Carbon-isotope stratigraphy across the Permian–Triassic boundary: A review

Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy

Carbonate platforms changed substantially in spatial extent, geometry, composition and palaeogeographical distribution through the Phanerozoic. Although reef construction and carbonate platform development are intimately linked today, this was not the case for most of the Phanerozoic. Carbonate production by non-enzymatic precipitation and non-reefal organisms is mostly responsible for this decoupling. Non-reefal carbonate production was especially prolific during times of depressed reef growth, balancing losses in reef carbonate production. Palaeogeographical distribution and spatial extent of Phanerozoic carbonate platforms exhibit trends related to continental drift, evolutionary patterns within carbonate platform biotas, climatic change and, possibly, variations in ocean chemistry. Continental drift moved large Palaeozoic tropical shelf areas into higher latitudes, thereby reducing the potential size of tropical platforms. However, the combined global size of carbonate platforms shows no significant decline through the Phanerozoic, suggesting that availability of tropical shelf areas was not a major control of platform area. This is explained by the limited platform coverage of low-latitude shelves (42% maximum) and occasional high-latitude excursions of platform carbonates. We speculate that reduced tropical shelf area in the icehouse tropics forced the migration of the many carbonate-secreting organisms into higher latitudes and, where terrigenous input was sufficiently low, extensive carbonate platform could develop.

Patterns of Phanerozoic carbonate platform sedimentation

Pelagic deposits at Abadeh represent a complete biostratigraphic record across the Permian/Triassic boundary (PTB). The presumed water depth during deposition of these sediments was between 60 and 90 m. Similar to other Permian/Triassic boundary sections, the succession at Abadeh is characterised by a negative carbon isotope shift of approximately 4‰. The values start to decrease in the lower C. changxingensis - C. deflecta s.l. Zone, reach −0.12‰ (V-PDB) in the uppermost Permian just below the PTB, remain low to the early I. isarcica Zone (−0.32‰) and increase subsequently in the upper I. isarcica Zone. For the time interval of the PTB negative carbon isotope excursion, between the C. iranica and the I. isarcica Zones, no correlation exists between the δ13Ccarb and the δ18Ocarb. The above observations argue against the conclusion of Heydari et al. (2001) that the carbon isotope event at the P/T transition is an alteration artefact and not a global signal. The decrease in δ13Ccarb is accompanied by a ~5‰ (and potentially up to 10‰) increase in δ34SSSS. Together, these features are thought to reflect a complex global event, notably the development of widespread anoxic oceans with anoxic bottom layers rising onto the shelves. For the carbon isotope drop, other factors, such as the collapse of ocean primary productivity may also have played a role. The 87Sr/86Sr ratios of Dzhulfian seawater show only a minor increase from 0.70705 to 0.70710, reaching 0.70720 in the Dorashamian. The increase becomes steeper in the Early Triassic reaching 0.70754 in the N. dieneri Zone. The rise of the strontium isotope values is thought to be related to enhanced continental weathering under humid climatic conditions in the uppermost Permian (C. meishanensis - H. praeparvus Zone) and the lack of a dense land vegetation in the Early Triassic, prior to the Spathian (Upper Olenekian).

Carbon, sulfur, oxygen and strontium isotope records, organic geochemistry and biostratigraphy across the Permian/Triassic boundary in Abadeh, Iran

Early Triassic carbon isotopes are measured based on 1422 carbonate bulk samples from 10 Lower Triassic sections in different palaeogeographic settings in South China. Early Triassic fluctuation of d 13 C is used as a proxy for environmental change to discuss the devastation and restoration of marine ecosystems following the biggest Phanerozoic mass extinction at the end of the Permian. Early Triassic d 13 C profiles derived from various depositional settings in South China yield comparable excursion patterns. A dramatic negative shift of d 13 C across the Permian/Triassic boundary is followed by a moderate increase in d 13 C values throughout the Induan. A positive d 13 C anomaly occurs near the Induan/Olenekian boundary, followed by a Smithian interval of lower d 13 C values. A distinct positive shift in d 13 C coincides with the Smithian/Spathian boundary, and is followed by a high Spathian plateau of d 13 C values. Thus the distinct d 13 C anomalies coincide well with key stratigraphic boundaries. Early Triassic intervals of low d 13 C values correspond to a decline in benthic generic diversity in South China and intervals of high d 13 C values are coupled with an increase in biodiversity. The Early Triassic fluctuations of d 13 C profiles are good proxies for perturbations of ecologic environments, indicating multiple phases of devastation–restoration of marine ecosystems punctuated by several unexpected extraneous events such as volcanic activity. The initial restoration of marine ecosystems may have occurred earlier in the Induan and the ecosystems were subsequently destroyed during the Smithian. Biotic and environmental recoveries occurred at the Spathian, while the ecosystems eventually returned to normal levels in the early Middle Triassic. Copyright # 2007 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/gj.1084

Early Triassic carbon isotope excursions from South China: proxies for devastation and restoration of marine ecosystems following the end‐Permian mass extinction

A Ramovš

Papers

The Permian–Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations