The following is a guest blog post by Phoebe A. Cohen, Ph.D. Phoebe is currently the Education and Outreach Lead and Post-Doctoral Associate at the MIT NASA Astrobiology Program at MIT. Starting in July she will be Professor of Geoscience at  Williams College. Phoebe is also the daughter of Board Vice President Carl M. Cohen.

 

 

More than we bargained for? 570 million years of history beneath the Durant Kenrick House.

I grew up on Magnolia Avenue, in an old Victorian house where my parents still live today. It was a wonderful place to be a kid – a golf course for sledding just down the street, a big back yard for playing, and an old red house on the other side of the block. That red house is the Durant-Kenrick house, and when I was growing up it was hard to get a look inside. We did go in at least once however, and I remember the creaking floorboards, a huge wooden chest that I imagined was filled with wondrous treasures, low ceilings, and a sense that I was walking into a past world about which I knew very little but desperately wanted to know more.

Cut to about 15 years later and I returned to the Boston area to start a PhD program in paleontology at Harvard University. I had never been very much into rocks and fossils as a kid, being more interested in wolves and foxes, but my fascination with the history of life on Earth had grown during my undergraduate years to the point where I decided to devote my professional life to the study of Earth’s past. As I progressed through my graduate work, my academic focus fell squarely on a time known as the Neoproterozoic – a time period ranging from 1,000 million years ago when the world’s continents were barren and life in the oceans was diverse but microscopic, to 543 million years ago, when the world’s oceans rather suddenly became replete with animals and plant life, an event known as the Cambrian explosion.

In my time as a scientist, I have traveled all over the world - Namibia, the Yukon, Australia, Death Valley, Newfoundland  - to study  Neoproterozoic rocks and the fossils they contain –. So in essence, not much had changed – I still wanted to learn about the past, I had just ended up WAY further back in time than the age of the DK house -- or so I thought.

About three weeks ago my boyfriend Zeke and I went to Newton to my parent’s house for a Sunday dinner consisting of my dad’s incredible thin crust style pizza. Over the crunch of crusts, my dad gave us an update on the ‘DK’ project and the classroom and storage space that was being built. He mentioned that  in the course of digging the foundation for the classroom and storage space, the construction crew had encountered  a minor  inconvenience – a deep layer of rock starting just 2 feet below the surface! . My dad started describing the rock to me, and my brain started to tingle. Once he showed me a few photos he had snapped, I was hooked. What he was showing me were sedimentary rocks. The reason this was so exciting to me is that of the three major rocks type, igneous, metamorphic, and sedimentary, the latter is the only kind that can preserve fossils. So my mind suddenly wandered to the possibility that the rocks underneath the DK house, and thus underneath my own childhood home, might contain fossils.

A portion of the excavation, showing at the top about 18" of earth with sedimentary rock below.

The next day I set to work researching the geology of Newton and the Boston area. At some point I’m sure I learned something about this, but it might not have been since the 7^th grade, the last time most students in Newton take Earth Science. I had been lucky enough to take a half-semester elective course on geology and astronomy at Newton North, but even that was ages ago. Since I had become a professional scientist, I had never thought about what rocks might lie under my childhood home – until now.  What I discovered after a little time spent on Google Scholar and the United States Geological Survey website shocked me. Not only were the rocks under my neighborhood sedimentary, they were Neoproterozoic. So it turns out that the rocks that I have devoted my professional career to studying are the very same rocks that lay beneath the house where I spent the first 18 years of my life.

The rocks are in a formation that is known as the Cambridge argillite or the Cambridge slate. An argillite is basically a big mass of mud and clay deposited at the bottom of an ancient ocean basin that has, over millions of years, turned into rock. The DK  argillite is a part of a larger group of rocks known as the Boston Bay Group, which consists of familiar-sounding units including the Brookline and Dorchester members. Radiometric dating of these rocks by scientists from Wellesley College and MIT shows that the Cambridge Argillite is between 570 and 595 million years old – from the latest part of the Neoproterozoic. In general, the rocks of the Boston Bay Group record a period of dramatic change in Earth’s history during which levels of oxygen in the world’s oceans began to rise, which set the stage for the radiation of animal life in the Cambrian.

Much to my delight, it turns out that decades ago a young graduate student at Harvard University in the same lab that my advisor eventually got his degree from had looked at some of these rocks for evidence of microscopic fossils and found some! Admittedly the fossils they found are somewhat scrappy and not earth shattering, but nonetheless, I was thrilled. I quickly called my dad and asked that he go over to the build site before they poured the concrete foundation to get me some samples of the rock. So now there is a (very heavy) bag of Cambridge argillite sitting in my office. This fall I plan to take small pieces of the rock and dissolve it in hydrofluoric acid (HF). . The HF breaks apart the rock itself but leaves behind organic matter, including any organic-walled microfossils that might be inside. These microfossils likely represent the tough external coatings of single-celled algae and other protists, evidence of the microscopic communities that thrived in the same oceans where the first early animals were beginning to evolve.

It still amazes me that I never realized what rocks lay beneath the bucolic tree-lined streets of my own neighborhood until just recently. It reminds me that no matter how far you travel and how much you think you know, there is always more to learn and more to explore. This discovery also makes me wonder what magic the DK house and the rocks it sits on worked during my childhood years that helped to put me where I am today – a woman fascinated by reconstructing the past.

Phoebe A. Cohen