Taking the heat out of British Jurassic septarian concretions

Septarian carbonate concretions in marine mudrocks contain calcite cements that should represent evolving conditions from ambient temperature, shallow subsurface environments to warmer, burial diagenetic conditions. Clumped isotope results British Middle and Upper Jurassic indicate most concretion body calcites formed at temperatures between 9 ± 5°C 18 pore waters with ?18O values 0.2 1.1‰ ?2.2 1.1‰VSMOW. Early diagenetic, brown, fibrous fracture mostly 15 19 5°C, again marine-derived fluids compositions ?0.5 0.3 1.2‰VSMOW. Two of these showed evolution warmer more evolved growth, indicating transition deeper Later sparry gave variable but all indicated involvement meteoric fluids. The highest clumped temperature (43 6°C) is within error the 50°C regional maximum estimate. These are consistent published geological stable constraints on formation septarian highlight their potential as robust archives benthic palaeotemperatures. Some differ data an earlier study reported higher for early cement fringes probably due methodological differences.