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The Mid-Cretaceous Super Plume, carbon dioxide, and global warming

Ken Caldeira and Michael R. Rampino · Geophysical Research Letters 18, 987-990 · 1991

Key finding. A carbonate-silicate cycle model with four different silicate-weathering formulations gives mid-Cretaceous atmospheric CO2 of 3.7 to 14.7 times the pre-industrial value of 285 ppm from super-plume tectonics, implying 2.8 to 7.7 degrees Celsius of warming — but CO2 released from oceanic plateaus alone is unlikely to account for more than 20% of that CO2 increase.

Two stacked line graphs against geologic time in millions of years, running from 60 on the left to 140 on the right. The upper panel plots atmospheric CO2 in parts per million, with four curves labelled Walker et al. 1981, Berner et al. 1983, Volk 1987, and Berner 1990; all rise from about 300 ppm at 60 million years to peaks between roughly 1000 and 4200 ppm near 115 million years. The lower panel plots temperature change in degrees Celsius, with the same curves peaking between about 3 and 8 degrees.
Estimated mid-Cretaceous CO2 and temperature under four different formulations of how silicate weathering responds to CO2. All four peak near 115 million years ago, but they disagree by a factor of four about how high CO2 went — which is why the paper reports a range rather than a value. Figure 3 from Caldeira and Rampino (1991), Geophysical Research Letters 18, 987-990. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

The mid-Cretaceous, about 120 to 100 million years ago, was 6 to 14 degrees Celsius warmer than today. A super plume rising from near the core-mantle boundary and erupting beneath the Pacific — building the Ontong-Java Plateau — had been proposed as the cause, through the CO2 it degassed.

That was a qualitative suggestion. This paper asked how much warming the plume's carbon could actually deliver, given that silicate weathering removes CO2 faster as the planet warms.

What did we find?

The calculation is run with four different formulations for how silicate-rock weathering responds to atmospheric CO2, so the spread of answers reflects genuine disagreement about the negative feedback rather than one modelling choice.

Super-plume tectonics — faster sea-floor generation and subduction, not just the plateau eruption — gives CO2 between 3.7 and 14.7 times pre-industrial, and 2.8 to 7.7 degrees Celsius of warming above today's global mean.

The plateau itself is a small part of it. CO2 from oceanic plateau eruptions alone is unlikely to have been directly responsible for more than 20% of the mid-Cretaceous CO2 rise; the rest comes from ridge degassing and subduction-zone decarbonation tracking the higher sea-floor generation rate.

Geography contributes about 4.8 degrees Celsius independently, through altered continental positions and higher sea level.

Together, palaeogeography and super-plume CO2 give 7.6 to 12.5 degrees Celsius, which sits inside the 6 to 14 degrees previously estimated for mid-Cretaceous warmth — so the hypothesis survives quantification.

Why does it matter?

It converted a plausible story into a number, and the number came out compatible with the geological record. That is the useful outcome either way: a mechanism that could not produce enough warming would have been eliminated.

The 20% result is the part that changed the framing. The dramatic event — the plateau eruption — is not the main carbon source; the sustained increase in sea-floor spreading that accompanies it is.

It is an early demonstration that deep-Earth processes and surface climate have to be modelled as one system, which is the approach the group's later palaeoclimate work continues to take.

Citation

Ken Caldeira and Michael R. Rampino (1991). The Mid-Cretaceous Super Plume, carbon dioxide, and global warming. Geophysical Research Letters 18, 987-990.

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