Stable Thermocline Research Articles

Optimized flow distributor for stabilized thermal stratification in a single-medium thermocline storage tank: A numerical and experimental study

This paper investigates the impact of flow injection on the temperature stratification inside single-medium thermocline storage tanks. Special focus is given on how to maintain the stable thermocline evolution during the dynamic charging or discharging operation, by using a new type of fluid diffuser, namely the Ring-Opening Plate Distributors (ROPDs). Firstly, the ROPDs are designed and the size distribution of the ring-shape openings is optimized by a simulation-based optimization algorithm. This diffuser concept has then been validated experimentally, showing its effectiveness in improving the charging and discharging efficiency of a laboratory-scale storage tank by up to 14.5% and 19.8%, respectively.Systematic experiments are performed to examine the effects of the injecting flow rate and temperature on the local transient temperature profiles and on the energy and exergy efficiencies of the thermocline tank. The stability range of operational parameters, characterized by the bulk Froude number (Fr < 3), has been determined for quasi-undisturbed advance of the thermocline. Beyond this stability range, simple tube-type diffuser is no longer capable of mitigating the impact of inflowing thermal jet. Additional structuration and optimization of the diffuser geometry become essential to stabilized the thermal stratification and to maintain a good thermal performance of the storage tank.

Experimental investigation on single-medium stratified thermal energy storage system

The demand for renewable energy sources is limited by their inherent intermittent nature. The thermal energy storage technique overcomes this shortcoming by allowing storage of naturally occurring energy (e.g. solar) during “non-peak hours”. The article presents an experimental investigation of the sparsely studied ‘single medium thermocline’ (SMT) based single tank sensible thermal energy storage (TES). The work aims to reconsider its potential by minimizing the fluid dynamic perturbations during the charging cycle which disrupt the stability of the thermocline leading to undesirable energy losses. The article discusses the effect of Atwood number (density stratification) and mean temperature on TES effectiveness as well as providing the appropriate thermodynamic insights. Implementation of the experimental procedures and strategies discussed in this article demonstrates a stable thermocline which persists for more than 6 h (hours) with minimal energy degradation.

Refining the interpretation of oxygen isotope variability in free-swimming organisms

Serially sampled oxygen isotope ratios (δ18O) from fossil and modern cephalopods may provide new insight into the behavior and longevity of individuals. Interpretation of these data is generally more difficult than similar data from bivalves or brachiopods because the measured δ18O from shell combines both seasonal change and depth change over the life of an individual. In this paper, a simple null model is presented combining the three fundamental controls on a measured δ18O profile in a free-swimming organism: swimming behavior, seasonal water column change, and time averaging in sampling. Model results indicate that seasonal variability in δ18O in a free-swimming organism can be interpreted in locations with strong seasonality through most of the swimming range but is complicated by swimming velocity and is sometimes best expressed by changes in δ18O variance rather than simple sinusoidal patterns. In other locations with a stable thermocline or seasonal ranges in only a small portion of the water column, no variability caused by seasonality would be expected. Furthermore, large ranges of δ18O (~ 4‰) are possible within and between individuals in settings with persistent thermoclines like the tropics, depending on the swimming depth limits and behavior of individuals. These results suggest that future interpretation of serially sampled δ18O should consider seasonal water column variation from either modern or modeling sources in addition to comparison to co-occurring benthic and planktonic organisms. Additionally, this modeling casts doubt on the promise of isotope sclerochronology alone as a growth chronometer in ammonites and other free-swimming fossil organisms and highlights the need for other methods of quantitatively determining age.

