Extraction of dissolved organic carbon from carbonate-rich lake sediments

Background/Question/Methods

Dissolved organic carbon (DOC) is the largest pool of reduced carbon for most lakes. Shallow lakes are important in C cycling but it is uncertain if they function as sources or sinks in the carbon cycle and even less is understood about DOC dynamics in this context. We have been investigating DOC burial in lake sediments mediated by calcium carbonate precipitation in hard water shallow lakes in western Minnesota. We performed extractions on a sediment core from Jennie Lake, a shallow alkaline lake with high carbonate burial rates, in order to liberate DOC from the carbonate matrix. We used deionized water to access the loosely-bound fraction and hydrochloric acid (HCl) to oxidize the carbonate and release DOC bound within the matrix. The sediment core was partitioned into 2-cm increments and samples were added to water and HCl separately before being placed on a shaker table. 

Results/Conclusions

The amount of extractable DOC rapidly decreased from the surface sediments to 4 cm deep in the water and HCl treatments with a 36.0% and 38.2% reduction respectively. At each depth interval the average amount of DOC extracted was higher in HCl than deionized water, which suggests that a fraction of the extractable DOC pool can only be released under very acidic conditions. However, at approximately 20 cm deep the amount of DOC being extracted remained relatively uniform down to 40 cm. SUVA₂₅₄ values from the deionized water extractions were positively correlated with depth (R² = 0.916) and increased linearly, suggesting that the extractable DOC becomes more recalcitrant with depth and age if we assume consistent sediment accumulation on an annual timescale. This assumption correlates with the C:N ratio, which was also found to increase with depth indicating a preferential decomposition of the more labile fraction of DOC. Our findings suggest that carbonate precipitation may act as an important process for the sequestration and burial of DOC.