A paired apatite and calcite clumped isotope thermometry approach to estimating Cambro-Ordovician seawater temperatures and isotopic composition

The secular increase in delta O-18 values of both calcitic and phosphatic marine fossils through early Phanerozoic time suggests either that (1) early Paleozoic surface temperatures were high, in excess of 40 degrees C (tropical MAT), (2) the delta O-18 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 delta O-18 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 degrees C) and seawater oxygen isotope ratios similar to today's ratios. (C) 2017 Elsevier Ltd. All rights reserved.