Studying early life and evolution through fossils
Peter Liberty, a current PhD candidate working under the supervision of Prof. Marc Laflamme, is a paleontologist studying early life and evolution through fossils.
In order to determine the rate of changes in life history and determine if certain environmental changes (e.g. sea level, atmospheric oxygen, etc.) impact the evolutionary history of Earth, the exact ages of the rocks containing that information must be known. This is typically done through radiometric dating, a technique that measures the decay of radiogenic elemental isotopes to stable isotopes. As these isotope systems have known rates of decay, the mother and daughter isotope ratios are an indication of how much time has passed since the system became closed. Carbon dating is perhaps the most famous example of this, which can be used to date organic substances up to 50,000 years ago.
Uranium-lead (U-Pb) dating is the current international standard for determining the ages of rocks. Thanks to its ability to compare multiple radiometric decay pathways at once, it can “self-check” and ensure more accurate results, and is accurate when determining ages in the millions to billions of years. This dating typically performed on zircons, small crystals that form deep underground and have a variety of properties that make them ideal for testing: they are temperature resistant, pressure resistant, relatively easy to find, and at the time of formation they accept uranium into their crystal matrix while rejecting lead, which means that all lead present in a modern sample is a result of radiogenic uranium decay. However, fossils are found in sedimentary rocks rather than the igneous and metamorphic rocks zircons are found in. As such, zircons are of limited use in providing direct ages for fossils. If volcanic ash layers are close to a rock layer containing fossils they can be used as an indication of a minimum/maximum age, but time spans between rock layers can represent thousands to millions of years.
With the funding received from the CPS Research Visit Fellowship program that is supported by the Vice Dean, Graduate’s office, Peter was able to go to the Université du Québec à Montréal (UQAM) to work with Professor Joshua Davies and his research group to try experimental carbonate U-Pb dating on fossils. Fossil shells are typically made of calcium carbonate (CaCO3), a mineral that, like zircons, accepts uranium and rejects lead at the time of formation. However, lead is a highly mobile element during chemical alteration of a rock, which can occur from interactions with heat, pressure, or water. As such, only exceptionally preserved fossils can be dated using this method.
Peter analysed samples lent from a colleague at the University of Tennessee Martin, as well as samples he collected from the field in Namibia and from the University of Toronto collections, using cathodoluminescent (CL) microscopy and a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at UQAM. Peter will be presenting his results at conferences later this year and will use them as the basis to publish a future geochronology paper on carbonate fossil U-Pb dating.
