Carbon,Oxygen And Sulfur Isotopes Of Extreme Environmental Shells And Cold Seep Carbonates In The Mid-Okinawa Trough

In this study,we focus on bivalve shells of cold seep and hydrothermal vent,and cold-seep carbonates in the Mid-Okinawa Trough.Mineral composition,carbon-oxygen isotopes of carbonate-associated sulfate(CAS)were analysed.Combined with the special geological background in the Mid-Okinawa Trough and the previous research results,the aims of this study are to:explore the potential of environmental restoration by biological shells of cold seep and hydrothermal vent,and cold seep carbonate,as well as their indicating significance.The results are summarized as follows:The mineralogical study shows that the components of the shells in the cold seep and hydrothermal vent are aragonite and calcite,and calcite is the main component.The microstructure of the shells in the two types is similar without diagenetic alteration.In contrast,the fossil shell in the cold seep is composed of calcite and a small amount of aragonite,and the microscopic structure shows that diagenetic alteration has occurred.Studies of carbon and oxygen isotopes show that,the carbon isotope value of fresh cold seep shells(-2.3?3.6‰,V-PDB)is lower than that of hydrothermal vent shells(1.7?2.7‰,V-PDB),The range of the cold fossil shells carbon isotope value(-2.3?3.6‰,V-PDB)is wide,but the lowest value is obviously lower than that in hydrothermal zone.It shows that the carbon source of cold seep shell is different from that of hydrothermal shell,which is consistent with the negative carbon isotope of cold seep fluid and cold seep carbonate.Isotopic composition of cold seep fresh shell(-2.3?3.6‰,V-PDB)and fossil shell(-2.3?3.6‰,V-PDB)are different significantly,but carbon-oxygen isotope values of both fresh and fossil shell have high correlations(R²=0.61),which indicente that the two types of shell carbon-oxygen isotopes share the same source,but the principal causes of differences is period activities of cold seep and changes of methane leakage flux.The carbon and oxygen isotopic composition of hydrothermal vent shells show that the biological shells are affected by biological growth rates.The faster the biological growth rate is,the more negative the isotope value will be.The main carbon source of the cold seep carbonate in the study area is biogenic methane,while the oxygen isotope research shows that the main source of oxygen isotope of the carbonate is the deep hydrate decomposition.The wide fluctuation range of the carbon and oxygen isotopes may be attributed to the late diagenetic alteration or other carbon source.Studies of carbonate-associated sulfate show that,the?³⁴S_CAS of bivalves in the cold seep are higher than those in the hydrothermal vent,which is consistent with the higher sulfur isotopic value of pore water sulfate in the cold seep,indicating that bivalves living at the sediment-seawater interface are mainly affected by pore water sulfate and seawater sulfate.The?¹³C and?³⁴S_CAS are significantly negatively correlated,indicating that the higher the methane flux is,the more negative the?¹³C and the higher the?³⁴S_CAS are,and the closer of the SMTZ interface is to the seabed.Although the carbon-oxygen isotopes of carbonate are affected by diagenetic alteration greatly,the?¹⁸O_CAS/?³⁴S_CAS of the seep carbonates is 0.36,which indicates that the CAS of carbonate rocks in this study area faithfully records the geochemical information and is not affected by diagenetic alteration.In this study,the values of carbon isotope is positively correlated with the contents of CAS,while the values of oxygen isotope is negatively correlated with the contents of CAS.The value of carbon isotope is negatively correlated with the sulfur isotope of CAS,which show that the higher the methane fluxes in the seabed cold seep,the lower the contents of CAS,the lower the values of carbon isotopes,and the higher the value of oxygen isotopes.