Variations of Zn isotopes in phosphorites and their implications

Variations of Zn isotopes in phosphorites can play an important role in elucidating the primary productivity and temporal dynamics of the marine environment. However, the controlling factors associated with Zn isotope fractionation in phosphorite deposits are not well documented, which limits the utility of Zn isotope as a proxy to study modern and paleo-ocean productivity. In this research, several geochemical proxies derived from trace elements are used to monitor the effect of various parameters on the evolution of δ⁶⁶Zn in Cenozoic and Permian phosphorite deposits in detail. Lower δ⁶⁶Zn values in phosphorites indicate reduced productivity, while higher δ⁶⁶Zn values suggest increased productivity, although this relationship may be influenced by other moderating factors. The study finds that the Peru Margin (Cenozoic) and Retort Member (Permian) phosphorites exhibit lower δ⁶⁶Zn values, acting as sinks for lighter δ⁶⁶Zn. In contrast, phosphorites from the Blake Plateau align with modern deep ocean δ⁶⁶Zn values, while those from the California Borderland, Chatham Rise, and open-ocean seamounts (Cenozoic) and Meade Peak Member (Permian) show higher δ⁶⁶Zn values, serving as sinks for heavier δ⁶⁶Zn isotopes. These variations of Zn isotopes in phosphorites could be controlled by productivity and redox conditions. Based on our dataset, we suggest that in addition to organic-rich shales, FeMn oxides, and carbonates, phosphorites may also represent another potential important sink for both lighter and heavier Zn isotopes.