calibration of modern sedimentary δ2h plant wax water relationships in greenland lakes abstract sedimentary plant wax distributions and isotopic compositions are powerful widely applied paleoenvironmental proxies however there is conflicting evidence on the behavior of these proxies at high latitude sites where extreme climate and light conditions may uniquely influence plant physiology and growth here we present modern sedimentary n alkane and n alkanoic acid abundances and compound specific 2h and 13c isotope values from a 22 lake transect extending from northwest to southernmost greenland covering a large latitudinal and climatic gradient sedimentary plant waxes are similar in abundance and carbon isotopic composition across the transect suggesting no major differences in biologic sources there are strong correlations r 0 8 0 9 between 2h values of many long chain sedimentary waxes and those of modelled precipitation with n alkanes more tightly correlated to precipitation than n alkanoic acids data presented here also demonstrate that 2h values of mid chain sedimentary waxes do not strongly correlate to the 2h values of lake water when it decouples isotopically from precipitation i e in glacier fed and evaporatively enriched lakes this calls into question the common interpretation that mid chain sedimentary waxes can be ascribed to aquatic plants we contextualize our greenland data with an updated global dataset of 2h values of modern sedimentary waxes and precipitation this update adds 100 lakes from recently published literature to the seminal review presented by sachse et al 2012 this large new compilation suggests a global average apparent fractionation including arctic data between n c28 alkanoic acids and annual precipitation ec28 ann of 99 and between n c29 alkanes and annual precipitation ec29 ann of 121 the latter value is remarkably consistent with the value first reported by sachse et al 2012

deconstructing atlantic intertropical convergence zone variability influence of the local cross equatorial sea surface temperature gradient and remote forcing from the eastern equatorial pacific 1 we investigate causes of interannual variability in atlantic intertropical convergence zone itcz convection using a monthly mean global precipitation data set spanning 1979 1999 starting from the hypothesis of two dominant influences on the itcz namely the cross equatorial gradient in tropical atlantic sea surface temperature sst and the anomalous walker circulation due to the rearrangement of tropical pacific convection associated with the el nino southern oscillation we analyze anomaly composites over the 1979 1999 period that best isolate the effects of each mechanism our results suggest that to first order a strong anomalous walker circulation suppresses precipitation over the tropical atlantic whereas an anomalous warm north cool south sst gradient shifts the meridional location of maximum itcz convection anomalously north we examined the processes underlying each of the two mechanisms for the anomalous walker circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial pacific for the sst gradient mechanism our results confirm previous studies that link convection to cross equatorial winds forced by meridional sst gradients we find that positive surface flux feedback brought about through the cross equatorial winds is weak and confined to the deep tropics on the basis of the results of this and other studies we propose an expanded physical picture that explains key features of atlantic itcz variability including its seasonal preference its sensitivity to small anomalous sst gradients and its role in the context of tropical atlantic sst gradient variability index terms 4215 oceanography general climate and interannual variability 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3354 meteorology and atmospheric dynamics precipitation 1854 3374 meteorology and atmospheric dynamics tropical meteorology 4522 oceanography physical ocean optics keywords tropical atlantic precipitation climate variability el nino southern oscillation ocean atmosphere interaction

an abrupt weakening of the subpolar gyre as trigger of little ice age type episodes we investigate the mechanism of a decadal scale weakening shift in the strength of the subpolar gyre spg that is found in one among three last millennium simulations with a state of the art earth system model the spg shift triggers multicentennial anomalies in the north atlantic climate driven by long lasting internal feedbacks relating anomalous oceanic and atmospheric circulation sea ice extent and upper ocean salinity in the labrador sea yet changes throughout or after the shift are not associated with a persistent weakening of the atlantic meridional overturning circulation or shifts in the north atlantic oscillation the anomalous climate state of the north atlantic simulated after the shift agrees well with climate reconstructions from within the area which describe a transition between a stronger and weaker spg during the relatively warm medieval climate and the cold little ice age respectively however model and data differ in the timing of the onset the simulated spg shift is caused by a rapid increase in the freshwater export from the arctic and associated freshening in the upper labrador sea such freshwater anomaly relates to prominent thickening of the arctic sea ice following the cluster of relatively small magnitude volcanic eruptions by 1600 ce sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events a necessary causal link between the volcanic cluster and the spg shift can therefore be excluded instead preconditioning by internal variability explains discrepancies in the timing between the simulated spg shift and the reconstructed estimates for the little ice age onset

biomarkers as proxies for plant inputs to peats an example from a sub boreal ombrotrophic bog lipid distributions in modern peat forming plants were determined and compared to biomarkers recovered from a ca 2800 year old peat deposited in a dutch ombrotrophic bog the peat section spans a well constrained shift in local hydrology and vegetation and potential molecular proxies for plant inputs were directly tested against the macrofossil record based on the analyses of modern plants low molecular weight lmw n alkanes c23 c25 and high molecular weight n alkanes c33 are proposed as potential biomarkers for sphagnum and non sphagnum inputs respectively consistent with this in the bargerveen peat core lmw n alkane abundances increase and hmw n alkane abundances decrease coincident with the shift to sphagnum dominated peat additional potential proxies include the abundances of c22 alkanedioic acid and phytenyl phytenoate as sphagnum indicators and the abundances of the triterpenoids taraxer 14 ene and taraxast 20 ene as biomarkers for ericaceae rootlets

florida straits density structure and transport over the last 8000 years 1 the density structure across the florida straits is reconstructed for the last 8000 years from oxygen isotope measurements on foraminifera in sediment cores the oxygen isotope measurements suggest that the density contrast across the florida current increased over this time period the magnitude of this change corresponds to an increase in the geostrophic transport referenced to 800 m water depth of 4 sverdrups sv over the last 8000 years the spatial and seasonal distribution of incoming solar radiation due to changes in the earth s orbit has caused systematic changes in the atmospheric circulation including a southward migration of the intertropical convergence zone over the last 8000 years these changes in atmospheric circulation and the associated wind driven currents of the upper ocean could readily account for a 4 sv increase in the strength of the florida current we see no evidence in our data for dramatic changes in the strength of the atlantic meridional overturning circulation over this time period

comparison of observed hf radar and adcp measurements and computed tides in the north channel of the irish sea abstract a three dimensional high resolution grid of order 1 km model of the north channel of the irish sea incorporating a one equation turbulence energy submodel to parameterize vertical mixing is used to compute the m2 s2 n2 k1 and o1 tides elevations and currents are compared with observations with particular emphasis on a detailed comparison with current profiles recorded by two acoustic doppler current profilers adcps and hf radar measurements of the surface current the comparison with the hf radar shows small scale spatial variations in both modeled and observed currents superimposed upon the larger scale tidal currents these small scale changes appear to be associated with variations in bottom topography and can only be resolved by using fine grid models or detailed measurements computed m2 current profiles derived from a multiconstituent calculation are in excellent agreement with profiles measured by the adcps however in a single constituent m2 calculation the magnitude of the

current measurement technology for near shore waters abstract this paper summarises recent experiences in scawvex and elsewhere in measuring near shore currents together with subsequent techniques of data analyses in situ instrumentation described includes mechanical rotors electromagnetic and adcp acoustic doppler current profiler deployed on sea bed frames in towed mode and mooring lines surface observations from the ocean surface current radar oscr h f radar system are also included deployments are in estuaries along open coasts and in tidal straits all involving strongly tidal regimes around the uk coasts the characteristics including specific capabilities and limitations of each instrument are listed synoptic comparisons are used to illustrate those components such as the depth mean principal tidal constituents which show good agreement and to highlight other components e g vertical profiles of residuals most sensitive to the system adopted a coordinate system suitable for analysing net fluxes in shallow estuary with large tides is described associated aspects which impact on planning current meter deployments are outlined

