Dataset: Global estimates of Thorium 234 based particulate organic carbon export, 2004 (U.S. JGOFS Synthesis & Modeling Phase project results)

Research Topic: Magnitude, Variability and Controls of Particulate Export in the Upper Ocean

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.