After discussing orbital forcing of climate and how we can judge its effects in the geological record, I thought I should return to talking about hominins, but not without a smooth segue between the two. Two more proxies can also indirectly tell us what ancient climate was like, and these are are isotopes of carbon, C, and the metal strontium, Sr.
This global time-travelling adventure starts in the town of Monterey, California where an outcrop of an oil-rich rock formation caught the interest of a bunch of researchers. Oil is the remains of ancient diatoms (a group of algae) which have been under very high pressure for millions of years, and the layers of sediment in the rock itself also record the amount of algae that is present in the ocean at any one time. The layers of the Monterey Formation which formed 20 – 15 million years ago have particularly high levels of these algae. These simple plants take in carbon dioxide from the atmosphere via photsynthesis, and their subsequent death and deposition on the sea floor locks this carbon away (Raymo, 1994).
Two questions arise here: what causes these great algal blooms? And: what do carbon and strontium isotopes have to do with it?