A comparison of lignin and stable carbon isotope compositions in quaternary marine sediments

Abstract

Organic matter in four Quaternary sediment cores from the Gulf of Mexico and one core from the Washington State coast have been analyzed for lignin and stable carbon isotope compositions. Holocene sequences of all five cores contain organic matter with high relative abundances of 13C ( δ 13 C = −19.0 to −22.5% versus PDB ) and low lignin concentrations, both of which are consistent with a marine origin. Distinctly lower 13C concentrations ( δ 13 C = −24.0 to −25.5% ) occur in underlying glacial-age sequences from four of the five cores, including the core from the Washington coast where such trends are previously unreported. Although the carbon isotopic compositions of these Pleistocene sediments are typical of predominantly land-derived organic matter, they contain only about 5% of the lignin found in modern sediments of similar δ 13 C from adjacent continental shelves. The lignin-poor organic matter in the glacial-age deposits appears to be either marine-derived or terrigenous material that likely was depleted in vascular plant debris at the time of deposition.