Biomaterials and their isotopes through geologic time

Calcitic and phosphatic biomaterials provide a critical record of Earth's past ocean and climate conditions. Here we explore the nature of the materials including their structure and isotopic composition. Specifically we consider the secular increase in δ¹⁸O values of both calcitic and phosphatic marine fossils through time. This trend suggests either that 1) early Paleozoic surface temperatures were high, in excess of 40°C (tropical MAT), 2) the δ¹⁸O value of seawater has increased by 7–8‰ VSMOW through Paleozoic time, or 3) diagenesis has altered secular trends in early Paleozoic samples. Carbonate clumped isotope analysis, in combination with petrographic and elemental analysis, can deconvolve fluid composition from temperature effects and therefore determine which of these hypotheses best explain the secular δ¹⁸O increase. Clumped isotope measurements of a suite of calcitic and phosphatic marine fossils from late Cambrian- to Middle-late Ordovician-aged strata–the first paired fossil study of its kind–document tropical sea surface temperatures near modern temperatures (26-38°C) and seawater oxygen isotope ratios similar to today’s ratios.