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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.



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  levels in future. It was estimated that the anthropogenic carbon emissions must be reduced in order to stabilize CO2 in atmosphere at various  levels ranging from 350×10-6 to 750×10-6. All the stabilization scenarios require a substantial future reduction in emissions (Jin and Shi, 2001).

Song et al. (2002) reported that the relation between partial pressure of CO2 ( water ) and temperature (T) in the surface water was obtained from the simulated experiments of laboratory, which showed the formula water6.62T+221.03. The relative error between the estimated water and the measured values is lower than 4.5%. The air-sea flux seasonal distributions and strength of source/sink of CO2 in the East China Sea were obtained for the first time based on the data of surface seawater temperatures and partial pressure of atmosphere. The seawater could take in CO2 from atmosphere in the Bohai Sea, the Yellow Sea and the East China Sea and the flux values are higher in winter than those in spring. And in summer, the situation is reverse and CO2 is released to atmosphere. In autumn, the seawaters can take in CO2 in the Bohai Sea and the northern Yellow Sea, but releases CO2 to atmosphere in the East China Sea and the southern Yellow Sea. The minimum and maximum of air-sea flux of adsorbed CO2 appear in autumn of the northern Yellow Sea (5.3 gC/(m2·a)) and in winter of the Bohai Sea (106.0 gC/(m2·a))respectively, and the minimum and maximum of released CO2 appear in summer of the northern Yellow Sea(–1.9 gC/(m2·a))and the East China Sea(–18.8 gC/(m2·a)) respectively. The annual mean fluxes from seawater to air  are 36.8, 35.2, 21.0 and 3.5 g C/(m2·year) in the Bohai Sea, the northern Yellow Sea, the southern Yellow Sea and the East China Sea, respectively (the Yellow Sea flux is 23.7 gC/(m2·a)). The East China Sea is the net sinks of atmospheric CO2 in spring and in winter, which can take in 7.69 and 13.56 million tons carbon, respectively, and is the source releasing CO2 to air at 4.59 million tons carbon. The Bohai Sea and the northern Yellow Sea are the sink of atmospheric CO2 and can take in 0.27 million tons carbon. The southern Yellow Sea and the Eastern China Sea are the source of CO2 , which release to air 3.24 million tons carbon in autumn. As a result, the net carbon sink strength of the East China Sea is 3.24 million tons carbon in autumn. The annual mean sink strength of atmospheric CO2 in the seas east of China is 13.69 million ton carbon per year.

In conclusion, in the past 4 years, the studies on exchanges of carbon between air and water in Chinese marginal seas have made some progresses, especially on exchanges of CO2. Various ocean carbon cycle models from different points of view have been proposed and applied for studying the carbon cycles between air and water.



