Continental configurations have come and gone over Earth’s history. From the steady cycling through supercontinental arrangements to the distributed scattering of numerous continents separated by oceans today, the geography of our world is constantly changing. Critically, these configurations have influenced the global distribution of heat, water (ocean, continental precipitation and runoff), and biology through the variable heating of landmasses and periodic impedance of ocean currents. In this way, the geography of our world can influence biogeochemical cycles, such as the carbon cycle, that strongly influence Earth climate. Atmospheric CO2 has also cycled dramatically over the last ~540 million years. In their newest article published in Geological Magazine, authors Goddéris and Donnadieu explore the control of palaeogeography on atmospheric CO2 by describing how it shifts the relative balance between global CO2 consumption and production.

The formation of soils through the chemical weathering of silicate materials is a key process controlling atmospheric CO2 consumption. More weathering and soil formation leads to lower atmospheric CO2. As temperature and precipitation increase, the amount and rate of chemical weathering generally increase such that temperate environments weather promote more weathering than colder, drier regions. Over time, the shifting of landmasses to different latitudes and longitudes alters the distribution of particular environments, where and how much soil is formed, and therefore the overall CO2 balance.

Goddéris and Donnadieu consider this indirect influence for several palaeogeographic configurations. Using simulations produced by global climate-carbon model GEOCLIM, the authors calculate steady-state atmospheric CO2 levels for 22 Phanerozoic time intervals by balancing the calculated CO2 consumption via silicate weathering and the solid Earth degassing through magmatic activity. The CO2 consumption is determined for continental grid cells spaced every 7.5° longitude x 4.5° latitude. In modified versions of GEOCLIM, the authors also consider shielding effects of thick soils, the relative balance between physical and chemical weathering rates, and variable magmatic degassing.

Remarkably, almost all simulations suggest identical “source” or “sink” controls over particular intervals of geologic time. “Palaeogeographic forcing is a main driver of the climatic evolution from the beginning of the Phanerozoic to the beginning of the Jurassic, through the modulation of the atmospheric CO2 consumption by continental silicate weathering,” the authors assert. From the Jurassic to just about 50 million years ago, vigorous magmatic degassing counteracted cooling trends promoted by a dispersed continental arrangement to force the planet into warm ‘hothouse’ conditions that characterize the late Mesozoic and early Cenozoic. Not until the dramatic uplift, erosion, and weathering of the Himalaya by ~50 Ma could CO2 consumption prevail over CO2 production, and shift global climate back to icehouse conditions that still exist today.

The full article “A sink- or a source-driven carbon cycle at the geological timescale? Relative importance of palaeogeography versus solid Earth degassing rate in the Phanerozoic climatic evolution” by Yves Goddéris and Yannick Donnadieu is available to download free of charge for a limited time here. Related research published last year by the authors and colleagues appears in Nature Geoscience and were presented at the recent European Geoscience Union Meeting on 12 April 2018 .

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