ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1997

Description: This thesis is a study of the physical mechanisms that ventilate the subtropical thermocline of the eastern North Atlantic. The starting point is an analysis of the existent historical database of natural and anthropogenic tracers, with special emphasis on 3He and tritium, that can be used to infer rates of ventilation. If the flow is predominantly advective, the temporal evolution of coupled transient tracers can be used to define a tracer age which measures the elapsed time since a water parcel was resident in the surface mixed layer. A principle finding is that the observed tracer age shows a large and systematic change over time. Tritium-3He age in the eastern Atlantic thermocline is seen to increase over time; the magnitude of the change is greatest for the deeper, more slowly ventilated layers of the thermocline. The first hypothesis examined is that the observed shift in the tracer age field is the manifestation of a slackening of the physical ventilation. A time series of the meridional geostrophic velocity shear in the eastern Atlantic shows no indication of a change in the strength of the large-scale circulation. Uncertainty of the geostrophic calculation due to data sparsity and mesoscale eddy contamination prevents conclusive rejection of the hypothesis of a changing circulation. There are other tracers which offer useful clues: comparison of the tritium-3He age field with dissolved oxygen reveals a temporal trend in the property-property correlation. The spatial structure of the oxygen field, however, shows no long-term evolution over time. From this line of evidence it is concluded that the physical ventilation of the thermocline has not altered over time and, therefore, the temporal change in the tritium-3He age field must be the signal of the tritium invasion itself. A second hypothesis, which analysis shows is more consistent with the observations, is that the changing tracer age is a consequence of mixing effects in the ventilation of 3He and tritium. Numerical simulations of the thermocline ventilation of 3H and 3He are performed to examine the steadiness of the tracer age field under different advective-diffusive regimes. A one-dimensional model is constructed based on the assumption that the totality of the fluid in the thermocline derives from subduction out of the surface mixed layer. The temporal behavior of the tracer age field is found to be dependent on the radiotracer Peclet number, which measures the ratio of the diffusive and advective time scales. In a model with steady circulation, the observed temporal behavior of the tracer age field can be reproduced only when the effects of lateral mixing play a significant role in the process of ventilation. The vertical structure and magnitude of the implied lateral diffusivity are, however, inconsistent with other observations. The numerical simulations are next extended to two-dimensions to allow for the presence of a pool of unventilated, re-circulated water within the anti-cyclonic, subtropical gyre. Comparison of the model with the observed transient tracer field in the lower thermocline shows consistency with conventional estimates of lateral mixing rates only when the diffusively ventilated "pool" region extends across the entire zonal domain of the gyre. In contrast, the transient tracer fields in the upper portion of the thermocline are best reproduced when the isopycnal surfaces are ventilated by advection directly from the surface mixed layer. The results obtained here are consistent with numerical simulations which reveal a prominent role for mesoscale eddies in the ventilation of the thermocline.

Description: This research was supported by Office of Naval Research AASERT contract number N00014-95-l-0824.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1989