Summer Distribution of Total Suspended Matter Across the Baltic Sea

There are three optical in-water components that, besides water itself, govern the under-water light field: phytoplankton, total suspended matter (TSM) and colored dissolved organic matter (CDOM). In essence, it is the spectral absorption and scattering properties of each optical component that govern the underwater light field, and also the color of the sea that we can perceive, and that can also be measured remotely from space. The Baltic Sea is optically dominated by CDOM, apart from cyanobacteria blooms that often cover most of the Baltic proper during summer. Remote sensing images of TSM reveal large-and mesoscale features and currents, especially in the Southern Baltic, which are influenced both by atmospheric Rossby waves and the Coriolis force. In coastal waters, the optical properties are strongly influenced by inorganic suspended matter, which may originate from coastal erosion and from run-off from land, streams and rivers. In this paper, we evaluate the distribution of TSM across the Baltic Sea using remote sensing data and statistically compare the TSM loads in the different Helsinki Commission (HELCOM)-defined basins. The TSM loads during summer vary substantially in the different basins, with the south-eastern Baltic overall being most influenced by cyanobacteria blooms. The Gdansk basin and the Gulf of Riga were distinguished both by relatively high TSM loads with high standard deviations, indicating strong fluvial input and/or resuspension of sediments. We also evaluate a coastal TSM transect in Himmerfjarden bay, which is located at the Swedish East coast in the Western Gotland Basin. The effect of wind-wave stirring on the distribution of TSM from source (shore) to sink (open sea) can be assessed using satellite data from European Space Agency’s (ESA) MEdium Resolution Imaging Spectrometer (MERIS) mission (2002-2012) with 300 m resolution. The TSM transect data from areas with low wind exposure and a stable thermocline showed a gradient distribution perpendicular to the coast for summer seasons 2009, 2010, 2011 and a 3-year summer composite, confirming a previous bio-optical study from the Western Gotland basin.

The experimental investigation of the thermal stratification in a solar hot water tank

Thermal storage technology is an important approach to improve the efficiency of solar energy utilisation. To analyse the thermal stratification in a hot water storage tank at an initial temperature of 80 °C and at an inlet temperature of 5 °C, this study thoroughly analysed the impact of the position of PCM balls on the thermal stratification for various flow rates (1, 3, 5, 7, and 9 L/min) under different position. Performance parameters, including the Richardson number, MIX number, and exergy efficiency, were involved in the evaluation. The study was further extended to explore the applicability of Fill Efficiency (FE) as a performance parameter of thermal stratification within a storage tank. The experimental results indicated that, for the same inlet flow rate, an improved thermal stratification of the tank was achieved as the positions of the balls became closer are to the inlet, and the balls with a small diameter also showed remarkable potential for improving the thermal stratification of the water tank. When the flow rate increased, the cold and hot water mixing intensified, and the thermocline thickness in the tank increased. Thus, the thermal stratification weakened. Furthermore, when the water-release process progressed, the mixing of the hot and cold water in the water tank tended to be stable, and formed a stable thermocline. Finally, when the inlet flow rate was less than 7 L/min, the thermal stratification of the water tank exceeded that of the PCM water tank, while for an inlet flow rate in excess of 7 L/min, the thermal stratification of the PCM water tanks was superior to that of the water tank in the case when the balls was on the fourth layer.

An experimental investigation on thermal stratification characteristics with PCMs in solar water tank

Thermal storage technology with phase-change materials (PCMs) is an important approach for improving solar energy utilisation efficiency. In this study, for analysing the stratification of a thermal tank with PCMs at an initial water temperature of 353.15 K and inlet water temperature of 278.15 K, a thermal storage tank containing sodium acetate trihydrate with a phase change temperature of 325.15 K and super-cooling temperature below 278.15 K was developed. This study thoroughly investigated the effect of the positions of the PCMs on thermal stratification characteristics at various flow rates (0.06, 0.18, 0.3, 0.42, and 0.54 m3/h) and with increasing dimensionless time. This study further examined the fill efficiency, which was compared with the exergy efficiency, MIX number, and Richardson number to characterise the stratification of the thermal tank. The experimental results demonstrated that when the temperature of the water storage tank increased from 278.15 K to 353.15 K, the energies of the water tank and PCM tank were 18.81 and 19.34 MJ, respectively. At the same inlet flow rate, increasing the PCMs close to the inlet resulted in improved thermal stratification of the tank. With high flow rates, the cold–hot water mixing intensified and the thermocline thickness in the tank increased, thereby weakening the thermal stratification. Moreover, as the water-release process progressed, the cold–hot water mixing in the water tank tended to be stable, thereby forming a stable thermocline. The thermal stratification of the ordinary tank was superior to that of the PCM tank. However, as the PCMs were located at the bottom of the water tank, the thermal stratification was optimal when the inlet flow rate was higher than 0.42 m3/h.

