Oxygenation of the Baltoscandian shelf linked to Ordovician biodiversification

The Great Ordovician Biodiversification Event (GOBE) saw substantial increases in marine biodiversity and ecosystem complexity, but the drivers of the most rapid pulses of taxonomic diversification remain debated. Hypotheses implicate climate cooling, oxygenation, tectonics and sea-level change. Although models and some geochemical data suggest rising oxygen levels could have facilitated biodiversification, direct marine redox proxy data with adequate spatio-temporal resolution for the Ordovician are sparse. This study investigates whether progressive oxygenation of shelf seas was linked to biodiversification by generating a high-resolution, multi-site iodine-to-calcium (I/Ca) record from Early–Middle Ordovician carbonates across the Baltoscandian shelf, aiming to reconstruct local to regional redox evolution and compare it with biodiversity and environmental trends.

Previous work documents long-term Ordovician climate cooling inferred from conodont oxygen isotopes and other proxies, which may have created more hospitable marine conditions and enhanced oxygen solubility. Global compilations and models show co-variation between atmospheric O2 and biodiversity through the Phanerozoic, and studies have proposed oxygenation as a driver of the GOBE. However, early Paleozoic atmospheric O2 levels remain uncertain and often discordant among models and empirical datasets. Traditional redox proxies frequently capture only strongly reducing conditions and global signals, limiting their ability to resolve subtle, local redox gradients on shelves. Iodine in carbonates has emerged as a sensitive, locally informative redox proxy, with I/Ca thresholds from modern systems indicating >2.6 µmol mol−1 for well-oxygenated conditions and <1.5 µmol mol−1 for oxygen-limited settings, though extrapolation beyond local-regional scales requires caution. Against this background, a detailed, region-wide I/Ca dataset from Baltoscandia can test proposed links among climate cooling, ocean circulation, shelf ventilation and biodiversification.

Field sampling was conducted at multiple localities across the Baltoscandian shelf representing a shore-to-basin transect through Early–Middle Ordovician strata. Representative subsamples were selected for thin-section petrography (screening) and powdered for geochemistry by low-speed drilling of fresh, macroscopically well-preserved carbonate using diamond-tipped microdrill bits cleaned with ethanol and dried between samples. Between 2 and 4 mg of carbonate powder per sample was dissolved in trace-metal grade 3% HNO3 and diluted with ultrapure 2% HNO3 to a matrix of 50 ± 5 ppm [Ca]+[Mg]. Iodine-to-(Ca+Mg) analyses were performed on an Agilent 7500cs ICP-MS within 2 hours of acidification to minimize iodine loss. The precision for 127I is typically better than 2%; blank standard deviation is typically <300 cps, and sensitivity for a 1 ppb standard is ~60,000–120,000 cps depending on daily setup. Calibration standards were prepared by serial dilution of a 10 ppm iodide ICP-MS standard to match ~50 ppm Ca+Mg matrix. Long-term accuracy was monitored via replicate analyses of reference materials (KL1–2, KL1–4) and is ±0.5 µmol mol−1. Petrographic screening helped identify potential diagenetic overprint. For spatio-temporal reconstructions, I/Ca series were subdivided by biostratigraphy, and average values were calculated for the shortest correlatable intervals. Maps were generated in Esri ArcGIS Pro 2.6 using Inverse Distance Weighted interpolation (Geostatistical Analyst; Power = 2.5). Localities lacking relevant strata were omitted from individual runs; paleotopography was not imposed due to data limitations. All datasets were tied to biozonation and the Geologic Time Scale 2020 for stratigraphic alignment.

