How the Earth stores records of the past

When human data doesn’t go back far enough, researchers turn to natural archives.

 

When researchers want to understand the history of the environment, they extrapolate data from oral histories or peruse digitized weather observations, aging diaries and farmers’ journals. But these records are spotty, subjective and only go back a few centuries. So scientists also look to the Earth itself and the vast timeline hidden therein — the natural archive. 

These storehouses of earthly data — ice cores, seafloor sediments, oyster shells, even road cuts through layers of rock — offer a far deeper look back in time than human records. A researcher can extrapolate water-quality data by examining river sediment, or determine forest fire frequency and magnitude through the rings of a tree.

As with any archive, scientists must learn to read the natural archive’s language in order to understand what it is saying. Translations constantly evolve, meaning interpretations do as well. And like other archives, the natural archive is fragile: Just as a library can be destroyed by fire, so too can the massive museum of arboreal memories that essentially comprises a forest.

ARBOREAL MEMORIES

In 1922, when federal and state officials divvied up the Colorado River, they based their negotiations on a few decades’ worth of streamflow gauge records. They estimated that 15 million acre-feet of water flowed each year past the “fulcrum point” of Lees Ferry, just below the current location of Glen Canyon Dam in Page, Arizona. 

But when drought gripped the region in the 1930s and streamflows dropped, some wondered whether it was a reliable number. To find out, University of Arizona graduate student Edmund Schulman turned to the trees. 

Scientists have long known that a tree grew a new ring about every year, and that those rings provided a record of environmental conditions. The rings’ width corresponds with precipitation levels and temperature, while blackened scars reveal past fires. By correlating recent years’ growth with weather observations, researchers can “calibrate” tree rings, enabling them to reconstruct past climate.

Using conifer core samples taken throughout the Colorado River watershed, Schulman roughly reconstructed several centuries of climate, and found the first two decades of the 20th century — the baseline for the Colorado River Compact — were far wetter than the 600-year average. Water managers weren’t too worried because the states were nowhere close to using up the existing supplies. Eventually, however, as the region’s population and water consumption increased, the river no longer reliably delivered its 15 million acre-feet, and folks took notice. University of Arizona researchers again sampled tree cores to reconstruct past flows. Their findings were alarming: The average flow at Lee’s Ferry for the past four centuries was as much as 3 million acre-feet — hundreds of billions of gallons — lower than the amount allotted to the seven Colorado River Basin states and Mexico.

PACKRAT MEMORIALS

Anyone who has opened the hood of their car in the morning and found a massive pile of twigs and chunks of rubber, plastic and wire where, only hours ago, a functioning engine sat knows that packrats are extremely efficient — and occasionally unwelcome — collectors of stuff. But the little cuties are also some of nature’s most effective archivists, though instead of using climate-controlled vaults, they preserve the historical record with their pee.

If you found your car disabled and occupied by a packrat and decided to just surrender, close the hood and wait a few thousand years, you might return to discover an engine-compartment-sized, lumpy, charcoal-brown chunk of dried packrat urine and poop, studded with all the little hoarder’s treasures. If you were to excavate the chunk of said urine — also known as a midden — you would find the parts of your car, along with bones, twigs and those candy bar wrappers you guiltily stashed under the seat, all of it remarkably well-preserved. 

By carbon-dating preserved plant debris and fecal matter from packrat middens, scientists have been able to determine, for example, that the flora in Joshua Tree National Park hasn’t changed dramatically in the last 15,000 years. The University of New Mexico researchers who traced pollen dispersal in radiocarbon-dated packrat middens found that ponderosa pine forest gave way to piñon in the Chaco Canyon region some 8,000 years ago, courtesy of a climatic shift. Middens even preserve DNA and provide a detailed window into communities of plants, animals, bacteria and fungi present millennia ago.

SILT REMEMBRANCE

When Glen Canyon Dam began backing up the Colorado River in 1963, it also began compiling a massive storehouse of water-quality data for the 100,000 square-mile upper watershed. This archive is not the water in Lake Powell, but rather the silt, which is piling up at a rate of about 1.6 billion cubic feet — enough to fill nearly 19,000 Olympic-size swimming pools — per year.

