Ancient biologically-formed, 2 billion year old rocks; Marilyn, Richmond Gulf, 2011 |
Since the
early 1970s, scientists have been measuring the amount and nature of organic
carbon from almost 4 billion-year-old Precambrian rocks. Geologists searched
the world over for older and older rocks resting exposed on Earth’s surface
that might contain evidence of the first signs of life. Greenland’s coasts have
a couple of small outcrops with some of these oldest rocks. For example, the
Isua formation was thought to have formed in a sedimentary environment 3.85
billion years ago. Not everyone agrees these were originally sedimentary rocks,
however. The other outcrop—the Akilia formation—is very small in scope with
similar disagreement as to the rocks’ origin. In far northern Canada on the
shores of Hudson Bay, a third very old section of rocks with a nearly
unpronounceable name of Nuvvuagittuq is also thought to be almost 4 billion
years old. These ancient rocks are important because they hold clues to the
origin and timing of life on Earth—something critical to searching for life on
other planets or moons. If we can’t find the necessary evidence of life here on
Earth, how will find it using rovers on Mars or spectra of far distant
extrasolar planets?
In the 1980s, Cyril Ponneperuma and his
student, Cliff Walters, of the University of Maryland, examined the organic
geochemistry of the Isua rocks to find evidence of the first living organisms. Professor
Ponneperuma wanted Cliff to discover something revolutionary. Cliff struggled
at the University of Maryland to find any molecules that did not look like
modern contamination, but his professor pressured him to “discover” something
big. Fortunately for Cliff, he sought out the wisdom of Geophysical Lab’s Tom
Hoering. Hoering’s reputation for careful, exacting work was well known in the
geochemistry community, particularly after he debunked an earlier study on
“Precambrian” hydrocarbons, which turned out to be ink from the newspapers
wrapping the rock specimens. Walters, working with Tom, finally concluded that
any molecular signals in these samples were contamination. He went on to become
a very successful petroleum organic geochemist at Exxon Mobil, having learned
from Tom Hoering about stringent lab procedures.
A decade later, UCLA scientists used
more sophisticated instrumentation to measure carbon isotope signals directly
in the rocks (i.e. in situ)
with an instrument called an ion probe, a multi-million dollar combination mass
spectrometer and microscope. A beam of
strong ions, charged particles, bombard the polished surface of a rock sample.
Elements from the rock are sputtered off the surface then accelerated through a
high vacuum flight tube where they are separated in a strong magnetic field
and then counted by ultrasensitive electronics. The ion probe was promoted as
the solution to answering the question of whether the carbon in ancient rocks
was indigenous to the sample or was caused by contamination later in the rock’s
nearly 4 billion year history. The UCLA group measured carbon isotope signals in
Isua samples, concluding that they were in the range of similar measurements
from much younger samples that every one in this scientific community agrees
are formed by living organisms.
Stromatolites, Belcher Islands, Canada, 2011 |
The problem with the ion probe
measurements is that there were no comparable carbon isotope standards by which
to compare the carbon isotopes of the sample. As time went on, ion probe users
realized they needed to be much more careful about how their instruments were
standardized. At the Carnegie, scientists there worked on this problem and
solved it by testing a suite of rock types by conventional isotope techniques
with measurements made by the ion probe. This is now standard protocol. Dominic
Papineau, a postdoctoral fellow at the Geophysical Laboratory in the mid-2000s,
used this approach for accurate and precise carbon isotope analyses of the Akilia
rocks from southwestern Greenland.
Dominic Papineau, a French Canadian,
was a graduate student at the University of Colorado training with Stephen
Mojzsis, the senior author on the original ion probe paper while he was a
student at UCLA and now a professor in Colorado. Steve Mojzsis has quite a
reputation for speaking his mind at scientific conferences. A bright,
well-spoken man, he can argue a point with great skill, which he does. Dominic
wrote to me midway in his doctoral work and asked if would be on his
dissertation committee. I readily agreed. Dominic, learning from his professor,
tried to emulate Mojzsis, but as a student, he wasn’t ready to take on senior
scientists in public. Papineau came to the Geophysical Lab as a postdoc working
with me and opened many new doors for research and collaboration. With a bit of
a swagger, he worked hard trying to deal with opinions and speculations about
the Earth’s oldest rocks. Unfortunately, Dominic has a reputation in the
community that is tainted by a scandal that he was involved in during a field
trip to Canada that Dominic led for Boston College students in 2012. He was
able to prove his innocence, but he lost his position at Boston College and is
now working at the University College London. I stood by him during this
difficult time. We remain colleagues to this day.
