Quantification and characterization of dissolved organic carbon and iron in sedimentary porewater from Green Bay, WI, USA

Abstract

Both dissolved organic carbon (DOC) and iron play an important role in biogeochemical processes in lacustrine benthic environments. Moreover, recent evidence has shown that both substances can act as active reductants in the redox transformation of organic pollutants. This paper examines the nature and abundance of DOC and dissolved ferrous iron (FeII) in porewaters from a sediment core collected in Green Bay, WI, USA. The concentration of dissolved FeII and the abundance, absorbance at 280 nm (A280 nm), molar absorptivities (e280 nm), molecular weights, and polydispersities of DOC were measured as a function of depth in porewaters. Dissolved FeII concentrations increased from 3.6 μM near the sediment–water interface to 163 μM at a depth of 11 cm, then gradually declined. The DOC distribution varied with sediment depth, with the greatest variation in porewater DOC content and properties occurring in the transitional zone between oxic and suboxic conditions. The down-core porewater DOC profile was characterized by an increase in DOC concentration with depth from 0.64 mM OC at 1 cm to 1.23 mM OC at 13 cm, below which it remained relatively constant. A strong correlation was observed between FeII and DOC concentrations, suggesting that these constituents co-accumulate in these porewaters. The correlation between the DOC concentration of the porewaters and A280 nm was significant, making this parameter a good predictor for DOC concentrations in these waters. The molecular weight distributions of the porewater DOC were primarily monomodal, with relatively low polydispersivities. Weight-average molecular weights ranged from 1505 to 1949 Da. This data set is unique in that it is the first detailed study of a relatively highly resolved DOC profile of benthic porewater in surficial sediment from the Laurentian Great Lakes.