Magnetic Properties Of Core Sediments From The Northern Slope Of The South China Sea And Their Environmental Significance

South China Sea(SCS)is the largest marginal sea in the tropical region of the western Pacific.Its thick hemipelagic sediments and relatively deep carbonate compensation depth(CCD)make SCS sediments favorable archives to retrieve high-resolution records of environmental changes of the region.Despite the fact that lots of achievements about the paleoceanographic and paleoenvironmental evolution have been obtained on the glacial-interglacial time scales over the past decades using multiple proxies including stable isotopes,spores and pollen,foraminifera,nannofossils and so on,magnetic proxies have not been used extensively on studying the SCS sediments,particularly on the short-timescale evolution of the SCS.This thesis investigates the magnetic properties of four sediment cores from Shenhu Water and Dongsha Water in the northern slope of the SCS to examine their environmental significance.Core GHE24L and Core GHE27L were collected from Shenhu Water in the northern SCS.Core GHE24L is 5.96 m long and Core GHe27L is 5.73 m long.Both cores consist of light grey mud and silty mud,and haven't been affected severely by reductive diagenesis.A total of 584 samples were collected from the two cores and a series of magnetic parameters have been measured including magnetic susceptibility(x),anhysteretic remanent magnetization(ARM),isothermal remanent magnetization(IRM)acquisition curve,saturation IRM(SIRM),magnetic hysteresis and so on.The results show that pseudo-single domain magnetite grains dominate the sediments.Also,sediments of the core GHE24L and GHE27L have not been affected by reductive diagenesis and appear to have faithfully recorded environmental changes of the study area for the past?20 ka.Changes in the content and grain size of magnetic minerals of the sediments were mainly controlled by the terrigenous input,which is in turn governed by climate changes by modulating weathering processes.By comparing the variations of different magnetic parameters of the core GHE24L with the ?18O of the nearby core 17940,which is very well-dated and deemed as a reference for the northern SCS,we found that changes in magnetic characteristics of the sediments record three distinct stages of evolution.Stage?,from 20.80 to 18.74 ka,is characterized by high ?,SIRM,and low ?arm/?and ?arm/SIRM,indicating enhanced terrigenous input and the dominance of coarser component of magnetic grains during the LGM.Stage ?,from 18.74 to 11.31 ka,is characterized by fluctuations of various magnetic parameters,particularly those indicative of grain size variations.These magnetic parameters recorded major climatic shifts of this region including the Heinrich 1 event,B ?lling-Aller ?d warm event,Younger Dryas event during Stage ?.Stage ?,from 11.31 to 0.36 ka,recorded strengthening of Asian Summer Monsoon during early Holocene and the surface circulation in South China Sea,which may have carried coarse sediments from offshore Taiwan to the northern slope of the South China Sea,leading to gradually coarsening upward during the Mid-to Late Holocene.The magnetic results from Shenhu water not only provides a basis for the SCS' paleoenvironmental reconstruction from Last Glacial Maximum,but also can provide some constraints for exploring the coupling link about land-sea interactions of the Earth system.Core DH-CL12 and Core DH-CL13 were retrieved from the Dongsha Water of the northern SCS where gas hydrate has been recognized.In this region,gas hydrate disassociation led to the production of methane that migrates upward through sediments.Once methane meet with the downwards dissipating sulfate from the sea water at certain depths,known as sulfate-methane transition zone(SMTZ),a chain of chemical reactions take place,leading to reductive diagenesis.Iron oxides in sediments such as magnetite and hematite can be transformed into iron sulfides such as pyrite and greigite.In Core DH-CL12,SMTZ occurs at 7.20?11.50 m.In this section,lots of magnetite were transferred into pyrite,almost all of the little hematite in sediments is also involved in the transformation.Since fine grain magnetite is more easily transformed into pyrite than coarse grain magnetite,residual coarser magnetite became relatively more abundant.A significant reduction of magnetite and rapid increase in pyrite made pyrite become the dominant magnetic minerals in this section.An increase in the of relative abundance of hematite in the residual iron oxides results in the increase in the remanence coercivity of the samples from this zone.The magnetic characteristics of Core DH-CL13 are similar to those of Core DH-CL12.SMTZ occurs at 5.68?8.20 m,which is comparable with 7.2?11.50 m of Core DH-CL12.The main magnetic minerals are pyrite and some greitite,and magnetite is very rare in this section.Magnetite is totally transferred into iron sulfides so that SD greitite grains dominates the ferrimagnetic phases.The obvious difference between the 0.10?3.00 m and 3.00?5.68 m segments in Core DH-CL13 probably results from both the reductive diagenesis and terrigenous inputs which are controlled by the climate and environment changes.Comparison of SMTZs of Core DH-CL12 and DH-CL13 suggests that iron oxides such as magnetite and hematite were transferred into iron sulfite such as pyrite,greigite in these zones and using X?XARM?SIRM and other magnetic parameters as the indicator of reductive diagenesis may not be always reliable for the existence of greigite.Compared with the traditional magnetic parameters,HIRM is more suitable to be used as an indicator of the reductive diagenesis because greigite does not influence this parameter.