• I/Ca data record progressive oxygenation across the Baltoscandian shelf from the late Floian through the middle Darriwilian, with spatial and temporal coherence among sections.
• Basal Floian I/Ca spans ~2–5 µmol mol−1, indicating heterogeneous oxygenation; the subsequent Floian shows low values (~1–2), followed by a pronounced rise to a peak (~2.5–14.5) at the Floian–Dapingian boundary, coincident with regional carbonate expansion and relatively low sea level.
• Early Dapingian I/Ca declines region-wide, then increases through the Dapingian; sustained high values (maxima ~5–7.5 µmol mol−1) persist from upper Dapingian through middle Darriwilian (~5 Myr), especially on the mid-shelf, indicating prolonged well-oxygenated conditions (>2.6 µmol mol−1 threshold).
• A temporary I/Ca drop occurs near the Lenodus variabilis–Yangtzeplacognathus crassus conodont zonal boundary, coinciding with the Swedish “Täljsten” interval characterized by microbial fabrics and opportunistic macrofauna, consistent with locally reducing, restricted conditions.
• Upper Darriwilian strata return to relatively low I/Ca and oxygen-limited conditions; outer shelf black shale deposition resumes as sea level rises.
• Map projections show oxygen-rich waters encroaching onto Baltica from the Iapetus-facing margin, migrating generally south- and westward (modern coordinates), implicating broader-scale oceanographic forcing and currents as sources.
• I/Ca trends co-vary with regional and global biodiversity curves (e.g., graptolites, brachiopods, ostracods, conodonts), with pronounced biodiversity increases during intervals of elevated I/Ca, and inverse relation to sea-surface temperature proxies, supporting a role for cooling-enhanced oxygenation in catalyzing biodiversification.
• Peak I/Ca at the Floian–Dapingian boundary aligns with regionally extensive hardgrounds exhibiting exceptional bioerosion, suggesting deeper oxygen penetration into the seafloor and expansion of ecospace and infaunal behavior.

The multi-site I/Ca record demonstrates that ventilation and oxygenation of the Baltoscandian shelf progressed through the late Floian to middle Darriwilian, aligning with major pulses in marine biodiversification. The spatial pattern—oxygen-rich waters advancing from the Iapetus side—implies that regional redox evolution was coupled to broader oceanographic circulation rather than solely to local shallowing. The coincidence of sustained high I/Ca with a marked sea-level fall, cooler sea-surface temperatures, and the proliferation of skeletal fauna suggests synergistic drivers: climate cooling increased oxygen solubility and strengthened circulation, while lowered sea level enhanced mixing and shelf ventilation. These changes expanded habitable ecospace, reduced physiological stress from hypoxia, and enabled more energetically demanding lifestyles, fostering taxonomic diversification across benthic, nektic, and planktic clades. Temporary redox deterioration (e.g., Täljsten interval) and the late Darriwilian decline in I/Ca highlight dynamism and regional heterogeneity, consistent with global records showing non-synchronous redox evolution. Overall, the results substantiate oxygen availability as a key control on Ordovician ecosystem restructuring in Baltoscandia, linked to climate and circulation changes.

An extensive, biostratigraphically constrained I/Ca dataset from Baltoscandian carbonates reveals progressive shelf oxygenation that peaked from the upper Dapingian to middle Darriwilian and closely tracked major biodiversification pulses. Oxygen-rich waters sourced from the Iapetus Ocean, combined with climate cooling and sea-level fall, ventilated the shelf, expanded ecospace, and facilitated ecosystem reorganization. These findings strengthen the mechanistic link between climate-driven ocean ventilation and early Paleozoic biodiversity increases. Future work should (i) develop comparable I/Ca transects in other paleocontinents to test global synchronicity and sources, (ii) integrate multi-proxy redox and circulation models to resolve mechanisms and thresholds, and (iii) refine temporal resolution across critical intervals (e.g., Täljsten) to capture short-term redox dynamics and biotic responses.

• I/Ca is a local-to-regional redox proxy; extrapolation to global conditions is inappropriate without complementary datasets.
• Potential diagenetic alteration may affect specific horizons despite petrographic screening.
• Lack of directly comparable Ordovician I/Ca datasets from other regions limits global contextualization.
• Mapping reconstructions used IDW interpolation without incorporating paleotopography or barriers due to data limitations, potentially smoothing spatial heterogeneity.
• Thresholds for oxygenation are calibrated from modern systems and may not map perfectly onto ancient conditions.
• Global ocean did not evolve uniformly; observed patterns may be region-specific and time-transgressive.