STABLE ISOTOPES AND TRACE SUBSTANCES: THEIR USES IN
OCEANOGRAPHY AND CLIMATE RESEARCH ON VARIOUS
TIMESCALES
Location: Arts Building 120 LT
Location of Posters: Arts Building 126 LR2/101LR4
Wednesday 21 July AM
Concurrent Poster Session
Introduction 0900
P07/L/01-A3 0905
CIRCULATION FEATURES IN THE ARCTIC OCEAN REVEALED BY THE NUCLEAR FUEL REPROCESSING TRACERS, 129I AND 137CS
John N. SMITH - Marine Environmental Sciences Division, Bedford Institute of Oceanography, Dartmouth, N. S., Canada B2Y 4A2
The development during the past 10 years of analytical techniques to measure 129I by accelerator mass spectrometry has led to recent advances in its use as an oceanographic tracer, particularly in the Arctic Ocean. This stems from the fact that large quantities of 129I (t1/2 = 16 x 106 y) have been discharged from the Sellafield (UK) and La Hague (France) nuclear fuel reprocessing plants into the Irish Sea and English Channel, respectively since the 1960s. Together with 137Cs (t1/2 = 30 y), derived mainly from Sellafield, the 129I reprocessing signal is transported into the North Sea and Norwegian Coastal Current and then enters the Arctic Ocean through Fram Strait and the Barents Sea where both tracers reflect the circulation of Atlantic-origin halocline and intermediate water. Measurements of 129I and 137Cs, combined with a knowledge of the historical record of reprocessing plant discharges, can be used to identify a given year of transport through the Norwegian Coastal Current (NCC), thereby permiting the determination of a transit time from the NCC to a given sampling location, similar to a ventilation age determined using atmospherically-derived tracers such as tritium and chlorofluorocarbon compounds (CFC’s). One difference is that the release rate of 129I from La Hague and the dynamic range of the tracer pair for transit time measurements is rapidly increasing while the input functions for many ventilation tracers are leveling off or decreasing owing to global constraints placed on their production and usage.
Measurements of 129I and 137Cs conducted on seawater samples collected during icebreaker and US Navy nuclear submarine cruises (SCICEX program) to the Central Arctic Ocean since 1993 provide a basis for evaluating the utility of these tracers. 129I results in halocline waters clearly delineate the front between Atlantic-origin water having an elevated 129I reprocessing signal (> 100 x 107 at/l) and Pacific-origin water labeled mainly by fallout (< 5 x 107 at/l), that is aligned with the Mendeleyev Ridge. Low 129I levels (< 2 x 107 at/l) measured in Atlantic and intermediate water in the Northern Canada Basin and over the Alpha Ridge indicate that ventilation rates are extremely low …
P07/W/06-A3 0935
USING CHEMICAL TRACERS TO ASSESS OCEAN MODELS
Matthew H. ENGLAND Centre for Environmental Modelling and Prediction (CEMAP), School of Mathematics The University of New South Wales, NSW 2052 Australia and Ernst Maier-Reimer Max Planck Institut fur Meteorolgie, Hamburg, Germany
Geochemical tracers can be used to assess the simulated circulation in ocean models. Tracers that have been used in this context include tritium, chlorofluorocarbons, natural and bomb-produced radiocarbon; and, to a lesser extent, oxygen, silicate, phosphate, isotopes of organic and inorganic carbon compounds and certain noble gases (e.g., helium and argon). This paper reviews the use of chemical tracers in assessing the circulation and flow patterns in global and regional ocean models. It will be shown that crucial information can be derived from chemical tracers that cannot be obtained from temperature-salinity (T-S) alone. In fact, it turns out that a model with a good representation of T-S can have significant errors in simulated circulation, so checking a model's ability to capture chemical tracer patterns is vital. Natural chemical tracers such as isotopes of carbon, argon, and oxygen are useful for examining the model representation of old water-masses, such as North Pacific and Circumpolar Deep Water. Anthropogenic or transient tracers, such as tritium, chlorofluorocarbons, and bomb-produced 14C are best suited for analysing model circulation over decadal time-scales, such as thermocline ventilation, the renewal of Antarctic Intermediate Water, and the ventilation pathways of North Atlantic Deep Water and Antarctic Bottom Water. Tracer model studies have helped to reveal inadequacies in the model representation of certain water-mass formation processes; for ex- ample, convection, downslope flows, and deep ocean currents. They show how coarse models can chronically exaggerate the spatial scales of open ocean convection and deep currents, while underestimating deep flow rates and diffusing downslope flows with excessive lateral mixing. Higher resolution models typically only resolve thermocline ventilation because of shorter integration times, and most resort to high-latitude T-S restoring to simulate reasonable interior water-mass characteristics. This can be seen to result in spuriously weak chemical tracer uptake at high latitudes due to suppressed convective overturn and vertical motion. Overall, the simulation of …
P07/W/17-A3 0955
WATER MASS FORMATION AND CIRCULATION IN THE SOUTHERN AND SOUTHERN INDIAN OCEANS: RESULTS FROM STABLE OXYGEN ISOTOPES
M.P.MEREDITH, K.J.Heywood, R.D.Frew and P.F.Dennis
