Rocks containing ancient organic carbon are quietly releasing CO₂ as they weather, and this process could be a major but overlooked feedback that amplifies global warming for thousands of years. Climate models currently ignore this mechanism because the standard technique for measuring rock organic carbon oxidation—radiocarbon dating—cannot see beyond 60,000 years. This leaves a critical blind spot: Earth’s history includes several abrupt warming events tens of millions of years ago, and we have no way to test whether rock-derived CO₂ helped drive and sustain them. Without that knowledge, projections of future warming may be systematically underestimating the planet’s sensitivity to CO₂. This project will develop a new tool based on fossil lipids—stable organic molecules that survive in rocks and sediments for millions of years. By measuring these lipids in modern weathering profiles, rivers, and marine sediments, the team will calibrate a proxy that can quantify CO₂ release from rock oxidation during past warming events. They will then apply it to three ancient hyperthermals of different magnitudes. If successful, the work will reveal whether rock organic carbon oxidation is a universal, temperature-sensitive feedback that has operated during every major warming event. That would force climate models to incorporate a process they currently ignore, potentially revising upward the projected duration and intensity of anthropogenic warming. This is fundamental science with no immediate practical application, but understanding this feedback could reshape how we assess long-term climate risk—much as discovering the permafrost carbon feedback did a decade ago.
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The magnitude of future anthropogenic warming is uncertain and will depend upon the strength of different carbon cycle feedback mechanisms. These are processes in the Earth system that alter the concentration of atmospheric CO2 and can either amplify warming (a positive feedback) or reduce warming (a negative feedback). Future climate change projections suggest that the combined effect of known climate feedback mechanisms is to amplify CO2-driven global warming. However, climate models are blind to the ‘unknown unknowns’ - these are the things we know little about but have the potential to take future climate into unimagined directions. The geological record captures the response of the climate system to all feedbacks in operation, including those we don’t know about. Earth’s history includes several short-lived warming events (‘hyperthermals’) that occurred tens of millions of years ago and can provide unique insights into the unknown feedbacks that may operate in the future. Exposure of rock organic carbon at the Earth’s surface can release carbon dioxide (CO2) and may have been an important carbon cycle feedback during hyperthermals, potentially enhancing and prolonging global warming. However, this feedback is not included in state-of-the-art Earth system models. We hypothesise that rock organic carbon oxidation is a universal positive feedback mechanism associated with rapid warming events and is capable of prolonging global warming for thousands of years. However, the lack of empirical data means that our knowledge of how this feedback operates during past warming events remains a major gap in our understanding, with potentially important consequences for the magnitude of future global warmth. Crucially, the techniques that have been developed to measure rock organic carbon oxidation in the modern environment (radiocarbon) cannot be applied beyond the last 60,000 years. We will address this knowledge gap using a novel tool based on lipid analysis. Lipids are organic molecules that are essential to life and produced by all biological organisms. During burial, lipids undergo structural transformations and are transformed into more stable compounds. These so-called ‘fossil’ lipids have been used to trace input of old, reworked organic carbon into aquatic environments. However, we lack a mechanistic understanding of how ‘fossil’ lipids translate into oxidative weathering fluxes and ultimately CO2 release. To make this crucial step forward, we will (i) measure ‘fossil’ lipids in contemporary environments and (ii) develop new metrics to quantify CO2 release via rock organic oxidation. This knowledge will be applied to several hyperthermal events to quantify rock organic oxidation and CO2 release during global warming events for the first time. To develop a proxy for oxidative weathering, we will measure fossil lipids in modern shale weathering profiles and calibrate this against in-situ contemporary measurements of oxidative weathering (WP1). To enable successful application in the geological record, we will explore how the ‘fossil’ lipid signature is modified along the land-to-sea continuum by measuring lipids in modern rivers (WP2) and marine sediments (WP3). To determine whether rock organic carbon oxidation is an intrinsic feedback associated with rapid warming events, we will apply our proxies to three past warming events associated with different magnitudes of warming (WP4), helping to constrain the temperature sensitivity of this positive climate feedback mechanism. Our data-driven approach will reveal how rock OC oxidation operates during past warming events and will evaluate whether Earth system models currently underestimate CO2-driven warming in past warm climates.
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