Oxygen isotope ratio precipitation

Proceedings of the National Academy of Sciences Figure 1. Deuterium isotope ratio vs. oxygen isotope ratio for world-wide precipitation (IAEA data), showing the slope of eight (31). 

Figure 25. Oxygen isotope ratio in world-wide precipitation vs. monthly average air temperature, showing that for every point on the line in Figure 1, there is a corresponding average air temperature.

The oxygen isotope ratio of ice cores is mostly controlled by a different influence, however the temperature of the clouds from which the snow fell. When water vapour condenses to water or ice, isotope sifting occurs just as it does during evaporation - but in reverse the lighter isotope stays behind. The last precipitation to leave a cloud - the snow that falls over the poles - is therefore enriched in O. The amount of enrichment turns out to depend on how cold it is over the ice sheet. So ice-core isotope records show us how atmospheric temperatures have changed over time. 

Hydrogen- or oxygen-isotopic ratios of water from precipitation have long been used in estimating past Earth-surface temperatures. A strong correlation is observed between surface temperatures and modem or recent deuterium hydrogen (D H) and ratios... 

The oxygen isotope ratios in carbonates are a function of both temperature and the 8 Ow of the surrounding seawater. Since the early equation given by Epstein et al. (1953), many equations have been published, which substantiated the potential of oxygen isotope paleothermometry for biogenically precipitated calcite. The first equation based on laboratory experiments with planktonic foraminifera was generated by Erez and Luz... 

Figure 7. Oxygen and deuterium isotope ratios vs. temperature in Austrian precipitation (26).

The importance of oxygen and hydrogen isotope ratios in the water cycle was first demonstrated by Dansgaard (1964), who showed that a simple linear relationship exists between < 180 in precipitation and the average annual air temperature. Through the isotopic analysis of a large number of meteoric water samples (rainfall and surface water) collected at different latitudes, Craig (1961) had previously demonstrated that a simple relationship existed between <5180 and < D in precipitation ... 

Most hot spring waters have deuterium contents similar to those of local precipitation, but are usually emiched in as a result of isotopic exchange with the country rock at elevated temperatures. The magnitude of the oxygen isotope shift depends on the O-isotope composition of both water and rock, the mineralogy of the rock, temperature, water/rock ratio, and the time of interaction. 

If we subtract 1 and multiply by 1000 on both sides, the left side of the equation is equal to the definition of of O2 with respect to the standard SMOW, and the right side is +20%o. This value is lower than the measured value of 23.5%o because roughly half of the oxygen production by photos5mthesis occurs on land and about half in the ocean. O2 from terrestrial photosynthesis is 4%o-8%o heavier than SMOW because evaporation exceeds precipitation in the water of leaves, where most of the photosynthesis of the terrestrial biosphere occurs. The mean 8 0 from oceanic and terrestrial photosynthesis is thus +22%o to +24%o. If one assumes that the partitioning of photosynthesis between the land and ocean remained the same and fractionation factors did not change in the past, then the sole reason for changes in the 8 0-02 of the atmosphere trapped in bubbles of the ice is due to the variation in the isotope ratio of seawater. 

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