Formation temperature of biomaterials through geologic time*

The well-known secular increase in δ¹⁸O values of both calcitic and phosphatic marine fossils through early Phanerozoic time suggests three end-member hypotheses 1) early Paleozoic surface temperatures were high, in excess of 40C (tropical MAT), 2) the δ¹⁸O value of seawater has increased by up to 7–8‰ VSMOW over Paleozoic time, or 3) early Paleozoic samples are more significantly altered than younger materials, with the secular trend reflecting these differences in post-depositional processes. Carbonate clumped isotope analysis, in combination with petrographic and elemental analysis, has the capacity to deconvolve fluid composition from temperature effects and therefore to determine which of these hypotheses is best supported. A sample suite including calcitic, aragonitic and phosphatic marine fossils from the last 500 million years indicate surface temperatures in tropical seas were similar to slightly warmer than today (avg. 26-38C) in the early Paleozoic, while the oxygen isotope budget of seawater appears to be similar to that of today.