The Dynamics of Ice Age Megafloods: Insights from an Extended Sea-Level Equation

Abstract:

Rising global mean sea level during deglacial periods is frequently accompanied by episodes of megaflooding. During the last deglaciation, numerous megafloods were initiated by glacial lake outbursts or overtopping of sills that had previously isolated regions of low sea level. Notable examples include the reflooding of the Black Sea, the Caspian Sea, and the Persian Gulf. Evidence for flooding in previous deglacials has also been identified in the English Channel, indicating outburst flooding across the Dover Strait from a large meltwater lake. Despite stratigraphic and paleontological evidence associated with these flooding events, the timing and nature of flooding often remains uncertain. The so-called “sea-level equation”, developed over the last 40 years in the ice-age dynamics literature, has been used to explore Earth’s sea-level response to mass flux from continental ice sheets and glaciers, but cannot be directly applied to model these flooding events. To address this issue, we present a generalized theory governing gravitationally self-consistent, spatio-temporal sea-level changes within an ocean-plus-lake system that is intermittently connected by water mass flux across a sill, and apply it to megaflooding events to gain insight into their evolution. An important result from this modelling is that floods tend to self-perpetuate; that is, a previously isolated basin will become fully flooded in a very short time. Once the flooding initiates, it creates a load which acts to depress the crust and gravitationally attract water from the open ocean, and both effects raise sea-level at the sill. Thus, more water becomes available to overtop the sill and a positive feedback loop is generated. By drawing example scenarios from the interglacial floods described above, we explore the conditions required to generate this feedback mechanism and its implications for the nature of catastrophic flooding events.