Measurements of the stable isotopes of oxygen made from samples collected on the Antarctic Deep Outflow Experiment (ADOX) cruises in the Southern Ocean and southern Indian Ocean, February to March 1993 and 1994, are discussed. The data are used in conjunction with hydrographic data to infer characteristics of the formation and mixing of water masses found in the region. The isotopically heaviest waters of the survey were found at the surface of the Madagascar Basin, a consequence of evaporation-induced enrichment of the heavier molecule. The isotopically lightest waters were found on the continental shelf of Antarctica, adjacent to the Princess Elizabeth Trough (PET); these waters are made isotopically light by the injection of around 1% of glacial ice melt, and are probably advected to the region from further east by the current associated with the Antarctic Slope Front. They appear to be locally disassociated from the Antarctic Surface Water and Winter Water (WW) further north in the PET. The WW of the Enderby Basin is isotopically lighter than the PET WW, and also fresher, indicating the presence of an additional component of glacial ice melt or high-latitude precipitation. North of the Antarctic Circumpolar Current (ACC), the delta 18-O of the surface waters show a strong correlation with salinity, but extrapolate to an apparent freshwater endmember which is too isotopically light to be reasonable; advection and mixing of the water masses dominate over the local water balance at this location. The Subantarctic Mode Water of the southern Indian Ocean lies on the line of the surface waters in salinity-delta 18-O space, a consequence of its formation at the surface of the region by deep convection. The Antarctic Intermediate Water also lies on the same line, but this is somewhat coincidental since it does not originate in the region. Consequently, the observation does not, in fact, imply formation by local deep convection. There is no evidence in the data from the two repeated surveys of the Crozet-Kerguelen Gap to suggest interannual variability in the amount of glacial ice melt contributing to Antarctic Bottom Water (AABW) formation in the Weddell Sea. A previous hypothesis based on a subset of the data used here…
P07/W/08-A3 1015
TRACER STUDIES OF ARCTIC SEA-ICE GROWTH AND ABLATION
H. EICKEN (Geophysical Institute, University of Alaska, Fairbanks, AK 99775-7320, USA,
Email: hajo.eicken@gi.alaska.edu), S. Pfirman (Barnard College, Columbia University, New York, NY 10027-6598, USA), H. R. Krouse (University of Calgary, Calgary, T2N 1N4, Canada), C. Haas, J. Freitag, A. Mackensen (Alfred Wegener Institute, D-27515 Bremerhaven, Germany)
In conjunction with ice sampling, stable-isotope and fluorescent tracer studies can yield substantial insight into the processes controlling growth and ablation of the Arctic sea-ice cover. Similar to drifting buoy measurements, the isotopic record of sea-ice cores extracted from the pack ice may provide time series of parameters relevant in the context of ocean-ice-atmosphere interaction. Drawing on field studies the following research will be discussed.
(1) Stable-isotope measurements (delta-18O and delta-D) allow discrimination between the contribution of meteoric (precipitation, snow meltwater, river water) and marine waters to the total ice volume. In the Eurasian Arctic, with its substantial inflow of river water onto the shelves, this helps in identification and tracking of coastal ice into the deep basins. Based on a linear-mixing/fractionation model, the stable-isotopic composition of sea ice has furthermore been utilized to reconstruct the formation and growth of sediment-laden ice in the Laptev Sea.
(2) For a given parent water-mass composition, the growth-rate dependent fractionation of stable isotopes during ice accretion allows a reconstruction of the growth history and - through integration of surface-energy balance data - the oceanic heat flux at the base of the ice cover. In the Transpolar Drift, where isotopic gradients in the surface water are associated with the entrainment of Atlantic water into the Arctic halocline, the isotopic ice-core record may provide insight into the spatial and temporal variability of entrainment processes.
(3) The pronounced isotopic contrast between snow and ice meltwater and the strong seasonality of the melt signal can furthermore be utilized in studies of meltwater dispersal in the ice cover and the underlying water layers. Recent work at the SHEBA field site has demonstrated the great potential of fluorescent tracers in determining lateral and vertical advection of meltwater and associated heat transport through the ice cover on time scales ranging from hours to several weeks. It was shown that hydraulic properties and processes may play an important role in controlling the evolution of summer ice albedo.
P07/W/14-A3 1035
THE APPLICTION OF RADOTRACEDRS TO A STUDY OF BLACK SEA CIRCULATION: VALIDATION OF NUMERICAL SIMULATIONS AGAINST OBSERVED WEAPON TESTING AND CHERNOBYL 137CS DATA
J. V. STANEVA, E. V. Stanev (both at the University of Sofia, 5 J. Bourchier Street, Sofia 1126, Bulgaria, Fax: +359-2-9625276; e-mail: joana@phys.uni-sofia.bg); Ken. O. Buesseler (Department of Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Clark-447, Woods Hole, MA 02543 USA); Hugh D. Livingston (IAEA Marine Environment Laboratory, Monaco)
In this paper, we use the distribution of the artificial radionuclide, 137Cs, to investigate mixing and ventilation in the Black Sea. Time-series data of vertical radionuclide distributions are combined with model simulations in order to follow the magnitude and depth of penetration of surface oxic water into intermediate depths, below the oxic/anoxic interface. Simulated data are produced by a three-dimensional circulation model that includes a new parameterization of the Bosphorus inflow/plume to simulate the effects of Mediterranean water on internal mixing. A radioactive tracer model for pre-Chernobyl 137Cs (weapon testing 137Cs) and Chernobyl 137Cs is coupled to the circulation model. The model results are compared to field data collected between 1986 and 1992. The main output from the simulations is the identification of the contribution of entrainment in the mixing of surface waters into subsurface layers. The trend of tracer penetration into the deeper layers following isopycnal surfaces is well demonstrated in the model and is consistent with the known circulation and physics of the Black Sea. The correlation between the activities of radionuclides and salinity, found in the field data, is supported by the simulations. These model results illustrate that the time-space abundance of the existing field data is sufficient for reconstructing the distribution of tracers in space and time, provided reliable estimates of Black Sea circulation exist.