Description: The general theme of this thesis is the study of systematic mathematical techniques for determining the ocean circulation from classical hydrographic data. Two aspects of this theme are analyzed. The first is finding an efficient representation of hydrographic structure so as to make it most useful and informative. The second is application of inverse methods to the data to determine ocean circulation. Both subjects are examined in the North Atlantic Ocean. The efficient representation is examined in terms of empirical orthogonal functions (EOFs) among the variations in vertical hydrographic profiles. The data used are of a new set of high quality hydrography, all obtained in the early 1980s. Common EOFs are examined among temperature, salinity, oxygen, phosphate, silicate, and nitrate. The EOFs identify a fundamental simplicity in the spatial distributions of t hese properties. Although the volume of numbers involved in the raw data is large, the significant degrees of freedom are only six in space and two among the six properties; temperature and salinity are represented by one mode, while the nutrients by another. The modal structure reflects some underlying simplicity in ocean physics. EOFs form a quantitative basis from which models of the ocean's hydrographic structure can be constructed for various degrees of complexities. As for the second aspect, two applications of inverse methods are explored on small regional scales. The first problem addressed concerns the circulation inside a 12° square located in the eastern basin over the axis of the Mediterranean Water tongue. The study is based on an ocean model constructed by mapping the modes identified in the first half of the thesis over the entire North Atlantic Ocean. A combination of box model inverse and β-spiral method is used to determine the geostrophic reference level velocities. The circulation consists of an anticyclonic circulation near the surface, which is part of the eastern half of the wind-driven subtropical gyre. The flow at depth is weak, and is a cyclonic circulation around the core of the Mediterranean Water tongue. In the second inverse problem, we examine a decaying warm-core ring. Observations of a warm-core ring are used to formulate a model for diagnosing the physics of ring change over a two month period. About 30 hydrographic casts and acoustic doppler current measurements are used to generate estimates of an equivalent radially symmetric ring with radial contrasts of stratification, temperature, salinity, azimuthal velocity, angular momentum, and potential vorticity. A series of related models are inverted for the ring circulation and mixing coefficients. The circulation is insensitive to the model details, is well-resolved, and is a radial outflow and upwelling. Eddy coefficients are only partially resolved; determining the mixing with any degree of confidence appears to require a much more elaborate data set than the one available.

Description: This research was funded in part by the Office of Naval Research (Secretary of the Navy Chair) and the National Science Foundation under grant OCE 85-21685.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1998

Description: Planktonic protozoan grazers have the potential to significantly affect the chemistry of particle-associated trace metals. This is due both to the importance of protists as consumers of bacterial-sized particles, and to the unique low-pH, enzyme-rich microenvironment of the grazer food vacuole. This thesis examines the role of protozoan grazers in the marine geochemistry of strongly hydrolyzed, particle-reactive trace metals, in particular Th and Fe. A series of tracer experiments was carried out in model systems in order to determine the effect of grazer-mediated transformations on the chemical speciation and partitioning of radioisotopes C9Fe, 234Th, 51Cr) associated with prey cells. Results indicate that protozoan grazers are equally able to mobilize intracellular and extracellular trace metals. In some cases, protozoan regeneration of trace metals appears to lead to the formation of metal-organic complexes. Protozoan grazing may generate colloidal material that can scavenge trace metals and, via aggregation, lead to an increase in the metal/organic carbon ratio of aggregated particles. Model system experiments were also conducted in order to determine the effect of grazers on mineral phases, specifically colloidal iron oxide (ferrihydrite). Several independent techniques were employed, including size fractionation ors9Fe-labeled colloids, competitive ligand exchange, and iron-limited diatoms as "probes" for bioavailable Fe. Experimental evidence strongly suggests that protozoan grazing can affect the surface chemistry and increase the dissolution rate of iron oxide phases through phagotrophic ingestion. In further work on protozoan-mediated dissolution of colloidal Fe oxides, a novel tracer technique was developed based on the synthesis of colloidal ferrihydrite impregnated with 133Ba as an inert tracer. This technique was shown to be a sensitive, quantitative indicator for the extent of ferrihydrite dissolution/alteration by a variety of mechanisms, including photochemical reduction and ligand-mediated dissolution. In field experiments using this technique, grazing by naturally occuring protistan assemblages was shown to significantly enhance the dissolution rate of colloidal ferrihydrite over that in non-grazing controls. Laboratory and field results indicate that, when integrated temporally over the entire euphotic zone, protozoan grazing may equal or exceed photoreduction as a pathway for the dissolution of iron oxides.

