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Sediment Cores

Every year, little particles drift their way down to the depths of Lake Mendota. Sometimes they are the shadows of once-living beings––algae and milfoil and zooplankton––falling to the bottom where parts will decompose, be eaten, and finally rest. Waves and wind also pull a different form of matter to the bottom: sandy beaches, eroded rocks, unstable banks, loose dirt. These inorganic materials combine with the once-living to create a history of the waterways settled into the lakebed––a process called sedimentation.

We can read this history. Using tools like the piston-operated corer or the freeze corer, scientists plunge these instruments into the lakebed to remove cylinders of sediment, ranging anywhere from a meter to ten meters tall. Each sediment core is a timeline of the lake’s life––it contains the changing levels of chemicals and nutrients of Lake Mendota, gives us clues to the lake’s food web over the years, and captures pollen grains that help us envision what plants were growing along the shores. Cores show us what was occurring before our lives, before the institutional study of limnology (freshwater) was developed along the shore, before the Ho-Chunk began farming corn and squash, before humans were even here at all.

In November of 1968, a 990cm sediment core was taken in Lake Mendota. This nearly 10 meter-long core captured the sediment of almost 15,000 years. That means that at the deepest point of this core, the last glacier was still retreating across Wisconsin and mammoths and mastodons were still roaming the terrain.

Over the past 15,000 years, the climate has been slowly warming, which has shifted Wisconsin through the different biomes pictured above. This shift is not to be confused with the very rapid climatic warming in recent centuries due to human activity. Illustration by Julia Buskirk.

For the majority of this core––928 cm of it to be exact––the sediment is a pale color. This rock is called “buff marl” by geologists, a rock that’s a combination of clays and limestone. These 928cm of buff marl represent roughly 14,800 years of Lake Mendota's life. Only a centimeter of sediment accumulated on the lakebed every 16 years, as humans moved into the area after the last glacier, as the climate slowly warmed, as boreal forest biomes shifted to hardwoods shifted to prairie.

About three fourths of the way down the core, somewhere around 9,500 B.C.E., hundreds of fluted spear points were chiseled. This gives us the timeframe of when the first humans had officially made Four Lakes their home. In the next meter of core, trade networks were developed throughout the continent, bringing North Michigan copper, Wyoming obsidian, and Atlantic seashells to the shores of Glacial Lake Yahara, the large continuous lake the last glaciers had left behind. By 5,000 B.C.E. though, less than half way down the core, this large waterway had settled into the familiar shapes of Mendota, Monona, Waubesa, and Kegonsa.

It was around these Four Lakes that the ancestors of the Ho-Chunk would build the first conical mounds, about a quarter from the top of the core in 500 B.C.E. In the coming thousand years, Ho-Chunk created Effigy mounds in the shapes of animals and spirits to celebrate their gods and honor their dead. They grew squash, corn, potatoes, pumpkins, and other goods. Their camps developed into large communities around the shores of Mendota and the other linked lakes.

And then there’s the top 62cm of this core. The paleness of buff marl begins to get darker for a few centimeters, before turning to a near-black sludge. This black “gyttja,” (said like "yih-chah"), a sediment seeped in organic matter, makes up the remaining core. Radiocarbon dating estimates the age of the first gyttja deposits to be at the beginning of the 1800s––the onset of Euro-American colonization.

The drastic color change marks a drastic shift in lifeways. In the early 1800s, the first white colonizers broke soil with steel plows for expanded agriculture, propelling the rich surrounding soil of Mendota to erode at dramatic rates. These Euro-American newcomers quickly came into conflict with the indigenous Ho-Chunk people.

Colonizers were pushing the newly formed United States westward; their deadly beliefs of racial and cultural superiority dehumanized the Ho-Chunk people and justified expansion in their eyes. The U.S. government perpetuated this ideology and supported colonization through policy––in 1832, the United States forced the Ho-Chunk to cede their land under duress, without prior free and informed consent. In the coming decades, the U.S. sanctioned the violent removal of many of the Ho-Chunk people in their communities across Dejope. This state-sanctioned violence encouraged soldiers and settlers alike to destroy Ho-Chunk homes and communities, forcing many of them to a reservation in Nebraska. Those who remained watched the names of their lakes and rivers erased and changed, and witnessed the destruction of their ancestor's effigy mounds as many were dug up or built over to make way for the growing Madison metropolis.

These newcomers lacked the thousands of years of relationship and history that humans held with the Four Lakes land. The violence and erasure inflicted upon the Ho-Chunk people, even as they continued to return to their homeland, was also the disregard of deep ecological knowledge that comes from a millennia of living here. It was almost inevitable, then, that this violation of human rights quickly evolved into the violation of the rest of the Yahara Watershed.

Many of the nearby hardwood forests were clearcut, a reality shown in the lake cores by a sudden increase in preserved ragweed or ambrosia pollen, a now-common weed, which took advantage of cleared forests and proliferated. Residents dispelled raw waste into Lake Mendota for many years as the city of Madison grew. This method of sewage treatment fortunately stopped within the coming decades, but while the lakes were no longer burdened with excess phosphorus and nitrogen from human waste, these nutrients began leaching into the waterways from a new source. Synthetic fertilizer, high in phosphorus and nitrogen for plant growth, came to dominate industrialized farming practices. These nutrients leached into the waterways, promoting algae to grow at extremely high rates. The algal blooms in Mendota continue to this day.

A map of Madison's urban growth, 1856. A mere half-century of Euro-American colonization saw drastic urban development. Photo from the Wisconsin Historical Society.

In just a few decades, as consistent nutrient levels, sedimentation, and lake production gave way to 200 years of dramatic change, the buff marl turned black. The combination of erosion and increased production by plants and animals darkened these sediments and accelerated their deposits. Today, one centimeter of the Lake Mendota lakebed takes only two years to accumulate, instead of the historic 16.

It’s a dizzying reminder of the time scales we’re a part of, of the rapid ecological crisis we’ve catalyzed. And it’s also a painful and necessary reminder of the origins of the city I call home, the histories I don’t learn, the initial violence I’m only several centimeters of sediment removed from.

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Stay tuned to learn about more recent applications of sediment cores as I delve into invasive species and the rapid changes they’re also inflicting on waterways. And we’ll get to the hopeful stuff too, if you stick around! We need to know the issues before we can delve into any of the solutions.

About the Author & Project

Julia Buskirk has always loved lakes, but after doing research for the Center for Limnology at Trout Lake Station last summer, she is particularly interested in the stories science tells about them. As a Conservation Biology and English double major, she looks forward to bringing these stories outside of a scientific realm and sharing them with anyone who cares about our water.

Thanks to UW-Madison's Hilldale Undergraduate/Faculty Fellowship for funding this project, to Joshua Calhoun (Assoc. Prof., English Department; Affiliate Faculty, Nelson Institute for Environmental Studies) for advising this project, & to the Holding History program for hosting this bookish, bloggish conversation about waterways.

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