to compare the influence of conductivity and susceptibility to in phase and quadrature components at different frequencies both parameters substantially affect the em response however the influence of susceptibility decreases more with depth and offers lower depth resolution than that of conductivity a 1 d inversion algorithm to reconstruct vertical conductivity distributions was developed from existing non linear inversion methods using apparent conductivity and apparent susceptibility recovered from simultaneous half space inversion as a priori information this algorithmwas tested on synthetic and real marine emdata from a commercial multifrequency concentric loop emi system gem 3 the results indicate that our inversion algorithm yields meaningful results down to approximately 3 m depth under typical shallowmarine conditions the comparison of inversion results recovered with 1 d and 2 d constraints showed that combining lateral with vertical constraints substantially improves the resolution of the inversion outputs field data from the nw iberian shelf was calibrated according to a processing flow specifically designed for underwater conditions and analysed inversion outputs are in good agreement with ground truthing stratigraphic investigations and deliver relevant clues on past and present sediment dynamics

a late quaternary sedimentary shelf system under hyperarid conditions unravelling climatic oceanographic and sea level controls golfe d arguin mauritania nw africa abstract the sedimentation on the mauritanian shelf off the western saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping shallow acoustic profiling and the analysis of sediment cores coarse grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the atlantic ocean the sheltered golfe d arguin in contrast is characterized by muddy shoals in its inner area and locally restricted and wedge shaped muddy depocenters which extend contour parallel in mid shelf settings in both cases cross shelf currents control sediment distribution and accumulation not location or amount of primary eolian input a major discontinuity on the outer shelf is located in 85 110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface tidal and open shelf deposits this succession has formed during temporary lower sea levels along the last overall regression mis 4 and mis 3 the last glacial maximum and during the past deglacial sea level rise significant sediment deposition and preservation is limited to very restricted areas concerning the overall shelf architecture sea level was the main factor enabling or suppressing deposition in terms of 1 creating local erosional swales which have favored subsequent trapping and preservation of sediments 2 determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions 3 interacting with the seafloor morphology and by this controlling timing of initiation and growth geometry of mud wedges these mud wedges have formed since ca 9 cal ka under a continuous sedimentation regime these bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves in general sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input strong ravinement activity during changing sea levels significant bypassing and winnowing processes and a low preservation potential of the non cohesive materials these conditions lead to the development of almost sediment starved systems with low subsidence rates over longer geological timescales

depositional history of estuarine infill during the last postglacial transgression gulf of cadiz southern spain abstract the late pleistocene and holocene evolution of the estuaries in the gulf of cadiz is interpreted for the first time using drill cores logs trenches and 38 new radiocarbon data and the results compared with the shelf the odiel tinto and guadalete rivers deposited conglomerates during a highstand that did not reach the present sea level dated at ca 25 30 ka isotopic stage is 3 corresponding to a relatively humid period in the area rivers incised these coarse grained deposits during the last main lowstand at ca 18 ka when sea level dropped to 120 m and the coastline lay 14 km seawards from the present the erosional surface is a sequence boundary and the flooding surface of the postglacial eustatic rise overlain by the valley fill deposits of the transgressive and highstand phases of the last fourth and fifth order depositional sequences recognised in the shelf the first marine influence in the estuaries during the transgression occurs at 25 30 m at ca 10 000 years bp according to fossil assemblages the transgressed basins changed from brackish to more open marine as the sea rose until ca 6500 years bp when it reached the maximum flooding and the sandy estuarine barriers ceased to retrograde toward the muddy central basins then the rate of eustatic rise decreased drastically and the estuarine filling followed a two fold pattern governed by the progressive change from vertical accretion to lateral centripetal progradation at ca 4000 years bp the fluvial input surpassed the already negligible rate of rise causing partial emergence of tidal flats and spit barriers in the largely filled estuarine basins prevalence of coastal progradation upon vertical accretion at ca 2400 years bp caused accelerated expansion of tidal flats and rapid growth of the sandy barriers further changes since the 16th century reflect widespread anthropic impacts

