The bioavailability of nitrogen and phosphorus is tightly coupled in the modern ocean, a concept known as the ’nitrostat’. Phosphorus is considered the ultimate limiting nutrient on geological timescales, while nitrogen supply is biologically modulated via nitrogen fixation and denitrification. This balance maintains relatively constant N:P ratios in seawater. However, the strong redox dependence of both N and P cycling, as well as the dependence on specific enzymes, suggest that the balance of the marine ‘nitrostat’ may have evolved over Earth’s history in response to oxygenation and biological innovation. Here, we review what is known about N and P biogeochemistry across the Great Oxidation Event (GOE) and discuss cause-effect relationships between the evolving N:P balance and primary production. A key remaining uncertainty is when exactly life transitioned from energy- to nutrient-limitation. However, it is likely that a marine ‘nitrostat’ was established at least locally or temporarily from the Neoarchean onwards. This implies increasing biological control over water chemistry, which may in turn have contributed to stabilizing O2-producing ecosystems and thus acted as a positive feedback for the GOE.
Bastnäsite is a major rare earth element (REE) host in a range of geological environments and typically contains low, yet measurable concentrations of U, Th, and Pb. Despite this, bastnäsite is seldom used as a robust geochronometer because of typically low U content, variable Th/U ratios, compositional heterogeneity, and ill-constrained closure temperatures. Here, coupled U–Pb and Th–Pb geochronology is applied to texturally and compositionally distinct bastnäsites from Oak Dam West, South Australia, a Mesoproterozoic iron oxide-copper–gold system. The variations of composition and texture hint at formation during distinct geological events. Complementary hematite U–Pb geochronology provides an independent framework to assess reliability of bastnäsite ages. Millimetre- to centimetre-sized pods of fine-grained low-Th bastnäsite from deep low-sulphide hematite breccias yield U–Pb ages around 1590 Ma, consistent with coexisting hematite. High-Th bastnäsite from shallow levels of the orebody occur as 50–400 μm rosette-like aggregates with Th–Pb ages between 718 and 681 Ma. These younger ages are interpreted to record a fluid-driven event that remobilised and reprecipitated REE and U. Rosette bastnäsite is also distinguished by positive Ce anomalies and associations with oxidised alteration mineral assemblages. Petrographic, geochemical, and geochronological evidence support a model in which oxidised fluids, plausibly linked to Cryogenian subglacial meltwater infiltration in the upper orebody along reactivated structures promoting dissolution–reprecipitation of earlier REE mineralisation. Coupled U–Pb and Th–Pb bastnäsite geochronology is a powerful tool to resolve multistage histories in REE-bearing ore systems and highlights the potential for Snowball Earth-related fluid circulation to drive oxidative metal remobilisation in buried ores.
Retrodeformation of Ediacaran fossils currently relies on the assumption that basal discs—a morphological feature common to diverse taxa—were originally circular. Despite their widespread application as tectonic strain markers, this foundational premise remains unverified. This study reassesses the reliability of the assumption of initial circularity by analysing a population of 287 basal discs from the E. M. Coombs Surface in Newfoundland, Canada. Using geometric measurements and statistical analyses, three hypotheses regarding the pre-deformation shape of the discs were tested: uniform circularity, uniform ellipticity, and heterogeneous ellipticity. Results support the hypothesis that, prior to deformation, the studied fossil disc population exhibited heterogeneity in ellipticity, ranging from circular to eccentric morphologies. These findings demonstrate that individual basal discs are unreliable tectonic strain markers, highlighting the need for alternative methods. Practical guidelines are proposed to circumvent this issue and improve the robustness of deformation assessments in Ediacaran fossil assemblages.
The ability of animals to biomineralize was one of the fundamental biological innovations of the Ediacaran, changing eco-evolutionary relationships, the carbon cycle and sedimentary styles. However, environmental factors that may have influenced the early radiation of skeletal fauna remain poorly understood. We present multiproxy data (carbonate-hosted Li and Sr isotopes) from the late Ediacaran Tamengo (Brazil) and Dengying formations (China), which capture the early record of animal biomineralization. We aim to investigate the role of continental weathering on changes in seawater chemistry, which may have primed environments for biomineralizers. Our results show that these basins were strongly affected by regional runoff during low sea level, while highstand intervals favoured mixing with global seawater. Deviations of lithium and strontium isotope compositions from coeval seawater values record regional weathering processes that reveal latitudinal climatic controls. Mid- to high-latitude basins show low δ7Li (<10 ‰) values and radiogenic 87Sr/86Sr during semi-restricted phases, indicating congruent weathering under cooler, drier conditions. Low-latitude basins record high δ7Li (>20 ‰) values and radiogenic 87Sr/86Sr under similar sea-level settings, reflecting incongruent weathering and soil formation in warmer, more humid climates. Across all paleolatitudes, congruent weathering regimes were closely coupled to seawater chemistry: low δ7Li values are concomitant with or precede strata that display elevated Sr/Ca and δ13C values. This suggests that an increased flux of ions via weathering could have caused elevated seawater alkalinity, thereby stimulating bioproductivity and deoxygenation. The tubular skeletal metazoan Cloudina colonized the substrate during episodes of congruent weathering, which may indicate that chemical weathering played an important role in the dispersion of the earliest biomineralizers.
