Four Million Year Long Marinoan Snowball Earth - Adrian Tasistro-Hart

Four Million Year Long Marinoan Snowball Earth
- Adrian Tasistro-Hart
University of California, Santa Barbara
The two Snowball Earth glaciations of the Cryogenian are among the most dramatic climatic events in Earth history and accompany major shifts in evolution and geochemical cycling. The chronology for the Sturtian, the first snowball glaciation, is well-constrained on several continents from ca 717--662 Ma. The termination of the Marinoan, the second snowball glaciation, is also constrained on several continents to ca 635 Ma. The onset, however, remains poorly constrained from 651--639 Ma. The date of onset determines both the duration of the Marinoan as well as the Middle Cryogenian period between the Sturtian and Marinoan. Uncertainty in onset limits our ability to test hypotheses for possible triggers for the Marinoan, as well as to develop age models for the Middle Cryogenian.
We present four new CA-ID-TIMS U-Pb zircon ages from the syn-Marinoan Ghaub Formation at Fransfontein Ridge in Namibia. The oldest age of 638.93+/-0.32 Ma originates in stratified glacial sediments 3.8 m above a conformable and gradational contact with the underlying non-glacial Franni-aus Formation. We interpret this age to constrain the onset of local glaciomarine sedimentation at the beginning of the Marinoan, implying a 4 My duration.
These ages also constrain the timescales of grounding line (GL) advance-retreat cycles preserved in the Ghaub Fm, which are not limited to the late stages, but occur throughout glaciation. By comparison with orbitally-forced Cenozoic glaciomarine stratigraphies, we argue that Ghaub Fm GL cycles are inconsistent with Cenozoic-style sensitivity to obliquity and precession.