Dominic and Marilyn, Canada, 2011 |
As Dominic began his studies on the
Akilia rocks, I became aware of all of the strong opinions and controversies
with these particular samples. Several authors published on the Akilia “rocks”,
however, there are only a handful of specimens from this location and no real
outcrop that can be studied by the community. Therefore, few samples can be shared
among labs. Speculation and debate about
what type of rocks these are and how they were formed abounds. Dominic obtained
a couple of the Akilia specimens from his PhD advisor Stephen Mojzsis and we
entered the scientific fray. My first paper with Dominic and Steve was based
primarily on microscopic analyses using Raman spectroscopy, transmission
electron microscopy, and Synchrotron X-ray based microscopy, all highly
technical instruments available at national and high-end scientific research
labs like the Carnegie. In this paper, Papineau studied graphite, a high
temperature pure carbon form, found in association with apatite crystals,
phosphate minerals common in many types of rocks.
Andrew Steele and I encouraged him to
quantify the occurrences of graphite-apatite pairs rather than loosely
describing them. Were their common features? Were there only one or two within
a thin section? Did they all look the same? Papineau found that about one-sixth
of the apatite crystals were associated with a graphite coating. Raman
spectroscopy determined that the graphite in these rocks was crystallized at
very high temperatures (>650°C) during metamorphism in the deep Earth. The
carbon was severely altered in its composition making it impossible to
determine if it was originally made by a living organism or from non-living, geological
processes that produce the thick deposits of graphite that is mined to make
pencils. In a second publication with
these samples, we found that the carbon isotope patterns were quite different
than the earlier ion probe measurements. We were stuck concluding that we were
unable to pin a biological origin to the graphite in these old rocks. It was a
disappointing, unsensational conclusion, but correct. Papineau had gone the
extra mile to produce good primary data. As we learn more, perhaps there will
be a new instrument to provide a more definitive answer as to whether life
originated 3.85 billion years ago or much later by 3.5 billion years ago.
While doing this research, I learned
first hand about “having a dog in this fight” as it refers to scientific
findings. With very few exceptions, for example, the majority of scientists
understand global climate change. Non-scientists might still have some doubt,
but for me this is “settled” science. Early Earth scientists can be very opinionated
and have no problem arguing one way or another about when life arose on Earth.
They have “dogs in the fight.” I remain hopeful that as new techniques are
developed the community will come together.
Traveling the world to find old rocks
The work with Papineau took me to
several locations around the globe to examine Precambrian rocks in the field.
We traveled to Ontario and Quebec to study banded iron formations (BIFs) on a
NASA Astrobiology Institute -sponsored field trip in which a diverse team of
scientists argued in the field about the levels of oxygen on early Earth,
formation of band iron formations, and isotopic compositions of
billion-year-old rocks. One memory I have of this trip is of geologist Dick
Holland, a distinguished professor at Harvard University, and Hiroshi Ohmoto, a
strongly opinionated professor Penn State, standing on a BIF and speaking into a
bullhorn to young astrobiologists, to give their perspectives on all of these
topics.
Marilyn and Verena Starke, Canada NAI trip |
My next trip with Dominic was to
Rajasthan, India, where we sampled stromatolites containing commercial grade
phosphates from the Aravalli Supergroup. I arrived in India via Mumbai airport
and was immediately staggered by the density of human beings. Traffic moves in
both directions even on separated freeways! Trash is everywhere—in scared
temples, in fancy neighborhoods, as well as places with shacks made out of
sheet metal and cardboard. Open pit mining, which has largely disappeared in
much of the United States, was happening in every town we traveled through. On
the plus side many people, even in dense cities, had small vegetable
gardens. We were frequently invited into
people’s homes for tea, something rarely done in America. Food was outstanding.
My only gastrointestinal challenge came from eating at a deserted, tourist
restaurant. The rock outcrops were spectacular and no one bothered us.
Himani Chobisa, my assistant, and Marilyn, India 2009 |
This trip to India and my first
in-depth field trip to examine stromatolites in a natural setting was a
remarkable experience. Standing on outcrops that extended for several
kilometers and that had been formed almost entirely by the actions of microbes
was a highlight for me as a biogeochemist who was brought into the field by the
early work of Barghoorn and others from the 1970s. My challenge was to inspect
the rocks in the field and couple observations with my more reductionist
approach based on isotopic measurements in the laboratory. We traveled with two
Indian specialists, Professor Roy and Professor Ritesh Purohit, who had studied
the geology of these formations for many years. Based on the samples we
collected from India, we published a series of papers on the development of the
Earth’s early nitrogen cycle. Based on these 2.15 billion years old samples, we
linked the carbon cycle to a robust nitrogen cycle at the time when atmospheric
oxygen increased 2.4 billion years ago. Microbes, primarily cyanobacteria, were
the producers of oxygen at that time. Not only did we measure high concentrations
of organic carbon in these rocks, but their carbon isotope values
were highly variable. Extreme variability in carbon isotopes is indicative of
swings from low to high primary productivity by photosynthetic organisms.
