A part of my John Everett series – read more: 0/I – II.0 – II.5 – II.75 – III.0 – III.3 – IV.0 – IV.4 – IV.8 – V – VII – VIII – Full Report
We’ve seen that Dr. Everett’s discussion of paleogeochemistry fails to consider both rates of change and the geological record of ocean acidification. There is one last talking point in this section which requires comment:
“From 50-600 million years ago, atmospheric CO2 levels were usually 2-20 times higher than at present. […] This included the age of the dinosaurs, when life was so prolific that we still use its carbon, limestone and chalk.”
Limestone and chalk, like corals and coccoliths, are made out of calcium carbonate. Many deposits of calcium carbonate occurred when there was much more carbon dioxide in the air. The Cretaceous is named after chalk deposits like the White Cliffs of Dover; CO2 levels during the Cretaceous were over 1000 ppm, compared to current levels around 390 ppm. If the ocean deposited calcium carbonate en masse during the high-CO2 Cretaceous, why should we expect it to become hostile to carbonates now?
The chalk cliffs of Dover, massive deposits of calcium carbonate from the high-CO2 Cretaceous. Is this a paradox? Not really. Click for sauce.
The answer lies, again, in time scales.
Over short time scales, like those on which acidification is currently occurring, the saturation state of calcium carbonate is determined by pH, which is controlled by CO2. However, on longer time scales, it’s controlled by another factor. As this article explains:
“Hence, the key, but rather counterintuitive result, is that on long time scales, ocean pH and atmospheric CO2 are decoupled from carbonate mineral saturation state, which is dictated primarily by weathering (in conjunction with the major cation [Ca2+, Mg2+] content of the ocean). Actually, saturation is not entirely decoupled geologically from pH and CO2, as all things being equal, at high CO2 (and a warmer climate), enhanced weathering requires higher carbonate burial and hence higher ocean saturation. Thus, the presence of “carbonate factories” with widespread CaCO3 production and burial is entirely consitent with a high CO2, low pH world. […] Only in significant and geologically “rapid” departures from steady-state carbon cycling will both pH and saturation fall together…” (my emphasis)
In other words, over a long timeline, it’s the calcium that determines calcium carbonate favorability. Over short timelines, it’s the pH- and CO2 emissions are altering the pH on a short timeline.