RISING FROM THE ASHES: LINKING ANDEAN VOLCANISM, DIATOMS, AND MARINE MAMMAL DIVERSITY AND EXTINCTION WITH THE LATE MIOCENE COOLING EVENT

The Late Miocene Cooling event (LMC, 7.6 to 5.4 Ma) marks a short-lived but significant drop in global temperatures that has been noted in marine and terrestrial records, foreshadowing the onset of Plio-Pleistocene bipolar glaciation. The cause of this event is unclear but is thought to be related to a dramatic drop in CO₂ that was associated with the extinction of a significant number of marine megafauna and the establishment of modern marine mammal communities. On land, this event is associated with an increase in the global abundance of C₄ grasses and expansion of arid conditions in terrestrial environments. Chemical weathering of Ca-silicate rocks and tectonic degassing are some of the primary hypotheses to explain global Cenozoic cooling via CO₂ drawdown, with the Himalayas and Tibetan Plateau often identified as the most likely source material. Here, we propose an alternative hypothesis, that vigorous volcanism from the Andes of western South America, enhanced the biological pump though ocean fertilization triggering global cooling during the LMC.

We compiled an exhaustive dataset of >4000 magmatic ages from the Central Andes and identify a significant increase in frequency of volcanic events from 9 to 5 Ma with a peak at 7.0 Ma. The role of volcanism on global climate remains debated but is most often argued to contribute to global warming through CO₂ release. However, active Andean volcanism and mountain building in the Late Miocene would also have fertilized marine surface waters with silica and iron, enhancing diatom productivity and intensifying the biological pump, significantly contributing to carbon burial and CO₂ drawdown in the Cenozoic. Comparison of volcanic activity in the Central Andes with benthic and planktic isotopic records (δ¹³C, δ¹⁸O), diatom abundance data, and marine sediment chemical alteration indices from deep sea cores show a strong correlation, supporting a connection between Cenozoic Andean volcanism and major climatic and biotic events at the LMC. We also note a correlation between Andean volcanism, algal blooms, and extinction suggesting a feedback relationship between volcanism, biodiversity, and the carbon cycle. We conclude that volcanism is a key driver of climate change and biotic events over geological times through its ability to contribute both positively and negatively to global temperatures.