Tracking Eukaryotic Production and Burial Through Time with Zinc Isotopes

Abstract:

Zinc is an important, often co-limiting nutrient for eukaryotes in the oceans today. Given the importance of Zn in the modern oceans, we developed a Zn isotope approach to track the extent of Zn limitation and eukaryotic production through Earth’s history. Specifically, we use the isotopic systematics of the pyrite (δ⁶⁶Znpyr), rock extracts (bitumen) and kerogen pyrolysate (δ⁶⁶Znorg) within euxinic black shales. We show that δ⁶⁶Znpyr of euxinic core-top muds from the Cariaco basin capture the global deep seawater signature, validating its use as a seawater proxy. Additionally, we propose that Δ⁶⁶Znpyr-org can be used to track surface water zinc bioavailability. Detailed studies of short-lived oceanic anoxic events such as Cretaceous OAE2, which punctuate an otherwise dominantly oxic Phanerozoic world, exhibit dramatic shifts in seawater δ⁶⁶Zn and organic bound zinc. Such perturbations are consistent with the demise of eukaryotes under a nitrogen stressed regime, in which cyanobacteria carry the competitive advantage. Contradictory to previous models, however, our data suggest that zinc remained largely bioavailable throughout these anoxic intervals despite significant drawdown of the global reservoir. The framework developed from studies of the modern, Cenozoic, and Mesozoic can be used to track the Precambrian evolution of the marine Zn cycle and the rise of eukaryotic algae to ecological dominance.