Preprints
https://doi.org/10.5194/egusphere-2023-23
https://doi.org/10.5194/egusphere-2023-23
 
12 Jan 2023
12 Jan 2023
Status: this preprint is open for discussion.

The hidden role of dissolved organic carbon in the biogeochemical cycle of carbon in modern redox-stratified lakes

Robin Havas1, Christophe Thomazo1,2, Miguel Iniesto3, Didier Jézéquel4, David Moreira3, Rosaluz Tavera5, Jeanne Caumartin6, Elodie Muller6, Purificación López-García3, and Karim Benzerara6 Robin Havas et al.
  • 1Biogéosciences, CNRS, Université de Bourgogne Franche-Comté, 21 000 Dijon, France
  • 2Institut Universitaire de France, 75005 Paris, France
  • 3Ecologie Systématique Evolution, CNRS, Université Paris-Saclay, AgroParisTech, 91190 Gif-sur-Yvette, France
  • 4IPGP, CNRS, Université Paris Cité, 75005 Paris, and UMR CARRTEL, INRAE & USMB, France
  • 5Departamento de Ecología y Recursos Naturales, Universidad Nacional Autónoma de México, México
  • 6Sorbonne Université, Muséum National d'Histoire Naturelle, CNRS, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), 75005 Paris, France

Abstract. The dissolved organic carbon (DOC) reservoir holds a critical role in the C cycle of marine and fresh water environments because of its large size and involvement in many biogeochemical reactions. Despite poor constraints, its importance in ancient Earth’s C cycles is also commonly invoked. However, DOC remains rarely quantified and characterized in modern stratified analogs. Here, we investigated the DOC reservoirs of four redox-stratified alkaline crater lakes from Mexico. To achieve this, we analyzed the concentrations and isotopic compositions of DOC throughout the four water columns and compared them with existing data on dissolved inorganic and particulate organic C reservoirs (DIC and POC). The four lakes have high DOC concentrations with important variability between and within the lakes (averaging 2 ± 4 mM; 1SD, n=28; representing from ~ 15 to 160 times the amount of POC). δ13CDOC signatures also span a broad range of values from -29.3 to -8.7 ‰ (with as much as 12.5 ‰ variation within a single lake). The prominent DOC peaks (up to 21 mM), together with their associated isotopic variability, are interpreted to reflect oxygenic and/or anoxygenic primary productivity through the release of excess fixed-carbon in three of the lakes (Atexcac, La Preciosa and La Alberca de los Espinos). By contrast, the variability of [DOC] and δ13CDOC in Lake Alchichica was mainly explained by partial degradation of organic matter and accumulation of DOC in anoxic waters. Overall, DOC records detailed metabolic functioning such as active DIC-uptake and DIC-concentrating mechanism that cannot be inferred from DIC and POC analyses alone but that are critical to understand carbon fluxes from the environment to the biomass. Extrapolating our results to the geological record, we suggest that anaerobic oxidation of DOC may have caused the very negative C isotope excursions in the Neoproterozoic, but it is unlikely that a large oceanic DOC reservoir overweight the associated DIC reservoir. Overall, this study shows how the analysis of DOC in modern lakes deepens our understanding of the C cycle in stratified environments and how it can help to size boundary conditions to the Earth’s past oceans.

Robin Havas et al.

Status: open (until 23 Feb 2023)

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  • RC1: 'Comment on egusphere-2023-23', Anonymous Referee #1, 24 Jan 2023 reply

Robin Havas et al.

Robin Havas et al.

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Short summary
Dissolved organic carbon (DOC) is a reservoir of prime importance in the C cycle of both continental and marine systems. It has also been suggested to influence the past Earth climate, but is still poorly characterized in ancient Earth's-like environments. In this paper we show how DOC analyses from modern redox-stratified lakes can evidence specific metabolic reactions and environmental factors and how it can help us to interpret the C cycle of specific periods in the Earth's past.