研究業績

Multiple short-term (<105 years) global warming events (“hyperthermals”) occurred throughout the early Eocene. Chemical weathering of continental silicate rocks is considered to have played a key role in recovery from these transient global warming events via consumption of excess atmospheric CO2. However, although evidence of this negative feedback has been reported for the Paleocene–Eocene Thermal Maximum (PETM), the effectiveness of chemical weathering during other transient hyperthermals remains poorly constrained. Here, we newly report early Eocene marine Os isotope records including Eocene Thermal Maximum 2 (ETM2), the I1 event, and ETM3 at Ocean Drilling Program Site 752 in the southern Indian Ocean. We demonstrate that marine 187Os/188Os increased during the hyperthermal events that were less severe than the PETM. The intensity of continental silicate weathering increased by 18%–22%, 17%–21%, and 13%–16% during ETM2, the I1 event, and ETM3, respectively, compared to the background conditions. Based on a reconsidered formulation of an Os mass-balance model, the intensity of continental silicate weathering during the PETM was reassessed to be a 34%–42% increase relative to the pre-PETM level. Thus, the continental chemical weathering flux during the Eocene hyperthermals may have been up to half as large as that of the PETM. It is noteworthy that the ordering of the changes in continental silicate weathering did not agree with the ordering of the seawater temperature rise among these events. This weathering–warming inconsistency might be attributable to (1) potential underestimation of continental silicate weathering due to the emplacement of Large Igneous Provinces (LIPs) and related basalt weathering, such as the North Atlantic Igneous Province and Siletzia in western North America, (2) changes in the background level of weathering intensity, and/or (3) a secular decrease in continental weatherability through the long-lasting early Eocene warming trend.