Exploring the potential of clumped isotope thermometry on coccolith-rich sediments as a sea surface temperature proxy

Understanding past changes in sea surface temperatures (SSTs) is crucial; however, existing proxies for reconstructing past SSTs are hindered by unknown ancient seawater composition (foraminiferal Mg/Ca and delta O-18) or reflect subsurface temperatures (TEX86) or have a limited applicable temperature range (U-37(k')). We examine clumped isotope (Delta(47)) thermometry to fossil coccolith-rich material as an SST proxy, as clumped isotopes are independent of original seawater composition and applicable to a wide temperature range and coccolithophores are widespread and dissolution resistant. The Delta(47)-derived temperatures from <63, <20, <10, and 2-5 mu m size fractions of two equatorial Pacific late Miocene-early Pliocene sediment samples (c1; c2) range between similar to 8 and 29 degrees C, with c1 temperatures consistently above c2. Removing the >63 mu m fraction removes most nonmixed layer components; however, the Delta(47)-derived temperatures display an unexpected slight decreasing trend with decreasing size fraction. This unexpected trend could partly arise because larger coccoliths (5-12 mu m) are removed during the size fraction separation process. The c1 and <63 mu m c2 Delta(47)-derived temperatures are comparable to concurrent U-37(k') SSTs. The <20, <10, and 2-5 mu m c2 Delta(47)-derived temperatures are consistently cooler than expected. The Delta(47)-U-37(k') temperature offset is probably caused by abiotic/diagenetic calcite present in the c2 2-5 mu m fraction (similar to 53% by area), which potentially precipitated at bottom water temperatures of similar to 6 degrees C. Our results indicate that clumped isotopes on coccolith-rich sediment fractions have potential as an SST proxy, particularly in tropical regions, providing that careful investigation of the appropriate size fraction for the region and time scale is undertaken.