[S-GC33] 固体地球化学・惑星化学

[SGC33-P08] C-isotope anomalies in Cambrian carbonates and beyond: is it due to diagenesis and/or oxidation of dissolved organic carbon?

*Arshad Ali¹、Mohamed El-Ghali¹、Mohamed Moustafa¹、Iftikhar Abbasi¹ (1.Sultan Qaboos University)

キーワード：Buah Formation, Oman, Diagenesis, Ediacaran-Cambrian Transition, Carbon Isotopes

The Nafun Group, situated above the Marinoan cap carbonate (Hadash Formation), is extensively exposed in both the Jabal Akhdar (JA) and the Huqf region of east-central Oman. It consists of two 'grand cycles', including siliciclastic to carbonate deposits, each initiated by distinguished transgressions. These cycles include the Masirah Bay/Khufai formations and the Shuram/Buah formations [1]. The Neoproterozoic geological record often shows occurrences of negative δ¹³C excursions, commonly associated with the Shuram Formation in Oman. However, it remains uncertain whether these excursions are primary, diagenetic, or have a local or global origin [2]. According to a mathematical model, it has been hypothesized that there could be a considerable fluctuation in the isotopic record due to changes in the fractionation linked to organic production or in the size of the dissolved organic carbon reservoir in ocean water [3]. In Neoproterozoic carbonates, these negative carbon excursions occur because of pulses in the remineralization of a significant oceanic organic carbon pool, which is intensified by glaciation, or due to the biomineralization of the same, increased by the presence of early metazoans [3]. However, the concrete identification of such a carbon pool remains challenging. Among the potential hypotheses, diagenesis has been suggested as a plausible explanation for the exceptionally low δ¹³C values (-12‰) in the Shuram Formation [4]. Recently, the utilization of Ca and Mg isotopic data has been used to argue for the primary origin of the carbon pool in Ediacaran Period [5]. The Shuram anomaly unequivocally shows that mantle input (δ¹³C = -5‰; [6]) is insufficient to produce δ¹³C values of -12‰, indicating that organic matter (δ¹³C = -25‰ to -30‰; [7]) oxidation during remineralization is most likely involved. The record of diagenesis observed in the most recent 500 Myr of Earth history provides evidence for only modest alteration of primary signals. The non-steady-state model illustrates that prolonged and severe negative perturbations are plausible, probably associated with the evolution of the biosphere during the Neoproterozoic [3]. The gradual recovery of the negative anomaly may reflect a slow remineralization process. To understand the principles and control mechanisms of release, migration, and distribution of elements during chemical weathering process, it is helpful to investigate the geochemical and isotopic characteristics of rocks. The Mn/Sr ratio of carbonate rocks serves as a widely recognized proxy for diagenetic alteration [8]. Some researchers propose that the ratio should be < 2, particularly when considered in conjunction with δ¹⁸O > −10‰ [9-10]. The δ¹³C ranges are as follows: M1-Huqf from +2 to -4‰, M2-Huqf from +1 to +2‰, WAK-JA from +2 to -8‰, and WKK-JA from 0 to -6‰. A cross plot between δ¹³C and Mn/Sr includes data from sections of the Buah Formation and the Shuram Formation (Oman, as well as the Ediacaran-Cambrian Transition (ECT) in South China (Image 1). The majority of JA, Shuram, and ECT data falls within the Mn/Sr < 2 range, indicating unaltered conditions, as supported by their δ¹⁸O > −10‰ [11-13]. However, the δ¹³C variation is widespread, except for a restricted range in ECT (China). This indicates that δ¹³C negative excursions cannot be solely attributed to the primary Ediacaran surface environment, as suggested in Ca and Mg isotopic studies on Wonoka carbonates [5]. Our dataset from Cambrian carbonates indicates a combination of processes, including the oxidation of organic carbon mixing with another exogenous organic source through remineralization [14] and diagenesis in a marine environment. This is evident in the Buah carbonates of the Huqf region, which are highly altered, showing a minimum δ¹³C value of -4‰, in contrast to a few JA carbonates with δ¹³C value of -8‰ are mostly unaltered or mildly altered (Image1).

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Keywords: Buah Formation, Oman, Diagenesis, Ediacaran-Cambrian Transition, Carbon Isotopes