ERSC 3P12 Lecture 13: 13. Cyclostratigraphy II
29 May 2016
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Geological records tuned by northern summer insolation (at 65 n). 16o is normal isotope of oxygen (99. 756%) has 8 protons and 8 neutrons. 18o (much rarer) has 8 protons and 10 neutrons, so slightly heavier. Both are stable isotopes do not decay. When sea water evaporates from the surface, the lighter isotope is preferentially removed in the water vapour. Interglacial conditions: 16o returns to oceans as river runoff etc. Glacial conditions: 16o gets preferentially trapped in polar ice caps. Foraminifera record the concentration of 18o in the tests by the formula: 18o = 1000 x (18o/16o sample 18o/16o standard)/ (18o/16o standard) Ice sheets enriched in lighter isotope (16o) so glacial seas are richer in 18o. But colder temps also favour heavier isotope in forams! So use benthic forams because bottoms of oceans are a relatively constant ca. Foraminiferal 18o/16o therefore must reflect global ice volume. Note long-term temperature decline from early late pliocene, and especially from.
