Evolution of the Global Carbon Cycle and Climate Regulation on Earth

The existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon-silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long-term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long-term changes in atmospheric pCO(2). We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO(2) regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off-axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean-atmosphere system via reverse weathering-that in turn acted to elevate ocean-atmosphere CO2 levels.