Constraints on silicate weathering and ocean circulation during the MECO from silicon isotopes - Preprint

[Geoengineering News]

https://essopenarchive.org/doi/full/10.22541/essoar.15002760/v1

Authors: Ms. Ruchi, Dr. Michael James Henehan, Dr. Patrick Frings, Ollie Laub et al.

05 May 2026

Primary Abstract

The Middle Eocene Climate Optimum (MECO, ~40.2 Ma) is an interval of gradual warming lasting for ~300-500 kyr followed by rapid recovery that shows puzzling features such as conflict with our current understanding of the long-term carbon cycle, leading the MECO to be termed a “carbon cycle conundrum”. Excursions in osmium (Os) and lithium (Li) isotope ratios over the MECO have led to the hypothesis that the MECO arose from a reduced efficiency of the Earth’s silicate weathering feedback. However, such a mechanism is hard to reconcile with both the pace of CO2 injection and subsequent drawdown of atmospheric CO2 suggested by boron (B) isotope records. To test if the persistence of carbonate dissolution during the warming is associated with a diminished silicate weathering feedback, we generated the first silicon (Si) isotope records through the MECO. Si isotope ratios measured in surface-dwelling radiolarians suggest that the MECO was characterised by stable Earth surface weatherability. Coeval values in benthic sponge spicules point to a reorganisation of ocean circulation coinciding with peak warming and the brief CO2 rise indicated by B isotopes. Our results indicates that the silicate weathering feedback is not uniform, and its response to climatic or carbon-cycle perturbations likely varied through geological time. We suggest that ocean circulation could have played a significant role in global carbon cycling by enhancing outgassing of CO₂-rich subsurface waters, redistributing reverse weathering loci, and/or promoting sea-level rise through loss of labile ice, thereby shifting carbonate deposition onto shelves and sustaining carbonate dissolution.

Source: ESS OPEN ARCHIVE