Marilyn in Rajasthan, India 2009 |
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 (Ohmoto et al., 2008) where a
diverse team of scientists argued in the field about the levels of oxygen on
early Earth, formation of banded iron formations, and isotopic compositions of billion
year old rocks. One memory I have of this trip is of Dick Holland, Harvard
University, and Hiroshi Ohmoto, Penn State, standing on a BIF and speaking with
a bullhorn to young astrobiologists, to give their perspectives on all of these
topics. My next trip with Dominic was to Rajasthan, India, where we sampled
stromatolites containing commercial grade phosphates from the Aravalli
Supergroup. My 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. Fieldwork
in India, as opposed to other places I had worked, was never conducted without
close watchfulness from local people. 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 buffaloes, and cows observing our activities.
Dominic Papineau, Marilyn, Prof. Roy, and Ritesh Purohit (left to right), India standing on stromatolites billions of years old |
Based on the samples we collected from
India, we published a series of papers on the development of the Earth’s early nitrogen
cycle (Papineau et al., 2013). 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.
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 Umijaq on a Sunday afternoon, making our way across Hudson 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.
Wouter Bleeker lecturing on Precambrian geology |
Stromatolites on Belcher islands, 2010 |
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 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 about the ecosystem in which they
live.
Inuit crew: Captain on left, Marilyn on right |
The 2nd week of our expedition took us back towards 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 3rd
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, Dr. Wouter
Bleeker, took samples for dating, as there are only a handful of dates from
this whole area.
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.
Marilyn and Chris, Belcher Islands |
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 quite strong;
individuals 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.
No comments:
Post a Comment