Cold low-latitude Ordovician paleotemperatures may be in hot water

Seawater’s oxygen-isotope (18O/16O) evolution is fundamental to oxygen-isotope (δ18O) paleothermometry and thus to understanding Earth’s climate, habitability, and biological evolution. Thiagarajan et al. (1) seawater δ18O in the Early Phanerozoic based on model-adjusted clumped and oxygen isotope analyses of fossils and especially fine-grained carbonate sediments. We are concerned, however, with the choice of fine-grained carbonate sediments as clumped and oxygen isotope archives, the assumption of closed-system diagenesis in the studied geologic units, and the geologically untenable cold paleotemperatures obtained. In addition, we believe that the paper is mistitled in that the reconstruction of Phanerozoic climate appears only in supplement.