An assessment of climate change and stratification in Lake Kariba (Zambia–Zimbabwe)

AbstractLake Kariba is a monomictic reservoir, isothermal in winter (June–August), and stratified for the rest of the year. Its temperature has increased since the 1990s, with some researchers suggesting it now has a stronger and more stable thermocline as a result. It was also claimed that the depth of the epilimnion had decreased, and nutrient depletion has led to a decrease in the depth of the euphotic zone, algal biomass and primary productivity. A re‐analysis of the data suggests this view is incorrect. The epilimnion is more homogenous now than in the 1980s, with less vertical variation in temperature and dissolved oxygen in the upper 20 m of the lake. The depth of the euphotic zone has not changed since the 1960s, and the algal biomass and primary productivity have increased. The thermocline appears to have weakened with temperature and density gradients being much smaller than in the 1980s because of a more rapid rate of warming in deeper water. The reasons for this phenomenon are not clear, although evaporative cooling at the surface may contribute to it. There was a significant increase in Birgean heat budgets over time, although there was none for stability. This may be explained by the fact that the lake is monomictic, cooling to 22°C in winter, with a new cycle of stratification beginning each year, and it may be affected by variable factors such as evaporation, rainfall, river flow and internal wave movements.

Microbial metatranscriptomes from the thermally stratified Gulf of Aqaba/Eilat during summer

The water column in the oligotrophic Gulf of Aqaba/Eilat experiences distinct seasonal cycles with the cooling air and water temperatures of late fall and winter destabilizing the thermocline and forming mixed layer depths reaching 300 to 700m. As air temperatures warm thermal re-stratification results in a stable thermocline throughout the summer which physically separates a photic, nutrient-poor surface layer from an aphotic, nutrient-rich deep layer. Here we present the first metatranscriptome dataset, and its taxonomic assignments, sampled from three depths of the 700m deep Station A in the Gulf of Aqaba during the summer stratification (surface – 10m, deep chlorophyll maximum (DCM) – 85m, deep aphotic zone −500m). Intensive transcriptional activity was attributed to Prochlorococcus - the most abundant photosynthetic organism in the RNA-seq dataset - both at the surface and at the DCM. In contrast, cDNA reads related to picoeukaryotic algae were detected almost exclusively at the DCM. The metatranscriptomes presented here provide a basis for examining the seasonal differences in microbial gene expression by comparison with the published metatranscriptomes sampled during the winter deep-mixing from the same station.

Thermocline characteristics of molten-salt thermal energy storage in porous packed-bed tank

Based on the local thermal equilibrium theory in porous media, a two-dimensional numerical model is developed to investigate the heat storage and heat release processes of a molten-salt thermocline in a porous packed-bed tank. The numerical model is validated by comparing the simulation results with the experimental data. The performance of the heat storage system is analyzed using the thermocline thickness and the heat storage efficiency with a focus on energy balance. The results show that stable thermocline layers can form during heat storage or heat release processes. Because of the existence of the thermocline and replaced porous fillers, the heat storage efficiency in a porous packed-bed tank is slightly lower than that of pure molten-salt thermocline heat storage. The volumetric heat capacity of porous media is a key factor in heat storage performance. Improving the performance of the porous filler and setting the condition of a reasonably low flow rate, it can enable optimization control of the thermocline evolution in heat storage/release processes and improve the thermal efficiency of the heat storage system, which reveals directions for further research.

