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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.

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ABSTRACT. In the past three centuries, human perturbations of the environment have affected the biogeochemical behavior of the global carbon cycle and that of the other three nutrient elements closely coupled to carbon: nitrogen, phosphorus, and sulfur. The partitioning of anthropogenic CO 2 among its various sinks in the past, for the present, and for projections into the near future is controlled by the interactions of these four elemental cycles within the major environmental domains of the land, atmosphere, coastal oceanic zone, and open ocean. We analyze the past, present, and future behavior of the global carbon cycle using the Terrestrial-Ocean-aTmosphere Ecosystem Model (TOTEM), a unique process-based model of the four global coupled biogeochemical cycles of carbon, nitrogen, phosphorus, and sulfur. We find that during the past 300 yrs, anthropogenic CO 2 was mainly stored in the atmosphere and in the open ocean. Human activities on land caused an enhanced loss of mass from the terrestrial organic matter reservoirs (phytomass and humus) mainly through deforestation and consequently increased humus remineralization, erosion, and transport to the coastal margins by rivers and runoff. Photosynthetic uptake by the terrestrial phytomass was enhanced owing to fertilization by increasing atmospheric CO 2 concentrations and supported by nutrients remineralized from organic matter. TOTEM results indicate that through most of the past 300 yrs, the loss of C from deforestation and other land-use activities was greater than the gain from the enhanced photosynthetic uptake. During the decade of the 1980s, the terrestrial organic reservoirs were in rough carbon balance. Organic and carbonate carbon accumulating in coastal marine sediments is a small but significant sink for anthropogenic CO 2 . Increasing inputs of terrestrial organic matter and its subsequent oxidation in the coastal margin (increasing heterotrophy) were significant sources of CO 2 in coastal waters in the 20th century. However, the coastal ocean did not evolve into a greater net source of CO 2 to the atmosphere during this period because of the opposing pressure from rising atmospheric CO 2 . Since pre-industrial time (since 1700), the net flux of CO 2 from the coastal waters has decreased by 40 percent, from 0.20 Gt C/yr to 0.12 Gt C/yr. TOTEM analyses of atmospheric CO 2 concentrations for the 21st century were based on the fossil-fuel emission projections of IPCC (''business as usual'' scenario) and of the more restrictive UN 1997 Kyoto Protocol. By the mid-21st century, the projected atmospheric CO 2 concentrations range from about 550 ppmv (TOTEM, based on IPCC projected emissions) to 510 ppmv (IPCC projection) and to 460 ppmv (TOTEM, based on the Kyoto Protocol reduced emissions). The difference of about 40 ppmv between the IPCC and TOTEM estimates by the year 2050 reflects the different mechanisms within the C-N-P-S cycles on land that are built into our model. The effects of the reduced emissions prescribed by the Kyoto Protocol begin to sho...

[1] There is little doubt that human activities such as burning of fossil fuels and land use practices have changed and will continue to change the cycling of carbon in the global coastal ocean. In the present study, two biogeochemical box models were used to investigate the consequences of increasing atmospheric CO 2 and subsequent ocean acidification and increasing riverine transport of organic matter and nutrients arising from human activities on land on the global coastal ocean between the years 1700 and 2300. Numerical simulations show that the net flux of CO 2 between coastal ocean surface water and the atmosphere is likely to change during this time from net evasion to net invasion owing to increasing atmospheric CO 2 , increasing net ecosystem production arising from increasing nutrient loading to this region, and decreasing net ecosystem calcification due to lower carbonate ion concentration and subsequent lower surface water saturation state with respect to carbonate minerals. Model calculations show that surface water saturation state with respect to calcite will decrease 73% by the year 2300 under a business-as-usual scenario, which in concert with increasing temperature will cause overall biogenic calcification rate to decrease by 90%. Dissolution of carbonate minerals increased by 267% throughout the model simulation. This increase was in part due to increased invasion of atmospheric CO 2 , but mainly due to greater deposition and remineralization of land-derived and in situ produced organic matter in the sediments, producing CO 2 that caused pore water pH and carbonate saturation state to decrease. This decrease, in turn, drove selective dissolution of metastable carbonate minerals. As a consequence, the relative carbonate composition of the sediments changed in favor of carbonate phases with lower solubility than that of an average 15 mol% magnesian calcite phase. Model projected changes in surface water carbonate saturation state agree well with observations from the Hawaiian Ocean Time series and the calculated air-sea CO 2 exchanged agrees well with a recent independent estimate of this flux derived from measurements from diverse coastal ecosystems scaled up to the global coastal ocean area.

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