Seminar by Dr. Megumi Chikamoto

Utah State University / University of Hawaii

Controlling calcium carbonate preservations during the Paleocene-Eocene Thermal Maximum

Megumi O. Chikamoto (University of Hawaii), Yoshi Chikamoto (Utah State University), Akira Oka (University of Tokyo), and Axel Timmermann (Pusan National University)

Ocean biogeochemistry in paleoclimate history exhibits abrupt changes in calcium carbonate preservations after CO2 emissions. During the Paleocene-Eocene Thermal Maximum (PETM) period, for example, the calcium carbonate (CaCO3) content was dropped abruptly after the CO2 emissions corresponding to the atmospheric CO2 level of 3000–4600 ppmv.  This decrease in CaCO3 could be explained by ocean acidification because laboratory experiments show that more acid water due to the CO2 injection enhances the dissolution of CaCO3 shell of phytoplanktons. From the microscope analysis for sediments in the North Atlantic, however, there is no evidence of CaCO3 dissolution after the CO2 emissions at the onset of PETM. This observational evidence arises a question how the CaCO3 preservations are controlling by biological production in association with the CO2 emissions. 

To evaluate a role of biological production in CaCO3 preservations, we conduct the PETM simulations using an earth system model of intermediate complexity. In response to the CO2 emissions, the atmospheric CO2 rise initially reduces and then recovers biological production of CaCO3. This is because the surface nutrient declines via strong ocean stratification and turns to increase by the regeneration of meridional overturning circulation. This variability is obtained only in the experiments under the Eocene topography, in which the locations of deepwater formation switches from North to South in relation to higher CO2. The same experiments, but under the modern topography, on the other hand, present relatively stable meridional overturning circulation, preventing large CaCO3 variability. This result is also consistent with the variations in proxy records on interglacial warming of the late Pleistocene. These idealized experiments suggest that the drastic drop in CaCO3 preservation on the PETM is affected by the reduction in biological productivity that depends on the Paleogeographic conditions.

Jasmine - Seminar Room (1010), 10th floor, M Building