New constraints on phosphate concentration and temperature in shallow late Tonian seawater

Although it is difficult to reconstruct Earth surface temperatures during Neoproterozoic time, sedimentological and paleomagnetic evidence demonstrate a dynamic climate, featuring two global “Snowball Earth” glaciations. The recent observation of petrographic fingerprints of ikaite, a mineral that typically forms in near-freezing sedimentary environments, in late Tonian strata was interpreted as evidence that low-latitude shallow marine environments were cold millions of years prior to the Cryogenian Period. Meanwhile, other recent work has demonstrated that elevated phosphate concentration ([DIP]) can inhibit calcite nucleation (perhaps enabling ikaite to form and persist at warmer temperatures) and that late Tonian carbonates formed in phosphate-rich seawater. So, was late Tonian seawater cold, or was it phosphate-rich? To address this question, we combined measurements of carbonate-associated phosphate and ooid-size-based pH constraints to reconstruct seawater [DIP] values for one snapshot of time in the late Tonian Period. Our seawater [DIP] estimates range from 3.8 µM to 7.8 µM, substantially elevated relative to modern shallow seawater and consistent with inferences from previous approaches. Our estimates are below values at which calcite nucleation inhibition has been observed, suggesting that elevated phosphate is an insufficient explanation for the ikaite forming in warm conditions and supporting the hypothesis that late Tonian climate was cool.