Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale

TL;DR

This study uses $^{81}$Kr and noble gases to date groundwater in the Baltic Artesian Basin, revealing complex mixing of water sources and flow dynamics over the past million years, including glacial influences.

Contribution

It introduces a novel application of $^{81}$Kr and noble gases to characterize and date deep groundwater and brines over geological timescales in the Baltic Artesian Basin.

Findings

Groundwater ages range from 300,000 to over 1.3 million years.

The system is controlled by mixing of meteoric water, glacial meltwater, and ancient brine.

Brine originates from evaporated seawater modified by water-rock interactions.

Abstract

Analyses for $^{81}$Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water $(\delta^{18}\text{O} \approx -10.4\unicode{x2030})$ with a poorly evolved chemical and noble gas signature, glacial meltwater $(\delta^{18}\text{O} \leq -18\unicode{x2030})$ with elevated noble gas concentrations, and an old, high-salinity brine component $(\delta^{18}\text{O} \geq -4.5\unicode{x2030}, \geq 90 \text{g Cl}^{-}/\text{L})$ with strongly depleted atmospheric noble gas concentrations. The $^{81}$Kr measurements are interpreted within this mixing framework to estimate the age of the end-members. Deconvoluted $^{81}$Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA 3 instrument of 1.3 Ma. The radiogenic noble gas components $^{4}$He* and $^{40}$Ar* are less conclusive but also support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system.