DISSOLVED ORGANIC CARBON AND MARINE SNOW IN THE PROKARYOTE DOMINATED PRECAMBRIAN OCEANS

Production of fecal pellets from eukaryote grazers was probably not of importance until the end of the Neoproterozoic. Today, discrete fecal pellets constitute 10-30% of organic matter collected in sediment traps. The rest of the taped organic matter is marine snow, of which up to ~40% may be fecal pellets. Thus, fecal pellets are an important component of the vertical transport of carbon in the present ocean. The question is then, how organic carbon was cycled and sedimented to the ocean floor in the prokaryote-dominated Precambrian ocean, before the evolution of grazing eukaryotes and the production of fecal pellets?

In the Archean and Early proterozoic oceans where the food-web most likely was characterized by only lateral competition without grazers, carbon export from the euphotic zone was probably driven by DOC advection and/or aggregation of prokaryote cell material. The formation and sinking properties of aggregates would have depended on the concentration of bacteria, dissolved organic carbon (DOC), particle stickiness and inorganic material.

Formation of aggregates and marine snow in a prokaryote-dominated world are modeled using aggregation kernels, and simple growth models for cyanobacteria (nutrient and light limited). The modeled mean settling velocity scale linearly with the initial particle size. As a result the organic particle size spectra would have been skewed toward smaller sizes and settling velocities in an ocean dominated by relatively small cyanobacteria (c.f. Prochlorococcus ). However, the mean settling velocities only would have been slightly less than today, if the ocean was dominated by relatively large cyanobacteria (c.f. filamentous or colonial Trichodesmium). Modest additions of metabolically derived inorganic material from anoxygenic phototrophs enhance settling velocities significantly, which could have promoted a two layer ecosystem in the prokaryote-dominated world, where anoxygenic phototrophs in the lower layer dominated the vertical carbon flux. The possible size spectra are included in a settling and degradation model embedded in a simple vertically resolving ocean model, which enable us to approach a quantification of the sizes of the different carbon pools in the ocean and vertical chemical structure of a prokaryote-dominated world.