Final answer:
Global temperatures are deduced from deep-sea cores by analyzing oxygen isotope ratios in ocean sediments and comparing these with ice core data. These methods use proxies to infer past temperatures and reveal the Earth's climate history, including long-term temperature cycles.
Step-by-step explanation:
Scientists determine global temperatures from deep-sea cores by analyzing the composition of the sediments, and in particular, the oxygen isotope ratios. The concentration of oxygen isotopes in the shells of small ocean organisms that settled on the ocean floor (like foraminifera) serves as a proxy for past temperatures. The ratio of oxygen-18 to oxygen-16 (expressed as ‰0′18O) varies with the temperature of the ocean water in which the organisms lived. Higher concentrations of ‰0′18O generally indicate higher temperatures. Additionally these deep-sea sediment cores are compared with hydrogen isotope ratios found in ice cores from Antarctica. Both sets of data provide a second-hand view of the Earth's past climate known as a proxy. By analyzing these proxies scientists can reconstruct the Earth's climate history.
In particular the presence of left- or right-coiling foraminiferal fossils provides clues about the relative temperatures of seawater at different times. If predominantly right-coiling fossils are found within a sediment layer, it suggests warmer ancient water temperatures. The periodicity in temperature variations, often seen in ice core data and reflected in deep-sea sediments, align with other forms of proxy evidence, such as tree rings and glacier sizes. This composite picture reveals how the Earth's climate has changed over time, showing patterns like the roughly 100,000-year cycles dominated in the last million years.