Paleoceanographic changes during the Albian–Cenomanian in the Tethys and North Atlantic and the onset of the Cretaceous chalk

During the mid-Cretaceous the Earth was characterized by peculiar climatic and oceanographic features, such as very high temperatures, smooth thermal meridional gradient, long-term rising sea level, and formation of oceanic gateways and seaways. At that time widespread deposition of micritic pelagic limestones, generally called chalk, occurred in deep pelagic settings as well as in epeiric seas, both at tropical and at high latitudes. The origin of such extensive chalk deposition in the mid-Cretaceous is a complex and still controversial issue, which involves the interaction of several different factors. In this work we address this topic from the paleoceanographic perspective, by investigating the contribution of major oceanic circulation changes.We characterize several stratigraphic sections from the Tethys and North Atlantic with litho-, bio-, and carbon isotope stratigraphy. Our data show a change between two different oceanic circulation modes happening in the Late Albian. The first is an unstable mode, with oceanographic conditions fluctuating frequently in response to rapid environmental and climatic changes, such as those driven by orbital forcing. The second mode is more stable, with better connection between the different oceanic basins, a more stable thermocline, more persistent current flow, better defined upwelling and downwelling areas, and a more balanced oceanic carbon reservoir. We propose that under the mid-Cretaceous paleogeographic and paleoclimatic conditions this change in oceanic circulation mode favored the beginning of chalk sedimentation in deep-water settings.

Water Mass Characteristics and Stratification at the Shallow Sunda Shelf of Southern South China Sea

CTD data obtained from two oceanographic cruises during June and October 2012 were used to define the water mass characteristics and degree of stratification at the shallow Sunda shelf located at the southern South China Sea. The water masses during both cruises showed characteristics similar (southwest monsoon) to those observed in the adjacent regions. A clustering method was used in which three water masses were classified from the composite dataset. There are WM1 (T 29°C - 31°C, S 32 - 33.5 psu, & σT, ~19.5 - 20.7 kg/m3), WM2 (T 25°C - 29°C, S 32.8 - 33.8 psu, & σT, ~22.3 - 23 kg/m3) and WM3 (T 23°C - 25°C, S33.5 - 34.0, & σT ~22.3 - 23.3 kg/m3). Even though the water masses found were characterized under southwest monsoon characteristics, the degrees of stratification obtained varied between these cruises. The stability of the stratification also plays roles in the distribution of the water masses in the water column. WM2 was found in thermocline layer and most dominant in June compared to in October. In June, WM3 was found under the thermocline layer and absent in October. The stable thermocline caused the denser WM3 cannot mixed with WM2. Higher temperatures and weaker winds during June may have caused the strong stratification, while decreasing temperature and stronger winds of an upcoming northeast monsoon enhanced the vertical mixing during October.

Numerical simulation and verifications on thermal stratification in a stratified reservoir

Taking Jinpen Reservoir in Xi'an as a study case,a numerical simulation method for thermal structure of a stratified reservoir was developed and validated against the field data. The development and characteristics of the thermocline under various conditions of short wave radiation and bulk extinction coefficient for short wave radiation was numerically investigated by using a commercially-available software of Fluent. The net total surface heat transfer is positive in spring and summer,while negative in autumn and winter. The long wave radiation mainly contributes to the net total surface heat transfer,while the short one controls the development of the thermocline. The depth of the thermocline increased and the temperature gradient over the thermocline decreased as the bulk extinction coefficient decreased,and the bulk extinction coefficient is positively related to the algae concentration. There was a maximum value of the surface short wave radiation for a certain reservoir,the temperature difference between the top and bottom of the thermocline increased when the short wave radiation was below the maximum radiation,but it decreased when the short wave radiation was beyond the maximum radiation. Moreover,a heat balance and a stable thermocline can't be easily developed under higher short wave radiation.

Foraminiferal faunal evidence for Glacial–Interglacial variations in the ocean circulation and the upwelling of the Gulf of Tehuantepec (Mexico)

Abstract Three marine sediment cores from the Gulf of Tehuantepec (Eastern Tropical North Pacific) were used to determine variations in upwelling intensity and surface circulation patterns over the last 26 kyr BP. Changes in foraminiferal assemblages at glacial-interglacial timescale indicate a sequence of three major palaeoceanographic events in the study area: (1) The Late Pleistocene (including the Last Glacial Maximum, LGM), dominated by a Neogloboquadrina dutertrei–Globorotalia menardii assemblage, indicative of a stable thermocline, stratified water column and greater persistence of warm equatorial waters in the region. (2) The Termination 1, dominated by a Globigerinita glutinata assemblage, which indicates mixed surface water and transitional conditions between upwelling and stratified water column episodes. Changes in sedimentation rates and a clear reduction of the faunal abundances at this time, could be the result of eustatic sea level change. (3) The Holocene, dominated by a Globigerina bulloides–Globigerinoides ruber assemblage, indicates periods of elevated productivity, intense upwelling and the establishment of the modern ocean conditions. These findings are consistent with climate model results that suggest that regional surface ocean and upwelling conditions are mostly linked to changes in atmospheric circulation over the Laurentide Ice Sheet and shifts in the latitudinal position of the Intertropical Convergence Zone.