P07/L/02-A3 1125
REGIONAL AND INTERANNUAL DATA ARCHIVED BY DRIFTING ARCTIC SEA ICE
S. PFIRMAN (Barnard College, Columbia University, New York, NY 10027-6598, USA, Email: spfirman@barnard.columbia.edu); H. Eicken (Geophysical Institute, University of Alaska, Fairbanks, AK 99775-7320, USA); R. Colony (International ACSYS Office, Postboks 5072 Majorstua, N-0301 Oslo, Norway); P. Schlosser (Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964); I. Rigor (University of Washington, Polar Science Center, 1013 NE 40th Street, Seattle, WA, 98105); R. Mortlock (Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964); D. Bauch (GEOMAR, Kiel, Germany)
Arctic sea ice drift trajectories coupled with measurements of d18O values in cores from multiyear ice floes can provide information on regional and interannual variations in sea surface d18O values. The isotopic composition of newly formed sea ice is influenced by the parent water d18O composition and fractionation during ice formation. Under floe ice accretion can be estimated by an ice growth model. Drifting sea ice trajectories are reconstructed from the International Arctic Buoy Programme database. Stepping back along the trajectory, sea surface d18O values are derived from either the measured profiles or a gridded compilation of measured sea surface d18O values. Comparison of the two approaches yields information on regional and interannual variations in sea surface d18O along the drift trajectories as well as ice accumulation and hence heat flux along the drift track. Errors include:1) errors in ice floe trajectories, 2) volume averaging in core samples, and 3) misrepresentation of energy balance terms at the upper or lower surface.
P07/W/15-A3 1145
ADDITION OF GLACIAL MELTWATER TO ANTARCTIC SHELF WATERS DERIVED FROM HELIUM ISOTOPES
Peter SCHLOSSER (peters@ldeo.columbia.edu; Department of Earth and Environmental Sciences, Columbia University, NY; Lamont-Doherty Earth Observatory of Columbia University), Roland Hohmann, Andrea Ludin, and Ralf Weppernig (all at Lamont Doherty Earth Observatory, POB 1000, Palisades, NY, 10964)
The ice shelves surrounding Antarctica contain about 10 percent of atmospheric air. During melting of the glacial ice a significant fraction of this air is dissolved in the sea water. Helium has the lowest solubility among the gases contained in air. Therefore, the melt water is highly supersaturated in helium. The helium signal imprinted by addition of glacial meltwater can be detected in large portions of the shelf region of the Southern Ocean.
Here we present He-4 data from all sectors of the Southern Ocean and interpret them in terms of addition of glacial meltwater to the shelf waters, as well as the deep and bottom waters. An attempt is made to quantify the flux of glacial meltwater into the individual water masses around Antarctica. Where available, the helium data are compared to stable isotope ratios of water.
P07/E/04-A3 1205
ANALYZING ANTARCTIC BOTTOM WATER FORMATION IN A GLOBAL ICE-OCEAN MODEL USING PASSIVE TRACERS
Jean-Michel CAMPIN and Hugues Goosse (Institut d'Astronomie et de Geophysique G. Lemaitre, Universite Catholique de Louvain, 2 ch. du cyclotron, 1348 Louvain-la-Neuve, Belgium, email: campin@astr.ucl.ac.be and hgs@astr.ucl.ac.be)
Antarctic Bottom Water (AABW) is formed near Antarctica. In those regions, the brine released on the continental shelf during ice formation is responsible for an increase of the water salinity. After mixing with ambient water (mainly Circumpolar Deep Water) at the shelf break this salty and dense water sinks along the shelf slope and invades the deepest part of the global ocean. Ocean General Circulation Models generally fail to represent this mechanism correctly. Two potential causes of this problem are a too simple parameterization of the sea-ice influence and the difficulty to simulate the downward movement of the dense water along the slope in large-scale models. In the present study, AABW formation is analyzed in a global coupled ice-ocean model which takes into account these two processes: the sea-ice component has a representation of both thermodynamic and dynamic processes and the ocean component includes an explicit parameterization of downsloping flows. The investigation of the T-S properties of the water masses in the Southern Ocean shows that the model is able to reproduce the main characteristics of AABW although regional differences are not well represented. In addition, the CFC concentrations computed by the model compare favorably with observations. They depict the pathway of the newly formed bottom water and offer a quantitative estimate of the recent ventilation of the Southern Ocean. The simulated natural C14 distribution extends this estimation to longer time scales and provides supplementary information concerning the water masses involved in AABW formation.
P07/W/11-A3 1225
AN OXYGEN ISOTOPE DATA SET FOR MARINE WATERS: DEEP CONVECTION IN THE NE PACIFIC?
Grant R. BIGG (School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K., e-mail g.bigg@uea.ac.uk); Eelco J. Rohling (School of Ocean and Earth Sciences, University of Southampton, Southampton Oceanography Centre, Southampton SO14 3ZH, U.K., e-mail:ejr@soc.soton.ac.uk)
The proportion of 18O in a sample of seawater is an excellent tracer of its past history as, away from the surface, it is conservative and also non-dynamical. The range of values in source waters is also large, and the accuracy achievable in modern measurement high. Here we bring together for the first time a global dataset of over 6000 individual measurements from the past 40 years. The properties of this dataset are described. Noteworthy features include the hitherto unnoticed, but distinctive, contribution of Pacific intermediate water to the 18O:salinity relationship, and different origins of the deeper waters of the Atlantic and Pacific Oceans. As an illustration of the contribution that this dataset could make to oceanography we examine evidence in the 18O distribution for at least occasional deep convection in the Gulf of Alaska of the North-east Pacific.
