Temporal variability in terrestrially-derived sources of particulate organic carbon in the lower Mississippi River and its upper tributaries

A lack of correlation between POC and lignin phenol abundances (Λ8) was likely due to dilution effects from autochthonous production in the river, which has been shown to be considerably higher than previously expected. The range of δ13C values for p-hydroxycinnamic and ferulic acids in POC in the lower river do support that POM in the lower river does have a significant component of C4 in addition to C3 source materials. A strong correlation between δ13C values of p-hydroxycinnamic, ferulic, and vanillyl phenols suggests a consistent input of C3 and C4 carbon to POC lignin while a lack of correlation between these same phenols and POC bulk δ13C further indicates the considerable role of autochthonous carbon in the lower MR POC budget. Our estimates indicate an annual flux of POC of 9.3 × 108 kg y−1 to the Gulf of Mexico. Total lignin fluxes, based on Λ8 values of POC, were estimated to be 1.2 × 105 kg y−1. If we include the total dissolved organic carbon (DOC) flux (3.1 × 109 kg y−1) reported by [Bianchi T. S., Filley T., Dria K. and Hatcher, P. (2004) Temporal variability in sources of dissolved organic carbon in the lower Mississippi River. Geochim. Cosmochim. Acta 68, 959-967.], we get a total organic carbon flux of 4.0 × 109 kg y−1. This represents 0.82% of the annual total organic carbon supplied to the oceans by rivers (4.9 × 1011 kg).