Carbon evolution and mixing effects on groundwater age calculations in fractured basalt, southwestern Idaho, U.S.A

Authors: SCHLEGEL, MELISSA - Idaho State University; SOUZA, JENNIFER - Idaho State University; WARIX, SARA - Idaho State University; MURRAY, ERIN - Us Geological Survey (USGS); GODSEY, SARAH - Idaho State University; SEYFRIED, MARK - Retired ARS Employee; Cram, Zane; LOHSE, KATHLEEN - Idaho State University

Submitted to: Frontiers in Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2024
Publication Date: 7/15/2024
Citation: Schlegel, M., Souza, J., Warix, S., Murray, E., Godsey, S., Seyfried, M.S., Cram, Z.K., Lohse, K. 2024. Carbon evolution and mixing effects on groundwater age calculations in fractured basalt, southwestern Idaho, U.S.A. Frontiers in Water. 6. Article 1388465. https://doi.org/10.3389/frwa.2024.1388465.
DOI: https://doi.org/10.3389/frwa.2024.1388465

Interpretive Summary: Carbon cycling in groundwater is an important part the global carbon cycle, as it plays a crucial role in mineral weathering and carbon storage, especially in silicate minerals. Though silicate rocks account for 41% of global land area, few studies have investigated the role of groundwater in carbon cycling in fractured basalts. Determining rates of these processes requires an understanding of carbon evolution and age tracers such as 14C. Using isotopic compositions from 6 springs and 10 wells, we trace carbon and carbon dioxide (CO2) into groundwater of the semiarid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Findings show that carbon systems can change in conjunction with soil and climate fluctuations, and that care should be taken when interpreting carbon evolution over time. Characterizing the deep groundwater in a semiarid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling and is significant in understanding global carbon cycles.

Technical Abstract: Using hydrochemical and isotopic compositions of springs and wells, we trace carbon from critical zone carbon dioxide (CO2) into groundwater of the semiarid Reynolds Creek Experimental Watershed - Critical Zone Observatory, southwestern Idaho, USA. Dissolved inorganic carbon (DIC) concentrations, pH and stable isotope tracers of carbon for DIC (d13C-DIC), are used to show that most groundwater evolves under open system conditions, moving carbon into the groundwater and acting as a carbon sink. However, one sample (-10.94‰ d13C-DIC, 6,350 14C years before present (yrs. BP)) may have evolved under closed system conditions with a higher partial pressure of critical zone CO2 than present-day soils. By characterizing the carbon cycle, we show that: (1) carbon evolution is primarily under open-system condition; (2) shallow groundwater samples are generally less mixed and more recent (10 to 70 3H yrs. BP) than deeper groundwater samples (1,469 to 6,350 14C yrs. BP); and (3) the older portion of the groundwater may be even older than the calculated 14C ages, as indicated by the mixing of age tracers in intermeiate wells. Our global conception of the deep critical zone should include carbon cycling of critical zone CO2 in old groundwater. Characterizing the deep critical zone in a semiarid weathered silicate watershed improves our global understanding of carbon, nutrient and water cycling.