Cadmium isotopic evidence for a collapse of primary productivity during the Hirnantian glaciation and mass extinction

Abstract

The Late Ordovician witnessed mass extinctions (LOME) and carbon cycle dynamics, such as Hirnantian glaciation and positive carbon isotope excursion (HICE). Model results suggested that both climatic cooling and HICE were driven by high primary productivity and increased organic carbon burial, which, however, is inconsistent with the decline in organic carbon burial revealed by sedimentary organic carbon data. The evolution of marine primary productivity is critical to reconcile these contradictions but is poorly understood. Cd isotope is widely used as a paleo-nutrient proxy for past nutrient utilization and primary productivity. Here, we present new Cd isotopic records paired with organic carbon isotope and major and trace elements covering the late Katian, Hirnantian, and early Rhuddanian stages (ca. 447-442 Ma). The seawater δ^114/110Cd values of 0.25 ‰ ± 0.13 ‰ during the late Katian and of 0.48 ‰ ± 0.16 ‰ during the early Rhuddanian, comparable to those of modern deep ocean (+ 0.25 ‰ to +0.45 ‰), may reflect near-modern nutrient utilization and productivity levels. In contrast, lower seawater δ^114/110Cd of 0.08 ‰ ± 0.07 ‰ during the Hirnantian glaciation may indicate a low primary productivity. Our new finding of low organic production and burial is consistent with geological records of low organic carbon burial rate and further supports that the Hirnantian glaciation and HICE may not be driven by increased organic carbon burial but require other factors. Low primary productivity may be caused by a collapse of primary producers, which may exert food and energy stress on marine ecosystems and likely contribute to LOME. Our work provides new insights into carbon cycle dynamics and LOME from the perspective of primary producers.