Magnetic Properties of Holocene Tidal Flats in the Yangtze Delta and Their Paleoenvironmental Implications

Key words: 
• Holocene /
• tidal flat /
• magnetic properties /
• hematite /
• goethite /
• the Yangtze Delta

Abstract: In this paper, a core (NT, 60.9 m in length) in the northern part of the Yangtze River Delta, from Nantong, Jiangsu province, were subjected to environmental magnetic analysis. In combination with analysis of lithology, particle size and diffuse reflectance spectroscopy (DRS), this paper discusses the magnetic properties of tidal deposits formed in early and late Holocene and its paleoenvironmental implications. Core NT can be divided into six depositional units, namely U1 to U6, from bottom to top, among which the lower part of U2 (49.9~44.8 m) and U6 (7.5~0.3 m) are interpreted to be tidal flat facies. They are characterized with lower S-ratios and higher hard isothermal remanent magnetization (HIRM) and SIRM/χ values, which indicates higher concentration and proportion of antiferromagntic minerals such as hematite and goethite. In combination with DRS, it is found that the enriched hematite and goethite occur in the upper part of U6, while only hematite is enriched in the lower part of U2. Such a difference in magnetic mineralogy is related to the sedimentary environments when these sediments are deposited. Sediments in U2 were formed during late stage of late Pleistocene to early Holocene and has been in subaerial condition for a long period, which favors hematite formation and preservation. With the rising sea-level during early Holocene, the salt marsh accreted vertically and eventually inundated, which does not favor the formation of goethite. In contrast, sediments in U6 were formed during the late Holocene. The oscillating redox condition favors the formation of goethite. Abundant maghemite and hematite were formed due to pedogenesis after land formation at a later stage. This study shows that sediments in salt marsh formed in different stages of the Holocene exhibits different magnetic properties. As a result, magnetic properties of sediments can provide information about the evolution of tidal flat, which has important implication for delta paleoenvironmental reconstruction.