Friday, August 30, 2019

Around the world travel to find old rocks

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.

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