coupled north atlantic slope water forcing on gulf of maine temperatures over the past millennium to investigate ocean variability during the last millennium in the western gulf of maine gom we collected a 142 year old living bivalve arctica islandica l in 2004 and three fossil a islandica shells calibrated 14cams 1030 78 ad 1320 45 ad 1357 40 ad for stable isotope and growth increment analysis a statistically significant relationship exists between modern gom temperature records shell isotope derived 30 m r 0 79 p 0 007 prince 5 50 m r 0 72 p 0 019 boothbay harbor sst r 0 76 p 0 011 and labrador current lc transport data from the eastern newfoundland slope during 1993 2003 in all cases as lc transport increased gom water temperatures decreased the following year decadal trends in the north atlantic oscillation nao and the atlantic multidecadal oscillation amo influence gom water temperatures in the most recent period with water temperatures decreasing during nao and amo negative modes most likely linked to lc transport and gulf stream interaction mean shell derived isotopic changes 18oc during the last 1 000 years were 0 47 and likely reflect a 1 2 c cooling from 1000 ad to present based on these results we suggest that observed cooling in the gom during the last millennium was due to increased transport and or cooling of the lc and decreased gulf stream influence on the gom

transport potential vorticity and current temperature structure across northwest providence and santaren channels and the florida current off cay sal bank currents and temperatures were measured using pegasus current profilers across northwest providence and santaren channels and across the florida current off cay sal bank during four cruises from november 1990 to september 1991 on average northwest providence 1 2 sv and santaren 1 8 sv contribute about 3 sv to the total florida current transport farther north e g 27 n partitioning of transport into temperature layers shows that about one half of this transport is of 18 c water 17 c 19 5 c this can account for all of the excess 18 c water observed in previous experiments this excess is thought to be injected into the 18 c layer in its region of formation in the northwestern north atlantic ocean due to its large thickness potential vorticities in this layer in its area of formation are very low in our data lowest potential vorticities in this layer are found on the northern end of northwest providence channel and are comparable to those observed on the eastern side of the florida current at 27 n on average a low potential vorticity 18 c layer was not found in the florida current off cay sal bank

climate change on the yucatan peninsula during the little ice age we studied a 5 1 m sediment core from aguada x caamal 20 36 6 n 89 42 9 w a small sinkhole lake in northwest yucatan mexico between 1400 and 1500 a d oxygen isotope ratios of ostracod and gastropod carbonate increased by an average of 2 2 and the benthic foraminifer ammonia beccarii parkinsoniana appeared in the sediment profile indicating a hydrologic change that included increased lake water salinity pollen from a core in nearby cenote san jose chulchaca showed a decrease in mesic forest taxa during the same period oxygen isotopes of shell carbonate in sediment cores from lakes chichancanab 19 53 0 n 88 46 0 w and salpeten 16 58 6 n 89 40 5 w to the south also increased in the mid 15th century but less so than in aguada x caamal climate change in the 15th century is also supported by historical accounts of cold and famine described in maya and aztec chronicles we conclude that climate became drier on the yucatan peninsula in the 15th century a d near the onset of the little ice age lia comparison of results from the yucatan peninsula with other circum caribbean paleoclimate records indicates a coherent climate response for this region at the beginning of the lia at that time sea surface temperatures cooled and aridity in the circum caribbean region increased

dynamically constrained projection for subsurface current velocity 1 an approach is described that projects surface current observations downward to obtain subsurface current structure consistent with the interior current dynamics the projection approach is unrestricted by water depth however this study emphasizes well mixed constant density flow in water depths appropriate for mid and outer shelf where viscous and inertial processes are comparably important by means of twin experiments in which the projected current profiles are compared to the known simulated current profiles it is shown that both the projection time step and the time domain affect the accuracy of the projection at minimum the time domain needs to span the dominant period of current oscillation while the time step resolves this oscillation when both are achieved the projected current profiles converge to the simulated profiles and this convergence can be faster than that by assimilating surface observations to correct model spin up from an inaccurately known initial condition furthermore the projection is shown to be robust in the presence of data noise and with appropriate weighting of the data constraints the effect of noise on the projection accuracy can be minimized in the present projection problem the sea surface slope is assumed unknown and obtained together with the current profile the use of variable eddy viscosity in velocity projection is illustrated as well with an iterative procedure

environmental influences over the last 16 ka on compound specific δ13c variations of leaf wax n alkanes in the hani peat deposit from northeast china compound specific carbon isotope ratios 13c of leaf wax n alkanes c21 c33 odd carbon numbered n alkanes were measured in the hani peat sequence from northeast china these data were compared with lipid biomarker compositions to assess changes in local vegetation and paleoclimate for the last 16 ka the 13c values of n alkanes range between 36 6 and 30 7 showing that the compounds originate from c3 plants much larger variations 5 4 in the n alkane 13c values than those of atmospheric co2 during the last 16 ka 0 5 indicate that the isotopic values were affected by environmental factors in addition to the postglacial 13c variations in the atmospheric reservoir the stratigraphic records of 13c reveal decoupled fluctuations among the individual n alkanes particularly between 15 5 and 11 4 ka synchronous excursions in the 13c offsets among individual n alkanes 13c and lipid biomarker paleoplant proxies paq and c23 c31 and c27 c31 from 14 9 to 13 2 ka and 12 7 to 11 6 ka suggest that vegetational changes are the most likely causes for the decoupled 13c variations parallel fluctuations of the 13c values of terrestrial higher plant derived c29 and c31n alkanes and the n alkane average chain length acl from 11 to 6 ka indicate that the 13c variations responded to net evaporation changes negative shifts in the n alkane 13c values coinciding with the acl decreases at 10 5 9 3 ka and 8 1 ka indicate the short term onset and fluctuations of the summer monsoon strength in eastern china during the early holocene

impact of early diagenesis on distributions of sphagnum n alkanes in peatlands of the monsoon region of china abstract sphagnum species are important peat forming plants in wetlands the world over their n alkane distributions are typically characterized by a predominance of mid chain c 23 and c 25 alkanes a property that has been employed in paleoclimatic reconstruction based on peat sequences however knowledge of the fate of sphagnum derived n alkanes during the very early stage of diagenesis remains limited this study compares the n alkane differences between samples of fresh plants and litter collected from four peatlands across the monsoon region of china the results clearly show that the distributions of sphagnum derived n alkanes differ between the plants and the litter with proportional decreases in c 23 and c 25 n alkanes and increases in c 29 and c 31 homologs in the litter other than the influence of species shifts changes in the n alkane distributions between the fresh plants and litter samples may be caused by preferential degradation of mid chain n alkanes c 23 and c 25 relative to their longer chain homologues c 29 and c 31 during the early diagenesis of sphagnum n alkanes this hypothesis is supported by decreases in cpi values and positive excursions of 13 c values of c 23 and c 29 n alkanes from the plant to the litter samples regardless of what the exact mechanisms for the changes may be the p aq ratio should be applied with caution as a paleoclimatic proxy in peat deposits especially those located in warmer climates or deposited during former warmer stages relatively low p aq values may result from elevated microbial degradation of sphagnum alkanes in such conditions and hence fail to record the former plant assemblages accurately

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

late holocene variability of upper north atlantic deep water temperature and salinity 1 magnesium calcium ratios in benthic foraminifera cibicidoides pachyderma from a sediment core on the laurentian slope 1854 m exhibit strong millennial scale fluctuations during the past 4000 years we convert these data to seawater paleotemperatures using a new monospecific linear equation results suggest that the temperature of upper north atlantic deep water dominated by labrador seawater today has varied by at least 2 c during the late holocene millennial scale coolings coincide with previously identified periods of increased drift ice and regional glacier advances including the little ice age paired oxygen isotope measurements indicate that salinity and perhaps density were reduced during the cold periods we discuss possible mechanisms for transmitting this cold fresh signal from surface waters to intermediate depths our reconstructed late holocene ranges in upper north atlantic deep water properties greatly exceed those of the instrumental record and imply that large changes may be yet to come

