Stoneman Lake (posted September 18, 2019)
The above picture illustrates some reasons why I chose Stoneman Lake for my research while a graduate student at Northern Arizona University in Flagstaff – it’s gorgeous, its tranquil, and its secluded! This lake scene is looking eastward toward the volcano complex, Lake Mountain, in the background. I conducted geologic and botanical investigation at this lake for my master’s thesis. It was titled “A Geo-botanical Study of Stoneman Lake, Wet Beaver Creek Experimental Drainage Basin, Coconino County, Arizona”.
To my knowledge, I was the first geologist to conduct research on Stoneman Lake (1970). From 1970 to 2014 the scientific community showed little interest in this remote body of water, but that began to change due to the contentious debates over “climate change”. Stoneman Lake has recently come under the spotlight because its greater than 300-foot-deep sedimentary record, being a completely closed system, offers a unique, uninterrupted record of the climate in southwestern U.S. covering the last two million years. Since 2014, researchers from Arizona, New Mexico, Minnesota, Nevada, Pennsylvania, Spain, and Scotland have published papers on Stoneman Lake.
A Brief Overview of Stoneman Lake
If you vacation in Northern Arizona, Stoneman Lake is a little off the beaten path but well worth a visit. As the Black Canyon Highway ascends from the Verde Valley toward Flagstaff, look for the Stoneman Lake exit. After a dozen or so miles of back roads, you arrive at the west rim overlook (above view). All the rocks within your sight are volcanic, ranging from dark lava (basalt) to ash and cinders. The west rim is over 100 feet lower in elevation than its surrounding rim-rock; this is because the lake overflowed during cold seasons within the ice age, eroding the lava-rock to form notches and terraces.
Geologists believe lava first began pouring out from the base of Lake Mountain through north-trending fissures around 8 million years ago. At least four or five times over the next 6 million years the volcano was active; we know this because each layer of lava that flowed is composed of a different suite of minerals. Research data estimates the last flow occurred around two million years ago. Shortly after that, the lava beds collapsed, forming the Stoneman Lake caldera. The collapse is thought to have been caused by magma retreating through a southwest-trending chamber at depth. At least two layers of volcanic ash (called tephras) are embedded in the lake sediments, one at 30.8 meters (101 feet) and the other at 36.3 meters (119 feet), so we know (based on sedimentation rates and radio carbon dating) this marks eruptions that occurred after the lake caldera formed, one 720,000 years ago and the other 802,000 years ago. The estimated total thickness of sediments in the lake is at least 300 feet, which would easily make it over two million years old.
Natural springs on the eastern shore are the primary source of inflowing water. The large trees on top of and inside the crater are Ponderosa Pine, Alligator Bark Juniper, and Utah Juniper; based on annual tree rings, some of the Juniper trees are over 1000 years old! Beneath that scenic lake are sediments filled with fossils of clay varves, paleomagnetic “excursions”, pollen, diatoms, mollusks, and the bones and scales of fish; when analyzed, these things provide us with accurate dates and a clear picture of the climate changes over time.
Fossil Pine Pollen in Stoneman Lake
Fossil Diatoms in Stoneman Lake
If we want to be able to accurately predict the impact of climate change in our future, we need to understand what it looked like in the past; Stoneman Lake provides researchers with lots of climate change data back to two million years ago. I assume the reader is familiar with pollen; all plant species can be identified by their unique pollen profile. Fossil pollen in the lakes and soils gives us a picture of changes in vegetation over time, reflecting the climatic environment in which they thrive. For example, when we see cactus pollen, we anticipate hot, dry desert environment; willow and cypress indicate lots of available water; Bristlecone Pine live in alpine settings at elevations over 10,000 feet. The pollen record of Stoneman Lake over time demonstrates alternating climate cycles of megadrought (multi-decade droughts), wetland (shallow water marshland), and lacustrine (lake levels from a few feet to hundreds of feet deep).
Over 85 species of diatoms are preserved in the Stoneman Lake deposits. Diatoms (see above), also known as plankton, are single-celled, microscopic algae found in the oceans, waterways and soils of the world. Besides being known as “whale food”, they generate over 20 percent of oxygen we breath, and take in over 6.7 billion tons of silicon per year. Diatoms are very picky about where they live; some prefer deep acidic lake water while others like shallow, warm ocean lagoons; still others thrive in alkaline marshes. Diatoms present an accurate picture of their environment, which is a direct reflection of the climate. They are key indicators of Stoneman Lake’s paleoclimate changes.
Nearly daily you can turn on the news and hear someone’s opinion about climate change. One thing is certain, our planet has gone through repeated climate change over its billions of years, is currently undergoing climate change, and will continue to do so far into the future. At least 20 glacial (and interglacial) cycles are preserved in the sediments of Stoneman Lake. In general, we have learned (from study of Stoneman Lake) that the climate in southwest U.S. at the peak of each glacial epoch was surprisingly less extreme than originally speculated; this could bode well for mankind when (not if) we enter our next ice age. Also, the transition periods of these climate change cycles appear to be thousands of years, not ‘suddenly’ like the 12 years (doomsday scenario) some of our politicians are predicting. Earth is surely marching toward its next ice age, despite man’s best efforts (sarcasm intended) to delay it by releasing CO2 into the atmosphere. Our current level of CO2 is 405 parts per million (ppm). If we are to feed the 7.5 billion, and growing, humans on earth into the future, finding ways to increase CO2 levels might help offset the pending cooling of our planet.
The Creator foresaw this eon ago. He stored up abundant fossil fuel and gave us methane farting cows to keep us well stocked with a supply of greenhouse gas in preparation for the billions of humans to be fed and to recreate on his planet. CO2 is plant food!
Bonus: Where did Stoneman Lake get its name?
The lake was named in honor of General George Stoneman (1822 – 1894), a Union officer in the Civil War, and roommate with Stonewall Jackson at West Point. He was commander of the Union cavalry under General Hooker at the disastrous (for the North) Battle of Chancellorsville. He was replaced by General Sickles of Battle of Gettysburg fame, then re-emerged under General Sherman in the march through Atlanta. After the Civil War he moved to Arizona where in 1870 he took command in the “Apache Wars” to fight the renegade Cochise. He later became the 15th governor of California.
General George Stoneman
Picture credit: Pinterest.nz
Definitions:
Paleomagnetic Excursions: Magnetic excursions are brief, global, millennial-scale or centennial-scale, directional anomalies of the geomagnetic field that occur within a “chron”. The earth’s geomagnetic poles have reversed 183 times (North and South Pole reversal) over the last 83 million years, an average reversal every 453,000 years. This period is called a “chron”. The most recent chron, the one we live in called the Brunhes-Matuyama Reversal, happened 780,000 years ago, so we’re overdue for another chron. Within each chron may occur any number of “excursions” lasting hundreds to a few thousand years.
Caldera: a large cauldron-like hollow that forms shortly after the emptying of a magma chamber/reservoir in a volcanic eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is lost. The ground surface then collapses, leaving a massive depression, often described as a crater or sinkhole.
Diatom image credit: microscopesandmonsters.wordpress.com
Pine pollen image credit: botit.botany.wisc.edu
