High- vs. low-rate of sea level change fluvial floods: Past analogues for future forecast

Abstract

Projections based on the Intergovernmental Panel on Climate Change scenarios predict a clear rise of the sea level in the near future, together with a decrease in frequency and magnitude of fluvial floods, which are one of the main sources of sediments for some coasts, as response to a decrease in rainfall.

We present a fluvial flood series for the last 17,000 yrs. obtained from cores retrieved in a former restricted embayment by using a multiproxy approach (geochemistry, sedimentology, stratigraphy, palynology), with sub-decadal time-resolution. Flood subfacies are defined by their siliciclastic and saline components. Time distribution reveals that fluvial floods controlled sedimentation rates. Comparison against other records of the Southern Iberian Peninsula allows to identify common periods. For the end of the Pleistocene, low frequency floods are associated to Meltwater Pulses or Heinrich events. During early Holocene, there is a noticeable increase in flood frequency, reaching their maximum at the Holocene Thermal Maximum. The mid- to late-Holocene is characterized by a flood frequency clearly lower than the early Holocene one, but higher than the late Pleistocene one.

Millennial scale changes in insolation controlled atmospheric moisture which, in turn, determined evolution of floods. Centennial scale changes in irradiance were responsible of shorter-term flood episodes over moisture changes.

These results do not fully agree with the expected evolution of floods under IPCC projected scenarios, so further research is needed to improve our knowledge about flood behaviour.