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[BCG06-10] Secular variation of seawater phosphate through geologic time
★Invited Papers
Keywords:Precambrian, Phosphorus, Carbonate-associated phosphate
Phosphorus is one of the bio-essential elements and serves as a limiting nutrient of primary productivity at geological timescales in the modern ocean. It is therefore widely believed that its availability in seawater would directly affect the carbon cycle, and thereby the redox state of the atmosphere and oceans through Earth’s history. Despite its importance, phosphate levels of seawater through geologic time are poorly constrained; contradicting views have been proposed from the P/Fe ratios in iron-rich rocks (e.g., Bjerrum & Canfield, 2002; Jones et al., 2015; Konhauser et al., 2007; Planavsky et al., 2010)and from whole-rock P contents in shales (Reinhard et al., 2017).
Recently, a more suitable proxy for reconstructing the secular variation has been newly developed, which utilizes carbonate-associated phosphate (CAP) because carbonate minerals can incorporate P linearly to the total P content of the coexisting solution in equilibrium. There are two types of methods to analyze the CAP contents: a weak acid leaching method and an in-situ analysis, respectively. The former is often used for CAP analysis (e.g., Dodd et al., 2021; Ingalls et al., 2022); 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 P through geologic time. We then conducted an in situ analysis of the CAP contents of carbonate minerals from Neoarchean to Phanerozoic using LA-ICP-MS/MS.
Textures such as ooids, micritic structures, isopachous cement, and spar cement were observed in our thin sections, and we conducted in-situ analyses for each texture. The observed CAP values within each sample, expressed as P/ (Ca + Mg) mmol/mol, varied systematically with the textural classification. Thus, the CAP values of the points that preserved the primary textures derived from seawater precipitates were used for the estimation. 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 ooid (Ingalls et al., 2022). The CAP values of the Archean carbonate minerals obtained in this study range from 0.03 to 0.09, whereas those by the acid leaching method range from 0.01 to 0.47 (Ingalls et al., 2022). The CAP values in most Proterozoic carbonates are higher than those of modern ooids, and gradually increase from 0.03–0.11 at 1.8 Ga Duck Creek Fm. to 0.49–0.88 at 0.75 Ga Backlundtoppen Fm.
The obtained variation in the CAP values with different ages was interpreted as the secular variation of seawater P contents through geologic time. The higher CAP values in the Precambrian than the modern values are inconsistent with the “Phosphate Crisis” hypothesis (Bjerrum & Canfield, 2002; Jones et al., 2015), which assumes that severe depletion in seawater P suppressed the Precambrian biosphere. The gradual increase in seawater P content in the Proterozoic is coincident with the rise in carbon isotope values of carbonate, namely the growing biomass. The concomitant rise indicates that the P influx into seawater should always be larger than the biomass consumption, suggesting that P was not a limiting nutrient of primary production in the Precambrian.
References:
Bjerrum and Canfield, 2002. Nature 417, 159–162; Dodd et al., 2021. Geochim. Cosmochim. Acta. 301, 48–69; Ingalls et al., 2022. Geophys. Res. Lett. 49, e2022GL098100; Jones et al., 2015. Geology 43, 135–138; Konhauser et al., 2007. Science 315, 1234; Planavsky et al., 2010. Nature 467, 1088–1090; Reinhard et al., 2017. Nature 541, 386–389.
Recently, a more suitable proxy for reconstructing the secular variation has been newly developed, which utilizes carbonate-associated phosphate (CAP) because carbonate minerals can incorporate P linearly to the total P content of the coexisting solution in equilibrium. There are two types of methods to analyze the CAP contents: a weak acid leaching method and an in-situ analysis, respectively. The former is often used for CAP analysis (e.g., Dodd et al., 2021; Ingalls et al., 2022); 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 P through geologic time. We then conducted an in situ analysis of the CAP contents of carbonate minerals from Neoarchean to Phanerozoic using LA-ICP-MS/MS.
Textures such as ooids, micritic structures, isopachous cement, and spar cement were observed in our thin sections, and we conducted in-situ analyses for each texture. The observed CAP values within each sample, expressed as P/ (Ca + Mg) mmol/mol, varied systematically with the textural classification. Thus, the CAP values of the points that preserved the primary textures derived from seawater precipitates were used for the estimation. 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 ooid (Ingalls et al., 2022). The CAP values of the Archean carbonate minerals obtained in this study range from 0.03 to 0.09, whereas those by the acid leaching method range from 0.01 to 0.47 (Ingalls et al., 2022). The CAP values in most Proterozoic carbonates are higher than those of modern ooids, and gradually increase from 0.03–0.11 at 1.8 Ga Duck Creek Fm. to 0.49–0.88 at 0.75 Ga Backlundtoppen Fm.
The obtained variation in the CAP values with different ages was interpreted as the secular variation of seawater P contents through geologic time. The higher CAP values in the Precambrian than the modern values are inconsistent with the “Phosphate Crisis” hypothesis (Bjerrum & Canfield, 2002; Jones et al., 2015), which assumes that severe depletion in seawater P suppressed the Precambrian biosphere. The gradual increase in seawater P content in the Proterozoic is coincident with the rise in carbon isotope values of carbonate, namely the growing biomass. The concomitant rise indicates that the P influx into seawater should always be larger than the biomass consumption, suggesting that P was not a limiting nutrient of primary production in the Precambrian.
References:
Bjerrum and Canfield, 2002. Nature 417, 159–162; Dodd et al., 2021. Geochim. Cosmochim. Acta. 301, 48–69; Ingalls et al., 2022. Geophys. Res. Lett. 49, e2022GL098100; Jones et al., 2015. Geology 43, 135–138; Konhauser et al., 2007. Science 315, 1234; Planavsky et al., 2010. Nature 467, 1088–1090; Reinhard et al., 2017. Nature 541, 386–389.