ALBERT

Description: The Aegean water masses and circulation structure are studied via two large-scale surveys performed during the late winters of 1988 and 1990 by the R/V Yakov Gakkel of the former Soviet Union. The analysis of these data sheds light on the mechanisms of water mass formation in the Aegean Sea that triggered the outflow of Cretan Deep Water (CDW) from the Cretan Sea into the abyssal basins of the eastern Mediterranean Sea (the so-called Eastern Mediterranean Transient). It is found that the central Aegean Basin is the site of the formation of Aegean Intermediate Water, which slides southward and, depending on their density, renews either the intermediate or the deep water of the Cretan Sea. During the winter of 1988, the Cretan Sea waters were renewed mainly at intermediate levels, while during the winter of 1990 it was mainly the volume of CDW that increased. This Aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects: (i) during the winter of 1990 the position of the front between the Black Sea Water and the Levantine Surface Water was displaced farther north than during the winter of 1988 and (ii) heavier waters were formed in 1990 as a result of enhanced lateral advection of salty Levantine Surface Water that enriched the intermediate waters with salt. In 1990 the 29.2 isopycnal rose to the surface of the central basin and a large volume of CDW filled the Cretan Basin. It is found that, already in 1988, the 29.2 isopycnal surface, which we assume is the lowest density of the CDW, was shallower than the Kassos Strait sill and thus CDW egressed into the Eastern Mediterranean.

Description: Published

Description: 1841-1859

Description: JCR Journal

Description: reserved

Description: In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

Description: This work has been funded by the ENACT Project (Contract EVK2-CT2001-00117) for A. Bellucci and P. Di Pietro, and partially by the ENSEMBLES Project (Contract GOCE-CT-2003-505539) for A. Bellucci.

Description: Published

Description: 3785-3807

Description: 3.7. Dinamica del clima e dell'oceano

Description: JCR Journal

Description: reserved

Description: Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surface temperature impact of decadal variations in the MOC is now apparent, and variations of surface air temperatures in the North Atlantic Ocean are also potentially predictable on interannual to decadal time scales, albeit with potential skill levels that are less than those seen for MOC variations. This intercomparison represents a step forward in assessing the robustness of model estimates of potential skill and is a prerequisite for the development of any operational forecasting system.

Description: Published

Description: 1195-1203

Description: JCR Journal

Description: reserved

Description: Skill in ensemble-mean dynamical seasonal climate hindcasts with a coupled land-atmosphere model and specified observed sea surface temperature is compared to that for long multi-decade integrations of the same model where the initial conditions are far removed from the seasons of validation. The evaluations are performed for surface temperature and compared among all seasons. Skill is found to be higher in the seasonal simulations than the multi-decadal integrations except during boreal winter. The higher skill is prominent even beyond the first month when the direct influence of the atmospheric initial state elevates model skill. Skill is generally found to be lowest during the winter season for the dynamical seasonal forecasts, equal to that of the long integrations, which show some of the highest skill during winter. The reason for the differences in skill during the non-winter months is attributed to the severe climate drift in the long simulations, manifest through errors in downward fluxes of water and energy over land and evident in soil wetness. The drift presses the land surface to extreme dry or wet states over much of the globe, into a range where there is little sensitivity of evaporation to fluctuations in soil moisture. Thus, the land-atmosphere feedback is suppressed, which appears to lessen the model’s ability to respond correctly over land to remote ocean temperature anomalies.

Description: Center for Ocean-Land-Atmosphere Studies

Description: Published

Description: When the U. S. Bureau of Fisheries, in cooperation with the Museum of Comparative Zoology, commenced the oceanographic survey of the Gulf of Maine in the summer of 1912 (Bigelow, 1925-1927), it was in the hope that this might later be extended to the coastal waters thence southward; eventually even as far as the Gulf of Mexico. Cruises carried out in connection with investigations of the biology of the mackerel, by the Fisheries' steamer "Albatross II" from 1927 to 1932, supplemented by those of the research ship "Atlantis" of the Woods Hole Oceanographic Institution, have made it possible to extend the detailed examination of the physical oceanography of the continental shelf as far as the offing of Chesapeake Bay, and to the offing of Cape Hatteras for some of the months. The present account of the temperature of the region will, it is hoped, be followed shortly by corresponding accounts of salinity, of circulation and of the dominant planktonic communities.

