| Popis: |
The severe “Snowball Earth” glaciations proposed to have existed during the Cryogenian period (720 to 635 million years ago) coincided with the breakup of one supercontinent (Rodinia) and assembly of another (Pannotia). The presence of extensive continental ice sheets should theoretically lead to a tidally energetic Snowball ocean due to the reduced ocean depth, as was the case during the last glaciations, but the theory of the supertidal cycle suggests that the supercontinent paleogeography should lead to weak tides because the surrounding ocean is too large to host tidal resonances. So which theory is correct? Using an established numerical global tidal model and 22 paleogeographic reconstructions spanning 750-600Ma, we show that the Cryogenian ocean hosted diminished tidal amplitudes and associated energy dissipation rates, reaching 10-50% of today’s rates, during the Snowball glaciations. In contrast, the tides were more energetic during the ice-free periods, and we propose that the near-absence of Cryogenian tidal processes may have been one contributor to the prolonged glaciations if these were near-global. These results also constrain lunar distance and orbital evolution throughout the Cryogenian and highlight that simulations of past oceans should include explicit tidally driven mixing processes. |