postglacial sea level rise and sedimentary response in the guadiana estuary portugal spain border abstract the study of sedimentological features foraminifera and molluscan fauna in samples recovered from four rotary boreholes permitted after 14 c dating the first assessment of the post glacial transgression in guadiana river estuary the holocene sequence is underlain by coarse sediments from delta fan enclosing continental swamp deposits dated 16 980 year bp the holocene sedimentary sequence was deposited either directly on this delta fan gravels or on top of fluvial sands deposited probably between 11 000 and 10 000 yr bp an accelerated phase of the estuary infilling by clayey sediments containing the salt marsh foraminifer trochammina began ca 9800 year bp when sea level was about 39 m below present between 7500 and 7000 year bp the central part of the estuary started to accommodate coarser sediments partially introduced from the continental shelf the first phase of holocene sea level rise at a rate of 0 85 m century terminated ca 6500 year bp since then lagoonal sediments in the vicinity of the estuary have been enclosed behind sand spits and predominantly sandy sedimentation was initiated within the estuary after a second phase of slower rise at the rate of 0 3 m century which lasted until ca 5000 year bp the sea approached the present level

sedimentary architecture of a low accumulation shelf since the late pleistocene nw iberia abstract continental shelves represent areas of highest economical and ecological importance nevertheless these sedimentary systems remain poorly understood due to a complex interplay of various factors and processes which results in highly individual construction schemes previous studies of sedimentary shelf systems have mainly focused on a limited number of cores retrieved from holocene fine grained depocentres as such the relation between shelf architecture and sedimentary history remains largely obscure here we present new data from the nw iberian shelf comprising shallow seismic profiles a large number of sediment cores and an extended set of radiocarbon dates to reveal the late quaternary evolution of a low accumulation shelf system in detail on the nw iberian shelf three main seismic units are identified these overly a prominent erosional unconformity on top of the basement the lowermost unit 1 is composed of maximal 75 m thick late tertiary to pleistocene deposits the youngest sediments of this unit are related to the last glacial sea level fall unit 2 was controlled by the deglacial sea level rise and shows a maximum thickness of 15 m finally unit 3 comprises deposits related to the late stage of sea level rise and the modern sea level highstand with a thickness of 4 m in mid shelf position two pronounced seismic reflectors separate these main units from each other their origin is related to 1 exposure and ravinement processes during lower sea level and 2 to reworking and re deposition of coarse sediments during subsequent sea level rise according to the sediment core ground truthing sediments of the late tertiary to pleistocene unit predominantly display homogenous fine sands with exceptional occurrences of palaeosols that indicate an ancient exposure surface fine sands which were deposited in the run of the last sea level rise show a time transgressive retrogradational development the seismic reflectors bounding the individual units appear in the cores as 0 1 to 1 m thick deposits consisting either of shell gravels or siliceous coarse sands with gravels the modern sea level highstand stage is characterised by zonal deposition of mud forming a mud belt in mid shelf position and sediment starvation on outer shelf zones radiocarbon ages indicate that this mud belt was the main depocentre for river supplied fine material on the nw iberian shelf at least over the past 5 32 ka bp the initial onset of this depocentre is proposed to be related to a shift in the balance between rate of sea level rise and amount of terrigenous sediment supply various other stratigraphical shelf reconstructions reveal analogies in architecture which indicate that timing and shaping of the individual units on low accumulation shelves is fundamentally controlled by eustatic sea level changes other factors of local importance such as differential elevation of the basement and the presence of morphological barriers formed by rocky outcrops on the seafloor have additionally modifying influence on the sedimentary processes

variations in sediment texture on the northern monterey bay national marine sanctuary continental shelf abstract the storm protected continental shelf of monterey bay part of the monterey bay national marine sanctuary north central california is subject to abundant episodic sediment input from fluvial sources north of monterey bay conditions of reduced sediment supply combined with the exposed nature of the shelf provide an effective laboratory for studying the contrasting effects of storm versus fluvial dominated conditions on modern sedimentation textural analyses performed on surface sediment samples collected from more than 380 box cores and multicores document the existence of a clearly defined mud belt occupying the mid shelf throughout the region inshore sands combined with these mid shelf muds represent deposits from modern sedimentation processes in monterey bay where episodic fluvial input from winter storms dominates sedimentation the mid shelf mud belt extends across the shelf to the shelf break north of monterey bay where sediment loads are reduced and both oceanographic and storm processes dominate the mid shelf mud belt is bordered by relict sediments occupying the outer shelf in the study area mass accumulation rates established by radiochemical studies support the contention that storm induced along shelf processes result in northward transport of sediment within the mud belt the continuity of transport however is interrupted by topographic highs which are barriers or inhibitors to sediment transport created by wrench style tectonics associated with the san andreas fault system

