Manipulating the extra cellular fluid which is around a pH of 9 enabling the conversion of CHO3 ( bicarbonate )to CO3( carbonate) takes energy and may be assisted by increased photosytnetic activity related to the temperature manipulations in the paper or otherwise in tanks where nutrients are relatively high.
Curious about the source for the 7.84 high point for calcification noted.
To the OPS question: Calcium carbonate does begin to dissolve at 7.7; so 7.7 is a low end threshold to be avoided IMO.nsw is generally around 8.1. Personally, I prefer 8.2 to 8.3 in my system.