Shallow‐Marine, Benthic Ecosystems Show Compositional Shifts in Response to the Paleocene‐Eocene Thermal Maximum (PETM) on the Adriatic Carbonate Platform.

The response of shallow‐marine ecosystems to the Paleocene‐Eocene Thermal Maximum (PETM) is understudied, and analyses that do exist typically focus on larger benthic foraminifera and coral‐algal reef mound evolution. Here we investigate the dynamics of benthic marine assemblages across the PETM in the Kozina and Čebulovica sections (Slovenia) on the Adriatic Carbonate Platform. Our results show significant ecological changes associated with the PETM with a protracted recovery into Shallow Benthic Zone (SBZ) 6. The compositional change corresponds to the reduced dominance of red algae, a turnover in the dominant larger benthic foraminifera, and increased assemblage homogeneity. These changes are not associated with a lithofacies transition, as a shift from reef mounds to a foraminiferal‐dominated carbonate platform occurs prior to the Paleocene/Eocene boundary. The investigated sections also show a transition from coral‐algal mounds to microbial mounds during SBZ 4, but the main difference between these reef types is the increased dominance of the microbes. Moreover, there is no unequivocal evidence that either reef type persisted into the Eocene in this region. Whilst the PETM is not an extinction event, except for deep‐sea benthic foraminifera, the ecological changes recorded show that the ecosystem responses at the PETM are consistent with other hyperthermal events. Plain Language Summary: Today, climate warming and its impacts on ecosystems threaten global biodiversity, but how life on Earth will respond to climate change on centennial to millennial scales is not well‐understood. To better understand the threats to global biodiversity and ecosystem services we can study similar events that have happened in the past, such as the Paleocene‐Eocene Thermal Maximum (PETM) when global temperatures rose by 5–8°C in less than ten thousand years. In our study, we tried to understand how the PETM impacted shallow‐water, ocean ecosystems from the Adriatic region. Our study found that the PETM caused significant changes in the composition of these ocean ecosystems with the loss of red algae, loss of reefs, and a change in the most common unicellular organisms called foraminifera. We also found no signs of recovery. Even though the PETM was not a mass extinction, it did cause significant impacts on ocean ecosystems. Key Points: Significant changes in benthic marine assemblages in response to the PETMPETM caused the extirpation of red algae, a turnover in larger benthic foraminifera, and a protracted recoveryDisappearance of patch reefs and a change in reef type occur prior to the PETM [ABSTRACT FROM AUTHOR]