PROPOSAL FOR A NEW SCOR WORKING GROUP #116

Sediment Trap and ²³⁴Th methods for Particulate Organic Carbon Export in the upper ocean: Current Status

Background and significance

One of the primary goals of the Joint Global Ocean Flux Study (JGOFS) is to obtain a better understanding of the cycling of carbon and associated biogenic elements in the ocean. A major component of this study is the determination of the fluxes of biogenic matter through the ocean water column, especially from the base of the photic zone to ocean interior. This flux, termed as " export production" includes settling particulate matter of plant and animal origin and advective/diffusive transport of Dissolved Organic Matter (DOM) produced by the decomposition of organic matter in the photic zone. The removal of carbon from the photic zone, is at least partly replenished by drawdown of atmospheric CO₂. Thus knowledge of export production and its variability is important to assess the role of oceans in sequestering atmospheric CO₂. Further, at steady state, export production is expected to be the same as new production, a key parameter in marine biogeochemical cycling models. Thus the determination of export production of carbon and its relation to primary and new production has gained considerable importance in JGOFS type studies.

Two methods are commonly used to measure export production; (i) sediment trap and (ii) ²³⁴Th:²³⁸U disequilibrium. Both these are designed to quantify export associated with the vertical flux of settling particulate biogenic materials (i.e. advective/diffusive transport of DOM and lateral transport are not included).

Sediment traps are commonly used in oceanographic research for the measurement of settling fluxes of particles and their collection for chemical and biological analyses. In spite of their wide use, there are concerns about their ability to quantitatively collect unbiased samples of settling particles. These concerns are heightened in traps deployed at the base of the photic zone where current velocities could be quite high. Further, contamination from "Swimmers" and solubilisation of POC in traps could contribute to additional uncertainties in flux data.

The status of sediment trap sampling in surface waters to determine settling flux of particles, especially organic carbon was discussed during the first International JGOFS Scientific Symposium held at Villefranche, 1995 and a report of this is available on the web (http://www-ocean.tamu.edu/JGOFS/contents.html).

The second method used for determining settling fluxes is based on ²³⁴Th:²³⁸U disequilibrium. ²³⁴Th, a short lived radionuclide, (t½ 24.1 days) is produced continuously in sea water at a nearly uniform rate from the decay of ²³⁸U. ²³⁴Th thus produced gets quickly attached to particles and is removed out of surface water with the settling particles. This preferential removal of ²³⁴Th causes its activity to be deficient relative to its parent ²³⁸U in the upper water column (~150-200 m). The extent of this deficiency, in conjunction with data on POC/²³⁴Th in particulate material has been used to determine the export production in many of the JGOFS process study regions, in the North Atlantic, Equatorial Pacific and the Arabian Sea. The accuracy of this method depends on (i) how well the ²³⁴Th deficiency can be determined and (ii) the reliability of POC/²³⁴Th used in calculation. The POC/²³⁴Th ratio shows considerable variation among various particle classes and over short time intervals of a few weeks.

During the recent International Scientific Symposium on "Biogeochemistry of the Arabian Sea: Synthesis and Modelling" held at Bangalore - January 18-20, 1999, it was felt that there is a need to better understand the reliability of export flux determined based on traps and ²³⁴Th studies and how they intercompare.

Current Terms of Reference

Considering this suggestion and the importance of export production in biogeochemical models, we propose, that SCOR establishes a working group on "Sediment Trap and ²³⁴Th methods for carbon Export Flux Determination: Current Status" with the following terms of reference.

1 To explain the terms export production and new production and their inter-relation. How does the carbon flux determined using traps and ²³⁴Th relate to export production?

2 To review the current status of carbon export flux determination using moored and floating sediment traps, their advantages and problems, associated uncertainties and their magnitudes. To suggest suitable trap designs and necessary protocols to get reliable flux data.

3 To review the basis of ²³⁴Th based carbon export flux measurements, models, assumptions and parameters used in the calculations. To assess the reliability of these assumptions/parameters, the sources and magnitudes of associated uncertainties. (For example: How do the time scales of sampling, temporal variability in ²³⁴Th fluxes, POC/²³⁴Th ratio in different particulate pools affect the flux data?).

4 To intercompare the carbon export fluxes determined by trap and ²³⁴Th methods. If they differ, what are the main causes of discrepancy and how can they be resolved?

5 To suggest experimental design and protocols to be followed to obtain quantitative and reliable carbon export fluxes based on the above methods. Can ²³⁴Th serve as a global survey tool to determine carbon export fluxes?

6 To prepare a final report within ..4 years and interim report within 2 years.