Carbon dioxide (CO2) emissions and associated climate change are thought to have caused a number of widespread marine anoxia and mass extinction events in the geologic past. However, how marine ecosystems respond to different CO2 emission patterns remains an important unresolved question. The geologic records of the Permian Period, which witnessed two mass extinctions associated with volcanic eruption (thus CO2 emissions) but with vastly different biological responses, provide a unique window to address this issue. Here, we present a long-term uranium isotope (8238U) record using marine limestones covering the latest Early Permian through Middle to Late Permian. The 8238U values show two episodes of low values in the middle Capitanian and late Changhsingian, indicating two periods of expansion of marine anoxia during the Permian Period. We use a uranium isotope mass balance model to quantify the anoxic seafloor areas, and we further use a carbon cycle model (LOSCAR, Long-term Ocean Sediment Carbon Reservoir) based on observed 813C of marine carbonates, sea surface temperature records, and ocean surface pH data to quantify the carbon emission rates across the two biocrises. The uranium isotope mass balance model reveals that the anoxic seafloor area is three times larger during the end-Permian mass extinction (EPME, covering -35% of the seafloor areas) than that during the end-Guadalupian event (EGE, covering -10% of the seafloor areas). The CO2 emission rates across the two biocrises modeled from the LOSCAR model show that the carbon emission rate across the EPME was at least five times faster than that during the EGE, with the best-fit 813C values of the input sources ranging from -8 to -12%o, indicating a predominant volcanic CO2 source during the EPME, and close to -25%o during the EGE. Comparing model results and observed proxy data led to the suggestion that the more severe ecosystem responses during the EPME, including higher extinction rate and larger extent of seafloor anoxia, are closely linked to the faster carbon emission rates compared to the EGE.(c) 2022 Elsevier B.V. All rights reserved.