34-1 Carbonate-associated phosphate and the lack of covariation across Paleoproterozoic carbon isotope excursions: Evidence from the Onega Basin (Russia) and Francevillian Basin (Gabon)

Session: Geobiology and Astrobiology in Modern and Ancient Environments: From Microbial Interactions to Planetary Exploration

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Marine phosphorus availability is hypothesized to regulate primary productivity, organic carbon burial (f_org), and the extent of Paleoproterozoic δ¹³C excursions. Carbonate-associated phosphate (CAP) provides a direct proxy for paleo-marine phosphate levels. This study tests the “nutrient-control” hypothesis by combining new CAP data with δ¹³C_carb, δ¹³C_org, ΔC, TOC, and isotope-mass-balance estimates of f_org at stratigraphic resolution across two different Paleoproterozoic successions: the Francevillian FB Formation (LST12 core, Gabon) and the Zaonega Formation (OnZAP cores, Russia). CAP levels are consistently high, exceeding modern carbonate values by more than an order of magnitude. However, nearest-neighbor correlations between CAP and δ¹³C_carb, δ¹³C_org, ΔC, TOC, and f_org lack statistical significance. During positive δ¹³C conditions, ΔC widens and f_org increases, whereas negative δ¹³C conditions display more variable ΔC and low f_org, despite high CAP. These patterns remain consistent regardless of δ_in, carbonate content thresholds, or the choice of pairing window. Globally, CAP data show that the Paleoproterozoic exhibits a unique combination of modest median values and an extended upper range, indicating episodes of high-phosphate pulses rather than persistent high background levels. Overall, the findings suggest that nutrient-carbon-oxygen feedbacks during the Paleoproterozoic heavily depended on the state, with phosphorus setting the upper bound on gross primary productivity. Still, the isotopic signature only appeared when export fluxes were efficient, and preservation conditions promoted long-term sequestration. CAP reflects the biosphere’s latent capacity to oxygenate the planet, not its inevitable translation in the carbon-isotope record.

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