A Study On The Composition And Distribution Of GDGTs Compounds In The Surface Sediments From Offshore Region Surrounding The Leizhou Peninsula And Their Geochemical Significance

The offshore is a sea area where the interaction between the sea and the land,the input of terrestrial organic matter and the sedimentation are very strong.To study the composition and distribution of biomarkers with biogenic environmental specificity in the sediments of this sea area,and to explore the theoretical and practical application significance of these biomarkers in the source input of sedimentary organic matter,regional ocean and terrestrial climate and environmental conditions.This master's thesis takes glycerol dialkyl glycerol tetraethers(GDGTs)in the surface sediments of the seabed in the coastal waters of the Leizhou Peninsula as the research object.By analyzing the composition abundance and regional distribution characteristics of this type of special biomarkers in the sediments,the sources of sedimentary organic matter reflected by the GDGTs compounds and related parameters in the offshore seabed sediments and the climate and environmental status of the regional ocean and terrestrial source areas are studied.Discuss the applicability of GDGTs related indicators in the offshore environment of the peninsula.The paper has achieved the following results:1.The branched-chain glycerol dialkyl glycerol tetraether compounds(brGDGTs)in the surface sediments near Leizhou Peninsula are mainly acyclic and low-methyl-number compounds,of which acyclic tetramethyl GDGT-Ia has the highest content.The composition and abundance of brGDGTs have obvious regional distribution characteristics.The areas with higher abundance are mainly distributed in the northeastern sea area of the peninsula,the northwest sea area and the west side of the Qiongzhou Strait.In general,the abundance of brGDGTs in the western sea area is higher than that in the eastern sea area.Except for the waters in the southwest of Leizhou Peninsula,all components of brGDGTs have diminishing offshore distribution characteristics in other high-abundance sea areas.The numerical ranges of the cyclization index TMR and DC of the brGDGTs compounds in the surface sediments of the Leizhou Peninsula are 0.38?0.85 and 0.17?0.34,respectively,and the average values are 0.57 and 0.22,respectively.The high value of the cyclization index in the northwest sea area indicates that there is an input of brGDGTs that are spontaneous in the water body in the northwest sea area.2.The high value area of terrestrial input index BIT in the offshore waters around Leizhou Peninsula mostly overlaps with the high value area of acyclic brGDGTs compounds,indicating that the BIT indicator in the offshore surface sediments of Leizhou Peninsula has a certain provenance indicator function.Excluding the sediments in the western and southern seas,the CBT-p H of brGDGTs in the eastern seas of the peninsula is similar to the soil p H of the peninsula,indicating that CBT can be used to study the paleoenvironment reconstruction of the provenance area in the eastern sea of Leizhou Peninsula.In the eastern sea area of the peninsula,the peninsula's annual average temperature(MAT)calculated according to the MBT/CBT index is relatively low,which may be related to the input of remote soil brGDGTs brought by rivers,especially the Pearl River.The reconstructed temperature mainly reflects the average temperature of the river basin.3.A total of six types of isoprene glycerol dialkyl glycerol tetraether compounds(isoGDGTs)were detected in the surface sediments of the coastal waters of the Leizhou Peninsula,including GDGT-0,GDGT-1,GDGT-2,GDGT-3,Crenarchaeol and its isomers(Crenarchaeol').The total concentration of isoGDGTs compounds is between 952.01-3571.84pg/g,and the average content is 1945.59pg/g.Among isoGDGTs compounds,Crenarchaeol has the highest content,accounting for 72.05% of the total isoGDGTs content on average,followed by GDGT-0,and the lowest content is Crenarchaeol',with an average content of only 1.00%.The compound composition and abundance of isoGDGTs have obvious regional distribution characteristics,and the areas with higher abundance are mainly located in the sea area near Liusha Bay.4.The range of correlation ratios calculated according to the relative abundance of isoGDGTs compounds on the mass spectrum are respectively,GDGT-0/Cren ratio is less than 2,MI index is less than 3,GDGT-2/(GDGT-1+GDGT-2+GDGT-3+Cren')ratio is less than 45%and Cren/Cren'ratio is greater than 25.These ratio parameters indicate that the isoGDGTs compounds are mainly derived from the marine plankton Species Archaea Group I.Ia.The concentration of isoGDGTs in the surface sediments off the Leizhou Peninsula is negatively correlated with the temperature of the surface and bottom water bodies in summer,and is positively correlated with the salinity of the water body;while the temperature of the sediments is negatively correlated,and positively correlated with the TOC and TN of the sediments.This shows that the archaeal community will be affected by the environmental conditions of water bodies and sediments.5.According to the TEX^H₈₆index,the surface seawater temperature of the coastal waters around Leizhou Peninsula ranges from 18.06°C to 26.00°C,with an average value of 22.38°C.The average value of SST in the eastern sea area is 20.64°C,and the average value of SST in the western sea area is 23.61°C.Compared with the actual annual average surface sea temperature,the actual surface sea temperature in the eastern peninsula is 4.36?higher than the TEX^H₈₆-SST value;the actual surface sea temperature in the western peninsula is 1.39?higher than the TEX^H₈₆-SST value.This difference may mainly reflect that the surface seawater temperature calculated based on the composition of the sediment GDGTs represents the average temperature of the present to the past period of time,while the current actual temperature is only the average temperature of the short period in recent years.