Beyond quartz - Revisiting whole-rock in situ cosmogenic C-14

The Earth is continuously bombarded by cosmic rays from space, which yield cascades of subatomic particles in the atmosphere. Some of these particles have high enough energies to induce nuclear reactions with elements in minerals near Earth’s surface, resulting in the production of cosmogenic nuclides. The concentrations of these nuclides reflect the exposure history of the surface as well as the rates of various natural near-surface processes operating on the landscape. One of the most powerful applications of these cosmogenic nuclides is dating the timing of advances and retreats of glaciers and ice sheets, as glacial erosion of the top two meters of rock is enough to reset the cosmogenic nuclide ‘clock’. Polar regions such as Antarctica, however, often experience glacial advances and retreats with limited erosion – concentrations of stable nuclides in those regions can thus reflect multiple episodes of exposure and burial, perhaps over hundreds of thousands of years or more, concealing the most recent exposure signal of glacial retreat. Carbon-14 is also produced in situ and measured in quartz, but decays much more rapidly, reflecting only surface exposure histories over the last 30,000 years or so. As such, in situ Carbon-14 measurements in quartz have been applied in concert with longer-lived cosmogenic nuclides in a growing number of polar landscapes to ‘see through’ multiple exposure/burial periods. However, while quartz is common at Earth’s surface, there are significant regions globally where quartz-poor landscapes currently prevent in situ Carbon-14 from being measured. The work proposed here thus seeks to expand in situ Carbon-14 applications into quartz-poor settings in Antarctica, and by extension globally, by developing reliable techniques for extracting the in situ Carbon-14 signal from quartz-poor whole-rock samples and investigating how different whole-rock compositions can impact its production. Results will help to fill key knowledge gaps in Antarctic glacial history and broaden the global reach of in situ Carbon-14 applications. Delineating the former extents and rates of change of Antarctic ice sheets and glaciers is critical to better place ongoing ice dynamics into past spatial and temporal contexts, particularly for the period since the Last Glacial Maximum and through subsequent deglaciation. In situ cosmogenic Carbon-14 in quartz is rapidly becoming crucial in this effort given its sensitivity solely to exposure histories younger than 30 thousand years. It is now widely applied in quartz-bearing rocks across Antarctica, but numerous sites with ambiguous ice histories remain because quartz-poor silicate lithologies currently preclude its use. Development of robust extraction methods for such targets, paired with improved constraints on in situ Carbon-14 production systematics in whole-rock silicates, will help fill key knowledge gaps in Antarctic glacial history. Pilot studies using the automated PRIME Lab in situ Carbon-14 extraction system with protocols modified for whole-rock silicates, and MATLAB code developed to calculate compositionally dependent in situ Carbon-14 production rates, demonstrate the potential for applying in situ Carbon-14 in quartz-poor terranes and motivate further study. Well-characterized archival calibration samples in-hand from the western US and Scotland from the NSF-funded CRONUS-Earth project, with deeply shielded counterparts, will be used to develop reliable laboratory methods for extracting in situ Carbon-14 from whole-rock samples, constrained by corresponding existing quartz results. Independently well-dated western US mafic volcanic calibration samples will augment these data and explore the possibility of atmospheric Carbon-14 incorporation into lavas. The resulting protocols will then be validated by leveraging mafic altitude transect samples in-hand from western Dronning Maud Land, Antarctica, bracketed by existing NSF-funded quartz-based deglaciation chronologies, extending in situ Carbon-14 to quartz-poor terranes. This proposal will thus ensure a firm empirical foundation for applications of whole-rock in situ Carbon-14 within a consistent analytical framework, reducing lithologic barriers and improving not only our understanding of post-Last-Glacial-Maximum Antarctic glacial history but of late Pleistocene-Holocene Earth surface processes globally. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.