Apatite (U-Th)/He thermochronometric data show rapid late Eocene incision in the central Transantarctic Mountains

The Eocene-Oligocene climatic transition and the development of a permanent Antarctica icesheet at ca. 34 Ma is well documented by benthic foraminiferal oxygen isotope and other paleoclimate records, but when and the extent to which icesheet development impacted the subglacial landscape of Antarctica is poorly defined. One difficulty in constraining the timing of landscape development stems primarily from the fact that data from low temperature thermochronometers capable of recording signals of landscape change are often too dispersed to allow precise dating of cooling due to incision and erosion. Here we present new apatite (U-Th)/He data (n=116) from 19 samples across two vertical transects on Barnes Peak and the Cloudmaker, near the Beardmore Glacier in the central Transantarctic Mountains, and consider the extent to which the dispersed ages can be interpreted to robustly constrain the timing of cooling. We augment routine (e.g., n≈5) analyses with several large-n (e.g., n≈25) analyses, and show that old-skewed apparent-age distributions exhibit young peaks that coincide with the first quartile of the respective distribution. We further quantify the likelihood that estimates of the first quartile in small-n sample sets would accurately predict the first quartile of an old-skewed distribution of a hypothetical apatite population. We argue that the minimum and the first quartile of each sample can be used to constrain intrasample age variation, appropriately filtering out the noise of most sources of error in the method, and that its use is justified both by the principles of apatite helium dating (because almost all sources of error will skew ages to be older) and the empirical age distributions in our samples. Applying this to our dataset yields a clear age-elevation relationship that suggests significant incision in the Transantarctic Mountains occurred at or by the onset of glaciation, at ca. 36-34 Ma.