Early-Middle Permian carbon-isotope stratigraphy of marine carbonates in the northern edge of the South China: implications for global correlation

Abstract

The Early-Middle Permian δ 13 C carb variations are recorded by the marine carbonates of the Luduba section in the northern margin of the South China Block. The investigated interval (~ 300 m) spans the Chihsia and Maokou formations. Preservation of the investigated micritic carbonates was evaluated by multiple petrographic and geochemical screening tools. The δ 13 C carb and δ 18 O values of near-primary micrites range from 2.3 to 5.0‰ (VPDB) and from − 6.4 to − 3.4‰ (VPDB), respectively, showing insignificant correlation (R 2 = 0.13). Similarly, the Mn/Sr ratios show insignificant correlation with δ 13 C carb or δ 18 O ratios. In addition, the Al and Sr concentrations exhibit insignificant correlation, which suggests preservation of at least near-primary δ 13 C carb signatures that can be utilized to construct a reliable C-isotope profile of the equatorial eastern Paleo-Tethys for global correlations. The C-isotope profile exhibits distinct short-time negative shifts superimposed on the general long-term trend of increase in δ 13 C carb values. The long-term trend likely reflects a consistent global sea-level rise during the Early Guadalupian, which was associated with one of the largest Permian transgressions. The general sealevel rise was disrupted by few minor short-term sea-level low-stands associated with negative δ 13 C carb shifts (~ 1-2 ‰) on the C-isotope profile, which are associated with similar minor positive 87 Sr/ 86 Sr shifts (~ 0.0005 each) on the Sr isotope profile and with Al enrichment, possibly caused by the combination of global eustasy and regional tectonics in South China. Two negative δ 13 C carb shifts have been recorded in the lower and upper Chihsia Formation that can be correlated with the Early Kungurian and the Kungurian/Roadian boundary, respectively. Also, the Maokou Formation records a middle negative shift that inferred correlates with the Early Wordian. These three negative δ 13 C carb shifts are globally comparable with those documented by both the eastern Paleo-Tethys and western Panthalassa Early-Middle Permian sedimentary records and can be utilized as chemostratigraphic markers. The general increase in the δ 13 C carb values peaks at ~ 5 ‰ near the top of the Maokou Formation that resembles the Kamura event recorded in Tethy but there are issues with the origin and timing of this positive δ 13 C carb excursion. The petrological characteristics of the Luduba section carbonates support the suggested scenario of the general long-term trend of sea-level rise.