Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation

Abstract

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 δ 13 C 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 δ 13 C pose a viable mechanism to induce bulk-rock δ 13 C 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 proPublished by Copernicus Publications on behalf of the European Geosciences Union.

Permian Large Igneous Provinces and Their Paleoenvironmental Effects

“…δ 18 O apatite changes from Meishan, Shangsi, Daijiagou, Liangfengya (Chen et al, 2016b) and Penglaitan (Shen et al, 2019) in South China, representing different environmental settings (e.g., carbonate platform, upper slope, lower slope, and ramp), demonstrated a consistent pattern of seawater temperature changes around the end-Permian mass extinction. Such pattern has also been observed at the Abadeh, Kuh-e-Ali Bashi, and Zal sections in Iran (Korte et al, 2004;Chen et al, 2013;Schobben et al, 2017). Using the Meishan profile as an example, δ 18 O apatite values fluctuate around ~21‰ for most of the Changhsingian, until a sudden decrease from 21.0‰ in Bed 25, to 18.6‰ in Bed 27a, and 17.9‰ in Bed 28.…”

“…A prominent negative carbon isotope excursion (NCIE) around the PTB has been found in a plethora of marine and nonmarine sections worldwide and confirmed as a global signal (Korte & Kozur, 2010;Shen et al, 2013;Schobben et al, 2017;Cui et al, 2017). Figure 18.2 shows seven intervals for the 1-Myr span (252.3-251.3 Ma) of δ 13 C carb variations, using data from the Meishan section (Chen & Xu, 2019) for the correlation with the timeline of the Siberian Traps magmatism (Burgess et al, 2017).…”

Pre–mass extinction decline of latest Permian ammonoids

“…before the main extinction pulse in China. This is also the time of onset of a long-term negative carbonate δ 13 C excursion at a global scale (Korte and Kozur, 2010;Schobben et al, 2017).…”

Aras Valley (northwest Iran): high-resolution stratigraphy of a continuous central Tethyan Permian–Triassic boundary section

“…However, the degree to which stratigraphically relevant information is captured in δ 13 C carb records can vary and depends on many factors, such as carbonate mineralogy changes (Brand et al, 2012), diagenetic resetting (Marshall, 1992;Schobben et al, 2016), sedimentary continuity in conjunction with the sampling resolution, and bioturbation intensity (Schobben et al, 2017). As such, we refrained from an overly detailed description of the here-produced δ 13 C carb record, and we only describe first-order trends over broad stratigraphic intervals (Schobben et al, 2019). The first-order stratigraphic pattern in the Aras Valley sequence is compatible with δ 13 C records from various regions (e.g.…”

“…The release of greenhouse gas by intrusive volcanism provides, however, a promising prospect in the search for a common driver of the negative C isotope inflection and the causes behind this mass extinction event. In addition, this volcanically induced carbon gas release might have generated a geochemical anomaly that can serve as a first-order feature in a chemostratigraphic scheme to correlate PTB beds from across the globe (Korte and Kozur, 2010;Schobben et al, 2019). Hence, the negative carbonate-carbon isotope excursion as recorded in the Aras Valley succession is supportive of a relatively complete sequence of the PTB interval.…”