Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran

Abstract

Sedimentary successions across the Permian-Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ 18 O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ 13 C and δ 18 O, as well as conodont δ 18 O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C-O isotopes and Mn-Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ 13 C-δ 18 O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ 13 C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of -1.2(±0.4)‰. This suggests that the first-order δ 13 C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‗Boundary Clay' in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by -2.1(±0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~50-80°C and water/rock ratios of <10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon --at a global scale -that has received relatively little attention to date.

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

“…Our new data are complemented by published bulk-carbonate δ 13 C data from multiple P-Tr localities in Central Iran (Abadeh (Heydari et al, 2000;Korte et al, 2004a;Horacek et al, 2007;Korte et al, 2010;Liu et al, 2013), Shahreza (Korte et al, 2004b;Heydari et al, 2008;Richoz et al, 2010) and north-western Iran (Ali Bashi mountains and Zal; Baud et al, 1989;Korte et al, 2004c;Kakuwa and Matsumoto, 2006;Horacek et al, 2007;Richoz et al, 2010;Schobben et al, 2016). For a more global perspective and a comparative approach, we additionally extracted published data from the P-Tr Global Boundary Stratotype Section and Point (GSSP) at Meishan, China (Baud et al, 1989;Chen et al, 1991;Xu and Yan, 1993;Hoffman et al, 1998;Jin et al, 2000;Cao et al, 2002;Gruszczyński et al, 2003;Zuo et al, 2006;Kaiho et al, 2006;Huang et al, 2007;Riccardi et al, 2007;Xie et al, 2007).…”

“…Moreover, the imprint of 12 C-depleted carbon on latest Permian carbonate rock produced by the introduction of authigenic carbonate has been connected to a systematic carbon isotope offset between bulk rock and brachiopod shells (Schobben et al, 2016). Diagenetic and authigenic carbonate sources can, however, result in a range of carbon isotope values relating to the specific microbial community and sedimentary environment (Irwin et al, 1977).…”

“…Lowstand Transgression Abadeh Abadeh (Korte et al, 2004) Abadeh Abadeh (Richoz et al, 2010) Abadeh 2 (Korte et al, 2010) Ali Bashi (Kakuwa and Matsumoto, 2006) Ali Bashi Ali Bashi (Richoz et al, 2010) Ali Bashi 1 (Baud et al, 1989) Ali Bashi 1 (Schobben et al, 2016) Ali …”

“…This fraction of 0.2 is considered to be the diagenetic carbonate fraction (f dia ) of the total rock volume (e.g., Schobben et al, 2016). Specific metabolically produced DIC δ 13 C values are assigned for organoclastic microbial sulfate reduction, AOM and organoclastic methanogenesis (see Table 2).…”

“…Foremost, the reactive and biologically active upper section of the sediment column can be envisioned as leaving an imprint on carbonate chemistry, including the carbon isotope composition. This carbon isotope signal will be retained in postdepositional carbonate cements and is controlled by organic carbon (OC) fluxes, pH, alkalinity and the nature of the (sub-)sea-floor microbial communities (Melim et al, 2002;Reitner et al, 2005;Wacey et al, 2007;Birgel et al, 2015;Schobben et al, 2016;Zhao et al, 2016).…”