One of the research interests of the co-authors of this paper, recently published in Historical Biology, is understanding the timing and driving mechanisms that influenced the evolution of life from marine, lacustrine, or otherwise wet environments towards land. Evaluating the rate and character of the early land colonization by life requires the precise dating of early land biotas.
Sedimentary rocks exposed in the United Kingdom mark the appearance of significant changes in life in Earth's history. These include the speculated first air-breathing land animal and first vascular land plants exposed in the rock record. Based on biostratigraphy, which focuses on correlating and assigning relative ages of rock strata using fossil assemblages contained within them, these events occurred in the Silurian times (between 443.7 and 416 million years ago, Ma). These were critical times that saw the appearance of air-breathing land animals, the emergence of land plants, and several new species of fish and marine life.
Rocks that contain these markers of changes in Earth's life history, however, have few absolute ages. This missing time limits our understanding of not only when life began to appear in drier environments on Earth, but also what drove it to seek these conditions.
The co-authors of this paper remedy this situation by tackling the precise radiometric dating of sedimentary rock layers in the UK that have recognized the importance of identifying major life changes. We extract and date zircon (ZrSiO4), a radioactive mineral common in sedimentary rocks, and often used to understand rock histories.
How is this research conducted?
STEP ONE: We need rocks. For most of our research, we ask field geologists or paleontologists who we know have worked in areas that contain important outcrops (exposed rocks) so they are not subjected to further exploitation. We send emails. Rocks that been studied by paleontologists are ideal because they directly link any ages we obtain to those who understand the significance of the ancient life contained within them. We work with the British Geological Survey (Environmental Science Center, Nottingham) and the Natural History Museum in London (Dr. Giles Miller). We reach out to geologists who publish papers regarding the areas of our interest.
Unfortunately, we find time and time again that universities throw out rocks of geologists after they retire or pass away. Here is a message from one of our colleagues:
"All the original material including type specimens were subsequently cleared from his room at the University of XX and their whearabouts are still unknown."
So, we conduct fieldwork and collect rocks. We rely on published work to tell us where outcrops of interest are located. These can be anything from field guides, 19th-century papers, or recently published results. We sample responsibly following guidelines established by the UK and the geology community.
Sedimentary rock layers are deposited horizontally. These have been tilted 90 degrees. We target the fine-grained layers between the thicker layers as those sometimes are ash from ancient volcanoes. That ash contains the radioactive mineral zircon.
Brookfield sampling the ash layers. We use a small rock hammer to chip away and place the rocks in ziplock bags that I will show you in photographs in the next section.
We need to understand if the rocks contain information about early life. We spend time examining their mineralogy and textures.
Some of the rock layers are exposed beautifully, and we have wonderful weather.
Other days, we spend long hours hiking in terrible weather only to discover the rocks likely don't contain the information we need.
Some rock layers we are interested in are not in beautiful areas but are located behind dumpsters or in car parking lots.
Other times, we hike for hours to find small sections exposed by river erosion. Where we parked our car is circled. We walk on trails, like the picture below, or we make our own trails or use ones that have been created by sheep.
Sometimes we hike and find nothing.
Other times we get lucky and find beautiful rock layers exposed.
We even take advantage of new exposures of rock layers, like these that we stumbled upon as a new road was being constructed.
We have a wonderful time in the field, taking breaks at castles and local restaurants. The photograph above is the Island of Kerrera in Scotland.
Brookfield with wife Kate. The team's official field assistant.
STEP TWO. We need zircons. We extract and document tiny zircon grains from each rock, using techniques pioneered by Suarez et al. (2017). The photograph below shows the rock samples we collected this past field season. Removing and isolating zircons from these rocks is difficult because clay clings to the mineral, making extraction challenging. The samples will go from solid rock to sediment as we pass through several steps in the process.
Eventually, we remove the zircon from their rock host. We mount them for analysis using electron, laser, and ion beam instruments. This painstaking work ensures that we don't lose any of the zircon grains. You can see two mounts in progress in the picture below. Can't see the zircons? They are there, in the center of the circle. We pour epoxy in the ring, and they become encased in a mount.
We polish and image each grain to see their zoning. Sometimes, these grains are spectacular. We used a Scanning Electron Microscope at UT Austin's Bureau of Economic Geology to get color cathodoluminescence images of these zircons from the Island of Kerrera. These grains are as small as the width of your hair. Note that some of the zircons have laser pits in them where we dated them.
STEP THREE. We need ages. We date zircons using a variety of approaches and different lab facilities. We have used the UT Austin's Dept. of Geological Sciences Laser Ablation Inductively Coupled Plasma Mass Spectrometry facility. This facility uses a laser that is aimed at the zircon and radioactive isotopes are ablated from the grains and used to generate an age. We date the zircons using the U-Pb method. We also have used two secondary ion mass spectrometry (SIMS) facilities at UCLA and at Heidelberg University (Germany). I show how we work in these facilities below.
We coat our epoxy mounts in gold and place them in the SIMS. This image shows our mount, along with several others in the UCLA ion microprobe.
Although we get ages from the SIMS or laser instruments, these results are usually too imprecise for us to publish. We use these instruments only to screen for the youngest age. The last step in the analysis process is to get higher-precision age data and to confirm our results. For that, we send our samples to the Scottish Universities Environmental Research Centre (SUERC) at the University of Glasgow, Scotland. They pluck the youngest zircon grains from the epoxy mounts and use chemical abrasion-inductively coupled-thermal ionization mass spectrometry (CA-ID-TIMS) to get the highest-precision ages possible.
When and why life emerged from the sea is of interest to us as humans as we wonder about our own origins. We recognize that the research in the Historical Biology paper has been cited in several news articles and YouTube videos. We appreciate the interest and your comments. The project has involved a team researchers from the US, Scotland, and Germany as well as a contingent of students from UT Austin's Jackson School of Geosciences, including Stephanie Suarez, who is now a graduate student at the University of Houston.
Graduate student Stephanie Suarez.
What is next?
Our future plans are to continue to work on several critical sections in the UK using the approaches described above. We had a fantastic field season in 2019, and are processing these samples for future dating. Although we are working on the question of when did life emerge, we are also focusing attention to the why. Stay tuned.
(Funding sources: Max Kade Foundation, Jackson School of Geosciences, UT Austin Faculty Innovation Center, UT Austin International Office (Global Classrooms), SIMS instruments are funded by the US National Science Foundation and the Deutsche Forschungsgemeinschaft - German Research Foundation. We appreciate collaborative support from SUERC.)