Implication of POC/²³⁴Th Ratios in Oceanic Particulate Matter

An Approach to Particle Aggregation

Abstract

   ²³⁴Th has been widely applied as a tracer of particulate organic carbon (POC) fluxes in the upper ocean. Fundamental to this approach is the determination of ²³⁴Th fluxes from water column measurements of the ²³⁴Th-²³⁸U disequilibria, and the conversion of ²³⁴Th flux to POC export, using the measured POC/²³⁴Th ratio on particles. As such, POC/²³⁴Th ratios are one of the most critical factors in quantifying the carbon export flux in ocean interior when using this approach. However, the POC/²³⁴Th ratios show significant temporal and spatial variations, but cannot be predicted at this time. Therefore, it is important to elucidate factors controlling the variations of the POC/²³⁴Th ratios. To achieve this purpose, we should understand the chemical interactions between POC and ²³⁴Th. In the open ocean, POC/²³⁴Th ratios have been determined together with other oceanographic parameters. We examined here the relationship between POC/²³⁴Th and primary production. The POC/²³⁴Th ratios were linearly related to logarithmic values of primary production. Taken into account the complexation between surface ligand on particulate organic matter (POM) and ²³⁴Th, a complexation model suggests that the size of particles adsorbing ²³⁴Th is related to primary production; in the equatorial Pacific, the size of particles adsorbing ²³⁴Th apparently decreases with increasing primary production, whereas opposite phenomenon occurs in the North Atlantic. Since the POC/²³⁴Th ratios were determined in filtered particulate matter, this finding suggests that aggregation of small particles would be dominant in the equatorial Pacific, which can be explained by a chemical aggregation model.