| 其他摘要 | Bivalve shells offer a great potential as environmental proxies. Bivalves are beneficial in that they can provide high resolution records of environmental conditions (annual、seasonal and even weeks) and have a wide geographical distribution. In addition, bivalve shells are well represented in the fossil record since the Cretaceous, potentially allowing records of environmental conditions to be extended into the past.
Environmental information can be extracted from bivalve shell geochemistry (e.g., stable isotopes and elemental composition); the research of modern bivalve shell in situ is to confirm the potential and their environment significance of those environmental proxies. According to the theory of uniformitarianism, i.e., past is equal to the present, these understanding will not merely be to the modern shell itself, its principle and equations might use in the fossil shell. Therefore, the modern shell research is the foundation and prerequisite to use the fossil shell to recognite and extract the pale climate and pale environmental information.
Nevertheless, Bivalve is much less studied than corals and foraminifera and is largely limited to isotopic studies. This is probably due to the fact that the literature has been contradictory regarding the faithfulness of elemental proxies in bivalves. It is the aim of this dissertation to increase our knowledge of proxy incorporation in bivalve shells. We studied on the modern freshwater bivalve corbicula (Corbicula fluminea Müller, 1774) and achieved some results, obtained some meaningful conclusions, they mainly include:
1 Carbon and Oxygen Isotope Composition and Its Significance of Different Cross-Sections of Corbicula Shell
When measured the isotope composition of shell in situ, the different isotope composition between different shell cross sections will affect the environment significance judgments of shell isotope and have the adverse effect on the environment information extraction. This article has investigated carbon and oxygen isotope of the corbicula shell, the results shows: The carbon isotope profiles not obvious change in different cross sections, Therefore it can not consider the difference between cross sections, and can extend the length of sample path in the sample process to obtain the enough powder to determine the carbon isotope. The oxygen isotope profiles, however, existing difference between cross sections, so it should consider the difference oxygen isotope composition between cross sections, and in the sample process, the length of sample path should control in certain scope. Finally, according to the isotope difference between cross sections, the article had determined how to select the study cross section during the analysis of the shell.
2 Confirm the Potential and Environment Significance of Environmental Proxies
We conclude that Corbicula fluminea precipitates its shell in oxygen isotope equilibrium with the ambient water and that winter values are not represented due to growth cessation. Therefore, shells of Corbicula fluminea provide reliable archives of paleoclimate and paleoenvironmental conditions.Comparison of the measured δ18Oar against the calculated values indicates that shell growth occurred from late May to early November. To evaluate potential triggers, we compared the water temperature records of shell growth cessation and onset points. Temperatures in late May (May 23, 2006) was 18 °C and every similar to 17 °C in early November (November 7, 2006). Therefore, growth cessations and onset of shell was controlled by water temperature change, for Corbicula fluminea, the temperature threshold is 17 °C, which is higher than the temperature threshold of 12 °C for unionidae.
Shell δ13C values is more negative than predicted δ13C values, and we observed that all the shells followed lower δ13C values trend with increasing age, therefore the incorporation of metabolic carbon is the most likely cause of the negative offset in shell δ13C. The variable offset from equilibrium δ13C values precludes the direct use of Corbicula fluminea shell in the study of the δ13CDIC in ancient bodies of water.
3 Metabolic Effects on Shell Carbon Isotopic Composition (Raise Experiment)
Corbicula fluminea (Muller,1774) was breed in the lab and selected two shell sample (shell height, A=13 mm, B=9 mm), determined carbon isotope of the increment shell (aragonite, CaCO3) and the host water, the research indicated: δ13CDIC rises from -5.24 ‰ to 1.41 ‰, shell carbon isotopes also along with it ascension, indicate the influence of δ13CDIC on the shell carbon isotope; δ13CA arrange from − 4.76 ‰ to −2.09 ‰; δ13CB from − 8.49 ‰ to −2.89 ‰; Compares with the value of the predicted equilibrium value, the δ13C of shell A and B are both negative, indicated the shell has used the metabolism carbon in the forming process. According to computation, shell A used metabolism carbon proportion (the M value) from 24% to 43%, the mean value is 33%; shell B is 33% to 75%, the mean value is 58%. The M value assumes the drop change along with the biological growth, these shows that during the experiment, corbicula mainly increase the absorption and use the DIC to satisfy the demand of material quantity.
4 Metabolic Effects on Shell Carbon Isotopic Composition (Field Experiment)
δ13Ctissues of individual sample is very similiar to the sample that mix with individuals that have same shell height, showed the difference of δ13Ctissues between corbicula individual that have same shell height is very small, therefore it’s not necessary to determinate every corbicula individual. While δ13Ctissues between groups exist certain difference, along with shell grow, δ13Ctissues change tendency first reduces and then increase, may be indicated that feeding habits (foods) of corbicula be able to have certain change in the growth process. δ13Ctissues had not demonstrated remarkable difference between the hydrochloric acid processing sample and the raw sample; this suggested that the content and influence of inorganic carbonate in the sample is very small. Corbicula and anodonta in δ13Ctissues extremely close, but shows remarkable differences with the viviparus, this indicated that the feeding habits of bivalve (Corbicula and anodonta) existence obvious difference with gastropoda (viviparous).
Strong ontogenic decreases in δ13Cshell are evident in all samples, and this phenomenon is caused by an increase in the amount of respiratory CO2, therefore, the anti-correlation betweenl δ13Cshell and shell height may consider as the indicator of vital effect (carbon isotope).
The percent metabolic C (M) incorporated into the Corbicula shell in HuaXi River was significantly higher, with a range from 19.8% to 26.8%, than has been found in other bivalve shells, which usually contain less than 10%. There is a positive linear relationship between shell height and M（M = 0.39H + 17.36（n = 18, R2 = 0.74））. Between the different species as well as the same species in the different area, the regression equation of M and shell height are different, therefore using the modern shell to establishes regression equation of M and shell height between is the precondition and foundation to withdraw the water information though the ancient shell fossil.
5 Carbon Isotope of Corbicula Shell Indicated Different Environmental Condition
The influence of metabolism effects on bivalve shell carbon isotope was confirmed by a larger numbers of studies, this phenomenon causes more complex and difficult about the explanation and application of carbon isotope. We raised the freshwater bivalve corbicula in the aquariums, with the aim to determine whether metabolism effects already blurred the shell record to the DIC. The datum shows that: there has a big different between carbon isotopic composition of the shell, which from different aquarium (different experimental condition), although it’s not represented the real δ 13CDIC value, but has reflected the different environment information; therefore the shell carbon isotope may serve as the qualitative tool to direct different environment condition. As a result of the biological metabolism influence, corbicula shell carbon isotope composition have a constant offset of 6.0 ‰ more negative than predicted inorganic aragonite δ 13C, this constant offset is likely to reflected shell sample quantity and the size distribution question in the experiment and can not prove the shell carbon isotope is the quantitative proxy of δ 13CDIC. To sum up, although metabolic carbon can complicate the δ13Cshell profile, δ13Cshell can still be a useful indicator of environmental conditions. |
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