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 similar to 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 similar to 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.