Brackish Conditions Revealed by Clumped Isotope Thermometry of Bioclastic Carbonates and Early Concretions, Barton Clay, UK

Carbonate concretions in clastic rocks and their associated septarian fracture fills can act as ';time capsules' for diagenetic processes, as they record changes in pore fluid chemistry during their growth. The septarian fractures and concretion matrix of near-shore marine concretions commonly have δ¹⁸O values lighter than would be expected for purely marine water, a phenomenon often explained by precipitation from mixed meteoric-marine water. However, the light δ¹⁸O may also be due to diagenetic reactions or precipitation at higher temperatures during burial. Even in formations that have only undergone shallow burial (<50 m) it can be difficult to distinguish the cause. Here, we apply the carbonate clumped isotope palaeothermometer to well-preserved aragonitic gastropods, calcite septarian fracture fills and matrix cement of concretions from the Upper Eocene Barton Clay, UK, a formation that has not undergone significant burial. The results indicate precipitation at low temperatures and we use these to back-calculate the parent pore fluid δ¹⁸O during the concretion and fracture fill precipitation, as well as isolating the marine δ¹⁸O composition using the gastropods. Initial results indicate that the back-calculated δ¹⁸O composition of the marine fluid (-2.9‰) is similar to the parent fluid of some septarian fracture fills (-3.1‰). Therefore, at least in the Barton Clay, the concretions were very early features and their δ¹⁸O appears to reflect the δ¹⁸O depletion of near-shore brackish waters, which may be explained by freshwater input into the coastal waters. We gratefully acknowledge a BP-EPSRC Case Studentship for funding this project.