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

testing the utility of paleonutrient proxies cd ca and zn ca in benthic foraminifera from thermocline waters 1 reconstruction of oceanic nutrient concentrations in the past provides information about nutrient cycling and physical circulation and the role these processes have played in past climate changes the cd ca and zn ca of benthic foraminifera have been used successfully to reconstruct past deep ocean nutrient concentrations in this study we test the ability of benthic foraminiferal cd ca and zn ca to detect changes in thermocline nutrient concentrations using a set of 31 multicores collected in the florida straits multicore sites span depths of 173 751 m estimated seawater cd concentrations of 0 02 0 42 nmol kg 1 and estimated seawater zn concentrations of 0 3 3 1 nmol kg 1 cd ca in several taxa of benthic foraminifera captures the regional differences and general shape of the nutricline on either side of the florida straits indicating that it is capable of resolving small changes in upper ocean nutrient concentrations in the past we estimate that the upper ocean partition coefficient for calcitic cd ca may be slightly higher than the established value low seawater zn concentrations combined with low partition coefficients and slight laboratory contamination severely complicated the evaluation of benthic foraminiferal zn ca however at least one species c pachyderma incorporates zn with a relatively high partition coefficient 22 minimizing these complications c pachyderma zn ca reflects the regional differences and general shape of the nutricline on either side of the florida straits and demonstrates potential for thermocline paleoceanography

observations of near inertial surface currents off oregon decorrelation time and length scales 1 high resolution km in space and hourly in time surface currents observed by an array of high frequency radars off oregon are analyzed to quantify the decorrelation time and length scales of their near inertial motions the near inertial surface currents are dominantly clockwise with amplitudes of 9 12 cm s 1 however they appear asymmetric and elliptical as a result of counterclockwise inertial motions with magnitudes in a range of 2 5 cm s 1 the decorrelation time and length scales are computed from the decay slope of the near inertial peak and the spatial coherence in the near inertial frequency band respectively decorrelation time scales of clockwise near inertial motions increase from 2 days nearshore within 30 km from the coast to 6 days offshore and their length scales increase from 30 to 90 km seaward possibly due to coastal inhibition the local spatial coherence has an exponentially decaying structure for both clockwise and counterclockwise rotations and their phases propagate northwestward offshore for clockwise and northeastward onshore for counterclockwise rotations

surface to subsurface velocity projection for shallow water currents sea surface currents in coastal oceans are accessible to continuous direct observations by shore based high frequency doppler radar systems inferring current structure in shallow water from such surface current observations is attempted the approach assumes frictionally dominated flow and vertically varying current velocity on the scale of the ekman boundary layer the approximation of the velocity variation with depth is consequently derivable in terms of orthogonal basis functions from the sea surface kinematic and dynamic boundary conditions specifically the viscous momentum and shear equations evaluated at the sea surface the inference procedure developed is demonstrated with sea surface data obtained in the coastal high resolution remote sensing experiment on the continental shelf off cape hatteras despite uncertainties in the surface measurements qualitative agreement is obtained between the inferred subsurface current and the current measured in situ the sensitivity of the inference to the measurement uncertainties as well as to the model assumptions is investigated and the inferred result is found to be generally robust

transports through the straits of florida abstract transports were calculated for four sections of the florida current from key west to jupiter florida using a moored current meter array and voltages from cross channel telephone cables at the western and northern ends of the straits of florida in addition moored arrays were used to estimate transport through the northwest providence santaren and old bahama channels that connect the florida current to the southwestern part of the north atlantic ocean transport measurements were obtained for an 11 month period from december 1990 to november 1991 mean transports of 25 sv 1 sv 106 m3 s 1 for the flow across the western ends of the straits which agree quite well with recent estimates of 23 8 1 sv entering the gulf of mexico through the yucatan channel were obtained from both the key west to havana cable and the moored array this estimate is about 5 sv less than the generally accepted transport through the northern end of the straits at 27 n this difference was partially accounted fo

measurement of surface currents in liverpool bay by high frequency radar the backscattering of high frequency radar echoes from the sea surface can yield useful information about ocean waves and surface currents1 3 the rutherford appelton laboratory ocean surface current radar oscr system is based on the codar system4 but differs significantly in that it uses a beam forming receive antenna to determine the direction of the backscattered signals instead of using the codar direction finding system oscr derives a radial component of surface velocity by measuring the shift in recorded spectra associated with advancing and receding waves in this survey of surface currents in liverpool bay using only oscr measurements were made first for 7 days from the lancashire coast followed 1 month later by similar measurements from the wirral coast these were combined to compute current ellipses for the predominant m2 tidal constituent spatial distributions of the ellipse parameters agree well with values obtained from a numerical model and from moored current meters this new observational technique should provide data on the fine structure of near shore surface currents for use in studies of sediment transport and pollutant dispersal

a geostrophic transport estimate for the florida current from the oxygen isotope composition of benthic foraminifera we present a new method for the quantitative reconstruction of upper ocean flows for during times in the past for the warm t 5 c surface ocean density can be accurately reconstructed from calcite precipitated in equilibrium with seawater as both of these properties increase with decreasing temperature and increasing salinity vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera the net volume transport between two vertical density profiles can be calculated using the geostrophic method using benthic foraminifera from surface sediment samples from either side of the florida straits florida keys and little bahama bank we reconstruct two vertical density profiles and calculate a volume transport of 32 sv using this method this agrees well with estimates from physical oceanographic methods of 30 32 sv for the mean annual volume transport we explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater

remotely sensed surface currents in monterey bay from shore based hf radar coastal ocean dynamics application radar near surface currents in monterey bay derived from a network of shore based hf radars are presented for august december 1994 and compared with those from april to september 1992 focus is placed on the low frequency 2 to 30 day period motions in the remotely sensed data and on comparison of radar derived currents with moored current and wind observations ship based acoustic doppler current profiler observations satellite based surface temperature imagery and surface drifter velocities the radar derived picture of the late summer mean flow is very similar in the two realizations and is consistent with historical data flow is equatorward in the outer part of the bay poleward in a narrow band nearshore and very sluggish in the middle of the bay low pass filtered time series of radar derived currents are highly correlated with moored current observations and with winds in the outer part of the bay the vector time series are also coherent across a broad frequency band with currents typically in phase between 1 and 9 m depths and with 1 m currents typically 40 60 to the right of the wind overall these results confirm the utility of coastal ocean dynamics applications radar codar type hf radars for the study of coastal surface currents out to ranges 50 km from shore particularly for highly averaged fields data variability and comparison with in situ observations for high frequency 1 to 48 hour period motions point to the need to better characterize and minimize sources of error in the radar observations

morphodynamic modelling of intertidal sediment transport in morecambe bay abstract the construction of a morphodynamic model for the intertidal region of morecambe bay england and its calibration and validation using extensive sediment volume change data measured by remote sensing is described a properly validated sediment transport model of the bay would be of interest for coastal defence management and economic purposes the morphodynamic model consisted of a set of linked modules for predicting 2d depth averaged tidal currents waves sediment transport rates and sediment budgets the intertidal bathymetry was updated at regular intervals and the currents waves and sediment transport patterns recomputed using the new bathymetry tidal input filtering using representative spring and neap tidal cycles was used to reduce computational requirements sediment volume changes during the period 1992 1997 were measured over the intertidal zone at a resolution of 240 m using the waterline method employing remote sensing and hydrodynamic modelling the total volume of sediment eroded was 102 16 106 m3 and the total volume accreted was 107 15 106 m3 tidal asymmetry is shown to be the dominant agent of sediment transport in the intertidal zone with waves being of secondary importance a close correlation was found between the computed directions of tidal asymmetry and observed sediment transport paths it was necessary to use the remotely sensed data for calibration of the sediment transport model as well as for validation the model showed some limited capability for predicting gross changes in bathymetry but there were substantial differences between the predicted and observed patterns of sediment movement the possibility of improvement by a further incorporation of the spatially and temporally extensive field data into the modelling process is discussed

