Stable isotopes of the water molecule have been widely used for palaeoclimatic reconstructions. Most of the studies were concerned with the oxygen isotopic composition but high precision deuterium measurements allowed co-isotopic studies to be performed, both in S18O and in SD. S18O is defined as the relative range between the 18O/16O ratio in a sample and the 18O/16O ratio of standard seawater, expressed in parts per mil. SD is similarly defined as the relative range between the 2H/1H ratio in a sample and 2H/1H ratio of standard sea water, also expressed in parts per mil. 2H is also called D (for deuterium). The plotting of SD against S18O opens up the possibility to visualize important processes. Deuterium and oxygen-18 concentrations in precipitation (rain or snow) are linearly related. The equation expressing this fact is very well obeyed. Thus has been defined the meteoric water line, usually expressed as SD = 8 S18O + d, where the independent term d is called the deuterium excess. The value of d for a sample is thus its SD value minus eight times its S18O value. It is clear that for samples aligned along a meteoric water line (MWL), d is constant (actually equals 10 on the Global Meteoric Water Line). There are, however, situations where the slope of the linear relationship between SD and S18O is different from 8. In such cases, d is not constant along the line. If the slope is less than 8, the deuterium excess is usually correlated with SD or S18O, decreasing with increasing S values and vice versa.

Meteoric ice can be defined as ice resulting from the meta-morphism of snow or more generally of solid precipitation. Such ice is by far the most important constituent of glaciers and ice sheets. A meteoric water line is usually displayed for such ice with a well-defined value of deuterium excess. By contrast, samples of ice resulting from water freezing can be aligned in a SD-S18O

diagram on a straight line with a slope lower than 8. This slope is the signature of a freezing process and will be considered later in this paper. Such ice not directly derived from solid precipitation is called here non-meteoric ice. The significance of the deuterium excess concept in non-meteoric ice is thus completely different from that in meteoric ice. The purpose of this paper is to review the environmental significance of deuterium excess in both situations. The first one is developed in the case of the Vostok and EPICA deep-ice cores. The second one is illustrated in different case studies where ice is formed by water freezing.

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