Study gives new answer for muddy mystery

The origin of quartz silt shouldn’t be taken for granite.

Chattanooga black shale with quartz-filled cysts (bright spots). Schieber

Geologists interested in the life and times of ancient oceans have long assumed that tiny particles of quartz found in mudstone under former seas got there from sources on land, including granite, limestone, and sandstone.

That’s a reasonable assumption, says geologist Jürgen Schieber of the University of Texas at Arlington. Only it’s wrong, he contends. In the Aug. 31 Nature, a team led by Schieber shows that quartz silt may have instead emerged from dissolved skeletons of tiny organisms, radiolaria and diatoms, that lived 370 million years ago.

If quartz grains entered the ocean as geological detritus carried from land by rivers or wind, the mineral should be more abundant near prehistoric coastlines. It isn’t, Schieber says.

He found comparable amounts of quartz silt in rocks from ancient coastlines and in mudstone formerly hundreds of miles out to sea.

Schieber’s team examined micrometer-scale quartz grains using scanning cathodoluminescence, a type of electron microscopy good for detecting textural differences. The textures of quartz grains depend on where and how the grains form, Schieber says. Most of the team’s mudstone grains shared few textural features with detrital quartz.

Measurements of oxygen isotopes provided more detail. The ratio of oxygen isotopes in minerals undergoing chemical change depends on the temperature of the transformation. The isotope data were more consistent with an icy, deep-ocean birthplace for the quartz grains sampled than a fiery origin in continental rock.

The surprising revision Schieber proposes for the origin of quartz silt may solve another mystery. Geologic evidence shows that diatoms date back as far as the Jurassic period, around 180 million years ago, Schieber says. Yet DNA evidence from living diatoms suggests their earliest progenitors predate that time. The silt could be the missing geologic testament to an earlier profusion of the plankton, he says.

In an article accompanying Schieber’s paper, Alan Kemp of the Southampton Oceanography Centre in England warns that the molecular evidence prescribing an earlier appearance for diatoms is questionable. The quartz may have derived not from diatoms, but from the silica coatings of the diatoms’ algaelike ancestors.

Regardless of the organisms involved, Schieber’s study calls into question models of early climate, which assume sufficient wind to blow quartz silt out to sea, Kemp says. And it means the size of quartz grains in mudstone may be a poor basis for reconstructing the geography of ancient seas.

Schieber points out a few more surprises. For one thing, he says, there must have been lots of diatoms to produce the amount of quartz silt seen today. “It must have been raining skeletons–tiny skeletons, of course,” he says. What’s more, he adds, to support so much life, the waters may have been warmer than previously believed.

Abhijit Basu, a geologist at the University of Indiana at Bloomington, admires Schieber for “trying to figure out how nature works without any regard to the establishment.” Although he acknowledges his work goes against the grain, Schieber says no one knows now what the repercussions might be.

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