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 February 2006

Description: Between 9º-25º E on the ultraslow-spreading Southwest Indian Ridge lie two sharply contrasting supersegments. One 630 km long supersegment erupts N-MORB that is progressively enriched in incompatible element concentrations from east to west. The second 400 km long supersegment contains three separate volcanic centers erupting EMORB and connected by long amagmatic accretionary segments, where mantle is emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted. Rather than a major break in mantle composition at the discontinuity between the supersegments, this sharp contrast in geometry, physiography, and chemistry reflects “source” versus “process” dominated generation of basalt. Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate, lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity) provides a unique opportunity to compare the influence of spreading geometry and rate on MORB generation. What had not been well established until now is the importance of melting processes rather than source at spreading rates 〈 20 mm/yr. Along the orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting effectively sample the bulk mantle source, while on the obliquely spreading supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the bulk source is not uniformly sampled, and thus “process” rather than “source” dominates melt chemistry.

Description: The main body of work consisting of major element, trace element, and isotopic data acquisition and interpretation (Chapter 2 & 3) was funded by H. Dick’s grant from the National Science Foundation-OCE 9907630. National Science Foundation-OCE 0137325 supported the U-series work described in Chapter 4. The published work of Chapter 5 was funded by National Science Foundation-EAR 9804891, NSF-OCE 9416620, and NSF-OCE 0096634.

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 November 1980

Description: Plankton samples have been carefully collected from a variety of marine environments under the rigorous conditions necessary to prevent contamination for major and trace-chemical analysis. Immediately after collection, the samples were subjected to a series of physical and chemical leaching-decomposition experiments designed to identify the major and trace element composition of particulate carrier phases. Elements examined through some or all of these experiments were: C, N, P, Mg, Ca, Si, Fe, Mn, Ni, Cu, Cd, Al, Ba, and Zn. Emphasis was placed on the identification of trace element/major element ratios in the biogenic materials. The majority of the trace elements in the samples were directly associated with the non-skeletal organic phases of the plankton. These associations included a very labile fraction which was rapidly released into seawater and a more refractory fraction which involved specific metal-organic binding. Calcium carbonate and opal were not significant carriers for any of the trace elements studied. A refractory phase containing Al and Fe in terrigenous ratios was present in all samples, even from the more remote marine locations. The concentration of this carrier phase within the plankton samples varied in proportion to the estimated rate of supply of terrigenous matter and in opposition to the rate of production of the biogenic particulate matter. The aluminosilicates contributed insignificant amounts to the other trace elements studied. A trace concentration of particulate Al was identified which was more labile and associated with the organic fractions of the samples. Variations in the surface water concentrations of dissolved Cu, Ni, Cd, and Zn with respect to P are compared to the ratios measured in the plankton samples and their regeneration products. The trace element/major element ratios ,in the residual plankton materials can be combined with estimates of the carrier fluxes to account for the transport of trace elements required to maintain their deep enrichment. A variety of processes determining the geochemical cycles of specific trace elements were identified. As much as 50% of the Cd, Ni, Mn, and P are rapidly released from plankton and recycled within the surface ocean. During this process, the metal/P ratio in the residual particles must decrease by 10-30% for Cd and increase by a factor of 2-4 for Ni and Cu to balance their deep enrichments. Although Mn is taken up and regenerated by plankton, the magnitude of this process is small with respect to other non-biogenic Mn fluxes and has very little influence on its dissolved distribution. The Ba content of all known surface carriers is insufficient to account for the deep enrichment of Ba. A secondary concentration process results in the formation of significant particulate Ba within the upper thermocline.

