Littoral bioerosion, a tool for the study of sea level variations during the Holocene

Field study ofbioeroded littoral notches in several regions ofthe world was used to test the various values of bioerosion rates proposed in the literature. Data obtained, which are somewhat lower than values currently indicated, were used for suggesting answers to two pending questions in the history of the late Pleistocene and Holocene sea level rise. The first questioninvolvesthe existence and duration ofstillstands during the Holocene transgression. Numerous littoral notches, produced by submersion at constant depths, allow us to propose that cumulative stillstand time mayhave been a few thousand years. This implies that stillstands were followed by episodes of very rapid sea level rise and that sea level curves are more stepped than is generally indicated by coral reef studies. The other problem involves the existence of fossil lines of elevated corals and vermetids along several tropical coasts (Brazil, west Africa, eastern Australia) which are currentlyexplained by a slowhydro-isostatic rebound ofthe crust. The general weakness ofbioerosion on such elevated biogenic remains from Brazilian and west African shores, means that these remains were not submitted to mid-littoral bioerosion for long periods, i.e., that corresponding sea level fluctuations were too rapid to be explained by the hydro-isostatic rebound theory alone. They mayinstead have been caused by a combination ofsloweustatic rise and/or hydro-isostatic rebound with rapid shifting ofthe sea surface, originating from fluctuations ofgeostrophic currents or water masses.