Photosynthetic fractionation of carbon: A biological driver for the Neoproterozoic Shuram carbon isotope excursion

IF 3.2 2区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Precambrian Research Pub Date : 2025-02-01 DOI:10.1016/j.precamres.2025.107702

Euan N. Furness , Emily G. Mitchell

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Abstract

The Neoproterozoic Shuram excursion (574 to 567 Ma) is the single largest carbon isotope excursion in the past two billion years, yet its cause is still unclear. Previous suggestions have been contentious: suggestions of a diagenetic origin for the excursion cannot explain coeval shifts in organic carbon isotopes, but suggestions that the excursion represents a dramatic perturbation to the carbon cycle have faced criticism on the basis of the considerable flux of oxidants required to drive the observed change. Here, we present a third potential contributing factor: a change in the carbon isotope fractionation of photosynthesis. We investigate the explanatory power of this mechanism using a carbon box model, with comparison to empirical isotopic measurements from the Doushantuo Formation in China and the Shuram and Buah formations in Oman. Our results suggest that a ∼20 ‰ decrease in photosynthetic carbon isotope fractionation over the course of the Shuram excursion is sufficient to drive observed changes in both carbonate and organic carbon isotope ratios. Our modelling shows that this driver frequently outperforms other potential drivers of the Shuram excursion in terms of fit of model results to empirical data. Therefore, we suggest that a change in carbon isotope fractionation may have contributed to the Shuram excursion. However, this mechanism does not explain other contemporaneous phenomena, such as apparent changes in marine sulphur chemistry. Consequently, we suggest that a change in fractionation is unlikely to have been a sole driver of the excursion. A change in fractionation could have arisen either due to changes in the abiotic environment, which could have influenced producer physiology or partitioned carbonate and organic carbon formation, or as a consequence of the evolution of the first macroscopic animal communities in the Ediacaran, which could have modified plankton communities through feeding pressure, impacting the abiotic environment on a global scale.

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碳的光合分馏：新元古代Shuram碳同位素偏移的生物驱动

新元古代Shuram偏移（574 ~ 567 Ma）是过去20亿年间最大的单次碳同位素偏移，但其原因尚不清楚。以前的建议是有争议的：关于偏移的成岩起源的建议不能解释有机碳同位素的同期变化，但是关于偏移代表了碳循环的戏剧性扰动的建议面临着批评，因为驱动观察到的变化所需的氧化剂的相当大的通量。在这里，我们提出了第三个潜在的影响因素：光合作用的碳同位素分馏的变化。我们利用碳盒模型研究了这一机制的解释力，并与中国陡山沱组和阿曼Shuram和Buah组的经验同位素测量结果进行了比较。我们的研究结果表明，在舒拉姆偏移过程中，光合作用碳同位素分异减少了~ 20‰，这足以驱动观察到的碳酸盐和有机碳同位素比值的变化。我们的建模表明，就模型结果与经验数据的拟合而言，该驱动程序经常优于Shuram偏移的其他潜在驱动程序。因此，我们认为碳同位素分异的变化可能促成了舒拉姆偏移。然而，这一机制不能解释其他同时期的现象，如海洋硫化学的明显变化。因此，我们认为分馏的变化不太可能是偏移的唯一驱动因素。分异的变化可能是由于非生物环境的变化，这可能影响了生产者的生理或碳酸盐和有机碳的形成，或者是由于埃迪卡拉纪第一个宏观动物群落的进化，这可能通过摄食压力改变了浮游生物群落，在全球范围内影响了非生物环境。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Precambrian Research 地学-地球科学综合

CiteScore

7.20

自引率

28.90%

发文量

325

审稿时长

12 months

期刊介绍： Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.