boron and carbon isotope, Phytane, Ice cores, Corals, Lake and ocean sediments etc.
Explanation:
Drillings in ice cores like within ice sheets of Greenlands, Antarctica started in 1956. Collecting core samples of 4 and a half inches and use of molecules of water O 16 and 18 that helps to determine the past atmospheric carbon.
Corals, skeletal rings have bands of paleo-climatological information and as their body is made of the calcium carbonates these limes creating organisms help to determine the atmospheric concentration of carbon on prehistoric earth.
The use of oxygen isotope in lakes and oceans has given data related to the content of ocean sediments, and values deposited in clay and sand.
The use of phytane a diterpenoid alkane that breaks down chlorophyll is used to estimate the ancient Co2 level.
