长江水下三角洲浅表沉积层中的生物扰动构造

BIOTURBATION IN SURFICIAL SEDIMENTS OF SUBAQUEOUS CHANGJIANG (YANGTZE) RIVER DELTA

为查明长江水下三角洲生物扰动类型、特征和分布规律,探讨生物扰动构造的可能影响因素,使用箱式采样器获取包括三角洲前缘、前三角洲、过渡带以及正常浅海的无扰动沉积岩心,利用高分辨X-ray成像技术对浅表沉积层中的生物扰动构造进行了研究。结果表明,长江水下三角洲发育虫孔构造和挖掘构造,虫孔构造直径0.5—2mm,长度数厘米至数十厘米,多数虫孔垂直于层理发育,挖掘构造形态呈漏斗状,其内沉积物混合强烈;生物扰动构造具有明显的分带性,三角洲前缘生物扰动指数<1,前三角洲及过渡带扰动指数为2—3级,最高达4级,而紧靠前三角洲的正常浅海生物扰动指数为2级。生物扰动构造的发育主要受上覆水体盐度、悬浮体含量制约,在盐度接近正常浅海水体盐度、悬浮体含量中等的前三角洲、过渡带是生物扰动构造发育的有利环境。研究未发现底质沉积物类型、底质环境因子对生物扰动构造的明显制约作用。

The structure, spatial distribution, and controlling factors of bioturbation in subaqueous Changjiang （Yangtze） River Delta were studied. Serial undisturbed surficial sediment cores covering the sub-environments of the subaqueous delta were collected using box sampler and then examined with high-resolution X-ray photograph method. The results show that biogenic tubes and borrows are two major forms of bioturbation. The tubes are 0.5-2.0mm in diameter, and several to tens centimeters in length; most of them are perpendicular to laminations. The borrows are funnel-shaped, in which sediments are strongly disturbed. The bioturbation distributed in zonation and intensified in the deltaic region from delta front, prodelta, and transition zone, which was mainly controlled by bottom water salinity and turbidity. Having moderate total suspended matter supply and close to shallow sea water salinity, the prodelta and transition zone favor the development of bioturbation. In addition, no close relations of bioturbation to surface sediment type and pH or Eh state were found.