PHOSPHATE IN BIF'S: IMPLICATIONS FOR THE ARCHEAN BIOSPHERE

Phosphorus is a principal nutrient potentially limiting rates of carbon production by photosynthesis. We hypothesize that in the Archean and early Proterozoic the limited availability of phosphate considerably suppressed rates of photosynthesis by cyanobacteria. Our hypothesis builds upon the strong absorption between phosphate and freshly precipitated Fe oxide minerals, as would have occurred during the period of active banded iron formation (BIF) deposition. Using observations from present mid-ocean ridges we calculate the ancient ambient water phosphate concentrations from phosphate/iron ratios of BIF's and the calculated adsorption constant.

The calculations indicate that Archean and early Proterozoic ocean water had phosphate concentrations of only 5-10% of the present day deep ocean concentration. Consequently the production of organic matter in the Archean ocean probably was limited compared to the contemporaneous ocean. We furthermore show that a flux of total reactive iron to the ocean equal to the present-day flux would have been able to adsorb a very large fraction of the reactive phosphate flux from land and thereby limited the burial of organic matter phosphate.

A simple biogeochemical ocean model is used to explore the interaction between phosphate, organic matter synthesis, iron concentrations and atmospheric oxygen levels. The two box ocean model includes a parameterization of shelf and deep ocean carbon burial. The parameter space of phosphate and iron delivery and other poorly constrained model components are investigated. The modeled Archean deep ocean contained between 1 to 20 µM Fe²⁺ and was in steady state equilibrium with an atmosphere oxygen content of less than 0.5% of the present level. Transient model calculations relevant to the understanding of the rapid oxygenation at 2.2 to 1.8 Ga will be presented.