Quantitative measurements of dates and rates of events and processes provide some of the most fundamental foundations of earth and planetary science. Geochronology establishes the basis for our understanding of phenomena ranging from condensation of the solar nebula 4.6 billion years ago to the nature and pace of climate change in the last ten thousand years. While geochronology is an essential tool for nearly any field of earth and planetary science, it is also important to recognize it as a rapidly evolving and dynamic field in its own right. In the last few decades, major theoretical and technical advances in geochronology have influenced the research directions in earth and planetary research and directly driven some of the most important breakthroughs. These include advances in tectonic geomorphology, ore and petroleum migration, and climate dynamics. As geochronology continues to evolve, it will continue to not only inform and support the diverse fields that draw from it, but play a major role in influencing their courses and interactions. The AGC seeks to enhance collaboration, innovation, and leadership in geochronologic research, with the goal of establishing a distinguished center of excellence and motivating outstanding fundamental and applied research in earth and planetary science.
The AGC was formally conceived and proposed to the University of Arizona and Arizona Board of Regents in January 2007. It seeks to coordinate research and outreach efforts among several outstanding geochronology groups at the UofA, capitalizing on extent active research programs in:
Cosmogenic isotopes: including (14C, 10Be, 26Al) for dating biologic materials, artifacts, and extraterrestrial materials, and exposure age and sedimentation and erosion rate studies on timescales of 102 to 106 yr.
The radiogenic isotope systems U/Pb, Rb/Sr, Lu/Hf, Sm/Nd, and Re/Os, for dating rocks and minerals over timescales of 106 to 109 yr, and tracing sources and cycles of hydrocarbons, economic ores, groundwater, and environmental contaminants. Much of the initial work establishing the now widely used Lu/Hf system was done at the UofA.
The radiogenic noble-gas isotope systems 40Ar/39Ar and (U-Th)/He for dating formation ages of rocks and minerals over timescales of 104 to 109 yr, and establishing time-temperature histories of rocks through the shallowest parts of the earth’s crust.
Uranium- and Thorium-series isotope systems, with an enormous range of applications from magmatic processes to climate reconstructions over timescales of 102-106 yr.
Dendrochronology, for combining precise absolute dating with climate dynamics, geologic events, and human history, over timescales of 100-104 yr.
Specific research foci contributing to the AGC’s larger mission:
Integrated Studies of Mountain Belts
Mountain belts provide natural laboratories for studying a wide range of geologic processes such as plate tectonics, crustal magmatism and metamorphism, erosion and sedimentation, and climate phenomena. Interdisciplinary geochronologic approaches that measure ages and rates and trace mass transfer among reservoirs are essential requirements for any meaningful understanding of these processes and the larger scale behavior of mountain belts, the lithosphere, and plate tectonics. Highly integrated large-scale study of mountain belts with essential geochronologic components is, and has been for several decades, a distinctive strength of UofA Geosciences. An example of this approach is a recent successful grant for integrated study of the central Andes involving upwards of a dozen PIs in Geoscience, whose fields range from seismology to climate dynamics. Essential components are the basic understanding of spatial and temporal patterns (and mantle vs. crustal contributions) of magmatism from U/Pb dating and Sm/Nd and Lu/Hf isotope systems, metamorphism and crustal deformation from Rb/Sr and 40Ar/39Ar dating, erosion and tectonic exhumation from (U-Th)/He dating and cosmogenic isotopes, sedimentation and volcanic stratigraphy from 40Ar/39Ar and U/Pb dating, and paleoalitmetry and climate change through time, from 14C, U-series, and cosmogenic dating, and tree-ring studies. Integrative research using all of these techniques is necessary to understand some of the most pressing and scientifically rich problems surrounding orogenic dynamics and lithosphere-atmosphere interactions. These include the nature and extent of dynamic coupling between erosion and tectonics, and links between punctuated episodes of crustal shortening, uplift, magmatism, and lithospheric foundering (crustal-scale “dripping” of material into the mantle) on 106-107 yr timescales.
