Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation
Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation
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DOI：
10.5194/cp-13-1635-2017
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发表时间：
2017-11
影响因子：
4.3
通讯作者：
Martin Schobben;S. V. D. Velde;Jana Gliwa;L. Leda;D. Korn;U. Struck;C. Ullmann;V. Hairapetian;A. Ghaderi;C. Korte;R. Newton;S. Poulton;P. Wignall
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作者：
Martin Schobben;S. V. D. Velde;Jana Gliwa;L. Leda;D. Korn;U. Struck;C. Ullmann;V. Hairapetian;A. Ghaderi;C. Korte;R. Newton;S. Poulton;P. Wignall
抽象的。整体碳酸盐碳同位素比值是研究古生物地球化学碳循环的一个广泛应用的指标。时间碳同位素趋势是一种主要的地层学工具，其内在假设是，块状泥晶碳酸盐岩是海水溶解无机碳同位素组成的忠实地球化学记录。然而，块状碳酸盐岩也容易包含成岩信号。本研究的目的是从伊朗和中国南部海相碳酸盐岩层序中穿越二叠系-三叠系界线的碳同位素记录中的成岩信号中解开主要趋势。通过汇集最新生产和发布的碳同位素数据，我们确认了全球存在价值枯竭的一阶趋势。然而，在这一地球化学记录上叠加了大量的散布。此外，我们观察到这种残留的δ13C变异性的幅度有时间趋势，这在两个研究地区是可以重现的。我们认为，海底微生物群落及其对方解石成核和周围孔隙水中溶解的无机碳δ13C的控制是诱发块状岩石δ13C变异性的一个可行机制。数值模型计算表明，早期成岩碳酸盐岩稳定和伴生碳同位素蚀变可受有机质供应和随后的微生物再矿化控制。晚二叠世底栖生物的重大生物衰退促进了非均质有机碳积累的空间增加。结合较低的海洋硫酸盐，这导致了不同程度的碳同位素叠印。模拟的时间序列表明，有机碳的空间分散度相对于平均值增加50%，此外，微生物方解石成核形成的胶结物取样的可能性增加到10个样品中的1个，足以引发碳同位素变化的观测信号。这些发现限制了基于全岩样品的二叠纪-三叠纪碳同位素化学地层学的应用，这种方法似乎不如经典的生物分带测年方案那么精确。另一方面，在显生界大规模灭绝的同时，碳同位素变异性增加的这一信号可能提供了关于受抑制的生物扰动下空间异质(次)海底微生物群落所介导的局部碳循环的信息。
Abstract. Bulk-carbonate carbon isotope ratios are a widely applied proxy for investigating the ancient biogeochemical carbon cycle. Temporal carbon isotope trends serve as a prime stratigraphic tool, with the inherent assumption that bulk micritic carbonate rock is a faithful geochemical recorder of the isotopic composition of seawater dissolved inorganic carbon. However, bulk-carbonate rock is also prone to incorporate diagenetic signals. The aim of the present study is to disentangle primary trends from diagenetic signals in carbon isotope records which traverse the Permian–Triassic boundary in the marine carbonate-bearing sequences of Iran and South China. By pooling newly produced and published carbon isotope data, we confirm that a global first-order trend towards depleted values exists. However, a large amount of scatter is superimposed on this geochemical record. In addition, we observe a temporal trend in the amplitude of this residual δ13C variability, which is reproducible for the two studied regions. We suggest that (sub-)sea-floor microbial communities and their control on calcite nucleation and ambient porewater dissolved inorganic carbon δ13C pose a viable mechanism to induce bulk-rock δ13C variability. Numerical model calculations highlight that early diagenetic carbonate rock stabilization and linked carbon isotope alteration can be controlled by organic matter supply and subsequent microbial remineralization. A major biotic decline among Late Permian bottom-dwelling organisms facilitated a spatial increase in heterogeneous organic carbon accumulation. Combined with low marine sulfate, this resulted in varying degrees of carbon isotope overprinting. A simulated time series suggests that a 50 % increase in the spatial scatter of organic carbon relative to the average, in addition to an imposed increase in the likelihood of sampling cements formed by microbial calcite nucleation to 1 out of 10 samples, is sufficient to induce the observed signal of carbon isotope variability. These findings put constraints on the application of Permian–Triassic carbon isotope chemostratigraphy based on whole-rock samples, which appears less refined than classical biozonation dating schemes. On the other hand, this signal of increased carbon isotope variability concurrent with the largest mass extinction of the Phanerozoic may provide information about local carbon cycling mediated by spatially heterogeneous (sub-)sea-floor microbial communities under suppressed bioturbation.