The Secchi disk is one of the simplest and least expensive instruments for measuring the rate of attenuation of light in natural water bodies. Because this rate of attenuation is often closely related to the concentration of suspended sediment in the water, disk measurements may, in some situations, provide a useful and inexpensive alternative to the standard gravimetric methods for determining sediment concentration. Published empirical studies indicate that satisfactory relationships between Secchi disk transparency and concentration can be established but that these relationships differ from area to area and become more complex as turbidity increases. Results presented in this paper based on data from the Mackenzie delta, N.W.T. support these conclusions. These results cover a range of turbidities which extends well above those previously examined. Physical interpretation of these empirical results is made difficult by the large number of uncontrolled variables associated with Secchi disk readings. This interpretation is facilitated by the introduction of objective attenuation coefficients (more variables controlled) as variables intermediate between transparency and concentration. The two coefficients most commonly used are the volume attenuation coefficient α and the diffuse attenuation coefficient for downwelling irradiance Kd. Empirical relationships between either of these objective attenuation coefficients and disk transparency are reciprocal in form but depart from the simple reciprocal model at high sediment concentrations. Tyler (1968) has shown theoretically that transparency is dependent upon both α and Kd. Since the ratio α/Kd increases with turbidity, the observed departures from the simple reciprocal model can be explained. Burt (1954, 1955) has developed a theoretical expression relating α and sediment concentration in which he shows that α and thus Secchi disk transparency are dependent upon total particle surface - ie., on particle size as well as concentration. This dependence on particle size provides an explanation for the increasing complexity of empirical transparency-concentration relationships as the turbidity range considered widens and for the change in relationships with location. Tyler's and Burt's expressions have been combined in this paper to enable estimations of particle size to be made given only an empirical transparency-concentration relationship and knowledge of the ratio α/Kd. The resulting equation has been applied to the Mackenzie delta data and, considering the large number of untested assumptions made, the particle size estimations are remarkable close to those obtained using Postma's (1961) purely empirical relationship. Secchi disk measurements, then, can provide a surprising amount of information about sediment concentration and particle size. There are, of course, many limitations to the method and the theory associated with it is largely untested. Within the context of these limitations, however, the disk can be very useful. It is simple to use in the field, is remarkable insensitive to changes in the underwater light field and is not greatly influenced by observer bias / Arts, Faculty of / Geography, Department of / Graduate

Marine sediments are a storehouse of the geochemical, biological, and physical changes in the ocean over thousands to millions of years. Intensive study of the Atlantic Ocean has well constrained the role of this basin in global climate change, but the vast Pacific Ocean, deeper and more corrosive to carbonate, has remained more elusive. This thesis leverages a new suite of sediment cores collected on the Juan de Fuca Ridge in the East Subarctic Pacific Ocean (~45˚N, 135˚W) to better understand how the paleoceanographic history of this region has evolved over the past 500kyr. In Chapter 1, I developed age models for multiple cores using benthic δ18O and physical properties of sediment as stratigraphic markers. Despite the proximity of the cores (within 50km2), the sedimentation rates varied by an order of magnitude, likely reflecting remobilization of sediment caused by the high relief of the mid-ocean ridge bathymetry. In Chapter 2, I analyzed uranium series disequilibria in the sediment in order to investigate the processes generating the highly variable sedimentation rates. This chapter presents evidence that the particle flux settling through the water column (based on excess 230Th) is relatively constant at six different sites, and the variability in sedimentation rates is largely driven by lateral sediment remobilization along the rough bathymetry of the ridge. Chapter 3, entitled “Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean”, presented high-resolution x-ray fluorescence records of metal diagenesis in response to changing oxygen conditions in the sediment. This study is the first to show strong evidence for low sedimentary oxygen conditions during interglacial periods in the North Pacific, which we suggest may be linked to hydrothermal sulfide deposition. In Chapter 4, I returned to uranium series disequilibria by utilizing 231Pa/230Th records from the Juan de Fuca Ridge to reconstruct productivity in the East Subarctic Pacific Ocean over the last 200kyr. Productivity across much of the Subarctic Pacific is low during glacial periods and high during interglacial periods, which is usually associated with changes in stratification. I investigated several different mechanisms for increasing stratification during glacial periods, and conclude that a combination of surface freshening, weak winds, and reduced subsurface nutrient concentrations likely created the stratification that led to low glacial productivity. Finally, in Chapter 5, “Dust deposition in the East Subarctic Pacific on glacial-interglacial timescales”, I reconstructed the patterns of dust fluxes in the East Subarctic Pacific Ocean over the last 500kyr to assess the climatic effects on the spatial distribution of dust in the North Pacific Ocean. I predict that migration of the westerlies would have caused a shift in dust provenance away from Asian dust and towards higher North American contributions during glacial periods. Although lithogenic endmembers are currently poorly constrained in this region, I present some evidence for variable provenance over time that may be consistent with the influence of the westerlies on dust fluxes in the East Subarctic.

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No description available.

Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Includes bibliographies. / Sc.D.

No description available.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1980. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 227-236. / by Mary Josephine Richardson. / Ph.D.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographies. / by Wilford D. Gardner. / Ph.D.

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, and the Woods Hole Oceanographic Institution), 2003. / Includes bibliographical references. / Prevailing wisdom holds that the vertical distribution of molybdenum (Mo) in the open ocean is conservative, despite Mo's important biological role and association with Mn oxides and anoxic sediments. Mo is used in both nitrogenase, the enzyme responsible for N2 fixation, and nitrate reductase, which catalyzes assimilatory and dissimilatory nitrate reduction. Laboratory culture work on two N2 fixing marine cyanobacteria, Trichodesmium and Crocosphaera, and a marine facultative denitrifier, Marinobacter hydrocarbanoclasticus, showed that Mo cell quotas in these organisms were positively correlated with Mo-containing enzyme activity. Mo concentrations in Crocosphaera dropped almost to blank levels when not fixing N2 suggesting daily synthesis and destruction of the entire nitrogenase enzyme and release of Mo. Trichodesmium cultures, however, retained a pool of cellular Mo even when not fixing N2. Colonies of Trichodesmium collected in the field have Mo:C tenfold higher than seen in culture, these Mo:C ratios were reflected in SPM samples from the same region. Fe:C ratios for Trichodesmium were between 12-160 pmol:mol in field and culured samples. The Fe:C ratio of Crocosphaera was established to be 15.8 =/+ 11.3 under N2 fixing conditions. Mo cellular concentrations in cultured organisms were too small to significantly influence dissolved Mo distributions, but may slightly affect Suspended Particulate Matter (SPM) distributions. Mean SPM Mo:C ratios were slightly elevated in regions of N2 fixation and denitrification.. A high precision (=/+ 0.5%) isotope dilution ICP-MS method for measuring Mo was developed to re-evaluate the marine distribution of Mo in the dissolved and particulate phase. / (cont.) Mn oxides were not found to significantly influence either the dissolved or SPM Mo distribution. Dissolved Mo profiles from the Sargasso and Arabian Sea were conservative. However, dissolved Mo profiles from the Eastern Tropical Pacific showed both depletion and enrichment of dissolved Mo possibly associated with interaction of Mo with coastal sediments. Dissolved Mo profiles in several California Borderland Basins showed 1-2 nM Mo depletions below sill depth. A more focused study of water column response to sediment fluxes using the high precision Mo analyses is necessary to determine whether these phenomena are related. / by Caroline Beth Tuit. / Ph.D.