Description: Portions of the data in this research have been collected under the NSF grant DES 75-03826 and ONR grant N00014-80-C-0273. I have been supported for the last three years on an NSF National Needs Graduate Traineeship to the MIT-WHOI Joint Program. Ship time was generously provided by various principal investigators of the NSF Galapagos Hydrothermal research program and the NSF-IDOE MANOP program.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Philosophy and the Woods Hole Oceanographic Institution November 1980

Description: Suspended particulate matter was collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (Site E), the north equatorial Pacific (Site P), and the Panama Basin (STIE Site). Additional samples of suspended particles were obtained by in situ filtration at Site F., at the STIE Site, and in the Guatemala Basin. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P, at a second site in the Sargasso Sea (Site D), at the STIE Site and in the Guatemala Basin. Sediment samples were obtained from cores taken near Sites E and P. Results have shown unequivocally that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. This behavior could not have been predicted from the known physical chemistry of Th and Pa. Dissolved 230Th/231Pa activity ratios were 3-5 at Sites P and D and 3-8 at the STIE Site. In contrast, unsupported 230Th/231Pa ratios were 22-35 (average 29.7 for 7 samples) in sediment-trap samples from greater than 2000 m at Sites S2, E and P. Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. In contrast to the open ocean samples described above, samples collected by six sediment traps at depths of 667-3791 m in the Panama Basin had unsupported 230Th/231Pa ratios of 4-8, and the deepest samples had the lowest ratios. Fractionation of Th and Pa that was observed at the three open ocean sites either does not occur or occurs to a very limited extent in the Panama Basin. Particulate 230Th/231Pa ratios were negatively correlated with the concentration of suspended particles. However, variable scavenging rates, as indicated by variable particle concentration, do not completely control the ratio at which Th and Pa are scavenged from solution. Major biogenic and inorganic components of trapped material were found in approximately the same proportions in the STIE samples and in samples from Sites E and S2. Lower 230Th/231Pa ratios found in the STIE samples must therefore result from subtle changes in the chemical properties of the particles. Consideration of 230Th/23lPa ratios in several depositional environments indicates that no single factor controls the ratio at which Th and Pa are adsorbed from seawater. Fluxes of 210Th and 231Pa were less than their rates of production in the overlying water column in every trap at Sites S2, E, and P. In the Panama Basin, fluxes measured with the same traps were greater than or equal to their rates of production. These results are a strong indication that even extremely reactive elements such as Th and Pa are redistributed within the oceans. Redistribution occurs because variable scavenging rates in different environments set up horizontal concentration gradients. Horizontal mixing processes produce a net horizontal transport of Th and Pa from areas of 1ow scavenging rates to areas of high scavenging rates. Protactinium is redistributed to a greater extent than Th. Fluxes of 230Th can be used to set lower limits for horizonttal transport of Pa even when absolute trapping efficiencies of the sediment traps are not known. Less than 50% of the Pa produced at the open ocean sites is removed from the water column by scavenging to settling particles. The remainder is removed by horizontal transport to other environments. At Sites E and P, 230Th/231Pa ratios were identical in the deepest sediment trap sample and in surface sediments. However, 230Th/232Th and 231Pa/232Th ratios were 2.5 times higher in trapped particles than in surface sediments. The 230Th/232Th ratios were 5.5 times higher in particles filtered at 3600 m and 5000 m at Site E than in surface sediments. This observation is best explained by dissolution of most of the 230Th and 231Pa scavenged by settling particles during remineralization of labile biogenic phases. The behaviors of certain other radioisotopes were also studied. 232Th is present only in detrital mineral components of trapped material. Concentrations of 232Th in trapped particles correlate closely with Al and K, at ratios approaching that of average shale or crustal abundances at Site E and P and basalts at the STIE Site. High specifìc activities of 228Th and 239+240Pu were found in sediment trap samples throughout the water column at Sites E and P and in the Panama Basin. The dominant source of these isotopes is near the sea surface and also near the sea floor in the case of 228Th. Thus it appears that the bulk of the trapped material is recently derived from the sea surface where it incorporates these isotopes,with little loss during rapid transit through the water column. A bioauthigenic form of particulate uranium is produced at the sea surface and remineralized in the deep ocean along with its labile carrier phase(s). This flux of uranium to the deep ocean is 0.25-1.0 dpm/cm2103 years, which is insufficient to cause a measurable concentration gradient in the uranium distribution within the mixing time of the oceans. Increased concentrations and fluxes of particulate uranium were not found in the eastern equatorial North Pacific under areas of an intense oxygen minimum. Therefore, reduction of uranium to the tetravalent state with subsequent scavenging to settling particles in oxygen minima is not a mechanism removing uranium from the oceans.

