Sky water. It needs no fence. Nations come and go without defiling it. It is a mirror which no stone can crack, whose quicksilver will never wear off, whose gilding Nature continually repairs.

-Henry David Thoreau

Years ago, Crawford Lake revealed that Indigenous groups had lived locally in the area, pre-contact. Now, scientists suspect the mud at the bottom of the lake could contain the benchmark that defines the era in which we currently live. Known as the Anthropocene, it is a newly proposed epoch of geological history spanning the last 70 years or so. The proof, scientists hope, lies in what they can extract from the mud at the bottom of the lake. It contains a vivid record of the past human migrations to the area, and earth scientists believe it can act as a gold standard to which subsequent studies are compared.

Scientists are working to define the Anthropocene as the period in Earth’s history in which human activity has been the most dominant influence on the planet. In order to define it, a single reference point needs to be identified by one or more stratigraphic signals. Once this point is adopted, then it can be correlated to strata around the world. Given any geological time interval tends to use rock strata to slice up Earth’s history, the Anthropocene is a little different due to its short duration and the fact that it is ongoing, which means we cannot use rock strata for comparison as there hasn’t been enough time for rocks to form. We need something a little more time sensitive, which the accumulated mud at the bottom of Crawford Lake can provide.

All of the research conducted at Crawford Lake has proven it to be a perfect mirror through which to view a new epoch but scientists have yet to decide on which marker to use to define the Anthropocene. One of the markers being considered as a candidate to define the onset of the Anthropocene is the Radiocarbon “bomb spike” due to atmospheric nuclear testing that was conducted during the fifties and early sixties. The Radiocarbon isotope (14C) is naturally occurring but the signal is greatly distorted by production of 14C in the many atmospheric nuclear tests. As this signal is worldwide, it represents a possible stratigraphic marker for the Anthropocene, and one that we could confidently date at Crawford Lake.

Crawford Lake is a meromictic lake, which means that the water in the deep central basin is devoid of oxygen and permanently stratified. This is due to the great depth of the lake, compared to the small surface area. The permanent stratification prevents the lower, colder and denser layer from mixing with the upper, warmer and circulating layer, so the lake doesn’t experience the spring and fall turnover—when the two layers mix—, the way that most lakes do. Most lakes are the shape of a bowl, whereas Crawford Lake looks more like an inverted traffic cone, which further protects it from the churning power of the wind. Due to the lack of oxygen, no living thing exists in the deep basin to stir up the falling sediment that accumulates. The lack of circulation within the lake and the absence of any bottom-dwelling creatures both result in sediment layers being well-preserved.

In 1970, it was discovered that the sediment in the lake was “banded” with alternating dark and light layers. One dark and one light layer together form what is called a varve, which is an annual layer of sediment, similar to an annual tree ring with clearly visible boundaries. Since the varves form on a yearly basis, scientists can use an undisturbed sediment sample containing the present year layer still forming, to count back to each individual year. This is known as an absolute chronology, where each year is confidently known. Many chronological studies depend on records with a relative chronology —where the only relationship known is that layers nearer to the top are younger —which isn’t sufficient for the Anthropocene’s short duration. This makes the sediments at Crawford ideally suited to study the Anthropocene.

To see the undisturbed sediment up close, we use a freeze corer, also known as the “frozen finger”. This technique has been used to recover sediment from Crawford Lake since the seventies. The device is a wedge-shaped steel box with two smooth metal faces and two faces used for handling. The corer, which is hollow, is filled with a slurry of dry ice and ethanol. This causes the surface of the corer to be extremely cold—between -70°C and -80°C—so that when the corer is lowered into the underlying sediment and left in place, the sediment flash freezes to the metallic surface and can be recovered without disturbance or damage to the layers, preserving the absolute chronology.

Soon after the annual mud layers were discovered at Crawford Lake in 1970, the upper portion of the sediment revealed a clear timeline of European farming. A decrease in tree pollen, for instance, indicated the clearing of forests, and an increase in grass and weed pollen indicated land disturbance as a result of farming beginning in the early 1820s. Following this, in the 1950s, there is a rise in woody tree pollen as more individuals left farming to move to the cities where stable work was easier to come by. This is how, through the mud at the bottom of the lake, we get a glimpse of human movement.

When scientists looked back even further into the mud layers, they found a second period of farming, indicated by six hundred-year-old corn pollen. The First Nations farming period began in the early fourteenth century and lasted about 300 years, after which the Wendat dispersed out of southern Ontario. After the corn pollen was discovered, archaeologists began studies of the area around the lake and found the remains of six longhouses about two hundred metres from the lake, with dimensions that suggest a village of around four hundred and fifty people. Additional surveys of the area found five more longhouses and a number of other villages. Further studies of the mud layers at the bottom of Crawford has added to the history of human occupation and movement, including records of charcoal (remains of fire) and eutrophication (increased nutrients to the lake due to farming). Through previous studies, scientists have discovered humans defining their own environment—and possibly, a new era.

Other indicators being considered include large volcanic eruptions, including Tambora and Krakatoa—as they occurred lock-step with the industrial revolution—and the arrival of technologically-produced fossils, such as plastics and silicon-based computer components —but what better signal to define the epoch in which humanity has been the dominant influence on the planet than the record left behind from the testing of weapons meant to obliterate opposing populations in the aftermath of the Second World War?

The opening musings of Thoreau on Walden pond as a timeless mirror are even truer for Crawford Lake. It has stood watch for millennia, with its azure waters retaining the memory of the many people who have called its shoreline home. Through the lake, scientists can see how inhabitants of the land have changed from simple farmers to a civilization capable of changing the Earth itself. There is something poignant about using an unassuming lake to define a geological epoch in which humanity reigns supreme. Somehow we think that the man who went in the woods to live deliberately, and ended up falling for the sky water of Walden pond, would approve.

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Last modified: October 17, 2018

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