Rapid flood intensification and environmental response of the Lower Meuse during the Allerød-Younger Dryas climate transition

Abstract

Lateglacial climatic oscillations exerted profound impacts on the Meuse fluvial system. In the Lower Meuse (southern Netherlands), geomorphological studies in the last decades mainly centred on Lateglacial vegetation evolution, channel pattern changes and river terrace formation. Little information has been reported about the paleohydrology and its relation with the regional climate conditions. This study investigates a sediment core that contains flood sediments deposited from the early Allerød up to the middle Holocene. We conducted grain size analysis, thermogravimetric analysis (organic matter and calcium carbonate content), pollen and macrofossil analysis, and determined the oxygen and carbon stable isotope ratios of biogenic carbonate (opercula of the freshwater gastropod Bithynia tentaculata). The chronology of the core is based on AMS 14C dating and pollen biostratigraphical correlation. The pollen and macrofossil studies reveal that the core site was a lake environment during the Allerød and Younger Dryas periods. The oxygen isotope record in conjunction with organic matter and carbonate content are believed to have captured the intra-Allerød Cold Period (IACP). The synchronous variation of oxygen and carbon isotopes with calcium carbonate content indicates a dominant evaporation effect on carbonate chemistry of the lake environments during the warm Allerød interstadial. End-member modelling decomposes the grain-size distributions into two sandy end members (bed load) and two silty-clayey end members (suspended load). In order to highlight the flood signal, we constructed two flood energy indexes (FEI-1 and FEI-2) that reflect the coarseness of the suspended load and bed load, respectively. Both indexes show a relatively high flood condition during the IACP, followed by a low flood phase in the Late Allerød and quickly intensified flood conditions at the onset of the Younger Dryas. In the second phase of the Younger Dryas, deposition of sandy aeolian sediments to the core site complicates the paleoflood identification using FEI-2 (bed load). This study shows a high sensitivity of the hydrological process and sedimentary environment at the Lower Meuse to the regional climate system.