Zinc isotope perspective on global carbon cycling during the onset of the late Paleozoic icehouse

The onset of the late Paleozoic icehouse coincided with a large global carbon-cycle perturbation (represented by the mid-Tournaisian carbon isotope excursion [TICE]), although the underlying cause of this isotopic event remains uncertain. Zinc (Zn), an essential micronutrient for plankton, has an isotopic composition (δ⁶⁶Zn) in seawater that is sensitive to subtle fluctuations in marine carbon (C) cycling. Here, we investigated C cycling during the TICE using paired carbonate δ⁶⁶Zn-δ¹³C and organic δ¹³C records from two widely spaced sections in South China. These records reveal coupling between δ⁶⁶Zn and δ¹³C over an ∼4 m.y. interval in the form of two positive excursions (Peaks I and II) separated by a negative shift (interpeak). Peak I is attributed to enhanced marine primary productivity (MPP), which was stimulated by increased micronutrient supply linked to enhanced silicate weathering and/or upwelling. The interpeak may record oxidation of remobilized organic matter from continental shelves exposed by glacio-eustatic sea-level fall, which, in turn, promoted MPP again and resulted in Peak II. Thus, the TICE records complex climate−carbon cycle feedbacks that may have led to large-scale organic carbon burial and driven stepwise cooling, marking the onset of a sustained late Paleozoic icehouse climate.