Past and present of sediment and carbon biogeochemical cycling models

Abstract

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 CO 2 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 CaCO 3 precipitation. Due to rising atmospheric CO 2 concentrations because of fossil fuel combustion and land use changes, the direction of the air-sea CO 2 flux has reversed, leading to the ocean as a whole being a net sink of anthropogenic CO 2 . The present thickness of the surface ocean layer, where part of the anthropogenic CO 2 emissions are stored, is estimated as of the order of a few hundred meters. The oceanic coastal zone net air-sea CO 2 exchange flux has also probably changed during industrial time. Model projections indicate that in preindustrial times, the coastal zone may have been net heterotrophic, releasing CO 2 to the atmosphere from the imbalance between gross photosynthesis and total respiration. This, coupled with extensive CaCO 3 precipitation in coastal zone environments, led to a net flux of CO 2 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 CO 2 and storage of carbon in the coastal ocean, despite the significant calcification that still occurs in this region. FurCorrespondence to: A. Lerman (alerman@northwestern.edu) thermore, evidence from the inorganic carbon cycle indicates that deposition and net storage of CaCO 3 in sediments exceed inflow of inorganic carbon from land and produce CO 2 emissions to the atmosphere. In the shallow-water coastal zone, increase in atmospheric CO 2 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.

Air–sea exchanges of CO<sub>2</sub> in the world's coastal seas

“…Although efforts have been made to evaluate the parameters that affect the carbon cycle and to model past and future 6534 C.-T. A. Chen et al: Air-sea exchanges of CO 2 in the world's coastal seas changes in carbon fluxes (Friederich et al, 2002;Fransson et al, 2006;Macpherson et al, 2008;Ver et al, 1999b, a;Mackenzie et al, 2000Mackenzie et al, , 2004Thomas et al, 2007;Borges, 2011), whether the size of the estuarine source and the continental shelf sink for CO 2 changes with time has not been determined because too few data are available. As stated above, the fact that the shelves are a sink rather than a source of CO 2 has only been established in the last few years.…”

“…In any case, the present data clearly demonstrate that the coastal oceans are a sink of CO 2 . Whether a threshold has been crossed or whether the metabolism of the ecosystem has been changed cannot yet be determined (Mackenzie et al, 2004).…”

Potential and limitations of finite element modelling in assessing structural integrity of coralline algae under future global change

“…1), non-geniculate free living (live and dead) coralline red algae (Foster, 2001), are an extremely diverse group of benthic calcifying organisms found from the polar to the tropical regions and low intertidal zones to 150m deep (Foster, 2001). They are major contributors to the global inorganic carbon budget in shallow water ecosystems (Mackenzie et al, 2004). The largest rhodolith bed found on the Abrolhos shelf contributes roughly 5 % to the global calcium carbonate budget (Amado-Filho et al, 2012).…”

Influence of seasonal monsoons on net community production and CO<sub>2</sub> in subtropical Hong Kong coastal waters

“…The organic matter inputs from riverine outflow and domestic sewage effluent increase the occurrence of hypoxia or anoxia and result in high CO 2 release in some estuarine and coastal waters (Ducklow and McCallister, 2004;Diaz and Rosenberg, 2008;Borges et al, 2006), while the enhanced inorganic nutrient fluxes increase primary production and consequently oxygen production and the CO 2 sink (Mackenzie et al, 2004;Gypens et al, 2009). The ratio of dissolved inorganic nutrient to labile organic matter will determine how the CO 2 flux will be evolved (Gypens et al, 2009;Borges and Gypens, 2010).…”