|Abstract or Summary
- 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
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.