gulf stream density structure and transport during the past millennium the gulf stream transports approximately 31 sv 1 sv 10 6 m 3 s 1 of water and 1 3 10 15 w of heat into the north atlantic ocean the possibility of abrupt changes in gulf stream heat transport is one of the key uncertainties in predictions of climate change for the coming centuries given the limited length of the instrumental record our knowledge of gulf stream behaviour on long timescales must rely heavily on information from geologic archives here we use foraminifera from a suite of high resolution sediment cores in the florida straits to show that the cross current density gradient and vertical current shear of the gulf stream were systematically lower during the little ice age ad 1200 to 1850 we also estimate that little ice age volume transport was ten per cent weaker than today s the timing of reduced flow is consistent with temperature minima in several palaeoclimate records implying that diminished oceanic heat transport may have contributed to little ice age cooling in the north atlantic the interval of low flow also coincides with anomalously high gulf stream surface salinity suggesting a tight linkage between the atlantic ocean circulation and hydrologic cycle during the past millennium

persistent intensification of the kuroshio current during late holocene cool intervals abstract the kuroshio current kc which redistributes heat and moisture from the tropics toward mid latitudes considerably affects ocean atmosphere land interactions along its pathways however our knowledge about the kc strength and associated forcing based on investigations of instrumental data and model simulations is controversial impeding assessment of its possible change in the near future in this study we investigated kc strength over the past 4400 yr by analyzing alkenone u 37 k sea surface temperature sst signal in a sediment core under the influence of the yellow sea warm current yswc a branch of the kc in the western north pacific along with a coastal sst record over the last 1000 yr outside of the yswc effect our sst record in the warm current pathway opposite to the patterns of coastal sst and solar irradiance changes at multi centennial timescales effectively manifests variations of yswc strength in conjunction with published records we show a remarkably intensified yswc and probably the kc as well during the cool interval between 1400 1850 ad the little ice age lia we suggest that cold climate conditions perhaps in relation to strengthened east asian winter monsoon and pacific walker circulation were more favorable for kc enhancement

a 250 year decadally resolved radiocarbon time history in the gulf of maine reveals a hydrographic regime shift at the end of the little ice age

lipid biomarker δ13c and plant macrofossil stratigraphy of a scottish montane peat bog over the last two millennia abstract seven horizons of a peat core covering the last 2000 years from moine mhor a blanket bog in the cairngorm mountains scotland have been examined for the carbon number distributions of the long chain hydrocarbon alcohol and acid components using gas chromatography gc and gas chromatography mass spectrometry gc ms compound specific 13 c values for individual n alkanes were obtained using gas chromatography isotope ratio mass spectrometry gc irms the lipid biomarker distributions and the 13 c values have been compared with those of eleven species of living plant dominant at the contemporary surface of the bog the observed lipid stratigraphy shows only partial agreement with that calculated using macrofossil abundance data and the lipid distributions for the living taxa a result which reflects the inherent uncertainties in both the lipid biomarker and the macrofossil approaches to palaeoenvironmental stratigraphy the carbon isotope values for the individual n alkanes of the plants 27 6 to 36 6 and of the peat layers 29 to 31 7 are as expected for the c 3 photosynthetic pathway however the n alkanes from the surface samples were more depleted by 2 in 13 c than those from the rest of the core a negative shift which may in part reflect the shift 1 2 in 13 c estimated for co 2 in a pre industrialised to an industrialised atmosphere

a new view of near shore dynamics based on observations from hf radar abstract since 1984 the oscr hf radar system has been used in over 50 deployments to measure near shore surface currents for both scientific and engineering applications the enhanced scope resolution and accuracy of these measurements have yielded new insights into the tidal wind and density driven dynamics of the near shore zone tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics coherent patterns of wind forced currents ahve been determined with strong evidence of a slab like surface response in one deployment with offshore winds blowing over relatively deep water this slab rotated clockwise at near inertial frequency strong up to 20cm s 1 persistent surface residual currents are commonly observed these are almost certainly generated by small horizontal density gradients these observed surface residuals provide ideal data for rigorous testing of 3 d numerical models with a threatened rise in sea level hf radar is well suited for observing the expected changes in the dynamics of near shore regions continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents future applications may extend beyond the near shore region to measurements along the shelf edge in oceanic gyres and for beach processes

on the accuracy of hf radar surface current measurements intercomparisons with ship based sensors high frequency hf radar systems can provide periodic two dimensional vector current estimates over an area approaching 1000 km as the use of these hf systems has gained wider acceptance a number of attempts have been made to estimate the accuracy of such systems however comparisons of hf radar current estimates with in situ sensors are difficult to interpret since hf systems measure currents averaged over an area of 1 km 2 and to a depth of only 50 cm while in situ sensors measure currents at a point and somewhat greater depths 1 to 10 m previous studies of the accuracy of hf radar technology have thus attributed the differences observed between hf radar and in situ sensors to an unknown combination of vertical shear horizontal inhomogeneity in situ instrument errors and hf radar system errors this study examines the accuracy of hf radar current measurements using data from the 1993 high resolution remote sensing experiment conducted off cape hatteras north carolina data from four shipborne in situ current meters are compared with data from an ocean surface current radar oscr a commercial current measuring radar we attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location the results suggest an upper bound on the accuracy of the oscr derived radial currents of 7 to 8 cm s the measurement of near surface currents in the coastal environ ment is an important problem

effect of lower sea level on geostrophic transport through the florida straits during the last glacial maximum 1 we investigate the effect of a 120 m sea level drop on transport through the caribbean sea and the florida straits during the last glacial maximum lgm relative to the present using the regional ocean modeling system roms a geostrophic transport estimate for the florida straits suggests the lgm florida current was weaker than today by one third assuming that the velocity at the bottom of the channel was as small as it is today this is consistent with a decrease in the north atlantic overturning circulation but there are other possible reasons for a flow decrease it is possible that a shallower lgm florida straits sill depth could cause the diversion of some flow from the florida current our model results show that the volume transport through the florida straits is slightly reduced in a lower sea level model simulation when compared to a control sea level simulation 34 8 2 0 sv versus 39 8 2 3 sv the difference in transport is of the order of 5 sv likely representing a maximum limit to the lgm flow reduction due to sea level change therefore the change in sill depth between the lgm and the present is unlikely to have been a cause of the entire observed flow reduction we also use the model output to demonstrate that transport through the florida straits can be accurately calculated from ocean margin density data at the core locations in the work of lund et al 2006 and lynch stieglitz et al 2009 provided that a sufficiently deep reference level is chosen

