Millennial-scale climatic fluctuation in the fluvial record during MIS3: Very high-resolution seismic images from NE Hungary

Abstract

Alluvial architectures of a meandering river existing in MIS3 were observed on very high-resolution (VHR) single-channel waterborne seismic profiles, 20–30m below the Tisza River in the Pannonian Basin (Hungary). The study investigated the spatial and temporal variations of two, more than 2km-long continuous series of inclined reflections interpreted as laterally accreted point bar complexes. The phases of natural meander migration were reconstructed in 3D based on the changes in the geometry and dip of the inclined reflections. A channel-forming discharge curve extending over approximately 2.5ky was calculated by using paleo-width and depth data derived from the lateral accretion surfaces.Systematic analysis of the geometrical variations coupled with the discharge curve evaluation on each point bar complexes indicates millennial-scale discharge fluctuations 40–50ky ago, to that the river responded principally by incision and infilling. The primary periodicity, comparable to the sub-Milankovitch cycles, is superimposed by shorter periods: ca. 500-yearcycles reflect the phases of unidirectional meander development, while the smallest ones reflect the recurrence interval (150–200years) of the highest floods. River-bed incisions happened step-by-step related to extreme floods, when the meander development also changed. The smaller-scale and more rapid fluctuations within a development unit were represented in variations of the channel width. Although the poor geochronology of the sandy fluvial deposits cannot allow any direct correlation to the climatostratigraphic events, the millennial-scale climate variations of MIS3 were pronouncedly characteristic in the discharge fluctuations.