Description: This work was financially supported by a Department of Defense ONR-NDSEG Graduate Fellowship, Office ofNaval Research AASERT Award (N00014-94-1-0711), and the National Science Foundation EGB Program (OCE-9523910).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1995

Description: Data from fifteen globally distributed, modern, high resolution, hydrographic oceanic transects are combined in an inverse calculation using large scale box models. The models provide estimates of the global meridional heat and freshwater budgets and are used to examine the sensitivity of the global circulation, both inter and intra-basin exchange rates, to a variety of external constraints provided by estimates of Ekman, boundary current and throughflow transports. A solution is found which is consistent with both the model physics and the global data set, despite a twenty five year time span and a lack of seasonal consistency among the data. The overall pattern of the global circulation suggested by the models is similar to that proposed in previously published local studies and regional reviews. However, significant qualitative and quantitative differences exist. These differences are due both to the model definition and to the global nature of the data set. The picture of the global circulation which emerges from the models IS a complex, turbulent flow. When integrated across ocean basins not one, but two major cells emerge. The first connects an Atlantic overturning cell (estimated at 18± 4x 109 kg s- 1) to the Southern Ocean where the Antarctic Circumpolar Current carries lower deep waters to the Indian and Pacific basins where they are converted to upper deep and intermediate waters before returning to the Atlantic. The second cell connects the Pacific and Indian Basins to the north and south of Australia. In t his cell deep waters pass into the Pacific and return within the Indian Basin as intermediate waters after passing through the Indonesian Passages. The two cells are found to be independent of one another, i.e. within the models, the Indonesian Passages do not represent a significant element in a net global circulation. While there is ample evidence of westward flow around the southern tip of South Africa which would support a "warm" water path scenario, the variability of flow in this region, rich with eddies makes hydrography a poor estimator of the relative strengths of the controversial "warm" and "cold" water paths. All existing estimates of Indonesian Passage throughflow, including the smallest (O x 106 m3 s-1) and the largest (20 x 106 m3 s-1), are consistent with the model constraints. When the Pacific- Indian throughflow is not constrained, the model produces an estimate of 11 ± 14x 109 kg s-1. The model heat flux estimates are both significantly different from zero and quite robust to changes in initial assumptions, with the exception of the choice of wind field. Although in this work it was not possible to compute freshwater fluxes which were significantly different from zero, future inclusion of salinity anomaly constraints along with terms describing vertical diffusion may yet make it possible to compute significant freshwater :flux estimates from hydrography.

Description: This research was partially funded by a NASA Global Change Fellowship and was also supported by NASA under contract NAGW-1048 and NSF under contract OCE-9205942.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1990