It is especially important to study carbon cycles in the East China Sea, which is a marginal sea with its coastal regions affected significantly by human activities. It includes typical continental shelf, continental slope and semi-deep-sea regions. Studies on DOC and POC along a cross-shelf transect in the southern East China Sea showed that the DOC concentrations were higher (>85 mmol/L) in the inner shelf and slope waters but lower (ca. 65 mmol/L) around the shelf break, where the Kuroshio upwelling occurred. Such a distribution pattern showed a little temporal variations. The coastal water contained less colloidal organic carbon (COC) than that in the oligotrophic slope water, suggesting a lower production rate and /or a higher breakdown rate in the coastal water. The POC distribution showed a decreasing trend from the inner shelf to the slope with a local maximum at the shelf break, where POC was enriched due to the enhanced primary productivity induced by upwelling. The average POC content was the DOC of about 1/10. There was a maximum of POC in the mid-depth over the slope, which indicates the lateral transport of POC going offshore from the shelf. The net POC exports of DOC and POC from the shelf are estimated to be 414 and 106 Gmol C/a, respectively (Chen, J.F., 1999). Heterotrophic bacterial biomass, production and turnover rates were investigated in transect across the continental shelf of the southern East China Sea during spring and autumn. In the coastal and upwelling areas, bacterial biomass is 350-1200 mg C/ m2production is 28-329 mg C/ m2and the averaged turnover rates are 0.09-0.22 /dwhich are at least 2-fold of those in the Kuroshio waters. Production and turnover rates were positively correlated with primary production (90-2133 mg C/ m2) and particulate organic carbon (POC, 1415-4682 mg C/ m2). Dissolved organic carbon from the non-phytoplankton and allochothonous sources might play a significant role in supporting bacterial carbon demand in the shelf area of the East China Sea (Shiah, Liu and Kao, 2000). The southern East China Sea continental shelf region is characterized by relatively low organic carbon concentration with a fast sedimentation rate. The Organic carbon concentration ranged from 0.3 to 0.6 wt% and the sedimentation rate from 0.2 to 0.7 cm/a. In addition, the normal marine S/C ratios were observed. Up to 96% of pyrite-sulfur was reoxidized before its final burial. The sulfate reduction rate and the pyrite-sulfur burial rate increased linearly with the increase of the organic carbon burial rate, which indicates that the organic carbon deposition controlled the pyrite formation in the East China Sea continental shelf sediments. The organic carbon utilized by the sulfate reduction and its burial represented a significant but relatively small fraction of the primary production in the studied East China Sea region (Lin et al., 2000). According to carbonate and the related parameters, the low-temperature, low-salinity water mass in the summertime northern East China Sea originates from the Yellow Sea Cold Water, which is formed farther northward. There is no apparent annual variation in the carbonate parameters in the Kuroshio east of the shelf break. The partial pressure of CO2 calculated from the pH, TA or TCO2 data in this study shows that the surface water in the shelf area is undersaturated with CO2 in spring and summer. Taking the above data combined with the other data collected in different seasons into consideration, it is shown that the shelf area of the East China Sea is indeed a net sink for atmospheric CO2, it absorbs as much as 0.013-0.030 Gt C/a (Wang S.L. et al., 2000). The vertical fluxes and the molar ratios of carbon of the suspended particulate matter in the Yellow Sea were studied based on the analysis of the suspended particulate matter, it was showed that most of the particulate organic matter in the Yellow Sea water column came from marine life rather than the continent, the vertical fluxes of POC in the Yellow Sea are much higher than those in other seas over the world. There was high primary production in this region (Wang B.D. et al., 2002). 

The average carbon biomass of Sybechococcus in the East China Sea ranged from 0.09 gC/m2 (early spring) to 0.90 gC/m2 (autumn). The upward flux of nitrate into the euphotic zone in the South China Sea was calculated by the coupled Ra-nitrate approach, and further converted into a new production of 4.4 mmol C/ (m2·d) based on a Redfield ratio of 6.6 for C: N. The 234Th-238U disequilibrium and the measured ratio of POC to particulate 234Th yield a POC export flux of 5.7 mmol C/ (m2·d), it is consistent with the new production calculated by nutrient budget. Based on the 234Th-228Ra disequilibrium, POC export flux was estimated to be 1.7 mmol C/ (m2·d), significantly lower than the derived new production. The discrepancy can be caused by the uncertainty of the DOC transport or accumulation and of the data obtained in different way and in different seasons and durations (Cai et al., 2002; Kuo-Ping C, et al., 2002).

The Donghu Lake is a typical shallow eutrophic lake along the Changjiang River's middle reaches. The mean concentrations of DOC were 15.11±3.26, 15.19±4.24, 14.27±3.43, 13.31±3.30 mg/L in four stations during 1996-1997, respectively. The DOC concentrations of the study area were very similar to those in other lakes along the Changjiang River's middle reaches. The mean POC of the whole lake was 5.01mg/L due to the large amount of organic detritus from both local origin and allochthonous origin. A significant linear relation was found between POC and chlorophyll a at all of 4 stations, which presumably displays that phytoplankton and its exudates and metabolic products are the main contributors to the POC in the water column. DOC/POC (mean value of 4.40) indicated that the organic detritus would be the most important component of the particulate organic matter. Phytoplankton is also a factor to dominating the particulate organic matter in the Donghu Lake (Liu X.J., et al., 2000).

