Earth & Life (http://www.geofinds.com), 2006-10-1, Vol.1. , No.1: 60-68

Oxygen isotope composition (δ¹⁸O) of Late Carboniferous sea water^* 

Wu Ya Sheng¹

1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029 wys@mail.igcas.ac.cn

Abstract  Carbonate samples from a Late Carboniferous reef in southwest China yield б¹⁸O of –2.39‰. If the paleotemperature of the sea water on the reef then was 25~29℃, as it is sure for all other typical reefs, the б¹⁸O of the sea water can be calculated from Craig’s formula, being –0.58~0‰, typically 0.14‰. With the б¹⁸O of Late Carboniferous oceans known, the paleotemperature of the Late Carboniferous Texas sea can be determined, to be 27.4℃. Paleotemperatures of other Late Carboniferous seas can be determined in the same way. Since reefs occur in most geological periods of Phanerozoic, the б¹⁸O of the sea waters of these periods can be determined in this way, which would enable the restoration of the paleolatitude, the oceanic stratifying  of these periods, as well as the geochemical evolution of coeans and quantitative establishment of paleoclimatic history.

 Keywords: δ¹⁸O; oxygen isotope; Late Carboniferous; paleotemperature; paleoclimate; reefs

1. Introduction

As early as in 1953, Epstein proposed the following formula to determine the paleotemperatures of ancient sea water.

        T=16.5 – 4.3 (б¹⁸O_c-б¹⁸O_w) + 0.14(б¹⁸O_c-б¹⁸O_w)²

where б¹⁸O_c, б¹⁸O_w means the б¹⁸O (PDB) of carbonate and that of the sea water in which the carbonate formed. Then, in 1965, Craig revised the formula as

         T=16.9-4.2(б¹⁸O_c-б¹⁸O_w)+0.13(б¹⁸O_c-б¹⁸O_w)²

Craig’s formula should serve as a good way to determine paleotemperatures of ancient sea waters. Unfortunately, however, it can not yield satisfying paleotemperatures because the fact that the б¹⁸Ow of ancient sea waters, as a precondition, was actually unknown: up to now, there are no effective methods available to determine the б¹⁸Ow of ancient sea water.

In order to calculate paleotemperatures of ancient sea waters, some authors bravely assumed that the б¹⁸O_w of the ancient seas in the periods without ice-sheet to be –1.2‰ and that in the periods with ice-sheet to be 1.2‰. However, as we know, the ice-sheet volume of the Earth was not constant in geological history; it has being changed with global climates. Any change in ice-sheet volume will cause a change in the total volume of oceans and thus a change in the content of oxygen-18 in the oceans (i.e., change of б¹⁸O_w).

*Supported by National Natural Scientific Foundation of China (No. 40472015) and the State Key Laboratory of Modern Paleontology and Stratigraphy (No. 053102), as well as the Key Laboratory for Minerals and Resources, Chinese Academy of Sciences.

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Content of Vol. 1 No.1