Graduate Thesis Or Dissertation
 

A study of nutrient dynamics in the Atlantic Ocean

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  • During the GEOSECS cruise of the R/V KNORR, July 1972-April 1973, a very complete and high quality nutrient data set was acquired for the Atlantic Ocean. One hundred and twenty-one hydrographic stations were occupied throughout the Atlantic providing an internally consistent picture of the nutrient dynamics for this ocean. The dynamic and biological controls on the nutrient distribution were viewed by means of horizontal distribution patterns, vertical profiling, and statistical modeling of relationships between oxygen, potential temperature, salinity, and nutrients. The general conclusions are summarized as follows: 1. The nutrient concentrations in the Atlantic exhibit the interplay at all depths of nutrient rich waters of South Atlantic origin with nutrient poor waters of the North Atlantic. This interrelationship of the two water sources manifests itself in numerous extrema (maxima and minima) in the water column. 2. For intermediate and deep waters, the strong predominance of lateral transport over processes of vertical dissipation are apparent in the Atlantic. Identifiable water types with only small variations of potential temperature (θ), salinity (S), and preformed nutrients can be characterized thousands of miles from their region of origin. 3. Silicate distribution in the Atlantic exhibits very marked gradients between waters of South and North Atlantic origin. Variations of up to 100 μm/kg occur where salinity differences are less than 0.3‰. Great potential exists for the use of silicate as a water mass tracer for Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). 4. The deep and bottom water nutrient distribution can be explained purely from hydrodynamic considerations. Nutrients, dissolved oxygen (O₂), and apparent oxygen utilization (AOU) behave like conservative parameters. The rates of oxidation in deep water are slow relative to the physical processes of mixing and advection, 5. The total organic carbon (TOG) is relatively invariant below a few hundred meters. Significant variation at the cores of NADW, AAIW, and at the ocean bottom is indistinguishable at the present analytical capability. This supports the observation of very low rates of oxidation in the abyssal waters of the Atlantic. 6. The use of statistical models of O₂ as a function of θ or S and a nutrient are consistent with θ-S diagrams in distinguishing the influence of various water types. In addition, a subsurface water type is seen in temperate and equatorial regions which is due to biochemical activity. This water type corresponds to the portion of the water column where rapid oxidation of organic carbon ceases. It is characterized by a low preformed nutrient concentration but a relatively high oxidative nutrient portion. 7. Statistical modeling for a series of stations in the Drake Passage shows the extent of biological depletion across the Passage and points out the influence of an oxygen rich bottom water in the southern reaches of the Drake Passage. This is bottom water from the South Scotia Sea observed by other authors. 8. An apparent breakdown of Redfield's ratio for the Δ O₂: Δ PO₄ and the Δ O₂:Δ NO₃ in the bottom waters of the Atlantic is seen. My analysis indicates that the variation is due not to an inconsistency in the Redfield ratio but to the very low rates of oxidation at great depths. Nearly all the variation in the oxygen content of the deep water at an equatorial station and a station in the Drake Passage can be explained by the use of a conservative variable such as θ or S. Significant oxidation larger than the analytical errors of the GEOSECS methods cannot be seen for the stations considered at present.
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