Abstract: Nearly all the literatures about Palaeoproterozoic positive δ¹³C excursion in the carbonate sequences on the earth during 2.20-2.06 Ga, which is called as Lomagundi Event, are reviewed. Hypotheses including biology evolvement, the rise of atmospheric oxygen, the Palaeoproterozoic ice age, super-continent break-up and evaporation, are proposed to interpret the Lomagundi Event in those literatures. Based on statistics of relations between the positive δ¹³C excursion and each geological phenomenon according to the data from those literatures , the authors conclude that those hypotheses are all reasonable, but different importance. The carbon isotope fractionation theory shows that the developing magnitude of buried organic matter will increase carbon isotope ratio of carbonate carbon reservoir. Biology evolvement can raise oxygen amount in atmosphere. Warm environment after the Palaeoproterozoic ice age and Kenorland super-continent break-up can both offer good conditions for biology developing. Thus, these evolutions can increase amount of buried organic matter and then cause positive carbon isotope excursion in carbonate carbon reservoir. Meanwhile, evaporation distributed over the world during the time may also cause positive δ¹³C excursion in local restricted environment, so the authors should research depositional environment case by case when considering the positive δ¹³C excursion. The authors analyzed original data of Mn/Sr, δ¹³Cand δ¹⁸O from North America and Fennoscandian Shield to judge effect of weathering, diagenesis and metamorphismon carbon isotope data of the samples. The result shows δ¹³C and δ¹⁸O data of samples are reduced by weathering in some case.

Key words: positive δ¹³C excursion, Palaeoproterozoic era, Mn/Sr, the rise of atmospheric oxygen

摘要： 归纳总结了前人关于全球古元古代(2.20 ～2.06 Ga)碳酸盐岩碳同位素正异常现象的基础数据及资料。在此基础上, 对前人关于该正异常成因的假说(生物演化- 大气充氧说、古元古代冰期期后说、超级地幔柱- 超大陆裂解说及蒸发环境说等)进行了评估, 认为以上假说虽然都有地质证据支持, 但其重要性还是有些差异。根据碳同位素分馏原理, 有机物的繁盛会使碳酸盐碳储库的同位素变正。生物演化导致大气充氧、古元古代冰期后形成的温暖环境和Kenorland超大陆裂解均为生物的繁盛提供了有利条件, 从而使碳酸盐碳储库的同位素变正。另外, 蒸发环境在全球多有分布, 它可能是造成部分局域封闭式环境碳同位素正异常的原因, 所以应对发现正异常的各区域进行详细的沉积环境分析。以4个区域的数据为对象, 分析了古元古代碳酸盐岩碳同位素正异常数据的可靠性; 通过Mn/Sr, δ¹³C和δ¹⁸O三者的关系图, 判断了沉积期后作用对于数据的改造程度。

关键词: 古元古代, Mn/Sr, 充氧事件, 碳同位素正异常