Friday, February 14, 2020

Earth’s Earliest Signs of Life: If we found it, could we recognize it?

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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