Description: As a primary feature of ocean circulation and a key component of the global carbon cycle, changes in the thermocline must be accounted for if we are to understand the processes involved in Quaternary climatic fluctuation. Toward this goal, this thesis contains studies of the modern and glacial thermoclines at the Bahama Banks and it presents an novel approach to determine sea level based on the flux of 230Th and 231Pa from thermocline waters to the seafloor. In the first chapter, the hydrography of the modern thermocline in Northwest and Northeast Providence Channels, Bahamas, is investigated using CTD data. Potential temperature- salinity relationships demonstrate that the deep waters and most of the thermocline waters in these channels originates in the Sargasso Sea. Cross channel sections of water properties suggest the following: (1) water from the shallow core of the Deep Western Boundary Current (Fine and Molinari, 1988) may circulate along the channel margins, and (2) where the western end of Northwest Providence Channel opens to the Florida Straits, shallow flow is toward the straits in the southern portion of the channel and away from the straits in the northern portion. In the next two chapters, changes in the temperature and nutrient structures of the thermocline from the last glaciation to the recent Holocene are inferred from isotopic variations of the planktonic foraminifera Globigerinoides ruber (212-250μm) and G. sacculifer (300-350μm) and the benthic foraminifera Planulina wuellerstorfi, P. ariminensis, P. foveolata and Cibicidoides pachyderma (〉250μm) in a suite of cores from the margins of Little and Great Bahama Banks. During the last glaciation, δ18O values were from 1.4 to 1.9 per mil greater than during the recent Holocene. Based on the δ18O/sea-level model of Fairbanks (1989), we estimate that the upper 1500 m of the water column was cooler by at least 1°C- the deepest waters were several degrees cooler. The temperature gradient (dT/dz) was steeper and the base of the thermocline appears to have stayed at about the same depth or risen slightly. At all depths in the thermocline, δ13C was greater during the last glaciation than during the recent Holocene by at least 0.1-0.2 per mil and as much as 0.6 per mil in the lower thermocline. While recent Holocene δ13C reaches minimum values in the lower thermocline (the poorly-ventilated oxygen minimum/phosphate maximum layer), this feature was not present during the last glaciation. These data show that the concentrations of nutrients throughout the thermocline were reduced and that there was no oxygen minimum layer, indicating greater, more uniform ventilation of thermocline waters. These results are consistent with our understanding of the physics of thermocline circulation and evidence for hydrographic conditions at the ocean surface during the last glaciation, indicating a direct response of thermocline circulation to changes in climate. Cooler thermocline waters reflect cooler surface ocean temperatures at mid-latitudes where thermocline isopycnal surfaces outcrop. Increased, more uniform ventilation of the glacial thermocline is consistent with both more vigorous glacial winds leading to increased Ekman pumping and all isopycnal surfaces of the thermocline outcropping in the area of Ekman downwelling. Taken together with previous studies of intermediate-depth waters, these data document that the entire upper water column of the North Atlantic was depleted in nutrients during the last glaciation. A final suggestion of the third study is that Mediterranean and southern source waters contributed little to deeper intermediate-depth waters in the North Atlantic. The fourth chapter presents two new approaches to reconstruct the sea-level history based on the fluxes of 230Th and 231Pa to the seafloor. The approaches rely on the fact that fluxes of these nuclides to a site on the seafloor are proportional to the height of the water column above the site. Consequently, a change in sea level causes changes in the 230Th and 231Pa fluxes which, at shallow sites, are large fractions of the total fluxes. Past sea level can be reconstructed using either the record of nuclide accumulation in a single core of sediment, or nuclide concentrations in synchronously deposited sediment samples from cores collected over a range of water depths. Importantly, this record of sea level is both continuous (not just high stands) and independent of the assumptions of constant seawater temperature or uplift rate required by some other approaches.

Description: This work was supported by NSF grant number OCE-8813307 to W. Curry, NSF grant number OCE-8800693 to J. Broda and W. Curry and the Education Office of the MIT/WHOI Joint Program in Oceanography.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1988

