Thematic Articles

The preservation of organic carbon (OC) in marine sediments is a fundamental control on Earth’s long-term carbon cycle and climate. Globally, less than 2% of carbon fixed by primary producers is ultimately buried, yet specific environments and geological intervals exhibit markedly enhanced preservation. These variations reflect changes in organic matter composition, mineral associations, microbial activity, geochemical conditions, temperature, and sediment transport. Planetary-scale changes in climate, tectonics, continental configuration, biological evolution, and ocean circulation have repeatedly altered these controls, promoting enhanced OC burial during key periods of Earth history. OC preservation has thus acted as an important stabilizing feedback following major carbon-cycle perturbations. Here, we examine these mechanisms and their significance through time.

The ocean’s buffering capacity, or alkalinity, regulates the amount of atmospheric CO₂ the ocean can sequester. Typically, it is assumed that the formation of carbonate minerals is the only sink for ocean alkalinity. However, in recent years, the formation of alumino-silicate phases in the seabed via reactions that consume alkalinity and produce CO₂ (reverse weathering) has been shown to be significant in the modern ocean and is thought to provide a control on the long-term C cycle. Evolutionary changes in the modes of carbonate production and the availability of certain seawater constituents are also important controls on CaCO₃ formation beyond the flux of alkalinity into the ocean. Here we explore the links between biogeochemical cycles, seawater chemistry, and alkalinity sinks.

Seawater circulation through oceanic crust acts as an essential sink for CO₂ and affects the alkalinity budget of the ocean. Seafloor weathering and ridge flank hydrothermal activity contribute to modern carbon sequestration by taking up carbon at a rate < 0.5 Tmol y−1. In addition, these processes release < 1 Tmol y−1 alkalinity to the ocean. During warmer eras in Earth history, the carbon uptake rates were considerably higher. Estimates range between 2.1 and 3.4 Tmol y−1 during the Cretaceous and Jurassic. The more intense carbonation of the seafloor in the Mesozoic is due to higher temperatures and less pelagic sedimentation in the deep ocean. Accelerated rates of reaction between seawater and basalt and prolonged durations of exposure of igneous crust to seawater led to more intense basalt alteration and carbonate formation within the crust. The interactions between oceanic crust and seawater hence profoundly influence global carbon cycling on long time scales.

Conventional wisdom on Earth’s long-term habitability relies on a negative feedback loop among climate, silicate weathering, and atmospheric CO₂ concentrations. Essentially, climate modulates the rate of CO₂ consumption associated with silicate weathering, which in turn stabilizes climate through the greenhouse effect of atmospheric CO₂. This review assesses the efficacy of continental weathering as a climate stabilizer, including the supply of bedrock available for weathering, the influences of atmospheric oxygenation and land-plant colonization on weathering efficiency, and CO₂-emitting weathering processes that may counteract the negative feedback associated with silicate weathering. Basalt weathering plays a key role in mitigating these constraints, as it can sustain a large weathering-derived CO₂ sink and has exhibited a high sensitivity to climatic changes through much of the Earth’s history.

Igneous and metamorphic processes play a critical role in the geological carbon cycle and Earth’s long-term habitability by transferring carbon between rocks and the ocean–atmosphere system. The magnitude of these carbon fluxes, both in the present day and throughout Earth’s history, remains poorly constrained. Traditional models link carbon degassing to riverine bicarbonate fluxes, but these approaches rely on the questionable assumption that the modern system is in steady-state. Here, we summarize the current state of research on quantifying igneous and metamorphic carbon fluxes using direct measurements, geochemical proxies, and ancient rock records. We also examine the spatial and temporal variability of these processes, which is crucial for understanding their influence on Earth’s carbon cycle over geological timescales.

The foundation of our understanding of the geological carbon cycle, and how this acts as Earth’s “thermostat,” was articulated in a seminal paper in 1981 (Walker et al). They suggested that silicate weathering on the continents acts as a stabilizing feedback on the carbon cycle such that increased atmospheric pCO₂ leads to increased weathering rates and hence increased removal of CO from the atmosphere. This “textbook model” is at the core 2 of most models of long-term biogeochemical cycles. We summarize evidence that there are many other processes in the geological carbon cycle that may be equally or more important than those in the Walker model. We argue there is a need to move beyond the textbook model in both teaching and research.