Enhanced clay formation key in sustaining the Middle Eocene Climatic Optimum

Abstract

The Middle Eocene Climatic Optimum (around 40 million years ago) was a roughly 400,000-year-long global warming phase associated with an increase in atmospheric CO2 concentrations and deep-ocean acidification that interrupted the Eocene’s long-term cooling trend. The unusually long duration, compared with early Eocene global warming phases, is puzzling as temperature-dependent silicate weathering should have provided a negative feedback, drawing down CO2 over this timescale. Here we investigate silicate weathering during this climate warming event by measuring lithium isotope ratios (reported as δ7Li), which are a tracer for silicate weathering processes, from a suite of open-ocean carbonate-rich sediments. We find a positive δ7Li excursion—the only one identified for a warming event so far —of ~3‰. Box model simulations support this signal to reflect a global shift from congruent weathering, with secondary mineral dissolution, to incongruent weathering, with secondary mineral formation. We surmise that, before the climatic optimum, there was considerable soil shielding of the continents. An increase in continental volcanism initiated the warming event, but it was sustained by an increase in clay formation, which sequestered carbonate-forming cations, short-circuiting the carbonate–silicate cycle. Clay mineral dynamics may play an important role in the carbon cycle for climatic events occurring over intermediate (i.e., 100,000 year) timeframes.

Astronomical Pacing of Middle Eocene Sea‐Level Fluctuations: Inferences From Shallow‐Water Carbonate Ramp Deposits

“…Given a drop in surface ocean pH of ∼0.2 during the MECO (Henehan et al., 2020), the spectacular skeletal preservation of many PB fossils (Brachert et al., 2022; Merle, 2008) may therefore allow us to examine the impact on epicontinental shelf ecosystems. In particular, currently it is unclear whether the MECO surface ocean was buffered at constant carbonate saturation state by silicate weathering (Henehan et al., 2020), or whether a reduced weathering feedback (Krause et al., 2023; van der Ploeg et al., 2018) meant that this drop in ocean pH also coincided with a drop in ocean carbonate saturation, and thereby “true” ocean acidification (Hönisch et al., 2012). Future studies of calcification patterns in the PB over the MECO could therefore inform as to the dynamics of the silicate weathering feedback over this enigmatic event, and in doing so help to constrain atmospheric CO 2 levels at this time (Henehan et al., 2020).…”

Assessment of Chalk as an Archive for the Lithium Isotope Composition of Seawater

“…Clay minerals are a sink for Li whereby the light Li isotope ( 6 Li) is preferentially incorporated, driving river water to isotopically heavier compositions, which is a typical signal of incongruent weathering (e.g., Bouchez et al., 2013; Dellinger et al., 2015; Pogge von Strandmann & Henderson, 2015). If no or only little secondary clay mineral formation occurs, the Li isotope composition of the river water reflects the composition of the source rock (lower δ 7 Li values), which is taken as a typical signal of congruent weathering (e.g., Dellinger et al., 2015; Krause et al., 2023).…”