Han et al.(1998) studied the component of OC and carbon cycle in Daya Bay and the Zhujiang River estuary. They classified OC into the dissolved OC (DOC), the particulate OC (POC) and the sedimentary OC (SOC). It is found that both the East China Sea and the Taiwan Strait are reservoirs of CO2 in winter, and the Taiwan Strait is a weaker source of CO2 in spring. A research into DOC and COC in the Zhujiang River estuary shows that COC accounts for DOC of 3%-32%, the maxima are lower than these of low salinity regions (<5%) not only in winter but also in summer (Chen, J.F., 1999). Colloid matter is formed on the spot where its found (Cai, et al., 1999). The phytoplankton biomass and primary productivity, and their annual variations and the photosynthetic carbon flow in the Taiwan Strait were investigated during the cruises in Aug., 1997, Feb.- Mar. and Aug., 1998, and Aug., 1999. Their results showed that nanophytoplankton (NANO) and picophytoplankton (PICO) dominated the community, with a contribution of 34%-48% and 34%-40%, respectively , microphytoplankton (MICRO) contributed only 12%-27%. Seasonal and annual variations occurred for size-structure and size-fractionated phytoplankton biomass. PICO dominated the phytoplankton productivity with 45%-50%, both NANO and MICRO contributed 19%-32%. The photosynthetic carbons (PC) of 25% were incorporated into microbial food web (MFW) via secondary production by heterobacteria, PC of 36% into MFW via gazing by heteroflagellate. Thus, PC of approximately 60% went into MFW via the two paths, which indicated that MFW would play an important role in transformation of organic carbon in the Taiwan Strait (Han, W.Y., 1998; Huang, et al., 2002).

Hong and Wang (2001) studied the biogeochemical processes of biogenic elements in the Taiwan Strait based on the marine dynamics, the coupling of physical, chemical and biological processes, and the contribution of microplankton to carbon cycle. The results show that in the large spacial and temporal scale, biogeochemical cycling of carbon and phosphate is regulated by marine dynamics in this area. It is shown that the southern area of the Taiwan Strait is a strong source of CO2 in the air in summer, the dissolved organic carbon is the major organic species and the 60% particulate organic carbon comes from continent. Also, the Nano- and pico-phytoplanktons dominated phytoplanktons in this sea area. Their contributions to biomass and primary productivity were over 60% and 80% respectively. Most of the primary productivities were consumed by bacteria and by hetero-dinoflagellates.

The POC concentration in Xiamen Bay ranged from 14.4 to 34.6 mmol/m3 with an average value of 21.6 mmol/m3. The contributions of living organic carbon (phytoplankton) and organic detritus were estimated at about 8%-26% and 74%-92% of the TPOC, respectively. The profiles of POC showed a gradual decrease with the depth, and its temporal variation showed that the POC concentrations in daytime are higher than those in nighttime. Both features suggested that POC would be closely related to biological processes in Xiamen Bay. The primary production in the study area varied up to 5 times in 1 d, which was consistent with that of the biomass. Primary production also decreased with depth, which was coincident with the patterns of chlorophyll-a and POC. In the meantime, the effect of incubation time on the determination of primary production was studied. The primary production calculated from short-time incubation (2h) is higher than that from long- time incubation (24h), indicating that some of the new fixed carbon are preferentially respired and then excreted. Based on the particulate export fluxes from disequilibria and the POC/PTh radtios in the particles, the POC export flux from euphotic zone is estimated at 16.0 mmol/ (㎡·d), in which the export fluxes of living organic carbon and detrital organic are 2.7 and 13.3 mmol/ (㎡·d), respectively. The ratio between POC export and primary production, referred as the ratio, is 0.31. Both POC export fluxes and the ratios are consistent with the predictive value from the relative formula presented by Aksnes and Wassmann in 1993, but not with other models. The residence time of POC in the euphotic zone was estimated at 11d, indicating a rapid regeneration rate of POC in study region (Chen, Huang and Qiu, 2000).

The DOC concentration was 142-239 mmol/L in the freshwater taken in March 1997 from the four Zhujiang River tributaries flowing into the Lingdingyang estuary. High concentration was observed in the Human tributary located near Guangzhou. The rapidly increased DOC concentration at low salinities (~5) may be attributed to the exchange between macroparticulate and dissolved organic matter during the early stage of estuarine mixing. The DOC concentration overall followed the mixing line until salinity 25, where the deep Bay is located and where DOC was elevated. The elevated DOC may suggest a local organic source from Shenzhen. COC in the study area ranged from 5 to 85mmol/L, representing DOC of 3% to 32%. The highest COC percentage was found at low salinities (<5) in both summer and winter. This suggest that the terrestrial organic matter be an important factor controlling the carbon cycling, and so be the hydrokinetic factors, seasons and salinity (Dai, et al., 2000).