Himani and Marilyn, in the airport, 2009 |
Fieldwork in India, as well as in
Ethiopia, was never conducted without close watchfulness from local people. Working
in India was a different experience than Australia, Belize or Svalbard. The
absence of personal space in India was something I’d not experienced before.
People carried out their “bathroom” activities on the side of highways, as an
example. When we were collecting rock samples, folks did not ever interfere,
but they were within a few feet of where we were working. At the end of a 1 to
2 hour sampling, our field area would be lined with about 20 to 30 men, women,
and children along with goats, water buffalos, and cows observing our
activities.
Field work in Rajasthan, India 2009 |
My second major field trip with Dominic
Papineau was fascinating for its spectacular geology, the remoteness of the
location, and the chance to interact with native people of northern Quebec.
From a small village on the eastern shore of Hudson Bay we chartered a fishing
boat, the Kakivak, in July 2011, that
was crewed by Inuit men. My husband accompanied me and 13 other
scientists along with five Inuit crew for a two-week adventure on Hudson Bay.
We set sail from the small village of Umijaq on a Sunday afternoon, making our
way across the Bay to the Belcher Islands.
These islands are special for several reasons. First, they are very
remote, and scientists have visited them only sporadically over the past 100
years. Robert Flaherty described the geological formations in 1918. Our target
samples were 1.875 billion-year-old stromatolites that had first been found in
the early 20th century. Scientists at that time realized how special these rocks
were and found evidence for the remains of microorganisms that lived on the
early Earth. We returned to several of these sites, spending three days at one
of the most spectacular stromatolite sections that I have ever seen.
Loading the Kakivak in Umiaq, 2011 |
Second, the
islands are special because they are biologically pristine. This was the second
time I was able to study and sample tundra vegetation. As the temperatures of
Arctic and tundra areas increase due to climate change, plants will respond
with longer growing seasons, making it important to develop records of present
day communities and the processes that influence them. I was able to collect
about 75 specimens from the Belcher Islands for my herbarium collection that
may—some day—serve as an historic record of what the plant life was like in the
early 21st century.
People other than the Inuit rarely
visit the Belcher Islands, as there is little to no support for ecotourism in the
area. We were fortunate to be able to experience Inuit culture including native
fishing. Periodically, the crew fished while we were out examining rocks. They
caught Arctic char which they shared with us: the muscle, Canadian sushi, went
to the scientists and the rest of the fish--tongue, liver, intestines, skin,
heart--was consumed raw with great relish by the crew. The Inuit understand in
a very fundamental way the ecosystem in which they live.
The second
week of our expedition took us back toward the mainland. We traveled to the
Nastapoka Islands that form an arc parallel to the coastline, a part of the
Hudson Bay considered by some to be a remnant crater from a meteorite impact.
Our scientific party scoured several of these islands looking for evidence of
shocked rock strata indicative of such an impact. We were unable to find
samples of this nature, but could see correlations between these rocks and
those on the Belcher Islands. Our third destination was the Richmond Gulf, an
unbelievably beautiful body of water with high mountains, cliffs, and crystal
clear waters. Our team scoured at least 7 different sites with numerous
outcrops to compare the stratigraphy here with that on the Belcher Islands.
Canadian Geological Survey scientist Wouter Bleeker took samples for dating, as
there are only a handful of dates from this whole area.
Chris, Wouter Bleeker, Marilyn--after two weeks of no showers |
In the Richmond Gulf, we were treated
to a sighting of beluga whales, small white whales considered a delicacy by the
Inuit. The pod of about 20 belugas swam into the inlet where we were moored,
diving, jumping, and hunting for the abundant Arctic char. Our Inuit crew
watched them carefully, but decided not to hunt owing to the fact that we had
15 people on one small boat.
Almost 600 kg of rocks were shipped
back to the United States and Ottawa for further analysis. The expedition was a
lifetime experience for all of us, as we were privileged to seeing places,
rocks, and people that very few people will ever have the opportunity to
experience. The results from this trip are currently being written up for a
publication, spear-headed by Papineau, on the nature of concretions found in
Paleoproterozoic rocks and what they mean in terms of organic carbon cycling.
Studies on isotopic compositions of
Earth’s earliest sedimentary rocks are going to feed into studies that will
consume the astrobiological community when samples from Mars are finally
returned to Earth. It is vitally important for the scientific community to
continue to carefully study biosignatures on the Earth weighing what is a
definite biosignature versus an ambiguous one.
The personalities that study Earth’s oldest rocks are strong and hold
strong opinions. There is a constant push and pull to announce the first
evidence of life on Earth, similar to the desire to find the signs of life on
Mars.
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