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Geochronology: It's About Time OPINION the potassium- argon (K- Ar) and uranium- thorium/helium (U- Th/He) methods to rocks and minerals has advanced our understanding Geochronology: It’s About Time of the pace of tectonic processes. Together with studies of nuclides produced by cosmic rays, this understanding has revolutionized the study of landscape evolution. The advent of carbon- 14 dating radically altered our understanding of prehistoric human migration. However, this isotope’s 5700-year half-life led to an apparent concen- tration of events 30,000 to 40,000 years ago. This pileup only relaxed to include much older ages after the development of optically stimu- lated luminescence dating, which brought quantitative chronologies to systems that were historically difficult to date. Coupled with enormous advances in uranium series dating, these techniques focused on the Pleistocene have been essential for calibrating glacial- interglacial cycles from climate records. Taking Stock: Where Are We Now? George Gehrels A visionary program (EarthScope, http://www A zircon crystal (approximately 0.2 millimeter long) from the Coast Mountains in western Canada shows zonation that .earthscope.org) begun in 2002 has been spec- records multiple stages of crystallization. The ages from this sample range from more than 116 million years old at their tacularly successful in revealing the three- cores to 58 million years old for outer rims. Zircon plays an important role in radiometric dating because it is a com- dimensional structure within the North Amer- mon mineral in crustal rocks; it contains trace amounts of uranium and thorium, which decay to lead with reasonably ican continent. However, fulfilling its goal of well-constrained half-lives; and it preserves a record of geological processes despite younger metamorphism. understanding the evolution of the North American continent will require a major effort led by geochronologists. ime is at the heart of Earth sciences; million years into billions of years. The ability The National Research Council [2012] report every significant advance in geochro- to date young basalts revealed a geomagnetic New Research Opportunities in the Earth Sciences T nology has produced a paradigm- time scale that led directly to the plate tectonic recognized the central role that geochronology shifting breakthrough in our understanding of revolution. plays in the geosciences (Figure 1) and identi- Earth’s history. Without quantitative knowl- The development of high- precision fied pressing instrumentation and facilities edge of absolute and relative time, no modern uranium-lead (U- Pb) zircon dating is currently needs for fostering research and education. In discipline with a historical focus could func- revolutionizing our understanding of magmatic response, with the support of the National tion. time scales as well as the tempo of sediment Science Foundation, we led a yearlong consul- However, when we conducted a broad con- accumulation and biologic change. In situ U- Pb tation with consumers and producers of geo- sultation with geochronology experts and dating challenges our assumption that early chronology to understand their aspirations researchers who rely upon geochronological Earth was an arid world that was hostile to life. and the challenges they face. data, we found a strong sense that the field The recognition that major extinction events We surveyed nearly 300 U.S.-based geo- has been orphaned by the national science are coeval with the formation of large igneous chronology producers on their views regarding support structure and weakened by the wide- the role of geochronology in innovation, spread view that it is a “tool” rather than a Geochronology has transformative science, facility support, syn- scientific challenge. ergistic research, and the status of decay con- Every discipline that benefits from geo- provided information that stants. With these results as a guide, we chronology should participate in the steward- drastically changes our hosted meetings at the 2014 Goldschmidt ship and development of this field, including Conference, the 14th International Conference scientific efforts and financial support. In understanding of natural on Thermochronology, and the 2014 Geologi- return, geochronologists must address the cal Society of America Annual Meeting. The research priorities of the disciplines they sup- phenomena. conclusions drawn from these interactions port and provide enhanced user access to data. were recently published in the report “It’s About Time: Opportunities and Challenges Radical Changes in Perception provinces and asteroid impacts is changing our for U.S. Geochronology” [Harrison et al., 2015]. In one instance after another, geochronology understanding of the processes of species evo- has provided information that drastically lution and highlights the dependency of Data Disconnect changes our understanding of natural phe- Earth’s living systems on extraterrestrial Our field has changed dramatically over the nomena. The earliest mineral dates, deter- inputs. past 20 years. Many scientists now focus on mined more than 100 years ago, catapulted By quantifying variations in rock tempera- dating youthful features at or near Earth’s Earth’s estimated age from 10 million to 100 ture and depth through time, application of surface using measurement methods that 12 // Eos 1 March 2016 OPINION didn’t exist a generation ago. In situ methods into disciplines that require have produced massive amounts of new data different constraints on tim- for those interested in deeper time or deeper ing and rates. As geochro- Earth, whereas traditional methods have nology spread into these gained an exquisite degree of precision as they emerging fields, it often have matured. failed to become firmly Despite these advances, our consultations rooted within those cultures. revealed several troubling paradoxes. Routine analyses could be Although more instruments than ever are supported through existing being employed for Earth science research in programmatic funding, but the United States, the geochronologic commu- new fields lacked the tradi- nity still has enormous demand for more data. tion of sustaining the devel- Individual laboratories produce more, and opment of geochronologic higher quality, data than ever, but many inter- protocols. ested parties feel that costs are prohibitive or For our community to that bottlenecks in the process preclude their truly prosper, we must make participation. The techniques and instruments the case across the geosci- in use today are far more sophisticated than ences that stewardship of those in the recent past, but many are con- geochronology is the cerned by the lack of progress in putting responsibility of all disci- proven advances in geochronologic instru- plines that use its products. mentation to use. This stewardship includes Our consultations point to a lack of optimi- the need to support high- Fig. 1. Geochronology plays a central role in all historical aspects of the Earth zation between geochronology producers and risk research and develop- and planetary sciences but has no disciplinary home within the federal fund- consumers. We believe this mismatch is ment of equipment, meth- ing umbrella. driven by misdirected incentives in combina- ods, and applications. In tion with a pervasive view that geochronology return, the geochronology is merely a tool rather than a discipline in its community needs the ana- own right. lytical and staffing resources to address the unanswered scientific questions of our time Looking back, it is easy to see how this research priorities of supportive disciplines (e.g., when life began on Earth) and would occurred. To address the specific needs of a and to provide enhanced user access to permit the goal of EarthScope—to understand particular Earth sciences discipline, geochro- data. the four- dimensional structure of North nologists developed novel methods to address America— to be fully realized. their challenging scientific problems. In the Grand Challenges Our report points to the need to support process, individual geochronologists became Geochronology is poised to make unprece- foundational and potentially high-risk devel- experts in an increasing diversity of special- dented leaps in its capacity to stimulate trans- opment of new geochronological methods, the ized fields. This specialization, in turn, has formative research. We envision four grand need for greater cooperation among existing separated the providers of geochronologic challenges for the coming decade: laboratories, and cooperative interactions with information from those who apply this infor- • age precision and accuracy of ±0.01% allied scientists. We invite you to download a mation: In some cases, these information con- from the Cenozoic to the Hadean, which copy of our report (http://bit.ly/Geochron_rpt) sumers have never actually participated in the requires creating methods and mass analyzers and join the conversation on the future of U.S. analyses that provide them with their data. of unprecedented sensitivity and resolution geochronology at http://bit.ly/Geochron Even within geochronology, researchers with vastly improved decay constants _ feedback. perceive different issues facing them. For • continuous temporal coverage through- example, some researchers are frustrated by out the Quaternary—from 1 week to 1 million References the low accuracy with which decay constants years—of processes key to today’s societal Harrison, T. M., S. L. Baldwin, M. Caffee, G. Gehrel,
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