Wednesday 21 July PM
P07/E/01-A3 1400
ENVIRONMENTAL CONTROL ON MEDITERRANEAN SALINITY AND DELTA18O
Eelco J. ROHLING (School of Ocean and Earth Science, Southampton University, Southampton Oceanography Centre, Southampton SO14 3ZH, United Kingdom, email E.Rohling@soc.soton.ac.uk)
A simple box-model presenting the long-term average Mediterranean is developed that resolves, within one system, for change in both salinity (S) and delta18O. In spite of its basic configuration and schematic input, the model successfully approximates: (1) present-day Atlantic-Mediterranean salinity and delta18O contrasts (using relative air humidity of 70%); (2) milennium-scale variability in Mediterranean inflow volume as also inferred from micropaleontological records; and (3) general changes in basin-average planktonic foraminiferal carbonate delta18O since the last glacial maximum. This justifies the use of the model to explore the nature and principal causes of temporal changes in Mediterranean S and sea-water delta18O (SM and dM, respectively). The model indicates that dM shows temporal variability of much greater amplitude than SM. Relative to the present, reversed Atlantic-Mediterranean delta18O gradients may have occurred, while present-day type salinity gradients were maintained, albeit somewhat weakened. A strong temporal variability is observed of the non-proportionality between dM and SM responses to forcing by an internally consistent set of environmental changes. Consequently, the results suggest that oxygen isotope results cannot be used to reliably approximate paleosalinity change in the Mediterranean. However, oxygen isotope residuals still hold vital information on interactions between the freshwater cycle and advective processes, and so remain an important element in studies of the (paleo-)circulation state of the basin. From foraminiferal carbonate delta18O records, it has been inferred that distinct dM depletions occurred during sapropel deposition. The model results immediately imply that such depletions are possible only if run-off at those times was isotopically more depleted than today. The model results also allow speculation on the nature of particularly strong depletions such as observed in sapropel S5 from the penultimate interglacial maximum. These can no longer be interpreted as immediate evidence of major freshening of Mediterranean surface waters, but might equally likely have resulted from slight (of order 5%) increases in relative air humidity over the basin, relative to the present.
P07/E/03-A3 1430
HOLOCENE MONSOONAL AND UPWELLING RECORDS DERIVED FROM THE MARGIN SEDIMENTS OF THE EASTERN ARABIAN SEA
B.L.K. SOMAYAJULU, A. Sarkar, R. Agnihotri, R. Ramesh, K. Dutta, R. Bhushan. Oceanography & Climate Studies Area, Physical Research Laboratory, Ahmedabad 380 009, India, e-mail: soma@prl.ernet.in) A.J.T. Jull, G.S. Burr (NSF Arizona AMS Facility, The University of Arizona, Tuscon, AZ 85721, USA, e-mail: AMS@physics.arizona.edu)
Eastern continental margins of the Arabian Sea viz. West Coast of India experience widely varying annual rainfall, ~30 cm at 22oN to ~250 cm at ~9 oN peaking to ~400 cm at ~13oN. Between ~200 m and ~1000 m, the waters are perennially anoxic and upwelling induced biological productivity results in high concentrations Corg (~2% - ~6%) in the underlying sediments. Five of the ten gravity cores dated by AMS 14C on planktonic foraminiferal separates yielded fairly uniform sedimentation rates (4-100 cm/k yr) three of which have so far been studied for paleomonsoonal and productivity and/or upwelling trends using d18O, d13C, Corg, Sr/Al, Ba/Al as proxies.
We infer that the Evaporation-Precipitation (E-P), decreased steadily during the last 10 Ka, indicated by d18O of G. sacculifer, after correcting for SST variations, which are minor. Such a trend is also shown on Corg (%) and Sr/Al, indicators of productivity. Significant periodicities of 700 and ~1450 years seen in d18O record indicating possible teleconnection with the North Atlantic Oscillation.
P07/W/09-A3 1450
TESTING ASSUMPTIONS FOR STABLE ISOTOPE PROXY DATA: WHAT CAN MODELS TELL US?
Gavin SCHMIDT (NASA Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University, 2880 Broadway, New York, NY 10025, USA, Email:gschmidt@giss.nasa.gov)
The ratio of oxygen isotopes (O-18 to O-16) in foraminiferal carbonate deposits in ocean sediments and corals is a record of both the ambient ratio of the oxygen isotopes in the surrounding seawater and the temperature-dependent fractionation that occurs as the carbonate precipitates from the seawater. In addition, the record contains influences of thermocline or seasonality changes on planktonic foraminifera ecology. All these factors are functions of climate.
We attempt to untangle these competing effects using results from the GISS global ocean model with water isotope tracers, in conjunction with simple ecological models that forward model the isotopic signals recorded in the carbonate. Models of planktonic foraminiferal abundance estimate the distribution in the vertical over time as a function of monthly column temperature, season, light intensity and density stratification. Carbonate is assumed to precipitate in isotopic equilibrium contemporaneously with abundance. Comparisons with core-top data show reasonable matches when conditions typical for N. pachyderma (l), G. bulliodes and G. ruber (white) are used.
Time series of these virtual proxy data as the model climate changes are then examined to investigate the accuracy of derived climatic variables (SST, SSS) and the sensitivity of the results to changes in the assumptions concerning foraminiferal growth and seawater isotopic variation. Error estimates for paleo-temperatures and paleosalinity records are given for different regions. Errors due to measurements are generally negligible, however, errors due to the "goodness of fit" of various regressions used in calculations can be significant. In regions such as the North Atlantic, co-variance of d18Ow and temperature are the biggest source of error in paleo-temperature reconstructions. For paleo-salinity calculations, the largest errors occur because of the differences between temporal and spatial d18O: salinity relationships and through the use of non-concurrent temperature measurements.
P07/W/10-A3 1510
HIGH-RESOLUTION CORAL D14C RECORDS FROM THE TROPICAL AND SUB-TROPICAL PACIFIC: RECONSTRUCTING OCEAN DYNAMICS.
T.P. GUILDERSON (Center for AMS, LLNL, Livermore CA), D.P. Schrag (Harvard Univ., Cambridge MA), M. Kashgarian, and J. Southon both at (Center for AMS, LLNL, Livermore CA).
