Young, Old, and Weathered Carbon-Part 1: Using Radiocarbon and Stable Isotopes to Identify Carbon Sources in an Alkaline, Humic Lake

Evelyn M Keaveney,Paula J Reimer,Robert H Foy

doi:10.2458/azu_rc.57.18354

Evelyn M Keaveney, Paula J Reimer + Show 1 more

Open Access

https://doi.org/10.2458/azu_rc.57.18354

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Abstract

This article presents a case study of Lower Lough Erne, a humic, alkaline lake in northwest Ireland, and uses the radiocarbon method to determine the source and age of carbon to establish whether terrestrial carbon is utilized by heterotrophic organisms or buried in sediment. Stepped combustion was used to estimate the degree of the burial of terrestrial carbon in surface sediment. Δ14C, δ13C, and δ15N values were measured for phytoplankton, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). Δ14C values were used to indicate the presence of different sources of carbon, including bedrock-derived inorganic carbon, “modern,” “recent,” “subsurface,” and “subfossil” terrestrial carbon in the lake. The use of14Cin conjunction with novel methods (e.g. stepped combustion) allows the determination of the pathway of terrestrial carbon in the system, which has implications for regional and global carbon cycling.

Highlights

The structure and function of aquatic ecosystems are strongly influenced by carbon imported from terrestrial catchments
Stepped combustion was used to estimate the degree of the burial of terrestrial carbon in surface sediment. ∆14C, δ13C, and δ15N values were measured for phytoplankton, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). ∆14C values were used to indicate the presence of different sources of carbon, including bedrock-derived inorganic carbon, “modern,” “recent,” “subsurface,” and “subfossil” terrestrial carbon in the lake
The ∆14C values of DIC, DOC, and POC collected from Lower Lough Erne in February indicate that there is a significant difference in age of carbon among these three pools: enriched DOC, depleted POC, and intermediate DIC values

Summary

Introduction

The structure and function of aquatic ecosystems are strongly influenced by carbon imported from terrestrial catchments. Temperature, and greater mobility of DOC lead to higher export of catchment terrestrial carbon (Vuorenmaa et al 2006). Significant loading of terrestrial carbon (TC) metabolized in the food web leads to increased pCO2 values and elevated CO2 flux to the atmosphere (Gupta et al 2008), while a lengthened photoperiod can increase photomineralization of DOC and elevate bacterial production (Kritzberg et al 2006; Koehler et al 2014). Diagenesis of carbon can lead to decreased nutrient quality, determining the likelihood of metabolization in the food web or alternatively burial in sediment. Autotrophic Production It has been demonstrated that δ13C values of aquatic photosynthetic plants and plankton utilizing inorganic carbon in lakes are variable due to differences in carbon availability and kinetic fractionation (Farquhar et al 1989; McConnaughey et al 1997). Isotopic food web studies require separation between different sources, but baseline autotrophic δ13C signals are sufficiently variable to overlap with other end members (Ishikawa et al 2013)

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