Partitioning of organic matter in continental margin sediments among density fractions

Hydrodynamic processes sort and redistribute organic matter (OM) and minerals on continental margins. Density fractionations were conducted on sediments from diverse margins (Mexico margin, Gulf of Mexico, Mississippi River delta, Eel River margin) to investigate the nature, provenance and age of OM among density fractions. Mass. elemental (C and N), lignin, and surface area distributions, as well as stable carbon and radiocarbon isotopic compositions were measured, The lowest density fractions (<1.6 g cm(-3)) contained the highest organic carbon (OC) (up to 45%) and lignin concentrations (up to 8 mg g(-1)) due to abundant woody debris, whereas high density fractions (>2.5 g cm(-3)) were OC-poor (%OC < 0.5) mineral material. Most sediment mass was found in the mesodensity fractions (1.6 to 2.5 g cm(-3)) that contained the highest proportion of OC (up to similar to 75%) for each sediment. Stable carbon isotope compositions (delta C-13 similar to 25.5 parts per thousand to -22.9 parts per thousand) show terrigenous OC as a significant component of density isolates from the river-dominated sediments (Gulf of Mexico, Mississippi River, and Eel margin), whereas the Mexico margin, least influenced by riverine input, was dominated by autochthonous marine OC (delta C-13 similar to - 21.5 parts per thousand). Radiocarbon compositions of density fractions indicate significant pre-aged OC (Delta C-14 as low as - 900 parts per thousand) in river-influenced sediments but not on the Mexico margin (Delta C-14 > -200 parts per thousand). Ratios of vanillic acid to vanillin (Ad/Al)v among lignin oxidation products increase with increasing particle density suggesting variable lignin sources or selective degradation of lignin among the different density fractions. (C) 2009 Elsevier B.V. All rights reserved.