环礁沉积体系的过程-产物关系: 勘察式模拟初探

海洋沉积体系的定量模拟可分为仿真和勘察式模拟,后者的目标是建立沉积特征的连续谱、凝练新的科学问题。本文提出一个勘察式模拟的方法论框架,即根据控制方程确定自变量的定义域,进而由模拟计算给出应变量的值域,并结合物质收支平衡原理和“圆台状环礁”几何模型,以此方法预估环礁及其海底阶地物质输运和堆积格局。环礁是珊瑚礁的独特类型,其礁盘区提供了礁体生长和礁外坡、海底阶地堆积的几乎全部物源。结果表明,环礁自身生长受控于生物碎屑生产,而海底阶地的范围和沉积速率不仅决定于离礁悬沙通量,而且受到环礁外水深和海盆环流的制约。悬沙输运和重力流过程形成常态沉降和水下滑坡的交替沉积。此模型所预测的环礁沉积体系高程-面积曲线与南海环礁的实际曲线相对照,可获得环礁演化的机制信息。模拟结果还提示了需进一步研究的科学问题,如环礁生长规模及其控制机制、环礁沉积体系中周期性沉积的时间尺度、环礁外坡由于珊瑚生长自组织机制形成的重力流事件与地震等极端事件的对比和机制识别、环礁群的海底阶地沉积记录多样性、海面变化对环礁演化的影响和沉积记录、环礁人居环境安全等。

PROCESS-PRODUCT RELATIONSHIPS OF ATOLL DEPOSITION SYSTEMS: A PRELIMINARY TESTING OF EXPLORATORY MODELING

The quantitative modeling of marine sedimentary system can be divided into simulation and exploratory approaches, and the latter aims at establishing a continuous spectrum of sedimentary pattern and identifying new scientific questions. In the present contribution, a methodological framework of exploratory modeling was proposed to determine the definition domain of the independent variables contained in the governing equations, and then to define the range of dependent variables. In combination with the sediment budgeting principle and an assumption for the atoll geometry, the sediment transport and accumulation pattern over the atoll proper and its submarine terrace are predicted. Atoll is a unique type of coral reef, and its top part provides almost all the source of materials for reef growth and accumulation on reef slope and over the terrace. Results show that the growth of atolls is controlled by bioclastic production, and the extent and deposition rate at the terraces are controlled by the suspended sediment flux off the atoll, which is in turn controlled by water depth outside the atoll and the circulation of the ocean basin. The process of suspended sediment transport and gravity flow forms the alternating deposits. The hypsometric curves predicted by the model can be compared with the actual curves, to reveal the various mechanisms of the atoll evolution. Furthermore, the modeling output implies a number of scientific questions for further research, e.g., the magnitude of atoll growth and its control mechanisms, the temporal scale associated with the cyclisity of the deposits, the differentiation between the sediment gravity flows due to the coral self-organization mechanism and those due to earthquakes and other extreme events, the diversity of atoll-terrace sedimentary records in relation to the information on atoll evolution and environmental changes, and the influence of sea level changes on the atoll living conditions.