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.
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 7th 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
