Particulate Matter in Surface Seawater: Sources, Chemical Composition, and Affinity for the Sea-Air Interface

An adsorptive bubble separation technique was used to examine the ability of bubbles to transport particulate matter in surface seawater samples. A substantial fraction of the particulate organic carbon (POC) and particulate organic nitrogen (PON) could be recovered in the froth generated by this technique from both estuarine and open-ocean surface waters. Recovery values were lower in samples taken during periods of high productivity. C/N ratios of the particulate matter recovered in the froth were generally not significantly different from the particulate matter not recoverable by this technique. Budget calculations indicated that there was no significant conversion of dissolved organic matter to particulate organic matter during the flotation process although large macroscopic particles were observed to form quite readily in the froth. This bubble induced formation of large particles may be a significant process leading to the transport of large particle POC to deep waters. The recovery of particulate trace metals (PTM) from estuarine and open-ocean surface samples was generally higher than that for POC and PON, suggesting that the particles transported by bubbles were richer in PTM than those remaining behind. PTM recovery values in open-ocean samples were found to be similar to those observed in Narragansett Bay. This observation was consistent with the results observed for POC and PON. However, interpretation of the results of the open-ocean flotation experiments was difficult due to the unexpectedly low PTM concentrations observed in these samples and the consequently greater problem presented by contamination. Particulate trace metal concentrations of the Sargasso Sea were lower than or equivalent to the lowest concentrations previously reported in the literature for open-ocean surface waters. Examination of the relative variations in PTM with respect to particulate aluminum and carbon led to the conclusion that organic matter was the probable regulator of PTM abundance in open-ocean surface waters and was important in this respect for continental shelf and slope waters as well. Bubble transport fluxes of POC and PTM in the Sargasso Sea were estimated on the basis of the flotation experiment results. Bubble transport was found to be capable of producing observed surface microlayer concentrations of POC and PTM in a matter of minutes, suggesting the importance of bubble transport in this respect. Bubble transport rates were compared with atmospheric deposition rates of PTM which were estimated using published atmospheric trace metal concentrations in marine air sampled at Bermuda over a two year period. The ratio of