Seawater temperatures during the early to middle Ediacaran: Phosphate oxygen isotope records

Abstract

Early multicellular eukaryotes are hypothesized to have first evolved in deep-water (i.e., subphotic) environments with a narrow temperature range and low oxygen levels (pO₂). However, seawater paleotemperature estimates for the Ediacaran Period remain poorly known. To address this issue, we measured the oxygen isotopic compositions of phosphate (δ¹⁸O_P) in fine-grained marine siliciclastic sedimentary rocks from two slope sections of the Doushantuo Formation (∼635–551 Ma) in the Nanhua Basin, South China. The bulk δ¹⁸O_P values (∼19–26 ‰) found in early-middle Ediacaran phosphatic marine shales represent the oldest known records of modern seawater-like δ¹⁸O_P values. These values are inferred to record original seawater signatures and suggest that subtropical surface temperatures had a modern-like range of 15–30 °C. Based on these estimates, we propose that the extreme greenhouse conditions of the earliest Ediacaran following the Marinoan Ice Age were of quite short duration (<∼0.1 Myr). Subsequently, the oldest known metazoan fossil assemblage, the Lantian Biota (∼602 Ma), thrived in a deep-water environment characterized by low oxygen levels and a relatively cold, narrow temperature range (∼7–8 ± 2 °C). In contrast, the slightly younger Weng'an Biota (∼587 Ma) inhabited shallower waters with higher oxygen levels and a broader temperature range (16–30 °C). Finally, our evidence shows that the oxygen isotopic composition of phosphate in fine-grained siliciclastic marine sedimentary rocks can serve as a valuable proxy for reconstructing the temperature of ancient seawater. Such paleotemperature information, combined with oxygen concentration data, is crucial for understanding the origin and evolution of early animal life.