The influence of subsurface thermal structure on the diving behavior of northern fur seals (Callorhinus ursinus) during the breeding season

In the heterogeneous marine environment, predators can increase foraging success by targeting physical oceanographic features, which often aggregate prey. For northern fur seals (Callorhinus ursinus), two prevalent oceanographic features characterize foraging areas during summer in the Bering Sea: a stable thermocline and a subsurface “cold pool”. The objective of this study was to examine the influence of these features on foraging behavior by equipping fur seals from St. Paul Island (Alaska, USA) with time-depth recorders that also measured water temperature. Foraging bout variables (e.g., mean dive depth and percent time diving in a bout) were compared with respect to subsurface thermal characteristics (thermocline presence and strength and cold pool presence). Over 74% of bouts occurred in association with strong thermoclines (temperature change > 5°C). Few differences were found for dive behavior in relation to the presence of a thermocline and the cold pool, but for epipelagic bouts, a strong thermocline resulted in increased bottom times, number of dive wiggles, and percent time diving when compared to moderate thermoclines. There was also a positive relationship between mean dive depth and thermocline depth. The combination of increasing foraging effort in areas with strong thermoclines and diving to depths closely related to the thermocline indicates this feature is important foraging habitat for northern fur seals and may act to concentrate prey and increase foraging success. By recognizing the environmental features northern fur seals use to find prey, managers will be better equipped to identify and protect foraging habitat that is important to northern fur seals, and possibly other marine predators in the Bering Sea.

TAXONOMY AND STABLE ISOTOPE PALEOECOLOGY OF WELL-PRESERVED PLANKTONIC FORAMINIFERA FROM THE UPPERMOST OLIGOCENE OF TRINIDAD

Outcrops of the Cipero Formation that are currently exposed near San Fernando, Trinidad, are described. These contain exceptionally well-preserved planktonic foraminifer assemblages of latest Oligocene and earliest Miocene age. The full range of included species is documented, including some rarely reported forms. Details of foraminifer test construction and wall ultrastructures are examined using scanning electron microscopy of whole and dissected tests. Observations include (i) the presence of micrometer-scale spikes – confirmed by examination of comparable material from the Oligocene of Puerto Rico – on the interior test surface of Cassigerinella chipolensis, (ii) detailed investigation of the calcite crust on tests of Catapsydrax dissimilis and Turborotalita quinqueloba, (iii) well-preserved wall microstructure on tests of Paragloborotalia pseudokugleri, and (iv) confirmation of a radially crystalline wall in Tenuitella and Globigerinita that compares closely with Miocene Globigerinatella from ODP Site 871 (western Pacific Ocean). Multi-species oxygen and carbon stable isotope measurements divide the assemblage into various ecological groups, including mixed layer, thermocline, and sub-thermocline dwellers. The fossil assemblage and isotope data indicate an oligotrophic open-ocean setting with a well-defined and seasonally stable thermocline with sea surface temperatures of about 27°C (similar to modern) and sub-thermocline waters of about 13°C. The mixed layer was dominated by species of Globigerina, Paragloborotalia pseudokugleri, Cassigerinella, and Globigerinita. The base of the mixed layer and upper thermocline was inhabited by Turborotalita, Paragloborotalia, larmeui, and Tenuitella. The deep thermocline was inhabited by the large species Dentoglobigerina venezuelana and D. rohri. Sub-thermocline intermediate water was occupied by Catapsydrax dissimilis.

