The kaolinite shuttle links the Great Oxidation and Lomagundi events

The study investigates how enhanced terrestrial weathering after the Great Oxidation Event (GOE) could have driven a prolonged perturbation to the global phosphorus (P) cycle that fueled high marine primary productivity during the Lomagundi Event (LE), the most pronounced, long-lived positive carbon isotope excursion in Earth history. Because organic carbon burial enriches marine dissolved inorganic carbon in 13C, the sustained positive δ13Ccarb excursion implies increased primary productivity and nutrient supply. The authors posit that P was the ultimate limiting nutrient and that explaining >100 Myr of elevated productivity requires a sustained increase in bioavailable P delivery to the oceans. They hypothesize that kaolinite produced by intense acidic weathering post-GOE acted as a shuttle: efficiently adsorbing P in acidic freshwater and releasing it upon entering marine conditions, thereby linking increased continental weathering to augmented coastal primary productivity during the LE.

Prior explanations for the LE focus on increased P solubilization due to acid rock drainage from oxidation of sulfide minerals (e.g., pyrite) and siderite, elevated atmospheric CO2, and intensified weathering of nutrient-bearing minerals (notably apatite), all processes expected after the GOE. In modern systems, most riverine P is transported in particulate form (e.g., adsorbed to Fe(III)- and Al(III)-oxyhydroxides or clays, or organically complexed), with much of it settling in estuaries. During the Paleoproterozoic, ferric oxyhydroxides may have been solubilized in sulfide- and siderite-rich soils, making clays a potentially important vector for P delivery. Kaolinite and halloysite dominate highly weathered soils today, while smectites/illites are typical of moderate weathering and dissolve more readily under acidic conditions. Geological context suggests increased kaolinite formation post-GOE due to expanded continental area, warm climates following Huronian glaciations, and intense acid fluxes from pyrite/siderite oxidation. Evidence of kaolinite occurs in Lomagundi-age shales (e.g., Francevillian Group) and in Paleoproterozoic paleosols that show Al-richness consistent with kaolinite formation, although burial metamorphism often alters original clay assemblages.

• Paleosol geochemical analysis: Compiled a database of eight relatively complete paleosols (2.76–1.70 Ga) and additional records to assess weathering intensity using a CNM-A-K ternary framework (CaO+Na2O+MgO; Al2O3; K2O, with corrections for non-silicate Ca and Mg and K-metasomatism). Assessed Al-richness as a proxy for kaolinite abundance across time bins (pre-GOE, GOE/LE, post-LE). Calculated paleolatitudes to assess potential bias.
• Mineralogical characterization: X-ray diffraction (Bruker D8 Advance, Rietveld analysis) on mudstones from LST-12 (Francevillian Group, Gabon) and OPH (Zaonega Formation, Russia) to evaluate presence of kaolinite and other clays.
• Equilibrium adsorption experiments: Measured phosphate adsorption onto kaolinite, illite, and montmorillonite at phosphate concentrations of 1–30 µM under varying pH (4, 6, 8) and ionic strength (0.01 M and 0.56 M NaCl) conditions. Prepared 1 g/L clay suspensions; monitored pH; analyzed phosphate by ICP-MS.
• Dynamic adsorption/desorption experiments: Simulated freshwater (pH 4, IS 0.01 M) then switched to marine conditions (pH 8, IS 0.56 M) in a stirred beaker with 1 g/L kaolinite and ~5 µM phosphate to track time-dependent desorption over 3 days. Conducted analogous experiments with hematite (Fe2O3) and gibbsite (Al(OH)3) as comparisons.
• Cyanobacterial growth experiments: Cultured Synechococcus sp. PCC7002 in modified A+ medium with limiting phosphate (5 µM). After reaching stationary phase, split into triplicates and amended two with P-bearing kaolinite at 1 g/L and 2 g/L; monitored growth via OD750 and chlorophyll-a, converting to cell density via established calibration. Growth tracked to day 14.
• Data and code management: Data deposited at Mendeley Data; code archived on Zenodo.