Despite the South China Sea as a reservoir of CO2, its area is large, there are many upwellings favorable to transferring CO2 from the lower layer to the upper layer (Chen. J. et al., 1998; Dai, et al., 2001). Based on the organic carbon concentrations, the stable isotope analyses, the analyses of the distributions of benthic foraminifera in gravity and the piston cores from two observed stations of the northern and southern continental slopes of the South China Sea, the changes in the surface paleoproductivity and the variations of the East Asian Monsoon over the last 40000 years have been evaluated. The distribution patterns and accumulation rates of some deep-sea benthic foraminiferal species may be primarily controlled by the organic carbon flux to the seafloor in the South China Sea. Two major subgroups of these species serve as proxy to distinguish two different ranges of organic carbon fluxes (>2.5 g C/ (㎡·103 a) and >3.5 g C/ (㎡·103a)). When organic carbon flux increases to above 3.5 in the southern South China Sea during the Last Glacial Maximun and in the northern South China Sea during the early Holocene, a group of detritus feeders such as Bulimina aculeate and Uvigerina dominates over the others. However, the suspension feeders such as cibicidoides wuellerstorfi and Chilostomella ovoidea gradually become more important than detritus feeders as soon as the organic carbon flux decreases to 2.5-3.5 g C/(㎡·103a). During the LGM, the high organic carbon flux and the increased abundances and accumulation rates of B.aculeate and U.peregrina in the southern South China Sea are mainly caused by the enhanced NE winter monsoon-driven upwelling and the associated productivity, and partly by the increased input of terrigenous nutrients as a result of the lowered sea level. However, during the first part of the Holocene, around 10000 a B.P., the remarkably increased abundances and accumulation rates of B.aculeate and U.peregrina especially in the northern South China Sea, together with the high organic flux, point to increasing productivity, probably driven by a maximum intensity in the SW summer monsoon (Jian, Wang and Kienast, 1999). Carbon biogeochemical processes in the special environment also have been studied. For example, the hydrothermal vent communities are quite different from the typical deep-sea communities in many aspects, whose foodchain structure is chemosynthetic bacteria, which feed on reduced inorganic chemicals like hydrogen sulfide in order to synthetize OC, to provide energy for macroanimals by means of endosymbioses (Li and Hou,1999).

The DOC and POC at 12 stations in the Yantai Sishili Bay in May, August, November of 1997 and March and May of 1998 were investigated. The DOC concentrations varied from 1.14 mg/L to 5.35 mg/L. The average values at all stations in each cruise varied from 1.52 mg/L to 2.12 mg/L. The POC concentrations varied from 0.049 mg/L to 1.411 mg/L. The average values of POC in each cruise varied from 0.159 mg/L to 0.631 mg/L. Horizontal distribution of DOC was influenced by several factors, such as terrestrial input, organism activity, temperature, aquiculture environment, etc. The higher POC concentration occurred along the coast. The vertical distribution of DOC and POC changed obviously in spring and summer, but not in autumn and winter. The DOC concentration was the highest in summer and POC in spring; both DOC and POC were the lowest in winter. The seasonal variation of DOC was consistent with that of primary productivity. The seasonal variation of POC was consistent with that of chlorophyll-a. There was the significant seasonal variation trend of C/ N ratios in the dissolved organic matter, but the C/N ratio of particulate organic matter had no significant trend (Zhao et al., 2001). 

The carbon biogeochemistry is a science that studies the correction and role of carbons that affect lives and their environments by exploring carbon's transferring and recycling.  Biogeochemical abundance, currents, coupling and field are its four elementary ideas. Transferring and recycling of CO2, CH4 and N2O etc. have been paid close attention to because of abnormal global climates. Li C.S. (2001) adopts a DNDC model to study the biogeochemical factors and processes associated with C and N cycling, and to simulate the interactions among global climate changes, human activities and terrestrial ecology. DNDC represents the denitrification and decomposition  which are two main reactions to cause C and N transferring from soil to atmosphere. This model has been used to forecast fertility of soil and emission of greenhouse gas in some countries. There are some reports about biogeochemical cycling models of C and N abroad, but so far, there are no such a systematic or comprehensive model which can simulate C cycle and N cycle in the atmosphere-ocean-land system. To develop such a comprehensive model needs to set up a multi-disciplinary project in which all scientists from marine chemistry, biology, geology and physics, etc. make a joint study.

