A TALE OF TWO ARCS: STARTING AND STOPPING SNOWBALL EARTH

The occurrence of repeated, severe, global glaciations during Precambrian time presents a climatic enigma to explain, which is particularly deepened as the two intervals are further distinguished by the two most significant rises in atmospheric oxygen in Earth history. The “snowball Earth” hypothesis has withstood testing better than alternative theories such as high-angle obliquity of the planet. Deglaciation is commonly attributed to the build up of carbon dioxide emissions of ongoing volcanic eruptions. There is, however, little consensus over the initiation of snowball conditions, that is, before the runaway albedo effect took effect. Here we present a holistic model for snowball Earth incorporating the regulation of arc magmatism. Oxygen and hafnium isotopes of zircon, well-dated proxies for magmatic compositions, demonstrate that snowball intervals were (i) preceded by significant mantle-derived ocean arc magmatism, (ii) accompanied by gaps in the magmatic record that reflect the lack of preservation of ocean arc magmatism, and (iii) ended by significant crustal magmatism. Ocean arc magmatism is relatively low in carbon dioxide flux and enhanced chemical weathering, particularly basaltic weathering of volcanic atolls and volumetric large igneous provinces emplaced before continental breakup, consumes carbon dioxide, collectively inducing severe glaciation. Then, as drifting continents eventually override the ocean arc system, a subsequent shift to dominantly continental arc magmatism can explain, as before, glacial termination. The weathering of the extensive arc system required of this model would have provided phosphorous as nutrients to fuel oxygenic photosynthesis, offering a mechanism for the enigmatic correlation between sharp rises in atmospheric oxygen and the snowball glaciations.