渤海湾全新世贝壳堤和牡蛎礁的古环境

贝壳堤和牡蛎礁是渤海湾沿海全新世的两类重要地质现象。本文运用沉积学、年代学(ＡＭＳ１４Ｃ测年和古地磁)和生物碳酸钙氧、碳同位素测定等方法，研究了贝壳堤、牡蛎礁及相关泥质沉积物的形成和地层序列，考订了各类海面标志点的地质意义和１４Ｃ数据的可靠程度，校正了文献中的１４Ｃ数据，测定了区内全新世晚期软体动物壳体的δ１３Ｃ平均值，以及δ１８Ｏ水／盐度和δ１３Ｃ贝壳／Ｔ等基础数据，绘出了地区性相对海面变化的时间－深度图，研究了新构造活动对相对海面变化的影响。基于上述研究，进一步探讨了本区约４０００ｃａｌＢ．Ｐ．以来的环境变化及其原因。

PALAEOENVIRONMENT OF THE HOLOCENE CHENIERS AND OYSTER REEFS IN THE BOHAI BAY, CHINA

The major objective of this study is to understand the background of the present-day climatic and environmental changes. The study area is located at the low-lying coast of Bohai Bay, which can be divided into Chenier Plain in the south and Oyster Plain in the north.Sedimentostratigraphy The cheniers, as a product of polygenetic coastal pro cesses, have experienced complex changes. Their lower part, composed of fine shelly fragments and mud, at fore-base began to build up in shallow subtidal to intertidal zone by wave/tidal-currents. The middle part was formed by vertical accretion and/or flattening of storm-surge heaping/ overwashing at intertidal to supratidal zone. The eolian accumulation which was most active during the LIA (Little Ice Age) caused the height of the young cheniers to increase through dune building-up processes. Some soil horizons marked relative calm periods.The Oyster reefs consist of mainly Crassostrea gigas. Against ambient muddy substratum, the upper part of the reefs may get into intertidal zone. Some intercalated layers, consisting of marine mud admixed with a great amount of broken and articulated oyster shells, were periodically sandwiched into the vertical buildingup of the reef in Biaokou site. Overlying the reefs are brownish massive mud covered by rivers with abundant supply in coastal area.Isotopestratigraphy Feasibility study, concerning huge size (10-40cm in height), consecutive microgrowth-layers and the primary low -Mg composition of the layers, indicates that the C .gigas shells are a proper objective for isotope analysis.Intriguing results include: remarkable seasonal variations of the shell δ18O and δ13C composition, covariance and decoupling between the δ 18O and δ13 C, the lower limit of water-temperature for shell deposition (ca.11℃ ), the regional ratios of δ18Ow/ salinity of the Bohai Bay (+0.19‰-+0.39 ‰ PDB/per mil salinity and the south Holland (+0.16‰-+0.27‰ PDB/per mil salinity)respectively, and (as a working hypothesis) a temperature effect for δ13Cshell (-0.1‰ PDB/℃), etc. Based on these" pretreatment" studies, some palaeoenvironmental changes have been realized: (1) Under the same lower water temperature, the maximum mean of δ18O are remarkably different. This is mainly caused by salinity change: ca. 20‰ for oyster BK 14 at ca. 6500 cal B. P. and 25‰, for BK1 at ca. 4000calB. P. and 30‰ , for BHI at 2 350cal B. P .;(2) Vertical distribution of the animal in a water column has been determined by connecting δ13C with phytoplankton bloom and /or decomposition of the dead phytoplankton; (3) Much lighter δ18O of the fossil shells suggest small and isolated palaeowater masses, with less thermo-stability, in inner bay conditions,which were different from the present-day open bay/estuary environments for the living analogues; (4 ) Quick shift to heavy δ13C after winter indicates that phytoplankton bloom occurred at the early spring due to a fast-rise of water temperature. This was dominant before 4000 cal B. P. However, the slow-rise was a remarkable phenomenon of BHI at 2350 cal B.P.Chronostratigraphy Detailed x-ray diffraction analyses and δ13c composition measurement have been done for common shells in the area. The regional average of δ13C values for the common shells are calculated as -0.48 ‰, PDB for aragonitic shells and -3.18‰, for calcitic shells, and an average value for both aragonite and calcite is -2.68‰, PDB. As a result, two relevant shells,Umbonium and Terebridae were recommended for 14C dating.Sea-level change and neotectonics Two essential neotectonic systems are emphasised as the NNW and NWW fault zones. The former controlled the stretch of the chenier chains and the Chenier Plain, while the latter dominated the distribution of oyster reefs and the Oyster Plain. The regional eustatic sea-level has reached the present-day altitude at ca. 7000-6000 cal B. P.Conclusion The first remarkable aridity event in the area occurred after BK1 at ca. 4000 cal B. P. It was a symbol of the first order turning-point in the local Holocene development.