geochemical tracers of northern portuguese estuarine sediments on the shelf the northern portuguese coastal zone is drained by large iberian rivers along which there is intensive industrial agricultural and urban activity offshore of the two main river basins of the douro and minho two fine grained sedimentary formations can be characterised a geochemical study of this sediment cover has been performed focussing in particular on the fine deposits and the adjacent estuaries the chemical composition of superficial sediments collected from the northern portuguese shelf and from the river basins of the minho and douro rivers was determined for major minor and trace elements the rare earth elements ree composition was also determined in selected samples statistical analyses of these data for the shelf sediments allow us to identify elements with similar behaviours and the different sediment types along the shelf and to evaluate of their origins downcore profiles of the chemical composition of the fine sediments reveal no evidence for changes induced by recent human activities to assess the influence of the rivers on the fine sedimentary formations the elemental distribution patterns of selected sediments have been compared with the estuarine sediments lithogenic elements composition normalised to al and the shale normalised ree distributions showed there are similarities between the douro river sediments and the sediment composition of the douro and galicia mud patches however the galicia mud field showed no relationship in its chemical composition to sediments derived from the minho river nor was any evidence for heavy metal contamination originating from the douro estuary detected in anthropogenic element al ratios in the shelf sediments this may be because of processes associated with resuspension complexation and bioturbation that occur during sediment transport and deposition nevertheless the douro river seems to be the main continental source of fine sediments being deposited on the northern portuguese shelf

cross shore variation of wind driven flows on the inner shelf in long bay south carolina united states and exceeded 0 1 n m 2 vertical mixing occurred the two layered flow pattern disappeared and currents were directed alongshore with the wind at all sites and throughout the water column in the fall near bed flows close to the shore water depth 7 m were often reduced compared to or opposed those measured farther offshore under southwestward winds a simplified analysis of the depth averaged alongshore momentum balance illustrates that the alongshore pressure gradient approached or exceeded the magnitude of the alongshore wind stress at the same time that the nearshore alongshore current opposed the wind stress and alongshore currents farther offshore in addition the analysis suggests that the wind stress is reduced closer to shore so that the alongshore pressure gradient is large enough to drive the flow against the wind

mapping the magnetic susceptibility and electric conductivity of marine surficial sediments by benthic em profiling distribution accumulation and diagenesis of surficial sediments in coastal and continental shelf systems follow complex chains of localized processes and form deposits of great spatial variability given the environmental and economic relevance of ocean margins there is growing need for innovative geophysical exploration methods to characterize seafloor sediments by more than acoustic properties a newly conceptualized benthic profiling and data processing approach based on controlled source electromagnetic csem imaging permits to coevally quantify the magnetic susceptibility and the electric conductivity of shallow marine deposits the two physical properties differ fundamentally insofar as magnetic susceptibility mostly assesses solid particle characteristics such as terrigenous or iron mineral content redox state and contamination level while electric conductivity primarily relates to the fluid filled pore space and detects salinity porosity and grain size variations we develop and validate a layered half space inversion algorithm for submarine multifrequency csem with concentric sensor configuration guided by results of modeling we modified a commercial land csem sensor for submarine application which was mounted into a nonconductive and nonmagnetic bottom towed sled this benthic em profiler neridis ii achieves 25 soundings second at 3 4 knots over continuous profiles of up to a hundred kilometers magnetic susceptibility is determined from the 75 hz in phase response 90 signal originates from the top 50 cm while electric conductivity is derived from the 5 khz out of phase quadrature component 90 signal from the top 92 cm exemplary survey data from the north west iberian margin underline the excellent sensitivity functionality and robustness of the system in littoral 0 50 m and neritic 50 300 m environments susceptibility versus porosity crossplots successfully identify known lithofacies units and their transitions all presently available data indicate an eminent potential of csem profiling for assessing the complex distribution of shallow marine surficial sediments and for revealing climatic hydrodynamic diagenetic and anthropogenic factors governing their formation

