50 Million Years Of Climate Change: Some Plots And Illustrations
If you’re here, you’ve probably either just listened to our episode on 50 Million Years of Climate Change, or you’re about to.
Because a picture is definitely worth 1,000 words, this page has some sketches and plots to help you visualize some of the things explored in the episode.
The first set of plots show average global air temperatures and atmospheric concentrations of carbon dioxide over the last 50 million years, as reconstructed from environmental signals trapped in the calcium carbonate “tests” of foraminiferans. In the upper plot, temperatures have been reconstructed from the ratios of the stable isotopes of oxygen (i.e., 18O/16O) of the calcium carbonate (CaCO3). In the lower plot, atmospheric concentrations of carbon dioxide, have been reconstructed from the stable isotopic composition of boron (i.e., 11B/10B) trapped within the calcium carbonate as it formed.
The air temperature values are from Westerfeld et al (2020) An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science 369: 1383–1387, based on the algorithms given in Hansen et al (2013) Climate sensitivity, sea level and atmospheric carbon dioxide. Philosophical Transactions of the Royal Society A 371:20120294. The carbon dioxide reconstruction comes from Rae et al (2021) Atmospheric CO2 over the last 66 million years from marine archives. Annual Review of Earth and Planetary Sciences 49: 609–641.
Next, I’d like to impress you with my fabulous artistic talents, by showing you my not-to-scale sketch of the processes behind plate tectonics.
Although it’s a cycle, with no clear beginning or end, we’ll start with the upwelling of hot mantle that pushes magma up through the oceanic lithosphere and crust, that, when it reaches the top and, cooled by seawater, solidifies, creates a wedge of new rock. Not only does this formation of new seafloor eject carbon dioxide (among other things) from the mantle into the deep ocean, it shoves the older ocean crust and lithosphere out to the side, away from the ridge.
At the edge of the ocean, where the oceanic crust and lithosphere push against continental crust and lithosphere, something has to give. In the case of subduction, being denser, the oceanic lithosphere and crust and the sediments stuck on them (and all the carbon contained in all of those materials) get pushed under the continent and begin a slow descent into the mantle. During this subduction, the subducting slab of material partially melts, fueling volcanos, such as those along the Pacific’s Ring of Fire, returning some of the carbon to the atmosphere as carbon dioxide. Over time, the rest of the slab (and the remaining carbon) recycles into the mantle, closing the circuit of the mantle convection cell and the geological carbon cycle.
Here’s a graph of the amount of calcium carbonate being added every year to the deep sea taking off like a rocket over the last 50 million years, reflecting the increasing input of carbonate and bicarbonate ions to the ocean exactly as carbon dioxide is disappearing from the atmosphere. That’s the solid line. In case you want to be convinced that more carbon is leaving than returning via volcanoes, the dotted line traces the inputs of carbon dioxide to the surface Earth system via arc volcanoes and mid-ocean ridge volcanoes, as reconstructed via modeling.
These data come from Müller RD, et al (2022) Evolution of Earth’s plate tectonic conveyor belt. Nature 605: 629–639, based in part on Dutkiewicz A, et al (2019) Sequestration and subduction of deep-sea carbonate in the global ocean since the Early Cretaceous. Geology 47: 91-94.
And, if you want to see where the continents were 66 million years ago when the asteroid wiped out the dinosaurs there’s a great image of that here.