Taking the heat out of British Jurassic septarian concretions

Abstract

Abstract 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 from British Middle and Upper Jurassic concretions indicate that most concretion body calcites formed at temperatures between 9 ± 5°C and 18 ± 5°C from marine pore waters with δ18O values between 0.2 ± 1.1‰ and −2.2 ± 1.1‰VSMOW. Early diagenetic, brown, fibrous calcite fracture cements mostly formed at temperatures between 15 ± 5°C and 19 ± 5°C, again from marine‐derived pore fluids with δ18O compositions between −0.5 ± 1.1‰ and 0.3 ± 1.2‰VSMOW. Two of these cements showed evolution to warmer temperatures and more evolved pore fluids with growth, indicating transition to deeper burial conditions. Later diagenetic, sparry calcite cements gave more variable temperatures but all indicated involvement of meteoric pore fluids. The highest clumped isotope temperature (43 ± 6°C) is within error of the 50°C regional maximum burial temperature estimate. These results are consistent with published geological and stable isotope constraints on the formation of Jurassic septarian concretions and highlight their potential as robust archives of marine benthic palaeotemperatures. Some of these results differ from clumped isotope data in an earlier study that reported higher temperatures for concretion body and early diagenetic fibrous cement fringes probably due to methodological differences.

Keywords

• carbonate cements
• clumped isotopes
• Jurassic
• palaeotemperature
• septarian concretions