169-3 Elevated Marine Phosphate Availability During the Tonian Period

Session: Phantastic Phosphorus and Its Historical Biogeochemistry

Presenting Author:

Miquela Ingalls

Authors:

Ingalls, Miquela¹, Urist, Amanda², Trower, Lizzy³, Crockford, Peter⁴

(1) Pennsylvania State University, University Park, PA, USA, (2) Pennsylvania State University, University Park, PA, USA, (3) University of Colorado, Boulder, CO, , (4) Carleton University, Ottawa, Ontario, Canada,

Abstract:

Phosphorus (P) is considered the ultimate limiting nutrient for Earth’s biosphere over geological time, but ancient seawater phosphate ([PO₄^3-]) concentrations remain poorly constrained. Reconstructing Proterozoic P availability is critical for understanding whether biological expansion, eukaryotic diversification, and shifting primary producer communities during the Neoproterozoic Era were facilitated by relaxed nutrient limitation. Carbonate-associated phosphate (CAP) provides a semi-quantitative proxy for dissolved inorganic phosphate ([DIP]) in ancient shallow marine environments. We analyzed CAP across four Tonian-aged carbonate platforms—the Little Dal Group (~1.00-0.78 Ga, Canada), Akademikerbreen Group (~0.82-0.74 Ga, Svalbard), Otavi Group (~0.75-0.72 Ga, Namibia), and the Beck Spring Dolomite (~0.78-0.72 Ga, Death Valley, USA) and Chuar Group (~0.77-0.74 Ga, Grand Canyon, USA)—and three mid-Proterozoic platforms as a baseline comparison—Rocknest Formation (~1.90 Ga, Canada), Bylot Supergroup (~1.05 Ga, Canada), and Crystal Springs Formation (~1.08 Ga, USA). We integrated these into a global dataset of >1,600 CAP measurements from the Neoarchean to the Cenozoic. 

Neoproterozoic CAP concentrations represent peak global values across Earth history even within the uncertainties of differing analytical approaches and the context of global carbon cycle perturbations. In addition, we used ooid-size-based estimates of pH (8.49-8.68) and CAP values to reconstruct seawater [DIP] of 1.4-14.1 µM for the Akademikerbreen Group, 3.8 to 7.0 µM for the Beck Spring Dolomite, and 6.5 to 9.8 µM for the Chuar Group, exceeding modern P-limited ocean surface levels by up to sevenfold. Together, these findings have implications for both global climate and biogeochemistry. Reconstructed [DIP] falls below the threshold for calcite nucleation inhibition, indicating elevated [DIP] alone cannot account for ikaite persistence in warm waters supporting a cool late Tonian climate. In addition, CAP variations across the Bitter Springs carbon isotope anomaly and globally distributed platforms demonstrates phosphate availability reduced biosphere nutrient limitation and sustained primary productivity prior to animal radiation.

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