• Geological evidence: Kaolinite occurs in Lomagundi-age shales (Francevillian Group) and tentatively in Zaonega Formation shales despite metamorphism. Paleosol records show Al-richness peaking between 2.4–2.0 Ga (GOE/LE), implying widespread kaolinite formation and intense acidic weathering.
• Adsorption behavior: Kaolinite exhibits strong, pH-dependent P adsorption under acidic freshwater conditions due to positively charged sites (pHPZNC ~4.7) and bidentate mononuclear surface complexation. Under marine conditions (higher pH and ionic strength), adsorption capacity drops substantially.
• Dynamic experiments: At ~5 µM initial phosphate, ~95% of P adsorbed onto kaolinite at pH 4, IS 0.01 M. Upon transition to pH 8, IS 0.56 M, 20% of the pre-adsorbed P desorbed into solution over 3 days. In contrast, hematite and gibbsite adsorbed ~100% of P in freshwater but showed negligible P release under marine conditions.
• Comparison with other clays: Illite and montmorillonite can also shuttle P from rivers to oceans due to changing aqueous conditions, but release magnitudes are lower than for kaolinite, highlighting kaolinite's superior P-shuttling capacity.
• Bioavailability and productivity: In continuous culture of Synechococcus sp. PCC7002, without clay input, cell density declined to ~5×10^6 cells/mL (55% of initial) by day 14. With 1 g/L P-bearing kaolinite, final density was ~8×10^6 cells/mL (97% of initial). With 2 g/L kaolinite, density reached ~1.2×10^7 cells/mL (139% of initial), more than twice the blank, demonstrating that desorbed P is bioavailable and enhances primary productivity.
• Conceptual outcome: A kaolinite-mediated P shuttle introduces a nonlinear increase in P fluxes to the ocean with intensified terrestrial weathering, providing a mechanism to sustain elevated productivity during the LE.

The findings support a model in which intense acidic weathering after the GOE produced abundant kaolinite capable of adsorbing phosphate in rivers and releasing it upon entry to marine environments, thereby enhancing nearshore bioavailable P and primary productivity. This kaolinite shuttle likely acted alongside other P-fertilization mechanisms, including diagenetic release from sediments and reductive dissolution of ferric oxyhydroxides below a redoxcline with upwelling return of P. While smectites and illite could contribute, kaolinite's higher release under marine conditions makes it particularly effective. The model explains the sustained high δ13Ccarb values during the LE via increased organic carbon burial driven by P-fueled productivity. The shuttle concept may also apply to other major carbon cycle perturbations, such as the PETM, where evidence indicates spikes in kaolinite formation and enhanced weathering, potentially promoting productivity and facilitating carbon cycle recovery. A key implication is nonlinearity in the P cycle response to weathering intensity: once kaolinite-rich weathering regimes become widespread, P delivery to oceans can increase disproportionately, helping explain the uniqueness of the Lomagundi Event.

This study introduces and substantiates the kaolinite phosphorus shuttle as a mechanistic link between intensified post-GOE weathering and the prolonged productivity and organic carbon burial that define the Lomagundi Event. Empirical adsorption/desorption experiments and cyanobacterial growth assays show that kaolinite efficiently captures P in acidic freshwater and releases a bioavailable fraction under marine conditions, enhancing nearshore primary productivity. Geological and geochemical evidence indicates widespread kaolinite formation during 2.4–2.0 Ga. The proposed shuttle provides a non-linear pathway for boosting P fluxes to the ocean during intervals of intense weathering and may operate during other climatic and biogeochemical upheavals (e.g., PETM). Future research should focus on quantifying the spatial-temporal extent of kaolinite formation in deep time despite diagenetic overprints, integrating sedimentary P cycling feedbacks, and constraining the relative contributions of different particulate phases to P delivery across diverse paleoenvironmental settings.

• Preservation bias: Burial metamorphism and diagenetic transformation of clays hinder accurate quantification of kaolinite abundance in the rock record, making it difficult to estimate how common kaolinite was globally.
• Proxy constraints: Al-richness in paleosols serves as an indirect proxy for kaolinite and can be influenced by processes like K-metasomatism (addressed by corrections) and diagenetic chlorite formation.
• Partial mechanism: Kaolinite shuttling is not the sole pathway for bioavailable P; sedimentary diagenesis and redox-dependent Fe cycling can also release P, complicating partitioning of sources.
• Experimental scope: Laboratory conditions (specific pH, ionic strength, and concentrations) simplify natural variability in estuarine mixing, residence times, flocculation, and organic complexation that affect P desorption and bioavailability.
• Marine modulation: Changes in terrestrial P inputs are modulated by marine P cycling (e.g., Fe-P coupling, upwelling), which may vary regionally and temporally and are not fully constrained here.