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

the distribution of long chain n alkan 2 ones in peat can be used to infer past changes in ph abstract long chain c21 c33 n alkan 2 ones are biomarkers ubiquitous in peat deposits however their paleoenvironmental significance lacks constraints here we evaluate the influence ph exerts on the occurrence of long chain n alkan 2 ones in chinese peats a comparison of the distribution in a collection n 65 of modern peat samples with different ph ph values 4 4 8 6 from china demonstrates that their distribution is significantly different in acid compared to alkaline peat this difference can be explained by the ph control on the conversion of n alkan 2 one precursor compounds n alkanes and fatty acids transfer functions between ph and n alkan 2 one ratios were established using linear and logarithmic regression models we then applied these proxies to reconstruct variations of paleo ph in the dajiuhu peat sequence to identify the history of peatland acidification over the last 13 kyr we find significant changes in paleo ph during the deglaciation early holocene and relate these to times of dry climate in the region the drought induced peat acidification is supported by observations from modern drying events in the peatland we propose that long chain n alkan 2 ones in peats have potential to trace paleo ph changes across the deglaciation and holocene although further research from different peatlands and time periods is still needed

assessing paleohydrologic controls on the hydrogen isotope compositions of leaf wax n alkanes in chinese peat deposits abstract the high concentrations of lipid biomarkers in peat and the close relation between peatland development and hydrological conditions merit further assessment of the paleohydrological proxy potential of the 2h 1h ratio of leaf wax n alkanes 2halk in peat deposits in the context of the dynamics of the east asian summer monsoon detailed investigation of modern processes is an important way to improve our understanding of the controls on 2halk records from chinese peat deposits this review first synthesizes the results of investigations of modern processes in the dajiuhu peatland of central china complemented by results from other peatlands and nearby non peatland sites to evaluate how the 2h 1h ratio changes from source water to leaf water and then to lipids in leaf waxes and ultimately to peat deposits the results reveal that the precipitation 2h values and the peatland plant types are the two major factors that control the 2halk signals preserved in peat at dajiuhu comparison of results from holocene 2halk sequences from other chinese peat deposits illustrates how peat 2halk values respond to paleoclimate changes on centennial to millennial timescales throughout the east asian summer monsoon region finally proposed incubations of peat forming plants in controlled chambers that could assess the response of plant 2halk values to environmental stresses are described as an example responses of the compound specific biomarker 2h and 13c values of sphagnum the dominant plant in the dajiuhu peatland could provide important information on the symbiotic relation between sphagnum and microbes it is expected that experiments such as these that combine the carbon and hydrogen isotope compositions of multiple lipids will yield important new insights into the responses of peatland ecosystems to climate changes

late holocene 0 2 4 ka bp surface water temperature and salinity variability feni drift ne atlantic ocean abstract planktonic foraminiferal mg ca ratios and oxygen isotopic compositions of a spliced sediment record from feni drift ne atlantic ocean box core m200309 and piston core enam9606 trace late holocene sea surface temperature sst and salinity changes over the past 2400 years at this location the variability of sst and oxygen isotopic composition of seawater 18 o w reflects variable northward advection of warm and saline surface waters which appears linked to climate variability over the adjacent european continent our records reveal a general long term cooling trend superimposed on this overall trend partly higher temperatures and salinities from 180 to 560 ad and 750 to 1160 ad may be ascribed to the roman and medieval warm periods respectively subsequently our record displays highly variable surface water conditions the main little ice age sst minimum is restricted to the 15th and 16th centuries ad pervasive multidecadal to centennial scale variability throughout the sedimentary proxy records can be partly attributed to solar forcing and or variable heat extraction from the surface ocean caused by shifts in the prevailing state of the north atlantic oscillation nao high salinities in the 17th and 18th centuries are considered to reflect tropical anomalies linked to a southward shift of the intertropical convergence zone propagating across the north atlantic ocean

exploring globorotalia truncatulinoides coiling ratios as a proxy for subtropical gyre dynamics in the northwestern atlantic ocean during late pleistocene ice ages we explore the use of the coiling direction of planktic foraminifer globorotalia truncatulinoides in sediment cores from the northwestern subtropical atlantic ocean as a proxy for variations in the intensity of the western boundary of the subtropical gyre over the past 280 kyr core top sediments from the study region are dominated by the left coiling variety consistent with the deep permanent thermocline at the study sites knr140 37pc and ocean drilling program site 1059 downcore g truncatulinoides sinistral maxima occur in conjunction with 14 out of the 25 northern and southern hemisphere precession maxima contained in the study interval the agreement between the dominance of left coiling tests and the precession index of the southern hemisphere in particular supports a link between a deep thermocline in the northwestern subtropical atlantic and northward flow of equatorially sourced warm surface currents a situation analogous to the late holocene interglacial marine oxygen isotope stage mis 5 lacks g truncatulinoides s minima attesting to the relative stability of the western boundary during an interval of prolonged global warmth g truncatulinoides s disappear during the glacial extremes of mis 2 6 and 8 implying a weaker western boundary current at these times our results support that the coiling direction of this species is sensitive to variations in hydrography of the western boundary of the subtropical gyre because of the association between g truncatulinoides s and precession maxima in both hemispheres results support the importance of oceanic heat transport in half precession climate variability in the north atlantic

tidal currents in baie de seine comparison of numerical modelling and high frequency radar measurements abstract numerical modelling and hf radar technique are two developing methods for studying coastal circulation they both have a wide space coverage and similar time and space discriminations respectively a few minutes and a few km2 it is therefore particularly interesting to compare their results an experiment using two high frequency radars was made in western baie de seine at the end of summer 1982 and three days of these measurements covering a large range of tidal coefficients have been compared with the corresponding results of a two dimensional modelling of the whole bay characteristics of this model as well as the main methodological and technical points concerning hf radar measurements especially a discussion of the measurements errors evaluated to be 3 to 7 cm s 1 on each radial component are presented a comparison is made at 6 points for the current vector and at 20 points for one of the radial components it concerns the half day mode amplitude phase hodograph in most of the area no significant phase lag is observed the agreement is to within ca 10 cm s 1 for the amplitude and to within less than 10 for the bearing of tidal ellipses with smaller intervals for smaller tides this agreement shows that the intrinsic accuracy of the model is comparable to that of other sensors such as current meters what is even more satisfactory is that the model does not use as boundary conditions any measurements specific to the period studied except for sea level forecasts which are given by a greater scale physical model a significant difference is observed only close to the coast where the strong gradient of the current velocity referred to the discrimination scale is unfavourable both to the model and to the measurements

calibration of mg ca thermometry in planktonic foraminifera from a sediment trap time series 1 paired mg ca and 18o measurements on planktonic foraminiferal species g ruber white g ruber pink g sacculifer g conglobatus g aequilateralis o universa n dutertrei p obliquiloculata g inflata g truncatulinoides g hirsuta and g crassaformis from a 6 year sediment trap time series in the sargasso sea were used to define the sensitivity of foraminiferal mg ca to calcification temperature habitat depths and calcification temperatures were estimated from comparison of 18o of foraminifera with equilibrium calcite based on historical temperature and salinity data when considered together mg ca mmol mol of all species except two show a significant r 0 93 relationship with temperature t c of the form mg ca 0 38 0 02 exp 0 090 0 003 t equivalent to a 9 0 0 3 change in mg ca for a 1 c change in temperature small differences exist in calibrations between species and between different size fractions of the same species o universa and g aequilateralis have higher mg ca than other species and in general data can be best described with the same temperature sensitivity for all species and pre exponential constants in the sequence o universa g aequilateralis g bulloides g ruber g sacculifer other species this approach gives an accuracy of 1 2 c in the estimation of calcification temperature the 9 sensitivity to temperature is similar to published studies from culture and core top calibrations but differences exist from some literature values of pre exponential constants different cleaning methodologies and artefacts of core top dissolution are probably implicated and perhaps environmental factors yet understood planktonic foraminiferal mg ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures using g ruber pink g ruber white g sacculifer n dutertrei p obliquiloculata etc the existence of a single mg thermometry equation is valuable for extinct species although use of species specific equations will where statistically significant provide more accurate evaluation of mg ca paleotemperature

