Reply on RC3

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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&rsquo;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 &plusmn; 4 mM; 1SD, n=28; representing from ~ 15 to 160 times the amount of POC). &delta;¹³C_DOC signatures also span a broad range of values from -29.3 to -8.7 &permil; (with as much as 12.5 &permil; 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 &delta;¹³C_DOC 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&rsquo;s past oceans.