sources of organic carbon in the portuguese continental shelf sediments during the holocene period organic c oc and total n tn concentrations and stable isotope ratios 13 c in muddy deposit sediments of the northern and southern portuguese continental shelf were used to identify sources of fine sized organic matter 63 m during the holocene period sedimentary columns off the guadiana core crida 05 tagus core md 992332 and douro core ksgx 57 estuaries are characterised by elemental and isotopic values that reflect distinct sources of organic matter oc tn and 13 c ranging respectively from 8 5 to 21 and from 22 4 to 27 intense supplies to the guadiana continental shelf of fine terrigenous particles during the younger dryas event are closely linked with higher oc tn values and lower 13 c ratios during the postglacial transgression phase an increasing contribution of marine supplies up to 80 occurred higher 13 c up to 22 4 values and low oc tn ratios down to 8 5 are found as the sea level approaches the current one the upper holocene records emphasize the return to enhanced terrestrial supplies except for the little climatic optimum between the 11th and 15th centuries ad this climatic event is especially obvious in the three cores as a return to marine production and a decrease in terrestrial sediment supply to the continental shelf the return to a cooling event the little ice age between the 15th and 19th centuries ad is mirrored by decreased terrigenous supplies in core ksgx 57 gradually increasing sedimentation in estuaries as well as formation of coastal dune fields have been hypothesized on the basis of increasing 13 c and decreasing oc tn and oc tn values

recent and modern marine erosion on the new jersey outer shelf abstract recent chirp seismic reflection data combined with multibeam bathymetry backscatter and analysis of grab samples and short cores provide evidence of significant recent erosion on the outer new jersey shelf the timing of erosion is constrained by two factors 1 truncation at the seafloor of what is interpreted to be the transgressive ravinement surface at the base of the surficial sand sheet and 2 truncation of apparently moribund sand ridges along erosional swales oriented parallel to the primary direction of modern bottom flow and oblique to the strike of the sand ridges these observations place the erosion in a marine setting post dating the passage of the shoreface ravinement and the evolution of sand ridges that form initially in the near shore environment also truncated by marine erosion are shallowly buried fluvial channel systems formed during the last glacial maximum and filled during the transgression and a regional reflector r that is 40 kyr depths of erosion range from a few meters to 10 m the seafloor within eroded areas is often marked by ribbon morphology seen primarily in the backscatter data as areas of alternating high and low backscatter elongated in the direction of primary bottom flow ribbons are more occasionally observed in the bathymetry where observed crests exhibit low backscatter and troughs exhibit high backscatter sampling reveals that the high backscatter areas of the ribbons consist of a trimodal admixture of mud sand and shell hash with a bimodal distribution of abraded and unabraded sand grains and microfauna the shell hash is interpreted to be an erosional lag while the muds and unabraded grains are in this non depositional environment evidence of recent erosion at the seafloor of previously undisturbed strata the lower backscatter areas of the ribbon morphology were found to be a well sorted medium sand unit only a few 10 s of cm thick overlying the shelly muddy sandy material concentrations of well rounded gravels and cobbles were also found in eroded areas with very high backscatter and at least one of these appears to be derived from the base of an eroded fluvial channel seafloor reworking over the transgressive evolution of the shelf appears to have switched from sand ridge evolution which is documented to 40 m water depth to more strictly erosional modification at greater water depths we suggest that this change may be related to the reduction with water depth in the effectiveness of sediment resuspension by waves resuspension is a critical factor in the grain size sorting during transport by bottom currents over large bedforms like sand ridges otherwise we speculate displacement of sand by unidirectional currents will erode the seafloor

oxygen isotope variations in phosphate of biogenic apatites iii conodonts abstract the method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite down to 10 mg this extension enables one to analyze hand picked conodont samples with an analytical reproducibility better than 0 5 1 46 samples of conodonts ranging from the ordovician to the pennsylvanian of north america were analysed some insoluble phosphatic residues ichthyoliths and inarticulate brachiopods of the same time range were also measured the range of the 18o values of the analysed conodonts is between 15 and 19 it shows a general trend of decreasing d18o with increasing age from an average value of about 19 in the pennsylvanian to 17 in the ordovician this trend parallels that previously detected in marine phosphorites for the time range between the devonian and pennsylvanian our data are in agreement with independent paleoclimatic information specifically we detect maximum18o enrichment at the end of the pennsylvanian and minimum enrichment at the end of the devonian the difference between these two extremes is equivalent to about 10 c from about 40 to 30 c assuming constant isotopic composition of ocean water the success of oxygen isotopic analysis of conodonts raises the possibility of their use in paleozoic paleo oceanography in a similar way to foraminifera in the mesozoic cenozoic

