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214-8 Secular variation of seawater phosphate through geologic time

Session: The Neoproterozoic Earth and Life Co-evolution, Part II

Presenting Author:

Ryosuke Nagao

Authors:

Nagao, Ryosuke¹, Yoshida, Satoshi², Adachi, Natsuko³, Ueno, Yuichiro⁴, Komiya, Tsuyoshi⁵, Hirata, Takafumi⁶

(1) Department of Earth and Planetary Sciences, Graduate School of Science, The University of Tokyo, Tokyo, Japan, (2) Department of Solar System Sciences, Japan Aerospace Exploration Agency, Kanagawa, Japan, (3) Department of Geosciences, Graduate School of Science, Osaka Metropolitan University, Osaka, Japan, (4) Department of Earth and Planetary Sciences, Institute of Science Tokyo, Tokyo, Japan; Earth-Life Science Institute, Institute of Science Tokyo, Tokyo, Japan, (5) Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, Japan; Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan, (6) Geochemical Research Center, The University of Tokyo, Tokyo, Japan,

Abstract:

Phosphorus is one of the bio-essential elements and serves as a limiting nutrient of primary productivity at geological timescales in the modern ocean. Therefore, it is widely believed that its availability in seawater directly affects the redox state of the atmosphere and oceans throughout the Earth’s history. Carbonate-associated phosphate (CAP) is one of the prominent proxies to quantify dissolved inorganic phosphate (DIP) levels in shallow oceans. A weak acid leaching method is often favored for CAP analysis; however, the method cannot distinguish among different generations of carbonate minerals within carbonate rocks. We classified carbonate minerals into some generations and formation processes based on their textures to quantitatively estimate the secular variation of seawater DIP content through geologic time. We then conducted in situ CAP analysis of carbonate minerals from Neoarchean to Phanerozoic using LA-ICP-MS/MS.

The observed CAP values within each sample, expressed as P/ (Ca + Mg) mmol/mol, varied systematically with the textural classification. Thus, CAP values from primary seawater-derived textures were used as proxies for the ambient DIP. The CAP values from the Ordovician to Devonian range from 0.03 to 0.09, equivalent to the previously reported values of 0.03–0.08 for modern ooids. The CAP values of the Archean carbonate minerals from the 2.7 Ga Tumbiana Fm. range from 0.03 to 0.09. The CAP values in most Proterozoic carbonates are higher than those of modern ooids, and gradually increase from 0.03–0.11 at the 1.8 Ga Duck Creek Fm to 0.49–0.88 at the 0.75 Ga Backlundtoppen Fm.

These results suggest high phosphate concentrations in Precambrian seawater and a secular increase in seawater P levels through time. The elevated Precambrian CAP values contradict the “phosphate crisis” hypothesis. The gradual rise in seawater P content in the Proterozoic parallels the positive shift in carbonate carbon isotope values, indicative of growing biomass. This concomitant increase indicates that P input consistently exceeded biological demand, suggesting that P was not a limiting nutrient for primary production in the Precambrian. However, this enhanced P input may have served as a nutrient reservoir for Neoproterozoic oxygenation of Earth’s surface environment.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5621

Secular variation of seawater phosphate through geologic time

Description

Session Format: Oral

Presentation Date: 10/21/2025

Presentation Start Time: 03:35 PM

Presentation Room: HBGCC, 304A

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