Hydraulic Management Drives Heat Budgets and Temperature Trends in a Mediterranean Reservoir

AbstractBy contrast to the more regular and predictable temperate lakes, heat budgets and temperature dynamics in Mediterranean reservoirs are characterized by a marked interannual variability. In the present paper, the heat content, annual Birgean heat budget (ABHB), and thermal structure of Sau Reservoir were examined during a period of hypolimnetic withdrawal between 1980 and 1985, and during a period of withdrawal at intermediate depths between 1996 and 2003. The two study periods were also characterized by a wide range of stored water volume fluctuations. Results were used to develop and validate an empirical model to predict annual and monthly heat dynamics statistics and mixed layer depth as a function of hydraulic management parameters such as water volume and selective withdrawal depth. During the hypolimnetic withdrawal period elevated ABHB and deep mixed layer depths were recorded in the reservoir, which behaved as a heat trap. By contrast, intermediate depth withdrawal promoted a shallower and more stable thermocline, thus increasing the cold hypolimnetic water volume and decreasing heat content and ABHB. The study reveals that hydraulic management constitutes the main driver of the heat and thermal dynamics in reservoirs with multiple withdrawal outlets. By contrast with the increasing temperature trends recorded in many natural lakes, the hydraulic management in Sau Reservoir induced a progressive reduction in water temperature and heat content in the system, thus partially counteracting the possible deleterious effects of global warming. Our intensive study in a single, highly‐dynamic ecosystem constitutes a new approximation to the study of thermal structure and heat dynamics in water bodies. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Direct and large eddy simulations of a bottom Ekman layer under an external stratification

A steady Ekman layer with a thermally stratified outer flow and an adiabatic boundary condition at the lower wall is studied using direct numerical simulation (DNS) and large eddy simulation (LES). An initially linear temperature profile is mixed by turbulence near the wall, and a stable thermocline forms above the mixed layer. The thickness of the mixed layer is reduced by the outer layer stratification. Observations from the DNS are used to evaluate the performance of the LES model and to examine the resolution requirements. A resolved LES and a near-wall model LES (NWM–LES) both compare reasonably well with the DNS when the thermal field is treated as a passive scalar. When buoyancy effects are included, the LES mean velocity and temperature profiles also agree well with the DNS. However, the NWM–LES does not sufficiently account for the overturning scales responsible for entrainment at the top of the mixed layer. As a result, the turbulent heat flux and the rate of change of the mixed layer temperature are significantly underestimated in the NWM–LES. In order to accurately simulate the boundary layer growth, the motions responsible for entrainment must either be resolved or more accurately represented in improved subgrid-scale models.

Sea Ice-Upper Ocean Ecosystems and Global Changes in the Arctic

In the Arctic, a clear temperature trend toward warming has been observed during the last two decades. The warming has led to a reduction in the area and thickness of the Arctic sea ice cover because of increased melting. Melt water is accumulated within the 0- to 30-m surface layer, markedly changing its hydrological and hydrochemical characteristics: this water is warmer and fresher, with lower nutrient concentrations. A stable thermocline at depths of 30–35 m weakens vertical mixing of water and hampers active exchange between nutrient-enriched water lying below the thermocline and nutrient-poor water under ice. This affects the qualitative and quantitative composition of upper-ocean phyto- and zooplankton. The changes in the climate and the ice environment cause marked alterations in the composition and structure of the biological communities of sea ice and under-ice surface water. Comparative analysis of materials collected in the anticyclonic Arctic Gyre at the ice stations Severnyi Polyus-22 (1975– 1981) and SHEBA (1997–1998) shows that over the last two decades the number of species of ice diatoms markedly decreased and the role of freshwater algae increased. The number of nematodes, copepods, amphipods, and turbellarians, which were the dominant groups in perennial ice, also declined sharply. The results suggest that the sea ice-upper ocean ecosystem is changing from typically marine to brackish-water. The main reason for these changes is probably global warming in the Arctic.

