PARTICULATE AND DISSOLVED ORGANIC CARBON IN OCEANS 215 



Menzel 35 examined the process of particle formation with some improve- 

 ments and modifications of earlier methods: (a) he found that his laboratory air 

 supply was contaminated and developed a method for precombusting the air; (b) 

 on second and subsequent filtrations, there was additional accumulation of 

 carbon on filters, and he filtered his samples three times before bubbling; (c) he 

 replicated samples and compared them with unbubbled controls. Under the 

 conditions of his experiments, there was no significant particle formation. 



Batoosingh, Riley, and Keshwar reexamined the problem, comparing 

 methods used by previous investigators. Large and significant yields were 

 obtained after filtration through coarse filters; little or none was obtained after 

 filtration through fine filters (0.2 /i) or triple filtration as practiced by Menzel. 

 Thus it would appear that filter-passing particles are important as nuclei for the 

 aggregation process, although the flakes that are formed are smooth and rather 

 featureless and appear to be more than a mere condensation of particles. 



Initial formation is rapid, but it approaches an asymptote after a day or so of 

 continuous bubbling, and the maximum concentration attained is more or less 

 similar to the original amount of naturally occurring organic carbon in the 

 sample. 



However, some of the experimental methods ' have involved removal of 

 particles during the bubbling period, and in such cases the total yield has greatly 

 exceeded the amount that can be accumulated in suspension during ordinary 

 experimental bubbling. This accumulation appears to inhibit further formation 

 unless part of the yield is removed, but the mechanism for achieving this 

 steady-state situation remains an enigma. 



As indicated earlier, there is experimental evidence of particle formation 

 mediated by UV light. This is thesis work in preparation by John R. Wheeler, 

 and results are incomplete, but he has added lipids and amino acids to artificial 

 seawater, and, after irradiation at approximately the intensity of UV in natural 

 sunlight, there was chemical evidence of an increase in molecular weight, and 

 microscopically visible particles were recovered. These appeared to be aggregates 

 of smaller particles rather than flakes of the kind produced by bubbling. 



These two processes obviously are limited to surface and near-surface waters. 

 There is evidence of a third process that theoretically can take place anywhere in 

 the water column. Sheldon, Evelyn, and Parsons 7 reported that small particles 

 form spontaneously in filtered seawater, gradually shifting toward a maximum 

 concentration with a particle size of about 4 n at the end of 5 days. Refiltration 

 was followed by further particle formation, and this cycle could be repeated 

 several times. These investigators apparently thought that an equilibrium 

 concentration was reached at the end of 5 days, with a carbon value that could 

 be roughly estimated as about 25 Mg/hter, although the actual measurements 

 were made with a Coulter Counter. 



Riley 1 examined this process over a longer period and found further 

 increases that were largely associated with bacterial growth but were far in 

 excess of any reasonable estimates of bacterial biomass. Filtration through 