reconstructing past seasonal to multi centennial scale variability in the ne atlantic ocean using the long lived marine bivalve mollusc glycymeris glycymeris the lack of long term highly resolved annual to sub annual and absolutely dated baseline records of marine variability extending beyond the instrumental period last 50 100 years hinders our ability to develop a comprehensive understanding of the role the ocean plays in the climate system specifically without such records it remains difficult to fully quantify the range of natural climate variability mediated by the ocean and to robustly attribute recent changes to anthropogenic or natural drivers here we present a 211 year 1799 2010 ce all dates hereafter are common era seawater temperature swt reconstruction from the northeast atlantic ocean derived from absolutely dated annually resolved oxygen isotope ratios recorded in the shell carbonate 18oshell of the long lived marine bivalve mollusc glycymeris glycymeris the annual record was calibrated using sub annually resolved 18oshell values drilled from multiple shells covering the instrumental period calibration verification statistics and spatial correlation analyses indicate that the 18oshell record contains significant skill at reconstructing northeast atlantic ocean mean summer swt variability associated with changes in sub polar gyre spg dynamics and the north atlantic current reconciling differences between the 18oshell data and corresponding growth increment width chronology demonstrates that 68 of the variability in g glycymeris shell growth can be explained by the combined influence of biological productivity and swt variability these data suggest g glycymeris can provide seasonal to multi centennial absolutely dated baseline records of past marine variability that will lead to the development of a quantitative understanding of the role the marine environment plays in the global climate system

on tide induced lagrangian residual current and residual transport 2 residual transport with application in south san francisco bay california the transports of solutes and other tracers are fundamental to estuarine processes the apparent transport mechanisms are convection by tidal current and current induced shear effect dispersion for processes which take place in a time period of the order of a tidal cycle however as emphasis is shifted toward the effects of intertidal processes the net transport is mainly determined by tide induced residual circulation and by residual circulation due to other processes the commonly used intertidal conservation equation takes the form of a convection dispersion equation in which the convective velocity is the eulerian residual current and the dispersion terms are often referred to as the phase effect dispersion or sometimes as the tidal dispersion the presence of these dispersion terms is merely the result of a fickian type hypothesis since the actual processes are not fickian thus a fickian hypothesis obscures the physical significance of this equation recent research results on residual circulation have suggested that long term transport phenomena are closely related to the lagrangian residual current or the lagrangian residual transport in this paper a new formulation of an intertidal conservation equation is presented and examined in detail in a weakly nonlinear tidal estuary the resultant intertidal transport equation also takes the form of a convection dispersion equation without the ad hoc introduction of phase effect dispersion in a form of dispersion tensor the convective velocity in the resultant equation is the first order lagrangian residual current the sum of the eulerian residual current and the stokes drift the remaining dispersion terms are important only in higher order solutions they are due to shear effect dispersion and turbulent mixing there exists a dispersion boundary layer adjacent to shoreline boundaries an order of magnitude estimate of the properties in the dispersion boundary layer is given the present treatment of intertidal transport processes is illustrated by an analytical solution for an amphidromic system and by a numerical application in south san francisco bay california the present formulation reveals that the mechanism for long term transport of solutes is mainly convection due to the lagrangian residual current in the interior of a tidal estuary this result also points out the weakness in the tidal dispersion formulation and explains the large variability of the observed values for tidal dispersion coefficients further research on properties of the dispersion boundary layer is needed

florida current surface temperature and salinity variability during the last millennium surface water d 18 ow near dry tortugas increased 0 4 during the course of the little ice age lia 1200 1850 a d equivalent to a salinity increase of 0 8 1 5 on the great bahama bank where surface waters are influenced by the north atlantic subtropical gyre d 18 ow increased by 0 3 during the last 200 years although a portion 0 1 of this shift may be an artifact of anthropogenically driven changes in surface water sco2 the remaining d 18 ow signal implies a 0 4 1 increase in salinity after 200 years b p the simplest explanation of the d 18 ow data is southward migration of the atlantic hadley circulation during the lia scaling of the d 18 ow records to salinity using the modern low latitude d 18 ow s slope produces an unrealistic reversal in the salinity gradient between the two sites only if d 18 ow is scaled to salinity using a high latitude d 18 ow s slope can the records be reconciled variable atmospheric 14 c paralleled dry tortugas d 18 ow suggesting that solar irradiance paced centennial scale migration of the inter tropical convergence zone and changes in florida current salinity during the last millennium citation lund d c and w curry 2006 florida current surface temperature and salinity variability during the last millennium paleoceanography 21 pa2009 doi 10 1029 2005pa001218

late holocene variability in florida current surface density patterns and possible causes 1 planktonic foraminiferal 18o time series from three well dated high sedimentation rate cores near the florida keys 24 4 n 83 3 w exhibit repeated centennial to millennial scale oscillations during the late holocene isotopic shifts of 0 2 0 3 over the past 5200 years represent changes in sea surface temperature sst of 1 0 1 5 c or salinity variability of 1 2 psu the largest significant isotopic events are centered at approximately 200 2000 3200 and prior to 4000 calendar years bp high florida current 18o during the little ice age lia correlates with published records of high 18o in the sargasso sea and low sst off the coast of west africa an interval of generally low 18o in the florida straits from 1800 to 500 years bp is synchronous with the medieval warm period off west africa but leads low 18o in the sargasso sea by several hundred years synchronous cooling across the subtropical gyre during the lia is difficult to explain using interannual north atlantic oscillation patterns but may be consistent with the simulated effects of reduced solar irradiance at frequencies between 1 1000 and 1 300 years during the late holocene florida current 18o is coherent with a published estimate of 14c production rate radiocarbon production seems to lead 18o at these frequencies but uncertainty in the phase calculation precludes a clear lead lag relationship at frequencies lower than 1 300 years florida current 18o is coherent and in phase with atmospheric 14c the coherence of 14c and 18o at periods 1000 years implies oceanic circulation may play a role in modulating atmospheric radiocarbon on millennial timescales

coherent high and low latitude climate variability during the holocene warm period a faunal record of sea surface temperature sst variations off west africa documents a series of abrupt millennial scale cooling events which punctuated the holocene warm period these events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling the most recent of these events was the little ice age which occurred between 1300 to 1850 a d when subtropical ssts were reduced by 3 to 4 c these events were synchronous with holocene changes in subpolar north atlantic ssts documenting a strong in phase link between millennial scale variations in high and low latitude climate during the holocene

weaker gulf stream in the florida straits during the last glacial maximum as it passes through the florida straits the gulf stream consists of two main components the western boundary flow of the wind driven subtropical gyre and the northward flowing surface and intermediate waters which are part of the global conveyor belt compensating for the deep water that is exported from the north atlantic ocean1 the mean flow through the straits is largely in geostrophic balance and is thus reflected in the contrast in seawater density across the straits2 here we use oxygen isotope ratios of benthic foraminifera which lived along the ocean margins on the boundaries of the florida current during the last glacial maximum to determine the density structure in the water and thereby reconstruct transport through the straits using the geostrophic method a technique which has been used successfully for estimating present day flow3 our data suggest that during the last glacial maximum the density contrast across the florida straits was reduced with the geostrophic flow referenced to the bottom of the channel at only about two thirds of the modern value if the wind driven western boundary flow was not lower during the last glacial maximum than today these results indicate a significantly weaker conveyor belt component of the gulf stream compared to present day values whereas previous studies based on tracers suggested that deep waters of north atlantic origin were not widespread during glacial times indicating either a relatively weak or a shallow overturning cell our results provide evidence that the overturning cell was indeed weaker during glacial times

