Human History Recorded In Mud

Human History Recorded In Mud

11:10am Nov 04, 2016
The footprint surface is in the foreground, and we are looking at Oldoinyo L’engai in the background
This is a photograph looking south. The footprint surface is in the foreground, and we are looking at Oldoinyo L’engai in the background (it is an active volcano, but also the likely volcano that produced the footprinted sediments). Along the right side of the photo, you can see a steep cliff and that is the East African Rift Scarp (the zone where the African continent is splitting apart).
Cynthia M. Liutkus-Pierce, from Liutkus-Pierce et al., 2016 Figure 2

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Your community emerges after a soaking rain. The merciless sun burns off the clouds, disentangling them from smoke, newly wafting from a volcano on the opposite shore of a great salty lake. You join a group, mostly women and children, to forage for food and clean water, working fast because you know it will all dry up quickly.

Dr. Cynthia Liutkus-Pierce Interviews for SciWorks Radio at the ASU studios. Credit: David Blanks

Days later, the volcano erupts, unleashing a blizzard of ash atop the now-parched land.

Soon after, rain and volcanic mud-swollen rivers and streams inundate the lake, flooding the area, and leaving a new layer of mud atop the hapless region.

This is one likely scenario for a group of humans in Tanzania’s Great African Rift Valley, along the southern shore of the salty Lake Natran, tens of thousands of years ago.

It’s a site called Engare Sero, and it sits right at the base of an active, carbonatite volcano called Oldoinyo Lengai.

That’s Dr. Cynthia Liutkus-Pierce, Associate Professor of Geology at Appalachian State University. She is the lead author of a recent paper published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, investigating a site of fossilized human footprints.

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The preservation is astounding. You can imagine when you kind of step into mud, it kind of slurps up between your toes? We see all of that. And, even between successive footprints of the same individual, you can see drips of mud that was kind of dropping off their feet as they were moving and walking.

Credit Cynthia M. Liutkus-Pierce.  This photograph is of one of the footprints at Engare Sero.  You can see how “slurpy” the sediment was, and how the mud almost squishes up around the foot of the print maker.  This tells us that the sediment was very goopy when the footprints were made.  But, you can also see the large linear cracks on the surface of the sediment…  this tells us that, eventually, after the footprints were made, the soft sediment dried out (creating the large mud cracks) which likely helped to preserve the footprints.

At Engare Sero, we have over 400 individual human footprints. It seems that most of the footprint makers were women and children, with maybe only one or two adult males in the group.

There seems to be two different groups of people, and they’re walking in opposite directions. At least two of the individuals were running. This was probably some sort of a foraging party going out and looking for either food or water or shade.

There’s also evidence of fossilized plants and animals, including zebra hoofprints that show the animal sliding a few inches in the slippery mud.

In general, fossilization is rare and unlikely. You have to have some special circumstances.  

You would have the muddy debris flow come down, the humans would walk through it, and kind of leave those trackways. The area had to dry out. We also know that the hardened footprint's ash had to be covered very soon after by another deposit.

To date the site, the team relied on crystals formed in an eruption, but this dry volcanic mud is a mixture from many eruptions over a great span of time.

We had to date numerous crystals and find out which one was the youngest. These debris flows can pick up crystals of any age. The flow had to come after those crystals were deposited, and so you have to find the youngest crystal within the flow, and then say ok, the flow is younger than that.

So we have our “it can’t be older than” date: 19,000 years.

The youngest possible age is determined from the overlying lake deposit.

The oxygen isotopes of the calcite in that overlying unit show that they had to come from a saltwater source. That overlying unit had to be in place by the last time Lake Natron expanded and flooded the site. That was about 10 to 12-thousand years ago.

So that gives me a very nice tighter bracket -  about 10 to 19-thousand years ago - for the footprints to be deposited and preserved.

Dr. Liutkus-Pierce’s team sleuthed out about a nine-thousand year range, giving a snapshot in time of the life, culture, and even gender roles of our not-so-distant ancestors. And there’s still more at this site to be uncovered.

When we first got there in 2008, there were tire tracks over the footprints’ surface. The Tanzanian government has now barricaded off the human-footprinted surface. But we are very concerned about trying to come up with a conservation and a preservation proposal to make sure that this area is highlighted for its anthropological significance, but also kept safe.

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