Sedimentary record associated with atoll growth: exploratory modeling approach
Shu Gao
21st International Sedimentological Congress
Theme 8. Modern Sedimentary Process
Invited presentation, Session T8-2
24th August, 2022
Good morning, afternoon and evening, ladies and gentlemen. I am honored to have the opportunity to talk about exploratory modeling and atoll sedimentary systems.
Generally speaking, marine sediment dynamics is concerned with the understanding of the dynamic processes, the establishment of the process-product relationship, and a sound interpretation of sedimentary records.
For these tasks, modeling plays an important role. There are two types of modeling, i.e., simulation and exploratory modeling, with the latter being associated with the definition domains of the variables of both processes and products.
Here I would like to use an atoll system to demonstrate the application of exploratory modeling. Atolls represent a clearly defined source-to-sink system, involving both shallow- and deep-water processes, and they are widely distributed in tropical regions in the western Pacific, Atlantic and Indian Oceans, and in deep water marginal seas. For instance, atolls cover an area of more than 8000 km2 in the South China Sea. Here are two photographs, showing an aerial view of an atoll, and a part of a giant atoll, in the South China Sea.
The procedure of the modeling has five steps: (1) to identify the variables representing the processes; (2) to determine the definition domains of the controlling variables; (3) to calculate the responsive variables of the system; (4) to evaluate what is possible or not possible in the system; and (5) to look for new clues of critical scientific problems for further studies.
There are two assumptions for the analysis. The first is that mass conservation can be represented by a budgeting equation, which describes the relationship between atoll growth and material balance. The material supply is due to coral biological production, whereas the loss is due to wave induced erosion, biological erosion, transport of fine-grained sediment, and sediment gravity flow. For example, abrasion on the reef surfaces and parakeet fish feeding can cause remarked erosion.
The second is about the atoll geometry: a shape of circular truncated cone is assumed, which can be described by surrounding water depth, radius at the top, and radius at the bottom.
On such a basis, the sedimentation patterns of the atoll and the surrounding submarine terrace can be modeled. In the modeling, the parameters of atoll size, water depth, biological production, material escape ratio, transport distance and frequency of occurrence and magnitude of sediment gravity flow are allowed to vary within their definition domains.
A (Fortran) computer programme is used to carry out the calculations. To let you taste the flavour of the model output, I wish to present some representative results.
Firstly, §the growth of an atoll with an initial radius of 500m is sensitive to water depth. At 1000 and 2000 m depths, which are typical for the South China Sea atolls, the growth values are 6 and 2m, respectively, on the Holocene time scale. Because the growth is too small, one would wonder perhaps a large atoll of several dozens of km in radius might not have developed from a small atoll like this one.
Secondly, the deposition rate of fine-grained sediment over the submarine terrace is generally low. For the water depth of 1000m, maximum rates occur near the atoll slope, being of the order of 7 cm per 10 thousand years. Apparently, only if the terrace receives material input from several adjacent atolls can the rate be made larger.
Finally, sediment gravity flow deposits over the submarine terrace are also small in scale. This is because in the calculation the time scale of sediment gravity flow event is based upon the life cycle of the polyparium of corals, taken as 400 years on average. As a result, the magnitude of the event is below one million m3. Therefore, the resultant deposits over the submarine terrace are not thick. This implies that any larger event, if found, should be related to earthquakes, submarine landslides, or long-term accumulation effects, rather than coral growth itself.
The model output enables us to seek new clues of the system behaviour. For instance, cyclicity of atoll sedimentary record at different time scales may provide information on water column environmental changes, as is indicated by the red colored lines in the left diagram. This information is also particularly important for ancient periodic deposits such as those beyond annual layers in early Triassic thin-bedded limestone deposits, shown on the wright side.
Some other examples of scientific questions derived from the present investigations are listed here. They are relevant to the interpretations of sedimentary record in terms of mechanisms and environmental consequences.
Here is the brief summary of my presentation. Thank you for your attention, and I am looking forward to discussing the various issues with you.
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