Boron concentrations and isotopic compositions in methane-derived authigenic carbonates: constraints and limitations in reconstructing formation conditions

Wei Li Hong*, Aivo Lepland, Kalle Kirsimäe, Antoine Crémière, James W.B. Rae

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The boron content and isotopic composition (δ11B), of marine carbonates have the potential to constrain CO2 chemistry during carbonate growth conditions. However, obtaining and interpreting boron compositions from authigenic carbonates in geological archives present several challenges that may substantially limit their application. In particular, contamination from non-carbonate phases during sample preparation must be carefully avoided, and a variety of controls on boron composition during authigenic growth conditions must be evaluated. To advance understanding of the use and limitations of boron in authigenic carbonates, we present data and modelling results on methane-derived authigenic carbonate (MDAC), a by-product of microbially mediated anaerobic oxidation of methane, taken from three cold seep sites along the Norwegian margin. We present a novel sequential leaching method to isolate the boron signals from the micritic (Mg-calcite) and cavity-filling (aragonitic) MDAC cements in these complex multi-phase samples. This method successfully minimizes contamination from non-carbonate phases. To investigate the factors that could potentially contribute to the observed boron signals, we construct a numerical model to simulate the evolution of MDAC δ11B and B/Ca ratios over its growth history. We show that diagenetic fluid composition, depths of precipitation, the physical properties of sediments (such as porosity), and mineral surface kinetics all contribute to the observed boron compositions in the different carbonate cements. While broad constraints may be placed on fluid composition, the multiple competing controls on boron in these diagenetic settings limit the ability to place unique solutions on fluid CO2 chemistry using boron in these authigenic carbonates.

Original languageEnglish
Article number117337
Number of pages10
JournalEarth and Planetary Science Letters
Volume579
Early online date23 Dec 2021
DOIs
Publication statusPublished - 1 Feb 2022

Keywords

  • Boron
  • Early diagenesis
  • Methane-derived authigenic carbonate
  • Transport-reaction modelling

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