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

doi: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.

Highlights

Cold seeps are common seafloor features along both passive and active continental margins (Suess, 2014)
Note that the investigated methane-derived authigenic carbonate (MDAC) formed in seep environments experiencing several fluid flow events during the past 42 to 156 kyr (Himmler et al, 2019), so while the modern fluids serve as a guide for the boron systematics in such settings, they do not necessarily reflect the fluid composition at the time of MDAC formation
Through a newly developed sequential leaching protocol, we isolate boron signals from MDACs to constrain the fluid condition during carbonate precipitation in three cold seeps along the Norwegian margin

Summary

Introduction

Cold seeps are common seafloor features along both passive and active continental margins (Suess, 2014). The seepage and consumption of methane through anaerobic and aerobic microbial activities make cold seeps oases for life in nutrient-limited pelagic environments (Boetius et al, 2000). Precipitation of authigenic minerals in cold seeps, such as carbonates and sulphides, has been suggested to influence the global carbon and sulphur cycles (Reeburgh, 2007). Methane-derived authigenic carbonates (MDACs) are commonly found from cold seeps (Loyd et al, 2016; Crémière et al, 2016b; Thiagarajan et al, 2020) where intensive anaerobic oxidation of methane (AOM) (Boetius et al, 2000) increases alkalinity (Eq (1)) and stimulates carbonate precipitation with seawater calcium (Eq (2); Luff and Wallmann, 2003): AOM : CH4 + SO24− → HCO−3 + HS− + H2O (1).

Methods

Results

Conclusion