Description: Inverse methods are applied to historical hydrographic data to address two aspects of the general circulation of the Atlantic Ocean. The method allows conservation statements for mass and other properties, along with a variety of other constraints, to be combined in a dynamically consistent way to estimate the absolute velocity field and associated property transports. The method is first used to examine the exchange of mass and heat between the South Atlantic and the neighboring ocean basins. The Antarctic Circumpolar Current (ACC) carries a surplus of intermediate water into the South Atlantic through Drake Passage which is compensated by a surplus of deep and bottom water leaving the basin south of Africa. As a result, the ACC loses .25±.18x1015 W of heat in crossing the Atlantic. At 32°S the meridional flux of heat is .25±.19x1015 W equatorward, consistent in sign but smaller in magnitude than other recent estimates. This heat flux is carried primarily by a meridional overturning cell in which the export of 17 Sv of North Atlantic Deep Water (NADW) is balanced by an equatorward return flow equally split between the surface layers, and the intermediate and bottom water. No "leak" of warm Indian Ocean thermocline water is necessary to account for the equatorward heat flux across 32°S; in fact, a large transfer of warm water from the Indian Ocean to the Atlantic is found to be inconsistent with the present data set. Together these results demonstrate that the Atlantic as a whole acts to convert intermediate water to deep and bottom water, and thus that the global thermohaline cell associated with the formation and export of NADW is closed primarily by a "cold water path," in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage. The second problem addressed concerns the circulation and property fluxes across 24°and 36°N in the subtropical North Atlantic. Conservation statements are considered for the nutrients as well as mass, and the nutrients are found to contribute significant information independent of temperature and salinity. Silicate is particularly effective in reducing the indeterminacy of circulation estimates based on mass conservation alone. In turn, the results demonstrate that accurate estimates of the chemical fluxes depend on relatively detailed knowledge of the circulation. The zonal-integral of the circulation consists of an overturning cell at both latitudes, with a net export of 19 Sv of NADW. This cell results in a poleward heat flux of 1.3±.2x1015 Wand an equatorward oxygen flux of 2900±180 kmol S-l across each latitude. The net flux of silicate is also equatorward: 138±38 kmol s-1 and 152±56 kmol s -1 across 36°and 24° N, respectively. However, in contrast to heat and oxygen, the overturning cell is not the only important mechanism responsible for the net silicate transport. A horizontal recirculation consisting of northward flow of silica-rich deep water in the eastern basin balanced by southward flow of low silica water in the western basin results in a significant silicate flux to the north. The net equatorward flux is thus smaller than indicated by the overturning cell alone. The net flux of nitrate across 36°N is n9±35 kmol 8- 1 to the north and is indistinguishable from zero at 24°N (-8±39 kmol 8-1 ), leading to a net divergence of nitrate between these two latitudes. Forcing the system to conserve nitrate leads to an unreasonable circulation. The dominant contribution to the nitrate flux at 36°N results from the correlation of strong northward flow and relatively high nitrate concentrations in the sub-surface waters of the Gulf Stream. The observed nitrate divergence between 24°and 36°N, and convergence north of 36°N, can be accounted for by a shallow cell in which the northward flow of inorganic nitrogen (nitrate) in the Gulf Stream is balanced by a southward flux of dissolved organic nitrogen in the recirculation gyre. Oxidation of the dissolved organic matter during its transit of the subtropical gyre supplies the required source of regenerated nitrate to the Gulf Stream and consumes oxygen, consistent with recent observations of oxygen utilization in the Sargasso Sea.

Description: This research was supported by NASA under contract NAG5-534 and NSF under contract OCE-8521685.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1988

Description: This thesis is a study of the effect of geothermal sources on the deep circulation, temperature and salinity fields. In Chapter 1 background material is given on the strength and distribution of geothermal heating. In Chapter 2 evidence for the influence of a hydrothermal system in the rift valley of the Mid-Atlantic Ridge on nearby property fields and a model of the flow around such a heat source are presented, with an analysis of a larger-scale effect. Results of an analytical model for a heat source on a β-plane in Chapter 3 show how the response far from the source can have a structure different from the forcing because of its dependence on two parameters: a Peclet number (the ratio of horizontal advection and vertical diffusion), and a Froude-number-like parameter (the ratio of long wave phase speed to background flow speed) which control the relative amount of damping and advection of different vertical scales. The solutions emphasize the different behavior of a dynamical field like temperature compared to tracers introduced at the source. These ideas are useful for interpreting more complicated solutions from a numerical model presented in the final chapter.

Description: This study was supported by the National Science Foundation grants OCE8515642 to T. Joyce and OCE82-13967 to B. Warren, and by the WHOI-MIT Joint Program Ocean Ventures Fund.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1984

