Phanerozoic cycles of sedimentary carbon and sulfur ( isotopes / sediment cycles / natural oxygen reservoirs )

A reservoir model of a Recent steady-state sedmaterial transport, such as gas exchange, deposition from the imentary system in which the reduced sulfur and oxidized sulfur ocean, and dissolution and weathering of rocks that produce reservoirs were coupled with the oxidized carbon and reduced inputs to the ocean. Despite the variety ofreservoirs and fluxes carbon reservoirs was constructed. The time curve of the sulfur in Fig. 1, this model depicts the biogeochemical cycles of only isotope ratios of the sedimentary sulfate reservoir was used to three main elements--carbon, sulfur, and oxygen-and the andrive the model back to the beginning of Cambrian time (600 milcillary roles of several other elements (Fe, Mg, Ca, Si), through lion years ago), producing the reservoir sizes and isotope values which the C, S, and 0 reservoirs and fluxes are connected to and material fluxes of the carbon-sulfur system. The predicted one another. The individual transfers between the reservoirs values of carbon isotope ratios of the carbonate reservoir agree in Fig. 1can be represented by a net balance equation for the well with observed values, showing that the model is basically transfer fluxes (1) written in the form of a stoichiometrically sound. Some general conclusions from this success are (i) material balanced chemical reaction: f l u rates in the carbon-oxygen-sulfur system of the geologic past (averaged over tens of millions of years) lie within about a factor of 2 of Recent rates. (ii) The oxidation-reduction balances of Phanerozoic time were dominated by reciprocal relationships between carbon and sulfur compounds. (iii) The rate of production of atmospheric oxygen by storage in sediments of organic carbon of photosynthetic origin increased from the Cambrian Period to the The process of accumulating 8 (8 is arbitrary and for illustraPermian Period and declined somewhat from the Permian Period tive purposes) molar units ofCa and SO, in gypsum or anhydrite to the Present. (ic) The storage of oxygen in oxidized sulfur comcan be followed descriptively as follows. The sulfur stored in pounds kept pace (within the limits of the data) with oxygen proSO, must come from oxidation of reduced sulfur from the FeS, duction. (u)Transfer of oxygen from CO, to SO, from the Camreservoir, which requires 15 0, and produces 2 Fe,O, and 16 brian to the Permian Period was several times the Recent free H+ along with the required 8 SO4'-. The replacement of 15 0, oxygen content of the atmosphere. consumed from the atmosphere is accounted for in part (8 0,) by photosynthesis of 8 CO, released from the CaCO, reservoir In recent years attempts have been made to interpret the bioby storage of 8 Ca2+ in gypsum. Production of 8 0, from 8 CO, geochemical processes in the Earth's surface environment also requires burial of 8 C in organic carbon. The other 7 0, within a framework of cycles. A conceptual model of a cycle of required for SO: must come from photosynthesis of CO, one or more elements is generally a system of reservoirs repyielded by the MgCO, component of carbonate rocks (shown resenting different compartments of the environment and as the MgCO, reservoir in Fig. 1).The 7 0, are released by fluxes transporting materials among them. transfer of Mg from carbonate to silicate, that is, from dolomite Most criteria that might be used to indicate secular trends to clay. This CO, cannot be derived from CaCO, conversion to of the sizes of sedimentary reservoirs or fluxes of the global sedCaSiO, because Ca is stored in large quantities in sediments imentary system indicate that reservoir sizes and fluxes change only as carbonate or sulfate. Calcium silicates do not form in the with time, but that there are no pronounced trends. One might surface environment. The carbon of the MgC0,-derived CO, liken the reservoir system changes to those of the water content is stored as organic C. of a number of aquaria, linked by pipes, that would take place The preceding brief description of the cycle seems to indicate if the aquaria were mounted in the hold of a ship. Water would that gypsum deposition may be the driving force of the system. move among the reservoirs as the ship pitched and rolled, but Conversely, Eq. 1 also results from the assumption of accuone could conceive of a "mean steady state," the contents of the mulation of organic carbon without change in the composition reservoirs at the mean position of the ship. We visualize the of ocean or atmosphere. Briefly, storage of 15 CH,O requires reservoirs of sedimentary materials during the last half-billion 15 C from carbonates and produces 15 0, in the ocean plus atyears as conforming to such a model-some reservoirs changing mosphere. The sink for the free 0, is oxidation ofreduced sulfur their masses by factors of two or three, perhaps, but with no and iron. change in t h e total mass of all reservoirs o r in t h e ocean-atmosphere system that is the medium of transfer, and CARBON-SULFUR MODEL a general oscillation of mass of a given reservoir around a mean value. Fig. 1 shows the stoichiometry required for increase of Essential data and constraints of the model mass of the CaSO, reservoir. The transfer involves 10 reserThe system of Fig. 1 can be used as the basis for a mathematical voirs: atmosphere, ocean, and eight reservoirs ofmineral phases model of the carbon-sulfur cycle. Starting with the modern, but and organic matter. The arrows between the reservoirs indicate prehuman, carbon-sulfur system, the system is driven backThe publication costs of this article were defrayed in part by page charge * Present address: Dept. of Marine Science, Univ. of South Florida, St. payment. This article must therefore be hereby marked "adcertisePetersburg, FL 33701. ment" in accordance with 18 U. S. C. $1734 solely to indicate this fact. + To whom reprint requests should be addressed.