We have generated several new near-monthly resolution, coral-based, post-bomb radiocarbon (D14C) time series from the subtropical and tropical Pacific. In general, the subtropics (Hawai'i, Rarotonga) have higher D14C reflecting the stability of the gyres and high air-sea exchange. D14C in the eastern equatorial Pacific (Galapagos) are lower and reflect the subsurface pathway of the Equatorial Undercurrent and entrainment of deeper thermocline waters which feed the upwelling in this region. Radiocarbon values in the "warm pool" region (Nauru) are intermediate between the higher subtropics and those in the east. The long-term trend reflects the invasion of bomb- 14C into the surface ocean. The post-bomb maxima in the subtropics is reached in the early 1970s whereas at Nauru and Galapagos it is delayed by 10 years. The delay is a consequence of the subsurface history of waters upwelling in the east and the subsequent advection and mixing of these water in the west. Superimposed upon the long term trend is seasonal to interannual variability. The patterns of radiocarbon variations in the Galapagos timeseries can be explained in terms of variability in the intensity of upwelling and changes in source water feeding the upwelling. During ENSO warm events, the depth of the thermocline increases in the east so that Ekman pumping no longer brings water from the undercurrent to the surface. This creates high D14C anomalies in the coral timeseries which are most pronounced from July through September. Large interannual variability is observed in the Nauru record and also follows ENSO. During ENSO warm phases D14C is higher reflecting the reduction of low-14C water upwelling in the east and the invasion of subtropical water into the western equatorial tropical Pacific. Warm events tend to be sharply terminated with sharp overshoots. These series of coral records demonstrates the potential for using radiocarbon time series for documenting variability in Pacific shallow circulation over interannual and decadal timescales. Timeseries such of these hold great promise to augment onetime surveys such as GEOSECS and WOCE.
P07/W/19-A3 1550
RIVER, SEA-ICE, AND PACIFIC WATER MASS COMPONENT SEPARATION FROM ATLANTIC WATER IN THE ARCTIC OCEAN USING delta O-18 AND PO4*
Brenda EKWURZEL (ekwurzel1@llnl.gov ), Peter Schlosser (peters@ldeo.columbia.edu), Rick G. Fairbanks (fairbank@ldeo.columbia.edu), and Rick Mortlock (mortlock@ldeo.columbia.edu), (all at Lamont-Doherty Earth Observatory, POB 1000, Palisades, NY, 10964, present address for Ekwurzel: Lawrence Livermore National Laboratory, POB 808, Livermore, CA 94551)
Immense river discharge combined with the small ocean basin volume gives the Arctic Ocean the distinction of having the highest percentage of river runoff of all the world ocean basins. The resulting surface mixed layer and highly stratified halocline form a low salinity layer of water with temperatures close to the freezing point that overlie the more saline and warmer layer of Atlantic origin. Three freshwater sources maintain the halocline: river runoff, sea ice meltwater, and the low salinity Pacific Water entering through the Bering Strait.
Identifying the freshwater components in the halocline provides a clue to possible causes for the diminished halocline observed over the past few years. We use delta O-18, PO4* (= PO43- + O2/175 - 1.95 micro mol/kg), and salinity to quantify the spatial and temporal distribution of freshwater sources in the halocline. Fractionation between the heavier and lighter oxygen isotopes due to temperature and distance from the source area results in significant depletion of O-18 in Arctic River runoff relative to Standard Mean Ocean Water. The phase transition during sea ice formation fractionates the oxygen isotopes and can be used to separate the sea ice component. The large range between the nutrient-rich Pacific Water PO4* (~ 2.4 micro mol/kg) and the three other components (range: 0.1-0.7 micro mol/kg) make it a good tracer for the Pacific Water component.
River runoff circulation in the Kara Sea in 1993 exhibited a cyclonic circulation from the Ob and Yenisey rivers, plus a net eastward flow toward the Laptev Sea. River runoff exhibited a net eastward surface circulation in the Laptev and East Siberian Sea, with a northeastward component flowing toward the Lomonosov Ridge and Mendeleyev Ridge region in 1994…
P07/W/02-A3 1620
CONSTRAINING OCEAN GENERAL CIRCULATION MODELS USING CFC OBSERVATIONS
Suzanne GRAY (Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK, Email:S.L.Gray@rdg.ac.uk) Thomas Haine (Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, UK).
We have developed and tested an inverse method, based on Green's functions, for using transient tracer observations to constrain Ocean General Circulation Models. The method has been tested using a North Atlantic configuration of MICOM (the Miami Isopycnic Coordinate Ocean Model) with 4/3 degree resolution. A substantial quantity of North Atlantic CFC-11 and CFC-12 data has been collated.The fluxes of these tracers into the ocean and the errors in the fluxes and in the tracer data have also been estimated.
Systematic differences can be seen between the observed CFC concentrations and an optimal prediction of these concentrations by the model. The errors in these predictions (due to errors in the assumed tracer flux into the ocean) and in the observations are incorporated into the inverse calculation to yield a quantitative measure of the degree of fit between the predictions and the observations. A poor fit is a consequence of deficiencies in the model circulation and may be due to processes which are not well parameterized in the model. We find that the model circulation
is inconsistent with the CFC dataset, the estimated tracer fluxes, and the estimated errors on scales of order a few 100 km.
P07/W/03-A3 1640
STABLE ISOTOPES OF OXYGEN IN SEAWATER IN THE NORDIC SEAS AND NORTHEAST ATLANTIC
Karen J. HEYWOOD, Michael P.Meredith, Timothy A. Winters, Paul F.Dennis (School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, email: k.heywood@uea.ac.uk)
We present analysis of the stable isotopes of oxygen (O18 and O16) in seawater samples collected in 1991, 1996, 1997 and 1998 in the Nordic Seas and Northeast Atlantic. Most of the work has been undertaken through the EU MAST programme VEINS (Variability of Exchanges in Northern Seas).
In Fram Strait, preliminary calculations of the flux of meteoric water and sea ice melt out of the Arctic show a strong meteoric outflow, and negative sea ice meltwater. Negative values imply sea ice formation, equivalent to 4 m of ice. Just southwest of Svalbard, observations in the Storfjord plume of bottom water indicate that while the surface water is predominantly fresh due to sea ice melt, the bottom water is fresh due to mainly meteoric imput (probably glacial melt from Svalbard).
