Oxygen isotope enrichment of sea water by evaporation [antikvár]

Geochimica et Cosraochimica Acta, 1966, Vol. 30, pp. 801 to 814. Pergamon Press Ltd. Printed in Northern Ireland Oxygen isotope enrichment of sea water by evaporation R. M. Lloyd* Shell Development Company (A Division of Shell Oil Company), Exploration and Production Research Division, Houston, Texas (Received 8 August 1965) Abstract—An evaporation mechanism proposed by Sverdrup is used as a model to interpret the changes in isotopic composition that occur during the evaporation of sea water. The relative himiidity at the site of evaporation is shown to be an important factor in determining the net isotopic em-ichment, especially at very high salinities. From these results it is proposed as a working hypothesis that brines originating by evaporation of sea water in a humid coastal environment mil attain values no higher than 6%„ Introduction In the pioneer work of Epstein and Mayeda (1953) on the oxygen isotope composition of natural waters it was shown that well mixed open ocean water has an almost constant (50^® composition averaging 0%^. It was also shown that meteoric waters (rain, snow, ice, fresh water lakes and streams) tend to have values more negative than average ocean water while waters which have undergone evaporation tend to be isotopically enriched, i.e., to show an increase in <50^®. Meteoric waters have been studied in detail by Dansgaaed (1953, 1954), Epstein (1956), Epstein and Sharp (1959), and Craig (1961a) and the processes which lead to variations in their isotopic composition are fairly well known. Waters which are isotopically enriched by evaporation have received less attention. The work of Epstein and Mayeda (1953) and Lowenstam and Epstein (1957) covers only a narrow range of evaporation as indicated by small increases in sea water salinity (Fig. 2). They concluded that evaporation in nature occurs by the removal of vapor that is in isotopic equilibrium with the liquid phase. However, a recent study by Craig et al. (1963) on fresh water evaporation has shown that: (a) The removal of vapor is characterized by a kinetic isotope effect rather than by the loss of equilibrium vapor from the system. (b) The exchange of water molecules between the liquid phase and atmospheric vapor predominates over the expected isotopic enrichment from net loss of vapor by evaporation. As a result, instead of a continuous increase in a stationary isotopic composition is reached when evaporation has reduced the liquid phase to some 15 to 25 per cent of its original mass. These conclusions were derived by studying the HDO and HjOi® relationships in a series of open air and laboratory evaporation experiments. A similar series of experiments were performed by the present witer on sea water to shed light on peculiar versus salinity relationships noted in naturally * EPR Publication No. 402.