determination of oxygen isotope fractionation between water and phosphate from living lingulids potential application to palaeoenvironmental studies abstract lingulids are inarticulate brachiopods with phosphatic shells which live exclusively in marine waters their distribution is generally restricted to the continental shelf within the 40 belt from temperate to equatorial areas they show a range of morphological physiological and behavioral features that have remained remarkably constant since the cambrian the oxygen isotope fractionation between water and phosphate from living lingulids was determined as a tool to investigate the temperature and or salinity of past coastal seawater oxygen isotope measurements were performed according to the silver phosphate method on a collection of lingulids coming from well known and varied marine environments a global 18o variation of 3 5 was found for these lingulids which proves their good sensitivity to environmental factors namely the oxygen isotope composition and the temperature of seawater oceanographic data provided average temperature and salinity values in the living sites of the studied specimens least squares treatment of the data gave the following fractionation equation t c 112 2 15 3 4 20 0 71 18o po4 18o h2o which is similar to the equations determined by longinelli and nuti 1973a and kolodny et al 1983 for other phosphate bearing invertebrates and fishes all these results suggest that a unique equation describes the phosphate water fractionation of many marine organisms except for mammals

clay minerals from the sedimentary cover from the northwest iberian shelf abstract the northern iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime waves and tides and receiving inputs of continental sediments via riverine discharges the principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system the distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport the mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments those deposits that are directly influenced by riverine discharges have higher contents of kaolinite 20 whereas those that are not have higher contents of illite 80 the available data indicate no significant quantities of terrigenous particles are being discharged from the spanish rias therefore we conclude that physical processes are controlling the clay mineral distributions and that despite contributions from the minho river the main source of fine detrital particles to the shelf region is the douro river discharge these particles settle on the middle shelf below the 60 m isobath during storm events these particles are re suspended and advected northwards to the galician shelf or into deeper domains thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off shelf

the temperatures and oxygen isotope composition of early devonian oceans the oxygen isotope composition of pre carboniferous 360 myr old marine carbonates cherts and phosphates has been reported1 8 to be lower than that of later post devonian samples according to one explanation1 3 this is a reflection of warmer generally 40 c pre carboniferous oceans relative to modern oceans alternatively it has been proposed5 7 that the pre carboniferous oceans were depleted in 18o by 2 smow relative to modern oceans here i report high 18o values 1 9 to 2 9 pdb in normal shallow marine limestones and three brachiopod shells from the lower devonian haragan and bois d arc formations in south central oklahoma i interpret these values as near primary and therefore constraining the temperature and oxygen isotope composition of early devonian sea water to 25 7 c and 0 1 smow respectively in conjunction with similarly high 18o values obtained from older ordovician and silurian samples8 these results imply that the temperature and oxygen isotope composition of pre carboniferous oceans may at least during some time intervals have been similar to those of modern oceans

natural and human induced holocene paleoenvironmental changes on the guadiana shelf northern gulf of cadiz three contrasting sedimentary environments on the continental shelf off the guadiana river northern gulf of cadiz were integrated in a chronological framework and analysed in terms of sedimentology and benthic foraminiferal assemblages to understand the holocene paleoenvironmental evolution the analysed environments differ in terms of their depositional regimes and benthic foraminiferal assemblages however a dominant fluvial origin of the sand fraction was observed in all three environments holocene sedimentary processes were mainly controlled by natural sea level changes and climate variations and human induced processes e g deforestation agriculture along four evolutionary stages the three older stages were mainly influenced by natural processes such as sea level variations and fluvial inputs whereas the most recent stage reflects a combination of climatic and human induced processes a deepening of sedimentary environments related to a period of rapid sea level rise strongly influenced by river discharges occurred from c 11 500 to c 10 000 cal yr bp a reduction in sediment export to the shelf as a result of the continuous and rapid sea level rise and enhanced estuary infilling reflects the second stage from c 10 000 to c 5000 cal yr bp the beginning of the third stage from c 5000 to c 1500 1000 cal yr bp is marked by a sea level slowdown and the relatively stable climate and environmental conditions the fourth stage from c 1500 1000 cal yr bp to recent times reflects the intensification of human induced processes and climatic variability in the guadiana river basin this stage also reflects modern depositional conditions with the formation of a proximal prodeltaic wedge and a distal muddy body

