Characteristic Of Greigite In Holocene Deposits From The Yangtze Delta And Ningshao Plain And Its Paleoenvironmental Implications

Ferrimagnetic greigite is widespread in the sedimentary environment,which is an important product of the C-Fe-S cycle.Despite abundant reports of greigite occurrence in marine and lacustrine deposits,its formation mechanisms in deltaic deposits remain poorly studied.In this study,cores were taken from paleo-incised valley of the Yangtze delta,including the Chongming Island（CM01 and CM02）and Hengsha Island（HSD）,and the inter-fluvial terrace including the Yaojiang Valley（YJ1503）and the Hangjiahu Plain（SL）.Combined Environmental magnetism,AMS¹⁴C dating,OSL dating,granularity,geochemical elements,diffuse reflectance spectroscopy（DRS）,scanning electron microscopy（SEM）and microfossils analysis were integrated to identify magnetic mineral assemblages in different sedimentary units.This study aims to reveal the sedimentary environment of greigite occurrence and its formation mechanism and its paleoenvironmental implications.The main conclusions are as follows:1.Greigite occurs mainly at the boundary of sedimentary facies transition with marked salinity change in the Yangtze River delta.Greigite-bearing layers occur mainly in rapidly accreting tidal flats formed during periods of rapid sea-level rise in the early Holocene,and to a lesser extent at the boundary where the shallow marine facies changes to the deltaic facies.In comparison,the signal of greigite is stronger than the former.Similarly,greigite bearing layers from core YJ1503 in Hangzhou Bay and core SL in the Hangjiahu Plain also mainly occur in tidal flat environment of early Holocene.2.SEM results show that the greigite particles are small（<1μm）,mainly showing irregular shapes,round spheres and flakes.Pyrite particles are larger（2-10μm）,showing either framboid or cluster.The individual particle shows morphology of pentagonal dodecahedra,octahedra,and cubes etc.3.Greigite particles from cores YJ1503 and SL are finer than those of cores CM01,CM02 and HSD.The FORC diagram shows that greigite from cores YJ1503and SL only show a single concentric signal with coercivity range of 20-60 m T.However,the FORC diagram shows that greigite from cores CM01,CM02 and HSD show both single and double concentric signal,with coercivity range of 40-80 m T.4.There is no significant correlation between TOC content and the strength of greigite signal.Low-moderate salinity and high sedimentation rate environment favor greigite formation.The following model of greigite formation is proposed:（1）Greigite-bearing layers preserved mainly in rapidly accreting tidal flats formed during periods of rapid sea-level rise in the early Holocene,and to a lesser extent in the estuarine-shallow marine and tidal river environment;（2）In estuarine-shallow marine and tidal river environment with low deposition rate（<4 m/ka）,greigite signal is weak regardless of salinity increase or not;（3）In accreting tidal flats,greigite signal increases with increasing deposition rate.When the deposition rate of tidal flats is higher than 9 m/ka,greigite signal increases with increasing salinity.5.Identifying greigite-bearing layer could distinguish accreting and progradational tidal flat.The former is marked by high frequency occurence of greigite signal,while the latter is not significant.Centennial-scale alternations of greigite and non-greigite bearing layers is common in the accretionary tidal flat,which could reflect the high frequency fluctuation of the redox environment.In core YJ1503,iron sulfide bearing layers reflect waterlogged condition whereas layers containing antiferromagnetic minerals reflect drier periods of soil formation.6.Variation of magnetic mineral assemblage in core YJ1503 can indicate paleoenvironmental change with a higher resolution.Centennial scale fluctuations of magnetic mineralogy around the 8.2 ka cooling event and 7.5-6.6 cal ka BP reflect frequent and unstable environments,with limited Neolithic occupation.A strong drought event occurs around 6.0 cal ka BP,which is preceded by an extensive marine inundation event in the study area,which interrupted continuous rice farming.Another extended drought event（4.2-3.0 cal ka BP）coincides with the collapse of Liangzhu culture（5.3-4.2 cal ka BP）and subsequent replacement by less advanced cultures in the region.This study deepens the understanding of early diagenesis of magnetic minerals.It provides a theoretical basis for identifying sedimentary facics and a new approach of extracting centennial-scale hydro-morphological processes information.