Geochemistry

Well-Preserved Brines

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Science  24 Oct 2003:
Vol. 302, Issue 5645, pp. 535
DOI: 10.1126/science.302.5645.535b

Sedimentary basins are geological depressions in which sediments have accumulated, and in which most of the world's petroleum reserves are found. The deepest parts of many Phanerozoic sedimentary basins, such as the Gulf of Mexico and the North Sea, are saturated with brines rich in Ca, poor in SO4, and of uncertain origin. One popular theory about how these brines formed is that ancient seawater, first concentrated by evaporation, reacted in a complicated fashion with the surrounding rocks, losing MgSO4 and gaining CaCl2 in the process. This scenario is based on the assumption that the chemical composition of Phanerozoic seawater was like that of modern seawater.

Lowenstein et al. challenge this theory by suggesting that the chemistry of Phanerozoic brines reflects the composition of the seawater from which they formed, thereby eliminating the necessity of invoking a series of unlikely chemical transformations, as well as avoiding associated mass balance problems. They base this conclusion on data that show that the oceans have alternated between MgSO4-rich and CaCl2-rich compositions over the past 550 million years, and they discuss their proposal in the context of brines in the Illinois basin. — HJS

Geology 31, 857 (2003).

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