Sedimentary balance through geological time.

Balance in the sedimentary cycle is maintained by the sum total of weathering, transport, deposition of sediments, and their reincorporation into the continental crust. Biological activity and primarily photosynthesis strongly affect the fluxes and deposition of C-, S-, Fe- and P-containing sediments. The limiting-nutrient role of P is to a variable degree related to mineral solubilities and competition of inorganic sinks (clays, Fe oxides) for dissolved phosphate. The efficiency of P utilization by organisms is greater in land plants than in marine phytoplankton, and forms a large reservoir of P. Changes in its size can constitute significant inputs of P into oceanic biota. Photosynthetic fixation of C and its net storage in sediments control four major fluxes of the global sedimentary cycle: organic matter, limestones, sulphate and sulphide. Changes in the net deposition rate of organic C produce changes in all the other fluxes in the cycle. The magnitude of such changes depends on the nature of reduced iron-mineral sinks for oxygen and the nature of the CO₂ sources replenishing CO₂ consumed in photosynthesis. A negative correlation between the isotopic abundance ratios ¹³C/¹²C in carbonates and the ³⁴S/³²S in sulphates since the Late Proterozoic corroborates the dependence of the sedimentary fluxes on one another. -P.Br.