Carbonate sedimentary rocks simultaneously record information about the physical, biological and chemical conditions in which they formed. Consequently, they represent rich archives of the history of Earth's surface, especially when geochemical or paleobiological time series are anchored firmly with field observations and insight (figure right, displaying chemostratigraphy from the famous outcrop of the Taconic Unconformity in Catskill, NY; panorama courtesy of Adam Maloof). The backbone of the carbonate chemostratigrapher's toolkit are measurements of δ13C, used as a proxy to study the behavior of the global carbon cycle. Despite decades of work on this subject, geologists and geochemists still argue about the meanings of these records, and how they should be interpreted in the context of Earth history. What are the causes of positive and negative δ13C excursions? Is the dominant signal in these records the operation of the global carbon cycle, or local processes, such as diagenesis or local carbon cycling? What can δ13C tell us about other biogeochemical cycles, such as atmospheric oxygenation? Of increasing importance is the integration of these datasets with Ca and Mg isotope datasets, as these represent the other main chemical constituent of limestone and dolostone rocks. Our research projects in this field has focused on a variety of time periods with anomalous carbon cycle behavior, including the Middle and Late Ediacaran, Ordovician, Devonian and Middle and Late Triassic. δ13C and δ18O measurements are made in SEOS at the University of Victoria on a Sercon 20-22 gas-source MS with a Gas Box 2 front end.
In the study of Earth history, the importance of time cannot be overstated. Geochronology and the development of rigorous age models allows the timing, order and rates of Earth system change to be quantified, thereby shedding light on the fundamental processes that drive Earth system evolution. Within the sedimentary rocks we study, our group is always on the look out for ash falls amenable to U-Pb dating on zircon using isotope-dilution thermal ionization mass spectrometry (ID-TIMS), in collaboration with labs such as the Princeton Radiogenic Isotopes Lab. In modern geochronological laboratories, low Pb-blank levels (< 1 pg per analysis) allows for the dating of single crystals at high precision. At the University of Victoria, we are able to extract zircons from field samples using standard magnetic and heavy liquid separation techniques, as well as anneal the heavy separates to prepare them for clean lab work. Chemical abrasion follows the technique of Mattinson (2005). After chemical abrasion and washing, grains are spiked with EARTHTIME tracer solution (Condon et al 2015; McLean et al 2015) and dissolved fully in HF + trace HNO3 (figure above, left panel). These techniques allow for high accuracy and precision dates (0.05-0.075% uncertainty (2σ) on the measured age) and form the gold standard for time scale geochronology. Our research has contributed new ages to better constraints the Ordovician-Silurian and Silurian-Devonian boundaries. Chemistry cartoon courtesy of Blair Schoene. Mass spectrometer photographs courtesy of Brenhin Keller.
The sedimentary rock reservoir both records and influences changes in Earth's surface environment. Geoscientists extract data from the rock record to constrain long-term environmental, climatic and biological evolution (see U-Pb geochronology and carbonate sedimentology sections above), with the understanding that geological processes of erosion and rock destruction may have overprinted some aspects of their results. It has also long been recognized that changes in the mass and chemical composition of buried sediments, operating in conjunction with biologically catalyzed reactions, exert a first-order control on Earth surface conditions on geologic timescales. Thus, the construction and destruction of the rock record has the potential to influence both how Earth and life history are sampled, and drive long-term trends in surface conditions that otherwise are difficult to affect. We explore these connections through geological data synthesis using Macrostrat, a geospatial database that describes the age and properties of rocks in the upper crust, and GeoDeepDive, a digital library designed to support scientific data mining. The most basic calculation that can be derived from these resources is the quantification of rock abundance vs. rock age - for a given point in time at a given region in space, does rock exist? In the movie on the right, this quantity is shown for North America sediments (1 billion years ago to present). Does the rock record get worse with age, as is commonly thought? How do records from one region of the globe compare to other regions? How do such time series compare to other, significant events in Earth history, such as atmospheric oxygenation and biological evolution? We continue to pursue data synthesis approaches to Earth history, including through the geographic expansion of Macrostrat (such as to Australia) and its enrichment with GDD-extracted data (such as evaporite mineralogy).