Biogeochemical processes captured by carbon isotopes inredox-stratifiedwater columns: a comparative study of four modern stratified lakes along analkalinity gradient

Redox-stratified water columns are a prevalent feature of the Earth'shistory, and ongoing environmental changes tend to promote a resurgence ofsuch settings. Studying modern redox-stratified environments has improvedour understanding of biogeochemical processes and element cycling in suchwater columns. These settings are associated with peculiar carbonbiogeochemical cycling, owing to a layered distribution of biologicalprocesses in relation to oxidant availability. Metabolisms from distinctbiogeochemical layers are diverse and may differently imprint thesedimentological record. Paired carbon isotope compositions of organicmatter and carbonates, which are commonly used to characterize theseecological dynamics, can thus vary from one stratified environment toanother. Changes in the organic/inorganic carbon sources and mass balancecan further complicate the isotopic message in stratified environments.Better understanding of these multifaceted carbon isotope signals requiresfurther evaluation of how the processes occurring in redox-stratified watercolumns are transferred to the sediments. We therefore characterized andcompared the isotopic signatures of dissolved inorganic carbon (DIC),carbonate, and organic matter reservoirs at different depths in the watercolumn and upper sediments of four stratified Mexican lakes that follow agradient of alkalinity and salinity. Comparing these systems shows strongdiversity in the carbon isotope signals of the water column and sediments.Differences in inorganic carbon isotope signatures arise primarily from thesize of the DIC reservoir, buffering the expression of redox-dependentbiological processes as alkalinity increases. Combining this isotopicdataset with water column physicochemical parameters allows us to identifyoxygenic photosynthesis and aerobic respiration in the four lakes studied,while anoxygenic photosynthesis is evidenced in only two of them.Sedimentary organic matter does not originate from the same water columnlayers in the four lakes, highlighting the ecological variability that canstem from different stratified water columns and how it is transferred ornot to the sedimentary record. The least alkaline lake shows higherisotopicvariability and signatures typical of methanogenesis in the sedimentporewaters. This metabolism, however, does not leave diagnostic isotopicsignatures in the sedimentary archives (organic matter and carbonates),underlining the fact that even when alkalinity does not strongly buffer theinorganic carbon reservoir, a comprehensive picture of the activebiogeochemical carbon cycling is not necessarily transferred to thegeological record.