Tonian Low‐Latitude Marine Ecosystems Were Cold Before Snowball Earth.

Precambrian marine carbonate strata are commonly assumed to have formed in warm‐water carbonate factories due to the temperature dependence of non‐skeletal carbonate precipitation rates. However, some climate models and geological observations suggest that global climate was cool for tens of millions of years prior to the onset of Snowball Earth glaciation at ∼717 Ma, in conflict with common interpretations of pre‐glacial carbonates as warm‐water carbonate factories. We report the occurrence of guttulatic microfabric—a petrographic fingerprint of ikaite, a carbonate mineral that only forms in cold sedimentary environments—in the Beck Spring Dolomite, a carbonate succession deposited in a low‐latitude shallow marine environment between ∼780 and 730 Ma. This interpretation of pre‐glacial carbonate factories aligns cold conditions with vase‐shaped microfossils, possible algal fossils, and molecular clock dates for crown‐group metazoans. Our observations indicate that these marine ecosystems were able to thrive in cold low‐latitude environments millions of years before the Snowball glaciations. Plain Language Summary: Between 717 and 635 million years ago, Earth experienced two dramatic global glacial events, known as "Snowball Earth" glaciations, during which ice covered the oceans all the way to the equator. Geoscientists are still seeking to fully understand what caused these extreme climate events and how life on Earth survived them. Although geochemists have a variety of tools to reconstruct the temperature of ancient oceans, these methods are difficult to apply in rocks this old because primary signals have been too altered. Instead, we looked for a key microscopic fingerprint ("guttulatic microfabric") of a type of calcium carbonate mineral ("ikaite") that only forms in cold‐water environments. Previous work had proposed that we might expect to find evidence of this cold‐water carbonate mineral associated with a specific type of sediment called "giant ooids." We found abundant evidence of guttulatic microfabric in sedimentary rocks containing giant ooids that formed in a low‐latitude shallow marine environment millions of years before the onset of global glaciation. Our observations suggest that Earth's climate was cold before the onset of global glaciation, which could mean that marine organisms were accustomed to cold conditions well before the Snowball glaciations. Key Points: Guttulatic microfabric is a characteristic fingerprint of ikaite, a mineral that forms only in cold‐water depositional environmentsWe report guttulatic microfabrics in grains and cements associated with giant ooids in the Tonian Beck Spring DolomiteOur findings demonstrate that global climate was cold millions of years before the onset of the Sturtian glaciation [ABSTRACT FROM AUTHOR]