The theoretical study to imitate the carbon cycling test in laboratory has also been conducted. Wang, Zhao and Zhang (2000) find that pH can affect adsorption of DOC on goethite. The adsorption percentage of DOC shows a maximum at pH 5-6, and above 50% of it at pH 8.1. It shows that the adsorption can affect distributions of DOC in seawater.

A simple dynamical box-model is constructed in order to test the seasonal variation features of phytoplankton, zooplankton, DIN, DIP, DOC, POC, as well as dissolved oxygen (DO) in the northern part of Jiaozhou Bay in 1995. The annual variations of the phytoplankton production show two high value periods (Mar. to Apr. and July to Aug.) and two low value periods (May to Jul. and after Oct.). DOC shows the common features: it is high in summer and low in winter (Wu, Z.M., et al.,1999) . Yu, et al. (1999) sets up a hydrodynamical model in Jiaozhou Bay. Their model equations describing the cycles of phytoplankton and OC in the ocean are as follows

dp/dt= B 1B 2B 3B 4B 6– B7
dZ/dt= B 4B 8B 9B 10
Particulate organic carbon(POCCmg/m3)
d[POC]/dt= B 6 + B 8+ B 10B 12B13B14
Dissolved organic carbon(DOCCmg/m3)

d[DOC]/dt= B2 +B13 B 15 +QDOC


where B 1 stands for photosynthesis; B 2,external secretion; B 3, exhalation; B 4, zooplankton assimilating; B 6, natural death; B 7, settling; B 8, dejection; B 9, excretion; B 10, natural death including assimilation; B 12, decomposed by bacterium into inorganic matter; B 13, decomposed by bacterium into DOC; B 14, POC settling; B 15, DOC decomposed by bacterium into inorganic matter. QDOC is the discharge of terrestrial wastewater and the dissolution in sediment.

The methods of getting colloid carbon (COC) analyzed and separated also have been studied in this stage. The cross flow-over filter technology (CFF) is a common means that separates COC from the total dissolved organic carbon (TDOC). COC and TDOC are usually determined by the high temperature combustion (HTC) and the UV/persulfate method (Wang J.T. et al., 2000 ).

It may be found from the above review that the main aspect of the marine carbon cycling studies in China in recent years is the carbon geochemistry in the China Seas, which includes the forms, transfers, distributions, and changes of carbon, the biological productions and the models of carbon cycling, etc. So far, the comprehensive or systematic studies on carbon cycles of China marginal seas, especially, the process studies of carbon cycles in the carbon biology-chemistry-dynamics system, have not been reported, which should be devoted deeply in the coming years.



In order to gain a deeper insight into the ocean carbon cycle, it is very important to study the influences of marine sediments and soils from rivers and lands on carbon cycles. The potential of soil and sediment providing DOC for natural waters depends on the content and the sorption coefficient of the soluble organic carbon (Shu and Lin, 2000). Soil from river is an important factor in regional and global carbon budgets because it serves as a reservoir of large amount of OC. The d13C isotope analysis is the particulate organic matters from the Changjiang Drainage Basin has shown that the particulate organic matter input from the southern tributary mainly comes from the higher plants, and the particulate organic matter input from the northern tributary mainly comes from soils with the higher organic matter, and less depends on the higher plants (Wu Y. et al., 2000). Soils from the land also affect the global carbon cycles. Duan et al. (2002) showed that the total storage of organic carbon in the 0-50 cm soil layer of the desertified lands is 855Mt. In the last 40 years, the total CO2 amount released by the land-desertification processes to the atmosphere is 150 Mt C, while the CO2 amount sequestered from the atmosphere by the anti-desertification processes is 59 Mt C. Hence, the net CO2 amount released from the desertified lands of China is 91 Mt C, which indicated that CO2 sequestered by the anti-desertification reversing processes in the desertified land had greater potential than that in the other soils.