Convection driven by differential heating at a horizontal boundary

We report laboratory and numerical experiments with the convective circulation that develops in a long channel driven by heating and cooling through opposite halves of the horizontal base. The problem is similar to that posed by Stommel (Proc. Natl Acad. Sci. vol. 48, 1962, p. 766) and Rossby (Deep-Sea Res. vol. 12, 1965, p. 9; Tellus vol. 50, 1998, p. 242), where flow forced by a linear temperature variation along the ocean surface or the base of a tank presented a demonstration of the smallness of sinking regions in the meridional overturning circulation of the oceans. In contrast to the previous experiments, we use small aspect ratio, larger Rayleigh numbers, piecewise uniform boundary conditions and an imposed input heat flux. The flow is characterized by a vigorous overturning circulation cell filling the box length and depth. A stable thermocline forms above the cooled base and is advected over the heated part of the base, where it is eroded from below by small-scale three-dimensional convection, forming a ‘convective mixed layer’. At the endwall, the convective mixing is overshadowed by a narrow but turbulent plume rising through the full depth of the box. The return flow along the top of the box is turbulent with large slowly migrating eddies, and occupies approximately a third of the total depth. Theoretical scaling laws give temperature differences, thermocline thickness and velocities that are in good agreement with the experimental data and two-dimensional numerical solutions. The measured and computed density structure is largely similar to the thermocline and abyssal stratification in the oceans.

Cd/Ca in Planktonic Foraminifera of the Glacial Southern Ocean

Since the industrial revolution, 200 years ago, atmospheric carbon dioxide levels have risen by 90 ppmV, an amount equalling the natural variation between the last glacial maximum and pre-industrial values. To predict the fate and influence of anthropogenic carbon dioxide, it is vital to understand the natural controls on the glacial-interglacial variations in atmospheric composition. The biological pump is known to be an important draw-down of carbon dioxide today due to the complete removal of nutrients from the euphotic zone of most surface waters and regeneration at depth. An exception exists in the Southern Ocean today, where the surface waters have high phosphate concentrations indicating incomplete removal of the nutrients either due to very rapid upwelling rates or iron limitation. There is huge potential for glacial carbon dioxide draw-down if the efficiency of the biological pump had been enhanced in the glacial Southern Ocean. Particle proxies suggest that export productivity increased in the glacial subantarctic Southern Ocean (1) but there has been no clear evidence of a lowering of the glacial surface-water nutrients (2). fil3c in planktonic foraminifera has been found to be unreliable as a nutrient tracer in surface waters due to a number of factors such as susceptibility to isotopic fractionation during air-sea exchange (3). Cd/Ca in foraminifera, a reliable deep-water nutrient tracer, has been uncalibrated for surface waters and is often present at levels too low for determination by existing methods. We have developed a new isotope dilution, mass-spectrometric method for low-level determination of Cd/Ca in foraminifera which has allowed reliable and accurate planktonic analyses to be made. We have investigated the application of Cd/ Ca in planktonic foraminifera for the reconstruction of past surface water phosphate levels. Down-core records of Cd/Ca in G.Bulloides covering the last complete glacial cycle have been obtained from a latitudinal transect of cores spanning the present day Subtropical Convergence Zone (SCZ), at approximately 40~ and the Antarctic Polar Front (APF), at approximately 50~ in the Indian Ocean sector of the Southern Ocean. In each core there is a striking correlation between the Cd/Ca and the 5180 of G.Bulloides over the last glacial cycle indicating low surface water nutrient levels during the glacials and high nutrient levels during the interglacials. The greatest reduction in nutrient levels occurs in the subantarctic zone of the Southern Ocean, with little change south of the APF, consistent with the export productivity conclusions. There are two possible explanations for this glacial lowering of surface water nutrients. Firstly the rate of upwelling remains constant but the efficiency of nutrient removal is increased, possibly by iron fertilization due to an increased atmospheric dust fallout in glacial times. Secondly, the glacial Southern Ocean water column became stratified with a stable thermocline altowing the surface water productivity to deplete the nutrients without constant replenishment from upwelling. It is important to distinguish between these two mechanisms as each has a differing potential for glacial draw-down of atmospheric carbon dioxide. A simple box-model following some of the concepts in (4) has been run to resolve which of these two mechanisms is more consistent with the new surface water nutrient data and the existing export productivity conclusions.