Magnetic properties of tidal flat sediments on the Yangtze coast, China: Early diagenetic alteration and implications

Abstract

Measurements of magnetic properties, total organic carbon (TOC) and total sulphur (TS) were performed on recent tidal flat sediments from the Feng-Xian (FX) and Bei-Bu-Gang (BBG) areas of the Yangtze coast and on Holocene tidal flat sediments from core SL67 from the southern Yangtze delta plain, China. The results indicate that greigite has likely formed in the recent upper and middle tidal flat sediments of cores FX and BBG, which are enriched in TOC and TS. Greigite is also present in association with pyrite in the early to middle Holocene saltmarsh and tidal flat sediments of SL67. The abundance of greigite in the early Holocene basal saltmarsh sediments of core SL67, along with the extremely low values of TS/TOC, suggest the presence of either a limited sulphate supply from seawater or a major terrestrial source of organic matter that decomposed slowly. The presence of greigite in mid-Holocene lower tidal flat sediments that contained low amounts of TOC indicates an upward diffusion of CH4 and H2S from the underlying upper tidal flat sediments. A comparison between the sediments of core SL67 and those of the modern tidal flat suggests that early diagenesis (including selective dissolution and pyritisation) continued after the sediments were buried during the Holocene, resulting in the depletion of soil-derived superparamagnetic (SP) particles and the enhanced magnetic properties because of authigenic greigite. However, antiferromagnetic magnetic minerals have not been altered by early diagenesis. Thus, magnetic parameter hard isothermal remanent magnetisation (HIRM) which preserved the primary magnetic signals reflects fluctuations in the detrital mineral input to the core site and records a two-stage 8.2 ka cooling event.

Magnetic mineral diagenesis

“…Prolonged greigite growth within tidal flat sediments of the Yangtze estuary has been associated with the refractory nature of slowly degrading terrestrial organic matter and ongoing sulphidization (Chen et al, 2015). While such sediments are not widely targetted for detailed mineral magnetic investigations, their sensitivity to sea level change makes them a worthy target for environmental analysis.…”

Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period): two new Global Boundary Stratotype Sections and Points (GSSPs) and three new stages/subseries

“…Also shown are other deep drill sites, including GRIP and DYE-3 cores used in the construction of the GICC05 time scale upon which the chronology of the NGRIP1 and NGRIP2 records are based. around 8.2 ka, including those from speleothems in southern Asia, China, Brazil and California (Cheng et al, 2009;Wu et al, 2012;Oster et al, 2017); lake sequences in the Levant (Vossel et al, 2018), tropical Africa (Gasse, 2000), northern India (Dixit et al, 2014) and the Tibetan Plateau (Zhang and Mischke, 2009); pollen records from the Mediterranean (Pross et al, 2009;Peyron et al, 2011); ice cores from eastern Africa (Thompson et al, 2002); pollen data from Siberia (Velichko et al, 1997); tidal flat sediments from China (Chen et al, 2015); and marine records from the north-west Pacific (Hua et al, 2008). The 8.2 ka event may also be recorded in lake sediment successions in East Antarctica (Cremer et al, 2007) and New Zealand (Augustinus et al, 2008).…”

Paleoenvironmental reconstruction of the radial sand ridge field in the South Yellow Sea (east China) since 45ka using the sediment magnetic properties and granulometry

“…Magnetic measurements provide a means of characterizing the mineralogy, concentration, and grain size of magnetic minerals contained in sediments in a rapid, economical and non-destructive way (Thompson and Oldfield, 1986). Therefore, the environmental magnetic method has been widely applied to characterize sediments and extract information regarding sediment provenance (Bloemendal et al, 1992;Jenkins et al, 2002;Liu et al, 2003;Booth et al, 2005;Wang et al, 2009;Ouyang et al, 2014), sedimentary dynamics (Rosenbaum et al, 1996;Yamazaki and Ioka, 1997;Watkins et al, 2007;) and post-depositional diagenesis (Demory et al, 2005;Karlin and Levi., 1983;Liu et al, 2004;Rowan et al, 2009;Chen et al, 2015).…”