Past and present of sediment and carbon biogeochemical cycling models

Abstract. The global carbon cycle is part of the much more extensive sedimentary cycle that involves large masses of carbon in the Earth's inner and outer spheres. Studies of the carbon cycle generally followed a progression in knowledge of the natural biological, then chemical, and finally geological processes involved, culminating in a more or less integrated picture of the biogeochemical carbon cycle by the 1920s. However, knowledge of the ocean's carbon cycle behavior has only within the last few decades progressed to a stage where meaningful discussion of carbon processes on an annual to millennial time scale can take place. In geologically older and pre-industrial time, the ocean was generally a net source of CO₂ emissions to the atmosphere owing to the mineralization of land-derived organic matter in addition to that produced in situ and to the process of CaCO₃ precipitation. Due to rising atmospheric CO₂ concentrations because of fossil fuel combustion and land use changes, the direction of the air-sea CO₂ flux has reversed, leading to the ocean as a whole being a net sink of anthropogenic CO₂. The present thickness of the surface ocean layer, where part of the anthropogenic CO₂ emissions are stored, is estimated as of the order of a few hundred meters. The oceanic coastal zone net air-sea CO₂ exchange flux has also probably changed during industrial time. Model projections indicate that in pre-industrial times, the coastal zone may have been net heterotrophic, releasing CO₂ to the atmosphere from the imbalance between gross photosynthesis and total respiration. This, coupled with extensive CaCO₃ precipitation in coastal zone environments, led to a net flux of CO₂ out of the system. During industrial time the coastal zone ocean has tended to reverse its trophic status toward a non-steady state situation of net autotrophy, resulting in net uptake of anthropogenic CO₂ and storage of carbon in the coastal ocean, despite the significant calcification that still occurs in this region. Furthermore, evidence from the inorganic carbon cycle indicates that deposition and net storage of CaCO₃ in sediments exceed inflow of inorganic carbon from land and produce CO₂ emissions to the atmosphere. In the shallow-water coastal zone, increase in atmospheric CO₂ during the last 300 years of industrial time may have reduced the rate of calcification, and continuation of this trend is an issue of serious environmental concern in the global carbon balance.