Weathering

A feedback between Earth surface weathering and climate is thought to be fundamental in maintaining Earth’s habitability over long timescales, but investigating this control in the modern world is difficult. The geologic record of cycles between glacial and interglacial conditions of the last 2.6 million years allows us to study weathering feedback in action. A suite of mineral, element and isotope proxies have been applied to address how weathering rates have varied over glacial cycles. Despite evidence for substantial local changes, the emerging answer at a global scale seems to be, “not very much”.

Rising levels of atmospheric carbon dioxide (CO2) are driving increases in global temperatures. Enhanced weathering of silicate rocks is a CO2 removal technology that could help mitigate anthropogenic climate change. Enhanced weathering adds powdered silicate rock to agricultural lands, accelerating natural chemical weathering, and is expected to rapidly draw down atmospheric CO2. However, differences between enhanced and natural weathering result in significant uncertainties about its potential efficacy. This article summarizes the research into enhanced weathering and the uncertainties of enhanced weathering due to the key differences with natural weathering, as well as future research directions.

Since land plants emerged from swampy coastlines over 400 million years ago, they have played a fundamental role in shaping the Earth system. Roots and associated fungi increase rock weathering rates, providing access to nutrients, while altering atmospheric CO2. As soils weather, the dissolution of primary minerals forces plants to rely on recycling and atmospheric deposition of rock-derived nutrients. Thus, for many terrestrial ecosystems, weathering ultimately constrains primary production (carbon uptake) and decomposition (carbon loss). These constraints are most acute in agricultural systems, which rely on mined fertilizer rather than the recycling of organic material to maintain production. Humans now mine similar amounts of some elements as weather out of rocks globally. This increase in supply has myriad environmental consequences.

Since land plants emerged from swampy coastlines over 400 million years ago, they have played a fundamental role in shaping the Earth system. Roots and associated fungi increase rock weathering rates, providing access to nutrients, while altering atmospheric CO2. As soils weather, the dissolution of primary minerals forces plants to rely on recycling and atmospheric deposition of rock-derived nutrients. Thus, for many terrestrial ecosystems, weathering ultimately constrains primary production (carbon uptake) and decomposition (carbon loss). These constraints are most acute in agricultural systems, which rely on mined fertilizer rather than the recycling of organic material to maintain production. Humans now mine similar amounts of some elements as weather out of rocks globally. This increase in supply has myriad environmental consequences.

Earth’s climate is buffered over long timescales by a negative feedback between atmospheric CO2 level and surface temperature. The rate of silicate weathering slows as the climate cools, causing CO2 to increase and warming the surface through the greenhouse effect. This buffering system has kept liquid water stable at Earth’s surface, except perhaps during certain ‘Snowball Earth’ episodes at the beginning and end of the Proterozoic. A similar stabilizing feedback is predicted to occur on rocky planets orbiting other stars if they share analogous properties with Earth, most importantly an adequate (but not overly large) abundance of water and a mechanism for recycling carbonate rocks into CO2. Periodic oscillations between globally glaciated and ice-free climates may occur on planets with weak stellar insolation and/or slow volcanic outgassing rates. Most silicate weathering is thought to occur on the continents today, but seafloor weathering (and reverse weathering) may have been equally important earlier in Earth’s history.

Weathering is the chemical and physical alteration of rock at the surface of the Earth, but its importance is felt well beyond the rock itself. The repercussions of weathering echo throughout the Earth sciences, from ecology to climatology, from geomorphology to geochemistry. This article outlines how weathering interacts with various geoscience disciplines across a huge range of scales, both spatial and temporal. It traces the evolution of scientific thinking about weathering and man’s impact on weathering itself—for better and for worse. Future computational, conceptual and methodological advances are set to cement weathering’s status as a central process in the Earth sciences.