Results from Denmark Strait show that between August 1972 (GEOSECS data) and August 1991, the core of Labrador Sea Water (LSW) had both freshened and ecreased in O18, due to changes in the formation properties of LSW. Because there is a change in O18, the freshening of LSW must be due to the inclusion of meteoric water precipitation or runoff from land/ice) and not due to changes in sea ice melt quantities or locations. In the deep water overflows, the O18 data indicate that the water has undergone sea ice freezing processes during its formation, showing that sea ice formation is an important component of the deep convection process.
In contrast, deep waters in the Faeroe-Shetland Channel are diluted with fresh water or meteoric origin (probably Arctic precipitation) and no influence of sea ice freezing processes is required. Isotope ratios in the European Shelf Edge Current, observed in the Faeroe-Shetland Channel and the Rockall Trough, imply influence of water from the Mediterranean.
P07/W/05-A3 1700
AN ESTIMATE OF THE ANTHROPOGENIC OFFSET OF OCEANIC DELTA C-13 (DIC) BASED ON THE VENTILATION OF THE CALIFORNIA CURRENT AT 42°N.
J.D. ORTIZ 1, A.C. Mix2, P. Wheeler2, and R.M. Key3, 1Lamont-Doherty Earth Observatory of Columbia University, 2College of Oceanic and Atmospheric Sciences, Oregon State University, 3Department of Geosciences, Princeton University
Nutrient and delta C-13 (DIC) measurements from depth profiles down to 2,500 m at four stations across the California Current (42°N) constrain the magnitude of the oceanic anthropogenic delta C-13 9DIC) shift (denoted Delta delta C-13a-p). At these stations, nitrate, rather than phosphate, provides the strongest correlation to delta C-13 (DIC). Values of Delta delta C-13a-p vary systematically as a function of water mass, similar to the pattern of Delta C-14 and Tritium penetration for these waters. Near surface waters of potential density (sq) < 25.1 and within the Shallow Salinity Minimum (sq 25.1 to 26.5) have a mean Delta delta C-13a-p offset of -0.6 +/- 0.24. This estimate is within the range of surface water values cited by Beveridge and Shackleton [1994] and suggests that near-surface waters obtain their anthropogenic signature by direct contact with the atmosphere, while waters of the Shallow Salinity Minimum (SSM) obtain their anthropogenic signature through winter ventilation of outcropping isopycnal surfaces. Below the SSM, a simple scaling argument suggests that the anthropogenic signature is transmitted diffusively across the North Pacific Intermediate water (NPIW). Our results suggest the magnitude of Delta delta C-13a-p in well ventilated waters is essentially constant, and decreases diffusively into less well ventilated waters.
P07/W/18-A3 1720
HELIUM-3 BALANCE OF THE UPPER WATERS OF THE WEDDELL SEA: IMPLICATIONS FOR OCEANIC HEAT FLUXES
Roland HOHMANN and Peter Schlosser (Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades, NY 10964-8000, USA, email: hohmann@ldeo.columbia.edu)
To a large extent the hydrographic structure of the upper waters of the Weddell Sea is the result of atmosphere/ice/ocean interaction. During winter, a seasonal ice cover develops over the marginally stable ocean surface layer. Brine release during sea ice formation induces convection in the Winter Mixed Layer (WML) which leads to the erosion of the pycnocline that separates the WML from the underlying Weddell Deep Water (WDW) and to subsequent entrainment of relatively warm, salty, low-oxygen and 3He-rich WDW into the WML. In summer, after the melting of the sea ice cover, warm and fresh Antarctic Surface Water (AASW) is formed by addition of sea- ice meltwater to the WML water and by solar heating. The remnant water of the WML is preserved as Winter Water (WW) and centers around 80 to 100 m depth. It is not possible to determine the rate of WDW entrainment into the WML directly from temperature and salinity, because the heat transferred into the WML by WDW entrainment is lost to the atmosphere by heat flux through the sea ice cover and leads, and the mixed layer salinity is altered by sea ice formation and melting. An alternative method is provided by the evaluation of gas balances (e.g. 3He) of the upper layers in the ice-covered regions. If we assume that the elevated 3He concentrations of the WML are chiefly the result of entrainment of 3He-rich WDW into the WML and that gas exchange is to a large extent suppressed by the sea-ice cover, we can convert the observed 3He excess into the apparent fraction of WDW contained in the WML and calculate entrainment rates and heat fluxes. Data obtained from samples collected during the Winter Weddell Sea Project (ANT V/2, 1986), the drift of Ice Station Weddell (ISW, 1992), and the Antarctic Zone Flux Experiment (ANZFLUX, 1994) provide a consistent picture of the entrainment of WDW into the WML and the oceanic heat fluxes in the central Weddell Sea during winter. The fraction of WDW in the WML derived from the 3He excess in the surface layer increases with latitude and reaches 20–30% at 68°S…
P07/W/07-A3 1740
WATER MASS ANALYSIS IN THE WESTERN EQUATORIAL PACIFIC USING ANTHOPOGENIC TRACERS
Olaf KLATT, Birgit Klein (both at Institute of Environmental Physics, Department of Tracer Oceanography, P.O. Box 330440, 28334 Bremen, Germany, Email oklatt@physik.uni-bremen.de)
During R/V Sonne cruise 113 in late 1996, in the Western Equatorial Pacific, hydrographic properties were measured as well as (anthropogenic) tracers (Chloroflourocarbons (including CCl4), Tritium, Helium). The goal of the tracer analysis is to further our knowledge about intermediate and deep waters in this area of investigation. The anthropogenic origin of CFCs and tritium makes them useful tools to investigate water mass formation and spreading. Making use of the different input-function of the above mentioned tracer one can distinguish temporal and spatial differences in the spreading of water masses. Whereas CFCs allow a temporal differentiation, tritium measurements allow a spatial distinction of the formation areas. Helium marked the water masses at the bottom of the ocean due to the geothermal activity of the East Pacific Rise, hence it is qualified for the deduction of spreading of deep water. We found elevated CCl4 concentrations at the bottom north of the Caroline Seamounts. The presence of higher CCl4 concentrations at the bottom is interpreted as a signature of the spreading of Lower Circumpolar Deep Water (LCDW) in a western boundary current. It has been postulated that a deep boundary current should exist north of the Caroline Seamounts in which a faster advection of LCDW could be expected. The tracer measurements indicate the boundary current must include a component of LCDW which is younger than 80 years.The different behaviour of CFC and tritium leads to a large dynamical range with respect to the ratio of CFC-11 and tritium. By means of this ratio we are able to separate intermediate water masses of North Pacific origin from those of South Pacific origin.
