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We propose to examine, within existing JGOFS and related international
data sets, the relationships between export and production and the special
role that food web processes may have in controlling upper ocean export.
The results thus far indicate that the relative rates of C fixation and
C removal via sinking particles vary widely as a function of local food
web dynamics (Buesseler, 1998). Given the large number of studies which
now use 234Th as a proxy for POC export,
we can start to compile global maps of POC export from the upper ocean.
We will look directly at the full range of 234Th
export data to assess trends between seasonal, episodic, or regional flux
variability and a suite of physical and biological parameters. Because
a significant portion of export may occur during short pulses or events,
our goal is to help explain export variability in order to better model
long-term mean export over larger time or space scales.
If common mechanisms can be found for the variations of production and
export, these could then be incorporated into more reliable models of the
global carbon cycle. While much of the ocean is characterized by low relative
POC export, sites of high export are most often characterized by food webs
dominated by large phytoplankton, in particular diatoms. If this result
holds, models that attempt to predict new and export production from surface
chlorophyll or production alone will not resolve the local carbon balance
or allow one to model export controls. A mechanistic understanding of the
underlying export processes is crucial to being able to incorporate this
understanding into models, particularly if we desire to predict fluxes
in future climate states.
We are closing in on the rate of exchange of CO2
between the ocean and atmosphere, a crucial flux for JGOFS. The
partial pressure of CO2 at the surface is determined
by thermodynamic effects on partial pressure and the balance of carbon
fluxes into and away from the surface ocean. Determining the removal terms
for carbon is one of the most challenging tasks; however, without measuring
them correctly, our global models and global synthesis will always be tenuous.
This is why understanding export is one of the highest priorities for SMP.
We have to know what leaves the surface to balance against what is mixed
into the surface or added by nitrogen fixation or atmospheric deposition.
We also have to understand how this changes with time, ecosystem structure
and physical regime. This proposal results in a global synthesis of the
thorium derived export data. This will lead to a better understanding of
carbon fluxes and parameterization of export that can be inserted into
other models.
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