Project I.2: Geomagnetic reversal boundaries of the last 3 Myr: what is their real age? (Tiffany Rivera supervised by Michael Storey)

Objective: Our aim is to intercalibrate high-precision 40Ar/39Ar ages with refined astronomically derived values (Project I.1) for the youngest part of the geological record (the last 3 Myr). Focus will be in particular on dating geomagnetic reversal boundaries by both methods in view of existing inconsistencies with previously proposed intercalibrations. The outcome of this project will directly be applied to determine phase relations between astronomical forcing and climate response and constrain the tuning of pre-Neogene successions (Projects II.1 and III.1).

Background: In the (early) nineties, numerous studies provided new 40Ar/39Ar ages that were consistent with results of the upcoming astronomical method for dating geomagnetic reversal boundaries of the last 3.0 Myr, thereby confirming the then discrepant astronomical ages. However, the intercalibration of the 40Ar/39Ar and astronomical method based on late Miocene successions (see textbox: Kuiper, 2004; Kuiper et al., 2007) is not consistent with the outcome of most of these studies. This project aims to rigorously test the intercalibration of the 40Ar/39Ar and astronomical methods by solving the discrepancies with these previous studies. This outstanding and unsettled issue has to be solved contemporaneous with the application of the intercalibration under task II and III. The focus of project I.2 is on reversal boundaries of the last 3 Myr.

Research strategy and methodology: High precision 40Ar/39Ar dating will be carried out on (i) sanidine and biotite derived from volcanic ash layers that are intercalated in astronomically dated sequences that record geomagnetic reversal boundaries and cryptocrons from the last 3 Myr and and (ii) lavas erupted at the time of field reversals. Potential targets come from deep-sea piston cores (e.g., KCO1B: Lourens, 2004), and from continental and lava successions (e.g., east Africa: Deino et al., 2006)). Improved and consistent correlation of 40Ar/39Ar ages with astronomical cycles requires improved precision on 40Ar/39Ar dates, particularly for Pleistocene-aged samples. The recent development of commercial multi-collector Ar mass spectrometers offers the possibility of higher precision argon isotopic measurements, as multicollection reduces the errors introduced through signal decay and sequential measurement of different isotopes species on a single collector instrument. This project will exploit this technological breakthrough by undertaking 40Ar/39Ar dating using the newly commissioned multi-collector noble gas mass spectrometer (Noblesse) at QUAD-Lab.

Feasibility, and innovative aspects and relevance: Adoption of such an empiric cross-calibration may lead for the first time to internally consistent 40Ar/39Ar, U/Pb and astronomical time scales. Further, an astronomically calibrated standard in the 40Ar/39Ar geochronology significantly reduces the absolute uncertainties to <= 0.3% allowing the method to identify the correlative 405 kyr eccentricity cycle in the first-order tuning of pre-Neogene successions.