Wednesday 21 July AM
P07/W/01-A3 Poster 0900-01
210Pb BALANCE IN THE SEDIMENT AND WATER COLUMNS OF THE EAST SEA
Deok-Soo Moon (Chungnam National University, Dept. of Oceanography, Taejon, Korea, 305-764; E-mail : s_dsm@hanbat.chungnam.ac.kr) and Kee-Hyun Kim (Chungnam National University, Dept. of Oceanography, Taejon, Korea, 305-764; E-mail: khkim@hanbat.chungnam.ac.kr)
The balance of radioactive lead in the East Sea was constructed by comparing the deficiency of 210Pb in given water column (its supply from the atmosphere and in situ decay of 226Ra) with inventories of excess 210Pb in underlying sediments. The scavenging effectiveness defined as the ratio of 210Pb deficiency in water column to total supply into sediment column, was estimated to be 82-98 % in regions as the shelf of Korea Strait and upper slope of Ulleung Basin, and less than 82 % in the central Ulleung Basin. These high scavenging effectiveness in coastal regions indicates that 210Pb adsorbed onto sinking particles is actively removed from water column by scavenging. A estimation of the relative importance of vertical and lateral processes in 210Pb scavenging is derived from the ratio of sediment 210Pbex inventories to the deficiency of 210Pb in the overlying water column. The 210Pbex fluxes in coastal sediment were deposited in the range of 116-170 %, compared to the values estimated from 210Pb deficiency relative to 226Ra profiles with the atmospheric flux. This is interpreted as evidence that the sea floor of these regions are supplied much fine sediment in the past 100 years compared to other regions. The increased 210Pb deliveries to these areas located in outflow of the Nakdong river is a consequence of sediment focusing in the last century, rather than surface productivity. In contrast, sediments in the central Ulleung Basin were accounted for only 35 - 70 % of the 210Pb scavenged from the overlying water column, and the remainer were transported to sinks outside the area. The data coverage is insufficient to permit a quantitative mass balance to be constructed, but cores taken in areas of frequent bottom disturbances and underlying the relatively high productivity waters have surpluses of 210Pbex.
P07/W/04-A3 Poster 0900-02
FRESHWATER BALANCE OF THE LABRADOR SEA
Samar P. Khatiwala, Peter Schlosser, Richard G. Fairbanks, and Richard Mortlock
Lamont-Doherty Earth Observatory of Columbia University, Palisades NY 10964, USA
Oxygen isotope (H2O18/H2O16)/salinity data are used to identify freshwater sources to the Labrador Sea. The dominant freshwater source for the low-salinity Labrador and West Greenland Currents is runoff into the Arctic Ocean. It is estimated that 2-3.5 m of freshwater is extracted from the water column on the Labrador Shelf to form sea ice, a process which significantly modifies the isotope-salinity characteristics of waters on the Labrador Shelf, as well as further downstream on the Scotian Shelf, Gulf of Maine, and Middle Atlantic Bight. In the interior Labrador Sea, low-salinity water originating in the Arctic is diffusively mixed in from the boundaries and is of primary importance in freshening the upper 200m. Sea ice meltwater is added to the surface ocean by local melting of sea ice, and its influence is restricted to the upper 10-15m. We also examine the impact of interannual variations in sea ice formation on (a) the strength and T/S properties of the Labrador Current, and (b) exchange rates between the Shelf and open ocean. These results have important implications for convection in the Labrador Sea.
P07/W/12-A3 Poster 0900-03
TRANSPORT OF ICELAND SCOTLAND OVERFLOW WATER FROM THE ICELAND BASIN TO THE WESTEUROPEAN BASIN
Uli FLEISCHMANN, Reinhold Bayer, Alfred Putzka
Increased values of freon (CFC-11) and tritium at the western flank of the Mid Atlantic Ridge at 48° N (WHP A2 section) hint to newly ventilated Iceland Scotland Overflow Water (ISOW). This water has passed the Gibbs Fracture Zone (52° N) and flows topographically leaded towards the south along the Mid Atlantic Ridge. By comparison of the tracer concentrations in the eastern and western part of the eastern basin a surplus of tracers is determined. This surplus is used to calculate a CFC-11/tritium ratio age for the ISOW, which is compared with the CFC-11/tritium ratio age of the ISOW on the section WHP A1 (58° N) distance of the two sections, the area covered by the measurements with a tracer surplus on the WHP A2 section and the fraction of ISOW resulting from the ratio dating on A2 result in a lower limit for the transport of newly ventilated ISOW past the Gibbs Fracture Zone of (1.08±0.20) Sv.
P07/W/13-A3 Poster 0900-04
ASSIMILATING A GLOBAL OCEAN OXYGEN-18 ISOTOPE DATA SET INT0 AN OCEAN GENERAL CIRCULATION MODEL
Martin WADLEY and Grant Bigg (both at School of Environmental Sciences, University of East Anglia, Norwich, UK, email: m.wadley@uea.ac.uk) Eelco Rohling (Southampton Oceanography Centre, Southampton, UK)
A global ocean oxygen-18 isotope data set (Bigg and Rohling, 1999) has provided a new method of validating ocean general circulation models (OGCMs). An OGCM is forced at the surface to the observed oxygen-18 isotope distribution, and the resulting modelled sub-surface distribution compared with observations. In regions of agreement between the modelled and observed values the OGCM provides a valuable tool for interpolating between sparse observations. However, in regions of disagreement between the modelled and observed oxygen-18 distributions, the use of this extra passive oceanic tracer provides a useful tool for understanding the weaknesses of the OGCM, and evaluating improvements.