Integrated Study of Pleistocene and Holocene Climate Records
One of the most exciting, rapidly expanding, and important areas of geochronology today is its application to climate records during the Pleistocene ice ages and in more recent (Holocene) time. Some powerful influences on our climate today, such as the El Niño—Southern Oscillation (which gives Arizona half or more of its annual rainfall), or the flow of warm tropical water into the northern Atlantic Ocean that warms Europe, may have been radically different in the past. There may be tipping points in global temperature change, when large shifts in climate or ocean circulation occur in response to very small additional warming. Understanding that past history of global changes during the Pleistocene and Holocene should enable better prediction for humans as global temperature creeps upwards in the coming century. Integrating climate records across huge distances and latitudinal gradients is critical for understanding past (and future) climate change, and requires much precise geochronologic work. The three techniques best suited to this task—14C and U-series radioisotopic dating and tree-ring dating —are especially strong at the UofA. Carbon-14 dating has been a distinctive strength of UofA for decades, and recent innovations here in sample extraction and measurement now make it one of the best facilities in the world for this method. U-series dating has recently (since 1997) been developed at the UofA. The modern field of dendrochronology was created at the UofA early in the 20th century, and the Laboratory of Tree-Ring Research remains a world leader in tree-ring applications to a broad range of fields, including climatology, ecology, and geology. A large cadre of climate researchers at UofA focuses on modeling, coral or cave records, radiocarbon and tree rings, making it an internationally recognized center for understanding global Pleistocene/Holocene climate connections. U-series dating allows rigorous dating of carbonate deposits, like coral and cave calcite, back to 400,000 years before the present, encompassing all of the last three glacial/interglacial oscillations, and complementing 14C extremely well. Tree-ring dating currently provides seasonal and annual resolution dating of woody materials back through most of the Holocene (9,000 plus years before present), and there is great potential to extend these records to 20,000 years or longer with buried and submerged wood that is now being extracted in various places around the world. There are opportunities to use such records for extension of the calibration of radioisotopic dating, with payoffs in improved understanding of climatic phenomena, such as rapid climate changes of the past.
Technical Innovation in Geochronology
Development of new analytical technology has always played an important role in geochronology. The advent of both multi-collector ICP-MS and SIMS technology, for example, revolutionized geochronology, and also attracted much favorable attention to the labs and departments that achieved these developments. Routine analyses with standard geochronologic techniques may serve conventional applications, but innovation in methods and instrumentation has the potential to produce establish real research leadership. In this light, a long-term priority for the AGC is pursuing directions and funding for technological advances in geochronology, by encouraging and facilitating interactions between geochemists, engineers, and researchers with innovative approaches. Two areas for focused attention are experimental development of small-scale accelerator mass spectrometry, and in-situ geochronologic devices suitable for field or spacecraft use. Small, self-contained geochronologic analytical systems would be valuable both for use in situ on planetary surfaces and in remote terrestrial environments. Although such a system could not hope to compete with the kind of laboratory-based analyses of which AGC is capable, they could revolutionize analyses of planetary surfaces in cases where samples will not be returned for many years or decades and the absolute calibration of stratigraphic chronologies is in doubt, as is the case for Mars. For terrestrial applications, the use would be as a reconnaissance tool, enabling geoscientists in remote or inaccessible regions to quickly establish basic chronologic relationships in the field, and choose the best samples to return for laboratory analysis. Tim Swindle in UofA’s LPL has done extensive development work on a noble gas-based system, for K/Ar analyses or (for planets without an atmosphere or magnetic field) cosmic-ray-exposure ages of surfaces. Based on test measurements, uncertainties of 20% or less (far worse than laboratory measurements but useful for either the martian surface or many terrestrial locations) should be possible. Under the acronym AGE (Argon Geochronology Experiment), this has already attracted more than $2 million in external funding. Although an instrument for spacecraft applications would be different in some details from a “backpack” instrument, the basic traits of low mass, volume, and power consumption would be common to both.