226 / 2025-04-15 14:48:07

Calcium isotope constraints on the marine carbon cycle during the Cambrian ZHUCE event

Cambrian, Carbon isotope excursion, Calcium isotopes, Diagenesis, Carbon cycle

Theme 3: Earth-Life Co-evolution Through Time > Session 3b: Evolution of the Earth-Life System During the Neoproterozoic-Paleozoic Transition

The early Cambrian Stage 2 records a prominent positive carbon isotope excursion known as ZHUCE (ZHUjiaqing Carbon isotope Excursion) in South China. Although studies have shown that ZHUCE is a globally synchronous event, the carbon isotope variation of the ZHUCE raises the doubt whether it records the changes of the global carbon cycle. This is partly attributed to possible local or regional influences including diagenesis and variations in carbonate mineralogy (e.g., primary minerals as aragonite vs calcite) that deviate δ¹³C signals of shallow marine carbonates from the global dissolved inorganic carbon (DIC) reservoir. Here, we use paired data of δ¹³C and δ^44/40Ca as well Sr/Ca from a carbonate-dominated, bio-stratigraphically well constrained Laolin section in eastern Yunnan, located on the Yangtze platform, South China to constrain the potential for local diagenetic processes in the decoupling δ¹³C signals from the open marine facies. Our data reveal a strong negative covariation between δ^44/40Ca and δ¹³C, and a moderate negative covariation between δ^44/40Ca and Sr/Ca during the ZHUCE interval. These patterns can in part be explained by the fluid-buffered early marine diagenetic process. However, numerical modeling results suggest that this process alone cannot account for the entire shift observed in δ¹³C, with the remaining portion of its stratigraphic trend likely driven by δ¹³C changes of the DIC reservoir in ambient water mass. The ZHUCE plateau (~3.5‰) at Laolin section corresponds with notably light δ^44/40Ca (as low as -1.33‰) and high Sr/Ca values (as high as 6.34) implying the least altered part of the section and consistent with aragonite as a high proportion of the primary mineralogy. However, this peak δ¹³C value is much lower than that of the Xiaotan section (~7‰ in average). Lateral comparison of paired δ¹³C, δ^44/40Ca and Sr/Ca values of these two sections suggests that neither changes in primary mineralogy nor early marine diagenesis can properly explain this discrepancy. We therefore propose that spatial variation in δ¹³C of the platform water masses exists and serves as an important contributor to such δ¹³C_carb differences. This study exemplifies that despite the global nature of the ZHUCE event, its expression in carbonate platform settings would have been modulated by the processes considered here.