Because silt accumulates at a fairly consistent rate, core samples of it provide a sort of timeline of upstream water chemistry. Some events, such as large tailings-pile breaches, are even visible to the naked eye, appearing as yellowish-gray strata.  

In 2018, U.S. Geological Survey scientists set out to unearth Lake Powell’s sedimentary memories, with a special focus on the San Juan River Delta, since that’s where a portion of the 540 tons of metal spewed by the 2015 Gold King Mine spill residue would have settled after a nearly 300-mile river journey from southwestern Colorado. And, indeed, about 10 feet down, scientists conducting the chemical analysis found a spike in zinc and other metals. But some 35 feet deeper, they discovered an even bigger spike, likely correlating with tailings pond spills and other mining pollution from the 1970 and ’80s. We may not remember, but the silt never forgets.

DUST DEVELOPMENTS

Every spring, when the winds kick up and the mercury climbs, the glittering white snow blanketing the San Juan Mountains of southwestern Colorado takes on a reddish-brown hue. This is dust, snatched by spring winds from cattle-trampled pastures and public lands to the west, and carried to the steep slopes of the high country, where it’s deposited with the snow, or, in its absence, on the ground.

When dark-colored dust (or ash, or carbon or what have you) coats the snow, it reduces the albedo, causing the snow surface to absorb more solar energy and melt more quickly. That throws off the natural calendar that tells plants when to sprout and bloom and pushes spring runoff earlier into the year. Reduced albedo enhances evapotranspiration and snow sublimation — meaning the snow melts and vaporizes in one fell swoop — reducing the total volume of runoff and depleting rivers of their flow.

While anecdotal accounts of these aeolian dust events can be found throughout history, researchers only began systematically recording them a few decades ago. In order to get a better grasp on dust trends over time, University of Colorado researchers in 2008 examined the sediment that had built up over nearly six millennia at the bottom of lakes high in the San Juan Mountains. They concluded that most of the dust deposited there comes from the Colorado Plateau, that dust picked up at about the same time as white settler-colonists arrived in the mid-1800s, and that volumes and frequency peaked in the first few decades of the 20th century. This seems to leave little doubt about the cause: It’s the results of the newcomers’ land-disturbing ways — all that mining, development, tilling, logging, and, perhaps most dust-raising of all, cattle grazing.

ICE CORE RECALL

Between 1950 and 2011, weather observations showed that a pattern known as the Aleutian Low had strengthened, causing precipitation at several Alaska coastal weather stations to rise significantly. Scientists thought it might indicate a changing climate. But the Aleutian Low has been known to shift for decades at a time, and the six decades of observations weren’t enough to show whether the change was caused by natural variability or a long-term climatic shift.   

So, in 2017, Dartmouth researchers headed to Denali National Park and the Mount Hunter summit plateau, where snow piles up at mind-blowing rates — an average of more than one meter of water per year — and doesn’t melt. They drilled more than 200 meters into the ice and snow and extracted long cylindrical cores. By tracking seasonal chemical oscillations, they could delineate each annual layer in the core, creating a timeline, analogous to a tree’s rings, from which they could estimate precipitation levels for each year.

The findings were striking: The precipitation increase that showed up on the observed records after 1950 actually began more than a century earlier, in 1840. And while there had been multi-decadal variations in precipitation over the previous 1,100 years, the ice core confirmed that the 1840-2011 increase marked a distinct and unprecedented shift in the climate.

Ice cores were first drilled for research purposes in 1955 in Greenland. Since then, hundreds of cores — including one nearly two miles long — have been drilled in Greenland and Antarctica. Some go back as far as 800,000 years and can be used to track atmospheric carbon dioxide concentrations, dust or carbon and temperature shifts over time.

Jonathan Thompson is a contributing editor at High Country News. He is the author of Sagebrush Empire: How a Remote Utah County Became the Battlefront of American Public Lands. Email him at [email protected] or submit a letter to the editor. See our letters to the editor policy.

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