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Suppressed upwelling during the Cambrian–Ordovician transition revealed by coupled Cr–Cd isotope systematics
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DOI：10.1016/j.gca.2026.07.010.png)
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Abstract
En 中文
Biodiversity remained at a plateau between the Cambrian Explosion and the Great Ordovician Biodiversification Event, marked by high rates of both extinction and origination. Widespread oceanic anoxia and transgression-driven upwelling of anoxic deep waters have been widely invoked to explain these biotic perturbations, yet how elevated origination rates were sustained under such environmentally stressful conditions remains poorly understood. Here we present coupled chromium (δ53Cr) and cadmium (δ114Cd) isotope data, together with trace-element records, from marine carbonates at the Cambrian–Ordovician Global Stratotype Section and Point (GSSP) at Green Point, eastern Laurentia. Across the transgressive interval, carbonates record a positive δ53Cr excursion, reflecting enhanced internal marine Cr cycling driven by expanded water-column anoxia. In contrast, a negative δ114Cd excursion records a shift in surface-water sourcing rather than increased nutrient supply or primary productivity. Persistently negative Ce anomalies indicate that surface waters remained oxygenated throughout transgression, despite intensified anoxia at depth. Comparison with coeval carbon-isotope records further suggests that upwelling, where present, was restricted to outer-shelf environments and did not significantly impact shallow surface waters. Together, these observations demonstrate that deep-water anoxia expanded during sea-level rise, but its chemical influence was largely decoupled from the surface mixed layer owing to suppressed upwelling. We propose that enhanced water-column stratification under greenhouse climate conditions limited vertical exchange, confining anoxic waters to depth while preserving ventilated and habitable shallow-marine environments. This physical decoupling provides a mechanistic explanation for how high origination rates could be maintained during intervals of recurrent anoxia, facilitating rapid ecological recovery and setting the environmental stage for the Great Ordovician Biodiversification Event.
Keywords:
Cambrian–Ordovician transition
Cadmium isotopes
Chromium isotopes
Oceanic anoxia expansion
Suppressed upwelling
Marine ecosystem resilience
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