no vital effect on δ18o and δ13c values of fossil brachiopod shells middle devonian of china biogenic fractionation vital effect of carbon and oxygen isotopes during metabolism is ubiquitous among marine organisms whether there is vital effect on the isotopic compositions of fossil brachiopods represents one of the most controversial issues in the carbonate stable isotope geochemistry furthermore the problem involves not only the feasibility of the utilization of fossil brachiopod shells as analysis samples for obtaining the original carbon and oxygen isotopic compositions of carbonates but also the credibility of the data and conclusions already achieved in the study field with fossil brachiopods from middle devonian of china this study investigates the vital effect by comparisons of 18o and 13c between the fossils of the same original living environment and the shell s preservation at levels of species genus and family no systematic differences of 18o and 13c between fossils were found which led to the conclusion that no detectable vital effect occurred in the fossil brachiopod shells the argument that the fossil brachiopod possibly has biogenic fractionation with the stable isotopes of carbon and oxygen resulted likely from studies of fossils that are different in the original living environment or the state of preservation

holocene evolution of mud depocentres on a high energy low accumulation shelf nw iberia the high energy low accumulation nw iberian shelf features three confined holocene mud depocentres here we show that the evolution of such depocentres follows successive steps the flooding of inner shelf zones and river catchment areas by the late deglacial sea level rise provided the precondition for shelf mud deposition following this the holocene deceleration of the sea level rise caused a rapid refill of the accommodation space within river valleys subsequently the export of major amounts of fines was initiated the initial onset and loci of shelf mud deposition were related to deposition favouring conditions in mid shelf position or to the presence of morphological highs which act as sediment traps by providing protection against stronger hydrodynamic energy the detailed reconstruction of the holocene depocentre evolution shows for the first time that the expansion of such shelf mud deposits cannot only occur by linear growth off the associated sediment source rather they might develop around centres that are fully disconnected from the source of original sediment supply and expand later into specific directions based on these differences and on the connection of the individual mud depocentres to the material source we propose a conceptual subdivision of the group mid shelf mud depocentres

distal mud deposits associated with the pearl river over the northwestern continental shelf of the south china sea abstract continental shelf mud deposits represent an important archive of the holocene earth history such deposits are usually associated with large rivers that transport a large quantity of suspended sediment the pearl river has formed a distal shore parallel holocene mud deposit on the northwestern continental shelf of the south china sea investigations into its spatial distribution and evolutionary history are necessary for an improved understanding of such sedimentary systems seabed sediment sampling in situ tidal cycle measurements seismic survey and the associated data analyses reveal that the distal mud deposit associated with the pearl river occupies an area of approximately 8 87 10 3 km 2 with a modern deposition rate of 1 27 to 5 66 mm yr 1 evidence from sediment balance environmentally sensitive components sediment transport processes and regional shelf circulation patterns indicates that the pearl river is a major sediment source for the mud deposit this distal mud deposit represents an initial stage of its development the fining upward pattern and the negative skewness values in the layer are indicative of the pearl river influence and the 210 pb age is younger than 10 2 yr the main body of the holocene deposit of the region is not related directly to the modern pearl river sediment for most of the holocene periods the pearl river has been trapping the sediment to fill its extensive estuarine waters and therefore the formation of the distal mud is a recent event a regional characteristic is that the timing of the formation of distal muds associated with large rivers depends on the estuarine morphological evolution and the quantity of sediment discharge

oxygen isotope evolution of biogenic calcite and apatite during the middle and late devonian oxygen isotope ratios of well preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the middle and late devonian by assuming an oxygen isotopic composition of 1 v smow for devonian seawater the oxygen isotope values of eifelian and early givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 c late givetian and frasnian palaeotemperatures calculated from 18o values of conodont apatite are close to 25 c in the early frasnian and increase to 32 c in the latest frasnian and early famennian oxygen isotope ratios of late givetian and frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite 18o values and give unrealistically warm temperatures ranging from 30 to 40 c diagenetic recrystallization of shell calcite different habitats of conodonts and brachiopods as well as non equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the 18o depletion of brachiopod calcite moreover the 18o depletion of brachiopod calcite with respect to equilibrium 18o values calculated from conodont apatite is too large to be explained by a change in seawater ph that might have influenced the oxygen isotopic composition of brachiopod calcite the realistic palaeotemperatures derived from 18o apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of palaeozoic oceans more faithfully than brachiopod calcite and do not support the hypothesis that the 18o 16o ratio of devonian seawater was significantly different from that of the modern ocean

