Constraining Silicate Weathering During the Middle Eocene Climatic Optimum (MECO) Using the Ge/Si Ratio of Siliceous Microfossils

Silicate weathering is a critical component of the climate system, acting as a stabilizing feedback to atmospheric CO2 levels. During past climate perturbations, silicate weathering is thought to be the main driver of climatic recovery. However, during the Middle Eocene Climatic Optimum (MECO, ~40 Ma), this feedback apparently failed to stabilize climate over the expected timescales, leading to a prolonged state of warming that lasted much longer than comparable transient events. We aim to characterize the behavior of silicate weathering during the MECO by performing germanium/silicon (Ge/Si) analysis on isolated siliceous microfossils collected during IODP Expedition 371 at Site U1511, located on the Tasman Abyssal Plain. Germanium is found in trace amounts in siliceous microfossils by substituting for silicon due to its similar chemical properties. The lack of discrimination against Ge during uptake results in the Ge/Si ratio reflecting that of the surrounding seawater. The modern open ocean Ge/Si (0.7 μmol/mol) is controlled by two primary fluxes: continental silicate weathering and deep-sea hydrothermal fluids. These two sources have pronounced differences in their Ge/Si, with continental weathering products having a relatively low Ge/Si of approximately 0.6 μmol/mol and hydrothermal fluids having a relatively high Ge/Si of 8 to 14 μmol/mol. We will use how Ge/Si values changed across the MECO as a paleo-weathering proxy as an increase in continental weathering relative to hydrothermal input would result in a decrease in seawater Ge/Si while a relative decrease would result in higher Ge/Si ratios. Alternatively, there might be no change in the Ge/Si, indicating no changes in silicate weathering. By determining what changes, if any, in silicate weathering occurred during the MECO, we will confirm or reject previously proposed hypotheses that aim to explain the discrepancy between the proxy evidence and model predictions of the MECO. This study will deepen our understanding of the response of silicate weathering to warming events, providing context to how the Earth will respond to anthropogenic CO2 emissions in the future.