Description: The distribution and feeding behavior of bacterivorous micro flagellates (2-20 μm protozoa) and their ingestion by copepods were examined in an attempt to assess the importance of these protozoa as a trophic link between planktonic bacteria and zooplankton. The abundance of microflagellates relative to other picoplankton (0.2-2.0 μm) and nanoplankton (2-20 μm) populations in water samples in the North Atlantic and in Lake Ontario and on macroaggregates in the North Atlantic was determined using direct microscopical and culture estimation techniques. Seasonal, vertical and geographical changes in the density of microflagellates were generally not greater than one order of magnitude. Microscopical counts of heterotrophic nanoplankton (presumably microflagellates) typically ranged from a few hundred to a few thousand m1-1 for a variety of planktonic environments. They constituted approximately 1/3 to 1/2 of the nanoplankton in the euphotic zone and dominated the nanoplankton in the aphotic zone. Most Probable Number (MPN) estimation of the density of bacterivorous protozoa indicated that microflagellates were, on average, an order of magnitude more abundant than bacterivorous ciliates and amoebae. MPN and direct microscopical counts of microflagellates differed by as much as 104. This discrepancy was smaller in eutrophic environments (e.g. Continental Shelf and Lake Ontario) and on macroscopic detrital aggregates. All microbial populations enumerated were highly concentrated on macroscopic detrital aggregates relative to their abundance in the water surrounding the aggregates. Enrichment factors (the ratio of abundance of a population on a macroaggregate to its abundance in the surrounding water) increased along a eutrophic-to-oligotrophic gradient because of the combined effects of an increased abundance of microorganisms on macroaggregates in oligotrophic environments and a decreased abundance in the surrounding water in these same environments. Average enrichment factors for direct microscopical counts of heterotrophic nanoplankton (range = 17-114) were not as large as enrichment factors observed for MPN estimates of the number of bacterivorous microflagellates (range = 273-18400). Microflagellates numerically dominated the bacterivorous protozoa cultured from macroaggregates by one to two orders of magnitude, but ciliates and amoebae were also highly enriched on macroaggregates. Microenvironments are therefore a potentially important aspect for the ecology of planktonic microorganisms. Observations on the microbial colonization of mucus sloughed by ctenophores and discarded appendicularian houses suggest that these materials may be important sources of macroaggregates. Batch and continuous culture experiments were conducted with clonal cultures of microflagellates to test their ability to grow on various types and densities of bacteria. The doubling time of Monas sp. 1 ranged from 43 hr (when fed the cyanobacterium Synechococcus Strain WH 8101) to 6.9 hr (when fed the heterotrophic bacterium Serratia marinorubra). Cell yields (i.e. the conversion of bacterial biomass into protozoan biomass) of Monas sp. 1 fed two species of heterotrophic bacteria were greater than yields for the microflagellate fed two species chroococcoid cyanobacteria (range = 7-68%). Cell yields of two other species of microflagellates (Monas sp. 2 and Cryptobia maris) were 48% and 61%, respectively, on the bacterium Pseudomonas halodurans. Microflagellates grew in continuous culture at concentrations of bacteria which were lower than bacterial densities required for the growth of ciliates as shown by other investigations. Therefore, microflagellates appear to be well-adapted for grazing bacterioplankton. Microflagellates were also investigated for their ability to graze bacteria attached to particles. Bodo nanorensis and Rhynchomonas nasuta both showed a marked ability to graze attached bacteria and a limited ability to graze unattached cells. These results suggest that microflagellates may also be important consumers of bacteria attached to particles in the plankton and may explain the highly elevated densities of microflagellates on macroaggregates. Grazing experiments performed with the copepod Acartia tonsa indicated that heterotrophic microflagellates were ingested by the copepods at rates comparable to the ingestion of phytoplankton of similar size. The presence of heterotrophic microflagellates did not depress filtration rates of the copepods, and one species (Cryptobia maris) appeared to be selectively grazed. Survival of A. tonsa on a diet of heterotrophic microflagellates was similar to survival on a diet of phytoplankton and was significantly longer than survival of starved Controls or copepods fed only bacteria. Due to their ability to grow at in-situ densities of planktonic bacteria, their relatively high cell yields, and their acceptability as food for zooplankton, it is concluded that bacterivorous microflagellates may constitute an important trophic link between bacteria and zooplankton. This link may provide a mechanism whereby organic material and energy from the detrital food chain can be returned to the classical phytoplankton-copepod-fish food chain.

Description: This research was supported by National Science Foundation grants OCE80-2444l and OCE82-l4928 and Ocean Industry Program grant 4473 awarded to Dr. Laurence P. Madin, NSF Doctoral Dissertation grant OCE8l-l299l, the Woods Hole Oceanographic Institution Education Program and the Wood Hole Oceanographic Institution Biology Department.