Examining uncertainties in major-ion reconstructions of seawater from halite fluid inclusions

Abstract

Fluid inclusions of evaporitic brines within halite deposits are one of our most valuable records for reconstructing seawater composition over geological timescales. They have been used to generate estimates of major element and even some trace element concentrations in seawater over time; these reconstructions are widely used as constraints for a variety of applications. The uncertainties in this method of paleoseawater reconstruction are well known to those who perform the measurements and generate the reconstructions, but are not commonly appreciated outside of the community that works on halite fluid inclusions. This contribution seeks to critically examine those uncertainties and highlight the artifacts that exist within familiar seawater reconstructions. The greatest uncertainties stem from the assumption that gypsum saturation has remained constant, which affects reconstructed calcium and sulfate concentrations. The reconstructed magnesium record also shows unrealistic levels of scatter and is likely unreliable, and this scatter is transferred to reconstructed sodium concentrations because of how the requirement of charge balance is handled. Independent proxy constraints on aspects of seawater chemistry are subject to their own assumptions and uncertainties and are generally not precise enough to offer robust alternative reconstructions. However, an alternative approach that uses the fluid inclusion record in a more limited way, along with calibrated stoichiometric relationships among major ions over the Neogene, shows some promise and should be explored further. It is possible that future work combining the most robust features of the fluid inclusion record, new independent constraints on major ion concentrations, and reasonable ion-exchange reactions or reaction networks could be used to develop improved estimates for major-ion concentrations in seawater over Earth history. Despite the weaknesses of the fluid inclusion archive, it remains at present the source of our best available constraints for the secular evolution of seawater chemistry, but it should also be used carefully and with awareness of its inherent uncertainties.