新元古代南华盆地Sturtian冰川沉积物中碳同位素负偏及其地质意义

Negative Carbon Isotope Excursions of the Statian Glacial Sediments in the Neoproterozoic Nanhua Basin and Their Geological Significance

【目的】新元古代Sturtian冰期(~717~660 Ma)在全球范围内广泛发育,在南华盆地中也有良好的记录,但是目前对冰期的碳循环过程研究较少。为了解Sturtian冰期期间的碳循环过程,对冰期期间沉积的铁丝坳组碳同位素进行系统的研究。【方法】以黔东松桃地区高地ZK2115钻孔为研究对象,对铁丝坳组进行高分辨率的有机碳和无机碳同位素(δ13Ccarb和δ13Corg)测试分析。【结果】铁丝坳组δ13Ccarb值介于-9.29‰~-3.37‰(平均值为-7.24‰),具有明显的负偏,δ13Corg值介于-33.63‰~-23.35‰(平均值为-29.29‰)。此外,δ13Ccarb和δ13Corg具有明显的正相关关系,表明地层中的无机碳同位素组成不受成岩作用的影响,δ13Ccarb代表了Sturtian冰期期间南华盆地中溶解无机碳(Dissolved Inorganic Carbon,DIC)库的碳同位素组成,并且地层中的有机质主要是通过光合作用形成的,有机质中固定的碳主要来源于海水中的DIC库。【结论】结合已发表的铁丝坳组总有机碳(Total Organic Carbon,TOC)数据(平均值为0.2%),这意味着在冰期这种极端气候条件下,光合作用虽然仍在进行,但是其速率比较缓慢,产生有机质的量相对较少。对Sturtian冰期沉积物中碳同位素组成的研究,有助于进一步探讨冰期的碳循环过程,同时也为深入理解极端气候条件下的古海洋环境及生物地球化学循环提供依据。

[Objective]The Neoproterozoic Struttin Glaciation(~717660 Ma)developed widely across the world and was well recorded in the Nanhua Basin,but the study of carbon cycling during this period is lacking.To investigate the carbon cycle process during the Sturtian Glaciation,the carbon isotope compositions of the Tiesi'ao Formation precipitated during the glacial interval were systematically studied.[Methods]This study focuses on drillcore ZK2115,located in Gaodi Manganese Deposit,Songtao area,eastern Guizhou province,China.We analyzed highresolution organic and inorganic carbon isotopes(δ13Corg andδ13Ccarb)from the syn-Sturtian Tiesi'ao Formation.[Results]The results show that the Tiesi'ao Formation is characterized by negative carbon isotope excursions,ranging between-9.29‰and-3.37‰(mean-7.24‰).Theδ13Corg values vary between-33.63‰and-23.35‰(mean-29.29‰).There is a positive correlation between theδ13Corg andδ13Ccarb values,indicating that the inorganic carbon isotope compositions are not affected by diagenesis and can be used to reflect the original carbon isotope signals of the Dissolved Inorganic Carbon(DIC)reservoir in the Nanhua Basin.Organic matter preserved in the Tiesi'ao Formation was generated through photosynthesis,and the carbon fixed in the organic matter was sourced from the DIC reservoir in seawater[.Conclusions]Combined with the low Total Organic Carbon(TOC)content(mean 0.2%)of the Tiesi'ao Formation,our results indicate that photosynthesis still occurred under the extreme glacial conditions at a low rate,although only a small amount of organic matter was generated.The study of carbon isotope compositions in glacial sediments can be used to explore carbon cycling during the Sturtian Glaciation,and to provide evidence for the paleomarine environment and biogeochemical cycle under extreme climatic conditions.