<P> In addition to oxygen isotopes, water contains hydrogen isotopes - H and H, usually referred to as H and D (for deuterium) - that are also used for temperature proxies . Normally, ice cores from Greenland are analyzed for δ O and those from Antarctica for δ - deuterium . Those cores that analyze for both show a lack of agreement . (In the figure, δ O is for the trapped air, not the ice . δD is for the ice .) </P> <P> Air bubbles in the ice, which contain trapped greenhouse gases such as carbon dioxide and methane, are also helpful in determining past climate changes . </P> <P> From 1989 - 1992, the European Greenland Ice Core Drilling Project drilled in central Greenland at coordinates 72 ° 35' N, 37 ° 38' W. In their project, ice at a depth of 770 m were 3840 years old; 2521 m were 40,000 years old; and 3029 m at bedrock were 200,000 years old or more . However, ice cores can reveal the climate records for the past 650,000 years . </P> <P> Location maps and a complete list of U.S. ice core drilling sites can be found on the website for the National Ice Core Laboratory: http://icecores.org/ </P>

Which of the following proxies have been used to reconstruct temperatures records in paleo climates