late holocene sea and land level change on the u s southeastern atlantic coast abstract late holocene relative sea level rsl reconstructions can be used to estimate rates of land level subsidence or uplift change and therefore to modify global sea level projections for regional conditions these reconstructions also provide the long term benchmark against which modern trends are compared and an opportunity to understand the response of sea level to past climate variability to address a spatial absence of late holocene data in florida and georgia we reconstructed 1 3 m of rsl rise in northeastern florida usa during the past 2600 years using plant remains and foraminifera in a dated core of high salt marsh sediment the reconstruction was fused with tide gauge data from nearby fernandina beach which measured 1 91 0 26 mm year of rsl rise since 1900 ce the average rate of rsl rise prior to 1800 ce was 0 41 0 08 mm year assuming negligible change in global mean sea level from meltwater input removal and thermal expansion contraction this sea level history approximates net land level subsidence and geoid change principally from glacio isostatic adjustment historic rates of rise commenced at 1850 1890 ce and it is virtually certain p 0 99 that the average rate of 20th century rsl rise in northeastern florida was faster than during any of the preceding 26 centuries the linearity of rsl rise in florida is in contrast to the variability reconstructed at sites further north on the u s atlantic coast and may suggest a role for ocean dynamic effects in explaining these more variable rsl reconstructions comparison of the difference between reconstructed rates of late holocene rsl rise and historic trends measured by tide gauges indicates that 20th century sea level trends along the u s atlantic coast were not dominated by the characteristic spatial fingerprint of melting of the greenland ice sheet

estimation of 3 d current fields near the rhine outflow from hf radar surface current data hf radar systems are designed to measure spatially variable sea surface currents a methodology was developed to complement these data with information about the current variability over the water column in a stratified shallow sea current profiles were estimated using a diagnostic model driven by surface current measurements from an hf radar system and by sea surface slopes derived from tide gauge data the structure of the model has a physical basis but its parameters were derived from an analysis of in situ current profile measurements application of the model to hf radar data from the scawvex rhine outflow experiment showed fair agreement with in situ current data as applications estimation and tidal analysis of current fields are demonstrated

an assessment of local coastal dynamics observed with high frequency radar an assessment is made of the dominant processes governing the circulation in a 30 km by 40 km area off the coast of cape hatteras north carolina where surface current velocity vectors have been obtained using shore based high frequency radars in the high resolution remote sensing experiment in 1993 although the currents in this region are constantly under the influence of winds and tides frequent intrusion of energetic flows into the area apparently mostly of gulf stream origin is shown to have a strong influence on the current variability analysis of vorticity and divergence shows that the flow intruding into the area is strongly rotational with positive vorticity this rotational motion is not in geostrophic balance with the surface pressure gradient and an inertial oscillation is often excited as a result with vorticity and divergence oscillating 90 out of phase on average the geostrophic imbalance generates a net surface flow divergence which in turn induces negative vorticity that more than offsets the positive vorticity advected into the region however vertical viscous transfer of positive vorticity from subsurface to surface is shown to be required to complete the surface vorticity balance the same energetic cyclonic shear flows that intrude into the area are suggested as the source of the subsurface vorticity as these flows of gulf stream origin are known to be subducted under buoyant shelf water

a 4500 year reconstruction of sea surface temperature variability at decadal time scales off north iceland marine paleo records acquired at high temporal resolution provide critical data for testing numerical climate models and help to understand processes underlying ocean variability this study presents a unique 4500 year reconstruction of sea surface temperature sst obtained from alkenones in the north atlantic polar front area off north iceland at an average temporal resolution of 4 5 years spectral analysis of this signal shows dominant multidecadal oscillations which occurred with a stronger amplitude between 2500 and 4200 years bp hand in hand with fluctuations of bottom currents indicated by paleomagnetic proxies contemporaneous large excursions of the inter tropical convergence zone itcz are also recorded by the distant cariaco titanium time series suggesting a link with low latitude atlantic climate we speculate that the oscillations reflect changes of the meridional overturning circulation moc induced by increased enso activity

labrador current variability over the last 2000 years the ice loaded labrador current lc is an important component of the western north atlantic circulation that influences the position and strength of the northern limb of the north atlantic current nac this flow of cold and fresh polar waters originating from the arctic has a marked impact on the north atlantic climate yet little is known about its variability beyond the instrumental period in this study we present the first sub decadal alkenone based 2000 year long sea surface temperature sst records from the western labrador sea a climatically crucial region at the boundary between the lc and the nac our results show a clear link between the lc strength and the northern annular mode nam with a stronger nam and a more vigorous lc during the medieval climate anomaly mca this suggests enhanced lc activity upon future global warming with implications for the atlantic meridional overturning circulation amoc

remote radar measurement of shelf currents off key largo florida u s a abstract the coastal shelf of the florida keys is characterized by shallow and highly variable topography where currents are influenced by tides wind and the very energetic offshore florida current system the transient dynamics of variable scale are not easily captured with traditional fixed mooring observation platforms consequently an experiment combining moorings acoustic doppler current profiler adcp transects and a shore based high frequency hf radar system oscr was conducted to monitor the spatial and temporal variability of the flow field over the shelf and off shore deeper waters both the wind and tide driven flows in the middle to inner shelf called hawk channel and two offshore eddy events of the florida current were sampled during the experiment qualitative agreement between oscr current meter moorings and adcp transects was good in the shelf however at the inshore edge of the florida current there were significant differences the adcp captured the strong horizontal shear at the shelf edge whereas oscr smoothed the front over two kilometres due to its measurement cell size these differences in the spatial sampling resolution in the high shear region prevented direct quantitative comparisons although there were significant local variations linked to topography a major portion of the subtidal flow in the shelf was related to wind within the shelf both cross shelf and along shelf flows were highly spatially correlated with r 0 82 and r 0 86 respectively two sub mesoscale eddies that had characteristics consistent with the spin off eddies described by lee 1975 were observed to advect at velocities ranging from 0 53 ms 1 to 0 80 ms 1 along the inshore edge of the florida current the two eddies had along shelf length scales from 19 km to 47 km and cross shelf scales of 15 km and 25 km each eddy consisted of a single cyclonic vortex rotating with tangential velocities of 0 30 ms 1 and 0 50 ms 1 respectively the eddies had a strong signature at the shelf break however they did not have any observable influence on flows over the inner shelf

short and long term changes in estuary capacity changes in capacity have been examined in the lune ribble mersey and humber estuaries to assess the role played by land derived sediments in coastal accretion the engineering concept of dynamic equilibrium is introduced and shown to imply that land derived sediments should contribute exclusively to coastal accretion the various factors responsible for changing estuary capacity are also introduced capacity changes over the last 100 years show that all the estuaries considered have suffered large scale siltation with the majority of the sediment being derived from coastal and offshore sources in the case of the lune ribble and mersey estuaries engineering works have been responsible for large permanent changes in capacity the situation in the humber is less clear and siltation is probably a result of natural infilling the estuary having been over deepened by glacial and fluvial action in the past a detailed study of the mersey suggests that the estuary was also over deepened and over widened and has only been tidal for the last 9000 years or so calculations suggest that a new equilibrium will be achieved in about 250 years time and that the estuary may then be in a state of dynamic equilibrium the much smaller lune estuary may already have achieved this state although detailed evidence is scarce

lipid distribution in a subtropical southern china stalagmite as a record of soil ecosystem response to paleoclimate change abstract lipid extracts from a 61 7 cm long subtropical stalagmite in southern china spanning the period of ca 10 000 21 000 yr ago as constrained by u th dating were analyzed using gas chromatography mass spectrometry the higher plants and microorganisms in the overlying soils contribute a proportion of n alkanes identified in the stalagmite the occurrence of lmw lower molecular weight n alkanols and n alkan 2 ones in the stalagmite was mainly related to the soil microorganisms we suggest that hmw higher molecular weight n alkanols and n alkan 2 ones identified in the stalagmite originate from soil organics and reflect input from contemporary vegetation shifts in the ratio of lmw to hmw n alkanols or n alkan 2 ones indicative of the variation of soil ecosystems e g microbial degradation of organic matter and or the relative abundance of soil microorganisms to higher plants are comparable with the subtropical alkenone sst sea surface temperature record of the same period the similar trends seen in the 13c data and the lipid parameters in this stalagmite imply that the overlying soil ecosystem response to climate might be responsible for the variation of 13c values

