As our climate warms, scientists are looking to ancient climate events to predict future changes. One such event is the Miocene Climate Optimum (MCO), which occurred between 17 and 15 million years ago. The MCO is associated with significant volcanic activity, particularly the Columbia River Basalts in the Northwestern US, which released large amounts of CO2. This volcanic CO2 is believed to have contributed to the global warming of the time. However, a new study led by Jennifer Kasbohm from the Carnegie Science’s Earth and Planets Laboratory challenges the idea that these volcanic eruptions directly triggered the warming.
Kasbohm’s study is the first to use high-precision radiometric dating on ocean sediment cores to create a more accurate timeline of the MCO. This method, which uses uranium-lead dating of zircon crystals found in volcanic ash, allowed the team to more precisely date both the volcanic eruptions and the warming event. Surprisingly, the findings showed that warming during the MCO began around 200,000 years before the Columbia River Basalts erupted, raising questions about the previously assumed direct link between the eruptions and the warming.
While the eruptions did not initiate the warming, the study found that they coincided with the peak temperatures of the MCO. This suggests that the volcanic activity did play a role in sustaining the extreme warmth during the period, but other factors must have driven the earlier temperature rise. One possibility is that CO2 was gradually released from basalt magma trapped in the Earth’s crust, which stalled and formed “sills” before the magma could erupt. As these sills cooled, they could have emitted greenhouse gases over hundreds of thousands of years, contributing to the pre-eruption warming.
Kasbohm’s research also lends support to the idea that volcanic activity may not always follow a straightforward cause-and-effect relationship with climate events. Other studies of Large Igneous Provinces, such as those linked to the end-Permian and end-Triassic extinctions, have shown similar mismatches in timing between volcanic activity and climate effects. The study further suggests that tectonic activity, including the production of ocean crust, may have contributed additional CO2 to the atmosphere during the MCO, adding complexity to the factors driving global temperature changes.
In addition to shedding new light on the MCO, this study is significant for its technological advancements. The high-precision radiometric dating technique used on ocean sediments is a breakthrough, enabling scientists to more accurately date climate records. By comparing their results with previous estimates based on Earth’s orbital cycles, the study confirmed the reliability of astronomical models used to understand past climate patterns. This could have wide-reaching implications for future studies of geological climate records, extending well beyond the Miocene.
Kasbohm’s work also highlights the potential for further discoveries from existing ocean cores. Many cores collected over decades remain underutilized, but with the success of this study, there is renewed interest in applying these advanced dating techniques to older samples. As the ocean drilling ship JOIDES Resolution is set to be decommissioned in 2024, the study underscores the importance of continuing to analyze the data already collected.
Overall, this research marks an important step forward in understanding the complex factors that drove the MCO. While volcanic activity did contribute to the peak warming, the study’s findings indicate that earlier warming was caused by other mechanisms, emphasizing the need for continued exploration of Earth’s deep climate history as we face similar challenges today.