摘要
本文介绍了测定实验室养殖的介形虫Ｃａｎｄｏｎａｒａｗｓｏｎｉ和水的氧同位素分馏系数，结果表明，这个种属的介形虫壳的氧同位素组成是它所生成环境水的氧同位素组成和水温的函数。因此，介形虫壳的氧同位素组成是一个很好的水化学环境指示计。本文研究了Ｃａｎｄｏｎａｒａｗｓｏｎｉ的脱壳温度范围，发现它可以在很宽温度范围内脱壳及生长新壳。由于季节温度变化很大，所以用介形虫壳的地球化学指标研究古环境的变化时，每个样品都要采集较多的个体介形虫壳，以使其分析结果能代表年平均温度。
The calcite shells of ostracodes are an important source of biogenic carbohate that can be analyzed for its isotopic composition and trace element ratios to provide an account of chemistry and temperature changes in closed-basin lakes. The variations in lake chemistry and temperature can in turn provide high-resolution records of hydrologic and climatic change in the study area.Accurate interpretation of the geochendcal record from fossil ostracodes requires a solid understanding of ostracode isotope-chemistry and life-history. To explore these issues, ostracodes were cultured in the laboratory under controlled water chendstry and temperature. The shells of ostracode, Candona rawsoni, formed at experimental temperatures are not in isotopic equilibrium with water, but had a constant offset from equilibrium oxygen-isotope fractionation of inorganic carbonates. The oxygen-isotope fractionation factors of C. rawsoni have been calculated as 1.0305 at 25 ℃ and 1.0322 at 15 ℃. The shells are enriched by about 1%0 in δ18 O compared to inorganic carbonates, but are close to marine biogenic aragonites with a slightly different temperature dependence.The individual adult ostracodes collected monthly from Roslyn Lake,, South Dakota, and Coldwater Lake, North Dakota, showed large variations in δ18 O,Mg /Ca and Sr /Ca within same collections.Late-instar. juveniles, on the other hand, showed nearly uniform ratios for the entire summer(late May to late September). A comparison of these results with the variations in the lake water suggests that penultimate instar juveniles molt from spring to fall panning a wide range of temperatures and water chemistry but instars molt during a specific time of the year,most likely mid-summer. For the purpose of paleoclimate reconstruction,it is clear that a large number of fossil shells should be analyzed for each stratigraphic interval to reduce the seasonal noise. Moreover, late-instar juveniles may be a better choice for such reconstruction since they provide consistent record of mid-summer conditions.The geochemistry of ostracede shells and bulk carbonates in a 18.84-meter longsediment core documents at century-scale resolution the evolution of water chemistry in ChldWater inke, providing a continuous paleohydrologic record of Holocene climate change in the northern Great Plains. A combination of δ18 O,δ13 C, Mg /Ca and Sr /Ca in ostracode calcite aided by Sr /Ca in bulk carbonates was used for paleoclimatic reconstructions.A fresh-water phase in the early Holocene was followed by a sharp increase in salinity between 10790 and 8900 yr B. P. The climate was predominately dry during the late part of the early Holocene and most of the huddle Holocene (8900 ～5000yr B.P.), when the lake was very sensitive and re-corded a series of dry and wet oscillations. Maximum salinity occurred at around 5500 yr B. P., and was followed by a gradual decrease between 5000 and 2400 yr B.P.From 2400 yr B. P., generally wet conditions interrupted by a series of lesser salihity and temperature oscillations lasted until 600 yr B. P., and a warm and dry climate returned at about the time of the Little Ice Age.
出处
《第四纪研究》
CAS
CSCD
1996年第4期345-352,共8页
Quaternary Sciences
