Article in Earth and Planetary Science Letters

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, 2022. Boron concentrations and isotopic compositions in methane-derived authigenic carbonates: Constraints and limitations in reconstructing formation conditions. Earth and Planetary Science Letters: 579, 117337.

https://doi.org/10.1016/j.epsl.2021.117337

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.

Fig. 1. (a) A map showing the locations of the seep sites investigated. CFC and MC stand for cavity-filling cement and micritic cement, respectively. Sample 23C-M is a consolidated siliciclastic sediment sample from Vestnesa Ridge. (b)-(e) Photos showing the locations where the micro-drilled samples were obtained from each of the four MDAC slabs. Dotted squares mark the sampling locations for MCs while the solid squares mark those for the CFCs. The carbonate cements, though appearing at adjacent locations, are not necessary formed under the same fluid seepage episode. The white bars are one centimetre. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)