Yuan et al. (2002) made a study of OC in the core sediments in the 6 stations located in 3 typical regions of the Bohai Sea, i.e., the region of Bohai Bay, the region off the Huanghe estuary and the region of Liaodong Bay. And they further approached the profile distribution of OC and the influences of redox environments (Eho, Eso and Fe3+/Fe2+ ratio) on OC in the Bohai Sea sediment. The organic carbon content in the sediments with natural grain size is 0.38%-0.86% in the Bohai Sea. Its variations are greater in the surface and subsurface layers, and less in the deep layerwhich is due to the organic carbon diagenesis mechanism in sediments and the sediments origin. A correlation analysis shows that the sedimentary reduction in the middle layer is greater than that in the surface layer, where the oxidizing and reducing environments coexist, the organic matter is oxidized, the OC concentrations tend to decrease, the Fe3+/Fe2+ ratios also tend to decrease, a significant negative correlation exists between the OC concentrations and the Fe3+/Fe2+ ratios. In the bottom layer, the reduction takes precedence, a great deal of OC can not be oxidized and reserved, and the Fe3+/Fe2+ ratios also tend to decrease, which results in a more significant negative correlation between the OC concentrations and the Fe3+/Fe2+ ratios. In the surface layer, the correlation of OC and Fe3+/Fe2+ is complex and nonlinear because of some biochemical and physical factors. The surface and subsurface OC sediments are controlled by material sources and physical disturbances. The middle and bottom OC sediments are controlled by redox environments, which leads to the significant mineralization and the OC concentration gradually decreases. It is evident that the OC contents in the different layers are regulated by material sources, settling environments, redox processes and different biological and chemical processes. It is found from the observed profiles in Liaodong Bay that the OC contents from the surface and subsurface sediments show vertical laminating distributions, which are due to the gradual settling and depositing. The OC contents below the 25cm sediment surface show level-gradient distribution, which may be due to the special depositing event such as great flood scouring and depositing.

The decomposition constant of OC in the core sediment of the Bohai Sea is 0.00479/a, and the decomposition rates of biogenic elements C, N, P, Si have the sequence N>P>C>Si. The OC/TN ratio is much lower than OC/ON, which indicates that the sediment preserves plenty of inorganic nitrogen (IN) and/or fixed nitrogen, and the decrease of ratio OC/ON with depth is due to the ON reservation in sediments (Song, Ma and Lü, 2002) . It is clear that the function of sediment in carbon cycles is well correlative with the other biogenic elements. It is necessary to study the influences of the relevant elements on carbon cycles, in order to gain a deep insight into the functions of sediments in ocean carbon cycles.

Radiocarbons of carbonate (PIC) and of organic carbon (POC) in the sediment trap samples from the Okinawa Trough were measured by AMS. Concentrations of C14 in PIC and POC (∆C14-PIC and ∆C14-POC) ranged approximately from +40‰ to –80‰ and its average approximately is –32‰ over the fully 2 years. These values are much lower than those of the dissolved inorganic carbon (∆C14-DIC) in the upper 200m of the water column (+100‰ on an average). The variations in ∆C14-PIC and ∆C14-POC were positively correlated with concentrations of inorganic and organic carbons, respectively, and negatively correlated with concentration Al. This suggests that variabilities in ∆C14-PIC and ∆C14-POC be associated with the input of lithogenic materials, and they had a seasonal variation during 2 years with lower values in winter and higher values in summer (Makio et al., 2000).

The contents of OC in the two sediment cores from the Nansha Islands region average 0.7% and 0.53%, respectively, and are higher than those of the other sedimentary environments in this region. The distributions of various lipid compounds indicate that most of the sedimentary organic matter in the two cores is derived from marine plankton and bacteria, with a smaller amount of land-derived organic matter. It is evident that the relative abundance of shorter chain lipids decreases with depth, as is the biochemical conversion of stenols to stanols in some samples (Duan Y., 2000).

Apparently, there are many studies about the distribution, content, transfer and transformation of carbon in sediment or soil to be performed. As one of three interdependent basic links in sediment (including soil-water-atmosphere systems), marine sediment plays an important role in oceanic or global environments. From now on, more attention should be paid to the research into the functions of sediments in carbon biogeochemical cycles (Sun Y.M., 2002).

It is expected that in the coming years Chinese scientists will lay emphasis on the research into the carbon-fixed mechanism of marine organisms, the carbon exchange processes between the China marginal seas and its adjacent oceans, the coupled mechanism of dynamical transformation and biochemistry, and the functions of sediments in carbon cycles, etc. The Chinese Academy of Sciences has carried out a new project “carbon sinks and sources study in the China land and marginal sea ecosystems” since 2001, which will make a great progress in the study of Chinese marine carbon biogeochemical cycle processes, and will play an important role in studying the functions of oceans in global carbon cycles.


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