Tales written in the sand
Writing records in the sand usually consigns them to oblivion. That's not so with the climate records Nebraska's Sand Hill dunes encode. They clearly show that the region suffered a prolonged drought during the so-called Medieval Warm Period 800 to 1,000 years ago.
Venkataramana Sridhar at the University of Nebraska in Lincoln and colleagues call it "a historically unprecedented large-scale shift of [atmospheric] circulation that removed the source of moisture during the growing season." Moreover, it wasn't a one-time extreme event. They warn that the dune record indicates that the region "is prone to droughts far more severe than those in the historical record."
Their account of how they decoded the dune record, published in Science a few weeks ago, is just the latest example of scientists' efforts to read geophysical information "written" in sand. Much of the research focuses on what you could call the "fine print." These are ripple patterns, whose features are measured in centimeters. They are left in sand by wind or running water. You may have stepped on some of them on a sandy beach. They help scientists understand erosion in the Grand Canyon and how water may have flowed on Mars. Reviewing the history of this research last month in Eos, a weekly newspaper of geophysics, David Rubin of the US Geological Survey in Santa Cruz, Calif., noted that the Martian studies show that what was literally a down-to-Earth subject now has "attained a cosmic significance."
Some of the records are ephemeral, as wind or water changes the patterns. Other accounts are written in sandstone. Ripples on Mars may have been undisturbed for billions of years. The Nebraska dunes retain information for centuries because they are stabilized by vegetation. They're a "large print" database whose significant features are measured in meters and kilometers. Their form and orientation reflect the winds that shaped them.
Today, the winds of spring and summer blow from the south or the southeast over that region. They bring moisture from the Gulf of Mexico for the growing season, sustaining vegetation that has stabilized the dunes since the Medieval Warm Period. Back then, dry southwesterly winds replaced the southerly moist flow we now take for granted. With little vegetation to stabilize them, the sand dunes were easily shaped by the southwesterly wind. Their shapes and orientation were locked in place when the winds changed and renewed moisture allowed vegetation to regrow. This preserved the climate record the Nebraska geophysicists now can read.
Ripple records can show both what has happened in the past and what is going on today. In the Grand Canyon, changing ripple patterns show details of water flow. They help guide planning to combat undesirable erosion. On Mars, the "fine print" is microscopic as scientists look for signs of the action of water on microscopic features in the sand. This helps determine whether wind or water formed the ripples.
British geologist Henry Clifton Sorby started that kind of research in 1859. Dr. Rubin notes that a microscopic imager on the Martian rover Opportunity is a modern extension of Sorby's work. He quotes Sorby as saying people laughed at him for trying "to examine mountains with microscopes." Rubin adds, "Imagine Sorby's surprise if he learned that microscopy was essential to examining mountains on other planets."