Grain-size component dependent storage threshold of orbital cycles in alluvial stratigraphy caused by autogenic dynamics

Abstract

Numerical forward modelling and laboratory experiments suggest that autogenic factors in the sediment routing system serve as long-pass filters, preserving only orbital cycles with a period exceeding the compensation timescale, T_c, or thickness in the depth domain exceeding the compensation depth scale, H_c. For a specific orbital cycle with a certain period, this preservation in alluvial strata occurs unless it exhibits a sufficiently large amplitude. This study stratigraphically confirms, for the first time, the long-pass filtering of autogenic dynamics using elemental data from the alluvial–lacustrine Sifangtai and Mingshui formations in the Songliao Basin. Spectral analysis of the Si and Zr series in coarse-grained sediments reveals no cyclic signal with thicknesses below the estimated lower limits of H_c. This implies that the spatial storage threshold for orbital cycles in proxies of the coarse-grained sediment component is equal to or less than H_c. However, cyclic signals of obliquity and precession with smaller thicknesses are identified in Ti, Fe and Al enriched in the fine-grained sediment components of the stratigraphy. Notably, previously reported proxies preserving high-frequency orbital cycles are derived from fine-grained sediment components, differing from the sedimentation rate series used in the reported experimental studies. Therefore, the authors hypothesize a grain-size component-dependent storage threshold, suggesting that the storage threshold of orbital cycles in proxies associated with fine-grained components is lower. This hypothesis arises from the weaker effect of autogenic dynamics on the content of fine-grained sediment components transported to the sampling site by a suspended load compared to coarser components that are subjected to stronger autogenic dynamics within or near channels. The hypothesis and model presented propose a dynamic process elucidating the nuanced roles of autogenic dynamics in preserving orbital cycles. This perspective, considering sediment composition, inspires prioritizing proxies enriched in the fine-grained fraction for identifying allogenic cycles in alluvial strata.