Geochemical model of δ18O of pedogenic calcite versus latitude and its application to Cretaceous palaeoclimate
✍ Scribed by Greg H. Mack; David R. Cole
- Publisher
- Elsevier Science
- Year
- 2005
- Tongue
- English
- Weight
- 170 KB
- Volume
- 174
- Category
- Article
- ISSN
- 0037-0738
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✦ Synopsis
In the absence of a systematic, global data base on the y 18 O composition of modern soil calcite, predicted isotopic values are calculated as a function of latitude between 608 and 208, using y 18 O of summer and winter meteoric water, ambient soil temperature equal to either measured values in Asia or mean annual temperature, and 0%, 10%, 20% and 30% evaporation of soil water. The results show overall higher isotopic values for summer versus winter precipitation, but steeper, positive slopes for the winter-precipitation curves. Where soil temperature is equated to mean annual temperature, the summer-precipitation curves are nearly flat at high latitudes and have a negative slope at low latitudes. In all cases, the y 18 O values increase with increasing percent evaporation of soil water.
Oxygen isotopic data for Early (Aptian) and Late (Maastrichtian) Cretaceous palaeosol calcite as a function of palaeolatitude of the Western Interior of North America plot in the modern winter-precipitation field, which, along with vertic features, may suggest a winter-wet palaeoclimate. Steeper, positive slopes for the Cretaceous curves compared to the modern curves may be attributable to greater latitudinal gradients in mean annual temperature and soil temperature in the Cretaceous compared to today, and/or to a greater degree of evaporation in Cretaceous soils at low latitudes compared to the modern earth. Distinguishing between these possibilities will require a more complete understanding of the variables that affect oxygen isotopes in pedogenic calcite, as well as better Cretaceous terrestrial palaeoclimate proxies with which to compare the isotopic data.