TRACKING SUBTLE VARIATIONS OF NEOPROTEROZOIC OCEANIC OXYGENATION USING VANADIUM ISOTOPES: IMPLICATIONS FOR EARLY ANIMALS

Reconstructing the redox conditions of the Neoproterozoic oceans is critical to understanding the emergence and evolution of early animals. The causal relationship between oxygenation and evolution of early animals is highly debated in part because discerning low oxygen conditions have been difficult. To unravel this debate, it is important to identify the prevalence of seafloor with low but sufficient levels of bottom water oxygen (e.g., 10 – 20 mM) to meet the metabolic requirements of early animals. However, current paleoredox proxies such as Mo and S isotopes, and iron speciation cannot fingerprint this type of redox conditions reliably.

Vanadium isotopes (⁵¹V/⁵⁰V) have the potential to resolve subtle variations of low bottom water oxygen conditions. Vanadium is considered conservative due to its long marine residence time of ~50 to 100 kyr and generally constant seawater concentrations globally in the open oceans. A linear correlation is observed between authingic V isotopic compositions in marine sediments and bottom water oxygen conditions from a modern global dataset. Specifically, modern marine sediments deposited from bottom waters with oxygen content below 10 mM have V isotopic compositions higher than –0.7‰ and sediments deposited from bottom waters with oxygen concentrations above 10 mM have more negative values. Therefore, variations of ⁵¹V/⁵⁰V in marine sediments have the potential to track minor changes of bottom water redox conditions.

We will present new ⁵¹V/⁵⁰V values for several Neoproterozoic and Early Cambrian (635 – 520 Ma) black shale deposits from different water depths (shallow shelf, distal slope, and basin) to constrain the spatiotemporal oxygenation of the oceans and provide insights into the link between low oxygen redox conditions and the emergence and evolution of early animals. Importantly, our work is interpreted in combination with previous paleoredox characterizations using proxies such as Fe speciation, abundances of redox-sensitive elements, and Mo and Tl isotopes. Our results show variable V isotopic compositions (–1.1 to –0.4 ‰) in black shales, suggesting considerable spatiotemporal fluctuations in bottom water redox conditions over the Neoproterozoic. This work will provide the foundation to explore the link between bottom water oxygen variations and animal evolution in the future.