The effectiveness of the Paleoproterozoic biological pump; a delta (super 13) C gradient from platform carbonates of the Pethei Group (Great Slave Lake Supergroup, NWT)

Samples of carbonate cements collected along a depth transect of the Pethei Platform, a 1.9 Ga stromatolitic reef, reveal a small ( approximately 0.5ppm) carbon isotope gradient between shallow and basinal facies. The magnitude of this gradient would conventionally be interpreted as indicating low export of organic matter from ocean surface waters, but steady-state simulations using a two-box model of the Paleoproterozoic ocean suggest that the small carbon isotope gradient could instead be due to high partial pressures of carbon dioxide in the Paleoproterozoic atmosphere, which would increase the ocean's dissolved inorganic carbon content and damp the effects of biological pumping. If the Paleoproterozoic atmosphere were indeed enriched in atmospheric CO (sub 2) in compensation for a less luminous Sun, these results indicate that the planktonic biota was a significant component of the Precambrian global carbon and nutrient cycles, affecting if not regulating (as today) the composition of both surface and deep ocean waters.