Soru

Neoproterozoic glacial deposits (such as diamictites) are found globally, even at paleo-equatorial latitudes. Scientists have proposed four conflicting models to explain these geological observations.

Model 1 (Snowball Earth)
This model proposes that the Earth’s surface was entirely frozen, from pole to pole. A runaway ice-albedo feedback triggered complete glaciation. Because the oceans were sealed by ice, the hydrological cycle stopped, preventing chemical weathering of silicate rocks. Volcanic outgassing of $CO_2$ accumulated in the atmosphere until it reached extremely high levels ($~0.1\text{ bar}$), triggering a hyper-greenhouse effect that rapidly melted the global ice sheet.

Model 2 (Slushball Earth)
This model argues that complete global glaciation would have driven Neoproterozoic life to extinction, which is not supported by the fossil record. Instead, Model 2 proposes a dynamic equatorial ocean belt of open water or thin, slushy ice. Glaciation was stabilized before runaway feedback occurred, primarily due to negative feedbacks from tropical cloud cover. The hydrological cycle continued at a reduced rate, allowing slow silicate weathering to continue and requiring less extreme atmospheric $CO_2$ accumulation to initiate melting.

Model 3 (Zipper Rift)
This model contests the global nature of these glaciations, proposing instead that the deposits are regional. During the breakup of the supercontinent Rodinia, active continental rifting created localized, high-elevation mountain ranges along rift margins. Glaciers formed on these alpine highlands at low latitudes, and the resulting glacial debris (diamictites) was deposited in adjacent, rapidly subsiding rift basins. The apparent global distribution is an artifact of sequential rifting events occurring at different times across the globe, rather than a synchronous global ice age.

Model 4 (High Obliquity)
This model proposes that the Earth’s rotational axis had a tilt greater than $54^\circ$ during the Neoproterozoic. At such high tilt angles, the equator receives less solar radiation annually than the poles, making low-latitude regions colder than high-latitude regions. This setup explains why glaciers formed preferentially at the equator while polar regions remained ice-free, without requiring global ice sheets or anomalous carbon cycle states.

Based on the models presented, match each mechanistic prediction or assumption on the left with the correct scientific model on the right.