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

holocene multidecadal to millennial scale variations in iceland scotland overflow and their relationship to climate the nordic seas overflows are an important part of the atlantic thermohaline circulation while there is growing evidence that the overflow of dense water changed on orbital time scales during the holocene less is known about the variability on shorter time scales beyond the instrumental record here we reconstruct the relative changes in flow strength of iceland scotland overflow water isow the eastern branch of the overflows on multidecadal millennial time scales the reconstruction is based on mean sortable silt ss from a sediment core on the gardar drift 60 19 n 23 58 w 2081 m our ss record reveals that the main variance in isow vigor occurred on millennial time scales 1 2 kyr with particularly prominent fluctuations after 8 kyr superimposed on the millennial variability there were multidecadal centennial flow speed fluctuations during the early holocene 10 9 kyr and one prominent minimum at 0 9 kyr we find a broad agreement between reconstructed isow and regional north atlantic climate where a strong weak isow is generally associated with warm cold climate we further identify the possible contribution of anomalous heat and freshwater forcing respectively related to reconstructed overflow variability we infer that ocean poleward heat transport can explain the relationship between regional climate and isow during the middle to late holocene whereas freshwater input provides a possible explanation for the reduced overflow during early holocene 8 10 kyr

differential hydrogen isotopic ratios of sphagnum and vascular plant biomarkers in ombrotrophic peatlands as a quantitative proxy for precipitation evaporation balance abstract we have developed a new approach to quantitatively reconstruct past changes in evaporation based on compound specific hydrogen isotope ratios of vascular plant and sphagnum biomarkers in ombrotrophic peatland sediments we show that the contrast in h isotopic ratios of water available to living sphagnum top 20 cm and in the rooting zone of peatland vascular plants can be used to estimate the fraction of water remaining after evaporation vascular plant leaf waxes record h isotopic ratios of acrotelm water which carries the d h ratio signature of precipitation and is little affected by evaporation whereas the sphagnum biomarker c 23 n alkane records h isotopic ratios of the water inside its cells and between its leaves which is strongly affected by evaporation at the bog surface evaporation changes can then be deduced by comparing h isotopic ratios of the two types of biomarkers we calibrated d h ratios of c 23 n alkane to source water with lab grown sphagnum we also tested our isotopic model using modern surface samples from 18 ombrotrophic peatlands in the midwestern united states finally we generated a 3000 year downcore reconstruction from minden bog michigan usa our new record is consistent with records of other parameters from the same peatland derived from different proxies and allows us to differentiate precipitation supply and evaporative loss

hf radar observations of surface circulation off bodega bay northern california usa 1 two high frequency radar stations codar were installed along the northern california coast in may 2001 comparisons of radar data with acoustic doppler current profiler adcp current data and near surface drifter tracks indicate considerable agreement with minimum rms differences of order 0 05 0 15 m s and average drifter hf radar track separation rates of 5 3 km d radar data resolve the three main sources of intraseasonal current variability in the area 1 upwelling relaxation dynamics 2 tidal and diurnal forcing and 3 inertial currents subtidal fluctuations are the largest component of variability accounting for 45 75 of the variance wind driven dynamics are the dominant source of this subtidal variability 67 of subtidal variability both upwelling and relaxation periods exhibit consistent patterns of surface velocity with nearshore currents being slower and more poleward than offshore currents which are strongly equatorward analysis of tidal and inertial variability indicate that current fluctuations are polarized toward clockwise rotation and are generally weaker and more linearly polarized near the coast m2 tidal current ellipses switch direction of rotation at the shelf break suggesting the presence of internal tidal waves currents in all frequency bands are deflected and accelerated around pt reyes and there are indications of increased current variability and changes in flow direction near cordell bank the presence of considerable cross shore and alongshore gradients in the strength and direction of surface flow patterns and in particular weak poleward currents over the inner shelf could have important consequences for plankton retention in the area

peatland evolution and associated environmental changes in central china over the past 40 000 years central china has experienced stronger summer monsoon during warm periods such as marine isotope stages mis 1 and 3 and weaker summer monsoon during cool periods such as mis 2 the evolution history of dajiuhu subalpine peatland in central china can help investigate how the expansion and shrinkage of peatland were associated with monsoonal strength over the last glacial interglacial cycle here we apply bulk organic carbon and molecular biomarkers hopane and n alkane to reconstruct the evolution history for the dajiuhu peatland over the past 40 000 yr the results indicate fluctuations between lacustrine and peat like deposition during mis 3 steady lacustrine deposition during mis 2 and peatland initiation and expansion during mis 1 in the dajiuhu peatland therefore at the glacial interglacial scale warmer summer and cooler winter conditions in interglacial periods are crucial to trigger peat deposition whereas reduced evaporation in glacial period instead of decreased monsoonal driven precipitation would have played a predominant role in the regional effective moisture balance however within the holocene mis 1 monsoonal precipitation changes appear to be the main controller on millennial scale variations of water table level of the dajiuhu peatland

oceanographic changes through the last millennium off north iceland temperature and salinity reconstructions based on foraminifera and stable isotopes abstract temperature and salinity reconstructions for two 1000 year high resolution sedimentary records located at the boundary between atlantic and arctic surface waters on the north icelandic shelf are based on transfer functions and oxygen isotopes for planktonic and benthic foraminifera there is a general increase of arctic water indicator species at the transition from the medieval warm period into the little ice age lia and a subsequent return of atlantic water indicator species towards the end of the lia and in the 20th century the timing of the reconstructed temperature changes both at the beginning and at the end of the lia appears to be slightly different for the different water masses the earliest temperature change is seen in the bottom and subsurface waters where a cooling is reconstructed as early as ad 1150 1200 at both locations whereas previously published diatom based and alkenone based sea surface temperature reconstructions show a change at ad 1300 coinciding with the air temperature shift in the area our results show the need of a thorough understanding of the oceanography in the study area as well as the different living habitat for the biological proxies used for the temperature estimates

postglacial climate change record in biomarker lipid compositions of the hani peat sequence northeastern china abstract the peat sequence at hani in northeastern china accumulated over the past 16 cal kyr in a percolation mire in which rain water and ground water seeped through the peat system the molecular compositions of n alkanes n alkanols and n alkanoic acids extracted from the hani peat sequence reveal different responses to the progressive evolution of climate and changes in the nature of the peat forming vegetation long chain length components that originate from the waxy coatings of subaerial vascular plants dominate the n alkane distributions throughout the hani peat sequence the paleoclimate integrity of these biomarker molecules appears to be well preserved most of the n alkanol distributions are similarly dominated by long chain components that indicate their origins from subaerial plants in contrast n alkanoic acid distributions are dominated by secondary components that record the importance of post depositional microbial activity in this peat sequence which evidently can be extensive in a percolation mire elevated n alkane paq values and c23 c29 ratios which are both molecular proxies for water loving plants record an especially moist local climate in the bolling allerod 14 5 to 12 9 ka younger dryas 12 9 to 11 5 ka and pre boreal 11 5 to 10 5 ka portions of the hani peat sequence depressed paq values and c23 c29 ratios and larger n alkane average chain length values indicate that the holocene climatic optimum 10 5 to 6 ka was a period of warmer climate with lower effective precipitation which contrasts with evidence of wetter climates in most of east asia

palaeoclimate records in compound specific δd values of a lipid biomarker in ombrotrophic peat compound specific d values recorded by means of gas chromatography thermal conversion isotope ratio mass spectrometry gc tc irms of the biomarker n alkane n tricosane n c23 representative of the dominant sphagnum species in a 40 cm peat profile from bolton fell moss cumbria uk correlate with vegetation changes in the past 200 years age depth model based on 210pb dating the bog vegetation is sensitive to climate change correlating with the global scale cooler period of the later 19th and early 20th centuries the correlation with meteorological records suggests compound specific d values of lipid biomarkers have potential for use as a new climate proxy