Paper No. 11
Presentation Time: 10:30 AM
COSMOGENIC NOBLE GASES AS POTENTIAL PALEOALTIMETERS
38Ar and 3He data from Antarctica and Bolivia show a distinct increase in the cosmogenic 3He to 38Ar ratio (3Hec:38Arc) with increasing sample elevation. Apatites sampled from 1.6 to 4 km suggest a sea level ratio of 2, with an increase to ~6 at 4 km elevation. This variability has three possible sources: 1) Significant production of cosmogenic 36Clc and/or 36Arc; 2) Erosion; or 3) An increase in 3Hec production with elevation relative to 38Arc production. Calculation of 38Arc includes the implicit assumption that 36Ar produced from cosmogenic or other processes is minimal relative to 36Ar originating from air. 36Cl decays to 36Ar and is produced through both cosmic ray neutron and muon processes; cosmogenic 36Arc is also produced directly. Analysis of these data, however, indicates that excess 36Ar cannot fully explain the observed trend. Second, the flux of cosmogenic muons increases with depth in rock. Erosion of an exposed surface would result in an apparent increase in 38Arc production relative to 3Hec. Our data point to an opposite relationship, with the highest 3Hec:38Arc occurring at the site with the most effective erosion. Finally, 3Hec:38Arc can increase with elevation if muogenic production of 38Ar is significant. Theoretical evaluation of direct and indirect 38Arc production from muons on Ca suggests ~40 atoms g-1 yr-1. Model analysis of data thus far produces a best-fit where production from fast neutrons is ~12 atoms g-1 yr-1 and from negative muons (via capture or fast processes) is ~38 atoms g-1 yr-1, suggesting that noble gases could become central components of a quantitative paleoaltimetric technique.