Tonian Carbonates Record Phosphate‐Rich Shallow Seas

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2023-05, Vol.24 (5), p.n/a
• Main Authors: Roest‐Ellis, Sascha, Richardson, Jocelyn A., Phillips, Brian L., Mehra, Akshay, Webb, Samuel M., Cohen, Phoebe A., Strauss, Justin V., Tosca, Nicholas J.
• Format: Article
• Language: English
• Subjects: Algae; Analytical methods; Availability; carbonate; Carbonate minerals; Carbonate rocks; Carbonates; Climate; Continental margins; Distribution; eukaryote; Fluorescence; Geochemistry; GEOSCIENCES; Life on Earth; Magnetic resonance; Mineral nutrients; Mudstone; neoproterozoic; nitrogen; NMR; Nuclear magnetic resonance; Nutrients; phosphate; Phosphates; Primary production; Seawater; Shallow water; Speciation; Spectroscopy; Statistical analysis; Statistical methods
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC010974

The early‐middle Neoproterozoic is thought to have witnessed significant perturbations to marine P cycling, in turn facilitating the rise of eukaryote‐dominated primary production. However, with few robust constraints on aqueous P concentrations, current understanding of Neoproterozoic P cycling is generally model‐dependent. To provide new geochemical constraints, we combined microanalytical data sets with solid‐state Nuclear Magnetic Resonance, synchrotron‐based X‐ray Absorption Near Edge Structure spectroscopy, and micro‐X‐ray Fluorescence imaging to characterize the speciation and distribution of P in Tonian shallow‐water carbonate rocks. These data reflect shallow water phosphate concentrations 10–100× higher than modern systems, supporting the hypothesis that tectonically‐driven influxes in P periodically initiated kinetically‐controlled CaCO3 deposition, in turn destabilizing marine carbonate chemistry, climate, and nutrient inventories. Alongside these observations, a new compilation and statistical analysis of mudstone geochemistry data indicates that, in parallel, Corg and P burial increased across later Tonian continental margins until becoming decoupled at the close of the Tonian, implicating widespread N‐limitation triggered by increasing atmospheric O2. Plain Language Summary As an essential nutrient for all life on Earth, changes in the availability of seawater phosphate over geological timescales are thought to have regulated biospheric productivity and ecosystem structure. Increases in marine phosphate availability in the Neoproterozoic Era (1,000–538 million years ago), inferred from the total concentration of P in mudstones, are thought to have driven the emergence of eukaryotic algae. However, the causes are not well understood because the concentration of P in mudstones is difficult to relate to seawater phosphate concentrations. To address this problem, we applied new analytical methods to probe the distribution and mineral hosts of phosphate in shallow water carbonate rocks, which permits new estimates for ancient seawater phosphate concentrations. These results reflect shallow water phosphate concentrations several times higher than the modern ocean. Elevated phosphate may have strongly impacted the rate and tempo of carbonate mineral production from Neoproterozoic seas, consistent with geological evidence. This may have driven fluctuations in seawater carbonate chemistry and climate, impacting other critical nutrients such as dissol