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[BCG06-02] Phosphorus cycles in the Precambrian and its implication for the Earth's oxygenation
Keywords:Precambrian, Phosphorus, Independent component analysis
Phosphorus (P) is a bio-essential nutrient. Its cycling in the ocean, particularly its vertical [P] profile, can serve as an indicator of biomass abundance and, consequently, oxygen production(Broecker & Peng, 1982). Although phosphorus cycling plays a crucial role in modeling the evolution of the Earth's atmosphere, its vertical distribution in the Precambrian ocean remains poorly understood.
Phosphate levels in seawater over geologic time are typically estimated using P/Fe ratios in iron-rich rocks (e.g., Bjerrum & Canfield, 2002; Jones et al., 2015; Konhauser et al., 2007; Planavsky et al., 2010), whole-rock P content in shales (Reinhard et al., 2017), and carbonate-associated phosphate (CAP), which reflects phosphate incorporated into carbonate minerals via defects (Dodd et al., 2021). While CAP provides a direct record of shallow ocean [P], the P content in banded iron formations (BIFs) and shales serve as proxies for deep-sea P reservoirs and Redfield ratio, respectively. However, distinguishing primary chemical precipitates, which preserve the ocean’s phosphate concentration at the time of deposition, from detrital and other various contaminants for BIFs and shales, remains challenging. We applied independent component analysis (ICA) to BIFs and shales to estimate the compositions of the primary chemical precipitate end-members of iron-hydroxide, specifically, P adsorbed onto the iron-hydroxides. We propose a novel approach to reconstruct vertical [P] profiles and infer biomass size in the Precambrian ocean by integrating CAP records with ICA results from BIFs and shales.
The CAP values (P/Ca+Mg, mmol/mol) in Precambrian carbonates are generally higher than their Phanerozoic counterparts (Ordovician to Devonian range: 0.03–0.09) and show a gradual increase from 0.03–0.11 in the 1.8 Ga Duck Creek Formation to 0.49–0.88 in the 0.75 Ga Backlundtoppen Formation. These records contradict the ‘Phosphate Crisis’ hypothesis (Bjerrum & Canfield, 2002; Jones et al., 2015), which suggests severe depletion of seawater P content in the Precambrian biosphere. Instead, the gradual increase in seawater P content throughout the Proterozoic coincides with rising carbonate carbon isotope values, indicating an expanding biomass. The P/Fe ratios of the iron oxide end-members extracted using ICA were also calculated. A comparison between CAP values and P/Fe ratios over geologic time allows semi-quantitative estimates of Precambrian biomass size.
References:
Broecker and Peng, 1982. Eldigio Press, 1-690; 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.
Phosphate levels in seawater over geologic time are typically estimated using P/Fe ratios in iron-rich rocks (e.g., Bjerrum & Canfield, 2002; Jones et al., 2015; Konhauser et al., 2007; Planavsky et al., 2010), whole-rock P content in shales (Reinhard et al., 2017), and carbonate-associated phosphate (CAP), which reflects phosphate incorporated into carbonate minerals via defects (Dodd et al., 2021). While CAP provides a direct record of shallow ocean [P], the P content in banded iron formations (BIFs) and shales serve as proxies for deep-sea P reservoirs and Redfield ratio, respectively. However, distinguishing primary chemical precipitates, which preserve the ocean’s phosphate concentration at the time of deposition, from detrital and other various contaminants for BIFs and shales, remains challenging. We applied independent component analysis (ICA) to BIFs and shales to estimate the compositions of the primary chemical precipitate end-members of iron-hydroxide, specifically, P adsorbed onto the iron-hydroxides. We propose a novel approach to reconstruct vertical [P] profiles and infer biomass size in the Precambrian ocean by integrating CAP records with ICA results from BIFs and shales.
The CAP values (P/Ca+Mg, mmol/mol) in Precambrian carbonates are generally higher than their Phanerozoic counterparts (Ordovician to Devonian range: 0.03–0.09) and show a gradual increase from 0.03–0.11 in the 1.8 Ga Duck Creek Formation to 0.49–0.88 in the 0.75 Ga Backlundtoppen Formation. These records contradict the ‘Phosphate Crisis’ hypothesis (Bjerrum & Canfield, 2002; Jones et al., 2015), which suggests severe depletion of seawater P content in the Precambrian biosphere. Instead, the gradual increase in seawater P content throughout the Proterozoic coincides with rising carbonate carbon isotope values, indicating an expanding biomass. The P/Fe ratios of the iron oxide end-members extracted using ICA were also calculated. A comparison between CAP values and P/Fe ratios over geologic time allows semi-quantitative estimates of Precambrian biomass size.
References:
Broecker and Peng, 1982. Eldigio Press, 1-690; 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.