Abstract
AbstractSeptarian 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.
Highlights
There is considerable interest in the application of clumped isotope palaeothermometry for reconstructing temperatures from carbonate rocks and fossils from a range of Earth surface palaeoenvironmental and diagenetic settings (Eiler, 2007, 2011; Ghosh et al, 2006; Huntington et al, 2011)
This study was followed by clumped isotope palaeothermometry on septarian concretions (Dale et al, 2014; Loyd et al, 2014) of various ages, attractive targets because a single decimetre-sized concretion can contain a variety of carbonate cements that record changes in the geochemical environment and temperature from initial formation through progressive burial in sedimentary basins over millions of years
New clumped isotope data from British Jurassic septarian concretions demonstrate: 1. That earliest-formed concretion body calcites mostly precipitated at temperatures between 9 ± 5°C and 18 ± 5°C, from fluids with δ18O values between 0.2 ± 1.1‰ and −2.2 ± 1.1‰Vienna-standard mean ocean water (VSMOW)
Summary
There is considerable interest in the application of clumped isotope palaeothermometry for reconstructing temperatures from carbonate rocks and fossils from a range of Earth surface palaeoenvironmental and diagenetic settings (Eiler, 2007, 2011; Ghosh et al, 2006; Huntington et al, 2011). This study was followed by clumped isotope palaeothermometry on septarian concretions (Dale et al, 2014; Loyd et al, 2014) of various ages, attractive targets because a single decimetre-sized concretion can contain a variety of carbonate cements that record changes in the geochemical environment and temperature from initial formation through progressive burial in sedimentary basins over millions of years. Both studies reconstructed cementation temperatures and the fluid types involved in concretion cementation. Clumped isotope data show that negative δ18O values in sparry calcite cements filling septarian fractures may not indicate meteoric water input (Dale et al, 2014), they often do (Loyd et al, 2014)
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