Biogeochemical processes captured by carbon isotopes in redox-stratified water columns: a comparative study of four modern stratified lakes along an alkalinity gradient

Abstract

Redox-stratified water columns are a prevalent feature of the Earth's history, and ongoing environmental changes tend to promote a resurgence of such settings. Studying modern redox-stratified environments has improved our understanding of biogeochemical processes and element cycling in such water columns. These settings are associated with peculiar carbon biogeochemical cycling, owing to a layered distribution of biological processes in relation to oxidant availability. Metabolisms from distinct biogeochemical layers are diverse and may differently imprint the sedimentological record. Paired carbon isotope compositions of organic matter and carbonates, which are commonly used to characterize these ecological dynamics, can thus vary from one stratified environment to another. Changes in the organic/inorganic carbon sources and mass balance can further complicate the isotopic message in stratified environments. Better understanding of these multifaceted carbon isotope signals requires further evaluation of how the processes occurring in redox-stratified water columns are transferred to the sediments. We therefore characterized and compared the isotopic signatures of dissolved inorganic carbon (DIC), carbonate, and organic matter reservoirs at different depths in the water column and upper sediments of four stratified Mexican lakes that follow a gradient of alkalinity and salinity. Comparing these systems shows strong diversity in the carbon isotope signals of the water column and sediments. Differences in inorganic carbon isotope signatures arise primarily from the size of the DIC reservoir, buffering the expression of redox-dependent biological processes as alkalinity increases. Combining this isotopic dataset with water column physicochemical parameters allows us to identify oxygenic 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 column layers in the four lakes, highlighting the ecological variability that can stem from different stratified water columns and how it is transferred or not to the sedimentary record. The least alkaline lake shows higher isotopic variability and signatures typical of methanogenesis in the sediment porewaters. This metabolism, however, does not leave diagnostic isotopic signatures in the sedimentary archives (organic matter and carbonates), underlining the fact that even when alkalinity does not strongly buffer the inorganic carbon reservoir, a comprehensive picture of the active biogeochemical carbon cycling is not necessarily transferred to the geological record.