origin of inner shelf mud deposit in the southeastern yellow sea huksan mud belt abstract a variety of previous sedimentological results together with newly obtained hydrodynamic data are summarized and reinterpreted with respect to the most likely origin of the huksan mud belt hmb in the southeastern yellow sea the hmb consists of two mud units that have similar grain texture and clay mineralogy but can be differentiated on seismic profiles because the lower unit is generally harder than the watery upper unit the hydrographic measurements together with satellite imagery clearly indicate that the suspended plume covering the hmb originates from the west coast of korea the suspended plume becomes enhanced by winter monsoon winds beginning in late fall most of the studies of clay mineralogy geochemistry and isotherm distribution also suggest that the hmb has been derived from the west coast of korea particularly from the keum river an approximate evaluation of the depositional time for the hmb based on its dimensions and the discharge rate of the keum river suggests a time duration of the order of 14 000 years hydrodynamic measurements across two transects over the northernmost hmb reveal that suspended matter travels southwest toward the main body of the hmb in a well constrained corridor ultimately connected with the keum river time series 12 5 h hydrodynamic measurements at a site over the hmb suggest that the winter season suspended plume carries concentrations of suspended matter one to two orders of magnitude higher than those of the summer silt grains of 5 6 well sorted through numerous resuspension events settle out onto the hmb forming distinctive silt laminae alternating with clay rich laminae

δ18o and δ13c values of modern brachiopod shells abstract researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater isotopic variability at the intraspecimen and intertaxon levels has not been examined without such data for modern brachiopods similar data from ancient brachiopods cannot be accurately interpreted in this study a survey is made of 18 o and 13 c values of terebratulid rhynchonellid thecideidine and craniacean brachiopods from antarctica the bay of fundy curacao japan new zealand norway puget sound palau sicily and south africa this suite of samples provides a wide range of taxonomic levels temperatures salinities and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite new data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater calcite from the primary layer and specialized shell structures hinge brachidium foramen interarea muscle scars are depleted in both 18 o and 13 c a characteristic of biological fractionation or vital effects often found in other calcerous marine organisms our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods secondary layer calcite the material most often analyzed in ancient brachiopods has higher 18 o and 13 c values which approach and sometimes correspond with predicted equilibrium values therefore secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans these data come with caveats large intraspecimen variability in the 18 o values of secondary layer calcite 1 in some samples limits the use of brachiopods as precise indicators of the 18 o value or temperature of ancient seawater the 18 o and 13 c values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures indicating a measurable vital effect in most cases measured 18 o values are lower than the calculated equilibrium values dictated by seasonal variations in the 18 o value and temperature of seawater ontogenetic variations in the 13 c values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data modern brachiopods occur in a wide range of seawater salinities and temperatures therefore the relation between salinity and the 18 o value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer 18 o values errors of up to 15 c are found if ambient seawater is assumed to have a constant 18 o value e g 0 smow and the 18 o values of secondary layer calcite are used to calculate temperature coupled with vital effects and intraspecimen variability these errors can obscure the resolution of secular variations in the rock record

environmental and physiological influences on isotopic and elemental compositions of brachiopod shell calcite implications for the isotopic evolution of paleozoic oceans abstract brachiopods from the demissa bed middle devonian hamilton group biologically regulated the incorporation of mg sr and na into their shell calcite significant taxonomic differences in the elemental contents of three species athyris spiriferoides mediospirifer audacula and mucrospirifer mucronatus may be related to differences in calcification processes in contrast no significant difference or vital effects were observed between the isotopic values of mediospirifer audacula and athyris spiriferoides from erie 18o 0 03 p 0 949 13c 0 76 p 0 083 and genessee counties 18o 0 39 p 0 471 13c 0 06 p 0 854 this suggests that these brachiopods did not exert a biological control over their isotopic compositions and that their shell calcites reflect ambient physicochemical conditions isotopic compositions in unaltered shell calcites of brachiopods from genessee county which was close to the basin depocentre are heavy for carbon 13c x 5 01 pdb and oxygen 18o x 2 85 pdb compared to the species sampled at the basin s edge erie county 13c x 2 79 pdb 18o x 3 83 pdb there is a significant separation in isotopic values between the deeper and shallower water brachiopods of the basin 18o 0 98 p 0 0005 13c 2 22 p 0 0005 the 18o variation suggests a temperature salinity change with water depth whereas the change in 13c composition probably records an enrichment depletion of organic matter with water depth this observation has significant implications for paleozoic ocean isotopic evolution studies because many global changes in marine 18o and 13c are based on isotopic shifts of similar magnitude