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5. Summary and conclusions
On the five cruises from May 2004 to March 2005 to the SEATS Station, the observed d13C value of suspended POC varied from 24.2% to 21.3% in the top 20 m, from 25.1% to 21.5% in the 21–100 m layer, and from 25.9% to 23.2% in the 101–200 m layer. The organic carbon in sediment trap samples collected from September 2001 to May 2002 in northern SCS had d13C values ranging from
25.0% to 21.0% for traps deployed between 374 and 1248 m and from 23.7% to 21.9% for traps deployed between 1925 and 3250 m. The C/N ratios of sediment trap samples reached as high as 18, indicating contribution of terrigenous organic mat-ter, which occurred mainly in the depth range of 450–1300 m. A mixing model depicting the relation-ship between the d13C value and C/N ratio of POM revealed that the organic matter in sediment trap samples were mixtures of terrigenous organics with d13C value of 25.5% and C/N ratio of 22 and marine organics with d13C values from 22.5% to
21.3% and C/N ratios from 5.6 to 7.6. The same mixing model revealed that the suspended POM from the SEATS Station contained minor amount of terrigenous organic matter, and the observed range of d13CPOCvalues reflected the actual isotopic variability of marine organic matter.
Simulation of the carbon isotopic composition of phytoplankton in the surface water based on observed values of hydrographic and chemical variables may match the observed range of d13CPOC
values in the top 20 m, while variations in the biological parameters, including the specific growth rate, enzymatic isotope fractionation during carbon fixation, cell size and cell wall permeability, within the normal ranges may have contributed signifi-cantly to the observed isotope variability. If the same processes controlling the carbon isotopic composition of phytoplankton in the surface water worked in the entire euphotic zone of about 100 m thick, isotopically very light POC with d13CPOC as low as 28% could have been produced in the subsurface layer due to the exponentially decreasing specific growth rate. Therefore, the observed de-creasing trend of d13CPOC below the surface layer down to 200 m could have resulted from addition of the isotopically very light POC produced in the subsurface water to the isotopically normal POC produced in the surface layer.
The marine component in the sinking POM collected in sediment traps had a mid d13C value
(22.1%) coinciding with the concentration weighted mean d13CPOC value in the top 20 m, suggesting the surface layer as the dominant source of sinking POC. However, the inferred d13C values of the marine end-member of the sinking POM are significantly lower than those (22.8% to 20.1%) of the POM in the nepheloid layer above seafloor in the SCS, which are in turn lower than those (21.3% to 19.0%) of organic matter in surficial sediments from the SCS. It cannot be excluded that preferential removal of 12C during degradation of POM accounts for the progressive enrichment of
13C during settling and deposition of organic matter. However, according to a calculation, the relative depletion of13C in the more recent samples, namely the POM in the surface water and the nepheloid layer, with respect to the organic carbon in surficial sediments is attributable to two factors:
(1) the drop of 1.6% in the isotopic composition of DIC attributed to the Suess effect, and (2) the increasing concentration of aqueous CO2and other related changes in carbonate species that favor isotopic fractionation against incorporation of 13C during carbon fixation. The overall effect is more than enough to account for the observed isotopic differences. Therefore, this study supports the notion that the isotopic composition of deposited organic carbon in the SCS can faithfully reflect the CO2 chemistry in the surface water, but consider-able isotopic variation may be induced by changes in hydrographic and biological conditions. In other words, isotope fractionation during degradation of POM may not be ruled out as a possible mechanism for the observed vertical variation in the d13CPOC
value, but its importance in controlling the d13C value of deposited organic carbon is not warranted.
More studies focused on the production of POM in the surface water and relationship between the freshly produced POM in the upper water column and the sinking/deposited organics are warranted for a better understanding of the factors controlling the isotopic composition of the sedimentary organic carbon.
Acknowledgments
This study was supported by the Grant NSC 94-2611-M-008-002 from the National Science Council of the Republic of China. We thank the captains and crew of the R/V Ocean Researcher I and R/V FR-1 for sampling assistance. We thank the Sediment Trap Laboratory (NCOR) for providing
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trap samples obtained under the Grant NSC 90-2119-M-110-003. This is NCU-IHS Contribution
#53.
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