Molybdenum isotope records of the Chilpi Group, Bastar Craton, India: Implications on atmospheric and shallow-seawater oxygen content in the Orosirian period

IF 7.2 1区地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY

Gondwana Research Pub Date : 2025-06-12 DOI:10.1016/j.gr.2025.05.020

Prasanta Kumar Mishra , Sarada Prasad Mohanty , Kosuke T. Goto , Susanta Kumar Mishra

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Abstract

The Chilpi Group (2050 – 1850 Ma), central India, preserves iron formations of the Paleoproterozoic Era. The PO₂ value of 10^-3 – 10^-5 times the present atmospheric level (PAL) during deposition of the Chilpi Group was reported from the analyses of the geothermometry and oxygen fugacity variations using the mineral chemistry of the rare association of magnetite-greenalite-cronstedtite-siderite in the iron-rich rocks. To validate the results obtained from the mineral chemistry, and to understand the evolution of ocean redox conditions and low PO₂ value, we measured the Mo concentration and Mo isotope data of iron-formation samples. The analytical results of δ^98/95Mo_NIST values have a range from –0.2 to 1.2 ‰, similar to the sediments deposited below anoxic (ferruginous – non-euxinic) water columns. The δ^98/95Mo_NIST data calculated for adsorption removal of Mo from the modern seawater value of 2.09 ‰ show a negative offset (Δ ^98/95Mo) of –0.7 ‰ for ferrihydrite adsorption model and –0.97 ‰ for magnetite adsorption model. The range of Δ ^98/95Mo value of the Chilpi iron-formations is between –0.6 ‰ of 2.5 Ga iron-formations and –1.2 ‰ of 1.85 Ga iron-formations. A comparable range of δ^98/95Mo_NIST (–0.3 to 1.8 ‰) and Δ ^98/95Mo of –0.8 ‰ for ferrihydrite adsorption model for the iron-formations of the Animikie Basin, Lake Superior region, corroborate the observations. The results indicate a low value of the paleo-seawater (δ^98/95Mo_NIST = 1.12 ‰) during the Orosirian Period, interpreted to be the result of a low oxygen content of the atmosphere and shallow sea and inefficient Mo-removal from the seawater. The lowered paleoproductivity due to the low level of atmospheric oxygen (between 10^–3 and 10^–5 PAL) and a reduced content of Mo, a bio-essential element, in seawater during the late Paleoproterozoic Era might have an impeding effect on the development of eukaryotes.

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印度Bastar克拉通Chilpi群的钼同位素记录：对奥陶纪大气和浅海氧含量的指示

印度中部的Chilpi群（2050 - 1850 Ma）保存着古元古代的铁构造。利用富铁岩石中罕见的磁铁矿-绿绿石-菱铁矿组合的矿物化学分析，分析了赤皮群沉积时期的PO2值为现在大气水平（PAL）的10-3 ~ 10-5倍。为了验证矿物化学的结果，了解海洋氧化还原条件和低PO2值的演化，我们测量了铁形成样品的Mo浓度和Mo同位素数据。δ98/95MoNIST的分析结果在- 0.2 ~ 1.2‰之间，与缺氧（含铁-非含氧）水柱下沉积的沉积物相似。现代海水吸附去除Mo值为2.09‰的Δ 98/ 95monist数据显示，水合铁吸附模型为-0.7‰，磁铁矿吸附模型为-0.97‰，负偏移（Δ 98/95Mo）。赤陂铁组的Δ 98/95Mo值介于2.5 Ga铁组的-0.6‰和1.85 Ga铁组的-1.2‰之间。对苏必利尔湖地区Animikie盆地铁层吸附模型的Δ 98/ 95monist（-0.3 ~ 1.8‰）和Δ 98/95Mo（-0.8‰）进行了比较，证实了上述观测结果。结果表明，古海水的δ98/95MoNIST = 1.12‰，是由于大气和浅海的氧含量较低，海水中mo的去除效率较低所致。古元古代晚期大气氧含量低（10-3 ~ 10-5 PAL），海水中生物必需元素Mo含量降低，导致古生产力下降，可能对真核生物的发育有阻碍作用。

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来源期刊

Gondwana Research 地学-地球科学综合

CiteScore

12.90

自引率

6.60%

发文量

298

审稿时长

65 days

期刊介绍： Gondwana Research (GR) is an International Journal aimed to promote high quality research publications on all topics related to solid Earth, particularly with reference to the origin and evolution of continents, continental assemblies and their resources. GR is an "all earth science" journal with no restrictions on geological time, terrane or theme and covers a wide spectrum of topics in geosciences such as geology, geomorphology, palaeontology, structure, petrology, geochemistry, stable isotopes, geochronology, economic geology, exploration geology, engineering geology, geophysics, and environmental geology among other themes, and provides an appropriate forum to integrate studies from different disciplines and different terrains. In addition to regular articles and thematic issues, the journal invites high profile state-of-the-art reviews on thrust area topics for its column, ''GR FOCUS''. Focus articles include short biographies and photographs of the authors. Short articles (within ten printed pages) for rapid publication reporting important discoveries or innovative models of global interest will be considered under the category ''GR LETTERS''.