P07/W/16-A3 Poster 0900-05
A RESISTANCE METHOD OF TRACER DATA ANALYSIS.
Mikhail Vladimirovich ANISIMOV, P.P Shirshov Institute of Oceanology, Nakhimovskiy pr.,36, Moscow 117851, Russia
A one-dimension task of radioactive tracer transfer is considered. The purpose is to find a space distribution of transfer coefficient, when discrete observatons of concentration are available. The base is using of a "resistance" value, which is bound with transfer coefficient like an electrical resistance with conductivity. The method seems to be simple and flexible and applied to RN-222 data in the ocean.
P07/E/02-A3 Poster 0900-06
STRUCTURE AND VARIABILITY OF THE THERMOHALINE FIELDS AND THE CHARACTERISTICS OF THE CURRENTS IN THE ARGENTINE ISLANDS (THE WESTERN SHELF OF THE ANTARCTIC PENINSULA) IN MARCH 1998
Juriy ARTAMONOV, Pavel Lomakin, Irina Orlova, Juriy Popov, Vladimir Ukrainsky (all at Marine Hydrophysical Inst.,2 Kapitanskay str., Sevastopol, 335000, Crimea, Ukraine; email:ocean@mhi2. sebastopol.ua. Tell.: 380 (0692)522236, Fax: 380 (0692) 444253).
The thermohaline structure and the characteristics of the currents in the region of the Argentine Island shelf on the base of CTD- and current observations during the Second Ukraine Antarctic Expedition (March 1998) are analyzed. It is shown that the vertical thermohaline structure had 2-layer stratification, which is typical for the Antarctic shelf water in the autumn season. Tidal activity is the essential factor induced variability in the thermohaline fields. Part connected with this phenomenon in the summary dispersion of the temperature and salinity on the synoptic temporal scale is estimated as 50 – 70%. Tidal currents are produced by the semi-diurnal and the diurnal tidal waves. Semi-tidal waves spread from NNW to SSE, diurnal waves – from WNW to ESE. Tidal and remainder currents were very week. Theirs speeds were not more then 2-4 sm/s. Reverse and elliptic tidal currents were fixed.
P07/W/20-A3 Poster 0900-07
FLOW THROUGH BATHYMETRIC CHANNELS AT THE MOUTHS OF ESTUARIES
R. P. Mied, R. A. Handler, and T. Evans, Naval Research Laboratory Remote Sensing Division Washington, D.C.20375
In-situ observation and remote sensing imagery indicate the presence of velocity convergences located over bathymetric channels in the mouths of tidal estuaries. In the Chesapeake Bay for example, there are three such features which extend partially or completely through the mouth of the Bay. As such, they are riven by strong tidal forcing, and convergent features are formed over them. In this talk, we present numerical simulations performed to investigate these velocity structures in an estuary with a channel in an otherwise flat bottom.
The equations of motion are solved using a fully spectral code in the vertical plane (x-z) on a rotating earth. No along-channel flow variations (in the y direction) are permitted The bottom bathymetry is generated by using a unique virtual surface approach (Goldstein et al., J. Comp. Phys.,105, 1993) in which the no-slip bottom is generated using feedback forcing. A Gaussian-shaped channel is employed to simulate typical bathymetry observed in estuarine regions. In the along-channel direction, a constant pressure gradient is imposed and the flow is integrated until a steady state results.
The simulations are performed at high Rossby number (of order unity) based on the width of the groove and a typical surface velocity. Simulations show the development of a localized surface jet co-located with a bottom feature. This bottom feature takes the form of a recirculation cell, and results from the coupling of the along-channel flow with the motion in the vertical plane. The associated across-channel surface flow in the vicinity of the jet exhibits convergent and divergent regions, which correlate reasonably well with features observed in radar imagery. Their position and strength are seen to vary with the along-channel Reynolds number, Ekman layer thickness, and aspect ratio of the channel.
P07/L/03-A3 Poster 0900-08
THE TRANSFER OF 99TC FROM SELLAFIELD TO THE ARCTIC
Peter Kershaw^a, Lars Foyn^b, Gordon Christensen^c, Kins Leonard^a, Hilde Elise Heldal^b, Per Varskog^c.; Centre for Enviromental, Fisheries and Aquaculture Science (CEFAS), Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK, ^b The Institute of Marine Research, Nordnengaten 50, P.O. Box 1870, N-5024 Bergen, Norway, ^c Institute for Energy Technology, P.O. Box 25, Kjeller, Norway
Substantial increases in the discharge of 99Tc occurred in the mid-1990's, from the Sellafield nuclear fuel reprocessing plant in the UK, against the overall trend of most other radionuclides. 99Tc is long-lived radionuclide which normally is considered to behave conservatively in seawater and is readily taken up by by the brown seaweed Fucus sp. The "pulsed" release of 99Tc in 1994-96 has provided a new opportunity to study transport pathways and transit times from the Irish Sea to other parts of the NE Atlantic and Arctic Oceans. The distribution of 99Tc has been mapped from a series of RV cruises, extending from the Irish Sea to Fram Strait and the Barents Sea. A comparison is made between the response of seawater concentrations and those in Fucus vesiculosus (used as a bio-accumulator) to variations in the release rates. Transit times within UK waters and the North Sea from the 99Tc data were significantly shorter than rates reported previously using other radiotracers. The possible reasons for this are discussed, together with the scope for continuing to follow this tracer as it enters the Eurasian Basin.