A robust geochronology of the Yangtze River Delta based on magnetostratigraphy and cyclostratigraphy of sediment core ZKA2

Abstract Studying the evolution of sea-level fluctuations, climate oscillations and tectonic movements in the Yangtze River Delta requires a reliable chronology. Here, we present the results of a comprehensive study of the magnetostratigraphy and cyclostratigraphy of a 295-m-long sediment core (ZKA2) recovered from Nantong City, Jiangsu Province, China. Anisotropy of magnetic susceptibility (AMS) studies reveal that the petrofabrics are mainly primary. Based on rock magnetic and thermal demagnetization analyses, the major magnetic minerals in the studied sediments are magnetite, hematite and greigite, but only magnetite and hematite carry the primary remanence. The magnetostratigraphy reveals seven reversed polarity intervals, nine normal polarity intervals, and one interval of mixed polarity. Variations in natural gamma-ray activity reflect variable continental weathering in the sediment source regions and are paced by Earth orbital cycles. Notably, a prominent 405-kyr long eccentricity cycle and ~100-kyr short eccentricity cycle are evident, which allow us to constrain the basal age of core ZKA2 to ~3.1 Ma. The magnetostratigraphic and cyclostratigraphic results together enable us to define the depths of key chronostratigraphic and magnetostratigraphic boundaries. They include the Late/Middle Pleistocene boundary at ~18-m depth, the Brunhes/Matuyama boundary at ~88-m depth, and the Matuyama/Gauss boundary at ~254-m depth. According the new chronology, we find that although clear long eccentricity (~405 kyr) and short eccentricity cycles (~100-kyr) cycles are evident in the magnetic susceptibility record, they are not well matched with cycles of natural gamma-ray activity. This suggests the source and post depositional process of the magnetic particles is complicated, and it should be cautious to choose magnetic properties for cyclostratigraphic analysis.