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Long-term control of atmospheric carbon dioxide: low-temperature seafloor alteration or terrestrial silicate-rock weathering?

Ken Caldeira · American Journal of Science 295, 1077-1114 · 1995

Key finding. Silicate-mineral dissolution rates are nearly independent of pH over the range 5 to 8, and deep-ocean pH is above 7.5, so low-temperature seafloor basalt alteration is not likely to be an effective feedback on atmospheric CO2; terrestrial silicate-rock weathering consumes CO2 several orders of magnitude more sensitively to changes in atmosphere and ocean CO2, and is therefore the primary long-term control.

Two stacked line graphs of alkalinity flux in units of 10 to the 12 equivalents per year, from 0 to 50. The upper panel plots against atmospheric CO2 concentration relative to 300 parts per million on a logarithmic axis from 0.1 to 100; the silicate weathering on land curve climbs from about 4 to 44, while the two seafloor basalt dissolution curves rise only from about 5 to 10. The lower panel plots against deep ocean pH from 6 to 9; silicate weathering falls steeply from about 47 to 2, while the basalt dissolution curves fall only from about 11 to 6.
The comparison the paper turns on. Terrestrial silicate weathering responds strongly to both CO2 and ocean pH, while seafloor basalt dissolution is nearly flat across the whole range — and a flux that barely changes cannot regulate what it is supposed to be regulating. Figure 2 from Caldeira (1995), American Journal of Science 295, 1077-1114. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Something has held atmospheric CO2 within habitable bounds over hundreds of millions of years. The usual candidate is silicate-rock weathering on land, which speeds up as the planet warms.

A competing proposal held that the real control is carbonate accumulating in ocean-floor basalt during low-temperature alteration, with the rate set by deep-ocean hydrogen-ion and carbon concentrations. This paper asked which of the two actually provides the feedback.

What did we find?

The earlier seafloor-alteration proposal made basalt dissolution proportional to deep-ocean hydrogen-ion and total carbon concentrations. This paper builds an improved parameterization from laboratory dissolution data and puts it in a global carbon-cycle model.

The experimental result that undoes the mechanism is the flat pH dependence. Silicate minerals dissolve at nearly the same rate anywhere between pH 5 and pH 8 in a wide variety of experiments, and the deep ocean never leaves the upper end of that range.

Comparing sensitivities directly, CO2 consumption by terrestrial weathering responds to changes in atmosphere and ocean CO2 several orders of magnitude more strongly than CO2 consumption by low-temperature seafloor alteration.

The conclusion does not delete the seafloor from the carbon budget. Ocean-basement carbonate accumulation should still be included in global carbon budgets, and it may affect atmospheric CO2 indirectly by changing how much CO2 is degassed during subduction-zone metamorphism.

Why does it matter?

It settled which of two candidate thermostats actually regulates atmospheric CO2 over geological time, and it did so on the strength of laboratory dissolution kinetics rather than model tuning.

The reasoning is a clean example of a general point: a process can move a large amount of carbon and still be useless as a feedback. What makes a feedback is sensitivity to the thing it is supposed to regulate, and a rate that barely varies cannot regulate anything.

Terrestrial weathering remaining the control is why continental configuration, mountain building, and land vegetation stay central to explaining Earth's long-term climate.

Citation

Ken Caldeira (1995). Long-term control of atmospheric carbon dioxide: low-temperature seafloor alteration or terrestrial silicate-rock weathering?. American Journal of Science 295, 1077-1114.

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