CSDMS 2011 annual meeting poster Wonsuck Kim
Sea-level rise, depth-dependent carbonate growth, and the paradox of drowned platforms
A mathematical model of carbonate platform sedimentation is presented in which the depth-dependent carbonate growth rate determines the depositional rate of a platform top responding to relative sea-level rise. This model predicts that carbonate platform evolution is primarily controlled by the initial water depth and the sediment production rate at the initial depth, rather than by the maximum potential production rate and imposed rate of relative sea-level rise. A long-standing paradox in the understanding of drowned carbonate platforms in the geological record is based on comparing relatively slow long-term rates of relative sea-level rise with maximum growth potentials of healthy platforms. The model presented here demonstrates that a carbonate platform could be paradoxically drowned by a constant relative sea-level rise when the rate is still less than the maximum carbonate production potential. This does not require other external controls of environmental change, such as nutrient supply or siliciclastic sedimentation. If the rate of relative sea-level rise is higher than the production rate at the initial water depth, the top of the carbonate platform gradually drops below the active photic zone and drowns even if the rate of relative sea-level rise is lower than the maximum carbonate accumulation growth potential. This result effectively resolves the paradox of a drowned carbonate platform. Test runs conducted at bracketed rates of relative sea-level rise have determined how fast the system catches up and maintains the “keep-up” phase, which is a measure of the time necessary for the basin to respond fully to the external forcing. The duration of the “catch-up” phase of platform response (termed carbonate response time) scales with the initial seawater depth and the platform-top aggradation rate. The catch-up duration can be significantly elongated with an increase in the rate of relative sea-level rise. The transition from the catch-up to the keep-up phases can also be delayed by a time interval associated with ecological reestablishment after platform flooding. The carbonate model here employs a logistic equation to model the colonization of carbonate-producing marine organisms and captures the initial time interval for full ecological reestablishment. The increase in delay time due to the carbonate response time and self-organized processes associated with biological colonization, implies a greater likelihood of autogenic origin for high-frequency cyclic strata than has been previously estimated.
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