“Don’t Drink the Dirty Chai” - An Examination of Macrofossils in the Grinnell College Peat Deposit

 Isabella Vergara, Hayden Bhavsar, Anna Lipari, Nate L’Esperance

Driving down Interstate 80, or almost anywhere else in Iowa, what do you see? It’s often fields, sometimes rolling, with sparse groups of deciduous trees poking out from the endless expanse of corn and soybeans. To most, the lack of, well, anything at all, creates a bland impression of the Hawkeye State. However, what if we were to tell you it wasn’t always this brown slab of corn? Would you ever assume that at some point, Iowa was a coniferous swampland akin to the vast forests of the United States’ Northwoods regions or Western Ontario? Grinnell Professor Ben Graham has helped to prove that Iowa indeed has an interesting, decidedly very different past.

Way back in the ancient, bygone, prehistoric era of 1960, Professor Ben Graham began to inspect the excavation site of what would eventually become a new Fine Arts center for Grinnell College in Grinnell, Iowa. After digging a mere 2.5 feet below the excavation floor, he began to find a 3 foot thick layer of dark brown, and loose organic matter. Graham found this layer to have an abnormally high water content of 82.4% at a depth of 16 feet, relative to 26% above to 28% below this layer (Graham, 1962.) Later, this layer was dated to 26-27,000 years before present. The layer consisted of peat, a type of organic soil resulting from prehistoric wetlands (Graham, 1962.) Peat originates from acidic freshwater bogs. Because of the acidic, low-oxygen conditions, most dead organic matter that fell into the water became preserved, as the waters would not decompose the debris (Xintu, 2009.) As time passed and more earth filled in over bogs, the resulting preserved organic material hardened into peat deposits. 

Due to the lack of decomposition, Ben Graham found that his excavated peat samples were full of preserved bits of leaves, seeds, stems, and pollen from many prehistoric plants. Formally, these preserved bits are called plant macrofossils. These macrofossils are classified as “remains large enough to be visible without a microscope” (NOAA.) We set out with the objective of indiscriminately extracting these plant macrofossils from the peat samples. What we find in the peat samples may give us more insight into what kind of plants may have existed in place of Grinnell College many tens of thousands of years ago. Similar extractions of the peat have yielded interesting results in 1962 and 2018, which we hope to build on with new techniques that may allow us to find more macrofossils.

As a class, we decided to pick one chunk of peat to share among team pollen, team wood, and us, team macrofossil. We placed small lumps of our peat into beakers with either purified water or KOH (an acidic solution intended to remove compounds that might interfere with imaging) and warmed them to dissolve the chunks into a coffee-colored sludge that we fondly described as a dirty chai. 

Figure 1: Left - the “Dirty Chai” consisting of broken up peat samples. We are testing deionized water and an acidic solution, KOH, for breaking up samples. Right - Anna and Nate searching through the samples for macrofossils with bronze fine mesh sieves (106 and 250um, shown in the bottom right corner). Macrofossils were obtained with tweezers and prepared on microscope slides.

Then, we gently sieved through the sludge under a microscope, to look for any little bits of plant material we could see. If we happened upon anything that looked plant-like, we used a pair of tweezers to gently place the plant bits on slides with a drop of tacky glue, so we could visualize them under our scopes later (Mauquoy et al., 2010). Once our tacky glue had dried, we took pictures of each of our slides and tried to describe and identify the plant material we were seeing based on published macrofossil keys (Lévesque et al., 1988). Then, we were able to compare our findings to the slides that students from previous years had made. 

In 2018, team macrofossil found what they thought was rhizome epidermis from the bog plant Scheuchzeria palustris. Our collection was much different from the 2018 team’s. We found a variety of plant parts amongst the peat. While we were unable to use these pieces to figure out with certainty what kinds of plants grew in the area since they were so small, we made informed guesses about the types of tissue we unearthed. Many of the macrofossils we found were wood fragments that required maceration for identification, a process which our group was not equipped to complete. Despite the abundance of unidentifiable wood, we found a higher volume of plant tissue compared to team Macrofossil of 2018. It’s possible that our fragment of peat had a higher concentration of plant material than the other, or that we tended to isolate smaller fragments than the other team, as there may be differences in our sampling and searching methods despite our efforts to replicate their work. 

In addition to wood, we found charcoal pieces (courtesy of Team Wood) which suggest the presence of fire in the peat bog. We also found significant amounts of epidermis tissue from unknown plants (figures 3 and 4). These samples tended to be very small, without distinguishing characteristics that pointed towards a specific species. We saw two main categories, tissue with short, square-ish cells and tissue with more elongated cells. The lack of stomata, donut shaped cellular holes that allow for gasses to move in and out of the plant, suggests that much of this tissue is bark or root tissue. We did find a few samples which we believe to be leaf tissue with stomata (see figure 4). Other interesting finds include a fossil resembling an amoeba (figure 5), a partial winged seed (figure 5), and what might be some sort of grass sheath (figure 1). 

Figure 2: A ring-shaped tissue fragment possibly from a grass sheath of Scheuchzeria palustris. The elongated cells and shape ring are vaguely similar to leaf scars on S. palustris. Left: 40x, Right: 400x magnification. 

Figure 3: Pieces of fibrous plant tissue with elongated cells. It is possible these come from some kind of marsh grass. 40x magnification. 

Figure 4: Plant tissue with short cells. The bottom right image shows tissue with stomata, donut shaped holes that allow for gasses to move in and out of the plant (shown in the bottom right corner at 400x). This piece probably came from a leaf or stem since these are the areas where stomata are present. Left image and top right at 40x magnification. 

Figure 5. Odds and ends. Left - Tissue fragment, possibly a piece of a winged or wind-dispersed seed. 40x magnification. Top right - a piece of charcoal found by Team Wood. 40x magnification  Bottom right - potentially a testate amoeba, evidenced by what appears to be a shell. 400x magnification.

Though we can’t draw any strong conclusions, our macrofossils hint at changes in prehistoric plant life. Our peat was formed around 27,000 years ago, but we don’t know the range of time covered. Team macrofossils in 2018 might have found different fragments because they extracted macrofossils from a piece of peat that formed when the area was a different kind of environment. Maybe they were looking at a piece from when it was a fen while our piece was from when it was a bog. In this case our findings might show environmental change. For instance, the presence of charcoal in our peat shows evidence of fire. And while we have been unable to make any genus-level identifications, this does say something about the diversity of plant life that once survived in this area– a far cry from the endless fields of corn that seem to dominate today’s Iowa, and a hint at the multiplicity of species that still exist today in the gaps between. 

Works Cited 

Lévesque, P.E.M., Dinel, H., Larouche, A. (1988). Guide to the identification of plant macrofossils in Canadian peatlands. Canadian Government Publishing Centre. 

Mauquoy, D., Hughes, P., van Geel, B. (2010). A protocol for plant macrofossil analysis of peat deposits. Mires and Peat 7(6), 1-5. 

National Oceanographic and Atmospheric Administration (NOAA). Plant Macrofossil. National Centers for Environmental Information.

Xintu, Liu. (2009). Conditions of Peat Formation. Coal, Oil, Shale, Natural Bitumen, Heavy Oil and Peat - Vol II. Encyclopedia of Life Support Systems (EOLSS)