Description: Financial support for parts of this work have come from many sources, including: National Science Foundation Grants OCE-7826318, OCE-7825724, and OCE-7727004; Department of Energy Contract EY-76-S-02-3566; a Cottrell Research Grant from the Research Corporation; the WHOI Ocean Industries Program; a fellowship from the WHOI Education Office, and the Paul Fye Fellowship

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 October, 1976

Description: Considerable geological and geophysical evidence now exists to support the hypothesis that seawater circulates through freshly intruded basalt at the mid-ocean ridges. As a consequence of this process, reactions between basalt and seawater take place at elevated temperatures. The mineralogy and chemistry of hydrothermally altered pillow basalts dredged from the Mid-Atlantic Ridge, and belonging to the greenschist facies, have been studied in order to determine the mineralogical changes that result from hyrdrothennal alteration, and to assess the chemical fluxes that result from these reactions in terms of their possible significance in elemental geochemical budgets as potential sources and sinks for elements in seawater. Where possible, pillow basalts were studied that showed varíous degrees of a1teration within a single rock. Such samples provide the best evidence that they have been affected by hydrothermal alteration, rather than regional burial metamorphism, and provide the most useful information for elemental flux calculatìons. During hydrothermal alteration, plagioclase is generally albitised, sometimes with the formation of epidote, and albite may be subsequently a1tered to chlorite. Plagioclase, in association with skeletal clinopyroxene, alters to chlorite and epidote. Olivine is pseudomorphed by chlorite, and clinopyroxene alters to actinolite. The glassy matrix alters to an intergrowth of actinolite and chlorite. Vein minerals irclude chlorite, actinolite, epidote, quartz, and sulphides. On the basis of their minaralogy, the samples may be subdivided into chlorite-rich (〉15% chlorite and 〈15% epidote) and epidote-rich (〉15% epidote and 〈15% chlorite) assemblages. The chlorite-rich assemblages lose CaO and gain MgO, while the epidote-rich samples show very little change in composition compared with their basalt precursor. The epidote-rich samples are more oxidised than their precursors, while the chlorite-rich rocks can be further suhdivided into those that maintain the same proportions of fetrous and ferric iron, and those that show an increase in ferrous iron due to the precipitation of pyrite. The major chemical changes that occur during hydrothermal alteration of pillow basalts are uptake of MgO and H2O, and loss of SiO2 and CaO. The concentrations of Na2O and K2O are apparently not greatly changed, although. they do show some variations in the core-to rim analyses. Consideration of the elemental fluxes in terms of steady-state geochemical mass balances indicates that hydrothermal alteration provides a sink for Mg, which is extremely important in solving the problem of apparent excess magnesium input to the oceans. The amount of calcium that is leached from the rock may be of significance in the geochemical budget of calcium. The concentration of silica in the circulating fluid is probably controlled by the solubility of quartz, and considerable redistribution of silica takes place within the basaltic pile. The changes in the redox conditions during hydrothermal alteration do not affect the present-day oxidation states of the atmosphere and hydrosphere. Trace element analyses indicate that copper and strontium are leached out of the rock and migrate in the circulating fluid, with local precipitation of Cu as sulphides in veins. Li, B, Mn, Ba, Ni and Co show sufficient variation in concentration and location within the altered basalts to indicate that some leaching does take place, and hence hydrothermal alteration of basalts could produce a metal-enriched solution, which may be important in the formation of metalliferous sediments at active mid-ocean ridges.

Description: Most of this work was supported by the National Science Foundation Grants OCE-74-2297l and DES-75-l6596.

Description: Submitted in partial fulfillment of the requirements for Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2005

Description: A moored profiler record from the western tropical North Atlantic provides the first continuous time series of temperature, salinity and velocity profiles in a thermohaline staircase. Variations in the intensity of layering and the evolution of layer properties are well documented during the 4.3 month record. Such staircases are the result of strong salt fingering at the interfaces between the mixed layers, and these data provide unique insights into the dynamics of salt fingers. In particular, a striking linear correlation between the temperature and salinity of the layers may be interpreted as resulting from vertical salt finger flux divergences. Data from this record allow new interpretations of previous work on this topic by McDougall (1991).

Description: This research was supported by the National Science Foundation under grants OCE-0081502 and OCE-0350743