CoNTESTA · A geologic approach to resolving critical uncertainties in the impact of geomagnetic variance on in-situ cosmogenic nuclide production

A geologic approach to resolving critical uncertainties in the impact of geomagnetic variance on in-situ cosmogenic nuclide production

Glaciers respond to changes in climate and leave a record of these changes on the landscape in the form of moraines. Given chronologic control, moraines can be used to reconstruct past climate variability. Recent advances in cosmogenic nuclide (CN) surface-exposure dating now enable just this, establishing these landforms as a key climate proxy. Where glaciers have left a detailed record of past changes in their extent, these geomorphic markers can be used to determine the timing and magnitude of past climate change. However, to make full use of the glacial-geomorphic record for climate reconstruction we must overcome a fundamental methodological uncertainty in cCN surface-exposure dating: the uncertain rate of change of CN production over time. The premise of surface-exposure dating is that glacially transported clasts originate from erosion at glacier beds, where they have experienced minimal exposure to the incoming cosmic-ray flux to Earth’s surface. Transported to the ice margin, the clasts become exposed to cosmic rays and CN accumulation begins via spallation reactions with target atoms in the mineral lattice. Measuring CN concentrations in glacial boulders thus provides an ‘exposure’ age for a moraine, which in turn provides valuable climate data. While straightforward in principle, the viability of this method relies on accurate constraint of the CN production rate, which can be calculated by measuring CN concentrations in rock surfaces of known age. Yet owing to differences in geomagnetic strength and atmospheric thickness, production rates vary widely among sites. As it is impossible to calibrate a production rate for each sample, ‘scaling schemes’ have been developed to correct for differences in elevation and location, enabling the extrapolation of production rates from a calibration site to distal points and periods. Although atmospheric attenuation aligns with air pressure in a predictable manner, geomagnetic variability and its impact on CN production are poorly resolved, especially in the tropical latitudes where the geomagnetic field is strongest. As a result, calculated exposure ages can differ considerably depending on the scheme used. Cosmogenic helium-3 (He-3) is produced by reactions between incoming cosmic rays and atoms within minerals in rock surfaces. Certain minerals such as pyroxene, common in volcanic rocks, retain this He-3 within their crystal lattice. The production rate of cosmogenic He-3 is so far constrained by only a small number of calibration studies, few of which are from the tropics. The objectives of CoNTESTA are to exploit recent advances in He-3 surface exposure dating and Ar/Ar geochronology to establish multiple He-3 production rate calibrations of varying age from the tropics in order to ascertain the viability of discrete scaling schemes, and ultimately to resolve the natural capacity of Earth’s climate for change. The Central Volcanic Zone (CVZ) of Peru contains lava flows of varying age that are compositionally appropriate for both He-3 surface-exposure dating and 40Ar/39Ar dating. Dating these lavas using two independent means provides a series of discrete nuclide production rate calibrations of varying age, data required for establishing the viability of scaling schemes both in the tropics and around the globe. The primary objectives of CoNTESTA are to: 1) Identify lava flows in the CVZ appropriate to target for surface-exposure dating. 2) Establish independent age control on sampled lavas using 40Ar/39Ar dating at the Lamont-Doherty Earth Observatory (LDEO), United States, or, for younger lavas (< 30 kyr), using both 40Ar/39Ar dating and radiocarbon dating of overrun organic materials 3) Measure cosmogenic He-3 in sampled lava surfaces. This objective involves the ultimate measurement of He-3/He-4 ratios at the Noble Gas Mass Spectrometry Laboratory at Le Centre Recherches Pétrographiques et Géochimiques (CRPG) in Nancy, France. 4) To quantify the impact (magnitude and sign) of geomagnetic field variance on CN production over time.

Data: CORDIS, © European Union