SUN Yunming and SONG Jinming
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
Biogeochemical process of marine carbon cycle is one of the key links controlling the global change. With the developing of the key international plans of JGOFS, GLOBEC, SOLAS, etc. in recent ten years, the carbon cycle study has made a great progress. It may be said that the biogeochemical process of marine carbon cycle had been understood more systematically than ever before. Especially the marine biological pump process and the mechanism of CO2 absorbed in oceans have been quantitatively recognized and understood. This report focuses mainly on the progress of biogeochemical processes of marine carbon cycles from 1998 to 2002 in China. It includes 3 parts, i.e. the CO2 fluxes and processes between atmosphere and seawater; carbon and its biogeochemical cycles; functions of sediment and soils around estuaries in marine cycles.
I . CO2 FLUXES AND PROCESSES BETWEEN ATMOSPHERES AND SEAWATER
The aquatic ecosystem, especially the ocean, is totally a huge CO2 reservoir .According to the recent estimation, human behavior contributes CO2 of 5.5×109 t to atmosphere annually, in which about 2.0×109 t is absorbed by the ocean, accounting for 35% of total discharge, and about 0.7×109 t is absorbed by terrestrial ecological system, accounting for 13%. It is showed that the ocean and land hold about half of CO2 from human activity, and another half of CO2 is emitted into atmosphere. It is clear that the ocean could weaken greenhouse effect from CO2, and plays an important role in regulating the levels of atmospheric CO2, and hence global climate. The study of biogeochemical processes of marine carbon cycling has become the key issue of studying marine carbon cycle and global climate change, and it also will be an important work of the future international oceanography in the 21st century (Tao 1998; Wang S.L. et al., 2000).
Carbon in the oceans mainly exists in the forms of CO32- and HCO3-. The in most of seawaters is about 2 mmol/kg and about ten times of the dissolved organic carbon (DOC), and much higher than the particulate organic carbon (POC). The ocean's role in regulating the uptake capacity of CO2 and the carbon exchange between atmosphere and ocean depends on the mixed layer carbonate chemistry, the advection transfer of carbon dissolved in seawaters, the CO2 diffusion across the water-air interface, the various biological processes and settling of organic carbon from biological production, and the dissolving and settling of carbonates around the sediment-seawater interface, ect. Many models have been established and developed in order to evaluate the CO2 sink in the oceans. The net ocean sink is estimated at a range from 1.2 to 2.4 GtC/a based on the box model and the general circulation model, which is generally accepted as 2.0 GtC/a. CO2 in the atmosphere is driven by biological pump into the ocean. In the marine ecosystem CO2 is then changed into OC due to biological carbonates of biological photosynthesis in the mixed layer, and is further transferred from surface to the deep layer, which is the main processes of marine carbon cycles (Chen and Tsunogai, 1998; Fang et al., 2001; Yu et al., 1999; Zhang, Wang and Chen, 2000).
In order to gain a deeper insight into global carbon cycle, the first thing is to study the variations of CO2 in the surface water and the differences of PCO2 (D ) between the sea and the air. The changes of total dissolved CO2 ( )in the surface water in the tropical Pacific
(10°S, 20°N; 120°E, 90°W) during the El Niño and the La Niña events have been numerically simulated using a 3D global ocean carbon cycle model with biological pump. The results showed that the changes of the total dissolved CO2 and the partial pressure difference between the sea and the air(DPCO2 ) in the northwest Pacific (0-20°N, 120°-150°E) and in the central and east equatorial Pacific (10°S, 10°N; 150°E, 90°W) were noticeable. During the El Niño events, the
changes of in the surface water increased in the northwest Pacific and decreased in the
central and east equatorial Pacific; there were opposite changes in both regions during the La Niña events (Xing and Wang, 2001).
A 3D global ocean carbon cycle model with the ocean biological pump was developed. In this model, the atmosphere is represented as a well-mixed box of CO2, where CO2 from the surface water is exchanged. The carbon cycle model has been numerically integrated for 1200 years and finally reached a quasi-equilibrium state. Under the quasi-equilibrium state condition of the model, the computed , alkalinity, the dissolved oxygen concentration in seawaters, the distribution of new production and the differences of between the sea and the air are close to the observed results. CO2 absorbed by the sea is 42% and 7% with and without the ocean biological pump, respectively, which shows that there are significant effects of the ocean biological pump on the capacity of ocean absorbing CO2 in the air. A 3D ocean carbon cycle model and a simple terrestrial biosphere model were used to simulate the anthropogenic CO2 uptake by the ocean and terrestrial biosphere under the IPCC (Intergovernmental Panel on Climate Change) scenarios to predict the atmospheric