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Digging at the dolomite dilemma.

In geology, the present is often the key to the past. One curious exception to this rule is dolomite, a magnesium-calcium carbonate (limestone). The "dolomite problem," as it has come to be called, arises from data collected earlier this century indicating that considerably more dolomite formed on the ancient earth than is forming today. Why this is so, and how the mineral is created, even in modern settings, are subjects of lengthy debate.

A new paper by David N. Lumsden at Memphis (Tenn.) State University presents the first measurements of dolomite deposited in deep-sea sediments over the last 150 million years. These data verify the earlier indication that dolomite production has decreased with time. But what isnew is Lumsden's finding that, superimposed on this general decline, there are large fluctuations in the dolomite levels. And these fluctuations appear to be in tune with the rising and falling of the global sea level -- a discovery that may help scientists refine their thinking about how dolomite is made.

"... there probably is no mineral ... about which so much has been written and about which we know so little. Lumsden has pulled together the first decent data set, which shows some very interesting relationships," writes Bruce Wilkinson of the University of Michigan in Ann Arbor in the November GEOLOGY, in which Lumsden's paper appears.

Lumsden examined the reported dolomite content of 844 marine sediment samples cored during the Deep Sea Drilling Project from 127 sites in the Atlantic Ocean, Pacific Ocean, Mediterranean Sea, Red Sea, Black Sea and Gulf-Caribbean. In general, he found peaks in dolomite production at about 130 million years ago (Ma), 110 Ma, 90 Ma, 50 Ma and 10 Ma. The similarities of dolomite records at widely spaced sites convinced him that the fluctuations in dolomite concentration were not random, but were due to the same global cause. Lumsden believes that cause is tied to sea level changes; periods of low dolomite formation appear to have corresponded to low sea levels, and dolomite peaked at high sea levels.

Lumsden suspects that the link between sea level and dolomite production lies with shallow-water shelves, which are though to provide a favorable environment for dolomite formation. According to one theory, the high evaporation rates in these areas selectively enrich the remaining water in magnesium, which can then change calcium carbonates in the underlying sediments into dolomite. At low sea levels these areas dry up, while at high sea levels more shallow water is created. The general trend of decreasing dolomite also indicates that shallow marine environments were more plentiful 150 million years ago than now -- an idea also compatible with the existing picture of the Atlantic seafloor being higher at that time.

Changes in sea level may also affect the chemistry of the oceans and atmosphere, which in turn influences the production rates of dolomite. In fact, Wilkinson and others have proposed that chemistry changes over the last 600 million years have been responsible for variations in the relative abundance of two different structural forms of calcium carbonate called calcite and aragonite. Lumsden presents "the first data on dolomite that we can try to relate to that general scenario," says Wilkinson. At this point, Lumsden says, it's still hard to sort out the roles different factors play in dolomite production, but he believes his data indicate that sea level changes, and not geochemistry, are the dominant control.

Whatever the exact forces driving dolomite formation, Lumsden's recent marine data and any future studies of the much older continental record of dolomite are likely to help scientist unravel the dolomite problem. And understanding how dolomite is created may have benefits beyond satisfying scientific curiosities. A significant portion of the world's oil lies in dolomite rocks, and without a conceptual model of how these rocks form, geologists cannot very well predict where oil companies should drill, says Lumsden. "The person who solves the question of the origin of dolomite," he says, "might wel ... improve our ability to exploit those reservoirs."
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Author:Weisburd, Stefi
Publication:Science News
Date:Nov 30, 1985
Words:664
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